Kontonikas-Charos Et Al. 2014

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Albitization and redistribution of REE and Y inIOCG systems: Insights from Moonta-Wallaroo,

Yorke Peninsula, South Australia

ARTICLE in LITHOS NOVEMBER 2014

Impact Factor: 4.48 DOI: 10.1016/j.lithos.2014.09.001

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Alkis Kontonikas-Charos

University of Adelaide

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Cristiana L. Ciobanu

University of Adelaide

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Albitization and redistribution of REE and Y in IOCG systems: Insightsfrom Moonta-Wallaroo, Yorke Peninsula, South Australia

Alkis Kontonikas-Charos, Cristiana L. Ciobanu , Nigel J. CookCentre for Tectonics, Resources and Exploration, School of Earth and Environmental Sciences, University of Adelaide, 5005 SA, Australia

a b s t r a c ta r t i c l e i n f o

Article history:

Received 2 November 2013

Accepted 1 September 2014Available online 16 September 2014

Keywords:

AlbitizationRare earth elementsIOCG depositsFeldsparMoonta-WallarooOlympic Province

Trace element concentrations, particularly rare earth elements and yttrium (REY) in feldspars and accessoryminerals, have been determined in a suite of albitized igneous, metasedimentary and metasomatite rocks from

the Moonta-Wallaroo district, Olympic CuAu Province, South Australia. Results show that changes in REY-fractionation trends and concentrations in feldspars and common accessories are associated with key texturesin albite-bearing associations from different lithologies. In granitic rocks, pseudomorphic replacement ofpre-existing feldspars is typied by porous albite with cleavage-oriented intergrowths of sericite and pore-attached hematite. These observations are comparable with albitization features of granitic terranes elsewhere.A mineral association (albite-sericite chlorite), similar to that from granitoids, is observed as pervasive spotsin limestone, inferring prograde skarnoid reactions at low uid/rock ratio in an impure carbonate. Inmetasedimentaryand metasomatiterockswithcomparable Na2O content(~56 wt.%),ne-grained granoblasticalbite suggestsgrowthunder highuid/rock ratios irrespectiveof lithology. In suchcases, albite with the highestREY content (REY ~ 200 ppm) accounts for the entire REY budget, e.g., in albitebiotite-schist with the lowestabundance of accessory minerals. Nanoscale investigation conrms this albite to be a REY carrier (elementsincorporated within the crystal lattice); no pore-attached inclusions are observed. In contrast, albite with thelowest REY-concentration (~ 14 ppm) is encountered in the metasomatite. In such rocks, recording the highestREY (~1000 ppm) in whole-rock, partitioning of REY is favoured among the abundant accessories (titanite,apatite) and calc-silicates (actinolite, clinozoisite) rather than albite. Comparable low-REY albite is also found

in granitoid-derived albitite (Na2O ~ 5 wt.%), in which abundant accessories and discrete REY-minerals formedduring albitization account for the highREY content (~700 ppm) in whole rock.The role of coupled dissolutionreprecipitation reactions (CDRR) is critical for REY (re)distribution withinalbitized igneous rocks, where REY-release from early magmatic accessories and/or feldspars assists REY-enrichment into late albite. The presence of abundant nanopore-attached inclusions in plagioclase demonstratesthe nanoscale nature of CDRR-driven albitization in granitoids, consistent with published experimental work onaltered granites. Such porosity offers sites for REYentrapmentseen within discreteREY-minerals in new-formedK-feldspar. Similarly, release and uptake of REY, concurrent with albitization, is seenin formation of coarser REY-minerals (xenotime, bastnsite, synchysite) during CDRR-driven replacement of accessory FeTi-oxides bysymplectites of chlorite and hematite.Based on the differencesidentied between the albitization pathways in igneousand metasedimentaryrocks, wediscuss how albitization proceeds via a series of complex uidmineral reactions, each involving the redistribu-tion, accumulation andretention of REY. These reactions arecritical fordeningthe endowment anddeportmentof REYin rocks that have undergonesodicalteration. Contraryto previous models,albitizationappearscontrolledby pH rather than redox conditions. Despite regional differences in local geological environment and alterationstyle across the Olympic CuAu Province, albitization, the initiation of hydrothermal alteration, is a pre-

requisite stage for REY-enrichment in Iron-OxideCopperGold (IOCG) systems. REY distribution patterns infeldspars may thus have value in mineral exploration as criteria enabling alteration associated with mineraliza-tion to be distinguished from the regional background. Strong albitization withoutsuperposition of later potassicalteration may not, however, be automatically linked to the formation of giant IOCG deposits. Albitization en-hances rock permeability and in a stronglyfaulted structural environment without a suitabletrap, hydrothermaluids may be more readily lost from the system.

2014 Elsevier B.V. All rights reserved.

Lithos 208209 (2014) 178201

Corresponding author.E-mail address: [email protected](C.L. Ciobanu).

http://dx.doi.org/10.1016/j.lithos.2014.09.001

0024-4937/ 2014 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

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1. Introduction

Iron oxide copper-gold (IOCG) mineralization is presently consid-ered part of a broad group of deposit types that form within ore systemsspanning from the Archean to the Phanerozoic(Barton,2014; Grovesetal., 2010; Hitzman et al., 1992; Williams, 2010; Williams et al.,2005).Formation models for these deposits are still much debated,particularly regarding the role of igneous rocks, the sources of metals

and

uids, and geodynamic settings (e.g.,Bartonand Johnson, 2004;Chiaradiaet al., 2006; Groves et al., 2010; Pollard, 2006; Williamsetal., 2005).

