low‐angle shear zones in central otago, new zealand—their regional extent and economic...
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Low‐angle shear zones in Central Otago, NewZealand—their regional extent and economicsignificanceColin N. Winsor a ba Geology Department , University of Otago , P.O. Box 56, Dunedin, New Zealandb Winsor & Associates , 28 Wildwood Drive, SA, Australia , 5109Published online: 23 Mar 2010.
To cite this article: Colin N. Winsor (1991) Low‐angle shear zones in Central Otago, New Zealand—their regionalextent and economic significance, New Zealand Journal of Geology and Geophysics, 34:4, 501-516, DOI:10.1080/00288306.1991.9514486
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New Zealand Journal of Geology and Geophysics, 1991, Vol. 34: 501-5160028-8306/91/3404-0501 $2.50/0 © Crown copyright 1991
501
Low-angle shear zones in Central Otago, New Zealand—their regional extentand economic significance
COLIN N.WINSOR
Geology DepartmentUniversity of OtagoP.O. Box 56Dunedin, New Zealand*
*Present address: Winsor & Associates, 28 Wildwood Drive,SA., Australia 5109.
Abstract A relation between low-angle shear zones andfolds is established in the dynamically deformed Otago Schist.These events occurred during the Cretaceous towards theclose of the Rangitata Orogeny (D3_4). Prominent low-anglenorth-dipping shears are often subparallel to F3 axial planes.South-dipping mesoshears are present as an antithetic set.Thrusting was initiated on the shear zones after F3 folding,accompanying continued regional compression.
In western sections, two sequential F3 fold subphases aredistinguished, but to the east the earlier phase is weaklypreserved. The existence of two post-S2 folding events, anddominance of north-dipping shears, is important in relation topotential sites of gold mineralisation. Structural factorsinfluencing the development of dilation sites associated withgently dipping macroshears include the F3 axial planegeometry, variations in fold intensity, and lithology-shearstrength control. The lack of widespread low-angle macro-shears is believed to be due to the dominantly segregated,coarsely layered nature of the Otago Schist and the localisationof F3b folds.
Keywords shears zones; quartz veins; structural control;gold mineralisation; fold generations; Otago Schist
INTRODUCTION
The Otago Schist within the Haast Schist of the South Island,New Zealand (Fig. 1), is a region of multiple-deformed andmoderately metamorphosed quartzofeldspathic, turbiditic, andvolcanic rocks (Bishop et al. 1985; Craw 1985). Correlationcriteria in the progressively shortened and uplifted belt cannotbe applied with as much certainty as is usually justified. As aresult of correlation problems previously described by otherauthors (see Means 1963, 1966; Wood 1963, 1978; Brown1968; Craw 1985; Turnbull 1988), the approach recom-mended is to assign the most recent events first.
Despite examination of the internal structure of a prom-inent low-angle shear zone—the Hyde-Macraes Shear Zone(HMSZ)—by Teagle (1987) and Teagle et al. (1990), the
G90012Received 20 March 1990; accepted 5 March 1991
present study is the first regional analysis of semibrittle eventsand their relation to F3 folds (previously designated theManorburn Generation; Norris 1977). Gold mineralisation iswidespread and has been extensively studied (see Williamson1934; Williams 1974; Henley et al. 1976; Paterson 1986;Craw & Norris 1988; McKeag & Craw 1989; McKeag et al.1989). Often a structural control has been suggested; the lodesdeveloped during progressive uplift of the Otago Schist. Littleattempt has been made to provide a coherent interpretation toassist in the discovery of additional mineralised sites.
Three tectonostratigraphic terranes (the Torlesse, Caplesand Aspiring) are identified in the Otago Schist (Tumbull1981; Bishop et al. 1985; Norris & Craw 1987). The areaunder examination is mainly in the Torlesse Terrane, althougharea 2 (Fig. 1) is near a terrane boundary. Metamorphic gradeis in the greenschist facies, chlorite zone, but increasing to thenorthwest. To avoid confusion and simplify description, I haveadopted nongenetic field-based terms to distinguish rocktypes, similar to the massive and layered schist types used byBrown (1968). They are "coarsely, moderately and finelylayered", applying the width of quartzofeldspathic bands,which are metamorphic segregations or layer parallel veins.Coarsely layered schist is mainly quartzose, with bands >1 cm;moderately layered schist has bands >5 mm to <1 cm, andfinely layered schist is micaceous, with bands <5 mm.
The Otago Schist was progressively shortened by fourevents during the Jurassic to Late Cretaceous (Harper &Landis 1967; Coombs et al. 1976) of the Rangitata Orogeny.These events are correlated by Craw (1985, fig. 20). Initialphases resulted in transposition and isoclinal folding (Brown1963, 1968; Means 1963; Wood 1963, 1978; Bishop et al.1976), but distinction is difficult as fabrics grade into oneanother. The metamorphic climax was after F ^ , pre-F3 (Craw1985), with F3 folds exhibiting marked variation in morph-ology. In eastern Otago, a thrusting event is recognised alongthe gently northeast dipping HMSZ (Teagle et al. in press),with associated gold mineralisation assigned to D4 (McKeag& Craw 1989).
