Uranium systems processes in the Crocker Well Suite, South Australia · 2015-12-03 · Uranium...

Anthony Schofield

Record

2011/45

Uranium systems processes in the Crocker Well Suite, South Australia

GeoCat # 72058

APPLYING GEOSCIENCE TO AUSTRALIA’S MOST IMPORTANT CHALLENGES

G E O S C I E N C E A U S T R A L I A

Uranium systems processes in the Crocker Well Suite, South Australia GEOSCIENCE AUSTRALIA RECORD 2011/45 by Anthony Schofield

1. Minerals and Natural Hazards Division, Geoscience Australia

Department of Resources, Energy and Tourism Minister for Resources and Energy: The Hon. Martin Ferguson, AM MP Secretary: Mr Drew Clarke Geoscience Australia Chief Executive Officer: Dr Chris Pigram

© Commonwealth of Australia (Geoscience Australia) 2011 With the exception of the Commonwealth Coat of Arms and where otherwise noted, all material in this publication is provided under a Creative Commons Attribution 3.0 Australia Licence (www.creativecommons.org/licenses/by/3.0/au/) Geoscience Australia has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not solely rely on this information when making a commercial decision. ISSN 1448-2177 ISBN 978-1-921954-60-3 print 978-1-921954-59-7 web GeoCat # 72058 Bibliographic reference: Schofield, A., 2011. Uranium systems processes in the Crocker Well Suite, South Australia. Geoscience Australia Record, 2011/45, 39p.


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Contents Executive summary ..................................................................................................1

Introduction..............................................................................................................2

Geological setting ....................................................................................................3 The Crocker Well Suite .............................................................................................................. 3 Uranium and thorium mineralisation in the Crocker Well Suite ................................................ 3

Petrography..............................................................................................................6 Granitic rocks.............................................................................................................................. 6 Pegmatitic rocks.......................................................................................................................... 8 Breccias....................................................................................................................................... 8 Textural relationships of uranium- and thorium-bearing minerals ............................................. 8

Major uranium- and thorium-bearing phases.......................................................................... 8 Minor uranium- and thorium-bearing phases.......................................................................... 9

Geochemistry ......................................................................................................... 11 Method...................................................................................................................................... 11 Geochemically-defined rock groups ......................................................................................... 12 Main granitic rocks ................................................................................................................... 12 Diorites and mafic granodiorites............................................................................................... 16 High-sodium granitic rocks....................................................................................................... 17 Pegmatites................................................................................................................................. 17 Breccias..................................................................................................................................... 21 Uranium and thorium geochemistry ......................................................................................... 21

Fluid inclusions ..................................................................................................... 22 Fluid inclusion description........................................................................................................ 22 Inclusion compositions from freezing data ............................................................................... 24 Fluid compositions.................................................................................................................... 25

Discussion ............................................................................................................. 28 Magmatic evolution in the Crocker Well Suite......................................................................... 28 Fluid composition ..................................................................................................................... 30 Potential depositional controls .................................................................................................. 32 Mineral systems processes at Crocker Well.............................................................................. 32

Conclusions ........................................................................................................... 33

Acknowledgements ................................................................................................ 34

References ............................................................................................................. 35

Appendix 1: Geochemical data .............................................................................. 39


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Executive summary Magmatic-related uranium systems represent an uncommon, yet significant, family of uranium mineral systems. Despite the extremely large size of some magmatic-related uranium deposits, the key processes controlling uranium deposition are poorly understood compared to other more common uranium deposit styles. Petrographic, geochemical and fluid inclusion studies have been undertaken at the Crocker Well granite-hosted uranium deposit in South Australia in order to better constrain the key uranium mineralisation processes in this magmatic-related system. The results of this study allow a genetic model for the Crocker Well deposit to be proposed. Uranium mineralisation is interpreted to be associated with the intrusion of a volatile- and sodium-rich pulse of magma, or with the localised release of a highly sodic fluid from the main granitic rocks in the area. Exsolution of volatile-rich fluids partitioned uranium and thorium preferentially into a magmatic-hydrothermal fluid phase, and initiated the creation of fractures, veins and breccia zones. These acted as fluid flow pathways for the magmatic fluid. Uranium deposition likely occurred as a result of temperature and/or pressure decrease. This genetic model has been translated into a number of mappable criteria which may be used in prospectivity studies for magmatic-related uranium systems. The presence of evidence suggesting the potential to generate a uranium-bearing magmatic fluid, evidence of permeable structures contemporary with igneous activity, and evidence of favourable host rocks are suggested as useful criteria.


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Introduction Magmatic-related U systems represent one of three end-member families of U system defined by Skirrow et al. (2009; Fig. 1). A number of significant magmatic-related U systems are known globally, including the intrusive-related deposits at Rössing in Namibia (e.g., Berning et al., 1976), Ross Adams in Alaska (Thompson et al., 1982; Thompson, 1988) and Kvanefjeld in Greenland (e.g., Sørensen, 2001), and the volcanic-related deposits associated with the Streltsovka Caldera in Russia (e.g., Chabiron et al., 2003). Known magmatic-related U mineralisation is rare in Australia. Volcanic-related U systems are mostly known from north Queensland, while intrusive-related U mineralisation is almost entirely restricted to South Australia (McKay and Miezitis, 2001). Compared with more commonly targeted U mineral systems, such as those occurring in basinal or surficial geological settings, mineral system processes for magmatic-related deposits are poorly understood, adding significant exploration risk. In particular, depositional controls for U in magmatic-related systems are problematic and poorly constrained (Skirrow et al., 2009). Potential depositional mechanisms include a number of physical and chemical factors, such as fluid mixing, boiling, reduction and changes in temperature-pressure conditions. Magmatic-related U mineralisation in the Crocker Well area of South Australia is spatially and genetically associated with sodic granitic rocks of the Crocker Well Suite (King, 1954; Campana and King, 1958; Ashley, 1984; McKay and Miezitis, 2001; Wilson and Fairclough, 2009; Fig. 2). This report details an investigation undertaken in the Crocker Well area (predominately around the Main Eastern prospect) to better understand the key processes controlling mineralisation there, and particularly those relating to U deposition. Through this, an understanding of where U deposition may be expected, and the processes involved, may be established and translated into mappable exploration criteria. These criteria may then be applied to other potential magmatic-related systems and assist in the identification of prospective regions for magmatic-related U deposits.

Figure 1: Ternary U systems diagram showing the three end-member families of U mineralising systems (modified from Skirrow, 2009).


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Geological setting The Crocker Well area is located in the Olary Domain of the Paleoproterozoic to Mesoproterozoic Curnamona Province. Rocks of the Curnamona Province have been subdivided into two broad stratigraphic packages: the late Paleoproterozoic Willyama Supergroup metasedimentary, metavolcanic and intrusive rocks, and an overlying package consisting of early Mesoproterozoic volcanic, sedimentary and granitoid rocks (see Kositcin, 2010 and references therein). Three Mesoproterozoic granitic suites have been identified in the Olary Domain: 1) two-mica ‘regional granites’ of the Bimbowrie Suite, 2) sodic to biotite-only granites of the Crocker Well Suite, and 3) granodiorites to diorites coeval with the Crocker Well Suite and, locally, other peraluminous granites (see Fricke, 2008 and references therein). The Bimbowrie Suite, emplaced between ~1592 Ma and 1580 Ma (Fricke, 2008), only crops out in the Olary Domain, but is interpreted to continue below cover across a large portion of the central Curnamona Province (Fig. 2). The Bimbowrie Suite is dominantly comprised of strongly peraluminous two-mica granites with S-type affinities, although some components show weakly metaluminous compositions (Fricke, 2008). Field relationships show that the Bimbowrie Suite has diffuse contacts with the surrounding metasediments, which together with Nd isotopic constraints suggests that it was derived via melting of surrounding Willyama Supergroup metasedimentary rocks (Barovich and Ashley, 2002; Fricke, 2008). THE CROCKER WELL SUITE Granites of the Crocker Well Suite are generally medium- to coarse-grained and massive, ranging from tonalite to monzogranite, and accompanied by abundant alaskitic pegmatites (Ashley, 1984; Fricke and Conor, 2010). They are characterised by high Na, and are metaluminous to peraluminous in bulk composition (Ashley, 1984; Fricke, 2008; Fricke and Conor, 2010). Despite having high Na, there is little visual evidence of extensive alteration (Ashley, 1984). A number of plutons make up the Crocker Well Suite, and these display geochemical and lithological zonation (Fricke and Conor, 2010). The age of magmatism at Crocker Well is given by Ludwig and Cooper (1984), who reported a pooled zircon U-Pb age of 1579.2 ± 1.5 Ma for two samples of unmineralised leucoadamellite and adamellite. Alaskites from the Crocker Well Original prospect yield an age indistinguishable to the main granitic rocks (Ludwig and Cooper, 1984). Granites of the Crocker Well Suite exhibit comagmatic field relationships with each other and with other more primitive rocks in the Crocker Well region (Barovich and Foden, 2002; Fricke, 2008). Ashley (1984) considered the granites of the Crocker Well Suite to be derived from anatectic melting of sodic felsic gneisses present in the area. However, the relationship between the felsic intrusives and diorites, together with Nd isotopic evidence, suggests that the Crocker Well Suite had some mantle input (Barovich and Foden, 2002; Fricke, 2008). Ashley (1984) considered the depth of magma emplacement to be approximately 7-10 km. URANIUM AND THORIUM MINERALISATION IN THE CROCKER WELL SUITE Uranium and Th mineralisation in the Crocker Well area was initially detected in 1951 during an airborne radiometric survey flown by the Department of Mines, South Australia (now the Department of Primary Industries and Regions, South Australia), and was the subject of follow-up fieldwork during the 1950’s (McKay and Miezitis, 2001). Other U-Th prospects also occur in the area, including the Mount Victoria prospect approximately 10 km north of Crocker Well (McKay and Miezitis, 2001; Wilson and Fairclough, 2009; Fig. 3).


