A soil geochemistry orientation survey for uranium at Koongarra, Northern Territory

A soil geochemistry orientation survey for U at Koongarra was designed to determine optimum conditions for future U exploration in the area. Soil samples were collected at various depths from auger holes drilled along two traverses over the Koongarra No. 1 orebody, along a single traverse over suspected mineralization at nearby Anomaly A, and at three background localities. Rock samples collected from surface outcrop, costeans, and drill core were used to investigate any elemental associations with the ore or primary dispersion which could then be traced in the overlying soils.The results showed that Cu and Pb are potentially suitable pathfinder elements, where U data are not definitive, while Co, Ni and Be also provide significant information. The optimum sample depth was 1.2 m. For a Koongarra-sized target the maximum sample spacing should be 30 m on lines 200 m apart, provided every anomalous sample is followed up with closer spaced sampling around it. Anomalies detected in alluvial soil deeper than 1.2 m were due to hydromorphic dispersion. Some of the general distribution patterns for individual elements may be related to soil-type variations.     

Uranium/daughter disequilibrium in the Koongarra uranium deposit,Australia

Fifty five samples from the Koongarra uranium deposit have been analysed for radiometric disequilibrium. The deposit is situated in the Northern Territory, Australia and consists primarily of steeply dipping pitchblende veins overlain by a dispersion fan of weathered ore. The disequilibrium ratios, which assess the degree to which the emitted gamma radiation represents the uranium content of a sample, show the pitchblende ore to be at, or close to, equilibrium. The host wall rocks, which have a low uranium grade, and some samples from the dispersion fan have low ratios indicating radium enrichment. Analysis of Variance of the data was performed to determine if significant disequilibrium trends were associated with sample location, grade, rock type or grouping by ore type. A significant trend was found in samples from the dispersion fan where the disequilibrium ratio increases in the direction of the groundwater flow, indicating movement of uranium in these waters. No conclusive evidence was found for movement of uranium from depth to the dispersion fan and the fan is apparently a result of weathering processes on a previous upward extension of the primary pitchblende veins. Redistribution of both uranium and radium has occurred at depth within primary ore zones and host wall rocks.     

Age and paragenesis of mineralisation at Coronation Hill uranium deposit,Northern Territory,Australia

Coronation Hill is a U?+?Au?+?platinum group elements deposit in the South Alligator Valley (SAV) field in northern Australia, south of the better known unconformity-style U East Alligator Rivers (EAR) field. The SAV field differs from the EAR by having a more complex basin-basement architecture. A volcanically active fault trough (Jawoyn Sub-basin) developed on older basement and then was disrupted by renewed faulting, before being buried beneath regional McArthur Basin sandstones that are also the main hanging wall to the EAR deposits. Primary mineralisation at Coronation Hill formed at 1607?±?26 Ma (rather than 600–900 Ma as previously thought), and so it is likely that the SAV was part of a single west McArthur Basin dilational event. Most ore is hosted in sub-vertical faults and breccias in the competent volcanic cover sequence. This favoured fluid mixing, acid buffering (forming illite) and oxidation of Fe²⁺ and reduced C-rich assemblages as important uranium depositional mechanisms. However, reduction of U in fractured older pyrite (Pb model age of 1833?±?67 Ma) is an important trap in diorite. Some primary ore was remobilised at 675?±?21 Ma to form coarse uraninite?+?Ni-Co pyrite networks containing radiogenic Pb. Coronation Hill is polymetallic, and in this respect resembles the ‘egress’-style U deposits in the Athabascan Basin (Canada). However, these are all cover-hosted. A hypothesis for further testing is that Coronation Hill is also egress-style, with ores formed by fluids rising through basement-hosted fault networks (U reduction by diorite pyrite and carbonaceous shale), and into veins and breccias in the overlying Jawoyn Sub-basin volcano-sedimentary succession.     

