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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
*Corresponding Author 17
18
2 AngloGold Ashanti Ltd 19
Level 13, St Martins Tower 20
PO Box Z5046, Perth WA 6831 21
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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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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