IOCG systems are typied by zoned, broad alteration haloes com-prising early, barren albite, and, in most cases, a late, ore-hosting typeof alteration. Albite-bearing alteration is part of a broader sodic(-calcic)alteration spectrum with various mineralogical expressions. The lateore-hosting alteration can be broadly subdivided into deeper potassic(K-feldspar biotite) and upper hydrolytic (sericite chlorite carbonate) alteration, where magnetite and hematite are dominantFe-oxides, respectively. The sericitic alteration may result from break-down of pre-existing potassium feldspars, e.g., in granitic rocks or theirvolcanic equivalents, by interaction with acidic uids (e.g.,Hitzmanetal., 1992). Other systems, associated with carbonate-rich protoliths,feature instead calc-silicate alteration and in this case the ore is hostedwithin skarn assemblages. Based on gains or losses of differentcomponents,Barton (2014)considers skarn formation (referred to ascarbonate-hosted alteration) as distinct from the NaCa alteration ofaluminous igneous or sedimentary protoliths which can also result information of calc-silicates. A comprehensive account of variation inalteration styles in IOCG systems worldwide, and how they differ fromthose in other types of hydrothermaldeposits, is given by Barton(2014).

Albitization, by itself, does not dene an IOCG system. For example,widespread regional expression of albitization (NaCa-alteration) isalso recognized in seaoor hydrothermal systems (e.g.,Alt, 1999), orcan reect regional-scale metasomatism related to metamorphic reac-tions involving evaporite-derived uids (e.g.,Oliver et al., 2004). Large-scale metasomatic albitization of continental crust is recognized as aproduct ofuidrock interaction withcrustaluids, sometimes resulting

in regional-scale albitite terranes, e.g., Bamble, SE Norway (e.g.,Engviketal., 2008; Plmper and Putnis, 2009). Moreover, sodium metasoma-tism of gneisses, migmatites and granites relating to crustal-scale shear

zones can result in concentration of uranium, as for example, centralUkraine (Cuney et al., 2012) and also in IOCG-hosting terranes else-where (Montreuil et al., in press).

Modelling the role of sodic alteration in the formation of IOCGdeposits has been addressed in the Eastern Mt. Isa Block, Australia(Oliver et al., 2004). This is considered a landmark study as it providesa comprehensive approach (whole-rock geochemistry, stable isotopes,uid inclusions and numerical modelling) aimed at understanding

gains and losses of various elements and processes associated withuidrock interaction during sodic metasomatism.Oliver et al. (2004)studied the behaviour of a broad range of major and minor elements(Na, Fe, K, Ba, Rb, Ca, Pb, Zn, Cu etc.) but did not include either rareearth elements and yttrium (hereafter REY) or uranium (U). Themarked enrichment in REY and U relative to average crustal values is,however, a dening characteristic of the IOCG deposit class, for whichany genetic model must account(Hitzmanet al., 1992). This is despitethe fact that some IOCG terranes do not contain accumulations ofthese elements at the levels seen in N 9000 Mt Olympic Dam CuAuUdeposit, South Australia (0.26 kg/t U3O8, ~0.17 wt.% La and 0.25 wt.%Ce;Ehrig et al., 2013). The conceptualization of IOCG deposits (largetonnages, low Cu and Au grades and abundance of Fe-oxides in thebreccia host for the CuAuU ore) is based on Olympic Dam(Hitzmanetal., 1992), which contains the largest known resource of U on Earth(Cuney,2010; Hitzman and Valenta, 2005).

The Olympic IOCG Province, eastern Gawler Craton, South Australia(Skirrow et al., 2002, 2007), including the Olympic Dam deposit, isone of the archetypal Mesoproterozoic examples of giant IOCG prov-inces. Whereas Olympic Dam stands out by being hosted within alarge body of brecciated granite with multiple deposit-scale mineraland geochemical zoning (e.g.,Ehrig et al., 2013), other deposits andprospects in the province feature a range of alteration styles and metalendowments (e.g.,Hayward and Skirrow, 2010; Skirrow et al., 2002,2007). Alteration ranges from the end-membersericitehematitebreccia hosted type such as Olympic Dam and Prominent Hill, in thenorthern part of the province (Fig. 1a), with transition to skarn-hostedmineralization in thePunt Hill district, in thecentral part of the province(Reidet al., 2011), and mineralization on the Yorke Peninsula in the

south with Hillside as the best characterized example(Conoret al.,2010;Ismail et al., 2014). Unlike in other IOCG terranes in Australia(e.g., Cloncurry District), the Olympic Province also stands out by a

Fig. 1.a) Location of Moonta-Wallaroo region and IOCG deposits/prospects within the Olympic IOCG Province, Gawler Craton (adapted fromConor et al.,2010). Inset: Location of South

Australia. b) Geological sketch of basement stratigraphy and location of drill hole locations in the Moonta-Wallaroo region (adapted fromForbes, 2012).

