Peculiarities of some mafic-ultramafic- and ultramafic-hosted massive sulfide deposits fom the Main Uralian Fault Zone, southern Urals

Some Cu-rich, mafic-ultramafic- and ultramafic-hosted massive sulfide deposits from the southern segment of the Main Uralian Fault Zone (Ivanovka and Ishkinino deposits, southern Urals) show unusual characteristics. Their major features include: (i) relatively high Co (Ni, An), very low Zn and negligible Pb grades; (ii) a pyrrhotite-dominated mineralization, locally characterized by the presence of open- latticework aggregates of lamellar pyrrhotite with Mg-saponite Mg-chlorite and carbonate matrix; (iii) hydrothermal alteration of ultramafic host rocks into talc carbonate quartz chlorite and of mafic host rocks into chloritites; (iv) the presence of clastic facies with reworked sulfide and ultramafic or mafic components; (v) the widespread occurrence of sulfide-associated chromite; (vi) the specific mineralogy of Co, Ni, Fe and As, including sulfoarsenides, mono- and diarsenides, and Co-rich pentlandite and pyrite; (vii) the supra-subduction -zone geochemical signature of the host serpentinites and volcanic rocks. Although some of these features have been separately reported in certain modem ocean-seafloor and ophiolite-hosted fossil deposits, a true equivalent has yet to be found. Based on recognized partial analogies with a few modem seafloor examples, the arc tholeiitic-boninitic geochemical signature of sulfide-associated volcanic rocks and the highly refractory compositions of sulfide-hosted chromite relicts, the studied deposits are believed to have formed by seafloor-subseafloor hydrothermal processes in an oceanic island arc setting. Possible tectonostratigraphic correlation of sulfide-associated units with infant, non-accretionary arc volcanic units of the adjacent Magnitogorsk oceanic island-arc system suggests formation of the studied deposits during the earliest stages of Devonian subduction-related volcanism

Gold- and Silver-Rich Massive Sulfides from the Semenov-2 Hydrothermal Field, 13\ub031.13\u2032N, Mid-Atlantic Ridge: A Case of Magmatic Contribution?

The basalt-hosted Semenov-2 hydrothermal field on the Mid-Atlantic Ridge is host to a rather unique Cu-Zn\u2013 rich massive sulfide deposit, which is characterized by high Au (up to 188 ppm, average 61 ppm, median 45 ppm) and Ag (up to 1,878 ppm, average 490 ppm, median 250 ppm) contents. The largest proportion of visible gold is associated with abundant opal-A, which precipitated after a first generation of Cu, Fe, and Zn sulfides and before a second generation of Fe and Cu sulfides. Only rare native gold grains were found in earlier sulfides. Fluid inclusions in opal-A associated with native gold indicate precipitation at 300\ub0 \ub1 40\ub0C from fluids of salinity higher than that of seawater (3.5\u20136.8 wt % NaCl equiv). According to laser ablation-inductively coupled plasma-mass spectrometry analyses, invisible gold is concentrated in secondary covellite (23\u2013227 ppm) rather than in the primary sulfides (1,000 ppm) than all other sulfides (1 are more consistent with a mafic signature. Thermodynamic modeling of hydrothermal fluids produced by reactions between various proportions of seawater and basalt or peridotite at 350\ub0C shows that mineral assemblages broadly similar to those of the Semenov-2 deposit can precipitate from fluids produced in a mafic environment, but that Au and Ag minerals are not predicted to precipitate from such fluids over a wide temperature range. These results suggest that an additional contribution to the hydrothermal system is required in order to achieve saturation in precious metals. A magmatic input is suggested by the occurrence of plagiogranites and tonalites dredged on sea floor in the Semenov area, which could be a potential source of Au-rich magmatic fluids, and by mineralogical and geochemical similarities with magma-related, low- to intermediate-sulfidation epithermal systems, namely high Au and Ag grades, high Au/(Cu + Zn + Pb) and Au/Ag ratios, and presence of Ag, Bi, and Te minerals. The likely crucial role of silicic melts in producing high Au and Ag grades suggests that exploration for precious metal-rich, volcanic-hosted massive sulfide deposits should be primarily directed to sites in which evolved igneous rocks occur on sea floor. Both in modern and ancient mafic-hosted deposits, zones characterized by abundant deposition of silica could be good clues to the presence of significant gold