The relation between Au and U content and provenance has been evaluated
for a number of quartz-pebble conglomerates from the Archean
Witwatersrand and Early Proterozoic Huronian Supergroups, through
analysis of the oxygen isotope composition of siliceous detritus within
these rocks. Oxygen isotope compositions of quartz, quartzite, and chert
pebbles, as well as of individual sand-sized quartz grains in the matrix
of the conglomerates, were measured using conventional and laser
fluorination techniques. Significant heterogeneity in the delta(18)O
values of quartz and quartzite pebbles and quartz sands within
individual samples of these conglomerates, suggests that they have
preserved delta(18)O values of their source rocks. Preservation of
original delta(18)O values of individual sand grains in conglomerates
whose matrix displays a typical metamorphic texture implies limited
oxygen and silica mobility, and recrystallization by pressure solution
in the presence of small amounts of intergranular fluids.
Oxygen isotope analyses of quartz pebbles along the Main `'reef''
(conglomerate) of the West Rand Consolidated gold mine, Witwatersrand,
indicate that quartz-pebble delta(18)O values reflect local variations
in provenance. Yet, the overall delta(18)O distributions for quartz
pebbles and quartz sands from the Au- and U-bearing Main reef and the
directly overlying U-bearing Monarch `'reef'' (conglomerate) in the same
mine are identical, implying an overall similarity in their source
rocks. Comparison of the quartz-pebble delta(18)O distributions between
these two reefs and other Witwatersrand conglomerates, including
polymictic conglomerates from the Welkom area of the Witwatersrand
basin, moreover, suggests that the provenance was similar throughout
deposition of much of the Central Rand Group. Differences in the ore
content and Au/U ratio between conglomerates of the Central Rand Group
are compatible with differences in their sedimentology.
In contrast to conglomerates from the Witwatersrand Supergroup,
delta(18)O distributions of quartz pebbles from different conglomerates
throughout the Huronian Supergroup indicate distinct differences in
provenance. A erude positive correlation between Au content and
proportion of quartz pebbles with delta(18)O values greater than or
equal to 11.5 per mil exists for these conglomerates. The U-bearing
Matinenda Formation conglomerates in the Elliot Lake area have a very
low proportion of pebbles with delta(18)O values above 11.5 per mil.
Matinenda Formation conglomerates sampled closer to present outcrops of
Archean greenstone terranes, as well as conglomerates of the Lorrain and
Livingstone Creek Formations, have a higher proportion of quartz pebbles
with delta(18)O values greater than or equal to 11.5 per mil and also
higher Au contents.
Differences between the delta(18)O distributions for quartz sands and
quartz pebbles help to identify sediment sources. Absence of quartz
sands with delta(18)O greater than or equal to 11.5 per mil, but
presence of quartz pebbles with such values in the same sediment, may
indicate a source which yields little or no sand-sized material in
fluvial settings. Comparison of the delta(18)O values of quartz pebbles
and quartz sands in quartz-pebble conglomerates to delta(18)O values for
quartz from likely source rocks suggests that compositionally
homogeneous granites and related pegmatites are the dominant source of
uranium, whereas hydrothermal lode gold veins are the dominant source of
gold.