179A. Kontonikas-Charos et al. / Lithos 208209 (2014) 178201
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ks_Multiple_Feldspar_Replacement_Reactions_under_Subsolidus_Conditions?el=1_x_8&enrichId=rgreq-3a26798e-8abb-45fa-98a0-3a7881a90997&enrichSource=Y292ZXJQYWdlOzI4MDA0MjU3NjtBUzoyNzE1OTE0ODU0NzI3NjhAMTQ0MTc2Mzg2NDY3Nw==https://www.researchgate.net/publication/250273870_Albitization_of_granitic_rocks_the_mechanism_of_replacement_of_oligoclase_by_albite._The_Canadian_Mineralogist_(46)_1723-1737?el=1_x_8&enrichId=rgreq-3a26798e-8abb-45fa-98a0-3a7881a90997&enrichSource=Y292ZXJQYWdlOzI4MDA0MjU3NjtBUzoyNzE1OTE0ODU0NzI3NjhAMTQ0MTc2Mzg2NDY3Nw==https://www.researchgate.net/publication/250273870_Albitization_of_granitic_rocks_the_mechanism_of_replacement_of_oligoclase_by_albite._The_Canadian_Mineralogist_(46)_1723-1737?el=1_x_8&enrichId=rgreq-3a26798e-8abb-45fa-98a0-3a7881a90997&enrichSource=Y292ZXJQYWdlOzI4MDA0MjU3NjtBUzoyNzE1OTE0ODU0NzI3NjhAMTQ0MTc2Mzg2NDY3Nw==https://www.researchgate.net/publication/240336289_Origin_of_fluids_in_iron_oxide-copper-gold_deposits_Constraints_from_37Cl_87Sr86Sri_and_ClBr?el=1_x_8&enrichId=rgreq-3a26798e-8abb-45fa-98a0-3a7881a90997&enrichSource=Y292ZXJQYWdlOzI4MDA0MjU3NjtBUzoyNzE1OTE0ODU0NzI3NjhAMTQ0MTc2Mzg2NDY3Nw==https://www.researchgate.net/publication/240336289_Origin_of_fluids_in_iron_oxide-copper-gold_deposits_Constraints_from_37Cl_87Sr86Sri_and_ClBr?el=1_x_8&enrichId=rgreq-3a26798e-8abb-45fa-98a0-3a7881a90997&enrichSource=Y292ZXJQYWdlOzI4MDA0MjU3NjtBUzoyNzE1OTE0ODU0NzI3NjhAMTQ0MTc2Mzg2NDY3Nw==https://www.researchgate.net/publication/240336289_Origin_of_fluids_in_iron_oxide-copper-gold_deposits_Constraints_from_37Cl_87Sr86Sri_and_ClBr?el=1_x_8&enrichId=rgreq-3a26798e-8abb-45fa-98a0-3a7881a90997&enrichSource=Y292ZXJQYWdlOzI4MDA0MjU3NjtBUzoyNzE1OTE0ODU0NzI3NjhAMTQ0MTc2Mzg2NDY3Nw==https://www.researchgate.net/publication/240301739_Evolution_of_Uranium_Fractionation_Processes_through_Time_Driving_the_Secular_Variation_of_Uranium_Deposit_Types?el=1_x_8&enrichId=rgreq-3a26798e-8abb-45fa-98a0-3a7881a90997&enrichSource=Y292ZXJQYWdlOzI4MDA0MjU3NjtBUzoyNzE1OTE0ODU0NzI3NjhAMTQ0MTc2Mzg2NDY3Nw==http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-


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characteristic REE and U enrichment, irrespective of alteration style(e.g.,Skirrow et al., 2007).

The Moonta-Wallaroo area in the north-west of the Yorke Peninsulais known for past exploitation of small CuAu veins (Fig. 1). Mineraliza-tionin the Moonta and Wallaroo Mines is hosted within felsic porphyryrocks displaying potassic alteration (biotitemagnetite K-feldspar albite;e.g., Conor et al., 2010). Theareais considered prospective fornew,large-tonnage, lower-grade resources. The region is also an example of

an IOCG-hosting terrane in which regional-scale alkali alteration isrecognized in different lithologies (e.g., Conoret al., 2010; Cowleyetal., 2003), ranging from igneous to metasedimentary rocks. Theseinclude distinctive calc-silicate-bearing lithologies such as the OorlanoMetasomatite Formation (Conor, 1995), a descriptor for distinctive,highly-altered bodies of rock for which the sedimentary or granitic pre-cursor identity cannot be readily determined, as well as units withintheWandearah Formation of the Wallaroo Group comprising bandedfeldspar + calc-silicates and carbonate-rich units (Fig. 1b). Rocks ofthe Wallaroo Group are also recognized in the Punt Hill district and atHillside where they are likely protoliths for skarn formation.

Skirrow et al. (2002)suggested that the Moonta-Wallaroo districtrepresents a deeper crustal level of IOCG hydrothermal activity thanthe Olympic Dam district. Given the extreme REY-U-enrichment insericite-altered granitic rocks at Olympic Dam, we raise the questionof whether the initiation of such an event can be recognized in deeper,less altered rocks of the Moonta-Wallaroo district in which graniticand felsic volcanic rocks are abundant and albitization is present. Theapplicability of REY fractionation trends in minerals to track the tempo-ral evolution of an IOCG system from protolith through early and latemineralization stages has been demonstrated at Hillside(Ismailet al.,2014).Using the Moonta-Wallaroo area as a study case, our aim is touse this highly sensitive tool to track those mineral reactions associatedwith initiation of hydrothermal activity (albitization) relative to mineraltextures and their REE geochemistry in both granitic and sedimentaryrocks. Giventhe closeparageneticand geochemical correlationbetweenREY and U, this work also carriesimplicationsfor the distribution of U inIOCG systems.