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Figure 2: Solid geology map of the Curnamona Province, South Australia. Solid geology is modified from Cowley (2006). The region shown in Figure 3 is given by the boxed area.


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Mineralisation at Crocker Well consists of a number of individual prospects (Fig. 3). The main mineralisation consists of thorian brannerite (also called absite by early workers), with lesser davidite (King, 1954; Whittle, 1954; Campana and King, 1958; Ashley, 1984; McKay and Miezitis, 2001; Wilson and Fairclough, 2009). Uranium and Th mineralisation is associated with phlogopitic biotite, rutile, xenotime, fluorapatite, fluorite and blue quartz (King, 1954; Whittle, 1954; Ashley, 1984). Most mineralisation is restricted to veins, fractures and local phlogopite-rich breccia bodies (Ashley, 1984). Alaskitic pegmatites also host U-Th-rare earth element (REE) mineralisation (King, 1954; Campana and King, 1958; Ashley, 1984). The breccia bodies are considered to be non-tectonic (Campana and King, 1958). Faults appear to influence the location of mineralisation, although these controls are less evident at the Main Eastern prospect (Campana and King, 1958; Wilson and Fairclough, 2009). Mineralisation has been inferred to be genetically linked to the granitic rocks (King, 1954; Campana and King, 1958), and is interpreted to be magmatic-hydrothermal in origin, related to the exsolution of late-stage volatiles from the magma (Campana and King, 1958; Ashley, 1954). Uranium-lead geochronology data on brannerite from Crocker Well yield scattered and discordant results with intercepts at ~1579 Ma and 420-480 Ma, and are broadly consistent with the age of the host granite (Ludwig and Cooper, 1984). Davidite mineralisation at Mount Victoria and Radium Hill has interpreted ages similar to, or slightly older than the age of brannerite mineralisation at Crocker Well (Ludwig and Cooper, 1984).

Figure 3: Main U prospects in the Crocker Well area. Geology is modified from Geoscience Australia’s 1:1 000 000 surface geology of Australia map.


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Petrography A petrographic investigation was carried out on a number of unmineralised to weakly mineralised samples. Samples were selected in order to examine U mineralisation processes in the Crocker Well area. Lithologically, these include granitic rocks, felsic gneisses, amphibolite, pegmatitic rocks and breccias. In general, the rocks studied appear to be unaltered to weakly altered. GRANITIC ROCKS The granitic rocks studied are largely monzogranite and granodiorite, with one sample having a syenogranite composition. Most granites sampled during this study belong to the Na-rich granitic rocks group (see below). In hand sample, granitic rocks are medium- to coarse-grained and equigranular. Minerals observable in hand sample are limited to quartz, plagioclase, K-feldspar, biotite and rutile. Colour ranges from cream to pinkish hues, reflecting varying K-feldspar content. Mafic mineralogy is a minor component, ranging up to only a few percent of the rock, and is restricted to small (≤1 mm) biotite grains in the samples studied. The rocks are largely weakly foliated to unfoliated. Where deformation is more obvious, it is defined by preferential orientation of biotite crystals. In thin section, the granitic rocks are generally homogeneous (e.g., Figs. 4a and 4b). Quartz appears to be stressed and deformed, as evidenced by extensive recrystallisation and undulose extinction. Interlocking with quartz are feldspars which range in size and shape, from anhedral and irregular morphologies to smaller blocky grains. The dominant feldspar is albitic plagioclase. Energy dispersive X-ray analyses undertaken on a SEM show a very small peak for Ca, indicating that the anorthite component is minor. Despite the prevalence of albite and the overall Na-rich nature of the Crocker Well Suite, textural evidence of albitisation is not strong. Alkali feldspar (dominantly orthoclase) is also common, and occurs in subordinate quantities to plagioclase. It is anhedral and sericitised, giving a brownish, dirty appearance. Microcline occurs in minor quantities in patches interstitial to the other feldspars. Biotite is present as the sole major mafic mineral phase. It is commonly partially replaced by chlorite and secondary opaque minerals. Altered biotite is in places associated with patches of reddish-brown staining suggestive of hematite. Biotite occurs as aggregates of crystals occurring interstitially to quartz and feldspar grains and as ‘stringers’ that appear to be late in the crystallisation sequence. Zircon is commonly included in biotite (Fig. 4c), producing radiation damage haloes in the host grain. In some cases, small muscovite plates occur with the biotite aggregates. Biotite pleochroism (typically α = pale straw yellow, β = γ = chocolate brown) suggests crystallisation under oxidising conditions. Subhedral to anhedral, honey-brown rutile grains up to ~1 mm are a common accessory mineral, and are commonly intergrown with biotite (Fig. 4d). Other accessory phases observed optically are interstitial fluorite (interpreted to be magmatic-stage), opaques and fluorapatite. Several REE-bearing mineral phases were observed in granite at the Main Eastern prospect. Bastnäsite ((La, Ce, Y)CO3F) occurs interstitially to feldspar grains, and is either intergrown with, or replaces biotite. A related mineral, parisite (Ca(Ce, La)2(CO3)3F2), appears to be a magmatic-stage mineral. Allanite is also present. A number of U-Th-bearing phases were observed, including minor uraninite, thorite and an unidentified U-Pb mineral included in rutile. Other minerals identified in minor quantities using a SEM are barite, monazite, xenotime, and unidentified Sn- and Ni-Cr minerals.


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Figure 4: Photomicrographs of typical granite from the Crocker Well area. a) Typical granite texture in plane light; b) Typical granite texture from Fig. 4a with crossed polars; c) Cluster of biotite grains hosting zircon grains; d) Rutile grain intergrown with biotite. Qtz = quartz, kfs = K-feldspar, plag = plagioclase feldspar, bi = biotite, zrc = zircon, ru = rutile.

Figure 5: Typical texture from the granite breccia. a) Typical breccia texture; b) Exsolution of secondary opaque minerals along cleavage planes within biotite.


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PEGMATITIC ROCKS Pegmatitic rocks are common in the Crocker Well area, and alaskitic pegmatites host significant U-Th mineralisation (Ashley, 1984). In hand sample, the pegmatitic rocks are coarse-grained and broadly equigranular, consisting of quartz, feldspar and biotite. The pegmatitic rocks all have a similar appearance in thin section. Quartz grains show evidence of stress, showing undulose extinction and recrystallisation. Albite and K-feldspar are both present, with K-feldspar present in higher abundances. Feldspars have been extensively sericitised. Biotite is the sole mafic mineral present (α = pale straw yellow, β = γ = yellowy-brown). It is commonly partially replaced by chlorite and secondary opaques. Rutile and zircon bear a spatial association with biotite. Muscovite is present in minor quantities as an interstitial mineral phase. BRECCIAS Brecciated granitic rocks host most of the mineralisation at Crocker Well (Ashley, 1984). One weakly brecciated sample (20100706010) was collected from Crocker Well Junction. In hand sample, the rock consists of angular to subrounded clasts of relatively fresh granite, similar to that seen elsewhere, set in a fine-grained matrix dominated by quartz and biotite. In thin section, the matrix is conspicuous for its overall higher biotite content (Fig. 5a). Matrix biotite crystals are larger than those contained within the granite and occur as aggregates interstitial to recrystallised quartz and sericitised feldspar grains. Exsolution of secondary opaques along cleavage planes in biotite is commonly observed (Fig. 5b). Biotite pleochroism (α = pale straw yellow, β = γ = light chocolate brown) suggests crystallisation under oxidising conditions. Rare accessory zircon and rutile were observed in association with biotite. Under the SEM, biotite can be seen to host a number of thorite grains. These have caused extensive radiation damage to the host biotite crystal (Fig. 6d). Uranium-rich cheralite (generally Ca, Th (PO4)2, but in this instance containing appreciable U), occurring within fractures in albite, and an unidentified bladed Th-U mineral are the significant hosts to Th and U in the samples studied (Figs. 6e and 6f). Other accessory minerals identified using the SEM are fluorapatite, xenotime, an unidentified Ca-REE-fluorocarbonate, native Au and a Cu-Au alloy. TEXTURAL RELATIONSHIPS OF URANIUM- AND THORIUM-BEARING MINERALS A range of U- and Th-bearing minerals have been identified (Tab. 1). These are subdivided into two broad categories:

1. U- and/or Th-rich: thorite, uraninite, an unidentified Th-U mineral, and an unidentified U-Pb mineral; and

2. Minor U-Th component: bastnäsite, allanite, parisite, U-rich cheralite, Th-rich zircon, Th- and REE-rich zircon, and Th-rich monazite.

