Ore Deposits

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Ore depositsMichael A. MckibbenDepartment of Earth Sciences, University of California, Riverside

Introduction

Metallic ore deposits constitute the largest geochemical anomalies within the crust.

Their study has been critical to understanding the behavior of elements and isotopes in

mineral- and rock-forming processes, as well as to deciphering the geochemical

differentiation of the Earth through time. The study of ore deposits therefore influences

and draws upon virtually every subdiscipline in the Earth Sciences.

The four-year research review presented here cannot be comprehensive, given the

editor's limit of citing less than 100 research papers by U.S. authors or authors from

U.S. institutions. Instead, the intent is to provide the reader with synopses of a

representative spectrum of papers containing important advances in U.S. research on

ore deposits. In cases where difficult citation choices had to be made the more recent

papers on a particular subtopic are usually cited, because they lead the reader back to

earlier papers within the 4-year review period. Judicious use of the selected citations in

conjunction with standard literature searching tools should allow any reader to quickly

find most of the relevant literature on each subtopic.

The most recent quadrennial report on ore deposits was made byBurt[1991] for the

period from late 1986 to mid-1990, so the present review cites only publications

appearing between mid-1990 and mid-1994. Citations are made only to peer-reviewed

publications appearing in major journals, periodicals and books. Meetings abstracts,

conference and symposia proceedings, field trip guidebooks, open-file reports, and

other ``gray'' literature have not been cited.

Of the journals whose articles deal mainly with ore deposits and economic geology the

most important is Economic Geology, which at the time of this review had published

the fourth issue of 1994. Occasional Monographs on special topics are also issued. The

quarterly Newsletter of the Society of Economic Geologists, which appeared beginning

in April of 1990, is also a valuable source of current research, exploration, mining, and

environmental trends in the area of metallic mineral resources. Both the Society of

Economic Geologists and the Mineralogical Society of America periodically publish

review volumes that emphasize U.S. research on ore deposits. Other major journals that

sometimes contain articles about U.S. research on ore deposits include American

Journal of Science, American Mineralogist, Canadian Mineralogist, Chemical Geology,

Geochimica et Cosmochimica Acta, Geology, Journal of Geochemical Exploration, and

Mineralium Deposita. The U. S. Geological Survey (USGS) frequently publishes

bulletins, papers, monographs, circulars, and maps on ore deposits, as do many state

and county geological surveys.
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Status of Ore Deposits Research in the U.S.

Domestic mineral exploration is currently in decline owing to intertwined market,

legislative, environmental and political factors. Consequently many U.S. businesses,

universities and government agencies involved in mineral resources are reassessing and

readjusting their research and development priorities. Many research programs in

economic geology and mining are being downsized or phased out, and the number of

domestic students pursuing careers in economic geology is diminishing. Based on a

recent survey, Prof. Marco Einaudi of Stanford University estmates that only about 80

Ph.D. candidates are now enrolled in the discipline of economic geology at North

American universities [ M. T. Einaudi, pers. commun., 1994].

In spite of these trends, our national per capita consumption of mineral resourcescontinues to grow and that of much of the rest of the world is rapidly catching up with

ours. Those domestic mineral resources which we can still exploit must be extracted

more delicately and their carcasses restored more carefully to an acceptable

environmental state. We are increasingly dependent upon, and increasingly competing

with, the developing nations for their mineral resources. Within many of these nations,

political and environmental constraints on mineral exploration and development are

likely to grow with time.

In light of these constraints, our need to understand metallogenesis and the occurrence

of ore deposits, and our ability to exploit domestic and foreign mineral resources more

efficiently and carefully, must remain a high national priority. Otherwise we riskbecoming a vulnerable mineral-import dependent nation with no ability to exploit its

own resources in times of strife and no ready supply of domestic professionals who can

compete on the international scene. Unfortunately, our vulnerability is exacerbated by

the fact that the average U. S. citizen has little appreciation of the critical role that

mineral and energy resources play in their high standard of living. Efforts to correct this

situation must begin early in the educational process, a fact that some government and

industry agencies are now vigorously addressing.

General Books and Reviews

An excellent introductory text on mineral resources and economic geology was

produced byKesler[1994]. Compared with many earlier textbooks, he included more

emphasis on mineral economics, mining law, exploration, mining methods and the

environmental consequences of exploitation. The text is particularly suitable for an

introductory survey course on global mineral and energy resources for undergraduates

in the sciences and humanities. If every university Earth Science department offered

such a survey course, the ultimate result would be voters and decision-makers who have

a far greater understanding of global economics and politics and the role that mineral

and energy resources play in a nation's wealth.
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Studies of Specific Deposits or Districts

This first section highlights U. S. research on mineralized regions or districts, as well as

studies of specific mineral deposits.

Regional Metallogenesis and Mineral Exploration

A review of the economic geology of the U.S. was edited by Gluskoter et al. [1991] as

part of the Geology of North America Series of the Geological Society of America.

