Finding Deeply Buried Deposits Using Geochemistry

Finding deeply buried deposits using geochemistry

Eion M. Cameron1, Stewart M. Hamilton2, Matthew I. Leybourne3, Gwendy E.M. Hall4

& M. Beth McClenaghan4

1Eion Cameron Geochemical Inc., 865 Spruce Ridge Road, Carp, Ontario, K0A 1L0, Canada2Ontario Geological Survey, Sudbury, Ontario, P3E 6B5, Canada

3Department of Geosciences, University of Texas at Dallas, Richardson, TX 75083-0688, USA4Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada

ABSTRACT: It has become increasingly common for geologists to drill through100 m or more of cover in search for buried mineral deposits. Geochemistry is onetool applied to this search, using a variety of approaches, including selective leachingof soils to extract the mobile component of elements, and the measurement ofinorganic and organic gases. This paper provides an overview of some of the workcarried out by the project Deep-Penetrating Geochemistry, sponsored by theCanadian Mining Industry Research Organization (CAMIRO), and supported by26 Canadian and international companies and by the Ontario Geological Survey andthe Canadian Geological Survey. The objective was to provide the mining industrywith information relating to processes that may form anomalies at surface overburied deposits and to provide comparative data on methods used to detect theseanomalies.

Phase I of the project considered the theoretical and experimental framework forthe movement of material from deeply buried deposits to the surface; much of thisinformation has come from research on the containment of buried nuclear waste. Inarid or semi-arid terrain, with a thick vadose zone, advective transport, which is themass transfer of groundwater or air along with their dissolved or gaseousconstituents, is the only known viable means of moving elements to the surface;diffusion of ions in water or gases in air is orders of magnitude slower. Examples ofadvective transport are pumping of mineralized groundwater to the surface duringseismic activity and the extraction of air plus gas by barometric pumping. Bothmechanisms require fractured rock and the interpretation of the derived anomaliesrequires consideration of neotectonic structures. In wetter climates, where water liesclose to the surface, a variety of mechanisms have been proposed for creatinganomalies at the surface. Diffusion-based models again suffer from slow rates ofmigration. Electrochemical models show a cathodic zone at the top of a buriedsulphide conductor. Cations are attracted to the cathode, rather than to the surface,yet metals that most commonly migrate as cations are found to form anomalies at thesurface.

Phase II of the CAMIRO study involved field studies at ten test sites. The test sitesincluded buried porphyry deposits in northern Chile, a gold–copper deposit in theCarlin district of Nevada, and volcanogenic massive sulphide bodies covered byglacial sediments in the Abitibi greenstone belt of Ontario. In all cases anomalieswere found in soils above buried mineralization. It is suggested that anomalyformation is an episodic and cyclic process, in which batches of metal inwater-soluble form are introduced and the metal is then progressively incorporatedwith time into the secondary minerals of soil. Selective leaches have been developedto dissolve specific phases in the soil to detect these anomalies. We have comparedthe results for five selective leaches that are available from commercial laboratories:deionized water, ammonium acetate, hydroxylamine hydrochloride, Enzyme Leachand Mobile Metal Ion (MMI) plus one non-selective decomposition, aqua regia. Inaddition, the Institute of Geophysical and Geochemical Exploration laboratory inChina has supplied data for four sequential selective leaches: water-extractable,adsorbed, organic-bound and iron- and manganese-bound. The weakest leachesdissolve mainly the most recently introduced metals that remain in water-soluble

Geochemistry: Exploration, Environment, Analysis, Vol. 4 2004, pp. 7–32 1467-7873/04/$15.00 � 2004 AEG/Geological Society of London

form. Other leaches dissolve specific secondary minerals, such as carbonates, or ironand manganese oxides, which contain the introduced metals. The usefulness ofleaches that dissolve secondary minerals depends on the ratio of introduced(exogenic) metal that the minerals contain relative to that of endogenic origin derivedfrom the primary minerals of soils. Our results indicate that this ratio is variable fromsite to site, so that there is no universal ‘best’ leach for dissolving secondary mineralsin exploration surveys. For the test sites in Chile and Nevada, anomalies may haveformed incrementally over a period of a million years or more, which permittedmetals of exogenic origin to become incorporated into many secondary minerals. Forthese sites, some anomalies can be detected by aqua regia, although theanomaly/background contrast is less than for selective leaches. For the test sites inOntario, only a few thousand years have elapsed since glacial sediments weredeposited to conceal mineralization. Over this short period, metal of exogenic originhas been incorporated into only the most labile of secondary minerals and it is theleaches that dissolve these labile minerals that can successfully identify anomalies. Atthe two sites where the most detailed studies have been carried out, the Spencedeposit in Chile and Cross Lake near Timmins, we have found that the optimumsampling depth in soils is critical to detecting anomalies.

KEYWORDS: exploration, geochemistry, buried deposits, selective leaches, Chile, Nevada, Ontario

INTRODUCTION

The most frequently used geochemical method to identifyburied mineral deposits is selective leaching of soil samples.These leaches remove only a fraction of the metal that might bedissolved by aqua regia or by total dissolution, in the expec-tation that this fraction represents a more readily dissolved‘mobile’ phase, perhaps derived from an ore deposit. Leachesinclude the proprietary reagents, Enzyme Leach and MobileMetal Ion (MMI), and others that have been developed,principally by soil scientists, to dissolve specific secondaryminerals in soils, such as carbonates, or Fe and Mn oxides. Theleach solutions are most frequently analysed by inductivelycoupled plasma mass spectrometry (ICP-MS). This techniquehas shown progressive evolution, so that detection limits of rareelements have been lowered to levels thought unobtainable afew years ago. This has had the effect of increasing the numberof elements that can be measured with precision; now 50 ormore are being reported. This benefit also presents a challenge.With 50 elements at hand and the many ratios that may bederived from these primary data, it is not too difficult to find an‘anomaly’ over every known deeply buried deposit. The easewith which data may be obtained has led to the widespreadapplication of selective leaches, with some successes, but many‘dry’ holes and a poor understanding of the processes that mightbe involved in the creation of real or false anomalies.

This uncertainty caused a number of companies to proposea scoping study to the Canadian Mining Industry ResearchOrganization (CAMIRO) to see how they might better under-stand the processes that have caused anomalies to form abovedeeply buried deposits. This scoping study, Phase I of thisstudy, was carried out by Cameron over a six-month periodfrom October 1997. It was generously supported by 28companies, both Canadian and international. Several of thesecompanies provided data from their own orientation studiesover known deposits.

During the development of selective leach geochemistry,ideas have emerged about processes for the dispersion ofelements from buried targets, the most prominent being elec-trochemical dispersion. Research on dispersion mechanisms bythe exploration community, including researchers in universities

and government laboratories, has been limited by the availablefunding. In contrast, over the past two decades, tens or possiblyhundreds of millions of dollars have been spent examiningprocesses that might cause dispersion of toxic materials fromburied nuclear waste. These processes are entirely analogous tothose that might be involved in the dispersion of metals fromburied mineral deposits. Moreover, a substantial portion of thenuclear studies in the United States have been carried out in thearid terrain of the Southwest, similar to regions wherecompanies are targeting buried deposits.

The results of the Phase I scoping study encouraged afull-scale study to test methods and better understand pro-cesses. This was launched in 1999 as Deep-Penetrating Geo-chemistry, Phase II. The project was again sponsored byCAMIRO and supported by 26 Canadian and internationalcompanies. Test sites were selected in three regions, wherecompanies are actively exploring for deeply buried deposits: theAbitibi greenstone belt of Ontario, where thick glacial sedi-ments cover prospective rocks; the Carlin belt of Nevada,where the most productive cluster of Au deposits in NorthAmerica has prospective areas of basement covered by theCarlin Formation; and thirdly, northern Chile, where basementrocks containing porphyry mineralization are covered by thickpiedmont gravels. We looked at ten sites. In this account we willdiscuss five: (a) the Spence deposit, Chile, where 400 Mt of 1%Cu is covered by piedmont gravel; (b) the Gaby Sur deposit,Chile, where 400 Mt of 0.54% Cu is also covered by piedmontgravels; (c) the Mansa Mina deposit, where piedmont gravelsconceal 325 Mt of 1% Cu; (d) the Mike deposit, Nevada, where240 m of Carlin Formation conceals a copper–gold deposit; and(e) the Cross Lake prospect near Timmins where Zn-rich VMSmineralization of Archean age is covered by 30 to 50 m ofglaciolacustrine clay and sand.

For the Phase II studies Stew Hamilton of the OntarioGeological Survey (OGS) assumed the major role of workingon the Abitibi sites and came with substantial funding from thatorganization that permitted drilling of the overburden. BethMcClenaghan of the Geological Survey of Canada providedexpertise as a Quaternary geologist. Before the study com-menced, it was apparent that groundwater might have played animportant role in the formation of surface anomalies in

E.M. Cameron et al.8

northern Chile. Matthew Leybourne of the University of Texasat Dallas led the studies of groundwater geochemistry. MaryDoherty of BHP Minerals and International GeochemicalConsultants participated in the studies at Mike. The projectinvolved comparing different leaches, and Gwendy Hall of theGeological Survey of Canada took responsibility for theanalytical programme. We were fortunate to get the support ofall major commercial analysts in Canada, who provided analysesgratis and also communicated much of their knowledge: JohnGravel of Acme Laboratories, Eric Hoffman of Actlabs, BrendaCaughlin and Pat Highsmith of ALS-Chemex, Claude Massie ofBondar-Clegg, Robert Ellis of Gedex and Hugh De Souza ofXRAL. Moreover, Xie Xuejing of the Institute of Geophysicaland Geochemical Exploration (IGGE) in China providedanalyses by methods typically used in that country. Thus, formany of the samples there are nine sets of analyses to compare.

Starting in 1999, 38 reports on the Phase II project werereleased to sponsors on topics as different as biogeochemistry,hydrology and Pb isotopes at the Abitibi sites, stable isotopes ingroundwater and soils from Chile and soil gas tests in Nevada.This article touches on only a few of these topics and includesonly information for which a two-year confidentiality limit hasexpired.

PROCESSES FOR THE VERTICAL MIGRATION OFELEMENTS

Migration of dissolved solids in water

Solids dissolved in water may migrate within the water mass –diffusion – or may migrate as part of a moving water mass –advection. Diffusion, the first of these to be discussed, mayoccur as a result of a chemical or electrical gradients.

Diffusion through the vadose zone. For all but the wettest of areas,vertical transport of elements from a buried deposit to thesurface must include passage through the vadose or undersatu-rated zone. Except for hyper-arid climates, the vadose zone isnot dry. There is a film of water around mineral grains of therock. This film of water is in downward motion and serves torecharge to groundwater the water from precipitation remainingafter evaporation and run-off. The rate at which this water filmmoves can be estimated by measuring depth profiles ofelements and isotopes in the water film extracted from drillcore. Since it is a conservative element, Cl is widely used. Raincontains Cl, the content of which, per volume of precipitation,is relatively constant over time for a given area. With increasedaridity and therefore evapotranspiration, the Cl content of soilwater increases and the downward flux of moisture rechargingto the groundwater decreases.

Figure 1 shows data from core of the top of the unsaturatedzone from the Hueco Bolson region, near El Paso, Texas. Thisalluvial plain slopes 1 to 1.5% to the Rio Grande. Creosote andmesquite are common and root to 1 to 5 m depths. Meanannual precipitation is 28 cm. The unsaturated zone comprises0 to 15 m of silty to gravel loam, underlain by 140 m of claywith interbedded sand and silt. A discontinuous layer of calicheoccurs at 1 to 2 m. Chloride was measured after addingdeionized water to dried core samples. The content is low fromthe surface to 0.3 m, the result of downward leaching by rain.Below this, Cl increases as a result of evapotranspiration,forming a bulge with maximum concentrations at depths of 1.3to 4.6 m for different cores. This bulge occurs throughout thesouthwestern United States, just below the root zone (Phillips1994). It corresponds to a minimum in the moisture flux, i.e. amaximum in evapotranspiration, which represents a cessation

of recharge to the water table about 13,000 to 15,000 years ago,when the climate began to shift from cooler and wetter tothe current arid conditions. When precipitation is greater,profiles move slowly downwards as water and Cl are addedincrementally at the top, thus providing a view of changingmoisture fluxes with time. The 15 m profile shown is esti-mated to represent 30,000 years. Below the bulge, thedecreasing Cl contents indicate an increasing moisture fluxprior to 15,000 years.

Mathematical methods of relating Cl concentration tomoisture flux are given in Scanlon (1991). An advection–diffusion equation can be fitted to the data to evaluate therelative importance of these two factors on the mobility of Cl.The very large concentration gradients represented by the bulgeprovide a driving force for diffusion, which will tend to smooththe bulge. Above the peak in the bulge, the upward diffusivefluxes of chloride are 10�3 to 10�4 mm a�1. Below the peak,the downward fluxes are 10�3 to 10�5 mm a�1. The down-ward advective flux is 10�1 to 10�2 mm a�1, or two to threeorders of magnitude greater than the diffusive flux.

The moisture content has a direct effect on the migration ofdissolved material by diffusion. As the ground becomes drier,liquid flow paths become more tortuous and the degree ofliquid interconnection decreases. Some flow paths may deadend. Whereas water molecules may move in the vapour phaseacross air-filled pores, involatile solute molecules must followtortuous connected liquid pathways. There will be flow into‘dead-end’ paths, since water continues to move in the vapourphase. Non-volatile dissolved molecules can only escape fromdead-end paths by diffusion back out against the flow direction.Thus, involatile dissolved molecules, such as Cl, move moreslowly than water. This has been demonstrated by isotopestudies, including 36Cl and 3H, released during atomic bombtests. 36Cl was generated during sub-sea tests and fallout peakedin 1955–56. 3H was generated during atmospheric tests andpeaked later, in 1964–65. Once incorporated into soil moisture,these isotopes behave differently. 36Cl is non-volatile and isrestricted to liquid phase flow, whereas tritiated water, likenormal water, can move in either liquid or vapour phases.Scanlon (1992) measured the penetration of 36Cl and 3H intothe soils of the same region, Hueco Bolson, referred to above.The 36Cl/Cl ratio reaches a maximum at 0.5 m depth (Fig. 2).Although 3H peak fallout was eight years later than 36Cl, tritiumpenetrated deeper, below 1 m, indicating that faster migrationis possible in the vapour phase compared to dissolvedcomponents.

Electrochemical dispersion. Electrochemical cells are generatedaround most sulphide bodies as a result of their oxidation and

Fig. 1. Chloride content and estimates of moisture flux for profilesof unsaturated soils, Hueco Bolson, Texas. Scanlon (1991).

