白云鄂博矿床含矿白云岩的成因探讨

Origin of the Ore-bearing Dolomite in Baiyun Obo Ore Deposit

以白云鄂博矿区含矿白云岩的分布、野外地质产状等基础地质特征为基础，结合岩石学、矿物学和地球化学等一系列特征进行了研究，认为含矿白云岩既非典型的正常海相沉积碳酸盐岩，亦非典型的岩浆碳酸岩，它既有典型的海相沉积碳酸盐岩的某些特征，如其分布具有一定的时代性和层位性，呈层状、似层状或透镜状，与周围地层呈整合接触并与其同步褶皱和变形，具明显的层理等构造；同时又具有岩浆碳酸岩的某些特征，如沿深大断裂构造分布，下都有脉状体等。该白云岩还具有自己独特的特征，如具“眼式”构造，上都为层状，下部为脉状的“二元式”结构，它的C、O、Sr同位素特征介于海相碳酸盐和岩浆碳酸盐之间。上述特征都表明，白云岩为海底循环的热水流体喷溢沉积而成。

The superlarge Fe-Nb-REE ore deposit at Baiyun Obo is one of the most large Nb-REE ore deposits in the world, which is hosted by dolomite in the Baiyun Obo Formation of Middle Prpterozoic age. In this paper the tectonic setting, regional geology, regional distribution, occurrence, petrology, mineralogy and C-,O-,Sr-isotopic geochemistry of the ore-bearing dolomite in Baigun Obo, lnner Mongolia,China are studied. The ore-bearing dolomite is only located in the Baiyun Obo Formation of Middle Proterozoic age. It shows some features of normal marine sedimentary carbonate, such as special thoe-stratigraphic distribution, strati form and lens shaped, laminated and bedded structures, conformable contact and synchronous fold with wall rocks; and some features of magmatic carbonatite, such as distribution along deep fault, thin dolomite vein cutting wall rocks in heading wall, so the dolomite is neither the normal marine sedimentary carbonate northe magmatic ca-rbonatite.The dolomite distributed along regional fault in strati form, stratifold and lentiform was controlled by second tectonic basin. The dolornite thickness in center is much larger, and the thickness in marginal is much smaner or thin out,The dolomite displays special 'eye-type' structure,e. g massive strucure in the center part, banded and faminated in the margin.The dolomite shows 'two-end structuresn, e. g. strati form body is located in upper part and vein in lower part, in which the stratigraphic position of the vein dofomite is not overstep the stratigraphic position of the strati form dolomite. The ore-bearing dolomite have δ18O per mil values between +6. 4 and +16. 5, andδ18Ovalues between -4. 7 and +1. 4, which is located between the normal marine sedimentary and magmatic carbonates. 87Sr/86Sr data have confirmed this conclution.To sum up,authors believes that the dolomite was formed by hydrothermal venting which was circulating in submarine.