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Conference report

Volcanic processes in ore genesis

I. G. GASS

Two major themes pervaded this joint meeting of the Volcanic Studies Group and the Institute of Mining and Metallurgy. On the one hand were the 'aca- demics' who, using trace element and isotope geochemistry and even naked thermodynamics, erected hypotheses and models for the tectonic setting, origin and circulation of ore forming fluids. At the other extreme were those who pro- vided the critical evidence as to whether these data applied to a particular ore deposit. Fortunately, speakers such as J. P. Hunt, T. Sato and G. Constantinou with experience in both areas more than adequately bridged the all-too-common gulf between the two.

Stable isotope studies strongly suggest that the origin of the ore-carrying solutions was either sea-water, in the case of the massive sulphides formed at or near constructive margins, or meteoric waters for ore bodies such as porphyry coppers, emplaced above subduction zones. The role of magmatic waters is minimal in the former and minor in the latter; magmatic processes seem to provide the thermal energy and very little else. Both thermodynamic models of geothermal systems (J. W. Elder), or modification of stable isotope ratios (E. T. C. Spooner & T. H. E. Heaton) indicate that the circulation of the solutions that cause mineralization follow immediately after the magmatic thermal event and that they are short-lived in terms of thousands rather than millions of years and vigorous. There are therefore signs of a semi-quantitative break- through in the understanding of geothermal processes. This has been brought about by the use of a wide spectrum of analytical techniques on, for instance, the ophiolite massive sulphides of Cyprus and the porphyry copper of E1 Salvador. Their use elsewhere will enhance understanding of the processes involved in ore genesis.

Happily, the days when it was considered almost indecent for 'pure' academic scientists to concern themselves with ores are over. A period of fruitful collabo- ration between hard-headed practicality and high quality academic research is hopefully here to stay.

Summary of joint meeting of The Institution of Mining & Metallurgy and The Volcanic Studies Group, held in Burlington House 21-22 January 1976.

Global t e c t o n l c s - l l u l d s - o r e d e p o s i t s W. S. Fyfe

The requirements for the formation of an ore deposit are a suitable solvent, an energy source

for forced flow, a structure to focus flow and an appropriate site for deposition. Energy sources are normally supplied by gravity or igneous intrusions. Fluid sources are normally from the hydro- sphere, and halide-bearing fluids seem appropriate

Jl geol. Soc. Lond. vol. x32, x976, pp. 563-575 Printed in Northern Ireland.

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for most situations. Examples of such flow regimes can be drawn from the ocean ridge environment, the subduction environment, high-level plutons, faults-thrust-shear zone environments, burial metamorphism and lateritic weathering. Geochemical cycles require modification on account of the new importance attached to the linkage continental weathering, ocean chemistry, spilitization, subduction of spilite and sediments. New data that suggest more eclogite in the lithosphere make sediment subduction more plausible. Metal transport is no longer a chemical problem. New prospecting methods are focussed on evidence of mass relations and depositional environments. Extensive rather than intensive parameters should be used. Particular emphasis will be placed on gangue mineral volumes, stable isotopes and fluid inclusions.

Ident/ficatlon of ore-depos | t lon environment f r o m trace-e lement geochemis try J. A. Pearce & G. H. Gale

Geochemical studies of igneous rocks genetically associated with ore deposits can provide information on the tectonic environment of ore formation. This approach is applied to three types of deposit. Cyprus-type deposits (example used: Troodos Massif, Cyprus; Lansail prospect, Oman; I.~kken, Nor- way; York Harbour, Newfoundland). These massive Cu-Zn-bearing sulphide deposits are found in sequences of predominantly basic pillow lavas at the boundary between two distinct lava units. Evidence from Cyprus suggests that the lower of these units was erupted at a ridge axis, the upper away from the ridge axis.

'Stable' trace-element geochemistry shows that the lavas tend to have lower concentrations of Ti, Zr, Nb, Y, Ta, Hf, rare-earth elements and Cr compared with an average ocean-floor tholeiite (OFT). Also, the concentrations of these elements (except Cr) decrease up the lava sequence. Empirical studies show all gradations between apparent ocean-floor tholeiite and primitive island arc tholeiite characteristics; the island arc character increases up the sequence.

The deposits were not formed in a 'normal' mid-ocean ridge environment; if an equivalent of this environment exists today, it is in a marginal basin; water and perhaps copper from a subduction zone may have been involved in the genesis of the lavas; the ridge axis may have been slow-spreading and higMy faulted. Other massive sulphides (examples used include

Joma, Gjersvik, Roros, Norway; Boliden, Sweden; Parys Mountain, Wales; Buchans, Newfoundland) Geologically and geochemically, these deposits fall into two categories. Category ~ deposits are found within a sequence of submarine calc- alkali volcanics of island arc geochemical characteristic.s. These range from Cu-Zn-bearing types associated with basalts and andesites to Pb-Zn- Cu-Ag-bearing (Kuroko) types associated with acid rocks. 'Stable' trace-element geochemistry shows that, compared with an average OFT, the basic lavas have lower concentrations of Ti, Zr, Y, Nb, Ta, I-If, heavy rare-earth elements and Cr, but sometimes more La, U, Th and (in fresh rocks) alkali elements. Lavas associated with some Archaean deposits also had these characteristics. Category 2 deposits are less common. They are found in interbedded basic volcanic-sedimentary sequences. Compared with OFT these lavas contained higher concentrations of alkali elements, U, Th, La, Nb, Ta and (usually) Ti, Zr, I-If; they contained similar concentrations of Y, heavy rare-earth elements, Cr and Sc. These have a within-plate chemical character-indicating a continental margin or 'failed rift' environment.

On the basis of studies of unmineralized as well as ore-bearing lavas, most massive sulphide deposits are probably formed in island arc or marginal basin environments and may all be dependent in some way on the subduction process for their formation; no deposit has been found within lavas that have 'normal' ocean-floor basalt characteristics. Tin deposits (examples used from Cornwall, Malaysia, Bolivia, Nigeria). The environment of eruption of intermediate and acid igneous rocks can be deduced by use of diagrams based on the element Nb--such as SiO~ versus Nb. The acid- intermediate volcanics and granitic rocks from tin provinces have typically high Nb concentrations and classify either as within-plate or evolved volcanic arc settings. In contrast, igneous rocks associated with porphyry copper deposits usually have lower Nb concentrations (less than I o ppm in intermediate rocks). Petrogenetic calculations indicate that the tin may have a source in a part of the mantle that is enriched in incompatible trace elements, and is partially melted at 'hot-spots' or in rifting situations behind subduction zones. The tin is further concentrated during crystal fractionation processes as the magma rises through a great thickness of sialic crust; or, alternatively, tin-bearing fluids from the magma enrich the base of the sialic crust, which then undergoes partial melting.



