Occurrence and Enrichment Mechanism of Gold in the Qiuling Carlin-Type Gold Deposit,Zhen'an County,Shaanxi Province,China

Located in the western Qinling orogen,the Qiuling gold deposit is a typical Carlin-type gold deposit.Gold mineralization is hosted in calcareous siltstones,silty shales,and argillaceous limestones of the Upper Devonian Nanyangshan Formation and the Lower Carboniferous Yuanjiagou Formation.Pyrite and arsenopyrite dominate the metallic minerals,whereas quartz,calcite,and sericite are the main non-metallic alteration minerals.An integration of optical microscopy,scanning electron microscopy equipped with energy dispersive system,electron microprobe,and laser ablation ICP-MS was used to study the occurrence,distribution,and enrichment mechanism of gold in the Qiuling gold deposit.The results indicate that gold occurs as submicroscopic and,less significantly,microscopic species.The submicroscopic gold is present as nanoparticles of native gold(Au0) in the coarse-grained pyrite and solid solution(Au+) in the arsenian rims of zoned pyrite and arsenopyrite,with lesser amount of gold nanoparticles in the As-deficient core of zoned pyrite.The sub-microscopic gold particles in the core of zoned pyrite were likely formed during the diagenesis of the host rocks through adsorption,while the structure-bounded Au(Au0 and Au+) in fine-grained zoned pyrite and arsenopyrite may have been transported as sulfur and arsenic complexes in ore-forming fluids of the main hydrothermal stage.Microscopic gold grains are mostly less than 3 to 5 μm in size and are contained in the marginal areas fine-grained pyrite and included in hydrothermal sericite,quartz,and calcite.They formed likely due to local saturation of gold in the ore fluids or as a result of leaching of the submicroscopic gold by late-stage fluids and subsequent re-precipitation.