Although vergence changes are useful in the identificationof macrofolds throughout Central Otago, Means (1966) hasindicated that caution must be used because a foliation couldbe undulating prior to folding, or a conjugate system couldexist. Few criteria are available to distinguish these possi-bilities. For examples discussed here, increases in mesofoldintensity and tightness adjacent to probable vergenceboundaries support macrofold presence. Although anomalousvergence patterns may be produced by later deformation(Lisle 1988), locally a distinction is possible between asequential or synchronous interpretation. F3 folds can begenerally distinguished from sharp angular kinks, present inbands and related to Cenozoic (Kaikoura) block faulting. On aregional scale, the persistence of fold trends suggests norotation during late faulting.
Based on overprinting criteria and local vergence differ-ences (using the definition of Bell 1981), two post-S2 fold sets,
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502 New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
\l ±\ /Rise and Shine Shear ,
','• \ ' ' ' ,' ' ''' [ i , ' T°?*/ i,.« ! : 11 i ' ' ! ' i />\—} ' ' \ PCromwell Gorge
ICarricktown , \ ^ \ i , / , ' , i,
\ /45°S
I ; '
i I Patearos area
>. » \ \ \ \ \ \ ¡
\ \ \ \ N v S \ i ( Barewood area NN \ \ X y^
169° E 170°
SCALE10 20 40
Kilometres 46OS
Fig. 1 Portion of the Otago Schist displaying: (1) areas of known gold mineralisation, study areas 1,3,4, 5 & 7, Conroys Gully and WhitesReef; (2) sites of F3 vergence boundaries, study areas 1,4,5,6 and Blackstone Hill; and (3) regions of known low-lying macroshears includingthe Hyde-Macraes Shear Zone and areas 1 & 2. Quartz rod trend after Mortimer (1989).
F3a (Fig- 2A. B) an(* F3b. with rounded hinges, are distin-guished in west-central Otago. I have adopted this nomen-clature, as the two phases are not readily separable andprobably developed progressively. A previous study byTurnbull (1988) did not separate these events. F3a and F3b areinterpreted as discrete phases which developed sequentially
during progressive deformation (although not strictly in acontinuum in the sense adopted by Nicolas 1987), because:
(1) F3b mesofolds overprint F3a macrofolds;
(2) F3 a axes are subparallel to the mineral elongationdirection L2, while F3b axes are oblique except in eastOtago where F3b strain was more intense;
Fig. 2 Critical structural observations across the Rise and Shine Shear, in the Cromwell Gorge and Conroys Gully. A, A minor west- >•verging F3a fold in the Cromwell Gorge has a thick quartz vein (041/16W) subparallel to the axial plane of the fold. Fold axis plunges 10towards 216, and axial plane is oriented 015/16W. Early veins parallel to layering are offset by veins subparallel to the axial plane. B, North-verging F3a folds with axial plane oriented 108/18S and fold axis plunges 13 towards 080, which fold the penetrative cleavage S2, situatedsouth of the Rise and Shine Shear Zone. A south-dipping (097/28S) shear with no quartz filling is subparallel to the axial plane. Rock type isa moderately layered-segregated schist. C, A shallow north-dipping poddy quartz vein (O58/O8N) in the Cromwell Gorge, subparallel to F3baxial planes transects a parallel vein and a minor F3a fold: axial plane 073/22S, fold axis plunges 12 towards 263. Another vein is apparentoriented 083/52N, while a vein at a high angle to layering predates the cleavage and is shortened. Rock type is a coarsely layered schist. Scalein centimetres. D, Dilational offset in the Cromwell Gorge where a nonfibrous north-dipping quartz vein (081/13N) displaces a thin, south-dipping vein (073/13S), at a low angle to the penetrative cleavage (S2) oriented 113/18S. Rock type is a coarsely layered schist. Scale ruler is15 cm long. E, Dilational vein offset in the Cromwell Gorge where a nonfibrous north-dipping quartz vein (095/32S) displaces a south-dipping nonfibrous vein (104/32S). Veins are 2-3 cm wide, the penetrative cleavage S2 is oriented 094/20S. Scale ruler is 15 cm long.F, Subparallel quartz-filled mesoshears in the Cromwell Gorge form a mesoduplex which transects north-verging F3a folds whose axialplanes are oriented 129/32S and fold axis plunges towards 125. G, Imbricated schist between subparallel quartz-filled shears in CromwellGorge. Upper shear oriented 084/20N, lower shear 080/18N; indicating north-south thrusting. Orientation of adjacent penetrative cleavage139/1 OS. Scale ruler is 15 cm long. H, Mesoshear oriented 122/28S, with a 3 cm wide gouge zone, oriented subparallel to the axial plane ofF3 folds in Conroys Gully. Fold axis plunges 3 towards 121.
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Winsor—Low-angle shears, Otago 503
• • > ; •
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504 New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
X\ .
L e g e n d
Macroscopic F 3 , anticllna
Macroscopic F 3 , synclins".jOuarl j
Stralching dlrsctlon
Ouarlz rod
Cranulallon
38
5
~ <
Vargsncs F3,
Vsrgsncs F3&
Poat F3 fold
Mineralized shear
.5 Kilometres
32
N
26 /16
\ \
Fig. 3 Rise and Shine Shear Zone structural interpretation (structural elements without values are interpreted).
