The Buck and Pegli lithium deposits, southeastern Manitoba; the problem of updip fractionation in subhorizontal pegmatite sheets

      Two sheets of complex rare element pegmatites of the petalite subtype crop out in the eastern part of the Bernic Lake pegmatite group, Cat Lake-Winnipeg River field (Bird River subprovince, western Superior province of southeastern Manitoba. They dip west at 7 degrees to 12 degrees ; drilling reveals a downdip extent of 2,400 to 3,000 m, a north-south width of more than 250 m, and thickness varying from 2 to 16 m. Zoning, largely symmetrical, grades from primitive border-wall-core units updip into a complex border-wall-intermediate core-margin core sequence. Mineral assemblages diversify in this direction from K feldspar + albite + quartz + muscovite (+ or - Bi) to suites carrying beryl, petalite, spodumene, triphylite, cassiterite, columbite-tantalite, pollucite, traces of sulfosalts, and multigenerational sequences of amblygonite-montebrasite, tourmaline, Al to Li,Al micas and apatite. Besides extensive enrichment in Li, Be, Sn, Nb < Ta and Cs evident from the mineral assemblages of both pegmatites, strong updip gradients also are shown by fractionation of K feldspar and micas (K/Rb = 32.8-7.2, 27.3-3.6; Rb/Cs = 48.5-13.8, 31.2-7.3, respectively), beryl (Na/Li = 7.5-2.5, Cs = 0.18-1.2 wt %), and columbite-tantalite (Mn/(Mn + Fe) at. = 0.4-0.99, Ta/(Ta + Nb) at. = 0.22-0.60). In contrast, compositional gradients across the more or less concentric zoning of the pegmatite sheets are largely moderate, and relatively best expressed in the topmost parts. The strong compositional gradient along the extensive subhorizontal dip of the thin pegmatite sheets could not originate by thermogravitational convection, gravity-driven transport through the melt, or metasomatism in the solidified pegmatite. The geometry, attitude, and rapid consolidation of the pegmatite sheets provide strong arguments against internal diversification of a homogeneous melt alone. A trainlike intrusion from a layered magma chamber, or a flow differentiation during the intrusion of heterogeneous melt induced by internal differences in fluidity and density, or a combination of both seem to be a required component of the overall fractionation process.

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