Tin Mineralization of Gold-Bearing Seafloor Hydrothermal Sulfides at Ocean Core Complex, 12.4˚S, Central Indian Ridge

Abstract

The Zn-rich chimney (up to 50.1 wt.%) fragments were recovered from ocean core complex (OCC-4-1) at 12.4°S, in Central Indian Ridge. They are characterized by high concentrations of Sn (up to 1,720 ppm) and Au (up to 10.4 ppm). Similar to the modern ultramafic-hosted seafloor hydrothermal sulfides, the ores mainly contain sphalerite, isocubanite, chalcopyrite, pyrrhotite and marcasite, accompanied by minor galena and native gold. The native gold occurs predominantly as inclusions (< 3um in diameter) in sphalerite and isocubanite and contains low Ag contents of < 15 wt.%. No other gold-bearing minerals were observed but native gold. FeS contents of sphalerite are significantly variable from 6.7 to 42.7 mole % during ore-forming stages. Sphalerites formed in the early stage are depleted in FeS and most trace element contents relative to late-stage sphalerites. The late-stage sphalerites are particularly enriched in Sn which ranges up to 5.5 wt.%. Tin mineralization mainly occurs grain-boundaries between sphalerite and isocubanite, related to In and Ga coprecipitation. The Sn-enriched sphalerites show a negative correlation between Zn and Cu + Sn with the uniform Cu/Sn ratios (almost 2:1). Sn-bearing inclusions are notably absent based on TEM analysis. STEM-EDS mapping and FFT pattern analysis reveal that Sn is incorporated into sphalerite as a solid solution, not a discrete inclusion phase. We suggest that the Sn-enrithe modern ultramafic-hosted seafloor hydrothermal sulfides, the ores mainly contain sphalerite, isocubanite, chalcopyrite, pyrrhotite and marcasite, accompanied by minor galena and native gold. The native gold occurs predominantly as inclusions (< 3um in diameter) in sphalerite and isocubanite and contains low Ag contents of < 15 wt.%. No other gold-bearing minerals were observed but native gold. FeS contents of sphalerite are significantly variable from 6.7 to 42.7 mole % during ore-forming stages. Sphalerites formed in the early stage are depleted in FeS and most trace element contents relative to late-stage sphalerites. The late-stage sphalerites are particularly enriched in Sn which ranges up to 5.5 wt.%. Tin mineralization mainly occurs grain-boundaries between sphalerite and isocubanite, related to In and Ga coprecipitation. The Sn-enriched sphalerites show a negative correlation between Zn and Cu + Sn with the uniform Cu/Sn ratios (almost 2:1). Sn-bearing inclusions are notably absent based on TEM analysis. STEM-EDS mapping and FFT pattern analysis reveal that Sn is incorporated into sphalerite as a solid solution, not a discrete inclusion phase. We suggest that the Sn-enri