Regional gold-enrichment of conglomerates in Paleoproterozoic supergroups formed during the 2.45 Ga rifting of Kenorland

Abstract

We describe the presence of Witwatersrand-type gold mineralization in Paleoproterozoic strata indicating this style of deposit is not confined to the Archean. The discovery of basal and intraformational conglomerate-hosted gold mineralization in the Huronian Supergroup’s Mississagi Formation near Sudbury, Ontario, has indicated potential for Witwatersrand-type gold mineralization to occur also in early Paleoproterozoic basin fills on the Hearne, Wyoming and Karelian cratons. The Paleoproterozoic Hurwitz, Snowy Pass and Karelian supergroups, located on these cratons, are similar to the Huronian Supergroup on the Superior Craton, in terms of sequence and litho-stratigraphy, and maximum age of deposition. A review of the stratigraphy and past gold exploration from each supergroup has revealed each basin to contain similar fluvial poly- to oligomictic, pyritic conglomeratic strata with locally anomalous gold and/or uranium concentrations at stratigraphic levels that correlate to those of the Mississagi and Matinenda formations of the Huronian Supergroup.Examination of gold and gold-enriched pyrite from the Mississagi Formation conglomerates suggests that specific point sources existed for most of the gold therein. Each of the above basins forms part of a larger contiguous continental rift system prior to the breakup of the supercontinent Kenorland at around 2480Ma. As each basin fill was displaced to different geographic positions following this continental breakup and then subjected to separate orogenic and metamorphic overprints, the gold and uranium mineralization in each of the basins is unlikely to be of a common, post-depositional hydrothermal origin, but in all likelihood of a paleoplacer origin. It is suggested that this period of gold-enrichment is the result of truncation and reworking of underlying stratigraphically older units and/or from former gold deposits/occurrences and gold-enriched greenstone-dominated hinterlands. The omnipresence of detrital and/or syn-depositional pyrite in conglomerates of each basin reinforces the idea that a lack of atmospheric oxygen at the time of Kenorland rifting existed during their deposition and prior to the “great oxidation event”. Our comparative analysis of Kenorland rift graben fills reveals that gold potential, which can be determined by the presence of detrital pyrite as observed at the Pardo project, should exist in all stratigraphic equivalents of the Mississagi and Matinenda formations where preferentially reworked fluvial conglomerates rest on an erosional unconformity and where a corresponding Au-enriched Archean hinterland existed.