Conference report 565 Identification of the origin of ore-form;ng

solutions by use of stable isotopes S. M. F. Sheppard

The origin of most constituents (magmatic, leaching of country rocks, fluid source reservoir) of hydrothermal solutions is indeterminate because (I) during reactions of the fluid with the wallrocks, element concentrations are controlled by solubility and ion-exchange equilibria and (2) most elements, except notably H, C, O, S, Sr and Pb, do not have stable isotope ratios that vary measurably and that can be used as 'finger- prints' of their origin. The stable isotope ratios of hydrogen (D/H), oxygen (leOf180) and sulphur (z4S/S2S) vary in minerals and waters. Most unaltered igneous rocks, and waters of different origin--magmatie, metamorphic, meteoric, connate and ocean water--have characteristic D/H and tsO]tSO ratios. At equilibrium the isotopic composition of a hydrothermal mineral is controlled by the physico-chemical conditions of the solutions (T, fo2, pH), the isotopic composition of the fluids and exchanging rocks, and the mass of the element in the fluid relative to the rock (water/rock ratio). A combined oxygen and hydrogen isotope approach is necessary, in general, to determine the origin of water in magmatie and post-magmatic processes. Know- ledge of the origins of water and sulphur is fundamental to any theory of ore genesis. Waters of several origins are involved with ore deposits associated with volcanic and epizonal intrusive environments. Waters of a single origin dominate main-stage mineralization in some deposits: magmatic--Casapalca, Peru (Ag-Pb-Zn-Cu); rr~- te0r/c--Butte, Montana (Cu-Zn-Mn), epithermal deposits, e.g. San Juan Mts. District, Colorado (Ag-Au-Pb-Zn), Comstock--Goldfield--Tono- pah, Nevada (Ag-Au), Pachuca, Mexico (Ag-Au) ; sea water--Troodos, Cyprus (Fe-Cu), Kuroko, Japan (Fe-Cu-Pb-Zn-Ba), Echo Bay, NWT, Canada (U-Ni-Ag-Cu). Solutions of more than one origin are important in certain deposits: magmatic and m~teoric--porphyry copper and/or molybdenum deposits, North and South America; and present in many.

In the porphyry Cu-Mo deposits the initial major ore transportation and alteration processes (K-feldspar-biotite alteration) are magmatie- hydrothermal events occurring at 75o-5oo°C. These fluids are typically highly saline Na-K- Ca-C1 rich brines (more than 15 wt ~o equivalent NaC1). The convecting meteoric-hydrothermal system that develops in the surrounding country

rocks with relatively low water/rock ratios (less than 0. 5 atom ~o oxygen) subsequently collapses in on a waning magmatic-hydrothermal system at about 35o-2oo°C. These fluids generally have mod- erate to low salinities (less than x 5 wt ~o equivalent NaC1). Differences among these deposits are probably in part related to variations in the relative importance of the meteorlc-hydrothermal versus the magmatic-hydrothermal events. The sulphur comes from the intrusion and the country rocks.

Deposits where meteoric or sea water is the dominant constituent of the hydrothermal fluids come from epizonal intrusive and sub-oceanic environments where the volcanic country rocks are fractured or well jointed and highly permeable. Integrated water/rock ratios are typically high with minimum values of o. 5 or higher (atom ~o oxygen)--the magmatic water contribution is often 'drowned out'. Salinities are low to very low (less than to wt ~o equivalent NaC1), and temperatures are usually in the range 35o-t5o°C. The intrusion supplies the energy to drive the large-scale convective circulation system. The sulphur comes from the intrusion, the country rocks and/or the sea water.

Argillic alteration, occurring to depths of several hundred metres, generated during super- gene weathering in many of these deposits is isotopically distinguishable from hydrothermal clays.

C/rculatlon of water in the crust, deductions from studies of modern hydrothernml sys tems J. w . Elder

The dynamical nature of the mechanisms of operation of hydrothermal systems and their key parameters were described. A simple model suitable for a gross analysis of the role of water circulation on the solution, transportation and deposition of ores was considered in detail.

Nature and distribution of porphyry copper deposits J. P. Hunt

Efforts to explain the origin of porphyry coppers by global tectonic and igneous processes must account for the nature of these deposits and their distribution in space and time. Data from mine studies were reviewed, including texture, composition, size and shape of mineralized intrusives; amount and proportions of copper, sulphide sulphur, sulphate sulphur and associated minor metals; the evolution of mineralization and alteration with respect to magrnatic processes and the effects of later groundwater; the




timing and duration of mineralization with respect to associated vulcanism and geothermal activity; the size and position of co-magmatic calc-alkaline batholiths and depth of emplacement. Knowledge of the absolute age and world- wide distribution of porphyry coppers is also summarized with respect to position of convergent plate boundaries, the locations of giant deposits and gross amounts of copper deposited. The distribution of porphyry copper deposits with respect to seismicity, regional petrologic provinces and major structure features is obscure. Some fundamental genetic problems that are not yet resolved include the sources of both magmas and metals and possible relations, if any, to 'normal' igneous rock-forming processes and subduction zones.

Minera l i za t ion at destruct ive plate marg ins M. S. Garson & A. H. E. Mitchell

Three main types of destructive plate margin are recognized--island arc type, Andean type and collision type, each displaying distinctive mineralization.

The first, characterized by an ensimatic island arc magmatic belt developed within oceanic crust, has associated deposits which include diorite-type porphyry, copper and gold, Kuroko- type zinc-copper-lead sulphides, mercury, native sulphur-pyrite and Besshi-type cupriferous sulphides.

The second, characterized by an ensialic or Andean-type magrnatic belt developed within continental crust, has associated porphyry copper and molybdenum and, locally, tin-tungsten deposits and Chile-type strata-bound copper sulphides. Where complete, mineral zonation away from the plate margin into continental crust comprises iron, copper-molybdenum, silver-lead- zinc, tin-tungsten and antimony, the source of the metals possibly being related to progressively deeper levels in the underlying Benioff zone. A modification of this type of plate margin results from the formation of a marginal basin by outward migration of incipient Andean-type mountain belts bordered on the ocean side by a subduction zone. Granite plutons with associated tin, tungsten and fluorite are emplaced in this setting.

