Tracing Pb from Nolans Bore thorianite through Alice Springs thorite to radiogenic galena: EPMA and LA-ICP-MS study of time and space

Abstract

The Pb isotope composition of crustal rocks often varies in its proportions of radiogenic Pb, formed by the decay of Th and U. In most cases, it is impossible to trace this radiogenic Pb from its source, through dilution, to a reservoir dominated by common Pb. Nolans Bore is a Th-rich REE ore deposit in the Northern Territory, Australia, in which this progression is recorded in various minerals. We show 208Pb/204Pb ratios greater than 100 000 in thorianite and 10 000 in thorite, with subsequent dilution by common Pb recorded by stetindite and ekanite (208Pb/204Pb = 600–800) and, relative to common Pb, a strongly radiogenic signal contained in late-stage zeolite veins (208Pb/204Pb = 40–80). Pyrite intimately associated with thorite inherits a highly radiogenic 208Pb/204Pb ratio of c. 2000, and nearby galena crystallized during a regional fluid flow event (Alice Springs Orogeny) is likewise radiogenic at 208Pb/204Pb = 100–120. Microbeam chemical dating of primary thorianite records the magmatic formation of Nolans Bore at 1521 ± 54 Ma (2σ), and secondary thorite records the Alice Springs Orogeny at 359 ± 10 Ma (2σ).