Chapters on the geology of specific mineral commodities, mostly written by USGS

experts, covered the major metals and industrial minerals. Three large maps showed the

locations of all the deposits and districts discussed in the text.

Discovery of South Australia's giant Olympic Dam deposit, comprised of 2 billion

metric tons of hydrothermal Cu-U-Au-REE (rare-earth-element) ore within

[4] hematitic, granitic breccias, led to a realization that that similar mineralization may

be associated with K-rich granites in the Precambrian basement of the U. S.

midcontinent. The strategic and critical mineral resources of the midcontinental U.S.

were therefore evaluated by a group of USGS, state and industry geologists and the

results reported in a series of papers edited byPratt and Sims [1990] andDay and Lane

[1992]. In particular, the middle Proterozoic Pea Ridge deposit of southeast Missouri

was recognized to be an Olympic Dam type deposit. The authors summarized the

available data and developed exploration strategies for locating other Olympic Damtype deposits in the U. S. midcontinent. Other examples of the development of geologic

frameworks and exploration strategies for mineral deposits can be found in a series of

papers edited by Scott et al. [1993].

The USGS continued its efforts to develop concise descriptive and grade-tonnage

models of mineral deposits for use in exploration, as described in a series of papers

edited byBliss [1992]. New and revised models were developed, mainly for various

types of gold deposits. Worksheet templates were provided for ranking the potential of

specific occurrences or prospects using the framework of models developed so far. It

will be interesting to learn from the minerals industry how useful and successful the

models are in conducting exploration and mining.

Magmatic and Magma-Hydrothermal Ore Deposits

The origins of platinum group element (PGE) enriched horizons in mafic layered

intrusions are of great interest because such types of mineralization are the main

resources of PGE.Bird et al. [1991] described a Au-Pd bearing horizon (Platinova reef)

in the Middle zone of the Skaergaard intrusion of east Greenland. Based on textures, the

Au appeared to have been trapped at a late magmatic stage as immiscible metal droplets

within rims on cumulate silicates. They argued that three distinct fluids must have

coexisted at the time of formation of the reef: silicate, sulfide and gold-rich. Boudreauand McCallum [1992] reviewed evidence for PGE enrichments in the reefs of the
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Stillwater layered mafic intrusion in Montana. They proposed a model in which a Cl-

rich fluid phase exsolved from the intercumulus (interstitial to crystals) liquid and

leached PGE from sulfide inclusions in cumulate phases, transporting both PGE and Supward and re-depositing these elements in mineralization fronts analogous to those in

rollfront uranium deposits. These two studies are not contradictory; instead they show

that multiple processes within crystallizing mafic magmas can influence the ultimate

distribution of PGE observed within layered intrusions.

Base metal skarn (coarse calc-silicate) and porphyry deposits typically develop around

crystallizing granitic plutons that have been emplaced at moderate to shallow crustal

depths above subduction zones beneath their coeval volcanic arcs. A comprehensive

review and bibliography on the geology of Au-bearing skarns was provided by

Theodore et al. [1991]; they noted that most were calcic exoskarns (developed in wall-

rock) associated with intense retrograde hydrosilicate alteration.Newberry et al. [1991]

expanded and reinterpreted mineralogic, geochemical and isotopic data from the classic

Darwin Pb-Zn-Ag skarn deposit in California, showing it to consist of several

concentrically zoned sulfide skarn pipes whose ores precipitated in response to large

shifts in fluid temperature, pH and oxidation state. Moreover, contrary to earlier studies,

they showed that this deposit was genetically unrelated to the nearby, but older, Darwin

pluton.Dilles and Einaudi [1992] described the geology and geochemistry of an

exposed 5 km vertical section of hydrothermal alteration and mineralization associated

with Ann-Mason porphyry copper deposit, one of three such deposits related to the

Yerrington batholith in Nevada. From this unique section they were able to reconstruct

the flow-paths and thermochemical evolution of hydrothermal fluids which formed thedeposit. They identified a dike swarm emanating from a deep granitic cupola as being

responsible for the mineralization, and also identified argillic alteration in an adjacent

mountain range as representing the paleosurface environment of the deep hydrothermal

system.

Olympic Dam type deposits may represent the most significant new type of ore deposit

whose geology became well-documented during the review period. As a follow-up to

their 1990 paper on the Olympic Dam deposit in South Australia,Oreskes and Einaudi

[1992] reported fluid inclusion and stable isotopic data from the unusual Fe-rich

breccias and Cu-U-Au-Ag ores. They argued that primary magmatic fluids probably

deposited early magnetite, but that the mineralized hematitic breccias were formed fromthe influx of cooler fluids having a more surficial origin. As noted above, Proterozoic

Olympic Dam type deposits also occur in granite-rhyolite terranes of the U.S.

midcontinent, as described byNuelle et al. [1992] and Sidder et al. [1993]. These

USGS authors concluded that saline magma-hydrothermal fluids derived from Fe-rich

trachytes had initially emplaced Fe-silica ore and then subsequently boiled and

explosively emplaced rare earth element

[4] (REE) bearing breccias into rhyolitic tuffs within a shallow eroded caldera complex.