Re-evaluation of the petrogenesis of the Proterozoic Jabiluka unconformity-related uranium deposit, Northern Territory, Australia

The world class Jabiluka unconformity-related uranium deposit in the Alligator Rivers Uranium Field, Australia, contains >163,000 tons of contained U₃O₈. Mineralization is hosted by shallow-to-steeply dipping basement rocks comprising graphitic units of chlorite–biotite–muscovite schist. These rocks are overlain by flat-lying coarse-grained sandstones belonging to the Kombolgie Subgroup. The deposit was discovered in 1971, but has never been mined. The construction of an 1,150 m decline into the upper eastern sector of the Jabiluka II deposit combined with closely spaced underground drilling in 1998 and 1999 allowed mapping and sampling from underground for the first time. Structural mapping, drill core logging and petrographic studies on polished thin sections established a detailed paragenesis that provided the framework for subsequent electron microprobe and X-ray diffraction, fluid inclusion, and O–H, U–Pb and ⁴⁰Ar/³⁹Ar isotope analysis. Uranium mineralization is structurally controlled within semi-brittle shears that are sub-conformable to the basement stratigraphy, and breccias that are developed within the hinge zone of fault-related folds adjacent to the shears. Uraninite is intimately associated with chlorite, sericite, hematite ± quartz. Electron microprobe and X-ray diffraction analysis of syn-ore illite and chlorite indicates a mineralization temperature of 200°C. Pre- and syn-ore minerals extracted from the Kombolgie Subgroup overlying the deposit and syn-ore alteration minerals in the Cahill Formation have δ¹⁸O_fluid and δD _fluid values of 4.0±3.7 and −27±17‰, respectively. These values are indistinguishable from illite separates extracted from diagenetic aquifers in the Kombolgie Subgroup up to 70 km to the south and east of the deposit and believed to be the source of the uraniferous fluid. New fluid inclusion microthermometry data reveal that the mineralising brine was saline, but not saturated. U–Pb and ²⁰⁷Pb/²⁰⁶Pb ratios of uraninite by laser-ablation ICP-MS suggest that massive uraninite first precipitated at ca. 1,680 Ma, which is coincident with the timing of brine migration out from the Kombolgie Subgroup as indicated by ⁴⁰Ar/³⁹Ar ages of 1,683±11 Ma from sandstone-hosted illite. Unmineralized breccias cemeted by chlorite, quartz and sericite cross-cut the mineralized breccias and are in turn cut by straight-sided, high-angle veins of drusy quartz, sulphide and dolomite. U–Pb and ²⁰⁷Pb/²⁰⁶Pb ratios combined with fluid inclusion and stable isotope data indicate that these post-ore minerals formed when mixing between two fluids occurred sometime between ca. 1,450 and 550 Ma. Distinct ²⁰⁷Pb/²⁰⁶Pb age populations occur at ca. 1,302±37, 1,191±27 and 802±57 Ma, which respectively correlate with the intrusion of the Maningkorrirr/Mudginberri phonolitic dykes and the Derim Derim Dolerite between 1,370 and 1,316 Ma, the amalgamation of Australia and Laurentia during the Grenville Orogen at ca. 1,140 Ma, and the break-up of Rodinia between 1,000 and 750 Ma.     

Mapping Proterozoic unconformity-related uranium deposits in the Rockhole area, Northern Territory, Australia using landsat ETM+

The Rockhole area, Northern Territory, Australia, hosts a number of Proterozoic unconformity-related uranium deposits. The geology of the area features within Paleoproterozoic rocks of the Pine Creek Orogen, near the unconformity with overlying platform cover sandstone of the Paleo- to Mesoproterozoic McArthur Basin. Landsat Enhanced Thematic Mapper plus (ETM+) data was used in the Rockhole area to assist in mapping geological structures and lithology, and to identify anomalous concentrations of ferrous minerals, the product of alteration, which can be indicators of buried uranium mineralization. Several image-processing procedures were applied to the ETM+ data to identify, isolate and enhance mineralogical information as simple and complex false color composites. ETM+ 754 shown as red green and blue respectively was the best simple image. Overall, complex images based on Principal Component Analysis proved to be the most useful products. Sandstone, shale and siltstone, the target lithologies, Koolpin Formation, the target stratigraphic unit, and bleaching pattern due to the removal of iron(II) compounds, the target alteration pattern, were confidently mapped to provide information required by the mineral emplacement model, which ultimately identified areas of likely uranium mineralization. Thus the contrasting behavior of the two principle oxidation states of uranium and iron can be utilized to map/delineate bleached alteration zones associated with economic concentrations of uranium using multispectral sensors like Landsat or better hyperspectral sensors.     

