Accepted Manuscript

Structural Controls on gold mineralization on the margin of the Yilgarn craton, Albany–Fraser orogen: The Tropicana Deposit, Western Australia

T.G. Blenkinsop, M.G. Doyle

PII: S0191-8141(14)00026-1

DOI: 10.1016/j.jsg.2014.01.013

Reference: SG 3017

To appear in: Journal of Structural Geology

Received Date: 1 March 2013

Revised Date: 20 January 2014

Accepted Date: 26 January 2014

Please cite this article as: Blenkinsop, T.G., Doyle, M.G., Structural Controls on gold mineralization onthe margin of the Yilgarn craton, Albany–Fraser orogen: The Tropicana Deposit, Western Australia,Journal of Structural Geology (2014), doi: 10.1016/j.jsg.2014.01.013.

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• Tropicana is a world class gold deposit on the margin of the Yilgarn

craton

• Gold was precipitated in the Archean at greenschist facies within

granulite gneiss hosts

• Mineralization was governed by fluid flow in a network of shear zones

• The shear zones and ore bodies reflect the geometry of the host gneisses

• The entire history of five deformation events has affected gold

mineralization

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Gneissic banding and shear planes have similar orientations and control ore bodies

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Structural Controls on gold mineralization on the 1

margin of the Yilgarn craton, Albany–Fraser 2

orogen: The Tropicana Deposit, Western Australia 3

4

T. G. Blenkinsop1,2* and M. G. Doyle2 5

1 Economic Geology Research Unit 6

School of Earth and Environmental Science, 7

James Cook University, 8

Townsville QLD 4811, 9

Australia 10

2Present Address: 11

School of Earth and Ocean Sciences, 12

Main Building, Park Place, 13

Cardiff CF10 3AT, 14

United Kingdom 15

BlenkinsopT@Cardiff.ac.uk 16

*Corresponding Author 17

18

2 AngloGold Ashanti Ltd 19

Level 13, St Martins Tower 20

PO Box Z5046, Perth WA 6831 21

MDoyle@AngloGoldAshanti.com.au 22

Key words. Shear zone, Archean, Lode gold, orogen, Albany-Fraser, Tropicana 23

24

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Abstract 25

26

The Tropicana gold deposit is located adjacent to the margin of the Yilgarn craton in 27

the Albany–Fraser orogen, Western Australia. The deposit is hosted in granulite facies 28

quartzo-feldspathic gneisses of the Archean Tropicana Gneiss. Ore bodies comprise 29

biotite-pyrite alteration concentrated in shear zones that formed during NE-SW 30

shortening in the late Archean, and clearly postdate the formation and deformation of 31

high-grade gneiss fabrics (D1 and D2). The orientation of the ore bodies is controlled 32

by the shear zones that are in turn localised by the gneissic banding. Mineralization 33

also involved solution and coeval microfracturing and veining of more competent 34

pegmatitic units. The mineralizing event (D3) was followed by at least two further 35

deformations, which reactivated and overprinted the biotite fabrics with sericite and 36

chlorite, created new shear zones, and affected gold distribution. D5 consisted of 37

dextral shear on ~E-W shear zones, which subdivide the deposit into five major 38

structural domains. The importance of structurally controlled permeability at 39

Tropicana is similar in cratonic lode gold deposits, as is the protracted 40

deformation/fluid flow history. Like Renco mine in Zimbabwe, Tropicana gold 41

deposit was formed by hydrothermal fluid flow peripheral to the craton: economic 42

gold mineralization was clearly post-peak metamorphism. 43

44

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1. Introduction 45

46

Many Archean lode gold deposits have distinctive geological characteristics (e.g. 47

Robert and Brown, 1986; Groves et al., 1998; 2000; Wit and Vanderhor, 1998; 48

Goldfarb et al., 2001) including: 49

50

1) Discrete, high grade lodes, commonly with abundant quartz and 51

carbonate veining; 52

2) Greenschist-amphibolite facies peak metamorphism of the host rocks, 53

which slightly predates alteration and mineralization at similar or lower 54

grade metamorphic conditions; 55

3) A variety of supracrustal host rocks, although Fe-rich and competent 56

lithologies make especially favourable sites for mineralization; 57

4) Little mineralization in plutonic rocks; 58

5) A spatial association with felsic intrusions. 59

60

In addition to these general geological characteristics, the ore bodies all have in 61

common strong structural controls, which testify to the essential roles of 62

permeability and fluid flow in creating these hydrothermal ore bodies (e.g. Cox, 63

1999). The controls can be crudely classified in terms of the hosting structure as 64

breccias, faults and shear zones (e.g. Hodgson, 1989). In well-documented cases 65

there is evidence of reactivation of structures and multiple cycles of deformation 66

and fluid flow (e.g. Poulsen and Robert, 1989; Baker et al., 2010; Davis et al., 67

2010; Miller et al., 2010; Dirks et al., 2013). Increasingly these patterns are 68

interpreted in terms of stress and fluid pressure fluctuations associated with the 69

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earthquake cycles (Sibson et al., 1987, 1988; Robert et al., 1995; Cox and Ruming, 70

2004; Micklethwaite and Cox, 2004; 2006). 71

72

At a scale greater than individual deposits, it is well recognised that Archean lode 73

gold deposits are not found directly on craton-scale shear zones, but instead lie 74

in adjacent lower order structures (e.g. Kerrich, 1989; Vearncombe, 1998), 75

although a role for the first order features can be inferred from the distribution 76

of mining camps along them (e.g. Weinberg et al., 2004; Blewett et al., 2010a,b). 77