2. Geological background

2.1. IOCG mineralization and the Olympic CuAu Province

The Olympic CuAu Province (Hayward and Skirrow, 2010; Skirrowet al., 2002, 2007; Fig. 1a) is located in the Olympic Domain (Ferris et al.,2002) and extends for over 700 km, encompassing numerous prospectsalongside the Olympic Dam and Prominent Hill deposits (Ferris et al.,2002; Hayward and Skirrow, 2010; Skirrow et al., 2002, 2007). ThePaleo- to Mesoproterozoic basement of the Olympic Domain belongsto one of the mobile belts that surround the Late Archean granulitefacies core of the Gawler Craton. Two orogenic events are recognized:one associated with emplacement of the Donington granitoid suite at1.85 Ga; and the Kimban orogeny at 1.731.69 Ga (e.g.,Handet al.,

2007;Reid and Hand, 2012). These affected the basinal rocks of the(2.01.85 Ga) Hutchinson Group and the (1.761.74 Ga) WallarooGroup, respectively.

Major magmatism, interpreted as a Large Igneous Province by someauthors (e.g., Allen et al., 2008; Wade et al., 2012) took place at ~1.6 Ga.This comprised emplacement of the Hiltaba Intrusive Suite (HIS) andlarge-scale volcanism (Gawler Range Volcanics; GRV). Both intrusiveand extrusive rocks include felsic and mac components. Althoughdeformation is considered to have largely ceased with the Kimbanorogeny, evidence for subsequent greenschist- to lower amphibolitefacies metamorphism is recognized in Wallaroo Group rocks in theMoonta region. This, broadly termed the Kararan orogeny(Handet al.,2007;Reid and Hand, 2012), is considered synchronous with emplace-ment of the TickeraGraniteat 1598 7 to 1575 7 Ma (Conor, 1995;

Fanninget al., 2007;Fig. 1). Recognition of this metamorphic overprint

has been taken as an argument against the proposed anorogenic settingfor LIP magmatism, with implications for generation of IOCG systems(Skirrow, 2008).

Deposition of CuAu mineralization is consideredto be contempora-neous with the ~1.6 Ga magmatic event, based on dating of igneous andhydrothermal minerals (e.g.,Ciobanuet al., 2013; Jagodzinski, 2005;

Johnsonand Cross, 1995; Reid et al., 2013; Skirrow et al., 2007).Theregionis unconformably overlain by sequencesof Neoproterozoic,

Cambrian, Permian and Cenozoic sediments (e.g., Conoret al., 2010;MoralesRuano et al., 2002).

2.2. The Moonta-Wallaroo region

The Wallaroo Group is a diverse suite of siltstone-dominatedmetasedimentary,felsic and mac metavolcanicrock packages,includingthe Wandearah and Weetulta Formations(Conoret al., 2010; Cowleyetal., 2003). The Wandearah Formation comprises metasediments, anda range of feldspathic,calc-silicateand carbonaceous members, includingthe Doora and New Cornwall Members, whereas the rhyodaciticMoontaPorphyry Member and ~1740 Ma Wardang Volcanic Member form theWeetulta Formation (Cowley et al., 2003;Fig. 1).

The Hiltaba Suite comprises granites of which the Tickera and1583 7 Ma Arthurton Granites are the largest in the Moonta-Wallaroo district (Conor et al., 2010), as well as macultramacintrusive rocks, e.g., the 1583 3 Ma Curramulka Gabbronorite to thewest of the study area (Zang et al., 2007).Zang et al. (2007)denedboth the Tickera and Arthurton Granites as composite batholiths, withcompositions ranging from monzogranite and granodiorite to tonalite.Granitoids of the Tickera Granite are stated as I- and S-type, whereasthe Arthurton Granite is described as A-type (Cowley et al., 2003).

Alteration is widespread throughout a varied range of rocks acrossthe northern Yorke Peninsula(Conoret al., 2010). The link betweendistrict- to regional-scale alteration and IOCG mineralization in theGawler Craton was rst stressed byConor (1995) in the Moonta-Wallaroo district. A summary of the current view on alteration stagesacross the Olympic Province (Hayward and Skirrow, 2010), comprisingfour different types of alteration, highlights the Moonta-Wallaroo

district in the overall context of the Olympic IOCG Province.Early albite calc-silicate (actinolite-diopside) magnetite alter-

ation (aliasNa-Ca-Fe alteration) is recognized as km-scale zones in theMoonta-Wallaroo district and in the Mt. Woods Inlier. Geophysicalmodels infer large-scale magnetite alteration at deeper levels inthe Olympic Dam district. Calc-silicate (actinolite-clinopyroxene titanite) or scapolite-bearing assemblages occur locally and are consid-ered part of this alteration stage. Thisrstalteration stage is comparableto Na-Ca alteration seen in other IOCG domains in Australia (e.g., in theCloncurry district;Williams et al., 2005) and worldwide. The secondtype of alteration (biotite-magnetite or Fe-K) is characteristic of theMoonta-Wallaroo district and Mt. Woods Inlier. It is also considered aregional alteration signature in the deeper part of the Olympic Damdistrict based on aeromagnetic data (Raymond, 2003). Albite is consid-

ered to be stable during this alteration. Although only weak Cu-mineralization is generally associated with this stage, such alterationcan also host high-grade vein Cu-Au ores as seen by the mineralizationcharacter of the Moonta and Wallaroo Mines(Conoret al., 2010).

