(Sedimentary Exhalative) Processes 3. Summary

Home , Baryte, Chalcopyrite, Massive sulfide deposits, Ore, Ore genesis

1. Details of Module

Subject Name Geology Paper Name ECONOMIC GEOLOGY & MINERAL RESOURCES OF INDIA Module Name/Title SOME MAJOR THEORIES OF ORE GENESIS PART 2: ORIGIN DUE TO SURFACE (EXOGENIC) PROCESSES Module Id GEL-05-143 Pre-requisites Before learning this module, the users should be aware of  Origin of orthomagmatic deposits.  Genesis of igneous iron ore.  Origin of pegmatites. Objectives To understand:  the mode of occurrence or orthomagmatic iron deposits associated with intermediate to felsic volcanic rocks.  the mode of occurrence of pegmatites, their mineral wealth, depth- wise classification of pegmatites and diverse views on their origin. Keywords Felsic magmatism, orthomagmatic deposits, pegmatites.

2. Structure of the Module-as Outline: Table of Contents only (topics covered with their sub-topics)

1. Introduction 2. Volcanic-exhalative (sedimentary exhalative) processes 3. Summary

3.0 Development Team:

Role Name Affiliation National Co-ordinator Subject Co-ordinators Prof. M.S. Sethumadhav Centre for Advanced Studies (e-mail: Prof. D. Nagaraju Dept of Earth Science [email protected]) Prof. B. Suresh University of Mysore, Mysore-6 Paper Co-ordinator Prof. M.S. Sethumadhav Centre for Advanced Studies Dept of Earth Science University of Mysore, Mysore-6 Content Writer/Author(CW) Prof. B. Krishna Rao Former Professor, Department of studies in Geology, University of Mysore, Mysore-6 Content Reviewer (CR) Prof. A. Balasubramaian Centre for Advanced Studies Dept of Earth Science University of Mysore, Mysore-6

1. INTRODUCTION

Exogenic (surface) processes involved in the development of mineral deposits include:

i. Mechanical accumulation: This process involves concentration of heavy, durable minerals into placer deposits. Placer deposits are classified as (a) residual placers, (b) eluvial placers (concentration in a moving solid medium, generally formed upon hill slopes from mineral released from a nearby source rock), (c) stream or alluvial placers, beach placers and offshore placers (all the three are the concentration in a moving water medium), and (d)

Aeolian placers (concentration by air). Typical examples of Placer deposits are: Rutile and zircon sands of New South Wales, Australia and Trail Ridge, USA; Tin placers of Malaysia;

Gold placers of the Yukon, Canada; Precambrian palaeoplacer deposits of Au, Au-U and U, including the large Witwatersrand Au-U placer deposit.

ii. Chemogenic precipitation leading to the formation of sedimentary deposits. This processes takes place through precipitation of particular elements in suitable sedimentary environments, with or without the intervention of biological organisms. Typical examples of sedimentary deposits include Banded Iron Formations of Precambrian shields, manganese deposits of Groote Eylandt, Australia; manganese deposits in South Ukrainian Oligocene

Basin (contains about 70% of the World’s reserves of manganese ore); Zechstein evaporite deposits of Europe; Phosphate deposits of Florida, USA.

iii. Residual processes: Residual deposits are generated through the process of leaching from rocks of soluble elements leaving insoluble elements in the remaining material.

This category of deposits include high level or upland and low level peneplain type lateritic bauxites, auriferous bauxites and laterites, residual deposits of nickel, chromium, REE and titanium. Typical examples of residual deposits include nickel laterites of New Caledonia; high level leateritic bauxites deposits of India; Kaolin deposits in Nigeria; Residual deposit of

REE over carbonatites at Mount weld near Laverton, western Australia; and Anatase deposits in the overburden of alkalic rocks in Parana Basin, Brazil.

iv. Supergene enrichment of sulfide and oxide deposits: This process involves leaching of valuable elements from the upper parts of mineral deposits and thir precipitation at depth to produce higher concentrations. This process in fact generated economic-grade ores from sub economic ones in many porphyry-type base metal deposits.

v. Volcanic exhalative (= sedimentary exhalative) processes: This process involves exhalations of hydrothermal fluids at the surface, usually under submarine conditions and generally producing stratiform ore bodies. Deposits resulted from this process is known in literature as volcanic-associated massive sulfide (VMS) deposits. Typical examples of VMS deposits are: Base metal deposits of Meggan, Germany; Sullivan, Canada; Mount Isa,

Australia, Rio Tinto, Spain; Kuroko deposits of Japan: Black smoke deposits of modern oceans.