Although earlier workers (King, 1954; Whittle, 1954; Campana and King, 1958; Ashley, 1984) describe brannerite and davidite as the main U- and Th-bearing minerals, these were not encountered during this study. This is likely to be a function of the samples examined here coming from more distal parts of the ore system, rather than the brannerite and davidite found in the main ore zone. Major uranium- and thorium-bearing phases Thorite was present in a number of samples, and is the main U-Th-bearing mineral observed in this study. In the granite, thorite is intergrown with Fe-oxides (tentatively identified as hematite) and fluorite (Fig. 6a). The hematite appears to replace biotite, thus placing thorite late in the crystallising sequence. Thorite is also intergrown with rutile in the breccia, and also occurs as inclusions within biotite and albite (Fig. 6d). Where this is observed, extensive radiation damage haloes are developed


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within the host mineral (Fig. 6d). Replacement textures in the breccia and felsic gneiss indicate that thorite partially replaces biotite, zircon and fluorapatite. Thorite commonly has a mottled texture, suggesting late in-situ growth. Uraninite was only observed in one sample from Main Eastern prospect (Fig. 6b). The grains are small (~2 μm) and appear to be intergrown with, or replacing, biotite. Rutile hosts tiny (<1 μm) inclusions of an unidentified U-Pb mineral as blebs and small veins. The unidentified Th-U mineral observed in the breccia occurs interstitially to fluorapatite (Fig. 6f), and appears to be a primary magmatic mineral. Minor uranium- and thorium-bearing phases Textural relationships for minor U- and Th-bearing phases are given in Table 1. Minor U- and Th-bearing phases largely tend to be REE-rich, and are generally late in the crystallising sequence. They may be late magmatic-stage minerals, or may be replacement products. Table 1: Textural relationships for uranium- and thorium-bearing minerals

MINERAL HOST ROCK TEXTURAL FEATURES FIGURE

Thorite Granitic rocks, breccia and felsic gneiss

Intergrown with Fe-oxides (hematite), rutile and fluorite. Commonly included in biotite and albite. Observed replacing biotite, zircon and fluorapatite in places

Fig. 6a, Fig. 6d

Uraninite Monzogranite Intergrown with, or replacing, biotite

Fig. 6b

Unidentified Th-U mineral Breccia Interstitial to fluorapatite Fig. 6f

Unidentified U-Pb mineral Monzogranite Inclusion in rutile -

Bastnäsite Monzogranite Replaces biotite, suggesting late crystallisation. Occurs in association with fluorite

Fig. 6c

Ce-allanite Monzogranite Interstitial magmatic-stage mineral. Possibly intergrown with biotite. Occurs in association with fluorite

-

Parisite Monzogranite Late magmatic-stage mineral -

U-rich cheralite Breccia Occurs in late fractures in albite

Fig. 6e

Th-rich zircon Felsic gneiss Primary magmatic mineral, as well as possible replacement product of normal zircon

-

Th- and REE-rich zircon Felsic gneiss Possible replacement product of normal zircon

-

Th-rich monazite Felsic gneiss Small inclusion in ?hematite -


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Figure 6: SEM photomicrographs of U- and Th-bearing minerals from Crocker Well. a) Thorite intergrown with fluorite and Fe-oxides (hematite); b) Uraninite in biotite; c) Bastnäsite intergrown with biotite and fluorite; d) Large thorite crystal with radiation damage halo; e) Vein of U-rich cheralite in plagioclase; f) Bladed unknown Th-U mineral with fluorapatite. Qtz = quartz, fl = fluorite, th = thorite, feo = Fe-oxides (hematite), bi = biotite, plag = plagioclase feldspar, ba = bastnäsite, ch = cheralite, fap = fluorapatite, xe = xenotime.


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Figure 7: Geochemical sample localities. Geology is modified from Geoscience Australia’s 1:1 000 000 surface geology of Australia map.

Geochemistry Fourteen geochemical samples were analysed during this investigation from the Crocker Well and Mount Victoria areas (Fig. 7), and encompass a range of lithologies including amphibolite, breccia, granitic rocks and pegmatites. The samples analysed are unmineralised to weakly mineralised. These data are given in Appendix 1. Geochemical data for this investigation are supplemented by additional data from previous studies (Ashley, 1984; Fricke, 2008; Fricke and Conor, 2010). METHOD Rock samples were prepared for chemical analysis by jaw crushing followed by grinding of a sub-sample (50-70 g) in a tungsten carbide ring mill. Abundances of major and trace elements were determined by XRF and ICP-MS at Geoscience Australia, Canberra. Major and minor elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P and S) were determined by wavelength-dispersive XRF on fused disks using methods similar to those of Norrish and Hutton (1969). Precision for these elements is better than ±1% of the reported values. Arsenic, Ba, Cr, Cu, Ni, Sc, V, Zn and Zr were determined by pressed pellet on a wavelength-dispersive XRF using methods similar to those described by Norrish and Chappell (1977). Selected trace elements (Cs, Ga, Nb, Pb, Rb, Sb, Sn, Sr, Ta, Th, U, Y) and the rare earth elements were analysed by ICP-MS (Agilent 7500 CE with reaction cell) using methods similar to those of Eggins et al. (1997), but on solutions obtained by dissolution of fused glass disks (Pyke, 2000). Precisions are ±5% and ±10% at low levels (<20 ppm). Agreement between XRF and ICP-MS are within 10%. Loss on ignition (LOI) was by gravimetry after combustion at 1100C. Abundances of FeO were determined by digestion and electrochemical titration using a modified method based on Shapiro and Brannock (1962).


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GEOCHEMICALLY-DEFINED ROCK GROUPS Three separate groups have been identified based on geochemical trends: diorites and mafic granodiorites, main granitic rocks, and high-Na granitic rocks (Fig. 8). The high-Na granitic rocks have been subdivided into two categories. Most correspond to the tonalites and trondhjemites of Fricke and Conor (2010). Others are not associated with the tonalites and trondhjemites of Fricke and Conor (2010) and potentially represent main granitic rocks which have undergone sodic alteration, although, as stated above, textural evidence for albitisation is poor. The high-Na granitic rocks largely correlate with proximity to known prospects, although this is not universal. As mentioned previously, the diorites and mafic granodiorites display comagmatic field relationships with the other granitic units (Barovich and Foden, 2002; Fricke, 2008). Plots of major elements appear to support this, with these rocks lying on the same general trend as the main granitic rocks (Fig. 8). On selected trace element plots however, they appear to occur on a different trend to the main granitic rocks and the high-Na granitic rocks. MAIN GRANITIC ROCKS Granitic rocks constitute the major rock type in the Crocker Well and Mount Victoria area. Zircon saturation temperatures calculated according to Watson and Harrison (1983) are highly variable (672-916oC), but largely appear to yield moderate temperatures (median = 832oC). Silica contents for the main granitic rocks range from 65 to 74%. In general, the main granitic rocks in the Crocker Well area have moderate element abundances, with elevated Na2O, Th, U, Th/U, P2O5 and Nb. Major elements show well-defined negative linear trends for TiO2 (Fig. 9), FeOtotal and MgO, and broader negative trends for Al2O3, CaO, Na2O (Fig. 9) and P2O5 against SiO2. Potassium broadly correlates positively with increasing SiO2 (Fig.9). Overall, the data are more scattered below ~68% SiO2. Plots of trace elements against SiO2 typically show scattered trends. Diffuse negative trends are observed for light rare earth elements (LREE), some transition metal elements (TME; Sc, V), high field strength elements (HFSE; Y, Th, Nb) and large ion lithophile elements (LILE; Ba, Sr). Good negative correlations were observed for Zr (Fig. 9) and Hf, although there is considerable scatter below ~68% SiO2. The heavy rare earth elements (HREE), U (Fig. 9), Pb, Ta, Cr, Ni, Cu, Zn and Rb (Fig. 9) are scattered. On N-MORB-normalised multi-element plots, patterns generally exhibit enrichment in incompatible elements, with negative anomalies for Ba, Nb, Sr and Ti, and positive anomalies for Th-U and Pb (Fig. 10). Abundances of Ta span a considerable range of values, which is reflected in a variable Ta anomaly in multi-element plots. Rare earth element patterns (Fig. 11) show an enrichment of LREE over HREE, with moderate total REE abundances. All samples have a strong negative Eu anomaly (EuN/EuN* = 0.48-0.20), consistent with the removal of plagioclase from the melt or retention of residual plagioclase in the magma source region.


