Alligator Rivers Uranium Field Research Articles

Fluid Migration Regimes during the Formation of the Unconformity-Related Uranium Deposits of the Alligator Rivers Uranium Field, Australia

Fluid Migration Regimes during the Formation of the Unconformity-Related Uranium Deposits of the Alligator Rivers Uranium Field, Australia

Fluid migration regimes during the formation of the unconformity-related uranium deposits of the Alligator Rivers Uranium Field, Australia

Fluid migration regimes during the formation of the unconformity-related uranium deposits of the Alligator Rivers Uranium Field, Australia

Uranium mobility and deposition over 1.3 Ga in the Westmoreland area (McArthur Basin, Australia)

The Westmoreland area is located in the southern part of the McArthur Basin (Australia) and hosts several uranium (U) deposits and prospects. Previous studies proposed that U mineralisation formed under conditions similar to the unconformity-related U deposits of the Alligator Rivers Uranium Field (ARUF), located north of the McArthur Basin. Detailed mineralogical, geochemical and geochronological studies on mineralised intervals from the Redtree, Junnagunna, and Huarabagoo deposits, and integration with previous studies, identified at least six generations of uranium oxides. These formed between ca. 1680 and 350 Ma, highlighting protracted mobility of U over 1.3 Ga. Each generation of uranium oxide has a specific chemical composition, indicating variable physicochemical conditions for their formation throughout this period. Although the 1680-Ma mineralising event formed at the same time and with similar mineralising fluids as the unconformity-related U deposits from the ARUF, the physicochemical conditions differed between the two areas. Deposits of the Westmoreland area lack the typical Mg– and B–metasomatism of the unconformity-related U deposits in the ARUF: in the Westmoreland area, the chlorite is Mg-poor and no B minerals are observed, whereas the ARUF is marked by Mg-rich chlorite, between clinochlore and Mg–amesite, Mg–foitite, and alumino–phosphate–sulfate minerals. The chondrite-normalised REE patterns of uranium oxides are enriched in light rare earth elements (LREEs) or flat, compared with the typical bell shape of the unconformity-related U deposits. Chlorite thermometry indicates significantly higher temperature conditions (> 300 °C) than in the ARUF for the early U stages. Based on these results, mineralisation in the Westmoreland area is thus significantly different from the unconformity-related U deposits in the ARUF and in other sedimentary basins. Collectively, the data point towards atypical ore-forming processes for basin-related U deposits in the Westmoreland area, but show some similarities with those known in the U deposits of the Otish Basin (Quebec, Canada).

The Ranger uranium deposit, northern Australia: Timing constraints, regional and ore-related alteration, and genetic implications for unconformity-related mineralisation

The Ranger uranium deposit, northern Australia: Timing constraints, regional and ore-related alteration, and genetic implications for unconformity-related mineralisation

Relating unconformity-type uranium mineralization of the Alligator Rivers Uranium Field (Northern Territory, Australia) to the regional Proterozoic tectono-thermal activity: An illite K–Ar dating approach

