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

Ore Geology Reviews 20 (2002) 83 – 108 www.elsevier.com/locate/oregeorev

Geology of the Ranger Uranium Mine, Northern Territory, Australia: structural constraints on the timing of uranium emplacement K.A.A. Hein* Faculteit Aardwetenschappen, Universiteit Utrecht, Postbus 80021, 3508 TA Utrecht, The Netherlands Received 19 October 2001; accepted 7 March 2002

Abstract 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 F 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 ( f 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 F 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.

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Tel.: +31-30-253-5052; fax: +31-30-253-7725. E-mail address: [email protected] (K.A.A. Hein).

0169-1368/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 6 8 ( 0 2 ) 0 0 0 5 4 - 9

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[Precambrian Res. 40/41 (1988) 543] and others. Consequently, the absolute age of uranium mineralisation at the Ranger Mine is open. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Uranium; Ranger Mine; Palaeoproterozoic; Structure controls; Tectonics

1. Introduction The Ranger Uranium Mine is located within the Ranger Project Area, approximately 250 km east of Darwin in the Nimbuwah Domain of the Pine Creek Inlier (Fig. 1). The mine lies within the Alligator Rivers Uranium Field (ARUF) and close to the NE boundary between Kakadu National Park and the Arnhem Land Aboriginal Reserve. It comprises six uranium anomalies and two orebodies, namely, Ranger No 1 and Ranger No 3. The orebodies have a total reserve (total endowment) of 166,300 tonnes of U3O8 (Savory, 1994). Current operations include open cut mining of the No 3 orebody and rehabilitation of the No 1 pit. The Ranger No 1 and 3 orebodies have been classified as unconformity-related uranium deposits and, together with Nabarlek, Koongarra and Jabiluka, form a significant uranium resource (Wilde, 1988). These deposits have been the focus of considerable geological investigation over the preceding two decades, particularly with respect to orebody morphology and ore genesis (Hegge and Rowntree, 1978; Binns et al., 1980; Eupene, 1980; Ferguson et al., 1980b; Ypma and Fuzikawa, 1980; Ewers et al., 1983; Gustafson and Curtis, 1983; Ludwig et al., 1987; Wilde, 1988; Maas, 1989; Kendall, 1990; Hancock et al., 1990; Wilde and Noakes, 1990; Snelling, 1990; Solomon et al., 1994). These studies have demonstrated that the deposits commonly occur near, or within fault breccias, faults and/or shear zones in deformed and metamorphosed Palaeoproterozoic basement. They formed through the interaction of a weakly acidic, highly oxidised, saline fluid with a reduced, less saline fluid. Isotopic data suggest that the deposits have been reworked subsequent to deposition. The deposits of the Alligator Rivers Uranium Field are interpreted to have formed subsequent to erosion of the Palaeoproterozoic basement and deposition of the Palaeo- to Mesoproterozoic Kombolgie Formation

at about 1650 Ma, based on Sm – Nd ages for primary uranium mineralisation at Nabarlek and Jabiluka (Maas, 1989). However, the Ranger orebodies have been dated at 1737 F 20 Ma (Ludwig et al., 1987; U – Pb whole-rock age of 10 –15 cm long quarter splits of diamond drill core 3/83 from the Ranger No 3 pit), which suggests the deposits formed prior to deposition of the Kombolgie Formation. In August to October 1998, the Ranger No 1 and 3 pits became the focus of a detailed lithological and structural study by Energy Resources of Australia. In the course of that investigation, the geology of the pits was (re)mapped in order to establish the relationship between ore distribution, lithology and structure. The results provided structural constraints on the relative time of orebody formation, but raised questions about aspects of the currently accepted stratigraphy at the Ranger Mine. In this study, the lithologies and structure of the Ranger No 1 and 3 pits are described and correlated, and a stratigraphic and tectonic history for the Ranger Mine is established. These data are correlated with regional stratigraphic and structural data of Needham et al. (1980, 1988), Stuart-Smith et al. (1980), Needham and De Ross (1990), Johnston (1984) and StuartSmith et al. (1993) to arrive at a tectonic history and relative time of uranium ore formation.

2. Geological setting 2.1. Introduction The Ranger Uranium Mine is located in the northeast region of the Pine Creek Inlier (PCI) in the Nimbuwah Domain (Fig. 1). This region consists of Palaeoproterozoic geosynclinal and transitional sequences of the Kakadu Group, Cahill Formation (lower and upper), and Nourlangie Schist, which unconformably overlie Archaean base-

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Fig. 1. Location map of the Ranger Uranium Mine. The Arnhem Land Plateau lies to the east of the mine. The Ranger Project Area modified after Solomon et al. (1994).

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ment rocks of the Nanambu Complex ( f 2500 Ma) (Fig. 2). The geosynclinal and transitional sequences are intruded by granite (sensu lato) plutons and mafic dykes, and are crosscut by a suite of pegmatites veins. The sequences are obscured to the east of the mine, where they dip beneath Palaeo- to Mesoproterozoic sedimentary rocks of the Arnhem Land Plateau, and in the north and northeast where they dip beneath Mesozoic sedimentary rocks of the Bathurst Terrace. 2.2. Regional tectonics Between 1870 and 1780 Ma, the PCI underwent a major period of deformation known as the Top End Orogeny (Needham et al., 1988). The orogeny consisted of three compressional events; the Nimbuwah Event that has been correlated with the Barramundi Orogeny recognised throughout most of northern Australia by Etheridge et al. (1987), and the Maud Creek and Shoobridge Events, each of which has been interrupted by a phase of extensional tectonism. Johnston (1984) concluded that five deformation events were associated with compressional tectonism, viz., D1—westerly verging thrusts and isoclinal folds, D2—recumbent folding coincident with peak metamorphism, D3—upright, tight, N –NW trending folds, D4—westerly trending folding, and D5—kinking related to basement block movements. D1 – D2 are herein correlated with the Nimbuwah Event, while D3 – D4 are correlated with the Maud Creek Event. The principal phase of compression took place during the Nimbuwah Event (Stuart-Smith et al., 1993) at approximately 1870 Ma (Page et al., 1980, maximim age based on U – Pb zircon and Rb – Sr whole-rock data of granodiorite and tonalite of the Nimbuwah Complex that where deformed during this event). Compressional tectonism was accompanied by the development of early bedding-parallel thrusts, recumbent to upright folds (Stuart-Smith et al., 1993) and retrogressive metamorphism to lower to upper greenschist facies (Ferguson et al., 1980a). Compression was preceded by the intrusion of a suite of tholeiitic sills known as the Zamu Dolerite (Needham et al., 1988). The period of extensional tectonism following the Nimbuwah Event was accompanied by the develop-

