The ages of Zn-Pb deposits are exceptionally challenging to determine owing to the lack of suitable mineral chronometers and techniques. Here we present the first result for in situ LA-ICP-MS U-Pb dating of carbonates and barite from a Mississippi Valley−type (MVT) Zn-Pb deposit in South China. Hydrothermal dolomite in close textural and paragenetic association with Zn-Pb sulfides, and calcite and barite cement from the breccia ores, yield ages of 473.4 ± 2.7 Ma and 368.7 ± 3.1 Ma, respectively. Together with new in situ S-Pb-Sr isotope values, these data reveal an epigenetic Zn-Pb mineralization history, agreeing well with a model involving basinal brine accumulation and MVT Zn-Pb sulfide precipitation. Because carbonate is a common mineral in Zn-Pb deposits worldwide, and other minerals in such deposits suitable for isotope dating are generally absent, in situ U-Pb dating of gangue carbonates opens a new window for better defining the ore genesis of this globally important Zn-Pb deposit type and for tracking hydrothermal fluid flow in sedimentary basins.
Abundant fluorites occur in the Shihuiyao rare metal (Nb-Ta-Rb) deposit in Inner Mongolia of NE China, and they can be classified by their occurrence into three types. Type I occurs disseminated in greisen pockets of albitized granite. Type II occurs in the skarn zone between granite and carbonate host rocks, and it can be subdivided into different subtypes according to color, namely dark purple (II-D), magenta (II-M), green (II-G), light purple (II-P), and white (II-W). Type III are the fluorite-bearing veins in the silty mudstones. On the basis of petrography of the fluorites and their high contents of HFSEs (high field strength elements) and LILEs (large ion lithophile elements), strong negative Eu anomalies, and tetrad effects, we suggest that Type I fluorites crystallized in a late-magmatic stage with all the components derived from the granite. The high Y/Ho ratios suggest that the Type II fluorites crystallized in the early- or late-hydrothermal stage. The rare earth elements (REEs) characterized by various Eu anomalies of the Type II fluorites indicate a mixed origin for ore-forming metals from granite-related fluids and limestones, and the oxygen fugacity increased during fluid migration and cooling. Compared to the Type II fluorites, the similar trace element contents of the Type III suggest a similar origin, and remarkable positive Eu anomalies represent a more oxidizing environment. The Sr isotopic composition (87Sr/86Sr)i = 0.710861) of the Type I fluorites may represent that of the granite-derived fluids, whereas the (87Sr/86Sr)i ratios of the Type II (0.710168–0.710380) and Type III (0.709018) fluorites are lower than that of the Type I fluorites but higher than those of the Late Permian-Early Triassic seawater, suggesting a binary mixed Sr source, i.e., granite-derived fluids and marine limestones. Nevertheless, the proportion of limestone-derived Sr in the mixture forming the Type III fluorites is much higher than that of Type II. The rare metal Nb and Ta get into the granite-derived F-rich fluids by complexing with F and precipitate in the form of columbite-group minerals after the Type I fluorites crystallize. Most of Nb and Ta may have deposited as columbite-group minerals during the magmatic stage, resulting in no Nb-Ta mineralization in the hydrothermal stage when the Type II and III fluorites formed. Hence, the Type I fluorites in the Shihuiyao mining area can be used as an important exploration tool for the Nb-Ta mineralization.
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