Tracking redox controls and sources of sedimentary mineralization using copper and lead isotopes

Abstract

We show how the copper (Cu) and lead (Pb) isotope systems provide a powerful combination of methods for tracing the environmental conditions and sources of sedimentary copper mineralization. The mineralization was studied at three different locations in southern Israel and comprises copper sulphide concretions hosted by Lower Cretaceous sandstones that unconformably overlie Cambrian sediments. At one location, Timna Valley, the Cambrian sediments contain extensive copper and manganese ores, whereas at the two other locations (Amram and Rehavam (Elat) located south of Timna) the Cambrian sediments are not mineralized.The Pb isotopic ratios of the Lower Cretaceous copper minerals at Timna Valley show a tight group with non-radiogenic values and overlap those of epigenetic (Type B) manganese nodules (Bar-Matthews, 1987) in the underlying Cambrian rocks. This overlap establishes a genetic connection between the fluids that remobilized copper into the Lower Cretaceous sandstones and the fluids that formed the epigenetic manganese mineralization. The Pb isotopic compositions of the Lower Cretaceous sandstone hosted Cu-sulphide concretions at Amram and Rehavam, demonstrate that they formed from solutions that originated in the Timna Valley and then migrated southward while mixing with other, more radiogenic, Pb solutions. The Cu-isotopic compositions of copper sulphide concretions from all three Lower Cretaceous localities define a bimodal distribution with δ65Cu=−0.91±0.51‰ and −3.32±0.29‰. Using fluid speciation modeling calculations (Asael et al., 2009), calculated Eh values for representative conditions are 0.45 and 0.65V (logfO2≈−29 and −19bars), respectively. Overall, open system conditions are inferred for the regional mineralization, whereby copper-bearing solutions originating in the Timna Valley Cambrian rocks moved freely through the Lower Cretaceous sandstones. Redox (Eh) and salinity variations were the major regional parameters controlling the Cu isotopic composition variation, and the mobilization and precipitation of copper minerals. Organic matter decay and bacterial sulphate reduction locally influenced redox conditions at the site of Cu concretion formation.