Polymetallic massive sulfides at the modem...

E L S E V I E R Ore Geology Reviews 10 (1995) 95-115

ORE GEOLOGY REVIEWS

Polymetallic massive sulfides at the modem seafloor A review

Peter M. Herzig a,*, Mark D. Hannington b "Lehrstuhlf~r Lagerstiittenlehre, Institut fiir Mineralogie, Technische Universitiit Bergakaderaie Freiberg, Brennhausgasse 14,

D-09596 Freiberg, Germany b Geological Survey of Canada, 601 Booth Street, Ottawa, Ont. K1A OE8, Canada

Received 28 July 1994; accepted 1 May 1995

Abstract

Polymetallic massive sulfides on the modern seafloor have been found in diverse volcanic and tectonic settings at water depths ranging from about 3700 to 1500 m. These deposits are located at fast-, intermediate-and slow-spreading mid-ocean ridges, on axial and off-axis volcanoes and seamounts, in sedimented rifts adjacent to continental margins and in subduction-related back- arc environments. High-temperature hydrothermal activity and large accumulations of polymetallic sulfides, however, are known at fewer than 25 different sites. Several individual deposits contain between 1 and 5 million tonnes of massive sulfide (e.g., Southern Explorer Ridge, East Pacific Rise 13°N, TAG Hydrothermal Field) and only two deposits (Middle Valley and Atlantis 11 Deep, Red Sea) are known to contain considerably higher amounts of sulfides ranging between 50 and 100 million tonnes. This range ( 1-100 million tonnes) is similar to the size of many volcanic-associated massive sulfide deposits found on land. However, the vast majority of known sulfide occurrences on the modem seafloor amount to less than a few thousand tonnes and consist largely of scattered hydrothermal vents, mounds and individual chimney structures.

Recovered samples from about 25 deposits world-wide represent no more than a few hundred tonnes of material. The mineralogy of these samples includes both high ( > 300°-350°C) and lower-temperature ( < 300°C) assemblages consisting of varying proportions of pyrrhotite, pyrite/marcasite, sphalerite/wurtzite, chalcopyrite, bornite, isocubanite, barite, anhydrite and amorphous silica. Massive sulfide deposits in back-arc environments additionally may contain abundant galena, Pb--As-Sb sulfosalts (including jordanite, tennatite and tetrahedrite), realgar, orpiment and locally native gold. Close to 1300 chemical analyses of these samples indicate that the seafloor deposits contain important concentrations of Cu and Zn comparable to those of massive sulfide deposits on land. The sediment-hosted deposits, while being somewhat larger than deposits on the sediment- starved mid-ocean ridges, appear to have lower concentrations and different proportions of the base metals due to fluid-sediment interaction. Initial sampling of sulfides in the back-arc spreading centers of the West and Southwest Pacific suggests that these deposits have higher average concentrations of Zn, Pb, As, Sb and Ba than deposits at the sediment-starved mid-ocean ridges. Gold and silver concentrations are locally high in samples from a number of mid-ocean ridge deposits (up to 6.7 ppm Au and 1000 ppm Ag) and may reach concentrations of more than 50 ppm Au and 1. I wt% Ag in massive sulfides from immature back- arc rifts, that are dominated by felsic volcanic rocks. Precious metal contents of seafloor sulfides thus are well within the range of those found in land-based deposits.

Although massive sulfide deposits have been found at water depths as shallow as 1500 m, boiling of the hydrothermal fuids may prevent the formation of typical polymetallic massive sulfides at shallower depths (i.e., less than a few hundred meters), where the hydrostatic pressure is too low to prevent phase separation. In this case, mineralization with distinct epithermal characteristics and significant amounts of precious metals can be expected.

* Corresponding author.

0169-1368 /95 / $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO169-1 368 (95)00009-7

96 P.M. Herzig, M.D. Hannington / Ore Geology Reviews 10 (1995) 95-115

1. Introduction

Within the past 15 years, seafloor polymetallic sulfides have been found in a variety of volcanic and tectonic settings on the modern ocean floor at water depths ranging from about 3700 to 1500 m. Although only a small portion (less than 5%) of the world's ocean ridge system has been explored in detail, about 20 deposits have been located in the Pacific Ocean, four in the Atlantic and one each in the Indian Ocean and the Mediterranean Sea (Fig. 1). One of the largest deposits occurs in the Atlantis II Deep of the Red Sea.

Polymetallic sulfide deposits are found on fast-, intermediate- and slow-spreading mid-ocean ridges, on axial and o f f axis volcanoes and seamounts, in sedimented rifts adjacent to continental margins and in subduction-related back-arc environments (Fig. 2). Rona (1988), Rona and Scott (1993) and Hannington et al. (1994) have compiled data on more than 100 occurrences of hydrothermal mineralization on the seafloor, including Fe- and Mn-oxide deposits, nontronite

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deposits, disseminated sulfides, metalliferous sediments, massive polymetallic sulfide mounds and black and white smoker chimneys. High-temperature hydrothermal activity and large accumulations of polymetallic sulfides, however, are known at fewer than 25 different sites.

An evaluation of the economic significance of these deposits is limited by the lack of sufficient data concerning their distribution, size and bulk composition. Mapping and sampling has been carried out primarily by deep-towed camera surveys, dredging, submersible operations and, recently, by remotely-operated vehicles (ROV's). Most of these deposits have been examined in only two dimensions; their extent and composition at depth are poorly constrained. Some appreciation for the third dimension of large deposits may be acquired through deep drill holes (Ocean Drilling Program) or with small submersible rock drills (Ryall, 1987; John- son, 1991 ). However, systematic drilling of the deposits like that carried out during land-based exploration programs (e.g., many thousands of meters of drilling

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P.M. Herzig, M.D. Hannington / Ore Geology Reviews 10 (1995) 95-115 97

•-• Mid-Ocean Ridge

/~[ Volcanic-hosted ~. Intraoceanic J

Back-Arc Basin

Sediment-hosted ] Intracontinental

Seamounts

1 Intraplate

Off-axis

Fig. 2. Simplified diagram showing the diverse geological environments for the occurrence of seafloor hydrothermal systems. Polymetallic massive sulfide deposits have been found in all settings except for intraplate seamounts.

for a single deposit) is well beyond the scope of current seafloor research projects. The geologic setting of seafloor hydrothermal deposits is usually mapped within only limited areas (typically < 30 km 2) and the geology is described in terms of local features (types of lava flows, thickness and type of sediment cover, local structural elements). The broader tectonic settings of the deposits have been studied by means of high-resolution, multibeam echo-sounding and side-scan sonar imagery.

Land-based massive sulfides and polymetallic sulfide deposits on the seafloor are products of the same geological and geochemical processes and many anal- ogies can be drawn between modern examples and base metal deposits currently being mined (Franklin et al., 1981; Scott, 1985, 1987 Franklin, 1986; Koski, 1987b; Hannington et al., 1995a, b). Modern seafloor hydrothermal systems are excellent natural laboratories for understanding the genesis of volcanogenic massive sulfide deposits and this knowledge can be translated directly to the ancient geological record where evidence for the origin and nature of mineral deposits is often obscured by millions of years of geological history.

In this paper we review and discuss some of the characteristics of seafloor polymetallic sulfide deposits, including the regional and local tectonic setting, the type of occurrence, the spatial distribution and size, the mineralogical, bulk chemical and precious metal composition, the physical properties and the major factors

controlling the formation of these deposits at the seafloor.

2. Tectonic setting and spatial distribution of deposits

The formation of seaftoor polymetallic massive sulfides is intimately related to the heat regime associated with the formation of new oceanic crust. Massive sulfide deposits are known to occur in rather diverse tectonic settings, including divergent plate boundaries (i.e., mid-ocean ridges) and convergent, subduction- related plate boundaries, where sulfide formation takes place within the extensional environment of spreading centers in back-arc basins (Fig. 2). In both cases, volcanic-hosted and sediment-hosted deposits may form as a consequence of seawater circulation in the volcanic basement. Although the ore-forming process at mid- ocean ridges and back-arc rifts is almost identical, the composition of the volcanic rocks varies from mid- ocean ridge basalts (MORB) to calk-alkaline felsic lavas (andesite, rhyolite) which causes major differences in the composition of the sulfide deposits. This is evidenced by the mineralogical and chemical variability of massive sulfides forming at mid-ocean ridges (e.g., East Pacific Rise 21°N), in intraoceanic back-arc rifts developing in oceanic crust of the West and South- west Pacific (e.g., Lau Basin, North Fiji Basin, Manus


: : : :~ . : ~ i ! ~ ? . i i i : , i ~ ( : ~ . ~ ' ~ i ~ Mid-Ocean Ridge ~/~ (Type EPR 21°N)

lntraoceanic Back-Arc (Type Lau Basin)

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Fig. 3. Different environments of seafloor spreading associated with hydrothermal systems and polymetallic massive sulfide deposits.

Basin, Mariana Back-Arc) and at intracontinental rift zones forming in submarine fragments of continental crust (e.g., Okinawa Trough in the East China Sea) (Fig. 3).

A number of massive sulfide deposits have been found at submarine volcanoes occurring along or close to the axis of oceanic rift zones (Fig. 2). Hydrothermal activity is also closely associated with intraplate, hot- spot and island-arc related seamounts (Karl et al., 1988; Cheminee et al., 1991; Hekinian et al., 1993; Stiiben et al., 1992; McMurtry et al., 1993) and polymetallic sulfides with a complex chemical and mineralogical composition have been recovered from Palinuro Sea- mount in the Tyrrhenian Sea (Puchelt, 1986). Hydro- thermal mineralization is also associated with shallow-water alkaline island-arc volcanoes in the Southwest Pacific, which were found to have characteristics of epithermal gold mineralization known on land (Berger and Bethke, 1985; Hannington and Her- zig, 1993; Herzig et al., 1994).

Estimates of the heat and mass flux along the mid- ocean ridges require that high-temperature hydrothermal activity be a common feature in this environment (Rona, 1984, 1988). Total discharge of hydrothermal vents at oceanic ridges is estimated to be in the order of 5× 10 6 I/S (Wolery and Sleep, 1976), which requires that the total amount of water in the oceans is circulated through thermally active seafloor rift zones every 5-11 Ma (Wolery and Sleep, 1976). In order to account for the estimated annual global flux of hydrothermal fluid from the mid-ocean ridges, there would have to be at least one black smoker with a mass flux of approximately 1 kg/s and an estimated power of about 1.5 megawatts (Converse et al., 1984) for every

50 meters of ridge-crest (55 000 km in total), assuming that there is no component of diffuse flow. Of course, the number of known black smoker vents is extremely small by comparison and diffuse flow must account for a large part of the heat loss from the mid-ocean ridges. Low-temperature, diffuse flow is particularly important for off-axis hydrothermal circulation and may remove as much as 80% of the total heat produced at the ridge (Morton and Sleep, 1985; Wheat and Mottl, 1994). The high-intensity component of hydrothermal activity is not distributed uniformly along the mid-ocean ridges. High-temperature hydrothermal activity is often, but not always, focused along topographically elevated (i.e., shallow) portions of individual ridge segments, where crustal buoyancy is caused by the presence of a large magma reservoir (Bailard et al., 1981; Ballard and Francheteau, 1982; Francheteau and Ballard, 1983). High-resolution seismic reflection studies have indicated that these magma reservoirs commonly occur only 1-3 km below the seafloor (Detrick et al., 1987; Collier and Sinha, 1990). The occurrence of ridge crest topographic highs often coincides with the dominance of sheet flows versus pillow lavas and the presence of more fractionated volcanic rocks (Thompson et al., 1985). Conductive heat transfer from the frozen top of the magma chamber to deeply penetrating seawater drives the hydrothermal convection system which may give rise to black smokers at the seafloor (Cann and Strens, 1982). The crustal residence time ofconvecting seawater has been constrained to be three years or less (Kadko and Moore, 1988). At sites where many black smoker vents are active simultaneously for a longer period of time, large massive sulfide deposits may form (e.g., TAG Hydrothermal Field). Metal precipitation is a consequence of changing physico-chemical conditions during mixing of cold (about 2°C), oxygenated seawater and high-temperature, metal-rich hydrothermal fluids with a low pH and redox potential (cf., Hannington et al., 1995a).

