Tindle, Andrew G.; Breaks, Fred W. and Webb, Peter C.
Wodginite-group minerals from the Separation Rapids rare-element granitic pegmatite group, northwestern Ontario.
Canadian Mineralogist, 36(2),
Wodginite (ideally MnSnTa 2 O 8 ), and the rarer species, ferrowodginite (ideally Fe (super 2+) SnTa 2 O 8 ) and titanowodginite (ideally MnTiTa 2 O 8 ), have been 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 this locality. The pegmatites intrude a metavolcanic (greenstone) belt between the English River and Winnipeg River subprovinces of the Canadian Shield, where they are associated with a 2643+ or -2 Ma rare-element-enriched granitic intrusion, the Separation Rapids pluton. The tungsteniferous wodginite has an excess positive charge at the C site due to substitution of Ta by W, which is balanced by the presence of Fe (super 3+) and Mn (super 2+) at the B site via the substitutions: B [Sn (super 4+) ] + C [Ta (super 5+) ] <--> B [Fe (super 3+) ] + C [W (super 6+) ], B [Sn (super 4+) ] + 2 C [Ta (super 5+) ] <--> B [Mn (super 2+) ] + 2 C [W (super 6+) ], and 2 B [Sn (super 4+) ] + C [Ta (super 5+) ] <--> B [Mn (super 2+) ] + B [Ta (super 5+) ] + C [W (super 6+) ]. These new schemes of substitution result in hypothetical tungsteniferous end-members Mn (super 2+) 4 (Fe (super 3+) 2 Sn 2 ) (W 2 Ta 6 )O 32 , Mn (super 2+) 4 (Mn (super 2+) 2 Sn 2 ) (W 4 Ta 4 )O 32 , and Mn (super 2+) 4 (Mn (super 2+) Sn 2 Ta) (WTa 7 )O 32 . At Separation Rapids, the rare-metal oxides follow two distinct evolutionary paths: (i) ferrocolumbite --> ferrocolumbite + ferrowodginite --> ferrotantalite + ferrowodginite --> microlite-group minerals, and (ii) manganocolumbite --> manganocolumbite + wodginite --> manganotantalite + wodginite --> microlite-group minerals. Sequence (i) is considered to have arisen from a relatively F-poor magma, and sequence (ii), from a magma richer in F, where extreme enrichment in Mn was perhaps achieved through F-complexing. Wodginite-group minerals are most often found in the albite-enriched regions of the pegmatites. Magmatic fractionation is considered to be the major process controlling concentration of the rare elements. Titaniferous wodginite compositions cannot be explained by simple magmatic fractionation (from a Fe- or Mn-rich wodginite starting composition); localized interaction 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 wodginite with a spread of compositions comparable to that at Separation Rapids. On the basis of a striking similarity in geological setting, mineralogy and age of emplacement with the rare-element pegmatite groups commencing 40 km west in Manitoba (including Tanco), we contend that the Separation Rapids Pegmatite Group constitutes the eastern limit of the Cat Lake-Winnipeg River Pegmatite Field.
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