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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 on
the 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
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margin of the Yilgarn craton, Albany–Fraser
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orogen: The Tropicana Deposit, Western Australia
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School of Earth and Environmental Science,
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James Cook University,
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Townsville QLD 4811,
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Australia
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School of Earth and Ocean Sciences,
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Main Building, Park Place,
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Cardiff CF10 3AT,
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United Kingdom
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[email protected]
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*Corresponding Author
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Present Address:
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Economic Geology Research Unit
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T. G. Blenkinsop1,2* and M. G. Doyle2
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Level 13, St Martins Tower
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PO Box Z5046, Perth WA 6831
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[email protected]
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Key words. Shear zone, Archean, Lode gold, orogen, Albany-Fraser, Tropicana
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AngloGold Ashanti Ltd
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Abstract
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The Tropicana gold deposit is located adjacent to the margin of the Yilgarn craton in
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the Albany–Fraser orogen, Western Australia. The deposit is hosted in granulite facies
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quartzo-feldspathic gneisses of the Archean Tropicana Gneiss. Ore bodies comprise
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biotite-pyrite alteration concentrated in shear zones that formed during NE-SW
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shortening in the late Archean, and clearly postdate the formation and deformation of
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high-grade gneiss fabrics (D1 and D2). The orientation of the ore bodies is controlled
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by the shear zones that are in turn localised by the gneissic banding. Mineralization
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also involved solution and coeval microfracturing and veining of more competent
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pegmatitic units. The mineralizing event (D3) was followed by at least two further
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deformations, which reactivated and overprinted the biotite fabrics with sericite and
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chlorite, created new shear zones, and affected gold distribution. D5 consisted of
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dextral shear on ~E-W shear zones, which subdivide the deposit into five major
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structural domains. The importance of structurally controlled permeability at
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Tropicana is similar in cratonic lode gold deposits, as is the protracted
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deformation/fluid flow history. Like Renco mine in Zimbabwe, Tropicana gold
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deposit was formed by hydrothermal fluid flow peripheral to the craton: economic
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gold mineralization was clearly post-peak metamorphism.
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1. Introduction
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Many Archean lode gold deposits have distinctive geological characteristics (e.g.
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Robert and Brown, 1986; Groves et al., 1998; 2000; Wit and Vanderhor, 1998;
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Goldfarb et al., 2001) including:
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1) Discrete, high grade lodes, commonly with abundant quartz and
carbonate veining;
2) Greenschist-amphibolite facies peak metamorphism of the host rocks,
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which slightly predates alteration and mineralization at similar or lower
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grade metamorphic conditions;
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3) A variety of supracrustal host rocks, although Fe-rich and competent
lithologies make especially favourable sites for mineralization;
4) Little mineralization in plutonic rocks;
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5) A spatial association with felsic intrusions.
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In addition to these general geological characteristics, the ore bodies all have in
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common strong structural controls, which testify to the essential roles of
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permeability and fluid flow in creating these hydrothermal ore bodies (e.g. Cox,
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1999). The controls can be crudely classified in terms of the hosting structure as
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breccias, faults and shear zones (e.g. Hodgson, 1989). In well-documented cases
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there is evidence of reactivation of structures and multiple cycles of deformation
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and fluid flow (e.g. Poulsen and Robert, 1989; Baker et al., 2010; Davis et al.,
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2010; Miller et al., 2010; Dirks et al., 2013). Increasingly these patterns are
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interpreted in terms of stress and fluid pressure fluctuations associated with the
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earthquake cycles (Sibson et al., 1987, 1988; Robert et al., 1995; Cox and Ruming,
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2004; Micklethwaite and Cox, 2004; 2006).
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At a scale greater than individual deposits, it is well recognised that Archean lode
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gold deposits are not found directly on craton-scale shear zones, but instead lie
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in adjacent lower order structures (e.g. Kerrich, 1989; Vearncombe, 1998),
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although a role for the first order features can be inferred from the distribution
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of mining camps along them (e.g. Weinberg et al., 2004; Blewett et al., 2010a,b).
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At a global scale, the occurrence of gold provinces that contain giant or several
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world class gold deposits has been explained as the consequence of their
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formation in orogenies involving thin lithosphere or subducted oceanic crust
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(Bierlien et al., 2001; 2006) because of the greater likelihood of high
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asthenospheric heat input.
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This study describes the structural controls on Australia’s largest new gold
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discovery, the world class Tropicana deposit in Western Australia. The Tropicana
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deposit is located adjacent to the edge of the Archean Yilgarn craton in the
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Albany–Fraser orogen (Fig. 1), naturally leading to comparisons with the
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Archean lode gold deposits of the Yilgarn craton, and posing the question of
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whether it has formed in a similar way. The aims of this paper are to describe the
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structural controls on mineralization at Tropicana, to make a comparison with
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the classic deposits of the Yilgarn craton, and to highlight some remarkable
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comparisons between the deposit and the Renco gold mine in Zimbabwe. These
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comparisons cast light on the genesis of the Tropicana deposit.
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2. Geology of the Tropicana Deposit
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2.1 Regional Setting
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The Tropicana deposit is situated 41 km to the E of the easternmost magnetic
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expression of the Archean Yilgarn craton, in the Northern Foreland of the
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Albany–Fraser orogen (Spaggiari et al., 2011). The proximal part of the Yilgarn
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craton is the Yamarna Terrane of the Eastern Goldfields Superterrane (Pawley et
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al., 2012) (Fig. 1). On a regional scale, the boundary between the Northern
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Foreland and the craton has been interpreted as a major regional structure, the
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Cundeelee fault, which may have originated as a thrust (Spaggiari et al. 2011).
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Immediately to the W of the Northern Foreland, a thick sequence of Permo-
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Carboniferous sedimentary rocks overlies the craton, and is separated from the
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Northern Foreland around Tropicana by the Gunbarrel fault (Fig. 2), a steeply
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NW dipping normal fault which cuts the Cundeelee fault. There is no obvious
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continuity between the NNW trending structures on the Yilgarn craton in the
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Yamarna terrane (including the Yamarna shear zone) and structures in the
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Tropicana area of the Northern Foreland (Fig. 2) (e.g. Jones et al., 2006).
