Archaean oil migration in the Witwatersrand Basin of South Africa

Abstract

Abstract: The Late Archaean Witwatersrand Supergroup of South Africa hosts the largest known gold-uranium-pyrite ore deposits. Oil preserved in fluid inclusions in quartz grains in siliciclastic sedimentary rocks of that supergroup implies that hydrocarbon generation and migration occurred during the Archaean, and may have been involved in mineralization processes. Through reference to Phanerozoic analogues, oil-bearing fluid inclusions entrapped in healed microfractures in detrital quartz grains and in early syntaxial quartz-overgrowths imply, that the onset of oil migration coincided with early to intermediate stages of burial, while intra-granular porosity was still preserved. Multiple generations of oil migration are indicated by: (i) oil inclusions within early diagenetic cements at different levels in the stratigraphic succession; (ii) more than one type of oil in entrapment sites; (iii) oil entrapment in multiple stages of the quartz paragenetic sequence. Oil generation and migration are considered to have occurred throughout, and for some considerable time after, development of the Witwatersrand Basin, consistent with progressive burial and kerogen maturation in more than one tectonic regime. Oil-bearing fluid inclusions within detrital sandstone fragments suggest that oil migration also occurred in a sedimentary succession on the Kaapvaal Craton prior to 2.9 Ga. Oil in the Witwatersrand Supergroup was most likely derived from multiple source areas, with the principal source probably being shales within the lower Witwatersrand Supergroup. The hydrocarbon migration history of the basin has important implications for understanding the textural relationship between gold, bituminized oil and uraninite in the giant gold-uranium-pyrite ore deposits.