Illite particles of <0.4, 1–2 and <2μm size were separated from Kombolgie sandstones and basement rocks of the Alligator Rivers Uranium Field (Northern Territory, Australia) at the Jabiluka and Nabarlek deposits and at nearby exploration sites, on the basis of a previous X-ray diffraction and scanning electron microscope study. Dated by the conventional K–Ar method to constrain the regional tectono-thermal evolution, illite crystallized during successive Proterozoic tectono-thermal episodes at 1774±40Ma for one separate of a basement rock, and at ∼1656, 1600, 1554, 1457, 1425, 1371, 1355 and 1318Ma (mean-age peaks). Most of the studied host-rocks underwent hydrothermal processes, mainly from ∼1650 to 1300Ma, that monitored large-scale basin-derived brine flows initiated by contemporary tectonic pulses.Comparison with ages obtained before by U–Pb, 207Pb/206Pb, Sm–Nd and Rb–Sr on UO2, and Rb–Sr and 40Ar/39Ar on illite and other silicates of deposits comparable to those studied here, suggests that illitization was not always linked to U precipitation. However, significant amounts of illite crystallized between ∼1680 and 1520Ma (K–Ar individual ages) at the time U was deposited. This indicates that the unconformity-type U deposits formed probably during three major regional events between 1680 and 1520Ma (at 1656, 1600 and 1554Ma). Only detected away from U concentrations between ∼1457 and 1318Ma, the younger illitization episodes resulted probably from U-devoid hydrothermal fluids, although probably of basinal origin, during minor tectono-thermal events. Further U–Pb ages from 1300 to 500Ma obtained previously on UO2 are also considered as isotopic resets. Not linked to illitization, they were probably not connected to tectono-thermal activity and large-scale brine circulations. These late events appear to have played only a minor role in the U endowment of the region.This study demonstrates also that only specific tectono-thermal events of the 1680–1520Ma period in the ARUF region induced regional mineralizing brine circulations and subsequent U deposition, while younger illitization episodes until ∼1300Ma caused restricted fluid circulations and illitization episodes without U supply, as records of the further history of this U deposition. At a broader scale, this study confirms also the critical importance of the geodynamic and subsequent tectono-thermal conditions during this 140-Ma long period during which a variety of world-class metal deposits (U, but also Cu, Pb and Zn) formed in the Paleo-Proterozoic sedimentary basins of the North-Australian Craton.

Advances in understanding the Kombolgie Subgroup and unconformity-related uranium deposits in the Alligator Rivers Uranium Field and how to explore for them using lithogeochemical principles

The Alligator Rivers Uranium Field (ARUF) includes the mined and unmined Jabiluka, Ranger, Koongarra and Nabarlek unconformity-related uranium deposits and several small prospects including the newly discovered King River prospect. Uranium mineralisation is hosted by a variety of metamorphosed Nimbuwah Domain lithologies that are unconformably overlain by the Kombolgie Subgroup, a basin package of unmetamorphosed arenites and mafic volcanics. All of the uranium deposits and prospects preserve an identical alteration assemblage that is subdivided into a distal and proximal alteration zone. The distal alteration zone comprises an assemblage of sericite and chlorite that replace albite and amphibole. In some cases, this alteration can be traced >1000 m from the proximal alteration zone that is dominated by uraninite, hematite, chlorite and sericite. Uranium precipitated in the basement as uraninite at 1680 Ma at around 200°C from a fluid having δ18Ofluid values of 3.0±2.8‰ and δDfluid values of −28±13‰ VSMOW reflecting an evolved marine source. These geochemical properties are indistinguishable from those recorded by diagenetic illite and chlorite that were collected from the Kombolgie Subgroup sandstones across the ARUF. The illite and chlorite formed in diagenetic aquifers, and where these aquifers intersected favourable basement rocks, such as those containing graphite or other reductants, U was precipitated as uraninite. Therefore, it is proposed that the Kombolgie Subgroup is the source for fluids that formed the deposits. A post-ore alteration assemblage dominated by chlorite, but also comprising quartz±dolomite±sulfide veins cut the uranium mineralisation at all deposits and has historically been recorded as part of the syn-ore mineralisation event. However, these minerals formed anywhere between 1500 to 630 Ma from fluids that have distinctly lower δ18Ofluid values around 1.5‰ and lower δDfluid values around −45‰ reflecting a meteoric water origin. Despite unconformity-related uranium deposits having a large alteration halo, they remain difficult to find. The subtle alteration of albite to sericite several hundred metres from mineralisation occurs in isolation of any increase in trace elements such as U and radiogenic Pb and can be difficult or impossible to identify in hand specimen. Whole rock geochemical data indicate that Pearce Element Ratio (PER) analysis and General Element Ratio (GER) analysis may vector into this subtle alteration because it does not rely on an increase in trace elements to identify proximity to ore. PER and GER plots, Al/Ti vs (2Ca + Na + K)/Ti, Na/Al vs (Na + K)/Al, K/Al vs (Na + K)/Al and (Fe + Mg)/Al vs (Na + K)/Al provide a visual guide that readily distinguish unaltered from altered samples. A plot of (Na + K)/Al and (Fe + Mg)/Al on the x-axis against the concentration of trace elements on the y-axis reveals that U, Pb, Mo, Cu, B, Br, Ce, Y, Li, Ni, V and Nd are associated with the most intensely altered samples. The lithogeochemical vectors should aid explorers searching for uranium mineralisation in a prospective basin environment, but exploration must first focus on the characteristics of the basin to assess its mineralisation potential. A holistic model that describes the evolution of the Kombolgie Subgroup from deposition through diagenesis to formation of the uranium deposits in the underlying basement rocks is presented and has application to other basins that are considered prospective for unconformity-related uranium deposits. The model outlines that explorers will need to consider the thickness of the sedimentary pile, its lithological composition relative to depositional setting, the depth to which the sediments were buried during diagenesis and the degree of diagenesis achieved, which may be time dependant, before deciding on the prospectivity of the basin.

