Partitioning and distribution of silver in sediment-hosted Cu-Ag deposits: Evidence from the Ghanzi-Chobe Belt portion of the Kalahari Copper Belt

Abstract

The Khoemacau Mining (KCM) Cu-Ag deposits in northwestern Botswana are comprised of several mineralised zones, including from south to north: the Banana Zone (made up of the Chalcocite Zone, New Discovery, North Limb, South Limb Definition, and the North East Fold), Mango, Zone 5, Zone 5 North, Boseto, Zeta NE, and Zone 6. The KCM Cu-Ag deposits are characterised by two styles of mineralisation that include disseminated and structurally controlled styles of mineralisation, both typified by the scarcity of Ag-bearing minerals. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and electron probe microanalysis (EPMA) techniques were employed to (i) track the sources of Ag by assessing the Ag budget of the sulphide phases, (ii) assess the partitioning and distribution of Ag in coexisting sulphide phases, and (iii) outline the mode of occurrence of Ag in the host sulphides. Bornite, chalcocite, wittichenite, tetrahedrite-like mineral, covellite, and galena were observed to be Ag-endowed. Bornite and tetrahedrite-like mineral are the principal hosts of Ag and contain, on average, up to 2090 and 4107 ppm Ag, respectively. But when bornite coexists with chalcocite, Ag preferentially partitions into chalcocite (mean Ag concentrations range from 423 to 1050 ppm) than in bornite (mean Ag concentrations range from 201 to 309 ppm). In the chalcocite-wittichenite assemblage, Ag partitions equally in these phases with mean concentrations of 1890 and 1854 ppm, respectively. Galena from the North East Fold contains a significant amount of Ag (mean 477 ppm) than that one from Zone 5, which is Ag-poor (mean up to 87 ppm). Unlike the spatially associated sulphides, chalcopyrite, however, is a poor host for Ag (mean concentration 1.3–104 ppm). The LA-ICP-MS data demonstrated that Ag is essentially bound in the crystal lattice of bornite, chalcocite, wittichenite, tetrahedrite-like mineral, covellite, and locally galena, as shown by the homogeneous distribution of Ag in the elemental maps, small to medium standard deviations (averaging 2–44% of the mean) and relatively flat time-resolved depth profiles. However, large variations in the Ag concentration between laser spots (averaging 51–70% of the mean) from the same sample, as observed in some bornite II grains from the New Discovery and the North East Fold mineralised zones, may indicate the presence of Ag-rich micro to nanoparticles in bornite II. The initial mineralising fluid is thought to have been saturated in Ag and precipitated Ag-rich phases in the mineralised zones southwest of the KCM Property, including the Chalcocite Zone, New Discovery, North Limb, South Limb Definition, and the North East Fold. Subsequently, it became Ag-under-saturated with time as it precipitated Ag-poor phases in the mineralised zones northeast of the KCM Project (such as Zone 5). The positive correlations (R > 0.60) displayed between Ag and As, Se, Sb, Hg, Te, and Bi in the Cu-(Fe)-sulphides demonstrate that these elements could be used as pathfinder elements in sediment-hosted Cu-Ag deposit exploration.