At a global scale, the occurrence of gold provinces that contain giant or several 78

world class gold deposits has been explained as the consequence of their 79

formation in orogenies involving thin lithosphere or subducted oceanic crust 80

(Bierlien et al., 2001; 2006) because of the greater likelihood of high 81

asthenospheric heat input. 82

83

This study describes the structural controls on Australia’s largest new gold 84

discovery, the world class Tropicana deposit in Western Australia. The Tropicana 85

deposit is located adjacent to the edge of the Archean Yilgarn craton in the 86

Albany–Fraser orogen (Fig. 1), naturally leading to comparisons with the 87

Archean lode gold deposits of the Yilgarn craton, and posing the question of 88

whether it has formed in a similar way. The aims of this paper are to describe the 89

structural controls on mineralization at Tropicana, to make a comparison with 90

the classic deposits of the Yilgarn craton, and to highlight some remarkable 91

comparisons between the deposit and the Renco gold mine in Zimbabwe. These 92

comparisons cast light on the genesis of the Tropicana deposit. 93

94

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95

2. Geology of the Tropicana Deposit 96

97

2.1 Regional Setting 98

The Tropicana deposit is situated 41 km to the E of the easternmost magnetic 99

expression of the Archean Yilgarn craton, in the Northern Foreland of the 100

Albany–Fraser orogen (Spaggiari et al., 2011). The proximal part of the Yilgarn 101

craton is the Yamarna Terrane of the Eastern Goldfields Superterrane (Pawley et 102

al., 2012) (Fig. 1). On a regional scale, the boundary between the Northern 103

Foreland and the craton has been interpreted as a major regional structure, the 104

Cundeelee fault, which may have originated as a thrust (Spaggiari et al. 2011). 105

Immediately to the W of the Northern Foreland, a thick sequence of Permo-106

Carboniferous sedimentary rocks overlies the craton, and is separated from the 107

Northern Foreland around Tropicana by the Gunbarrel fault (Fig. 2), a steeply 108

NW dipping normal fault which cuts the Cundeelee fault. There is no obvious 109

continuity between the NNW trending structures on the Yilgarn craton in the 110

Yamarna terrane (including the Yamarna shear zone) and structures in the 111

Tropicana area of the Northern Foreland (Fig. 2) (e.g. Jones et al., 2006). 112

113

The Albany–Fraser orogen mantles the southern and western margins of the 114

Yilgarn craton over a distance of more than 1000 km. Mesoproterozic orogenic 115

events have been recognised at 1350 – 1260 Ma and 1215-1140 Ma (Clark et al., 116

2000), but more recently it has become clear that Paleoproterozoic events 117

including deposition of metasedimentary rocks and intrusion of granitic to 118

gabbroic intrusions, constitute a major part of the eastern Albany–Fraser 119

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orogeny in the Biranup zone (Kirkland et al., 2011). High grade deformation 120

occurred here at 1680 Ma, called the Zanthus Event within the Biranup orogeny, 121

and this geological history has been interpreted as representing the evolution of 122

an arc-backarc on the margin of the Yilgarn craton (Kirkland et al., 2011). 123

124

The Northern Foreland is defined as the reworked part of the Yilgarn craton 125

within the Albany–Fraser orogen (Myers, 1990). The intensity and grade of 126

reworking varies in the Northern Foreland from amphibolite-granulite facies in 127

the S to greenschist-amphibolite facies in the N (Spaggiari et al., 2011). Around 128

and approximately 200 km to the SW of Tropicana, the Northern Foreland 129

consists of a fault-bound assemblage of rocks with a common and distinct 130

geological history that we define as the Plumridge terrane. The Plumridge 131

terrane is approximately 27 km wide at Tropicana: to the E lies the Biranup Zone, 132

consisting of intensely deformed gneiss and metagabbro with Paleoproterozoic 133

ages (Bunting et al., 1976; Spaggiari et al., 2011). The contact between the two 134

zones has a curved NE trending shape in map view, which is overall convex to 135

the NW: it is interpreted as a thrust, herein referred to as the Black Dragon 136

Thrust (Fig. 2). The Black Dragon Thrust juxtaposes ca. 1820 Ma metagranite and 137

amphibolite rocks in the Black Dragon Domain of the Biranup Zone above 138

Archean gneissic rocks hosting the Tropicana deposit, herein referred to as the 139

Tropicana Gneiss (Fig. 2). Deformation in the Biranup zone is associated with the 140

Biranup orogeny (1710 – 1650 Ma), but there was also activity along the Yilgarn 141

margin at 1800 Ma, as indicated by the deposition of sedimentary rocks and 142

intrusion of granites of this age (Spaggiari et al., 2011). 143

144

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145

2.2 Host rocks 146

Neither the host rocks nor the ore body are exposed at Tropicana, being covered 147

by up to 15 m of Cretaceous to Recent sediments. All the data in this study are 148

based on the diamond drilling carried out to delineate the mineral resource. Core 149

was examined from 36 drillholes (Supplementary Material gives drillhole 150

locations), but this did not include any drillholes into the Boston Shaker or the 151

Havana Deeps domains. 152

153

The host rocks at Tropicana are gneisses dominated by garnet gneiss 154

(plagioclase, amphibole, garnet ± leucoxene, quartz) and quartzofeldspathic 155

gneiss (plagioclase, k-feldspar, quartz, biotite), with lesser amounts of 156

amphibolite, meta-ferruginous chert (quartz, grunerite), pegmatite and mafic 157

granulite. The pegmatites appear to be products of in situ partial melting at peak 158

metamorphism, which was at upper amphibolite to lower granulite facies (Doyle 159

et al., 2007; 2009). Compositional banding in the gneisses dips moderately to the 160

E to SE (Fig. 4). The hangingwall of the deposit is dominated by the garnet gneiss. 161

The gneisses and the ore bodies are cut by mafic dykes ascribed to the c. 1210 162

Ma Gnowangerup-Fraser Dyke Suite (Doyle et al., 2007), which are prominent 163

regional aeromagnetic features trending NE (Fig. 2). 164

165

2.3 Geochronology 166

The age of the host rocks regionally was inferred to be Archean (Bunting et al., 167

1976). This possibility has been strengthened by unpublished propriety 168

geochronological data (Doyle et al., 2009) and preliminary U-Pb zircon ages of 169

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2722± 15 Ma and 2643 ± 7 ma for a metagranite sample taken 7 km N of 170

