白云鄂博REE-Nb-Fe矿床的富钠岩石类型及成因分析

现代地质 ›› 2006, Vol. 20 ›› Issue (1): 151-164.

白云鄂博REE-Nb-Fe矿床的富钠岩石类型及成因分析

肖荣阁^1,2，刘敬党^1,2，费红彩³，王翠芝^1,2，黄晓凌^1,2

1中国地质大学地质过程与矿产资源国家重点实验室，北京100083；2中国地质大学岩石圈构造、深部过程及探测技术教育部重点实验室，北京100083； 3中国地质科学院矿产资源研究所，北京100037

收稿日期:2005-06-28 修回日期:2005-12-01 出版日期:2006-01-20 发布日期:2006-01-20
作者简介:肖荣阁，男，教授，博士生导师，1949年出生，矿床学专业，主要从事矿床地球化学与矿产评价预测工作。
基金资助:
国家自然科学基金项目（40073013）；2004 SEG Foundation Student Research Grant

Types and Genesis of Na-Rich Rocks in the Bayan Obo REE-Nb-Fe Deposit, Inner Mongolia, China

XIAO Rong-ge^1,2，LIU Jing-dang^1,2，FEI Hong-cai³，WANG Cui-zhi^1,2， HUANG Xiao-ling^1,2

1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing100083, China;
2Key Laboratory of Lithosphere Tectonics and Lithoprobing Technology of Ministry of Education, China University of Geosciences,
Beijing100083, China;
3Institute of Mineral Resources，Chinese Academy of Geological Sciences，Beijing100037，China

Received:2005-06-28 Revised:2005-12-01 Online:2006-01-20 Published:2006-01-20

摘要/Abstract

摘要：

白云鄂博稀土-铌-铁矿是世界最大的稀土多金属矿床之一，主要产于中元古界富钠岩石、白云岩和富钾板岩中。富钠岩石与含矿碳酸岩、富钾板岩一起在白云鄂博广泛分布，特别是在主矿和东矿南侧分布广泛，也产在含矿围岩和底盘围岩中。富钠岩石包括富钠长石岩、含钠闪石钠长岩脉、钠闪石岩脉及钠闪石碳酸岩脉和钠闪石化蚀变白云岩。富钠岩石在一个地质单元内出现，但成分略有不同，其岩石化学分析表现出富钠特征，大部分样品的w(Na₂O)在6%以上，w(Na₂O+K₂O)＞8%，w(Na₂O)＞w(K₂O)。富钠长石岩的里特曼指数δ为2.96～8.56，n(Al)/n(K+Na)＞1，n(Al)/n(K+Na+Ca)＜1，表明为弱碱性偏铝质岩石。富钠岩石以富集地壳不相容元素、亏损地幔元素为特征，其稀土总量低于矿化白云岩，但轻稀土元素高度富集，具弱负铕异常，与含矿白云质火山岩一致。根据岩石产状、组构及化学成分分析，富钠长石岩属于富钠火山岩，而其他含钠闪石岩脉则属于岩浆分异侵入的岩脉与岩浆热液充填岩脉。钠化交代以霓石化、钠铁闪石化和钠长石化出现为特征。另外，富钠流体与白云岩反应形成了钠铁闪石化白云岩，强烈的钠化蚀变作用主要出现在稀土强烈富集的主矿和东矿。比较蚀变与未蚀变岩石的化学成分，表明蚀变作用引起稀土的重新分配和弱负铕异常。钠铁闪石化白云岩的w(SiO₂)、w(TiO₂)、w(K₂O)和w(Na₂O)升高，w(P₂O₅)降低。钠化蚀变前的碳酸岩中稀土元素已经富集，热液蚀变引起稀土元素的活化和重新分配，但是没有提供新的稀土来源。

关键词: 富钠岩石, 钠长岩, 钠铁闪石脉, 钠化白云岩, 白云鄂博

Abstract:

The giant Bayan Obo REE-Nb-Fe deposit is the world's largest REE deposit, which is hosted mainly by Middle Proterozoic Na-rich rocks, dolomitic rock and K-rich slate. Na-rich rocks occured together with ore-bearing dolomitite and K-rich slate are distributed widely at Bayan Obo, especially in the south area of the Main orebody and East orebody，also occur in the ore-bearing country rocks and the footwall. Na-rich rocks include albitite, arfvedsonite-bearing albitite dyke, arfvedsonite dyke, and arfvedsonite-bearing carbonatite dyke and arfvedsonitized dolomitite. These all occur together in one geologic unit but differ slightly in their compositions. Whole rock analyses of Na-rich rocks have revealed Na-rich compositions. Na₂O contents of most of the analysed samples are much higher than 6 %. The Na₂O+K₂O content is more than 8 %, with Na₂O in excess of K₂O. The Rittmann index (δ) of the albitite is between 2.96 and 8.56，with Al/(K+Na) more than 1 and Al/(K+Na+Ca) less than 1, indicating that these are alkali metaluminous rocks. The Na-rich rocks are characterized by enrichment of crustal-derived incompatible trace elements, and depletion of mantle-derived elements. Rare earth elements, especially light REE in Na-rich rocks are extremely abundant. There is no distinct Eu anomaly. The total amounts of REE in Na-rich rocks are lower than that of mineralized dolomitic rocks. These characteristics are similar to those of the dolomitic volcanic rocks. Based on their occurrence, structure and chemical compositions, the albite rocks are inferred to have been Na-rich volcanic rocks, whereas the arfvedsonite-rich dykes are magmatic hydrothermal dykes and the dykes of magmatic differentiation.Sodic alterations at Bayan Obo mainly occur in the forms of fenitization, arfvedsonization and albitization. In addition, the reaction of Na-rich fluid with dolomitite has produced an arfvedsonitized dolomitite.The most intensive alterations at Bayan Obo occur in the Main and East orebodies, where REE have also been enriched. Compared to the compositions of unaltered and altered dolomitites in terms of bulk rock, trace elements and REE, alteration has caused REE differentiation and a weak negative Eu anomaly. The arfvedsonitized dolomitite is high in SiO₂, TiO₂, K₂O and Na₂O, and depleted in P₂O₅ compared to the protolith. It is indicated that REE had already been enriched in the original dolomitite prior to sodic alteration (fenitization). Hydrothermal process caused modification and redistribution of REE but did not add any significant new REE to the deposit.

Key words: sodic rock, albitite, arfvedsonite, arfvedsonitized dolomitite, Bayan Obo

中图分类号:

P581.1

肖荣阁刘敬党费红彩王翠芝黄晓凌. 白云鄂博REE-Nb-Fe矿床的富钠岩石类型及成因分析[J]. 现代地质, 2006, 20(1): 151-164.

XIAO Rong-ge，LIU Jing-dang，FEI Hong-cai，WANG Cui-zhi，HUANG Xiao-ling. Types and Genesis of Na-Rich Rocks in the Bayan Obo REE-Nb-Fe Deposit, Inner Mongolia, China[J]. Geoscience, 2006, 20(1): 151-164.

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