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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. [ABSTRACT FROM AUTHOR]

Understanding the various emplacement pathways of ultramafic‑carbonatite intrusive complexes and mapping of their global distribution are in an early stage. Discerning the pattern of their emplacement can reveal information about earth's dynamics and guide exploration for associated economic deposits. Here we apply multiple spatial analysis methods and synthetic aperture radar (SAR) data to assess the correlation between outcrops of ultramafic‑carbonatite intrusives and regional craton-scale fractures and assign targets for further exploration. Kernel density cluster, two-point azimuth, and the Fry method two-dimension spatial analyses are applied to 58 known outcrops of intrusives in the Avon Volcanic District (AVD), southeast Missouri USA. The results of the spatial analysis show linear trends among the outcrops with three primary orientations: northwest, northeast, and east-west and a strong correlation with regional fault zones. SAR data processing and analysis guided by the spatial results reveals rhombic lineament patterns with sides that parallel the northwest-northeast emplacement patterns of the AVD. These rhombic structures are related to possible extensional stepover duplexes, likely formed during Precambrian North American plate rifting along transfer faults. Reactivation of the Precambrian structures during high angle dip slip and wrench faulting that was penecontemporaneous with emplacement of the AVD in the Devonian likely provided pathways for the mantle-derived intrusives. These findings can guide exploration efforts and help us understand the emplacement of a unique intracontinental intrusive complex. [ABSTRACT FROM AUTHOR]

Here we report field, petrographic, and geochemical analyses of the southeast Missouri Avon Volcanic District intrusive rocks and present the first combined textural and geochemical evidence for the presence of a primary magmatic carbonatite phase among ultramafic dikes, pipes, and diatremes of olivine melilitite, alnöite, and calciocarbonatite. The δ 13 C VPDB values measured for primary calciocarbonatite as well as carbonates in olivine melilitite and alnöite rocks range from − 3.8‰ to − 8.2‰, which are within the typical range of mantle values and are distinct from values of the carbonate country rocks, 0.0‰ to − 1.3‰. The carbonate oxygen isotope compositions for the intrusive lithologies are in the range of 21.5‰ to 26.2‰ (VSMOW), consistent with post-emplacement low temperature hydrothermal alteration or kinetic fractionation effects associated with decompression and devolatilization. Metasomatized country rock and breccia-contaminated igneous lithologies have carbonate δ 13 C VPDB values gradational between primary carbonatite values and country rock values. Unaltered sedimentary dolomite breccia and mafic spheroids entrained by calciocarbonatite and the lack of microstratigraphic crystal growth typical of carbonate replacement, also exclude the possibility of hydrothermal replacement as the cause of the magmatic-textured carbonates. Rare earth element (REE) patterns for the alnöite, olivine melilitite, and carbonatite are similar to each other with strong light REE enrichment and heavy REE depletion relative to MORB. These patterns are distinct from those of country rock rhyolite and sedimentary carbonate. These data suggest that rocks of the Avon Volcanic District represent a single ultramafic-carbonatite intrusive complex possibly derived from a single mantle source. [ABSTRACT FROM AUTHOR]