ARTICLE

D. Craw á S.J. Windle á P.V. Angus

Gold mineralization without quartz veins in a ductile-brittle shear zone,Macraes Mine, Otago Schist, New Zealand

Received: 4 December 1997 /Accepted: 21 September 1998

Abstract Greenschist facies schist which hosts theMacraes Mine in East Otago, New Zealand has beenpervasively altered by post-metamorphic (lower green-schist facies) ¯uids over a 120 m thick section perpen-dicular to foliation. Metamorphic titanite has beenreplaced by rutile, and epidote has been replaced by avariety of metamorphic minerals including siderite,chlorite, muscovite and calcite. The early stages of thisalteration occurred during development of a ductilecleavage associated with kilometre scale recumbentfolding. The cleavage was widely overprinted by a sub-parallel set of spaced (mm scale) microshears which arelocally enriched in rutile and hydrothermal graphite.Strain was then concentrated into narrow (m scale)zones where more intensely deformed portions of therock are crossed and highly disrupted by closely spaced(100 lm scale) microshears. The highly strained rocksshow a combination of mylonitic and cataclasticmicrostructures, including crystal-plastic grain size re-duction and recrystallization of micas to form a newfoliation. Muscovite has grown at the expense of albitein the mylonitic cataclasites. Hydrothermal alterationwas accompanied by addition of pyrite, arsenopyrite andgold without development of quartz veins. Gold pre-cipitated with sulphides during reduction of the ¯uid byhydrothermal graphite. The whole altered rock sequencewas later cut sporadically by mesothermal quartz veinswhich contain gold, scheelite, rutile, pyrite and arseno-pyrite. This deposit displays a continuum of post-metamorphic processes and hydrothermal ¯uid ¯owwhich occurred during uplift of the schist belt.

Introduction

Gold mineralization has occurred in most metamorphicbelts of the world, resulting in mineralised zones whichcrosscut metamorphic fabrics and formed after the peakmetamorphic recrystallization (Bohlke and Kistler 1986;Bottrell et al. 1990; Forde 1991). The mineralizing eventstypically involved hydrothermal ¯uids depositing gold,quartz and sulphides in structurally controlled fracturesformed after the metamorphic belt had undergone someuplift into brittle regions of the crust (Henley et al. 1976;Cox et al. 1986; Forde 1991). Wall rock alteration isgenerally pronounced only on a small (1±10 m) scale,and some deposits have essentially no alteration adja-cent to the mineralized zones (Paterson 1986; Groveset al. 1989), although subtle alteration e�ects can extendto the kilometre scale (Phillips 1986). Some workers in-voke metamorphic processes for the source of the min-eralizing ¯uids (Henley et al. 1976; Kerrich and Fyfe1981; Bottrell et al. 1990), whereas other workers suggestthat almost ubiquitous post-metamorphic magmatismprovided the hydrothermal ¯uid (Pattrick et al. 1988;Burrows and Spooner 1989).

The Otago Schist of New Zealand provides a usefullaboratory in which to study schist-hosted gold miner-alization in the apparent absence of plutonic rocks(Williams 1974; Paterson 1982). Further, gold-bearingquartz veins have formed at several stages in the uplifthistory, from early uplift stages to near-surface levels(McKeag and Craw 1989; Craw and Norris 1991). Onedeposit (Invincible Lode) consists of cross-cutting quartzveins with wall rock alteration minerals essentiallyidentical to the host metamorphic minerals, implyingthat mineralization occurred under greenschist faciesconditions (Hay and Craw 1993). The wide range ofmineralization stages initiated in the latter stages ofmetamorphism helps to elucidate the relationship be-tween metamorphic processes and gold mineralization.

The present study examines post-metamorphic min-eralization under near-metamorphic conditions where

Mineralium Deposita (1999) 34: 382±394 Ó Springer-Verlag 1999

Editorial handling: J. Hedenquist

D. Craw (&) á S.J. WindleGeology Department, University of Otago,PO Box 56, Dunedin, New Zealand

P.V. AngusMacraes Mining Co. Ltd, PO Box 84,Palmerston, Otago, New Zealand

the rocks were too ductile to permit fracturing and veinformation. The host rock is an unusual schist-type whichhas had a long history of late- and post-metamorphicdeformation, from conditions which involve almostpervasive rock recrystallization, through more clearlypartitioned deformation. Gold mineralization withoutquartz vein formation occurred during the latter stagesof this late- and post-metamorphic deformation. Sub-sequently, strain became focused into a well-de®nedduplex fault system which hosted late stage gold-bearingquartz veins (Teagle et al. 1990; Angus 1993). Theseveins formed from ¯uids thought to be metamorphic inorigin (McKeag et al. 1989; Teagle et al. 1990). Thisstudy focuses on post-metamorphic processes whichoccurred before quartz vein formation while the rockswere still partly ductile. These processes represent astage in an apparent continuum of ¯uid ¯ow and hy-drothermal alteration from ductile metamorphism tobrittle faulting. The late-stage brittle structures andquartz vein hosted mineralization is not discussed exceptto quantify mineralization conditions.

