Multiple Isotopic Constraints on the Origin of the High-Grade Chaxi Gold Deposit in the Jiangnan Orogenic Belt, South China

Abstract

The Jiangnan orogenic belt in South China is known for its numerous gold deposits that are hosted by Precambrian low-grade metamorphic rocks that were reworked during Paleozoic and Mesozoic orogenic events, which resulted in multiple epochs of gold mineralization. The Chaxi gold deposit, located in southwestern Hunan in the central portion of the Jiangnan orogenic belt, is characterized by localized ultrahigh-grade gold mineralization (visually estimated to be up to several tens of percent). Despite its economic significance, the timing of formation, gold enrichment processes, and origin remain ambiguous; yet these are critical for both exploration and our understanding of gold metallogenesis in the Jiangnan orogenic belt. In addition to the pre-ore quartz-sericite-pyrite stage (stage I), several vein stages associated with primary gold mineralization and alteration have been identified at Chaxi: stage II quartz-chalcopyrite-electrum veins with pyrite alteration, stage III quartz-dolomite-polymetallic sulfides-native gold veins with sericite alteration, and stage IV quartz-dolomite-polymetallic sulfides-native gold veins with chlorite alteration. A supergene stage (stage VI) containing secondary gold mineralization is also present. The Ar-Ar age of sericite in stage I and the Re-Os age of molybdenite-galena intergrowths in stage II are 430.4 ± 2.7 Ma (plateau age; mean square of weighted deviates [MSWD] = 0.82) and 430.6 ± 1.1 Ma (weighted mean model age; MSWD = 0.38), respectively, demonstrating that the gold mineralization is related to the Paleozoic intracontinental orogeny and occurred ~10 m.y. after the metamorphic peak age. Ore-related sulfides from the primary mineralization stages and native gold from superenriched gold ores show negative to zero Δ199Hg values (–0.34 to 0‰), indicating that the ore-forming metals were sourced from the Precambrian metamorphosed volcanic-sedimentary rocks. The restricted Pb isotope signature of galena (207Pb/206Pb = 0.909–0.925) and Sr-Nd isotope compositions of apatite (Sri: 0.710215–0.710392; ɛNd(t): –6.0 to –3.6) further suggest that the ore-forming fluid likely originated from the Neoproterozoic metamorphic basement rocks, with no evidence of contributions from magmatic-hydrothermal fluids. This is consistent with the absence of igneous rocks at Chaxi and the distinct age and Pb isotope compositions between the gold mineralization and diabase in southwestern Hunan. During the mineralizing process, intense sulfidation of the wall rocks controlled the precipitation of compositionally homogeneous electrum (gold fineness of 738–774) in stage II. The continuously decreasing δ34S values of sulfides from stage II to stage IV (stage III: 7.94–18.78‰, stage IV: 2.03–10.90‰) may be a result of phase separation triggered by a fault valve cycle, an interpretation that is supported by the presence of hydrothermal breccias in stage III and stage IV, and by the fact that stage III veins were reopened and refilled by stage IV veins. Primary gold in stages III and IV occurs as heterogeneous native gold grains with varying gold-fineness patches, as well as homogeneous native gold and petzite intergrown with bismuth and tellurium minerals. The heterogeneous native gold may have been generated by instability of Au-Ag complexes as a result of phase separation, while the intergrowth of homogeneous native gold and petzite with bismuth and tellurium minerals may be related to the scavenging of gold by Bi-Te melts. Secondary enrichment may be crucial for the generation of superenriched gold mineralization at Chaxi, as suggested by the fact that native gold in superenriched gold ores has an elevated fineness (~960), a porous texture, and coexists with goethite. The formation of secondary gold may have resulted from the release of nanoscale inclusions from primary minerals to secondary minerals, as indicated by the presence of nanoscale gold in these minerals and mineral assemblages. The present study demonstrates that the early Paleozoic is a crucial epoch for forming orogenic gold deposits in the Jiangnan orogenic belt and provides new insights into gold superenrichment mechanisms in orogenic gold systems.