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The Albany–Fraser orogen mantles the southern and western margins of the
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Yilgarn craton over a distance of more than 1000 km. Mesoproterozic orogenic
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events have been recognised at 1350 – 1260 Ma and 1215-1140 Ma (Clark et al.,
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2000), but more recently it has become clear that Paleoproterozoic events
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including deposition of metasedimentary rocks and intrusion of granitic to
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gabbroic intrusions, constitute a major part of the eastern Albany–Fraser
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orogeny in the Biranup zone (Kirkland et al., 2011). High grade deformation
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occurred here at 1680 Ma, called the Zanthus Event within the Biranup orogeny,
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and this geological history has been interpreted as representing the evolution of
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an arc-backarc on the margin of the Yilgarn craton (Kirkland et al., 2011).
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The Northern Foreland is defined as the reworked part of the Yilgarn craton
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within the Albany–Fraser orogen (Myers, 1990). The intensity and grade of
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reworking varies in the Northern Foreland from amphibolite-granulite facies in
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the S to greenschist-amphibolite facies in the N (Spaggiari et al., 2011). Around
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and approximately 200 km to the SW of Tropicana, the Northern Foreland
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consists of a fault-bound assemblage of rocks with a common and distinct
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geological history that we define as the Plumridge terrane. The Plumridge
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terrane is approximately 27 km wide at Tropicana: to the E lies the Biranup Zone,
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consisting of intensely deformed gneiss and metagabbro with Paleoproterozoic
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ages (Bunting et al., 1976; Spaggiari et al., 2011). The contact between the two
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zones has a curved NE trending shape in map view, which is overall convex to
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the NW: it is interpreted as a thrust, herein referred to as the Black Dragon
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Thrust (Fig. 2). The Black Dragon Thrust juxtaposes ca. 1820 Ma metagranite and
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amphibolite rocks in the Black Dragon Domain of the Biranup Zone above
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Archean gneissic rocks hosting the Tropicana deposit, herein referred to as the
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Tropicana Gneiss (Fig. 2). Deformation in the Biranup zone is associated with the
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Biranup orogeny (1710 – 1650 Ma), but there was also activity along the Yilgarn
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margin at 1800 Ma, as indicated by the deposition of sedimentary rocks and
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intrusion of granites of this age (Spaggiari et al., 2011).
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2.2 Host rocks
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Neither the host rocks nor the ore body are exposed at Tropicana, being covered
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by up to 15 m of Cretaceous to Recent sediments. All the data in this study are
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based on the diamond drilling carried out to delineate the mineral resource. Core
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was examined from 36 drillholes (Supplementary Material gives drillhole
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locations), but this did not include any drillholes into the Boston Shaker or the
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Havana Deeps domains.
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The host rocks at Tropicana are gneisses dominated by garnet gneiss
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(plagioclase, amphibole, garnet ± leucoxene, quartz) and quartzofeldspathic
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gneiss (plagioclase, k-feldspar, quartz, biotite), with lesser amounts of
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amphibolite, meta-ferruginous chert (quartz, grunerite), pegmatite and mafic
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granulite. The pegmatites appear to be products of in situ partial melting at peak
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metamorphism, which was at upper amphibolite to lower granulite facies (Doyle
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et al., 2007; 2009). Compositional banding in the gneisses dips moderately to the
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E to SE (Fig. 4). The hangingwall of the deposit is dominated by the garnet gneiss.
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The gneisses and the ore bodies are cut by mafic dykes ascribed to the c. 1210
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Ma Gnowangerup-Fraser Dyke Suite (Doyle et al., 2007), which are prominent
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regional aeromagnetic features trending NE (Fig. 2).
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2.3 Geochronology
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The age of the host rocks regionally was inferred to be Archean (Bunting et al.,
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1976). This possibility has been strengthened by unpublished propriety
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geochronological data (Doyle et al., 2009) and preliminary U-Pb zircon ages of
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2722± 15 Ma and 2643 ± 7 ma for a metagranite sample taken 7 km N of
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Tropicana, which have been interpreted as ages of crystallization and
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metamorphism respectively (Spaggiari et al., 2011).
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The retrograde path from peak granulite facies metamorphism is constrained by
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a rutile U-Pb date of 2524 ± 8 Ma, interpreted to reflect cooling through 500-
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550°C (Doyle et al., 2013). A minimum age of 2515 ± 8 Ma for mineralization is
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suggested by biotite Ar-Ar analyses, which is consistent with late Archean Re-Os
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analyses of pyrite (Doyle et al., 2013). Discordance in zircons and monazites can
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be interpreted in terms of Pb loss in Stage II of the Albany–Fraser Orogeny
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(1215-1140 Ma: Kirkland et al., 2011).
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3. Ore Geometry and Style of Mineralization
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The resource at Tropicana occurs along a 5 km strike length trending overall NE,
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which can be divided into five structural domains from N to S: Boston Shaker,
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Tropicana, Havana, Havana Deeps and Havana South (Fig. 3). Low grade
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mineralization is also recorded to the S of these main areas, for example at
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Crouching Tiger prospect, and at other prospects regionally (Fig. 2). The five
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domains have en echelon arrangement. Within each domain the general
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mineralization envelope trends N to NE. The domains are separated by E to SE
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trending shear zones, such as the Boston Shaker shear zone between the Boston
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Shaker and Tropicana domains, and the Swizzler, Cobbler and Don Lino shear
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zones (Fig. 3). Most of these shear zones dip S. Map scale shears with a similar
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orientation also occur within the resource areas, as well in a NE direction (Fig. 3).
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Mineralization is concentrated in one to several sub-parallel tabular ore zones 2
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– 50 m thick which generally dip to the E to SE, within quartzofeldspathic gneiss
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(Fig. 4). Within these ore zones there are higher grade lenses. When viewed in
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section parallel to strike, ore zones show an inosculating pattern, separating
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lenses of unmineralised rock, and thickening and thinning (Fig. 5). The map view
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of the gold assay data x thickness (gram-metres) shows high grade ore shoots
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with slightly variable orientations between the domains. In Boston Shaker, the
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trend is SE, in the northern part of Tropicana, ESE; in Havana and Havana Deeps,
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SSE (Fig. 3).