Geology

Otago Schist

The Otago Schist is a Mesozoic metasedimentary belt whichranges in metamorphic grade from weakly cleaved prehnite-pumpellyite facies greywackes and argillites to thoroughly re-crystallized and multiply deformed upper greenschist faciespsammitic and pelitic schists (Brown 1967; Bishop 1972). Theschist belt is an amalgamation of Mesozoic terranes, the mostextensive of which is the Torlesse Terrane (Mortimer 1993). Mostof the gold-bearing vein systems occur in the central higher gradeportions of the schist belt which are sporadically biotite and/orgarnet bearing, and these are mainly hosted by Torlesse Terraneschist (Mortimer 1993).

Hyde-Macraes shear zone

This is one of the largest Mesozoic structures mapped in the OtagoSchist, traceable for 30 km along strike in east Otago, and is thehost for the Macraes mine centred on the Round Hill pit (Fig. 1).The shear zone varies in thickness along strike from 120 m atMacraes to <10 m at either end (Fig. 2), and the thickest partconsists of several stacked mineralized zones. For most of the shearzone's length there are well-de®ned upper and lower shears, theHangingwall Shear (top) and Footwall Shear (base) (Fig. 2). Be-tween these shears, the mineralized duplex system is hosted byIntrashear Schist, a distinctive graphitic schist which is pelitic inappearance (McKeag et al. 1989). The Macraes mine currently hasreserves of 60 million tonnes at about 1.58 gramme/tonne in thecentral portion of the shear zone near Macraes (Fig. 1), and ex-ploration is continuing along strike.

Hangingwall and Footwall Schist

The Hangingwall and Footwall Schists above and below the min-eralized Hyde-Macraes shear zone (Figs. 2, 3) are typical quartz-ofeldspathic Otago Schist. The rocks are lithologicallymonotonous, consisting of layers (1±10 m thick) of quartz andfeldspar rich psammitic and more micaceous pelitic schist whose

distinction is subtle. Minor other rock types occur but these arenormally detectable only in exceptional exposures such as in drill-core. Rare green metavolcanic horizons and associated metachertsoccur in the East Otago region but not in the immediate vicinity ofthe Hyde-Macraes shear zone. The Hangingwall and FootwallSchists are pervasively foliated with numerous deformed syn-metamorphic veins separating laminae, rich in micas, so that theschist is well segregated on the 1 cm scale. Biotite and garnet occurin similar schists in the Nenthorn Stream area, about 25 km to thesouthwest (Brown 1967), implying middle to upper greenschist fa-cies metamorphic conditions: about 400 °C and 4±5 kbar pressure,based on ¯uid inclusion and oxygen isotope thermometry andsphalerite geobarometry (Yardley 1982; Jamieson and Craw 1986).The metamorphic ¯uid was low salinity water which is unsaturatedwith CO2 even at low temperatures, although CO2 clathrates can bedetected in ¯uid inclusions, with homogenization temperatures upto +3 °C (Yardley 1982).

The schist contains the typical Otago Schist minerals quartz,albite, muscovite, chlorite, epidote and titanite, along with normalaccessories, tourmaline, apatite and zircon (Fig. 4). Quartz andalbite are concentrated in synmetamorphic veins or segregationswith minor micas, whereas the other minerals are mainly concen-trated in the intervening micaceous lamellae. Rare scattered dustygraphite (Landis 1971), is con®rmed by total organic carbon (TOC)analyses marginally above detection limit of 0.1% (see later). Pyriteis the main opaque mineral, occurring as scattered anhedral grainsin micaceous lamellae.

The schist is variably folded on the 1 cm to 1 m scales by opento isoclinal late metamorphic ductile folds. These folds have locallydeveloped a spaced axial surface cleavage with localised recrystal-lisation of muscovite and chlorite in fold hinges and along stronglyattenuated fold limbs. The folds formed throughout Otago duringkilometre scale late or post-metamorphic recumbent fold or nappeformation (Mortimer 1993). The large structures are generallyasymmetric, with well-de®ned upper limb and hinge zones andhighly deformed and attenuated lower limbs which are dominatedby a new near-pervasive cleavage (Turnbull 1981; Craw 1985). Apost-metamorphic fold of this type has been mapped immediatelyabove the Hyde-Macraes shear zone (Fig. 4; Paterson 1986; Craw

Fig. 1 Geological map of east Otago, New Zealand, showing thesetting of the Hyde-Macraes Shear Zone in the Otago Schist. Thepresent study describes the shear zone in the vicinity of Macraes,mainly focused on the Round Hill mine. Inset shows the Otago Schistbelt in the South Island of New Zealand

383

unpublished data). The fold is de®ned on changes in minor foldvergences and relationships between minor folds and the late stagecleavage (Fig. 4).

