Genesis of the Spar Lake strata-bound copper-silver deposit, Montana: Part I. Controls inherited from sedimentation and preore diagenesis

Mineable zones of the Spar Lake deposit occur where argentiferous copper sulfides and native silver 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 copper sulfides and native silver 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 five stratigraphic units of the upper member.Deduced depositional environments for the host sedimentary rocks include beach and near-shore 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.Mineral zonation has been mapped, and seven major associations, each named for its most abundant sulfide and/or most characteristic gangue cement, are recognized. Zones that appear to be developed on a regional scale include, in spatial order, pyrite-calcite, chalcopyrite-ankerite, and the lavender (hematitic) zone. Minor concentrations of base and precious metals occur along boundaries between the hematite and chalcopyrite-ankerite zones, and between the chalcopyrite-ankerite and pyrite-calcite zones; however, at the major concentrations of metals in the Revett Formation, including the Spar Lake deposit, additional mineral zones are developed between the chalcopyrite-ankerite and pyrite-calcite regional zones. Mineral zones at the deposit are, from northwest to southeast: pyrite-calcite, galena-calcite, chalcopyrite-calcite, bornite-calcite, chalcocite-chlorite, and chalcopyrite-ankerite. Gangue minerals, including carbonates, Fe-Ti oxides, chlorite, barite, authigenic feldspars, and apatite, are zonally distributed with boundaries parallel to the sulfide-mineral zone boundaries. Bornite-calcite and chalcocite-chlorite zones form ore grades in certain, but not all, quartzite intervals.As observed at unmetamorphosed deposits where sulfide-mineral zonation is similar, some pyrite-calcite-zone minerals were probably replaced by galena-calcite-zone minerals, which were probably succeeded, in sequence, by minerals of the chalcopyrite-calcite, bornite-calcite, and chalcocite-chlorite zones. This inferred paragenesis suggests that the chalcocite-chlorite zone is more proximal to the source of ore solutions than the galena-calcite or pyrite-calcite zones. The subeconomic chalcopyrite-ankerite zone, found farthest to the southeast at the Spar Lake deposit, was apparently even more proximal to the source than ore. Ore deposition took place during diagenesis from solutions that migrated upward and laterally through the sediments from a southeasterly source.The distributions of mineral zones and ore grades were controlled by two factors, one inherited from sedimentation and the other from preore diagenesis. All mineral zones spread out within coarser grained portions of quartzite units, suggesting that lateral migration of ore solutions was controlled by primary permeability of the sediments. However, high grades of copper and silver are found only in certain of the coarser grained beds. The distributions of higher grades suggest that ore mineral abundances reflect the abundances of preore diagenetic phases which were involved in the ore precipitation reactions. Preore reactant phases were evidently more abundant in sandstones deposited in subtidal(?) channels (the ore-grade middle quartzite beds) than in sandstones deposited in beach and nearshore slope environments (the lower quartzite beds). The identity of the reactant phases and the processes that resulted in their concentration at the site of later ore deposition remain unknown, although a preore sulfide- and hydrocarbon-bearing pore fluid appears to be the best hypothesis as to the identity of the reactants.