Tropicana, which have been interpreted as ages of crystallization and 171

metamorphism respectively (Spaggiari et al., 2011). 172

173

The retrograde path from peak granulite facies metamorphism is constrained by 174

a rutile U-Pb date of 2524 ± 8 Ma, interpreted to reflect cooling through 500-175

550°C (Doyle et al., 2013). A minimum age of 2515 ± 8 Ma for mineralization is 176

suggested by biotite Ar-Ar analyses, which is consistent with late Archean Re-Os 177

analyses of pyrite (Doyle et al., 2013). Discordance in zircons and monazites can 178

be interpreted in terms of Pb loss in Stage II of the Albany–Fraser Orogeny 179

(1215-1140 Ma: Kirkland et al., 2011). 180

181

182

3. Ore Geometry and Style of Mineralization 183

184

The resource at Tropicana occurs along a 5 km strike length trending overall NE, 185

which can be divided into five structural domains from N to S: Boston Shaker, 186

Tropicana, Havana, Havana Deeps and Havana South (Fig. 3). Low grade 187

mineralization is also recorded to the S of these main areas, for example at 188

Crouching Tiger prospect, and at other prospects regionally (Fig. 2). The five 189

domains have en echelon arrangement. Within each domain the general 190

mineralization envelope trends N to NE. The domains are separated by E to SE 191

trending shear zones, such as the Boston Shaker shear zone between the Boston 192

Shaker and Tropicana domains, and the Swizzler, Cobbler and Don Lino shear 193

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zones (Fig. 3). Most of these shear zones dip S. Map scale shears with a similar 194

orientation also occur within the resource areas, as well in a NE direction (Fig. 3). 195

196

Mineralization is concentrated in one to several sub-parallel tabular ore zones 2 197

– 50 m thick which generally dip to the E to SE, within quartzofeldspathic gneiss 198

(Fig. 4). Within these ore zones there are higher grade lenses. When viewed in 199

section parallel to strike, ore zones show an inosculating pattern, separating 200

lenses of unmineralised rock, and thickening and thinning (Fig. 5). The map view 201

of the gold assay data x thickness (gram-metres) shows high grade ore shoots 202

with slightly variable orientations between the domains. In Boston Shaker, the 203

trend is SE, in the northern part of Tropicana, ESE; in Havana and Havana Deeps, 204

SSE (Fig. 3). 205

206

Similar distinctions in orientations between the domains are seen in three-207

dimensional data by examining the orientations of modelled high grade lenses 208

(≥3 g/t) (Fig. 6). Tropicana is characterized by E to SE dipping ore bodies, 209

generally coaxial about an E-plunging line (29° → 087°), whereas the ore bodies 210

in Havana North dip between S and E, and have a common axis plunging to the 211

SSE (22° → 163°). 212

213

Gold grades in the ore zones at Tropicana are dominantly associated with 214

intervals of biotite-pyrite alteration that occur within quartzofeldspathic gneiss 215

with pegmatites. Biotite with pyrite and gold replaces metamorphic biotite and 216

amphibole, most commonly in millimetre wide shear zones defined by strong 217

fabrics consisting of elongate biotite and pyrite grains (Fig. 7a,b), but also in 218

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disseminated volumes. Higher gold grades are also associated with areas of 219

brecciation in pegmatites around shears, with shears containing biotite-sericite 220

and minor chlorite (Fig. 7c, d), and in areas with solution fabrics (see below; Fig. 221

8). Known occurrences of visible gold correspond with intercepts of >30 g/t in 222

1m composite assays. Visible gold is paragenetically late and typically localized 223

on muscovite fractures which cut across anatectic segregations, quartz veins and 224

gneissic bands and biotite-pyrite fracture fills. 225

226

4. Deformation History, Meso- and Microstructures 227

228

Table 1 gives a deformation history that can be inferred from drill core, 229

geophysics and deposit scale geometry. This section focuses on the detailed 230

evidence from the core pertaining to events which may be associated with gold 231

mineralization: the preceding history is outlined because it affects the deposit 232

geometry. 233

234

4.1 Gneissic Banding S1, F1 folds 235

The most commonly observed mesoscale structure in the cores is a gneissic 236

banding defined by variations of up to 20% in the proportions of quartz, 237

feldspars, biotite, amphibole and garnet on a mm to cm scale (Fig. 9a,b). The 238

gneissic banding is tight to isoclinally folded (Fig. 9a,b) with E to SE dipping 239

hinge surfaces and gently S plunging hinges (Fig. 10). Some of these folds are 240

rootless (Fig. 9b), suggesting that the gneissic banding is the product of early 241

deformation and high grade metamorphism, as indicated by leucosomes that are 242

generally parallel to the banding. 243

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244

4.2 D2 245

A fold on the scale of hundreds of m is suggested by W-dipping gneissic banding 246

in cores to the W of the deposit. The drill core data imply an asymmetric synform 247

in the footwall of the mineralization. Based on evidence from the nearest outcrop 248

at Hat Trick Hill (Fox-Wallace, 2010) and regional considerations (Spaggiari et al., 249

2011), this W verging fold is likely associated with a W to NW verging thrust 250

system that is developed regionally. It is possible that some of the S plunging 251

folds shown in Fig. 10 are F2 folds. 252

253

4.2 D3: Shear Zones 254

Quite distinct from the gneissic banding are localised zones of strong foliation 255

defined by biotite and pyrite, chlorite or sericite (Fig. 7b). Such shear zones are 256

typically mm to cm wide, and clearly cut across the gneissic banding in places, 257

although they are generally parallel to the banding. Asymmetric fabrics 258

indicating shear are common in such zones, and include SC and SC’ fabrics, sigma 259

porphyroclasts and oblique foliations (Fig. 9d). Lineations are very difficult to 260

observe because the foliation surfaces are not generally visible in the core. The 261

shear zones are commonly surrounded by zones of brecciation. 262

263

Shear zones containing biotite – pyrite only are distinct from those that may also 264

include chlorite or sericite: these minerals appear to overprint the biotite, so that 265

the shear zones containing biotite-pyrite are regarded as a third deformation 266