Intrashear Schist

The Intrashear Schist hosts the mineralized structures of the Hyde-Macraes shear zone, and generally lies between the well-de®ned

Hangingwall and Footwall Shears (Fig. 2). The Intrashear Schist isdistinctively di�erent in appearance from the adjacent schist bod-ies, being darker in colour and more ®ssile. Good exposures in minecuts show that the schist contains a variety of rock types indicativeof primary lithological variation, including psammitic and pelitic

Fig. 2 Cross section (modi®ed after Angus 1993) through the Hyde-Macraes Shear Zone at the Round Hill mine near Macraes (seeFig. 1), showing the metamorphic-hydrothermally altered IntrashearSchist sandwiched between the Hangingwall and Footwall Shears. Apod of similar altered schist lies above the Hangingwall Shear about1 km to the southeast (projected into this section). Myloniticcataclasite is most common immediately below the Hangingwall Shear

Fig. 3 Cross section through Otago Schist in east Otago through theRound Hill mine on the Hyde-Macraes Shear Zone (Fig. 1), showingthe ductile structures which dominate the metamorphic geology of thearea. The Hyde-Macraes Shear Zone is located in a highly strainedzone on the lower limb of a km scale recumbent fold. The earliestrecognizable fabric is a well-segregated metamorphic foliation (heavylines in A, B, C). The dominant fabric in the shear zone, a spacedcleavage (light lines), crosscuts the form surface (metamorphicfoliation) of the fold (A, B) and lies parallel to the axial surface ofthe fold. This late cleavage is strongly developed in B and D, butweakly developed in the large fold hinge (C)

384

schists, metaconglomerate (Craw and Angus 1993), and pale greenmicaceous horizons. Boundaries between these di�erent lithologiclayers are parallel to the foliation; layers are generally 1±10 m thickperpendicular to foliation and can be traced for tens to hundreds ofmetres laterally.

The Intrashear Schist generally consists of the same primaryminerals as the Hangingwall and Footwall Schists (Fig. 4) with theimportant di�erence that epidote and titanite are absent (as dis-cussed in more detail later). The schist is pervasively foliated andhighly segregated into quartz-albite rich and micaceous lamellae onthe 1 cm scale, similar to the metamorphic segregations in theHangingwall and Footwall Schists. Rare small (mm scale) folia-tion-parallel quartz-albite-muscovite-chlorite-calcite-rutile veinswith minor pyrite and chalcopyrite are dispersed through theIntrashear Schist and have been folded and boudinaged. The In-trashear Schist has been overprinted by more late- to post- meta-morphic chemical and structural e�ects (as described later) than theenclosing Hangingwall and Footwall Schists, and these featurescontribute to the di�erent appearance at all scales.

Mineralization

Mineralization without quartz veins

Gold mineralization occurred in the Intrashear Schist inthe latter stages of the late- and post-metamorphic de-

formation (Fig. 4), before development of cross-cuttingmineralized quartz veins (next paragraph). This stage ofgold mineralization occurred principally within 1±3 m ofthe Hangingwall Shear (Fig. 2) and is hosted by faultrocks (see ``mylonitic cataclasite'', later). This style ofmineralization contributed at least 10% of the gold inthe Intrashear Schist in the Macraes area as a whole, andforms higher proportions (up to 80%) locally.

Mineralized quartz veins

Large (decimetre scale) mineralized quartz veins haveformed in and adjacent to faults within the IntrashearPelite and the bounding shears. There are several gen-erations of mineralized veins, all of which containquartz, calcite, rutile, pyrite and arsenopyrite. Gold andscheelite are irregular in occurrence, but gold occurs inclose proximity to sulphides, and is commonly enclosedin sulphides. Some veins, principally within the Hang-ingwall Shear, have undergone ductile deformation andoccur as elongate boudin-like bodies along the hoststructures. Quartz has been dynamically recrystallized,

Fig. 4 Cartoon sketch of vari-ably altered Intrashear Schist,showing the terminology ofprincipal structural features andlisting the primary and second-ary minerals which occur ineach stage of post-metamorphicalteration and deformation, asdescribed in the text. Mineralsin brackets are rare and/or onlyoccur locally; minerals withquestion marks are suspected tooccur but there is no directevidence. Syn-metamorphicquartz veins occur, but therewas no quartz vein formationaccompanying post-metamor-phic processes

385

and extensive stylolite formation has occurred. Unde-formed veins are the most common, and occur in a widevariety of orientations. Many of the mineralized veinsare low-angle structures associated with faults withsouthwestward thrust movement sense, ®lling in localextensional sites (m scale) in a duplex thrust system.Shallow-dipping tension gashes occur also as part of thisduplex system (Teagle et al. 1990). Steeply-dipping veinarrays, or ``stockwork'' vein systems, have formed atextensional sites in lateral ramp structures associatedwith the main thrust system (Angus 1993). Many ofthese latter veins consist of delicately banded quartz.Some veins contain breccias which are partly or whollysilici®ed, and host rock is commonly silici®ed. Minorpost-mineralization cataclasis occurs along many veinmargins. The quartz veins are best developed alongshoots (hundreds of m long) at a high angle to the strikeof the shear zone and spaced about 500 m apart (Anguset al. 1997). Vein formation occurred in Late Jurassic-Early Cretaceous (Angus et al. 1997).

Fluid inclusions in veins associated with thrustfaulting homogenize at about 130±160 °C, and have ice-melting temperatures of )0.7 to )1.0 °C with undetect-able CO2 contents (McKeag and Craw 1989). Mineral-ization is inferred to have occurred near to, but above,the brittle-ductile transition, at about 350 °C and at adepth of ca. 10±12 km (McKeag et al. 1989; Teagle et al.1990).

Post-metamorphic evolution of Intrashear Schistbefore quartz vein formation

The Intrashear Schist has undergone post-metamorphicdeformation with associated mineralogical transforma-tions which have resulted in the distinctive nature of therocks of this zone. These transformations were accom-panied by gold mineralization without formation ofquartz veins, prior to the brittle fracturing that allowedlater quartz veins to form. The pre-quartz vein trans-formations are the main focus of this study, and areoutlined in the following sections. Terminology andmineral assemblages for these sections are summarizedin Fig. 4.

