Mineralogical And Geochemical Study Of Hydrothermal Precipitates From Edmond Field In Central Indian Ridge (CIR)

Texture, structure, mineralogy and geochemistry of hydrothermal precipitatesfrom slow-spreading Edmond hydrothermal field have been investigated in detail todiscuss the mineralizing episodes and conditions and the different factors controllingthe elemental variations.Edmond field has typically high temperature to low temperature hydrothermalprecipitates, incluing Fe-Cu sulfide, Zn-rich sulfide, fragments of Ca sulfate andSi-rich precipitates.The mineralizing processes in Edmond field is complex and show multiple-stagereworking of fluids. The formation of Zn-rich sulfides can be divided to sulfide andopal mineralising episodes, and in the former episode Fe sulide, Zn sulfide andamorphous silica formed sequentially. In Zn-rich sulfides here are abundant tube-likeremnants most of which have been in filled by Fe sulfide and amorphous silica.Specific chalcopyrite diseases have occurred along cleavage planes in euhedral pyritein Zn-rich sulfides, which are inferred as replacement origin. Though the formation ofFe-Cu sulfides can also be divided to two episodes, Fe-Cu sulfides experienceddepositional break and multi-stage reworking. The replacement of early sphalerite byhigh temperature Cu-rich fluids led to the formation of argentotetrahedrite-tennantiteand Ag-rich grains. Silicates and anglesite formed by the mixing between latehydrothermal fluids with seawater. Visible Au grains incluing natural gold andelectrum have been identified in Fe-Cu sulfides.During the deposition of sulfides in Edmond field, almost all elements wereleached from underlying baserocks. U content is low in both sulfides and fluids, andthe slight enrichment in Si-rich precipitates was ascribed to reduction of U(VI) toU(IV). The depletion of Ba and Pb in sulfides relative to continental deposits are dueto the depletion in Edmond baserocks. The distribution of Cu, Zn, Mo, Ag, As areinfluenced maily by temperature. Ga and Cd are controlled by mineralogical bindingand incorporated into sphalerite, while Ag, As, Pb, Au, In are partly influenced. Mndon't intend to incorporate into any kind of sulfides of be adsorbed. Although Ni isenriched in magmatic fluids, the concentration of Ni in sulfides is rather low perhapsdue to the subseafloor depositon. Ti, V, Zr, Nb are less solube and hard to be leachedfrom baserocks, which results in the low content in hydrothermal sulfides.Gold in enriched in both Zn-rich sulfids and Fe-Cu sulfides from Edmond field,indicating the prevalent enrichment in precipitates of Edmond field, that is, gold-richfluids may exist.