Mineable zones of the Spar Lake deposit occur where argentiferous Cu sulfides and native Ag formed cements and replaced certain earlier cements and clasts in the gently dipping middle quartzite beds of the upper member of the Revett Formation, middle Proterozoic Belt Supergroup. The Cu sulfides and native Ag are part of a large, zoned system of authigenic ore and gangue minerals at Spar Lake. Mineral zone boundaries of ore and gangue phases cross all 5 stratigraphic units of the upper member. Deduced depositional environments for the host sedimentary rocks include beach and nearshore slope environments for the lower quartzite beds and subtidal(?) channels for the middle quartzite beds. The deposit must be epigenetic because mineral zone boundaries cross every facies in the sequence of beach and nearshore slope sediments. The distributions of mineral zones and ore grades were controlled by 2 factors, one inherited from sedimentation and the other from preore diagenesis.-from Authors
Additional publication details
Genesis of the Spar Lake strata-bound copper- silver deposit, Montana: part I. Controls inherited from sedimentation and preore diagenesis ( USA).