Characteristics And Origin Of The Baocun Skarn Gold Deposit,Tongling,Anhui Province

Located in the northeastern Shizishan orefield, the Baocun Au deposit is a typical and representative independent skarn gold deposit in the Tongling area, with very high research value. Detailed orefield geology, ore deposit geology, mineralogy,and geochemistry studies have been carried out on the Baocun Au deposit. On the basis of these studies, its origin has been discussed.The field investigation indicates the occurrence of the orebodies of the Baocun Au deposit in the contact zone between quartz dioritic intrusion and Lower Triassic carbonate strata and the control by F1 and F2 faults and fZ fracture zone. Based on mineral assemblage and texture and structure, gold ores in the Baocun gold deposit can be divided into taxitic, brecciated and vein-banded types. The taxitic type of ores is characterized by an assemblage of magnetite and arsenopyrite, etching-skeletal and replacement remnant textures, and taxitic and disseminated structures, while the brecciated type of ores by an assemblage of galena and sphalerite, euhedral to subhedral granular textures, and brecciated and massive structures. In contrast, the vein-banded type of ores has an assemblage of colloform pyrite and various Bi-bearing minerals, anhedral grain crystal texture, and vein and banded structures.The field investigation and microscopic observation display the existence of typical zoning of wall rock alteration and mineralization in the Baocun gold deposit,from the intrusion to carbonate wall rock, occurring quartz diorite, cryptoexplosive breccia, diopside skarn, garnet skarn, skarn orebody, breccia orebody, cryptoexplosive breccia, and marble with hornfels. According to vein penetration relation and mineral assemblage, the formation process of the Baocun gold deposit can be divided into high temperature pneumatolytic, middle to high temperature hydrothermal, and low temperature hydrothermal periods. The high temperature pneumatolytic period can be further divided into dry and wet skarn stages. The dry skarn stage is characterized by the formation of garnet and diopside, while the wet skarn stage by the formation of tremolite, epidote, and magnetite. The middle to high temperature hydrothermal period includes only a stage of quartz-sulfide stage with the formation of varioussulfide and Bi-bearing minerals. The low temperature hydrothermal period has only a stage of carbonate stage marked by the formation of carbonate veins along fractures.The field investigation result shows that crystalline pyrite occurs as crystal grains in both of the taxitic ore and the brecciated ore, while the colloidal pyrite as nodules of 500 to 800 μm size in the vein-banded ore. The microscopic observation indicates the occurrence of small pyrite grains as a rim surrounding a colloidal pyrite nodule.This fact combined with the ratios of w(Fe)/w(S), w(Co)/w(Ni), and w(S)/w(Se)proves that the two types of pyrite in the Baocun Au deposit were not formed by sedimentation, but by hydrothermal mineralization. The major and rare earth elements analyses of garnet show an origin of the deposit by contact metasomatism.The obtained various data support the formation of the Baocun gold deposit by a complex mineralization process. The intrusion of quartz dioritic magma into the carbonated wall rocks in the Yanshanian period resulted in an intensive contact metamorphism, forming skarn with the taxitic type of ores. Then the upwelling of hydrothermal fluid along the fracture zone caused a hydrothermal cryptoexplosion,forming the brecciated type of ores. During the late period of mineralization,hydrothermal filling was responsible for the formation of the vein-banded type of ores.