Oxide minerals of the separation rapids rare-element granitic pegmatite group, northwestern Ontario

The newly discovered Separation Rapids pegmatite group, situated in mafic metavolcanic host-rocks that represent the eastern extremity of the Bird River metavolcanic-metasedimentary belt, contains Ontario's first occurrences of wodginite-group minerals (mainly wodginite and ferrowodginite), the pyrochlore-group minerals stibiomicrolite, stibiobetafite and yttropyrochlore, ferrotapiolite, and probably the first occurrence in North America of nigerite from a granitic pegmatite. This example of the rare-element class of granitic pegmatites hosts both beryl- and petalite-subtype pegmatites. Columbite-tantalite and cassiterite are the predominant oxide species. On the basis of columbite-tantalite compositions, the pegmatites have been divided into an Fe-suite and a Mn-suite. Both beryl and petalite pegmatites occur in each suite. On the basis of ferrocolumbite compositions, the associated Separation Rapids pluton is considered to be the parent of at least the Fe-suite of pegmatites. The Fe suite includes beryl pegmatites within and adjacent to the pluton, in which ferrocolumbite coexists with ferrowodginite, and, with increasing evolution, petalite-bearing pegmatites that contain ferrotantalite and wodginite. In individual pegmatites, columbite-tantalite variation is mainly in Ta/(Ta+Nb). Minor microlite, antimonian microlite and stibiomicrolite are found replacing earlier phases. Cassiterite is the final Nb-Ta-bearing oxide to crystallize. Pegmatites belonging to the Mn-suite follow a similar pattern of crystallization, with early manganocolumbite followed by manganotantalite, the latter coexisting with wodginite. Manganocolumbite within individual samples varies appreciably in Mn/(Mn+Fe), whereas the variation in manganotantalite is mainly in Ta/(Ta+Nb). In pods rich in "cleavelandite" and Li-mica within one of the beryl pegmatites, extreme Mn-enrichment has produced near-end-member manganotantalite and W-bearing wodginite. Microlite is an important late phase, which is either primary or forms as a replacement, mainly of wodginite. The presence of microlite, lithian mica and topaz in Mn-suite pegmatites (and aplites) indicates that they were derived from a more F-rich melt than that which produced the Fe-suite of pegmatites. Albitization also is more apparent in the Mn-suite of pegmatites. The wall zone of Marko's pegmatite, the largest body in the eastern subgroup and part of the Mn-suite, is unique in hosting titanowodginite, "ferrotitanowodginite", stibiobetafite and struverite. These Fe-, Ti- and Sb-phases are considered to have developed as a result of interaction of the pegmatite-forming melt with banded ironstones and Fe-Ti-rich metavolcanic host-rocks.