Geology of the Alligator Rivers Uranium Field, Northern Territory

The Alligator Rivers Uranium Field covers about 22500 km2 in the northeast of the Pine Creek Geosyncline and contains the Jabiluka, Ranger 1, Koongarra and Nabarlek uranium deposits. The area is distinct from other parts of the geosyncline in-hat metamorphic grade and degree of deformation is markedly greater. Uranium was discovered in 1970. Mapping and drilling in the region was carried out by the BMR between 1971 and 1978. The area between the South and East Alligator Rivers is dominated by the Nanambu Complex with granitoids that record Archaean and c.1800 Ma orogenic events, and Early Proterozoic accreted feldspathic and quartzitic gneiss and schist. The gneiss is in places gradational outwards into Mount Basedow Gneiss of the Kakadu Group, which is the lowest recognisable part of the Early Proterozoic metasedimentary sequence. The Munmarlary Quartzite is a quartz-rich equivalent of the gneiss on the western flank of the complex. The Cahill Formation overlies the Kakadu Group but apparently onlaps locally to rest directly on Archaean granite of the Nanambu Complex. The Cahill Formation is host to all the uranium deposits and most of the uranium prospects of the area, and is the prime guide to exploration. Uranium mineralisation lies in the lower member, it characterises by the presence of carbonaceous units and metacarbonate rocks, and includes interlayered mica schist, chloritised feldspathic quartzite, quartz schist, para-amphibolite and calc-silicate rock. The Cahill Formation is overlain, in places apparently unconformably, by the Nourlangie Schist, a monotonous sequence of mainly quartz-mica schist. Ferruginous schist with finely crystalline quartz bands exposed in the north may be a metamorphosed equivalent of the South Alligator Group; the latter is a distinctive, preferentially mineralised sequence widespread in the remainder of the Pine Creek Geosycline. East of the East Alligator River, the Early Proterozoic meta-sediments grade into metamorphically differentiated schist and gneiss of the Myra Falls Metamorphics. Some distinctive lithologies can be mapped as Yesisters' which are correctable with parts of the Kakadu Group and Cahill Formation. In the extreme east, pre-orogenic granitoid intruded at c.1870 Ma was metamorphosed and partly differentiated along with the metasediments to form the migmatitic Nimbuwah Complex; the migmatite ranges from granitic to granodioritic in the north, to tonalitic in places south of Nabarlek. The Top End Orogeny' probably began with the intrusion of the pre-deformation granitoid, and lasted until at least 1780 Ma. Pre-deformation tholeiitic dolerite sills are preserved throughout the area as strongly foliated amphibolite. Post-deformation granite-trondhjemite form small stocks in the south of the region and east and south of Nabarlek. The Middle Proterozoic Kombolgie Formation makes up a sub-horizontal sheet of sandstone and interbedded mainly andesitic volcanics. The contact between the Kombolgie Formation and the older rocks is a marked regional unconformity, representing a major erosional period. Small pockets of tuffaceous sediments along the contact are probably distal equivalents of the mainly felsic late Early Proterozoic volcanism centred in the South Alligator Valley Uranium Field. Minor phonolite and dolerite dykes were emplaced between about 1370-1200 Ma ago, concentrated in the Nanambu and Nimbuwah Complexes. Jabiluka, Ranger 1 and most uranium prospects are stratabound within the Early Proterozoic Cahill Formation. Nabarlek is hosted by the Myra Falls Metamorphics, which is in part a correlative of the Cahill Formation. The metacarbonate rocks near Jabiluka, Ranger 1 and Koongarra were derived from evaporites, indicating a precursor supratidal environment. This and the stratabound nature of the deposits suggest that a supratidal environment was a possible prerequisite for uranium concentration.

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