ment of a NW –SE trending, shallow rift graben in the vicinity of the Alligator River Valley (Fig. 1) and the emplacement of granite (sensu lato) batholiths between 1870 and 1860 Ma (Needham et al., 1988; Stuart-Smith et al., 1993; age range based on U – Pb zircon and Rb – Sr whole-rock data). This graben became the depositional basin for clastic – volcanoclastic sediments and volcanics that were subsequently folded and faulted during the second phase of compressional tectonism, the Maud Creek Event. The Maud Creek Event, dated at about 1850 Ma (relative age, Needham et al., 1988), was associated with the development of N – NW trending upright folds which were refolded by broadly spaced, open, westerly trending folds (Stuart-Smith et al., 1993). Faulting, uplift and erosion accompanied folding. The Maud Creek Event was succeeded by the emplacement of granite (sensu lato) plutons at between 1835 and 1820 Ma (Needham et al., 1988; Stuart-Smith et al., 1993). These granites are predominantly I-type in character (Etheridge et al., 1987; Wyborn, 1988), although Ferguson et al. (1980a,b) recorded the presence of S-type granites in the Central Domain of the PCI. Many of these granites are enriched in U (Solomon et al., 1994). Following emplacement of granite plutons, graben subsidence was once again initiated through rifting and downwarping of the peneplaned landscape. The Shoobridge Event (at approximately 1770– 1780 Ma), the final deformational event of the Top End Orogeny, was accompanied by the development of new and/or reactivation of NW trending regional shears and faults (Needham et al., 1988) throughout the PCI. The age of this deformation is given by Rb – Sr total-rock ages of granite (sensu lato) plutons in the Central Domain, and 1800 F 24 Ma ages recorded in K – Ar and Rb –Sr chronology of metamorphic rocks in the Alligator Rivers region (Stuart-Smith et al., 1993). The emplacement of anorogenic intrusions, such as the Lewin Springs Syenite, and the dykes and lopoliths of the Oenpelli Dolerite (dated at approximately 1690 Ma by Page et al., 1980; concordant Rb – Sr total-rock and mineral age), predate a period of extensive erosion, landscape denudation and intense chemical weathering (Needham and De Ross, 1990).

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Fig. 2. Stratigraphy of the Ranger Mine, Nimbuwah and Central Domains of the Pine Creek Inlier.

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Fig. 3. Summary geology map of the Ranger No 1 pit.

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Fig. 4. Summary geology map of the Ranger No 3 pit.

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2.3. Platform sequences The Palaeo- to Mesoproterozoic platform sedimentary rocks of the Arnhem Land Plateau lie to the east of the Ranger Uranium Mine (Fig. 1), and these bound the eastern margin of the PCI. During rifting, the sediments were deposited on a broad shelf or alluvial plain adjacent to a northerly trending graben (Ojakangas, 1997; Plumb et al., 1990) that occupied a position close to the eastern coastline of the present-day Arnhem Land peninsula. The graben became the locus for sediments of the Katherine River Group, of which the most prominent unit is the 0.6 – 1.0-km-thick Kombolgie Formation. This Formation consists of coarse quartz – sandstone, basal quartz – pebble to boulder conglomerate, minor polymict boulder conglomerate, and minor tuff and basalt lava (Bagas et al., 1982). The age of Nungbalgarri Volcanic Member (stratigraphically in the middle of the Formation), and therefore the sequence, is given as 1648 F 29 Ma (Page et al., 1980; Rb – Sr age). 2.4. Local geology The Ranger Uranium Mine is hosted by the Cahill Formation (Hegge and Rowntree, 1978; Stuart-Smith et al., 1980, 1993; Needham et al., 1988; Browne, 1990; Kendall, 1990), which comprises quartz schist, mica schist, para-amphibolite, calc-silicate and carbonate. In the Ranger Mine area, the Cahill Formation has been traditionally subdivided into several sequences (Fig. 2). From oldest to youngest, they are the following. (1) The Footwall Sequence—the sequence is part of the Archaean basement of the Nanambu Complex and is composed of schist, gneiss, and granite (sensu lato). It is exposed in the Ranger No 1 pit. (2) The Lower Mine Sequence ( f 300 m thick)— the sequence consists of thick carbonate units that are interbedded with schist and chert. The carbonate units overlie the Footwall Shear Zone and are divided into a lower and upper unit: these are separated by the Lenticular Schist. The schist consists of quartz, sericite and lenticular nodules of chlorite. (3) The Upper Mine Sequence ( f 500 m thick)— the sequence comprises quartz – feldspar – biotite schist and microgneiss, with rare discrete beds of carbonate. The sequence contains numerous faults

and thrusts: thrust stacking is toward the west (Kendall, 1990). (4) The Hangingwall Sequence—the sequence comprises micaceous quartz –feldspar schists that are intercalated with amphibolitic units. In the Ranger Project area, the Kombolgie Formation unconformably overlies the Cahill Formation. The unconformity crops out on batters in the western wall of the Ranger No 3 pit (Kendall, 1990). Uranium mineralisation is hosted in parts of the Lower and the Upper Mine Sequence, the Lenticular Schist, pegmatite veins and basic dykes (Kendall, 1990). Primary uranium mineralisation occurs as vein and disseminated uraninite with accessory coffinite and brannerite (Savory, 1994). Saleeite, skodowskite and torbernite occur as secondary minerals in the oxidized zone (Hegge and Rowntree, 1978). Alteration assemblages are dominated by Mg-chlorite, sericite and quartz, with subordinate pyrite and dolomite (Savory, 1994). The ore-bodies exhibit a strong structural control (Hegge and Rowntree, 1978; Johnston, 1984; Wilde, 1988; Solomon et al., 1994). In both the No 1 and 3 pits, they strike NNW – NNE with a moderate dip to the east (Figs. 3 and 4). In the No 1 pit, high-grade ore shoots (average 1% U308) form parallel vein sets, and these are associated with intense brecciation and chloritization of the wall rock (Kendall, 1990). A low-grade zone averaging 0.15% U3O8 surrounds the high-grade zone. In the No 3 pit high-grade ore defines a narrow NNW trending zone of intense brecciation of metasedimentary rocks (Fig. 4).

3. Research methods The Ranger No 1 and 3 pits were mapped at 1:1000 scale, where access to benches was possible, and the geology between the two pits was correlated. All station points were photographed and surveyed, as were significant lithological contacts. The combined data were compared with historic data and are summarised in Figs. 3 and 4. Structural data were analysed using GEORIENT 8.0 (Holcombe, 2000). A proportion of the geology of the No 1 pit has been reclassified in order to avoid confusion between stratigraphic, metamorphic and structural terminology.

