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Characteristics, origin and significance of Mesoproterozoic bedded clastic facies at the Olympic Dam Cu–U–Au–Ag deposit, South Australia
Accepted Manuscript Title: Characteristics, origin and significance of Mesoproterozoic bedded clastic facies at the Olympic Dam Cu-U-Au-Ag deposit, South Australia Author: Jocelyn McPhie Karin Orth Vadim Kamenetsky Maya Kamenetsky Kathy Ehrig PII: DOI: Reference:
S0301-9268(16)00050-4 http://dx.doi.org/doi:10.1016/j.precamres.2016.01.029 PRECAM 4448
To appear in:
Precambrian Research
Received date: Revised date: Accepted date:
16-7-2015 27-1-2016 27-1-2016
Please cite this article as: McPhie, J., Orth, K., Kamenetsky, V., Kamenetsky, M., Ehrig, K.,Characteristics, origin and significance of Mesoproterozoic bedded clastic facies at the Olympic Dam Cu-U-Au-Ag deposit, South Australia, Precambrian Research (2016), http://dx.doi.org/10.1016/j.precamres.2016.01.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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3 to 5 bullet points (maximum 85 characters, including spaces, per bullet point) Five bedded clastic facies associations have distinct composition and grain size.
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Bedded clastic facies contain zircons dated at 1600 Ma, 1740 Ma, 1900 Ma and 2500 Ma.
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Pre-lithification folds have gently northeasterly plunging fold axes.
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Structure and facies suggest deposition in a fault-controlled sedimentary basin.
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Characteristics, origin and significance of Mesoproterozoic bedded clastic facies at the
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Olympic Dam Cu-U-Au-Ag deposit, South Australia
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Jocelyn McPhiea, Karin Ortha, Vadim Kamenetskya, Maya Kamenetskya, Kathy Ehrigb
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7001, Australia
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School of Physical Sciences and CODES, University of Tasmania, Private Bag 79, Tasmania
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BHP Billiton Olympic Dam, Adelaide, South Australia 5001, Australia
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Jocelyn McPhie
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Karin Orth
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Vadim Kamenetsky < [email protected]>
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Maya Kamenestky
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Kathy Ehrig
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Corresponding author
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Jocelyn McPhie
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Abstract
Mesoproterozoic bedded clastic facies occur in the Olympic Dam Breccia Complex
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which hosts the Olympic Dam Cu-U-Au-Ag deposit, South Australia. Contacts of the bedded
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clastic facies with breccia of the Olympic Dam Breccia Complex are faulted and/or
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brecciated; fragments of the bedded clastic facies occur in the breccia complex. The bedded
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clastic facies comprise five main facies associations, four of which are organised into
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mappable units. These associations have distinct textures and components and have not been
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mixed. Major sources of detritus were felsic and mafic volcanic units and granitoids of the
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~1590 Ma Gawler Silicic Large Igneous Province. Archean and Paleoproterozoic zircons in
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well-sorted quartz-rich sandstone indicate that older Gawler Craton basement successions
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also contributed sediment to the bedded clastic facies. Accumulation of the bedded clastic
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facies was at least partly contemporaneous with rhyolitic explosive eruptions from vent(s)
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probably tens of km away because bubble-wall shards are present in tuffaceous mudstone
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facies. Deformation of the bedded clastic facies prior to final lithification produced folds that
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have diverse shapes and sizes and lack cleavage. The folds have a reasonably consistent
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gentle plunge to the northeast. These soft-sediment folds could be related to slumping on a
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northeast-striking paleoslope that dipped either to the northwest or southeast. The depocenter
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in which these facies accumulated may have been bounded by a combination of northeast-
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striking and northwest-striking faults. Because the bedded clastic facies occur at the top of
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the Olympic Dam Breccia Complex and contributed clasts to it, the fault-controlled
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sedimentary basin was in place when the breccia complex and the Olympic Dam ore deposit
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formed.
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Key words: Olympic Dam, bedded clastic facies, provenance, pre-lithification fold,
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sedimentary basin, Mesoproterozoic
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1. Introduction
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The supergiant Olympic Dam Cu-U-Au-Ag deposit is famous for its size and
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polymetallic character (e.g., Ehrig et al., 2012), and for being one of the deposits upon which
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the iron oxide-copper-gold (IOCG) ore deposit class is based (Hitzman et al., 1992). When
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first discovered but prior to major underground development, the breccia-dominated host
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succession to the deposit was interpreted to be largely sedimentary and was described in
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terms of two formations that were further divided into members (Roberts and Hudson, 1983).
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Extensive underground exposure and further drilling led to the recognition that the clastic
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host rocks are part of a breccia complex formed mainly by a combination of hydrothermal
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and tectonic processes (Olympic Dam Breccia Complex, ODBC; Reeve et al., 1990). Reeve
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et al. (1990) also reported “volcaniclastic rocks in diatreme structures” associated with mafic
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dykes and showed five such diatremes on their map (Reeve et al., 1990, Figure 2(a)). The
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diatremes were interpreted as the roots of maar volcanoes where hydrothermal and
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phreatomagmatic eruptions had taken place. Even though the mapped diatremes occupy less
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than 2% of the total area of the ODBC (~17 km2), Reeve et al. (1990) attributed much of the
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brecciation to eruption-related decompression. Domains of well-bedded, mostly fine-grained
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lithologies in the ODBC were thought to be “crater facies epiclastics”. The maar-diatreme
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interpretation has been developed and/or repeated in several subsequent papers on Olympic
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Dam and has been incorporated in genetic models for Olympic Dam and for IOCG deposits
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in general (e.g., Cross et al., 1993; Haynes et al., 1995; Johnson and Cross, 1995; Skirrow et
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al., 2002; Groves et al., 2010; Hayward and Skirrow, 2010).
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McPhie et al. (2011) proposed that the well-bedded, mostly fine-grained clastic
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lithologies in the ODBC were the remnants of a Mesoproterozoic sedimentary basin, using
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data on facies characteristics and relationships, and on the clastic components. Here we
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provide new data on the facies, stratigraphy, age and structure of the bedded clastic facies at
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Olympic Dam. Our data come from a selection of the ~1000 drill holes drilled between 2003
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and 2008 in the eastern two-thirds of the deposit; these drill holes are the most recent
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available. They represent ~60% of the total number of holes, and of the total meters drilled, at
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Olympic Dam, and have added substantially to knowledge and understanding of the deposit
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and its setting (Ehrig et al., 2012). These data are used to argue that the bedded clastic facies
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were deposited in a fault- bounded sedimentary basin. The presence of mappable internal
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stratigraphy cannot be accommodated by deposition of the bedded clastic facies in the
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isolated maar craters interpreted by Reeve et al. (1990). Because the fault-bounded basin was
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present when the Olympic Dam ore deposit formed, this setting ought to be considered in
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models of ore genesis.
