OXIDE MINERALS OF THE SEPARATION RAPIDS RARE-ELEMENT GRANITIC PEGMATITE GROUP, NORTHWESTERN ONTARIO

The newly discovered Separation Rapids pegmatite group, situated in ma fic metavolcanic host-rocks that represent the eastern extremity of th e Bird River metavolcanic - metasedimentary belt, contains Ontario's f irst occurrences of wodginite-group minerals (mainly wodginite and fer rowodginite), the pyrochlore-group minerals stibiomicrolite, stibiobet afite and yttropyrochlore, ferrotapiolite, and probably the first occu rrence in North America of nigerite from a granitic pegmatite. This ex ample of the rare-element class of granitic pegmatites hosts both bery l- and petalite-subtype pegmatites. Columbite-tantalite and cassiterit e are the predominant oxide species. On the basis of columbite-tantali te compositions, the pegmatites have been divided into an Fe-suite and a Mn-suite. Both beryl and petalite pegmatites occur in each suite. O n 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 adjac ent to the pluton, in which ferrocolumbite coexists with ferrowodginit e, and, with increasing evolution, petalite-bearing pegmatites that co ntain ferrotantalite and wodginite. In individual pegmatites, columbit e-tantalite variation is mainly in Ta/(Ta + Nb). Minor microlite, anti monian microlite and stibiomicrolite are found replacing earlier phase s. Cassiterite is the final Nb-Ta-bearing oxide to crystallize. Pegmat ites belonging to the Mn-suite follow a similar pattern of crystalliza tion, with early manganocolumbite followed by manganotantalite, the la tter coexisting with wodginite. Manganocolumbite within individual sam ples varies appreciably in Mn/(Mn + Fe), whereas the variation in mang anotantalite is mainly in Ta/(Ta + Nb). In pods rich in ''cleavelandit e'' and Li-mica within one of the beryl pegmatites, extreme Mn-enrichm ent has produced near-end-member manganotantalite and W-bearing wodgin ite. 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) indicate s that they were derived from a more F-rich melt than that which produ ced the Fe-suite of pegmatites. Albitization also is more apparent in the Mn-suite of pegmatites. The wall zone of Marko's pegmatite, the la rgest 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 de veloped as a result of interaction of the pegmatite-forming melt with banded ironstones and Fe-Ti-rich metavolcanic host-rocks.