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Geometry and petrography of stockwork veinswarms, Macraes mine, Otago Schist, New ZealandM. J. Begbie a & D. Craw ba Department of Geology , University of Otago , P.O. Box 56, Dunedin, New Zealand E-mail:b Department of Geology , University of Otago , P.O. Box 56, Dunedin, New ZealandPublished online: 22 Sep 2010.

To cite this article: M. J. Begbie & D. Craw (2006) Geometry and petrography of stockwork vein swarms,Macraes mine, Otago Schist, New Zealand, New Zealand Journal of Geology and Geophysics, 49:1, 63-73, DOI:10.1080/00288306.2006.9515148

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New Zealand Journal of Geology & Geophysics, 2006, Vol. 49Begbie&Craw—Stockworkveinswarms,Macraesmine: 6 3 - 7 30028-8306/06/4901-0063 © The Royal Society of New Zealand 2006

63

Geometry and petrography of stockwork vein swarms, Macraes mine,Otago Schist, New Zealand

M. J. BEGBIE

D. CRAWDepartment of GeologyUniversity of OtagoP.O. Box 56Dunedin, New Zealandmike.begbie@stonebow.otago.ac.nz

Abstract The Macraes gold deposit is located in a low-anglenortheast-dipping (c. 15°) shear zone, the Hyde-MacraesShear Zone (HMSZ) cutting greenschist facies metasedimentsof the Otago Schist. The shear zone is host to large volumes ofmineralised schist and relatively sparse quartz veins. Duringthe development of this through-going shear, the schist un-derwent episodes of hydrofracturing and fluid redistribution.As a result, parts of the HMSZ are pervaded by swarms ofquartz-filled pure extension and extensional-shear fractureswith gold and scheelite. The swarms comprise networks or"stockworks" of veins that strike northeast, are subvertical,and form subperpendicular to the shear zone fabric. Veinswarms are mostly restricted to more competent pods of mas-sive schist within the Intrashear Schist. Vein frequency acrossstrike within swarms is typically c. 1/m, with most veins5-10 cm thick and vertically continuous for up to c. 20 m.Many of the veins show delicately laminated internal texturesthat are parallel to the vein margins. These laminae indicateincremental growth and are defined by a variation in grain sizeor by thin slivers of wall-rock schist. Veins were initiated asbrittle structures, but have been subsequently deformed in aductile manner. Most veins are variably overprinted by plasticdeformation, although many primary structures are preserved.This deformation has produced subgrain structures, migra-tion of grain boundaries with sutured margins, and unduloseextinction of grains. These microstructural observationsindicate temperatures were between 300 and 400°C duringstockwork vein deformation. The geometry of stockworkveins indicate that localised regions of extension occurredwithin the HMSZ during continuous shortening. Localisedextension was mainly parallel to the structural trend; that is,it took place perpendicular to the direction of inferred thrust-ing. Transitions from shortening to local extension were mostlikely driven by changes in shear zone geometry (e.g., lateralor oblique ramps).

Keywords stockwork veins; gold; lateral ramps; Hyde-Macraes Shear Zone; Otago Schist

G05015; Online publication date 28 February 2006Received 15 April 2005; accepted 28 October 2005

INTRODUCTION

Gold-bearing veins in the Otago Schist are generally well-defined structures that occur in discrete fault zones (Williams1974; Paterson 1986; Hay & Craw 1993; MacKenzie &Craw 1993). The veins vary from centimetre scale to metrescale in thickness and can be traced along-strike for tens orhundreds of metres. Mineralised faults of this type occur ingroups separated by tens or hundreds of metres of unmin-eralised schist. Most of these faults have a northwest strikeand dip moderately to steeply northeastwards. More than200 such structures have been mined or seriously prospectedhistorically in Otago (Williams 1974; Paterson 1986; Craw& Norris 1991).

The Macraes mine of east Otago (Fig. 1) is developed ina regional structure, the Hyde-Macraes Shear Zone (HMSZ)(Teagle et al. 1990). This gently (15°) northeast-dippingshear zone contains variably mineralised schist and sparsequartz veins (Craw et al. 1999), and is therefore distinctlydifferent from other Otago mineralised vein systems. Somewell-defined but discontinuous quartz veins dip shallowly tothe northeast subparallel to the shear zone, and these wereexploited historically (Williamson 1939; Williams 1974). Themodern mine was developed initially at Round Hill (Fig. 1),where these shallow-dipping veins were relatively abundant.Subsequent mine development has led to exposure of min-eralised shear zone rocks that lack quartz veins (Craw et al.1999; Craw 2002).

