Abstract

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 crude positive correlation between Au content and proportion of quartz pebbles with delta 18 O values > or = 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 > or = 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 > or = 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.

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