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 Bernic Lake pegmatite group (which includes the Tanco pegmatite) is located in the Bird River greenstone belt, which is part of the Archean Superior Province. The Bird River greenstone belt has been historically described as a large, synclinal keel (Trueman, 1980; Cerný et al., 1981); however, mapping by the Manitoba Geological Survey has led to a re-interpretation of the volcanostratigraphic framework of the belt (Gilbert, 2006; Gilbert, 2007; Gilbert et al., 2008). Gilbert (2008) has subdivided the southern part of the Bird River greenstone belt into two distinct (northern and southern) panels, both of which are composed of ca. 2.75-2.72 Ga juvenile, arc-type metavolcanic and associated metasedimentary rocks. These two metavolcanics panels are separated by the fault bounded enclave of the Booster Lake Formation (<2712 ± 17 Ma, Gilbert, 2006), a turbiditic sequence with classic Bouma-type features, penecontemporaneous with clastic sedimentary rocks of the Flanders Lake Formation (Gilbert, 2006).
The Tanco pegmatite
The Tanco pegmatite is a sub horizontal, essentially undeformed, bilobate, saddleshaped 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 has been estimated as ~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 in southeastern Manitoba and has only been examined by drill core and underground mining exposures.
Zoning of the Tanco pegmatite
The Tanco pegmatite is a classic example of a complexly zoned pegmatite. In each of the different zones it is possible to find the different combinations of minerals in association including the ones containing the elements of economic interest: Ta, Li, and Cs. The zonation consists of nine discrete zones. The zones are substantive enough and distinct enough to be mapped but also individually characterized for mining purposes. The outer zones are concentric, whereas the inner zones are layered segments that are locally complex in shape.
Mineralogy
Mineralogy from Tanco is very diverse with more than 100 contained minerals listed in the literature (e.g. Cerný et al., 1996; 1998). Tanco has yielded four holotypes of 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).
Geochemistry
The Tanco pegmatite is a mineralized, peraluminous pegmatite body, belonging to the LCT family, Rare-Element-Li subclass, complex type, subtype petalite (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 highfield-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).