Uranium ore-forming systems of the - Geoscience Australia

Uranium ore-forming systems of the Lake Frome regionA critical process in uranium systems development in the region was uplift and exhumation ofuranium-rich Proterozoic basement. It is during these periods of exhumation, in three major periodsafter deposition of each of the Mesozoic Eromanga Basin, Paleocene-Eocene Eyre Formation, andMiocene Namba Formation (Fig. 3.1), that previously deeply weathered basement may have beensubject to leaching by oxidised meteoric waters and groundwaters, during climatically favourableperiods. In this hypothesis, major quantities of loosely bound (adsorbed) uranium residing withindeeply weathered relatively permeable uplifted basement may have been the source of uranium thatwas chemically transported into sediments of the Eromanga Basin and Callabonna Sub-basin.The fluids required to mobilise and transport large quantities of uranium in the near-surface parts ofsedimentary basins are necessarily highly oxidised, with minimum logfO 2 values several units abovethe hematite-magnetite buffer (see Skirrow et al., 2009, and references therein). One of theproblems in understanding uranium transport and deposition in general is how high oxidation statesare maintained during fluid flow through rocks with redox buffering capacity (Skirrow et al., 2009).For example, in section 7.4.2 it is shown that oxygen-saturated rainwater deeply circulated withinthe uranium-rich granites of the Mt Painter Inlier loses its capacity to transport significant uraniumas it is buffered to relatively low oxidation states. The hypothesis of uranium leaching from alreadydeeply weathered basement rocks solves this problem because the weathered rock is oxidised to theextent that oxygen-saturated meteoric waters may pass without being buffered to significantly lowerredox state, enabling uranium transport. Nevertheless, to remove adsorbed uranium and preventfurther local adsorption of uranium on iron oxides and oxyhydroxides the fluid chemistry must bedifferent to that during the process of adsorption. Uranium adsorption on iron oxides andoxyhydroxides is highly pH-dependant and occurs preferentially at pH values around neutral (Payneand Airey, 2006). Hence, acidic or highly alkaline oxidised groundwaters potentially may de-adsorburanium and transport it significant distances through the deeply weathered basement rock and intobasin sediments. Possible sources of acidity in the Lake Frome region are not clear, but mayinclude: oxidation of sulfide minerals; buffering by clay minerals; humic acid generation in organicrichenvironments; and high CO 2 content. Highly alkaline waters may occur in some playa lakeenvironments.This hypothesis allows some testable predictions to be made.1. Deeply weathered basement is predicted to contain uranium adsorbed on iron oxides andoxyhydroxides in areas not leached of uranium during mineralising events.2. A large proportion of the uranium in Proterozoic basement rocks is expected to be hosted byeither metamict minerals (zircon, monazite, allanite, etc) or by other readily leacheableuranium minerals such as uraninite (e.g., pre-existing uranium mineralisation).3. Uplifted and exhumed areas of deeply weathered basement are predicted to have been moreextensively leached of uranium than areas uplifted to a lesser extent. Of course mechanicalerosion may have removed some or all of the weathered profile (cf. Mt Gee area where thereis no evidence of Cenozoic weathering; Idnurm and Heinrich, 1993).4. Areas of deeply weathered basement that were subsequently leached of adsorbed uraniummay exhibit alteration by either acidic or strongly alkaline fluids. This alteration maypersist down-stream into mineralised areas. Alteration by acidic fluids will result indestruction of most silicate and carbonate minerals, including even clay minerals (cf. quartzcorrosion and replacement of kaolinite by uraninite and phosphate minerals at Four MileEast, Chapter 5).Page 103 of 151