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Similar distinctions in orientations between the domains are seen in three-
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dimensional data by examining the orientations of modelled high grade lenses
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(≥3 g/t) (Fig. 6). Tropicana is characterized by E to SE dipping ore bodies,
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generally coaxial about an E-plunging line (29° → 087°), whereas the ore bodies
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in Havana North dip between S and E, and have a common axis plunging to the
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SSE (22° → 163°).
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Gold grades in the ore zones at Tropicana are dominantly associated with
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intervals of biotite-pyrite alteration that occur within quartzofeldspathic gneiss
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with pegmatites. Biotite with pyrite and gold replaces metamorphic biotite and
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amphibole, most commonly in millimetre wide shear zones defined by strong
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fabrics consisting of elongate biotite and pyrite grains (Fig. 7a,b), but also in
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disseminated volumes. Higher gold grades are also associated with areas of
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brecciation in pegmatites around shears, with shears containing biotite-sericite
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and minor chlorite (Fig. 7c, d), and in areas with solution fabrics (see below; Fig.
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8). Known occurrences of visible gold correspond with intercepts of >30 g/t in
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1m composite assays. Visible gold is paragenetically late and typically localized
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on muscovite fractures which cut across anatectic segregations, quartz veins and
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gneissic bands and biotite-pyrite fracture fills.
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4. Deformation History, Meso- and Microstructures
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Table 1 gives a deformation history that can be inferred from drill core,
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geophysics and deposit scale geometry. This section focuses on the detailed
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evidence from the core pertaining to events which may be associated with gold
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mineralization: the preceding history is outlined because it affects the deposit
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geometry.
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4.1 Gneissic Banding S1, F1 folds
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The most commonly observed mesoscale structure in the cores is a gneissic
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banding defined by variations of up to 20% in the proportions of quartz,
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feldspars, biotite, amphibole and garnet on a mm to cm scale (Fig. 9a,b). The
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gneissic banding is tight to isoclinally folded (Fig. 9a,b) with E to SE dipping
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hinge surfaces and gently S plunging hinges (Fig. 10). Some of these folds are
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rootless (Fig. 9b), suggesting that the gneissic banding is the product of early
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deformation and high grade metamorphism, as indicated by leucosomes that are
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generally parallel to the banding.
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4.2 D2
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A fold on the scale of hundreds of m is suggested by W-dipping gneissic banding
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in cores to the W of the deposit. The drill core data imply an asymmetric synform
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in the footwall of the mineralization. Based on evidence from the nearest outcrop
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at Hat Trick Hill (Fox-Wallace, 2010) and regional considerations (Spaggiari et al.,
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2011), this W verging fold is likely associated with a W to NW verging thrust
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system that is developed regionally. It is possible that some of the S plunging
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folds shown in Fig. 10 are F2 folds.
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4.2 D3: Shear Zones
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Quite distinct from the gneissic banding are localised zones of strong foliation
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defined by biotite and pyrite, chlorite or sericite (Fig. 7b). Such shear zones are
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typically mm to cm wide, and clearly cut across the gneissic banding in places,
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although they are generally parallel to the banding. Asymmetric fabrics
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indicating shear are common in such zones, and include SC and SC’ fabrics, sigma
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porphyroclasts and oblique foliations (Fig. 9d). Lineations are very difficult to
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observe because the foliation surfaces are not generally visible in the core. The
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shear zones are commonly surrounded by zones of brecciation.
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Shear zones containing biotite – pyrite only are distinct from those that may also
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include chlorite or sericite: these minerals appear to overprint the biotite, so that
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the shear zones containing biotite-pyrite are regarded as a third deformation
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(D3), after the formation and folding of the gneissic fabric, but predating later
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overprinting by other phyllosilicates.
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4.3 D3: Solution Fabrics and Breccias
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Zones of intense solution fabrics are defined by wavy seams containing biotite and
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pyrite 1 – 2 mm wide between fractured quartz and feldspar layers 5 – 10 mm thick
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(Fig. 8). In places the fracturing is dense enough to be described as a breccia (Fig. 8b).
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The quartz and feldspar are fractured by mm long veins filled with calcite that form
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distinctive irregular shapes perpendicular to the stylolites (Fig. 8a). The calcite veins
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appear to be extensional and in places are markedly oblique to the solution seams.
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They are associated with ostensibly the same auriferous pyrite as the solution seams,
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since that pyrite can be seen as a component of the fracture fill in the carbonate veins
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(Fig. 8d), and biotite alteration in the seams extends into the fractures. The presence
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of biotite and pyrite suggests that this fabric may have formed during D3, although
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there is also a strong association with sericite in places. Significant gold grades were
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recovered from a stylolitic interval in core from drill hole TP202.
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4.4 D3 Folds
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Gentle folds of the lithological layering visible in the mine model plunge
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moderately SE and occur on the scale of whole domains. Folding with a similar
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orientation can be inferred from the distribution of poles to gneissic banding (Fig.
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11), and from some measurements of individual folds in core (Fig. 10). This
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folding postdates D1 and D2, and is ascribed to D3.
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4.3 D4, D5 Shear Zones and Folds
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Some biotite-pyrite shear zones are overprinted by fabrics defined by chlorite
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and sericite, which have distinct kinematics. Other shear zones contain sericite
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and chlorite only. A distinctive set of shear zones with biotite and
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sericite/chlorite dip S and SW and have dextral kinematics. Some of these later
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fabrics are folded into characteristically asymmetric folds on a 1 - 10 cm scale
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(Fig. 9c). These folds have been measured at the boundary between the
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Tropicana and Havana domains, near the Boston Shaker shear zone. Fold hinge
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surfaces dip S, with generally E to SE plunging hinges (Fig. 10) and Z
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asymmetries. The folds and the S dipping dextral shears are consistent with a
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late deformation event comprising dextral shear on S and SW dipping zones.
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Since they fold a sericite-chlorite fabric, this event (D5) probably postdates an
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intermediate event represented by sericite-chlorite shear zones in various
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orientations (D4).
