Regional- to Deposit-Scale Geologic Controls on Copper-Silver Mineralization in the Kalahari Copperbelt, Botswana

Abstract

The Kalahari Copperbelt in northwestern Botswana is characterized by structurally controlled, stratabound, mineralogically zoned copper-silver deposits hosted along a major redox boundary within a late Mesoproterozoic rift succession. Copper-silver mineralized rocks occur on the limbs and in the hinge positions of regional-scale folds that characterize the Pan-African Ghanzi-Chobe zone fold-and-thrust belt. Regional facies changes along the base of the transgressive marine D’Kar Formation, the host to the majority of mineralized rocks, delineate a series of synsedimentary basin highs and lows. The facies changes were identified through both lithostratigraphic analysis of drill holes and along-strike variations in magnetic lithostratigraphy, a technique that correlates the magnetic fabrics of second vertical derivative aeromagnetic maps with changes in lithostratigraphy. Basin highs controlled the development and distribution of favorable lithostratigraphic and lithogeochemical trap sites for later sulfide precipitation. Major facies changes across the Ghanzi Ridge area straddle a significant crustal structure identified in gravity datasets that appears to have influenced extensional activity during basin development. During basin inversion, the basin highs, cored by rheologically stronger bimodal volcanic rocks, localized strain within mechanically weaker rock types of the Ghanzi Group metasedimentary rocks, leading to the development of locally significant permeability and the formation of structural trap sites for mineralization by hot (250°–300°C), oxidizing, metalliferous Na-Ca-Cl brines. Structural permeability was maintained within trap sites due to silicification and/or feldspar alteration during progressive deformation and associated hydrothermal mineralizing events.

Diagenetic and epigenetic origins for Cu‐Ag mineralization in the Khoemacau Zone 5 deposit, Kalahari Copperbelt, northwestern Botswana

“…Generalized geologic map and Cu‐Ag deposits of the Ghanzi Ridge, Kalahari Copperbelt in Botswana (modified from Lehmann et al, 2015; Hall et al, 2018). Geology is inferred from known geology and geophysics.…”

“…Recent advances in understanding the geology and origin of Cu‐Ag mineralization in the KCB is a result of in‐depth studies on subjects spanning regional‐ to deposit‐scale characterization and delineation of the KCB (e.g., Schwartz et al, 1995; Gorman, 2013; Morgan, 2013; Shephard, 2014; Lehmann et al, 2015; Hall et al, 2018), depositional environment, geochronology (e.g., Modie, 1996, 2000; Hall, Hitzman, et al, 2018; Hall et al, 2021) and provenance and distribution of metals (e.g., Schwartz et al, 1995; Modie, 1996, 2000; Kelepile et al, 2017, 2020; Hall, Hitzman, et al, 2018). These studies altogether suggest that the litho‐structural control, permeability, metal source, sulfur source, and a reductant (in‐situ or mobile) were key elements for precipitation of the Cu‐Ag ores.…”

“…These studies altogether suggest that the litho‐structural control, permeability, metal source, sulfur source, and a reductant (in‐situ or mobile) were key elements for precipitation of the Cu‐Ag ores. It has also been documented that Cu‐Ag mineralization is associated with regional fold limbs and hinges, rendering the structural setting a key element in the exploration of potential ore trap sites (e.g., Hall et al, 2018).…”

“…Despite this progress, metallogenic models proposed for Cu‐Ag mineralization in the KCB remain contentious. These models include syngenetic‐diagenetic origin (e.g., Ruxton, 1986; Ruxton and Clemmey, 1986; Borg, 1995; Schwartz et al, 1996), purely epigenetic origin (Sillitoe et al, 2010; Maiden and Borg, 2011; Hall et al, 2018) and two (or more) stage models that require overprinting and/or remobilization of a diagenetic precursor or early hydrothermal base metal sulfides (Borg and Maiden, 1986, 1987, 1989; Borg, 1988a, 1988b, 1995; Walraven and Borg, 1992; Van der Heever et al, 2009; Hall et al, 2021). The disagreement between the proposed models is in part linked to challenges associated with documenting detailed in‐depth parageneses, particularly at the deposit scale.…”

“…The deposit was discovered in 2012 from a soil anomaly and it is found beneath ~40 m of Kalahari Sands cover on a limb of a regional scale anticline. Zn and Pb form anomalous halos around high Cu‐grade orebodies and have been identified as a practical exploration tool (Hall et al, 2018). Building on the previous extensive works on the KCB, this study provides local litho‐structural controls on mineralization, distribution, and textural features of sulfide mineralization, the nature of ore fluids, and stable isotopic constraints for the Zone 5 Cu‐Ag deposit.…”