(D3), after the formation and folding of the gneissic fabric, but predating later 267

overprinting by other phyllosilicates. 268

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269

4.3 D3: Solution Fabrics and Breccias 270

Zones of intense solution fabrics are defined by wavy seams containing biotite and 271

pyrite 1 – 2 mm wide between fractured quartz and feldspar layers 5 – 10 mm thick 272

(Fig. 8). In places the fracturing is dense enough to be described as a breccia (Fig. 8b). 273

The quartz and feldspar are fractured by mm long veins filled with calcite that form 274

distinctive irregular shapes perpendicular to the stylolites (Fig. 8a). The calcite veins 275

appear to be extensional and in places are markedly oblique to the solution seams. 276

They are associated with ostensibly the same auriferous pyrite as the solution seams, 277

since that pyrite can be seen as a component of the fracture fill in the carbonate veins 278

(Fig. 8d), and biotite alteration in the seams extends into the fractures. The presence 279

of biotite and pyrite suggests that this fabric may have formed during D3, although 280

there is also a strong association with sericite in places. Significant gold grades were 281

recovered from a stylolitic interval in core from drill hole TP202. 282

283

4.4 D3 Folds 284

Gentle folds of the lithological layering visible in the mine model plunge 285

moderately SE and occur on the scale of whole domains. Folding with a similar 286

orientation can be inferred from the distribution of poles to gneissic banding (Fig. 287

11), and from some measurements of individual folds in core (Fig. 10). This 288

folding postdates D1 and D2, and is ascribed to D3. 289

290

4.3 D4, D5 Shear Zones and Folds 291

Some biotite-pyrite shear zones are overprinted by fabrics defined by chlorite 292

and sericite, which have distinct kinematics. Other shear zones contain sericite 293

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and chlorite only. A distinctive set of shear zones with biotite and 294

sericite/chlorite dip S and SW and have dextral kinematics. Some of these later 295

fabrics are folded into characteristically asymmetric folds on a 1 - 10 cm scale 296

(Fig. 9c). These folds have been measured at the boundary between the 297

Tropicana and Havana domains, near the Boston Shaker shear zone. Fold hinge 298

surfaces dip S, with generally E to SE plunging hinges (Fig. 10) and Z 299

asymmetries. The folds and the S dipping dextral shears are consistent with a 300

late deformation event comprising dextral shear on S and SW dipping zones. 301

Since they fold a sericite-chlorite fabric, this event (D5) probably postdates an 302

intermediate event represented by sericite-chlorite shear zones in various 303

orientations (D4). 304

305

306

5. Kinematic Analysis 307

308

Shear zones were measured from cores into the Tropicana, Havana North and South 309

domains (Fig. 12). Kinematic analysis of shear zones was possible from SC and SC’ 310

fabrics and sigma clasts which could be used to identify the vorticity vector and hence 311

the shear direction as perpendicular to the vector. A kinematic analysis was performed 312

using linked Bingham axes (cf. Marrett and Allmendinger, 1990) and filtering the 313

results by the phyllosilicate mineralogy of the shear zones into biotite, biotite with 314

sericite and/or chlorite, and sericite/chlorite groups (Figs. 12, 13). In all cases the 315

linked Bingham axes from the kinematic analysis give one of two shortening 316

directions: NE or NW (Fig. 13). 317

318

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The biotite shear zones dip to the SW, S, SE and NE (Fig. 12). Kinematics vary 319

consistently with orientation: SW, S and SE dipping shears are sinistral, while E and 320

NE dipping shears are dextral (Fig. 12). The displacement pattern is kinematically 321

coherent, and consistent with NE horizontal shortening, which is also reflected by the 322

linked Bingham axes (Fig. 13). This orientation defines D4 kinematics. 323

324

The other shear zones have similar orientations to the biotite shears, but in all 325

orientations there is a variety of shear directions and senses (Figs. 12, 13), commonly 326

with contradictory shear senses on adjacent and sub-parallel shears. A particularly 327

common set of shears dips S to SW with mostly dextral normal kinematics: these are 328

common at the major breaks between the Tropicana and Havana North domains and 329

between the Havana North and South domains, (e.g. holes TFRC090D and TFD167) 330

(Figs. 11, 12), and they define the D5 kinematics with a NW shortening direction. The 331

same shortening direction is apparent from shears that have sericite/chlorite and no 332

biotite, which can be associated with D5 (Table 1). 333

334

Shear zones with biotite and sericite/chlorite show some overlap between shortening 335

and extensional quadrants, and the Bingham axes reflect either NE or NW shortening 336

(Fig. 14). This is consistent with the possibility that some of these shear zones have 337

been reactivated in D5 kinematics, while others preserve D3 shortening directions. 338

339

340

6. Discussion 341

342

6.1 Deformation History and Structural Controls on Ore bodies at Tropicana 343

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344

The structural/mineralization history at Tropicana is summarised in the cartoons 345

of Fig. 16. D1 included the formation of high-grade gneissic banding, partial melting, 346

and isoclinal folding (Fig. 16a). The kinematics of D1 could not be constrained by 347

observations in this study, and the event as defined here might include additional 348

complexities. Regional considerations suggest that D2 was a major event of W to NW 349

directed thrusting that probably created some of the major structures in the area such 350

as the Iceberg thrust (Fig. 16b). Tight to isoclinal folds in gneissic banding plunging S 351

to SE observed in the core cannot definitively be ascribed to D1 or D2, and it is 352

possible that they represent a progressive deformation event. 353

354

Gold mineralization at Tropicana is controlled by a system of biotite-pyrite shear 355

zones within a favourable lithological band of feldspathic gneiss that has a sheet dip 356

to the E to SE. The biotite shear zones are generally sub-parallel to gneissic banding, 357

but clearly postdate it, and are surrounded by diffuse bodies of mineralised breccia. 358

