The approximately 2.9 Ga old Witwatersrand Basin of South Africa hosts different types of pyrite grains. The pyrite grains are hosted in different quartz pebble conglomerate reefs including the Ventersdorp Contact Reef (VCR). The various reefs are accessible from numerous goldfields. Regardless of years of studying the various types of pyrite grains from the VCR, there is still a disagreement as to their origins. Numerous pyrite grains of different types, shapes, and textures have been identified in the VCR of the Kloof Gold Mine. The origin of the different types of pyrite grains have been attributed to various mechanisms. Consequently, different depositional models were suggested for the mineralisation of the pyrite grains. Nevertheless, the style of mineralisation and origin of the different types of pyrite grains require further work especially for the VCR. This study determined whether a single or multiple processes were responsible for the formation of the different pyrite grains.
The different types of pyrite grains including the rounded porous, rounded massive and angular massive grains were identified using a reflected light microscopy, scanning electron microscopy, and Röntec energy-dispersive x-ray spectroscopy. The above techniques were used to describe the shapes, sizes, and textures of the different types of pyrite grains in order to classify and group the pyrite grains for further investigation by chemical analysis. Identification and description of minerals associated with pyrite were conducted to determine the environment and conditions under which pyrite grains were formed. Genetic structures were identified in order to resolve whether the pyrite grains were transported or crystallised in situ. Different types of inclusions hosted in pyrite grains were identified and described to determine the nature of pyrite grains. Rounded zircon grains were identified in rounded massive pyrite grains, and the appearance the rounded zircon grains might suggest that the rounded massive pyrite grains formed later after the deposition of the Witwatersrand basin sediments. A particle induced x-ray emission, an electron microprobe, and a laser ablation inductively coupled plasma mass spectrometry were used to carry out chemical analysis of the major, minor and trace elements of the different pyrite grains. The chemical analyses were conducted to investigate and determine the chemical signatures and the composition of the different types of the pyrite grains to determine their processes of formation. Compositional zonation patterns and overgrowths of the pyrite grains were identified in order to determine whether the pyrite grains were formed through multiple mechanisms.
The compositional zonation patterns were observed in the rounded porous pyrite grains. The zonations suggest multiple stages of growth processes resulting in most porous parts of the pyrite being enclosed by nearly pure pyrite. The rounded pyrite grains are generally containing high concentrations of Ni and Co as opposed to angular pyrite grains. The Ni/Co ration is low in the angular pyrite grains, but high in the rounded grains. The high Ni/Co ratio in rounded grains may be related to low-temperature sedimentary processes, whereas the low Ni/Co ratio in angular pyrite grains may indicate hydrothermal source. The differences between rounded and angular pyrite grains can be interpreted as a result of different genetic history, and origin from different sources. Thus the different pyrite grains formed under different chemical and atmospheric conditions, and by different depositional mechanisms.