Stable isotopes can be used to ascertain the degree to which magmatic volatiles

contributed directly to volcanic-hosted ore deposits. Vennemann et al. [1993] used

stable isotopic data to infer a direct role for condensed magmatic fluids in genesis of thePueblo Viejo acid sulfate Au-Ag deposit (Dominican Republic), the world's largest bulk
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mineable deposit of this type. The late Cretaceous deposit was formed at shallow crustal

depth within a maar-diatreme setting. Although metals and fluids were derived largely

from magmatic vapors, it may be that shallow mixing with and cooling by convectivelycirculating meteoric or seawaters caused precipitation of the ores and acid alteration

assemblages. The Pueblo Viejo hydrothermal system may have been similar to modern

magma-hydrothermal systems such as White Island, New Zealand.

Hydrothermal Mineral Deposits

Recent U.S.-Canada research along the Gorda Ridge in the NE Pacific led to a special

Economic Geology issue on seafloor hydrothermal mineralization, edited byRona and

Scott[1993].Zierenberg et al. [1993] described Besshi-type massive sulfide deposits

forming on a sediment-covered spreading center, the axial Escanaba Trough on theSouthern Gorda Ridge. The deposits form along the margins of uplifted sediment fault

blocks generated by intrusion of MORB (mid-ocean ridge basalt) laccoliths. Because of

hydrothermal fluid interactions with sediments, the deposits are enriched in group IV, V

and VI elements, thermogenic hydrocarbons, and radiogenic Pb compared with those

deposits forming in sediment-free spreading centers.Doe [1994] analyzed and

discussed source rock control on the Zn, Cu and Pb contents of ocean-ridge

hydrothermal fluids; in particular the relatively low Pb contents of mid-ocean ridge

basalts lead to a predominance of Zn- and Cu-rich sulfide deposits in sediment-starved

ridge systems.

Studies of Mississippi Valley-type carbonate-hosted lead-zinc deposits continued toreveal the geologic, hydrologic and geochemical complexities of these fascinating

epigenetic ore deposits, which form the major U. S. resources of lead and zinc. There

are still many unresolved apects of the origin of these deposits. Several papers

presented various geochemical arguments for the influence of multiple fluid sources

and/or aquifers in the genesis of some deposits or districts (for example, compare and

contrast the databases and conclusions ofViets and Leach, [1990], Shelton et al.,

[1992], andKesler et al., [1994]). Structural and tectonic controls of ore genesis in the

Southeast Missouri lead belt were examined by several authors.Horrall et al. [1993]

suggested that much of the Cu, Co, Ni and siderophile element enrichments in the

southeast Missouri Pb-Zn district were derived by basinal brine leaching of alkali mafic

and ultramafic plutons occurring along the margins of the Reelfoot rift (New Madridseismic zone). Clendenin et al. [1994] argued that local and micro-structural controls on

fluid flow were important in localizing ore, and that stratigraphic units do not behave as

homogeneous aquifers as is commonly assumed in many numerical fluid-flow models.

Nonetheless, Garven et al. [1993] developed numerical simulations for Late Paleozoic

regional gravity-driven groundwater flow triggered by uplift after the Alleghanian

orogeny in the midcontinental region of North America. They used temporal and

geographic variations in uplift to explain variations in the timing and directions of

regional fluid flow and discharge, and the resulting genesis of carbonate-hosted Pb-Zn

mineral deposits.

Much like diamonds, gold continues to garner a level of interest that is out of proportion

to its true relative importance in technology and industry. Nonetheless, important
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contributions to the U. S. literature on hydrothermal gold deposits were made during the

review period. A volcano-tectonic framework for epithermal Au-Ag deposits in the

western United States was presented byBerger and Bonham [1990].Lipman [1992]reviewed how structures in calderas influence and localize ore deposition. The geology

and origin of the high-grade acid sulfate Cu-Au vein deposit at El Indio, Chile was

described byJannas et al. [1990]; they found that Au deposition actually occurred from

late, low-salinity geothermal fluids that were different from those of more magma-

hydrothermal affinity that deposited the enargite and alunite. Cunningham et al. [1991]

described a conceptual genetic model for the diversity of volcanic-dome hosted

precious metals deposits in Bolivia, which should prove more generally applicable.

In a paper with important exploration significance,Nelson [1990] evaluated the

geochemistry of jasperoids from Carlin-type sediment-hosted Au deposits in the

western United States. He found that elements characteristic of metalliferous marine

black shales can be used to distinguish ore-bearing from barren systems. Acid alteration

and oxidation are frequently cited as evidence of boiling of epithermal fluids in such

deposits, butKuehn and Rose [1992] showed that Au deposition at Carlin, Nevada, was

structurally and stratigraphically controlled and occurred well before widespread

oxidation, the latter of which is supergene (shallow weathering) in origin and not

caused by shallow boiling during ore formation.

