Sources and Evolution of the Ore-forming Materials in the Anqing Cu-Fe Deposit in Anhui Province-Geological and S,C, O Isotopic ConstraintsEI北大核心CSCD

The Anqing Cu-Fe deposit is one of the representative large Cu-Fe deposits along the Yangtze River in Anhui province, with controversial metallogenic mechanism. Based on the ore-forming geological characteristics, this paper focus on the sulfur, carbon and oxygen isotopic compositions of the ores and surrounding rocks, and discuss the sources and evolution processes of the ore-forming materials. The Cu-Fe deposit occurs in the contact zone between the early Yanshanian Yueshan diorite and Triassic marble, with clear horizontal zonings in the skarns and ore bodies. The garnet skarn and thick massive magnetite ore body commonly occur within the external contact zone, which have clear boundaries with the surrounding rocks; whereas the diopside skarns with disseminated copper sulfide commonly occur within the internal contact zone and show gradual and transitional relations with the diorite. The δ34S values of the ores range from -6.5‰ to 10.6‰, and show a V-shaped trend from the diorite to the outer marble. This compositional variation indicates that most of the sulfur may come from magma, with involvement of some pre-Triassic clastic strata sulfur and Triassic marine sulfates in the later stage. The δ13C values of the gangue minerals range from -5.5‰ to 2.0‰, which decrease from the external contact zone to internal contact zone, indicating that the carbons of the ore-forming fluids may be mainly derived from magma, with some Triassic carbonate stratigraphic carbon involved. The marbles nearby the orebody show δ18O values lower than those of the Triassic strata, indicating that they have been remolded by the low δ18O magmatic hydrothermal fluids. The magnetite have some magma filling geological features and extremely low δ18O value, may be the result from the filling of the high temperature iron-rich fluids along the contact zone and fault. This study shows that the ore-magma filling type and hydrothermal-metasomatic type ore bodies coexist in the Anqing Fe-Cu deposit. The immiscibility between iron oxide and silicate melt occurred in magma chamber, which resulted in the formation of iron-rich fluid. The fluid migrated upward and eventually precipitated in a favorable tectonic area or contact zone, and the magnetite ore bodies were formed in the outer contact zone. By the later fluid mixing, filling metasomasis, and water-rock reaction between the differentiated hydrothermal solutions and diorite, the copper ore bodies and the copper-bearing altered diorite were formed in the internal contact zone. © 2018, Science Press. All right reserved.