The third type of alteration (magnetite-K-feldspar actinolite carbonate), although a similar type of FeK metasomatism as thebiotitemagnetite, hasa distinct mineralogy andis typical of theOlympicDam district, in particular satellite prospects such as Acropolis, WirrdaWell and Murdie Murdie (e.g.,Davidsonet al., 2007). Relicts of similarassemblages are observed in many IOCG systems. As in the biotite-magnetite alteration, the magnetite-K-feldspar actinolite carbonatealteration may be associated with low-grade Cu-mineralization. This isalso suggested to be the equivalent of hydrothermal magnetite-sideritein deeper and outer parts of the Olympic Dam deposit (Hayneset al.,

1995;Reeve et al., 1990).

180 A. Kontonikas-Charos et al. / Lithos 208209 (2014) 178201
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The fourthstyleof alteration (sericitehematitechloritecarbonate)is not characteristic of the Moonta-Wallaroo district but is the dominantalteration style at Olympic Dam and also at Prominent Hill (Belperioetal., 2007).Hayward and Skirrow (2010)attribute deposition of themain CuAuU ores and REE-phosphates to this stage. Sericite isinterpreted to replace igneous and metamorphic K-bearing phasessuch as K-feldspar whereas chlorite replaces FeMg-silicates (amphi-boles, biotite). In cases where no precursor minerals are evident, such

alteration is considered to form in vein and breccia matrices. This alter-ation is interpreted as a form of H2OCO2metasomatism involvingstrong oxidation. The latter is inferred from the change in mineralassemblage from one containing ferrous iron (e.g., magnetite, amphi-bole) to another containing ferric iron (hematite, clinozoisite). Thisalteration type is interpreted (Hayward and Skirrow, 2010) as theequivalent of hydrolytic alterationin IOCG districts elsewhere (seeabove) but argue against use of this term since it has a strong geneticconnotation.

Based on dating in the Moonta-Wallaroo district and elsewhere inthe Yorke Peninsula,Conor et al. (2010)states that the main ingredientsin this sub-domain are a suite of HIS granites, regional greenschistfacies metamorphism, NaKCaFe metasomatism and polymetallicmineralization. This convergence is optimally seen at contacts withthe Wallaroo Group, where intensely partitioned deformation andmetasomatism have produced high-grade ores. There is, however,difculty in constraining the relative timing of metasomatism, intrusionand regional metamorphism.

3. Approach and methodology

Fifteen drillcore samples (Fig. 1,Table 1) previously collected from10 drillcores in the Moonta-Wallaroo region were studied.

An FEI Quanta 450 scanning electron microscope (SEM) with energydispersive X-ray spectrometry and back-scatter electron (BSE) imagingcapabilities (Adelaide Microscopy, University of Adelaide) was used.BSE imaging (accelerating voltage, 20 kV, and beam current of 10 nA)allowed for characterization of each sample in terms of signicanttextures and mineralogical relationships, and identication of suitable

areas for further microanalysis.Quantitative compositions of feldspars and accessory minerals

within representative samples were determined using a Cameca SX-Five Electron Probe Microanalyser (EPMA). Standards, X-ray lines,count times, typical minimum detection limits (mdl) are given inAppendix B.

Laser-Ablation Inductively-Coupled Mass Spectrometry (LA-ICP-MS)was used on selected samples to provide quantitative trace element

data (as spot analyses and element maps) for potassium feldspar, albite,rutile, titanite, apatite, zircon and calcite. This was performed on aResonetics M-50-LR 193-nm Excimer laser microprobe coupled to anAgilent 7700cx Quadrupole ICP-MS at Adelaide Microscopy, Universityof Adelaide. Full details of the analytical methods are given in ElectronicAppendix B.

Focussed ion beam-scanning electron microscopy (FIB-SEM) workwas carried out on a Dual Beam FEI Helios Nanolab 600 platform

(Adelaide Microscopy) allowing for cross-section imaging, as well ascutting, extraction and thinning of foils for transmission electronmicroscopy (TEM) study at site-specic locations in the sample. Proce-dures for cutting, extraction and thinning of TEM foils followed Ciobanuetal. (2011). Working on polished blocks, grain areas were selectedimmediately adjacent to LA-ICP-MS craters of interest. Slices removedfor TEM sample preparation were attached to a tungsten needle andtransported to the grid holder. Each slice was then sequentially thinnedfromboth sidesuntil it becamesufciently transparentfor TEM analysis.Final thinning (tob60100 nm) isdoneat 30 kVand (maximum) 93pA,and then cleaned at 5 kV or lower to remove material deposited ontothe surface.

The TEM study was performed on Philips 200CM and FEI Tecnai G2Spirit instruments operated at 200 and 120 kV, respectively (AdelaideMicroscopy). Both instruments are equipped with a Gatan digitalcamera and energy-dispersive X-ray analysis (EDAX) capabilities.

