Earth element is an important member of key metals. China's rare earth resource endowment is superior and mainly comes from Baiyun Obo, a rare super-large rare earth deposit in the world. However, due to the constraints of mine exploration objectives, rare earth mineralization theory and exploration technology, there have been different understandings of its massive metal enrichment mechanism, ore body spatial morphology and potential resources, which restrict the evaluation and effective utilization of rare earth resources. In order to clarify the formation mechanism of the Bayan Obo deposit and evaluate the potential resources of rare earth, the Institute of Geology and Geophysics of the Chinese Academy of Sciences deployed key projects and cooperated with Baotou Iron and Steel (Group) Co., Ltd. and its affiliated units to carry out detailed regional geological survey, revision of 1:5000 scale geological map, multi-method and multi-scale comprehensive geophysical survey and metallogenic research in Bayan Obo. Through the joint research of geology, geochemistry, geophysics and other disciplines, the evolution process of Bayan Obo carbonatite magma and the enrichment mechanism of rare earth have been revealed, the carbonatite emplacement mechanism and structural ore-control factors have been clarified, the three-dimensional shape of the ore-bearing geological body has been constructed, and the potential rare earth resources have been reassessed. (1) Baiyunebo area has experienced multiple tectonic movements. Before the emplacement of carbonate rocks, the early-middle Proterozoic sedimentary rocks (Baiyunebo group quartz sandstone, conglomerate, slate, etc.) in the mining area have undergone regional compressional tectonic action, and the horizontal strata have been replaced by structures to form gravel cake structure, mylonite, fold, etc. The newly formed nearly E-W trending and steep tectonic schistosity provides a favorable channel for the upwelling of carbonated magma of~1.3 billion years (Fig. 1). The distribution, attribution and relationship between early and late of the sedimentary rocks of the Middle Proterozoic Baiyunebo Group in the mining area need to be re-examined.
Fig. 1 Development history and carbonatite emplacement of the Mesoproterozoic Bayan Obo basin
(2) Baiyunebo H8 dolomite is igneous carbonate rock, which has obvious intrusive contact relationship with surrounding rock. Carbonate rock is the parent rock of rare earth mineralization and also the rare earth ore body. The accumulation of massive metals in Bayan Obo occurred in~1.3 billion years. Carbonic magma has a trend of evolution from iron-magnesium-calcareous, and the rare earth elements in carbonate rocks at different stages, especially light rare earth elements, show a trend of gradual enrichment. After the formation of the deposit, it underwent two transformations in the Early Paleozoic (450~400 million years) and the Late Paleozoic (280~260 million years) respectively. The transformation process led to the activation of rare earth and the formation of new minerals, but there was no obvious addition of foreign rare earth.
(3) The distribution of carbonate rocks revealed from the inversion results of magnetic anomalies has the basic characteristics of east-west distribution. The main ore and the east ore are the main areas of magnetic body distribution. The main ore and the east ore are connected carbonate rock distribution areas, and the development depth of carbonate rocks is large. The high magnetic anomaly body and low resistivity anomaly body reveal the three-dimensional distribution of carbonate rock (ore body) (Fig. 2). The carbonatite in Bayan Obo has an emplacement center and enjoys the same magma channel in the deep part. The center is located between the main ore and the east ore. After the emplacement of carbonated magma, the steep foliation formed by the replacement of the early structure is pushed to the west (west mine) and to the east (Huahua) respectively, and bifurcation and merging can occur (Fig. 3).
Fig. 3 Spatial distribution model of carbonatite in Baiyunebo deposit
(4) Baiyunebo carbonatite has a large volume and high degree of evolution, which is the key factor for its massive rare earth accumulation. Based on the obtained distribution range, volume and (minimum) density of carbonate rock mass (rare earth ore body), and using the 2% rare earth content of the whole rock of carbonate rock (based on the conservative average value obtained from the data over the years), it is estimated that the potential resource of shallow rare earth oxide in Baiyunebo mining area is 333 million tons, which is nearly 10 times the current recognized value of 36 million tons in Baiyunebo, The newly released global total proved rare earth resources (including Bayan Obo) by the United States Geological Survey is 2.78 times of 120 million tons.
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Post time: Mar-02-2023