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Fig. 5. (a) Granitic gneiss sequence on the western batters of the No 1 pit. (b) Granitic gneiss sequence in contact with the metavolcanic sequence. The contact is defined here by a D2 reverse shear zone. The contact is also normal faulted in D4.

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Fig. 6. Metavolcanic sequence. (a) The basal section of the metavolcanic sequence. It comprises 1 – 2 m interval of black volcanoclastic siltstone. Some beds are graded and exhibit basal scours that indicate younging is toward the east. (b) Resedimented hyaloclastite in the Ranger No 1 pit.

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Fig. 7. Metavolcanic sequence. (a) Hyaloclastite breccia. Breccia clasts in hyaloclastite units range from 1.0 mm to 10 cm in diameter. (b) Massive basalt that is jointed and crosscut by a small-scale reverse D2 shear. (c) Close up view of the small-scale reverse shear in (b).

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4. Lithologies 4.1. Introduction In this study, the lithologies of the No 1 and 3 pits have been divided into four broad sequences: (1) (2) (3) (4)

Granitic gneiss sequence. Metavolcanic sequence. Metacarbonate sequence. Clastic metasedimentary sequence.

These sequences have been folded, faulted and sheared, and are crosscut by east-trending granite (sensu stricto) dykes, ENE – NE trending pegmatite veins, and gently dipping N – NE trending mafic dykes (Figs. 3 and 4). Regional metamorphism is to greenschist facies and contact metamorphism is to hornblende-hornfels facies. Based on fault-shear geometries in the No 1 pit, sheared rocks that have been termed ‘‘lenticular schist (isch)’’ and ‘‘other schists (imsch)’’ by Hegge and Rowntree (1978) and Kendall (1990) and on historic mine geology maps, and which traditionally have been considered discrete stratigraphic units, are herein reclassified as shears. Together with the ‘‘Footwall Shear Zone (sz)’’ and ‘‘Upper Thrust Zone’’ of Savory (1994), they form an important fault-shear array and are therefore discussed elsewhere. 4.2. Granitic gneiss sequence The granitic gneiss sequence forms a basement to the rocks in the region and crops out on the western batters of the Ranger No 1 pit (Figs. 3 and 5). It consists of a medium grained assemblage of quartz, muscovite, and potash feldspar with accessory chlorite and rutile. The sequence ranges in morphology from schist to gneiss. It is crosscut by pegmatite veins and granitic (sensu stricto) dykes, and is chloritised in places. A crosscutting NNE-trending parallel foliation gives the sequence a blocky to slabby appearance.

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The granitic gneiss sequence forms part of the Archaean Nanambu Complex, which is dated at 2470 F 50 Ma based on U –Pb zircon and Rb –Sr whole-rock data by Page et al. (1980). The sequence is equivalent to the Footwall Sequence of Kendall (1990) and is unconformably overlain by a metavolcanic sequence. The unconformity trends NNW – NNE and is sheared and faulted. The sequence does not crop out in the Ranger No 3 pit. 4.3. Metavolcanic sequence Based on detailed mesoscopic studies of outcrop in the No 1 and 3 pits, an approximately 250 m thick metavolcanic sequence is recognised at the Ranger Mine. The sequence includes those units previously referred to as ‘‘massive chlorite rock (isch and imsch)’’, ‘‘intrusive chlorite rock (Mcr)’’ and ‘‘basic dykes (bd)’’ on historic mine geology maps of the Ranger No 1 pit. It is equivalent to the ‘‘Hanging Wall Schist’’, ‘‘Massive Chlorite Rock’’ and ‘‘Lower Mine Sequence’’ of Kendall (1990), and includes ‘‘chert’’, ‘‘jasperoidal chert’’ and ‘‘various breccias’’ of the Lower Mine Sequence. It also includes the ‘‘Lower Mine Sequence Chert’’ of Hegge and Rowntree (1978). As such, the metavolcanic sequence is well exposed on the lower western and central batters of the No 1 pit, but also crops out the upper western batters of the No 3 pit. The basal section of the metavolcanic sequence consists of a 1– 2-m interval of black volcanoclastic siltstone which is overlain by several gradational intervals of (1) chloritised, aphyric, massive basalt – dolerite, (2) rare autobreccia, and (3) coarse in-situ hyaloclastite breccia to resedimented hyaloclastite (Figs. 6 and 7). These units are intercalated with discontinuous beds of black interflow siltstone-shale, but also fine-grained black, grey or jasperoidal chert. In the No 1 pit, fine scale bed gradations and rare basal scouring within volcanoclastic siltstone indicate that the sequence is east facing (Fig. 6). Breccia clasts in hyaloclastite units range from 1.0 mm to 10 cm in diameter. In-situ hyaloclastite is rarely

Fig. 8. Metasedimentary sequence. (a) The clastic metasedimentary sequence consists of a basal quartzite unit that is overlain by thick beds of siltstone – sandstone F gritstone, thick to thin beds of siltstone – shale F sandstone and, finally, discrete beds of shale – siltstone. There is an overall fining upwards in the metasedimentary sequence. (b) F2 in the metasedimentary sequence of the Ranger No 1 pit. Fold vergence is toward the west. F2 is non-cylindrical, asymmetric and open to tight in profile.

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slabby and associated with tortoise shell jointing, while resedimented hyaloclastite is granular and matrix rich. Massive basalt – dolerite is block- to slabjointed. Chalcedonic quartz in this unit occurs as a lining or cavity-fill to vughs. With respect to an environment of deposition, the presence of in-situ and resedimented hyaloclastic breccia is indicative of a subaqueous setting for emplacement of the sequence (McPhie et al., 1993). The metavolcanic sequence is not magnetic. It is generally chloritised except within the weathering profile or adjacent to crosscutting faults and/or shears where it is strongly kaolinised. Hyaloclastite breccias are carbonated and/or ferruginized with a characteristic brick-red oxide staining. The metavolcanic sequence is unconformably or disconformably overlain by a clastic metasedimentary sequence and in part, by a metacarbonate sequence. It is crosscut by NE – ENE trending pegmatite veins in the No 1 pit and northerly trending mafic dykes in the No 3 pit. 4.4. Metacarbonate sequence A metacarbonate sequence has been recorded by Hegge and Rowntree (1978), Needham (1982), Gustafson and Curtis (1983), Wilde (1988) and Kendall (1990) in the vicinity of Ranger Mine, Jabiluka deposit and Koongarra deposit. Needham (1982) described the sequence as a 250-m-thick, massive crystalline carbonate that ranged in composition from calcitic dolomite to magnesite. He assigned the sequence to the base of the Cahill Formation. Gustafson and Curtis (1983) reported coarse granoblastic to porphyroblastic marble in drill intersections south of Jabiluka in which bedding was rare. They noted local brecciation and silicification of the sequence. Kendall (1990) described a carbonate sequence in the Ranger No 1 pit and termed it the Lower Mine Sequence. He split this sequence into three discrete units: lower carbonate unit, lenticle schist and upper carbonate unit, and placed them stratigraphically above a shear zone located at the contact between