Finding deeply buried deposits using geochemistry 9

are often detectable by negative self-potential (SP) anomaliesabove the bodies. A sulphide body can be considered aconductor immersed in an electrolyte, groundwater. Redoxdifferences exist between the more oxidized upper (cathodic)portion of the body and the more reduced deeper (anodic) part.Anodic corrosion takes place at depth between sulphide min-erals and the electrolyte, releasing electrons: MeS � M2+(aq) +S + 2e�. Electrons travel, usually upwards, through theconductive sulphide body to locations where oxygen or otheroxidants are available and take part in its cathodic reduction: O2+ 4H+ + 4e� � 2H2O. Because of the electrical potentialestablished between the cathodic and anodic sites, there iscurrent flow, carried by ions in the groundwater. Cationsmigrate to the cathode and anions to the anode. Cathodicreactions are usually shallow, close to the supply of oxidantsnear the water table. Water is required as a source of dissolvedoxidants and to permit the migration of dissolved cations andanions, which carry the current. One well-described cell is thatproduced by the massive nickel sulphide deposits at Kambalda,Western Australia (Fig. 3).

Much of the pioneer work in exploration geochemistry onelectrochemical cells involved sulphide bodies that reach closeto the surface. Air and infiltrating rainwater, saturated inoxygen, bathe the cathodic zone. Bolviken & Logn (1974)described an electrochemical cell formed by the Joma deposit,Norway, which is covered by 0.5 to 1.2 m of glacial sediments.There is a well-developed SP anomaly detectable at surface.Samples of till, taken as close as possible to bedrock, showedelevated concentrations of metals over the mineralization anddisplay higher conductivity in water–till slurries. Increasedconductivity supports the contention that free mobile ionsshould be more abundant near the upper cathodic zone. Similar

results were obtained by Govett (1975) for a variety oflocations, with unambiguous conductivity anomalies found overdeposits with shallow cover.

Smee (1983); Govett et al. (1984) developed similar modelsfor the formation of twin or ‘rabbit ear’ anomalies at thesurface above an oxidizing sulphide deposit. WhereasGovett’s model was for transfer through the vadose zone in aridterrain, Smee’s was for migration through saturated clay. InGovett et al.’s model, cations following the current pathaccumulate on either side of the sulphide body in a zone of highcurrent density near the base of the conductive weathered zone(Fig. 4), which is near to the water table. From these zones ofhigh concentration, cations move to the surface by chemicaldiffusion. But, as described in an earlier section, chemicaldiffusion within the water film coating grains in the vadose zoneis much slower than the downward advective flow of the waterfilm.

Fig. 2. Vertical profiles of 3H and 36Cl/Cl for unsaturated soils,Hueco Bolson, Texas. Scanlon (1992).

Fig. 3. Supergene alteration caused by electrochemical oxidation ofpyrrhotite–pentlandite ore in arid terrain. From Blain & Brotherton(1975); Thornber (1975).

Fig. 4. Conceptual model by for the electrochemically induceddispersion in arid terrain (Elura deposit, Australia). For Elura, thesulphide has been completely oxidized to a depth of 98 m and thewater table lies between 80 and 115 m. Diagram simplified fromGovett et al. (1984).


Smee’s model is for the Abitibi greenstone belt, where thick,>30 m, generally saturated, glaciolacustrine clays often covermineralization. Cations that concentrate at the base of the claysdiffuse upward. Diffusion coefficients of 1x10�10 and 1x10�11

m2 s�1 are typical ranges for non-reactive chemical species inclay. A diffusion front representing only one percent of theoriginal concentration would reach 10 m out from source after10,000 years. This is for one-dimensional diffusion, whereasdispersion from an orebody would be radial, resulting in lowerconcentrations at a given distance from the deposit. Clays mayact as a semi-permeable membrane, resisting the diffusion ofions (Schwartz 1974) or clay mineral surfaces may adsorb metalions. The limited nature effect of either advective or diffusiveflow in thick glaciolacustrine clays since their time of depositionis illustrated by the empirical data of Remenda et al. (1994). Theyfound that at depths of 20 to 30 m in clay from various sites inthe southern Canadian Shield, interstitial waters retained �18Ocompositions of �24 to �25‰ characteristic of surfacewaters at the time of deposition. These waters have been static,showing no evidence of equilibration with modern waters in thearea of �13 to �14‰. Thus, neither advection or diffusionhas been effective in moving these waters over the last c. 10 ka.Although electrochemical processes are an important means forreleasing ions from a buried sulphide body, there is difficulty inexplaining how the ions so released travel to the surface.

A different model was proposed by Hamilton (1998, 1999,2000) for saturated overburden in glaciated terrain where thesulphide subcrop is covered by tens of metres of glacialsediments. This model (Fig. 5) invokes transport along redoxgradients. Consumption of oxidants at the sulphide/sedimentinterface creates a reduced environment. Continued oxidationcan only occur if reduced species are removed. Redox differ-ences between the top of the body and the water table near thesurface, where oxygen is in contact with groundwater, producea vertical electrochemical gradient. Reduced species, such asHS� and Fe2+, migrate upward along this gradient andreactions between oxidized species moving in the oppositedirection dissipate charge away from the sulphide. As thelimited number of oxidizing agents between the top of the bodyand the water table are consumed, a reduced ‘column’ ispostulated to form between the body and the water table. At the

water table there is ready access to O2, permitting oxidation ofthe reduced material. This will produce a number of effects,including a decrease in pH and the concomitant dissolution ofcarbonate (Fig. 5). The reduced column, pH lows and dissolu-tion of carbonate predicted by this model have been docu-mented at the Cross Lake site discussed below and at otherlocations in the Abitibi greenstone belt. And elements presentin the mineralization, such as Zn and Cd, show clear anomaliesin the soils above. There are also negative SP anomalies,indicative of current movement in the conductor. There areconsiderable technical difficulties in fully evaluating the pro-cesses that occur within the reduced column. To create thefeatures seen at the surface at Cross Lake, where 30 m or moreof clay lie above the sulphide lens, and within a period of 8,000years since the clays were deposited, requires rates of migrationwell beyond that of chemical diffusion.

Advective flow. Water and its dissolved contents typically flowdown-gradient, not upwards. However, there are someexceptions. One of these is the effusion of groundwater at thesurface during earthquakes. In the Phase I scoping report byCameron (1998), this mechanism was given less than a page,because it was considered unlikely to produce significant surfaceanomalies. However, in the subsequent field programme inChile this appeared as an important process for anomalyformation.

Earthquake-induced surface flooding by groundwater haslong been recognized. Sibson (1981), p. 593) described ground-water movements along fault lines after earthquakes: “In aridterrain particularly, there are reports of changes in well waterlevel, spring flow and occasional dramatic effusions of ground-water immediately following moderate to large shallow earth-quakes”. Surface flows occurred in a desert area of Iran duringearthquakes in 1903 and 1923 (Tchalenko 1973). During themagnitude 7.3 Salmas earthquake of 1930 in NW Iran, waterwas expelled along the main fault trace and a broad area ofalluvial plain on the downthrow side became fissured andwaterlogged (Tchalenko & Berberian 1974). Nur (1974)noted outpourings of warm groundwater accompanying theMatsuchiro earthquake in Japan. Following the Hebgen Lakeearthquake of Montana in 1959, three rivers increased in flowby c. 50%, the increases continuing for several weeks throughdry weather (Muir-Wood 1994). The Kern County, California,magnitude 7.5 earthquake of July 1952 was followed by twomonths of drought. There were outpourings of groundwater inthe vicinity of faults, and increases in spring flow and well waterlevel, which continued for two months (Briggs & Troxell 1955).Sibson et al. (1975) used the term ‘seismic pumping’ to explainthis process. Pre-seismic extension produces fractures in thebrittle upper crust that provide pathways for groundwatermigration and storage. During earthquakes, stress fields becomecompressional, closing fractures and forcing groundwater to thesurface along faults. Given the low permeability of basementrocks, groundwater is slow to migrate to faults, resulting incontinued effusion of water for weeks following an earthquake.Groundwaters expelled from depth are typically old and saline,but may mix near-surface with less saline waters.

There are other mechanisms for the advective transfer ofgroundwater and solutes to the surface. One that was appliedearly in the history of exploration geochemistry is advectivetransfer by deeply rooted plants. Analysis of the plant materialprovides information on the composition of deep groundwater.In some arid areas, plants have been found to root as deep as100 m (Cannon & Starrett 1958). Also, capillarity can drawgroundwater from the water table to the surface, where itevaporates. However, the water table must be relatively close to

Fig. 5. Conceptual model from Hamilton (1999) showing the effectsof the dispersion of a ‘reduced column’ of metal and otherconstituents upwards through clay overburden from sulphide min-eralization exposed at the subcrop. The reduced material reacts withoxygen at and above the water table to form acid, which dissolvesand redistributes carbonate.


the surface, since the height over which the water can be drawnis dependent on the parameter known to soil scientists as ‘soilsuction’. Soil suction depends on the nature of the soil and thedegree of dryness. Clay soils exert greater suction than sand orsilt and dry soils more than damp. Fontes et al. (1986) describea site in the Sahara Desert where capillarity draws water to thesurface from a depth of 10 m. They suggest that 20 m may bea limiting depth for this process.

Migration of gases and the effects

Lovell et al. (1983); McCarthy et al. (1986) and others haveshown that gases such as carbon dioxide and methane are moreabundant in soils above some deposits, whereas oxygen isdepleted. Clark et al. (1997) have argued for the migration ofhalogens and volatile metal compounds to the surface. Chinesegeochemists (e.g. Xie et al. 1997) have suggested that a variety ofelements, Au, As and Sb, are carried to the surface in mobileform by gases, including very fine particulates, in the sub-micrometre to nanometre range. Hydrocarbons are currentlybeing measured in soils. The SGH method of Actlabs Ltd(Ancaster, ON, Canada) desorbs and measures hydrocarbons inthe C5 to C17 range from B-horizon soils. A similar method,soil desorption pyrolysis (SDP, St. Lucia, Queensland,Australia), has also been developed for hydrocarbon gases insoils. Similar to dissolved solids in water, gases may migrate bydiffusion through air or water, or advectively, as a result of themovement of a mass of air or water containing the gases. Forgases, as for dissolved solids in water, advective flow is a farmore rapid transport mechanism than diffusion.

Barometric pumping. Carrigan et al. (1996) carried out a simulatedunderground nuclear test. A charge of 1.3 � 106 kg of chemicalexplosives, equivalent to a 1 kt nuclear charge, was detonated ata depth of 400 m in bedded tuff at Rainier Mesa in the NevadaTest Site (NTS) (Fig. 6). Two bottles of gas were placed near tothe charge. One contained 3He and the other SF6. The amountsof gas involved were not large, for 3He, 1.3 m3 and for SF6,8 m3. The detonation chamber was close to a fault. After theexplosion, sampling sites were established at the surface todetect these gases. The first gas to be detected was SF6 alongthe fault at site OS6, 50 days after detonation, during a strongbarometric depression. The fault along where OS6 is sited is notthe one that runs close to the detonation cavity. Thereafter, SF6

was detected at sites OS1, OS2 and OS3. 3He was first detectedat the surface at OS6, 375 days after detonation.

These empirical observations are entirely contrary to migra-tion by gaseous diffusion. 3He has a much higher diffusivity thanSF6 and, if diffusion was the dominant mechanism, shouldreach the surface long before the other gas. The diffusivity ofSF6 is such that it would require 10 s to 100 s of years to reachthe surface, yet it has happened in days. Why? The reason is thatgaseous diffusion is overridden by a much faster mechanism,barometric pumping. The description of barometric pumpinggiven here is largely derived from Nilson et al. (1991), workcarried out at the NTS.

Barometric pumping refers to the process where cycles ofhigh and low barometric pressure first force air into the earthand then withdraw a mixture of the air plus gases that were inrock. For all practical purposes, barometric pumping appliesonly to fractured rock. For reasons discussed below, barometricpumping is not a significant process for non-indurated, unfrac-tured material, even if permeable. When the permeability ofNTS alluvium is measured on small samples in the laboratory orlarge volumes are measured in situ in the field by boreholeinjection, results are similar, in the range 1 to 15 darcy (D).When the permeability of NTS volcanic rock is measured oncore samples in the laboratory and again by borehole injection,results are very different, only 10�6 to 10�2 D on core, but 1to 15 D for borehole injection, the same as for the alluvium.The higher permeability of the bulk rocks is because fractures,not present in the small laboratory core samples, control thepermeability.

High barometric pressure forces air down fractures and intopore space in the rock around the fractures. Gases within therock, such as CO2 or hydrocarbons, mix with air by moleculardiffusion. When the barometric pressure drops, air in theporous rock, now containing the gases, returns to the fractureand, after several cycles of high and low pressure, reaches thesurface. Pumping occurs because the volume of air enteringrock porosity is much greater than the volume of air present inthe fractures. The rock porosity provides the ‘breathing volume’that permits large vertical movements during high pressure‘inhalation’ and low pressure ‘exhalation’. By contrast, innon-fractured permeable media, such as alluvium, air move-ments are piston-like and nearly reversible, so that there is littleupward transport of a gas of deep origin. For a fracturedpermeable medium, barometric transport can be several ordersof magnitude greater than molecular diffusion, whereas forunfractured soil and alluvium, molecular diffusion is moreimportant.

Using data from NTS volcanic rock, simulations were carriedout on the removal by barometric pumping of a deep gas. Withthe condition of fresh air from the surface to 200 m, then airplus radioactive gas from 200 m to the water table at 500 m,10% of the contaminated gas was removed in one year. In termsof geological processes this is rapid. The overall efficiency ofdeep gas transport to the surface is critically dependent on thespacing of fractures. For closely spaced fractures, say only a fewcentimetres or less, each fracture has only a small ‘breathingvolume’ of porous rock around it. The transport efficiencyincreases with increasing fracture spacing up to a few metres.But after about 10 m spacing there is no incremental benefitbecause the air can only penetrate a few metres in the halfperiod of about 100 hours that is usual between high and lowbarometric phases. Maximum transport efficiencies are reachedfor spacing in the range 2 to 10 m. Although it may initiallyappear counterintuitive, transport efficiencies are decreasedwith increasing molecular diffusivities of gases. This is becausea gas with high molecular diffusivity can more readily diffuse out

Fig. 6. Rainier Mesa, Nevada, showing the location of a cavity for a1kt explosion. Bottles of SF6 gas and 3He gas were set close to theexplosion. SF6 first reached the surface at site OS6 50 days after thedetonation during a barometric low. 3He arrived only after 325 days.From Carrigan et al. (1996).


of the upward-moving air/gas mixture in the fractures intoporous wall rock. This explains why in the bomb-simulationexperiments SF6 arrived at the surface more speedily than 3H,which has a higher molecular diffusivity. Diffusion of hydro-carbons out of the upwardly rising air/gas mixture into thesurrounding wall rock may explain a ‘reduced column’ of rockabove a leaking hydrocarbon reservoir.

