内蒙古大苏计和曹四夭大型钼矿床的发现及意义

大苏计矿床和曹四夭矿床是近几年在内蒙古中部地区找到的2处大型特大型钼矿床,产出规模分别达到20万吨和200万吨.钼矿化主要在太古宙变质岩及中生代石英斑岩、正长斑岩和花岗斑岩体内呈浸染状、网脉状和脉状产出,并且构成筒状、倒扣“碗状”和囊状矿体.两处矿床无论在产出环境和地质特征上,还是在矿物种属、元素组合和热液蚀变方面,均与美国克莱麦克斯钼矿床和黑龙江岔路口钼矿床较为相似,是中生代构造-岩浆活动的产物,属古大陆碰撞造山后伸展环境产出的斑岩型矿床.文章所提出的地壳4个演化阶段和3个成矿期次的成矿模式对于提高大苏计和曹四夭钼矿床理论研究水平,推动低山丘陵覆盖区大型隐伏钼矿床的找矿勘查工作深入进行,均具有重要意义.

Discovery of Dasuji and Caosiyao large-size Mo deposits in central Inner Mongolia and its geological significances

Located in Liangcheng uplift of Inner Mongolian massif (Axis) within the Northern China craton, the newly discovered Dasuji and Caosiyao deposits are large-size and superlarge Mo deposits in central Inner Mongolian, respectively. During the Mesozoic Indosinian and Yanshanian orogenies, intensive tectonic and igneous activities resulted in large-scale granitoid magmatism in the Dasuji and Caosiyao mineralized districts. The granitoid porphyry stocks and dyke swarms were emplaced in the Achaean metamorphic rocks of the Jining Group. There exist no differences in ore-bearing intrusions in the Dasuji and Caosiyao mineralized districts. The ore-bearing granitoid intrusions consist of quartz porphyry, syenogranite porphyry and granite porphyry. Rock-forming minerals are mainly quartz, microcline, microperthite and plagioclase associated with minor biotite and sericite. Accessory minerals are magnetite, apatite, zircon, sphene and allanite. The porphyry stocks are petrochemically characterized by relatively high SiO₂, K₂O, Rb, Nb and U, obviously low CaO, Fe₂O₃+FeO, MgO, Sr, Ba, Ta and REE, belonging to silicon-high, potassium-rich rock of calc-alkaline series. Molybdenum mineralization occurs in both Achaean metamorphic rocks and granitoid porphyry stocks as veins, veinlets and disseminated blocks. The molybdenum ore consists of pyrite, chalcopyrite, molybdenite, sphalerite, pyrrhotite, scheelite, wolframite, K-feldspar, quartz, sericite, chlorite and epidote. As the most important economic molybdenum mineral, molybdenite occurs in impregnation, radioflake, thin film, aggregated flakes and veinlet forms in the veins. Re-Os isotopic data of four molybdenite separate samples from the Dasuji deposit defined a correlation line corresponding to an age of (222.5±3.2) Ma. Meanwhile, the molybdenite samples from the Caosiyao deposit has yielded Re-Os model ages from 136 Ma to 130 Ma, with an average value of 134 Ma. The types of hydrothermal alteration observed both at Dasuji and Caosiyao are mainly silicification, sericitization, K-feldspathization, biotitization, fluoritization, chloritization, uralitization and carbonatization. Of these alterations, silicification and K feldspathization are well developed, and have intimate relationship with molybdenum mineralization. Preliminary studies show that both the Dasuji and Caosiyao deposits were formed during the Mesozoic Indosinian and Yanshanian extensional tectonic movements that resulted in the intensive igneous activities. The well-developed high K and SiO₂ granitoid porphyry stocks 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 both the Dasuji and Caosiyao deposits belong to the porphyry type medium-high temperature hydrothermal Mo deposit. The Dasuji deposit is believed to be a product of the Early Indosinian intraplate granitoid magmatism within the Liangcheng uplift. Moreover, the forming-processes of the Caosiyao deposit may be genetically related to the Yanshanian igneous activities in the same tectonic unit. The genetic model and mineral exploration criteria of the Dasuji and Caosiyao deposits can also be used as useful tools during the comprehensive evaluation of concealed Mo deposits in Inner Mongolian massif of the North China craton.