In addition, modern mining has exposed swarms of closelyspaced (metre scale), thin (centimetre scale), steeply dippingquartz veins containing scheelite and gold (Fig. 2). Theseveins strike northeast, at a high angle to the hosting structure(Angus 1993), and constitute a distinctly different style ofmineralisation from that observed elsewhere in Otago. Thevein swarms are locally interconnected and form a stockworkarray in some outcrops (Angus 1993; de Ronde et al. 2000).Despite their economic significance in the modern mine, andtheir potential for an exploration target elsewhere in Otago,these stockwork vein swarms have not been described in detailfrom a structural point of view. This paper provides structuraldata and analysis on these veins, and documents their scaleand extent through the exposed parts of the mineralised shearzone. The paper also interprets vein emplacement processesin the context of the structural evolution of the shear zone,using structural and petrographic observations on the veinsand their host rocks.

GENERAL GEOLOGY

The Otago Schist is composed predominantly of greenschistfacies metasediments with lesser amounts of metabasite(Craw 1984; Mortimer 1993a). The schist belt developedduring Mesozoic collisional amalgamation of at least twometasedimentary terranes—the volcanogenic Caples Terrane

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Begbie & Craw—Stockwork vein swarms, Macraes mine 65

NE

Fig. 2 A, B, Cross-sectional views of the general structural geometry through the Hyde-Macraes Shear Zone at Frasers Pit. Viewingdirection of photographs is parallel to the northwest strike of the shear zone. Intrashear Schist is enclosed between the Hangingwall Shear(exposed) and the Footwall Fault (not exposed). Photograph (B) is c. 250 m northwest along-strike from (A). C, Typical exposure ofmineralised stockwork veins. Veins are subvertical and form at high angle to the schist foliation. These veins are hosted in the metamor-phic-hydrothermally altered intrashear schist.

NW IntrashearSchist

SE

Stacked sequenceo f laminated

j fault-veins (quartz)

'stockwork'veins

(black)Lateralramp

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66 New Zealand Journal of Geology and Geophysics, 2006, Vol. 49

striations

/viewpointof Fig. 3B

sketch

averagefold axis

N = 38

Fig. 4 Lower hemisphere equal area stereonet of structural com-ponents shown in Fig. 3B from the Golden Point Pit. Striated shearsurfaces on low-angle fault-veins and the lateral ramp consistentlyindicate a top-to-the-southwest sense of movement.

alteration of the former. This alteration includes addition ofgraphite and sulfides, replacement of titanite by rutile, anddecomposition of epidote (Craw et al. 1999).

The HMSZ formed as a thrust system during late meta-morphic compression, and movement in the vicinity of theMacraes mine was directed towards the southwest (Teagle etal. 1990). The shear zone developed subparallel to the folia-tion, and the foliation was deformed into a duplex system,which resulted in the stacking and thickening that character-ises the central part of the zone (Fig. 2A,B). Shallow dippingfault-veins formed subparallel to the principal shears and theenclosing foliation during the thrusting (Fig. 3A,B, 4). Thesefault-veins are generally <<1 m thick, but can reach >2 mthickness (Fig. 3A,B). The fault-veins fill local extensionalsites (metre scale) in the duplex thrust system. Shallow-dip-ping tension gashes occur also as part of this duplex system(Teagle et al. 1990). In addition, steeper dipping (c. 40º)shears strike subparallel to the movement direction and at ahigh angle to the main northwest-striking shear zone. Theseshears form links between northwest-striking shears, andcontain similar mineralised rocks, including shear-parallelquartz veins. These linking shears have been interpreted aslateral ramp structures formed at steps in the main thrust zone(R. J. Norris pers. comm. 1992; Angus 1993). A quartz-veinedlateral ramp structure can be seen cutting and merging with athick foliation-parallel quartz vein in Fig. 3B (centre).

The Macraes mine is a large open-cut operation beingdeveloped in the HMSZ. The mine resource estimate in 2004was 3.9 Moz of gold in 87 Mt of ore at 1.4 g Au/t. This in-cluded reserves of 2.1 Moz gold in 47 Mt of ore, at 1.4 g Au/t.Gold occurs principally as micrometre-scale blebs in pyriteand arsenopyrite. These sulfide minerals replace silicates inIntrashear Schist, occur along brittle/ductile microshears, andare vein-forming minerals in low-angle fault-veins. Gold alsooccurs in steeply dipping vein arrays (stockwork veins; Angus1993) that are the subject of the present study.