Age dating for the TAG Field at the Mid-Atlantic Ridge 26°N has indicated a complex hydrothermal history (Lalou et al., 1990, 1993). Hydrothermal activity along this ridge segment first began about 130 000 years ago with the deposition of low-temperature Mn oxides. The onset of high-temperature activity with precipitation of massive sulfides can be traced back to about 40 000-50 000 years ago. The presently active site has experienced intermittent pulses of activity


every 5000-6000 years over the past 20 000 years. After an inactive period of about 4000 years, the present black smoker activity started about 50 years ago. This episodic high-temperature hydrothermal activity is probably related to the replenishment of the axial magma chamber (cf., Cann and Strens, 1982) by magma rising from the upper mantle to shallow crustal levels. Active and inactive barite-sulfide chimneys from the Mariana Trough have been dated to only 0.5- 2.5 years ( Moore and Stakes, 1990 ) while a large inac - tive sulfide chimney from the East Pacific Rise has an age of 60-80 years (Marchig et al., 1988) which is similar to the presently active chimneys at TAG.

Through detailed mapping of the ocean floor in areas of known hydrothermal activity, important geological controls on the occurrence of large deposits have been recognized. The largest sulfide deposits are not always found on the shallowest portion of the ridge segment or in the center of the axial valleys. Instead, they tend to occur on segments of ridge-crests undergoing major constructional volcanism followed by periods of tectonic activity. Here, hydrothermal fluids may be focused by faults along the outer margins of the axial valleys (Malahoff, 1982; Kappel and Franklin, 1989; Karson and Rona, 1990). These faults develop during periods of tectonic activity that alternate with periods dominated by volcanic eruptions. Smaller hydrother- real vents with little accumulation of massive sulfide are commonly found along eruptive fissures near the center of the central graben. Very large deposits may be forming on sedimented ridge-crests (e.g., Middle Valley, Northern Juan de Fuca Ridge and Escanaba Trough, Southern Gorda Ridge) which retain crustal heat longer than bare-rock ridge-crests and allow effec- tive precipitation of sulfides within the several hundred meters of sediment that cover the ridge crests (Koski, 1987b; Davis et al., 1992).

Although massive sulfides have been found at water depths as shallow as 1500 m, there may be important physical limitations to the depths at which massive sulfide deposits might form. In shallow waters, the pressure at the seafloor is insufficient to prevent boiling of the hydrothermal fluids. At 350°C, these solutions will begin to boil if the hydrostatic pressure drops below 160 bar (16 MPa) which is equivalent to about 1600 m of water depth (cf., Bischoff and Rosenbauer, 1984; Bischoff and Pitzer, 1985). In response to boiling, a portion of the dissolved metals will be deposited as

disseminated or vein mineralization beneath the seafloor (Drummond and Ohmoto, 1985). Phase-separated fluids emanating from hydrothermal vents on the seafioor are significantly depleted in dissolved metals (cf., Massoth et al., 1989; Butterfield et al., 1990). Formation of very large polymetallic massive sulfide deposits at the seafloor may be restricted to water depths of several hundred meters or more.

3. Size and type of the deposits

Estimation of the continuity of sulfide outcrops is difficult and the thickness of the deposits is poorly constrained. However, visual estimates for several deposits on the mid-ocean ridges (e.g., Southern Explorer Ridge, Galapagos Rift, TAG Hydrothermal Field, Seamount at East Pacific Rise 13°N) suggest sizes of 1-5 million tonnes. One of the largest deposits is found on a failed and heavily sedimented, but still hydrothermally active, ocean ridge. A systematic investigation of the Atlantis II Deep in the Red Sea has proven 94 million tonnes of metalliferous sediment hosted in a basin about 10 km in diameter (Mustafa et al., 1984). The deposit contains an average of 2 wt% Zn and 0.5 wt% Cu, in addition to 39 ppm Ag and 0.5 ppm Au (Nawab, 1984; Oudin, 1987). A pilot mining test which pumped metalliferous sediments from 2000 m depth has shown that this deposit can be successfully mined (Amann, 1982, 1985). Drilling carried out by the Ocean Drilling Program during Leg 139 also has indicated more than 96 m of massive sulfides at the Middle Valley site on the northern Juan de Fuca Ridge (Davis et al., 1992) which may point to a size which significantly exceeds previous estimates (i.e., about 50-100 million tonnes ). Underestimation of size might be also true for other deposits as drilling of land-based sulfide ore bodies has shown that the major part of mineralization commonly was formed by extensive alteration and replacement of volcanic rocks beneath the seafloor. This was confirmed recently as drilling of the TAG Hydrothermal Field during Ocean Drilling Program Leg 158 indicated that the accumulation of sulfides appears to be substantially a process of hydrothermal replacement of rocks in the upflow zone rather than direct precipitation on the seafloor (ODP Leg 158 Shipboard Scientific Party, 1995).


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Fig. 4. Compilation of gold grades and tonnages of land-based volcanogenic massive sulfide deposits world-wide (past production and current reserves, n = 372). Data are from Mosier et al. (1983), Cana- dian Mines Handbook (1987) and Divi et al. (1980).

Typical black smokers are estimated to produce about 250 tonnes of massive sulfide per year (Scott, 1992). Thus, a local vent field with a few black smokers can easily account for a small size sulfide deposit. Esti- mates of sizes between 1-100 million tonnes for individual massive sulfide deposits on the seafloor thus are within the range of typical volcanic-associated massive sulfide deposits on land (Fig. 4). However, most occurrences of seafloor sulfides amount to less than a few thousand tonnes and consist largely of scattered hydrothermal vents and mounds usually topped by a number of chimneys, with one or more large accumulations of massive sulfide. More than 60 individual occurrences are reported from an 8 km segment of Southern Explorer Ridge, but most of the observed mineralization occurs in two large deposits with dimensions of 250×200 m (Scott et al., 1991). The thick- nesses of the deposits are difficult to determine unless their interiors have been exposed by local faulting. Reports of explored dimensions of deposits based on visual estimates from submersibles may be accurate to only -I-50% of the distances given and commonly include weakly mineralized areas between larger, discrete sulfide mounds (thereby over-estimating the continuity of sulfide outcrop). Reports based on transponder navigated camera tracks are probably accurate to ___ 20%, but the extent of coverage is limited due to the slow tow-speeds and the narrow image. No other geophysical tools currently provide a good basis

for estimating the area of sulfide outcrop. High-resolution, deep-towed, side-scan sonar may be refined to provide more accurate information over larger areas.

Seafloor sulfide deposits typically consist of a con- solidated basal sulfide mound, underlain by a subseafloor stockwork (vein and disseminated mineralization; cf., Ocean Drilling Program Leg 158: TAG Hydrothermal Field) and abundant chimney structures, hydrothermal crusts, metalliferous sediments and accumulations of sulfide talus and debris (Fig. 5). Little is known about the growth and compositional variations of the mounds itself which typically accounts for most of the hydrothermal precipitates on the seafloor.

High-temperature black smokers and lower temperature white smokers up to 30 m high are by far the most spectacular features of active seafloor hydrothermal systems. However, they represent only the uppermost part of a deposit and usually develop on top of a hydrothermal mound which largely consists of massive sulfides. The mounds are continuously growing by circulation of hydrothermal fluids through the sulfide pile causing abundant recrystallization of sulfide minerals. Collapsed chimneys which eventually become part of the mound are readily replaced by new chimneys which may grow at a rate of up to 10 cm per day (Hekinian et al., 1983). The TAG mound at the Mid- Atlantic Ridge 26°N is a typical example of such a large active sulfide deposit. The mound is about 50 m high and has a diameter of 250°300 m (Rona et al., 1986; Thompson et al., 1988). Focused fluid discharge occurs at the high-temperature (350°-360°C) Black Smoker Complex at the central top of the mound; lower-temperature fluids (260-300°C) are venting from the "Kremlin", which is an area of white smoker activity. Diffuse discharge of clear, low-temperature (20 °- 30°C) hydrothermal fluids with locally high concentrations of dissolved silica is a common feature at TAG (Hannington et al., 1990a). These fluids are locally venting through the oxidized surface and apron of the mound and form "tetsusekiei-type" (cf., Kalogero- poulos and Scott, 1983) silicified Fe-oxides. These deposits commonly contain abundant filamentous bac- teria (Juniper and Fouquet, 1988; Hannington and Jon- asson, 1992).

At a few sites, normal faults have exposed the interior of sulfide mounds and the upper parts of the stringer or stockwork zone (Embley et al., 1988; Fouquet et al., 1993). Submersible mapping and sampling have

P.M. Herzig, M.D. Hannington / Ore Geology Reviews 10 (1995) 95-115

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shown a thermal zonation with high-temperature sulfides (e.g., chalcopyrite, isocubanite, pyrite) at the interior and lower-temperature precipitates (sphalerite, anhydrite, amorphous silica) at the margins of the mound (Hekinian and Fouquet, 1985), which is very similar to many ancient massive sulfide deposits on land. The stockworks usually consist of highly altered host rock with abundant vein-type mineralization. At the Vai Lili site in the Lau back-arc, at least two gen- erations of hydrothermal veins can be distinguished on

the basis of cross-cutting relationships (Fouquet et al., 1993).

Where hydrothermal fluids vent directly onto the seafloor, more than 90% of the total mass flux (i.e., a large portion of the metals) may be lost to a diffuse hydrothermal plume. For one small vent field at the East Pacific Rise at 21°N the total mass flux equals about 150kg/s and an estimated97% of the particulates or smoke in the black smoker plume are lost to seawater (Converse et al., 1984). The particles are usually dis-


persed by near bottom currents and may be deposited at great distances from the hydrothermal vents (Dymond et al., 1973; Cronan, 1976; Leinen and Stakes, 1979). These metals are incorporated by normal marine sediments and can be recognized only as geochemical anomalies in distal sedimentary sequences (Barrett et al., 1988). Detailed studies of trace element patterns in metalliferous sediments from different hydrothermally active seafloor regions have shown highly variable trace element levels which are mainly attributed to mixing between detrital sediments and hydrogenetic components (Boyd et al., 1993). Similar erratic patterns are usually found in metalliferous sediments in the geologic record (Kalogeropou- los and Scott, 1983) and considerably hinder geochemical exploration for massive sulfide deposits.

On most modern bare-rock, mid-ocean ridges the efficient accumulation of metals on the seafloor requires a physical or chemical barrier to the free venting of the hydrothermal fluids into the overlying water column. However, this may not be true for the formation of some ancient ore deposits in less well mixed and oxygenated oceans. Trapping of the vent fluids may be achieved partly by large sulfide, anhydrite or barite structures and mounds which form from smaller hydrothermal vents due to continuing growth, collapse and renewed growth of chimneys (Tivey and Delaney, 1986; Hannington and Scott, 1988a). Major volcanic edifices on the ridge-crest also provide insulation for ascending hydrothermal fluids and help to sustain a large, high-temperature fluid reservoir (Kappel and Franklin, 1989). Thick accumulations of inflated pillow lavas, as well as impermeable hydrothermal crusts or mounds, provide a suitable cap-rock and help to prevent dispersal of metals into the water column, thereby promoting the growth of larger deposits. In heavily sedimented rifts, the long-term heat retention afforded by a thick sediment cover, as well as the entrapment and insulation of vent fluids, may account for the large size of sediment-hosted deposits. Metals can be precipitated from the hydrothermal fluids beneath the sediment-seawater interface as a consequence of mixing with pore waters and reaction with and replacement of the host sediments. A cap of sediments can also serve as protection against submarine weathering and oxidation of the sulfides.