4. Petrography

Of the lithologies in the Moonta area illustrative of early regionalalkali metasomatism (albite K-feldspar), particularly albitization(Conoret al., 2010), three types have been studied here: altered felsicigneous rocks, intensively altered rocks of igneous origin and rocks ofmetasedimentary origin from the Wandearah Formation and OorlanoMetasomatite (Fig. 1). Key textures and mineral relationships aredepicted in BSE images in Figs. 25. Compositional data (EPMA) forfeldspars aregiven as Tables 2a and 2b. Datafor actinolite, clinozoisite, ac-cessory and REY-minerals are given as Appendix A, Tables 15.

4.1. Main rock types: primary and alteration features

4.1.1. Altered igneous rocks

Igneous rocks studied include two granitoids (Tickera Graniteand Arthurton Granite) and a felsic volcanic (rhyodacite; WardangVolcanic). The Tickera Granite is intensely deformed and maybe olderthan the Arthurton Granite, even though ages overlap (Cowley et al.,2003).

Table 1

Index of samples studied.

Sample ID Stratigraphy Rock type Main mineralogy Accessory minerals Minor/trace minerals

Kfs Ab Qz Chl Ap Rt Ttn Zrc Other REEm

227DDH1 Arthurton Granite Alkali monzogranite xx x xx xx x x x Hm, Chl Xn, Bast,227DDH2 Arthurton Granite Alkali monzogranite xx xx xx x x x x Hm, Chl Xen, Bast,193DDH1 Arthurton Granite Alkali monzogranite xx x xx xx x x Hm, Chl Xen, Mon33PBD1 Tickera Granite Variable monzogranite and quartz monzonite xx xx x x x x x x Hm, Chl, Cp Bast, Mon33PBD2 Tickera Granite Variable monzogranite and quartz monzonite xx xx x x x x x Hm, Chl, Cal Bast, Xen212DDH1 Wardang Volcanic Rhyodacite felsic volcanic xxx x xx x x x Hm, Chl Bast, Mon212DDH2 Wardang Volcanic Rhyodacite felsic volcanic xxx x xx x x x x Hm, Chl33DDH1 Undened Hiltaba Suite Highly albitised granite (albitite) x xxx xx xx x x x Hm, Chl Bast, Xen, Syn197DDH1 Moonta Porphyry Highly altered rhyodacite xxx x xx xx x x x Hm, Chl, Xen, Mon175DDH1 Doora Member Albitebiotite- schist xx xx x x x Bt, Mu, Hm, Chl, Mgh, Cp158DDH1 Wanderah Formation Chloritic schist xx x xx xx x x Hm, Chl190DDH1 New Cornwall Member Limestone x x x x Dol, Chl,190DDH2 New Cornwall Member Limestone x x x x x Dol, Chl, Bast,38PBD1 Oorlano Metasomatite Calc-silicate feldspar schist x xx x x x x Clz, Act, Hm, Chl, , Cp, Cal Bast,38PBD2 Oorlano Metasomatite Calc-silicate feldspar schist x xx x x x x Clz, Act, Hm, Chl, Cp Mon

Abbreviations:Ab albite,Act actinolite, Ap apatite, Bast bastnasite, Bt biotite,Cal calcite, Chl chlorite, Clz clinozoisite, Cp chalcopyrite, Dol dolomite,Hm hematite,Ilmilmenite, Kfs K-feldspar, Mgh maghemite, Mon monazite, Mt magnetite, Mu muscovite, Plag plagioclase, Rt rutile, Ser sericite, Syn synchysite, Ttn titanite,

Xen

xenotime, Zrc

zircon, REEm

REE minerals. Note on compositions: xxx =N

50%, xx = 20

50%, x = 5

20%, x =b

5%.

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Both granitoids are coarse-grained and consist of K-feldspar, low-Caplagioclase (An0.8-8;Tables 2a and 2b), quartz and chlorite (mostlyreplacing hornblende biotite) as main components, minor Fe- andTi-oxides, as well as abundant accessory minerals such as zircon andapatite. Although albite is presently the dominant plagioclase feldsparin both rocks, andesine (An~32), an intermediate member of the plagio-

clase series, is also present as relicts in the Tickera Granite (Tables 2aand 2b). Large (N2 mm) xenoblastic K-feldspar (orthoclase) of igneousorigin commonly displays zonation with respect to Ba content, areas ofsericitization, and varying degrees of porosity (Fig. 2a, b). In addition,exsolution-like lamellae of albite are preserved at variousscales withinthis K-feldspar, forming perthitic textures (Fig. 2a, c). Such perthitictextures could be an endproduct of feldspar crystallization fromgraniticmelts. In the Tickera Granite, perthites are more abundant andalso show changes in the albite morphology from lamellar to lens-shaped. Such modication can be attributed to diffusion and/or syn-deformational (re)crystallization of pre-existing exsolution perthites.Accessory minerals such as apatite and zircon are clustered aroundpockets of Fe-Ti-oxides (Fig. 2d).

