Structural and lithological controls on gold ...

New Zealand Journalmineralisation of Geology & in Geophysics, 2009, Vol. 52: 43–57 Mackie et al.—Gold Otago Schist 0028–8306/09/5202–0043 © The Royal Society of New Zealand 2009

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Structural and lithological controls on gold mineralisation at Oturehua on the northeastern margin of the Otago Schist, New Zealand C. Mackie D. J. Mackenzie D. Craw* Geology Department University of Otago PO Box 56 Dunedin 9054, New Zealand Corresponding author: [email protected]

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Abstract A set of more than 50 mineralised normal faults cuts lower greenschist facies rocks at the northeastern margin of the Otago Schist belt near Oturehua, central Otago. The faults strike northwest and dip steeply (>60°) northeast, and individual mineralised zones are traceable for up to 700 m along strike. The mineralised faults are closely associated with, and are generally parallel to, a swarm of northwest striking, steeply dipping extensional quartz ± ankerite veins (centimetre scale) that occur over an area of at least 25 km2 in the host schist. Mineralised faults contain 10–100 cm thick quartz veins that are traceable as interlinked structures for tens of metres along strike. Where faults cut massive quartz-feldspar-rich schist, the veins are planar, contain scattered sulfide minerals and visible gold, and there has been little alteration of essentially undeformed adjacent wall rock (centimetre scale). In contrast, muscovite-rich laminated schist host rock is extensively deformed and altered adjacent to mineralised faults (metre scale), with addition of quartz (silicification) and ankeritic replacement of metamorphic chlorite by kaolinite and/or sericitic muscovite. Veins adjacent to laminated (micaceous) schist contain abundant breccias with variably altered clasts. Widespread addition of auriferous sulfide minerals, mainly arsenopyrite, has occurred in the laminated schist alteration zones and breccias. The mineralised faults formed in the hinge of a northwest-trending post-metamorphic antiform, and the faults may have been localised by zones of weakness associated with kink folding in the hinge of the antiform. Hydrothermal fluids responsible for mineralisation were channelled by the normal faults. These faults developed by coalescence of extensional fractures in the schist rock mass during middle Cretaceous regional extensional unroofing of the Otago Schist belt. The location of these mineralised faults near the margin of the Otago Schist belt suggests a genetic relationship between mineralisation and the regional northwest-striking normal fault systems that cut and define the schist margin.

Keywords gold; structure; normal fault; alteration; vein; breccia; extension; Otago Schist INTRODUCTION The Otago Schist belt in southern New Zealand hosts numerous orogenic (mesothermal) gold deposits (Fig. 1). Numerous auriferous quartz vein systems were mined historically, although production was small (Williams 1974; Paterson 1986). The veins occur along faults and shear zones that strike northwest. Most of these mineralised structures occur within the central portion of the schist belt, and are hosted in upper greenschist facies rocks. Mineralised rocks are rare near the margins of the belt, and no auriferous veins occur in the greywackes beyond the schist margins. The Macraes mine, a currently active 6 million ounce deposit, is being developed in a mineralised shear (thrust) zone that is nearer the northeastern margin of the schist belt than most other mineralised zones (Fig. 1) (Mitchell et al. 2006). The Macraes deposit is hosted in lower greenschist facies rocks immediately above the post-mineralisation Footwall Fault, a Cretaceous extensional structure (Craw 2002; Mitchell et al. 2006). The mineralised zone at Macraes is foliation-parallel and >250 m thick measured perpendicular to foliation (Jones et al. 2007). The structurally uppermost portion of this mineralised zone is a set of auriferous quartz veins (Eastern Lodes, Angus et al. 1997) that are the nearest mineralised rocks to the northeastern margin of the schist belt in east Otago. A set of auriferous quartz veins occurs near the northeastern margin of the schist belt at Oturehua, to the northwest of the Macraes mineralised system. These veins appear to lie approximately along strike from the Macraes mine, but are even closer to the margin of the schist belt than the Eastern Lodes at Macraes (Fig. 1). The Oturehua veins are the subject of this study, where we investigate the structure and mineralogy of the veins and the structural controls of vein emplacement in the context of regional post-metamorphic structural evolution of the Otago Schist belt. We specifically address key similarities and differences between the Oturehua veins and the Macraes mineralised system along strike to the southeast. In addition, we compare structural controls on the Oturehua veins to those of veins in the core of the schist belt. GENERAL GEOLOGY

G08031; Online publication date 3 June 2009 Received 24 November 2008; accepted 24 March 2009