Geology of the Ranger Uranium Mine, Northern Territory, Australia: structural constraints on the timing of uranium emplacement

The geology of the No 1 and 3 pits at the Ranger Mine in the Pine Creek Inlier (PCI) of Australia is dominated by Palaeoproterozoic volcanic, carbonate and sedimentary sequences that unconformably overlie Archaean granitic gneiss of the Nanambu Complex (2470±50 Ma). These sequences are folded, faulted and sheared, and crosscut by east-trending granite (sensu stricto) dykes and pegmatite veins, and gently dipping N–NE trending mafic dykes of the Oenpelli Dolerite (1690 Ma). Regional metamorphism is to greenschist facies and contact metamorphism is to hornblende-hornfels facies.The rocks of the Ranger Mine have been subjected to at least two phases of ductile–brittle deformation (D2–D3) and one phase of brittle deformation (D4). These events were preceded by regional diastathermal or extension-related metamorphism (D1) and the development of an ubiquitous bedding-parallel cleavage (S1).D2 resulted in the development of NNE–NNW trending mesoscopic folds (F2) and a network of thrusts and dextral reverse shears. The modelled palaeo-stress directions for the emplacement of pegmatite veins suggests that they formed early in D2. D3 resulted in the development of WNW–NW trending mesoscopic folds (F3), a weakly defined axial planar cleavage (S3) and sinistral reactivation of D2 shears. D2–D3 are correlated with deformation during the Maud Creek Event of the Top End Orogeny (1870–1780 Ma), while the emplacement of granite dykes and pegmatite veins is correlated with emplacement of regional granites at 1870–1860 Ma.D4 is associated with brittle deformation and resulted in the development of normal faults and fault breccias during a period of east–west extension. This event is correlated with regional east–west extension during deposition of Palaeo- to Mesoproterozoic platform sequences.The sequence of tectonic events established in this study indicates that uranium-bearing ore shoots in the Ranger No 1 and 3 pits formed during extension in D4, and after emplacement of the Oenpelli Dolerite at 1690 Ma. However, the currently accepted 1737±20 U–Pb Ma age places the mineralising event at time of regional post-orogenic erosion, after the Top End Orogeny and before emplacement of the Oenpelli Dolerite and extension in D4. The U–Pb age is not consistent with Sm–Nd ages for primary uranium mineralisation at Nabarlek and Jabiluka at 1650 Ma [Econ. Geol. 84 (1989) 64] and does not concur with currently accepted regional tectonic data of Johnston [Johnston, J.D., 1984. Structural evolution of the Pine Creek Inlier and mineralisation therein, Northern Territory, Australia. Unpublished PhD Thesis, Monash University, Australia], Needham et al. [Precambrian Res. 40/41 (1988) 543] and others. Consequently, the absolute age of uranium mineralisation at the Ranger Mine is open.     

