阿尔泰南缘麦兹盆地萨吾斯铅锌矿床控矿铁闪石矽卡岩的40Ar／39Ar年代学研究

^(40)Ar/^(39)Ar geochronology of ore-controlling grunerite skarn from the Sawusi Pb-Zn deposit,Maizi basin of the southern Chinese Altay Mountains

萨吾斯矿床位于麦兹盆地东部,是一个新近发现和勘探的铅锌矿床。萨吾斯铅锌矿床赋存于下泥盆统康布铁堡组上亚组凝灰岩、流纹岩及其沉积碳酸盐夹层,矿(化)体直接受石榴黑云矽卡岩和铁闪石矽卡岩控制,表现出火山喷流-沉积成矿和矽卡岩矿床的双重特征。这两类矿床的重要区别之一是同生和后生成矿。对主要矽卡岩矿物铁闪石进行精确的ArAr年龄测定,将其与康布铁堡组流纹岩的年龄对比,有助于确定萨吾斯铅锌矿床的成因。高精度、高灵敏度激光40Ar-39Ar测年方法,为解决闪石类低K矿物的定年提供了新的手段。岩芯样品SW85中,铁闪石晶粒较大、晶形发育,与闪锌矿、碳酸盐矿物和石榴子石都具有平直的边界,而不是复杂交生,不含矿物和流体包裹体,代表了相对简单均一的系统。对SW85的铁闪石做了激光40Ar-39Ar测年,其结果为412±15Ma,与康布铁堡组流纹岩的SIMS锆石U-Pb年龄(401±2.7Ma)和别斯萨拉玢岩的SIMS锆石U-Pb年龄(401±3.1Ma)在误差范围内一致,表明矽卡岩和铅锌矿的形成与康布铁堡组流纹质火山岩的喷发及其碎屑岩的沉积是同时的,成因上密切相关。其次,矽卡岩呈层状产出,局限于康布铁堡组凝灰岩及其铁锰钙质碳酸盐夹层的范围内;石榴子石为铁铝榴石、锰铝榴石,闪石为铁闪石,缺失透辉石、钙铁辉石。上述特征与接触交代矽卡岩不同,后者以钙铝榴石、钙铁榴石、透辉石/钙铁辉石为特征,闪石为角闪石。接触交代矽卡岩矿床是后成矿床,它晚于岩浆侵入体的围岩。综上所述,萨吾斯铅锌矿是一个与康布铁堡组火山喷发和浅成侵入有关的喷流-沉积矿床。麦兹盆地的控盆断裂巴寨断裂从北西端向南东发展而成。阿巴宫-库尔提断裂和琼库尔断裂是克兰盆地的控盆断裂,它们在北西撒开、向南东收敛,具有羽状张剪性断裂的特征。因此,阿尔泰南缘在早泥盆世具有活动陆缘的局部拉张性质。西伯利亚板块与哈萨克斯坦-准噶尔板块的推错,在阿尔泰南缘产生局部张剪性应力场,从而形成了上述断裂以及早泥盆世火山-沉积盆地。

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 40Ar-39Ar 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 grnnerite separates represent a homogeneous system. A laser 40 Ar_39 Ar age determination was performed on the SW85 grunerites that yielded an isochron age of 412 ± 15Ma. The 40Ar/39Ar 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-Kuertifault 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.