Seasonal exhaustion of air and gas from the vadose zone. YuccaMountain, Nevada, rises 250 m above Solitario Canyon. Inwinter, air within the mountain is warm. This warm air rises andis replaced by cold, denser air entering at the base of the scarp.Air exhausts continuously, at a relatively high velocity, typically3 m s�1, from wells drilled near the crest of the mountain(Weeks 1987). In summer, the wells alternately intake andexhaust air several times a day, but at lower velocity. A farmerin Idaho drilled a 30 m dry well into the Snake River basalts,near the Snake River Gorge. He built a greenhouse over thewell, with the air exhausting from the well maintaining plantgrowth through the winter. This process for the advectivemovement of air plus gas has been described by Rose & Gow(1995).

Migration as bubbles and bubble-generated aerosols. Whereas baro-metric pumping provides a mechanism for the advectivetransfer of gases to the surface in arid terrains with a thick,fractured vadose zone, bubbles provide a means for advectivemovement through the saturated zone. Kristiansson &Malmqvist (1982) found that the radon migrated to the surfacemuch faster than can be accounted for by diffusion in ground-water. They suggested that radon was included in microscopicbubbles composed of nitrogen, argon, oxygen and methane.

Bubbles and bubble-generated aerosols may be involved inthe upward transport of the dissolved constituents of ground-water. The clue for this is metal-rich aerosols over the oceans.Some elements are greatly enriched in the marine atmosphererelative to their crustal abundance (Fig. 7). When waves break,air is trapped in seawater, which rises as bubbles. Surface-activesubstances are attracted to the bubble–water interface. Theseinclude bacteria, colloids, soluble organic molecules and dis-solved heavy metals (Piotrowicz et al. 1979a; Piotrowicz et al.1979b). As bubbles rise they ‘scrub’ the water of thesesubstances. At the surface of the ocean there is a surfacemicrolayer, enriched in the same surface-active materials. Whena bubble bursts, aerosols are formed from the bubble boundarylayer and from the surface microlayer. Experimental studies ofaerosol generation show strong fractionation of elements.Piotrowicz et al. (1979) found that Fe was not enriched, whereasCu, Cd and Zn were enriched by a factor of 200. Lead had the

greatest enrichment with a factor of c. 2000. Fractionation isincreased with increasing length of the water column throughwhich the bubbles pass. Metals are most strongly enriched insub-micrometre aerosols (Arimoto et al. 1990). Aerosols in thesize range 0.1 to 10 µm have a residence time in the loweratmosphere of about a week. The small size of these particlesfavours transfers from the solid to vapour phase and reactionsbetween the gases and solids, which may form volatile com-pounds

What are the possible relationships between oceanic aerosolsand ore deposits? Gases may perhaps be generated during theoxidation of sulphides or gases from deeper sources, e.g. themantle, and may rise up faults along which deposits lie. Somesamples of groundwater taken from the Spence copperporphyry deposit, Chile, were found to be saturated with CO2.The degassed CO2 can rise as bubbles through groundwater,scrubbing metals from the water en route. Mineralized ground-waters have concentrations of metals orders of magnitudegreater than sea water and bubbles are likely to scrub a greaterdepth of water than the one or two meters typical of wave-generated bubbles. Barometric pumping may serve to extractsub-micrometre aerosols from the water table, but this has notbeen demonstrated. Even where aerosol generation is minimal,bubbles, with a high concentration of metals in the boundarylayer, may deliver metals to the water table. This may be ofsignificance in wet areas, where the water table is close to thesurface.

The Osborne deposit, in northern Queensland is a copper–gold replacement deposit in lower Proterozoic metamorphicrocks, covered by 30 to 60 m of fractured sedimentary rock ofMesozoic age. Placer Australia Pty sponsored a series ofgeochemical tests over the deposit prior to mining, includingpSirogas collectors (Rutherford et al. in press). A plasticcollector containing a polystyrene film was buried in the soil ata depth of 50 cm. A hole in the base of the container allowedsoil air to circulate over the polystyrene film. After 30 to60 days, the collectors were recovered and the surface ofthe films analysed by Particle Induced X-ray EmissionSpectrometry (PIXE) for a variety of trace metals. The surfacesof the films were also examined by Scanning ElectronMicroscopy (SEM). These scans show circular areas of precipi-tate, 20 to 30 µm in diameter, which they attribute to theimpingement, then evaporation, of water or aerosol droplets.Most of the geochemical data from Osborne are erratic betweenadjacent sites (‘spikey’), perhaps reflecting transport upfractures, with the pSirogas results showing this strongly,including the strongly anomalous results for Cu (Fig. 8).

Mineralogical change in soils by gases. The passage of gas from a deepsource through soils may cause changes in the soils’ com-position or mineralogy. A possible, if extreme, example isshown by analyses of soils above underground nuclear testexplosions in Nevada (Hall et al. 1997). In these tests, deton-ations were at the bottom of a vertical shaft that had beenback-filled and cemented. For the described sites, explosionstook place at depths of 293 to 350 m, well above the watertable, at 450 to 530 m. Data shown on Figure 9 are from soilsamples taken across the Ville test site, where the explosioncaused a surface slump crater 120 m in diameter and 17 m deep.Analyses after a hydroxylamine extraction show a stronganomaly for I, extending for 300 m on either side of groundzero. Iodine is an element that forms a variety of volatilecompounds. Arsenic shows a weaker anomaly and also canform volatile compounds. The presence of anomalies forvolatile elements suggests the passage of these plus possiblecarrier gases through the soil. But also note the strong depletion

Fig. 7. Heavy metal enrichments in the marine atmosphere. Enrich-ment is measured relative to crustal abundance normalized to Al.Data from Rahn (1976) and for Hg from Crozat et al. (1973).


in the amount of Fe extracted. Analyses for the total contentsof Fe and other elements showed a flat pattern: there has beenno change in the bulk composition of the soils above thedetonation site; only the small fraction of selected elements thatcan be dissolved by a selective leach. The depletion in Fe shownby the selective extraction may be the result of gas changing themineralogical state of the Fe oxide minerals, such that Fe wasless easily dissolved by the hydroxylamine leach. The possibilitythat the observed changes may be the result of contaminationduring drilling have been discounted by more recent unpub-lished work where the nuclear device was placed using ahorizontal adit. Soils over the Ruby Star copper skarn deposit,Arizona (Fig. 10) are also depleted in Fe, when measured by aselective leach. Until more detailed studies are carried out, thecauses for these depletions must remain speculative.

Measurement of elements and gases in soils

Selective leaches. Most applications of geochemistry to explorationfor buried deposits involve the analysis of soils by selectiveleaches (Hall, 1998). The term selective leach was first appliedby soil scientists to reagents that could dissolve particularmineral phases, with minimal effect on other minerals. Inexploration geochemistry, selective leaches are used to dissolveminerals that may include a high proportion of the mobilephase of elements, including material derived from deposits.Soil-forming processes convert primary minerals derived fromrocks into minerals stable at low temperature. The elementfraction that comes from primary minerals is here consideredthe internal or endogenic phase. Elements from external sources,

including a mineral deposit, are termed the exogenic phase. Theexogenic phase is initially added to the soil in water-solubleform; as a result of soil-forming processes, this phase isprogressively incorporated into secondary minerals (Fig. 11).Each secondary mineral contains elements of both endogenicand exogenic origin. The relative amounts of these phases varybetween minerals and, for a given mineral, from place to place.

Given that the exogenic phase must first enter a soil in awater-soluble form, one approach is to use a weak leach thatdoes not attack any minerals, but dissolves water-soluble saltsand elements loosely adsorbed to mineral surfaces. Pure water,i.e. deionized water, is the most simple leach for this purpose,but suffers from poor analytical reproducibility. An alternative isthe Enzyme Leach. In our experience, this leach dissolves little

Fig. 8. Cu in soil air over Osbornedeposit, Line 21737N, as measured byCSIROGAS collectors. Deposit liesbelow 11200E to 11600E. Modifiedfrom Rutherford et al. (in press).

Fig. 9. Contents of Fe, I and As in soils collected along a traversecrossing ground zero (0 m on horizontal scale) above the Ville atomicbomb test, Nevada. Soils were extracted with hot hydroxylamine(HX Fe). From Hall et al. (1997).

Fig. 10. Iron and Cu by Enzyme Leach in soils over the Ruby Starcopper deposit, Arizona. Data are moving average of three sites.Deposit contains 100 Mt 1% Cu and consists of massive slumpblocks enclosed within fanglomerate and covered by 40 to 300 m ofgravel. Data from Kelley (1995).


more than deionized water (Fig. 12), but the analytical repro-ducibility is better. The other main proprietary leach, MobileMetal Ion (MMI), comes in several forms, two of which are anacidic MMI-A and a basic MMI-B. The MMI-A leach (targetingZn, Cu, Pb, Cd) dissolves more metal than the Enzyme Leach,which indicates that it is dissolving secondary minerals. How-ever, the formulations of the MMI solutions have not beenpublished and, because major elements are not determined bythe laboratories carrying out MMI analyses, it is not possible tointerpret which mineral phases are being dissolved.

The second approach is to use a leach that dissolves one ormore secondary minerals that contain a favourable ratio ofexogenic to endogenic material at a particular site. Ammoniumacetate at pH 5 dissolves carbonate minerals. For oxideminerals, hydroxylamine hydrochloride is widely used. ‘Cold’(room temperature) hydroxylamine in an acidic solution, pH c.1.5, (HX Mn) dissolves Mn oxides, whereas ‘hot’ (60 oC)hydroxylamine in a more strongly acidic solution (HX Fe)dissolves both Mn and Fe oxides. Because both hydroxylamineleach solutions are acidic, they will also dissolve carbonate, sothat the element phases extracted are the sum of those presentin carbonates and oxide minerals, plus, of course, that presentin water-soluble form. In general, the more acidic the leaches,the less selective they become. For organic material, such ashumus, sodium pyrophosphate is the most commonly usedleach, although it tends to dissolve large amounts of elements ofendogenic origin, unrelated to mineralization. Aqua regia shouldalso be mentioned. This is not a selective leach, but dissolvesmost secondary minerals and partially extracts some silicates.Because it dissolves a high proportion of endogenic phases, theanomaly to background contrast is generally low. However, itprovides useful information on the overall composition of thesoils, which assists in the interpretation of the selective leachdata. And it may give good results for mature anomalies, whereelements of external origin are no longer being introduced.

There are complexities involved in the use of selectiveleaches, which require consideration during survey planning,analysis and interpretation. When significant amounts of car-bonate are present in soils, this can partially neutralize leachsolutions that contain acid, so that their leaching capacity isdegraded. For example, 12–15% CaCO3 in samples can increasethe pH of a cold hydroxylamine (HX Mn) solution from c.1.5 to5.5 and MMI-A from c. 2.5 to 6. Element contents that mightbe extracted in the absence of carbonate may not be fully

extracted or are reprecipitated or adsorbed as the pH increases.Thus analytical laboratories should measure and report the finalpH of the leach solution. The ICP-MS is subject to spectral andnon-spectral interferences, which may affect the determinationof numerous elements, particularly those at low concentration(Hall 1992). For example, mass 105Pd is commonly used tomeasure Pd, but this is also the mass for the molecular band105(SrOH). Palladium is usually present in soils in the low ppbrange and its signal can be overwhelmed by SrOH formed in theplasma during analysis of soils of moderate to high Sr content.

Quality control on analyses is mandatory for surveys, usinghidden duplicates and ‘standards’, the latter being samples thathave been analysed in previous surveys. ‘Analytical duplicates’are separate vials of the same prepared sample, given differentnumbers and placed at distant positions in the submittedanalytical batch. ‘Field duplicates’ are separate samples collectedat the same site, but 2 to 5 m apart from each other. Afteranalysis, the analytical and field duplicates are separated intogroups and the standard deviation, s, and relative standarddeviation, RSD, are computed for each:

s2 = ~�~xi1 � xi2!2!/2N

RSD = 100 s/X where the squares of the differences betweenthe duplicate pairs are summed, then divided by 2N, N beingthe number of pairs, to produce the variance estimate s2. Themean of all duplicates, X, is then used to calculate RSD.

In Figure 13 RSDs are shown for 29 elements as determinedby the Enzyme Leach. The RSDs were computed from ten pairsof analytical duplicates of B-horizon soils from the Abitibi testsites. The median RSD is also shown. The median RSD of theanalytical duplicates for each leach provides a useful compari-son of analytical precision between different leaches. In Table 1,median RSDs for five leaches have been compiled. In general,the stronger the leach, extracting more of each element, thelower is the RSD. This is because it is more difficult toconsistently extract a small fraction of the element present thana large fraction. The amounts of elements extracted by thedifferent leaches, as shown in Figure 12, correlate with theRSDs given in Table 1. Other than deionized water, the RSDsobtained from the selective leaches are satisfactory. The vari-ance estimate, s2, for the field duplicates is the sum of theanalytical variance plus that due to sampling. Thus the RSDs forfield duplicates are higher than those for analytical duplicates

Fig. 11. Conceptual model showing thedistribution of exogenic and endogenicphases between soil-forming minerals.


(Table 1). Only in the case of the deionized water leach is theRSD for the analytical duplicates so high that it approaches theRSD of the field duplicates.

Measuring gases. Soil gases are commonly measured directly in thefield using instrumentation. This confers the advantage thatanomalies may be investigated immediately. The disadvantage isthat the instantaneous flux of gas so measured is subject tomany environmental variables that may change the estimatedconcentration. In a study of radon activities from sites inPennsylvania, Rose et al. (1990) found that readings from depthsbelow 70 cm varied by factors of 3 to 10 during the year fordifferent sites. At shallower depths, seasonal variability is evengreater. Temperature and moisture variations, rain, wind andbarometric change may all affect the measured concentration.Carbon dioxide shows variation due to seasonal, samplingdepth and environmental changes similar to that of radon. Theflux of CO2 in soils increases by between 1.5 to 3 times for

every 10oC increase in temperature and is also influenced by soilmoisture and the content of organic matter (Amundson &Davidson 1990). Carbon dioxide dissolves in water, formingH2CO3, HCO3

� and CO32�. Thus the presence or absence of

moisture can substantially influence the concentration of thisgas. An alternative, that reduces temporal variations, is tomeasure the integrated flux, such as is done with radon byburying alpha track film in the soil for days or weeks oradsorbing gaseous Hg on Au film. For metals in soil air, CSIROin Australia and BRGM in France place collectors in the groundfor c.100 days. In the BRGM method (Pauwels et al. 1999) themetals are collected on activated carbon, whereas in the CSIROmethod (Rutherford et al. in press) solids and vapour impingeon a thin plastic film. Organic gases are absorbed on clayminerals. By desorbing these gases, then measuring their relativeabundance, clues may be obtained to buried deposits. This is thebasis of the SGH (Soil Gas Hydrocarbons) technique of Actlabsand the SDP (Soil Desorption Pyrolysis) method of theUniversity of Queensland.