(3) south-dipping F3a axial planes are at a lower angle to themean S2 orientation than F3b planes, suggesting greatershortening of F3a folds;
(4) mesofolds of both subphases are not spatially restrictedbut occur throughout, even in the same locations;
(5) south-dipping quartz veins subparallel to F3a axial planesare folded about an F3b axis;
(6) northwest-northeast dipping veins subparallel to F3b axialplanes transect F3a folds; and
(7) differences in wavelength, amplitude, and interlimb anglesupport the separation; F3a are often tighter and of higheramplitude.
Although these points suggest a time order, local box foldsor conjugate kinks can indicate concurrent folding. Box foldsare rare however and the divergence in fold axes suggestssequential development. Conjugate kinks are describedelsewhere (Ramsay 1962; Kleist 1972; Tobisch & Fiske1976); their axial planes can be variably oriented or invariant,but fold axes usually coincide or are symmetrical about the
mineral elongation direction; this is not the case in CentralOtago. An explanation often considered is fault-related stressreorientation (e.g., Stubley 1989), although in the Otago Schistthis is unlikely as faults generally postdate macrofolds. Pfaff &Johnson (1989) have examined theoretical aspects of foldasymmetry and demonstrated that the sense of asymmetry ismainly determined by layer contact behaviour. Although foldsof opposed asymmetry can develop concurrently in differenthorizons, an alternative explanation is nonsynchronousdevelopment, which is amenable to the dynamic conditionsand supported by fold overprinting.
STRUCTURAL INTERPRETATION IN SELECTEDAREAS OF THE OTAGO SCHIST
Quartz veins containing gold mineralisation are widelydistributed in the Otago Schist (Fig. 1, including areas 1,3-5,7 and the HMSZ). For the purpose of this article, a macro-scopic shear zone is defined as one with strike length >5 km.F3 vergence boundaries are documented across the Rise and
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Winsor—Low-angle shears, Otago
A North of Rise and Shine Shear
505
54 poles
D South of Rise and Shine Shear E
20 folds
44 poles
Quartz rod
Contour max.great circle
F3a fold axis
F3 a axial plane
O
a
48 lineatlons
F3b fold axis •
F30 axial plane •
37 folds
Rotation path
Fig. 4 Rise and Shine Shear area, ductile structural data. A, Contoured poles to penetrative cleavage S2. Interval 1.9, 3.7, 7.4%, max.plane 125/22E. B, Contoured mineral lineation L2. Interval 1.9, 3.7, 7.4%, max. lineation 015/10. C, F3a fold axes and axial planes.D, Contoured poles to penetrative cleavage S2. Interval 2.2, 4.6, 9.1, 18.2%, max. plane 120/05N. E, Contoured mineral lineation L2.Interval 2.1, 4.2, 8.3%, max. lineation 124/00. F, F3b fold axes and axial planes.
Shine Shear Zone (RSSZ) area 1 (Paterson 1986); the HMSZ(Teagle et al. 1990; Winsor 1991); and in the Ophir region,area 4 (Means 1966). The only other known gently dippingmacroshear zone crops out in the Cromwell Gorge (area 2), asa diffuse zone separating coarsely layered quartzofeldspathicand finely layered micaceous schist (Turnbull 1988).
Rise and Shine Shear Zone, BendigoF3 vergence boundaries are present adjacent to this zone (Fig.3). The gendy northeast-dipping RSSZ transects F3 mesofoldswith shallow axial planes (Fig. 4). The rock away from theshear is coarsely to moderately layered, whereas it isbrecciated and siliceous in the zone. The penetrative foliation(52) north of the RSSZ dips gently north (Fig. 4A). To thesouth, S2 is subhorizontal (Fig. 4D) but in the RSSZ dipsshallowly southeast, and a nonpenetrative fracture cleavage(53) is subparallel to the zone. A mineral lineation (L2),defined by elongate micas and quartz, plunges north tonorthwest or southeast (Fig. 3, 4B, E). Locally, a northwest-trending mineral elongation transects a NNE trend (L2), andquartz rods are rotated in minor shear zones from thenorthwest to north.
Table 1 Morphology elements of mesofolds for sections of CentralOtago.
AreaNo./gen.
Wavelength(cm)
Amplitude(cm)
Interlimbangle (°)
RSSZ47 F3a folds7 F3b foldsCromwell Gorge33 F3a folds32 F3b folds
Ophir47 F3a folds28 F3b folds
Oturehua10 F3 folds
Patearoa24 F3 folds
Barewood20 F2 folds14 F3b folds
Blackstone Hill39 F 3 a folds12 F 3 b folds
15.1±10.3+1
9.4+122 .0±
18.9 ±18.1±1
3.8 +
6.7 +
1.5 ±12.8 +2.1
12.7+4.4 +
26.911.8
12.228.4
30.314.4
1.9
9.7
1.02.1
38.33.0
12.7±217 .2±
9.9+19.6±1
21.6+3119.9±
3.8±1
6.3+
1.6±3 .6±
15.7 +2 .7+
21.67.2
10.711.6
31.329.5
1.9
9.3
.92.3
42.42.0
58.2±26.168.6+1
3 0 . 4 13 6 . 4 1
5 1 . 5 153.0 +
21.71
20.31
33.6144.41
50.3152.41
26.117.0
26.831.7
20.720.3
27.0
23.9
9.518.3
16.818.0
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A North of Rise and Shine Shear
New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
C Rise and Shine Shear
D South of Rise and Shine Shear
52 poles
Contour max. great circle
Stretching direction
Pole to penetrative foliation (S2)
Quartz rod
S3 fracture cleavage Shear
Quartz vein
Quartz vein fibre
Vein relative timing
shear // Main shear
reverse movement
normal movement
A
O
8
•
•A-
1,2
—
Fig. 5 Rise and Shine Shear area brittle structural data. A, Contoured poles to quartz veins. Interval .9, 1.8, 3.5%, max. plane 096/76N. B, Poles to minor shears. C, Structural data from the shear zone. D, Poles to quartz veins. E, Contoured poles to minor shears.Interval 1.9, 3.9, 7.7%, max. plane 142/15W.