Metamorphosed Ore Deposits

Because of the obliterative effects of metamorphism, the primary origins of ore depositsfound in metamorphic rocks are often controversial.Slack et al. [1993] studied

mineralization in the large Broken Hill District of Australia, and demonstrated that the

metamorphosed base metal ores originally formed during interaction of hydrothermal

fluids with non-marine evaporitic sediments in a Proterozoic continental rift setting.

Gemmell et al. [1992] described a stratigraphically conformable Zn-Pb-Ag deposit in

Argentina whose geologic and isotopic features support an origin as a shallow marine

sedimentary exhalative sulfide deposit, rather than a contact metamorphic magma-

hydrothermal deposit as proposed earlier. A metamorphosed submarine hydrothermal

Mn deposit in North Carolina whose geochemical signatures survived metamorphism to

amphibolite facies conditions was described byFlohr[1992]. From a textural and

mineralogical standpoint, Craig and Vokes [1993] reviewed the effects ofmetamorphism on pyritic ores.

Sedimentary Mineral Deposits

A series of papers on manganese metallogenesis appeared in a journal issue edited by

Frakes and Bolton [1992]. They also reviewed the mode of origin of Phanerozoic

sedimentary manganese deposits and correlated their occurrences with variations in

ocean chemistry, sea level and paleoclimate. They concluded that extensive Mn

carbonate and oxide precipitation occurred during periods of regression; these periods

promote oxidation of seafloor organic matter, release of CO and global greenhousewarming.
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Some unusual Ni-Mo-PGE-bearing black shales in China were studied byMurowchick

et al. [1994], who concluded that they formed via venting of metalliferous hydrothermal

fluids into an anoxic, phosphogenic basin. Large variations in ion microprobe S/ Svalues for pyrite implied that bacteriogenic seawater sulfate reduction associated with

organic matter decomposition was an important mechanism for ore deposition.

Most of our domestic uranium resources occur in Tertiary non-marine sandstone

deposits that are thought to have formed by groundwater transport and deposition.

Sanford[1994] developed a four-layer finite difference model for the formation of

tabular sandstone uranium deposits. His results indicated that regional fluid flow was

gravity-driven, with discharge concentrated at lake shorelines or playa margins. Inferred

zones of mixed local and regional groundwater discharge were associated with the ore

zones; these data support a fluid interface mixing mechanism for ore deposition.

Precambrian conglomerates rich in detrital pyrite, uraninite and quartz continue to

challenge economic geologists as well as paleoclimatologists, because such a

combination of minerals cannot survive fluvial transport in our present oxygen-rich

atmosphere. Vennemann et al. [1992] found variable O values in adjacent quartz

pebbles and their contained fluid inclusions in Archean conglomerates from the

Witwatersrand (South Africa) and Huronian (Canada) districts. They concluded that the

pebbles preserved their predepositional oxygen isotopic compositions and fluid

inclusion chemistry. Both areas exhibited quartz pebble O modes consistent with

derivation from erosion of Archean granites and pegmatites. However, the

Witwatersrand pebbles exhibited a broader, heavier range in O values, suggesting anadditional source of quartz from erosion of Archean greenstone belt lode gold deposits.

This provenance difference may explain the presence of both Au and U in the

Witwatersrand ores, but only Au in the Huronian ores. The O values and fluid

inclusion characteristics of the quartz pebbles were inconsistent with previous proposals

for their derivation from Archean exhalative deposits.
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Ore-Forming Processes and Tools Used

in Their Study

This section highlights research that was centered more on general assessments of

processes of ore formation than on evaluating the genesis of specific deposits or

districts.

Ore Petrology and Phase Equilibria

Magmatic and Magma-Hydrothermal Ore-Forming Processes

Metals in Hydrothermal Fluids

Ore Mineral Precipitation

Hydrothermal Alteration

Low-Temperature Processes and Weathering

Light Stable Isotopes

Radiogenic and Heavy Isotopes

Fluid Inclusions

U.S. National Report to IUGG, 1991-1994

Rev. Geophys. Vol. 33 Suppl., 1995 American Geophysical Union

Ore Petrology and Phase Equilibria

Compared with other types of petrologists, ore petrologists spend a large fraction of

their microscope time looking at the ``opaques.'' However, with the decline in domestic

economic geology programs, fewer courses on reflected-light microscopy will probably

be available to geology students. This is unfortunate, because oxide and sulfide

minerals often record critical information on the conditions of rock genesis. Craig

[1990] andBarton [1991] reviewed examples of textures in ores and their interpretation.

