Origin and distribution of gold in the Huronian Supergroup, Canada — the case for Witwatersrand-type paleoplacers

Field observations and experimental results show that gold is mobile under a wide range of natural conditions in the surficial environment. However, the extent to which, and the form(s) in which gold was mobile in ancient placers remains speculative. Rather more convincing is the extent to which diagenetic and metamorphic processes have been active in redistributing the gold.Huronian paleoplacer gold deposits span a critical transition in Earth history, namely, the oxyatmoversion, evidence for which exists in the upper Gowganda Formation dated at 2.288 Ga. Prior to this transition, deposition of gold occurred under reducing atmospheric conditions, with transportation of the more finely-divided material possibly as organic-protected colloids, as has been suggested for the Witwatersrand. Following the oxyatmoversion, gold deposition will have been subject to secondary enrichment, like many Phanerozoic placer gold occurrences. For this reason, and on purely sedimentological grounds, upper Huronian strata ought to have as much potential for hosting economic deposits of gold as the basal units.A total of 121 Au and Au---U occurrences, including several past and presently producing mines from the Huronian Supergroup, are examined. These are classified according to whether mineralization is: in or adjacent to diabase dikes (11 cases); in (quartz, quartz—carbonate) veins (85 cases); stratiform (25 cases). Of the non-diabase-hosted occurrences, 41.3% occur in the Cobalt Group, 15.7% in the Quirke Lake Group, 24.9% in the Hough Lake Group and 9% in the Elliott Lake Group.Frequency of occurrence can be related to transgressive sedimentary cycles, with deposits concentrated in the Matinenda, Mississagi and Gowganda Formations, which immediately overlie the Archean—Huronian unconformity. Most of the deposits occur in the Gowganda Formation, although none of these is stratiform.In terms of Au content, there is a large overlap in class intervals of stratiform vein deposits. Vein deposits are, in general, richer than stratiform by a factor of 10. Selected stratiform deposits in the Matinenda, Mississagi and Serpent Formations are examined in light of available geological and geochemical data. In these deposits, anomalous gold values in dominantly quartzitic metasediments are accompanied by fine-grained pyrite and other heavy minerals, including uranium, which occurs in most, but not all cases. Metamorphic grade ranges from upper greenschist to lower amphibolite facies. A few of the stratiform occurrences are accompanied by accumulations of carbonaceous material, an association reminiscent of the Witwatersrand goldfields.Results of electron-microprobe study indicate that much of the gold in the Huronian metasediments occurs as low level concentrations in pyrite of morphologically different types, in arsenopyrite, chalcopyrite, and in pyrrhotite variously altered to marcasite. It is clear that Huronian paleoplacer gold deposits exist, but only in conditions much modified by diagenetic and metamorphic processes.