Table 2 Prominent vein (V) and shear (Sh) types in sections of the Otago Schist Belt and their relativetiming V1>2-
Area
RSSZ
Cromwell Gorge
Carricktown
Ophir
Oturehua
Patearoa
Barewood
Whites ReefConroys Gully
Blackstone Hill
Normal^2 -F3a
axes
V!
Vl,2_
v?ShViShv?——-—_v?v?
v7
Subpar.F3a axialplanes
ShViSh__ViSh_Sh__Vi
_-Sh_Sh
NormalF3baxes
V j
-
—
v?—v?___v?——__-—--
ConjugateF3b axialplanes
__-___
____Sh
v?
____-
Subpar.Yya axialplanes
_ShV2 (thrust)Sh_—
v?Sh
v?_V,Sh
v?Sh_-——Sh
Gentlynorth
dipping
v 2ShVl.,2ShV,ShV,Sh_Sh_ShminorSh
v?v?_Sh
Gentlysouth
dipping
Sh-Sh__
v2Sh__V,Sh_
—Sh_Sh
Vj „ 2 denotes the relative timing of vein dilation.For Vj ̂ both early and late veins are present.The relative timing of V? veins is unknown.
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Winsor—Low-angle shears, Otago 507
Legend
Stretching direction
Quartz rod
Crenulatlon
Macroscopic F3b
synciine
Vergvnc* F ;
Vergenc» F3
Post F3 told
Lakt margin
Kilometres
Fig. 6 Cromwell Gorge structural interpretation (structural elements without values are interpreted).
Based on vergence criteria, two fold groups F3a and F3b
which deform S2 are identified (Fig. 2). F3a axes exhibitvariable trends north of the shear (Fig. 4C), most plungingnorthwest-southeast or northeast with southwest-dippingaxial planes. Folds designated F3b have similar axis directions,but their axial planes usually dip northeast (Fig. 4C, F). Table1 shows fold differences revealing greater wavelength,marginally greater amplitude, and tighter interlimb angles forF3a folds adjacent to the RSSZ.
Quartz veins are mainly narrow (<5 cm) and nonfibrous,although some have fibres parallel to 1^. Two prominent setsare noted: (1) shallow north dipping, parallel to the RSSZ andF3b axial planes; and (2) vertical, normal to F3aaxes (Fig. 5 A,D), whereas in the RSSZ, veins are often vertical north-southstriking (Fig. 5C). Table 2 displays the continuity of veins andshear sets throughout the Otago Schist Belt, revealingwidespread occurrence of veins with similar geometries whichimply that Cenozoic block faulting has not significantlyreoriented structural elements. Vein offsets (Fig. 5A), suggestshallow north-dipping veins formed after east-west striking,subvertical veins. Two mesoscopic, moderately northeast orsouth dipping shear sets exist across the RSSZ (Fig. 5B, E),both exhibiting thrusting. Northeast-dipping mesoshearstransect F3a folds, and are subparallel to F3b axial planes andthe RSSZ, displaying thrusting in minor duplexes.
Cromwell GorgeCromwell Gorge (Fig. 6) is the site of a low-angle south-dipping macroshear zone (Turnbull 1988) subparallel to S2,
the gorge (Fig. 7A), and adjacent to an interpreted terrainboundary (Norris & Craw 1987) across which structures arereoriented. A mineral elongation (L2) and quartz rods trendnorth to NNE-SSW (Fig. 7B). Extreme variations exist inpost-S2 "ductile" (F3) fold orientations (Fig. 6, 7), with twovergence directions identified; that is, F3a folds verge NNW(Fig. 7C) with southeast-dipping axial planes, while F3b foldsverge southeast (Fig. 7F) with northwest-dipping axial planes.Features suggesting a fold chronology are F3a axes oftensubparallel to Lj, while F3b axes are oblique and folds are notspatially restricted. Also supportive are differences in foldmorphology, indicating F3a folds are slightly tighter, of shorterwavelength, and greater amplitude (Table 1).
