Texture, in-situ geochemical, and S isotopic analyses of pyrite and arsenopyrite from the Longshan Sb-Au deposit, southern China: Implications for the genesis of intrusion-related Sb-Au deposit

Abstract

Longshan is an important Sb-Au ore deposit (3.7 Mt @4.5 wt% Sb and 4.6 g/t Au) in the Xiangzhong metallogenic province (XZMP), southern China. The Sb-Au mineralization is characterized by vein ores comprising quartz, stibnite, native gold, auriferous pyrite, and arsenopyrite within the Neoproterozoic Jiangkou Formation slate. The paragenesis of the Longshan deposit can be divided into three stages, i.e., (1) stage 1 (pre-ore) composed of euhedral coarse to nodular sedimentary pyrite and deformed irregular quartz veins, (2) stage 2 marked by NE-striking and NW-dipping quartz, stibnite, pyrite, arsenopyrite, and minor native gold veins, and (3) stage 3 characterized by NW-striking and NE-dipping quartz, stibnite, pyrite, arsenopyrite, scheelite, and minor native gold veins. Seven subtypes of pyrite and four subtypes of arsenopyrite from the three stages are recognized based on mineral assemblages, textures, and geochemistry. Visible gold shows the complexity with three distinct categories, including Au1 from stage 2, Au2 and Au3 from stage 3. In this study, we integrated laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses to document textural, compositional, and S isotopic variation among texturally complex pyrite and arsenopyrite assemblages, and constrain the genesis of the Longshan Sb-Au deposit.Py1a and Py1b of sedimentary origin from stage 1 are enriched in δ34S (from 13.3 to 16.9‰, average 15.3‰, n = 8) and have low Au concentrations (from 0.43 to 4.10 ppm, n = 37). Two subtypes of pyrite (Py2a–Py2b) and arsenopyrite (Apy1a–Apy1b) from stage 2 are characterized by oscillatory zoning textures in backscattered electron (BSE) images and are obvious by their relatively high content of invisible Au (up to 351 and 1357 ppm for pyrite and arsenopyrite, respectively) and restricted range of δ34S values (from 2.1 to 4.7‰, average 3.7‰, n = 19). Three subtypes of pyrite (Py3a–Py3c) and two subtypes of arsenopyrite (Apy2a–Apy2b) from stage 3 have relatively low Au concentrations (up to 95.1 and 64.9 ppm for pyrite and arsenopyrite, respectively), and show a narrow variation in δ34S values (from –2.5 to 3.7‰, average 0.8‰, n = 22). These data suggest that magmatic-hydrothermal ore-forming fluids contribute to most of the Au and S budget in the Longshan Sb-Au deposit.The major differences between mineralization in terms of texture, trace elements, and S isotopic compositions of pyrite and arsenopyrite can reflect contrasting mechanisms of Au precipitation and provide implications for the genesis of this intrusion-related Sb-Au deposit. The new data underpin the importance of performing complementary in-situ mineralogical analyses to elucidate the source and evolution of ore-forming fluids and enable correct interpretation of the hydrothermal Sb-Au system architecture.