Centres of porphyry copper mineralization may be controlled locally by transform faulting at both types of plate margin, and Kuroko-type deposits in some island arcs may also be situated on transform faults.

Tectonic wedges of oceanic crust and upper mantle are scraped up with overlying sediments

and emplaced in outer arcs on margins of continents. These wedges may contain mineral deposits previously formed in the ocean, including Cyprus- type massive sulphides, podiform chromite, platinum, magnesite and asbestos.

The collision type of destructive plate margin is formed during the final stage of subduction of ocean floor between two continents, between two island arcs or between a continent and island arc. Slices of tectonically emplaced crust and upper mantle, forming ophiolites, are obducted or thrust on to the continental crust or inactive island arc on the subducting plate to form elon- gate belts at the junction of the two plates. These collision belts contain mineral deposits interpreted as having formed at oceanic ridges and in island arcs, and other related deposits may include tin-tungsten, iron-titanium ores associated with anorthosites in the lower continental crust, native silver-nickel-cobalt arsenides, gemstones and strata-bound uranium-vanadium-copper mineralization in molasse derived from associated mountain belts.

Igneous geology and evolution of hydrothermal sys t ems in some sub-volcanlc t in depos i t s ofBol iv laJ . H. Grant, C. Halls, & G. Avila

The tin deposits of the southern part of the Cordillera Oriental of Bolivia consist of complex vein systems and pervasive disseminated mineralization within zones of strong hydrothermal alteration in, or at the margins of, eruptive centres of late Tertiary age. At some centres erosion has removed the volcanic superstructure completely and only intrusive rocks are preserved (Llallagua, San Pablo de Morococala). At others, both the intrusives and coeval volcanics are preserved (Chorolque, Oruro, Potosi), whereas in some cases only the partially eroded volcanic superstructure is exposed and no unequivocally intrusive rocks are known (Chocaya, Tatasi).

In general, the geological relationships at all these deposits suggest that the mineralization is the product of hydrothermal systems generated in the inner, deeper regions of terrestrial strato- volcanoes.

The igneous rocks most closely associated with mineralization are strongly altered quartz porphyries. Their original composition was probably rhyodacite. Where stocks are preserved, they are complex bodies, often conical in vertical section, narrowing with depth. They show evidence of several phases of intrusion, igneous brecciation



Conference report 567

and explosive hydrothermal brecciation, which pre-date the formation of the major vein systems.

Silicate alteration assemblages include quartz- tourmaline, tourmaline-sericite and serieite-clay, which often show rough concentric zonation in that order outward from the centre. An outermost aureole of propylitic alteration is sometimes present where there are remnants of the volcanic superstructure. Alteration is pervasive and not controlled by the major veins. Low-grade cassiterite mineralization is dispersed throughout the inner zones.

Fluid inclusion studies at Chorolque show that the hydrothermal system was initiated by the separation of a highly saline brine, or melt, of complex chemistry, which formed at temperatures above 5oo°C. The pervasive quartz-tourmaline alteration, initial fracturing of the igneous rocks of the vent and the hornfelsing of the adjacent sedimentary rocks were accomplished while temperatures fell to below 45o°C and intermittent boiling took place. Widespread disseminated cassiterite was deposited during this phase. The earliest stages of growth of the major quartz- cassiterite veins took place at temperatures of around 4oo°C, from a fluid similar in composition to that of the earliest fluid, though substantially diluted (about 4 ° wt % NaC1 equivalent). Vein growth continued while temperatures fell to below ooo°C and salinity decreased. Most cassiterite deposition seems to have taken place in the temperature range 3oo-25o°C and may have been accompanied by a major decrease in the salinity of the fluids.

The data obtained from the fluid inclusion studies, taken in conjunction with the gross geometry of the mineralization, suggest that hydrothermal processes at the volcanic centres were initially controlled by the balance of con- fining lithostatic pressure and the pressure of the hydrous fluid residuum in the differentiated parts of the rhyodacite magma, leading to pervasive hydraulic fracturing, brecciation and alteration, of which the early generation of cassiterite mineralization was an integral part.

The upper levels of the mineralized volcanic structure appear to have stabilized while magmatic activity and hydrothermal generation at that level declined and the focus of generation retreated to greater depths from which mineralization was controlled by the interplay of tectonic stresses in the volcanic infrastructure and surrounding basement and the build-up of hydrothermal fluid pressure in linear vein-fault systems.

This concept of a bimodal style of minerali-

zation related to a retreating hydrothermal focus can lead to a clearer understanding of the xenothermal or 'telescoped' type of mineral and temperature zonation in sub-volcanic systems. Temperature data obtained from fluid inclusion studies show, however, that mineral paragenesis is not an unambiguous criterion of temperature zonation and that the two have to be considered independently before a combined synthesis becomes meaningful.

Geochemical model for Kuroko mineral/- zation T. Sato Kuroko deposits are stratabound massive

sulphide deposits associated with Miocene felsic volcanic rocks of the green tuff region of Japan. They are characterized by lenticular, massive Pb-Zn-Cu orebodies with abundant barite, resting directly on highly silicified stockwork zones. General vertical mineral zoning is distinct, but the zoning is concentric near the mineralizing centres. A model for physical and chemical processes involved in the submarine deposition of Kuroko orebodies is proposed, based on the assumptions that heavy metals dissolved in the ore fluid were complexed with chlorine and that the fluid was equilibrated with pyrite, chalcopyrite, sericite and kaolinite. By use of available thermodynamic data the ore fluid is calculated to have been cooled from about 25o°C at the bottom of the stockwork zone to about i5o°C at the top of the massive orebody, which shows a good agreement with fluid inclusion data. Gradients of pH and oxygen fugacity and initial heavy metal concentrations of the ore fluid are also calculated. The zoning pattern observed in Kuroko deposits is explained quantitatively by differential precipitation of ore metals from a polymetallic fluid that was discharged into and mixed with sea water. Application of the model to other stratiform massive sulphide deposits was attempted, and the results were discussed in terms of the difference of tectonic and geologic environments.

Stable isotope stud/es on Bougalnville and in Matupi Harbour, New Britain, Papua, New Guinea J. H. Ford, D. C. Green, J. R. Hulston, I. H. Crick & S. M. F. Sheppard Two case histories were presented in which

stable isotope ratios (D/H and xso/teO) place important constraints on the origins of ore- forming fluids in the Panguan porphyry copper deposit (upper Pllocene) and on the nature of the volcanic exhalative environment at Matupi Har- bour, near Rabaul.




