The Basil Cu–Co deposit, Eastern Arunta Region, Northern Territory, Australia: A metamorphosed volcanic-hosted massive sulphide deposit

Ore Geology Reviews 56 (2014) 141–158

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The Basil Cu–Co deposit, Eastern Arunta Region, Northern Territory,Australia: A metamorphosed volcanic-hosted massive sulphide deposit

Kelly Ann Sharrad a,⁎, Jim McKinnon-Matthews b, Nigel J. Cook a, Cristiana L. Ciobanu a, Martin Hand a

a Centre for Tectonics, Resources and Exploration, School of Earth and Environmental Sciences, University of Adelaide, 5005 SA, Australiab Mithril Resources Ltd., 58 King William Road, Goodwood, 5034 SA, Australia

⁎ Corresponding author.E-mail address: [email protected] (K.A. S

0169-1368/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.oregeorev.2013.08.008

a b s t r a c t
a r t i c l e i n f o
Article history:Received 20 March 2013Received in revised form 2 August 2013Accepted 14 August 2013Available online 3 September 2013

Keywords:Cu–Co depositHarts RangeMetamorphic overprintingSulphide petrographyOre genesisLarapinta EventVolcanic-hosted massive sulphide deposit

The Basil Cu–Codeposit, Harts Range, central Australia, is hosted by the RiddockAmphibolite, a sequence that hasbeen metamorphosed at upper-amphibolite- to granulite-facies conditions at 480–460 Ma (Larapinta Event),and subsequently reworked at amphibolite-facies conditions (450–300 Ma). As a result, many of the primarymineralization textures and other features that could characterise ore genesis have been obliterated. However,preserved textures and mineral relationships in the mineralized zone, allow some constraints to be placed onthe genetic history of the deposit using mineralogical, petrographic and geochemical studies of host rocks andsulphides.Results of this study permit at least two geneticmodels to be ruled out. Firstly, whole rock geochemistry and gar-net compositions suggest that the deposit is not a skarn system. Secondly, the lack of any significant Ni-signature,and the presence of abundant zircons in the host amphibolite (indicating that not all host rocks aremafic in com-position and/or magmatic in character), make an orthomagmatic Ni–Cu–(PGE) system unlikely. Alternatively,Basil is assigned to a volcanic-hosted massive sulphide (VHMS)-style of mineralization, formed on the seafloor,within basaltic and sedimentary host rocks, typical of deposits occurring in such settings. The lack of arecognisable hydrothermal alteration zone is consistent with either destruction of the alteration zone duringmetamorphism or detachment of the ore from alteration during later deformation.The occurrence of sulphide inclusions within garnet and amphibole indicates that the sulphides must be syn-metamorphic or earlier. Partitioning of trace elements between pyrite and co-existing pyrrhotite suggests that(re)crystallization occurred under equilibrium conditions. The composition of sphalerite coexisting with pyriteand pyrrhotite indicates crystallization at pressures of at least 10 kbar, consistent with peak metamorphismduring the Early Ordovician Larapinta Event. Zr-in-titanite geothermometry indicates peak temperatures of730–745 °C.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Contemporary exploration models that link the common char-acteristics of a certain ore type depend upon an understanding of themechanisms of ore formation and how these relate to the geological en-vironment in which they occur. A common problem in understandingthemetallogeny ofmineralizedmetamorphic terranes is uncertainty re-garding the timing of mineralization relative to tectonic and magmaticevents. Deformation and fluid–rock interaction associated with meta-morphism can destroy original deposit geometry or can result inremobilization of the ore and gangue minerals obscuring primary oretextures (Spry et al., 2000). In extreme cases, metamorphic overprintingmay also make it very difficult, or often impossible to determine theoriginal genetic type, the processes bywhich themineralization formed,and indeed whether initial deposition predates or is synchronous withmetamorphism (metamorphosed vs. metamorphogenic deposits; Spry

harrad).

ghts reserved.

et al., 2000, and references therein). TheBasil Cu–Codepositwasdiscov-ered by Mithril Resources Ltd. (Mithril; www.mithrilresources.com.au)in 2008. Themineralization is hostedwithin the RiddockAmphibolite ofthe Harts Range Group of the Irindina Province, in the Arunta Region ofcentral Australia. The deposit has undergone significant syn- to post-deposition tectonic and metamorphic overprinting, and prior to thisstudy, ore genesis processes were poorly constrained.

A key question demanding clarification is whether the Basil deposithas experienced the high-grade Early Ordovician Larapinta Event (480–460 Ma). If so, it should be considered whether ore genesis is related tomagmatism at ca. 520–500 Ma, or alternatively, to hydrothermal pro-cesses during the Larapinta Event itself. Conversely, if mineralizationwas not subjected to the Larapinta Event, consideration is required asto whether the deposit formed post-Larapinta, or was remobilizedfrom an earlier location. The difference between these two scenarioshas important implications for regional exploration models (i.e., strati-graphic vs. structural).

Preliminary work by Mithril has shown that the Basil deposit hascharacteristics consistentwith several conventional ore deposit models.

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142 K.A. Sharrad et al. / Ore Geology Reviews 56 (2014) 141–158

These include: submarine volcanic-hosted massive sulphide (VHMS)deposits, in which sulphides were formed syngenetically on the oceanfloor (Franklin et al., 2005); skarn deposits, where deposits are formedthrough replacement and alteration of limestone and calc-silicaterocks by hydrothermal fluids (Meinert, 1992); and Ni–Cu–(PGE)deposits, formed by segregation and concentration of liquid sulphidedroplets from mafic or ultramafic magma (Naldrett, 1999). No datahave yet been presented to date that would allow the deposit to beconfidently classified into either of these categories.

In this study, petrology and mineral chemistry of sulphides (pyrite,pyrrhotite and sphalerite) and host silicates are used to constrain pro-cesses involved in ore formation, their relative timing and conditions.We also address the impacts that metamorphism and deformationhave had on the ore system and whether there are structural controlson mineralization. Following characterization of the mineralization,specific genetic models are critically evaluated based on deposit geom-etry, whole rock geochemistry and mineral compositional data. Estab-lishment of a genetic model for Basil will assist in the developmentof conceptual models to be applied in future mineral exploration inthe eastern Arunta region, a region with considerable potential yet atpresent under-explored (Whelan et al., 2013).

2. Geological setting

2.1. Regional geology

The Neoproterozoic to Early Palaeozoic Harts Range Group (HRG) islocated in the Irindina Province in the southeastern part of the AruntaRegion, central Australia (Buick et al., 2005, 2008; Hand et al., 1999a;Maidment et al., 2013; Mawby et al., 1999; Scrimgeour, 2003;Scrimgeour and Close, 2011) (Fig. 1a). The oldest rocks in the HRGconsist of Palaeoproterozoic igneous and metamorphic rocks (e.g.,Maidment et al., 2005; Wade et al., 2008) that appear to correlatewith rocks of the Aileron Province to the west and northwest.Palaeoproterozoic rocks of the Aileron Province are structurally overlainby a succession of upper amphibolite- to granulite-facies metamorphicrocks that comprise the HRG (Buick et al., 2005; Joklik, 1955;Maidment et al., 2013). The HRG can be divided into two packages;metasedimentary units (divided into the Naringa, Stanovos, Irindina

Fig. 1. (a) Location and regional geology of the Harts Range area, modified after Miller et al. (19deposit. (b) Location of the Irindina Sub-basin at the time of the deposition of sediments (~530Inlier; CB— Canning Basin; GB— Georgina Basin; MI — Musgrave Inlier; OB— Officer Basin.Adapted from Buick et al. (2005).

and Brady Units), and locally voluminous mafic igneous units (Buicket al., 2005; Maidment et al., 2013) belonging to the Harts RangeMeta-igneous Complex of Sivell and Foden (1985), Claoué-Long andHoatson (2005) and Maidment et al. (2013). The Riddock Amphiboliteis the major part of the Harts Range Meta-igneous Complex, and in-cludes metabasic, meta-anorthositic and meta-ultrabasic rocks.

The Palaeoproterozoic rocks that underlie theHRG are dominated byfelsic orthogneisses and form a Palaeoproterozoic (Wade et al., 2008)basement to the Neoproterozoic–Cambrian HRG, which consists ofmetamorphosed sedimentary and mafic volcanic rocks (Buick et al.,2005). The HRG and the underlying Palaeoproterozoic rocks are dividedby a high-grade, low-angle shear zone, the Bruna Detachment Zone(Mawby et al., 1999), which was referred to earlier as the Harts RangeDetachment Zone by Ding and James (1985).

The protoliths to the HRG are interpreted to have been deposited inan E–W to SE–NW-trending rift, the Irindina sub-basin (Fig. 1b), whichwas located between the present-day Amadeus and Georgina basins(Buick et al., 2005; Maidment et al., 2013). Detrital zircons fromthe Irindina Gneiss have yielded a range of ages from c. 1300 Ma toc. 520 Ma (Buick et al., 2001, 2005; Maidment et al., 2013), givinga maximum depositional age of 520–500 Ma for deposition of thesedimentary and mafic igneous precursors of the HRG (Buick et al.,2005; Maidment et al., 2013). Mafic magmatism is considered to beboth extrusive and intrusive (Maidment et al., 2013), and may haveformed part of a larger magmatic province at ca. 520–500 Ma (Buicket al., 2005). Additional constraints are provided by the 480–460 Maages for metamorphic minerals (zircon, monazite, garnet) in theserocks.

The age of the Riddock Amphibolite is suggested to be Early toMiddle Cambrian in age (Buick et al., 2001). Metamorphic overgrowthrims on zircon from volcaniclastic units within the Harts RangeMetaigneous Complex yield ages of ~470 Ma, consistent with highgrade metamorphism associated with the Larapinta Event (Buick et al.,2001, 2005; Hand et al., 1999a,b; Mawby et al., 1999).

