The Tanco pegmatite deposit at Bernic Lake, in southeastern Manitoba, is a virtually completely hidden, essentially undeformed, subhorizontal, bilobate, saddle-shaped body ~1520 m long, 1060 m wide and up to ~100 m thick. The highly fractionated pegmatite of the LCT family, rare-element class, petalite subtype, consists of nine internal zones. The outer zones are concentric, whereas the layered inner zones are segmented and locally complex in shape. A 3D computer model representation of the pegmatite assisted in calculation of volumes and compositions of individual zones and of the whole pegmatite, based on 102 km of 1355 drill-hole intersections, underground observations, measured and estimated mineral modes of the zones, zone-specific compositions and densities of minerals, and ore grades. The early outer zones tend to show granitic modes, but the inner zones strongly deviate in proportions of feldspars, or by high enrichment in lithium minerals, and some of the zones are virtually monomineralic. The bulk mode of Tanco is granitic, except ~9 vol.% petalite (including spodumene + quartz pseudomorphs). Despite also being economically significant for Cs, Ta and Li, the pegmatite contains less than 1 vol.% of any other non-micaceous minerals, and less than 0.1% of most of them. The bulk chemical composition closely reflects the modal relationships: it is peraluminous, and granitic, with prominent 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. Zonal and bulk chemical composition is in rough agreement with the experimental thermal trough in the system Ab + Qtz + Ecr at 2 kbar P(H2O), but shifted toward the Ab apex by elevated B, P and F, and by the voluminous wall-zone. The composition of the wall zone (20) is remarkably similar to that of bulk Tanco, except for the elevated contents of rare elements in the latter that are concentrated in the intermediate and inner zones. In turn, zone (20) is very close to the composition of nearby, somewhat less fractionated peraluminous leucogranites that are parental to local genetically related pegmatite groups. The wall zone (20), which composes 44 vol.% of Tanco, represents by analogy a petrogenetic link between an undisclosed parent leucogranite and the Tanco deposit. These relationships document derivation of the Tanco pegmatite and its analogues elsewhere by igneous differentiation from a fertile leucogranitic magma.
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