Episodic mineralization at Yanshannian in the Woxi Au-Sb-W deposit, western Hunan, South China: Constraints from in-situ scheelite LA-ICP-MS U-Pb geochronology and element-isotope of pyrite

The Woxi deposit, located in the central Jiangnan Orogen, is a large gold deposit with significant antimony and tungsten mineralization. Recent research indicates that the deposit was formed primarily at Yanshanian, but the source of the mineralization materials and ore genesis are still controversial. By using SEM, EPMA, and LA-MC-IF-ICP-MS, this study analyzed the structures, elemental compositions and isotopic components of multi-stage pyrites, as well as U-Pb geochronology of scheelite. Within the Woxi deposit, three generations of five types of pyrite (Py) have been identified, i.e., pre-mineralization Py1, mineralization stage 2 Py2, and mineralization stage 3 Py3a, Py3b, and Py3c. The in-situ analysis of pyrite and the alteration characteristics of whole rocks indicate that, during mineralization, Au, As, and Cu were coupled, with Au mainly existing in the crystal lattice of Py3b as a solid solution (Au+), As substitution for S in pyrite was the primary factor promoting lattice gold enrichment, while strong water-rock reactions were responsible for the low Co/Ni ratios in the mineralizing stage. The in-situ trace elements and S-Pb isotopic compositions of the different generations of pyrite (delta S-34(V-CDT) values: Py2 range from -4.33 parts per thousand to -1.61 parts per thousand, Py3a range from -6.36 parts per thousand to 3.59 parts per thousand, Py3b range from -4.36 parts per thousand to 3.59 parts per thousand, Py3c range from -2.21 parts per thousand to -0.65 parts per thousand; the values of Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204: Py2 are 18.029, 15.618 and 38.653 respectively, and Py3a are 18.047 similar to 18.341, 15.594 similar to 15.698 and 38.781 similar to 39.398, respectively) and their variations in the zoned textural indicate significant differences in fluid properties and source areas during the second and third stages of mineralization. LA-IF-ICP-MS U-Pb dating of the third-stage white scheelite yields an age of 132.7 +/- 7.9Ma (1 sigma s, n=31, MSWD=0.43). Combined with the previous studies, our study revealed that the Woxi Au-Sb-W deposits went through two stage mineralization at Yanshanian, i.e., ca. 148 similar to 140Ma wolframite (with minor scheelite, pyrite and gold) and ca. 130Ma gold, anitimony and scheelite deposition. The earlier stage was formed in a relatively high-temperature reducing fluid produced by partial melting of underlying basement rocks. The later stage Au-Sb-W mineralization is dominated by pulsed mantle-derived (magma) fluids, and its metal may mainly come from the ore-rich hydrothermal solution evolved by the mixture of crust- and mantle-derived fluids. The pulsation of mineralization and fluid pressure fluctuation may be related to fault valve mechanism. The main precipitation gold mechanism is sulfidation and water-rock reaction, while fluid-mixing leads to deposition of stibnite and scheelite. The reactivation of enriched and metasomatic. Our research shows that the large-scale lithospheric extension caused by the episodic subduction and detachment of the lithospheric mantle enriched and metasomatic in multiple stages and the ancient Pacific plate is the key factor and tectonic background for the large-scale gold (polymetallic) mineralization in Woxi and other gold deposits in Jiangnan Orogen during Late Yanshanian.