Clay vein and its implication for uranium exploration activity in the northern part of the Alligator Rivers Uranium Field, northern Australia.

Clay vein and its implication for uranium exploration activity in the northern part of the Alligator Rivers Uranium Field, northern Australia.

Paleo-fluid composition determined from individual fluid inclusions by Raman and LIBS: Application to mid-proterozoic evaporitic Na–Ca brines (Alligator Rivers Uranium Field, northern territories Australia)

Detailed determination of the composition of individual fluid inclusions in the H 2O–NaCl–Ca(Mg)–Cl 2 system from microthermometry data may be difficult because (i) the melting of salt hydrates is often unclear, (ii) salts or salt hydrates are often metastable, and iii) eutectic temperatures are often difficult to relate precisely to a unique salt–water system. These problems can be circumvented by using a Raman microprobe to identify the salt hydrates and to measure the chlorinity, by using Laser Induced Breakdown Spectroscopy (LIBS) to determine cation ratios, and by cross-checking the data from these independent techniques at each step in the determination of the fluid inclusion composition. Data from individual inclusion analysis are then completed by crush–leach technique data on bulk samples. This methodology was applied to a case study, the mineralized faults and associated breccia located in the Alligator Rivers Uranium Field (middle Proterozoic Kombolgie Subgroup, Northern Territory, Australia). Faults are cemented by euhedral quartz containing a variety of fluid inclusions with a wide range of ice-melting temperatures, from near 0 °C down to about − 50 °C. The approach has permitted the identification of three types of fluids: an Na-rich brine with variable Mg concentrations, a Ca-rich brine with low Mg concentrations, and a lower salinity fluid. The Ca and Na-brines mixed with each other and a less saline end-member. On the basis of Cl/Br and cation ratios, the Na-brine is interpreted as a primary brine, resulting from the evaporation of seawater. It is inferred that the Na-brine evolved chemically during its interaction with the Ca-rich lithologies from Proterozoic basement, forming subsequently a Ca-brine through Na–Ca exchange reactions. Thus, the fault system, which affects both the base of the Kombolgie sandstone and its basement, represents active drainage zones where different fluid reservoirs were connected, and thus a place where fluid mixing was highly favored.

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 δ18Ofluid and δDfluid 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.

Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Deposits in the East Alligator Rivers Uranium Field, Northern Territory, Australia

Clay minerals associated with the unconformity-type uranium deposits of the East Alligator Rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the Kombolgie unconformity. Alteration develops a similar pattern at regional and smaller scales, with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity (e.g., faults, fractures, breccias). These discontinuities were mechanically active during the hydrothermal activity. The hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the Kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks. The time-space alteration sequence established in this study can be summarized as follows: (1) early crystallization of illite ± illite-smectite (I-S) mixed layer minerals + LREE-Sr–bearing aluminum phosphate-sulfates (APS) in sandstones above the unconformity, illite ± I-S mixed layer minerals + sudoite + APS on both sides of the unconformity and in fault gouges, and illite ± I-S mixed layer minerals + chlorite ± apatite deeper in the basement; and (2) late precipitation of chlorite in the open fracture network mainly centered below the unconformity. The main stage of uranium deposition was associated with chloritization in the basement rocks. Smaller particle size, lath-shaped or flaky morphology, common interstratification with expandable layers, lower crystallinity, and Mg-rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and/or chlorite. Hydrothermal illite can be distinguished from diagenetic and metamorphic illite-phengite by its poor crystallinity along the c-axis, interstratification with minor amounts of smectite layers, and a 1M polytype in which trans-vacant octahedral sites predominate (1 Mt). Chemically, the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher Mg content. The hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size, spherulitic habit, poor crystallinity along the c-axis, and common association with corrensite and/or chlorite-corrensite mixed layers. Most of the hydrothermal chlorite is Mg rich and has a structural formula close to that of clinochlore (XFe <0.20). Near the uranium deposits, the spherules of hydrothermal chlorite are chemically zoned (from XFe <0.10 to XFe <0.40), which is interpreted as the result of a change in the oxidation state of the hydrothermal solution during the chlorite crystallization. The space-time alteration sequence established for clay minerals and the observed transition of APS to apatite are interpreted as the result of an increasing degree of interaction of the basement rocks with infiltrating acidic and oxidizing sub-basinal brines. In turn, these fluids tend to be neutralized and reduced by their ongoing interaction with ferrous iron-bearing phyllosilicates (Fe chlorite and biotite) and iron sulfides of metamorphic origin. The ore deposits are surrounded by zones in which the degree of fluid-rock interaction was very high and chloritization is most abundant. The octahedral Mg content of the associated hydrothermal illite from the sandstone can be used as a guide to indicate the possible presence of uranium orebodies in the basement rocks, tens to hundreds of meters below. The crystal structures of the hydrothermal illite and chlorite suggest that these minerals crystallized at temperatures approximatively 30° to 50°C lower than those expected at the peak diagenesis of the Kombolgie sandstones near the unconformity.

Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Deposits in the East Alligator Rivers Uranium Field, Northern Territory, Australia

Clay Alteration Associated with Proterozoic Unconformity-Type Uranium Deposits in the East Alligator Rivers Uranium Field, Northern Territory, Australia

SIGNIFICANCE OF ALTERATION ASSEMBLAGES FOR THE ORIGIN AND EVOLUTION OF THE PROTEROZOIC NABARLEK UNCONFORMITY-RELATED URANIUM DEPOSIT, NORTHERN TERRITORY, AUSTRALIA—A DISCUSSION

Sir: In their recent paper on the Nabarlek uranium deposit, Polito et al. (2004) appear to have misunderstood Wilde and Wall’s (1987) description of outer and inner halo alteration. Wilde and Wall (1987) defined the outer alteration as pseudomorphous replacement of amphibole by chlorite, biotite by chlorite, and rutile (or anatase) and feldspar by finegrained white mica (sericite) and not as chlorite-hematite schist, chlorite-muscovite ± hematite schist and chloritesericite schist up to 1 km from the Nabarlek fault (Polito et al., p. 117). Contrary to the assertion by Polito et al. (2004, p. 117) that is ... relatively rare in the outer alteration zone, metamorphic rocks of the outer halo have been pervasively silicified beneath and along the margins of the Oenpelli Dolerite and quartz veining is common along pre-ore faults beyond the ore zone. The suggestion of Polito et al. (2004) that pervasive silicification was coincident with intrusion of the Oenpelli Dolerite lacks credible supporting evidence. An equally plausible interpretation is that the lithologic boundary between the competent and homogeneous Oenpelli Dolerite and its strongly foliated and inhomogeneous host rocks focused fluid flow during post-intrusion deformation. The inner halo as defined by Wilde and Wall (1987) extends for no more than 50 m from ore and is manifested as nearcomplete pervasive replacement of metamorphic quartz by fine-grained (clay-sized) chlorite and white mica involving extreme desilicification of the host rocks (Fig. 1a, b). In many samples from Nabarlek and other uranium deposits from the East Alligator Rivers uranium field there is evidence of preservation of metamorphic (and outer halo) phases, notably coarse metamorphic muscovite and biotite (often with interlayered chlorite) in pervasive fine-grained alteration products (Fig. 1a-d). Fig. 1. a. Photomicrograph, plane-polarized light, of altered schist from inner alteration zone at Nabarlek (DH96 60m). Ch …