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the granitic gneiss sequence and the overlying stratigraphy. In this study, a discrete carbonate unit was not evident in outcrop in the No 3 pit, and access to the eastern batters of the No 1 pit, where the unit was exposed, was restricted. Consequently, it has not been possible to confirm the gross lithological character of the carbonate sequence or to determine the nature of contacts. Regardless, summary cross-sections and fliche maps of the No 1 pit (unpublished ERA data) suggest that a laterally discontinuous metacarbonate sequence lies stratigraphically between massive chlorite rock (assigned to the metavolcanic sequence) and a metasedimentary sequence. Those data suggest that the metacarbonate sequence is approximately 75 m in thickness and may be shear bounded (Fig. 3). 4.5. Clastic metasedimentary sequence A clastic metasedimentary sequence dominates outcrop on northern, eastern and southern batters of the No 3 pit, while in the No 1 pit, it dominates outcrop on the upper eastern and southern batters (Figs. 3, 4, 7 – 9). The sequence is equivalent to the Upper Mine Sequence (Schist) and Hangingwall Sequence (Schist) of Hegge and Rowntree (1978) and Kendall (1990). The clastic metasedimentary sequence consists of a basal quartzite unit that is overlain by thick beds of siltstone –sandstone F gritstone, thick to thin beds of siltstone –shale F sandstone and, finally, discrete beds of shale – siltstone that historically have been termed the Hangingwall Sequence. There is an overall fining upwards in the metasedimentary sequence. The units are interbedded with volcanoclastic beds and carbonaceous shale or siltstone, particularly near the base. The package is weakly turbiditic. Fine scale bed grading, parallel laminations, and rare slumps are developed in some finer grained units and indicate that stratigraphic facing is toward the east in both open pits. The basal quartzite in the sequence is massive but exhibits weak planar or cross-beds. It is commonly faulted at the base where it contacts the metavolcanic sequence.

Fig. 9. (a) Granite (sensu stricto) dyke and pegmatite exposed on the western batters of the No 1 pit in crosscutting relationship to the granitic gneiss sequence. (b) Pegmatite vein from the southeastern batters of the No 1 pit with a narrow contact aureole that is expressed as a ferruginized rind about the veins. (c) Close up of a pegmatite vein. The veins are coarse grained and comprise the assemblage quartz – muscovite – plagioclase F chlorite F lepidolite F tourmaline F apatite.

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4.6. Granite dykes Granite dykes/dykelets are exposed on the western and southeastern batters of the No 1 pit in crosscutting relationship to the metavolcanic and metasedimentary sequences, respectively (Figs. 3 and 9). The dykes are coarse grained and comprise the assemblage potash feldspar, quartz and plagioclase, with accessory muscovite, chlorite and tourmaline. The dykes are near vertical and trend easterly. They are associated with a narrow contact aureole: the inner aureole is about 30 cm wide (hornblende-hornfels facies), while the middle aureole is 1 –2 m wide (epidote –albite facies). They do not crop out in the No 3 pit. 4.7. Pegmatite veins Conjugate pegmatite veins are exposed on batters throughout the No 1 and 3 pits in crosscutting relationship to the granitic gneiss, metavolcanic and metasedimentary sequences (Figs. 3, 4 and 9) where they are crosscut by reverse shears and normal faults. Type 1 pegmatite veins strike northeast and dip moderately southeast, while Type 2 pegmatite veins strike ENE and dip moderately northwest. They range from centimetres to metres in width and are typically sheeted in character. The pegmatites are generally coarse grained and comprise the assemblage quartz, muscovite and plagioclase, with accessory chlorite, lepidolite, tourmaline and apatite. They are associated with a narrow contact aureole that is expressed as a kaolinised and/or ferruginized rind about the veins. 4.8. Mafic dykes Mafic dykes are exposed on the southeastern batters of the No 1 pit and western batters of the No 3 pit in crosscutting relationship to folded and sheared metasedimentary and metavolcanic sequences (Figs. 3 and 4). They comprise porphyritic olivine dolerite, are massive and generally chloritised, but are not magnetic. They have been interpreted as part of the Oenpelli Dolerite (dated at approximately 1690 Ma by Page et al., 1980; concordant Rb –Sr total-rock and mineral age), given that (1) they crosscut deformed metasedimentary and metavolcanic sequences in the No 1 pit and therefore cannot be interpreted as part of the preorogenic Zamu Dolerite (Needham et al.,

1988), (2) they constitute porphyritic olivine dolerite and thus falls within the range of mafic rock types in the Nimbuwah Domain that characterise the Oenpelli Dolerite, as defined by Stuart-Smith and Ferguson (1978), and (3) they lie within the type locality (province) for the Oenpelli Dolerite as established by Stuart-Smith and Ferguson (1978).

5. Structure 5.1. Introduction 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 metamorphism (D1) to greenschist facies. D2 resulted in the development of NNE – NNW trending mesoscopic folds (F2) and a network of thrusts and dextral reverse shears. D3 is associated with the development of WNW – NW trending mesoscopic folds (F3), a weakly defined axial planar cleavage (S3), and sinistral reactivation of D2 shears. In contrast, D4 involved block displacement on normal faults during a period of east –west extension. 5.2. D1 At the Ranger Mine, D1 is characterised by a discrete parallel cleavage (S1) that is well developed in the granitic gneiss and metasedimentary sequences, but poorly developed in the metavolcanic sequence. The fabric is folded about NNE-trending mesoscopic D2 fold axes, refolded about NW-trending F3 fold axes, and drag rotated adjacent to D2 –D3 shears and D4 faults. S1 is typically bedding-parallel in the metasedimentary sequence, where it is associated with boudinage of competent lithologies in places. In both the granitic gneiss and metasedimentary sequences, S1 is defined not only by the alignment of platey minerals, including muscovite, chlorite and/or sericite, but also by the elongation of quartz and feldspar clasts. The character of S1 in the metacarbonate sequence is not known. S1 is not associated with fold or shear development in D1 as would be expected in contractional tectonics. This is interpreted to mean that S1 formed during