The main mineralization phase was associated with the biotite alteration, and the 359

shear zones formed with a NE shortening direction (Fig. 14). 360

361

The biotite-pyrite shear zones measured in the core have an approximate girdle 362

distribution around a SE-trending axis (Fig. 15). Although the shears have a variety of 363

orientations, they are not folded on the scale of the core. The SE trend is similar to 364

the direction of the high grade ore shoots visible on the map (Fig. 3) and to the 365

common intersection of high-grade ore shells in Havana (Fig. 6). Gentle folding of the 366

gneissic banding in this orientation is also apparent on a large scale (Fig. 11): these 367

are ascribed to an F3 generation of folds. 368

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369

The SE trend observed in the gram-metre plot, the high grade ore shells, the girdle 370

distribution of the shears and the orientations of gneissic banding, is a very significant 371

control on mineralization, which is consistent with fluid flow along the biotite-pyrite 372

shears. The observation that the biotite-pyrite shears are not folded and the similarity 373

of Figs. 11 and 15 suggest that their orientation was largely controlled by the gneissic 374

banding, to which they are generally parallel. The orientation of the gneissic banding 375

reflects D1 and D2, which imparted the moderate E to SE dip to the banding, and a 376

component of gentle folding superimposed in D3. The trends of the high grade shoots 377

are therefore parallel to common intersections of the biotite-pyrite shear zones and 378

hinges of F3 folds (Fig. 16c). 379

380

There are significant variations in these trends between Tropicana and the other 381

domains. In Tropicana, high grade ore shells dip more easterly than in Havana (Fig. 6), 382

giving an easterly trend to their intersection, which is also apparent in the gram-metre 383

plot (Fig. 3). At least two possibilities to explain this variation are: i) an initial 384

variation in geometry inherited from D1 and D2; or ii) Reorientation by D4 or D5 in 385

Tropicana, which is distinguished from the other domains by a higher density of late 386

shear zones. 387

388

Lower grade sericite/chlorite fabrics overprint the biotite-pyrite shears. SC and SC’ 389

fabrics were developed during this retrogression. The distinct group of S to SW 390

dipping shear zones observed in the cores with sericite/chlorite and dominantly 391

dextral normal kinematics near the junction of boundaries of the structural domains 392

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(Fig. 12) have a NW shortening direction, and define the D5 kinematics (Fig 16d). 393

The distinctive Z folds near the Boston Shaker shear zone are consistent with D5. 394

395

Notwithstanding the general history given above, there are examples of contradictory 396

shear senses, some even within the mineralising biotite-pyrite shears. These testify to 397

repeated reactivation, commonly in opposite shear senses, which is a hallmark of the 398

deposit. Much of the reactivation is consistent with D5 overprinting D3 structures, but 399

there are anomalous shear zones that do not fit in with this history: they could 400

represent the influence of D4. However, the kinematics of D3 and D5 appear to be 401

dominant (Fig. 14). 402

403

N trending shears observed in some cores (Fig. 12) are parallel to a change in 404

structural grain observed on a large scale around Tropicana and in the Plumridge 405

terrane compared to other parts of the Northern Foreland, where aeromagnetic trends 406

are NW and more comparable with adjacent trends in the Yamarna Terrane. This 407

inflexion may have been important to mineralization by bringing lithological bands 408

into a more favourable orientation for shearing. 409

410

411

6.2 Comparison with Archean lode gold deposits 412

413

In terms of its general geological properties, the Tropicana deposit has some 414

similarities but also significant differences from many Archean lode gold 415

deposits of the Yilgarn craton. Tropicana lacks the metre scale quartz carbonate 416

veining that is such a characteristic feature of many Archean lode gold deposits. 417

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Likewise, the upper amphibolite-granulite grade of metamorphism for the host 418

rocks is exceptional, while the occurrence of mineralization at greenschist facies 419

at Tropicana (Doyle et al., 2009) is more typical of lode gold deposits (e.g. 420

Vearncombe, 1998). The concentration of mineralization in the feldspathic 421

gneiss is comparable to preferential mineralization in some host rocks within 422

lode gold deposits, although the feldspathic gneiss itself is quite dissimilar 423

geochemically to the basaltic or andesitic volcanic hosts of many lode gold 424

deposits. The diffuse nature of many ore bodies at Tropicana, which occur in 425

volumes of altered rock that do not have discrete structural boundaries, is also 426

atypical of lode gold deposits, in which ore bodies are commonly confined by 427

vein, fracture, fault or shear margins. 428

429

However, the structural control by shear zones at Tropicana is similar to some 430

lode gold deposits, as is the role of solution and brecciation (e.g. Witt and 431

Vanderhor, 1998). Shear zones at Tropicana exist in many orientations (Fig. 12): 432

similarly orientated shear zones can have quite different kinematics (Fig. 13). 433

This feature is also typical of some lode gold deposits (see below) in which, as at 434

Tropicana, it is probably due to overprinting events, several of which may be 435

associated with gold mineralization and remobilization. The main phase of 436

economic gold mineralization at Tropicana is Archean and postdates D1 and D2 437

deformation events: in this respect it is also similar to Archean lode gold deposits 438

of the Yilgarn craton, although the timing of mineralization at Tropicana does not 439

appear to correspond with the majority of late Archean deposits on the craton. 440

Nevertheless, Tropicana fits well into the suggested categorization of those 441

deposits as “late orogenic, structurally controlled” (Witt and Vanderhor, 1998). 442

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443

It is difficult to establish how much new mineralization as opposed to 444

remobilisation may have been associated with D4 and D5 at Tropicana. 445

Remobilisation of gold is evidenced by clusters of visible gold localised in late 446

muscovite fabrics that overprint earlier biotite-pyrite fracture fills and grains. 447

Several detailed studies of large lode gold deposits show that they have 448

experienced more than one mineralizing event (e.g. the Golden Mile; Vielreicher 449

et al., 2010) or that reactivation has been a significant part of the deposit history 450

(e.g. St Ives: Miller et al., 2010; Renco Mine, Kolb and Meyer, 2002; Sunrise Dam: 451