The deposits of metalliferous sediment in the Red Sea are exceptional in size and character. Metals are

precipitated from stratified brine pools which are fed by hydrothermal vents at the bottom of deep, anoxic basins (Pottorf and Barnes, 1983; Zierenberg and Shanks, 1983). As a consequence of seawater circulation through Miocene evaporites, these metal-bearing brines have salinities which are many times greater than vent fluids on the mid-ocean ridges and, therefore, they tend to sink to the bottom of the basin rather than rise as a buoyant hydrothermal plume. Entrapment of the brines ensures that deposition of the metals is confined to the basin. The metals are deposited as a thin layer of metalliferous sediments and sulfides at the base of the brine pool and, in case of the Atlantis II Deep, may cover an area up to 40 km 2 (cf., Degens and Ross, 1969; Backer and Richter, 1973). In some areas (e.g., Kebrit and Shaban Deeps), however, sulfide chimneys have been found (Blum and Puchelt, 1991 ) and indicate local geyser-type discharge (Ramboz et al., 1988).

4. Mineralogy of the deposits

The mineralogical composition of seafloor sulfide deposits has been documented in a number of detailed studies on samples from various sites (e.g., Haymon and Kastner, 1981; Goldfarb et al., 1983; Haymon, 1983; Oudin, 1983; Koski et al., 1984; Davis et al., 1987; Kastner et al., 1987; Fouquet et al., 1988; Han- nington et al., 1991a, b; Fouquet et al., 1993). These studies have indicated distinct compositional differences between sulfide deposits found at sediment- starved and sediment-covered mid-ocean ridges and those forming in back-arc rift environments.

The mineral paragenesis of sulfide deposits at sediment-starved mid-ocean ridges (e.g., East Pacific Rise 21°N, Southern Explorer Ridge, TAG Hydrothermal Field) usually includes assemblages forming at temperatures ranging from about 300°--400°C to less than 150°C. High-temperature fluid channels of black smokers and the interiors of sulfide mounds commonly consist of isocubanite--chalcopyrite together with anhydrite, pyrrhotite, pyrite and locally bornite. In some high-temperature chimney interiors, a rare Mg- hydroxy-sulfate-hydrate has been documented ( "caminite", Haymon and Kastner, 1986). The outer portions of chimneys and mounds are made up of lower-temperature precipitates such as sphalerite/wurtzite, marcasite, pyrite and locally amorphous silica, which are

P.M. Herzig, M.D. Hannington / Ore Geology Reviews 10 (1995) 95-115

Table 1 Bulk mineralogical composition of seafloor polymetallic sulfides from mid-ocean ridges and back-arc spreading centers

103

po py/mc sph/wtz cpy/iso SiO2 anh ba ga ten tet ss ap/re

Mid-ocean ridges volcanic-hosted + + + + + + + sediment-hosted + + + + + + + + Back-arc ridges intraoceanic + + + + + + + intracontinental + + + + + +

+ + + + + +

po = pyrrhotite, py = pyrite, mc = marcasite, sph = sphalerite, wtz = wurtzite, cpy = chaleopyrite, iso = isoeubanite, SiO2 = amorphous silica, anh=anhydrite, ba=barite, ga=galenite, ten=tennantite (As), tet=tetrahedrite (Sb), ss=suifosalts (Pb-As-Sb), ap=orpiment (As), re = realgar (As). Data compiled in Petersen (1992).

also the principal minerals of low-temperature white smoker chimneys. Distinct compositional zoning reflecting strong gradients in fluid temperature and composition as described for individual sulfide chimneys has also been documented for large sulfide mounds (Hekinian and Fouquet, 1985). Anhydrite in the high-temperature assemblage is commonly replaced by later sulfides, late-stage amorphous silica and locally barite. At seafloor pressures, anhydrite dis- solves back into seawater when temperatures drop below about 150°C (Haymon and Kastner, 1981). The retrograde solubility of anhydrite is in part responsible for the instability and ultimate collapse of large inactive sulfide chimneys. Petrographic relationships and mineral intergrowths in chimneys and mounds from various hydrothermal sites have revealed complex replacement and recrystallization phenomena which reflect the highly dynamic and locally chaotic environment of sulfide formation at seafloor vents.

Mineral assemblages in sulfide deposits at sediment- covered mid-ocean ridges close to continental margins (e.g., Escanaba Trough, Southern Gorda Ridge and Guaymas Basin, Gulf of California) are locally rather complex and may contain sulfide minerals which oth- erwise are fairly rare. In this environment, hydrothermal fluids ascending from the basaltic basement interact with continent-derived turbiditic and hemipe- lagic sediments and thereby leach Pb, Ba and other elements from feldspar and other detrital components (cf., LeHuray et al., 1988). Mixing of these hydrothermal fluids with seawater may result in the precipitation of massive and disseminated sulfides within the sediments which commonly contain abundant galena in addition to Cu- and Zn-sulfides (Table 1). Locally,

sediment-hosted sulfide assemblages are very complex (e.g., Escanaba Trough) and include arsenopyrite, tetrahedrite, loellingite (FeAs2), boulangerite ((Pb, Z n ) 5 S b 4 S l l ) , stannite (Cu2FeSnS4), jordanite (Pb14As7S24), franckeite (Pb5Sb2Sn3S14) and native bismuth together with major amounts of barite and amorphous silica (Koski et al., 1984). Due to the strongly reduced nature of the hydrothermal fluids which have reacted with organic material in the sediment, pyrrhotite is a common constituent of these assemblages. Hydrothermally derived petroleum is also preserved locally in these deposits. However, fluids emanating from the sediments onto the seafloor are commonly strongly depleted in dissolved metals, pre- sumably due to sulfide precipitation within the sedimentary sequence.

Sulfide mineralization forming at back-arc spreading centers has some mineralogical characteristics which are similar to hydrothermal precipitates at sediment- starved mid-ocean ridges. In addition to high- and low- temperature mineral assemblages described for mid-ocean ridge sulfides, samples from the Lau back- arc contain variable amounts of tennantite together with galena, complex and locally non-stoichiometric Pb-As sulfosalts (i.e., gratonite, dyfrenosite, jordanite), barite, amorphous silica and native sulfur (Table 1). Com- monly, sphalerite is the dominating sulfide in these assemblages and anhydrite and pyrrhotite are rare. In addition, the first examples of visible primary gold in seafloor sulfides have been documented in samples of low-temperature white smoker chimneys from this environment (Herzig et al., 1990, 1993). The gold is relatively coarse grained (up to 18 micron) and occurs


as co-depositional inclusions in massive, Fe-poor sphalerite.

Massive sulfides forming in an environment where back-arc rifting takes place in submarine fragments of continental crust (i.e., Okinawa Trough) are characterized by abundant Ag-bearing galena, As- and Sb(- Ag) fahlore (tennantite, tetrahedrite), Ag-Sb-Pb sulfosalts, native sulfur, cinnabar and the presence of individual antimony sulfides (stibnite) and arsenic sulfides such as realgar and orpiment (Halbach et al., 1989, 1993). As in hydrothermal mineralization from the Lau Basin, barite and amorphous silica are abundant.

5. Metal contents of the deposits

Despite moderate tonnages in several seafloor deposits, recovered samples from about 25 deposits world- wide represent no more than a hundred tonnes of material. Based on existing data it is premature to com- ment on the economic significance of seafioor massive sulfides, but published analyses of sulfide samples indicate that these deposits may contain important concentrations of metals that are comparable to those found in ores from massive sulfide mines on land (Table 2). For example, most massive sulfide deposits that have been

mined in Canada have combined metal grades of about 6 wt% ( C u + Z n + P b ) (Franklin and Thorpe, 1982). Estimated concentrations of base metals in seafloor massive sulfides tend to be higher, which in part may be due to a strong bias in sampling.

Most samples of seafloor sulfides are recovered during submersible operations. A bias in the analytical data arises because sulfide chimneys which are relatively easy to sample are often the focus of study. However, they are unlikely to be representative of the bulk composition of the deposits as a whole (e.g., eleven analyzed samples from the Southern Juan de Fuca site have an average Zn content of greater than 34 wt%) and little is known about the interiors of larger sulfide mounds and the underlying stockwork zones. System- atic sampling of both high-and low-temperature assemblages across the surfaces of some large active mounds (e.g., TAG Hydrothermal Field, Explorer Ridge, Gal- apagos Rift) are more representative of the range of sulfide precipitates which comprise large deposits. Suf- ficient sampling, which has led to potentially realistic estimates of metal concentrations, has been achieved at only a few sites (e.g., Middle Valley, Explorer Ridge, Galapagos Rift) while quantitative assessment of contained metals has been possible only for the Atlantis II Deep in the Red Sea. Adequate information

Table 2 Bulk chemical composition of seafloor polymetallic sulfides from Mid-ocean ridges and back-arc spreading centers

Mid-ocean ridges Back-arc ridges

Volcanic_hosted ~ Sediment-hosted 2 Intraoceanic 3 IntracontinentaP

n 890 57 317 28 Fe (wt%) 23.6 24.0 13.3 7.0 Zn 11.7 4.7 15.1 18.4 Cu 4.3 1.3 5.1 2.0 Pb 0.2 1.1 1.2 11.5 As 0.03 0.3 0.1 1.5 Sb 0.01 0.06 0.01 0.3 Ba 1.7 7.0 13.0 7.2 Ag (ppm) 143 142 195 2766 Au 1.2 0.8 2.9 3.8

Explorer Ridge, Endeavour Ridge: Main Vent and High Rise Fields, Axial Seamount: ASHES and CASAM, Cleft Segment: N and S Fields, East Pacific Rise: 11°N, 13°N, 21°N, 7°30'S, 16°45'S, 18°30'S, 21°S, Galapagos Rift, TAG: Active Mound, Mir and Alvin Zones, Snake Pit,

Mid-Atlantic Ridge 24.5°N. 2 Escanaba Trough, Guaymas Basin. 3 Mariana Trough, Manus Basin, North Fiji Basin, Lau Basin: Kings Triple Junction, White Church, Vai Lili, Hine Hina Fields. 4 Okinawa Trough. Data compiled by Geological Survey of Canada and Freiberg University of Mining and Technology, Germany.


about the continuity of base and precious metal concentrations in the interiors of the deposits can only be provided by drilling, as recently successfully demon- strated at the Middle Valley site (Ocean Drilling Pro- gram Leg 139) and at the active TAG mound (Ocean Drilling Program Leg 158).

Comparison of close to 1300 chemical analyses of seafloor sulfides reveals systematic trends in bulk composition between deposits in different volcanic and tectonic settings (Table 2). The sediment-hosted massive sulfides (e.g., Escanaba Trough, Guaymas Basin), while being somewhat larger than deposits on the bare- rock mid-ocean ridges, appear to have lower concentrations and different proportions of the base metals. Massive sulfides from these deposits average 4.7 wt% Zn, 1.3 wt% Cu and 1.1 wt% Pb (n=57, Table 2). This reflects the influence of thick sequences of turbi- dite sediments on hydrothermal fluids ascending to the seafloor and possibly the tendency for widespread precipitation of metals beneath the sediment-seawater interface. Calcite, anhydrite, barite and silica are major components of the hydrothermal precipitates and may significantly dilute the base metals in sediment-hosted deposits. Although the Middle Valley deposit on the Southern Juan de Fuca Ridge is hosted by sediments, its bulk composition indicates a predominantly basaltic source for the metals with only minimal fluid-sediment interaction. On basaltic, sediment-free mid-ocean ridges, sulfides are precipitated largely around the vent site, resulting in small deposits, but high concentrations of metals. Deposits for which there are representative suites of samples (e.g., Explorer Ridge, Endeavour Ridge, Axial Seamount, Cleft Segment, East Pacific Rise, Galapagos Rift, TAG Hydrothermal Field, Snake Pit Hydrothermal Field, Mid-Atlantic Ridge 24.5°N site) have a narrow range of metal concentrations and average 11.7 wt% Zn and 4.3 wt% Cu, but have low concentrations (0.2 wt%) of Pb (n= 890; Table 2). Anhydrite, barite and silica are important constituents of some chimneys, but on average they account for < 20 wt% of the samples analyzed.

