Mineralogical Study of Gold-Bearing Arsenian Pyrite in the Laojunmiao Gold Deposit, South Qinling, and Its Geological Significance

Objective‌ The Laojunmiao gold deposit, located between the Gongguan and Qingtongou super-large Hg

-Sb deposits, is a medium-size gold deposit recently discovered in the South Qinling Orogenic Belt .

Methods The gold mineralization mainly hosted in the clastic-carbonate rocks of the Upper Devonian Xichahe Formation. Controlled by stratigraphy and structure, the orebodies are distributed along the EW-trending structural fracture zone, and the mineralization is relatively concentrated at the intersection of faults.

Results However, there is still a lack of systematic research on the distribution pattern of gold in minerals and the types of deposits. Based on field investigations, mineralogy observations, backscattered electron (BSE) imaging, and electron probe micro probe analysis (EPMA), the compositions of the gold-bearing minerals and the occurrence state of Au in the Laojunmiao gold deposit are determined, and the deposit's genesis was discussed. The results show that the mineralization process of the Laojunmiao gold deposit can be divided into three stages: sedimentary-diagenetic stage, hydrothermal mineralization stage, and supergene oxidation stage. The gold mineralization during the hydrothermal stage is predominantly fine-grained disseminated type, which can be subdivided into three substages: Quartz + Arsenic-bearing pyrite stage (Ⅰ), Quartz + Arsenic-bearing pyrite + Arsenopyrite + Sphalerite stage (Ⅱ), and Quartz + Calcite stage (Ⅲ). Arsenic-bearing pyrite and arsenopyrite are both gold-bearing minerals, but pyrite is the primary gold-bearing mineral. Both sedimentary-stage pyrites (Py1-Py2) and hydrothermal-stage pyrites (Py3 and Py4) contain gold. The gold content of hydrothermal-stage pyrites is significantly higher than that of sedimentary-stage pyrites, and there is a positive correlation between Au and As. Gold primarily exists as invisible gold. Au in sedimentary-stage pyrites (Py1 and Py2) occurs primarily as gold nanoparticles (Au0); while gold in hydrothermal-stage pyrites (Py3 and Py4) and arsenopyrite (Apy) occurs as lattice gold (Au+).

Conclusion The results indicate that the Laojunmiao gold deposit show similargeological characteristics, metallogenic conditions, and gold occurrence modes with typical Carlin-type gold deposits. Compared with the δFe-δS, δFe/δS-As, and (Fe+S)-As systems of various genetic types of gold deposits in China, the pyrite mineralization during the mineralization period all falls within the Carlin-type gold deposit region. Therefore, the Laojunmiao gold deposit could be classified as a Carlin-type gold deposit.