A quantitative analysis of alteration in the Carbon Leader Reef in the West Wits Line Goldfield: an application of Shortwave Infrared Reflectance Spectroscopy

Alteration within the Witwatersrand Basin has been of great interest and widely researched for many years with a focus on depositional and post-depositional processes that lead to the occurrence of the alteration. Numerous methodologies are used to investigate alteration of orebodies globally however hyperspectral scanning is a relatively new methodology that is gradually being introduced to determine the mineralogical characteristics of borehole core and hand specimens. Spectral scanning is quicker than using conventional and time-consuming methods such as petrological studies and geochemical analyses. A quantitative analysis of alteration within the Carbon Leader Reef in the AngloGold Ashanti West Wits Line Goldfield was carried out using several methodologies including macroscopic observations, petrology, geochemical analyses, gold assay as well as shortwave infrared reflectance spectroscopy (hyperspectral scanning). These methodologies were undertaken to better understand alteration and the associated gold grade across the West Wits Line (comprising Savuka Mine in the west, TauTona Mine in the east and Mponeng Mine south of Savuka and TauTona Mines). The immediate overlying hangingwall and underlying footwall units were also investigated. A comparison of the methodologies confirmed that more than one alteration event has occurred. This was noticeable in thin section and was verified by means of spectral scanning. A combination of rounded grains including gold, pyrite, chromite, uraninite and zircon were noted throughout the Carbon Leader Reef as well as within the hangingwall/footwall units and were suggestive of a detrital origin. Euhedral grains of tourmaline and pyrite were noted in a few of the reef samples representative of the north-east to central areas of the West Wits Line. Euhedral pyrite grains were seen in samples where pyrite had recrystallised and developed on the outer edges of a pre-existing detrital pyrite grain. Rounded to sub-rounded inclusions of gold and chalcopyrite within euhedral pyrite grains were also observed in thin section, suggestive of a secondary origin for the recrystallised pyrite, gold and chalcopyrite. Rutile that has undergone hydrothermal alteration results in the forming of leucoxene. Anhedral to euhedral grains of leucoxene were noted throughout the Carbon Leader Reef samples, further confirming that (secondary) hydrothermal alteration has taken place. Carbon (masses/flyspeck) was often noted to occur within the reef associated with leucoxene and rutile, and in a few samples associated with detrital gold and uraninite. Carbon was also noted to occur, infilling fractures or veinlets within uraninite grains. The close association of carbon with detrital uraninite grains suggests remobilisation of carbon has occurred. Carbon infilling fractures within uraninite grains suggests the carbon was introduced after the deposition of the uraninite grains. The spectral scanning data revealed variations in the alteration mineralogy. In most of the samples there was a dominant alteration mineral, with lesser amounts of different alteration products cross-cutting each facies type. This suggested that the alteration involved complex processes, possibly related to multiple injections of fluid phases during post-depositional fluid fluxes through the sedimentary package. This also suggested that several hydrothermal fluid (alteration) events may have taken place to alter the mineralogy of the host rock over a period of time. The rounded clasts within the samples suggest transportation from a source area has occurred. Following deposition, several pulses of hydrothermal fluids through time altered the mineralogy of the host rock, allowing for the formation of secondary mineral phases such as rutile, tourmaline and recrystallised pyrite. At the AngloGold Ashanti West Wits Line, a Carbon Leader Reef facies model based on three facies types (the No. 1 Facies, No. 2 Facies and No. 3 Facies) is used to differentiate between sedimentological variations as well as the occurrence of carbon within the Carbon Leader Reef. The facies model however does not provide much information on the mineralisation within the Carbon Leader Reef. As a result, the Carbon Leader Reef estimation domain model was developed and implemented by AngloGold Ashanti at its West Wits Line. The estimation domain model encompasses Carbon Leader Reef mineralogical characteristics, alteration mineralogy, sedimentology facies, structural influences as well as trend analyses with the variations of grade and channel width. The estimation domain model however does not account for the occurrence or absence of carbon within the Carbon Leader Reef. The current research findings suggest that variations in the mineralogy of the Carbon Leader Reef do not align fully with the Carbon Leader Reef facies model nor with the Carbon Leader Reef estimation domain model. A new Carbon Leader Reef facies model, specific to the West Wits Line, has been proposed to include a No. 3 Facies sub-type. The No. 3 Facies sub-type was based on samples that did not align fully with the No. 3 Facies definitions (i.e. samples with a thin basal carbon seam and/or higher than average gold grades of the No. 3 Facies). The samples used within this research also indicated that the TauTona, Driefontein and Peggy estimation domains need to be re-evaluated and revised on a continuous basis as more information becomes available. Using the results from the various methodologies applied in this research, three major alteration zones have been identified based on alteration styles including: pyrophyllitic alteration, chloritisation and sericitic alteration. The pyrophyllitic alteration zone was based on pyrophyllite as a major alteration mineral and extended from the north-eastern to the southern portion of TauTona Mine. The chloritisation zone, based on a combination of chlorite and chloritoid as the major alteration minerals, extended across the West Wits Line from east to west. The sericitic alteration zone, based on sericitic muscovite as a major alteration mineral, exhibited was similar distribution range to that covered to the chloritisation zone, however, it extended further north-east and further south than the chloritisation zone. The pyrophyllitic alteration zone was noted to coincide with the Pretorius Fault Zone. The zone of chloritisation was noted to coincide with frequent, smaller faults and fewer intrusions, and the sericitic alteration zone was noted to coincide with a frequent occurrence of intrusions with lesser faults. Based on geothermometric techniques, an estimated timeline for alteration was proposed. It was suggested that the Pretorius Fault Zone pre-dates the occurrence of the smaller and more frequent faulting events with fewer intrusions. The youngest events correspond to abundant intrusions associated with lesser faulting. Overall, this suggested that the intrusions are younger than the faults across the West Wits Line, with the Pretorius Fault Zone having occurred before the smaller faults and associated intrusions. Spectral scanning was able to assist in determining alteration within the host rock as reliably as petrological and geochemical analyses. However, spectral scanning could not accurately predict gold grade within the Carbon Leader Reef or the hangingwall and footwall units. Spectral scanning presents accurate results, and this contributes to the easy identification of sedimentological contacts where macroscopic observations are not as obvious. Analysis of the alteration within the Carbon Leader Reef, as well as the hangingwall and footwall units, showed that there was very little correlation between alteration type and the associated gold grade. The presence of organic matter (i.e. carbon) in samples is commonly noted to affect the spectral scanning outcomes. Additional research should be carried out using a reef, such as the Ventersdorp Contact Reef, that is non-carbon bearing to determine if there is a correlation between alteration type and associated gold, where the presence of carbon cannot influence the alteration readings of the spectral scans. The presence of carbon and/or uraninite within the host rock, however, was a consistent indicator for the presence of gold grade. Based on these results, it could be concluded that favourable gold grade is independent of the type of alteration and is more dependent on the presence of carbon and/or uraninite within the host rock
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in the fulfilment of the requirements for the degree of Master of Science, 2020