Significance of Alteration Assemblages for the Origin andEvolution of the Proterozoic Nabarlek Unconformity-Related Uranium Deposit,Northern Territory, Australia

The Proterozoic Nabarlek unconformity-related uranium deposit in the Alligator Rivers uranium field is hosted by Paleoproterozoic amphibolite-grade, metamorphosed semipelitic sedimentary rocks and amphibolite schist. High-grade ore is confined to the Nabarlek fault, a reverse fault/shear zone that crosscuts a series of interbedded muscovite-quartz-biotite schists and amphibolite. Petrographic studies on polished thin sections combined with electron microprobe analyses, X-ray diffraction, fluid inclusion data, O-H and U-Pb isotope values, as well as 40 Ar/ 39 Ar dating have identified up to eight significant fluid events beginning with the precipitation of early quartz veins during uplift of the Myra Falls Metamorphics at 1830 Ma and ending with limited uraninite recrystallization during reactivation of the Nabarlek fault between 1380 and 750 Ma. Quartz veins that likely formed toward the end of the Top End orogeny represent the earliest recorded fluid event. Fluid inclusion data and δ 18 O and δ D values indicate that these veins formed from basement-derived fluids that may have been heated by a cooling Nabarlek Granite during circulation through reverse faults/shear zones. The next fluid event, represented by fine-grained sericite and chlorite occurred when fluids passed into these faults and altered the metamorphic minerals following the exhumation of the basement and deposition of the Kombolgie Subgroup at ca. 1760 Ma. The intrusion of the Oenpelli Dolerite at ca. 1720 Ma resulted in the local remobilization of silica and the precipitation of quartz associated with minor pyrite and dolomite around the reverse faults. Uranium mineralization is associated with an inner and outer alteration halo that extends as far as 1 km from the Nabarlek fault. Alteration in the outer halo began as early as 1700 Ma and is dominated by chlorite and sericite, which formed when a 200°C fluid flowed into the Nabarlek fault from the overlying Kombolgie Subgroup. U-Pb and 207 Pb/ 206 Pb dating reveals that massive uraninite precipitated at ca. 1640 Ma and formed together with illite and hematite at ca. 200°C. Chlorite was not coeval with uraninite precipitation. Stable isotope values indicate that the pre- and synore alteration assemblage formed from basinal brines with δ 18 O fluid and δ D fluid values of 2 ± 2 and –25 ± 10 per mil, respectively. Reactivation of the Nabarlek fault at ca. 1360, 1100, and 900 Ma is indicated by U-Pb and 207 Pb/ 206 Pb dating of uraninite. These ages correlate with the intrusion of the Maningkorrirr phonolitic dikes and the Derim Derim Dolerite at ca. 1316 ± 40 and 1324 ± 4 Ma, respectively, the amalgamation of Australia and Laurentia during the Grenville orogen at ca. 1140 Ma, and the breakup of Rodinia between 1000 and 750 Ma. Fluid incursions associated with these events precipitated much of the chlorite that has previously been related to uraninite precipitation. Drusy quartz veins that host high-salinity fluid inclusions and sulfides, particularly galena, also formed after the initial uraninite-forming event. Finally, erosion of the Kombolgie Subgroup and subsequent weathering of the deposit resulted in the recent formation of kaolinite and numerous secondary uranium minerals. These data constrain individual events more precisely than previous studies and thus advance the current genetic model to a level that takes into account the multiple stages of fluid overprinting that occurred over a period of at least 800 m.y.

