Abstract

The How mine is situated in the southeastern quadrant of the Bulawayo greenstone belt within sediments and volcaniclastics of the late Archean Dacitic Greenstone Formation of the Upper Bulawayan Group. Sited on the northern limb of a northeast-verging syncline, the mine comprises several ore zones which occur as an en echelon array of parallel, steeply plunging linear shoots. These shoots are confined within an extensional right-stepping, northerly trending strike-slip duplex. Faulting has followed preferred lithologic contacts.Mineralization is dominated by a pyrite-gold association and occurs as disseminations within altered mylonites (felsites) and tuffs, as fabric replacement within tuffs, and associated with veins in siltstones and iron-formations. Accessory magnetite in tuffs and magnetite layers in iron-formation are replaced by pyrite, and ore textures suggest an epigenetic origin to the mineralization. Alteration is widespread and dominated by carbonation, sulfidization, and propylitic alteration.Permeability within the duplex was generated by slip and dilation of the synclinal axialplanar cleavage, which lies at a high angle to the strike of the duplex. Early ductile deformation is overprinted by later brittle faulting which cuts early mineralization. The linear ore zones parallel the intersection lineation between the preexisting cleavage and the duplex fault-shear zones. In the north area of the mine, ore zones are developed within the fault-bounded tuff unit (e.g., the north 180 ore zone), but in the south the ore zones parallel the Hanging-wall fault-shear zone and transgress major lithologic contacts.

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