2.2. Metamorphism

Two major tectonic and/or magmatic events are recognised in theHRG. The first phase of high-grade metamorphism corresponds to the

97), Hand et al. (1999a) and Buick et al. (2001). Black star marks the location of the BasilMa) that are the protoliths to the Harts Range Group. AB— Amadeus Basin; AI— Arunta

143K.A. Sharrad et al. / Ore Geology Reviews 56 (2014) 141–158

480–460 Ma Larapinta Event (Buick et al., 2005; Maidment et al., 2013;Mawby et al., 1999), which involved regional amphibolite- to granulite-facies metamorphism of the HRG (Buick et al., 2005; Maidment et al.,2013; Mawby et al., 1999; Miller et al., 1997). The 450–300 Ma AliceSprings Orogeny (ASO; Collins and Teyssier, 1989), which resulted inupper-amphibolite-facies metamorphism and exhumation of the HRG(Buick et al., 2008; Haines et al., 2001; Hand et al., 1999b; Mawbyet al., 1999).

Sedimentary andmafic volcanic rocks of the HRGwithin the Irindinasub-basin were metamorphosed at amphibolite–granulite facies duringthe 480–460 Ma Larapinta Event (Buick et al., 2005; Maidment et al.,2013). Peak metamorphic conditions were 8–10 kbar and 800–850 °C(Hand et al., 1999b;Mawby et al., 1999; Miller et al., 1997). Subsequentto peak metamorphism, decompression and cooling to 6 kbar and~700 °C (Mawby et al., 1999; Miller et al., 1997) was associated withthe formation of recumbent, mylonitic shear fabrics that developed dur-ing NE–SW-directed tectonic movement during or after the LarapintaEvent (Hand et al., 1999b; Mawby et al., 1999). The Larapinta Eventwas also accompanied by the intrusion of mafic to ultramafic dykes,plugs and sills, and the deposition of fine-grained sedimentary rocksin the surrounding Amadeus and Georgina basins (Hand et al, 1999a;Mawby et al., 1999).

Between the Larapinta Event and the ASO, the prevailing tectonicregime appears to have switched from regionally extensional to com-pressional, which resulted in the south-directed transport of highlymetamorphosed HRG units over Palaeoproterozoic units along theBruna Detachment Zone (Mawby et al., 1999). Upper-amphibolite-facies mylonitic fabrics within the Bruna Detachment Zone give anSm–Nd age of 449 ± 10 Ma (Mawby et al., 1999).

The ASO consisted of a long-lived period of intra-plate reworking(Buick et al., 2001; Haines et al., 2001; Hand et al., 1999a) that attainedpeak metamorphic P–T conditions of 6 kbar and 600 °C (Hand et al.,1999a). The ASO involved long-lived, episodic intra-plate reworking(Buick et al., 2008; Haines et al., 2001; Raimondo et al., 2011). TheASO was responsible for the exhumation of amphibolite- to granulite-facies rocks of the eastern Arunta Region, which are preserved withina polyphase, predominantly south-vergent, thrust system (Buick et al.,2008). This tectonismmarked the end of tectonic activity in the AruntaRegion. The cooling history of the southeastern Arunta Region from 400to 300 Ma during the ASO suggests that exhumation of the AruntaRegion occurred in a series of pulses, with relatively rapid exhumationfirst occurring at 400 Ma and again at approximately 350 and 310 Ma(Dunlap et al., 1995; Hand et al., 1999a).

2.3. Geological setting of the Basil deposit

Awide variety of differentmineralization types have been identifiedby Mithril Ltd. within its tenement package in the eastern AruntaRegion. These include prospects grouped as magmatic-hosted Ni–Cu–PGE sulphides, vein-hosted gold mineralization, Iron-Oxide–Copper–Gold (IOCG) mineralization, and Cu–Co mineralization of uncertain ge-netic type (Fig. 2a; see also Whelan et al., 2013).

The Cu–Codominant Basil deposit, of uncertain genetic type, is locat-ed in the centre of the tenement area (Fig. 2b), and is one of the largestmineral deposits identified in the eastern Arunta Region. Surfaceminer-alization, in the form of gossanous horizons highly anomalous in Cu–Co ± Zn ± Ag, within the amphibolite host (Fig. 3) has been identifiedover 10 km of strike, and is up to 50 m in thickness. Diamond and re-verse circulation drilling completed by Mithril over the southern por-tion of the Basil mineralization has identified a JORC-compliantinferred resource estimated at 26.5 Mt @ 0.57% Cu, 0.05% Co (MithrilResources Ltd., 2012).

The Basil Cu–Co deposit is hosted in the Riddock Amphibolite(Figs. 1a and 3). The Riddock Amphibolite (Shaw et al., 1991; Sivelland Foden, 1985), belongs to the Harts Range Metaigneous Complex(Maidment et al., 2013), and is traceable for N120 km within the HRG

(Fig. 1a). Maidment et al. (2013) note that the mafic units in the HartsRange Metaigneous Complex are not exclusively intrusive but also in-clude extrusive or volcaniclastic units; some massive amphiboliteunits have not yielded any significant zircon. The deposit is associatedwith Fe-rich metapelites, gneisses, with marble, calc-silicate rock andvoluminous metabasic rocks (Maidment et al., 2013). The mineraliza-tion at Basil is proximal to the regional-scale Basil Fault (Korsch et al.,2011). This may correspond to the Bruna Detachment Zone, which hasbeen interpreted by Korsch et al. (2011) from a deep seismic reflectionsurvey data (Nakamura et al., 2011) to sole out at a depth of ~10 kmdepth, eventually meeting the steeper Mount Mary Fault zone to thenorth, which is interpreted to continue to the base of the crust(Moho) (Fig. 4). It is unknown if the Basil Fault has any control on themineralization (Fig. 2a, b). Drilling of the mineralization shows thatmineralization comprises disseminated to semi-massive sulphidescomprising pyrite–pyrrhotite–chalcopyritewith subordinate sphalerite.

The morphology of the Basil deposit itself is irregular on individualcross-sections but generally stratiform. Mineralization mapped at sur-face also appears to suggest the mineralization could be stratabound,i.e., restricted to garnet-bearing amphibolite and capped by amphibo-lites in which garnet is rare or absent. Fig. 5 shows a detailed log ofdrill hole LB035DD, with lithologiesmarked and samples located. An in-terpretation of the relationship between zones of mineralization in thisdrill hole and that in drill hole LB027DD based on assay results is shownschematically on Fig. 6. This shows, broadly, the presence of two strong-ly mineralized zones, each up to 50 m in thickness, and one parallel,weakly-mineralized zone in the hanging wall, all dipping to the NW at~50°.

Unpublished consultant reports for Mithril havemade some sugges-tions regarding possible genetic models. In one petrographic study, theBasil deposit was tentatively interpreted as a skarn based on the abun-dance of garnet-rich amphibolite. A second report suggested that themineralization represented a metamorphosed magmatic Cu-sulphidedeposit, comparable to others in the same tenement area.

3. Methodology

Thirty-seven samples were collected from two diamond drill holes(LB027DD and LB035DD) intersecting intervals of mineralization. Thedrill cores were logged to define lithological units and the samplesprepared as either 1″ polished blocks to study the ore mineralogy, orpolished thin sections to study the host rock mineralogy (Appendix A).Optical and Scanning Electron Microscopy was used to obtain mineral-ogical and textural information. Representative whole rock major- andtrace-element data was obtained using ICP-MS or ICP-AES at GenalysisLaboratory Services (Intertek), Adelaide. Analytical details are given inSharrad (2012).

Electron microprobe analysis of pyrrhotite, sphalerite, and garnetwas undertaken using a CAMECA SX-51 instrument with wavelengthdispersive spectrometers (Adelaide Microscopy, University of Adelaide).Work focussed on determining the compositional variation in pyrite,pyrrhotite and sphalerite, since the composition of these minerals canyield valuable information about equilibrium crystallization and, poten-tially, conditions of peak metamorphism. Analysis transects of garnetwere undertaken to determine the presence or absence of metamorphicgrowth zonation or modified patterns of compositional zoning withinsingle grains.

Laser-Ablation Inductively Coupled Plasma Mass Spectroscopy(LA-ICP-MS) spot analysis of pyrite and pyrrhotite was undertakenusing the Agilent HP-7500 Quadrupole ICPM instrument at AdelaideMicroscopy. The instrument is equipped with a New Wave UP-213 Nd:YAG laser ablation system equipped with MeoLaser 213 software. Datareduction was performed using Glitter software (Van Achterberghet al., 2001). Measurements were recorded for isotopes Ag, Au, Bi, Co,Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Te, Tl, V and Zn.

Fig. 2. (a) Sketchmap showing the location ofMithril Resources' tenementswithin theHarts Range. Baldrick andBlackadder aremagmatic-hostedNi–Cu sulphide deposits. Yoda and Tibbsare vein hosted Au deposits. Basil is an unclassified Cu–Co deposit. Adapted from Mithril Resources Ltd. (2012). B) Location of the drilling programme executed on the Basil prospect byMithril Resources. LB027DD and LB035DD are the drill holes from which all samples were selected. These drill holes are located in Rotten Hill, which was the first piece of evidence thatthere was mineralization on this tenement. Adapted from Mithril Resources Ltd. (2012).


LA-ICP-MS elementmappingwas undertaken on representative gar-net grains from garnet-rich samples. Mapping was conducted using aResonetics M-50-LR 193-nm Excimer laser microprobe coupled to anAgilent 7700cx Quadrupole ICP-MS instrument. This same instrumentwas used for the determinations of Zr in titanite. TheNIST-610 referencestandard was used with Ca as the internal standard and theMass-1 ref-erence material (Wilson et al., 2002) for calibration. The methodologyfor this instrument has been outlined elsewhere (e.g., Cook et al., 2013).