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Figure 8: Geochemical plots showing the main rock groupings identified in this study in the Crocker Well Suite. Data for this and other figures are from this study, Ashley (1984), Fricke (2008) and Fricke and Conor (2010).


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Figure 9: Selected major and trace element plots for the main granitic rocks.


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Figure 10: Multi-element plots for the main rock groups identified in the Crocker Well Suite. Dashed lines show the median for each group, as well as the 25th and 75th percentile thresholds. Samples are normalised to N-MORB of Sun and McDonough (1989).

Figure 11: Rare earth element plot for the main granitic rocks. The shaded area encompasses the 25th to 75th percentile range of all data from previous studies (n = 51). Values are normalised to C1 chondrite of Sun and McDonough (1989).


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DIORITES AND MAFIC GRANODIORITES The diorite and mafic granodiorite group has a range of 55-67% SiO2. Major element plots show a spread of values beyond that expected for ordinary magmatic processes. Plotted against decreasing FeOtotal, TiO2, MnO, MgO, CaO (Fig. 12) and P2O5 show scattered negative correlations, while SiO2 and K2O diffusely increase. Scattered trends are observed for Al2O3 (Fig. 12) and Na2O. Trace element patterns are generally scattered (Fig. 12), although Rb is observed to diffusely increase with decreasing FeOtotal (Fig. 12) while Sr decreases. Rare earth element patterns show an enrichment of LREE over HREE, with generally flat HREE, and have a negative Eu anomaly (EuN/EuN* = 0.76-0.35). Some members of this group appear to have undergone some modification, as suggested by the high ASI for rocks of this composition (ASI = 0.73-1.18) and highly scattered element concentrations. Similarly, concentrations of U and Th are higher than expected for rocks of this composition (up to 24 and 80 ppm respectively).

Figure 12: Selected major and trace element plots for the diorites and mafic granodiorites.


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1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

Ro

ck/c

ho

nd

rite

Figure 13: Rare earth element plot for the diorites and mafic granodiorites. Values are normalised to C1 chondrite of Sun and McDonough (1989). HIGH-SODIUM GRANITIC ROCKS As mentioned above, most of the high-Na granitic rocks correspond to the tonalites and trondhjemites of Fricke and Conor (2010), although concentrations of some elements (eg. U, Th, Ga) are significantly higher than expected for such rocks. Relative to the main granitic rocks, TiO2, FeOtotal, MgO, CaO, Na2O, Na2O/K2O, P2O5, Th, U, Nb, Ta, Ga and F are higher, while K2O, Ba and Rb are lower (Fig. 14). The highest values (up to 8.33 wt.% Na2O) may reflect cryptic sodic alteration. The high-Na granitic rocks span a SiO2 range between 62-80% (mostly 68-75%). Harker variation diagrams for most elements do not define strong trends. Multi-element plots resemble those for the main granitic rocks, except for abundances of REE, Ta and K (Fig. 10). Tantalum and K are anomalous with respect to the main granitic rocks (positively and negatively anomalous respectively), while the REE are lower and parallel the main granitic rock trend (except Sm, which shows a weak negative anomaly). Rare earth element patterns show slight enrichment of LREE relative to HREE and a negative Eu anomaly (EuN/EuN* = 0.68-0.26; Fig. 15). PEGMATITES Pegmatitic rocks (including alaskites) are common in the Crocker Well area. Field traverses using hand held scintillometers show that these frequently have elevated U relative to their granitic host rocks. The pegmatitic rocks are highly felsic (>72% SiO2) and have high Na2O and low K2O (Fig. 16). Both Th and U (Fig. 16) are moderate to high, and Th/U ratios are highly variable (0.36-13.0). Rare earth element abundances are very low, and REE patterns are characterised by enrichment of LREE relative to HREE (La/YbN = 10.78), a slight Eu anomaly (EuN/EuN* = 0.75), and slight enrichment of HREE (Er, Yb, Lu) relative to MREE (Fig. 17). The pegmatitic rocks show the same compositional characteristics as the high-Na granitic rocks, although F is lower (Fig. 16).


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Figure 14: Selected plots showing the difference in elemental abundance between the high-Na and main granitic rocks.


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Figure 15: Rare earth element plot for the high-Na granitic rocks. The shaded area encompasses the 25th to 75th percentile range of all data from previous studies (n = 36). Values are normalised to C1 chondrite of Sun and McDonough (1989).

Figure 16: Selected major and trace element plots for the pegmatitic rocks.


20

1

10

100


Ro

ck/c

ho

nd

rite

Figure 17: Rare earth element plot for the pegmatite sampled in this study. Values are normalised to C1 chondrite of Sun and McDonough (1989).

1

10

100


Ro

ck/c

ho

nd

rite

Figure 18: Rare earth element plot for the breccia sampled in this study. Values are normalised to C1 chondrite of Sun and McDonough (1989).


21

BRECCIAS Breccia sampled during this study, together with alaskitic breccias reported by Ashley (1984), show high MgO, F, Th and moderate to high U relative to the granitic rocks. The Fe3+/Fe2+ ratio for the breccia sampled in this study is higher than that of the granitic rocks (1.58), suggesting conditions under which brecciation occurred were oxidising. Rare earth element patterns (Fig. 18) show low levels of REE, with LREE enriched relative to HREE (La/YbN = 3.5). The most notable feature on the breccia REE plot is a pronounced negative Eu anomaly (EuN/EuN* = 0.29) and a weak positive Ce anomaly, which is consistent with oxidation. Multi-element plots show negative anomalies for Ba, Nb-Ta, La-Ce, Sr and Ti, and positive anomalies for Th-U and P (Fig. 19). URANIUM AND THORIUM GEOCHEMISTRY Uranium and Th are generally elevated relative to average upper continental crust, but are highly variable within the groups present. Uranium does not appear to correlate with any other elements, suggesting either that the U content has been disrupted by subsolidus processes (e.g., weathering or alteration), or that U concentration has been affected by magmatic processes other than fractional crystallisation. Thorium appears to correlate weakly with LREE in the main granitic rocks, suggesting Th content is controlled by the removal of REE-bearing phases from the melt (e.g., xenotime, bastnäsite, monazite, allanite, parasite, cheralite). Ratios of Th to U are highly variable. Again, this may represent modification of magmatic Th/U ratios during and/or after magmatism.

Figure 19: Multi-element plot for the breccia sampled in this study. Samples are normalised to N-MORB of Sun and McDonough (1989).