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5. Kinematic Analysis
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Shear zones were measured from cores into the Tropicana, Havana North and South
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domains (Fig. 12). Kinematic analysis of shear zones was possible from SC and SC’
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fabrics and sigma clasts which could be used to identify the vorticity vector and hence
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the shear direction as perpendicular to the vector. A kinematic analysis was performed
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using linked Bingham axes (cf. Marrett and Allmendinger, 1990) and filtering the
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results by the phyllosilicate mineralogy of the shear zones into biotite, biotite with
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sericite and/or chlorite, and sericite/chlorite groups (Figs. 12, 13). In all cases the
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linked Bingham axes from the kinematic analysis give one of two shortening
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directions: NE or NW (Fig. 13).
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The biotite shear zones dip to the SW, S, SE and NE (Fig. 12). Kinematics vary
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consistently with orientation: SW, S and SE dipping shears are sinistral, while E and
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NE dipping shears are dextral (Fig. 12). The displacement pattern is kinematically
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coherent, and consistent with NE horizontal shortening, which is also reflected by the
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linked Bingham axes (Fig. 13). This orientation defines D4 kinematics.
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The other shear zones have similar orientations to the biotite shears, but in all
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orientations there is a variety of shear directions and senses (Figs. 12, 13), commonly
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with contradictory shear senses on adjacent and sub-parallel shears. A particularly
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common set of shears dips S to SW with mostly dextral normal kinematics: these are
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common at the major breaks between the Tropicana and Havana North domains and
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between the Havana North and South domains, (e.g. holes TFRC090D and TFD167)
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(Figs. 11, 12), and they define the D5 kinematics with a NW shortening direction. The
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same shortening direction is apparent from shears that have sericite/chlorite and no
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biotite, which can be associated with D5 (Table 1).
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Shear zones with biotite and sericite/chlorite show some overlap between shortening
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and extensional quadrants, and the Bingham axes reflect either NE or NW shortening
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(Fig. 14). This is consistent with the possibility that some of these shear zones have
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been reactivated in D5 kinematics, while others preserve D3 shortening directions.
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6. Discussion
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6.1 Deformation History and Structural Controls on Ore bodies at Tropicana
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The structural/mineralization history at Tropicana is summarised in the cartoons
of Fig. 16. D1 included the formation of high-grade gneissic banding, partial melting,
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and isoclinal folding (Fig. 16a). The kinematics of D1 could not be constrained by
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observations in this study, and the event as defined here might include additional
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complexities. Regional considerations suggest that D2 was a major event of W to NW
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directed thrusting that probably created some of the major structures in the area such
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as the Iceberg thrust (Fig. 16b). Tight to isoclinal folds in gneissic banding plunging S
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to SE observed in the core cannot definitively be ascribed to D1 or D2, and it is
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possible that they represent a progressive deformation event.
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Gold mineralization at Tropicana is controlled by a system of biotite-pyrite shear
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zones within a favourable lithological band of feldspathic gneiss that has a sheet dip
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to the E to SE. The biotite shear zones are generally sub-parallel to gneissic banding,
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but clearly postdate it, and are surrounded by diffuse bodies of mineralised breccia.
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The main mineralization phase was associated with the biotite alteration, and the
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shear zones formed with a NE shortening direction (Fig. 14).
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The biotite-pyrite shear zones measured in the core have an approximate girdle
363
distribution around a SE-trending axis (Fig. 15). Although the shears have a variety of
364
orientations, they are not folded on the scale of the core. The SE trend is similar to
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the direction of the high grade ore shoots visible on the map (Fig. 3) and to the
366
common intersection of high-grade ore shells in Havana (Fig. 6). Gentle folding of the
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gneissic banding in this orientation is also apparent on a large scale (Fig. 11): these
368
are ascribed to an F3 generation of folds.
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The SE trend observed in the gram-metre plot, the high grade ore shells, the girdle
371
distribution of the shears and the orientations of gneissic banding, is a very significant
372
control on mineralization, which is consistent with fluid flow along the biotite-pyrite
373
shears. The observation that the biotite-pyrite shears are not folded and the similarity
374
of Figs. 11 and 15 suggest that their orientation was largely controlled by the gneissic
375
banding, to which they are generally parallel. The orientation of the gneissic banding
376
reflects D1 and D2, which imparted the moderate E to SE dip to the banding, and a
377
component of gentle folding superimposed in D3. The trends of the high grade shoots
378
are therefore parallel to common intersections of the biotite-pyrite shear zones and
379
hinges of F3 folds (Fig. 16c).
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There are significant variations in these trends between Tropicana and the other
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domains. In Tropicana, high grade ore shells dip more easterly than in Havana (Fig. 6),
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giving an easterly trend to their intersection, which is also apparent in the gram-metre
384
plot (Fig. 3). At least two possibilities to explain this variation are: i) an initial
385
variation in geometry inherited from D1 and D2; or ii) Reorientation by D4 or D5 in
386
Tropicana, which is distinguished from the other domains by a higher density of late
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shear zones.
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Lower grade sericite/chlorite fabrics overprint the biotite-pyrite shears. SC and SC’
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fabrics were developed during this retrogression. The distinct group of S to SW
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dipping shear zones observed in the cores with sericite/chlorite and dominantly
392
dextral normal kinematics near the junction of boundaries of the structural domains
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393
(Fig. 12) have a NW shortening direction, and define the D5 kinematics (Fig 16d).
394
The distinctive Z folds near the Boston Shaker shear zone are consistent with D5.
395
Notwithstanding the general history given above, there are examples of contradictory
397
shear senses, some even within the mineralising biotite-pyrite shears. These testify to
398
repeated reactivation, commonly in opposite shear senses, which is a hallmark of the
399
deposit. Much of the reactivation is consistent with D5 overprinting D3 structures, but
400
there are anomalous shear zones that do not fit in with this history: they could
401
represent the influence of D4. However, the kinematics of D3 and D5 appear to be
402
dominant (Fig. 14).
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N trending shears observed in some cores (Fig. 12) are parallel to a change in
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structural grain observed on a large scale around Tropicana and in the Plumridge
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terrane compared to other parts of the Northern Foreland, where aeromagnetic trends
407
are NW and more comparable with adjacent trends in the Yamarna Terrane. This
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inflexion may have been important to mineralization by bringing lithological bands
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into a more favourable orientation for shearing.
