Is the Bayan Obo ore deposit a micrite mound? A comparison with the Sailinhudong micrite mound

The origin of the Bayan Obo ore deposit, the largest REE deposit in the world, has long been debated and various hypotheses have been proposed. Among them is that the Bayan Obo ore deposit is correlated with and has the same origin as the Sailinhudong micrite mound in the southern limb of the Bayan Obo synclinorium. To test this model, the Bayan Obo ore deposit and the Sailinhudong micrite mound are systematically compared for their geological features, elemental geochemistry, and C, O, and Mg isotopic geochemistry. We show that the Bayan Obo ore deposit and the Sailinhudong micrite mound are both calcareous, lens-like in shape, lack bedding features, and are both hosted in a sedimentary formation that consists of clastic sediments and carbonates, unconformably overlying the Archaean–Palaeoproterozoic crystalline basement. However, their geochemical characteristics differ markedly. Compared with the Sailinhudong micrite carbonates, the Bayan Obo ore-hosting dolomite marbles are strongly enriched in LREEs, Ba, Th, Nb, Pb, and Sr, and have very different (PAAS)-normalized REE patterns. Sailinhudong micrite carbonates have higher δ13CPDB and δ18OSMOW values, falling into the typical sedimentary field, but the Bayan Obo ore-hosting dolomites are isotopically intermediate between primary igneous carbonatite and typical sedimentary limestone. The δ26 Mg values of the Sailinhudong micrite carbonates are lighter than those of normal Mesoproterozoic sedimentary dolostone, while those of the Bayan Obo ore-hosting dolomite marble are isotopically heavier, similar to δ26 Mg of mantle xenoliths and Bayan Obo intrusive carbonatite. We conclude that the Bayan Obo ore deposit is not correlated with the Sailinhudong micrite mound; it is neither a micrite mound nor an altered micrite mound.白云鄂博矿床（Bayan Obo ore deposit）是全球规模最大的稀土元素（REE）矿床，其成因长期以来备受争议，学界已提出多种假说。其中一类假说认为，该矿床与白云鄂博复向斜（Bayan Obo synclinorium）南翼的赛林呼东微晶丘（Sailinhudong micrite mound）具有成因关联且同源。为验证该假说模型，本研究针对二者的地质特征、元素地球化学特征及碳、氧、镁同位素地球化学特征开展了系统对比。研究表明，二者均为钙质透镜状地质体，缺失层理构造，且均赋存于由碎屑沉积与碳酸盐岩组成的沉积地层中，不整合覆于太古宙-古元古代结晶基底之上。但二者的地球化学特征存在显著差异：相较于赛林呼东微晶碳酸盐岩，赋矿白云岩大理岩的轻稀土元素（LREEs）、钡（Ba）、钍（Th）、铌（Nb）、铅（Pb）与锶（Sr）强烈富集，且PAAS标准化稀土配分模式差异显著。赛林呼东微晶碳酸盐岩的δ¹³C_(PDB)与δ¹⁸O_(SMOW)值更高，落入典型沉积域范围；而白云鄂博赋矿白云岩的同位素组成介于原生火成碳酸岩（igneous carbonatite）与典型沉积灰岩之间。赛林呼东微晶碳酸盐岩的δ²⁶Mg值低于正常中元古代（Mesoproterozoic）沉积白云岩，而白云鄂博赋矿白云岩大理岩的δ²⁶Mg值更高，与幔源包体（mantle xenoliths）及白云鄂博侵入型碳酸岩（intrusive carbonatite）的δ²⁶Mg值相近。综上，白云鄂博矿床与赛林呼东微晶丘并无成因关联，其既非微晶丘，亦并非蚀变微晶丘。