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摘要
The Sawusi Pb-Zn deposit, newly discovered and being prospected in the east of the Maizi basin along the southern Chinese Altay Mountains, occurs within tuff, rhyolite, and intercalated carbonates of the upper section of the Early Devonian Kangbutiebao Formation. However, the Pb-Zn ore and mineralization is directly controlled by the garnet-biotite and grunerite skarns. These features resemble both the volcanic exhalative-sedimentary deposit and skarn deposit. One of the critical differences between these two types of deposits is that the former is syngenetic, whereas the latter is epigenetic. A precise determination of the age of skarn minerals and comparison of the age with that of the rhyolite of the Kangbutiebao Formation is conducive to the elucidation of ore genesis. The laser Ar-40-Ar-39 dating method provides a new means to date low-K minerals such as amphiboles. In the borehole sample SW85, grunerite, with relatively large size and well-developed crystal form, exhibit regular boundaries, instead of complex intergrowth, with coexisting garnet, carbonates, and sphalerite. Mineral and fluid inclusions are not observed within the grunerite. Hence, SW85 grunerite separates represent a homogeneous system. A laser Ar-40-Ar-39 age determination was performed on the SW85 grunerites that yielded an isochron age of 412 +/- 15Ma. The Ar-40/Ar-39 age is identical to the SIMS zircon U-Pb age (401 +/- 2.7Ma) of rhyolite of the Kangbutiebao Formation, and that (401 +/- 3.1Ma) of the Biesisala porphyrite, Sawusi deposit, within the analytical errors. This result demonstrates that the skarn formation and Pb-Zn ore deposition were concurrent with the volcanic eruption and sedimentation of the Kangbutiebao Formation. In addition, the skarn is stratiform and is bounded within tuff and intercalated Fe-Mn-Ca carbonates of the Kangbutiebao Formation. The skarn consists of almandine, spessartine, grunerite, with a notable absence of diopside and hedenbergite. The above characteristics of the stratiform, strata-bound skarn in the Sawusi contrast sharply with the contact metasomatic skarn whose mineral assemblage is typically composed of grossular, andradite, diopside/hedenbergite, and hornblende. Especially, the contact metasomatic deposit is epigenetic, and its age is younger than the country rock of the magma intrusion. In conclusion, the Sawusi Pb-Zn deposit is volcanic exhalative-sedimentary in genesis related to the rhyolite eruption and shallow intrusion of the Kangbutiebao Formation. Formation of the Bazhai fault that controlled the Maizi basin started from the northwest and continued to the southeast. The Abagong-Kuerti fault and Chonghuer fault that controlled the Kelan basin converge to the southeast and show the pinnate arrangement. So, these faults were transtensional faults during the Early Devonian that were generated by the differential compression between the Siberian plate and the Kazakhstan-Junggar plate.
- 出版日期2014-6
- 单位中国科学院大学; 中国科学院地质与地球物理研究所; 中国科学院地质与地球物理研究所兰州油气资源研究中心