On a broad scale, vent fluid composition at all of the bare-rock mid-ocean ridge sites are remarkably similar, reflecting the high-temperature reaction of seawater with a uniform basaltic crust at greenschist faCies conditions (e.g., Bowers et al., 1988, Campbell et al., 1988; Von Damm, 1988, 1990). However, some of the vent fluids differ by more than a factor of 10 in chloride and

H2S content and by several pH units for the highest temperature endmember fluids. This is clearly important in terms of the fluids' capacity to transport metals and represents a controlling factor on deposit composition. Large variations in base metal concentrations between deposits on the mid-ocean ridges for the most part reflect differences in the conditions of formation of the deposits. For example, zinc-rich deposits at Axial Seamount and the Southern Juan de Fuca site have formed at lower average temperatures ( < 300°C) than Cu-rich deposits ( > 300°C) elsewhere on the mid- ocean ridges.

Compared to samples from sediment-starved mid- ocean ridges, massive sulfides forming in basaltic to andesitic environments of intraoceanic back-arc spreading centers (e.g., Mariana Trough, Manus Basin, North Fiji Basin, Lau Basin) are characterized by elevated average concentrations of Zn (15.1 wt%), Pb ( 1.2 wt%) and Ba ( 13.0 wt%), but low contents of Fe ( 13.3 wt%, n = 317; Table 2). Polymetallic sulfides in the Okinawa Trough, where rhyolites and dacites are a product of back-arc rifting in submarine continental crust (Halbach, 1989), have low Fe contents (7.0 wt%) but are enriched in Zn ( 18.4 wt%) and Pb ( 11.5 wt%) and have high concentrations of Ag (2766 ppm, maximum 1.1 wt%), As (1.5 wt%) and Sb (0.3 wt%, n=28; Table 2). High Sb and As contents are accounted for by the presence of tetrahedrite, stibnite and As-sulfides (i.e., realgar and orpiment) in these assemblages.

The bulk composition of seafloor sulfide deposits in various tectonic settings is a consequence of the nature of the volcanic source rocks from which the metals are leached (cf., Doe, 1994). Potential source rocks identified in the different tectonic environments range from MORB and clastic sediments at the mid-ocean ridges, to lavas of intermediate composition (basaltic andesite, andesite) in intraoceanic back-arcs and felsic volcanics (dacite, rhyolite) which are typical for young intracontinental back-arc rifts. These compositional variations are reflected by differences in the composition of the respective vent fluids. For example, chemical analyses of endmember fluids from the Vai Lili Hydrothermal Field which occur in andesites of the Valu Fa Ridge in the southern Lau Basin indicate much lower pH and higher concentrations of Zn, Pb, As and other elements compared to typical mid-ocean ridge fluids (Table 3). Massive sulfides from the Okinawa Trough (Halbach


Table 3 Composition of hydrothermal fluids at mid-ocean ridges (MOR: East Pacific Rise 21°N) and intraoceanic back-arc spreading centers (Back-arc: Lau Basin)

MOW Back-arc 2

T (°C) 350 334 pH 3.6 2.0 Zn (ppm) 5.5 196 Cu 1.4 2.2 Ba 1.4 5.4 As (ppb) 17 450 Pb 54 808

1: Von Damm et al. ( 1985); 2: Fouquet et al. (1993).

et al., 1989) are even more enriched in Pb than massive sulfides from the Lau Basin, which is likely a consequence of the high Pb contents of rhyolites, andesites and sediments in the source region and the characteristics of the hydrothermal fluids generated in this environment. The composition of these lavas is explained by the presence of about 20 km thick continental basement (Sibuet et al., 1987) in the melting region. How- ever, a certain degree of interaction of the hydrothermal fluids with about 20-30 m thick sediments locally cov- ering the felsic volcanics is likely (Halbach et al., 1993). High Pb and Ba contents of sediment-hosted seafloor sulfides simply reflect the elevated Pb and Ba contents of individual components in the sediment (e.g., feldspar). Similar trends in the bulk composition of massive sulfide deposits are widely recognized in ancient terrains (e.g., Franklin et al., 1981; Ohmoto and Skinner, 1983; Fouquet et al., 1993).

6. Occurrence and distribution of gold

Gold concentrations are locally high in samples from a number of seafloor deposits on the mid-ocean ridges (Hannington et al., 1986; Hannington and Scott, 1988b, 1989; Hannington et al., 1991a, b) and in particular in samples from the back-arc spreading centers (Herzig, 1991; Herzig et al., 1993). Average gold contents for deposits on the mid-ocean ridges range from < 0.2 ppm Au up to 2.6 ppm Au, with an overall average of 1.2 ppm Au (Hannington et al., 1991a, b; cf., Table 2). In volcanic-dominated, sediment-free deposits, high-temperature (350°C) black smoker chimneys composed of Cu-Fe-sulfides typically contain ~ 0.2

ppm Au, which is similar to the gold content of associated plume particulates The 350°C endmember fluids contain about 100-200 ppt Au in solution (Hannington et al., 1991a). At the black smoker chimneys, much of the gold is lost to a diffuse hydrothermal plume (cf., Hannington and Scott, 1988b). The gold content of massive sulfides from the interior of hydrothermal mounds and in stockwork zones appears to be similar to the gold content of the high-temperature chimney assemblages (Hannington et al., 1990b). The highest concentrations of gold (up to 6.7 ppm Au) typically occur in lower-temperature (<300°C), sphalerite- dominated assemblages with sulfosalts and late-stage barite and amorphous silica (Axial Seamount: Han- nington et al., 1986; Hannington and Scott, 1989). Local enrichment of more than 40 ppm Au (e.g., TAG Hydrothermal Field; Hannington et al., 1995b) is a consequence of remobilization and reconcentration (hydrothermal reworking) of gold during sustained venting of hydrothermal fluids through the sulfide mounds (i.e., zone refining). The gold contents of sulfides from deposits in sedimented rifts (e.g., Guaymas Basin) are typically <0.2 ppm Au. Here, the interaction of hydrothermal fluids with organic-rich sediments causes strongly reducing conditions which limit the amount of gold that can be transported in hydrothermal solutions. However, Cu-rich sulfides from the Esca- naba Trough are an exception as some samples contain up to 10 ppm Au with an average of 1.5 ppm Au. This may be explained by an enriched source in the underlying sediments (Koski et al., 1988; Zierenberg et al., 1990; Zierenberg et al., 1993). The metalliferous muds in the Atlantis II Deep have bulk gold contents of about 0.5 ppm Au (Nawab, 1984), but sulfide-rich horizons have gold contents from < 0.5 up to 4.6 ppm Au and average close to 2 ppm Au (Oudin, 1987). The total gold content of the deposit has been calculated to about 45 tonnes Au (Mustafa et al., 1984).

Polymetallic sulfides from a number of back-arc spreading centers have revealed particularly high concentrations of gold between 3-30 ppm Au (Herzig et al., 1993). Gold appears to be most abundant in sulfides associated with immature seafloor rifts in continental or island arc crust. These settings are dominated by calc-alkaline volcanics including andesites, dacites and rhyolites (e.g., Okinawa Trough, Lau Basin, Manus Basin). Polymetallic sulfides from the Valu Fa Ridge in the Lau back-arc have gold contents of up to 29 ppm


Au (close to 1 oz per ton) with an average of 3.3 ppm Au (n = 75). These samples are among the most gold- rich hydrothermal precipitates yet reported from the modern seafloor and they contain the first known examples of visible primary gold (up to 18 micron) in polymetallic sulfides at active vents (Herzig et al., 1990, 1993). In the Okinawa Trough, gold-rich sulfide deposits with up to 24 ppm Au (average 3.8 ppm, n=28) occur in rifted continental crust and resemble Kuroko- type massive sulfides (Halbach et al., 1989, 1993; Urabe et al., 1990). Preliminary analyses of sulfides reported from the Central Manus Basin indicate average gold contents of 30 ppm Au (n = 10) and maximum concentrations of more than 50 ppm Au (W. Tufar, pers. commun., 1991 ). The average gold content of one chimney in the Eastern Marius Basin is 15 ppm with a maximum of 54.9 ppm Au (n=26; Binns, 1994). High gold contents up to 21 ppm Au also have been found in barite chimneys in the Western Woodlark Basin, where seafloor spreading propagates into continental crust off Papua New Guinea (Binns et al., 1991, 1993).

Sulfide deposits related to mature back-arc spreading centers associated with MORB-type volcanics (e.g., North Fiji Basin, Mariana Trough) have gold contents of only 0.1--4.3 ppm Au which are more similar to sulfide deposits on the mid-ocean ridges. Preliminary data suggest that the gold contents of back-arc lavas are not significantly different from those of ordinary MORB, and, therefore, these rocks probably do not represent an enriched source. However, the source-rock geochemistry is an important factor in controlling the composition of the hydrothermal fluids and their ability to carry gold and the gold content may be related to the buffering of the hydrothermal fluids during water-rock interaction. For example, the oxidation state of vent fluids on the mid-ocean ridges is strongly buffered by reaction with abundant FeO-bearing minerals in the rocks, and the ability of these vent fluids to become saturated with low concentrations of gold at high temperatures is a consequence of their low aO2 and strong redox buffering capacity. In contrast, vent fluids derived from the high temperature reaction of seawater with more felsic lavas tend to be more oxidized and have a lower redox buffering capacity because of the lower abundance of FeO-bearing minerals in the rock. These more oxidized solutions can carry more gold and may become saturated easily following a relatively

small amount of conductive cooling, mixing, or oxidation of H2S which may lead to the more efficient precipitation of gold. These observations imply that factors such as rock-buffering of the hydrothermal fluids may be as important as source considerations in generating gold-rich sulfides (Herzig et al., 1993).

Oxidation of massive sulfide deposits by oxygen- rich seawater at the seafloor may cause local but significant enrichment of gold. Supergene processes in the TAG Hydrothermal Field have resulted in high gold contents in secondary sulfides (up to 16 ppm Au, Han- nington et al., 1988) and submarine gossans (up to 23 ppm Au, Herzig et al., 1988, 1991 ) consisting of amorphous Fe-oxides, jarosite and atacamite together with minor goethite and amorphous silica. This paragenesis, and the secondary enrichment of gold, is similar to that observed in Fe-oxide gossans exposed to supergene weathering on land.

Silver concentrations in seafloor massive sulfides are commonly several hundred ppm Ag. In one back-arc deposit ( Okinawa Trough), however, silver concentrations reach several thousand ppm Ag with a maximum of 1.1 wt% Ag (Halbach et al., 1993). In general however, average concentrations of gold and silver in all of the seafloor deposits are well within the range of precious metal grades found in land-based deposits. Locally, small grains of native silver have been identified close to mineralized worm tubes, which may be due to precipitation of silver by microbial processes (cf., Zierenberg and Schiffman, 1990).

7. Physical properties of the deposits

The physical properties of seafloor sulfide deposits are an important consideration for future evaluation of their mineability and the recoverability of their contained metals. The bulk dry density of sulfide chimneys, hydrothermal crusts and sediments from the outer surface of the deposits are very low. Sulfide chimneys from East Pacific Rise 21°N have a dry density of only 1-2 g/cm 3 and an in-situ water content of 25-50% (Crawford et al., 1984). Higher densities due to com- paction, open-space filling and hydrothermal recrystallization of the sulfides can be expected in the interiors of the mounds. Most recovered samples are fine- grained, complex intergrowths of sulfide minerals and gangue (silica, barite, anhydrite). Their fine-grained


nature is partly a consequence of the manner in which the sulfides are precipitated from the hydrothermal fluids. Rapid quenching of the solutions as they mix with seawater results in rapid nucleation of minerals and limited growth of large crystals. Particle size analysis of sulfide chimneys from East Pacific Rise 21°N, crushed to their individual grains, shows that 93% of the material is in the 10 micron-to-1 millimeter range (silt-to-sand sized particles) (Crawford et al., 1984). Analyses of samples from the East Pacific Rise at 1 I°N and the Southern Explorer Ridge also indicate a range of grain sizes for pyrite, sphalerite and chalcopyrite from 1-600 microns with averages of 22-37 microns; 90-95% of the particles in the - 4 0 0 mesh range (smaller than 37 microns) are free of interlocking (Alton et al., 1989 ). Some degree of natural coarsening of the sulfides may occur in large deposits, where early- formed minerals are continuously recrystallized by hydrothermal reworking.

