巴西砂矿金刚石产地特征及比较研究

Origin Characteristics of Alluvial Diamonds in Brazil and Its Comparative Study

金刚石产地来源的确定是国际性难题,其中由于砂矿金刚石可能存在搬运过程的混合,其产地来源的确定更加具有争议性。巴西金刚石绝大部分来源于砂矿,从成矿地质背景、形貌和颜色、内部结构、矿物包裹体、微量元素、C同位素组成分布等六个要素总结分析了巴西金刚石研究的相关成果,为国际砂矿来源金刚石产地辨识提供参考。研究结果显示,巴西大部分矿区金刚石的特征与世界范围其它矿区的相似,以橄榄岩型为主,难以相互区分。但其中Juina地区和Machado河产地的金刚石具有独特的氮含量(无氮的Ⅱ型为主)和聚集状态,内部结构、矿物包裹体组合及温压环境特征均显示出深部地幔来源的特点,和巴西其它产地具有一定的区分度。与世界范围不同产地砂矿来源金刚石的基本特征对比显示,不同国家具体地区砂矿金刚石在上述不同要素的组合上存在一定的差异性,但总体上要借助现有资料进行产地来源的准确判断还非常困难,需要进行更深入细致的工作。

It has been an international challenge to determine diamond origin, especially of the alluvial diamonds, because they are usually mixed with other stuffs during transportation. In this paper, the essential characteristics and related research achievement of alluvial diamonds in Brazil are summarized concerning six perspectives, including geologic information, morphology and colour, internal structure, mineral inclusions, trace elements and C i sotopic composition, which may offer a reference for origin of the alluvial diamonds world wide. The study shows that the most diamonds in Brazil are similar to the other diamonds worldwide and it＇s hard to distinguish because they both largely contain peridotite. However, the diamonds from the Juina area and Machado River are distinguished in terms of their nitrogen content and aggregation（a large proportion of type Ⅱ ）. And their internal structures, mineral inclusions and associations, forming environment are results of inner mantle,which are different from other deposits in Brazil. Meanwhile, the study on comparison a mong alluvial diamonds from different areas worldwide, shows that alluvial diamonds of dif ferent areas in different countries may have a certain difference in those perspective men tioned above, but it＇s still very difficult to discriminate diamond origins accurately with the existing data, so some in-depth and meticulous work need to be done.