Study Of Ore Formation Conditions And Ore Deposit Models In Tongling Area, Anhui Province, China

Tongling area is one of the important Cu-Au deposit cluster areas in China. This paper focuses on ore formation conditions, ore deposit models, and prospecting targets in Tongling area. As a case study, the author analyses the forming process of the Xinqiao Cu-Au-Fe-S deposit—a typical and complex ore deposit in this area.The research on sedimentary environments in the Late Carboniferous shows that the layered ore deposits occurring in the Upper Carboniferous limestones are not formed by sedimentation. In this period, there developed a lot of small-sized hematite beds and manganese wad beds in Tongling area, but there were not advantage depositional conditions for forming the layered ore deposits, especially the large-sized ones.According to values of element concentration factor(K'), the author divides the elements in sedimentary strata into four kinds: (1)rich elements(K'>1), (2)impoverish elements(K'<1), (3)stable elements(K'≈ 1), and (4)astable elements(K'>1 and K'<1). Most elements in Silurian-Triassic strata in Tongling area belong to rich elements and astable elements. It is thermal fluid's activity that leads to this result. Thermal fluid precipitates its compositions in strata and enriches these elements;and it also makes strata elements active and causes a local enrichment and impoverishment of elements in strata. Generally, the elements in strata do not show a whole impoverishment, so the strata do not provide any material to form the layered ore deposits in Tongling area.The alkali-silicic fluid with a high temperature is an important ore fluid. It can act with igneous rocks and make potash-alteration, sodium-alteration, silicatization and skarnization happen in the rocks. By these actions, the elements (Fe, Cu, Au, Ag, Pb, Zn etc.) in igneous rocks can enter the fluid, causing the fluid rich in mineral elements. With decrease of temperature, the fluid precipitates these elements and forms pyrite, chalcopyrite, native gold, native silver, galenite, sphalerite etc. in advantage spaces that make up many different ores. This fluid can act many times, and the mineralization can happen many times, that makes the ore deposits in Tongling area being of certain cyclicity and rhythmicity.The NE-orientation folds and faults are the main ore deposit-hosted structures in Tongling area. All the large-sized ore deposits in this area occur in them. The study of the strata demonstrates that these structures are formed in T₂-J₁ and are the results of the Indo-China movement.The microscopic observations of the melnikovites in the Xinqiao deposit show that they are formed by hydrothermal activities. The Pb and S isotopic compositions of the melnikovites andthe pyrite nodules in strata imply a lower crust or mantle material source. The melnikovites are also affected by later fluid. The siderites in the Xinqiao deposit also show a hydrothermal genesis. They occur in strata pores in comb and ptygma shapes. The pore larger, the grain of siderite bigger. The C-isotopic composition suggests that the siderites are of a magmatic carbon source. So the siderites may be formed by the magmatic fluid. The Fe-Si rock in the Xinqiao deposit are composed of fine-grained quartzs and hematites. The Fe content in the rock has a negative correlation with the Si content, indicating that a silicic fluid acts with orebodies to form the Fe-Si rock. The H-0 isotopic compositions of inclusion water in different quartzs in the Xinqiao deposit suggest that the ore fluid mainly comes from magma. All studies of the Xinqiao deposit demonstrate that this ore deposit is mainly formed by the magmatic fluids.On the basis of research in Tongling area, the author establishes an ore deposit model of two-stage fluid activities: the early ore fluid related to basaltic magma forms the pyrite sheets;the later alkali- silicic fluid forms the Cu and Au ore bodies in the former pyrite sheets. The author also establishes an ore prospecting model of two elements (structure and pluton): taking the coupling place of large space caused by tectogenetic movement and pluton affected by alkali-silicic fluid as the prospecting target. According to this model, four prospecting targets are proposed for future geological study in Tongling area: (l)the West Longjishan district, (2)the North Baocun district, (3)the South Yaojiawan district, and (4)the Mingshan district.