A case study of the application of a microcomputer-based favorability evaluation system: uranium exploration in the Alligator Rivers uranium field, Northern Territory, Australia : T. Kobayashi, T. Semba, Y. Yamada, S. Williams & Y. Kato, Mathematical Geology, 26(8), 1994, pp 937–960

A case study of the application of a microcomputer-based favorability evaluation system: uranium exploration in the Alligator Rivers uranium field, Northern Territory, Australia : T. Kobayashi, T. Semba, Y. Yamada, S. Williams & Y. Kato, Mathematical Geology, 26(8), 1994, pp 937–960

A case study of the application of a microcomputer-based favorability evaluation system: Uranium exploration in the Alligator Rivers Uranium Field, Northern Territory, Australia

We have attempted to develop an expert system to evaluate the favorability of uranium exploration projects using the “Data-directed Numerical Method (DdM).” We developed the system for the Macintosh personal computer, so as to facilitate the iterative processes of trial and error inherent in the DdM. In a detailed case study, we use the above system to evaluate the favorability of the Alligator Rivers Uranium Field, Northern Territory, Australia. The favorability of the target area is evaluated by considering several exploration parameters related to the geological factors controlling ore formation. The selection of appropriate exploration parameters is important to the success of the method. We then selected 15 model cells to validate each exploration parameter and also to design the exploration model by the application of the chi-square test. The result of the favorability evaluation, as displayed by the developed system, indicates that our designed exploration model is acceptable. By employing the above processes we conclude that the system using the DdM is applicable as an exploration tool and is useful in establishing an exploration model.

The calculated solubility of platinum and gold in oxygen-saturated fluids and the genesis of platinum-palladium and gold mineralization in the unconformity-related uranium deposits

Thermodynamic calculations on the solubility of platinum and gold indicate that saline (1 m NaCl), fluids saturated with atmospheric oxygen can transport geologically realistic concentrations of platinum-group-elements (PGE), gold, and uranium as chloro-complexes. A number of calculations involving fluid-rock interaction suggest that the oxygen-saturated fluids flowing through rocks containing quartz, muscovite, kaolinite, magnetite and hematite, initially oxidize any magnetite to hematite, allowing subsequent batches of ore fluids to retain their high oxidation state. During their migration through the aquifer, the oxidizing fluids would move the oxidation-reduction interface deeper into the aquifer, leaching and redepositing platinum and gold. The redissolution of earlier precipitated platinum and gold depends on the fluid/ rock ratio and the associated increase in the oxidation state. Therefore, lowering of fluid/rock ratios and/or mixing of the oxidized fluids with a large amount of reduced fluid will precipitate uranium, PGE, and gold. It is suggested that this model can explain the genesis of gold and PGE mineralization in the unconformity-related uranium deposits of the Alligator Rivers Uranium Field in the Northern Territory, Australia.

Nd-Sr isotope constraints on the age and origin of unconformity-type uranium deposits in the Alligator Rivers uranium field, Northern Territory, Australia

