Effect and significance of contamination-related compositional melt variations of the Tweerivier and Bulhoekkop carbonatites, South Africa, with special reference to their REE mineralization

南非 Tweerivier 和 Bulhoekkop 碳酸盐岩与污染相关的成分熔体变化的影响和意义，特别是其稀土矿化

• 批准号: 437542348
• 负责人: Privatdozent Dr. Benjamin Walter
• 金额: --
• 依托单位: Rhodes University
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2019
• 资助国家: 德国
• 起止时间: 2018-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/437542348?language=en
• 关键词: Effect; significance; contamination; related; compositional

Carbonatites represent important rare earth elements (REE) deposits, which is reflected by the fact that REE exploration projects worldwide are dominated by carbonatites and accompanying rock types. The extraordinary REE enrichment in carbonatitic melts is generally attributed to a combination of parameters, including low-degree partial melting of enriched mantle sources, crystal fractionation, separation of fluid phases and hydrothermal alteration. However, the effect of contamination by crustal and cogenetic rocks, as a potential first order process is only weakly constrained. Nevertheless, our recent study on Kaiserstuhl carbonatites clearly provided evidence that the interaction of carbonatite magma with silicate wall rocks can enable strong REE enrichment in apatite via the britholite substitution mechanism involving Si. Based on the results from the Kaiserstuhl, the role of variable contaminants needs to be tested in detail to show if this process is of general importance in carbonatitic systems.This project will investigate the impact of carbonatite-country rock interaction on REE enrichment in carbonatites. The study will be performed using field examples from the Tweerivier and Bulhoekkop carbonatites (South Africa) which are known to bear a large variability of crustal xenoliths (BIF, sedimentary dolomites, granites, anorthosites and gabbros) and contain variable amounts of silicate minerals (e.g., amphibole, clinopyroxene, mica, tremolite) that may indicate crustal contamination.To test this hypothesis, whole rock data (XRF and ICP-MS), textural, mineral chemical and isotopic data (microscopy, EMPA, C and O-isotope systematics) will be carried out for the different carbonatites samples to study the effect of contamination by Fe, Mg, Al and Si-rich lithologies. The expected results will allow for (1) the identification of the influence of contamination on the REE pattern of the residual melt, and (2) the identification of mineralogical and compositional variation of major mineral phases.

碳酸盐岩是重要的稀土矿床，世界范围内的稀土勘探项目以碳酸盐岩及其伴生岩类型为主，反映了这一点。碳酸盐岩熔体REE的异常富集一般归因于富集地幔源的低程度部分熔融、晶体分馏、流体相分离和热液蚀变等参数的综合作用。然而，地壳和同生岩石污染的影响，作为一个潜在的一级过程，只有弱约束。然而，我们最近对Kaiserstuhl碳酸盐岩的研究清楚地提供了证据，证明碳酸盐岩岩浆与硅酸盐围岩的相互作用可以通过包含Si的橄榄岩取代机制使磷灰石中REE富集。根据Kaiserstuhl的结果，可变污染物的作用需要进行详细的测试，以表明这一过程在碳酸盐岩系统中是否普遍重要。本项目将研究碳酸盐岩-乡村岩石相互作用对碳酸盐岩中稀土元素富集的影响。这项研究将利用Tweerivier和Bulhoekkop碳酸盐岩（南非）的实地实例进行，这些碳酸盐岩已知具有地壳捕虏体（BIF、沉积白云岩、花岗岩、斜长岩和辉长岩）的巨大变化，并含有可能表明地壳污染的不同数量的硅酸盐矿物（例如角闪孔、斜辉石、云母、透闪石）。为了验证这一假设，将对不同的碳酸盐样品进行全岩数据（XRF和ICP-MS），结构，矿物化学和同位素数据（显微镜，EMPA， C和o同位素系统），以研究富铁，Mg， Al和si岩性污染的影响。预期结果将允许(1)识别污染对残余熔体稀土模式的影响，以及(2)识别主要矿物相的矿物学和成分变化。