Sulfur, Carbon, and Oxygen Isotope Constraints on Fluid Sources at the Tamdroust Cu Ore Deposit (Central Anti-Atlas)

Abstract

The Tamdroust copper ore deposit, located within the Bou Azzer–El Graara inlier (Central Anti-Atlas, Morocco), exemplifies Lower Cambrian carbonate–siliciclastic–hosted copper mineralization formed through the combined effects of stratigraphic, structural, and hydrothermal processes. The deposit lies within the Lower Cambrian Igoudine and Amouslek formations of the Tata Group. It is controlled by a major fault system trending N110°–N150°, which served as the main pathway for metalliferous fluids during Hercynian tectonic reactivation. Copper mineralization predominantly occurs in reduced green siltstones and dolostones deposited on a shallow, mixed carbonate–siliciclastic marine platform influenced by episodic terrigenous input. Two main styles of mineralization are recognized: (i) disseminated sulfides, including fine-grained bornite, chalcopyrite, and pyrite dispersed within permeable host rocks; and (ii) vein and veinlet stockworks along interconnected fracture corridors associated with the major fault zone. Textural and petrographic studies reveal a multi-stage paragenetic sequence evolution that comprises: (1) early disseminated and veinlet-type bornite–chalcopyrite–pyrite associated with quartz–calcite; (2) hydrothermal enrichment along faults marked by bornite replacement by chalcocite with digenite and covellite; and (3) supergene weathering producing native copper and secondary carbonates. Stable isotope geochemistry offers crucial insights into the origin and development of mineralizing fluids. Sulfur isotope compositions of bornite (δ³⁴S ≈ +10.2‰) suggest a mixed sulfur reservoir primarily formed by thermochemical sulfate reduction (TSR) of evaporitic sulfates, aligning with the presence of Lower Cambrian evaporite-rich formations. Carbon and oxygen isotope values measured in hydrothermal calcite (δ¹³C = –3.6 to –2.6‰ VPDB; δ¹⁸O = –15.8 to –15.2‰ VPDB, equivalent to +14.7 to +15.3‰ VSMOW) indicate moderate-temperature (~150–160°C) hydrothermal fluids originating from mixed meteoric–basinal brines that have isotopically equilibrated with carbonate–evaporite host rocks. The δ¹³C signatures further point to a dominant marine carbonate source with no significant biogenic carbon contribution, while minor meteoric or atmospheric mixing remains possible. These findings support a model of fluid–rock interaction in a mesothermal hydrothermal setting, where brines, partially modified by evaporites, played a key role in copper transport and sulfide formation. The spatial distribution of ores highlights the significance of redox-controlled mineralization, with the most notable mineral deposits forming at the boundary between oxidized hematite-bearing red beds and reduced green siltstones and carbonates. This redox boundary served as a chemical trap, allowing TSR-driven production of reduced sulfur species and subsequent copper sulfide deposition. In summary, geological, structural, and isotopic evidence indicate that the Tamdroust deposit is a carbonate-hosted copper system of epigenetic stratabound type in Cambrian evaporitic settings, formed during the Hercynian reactivation of Cambrian sedimentary basins. The Tamdroust system exhibits strong similarities with other Cambrian Cu ore deposits in the Anti-Atlas, particularly Jbel N’Zourk and Jbel Laassal, supporting a regional metallogenic model involving fault-controlled brine flow, evaporite involvement, and redox-driven sulfide formation. These findings offer a predictive framework for future copper exploration, focusing on structurally controlled brine pathways and redox boundaries as primary targets across the Central Anti-Atlas.