Detailed account on (a) placer deposits (b) sedimentary deposits, (c) residual deposits and (d) supergene enrichment of sulfide and oxide deposits are provided in four lessons among the series of lessons on Economic geology and mineral resources of India (GEL-5).

This lesson provides details on volcanogen exhalative process and massive sulfide deposits derived from this process.

2. VOLCANIC – EXHALATIVE (SEDIMENTARY EXHALATIVE) PROCESSES

Volcanic – associated massive sulfide deposits frequently show a close spatial relationship to volcanic rocks, but this is not the case with all the deposits, e.g. Sullivan,

Canada (Fig. 1)(Fig.2.14. Page 34 Evans) which is sediment-hosted and this and similar examples are referred to commonly as sedex (sedimentary-exhalative) deposits. In the volcanic-associated types the principal constituent is usually pyrite with varying amounts of copper, lead, zinc and baryte; precious metals together with other minerals may be present.

For many decades they were considered to be epigenetic hydrothermal replacement orebodies

(Bateman 1950). In the 1950s, however, they were recognized as being syngenetic, submarine-exhalative, sedimentary orebodies, and deposits of this type have been observed in the process of formation from hydrothermal vents (black smokers) at a large number of places along sea-floor spreading centres (Rona 1988). These deposits are now referred to as volcanic-associated (or volcanogenic) massive sulfide deposits.

The ores with a volcanic affiliation show a progression of types. Associated with basic volcanics, usually in the form of ophiolites and presumably formed at oceanic or back- arc spreading ridges, we find the Cyprus types massive sulfide deposits. These are essentially cupriferous pyrite bodies. Thcy are exemplified by the deposits of the Troodos Massif in

Cyprus and the Ordovician Bay of Islands Complex in Newfoundland (Canada). Associated with the early part of the main calc-alkaline stage of island arc formation are the Besshi-type massive sulfide deposits. These occur in successions of mafic volcanics in complex structural settings characterized by thick greywacke sequences. They commonly carry zinc as well as copper and are exemplified by the Palaeozoic Sanbagwa deposits in Japan, and the

Ordovician deposits of Folldal in Norway. The more felsic volcanics, developed at a later stage in island arc evolution, have a more varied metal association. They are copper-zinc-lead ores often carrying gold and silver. Large amounts of baryte, quartz and gypsum may be associated with them. They are called Kuroko-type deposits after the Miocene ores of that name in Japan, but similar deposits in the Precambrian are known as Primitive-type. All these different type massive sulfide deposits are normally underlain in part by a stockwork up which the generating hydrothermal solutions appear to have passed (Fig. 2) (Fig.2.20 Page,

39 Evans).

There is today wide agreement that these deposits arc submarine-hydrothermal in origin, but there is a divergence of opinion as to whether the solutions responsible for their

formation are magmatic in origin or whether they represent Circulating sea water. To understand this let us look at the evidence from hydrothermal activity on the ocean floors.

Hydrothermal mineralization at sea-floor spreading centres was first discovered in the

Red Sea in the mid 1960s, but the resultant deposits do not appear to be true modern analogues of ancient volcanic and sediment-hosted massive sulfide deposits. Since then various forms of hydrothermal mineralization have been found at many sites along spreading centres with the black smoker type producing obvious analogues of ancient massive sulfide deposits (Rona 1988).