STOCKWORK VEINS

Description and geometryStockwork veins occur in localised swarms that are confinedto the Intrashear Schist (Fig. 2A,B). Individual swarms rangefrom c. 100 to 2000 m2 in area and consist of numerous(10-100) subparallel veins. Most of these veins formed sub-perpendicular to the shallow northeast-dipping shear fabricof the Intrashear Schist (Fig. 2C). Stockwork veins occupyapproximately planar to sinuous, smooth sided fractures.These mineral-filled fractures are typically traceable for1-5 m vertically, although some can be traced for up to 20 m.Most veins filling fractures are 5-10 cm thick, but can be upto 50 cm thick. The thickness of veins is variable, as veinscommonly tend to pinch and swell along the strike and dip(Fig. 5). Most of the veins terminate as gradually taperingstructures in two dimensions, or are truncated and offset bylater structures (Fig. 3B, 5). Vein frequency across strikewithin swarms is c. 1/m, although vein spacing is not uniform.Stockwork veins are locally deformed by small wavelength(<25 cm), low amplitude (<10 cm) open style folds (Fig. 6).Fold axes typically plunge steeply to the northeast, althoughsubhorizontal fold axes occur as well.

The Intrashear Schist hosting the stockwork veins is highlyanisotropic. It contains lensoidal and folded bodies of morecompetent massive schist embedded within relatively incom-petent fissile schist. Massive schist pods are up to several tensof metres in their longest dimension. The fissile schist has awell-developed, closely spaced (c. 1 mm) penetrative folia-tion whereas the massive schist only has a weakly developedfoliation, typically spaced on the decimetre to centimetrescale. Foliation developed in the fissile material often deflectsaway from the massive schist pods, but invariably crosscutsor terminates at the lithological contacts. Most stockworkveins occur in and near massive Intrashear Schist pods, butsome extend into underlying and overlying fissile schist aswell. The extent of vein swarms is essentially controlled bythe dimensions of the massive schist pods. For example,Fig. 7 illustrates a swarm of northeast-striking, subverticalstockwork veins. These veins have almost entirely developedin the massive schist pods with only a small number in thefissile schist. Those veins developed in the fissile schist ap-pear to be almost an order of magnitude smaller in lengthand width. Similarly, Fig. 5 shows a series of folded podsof massive schist enclosed by more highly deformed fissileschist. Stockwork veins that crosscut the massive schist aresubvertical and strike consistently northeast, whereas veinsin the fissile schist typically dip moderate to steeply to eitherthe northwest or southeast. Veins that pass continuouslythrough massive schist into fissile schist or vice versa com-monly deflect to a shallower or steeper dip, respectively, asthey transect the lithologic boundary.

In general, stockwork veins systematically strike northeastand are subvertical (Fig. 8A), although infrequently veinshave shallower dips (Fig. 3, 5). At individual swarms, veinscommonly show a fairly restricted range in strike (±20° fromthe average) and dip (±5° from the average) values (Fig. 7).The stockwork veins cut steeply across the structural trendof HMSZ shears, low-angle veins, and folds associated withHMSZ thrusting (Fig. 4, 8B). There are mutual crosscuttingrelations between low-angle fault-veins/tension gashes andstockwork veins (Fig. 3A,B, 9). For example, Fig. 3 shows aseries of low-angle fault-veins that are subparallel to the lo-cal foliation with the exception of a locally developed lateral

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Begbie & Craw—Stockwork vein swarms, Macraes mine 67

SE • - - N W

fissileschist

metamorphic"segregation

veins

Fig. 5 Field sketch illustrating the influence of competence (tensile strength) contrast on the development of stockwork veins. Pureextension veins (subvertical) open in the relatively higher tensile strength massive schist whereas extensional shear veins initiated in thefissile schist. Inset: A lower hemisphere equal area stereonet of stockwork veins.

ramp. Stockwork veins are strike-parallel with the lateralramp, but with different dips. Slip lineations on fault-veinshear surfaces are common and where present are consis-tently parallel to the strike of stockwork veins (Fig. 4). Thelow-angle veins cut and are crosscut by stockwork veins. Thestockwork veins are locally deformed (folded and/or rotated)and variably offset by flat veins. Slivers of wallrock entrainedwithin the flat veins often contain portions of truncated stock-work veins. Some of these veins have relatively shallow dips,but still maintain a high angular relationship to the foliation.Veins within wallrock slivers were most likely rotated withprogressive deformation.

Internal structure and petrographyStockwork veins consist of quartz, muscovite, scheelite,calcite, rutile, gold, and sulfide minerals (pyrite and arseno-pyrite). Grain size varies from micrometre scale to millimetrescale (Fig. 10A-D). Quartz is the dominant vein mineral, butscheelite is locally an important constituent (Fig. 10A,E).Muscovite grain size (millimetre scale) is similar to, or coarserthan, that of the host schist. Sulfide mineral grains are com-monly scattered through the quartz and are generally spatiallyseparate (at the millimetre scale) from scheelite.

Many of the veins show finely laminated (millimetre-cen-timetre scale) internal textures that are parallel to the veinmargins (Fig. 11 A). These laminae are defined by either varia-tions in vein mineral grain size, thin slivers of wallrock schist,or thin seams of scheelite (Fig. 10A,E). Individual quartzlaminae range from <1 mm to c. 30 mm in thickness and arecomposed of anhedral to subhedral quartz (Fig. 10A,C,F).