At Panguna the overlap in 6D values between local meteoric water and inferred primary magmatic waters poses problems in the interpretation of secondary biotite and sericite isotope data. The range of hydrogen and oxygen isotope ratios of the secondary biotites associated with the mineralized areas is essentially identical to those from other magmatic hydrothermal deposits, including porphyry copper deposits. The sericites from alteration haloes adjacent to mineralized quartz veins have ~ xsO values of 5.4 to 8.6 ~ and 6D values of --6t to - 77 . A combination of thermodynamic and fluid inclusion techniques indicates that ~-~5oo°C is a realistic average temperature for much of the mineralization- quartz veining-biotitization process. The isotopic composition of the fluids causing this mineralization and alteration was dD ~ --20 to - 4 5 ~ and 61sO " " 6 to 7 %. I f a meteoric water component existed at this stage of alteration, meteoric water/rock ratios were extremely small (less than 0.05 at ~ ) .

Assuming that sericitization took place at temperatures down to 3oo°C, as suggested by the fluid inclusion data, then fluids responsible for this process had a composition of 6D = --3 ° to 5o% and d} t s o of ' ~ 3 - 5 ~ . These fluids probably would be largely of meteoric-hydrothermal origin, produced by t s o shifting of groundwaters during the formation of the propylitic zone and with a water/rock ratio of about o. x at ~ . Quartz d 1sO values from veins associated with the different alteration assemblages increase away from intrusive centres, consistent with an out- wards temperature decline, a decrease in ~ xso content of associated water and a progressive increase in the meteoric water component associated with the later and (?) lower-temperature sericitization. There is no isotopic evidence to suggest that sea waters or connate waters formed a recognizable component of the hydrothermal system at any stage. Calculations with possible 6 1sO depletions of o.5-I .o % in whole rocks from the Panguna region indicate that a meteoric leaching model for the genesis of the Panguna copper depoeit is theoretically possible, despite the small water/rock ratios involved.

The area around Matupi Harbour includes the dormant volcanoes Rabalanakala and Tavur- vur, set in loosely consolidated, Quaternary ash deposits. Tavurvur last erupted in x94t and x942 and has since reverted to a quiet solfataric con- dition. D/H and xsO/aeO ratios were determined on fumarole condensates, hot springs and meteoric waters. The data are grouped into distinct areas

close to the meteoric water line and demonstrate that the thermal springs away from the shoreline are of meteoric origin and that sea water only enters the springs at the shoreline. I f allowance is made for the vapour-liquid separation at the temperature of collection (Ioo°C), the fumarole results from Tavurvur may be interpreted as steam separating from a water phase at N x oo°C. The calculated water-phase isotopic compositions suggest that evaporation and possibly a small amount of mixing with sea water are involved.

These conclusions are in conflict with those drawn from anion ratio and trace metal contents inferred by previous authors to be consistent with an hypothesis of modified sea-water origin. We contend that the chemistry is a reflection of the later history of these acid, mineralized, geothermal waters and does not necessarily give a correct picture of their ultimate origin. The markedly enhanced Fe, Mn and Zn values of the Matupi springs are purely a function of the leaching potential of geothermal fluids at ele- vated temperatures and of the chemistry of the porous rocks through which they pass.

Modern subm~_rine h y d r o t h e r m a l minera l - izat lon: e x a m p l e s f r o m Santorinl and the Red Sea D. S. Cronan, P. A. Smith & R. D. Bignell

Submarine hydrothermal mineralization is known to occur on parts of the World Mid- Ocean Ridge System, and in association with island arc volcanism.

Two areas of submarine hydrothermal activity have been examined---one in the Atlantis II Deep of the Red Sea and the other off the volcano of Santorini in the Cyclades Volcanic Arc. In each case hydrothermal solutions are mixing with sea water, resulting in the precipitation of a variety of phases as the physico-chemical pro- perties of the mixture change.

Early-formed precipitates from the Atlantis I I Deep brines include sulphides and silicates. On continued mixing, iron is oxidized to the Fe wt- state and precipitates as ferric oxide and hydroxide. Finally, Mn ~+ is oxidized to Mn 4+ and can scavenge considerable quantities of trace metals out of solution on precipitation. Manganese precipitated in the waters over the Deep is probably redissolved on falling back in- to the brine. By contrast, manganese and some minor dements that escape from the Deep can precipitate up to x o k m from it.




Sulphides occur locally as a minor component of the newly precipitated sediments off Santorini, which are principally composed of iron oxides. Iron precipitates close to the hydrothermal outlets and decreases in concentration away from the volcanic source. By contrast, manganese is low in the sediments dchest in iron, but increases in concentration more than twenty- fold away from the iron-rich zone. The distribution of certain minor elements in the sediments follows that of Mn (e.g. Zn) and others Fe (e.g. Mo), suggesting that, here too, scavenging may be a control on their distribution.

The selective dispersion of hydrothermally introduced metals on mixing with sea water in these two environments results in a geochemical zonation in sediments away from the metal source in each case. The principal fractionation is between Fe and Mn, but some minor elements also show distinctive concentration gradients. These observations may be of great potential value in geochemical exploration for submarine hydrothermal orebodies, as the extent of the dispersion of the most mobile elements can be many times the extent of the deposit itself.

Ancient volcanic orebodies sometimes show similar selective dispersion haloes to those described heremindicating that in the modem environment we may be observing a not un- common volcanic ore-forming process in operation.

Or lg /n and emplacement of opldoHtes I. G. Gass

The most widely accepted definition of an ophiolite was that erected in x972 by the Geo- logical Society of America's Penrose conference on ophiolites. This describes an ophiolite as an assemblage of basic and ultrabasic rocks, a complete assemblage being:

Basic volcanics (commonly pillowed)

merging into Sheeted basic dyke complex

(Trondhjemites (Gabbros

Layered cumulates (Olivine gabbros (*Peridotites (*Dunites (with chromite)

Harzburgite (sometimes lherzolite) with tectonized fabric

(*commonly serpentinized)

Although the definition is purely descrip- tive and no genetic implications are intended or implied, the presence of radiolarian sediments and the pillowed form of the basic volcanics strongly suggest that ophiolites were formed in a submarine environment. Furthermore, this environment must be extensional to explain the presence of sheeted dyke complexes, and one where large volumes of basic magmas can occur close to the surface. Few would question the oceanic setting for ophiolite genesis, and many, pointing to the need for an extensional environment, see only the oceanic lithosphere as produced at constructive plate margins as fitting these requirements.