4. Results

4.1. Lithology

Fourmain lithological-(structural) units, all belonging to the RiddockAmphibolite (Table 1) are observed in the drill holes intersectingminer-alization: amphibolite (Fig. 7a–b); garnet-rich amphibolite (Fig. 7c–d);

clinopyroxene–magnesiohornblende schist (Fig. 7e–f); and weakly-foliated amphibolite (Fig. 7g). The abundance and diversity of mineralsvary within each of these units. A superimposed chlorite/calcite alter-ation assemblage (Fig. 7h) is restricted to the amphibolite and weakly-foliated amphibolite lithologies.

In the zone of mineralization, the gangue mineral assemblageincludes magnesiohornblende and andesine (confirmed by scanningelectron microscopy energy-dispersive X-ray analysis), quartz, garnet,hematite, ilmenite, and magnetite. Sulphides comprise as much as 60%of the sampled rocks by volume. Pyrite and pyrrhotite are present inapproximately equal abundance in many samples; others are eitherpyrite- or pyrrhotite-dominant. There are also very distinct boundariesbetween pyrite- and pyrrhotite-dominant zones, notably in a transitionfrom pyrite- to pyrrhotite-dominant mineralization in drill holeLB035DD (Fig. 8a). Chalcopyrite comprises up to 3% of the total sul-phides. The sulphides tend to be disseminated throughout the rock,

image of Fig.�2

Fig. 3. (a) Photograph showing LB035DD looking along strike of the drill hole towards SW. (b) Photograph showing the Riddock Amphibolite in outcrop close to drill holes LB027DD andLB035DD. (c) Photograph of outcrop of Riddock Amphibolite with coarse-grained garnet. (d) Photograph showing the surface expression of themineralization zone as a gossan. This pho-tograph was taken at Rotten Hill, where the two drill holes of interest are located.


locally giving it a semi-massive appearance. Rounded, compositefragments, up to 15 cm in diameter, are commonly observed inheavily mineralized sections of the core (Fig. 8b). These are dominat-ed by a medium- to coarse-grained magnesiohornblende–magnetiteassemblage and more rarely are quartz dominant. These fragmentsbear little or no resemblance to the host amphibolite and their sourceis unknown. Thin (0.5–1.0 cm), carbonate- or quartz-dominant vein-ing, which also hosts sulphides (pyrite, pyrrhotite and chalcopyrite)cross-cuts (at 60–90°) the dominant foliation defined by metamor-phic minerals such as hornblende. This suggests emplacement orremobilization of at least part of the mineralization during the latestages of metamorphism (Fig. 8c). The late veins could also signifi-cantly post-date metamorphism based on the textural evidencealone.

4.2. Mineralogy and petrography

Hornblende in the amphibolite is brown or green in colour, withSEM-EDAX analysis indicating that compositions corresponding to

Fig. 4.Migrated seismic section fromGeorgina–Arunta deep seismic reflection line 09GA-GA1 shThe Basil Prospect (black star) is in direct contact with the Basil Fault.

magnesiohornblende [Mg/(Mg + Fe) = 0.6–0.8] are dominant. Biotiteoccurs in some samples, but is generally rare. Titanite is abundant, fine-grained (b100 μm), and occurs as inclusions in allminerals but is partic-ularly concentratedwithinmagnesiohornblende (Fig. 9a, b). Zircon alsooccurs as inclusions in all minerals, and is generally finer-grained thanthe titanite (Fig. 9b, c).

The clinopyroxene–amphibole schist contains clinopyroxene,magnesiohornblende, quartz, andesine, titanite, with minor scapolite,orthopyroxene, calcite and dolomitic carbonate. Scapolite is rare andwas only seen in one of the samples (KS-3). In hand sample, this unit dis-plays a distinct fabric definedby clinopyroxene andmagnesiohornblende.In terms of mineral abundance, of the two samples of clinopyroxene–amphibole schist, one (KS-3) is rich (25 vol.%) in clinopyroxene, andthe other (KS-18) rich (15 vol.%) in orthopyroxene.

Theweakly-foliated amphibolite is similar in composition to the am-phibolite. Main minerals include magnesiohornblende, clinopyroxene,andesine, titanite and quartz. Clinopyroxene is observed in all samplesof weakly-foliated amphibolite. Titanite occurs as single grains up to300 μm in length.

owing interpretation and key provinces, adapted and simplified fromKorsch et al. (2011).

image of Fig.�3

image of Fig.�4

Fig. 5. Descriptive drill core log (LB035DD), with lithologies, presence and abundance of sulphides, structural information and sample locations (KS-14 to -36). Abbreviations:cpy — chalcopyrite, po — pyrrhotite, py — pyrite.


The garnet-rich amphibolite consists of up to 80% garnet, plus lessermagnesiohornblende, pyrrhotite and chalcopyrite, quartz as inclusionsin garnet and small (~1%) amounts of late-stage chlorite. Garnet grains

Fig. 6. Diagrammatic interpretation of the zones of mineralization that present in drill holes LBmineralization.Cross-section adapted from Mithril Resources Ltd. (2012).

are coarse, ranging from 2 to 3 mm in diameter, rarely asmuch as 5 cm.They contain inclusions of quartz, magnesiohornblende and sulphides.Apatite is aminor accessory phase (Fig. 9d). There is no optical zonation

027DD and LB035DD. Drill hole LB046DD, furthest to NE, only picks up a narrow zone of

image of Fig.�5

image of Fig.�6

Table 1Summary description of lithological unit in drill holes LB027DD and LB035DD.

Lithological unit Minerals and abundance Grain size Comments

Lithological unit Assemblage and relative abundance Grain size Comments

Amphibolite Magnesiohornblende (40%) Coarse-grained Magnesiohornblende varies from brown to green in colour, suggesting varyingMg/(Mg + Fe). Titanite is pervasive, occurring as inclusions in all minerals.Minor sulphides (mostly pyrite or pyrrhotite) are present. Minor garnet is notedin some samples. Zircon is included in magnesiohornblende and in titanite.

Andesine (25%) Medium-grainedQuartz (10%) Medium-grainedAccessory minerals:Titanite (10%) Fine-grainedIlmenite (10%) Fine-grainedZircon (3%) Very fine-grainedApatite (2%) Very fine-grainedRutile (1%) Very fine-grained

Garnet-rich amphibolite Garnet (60%) Very coarse-grained Magnesiohornblende is green. Quartz,magnesiohornblende and sulphides occuras inclusions in garnet. There is no evidence of optical zonation or orientation ofgrains. Sulphides often form a matrix enclosing garnet.

Magnesiohornblende (20%) Coarse-grainedQuartz (7%) Fine-grainedAccessory minerals:Ilmenite (10%) Medium-grainedApatite (2%) Very fine-grainedChlorite (1%) Fine-grained

Clinopyroxene–amphibole schist Magnesiohornblende (25%) Coarse-grained Clinopyroxene and brown–green magnesiohornblende are present in equalabundance. Titanite is pervasive and present as inclusions in all minerals; zirconis abundant.

Clinopyroxene (25%) Medium-grainedAndesine (15%) Medium-grainedOrthopyroxene (15%) Medium-grainedQuartz (10%) Fine-grainedAccessory minerals:Titanite (10%) Fine-grainedScapolite (2%) Fine-grainedDolomitic carbonate (2%) Medium-grainedCalcite (1%) Medium-grained

Weakly-foliated amphibolite Magnesiohornblende (40%) Coarse-grained Very weak foliation. Magnesiohornblende is brown. Titanite is abundant andfound as inclusions in all minerals.Clinopyroxene (20%) Medium-grained

Andesine (20%) Medium-grainedQuartz (10%) Fine-grainedAccessory minerals:Titanite (10%) Fine grained

Carbonate/chlorite altered rock Chlorite (60%) Coarse-grained Both calcite and chlorite are homogeneous and pervasive. Abundant magnetiteis fine-grained.Calcite (30%) Medium-grained

Magnetite (10%) Fine-grained


or evidence of retrograde reactions between the garnet and other min-erals. The presence of irregularly-distributed magnesiohornblende in-clusions infers co-crystallization of the garnet-hornblende assemblageduring metamorphism.

Euhedral pyrite mostly occurs in the pyrrhotite-rich samples(Fig. 10a); these grains are rimmed by pyrrhotite and chalcopyrite.Chalcopyrite and pyrrhotite are often seen intergrownwith one another(Fig. 10a).

Chalcopyrite is usually only found in association with pyrite or assmall, isolated blebs in the amphibolite. There also appears to be a sec-ond generation of pyrite (Fig. 10b) which is in very low abundance(b0.5 vol.%) throughout the samples. A later pyrite occurs as a rimaround other minerals, including magnetite, and is very fine-grainedwith respect to the surrounding minerals. It is distinguishable fromthe earlier generation of pyrite due to: (i) its lack of euhedral morphol-ogy; (ii) it does not occur as blebs in pyrrhotite-rich samples; and (iii) itdoes not have a massive, matrix-supporting texture in pyrite-richsamples.

Most importantly, in more sulphide-rich, semi-massive samples, thegangue minerals appear to be enclosed within coarse-grained sulphide(Fig. 10c). Exsolution of ilmenite within hematite is also observedwithin the zone of mineralization (Fig. 10d). Pyrrhotite, chalcopyrite,hematite, ilmenite and quartz form inclusions within garnet (Fig. 10e).Although the pyrrhotite and hematite do not share mutual boundaries,their presence, in the same grain, indicates evolution of fO2/fS2 duringgarnet growth. The presence of sulphide inclusions indicates that theywere present during garnet growth. Mineralized samples containminor amounts of baryte, generally filling fractures within pyrite, aswell as sub-200 μm-sized sphalerite and consistently very fine-grained (b10 μm), but relatively abundant, molybdenite (Fig. 10f). No

coarser molybdenite was observed which may have been suitable forRe–Os geochronology.

