Genesis and evolution of framboidal pyrite and its implications for the ore-forming process of Carlin-style gold deposits, southwestern China

摘要

Southwestern Guizhou, an indispensable component of the "golden triangle" area of southwestern China, is famous for its numerous Carlin-style gold deposits. Unlike other sulfides in the deposits that have been well studied, the genesis of framboidal pyrite is still controversial because of its tiny size. Here, we first determined the chemical and in situ sulfur isotopic compositions of framboidal pyrites from three typical Carlin-style gold deposits (namely Shuiyindong, Taipingdong and Nibao) using secondary electron image (SEI), back scattered electron (BSE), electron probe micro analysis (EPMA), and nanometer-scale secondary ion mass spectroscopy (Nano-SIMS) methods. Our EPMA data show that the framboidal pyrites contain a minor amount of trace elements (Ni 0.344 wt%, Co = 0.017-0.295 wt%, Cr = 0.009-0.292 wt% and As 1.479 wt%). Incorporation of the trace elements into the framboidal pyrites resulted in a slight decrease in S and Fe concentrations compared to the theoretical values. The in situ delta S-34 values of the framboidal pyrites, ranging from -53.2 parts per thousand to -30.9 parts per thousand, are in good agreement with the sulfur isotopic fractionation triggered by the bacterial sulfate reduction (BSR) process in an open-SO42- ocean condition. Combined with SEI, BSE, and Nano-SIMS high-resolution multiple elemental mapping, four consecutive processes responsible for their genesis and evolution are summarized as follows: (1) initial microcrystal nucleation via the BSR process, (2) abundant microcrystal aggregation to form individual framboid and multiple-framboids during sedimentation or diagenesis, (3) partial replacement and recrystallization of preexisting framboids by As-rich ore fluids, and (4) eventual framboidal pyrite transformation to larger euhedral grains via continuous recrystallization. These processes not only present compelling mechanisms responsible for the variations in morphology and chemical composition of the framboidal pyrites, from the initial framboid with a low (AsS)-As-75-S-32 signal (Py1) to the framboid with a recrystallization origin (Py3), but also indicate that their genesis and evolution may have an intimate correlation with the formation of Carlin-style gold deposits.