The technology for treating fine-grained ores is poorly developed, and fine grinding of the kind required to liberate individual grains of pyrite, sphalerite and chalcopyrite is energy intensive. Significant losses of the ultra-fine fraction (particles < 10 microns in size) are commonly encountered in processing of fine- grained ores from land-based deposits. Some deposits are economic only because reconcentration of metals during structural deformation and thermal metamor- phism has upgraded the ore and increased the grain size of the minerals of interest to a range amenable to processing. Flotation is the most important method in current use for the production of sulfide concentrates at base-metal mines on land, but there are also serious limitations to the effectiveness of this technique in sep- arating fine-grained particles ( < 10 microns). Ultra- fine particle flotation with seawater has been tested to produce a bulk sulfide concentrate from the Red Sea muds (Amann, 1985, 1989), but other novel methods of mineral processing may be warranted. Fine grinding and high inductance magnetic separation of seafloor massive sulfides has been proven to yield a suitable copper and zinc concentrate with a recovery rate of 81% (Alton et al., 1989). The complex polymetallic nature of the sulfides may require developments in extractive metallurgy before they could be adequately treated (e.g., hydrometallurgical processes such as oxi- dative pressure leaching).

Gold and silver are recovered as a casual by-product of base metal mining on land, most commonly during the smelting of copper, zinc and lead concentrates. However, much of the gold in some massive sulfide deposits is associated with pyrite and, therefore, not recovered with the base metals. The recovery of silver depends largely on its mineralogical siting in the ore, usually as a trace constituent contained in mineral inclusions in galena, chalcopyrite, or sulfosalts such as tetrahedrite. Gold is recovered as free grains of native metal or electrum and usually reports to the copper concentrate during flotation. Current milling practices, however, recover only a fraction of the total contained gold in most massive sulfide ores (as little as 60% in some cases). This poor recovery is a consequence of the uniformly small grain size of gold particles (typically < 10 microns) which are not adequately liberated by conventional methods of mineral processing. In primary (i.e., not oxidized, not recrystallized) sulfides from the seafloor, native gold (up to 18 microns) has been documented in only one deposit. Hydrothermal reworking of sulfide mounds at TAG has produced relatively coarse-grained secondary gold up to 4 microns in diameter (Hannington et al., 1995a, b). The recovery of fine-grained gold from massive sulfides without compromising the recovery of copper and zinc and at a reasonable cost (determined by energy requi- rements for fine-grinding) represents a major challenge to mineral processors and metallurgists and may be an important consideration in the possible development of seafloor polymetallic sulfides as a precious metal resource. The observation of coarse-grained, secondary gold in Fe-oxide gossans from the Mid-Atlantic Ridge suggests that seafloor deposits enriched by supergene processes may be a preferred short-term target for future economic consideration. Serious legal and envi- ronmental issues have to be addressed before mining of seafloor massive sulfides could happen and this is unlikely to be the case within the next 20 years (Scott, 1992).

8. Modern versus ancient deposits

It has been widely accepted that seafloor hydrothermal systems consisting of oxidized sulfide and Fe-oxi- hydroxide sediments, sulfide talus and debris, black and white smokers, and a massive sulfide mound which is

P.M. Herzig, M.D. Hannington /Ore Geology Reviews 10 (1995) 95-115

Table 4 Modern versus ancient voicanogenic massive sulfide (VMS) deposits

109

Modern Ancient

Mid-ocean ridges (e.g., EPR 210N, TAG) Ophiolitic Cu-Zn VMS (Cyprus, Oman) lntraoceanic back-arcs (e.g., Eastern Marius Basin) Archean Zn--Cu VMS (Noranda) Intracontinental back-arcs ( e.g., Okinawa Trough ) Phanerozoic Zn-Pb-Cu VMS ( Kuroko )

usually underlain by a funnel-shaped stockwork are modern analogs of many ancient massive sulfide deposits on land. In particular, seafioor sulfide deposits on the mid-ocean ridges (e.g., East Pacific Rise 21°N, TAG Hydrothermal Field) are interpreted as analogs of ophiolite-hosted Cu-Zn massive sulfides in Cyprus and Oman (Table 4; cf., Herzig and Friedrich, 1987, Herzig, 1988; Richards et al., 1989; Embley et al., 1988; Oudin, 1983; Haymon et al., 1984; Ixer et al., 1984; Koski, 1987a), although chimneys have only rarely been recognized in the geologic record (Oudin and Constantinou, 1984). Most of the ore from these deposits has been mined from a zone of massive pyrite and chalcopyrite deposited as a central mound on the paleo-seafloor. A significant portion of the recoverable base metals also occurs as stockwork mineralization and replacement ore beneath the seafloor (e.g., Turner- Albright deposit, Oregon). A number of deposits in the Troodos ophiolite, Cyprus (e.g., Limni, Pitharokhoma, Agrokipia B) contained several million tonnes of cupriferous stockworkore (Bear, 1963; Spooner, 1980; Herzig, 1988; Richards et al., 1989).

Back-arc sulfide deposits in the West and Southwest Pacific such as in the Okinawa Trough more closely resemble Phanerozoic Zn-Pb--Cu deposits of the Kuroko- or Iberian Pyrite Belt-type (cf., Halbach et al., 1989; Fouquet et al., 1993). Massive sulfides in the Eastern Manus Basin, where back-arc rifting has produced felsic volcanic rocks ofdacitic composition have been compared to Archean Zn-Cu massive sulfide deposits such as those in the Noranda district of Canada ( Scott and B inns, 1992, 1993). On a world- wide basis, less than about 25% of massive sulfide deposits in the geologic record have formed in exclusively basaltic rocks; more than 55% are associated with felsic rocks (Rona, 1988). This suggests that young back-arc spreading centers have been a particular important setting for massive sulfide formation through time.

By analogy with land-based massive sulfides, large individual deposits on the ocean floor are expected to

be rare. Discrete mounds (e.g., Magic Mountain, Southern Explorer Ridge; TAG Mound, TAG Hydro- thermal Field) may account for a large fraction of the total metal deposited in a given area, but small vent fields and isolated chimneys are much more common occurrences. In most mining districts on land, a significant proportion of the total metal reserves are usually contained within one large deposit. Empirical observations of grade and tonnage statistics for Cyprus-type massive sulfides show that few deposits ( < 15%) contain more than 10 million tonnes of ore and about 50% of the deposits contain more than I million tonnes (cf., Fig. 4; Mosier et al., 1983; Cox and Singer, 1986). Sawkins (1990) noted that most of the past production of copper from Cyprus-type massive sulfides world- wide is from just two deposits (Mavrouvoni: 15 million tonnes and Skouriotissa: 6 million tonnes). In the Tro- odos ophiolite, over 90 deposits are known, but most contain < 100 000 tonnes of massive sulfide (Sawkins, 1990). Similarly, grade-tonnage relationships for different base metal mining districts in Canada indicate that the single largest deposit in a given area typically contains 60-70% of the total metal reserves for that district; the second largest deposit may contain only 10-20% with a small fraction of the total metal in all of the remaining deposits (Boldy, 1977; Sangster, 1980). The average size of volcanic-associated massive sulfide deposits mined in Canada is about 1 million tonnes, but the statistics are obviously biased by the fact that recorded tonnages refer only to the deposits of sufficient size to be mined. Vast numbers of small sulfide occurrences like those on the seafloor are known to exist in the geologic record, but these are often not important enough to be included in published reserves.

Very large accummulations of massive sulfides similar to those in the Iberian Pyrite Belt with a combined tonnage of 750 million tonnes have not yet been found at the modern seafloor. It might be speculated that heavily sedimented back-arc spreading centers are environments which permit the formation of sulfide deposits


with a size comparable to that of the large sulfide districts on land. Preferred targets may be sedimented back-arc riffs such as the Andaman Sea in the eastern Gulf of Bengal, and the northern Aegean Trough in the Mediterranean between Greece and Turkey. However, sulfide accumulations of the size of the Iberian Pyrite Belt may be unique and thus are not necessarily to be found at the modern seafloor.

On a world-wide basis, ancient volcanogenic massive sulfide deposits contain about 3000 tonnes of gold metal (estimated from past production and present reserves; Mosier et al., 1983). The average land-based massive sulfide deposit typically has a gold content of about 1.2 ppm Au and a size of about 5-10 million tons (Fig. 4). High gold grades of significantly more than 10 ppm are documented and usually occur in sulfide deposits which are associated with felsic volcanic rocks. In most cases these gold-rich deposits were formed in back-arc or island-arc settings similar to that of gold-rich massive sulfides at the modern seafloor.

The discovery of gold-rich seafloor gossans in the TAG Hydrothermal Field refutes the conventional wis- dom that supergene enrichment of gold is exclusively a subaerial process and suggests that high concentrations of secondary gold may occur in ancient deposits that have never been exposed to near-surface weathering on land (Hannington et al., 1988). This has been confirmed by the discovery of high concentrations of gold up to 28 ppm in 80 million year old seafloor gossans ("ochres" ) which are associated with Creta- ceous massive sulfide deposits in Cyprus (Herzig et al., 1991 ).

9. Conclusions and other considerations

Even after more than a decade of intensive research on seafloor massive sulfides, there is a continuing lack of sufficient data concerning their distribution, size and bulk composition. As a result, it is premature to judge the economic significance of seafloor polymetallic sulfides.

The immediate rewards of seafloor research, however, lie in the vast improvement of our understanding of how ancient polymetallic massive sulfide deposits have been formed. A major advantage of the study of modern seafloor hydrothermal systems is the actual knowledge of the composition of endmember ore-

forming fluids which is a prerequisite for geochemical modelling and mass balance calculations. Furthermore, detailed studies on the regional setting and the geochemistry of seafloor deposits in the Southwest Pacific have identified important tectonic and petrologic controls on the occurrence of economically more attractive (e.g., gold-rich) massive sulfide deposits in the geologic record. These systematics now may be applied to a better and more reliable assessment of promising tectonic settings, resulting in better focused exploration models for land-based mineral deposits.

Active hydrothermal systems on the seafloor will continue to be excellent natural laboratories for the study of ore deposit genesis, in particular in the light of recent drilling of the active TAG mound at the Mid- Atlantic Ridge (cf., ODP Leg 158 Shipboard Scientific Party, 1995; Herzig et al., 1995) which provided much needed information about the extent of these systems beneath the seafloor. Furthermore, the study of recent volcanic eruptions and megaplumes at the East Pacific Rise 9°-10°N (Haymon et al., 1991 ) and the Coaxial segment of the Juan de Fuca Ridge (Embley et al., 1993) will become important for understanding the relationships between the generation of new oceanic crust, the development of hydrothermal systems and the formation of massive sulfide deposits (Hannington et al., 1995a, b). This information can be translated directly to the ancient geologic record where evidence for the origin and nature of mineral deposits is often obscured by millions of years of geological history. Early work on the seafloor focused on a limited range of deposits (e.g., Cyprus-type massive sulfides on basaltic mid-ocean ridges), but recent exploration adjacent to continental margins and island arcs has confirmed that a wide range of deposit types exist in different volcanic and tectonic settings. Recent exploration activities in shallow-marine island arc environments of the West and Southwest Pacific have lead to the discovery of a new type of seafloor hydrothermal system with strongly elevated gold contents of 40-70 ppm (Tabar-Feni Arc: Herzig et al., 1994; Izu-Oga- sawara Arc: Tsunogai et al., 1994; Watanabe et al., 1994), and distinct epithermal characteristics. Future seafloor research most likely will focus on extensional zones within active volcanic arcs (small interarc rift basins, rifting of active arcs), where true rhyolite volcanism and associated massive sulfide deposits can be expected.