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6.2 Comparison with Archean lode gold deposits
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In terms of its general geological properties, the Tropicana deposit has some
415
similarities but also significant differences from many Archean lode gold
416
deposits of the Yilgarn craton. Tropicana lacks the metre scale quartz carbonate
417
veining that is such a characteristic feature of many Archean lode gold deposits.
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Likewise, the upper amphibolite-granulite grade of metamorphism for the host
419
rocks is exceptional, while the occurrence of mineralization at greenschist facies
420
at Tropicana (Doyle et al., 2009) is more typical of lode gold deposits (e.g.
421
Vearncombe, 1998). The concentration of mineralization in the feldspathic
422
gneiss is comparable to preferential mineralization in some host rocks within
423
lode gold deposits, although the feldspathic gneiss itself is quite dissimilar
424
geochemically to the basaltic or andesitic volcanic hosts of many lode gold
425
deposits. The diffuse nature of many ore bodies at Tropicana, which occur in
426
volumes of altered rock that do not have discrete structural boundaries, is also
427
atypical of lode gold deposits, in which ore bodies are commonly confined by
428
vein, fracture, fault or shear margins.
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However, the structural control by shear zones at Tropicana is similar to some
431
lode gold deposits, as is the role of solution and brecciation (e.g. Witt and
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Vanderhor, 1998). Shear zones at Tropicana exist in many orientations (Fig. 12):
433
similarly orientated shear zones can have quite different kinematics (Fig. 13).
434
This feature is also typical of some lode gold deposits (see below) in which, as at
435
Tropicana, it is probably due to overprinting events, several of which may be
436
associated with gold mineralization and remobilization. The main phase of
437
economic gold mineralization at Tropicana is Archean and postdates D1 and D2
438
deformation events: in this respect it is also similar to Archean lode gold deposits
439
of the Yilgarn craton, although the timing of mineralization at Tropicana does not
440
appear to correspond with the majority of late Archean deposits on the craton.
441
Nevertheless, Tropicana fits well into the suggested categorization of those
442
deposits as “late orogenic, structurally controlled” (Witt and Vanderhor, 1998).
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It is difficult to establish how much new mineralization as opposed to
445
remobilisation may have been associated with D4 and D5 at Tropicana.
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Remobilisation of gold is evidenced by clusters of visible gold localised in late
447
muscovite fabrics that overprint earlier biotite-pyrite fracture fills and grains.
448
Several detailed studies of large lode gold deposits show that they have
449
experienced more than one mineralizing event (e.g. the Golden Mile; Vielreicher
450
et al., 2010) or that reactivation has been a significant part of the deposit history
451
(e.g. St Ives: Miller et al., 2010; Renco Mine, Kolb and Meyer, 2002; Sunrise Dam:
452
Baker et al., 2010). The evidence for reactivation at Tropicana is a point of
453
comparison with the Archean lode gold deposits.
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The timing of D3 and the main mineralizing event (Tropicana event) is well
456
constrained to the late Archean by Ar-Ar dating of biotite and Re-Os dating of
457
pyrite (Doyle et al., 2013). From a regional perspective there are at least three
458
possibilities for the reactivation recorded by D4 and D5 at Tropicana. The
459
Zanthus event of the Biranup orogeny occurred at c. 1680 Ma, and the two stages
460
of the Albany–Fraser orogeny occurred at 1345–1260 Ma (Stage I) and 1215–
461
1140 Ma (Stage II) (Kirkland et al., 2011). The Eastern Biranup zone, in contact
462
with the Northern Foreland to the E of Tropicana, has no geochronological
463
evidence for Stage I of the Albany–Fraser orogeny (Kirkland et al., 2011), which
464
is consistent with a lack of evidence for this event in zircon or monazite.
465
However, low grade deformation and fluid flow related to this event can not be
466
excluded. Thrust emplacement of the Eastern Biranup Zone over the Northern
467
Foreland is likely to have occurred in the later stage of the Albany–Fraser
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orogeny, which most probably correlates with the clearest evidence of
469
reactivation at Tropicana, in D5.
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6.3 Comparison with Renco Deposit, Zimbabwe
473
Renco gold mine in Zimbabwe is in a granulite terrane (the Northern Marginal
475
Zone) 10 km from the contact between the granulites and the Zimbabwe Archean
476
craton. The mine is hosted by a late Archean enderbite intrusion (Blenkinsop et
477
al., 2004). The mineralization lies in shear zones that dip moderately away from
478
the craton, and in steeper linking shear zones (Kisters et al., 1998).
479
Mineralization probably occurred in upper amphibolite facies conditions at the
480
end of the Archean (Kolb et al., 2000), although evidence for a lower grade of
481
mineralization suggests that there was a second, possibly Paleoproterozoic event
482
(Frei et al., 1999; Blenkinsop et al., 2004). Reactivation is also evidenced by low
483
grade fabrics in the shear zones (Kolb et al., 2003). Within the shear zones that
484
host the mineralization, two domains are distinguished: quartz-feldspar-biotite-
485
hornblende mylonites, which surround lithons of k-feldspar-quartz-biotite-
486
garnet-sulphide. Fractures in lithons are filled by sulphides, and the majority of
487
the grade is concentrated in them (Kisters et al., 2000).
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The similarity in the position of Renco and Tropicana as gold deposits in
490
granulite terranes relative to their adjacent cratons is striking, and this is
491
reinforced by the common geometry of the ore bodies in zones that dip
492
moderately away from the craton. A network of shear zones is the critical
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hydrogeological structure in both cases, and the fractured lithons of Renco have
494
their counterpart in the brecciated pegmatite bodies of the Tropicana deposit.
495
Both deposits also have evidence for lower metamorphic grade overprints on
496
granulite facies host rocks.
