Mineralization Of Polymetallic Sulifdes On Ultra-slow Spreading Southwest Indian Ridge At49.6°E

To understand the hydrothermal mineralization along ultra-slow spreadingridges, the first discovered acitive hydrothermal field, Southwest Indian Ridge (SWIR)49.6oE field, was selected to be investigated. Mineralogical, geochemical, and isotopicmethods were carried out to identify the characteristic and precipitate mechanism ofsulfides and oxides from this field. The results show that the sulfides in this field canbe classified into two distinct types according to the mineral assemblages, which areZn-rich ore and Fe-rich ore. The Zn-rich type ore is dominated bysphalerite-pyrite-chalcopyrie assemblages and the Fe-rich type is bypyrite-marcasite-isocubanite. Two mineralization stages were identified by thetextures and chemistry of the sulfide ores. Stage I is associated with relatively lowtemperature Zn-rich sulfide assemblages and stage II is to higher ore-formingtemperature Fe-rich sulfides. The transform of temperature from lower to higherreflects that episode emissions of hydrothermal fluids are there in this field. The firstoccurrence of native gold in sulfide from ultra-slow spreading ridge was reported inthis study. Based on the association of gold and sphalerite iron content, transportmechanism of gold in the fluid was qualified as Au(HS)0. The reworking process inSWIR49.6oE hydrothermal field was documented by the relationship between Au andAs, Pb. The low content of PGE and no Rh anomaly indicate that the magmatic fluidinput into the hydrothermal system was negligible. The sulfur isotope and Pb-Srisotope reveal that the metals in sulfide were derived mainly from substrate rock.Combined with the geological setting of Southwest Indian Ridge, it is can be inferredthat the hydrothermal activity in SWIR49.6oE field is controlled by magma supply asother fast spreading East Pacific Rise and slow spreading Mid-Atlantic Ridges.