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2. Regional Geological Setting of Olympic Dam Olympic Dam is on the eastern edge of the Gawler Craton in South Australia (Fig. 1).
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The Gawler Craton comprises Meso- to Neoarchean complexes surrounded by
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Paleoproterozoic to Mesoproterozoic successions (Swain et al., 2005; Fanning et al., 2007;
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Fraser et al., 2010). In the southern and eastern Gawler Craton, the Paleoproterozoic
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sedimentary and igneous units were deformed during the Cornian Orogeny (around 1850 Ma,
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Hand et al., 2007), and the Kimban Orogeny (1730 – 1690 Ma, Hand et al., 2007).
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Voluminous igneous units dominate the Mesoproterozoic successions of the Gawler Craton. In the central and eastern Gawler Craton, the principal units are the Hiltaba Suite
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(1595-1575 Ma; Hand et al., 2007) and the Gawler Range Volcanics (~1591 Ma, Fanning et
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al., 1988) which together constitute the Gawler Silicic Large Igneous Province (SLIP; Allen
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et al., 2008). The lower Gawler Range Volcanics include mafic and felsic lavas, and felsic
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ignimbrites and other pyroclastic facies (Giles, 1988; Blissett et al., 1993; Allen et al., 2008;
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Agangi et al., 2012). The upper Gawler Range Volcanics are dominated by three large-
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volume rhyolite lavas (Allen and McPhie, 2002; Allen et al., 2008).
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The Gawler Range Volcanics and younger units are flat-lying or gently dipping and
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have not been metamorphosed. Regional deformation roughly synchronous with the Gawler
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SLIP (1590-1570 Ma, Hand et al., 2007) was limited to movement on faults and shear zones
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in response to northwest-southeast directed shortening. Similarly, younger deformation
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events (Kararan Orogeny, 1570-1540 Ma, Hand et al., 2007; Coorabie Orogeny, 1470-1450
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Ma, Direen et al., 2005; Hand et al., 2007) involved reactivation of fault and shear zones,
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mainly in the northern and western parts of the Gawler Craton.
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The eastern Gawler Craton in the region of Olympic Dam is overlain by the Pandurra Formation which is composed of continental sedimentary rocks (Cowley, 1993) that are
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neither deformed nor metamorphosed. The age estimate of the Pandurra Formation is
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1424±51 Ma (Rb-Sr date, Fanning et al., 1983). The Pandurra Formation is overlain by
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Neoproterozoic to Cambrian sedimentary rocks of the Stuart Shelf (Preiss, 1993). East of the
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Torrens Hinge Zone, the sedimentary formations belong to the Adelaide Geosyncline and
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they are markedly thicker. The Gairdner Dyke Swarm comprises a 180-250 km-wide,
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northwest-striking group of mafic dykes which intruded the eastern and northern Gawler
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Craton around 825 Ma (Wingate et al., 1998).
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3. Geology of Olympic Dam
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The Olympic Dam Cu-U-Au-Ag ore deposit occurs in the Olympic Dam Breccia
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Complex (ODBC) which is surrounded by and has gradational contacts with the Roxby
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Downs Granite (Reeve et al., 1990). The Roxby Downs Granite has an age of 1594+/- 5 Ma
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(U-Pb in zircon, Jagodzinski, 2014) and is part of the Burgoyne Batholith, a member of the
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Hiltaba Suite; it is surrounded by the Paleoproterozoic Hutchison Group, Donington Suite
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and the Wallaroo Group. Both the Roxby Downs Granite and the ODBC are unconformably
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overlain by 300-350 m of Neoproterozoic and Cambrian Stuart Shelf sedimentary formations.
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The Gawler Range Volcanics have been logged in drillholes at nearby prospects (Wirrda
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Well, Snake Gully, Acropolis; all <25 km from Olympic Dam) but only a single unit of
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almost intact Gawler Range Volcanics has been identified at Olympic Dam (strongly altered
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olivine-phyric picrite; Ehrig et al., 2012). The Pandurra Formation is not present at Olympic
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Dam, but it occurs to the west, north and south.
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The main protolith of the ODBC was the Roxby Downs Granite and granite clasts are weakly to intensely hematite-altered clasts (Reeve et al., 1990). In the granite-rich breccia,
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granite clasts are weakly to moderately altered whereas in the hematite-rich breccia, the
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granite clasts are strongly to intensely altered and a hematite-rich, fine-grained matrix is
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present (Reeve et al., 1990). The ore minerals (Cu sulfides and uraninite, coffinite and
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brannerite) are typically fine-grained and disseminated in the hematite-rich breccia of the
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ODBC. There is a gross zonation in the Cu sulfides from chalcopyrite at depth through
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bornite to chalcocite at the shallowest levels (Reeve et al., 1990).
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In the southern mine area, the ODBC also includes large domains of polymictic breccia
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composed of feldspar-phyric clasts of the Gawler Range Volcanics, and two areas of bedded
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clastic facies (Fig. 2A). The distribution of the bedded clastic facies shown in Figure 2A is
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constrained by intersections in 302 diamond drill holes. The northern area of bedded clastic
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facies is 600 m long, 370-380 m wide and 210 m in vertical extent below the unconformity
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with the Stuart Shelf sedimentary formations. It reaches the deepest point below the
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unconformity in the northwest where the basal contact slopes shallowly to the northwest. The
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southern area of bedded clastic facies is 750 m south of the northern bedded clastic facies. It
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comprises one main domain and nearby outliers to the northeast (northern outlier) and
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southwest (western outlier). The main domain of the southern bedded clastic facies is
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elongate east-west and is 1300 m long, up to 370-380 m wide and 700 m in vertical extent
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below the unconformity with the Stuart Shelf sedimentary formations. The contact of the
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southern bedded clastic facies with breccia of the ODBC is deepest at the western end and
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shallower to the east. Intervals of the bedded clastic facies have been internally deformed and
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they have both faulted and gradational contacts with the ODBC. At the gradational contacts,
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the bedded clastic facies are commonly brecciated and separated from hematite-rich breccia
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or granite-rich breccia by intervals composed of fragments of the bedded clastic facies in
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hematite-rich breccia or granite-rich breccia.