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The timing of Renco mineralization is considered to be post-peak granulite facies
499
metamorphism in the late Archean, on a retrograde path but within 100°C of
500
peak metamorphism (Kolb and Meyer, 2002). The post-peak conditions and
501
timing are another similarity to Tropicana. The timing and geology of these
502
deposits suggest a possible link to granulite facies metamorphism in as much as
503
the deposits can be interpreted as forming in the retrograde parts of an orogenic
504
cycle that reached peak granulite facies. However, Kerrich (1988) has shown that
505
there is unlikely to be a direct geochemical link between granulite formation and
506
lode gold deposits: additional fluid input is probably required to explain
507
characteristic Large Ion Lithophile (LILE) enrichment of alteration associated
508
with mineralization compared to the depleted LILE characteristics of Archean
509
granulites.
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513
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7. Conclusions
514
Tropicana gold deposit in the Albany Fraser orogen on the margin of the Yilgarn
515
craton was formed by fluid flow through a network of biotite-pyrite-bearing
516
shear zones, initially in an event of NE shortening (D3). Permeability was created
517
in coarser, more competent pegmatitic layers by fracturing, and was
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518
accompanied by solution transfer along solution seams. Shear zones were
519
reactivated to provide a record of complex kinematics, including two retrograde
520
events (D4 and D5). D5 involved dextral shear on S-SW dipping surfaces.
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The most important geometrical control on mineralization at Tropicana is the
523
permeability created by the biotite-pyrite-bearing shear zones. The orientation
524
and location of these in turn were largely dictated by the orientation of the
525
gneissic banding in the favourable horizon of host rocks, which reflects two
526
previous deformation events and gentle folding in D3. High grade ore shoots
527
formed parallel to the common intersection direction of the shear zones, which
528
was also parallel to F3 fold hinges. Variations in the direction of the ore shoots
529
could reflect variations in the initial geometry of the gneisses, and/or later
530
deformation. A consideration of the entire geological history is necessary to
531
understand the deposit geometry.
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532
The style of mineralization at Tropicana is different from many Archean lode
534
gold deposits of the Yilgarn craton in as much as no metre scale
535
quartz/carbonate veins, fractures or faults seem to have played an important
536
role in mineralization. Nevertheless, the Tropicana deposit has a strong
537
structural control, in common with the Archean lode gold deposits, because all
538
these deposits were formed by hydrothermal fluid flow along structural
539
permeability. Mineralization at Tropicana occurred at greenschist facies in
540
granulite facies host rocks, clearly post-dating peak metamorphism.
541
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Renco mine in Zimbabwe and Tropicana gold deposit have several
543
characteristics in common, including their formation within granulites at the
544
margins of well-mineralized Archean cratons, probably within the late Archean.
545
It is possible that both could have been formed by fluid flow driven by heat
546
sources reflecting the waning of granulite facies metamorphism in orogenic belts
547
on the periphery of their respective Archean cratons.
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Acknowledgements
551
Independence Group NL and AngloGold Ashanti Ltd are thanked for permission
552
to publish the manuscript. The authors acknowledge the significant contribution
553
of past and present AngloGold Ashanti geologists and the exploration team who
554
have contributed to the discovery, delineation and understanding of the mineral
555
deposit. In particular, we are grateful for the assistance of Jeni Savage and Jovo
556
Przulj in completing the study.
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707
708
709
AC
C
705
EP
702
pp. 187-227.
Weinberg, R.F., Hodkiewicz, P.F., Groves, D.I., 2004. What controls gold distribution
in Archean terranes? Geology 32, 545.
Witt, W.K., Vanderhor, F., 1998. Diversity within a unified model for Archaean gold
mineralization in the Yilgarn Craton of Western Australia: An overview of the
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710
late-orogenic, structurally-controlled gold deposits, Ore Geology Reviews 13, 29-
711
64,
712
714
RI
PT
713
Figures
715
Fig. 1. Location of Tropicana gold deposit in the Northern Foreland of the
717
Albany–Fraser orogen (partly based on Spaggiari et al., 2011, Pawley et al., 2012).
SC
716
M
AN
U
718
Fig 2. Simplified interpretive geological map of the Tropicana region. The Black
720
Dragon thrust separates the Archean Tropicana Gneiss in the Plumridge terrane
721
from lower metamorphic grade rocks of the Paleoproterozoic Biranup zone.
722
Other NNE to NE trending structures separate further subdivisions of the
723
Albany–Fraser orogen.
TE
D
719
724
Fig. 3. Structural domains and mesoscopic shear zones of the Tropicana
726
Deposit, superimposed on a grade (g/t) X thickness (m) plot. GDA/UTM grid.
EP
725
727
Fig. 4. Schematic EW cross section of the Tropicana Deposit (based on Doyle et al.,
729
2007).
730
AC
C
728
731
Fig. 5. NE-SW section (true scale). Blue shapes are delimited by > 3g/t. “Principal
732
lineation” refers to the high grade ore shoots seen in the gm plot of Fig. 3. High
733
grade ore bodies pinch and swell.
734
ACCEPTED MANUSCRIPT
Fig. 6. Stereoplots of poles to high grade (>3 g/t) ore shells. A distinct difference
736
is noted between the Tropicana and Havana domains. Red squares are
737
eigenvectors of the distribution of poles. The minimum eigenvector is labelled
738
with its trend and plunge; a great circle connects the minimum and intermediate
739
eigenvectors. All stereoplots are lower hemisphere, equal area.
740
RI
PT
735
Fig. 7. Styles of Mineralization in the core.
742
a)
743
160 m.
744
b)
745
pyrite gives clear top-to-the left sense of shear. Core TPD361A, 161 m, XPL
746
(Cross-polarised light).
747
c)
748
gives clear shear sense.
749
d)
750
sericite and pyrite give clear top-to-the-right shear sense. Core TPD067, 143.3 m,
751
XPL.
M
AN
U
Biotite-pyrite shear zone sub-parallel to gneissic banding, core TPD366,
Biotite-pyrite shear zone (thin section). Foliation defined by biotite and
TE
D
Chlorite-sericite shear zone, core TPD167, 163.2 m. Asymmetric clast
Sericite shear zone (thin section). Spectacular SC fabrics defined by
EP
752
SC
741
Fig. 8. Styles of mineralization in the core.