Acknowledgements

Our research on modern and ancient seafloor hydrothermal systems has received support by the Alexander von Humboldt-Foundation, the German Federal Min- istry for Research and Technology, the German Research Foundation, the European Union, NATO Sci- entific Affairs Division, the Marine Minerals Technol- ogy Center (U.S Bureau of Mines), and Natural Resources Canada. Thanks are due to Rob Zierenberg, Garry McMurtry and Sven Petersen for their construc- tive reviews of this paper.

References

Alton, M.C., Dobby, G.S. and Scott, S.D., 1989. Potential for processing seafloor massive sulfides by magnetic separation. Mar. Min., 8: 163-172.

Amann, H., 1982. Technological trends in ocean mining. Philos. Trans. R. Soc. London, A(307)1499: 377--403.

Amann, H., 1985. Development of ocean mining in the Red Sea. Mar. Min., 5: 103-I 16.

Amann, H., 1989. The Red Sea Pilot Project: Lessons for future ocean mining. Mar. Min., 8: 1-22.

B~icker, H. and Richter, H., 1973. Die rezente hydrothermal sediment~e Lagerstiitte Atlantis II Tief im Roten Meer. Geol. Rundsch., 62: 697-740.

Ballard, R.D. and Francheteau, J., 1982. The relationship between active sulfide deposition and the axial processes of the mid-ocean ridge. Mar. Technol. Soc. J., 16: 8-22.

Bailard, R.D., Francheteau, J., Juteau, T., Rangan, C. and Normark, W., 1981. East Pacific Rise at 21°N: The volcanic, tectonic, and hydrothermal processes of the central axis. Earth Planet. Sci. Lett., 55: 1-10.

Barrett, T.J., Jarvis, I., Longstaffe, F.J. and Farquhar, R., 1988. Geo- chemical aspects ofhydrothermal sediments in the eastern Pacific Ocean: an update. Can. Mineral., 26: 841-858.

Berger, B.R. and Bethke, P.M. (Editors), 1985. Geology and Geo- chemistry of Epitbermal Systems. Reviews in Economic Geol- ogy, 2:298 pp.

Bear, L.M., 1963. The Mineral Resources and Mining Industry of Cyprus. Ministry of Commerce and Industry, Nicosia, Cyprus. Cyprus Geol. Surv., Dep. Bull., 1:208 pp.

Binns, R.A., 1994. Submarine deposits of base and precious metals in Papua New Guinea. In: R. Rogerson (Editor), Proc. PNG Geology, Exploration and Mining Conf. 1994. Aastralas. Inst. Min. Metall., pp. 71-83.

Binns, R.A., Boyd, T. and Scott, S.D., 1991. Precious metal barite spires from the Western Woodlark Basin, Papua New Guinea. Geol. Assoc. Can., Progr. Abstr., 16: A12.

Binns, R.A., Scott, S.D., Bogdanov, Y.A., Lisitzin, A.P., Gordeev, V.V., Gurvich, E.G., Finlayson, E.J., Boyd, T., Dotter, L.E., Wheller, G.E. and Muravyev, K.G., 1993. Hydrotbermal oxide

and gold-rich sulfate deposits of Franklin Seamount, Western Woodlark Basin, Papua New Guinea. Econ. Geol., 88: 2122- 2153.

Bischoff, J.L. and Rosenbauer, R.J., 1984. The critical point and two- phase boundary of seawter, 200-500°C. Earth Planet. Sci. Lett., 68: 172-180.

Bischoff, J.L. and Pitzer, K.S., 1985. Phase relations and adiabats in boiling seafloor geothermal systems. Earth Planet. Sci. Lett., 75: 327-338.

Blum, N. and Pucheit, H., 1991. Sedimentary-hosted polymetallic massive sulfide deposits of the Kebrit and Shaban Deeps, Red Sea. Mineral. Deposita, 26: 217-227.

Boldy, J., 1977. Uncertain exploration facts from figures. Bull. Can. Inst. Min. Metall., 70: 86-95.

Bowers, T.S., Campbell, A.C,, Measures, C.I., Spivack, A.J. and Edmond, J.M, 1988. Chemical controls on the composition of vent fluids at 13°N -11 °N and 210N, East Pacific Rise. J. Geophys. Res., 93: 4522-4536.

Boyd, T., Scott, S.D. and Hekinian, R., 1993. Trace element patterns in Fe-Si-Mn oxyhydroxides at three hydrothermally active seafloor regions. Resour. Geol., Spec. Issue, 17: 83-95.

Butterfield, D.A., Massoth, G.J., McDuff, R.E., Lupton, J.E. and Lilley, M.D. 1990. Geochemistry of hydrothermal fluids from Axial Seamount hydrothermal emmissions study vent field, Juan de Fuca Ridge: subseafloor boiling and subsequent fluid-rock interaction. J. Geophys. Res., 95: 12895-12921.

Campbell, A.C., Bowers, T.S. and Edmond, J.M., 1988. A time- series of vent fluid compositions from 21°N EPR (1979, 1981, 1985), and the Guaymas Basin Gulf of California ( 1982, 1985). J. Geophys. Res., 93: 4537-4539.

Canadian Mines Handbook, 1987-1988. C.D. Gardiner (Editor). Notthern Miner Press Ltd., Toronto, Ont.

Cann, J.R. and Strens, M.R., 1982. Black smokers fuelled by freezing magma. Nature, 298: 147-149.

Cheminee, J.L., Stoffers, P., McMuttry, G., Richnow, H., Puteanus, D. and Sedwick, P., 1991. Gas-rich submarine exhalations during the 1989 eruption of Macdonald Seamount. Earth Planet. Sci. Lett., 107: 318-327.

Collier, J. and Sinha, M., 1990. Seismic images of a magma chamber beneath the Lau Basin back-arc spreading centre. Nature, 346: 646-648.

Converse, D.R., Holland, H.D. and Edmond, J.M., 1984. Flow rates in the axial hot springs of the East Pacific Rise (21°N): implications for the heat budget and the formation of massive sulfides. Earth Planet. Sci. I.~tt., 69: 159-175.

Cox, D.P. and Singer, D.A. (Editors), 1986. Mineral Deposit Mod- els. U.S. Geol. Surv. Bull., 1693:379 pp.

Crawford, A.M., Hollingshead, S.C. and Scott, S.D., 1984. Geo- technical engineering properties of deep-ocean polymetallic sulfides from 21°N, East Pacific Rise. Mar. Min., 4: 337-354.

Cronan, D.S., 1976. Basal metalliferous sediments from the eastern Pacific. Geol. Soc. Am. Bull., 87: 928-934.

Davis, E.E., Goodfellow, W.D., Bornhold, B.D., Adshead, J., Blaise, B., Villinger, H. and Le Cheminant, G., 1987. Massive sulfides in a sedimented rift valley, Northern Juan de Fuca Ridge. Earth Planet. Sci. Lett., 82: 49-61.


Davis, E.E., Mottl, M.L. and Fisher, A.T. et al., 1992. Proc. ODP, Initial Reports, Vol. 139. College Station, Tex. (Ocean Drilling Program), 1026 pp.

Degens, E.T. and Ross, D.A. (Editors), 1969. Hot Brines and Recent Heavy Metal Deposits in the Red Sea. Springer Verlag, New York, N.Y., 600 pp.

Detrick, R.S.P., Buhle, E., Vera, J., Mutter, J., Oreutt, J., Madsen, J. and Brocher, T., 1987. Multichannel seismic imaging of a crustal magma chamber along the East Pacific Rise. Nature, 326: 35- 41.

Divi, S.R., Thorpe, R.I. and Franklin, J.M., 1980. Use of discriminant analysis to evaluate compositional controls of stratiform massive sulfide deposits in Canada. Can. Geol. Surv., Pap. 79-20.

Doe, B.R., 1994. Zinc, copper, and lead in mid-ocean ridge basalts and the source rock control on Zn/Pb in ocean-ridge hydrother- real deposits. Geochim. Cosmochim. Acta, 58: 2215-2223.

Drummond, S.E. and Ohmoto, H., 1985. Chemical evolution and mineral deposition in boiling hydrothermal systems. Econ. Geol.. 80: 126-147.

Dymond, J., Corliss, J.B., Heath, G.R., Field, C.W., Dasch, E.J. and Veeh, H.H., 1973. Origin of metalliferous sediments from the Pacific Ocean. Geol. Soc. Am. Bull., 84: 3355-3372.

Embley, R.W., Jonasson, I.R., Perfit, M.R., Franklin, J.M., Tivey, M.A., Malahoff, A., Smith, M.F. and Francis, T.J.G., 1988. Sub- mersible investigation of an extinct hydrothermal system on the Galapagos Ridge: sulfide mounds, stockwork zone, and differ- entiated lavas. Can. Mineral., 26:517-540.

Embley, R.W., Chadwick, W.W., Jonasson, I.R., Petersen, S., But- terfield, D., Tunnicliffe, V. and Juniper, K., 1993. Geological inferences from a response to the first remotely detected eruption on the mid-ocean ridge: Coaxial Segment, Juan de Fuca Ridge (abstr.). EOS, Trans. Am. Geophys. Union, 74: 619.

Fouquet, Y., Auclair, G., Cambon, P. and Etoubleau, J., 1988. Geo- logical setting, mineralogical, and geochemical investigations on sulfide deposits near 13°N on the East Pacific Rise. Mar. Geol., 84: 145-178.

Fouquet, Y., Von Stackelberg, U., Charlou, J.L., Erzinger, J., Herzig, P.M., Miihe, R. and Wiedicke, M., 1993. Metallogenesis in back- arc environments: The Lau Basin Example. Econ. Geol., 88: 2154-2181.

Francheteau, J. and Ballard, R.D., 1983. The East Pacific Rise near 21 °S: inferences for along-strike variability of axial processes of the mid ocean ridge. Earth Planet. Sci. Left., 64: 93.

Franklin, J.M., 1986. Volcanic associated massive sulphide deposits - -an update. In: C.J. Andrew et al. (Editors), Geology and Gen- esis of Mineral Deposits in Ireland. Irish Assoc. Econ. Geol., pp. 49-70.

Franklin, J.M. and Thorpe, R.I., 1982. Comparative metallogeny of the Superior, Slave and Churchill provinces. Geol. Assoc. Can., Spec. Pap., 25: 3-90.

Franklin, J.M., Lydon, J.W. and Sangster, D.F., 1981. Volcanic- associated massive sulfide deposits. Econ. Geol., 75th Anniv. Vol.: 485-627.

Goldfarb, M.S., Converse, D.R., Holland, H.D. and Edmond, J.M., 1983. The genesis of hot spring deposits on the East Pacific Rise, 21°N. Econ. Geol. Monogr., 5: 184-197.

Halbach, P., 1989. Erstentdeckung yon Massivsulfiden in einem intrakontinentalen Back-arc Becken. Geowissenschaften, 6: 157-161.

Halbach, P,, Nakamura, K., Wahsner, M., Lange, J., Sakai, H., Kase- litz, L., Hansen, R.D., Yamano, M., Post, J., Prause, B., Seifert, R., Michaelis, W., Teichmann, F., Kinoshita, M., Marten, A., Ishibashi, J., Czerwinski, S. and Blum, N., 1989. Probable modem analogue of Kuroko-type massive sulphide deposits in the Okinawa Trough back-arc basin. Nature, 338: 496-499.

Halbach, P., Pracejus, B. and Miirten, A., 1993. Geology and mineralogy of massive sulfide ores from the Central Okinawa Trough, Japan. Econ. Geol., 88: 2210-2225.

