Sulfide hydrothermal

Ocean Basins. Ocean basins are primarily formed from oceanic basalts and maybe interspersed with continental remnants, ridges, seamounts, or volcanic islands rising from the depths. Average water depth is around 4000 m but the most significant mineralization is generally found at 5000 m for manganese nodules, 4000 m for biogenic oozes, and 3000 m for hydrothermal metalliferous sulfides. The area is poorly explored, however. 

Ocean Basins. Known consohdated mineral deposits in the deep ocean basins are limited to high cobalt metalliferous oxide cmsts precipitated from seawater and hydrothermal deposits of sulfide minerals which are being formed in the vicinity of ocean plate boundaries. Technology for drilling at depth in the seabeds is not advanced, and most deposits identified have been sampled only within a few centimeters of the surface. 

Fig. 2. Distribution of ( ) known and (o) suspected metalliferous sulfide deposits and active hydrothermal vents in the Pacific Ocean (42).

Deposits which are forming are frequentiy characterized by venting streams of hot (300°C) mineralized fluid known as smokers. These result in the local formation of metalliferous mud, rock chimneys, or mounds rich in sulfides. In the upper fractured zone or deep in the rock mass beneath the vents, vein or massive sulfide deposits may be formed by the ckculating fluids and preserved as the cmstal plates move across the oceans. These off-axis deposits are potentially the most significant resources of hydrothermal deposits, even though none has yet been located. 

Copper ore minerals maybe classified as primary, secondary, oxidized, and native copper. Primaryrninerals were concentrated in ore bodies by hydrothermal processes secondary minerals formed when copper sulfide deposits exposed at the surface were leached by weathering and groundwater, and the copper reprecipitated near the water table (see Metallurgy, extractive). The important copper minerals are Hsted in Table 1. Of the sulfide ores, bornite, chalcopyrite, and tetrahedrite—teimantite are primary minerals and coveUite, chalcocite, and digenite are more commonly secondary minerals. The oxide minerals, such as chrysocoUa, malachite, and azurite, were formed by oxidation of surface sulfides. Native copper is usually found in the oxidized zone. However, the principal native copper deposits in Michigan are considered primary (5). 

Such reactions are discussed at appropriate points throughout the book as each individual compound is being considered. A particularly important set of reactions in this category is the synthesis of element hydrides by hydrolysis of certain sulfides (to give H2S), nitrides (to give NH3), phosphides (PH3), carbides (C Hm), borides (B Hm), etc. Useful reviews are available on hydrometallurgy (the recovery of metals by use of aqueous solutions at relatively low temperatures), hydrothermal syntheses and the use of supercritical water as a reaction medium for chemistry. 

In recent years, many hydrothermal solution venting and sulfide-sulfate precipitations have been discovered on the seafloor of back-arc basins and island arcs (e.g., Ishibashi and Urabe, 1995) (section 2.3). Therefore, it is widely accepted that the most Kuroko deposits have formed at back-arc basin, related to the rapid opening of the Japan Sea (Horikoshi, 1990). 

Kaolin minerals (kaolinite, dickite, nacrite), pyrophyllite and mica-rich mica/smec-tite mixed layer mineral occur as envelopes around barite-sulfide ore bodies in the footwall alteration zones of the Minamishiraoi and Inarizawa deposits, northern part of Japan (south Hokkaido) (Marumo, 1989). Marumo (1989) considered from the phase relation in Al203-Si02-H20 system that the hydrothermal alteration minerals in these deposits formed at relatively lower temperature and farther from the heat source than larger sulfide-sulfate deposits in the Hokuroku district. 

Heavy Rare Earth Element). Therefore, it is considered that negative Ce and positive Eu anomalies in hydrothermally altered volcanic rocks, Kuroko ores, and ferruginous chert and LREE enrichment in the Kuroko ores have been caused by hydrothermal alteration and precipitations of minerals from hydrothermal solution responsible for sulfides-sulfate (barite) mineralization. 

Positive Eu anomaly is observed for hydrothermal solution issuing from the hydrothermal vent on the seawater at East Pacific Rise (Bence, 1983 Michard et al., 1983 Michard and AlbarMe, 1986). Guichard et al. (1979) have shown that the continental hydrothermal barites have a positive Eu anomaly, indicating a relatively reduced environment. Graf (1977) has shown that massive sulfide deposits and associated rocks from the Bathurst-Newcastle district. New Brunswick have positive Eu anomalies. These data are compatible with positive Eu anomaly of altered basaltic rocks, ferruginous chert and Kuroko ores in Kuroko mine area having positive Eu anomaly and strongly support that Eu is present as divalent state in hydrothermal solution responsible for the hydrothermal alteration and Kuroko mineralization. 

Sato (1973) and Ohmoto et al. (1983) calculated the amounts of sulfides precipitated due to the mixing of ascending hydrothermal solution and cold seawater. Their calculations showed that the calculated ratios of the amounts of minerals precipitated are generally consistent with those in Kuroko ore deposits. 

Amorphous silica and barite precipitate simultaneously from white smoker in midoceanic ridge hydrothermal system (Edmond et al., 1979). It is inferred that amorphous silica precipitates in the chimney at a later stage than sulfides and sulfates (anhydrite and barite) which constitute chimneys from which black smoker is emerging. 

