Petrological, structural and magnetic studies of a layered basic intrusion, Bird River Sill, Manitoba

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Trueman, David Lawrence.
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The Bird River Sill, situated in southeastern Manitoba is a layered ultramafic to basic instrusive. In this sill 45 layers consisting of serpentinized periodotite interlayered with chromite bearing layers, pyroxenite, gabbro, anorthositic gabbro, anorthosite, hornblende gabbro, and porphyritic gabbro form a thickness of approximately 2,000 feet. Primary structures in the chromite bearing layers and in the gabbros are evidence of thermal or density currents in the magma during crystallization and cryptic variation of anorthite percent in the plagioclase shows an inverted trend indicating crystallization downwards from the roof as well as accumulation on the floor of the magma chamber. Complex faulting across and parallel in strike to the layering of the Sill has resulted in apparent change in thickness of, and omission of rock units in the Bird River Sill. Magnetic susceptibility study and a vertical field magnetic survey indicates a lack of magnetite in the gabbroic rocks. The consequent magnetic signature can thus be utilized with a knowledge of the geology to prospect for non-magnetic chromite in the Bird River Sill.