Ore-forming process and ore genesis of the Wangu gold deposit in the Jiangnan orogenic Belt, South China: Constraints from pyrite textures, trace elements and in-situ sulfur isotopes composition

Abstract

The Jiangnan Orogenic Belt is an Au–Sb–W polymetallic metallogenic belt located between the Yangtze Block and Cathaysian Block, with a total proven gold resource exceeding 970 tons (t). The Wangu gold deposit (85 t Au @ 6.8 g/t) is one of the most representative gold deposits in northeastern Hunan Province, which is located in the central of the Jiangnan Orogenic Belt. Similar to many gold deposits in this region, multi-stage tectonic-magmatic-thermal events resulted in a complicated and confusing metallogenic process and genesis. Six pyrite types (Py-1 to Py-6) from five mineralization stages (Stage Ⅰ to Ⅴ) were systematically classified through detailed field investigation and sample observation in the Wangu gold deposit. Coarse-grained pyrite (Py-1) disseminated in slate has more dissolution holes in its core and a relatively homogeneous edge; milky white quartz veins crosscut slate, with a small amount of medium to fine grained pyrite (Py-2) disseminated in coarse-grained quartz; quartz-scheelite veins crosscut milky white quartz, but no other sulfides; medium- to coarse-grained pyrite (Py-3a) disseminated in altered slate far from the auriferous quartz veins is replaced by arsenopyrite, while the medium-grained pyrite (Py-3b) and arsenopyrite near the auriferous quartz veins are replaced by chalcopyrite, sphalerite, and galena; medium- to fine-grained pyrite (Py-4), arsenopyrite, and quartz-sericite veins crosscut quartz-scheelite veins; fine-grained quartz-pyrite (Py-5) veins crosscut quartz and sericite, typically having a granularity of less than 30 μm; calcite, quartz and pyrite (Py-6) veins crosscut quartz, muscovite and chlorite. The Au and As contents of Py-1, Py-2 and Py-6 are much lower than those of Py-3 (a, b), Py-4, and Py-5, and Au mineralization mainly occurs in altered slate and quartz-sulfide veins. Almost all types of pyrites have a lower Co/Ni ratio (<1.0) and a higher As/Sb ratio (>20), and are closely related to quartz, sericite, chlorite, calcite, and other hydrothermal minerals, indicating that they are of mainly hydrothermal origin. The presence of a large number of gold-bearing sulfides in altered slate indicate that wall-rock sulfidation and fluid boiling are the main precipitation mechanisms of Au in the Wangu goldfield. The δ³⁴S values of Py-1 (–11.33 to –10.42 ‰), Py-3 (3a: –9.68 to –6.98 ‰; 3b: –10.14 to –8.48 ‰) and Py-4 (–11.48 to –6.75 ‰) are similar, indicating that their sulfur may have been derived from metamorphic sedimentary strata (similar to the Lengjiaxi Group), and the relatively high δ³⁴S values (–5.69 to –2.66 ‰) of Py-2 may imply that its sulfur is predominantly from the deep ore-causative magmas (or magmatic sulfur involved). However, the lowest δ³⁴S values (–38.49 to –37.53 ‰) of Py-6 may reflect the reduction of metamorphic sulfur by bacteria. Py-1 and Py-2 (Au-poor and As-poor) have different sulfur sources, while Py-3 (a, b), Py-4, and Py-5 (Au-rich and As-rich) have similar sulfur sources. It cannot be ruled out that deep magmatic activity plays an important role in the activation and migration of Au from metamorphic sedimentary strata (such as providing heat sources), but many geological and geochemical characteristics indicate that it may belong to an orogenic origin.