Black smokers were discovered in the late 1970s during ocean floor investigations using a submersible. They are plumes of hot, black, sometimes white, hydrothermal fluid issuing from chimney-like vents that connect with fractures in the sea floor. The black smoke is so coloured by a high content of fine-grained metallic sulfide particles and the white by calcium and barium sulfates. The chimneys are generally less than 6 m high and are about 2 m across. They stand on mounds of massive ore-grade sulfides (Fig.3)(Fig. 4.11, page 72

Evans) that occur within the grabens and on the flanks of oceanic ridges. Ten of the largest mounds in the eastern axial valley of the southern Explorer Ridge (about 350 km west of

Vancouver Island) average 150 m across and 5 m thick and are estimated to contain a total of

3-5 million tons of sulfides. The largest mound-chimney deposit so far found is the TTG mound on the Mid-Atlantic Ridge at 26°N, which is estimated to contain 4.5 million tons of sulfides. The mineralogy of the mounds is similar to that of massive sulfide deposits on land with high temperature copper-iron sulfides beneath lower temperature zinc- and iron-rich sulfides, baryte and amorphous silica. Silver-bearing sulfosalts with minor galena occur in the lower temperature (< 300°C) Zn-Fe assemblages rather than the Cu assemblages. Gold values ranging up to 16.4 ppm have been found.

Growth of a sulfide chimney commences with the precipitation of anhydrite from the cold sea water around the hot ascending plume, forming a porous wall that continues to grow upward during the life of the plume. Most of the hydrothermal fluid flows up the chimney to discharge as a plume into the surrounding sea water, but a small proportion flows through the porous anhydrite wall. In doing so the fluid moves rapidly from high temperature (> 300°C), acidic (pH~3.5) and reduced (H2S >> SO4) conditions to conditions close to those of normal sea water at oceanic depths (2°C, alkaline, pH ~7.8, oxidized, SO4 >> H2S) as it meets sea water penetrating the chimney from the outside. Thus, whilst the anhydrite chimney top grows upwards, the walls of the lower part thicken by the precipitation of sulfides in the interior portions and anhydrite on the outside (Fig. 3)(already quoted). This process leads to a concentric zoning with chalcopyrite (± isocubanite ± pyrrhotite) on the inside, an intermediate zone of pyrite, sphalerite, wurzite and anhydrite, and an outer zone of anhydrite with minor sulfides, amorphous silica, baryte and other minerals. The zoning is probably caused by the temperature decrease across the wall rather than other factors.

After growing upwards at a rate of perhaps 8-30 cm a day, chimneys eventually become unstable and collapse forming a mound of chimney debris mixed with anhydrite and sulfides upon which further chimney growth and collaps occur. Once a mound has developed it grows both by accumulation of chiney debris on its upper surface and by precipitation of sulfides within the mound. The covering of chimney debris performs the same role as the vertical porous anhydrite chimney walls producing sulfide and silica precipitation in the outer part of the mound. This decreases the permeability of the mound and forms a crust that constrains fluid escape and leads to considerable circulation of high temperature solutions within the mound. The isotherms within the mound then rise, leading to the replacement of lower temperature mineral assemblages by higher temperature ones, thus producing a similar

zoning to that in the chimneys and that of volcanic-associated massive sulfide deposits found on land.

Volcanic-associated massive sulfide (VMS) deposits may have the mound shape of modem massive sulfide deposits or they may be bowl shaped. The latter type probably developed when hydrothermal solutions, more saline (denser) than the surrounding sea water, vented into a submarine depression (Fig. 4)(Fig. 4.12 page 73, Evans). Many Cyprus-type deposits appear to have developed in this way and the available fluid inclusion data is consistent with this model.

A most surprising feature of modem submarine hydrothermal vents is the associated prolific biota and their food chain based on chemosynthetic bacteria. The megafauna is varied and characteristic and some individuals, particularly tube casts of vestimentifera have been preserved in chimney sulfides. The remains of ancient vents have been found in the

Cretaceous deposits of Cyprus, in similar deposits in the Oman where traces of this characteristic fauna are also present.

This brief summary of the formation of black smoker deposits shows that our knowledge of their mode of formation confirms the model (Kuroko model) put forward for the genesis of volcanic-associated massive sulfide deposits. The principal stages of development of this model are as follows (Fig. 5)(Fig. 4.13, Page 74 Evans).