Barton illustrated how careful interpretation of disequilibrium textures can reveal many

aspects of mineralizing processes, including the duration of geological processes. Craig

and Vaughan [1994] produced a second edition of their widely-used textbook on ore

microscopy and petrography; perhaps a future edition will contain more examples

illustrating the growing importance of elemental and isotopic microanalytical methods

in ore petrology.Murowchick[1992] summarized textural criteria that can be used in

assessing the ancestry of pyrite and marcasite, as well as the pH and temperature of

their formation.

Economic geologists continue to integrate the use of new microanalytical techniquesinto ore petrology. The microscopic distribution of metals, ligands and their isotopes
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within and among crystals in ores is relevant not only to understanding ore genesis, but

also to ore beneficiation and processing. For example, because the specific crystal

chemical hosts for Au in sediment-hosted disseminated gold deposits are uncertain,Arehart et al. [1993a] used backscattered electron and secondary ion imaging

techniques to document a direct correlation of Au with metastable arsenian pyrite on a

microscopic scale. They proposed that Au is present as Au in solid solution, having

been deposited from aqueous bisulfide complexes by coupled Au oxidation and As

reduction. Such data have important ramifications for processing and extracting the

gold from these types of ores.

Sphalerite is the only common ore mineral whose composition can record the pressure

of mineralization; it is thus an important geobarometer. Toulmin et al. [1990] reviewed

the basis and status of the sphalerite geobarometer, noting that experimental and

theoretical discrepancies at low temperatures and high pressures need to be resolved,

but also that the geobarometer could be successfully applied to equilibrium sphalerite-

pyrrhotite-pyrite assemblages which have not suffered retrograde effects. The phase

equilibria experiments ofLusk et al. [1993] have since filled in an important pressure-

temperature region relevant to this geobarometer.

Magmatic and Magma-Hydrothermal Ore-Forming

Processes

Using microbeam analytical techniques,Lowenstern et al. [1991] andLowenstern

[1993] discovered Cu sulfides in CO - and Cl-bearing vapor bubbles in melt inclusionswithin phenocrysts in pantellerites and rhyolites, thus demonstrating that melt Cu could

be strongly partitioned into an early magmatic vapor phase in

[4] phenocryst-poor magmas. The possibility of strong partitioning of Cu into an early

vapor phase, prior to extensive crystallization of phases that would otherwise remove

Cu from the melt, means that crystallization-induced volatile saturation (second boiling)

is not necessary for the creation of metal-rich fluids in shallow H O- or CO -rich silicic

magma chambers. They also argued that volcanic contributions of Cu to the atmosphere

may be more significant than previously thought.Meeker et al. [1991] identified

crystalline elemental gold and gold chloride particles being emitted from Mount Erebus

in Antarctica. This plus consistent Au/Cl ratios of aerosols from the volcano suggested

that the gold is transported as a chloride gas species. Transport of trace metals in

volcanic gases from Mount St. Helens was modeled by Symonds and Reed[1993], who

likewise concluded that most were volatilized from shallow magma as simple chlorides

and deposited as sublimates upon cooling as oxides, sulfides, halides, tungstates and

native elements.

Rye [1993] summarized the evolution of magmatic-hydrothermal ore-forming fluids

based on many years of stable isotopic research on such ore deposits. He reviewed

evidence for high-level interactions of deep magmatic components with shallow wall-

rock and meteoric waters, and emphasized the episodic, successive input of deep

magmatic volatiles and evolved brine into shallower crustal levels to generate acidalteration and ore deposition. His paper and the review papers by Giggenbach [1993]
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andHedenquist and Lowenstern [1994] are perceptive, complementary evaluations of

the processes that form magma-hydrothermal ore deposits.

Metals in Hydrothermal Fluids

Both experiment and empirical observation contributed to advances in knowledge about

the geochemistry of metalliferous hydrothermal fluids.Hemley et al. [1992] studied the

solubility of Fe, Pb, Zn and Cu sulfides in chloride solutions that were rock-buffered in

pH, fS and fO from 300-700 C and 0.5-2.0 bars (5 x 10 to 2 x 10 Pa).Hemley and

Hunt[1992] applied the results to conclude that for quasi-adiabatic transport conditions,

the pressure effect on rock-buffered solubilities compensates for the temperature effect,

allowing metal transport over long distances from deep-seated crystallizing plutons. The

outward Cu-Zn-Pb zoning typically seen around mineralized plutons forms in acomplex manner dictated by the intersection of transport pathways with metal sulfide

saturation surfaces, caused by thermal and chemical changes and their temporal

variations.Hemley and Hunt[1992] also present an insightful discussion of paragenesis

and zoning in space and time that should be read by everyone interested in ore genesis.

Other experimental studies of hydrothermal base

[4] metal solubility and speciation include those ofSeyfried and Ding[1993] on the

relative solubilities of Fe and Cu in Na-K-Cl fluids and Ohmoto et al. [1994] on the

solubility of pyrite in Na-Cl solutions, and references cited therein. Platinum-group

elements and gold were the focus of experimental studies by Wood et al. [1994],Berndt

et al. [1994] and references cited therein.

