Gold potential of the Roraima Supergroup in Guyana

Abstract The predominantly siliciclastic, up to 2.8 km thick Palaeoproterozoic Roraima Supergroup in the Guiana Shield, cropping out in the border region of Guyana, Venezuela and Brazil, bears many sedimentological similarities to the 2.90–2.79 Ga Central Rand Group in the Witwatersrand Basin of South Africa, world-famous for hosting the largest known concentration of gold. Many decades of artisanal mining of gold (and diamonds) derived from the Roraima Supergroup underscore its economic potential. To assess this, an extensive exploration programme was conducted, involving 61 boreholes, some of which have reached the pre-Roraima basement. Detailed sedimentological, stratigraphic, petrological, geochemical and geochronological analyses of the drill cores led to a new informal lithostratigraphic scheme for this supergroup. Its lower part (1958 ± 19 to 1901 ± 1 Ma), for which a continental foreland basin is suggested as depositional setting, contains numerous sequence boundaries in the form of erosion surfaces, several of which with gold palaeoplacer potential (Au grades and contents of up to 3.8 g/t and 135 cm.g/t). The source of detrital gold (micro-)nuggets in alluvial fan and braided fluvial conglomerates within that succession can be traced back to eroded orogenic-type gold deposits in the 2.21–2.06 Ga Transamazonian granitoid-greenstone belt to the northeast. Overall, our results attest a significant potential for discovery of Witwatersrand-type gold deposits. Their expected Au endowment would be smaller, however, because of the younger age and inferred gold source - comparable with the Palaeoproterozoic Tarkwa deposits in Ghana.