The in-situ LA-ICPMS analyses of trace element compositions of gold-bearing pyrites in the Chahuazhai gold deposit,SE Yunnan and their geological significances
The Youjiang Basin is one of the most concentrated areas of Carlin-type gold deposits in the world,and the SE Yunnan area on the southern margin of the basin is its important part.The Chahuazhai gold deposit is a representative gold deposit which occurred near the unconformity of Indosinian movement in the SE Yunnan.Pyrite is the main gold-bearing mineral with obvious zoning characteristics.This deposit provides an ideal example for understanding the Carlin-type gold mineralization in this area.In this paper,based on the field investigation and optical microscopy observation,and application of the in-situ laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)analytical technique,trace element compositions of pyrites in different stages have been analyzed,in order to summarize gold enrichment characteristics and key controlling factors of the gold mineralization in different stages of the Chahuazhai gold deposit.The results show that there are three types of pyrites in the deposit.They include the sedimentary originated pyrite(Py 1),including framboidal pyrite(Py1a)and the core of zonal pyrite(Pylb),the middle zone of zonal pyrite(Py2)with the metasomatic origin and the rim of zonal pyrite(Py3)with the hydrothermal origin.Especially,the Py1b is enriched in Sb,Pb,Zn and other elements,whereas the Py2 is enriched in As,Co,and Ni.The Py3 has high contents of Au,As,Cu,and Sb,with relative slightly high contents of Se,Hg,and Tl.The comparison of those three types of pyrites shows that the Py2 has the highest As content,while the Py1b has similar or relative slightly low contents of trace elements comparing to the Py2 with the exception of As.The Py1b and Py2 have Co/Ni ratios around 0.1,indicating that the Py2 was formed by hydrothermal metasomatic transformation of the Py1b.The Py3 has significantly different trace element compositions to those of the Py1b and Py2,indicating its hydrothermal origin.Moreover,the Py3 has relatively low Co and Ni contents comparing to the Py1b and Py2,with Co/Ni ratios ranging from 0.01 to 0.1,indicating that the ore-forming fluid has relatively low temperature without enriched Co and Ni.Co and Ni in the Py3 were mainly inherited from the Py1b and Py2.Au mainly occurred in lattice of the Py3 of the deposit in association with As,Sb,Cu,Se and other elements.In summary,the ore-forming fluid of the Chahuazhai gold deposit is a low-temperature hydrothermal fluid enriched in Au,As,Cu,Sb,Se,Tl and other elements.After this ore-forming fluid reached the ore-bearing strata,the fluid had metasomatized with the Py1b to have formed the high arsenic Py2 on surface of the Py1b.Then Au was adsorbed from the fluid.onto surface of the Py2 in association with the formation of Py3,which enriched in solid solution stated Au,in the metallogenic process.
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