As a complement to experimental studies,McKibben et al. [1990] described similar

dissolved concentrations but contrasting precipitation mechanisms of gold and PGE in

boiling hot brines within geothermal wells, thus providing empirical evidence for

significant differences in the transport mechanisms of these two metals in natural

hydrothermal brines.Peters [1993] described the connate origins and Au-Ag-Hg-

hydrocarbon contents of hot spring waters in the California Coast Ranges, and related

them to the genesis of hot spring precious metals deposits such as McLaughlin.

Ore Mineral Precipitation

The precipitation mechanisms and stabilities of iron sulfide minerals are not completely

understood. The conditions and rates of precipitation of marcasite and pyrite from

hydrothermal solutions were studied experimentally by Schoonen and Barnes [1991]

and Graham and Ohmoto [1994], who found that these minerals form via precursors of

crystalline FeS or liquid S.

Relying on a computational approach,Bowers [1991] developed a model for the

deposition of Au and other metals during pressure-induced fluid immiscibility. Her

model used the EQ3/6 speciation and mass transfer software package with extensions

accounting for O, H and S isotopic fractionation, plus the Redlich-Kwong equation ofstate for the P-V-T propoerties of H O-CO mixtures. She found that fluid immiscibility
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(volatile loss) could induce metal deposition under a variety of conditions. However,

she also showed that the influence of volatile loss on metal deposition must be

evaluated in the context of realistic constraints: the effects of volatile loss on fluid pHand redox state must be evaluated in light of the buffering capacity of the entire fluid-

rock system.

Hydrothermal Alteration

Feldspar hydrolysis is a common pervasive type of hydrothermal alteration. The

thermodynamics of hydrothermal alkali feldspar-mica-aluminosilicate equilibria were

evaluated by Sverjensky et al. [1991], who derived an internally consistent set of

thermodynamic data for these minerals and relevant aqueous species that will be useful

in modeling fluid-rock interactions. Revised values for the dissociation constant of HCl,an important source of acidity, were also derived.

The spatial scale of hydrothermal circulation and alteration in crustal rocks is important

because of its implications for the volume of rock that can be leached of metals to form

concentrated ores. Cathles [1993] used O isotopic data on altered rocks to conclude that

a major, long-lived hydrothermal convection cell, centered around a pluton in the

Noranda district, Quebec, had penetrated to depths of greater than 8 km. The most

intense and coherent zones of O depletion coincided with the highest tonnage massive

sulfide deposits.

Hydrothermal alteration of oceanic crust is important to understanding the formation ofseafloor massive sulfide deposits, the geochemical cycle of sulfur in the oceans, and

ultimately the origin of magmatic sulfur erupted from volcanic arcs over subduction

zones. Sulfur mass balance and isotopic systematics accompanying hydrothermal

alteration of oceanic crust by seawater were developed byAlt[1994], based on studies

of ophiolite complexes. Sulfur is redistributed from the lower dike and gabbro sections

to the upper dike section, and additional sulfur is added to the upper dike section by

reduction of convecting seawater sulfate. However, this latter gain is balanced by

oxidative loss of sulfur from the volcanic section. These processes result in exchange of

crustal sulfur for seawater sulfur, resulting in enrichment of S in altered crust.

Low-Temperature Processes and Weathering

Weathering of exposed or near-surface ores can result in the redistribution and zoned

concentration of valuable metals.Lichtner and Biino [1992] developed a numerical

model for metasomatic supergene enrichment of porphyry copper protore. They were

able to reproduce elemental zoning seen in the field, particularly the high Cu grades

seen in the upper zone of enrichment blankets formed by weathering.

Using transmission electron microscopy,Ilton and Veblen [1993] showed that

inclusions of native copper found in mica in rocks associated with porphyry copper

deposits were formed during weathering. Previously, such copper had been thought to
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be the product of primary magma-hydrothermal mineralizing processes related to the

emplacement of copper-rich magma.

The timing of ore deposit weathering can also be used to evaluate paleoclimates,

because some weathering products are rich in potassium and retain its radiogenic decay

products. Vasconcelos et al. [1994] used laser-heating Ar/ Ar dates on the mineral

jarosite, formed during progressive weathering of sulfide ores, to identify a global late

Miocene oxidation and weathering event responsible for weathering and supergene

enrichment of several ore deposits.