754
a)
755
with carbonate-filled extension fractures. TPD202, 281.9 m
756
b)
757
sericite-pyrite veins. TPD202B, 282.9 m, XPL.
758
c)
759
shaped patches of carbonate. TPD 202, 285.8 m
AC
C
753
Solution seams with biotite anastomosing between feldspar-rich lithons
Fragmentation and solution accompanying formation of carbonate-
More discrete formation of biotite lined solution seams and irregularly-
ACCEPTED MANUSCRIPT
760
d)
Extension microfracture in perthite grain, with filling of pyrite and
761
carbonate. TPD202C, 283.9 m, XPL
762
Fig. 9. Structural features in core.
764
a) F1 folds in gneissic banding. Yellow lines marks form surface, yellow spots are
765
hinges and red line is hinge surface trace. TPD251, 222.7 m.
766
b) Tight F1 fold hinge of gneissic banding, TPD202, 314.1 m.
767
c) Asymmetric F5 fold of gneissic banding and shear foliation. TPRC092D, 298.7 m.
768
d) SC’ defined by chlorite and sericite. Marking according to the scheme of
769
Blenkinsop and Doyle (2010). TPD262, 162m.
M
AN
U
SC
RI
PT
763
770
Fig. 10. Orientations of fold hinges and hinge surfaces, located relative to
772
drillholes and structural domains. Folds in cores MBRC019D, TPRC607D,
773
TFD137, TFRC501D, TPD202, TPD261 and TPD366 plunge moderately S to SE
774
with E to SE dipping hinge surfaces. These are interpreted as F1 or F2 folds,
775
because they are tight to isoclinal. Folds in cores TFRC090D and TFRC092D
776
plunge moderately E to SE with S-dipping hinges surfaces. Many of these have
777
dextral vergence (turquoise colour). These folds are located adjacent to the
778
Boston Shaker shear zone and are F5 folds. Alternatively those folds which are
779
symmetric could be F3.
EP
AC
C
780
TE
D
771
781
Fig. 11. Poles to gneissic banding (n = 1510) in the Havana domain. Cylindrical
782
best fit gives a fold hinge plunging 35° to 115°. Kamb contours with a contour
783
interval of 6σ, 3σ significance level.
784
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Fig. 12. Geographic distribution of shear zones in ball-and-string plots. Great
786
circles indicate shear planes: arrows indicate hangingwall movement. In most
787
plots there are sub-parallel shears with different movement directions, testifying
788
to reactivation.
RI
PT
785
789
Fig. 13. Tangent lineation plots of shear zones separated by structural domain
791
and by phyllosilicate mineralogy. Arrows indicate footwall movement, plotted at
792
the pole to the fault. Dextral components in green and sinistral in red.
SC
790
M
AN
U
793
794
Fig. 14. Kinematic analysis of shear zones separated by structural domain and by
795
phyllosilicate mineralogy. Red and blue dots are shortening and lengthening axes
796
for individual shear zones respectively; 1, 2, and 3 are the linked Bingham axes
797
for the distributions shown.
TE
D
798
Fig. 15. Poles to shear zone orientations in four structural domains, with
800
eigenvectors to the distributions shown as black squares. Great circle links
801
maximum and intermediate eigenvectors. Kamb contours, Contour interval 2σ,
802
significance level 3σ.
AC
C
803
EP
799
804
Fig. 16. Cartoon of the structural evolution of the Tropicana Gold Deposit
805
a)
D1 is preserved as asymmetric folds and gneissic banding
806
b)
D2: thrusting (orange surfaces) and folding is inferred from regional
807
considerations
808
c)
D3: the main mineralizing event, due to NE shortening
809
d)
D5: The main reactivation
ACCEPTED MANUSCRIPT
Meso-micro
structures
Kinematics
High
D3
SE plunging
folds
Gneissic
Banding
Isoclinal
folds
Shear zones
Solution
fabrics
Breccias
?
NW
shortening
2640-2524
2640-2524
NE Shortening
Biotite
Pyrite
Sericite
Carbonate
Chlorite
Timing
M
AN
U
Low
D4
D5
Shear zones
Shear zones
?
2524-2515
?
EP
TE
D
Table1. Deformation and mineralization history at Tropicana Gold Mine
AC
C
Shear zones
Asymmetric
folds
Dextral E-NE
shearing
SC
Au:
D2
Thrusts
Folds
RI
PT
D1
Macro
structure
? 1215-1140
a
Yamar r
123°
ACCEPTED MANUSCRIPT
Lin
e
ea
e
YILGARN CRATON
Norseman
p
tu
ut
lt
au
F
ac
rd
Je
32°
M
AN
U
Youanmi
Terrane
SC
Kalgoorlie
Terrane
32°
RI
PT
Kalgoorlie
Kurnalpi
Terrane
30°
Ea
Kanowna Belle
Mine
Gu
n
ba
rre
1000 km
lF
30°
ste
AFO
120°
Tropicana
a
PPl ult
luumm
rn
brri
Fr
Bi
didg
gee
ra
as
nu
T
er
p Z Teerrraa
Zo
nne
on
ne
e
Ta
sm
an
South
Australian
Craton
Area of
Fig 2
e
Eastern Goldfields Superterrane
Burtville
Terrane
on
MP
rZ
West
Australian
Craton
Voodoo Child
Sh
PO
North Australian
Craton
Yamarna
Terrane
ne
p
South
West
Terrane
nu
ira
Zo
TE
D
lB
ra
t
en
C
34°
Archean
AC
C
120°
EP
He
e
ult Zon
yne Fa
-Che
ywood
Esperance
Eucla
Basement
34°
100 km
123°
Paleoproterozoic
Metasedimentary rocks
Nornalup Zone
Tropicana Gneiss
Fraser Range Metamorphics
Major faults
Terrane boundary
Geological boundary
Munglinup Geniss
Undivided
Gwynne Creek Gneiss
Coastline
Biranup Zone
Albany-Fraser bedrock interp.cdr
AC
C
EP
TE
D
M
AN
U
SC
RI
PT
ACCEPTED MANUSCRIPT
10 km
AC
C
EP
TE
D
M
AN
U
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 3. Structural domains and mesoscopic shear zones of the Tropicana
Deposit, superimposed on a grade (g/t) X thickness (m) plot. GDA/UTM grid
ACCEPTED MANUSCRIPT
E
W
RI
PT
Transported
M
AN
U
SC
RL
300
BOCO
TE
D
200
100
Ore zone
Saprolite
Pebbly
sandstone
Basalt dyke
Garnet gneiss
Amphibolite
AC
C
EP
Hanging
wall
Meta Ferruginous
chert
Quartzo-feldspathic
gneiss
Fig. 4. Schematic EW cross section of the Tropicana Deposit (based on Spaggiari et al. 2011).
