Metallogenesis of the Hengjiangchong gold deposit in Jiangnan Orogen, South China

Abstract

The Hengjiangchong gold deposit is located in northeastern Hunan of the central Jiangnan Orogen, South China. Distribution of auriferous sulfide–calcite–quartz vein-type orebodies are controlled by NW-/WNW-trending ductile shear zones, and hosted in the Lengjiaxi Group (Gp.) low-grade metamorphic sequences and the Hengjiangchong granite. Ore minerals include mainly pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, and native gold, whilst the major alteration styles include silicic, sericite, carbonate and chlorite. Alteration/mineralization can be divided into three stages: quartz–calcite–pyrite–arsenopyrite mineralization (Stage 1), quartz–calcite–polymetallic sulfide mineralization (Stage 2), and quartz–calcite ore-barren alteration (Stage 3). Two types of fluid inclusion (FI) are present in the auriferous sulfide–calcite–quartz ore veins: CO2-bearing (C) and H2O-rich (W) type. Petrographic and microthermometric analyses of the FIs yielded homogenization temperatures for Stage 1, 2, and 3 to be 254–377, 191–339, and 134–223 °C, respectively, with corresponding salinities of 2.22–10.37, 2.23–9.98, and 1.56–4.94 wt% NaClequiv. Pressures of Stage 1 and 2 mineralization are estimated to be 280–370 and 170–300 MPa, respectively. δ18O and δD values are determined to be 9.8–10.1‰ and −70.2 to −68.7‰ (Stage 1), 7.4–8.1‰ and −72.4 to −71.2‰ (Stage 2), and 2.7 to 2.9‰ and −79.1 to −73.0‰ (Stage 3), respectively. These results indicate that the primary ore-forming fluids were derived from a metamorphic source. For the auriferous sulfides, their δ34S values are of −15.4 to −7.5‰, whilst their 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values are of 38.663–44.861, 15.637–15.769, and 18.301–20.936, respectively. Both the stable and radiogenic isotopic data indicate that ore-forming fluids and metals were derived from a deeper and higher metamorphic grade source (e.g. underlying metamorphosed rocks). Fluid immiscibility and fluid–rock interactions were likely critical for the gold ore precipitation. The Hengjiangchong deposit exhibits many features of orogenic gold deposits, such as the structural control on orebody distribution, alteration and mineralization styles, and FI microthermometric and H–O–S–Pb isotopic features. Therefore, the Hengjiangchong is best classified as an orogenic gold deposit.