Carbonatite-hosted niobium deposit at Aley, northern British Columbia (Canada): Mineralogy, geochemistry and petrogenesis

摘要: Abstract The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomite carbonatites of anorogenic affinity emplaced Lower Paleozoic sedimentary carbonate rocks the Devonian. Primary mineralization consists pyrochlore (commonly comprising a U–Ta-rich F-poor core) ferrocolumbite developed as discrete crystals replacement products after pyrochlore. These phases associated heavy minerals (apatite ± magnetite ± zircon ± baddeleyite) precipitated early magmatic history probably formed laterally extensive cumulate layers up to at least 1.5 m thickness. Fractionation copious amounts reflected chemical composition their constituent minerals, which show large variations Nb/Ta value, but near-chondritic Zr/Hf ratio. Alkali-rich metasomatic (in particular, fenites glimmerites) with are barren; bulk these contained rutile, phlogopite and, much lesser extent, amphibole. When passive margin North America became zone plate convergence Cretaceous, host were strongly deformed, manifested structures textures indicative grain comminution, ductile flow, folding locally, brecciation. structure continuity units enriched profoundly affected processes. Interaction crustal fluids complex chemistry resulted dolomitization, fersmite, development hydrothermal parageneses consistent lower greenschist-facies conditions. At late evolutionary stages, was mobilized only very limited extent sequestered variety (fersmite, euxenite, Mg-rich Nb-bearing rutile) typically occurring scarce minute dolomite, quartz chlorite. Progressive enrichment deformed C O isotopes relative primary calcite, coupled changes trace-element phases, indicate that equilibrated wall-rock hosting capable transporting F−, (PO4)3 −, U, Th rare-earth elements, not Nb.