内蒙古乌拉特中旗大乌淀石墨矿成因特征分析

Genetic features of Dawudian graphite deposit in Urad Middle Banner,Inner Mongolia

石墨的矿物学特征研究是揭示石墨矿化成因机制的重要途径。在对内蒙古新发现的大乌淀特大型晶质石墨矿进行成因分析时,通过电子显微镜、扫描电镜、拉曼光谱、X射线粉晶衍射及碳同位素等石墨矿物特征分析手段,证实其石墨矿物已具备完整晶型,晶体片径长度20~70μm,厚度30~50nm,石墨矿化度在75以上。另外,结合矿床地质及矿物组合等方面的证据,初步判定该石墨矿具有热接触变质成因特征。矿床形成于中温低压弱还原环境中,矿化变质温度为520~600℃。在早二叠世中酸性岩浆热液的烘烤作用下,中元古界白云鄂博群尖山组地层中的有机碳发生变质结晶形成石墨矿化,富铝质成分重结晶为红柱石。石墨矿化品质与距离岩体远近、热传导系统圈闭性、后期变质作用叠加及原岩碳质成分的结构含量等因素有关。

The mineralogical characteristic of graphite is the key to unveiling its genetic mechanism. In analyzing the genetic features of the Dawudian deposit, a giant flake graphite deposit newly discovered in Inner Mongolia, a full range of analytical methods including electron microscopy, scanning electron microscopy, Raman spectrometry, X-ray diffractometry and carbon isotope analysis were used. The graphite mineral is shown to be of complete crystalline form with a typical length and thickness of 20 - 70 μm and 30 - 50 nm, respectively, and a graphitization degree at greater than 75. Combined with evidence from ore geology and mineral assembly, the genesis of the deposit is congruous with a mechanism of thermal contact metamorphism. The graphitization happened in a weak reduction and low pressure environment, and the metamorphic temperature was about 520 to 600 ℃. Under the thermal baking of the Late Carboniferous acidic magma and embodied in the Middle Proterozoic Jianshan Formation of the Bayan Obo Group, the organic carbonaceous materials metamorphosed into graphite while the M-rich materials recrystallized into andalusite. The quality of graphitization was constrained by the distance from intrusive bodies, trap condition of the heat conduction system, later metamorphisms recombination, as well as structures and compositions of the carbonaceous materials in the original sedimentary rocks.