FIELD TRIALS

Study areas and methods

Phase II of the CAMIRO Deep-Penetrating Geochemistryproject was to carry out studies at test sites where knownmineralization is covered by 30 to 240 m of rock or consoli-dated or unconsolidated overburden. Sites were chosen in theAbitibi greenstone belt of Ontario, the Carlin district ofNevada, and the Atacama Desert of northern Chile. Thecharacteristics of these sites are summarized in Table 2 and theindividual studies carried out at each site are listed in Tables 3and 4. These three regions were chosen because of theirgeological and climatic contrasts. The Atacama is the driestlarge desert area of the world and is host to some of thelargest and lowest-cost Cu deposits. Carlin is the most produc-tive district for Au in North America and has a semi-aridclimate. The Abitibi, a glaciated region with a boreal coolhumid climate, contains many major lode Au and volcanogenicmassive sulphide (VMS) deposits.

Work in Chile and Nevada was entirely funded by companysponsors through CAMIRO. Much of the work in Ontario wasfunded by the Ontario Geological Survey (OGS). Reports onindividual studies were provided to the company sponsors onan ongoing basis starting in 1999. In November 2001, aCD-ROM containing 31 reports and seven database files wasdistributed. These results were reviewed at a meeting held in

Fig. 12. Box-whisker plots showing the amounts of elementextracted by deionized water, Enzyme Leach, MMI-A, ammoniumacetate at pH 5 (AA5), cold hydroxylamine (HX Mn) and aqua regia.Top of the diagram, Cu in 63 samples of soil from a traverse acrossthe Spence copper deposit. Bottom of diagram, Zn in 54 samples ofB-horizon soils from two traverses across VMS mineralization atCross Lake, Ontario.

Fig. 13. Relative standard deviation (RSD) for ten pairs of analyticalduplicates of B-horizon soils from the Abitibi belt analysed byEnzyme Leach.


Toronto in December 2001 when sponsors approved furtherwork. The additional results were distributed as seven reports inOctober 2002, including isotope studies in Chile, detailed trenchsampling at Cross Lake, and data provided by IGGE in Beijing.Project results are subject to a two-year confidentiality periodafter initial distribution to sponsors. Because of this restriction,some of the results from the November 2001 release and noneof the data from the October 2002 release could be consideredfor inclusion in this overview report. Work funded by the OGSwas not subject to the confidentiality agreement and theseresults have been presented in OGS publications and elsewhere,which are listed in the references. Because of the volume ofinformation obtained during Phase II of the project, onlyselected parts of the work are described here.

Analytical methods. Analysis of soils was on material sieved to <80mesh (<0.177 mm). For the deionized water extraction, 20 ml ofwater were added to 2.5 g sample and tumbled in a roller for3 min every 15 min for 1 hour. After settling for 10 min, theleach was centrifuged for 8 min at 2500 rpm, then decanted.Enzyme Leach and MMI-A are patented and proprietary leaches,respectively, described by Hall (1998). For ammonium acetate atpH 5 (AA5),which dissolves carbonate minerals, 1 g sample wasdigested by 40 ml of 1.0M NH4OAc/pH 5.0 for two hours,then centrifuged and the solution diluted with water 50:1. Coldhydroxylamine in weakly acidic solution dissolves Mn oxides (HXMn). For this, 1 g of sample is treated with 25 ml of a solutionwith 0.1M NH2OH:HCl in 0.04M HNO3 for 30 min atroom temperature. For a limited number of samples involving

Table 1. Relative Standard Deviations (RSD) for ten pairs of analytical duplicates and ten pairs of field duplicates taken from three test areas in the Abitibi greenstone belt in 1999

Analytical method Number of elements for median RSD Median RSD for analytical duplicates Median RSD for field duplicates

Aqua regia 52 7 11Cold hydroxylamine 35 11 22Enzyme Leach 29 21 38Deionized water leach 49 56 60MMI 7 (14)* (53)*

Samples of B-horizon soils are composites of sub-samples from five auger holes within a 2 m radius. Duplicate composite samples were collected at a 4 m distancefrom the first sample.*RSDs for MMI are not directly comparable with that of the other methods. MMI samples are single (not composite) samples taken by auger at a fixed interval10–25 cm below the Ao-horizon. Analytical RSDs for the MMI samples are derived from in-house laboratory duplicates, not blind duplicates as with othermethods.

Table 2. Summary of sites studied during CAMIRO Deep-Penetrating Geochemistry Phase II project, 1999–2001

Location Deposit type Age Grade Cover Cover thickness Cover age

ChileSpence Cu Porphyry Paleocene 400 Mt 1.0% Cu Piedmont gravel 3–180 m MioceneGaby Sur Cu Porphyry Oligocene 400 Mt 0.54% Cu Piedmont gravel 20–40 m MioceneMansa Mina Cu Porphyry Oligocene 325 Mt 1.0% Cu Piedmont gravel 50–>300 m MioceneTamarugal False Anomaly n/a n/a Piedmont gravel 300 m Miocene

NevadaMike Cu-Au Pre-Tertiary 150 Mt 0.25%

Cu, 0.71 g/t AuCarlin formation 150–250 m Miocene

AbitibiCross Lake Line 6 VMS Archean n/a Clay 30 m QuaternaryCross Lake Line 40 VMS Archean n/a Clay and sand 52 m QuaternaryHalf-Moon Lake VMS Archean n/a Clay 12–15 m QuaternaryMarsh Zone Gold Archean n/a Clay and peat 10–27 m QuaternaryTillex Copper Archean 1.38 Mt 1.6% Cu Clay, till, peat 30 m Quaternary

Table 3. Analytical methods for B-horizon and equivalent soils

Location Deionizedwater

EnzymeLeach

MMI-A Amm.acetate

Hydroxylamine Aqua regia WEM AEM OBM FMM

ChileSpence U U U U U U U U U U

Gaby Sur U U U U U U U U U

Mansa Mina U U U U U U U U

Tamarugal U U

NevadaMike U U U U U U U U U

AbitibiCross Lake Line 6 U U U U U U U U U U

Cross Lake Line 40 U U U U U U U U U U

Half-Moon Lake U U U U U U U U U

Marsh Zone U U U U U U U U U

Tillex U U U

WEM=Water-extractable, AEM=adsorbed extractable metal, OBM=organic-bound metal, FMM=iron- and manganese-bound metal


sequential leaches at Spence and Cross Lake, we also usedammonium acetate at pH 7 (AA7) and a more aggressive hydroxy-lamine leach (HX Fe), which dissolves Fe as well as Mn oxides:hot 0.25M NH2OH:HCl in 0.25M HCl. An aqua regia leach wasalso used; this dissolves all secondary minerals in soils. Afterdissolution all leach solutions were analysed by ICP-MS and, forelements at high concentrations, by ICP-ES. It should be notedthat because the MMI-A and hydroxylamine are acidic, they alsodissolve carbonates. Thus, in a non-sequential leach mode, HXMn dissolves the water-soluble salts and elements withincarbonates, as well as elements present in Mn oxides.

The WEM (water-extractable metal), AEM (adsorbedextractable metal), OBM (organic-bound metal) and FMM(iron- and manganese-bound metal) methods were carried outat the IGGE in Beijing. These leaches were applied sequentially.After the first leach, the residue is treated with the second leachand this is repeated through the third and fourth leaches. Thefirst leach (WEM) is deionized water. The next leach (AEM) isammonium citrate, which removes adsorbed elements. Thethird leach (OBM) is sodium pyrophosphate to extract organic-bound material. The final leach (FMM) is a mixture ofammonium citrate and hydroxylamine hydrochloride to dissolveFe and Mn oxyhydroxides.

Studies in Chile

Study areas in Chile are shown in Figure 14. Spence, Gaby Surand Mansa Mina are deposits with substantial reserves ofcopper, which are being prepared for mining or are the subjectof feasibility studies. All are covered by piedmont gravels, whichare consolidated rocks of Miocene age. At all sites, there aredistinct geochemical anomalies in soils above the deposits. Thefourth study area is Tamarugal, 50 km NW of Chuquicamata.Here there is a extensive anomaly, >100 km2, with highcontents of porphyry indicator elements, Cu, Mo, Re, As andSe. Drilling by Noranda through 300 m of piedmont gravelsshowed that the basement rocks below the gravels are barren.The study of this false anomaly was undertaken to understandthe processes that led to its formation. In this overview paperwe describe selected aspects of our study at the Spence, GabySur and Mansa Mina sites.

Spence Deposit, Chile. The Spence deposit was discovered byRioChilex in 1996–97 by grid drilling through piedmont gravelsof Miocene age. The deposit displays a typical supergene-enriched sequence and is associated with three dacite porphyrystocks (Fig. 15) intruded along a NE axis into andesite. Sulphide

Table 4. Other methods used at the test sites

Location Ground-water

Soil gasCO2, O2

Soil:stable

isotopes

Soil:Pb

isotopes

Metals insoil gas

Soil pH,Cond

Soil CO3 Soil SO4 Biogeochemistry Humus,Na

pyrophosphate

Humus,aqua regia

ChileSpence U U U U U

Gaby Sur U U U

Mansa Mina U U

Tamarugal U U U U

NevadaMike U U U U

AbitibiCross Lake Line 6 U U U U U U U U U U

Cross Lake Line 40 U U U U U U U U U

Half-Moon Lake U U U U U

Marsh Zone U U U U U U

Tillex

Fig. 14. Map of northern Chile showing the location of majorporphyry copper and other copper deposits. The sites studied whereporphyry copper deposits are covered by piedmont gravels areSpence, Gaby Sur, and Mansa Mina. Another site examined isTamarugal, 50 km NW of Chuquicamata, where a strong anomaly insoils lies above barren basement covered by 300 m of gravel.


mineralization was formed at 57 Ma, followed by supergenealteration over a long interval from 44 to 28 Ma (Rowland &Clark 2001). Reserves recoverable by open-pit mining are 50 Mtof oxide ore with 1.4% Cu, 200 Mt of enriched sulphide orewith 1.3% Cu, and 150 Mt of primary sulphide ore with 0.6%Cu. The irregular deposit surface is covered by 30–180 m ofgravels.

Soil samples were collected at 10–20 cm depth along anundisturbed east–west traverse over the centre of the deposit(Fig. 15), where gravels are c. 100 m in thickness. Measurementsmade in the field during sampling on soil deionized waterslurries identified two zones of high conductivity, one directlyover the Spence deposit and another 1 km to the east, shown inFigure 15 as the Eastern fracture zone. High conductivity is dueto NaCl (Na is shown in Fig. 16), which elsewhere on thetraverse is present in only trace concentrations. The highconductivity zones also contain higher levels of CaCO3 andmany elements that are abundant in groundwaters of the region,including Br, I, Li, S and Sr plus mobile indicators of porphyrymineralization that dissolve as anions: As, Mo, Se and Re.Caliche is developed in the soils over the Eastern fracture zone,but is otherwise absent along the traverse. Trenches dug in thegravels of the high conductivity zones showed that they arefractured, with north-trending and east-trending sets of verticalfractures. At a background, low conductivity site, west of thedeposit, gravels are unfractured. The fracture zones are inter-preted to represent the upward propagation of basement faultsduring the c. 10 Ma interval since the piedmont gravels weredeposited (Cameron et al. 2002). Pampa over the deposit is bareof vegetation and slopes gently to the west; there are no

depressions to explain the saline zones. Sodium chloride isinvolatile, so the most reasonable explanation for its presence insoils together with other constituents of local groundwater isthat mineralized groundwater reached the surface through thefracture zones. Where the soil sampling traverse crosses thedeposit, the water table is at 60 to 70 m depth. Duringearthquakes in this seismically active region, mineralizedgroundwaters were pumped up the fracture zones and floodedthe surface (Cameron et al. 2002).

Sampling of groundwaters from 25 drill holes showed thatwaters from the deposit are saline (Leybourne & Cameron2000a). Waters flowing into the deposit from the east have lowcontents of Cu, <50 ppb, whereas in the deposit, contents canexceed 1000 ppb (Leybourne & Cameron 2000b). Downflow,Cu content falls to <20 ppb (Fig. 17). High contents of Cu ingroundwater resulting from dissolution of ore minerals areconfined to the area of the deposit because Cu2+ is readilyadsorbed by colloids, limiting its mobility. Copper in soils(Fig. 16) shows a similarly restricted spatial distribution, withanomalous amounts occurring in soils above the Spence frac-ture zone but not over the Eastern fracture zone. By contrast,porphyry indicator elements that migrate as anions are notadsorbed by colloids. Selenium, for example, which amounts to<50 ppb in waters upflow from the deposit, increases to asmuch as 700 ppb in the deposit, as a result of oxidation ofsulphide minerals of magmatic origin. This high level continuesin the downflow groundwater plume from the deposit (Fig. 17).Sulphur isotope data on the groundwater show that sulphidesare actively oxidizing (Leybourne & Cameron 2000a). Theenergy thus released appears to be stimulating bacterial activity,possibly including methanogenesis, since there is an unusuallywide range in �13C for dissolved inorganic carbon from �28 to+9‰ (Leybourne & Cameron 2000a; Cameron & Leybourne2002). Although groundwater sampling is rarely used in mineralexploration, Figure 17 indicates the possible utility of thismethod using the far-travelling nature of porphyry indicatoroxyanions.

The ratio of filtered to unfiltered groundwater samples(Fig. 18) is an effective measure of the mobility of elements ingroundwater. Iron and Al readily precipitate as hydroxides toform suspended colloidal particles, which are removed alongwith adsorbed ions during filtering. These colloids have anegative charge, which attracts metal cations such as Pb, Cu, Ceand Y, fixing these cations and restricting their movement.Other metals – Re, Mo, V, As, Se – form oxyanions, withnegative charge, that are not attracted to the negatively chargedsuspensates. These metals are mobile as anions, along withconservative cations, such as Na, Ca and Sr. The right-hand sideof Figure 18 is dominated by cations, which either formcolloidal particles or are fixed to these particles, limiting theirmobility. The left-hand side of the figure comprises metals thatform oxyanions or by conservative cations, e.g. Ca2+ and Na2+,neither of which are adsorbed by negatively charged suspendedcolloidal particles.

