Abstract

The geological model (lithology and structure) developed for use in geotechnical engineering projects at Finsch Mine, Northern Cape Province, is described in this paper. The lithology model is presented as a detailed stratigraphic column. This study provides new lithofacies descriptions and suggests formation boundary positions for the upper Campbellrand Subgroup stratigraphy exposed by the open pit and underground tunnel network at the mine. The Klippan Formation lagoonal facies, the Kogelbeen Formation subtidal facies and the Gamohaan Formation microbialite facies are interpreted as a transgression sequence deepening into the basinal banded ironstone formation of the Asbestos Hill Subgroup.

The structural model is presented as a three-dimensional computer model, and as a summarized model of the structural domains. The structural model is of a northwest to north-northeast varying strike-slip system with “negative flower structure” geometry. Two phases of open folding have developed one trending east-southeast and the other parallel to the faults. Four sub-vertical joint sets are described. The structures are identified as an attenuated expression of the late-stage events of the Namaqua-Natal orogeny at circa 1Ga. The authors suggest that the latest phase of deformation was caused by oblique collision tectonics expressed initially as compression from the south-southwest producing transtensional north-northwest strike-slips faults with a dextral shear sense. Compression may have then swung to the west-southwest producing fault-parallel folds and a conjugate joint set.

The described structures were utilized during kimberlite emplacement, including the early intrusion of northeast striking kimberlite dykes. The mechanics of kimberlite emplacement are not discussed, but this work provides important data for such future research.

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