Insights into the genesis of orogenic gold deposits from the Zhengchong gold field, northeastern Hunan Province, China

Abstract

The Zhengchong gold deposit is located in the central Jiangnan Orogen Belt, where gold ore occurs mainly as quartz veins, altered slates, and minor breccias hosted by shear zones in the Lengjiaxi Group. Based on its vein crosscutting relationships, mineral assemblages, and paragenetic sequences, mineralization in this deposit is divided into four stages: Stage 1: quartz(Q1)–pyrite stage, Stage 2: quartz(Q2)–pyrite–arsenopyrite stage, Stage 3: quartz(Q3)–pyrite–polymetallic sulfide mineral stage, and Stage 4: quartz(Q4)–carbonate stage. Two types of fluid inclusions have been recognized in the quartz grains: CO2-bearing fluid inclusions (C-type) and aqueous fluid inclusions (W-type). Fluid inclusions in hydrothermal quartz yield homogeneous temperatures and salinities of 312–406 °C and 4.65–9.21 wt% NaClequiv (Stage 1), 252–324 °C and 2.20–8.10 wt% NaClequiv (Stage 2), 193–296 °C and 2.24–14.04 wt% NaClequiv (Stage 3), and 127–209 °C and 2.57–6.59 wt% NaClequiv (Stage 4). The co-occurrence of these inclusion types despite their variable fluid salinities can be explained by fluid boiling during Stages 2 and 3 of mineralization, possibly as a result of episodic pressure decreases linked to fault-valve activity. Laser Raman spectroscopic analysis results reveal that small quantities of CH4 and N2 are present in the Stage 2–3 fluid inclusions. Fluid inclusion data and microthermometric results show that the ore fluids were low-salinity and CO2-rich, yielding both CO2 inclusions (C-type) and aqueous inclusions (W-type). The trapping pressures estimated from the C-type inclusions are 80–180 MPa for Stages 2 and 3, yielding gold mineralization depths of ∼7–8 km. The mineralizing fluids had calculated δ18OH2O values ranging from +3.7‰ to +8.2‰ and δD values from −75.2‰ to −61.8‰, indicating derivation mainly from metamorphic fluids. The sulfide minerals yield δ34S values of −8.9 to −0.1‰ and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 17.83–18.12, 15.67–15.76, and 38.03–38.62, respectively, which differ significantly from the isotopic composition of the granodiorite (δ34S = −2.4‰ to +1.0‰; 206Pb/204Pb = 18.57–19.28; 207Pb/204Pb = 15.69–15.76; 208Pb/204Pb = 38.97–40.34). However, the S–Pb isotopic compositions of the sulfide minerals are similar to those of slates of the Lengjiaxi Group in the study area. The Zhengchong gold deposit probably originated from a metamorphic fluid system characterized by low salinity, low density, and CO2 enrichment with limited admixture of magmatic fluids. Based on similarities to other orogenic gold deposits, we infer that the Zhengchong orebody represents an orogenic gold deposit formed during the Late Ordovician Jiangnan Orogeny caused by collision of the Yangtze and Cathaysia blocks.