Field relationships and chemical and mineral composition of the tectonized peridotite suggest that it is material from which a liquid basaltic fraction has been fused and removed to form the higher levels of the ophiolite sequence. It seems that the magma body collects at shallow depth (x-2 km) below the sea/rock interface. In an extensional environment magma is tapped off, injected as dykes and extruded as pillow lavas. At the same time, the magma remaining within the chamber fractionates on cooling to give a layered sequence ranging upwards from dunites with chromite, through peridotites and olivine gabbros to gabbros and minor soda-rich granites.

The model erected to explain the formation of ophiolites and that deduced from geophysical studies for processes at constructive margins are so similar that the observational ophiolite data have been used to elaborate the skeletal geophysical model for present-day constructive margin processes. So long as a four-layer sequence of lavas, dykes, cumulate plutonics and tectonized peridotite is produced, however, the loca- tion of the constructive margin within the ocean basin is irrelevant, as is the size of the ocean basin. Ophiolites could equally well be produced at major constructive margins, such as the mid- Atlantic Ridge or East Pacific Rise, in minor 'oceans', such as the Red Sea or Gulf of Aden, at a behind-arc spreading axis, such as that in the Scotia Sea, or in marginal seas similar to those of the Western Pacific. By the same reason- ing, ophiolites could be produced near to or far from continents and island areas from which they can be separated by a destructive plate boundary. Indeed, most ophiolites seem to have originated in small behind-arc ocean basins.

As it spreads away from the constructive margin, the newly created lithosphere undergoes certain changes, seemingly brought about by the



57 ° Conference report

circulation of vast quantities of sea water through the still hot oceanic crust. This process imposes a horizontally disposed low-pressure/high-temperature zeolite-greenschist-amphibolite facies metamorphism on the basic rocks from which the circulating brines remove transition metals (e.g. Cu, Fe, Mn). On returning to the rock/water interface, these solutions react with the sea water to precipitate metal-rich sediments.

The precise mechanism(s) whereby part of an oceanic plate is emplaced on to a continental margin or island arc is (are) not known. What should normally go down goes up. The special term obduction, the converse of subduction, has been coined to describe the overall process. The least deformed ophiolite sequences appear to be vertically uplifted pieces of ocean floor that have not undergone much horizontal deformation. Most other ophiolites are much more deformed. Their gross structure is commonly that of a series of complexly deformed tectonic sheets stacked one on top of the other. Most of these processes are poorly understood. Nevertheless, they probably involve hydration reactions in the mantle, gravity sliding and/or tectonic pushing, together with other processes as yet unknown.

Geological setting of the Skorovas orebody withln the allochthonous meta-volcanic strat/graphy of the Gjersvlk Nappe, Cen- tral Norway C. Halls, A. Reinsbakken, I. Ferriday, A. Haughen & A. Rankin

The Skorovas orebody is one of the chief stratiform base-metal deposits within the allochthonous greenstone belt of the Central Nor- wegian Caledonides. It is contained in the volcanic level of a complex eruptive association of lower Ordovician age defined by Foslie and Oftedahl as the Gjersvik Nappe. The rocks of this nappe are contained as a depressed segment of the larger Krli Nappe and defined to the north and south, respectively, by the Brrgefjell and Grong-Olden basement culminations. The principal components of this nappe are a plutonic infrastructure of composite gabbroic intrusions within which has been emplaced a series of dioritic to granodioritic (trondhjemitic) bodies, which form the roots of a consanguineous submarine poly- genie volcanic sequence. The eruptive rocks are overlain unconformably by a sequence ofpolymict conglomerates and calcareous flysch sediments, the composition of which suggests immediate derivation by erosion from the underlying igneous complex.

Pre-tectonic segregations, veins and vesicle fillings of epidote, albite, chlorite, carbonate and quartz related to primary volcanic flow structures in the lava pile provide evidence of pervasive in-situ sea-floor metamorphism, and this interpretation is verified by the abundance of nearly monomineralic epidote clasts in the derived conglomerates.

The relationship of the eruptive and sedimentary suites is interpreted in terms of the evolution of an ensimatic island arc of lower Ordovician age, which underwent uplift and erosion prior to emplacement on the Fennoscan- dian basement during the climactic stages of collision tectonism of the Caledonian orogeny in Silurian times.

The entire igneous and sedimentary assemblage has been affected by the tectonic stages of allochthonous emplacement, but the gross differences in competence between the component lithologies has resulted in a particularly heterogeneous style of deformation in which folding, componental sliding, fracturing and penetrative metamorphic refabrication have been governed largely by the geometry of the most competent lithologies, notably gabbro, diorite and granodiorite (trondh- jemite) intrusives and, within the extrusive sequence, compact dacitic flows and their spili- tized aphanitic equivalents (keratophyres). The heterogeneous pattern of deformation is resolved in terms of two main stages of folding complicated by componental sliding movements.

Mineralization occurs at two levels in the eruptive sequence. The layered gabbros and lensoid metagabbros of the plutonic infrastructure contain small cumulus bodies of nickel, copper- and platinum-bearing pyrrhotite-pyrite-magnetite ore of magmatic derivation. Mineralization of this type is at present only known in sub-economic quantities.

The Skorovas orebody, in common with other widely dispersed volcanic exhalites in the Gjersvik Nappe, occurs within the volcanic sequence at a level marked by episodes of explosive dacitic volcanism and associated fumarolic activity. The Skorovas orebody consists of approximately I o oooooo tons of massive and disseminated predominantly pyritic ore with an approximate average grade of z.3% Zn and I . o ~ Cu, together with trace amounts of Pb, As and Ag. The complex lensoid geometry of the orebody is resolved in terms of the disjunction of a single stratiform unit by tight isoclinal folding and componental movements, probably involving both translation and rotation. Enrichment ofsphaerite, chalcopyrite




and, locally, galena within the magnetite-pyrite ores at the stratigraphic top and margins of the ore lenses is interpreted as a primary feature. The banded magnetite-pyrite ores are commonly associated with magnetitic cherts or jaspers and are thus transitional in aspect to the thin, iron- and silica-rich, base-metal-depleted, exhalative sedimentary horizons that occur extensively within the extrusive sequence of the Gjersvik Nappe. These are interpreted as the products of settling of colloidal iron and silica hydrosols following explosive dispersal into an oxidizing submarine environment. They are valuable time-stratigraphic markers and indicators of way-up in complicated structures and are a potentially valuable tool in exploration for massive sulphide bodies formed in limited reducing environments.

