The Characteristics And Genesis Of The Xinqiao Cu-S-Fe Deposit, Tongling, Anhui Province

Xinqiao Cu-S-Fe deposit is a typical superimposed deposit that is representative in the Tongling ore deposit concentrated area. Based on previous studies on this deposit and fieldwork, this study mainly focuses on the characteristics of ore deposit geology, mineralogy and geochemistry of this deposit and intends to clarify the metallogenic conditions and formative processes.Geological investigation indicates the distribution of major orebodies in the detached fracture zone between the Wutong Formation and the Huanglong Formation and the contact zone between Jitou intrusion and carbonate strata. Occurrence of the ore bodies were controlled by two different groups of faults. One is an interlayer fracture zone nearly parallel to the strata strike, and the other is a NW trend cross fault perpendicular to the strata strike. Data of ore type, mineralogical assemblage,orebodies texture and structure advocate the division of the ore bodies into three different types :stratabound sulfide, stratabound skarn, and contact metasomatic skarn types. The stratabound sulfide type of ore bodies is mainly characterized by the existence of stratabound net-veined, layered, and brecciated-type copper-bearing pyrite ores. In contrast, the stratabound skarn type of ore bodies is featured by the existence of massive copper-bearing magnetite and layered copper-bearing magnetite-pyrite ores, and the contact metasomatic skarn type of ore bodies by the occurrence of copper-containing skarn ores.Microscopic observation and field investigation show the occurrence of difference in alteration among the three types of ore bodies. The stratabound sulfide type of ore bodies essentialy includes a lower temperature mineral association of alteration, which is characterized by silicification and carbonation and has no obvious zoning. The stratabound skarn type of ore bodies includes a medium temperature mineral association of alteration, which is featured by serpentinization and tremolization and has an evident zoning controlled by composition of primary sedimentary rocks. The contact metasomatic skarn type of ore bodies consists mainly of a high temperature mineral association of alteration, which is characterized bygarnetization, diopsidization, actinolitization, and epidotization and has a typical contact metasomatic skarn zoning.According to vein penetration relation and mineral assemblage, the formation process of the Xinqiao deposit could be classified into primary sedimentary and magmatic-hydrothermal periods. In the primary sedimentary period of mineralization occurred the stratabound sulfide type of ore, which is compositionally predominated by sulfur and mineralogically characterized by colloform and oolitic pyrite. In the magmatic hydrothermal period of mineralization existed the stratabound and contact metasomatic skarn types of ore, which is compositionally predominated by copper and characterized by development of penetration and replacement texture and re-crystallization structure. The magmatic hydrothermal period of mineralization can be further divided into dry skarn stage, wet skarn stage, oxide stage, quartz-sulfide stage, and carbonate stages.Petrographic observation proves the occurrence of two types of pyrite with different shapes, i.e., colloform pyrite and crystalline pyrite. The colloform pyrite has layered and oolitic structures, and the crystalline pyrite displays replacement remnant and euhedral crystal textures. This fact combined with their ratios of Fe/S and Co/Ni indicates the formation of the early by sedimentation and the late by hydrothermal activity. Trace and rare earth element analyses of magnetite are consistent with the genesis of the Xinqiao deposit by superimposition of hydrothermal replacement on sedimentation. Major and rare earth elements analyses of garnet support an origin of the high temperature skarn in the deposit by typical contact metasomatism.The obtained ore deposit geological, mineralogical and ore deposit geochemical data support an origin of of the Xinqiao Cu-S-Fe deposit by superimposition of hydrothermal replacement on sedimentation. The stratabound sulfide type of ore characterized by colloform pyrite was formed in the Paleozoic period of sedimentary mineralization. In the Mesozoic period of magmatic hydrothermal mineralization,contact replacement which was occurring in contact zone between the intrusion and carbonate strata give rise to the formation of the contact metasomatic skarn type of ore, and modification of the previous stratabound sulfide type of ore by magmatichydrothermal fluid migrating along interlayer fracture zone at the same time led to the formation of the stratabound skarn type of ore.