Secondary (post-metamorphic) minerals

The Intrashear Schist contains several minerals whichhave crystallized intimately with the primary mineralsthroughout the schist. Rutile is the most prominent ofthese minerals as it is nearly opaque, thus contributingto the dark colour of many samples. Rutile is the prin-cipal titanium mineral, rather than titanite which isubiquitous elsewhere in the Otago Schist. Titanite is veryrare in Intrashear Schist, and occurs only as small rag-ged grains in less-deformed rocks such as metacon-glomerate. Calcite and siderite are common in

Intrashear Schist, as foliation-parallel veinlets and alongmica cleavages. Kaolinite occurs interlayered withmuscovite or as scattered grains in many samples. Pyriteand chalcopyrite occur in deformed metamorphicquartz-albite segregation veins.

Post-metamorphic cleavage development

The pervasive metamorphic foliation has been tightlyfolded and disrupted by development of a near-pervasivepost-metamorphic cleavage (Fig. 5). The new cleavage is

Fig. 5A, B Photomicrographs of Intrashear Schist in polarized lightwith partially crossed polars, showing mineralogy and microstruc-tures. A Intrashear Schist with quartz and albite (clear grains, about0.1±0.2 mm) forming metamorphic segregations which have beenfolded and disrupted by development of post-metamorphic cleavage(parallel to heavy lines in top left, generally near horizontal in thisview) de®ned by muscovite and chlorite ¯akes (grey, elongate). Earlyfoliation accompanying the segregations has also been disrupted, andremnants (parallel to dashed lines in top left) are at an angle to the newcleavage. Black microshears (largest and most continuous is at top)contain ®ne grained rutile and graphite and lie parallel to the post-metamorphic micaceous cleavage. Horizontal ®eld of view is 4 mm. BMylonitic cataclasite developed in schist originally similar to that in A.The rock is dominated by closely spaced black microshears whichcontain hydrothermal rutile, pyrite, graphite and minor arsenopyrite.These minerals, with associated recrystallized muscovite (light colour)form a near-pervasive syn-mineralization fabric. This fabric anasto-moses around oriented lenses of ®ne grained recrystallized andintergrown muscovite and quartz (clear, about 0.01 mm grains) whichhave replaced albite. Horizontal ®eld of view � 2 mm

386

essentially parallel to the earlier foliation except wherefold hinges are preserved and there is a high angle ofintersection. In micaceous lamellae, the new cleavagedominates the fabric of the rock, and the earlier foliationis only locally preserved in millimetre scale lenses ofunrecrystallized rock. The new cleavage is de®ned bycoarsely crystalline (100 lm) muscovite and chlorite insheafs up to 100 lm wide. This new cleavage is struc-turally equivalent to the spaced cleavage formed duringpost-metamorphic folding of the Hangingwall andFootwall Schists, as described already. The cleavage inthe Intrashear Schist is more strongly developed as itformed in a high-strain zone on the lower limb of therecumbent fold shown in Fig. 3.

Black microshears

These microshears are thin (about 20±50 lm wide)shears which cross many rocks in the Intrashear Schistwith a spacing of about 1±2 mm. The microshears aregenerally curved, discontinuous and anastomosing.They are most common within micaceous lamellaewhere they occur along the late cleavage described in theprevious section. The microshears also occur along themargins of metamorphic quartz veins or quartz-albitesegregations adjacent to micaceous lamellae.

It is di�cult to resolve (by light microscopy or elec-tron optically) the minerals which occur within themicroshears due to ®ne grain size which is generally lessthan the thickness of a standard thin section (30 lm).However, microprobe analyses show that some portionsare made up of recrystallized muscovite (and, locally,chlorite) which has grown across the post-metamorphiccleavage at a low angle and has a ®ner grain size (20±50 lm). Calcite and siderite occur locally as thin veinletsalong microshears or as microcrystalline material lyingparallel to the cleavage of micas in the microshears andimmediately adjacent schist. Rutile is a common con-stituent as 30±50 lm subhedral crystals, and pyriteforms small ragged anhedral grains scattered along someblack microshears.

Most of the colouration of the black microshears isdue to very ®ne grained (micron scale) dusty opaquegraphitic material. This material was observed clearlyonly in specially prepared ultrathin polished sections(<5 lm thick). The graphitic material is soft and pol-ishes poorly, has low re¯ectance (<10%), distinctlylower than adjacent rutile grains in the microshears, andforms thin crystals with elongate sections (1±5 lm scale).Similar material was examined optically (light andelectron) by McKeag et al. (1989) and has green lightre¯ectance of 6±7% in oil.