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deposition and accumulation of a thick stratigraphic pile, probably during a period of crustal extension. Under these conditions, clastic sediments and carbonates exhibit pore-volume reduction, while sediments composed of platey minerals develop a strong planar bedding-parallel fabric (Jones, 1994). Progressive sediment loading may be accompanied by the development of mineral assemblages and deformation textures indicative of low- to medium-grade metamorphism and for certain geometries of crustal extension, amphibolite to granulite facies metamorphism (Sandiford and Powell, 1986; Robinson, 1987; Robinson and Bevins, 1989; Choquette and James, 1990; Maltman, 1994). The inference here is that the deposition and accumulation of a thick stratigraphic pile may have led to the development of a strong beddingparallel cleavage at the Ranger Mine (and regionally) concomitant to medium grade metamorphism, but in the absence of contractional tectonics: i.e., during extension related metamorphism or regional diastathermal metamorphism. 5.3. D2 In the Ranger Mine, D2 is associated with the formation of NNE trending folds, NE –NNW trending shears with C-S2 fabric, and an array of quartzcarbonate veins that both crosscut bedding and are bedding-parallel. Folds formed during D2 are non-cylindrical, asymmetric and open to tight in profile (Fig. 8). They are well developed in the metasedimentary sequence, are only resolved as a gentle warping in the metavolcanic sequence, and are absent in the granitic gneiss sequence. The folds trend NNE – NNW and have an average half wavelength and amplitude of 4 and 3 m, respectively. They are discontinuous along the fold axis trend and generally do not exceed 10 m in trend length. The western limb of F2 is slightly overturned (unless rotated adjacent to faults or shears), the asymmetry being suggestive of a westerly direction to tectonic transport during D2. The mean strike and dip of the F2 axes and the plunge of F2 has been calculated using the distribution of poles to S0 – S1. In the No 1 pit, the mean strike and dip of F2 axes is 030j/39jE, with a mean fold plunge (beta axis) of 7j toward 039j, while, in the No 3 pit, the mean strike and dip of F2 axes is

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356j/34jE, with a mean fold plunge of 17j toward 023j (Fig. 10A,B). The spread of poles around the primitive circle is the result of refolding about NWtrending F3 fold axes and drag rotation adjacent to shears and faults. Based on the arrangement of quartz-carbonate veins, folding in D2 appears to have been flexural slip in style. Veins that crosscut the limbs of F2, terminate against bedding-parallel quartz-carbonate veins and are rotated towards the fold hinge: i.e., they are S-shaped on the left limb of the fold and Zshaped on the right limb of the fold. In addition, bedding-parallel quartz-carbonate veins are folded about F2 axes and boudinaged in the plane of bedding. The arrangement suggests that the veins were formed and deformed progressively during fold formation in D2. D2 is associated with the development of a series of NNE –NNW trending, easterly-dipping thrusts and NE-trending dextral reverse shears. These include the Footwall Shear Zone, Lenticular Shear and Upper Thrust Zone of Kendall (1990) and Savory (1994). Displacement is reverse (typically dextral) as indicated on slickenfibres and accretion steps: i.e., SE blocks have been stacked over NW blocks. There is some stratigraphic repetition across the shears, particularly in the metasedimentary sequence. The trace of the D2 thrusts is slightly curved from NNW in the No 3 pit to north in the No 1 pit (Figs. 3 and 4). The arrangement of thrusts and shears in the No 1 pit describes a dextral reverse shear-array. The shears are associated with a narrow C-S2-type cleavage and/or zones of brecciation (B2) in crosscutting relationship to S1. In the metavolcanic sequence, CS2 is resolved as narrow zones of fracture cleavage (Fig. 7). In the metasedimentary sequence, C-surfaces develop a graphitic or mirror-like sheen in places (locally termed ‘‘Cleopatra’s mirror’’). The general NNE – NNW trend of F2, in conjunction with a NE –NNW thrust-array, suggests that the maximum compressive stress (r1) operating during D2 was approximately orientated east –west. Since the asymmetry of F2 defines a westerly vergence, a westerly direction to tectonic transport during D2 is concluded. It is probable that folding, the development of thrusts and shears, block stacking and stratigraphic repetition were conjoint with east – west shortening. The development of relationship between

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(1) quartz-carbonate veins and folds and (2) folds, thrusts and dextral reverse shears, suggests that the rocks of the Ranger Mine were folded prior to failure.

It is therefore concluded that D2 was progressive, with crustal shortening being initially achieved by flexural slip-type folding, and later, by failure on a series of NNE –NNW trending thrusts and NE trending shears. Failure was concomitant with imbrication and stacking, and closure of fold profiles. 5.4. D3 D3 macroscopic and mesoscopic (parasitic) folds are restricted to the metasedimentary sequence of both open pits. Macroscopic folds crop out on the eastern and central batters of the No 3 pit where they trend ENE and plunge gently eastward. They are asymmetric, rounded and open in profile, with an estimated half wavelength of 100 – 300 m and unknown amplitude. Mesoscopic F3 crop out throughout the No 1 and 3 pit where they trend ENE – ESE and plunge moderately eastward or westward. They are asymmetric, non-cylindrical, and open to tight in profile, with an estimated half wavelength and amplitude of 5 –10 and 3 cm, respectively. They are associated with a weakly defined axial planar cleavage (S3) that dips steeply south or north. A southerly direction of tectonic transport is inferred from the fold asymmetry and the overall trend of S3. 5.5. D4 A period of brittle deformation, D4, is recognised from normal slip on fault planes that parallel or subparallel D2 shears. The faults form three sets: Set 1— NE trending and moderately SE dipping, Set 2— WNW – NW trending and moderately NE dipping, and Set 3—northerly trending and moderately east dipping. Normal offset is indicated by (1) down-step slickensides and slickenfibres, (2) offsets on bedding, pegmatites, and mafic intrusions, and (3) inferred

Fig. 10. Equal area stereographic projections. (A) The mean strike and dip of F2 is 030j/38jE, with a mean fold plunge of 7j toward 039j in the Ranger No 1 pit. (B) The mean strike and dip of F2 axes is 352j/30jE, with a mean fold plunge of 22j toward 036j in the Ranger No 3 pit. (C) The mean orientation of down-step slickensides on D4 faults is 39j toward 100j.

