STRUCTURAL CONTROLS ON MINERALIZATION, BOSETO COPPER DEPOSITS, NORTHWEST BOTSWANA

The Boseto sedimentary-rock hosted copper deposits in the southern foreland of the Proterozoic Damara Orogen in northwest Botswana are characterized by copper sulfides hosted predominantly by structural fabrics. Two deposits, Plutus and Zeta, occur on the northwest limbs of anticlines. The ore horizon occurs at a redox boundary between hematitic metasandstones and shallow marine limestone, metasiltstone, and phyllite. Shearing was focused along this boundary during flexural slip folding due to a contrast in rock strength. Structural fabrics that host the majority of copper sulfides include cleavage-foliation, veins, and shear bands. Disseminated copper sulfides are hosted in axial planar cleavage of micro-scale folds in algal limestone. Cleavage-foliation parallel veinlets and veins occur throughout the ore zone. Veins are characterized by early quartz precipitation followed by calcite-sulfide-(chlorite-sericite). Two main orientations of veins occur at Boseto: bedding parallel and transverse. The orientations are due to a change in σ3 from vertical to horizontal (normal to σ1) during folding. Bedding parallel quartz veins are commonly sheared and brecciated with a matrix of copper sulfides. Shear bands occur at lithological contacts and commonly contain anastomozing stringers of copper sulfides. Higher-grade zones occur within and at the margins of shear zones, indicating ore fluids were channeled along shear zones during deformation. Three periods of deformation are recognized at Boseto. D1 formed tight upright doubly-plunging folds with a dextral shear component. Local ductile folding of beds, veins and main stage mineralization are associated with D1. D2, which occurred during orogenic collapse formed brittle-ductile structures with normal shear sense including late mineralized en echelon veins, boudined veins with copper sulfide tails, winged sericitic mini-boudins in mylonitic breccias, and bed parallel offsets of mineralized veins. Post-mineralization brittle deformation (D3) is represented by Karoo-age (early Mesozoic) grabens and associated mafic dikes.