Tracing seawater evaporation and its role in the formation of sediment-hosted stratiform copper deposits

Abstract

This study investigates the preserved fluid contents of different generations of mineralised and unmineralised vein minerals from multiple different deposits across the Zambian Copperbelt to define the fluid physicochemical characteristics of the fluids and investigate the importance of fluid processes through the basin history. An investigation into REE and trace element and sulphur isotope signatures of sulphides was conducted to
investigate sulphide formation mechanisms. Re-Os dating of sulphides from the Domes Region to provide temporal constraints on mineralisation processes in this area. A textural study on uranium mineralisation at the Lumwana deposit was undertaken to understand the relationships between uranium and sulphide mineralisation.

Variable Cl/Br of fluids indicates they were derived from evaporation of seawater, deposition of evaporite sequences and the subsequent dissolution of these evaporite sequences, with bittern brines dominant earlier in the basin history, and dissolution of halite more important during compression and orogenesis. Cation contents of fluids record the development of alteration assemblages caused by the movement of these brines at temperatures typically >200C and salinities >30 wt% NaCl equiv. Stable isotope data records the role of organic reductants and fluid-host rock equilibration through basin history. Rare earth element signatures of sulphides further record the importance of the development of the alteration assemblage on the changing chemistry of the hydrothermal fluids, whilst trace element concentrations suggest the metal budget in the fluids reflects source rock variation. Sulphur isotope data suggests the main mechanism of sulphide formation was thermochemical sulphate reduction of seawater sulphate, consistent with the temperatures of the fluids reported here. Re-Os dating records the importance of orogenesis on the timing of mineralisation in the Domes Region, and are consistent with published Re-Os dating of sulphides and U-Pb dating of uranium minerals. Textural relationships indicate initial uranium mineralisation at the Lumwana deposit pre dates sulphide mineralisation, and final movement on the shear zone, but has undergone a remobilisation event later in the deposit history.

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ORCID for S. Roberts: orcid.org/0000-0003-4755-6703

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