4.3. Whole rock geochemistry

Whole rock geochemical analysis was carried out on two samples ofeach rock unit in an attempt to define the presence or absence of hydro-thermal alteration. Although complicated by the lack of unequivocallyunaltered rocks in the available sample suite, comparison of major ox-ides and trace elements in the different units, including fresh amphibo-lites, indicated no substantial differences among them, suggesting thatthere is no obvious alteration halo present. The fresh amphiboliteswere taken close to the tops of drillcores, i.e., well above themineralizedintervals. Notably, there is no indication of any significant calcic, potas-sic or sodic alteration in the rocks enclosing the ores. Rather, theminer-alization zone is characterised by amarked decrease inmobile elementssuch as Na, K and Sr, but only negligible change in characteristically im-mobile elements such as Zr and Ti. Geochemical differences betweenthe host rock and the mineralization zone are summarized in Table 2.We also include data from Hoatson et al. (2005) for samples ofunaltered Riddock Amphibolite, which do not significantly differ fromthe rocks reported here, including for oxides often associatedwith alter-ation (Na2O, K2O, CaO).

4.4. Electron probe microanalysis (EPMA)

The stoichiometry of pyrrhotite in terms of the Fe:S ratio was deter-mined by electron probemicroanalysis (EPMA) since the Fe:S ratio can,in some cases, be used to provide information on the crystallization his-tory of a rock in the context of the system Fe–S (e.g., Becker et al., 2010).

Fig. 7. Transmitted light photomicrographs illustrating important silicate and sulphide mineral textures. (a) The occurrence of titanite as inclusions in magnesiohornblende (KS-15).(b) Brown and green hornblende (KS-2). (c) Sulphides forming a matrix around garnet grain (KS-37). (d) Inclusions of sulphide, hornblende and quartz in garnet grain(KS-36). (e) Relatively coarse-grained mineral assemblage of clinopyroxene, hornblende and quartz (KS-3). (f) Occurrence of zircon (KS-18). (g) Occurrence of clinopyroxene (KS-4).(h) Chlorite alteration accompanied by magnetite (KS-8). Abbreviations: Chl = chlorite; Cpx = clinopyroxene; Grt = garnet; Hbl = hornblende; Mag = magnetite; Qtz = quartz;Sul = sulphide; Ttn = titanite; Zrc = zircon.


The dataset based on the analysis of 43 grains in six representative sam-ples (Table 3) shows that the composition is approximately Fe0.9S (i.e.,Fe8S9), with remarkably little variation within, or between texturallydistinct sub-populations. The composition of pyrrhotite occurring as in-clusionswithin garnet is remarkably similar to that of the pyrrhotite oc-curring freely in the ore.

EPMA was performed on sphalerite to determine the Fe content. Ifbuffered by both pyrite and pyrrhotite, as is the case for the Basil depos-it, the mol% FeS content of sphalerite can be an efficient geobarometer(Scott, 1973, 1976), giving an estimate of the pressure conditions atwhich the sphalerite (and implicitly other sulphides) formed orrecrystallized. The mol% FeS content of sphalerite in 12 grains from

image of Fig.�7

Fig. 8. (a) Distinct boundary between coarse-grained pyrite (left) and fine-grained pyr-rhotite (right). (b) Disseminated texture of sulphides. The amphibolite (dark grey) has asulphide matrix. Red circle highlights a small, rounded fragment. (c) Late calcite–pyrite–pyrrhotite vein cross-cutting the foliation of the amphibolite.


four samples ranges from ~10 mol% up to N25 mol% (Table 4). Thesphalerite geobarometer is, however, only temperature-independentup to ca. 600 °C, which is lower than any estimate that might be madeon the basis of the gangue assemblage (hornblende–clinopyroxene–orthopyroxene) in the clinopyroxene–amphibole schist. However, as-suming that therewill have been some re-equilibration of the sphaleritedown to about 600 °C, the lower mol% FeS values (9–13 mol% FeS) arestill consistent with pressures exceeding 10 kbar. The sphalerite withhigher mol% FeS (N15 mol% FeS) most likely represents further equili-bration upon retrograde cooling, or alternatively, the lack of equilibriumbetween sphalerite, pyrite and pyrrhotite (Cook et al., 1994). Fig. 11shows individual mol% FeS values are superimposed on the FeS vs. tem-perature diagram of Scott (1976) at a nominal 580 °C. The sphaleritecontains few other substituted elements. Cadmium, a common minor

constituent of sphalerite, was typically from below theminimumdetec-tion limit (bmdl) up to 0.24 wt.% (Table 4).

EPMA data for 120 garnet grains (Table 5) indicates significant al-mandine and pyrope components (~60 mol% almandine content,~30% pyrope, ~10%grossular and veryminor spessartine), compositionsconsistent with their appearance and paragenesis, strongly indicatingthat these are metamorphic garnet. Skarn garnets are, in contrast,typically calcic with high grossular (Ca–Al) and andradite (Ca–Fe) com-ponents (Meinert, 1992). There is negligible variation in major compo-nents across the four large garnet grains analysed and there is no core-to-rim compositional variation.

4.5. Laser-Ablation Inductively-Coupled Plasma Mass Spectroscopy(LA-ICP-MS) spot analysis

LA-ICP-MS multi-element spot analysis of pyrite and pyrrhotite infour representative mineralized samples was undertaken to identifywhich trace and minor elements are present in these minerals and, ifso, whether they provide additional petrogenetic information. The fullLA-ICP-MS dataset is given in Sharrad (2012). Pyrite contains up to7300 ppm Co and low but detectable concentrations of Mn, Ni and Se;most other trace elements were at or close to their minimum detectionlimits (Table 6). Variation in Co within and between samples canbe attributed to a subtle compositional zoning. Flat time-resolveddepth profiles for Co suggest the element is present in the crystal lattice,consistent with solid solution in the system FeS2–CoS2–NiS2 (Hawleyand Nichol, 1961). Cobalt, Ni and Mn are all elements commonlyfound substituting for Fe in pyrite, or for S in the case of Se(Winderbaum et al., 2012). Upon metamorphic recrystallization, incor-porated trace and minor elements are commonly released (Cook et al.,2013).

Comparative analysis of pyrrhotite in six samples shows that a sim-ilar range of trace elements is present: Co (although lower than in py-rite); and minor Mn and Se. The main exception in comparison withpyrite is Ni, which occurs at concentrations up to 645 ppm (Table 7).There is some variation within and between samples, but in general,the Ni concentration is fairly homogeneous. The flat time-resolveddepth profiles for Co and the other elements suggest that all these ele-ments are present in the sulphide lattice.

Fig. 12 depicts the contents of Ni and Co in pyrite and pyrrhotite. Thetwo Fe-sulphides define two very distinct groups: pyrite is enriched inCo (up to 7000 ppm) with low Ni content (b100 ppm); and pyrrhotiteis enriched in Ni (up to 700 ppm) and only moderately enriched in Co(up to 1000 ppm).

4.6. LA-ICP-MS element mapping

LA-ICP-MS element maps were obtained for two selected garnetgrains in sample KS-37 to determine if there was any subtle composi-tional zoning that was not identified by the EPMA study. Specifically,mapping targeted trace elements likely to be concentrated in the rimof the garnet relative to the core, and which might reflect fluid evolu-tion. The maps (Fig. 13) show that the Heavy Rare Earth Elements(HREE; Dy, Er, Lu, Y and Yb), as well as Cr and V, are notably enrichedin the rims relative to the core. The major elements which often definecompositional zoning in metamorphic garnet, such as Al, Ca and Fe(Argles et al., 1999) displayed no apparent zoning (Fig. 13). Other ele-ments mapped, including various Light Rare Earth Elements (LREE; Ce,Nb, Nd, Eu), Sn, Ti and Zr, were present in concentrations at or belowthe detection limits.

4.7. Zr-in-titanite geothermometry

To establish conditions of peakmetamorphism of the host amphibo-lites, Zr concentrations in the abundant titanite were determined by LA-ICP-MS for two samples (KS2 and KS15). Mean Zr concentrations in the

image of Fig.�8

Fig. 9. Back-scatter electron images illustrating relevant relationships between minerals. (a) Abundant inclusions of titanite within garnet and amphibole (KS-36). (b) Inclusions ofzircon within titanite (KS-15). (c) Inclusion of zircon within pyrite (KS-15). (d) Occurrence of apatite grains as inclusions within hornblende, which is included within garnet (KS-36).Abbreviations: Ap = apatite; Hbl = hornblende; Grt = garnet; Py = pyrite; Qtz = quartz; Ttn = titanite; Zrc = zircon.


two samples are 341 ± 37 ppm (n = 26) and 428 ± 37 ppm(n = 18), respectively. Application of the Zr-in-titanite geothermometerfollowing the calibration of Hayden et al. (2008) gives temperatures of730–745 °C at a pressure of 8–10 kbar (Fig. 14). This is consistent withconditions of 0.9–1.0 GPa and 800 °C given by Hand et al. (1999a),Mawby et al. (1999), Buick et al. (2001, 2005) and Maidment et al.(2013).

5. Discussion

5.1. Metamorphic paragenesis

Petrographic examination of the mineralization and its host rocksshows that sulphides occur as inclusions within rock-forming silicates(Fig. 10e), and thatminerals belonging to themetamorphic assemblage,particularly garnet, are also observed as inclusions in the sulphides(Fig. 10c). The occurrence of pyrrhotite–chalcopyrite inclusions withingarnet and magnesiohornblende, in particular, is suggestive of theentire package having undergone regional metamorphism at upper-amphibolite to granulite-facies (e.g., Vokes, 1969). This observationallows us to confidently interpret the sulphides as either be syn-metamorphic in origin, or alternatively, that they are part of a pre-metamorphic event and underwent recrystallization during metamor-phism. This would have occurred either during the ca. 480–460 MaLarapinta Event (Buick et al., 2005; Mawby et al., 1999), or the 450–300 Ma Alice Springs Orogeny (Buick et al., 2008; Hand et al., 1999b;Mawby et al., 1999).