N

_040/3_5SE(foliation)

massiveschist

- fold axisorientation 1m

Fig. 6 Field sketch illustrating a set of folded stockwork veins.

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68 New Zealand Journal of Geology and Geophysics, 2006, Vol. 49

NWmassiveschist

^ 3 : X > - fissile <-—_j~_ - schist

,<? c -

__ 340/15E __ .(foliation)__ 1— —

massiveschistmetamorphic

segregation vein(quartz, albite,

muscovite,chlorite)

12°-305(fold axis)

flat vein188/05W

Stockwork and flat veinsare mineralogically identical

(quartz, muscovite, scheelite,_ .calcite, rutile, gold, sulfides)

- - - - - - - 340/35E(foliation)

~~ — ~- — t — •»

0 0.4 m

N = 19

Fig. 7 Fieldsketchillustratingtheinfluence of competence (tensilestrength) contrast on the develop-ment of stockwork veins. Mostsubvertical stockwork veins (pureextension) formed in the more com-petentmassive schist with relativelyfew developed in the less competentfissile schist. The fissile materialappears to have taken up strain byfoliation-parallel shear.

Fig. 8 Lower hemisphere equalarea stereonets of: (A) poles tostockwork veins, and (B) fold axesof thrust-related folding.

Small fluid inclusions (<4 µm) are abundant and mainly formalong healed microfractures, of which multiple generationsoccur (Fig. 10B). Some veins contain breccias, which arepartly or wholly silicified (Fig. 1 1B). Vein breccias are com-posed of angular to subangular clasts of wall-rock and/or veinmaterial. Stockwork veins are crosscut and variably offset(<5 mm) by networks of thin (c. 1—2 mm), shallow dipping tosubhorizontal late calcite veins. Stylolite-like solution seamsare a common feature in the veins and are characterised bylocal concentrations of sulfides and muscovite (Fig. 10F).These solution seams are thin (<0.5 mm), relatively discon-tinuous, and have wriggly to smooth appearances. They aretypically oriented subparallel to the vein margins. Minorpost-mineralisation cataclasis has occured along many of thevein margins.

Quartz grains usually exhibit intense undulose extinction,kink and deformation bands, as well as deformation lamel-lae (Fig. 10C,D). Most of the veins' primary microstructuresand textures are largely preserved. The high angle grainboundaries are sutured with a very small wavelength (seri-ate-interlobate in shape) and tiny new grains (subgrains)

< Fig. 9 Field sketch showing a subvertical stockwork vein cross-cutting a low-angle vein. Flat veins and stockwork veins showmutual crosscutting relationships. Flat veins are differentiated fromfoliation-parallel metamorphic segregation veins by mineralogyand geometry.

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Begbie & Craw—Stockwork vein swarms, Macraes mine 69

Fig. 10 Photomicrographs illustrating the internal characteristics of stockwork veins (all crossed-polarised light except for E). A, Lamina-tions are defined by slivers of wallrock, bands of scheelite, or more commonly by variations in quartz grain size. B, Acicular rutile needlesin quartz crystals. Vertical planes are trails of fluid inclusions. C, Crystal plastic deformation of quartz indicated by pronounced unduloseextinction, deformation bands, and deformation lamellae. Grain boundaries are irregular due to grain boundary migration. Note develop-ment of subgrains. D, Relic of a quartz fibre, nearly completely replaced by new recrystallised grains. These grains are polygonised andc. 25 µm in size. E, Laminations of quartz and scheelite crosscut and variably offset by late calcite veins. F, Activity of pressure solutionis indicated by formation of stylolitic mica and sulfide seams.

with a diameter of c. 20-30 µm are formed along pre-existing (Fig. 10D). The shape of some recrystallised grains is nearlygrain boundaries (Fig. 10C). Weakly strained remnants of the equant, suggesting complete recovery (Passchier & Trouworiginally millimetre-sized quartz grains and rare quartz fibres 1998). Undulatory extinction is also observed in some of theare often preserved within a matrix of recrystallised grains recrystallised grains.

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70 New Zealand Journal of Geology and Geophysics, 2006, Vol. 49

Fig. 11 Internal structure of stockwork veins. A, Hand specimen of banded/laminated vein. Laminations are oriented parallel to thevein wall and crosscut at high angle by late carbonate veins. B, Hand specimen of laminated/brecciated vein. Dashed white line indicatesvein/wallrock boundary.