22

Fluid inclusions Reconnaissance fluid inclusion studies were undertaken on six samples from the Crocker Well area in an attempt to constrain the characteristics of the primary magmatic fluids, as well as fluids thought to be directly involved in U-Th mineralising processes. Sample details are given in Table 2. Inclusions were studied using petrographic, microthermometric and laser Raman microprobe methods. Microthermometric data were obtained using a Linkam MDS 600® heating–freezing stage. The Linkam stage was calibrated with a series of synthetic fluid inclusions of known composition. A heating rate of 2-5°C/min was used to record phase changes below 30oC and 20-50°C/min above 30oC, yielding uncertainties of approximately ± 2oC and ± 5-10oC above and below 30oC respectively. Laser Raman spectra of fluid inclusions were recorded on a Dilor SuperLabram spectrometer equipped with a holographic notch filter, 600 and 1800 g/mm gratings, and a liquid N2 cooled, 2000x450 pixel CCD detector. The inclusions were illuminated with 514.5 nm laser excitation from a Melles Griot 543 Series Ar ion laser, using 5 mW power. Data were accumulated over a single 30 second period. A 100x Olympus microscope objective was used to focus the laser beam and collect the scattered light. The focused laser spot on the samples was approximately 1 μm in diameter. Wavenumbers are accurate to ± 1 cm-1 as determined by plasma and Ne emission lines. For the analysis of CO2, O2, N2, H2S and CH4 in the vapour phase, spectra were recorded from 1000 to 3800 cm-1 using a single 20 second integration period per spectrum. The detection limits are dependent upon the instrumental sensitivity, the partial pressure of each gas, and the optical quality of each fluid inclusion. Table 2: Samples investigated for fluid inclusion characteristics

SAMPLE LOCALITY SAMPLE DESCRIPTION

20100706005 Main Eastern prospect High-Na granitic rock from the Main Eastern prospect. Thorium and F contents are elevated

20100706006 Main Eastern prospect High-Na granitic rock from the Main Eastern prospect. Uranium, Th and F contents are elevated

20100706010 Crocker Well Junction Granite breccia containing high Th and F. Fluid inclusions were analysed from the biotitic matrix

20100706016 Crocker Well Central Late(?) high-U quartz-feldspar-biotite-rutile-hematite vein. Green staining suggests elevated Cu content

20100706017 Main Eastern prospect Typical granitic rock sampled from drill hole DDEC59 from 95.3-95.4 m

20100706019 Main Eastern prospect Weakly brecciated granitic rock sampled from drill hole DDEC002 from 64.8-64.9 m

FLUID INCLUSION DESCRIPTION Fluid inclusions were not abundant in the samples studied in this investigation. Inclusions selected for this study are interpreted to have predominately pseudosecondary origins, although some interpreted primary inclusions were observed (Fig. 20). Inclusions interpreted to be secondary were not analysed or were rejected. While the inclusions analysed appear to be primary or pseudosecondary in origin, the timing of growth of the host quartz is unconstrained and therefore the data presented below should be used with caution. The origins of the fluid inclusions were determined using the empirical criteria proposed by Roedder (1984). Evidence of crystal growth direction and zoning was not observed in the inclusion-bearing grains. A summary of the most useful


23

criteria for determining fluid inclusion origins is given in Table 3. All inclusions analysed occur in quartz, and care was taken to avoid the most extensively recrystallised grains. Most fluid inclusions in the granitic rocks (20100706005, 20100706006 and 20100706017) contain liquid, vapour and one or three daughter minerals, although simple two phase inclusions are also present. A small subset of inclusions had up to five daughter minerals. Inclusions with greater than three daughter minerals are rare in the granitic rocks, and it is likely that some of the daughter minerals may have been accidentally trapped. Vapour contents vary in the range of 5-30% of the total inclusion volume, with 10-20% being the most common. Inclusions in the granitic rocks are generally quite small (<10 μm), although the more crystal-rich inclusions appear to be slightly larger (~15 μm). Mineral phases occupy a variable volume of the total inclusion, but the percentage may be quite large (2-80% of inclusion volume). Inclusions from the breccia (20100706010) were small (<10 μm), and contained a single daughter phase. A small (~10% volume) vapour bubble was present. Inclusions in the weakly brecciated granitic rock (20100706019) have two modes of occurrence: 1) daughter mineral-rich (usually three or four daughter minerals) and 2) single daughter mineral. Hematite, magnetite and rutile were identified by Raman microprobe in the daughter mineral-rich inclusions, along with a tentative optical identification of halite. It is not possible from the limited number of observations of the daughter-rich inclusions to determine whether or not the minerals are accidentally trapped, although the presence of Fe-oxides is fairly consistent across the inclusions observed. Rutile on the other hand was only confirmed to occur within a single inclusion. The vapour bubble typically constituted 5-10% of the inclusion volume. Inclusions were usually approximately 10 μm in size. The high-U vein sampled from Crocker Well Central (20100706016) is conspicuous for its generally larger inclusions (up to 41 μm). The vapour bubbles in this sample were small (<5% of inclusion volume). Some inclusions contain numerous daughter minerals, while some contained only a single phase. Muscovite, calcite and hematite were identified by Raman microprobe analysis. Table 3: Summary of the most useful criteria applicable to the samples used in this study for determining fluid inclusion origin (after Roedder, 1984)

PRIMARY SECONDARY PSEUDOSECONDARY

Occurrence as a single inclusion in an otherwise inclusion-free crystal

Occurrence in planar groups outlining healed fractures

Inclusion trails terminate in crystal

Isolated occurrence away from other inclusions (distance approximately >5 times inclusion diameter)

Very thin and flat Generally more equant inclusions than for secondary origins

Occurrence as part of a random three dimensional distribution throughout the crystal


24

Figure 20: Examples of typical fluid inclusions encountered in this study. a) Interpreted primary fluid inclusion containing a vapour bubble and two daughter minerals; b) Secondary inclusion trails in apatite; c) Daughter mineral-rich inclusion containing hematite, an unidentified clear mineral, and possibly a mica; d) Daughter mineral-rich inclusion containing a number of interpreted accidentally trapped minerals. INCLUSION COMPOSITIONS FROM FREEZING DATA Due to the overwhelmingly small size and poor quality of the inclusions observed, the heating and freezing data are subject to a high degree of uncertainty. Initial eutectic melting occurred mostly in the range of -66oC to -30oC. A small subset of CO2-bearing inclusions (see below) had lower initial eutectic temperatures (-91oC to -83oC). The freezing data may be broadly grouped into four main recognisable phase melting events, at approximately -58o to -55oC, -25oC to -20oC, -12oC to -9oC, and 2o to 21oC (the latter are dominantly between 16o and 18oC). Other melting events may have occurred, but were too subtle to recognise. Melting temperatures at around -58oC to -55oC were confined to a single quartz grain in one of the high-Na granitic rocks (20100706006). These temperatures are close to the melting point of CO2, which may have been lowered by the presence of other gases. Significantly, dissolved CO2 was confirmed in these inclusions by Raman microprobe analysis. Final phase melting at -25oC to -20oC represents the dominant population of melting temperatures, and is present in almost all of the samples analysed. Melting in this range is similar to, but slightly lower than, the H2O-NaCl eutectic (-21.2oC). The final phase melting event at around -12oC to -9oC is close to the eutectic melting temperature of H2O-KCl (-10.6oC), although it is mostly slightly lower. These lower than expected temperatures may be due to metastability of NaCl and KCl phases, or the presence of additional components (e.g., MgCl2) which prevent accurate determinations of fluid compositions (e.g., see Sterner et al., 1988).


25

The origin of the range of poorly reproducible positive temperatures for final phase melting between ~4o and 14oC is uncertain. Temperatures towards the lower end of this spectrum (i.e., <10oC) may represent clathrates or metastable behaviour. There is a well-represented population of final melting temperatures at approximately 18o to 21oC. This is close to the melting temperature of sylvite, as given by Zhang and Frantz (16.5oC; 1987). Positive melting temperatures were observed in all samples except 20100706005. Few homogenisation and decrepitation temperatures were obtained. This was due to the propensity for the studied inclusions to decrepitate at even moderate temperatures. FLUID COMPOSITIONS Based on the freezing data, two broad inclusion types may be recognised (Tab. 4; Fig. 21). The first type contains two or three phase melting events in the -58oC to -55oC, -25oC to -20oC and the -12oC to -9oC range. This has tentatively been identified as a CO2-bearing fluid, with possible additional KCl ± NaCl, although true fluid compositions remain uncertain at present. Inclusions belonging to this population were only encountered in sample 20100706006. No homogenisation temperatures were measured for the CO2-bearing fluid, but three decrepitation temperatures were recorded at 365.1oC, 388.6oC and 454.2oC. Compositions for the second type are uncertain. Phase melting temperatures of -25oC to -20oC and the -12oC to -9oC are close to the eutectic temperatures for H2O-NaCl and H2O-KCl, but are largely too low to be accurately correlated to these salt phases. As stated above, this may reflect the presence of other unobserved phases in the inclusions or metastability, which could have modified the melting temperatures. Despite the uncertainty, the second inclusion type probably represents NaCl-dominated fluids, based on the magnitude of the -25oC to -20oC melting event (Tab. 4). The NaCl-dominated fluid type possesses one or two recognisable phase melting events, with the -25oC to -20oC range occurring by itself, or accompanied by a -12oC to -9oC or 2o to 21oC event. Two sub-types of the NaCl-dominated fluid type are tentatively recognised:

1. H2O-NaCl ± KCl fluids (samples 20100706005, 20100706006, 20100706010, 20100706016, 20100706017 and 20100706019)

2. H2O-NaCl-unknown fluids (samples 20100706006, 20100706010, 20100706016 and 20100706019)

Owing to the unknown exact composition of the fluids and the problematic final phase melting at temperatures lower than the eutectic values for the interpreted compositions, it is not possible to calculate salinities for most of the inclusions analysed. Using the H2O-NaCl-MgCl2 system of Dubois and Marignac (1997), a handful suitable inclusions from the H2O-NaCl ± KCl subgroup have NaCl contents of 10.5 to 10.6 wt.% and MgCl2 of 3.4 to 4.6 wt.%. Taking only the final melting temperature, the same inclusions have 14.2 to 17.6 wt.% NaCl equivalent according to Bodnar (1993). These are similar to values for the CO2-bearing population if only the last phase melting temperature (-12oC to -9oC) is considered. Only two inclusions from the H2O-NaCl ± KCl subgroup were suitable for salinity calculation in the H2O-NaCl-KCl system (ice + hydrohalite; Bakker, in press), owing to melting temperatures lower than the expected H2O-KCl eutectic. Results for these vary greatly, with values of 17.6 and 11.8 wt.% NaCl and values of << 1 and 3.2 wt.% KCl. The other inclusions exhibited only one recognised melting event and have temperatures too low to be modelled by the H2O-NaCl system, and as such salinity calculations are not possible.


