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

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. In-situ sulfur isotopic analyses showed that the delta S-34 values range from -15.4 parts per thousand to 6.1 parts per thousand with a median of 2.0 parts per thousand for Pyl, from -1.8 parts per thousand to 7.1 parts per thousand with a median of 1.4 parts per thousand for Py2, and from -3.0 parts per thousand to 6.2 parts per thousand with a median of 3.9%o for Py3. Intensive spot analyses showed a slight fluctuation in S isotopic values within a short distance in pyrite subzones. The delta S-34 values of Py2 and Py3 dropped into a narrow range from -3 parts per thousand to 6 parts per thousand with a median of 3.3 parts per thousand. Considering the composition and isotopic characteristics, Py1 is classified as pre-ore pyrite and Py2 and Py3 as ore pyrites with Py3 representing the Au climax event. The Au state and distribution in the pyrite of the Taipingdong deposit imply that the ore-forming fluid was unsaturated for Au, but occasionally hosted metal nanoparticles. The Au climax event occurred towards the end of the fluid evolution. The decoupling of As and Au in the ore pyrites indicate that the Au-carrying capacity of the pyrite was limited by the higher concentration of Cu ions in the ore-forming fluids. The S isotopic data of the Taipingdong deposit is similar to the available in-situ S isotopic range of -5 parts per thousand to 5 parts per thousand of the representative Au deposits in the basin except for the Lannigou (Zhenfeng) Mine. The S isotopic deviation of pyrite in the Huijiabao ore field to the typical magmatic fluid (+/- 0 parts per thousand) may be the result of a mixture of hydrothermal and basin fluid to a certain extent. This feature combined with the intensive fluctuation of trace elements and isotopic compositions over a short duration accord with the characteristics of magmatic hydrothermal fluid. Combined with the strong silicification characteristic of the host rock and the previous geophysical works in the ore district, a magmatic hydrothermal model might be suitable for the Taipingdong gold deposit and even other Carlin-type gold deposits in the Youjiang basin.