白云鄂博矿区赋矿“白云质大理岩”成因的稳定同位素证据

The Genesis of the Ore Hosting “Dolomitic Marble” in the Bayun Obo Deposit, Inner Mongolia,China:Constrained by Isotopic Results

内蒙古白云鄂博铁、稀土、铌矿床是举世闻名的超大型稀土矿床。其矿床地质特征和成矿地质环境十分独特 ,在国内外引起广泛注意。关于矿床的成因争论很多 ,其中 ,赋矿“白云质大理岩”的成因最为人们所关注 ,成为争论的焦点。本文在前人工作的基础上补充了S、C、O同位素研究。综合现有资料得出 :“白云质大理岩”的高REE和Nb含量 ,REE分布型式 ,低87Sr/ 86Sr初始比值和接近岩浆岩的Si同位素组成 ,为其岩浆成因提供了证据。“白云质大理岩”中碳酸盐和硅酸盐矿物的高δ18O值及全岩和重晶石硫的δ3 4 S值表明 ,岩石与海水发生过明显的交换作用 ,进一步为岩石的海底火山成因提供了证据。“白云质大理岩”中碳酸盐矿物的δ13 C值也可以用碳酸岩受到海水碳酸盐混染加以解释。由此看来 ,“白云质大理岩”

The Bayan Obo Fe-REE-Nb ore deposit, Inner Mongolia, China is a super large REE deposit. This deposit attracts attentions of many geologists and geochemists for its unique geological characters and geotectonic background. However, there are quite a number of controversies on various aspects of its genesis, such as when the ore deposit was formed, where the ore forming materials came from and what conditions and geotectonic environment the deposit was formed. Among them, the genesis of ore bearing “dolomitic marble” is a focus point of debating. Isotopic methods are important tools for study on ore deposits. Therefore, supplemental stable isotope investigation was undertaken in this study to reach more information on the forming conditions of “dolomitic marble”. The high contents of REE and Nb, the REE distribution pattern, the low initial 87Sr/ 86Sr ratio and low δ 30Si values of trace silica in “dolomitic marble” all indicate its magmatic origin. The high δ 18O values of carbonate and silicate minerals, and the relatively high δ 34S values indicate that this rock had experienced isotopic exchange with sea water, implying their submarine volcanic origin. The δ 13C values observed in carbonate and silicate minerals can also be explained by carbonatite assimilated by oceanic carbonate. Therefore, based on its isotopic characters, the “dolomitic marble” is more likely of volcanic carbonatite rock formed during middle Proterozoic era.