Molecular marker and stable carbon isotope analyses of carbonaceous Ambassador uranium ores of Mulga Rock in Western Australia

Mulga Rock is a multi-element deposit containing uranium hosted by Eocene peats and lignites deposited in inset valleys incised into Permian rocks of the Gunbarrel Basin and Precambrian rocks of the Yilgarn Craton and Albany-Fraser Orogen. Uranium readily adsorbs onto minerals or phytoclasts to form organo-uranyl complexes. This is important in pre-concentrating uranium in this relatively young ore deposit with rare uraninite [UO2] and coffinite [U(SiO4)1-x(OH)4x], more commonly amorphous and sub-micron uranium-bearing particulates. Organic geochemical and compound-specific stable carbon isotope analyses were conducted to identify possible associations of molecular markers with uranium accumulation and to recognize effect(s) of ionizing radiation on molecular markers. Samples were collected from the Ambassador deposit containing low (<200 ppm) to high (>2000 ppm) uranium concentrations. The bulk rock C/N ratios of 82 to 153, Rock-Eval pyrolysis yields of 316 to 577 mg hydrocarbon/g TOC (Hydrogen Index, HI) and 70 to 102 mg CO2/g TOC (Oxygen Index, OI) are consistent with a terrigenous and predominantly vascular plant OM source deposited in a complex shallow water system, ranging from lacustrine to deltaic, swampy wetland and even shallow lake settings as proposed by previous workers. Organic solvent extracts were separated into saturated hydrocarbon, aromatic hydrocarbon, ketone, and a combined free fatty acid and alcohol fraction. The molecular profiles appear to vary with uranium concentration. In samples with relatively low uranium concentrations, long-chain n-alkanes, alcohols and fatty acids derived from epicuticular plant waxes dominate. The n-alkane distributions (C27 to C31) reveal an odd/even preference (Carbon Preference Index, CPI=1.5) indicative of extant lipids. Average δ13C of -27 to -29 ‰ for long-chain n-alkanes is consistent with a predominant C3 plant source. Samples with relatively higher uranium concentrations contain mostly intermediate-length n-alkanes, ketones, alcohols, and fatty acids (C20 to C24) with no preferential distribution (CPI~1). Intermediate length n-alkanes have modest carbon isotope enrichment compared to long-chain n-alkanes. These shorter-chain hydrocarbons are interpreted to represent alteration products. The diversity and relative abundance of ketones in highly mineralised Mulga Rock peats and lignites are not consistent with aerobic and diagenetic degradation of terrigenous OM in oxic environments. Moreover, molecular changes cannot be associated with thermal breakdown due to the low maturity of the deposits. It is possible that the association of high uranium concentrations and potential radiolysis resulted in the oxidation of alcohol functional groups into aldehydes and ketones and breakdown of highly aliphatic macromolecules (i.e. spores, pollen, cuticles, and algal cysts). These phytoclasts are usually considered to be recalcitrant as they evolved to withstand chemical and physical degradation. Previous petrographic analyses show that spores, pollen and wood fragments are preferentially enriched in uranium. Their molecular compositions are feasible sources of short- to intermediate-length n-alkanes that dominate the mineralised peats and lignites.