[en] Recent geological mapping has indicated changes to the stratigraphy of the Pine Creek Geosyncline. The new stratigraphy and interpreted depositional environments are examined in relation to the distribution and genesis of stratabound mineral deposits. Basinward correlations are made with near-shore carbonate and psammite-rudite units in the Rum Jungle region. Most other units in the same region are condensed, indicating long-lived supratidal, intertidal or shallow conditions during most of the depositional cycle. Units containing most of the mineralisation represent the earliest near-shore developments of strongly reducing partly pelitic and evaporitic conditions and contain mainly uranium and base metals. Areas of potential mineralisation include near-shore environments in the north, and carbonate reefs along growth faults. Two suites of postorogenic felsic volcanics and related sediments deposited in shallow water within and around northwest and east-northeast rift systems, overlie the metasediments of the Pine Creek Geosyncline in the south. The suites have potential for volcanogenic deposits, mostly of uranium, gold and copper

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[en] The Lower Proterozoic metasediments of the Pine Creek Geosyncline form a chronostratigraphic sequence of mainly greenschistand amphibolite-grade shallow-marine to supratidal pelites, psammites, carbonate rocks, and volcaniclastics which in places rests unconformably on Archaean basement. Granite and later dolerite intrude the sequence, and are associated with the major orogenic 1800-m.y. event which regionally metamorphosed the sediments. Most mineralisation is stratabound, and can be related at least partly to volcanic activity. Uranium mineralisation is mainly confined to particular carbonate-rich horizons near basement. Specialist studies indicate that uranium was leached from its souce rock, and probably carried as carbonate complexes in highly saline fluids at between 100 and 350 deg. C. Precipitation took place by redox reactions in breccia zones in carbonate rocks; these zones were formed by carbonate solution or diapiric movement of evaporites which preceded the carbonate

[en] The uranium deposits of Ranger 1, Koongarra, Jabiluka One and Two, and Nabarlek are in the Alligator Rivers Uranium Field, the northeastern part of the Pine Creek Geosyncline. Lower Proterozoic metasediments, which were metamorphosed mainly to amphibolite-grade and multiply isoclinally folded at about 1800 Ma, host much of the uranium and overlie or grade into the Archaean to Lower Proterozoic granitoid Nanambu Complex. In the northeast of the Field the metasediments grade into schist and gneiss forming the outer parts of the Lower Proterozoic Nimbuwah Complex; the inner parts of this Complex contain granodioritic and tonalitic migmatite and granitoid rocks which were emplaced before the 1800 Ma event. The metasediments are intruded by pre-orogenic and post-orogenic tholeiitic dolerite, by synorogenic granite, and by later minor phonolite and dolerite dykes. All but the minor dykes are overlain with marked unconformity by Carpentarian (Middle Proterozoic) sandstone with basalt flows, which conceals older rocks over most of the southeastern half of the area. The pre-Carpentarian (pre-Middle Proterozoic) rocks are deeply weathered and lateritised and are covered extensively by Mesozoic and Cainozoic sediment. The uranium is mainly contained in the lower member of the Cahill Formation, comprising mica quartz schist, magnesite and carbonaceous schist, which is chloritised around the uranium occurrences and along faults, shears and some stratigraphic breaks. The ore zones are located in breccia. The stratabound nature of the ore suggests that it has formed partly syngenetically; however, epigenetic processes appear essential for the development of such high-grade deposits. (author)

[en] New stratigraphic names and correlations are given for parts of the Early Proterozoic Pine Creek Geosyncline metasedimentary sequence and overlying felsic volcanics of the Darwin-Katherine region. They have significant implications for the stratigraphic distribution of uranium mineralisation in the Rum Jungle, Alligator Rivers and South Alligator Valley uranium fields

[en] The host rock at the Coronation Hill U-Au mine is a debris flow conglomerate, developed in a high-energy fluvial environment during deposition of the Coronation Sandstone of the El Sherana Group. Mineralisation took place by movement of low-temperature fluids from the U-enriched volcanics into the conduit sandstone and eventually into the reduced debris flow conglomerate and carbonaceous shale

[en] The Pine Creek Geosyncline comprises about 14km of chronostratigraphic mainly pelitic and psammitic Lower Proterozoic sediments with interlayered tuff units, resting on granitic late Archaean complexes exposed as three small domes. Sedimentation took place in one basin, and most stratigraphic units are represented throughout the basin. The sediments were regionally deformed and metamorphosed at 1800Ma. Tightly folded greenschist facies strata in the centre grade into isoclinally deformed amphibolite facies metamorphics in the west and northeast. Pre and post-orogenic continental tholeiites, and post-orogenic granite diapirs intrude the Lower Proterozoic metasediments, and the granites are surrounded by hornfels zones up to 10km wide in the greenschist facies terrane. Cover rocks of Carpentarian (Middle Proterozoic) and younger ages rest on all these rocks unconformably and conceal the original basin margins. The Lower Proterozoic metasediments are mainly pelites (about 75 percent) which are commonly carbonaceous, lesser psammites and carbonates (about 10 percent each), and minor rudites (about 5 percent). Volcanic rocks make up about 10 percent of the total sequence. The environment of deposition ranges from shallow-marine to supratidal and fluviatile for most of the sequence, and to flysch in the topmost part. Poor exposure and deep weathering over much of the area hampers correlation of rock units; the correlation preferred by the authors is presented, and possible alternatives are discussed. Regional geological observations pertinent to uranium ore genesis are described. (author)

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[en] Renewed interest in uranium exploration in northern Australia in the late 1960s led to the discovery of one of the most important uranium fields in the world. To obtain an understanding of the geological setting of the uranium deposits, and to provide guidelines for future exploration, the Bureau of Mineral Resources (BMR) undertook an investigation which included semi-detailed mapping, aided by geophysical surveys, isotopic dating, and stratigraphic drilling. From the results so far obtained and from the important data made available by mining companies engaged in exploration in the region, a pattern of uranium mineralization can be recognized, particularly in relation to the Lower Proterozoic sedimentation of the region. (author)