Metallogeny of the Jinming gold deposit in northeast Hunan, Jiangnan Orogen: Constraints from in situ sericite Rb-Sr dating, pyrite trace elements and S isotope geochemistry

Abstract

The newly discovered Jinming gold deposit in northeastern Hunan province (Eastern China) is tectonically located in the Jiangnan Orogen. The fault-controlled auriferous quartz ore veins are primarily developed in the low-grade metamorphosed Neoproterozoic Lengjiaxi Group. The alteration/mineralization comprises stage I quartz-pyrite mineralization, stage II quartz-native gold-polymetallic sulfide mineralization, and stage Ⅲ quartz-carbonate veining. Our in situ sericite Rb-Sr dating on stage II ore yielded an Early Devonian age of 412.4 ± 8.4 Ma (MSWD = 1.1). Five types of pyrite were identified, i.e., Py1 (Py1a and Py1b) in stage Ⅰ, and Py2 (Py2a, Py2b, and Py2c) in stage Ⅱ. Py1 has relatively low trace element contents and narrow δ³⁴S range (Py1a: −5.53 to −4.82 ‰, Py1b: −5.29 to −4.72 ‰), whereas Py2 is Au-As-Co-Ni rich with a broader δ³⁴S range (Py2a: −6.00 to −4.94 ‰, Py2b: −13.80 to −7.99 ‰, Py2c: –23.45 to −18.70 ‰). The distinct trace element and sulfur isotope features in Py1 and Py2 imply major physicochemical changes in the ore-forming fluids from stage I to II. At Jinming, gold occurs as native gold and refractory gold (Au⁺) in pyrite and arsenopyrite from stage II, and the invisible gold is primarily found as solid solution gold (Au⁺) within the lattice structure of pyrite. A marked positive correlation between the concentrations of Au and arsenic (As) in pyrite suggests a strong relationship between Au enrichment and As content. The pyrite’s element and sulfur isotope features imply a deep metamorphosed strata source for the ore material at Jinming. Geological and geochemical evidence suggests that the Jinming gold deposit is best classified as an orogenic type, associated with the Caledonian intracontinental orogeny.