Ore characteristics of the Yangshan gold deposit in Gansu Province and its genetic significance

The Yangshan gold deposit in Gansu Province is located in the east of Songpan-Ganzi fold system and is a large-sized disseminated gold deposit. Field investigation reveals that the orebody occurs in the internal or external contact zones between plagioclase granite porphyry dikes of Yanshannian period and phyllite of Shanhekou Formation of Middle Devonian period as well as in the shatter zone of the wall rocks. Microscope identification, electron microprobe analysis and ore composition analysis were carried out to study the ore fabrics, mineral compositions and gold occurrence in the ore systematically and thoroughly. The authors found that the pyrite of hydrothermal genesis was the main gold-bearing mineral and the pyrite was genetically formed by biochemical sedimentation which had played an important role in the formation of mineral deposits and enrichment of metallogenic materials; the authors also found that the source bed was Middle Devonian Shanhekou Formation. The ore texture and structure show that the formation of the gold deposit experienced sedimentation and hydrothermal mineralization with multiple stages during the ore-forming process. Studies of stable isotopes show that the major metallogenic matters came from the source bed and Yanshannian magmatite. The result indicates that the deposit is a stratabound gold deposit related to medium-low temperature magmatic hydrothermal activities.