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Mafic dykes intrude the ODBC and the Roxby Downs Granite (Reeve et al., 1990). Relatively fresh plagioclase-pyroxene-phyric basalt and dolerite dykes belong to the ~825 Ma
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Gairdner Dyke Swarm (Huang et al., 2015). Variably altered olivine-phyric, aphyric and
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doleritic dykes are considered to be of GRV age (~1590 Ma; Johnson and McCulloch, 1995;
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Huang et al., in review). In the northern mine area, these dykes are intensely altered and
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commonly mineralised.
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Ehrig et al. (2012) summarised a large amount of underground mapping and drill core
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data relating to five main sets of steeply dipping faults at OD, four of which (north-,
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northwest-, west-northwest-, and northeast-striking) were present before and/or during
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formation of the Olympic Dam deposit. Some of the northwest- and northeast-striking fault
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sets are regionally continuous, and have been mapped beyond the Olympic Dam area (e.g.
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Elizabeth Creek fault zone and Todd Dam fault zone; Hayward and Skirrow, 2010).
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Displacement directions in the vicinity of Olympic Dam are difficult to determine but normal
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displacement predominates. Hayward and Skirrow (2010) presented evidence for normal
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displacement on two regional northwest-striking faults that occur to the southwest (Elizabeth
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Creek fault zone, ~40 km from OD) and northeast (Gregory fault zone, ~60 km from OD).
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4. Facies Associations in the Bedded Clastic Facies
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The bedded clastic facies stand out markedly from the rest of the ODBC because they
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are consistently well bedded and dominated by fine grain sizes (mainly <2 mm). The rest of
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the ODBC lacks consistent bedding and is dominated by coarser grain sizes (fragments
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commonly >1 cm). Five facies associations defined on the basis of color, components, grain
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size and bedding characteristics are present in the bedded clastic facies: interbedded
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sandstone and red mudstone, well-sorted quartz-rich sandstone, green sandstone and
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mudstone, polymictic volcanic-clast conglomerate, and thinly bedded green and red
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mudstone.
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4.1 Interbedded sandstone and red mudstone The interbedded sandstone and red mudstone facies association comprises up to 50% of
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the bedded clastic facies and is present in both the northern and the southern bedded clastic
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facies areas. This association mainly consists of red mudstone intercalated with coarse to
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fine-grained, thinly bedded (3-10 cm thick) and laminated white, yellow and pink sandstone
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(Fig. 3A,B). Some thicker (30 cm) sandstone beds and 50-cm-thick polymictic, granule to
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pebble conglomerate and breccia beds are also present.
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The interbedded sandstone and red mudstone facies association is thickest at the northern edge of the southern bedded clastic facies, where it reaches 90 m (true thickness,
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measured in drill core) near the unconformity and tapers to 50 m at depth. It is significantly
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thinner (10-20 m thick) at the southern edge of the southern bedded clastic facies where it is
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in contact with granite-rich breccia of the ODBC (Fig. 2B). In the east, the interbedded
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sandstone and red mudstone facies association forms an outlier north of the southern bedded
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clastic facies. In the west, it comprises 25% of the western outlier of the southern bedded
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clastic facies. In the northern bedded clastic facies, the interbedded sandstone and red
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mudstone facies association makes up 5-20% of the bedded clastic facies, commonly as <5-
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m-thick intervals intercalated with the polymictic volcanic-clast conglomerate facies. It may
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be more abundant in the northern bedded clastic facies, but this area is intensely hematite-
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altered and discriminating among the various fine-grained thinly bedded facies associations is
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problematic.
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The thicker (30 cm) sandstone beds in the interbedded sandstone and red mudstone
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facies association are typically internally massive, parallel laminated or graded, and tabular
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though some are lenticular (Fig. 3C, D). Sandstone beds also show flame structures (Fig. 3B),
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ball-and-pillow structures, and cross-beds and cross-laminae defined by dark bands of grey
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hematite (Fig. 3C, Fig. 4A-D). The red mudstone intervals are planar laminated to very thinly
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bedded. Local disharmonic folds are common (Fig. 4E), as are mm- to cm-scale bed offsets
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along faults. The polymictic conglomerate and breccia beds in this facies association
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generally display normal grading upwards into parallel laminated sandstone.
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Granitoid-derived quartz, granitoid clasts and feldspar are the dominant recognisable
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components in the sandstone (Fig. 3E); hematite, zircon, tourmaline, titanite and rutile are
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also present. Clasts are angular to subangular. Some laminae are defined by concentrations of
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detrital hematite and zircon (Fig. 4A-D). Quartz, fine-grained white mica and hematite are
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common between the sand grains; interstitial sulfides (pyrite, chalcopyrite, bornite and
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chalcocite), florencite and Pb telluride are present locally. The red mudstone is almost
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entirely composed of red hematite, grey hematite and minor magnetite; scattered fine quartz,
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zircon, white mica and Fe-chlorite are also present. Clasts in the polymictic conglomerate and
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breccia in this association include grey and red fine-grained hematite, feldspar-phyric and
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mafic volcanic fragments, granite and granophyric clasts.
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4.2 Well-sorted quartz-rich sandstone
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Well-sorted quartz-rich sandstone has been identified in one drill hole (RD2751) in the
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northern bedded clastic facies area and one drill hole in the southern bedded clastic facies
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area (RD1628). In both drill holes, the sandstone is present as a single ~80-m-thick interval
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(RD2751, 848.7 m to 928.3 m; RD1628, 340.6 to 416.7 m) and is almost entirely brecciated
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(Fig. 5A). Only the topmost ~2.3 m (Fig. 5B) and another ~2-m-thick interval at ~898 m in
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RD2751 appear to be intact (although both could be large clasts). The clasts of well-sorted
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quartz-rich sandstone are internally massive or diffusely stratified and some are cross-
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stratified (Fig. 5D, E). The principal component of the sandstone is sub-rounded quartz; mm-
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size muscovite grains, tourmaline and fine-grained red hematitic fragments are also present
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(Fig. 5C). The quartz grains have undulose extinction, subgrains, and abundant fluid
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inclusions, and some are poly-grain aggregates with annealed, triple-junction boundaries. In RD2751, the brecciated well-sorted quartz-rich sandstone has a sharp upper and a
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more gradational lower contact with polymictic breccia of the ODBC (Fig. 5A). Higher in
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the same hole, the thinly bedded green and red mudstone facies association occupies an
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interval that is ~165 m thick. None of the nearby holes on the same section (~50 m southwest,
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or ~250 m northeast) includes the well-sorted quartz-rich sandstone facies.
