Gold and sulfur sources of the Taipingdong Carlin-type gold deposit: Constraints from simultaneous determination of sulfur isotopes and trace elements in pyrite using nanoscale secondary ion mass spectroscopy

摘要

The Youjiang basin in southwestern China is known for the occurrence of abundant Carlin-type gold deposits similar to those of northern Nevada in the U.S. Among them, Taipingdong is a representative deposit nearby the super-large Shuiyindong Mine in the Huijiabao ore field. Though some data of in-situ sulfur isotopes and trace element geochemistry of pyrite have been collected recently, the ore-forming fluid source and genesis model of the deposits remain under debate. The mineral chemistry and isotopic compositions of Au-carrying pyrite in the Taipingdong gold deposit were studied in detail using nanoscale secondary ion mass spectroscopy (NanoSIMS) in this study to further understand the aforementioned issues. Petrographic observations identified two types of pyrite: syn-sedimentary framboidal pyrites and hydrothermal pyrites. Back-scattered electron observations and electron microprobe analysis elemental mapping showed that the latter have three generations, termed as As-poor Pyl (the core), As-rich Py2 (the middle overgrowth), and As-weak Py3 (the outmost overgrowth zone), respectively. NanoSIMS high-resolution multiple elemental maps showed the following: 1) Pyl is poor in As and other trace elements; Py2 is rich in As and Cu and relatively rich in Se; and Py3 is rich in Au and relatively rich in As, Cu, and Se. 2) Gold is heterogeneously distributed as solid solution and nanoparticle, focusing on the outermost rim of the zoned pyrite (Py3). Other metal nanoparticles of Cu, Se, and Pb were identified in Py2 or Py3. 3) Arsenic elemental mapping showed dense mineral growth of pyrite in the subzones. 4) Line profile analyses of trace elements showed high Cu contents in Py2 and Py3 and that As is positively correlated with Cu but not with Au.