The results of an Sm-Nd and Rb-Sr isotope study on the large and rich unconformity-type uranium-(gold-platinum-group elements) deposits in northern Australia indicate identical Sm-Nd ages for primary uranium mineralization at Nabarlek (1,616 + or - 50 m.y.) and Jabiluka II (1,614 + or - 132 m.y.), in excellent agreement with new and previously published Rb-Sr data on U ores and altered host rocks. In contrast, the Sm-Nd ages are older than published U-Pb ages which range from 900 to 1,437 m.y. Sm-Nd data on uraninites from Koongarra are strongly scattered, but the geologic setting and Nd model ages for the least disturbed samples suggest an age between 1,600 to 1,650 m.y., identical to the age of primary mineralization at Nabarlek and Jabiluka II. The narrow range of Sm-Nd ages for these three deposits (1,600-1,650 m.y.) indicates that mineralization took place soon after the deposition of unconformably overlying sandstones and volcanics of the 1,650-m.y.-old Kombolgie Formation.Relatively high initial epsilon Nd values and low 87 Sr/ 86 Sr ratios in the ores indicate that ore fluids derived their metal content from two main sources, a preunconformity source comprising the local wall rocks (lower Proterozoic metasediments and amphibolites) and a postunconformity source including the volcano-sedimentary rocks of the Kombolgie Formation which form part of the evaporite-bearing McArthur Group. At Nabarlek, the Kombolgie Formation isotopic signature is obscured by the intensive wall-rock alteration involving isotopically similar amphibolites and dolerites, but mass balance calculations indicate major contributions of U and heavy rare earth elements from sources outside the alteration zone. The most likely external metal sources are again the postunconformity volcanics and sandstones. In contrast, isotopically more evolved sources such as U-rich late Archean basement gneisses and post-tectonic high-level granites are less probable metal sources on the basis of their much lower epsilon Nd and higher 87 Sr/ 86 Sr values, suggesting that the ore-forming hydrothermal activity did not penetrate the metamorphic basement except in the mineralized shear zones.The new data lend support to models of ore deposition that involve reaction of hot, oxidized, U-bearing formation-meteoric waters, whose source is the postunconformity rocks (e.g., the Kombolgie Formation), with reducing metasediments of the lower Proterozoic basement. Highly altered volcanics within the Kombolgie Formation are favored as major U sources.The origin of the mineralization at Ranger may have involved different processes. Recent U-Pb data indicate a 1,737-m.y. age for mineralization at this deposit, significantly earlier than that of the other deposits and, most importantly, earlier than that of deposition of the Kombolgie Formation. The Sm-Nd data for Ranger uraninites do not form an isochron but indicate a possible age range from 1,600 to [asymp] 1,750 m.y. Data resolution is insufficient to allow reliable distinction between a pre- or post-Kombolgie age. However, Nd-Sr initial ratios for Ranger point to unreasonably high epsilon Nd and high 87 Sr/ 86 Sr value sources when calculated for any age younger than 1,650 m.y., the age of Kombolgie sedimentation and basin formation. In contrast, uraninite Nd-Sr initial ratios at 1,737 m.y. are easily explained as mixtures involving Nd and Sr leached from various lower Proterozoic and late Archean sources. The data are thus more compatible with a pre-Kombolgie origin for uranium mineralization at Ranger.