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4.3 Green sandstone and mudstone
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The green sandstone and mudstone facies association is a distinctive part of the bedded
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clastic facies in the southern mine area at Olympic Dam. It is present in the center of the
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southern bedded clastic facies and the western outlier, is up to 100 m thick and can be traced
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over 1.3 km along strike (Fig. 2). This association is also present in the northern bedded
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clastic facies but is much thinner (<20 m).
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This association consists of interlaminated green sandstone and mudstone (Fig. 6A), 1-
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m-thick green sandstone beds, and a few 10-m-thick beds of green sandstone (e.g. RU38-
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2626, 400-430 m). Most of the thick sandstone beds contain green mudstone intraclasts (Fig.
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6B). The sandstone and mudstone have sharp contacts, although some sandstone beds grade
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to mudstone. Laminae and beds are laterally continuous and of even thickness.
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Altered mafic igneous clasts are the main component of the green sandstone. They have
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porphyritic, doleritic and vesicular textures (Fig. 6C). A mafic composition is suggested by
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the presence of chlorite-pseudomorphed olivine crystals that contain chromite inclusions
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(Fig. 6C) and free chromite crystals (Fig. 6D). Other clast types include feldspar-phyric felsic
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volcanic clasts, granitoid fragments and granitoid quartz, minor volcanic quartz, tourmaline
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and zircon. The cement between the clasts in sandstone includes quartz, carbonate, fine-
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grained white mica and chlorite. Sulfide minerals, including pyrite, chalcopyrite, bornite and
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chalcocite, occur together with siderite, barite, fluorite and apatite between the grains (Fig.
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6E).
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The green colour of the mudstone is caused by phyllosilicates, mainly chlorite, though fine-grained white mica is also present. Angular grains of quartz and feldspar are dispersed
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through the mudstone. Accessory minerals include zircon and titanite.
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4.4 Polymictic volcanic-clast conglomerate
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The polymictic volcanic-clast conglomerate is present in the northern bedded clastic
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facies, the western portion of the southern bedded clastic facies and in the western outlier
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(Fig. 2). In the northern bedded clastic facies, it is 70 m thick and has fault contacts with both
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the thinly bedded green and red mudstone facies association and the ODBC. At the western
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end of the southern bedded clastic facies and in the western outlier, it forms 3-m-thick beds
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within intervals of the green sandstone and mudstone. In the southern bedded clastic facies, it
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is 10-20 m thick and separates the interbedded sandstone and red mudstone facies association
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(above) and the green sandstone and mudstone facies association (below).
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The association consists mostly of polymictic volcanic-clast conglomerate and subordinate polymictic volcanic sandstone and mudstone. The conglomerate is characterized
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by a polymictic assemblage of pink-red hematite-altered or green chlorite-altered volcanic
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cobbles and pebbles (Fig. 7). Most of the clasts are feldspar-phyric and felsic in composition,
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and very similar to feldspar-phyric rhyolites in the Gawler Range Volcanics. A minor
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proportion (<1%) of the felsic volcanic clasts are sparsely quartz-phyric. Some clasts have a
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mafic composition reflected by the presence of pseudomorphs of skeletal olivine phenocrysts
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that contain chromite. Granite and minor hematite clasts are also present. Most clasts are well
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rounded, although some clasts are angular and others have irregular margins (Fig. 7). The
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conglomerate is typically clast-supported, but locally matrix-supported. Between the clasts is
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sandstone composed of polymictic volcanic fragments (Fig. 7), and less abundant feldspar,
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chromite, and rare quartz and pseudomorphs of olivine. The matrix has a patchy green or
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purple colour due to being chlorite- or hematite-altered, respectively.
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In the northern bedded clastic facies, the polymictic volcanic-clast conglomerate is interbedded with 1-2-m-thick intervals of polymictic volcanic sandstone and mudstone. The
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polymictic volcanic sandstone is very similar to the sandstone matrix between cobbles and
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pebbles in the polymictic volcanic-clast conglomerate. The sandstone matrix is composed of
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0.5-2 mm felsic and mafic volcanic fragments and subordinate feldspar, chromite,
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pseudomorphs of olivine and rare quartz. In the mafic clasts, pseudomorphs of strongly
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chlorite-altered, skeletal olivine contain chromite.
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4.5 Thinly bedded green and red mudstone
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The thinly bedded green and red mudstone facies association comprises up to ~20% of
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the northern bedded clastic facies. It is also a minor component at the eastern end of the
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southern bedded clastic facies.
This association is characteristically very fine grained (<1 mm) and delicately
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laminated. It consists predominantly of laminated red mudstone (Fig. 8A) and interlaminated
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green and red mudstone (Fig. 8B), and minor laminae and thick beds of pale grey, pale brown
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and purple fine-grained sandstone. Red mudstone consists of fine-grained hematite and white
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mica and the fine-grained sandstone beds contain quartz, feldspar, and felsic volcanic clasts.
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Some fine-grained sandstone beds are normally graded. Clasts in the sandstone are angular or
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subangular. Laminae and thicker beds are laterally continuous and of even thickness.
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In the northern bedded clastic facies, this association includes thin beds and laminae of
298
siliceous mudstone, ironstone, barite and felsic tuffaceous mudstone. Some of the siliceous
299
mudstone laminae are red due to the presence of fine hematite (Fig. 8C). Cracks
300
perpendicular to bedding, and local cm- and m-scale folds and faults are present (Fig. 8B).
13 Page 13 of 47
301
Some laminae and thin beds of green mudstone contain sparse chromite grains, and others
302
contain magnetite rimmed by hematite. The felsic tuffaceous mudstone is pale green, and occurs in internally massive laminae
304
and thin to thick beds. The tuffaceous origin is demonstrated by the presence of bubble-wall
305
shards, the shapes of which are very well preserved even though the original volcanic glass is
306
entirely altered (Fig. 8D). The tuffaceous mudstone probably has a felsic composition
307
because it contains fine, angular volcanic quartz (Fig. 8E) and zircon.