The different behaviours of cations and anions are alsoevident at the surface, after flooding by mineralized ground-waters. The rare rains of the region redistributed elements thatarrived with the groundwaters. Copper as Cu2+ is readilyadsorbed, then incorporated into soil minerals; it is retained inthe top 20 cm of a vertical soil profile over the deposit(Cameron et al. 2002), whereas elements that form anions, suchas As, and conservative cations, like Na, are removed to depthby the rain (Fig. 19).

Figure 20 shows the proportions of Cu contained in differentmineral phases within the same soil profile as shown in Figure19. Samples were treated sequentially by four leaches. The first

Fig. 15. Outline map of the basement geology of the Spence deposit,based on drilling carried out by RioChilex. The basement is coveredby 30 to 180 m of piedmont gravels. On the soil sampling traverse,the gap immediately to the east of the deposit is occupied by a roadand an aqueduct. UTM coordinates are in metres.


was ammonium acetate (AA5), to dissolve Cu present incarbonate. The filtered residues from AA5 were treated withcold hydroxylamine (HX Mn) to dissolve Mn oxides. Theresidues from HX Mn were treated with hot hydroxylamine(HX Fe) for Fe oxides. The final leach in the sequence was aquaregia. Anomalous quantities of Cu are confined to the top20 cm of the profile; the total concentrations of Cu extractedfrom samples below 20 cm are similar to the contents of Cu byaqua regia in the 10–20 cm interval in background areas awayfrom the deposit (Fig. 16). Separate, i.e. non-sequential, analysesof the samples were done by the Enzyme Leach to provide ameasurement of water-soluble Cu. The amounts extracted bythe Enzyme Leach are small relative to Cu sequestered inminerals, but also show the enrichment of water-soluble Cu inthe upper part of the profile. This profile, like most othersobserved at Spence, shows no clear differentiation into horizonseither by colour or composition; the soils are reddish-brownthroughout and are not layered with calcrete. The plots to theright of Figure 20 represent the amounts of the different hostphases extracted during the sequential leaching: Ca representingcarbonate, Mn representing manganese oxides an Fe represent-ing iron oxides. Carbonate and iron oxides increase down the

profile, whereas manganese decreases. These plots indicate thatthe higher contents of Cu present in the upper 20 cm of theprofile are unrelated the abundance of the host phases. Theresults of the sequential leaching experiment is an illustration ofthe conceptual model of Figure 11, where Cu derived from thedeposit via the intermediary of groundwater has becomedistributed through a variety of secondary minerals present inthe upper 20 cm of soil, including those only soluble in aquaregia. In the upper 20 cm of the profile all measured phases areenriched in Cu.

Gaby Sur. Codelco’s Gaby Sur porphyry deposit, containing400 Mt at 0.54% Cu, is 43 km east of Spence. It lies at analtitude of 2700 m in a broad valley. Like Spence, the climate ishyper-arid. Supergene-enriched copper oxides, up to 180 mthick, are underlain by hypogene sulphides (Fig. 21). Thedeposit lies in a graben, delimited by high-angle boundary faultsand is covered by up to 40 m of gravel, deposited at c. 9.6 Ma(Camus 2001). The upper 20 m of the gravel is cemented bycalcrete, making it impermeable to water where unfractured.Water is found only below the basement unconformity, i.e. at>40 m depth, where drill holes intersect major faults.

Fig. 16. Plots of Na extracted by deionized water and Cu by deionized water, Enzyme Leach, MMI, ammonium acetate, hydroxylamine (HXMn) and aqua regia from an east–west traverse across the Spence copper deposit, Chile. Horizontal scale (eastings) in metres. The backgroundconcentration of Cu away from the deposit represents the endogenic component and that over the deposit is a mixture of the exogenic andendogenic phases. The aqua regia leach shows the highest proportion of the endogenic phase and, hence, the poorest anomaly to backgroundcontrast.


Soils were sampled along an undisturbed east–west traverseacross the deposit, with samples at 50 m intervals over thedeposit and 100 m intervals on the flanks. The soils are more

complex than those at Spence due to the presence of calcreteand gypcrete. The sampling procedure was different fromSpence and followed the Codelco procedure of takingred-coloured, Fe-rich soil from within the top 10–30 cm of theprofile. In the soils over both boundary faults there are distinctanomalies for water-soluble Na (Fig. 21) and Cl and forelements similar to those found to be anomalous at Spence: As,I, Mo, Se and Re. In soil pits, the calcrete-cemented gravels arefractured in these saline zones, also similar to Spence. Thesefeatures are attributed to pumping of mineralized groundwaterto the surface up the boundary faults and through the overlyingfractured gravels during earthquake activity, followed byevaporation (Cameron et al. 2002).

Analyses for Cu by different extractions are shown inFigure 21. The results by the weakest leaches, deionized waterand the Enzyme Leach, show clear anomalies over bothboundary faults, with a high anomaly to background contrast.These anomalies are coincident with the peaks for Na (and Cl).The similarity in the amounts of Cu extracted by deionized

Fig. 17. Copper and Se contents ofgroundwaters from within and near theSpence deposit, Chile. Squares are drillholes where groundwater was sampled,with analyses as shown.

Fig. 18. Ratio of concentrations of elements in filtered to unfilteredgroundwaters, Spence deposit.

Fig. 19. Distribution of Cu, Na and As in a vertical soil profile over the Spence deposit from the surface to 100 cm depth. There are no readilydiscernible soil horizons. Extraction by hydroxylamine (HX Mn). Prior to sampling the profiles, the top few centimetres of the surface wasremoved so as not to include any contaminated material.


water and Enzyme Leach indicates that the latter is extractingmainly water-soluble Cu. The anomalous zones above the faultsthat bound the deposit are poorly defined by the strongerleaches. Ammonium acetate at pH 5 (AA5) dissolves Cu presentin carbonate minerals. Background concentrations of Cu by theAA5 leach are an order of magnitude higher than for the twoweaker leaches that extract water-soluble material only andthese additional amounts of Cu obscure the Cu derived fromthe mineralized groundwaters that reached the surface. Copperpresent in carbonate is thus largely of endogenic origin, derivedfrom primary minerals in the gravel soils. The cold hydroxy-lamine leach (HX Mn) dissolves Mn oxides, plus carbonateminerals, plus water-soluble salts. Cumulatively the first two ofthese phases have a high content of Cu of endogenic origin,giving a background concentration of Cu approximately 50times higher than Cu extracted by deionized water or theEnzyme Leach. Again, this obscures the Cu of exogenic origin.For the aqua regia extraction, the background concentration ofCu reaches 2000 times that of the deionized water and theEnzyme Leach, again obscuring Cu derived from the deposit viamineralized groundwaters. Note, however, that one sample ofsoil situated at 517450E, above the west boundary fault, hashigh contents of Cu by both hydroxylamine (HX Mn) and aquaregia. This high concentration suggests that, like Spence, therehas been a long period of pumping of mineralized groundwatersto the surface and the incorporation of Cu of exogenic origininto secondary minerals.

Mansa Mina. Mansa Mina is a faulted slice of porphyrymineralization, possibly from the nearby Chuquicamata deposit

(Fig. 22). The fault that cuts the deposit, and forms its easternboundary, is the West Fault, a major strike-slip fault of regionalextent that may have had an influence in localizing theimportant cluster of porphyry deposits in the Chuquicamataarea (Fig. 22). Mansa Mina is an elongate, steeply dippingdeposit (Fig. 22), containing a mainly hypogene assemblage.Sulphides account for 300 Mt at 0.95% Cu and oxides 25 Mt at1.11% Cu. The deposit is up to 300 m in width and over 1000 min vertical extent and comprises four steeply dipping panels thatare delimited by faults subsidiary to the West Fault (Sillitoeet al. 1996). The easternmost panel is barren, comprisingchloritized and pyritized andesitic flows. Panels 2 and 3 are ofsericitized granodiorite containing pyrite-poor, copper porphyrymineralization. Panel 4 is also highly altered granodiorite, withsericite and advanced argillic alteration, with a enargite- andbornite-rich high sulphidation assemblage. There has beensupergene alteration, forming a leached zone and a partiallyoxidized zone above the sulphide zone. The upper part of thesulphide zone has been enriched by chalcocite. The deposit isentirely covered by Miocene gravels, the minimum thickness ofthese being 50 m on the west side of the West Fault and severalhundred metres on the east side. Depth to the water table is notknown.

Mansa Mina lies between the towns of Calama andChuquicamata and immediately east of the highway linkingthese centres. Only a limited length of undisturbed ground wasavailable for a sampling traverse across the deposit (Figs 22, 23)between the highway and gravel dumps to the east. Sampleswere taken at irregular intervals to avoid vehicle tracks. Theground slopes gently from north to south. Soils are marked by

Fig. 20. Vertical soil profile directly above the Spence deposit. On the left are the amounts of Cu in ppm extracted sequentially by: (1) ammoniumacetate (AA5) dissolving carbonate, (2) cold hydroxylamine (HX Mn) dissolving manganese oxides, (3) hot hydroxylamine (HX Fe) dissolvingFe oxides, (4) aqua regia. In the centre is Cu in ppb by Enzyme Leach, extracting water-soluble material. On the right is (a) Ca extracted byammonium acetate (AA5), (b) Mn extracted by cold hydroxylamine (HX Mn), (c) Fe extracted by hot hydroxylamine (HX Fe).


strong development of both gypcrete (near the surface) andcalcrete, which first occurs at depths between 20 and 40 cm.Samples were taken immediately above the calcrete horizon tominimize inclusion of gypcrete. Results of Enzyme Leachanalyses show a strong ‘spike’ anomaly for Cl (Fig. 23) and Nain the soil immediately above the West Fault. There is acoincident ‘spike’ anomaly for Se (Fig. 23). All samples containS, because of the presence of gypcrete, but the gypcrete has alow Se content, and the median ratio for Se�1000/S acrossthe entire traverse is 0.07, in the range of ocean water. For thesample directly above the West Fault the ratio is 1.9, in therange of the oxidation products of igneous sulphide. The samesample is anomalous in Re (31 ppb by Enzyme Leach, 48 ppbby aqua regia), an indicator element for copper and copper–molybdenum porphyries, since it mainly occurs as an impurityin molybdenite. Other elements that are strongly anomalous inthis sample are Br and I, typical constituents of groundwaters ofthe region, and K, Rb and Tl, which are abundant in sericiticalteration. An anomaly for Mo (Fig. 23) and Cu are displaced60 m to the east. The results suggest that mineralized ground-water has been pumped up the West Fault during seismicactivity, followed by its evaporation at the surface.

Fig. 21. Plots of Na extracted by deionized water and Cu by deionized water, Enzyme Leach, ammonium acetate, hydroxylamine and aqua regiafor soils from an east–west traverse across the Gaby Sur copper deposit, Chile. Horizontal scale (eastings) in metres.

Fig. 22. On the left is an east–west cross-section across the MansaMina deposit after Sillitoe et al. (1996). The unpatterned area betweenthe sulphide zone and the fault is barren rock. On the right is a mapshowing the spatial relationship between the West Fault and thecluster of copper porphyry deposits in the Chuquicamata area. Forlocation of area shown, see Figure 15.


Summary of observations and interpretation at Chilean sites. At all threesites there is evidence of saline groundwater having reached thesurface and evaporated. In addition to Na and Cl, zones in soilsare enriched in elements typical of groundwater in the region, Iand Br, and porphyry indicator elements, Se, Re, Mo and Cu.Over the Spence deposit enrichment occurs above a fracturezone in the gravels, and above a water table at 60–70 m depth.At Gaby Sur the groundwater signature is seen in soils abovethe boundary faults of the deposit; the water table is at >40 mdepth. At Mansa Mina the groundwater signature occursdirectly above a major fault in the basement below the gravelcover. The depths of the present water table at Spence andGaby Sur are too great to permit groundwater to be raised tothe surface by capillarity. In a coarse-grained medium, such asgravel, soil suction can draw water up only a few metres at best.Moreover, given the permeable nature of gravel, groundwaterraised by capillarity would extend over a wide area and not be

localized by structural features. The evidence is consistent withgroundwater having been forced to the surface up fracturezones by earthquake-induced (seismic) pumping.

Studies in Nevada

More than 40 Au occurrences within the Carlin Trend innortheastern Nevada (Fig. 24) form the largest accumulation ofmineable Au in North America. One of these is the Mikedeposit.

Mike Deposit. This deposit, hosted by sedimentary rocks ofPalaeozoic age, was discovered by Newmont Corporation in1989 while drilling on the predicted extension of the NW-trending Good Hope fault (Fig. 25) into an area with a thickcover of Carlin Formation of Tertiary age. The intersection ofthis fault with NE-trending faults was the primary structuralcontrol for the previously discovered Tusk and Gold Quarrydeposits. The Good Hope fault is the boundary between twoportions of the Mike deposit: the Main Mike, containing43.2 Mt of 0.034 oz/t Au and 76 Mt of 0.22% Cu; and the WestMike containing 110 Mt of 0.025 oz/t Au and 74 Mt of 0.28%Cu.

Copper mineralization consists of a sub-horizontal supergeneoxide and sulphide blanket up to 120 m thick that mainlyunderlies, but also overlaps, Au mineralization. Supergeneprocesses may have caused the mobilization of Cu to greaterdepths than Au. Teal & Branham (1997) note enrichments inthe ore of Cu, Au, Zn, Ag, Bi, Mo and Te; in addition, Cd andSe may be enriched. The deposit is covered by up to 240 m ofCarlin Formation of Eocene age comprising piedmont gravel,finer clastic sediments, waterlain tuff and a basal conglomerateand regolith that contains mineralized (oxidized) clasts. Aperched water table in the Carlin Formation is c.50 m below thesurface and a second aquifer is in the Palaeozoic rocks (Jackson,2000).

The simplest mechanism for generating geochemicalanomalies through 240 m of post-mineral cover is by the

Fig. 23. Analyses of Cl, Se and Mo by Enzyme Leach in soil samplesfrom a west–east traverse across the Mansa Mina deposit at7526500N.