Favourable Horizon
Footwall
ACCEPTED MANUSCRIPT
Continuity at > 0.1 g/t
SC
RI
PT
Principal
lineation
AC
C
EP
TROPICANA
TE
D
swell
M
AN
U
pinch
500m
HAVANA
NORTH
See Enlg. above
250m
Fig. 5. NE-SW section (true scale). Blue shapes are delimited by > 3g/t. “Principal lineation”
refers to the high grade ore shoots seen in the gram x metre plot of Fig. 3. High grade ore
bodies pinch and swell.
ACCEPTED MANUSCRIPT
Tropicana
Havana
SC
22° /163°
RI
PT
29°/087°
AC
C
EP
TE
D
M
AN
U
Fig. 6. Orientations of poles to high grade (>3 g/t) ore shells. A distinct
difference is noted between the Tropicana and Havana domains.
ACCEPTED MANUSCRIPT
a)
RI
PT
b)
c)
SC
25 mm
EP
25 mm
TE
D
M
AN
U
d)
1 mm
1 mm
AC
C
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 sense
d) 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.
ACCEPTED MANUSCRIPT
a)
25 mm
c)
d)
1 mm
EP
25 mm
TE
D
M
AN
U
a
)
SC
RI
PT
b)
1 mm
AC
C
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 m
b) 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 m
d) Extension microfracture in perthite grain, with filling of pyrite and carbonate. TPD202C, 283.9
m, XPL
ACCEPTED MANUSCRIPT
a) a)
SC
RI
PT
b)
15 mm
M
AN
U
25 mm
d)
AC
C
EP
TE
D
c)
50 mm
25 mm
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 m
b) Isoclinal F1 fold hinge of gneissic banding, TPD202, 314.1 m
c) Asymmetric F5 fold of gneissic banding and shear foliation. TPRC092D, 298.7 m
d) SC’ defined by chlorite and sericite. Marking according to the scheme of
Blenkinsop and Doyle (2010). TPD262, 162m
TPRC090D, TPRC092D
TPRC607D, TFD137
Equal Area
Lower Hemisphere
Equal Area
Lower Hemisphere
ACCEPTED MANUSCRIPT
Tropicana
N = 29
N=6
RI
PT
Havana
North
Equal Area
Lower Hemisphere
M
AN
U
Havana South
SC
Equal Area
Lower Hemisphere
N = 52
TE
D
MBRC019D
N = 20
TFRC501D, TPD202,
TPD261, TPD366
AC
C
EP
Fig. 10. Orientations of fold hinges and hinge surfaces, located relative to drillholes and structural
domains. Folds in cores MBRC019D, TPRC607D, TFD137, TFRC501D, TPD202, TPD261 and
TPD366 plunge moderately S to SE with E to SE dipping hinge surfaces. These are interpreted as F1 or
F2 folds, because they are tight to isoclinal. Folds in cores TFRC090D and TFRC092D plunge
moderately E to SE with S-dipping hinges surfaces. Many of these have dextral vergence (turquoise
colour). These folds are located adjacent to the Boston Shaker shear zone and are F5 folds. Some of
these folds which are not asymmetric could also be F3.
M
AN
U
SC
RI
PT
ACCEPTED MANUSCRIPT
AC
C
EP
TE
D
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.
Fig. 12. Geographic distribution of shear zones in ball-and-string plots. Great circles indicate shear planes:
arrows indicate hangingwall movement. In most plots there are sub-parallel shears with different movement
directions, testifying to reactivation.
ACCEPTED MANUSCRIPT
TPD067, 068
TFD137
TPRC607D
TPD261, 262
RI
PT
TFD115
TROPICANA
M
AN
U
SC
TPRC090D
HAVANA
TE
D
TPD302
TFD134, 135
EP
TPD345
TPD365, 366
AC
C
HAVANA
SOUTH
TFD167
TPD103, 108, 109
TPD098, 157
TPD360A
Biotite
ACCEPTED MANUSCRIPT
Biotite+
Sericite/Chlorite
RI
PT
Tropicana
M
AN
U
SC
Havana
North
EP
TE
D
Havana
South
AC
C
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.
ACCEPTED MANUSCRIPT
Biotite+
Biotite
Sericite/Chlorite
Tropicana
3
3
2
Havana
North
3
SC
3
1
1
2
2
3
1
2
3
EP
1
2
2
M
AN
U
3
TE
D
Havana
South
1
RI
PT
1
1
2
AC
C
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
Tropicana
Havana North
ACCEPTED MANUSCRIPT
Crouching Tiger
AC
C
EP
TE
D
M
AN
U
SC
RI
PT
Havana South
a) D1
b) D2
ACCEPTED MANUSCRIPT
HSI
F1
FOLD
HINGE
N
c) D3
M
AN
U
F3
FOLD HINGE
EP
TE
D
Fig. 16. Cartoon of the structural evolution of the Tropicana Gold Deposit
a)
D1 is preserved as isoclinal folds and gneissic banding
b)
D2: thrusting and folding is inferred from regional considerations
c)
D3: the main mineralizing event, due to NE shortening
d)
D5: The main reactivation
AC
C
REACTIVATION
OF THRUSTS AS NORMAL FAULTS
SC
d) D4, D5
RI
PT
HINGE
SURFACE
A
AN
IC
OP
TR
A
N
VA
HA
F5
FOLD HINGE
Supplementary Material for Blenkinsop and Doyle 2013.
Locations of Drillholes used in this study
AC
C
EP
TE
D
M
AN
U
SC
RI
PT
ACCEPTED MANUSCRIPT

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