Schist host rock The Otago Schist belt was formed during tectonic juxtaposition of Mesozoic metagreywacke terranes in the Jurassic (Mortimer 1993). The central and northeastern portions of the belt were derived by metamorphism and textural reconstitution of

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New Zealand Journal of Geology and Geophysics, 2009, Vol. 52

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Torlesse Terrane quartzofeldspathic greywacke and argillite (Mackinnon 1983; Mortimer & Roser 1992). Metamorphic grade on the northeastern side of the schist belt ranges from prehnite-pumpellyite facies and pumpellyite-actinolite facies rocks on the margin (referred to as sub-greenschist facies in this study), through lower greenschist facies, to upper greenschist facies (biotite and garnet bearing) in the core of the belt (Fig. 2) (Mortimer 1993, 2000). Schist foliation and textural reconstitution become more developed from pumpellyiteactinolite facies rocks through to upper greenschist facies, in approximate parallelism with metamorphic grade increase (Turnbull et al. 2001). Textural reconstitution is complete and foliation is pervasive in greenschist facies rocks. The pervasive foliation is almost horizontal over most of the schist belt in Otago, except where post-metamorphic deformation has disrupted it, as at Oturehua in this study. The foliation is composite, incorporating several generations of synmetamorphic isoclinal folds and associated fold axial surface foliations. A prominent quartz rodding lineation has developed from disrupted synmetamorphic fold hinges, and this lineation has a broadly consistent trend over large areas of schist (Fig. 2) (Mortimer 1993; MacKenzie & Craw 2005). Most metamorphic zonal boundaries are defined by postmetamorphic structural discontinuities (Fig. 2) (Craw 1998; Mortimer 2000). These structural discontinuities separate domains with differing rock types, differing synmetamorphic fold styles, and differing orientations of structural elements such as quartz rodding lineation (MacKenzie & Craw 2005). All greenschist facies rocks in Otago have the same mineral assemblage: quartz, albite, muscovite chlorite, epidote, calcite, and titanite. Identification of pre-metamorphic rock types is difficult, as different protoliths result in only subtle differences in proportions of metamorphic minerals. Nevertheless, two end-member schist types are recognisable at outcrop scale on the basis of differences in fissility and foliation development, and these are mappable at 1–20 m scale (Brown 1968; Petrie

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Fig. 1 Location map of the Otago Schist belt (greenschist facies rocks in white), showing the principal structural features relevant to this study, including Cretaceous normal faults (heavy black lines; FWF = Footwall Fault, WF = Waihemo Fault) and principal gold-bearing structures (dark grey lines). Dashed box shows the location of Fig. 2. Inset: Location of the study area in the South Island of New Zealand.

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Fig. 2 Geological map of North Rough Ridge and surrounding areas, showing the principal post-metamorphic structures that affected the northeast margin of the Otago Schist belt (after Forsyth 2001 and MacKenzie & Craw 2005). Dashed box shows the location of the Oturehua area (Fig. 3A,B) that is the focus of this study.

& Craw 2005). Massive or feldspathic schist is typically relatively enriched in albite and has poorly developed foliation. In contrast, laminated or micaceous schist is more

