| 其他摘要 | Dating of Pb-Zn deposit is always a difficult problem to geologists at home and abroad. Due to the various types of Pb-Zn deposits, it is hard to figure out a general and effective method for dating these deposits. In recent years, Re-Os dating method has been widely applied to hydrothermal deposits. Although pyrite is common in Pb-Zn ores, it has multiple generations so that distinguishing pyrite from the ore-forming stage is difficult. Sphalerite and galena are the effective ore minerals for Re-Os dating of Pb-Zn deposits. However, Re concentration of sphalerite and galena is relative low and varies widely for different types of Pb-Zn deposits. Moreover, experimental studies have shown that the recovery of Re in sphalerite and galena samples is extremely low using routine anion exchange method. To solve the above problems, we carry out a systematic research using the re-usable Carius tube to determine Re and Os in sphalerite and galena. The revised method was used to date some typical Pb-Zn deposits. Combined with sulfur isotopes and trace element compositions of sulfides, the Re-Os results are used to reveal the genesis of the Pb-Zn deposits. The main conclusions are listed as follows. (1) By investigating the various factors affecting the recovery of Re, the key factor for loss of Re during chemical preparation is known. With respect to sphalerite, the abundant Zn2+ in matrix severely prevents the efficiency of anion exchange of (ReO4)-. However, the abundant Pb2+ in galena matrix does not significantly influences the anion exchange of (ReO4)-, and the white precipitation (mainly PbSO4) forming at the dissolving stage (by revise aqua regia) dose not lead to the loss of Re. Therefore, both PbSO4 and ZnSO4 in the medium of 2N HCl that severely lower the recovery of Re. (2) Experimental studies have shown that pre-dissolving samples by HCl or adding ~0.1 gram of Zn2+ or Fe3+ into the samples can significantly improve the recovery of Re in galena matrix, but these methods have some limits. Applying 0.8 N HNO3 medium for anion exchange not only can improve the recovery of Re in galena without any effects on determination of Os, but also can be suitable for 0.1-3.0 grams of sphalerite samples. (3) The revised method is applied to typical MVT Pb-Zn deposits in the famous Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province in southwest China. Pb-Zn sulfides in Shanshulin (杉树林), Tianqiao (天桥) and Jinshachang (金沙厂) deposits contain low Re of < 2.0 ng/g. However, sulfides from the Huize (会泽; Re~3.47-588 ng/g), Fule (富乐; Re~3.52-398 ng/g) and Maoping (毛坪; Re~1.29-89.05 ng/g) Pb-Zn deposits have relatively higher and more varied concentrations of Re, with relative lower common Os (< 0.1 ng/g), typical LLHR (low-level highly radiogenic) sulfides in these deposits. (4) The Maoping (毛坪) Pb-Zn deposit has a Re-Os isochron age of 55.2±3.5 Ma (2σ，MSWD=0.47). The light and dark sphalerites of the Fule (富乐) Pb-Zn deposit yielded isochron ages of 34.7±4.4 Ma (2σ，MSWD=2.9) and 20.4±3.2 Ma (2σ，MSWD=3.9), respectively. The Huize (会泽) Pb-Zn deposit has 187Re-187Os model ages ranging from 15.1 to 147 Ma, which indicates hydrothermactivities during Yanshanian and Himalayan periods. (5) Pyrites from the Huize (会泽) Pb-Zn deposit have higher Re concentration than sphalerites, and the fine-grained pyrites concentrate more Re than coarse-grained pyrites. The Ag, Sb, V, Co, Rb, Zr, Ba, Tl, Th, and U are also higher in fine-grained pyrites than coarse-grained pyrites. (6) For the Laochang (老厂) Pb-Zn deposit in Lancang (澜沧), the Re-Os isotope data of sphalerite and galena yield an isochron age of 308±25 Ma (2σ, MSWD=1.04) and initial 187Os/188Os of ~0.88±0.42. The Re-Os age of the Pb-Zn ore is in agreement with the age of the country rock (312~320 Ma；zircon U-Pb), indicating that the deposit is a typical VHMS type. The initial 187Os/188Os ratio indicates a mixed source of mantle and crust. |
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