Mylonitic cataclasite

Mylonitic cataclasite occurs principally adjacent to theHangingwall Shear, in the Intrashear Schist. This fault

rock falls at the boundary between mylonites and cat-aclasites (see Sibson 1977) as it has evidence for bothcrystal-plastic deformation and mechanical disruption.Mylonitic cataclasite involves an extreme form of blackmicroshear development (Fig. 5B), in that it is a rockwhich consists almost entirely of black microshears andassociated cataclastic zones, with ®ne grained (micronscale) recrystallized muscovite and chlorite which de®nea new foliation bending around disrupted primary schistporphyroclasts (mm to cm scale). Minor quartz recrys-tallization has occurred in porphyroclasts, to form poolsof ®ne grained (micron scale) annealed grains andnearly-annealed larger grains almost obscured by sub-grains. Ribbon quartz textures occur also. Some quartzgrains in small porphyroclasts have undergone pressuresolution on their margins, with localized redeposition ofquartz in pressure shadows. Primary albite grains arefractured, twinned, and have undulose extinction in less-deformed parts of this rock type, but in more intenselystrained rocks the albite has been totally replaced by ®negrained foliated muscovite intergrown with microcrys-talline quartz (Fig. 5B). Recrystallized zones in themylonitic cataclasite de®ne a foliation parallel to the latecleavage in adjacent non-mylonitic rocks.

Sulphide minerals, mainly pyrite and arsenopyrite,occur as euhedral grains up to 5 mm across inporphyroclasts and less-deformed portions of myloniticrock. These have been fractured near to mylonitic zones,and completely recrystallized at submicron scale in themost intensely deformed mylonitic cataclasite zones.Hydrothermal addition of ®ne grained (micron scale)pyrite accompanied replacement of albite by foliatedmuscovite (Fig. 5B), and this new pyrite formed eu-hedral crystals locally grown across microshears. Someof these crystals were subsequently fractured giving riseto trails of submicroscopic anhedral pyrite along reac-tivated microshears (Fig. 5B). Gold occurs as micronscale blebs in all generations of these sulphides, ascoarser grains (up to 50 lm) in fractures in the sulphides,and as submicron grains dispersed through ®ne grainedmylonitic rock. The latter gold is detectable only withelectron microprobe backscatter images (Youngson andCraw 1993). These occurrences of gold are in sulphideswhich replace mylonitic cataclasite, with no visiblequartz veins.

Cataclasite

Cataclasite occurs along fault zones such as the Hang-ingwall and Footwall shears, and other shears within theIntrashear Schist. Cataclasite is essentially crushed rockwhose grain size has been physically reduced withoutrecrystallization, and the mineralogy is identical to thatof the immediately adjacent rocks. This cataclasite post-dates mylonitic cataclasite described, but occurs in thesame locations, particularly in the Hangingwall Shear.

Cataclasite is also common but volumetrically in-signi®cant throughout the Intrashear Schist as thin

387

(<50 lm) zones along sheared foliation surfaces, with aspacing of 1±10 cm. The cataclasite is generally ac-companied by shiny black slickensides on these sur-faces. These polished surfaces consist of ®nelycomminuted host rock including sulphides and graph-ite. The latter minerals contribute to the black shinyappearance of these rock surfaces, particularly wherethe slickensides are developed on mylonite cataclasitefoliation surfaces.

Hydrothermal alteration of Intrashear Schist

Geochemical analyses

The Intrashear Schist has clearly been the locus of ex-tensive post-metamorphic ¯uid ¯ow before formation ofthe quartz veins which host most of the gold. The mostobvious geochemical changes accompanying this ¯uid¯ow are introduction of Au, As and S to the system.However, the more subtle petrographic changes de-scribed can be evaluated geochemically as well. Wholerock X-ray ¯uorescence analyses (see McKeag et al.1989) were conducted on powders taken from relativelyhomogeneous samples (1±10 kg) of Intrashear Schistincluding mylonitic cataclasite from drill core and mineworkings, for comparison to typical Otago Schist(Palmer et al. 1992).

Intrashear Schist samples, like all Otago Schist sam-ples, are inhomogeneous at the hand specimen scalebecause of extensive metamorphic segregation andmetamorphic vein formation. Hence, these samplescontain variable proportions of veins and phyllosilicatesegregations and analyses are correspondingly variablydiluted with quartz-rich material. Most of the quartz-rich material is metamorphic segregations, but someminor thin veinlets of late-stage mineralized quartz mayhave been included also. Representative sample analysesare presented in Table 1.

Immobile elements

Elements generally considered to be immobile duringmetamorphic-hydrothermal processes, Ti, Zr, Y, and Al(Dipple et al. 1990; Selverstone et al. 1991), are plottedagainst each other and in comparison to typical OtagoSchist (Fig. 6A, B). These plots show that most of theanalyzed samples lie within the ®eld of typical OtagoSchist. Both plots show a generally linear trend withapproximately zero intercept, as expected for immobileelements. Samples which de®ne the trend to lowercontents of all these immobile elements (Fig. 6A, B)have elevated silica which results in dilution of the otherelements.

Petrographic observations show that rutile is locallyenriched in black microshears compared to the unalteredrock between microshears. Hence, some local mobilityof Ti, at the millimetre scale, has occurred in many parts

of the Intrashear Schist. However, the consistent rela-tionship with Zr (Fig. 6A) suggests that Ti mobility didnot extend to the hand specimen scale.

Chromium enrichment at Macraes relative to typicalOtago Schist has been previously demonstrated(McKeag et al. 1989). A more extensive data set pre-sented here (Fig. 6C) con®rms this relative Cr enrich-ment which is similar in magnitude for both backgroundIntrashear Schist and mylonitic cataclasite. One addi-tional mylonitic cataclasite sample has Cr near1000 ppm, but this result was di�cult to reproduce withprecision and the sample is not plotted here. Relation-ships between Cr and other immobile elements (e.g. Zr,Fig. 6C) are poorly de®ned and inconclusive.

