Summary:
The Tanco pegmatite is part of the Winnipeg River-Cat Lake pegmatite field. This vast pegmatite field has been subdivided into two pegmatite districts, and subsequently into several different pegmatite groups according to their mineralogy, geochemistry and location (Cerný et al., 1981).
The Tanco pegmatite is a subhorizontal, essentially undeformed, bilobate, saddle-shaped body. The pegmatite is about 1520 m long, 1060 m wide, and up to ~100 m thick, thinning toward the edges. The volume of the pegmatite is ~21,850,000 m3, the mass is ~57,430,000 tonnes, and its average density is 2.63 g/cm3 (Stilling et al., 2006). It occurs mostly under Bernic Lake, southeastern Manitoba and it is most known by drill core and underground mining exposures. This highly fractionated pegmatite of the lithium-cesium-tantalum (LCT) family has an extensive mineralogy (more than 100 listed minerals) and it is zoned (consists of nine internal zones). The outer zones are concentric, whereas the layered inner zones are segmented and locally complex in shape.
Mineralogy from Tanco is very extensive with more than 100 minerals listed in the literature (e.g. Cerný et al., 1996; 1998). Tanco has also yielded holotypes of four new minerals: cernýte (Kissin et al,. 1978), tancoite (Ramik et al,. 1980), diomignite (London et al., 1987), titanowodginite (Ercit et al., 1992), Ercitite (Fransolet et al., 2000), Groatite (Cooper et al., 2009).
The Tanco pegmatite is a mineralized, peraluminous pegmatite body, belonging to the LCT family, Rare-Element-Li subclass, complex type, subtype petalite (updated classification by Cerný and Ercit, 2005).
The bulk mode of Tanco is close to a muscovite granite, with the exception of 8 wt.% petalite, 2.8 wt.% lithian micas, and 1 wt.% primary spodumene. The contents of all other accessory silicates and phosphates are only in tenths of a wt.%, and minerals of the high-field-strength elements account for mere hundredths to thousandths of a wt.% each. Accordingly, the bulk chemical composition of the pegmatite corresponds to that of a peraluminous, moderately silicic, high-phosphorus, Na>K granite, with enrichment in Li, Rb, Cs and F; moderate contents of Tl, Be, B, Ga, Sn, Nb and Ta, and remarkable depletion in Fe, Mn, Mg, Ca, Ba, Sc, Ti and Zr. A very high degree of fractionation is shown for the bulk pegmatite by the values K/Rb 4.7, K/Cs 9.3, Rb/Cs 2.0, Rb/Tl 137, Fe/Mn 0.63, Mg/Li 0.02, Al/Ga 917, Zr/Hf 2.6, Zr/Sn 0.21 and Nb/Ta 0.19 (Stilling et al., 2006).