CARBONATITE WEATHERING MINERALOGY

Recognition of carbonatites in humid regions is likely underrepresented. Global distribution maps of known occurrences show a dearth of carbonatites in regions with tectonic histories comparable to drier regions with dense occurrences, for instance the humid east coast of South America and the dry southwest African coast. This disparity is likely in part an artifact of weathering and dense vegetation. An important step in improving our understanding of carbonatite distribution and genesis is to understand the weathering mineralogy, which can be used as a proxy for the primary igneous assemblages. Documentation of weathering facies is especially important in regions with carbonate country rock where identification of igneous carbonates is challenging. To date, most studies of carbonatite weathering mineralogy have been focused on the rare earth element mineralization. Here we present X-ray diffraction analysis of carbonatite-related lithologies from the Avon Volcanic District in southeast Missouri, USA. The dominant minerals found to be associated with the weathering of carbonate-rich intrusives, are lizardite, vermiculite, and in extreme cases kaolinite and ferrous celadonite. These results are consistent with the presence of primary mafic minerals identified in some of the carbonatites. Lizardite, talc, and chlorite are formed as a result of low temperature mafic mineral alteration. Near-surface weathering of these minerals, along with ubiquitous phlogopite-biotite, can lead to magnesian vermiculite mineralization. Extensive laterization, found in one Avon location, leaves kaolinite, quartz, and celadonite as the residual mineralogy with the Al/Mg wt% oxide ratio reaching >4.5. The weathering products identified here may serve as important indicator minerals in carbonatite exploration in humid regions.