LA-ICP-MS spot analysis shows that Co is contained within pyrite(Table 3) and Ni is incorporated within pyrrhotite (Table 4). CobaltandNi are isomorphously substituted in the respective sulphide lattices,where Co2+ and Ni2+ replace Fe2+ (Hawley and Nichol, 1961). Underequilibrium conditions, Co is preferentially incorporated within pyriteand Ni will be preferentially taken up by pyrrhotite (Hawley andNichol, 1961). This relationship suggests equilibrium partitioning ofthe two elements between the two Fe-sulphides, presumably at thetime of metamorphic recrystallization. Coexistence of the two Fe-

sulphides at the time of trace element partitioning further suggeststhat the system was close to the pyrite–pyrrhotite buffer in fS2–fO2

space at that time (Hall, 1986). Sulphide recrystallization duringpeak metamorphism is also supported by the composition of at leastsome of the sphalerite in the ore. EPMA data show a sub-populationof sphalerite in equilibrium with pyrite and pyrrhotite with10–12 mol% FeS (Table 6), indicating metamorphic pressures at, orabove 10 kbar (Scott, 1976), consistent with burial at up to 30 km. Attemperatures of as much as 730–745 °C, as measured by Zr-in-titanitegeothermometry, and pressures of at least 10 kbar, sulphide recrystalli-zation can be expected (McClay and Ellis, 1983). Additionally, the com-posite chalcopyrite–pyrrhotite inclusions (Fig. 10a) may be a result ofequilibrium (re-)crystallization froman initial intermediate solid solutionphase in the system Cu–Fe–S (Cabri, 1973).

LA-ICP-MS spot analysis of pyrite (Table 3) has shown that, apartfrom Co, other trace elements common in pyrite, such as As, Se, or Mn,are either absent or present in only negligible concentrations. This ob-servation is also consistent with complete recrystallization of the sul-phide assemblage during metamorphism (McClay and Ellis, 1983).Any trace elements that had originally been incorporated in pyritewould be released to form discrete minerals, as for example shown forAu (Larocque et al., 1995); others such as Ni are partitioned intocoexisting pyrrhotite. So-called remobilizates (mineral assemblagesresulting from remobilization, and characteristically rich in elementssuch as Au, Ag and Sb which are most readily remobilized; e.g., Cooket al., 1998) were not observed in the present study.

Three key mineral textures are observed that further support thehypothesis that the Basil Cu–Co deposit is either a metamorphosed(i.e., it formed pre-metamorphism) or a metamorphogenic deposit(syn-metamorphic). These are: a lack of plastically-deformed (euhedral)pyrite; pyrite overgrowing garnet and; the presence of relict pyritewith-inmassive pyrrhotite-dominantmineralization (Fig. 10). All three obser-vations are consistent with metamorphic recrystallization of pyrite(Craig and Vaughan, 1994)where sulphide anneals duringmetamorphiccooling, forming ‘new’ euhedral grains. The presence of massive, ran-domly oriented pyrite and pyrrhotite indicates that recrystallization

image of Fig.�9

Fig. 10. Reflected light photomicrographs illustrating relevant ore textures. (a) Typical euhedral pyrite and characteristic intergrowths of chalcopyrite andpyrrhotite in thematrix enclosing thepyrite (KS-23b). (b) Fine-grained secondary pyrite at themargins of coarse-grained deformed pyrrhotite (KS-30). (c) Silicate inclusionswithin amatrix of pyrrhotite (KS-29). (d) Exsolution oftwo Fe–(Ti)-oxides (hematite and ilmenite) (KS-23b). Back-scatter electron images. (e) Inclusions of sulphide in garnet (KS-30). (f) Occurrence ofmolybdenite associatedwith amicrofracturein pyrite (KS-22). Abbreviations: Ccp = chalcopyrite; Grt = garnet; Hm = hematite; Ilm = ilmenite; Mo = molybdenite; Po = pyrrhotite; Py = pyrite.


must have continued during thewaning stages ofmetamorphism. Pyritetypically deforms either by plastic or brittle deformation and cannotgrow statically under strain (Barrie et al., 2010).

Further evidence for a pre- or syn-metamorphic origin of sulphidescomes from sulphide–silicate relationships. Pyrrhotite is observed as in-clusions within metamorphic garnet (Fig. 10) suggesting that the sul-phides must have been present at the time of garnet growth. Thisfinding is supported by themicroanalytical data showing that sulphidesincluded within garnet are compositionally identical to that elsewherein the mineralizing system (Table 5), even if there is a possibility thatthese sulphides re-equilibrated during retrograde conditions.

LA-ICP-MS mapping of garnet (Fig. 13) shows that the garnet rimsare enriched in HREE, as well as Cr and V. Garnet can preferentially in-corporate HREE relative to LREE and may show grain-scale composi-tional zoning with respect to HREE (e.g., cores that are stronglyenriched in HREE relative to the rims; Otamendi et al., 2002). For thezoning of the type observed in the Basil deposit to occur, a HREE-richfluid must have been available in the system during the final stageof garnet growth (presumably syn- or post-peak metamorphism). Apossible source of HREE would be the metamorphic breakdown ofamphibolite-facies minerals such as hornblende or biotite (Bingenet al., 1996) or most likely titanite, an excellent REE-host (Bingenet al., 1996; Hughes et al., 1997) as the system approaches granulite-

facies. Alternatively, HREE and Cr may have migrated to the rims via aprocess of solid-state diffusion, as has been demonstrated by Carlson(2012), albeit at significantly higher temperatures. Enrichment in Crand V may have been derived from the breakdown of magnetite orilmenite, which both may carry significant Cr or V (Toft et al., 1993).

5.2. Origin of the host rocks

The mineral assemblage of the amphibolite (Table 1) is observed tocontain abundant, very fine-grained zircon (Fig. 9b). The protolith forthe amphibolite that hosts the Basil Deposit has chemistry consistentwith formation in an intracontinental rift environment (Sivell andFoden, 1985), possibly as a sequence of submarine basaltic flows(Lawrence et al., 1987). Within-plate basalts are typically Ti–Zr-enriched over those from other settings, e.g., MORB (e.g., Pearce andCann, 1973), so the abundant zircon observed is not necessarily incon-sistent with zircon being a primary magmatic mineral. Alternatively,the observed zircons could have a detrital origin. Such an interpretationis supported by the age of zircon cores reported in the Riddock Amphib-olite (734 ± 44 Ma; Claoué-Long and Hoatson, 2005) which is taken asa maximum depositional age for this unit. Other units within the HartsRange Meta-Igneous Complex have yielded detrital zircon ages indicat-ing Early to Middle Cambrian deposition (Buick et al., 2005; Maidment

image of Fig.�10

Table 2Whole rock geochemistry of each rock unit recorded in the two sampled drill holes.

Minimumdetectionlimit

Amphibolite(mean)

Clinopyroxene–amphiboleschist (mean)

Gneiss(mean)

Garnet-bearingamphibolite(mean)

Garnetrich zone(mean)

Pyrrhotite-richsemi-massivesulphide(mean)a

Pyrrhotite–pyrite-rich semi-massivesulphide (mean)a

Pyrite-richsemi-massivesulphide(mean)a

Ore zonerelative tohost rock

Riddock amphibolite(mean of 2; Hoatsonet al., 2005)

Oxides (wt.%)SiO2 0.03 49.74 52.18 54.32 49.86 45.03 49.23 45.83 46.12 Similar 49.08Al2O3 0.02 14.51 13.93 15.91 14.21 14.49 12.15 8.28 5.37 − 14.78K2O 0.02 0.22 0.17 0.19 0.27 0.05 0.11 0.10 0.05 Similar 0.15Na2O 0.02 3.45 3.40 3.84 3.10 0.58 1.20 1.28 0.60 − 3.21CaO 0.02 10.62 14.98 11.45 10.46 3.91 7.76 5.64 5.31 − 10.74Fe2O3 0.02 11.98 6.23 6.20 12.86 27.66 42.73 44.84 45.12 + 12.07b

MgO 0.02 6.57 6.73 6.41 6.41 6.99 7.11 4.69 2.86 Varied 7.48MnO 0.02 0.16 0.10 0.10 0.24 0.35 0.40 0.21 0.25 + 0.19P2O5 0.03 0.18 0.15 0.08 0.20 0.07 0.10 bmdl 0.08 Similar 0.17TiO2 0.02 1.85 1.62 0.91 1.87 1.52 1.69 0.97 0.69 Similar 2.00

Trace elements (ppm)Ag 0.05 0.06 0.001 0 0.04 0.08 2.0 1.2 1.6 + –

As 0.50 11.5 10.2 11.0 14.0 7.9 11.8 16.7 21.9 + –

Ba 0.50 41.3 101 42.5 60.8 14.0 33.1 287 53.1 Varied 34Bi 0.01 0.03 0.001 0.05 0.12 0.07 1.4 2.4 3.0 + –

Cd 0.02 0.08 0.01 0.1 0.23 0.62 3.2 2.6 4.4 + –

Ce 0.50 18.9 14.4 12.8 17.5 4.8 14.1 8.5 7.9 Similar 16.7Co 0.1 51.5 20.3 34.8 49.5 77.3 548 930 1465 + –

Cr 20 190 246 197 195 182 177 97.2 61.4 − 237Cs 0.05 0.04 0.09 0.08 0.08 0.18 0.19 0.58 0.12 + −Cu 1.0 59 10 52 147 482 5643 4203 1657 + –

Dy 0.05 6.9 6.4 3.9 6.7 6.7 5.8 3.0 2.5 − 17.5Er 0.05 4.2 4.0 2.4 4.0 4.4 3.8 1.9 1.59 − 4.6Eu 0.05 1.7 1.3 0.96 1.6 1.2 1.2 0.67 0.63 Similar 1.8Ga 0.10 20.3 17.6 16.2 19 14.5 19.6 17.4 15.2 Similar 19.7Gd 0.05 6.2 5.6 3.4 6.0 5.2 4.8 2.5 2.2 − 7.0Ge 0.05 0.68 0.96 0.88 0.74 0.23 0.88 0.71 0.74 Similar –