1. Precipitation of fine-grained sphalerite, galena, pyrite, tetrahedrite, baryte with minor chalcopyrite (black ore) by the mixing of relatively cool (~200°C) hydrothermal solutions with cold sea water.

2. Recrystallization and grain growth of these minerals at the base of the evolving mound by hotter (~250°C) solutions, together with deposition of more sphalerite, etc.

3. Influx of hotter (~300°C) copper-rich solutions which replace the earlier deposited minerals with chalcopyrite in the lower part of the deposit (yellow ore). Redeposition of these replaced minerals at a higher level.

4. Still hotter, copper undersaturated solutions then dissolve some chalcopyrite to form pyrite-rich bases in the deposits.

5. Deposition of chert-hematite exhalites above and around the sulfide deposit.

Similar exhalites will also have formed during earlier stages. Silica is slow to precipitate, it needs silicate minerals to nucleate on and so, although much may be deposited in the underlying stockwork, the rest is mainly carried through the sulfide body to form exhalites above it.

Note that an evolution with time of the hydrothermal solutions is postulated from cooler to hotter and back to cooler and that the solutions of stages (2 - 4) cool as they move upwards and outwards through the sulfide mound to produce stage 1 type mineralization at the top of the deposit; (see Fig. 5)(already provided). The most important evidence on which this model is based, mineral replacement textures and fluid inclusion filling temperature trends, comes from young unaltered deposits and generally is difficult or impossible to find in older (Palaeozoic and Precambrian) deposits, which usually have suffered some degree of deformation and metamorphism.

Most workers agree at least on the broad outline of the above mechanisms for the formation of VMS deposits, but they disagree on the origin of the formative solutions, with the majority, favouring the hypothesis that they are composed of sea water which has circulated deep into the crust, where it has been heated, become more saline and dissolved base and precious metals from the volcanic and other rocks it has passed through, and then risen along permeable zones to be exhaled on the sea floor as sketched in Fig.6 (Fig. 4.17, page 77, Evans). These workers point, in particular, to the evidence from isotopic studies,

which is not unequivocal as we shall see. Before discussing the isotopic data the increase in salinity must be accounted for. Rock hydration reactions by consuming water will concentrate chloride in the pore solutions, phase separation of the NaCl-H2O fluid under high

PT conditions can produce a brine with salinities two to three times that of sea water and other processes, including dissolution of evaporites, can produce the same result.

Different types of water have characteristic hydrogen (D/H) and oxygen (18O/16O) isotopic ratios. Using these ratios it has been shown that various types of water were involved in general mineralization processes. Magmatic fluids were dominant in some cases, and in others initiated the mineralization and wall rock alteration only to be swamped by convective meteoric water set in motion by hot intrusions or other heat sources. In massive sulfide deposits, much isotopic evidence favours sea water as the principal or only fluid. The possibility must be borne in mind, however, that the sea water was a late addition to a magmatic hydrothermal system and that it has overprinted the pre-existing magmatic values.

It is worthwhile examining the evidence in some depth as isotopic studies are now being applied to all types of mineral deposits (Valley et al. 1986).

Variations in the isotopic ratios of hydrogen and oxygen are given in the δ notation in parts per thousand (per mil, ‰) where:

In the above formula for hydrogen, = and R = D/H; for oxygen, = 18δO and R

= 18O/16O.

The standard for both hydrogen and oxygen is standard mean ocean water or SMOW.

In nature, D/H is about 1/7000 and 18O/16O is about 1/500. These values are measured directly on natural substances, such as thermal waters, formation waters in sediments and fluid inclusions, or they are determined indirectly using minerals after removal of all the absorbed water. In the latter case the isotopic composition of the mineral is not that of the

fluid with which it was in contact at the time of crystallization or recrystallization. The δ values for the fluid have to be calculated from the mineral values using equilibrium fractionation factors determined by laboratory experiment or from studies of active geothermal systems. A temperature fractionation effect also occurs, so the temperature must be known (from fluid inclusion studies, etc.) in order to determine the isotopic composition of the water in equilibrium with the mineral. For example, in Fig.7 (Fig.4.14, Page 76, Evans) raising the temperature from 10 to 200°C gives rise to isotope exchange and re-equilibrium such that the isotopic composition of the water changes from Y1 to X1 whilst that of the coexisting kaolinite changes from Y2 to X2 . In other words rock·- water reactions cause a shift in the δ values of both the circulating meteoric water and the which it is in contact, with the result that the water is enriched in 18O as the temperature rises.

