Modern Seafloor Sulfide Hydrothermal Systems--A Comparison of Pacific and Mid-Atlantic Environments

摘要

Mid-ocean ridges host approximately 65 per cent of about 100 known sites of high-temperature venting and polymetallic sulfide deposits; most of them are located at the East Pacific Rise (EPR), while more diverse mineralisation styles occur at the Mid-Atlantic Rise (MAR). Seafloor hydrothermal activity at the EPR are related to MORB while different substrates, including MORE and ultramafic intrusive rocks, are associated with 'black smokers' at the MAR. Depth of hydrothermal activity ranges from 0.7 to 4.3 km. High-temperature vents precipitating Cu-Fe sulfides (chalcopyrite and isocubanite, rare bornite), pyrrhotite, pyrite (± marcasite), and sphalerite, and low-temperature edifices precipitating marcasite (with pyrite), sphalerite, barite, and subordinate chalcopyrite (pyrrhotite and isocubanite are absent) occur in both environments. There are considerable differences in Fe, Zn, Cu, Ag, Au and trace element contents in ores and in the FeS contents in Zn-sulfides in these deposits. A comparison with anhydrite and sulfide minerals precipitating from modern black smoker fluids sulfide reveals many similarities but also some important differences. Fluids with salinity values above and below that of seawater at 400°C down to 130 - 170°C deposited sulfide minerals. Mantle-derived He prevails in fluids trapped by sulfides in EPR 9°N and 21°N hydrothermal systems. Hydrothermal fluids in MAR and back-arc systems are richer in radiogenic ~4He. The sulfur isotope composition varies in the different hydrothermal fields and within zones of a chimney. EPR sulfides display narrower range of ?~(34)S values, while MAR and back-arc sulfides can be depleted and enriched in the heavier ~(34)S isotope. The most important finding is that fluid temperature, salinity, R/Ra ratios and ?~(34)S values ranged more significantly in the polymetallic sulfide deposits than when measured directly on venting fluids.