Application of pyrite trace element chemistry to exploration for SEDEX style Zn-Pb deposits: McArthur Basin,Northern Territory,Australia

Sedimentary pyrites in black shales contain abundant trace elements that provide information on the chemistry of the seawater at the time of sedimentation. This study focuses on the Barney Creek Formation (~ 1640 Ma) in the McArthur Basin in the Northern Territory of Australia, which is host to one of the world's largest SEDEX Zn-Pb-Ag deposits, and several smaller deposits. Fine-grained sedimentary pyrite has been sampled from three drill holes through the Barney Creek Formation at various distances from SEDEX mineralisation. Samples were selected through the stratigraphy of each hole and analysed by LA-ICPMS for a suite of 14 trace elements. The data show that sedimentary pyrite at the base of the Barney Creek Formation, closest (within 1 km) to SEDEX mineralisation, is strongly enriched in Zn and Tl by one to two orders of magnitude compared to the global average for sedimentary pyrite. In contrast sedimentary pyrite from the hole furthest from SEDEX mineralisation (~ 60 km) contains mean Zn and Tl values equal to, or less than, the global average. Based on the three drill hole pyrite data sets it is concluded that trace elements that are contributed to the basin during hydrothermal exhalation, and adsorbed into contemporaneous sedimentary pyrite, are principally Zn, Tl, Cu, Pb, Ag and As. In contrast, trace elements that are adsorbed into sedimentary pyrite from background seawater are principally Mo, Ni, Co, Se and As. These differences have enabled the development of a SEDEX fertility diagram for sedimentary basins, based on the composition of sedimentary pyrite, that distinguish high Zn, but barren shales, from high zinc SEDEX-related shales. In parallel with the increase in Zn and Tl in sedimentary pyrite approaching mineralisation there is a decrease in Ni, Co and Mo. This means that the ratios Zn/Ni and Tl/Co are particularly good pyrite vectors to SEDEX mineralisation in the McArthur Basin, varying over 4 to 6 orders of magnitude from barren shales to mineralised shales. It is speculated that the reason for the reverse relationship between Ni, Co and Zn, Tl may be caused by hydrothermal exhalations into the water column that effect the ion-exchange pyrite surface complexation processes that alter the uptake of these elements into sedimentary pyrite.Another important conclusion of this study is that hydrothermal exhalations into a sedimentary basin may affect the redox sensitive trace element chemistry of sedimentary pyrite and therefore the trace element chemistry of pyritic black shales. Nickel, Co and Mo all decrease in proximity to hydrothermal vents that form SEDEX deposits, whereas Zn, Tl and Pb increase. Selenium and bismuth are the only redox sensitive trace elements that appear to be unaffected by hydrothermal activity in the McArthur Basin. This has implications on how trace element concentrations of black shales and pyrite are used to reflect past global ocean chemistry.     

Groundwater regimes and isotopic studies,Ranger mine area,Northern Territory

Three types of groundwater occur in the area of the Ranger mine. Type A groundwater occurs in the loose sands and gravels occupying the present day stream channels, Type B in the weathering profile and Type C occurs in relatively fresh fractured bedrock occupying open fractures and other cavities. The three types of groundwater can be distinguished both chemically and isotopically. Light stable isotope data suggest that most early rains are lost by evapotranspiration and have no imprint on the groundwater. Later in the wet season, the ground is saturated and groundwater recharge occurs on a regional scale. This younger groundwater sits on the older waters. Mixing is probably minimal as before any large scale mixing could occur, most younger waters are lost by evapotranspiration. Stable isotope data suggest that Type B groundwater in certain areas has some connection with evaporated surface water bodies. Stable isotope measurements for the pollution monitoring bores around the tailings dam do not indicate any connection with the polluted pond waters at the time of sample collection.     

3D representation of geochemical data, the corresponding alteration and associated REE mobility at the Ranger uranium deposit, Northern Territory, Australia

