Metallogeny of the Xiaotongjiapuzi gold deposit, Liaodong Peninsula (Eastern China): Perspective from sulfide trace element geochemistry and sulfur isotopes

Abstract

Most gold ores have variable sulfur isotopic and trace elemental compositions at micro-/nano-scale, which makes the understanding of the gold formation process more challenging. The advancement of in-situ analytical techniques offer opportunity to gain insight into gold mineralization processes and ore-material source. The Xiaotongjiapuzi is a representative intrusion-related gold deposit in the Qingchengzi orefield of the Liaodong Peninsula, Eastern China. Alteration and gold mineralization comprise four stages: (I) quartz-pyrite-Au, (II) quartz-pyrite-biotite-Au, (III) Au-barren quartz-pyrite, and (IV) post-ore quartz-calcite. Detailed scanning electron microscope (SEM) backscattered electron (BSE) imaging, electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) elemental and LA–multi-collector (MC)–ICP–MS sulfur isotope analyses were conducted to reveal the ore-material source and mineralization process. Three texturally distinct auriferous pyrite generations were identified: Py1 and Py2 (in both stage I and II) and Py3 (in stage II and III). Py1 is porous and weakly fragmented, poorly crystallized, and overgrown by Py2. Py2 is commonly intergrown with arsenopyrite and marcasite, whilst Py3 overgrew around Py2 or replace it.Gold exists mainly as solid solution (Au+) in the pyrite lattice and nanoparticles (Au0), as well as (rare) micron-scale inclusions along the Py2-Py3 interface. Py2 has the highest gold content (mean 47 ppm, max 344 ppm), especially when intergrown with marcasite (Mrc1). The δ34S values of pyrite (5.71–15.31‰, n = 18) and marcasite (-19.59 to 7.45‰. n = 6) are the highest and lowest, respectively, and the values increase steadily from Py1 to Py3 (Py1: 5.71–7.54‰ → Py2: 7.44–9.55‰ → Py2 + Mrc1: 12.88–13.92‰ → Py3: 15.17–15.31‰). The Paleoproterozoic Dashiqiao and Gaixian formation host rocks contain large amount of sulfate minerals (e.g., barite and gypsum), and the coeval global sulfate has heavy sulfur isotope enrichment, suggesting that sulfate reduction may have provided H2S-rich fluids [Au(HS)2- and/or Au(HS)0] for the Mesozoic mineralization. For the multistage gold enrichment at Xiaotongjiapuzi, we suggest that the Paleoproterozoic sedimentary and metamorphic rocks may have formed the initial sulfur reservoir, which was then reactivated by Mesozoic magmatic-hydrothermal events.