Precious metal mineralization in the seafloor hydrothermal vent on the western part of TA25 caldera

Abstract

Sulfide samples from the TA25 hydrothermal vent fields (176°50W/24°25S, Tonga arc) are enriched in precious and/or trace elements such as Au (up to 35 ppm), Ag (up to 1,970 ppm), Sb (up to 4,730 ppm), Hg (up to 666 ppm) and As (up to 1,420 ppm). In particular, the bulk composition reveals that Au and Ag are strongly associated with each other and show positive correlation with Zn. The Au/Ag ratios of narrow range (close to 0.04) indicate that fractionation of between Au and Ag is considerably low. Two distinct mineralization stages are observed: (1) early stage sulfates was followed by assemblage of pyrite + sphalerite + chalcopyrite + sulfosalt. (2) late stage silver-bearing sulfosalts were overlapped. Detailed mineral chemistry shows that there are systematic distribution of trace elements within sphalerite grain during different ore stage. The overall degree of Ag, As and Sb enrichment within sphalerite grains from core to rim indicate that fractionation of ore-forming fluid plays an important role in concentration of precious element. Considering bulk composition and mineral chemistry, the mineralization of both Au and Ag is mainly controlled by later hydrothermal fluid which causes deposition of Au-Ag-bearing sulfosalts of invisible phase within sphalerite grain.1,420 ppm). In particular, the bulk composition reveals that Au and Ag are strongly associated with each other and show positive correlation with Zn. The Au/Ag ratios of narrow range (close to 0.04) indicate that fractionation of between Au and Ag is considerably low. Two distinct mineralization stages are observed: (1) early stage sulfates was followed by assemblage of pyrite + sphalerite + chalcopyrite + sulfosalt. (2) late stage silver-bearing sulfosalts were overlapped. Detailed mineral chemistry shows that there are systematic distribution of trace elements within sphalerite grain during different ore stage. The overall degree of Ag, As and Sb enrichment within sphalerite grains from core to rim indicate that fractionation of ore-forming fluid plays an important role in concentration of precious element. Considering bulk composition and mineral chemistry, the mineralization of both Au and Ag is mainly controlled by later hydrothermal fluid which causes deposition of Au-Ag-bearing sulfosalts of invisible phase within sphalerite grain.