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元素(REE)矿床，这反映在世界各地的稀土勘探项目都以碳酸岩及其伴生岩石类型为主。碳酸盐岩熔体中稀土元素的异常富集度一般归因于多种参数的综合作用，包括富集源的低程度部分熔融、晶体分馏、流体相分离和热液蚀变。然而，作为一种潜在的一级过程，地壳和共生岩石污染的影响只受到很弱的限制。然而，我们最近对凯泽斯图尔碳酸盐岩的研究清楚地证明，碳酸盐岩岩浆与硅酸盐围岩的相互作用可以通过含硅的沸石替代机制使稀土元素在磷灰石中强烈富集化。基于Kaiserstuhl的结果，需要详细测试可变污染物的作用，以表明这一过程在碳酸盐岩系统中是否具有普遍重要性。该项目将调查碳酸岩-围岩相互作用对碳酸盐岩中稀土元素富集的影响。这项研究将使用Tweerivier和Bulhoekkop碳酸盐岩(南非)的野外实例进行，这些碳酸盐岩已知具有较大的地壳包体(BIF、沉积白云岩、花岗岩、斜长岩和辉长岩)的变异性，并含有可能指示地壳污染的不同数量的硅酸盐矿物(例如角闪石、单斜辉石、云母、透闪石)。为了验证这一假设，将对不同的碳酸盐岩样进行全岩数据(XRF和ICPMS)、结构、矿物化学和同位素数据(显微镜、EMPA、碳和氧同位素系统学)，以研究富铁、镁、铝和硅同位素岩性的影响。预期结果将有助于(1)确定污染对残余熔体REE模式的影响，以及(2)确定主要矿物相的矿物学和成分变化。