Baker et al., 2010). The evidence for reactivation at Tropicana is a point of 452

comparison with the Archean lode gold deposits. 453

454

The timing of D3 and the main mineralizing event (Tropicana event) is well 455

constrained to the late Archean by Ar-Ar dating of biotite and Re-Os dating of 456

pyrite (Doyle et al., 2013). From a regional perspective there are at least three 457

possibilities for the reactivation recorded by D4 and D5 at Tropicana. The 458

Zanthus event of the Biranup orogeny occurred at c. 1680 Ma, and the two stages 459

of the Albany–Fraser orogeny occurred at 1345–1260 Ma (Stage I) and 1215–460

1140 Ma (Stage II) (Kirkland et al., 2011). The Eastern Biranup zone, in contact 461

with the Northern Foreland to the E of Tropicana, has no geochronological 462

evidence for Stage I of the Albany–Fraser orogeny (Kirkland et al., 2011), which 463

is consistent with a lack of evidence for this event in zircon or monazite. 464

However, low grade deformation and fluid flow related to this event can not be 465

excluded. Thrust emplacement of the Eastern Biranup Zone over the Northern 466

Foreland is likely to have occurred in the later stage of the Albany–Fraser 467

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orogeny, which most probably correlates with the clearest evidence of 468

reactivation at Tropicana, in D5. 469

470

471

6.3 Comparison with Renco Deposit, Zimbabwe 472

473

Renco gold mine in Zimbabwe is in a granulite terrane (the Northern Marginal 474

Zone) 10 km from the contact between the granulites and the Zimbabwe Archean 475

craton. The mine is hosted by a late Archean enderbite intrusion (Blenkinsop et 476

al., 2004). The mineralization lies in shear zones that dip moderately away from 477

the craton, and in steeper linking shear zones (Kisters et al., 1998). 478

Mineralization probably occurred in upper amphibolite facies conditions at the 479

end of the Archean (Kolb et al., 2000), although evidence for a lower grade of 480

mineralization suggests that there was a second, possibly Paleoproterozoic event 481

(Frei et al., 1999; Blenkinsop et al., 2004). Reactivation is also evidenced by low 482

grade fabrics in the shear zones (Kolb et al., 2003). Within the shear zones that 483

host the mineralization, two domains are distinguished: quartz-feldspar-biotite-484

hornblende mylonites, which surround lithons of k-feldspar-quartz-biotite-485

garnet-sulphide. Fractures in lithons are filled by sulphides, and the majority of 486

the grade is concentrated in them (Kisters et al., 2000). 487

488

The similarity in the position of Renco and Tropicana as gold deposits in 489

granulite terranes relative to their adjacent cratons is striking, and this is 490

reinforced by the common geometry of the ore bodies in zones that dip 491

moderately away from the craton. A network of shear zones is the critical 492

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hydrogeological structure in both cases, and the fractured lithons of Renco have 493

their counterpart in the brecciated pegmatite bodies of the Tropicana deposit. 494

Both deposits also have evidence for lower metamorphic grade overprints on 495

granulite facies host rocks. 496

497

The timing of Renco mineralization is considered to be post-peak granulite facies 498

metamorphism in the late Archean, on a retrograde path but within 100°C of 499

peak metamorphism (Kolb and Meyer, 2002). The post-peak conditions and 500

timing are another similarity to Tropicana. The timing and geology of these 501

deposits suggest a possible link to granulite facies metamorphism in as much as 502

the deposits can be interpreted as forming in the retrograde parts of an orogenic 503

cycle that reached peak granulite facies. However, Kerrich (1988) has shown that 504

there is unlikely to be a direct geochemical link between granulite formation and 505

lode gold deposits: additional fluid input is probably required to explain 506

characteristic Large Ion Lithophile (LILE) enrichment of alteration associated 507

with mineralization compared to the depleted LILE characteristics of Archean 508

granulites. 509

510

511

7. Conclusions 512

513

Tropicana gold deposit in the Albany Fraser orogen on the margin of the Yilgarn 514

craton was formed by fluid flow through a network of biotite-pyrite-bearing 515

shear zones, initially in an event of NE shortening (D3). Permeability was created 516

in coarser, more competent pegmatitic layers by fracturing, and was 517

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accompanied by solution transfer along solution seams. Shear zones were 518

reactivated to provide a record of complex kinematics, including two retrograde 519

events (D4 and D5). D5 involved dextral shear on S-SW dipping surfaces. 520

521

The most important geometrical control on mineralization at Tropicana is the 522

permeability created by the biotite-pyrite-bearing shear zones. The orientation 523

and location of these in turn were largely dictated by the orientation of the 524

gneissic banding in the favourable horizon of host rocks, which reflects two 525

previous deformation events and gentle folding in D3. High grade ore shoots 526

formed parallel to the common intersection direction of the shear zones, which 527

was also parallel to F3 fold hinges. Variations in the direction of the ore shoots 528

could reflect variations in the initial geometry of the gneisses, and/or later 529

deformation. A consideration of the entire geological history is necessary to 530

understand the deposit geometry. 531

532

The style of mineralization at Tropicana is different from many Archean lode 533

gold deposits of the Yilgarn craton in as much as no metre scale 534

quartz/carbonate veins, fractures or faults seem to have played an important 535

role in mineralization. Nevertheless, the Tropicana deposit has a strong 536

structural control, in common with the Archean lode gold deposits, because all 537

these deposits were formed by hydrothermal fluid flow along structural 538

permeability. Mineralization at Tropicana occurred at greenschist facies in 539

granulite facies host rocks, clearly post-dating peak metamorphism. 540

541

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Renco mine in Zimbabwe and Tropicana gold deposit have several 542

characteristics in common, including their formation within granulites at the 543

margins of well-mineralized Archean cratons, probably within the late Archean. 544

It is possible that both could have been formed by fluid flow driven by heat 545

sources reflecting the waning of granulite facies metamorphism in orogenic belts 546

on the periphery of their respective Archean cratons. 547

548

549

Acknowledgements 550

Independence Group NL and AngloGold Ashanti Ltd are thanked for permission 551

to publish the manuscript. The authors acknowledge the significant contribution 552

of past and present AngloGold Ashanti geologists and the exploration team who 553

have contributed to the discovery, delineation and understanding of the mineral 554

deposit. In particular, we are grateful for the assistance of Jeni Savage and Jovo 555