Cr occurs in the phyllosilicate layers in Otago Schist,principally in chlorite (Hay and Craw 1993), so sampleCr content is related to the proportion of phyllosilicatesegregations in the sample. To allow for this e�ect, wehave normalized Cr with MgO (Fig. 7A), and Ni isplotted in a similar way for comparison. Both Cr/MgOand Ni/MgO are strongly enriched in Intrashear Schistcompared to typical Otago Schist, con®rming the en-richment of Cr (and Ni) in Intrashear Schist (describedalready), and there is little di�erence between back-ground Intrashear Schist and mylonitic cataclasite. Thehigh Cr may be a primary lithological feature due tohigh sedimentary Cr, as suggested by the presence offuchsitic micas in conglomerate clasts in the IntrashearSchist (Craw and Angus 1993). Alternatively, the Crmay have been introduced hydrothermally by latemetamorphic auriferous ¯uids as has been reportedelsewhere in Otago (Hay and Craw 1993). The parallelenrichment of Cr and Ni (Fig. 7A) suggests that theformer explanation is more likely, but more geochemicalwork is needed to con®rm this.

Mobile elements

Potassium and sodium concentrations in backgroundIntrashear Schist are similar to typical Otago Schistexcept for the samples diluted with quartz (Fig. 6D, E).However, some mylonitic cataclasite samples havehigher potassium and lower sodium than the rest of theIntrashear Schist (Fig. 6D, E). Sodium depletion is themost pronounced of these trends, with the myloniticcataclasite de®ning a separate ®eld in Fig. 6E. Thechanges in alkali contents of mylonitic cataclasite sam-ples re¯ects the replacement of albite by muscovite seenpetrographically in these rocks (described already).Calcium concentrations in Intrashear Schist generallyfall within the typical Otago Schist ®eld, and minordeviations re¯ect petrographically observed variations incalcite content.

The changes in alkalis documented above can becombined to provide a geochemical alteration index,K2O/Na2O, which increases with increasing hydrother-mal alteration. This is used to document the di�erencesbetween background Intrashear Schist and mylonitic

388

Table

1Geochem

icalanalysesofIntrashearSchist,Macraes

mine,

withatypicalOtagoSchistpelitic

rock

from

thehangingwallatMacraes

forcomparison

Sample

BB3679

BB1331

IM540.00

IM53014c

BB488

RH001

BB3540

IM53014A

IM53014b

BB3662

BB488

Hanging

wall

Rock

type

I.Schist

I.Schist

I.Schist

I.Schist

Mylon.

Cat.

Mylon.

Cat.

Mylon.

Cat.

Mylon.

Cat.

Mylon.

Cat.

Mylon.

Cat.

Mylon.

Cat.

Pelitic

SiO

261.06

63.87

60.80

66.00

68.08

63.74

68.60

58.23

71.82

64.09

68.08

67.53

TiO

20.70

0.70

0.81

0.65

0.60

0.63

0.57

0.79

0.49

0.62

0.60

0.68

Al 2O

314.60

14.81

15.03

15.54

14.04

13.95

14.10

17.94

11.85

14.17

14.04

14.76

Fe 2O

3(tot)