308
5. Stratigraphic Relationships in the Bedded Clastic Facies
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All five facies associations are present in both the northern and southern bedded clastic
310
facies areas. Many contacts between the facies associations are faults. In the northern bedded
311
clastic facies, the main interval of the polymictic volcanic-clast conglomerate appears to
312
overlie the thinly bedded green and red mudstone. Stratigraphic relationships are somewhat
313
better preserved in the southern bedded clastic facies area (Fig. 2). Interbedded sandstone and
314
red mudstone is overlain by green sandstone and mudstone or thinly bedded green and red
315
mudstone. In the eastern part of the southern bedded clastic facies, thinly bedded green and
316
red mudstone occurs gradationally between interbedded sandstone and red mudstone below
317
and green sandstone and mudstone above. In the western part of the southern bedded clastic
318
facies, the green sandstone and mudstone is overlain by polymictic volcanic-clast
319
conglomerate (RU38-2626, 400 to 355 m, Fig. 2C; RU38-2625, 470 to 380 m). Two
320
intervals of interbedded sandstone and red mudstone occur along the northern and southern
321
margins of the southern bedded clastic facies and an interval of green sandstone and
322
mudstone occurs in between (Fig. 2A, B). It is unclear if there are two units of interbedded
323
sandstone and red mudstone, one below and one above the green sandstone and mudstone, or
324
if the two interbedded sandstone and red mudstone intervals are at the same stratigraphic
325
level and repeated by faults and/or folds.
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326 327
6. Structure of the Bedded Clastic Facies Bedding data for the bedded clastic facies presented here come exclusively from oriented drill core. Folds are evident in many logged intervals of the bedded clastic facies
329
(e.g. Fig. 4E) though no fold axes were directly measured. Folds do not have associated
330
cleavage or other foliations, even in mudstone (Fig. 4F). The folds are variable in size.
331
Wavelengths range from 1-2 cm to several metres, and are evident from changes in bedding
332
orientation, and in some cases, changes in younging direction. Concentric folds occur in some
333
sandstone beds that are thicker than 10 cm (Fig. 4E). Disharmonic folds are most abundant in
334
fine-grained laminated facies.
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The internal structure of the northern bedded clastic facies is consistent with the
336
presence of roughly cylindrical folds with gentle northeasterly plunges (Fig. 9A). The
337
exceptions are beds in the upper part of RD3450 (not plotted in Figure 9A) in the northern
338
bedded clastic facies, where all beds are subvertical and there is no consistent strike direction.
339
A similar but less well defined northeasterly fold plunge is present in the bedding data for the
340
southern bedded clastic facies (Fig. 9B). In detail, the internal structure is complex and beds
341
are not necessarily parallel to the boundaries of the facies associations. For example, in the
342
western part of the southern bedded clastic facies, the boundaries of the facies associations
343
dip 70⁰ to the north whereas the beds strike northeasterly, are subvertical near the top and less
344
steeply dipping with depth, dipping to the southeast, and the beds young to the south.
345
In the bedded clastic facies, faults can be marked by juxtaposition of unrelated
346
lithologies (e.g. granite against hematite-rich bedded clastic facies in RD3449 at 590 m, in
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the northern bedded clastic facies), abrupt changes in bedding orientation (Fig. 10A), and/or
348
one- to several-metres of fractured and closely jointed core associated with strongly hematite-
349
and sericite-altered zones. Some faults have been intruded by mafic dykes (Fig. 10B).
350
Fracture zones on cm-scale may be marked by rotated angular clasts healed with minerals
351
such as hematite, barite and siderite (Fig. 10C). Narrow fractures (<5 cm) also displace beds
352
by mm to cm (Figs 8B, 10C).
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Many internal faults are parallel to bedding or at 90⁰ to bedding (Fig. 9C). Early
354
northeast-striking faults separate the facies associations in the southern bedded clastic facies
355
and are possibly offset along northwest-striking transfer faults. Northeast-striking faults
356
define the boundaries of the green sandstone and mudstone in the western outlier. Northwest-
357
striking faults truncate the northern bedded clastic facies in the southwest and the northeast
358
(Fig. 2). Faults striking northwest and west also displace the facies associations within the
359
northern bedded clastic facies. Steep east-west striking faults separate the southern bedded
360
clastic facies from the ODBC (Fig. 2).
361
7. Composition of the Bedded Clastic Facies
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As predictable from the componentry, the green sandstone and mudstone facies
363
association differs in composition from the interbedded sandstone and red mudstone facies
364
association. The green sandstone and mudstone has higher Ti, Mg, Mn, Li, Cr, Sc, Ni and V
365
(Fig. 11, Supplementary Data). Most green sandstone and mudstone samples also contain
366
more abundant P2O5 (>0.2 wt.%) and have a higher volatile content (LOI) than samples of the
367
interbedded sandstone and red mudstone. Most of the green sandstone and mudstone samples
16 Page 16 of 47
have TiO2 >0.68 wt.% and >300 ppm Cr. Sandstone from the polymictic volcanic-clast
369
conglomerate, sampled from the western part of the southern bedded clastic facies, has a
370
similar composition to the green sandstone and mudstone.
371
8. Zircon Geochronology
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The well-sorted quartz-rich sandstone facies contains four populations of zircons. The
best defined populations have ages of ~1600 Ma and ~1740 Ma; less pronounced peaks occur
374
at ~1900 Ma and 2500 Ma (Fig. 12A).
cr
373
All zircons sampled from felsic volcanic clasts in the polymictic volcanic conglomerate
376
and from sandstone of both the green sandstone and mudstone and interbedded sandstone and
377
red mudstone display a cluster of ages around 1600 Ma (Fig. 12B, C). Jagodzinski (2014)
378
reported a SHRIMP U-Pb zircon age of 1592 +/- 4 Ma for sandstone from the green
379
sandstone and mudstone facies association in the southern bedded clastic facies. A few
380
younger and older ages are present in these samples but cannot be considered reliable.
381
Zircons from granite clasts in the polymictic volcanic-clast conglomerate have the same age.
382
9. Discussion
383
9.1 Depositional processes
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Planar laminated to thinly bedded mudstone is an important and locally dominant facies
385
in the bedded clastic facies. It accounts for ~35% of the interbedded sandstone and red
386
mudstone, ~55% of the green sandstone and mudstone, and ~75% of the thinly bedded green
387
and red mudstone. The fine grain size and planar beds indicate subaqueous deposition from
388
suspension and dilute muddy turbidity currents (e.g., Stow, 1986). The presence of pristine
389
bubble-wall shards in the felsic tuffaceous mudstone is best accounted for by settling from
390
suspension in a quiet subaqueous setting; shards deposited on land are abraded and mixed
391
with other components, or entirely destroyed (e.g., Fisher and Schmincke, 1984).