Superimposed hydrothermal alteration is expressed in three

assemblages resulting from pseudomorphic replacement of pre-

existing minerals within the two granitoids. Firstly, igneous feldsparsare replaced by widespread coarse albite displaying characteristicporosity. The andesine in the Tickera Granite occurs as relicts (Fig. 2e),illustrating replacement of igneous plagioclaseby albite. The hydrother-mal nature of the albite can be inferred from its textures such as thepresence of abundant pores in both granitoids, as well as nucleation of

sericite, hematite and discrete REY-U-minerals within such pores.Formation of albite + sericite on behalf of pre-existing andesine is alsomarked by the appearance of a new K-feldspar generation (Fig. 2e; seebelow). Comparable replacement among feldspars is seen in graniticrocks elsewhere(Engviket al., 2008; Plmper and Putnis, 2009). Sec-ondly, pre-existing mac minerals are replaced by chlorite + hematiteassemblages. Thirdly, accessory Fe- and FeTi-oxides(ilmenite and mag-netite) are replaced by hematite (Fig. 2d) and symplectites consisting ofrutile + chlorite hematite. The coarser REY-minerals are found withinsuch symplectites (see below). In the Tickera Granite, relict ilmenite ispreserved within the symplectite, and is more abundant. This is consis-tent with the relatively reduced character of the Tickera Granite relativeto the Arthurton Granite (e.g.,Cowley et al., 2003).

The rhyodacitic Wardang Volcanic is predominantly composed of

K-feldspar, quartz, minor albite, and contains abundant hematite and

Fig. 2.Back-scatter electron (BSE) images showing petrographic aspects of igneous rocks: Arthurton and Tickera Granites (a e); felsic rhyodacite, Wardang Volcanics (fg); albitite (h);Moonta Porphyry (i). (a) Coarse, xenoblastic orthoclase displaying varying degrees of porosity and domains of perthite. (b) Zonation with respect to Ba content in the K-feldspar.(c) Exsolution-like lamellae of albite preserved within K-feldspar. (d) Accessory apatite clustered around pockets of FeTi-oxides. (e) Large relict plagioclase (~ An30) containing smallinclusions of K-feldspar and replaced by an intergrowth of albite and sericite. (f) Porphyritic and ow banding fabrics in felsic volcanic. Note abundant hematite and apatite.(g) Potassium feldspar aggregates with minor perthitic domains, varying degrees of porosity with rims of hematite. (h) Ca-richer domains within porous albite; note cleavage-orientedsericite lamellae. (i) Corroded, relict K-feldspar surrounded by an overgrowth of second-generation K-feldspar characterized by more abundant pores. Abbreviations: Ab albite;Ap apatite; Chl chlorite; Hm hematite; Kfs K-feldspar; Mt magnetite; Qz quartz; Rt rutile; Ser sericite; Mt magnetite; Zrc zircon.

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accessory apatite. This rock has an overallne-grained, porphyritic andow-banded texture (Fig. 2f). Potassium feldspar aggregates commonlydisplay minor perthitic domains and varying degrees of porosity(Fig. 2g). In addition, K-feldspar grain boundaries are also rimmed byhematite. In contrast to the granites, albitization is relatively minorand advanced hematite alteration prevails.

4.1.2. Intensely altered igneous rocks

Two pervasively altered igneous rocks were studied: an albitite froman undened Hiltaba Suite granite; and a porphyritic felsic volcanic(rhyodacite) attributed to the Moonta Porphyry. These are representa-tive of distinct alteration sub-types, i.e., intense albitization andK-feldspar + silicication, respectively. Although relict textures arestill recognisable, the dominant feldspar within each is of replacementorigin (see below).

The albitite is mainly composed of albite (An1;Tables 2a and 2b),quartz, K-feldspar and chlorite, and contains abundant accessoryminerals clustering Fe-Ti-oxides. The albite hosts homogeneous, Ca-richer (12 wt.% CaO) domains enclosed by areas of increased porosityand intergrowths with sericite (Fig. 2h). This feature is comparable to

plagioclase relationships in the Arthurton Granite.

The highly altered felsic volcanic representing the Moonta Porphyryis similar to the aforementioned rhyodacite with respect to ne-grainsize, ow banding, and porphyritic texture (Fig. 2i). The MoontaPorphyry, however, features strong silicication, which is seen aslayering within the K-feldspar-rich domains. Importantly, albite isonly present in trace amounts. In both rocks, Fe-oxides (magnetite

and hematite), are signicant components, either as dusty inclusionsunderlining the breakdown of mac minerals to chlorite, or as large,fractured porphyroblasts. The latter texture suggests precipitationwithin strain shadows of larger K-feldspar. The cores of relict igneousK-feldspar are surrounded by pore-bearing overgrowths with mutualboundaries suggesting inwards-directed corrosion and replacement(Fig. 2i).

4.1.3. Metasedimentary rocks and Oorlano metasomatite

Three banded rocks were studied: albitebiotite-schist (DooraMember); chlorite-schist (Wandearah Formation); and calcsilicate-schist (Oorlano Metasomatite Formation). In addition to the threeschists, a limestone (New Cornwall Member) with incipient but perva-sive dolomitization and albitization was also studied. The banded rocks

all contain feldspars as major components but differ in terms of mac

Fig. 3. BSEimagesshowingpetrographicaspects of thealbitebiotite-schist(ac), chlorite-schist(de), OorlanoMetasomatite(fi) andlimestone(jl).(a andb) Fine- to medium-grainedlaminar layers of albite + biotite muscovite + K-feldspar + quartz de ning a schistose fabric. Note disseminations of Fe-oxides. (c) Distinctive Ba-zonation and domains of neperthitic textures. (d)Banding in thechlorite-schist expressed by differing proportionsof chlorite; notealbite in crosscutting vein. (e) K-feldsparporphyroblastshowing domains sugges-tive of multiple stages of syn-deformationalgrowth(variation in theintensityof porosityand Ba-content). (f)Alternatingbandsin theOorlano Metasomatitedominatedby feldspars (bothalbite and K-feldspar) or calc-silicates (actinolite and clinozoisite). (g) Disequilibrium replacement textures between K-feldspar, albite and calc-silicates. Note oscillatory zoning inclinozoisite. (h) P ervasive albite as spots in impure limestone. (i) Dolomite + quartz-bearing domains within the limestone. Abbreviations: Ab albite; Act actinolite; Bt biotite;Cal calcite; Chl chlorite; Dol dolomite; Ep epidote; Kfs K-feldspar; Mt magnetite; Mu muscovite; Qz quartz; Rt rutile.