Interrogation and 3D visualisation of multiple multi-element data sets collected at the Ranger 1 No. 3 uranium mine, in the Northern Territory of Australia, show a distinct and large-scale chemical zonation around the ore body. A central zone of Mg alteration, dominated by extensive clinochlore alteration, overprints a biotite–muscovite–K-feldspar assemblage which shows increasing loss of Na, Ba and Ca moving towards the ore body. Manipulation of pre-existing geochemical data and integration of new data collected from targeted ‘niche’ samples make it possible to recognise chemical architecture within the system and identify potential fluid conduits. New trace element and rare earth element (REE) data show strong fractionation associated with the zoned alteration around the deposit and with fault planes that intersect and bound the deposit. Within the most altered portion of the system, isocon analysis indicates addition of elements including Mg, S, Cu, Au and Ni and removal of elements including Ca, K, Ba and Na within a zone of damage associated with ore precipitation. In the more distal parts of the system, processes of alteration and replacement associated with the mineralising system can be recognised. REE element data show enrichment in HREE centred about a characteristic peak in Dy in the high-grade ore zone while LREEs are enriched in the outermost portions of the system. The patterns recognised in 3D in zoning of geochemical groups and contoured S, K and Mg abundance and the observed REE patterns suggest a fluid flow regime in which fluids were predominately migrating upwards during ore deposition within the core of the ore system.     

Dolomitization and the genesis of the Woodcutters lead-zinc prospect,Northern Territory,Australia

Conclusion Roberts' mechanism of ore genesis in this paper is plausible and is worthy of consideration. It could well apply to situations such as the Missouri-type deposits, although Gerdemann and Myers (1972) might disagree. One cannot deny that dolomitization and diagenetic processes have occurred at Woodcutters, but in view of the folding and the time-relationships exhibited by the rocks, it would appear that these are not the ultimate ore-forming processes.It is possible that the mechanism of ore genesis proposed by Roberts might apply more fully to the Bulman deposits east of Woodcutters (Patterson, 1965) where the resemblance to a Missouritype situation is stronger.     

Dolomitization and the genesis of the Woodcutters lead-zinc prospect,Northern Territory,Australia

The Woodcutters L. 5 lead-zinc prospect in the Northern Territory, Australia, occurs in the Golden Dyke Formation, a sequence of carbonaceous siltstone, dolomite, and greywacke forming part of the Lower Proterozoic Goodparla Group, which was deposited on an Archaean granitic basement. An attempt has been made to show how those factors which are considered to have been significant in the formation of dolomite were also important in the genesis of the Woodcutters deposit. These factors are: 1. An evaporitic environment which favoured dolomite formation concentrated lead and zinc in the overlying solutions. 2. The base metals were further concentrated, and fixed in the sediments, by co-precipitation with the precursors of dolomite, Mg-calcite and/or aragonite. 3. The formation of dolomite during diagenesis resulted in either a structural change if the precursor was aragonite, or an ordering if the precursor was Mg-calcite. The dolomite could not accommodate the relatively large amount of base metal associated with its precursors, and as a consequence, during dolomitization these were released to the pore solutions. The metals in the pore solutions possibly complexed with organic materials such as those from the degradation of algal protein, and so remained in solution during lithification. During folding, the metal-enriched solutions were transported to fractures, and metal sulphides precipitated when the organic complexes became unstable. After lithification the carbonate-quartz-sulphide veins were zones of weakness along which shearing took place, probably over a considerable period of time. This shearing, as well as slight rise in temperature, resulted in fracturing, recrystallization, and reaction between the first-formed simple sulphides to produce the ore in its present form.

