Pyrochlore and columbite-tantalite as indicator minerals for specialty metal deposits

Abstract

Pyrochlore supergroup and columbite-tantalite series minerals can be used as indicators for specialty metal deposits. Their mineral chemistry can be used to target specific specialty metal deposit-types and provide a vector towards mineralisation. A ternary Nb-Ti-Ta discrimination diagram for the pyrochlore supergroup shows that minerals from carbonatites and related deposits occupy the pyrochlore field and extend slightly into the betafite and microlite fields (>35% Nb, <65% Ti, and <45% Ta). Those from peralkaline complexes define a narrow zone paralleling the Nb-Ti join (>40% Nb, <60% Ti, and <10% Ta). Pyrochlore supergroup minerals from Li-Cs-Ta enriched (LCT) pegmatites occupy the microlite field (<90% Nb, ≤40% Ti, and > 15% Ta) whereas those from peraluminous granites plot in the microlite field along the Nb-Ta join, extending slightly into the pyrochlore field (<65% Nb, <10% Ti, and >35% Ta) on the Nb-Ti-Ta ternary diagram. The discrimination diagram for pyrochlore supergroup minerals is based on 349 new electron microprobe analyses from British Columbia and Québec, Canada, combined with 879 published analyses from 50 deposits worldwide. A columbite-tantalite series discrimination quadrilateral indicates that minerals from carbonatites and related deposits are Nb-rich, with Mn/(Mn+Fe) and Ta/(Ta+Nb) atomic ratios ≤ 0.25 and ≤ 0.20, respectively. Columbite from peralkaline complexes plot along the FeNb 2 O 6 –MnNb 2 O 6 join of the quadrilateral and have Mn/(Mn+Fe) ratios of ≤ 0.65 and Ta/(Ta+Nb) ratios of ≤ 0.15. Fe-Nb-rich varieties within this zone slightly overlap with those from carbonatites. Columbite-tantalite series minerals from Li-Cs-Ta (LCT) pegmatites and peraluminous granites are not well constrained but do slightly overlap with columbite-(Fe) from carbonatites. A minor overlap exists between the fields from carbonatites, peralkaline complexes, LCT pegmatites, and peraluminous granites near the columbite-(Fe) corner of the quadrilateral. The discrimination diagram for columbite-tantalite series minerals is based on 153 new electron microprobe analyses from British Columbia combined with over 2500 analyses complied from the literature. Combining the two discrimination diagrams presented here enhances their effectiveness to explore for specialty metal deposits.