Light Stable Isotopes

Microanalytical techniques developed within the past decade (ion microprobes, lasermicroprobes) are allowing not only elemental analysis on a fine scale, as noted above,

but also stable isotopic analysis on a fine (micron) scale. In particular, microbeam

studies of the zonation of S isotopes within and among individual crystals in sulfide

ores are providing important clues to deposit origins. Using secondary ion microprobe

mass spectrometry,McKibben and Eldridge [1990] found that hydrothermally altered

rhyolites within the Valles

[4] Caldera contained strongly S/ S-zoned authigenic pyrite crystals (enriched cores,

depleted rims) at depths coinciding with elevated Au contents. They concluded that

boiling and oxidative H S destruction had caused Au deposition coincident with

progressive Rayleigh S isotopic depletion in the growing crystals. The micron-scale

isotopic zoning may have recorded a large-scale geologic event, breaching of thecaldera wall and draining of the former caldera lake, which triggered the boiling and Au

deposition.Arehart et al. [1993b] also found large variations and late-stage depletions

in S/ S values for arsenian pyrite in fine-grained ores from the Post/Betze sediment-

hosted disseminated gold deposit in Nevada.

Several important advances were made based on conventional (bulk sample) analyses of

stable isotopes in ore deposits. An elegant paper byRye et al. [1992] worked out the

stable isotopic systematics of acid sulfate alteration. The characteristic mineral alunite

contains four stable isotope sites in its crystal structure, making it a very useful mineral

for reconstructing mineralizing conditions and processes. A companion paper by

Stoffregen et al. [1994] reported experiments that determined O and H fractionationfactors between alunite and water. Ohmoto et al. [1990] reviewed the sulfur isotopic

systematics of modern marine sediments and sediment-hosted base metal deposits.

Radiogenic and Heavy Isotopes

As with stable isotopes, researchers continue to ``push the sample size envelope'' for

radiogenic isotopes.Brannon et al. [1992] andNakai et al. [1993] used Rb/Sr isotopic

data on sphalerites to date the age of mineralization of several Mississippi Valley-type

Pb-Zn ore deposits; the technique is complicated by differential brine-mineral

partitioning of Rb and Sr and the consequent need to remove inclusion fluids prior toanalysis. Nonetheless, the diverse age determinations from different deposits suggest
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that the timing of Paleozoic orogenic activity and resulting regional brine migrations in

North America, thought to be responsible for forming these types of ore deposits, may

be more complicated than was previously assumed. Chelsey et al. [1994] used Sm-Ndisotopic data on fluorites to date the age of mineralization in the Illinois-Kentucky

fluorite district; they obtained a Permian age identical to that obtained by Brannon et al.

[1992] for Upper Mississippi Valley sphalerites, supporting a model for large-scale

fluid movement from the Illinois basin related to the Alleghenian-Ouachita orogenies in

North America.

Re-Os isotope systematics are proving useful in ore deposit studies because of the Os

isotopic contrast between mantle and crust, the occurrence of Re in molybdenite, and

the occurrence of Os in PGE deposits.Marcantonio et al. [1994] used Re-Os, Nd-Sm,

Rb-Sr and O isotopic systematics to demonstrate that primary magmatic PGE

mineralization in the Wellgreen intrusion, Yukon Territory, had been overprinted by

post-crystallization hydrothermal processes which remobilized radiogenic crustal Re

and Os from sedimentary wall-rock sources. They questioned earlier studies which had

concluded that radiogenic or variable Os isotopic compositions in magmatic PGE

deposits must reflect mantle heterogeneities or crustal assimilation, rather than

hydrothermal remobilization from radiogenic crustal sources after magma

emplacement.McCandless and Ruiz[1993] applied Re-Os isotopic systematics to

determine the ages of molybdenite-bearing porphyry base metal deposits associated

with the Laramide orogeny in Arizona. They used a variety of analytical techniques to

identify molybdenites which had not been affected by post-crystallization

remobilization. They found that in each deposit ore deposition consistently occurred inthe late stages of magmatic activity. Two narrow episodes of mineralization were

delineated: 74-70 million years ago (largely within older Precambrian basement) and

60-55 million years ago (largely within younger Precambrian basement). The

synchroneity of this widespread mineralization implied that some type of fundamental

crust-mantle interaction resulted in regional genesis of the metal-enriched magmas

responsible for the deposits. Walker et al. [1994] applied Re-Os, Nd-Sm and Pb isotopic

systematics to magmatic Cu-Ni-PGE sulfide ores and associated igneous rocks from

three Permian Noril'sk-type deposits in Siberia. They found that the isotopic data

required a hot-spot type asthenospheric mantle source for the primary igneous melts and

PGE, with little or no crustal contribution for these elements.

Fluid Inclusions

One of the fundamental assumptions in using fluid inclusion data to study mineral

genesis is that the inclusions have behaved as closed systems since their formation.

