Copper isotope fractionation in sedimentary copper mineralization (Timna Valley, Israel)

Copper isotopes (⁶⁵Cu/⁶³Cu) are potentially powerful new geochemical proxies for oxidation-reduction processes and metallic cycling. This research presents a Cu-isotope study of mineralization in historically mined stratiform sediment-hosted copper (SSC) ore deposits of the Precambrian and Cambrian rocks of the Timna Valley, southern Israel. These deposits provide a natural laboratory for studying isotopic fractionations between Cu-sulphides and Cu(II) minerals (copper carbonates, hydroxides and silicates), formed during sequential cycles of low-temperature alteration of igneous copper porphyries, marine sedimentary diagenesis, and epigenetic mobilization in sandstones. Isotopic measurements were made using MC-ICP-MS after ion chromatographic separation of the copper from matrix elements. In accord with abiogenic experimental studies showing that there should be a negative isotopic fractionation between reduced and oxidized copper minerals, δ⁶⁵Cu values of Cu-sulphides are significantly lower (- 3.4 to - 1.2‰) than coexisting Cu(II) carbonates and hydroxides (- 1.2 to 0.5‰). Cu(II) silicates, which should only show a very small isotopic fractionation relative to parent Cu (II) solutions, give average δ⁶⁵Cu values of 0.09 ± 0.24‰; consistent with the fact that the primary source of sedimentary copper was the Precambrian igneous rocks. Isotopic zoning profiles in Cu-sulphides of the Cambrian dolomites suggest they were formed through the interaction of small disconnected Cu solution reservoirs with H₂S formed by bacterial reduction of sulphate containing pore waters. Mass-balance modeling, based on the measured Cu-isotope compositions and experimental fractionation factors, shows that the main copper reservoir is the Cambrian sandstone-shale sequence and that the Cu-sulphide reservoirs are relatively small. Thus, most of the copper transport occurred in relatively oxidized conditions. The calculated reservoir sizes are in agreement with field observations and confirm that copper isotopes are able to trace both the oxidation-reduction cycles and mass transfer during sedimentary copper mineralization.