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from horizontal rotation of bedding. The mean downthrow block is situated ESE (relative to an up-throw block situated WNW). The mean orientation of downstep slickensides is 39j toward 100j (Fig. 10C): they smear up-direction slickensides on D2 – D3 reverse shears in some places. The faults are narrow zones of disruption (centimetre scale) or brecciation (B4) that are bounded by discrete fault margins. The faults are chloritised and decorated with a lithified mill breccia that is composed of centimetre- to millimetre-sized fragments of the wall rock in a chloritised rock flour. The faults are principally located at sites of competency contrast and, in particular, where the metavolcanic sequence contacts other lithologies including:

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dextral, while for type 2 pegmatites the offset is sinistral. The principal palaeo-stress directions (r1, r2 and r3) that operated at the time of formation of the pegmatite veins can be deduced from their geometry. The orientation of r2 is parallel to the intersection line of the vein types (Hobbs et al., 1976) and perpendicular to the r1r3 plane. r1 is located parallel to the bisector of the acute angle between the vein types. From this, the palaeo-stress orientations have been estimated: r1 plunged moderately toward the WSW; r2 plunged moderately ENE; r3 plunged gently NNW. The r1 direction is similar to that estimated for D2. They are interpreted to have formed early in D2.

6. Discussion (1) The unconformity at the contact of the granitic gneiss sequence and the metavolcanic sequence, (2) The contact of pegmatite veins and the metavolcanic sequence, and (3) The contact of the metavolcanic and metasedimentary sequence. Within the metasedimentary sequence, D4 normal faults reactivate D2 reverse shears. In the No 1 pit, a mafic dyke that has been interpreted by Kendall (1990) as part of the Oenpelli Dolerite (approximately 1690 Ma) is crosscut by these faults. The mean orientation of down-step fibres and the mean current position of the down-throw block suggests that D4 was associated with east –west extension.

5.6. Pegmatite veins Conjugate pegmatite veins are exposed on batters throughout the No 1 and 3 pits in crosscutting relationship to the granitic gneiss, metavolcanic and metasedimentary sequences. They are crosscut by D2 reverse shears and D4 normal faults. Type 1 pegmatite veins strike NE and dip moderately SE, while type 2 pegmatite veins strike ENE and dips moderately NW. The acute angle between the pegmatite vein types is 58j and their intersection plunges moderately to the NE. They exhibit offset of their host rocks in the plane of the vein. For type 1 pegmatites, this offset is

6.1. Introduction The geological interpretation described in this study is markedly different from that of previous studies of the Ranger Mine. Important differences include the following. (1) The ‘‘Lenticular Schist (isch)’’ and ‘‘other schist (imsch)’’ interpreted in historic mine data and by Hegge and Rowntree (1978) and Kendall (1990), as discrete stratigraphic units are fault rocks produced by shearing. (2) The contact between the granitic gneiss sequence and overlying sequences is an unconformity that trends NNE – NNW. In part, the unconformity is sheared and faulted. (3) A metavolcanic sequence is recognized in the place of the lower carbonate unit of the Lower Mine Sequence of Hegge and Rowntree (1978) and Kendall (1990), and others. The metavolcanic sequence has a distinct volcanic facies and comprises basalt, hyaloclastite and autobreccia. It is generally chloritised, kaolinised adjacent to faults and shears, and locally carbonated and ferruginized. (4) The breccia types described by Gustafson and Curtis (1983), Binns et al. (1980), Maas (1989), and others can be divided into two groups: volcanic breccias and tectonic breccias. Volcanic breccias include hyaloclastite and autobreccia, and are confined to the metavolcanic sequence. Tectonic breccias, B2 and B4, are associated with failure of hangingwall – footwall

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rocks adjacent to shears and thrusts in D2, and normal faults in D4. (5) A previously unreported set of granite (senso stricto) dykes crosscuts the sequences of the Ranger Mine in the No 1 open pit. (6) Pegmatite veins form a conjugate set. Consequently, the division of pegmatites by Kendall (1990) into four categories based on colour and quartz content is invalid. The modelled palaeo-stress directions that operated at the time of formation of the pegmatite veins suggest that they formed during a period of east –west compression similar to that in D2. (7) The Kombolgie Formation does not crop out west of the No 3 orebody as indicated by Hegge and Rowntree (1978) and Kendall (1990), and others, and does not crop out on batters on the western wall of the No 3 pit. It has been suggested that a quartzite unit in this position may represent a down-faulted block of Kombolgie Formation. However, the quartzite unit is not fault-bounded and/or down-faulted. Rather, it disconformably overlies hyaloclastite and volcanoclastic metasedimentary rocks of the metavolcanic sequence, and is overlain by chloritised siltstone and shale of the metasedimentary sequence. The unit is boudinaged in the plane of S1 and crosscut by D2 shears. The contact between the quartzite and metasedimentary rocks is folded by F3. Consequently, the unit cannot represent a down faulted block of the Kombolgie Formation. It is herein correlated with the basal quartzite of the metasedimentary sequence. The geology described in this study has also provided data regarding the structural evolution of the mine. The most important findings are the following. (1) The rocks of the Ranger Mine have undergone at least three phases of regional deformation, D2 – D4, and these were preceded by diastathermal metamorphism to greenschist facies in D1. (2) D1 is characterised by a discrete parallel cleavage (S1) that is well developed in the granitic gneiss and metasedimentary sequences, but poorly developed in the metavolcanic sequence. The fabric is interpreted to have formed during deposition of a thick stratigraphic pile concomitant to crustal extension. (3) D2 is associated with the formation of NNE – NNW trending folds, NE – NNW trending shears and thrusts, a C-S2 fabric, and an array of quartz-carbonate veins that are both crosscutting to bedding and bedding-parallel. The arrangement of quartz-carbonate

veins suggests that folding in D2 may have been flexural slip in style. (4) East – west trending macroscopic and mesoscopic (parasitic) folds formed during D3 and a period of NE – SW shortening. (5) A period of east – west extension, D4, is recognised from normal slip on fault planes that parallel or sub-parallel D2 shears. The faults are narrow zones of disruption or brecciation (B4) that are bound by discrete fault margins. (6) Mafic dykes are crosscut by D4 faults. These combined data have facilitated the reinterpretation of the stratigraphy and structures of the Ranger Mine, and enabled the development of an integrated tectonic model for ore genesis. This model accords with the regional tectonic model for the evolution of Pine Creek Inlier of Needham et al. (1988). 6.2. Correlation of regional and local stratigraphy In this study, the lithologies of the Ranger Mine have been divided into four broad sequences that correlate with units of the Namoona and Mount Partridge Groups of the Central Domain of the Pine Creek Inlier (Fig. 11). As described by Needham et al. (1988), the Namoona Group comprises a sequence of carbonaceous pelites with interbeds of calcareous sandstones and transitional evaporitic facies known as the Masoon Formation. These interfinger in places with volcanic breccias and lavas of tholeiitic and continental tholeiitic affinity (Stuart-Smith et al., 1980; Needham et al., 1988) in the upper part of the Namoona Group. The volcanic breccias and lavas have been termed the Stag Creek Volcanics. Unconformably overlying the Namoona Group is the Mount Partridge Group which consists of two conformable formations: (1) the Mundogie Sandstone which comprises a sequence of conglomerate, arkose, sandstone and siltstones, and (2) the Wildman Siltstone which is composed of siltstone, black carbonaceous laminated shale and minor sandstones. The Wildman Siltstone has been interpreted as the distal and vertical subtidal gradation of the fluviatile fans of the Mundogie Sandstone (Stuart-Smith et al., 1980). At the Ranger Mine, a metavolcanic sequence is overlain by a metacarbonate sequence, which, in turn is unconformably overlain by a metasedimentary se-

