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 delta-O-18 values of quartz pebbles within
any one sample typically vary by approximately 4 part per thousand or
more, but occasionally by as much as 8 parts per thousand, even for
adjacent pebbles within the same hand specimen. In addition, adjacent
quartz pebbles of widely contrasting delta-O-18 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 delta-O-18
values of quartz pebbles from the uranium-bearing Huronian ores are
normally distributed about a mean of 10.2 parts per thousand; several
outliers have delta-O-18 values <6 part per thousand and one has a
delta-O-18 of 14.6 parts per thousand. In contrast, values of the
pebbles from the gold- and uranium-bearing ores of the Witwatersrand
define a platykurtic distribution skewed toward higher delta-O-18 values
(mean 11.4 parts per thousand). Comparison with delta-O-18 values of
quartz from Archean granites, pegmatites, and mesothermal greenstone
gold veins, i.e., delta-O-18 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
delta-O-18 values of chert pebbles (9 parts per thousand to 11.5 parts
per thousand) relative to those expected for Archean and Proterozoic
marine cherts (commonly greater-than-or-equal-to 17 parts per thousand)
effectively exclude marine cherts, and, therefore, auriferous iron
formations and exhalatives, as likely sources of gold.