The role of sulfate-rich fluids in heavy rare earth enrichment at the Dashigou carbonatite deposit, Huanglongpu, China
The role of sulfate-rich fluids in heavy rare earth enrichment at the Dashigou carbonatite deposit, Huanglongpu, China
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DOI：
10.1180/mgm.2019.78
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发表时间：
2020-02-01
影响因子：
2.7
通讯作者：
Yardley, Bruce
中科院分区：
文献类型：
--
作者：
Cangelosi, Delia;Smith, Martin;Yardley, Bruce
黄龙铺碳酸岩位于秦岭造山带西北部。大石沟方解石碳酸岩脉是一种特殊的碳酸岩脉，其特征是重稀土元素相对轻稀土元素富集，稀土配分模式平坦，而且大多数碳酸岩脉的核部为石英。它们也是镁橄榄石的经济集中地。方解石碳酸岩脉根据热液改造程度的不同，表现出两种不同的矿物学特征，并反映在稀土元素的分布和富集上。蚀变方解石碳酸岩中的稀土元素主要以独居石-（Ce）为主的岩浆稀土矿物形式存在，其LREE/（HREE+Y）比值为9.9 ~ 17。在热液蚀变方解石碳酸岩中，岩浆独居石-（Ce）部分被HREE富集的第二相完全取代，岩石的Sigma LREE/（HREE+Y）比值为1.1至3.8。石英透镜体中的流体包裹体中保存着控制稀土元素再分配的流体。大部分石英缺乏流体包裹体，但被两个后来的热液石英世代切割，两者都含有富含硫酸盐的流体包裹体，硫酸盐通常以多个捕获的固体存在，以及在溶液中。流体包裹体中硫酸盐的总含量为6 ~>33 wt.% K（2）SO（4）等价。我们解释这些非均质流体包裹体是富硫酸盐流体与方解石碳酸岩脉反应的结果。HREE最终富集的原因是：（1）后期岩浆方解石的HREE逐步富集形成了HREE富集源，（2）REE-SO（4）（2-）络合作用使REE不发生分馏而重新分配;次生稀土成矿以重稀土富集的氟碳酸盐等矿物为主，磷钇矿-（Y）和磷钇矿-（Y），其晶体结构倾向于HREE。
The Huanglongpu carbonatites are located in the north-western part of the Qinling orogenic belt in central China. Calcite carbonatite dykes at the Dashigou open pit are unusual due to their enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE), leading to a flat REE pattern, and in that the majority of dykes have a quartz core. They also host economic concentrations of molybdenite. The calcite carbonatite dykes show two styles of mineralogy according to the degree of hydrothermal reworking, and these are reflected in REE distribution and concentration. The REE in the little-altered calcite carbonatite occur mostly in magmatic REE minerals, mainly monazite-(Ce), and typically have Sigma LREE/(HREE+Y) ratios from 9.9 to 17. In hydrothermally altered calcite carbonatites, magmatic monazite-(Ce) is partially replaced to fully replaced by HREE-enriched secondary phases and the rocks have Sigma LREE/(HREE+Y) ratios from 1.1 to 3.8. The fluid responsible for hydrothermal REE redistribution is preserved in fluid inclusions in the quartz lenses. The bulk of the quartz lacks fluid inclusions but is cut by two later hydrothermal quartz generations, both containing sulfate-rich fluid inclusions with sulfate typically present as multiple trapped solids, as well as in solution. The estimated total sulfate content of fluid inclusions ranges from 6 to >33 wt.% K(2)SO(4 )equivalent. We interpret these heterogeneous fluid inclusions to be the result of reaction of sulfate-rich fluids with the calcite carbonatite dykes. The final HREE enrichment is due to a combination of factors: (1) the progressive HREE enrichment of later magmatic calcite created a HREE-enriched source; (2) REE-SO(4)(2- )complexing allowed the REE to be redistributed without fractionation; and (3) secondary REE mineralisation was dominated by minerals such as HREE-enriched fluorocarbonates, xenotime-(Y) and churchite-(Y) whose crystal structures tends to favour HREE.