K.A.A. Hein / Ore Geology Reviews 20 (2002) 83–108 Fig. 11. Correlation between regional geology of the Central Domain and local geology from the Ranger Mine. The sequences of the Ranger Mine can been correlated with units of the Namoona and Mount Partridge Groups of the Central Domain of the Pine Creek Inlier. 103

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quence. The metasedimentary sequence exhibits an overall fining upwards in progression. The metavolcanic and metacarbonate sequences may be correlated with the Stag Creek Volcanics and intercalated carbonate beds of the upper Namoona Group, as shown in Fig. 11. The basal quartzite, siltstone – sandstone F gritstone beds, siltstone– shale F sandstone beds and finally shale – siltstone beds of the metasedimentary sequence may be correlated with the Mundogie Sandstone and Wildman Siltstone of the Mt Partridge Group. The unconformity between the metavolcanic – metacarbonate sequences and the metasedimentary sequence may be correlated with the unconformity between the Namoona Group and Mt Partridge Group, which marks a periods of provenance rejuvenation and gentle warping of the older Namoona Group sedimentary rocks (Stuart-Smith, 1987; Needham et al., 1988). 6.3. Correlation of regional and local tectonic events Based on data by Johnston (1984), Needham et al. (1988), Page and Williams (1988), Plumb et al. (1990) and Stuart-Smith et al. (1993), D1 –D4 at the Ranger Mine have been correlated with regional tectonic events of the Top End Orogeny of the PCI (1870 – 1780 Ma), as shown in Table 1. D1 is correlated with extension and the development of a thick sedimentary pile during subsidence of regional-scale NNW-trending half grabens (Needham et al., 1988). Diastathermal metamorphism of the pile has not previously been recognised. D2 – D3 at the Ranger Mine is correlated with regional D3 – D4 (Maud Creek Event at 1850 Ma) of the central Domain of the PCI. With respect to fold style, F2 at the Ranger Mine is similar to the regional F3, which were formed during a period of east –west shortening (Johnston, 1984). Similarly, F3 at the Ranger Mine is similar to the regional F4, which was formed during a period of north – south shortening. The emplacement of granite (sensu stricto) dykes and pegmatite veins at the Ranger Mine is correlated with the emplacement of regional (sensu lato) granites at 1870 – 1860 Ma (Stuart-Smith et al., 1993). The subsequent emplacement of mafic dykes can be correlated with the emplacement of the regional dyke swarms of the Oenpelli Dolerite at approximately 1690 Ma.

Extensional tectonics and normal faulting in D4 at the Ranger Mine can be correlated with east – west extension during deposition of the Palaeo- to Mesoproterozoic platform sequences. 6.4. Timing of uranium mineralisation In the No 1 pit, NNW – NNE trending ore shoots are hosted by metavolcanic – metacarbonate and metavolcanic – metasedimentary sequences, or lie at the contact of those sequences. In No 3 pit, NNW-trending ore shoots are hosted in the metasedimentary sequence. Uranium ore is also hosted by pegmatoids and mafic dykes (Browne, 1990; Kendall, 1990). These ore positions are commonly coincident with zones of tectonic breccia and intense chloritization of the wallrock, but importantly with either D2 shears or D4 faults. These data provide structural and tectonic constraints on the age of ore formation. The age of formation of thrust-related tectonic breccias (B2) in D2 is constrained to the Top End Orogeny of the Pine Creek Inlier at 1870 –1780 Ma (Needham et al., 1988) based on a reasonable correlation of D2 and the Maud Creek Event (Fig. 11, Table 1). The currently accepted U – Pb whole-rock age for ore formation at the Ranger Mine is 1737 F 20 Ma based on U – Pb whole-rock analysis of selected samples from diamond drill core (DDH 3/83) from the No 3 pit (Ludwig et al., 1987). The age places the mineralising event after the Top End Orogeny and after the Maud Creek Event (1850 Ma). Hence, the association of ore shoots and D2 shears is not significant with respect to timing. The alternative is that ore formation was associated with the development of normal faults and faultrelated tectonic breccias (B4) in D4. The age of formation of fault-related breccias (B4) in D4 may be constrained, in part, by the age of a mafic dyke that is crosscut by D4 normal faults in the No 1 pit. The dyke has been interpreted as part of the Oenpelli Dolerite that was emplaced at a shallow depth (1– 2 km) at 1690 Ma (Page et al., 1980) after a long period of post-orogenic erosion (Needham et al., 1988). This interpretation is reasonable given that (1) the mafic dyke crosscuts deformed metasedimentary and metavolcanic sequences in the No 1 pit and therefore, in relative chronology, cannot form part of the preorogenic Zamu Dolerite (Needham et al., 1988), (2) the

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Table 1 Comparison of regional and local tectonic events

mafic dyke comprises porphyritic olivine dolerite and thus falls within the range of mafic rock types in the Nimbuwah Domain that characterise the Oenpelli Dolerite, as defined by Stuart-Smith and Ferguson (1978), and (3) the mafic dyke lies within the type locality (province) for the Oenpelli Dolerite as established by Stuart-Smith and Ferguson (1978). It follows that, since the faults crosscuts the dyke, the faults must have formed after emplacement of the dyke at approximately 1690 Ma and, if the mineralisation is hosted in D4 faults and breccias, the ore must also have formed after emplacement of the dyke at approximately 1690 Ma. This conclusion would be in disagreement with the currently accepted 1737 F 20 Ma U – Pb age for ore formation at the Ranger Mine. Therein lies a problem. The 1737 F 20 Ma U –Pb whole-rock age of Ludwig et al. (1987) places the

mineralising event after the Top End Orogeny at a time of regional post-orogenic erosion and denudation (which does not seem plausible). The U –Pb age is not consistent with Sm – Nd ages for primary uranium mineralisation at Narbalek and Jabiluka at 1650 Ma of Maas (1989), and does not concur with currently accepted regional tectonic data of Johnston (1984), Needham et al. (1988), and others. It is therefore suggested that the U –Pb age is not the absolute age of uranium mineralisation at the Ranger Mine, but instead an average U – Pb age for wallrock and ore, particularly because the samples used to date the Ranger ore comprised whole-rock quarter-core splits (Ludwig et al., 1987). The absolute age of mineralisation at the Ranger Mine is therefore not known. Given this, the uranium ore shoots in the Ranger No 1 and 3 pits are interpreted to have formed during

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D4 (local), when the crust was extended in an east – west direction. It is likely that extension accompanied crustal decompression and the aggressive expulsion of hot saline brines that ultimately resulted in brecciation (B4) and alteration of the wall rock, and the deposition of uranium ore.

