ALLIGATOR RIVERS ANALOGUE PROJECT ISBN 0-642-59928-9 DOE/HMIP/RR/92/072 SKI TR 92:20-2 FINAL REPORT VOLUME 2 GEOLOGIC SETTING i 1992 AN OECD/NEA INTERNATIONAL PROJECT MANAGED BY: AUSTRALIAN NUCLEAR SCIENCE AND TECHNOLOGY ORGANISATION r<6 ALLIGATOR RIVERS ANALOGUE PROJECT FINAL REPORT VOLUME 2 GEOLOGIC SETTING A. A. Snelling' 'Consultant Geologist, P.O. Box 302, Sunnybank, Qld, 4109, Australia PREFACE The Koongarra uranium ore deposit is located in the Alligator Rivers Region of the Northern Territory of Australia. Many of the processes that have controlled the development of this natural system are relevant to the performance assessment of radioactive waste repositories. An Agreement was reached in 1987 by a number of agencies concerned with radioactive waste disposal, to set up the International Alligator Rivers Analogue Project (ARAP) to study relevant aspects of the hydrological and geochemical evolution of the site. The Project ran for five years. The work was undertaken by ARAP through an Agreement sponsored by the OECD Nuclear Energy Agency (NEA). The Agreement was signed by the following organisations: the Australian Nuclear Science and Technology Organisation (ANSTO); the Japan Atomic Energy Research Institute (JAERI); the Power Reactor and Nuclear Fuel Development Corporation of Japan (PNC); the Swedish Nuclear Power Inspectorate (SKI); the UK Department of the Environment (UKDoE); and the US Nuclear Regulatory Commission (USNRC). ANSTO was the managing participant. This report is one of a series of 16 describing the work of the Project; these are listed below: No. Title Lead Author(s) 1 Summary of Findings P Duerden, D A Lever, D A Sverjensky and L R Townley 2 Geologic Setting A A Snelling 3 Geomorphology and Paleoclimatic History K-H Wyrwoll 4 Geophysics, Petrophysics and Structure D W Emerson 5 Hydrogeological Field Studies S N Davis 6 Hydrogeological Modelling L R Townley 7 Groundwater Chemistry T E Payne 8 Chemistry and Mineralogy of Rocks and Soils R Edis 9 Weathering and its Effects on Uranium Redistribution T Murakami 10 Geochemical Data Bases D G Bennett and D Read 11 Geochemical Modelling of Secondary Uranium Ore D A Sverjensky Formation 12 Geochemical Modelling of Present-day Groundwaters D A Sverjensky 13 Uranium Sorption T D Waite 14 Radionuclide Transport C Golian and D A Lever, 15 Geochemistry of 239Pu, 291, "Tc and 36CI J T Fabryka-Martin 16 Scenarios K Skagius and S Wingefors (i) I CONTENTS EXECUTIVE SUMMARY (iv) ACKNOWLEDGMENTS (vi) 1 INTRODUCTION 1 1.1 The Locational Setting 1 1.2 Uranium Orebodies as Natural Analogues 1 2 GEOLOGIC DESCRIPTION 4 2.1 Exploration History 4 2.2 Regional Geology 7 2.3 Local Geology 9 2.4 Structure 14 2.5 Distribution of the Mineralisation 15 2.6 Hydrothermal Alteration 17 2.7 Primary Ore Mineralogy and Chemistry 18 2.8 Age and Genesis of the Primary Ore 20 2.9 The Secondary Mineralisation 21 3 THE DEVELOPMENT OF THE SOLID GEOLOGY 22 3.1 The Archaean Nanambu Complex Basement (2500-2200 My) 23 3.2 Sedimentation - The Cahill Formation (2200-1900 My) 23 3.3 Dolerite Intrusion (1900-1885 My) 24 3.4 Metamorphism and Deformation (1870-1800 My) 25 3.5 Erosion (1800-1650 My) 26 3.6 Sedimentation -The Kombolgie Formation (1650-1610 My) 26 3.7 Faulting (Approximately 1600 My) 27 3.8 Mineralisation-The Formation of the Koongarra Primary Ore (1600-1550My) 30 3.9 Erosion with Geological Stability (1500-135 My) 31 3.10 The Site Today 32 4 SAMPLING FOR THE NATURAL ANALOGUE STUDY 43 4.1 Reference Grid Used for Drilling and Sampling 43 4.2 Pre-ARAP Drilling and Uranium Analyses 44 4.3 ARAP Sampling and Drilling 46 4.4 Groundwater Measurements and Sampling 59 5 SUMMARY 62 6 REFERENCES 62 (ii) APPENDIX 1 Analyses of Koongarra Chlorites 68 APPENDIX 2 Representative Analyses of Koongarra Uraninites 90 APPENDIX 3 Representative Analyses of Koongarra Sulfides 93 APPENDIX 4 Noranda Bulk Drill Core Assays 97 APPENDIX 5 Representative Analyses of Koongarra Uranyl Oxides and Silicates 104 APPENDIX 6 Representative Analyses of Koongarra Uranyl Phosphates 108 APPENDIX 7 Logs of ARAP Drill Holes 110 (iii) I EXECUTIVE SUMMARY The Koongarra uranium deposit is 225 km east of Darwin, capital city of the Northern Territory, Australia, in the area known as the Alligator Rivers Region. It is one of four major uranium deposits discovered in the region. In 1981 the US Nuclear Regulatory Commission (USNRC) began to sponsor research by the Australian Nuclear Science and Technology Organisation (ANSTO, then called the Australian Atomic Energy Commission) on aspects of all four uranium deposits in the region as natural analogues of radioactive waste repositories. As the USNRC-funded project progressed, efforts became almost entirely directed towards study of the Koongarra deposit, both because it has not yet been mined and so the ore is still in place, and because its No. 1 orebody comprises well-defined zones of primary, weathered