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Geology of the Boss Mountain Mine, British Columbia Soregaroli, Arthur Earl

Abstract

Detailed investigation of the Boss Mountain molybdenite deposits, which are on the northeast slope of Takomkane Mountain approximately 35 miles north-northeast of 100 Mile House, British Columbia, was undertaken to determine the origin of the deposits, controls of mineralization, effects of mineralization on the host rock, origin of the breccia bodies, and the relationship of these features to the Boss Mountain Stock. The deposits occur in granodiorite and porphyritic biotite granodiorite phases of the composite Takomkane Batholith near an epizonal Cretaceous quartz monzonite porphyry body, the Boss Mountain Stock. Molybdenite occurs in economic concentrations in two classes of deposits: 1) Breccia Deposits, which include fracture zones, and 2) Vein Deposits, which include both single and multiple systems. The sequence of ore formation, which includes rhyolite porphyry and rhyolite dyke emplacement, breccia formation, fracture development, mineralization, and alteration, is directly related to the oscillatory emplacement of the Boss Mountain Stock. The Boss Breccias, including Phase I Breccia, Quartz Breccia, and Phase III Breccia, were formed by pulsating magmatic activity acting on an irregularity on the side of the magma chamber (Boss Mountain Stock). The rock above this irregularity was fractured by magmatic advances. Withdrawal of magmatic pressure permitted collapse of the overlying fractured rock forming breccia bodies. Rhyolite porphyry dykes, apophyses of the Boss Mountain Stock, preceded and accompanied Phase I Breccia formation. Non-porphyritic rhyolite dykes cut Phase I Breccia and have been engulfed by later breccia phases. Four stages of rock alteration genetically related to the deposits, including (in chronological order): Stage 1, garnet-hornblende Stage 2, biotite; Stage 3, microperthite-chlorite-sericite; and. Stage k, chlorite-talc, have been identified and delineated. Stage 1 formed in mylonite zones around the Boss Mountain Stock, Stages 2 and 3 around centres of mineralization and Stage H- occurs in and near shear zones. Five periods of fracture development, each of which contains quartz veins of unique mineralogy and characteristics, were interspersed with breccia formation, alteration and mineralization. Mineralization accompanied breccia formation and fracture development. Molybdenite was introduced, during three separate periods of mineralization, two of which were separated by a barren stage that produced a complex mineral assemblage. Pyrite accompanies all stages of mineralization. Pleistocene (?) alkali basalt dykes related to Takomkane Volcano, which forms the twin summits of Takomkane Mountain, cut all rocks and ore structures. The volcanic rocks of the volcano contain xenoliths of granodiorite, glassy black augit and peridotite.

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