The structural and engineering geology of the Country Rock at Finsch Mine

Master Thesis


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University of Cape Town

This project was undertaken at Finsch Mine, De Beers Consolidated Mines Ltd., in order to collect the relevant information to build a database for the country rock. The country rock is comprised of the ~2.5 Ga sediments of the Griqualand West Basin: the banded ironstones and carbonates of the Ghaap Group. A stratigraphic column has been produced that correlates the country rock at Finsch Mine with the corresponding strata in the accepted stratigraphy of the Griqualand West Sequence. Periods of regression and transgression in the Early Proterozoic basin may have controlled the Ca/Mg ratio and the percentage of limestone deposition. Later hydrothermal dolomitization had a significant affect on the amount of secondary dolomite present in the country rock. The rock structures (faults, dykes ad fractures) in the country rock were mapped in detail. The data was placed into MicroStation (a CAD package), which now acts as a limited three dimensional GIS database for the mapped structures. The structural data was analyzed and a complete three dimensional structural model was produced. The faulting belongs to a strike-slip system formed by crustal shear late during the Namaqua-Natal orogeny (ca. l.l Ga). A complex structural/tectonic history is suggested whereby transtension is followed by a rotation of the tectonic stress vectors with resultant transpression. During transpression the fault-parallel jointing/cleavage is fanned by fault-parallel folds and a conjugate set of joints form as a result of WSW directed crustal shortening. Extensive line sampling of the rock discontinuities (joints) was undertaken. The characteristics of the joints were qualitatively described/mapped across the mine. The data from the line sampling was analyzed and used to quantify the qualitative descriptions. The mapped joint's characteristics are graphically presented on plans, described by the author as 'joint-nets'. Up to six joint sets have been observed and genetically linked to the structural history. Distinct homogeneous structural domains do not exist at Finsch mine, but the joints were used to crudely define some domains. The rock types in the carbonate succession were studied in detail and distinct different types were sampled and tested to obtain UCS, point-load strengths and Young's moduli. This project describes the process of Rock Mass Classification so that the reasons for the data collection and analyses are apparent to the reader. The data collected during this project suggests that the kimberlite pipe had a significant affect on the smrnunding country rock, maybe as far 20 to 30 metres from the pipe contact. It is suggested that the intrusion of the precursor kimberlite bodies and the main pipe caused some deformation of the country rock mass. Such a suggestion is contrary to the classical understanding of kirnberlite emplacement.