Origin and mineralization significance of the ore-hosting Guishan Formation at the Laowan Gold Deposit in Qinling-Dabie Orogen, Central China

Abstract

Mesozoic lode-gold deposits in the Qinling orogen are important gold reservoirs in China. The Laowan gold deposit is one of the largest one in this region. The ore bodies are fault-controlled, sulfide-rich lodes and disseminations that hosted in amphibolite and mica schist of the Guishan Formation intruded by a Cretaceous pluton at ca. 120 Ma. Long-standing controversies are whether the Guishan Formation provided ore-forming materials or just create a favorable setting for gold deposition. Here, we discussed the mineralogy and geochemistry of this rock formation to pinpoint its contributions to gold mineralization. Studies on metamorphic petrographic and whole-rock geochemical characteristics of the Guishan Formation were carried out to reveal the classification and the tectonic environment of metamorphic protolith. In-situ race element and sulfur-lead isotopic geochemical characteristics of pyrites from the ore samples are compared with those of the Guishan Formation to explore their relationships. Results shows that the amphibolite has tholeiite-like major and trace element compositions with a continental affinity, and the mica schist has geochemical characteristics similar to sedimentary rocks deposited in active continental forearc margin. Significantly enrichment of Au (avg.13 and 22 ppb), Ag (0.14 and 0.17), As (7.81 and 9.26), and Sb (0.47 and 0.39) contents (in ppm) in amphibolites and mica schists of the Guishan Formation suggests they might be a potential ore-forming material source of the gold deposit. The high Ni content (avg. 103 and 38.6 ppm), low Co/Ni ratios (0.41 and 0.5) and δ 34 S values (0.73-2.05‰ and 1.83-2.98‰) in pyrites from the pre-ore and early main-ore stages resemble those from the Guishan Formation (Ni=283 ppm, Co/Ni=0.4, δ 34 S=-0.1-+2.67‰). The similar Pb isotopic composition between the Guishan Formation and pyrite from the early stage of the gold mineralization also suggests a genetic relationship between the two. With the late mineralization stage, magmatic hydrothermal fluids dominated the gold mineralization.