Field, textural, and geochronological investigations into the intrusion dynamics and crystallization histories of the Separation Rapids rare-metal pegmatites, northwestern Ontario

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Ching, John Garnet
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The Separation Lake greenstone belt (SLgb) is host to a suite of potentially economic Neoarchean Lithium – Cesium – Tantalum (LCT) complex type, petalite subtype granitic pegmatites (Big Whopper, Big Mack, Snowbank, Glitter and Marko’s). Field observations on these pegmatites such as well-developed layering, ptygmatic folds, boudins, and mullions have in the past led investigators to infer that the pegmatites experienced pervasive high-strain deformation after emplacement. In this study, we reinterpret the field relations between the pegmatites and country rock, incorporate pegmatite microtextures, and date, by the U-Pb method, the time of crystallization to reassess the role of regional-scale deformation in the distribution and morphology of these pegmatites. Uranium-Pb ratios in monazite and titanite were measured, in-situ, by LA-ICP-MS to establish the timing of regional metamorphism and pegmatite intrusion. Monazite in the country rock yields the same age, within error, of the ca. 2650 Ma Separation Rapids pluton and suggests intrusion and metamorphism were coeval in the area. The range of monazite U-Pb ages of ca. 2637 Ma, ca. 2617 Ma, and ca. 2602 Ma indicate that the pegmatites crystallized after metamorphism / deformation and may not be related to one magmatic event. Field relationships and microtextural data indicate that deformation is induced locally (i.e., not regional) and results from emplacement mechanisms during pegmatite intrusion. The dominant fabric in the country rock is defined by the alignment of hornblende and is cut by the pegmatite. Moreover, all pegmatites have a metasomatic halo of biotite that overprints the S2 fabric in the country rock. Essentially, the pegmatites acted as viscous indenters into fluidized, weakened, ductile country rock. Recognizing the fact that deformation is not a result of regional-scale deformation has important implications for exploration including: 1) pegmatites need not intrude along structural corridors, 2) the dominant direction of propagation is now oriented subvertical with the surface expression of several pegmatites suggesting the bulk of the intrusions lies buried under the current erosional surface; and 3) undiscovered LCT pegmatites may lie within other segments of metavolcanic rocks along the English River – Winnipeg River subprovincial boundary, as similar emplacement ages across the SLgb and Bird River belt of eastern Manitoba (e.g. Big Mack at ca. 2637 Ma and Tanco at ca. 2640 Ma) may infer continuous mineralization along the boundary.
Pegmatite, Big Whopper, Big Mack, TANCO