Anomalous metal enrichment of Basin Brines in the Zambian Copperbelt: a comparison of fluid chemistry in contrasting sediment-hosted copper systems

Abstract

The Central African Copperbelt is the largest repository of sediment-hosted copper and cobalt on Earth. Both metals are essential components of developing battery technologies and the ‘green energy revolution’, and as such, understanding the processes through which they are sourced, transported and precipitated in sedimentary basin environments is fundamental to securing future supply. This thesis presents an investigation of the physicochemical characteristics of palaeofluids from three world-class Cu-Co-(U) deposits in the Zambian Copperbelt, as well as comparisons with considerably smaller systems in the Kalahari Copperbelt (Botswana) and Munster Basin (Ireland). Pre- to syn-kinematic veins in Zambian deposits host anomalously high temperature-salinity brines with unusually potassic compositions and concomitant enrichments in the principal ore-forming metals. Later fluids are distinctly lower temperature-salinity NaCl-dominant brines with reduced base metal concentrations. Samples hosting dominant populations of fertile ore fluids correspond with those returning halogen ratios typical of highly evaporated bittern brines, whereas barren, later fluids indicate a component of halite dissolution. Stable isotope analyses of kinematically ‘early’ versus ‘late’ veins, as well as alteration assemblages associated with temporally distinct vein sets and structures support a model whereby highly evolved, potassic, bittern brines resided in Cu-fertile clastic aquifers for prolonged periods, acquiring high temperatures (>300°C), salinities (>35 wt % NaCl+KCl equiv.) and Cu-Co concentrations (102-103 ppm) prior to emplacement of mineralised veins. Later, barren fluids dissolved evaporites at the onset of basin inversion and were often emplaced as discordant veins with associated sodic alteration assemblages. Fluids associated with smaller-scale mineralisation in the broadly contemporaneous Kalahari Copperbelt share similar physicochemical characteristics with late NaCl brines from the Zambian Copperbelt, with ultra-potassic, fertile ore fluids apparently absent outboard of the Katangan Basin in the Pan-African belt. Mineralised and barren vein sets at the Allihies redbed Cu deposit in the Munster Basin display contrasting fluid temperature and salinity characteristics. Primary inclusions in barren, pre-peak orogenic regional vein sets yield moderate homogenisation temperatures (150-190°C) and salinities (~13 wt % NaCl equiv.), with cation compositions indicative of fluid-rock interaction within Devonian Old Red Sandstone aquifers. Later mineralised veins host distinctly higher temperature (~260°C), moderate salinity (~9 wt % NaCl equiv.) fluids with slight Cu enrichments. Zambian Copperbelt ore fluids are here characterised as anomalously high temperature and salinity brines with correspondingly elevated base metal contents. Few other systems appear to have produced the requisite geologic and physicochemical conditions over sufficient time periods to generate similarly fertile basin brines.

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