Mackie et al.—Gold mineralisation in Otago Schist fissile, has a relatively high proportion of muscovite, and has abundant foliation-parallel quartz-rich segregations. These different schist rock types are locally interlayered on the 0.1–1 m scale as well. Post-metamorphic deformation The regional flat-lying schist foliation has been disrupted by several generations of post-metamorphic deformation. Intense mesoscopic folding occurs in curvilinear zones up to 1 km wide through the central part of the schist belt (MacKenzie & Craw 2005). These fold zones (Poolburn Generation) have variably developed fold axial surface cleavage with some cataclasis, and some displacement has occurred across these fold zones. The folds were formed in the latter stages of compressional deformation as the rocks were uplifted from ductile to brittle deformation regimes. This fold generation formed under similar conditions, and probably at similar stage in evolution of the upper greenschist facies schist core, as the gold-bearing Rise & Shine Shear Zone (Cox et al. 2006). Structurally similar ductile-to-brittle deformation accompanied gold mineralisation at the Macraes mine in the Hyde-Macraes Shear Zone, a regionally extensive structural feature in lower greenschist facies rocks. However, Macraes deformation and mineralisation occurred in Late Jurassic to Early Cretaceous, distinctly earlier than the Rise & Shine Shear Zone (Cox et al. 2006). Extensional deformation began in the Otago Schist in the middle Cretaceous and continued to the middle Tertiary (Bishop & Laird 1976; Adams & Raine 1988; Carter 1988; Gray & Foster 2004). Extension-related subsidence in the Late Cretaceous to middle Tertiary (late Paleogene) was accompanied by marine transgression and cutting of a widespread low-relief erosion surface, the Waipounamu Erosion Surface (Landis et al. 2008). At a regional scale, the principal deformational features are major Cretaceous normal faults (Fig. 1) that have helped to exhume the schist belt and have juxtaposed rocks of differing metamorphic grade (Deckert et al. 2002; Gray & Foster 2004; MacKenzie & Craw 2005). These normal faults have formed a rectilinear structural pattern of northwest- and northeast-striking structures (Fig. 1). The northwest-striking Waihemo and Hawkdun Faults (Fig. 2) separate low-grade schist (footwall) from greywacke (hanging wall) for much of their length. Middle Cretaceous sediments eroded from the Waihemo Fault scarp are preserved locally (Fig. 2). Another two northwest-striking Cretaceous normal faults—the Thomsons Gorge Fault and Footwall Fault (Fig. 1)—have juxtaposed lower greenschist facies rocks (hanging wall) against upper greenschist facies rocks (footwall) and have truncated preexisting mineralised shear zones (Deckert et al. 2002; Cox et al. 2006). Most of the gold-bearing veins in the Otago Schist belt occur in swarms of small normal faults formed during Cretaceous extension (MacKenzie & Craw 1993; Scott 2005; MacKenzie et al. 2006). The transcurrent plate boundary through New Zealand was initiated in the Miocene, and this resulted in a change back to compressional deformation in the Otago Schist belt (Cooper et al. 1987). Many parts of the rectilinear pattern of normal faults were reactivated as reverse faults. The Waihemo and Hawkdun Faults, in particular, developed new relief on the northeast (now upthrown) side, and shed sediments to the south on to the Otago Schist (Youngson et al. 1998). Subsequently, in the Quaternary, the schist foliation and overlying sediments were folded into broad northeast-trending antiformal ridges with intervening synformal basins (Fig. 2) (Jackson et al. 1996).

45 METHODS Outcrop is poor on North Rough Ridge, and exposed rock is variably oxidised to at least 10 m depth. Observations on the schist basement were largely confined to scattered tors, from which regional structural observations were made. Historic mine workings have left shallow surface cuts and short (10 m below the Waipounamu Erosion Surface. Hematite in schist at 67 m depth in Fig. 4 is synmetamorphic. The lower greenschist facies schist foliation at the northern end of North Rough Ridge has been folded into a broad

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antiform with a northwest-trending axis (Fig. 3B). Foliation dips moderately and uniformly southwest (10–20°) on the southern limb of this antiform, and gently northeastwards on the northern limb (Fig. 3B). The hinge zone is broad (c. 3 km wide) and the foliation in the hinge zone is irregular and warped on a 10 m scale (Fig. 3B, 4). This generation of folding predates the middle Cenozoic Waipounamu Erosion Surface, which truncates the northwest-trending antiformal structure. The combination of the northeast-trending Pleistocene antiform and the earlier northwest-trending antiform results in a broadly domal structure of foliation attitudes in the Oturehua area (Fig. 3B). Zones (10–50 m scale) with abundant kink folds occur sporadically within the disturbed zone in the hinge of the earlier antiform, and are apparently related to that larger structure. Kink folds in these zones trend northwest and have

Mackie et al.—Gold mineralisation in Otago Schist ��

Fig. 4 Summary log of core from drillhole UWWE08–07, which was drilled at 55° to horizontal towards the southwest, from the northeastern side of the West of England mineralised fault (Fig. 5B). Foliation is dashed and principal schist rock types are indicated.

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fold axial surfaces that dip steeply southwest. Kink folds are variably developed in these folded zones and range from round-hinged, shallow amplitude (millimetre–centimetre scale) structures to well-defined centimetre to metre scale

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kinks with angular hinges. Folds are asymmetric, with most observed kinks having longer southwestern limbs. The tighter kinks with angular hinges have weakly developed fold axial surface fractures.

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New Zealand Journal of Geology and Geophysics, 2009, Vol. 52 Fig. 5 Structural map of the Oturehua area at the north end of North Rough Ridge, showing the orientations of principal extensional structures related to vein formation. A, Map view of trends of intersection lineations between extensional veins and foliation (arrows), and trends of mineralised faults (black lines). Stereonet (lower hemisphere) shows extensional veins (lineations = triangles; vein planes = great circles). B, Map view of mineralised faults with historic mine names. Stereonet (lower hemisphere) shows orientations of mineralised faults (great circles).

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EXTENSIONAL VEINS Swarms of small (centimetre scale) discontinuous veins fill fractures throughout the lower greenschist facies rocks of North Rough Ridge (Fig. 5A). The veins are typically