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Zusammenfassung Die Woodcutters L. 5 Blei-Zink-Lagerstätte im Northern Territory, Australien, tritt in der Golden Dyke Formation auf, die aus kohlenstoffhaltigem Siltstein, Dolomit und Grauwacke besteht. Dieselbe bildet einen Teil der Unter-Proterozoischen Sedimente der Goodparla group, welche auf einem Archaischen Granit-Untergrund abgelagert wurden. In dieser Arbeit ist der Versuch gemacht worden, zu zeigen, wie diese Faktoren, welche also wichtig in der Bildung von Dolomit betrachtet werden, auch bedeutend in der Genesis der Woodcutters Lagerstätte waren. Diese Faktoren sind: 1. Die evaporitische Umgebung, welche die Bildung von Dolomit begünstigt, konzentriert ebenfalls Blei-Zink in den darüberliegenden Lösungen. 2. Die Metalle wurden weiter angereichert und mit den Sedimenten durch Co-Precipitation mit den Vorläufern des Dolomits, Mg-Calcit oder Aragonit, verbunden. 3. Die Bildung von Dolomit während der Diagenese führte entweder zu einem strukturellen Wechsel, wenn Aragonit der Vorläufer war, oder zu einem Einbau im Falle von Mg-Calcit. Der gut geordnete Dolomit war nicht in der Lage, die verhältnismäßig große Menge von Blei-Zink, verbunden mit seinen Vorläufern, zu behalten und demzufolge wurden diese während der Dolomitisierung an die Porenlösungen abgegeben. Die Metalle in den Porenlösungen, möglicherweise zusammengesetzt mit organischem Material, wie diejenigen von der Degradation von Algen Protein, verblieben löslich während der Konsolidation. Während der Faltung wurden die mit Metall angereicherten Lösungen zu Spalten transportiert und als Metall-Sulphide niedergeschlagen, als die organischen Komplexe instabil wurden. Nach der Verfestigung wurden die Karbonat-Quarz-Sulphid-Gänge Schwächezonen, entlang denen Scherung stattfand, wahrscheinlich über eine große Zeitspanne hinweg. Sowohl diese Scherung als auch ein leichter Temperaturanstieg verursachten Brüche, Rekristallisationen und Reaktion zwischen den zuerst geformten einfachen Sulphiden, um das Erz in seiner jetzigen Form zu bilden.

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Published by permission of the Director, Bureau of Mineral Resources, Geology and Geophysics, Canberra, Australia.     

Gold exploration using lead isotopes at Tennant Creek,Australia

Lead isotope analyses have been undertaken as part of a program to evaluate the potential of geochemical methods for use in exploration in the Tennant Creek goldfield. Earlier exploration in this area was based on magnetic geophysical methods. Economic Au, Cu and Bi mineralization usually occurs in magnetic magnetite-chlorite lenses or pods (“ironstones”) which may be only 30 m across. Several hundred ironstones are found in the Tennant Creek field, of which only nine have been significant producers. Despite complications arising from the low Pb and relatively elevated U contents of the ore, determination of Pb isotope ratios in drill core material allows discrimination between economic magnetic ironstones and “barren” ironstones of similar mineralogy. A target signature for the Th-derived Pb isotope ratio, ²⁰⁸Pb/²⁰⁴Pb, is specific for lode mineralization, although it does not discriminate between Au-rich and Cu-rich lodes. The target signature is commonly found not only in the central Au-rich magnetite-chlorite zone, but also in the outer “barren” talc-magnetite and carbonate zones, offering up to a two-fold increase in the size of the target. The Pb isotope signature is retained in hematite-rich surface ironstones (termed “gossans” here) and it appears possible, at this stage of the project, to discriminate between gossans derived from mineralized magnetite lodes and “barren” magnetite lodes. A discrimination between weakly-mineralized (either Cu or Au) and economic deposits is equivocal at this stage. The difference between target and sample ²⁰⁸Pb/²⁰⁴Pb ratios (Δ208/204) ranges from −3 to +2% for the economic lodes, through weakly mineralized and “barren” magnetic ironstones (commonly in the range −20 to −40%) to the country rock magnetite shales (about −40 to −70%). These data are consistent with a significant Pb component in the “barren” ironstones being derived locally from the magnetite shales. Hematite shales commonly associated with economic lodes may have acted as limited channelways for the ore fluids as the target signature is discernible for up to 50 m along the hematite shale bands from the outer chlorite zones of the economic lodes. Further trace element and isotopic work is necessary to elucidate the genesis of the mineralization.     