Fig. 24. Map showing the Carlin Trend (shaded) and some of the 40gold deposits found along this trend.


movement of fluids or gases up faults in this cover.Dohrenwend & Moring (1991) carried out photo-geologicalinterpretations of recent faulting in this region. Of severalcriteria used to distinguish faulting, the most relevant to theMike area is: “prominent alignments of linear drainageways,ridges and swales, active springs or spring deposits, and lineardiscontinuities of structure, rock type, and vegetation.” Thisfaulting, which they assign to early to middle Pleistocene time(0.13 to 1.5 Ma), with a mean orientation of 028o, results indissection of the surface along faults of greater than 10 m. Thetopography of the Mike area (Fig. 26) shows deeply incised drystream beds with an orientation close to 028o. Based on thistopography, two faults in the Carlin Formation are interpretedto intersect the sampled Line 3. The fault to the NW, ‘ChannelA’, is marked by a stream and floodplain. Channel B was dryduring our visits. Its east slope is precipitous, which mayindicate a fault scarp.

Climate is semi-arid, with sparse sagebrush vegetation. Soilswere collected along Line 3 from sites at 30 m intervals; at eachsite, samples were taken from a depth of 40–50 cm from fiveholes dug within a radius of 1.5 to 3 m. The sampling depth wasselected on the basis of earlier studies by Jackson (2000). Thefive samples were mixed to form a composite sample. Soils areimmature with a weak B-horizon below 15–30 cm. Relativelylittle carbonate was found along this line and soils are mostlylow in organic material, except in the alluvial soils around thestream that marks Channel A.

Figure 27 shows results for Cd, Cu, Mn and Au by aquaregia. There are anomalies for Cd in the alluvial soils centredover Channel A. However, the strongest anomaly for Cd is atChannel B, located in residual soils of the steep west-facing(scarp?) slope of the dry valley, not in the vicinity of thestream-bed. Copper and Au are anomalous only on Channel B.The highest value for Au by aqua regia, at 17 ppb, is 9 times themedian value of 1.9 ppb, the median being a reasonable proxyfor the local background. Selective leaches were also used. Theresults by hydroxylamine (HX Mn) are mostly below thedetection limit of 0.05 ppb Au. The MMI maximum value of6.2 ppb Au over Channel B is 14 times the median. Many of the

Enzyme Leach results for Au for this line are below thedetection limit, but this extraction also shows an anomaly onthe east side of Channel B. For Cu, the results by the fourextractions are shown in Figure 28. The Channel B anomaly isshown clearly by the three leaches that extract the most Cu –aqua regia, hydroxylamine (HX Mn) and MMI – but theanomaly is not apparent in the Enzyme Leach results.

In an earlier section we have discussed anomaly formation asan incremental process, with metal being introduced in water-soluble form and then progressively incorporated into thesecondary minerals of the soils. If the anomaly associated withChannel B is related to Pleistocene faulting of the CarlinFormation, then the anomaly has had up to 1.5 Ma to form.This time span may account for the strong response shown bythe aqua regia extraction, which dissolves most secondaryminerals in the soil. Anomalies can also, presumably, stopdeveloping when new batches of elements in water-solubleform are no longer introduced. This may account for theanomaly response of the Enzyme Leach, which extracts water-soluble metal, being lesser than that of the stronger leaches thatdissolve secondary minerals.

Other elements that show strong anomalies on the steepwest-facing slope of Channel B include Mo by aqua regia, Cd byhydroxylamine (HX Mn), Zn by MMI and by aqua regia, andHg by aqua regia. Elements that are anomalous in the soils arethose enriched in the ore, so that their ultimate source in thedeep mineralization is a reasonable supposition. Elements thatare not components of the ore, such as rare earth elements, donot have anomalous patterns in the soil. Jackson (2000), aftercollecting samples on a wide-interval grid over Mike, found thatseveral elements were depleted across the top of the deposit,notably Mn; this was confirmed by our sampling (Fig. 27).Jackson suggested that elements may be transported to thesurface laterally and vertically around the deposit to createa central low with flanking highs. Manganese oxide is aredox-sensitive mineral that may be dissolved in a reducingenvironment. A sulphide deposit is a large mass of reducedmaterial, which, when oxidized, must reduce an equivalent mass

Fig. 25. Distribution of copper mineralization in the Mike deposit inrelation to major faults in the basement, shown by dashed lines.Information courtesy R.G. Jackson and Newmont Corporation. SolidNNE lines show two interpreted Pleistocene faults that cut the Carlinformation and intersect the sampled Line 3 (see Fig. 27).

Fig. 26. Topography of the immediate Mike area, with contours at25 ft intervals. The dashed lines show interpreted Pleistocene faultsin the Carlin Formation, based on the criteria of Dohrenwend &Moring (1991), who defined an orientation of 028o, as shown bythe arrow. These interpreted faults intersect the sampled Line 3 atsites A and B.


of oxidized material. So a possibility exists that the Mn low overthe deposit is a remote product of the oxidation of Mikesulphides, i.e. a reduced chimney. But elements that are notredox sensitive also show a low over the deposit: Cs, Ti, Zr andAl. So facies change in the Carlin Formation to a sandiersediment overlying the deposit, and more enriched in resistate(heavy) minerals, may explain some of the geochemical results.

In addition to the analyses of the soils by selective leaches,other studies were carried out. In July 1999, a CO2 – O2 soil gassurvey was carried out at 159 sites on three lines by PatrickHighsmith and Mary Doherty. The sampling depth for theprobe was c. 40 cm and the principal sampling interval was30 m. Repeated measurements at a base station showed that soilgas concentrations were stable during the sampling period.Delta values were calculated relative to atmospheric concen-trations for these gases. For �CO2 the range was 0.08 to 0.93%,with a median of 0.46% and for �O2 the range was �0.1 to�0.8%, with a median of �0.35%. There is an inversecorrelation between �CO2 and �O2, which is consistent withthe consumption of O2 to produce CO2. The amount of CO2is lower than values reported by Lovell & Hale (1983) for soilgas over a variety of massive sulphide mineralization, but is

similar to data obtained over the Carlin Trend by McCarthy &McGuire (1998). Higher �CO2 values and corresponding lowervalues for �O2 were found both over the deposit and in areasremote from it, but the survey was not extended to where Line3 intersects Channel B, the area of strongest metal response.The low delta values and anomalous values away from thedeposit, make it is difficult to judge whether variation is due todeep mineralization, or near-surface features, or normal varia-tions of gas within soils.

Pauwels et al. (1999) described a method for the collection ofmetals in soil air using activated carbon held inside a bag ofGore-Tex within in a cylindrical plastic container. They carriedout orientation surveys in southern Spain, where measurementsof CO2, He and Rn showed that gas is emanating from fracturesthat cut ore bodies in the Iberian Pyrite Belt. Collectors wereinstalled in the soils and recovered after 100 days. Elution andanalysis of the activated carbon from the Iberian Belt showedmetal anomalies coincident with CO2 anomalies. Courtesy ofPhilippe Freyssinet of BRGM, similar collectors were installedon Line 3 at the Mike deposit in May 2000 and retrieved inAugust 2000. The collectors were placed at the top of a 50 cmlength of PVC pipe inserted into auger holes, leaving the

Fig. 27. Analyses for Mn, Cu, Au andCd by aqua regia in soils from Line 3across the Mike deposit. Note that thehorizontal axis is in feet, correspondingto the original Newmont survey.

Fig. 28. Analyses for Cu by aqua regia,hydroxylamine, MMI-A and EnzymeLeach in soils from Line 3 across theMike deposit. Note that the horizontalaxis is in feet, corresponding to theoriginal Newmont survey.


collectors above ground. Forty-four collectors, includingduplicate sites, were installed and three blank collectors wereretained within plastic bags. Two of these blanks were kept inthe field and one in the laboratory. After shipping to BRGM,the carbon was eluted and the solution analysed by ICP-MS.Contents above the detection limit were obtained for As, Cr,Co, Cu, Ni, Pb and Zn. However, in all cases, the contentsobtained from the collectors installed in the field were nodifferent from the blanks, i.e. no measurable amounts of metalwere detected in soil air. Similar collectors were installed in thepiedmont gravel soils of the fracture zone above the Spencedeposit and at a background location west of the deposit. Thesewere retrieved after one year. Elution and analysis of the carbonshowed that metals were no more abundant at the sites over thedeposit than at the background sites.

Note added in proof

Newmont carried out a major drilling program at the Mikedeposit from 1997 to 2000 and the results have been describedby Norby and Orobona (2002). The principal soil anomalies atlocation B (Fig. 27) are now known to occur at the surfaceintersection of the Nebulous fault, a NNE-trending post-Carlinfault. In addition to cutting the Carlin Formation, it cuts anddisplaces the gold–copper oxide zone, and a lower sulphidezone at c. 500 m depth now described by Norby and Orobona.The top of the sulphide zone is distinguished by a 60 m thickblanket of sphalerite-rich rock. Cadmium is a common traceconstitutent of sphalerite. This explains why Cd shows such aclear anomalous pattern (Fig. 27) above a gold–copper deposit.There is localized oxidation of the sulphide zone where it isintersected by the Nebulous fault. Other elements present in thesulphide zone, including Zn, Ag, As and Ni, are also anomalousin the soils along the scarp slope of the Nebulous fault. Theelements are interpreted to have moved 500 m up the fault fromthe sulphide zone. Recent isotopic studies by Dublyansky et al.(2003) at the Yucca Mountain nuclear waste disposal site, alsoin Nevada, have shown that fluids of deep-seated origin havemoved up several hundred metres through a thick vadose zonealong a permeable fault.

Studies in Ontario

Four sites were studied in the Abitibi greenstone belt of Ontario(Fig. 29). This belt is host to a number of world-scale deposits,

such as the Kidd Creek volcanogenic massive sulphide (VMS)deposit near Timmins, the Hollinger, Dome and McIntyre Audeposits at Timmins and the Lake Shore and Kerr-Addison Audeposits near Kirkland Lake. Much of the belt has a dis-continuous cover of glaciolacustrine clays and other fine-grained glacial sediments (Fig. 29). Most of the known depositsare exposed in windows in this glacial cover. There is reason toexpect that undiscovered deposits lie hidden beneath glacialsediments. We here discuss some of the results for one of thesites, Cross Lake.

Cross Lake. The Cross Lake VMS mineralization of Archeanage, located 50 km SE of Timmins, is similar in mineralogy,composition and age to the well-known Kidd Creek Zn–Cu–Agdeposit. Host rocks are felsic pyroclastic rocks, intruded byfeldspar porphyry and diabase dykes. The immediate host is tuff,lapilli tuff and chert, with sericitic and chloritic alteration. OnLine 6 (Fig. 30), sulphides in the mineralized lens are, in orderof abundance, pyrite, honey-coloured sphalerite, chalcopyriteand galena. Zinc dominates over Cu with assays as high as 26%compared to a maximum of 1.3% Cu. The width of the VMSsubcrop underlying Line 6 is about 25 m and is covered by 30 mof varved clay with silty laminae that was deposited in a glaciallake c. 8,000 years. The area is wet, with groundwater close toor at the surface in low areas.

Fig. 29. The western portion of the Abitibi greenstone belt (shaded)showing the location of the four study sites. Much of the area to thewest of Rouyn-Noranda and Matagami has a discontinuous cover offine-grained glaciolacustrine sediments.

Fig. 30. Geological cross-section along Line 6 at Cross Lake, basedon diamond drill hole data by Cross Lake Minerals Ltd andoverburden drilling by Ontario Geological Survey. Superimposed area spontaneous potential (SP), ORP measurements of redox con-ditions at 1.5 m below water table with a two-minute reading,carbonate content of B-horizon soils, and Zn content by MMIanalysis of soils taken at a depth of 10 to 25 cm below the base ofdecomposing vegetation.


A spontaneous potential (SP) survey showed a low of 15 mVdirectly over the sulphides (Fig. 30). Initial sampling at sites25 m apart of soils taken from a thick 20 cm interval of theB-horizon and also of humus (H), followed by analysis by avariety of methods, gave only weak base metal anomalies overthe mineralization. But there is a pH low for soil-water slurriesover the mineralization and strong peaks for Ca, Mg and CO3on the flanks of the pH low. Unweathered clay containscarbonate, but this is removed during the formation ofB-horizon soils. Thus the carbonate peaks in the soils flankingthe pH low is unusual for B-horizon soils in this clay terrain.

Separate soil samples were collected specifically for MMIanalysis over a constant interval 10 to 25 cm below decayingvegetation. These samples were mainly B-horizon with anadmixture of Ae and/or Ah soil at some sites. They showeddistinct Zn anomalies over the mineralization that were absentin the B-horizon samples collected over a thicker and deeperinterval from that horizon. These disparate results for two setsof samples collected at the same time from the same sitessuggested a depth control on Zn contents in the soils.Follow-up sampling over a restricted length of the traverse,centred on the mineralized interval, collected (a) the upper0–10 cm interval of the B-horizon and (b) the 10–20 cm intervalof the B-horizon. The results (Fig. 31) show that anomalouslevels of Zn are confined to the uppermost part of theB-horizon, being absent in samples taken immediately below.

Sequential leach analyses were carried out by the GeologicalSurvey of Canada on selected samples from the 0–10 cminterval of the B-horizon. These samples (Fig. 32, with locationsshown on Fig. 31) comprise sample ‘A’ from a site away fromthe mineralization and samples ‘B’ and ‘C’ directly above themineralized subcrop. The first leach in the sequence wasammonium acetate at pH 7 (AA7). This dissolves carbonateminerals plus water-soluble elements. There is very little car-bonate in the samples, as shown by total CO3 concentrations ofranging from 300 to 800 ppm for the three samples. Neverthe-less, this leach dissolves what carbonate is present, as shown byanalyses for Ca (Table 5).

The location of the samples is shown in Figure 32. Sample Ais from a background area, whereas samples B and C overlie themineralization. The abbreviated headings for the differentsequential leaches are explained in the text.

The Zn analyses by this leach show a strong anomaly/background contrast between samples B and C over themineralized subcrop compared to background sample A. Afterleaching with AA7, the sample residues were treated withammonium acetate at pH 5 (AA5). This is a stronger extractantfor carbonate, but little, if any, carbonate is inferred to haveremained after the previous treatment, since the amounts of Caextracted are low. This leach extracts more Zn that AA7 and theanomaly/background for AA5 is greater than for AA7. Thenext leach, cold hydroxylamine (HX Mn), extracts Mn oxides.This leach dissolves more Zn from samples B and C than frombackground sample A, but the anomaly/background contrast isless than either of the acetate leaches (Table 5). The next leach,hot hydroxylamine (HX Fe) dissolves Fe oxides. This extractssubstantially more Zn than any of the preceding leaches, butthis is endogenic material, not derived from the mineralization,and no anomaly is apparent over the mineralized zone. The fifthleach, aqua regia, presents a similar pattern, with much Znextracted, but no anomaly.

