内蒙古兴和县曹四夭特大型钼矿床辉钼矿Re-Os同位素年龄及地质意义

Re-Os Isotopic Age Dating of the Molybdenite Separated from the Caosiyao Giant Molybdenum Deposit,Xinghe County,Inner Mongolia,and Its Geological Significances

内蒙古兴和县曹四夭矿床是迄今为止在我国北方找到的最大规模钼矿床,钼的资源量超过200万吨。考虑到大规模地质勘查工作仍在进行中,金属资源量和储量仍有较大提升空间。为了进一步查明曹四夭矿床的形成时间,对主要钼矿体4件辉钼矿样品进行了Re-Os同位素分析,所获同位素年龄值分别为131.9±2.3Ma、131.2±2.3Ma、129.7±2.0Ma和128.6±2.4Ma,平均值为130.4±2.4Ma,与花岗斑岩体锆石U-Pb年龄(134～131Ma)相近。根据岩(矿)体同位素年龄数据,并且结合辉钼矿呈浸染状和团块状分布于花岗斑岩株中,并且与部分硫化物呈共生结构关系的地质事实,可以推测曹四夭矿区含矿花岗斑岩株及有关钼矿床的形成时间均为早白垩世,属燕山期构造—岩浆活动的产物。曹四夭钼矿床形成时代的厘定对于提高该矿床的理论研究水平和指导隐伏金属矿床的找矿勘查工作均具有重要意义。

Abatract： Located in Liangcheng uplift of Inner Mongolian massif （Axis） within the Northern China craton, the newly discovered Caosiyao deposit in Xinghe county, central Inner Mongolia is a giant molybdenum deposit. During the Mesozoic orogenies, intensive tectonic and igneous activities resulted in large-scale granitoid magmatism in the Caosiyao mineralized district. The granitoid porphyry stocks and dyke swarms were emplaeed in the Achaean metamorphic rocks of the Jining Group. Rock-forming minerals of the ore-bearing granitoid porphyry are mainly quartz, microeline, microperthite and plagioclase associated with minor biotite and sericite. Accessory minerals are magnetite, apatite, zircon, sphene and allanite. The zircon samples from the ore-bearing granitoid porphyry stock have yielded LA-ICP-MS U-Pb ages ranging from 134 Ma to 131 Ma, indicating the stocks and dykes may be formed during the Early Cretaceous tectonic activity. Molybdenum mineralization occurs in both Achaean metamorphic rocks and granitoid porphyry stocks as veins, veinlets and disseminated blocks. The molybdenum ore consists of pyrite, molybdenite, chalcopyrite, sphalerite, wolframite, K-feldspar, quartz, sericite, chlorite and epidote. As the most important economic molybdenum mineral, molybdenite occurs as impregnation, radioflake, thin film, aggregated flakes and veinlet. Re-Os isotopic age dating on four molybdenite samples from the main orebody gives model ages of 131.9 ± 2.3Ma, 131.2 ± 2.3Ma, 129.7 ± 2.0Ma and 128.6 ± 2. 4Ma, with an average value of 130. 4± 2.4Ma, which is similar to that of zircon U-Pb age of the ore-bearing granitoid porphyry. As the Re-Os model ages are in agreement with field geological evidences, and the molybdenite has co-existing relations with pyrite and chalcopyrite, it has been suggested that the Re-Os model ages represent the ore-forming time of the Mo mineralization occurring in the Caosiyao district and its neighboring area. Preliminary studies show that the Caosiyao deposit were formed during the Mesozoic Yanshanian extensional tectonic movements that resulted in the intensive igneous activities. It is believed to be a product of the Early Cretaceous intraplate granitoid magmatism within the Liangcheng uplift. The well-developed high K and SiO2 granitoid porphyry stocks and dykes might have acted as the ＂engines＂ of the ore-forming systems. Most of the Mo was derived from partial melting of the Precambrian continental crust. The ore-forming fluid was dominated by magmatic water at the early metallogenic stage, and was characterized by mixture with magmatic water as the major source at the main metallogenic stage. The combined geochemical data, field geological observations and petrological evidence indicate that the Caosiyao deposit belongs to the porphyry type Mo deposit. The genetic model and mineral exploration criteria of the Caosiyao deposit can also be used as useful tools during the comprehensive evaluation of concealed Mo deposits in Inner Mongolian massif of the North China craton. Key words： Re-Os isotopic age; molybdenite; ore-forming age; porphyry type molybdenum deposit; Caosiyao; Central Inner Mongolia