The first Witwatersrand gold deposits in the Orange Free State were discovered under younger cover rocks in the 1930's with the aid of drilling and geophysics. The Welkom gold deposits are found in the sedimentary rock sequences of the Central Rand Group, which represent unconformity bounded genetic packages. The structural configuration of the goldfield is one of a north to south trending synform that is split near it's axis by the De Bron and Homestead faults. The "B" Reef is a highly variable, erratically mineralised reef that lies on an unconformity at the base of the Spes Bona Formation. The "A" Reef Zone consists of several placers 1 including the "Reworked BPM" 1 the Witpan, the Uitsig, the Hanging Wall Grits and the Upper "A" Reef. The Beatrix Reef lies at the base of the Eldorado Formation on an unconformity surface overlying the Virginia Formation in the southern part of the Welkom Goldfield. The origin of gold in the Witwatersrand basin can be classified into the modified placer theory, the syngenetic theory and the epigenetic theory. From the distribution of basin edge unconformities it can be deduced that the Welkom fan depository was tectonically active on the western, southern and eastern margins during sedimentation. Palaeocurrent studies indicate that sediment was transported predominantly from the south and west. It is thought that the "B", "A" and Beatrix Reefs were all deposited in a braided stream environment. A multidisciplinary approach to ore evaluation of Witwatersrand deposits is considered to be the best method, where sedimentology, geostatistics and structural geology are used.

Ph.D. (Geology) / Shear zone controlled hydrothermal alteration zones in the northern Kaapvaal craton (NKC) are developed in host rocks of vastly different chemical composition and metamorphic grade. Some carry appreciable Au and base metals and some are barren. Alteration zones in three different distinctive crustal zones were examined in detail to determine the controls of these two types of alteration. 1. The Matok Complex is situated in the southern marginal zone (SMZ) of the Limpopo Belt (LB), close to the zone of rehydration. Two major stages of hydrothermal alteration could be identified in local shear zones, a pervasive propylitization and a subsequent vein controlled quartzalbite alteration. The two-stage alteration occurred sometimes between the emplacement of the Matok Complex (2670 Ma) and the intrusion of unaltered mafic dykes (1900 Ma). Calculated isotopic compositions of the hydrothermal fluids indicate that magmatic ± meteoric waters as well as juvenile C02 were responsible for the establishment of the alteration zones. The fluids most probably were late magmatic fluids associated with the Matok magmatism. The propylitic alteration was accompanied by introduction of small amounts of CU + Au and represents an alteration type identical to that developed in porphyry copper deposits. The subsequent quartz-albite alteration was caused by extremely saline fluids which depleted the rocks of all the major and trace elements with exception of Si, Al, Na and Zr. 2. This chemical alteration pattern' contrasts with those developed in two alteration zones associated with economic gold mineralization in greenstone belts of the NKC (Sutherland and Pietersburg belts). At the Birthday and Eersteling gold mines, a biotite-calcite-quartz alteration is developed. The chemical pattern of the alteration is...

