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The Old Dick and Bruce massive sulfide deposits are hosted by a 3-km-thick Lower Proterozoic bimodal submarine volcanic sequence. Both deposits are at the contact between metabasalt flows and metamorphosed volcaniclastic strata of the Bridle Formation and overlying massive quartz porphyry of the Dick Rhyolite. Mixed metavolcanic and metamorphosed volcaniclastic rocks that may be part of the Butte Falls Tuff or Hillside Mica Schist overlie the Dick Rhyolite. Diverse, widespread, and locally intense hydrothermal premetamorphic alteration is spatially associated with massive sulfide mineralization is noted in both the Bridle Formation and in the Dick Rhyolite. Alteration patterns and chert suggest that hydrothermal activity was simultaneous with deposition of massive sulfide minerals and emplacement of the Bridle Formation. The recurrent, long-lived hydrothermal activity was probably driven by heat from synvolcanic mafic and felsic hypabyssal sills intruding the Bridle Formation. The delineation of previously unmapped hydrothermal alteration patterns stratigraphically beneath the Old Dick and Bruce deposits suggests a recurrent, metal-rich exhalative system that could be mineralized at different stratigraphic levels.

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