Przulj in completing the study. 556

557

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64, 711

712

713

Figures 714

715

Fig. 1. Location of Tropicana gold deposit in the Northern Foreland of the 716

Albany–Fraser orogen (partly based on Spaggiari et al., 2011, Pawley et al., 2012). 717

718

Fig 2. Simplified interpretive geological map of the Tropicana region. The Black 719

Dragon thrust separates the Archean Tropicana Gneiss in the Plumridge terrane 720

from lower metamorphic grade rocks of the Paleoproterozoic Biranup zone. 721

Other NNE to NE trending structures separate further subdivisions of the 722

Albany–Fraser orogen. 723

724

Fig. 3. Structural domains and mesoscopic shear zones of the Tropicana 725

Deposit, superimposed on a grade (g/t) X thickness (m) plot. GDA/UTM grid. 726

727

Fig. 4. Schematic EW cross section of the Tropicana Deposit (based on Doyle et al., 728

2007). 729

730

Fig. 5. NE-SW section (true scale). Blue shapes are delimited by > 3g/t. “Principal 731

lineation” refers to the high grade ore shoots seen in the gm plot of Fig. 3. High 732

grade ore bodies pinch and swell. 733

734

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Fig. 6. Stereoplots of poles to high grade (>3 g/t) ore shells. A distinct difference 735

is noted between the Tropicana and Havana domains. Red squares are 736

eigenvectors of the distribution of poles. The minimum eigenvector is labelled 737

with its trend and plunge; a great circle connects the minimum and intermediate 738

eigenvectors. All stereoplots are lower hemisphere, equal area. 739

740

Fig. 7. Styles of Mineralization in the core. 741

a) Biotite-pyrite shear zone sub-parallel to gneissic banding, core TPD366, 742

160 m. 743

b) Biotite-pyrite shear zone (thin section). Foliation defined by biotite and 744

pyrite gives clear top-to-the left sense of shear. Core TPD361A, 161 m, XPL 745

(Cross-polarised light). 746

c) Chlorite-sericite shear zone, core TPD167, 163.2 m. Asymmetric clast 747

gives clear shear sense. 748

d) Sericite shear zone (thin section). Spectacular SC fabrics defined by 749

sericite and pyrite give clear top-to-the-right shear sense. Core TPD067, 143.3 m, 750

XPL. 751

752

Fig. 8. Styles of mineralization in the core. 753

a) Solution seams with biotite anastomosing between feldspar-rich lithons 754

with carbonate-filled extension fractures. TPD202, 281.9 m 755

b) Fragmentation and solution accompanying formation of carbonate-756

sericite-pyrite veins. TPD202B, 282.9 m, XPL. 757

c) More discrete formation of biotite lined solution seams and irregularly-758

shaped patches of carbonate. TPD 202, 285.8 m 759

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d) Extension microfracture in perthite grain, with filling of pyrite and 760

carbonate. TPD202C, 283.9 m, XPL 761

762

Fig. 9. Structural features in core. 763

a) F1 folds in gneissic banding. Yellow lines marks form surface, yellow spots are 764

hinges and red line is hinge surface trace. TPD251, 222.7 m. 765

b) Tight F1 fold hinge of gneissic banding, TPD202, 314.1 m. 766

c) Asymmetric F5 fold of gneissic banding and shear foliation. TPRC092D, 298.7 m. 767

d) SC’ defined by chlorite and sericite. Marking according to the scheme of 768

Blenkinsop and Doyle (2010). TPD262, 162m. 769

770

Fig. 10. Orientations of fold hinges and hinge surfaces, located relative to 771

drillholes and structural domains. Folds in cores MBRC019D, TPRC607D, 772

TFD137, TFRC501D, TPD202, TPD261 and TPD366 plunge moderately S to SE 773

with E to SE dipping hinge surfaces. These are interpreted as F1 or F2 folds, 774

because they are tight to isoclinal. Folds in cores TFRC090D and TFRC092D 775

plunge moderately E to SE with S-dipping hinges surfaces. Many of these have 776

dextral vergence (turquoise colour). These folds are located adjacent to the 777

Boston Shaker shear zone and are F5 folds. Alternatively those folds which are 778

symmetric could be F3. 779

780

Fig. 11. Poles to gneissic banding (n = 1510) in the Havana domain. Cylindrical 781

best fit gives a fold hinge plunging 35° to 115°. Kamb contours with a contour 782

interval of 6σ, 3σ significance level. 783

784

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Fig. 12. Geographic distribution of shear zones in ball-and-string plots. Great 785

circles indicate shear planes: arrows indicate hangingwall movement. In most 786

plots there are sub-parallel shears with different movement directions, testifying 787

to reactivation. 788

789

Fig. 13. Tangent lineation plots of shear zones separated by structural domain 790

and by phyllosilicate mineralogy. Arrows indicate footwall movement, plotted at 791

the pole to the fault. Dextral components in green and sinistral in red. 792

793

Fig. 14. Kinematic analysis of shear zones separated by structural domain and by 794

phyllosilicate mineralogy. Red and blue dots are shortening and lengthening axes 795

for individual shear zones respectively; 1, 2, and 3 are the linked Bingham axes 796

for the distributions shown. 797

798

Fig. 15. Poles to shear zone orientations in four structural domains, with 799

eigenvectors to the distributions shown as black squares. Great circle links 800

maximum and intermediate eigenvectors. Kamb contours, Contour interval 2σ, 801

significance level 3σ. 802

803

Fig. 16. Cartoon of the structural evolution of the Tropicana Gold Deposit 804

a) D1 is preserved as asymmetric folds and gneissic banding 805

b) D2: thrusting (orange surfaces) and folding is inferred from regional 806

considerations 807

c) D3: the main mineralizing event, due to NE shortening 808

d) D5: The main reactivation 809

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D1 D2 D3 D4 D5

Macro structure

Thrusts Folds

SE plunging folds

Shear zones

Meso-micro structures

Gneissic Banding Isoclinal folds

Shear zones Solution fabrics Breccias

Shear zones Shear zones Asymmetric folds

Kinematics ? NW shortening

NE Shortening ? Dextral E-NE shearing

Au: Biotite

High Pyrite

Sericite

Low Carbonate

Chlorite

Timing 2640-2524 2640-2524 2524-2515 ? ? 1215-1140

Table1. Deformation and mineralization history at Tropicana Gold Mine

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Eastern Goldfields Superterrane