Comparative data are given in Table 5 for separate (non-sequential) analyses by the Enzyme Leach and MMI-A. EnzymeLeach extracts less Ca, somewhat less Mn, and an equivalentamount of Fe to leach AA7. Less Zn is extracted by the EnzymeLeach than AA7, but the anomaly/background contrast ishigh. No major elements are reported with MMI-A analyses.However, the anomaly/background contrast for Zn by MMI-Ais excellent. The data shown in Table 5 and Figure 32 illustratethe effectiveness of weak leaches in selectively extracting theexogenic phase of an indicator element and separating thisfrom the much more abundant endogenic phase that is derivedfrom the primary minerals of the clay sediment, the parent ofthe soil.

Concurrent with the soil sampling, drilling was carried out bythe Ontario Geological Survey. Shallow boreholes were drilled

Fig. 31. Analyses for Zn and Cd byEnzyme Leach and Ca by ammoniumacetate on soil samples taken from theupper 10 cm of B-horizon and the10–20 cm interval of B-horizon alongLine 6 at Cross Lake. Also shown arepH values for soil-water slurries. TheVMS subcrop is beneath 30 m of siltyclay and extends from 175 m to 200 malong Line 6. Locations of samples A,B and C for the 0–10 cm interval of theB-horizon (see lower left plot) relate toFigure 32.


to 9 m depth to characterize the overburden and subsurfacegeochemical, pH and redox conditions. Lower redox values, asmeasured by ORP, outline a reduced column in the claysubstrate above the mineralization (Fig. 33). This feature isreplicated at the Marsh Zone site. Below the water table, pH isrelatively constant in a neutral to weakly alkaline range, e.g., at1.5 m below the water table the pH ranges from 7.1 to 7.8. Butat the water table and above there is a substantial drop in pHdirectly over the mineralization, to values as low as 5.0.

Fieldwork at Cross Lake provided an opportunity to testthe model of Hamilton (2000). The empirical observationssummarized in Figure 31 show a close similarity to thetheoretical predictions summarized in Figure 5. Oxidation ofthe sulphides at depth, suggested by SP lows, releases a risingcolumn of reduced material. Below the water table, access tooxygen and other oxidized species is restricted, but where thecolumn intersects the water table, the reduced species are

oxidized, e.g.: Fe2+ + YO2 + 2H2O h Fe(OH)3 + H+.Formation of hydrogen ions lowers the pH of the soils at andabove the water table, dissolving CaCO3, which migrateslaterally and precipitates on the flanks of the reduced zone inareas of higher pH. The reduced column must include basemetals present in the sulphide mineralization, most notably Znand Cd, which show as anomalous concentrations in a narrowzone at the top of the B-horizon (Fig. 31). The width of theacidic zone and anomalous levels of Zn and Cd in the upperB-horizon are 65 to 70 m, with sharp boundaries (Fig. 31).Given the estimated width of 25 m for the VMS lens at thesubcrop beneath 30 m of clay, the upward dispersion ofmaterial is fairly narrowly constrained. Substantial amounts ofreduced material are involved to create the zones of low pH anddissolve carbonate at the surface. Lead extracted from the soilsabove the mineralization is less radiogenic than that sampledaway from the mineralization (Cameron et al. 2001). This

Fig. 32. Zinc in ppb by sequential leaches of background sample A and samples B and C over a VMS subcrop below 30 m of clay. Samplesare from the upper 10 cm of the B-horizon, Line 6 at Cross Lake. Locations of samples are shown in Figure 31, Zn plot. Upward projectionof the VMS subcrop is indicated by the hatched block.

Table 5. Data for Ca, Mn, Fe and Zn for a series of sequential leaches and for two single leaches of samples from the 0–10 cm interval of the B-horizon on Line 6 at Cross Lake.

Sequential leaches Single leaches

Sample AA7 AA5 HX Mn HX Fe Aqua regia Enzyme Leach MMI-A

Ca (ppm)A 1710 288 654 294 1513 205 –B 560 106 62 65 1039 142 –C 490 84 213 135 1032 99 –Mn (ppm)A 1.2 5.1 48 64 100 1.1 –B 4.1 5.2 3.5 46 79 2.1 –C 2.2 2.5 2.0 39 76 0.9 –Fe (ppm)A 2.2 23 118 2800 11400 2.7 –B 14 164 344 4500 7900 15 –C 10 135 252 3600 7200 10 –Zn (ppb)A 42 103 303 8200 25000 7 64B 435 1500 970 7700 19100 270 1930C 317 1410 590 7400 18700 186 1670

The location of the samples is shown in Figure 32. Sample A is from a background area, whereas samples B and C overlie the mineralization. The abbreviatedheadings for the different sequential leaches are explained in the text.


suggests that Pb2+, as well as Zn2+ and Cd2+ are beingtransferred upwards in the reduced column, because of thenon-radiogenic nature of Pb present in VMS, compared to thatof clay. Trenching showed that tree roots do not penetratedeeper than the water table, indicating that roots are notinvolved in the advective transfer of mineralized groundwaterfrom depth to the surface. Given the short length of time sincethe glacial sediments were deposited, c. 8,000 years, and the lowrates cited above for chemical diffusion, diffusion cannot be theprincipal means for moving reduced material to the surface.

Despite the close coincidence between the Hamiltontheoretical model and empirical observations, the high Zn andCd concentrations in the soils above the reduced columncannot be easily accounted for by electrochemical processes.These elements are usually mobile as Zn2+ and Cd2+, which arenot redox-active; in this form their migration should notdetermined by redox gradients. Hamilton et al. (2001, 2002)reported further intriguing data from the drilling studies atCross Lake. Over the mineralized zone there is an upward‘bulge’ in the piezometric surface of close to 50 cm on bothlines, despite downward hydraulic gradients between the over-burden and bedrock piezometers. Near-surface groundwateralso shows an increase of c. 1�C over the mineralized zone;measurements that have been replicated at different times.Similar features have been noted at the Marsh Zone and othersites where reduced ‘columns’ are documented within otherwiseoxidized overburden. Much remains to be done to understandtransport mechanisms and to better define changes in tempera-ture and the piezometric surface that may be related tomineralization. This is being undertaken as part of a successorproject at Cross Lake led by G.E.M. Hall and funded by theOntario Mineral Exploration Technology programme and theGeological Survey of Canada.

CONCLUSION

In the Deep-Penetrating Geochemistry project we haveconsidered the theoretical basis for the upward movement ofmaterial from buried deposits and have carried out field tests inChile, Nevada and Ontario to examine transport processes andcompare methods for detecting anomalies. For arid or semi-aridareas, with a thick vadose zone, upward diffusion of dissolved

elements is orders of magnitude slower than the downwardmovement of water films. The only possibilities for the upwardtransport of elements are mass (advective) transfer of water plusdissolved constituents, or air plus indicator gases or aerosols.

The most striking example of advective transport is inChile where the earthquake-induced (seismic) pumping ofmineralized groundwater to the surface through fracture zonesis recognized. The assemblage of elements found in the soilsabove fracture zones is similar to that found in groundwatersand, at Spence, Cu in soils and Cu in groundwaters at60 m depth are only anomalous in the vicinity of the deposit.Faults or fracture zones are an essential conduit for bringingmineralized groundwater to the surface and we find a spatialcorrelation between anomalies in soils and neotectonicstructures, which appear to represent reactivation of moreancient faults. In Nevada, at the Mike deposit, covered by240 m of Carlin Formation, we also find strong anomaliesassociated with interpreted neotectonic structures, but here wehave no evidence of the transport mechanism.

The Abitibi region stands in diametric contrast to Chile andNevada: the glacial clay and sand that cover bedrock are only afew thousand years old, lack fracturing, and are water-saturatedto near the surface. Here too soils show clear signs of buriedmineralization and the empirical observations are remarkablysimilar to those predicted by the model of Hamilton. Thisinvolves oxidation of the buried sulphide mass and the upwardtransfer of reduced material, including base metals. At the watertable this reduced material reacts with O2, creating an acidicenvironment that dissolves and redistributes carbonateoriginally present in the clay. The substantial flux of reducedsubstances to the surface, the elevated concentrations of Zn andCd in the soils over mineralization, bulges in the piezometricsurface, and temperature increases of groundwater at the samelocations cannot be accommodated by diffusion processes;additional mechanisms are required to explain the empiricaldata.

Barometric pumping provides a means for the rapidtransport of air plus gas or aerosols. At the Mike deposit,Nevada, tests of the metal content of soil air using collectorscontaining activated carbon failed to detect greater amounts ofmetal than blank collectors sealed in plastic bags. Similar tests inthe piedmont gravel soils on the fracture zone above the Spencedeposit also failed to detect metal concentrations that weregreater than a nearby background area. At Mike, CO2 and O2measurements in soil air showed variation, but it was notpossible to conclude that these were related to the deposit.There is a need to carry out tests on air/gas mixtures releasedfrom fractured rock over known mineralization to identify theconstituents present.

By far the most widely used geochemical method for thediscovery of buried deposits is selective leach analyses of soils;this study has focused on these methods. We argue thatgeochemical anomalies in soils over buried mineralization formincrementally. Metals of external, exogenous origin enter thesoil in water-soluble form and are progressively incorporatedinto secondary minerals, such as carbonates, and iron andmanganese oxides. In the case of the Chile and Nevada testsites, the time available for anomaly formation may have beena million years, or longer. At these sites there has been time formetal of exogenous origin to be incorporated in a variety ofsecondary minerals. Thus the anomalies can be detected by astrong leach, aqua regia, which dissolves all secondary minerals,although the anomaly/background contrast is less than forselective leaches that target specific secondary minerals. In theclay-covered terrain of the Abitibi region, several thousandyears only have been available for anomaly formation. In this

Fig. 33. Model for development of anomalies through clay coverincorporating empirical observations from the Cross Lake test site.There is a reduced column rising above the sulphide subcrop throughthe clay cover. At and above the water table, metal ions in thisreduced column are oxidized by infiltrating oxygen, which generateshydrogen ions. The resulting low pH environment dissolves carbon-ates from C-horizon clay soils and this carbonate is reprecipitated onthe flanks of the zone of low pH. The Zn2+ and Cd2+ ions areinferred to have migrated upwards in the reduced column becauseanomalous amounts have accumulated in a narrow zone at the top ofthe B-horizon.


case metals from the mineralization have only been incorpor-ated into the most labile secondary minerals and anomalies overmineralization cannot be recognized by strong leaches.

There are two strategies for the choice of leaches: (a) measurethe water-soluble phase before it has entered secondary miner-als; or (b) selectively dissolve one or more secondary minerals.A deionized water extraction or the Enzyme Leach bothmeasure the water-soluble phase, but the latter gives betterreproducibility. The amounts of metals extracted by either leachare similar. To dissolve secondary minerals we have testedammonium acetate for carbonates, cold hydroxylamine (HXMn) for Mn oxides, hot hydroxylamine (HX Fe) for Fe oxides,and MMI-A for undetermined minerals. The usefulness of thesemethods for exploration purposes depends on the ratio ofexogenic metal to endogenic metal in the target mineral. Wherethe ratio is high, a good anomaly/background contrast isobtained, as for Cu in carbonates in Spence soils or for Znassociated with carbonates at Cross Lake. In another case, atGaby Sur, the amount of Cu of endogenic origin in carbonateis too high and obscures the Cu derived from the deposit. Otherthan the leaches that target the water-soluble phase, leachesusually measure the cumulative content of two or more phases.Ammonium acetate at pH 5 measures metal present in carbon-ate plus that which is water-soluble. Hydroxylamine andMMI-A are acidic leaches, so dissolve carbonate as well as, forexample, the Fe and/or Mn oxides which hydroxylamine istargeted to dissolve. Neutralization of leach acidity by carbonatereduces their ability to dissolve and hold metals in solution,whereas ammonium acetate is buffered at constant pH.

The depth at which soil samples are taken is critical. AtSpence, the principal indicator element, Cu, is confined to thetop 20 cm of the profile, whereas other porphyry indicatorelements, such as As and Re, which dissolve as anions, have beenremoved by rainfall below 40 cm. In the Abitibi region, at CrossLake, only sampling that includes a critical metal-accumulationhorizon near the top of the B-horizon proved effective.

As will be apparent to the reader, this study could not have beencarried out without the cooperation of many people in the miningindustry and in government. We thank Ollie Bonham and Jack Currieof RioChilex and Gordon Gray and Kelly O’Connor of Rio Algomfor many courtesies during our work at Spence. Sampling at GabySur was done collaboratively with Ricardo Venegas and AldoVénegas of Codelco. E.M.C. and M.I.L. were assisted in the field inChile with enthusiasm and humour by Daniel Salinas and AlexiRamirez. George Steele of Rio Tinto kindly gave logistical supportfor the Chile work. At Mike, Robert Jackson of Newmont Inc.provided a great deal of help and advice. Our sampling at Mike wascarried out together with Mary Doherty and Kevin Creel. The CO2gas sampling at Mike was by Patrick Highsmith and the collectorscontaining activated carbon were provided by Philippe Freyssinet,who also provided the analyses of the carbon. For the Cross Lakestudies we are indebted to Robert Middleton and Ian Millar-Tate ofCross Lake Minerals for providing maps and drill hole information.Brian Polk and Devin Cranston worked with us throughout thesampling programme in the Abitibi. Colin Dunn carried out thebiogeochemical studies at Cross Lake on a tight time frame nearthe end of the project. We thank Richard Alcock, Research Directorof CAMIRO; Bill Coker, chair of the CAMIRO GeochemistryCommittee; and 26 company sponsors for their support and encour-agement throughout the project. The fruitful collaboration withthe Ontario Geological Survey would not have been possible with-out the enthusiastic support of Cam Baker. The participation ofGeological Survey of Canada staff and facilities is gratefully ac-knowledged. Sample preparation was by Overburden DrillingManagement. Analyses were provided at no charge by JohnGravel of Acme Laboratories, Eric Hoffman of Actlabs, PatrickHighsmith of ALS-Chemex, Claude Massie of Bondar-Clegg,Robert Ellis of Gedex and Hugh DeSouza of XRAL and XieXuejing of IGGE.

REFERENCES

AMUNDSON, R.G. & DAVIDSON, E.A. 1990. Carbon dioxide and nitrogenousgases in the soil atmosphere. Journal of Geochemical Exploration, 38, 13–41.

