, Volume 31, Issue 6, pp 72-82
Date: 13 Dec 2012

The occurrence of gold in hydrothermal sulfide at Southwest Indian Ridge 49.6°E

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Abstract

Massive sulfide precipitates found in the Southwest Indian Ridge (SWIR) 49.6°E hydrothermal field are enriched in gold. Here, the mineralogy and mineral chemistry of these massive sulfides to constrain the process of gold precipitation were studied. Sulfide samples in this field include lower-temperature Zn-rich sulfides and relative higher-temperature Fe-rich sulfides. Zn-rich sulfides are dominated by sphalerite-pyrite-chalcopyrite assemblages, with concentrations of gold ranging from 9.08 to 17.0 µg/g. Fe-rich sulfides consist mainly of pyrite-marcasite-isocubanite assemblages, with gold concentrations from 2.17 to 3.79 µg/g. The significant enrichment in gold within the lower-temperature Zn-rich sulfides and the effective separation of Zn and Fe in hydrothermal precipitates at the surface of this field are here interpreted to reflect the strong temperature dependence of gold transportation and deposition within the sulfides. In Zn-rich samples, large amounts of isolated native gold grains were identified. They were found mainly as inclusions up to 8 µm in diameter, occupying porous cavities in sphalerite or in the elevated iron content rim of sphalerite. The fineness of the gold ranged from 810 to 830. Unlike previously published results on other hydrothermal fields, these data show a low gold fineness values in SWIR 49.6°E. The FeS content of sphalerite associated with gold grains ranged from 3.2 mole % to 18.9 mole %. This was higher than in other fields, indicating that the sulfur activity is relatively low during the gold precipitation process and that sulfur activity may be one of the main factors affecting gold fineness in the SWIR 49.6°E hydrothermal field. Evidence regarding gold fineness and sulfur activity suggests that gold was quite likely transported as AuHS0 rather than as a Au(HS) 2 complex.

Foundation item: The International Seabed Area 11th Five Year Plan Program of China Ocean Mineral Resources R&D Association under contract No. DYXM-115-01-2-1; the International cooperation program of the Ministry of Science and Technology under contract No. 2006DFB21620; the program of the National Natural Science Foundation of China under contract No. 40776034; Grant from the scientific research fund of the First Institute of Oceanography, SOA under contract No. GY02-2012G10.