In situ trace element and sulfur isotope of pyrite constraints the mineralization process of haoyaoerhudong gold deposit in Inner Mongolia, China

With more than 11,000 t of gold resources, the Central Asian orogenic belt is the most significant gold deposit belt in the world. The majority of the orogenic belt's gold resources are found in a layer of carbon-rich black shale. However, there is disagreement regarding the origin and metallogenic process of such a significant quantity of gold. The Haoyaoerhudong gold deposit is located where the northern margin of the North China Craton and the southern margin of the Central Asian orogenic belt converge. It is the most significant black shale gold deposit in the northern margin of North China Craton gold province. The pyrite that typically develops in the Haoyaoerhudong gold deposit has been categorized into five varieties through comprehensive field investigation and mineralogical research, which correspond to five metallogenic stages: Stage 1, sedimentary diagenesis; Stage 2, tectonic deformation; Stage 3, hydrothermal; Stage 4, hydrothermal transformation; Stage 5, late metallogenic. For pyrite in the previous four metallogenic stages, in situ LA-ICP-MS trace element analysis and pyrite sulfide isotope analyses were performed. The results suggest that: The average Au contents in the pyrite of sedimentary diagenesis stage is 0.098 ppm; the average Au contents in the pyrite of the tectonic deformation stage and hydrothermal stage, is below the detection limit mostly; the average Au contents in the pyrite of hydrothermal transformation stage is 0.12 ppm. The results indicate that only 22.4% more gold is present in hydrothermal transformation stage than in sedimentary diagenetic stage overall. It may be inferred that the gold enrichment of Haoyaoerhudong gold deposit mostly took place during the sedimentary diagenetic stage; subsequent brittle-ductile shear and hydrothermal fluid activity did not result in a further enrichment of gold. The sulfur isotope test results of various metallogenic stages in the deposit can be analyzed, and they are generally consistent. The delta 34S values range from +10.15% to +16.47%, with an average value of +13.02%. It suggests that there might be a single source of sulfur. According to extensive analysis, the Haoyaoerhudong gold deposit formed a relatively low-grade ore body during the sedimentary diagenesis stage, and the subsequent tectonic deformation stage and hydrothermal stage provided physical conditions for further activation of gold metal but did not bring corresponding material sources for mineralization.