Massive sulphide deposits of the Troodos Massif . G. Constantinou

The Cyprus massive sulphide deposits are restricted to and constitute and integral part of the pillow lava succession of the Troodos Ophio- lite Complex. Extensive geophysical, geological and geochemical data collected in the last 2o years suggest that this complex represents an uplifted fragment of oceanic lithosphere and underlying upper mantle, which evolved in the Mesozoic at divergent plate boundaries. The massive sulphide deposits consist of the basaltophilic elements S, Fe, Cu and, less commonly, Zn. They have been formed at the original sea water--lower Pillow Lavas or Basal Group interface and are characterized by distinct vertical zoning comprising, in downward succession, a zone of massive ore, a zone of sulphide with silica and the stockwork zone. The latter consists of veins and disseminations of pyrite in brecciated and/or pillow lavas which have undergone extensive silicification, chloritization and argillization. The veins represent fillings of fractures in the lavas by sulphides, whereas the disseminated pyrite is believed to have been essentially produced by sulphurization of the titanomagnetite of the basaltic lavas and it often contains inclusions of euhedral leucoxene. Part of the disseminated pyrite formed from the iron present in the glassy mesostasis and the ferro- magnesian minerals of the lavas.

Detailed optical, chemical and X-ray diffraction studies indicate that the sulphide ore-forming solution produces extensive mineralogical and chemical changes in the stockwork lavas. These are distinctly different from the mineralogical and chemical changes in the unmineralized lavas

produced by regional hydrothermal metamorphism. Mineralogical changes in the stockwork lavas include the complete pseudomorphism of the primary minerals plagioclase and pyroxene by quartz, the sulphurization of magnetite and the alteration of the glassy mesostasis to a mixture of chalcedony, chlorite, illite and less common kaolinite. The chemical changes in the stockwork lavas involved the dramatic decrease of Ca, Na and, to a lesser extent, A1 and K, and considerable increase of S and Fe. Magnesium decreases considerably in the intensely mineralized lavas, whereas it increases in the weakly mineralized lavas. Whereas the stockwork lavas are invariably intensely silicified, there was no appreciable removal or introduction of silica in these lavas. Silicification of these lavas was probably produced by the selective leaching of the bases from the primary minerals and the glass, leaving behind residual silica in the form of quartz and chalcedony. In the stockwork lavas Cu, Zn and Co increased, whereas Ni and Mn decreased compared to the unmineralized lavas. Field, chemical and mineralogical relationships suggest that the stockwork zones represent the roots of the massive ores and the channels through which the ore-bearing fluids have ascended to form the massive ores and the sea water--pillow lava interface. The persistent presence of illite and absence of montmorillonite as a hydrothermal alternation product in the stockwork zone, combined with experimental data and studies of the active geothermal areas, suggest that the temperature of the ore-forming fluids was 25o-26o ° and its pH 6.6-6.8. Recent detailed stable isotope studies suggest that these fluids were mostly heated marine waters, which leached the ore metals from the underlying Sheeted Dyke Complex.

Rare-earth element evidence for the genesis of the met~_!l|ferous sediments of Troodos, Cyprus. A. H. F. Robertson & A. J. Fleet

The origin of metalliferous sediments associated with active ocean ridges continues to be debated. The various types of metalliferous sediment that occur on and within the lavas of the Troodos Massif, Cyprus, which is accepted by many workers as a slice of Mesozoic ocean crust formed at a constructive plate margin, enables the ficld relationships of such sediments to bc studied at first hand. Umbers, which are chcmically and mincralogically comparable with thc mctallifcrous sediments of prescnt-day active ocean ridges, overlie the uppermost lavas of the massif. Beneath thc umbers there are cxtcnsivc zones of veined,




brecciated and chemically altered lavas, which are interpreted as channelways for ascending hydrothermal solutions, within which metalliferous sediments occur. Manganese-poor metalliferous sediments known as ochres occur intimately associated with the massive sulphide ores of Troodos.

The rare-earth element (REE) contents and, hence, relative fractionation of samples of these metalliferous sediments and of some associated lavas and sulphides have been determined by instrumental neutron activation analysis in an attempt to identify the sources of these materials and/or the different igneous and sedimentary geochemical processes that give rise to these materials. All the metalliferous sediments are enriched in the REE relative to chondrites and in the light relative to the heavy REE. In the case of umbers this is to a greater degree than normal pelagic days; and the bentonitic clays, which overlie the umbers and are thought to be continental in origin, are enriched.

All these sediments exhibit a marked negative Ce anomaly, the size of which decreases with the increasing lithogenous content of the sediments. Such a marked negative Ce anomaly is a characteristic of sea water and a feature of the East Pacific Rise metalliferous sediments, but contrasts with the positive anomaly of manganese nodules. The REE contents of the lavas analysed are difficult to interpret, but are enriched in La relative to the other REE, probably because of alteration.

The REE contents of the Troodos metalliferous sediments appear to have been incorporated into the sediments from sea water. This supports the idea that the major components of the sediments, iron and, in the case of the umbers, some manganese oxides and hydroxides, were derived by the pervasive leaching by sea water of the igneous rock sequence. The REE contents of the interstitial sediments imply deep penetration by sea water far into the pillow lava succession in agreement with the results of oxygen and hydrogen isotope analyses. All the differing types of metalliferous sediment may be interpreted as related to differing redox conditions and events in the evolution of the Troodos ocean ridge.

Isotopic evidence for the origin of xn/nerali- zatlon associated with the ophlo]itlc rocks of the Troodos ~/asslf , Cyprus. E. T. C. Spooner & T. H. E. Heaton

The information presently available--in particular, that derived from the use of isotopes as geochemical tracers--indicates a model for

mineralization and metamorphism of the ophiolitic rocks of the Troodos massif that involves hydrothermal convection of sea water. During water/rock interaction in the recharge part of the cycle of convective heat and mass transfer, the Pillow Lavas, Basal Group and Dyke Complex were hydrothermally metamorphosed in zeolite facies to amphibolite facies conditions. Base metals, in particular Fe, Mn, Cu, Zn, Pb, Ni, Co, Ag and Au, were leached, and, at the localized positions of discharge of hot fluid, sulphide ore deposits and/or ferromanganoan oxide sediments (umbers) were precipitated.

Evidence for sea-water interaction during downflow through Pillow Lavas, Basal Group and Dyke Gomplex. This sequence of rocks was strongly hydrothermally metamorphosed and hydrated (mean increase in H20 + content = approx. 2"2 wt.%). It therefore interacted with an aqueous fluid.