Hannington, M.D. and Scott, S.D., 1988a. Mineralogy and geochemistry of a hydrothermal silica-sulfide-sulfate spire in the caldera of Axial Seamount, Juan de Fuca Ridge. Can. Mineral., 26: 603- 626.

Hannington, M.D. and Scott, S.D., 1988b. Gold and silver potential of polymetallic sulfide deposits on the sea floor. Mar. Min., 7: 271-285.

Hannington, M.D. and Scott, S.D., 1989. Gold mineralization in volcanogenic massive sulfides: implications of data from active hydrothermal vents on the modem seafloor. Econ. Geol. Mon- ogr., 6: 491-507.

Hannington, M.D. and Jonasson, I.R., 1992. Fe and Mn oxides at seafloor hydrothermal vents. Catena Suppl., 21: 351-370.

Hannington, M.D. and Herzig, P.M., 1993. Shallow submarine hydrothermal systems in modem island arc settings. Geol. Assoc. Can., Progr. Abstr., 19: A40.

Hannington, M.D., Peter, J.M. and Scott, S.D., 1986. Gold in seafloor polymetallic sulfides. Econ. Geol., 81: 1867-1883.

Hannington, M.D., Thompson, G., Rona, P.A. and Scott, S.D., 1988. Gold and native copper in supergene sulfides from the Mid- Atlantic Ridge. Nature, 333: 64-66.

Hannington, M.D., Herzig, P.M., Thompson, G. and Rona, P.A., 1990a. Metalliferous sulfide--oxide sediments from the TAG hydrothermal field (26°N), Mid-Atlantic Ridge (abstr.). EOS, Trans. Am. Geophys. Union, 71: 1653.

Hannington, M.D., Herzig, P.M. and Alt, J.C., 1990b. The distribution of gold in subseafloor stockwork mineralization from DSDP Hole 504B and the Agrokipia B deposit, Cyprus. Can. J. Earth Sci., 27: 1409-1417.

Hannington, M.D., Herzig, P.M. and Scott, S.D., 1991a. Auriferous hydrothermal precipitates on the modem seafloor. In: R.P. Foster (Editor), Gold Metallogeny and Exploration. Blackie and Son Ltd., Glasgow, pp. 249-282.

Hannington, M.D., Herzig, P.M., Scott, S.D. Thompson, G. and Rona, P.A., 1991b. Comparative mineralogy and geochemistry of gold-bearing deposits on the mid-ocean ridges. Mar. Geol., 101: 217-248.

Hannington, M.D., Petersen, S., Jonasson, I.R. and Franklin, J.M., 1994. Hydrothermal activity and associated mineral deposits on the seafloor. Generalized Geological Map of the World. Geol. Surv. Can., Open File Rep. 2915C, Map 1:35,000,000 and CD- ROM.

Hannington, M.D., Jonasson, I.R., Herzig, P.M. and Petersen, S., 1995a. Physical and chemical processes of seafloor mineralization at mid-ocean ridges. In: S. Humphris, D. Fornari and R.


Zierenherg (Editors), Physical, Chemical, Biological, and Geo- logical Interactions within Hydrothermal Systems, Am. Geo- phys. Union Monogr. (in press).

Hannington, M.D., Tivey, M.K., Larocque, A.C., Petersen, S. and Rona, P.A., 1995b. Evidence for hydrothermal reworking of gold in sulfide deposits of the TAG Hydrothermal Field, Mid-Atlantic Ridge (submitted).

Haymon, R.M., 1983. Growth history of hydrothermal black smoker chimneys. Nature, 301: 695-698.

Haymon, R.M. and Kastuer, M., 1981. Hot spring deposits on the East Pacific Rise at 2 I°N: Preliminary descriptions of mineralogy and genesis. Earth Planet. Sci. Lett., 53: 363-381.

Haymon, R.M. and Kastuer, M., 1986. Caminite: a new magnesium- hydroxide-sulfate-hydrate mineral found in a submarine hydrothermal deposit, East Pacific Rise, 210N. Am. Mineral., 71: 819- 825.

Haymon, R.M., Koski, R.A. and Sinclair, C., 1984. Fossils of hydrothermal vent worms discovered in Cretaceous sulfide ores of the Samail ophiolite, Oman. Science, 223: 1407-1409.

Haymon, R.M., Lilley, M., Fornari, D. and Perfit, M., 1991. Pyrrho- tite-rich "ash" deposits associated with recent volcanic eruption on the EPR axis at 9050.56'N ( "Tubeworm BBC" site): a product of explosive hydrothermal exhalation (abstr.). EOS, Trans. Am. Geophys. Union, 72: 495.

Hekinian, R. and Fouquet, Y., 1985. Volcanism and metallogenesis of axial and off-axis structures on the East Pacific Rise near 13°N. Econ. Geol., 80: 221-249.

Hekinian, R., Fevrier, M., Avedik, F., Cambon, P., Charlou, H.D., Needham, H.D., Raillurd, J., Boulegue, J., Merlivat., L., Moinot, A., Manganini, S. and Lange, J., 1983. East Pacific Rise near 13°N: Geology of new hydrothermal fields. Science, 219:1321- 1324.

Hekinian, R., Hoffert, M., Larque, P., Cheminee, J.L., Stoffers, P. and Bideau, D., 1993. Hydrothermal Fe and Si oxyhydroxide deposits from South Pacific Intraplate volcanoes and East Pacific Rise Axial and off-axial regions. Econ. Geol., 88: 2099-2121.

Herzig, P.M., 1988. A mineralogical, geochemical, and thermal pro- file through the Agrokipia " B " hydrothermal sulfide deposit, Troodos Ophiolite Complex, Cyprus. In: G.H. Friedrich and P.M. Herzig (Editors), Base Metal Sulfide Deposits in Sedimentary and Volcanic Environments. SGA, Springer, Heidelberg, Spec. Publ., 5: 182-215.

Herzig, P.M., 1991. Marine Rohstoffvorkommen - - Polymetallische Sulfidmineralisationen am Meeresboden. Geowissenschaften, 6: 169-178.

Herzig, P.M. and Friedrich, G.H., 1987. Sulfide mineralization, hydrothermal alteration, and chemistry in the drill hole CY-2a, Agrokipia, Cyprus. In: P.T. Robinson, I.L. Gibson and A. Pan- ayioutou (Editors), Cyprus Crustal Drilling Project, Initial Reports, Holes CY-2 and 2a. Geol. Surv. Can. Spec. Pap., 85- 29: 103-138.

Herzig, P.M., Hannington, M.D., Scott, S.D., Thompson, G. and Rona, P.A., 1988. The distribution of gold and associated trace elements in modem submarine gossans from the TAG Hydro- thermal Field, Mid-Atlantic Ridge, and in ancient ochres from Cyprus. Geol. Soc. Am., Progr. Abstr., 20: A240.

Her-zig, P.M., Fouquet, Y., Hannington, M.D. and Von Stackelherg, U., 1990. Visible gold in primary polymetallic sulfides from the Lau back-arc (abstr.). EOS, Trans. Am. Geophys. Union, 71: 1680.

Herzig, P.M., Hannington, M.D., Scott, S.D., Maliotis, G., Rona, P.A. and Thompson, G., 1991. Gold-rich seafloor gossans from the Troodos Ophiolite and on the Mid-Atlantic Ridge. Econ. Geol., 86: 1747-1755.

Herzig, P.M., Hannington, M.D., Fouquet, Y., Von Stackelberg, U. and Petersen, S., 1993. Gold-rich polymetallic sulfides from the Lau Back Arc and implications for the geochemistry of gold in sea-floor hydrotherrnal systems of the Southwest Pacific. Econ. Geol., 88: 2182-2209.

Herzig, P., Hannington, M., McInnes, B., Stoffers, P., Villinger, H., Seifert, R., Binns, R., Liehe, T. and Scientific Party, 1994. Sub- marine volcanism and hydrothermal venting studied in Papua, New Guinea. EOS, Trans. Am. Geophys. Union, 75: 513-516.

Herzig, P.M., Humphris, S.E. and the Leg 158 Shipboard Scientific Party, 1995. Drilling the TAG Hydrothermal Field (abstr.). Eur. Union Geosci., Strasbourg 1995, EUG 8, Terra Nova, 7: 206.

Ixer, R.A., Alabaster, T. and Pearce, J.A., 1984. Ore petrography and geochemistry of massive sulfide deposits within the Samail ophiolite. Trans. Inst. Min. Metall. (Sect. B: Appl. Earth Sci.), 93: 114-124.

Johnson, H.P., 1991. Next generation of seafloor samplers. EOS, Trans. Am. Geophys. Union, 72: 65-66.

Juniper, S.K. and Fouquet, Y., 1988. Filamentous iron-silica deposits from modem and ancient hydrothermal sites. Can. Mineral., 26: 859-870.

Kadko, D. and Moore, W., 1988. Radiochemical constraints on the crustal residence time of submarine hydrothermal fluids: Endeav- our Ridge. Geochim. Cosmochim. Acta, 52: 659-668.

Kaiogeropoulos, S.I. and Scott, S.D., 1983. Mineralogy and geochemistry of tuffaceous exhalites (tetsusekiei) of the Fukuzawa Mine, Hokuroku District, Japan. Econ. Geol. Monogr., 5: 412- 432.

Kappel, E.S. and Franklin, J.M., 1989. Relationships between geologic development of ridge crests and sulfide deposits in the Northeast Pacific Ocean. Econ. Geol., 84: 485-505.

Karl, D.M., McMurtry, G.M., Malaboff, A. and Garcia, M.O., 1988. Loihi Seamount, Hawaii: a mid-plate volcano with a distinctive hydrothermal system. Nature, 335: 532-535.

Karson, J.A. and Rona, P.A., 1990. Block-tilting, transfer faults, and structural control of magmatic and hydrothermal processes in the TAG area, Mid-Atlantic Ridge 26°N. Geol. Soc. Am. Bull., 102: 1635-1645.

Koski, R.A., 1987a. Sulfide deposits on the seafloor: geological models and resource perspectives based on studies in ophiolite sequences. In: P.G. Telekl et al. (Editors), Marine Minerals: Resource Assessment Strategies. Proc. NATO Advanced Research Workshop, Series C, 194. Reidel, Boston, Mass., pp. 301-316.

Koski, R.A., 1987b. A comparison of sulfide deposits from modem sediment-covered spreading axes with Besshi-type deposits of Japan. In: G.R. McMurray, (Editor), Gorda Ridge: A Seafloor Spreading Center in the United States Exclusive Economic Zone. Springer, New York, N.Y., pp. 117-130.


Koski, R.A., Clague, D.A. and Oudin, E., 1984. Mineralogy and chemistry of massive sulfide deposits from the Juan de Fuca Ridge. Geol. Soc. Am. Bull., 95: 930-945.

Koski, R.A., Shanks, W.C. Ill, Bohrson, W.A. and Oscarson, R.L., 1988. The composition of massive sulfide deposits from the sediment-covered floor of Escanaba Trough, Gorda Ridge: implications for depositional processes. Can. Mineral., 26: 655-674.

Lalou, C., Thompson, G., Arnold, M., Brichet, E., Druffel, E. and Rona, P.A., 1990. Geochronology of TAG and Snakepit hydrothermal fields, Mid-Atlantic Ridge: Witness to a long and complex hydrothermal history. Earth Planet. Sci. Lett., 97:113-128.

Lalou, C., Reyss, J.L., Brichet, E., Arnold, M., Thompson, G., Fou- quet, F. and Rona, P., 1993. New age data for Mid-Atlantic Ridge hydrothermal sites: TAG and Snake Pit chronology revisited. J. Geophys. Res., 98:9705-9713.

LeHuray, A.P., Church, S.E., Koski, R.A. and Bouse, R.M., 1988. Pb isotopes in sulfides from mid-ocean ridge hydrothermal sites. Geology, 16: 362-365.

