Y. Bone | Geology Reports

USGS1995

The minimal response to contact metamorphism by the Devonian Buchan Caves Limestone, Buchan Rift, Victoria, Australia

A 2.2 m thick, Late Eocene (?) dike that intruded the Devonian Buchan Caves Limestone, near Murrindal, Victoria, has produced a narrow contact aureole only centimeters wide in the adjacent host rock. Mean solid bitumen reflectance ranges from about 2.4 to 2.7% and shows little change up to 6 m away from the dike contact. Carbon and oxygen isotope values also show little change except at the dike contact. The most detectable response of the rock to contact metamorphism is the proportion of solid bitumen grains with finely granular mosaic structure increase towards the dike. Also under cathodoluminescence (CL) excitation, the host rock is initially observed to be recrystallized to a uniform bright orange color. By 1 cm away from the contact, however, CL shows preservation of sharp boundaries in finely zoned burial cements. A regional study of thermal maturation and a geothermometer based on solid bitumen reflectance indicate paleotemperatures near 200°C were widespread in the Buchan Rift during peak burial near the time of the Devonian Tabberabberan Deformation. The lack of response of the Buchan Caves Limestone to contact metamorphism is attributed to: (1) prior heating to near 200°C; and (2) the fact that the dike intruded into cool, near surface, low-porosity rocks which may have been in the vadose zone.

eastern Victoria↗

USGS1990

Organic tissues, graphite, and hydrocarbons in host rocks of the Rum Jungle Uranium Field, northern Australia

The Rum Jungle Uranium field consists of at least six early Proterozoic deposits that have been mined either for uranium and/or the associated base and precious metals. Organic matter in the host rocks of the Whites Formation and Coomalie Dolomite is now predominantly graphite, consistent with the metamorphic history of these rocks. For nine samples, the mean total organic carbon content is high (3.9 wt%) and ranged from 0.33 to 10.44 wt%. Palynological extracts from the host rocks include black, filamentous, stellate ( Eoastrion -like), and spherical morphotypes, which are typical of early Proterozoic microbiota. The colour, abundance, and shapes of these morphotypes reflect the thermal history, organic richness, and probable lacustrine biofacies of the host rocks. Routine analysis of rock thin sections and of palynological residues shows that mineral grains in some of the host rocks are coated with graphitized organic matter. The grain coating is presumed to result from ultimate thermal degradation of a petroleum phase that existed prior to metamorphism. Hydrocarbons are, however, still present in fluid inclusions within carbonates of the Coomalie Dolomite and lower Whites Formation. The fluid inclusions fluoresce dull orange in blue-light excitation and their hydrocarbon content is confirmed by gas chromatography of whole-rock extracts. Preliminary analysis of the oil suggests that it is migrated, and because it has escaped graphitization through metamorphism it is probably not of early Proterozoic age. The presence of live oil is consistent with fluid inclusion data that suggest subsequent, low-temperature brine migration through the rocks. The present observations support earlier suggestions that organic matter in the host formations trapped uranium to form protore. Subsequent fluid migrations probably brought additional uranium and other metals to these formations, and the organic matter provided a reducing environment for entrapment.

Rum Jungle Uranium Field, northern Australia↗