That this fluid was sea water is shown by the fact that the altered rocks are strontium isotopically contaminated relative to fresh rocks. The age- corrected 87Sr/S6Sr ratios (nine analyses) of fresh gabbros and plagioclase concentrates from fresh gabbros define a narrow range from o-7o344 -+- 5 to o.7o386-4-4 (mean ----o-7o362 ~ x3; E & A standard = o'7o8oo). In contrast, STSr/S6Sr ratios for altered rocks vary from o.7o338 -4-Io to values as high as o.7o69 (mean of 17 determina- tions =o '7o49-+-9) . These data show that during hydrothermal metamorphism the rocks were also strontium isotopically contaminated. The only reasonable reservoir of isotopically heavy strontium was sea water (STSr/S6Sr = o'7o76 in Campanian-Maestrichtian time). Hence, sea water was the source of the hydrothermal fluid. This interpretation is confirmed by the fact that the isotopic composition of the strontium in an interstitial zeolite sample taken from about 8oo m below the top of the pillow lavas is statistically indistinguishable from Campanian sea water (zeolite STSr/SeSr = 0.70760 ~ 3).

The whole rock oxygen isotope ratios (6xsO) are also strongly modified relative to the ~tsO value of fresh deep-sea basaltic rocks (OtsO---- ~, +6%). Enrichments in the upper part of the sequence (highest ~xso = + 12"4o%) change to depletions deeper down (lowest olSO = + 3"31%).

Mineral and whole rock combined D/H and xso/xeO data, together with data on mineral assemblages, enable the H and O isotope compositions of the Troodos metamorphic waters to be calculated. During greenschist facies metamorphism of the Sheeted Intrusive Complex and the upper parts of the gabbros, at temperatures




between 200 and 45o°C, the OD value of the waters was close to 0%, demonstrating a sea- water origin with little or no detectable 'primary magmatic water'. CMorite and actinolite formed from sea-water fluids that had undergone an O isotope shift (from O up to ,~ +5%) through exchange with rocks, implying low effective water/ rock ratios (less than x, wt ratio). In contrast, vein epidotes formed from relatively non O isotope exchanged sea waters, implying high effective water/rock ratios (greater than t, wt ratio). If hornblendes in the gabbros formed at temperatures higher than 5oo°C, a 'primary magmatic water' component in the fluid is suggested.

Evidence for formation of ore de~Oosits during discharge of geochemically modified sea water. From D/H and xso/xeO data on cMorites and quartzes from the Troodos mine stockworks, the H isotope data again demonstrate that the discharging hydrothermal fluids were of sea-water origin, and lack of change in the O isotope composition of these fluids implies high effective water/rock ratios (greater than x, wt ratio). Stockwork hydrothermal alteration temperatures on the order of 25o-35o°(3 are suggested (i.e. higher than the surrounding zeolite facies). The data suggest that the stockwork fluids were isotopically similar to the deeper vein epidote fluids. 'Primary magmatic water', if present in the stockwork fluids, was so diluted that it is isotopicaUy undetectable.

The ~ S values of pyrite from massive ore and stockworks define a range from +3"o% to +7.0% (mean of t 9 analyses = +4"8%). Re- interpretation of these data according to the theory of sulphur isotope fractionation formulated by Ohmoto suggests that the combined sulphur in the ore deposits is a mixture of approximately one-third reduced sea-water sulphate (OwaS= , ~ + i 6 % in upper Cretaceous time;) and two-thirds remobilized basaltic sulphur (6~S of sulphides in deep-sea basalts = ~ + i %).

Conclusion. A sea-water hydrothermal convection system model for mineralization and metamorphism of the ophiolitic rocks of the Troodos Massif is reasonable.

The Planes-San Antonio pyritic deposit Rio Tinto, Spain; its nature, environment and genesis. D. Williams, R. L. Stanton & F. Rambaud

The San Antonio stratiform cuprifcrous deposit of Rio Tinto, discovered in the early 196os, is now being exploited as an eastward extension of the bedded pyrititc sheet in the old Planes mine,

which is there underlain by a stockwork feeder pipe. The Planes pipe consists of sulphur-rich-

copper-poor sulphide ore formed by the almost complete replacement of a stockwork conduit which traverses lower Carboniferous felsic pyro- ciastic rocks. Overlying the pipe, and extending far beyond its margin, is the genetically related Planes-San Antonio cuprifcrous pyrititc, a 'scdi- rnentary-volcanogcnic' deposit precipitated as a chemical sediment derived from sea-floor hot springs associated with the lower Carboniferous volcanic island arc of southwest Ibcria. Most of the pyritite bodies of Rio Tinto wcrc deposited directly above or close to their underlying feeder stockworks. The Planes--San Antonio pyrititc displays well preserved stratification, slump structures and intcrdigitatcd tuff bands, and was mainly precipitated comparatively distant from the nourishing Planes stockwork.

Across-layer base-metal zoning in the Planes- San Antonio sheet is usually characterized by an increase in copper, and to a lesser extent in zinc and lead, towards the base of the deposit. Much of this enrichment may have been due to late hypogcnc processes involving the ascent of juvenile-meteoric solutions which leached base metals from the underlying volcanic pile and deposited them as chalcopyrite, sphalcritc and galena by partial replacement of the lower part of the preexisting pyrite-rich stratiform sulphide sheet.

A notable feature of the pyrititc is the abundance of colloforrn and framboidal pyrite. The mean isotopic composition of the sulphide sulphur of the Plancs stockwork (6~aS~ +xo%) is distinctly heavier than that of the stratiform Planes-San Antonio pyritite (6alS ~_ +2"7%).

The cupriferous pyritic mass of Planes--San Antonio exhibits some of the features characteristic of pyritic deposits formed during the initial stages of arc development ('Cyprus' or 'ophiolite' type ores). It was actually formed, however, at a late stage of arc evolution, during a period of relative quiescence within the waning episodes of submarine felsic volcanism, and it displays enviromental traits analogous to those of the Japanese 'Kuroko' ores.

Volcanogenle , , ,h, erallzatton at Avoca, Co. Wicldow, Ireland and its regional ira. plications. J. W. Platt

The northeast-southwest-striking volcanic rocks that make up the lower Palaeozoic volcanic province of southeast Ireland arc, in respect of their cconornic geology, the poorest known




outcropping section of the mobile belt of the Appalachian-Caledonian orogen.