Geology of the Nabarlek uranium deposit, Northern Territory, Australia

The Nabarlek deposit is an unusually high-grade uranium orebody (1.84% U 3 O 8 , 12,000 metric tons) hosted by amphibolite and semipelitic schist of lower Proterozoic age. It is unusual in the Alligator Rivers uranium field in being hosted by amphibolite. The disposition of the ore is controlled by a prominent reverse fault, which is part of a larger zone of reverse faulting. The fault zone is nearly perpendicular to prominent metamorphic S 2 and S 3 fabrics; a geometry which might enhance the fluid-rock ratio. A striking feature of the deposit is the extent and intensity of hydrothermal alteration which extends more than 1 km from the ore. Alteration remote from the ore resulted in pseudomorphous replacement of metamorphic biotite, amphibole, and feldspar, under conditions of a low fluid-rock ratio and mass transfer. Ore is associated with a small desilicification zone within, and superimposed on, this larger envelope. The fluid-rock ratio and mass transfer, resulting mainly in silica loss, were high within this interior zone and produced intense pervasive alteration, leading to an assemblage of phengitic white mica, magnesian chlorite, and hematite. Large volumes of fluid were required to effect the massive desilicification (and deposit such a volume of uranium), given the current knowledge of the solubility of quartz and uraninite.Ore deposition (predominantly as uraninite) postdated intense phengitic white mica alteration. Chlorite, however, is the main gangue mineral, possibly reflecting a change in redox conditions accompanying ore deposition. A redox control on ore deposition is supported by the sparse ocurrence of host-rock graphite which evidently participated in CH 4 -forming reactions. Abundant hematite in the alteration halo suggests the presence of an oxidized fluid (log f (sub O 2 ) approx. -35 at 200 degrees C) which was thus suitable for transporting ore-forming quantities of uranium, probably as chloride complexes. Primary mineralization underwent incongruent dissolution to coffinite, in equilibrium with a vanadian illite, kaolinite, and secondary hematite assemblage (residual ore). Weathering of primary and residual ore produced yellow and green secondary uranium minerals, both pseudomorphous after earlier ore types and as crusting of open goethite-lined fractures.

Age of uranium mineralization at the Jabiluka and Ranger deposits, Northern Territory, Australia; new U-Pb isotope evidence

The Ranger and Jabiluka uranium deposits are the largest in the Alligator Rivers uranium field, which contains at least 20 percent of the world's low-cost uranium reserves. Ore occurs in early Proterozoic metasediments, below an unconformity with sandstones of the 1.65-b.y.-old Kombolgie Formation. This study has used U-Pb isotope data from a large number of whole-rock drill core samples with a variety of mineral assemblages and textures. Ranger samples indicate a well-defined age of 1,737 + or - 20 m.y., consistent with earlier suggestions of the antiquity of this deposit. This age is distinctly pre-Kombolgie, so the Ranger deposit cannot have been formed by processes requiring its presence. The age is consistent, however, with mineralization related to heating associated with either the emplacement of early postmetamorphic granites, or possibly, with intrusion of the nearby Oenpelli Dolerite. In contrast, data for the least-altered Jabiluka ores yield a concordia intercept age of 1,437 + or - 40 m.y.--significantly younger than the Ranger age and also younger than the Kombolgie Formation. This age may correspond to a regional thermal event, as indicated both by mafic dikes of roughly this age and a zircon lower intercept age from a nearby granite-gneiss. Thus our data indicate that there must be at least three distinct periods of major U mineralization in the Alligator Rivers uranium field (including the previously determined approximately 900-m.y. age of the Nabarlek deposit), despite the fact that all of the deposits share strong similarities in their host rock, mineralogy, alteration, and geologic setting.Both Ranger and Jabiluka reflect a common, profound isotopic disturbance at about 400 to 600 m.y. This disturbance, which was especially pronounced at Jabiluka, may correspond to the development of basins and associated basalt flows to the west and southwest, as suggested by Crick et al. (1980).

Post-Kombolgie metasomatism at Jabiluka, Northern Territory, Australia, and its significance in the formation of high-grade uranium mineralization in lower Proterozoic rocks

Diagenetic lithification of Kombolgie sandstones followed by a metasomatic event which produced veining and replacement. Metasomatism was controlled by brecciation and bedding within the sandstones. The high-grade uranium ores are interpreted to be the product of a low-temperature (100 degrees -200 degrees C) hydrothermal system of ground water which was driven by a regional heating event also manifested by phonolite and/or diabase intrusions. The Kombolgie sandstones were an essential part of the hydrologic system which formed the high-grade orebodies, after lithification of the lower part of the thick clastic section but possibly contemporaneous with diagenetic events in the upper part. The processes operating in the Alligator Rivers uranium field were essentially similar to those in the Athabasca District of northern Saskatchewan, the primary difference probably being that no organic material was remobilized into the overlying sandstone to act as a reductant in the Australian case.--Modified journal abstract.