Surface alteration of a melilitite-clan carbonatite and the potential for remote carbonatite detection

Abstract

The varied lithologic facies and mineralogy resulting from emplacement of syngenetic alkaline, ultramafic and carbonatite (AUC) intrusions are made more diverse by variable weathering and alteration. Ultramafic-carbonatite intrusive complexes are a source for many valuable minerals including diamonds and rare earth element minerals. The intrusive bodies are often difficult to detect in the field due to their paucity, weathering, vegetation, and, in some instances, similarity to country rock, especially in the case of carbonatites among sedimentary carbonates. Remote spectroscopic detection is used extensively for geologic mapping yet has not been applied to differentiating sedimentary and igneous carbonate weathering profiles. Here we document the alteration mineralogy of a newly authenticated melilitite-clan carbonatite occurrence in the Avon Volcanic District in southeast Missouri, USA. The presence of lizardite, vermiculite, phlogopite, and andradite in the weathered crust of calcic and dolomitic carbonatites differentiate them from sedimentary dolomites. We apply field and laboratory spectral measurements to determine the feasibility of humid region AUC remote sensing and classification. Automated humid region detection and classification of carbonatites among sedimentary carbonates is shown to be possible using ratios of absorption features in the 2000–2400 nm range as well as features centered near 680, 900, and 1100 nm due transition metal charge transfer and crystal field splitting in garnet, sheet-silicates, and spinel.