26

As mentioned above, there is very limited homogenisation temperature data, ranging between 181.8oC to 253.2oC (n = 7) for the broadly H2O-NaCl ± KCl fluids and 238.3oC 308.5oC (n = 3) for the H2O-NaCl-unknown fluids. Decrepitation temperatures for the broadly H2O-NaCl ± KCl and H2O-NaCl-unknown fluids are in the range of 370.4oC to 492.1oC (n = 6) and 358.1oC to 423.5oC (n = 4) respectively. These data are insufficient to develop a coherent fluid paragenesis. A small number of homogenisation temperatures were also obtained from interpreted secondary inclusions hosted in two apatite grains in samples 20100706017 and 20100706018. These inclusions had dominantly simple two-phase contents. Both of these samples had single, highly variable phase melting temperatures in the range of -23.3oC to -15.2oC and -26.4oC to -6.2oC respectively. Observed homogenisation temperatures for these two samples were between 236.9oC and 323.3oC (20100706017) and 241.7oC and 284.4oC (20100706018). Table 4: Fluid inclusion populations identified on the basis of freezing data. The temperatures given are for final phase melting

TYPE FIRST OBS.

MELTING (oC)

FINAL OBS. MELTING (

oC)

OTHER OBS. MELTING (

oC)

COUNT Th (oC) Td (o

C)

1. H2O-CO2-(NaCl)-KCl

-57.8 to -55.4 -11.7 to -9.7 -24.6 to -23.2 6 - 365 to 454

2.1. H2O-NaCl ± KCl -23.5 to -20.8*

-27.9 to -20.6 or -13.8 to -10.2

- 39 182 to 253 370 to 492

2.2. H2O-NaCl-unknown

-25.7 to -20.8 2.4 to 21.1 - 29 238 to 309 358 to 424

* Most inclusions belonging to the H2O-NaCl ± KCl subgroup exhibited only one melting event between -27.9oC and -20.6oC. A small number (4) of inclusions also had phase melting at -13.8oC to -10.2oC. The first observed melting listed here only corresponds to those inclusions from the H2O-NaCl ± KCl subgroup showing two melting events.


27

Figure 21: Populations of fluid inclusions identified from freezing data. a) Temperature data for interpreted CO2-bearing inclusions; b) and c) are interpreted sub-types of a NaCl-dominated fluid. b) H2O-NaCl ± KCl; c) H2O-NaCl-unknown.


28

Discussion MAGMATIC EVOLUTION IN THE CROCKER WELL SUITE Major and trace element variation within the main granitic rocks of the Crocker Well Suite reflect the removal of biotite, hornblende, plagioclase, magnetite, apatite and titanite from the melt during fractional crystallisation (Fricke and Conor, 2010). Removal of a REE-bearing phase has resulted in decreasing REE content with fractionation. Plotted on the Rb-Ba-Sr ternary plot of El Bouseily and El Sokkary (1975), the main granitic rocks define a clear fractionation trend from ‘normal granite’ to ‘differentiated granite’ (Fig. 22), although this reflects decreasing Ba and Sr mainly, since Rb shows little change with increasing SiO2 (Fig. 9). In contrast, the high-Na granitic rocks do not show any clear differentiation trends with the main granitic rocks and plot as a distinct group on the ternary plot of El Bouseily and El Sokkary (1975; Fig. 22). This, together with significant differences in elemental abundances, suggests that at least some of the high-Na granitic rocks may not belong to the same magma batch as the main granitic rocks. Fricke and Conor (2010) suggest that these may represent a separate intrusion to the main granitic rocks present in the rest of the Crocker Well Suite. The alaskitic pegmatites also potentially represent the same intrusive phase as the high-Na granitic rocks, as their chemistry corresponds closely. However, the spread of geochemical data within the high-Na granitic rocks is too large to be a product of purely magmatic processes. Therefore it is likely that the compositions have been partly modified by hydrothermal alteration processes. The presence of rocks with geochemical characteristics similar to those of the high-Na granitic rocks away from the sodic core of the pluton argues in favour of an origin due to sodic alteration. If such alteration has occurred, it is not texturally obvious, and is focussed strongly on the centre of the Crocker Well Pluton along with mineralisation. The parental magma to the main granitic rocks may be similar to the dioritic and granodioritic rocks present in the area (as suggested by Barovich and Foden, 2002; Fricke, 2008; Fricke and Conor, 2010). Many of the geochemical variation diagrams are consistent with the main granitic rocks being derived from the more mafic rocks, with a pronounced inflection at around 68% SiO2 (Fig. 23). However, a number of sample points also occur above this inflection and lie on a linear trend with other samples corresponding to the main granitic rocks (Fig. 23). Thus, either the dioritic and granodioritic rocks are not parental to the main granitic rocks, or samples above the inflection represent cumulates or restite-rich phases. Fricke and Conor (2010) suggest that assimilation of other material was also involved in the evolution of the Crocker Well Suite. The high-Na granitic rocks perhaps had a component similar to broadly contemporary alkaline magmas of the Billeroo alkaline magmatic complex just northeast of the Crocker Well Suite (Rutherford, 2007) in their source.


29

Figure 22: Ternary Rb-Ba-Sr diagram of El Bouseily and El Sokkary (1975) showing the main groups identified in the Crocker Well Suite. Symbols are the same as for Figure 8.

Figure 23: Possible magmatic evolution processes in the Crocker Well Suite.

0

50

100

150

200

250

300

55 60 65 70 75 80

SiO2 wt.%

La ppm

Diorites and maficgranodiorites

Main granitic rocks

High‐Na granitic rocks

High‐Na granitic rocks(tonalites andtrondhjemites)

Resite or cumulatecomponent?

Fractionationtrend?


30

FLUID COMPOSITION Fluid inclusion data obtained during this investigation are inconclusive. However, from the available data, some potential characteristics of the fluid are evident. The fluid inclusions analysed are dominated by melting temperatures in the range of -25oC to -20oC (Fig. 21). This fluid was likely moderately saline, and dominated by NaCl, with the melting point lowered by other fluid components. Melting at around -12oC to -9oC suggests KCl is present in the fluids. Analyses from a small number of inclusions suggest that some CO2 was present in the system. These fluid inclusion data are similar to those given by Williams-Jones et al. (2000) for hydrothermal fluorite-REE deposits in New Mexico. The presence of hematite daughter minerals in sample 20100706019 from the Main Eastern Prospect suggests that the fluid was oxidised. Similarly, the presence of hematite in a high-U vein from Crocker Well Central suggests the presence of an oxidised fluid phase. The presence of rutile in sample 20100706019 may be as a result of accidental trapping, since only one occurrence was observed. However, the association of mineralisation with rutile suggests that the mineralising fluid was high in Ti. Petrographic observations show that U-Th and REE minerals commonly occur in association with F- and P-rich mineral assemblages (Fig. 24). This potentially indicates that the magma could have exsolved an F- or P-rich fluid. Such a fluid is also likely to be Na-rich, as evidenced by scapolite alteration of plagioclase in hornblende-biotite tonalites at Crocker Well (Fricke and Conor, 2010). The co-crystallisation of U-, Th- and REE-rich mineralogy suggests that these elements were co-transported. Experimental studies have shown that U and Th will not partition significantly into a magmatic-hydrothermal fluid if H2O is the only volatile phase present (Keppler and Wyllie, 1990). Studies indicate that F, Cl and CO2 are able to form complexes with U over a range of temperatures. Thorium on the other hand only appears to complex with F (Keppler and Wyllie, 1990; 1991). Phosphate complexes are also effective at complexing U at approximately neutral pH conditions up to at least 300oC (see Cuney and Kyser, 2008 and references therein). Observations from a number of deposits worldwide support the experimental data for the transport of REE, U and Ti by fluoride (e.g., Gieré, 1990; Williams-Jones et al., 2000; Gagnon et al., 2004; Agangi et al., 2010), or phosphate (Gieré, 1990; Agangi et al., 2010) complexes. Fluid/melt partitioning is enhanced for U and Th with higher ligand concentrations and at higher oxidation levels (Keppler and Wyllie, 1991; Peiffert et al., 1994; 1996). Together, these experimental and geological constraints suggest that U, Th and REE at Crocker Well were likely transported as fluoride or phosphate complexes.