The isotopic compositions of the various types of water show useful differences. Sea water in general plots very close to SMOW (Fig.8)(Fig. 4.15 Page, 76 Evans) and shows very little variation. Meteoric water varies fairly systematically with latitude along the line shown in Fig. 8. Values for metamorphic and magmatic waters have been deduced from measurements on minerals. Formation water (interstitial water in sediments) may have been trapped during sedimentation or may have entered the interstices at a later time, when it may be of any origin or age. It can be measured directly and plots as shown in Fig. 5. Since many of the formation waters are richer in 18O than SMOW they cannot have resulted from simple mixing of meteoric and sea water. There must have been isotopic exchange with the sediments at elevated temperatures (shown by Fig. 7 to result in 18O enrichment of water, e.g., the change from Y1 to X1), addition of rising metamorphic water or some other process.

We are now in a position to look at some results of studies on volcanic-associated massive sulfide deposits. The data of course comes from associated gangue and wall rock alteration minerals and not from the ore minerals themselves.

These results arc plotted in Fig.9 (Fig. 4.16 Page, 77 Evans). Those for the Cretaceous

Cyprus stockwork deposits coincide exactly with the values of sea water, and Heaton &

Sheppard (1977) suggest a model involving deeply circulating sea water as sketched in Fig. 6.

They also present evidence that the associated country rocks were thoroughly permeated by sea water during their metamorphism into the greenschist and zeolitic facies. The Kuroko fluids show δ18O values commensurate with a sea water origin, but δD is depleted by 11-26‰ relative to sea water. Ohmoto & Rye (1974) concluded that sea water was the dominant source of the hydrothermal fluid but that is contained a small meteoric and/or magmatic contribution. As Kuroko deposits belong to the Island Arc environment, meteoric water could be involved. This would suggest a model, similar to that in Fig. 6, of circulating sea water becoming a concentrated brine at depth, dissolving copper and other metals from the rocks it traversed and carrying these up to the surface where they were precipitated as sulfides with sulfur derived from the sea water and/or the rocks through which it passed. A large number of other deposits give similar plots to that of the Cyprus stockworks (Fig. 9) and the positions of two, Rio Tinto (the world's largest, ~500 Mt) and Salgadhino from the Iberian Pyrite Belt

(Carboniferous). The D-68 Zone deposit, Noranda, Quebec (Archaean), plots closer to the magmatic water box than the Kuroko solutions and Ikingura et al. (1989) suggested that 25-

30% of magmatic water may have been present in the mineralizing fluids. The Blue Hill,

Maine (Siluro-Devonian) solutions and values for the Kosaka deposit, Japan plot even closer.

The ore solutions of the Blue Hill deposit, according to Munha et al. (1986) contained a 40% magmatic component.

Did the solutions responsible for the bulk of the mineralization in these bodies really vary as much in isotopic composition as these results suggest, or was there massive but variable overprinting by sea water in convection cells set in motion by nearby igneous intrusions that may have been the source of the magmatic water? Solomon et al. (1987) have

shown that during the cooling cycle, after deposit formation, sea water convection cells would remain in motion for a considerable time, long enough indeed to produce wall rock alteration on the later formed hanging wall rocks above the deposits! Perhaps, as Stanton

(1986) has suggested, the process is a hybrid one, a magmatic source producing concentrated ore solutions that move up mixing with large quantities of sea water at higher levels so acquiring hydrogen, oxygen and sulfur isotope characteristics near to those of sea water. The evidence is equivocal.

