Columbite-tantalite mineral chemistry from rare-element granitic pegmatites: Separation Lake area, NW Ontario, Canada

The Separation Lake area is host to the most important rare-element pegmatites in Ontario, Canada. They include the Big Whopper and Big Mack petalite pogmatite systems which potentially represent the world's second largest lithium deposit of this type. The pegmatites occur in two distinct clusters adjacent to the Separation Rapids pluton which is thought to be the source of the rare-elements. Beryl-type and complex-, petalite-subtype pegmatites are the most common and a few pegmatites have characteristics similar to the lepidolite-subtype. This study reveals that columbite-tantalite in the pegmatites has an extremely wide range of composition from primitive ferrocolumbite to evolved, almost end-member manganotantalite. Evidence is provided that melt evolution resulted in increased fluorine activity (as seen in microlite compositions) and that in situ fractionation of magma within individual pegmatites often led to the crystallization of rare-element-enriched, Li mica-fluorapatite-cleavelandite pods. Zonation patterns seen in backscattered electron images show primary compositions of columbite-tantalite were modified by secondary processes related to extreme fractionation and involving the late stage development of albitic units: in individual pegmatites. This alteration led to recrystallization of columbite-tantalite and produced compositions with lower Ta contents, but with little change in Mn content.