5.95

6.13

6.69

5.28

5.49

5.24

4.23

6.86

3.92

5.28

5.49

5.43

MnO

0.08

0.10

0.14

0.07

0.08

0.07

0.06

0.06

0.07

0.07

0.08

0.10

MgO

2.18

2.25

2.46

1.63

1.50

2.05

1.12

1.83

1.38

1.69

1.50

1.75

CaO

2.10

2.32

3.56

0.94

1.98

2.53

1.62

1.35

1.51

1.61

1.98

1.35

Na2O

1.06

2.46

3.20

2.60

1.42

0.54

1.96

0.30

1.82

1.81

1.42

2.64

K2O

3.61

2.41

1.59

2.97

3.44

3.62

2.56

5.27

2.21

2.85

3.44

2.30

P2O

50.17

0.17

0.24

0.15

0.15

0.12

0.10

0.17

0.13

0.14

0.15

0.19

Loi

7.01

3.21

3.60

3.13

4.71

6.33

4.40

6.64

2.83

6.30

4.71

3.17

Total

98.70

98.43

98.13

98.99

101.75

98.99

99.32

100.08

98.18

98.66

101.75

99.90

Rb

145

98

58

123

133

140

97

210

91

116

133

91

Sr

166

275

508

146

314

337

168

181

225

164

314

228

Y29

25

30

27

27

25

22

33

18

24

27

26

Zr

148

149

147

184

157

184

180

236

133

161

157

147

Pb

15

18

12

10

19

22

16

31

717

19

13

Th

11

10

712

11

12

916

812

11

11

Ni

25

28

16

21

20

23

17

61

18

23

20

20

Cu

26

18

21

20

25

20

10

28

515

25

33

Zn

95

88

103

80

79

77

67

87

63

85

79

99

Nb

11

10

911

10

10

913

911

10

9V

138

125

143

121

107

116

89

164

94

106

107

118

Cr

140

137

62

76

122

76

281

521

69

253

122

68

Ba

706

571

552

690

590

569

501

1108

455

574

590

620

La

19

22

18

22

20

23

17

35

18

21

20

51

Ce

47

53

50

55

53

63

48

92

48

49

53

81

Nd

28

24

22

23

30

28

21

54

22

22

30

nd

As

1702

38

28

54

2560

1037

30

6201

1370

300

2560

4S%

0.46

0.09

0.21

0.08

0.68

1.20

0.04

0.56

0.31

0.42

1.03

nd

W143

0111

213

76

623

2179

180

28

76

nd

Au

1.28

0.02

nd

nd

3.75

nd

0.01

nd

nd

0.24

3.75

nd

TOC

%0.47

0.35

0.10

0.26

1.03

0.30

0.71

0.56

0.33

0.42

1.03

nd

Oxides,sulphurandtotalorganiccarbon(TOC)are

percentages;trace

elem

entsare

inpartsper

million.Organiccarbon(TOC)wasdetermined

onaliquotsofsamplepowdersusinga

CarloErber

CHNSElementalAnalyzerafter

removingcarbonate

carbonwithHClat80

°Covernight.Gold

analyseswereobtained

by®re

assayfollowed

byatomic

absorption

spectrophotometry

(0.05ppm

detectionlimit).Abbreviations;I.Schist,IntrashearSchist,includingMyloniteCat.,mylonitic

cataclasite;nd,notdetermined;bd,below

detection;Loi,

loss

onignition;totdenotesallironquotedasferric

389

Fig. 6A±F Geochemical plots showing variations of key major andtrace elements for background Intrashear Schist (black diamonds) andmylonitic cataclasite from the Intrashear Schist (open squares). Rangeof data (ellipse) for typical Otago Schist (from Palmer et al. 1992) isindicated for comparison. A Zr versus TiO2, and B Y versus Al2O3

showing the general similarity between Intrashear Schist and typical

Otago Schist for elements generally considered to be immobile duringmetamorphism. C Zr versus Cr shows a general enrichment in Cr inIntrashear Schist. D Zr versus K2O, and E Zr versus Na2O, showdeviations of mylonitic cataclasite alkalis from typical OtagoSchist compositions. F Zr versus Ca shows little deviation fromOtago Schist

390

Fig. 7A±F Geochemical plots (symbols as for Fig. 6) for suspectedmobile elements; note the logarithmic scale on many axes. ACr and Ninormalized with MgO. B±E Show variation of total organic carbon(TOC), As, S, and Au with increasing degree of hydrothermalalteration of Intrashear Schist as indicated by increasing K2O/Na2O

(re¯ecting replacement of albite by muscovite). F Shows therelationship between Au content and rock silica content. TypicalOtago Schist range for E and F is indicated with heavy line below thehorizontal axis

391

cataclasite with respect to some elements suspected to bemobile from petrographic observations (Fig. 7B, C, D).The strong enrichment of graphitic carbon (TOC) inIntrashear schist compared to typical Otago Schist isapparent (Fig. 7B), particularly for mylonitic catacla-site. Selected black portions of mylonitic cataclasite (1±2 g samples) have up to 2.5 wt.% TOC.

Intrashear Schist is visibly enriched in sulphides, andthis is con®rmed with As and S analyses (Fig. 7C, D).All Intrashear Schist samples are enriched with As abovetypical Otago Schist levels (Fig. 7C). As contents ofmany samples are near 1 wt.% (10 000 ppm, Fig. 7C),similar to the S contents of the same samples, implyingthat most sulphide in these samples is arsenopyrite. Therest of the generally high sulphur content (Fig. 7D) isdue to pyrite, which is particularly enriched in somemylonitic cataclasite samples. Copper contents are low(ca. 5±50 ppm) in all analyzed samples (Table 1).

Gold enrichment has occurred in most of the Intra-shear Schist samples, particularly mylonitic cataclasite(Fig. 7E), and gold concentrations range from belowdetection (50 ppb) up to 11 ppm. This Au enrichmentoccurred without large additions of silica (i.e. no quartzveins), except for two Intrashear Schist samples(Fig. 7F) which also display corresponding dilution ofimmobile elements (Fig. 6A, B). The silica content ofmany of the Intrashear Schist samples is lower than intypical Otago Schist (Fig. 7F). Hence, some loss of silicamay have occurred during development of the shearzones and associated hydrothermal alteration and goldmineralisation. However, primary lithologic di�erencescannot be ruled out, and high Cr and Ni (Fig. 7A) mayre¯ect a high ma®c (low silica) component in the meta-sediments.

Discussion

Rutile formation

Rutile is rare as a metamorphic mineral in the OtagoSchist, and occurs only in some impure marbles whichare themselves rare in the schist belt (Craw 1984). Post-metamorphic hydrothermal alteration of titanite inschist has resulted in localised rutile formation atBrighton, about 70 km SE of Macraes (Craw et al.1982). Formation of rutile at the expense of titanite canresult from increasing CO2 content of the ¯uid, by de-creasing the temperature with a constant CO2 content,or some combination of these (Hunt and Kerrick 1977).Decreasing temperature has not resulted in widespreadrutile formation in the Otago Schist, but rutile formationat Macraes may have been encouraged by post-meta-morphic high strain restricted to the Intrashear Schist.We can conclude merely that post-metamorphic ¯uid¯ow accompanying ductile and semiductile deformationin the Intrashear Schist has resulted in pervasive alter-ation of titanite to rutile over a restricted interval up to120 m thick.