Thin (<5 cm wide) quartz veins are common, althoughsome subhorizontal veins are up to 30 cm wide. Twoorientations are noted (Fig. 7D): subhorizontal to gently northdipping, subparallel to F3a orF3b axial planes; and subvertical-northwest striking, normal to l^. Most veins make a moderateangle to S2, are subparallel to F3b axial planes (Fig. 7F), andexhibit thrusting. They displace foliation subparallel veins(see Fig. 2C, D), while veins normal to L2 are offset by thoseparallel to F3 a axial planes. An offset supporting foldseparation is apparent in Fig. 2E. Mesofractures displayvariable orientations most dipping gently south (Fig. 7E)subparallel to F3a axial planes, while a few are parallel to ¥•$,planes and transect F3a folds (Fig. 2F). Shear criteria indicatenormal movement on south-dipping fractures; this movementsense is inferred on the macroshear zone recognised throughthe Cromwell Gorge (Tumbull 1988 & pers. comm.). Some
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A Cromwell Gorge
New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
C
299 poles
Contour max. great circle
Quartz rod A
Vein relative timing 1,2
— F3, told axla O
F3a axial plane 3
Quartz vein fibre •&
Fig. 7 Cromwell Gorge structural data. A, Contoured poles to penetrative cleavage S2. Interval 1.3, 2.6, 5.3, 10.5%, max. plane 132/32S. B, Contoured mineral lineation V^. Interval 1.9, 3.8,7.7%, max. lineation 205/21. C, F3a fold axes and axial planes. D, Contouredpoles to quartz veins. Interval .3, .7,1.3,2.7%, max. lineation 068/025W. E, Contoured poles to minor shears. Interval 1.3, 2.6,5.2%, max.plane 050/35S. F, F3b fold axes and axial planes.
northwest-dipping fractures exhibit normal shear, whereassimilarly oriented quartz veins display thrusting in minorduplexes (Fig. 2G).
Carricktown area
The area extends across an F2 antiform (Fig. 8), which istransected by northwest-southeast trending F3 folds. Apenetrative foliation (S2) in the dominant coarsely layeredschist dips mainly east (Fig. 9A) but is folded about a north-east axis. L2 plunges moderately northeast (Fig. 9B), as domost quartz rods. A group of quartz veins subnormal to L^(Fig. 9D, Table 1) exhibit a fibrous filling with north-plungingfibres. Nonfibrous veins dip gently northeast, not parallel to F2
or F3b axial planes (cf. Fig. 9C, D). Apart from high anglenorthwest, north-striking reverse mesoshears, gently tomoderately northeast and east dipping shears are present (Fig.9E, Table 2).
Ophir areaThis is a F3a vergence boundary site (Fig. 10; Means 1966,fig. 4), where the rock type is mainly coarsely layered schist.
Although a northeast-trending vergence boundary is present, itis transected by northwest-plunging folds which deform thepenetrative S2 cleavage and are correlated with F3b in subareas1 and 2 (Fig. 1). In the Ophir area, fold overprinting isconsistent with the relative timing established elsewhere. S2 isgenerally subhorizontal (Fig. 11 A) but folded about asouthwest axis. A mineral elongation plunges shallowlyNNE-SSW (Fig. 1 IB), subparallel to F3a axes and quartz rods,although some north-plunging rods transect F3 folds. F3a
mesofold vergence changes define macrofolds (Fig. 10), withsubhorizontal to gently east dipping axial planes, butexhibiting variation about a north axis (Fig. 11C).
Two main sets of quartz veins are noted (Fig. 1 ID & Table2), one subhorizontal to gently north dipping oriented at a lowangle to F3a axial planes, and S2, with a massive quartz filling.Their maximum dips are shallowly northeast whereas mostF3a axial planes dip east. The other set comprises fibrous veins,subvertical northwest striking and subnormal to L2. A minorset is subnormal to F3b fold axes but at an angle to the mineralelongation direction. Shears have variable orientations (Fig.1 IE), most being subparallel to F3a axial planes.
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Fig. 8 Carricktown area struc-tural interpretation (structuralelements without values areinterpreted).
N
SCALE
.5 1Kilometres
Legend
Stretching direction
Quartz rod 22
Crenulation
Vergence F j
Vergence F3
Post F3 fold
Mineralized shear
Macroscopic F2 anticline — W"
Macroscopic F3b anticline * - - _
y.
38
/ 1 6
38
30 \
A
/ Carricktown (derelict stone buildings)
22
/ 30
Oturehua areaSteeply dipping mineralised veins are located in this region(Fig. 1), where S2 is subhorizontal (Fig. 12A) and L2 trendsnortheast-southwest, although deformed about a gently WSWplunging axis (Fig. 12B). Elongate quartz rods are subparallelto L2, but locally deformed to plunge north. F3 folds (Fig. 12C)which are generally larger than the other areas examined showgreat variability, plunging north, northwest, south, ornortheast, usually with northeast-dipping axial planes. Thinquartz veins are abundant with variable orientations (Fig.12D), although two main sets are distinguished (Table 2). Avariety of mesofractures dip moderately (Fig. 12E) anddisplay thrusting. Northeast-dipping fractures are subparallelto quartz veins and F3 axial planes.
Patearoa areaF3b folds dominate in this area, where there is a change inquartz rod trends, from the NNE to northwest (Fig. 1). S2 isgenerally subhorizontal although locally folded about anorthwest-plunging (F3) axis (Fig. 13 A). A mineral elongationgenerally plunges shallowly NNW-SE (Fig. 13B) subparallelto F3 axes. Elongate quartz rods and a coarse crenulation(amplitude 5 cm, width 1. 2 cm) are subparallel to thisdirection. Vergence changes suggest a local F3 macro-antiform with subhorizontal to gently southwest dipping axial
plane (Fig. 13C). Table 1 shows morphology elements of F3
folds revealing low amplitude, wavelength, and interlimbangles, thus suggesting greater D3 shortening in this region.