DISCUSSION

Sense of separation and mode of fracture

Stockwork veins were examined in the field and in orientedthin sections to determine their relative sense of separa-tion/displacement and, hence, the type of structure hostingthe vein. Three basic types or modes of brittle fracture mayoccur in intact rock (Hancock 1985). These are faults/shearfractures, pure extension fractures, and extensional-shearfractures (Fig. 12). Offset markers, such as earlier low-angleveins and/or metamorphic segregation veins, demonstratethat the opening vectors for stockwork veins are eitherperpendicular or oblique to the vein walls (Fig. 2C, 5, 9).Thus, stockwork veins occupy a mixture of pure extensionand extensional-shear fractures. Extensional-shear fracturesare hybrids of pure extension fractures and shear fractures.They have a combination of shear and extensional openingacross the fracture. Veins filling pure extension fractures aremost common. Fracture growth is interpreted to be a phe-nomenon associated with stable (quasi-static) propagationrather than dynamic (unstable), based on the observation ofa gradually tapering style of vein termination (Pollard &Aydin 1988).

Orientation of stress field

The geometric information from the stockwork veins pos-sibly reflects the local orientation of the principal stressaxes (σ1, σ2, σ3) at the time of fracture formation (Hancock1985). The plane of the stockwork veins (assumed to bepure extension fractures) during fracture initiation wouldhave contained the maximum and intermediate compressivestresses (σ1 - σ2 plane) with the least compressive stress (σ3)perpendicular to the vein if plane strain conditions prevailed(Fig. 12) (Hancock 1985). Figure 5 shows a mixture of bothquartz-filled extension fractures and extensional shear frac-tures. In this case the σ1 - σ2 plane will lie in the plane of theextension fractures, and the intersection of these extensionfractures with the extensional shear fractures approximatesσ2. Therefore, in this situation, the stockwork veins are in-ferred to have developed in an extensional regime with σ1

subvertical, σ2 subhorizontal oriented southwest-northeast,

and σ3 subhorizontal oriented northwest-southeast (presentco-ordinates) (Fig. 13A). However, this inferred stress re-gime for the most part is not in agreement with the commonsubvertical statistical intersection line produced betweenmost intersecting stockwork veins. Assuming plane strainconditions, this intersection line would approximate σ2, withσ1 subhorizontal oriented southwest-northeast and σ3 subho-rizontal oriented northwest-southeast (present co-ordinates)(Fig. 13B). Neither of these alternative interpreted stress fieldsare compatible with the overall stress field required for theHMSZ thrusting: low-angle fault-veins and tension gashes,where σ1 was subhorizontal oriented southwest-northeast,σ2 subhorizontal oriented northwest-southeast, and σ3 sub-vertical (Fig. 13C) (Teagle et al. 1990). This suggests thatthe remote stress field must have been locally perturbed bysome means to become suitably oriented for development ofsubvertical stockwork veins.

Mechanical constraints

The type of fracturing that occurred during formation of thestockwork veins also provides a number of constraints onfluid pressure (Pf) and stress conditions. Extension fracturescan only occur when the failure condition Pf = σ3 + T is metand the differential stress satisfies the condition (σ1 - σ3)< 4T (σ1 = maximum compressive stress, σ3 = minimumcompressive stress, T= rock tensile strength) (Secor 1965,1969). Extensional-shear fractures require the differentialstress to be 4T < (σ1 - σ3) < 5.6T, whereas shear fracturesrequire a condition in excess of this upper bound (Sibson1998). Development of the stockwork veins mostly by exten-sional fracturing indicates that the differential stress was low(<4T) and that fluid pressures, at least locally and intermit-tently, exceeded the sum of the least principal stress (σ3) andthe intact rock tensile strength (T) during formation (Secor1965). Thus, for the inferred depth interval of stockworkvein formation (c. 10 km; Craw et al. 1999; de Ronde et al.2000), the extension fracture failure condition requires thatfluid pressures during vein growth were suprahydrostatic.However, lithostatic fluid pressures would have been neces-sary for initiation of low-angle fault veins and to hold opensubhorizontal tension gashes (Sibson 1998). Also, the stress

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Begbie & Craw—Stockwork vein swarms, Macraes mine 71

Fig. 12 Illustration of the threemodes of macroscopic brittle frac-ture and associated principal stressdirections (Hancock 1985). '

Extensionfracture

Extensional-shearfracture Shear fracture

stockworkveins

HMSZ(top to the SW)

G 2

Fig. 13 A, B, The possible orientations of stress axes required to initiate subvertical stockwork veins. C, Inferred orientation of the remotestress field during formation of the Hyde-Macraes Shear Zone.

difference (σ 1- σ3) necessary for initiation and propagation ofthe fault-veins (shear fractures) would have been significantlylarger than for the extension fractures (Sibson 1998). Mutualcrosscutting relations between stockwork veins and low-anglefault/tension veins provide clear evidence that formation ofthe different structures requiring contrasting fluid pressureand stress conditions were intimately related and part of acyclically repeated process (cf., Cox 1995; Nguyen et al.1998; Sibson 2001).

