Petrography And Genesis Of Boka Au (Cu) Ore Deposit, Dongchuan, Yunnan Province

The Boka Au (Cu) poly-metallic ore deposit in the Dongchuan district of Yunnan Province is a perspective large-scale ore deposit. The genesis of the Boka ore deposit is closely related to the intrusive rocks in the research area. However, the petrography, magma composition and evolution, as well as its relationship with mineralization still lack systematic study.The Boka Au (Cu) poly-metallic ore-field is located in the northwest of Dongchuan district, Kunming city, and situated between the Jinsha River and Xiao River. Geographically, it is located in the north of Yunnan-guizhou plateau, separated from Sichuan province by the Jinsha River. Geotectonically, the studied area is located in the western margin of the Yangtze plate, to the east of the Deqin-MengLian Permian volcanic belt, and to the northwest of Majiang early Mesozoic suture zone in Vietnam. The research area is adjacent to the N-S rending Panxi rifting zone, with extensive development of folds and faults, and has experienced poly-phases of magma activity. Numerous copper, gold, iron and other minerals deposites have been found in this area. This study systematically studied the petrography and mineralography features, mineralization enrichment mode of the Boka poly-metallic mining area, and further discussed the formation mechanism of the ore deposit. A synthetic study of microscopy, chalcography, mineralization enrichment mode of the Boka Au (Cu) polymetallic ore deposits shows that the sodium rich diabase (gabbro) plutons are widely outcropped in the Boka area. On the top of sodium rich diabase (gabbro), they often experienced a higher degree of fractional crystallization, and gradually transited into hornblende albities, biotite albites and albites, etc. The Boka Au (Cu) ore bodies have close genetic relationship with albities or biotitealbities. From the albites to surrounding carbonaceous slate, Au (Cu) mineralization related wall rock alteration can be divided into two zones:(1) albite-quartz-errodolomite belt;(2) quartz-Fe-poor carbonatite belt. The transition zone between these two belts tend to be the locations of the highest sulphides and gold content, while the content of sulphides are apparently low in quartz and calcite veins that are far away from the albites.Alkali metal elements (e.g., K, Na) in the magma are also the dominant factors for the formation of the Au (Cu) ore deposits. In potassium and sodium rich hydrothermal solution, CO32-, Cl-, SO4-and F-ion often coexist with potassium and sodium ions. These acid radicals and Au, Cu ore-forming metal elements form the complex anion. The potassium and sodium ions, as cations, combined with complex anion and form complexes. Under the condition of high temperature and high pressure, these complexes are relatively stable. However, when the ore elements bearing solution rise to shallow positons along faults, lower temperature and pressure leads to hydrolysis of the complexes. Furthermore, potassium and sodium ions are engaged in alkali metasomatism with surrounding rocks. Therefore, the ore-forming metallic elements are released from complexes, this will lead to precipitation and enrichment of Ore-forming metallic elements. Therefore, the Boka Au (Cu) ore deposit was generated by a long-term evolution from diabase (gabbro) to albite magmatic hydrothermal mineralization system in Mesoproterozoic-Neoproterozoic. The genetic type of ore deposit can be attributed to magmatic hydrothermal Au (Cu) deposit.This study systematically studied the structural geology, petrology, mineral deposit multidisciplinary by methods of detailed field investigation, petrography and mineralography. This thesis gives a possible model for Au (Cu) mineralization, and provided valuable information for further theoretical explanation, including metal enrichment processes and building-ore dynamics.