Three main vein sets are apparent (Fig. 13D): close tohorizontal to gently southwest dipping narrow veins (i.e.,subparallel to F3 axial planes which display thrusting);subvertical steeply northwest dipping, northeast striking,subparallel to F3 axes; and subvertical northwest striking,near-normal to F3 axial planes with a fibrous filling. Minorshears have variable orientations (Fig. 13E), most dipshallowly northeast transecting F3 folds. One set is subparallelto F3 axial planes and another is conjugate.
Barewood areaThe dominantly coarsely layered schist in this area has beenmined for gold, tungsten, and antimony in steeply dippingveins. S2 is generally subhorizontal although folded about anorthwest axis (Fig. 14A). L2 plunges shallowly northwest(Fig. 14B), subparallel to F2 & 3 axes and elongate quartz rodsbut may be deformed to a northerly trend. Crenulations arefine to coarse, with gently north-northwest plunging fold axes.An early crenulation has axes plunging gently northwest,subparallel to L2, but is transected by one trending north. It isdifficult to distinguish F2 and F3 folds as they are few innumber, and have similar morphology and axis orientations
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7 folds
22 poles
Pol* to penetrative foliation (S2)
Stretching direction
Quartz rod
F2 fold axis
F2 axial plane
F3 fold axis
F3 axial plane
Quartz vein
Quartz vein fibre
Shear
reverse movement
Fig. 9 Carricktown area structural data. A, Poles to penetrative cleavage S2. B, Mineral lineation L^- C, F2 and F3 fold axes and axialplanes. D, Poles to quartz veins. E, Poles to shears.
(Fig. 14Q. Tight interlayer folds are, however, assigned F2,whereas F3 folds deform S2 and have moderate southwest-dipping axial planes. Table 1 compares the Barewood areafold morphology with that of other regions. Although therange of the two generations overlaps, the mean wavelength,amplitude, and interlimb is greater for F3b folds.
Three quartz vein sets can be separated in the Barewoodregion (Fig. 14D, Table 2):
Setl Northeast-southwest striking, subvertical, fibrousveins, normal to 1^.
Set 2 Veins are parallel to the mineralised Barewood Reef,dipping 60° northeast, subnormal to F3 axial planes.They are < 1.5 m wide and filled with nonfibrousquartz, upon which slickensides suggest dip and strike-slip movement. Shear criteria evident by drag andsecondary fractures indicate normal displacement.
Set 3 Thin nonfibrous veins, subparallel to F3 axial planes.
Dilation occurred (Fig. 14D) in subvertical northeast-striking set 1, before set 2 veins. Minor fractures dipmoderately north or northeast (Fig. 14E) and are present asanastomosing secondary shears adjacent to the BarewoodReef, indicating normal movement. A few fractures aresubparallel to F3 axial planes, but most are subnormal.Deformation adjacent to north-dipping quartz veins (sub-parallel to S2) suggests thrusting.
The structure at the Barewood Antimony Deposit (Fig.14F) is abnormal, as S2 dips moderately southeast (60°) and L2is deformed. A number of gently north dipping mesofracturesare present displaying evidence of low-angle reverse andnormal shear. Possible explanations for fabric reorientationare: Cenozoic block faulting, or post-F3 shearing. Thepresence of mesoscopic thrusts suggests reorientation due to ashallow-dipping shear zone. Although exposure is limited, thearea may represent part of a structural "horse" of undeter-mined lateral extent.
Other areas of interestWhites Reef: This is a region of finely layered schistadjacent to a suspected F3 vergence boundary. The penetrativecleavage (S2) is subhorizontal to gently southeast dipping,while L2 and quartz rods plunge NNE or north. A medium tocoarse north-northwest plunging crenulation represents F3
axes. As Table 2 shows, quartz veins adjacent to Whites Reefmaintain a similar orientation to the other areas examined.
Conroys Gully: The rock here is a coarsely to locally finelylaminated, brecciated, siliceous schist. S2 is subhorizontal,while \JI and quartz rods trends north-south. F3 folds plungenorthwest-southeast shallowly with gently (30°) southwestdipping axial planes. Veins are subvertical, northwest striking
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Winsor—Low-angle shears, Otago 511
Verg*nc« F$b
Crenulation
Fault position F
approximate
Fig. 10 Ophir area structural interpretation (structural elements without values are interpreted).
or gently southwest dipping, subparallel to F3 axial planes.Minor shears are subparallel to F3 axial planes (Fig. 2H) andexhibit thrust-related drag folds.
Blackstone Hill: An F3a synformal vergence boundary ispresent here across which L2 plunges gently northeast,subparallel to F3a axes. Where axial planes are steep, L2plunges steeply. The vergence boundary is transected bynorthwest-southeast trending post-S2 folds denoted as F3b.Table 1 depicts differences between phases, indicating F3a
folds exhibit more variability and greater wavelengths andamplitudes than elsewhere. Three quartz rod orientations areapparent, gently northeast, north, or northwest plunging.North and NNE plunging crenulations are present, of whichthe former transects F3a folds.
Variable F3a axes suggest later folding about a WNW axis.Axial planes vary from steep southwest to moderate (45°) eastdipping, possibly due to later deformation. The existence oftwo D3 subphases supports the idea that F3b folds deformed thevergence boundary, thus providing an explanation for theapparent anomaly. A pre-F3 fold or inhomogeneity during D3cannot be entirely discounted, but they are unlikely, becauseL2 is deformed across the region while north-plunging quartzrods maintain their orientation, implying deformation post L2,syn-F3b. Most quartz veins in the area are thin and subnormalto the mineral elongation direction. Two mesoshear sets aredistinguished, dipping moderately north and south, andintersecting at about 60°.