ARIMOTO, R., RAY, B.J., DUCE, R.A., HEWITT, A.D., BOLDI, R. & HUDSON, A.1990. Concentrations, sources, and fluxes of trace elements in the remotemarine atmosphere of New Zealand. Journal of Geophysical Research, 95,22389–22405.

BLAIN, C.F. & BROTHERTON, R.L. 1975. Self-potentials in relation to oxidationof nickel sulphide bodies within semi-arid climatic terrains. Institute MiningMetallurgy Transactions, Section B, 84, 123–127.

BOLVIKEN, B. & LOGN, O. 1974. An electrochemical model for elementdistribution around sulphide bodies. In: ELLIOT, I.L. & FLETCHER, W.K.(eds) Geochemical Exploration 1974. Special Publication, 2. Association ofExploration Geochemists, 631–648.

BRIGGS, R.C. & TROXELL, H.C. 1955. Effect of Arvin-Tehachapi earthquake onspring and stream flow. In: OAKESHOTT, G.B. (ed.) Earthquakes in KernCounty, California, during 1952. Part 1. Division of Mines Bulletin, 171.Department of Natural Resources, California, 81–98.

CAMERON, E.M. 1998. Deep-Penetrating Geochemistry. Report. Canadian MiningIndustry Research Organization (CAMIRO).

CAMERON, E.M. & LEYBOURNE, M.I. 2002. Isotopic compositions of oxygen andcarbon from carbonate and sulphur from sulphate in soils from the Spence and Gaby Surporphyry deposits and from Tamarugal, northern Chile. Report. Canadian MiningIndustry Research Organization (CAMIRO).

CAMERON, E.M., COUSENS, B.L. & HALL, G.E.M. 2001. A pilot study for the useof lead isotopes to determine the genesis of anomalies above buried VMS mineralization,Cross Lake. Report. Canadian Mining Industry Research Organization(CAMIRO).

CAMERON, E.M., LEYBOURNE, M.I. & KELLEY, D.L. 2002. Exploring fordeeply-covered mineral deposits: formation of geochemical anomalies innorthern Chile by earthquake-induced surface flooding of mineralizedgroundwaters. Geology, 30, 1007–1010.

CAMUS, F. 2001. Descubrimiento de porfidos de cobre en zonas cubiertas: El ejemplo deGaby Sur. 20th International Geochemical Exploration Symposium.

CANNON, H.L. & STARRETT, W.H. 1958. Botanical prospecting for uranium onLa Ventura Mesa. Sandoval County, New Mexico. United States Geological SurveyBulletin, 1009M, 391–407.

CARRIGAN, C.R., HEINLE, R.A., HUDSON, G.B., NITAO, J.J. & ZUCCA, J.J. 1996.Trace gas emissions on geological faults as indicators of undergroundnuclear testing. Nature, 382, 528–531.

CLARK, J.R., YEAGER, J.R., ROGERS, P. & HOFFMAN, E.L. 1997. Innovative enzymeleach provides cost-effective overburden/bedrock penetration. Abstract,. Exploration‘97.

CROZAT, G., DOMERGUE, J.L. & BOGUI, V. 1973. Etude de l’aerosol atmos-pherique en Cote d’Ivoire et dans le Golfe de Guinee. Atmosphere andEnvironment, 7, 1103–1116.

DOHRENWEND, J.C. & MORING, B.C. 1991. Reconnaissance photogeological map ofyoung faults in the Winnemucca 1o by 2o quadrangle, Nevada. Miscellaneous FieldStudies Map MF-2175. United States Geological Survey.

DUBLYANSKY, Y.V., SMIRNOV, S.Z. & PASHENKO, S.E. 2003. Identification ofthe deep-seated component in paleo fluids circulating through a potentialnuclear waste disposal site: Yucca Mountain, Nevada, USA. Journal ofGeochemical Exploration, 78, 39–43.

FONTES, J.Ch., YOUSFI, M. & ALLISON, G.B. 1986. Estimation of long-term,diffuse groundwater discharge in the northern Sahara using stable isotopeprofiles in soil water. Journal of Hydrology, 86, 315–327.

GOVETT, G.J.S. 1974. Soil conductivities: assessment of an electrochemicaltechnique. In: ELLIOT, I.L. & FLETCHER, W.K. (eds) Geochemical Exploration.Special Publication, 2. Association of Exploration Geochemists, 101–118.

GOVETT, G.J.S., DUNLOP, A.C. & ATHERDEN, P.R. 1984. Electrogeochemicaltechniques in deeply weathered terrain in Australia. Journal of GeochemicalExploration, 21, 311–331.

HALL, G.E.M. 1992. Inductively coupled plasma mass spectrometry ingeoanalysis. Journal of Geochemical Exploration, 44, 201–249.

HALL, G.E.M. 1998. Analytical perspectives on trace element species ofinterest in exploration. Journal of Geochemical Exploration, 61, 1–20.

HALL, G.E.M., VAIVE, J.E. & BUTTON, P. 1997. Detection of past undergroundnuclear events by geochemical signatures in soils. Journal of GeochemicalExploration, 59, 145–162.

HAMILTON, S.M. 1998. Electrochemical mass transport in overburden: a newmodel to account for the formation of selective leach geochemicalanomalies in glacial terrain. Journal of Geochemical Exploration, 63, 155–172.

HAMILTON, S.M. 1999. Summary of Fieldwork and Other Activities, 1999. Open FileReport, 6000. Ontario Geological Survey, Ontario, 421–426.

HAMILTON, S.M. 2000. Spontaneous potentials and electrochemical cells. In:HALE, M. (ed.) Geochemical Remote Sensing of the Subsurface. Handbook ofExploration Geochemistry. Elsevier, Amsterdam, 6, 81–119.

HAMILTON, S.M., CAMERON, E.M., MCCLENAGHAN, M.B. & HALL, G.E.M. 2001.Deep-Penetrating Geochemistry: Cross Lake Final Report. Report. CanadianMining Industry Research Organization (CAMIRO) Report.


HAMILTON, S.M., CAMERON, E.M., MCCLENAGHAN, M.B., HALL, G.E.M.,LEYBOURNE, M.A., SADER, J.A. & CRANSTON, D.R. 2002. Thick OverburdenGeochemical Methods: Studies over Volcanogenic Massive SulphideMineralization and Kimberlite. In: Summary of Fieldwork and Other Activities,2002. Open File Report, 6100. Ontario Geological Survey, Ontario,271–2717.

JACKSON, R.G. 2000. Mobile Ion Geochemistry Mike Epithermal Gold Deposit,Nevada, USA. Report. Canadian Mining Industry Research Organization(CAMIRO).

KELLEY, D.L. 1995. An investigation of piedmont geochemical exploration techniques forburied porphyry Cu deposits. Part 1. Soil Geochemistry and Biogeochemistry. Report.BHP Minerals.

KRISTIANSSON, K. & MALMQVIST, L. 1982. Evidence of non-diffusive transportof Rn-222 in the ground and a new physical model for the transport.Geophysics, 47, 1444–1452.

LEYBOURNE, M.I. & CAMERON, E.M. 2000. Composition of groundwaters at theSpence deposit: Part A: Major elements and stable isotopes. Report. CanadianMining Industry Research Organization (CAMIRO).

LEYBOURNE, M.I. & CAMERON, E.M. 2000. Composition of groundwaters at theSpence deposit: Part B—Metals associated with supergene Cu mineralization. Report.Canadian Mining Industry Research Organization (CAMIRO).

LOVELL, J. & HALE, M. 1983. Application of soil-air carbon dioxide andoxygen measurements as a guide to mineral exploration. Transactions Instituteof Mining and Metallurgy, 92, 28–32.

LOVELL, J.S., HALE, M. & WEBB, J.S. 1983. Soil air carbon dioxide and oxygenmeasurements as a guide to concealed mineralizations in semi-arid and aridregions. Journal of Geochemical Exploration, 19, 305–317.

MCCARTHY, H. & MCGUIRE, E. 1998. Soil gas studies along the Carlin Trend. Eurekaand Elko Counties, Nevada. Open File Report, 98-338. United StatesGeological Survey, 243–250.

MCCARTHY, J.H., LAMBE, R.N. & DIETRICH, J.A. 1986. A case study of soilgases as an exploration guide in glaciated terrain: Crandon massive sulphidedeposit, Wisconsin. Economic Geology, 81, 408–420.

MUIR-WOOD, R. 1994. Earthquakes, strain-cycling and the mobilization offluids. In: PARNELL, J. (ed.) Geofluids: Origin, Migration and Evolution of Fluidsin Sedimentary Basins. Special Publication, 78. Geological Society, London,85–98.

NILSON, R.H., PETERSON, E.W., LIE, K.H., BURKHARD, N.R. & HEARST, J.R.1991. Atmospheric pumping: a mechanism causing vertical transport ofcontaminated gases through fractured permeable media. Journal ofGeophysical Research, 96, 21933–21948.

NORBY, J.W. & OROBONA, M.J.T. 2002. Geology and mineral systems ofthe Mike deposit. In: THOMPSON, T.B., TEAL, L. & MEEUWIG, R.O. (eds)Gold Deposits of the Carlin Trend. Nevada Bureau of Mines and GeologyBulletin 111, 59–70.

NUR, A. 1974. Matsushiro, Japan, earthquake swarm: confirmation ofdilatancy-fluid diffusion model. Geology, 2, 217–221.

PAUWELS, H., BAUBRON, J.C., FREYSSINET, P. & CHESNEAU, M. 1999. Sorptionof metallic compounds on activated carbon: application to exploration forconcealed deposits in southern Spain. Journal of Geochemical Exploration, 66,115–133.

PHILLIPS, F.M. 1994. Environmental tracers for water movement in desertsoils of the American southwest. Journal of Soil Science Society of America, 58,15–24.

PIOTROWICZ, S.R., DUCE, R.A., FASCHING, J.L. & WEISEL, C.P. 1979a. Burstingbubbles and their effect on the sea to air transport of Cd, Cu, Fe, Pb andZn. EOS, 60, 276.

PIOTROWICZ, S.R., DUCE, R.A., FASCHING, J.L. & WEISEL, C.P. 1979b. Burstingbubbles and their effect on the sea to air transport of Fe, Cu and Zn. MarineChemistry, 7, 307–324.

RAHN, K. 1976. The chemical composition of the atmospheric aerosol. TechnicalReport. University of Rhode Island, USA.

REMENDA, V.H., CHERRY, J.A. & EDWARDS, T.W.D. 1994. Isotopic com-position of old ground water from Lake Agassiz: Implications for LatePleistocene climate. Science, 266, 1975–1978.

ROSE, A.W. & GOW, W. 1995. Thermal convection of soil air on hillsides.Environmental Geology, 25, 258–262.

ROSE, A.W., HUTTER, A.R. & WASHINGTON, J.W. 1990. Sampling variability ofradon in soil gases. Journal of Geochemical Exploration, 38, 173–191.

ROWLAND, M.D. & CLARK, A.H. 2001. Temporal overlap of supergene alteration andhigh-sulphidation mineralization in the Spence porphyry Cu deposit, II Region, Chile,Abstract. Annual Meeting. Geological Society of America.

RUTHERFORD, N.F., JOHNSON, C., GIBLIN, A.M., GRIFFIN, W.L., RYAN, C.J. &SUTER, G.F. in press. The pSirogas research project: an assessment of theGeogas exploration method in Australian terrains. Geochemistry: Exploration,Environment, Analysis.

SCANLON, B.R. 1991. Evaluation of moisture flux from chloride data in desertsoils. Journal of Hydrology, 128, 137–156.

SCANLON, B.R. 1992. Evaluation of liquid and vapor water flow in desert soilsbased on chlorine 36 and tritium tracers and nonisothermal flowsimulations. Water Resources Research, 28, 285–297.

SCHWARTZ, F.W. 1974. The origin of chemical variations in groundwaters froma small watershed in southwestern Ontario. Canadian Journal of Earth Science,11, 893–904.

SIBSON, R.H. 1981. Fluid flow accompanying faulting: Field evidence andmodels. In: SIMPSON, D.W. & RICHARDS, P.G. (eds) Earthquake Predication:An International Review. Maurice Ewing Series, 4. American GeophysicalUnion, 593–603.

SIBSON, R.H., MOORE, J.M. & RANKIN, A.H. 1975. Seismic pumping; ahydrothermal fluid transport mechanism. Journal Geological Society LondonSpecial Publication, 131, 653–659.

SILLITOE, R.H., MARQUARDT, J.C., RAMIREZ, F., BECERRA, H. & GOMEZ, M.1996. Geology of the concealed MM porphyry Cu deposit, ChuquicamataDistrict, northern Chile. In: CAMUS, F., SILLITOE, R.H. & PETERSEN, R. (eds)Andean Cu Deposits: New discoveries, Mineralization, Styles and Metallogeny.Special Publication, 5. Society of Economic Geology, 59–70.

SMEE, B.W. 1983. Laboratory and field evidence in support of the electro-chemically enhanced migration of ions through glaciolacustrine sediment.Journal of Geochemical Exploration, 19, 277–304.

TCHALENKO, J.S. 1973. The Kashmar (Turshiz) 1903 and Torbat-e Heidariyeh(South) 1923 earthquakes in Central Khorassan (Iran). Annali di Geofisica,26, 29–40.

TCHALENKO, J.S. & BERBERIAN, M. 1974. The Salmas earthquake of May 6th,1930. Annali di Geofisica, 27, 151–212.

TEAL, L. & BRANHAM, A. 1997. Geology of the Mike gold-Cu deposit, EurekaCounty, Nevada. In: VIKRE, P. et al. (ed.) Carlin-Type Gold Deposits FieldConference. Guidebook Series, 28. Society Economic Geology, 257–276.

THORNBER, M.R. 1975. Supergene alteration of sulphides, I. A chemical modelbased on massive nickel sulphide deposits at Kambalda, Western Australia.Chemical Geology, 15, 1–14.

WEEKS, E.P. 1987. Effect of topography on gas flow in unsaturated fracturedrock: concepts and observations. In: EVANS, D.D. & NICHOLSON, T.J. (eds)Flow and Transport Through Unsaturated Fractured Rock. GeophysicalMonograph, 42. American Geophysical Institute, 165–170.

XIE, X., WANG, X., XU, L., KREMENETSKY, A.A. & KHEIFETS, V.K. 1999.Regional orientation of strategic deep penetration geochemical methods inthe central Kyzylkum desert terrain, Uzbekistan. Journal of GeochemicalExploration, 66, 135–143.


Finding Deeply Buried Deposits Using Geochemistry

Documents

Transcript of Finding Deeply Buried Deposits Using Geochemistry