白云鄂博碳酸岩型REE-Nb-Fe矿床——一个罕见的中元古代破火山机构成岩成矿实例

白云鄂博碳酸岩型REE-Nb-Fe矿床是世界上最大的稀土矿床。稀土矿石产于整个白云石碳酸岩体和部分脉状碳酸岩中。对比世界上20余个火成碳酸岩地区的特征后发现,白云鄂博地区完全具备国外火成碳酸岩区的地质特征。在岩石、矿石组合上,本区也发育一套碳酸岩+超基性岩+碱性基性岩(含基性熔岩)+碱性岩+稀土矿石+铁矿石组合;在矿物组合上,以白云石为主,方解石次之,伴生一套碱性闪石、长石、霓石、磷灰石、萤石、磁铁矿、稀土矿物组合;在全岩化学成分、微量元素、稀土元素和Sr、Nd、Pb、C、O同位素上,这些岩石具有一定的亲缘关系,有着共同的来源;在岩体的形态与岩石组构上,它们以岩席、岩筒和脉岩的形式出现,并发育有强烈的熔离作用与流动构造;在区域构造上,发育隐伏穹窿构造、岩筒构造和巨型断裂汇聚构造。综合分析上述特征表明:白云鄂博地区具有中元古代破火山机构的痕迹,赋矿白云石碳酸岩体则是顺层侵入的火成碳酸岩体,东矿、主矿可能是一个火山颈构造控矿,而赋矿白云石碳酸岩体西南侧的苏木图矿床则是隐伏岩筒构造控矿。

Bayan Obo Carbonatite REE-Nb-Fe Deposit: A Rare Example of Neoproterozoic Lithogeny and Metallogeny of a Damaged Volcanic Edifice

The Bayan Obo REE-Nb-Fe Deposit is the largest REE deposit in the world. The ore is embedded in all the dolomitic carbonatites and some vein carbonatites. A comparative study on more than 20 areas of igneous carbonatites in the world suggests that the Bayan Obo region is identical to the others in geology. Petrographi-cally there is here an assemblage of carbonatite + ultrabasic+alkaline basic (including lava)+alkaline + REE ore +iron ore; and mineralogically an assemblage mainly of calcite and dolomite with alkali amphibole, felspar, ae-girite, apatite, fluorite, magnetite and REE minerals. The whole-rock composition, trace elements, REE and Sr, Nb, Pb, C and O isotopes show that the rocks have a relationship and a common source. The rocks occur as sheet, pipe and vein with strong liquation and flow structure. There are buried domes, pipes and intersecting faults. A synthetic analysis of the above features indicates that there are, in the Bayan Obo region, remains of Neoproterozoic damaged volcanic edifices, the host dolomitic carbonatites are igneous intrusions along the bed-ding , the Main and East ore bodies are controlled by two volcanic necks, and the Sumt Deposit to the southwest of the host dolomitic carbonatite is controlled by buried pipes.