Search

The Xujiashan antimony deposit is hosted by marine carbonates of the Upper Sinian Doushantuo and Dengying Formations in Hubei Province, South China. Our Sr isotopic data from pre- and syn-mineralization calcites that host the mineralization show that the pre-mineralization calcite displays a narrow range of Sr/Sr ratios (0.7096 to 0.7097), similar to the ratios of the Sinian seawater, and high Sr concentrations (2645 to 8174 ppm). In contrast, the syn-mineralization calcite exhibits low Sr concentrations (785 to 2563 ppm) and high Sr/Sr ratios (0.7109 to 0.7154), which is interpreted as the result of addition of radiogenic strontium during the antimony mineralization. The study of Sr isotopes suggests that their Sr component to the pre-mineralization calcite derived directly from the host rocks (i.e. the Sinian marine carbonates), while radiogenic Sr for the syn-mineralization calcite derived from the underlying Mesoproterozoic Lengjiaxi Group basement through hydrothermal fluid circulation along the major fault that hosts the mineralization. The Pb isotopic ratios of stibnite are subdivided into two groups (Group A and Group B), Group A is characterized by higher radiogenic lead, with Pb/Pb = 18.874 to 19.288, Pb/Pb = 15.708 to 15.805, and Pb/Pb = 38.642 to 39.001. Group B shows lower lead isotope ratios (Pb/Pb = 17.882 to 18.171, Pb/Pb = 15.555 to 15.686, and Pb/Pb = 37.950 to 38.340). The single-stage model ages of Group A are mainly negative or slightly positive values (-258 to 3 Ma), while those of Group B range from 636 to 392 Ma, with an average of 495 ± 65 Ma. In addition, there are positive linear correlations among Pb isotopic ratios. These results suggest that the lead of Group A stibnite was mainly derived from the Sinian marine carbonates, and that of Group B stibnite from the underlying Lengjiaxi Group basement. This conclusion is consistent with the results of the Sr isotopes. These results indicate that the Xujiashan deposit is not syngenetic sedimentary and in situ reworked origin as previously considered. The metal (mainly Sb) of this deposit was not only derived from the Sinian host rocks, but also partly derived from the underlying Mesoproterozoic Lengjiaxi Group basement. [ABSTRACT FROM AUTHOR]

The Furong tin deposit in the central Nanling region, South China, consists of three main types of mineralization ores, i.e. skarn-, altered granite- and greisen-type ores, hosted in Carboniferous and Permian strata and Mesozoic granitic intrusions. Calcite is the dominant gangue mineral intergrown with ore bodies in the orefield. We have carried out REE, Mn, Fe, and Mg geochemical and C, and O isotopic studies on calcites to constrain the source and evolution of the ore-forming fluids. The calcites from the Furong deposit exhibit middle negative Eu anomaly (Eu/Eu*= 0.311–0.921), except for one which has an Eu/Eu* of 1.10, with the total REE content of 5.49–133 ppm. The results show that the calcites are characterized by two types of REE distribution patterns: a LREE-enriched pattern and a flat REE pattern. The LREE-enriched pattern of calcites accompanying greisen-type ore and skarn-type ore are similar to those of Qitianling granite. The REE, Mn, Fe, and Mg abundances of calcites exhibit a decreasing tendency from granite rock mass to wall rock, i.e. these abundances of calcites associated with altered granite-type and greisen-type ores are higher than those associated with skarn-type ores. The calcites from primary ores in the Furong deposit show large variation in carbon and oxygen isotopic compositions. The δ13C and δ18O of calcites are −0.4 to −12.7‰ and 2.8 to 16.4‰, respectively, and mainly fall within the range between mantle or magmatic carbon and marine carbonate. The calcites from greisen and altered granite ores in the Furong deposit display a negative correlation in the diagram of δ13C versus δ18O, probably owing to the CO2-degassing of the ore-forming fluids. From the intrusion to wall-rock, the calcites display an increasing tendency with respect to δ13C values. This implies that the carbon isotopic compositions of the ore-bearing fluids have progressively changed from domination by magmatic carbon to sedimentary carbonate carbon. In combination with other geological and geochemical data, we suggest that the ore-forming fluids represent magmatic origin. We believe that the fluids exsolved from fractionation of the granitic magma, accompanying magmatism of the Qitianling granite complex, were involved in the mineralization of the Furong tin polymetallic deposit. [ABSTRACT FROM AUTHOR]

Geochemical and Sr-Nd-Pb isotopic data are presented for volcanic rocks from Zougouyouchaco (30.5 Ma) and Dogai Coring (39.7 Ma) of the southern and middle Qiangtang block in northern Tibet. The volcanic rocks are high-K calc-alkaline trachyandesites and dacites, with SiO2 contents ranging from 58.5 to 67.1 wt % The rocks are enriched in light REE (LREE) and contain high Sr (649 to 986 ppm) and relatively low Yb (0.8 to 1.2 ppm) and Y (9.5 to 16.6 ppm) contents, resulting in high La/Yb (29-58) and Sr/Y (43-92) ratios, as well as relatively high MgO contents and Mg no., similar to the compositions of adakites formed by slab melting in subduction zones. However, the adakitic rocks in the Qiangtang block are characterized by relatively low ϵNd(t) values (-3.8 to -5.0) and highly radiogenic Sr ((87Sr/86Sr)i = 0.706-0.708), which are inconsistent with an origin by slab melting. The geochemistry and tectonics indicate that the adakitic volcanic rocks were most likely derived from partial melting of delaminated lower continental crust. As the pristine adakitic melts rose, they interacted with the surrounding mantle peridotite, elevating their MgO values and Mg numbers. [ABSTRACT FROM AUTHOR]