Abstract

Oxygen isotope compositions of pebbles from late Archean to paleo-Proterozoic gold- and/or uranium-bearing oligomictic quartz pebble conglomerates of the Witwatersrand district, South Africa, and Huronian Supergroup, Canada, were determined in an attempt to define the nature of the source terrain. The δ18O values of quartz pebbles within any one sample typically vary by ∼4‰ or more, but occasionally by as much as 8‰, even for adjacent pebbles within the same hand specimen. In addition, adjacent quartz pebbles of widely contrasting δ18O values also preserve distinct isotopic signatures of their fluid inclusions. This overall heterogeneity suggests that the pebbles did not undergo significant oxygen isotope exchange after incorporation in the conglomerates. Therefore, oxygen isotope analyses of such quartz pebbles, in combination with a detailed investigation of their mineral and fluid inclusions, can provide a useful method for characterizing pebble populations and hence dominant sediment source modes. The δ18O values of quartz pebbles from the uranium-bearing Huronian ores are normally distributed about a mean of 10.2‰; several outliers have δ18O values <6‰ and one has a δ18O of 14.6‰. In contrast, values of the pebbles from the gold- and uranium-bearing ores of the Witwatersrand define a platykurtic distribution skewed toward higher δ18O values (mean 11.4‰). Comparison with δ18O values of quartz from Archean granites, pegmatites, and mesothermal greenstone gold veins, i.e., δ18O values of sources commonly proposed for the conglomerate ores, suggests that uranium is derived from a granitic source, whereas gold has a mesothermal greenstone gold source. Low δ18O values of chert pebbles (9‰ to 11.5‰) relative to those expected for Archean and Proterozoic marine cherts (commonly ≥17‰) effectively exclude marine cherts, and, therefore, auriferous iron formations and exhalatives, as likely sources of gold.

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