As already discussed, /02 of Kuroko ore fluids is considered to lie in the predominance field of reduced sulfur species from the following two reasons (1) Selenium content of sulfides is very low (Yamamoto, 1974) and (2) H2S is dominant in hydrothermal solution venting from back-arc basins (section 2.3) from which hydrothermal ore deposits being similar to Kuroko deposits form. 

The concentrations of base-metals (Cu, Fe, Pb, and Zn) in hydrothermal solution in equilibrium with sulfides (chalcopyrite, pyrite, galena and sphalerite) depend on several variables such as pH, ntQx- concentration, temperature, /WH2S, and fo2- The relation between the concentrations and these variables can be derived based on the chemical equilibrium for the following reactions. 

Therefore, it is likely that Ag-rich electrum and large amounts of sulfide minerals including argentite could precipitate due to CO2 loss and pH increase under low /sj and /02 conditions. Therefore, this mechanism (boiling and gas loss from the hydrothermal solution with different /sj, /o2> CO2 concentration and pH) could explain why the Ag content of electrum correlates with Ag/Au total production ratio (Fig. 1.124). 

Cathles, L.M. (1983b) An analysis of the hydrothermal system responsible for massive sulfide deposition in the Hokuroku basin of Japan. Econ. GeoL Mon., 5, 439-487. 

Graf, J.L. Jr. (1977) Rare earth element as hydrothermal tracers during the formation of massive sulfide deposits in volcanic rocks. Econ. Geoi, 72, 527-548. 

Imai, A. and Uto, T. (2001) Association of electrum and calcite and its significance to the genesis of the Hishikari low-sulfidation epithermal gold deposits, southern Kyushu, Japan. Proc. International Symposium on Gold and Hydrothermal Systems, pp. 83-88. 

Janecky, D.R. and Seyfried, W.E. Jr. (1984) Formation of massive sulfide deposits on oceanic ridge crests incremental reaction models for mixing between hydrothermal solutions and seawater. Geochint. Cosmochim. Acta, 48, 2723-2738. 

Shikazono, N. (1972b) Solubility of sulfide minerals in hydrothermal solution. Mining Geology, 22, 403-412 (in Japanese). 

Yamaoka, K. (1969) Metallic minerals of the Kuroko deposits in Northeast Japan. Proc. Symp. Mineral Constituents of Sulfide Minerals from Hydrothermal Deposits, Morioka, pp. 1-38 (in Japanese). Yamaoka, K. (1976) On the genetical problems of the vein-type deposits of the Neogene age in the inner belt of Northeast Japan. Mining Geology Special Issue, 7, 59-74 (in Japanese). 

Another interesting characteristic of the Osorezan hydrothermal system is that it is located at a volcanic front. This is different from low sulfidation epithermal Au-Ag veins... 

Pb, Zn) sulfides occur in deeper part as in Broadlands (New Zealand) and in Okuaizu (Japan). Pb and Zn sulfides occur dominantly at 400 m from the surface in Broadlands. In the boreholes of Broadlands, concentrations of precious and base metal elements in the boreholes change with depth as studied by Ewers and Keayse (1977), Au, As, Sb and T1 decrease with depth, but Ag increases with depth. In active geothermal systems in Japan, Au, Te, Se, T1 and Hg are enriched on the surface in the Osorezan hydrothermal system. 

Average content (in parts per million or percent) of selected metals and SIOt in 37 hydrothermal sulfide samples from the Sunrise deposit (lizasa et ah, 1999)... 

Fiji Transform Fault Extensional Relay Zone A (16°10 S, 177°25 E) 1869-2335 Short spreading ridge axis which displaces Fiji transform fault as interpreted by Jarvis et al. (1994). Hydrothermal sulfide impregnation in MORB-like ba.salt dredged from axial valley. M etite, pyrrhotite, chalcopyrite and opal on fracture surface. 

The pH of hydrothermal solution in sediment-hosted sites is higher than that of the other volcanic rock-hosted sites. This might be due to the interaction of hydrothermal solution with sediments (probably carbonates). If pH increases by the interaction with sediments, sulfides tend to precipitate at the subsurface environment due to an increase in pH. The low concentration of base metals in hydrothermal solution in sediment-hosted deposits could be explained by such subsurface depositions. 

The pH of hydrothermal solution of white smoker from which anhydrite is precipitating shows very low 2 for Lau Basin Vail Lili fluid. This low pH cannot be explained only by water-rock interaction process. One likely explanation is decreasing of pH due to precipitation of sulfides. The pH decreases by the following reaction,... 

S S data on H2S and sulfides from Okinawa Trough (Okinawa Izena) show a high S S value (- -8%o) (Sakai, H. pers. conun cited in Ishibashi andUrabe, 1995). 8 " S values from other districts are similar to those of midoceanic ridges -i-2.1%o to 4-3. l%o, Mariana Trough (Kusakabe et al., 1990) -f0.3%o to -t-2.2%o, Minami Ensei Knoll (Nedachi et al., 1992 4-2.l%o to 4-2.8%o, Kita-Bayonnaise caldera (lizasa et al., 1992) 4-0.9% to 4-1.2% (Kaikita caldera) (vein part) (Ishibashi and Urabe, 1995). S " S values from the Okinawa (Izena) sulfides are higher than any of S S data from midoceanic ridge sulfides and H2S of hydrothermal solutions. 

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