The effect of weathering on the distribution of uranium and associated elements at Koongarra, Northern Territory, Australia

Edis, R

The effect of weathering on the distribution of uranium and associated elements at Koongarra, Northern Territory, Australia

Date

1991-03

Authors

Edis, R

Publisher

Sydney University and Australian Nuclear Science and Technology Organisation

Abstract

A number of analyses were conducted to investigate the redistribution of U, and other elements, at the Koongarra U deposit following the intersection of ore material by weathering conditions. Aspects of the redistribution considered here include spacial patterns of redistributed U, associations between U, other elements and minerals, and transport of material by groundwater colloids. Techniques employed included:- thin section analyses (optical analysis, autoradiography, electron microprobe, and scanning electron microscopy); mineralogical and chemical analyses (x-ray diffraction, x-ray fluorescence, PIXE-PIGME, neutron activation, and infra-red spectrophotometry); groundwater colloid sampling and concentrating (tangential and stirred cell ultrafiltration); transmission electron microscopy for the examination of colloids and scanning electron microscopy for the examination of prefilters. Contact of the primary U ore by oxidising groundwater has resulted in the mobilisation of U, leading to increased U contents of the weathered rock at least 300 m down gradient. The most extensive transport seems to have occurred in the near surface, although deep weathered zone enrichment appears to account for most of the mobilised U. In the unweathered zone, there were at least two types of chlorite present, the major non-quartz host rock mineral. These were a coarse Fe-rich form and a fine grained Mg-rich form. The relative abundances of the two probably affects the kinetics of weathering, the oxidation and leaching of the ore, and the formation of highly sorptive secondary phases. Chlorite in high grade ore appeared to contain more Fe(ll) and Ni than low grade material. Fracture filling chlorite was found to sometimes host veins of pitchblende, suggestive of a relationship between this chlorite and ore genesis. Elements that correlated with U in the unweathered zone were Nb, MO, Pb, Cu, Ni, Sn, and Th. The zone of primary mineralisation was characterised by high concentrations of heavy rare earth elements (HREEs) compared to light REEs. Possibly carbonate rich waters, bearing U, HREEs, and other elements, were responsible for the initial formation of the ore. Fracture coatings contained much more U than the bulk rock. Apparent diffusivity of U into an unweathered quartz chlorite matrix was estimated to be about 10-18 m2sec-1 . Fracture coatings showed more extensive penetration of U. Veins of chlorite also showed greater penetration of U, and a greater sensitivity to alteration, with enrichment of U accompanied by Fe and general oxidation. Biotite grains appeared to be more susceptible to weathering than chlorite. The redistribution of U with weathering followed the distribution of Fe oxides. An association between Fe oxides and U was apparent from the early stages of weathering. In well weathered rock, U was associated with secondary U minerals and oxides, particularly of Fe and Mn. Manganese oxide, probably lithiophorite, had the highest concentrations of U, but Fe oxides were much more abundant and accounted for most of the redistributed U. Different forms of Fe oxide probably play different roles in migration retardation, with nodules providing long-term storage and finer dispersed forms providing more efficient scavenging. The location of Fe and Mn oxides at fissure surfaces places them in an ideal position for entrapment of dissolved species. Elements sensitive to localised zoning of Fe oxides were P, As, Mn, Ni, V, Zn, Cu, and U. Elements depleted from the primary mineralised zone as a result of weathering were U, Mg, S, Mn, Ca, and Li. In the weathered zone above the ore body, and in the zone of uranyl phosphates, U correlated with P and tended to follow the distribution of Fe.This suggests a three way association between U, P and Fe, and a mineralogical control to the U distribution. Further down gradient U correlated with Fe, suggestive of a sorption control. The high degree of heterogeneity in the distribution of Fe oxides may complicate the prediction of U transport by sorption modelling. Groundwaters were found to contain very little colloidal material, -with from about < 105 to about 108 particles/L. The highest load was in the vicinity of a graphitic hanging wall unit. Colloid types observed were mainly clays (kaolinite and altered chlorite), anatase, U oxide, Pb oxide, Mg silicates and Fe oxides. One Au particle was also observed.

Description

Physical copies held by ANSTO Library at DDC: 553.4932/13.

Keywords

Koongarra deposit, Uranium, Ground water, Weathering, Elements, Minerals, Electron microprobe analysis, Mineralogy, Chemical analysis, X-ray fluorescence analysis, Colloids, PIXE analysis, Rare earths, Chlorite minerals, Manganese oxides, Clays

Citation

Edis, R. (1991). The effect of weathering on the distribution of uranium and associated elements at Koongarra, Northern Territory, Australia. [Master's dissertion, Sydney University].