Sulfur isotope and trace element compositions of pyrite determined by NanoSIMS and LA-ICP-MS: new constraints on the genesis of the Shuiyindong Carlin-like gold deposit in SW China

The Shuiyindong Carlin-like gold deposit, the largest one of this type in China, is located in the Late Paleozoic-Early Mesozoic Youjiang basin. The pyrites in the studied ore bodies (No. I to V) can be divided into four main types plus several sub-types: (1) As-poor pyrites include Py1 (Py1a and Py1b), Py2, and Py3; and (2) As-rich pyrite Py4 (Py4a and Py4b). In situ LA-ICP-MS analyses show that the As-rich pyrite also contains higher Au, Cu, Sb, and Tl than the other pyrite types. In situ NanoSIMS analyses show the delta S-34 variation of Py1 (48.1 similar to 67.5 parts per thousand), Py2 (2.4 similar to 7.6 parts per thousand), Py3 (9.2 similar to 14.2 parts per thousand), and Py4 (- 3.0 similar to 6.6 parts per thousand), with the average values of 58.1 parts per thousand, 5.1 parts per thousand, 11.4 parts per thousand, and 3.1 parts per thousand, respectively. The large delta S-34 variations of As-poor pyrites may reflect a sedimentary source. However, the narrow ranges of delta S-34 values for the As-rich pyrite (Py4) may not reflect a sedimentary but a magmatic or metamorphic origin. A metamorphic sulfur source of Py4 is favored due to paucity of coeval magmatism in the region and decreasing variations of delta S-34 values from Py1 to Py4. Integrated transmission electron microscope (TEM) and NanoSIMS element mapping show that the distribution and concentration of Au are discordant with As and Cu at the submicron scale, also implied by the occurrence of some of Au as nano-submicron particles in Py4. Overall, our new data indicate that the ore fluids were not only enriched in Au but also in As, Cu, and S. Taking an alternative of fluid oxidation probably resulting in the decreasing delta S-34 values of Py4, our result of I to V orebodies indicates the mixing between the underlying metamorphic fluid and meteoric water. Integrated variations in delta S-34 and As-Au-Cu correlations across individual pyrite grains infer the physicochemical changes of aqueous and/or aqueous-carbonic fluids along migration paths.