Mineralogical Study And In Situ Sulfur Isotopic Characteristics Of Au-carrying Pyrites In Shuiyindong Gold Deposit,Guizhou Province

Carlin-type gold deposit,firstly found in Nevada in the 1960 s,is characterized by invisible gold in arsenian pyrite and arsenopyrite.Two "golden triangle" areas of Yunnan-Guizhou-Guangxi and Sichuan-Gansu-Shanxi around Yangtze craton have more than 1000 t gold.Shuiyindong gold deposit is a typical Carlin-type gold deposit in southern China,Several ten years of studying leads to a clear understanding on the geological features of the deposit.However,the occurrences of gold and sources of ore fluids remains unclear.This paper mainly reported the mineralogical characteristics and in situ sulfur isotopes of Au-bearing pyrites in Shuiyindong gold deposit.Pyrites in ore rocks have several morphological types,such as strawberry pyrites,euhedral pyrites and subhedral-anhedral pyrites.And euhedral and subhedral-anhedral pyrites have a common zoned texture,which consists of a core that formed during pre-ore stage and hydrothermal overgrowth rims during ore stage.Data from Electron Probe Micro Analysis(EPMA)show that pyrites contains As(0~10.884wt%),Co(0.046~0.447wt%),Cu(0~0.252wt%),Cr(0~0.312wt%)and other elements.A negative relationship between As and S indicates an substitution of As for S in Au-carrying pyrites.In the lgAu-lgAs plot,most of datapoints are below the Au solubility curve,reflecting invisible gold in Shuiyindong deposit is present as structurally bound Au+.Co/Ni ratio is more than 1,implying ore fluid may be derived from the deep magma.High-resolution multiple elemental mapping by Nano-SIMS reveals the uneven distribution of trace elements(eg.Au,As,Cu,Se)in pyrites,reflecting ore fluid enriched in Au,As and Cu.And due to episodic variation of ore fluids,the Au-rich rim is composed of several finer subzones marked by the banded pattern of Au,As and Cu.Au content in the core is extremely low,because it formed during the pre-ore stage.Meanwhile,we analyzed in situ sulfur isotopes of Au-carrying pyrites by using Nano-SIMS.?34S values of the cores ranging from 1.3‰ to 6.5‰ and the rims from-3.5‰ to 7.4‰ indicate that sulfur is originated from the deep magma likely contaminated by some sedimentary sulfur.Finally,a genetic model of Shuiyindong gold deposit can be concluded that the Au-As-Cu-Si-S-bearing magmatic fluids upwelled along the deep fault to fill or react with Fe-rich wall rocks in Huijiabao anticline,which ultimately leads to the deposition of Au.Additionally,the meteoric water maybe involved in the formation of Au-rich pyrites by leaching and dissolving some Au from the sedimentary strata.