Pitchblende and galena ages in the Alligator Rivers region,Northern Territory,Australia

U-Pb and Pb isotopic studies have been made of pitchblendes and associated galenas from four major deposits in the Alligator Rivers region of the Northern Territory, Australia. These are compared with previously-published data on deposits in the South Alligator Valley and in the Westmoreland area on the Queensland border. In all, four different times of possible mineral formation have been detected (1880 ± m. y., 1700 ± m. y., 800–900 m. y., 400–500 m. y.) Some deposits reveal only one time, others two; no deposit shows evidence for all four. The associated galenas are in general highly radiogenic (206/204 ratios range from 2000 to 29, 000); some can be interpreted as reflecting development in two completely different generations of pitchblende. The clearest evidence for solid-state formation of galena from presently-existing pitchblende is to be found at Nabarlek, where the major time of pitchblende formation (920 m. y.) bears no relationship to any known Rb-Sr or K-Ar age.     

Thrust-controlled, gold quartz-vein mineralisation at the Tom's Gully mine, Northern Territory, Australia

Metasedimentary and minor metavolcanic rocks of the Early Proterozoic Pine Creek Inlier rest unconformably on Late Archaean granitic basement. Three basin-wide, regional deformation events at ca.1885–1870 Ma are recognised: I) W- to NW-verging thrusts and recumbent folds (D₂), II) upright, open to tight, doubly-plunging, NNE- to NNW-trending folds (D₃), and III) open, upright, E-trending folds (D₄). In the centre of the Pine Creek Inlier, post-tectonic granites (1835–1820 Ma) are spatially, temporally and probably genetically associated with mesothermal gold-quartz vein deposits. The Tom's Gully deposit consists of a shallowly S-dipping quartz reef in graphitic shale and siltstone within the thermal aureole of the post-tectonic (1831 ± 6 Ma) Mt Bundey pluton. Gold mineralisation comprises two(?) SSW-plunging sulphidic ore-shoots which are intimately associated with brecciation and recrystallisation of early barren quartz. Where early quartz is absent from the thrust, gold mineralisation is not developed, indicating that this secondary brittle fracturing was essential to sulphide and gold deposition. The ore-shoots plunge parallel to the trend of D₃ fold axes. The reef is hosted by a D₂ thrust fault with transport to the NW. D₃ folds in the hangingwall and footwall decrease in amplitude toward the reef indicating that, during continued E-W compression, the thrust acted as a décollement zone. Field relationships and microstructural studies suggest that quartz and sulphide were deposited in a reactivated thrust during wrench shear along several NNE-trending faults associated with emplacement of the Mt Bundey pluton.     

The Granites gold deposits, Northern Territory, Australia: evidence for an early syn-tectonic ore genesis

The ore deposits of The Granites goldfield are shear-hosted within Palaeoproterozoic amphibolite facies metasedimentary rocks in the Tanami Region, Northern Territory, Australia. The ore bodies are located within a 5- to 35-m thick sequence of steeply dipping unit of metamorphosed iron-rich metasedimentary rocks. Deformation at The Granites was complex and is characterized by five successive deformation phases (D_1–5). Shear veins (central and oblique) are the dominant type of vein geometry, with minor development of extensional veins and reverse-fault related veins. Four generations of syn-tectonic veins, corresponding to D₁, D₃, D₄, and D₅, have been recognized and are comprised of quartz, quartz-carbonate, calc-silicate, and calcite. In addition, two generations of disseminated sulfide–arsenide mineralization, dominated by pyrrhotite, arsenopyrite, and loellingite, with minor pyrite, chalcopyrite and rare marcasite, formed syn-D₁ and syn- to post-D₃. Textural and structural evidence indicates deposition of gold was contemporaneous with the syn-D₁ veins and sulfide–arsenide mineralization. Four hydrothermal phases are proposed for the formation of the veins and disseminated sulfide–arsenide assemblages. The first phase (D₁) was responsible for transport and deposition of the majority of the gold. Minor remobilization and deposition of gold occurred during the D₃ and D₄ phases. Little is known about the nature of the D₁ ore fluid, although a relatively low sulfur content is indicated by the assemblage pyrrhotite–arsenopyrite–loellingite+rare pyrite. The growth of amphibolite facies metamorphic minerals andalusite and almandine garnet during D₁ indicates a high temperature for the fluid. The D₃ hydrothermal phase coincided with peak metamorphism. D₄ fluids were hypersaline, high temperature, CO₂-poor, and H₂S-poor. Editorial handling: L. Meinert     

Triact spicules in proterozoic rocks of the Northern Territory of Australia

Triact fragments, probably of sponge spicules, occur in a siliceous rock found in the Proterozoic succession along the coast of the Gulf of Carpentaria in the Northern Territory of Australia.     