The belt in Scandinavia and east North America has been relatively well studied in the economic sense. Southeast Ireland has been relatively unstudied owing to the history of marginality and presumed epigenesis of the Avoea orebodies (Avoca is the only operating base-metal mine in the Irish Caledonides) and more geological emphasis in Ireland being accord- ed the Carboniferous ore deposits. Studies of published information on the geology and plate tectonic relationships of Avoca-related Scandi- navian and North American ore deposits (e.g. Bathurst Cap, New Brunswick, Canada; Buchans, Newfoundland, Canada; Skorovass, Norway; Stekkenjokk, Sweden) have led to the diagnosis of Avoca orebodies as 'volcanogenic'.

The association of Avoca orebodies with a felsic, calc-alkaiine volcanic pile, their ore mineralogy, age and tectonic style classify Avoca in type (b) of Hutchinson's three-category classification ofvolcanogenic sulphide ore deposits: (a) Zn-Cu-Py (b) Pb-Zn-Cu-pyrite (c) Cu-pyrite. The deformational history and plate tectonic model association of the Avoca volcanogenic ores have been qualified. The oceanic plate of the lower Palaeozoic Proto-Atlantic Ocean was destroyed along subduction (or Benioff) zones at its margins. A subduction system striking northeast-southwest has been recognized, parallelling the Caledonian thrust front and defining the trace of the Caledonian mobile belt--this was located northwest of a zone now extending from the English Lake District to Co. Waterford in Ireland. The island arc vulcanism that has made up the calc-alkaline volcanic province of southeast Ireland developed along this subduction zone.

Subsequent tectonics (including regional metamorphism to lower greenschist facies) have left the fossil island ares of southeast Ireland exposed in two northeast-southwest-striking synclinal cores, one stretching from Tramore in Co. Waterford to the Irish Sea at Arklow in Co. Wicklow and another in Co. Wicklow, known as the 'Avoca belt' and hosting the Avoca mines in a complex of quartzo-feldspathic tufts, lavas and volcanic breccias, intercalated carbonaceous and dolomitized clastic sediments and related dioritic intrusions.

An understanding of the volcanogenic ore geology of the Avoca mines is the key to the assessment of the ore potential of the Lower Palaeozoic volcanic province of southeast Ireland. The Avoca mining area covers 2 miles of strike

within the Avoca belt. Two and possibly three separate volcanogenic ore systems are recognized. The orebodies are compositionally zoned.

Zone (a) A siliceous cMoritized stockwork of disseminated chalcopyrite (o .8o~ Cu) (stringer ore) passing laterally into a sericitized alteration envelope with footwail rhyolite (probably domical and containing magnetitic, potash-rich areas).

Zone (b) A massive sulphide facies overlying the stringer ore and itself overlain by carbonaceous shales and magnesium- rich tufts. The massive sulphide orebody is zoned into an underlying Pb-Zn-Cu- pyrite zone. Contacts between these zones and their wallrocks are sharp, but they tongue laterally into ferruginous, manganiferous, carbonate-rich shales and tufts, perhaps representing a back arc type of basinal sedimentary facies and containing units of the diagnostic New Brunswick style 'bird's eye' crystal tuff. The carbonates are mainly pervasive oolitic dolomite.

An Avoca volcanogenic ore model in its undeformed state is presented and is shown to have some resemblance to respective ore models proposed for the Rosebery orebody, Tasmania, and the classic Kuroko deposits of Japan. It is suggested that the oolitic dolomite-rich facies of Avoca may be the lower Palaeozoic equivalent of the Kuroko 'Sekkoko' or Gypsum zone. One deformed Avoca volcanogenic ore unit (West Avoca) is compared with the undeformed Avoea ore model. Strike attenuation, isoclinal folding and graphitization of carbonaceous horizons have caused dissociation of the overlying Pb-Zn-Cu- pyrite zone of the massive sulphide orebody from its counterpart Cu-pyrite zone. Strike-slip has moved the Pb-Zn-Cu-pyrite zone to the southwest to leave it largely enclosed in the oolitic dolomitized shale/tuff sequence.

Historically, Pb-Zn mineralization at Avoca has been avoided by producers. Modem mining (post I945) has been conducted almost entirely at West Avoca in the Cu-pyrite zone of the massive sulphide orebody and its underlying stringer zone. The mine has been described as a marginal copper mine, and the potential prize of the Pb-Zn-Cu- pyrite zone awaits evaluation to amend this situation.




The development and application of the Avoca volcanogenic ore model has led so far to the partial discovery of the dissociated Pb-Zn-Cu- pyrite orebody at West Avoca, with consequent implications for the volcanic province of southeast Ireland. The 'volcanogenic diagnosis' has also led to the discovery of extensive tonnages of stockwork ore in the East Avoca sector of the Avoca Mines.

The tendency of voleanogenic ore deposits of Avoca type to occur in clusters leads to further speculation on the ore potential of southeast Ireland and its generally unrecognized position of importance in the study of Caledonian volcano- genie sulphide ores.

Occurrence, origin and sign;f~cance of mechanically transported sulphide ores at Buchans, Newfoundland. J. G. Thurlow

The Buchans orebodies are high-grade, polymetallic massive sulphide deposits associated with Silurian (?) felsic volcanism of the northern Appalachians. Since 1928, 17 ooo ooo tons of ore have been mined, of which c 5o% consists of mechanically transported sulphide accumulations.

These deposits consist of angular (indurated) and wispy (plastic) sulphide fragments intimately mixed with a large variety of angular to rounded lithic fragments. Abundant evidence exists to suggest that these ores were transported as rapidly moving sub-aqueous debris flows triggered by surface volcanic explosions and by earth- quakes. They were preferentially deposited in palaeotopographic depressions along the path of debris flow and maintain economic grade for distances in excess of 2 km from their source. Other deposits appear to have moved as slowly advancing 'mud glaciers'. In most cases the source of these transported ores can be identified as in-situ massive sulphide deposits, but at least one sub- economic prospect has no known source orebody. Parameters defining the direction of transport were discussed. On the basis of the Buchans example and numerous other occurrences of transported ores it is suggested that transported ores in deformed terrains may be more common than has previously been suspected. In terms of exploration, economic sulphide accumulations may occur as transported orebodies well beyond the limits of stockwork mineralization and significant alteration.

The paper presented at this meeting will be published as a joint publication of The Geological Society of London and the Instruction of Mining and Metallurgy; further information can be obtained from M. J. Jones, Instruction of Mining and Metallurgy, 44 Portland Place, London WIN 4BR.

IAN GORDON GASS, Department of Earth Sciences, The Open University, Milton Keynes, MK76AA.



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