The Stratigraphy, Sedimentology and Gold Distribution of the VSS/Beatrix Composite Reef at H.J. Joel Gold Mine, Orange Free State, Goldfield, Republic of South Africa

Abstract The zone of economic interest at H.J. Joel Gold Mine is the VSS/Beatrix Composite Reef, a quartz-pebble conglomerate of Archean age. The reef occurs at the base of the Eldorado Formation, Central Rand Group, of the Witwatersrand Supergroup. At least three stratigraphically distinct conglomerates form this economic zone. In order of deposition these are the Footwall, Beatrix, and the VSS/Beatrix reefs. The Footwall and Beatrix reefs are equated with the Aandenk Formation with transport directions toward the northwest, whereas the VSS/Beatrix Reef is of Eldorado age, with transport directions toward the south. The change in the drainage direction was caused by a major basin-wide tectonic event that occurred prior to the deposition of the Eldorado Formation. The Beatrix Reef was formed in a proximal, braided, fluvial environment by a process of aggrading gravel bars between channelways. The VSS/Beatrix Reef was formed by the desegregation of pre-existing Beatrix Reef with the introduction of new material containing much shale detritus and is seen to have been deposited in a more distal, braided, fluvial environment, similar to the Beatrix Reef.
The average gold concentrations are different for each conglomerate; the Beatrix at 32.1 ppm Au, the Footwall at 24.4 ppm Au, and the VSS/Beatrix at only 12.4 ppm Au. A sedimentary gold concentrating process related to higher energy braid channels was primarily responsible for these high gold grades. Not only the basal unconformity surface, but also, internal degradation surfaces, were prime sites for concentration. An understanding of the spatial distribution of these differing conglomerates and their braid channels forming the economic reef zone is necessary in predicting ore grades for future mining.
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Summary: The Molopo Farms Complex is a major, layered ultramafic/rnaflc, early Proterozoic intrusion which is totally covered by sediments of the Karoo and Kalahari Formations and situated in southern Botswana and northeastern South Africa. The approximately 3000 m thick, generally fine- to medium-grained ultramafic and mafic cumulates are intrusive into equivalents of Transvaal Supergroup sediments and lavas. Multiphase deformation resulted in tilting and segmentation of the Complex into individual...
Publication: Exploration & Mining Geology
Author(s): F.J. REICHHARDT
Issue: 3
Volume: 3
Year: 1994
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Summary: The Selebi-Phikwe deposits are hosted by amphibolite to granulite fades gneisses of the Limpopo belt. The sulfide orebodies are generally stratiform and conformable to the foliation of the wallrock gneiss. Sulfide remobilization has occurred during metamorphism and has produced discordant sulfide veins, called piercement structures, which project into the hanging wall and foot-wall gneisses. These piercement structures are compositionally similar to the stratiform sulfides, consisting mainly...
Publication: Exploration & Mining Geology
Author(s): CHRISTOPHER OSTERMAN, RICHARD W. HUTCHINSON
Issue: 3
Volume: 3
Year: 1994
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Summary: Bedding-parallel thrust-hosted gold veins are developed in the Sabie-Pilgrim's Rest gold-field, eastern Transvaal, South Africa. The reefs display a complex history characterized by temporally and spatially overlapping events of similar mineralization and deformation. Recognition of the episodic reactivation of these low angle (5° to 10°) thrust faults implicates high fluid pressures (equalling or exceeding lithostatic pressures). Geochemical constraints limit the amount of fluid-rock...
Publication: Exploration & Mining Geology
Author(s): MICHAEL HARLEY, E. GUY CHARLESWORTH
Issue: 3
Volume: 3
Year: 1994
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Summary: Nodules of carbonaceous matter have been observed in several Archaean peraluminous granites adjoining the gold- and uranium-bearing Witwatersrand Basin. These nodules, which resemble solid bitumen, usually replace uraninite or uranothorite and are paragenetically late. They show many similarities to kerogen seams and nodules associated with the conglomerate horizons in the Witwatersrand Basin, including composition, maturation, intimate association with uraninite, and a late paragenesis. U-Pb...
Publication: Exploration & Mining Geology
Author(s): L.J. ROBB, P. LANDAIS CREGU, P.M. MEYER
Issue: 3
Volume: 3
Year: 1994
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Summary: The old Albert Silver Mine is a polymetallic deposit comprising concentrations of Cu-Pb-Zn-Ag-U-F within a set of sub-parallel quartz-hematite veins which occur within the fine-grained apical phase of the 2050 Ma old A-type Bushveld granites. Mineralization occurs as an early pyrite-chalcopyrite-arsenopyrite-galena-sphalerite-(argentiferous) tetrahedrite paragenesis followed by a later, more oxidized assemblage comprising chlorite-hematite-fluorite-pitchblende. Accumulation of metals appears...
Publication: Exploration & Mining Geology
Author(s): L.J. ROBB, V.M. ROBB
Issue: 3
Volume: 3
Year: 1994
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Summary: Gold mineralization in the Main Reef Complex at Sheba Gold Mine is hosted within shear zones in a sequence of shales and greywackes of the Fig Tree Group. Competency contrasts between these rocks and a steeply dipping chert unit provided a locus for the development of right lateral steeply dipping shear zones. A set of flatter dipping shear fractures crosscut these zones with their intersections defining a system of en echelon ore shoots.
Mineralization is both of vein type and disseminated...
Publication: Exploration & Mining Geology
Author(s): M.J. ROBERTSON E.G. CHARLESWORTH, G.N. PHILLIPS
Issue: 3
Volume: 3
Year: 1994
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Summary: Intrusive features of cupriferous noritoids from the deeply eroded Proterozoic granite-gneiss terrane of the Okiep Copper District in South Africa illustrate the emplacement of basic magmas into a mid- to lower-crustal environment. The small, easterly trending intermediate to basic intrusions of the so-called Koperberg Suite display a wide variety of geometries, occurrence and emplacement modes, including dike-, sill- and plug-like bodies. Coeval granulite-facies metamorphism, partial melting...
Publication: Exploration & Mining Geology
Author(s): ALEXANDER P.M. KISTERS, JACOBUS E. POTGIETER
Issue: 3
Volume: 3
Year: 1994
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Summary: The common style of gold mineralization in the Basal Reef and other auriferous conglomerates of the Witwatersrand Supergroup comprises clusters of gold grains occurring interstiti-ally to quartz and pyrite pebbles as well as gold accumulations attached to the surface of and infilling hairline cracks and fractures in round, compact, detrital pyrite. The latter textural association is generally interpreted as a result of remobilization of originally detrital gold during regional metamor-phism...
Publication: Exploration & Mining Geology
Author(s): P.M. MEYER
Issue: 3
Volume: 3
Year: 1994
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