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minerals, i.e., mica, dominantly biotite (albitebiotite-schist), chlorite(chlorite-schist), and actinolite + clinozoisite (Oorlano Metasomatite).Whereas in the albitebiotite-schist, albite is the dominant mineral(~50%), the proportion of K-feldspar to albite, and feldspars to calc-

silicates varies with banding in the Oorlano Metasomatite; albite is

very minor in the chlorite-schist. With the exception of the OorlanoMetasomatite, the banded rocks are considered to be of sedimentaryorigin and are relatively poor in accessory minerals.

The albitebiotite-schist (Fig. 3a, b) comprisesne- to medium-

grained laminated layers of albite + biotite + K-feldspar + quartz

Fig. 4. BSEimagesshowingaccessoryand REY-minerals.(a andb) Oscillatory zonation in apatite fromgranitoids showinginversecore-to-rim patterns; bright anddark shades correspondto higher- and lower-REY concentrations. (c) Patchy zones in apatite core with porous margin marked by REY-depletion (darkeron gure); note monazite partially replacing the apatitemargin.(d) Typicaltexture ofapatite in thechlorite-schistcomprisinga rounded,deformedREE-richcoresurroundedby a REE-poor rimsuggestiveof multiplestages of growth.(e) Highlyfractured, metamict zircon withoscillatory zoningtypical of thegranitoids. (f)Pseudomorphicreplacementof titanite by bastnsiteand calcite,Oorlano Metasomatite.(g) Cavitiesrimmedby rutile; note inclusions of xenotime, albitite. (h) Typical pseudomorphic replacement of accessory ilmenite by rutilechlorite symplectites in granitoids. Note abundant REY-minerals(xenotime and bastnsite)as well as zircon.(i) Coarse, corrodedrutilein the rhyodacite hosting xenotime. (j) Detail of bastnsite within rutile (Fig. 4h) showing compositional variationandeldsof dusty inclusions andpores.(k) Synchysite-(Ce) displaying oscillatoryzoning; notelamellarexsolutions of thorite.Abbreviations:Ap apatite; Bast bastnsite; Cal calcite;Chl chlorite; Mon monazite; Qz quartz; Rt rutile; Syn synchysite; Thr thorite; Ttn titanite; Xen xenotime; Zrc zircon.

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muscovite chlorite which dene the schistose fabric. Accessory min-erals include rutile ilmenite and trace apatite, zircon and monazite.Maghemite, pseudomorphing magnetite, is widespread throughoutthe rockbut forms coarse porphyroblastic aggregates within thecoarserlayers. The K-feldspar displays some common features with thegranitoids: distinctive Ba-zonation; and domains of ne perthitictextures (Fig. 3c). Minor alteration is also expressed by patchy chloritereplacing biotite and breakdown of ilmenite to rutile.

The chlorite-schist is also ane-to medium-grained rock. Bandingis

expressed by changes in the relative proportions of chlorite (Fig. 3d).Minor magnetite and more abundant apatite occur throughout. Albiteis mostly observed in crosscutting chlorite veinlets. K-feldspar displaysporphyroblast development (b200m) with domains suggestive ofmultiple stages of syn-deformational growth, as seen by variation inthe intensity of porosity and Ba-content (Fig. 3e).

The Oorlano Metasomatite is composed of alternating bands domi-nated by either feldspars (both albite and K-feldspar) or calc-silicates(actinolite and clinozoisite) (Fig. 3f). In contrast to the other bandedrocks, the Oorlano Metasomatite contains abundant titanite, apatiteand Fe(Ti)-oxides. Euhedral clinozoisite [Fe/(Fe + Al) ~0.3] andactinolite [Fe/(Fe + Mg)= 0.30.4]commonlydisplay internalchemicalzoning and occasional overgrowth textures. Some of the calc-silicatesare overgrown by feldspars; disequilibrium textures between K-feldspar

and albite are recognized at thener scale (Fig. 3g).

The limestone is porous, calcite-dominant, and also contains perva-sive quartz, dolomite, feldspar (mainly albite), chlorite, sericite andminor apatite (Fig. 3h). Some coarser domains within the otherwisene-grained rock tend to be dolomite-rich (Fig. 3i). The albite containspatchy domains enriched in Ca (up to 12 wt.%), as seen in the albititeabove.

4.2. Accessory and REY-minerals

As mentioned above, all rocks contain variable amounts of accessoryminerals. These include zircon, apatite, rutile, titanite and REY-minerals(bastnsite, xenotime, synchysite, monazite) (Fig. 4). Such minerals aremost widespread in the igneous rocks and Oorlano Metasomatite. Basedon textures, the observed REY-minerals relate to the package of mineralreactions involving albite formation rather than magmatic crystalliza-tion. In the Oorlano Metasomatite, t

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Kontonikas-Charos Et Al. 2014