Destruction of epidote

Like titanite, epidote is a ubiquitous mineral in the Ot-ago Schist. Epidote is a more complex mineral thantitanite, so it is not possible to identify a single break-down reaction. Several aluminous minerals (muscovite,chlorite, kaolinite) were crystallizing during post-meta-morphic deformation and hydrothermal alteration, andany or all of these may have been recipients of epidotecomponents. Calcium from epidote went into calcite, theprincipal Ca-bearing secondary mineral in the Intra-shear Schist. Alteration of epidote to calcite and kaoli-nite occurs in low-CO2 ¯uid under sub-greenschistconditions (Bird and Helgeson 1981). This type of al-teration was inferred by Craw et al. (1982) to havecaused localized kaolinitisation at Brighton (see earlier),and may have been responsible for similar alteration atMacraes.

Normal Otago Schist epidotes contain about 15 mole% of the iron end-member (Brown 1967), so the prod-ucts of epidote breakdown should include an iron min-eral. The common occurrence of siderite as a secondarymineral in the Intrashear Schist may be in part due tothe iron component of epidote, forming iron carbonateas well as calcium carbonate. It is notable that nearly allthe iron in the epidote structure is the oxidized form,Fe3+, whereas that in siderite is the reduced form, Fe2+.The only other iron minerals crystallizing during the latemetamorphic alteration are almost exclusively Fe2+-bearing (pyrite, chalcopyrite, chlorite). Hence, the hy-drothermal ¯uid which altered the Intrashear Schist mayhave also had a reducing e�ect on the rock.

Graphite formation

Petrographic evidence (already mentioned) suggests thatmost of the graphite was introduced into the IntrashearSchist during formation of the black microshears. Thus,the graphite is a hydrothermal mineral, deposited by thepost-metamorphic ¯uid discussed. This has resulted indistinct enrichment of the rocks in graphite. H2O-CO2

¯uids below about 400 °C will be in equilibrium withgraphite (Holloway 1984), although the amount ofgraphite may be small (e.g. typical Otago SchistTOC � 0.1%). To deposit greater quantities of graph-ite, an additional carbon component of the ¯uid is re-quired, and methane is one possible source (Rumble etal. 1986). However, there is as yet no direct evidence formethane in Intrashear Schist during this post-meta-morphic alteration.

Conclusions

A hydrothermal continuum

Evidence has been presented for structurally controlledhydrothermal activity in the Intrashear Schist in the

392

latter stages of metamorphism and ductile deformation,and this continued through to later brittle fracture-controlled vein systems. This combined structural andmineralogical data implies that hydrothermal activitypersisted while the schist belt was uplifted from nearmetamorphic depths (about 15±20 km, see earlier)through the brittle-ductile transition (Sibson 1977) tothe brittle region (about 10 km). The observations aresummarized in the generalized context of crustal level inFig. 8. The Intrashear Schist has hosted distinctly more¯uid ¯ow throughout that time than adjacent unalteredand unmineralized schist bodies. Fluid ¯ow was initiallypervasive, but became progressively more focused into asmaller number of more permeable structures. Hydro-thermal alteration was essentially isochemical in theinitial stages of Intrashear Schist alteration, but laterhydrothermal activity introduced increasing amounts ofgraphite and sulphides to the ¯uid ¯ow zones, andcaused replacement of metamorphic albite by musco-vite.

Gold mineralization

Signi®cant amounts of gold were deposited near thebrittle-ductile transition in mylonitic cataclasites withoutquartz veins (this study), and more gold was deposited inquartz veins (previous studies) in later brittle structures(Fig. 8). There is as yet no direct evidence for gold de-position in black microshears or earlier structures, butnearly all analyzed samples in the hydrothermally al-tered shear zone have anomalous concentrations of As,S and Au (Fig. 7C±F). Textural evidence suggests thatthe gold-bearing sulphides formed in the mylonitic cat-aclasite in the latter stages of hydrothermal alteration,and the sulphides crosscut and replace black micro-shears. Gold deposition occurred with sulphides due toreduction by graphitic schist (McKeag et al. 1989; Craw1992), and gold-bearing sulphides are commonly foundin or adjacent to graphitic portions of mylonitic cat-aclasites. The combination of structurally controlledpost-metamorphic ¯uid ¯ow and the introduction ofgraphite throughout the Intrashear Schist provided thebasis for deposition of gold during deformation. Themore voluminous later quartz veins are clearly con-trolled by brittle structures, but hydrothermal graphitein the adjacent Intrashear Schist may have played aminor role in localization of gold in these veins as well.

Acknowledgements This study was supported ®nancially and lo-gistically by Macraes Mining Co Ltd whose continuing enthusiasmfor applied research is gratefully acknowledged. Discussions withR. J. Norris, A. Reay, J. Scott, and S. White helped to crystallizeand clarify some of the ideas presented. B. Pooley, D. Walls andD. Weston provided high-quality technical assistance. Incisive re-views by N. Oliver and an anonymous referee, and constructivecomments from J. Hedenquist and D. Rickard greatly improvedthe manuscript.

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