31

Figure 24: Element map of the same area shown in Figure 6a. Fluorite (showing in F and Ca) is associated with Th and U present in thorite, illustrating the relationship between U-Th minerals and potential ligands. Red = high abundance, blue = low abundance.


32

POTENTIAL DEPOSITIONAL CONTROLS At Crocker Well, only a small proportion of U was directly crystallised from the melt as a magmatic mineral, with the majority of mineralisation hosted by breccias, veins and fractures (Campana and King, 1958; Ashley, 1984). The overall higher F in the breccia suggests that it experienced an influx of volatiles contemporaneously with the introduction of U and Th. Analysis of phlogopite from mineralised breccias and veins reveal a lower activity of F relative to phlogopite in the host granites (Ashley, 1984). If U was transported as a fluoride complex, then lowering F activity would cause precipitation from the fluid. Destabilisation of fluoride complexes may occur via the lowering of F activity due to the crystallisation of fluorite (e.g., via changes in Ca activity) or other F-bearing mineral phases, such as fluorapatite and F-rich biotite. The most likely mechanisms for fluorite deposition are cooling, changes in pressure, dilution of ore fluids, fluid mixing, and pH increase (Richardson and Holland, 1979). Thus, the association between U mineralisation and zones of greater permeability (i.e., faults, breccias, fractures) may serve a double purpose in providing a fluid flow pathway, and as a site where magmatic-hydrothermal fluids may undergo cooling and depressurisation. Alternatively, the influx of a Ca-rich fluid or interaction with Ca-rich rocks may precipitate fluorite, although evidence for these processes has not been observed in this study. Interaction of Na-rich fluids with calcic plagioclase during albitisation will release Ca, which may also trigger U deposition from a fluoride complex. MINERAL SYSTEMS PROCESSES AT CROCKER WELL Based on the above discussion and results from previous studies (especially Ashley, 1984), the following genetic model is proposed for U-Th mineralisation in the Crocker Well Suite:

1. The main granitic rocks were emplaced at a relatively deep crustal level as evidenced by the overall equigranular texture, lack of textural indicators of high-level intrusion (e.g., miarolitic cavities) and anatectic field relationships with the surrounding country rocks (Ashley, 1984; Fricke and Conor, 2010). Uranium and Th were concentrated in the melt during fractional crystallisation

2. A pulse of higher Na magma was injected into the magma chamber, as represented by the high-Na granitic rocks. These were possibly enriched in volatile components, including F and P, and caused volatile saturation and the exsolution of a volatile-rich fluid phase, similar to the model inferred for granite-hosted mineralisation at Ross Adams (Thompson et al., 1982; Thompson, 1988). Alternatively, fluid exsolution may have occurred due to the crystallisation of anhydrous minerals within the melt (i.e., ‘second boiling’; Candela, 1997). An alternative to the separate magma pulse hypothesis is that volatile-rich fluid exsolution altered the main granitic rocks to produce the geochemical characteristics of the high-Na granitic rocks. Exsolution due to pressure release (‘first boiling’) was not an active process, since the Crocker Well Suite is not interpreted to have been emplaced at high crystal levels

3. Uranium, Th and REE were partitioned into the magmatic fluid. The fluid was likely dominated by F and/or P and Cl ligands, as Th is transported together with U (Keppler and Wyllie, 1990; 1991)

4. Fluid release initiated fracturing and brecciation in the host rocks (Phillips, 1972), which acted as fluid flow pathways for the U- and Th-bearing fluid. Since granite emplacement occurred at moderate depths, the extensive hydrothermal alteration commonly associated with shallow-crustal granite-related mineral systems was not produced. Hydrothermal alteration at Crocker Well was limited to zones of increased permeability (i.e., veins and breccias; Ashley, 1984)

5. Uranium deposition occurred as a result of ligand destabilisation. This may have occurred due to a decrease in temperature or pressure within the fractured and brecciated zones, or an increase in Ca activity (e.g., via fluid-rock interaction or mixing of a Ca-rich fluid), or a combination of the above.


33

Conclusions While the processes involved in U mineralisation in many U systems are well understood, the key mineralising processes in magmatic-related U systems are poorly constrained. The granite-hosted Crocker Well U deposit in the Olary Domain of the Curnamona Province presents an excellent opportunity to study the mineral systems processes involved in magmatic-related U systems. From this study, the following conclusions may be drawn: Three geochemical groups of igneous rocks are present within the Crocker Well Suite. Most of

the rocks are members of the main granitic rocks group. Dioritic and granodioritic rocks may represent parental compositions to the main granitic rocks, or may be related to other granitic rocks in the Crocker Well Suite only to a limited extent. The high-Na granitic rocks appear to be only weakly related to the main granitic rocks, and may represent a different magma pulse. The high-Na granitic rocks are associated with U-Th mineralisation

The fluid involved in U mineralisation at Crocker Well was likely to have been moderately saline and NaCl-dominated, with potentially high F and P contents. It was also likely oxidised

Mineralisation is closely associated with permeable structures, such as breccias, veins and fractures. Faults appear to have exerted a spatial control on U distribution

Deposition of U and Th was probably facilitated by the destabilisation of F- and/or P-complexes within the fluid. This likely took place due to temperature or pressure reduction in the more permeable zones, and/or reaction with Ca-bearing rocks or fluids.

These observations suggest that the following mappable criteria may be useful for magmatic-related U exploration: Evidence suggesting the potential to generate a U-bearing magmatic fluid. Such a fluid will

require the presence of suitable ligands; thus F- and P-rich melts are more favourable. These characteristics may also be deduced if the rock has common magmatic-stage fluorite or apatite

Evidence of permeable structures contemporary with igneous activity. These serve a dual function, since they provide fluid flow pathways and act as potential depositional sites where temperature and pressure reductions may occur, and where fluid-rock reaction was most intense, or where Ca-rich fluids may mix with the magmatic fluid. Examples of such features include faults, breccia zones, veins and fracture networks

Evidence of favourable host rocks. If U is transported as a fluoride or phosphate complex, interaction with Ca-bearing lithologies may precipitate fluorite or apatite, resulting in complex destabilisation. Thus, mineralisation may also be found at a distance from the main igneous body. This process was not observed at Crocker Well.

The similarities between Crocker Well and other examples of intrusive-related U mineralisation globally (e.g., Ross Adams, Kvanefjeld), suggest that the processes described above may be applicable to a wide range of settings.


34

Acknowledgements Much appreciation goes to Richard Chopping for his able assistance in the field. The managers of Plumbago and Mount Victor Stations are thanked for allowing land access for fieldwork. Access to drill core samples from Crocker Well was facilitated by the helpful staff at the PIRSA core facility in Moonta. Bill Pappas and Liz Webber are recognised for rapidly processing and analysing geochemical samples. Assistance in fluid inclusion data acquisition and interpretation was provided by Terry Mernagh. Patrick Burke is thanked for assisting with SEM investigations. This record benefited from helpful reviews by David Champion and David Huston.


35

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Cowley, W.M., 2006. Solid Geology of South Australia: peeling away the cover. MESA Journal, 43,

4-15. Cuney, M. and Kyser, K., 2008. Recent and not-so-recent developments in uranium deposits and

implications for exploration. Mineralogical Association of Canada Shortcourse Series, Quebec City, 39.

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36

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38

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39

Appendix 1: Geochemical data Sample coordinates are given in GDA94, Zone 54.

Sam

ple

no.

