Tracing the sources of sedimentary Cu and Mn ores in the Cambrian Timna Formation, Israel using Pb and Sr isotopes

Abstract

Diagenesis and epigenetic remobilization frequently obscure the original depositional environment of sediments that have been affected by Cu- Mn- mineralization. Such is the case for the sedimentary Cu and Mn ores of the Cambrian Timna Formation in Southern Israel, where different interpretations of field, petrographic and geochemical data have led to conflicting genetic models. In this study we show how Sr and Pb isotopic data can help unravel the origins and complex processes involved in the sedimentary-hosted Cu- Mn- mineralization. Applying Pb and Sr isotopic systems to the various host rock types, two petrographical and chemically distinct types of Mn nodules (referred to as A and B) and reduced and oxidized Cu minerals of the Timna Formation indicates that two major solution types were involved. The first solution is depleted in Th and enriched with U, with a constant 208Pb/204Pb ratio and single U source as reflected by the correlation between 206Pb/204Pb vs. 207Pb/204Pb. The second solution has higher Th and lower U concentrations; thus 208Pb/204Pb ratios significantly vary whereas the 206Pb/204Pb and 207Pb/204Pb ratios remain relatively constant. The low Th/high U solution is associated with the deposition of Type A Mn nodules, whose 87Sr/86Sr ratio of 0.7093±0.0005 matches that of the Cambrian Sea. Thus, in keeping with previous petrographic and mineralogical evidence, the isotope data are consistent with primary Mn nodule formation under oxidizing conditions during early diagenesis. In contrast, the second high Th/low U solution is associated with the transformation of primary Type A to epigenetic Type B Mn nodules. During these later processes, U was leached from Type A Mn nodules to form secondary Pb enriched Type B nodules. The Pb isotope composition of these nodules clearly shows that the epigenetic solutions were significantly different from those of a major phase of Oligocene-Miocene Fe- mineralization that affected the bedrocks adjacent to the Dead Sea Transform. Cu- Mn- mineralization exhibits more complex isotopic signatures. Cu-sulfides (mainly covellite) deposited during early diagenesis under reducing conditions, and although their 87Sr/86Sr ratio in places is 0.709, as a whole, their Sr-isotope ratios reflect their location rather than the Cambrian sea water signature, as clearly evident from their Pb isotope compositions. During later epigenetic stages, copper sulfides were altered to form malachite and paratacamite, whose Pb isotope compositions show that the oxidizing solutions underwent fluid-rock interaction with their host rocks, particularly during the transport of Pb in lower pH chloride solutions.Overall, it is evident that the combined use of the Sr and Pb isotope systems allows clarification of the various primary and secondary processes involved in the Cambrian Timna mineralization Cu- Mn- mineralization, and particularly the important identification of the involvement of Cambrian sea water in the primary ore formation processes.