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392
Many of the sandstone beds that are present in all five facies associations are massive or normally graded, and dominantly tabular. These characteristics indicate deposition from
394
relatively dilute sandy turbidity currents (e.g., Lowe, 1982). Some sandstone beds in the
395
interbedded sandstone and red mudstone facies association are cross bedded (cross-bed sets
396
<5 cm thick; Fig. 3C, 4A), consistent with traction deposits related to waning turbidity
397
currents. The very thick, poorly sorted beds of polymictic volcanic-clast conglomerate imply
398
rapid deposition from high-particle concentration gravity currents such as debris flows (e.g.,
399
Lowe, 1982) and could indicate within-channel settings.
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The bed characteristics of the brecciated well-sorted quartz-rich sandstone facies are
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not preserved though the presence within clasts of cross-bed sets in the order of 8-10 cm
402
thick, the good sorting and rounding, and the absence of interbedded mudstone all imply
403
deposition of this facies involved sandy traction currents; such currents operate most
404
commonly in shallow subaqueous (above wave base; e.g., Elliott, 1986) or fluvial-alluvial
405
settings (e.g., Collinson, 1986). The relationship between this shallow or subaerial setting
406
and the below-wave base setting indicated by the rest of the bedded clastic facies has not
407
been resolved.
408
9.2 Provenance of the bedded clastic facies
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The five facies associations have distinctive components that reflect derivation from
410
different sources. A mafic igneous provenance is apparent for the green sandstone and
411
mudstone facies association. It contains Cr-spinel, pseudomorphs of olivine and abundant
412
mafic volcanic clasts. The high abundances of Sc, Cr, V and Ti are also consistent with a
413
mafic igneous provenance. The elevated Cr is caused by the presence of chrome spinel.
414
Analyses of chrome spinel (McPhie et al., 2011) separated from the green sandstone and
415
mudstone facies association at Olympic Dam indicated affinity with a wide variety of mafic
18 Page 18 of 47
416
igneous magmas. The small proportion of volcanic quartz and zircon indicates that a felsic
417
volcanic source also contributed to this association.
418
Sandstone of the interbedded sandstone and red mudstone has a dominant granitoid provenance. It contains abundant granitoid-sourced quartz, bands of heavy minerals including
420
hematite, rutile and zircon, and granite clasts (Fig. 3E, 4A-D). This association has higher Si,
421
Fe, As, Sb and Ba than the green sandstone and mudstone, but lower Ti, Sc, V, and Cr,
422
reflecting the stronger influence of felsic rocks and Fe-oxides in the source region. A felsic
423
volcanic source provided a small proportion of volcanic quartz to this association. The
424
zircons separated from samples of sandstone have an age of ~ 1600 Ma (Fig. 12B). Both the
425
Hiltaba Suite granites and/or the felsic volcanic units of the Gawler Range Volcanics, are
426
consistent with this age and are probable source rocks for zircon, but the overwhelming
427
abundance of granitoid-derived quartz implies that Hiltaba Suite granites (rather than
428
volcanic rocks) were the major source. Although much of the hematite in this association is
429
likely to be hydrothermal, some of it is detrital (Fig. 4A-D), and indicates that the source area
430
included ironstone.
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The feldspar-phyric volcanic clasts in the polymictic volcanic-clast conglomerate have
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432
textures, compositions and ages that indicate derivation from the felsic Gawler Range
433
Volcanics. The mafic volcanic clasts probably also came from the Gawler Range Volcanics,
434
but there are no age data available for the mafic clasts. The very minor proportion of granitic
435
clasts in the conglomerate is likely to come from Hiltaba Suite granites.
436
The well-sorted quartz-rich sandstone has zircon populations (Fig. 13A) that reflect
437
derivation from Gawler SLIP sources as well as older basement sources. The ~1740 Ma
438
population is the same age as metamorphic and granitoid units exposed in the northern and
439
southern Gawler Craton (e.g., Mount Woods Domain, Nawa Gneiss, Peak and Denison
440
Domain; Hand et al., 2007) and the ~2500 Ma population matches the age of the Sleaford and
19 Page 19 of 47
Mulgathing complexes (Hand et al., 2007). The high abundance of metamorphic- and
442
granitoid-derived quartz in this facies is consistent with the provenance indicated by the
443
zircon populations. The well-sorted quartz-rich sandstone is part of the ODBC but because it
444
has been found in two drill holes only and it is not interbedded with any of the other facies
445
associations, its significance has yet to be fully explained.
The provenance of very fine grains that dominate the thinly bedded green and red
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441
mudstone is diverse. The felsic tuffaceous mudstone would have been generated by
448
contemporaneous eruptions because delicate glass shards typically have very poor
449
preservation potential in surface environments. The shard shapes and association with quartz
450
fragments (Fig. 8C, D) indicate that the eruption was explosive, magmatic-volatile driven
451
(rather than phreatomagmatic), and probably rhyolitic (e.g., Fisher and Schmincke, 1984).
452
The uniformly fine grain size and good sorting imply that the shards and crystal fragments
453
settled from an ash cloud and that the Olympic Dam depositional site was far (tens, possibly
454
hundreds km) away from the source vent(s). In contrast, detrital chromite and magnetite in
455
some mudstone beds have a mafic igneous source. Some facies in this association are
456
unlikely to be detrital, and instead possibly consist of chemical and/or hydrothermal
457
components, especially the siliceous mudstone, ironstone and barite laminae; Oreskes and
458
Einaudi (1990) also suggested a hydrothermal origin for these components.
459
9.3 Depositional setting – a fault-bounded sedimentary basin at Olympic Dam
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The associations in the southern bedded clastic facies are mappable and continuous
461
through the area delineated as three separate maar-diatreme centers by previous workers (Fig.
462
2A, V3, V4, V5; Reeve et al., 1990; Johnson and Cross, 1995). One other interpreted maar-
463
diatreme center (Fig. 2 inset,V2) comprises faulted intervals of the green sandstone and
464
mudstone facies and the polymictic volcanic-clast conglomerate that may originally have
465
been continuous with the same facies associations in the rest of the southern bedded clastic
20 Page 20 of 47
facies ~200 m to the east. The V1 interpreted maar-diatreme center (Fig. 2A) consists of
467
intervals of polymictic volcanic-clast conglomerate and thinly bedded green and red
468
mudstone facies association that have faulted contacts with the ODBC, and appears to be a
469
down-dropped fault block. Although disrupted, it is clear that at least four of the associations
470
are interbedded; facies boundaries are gradational but distinct, and the associations are not
471
mixed as might be expected from deposition in one or more small closed craters.