KurnalpiTerrane

KalgoorlieTerrane

YouanmiTerrane

SouthWest

Terrane

EuclaBasement

Heywood-Cheyne Fault Zone

Central B

iranup Z

one

Jerd

acuttu

p Fault

YILGARN CRATON

BurtvilleTerrane

Fras

er Z

one

Eas

tern

Bira

nup

Zone

Gun

barrel

Fau

lt

Shear Zone

Yam

a rar

Plu

mbr

idge

Ter

rane

North AustralianCraton

SouthAustralian

Craton

Tasm

an L

ine

WestAustralian

Craton

PO

MP

AFO

1000 km

Paleoproterozoic

Nornalup Zone

Fraser Range Metamorphics

Gwynne Creek Gneiss

Biranup Zone

Metasedimentary rocks

Tropicana Gneiss

Munglinup Geniss

Archean

Major faults

Terrane boundary

Geological boundary

Coastline

100 km

34°

32°

30°

120° 123°

Kalgoorlie

Esperance

Norseman

Kanowna BelleMine

Albany-Fraser bedrock interp.cdr

34°

32°

120°

123°

30°

Plu

mrid

ge T

erra

ne

Area ofFig 2

Voodoo Child

T aropican

Undivided

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10 km

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Footwall

Favourable Horizon

Hangingwall

Pebblysandstone

Basalt dyke

Garnet gneiss

AmphiboliteQuartzo-feldspathicgneiss

Meta FerruginouschertSaprolite

Ore zone

W ETransported

BOCO

RL

300

200

100

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Continuity at > 0.1 g/t

Principallineation

pinch

swell

See Enlg. above

HAVANANORTH

TROPICANA

500m

250m

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29°/087°

22° /163°

Tropicana Havana

Fig. 6. Orientations of poles to high grade (>3 g/t) ore shells. A distinct difference is noted between the Tropicana and Havana domains.

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d)c)

25 mm

25 mm

1 mm

1 mm

Fig. 7. Styles of Mineralization in the core.a) Biotite-pyrite shear zone sub-parallel to gneissic banding, core TPD366, 160 m.b) Biotite-pyrite shear zone (thin section). Foliation defined by biotite and pyrite gives clear top-to-the left sense of shear. Core TPD361A, 161 m, XPL (cross-polarised light).c) Chlorite-sericite shear zone, core TPD167, 163.2 m. Asymmetric clast gives clear shear sensed) Sericite shear zone (thin section). Spectacular SC fabrics defined by sericite and pyrite give clear top-to-the-right shear sense. TPD067, 143.3 m, XPL.

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Fig. 8. Styles of mineralization in the core.a) Solution seams with biotite anastomosing between lithons of feldspar with carbonate-filled extension fractures. TPD202, 281.9 mb) Fragmentation and solution accompanying formation of carbonate-sericite-pyrite veins. TOD202B, 282.9 m, XPL.c) More discrete formation of biotite lined solution seams and amorphous patches of carbonate. . TPD 202, 285.8 md) Extension microfracture in perthite grain, with filling of pyrite and carbonate. TPD202C, 283.9 m, XPL

a)

b)

d)c)

25 mm

25 mm

1 mm

1 mm

a)

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c)

25 mm

25 mm

50 mm

15 mm

d)

a)

Fig. 9. Structural Features in core.a) F1 Folds in gneissic banding. Yellow lines marks form surface, yellow spots are hinges and red line is hinge surface. TPD251, 222.7 mb) Isoclinal F1 fold hinge of gneissic banding, TPD202, 314.1 mc) Asymmetric F5 fold of gneissic banding and shear foliation. TPRC092D, 298.7 md) SC’ defined by chlorite and sericite. Marking according to the scheme of Blenkinsop and Doyle (2010). TPD262, 162m

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Lower Hemisphere

N = 6

TPRC607D, TFD137

Equal Area

Lower Hemisphere

N = 52

MBRC019D

TPRC090D, TPRC092D

Equal Area

Lower Hemisphere

N = 29

Equal Area

Lower Hemisphere

N = 20

TFRC501D, TPD202,

TPD261, TPD366

Tropicana

Havana South

HavanaNorth

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Fig. 11. Poles to gneissic banding (n = 1510) in the Havana domain. Cylindrical best fit gives a fold hinge plunging 35° to 115°. Kamb contours with a contour interval of 6σ , 3σ significance level.

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TPRC090D

TPD067, 068

TPD261, 262

TPD302

TPD345

TPD365, 366

TPRC607D

TPD360ATPD098, 157TPD103, 108, 109

TFD137

TFD167

TFD134, 135

TFD115

TROPICANA

HAVANA

HAVANASOUTH

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Tropicana

HavanaNorth

Biotite+

HavanaSouth

Biotite

Fig. 13. Tangent lineation plots of shear zones separated by structural domain and by phyllosilicate mineralogy. Arrows indicate footwall movement. Dextral components in green and sinistral in red.

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1

2

3

Tropicana

HavanaNorth

Biotite+ Sericite/Chlorite

HavanaSouth

Biotite

1

2

3

2

13

1

2

3

1

2

3

1

2

3

1

2

3

Fig. 14. Kinematic analysis of shear zones separated by structural domain and by phyllosilicate mineralogy. Red and blue dots are shortening and lengthening axes for individual shear zones respectively; 1, 2, and 3 are the linked Bingham axes for the distributions shown

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Havana South Crouching Tiger

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N

FOLDHINGE

F1

HINGESURFACE

HSI

a) D1

c) D3

F3FOLD HINGE

d) D4, D5

F5FOLD HINGE

REACTIVATIONOF THRUSTS AS NORMAL FAULTS

HAVANA

TROPICANA

b) D2

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