Pb-Sr-Nd isotope constraints on the fluid source of the Dahu Au-Mo deposit in Qinling Orogen, central China, and implication for Triassic tectonic setting

摘要

The Dahu Au-Mo deposit is a structure-controlled lode system occurring in the northern Xiaoqinling terrane, Huaxiong Block, Qinling Orogen. This paper reports a new Sr-Nd-Pb isotope dataset obtained for ore sulfides and the hostrocks within the Taihua Supergroup, in an attempt to constrain the source of the ore-forming fluids from a new dimension. 16 sulfide samples yield I_(Sr) ratios of 0.70470-0.71312, with an average of 0.70854; ε_(Nd)(t) values between -13.5 and -18.1, with average of -15.1; and (~(206)Pb/~(204)Pb)_i, (~(207)Pb/~(204)Pb)_i and (~(208)Pb/~(204)Pb)_i ratios of 17.033-17.285,15.358-15.438, and 37.307-37.582, with averages of 17.162,15.405, and 37.440, respectively. 5 gneiss samples from the Taihua Supergroup yield I_(Sr) ratios of 0.70947-0.73201, averaging 0.72294; sNd(t) values of -20.0 to -31.1, averaging -25.1; and (~(206)Pb/~(204)Pb)_i, (~(207)Pb/~(204)Pb)_i, (~(208)Pb/~(204)Pb)_i ratios of 17.127-18.392, 15.416-15.604 and 37.498-37.814, with averages of 17.547,15.470 and 37.616, respectively. These data show that the ore sulfides have less radiogenic Sr-Nd-Pb isotope systematics than the hostrocks, and suggest that the ore-forming fluids, which interacted with the wallrocks to form ores, must be sourced from a depleted mantle or a depleted, subducted oceanic slab. In combination with the spatial scenario and geo-chemical signatures of the Triassic magmatites and mineral systems, we argue that in the Triassic the Mianlue Ocean was not completely closed, and that the northward oceanic plate subduction still survived along the Mian-Lue suture, which caused the Late Triassic magmatism and associated mineralization in Qinling Orogen, including the Au-Mo Dahu deposit