Leinen, M. and Stakes, D., 1979. Metal accumulation rates on the central equatorial Pacific during Cenozoic time. Geol. Soc. Am. Bull., 90: 357-375.

Malahoff, A., 1982. A comparison of the massive submarine polymetallic sulfides of the Galapagos Rift with some continental deposits. Mar. Technol. Soc. J., 16: 39-45.

Marchig, V., R/Ssch, H., Lalou, C., Brichet, E. and Oudin, E., 1988. Mineralogical zonation and radiochronological relations in a large sulfide chimney from the East Pacific Rise at 18°25'S. Can. Mineral., 26: 541-554.

Massoth, G.J., Butterfield, D., Lupton, J.E., McDuff, R.E., Lilley, M.D. and Jonasson, I.R., 1989. Submarine venting of phase- separated hydrothermal fluids at Axial Volcano, Juan de Fuca Ridge. Nature, 340: 702-705.

McMurtry, G.M., Sedwick, P.N., Fryer, P., Vonderhaar, D.L. and Yeh, H.-W., 1993. Unusual geochemistry of hydrothermal vents on submarine arc volcanoes: Kasuga Seamounts, Northern Mar- iana arc. Earth Planet. Sci. Lett., 114: 517-528.

Moore, W.S. and Stakes, D., 1990. Ages of barite-sulfide chimneys from the Mariana Trough. Earth Planet. Sci. Lett., 100: 265-274.

Morton, J.L. and Sleep, N.H., 1985. Seismic reflections from a Lau Basin magma chamber. In: D.W. Scholl and T.L. Vallier (Edi- tors), Geology and Offshore Resources of Pacific Island Arcs - - Tonga Region. Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, pp. 441-453.

Mosier, D.L., Singer, D.A. and Salem, B.B., 1983. Geologic and grade-tonnage information on volcanic-hosted copper-zinc-lead massive sulfide deposits. U.S. Geol. Surv., Open-File Rep., 83- 89:78 pp.

Mustafa, H.E., Nawab, Z., Horn, R. and Le Mann, F., 1984. Eco- nomic interest of hydrothermal deposits: Atlantis II project. Proc. 2nd Int. Seminar Offshore Mineral Resources. Brest, France, pp. 509-539.

Nawab, Z., 1984. Red Sea mining: a new era. Deep Sea Res., 3l: 813-822.

ODP Leg 158 Shipboard Scientific Party, 1995. Drilling an active hydrothermal system at the Mid-Atlantic Ridge. EOS, Trans. Am. Geophys. Union, in press.

Ohmoto, H. and Skinner, B.J. (Editors), 1983. The Kuroko and related volcanogenic massive sulfide deposits. Econ. Geol. Mon- ogr., 5:604 pp.

Oudin, E., 1983. Mineralogie de gisements et indices lies a des zones d'accretion oceaniques actuelles (ride Est Pacific et Mer Rouge) et fossile (Cyphre). Chronique Recherche Miniere, 470: 43-55.

Oudin, E., 1987. Geochemistry of submarine sulphides. In: P.G. Teleki et al. (Editors), Marine Minerals: Resource Assessment Strategies. Proc. NATO Advanced Research Workshop, Series C, 194. Reidel, Boston, Mass., pp. 349-362.

Oudin, E. and Constantinou, G., 1984. Black smoker chimney fragments in Cyprus sulphide deposits. Nature, 308: 349-353.

Petersen, S., 1992. Mineralogie und Geochemie goldfiihrender Mas- sivsulfide des Lau Back-Arc (Siidwest-Pazifik). M.Sc. thesis, Aachen University of Technology, Aachen, Germany, 92 pp.

Pottorf, R.J. and Barnes, H.L., 1983. Mineralogy, geochemistry, and ore genesis of hydrothermal sediments from the Atlantis II Deep, Red Sea. Econ. Geol. Monogr., 5: 198-223.

Puchelt, H., 1986. HYMAS I, Cruise41 o fR/V Sonne. Rep. German Federal Minist. Res. Technol., Bonn, 10 pp.

Ramboz, C., Oudin, E. and Thisse, Y., 1988. Geyser-type discharge in the Atlantis II Deep, Red Sea: Evidence of boiling from fluid inclusions in epigenetic anhydrite. Can. Mineral., 26: 765-786.

Richards, H.G., Cann, J.R. and Jensenius, J., 1989. Mineralogical zonation and metasomatism of the alteration pipe of Cyprus sulfide deposits. Econ. Geol., 84:91-115.

Rona, P.A., 1984. Hydrothermal mineralization at seafloor spreading centers. Earth Sci. Rev., 20: 1-104.

Rona, P.A., 1988. Hydrothermal mineralization at oceanic ridges: Can. Mineral., 26:431-465.

Rona, P.A. and Scott, S.D., 1993. Preface to Special Issue on Sea- Floor Hydrothermal Mineralization: New Perspectives. Econ. Geol., 88: 1935-1976.

Rona, P.A., Klinkhammer, G., Nelsen, T.A., Trefry, J.H. and Eld- erfield, H., 1986. Black smokers, massive sulfides and vent biota at the Mid-Atlantic Ridge. Nature, 321 : 33-37.

Ryall, P.J.C., 1987. Remote drilling technology. Mar. Min., 6: 149- 160.

Sangster, D.F., 1980. Quantitative characteristics of volcanogenic massive sulphide deposits. Bull. Can. Inst. Min. Metall., 73: 74- 81.

Sawkins, F.J., 1990. Metal Deposits in Relation to PLate Tectonics, 2nd ed. Springer, Berlin, 461 pp.

Scott, S.D., 1985. Seafloor polymetallic sulfide deposits: Modern and ancient. Mar. Min., 5: 191-212.

Scott, S.D., 1987. Seafloor polymetallic sulfides: Scientific curiosi- ties or mines of the future? In: P.G. Teleki et al. (Editors), Marine Minerals: Resource Assessment Strategies. Proc. NATO Advanced Research Workshop, Series C, 194. Reidel, Boston, Mass., pp. 277-300.

Scott, S.D., 1992. Polymetallic sulfide riches from the deep: Fact or fallacy? In: K.J. Hsu and J. Thiede (Editors), Use and Misuse of the Sea Floor. Wiley, New York, N.Y., pp. 87-115.

Scott, S.D. and Binns, R.A., 1992. An actively-forming, felsic volcanic-hosted polymetallic sulfide deposit in the southeast Manus back-arc basin of Papua New Guinea [ abstr.l. EOS, Trans. Am. Geophys. Union, 73: 626.


Scott, S.D. and Binns, R.A., 1993. Presently-forming hydrothermal seafloor deposits of Marius and Woodlark Basins, S.W. Pacific, as models for ancient ores. In: Fenoll Hach-Ali et al. (Editors), Current Research in Geology Applied to Ore Deposits, SGA Granada 1993, pp. 381-384.

Scott, S.D., Chase, R.L., Hannington, M.D., Michael, P.J., Mc- Conachy, T.F. and Shea, G.T., 1991. Sulfide deposits, tectonics, and petrogenesis of Southern Explorer Ridge, northeast Pacific Ocean. In: J. Malpas et al. (Editors), Ophiolites: Oceanic Crustal Analogues. Proc. Troodos '87, Nicosia, Cyprus. Geol. Surv., Dep. Minist. Agricult. Nat. Resour., pp. 719-733.

Sibuet, J.-C., Letouzey, J., Barbier, F., Charvet, J., Foucher, J.-P., Hilde, T.W.C., Kimura, M., Ling-Yun, C., Marsset, B., Muller, C. and Stephan, J.-F., 1987. Back-arc extension in the Okinawa Trough. J. Geophys. Res., 92: 14041-14063.

Spooner, E.T.C., 1980. Cu-pyrite mineralization and seawater convection in oceanic crust--the ophiolitic ore deposits of Cyprus. Geol. Assoc. Can., Sp. Pap., 20: 685-704.

Stiihen, D., Bloomer, S.H., Taibi, N.E., Neumann, Th., Bendel, V., Piischel, U., Barone, A., Lange, A., Shiying, W., Cuizhong, L. and Deyu, Z., 1992. First results of study of sulphur-rich hydrothermal activity from island-arc environment: Esmeralda Bank in the Mariana arc. Mar. Geol., 103: 521-528.

Thompson, G., Bryan, W.B., Ballard, R., Hamuro, K. and Melson, W.G., 1985. Axial processes along a segment of the East Pacific Rise, 10°-I2°N. Nature, 318: 429-433.

Thompson, G., Humphris, S.E., Schroeder, B., Sulanowska, M. and Rona, P.A., 1988. Active vents and massive sulfides at 26°N (TAG) and 23°N (Snakepit) on the Mid-Atlantic Ridge. Can. Mineral., 26: 697-712.

Tivey, M.K. and Delaney, J.R., 1986. Growth of large sulfide structures on the Endeavour segment of the Juan de Fuca Ridge. Earth Planet. Sci. Lett., 77: 303-317.

Tsunogai, U., Ishibashi, J., Wakita, H., Gamo, T., Watanahe, K., Kajimura, T., Kanayama, S. and Sakai, H., 1994. Peculiar features of Suiyo Seamount hydrothermal fluids, lzu-Bonin arc: differences from subaerial volcanism. Earth Planet. Sci. Lett., 126: 289-301.

Urabe, T., Marumo, K. and Nakamura, K., 1990. Mineralization and related hydrothermal alteration in Izena Cauldron (JADE Site), Okinawa Trough, Japan [abstr.]. Geol. Soc. Am., Progr. Abstr., 22: A9.

Von Damm, K.L., 1988. Systematics of and postulated controls on submarine hydrothermal solution chemistry. J. Geophys. Res., 93:4551-4561.

Von Damm, K.L., 1990. Seafloor hydrothermal activity: black smoker chemistry and chimneys. In: G.W. Wetherill (Editor), Annual Review Earth Planet. Sci., 18: 173-204.

Von Damm, K.L., Edmond, J.M., Grant, B. and Measures, C.I., 1985. Chemistry of submarine hydrothermal solutions at 21°N, East Pacific Rise. Geochim. Cosmnchim. Acta, 49: 2197-2220.

Watanabe, K., Shibata, A., Kajimura, T., Ishibashi, J., Tsunogai, U., Aoki, M. and Nakamura, K., 1994. Survey method about the submarine volcano and its sea-floor hydrothermal ore deposit-- a example of Suiyo Seamount in the lzu-Ogasawara Arc with the submersible "Shinkai 2000". J. Jpn. Soc. Mar. Surv. Tech- nol., 6: 29--44.

Wheat, G.C. and Mottl, M.J., 1994. Hydrothermal circulation, Juan de Fuca Ridge eastern flank: factors controlling basement water composition. J. Geophys. Res., 99: 3067-3080.

Wolery, T.J. and Sleep, N.H., 1976. Hydrothermal circulation and geochemical flux at mid-ocean ridges. J. Geol., 84: 249-275.

Zierenherg, R.A. and Schiffman, P., 1990. Microbial control of silver mineralization at a sea-floor hydrothermal site on the northern Gorda Ridge. Nature, 348: 155-157.

Zierenherg, R.A. and Shanks, W.C., 1983. Mineralogy and geochemistry of epigenetic features in metalliferous sediment, Atlantis II Deep, Red Sea. Econ. Geol., 78: 57-72.

Zierenherg, R.A., Koski, R.A. and Morton, J.L., 1990. Cu-Au-rich sediment-hosted massive sulfide deposits from the Escanaba Trough, Southern Gorda Ridge [ abstr.]. Geol. Soc. Am., Abstr. Progr., 22: A9.

Zierenberg, R.A., Koski, R.A., Morton, J.L., Bouse, R.M. and Shanks, W.C., 1993. Genesis of massive sulfide deposits on a sediment-covered spreading center, Escanaba Trough, Southern Gorda Ridge. Econ. Geol., 88: 2069-2098.

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