Hydrothermal alteration of organic matter in uranium ores, Elliot Lake, Canada: Implications for selected organic-rich deposits

Abstract

Organic matter in the uraniferous Matinenda Formation, Elliot Lake, is preserved in the forms of syngenetic kerogen and solid bitumen as it is in many of the Oklo uranium deposits and in the Witwatersrand gold-uranium ores. The Elliot Lake kerogen is a vitrinite-like material considered to be remnants of Precambrian cyanobacterial mats. The kerogen at Elliot Lake has reflectances (in oil) ranging from 2.63–7.31% Ro max, high aromaticity, relatively low (0.41–0.60) atomic H C ratios, and it contains cryptocrystalline graphite. Bitumen, present primarily as dispersed globules (up to 0.5 mm dia.), has reflectances from 0.72–1.32% Ro max, atomic H C ratios of 0.71–0.81, and is somewhat less aromatic than the kerogen. Overall similarity in molecular compositions indicates that liquid bitumen was derived from kerogen by processes similar to hydrous pyrolysis.The carbon isotopic composition of kerogen (−15.62 to −24.72%.), and the now solid bitumen (−25.91 to −33.00%.) are compatible with these processes. Despite having been subjected to several thermal episodes, ca. 2.45 Ga old kerogen of microbiological origin here survived as testimony of the antiquity of life on Earth. U-Pb isotopic data from discrete kerogen grains at Elliot Lake form a scattered array intersecting concordia at 2139 ± 100 Ma, correspond to the Nipissing event. U-Pb systems were totally reset by this event. Uranium and lead show subsequently partial mobility, the average of which is indicated by the lower concordia intersect of 550 ± 260 Ma. The migrated bitumen contains virtually no uranium and thorium but has a large excess of 206Pb, which indicates that the once liquid bitumen must have acted as a sink for mobile intermediate decay products of 238U. Emplacement of the Nipissing diabase may have been responsible for producing the bitumen and, indirectly, for its enrichment in 206Pb as a result of outgassing of 222Rn. Except for noble metals and arsenic, the kerogen contains higher abundances of trace elements than bitumen; REE distributions vary sympathetically and HREE depletion occurs in both organic matter types. Significant relative enrichment of noble metals in bitumen resulted from low temperature processes, a phenomenon manifest in various platinum group element-organic matter associations at other localities.