Hf 0.10 3.6 3.0 2.1 3.40 3.00 3.19 1.91 1.16 − 3.7Ho 0.02 1.4 1.3 0.80 1.36 1.41 1.23 0.60 0.52 Similar 1.6In 0.01 0.1 0.06 0.04 0.09 0.12 0.33 0.43 0.24 + –

La 0.20 7.3 5.3 4.7 6.30 1.65 6.78 4.09 3.72 Similar 5.2Li 0.10 4.3 6.5 5.4 4.25 8.55 6.61 6.51 5.72 Similar –

Lu 0.02 0.59 0.54 0.33 0.56 0.67 0.56 0.27 0.19 Similar 0.66Mo 0.10 0.58 0.15 0.40 0.60 1.25 7.50 11.3 21.9 + –

Nb 0.05 2.5 1.4 2.1 2.71 1.40 2.57 1.8 0.93 Similar 2.95Nd 0.10 14.5 12.1 9.3 14.0 5.4 10.2 6.1 5.5 Varied 14.2Ni 1.0 47.3 49.5 32.0 52.0 41.0 86.5 86.5 8.0 + 82Pb 5.0 1.5 2.5 3.0 9.0 5.5 13.5 14.7 17.9 + 1.9Pr 0.05 2.8 2.3 1.9 2.7 0.84 2.0 1.2 1.1 Similar 2.8Rb 0.10 3.4 1.7 1.8 3.1 1.6 3.9 4.7 0.86 + 5.2Re 0.002 0.002 0.001 0.001 0.001 0.004 0.04 0.05 0.07 + –

S 50.0 1944 113 1617 3958 13,701 152,500 197,850 251,050 + –

Sb 0.05 0.08 0.08 0.03 0.16 0.14 0.15 0.13 0.25 + –

Sc 10.0 42.5 44.0 41.5 43.5 45.0 35.9 21.3 15.0 − 48Se 0.50 0.3 0.00 1.8 0.25 2.3 14.8 19.5 16.1 + –

Sm 0.05 4.7 4.14 2.8 4.5 2.9 3.4 1.9 1.7 − 4.8Sr 0.20 262 400 297 205 16.8 127 135 124 − 215Tb 0.02 0.99 0.93 0.59 0.97 0.89 0.81 0.41 0.35 Varied 1.3Te 0.05 0.001 0.04 0.04 0.001 0.04 0.65 0.70 0.23 + –

Th 0.05 1.3 0.65 1.3 0.99 0.12 0.22 0.12 0.10 − 0.25Tm 0.05 0.61 0.58 0.34 0.58 0.65 0.56 0.28 0.24 Similar −V 10.0 308 295 202 328 272 273 175 131 Varied 342Y 0.50 40.0 38.2 22.8 38.6 41.7 35.41 18.4 15.5 − 44Yb 0.05 3.8 3.6 2.2 3.6 4.1 3.5 1.8 1.4 Varied 4.2Zn 1.0 92.3 22.0 36.0 158 267 587 605 800 + 69Zr 1.0 131 108 77.5 122 112 123 75.4 47.1 − 137

Each mean represents two analyses of the rock unit from the two drill holes.a Fe predominantly as sulphide.b Sum of FeO + Fe2O3.


et al., 2005) and zircon rims that give ages consistent with metamor-phism during the Larapinta event (461 ± 6 Ma; Claoué-Long andHoatson, 2005; 462.2 ± 5.4 Ma; Maidment et al., 2013). This is consis-tent with interpreted magma mingling relationships between maficrocks and ~520 Ma felsic intrusive rocks in the Stanovos Gneiss. Asnoted above, a mixed origin seems likely, comprising volcanic rocks, as-sociated volcaniclastic rocks and their intrusive equivalents. Geologicalmapping by Mithril in the vicinity of the Basil deposit have identified

thin (b0.5 m-thick), laterally continuous (tens of metres) quartzite ho-rizonswhich support a sedimentary protolith for at least a portion of theamphibolite host sequence.

Given that the Basil deposit occurs within successive sequences ofdeformed flows and detrital material, a possible scenario can be put for-ward in which initial extrusive rocks in the Irindina sub-basin were ac-companied by deposition of sedimentary material, as discussed above.The position of the Basil deposit with respect to the stratigraphy of the

Table 3Mean EPMA data for pyrrhotite grains in 6 representative samples.

Wt.% KS-11Mean(n = 14)

KS-25aMean(n = 15)

KS-30Mean(n = 13)

KS-32Mean(n = 15)

KS-23bMean(n = 13)

KS-23bMean(n = 11)

S 38.28 38.70 38.97 38.49 39.12 39.24Fe 60.83 60.24 60.55 60.67 58.65 58.49Co b0.01 0.11 b0.01 b0.01 0.11 0.10Ni 0.03 0.06 0.03 0.04 0.22 0.22Cu 0.02 0.01 0.01 0.02 0.02 0.05Zn 0.01 0.01 0.01 0.02 0.03 0.02As 0.07 0.04 0.05 0.06 0.06 0.05Ag 0.02 0.02 0.02 0.02 0.02 0.03Sb 0.01 0.01 0.01 0.01 0.03 0.01Pb 0.13 0.13 0.15 0.13 0.17 0.15Bi 0.10 0.09 0.09 0.10 0.12 0.09Total 99.50 99.42 99.89 99.56 98.55 98.45

Formulae (calculated to 1 S atom)Fe 0.893 0.874 0.872 0.885 0.857 0.852Co 0.000 0.002 0.001 0.000 0.002 0.001Ni 0.000 0.001 0.000 0.001 0.003 0.003Cu 0.000 0.000 0.000 0.000 0.000 0.001Zn 0.000 0.000 0.000 0.000 0.000 0.000Ag 0.000 0.000 0.000 0.000 0.000 0.000Pb 0.000 0.000 0.000 0.000 0.001 0.001Total 0.893 0.877 0.873 0.886 0.847 0.842As 0.001 0.000 0.001 0.001 0.000 0.001Sb 0.000 0.000 0.000 0.000 0.000 0.000Bi 0.001 0.001 0.001 0.001 0.000 0.000Pb 0.001 0.001 0.001 0.001 0.001 0.001Bi 0.000 0.000 0.000 0.000 0.001 0.000S 0.998 0.998 0.998 0.997 0.999 0.999Total 1.000 1.000 1.000 1.000 1.000 1.000

Fig. 11. Plot ofmol% FeS in sphalerite vs. temperaturemodified after Scott (1976) showingelectron probemicroanalysis of data for sphalerite (from Table 3) as red circles. A nominalpeak metamorphic temperature of 580 °C is assumed. We acknowledge that this is toolow but at higher temperature, mol% FeS is co-dependent on both temperature and pres-sure. Assuming equilibrium crystallization with pyrite and pyrrhotite, the lower values ofmol% FeS indicatemineralization pressures of N10 kbar. Sphaleritewith highermol% FeS issuggested to represent re-equilibration during the retrograde metamorphic path.


sub-basin is speculative. Given the evidence presented, and a conceptu-al model for deposit genesis involving sedimentation above seafloormassive sulphides, we can suggest that it is stratigraphically locatedwithin the lower sections of the basin, and is structurally located tens

Table 4Mean EPMA data for sphalerite in 4 representative samples.

Wt.% KS-33Mean(n = 3)

KS-29Mean(n = 7)

KS-31Mean(n = 6)

KS-23b(n = 1)

S 32.10 32.58 31.33 31.87Fe 6.43 11.23 10.48 6.97Co 0.01 0.01 0.05 0.02Cu 0.28 0.12 0.19 0.21Zn 57.40 54.84 56.60 57.58As 0.04 0.02 0.04 0.01Ag 0.02 0.02 0.01 0.01Cd 0.17 0.08 0.05 b0.01Pb 0.08 0.06 0.10 0.02Bi 0.13 0.09 0.07 0.15Total 96.64 99.09 98.92 96.87

Formulae (calculated to 2 a.p.f.u)Fe 0.12 0.20 0.19 0.13Co 0.00 0.00 0.00 0.00Cu 0.00 0.00 0.00 0.00Zn 0.88 0.81 0.85 0.88Ag 0.00 0.00 0.00 0.00Cd 0.00 0.00 0.00 0.00Pb 0.00 0.00 0.00 0.00Bi 0.00 0.00 0.00 0.00Total 1.00 1.01 1.04 1.01As 0.00 0.00 0.00 0.00S 1.00 0.99 0.96 0.99Total 1.00 0.99 0.96 0.99mol% FeS 11.50 19.19 17.67 12.40mol% ZnS 87.75 80.47 81.78 87.20mol% CdS 0.00 0.00 0.02 0.00

to hundreds of metres above the Proterozoic boundary. Moreover, ifthe burial metamorphism model for the Larapinta Event is correct, thehigh metamorphic grade of the ore-hosting succession is consistentwith deeper burial in the lower part of the basin. Further discussionon the burial mechanisms for the highly metamorphosed Harts RangeGroup, including a schematic reconstruction of the tectonic setting at

Table 5Electron probe microanalysis of 4 representative garnet grains in sample KS-23b.

