The petrophysics, geophysics and structure of the Koongarra site, Northern Territory

Abstract

This paper reports on part of a natural analogue study on the migration of radionuclides in the weathered zone at the Koongarra site in the Northern Territory of Australia. The unmined but intensively drilled Koongarra uranium deposit in the Northern Territory of Australia has been studied to evaluate the processes and mechanisms involved in the alteration of the primary uranium mineralisation and to model the formation of the secondary uranium orezone and the dispersion fan. The work has resulted in solid phase and groundwater databases, and hydrological data from field and laboratory measurements. These are being used to test a range of geochemical, hydrological and transport models. The Koongarra uranium deposit is situated in the Alligator Rivers Uranium Province of the Northern Territory of Australia, about 200 km east of Darwin. The uranium ore is located in steeply dipping, physically anisotropic, quartz chlorite Cahill Formation schists of early Proterozoic age and is near the base of a prominent southeast-facing sandstone escarpment. The upper 25 m of the schist is weathered. An upper clayey zone of intense weathering separates deeper partly weathered schist from the overlying Quaternary sands, silts and ferricretes. Directly to the northwest of the ore deposit, a major reverse fault, the Koongarra Fault, has upfaulted the schist against the middle Proterozoic Kombolgie Sandstone. Hydraulic properties of the fault zone have not been tested directly, but the fault was previously considered to be a barrier or partial barrier to lateral flow of groundwater. The site is currently undisturbed by mining, but the steel casings of over 100 exploratory drillholes proved to be a source of noise in magnetic and electrical surveys and the condition of the holes precluded the use of drillhole and cross-hole geophysics in the site studies. A number of different approaches have been made to characterise the site and to try to understand: how groundwater has moved and is moving through the orezone region; the source of the water ? natural recharge or flow across the fault; how the groundwater drains out of the site; whether fluid flow is through fractured or porous media or both; and in particular, how this has influenced the transport of uranium and the formation of a prominent dispersion fan.