M.Sc. (Geology) / Please refer to full text to view abstract

The Kalahari Goldridge mine is located within the Archaean Kraaipan Greenstone Belt about 60 km SW of Mafikeng in the Northwestern Province, South Africa. Several gold deposits are located within approximately north - south-striking banded iron-formation (BIF). Current opencast mining operations are focused on the largest of these (D Zone). The orebody is stratabound and hosted primarily in the BIF, which consists of alternating chert and magnetite-chloritestilpnomelane-sulphide-carbonate bands ranging from mm to cm scale. The ore body varies in thickness from 15 to 45 m along a strike length of about 1.5 km. The BlF is sandwiched between a sericite-carbonate-chlorite schist at the immediate footwall and carbonaceous meta-pelites in the hanging-wall. Further west in the footwall, the schists are underlain by mafic meta-volcanic amphibolite. Overlying the hanging-wall carbonaceous metapeiites are schist units and meta-greywackes that become increasingly conglomeratic up the stratigraphy. Stilpnomelane-, chlorite- and minnesotaite-bearing assemblages in the BlFs indicate metamorphic temperatures of 300 - 450°C and pressures of less than 5 kbars. The BIF generally strikes approximately 3400 and dips from 60 to 75°E. Brittle-ductile deformation is evidenced by small-scale isoclinal folds, brecciation, extension fractures and boudinaging of cherty BIF units. Fold axial planes are sub-parallel to the foliation orientation with sub-vertical plunges parallel to prominent rodding and mineral lineation in the footwall. Gold mineralization at the Kalahari Goldridge deposit is associated with two generations of subhorizontal quartz-carbonate veins dips approximately 20 to 40°W. The first generation consists of ladder vein sets (Group lIA) preferentially developed in Fe-rich meso bands, whilst the second generation consists of large quartz-carbonate veins (Group lIB), which crosscut the entire ore body extending into the footwall and hanging-wall in places. Major structures that control the ore body are related to meso-scale isoclinal folds with fold axes subparallel to mineral elongation lineations, which plunge approximately 067°E. These linear structures form orthogonal orientation with the plane of the mineralized shallowdipping veins indicating stretching and development of fluid - focusing conduits. A second-order controlling feature corresponds to the intersection of the mineralized veins and foliation planes of host rock, plunging approximately 008°N and trending 341°. G0ld is closely associated with sulphides, mainly pyrite and pyrrhotite and to a lesser extent with bismuth tellurides, and carbonate gangue. The ore fluid responsible for the gold deposition is in the C-O-H system with increased CH₄ contents attributed to localized hydrolysis reaction between interbedded carbonaceous sediment and ore fluid. The fluid is characterized by significant C0₂ contents and low salinities below 7.0 wt % NaCl equivalent (averages of 3.5 and 3.0 wt % NaCl equivalent for the first and second episodes of the mineralization respectively) . Calculated values of f0₂. ranging from 10⁻²⁹·⁹⁸ to 10⁻³²·⁹⁶ bars, bracket the C0₂-CH₄ and pyrite-pyrrhotite-magnetite buffer boundaries and reveal the reducing nature of the ore fluid at deposition. Calculated total sulphur content in the ore fluid (mΣs), ranges from 0.011 to 0.018M and is consistent with the range (10⁻³·⁵ to 10⁻¹M) reported for subamphibolite facies ore fluids. The close association of sulphides with the Au and nature of the fluid also give credence that the Au was carried in solution by the Au(HS)₂ - complex. Extensive epigenetic replacement of magnetite and chlorite in BIF and other meta-pelitic sediments in the deposit by sulphides and carbonates, both on meso scopic and microscopic scales gives evidence of an interaction by a CO₂- and H₂S-bearing fluid with the Fe-rich host rocks in the deposit. This facilitated Au precipitation due to changes in the physico-chemical conditions of the ore fluid such as a decrease in the mΣs and pH leading to the destabilization of the reduced sulphur complexes. Local gradients in f0₂ may account for gold precipitation in places within carbonaceous sediments. The fineness of the gold grams (1000*Au/(Au + Ag) ranges from 823 to 921. This compares favourably with the fineness reported for some Archaean BIFhosced deposits (851 - 970). Mass balance transfer calculations indicate that major chemical changes associated with the hydrothermal alteration of BIF include enrichment of Au, Ag, Bi, Te, volatiles (S and CO₂), MgO, Ba, K and Rb but significant depletion of SiO₂ and minor losses of Fe₂O₃. In addition, anomalous enrichment of Sc (average, 1247%) suggests its possible use as an exploration tool in the ferruginous sediments in the Kraaipan greenstone terrane. Evidence from light stable isotopes and fluid inclusions suggests that the mineralized veins crystallized from a single homogeneous fluid source during the two episodes of mineralization under the similar physicochemical conditions. Deposition occurred at temperatures rangmg from 350 to 400°C and fluid pressures ranging from 0.7 to 2.0kbars. Stable isotope constraints indicate the following range for the hydrothermal fluid; θ¹⁸H₂O = 6.65 to 10.48%0, 8¹³CΣc = -6.0 to -8.0 %0 and 8³⁴SΣs = + 1.69 to + 4.0%0 . These data do not offer conclusive evidence for the source of fluid associated with the mineralization at the Kalahari Goldridge deposit as they overlap the range prescribed for fluid derived from devolatization of deep-seated volcano-sedimentary piles near the brittle-ductile transition in greenstone belts during prograde metamorphism, and magmatic hydrothermal fluids. / KMBT_363 / Adobe Acrobat 9.54 Paper Capture Plug-in