Paper No. 61-8
Presentation Time: 2:45 PM
CARBONATITE-RELATED MINERALIZED SYSTEMS, THEIR FOOT PRINT AND SELECTED VECTORING METHODS
Carbonatite-related mineralizing systems are sources of REE, Nb (±Ta), apatite, vermiculite, Cu, Ti, fluorite, Th, U, Zr, Fe, and Ca-carbonate. Examples are Oka, St. Honoré and Aley (Nb; CAN), Mountain Pass (REE; USA), Palabora (Cu, baddeleyite, apatite, vermiculite; ZAF), Bayan Obo (Fe, REE; CHN), Amba Dongar (fluorite; IND), Araxa, Catalao, and Tapira (Nb, apatite, REE, Ti; BRA), Swartbooisdrift (sodalite; NAM), and Ice River (nepheline syenite, REE; CAN). Most carbonatites occur in continental extensional settings, others in zones of orogenic uplift, and in rare cases, oceanic environments. In extensional settings, carbonatites have a strong magnetic and radiometric signature. They occur as isolated pipes, sills, dikes, plugs, lava and pyroclastic flows or as components of alkaline complexes. Mineralized zones are circular-, ring-, irregular- or crescent-shaped, or form pipes, veins, and breccias. Weathering-enriched REE, Nb, and apatite deposits blanket protore. Fenitization (desilicification with the addition of Fe3+, Na, K, ± Ca) surrounds carbonatites or carbonatite complexes. It manifests itself by the presence of Na- and K-amphiboles, aegerine-augite, wollastonite, nepheline, mesoperthite, antiperthite, pale brown mica, and albite. On average, incorporation of fenitization into deposit models increases target size by 60%. Intensity of fenitization may be used to vector towards mineralization. Cone sheets, rings, and radial dikes (e.g. Tundulu, MWI) increase the footprint up to 120 times. Nb, REE, P, Ba, Sr, F, U, and Th are pathfinder elements used in geochemical exploration for carbonatites using soils, stream sediments, tills and vegetation; F, Th, and U are pathfinders in water. Portable XRF is able to determine concentrations of these elements (with the exception of F) in ores, fenitized zones, host rocks, soils, stream sediments, and tills. Pyrochlore, columbite, Na-amphiboles and possibly pyroxenes and fluorite are indicators in stream sediments. Carbonatites in orogenic settings are deformed or truncated. Cross-cutting and textural relationships are obscured by metamorphism; however, carbonatites retain high HFSE content and LREE/HREE ratio. Fenitization zones may be deformed and cut off from carbonatites. Carbonatites are excellent exploration targets.