Paleovalley-related uranium deposits in Australia and China: A review of geological and exploration models and methods

The features and similarities in the geology of paleovalley-related uranium mineralizing systems in Australia and China can be used to refine strategies for exploration. Paleovalley-related uranium resources include sandstone-, lignite- and calcrete-style deposits that are developed within the host sediments deposited in paleovalleys. The paleovalleys incise either crystalline bedrock or older sedimentary rocks, and uranium was deposited and concentrated by the influx of oxidized/reduced groundwaters flowing in aquifers within the paleovalley fill. The critical features of paleovalley-related uranium deposits include sediment and uranium sources, geological setting, depositional environment, age and relative timing of mineralization, aquifer characteristics, availability and distribution of reductants, and preservation potential of the uranium mineral system. This set of information provides a basis to establish the uranium mineralization model, which can then be used to assist with generating targets for uranium exploration and prospectivity analysis of a region. With respect to Sino-Australian examples, paleovalley-related uranium deposits form mostly around the margins of sedimentary basins and the mineralization is commonly hosted within channel fills contained within paleovalleys developed upon, or proximal to, Precambrian crystalline rocks that contain primary uranium sources. The deposits that have been well studied show remarkably similar factors that controlled the formation of paleovalley-related uranium deposits. Basement/bedrocks with above-background (2.8 ppm U) levels of uranium (10–100 ppm) that are linked to, and/or, incised by paleovalleys are associated with these deposits and are the inferred source of the uranium. In these regions, extensive fluvial systems developed particularly during Mesozoic and Cenozoic times, uranium from the bedrock was first dispersed into the sediments, and then concentrated to form deposits through successive chemical remobilization, precipitation and concentration. The deposits formed in continental or marginal marine environments, and commonly are associated with reduced lithologies, containing pyrite and dispersed organic matter and/or seams of lignite, or show evidence of infiltrated hydrocarbons. The mineralization is developed where oxidizing fluids (carrying dissolved U) reacted with reductants in the sediments. Geological, geophysical and geochemical features of the paleovalleys and related uranium deposits are used to construct models to understand host sediment distribution, fluid flow and ore genesis that can assist exploration for paleovalley-hosted uranium deposits. Precise geometric definition of the basin margin and paleovalley architecture is important in identifying exploration targets and improving the effectiveness of drilling. Refinements in remote sensing, geophysical and data processing techniques, in combination with sedimentological and depositional interpretations, provide an efficient approach for outlining the principal drainage patterns and channel dimensions. To help reduce risk, an exploration strategy should combine these technologies with a detailed understanding of the physicochemical parameters controlling uranium mobilization, precipitation and preservation.     

Geochemical assessment of an analogue site for an engineered landform at Ranger Uranium Mine, Northern Territory, Australia

 An essential element in identifying sites as analogues for the long-term development of soils and vegetation on engineered landforms of the rehabilitated Ranger Uranium Mine, Northern Territory, Australia, is the need to match rocks. Comparison of the geochemistry of rocks from the waste rock dump of Ranger Uranium Mine and the potential analogue site of Tin Camp Creek area, Western Arnhem Land, indicates that there are several sites along Tin Camp Creek that may be used as analogue sites. Detailed comparisons between the Ranger and Tin Camp Creek lithologies have been undertaken using a variety of techniques, including cluster analysis. It is demonstrated that at least 70% of the rocks being mined at Ranger have analogues in the Tin Camp Creek area. Received: 2 October 1996 · Accepted: 4 November 1997