(Wt.%) 71733a_02 71733a_06 71733b_02 71733b_03

Mean (n = 30) Mean (n = 30) Mean (n = 30) Mean (n = 30)

CaO 3.20 3.33 3.13 3.20FeO 27.09 28.51 28.75 28.89TiO2 0.02 0.02 0.02 0.04MgO 8.24 8.32 8.00 8.07SiO2 40.01 38.44 38.57 38.77MnO 0.51 0.50 0.66 0.67Cr2O3 0.01 0.01 0.01 0.02Al2O3 21.05 21.44 21.68 21.68Total 100.14 100.57 100.81 101.32

Formula (calculated to 16 atoms p.f.u.)Mg 1.91 1.92 1.84 1.85Fe 3.51 3.70 3.71 3.71Mn 0.07 0.06 0.09 0.09Ca 0.53 0.55 0.52 0.53Total 6.02 6.24 6.16 6.18Al (total) 3.85 3.91 3.95 3.93Si 6.10 5.94 5.96 5.96Al 0.10 0.06 0.06 0.04Total 6.14 5.99 5.98 5.98Al 3.81 3.86 3.93 3.91Ti 0.00 0.00 0.00 0.00Cr 0.00 0.00 0.00 0.00Total 3.81 3.86 3.93 3.91TOTAL 16.0 16.1 16.1 16.1

Mol% end-membersAlmandine 59.2 59.2 60.3 60.1Pyrope 30.8 30.8 29.9 29.9Spessartine 1.4 1.0 1.4 1.4Grossular 8.5 8.9 8.4 8.5

Each mean value given represents a 30-point transect across each garnet grain.

image of Fig.�11

Table 6Summary of LA-ICP-MS spot trace element analytical data for pyrite grains from 4 representative samples.

Element V Cr Mn Co Ni Cu Zn Se Mo Ag Sb Te Au Hg Pb Bi

KS-25bMean (n = 8) 0.13 1.1 8.4 4845 54 0.74 0.73 13.0 0.08 0.67 0.03 – – 0.14 0.05 0.05std 0.09 0.722 18.4 1851 129 0.46 0.56 11.1 0.08 0.12 0.03 – – 0.09 0.03 0.11Max 0.32 2.0 54 6754 373 1.7 1.7 39.0 0.25 0.37 0.09 b0.94 b0.09 0.29 0.13 0.31Min 0.05 0.32 0.35 945 5.5 0.25 0.30 3.7 0.02 0.20 0.01 b0.01 b0.01 0.06 0.20 b0.01

KS-11Mean (n = 12) 0.89 – 2.6 2420 12 1.17 0.73 5.8 0.64 0.04 0.04 0.3 0.03 0.13 0.2 0.47Std 2.6 – 2.0 1158 5.1 0.79 0.35 1.6 0.04 0.03 0.04 0.20 0.03 0.05 0.27 1.1Max 9.1 b2.6 5.8 4112 26 2.9 1.6 9.9 0.12 0.14 0.15 0.62 0.07 0.2 0.85 3.6Min 0.04 b0.38 0.27 500 5.5 0.37 0.46 3.5 b0.01 0.02 0.01 0.06 b0.01 0.04 0.02 b0.01

KS-09mean (n = 12) 0.21 – 3.1 4889 5.3 1.5 0.87 11.0 0.06 0.13 0.06 0.29 0.01 0.17 0.69 0.78Std 0.20 – 2.7 1183 1.6 0.89 0.42 3.2 0.03 0.34 0.08 0.29 0.02 0.04 1.6 2.6Max 0.63 b3.1 6.6 7384 8.4 3.5 1.5 17.0 0.11 1.2 0.29 1.1 0.06 0.24 5.7 9.2Min 0.04 b0.48 0.32 2990 3.5 0.57 0.28 7.4 0.03 0.02 0.01 0.07 b0.01 0.1 0.05 b0.01

KS-30Mean (n = 9) 0.17 – 3.3 1708 32 1.7 1.06 18.0 0.06 0.03 0.02 0.15 – 0.12 0.13 0.03Std 0.22 – 4.7 4183 54.4 2.2 2.4 26.8 0.12 0.02 0.03 0.26 – 0.09 0.22 0.13Max 0.31 b7.3 6.5 5181 84 4.3 4.1 42.0 0.19 0.04 0.05 0.37 b0.06 0.17 0.38 0.20Min 0.03 b0.5 0.38 95.0 3.4 0.45 0.28 3.9 0.01 0.02 0.01 0.02 b0.01 0.05 0.02 b0.01

“–” signifies the element was not detected above minimum detection limit in any spots.


the time, aswell as the relationship between theHarts Range Group andthe surrounding non-metamorphosed basin successions is given byMaidment et al. (2013).

The mineralogy of the garnet-rich amphibolite (Table 1) is consis-tent with several potential protoliths. Metamorphic garnet growth ispromoted by Al- and Ti-rich chemistry. This garnet-rich amphibolite

Table 7Summary of LA-ICP-MS spot analytical data for pyrrhotite from 6 representative samples.

Element V Cr Mn Co Ni Cu Zn

KS-6mean (n = 7) 0.87 – 4.6 484 179 3.4 1.7S.D. 1.1 – 4.6 278 102 3.2 1.9Max 3.1 9.8 13.0 746 292 9.4 5.7Min 0.02 0.35 0.18 181 63.0 0.08 0.12

KS-11Mean (n = 7) 0.30 – 37.0 13.0 254 3.6 –

S.D. 0.29 – 56.1 3.0 63.1 1.5 –

Max 0.90 b28 127 18.3 346 5.6 b5.4Min 0.10 b1.1 1.3 10.6 203 1.4 b0.58

KS-28Mean (n = 10) 0.27 4.6 32.0 493 226 2.0 1.4S.D. 0.16 3.8 89.8 122 47.8 1.5 0.92Max 0.49 12.0 288 757 315 5.1 3.9Min 0.09 1.0 1.2 385 176 0.68 0.80

KS-30Mean (n = 7) 0.13 – 11.0 118 297 1.9 1.3S.D. 0.06 – 24.8 305 122 0.63 0.41Max 0.25 b1.6 67.0 809 407 2.5 1.9Min 0.07 b0.73 1.10 0.64 35.0 0.68 0.84

KS-32Mean (n = 7) – 1.4 2.6 137 211 1.8 1.3S.D. 0.02 0.34 1.6 6.2 18.0 0.81 0.29Max 0.14 2.1 6.0 145 237 3.0 1.8Min 0.10 1.1 1.1 130 182 0.79 1.0

KS-25AMean (n = 5) 0.21 4.0 2.8 823 416 4.8 2.4S.D. 0.15 3.6 2.0 86.8 136 5.3 2.3Max 0.46 9.8 5.6 968 645 14.0 6.5Min 0.10 1.1 0.90 742 311 1.0 1.1

“–” signifies the element was not detected above minimum detection limit in any spots.

could have originally contained an abundance of aluminous mineralsand Ti-bearing minerals such as rutile or ilmenite. The presence oforthopyroxene in the clinopyroxene–amphibole schist (Table 1) is con-sistent with metamorphic overprinting of either amafic or calc-silicate-rich sedimentary protolith. The presence of scapolite may suggest deri-vation from evaporitic sediments (Ramsay and Davidson, 1970).

Se Mo Ag Sb Te Au Hg Pb Bi

17.0 0.13 0.27 0.02 0.19 0.01 0.11 2.1 0.3711.2 0.12 0.26 0.01 0.09 0.01 0.04 3.6 0.6433.0 0.37 0.74 0.04 0.29 0.02 0.16 10.0 1.84.5 0.04 0.03 0.00 0.08 0.00 0.05 0.02 0.01

18.0 0.41 1.6 0.02 – 0.01 1.4 1.7 1.67.4 0.88 3.7 0.01 – 0.01 0.07 2.0 3.3

28.0 2.4 9.9 0.04 b0.46 0.02 0.30 5.9 9.09.7 0.05 0.09 0.01 b0.07 0.01 0.09 0.20 0.04

13.0 0.11 0.40 0.02 0.29 0.01 0.20 0.29 0.247.4 0.07 0.53 0.01 0.12 0.02 0.17 0.38 0.42

30.0 0.24 1.8 0.04 0.42 0.06 0.62 1.2 1.47.10 0.01 0.01 0.01 0.02 0.01 0.02 0.01 0.01

11.0 0.08 0.09 0.04 – – 0.40 0.17 0.035.8 0.05 0.05 0.02 – – 0.23 0.18 0.03

19.0 0.19 0.20 0.09 b0.73 b0.02 1.4 0.56 0.074.5 0.04 0.04 0.02 b0.18 b0.01 0.12 0.04 0.01

13.0 0.20 0.41 0.03 0.43 0.01 0.20 0.44 0.083.5 0.07 0.48 0.02 0.16 0.00 0.05 0.49 0.15

16.0 0.29 1.3 0.05 0.62 0.01 0.26 1.4 0.425.7 0.11 0.04 0.01 0.22 0.01 0.13 0.05 0.01

17.0 0.25 0.82 0.03 0.61 0.01 0.23 0.41 0.114.6 0.08 1.4 0.01 0.62 0.00 0.18 0.42 0.13

22.0 0.35 3.4 0.04 1.7 0.01 0.53 1.1 0.3211.0 0.15 0.11 0.01 0.19 0.01 0.08 0.08 0.01

Fig. 12. Diagram depicting LA-ICP-MS spot analytical data for Ni and Co in pyrite and pyr-rhotite (logarithmic scales). Pyrrhotite has higher Ni values than pyrite, with lower con-centrations of Co,whereas pyrite is enriched in Cowith generally low concentrations of Ni.


5.3. Ore genesis

Compared to the rocks hosting the mineralization, the whole rockgeochemistry of themineralized zone itself shows a loss ofmoremobileelements (Ca, K, Na, and Sr) whereas the more immobile elements (Zrand Ti) are largely unchanged, even after considering the large volumesof sulphide in the rock. There is nomarked halo of potassic, sodic or cal-cic alteration enclosing the ore. This is consistent with hydrothermalfluids causing breakdown of alkali-bearing minerals. Importantly, thissuggests that there could be alterationmarkers that could be used in ex-ploration for mineralization similar to the Basil deposit. Determinationof the extent of this alteration, both laterally and vertically, would

Fig. 13. LA-ICP-MS elementmaps of ametamorphic garnet grain. Note that Y, HREE (Dy, Er, Yb aof the grain. In contrast, elements that usually define compositional zoning in metamorphic ga

however require analysis of a greater number of samples includingthose from well outside the deposit.