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The dominant facies of the bedded clastic facies indicate a predominantly low-energy, below wave base subaqueous depositional setting. The polymictic volcanic-clast
474
conglomerate is consistent with such a setting but reflects much higher energy depositional
475
event(s). This setting extended across the area now occupied by the bedded clastic facies in
476
the southern mine area at Olympic Dam and probably extended farther because the contacts
477
of the bedded clastic facies with the ODBC are faults and the top is the unconformity with the
478
overlying Neoproterozoic sedimentary formations. It is also clear that the bedded clastic
479
facies includes components from outside the Olympic Dam area. Confirming the presence of
480
remotely sourced components is an important argument against the former presence of maar
481
craters within which mainly locally sourced sediment would be likely to accumulate. By
482
“local”, we mean within the area now occupied by the ODBC.
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483
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McPhie et al. (2011) concluded that two components in the bedded clastic facies,
484
chromite and volcanic quartz, have no recognised local sources. The wide range in
485
composition of chromite grains in the green sandstone and mudstone facies association
486
implies that either (1) multiple sources in different tectonic settings contributed the chromite
487
grains to the Olympic Dam bedded clastic facies, or (2) the source was local but the tectonic
488
setting changed in a complex fashion over a short period of time (e.g., chromites in Mount
489
Etna lavas reflect a complicated succession of tectonic settings operating in less than 1
490
million years; Kamenetsky and Clocchiatti, 1996), or (3) chromite compositions are not
21 Page 21 of 47
useful as provenance indicators in ancient cratonic settings. The second alternative would
492
appear untenable given current understanding of the tectonic events affecting the Gawler
493
Craton in the Mesoproterozoic (e.g., Hand et al., 2007). The third alternative is also rejected
494
because there is no reason why the well-understood controls on chromite compositions in
495
modern settings (e.g., Kamenetsky et al., 2001) would not operate in ancient settings. Hence,
496
the first alternative remains the most plausible explanation of the chromite compositions
497
found in the bedded clastic facies at Olympic Dam. Furthermore, although the green
498
sandstone and mudstone facies association has a strong mafic volcanic provenance, and
499
records contemporaneous mafic volcanism, the overall fine grain size and the well-developed
500
tabular bedforms are not consistent with a vent-proximal depositional setting.
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491
The presence of volcanic quartz in the bedded clastic facies at Olympic Dam (McPhie
502
et al., 2011) can now be understood in the light of the recognition of quartz-bearing rhyolitic
503
ash (felsic tuffaceous mudstone facies, Fig. 8D, C) in the thinly bedded green and red
504
mudstone facies association. The ash was probably erupted far from Olympic Dam and
505
transported there in a regional ash cloud. Such ash clouds deposit fine pyroclasts in all
506
surface environments over very large areas (tens to hundreds of km2; e.g., Fisher and
507
Schmincke, 1984). Although the felsic tuffaceous mudstone is present as discrete beds only
508
in the thinly bedded green and red mudstone association, the small proportion of fine
509
volcanic quartz grains found in all the other associations is also likely to be pyroclastic in
510
origin and from a remote source.
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Intact felsic Gawler Range Volcanics units do not occur within the ODBC though such
512
units were probably originally present above the Roxby Downs Granite. The polymictic
513
volcanic-clast conglomerate is dominated by well-rounded clasts of felsic Gawler Range
514
Volcanics that must have been rounded outside the present area of occurrence of the
515
conglomerate because this and other bedded clastic facies mainly record below-wave-base
22 Page 22 of 47
516
subaqueous settings (where rounding does not occur). We therefore conclude that the source
517
of the well-rounded clasts of felsic Gawler Range Volcanics was outside the local area.
518
The Paleoproterozoic and Archean zircons in the well-sorted quartz-rich sandstone (Fig. 12A) also suggest that the provenance of the bedded clastic facies extended beyond the
520
local Olympic Dam area. Paleoproterozoic units (mainly Wallaroo Group; Skirrow et al.,
521
2007) are widespread to the north, east and south of Olympic Dam (Fig. 1) and the nearest
522
occurrences are less than 10 km away. However, the nearest Archean rocks are about ~30 km
523
west of Olympic Dam (Mulgathing Complex; Fraser et al., 2010).
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519
Further constraints on the depositional setting are provided by structural analysis of the
525
bedded clastic facies. The beds of the bedded clastic facies were deformed prior to complete
526
lithification because there is no associated cleavage, and the shapes and sizes of mesoscopic
527
folds are highly variable. In addition, no fold-forming deformation events are known to have
528
affected this part of the Gawler Craton since the Paleoproterozoic. Bed orientations in the
529
northern bedded clastic facies are consistent with the presence of gently northeast-plunging
530
folds (Fig. 9A); a similar though less well-defined relationship exists in the southern bedded
531
clastic facies (Fig. 9B). Given the evidence for the deformation being pre-lithification, the
532
fold axis orientation could be defining the strike of the original depositional slope, down
533
which unstable portions of the bedded clastic facies eventually slid. Down-slope soft-
534
sediment slumping does not fully account for the deformation of the bedded clastic facies, in
535
particular, the sections of sub-vertical beds and the ~20-30o plunge on the axis of the slump
536
folds; tectonic deformation focussed on faults probably contributed, also prior to complete
537
lithification. Nevertheless, the fold axis orientation could be indicating that the bedded clastic
538
facies accumulated on a paleoslope with a northeasterly strike; the dip direction (northwest or
539
southeast) is unknown. This strike direction is parallel to the mapped northeast-striking faults
540
known to have been present when the Olympic Dam hydrothermal system was active
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23 Page 23 of 47
(Hayward and Skirrow, 2010; Ehrig et al., 2012). Several authors have inferred the presence
542
of large northwest- or west-northwest-striking faults at Olympic Dam (e.g., O’Driscoll, 1986;
543
Roberts and Hudson, 1983; Reeve et al. 1990; Cross et al., 1993; Hayward and Skirrow,
544
2010). We propose that the depocenter within which the bedded clastic facies accumulated
545
was fault-controlled and bounded by a combination of northeast-striking and northwest-
546
striking faults (Fig. 13). The northwest-striking master fault orientation shown in Figure 13 is
547
not constrained by our data; this strike direction is consistent with the orientation of both
548
local and regional faults (Cross et al., 1993; Hayward and Skirrow, 2010) and of a
549
continental-scale lineament upon which the Olympic Dam deposit is located (O’Driscoll,
550
1986).
