Abstract

Several factors suggest that hematite-bearing sediments, composed principally of alluvial fan sediments, were the source of metal-transporting water and of metals in stratiform copper deposits hosted by low-energy sediments. First, such sediments are associated with all examples of larger stratiform copper deposits hosted by low-energy sediments. Second, a zonal sequence, corresponding to an approximate sulfide solubility product increase, is developed laterally into the basin from sandy sediments in contact with the hematite-bearing sediments and upward from the contact of the ore-hosting sediments with the hematite-bearing sediments. Third, the zonal sequence is developed basinward from pinchouts and wedge-downs of the hematite-bearing sediments, and copper mineralization is absent from host horizons where hematite-bearing sediments are not developed. Fourth, authigenic mineral assemblages, with the exception of hematite, sulfides, and organic carbon, are similar in both the host sediments and in the hematite-bearing sediments.The authigenic and evaporite mineral assemblages, and the nature of associated sediments, indicate that most occurrences of the hematite-bearing sediments were probably deposited in closed continental basins. Published sedimentological and paleogeographic analyses show that the Rotliegende, associated with the Kupferschiefer-hosted deposits, was deposited in a closed continental basin.No consistent composition of the hematite-bearing sediments is evident, although basalt flows or basalt-derived clastic components occur in the alluvial fan sediments associated with three of the larger stratiform copper deposits. Authigenic and evaporite minerals and compositions of modern continental closed basin waters show that the waters originally within the closed basins were Na + Cl + SO 4 types with variable Mg, Ca, and SO 4 contents, in equilibrium with hematite, and with a pH constrained either by carbonate minerals-atmospheric carbon dioxide or by quartz-microcline-muscovite. Chloride concentrations were likely to have been about six times seawater. Waters with higher Mg and lower Ca contents were likely to be associated with alluvial fan sediments containing basalt- or dolomite-derived clastic components. Water-rock ratios in the alluvial fan sediments were probably 33 percent or less.If the 50-cm hypothesis, i.e., that chalcocite in stratiform copper deposits hosted by low-energy sediments was precipitated as a result of bacterial sulfate reduction within 50 cm of the surface of the host sediment, is true, then metal-transporting waters were being released from the hematite-bearing sediments very early in their burial history. If so, the metal-transporting water was the original closed basin water, with a temperature of less than 50 degrees C.

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