From the evidence reported here, it is possible to discount some ofthe ore genesis models that were suggested above. A hydrothermalskarn style of deposit has been suggested due to the presence of agarnet-rich (60–80%) lithology, but the present study shows thatthese are not calcic garnets typical of skarn deposits (Meinert, 1992). In-stead, present data shows compositions typical of metamorphic garnet(~60 mol% almandine, ~30% pyrope, ~10% grossular; Table 5). More-over the absence of any other calc-silicates in the host rocks (e.g., diop-side–hedenbergite, wollastonite or actinolite) rules out a skarn originfor Basil.

A second alternative, based on the location of the deposit within anamphibolite, is that Basil is a magmatic sulphide Ni–Cu–(PGE) deposit,consistent with the perceived potential for such deposits within theRiddock Amphibolite (e.g., Hoatson et al., 2005) and documentation ofNi-enrichment in other prospects in the tenement area considered asorthomagmatic in origin (e.g., Blackadder). The lack of significant Ni(b100 ppm) or PGE in the zone of mineralization is strong evidence toconfidently dismiss this deposit model.

As an alternative, based on evidence presented here, the origin ofBasil as a VHMS-type deposit is plausible. The intraplate rift setting inwhich the HRG is interpreted to have been deposited offers a suitablesetting for such a style of mineralization to occur via seafloor exhalativeprocesses (Fig. 1b). In such a model, deposition of flows and clastic ma-terial would have taken place in the Irindina sub-basin, with metalsleached from the mafic-rich host sequence, lending the mineralizationits Cu-dominant character. An in-depth discussion of the type ofVHMS deposit is beyond the scope of the present paper. We acceptthat the setting shows some similarities with Besshi-type deposits, yetthe Cu-dominant signature, with negligible Ag is more typical ofCyprus-type deposits. Syn-metamorphic remobilization may have notonly shaped deposit geology, but also influenced its geochemical signa-ture, possibly with remobilized Ag–(Au)-rich ores yet to be discoveredoutside knownmineralization. Although the host setting differs, the rel-atively high Co-content is perhaps more reminiscent of so-calledAtlantic-type VMS deposits known from the South Urals (Herrington

nd Lu), Cr and V all show compositional zoning, with amarked enrichment around the rimrnet, such as Al, Ca, Fe and Si, are uniformly distributed.

image of Fig.�12

image of Fig.�13

Fig. 14. (a) Histograms of Zr content in titanite in the two samples as measured by LA-ICP-MS. (b) Schematic plot of Zr concentration against temperature with the estimated peak tem-peratures given at pressures of 8–10 kbar. Geothermometric calibration after Hayden et al. (2008).


et al., 2005; Prokin and Buslaev, 1999), with a greater contribution ofmetals from mafic/ultramafic rocks in the hydrothermally-leachedsuccession.

An inconsistency with the VHMS hypothesis is the lack of a markedhydrothermal alteration halo enclosing the deposit. Zoned alterationhalos and an exhalite “cap” are characteristic of most metamorphosedVHMS deposits (Franklin et al., 2005). We do, however, note that thehanging wall rocks are less altered than those beneath the ore body aswould be expected in a VHMS system. We interpret the lack of arecognisable hydrothermal alteration (feeder) zone as being consistenteither with destruction of the alteration zone duringmetamorphism, ordetachment of the ore from its subjacent alteration zone either follow-ing formation (e.g., accumulation in a brine pool distal to the ventand not as a proximal sulphide mound; e.g., as proposed for the Hellerand Rosebery deposits, Tasmania (Green et al., 1981; Solomon andQuesada, 2003), or during syn-metamorphic deformation (e.g. viashearing focussed along the mineralized zone; Cook et al., 1990). Thisis entirely plausible given that the strike of themineralization lies with-in, and is coincident with the Basil Fault. Given the relatively wide drillspacing, there is still ample scope for locating a feeder zone within theknown mineralization area, or beyond the current drilled area.

Further constraints on ore genesis might be obtained from re-examination of other prospects hosted within the Riddock Amphibolite(Selins and Virginia; Whelan et al., 2013) and other VMS-style pros-pects, including those of Proterozoic age such as Jervois; Lennartz,2012) elsewhere in the eastern Arunta region.

Samplenumber

Description Sampleinterval (m)

LB027DDKS-1 ‘Normal’ amphibolite 31.45–31.6KS-2 Low-Cu amphibolite 49.1–49.25KS-3 Epidote-amphibole schist 90.1–90.2KS-4 Weakly-foliated amphibolite 96.07–96.12KS-5 Amphibolite containing minor garnet 163.25–163.4KS-6 Semi-massive sulphide Po (50%) Py (8%) Cpy (1%) 184.45–184.48KS-7 Carbonate-altered amphibolite 196.9–197KS-8 Carbonate–chlorite–magnetite-bearing amphibolite 211–211.15KS-9 Semi-massive sulphide Po (30%) Py (30%) Cpy (1%) 215–214.17KS-10 “Clast” amphibole–magnetite in semi-massive sulphide 222–222.13KS-11 Semi-massive sulphide Po (45%) Py (12%) Cpy (3%) 234.7–234.85KS-12 Semi-massive sulphide Py (40%) Po (19%) Cpy (1%) 231.19–241.35KS-13 Amphibolite (base sulphides in alteration) 243.95–244.1KS-14 ‘Normal’ amphibolite 271.15–271.3KS-15 Low-Cu amphibolite 23.3–23.5KS-16 Average-Cu amphibolite 55.1–55.3

6. Conclusions

The Basil deposit has beenmetamorphosed at P–T conditions consis-tent with the Ordovician Larapinta Event, therefore it formed pre- orsyn-metamorphism. The timing of ore genesis thus remains speculativeand beyond the scope of the present contribution. Syn-metamorphicsulphide recrystallization occurred with equilibrium partitioning oftrace elements between pyrite and co-existing pyrrhotite. The composi-tions of sphalerite buffered by pyrite and pyrrhotite, and equilibriumpartitioning of Co and Ni between the Fe-sulphides, both point to re-crystallization at pressures of at least 10 kbar.

Mineralogical, petrographic and geochemical evidence allow varioussuggested genetic ore types to be disproved. Garnet compositions aretypical of those of metamorphic origin and not the product of a hydro-thermal skarn system. The lack of significant Ni and PGEs in themineral-ization makes a magmatic-hosted Ni–Cu–(PGE) deposit scenariounlikely. This leaves the possibility that the Basil deposit was originally

formed as a VHMS deposit by volcanogenic-exhalative processes at ornear the seafloor. However whole rock geochemistry showed no signifi-cant evidence for a marked halo of hydrothermal alteration associatedwith the mineralization which typically accompanies VMS deposits.

Further studies on the Basil deposit and its host rocks are required tobetter understand the style of deposit and implications for explorationpotential for economic accumulations of Cu–Co sulphide deposits inthe Riddock Amphibolite unit of the HRG. There is a clear need for reli-able geochronological data for the mineralization and immediate hostrocks, a better-defined sequence of deformation events and timing ofthese, and a study on the highly unusual rounded exotic clasts presentwithin the deposit. Detailed mapping of alteration, lithology and struc-ture will also be invaluable for understanding this deposit.

Acknowledgements

We gratefully thankMithril Resources for the financial and logisticalsupport. Particular thanks go to Graham Ascough, David Hutton andPatrick Lyons. David Maidment and an anonymous reviewer providednumerous comments that assisted us with the revision of this manu-script. We also thank Angus Netting and Benjamin Wade (AdelaideMicroscopy) for assistance with analytical work. This is TRaX contribu-tion 271.

Appendix A. Descriptive list of samples

image of Fig.�14

Appendix A (continued)

Samplenumber

Description Sampleinterval (m)

LB035DDKS-17 Amphibolite/carbonate alteration zone contact 112.2–112.5KS-18 Epidote-amphibole schist 129.6–129.77KS-19 Weakly-foliated amphibolite 141.4–141.55KS-20 Low-Cu Amphibolite 161.7–161.85KS-21 Amphibolite containing minor garnet 175.17–175.25KS-22 Amphibolite containing sulphides 187.05–187.2KS-23A Garnet-rich amphibolite (60% garnet) 203.8–204.0KS-23B Garnet-rich amphibolite (60% garnet) 203.8–204.0KS-24 Mela-amphibolite 213.5–213.67KS-25A Semi-massive sulphide, garnet and mela-amphibolite

clasts containing sulphides220.95–221.15

KS-25B Semi-massive sulphide, garnet and mela-amphiboliteclasts containing sulphides

220.95–221.15

KS-26 Amphibolite (garnet-quartz-rich, 1% sulphides) 228.85–229KS-27 Amphibolite 239.95–240.05KS-28 Semi-massive sulphides Po (40%) Py (17%) Cpy (2–3%) 246.1–246.27KS-29 Semi-massive sulphides Py (34%) Po (5%) Cpy (1%) 258.1–258.2KS-30 Semi-massive sulphides Po (38%) Po (10%) Cpy (2%) 264.45–264.7KS-31 Semi-massive sulphides Py (40%) Po (8%) Cpy (2%) 274.15–274.32KS-32 Semi-massive sulphides Po (20%) Py (12%) Cpy (3%) 291.15–291.35KS-33 Semi-massive sulphides Py (25%) Po (14%) Cpy (1%) 299.9–300.05KS-34 Amphibolite 303.55–303.75KS-35 Amphibolite 354.95–355.05KS-36 Garnet-rich amphibolite 340.4–340.5


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The Basil Cu–Co deposit, Eastern Arunta Region, Northern Territory, Australia: A metamorphosed volcanic-hosted massive sulphide deposit

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