Sulphur isotopic variation is reported in terms of δ34S representing the changes in

34S/32S. It is sometimes possible to differentiate between crustal and mantle sources of sulfur on the assumption that mantle values of δ34S are zero, and Ohmoto (1986) concluded that, unless assimilation of crustal sulfur has occurred, magmas generated in the mantle will have

δ34S values between -3 and +3‰. This may be an oversimplification and evidence is appearing which suggests that contamination of the upper mantle by subduction is producing compositional heterogeneities, e.g. Chaussidon et at. (1987) obtained values up to + 9.5‰ from sulfide inclusions in diamonds. However magmatic rocks from the mantle do, in general, have values close to zero. On the other hand much crustal sulfide has undergone biogenic fractionation giving δ34S values as high as + 30‰ and ocean water has δ34S ~ 20‰.

δ34S values of modern sulfide mounds vary from about 1.5-4‰, indicating a mainly mantle source with a smaller component derived from reduced sea water sulfur. This is interpreted by those favouring a circulating sea water hypothesis as indicating sulfur leaching from the consolidated volcanic substrate, but it does not rule out derivation of this sulfur from a consolidating magma that yielded a concentrated ore solution at some stage during its crystallization.

The same argument applies to ancient deposits. Thus sulfur isotopic data for the

Cyprus ores was reviewed by Spooner (1977) who pointed out that δ34S for the pyrite is

somewhat higher than that of the basaltic country rocks from which the sulfur may have been partly derived. This suggests an additional source of isotopically heavy sulfur. Spooner felt that this was probably the circulating Cretaceous sea water which would have had a value of

δ34S = + 16‰. Later studies on other deposits have produced parallel findings and similar interpretations have been advanced (Lydon 1989).

Other isotopic tracers, including strontium and lead, also allow for a circulating sea water hypothesis or a magmatic-hydrothermal interpretation, e.g., Sato (1977) who suggested a direct magmatic source for the lead in Kuroko deposits based on his work on lead isotopes.

As mentioned above there is wide acceptance of the hypothesis that the mineralizing fluids represent heated sea water that has leached metals from subjacent igneous rocks. It is therefore of value to note some of the arguments, that the minority have advanced in favour of a magmatic-hydrothermal origin.

Most persuasive and perhaps influential has been Stanton (1978, 1986 and many other papers). He has argued that the ore-rock relations are the opposite of what might be expected from the circulating sea water hypothesis. He pointed out firstly that Cyprus-type deposits are small (usually < 5 Mt) and associated with basaltic successions of crustal dimensions whilst

Kuroko (and Primitive) types range up to immense sizes (> 200 Mt) and are often associated with thin successions of andesites to rhyolitic rocks, e.g., at Rio Tinto the thickness of the subjacent volcanic rocks is in places less than 100 m thick (Williams et al. 1975) and indeed the volcanic sedimentary complex underlying the many enormous orebodies of the 250 km long Iberian Pyrite Belt is only 50-800 m thick with much of the succession being sedimentary; secondly that basalts contain approximately twice the amount of Cu-Zn-Pb as dacites; and thirdly that the basalts of the oceanic crust are accessible to leaching solutions over long periods compared with the andesites, dacites and rhyolites, which often are erupted in shallow water just before volcanism becomes subaerial.

Surely then, Stanton argues, the basalts, enormously larger in bulk, containing more base metals and exposed to leaching to a far greater degree than the andesite-rhyolite series, should have far larger and more numerous associated orebodies; but exactly the opposite is the case. He further argues that if the leaching hypothesis were correct, then Ni and Co should be as plentiful as Cu and Zn in basalt-associated orebodies as these elements occur in the same abundance in basalts and are leached out just as easily. The Sherlock Bay deposits of Western Australia and the ores of Saskatchewan are impressive evidence that Ni can be mobilized within a hydrothermal medium and precipitated as a sulfide. Drawing attention to the fact that massive sulfide deposits occur in clusters rather than randomly and are normally confined to just one or two horizons, Stanton suggested that the ore-forming exhalations result from some relatively brief and clearly defined event in an underlying magma chamber, such as the attainment of a particular stage in crystallization, a sudden decrease in confining pressure with consequent degassing, abrupt igress of external water or a combination of two or more of these.