DISCUSSION
Regional correlationCorrelation across the Haast-Otago Schist Belt is difficult, asearly fabrics are rotated at variable degrees towards theelongation direction. The penetrative fabric may not always beS2 as a particular phase might not have developed syn-chronously on a regional scale. In correlating fabrics, anassumption is made that the penetrative fabric is S2. Whileevidence of an earlier deformation is preserved micro-scopically it is only rarely seen at a larger scale.
When the geometry and nature of structures are examined,the prevalence of gently north dipping quartz veins andmeso-macroshears is clear (Table 2). Often shears exhibitthrusting, although normal movement is also evident. Shallownorth-dipping shears are regionally correlated and taken torepresent a thrusting event. Although low-angle quartz veinsare often subparallel to F3 axial planes, where these planes dipsouth or subhorizontally, a set of north-dipping veins andshears maintain their orientation.
Apart from the gently north dipping shears and quartzveins, moderate to shallow south-dipping "conjugate"mesoshears (but usually not veins) are common. Examples arepresent across the RSSZ (Fig. 5), in the Cromwell Gorge(Fig. 7), Ophir (Fig. 11), and Pataeroa areas (Fig. 12) andacross the HMSZ (Winsor 1991). They display both thrust andnormal movement and are taken to represent a post-thrustingconjugate system.
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317 poles
Contour max. great circle
Quartz rod A
axial planes
F 3 b fold axis *
Vein relative timing 1,2,3
Fig. 11 Ophir area structural data. A, Contoured poles to penetrative cleavage S2. Interval .7, 1.5, 3.0%, max. plane O8O/O8S.B, Contoured mineral lineation L2. Interval .8, 1.7, 3.3, 6.7%, max. lineation 205/5S. C, Contoured F3a and 34 F3b fold axes. Contourinterval 1.9,3.6,7.7%, max. plunge 10 towards 007. D, Contoured poles to quartz veins. Interval .3, .6,1.3,2.5,5.1%, max. plane 113/20N.E, Contoured poles to minor shears. Interval 1.7,3.4,6.8%, max. plane 176/30E. F, Contoured F3a and poles to 32 F3b axial planes. Contourinterval 1.9,3.9, 7.7,15.4%, max. plane 012/16E.
A regionally common fabric which is a result of theyoungest semiductile event (here assigned to D5) is a low-amplitude (<0.5 cm) crenulation plunging northeast-south-west. A fourth deformation (D4) resulted in mesofolds,microfolds, kinks, and quartz rods, with a north-south trendtransecting F3 folds and deforming earlier lineations. An earlyD3 shortening "subphase" developed in west-central Otago,with fold distinction based on vergence criteria and geometryin relation to L^. Although often distinct from Cenozoic fault-related kinks, F3 folds can resemble kinks dependent on stressdistribution and rock type (cf. Dewey 1969, p. 208).
In west-central Otago, it is unclear whether D3 rotated L2into parallalism with F3a axes or nucleated in this direction(i.e., formed L3; e.g., Cobbold & Watkinson 1981). In parts ofeastern Otago where the degree of F3b shortening is morepronounced with quartz rod trends (Fig. 1) reflecting this, L2 isrotated into parallelism with F3 B axes.
Significance in relation to gold mineralisationThroughout the Otago Schist Belt, shears maintain a con-sistent geometry and relationship to F3b and to a lesser extentF3a folds. Although north-dipping mesoshears are common,
there is only limited evidence of macroshears. Possiblereasons for this are that the last D3 folding phase (F3b, a morebrittle event), is more pronounced in east Otago and the rocktype is mainly coarsely layered, strongly segregated schistwith evenly dispersed narrow quartz veins. If a wide zonequartz or finely laminated schist was present, this wouldprovide a local weakness in the rock mass along which laterthrusting could be accommodated.
Mesoscopic and microshears will preferentially nucleateand develop macroshears when the effective stress resultingfrom tectonism is above a certain value, the shear strength isbelow a critical value, and anisotropies are oriented such thatshear failure can occur under brittle or semibrittle conditions(Mandl 1988). Failure planes will tend to be either structurallyor lithologically induced sites of material instability, due to theinherient weaknesses. In a strongly foliated, folded, gentlydipping, generally homogeneous schist, weaknesses are likelyalong attenuated fold limbs. Under brittle-ductile transitionconditions, the potential for macroshear development is not asgreat in a zone of coarsely layered schist as in finely layeredlower shear strength material. Coarsely laminated or psam-mitic schist lenses in a shear zone between adjacent finely
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Winsor—Low-angle shears, Otago
A Oturehua area B
513
10 loldt
Contour max. great circle
Quartz rod
F3 (old axis
F3 axial plane
Quartz vein
Quartz vein fibre
Shear
reverse movement
normal movement
Rotation path
32 poles
Fig. 12 Oturehua area structural data. A, Contoured poles to penetrative cleavage S2. Interval 1.6, 3.3, 6.6, 13.1%, max. plane152/15W. B, Contoured mineral lineation L2. Interval 1.9,3.9,7.7,15.4%, max. lineation 041/00. C, F3 fold axes and axial planes.D, Contoured poles to quartz veins. Interval .6,1.2, 2.4%, max. plane 123/27N. E, Poles to minor shears.
laminated incompetent schist, should allow the formation oframp structures (as suggested by Teagle et al. 1990) andfacilitate brittle failure in the shear zone, inducing potentialmineralisation sites. An extensive narrow zone (probably 50-150 m thick) of finely layered schist in dominantly coarselylayered competent schist has a higher likelihood for thenucleation of a macroshear, than a more homogeneous rocktype of greater shear strength. In a wider finely laminatedzone, the possibility of a "single" narrow shear is reduced, andthe structure would become a dispersed anastomosing zone.This is a feature of the HMSZ on its eastern extension.
