In situ Pb and bulk Sr isotope analysis of the Yinchanggou Pb-Zn deposit in Sichuan Province (SW China): Constraints on the origin and evolution of hydrothermal fluids

The Yinchanggou Pb-Zn deposit, located in southwestern Sichuan Province, western Yangtze Block, is stratigraphically controlled by late Ediacaran Dengying Formation and contains > 0.3 Mt of metal reserves with 11 wt% Pb + Zn. A principal feature is that this deposit is structurally controlled by normal faults, whereas other typical deposits nearby (e.g. Maozu) are controlled by reverse faults. The origin of the Yinchanggou deposit is still controversial. Ore genetic models, based on conventional whole-rock isotope tracers, favor either sedimentary basin brine, magmatic water or metamorphic fluid sources. Here we use in situ Pb and bulk Sr isotope features of sulfide minerals to constrain the origin and evolution of hydrothermal fluids. The Pb isotope compositions of galena determined by femtosecond LA-MC-ICPMS are as follows: Pb-206/Pb-204 = 18.17-18.24, Pb-207/Pb-204 = 15.69-15.71, Pb-208/Pb-204 = 38.51-38.63. These in situ Pb isotope data overlap with bulk-chemistry Pb isotope compositions of sulfide minerals (Pb-206/Pb-204 = 18.11-18.40, Pb-207/Pb-204 = 15.66-15.76, Pb-208/Pb-204 = 38.25-38.88), and both sets of data plotting above the Pb evolution curve of average upper continental crust. Such Pb isotope signatures suggest an upper crustal source of Pb. In addition, the coarse-grained galena in massive ore collected from the deep part has higher Pb-206/Pb-204 ratios (18.18-18.24) than the fine-grained galena in stockwork ore sampled from the shallow part (Pb-206/Pb-204 = 18.17-18.19), whereas the latter has higher Pb-208/Pb-204 ratios (38.59-38.63) than the former (Pb-208/Pb-204 = 38.51-38.59). However, both types of galena have the same Pb-207/Pb-204 ratios (15.69-15.71). This implies two independent Pb sources, and the metal Pb derived from the basement metamorphic rocks was dominant during the early phase of ore formation in the deep part, whereas the ore-hosting sedimentary rocks supplied the majority of metal Pb at the late phase in the shallow part. In addition, sphalerite separated from different levels has initial Sr-87/Sr-86 ratios ranging from 0.7101 to 0.7130, which are higher than the ore formation age-corrected Sr-87/Sr-86 ratios of country sedimentary rocks (Sr-87/Sr-86(200 Ma) = 0.7083-0.7096), but are significantly lower than those of the ore formation age-corrected basement rocks (Sr-87/Sr-86(200 Ma) = 0.7243-0.7288). Again, such Sr isotope signatures suggest that the above two Pb sources were involved in ore formation. Hence, the gradually mixing process of mineralizing elements and associated fluids plays a key role in the precipitation of sulfide minerals at the Yinchanggou ore district. Integrating all the evidence, we interpret the Yinchanggou deposit as a strata-bound, normal fault-controlled epigenetic deposit that formed during the late Indosinian. We also propose that the massive ore is formed earlier than the stockwork ore, and the temporal-spatial variations of Pb and Sr isotopes suggest a certain potential of ore prospecting in the deep mining area.