Geology, geochemistry, and genesis of the Dashui Carlin‐type gold deposit in the West Qinling orogenic belt, Gansu Province, China

Abstract

The Dashui gold deposit occurs in the south of West Qinling orogenic belt where is belonged to a famous gold triangle area named “Shan‐Gan‐Chuan” in Northwest China. The presence of native Au associated with hematite alteration is unique in China. The ore bodies at Dashui mainly occur in the limestone and dolomitic limestone units within the Lower Triassic Maresongduo Formation. The granodioritic dykes that intrude the Triassic sedimentary rocks are locally mineralized because of later carbonate alteration. The ore bodies are vein, lenticular, cystic, and dendritic in shape and are controlled by NW‐, NNE‐ (near N‐S‐), and E‐W‐trending faults. Gold mineralization occurs commonly at fault intersections and contact zones between the carbonate and magmatic rocks. The NNE‐ (near N‐S‐) trending faults and associated pinnate fractures are most favorable loci for gold deposition. The metallic minerals include native Au, pyrite, cinnabar, and marcasite, which are mostly overprinted by an oxidized mineral assemblage of hematite and limonite. Gold mineralization is closely associated with decalcification, silicification, and carbonate alterations. Gold content in altered samples correlates with Si enrichment and Ca and Mg depletions. Rare earth elements (REE) increase with Al2O3 and Fe2O3 in altered carbonate rocks, indicating that an enrichment of REE may be closely related to argillic alteration and supergene weathering. The REE patterns of altered carbonate rocks are more likely consistent with granodiorite instead of the ones of fresh carbonate rocks. Moreover, δ34S values of Dashui deposit ranging from −1.8‰ to 4.1‰ indicate that the deep‐sourced magmatic hydrothermal fluids may have worked during the early stage mineralization. Altered carbonate rocks show higher δ13C and lower δ18O values than those of the original host rock. Therefore, the REE, S, C, and O data suggest that the ore‐forming fluids may be magmatic derived and later interact and exchange with host rock and meteoric water. The Dashui gold deposit is characterized by structural and stratigraphic controls; Au–As–Sb–Hg geochemical association; alteration of decalcification, silicification, and argillization; low base‐metals; high Au/Ag ratio; and low δ18O values. These features are consistent with Carlin‐type Au deposits in Western USA, suggesting that the Dashui deposit is a Carlin‐type deposit that formed in the Middle Jurassic to Early Cretaceous (170 to 135 Ma).