Lithological controls on stockwork vein formation

The Intrashear Schist hosting the stockwork veins has in-herited a considerable degree of mechanical heterogeneityon the macroscale after ductile deformation and shearing ofpredominantly micaceous (fissile) and feldspathic (massive)units with contrasting rock competence. Differences in therheology of intrashear micaceous-dominant and feldspathic-dominant units cause the rocks to respond very differently toimposed deformational forces. Formation of the stockworkveins required differential stresses (σ1-σ3) < 4Tfor initiationof extension fractures and between 4 and 5.66T for extension-al-shear fractures in conjunction with suitable fluid pressures(Secor 1965; Etheridge 1983). Brittle failure criteria dictatethat the mode of fracture that formed in the Intrashear Schistdepended largely on the balance between differential stress(σ1 - σ3) and the local rock tensile strength (competence).Thus, different portions of the Intrashear Schist under thesame differential stress may have failed in pure extension,extensional-shear, or shear depending on whether they pos-sessed a relatively high or low tensile strength (competence)

(Fig. 7) (see Sibson 1996). For example, Fig. 5 shows brittlefracture of a series of folded pods of massive schist enclosedby highly deformed fissile schist. The more competent mas-sive schist has failed in pure extension while the less com-petent fissile schist has failed in extensional shear. This formof lithological control on vein formation for the most part isrelatively inconspicuous, but provides a suitable explanationfor near-synchronous initiation of both pure extension andextensional-shear fractures. Typically, the massive schist podsare the favoured unit to contain pure extension veins (sub-vertical stockworks) while the surrounding fissile materialis relatively devoid of stockworks (Fig. 7). The more fissilematerial takes up strain by foliation-parallel shearing.

Deformation regimes

Stockwork veins were initiated as brittle structures, althoughmost have been later deformed in a ductile manner, includingfolding (Fig. 6, 10). This suggests both brittle and brittle/ductile conditions occurred during and after stockwork veinformation. Brittle conditions may have predominated duringperiods of decreasing temperature and/or increasing strainrate (Lockner 1995). Microstructural observations indicateoverprinting of a weak to moderate plastic deformation thatis invariably indicative of deformation by dislocation creep(Hirth & Tullis 1992; Passchier & Trouw 1998). Recrystal-lisation textures indicate that temperatures were somewherebetween 300 and 400°C during stockwork vein formation(e.g., Passchier & Trouw 1998; Stöckhert et al. 1999). Thisrange is in agreement with temperatures inferred for thrust-related veins (McKeag et al. 1989).

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72 New Zealand Journal of Geology and Geophysics, 2006, Vol. 49

favoured regionfor initiating

stockwork veins

HangingwallSchist

Footwall- Schist

stress field

Fig. 14 Schematic block diagram showing a possible mechanismof lateral ramp deformation to explain the formation of subverticalstockwork veins in an overall regime of shortening.

Relationship of stockwork swarms to the evolution ofthe HMSZStockwork veins indicate that local extension perpendicular tothe strike of the HMSZ occurred within the Intrashear Schistsyn-kinematically with thrusting, as demonstrated by mutualcrosscutting relationships between low-angle fault-veins/ten-sion gashes and stockworks (see Fig. 3). Stockwork veinshave been previously interpreted as structurally the oldest ofthe mineralised veins in the HMSZ, and suggested to be ex-clusively crosscut by low-angle fault-veins (Angus et al. 1997;de Ronde et al. 2000). However, mutual crosscutting relationsbetween the two vein types demonstrates that thrust-relatedfault-veins, low-angle tension gashes, and subvertical stock-work veins were active penecontemporaneously in most plac-es. Formation of stockwork veins postdated the main phase ofductile deformation of the Intrashear Schist. Stockwork veinswere initiated as brittle structures, although some have beenfolded subsequently (Fig. 6). The formation of stockworkveins during thrusting would have required localised regionsof extension to have developed within the regional shortening-related stress field (Teagle et al. 1990). Stockworks would alsohave required regions of low differential stress to meet thebrittle failure criteria for extension fracturing (Sibson 1998).In some cases, transitions from shortening to local extensioncould be driven by changes in thrust geometry (e.g., Oliveret al. 2001; Bayona et al. 2003). For example, the develop-ment of lateral or oblique ramps may give rise to extensionin parts of the shear zone during continuous shortening.Lateral ramps are commonly observed along the HMSZ anddip c. 20-50° to the northwest (e.g., Fig. 3). These ramps aremostly subparallel to the strike of stockwork veins. Bendingof the Intrashear Schist over lateral ramps during progressivedeformation may have locally reoriented the stress field to besuitable to initiate stockwork veins (Fig. 14). Alternatively,changes in the relative horizontal velocity of the IntrashearSchist as different portions of the schist reached the frontalramps at different times may have caused localised torsionaleffects, thus changing and reorientating the local stress field.Once the immediate intrashear material has passed throughthe lateral ramp, the stress field may return back to the remote

thrust-related stress field. This scenario allows the stress fieldto switch orientation and account for the mutual crosscuttingrelations observed between different structures.

