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

碳酸盐岩是重要的稀土矿床，这反映在世界范围内稀土勘探项目以碳酸盐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是多种因素综合作用的结果，包括富集地幔源区的低程度部分熔融、结晶分异、流体相分离和热液蚀变。然而，地壳和同生岩石混染的影响，作为一个潜在的一级过程，只有弱约束。然而，我们最近对Kaiserstuhl碳酸岩的研究清楚地提供了证据，碳酸岩岩浆与硅酸盐围岩的相互作用可以通过涉及Si的Britholite替代机制使磷灰石中的稀土元素强烈富集。根据Kaiserstuhl的结果，需要详细测试可变污染物的作用，以表明该过程在碳酸岩系统中是否具有普遍重要性。本项目将研究碳酸岩-围岩相互作用对碳酸岩中稀土富集的影响。该研究将使用来自Tweerivier和Bulhoekkop碳酸岩（南非）的野外实例进行，已知这些碳酸岩具有很大的地壳捕虏体（BIF、沉积辉长岩、花岗岩、斜长岩和辉长岩）变异性，并含有不同量的硅酸盐矿物（例如，为了验证这一假设，将对不同的碳酸岩样品进行全岩数据（X射线荧光分析和ICP-MS）、结构、矿物化学和同位素数据（显微镜、EMPA、C和O同位素系统学），以研究富铁、镁、铝和硅岩性的污染效应。预期的结果将允许（1）残留熔体的稀土模式上的污染的影响的识别，和（2）主要矿物相的矿物学和成分变化的识别。

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.