Abstract
The Zhengchong gold deposit is located in the central segment of the Jiangnan Orogen in northeastern Hunan Province, South China. The host rocks of this deposit are the Neoproterozoic slates of the Lengjiaxi Group and granodiorite. The structures in the Zhengchong gold deposit are dominated by NE-trending reverse faults, which control the gold-bearing veins. The orebody consists of NE-trending laminated quartz veins and NW-trending quartz veins. The alteration styles include silicification, carbonatization, sulfidation, sericitization and chloritization. The Zhengchong gold mineralization can be divided into four stages: Quartz-pyrite (stage I), quartz-pyrite-arsenopyrite (stage II), quartz-polysulfide (stage III) and quartz-carbonate (stage IV). Three generations of hydrothermal pyrite were identified: Disseminated euhedral to subhedral cubes in altered wall-rock (PyI), euhedral to subhedral cubes inter-grown with arsenopyrite and tetrahedrite in quartz veins and wall-rock (PyII), and euhedral cubes with microinclusions (native gold, galena, sphalerite, chalcopyrite, tetrahedrite, and pyrrhotite) or metasomatic textures in sulfide-rich veins or veinlets (PyIII). PyII and PyIII are arsenian pyrite and represent the main Au-bearing minerals. PyI records the lowest concentrations of Au; PyII and PyIII record similar amounts of Au, Cu, Pb, Zn, and Bi, but PyIII is more enriched in Co, Ni, Te, and Se. The substitution of As, Se and Te for S and that of Co and Ni for Fe occurs by direct-ion exchange. Invisible gold is uniformly distributed within the arsenian pyrite, and visible gold fills microfractures in PyII or occurs as inclusions in PyIII. Co, Ni, Cu exhibit positive correlations with Au and a negative correlation between Au + Cu + Co + Ni and Fe reflect that Fe vacancies may have been a major cause of the precipitation of invisible Au and other metal elements in pyrite structure. There are systematic trace element differences between the three generations of pyrite (PyI, PyII, PyIII). The more Co, Ni and Se, Te substitution that occurred for Fe and S, respectively, the greater the increase in the Co/Ni ratio (<1) and the decrease in the Se/Te ratio (<10) in stage III, indicating that a more reduced, lower-temperature metamorphic hydrothermal fluid was present in stage III.
Highlights
Gold deposits in metamorphic terranes can be classified as orogenic gold deposits or reduced intrusion-related gold deposits [1,2,3]
We show that systematic differences exist between different generations of pyrite, and we use these data to better understand the evolution stages of ore systems and characterize their ore-forming processes
The ore minerals are mainly composed of pyrite and arsenopyrite; lesser amounts of galena, The ore minerals are mainly composed of pyrite lesser amounts of galena, sphalerite, chalcopyrite, tetrahedrite, pyrrhotite, rutileand andarsenopyrite; native gold formed during stage II and sphalerite, chalcopyrite, tetrahedrite, pyrrhotite, rutile and native gold formed during stage II and stage III (Figure 6)
Summary
Gold deposits in metamorphic terranes can be classified as orogenic gold deposits or reduced intrusion-related gold deposits [1,2,3]. Reduced intrusion-related gold deposits are intrusion-hosted, sheeted arrays of thin, low-sulfide quartz veins with Au–Bi–Te–W signatures [2,6]. Some models have explained the role of fluids in gold mineralization, including a decrease in solubility that occurs during drops in pressure associated with earthquakes in the case of quartz veins [9,10] and the destabilization of gold bisulfide complexes by a redox reaction that occurs between the transporting fluid and iron in the host rocks, which precipitates both pyrite and free gold [7,11].
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