CONCLUSIONS

Orientation, structural style, and relative age relationships ofstockwork veins with respect to mineralised thrust-relatedstructures provide important constraints on the structuralevolution of the shear zone. They demonstrate that extensionalstructures in the HMSZ formed while shortening was active.Localised extension was mainly parallel to the structuraltrend; that is, it took place perpendicular to the direction ofinferred thrusting. Stockwork veins are not representative ofthe regional stress field during fracture initiation. Regions ofextension suitable for initiating stockworks were most likelycreated by changes in the shear zone geometry (i.e., lateralor oblique ramps; Fig. 14). The swarms of stockwork veinswithin the Intrashear Schist were lithologically controlledby the dimensions and locations of more competent massiveschist pods. Identification of structures such as lateral rampsin mineralised shear zones may be ideal targets for explorationof stockwork vein swarms.

ACKNOWLEDGMENTS

This study was financially supported by the University of Otagoand the Foundation for Research, Science and Technology.Logistical support was provided by OceanaGold (New Zealand)Ltd, whose continuing enthusiasm for applied research is gratefullyacknowledged. In particular, Bill Yeo, Mark Mitchell, Lindsay Maw,and Lachlan Reynolds facilitated the field aspects of the study.Discussions with Rick Sibson and Richard Norris of the Universityof Otago Geology Department helped in development of the ideascontained herein. Reviews by Nick Oliver and an anonymousreviewer helped improve this paper.

REFERENCES

Angus PVM 1993. Structural controls on gold deposition in theHyde-Macraes Shear Zone at Round Hill, Otago, NewZealand. Proceedings of the Australasian Institute of Miningand Metallurgy New Zealand Branch 27th Annual Confer-ence. Pp. 85-95.

Angus PVM, de Ronde CEJ, Scott JG 1997. Exploration alongthe Hyde-Macraes Shear. Proceedings of the New ZealandMinerals and Mining Conference. Pp. 151-157.

Bayona G, Thomas WA, Van der Voo R 2003. Kinematics of thrustsheets within transverse zones: a structural andpaleomagnet-ic investigation in the Appalachian thrust belt of Georgia andAlabama. Journal of Structural Geology 25: 1193-1212.

Cox SF 1995. Faulting processes at high fluid pressures: an exampleof fault valve behavior from the Wattle Gully Fault, Victo-ria, Australia. Journal of Geophysical Research 100(B7):12841-12859.

Craw D 1984. Lithologic variations in Otago Schist, Mt Aspiringarea, northwest Otago, New Zealand. New Zealand Journalof Geology and Geophysics 27: 151-166.

Craw D 2002. Geochemistry of late metamorphic hydrothermalalteration and graphitisation of host rock, Macraes goldmine, Otago Schist, New Zealand. Chemical Geology 191:257-275.

Dow

nloa

ded

by [

Uni

vers

ity o

f B

irm

ingh

am]

at 1

3:16

12

Nov

embe

r 20

14

Begbie & Craw—Stockwork vein swarms, Macraes mine 73

Craw D, Norris RJ 1991. Metamorphogenic Au-W veins and regionaltectonics: mineralisation throughout the uplift history of theHaast Schist, New Zealand. New Zealand Journal of Geologyand Geophysics 34: 373-383.

Craw D, Windle SJ, Angus PVM 1999. Gold mineralization with-out quartz veins in a ductile-brittle shear zone, MacraesMine, Otago Schist, New Zealand. Mineralium Deposita34: 382-394.

Craw D, MacKenzie DJ, Petrie B 2004. Disseminated gold miner-alization in a schist-hosted mesothermal deposit, Macraesmine, Otago, New Zealand. Proceedings of the AustralasianInstitute of Mining & Metallurgy PACRIM Conference,Carlton, Victoria. Pp. 135-140.

de Ronde CEJ, Faure F, Bray CJ, Whitford DJ 2000. Round Hillshear zone-hosted gold deposit, Macraes Flat, Otago, NewZealand: evidence of amagmatic ore fluid. Economic Geol-ogy 95(5): 1025-1048.

Etheridge MA 1983. Differential stress magnitudes during regionaldeformation and metamorphism: upper bound imposed bytensile fracturing. Geology 11: 231-234.

Gray DR, Foster DA 2004. 40Ar/39Ar thermochronologic constraintson deformation, metamorphism and cooling/exhumation of aMesozoic accretionary wedge, Otago Schist, New Zealand.Tectonophysics 385: 181-210.