Hall et al. [1991] andMavrogenes and Bodnar[1994] showed that H diffusion into

and out of fluid inclusions during metamorphism or laboratory heating could

significantly modify the chemical compositions of the inclusions. Diffusion will be

most rapid at high temperatures and when the hydrogen fugacity difference between the

inclusion and its surroundings is large. Failure to recognize or expect diffusion

problems could result in flawed reconstruction of the formation conditions of some

minerals and ores.
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In spite of these diffusion problems, it is still possible to retain primary gas ratios in

fluid inclusions from unmetamorphosed low-temperature ores.Graney et al. [1991]

studied the gas compositions of fluid inclusions in epithermal jasperoids from variousgold deposits. They noted a correlation of high H S/CO and other gas parameters with

mineralized jasperoids, suggesting the utility of the technique for exploration.

In the case of aqueous fluids trapped as inclusions during boiling, knowledge of the

temperature of boiling and fluid composition can lead directly to an estimate of the

depth of formation if the P-V-T properties of the fluid are known. Many economic

geologists have use the pure H O system as a proxy to interpret data from low-salinity

fluid inclusions in epithermal gold deposits.Barton and Chou [1993] reviewed P-V-T

data for the H O-CO system and demonstrated that large errors in hydrostatic

paleodepth reconstructions of epithermal systems may occur if the presence of

significant amounts of CO in fluid inclusions is not recognized. For example, if one

observes the formation of CO clathrates upon freezing of an inclusion, then the

inclusion musthave formed under relatively high CO pressure. This pressure would

add at least 1 km to the paleodepth that would otherwise be estimated if one used P-V-T

data for boiling of pure water. Therefore, if CO is not detected because clathrate does

not form or is not recognized upon freezing, then large errors may result when

reconstructing the original depth of formation of the host minerals.

Kesler[1991],Bodnar[1992] andMcKibben et al. [1994] edited special journal issues

containing several other papers on U.S. research on fluid inclusions applied to ore

deposits.

Conclusions

It is ironic and unfortunate that the study of ore deposits---the largest geochemical

anomalies in the earth's crust---is in a tumultuous state at a time when our technological

abilities to collect, manipulate, depict and analyze geologic data are accelerating

rapidly. We have never been in a better technical position to understand the Earth's

geologic processes and history. Tremendous advances are being made in the

applications of ion- and laser-microprobes, supercomputers, and global satellite

positioning systems.

The current interest in global climate change is stimulating a renewed interest in those

mineral deposits whose occurrence may reflect past climatic and environmental

changes. Also, we are increasingly seeking more efficient ways of extracting,

consuming and recycling mineral resources, to minimize the impacts on our

environment. Given these conditions, there should be growing research opportunities

for geologists who have an understanding of mineral deposit genesis. Yet there is a

clear downturn in current domestic opportunities for those researchers whoseprimary

avocation is studying how and where mineral deposits form and applying that

knowledge to finding new resources. This downturn has been prompted by the evolving

and sometimes conflicting economic, political, environmental and legislative
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constraints on domestic mineral exploration and exploitation, in the context of an

increasingly global economy.

To prosper as a nation we must continue to produce trained experts who understand

how and where mineral deposits form, but this capability is threatened. The current

declines in domestic mineral exploration and the consequent decreased domestic

university enrollments in the discipline of economic geology will have several long-

term effects on U. S. research on ore deposits. Already taking place is a rapid de-

emphasis of pure economic geology research programs at many universities, companies

and government agencies. Many retiring senior economic geologists are either not being

replaced or are being replaced by geologists with different specialties (often some

aspect of environmental geology). Many excellent recent Ph.D.s are surviving on a

year-to-year basis in post-doctoral positions, instead of landing permanent faculty and

research positions.

Some economic geology research programs and their graduates will survive by shifting

their emphases to the environmental contamination and remediation aspects of mineral

resource exploitation. Others may conduct some ore deposits research under the banner

of paleoenvironmental research. In the near-term, many domestic graduates interested

in mineral exploration may find better employment opportunities overseas; those

seeking domestic careers will need to emphasize the environmental and legislative

aspects of resource exploitation and clean-up. An increasing number of graduate

students in economic geology will likely come from developing foreign countries,

where the need for trained individuals is great. Graduate students in U. S. economicgeology programs may be more likely to work on mineral deposits in foreign countries,

because of more active exploration and better access to new deposits. Studies of

domestic deposits can be hampered by lack of both access to deposits and industry

logistical support, prompted in part by liability and fiscal considerations.

Although the current domestic situation regarding ore deposits research is not rosy, the

long-term outlook is by no means bleak. Economic geologists must of necessity be

familiar with almost all aspects of the earth sciences, as well as with basic economics

and politics, and therein lies their credentials for making continued contributions to

science and human prosperity. Because ore deposits are the ultimate examples of

geochemical diferentiation and enrichment in the Earth's crust, they will always providefertile research ground for those who must understand the geochemistry of the elements

and petrogenesis. By studying ore deposits, we learn fundamental aspects of

geochemistry and geology that can be applied broadly to problems beyond the origin

and occurrence of mineral resources.

Acknowledgments. The author thanks the anonymous reviewers, as well as the editors,

for making the final version of this paper much more readable.

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