20100706001

20100706002

20100706003

20100706004

20100706005

20100706006

20100706007

20100706008

20100706009

20100706010

20100706011

20100706012

20100706013

20100706014

Sam

ple

ID

2110181

2110182

2110183

2110184

2110185

2110186

2110187

2110188

2110189

2110190

2110191

2110192

2110193

2110194

Eas

ting

6457050

6457435

6457569

6457556

6457694

6457680

6457660

6456920

6456743

6456958

6456963

6457480

6466023

6466199

Nor

thin

g386755

386726

386718

386728

389852

389890

389787

387028

386927

387025

387021

388853

390810

390632

Gro

up

Fels

ic g

nei

ssH

igh-N

a gra

nitic

roc

kH

igh-N

a gra

nitic

roc

kH

igh-N

a

gra

nitic

roc

kH

igh-N

a gra

nitic

roc

kH

igh-N

a

gra

nitic

rock

Am

phib

olite

Hig

h-N

a gra

nitic

rock

Mai

n g

ranitic

ro

cks

Gra

nite

bre

ccia

Pegm

atite

Hig

h-N

a gra

nitic

roc

kM

ain

gra

nitic

ro

cks

Mai

n g

ranitic

ro

cks

SiO

261.0

573.4

371.5

172.4

072.3

171.9

455.5

172.6

868.4

068.5

473.5

774.5

372.0

771.5

5TiO

20.9

70.1

90.1

10.2

20.3

00.3

21.4

00.2

30.5

00.1

40.0

50.1

40.1

60.2

6Al2

O3

15.6

514.8

013.8

514.4

814.1

814.2

114.8

314.4

214.3

614.7

013.4

313.0

514.1

714.4

9Fe

O2.9

20.4

60.0

80.1

70.4

60.3

03.3

60.2

90.7

30.3

60.0

90.1

20.1

50.6

9Fe

2O

33.4

5<

0.2

70.0

60.1

80.2

10.3

05.0

50.0

82.0

70.5

70.1

30.0

70.9

80.7

8Fe

2O

3to

t6.6

90.2

40.1

40.3

70.7

30.6

48.7

80.3

92.8

80.9

80.2

30.2

01.1

41.5

5M

nO

0.0

3<

0.0

05

<0.0

05

<0.0

05

<0.0

05

<0.0

05

0.0

7<

0.0

05

0.0

1<

0.0

05

<0.0

05

<0.0

05

<0.0

05

0.0

1M

gO

1.7

00.0

40.0

60.3

91.1

50.7

83.9

30.4

60.7

81.9

10.2

20.0

40.4

10.5

5CaO

2.1

30.9

51.0

30.9

11.1

91.0

15.0

00.8

41.2

71.2

00.6

61.1

90.4

30.6

3N

a2O

4.9

87.0

06.8

97.1

36.4

86.4

03.2

36.6

85.3

16.5

06.3

65.8

83.3

14.6

8K2O

3.5

11.5

60.9

51.1

81.5

41.5

02.5

31.5

52.8

41.9

41.1

41.3

94.8

34.2

0P2O

50.5

10.2

50.3

80.2

50.3

80.2

10.5

80.2

20.2

80.4

30.0

50.3

80.1

80.1

3SO

30.0

20.0

10.0

10.0

10.0

10.0

10.0

10.0

10.0

10.0

10.0

10.0

10.0

10.0

1LO

I1.2

80.7

00.5

00.6

20.9

60.8

81.9

40.7

21.1

51.0

80.6

80.6

41.1

80.9

5Tota

l98.5

199.1

595.4

197.9

599.2

097.9

097.8

298.1

997.7

797.4

296.3

897.4

397.8

999.0

1

F3381

278

380

895

3314

2338

7776

1116

2091

5031

638

427

577

731

Be

4.0

05.1

04.5

05.0

05.2

04.9

04.6

05.5

02.9

04.5

04.0

04.9

05.0

06.5

0Sc

5.0

0<

1<

12.0

02.0

04.0

017.0

2.0

07.0

04.0

01.0

0<

17.0

06.0

0V

102

6.0

03.0

09.0

020.0

18.0

201

13.0

37.0

30.0

4.0

03.0

08.0

021.0

Cr

10.0

4.0

04.0

03.0

06.0

08.0

077.0

4.0

04.0

011.0

3.0

04.0

05.0

05.0

0N

i6.0

0<

1<

1<

12.0

02.0

082.0

<1

1.0

04.0

0<

1<

1<

1<

1C

u35.0

5.0

04.0

03.0

04.0

01.0

0<

12.0

015.0

2.0

01.0

06.0

03.0

029.0

Zn

31.0

7.0

06.0

03.0

011.0

9.0

040.0

6.0

015.0

6.0

03.0

07.0

05.0

011.0

Ga

26.9

27.9

27.8

28.1

28.6

26.9

23.6

31.5

21.9

33.5

24.5

25.2

24.2

24.1

Ge

1.1

72.1

12.2

62.0

11.9

91.7

21.1

61.9

01.0

52.2

51.7

62.0

31.6

61.0

9As

0.9

00.8

00.8

00.8

01.0

01.1

01.3

01.0

01.4

01.1

00.9

01.1

01.1

01.2

0Rb

178

36.7

23.6

45.2

93.6

82.1

164

56.8

128

127

36.1

43.9

257

213

Sr

136

51.8

49.6

50.1

47.6

50.6

248

43.3

81.7

46.0

48.5

49.8

51.8

83.9

Y68.2

22.8

41.7

32.9

35.4

18.5

50.8

23.9

51.9

45.1

6.7

038.3

44.1

20.9

Zr

748

139

72.6

192

241

263

426

206

413

146

59.5

111

118

236

Nb

34.2

38.4

30.0

50.3

46.5

51.9

39.1

68.3

27.4

2.0

08.8

049.6

49.7

21.5

Mo

0.6

00.4

01.1

00.5

01.2

00.9

00.7

0<

0.3

0.8

01.0

00.8

00.5

01.8

01.1

0Ag

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

<0.6

Cd

0.2

40.0

70.0

50.1

10.1

60.0

60.2

30.1

70.1

10.0

30.1

10.1

10.0

30.1

0Sn

3.7

00.5

00.3

01.1

01.3

01.8

04.6

01.1

04.1

00.4

0<

0.1

0.8

08.0

02.1

0Sb

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

<0.8

Cs

0.9

10.0

60.0

50.1

50.4

00.4

11.4

90.2

00.5

10.6

10.1

10.0

61.0

10.6

9Ba

792

93.8

72.1

60.1

56.7

53.3

161

69.5

531

69.5

78.5

65.8

276

406

La49.4

12.5

22.1

21.7

20.4

17.4

74.0

14.4

110

18.2

10.0

17.4

26.1

72.3

Ce

171

22.0

50.8

47.3

46.5

43.8

176

33.7

233

47.9

17.3

38.3

58.6

145

Pr17.5

3.3

16.4

65.7

56.1

05.7

221.6

4.1

028.6

5.8

21.9

75.0

36.8

515.1

Nd

70.8

14.7

27.8

23.7

26.9

24.2

83.7

16.7

107

26.8

7.8

221.2

25.4

50.6

Sm

16.4

3.6

86.9

25.1

36.3

04.8

515.1

4.0

819.4

7.6

71.4

15.6

86.0

88.5

4Eu

1.7

60.5

40.6

00.5

40.6

10.5

42.4

00.5

21.8

30.7

20.3

20.6

20.5

51.0

7G

d12.8

3.7

96.4

44.8

55.8

93.6

811.5

3.8

113.3

7.4

21.2

55.5

45.6

25.7

5Tb

2.0

20.5

91.0

60.7

50.8

90.4

91.6

30.5

91.7

11.1

40.1

80.8

71.0

00.7

3D

y11.9

3.7

96.3

64.8

75.7

83.0

89.1

53.8

19.3

76.9

81.1

46.1

06.7

53.9

5H

o2.3

30.7

61.2

70.9

81.1

50.5

61.7

10.7

61.7

71.4

80.2

01.2

41.3

80.6

6Er

6.1

12.1

23.6

42.8

53.1

51.5

94.5

72.1

54.6

14.1

00.6

13.5

74.0

61.7

8Yb

6.3

92.0

43.5

43.0

53.2

21.6

14.7

52.4

54.2

93.7

20.6

73.5

84.5

81.6

3Lu

0.9

50.2

90.4

80.4

80.5

00.2

70.7

30.3

60.6

30.5

80.1

20.5

40.7

30.2

6H

f18.6

3.8

31.9

25.3

96.8

27.2

39.3

86.1

210.6

4.0

21.6

13.2

03.5

66.4

6Ta

2.5

17.3

26.3

97.4

86.0

74.6

82.3

29.2

61.4

30.2

31.3

48.5

65.6

81.6

5Pb

11.0

10.7

46.5

21.7

18.6

163

15.7

4.8

311.0

26.1

8.6

117.9

9.6

612.8

Bi

0.2

00.0

30.0

40.2

20.2

10.0

50.1

20.0

50.0

70.1

80.1

20.0

50.0

30.0

5Th

224

50.8

33.5

6.4

7164

46.8

28.8

7.8

994.8

482

13.9

40.9

28.8

56.1

U14.6

5.7

86.5

25.7

05.4

827.5

6.6

82.3

84.0

85.1

92.3

45.2

65.2

15.8

3

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