Sawkins (1986) considered that the isotopic evidence supports the involvement of magmatic water in the formation of these deposits and he feels that we must have a hypothesis of origin that accounts for the narrow time-stratigraphical interval during which such deposits form. He suggests a genetic model having many long-lived, magmatically driven convection cells of sea water, similar to that of Fig. 6, which are responsible for most of the fluid producing the hydrothermal-rich magmatic solutions from the heat source occurred during a short time interval.

3. Summary

Exogenic (surface) processes involved in the development of mineral deposits include

(1). Mechanical accumulation, (2) chemogenic precipitation leading to the formation of sedimentary deposits, (3) residual processes, (4) supergene enrichment (of sulfide and oxide

deposits) and volcanic-exhalative processes. This lesson provides detailed account on various aspects of volcanic-associated massive sulfide deposits.

Volcanic-associated massive sulfide deposits frequently show a close spatial relationship to volcanic rocks, but this is not the case with all the deposits, e.g., Sullivan

(Canada), which sediment-hosted and this and similar deposits are referred to commonly as sedex (sedimentary-exhalative) deposits

The volcanic-associated massive sulfide deposits (VMS) the principal constituent is usually pyrite with varying amounts of copper, lead, zinc and baryte; precious metals. These are syngenetic, submarine-exhalative, sedimentary orebodies, and deposits of this type have been observed in the process of formation from hydrothermal vents (black smokers) at a large number of places along sea-floor spreading centres.

The massive sulfide deposits are encountered in basic volcanics (ophiolites) formed at oceanic or back-arc spreading ridges, (Cyprus type) and in Precambrian and rhyolitic lavas of back-arc basins (?) (Primitive-type).

There is today wide agreement that the massive sulfide deposits are submarine- hydrothermal origin, but there is a divergence of opinion as to whether the hydrothermal solutions responsible for their formation are magmatic in origin or whether they represent circulating sea water.

Present day Black smokers issuing from chimney-like vents (plumes) and standing on sulfide mounds spreading centres are the analogues of ancient massive sulfide deposits.

The principal stages in the development of sulfide mounds beneath black smokers are:

1) Precipitation of fine-grained spalerite, galena, pyrite, baryte tetrahedrite with minor chalcopyrite by mixing of relatively cool (~200ºC) hydrothermal solutions with cold sea water,

2) recrystallization and grain growth of these minerals at the base of the evolving mound by hotter (~205ºC) solutions, together with deposition of more sulfides,

3) Influx of hotter (~300ºC) copper-rich solutions which replace the earlier deposited minerals with chalcopyrite in the lower part of the deposit. Redeposition of the replaced minerals at a higher level

4) slill hotter, copper undersaturated solutions then dissolve some chalcopyrite to form pyrite-rich bases in the mound.

5) Deposition of chert-hematite exhalites above and around the sulfide mound.

Similar exhalites will also have formed during earlier stages.

Several workers agree at least on the broad out line of the above mechanisms for the formation of VMS deposits, but they disagree on the origin of the formative solutions

(magmatic-hydrothermal verses seawater-hydrothermal option), with majority favouring the hypothesis that they are composed of sea water which has circulated deep into the curst, where it has been heated, became more saline and dissolved base and precious metals from the volcanic and other rocks it passed through, and then risen along permeable zones to be exhaled on the sea floor to generate VMS deposits.

Oxygen and hydrogen isotopic data of Cyprus-type stockwork deposits indicate their derivation from sea water-rich hydrothermal fluids. Hydrothermal fluids involved in the formation of Kuroko type deposits contained dominantly sea water it also had a small meteoric and/or magmatic-hydrothermal component.

Oxygen isotope data of several other massive sulfide deposits indicate precence of different proportions of magmatic – hydrothermal fluid component in the mineralizing fluids.

Sulfur isotope data also reveal similar scenario Sawkins (1986) considered that the isotopic evidence supports the involvement of magmatic water in the formation of these deposits and he feels that we must have a hypothesis of origin that accounts for the narrow

time-stratigraphical interval during which such deposits form. He suggests a genetic model having long-lived magmatically driven convection cells of sea water which are responsible for most of the fluid producing the hydrothermal – rich magmatic solutions from the heat source occurred during a short time interval.