A chronology for Late Quaternary weathering in the Murray Basin, southeastern Australia: evidence from 230Th/U dating of secondary uranium phosphates in the Lake Boga and Wycheproof granites, Victoria

Abstract

The Lake Boga Granite in northern Victoria contains a suite of well-crystallised secondary uranyl phosphates, including torbernite, saléeite, metanatroautunite and ulrichite. The minerals crystallised in miarolitic cavities and on joints, after dissolution of primary uraninite and fluorapatite by oxidising groundwater. A more restricted assemblage occurs in the nearby Wycheproof Granite. The 230Th/U dating method was used to reveal any links between formation ages of these minerals and climatic fluctuations during the Quaternary. Out of 104 analyses, 77 gave apparent ages less than about 500 ka, the upper limit beyond which secular equilibrium in the 230Th/U decay series is closely approached. The age distribution curve shows a broad peak around 400 ka, coinciding with a global interglacial period (Marine Isotope Stage 11). Thereafter, there is a slow tapering off towards younger ages, but with no direct correlation with interglacial peaks and with no ages less than 100 ka. Throughout this time interval (ca 500–100 ka), the southeastern Australian region was undergoing aridification, suggesting that regional rather than global climate was the more significant influence on uranium phosphate crystallisation. While caution needs to be applied when interpreting these results, the overall distribution pattern might be best explained by fluctuating weathering affecting a finite primary uranium source that was progressively depleted from about 400 ka and exhausted by around 100 ka. The study has also revealed that the Lake Boga and Wycheproof granites acted as natural reservoirs for the sequestration of uranium in phosphate-rich assemblages over periods of several hundred thousand years.