Hancock PL 1985. Brittle microtectonics: principles and practice.Journal of Structural Geology 7: 437-457.

Hay R, Craw D 1993. Syn-metamorphic gold mineralisation, In-vincible Vein, NW Otago Schist, New Zealand. MineraliumDeposita 28: 90-98.

Hirth G, Tullis J 1992. Dislocation creep regimes in quartz aggre-gates. Journal of Structural Geology 14: 145-159.

Lockner DA 1995. Rock failure. In: Rock physics and phase rela-tions—a handbook of physical constants. American Geo-physical Union Reference Shelf 3: 127-147.

McKeag SA, Craw D, Norris RJ 1989. Origin and deposition of agraphitic schist-hosted metamorphogenic Au-W deposit,Macraes, East Otago, New Zealand. Mineralium Deposita24: 124-131.

MacKenzie DJ, Craw D 1993. Structural control of gold-scheelitemineralisation in a major normal fault system, Barewood,eastern Otago, New Zealand. New Zealand Journal of Geol-ogy and Geophysics 36: 437-445.

Mortimer N 1993a. Geology of the Otago Schist and adjacent rocks.Institute of Geological & Nuclear Sciences 1:500 000 Geo-logical Map 7.1 sheet.

Mortimer N 1993b. Jurassic tectonic history of the Otago Schist,New Zealand. Tectonics 12: 237-244.

Mortimer N 2000. Metamorphic discontinuities in orogenic belts:example of the garnet-biotite-albite zone in the Otago Schist,New Zealand. International Journal of Earth Sciences 89:295-306.

Nguyen PT, Cox SF, Harris LB, Powell C 1998. Fault-valve behav-iour in optimally oriented shear zones: an example at theRevenge gold mine, Kambalda, Western Australia. Journalof Structural Geology 20: 1625-1640.

Oliver NHS, Ord A, Valenta RK, Upton P 2001. The role of rockrheological heterogeneity in fluid flow and ore genesis, withexamples and numerical models from the Mt Isa district,Australia. In: Tosdal R, Richards J ed. Structural controlson ore deposits. Society of Economic Geologists Reviews14: 51-74.

Passchier CW, Trouw RAJ 1998. Micro-tectonics. Berlin, Springer-Verlag. 289 p.

Paterson CJ 1986. Controls on gold and tungsten mineralisation inmetamorphic-hydrothermal systems, Otago, New Zealand.In: Keppie JD, Boyle RW, Haynes SJ ed. Turbidite-hostedgold deposits. Geological Association of Canada SpecialPaper 32: 25-39.

Petrie BS, Craw D, Ryan CG 2005. Geological controls on refrac-tory ore in an orogenic gold deposit, Macraes mine, NewZealand. Mineralium Deposita 40: 45-58.

Pollard DD, Aydin A 1988. Progress in understanding jointing overthe past century. Geological Society of America Bulletin100: 1181-1204.

Secor DT 1965. Role of fluid pressure in jointing. American Journalof Science 263: 633-646.

Secor DT 1969. Mechanics of natural extension fracturing at depth inthe Earth's crust. In: Baer AJ, Norris DK ed. Research in tec-tonics. Geological Survey of Canada Paper 68-52: 3-47.

SibsonRH 1996. Structural permeability of fluid driven fault-fracturemeshes. Journal of Structural Geology 18: 1031-1042.

Sibson RH 1998. Brittle failure mode plots of compressional andextensional tectonic regimes. Journal of Structural Geology20: 655-660.

Sibson RH 2001. Seismogenic framework for hydrothermal transportand ore deposition. In: Tosdal R, Richards J ed. Structuralcontrols on ore deposits. Society of Economic GeologistsReviews 14: 25-50.

Stöckhert B, Brix MR, Kleinschrodt R, Hurford AJ, Wirth A 1999.Thermochronometry and microstructures of quartz—acomparison with experimental flow laws and predictionson the temperature of the brittle-plastic transition. Journalof Structural Geology 21: 351-369.

Teagle DAH, Norris RJ, Craw D 1990. Structural controls on gold-bearing quartz mineralization in a duplex thrust system,Hyde-Macraes Shear Zone, Otago Schist, New Zealand.Economic Geology 85: 1711-1719.

Williams GJ 1974. Gold scheelite mineralisation in rocks of theNew Zealand geosyncline. In: Economic geology of NewZealand. Australasian Institute of Mining and MetallurgyMonograph 4: 41—59.

Williamson JH 1939. The geology of the Naseby Subdivision, Cen-tral Otago, New Zealand. Geological Survey Bulletin 39.

Dow

nloa

ded

by [

Uni

vers

ity o

f B

irm

ingh

am]

at 1

3:16

12

Nov

embe

r 20

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