Abstract:

The Longtougou gold metallogenic belt is located in Shanyang County, Shaanxi Province. Four gold deposits have been discovered in this belt since 21st century, containing at least 20 t Au. The orebodies are hosted in the middle Devonian Dafenggou Formation and occur as pyrite-quartz veins. In order to find out the ore-forming process, two typical orebodies were chosen for careful study in this paper. Mineralization process can be divided into three stages, including silicification stage(early stage),quartz-sulfides stage(main ore-forming stage) and carbonation stage(late stage). In the main ore-forming stage, pyritohedron pyrite is the major gold carrier mineral. Furthermore, it is found that Au is unhomogeneous and local enrichment in broken gold-bearing pyrite during the later tectonic activation stage. The trace element compositions of gold-bearing pyrite and sulfur isotope studies revealed that the Neo-proterozonic Yaolinghe Formation is mostly likely the dominant source of gold based on their similar REE patterns and sulfur composition. Besides, Yaolinghe Formation has higher Au concentration than any other geologic bodies in the Longtougou metallogenic belt. The Co/Ni ratios of the pyrite range from 1.14 to 1.83, indicating a deep magmatic origin for the mineralization. Ore-forming fluid is characterized by higher oxygen fugacity since the δEu positive anomalies preserves not only in ores but also in pyrite grain. The geologic thermometer of sulfur isotope shows that the temperature of mineralization is 200-260 ℃. Thus, we proposed that the gold deposits in this belt are possibly related to magmatic activity beneath the Longtougou anticline at late Indo-Chinese epoch. Hydrothermal fluid, which exsolved from the magma, extracted gold from Yaolinghe Formation and then migrated upward and precipitated in secondary fractures of Longtougou fault later.

Key words: Longtougou metallogenic belt, gold deposit, pyrite, element geochemistry, sulfur isotope