Mineralogy and chemistry of massive sulfides from the Logatchev hydrothermal field (14 degrees 45'N Mid-Atlantic Ridge)

A horizontal mineralogical-geochemical ore zonation with a southeast to northwest trending succession of Cu and Fe (Zn) main zones has been discovered in the Logatchev hydrothermal field (14 degrees 459N Mid-Atlantic Ridge). The major minerals are pyrite, chalcopyrite, bornite, isocubanite, digenite and chalcocite. Zn-sulfide, marcasite and covellite are less abundant. Rare minerals are native copper, zincian copper and cobalt pentlandite (first found in oceanic massive sulfides), Cu and Cu-Co-bearing Fe-disulfides. Two groups of the main sulfide assemblages with different copper contents are recognized: Group I comprises assemblages with primary sulfides (isocubanite, chalcopyrite, pyrite and Zn-sulfide; Cu up to 35 wt% in Cu-bearing minerals); Group II contains assemblages with secondary Cu-rich sulfides (bornite, digenite, chalcocite; Cu approximately 60 wt%) as well as rare minerals (cobalt pentlandite, native copper and zincian copper). The assemblages of Group I formed at a temperature of about 300 degrees C; the sulfur fugacity (log f s2 atm) at 300 degrees C is probably 9.9. Relations between minerals of this group are contradictory, attributed to non-equilibrium environments, and no unified sequence of crystallization of minerals has been determined. Minerals of Group II replace, rim or cross the assemblages of Group I. Group II minerals crystallized in a distinct sequence (cobalt pentlandite-bornite-digenite-chalcocite-native copper), with increasing copper content from the start to the end of the depositional sequence. This may correspond to pH from 7 to 8 near Eh = -0.2 at 250 degrees C. Minerals of this group were generally formed under hypergene alteration processes. Compositions of ore minerals were studied by means of electron microprobe analysis (EMPA) (150 analyses). Narrow composition fields were revealed in all main sulfides, which comprised known experimental data. ZnS contains 6.0 to 28.0 mol.% FeS, which is close to data known for other hydrothermal massive sulfides from the Mid-Atlantic Ridge (MAR). A common admixture in ZnS is Cu (0.16 to 1.32 wt%). Rare grains of unusual anisotropic Fe-disulfides, (Fe (sub 0.95) Cu (sub 0.05) )S 2 and Cu(Fe,Co) 9 S 20 , were found among the ore minerals.