7. Tectonic evolution: conclusions The tectonic evolution of the Ranger Mine is subdivided into seven tectonic events. 7.1. Event 1: Extensional tectonics, basin formation, and unconformable deposition of the metavolcanic, metacarbonate and metasedimentary sequences

lato) granites at 1870 –1860 Ma (Stuart-Smith et al., 1993). 7.4. Event 4: D2 A period of SE –NW shortening, D2, resulted in the development of NNE – NNW trending folds, NNW – NE trending thrusts and shears, a C-S2 fabric, and a quartz-carbonate veins array indicative of early flexural slip-type folding. A westerly direction to tectonic transport is indicated from an overall asymmetry of D2 folds. D2 is correlated with regional D3 (Maud Creek Event at 1850 Ma) of the central Domain of the PCI. 7.5. Event 5: D3

The lithologies of the Ranger Mine record the unconformable deposition of three Palaeoproterozoic sequences on an Archaean basement gneiss sequence. The sequences describe a progression from volcanic processes to sedimentary process that is consistent with early rifting of the Pine Creek Geosyncline, as described by Needham et al. (1988).

A period of NE – SW shortening, D3, resulted in the development of macroscopic and mesoscopic east – west trending and an axial planar cleavage (S3). D3 is correlated with regional D4 (Maud Creek Event at 1850 Ma) of the central Domain of the PCI.

7.2. Event 2: Diastathermal metamorphism in D1 (extension-related metamorphism)

The dykes have been interpreted as part of the Oenpelli Dolerite that has been dated at 1690 Ma by Page et al. (1980).

D1 is characterised by a discrete bedding-parallel cleavage (S1) in the absence of structures that would suggest crustal shortening (folds, faults, etc.). Based on regional studies by Ferguson et al. (1980a), metamorphic conditions attained lower to upper greenschist facies. Event 2 is correlated with extension and the development of a thick sedimentary pile during subsidence of a regional-scale NNW-trending half graben (Needham et al., 1988). 7.3. Event 3: Emplacement of granite (sensu strito) dykes and pegmatite veins Emplacement of granite (sensu stricto) dykes and pegmatite veins accompanied contact metamorphism to hornblende-hornfels facies. Modelled palaeo-stress orientations for formation of the pegmatites indicate they may have formed during approximately east – west shortening early in D2. Their emplacement is correlated with the emplacement of regional (sensu

7.6. Event 6: Emplacement of mafic dykes

7.7. Event 7: D4 extensional tectonics D4 is associated with brittle deformation and the development of normal faults and fault breccias (B4) during a period of east – west extension. This event is correlated with regional east – west extension during deposition of the Palaeo- to Mesoproterozoic platform sequences. D4 accompanied uranium mineralisation and the development of uranium-bearing ore shoots in the Ranger No 1 and 3 pits.

Acknowledgements I thank the staff of the Ranger Mine for assistance in collecting the data for this study, in particular G. Hall, P. Stockman, D. Warner, R. Lithgow and T. Thomas. T. Thomas helped survey station points in the pits. I am grateful to A. Lips for his constructive

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review of the initial draft of this article. D.I. Groves and P. Laznicka are thanked for their careful reviews of the final version. This work was initiated and facilitated through the Energy Resources of Australia. However, Energy Resources of Australia does not necessarily agree with certain aspects of this article. References Bagas, L., Whitehead, B.R., Salas, C., Mulder, C.A., Needham, R.S., Stuart-Smith, P.G., Amri, C., 1982. Geology of the Katherine Gorge National Park (1:100,000 scale map). Northern Territory Geological Survey. Binns, R., McAndrew, J., Sun, S.-S., 1980. Origin of uranium mineralisation at Jabiluka. In: Ferguson, J., Goleby, A.B. (Eds.), Uranium in the Pine Creek Geosyncline. Proceedings of the International Uranium Symposium of the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 375 – 395. Browne, A.L.L., 1990. Ranger 68 Uranium deposit. In: Hughes, F.E. (Ed.), Geology of the Mineral Deposits of Australia and Papua New Guinea. The Australian Institute of Mining and Metallurgy, Melbourne, pp. 795 – 797. Choquette, P.W., James, N.P., 1990. Limestones—the burial diagenetic environment. In: McIlreath, I.A., Morrow, D.W. (Eds.), Diagenesis, Reprint Series 4 Geoscience Canada. Geological Society of Canada, Newfoundland, Canada, pp. 75 – 111. Etheridge, M.A., Rutland, R.W.R., Wyborn, L.A.I., 1987. Orogenesis and tectonic process in the early to middle Proterozoic of Northern Australia Geodynamic Series 17. American Geophysical Union, Washington, pp. 131 – 147. Eupene, G.S., 1980. Stratigraphic, structural and temporal controls of mineralization in the Alligator Rivers Uranium province, Northern Territory, Australia. In: Ridge, J.D. (Ed.), Proceedings of the Fifth Quadrennial IAGOD Symposium, Volume 1, pp. 347 – 376. Ewers, G.R., Ferguson, J., Donnelly, T.H., 1983. The Nabarlek uranium deposit, Northern Territory, Australia: some petrologic and geochemical constraints on genesis. Economic Geology, 823 – 837. Ferguson, J., Chappell, B.W., Goleby, A.B., 1980a. Granitoids of the Pine Creek Geosyncline. In: Ferguson, J., Goleby, A.B. (Eds.), Uranium in the Pine Creek Geosyncline. Proceedings of the International Uranium Symposium of the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 73 – 90. Ferguson, J., Ewers, G.R., Donnelly, T.H., 1980b. Model for the development of economic mineralization in the Alligator Rivers uranium field. In: Ferguson, J., Goleby, A.B. (Eds.), Uranium in the Pine Creek Geosyncline. Proceedings of the International Uranium Symposium of the Pine Creek Geosyncline. International Atomic Energy Agency, Vienna, pp. 563 – 574. Gustafson, L.B., Curtis, L.W., 1983. Post-Kombolgie metasomatism at Jabiluka, N.T., Australia, and its significance in the formation

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