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Thick sections of the Gawler Range Volcanics occur at Acropolis and Wirrda Well,
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551
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541
~25 km southwest and ~25 km south of Olympic Dam, respectively, but no fully intact
553
sections of the Gawler Range Volcanics occur at Olympic Dam. Clasts derived from the
554
Gawler Range Volcanics are locally abundant in breccia of the ODBC, and are the major
555
component in the polymictic volcanic-clast conglomerate facies of the bedded clastic facies.
556
In general, the bedded clastic facies occur immediately below the unconformity with the
557
Neoproterozoic formations. These relationships suggest that the Gawler Range Volcanics
558
were originally present at Olympic Dam and underlay the bedded clastic facies. Thick bedded
559
clastic facies are known from only one other section of the exposed Gawler Range Volcanics
560
(McAvaney and Wade, 2015), so special circumstances must have existed in the vicinity of
561
Olympic Dam for such a thick section of bedded clastic facies to accumulate. A fault-
562
controlled depocenter as has been inferred here on the basis of the structure and facies
563
characteristics of the bedded clastic facies, would also account for the local presence of thick
564
clastic facies in a region otherwise dominated by widespread, topography-building felsic
565
volcanic units.
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566
We conclude that the maar-diatreme setting for the bedded clastic facies is untenable and does not adequately account for their facies characteristics, facies geometry, provenance
568
and internal stratigraphy. The inference that a fault-bounded sedimentary depocenter existed
569
at Olympic Dam is not new. Roberts and Hudson (1983) first recognised the bedded clastic
570
facies and relied on these facies in framing their interpretation of Olympic Dam deposit as “a
571
very unusual type of sediment-hosted mineralisation” (p. 815). Although Roberts and
572
Hudson’s (1983) sediment-hosted interpretation of Olympic Dam is no longer accepted, other
573
authors have concluded that a fault-bounded basin existed at Olympic Dam (e.g., Oreskes and
574
Einaudi, 1990; Hayward and Skirrow, 2010).
575
9.4 Relationships of the bedded clastic facies to the ODBC and the Olympic Dam ore deposit
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Although most contacts between the bedded clastic facies and the ODBC are faults,
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576
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567
some contacts are gradational; intact but folded intervals of the bedded clastic facies grade
578
into brecciated bedded clastic facies that grades into hematite-rich breccia containing clasts
579
derived from the bedded clastic facies. These relationships indicate that the bedded clastic
580
facies were present at the time of the formation of the ODBC. Furthermore, although not
581
considered “ore”, the bedded clastic facies contain the same Cu sulfide and other
582
hydrothermal minerals as the ore zones. It thus appears that the bedded clastic facies pre-date
583
the ODBC and the Cu mineralising event. Given that the ODBC contains macroscopic clasts
584
of the bedded clastic facies, it is plausible that some fine-grained (<1 mm) components of the
585
breccia were also derived from the bedded clastic facies. In particular, some of the fine
586
hematite in the ODBC could be recycled from hematite-rich sandstone and mudstone in the
587
bedded clastic facies, partly accounting for the marked iron anomaly at Olympic Dam.
588
10. Conclusions
589 590
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The bedded clastic facies comprises five main facies associations that reflect different sources - a granitoid-dominated source (interbedded sandstone and red mudstone), a
25 Page 25 of 47
Paleoproterozoic and older basement provenance (well-sorted quartz-rich sandstone facies), a
592
strongly mafic volcanic source (green sandstone and mudstone), a mixed felsic-mafic
593
volcanic source (polymictic volcanic-clast conglomerate), and a partly hydrothermal and
594
partly volcanic source (thinly bedded green and red mudstone). Zircons from the granitoid
595
and felsic volcanic-derived facies indicate a ~1600 Ma age for these sources.
ip t
591
The dominant facies are planar bedded and relatively fine grained, suggesting
597
deposition in a below-wave base subaqueous setting from suspension and dilute turbidity
598
currents. The five associations are distinct, four of them are interbedded, and at least three of
599
them are mappable across the southern mine area, implying that the original depocenter was
600
much larger than the area within which they are preserved. The internal stratigraphy and
601
characteristics of the bedded clastic facies are inconsistent with deposition in separate but
602
nearby maar craters.
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The two main areas of bedded clastic facies are strongly deformed but the absence of
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603
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596
cleavage and the disharmonic nature of mesoscopic folds suggest that the deformation
605
occurred prior to complete lithification. In these two areas, there is a consistent pattern of
606
gently northeast-plunging folds. The folds may have been produced by slumping of
607
unconsolidated sediment on a southeasterly or northwesterly dipping paleoslope in an
608
extensional basin bounded by northwesterly and northeasterly striking faults. The northern
609
and southern bedded clastic facies are down-faulted remnants of an originally more extensive
610
sedimentary succession. The bedded clastic facies were present when the ODBC formed and
611
Olympic Dam mineralising hydrothermal system was active. The fault-bounded sedimentary
612
basin that was present during mineralisation may have contributed to or otherwise influenced
613
the formation of the Olympic Dam ore deposit.
614
Acknowledgements
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26 Page 26 of 47
Chantelle Lower, Robert Scott, and Susan Belford provided assistance with the analysis of
616
drill core structural data. Discussions of our data and interpretations with Ron Berry, Ken
617
Cross and Sebastien Meffre are gratefully acknowledged. We thank Julie Hunt and an
618
anonymous reviewer for constructive comments that have helped improve the manuscript.
619
This research was funded by BHP-Billiton.
620
References
621 622 623
Agangi, A., Kamenetsky, V.S., McPhie, J., 2012. Evolution and emplacement of high-F rhyolites in the Mesoproterozoic Gawler silicic large igneous province, South Australia. Precambrian Research, v. 208-211, p. 124-144.
624 625 626
Allen, S.R., McPhie, J., 2002. The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A > 675 km3, compositionally zoned lava of Mesoproterozoic age. Geological Society America Bulletin, v. 114, p. 1592-1609.
627 628 629
Allen, S.R., McPhie, J., Ferris, G., Simpson, C., 2008. Evolution and architecture of a large felsic igneous province in western Laurentia: The 1.6 Ga Gawler Range Volcanics, South Australia. Journal of Volcanology and Geothermal Research, v. 172, p. 132-147.
630 631 632
Blissett, A.H., Creaser, R.A., Daly, S.J., Flint, R.B., Parker, A.J., 1993. Gawler Range Volcanics. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 106-124.
633 634
Collinson, J.D., 1986. Alluvial sediments. In: Reading, H.J. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, pp. 20-62.
635 636 637
Cowley, W.M., 1993. Cariewerloo Basin, