WODGINITE-GROUP MINERALS FROM THE SEPARATION RAPIDS RARE-ELEMENT GRANITIC PEGMATITE GROUP, NORTHWESTERN ONTARIO

Wodginite (ideally MnSnTa2O8), and the rarer species, ferrowodginite ( ideally Fe2+SnTa2O8) and titanowodginite (ideally MnTiTa2O8), have bee n discovered in rare-element granitic pegmatites of the complex type, petalite subtype, which occur in the Separation Rapids pegmatite field , northwestern Ontario. Tungsteniferous varieties of wodginite and an unnamed wodginite (''ferrotitanowodginite'') are also described from t his locality. The pegmatites intrude a metavolcanic (greenstone) belt between the English River and Winnipeg River subprovinces of the Canad ian Shield, where they are associated with a 2643 +/- 2 Ma rare-elemen t-enriched granitic intrusion, the Separation Rapids pluton. The tungs teniferous wodginite has an excess positive charge at the C site due t o substitution of Ta by W, which is balanced by the presence of Fe3+ a nd Mn2+ at the B site via the substitutions: (B)[Sn4+] + (C)[Ta5+] dou ble left right arrow (B)[Fe3+] + (C)[W6+], (B)[Sn4+] + 2(C)[Ta5+] doub le left right arrow (B)[Mn2+] + 2(C)[W6+], and 2(B)[Sn4+] + (C)[Ta5+] double left right arrow B[Mn2+] + B[Ta5+] + C[W6+]. These new schemes of substitution result in hypothetical tungsteniferous end-members Mn- 4(2+)(Fe23+Sn2)(W2Ta6)O-32, Mn-4(2+)(Mn22+Sn2)(W4Ta4)O-32, and Mn-4(2)(Mn2+Sn2Ta)(WTa7)O-32. At Separation Rapids, the rare-metal oxides fo llow two distinct evolutionary paths: (i) ferrocolumbite --> ferrocolu mbite + ferrowodginite --> ferrotantalite + ferrowodginite --> microli te-group minerals, and (ii) manganocolumbite --> manganocolumbite + wo dginite --> manganotantalite + wodginite --> microlite-group minerals. Sequence (i) is considered to have arisen From a relatively F-poor ma gma, and sequence (ii), from a magma richer in F, where extreme enrich ment in Mn was perhaps achieved through F-complexing. Wodginite-group minerals are most often found in the albite-enriched regions of the pe gmatites. Magmatic fractionation is considered to be the major process controlling concentration of the rare elements. Titaniferous wodginit e compositions cannot be explained by simple magmatic fractionation (f rom a Fe- or Mn-rich wodginite starting composition); localized intera ction of the pegmatite-forming magma with mafic metavolcanic rocks is proposed for their origin. Of wodginite-group minerals worldwide, only the ''giant'' Tanco pegmatite at Bernic Lake in Manitoba hosts wodgin ite with a spread of compositions comparable to that at Separation Rap ids. On the basis of a striking similarity in geological setting, mine ralogy and age of emplacement with the rare-element pegmatite groups c ommencing 40 lm west in Manitoba (including Tanco), we contend that th e Separation Rapids Pegmatite Group constitutes the eastern limit of t he Cat Lake - Winnipeg River Pegmatite Field.