Mandl (1988) recognised shear zone thickness to bedependent on lithology-shear strength variations, state ofstress during initial loading, and drainage conditions. Controlsrestricting development of low-lying shears in the OtagoSchist are the general homogeneous-competent (coarselylayered) nature of the Otago Schist, the dominance of anintense low-lying foliation, and the localisation of F3b folds toeastern Otago.
Many examples exist of shear zones in fold and thrust beltswhere there is a lithological-competency (shear strength)control to the zone's development (e.g., Evans 1989; Ferrill &Dunne 1989). Often in such terrains, competent crystallinerocks are thrust over incompetent carbonates along a regionaldecollement with the essentially unmetamorphosed carbon-ates becoming imbricated. For the mainly homogeneous
Otago Schist, although the lithological contrast is notextensive even at terrane boundaries, minor differences arepresent (e.g., from coarse to finely layered schist). It is alongthese ductility contrasts that low-angle macroshears willnucleate, enhanced by suitably oriented structures.
The Hyde-Macraes Shear Zone (Fig. 1) is a prominentmacroshear, containing gold mineralisation at particular sitesand exhibiting a complex relationship to F3b folds (Winsor1991). The main reason for this variable fold association is thedifferent degree of D3 strain evident in the upthrust plate. TheRSSZ displays similar structural elements (i.e., orientation,ramping, and minor shears) to the HMSZ (Teagle et al. 1990),suggesting a thrust origin. At locations along the HMSZ whereF3b antiforms are thrust up, greater mineralisation potential issuspected due to reactivation of fold-related weaknesses.These shear orientations would be favourably oriented forreactivation and dilation during later thrusting, but for theRSSZ there are no upthrusted F3b macrofolds to provide anenhanced host structure.
Areas of the Otago Schist Belt with shallow north-dippingF3 axial planes are considered well suited for the developmentof a low-lying north-dipping shear zone and associatedmineralisation. The idealised structural geometry to enhancethe development of a low-lying shear zone is illustrated in Fig.15A & B. Where F3 axial planes and S2 dips south, the north-dipping thrusts would be oriented at moderate angles to F3 fold
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24 folds
52 poles
Contour max. great circle
Quartz rod
F3 fold axis
F3 axial plane
S3 fracture cleavage
Quartz vein
Quartz vein fibre
Fig. 13 Patearoa area structural data. A, Contoured poles to penetratve cleavage S2. Interval 1.9, 3.7, 7.5, 14.8%, max. plane 095/ION. B, Contoured mineral lineation L^. Interval 2.2,4.4, 8.7,17.4%, max. lineation 168/00. C, F3 fold axes and axial planes. D, Polesto quartz veins. E, Poles to minor shears.
axial planes and would restrict dilation width. There should bean optimum low angle between the north-dipping thrusts andF3 axial planes for maximum shear zone dilation, which waslocally reached along the HMSZ. Apart from the area adjacentto the HMSZ, the only other occurrences of north-dipping F3b
axial planes are near the RSSZ, in the Cromwell Gorge (Fig. 6)and the Blackstone Hill area. No known gold lode is present inthe latter areas, and the dispersed thin nature of low-lyingshears and quartz veins present, suggests any mineralisationwould be evenly distributed up-dip and along strike. Thedominance of F3a folds commonly exhibiting south-dippingaxial planes, weak development of F3b folds, and the mainlycoarsely laminated rock type in each of these areas hasinhibited macroshear zone development except across theRSSZ.
The Otago Schist Belt was progressively shortened duringthe third regional deformation by two sequential foldingphases (F3a and F3b), which vary in intensity across the belt.Eastern portions of the belt were affected to a greater extent byF3b folds than western and central areas. Gently north dippingmacroshears and mesoshears, and quartz veins exhibitingthrusting, transect F3a and F3b folds, representing a significantshortening event after ductile deformation. Low-angle conju-gate fractures are present regionally, containing evidence ofinitial reverse movement followed by normal movement. Goldmineralisation sites associated with low-lying macroshears are
anticipated in areas characterised by dominant F3b macrofolds,which display gently north dipping axial planes and wherethe rock comprises a laterally extensive zone (c. 50-150 mwide) of finely laminated schist adjacent coarsely laminatedschist.
ACKNOWLEDGMENTS
R. J. Norris (Otago University) is thanked for providing usefuladvice during this project and critically reviewing the manuscript.A. F. Cooper (Otago University) is also thanked for reviewing thearticle. Fieldwork and discussions with D. J. McKenzie areacknowledged. N. Mortimer (DSIR Geology & Geophysics) isthanked for allowing publication of the quartz rod trends in Fig 1.Drafting by D. P. Winsor is greatly appreciated. The author was arecipient of a New Zealand U.G.C. Postdoctoral award at the time ofthis research.
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