Algal mats, load structures, and synsedimentary sulfides in Revett quartzites of Montana and Idaho

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Carbonate-rich patches in quartzites of the Revett Formation are due to algal mats in marine arenites of about a 1.3-b.y. age; some occur in growth positions on bedding planes, some on slumped foresets, but mostly they were disrupted by faster currents depositing heavy minerals as lag sediments. Discovery of stromatolites in the Revett Formation confirms the algal growths.Algal debris provided the food source for anaerobes which produced H 2 S for sulfide precipitation of copper (silver), lead, zinc, and iron in the zonal sequence typical of stratiform syngenetic sulfides. At the Troy mine, these sulfide zones cross the strata.Sulfides vary in grain size and aggregation and are classified into original fine disseminations, reworked aggregates formed during dewatering, and infilling of fractures and vein openings by lateral secretion. The relationship of fine sulfide disseminations to heavy minerals and to algal carbonate indicates deposition of minute sulfide grains with the sediment.A quartzite pillow which contained 24 sagged and folded layers, each layer with three sublayers of, first, heavy minerals, and then clean quartzite, both overlain by sulfide-rich quartzite, was examined and revealed the previously poorly understood mechanism of pillow growth contemporaneous with sedimentation-and in this example--also with sulfide precipitation.Better mineralization in arenites is ascribed, not to permeability, but to stronger currents depositing sand and causing increased mixing of the upper metal-bearing waters with the lower anoxic water, in which hydrogen sulfide precipitates selectively, first, copper and silver as minute sulfide grains. The sulfides are incorporated into the suspended sediment, which is then sorted by waxing and waning currents according to grain size and specific gravity into the varying proportions especially observed in the tripartite laminae of the 24 layers of the quartzite pillow described in detail.