primary and dispersion fan ore. Furthermore, results of extensive investigations by the mining companies, including hydrological data, and all the drill cores and samples, plus numerous vertical boreholes in and around the deposit, were also available. Thus the internationally sponsored Alligator Rivers Analogue Project (ARAP) focussed research on the dispersal of uranium from the weathered primary ore in the weathered rock zone to form the dispersion fan ore as the natural analogue most suitable for validation of models for radionuclide transport. Discovered in July 1970, the Koongarra deposit was delineated by diamond and rotary percussion drilling between 1970 and 1973 and concluded to consist of two orebodies, designated as the Koongarra No. 1 and No. 2 orebodies. The host rocks to the mineralisation are schists of the Cahill Formation, rocks that were once shales and siltstones deposited on the flanks of domes of older crystalline granitic rocks (the Nanambu Complex) about 2200 million years ago. Multiple severe folding accompanied the 550-630"C heat and 5-8 kb pressure of metamorphism between 1870 and 1800 million years ago. Subsequent uplift, weathering and erosion produced a new land surface on which thick layers of sandstone (the Kombolgie Formation) were then deposited probably between 1690 and 1600 million years ago. At Koongarra the host Cahill schists which dip at 550 to the south-east are now in reverse faulted contact with the younger Kombolgie sandstone, which is below the schists. The uranium mineralisation consists of uraninite veins and veinlets in crosscutting fractures and brecciated zones within a 50 m thick quartz-chlorite schist unit. Associated with the ore are minor volumes of sulfides, which include galena, chalcopyrite, bornite and pyrite, with rare grains of gold, clausthalite, gersdorffite- cobaltite and mackinawite. The orebodies consist of partially coalescing lenses that are elongated and dip at 550 broadly parallel to the reverse fault breccia which forms the footwall to the ore zone. The strongest mineralisation with grades in excess of 1%U over several metres are just below a distinctive sheared graphite- quartz-chlorite schist unit that forms the hanging wall to the ore. The No. 1 orebody is elongated over a distance of 450 m and persists to a depth of about 100 m. The width of the primary ore averages 30 m at the top of the (iv) unweathered schist, tapering out at the extremities. Secondary uranium mineralisation in the weathered schist zone, derived from decomposition and leaching of former primary ore, is present from the surface down to the base of weathering at 25-30 m, and consists of uranyl phosphate minerals, primarily saleeite. A tongue-like body of ore in the weathered schist has formed by the dispersion of uranium down-slope for about 80 m to the south-east - the dispersion fan ore. A distinct and extensive primary hydrothermal alteration halo has been ob~served about the mineralisation extending for up to 1.5 km from the ore. Uranium mineralisation is only present within the inner halo which extends to nearly 50 m from ore, and in which pervasive chlorite-dominant replacement of the schist fabric and removal of quartz has occurred. Consequently, chlorite (magnesium-rich) is the principal gangue mineral. The bulk rock geochemistry reflects this alteration and is characterised by magnesium enrichment and silicon depletion. The primary ore is enriched in copper, lead, sulfur, arsenic and vanadium, while nickel and cobalt form an enrichment halo about the deposit. The Sm-Nd isotopic data on uraninite suggests a 165D-1550 million year age for the primary ore, consistent with the observation that the mineralisation occupies the breccia zones generated by the post-Kombolgie reverse faulting. However, the secondary ore in the weathered zone appears to only have formed in the last 3 million years or less. The Koongarra deposit today is covered by sandstone debris talus and eluvial sand, broken and washed down from an escarpment of Kombolgie sandstone just behind the deposit. Beneath these surficial deposits is the weathered schist zone which is separated from the unweathered schists beneath by a transitional zone. The natural analogue that was the focus of ARAP is the formation of the dispersion fan from the weathered primary ore in the weathered schist. The Koongarra mine reference grid coupled with elevations referenced to Australian Height Datum was used to locate all samples collected and analysed during ARAP. Core and crushed core samples were selected from the inclined diamond drill holes that were drilled on cross-sections 30.5 metres apart through the No.1 orebody. At the south-western end of the orebody particularly and beyond, vertical percussion holes also provided drill chips for study. All company records on the holes, including assay and geological data, were made available to the research effort.