[en] The topography geology and economic geology are described. (R.L.)

[en] The East Alligator River region lies approximately 220 kilometers east of Darwin, Northern Territory, Australia. Uranium exploration in the region commenced in 1967, and in 1969 the surface expressions of the Koongarra and Ranger deposits were detected by airborne radiometric surveys. In 1970 similar surveys were conducted over the Nabarlek and Jabiluka areas with the former deposit exhibiting a high priority uranium anomaly. To date, approximately 350,000 metric tons of contained U;sub 3;O;sub 8; have been indicated as reasonably assured reserves and estimated additional resources in the East Alligator River region. Topography in the region consists of low-lying flood plains to the west while a deeply incided sandstone sequence forming the Arnhem Land Plateau rises abruptly to the east. The main uranium mineralizing events are dated at approximately 900 and 500 m.y. 10 refs

[en] Pancontinental Mining Limited acquired exploration rights over an area in the East Alligator River Region, Northern Territory, Australia, in 1970. Subsequently, Getty Oil Development Company Limited acquired a substantial minority interest in the property. The Jabiluka deposits were discovered during the course of exploration and are currently the largest of the four major uranium deposits in the East Alligator River Region. This region at present contains 24% of the western world's reasonably assured resources of uranium. The exploration techniques employed during primary and secondary exploration on the property between 1971 and 1975 and during the delineation of the Jabiluka deposits are discussed in detail. The case history illustrates the exploration philosophy which was successfully employed on the Jabiluka property. The philosophy encompasses the following points: The need for an assessment on the limits of airborne radiometric surveys; the necessity for detection and evaluation of point source anomalies; the necessity for exploration along extensions of favourable lithologies; and the desirability of modification of exploration techniques on different types of anomalies. Some aspects of this philosophy may be useful in exploration for similar stratabound uranium deposits in other areas. (author)

[en] The Jabiluka One and Two uranium deposits occur in Lower Proterozoic metasediments of the Cahill Formation. The observed part of the Cahill Formation exhibits four horizons which are favourable hosts for uranium mineralization. The host rocks are mainly chlorite and/or graphite schists and their brecciated equivalents which have undergone initial regional prograde metamorphism to amphibolite facies, then retrograde metamorphism to greenschist facies. Mineralization consists of uraninite, mainly filling open spaces and to a lesser extent in disseminated form. Chlorite alteration is intimately associated with the uranium mineralization. A portion of the Jabiluka Two deposit contains economic concentrations of gold. Although the deposits are generally stratabound, structural preparation appears to be the most significant ore control on a local scale. The Jabiluka case history illustrates an effective philosophy which was successful in exploration for stratabound uranium deposits in the East Alligator River Region. This philosophy encompassed the following points: (a) The value of regional appraisals in selection of a property; (b) The recognition of the detection limits of airborne radiometric surveys; (c) The importance of ground prospecting for low-order point-source radiometric anomalies which cannot be detected by airborne survey; (d) The importance of evaluating all anomalies and the flexibility to change priorities as further exploration results are obtained; (e) The necessity of establishing the controls on the mineralization before proceeding with further exploration; (f) The necessity of exploring extensions of favourable lithologies to test for periodicity of mineralization even where cover precludes surface expression; and (g) The desirability for modification of exploration techniques on different types of anomalies

[en] As has been pointed out by a number of workers, the development of quartz-pebble conglomerate uranium deposits was confined to the period of 2800-2200 million years (Ma). The first appearance of these deposits coincides with major igneous, sedimentary, biogenic and tectonic changes, and it is suggested that all these factors jointly contribute to the uniqueness of quartz-pebble conglomerate uranium deposits. On the other hand, vein-type uranium deposits made their appearance throughout time post-2200 Ma. However, over 90% of vein-type uranium deposits are found in rocks dated between 2200 and 1700 Ma. It is suggested that this skewness is real. As continental crustal development in post-2200 Ma times appears largely to follow uniformatarian lines, the only variable which could explain the concentration of vein-type uranium in the 2200-1700 Ma period appears to be a steadily evolving atmosphere. It is suggested that during these times the hydrosphere was sufficiently oxidizing for uranyl transport, but rapidly reducing conditions were met short distances into the lithosphere. Reduction resulted in precipitation of uranium of UO₂ from meteoric water into suitable structural traps which were largely developed during periods of prolonged erosion. Rapid development of impermeable cover rocks preserved the uranium deposits. (author)

[en] Development of quartz-pebble conglomerate uranium deposits was confined to the period 2,800-2,200 M.y. The first appearance of these deposits coincides with major igneous, sedimentary, biogenic and tectonic changes, and it is suggested that all of these factors jointly contribute to the uniqueness of quartz-pebble conglomerate uranium deposits. Vein-type uranium deposits made their appearance in the post-2,200 M.y. period. However, over 90% of vein-type uranium deposits are found in rocks dated between 2,200 and 1,400 M.y. It is suggested that this skewness is real. The only variable which could explain the concentration of vein-type uranium in the 2,200-1,700 M.y. period appears to be a steadily evolving atmosphere. It is suggested that during these times the hydrosphere was sufficiently oxidising for uranyl transport, but that rapidly reducing conditions were met short distances into the lithosphere. Reduction resulted in precipitation of uranium as UO₂ from meteoric water into suitable structural traps, which were largely developed during periods of prolonged erosion. The structural traps may also have been active during the early sedimentation of the middle proterozoic cover rocks. Rapid development of impermeable cover rocks preserved the uranium deposits

[en] This paper reviews briefly and discusses some of the material presented during this Symposium. Much of the Symposium centred around two main questions: 1) What are the characteristic features of the known uranium mineralization in the Pine Creek Geosyncline. 2) How did the known uranium mineralization in the Pine Creek Geosyncline form. It is convenient to begin by reviewing briefly the models which were proposed during the Symposium. The present authors have had to base this review and discussion paper on the papers presented at the Symposium. They do not in any way seek to pass judgement on these papers, but attempt to make constructive comments and draw attention to some problems. While the Symposium is a landmark in our progress towards gaining a full understanding of uranium mineralization in the Proterozoic rocks of northern Australia, we look forward to further work which will confirm or modify some of the concepts put forward. For the most part we will not refer to mineralization other than of uranium. Indeed, most of this paper will be concerned with the very rich uranium mineralization which occurs in the Alligator River Uranium Field