Early Cretaceous Granitoids And Gold-copper Mineralization At The Shizishan Orefield In Tongling

The shizishan orefield is a major copper, gold, iron and sulphur producer in the middle-lower reaches of the Yangtze River. Early Cretaceous granitoids in the orefield are closely related to the gold-copper mineralization. In this study, the granitoid suit ranging from pyroxene diorite to granodiorite and the gold-copper mineralization associated with them were sampled. Zircon U-Pb dating and Hf isotope analysis, whole rock Sr-Nd-Pb-0 isotopic geochemistry, mineral thermobarometry, H-O-isotope analyses of the ore-forming fluid as well as in-situ microanalysis of fluid inclusions are combined with numerical simulations to model petrogenesis and to reveal the circumstances favouring Au-Cu mineralization. This work could be considered in further researches on lithospheric dynamic evolutions and the associated metallogenesis.Weighted average 2O6Pb/238U age for the magmatic zircons in the Baimangshan, Qingshanjiao, Datuanshan and Nanhongchong plutons are 136.3±5.2 Ma (1σ), 138.8±1.6 Ma,138.7±1.3 Ma, and 141.2±1.6 Ma, respectively, while the Rb/Sr isochron ages of these plutons lie between 133 and 139 Ma. The results suggest that magmatic emplacement and crystallization mainly occurred at about 139 Ma and continued to 135 Ma, which overlapped the gold-copper mineralization event. Thus the magmatic and mineralizing event happened at Shizishan in Early Cretaceous.The granitoids with SiO2 content ranging from 51 wt.%to 64 wt.%are classified as high-potassium calc-alkaline granitoids for their high alkali content (Na20> K2O).They show enrichment of Rb, Ba, Sr and LREE, relative depletion of Nb, Ta and Ti, and absence of Eu anomaly. In addition, FeO, MgO, and CaO contents are negatively correlated to SiO2 content, while REE fractionation and Na2o are proportional to that across the plutons. These geochemical features commonly suggest that the plutons may have originated from a similar source and experienced consecutive magmatic precesses. Whole rock initial Sr-Nd-Pb isotopic compositions of these plutons lie in a narrow range, which indicate that material from thickened crust significantly contributed to the mantle-derived magma via partial melting. The variation of initial Sr and Nd isotopes could be explained by the Assimilation and Fractional Crystallization (AFC) according to simulations, and the ratio of the assimilated to the fractional crystallized is around 0.6.The magmatic zircons from the quartz diorites yield mean εHf((t) value of-19.8±3. 9 (1 σ), while those from the pyroxene diorite show notably higher mean εHf(t) value (-8.5±1.4). The inherited zircons from the quartz diorite yield mean εHf(t) value of-2.9±1.4, while those from the pyroxene diorite show similar mean εHf(5) value (-5.6 ±4.5). Zircon εHf(5) values combined with U-Pb ages recorded a multi-staged dynamic magmatic evolution in geological history. The presence of inherited zircons implies that the equilibrated magmatic temperature could be as low as the zircon saturation temperature. Mineral thermobarometry results reveal that the depths of the discrete magma chambers were about 23 km, and 10 to 2 km, with the equilibrated temperatures of 900℃ and 700℃-760℃, respectively. Triggered by the shift to extensional tectonic condition, the magma at the lower magma chamber emplaced into shallow level (about 2 to 10 km), then AFC and magma mixing became the dominating processes for the magmatic evolutions.Microthermometry analyses on the primary fluid inclusions yield predominant salinities of 18 wt.%-23 wt.%NaCl eq. and 32 wt.%-38 wt.%NaCl eq. for the Chaoshan gold deposit, of 5 wt.%-10 wt.%NaCl eq. for the Datuanshan copper deposit, and of 1 wt.%-14 wt.%NaCl eq. for the Dongguashan copper deposit. The principal homogenization temperatures are 315℃-335℃ and 255℃-275℃for the Chaoshan gold deposit,380℃-410℃and 280℃-310℃ for the Datuanshan copper deposit, and 300℃-325℃for the Dongguashan copper deposit. Based on petrographic observations during microthermomery experiment, it is suggested that multi-boiling processes may have occurred at the dominating physicochemical conditions.In the pre-mineralization stage of the Dongguashan copper deposit, the fluid contains pre-enriched copper (up to 3000 μg/g) and sulfur (above 1 wt.%) at P-T condition of above 11.0 MPa and 320℃ (Type I).In the early skarn mineralization stage, copper concentrations of the fluids further increase up to about 7000 μg/g, associated with up to 2 wt.% of sulfur at minimal P-T of 8.1-22.6 MPa and 291℃-380 ℃. In the late skarn stage, the mean salinities of the fluids increased up to 18 wt.%-41 wt.%NaCl eq. at the minimal P-T of 2.0 MPa and 239℃, respectively. The copper was effectively enriched and transported in the ore-forming fluids, which was possibly promoted by sulfur species. The none-precipitating elements are concentrated partly because of continuous boiling. Exceptionally high contents of lead, zinc, and detectable gold and molybdenum in the late fluids could be considered for future metal prospect in this orefield.Raman analyses indicate that the vapor phase of the fluid inclusions consists of minor amount of CO2, and possibly CH4. The hydrogen and oxygen isotopic compositions of the ore-forming fluid from all the three deposits show characteristics of magmatic origin. Close associations between the magmatic activities and the mineralization have been observed, such as spatial zonation in the field, ore metal affinity to diverse magmatic rock types, and copper background in the ore-introducing fluid approximating to the copper content in the causative magma. The gold-introducing fluids may have a similar origin as the copper-introducing ones. Gold and copper precipitated at different stages during fluid evolutions, which could be attributed to the different physicochemical conditions.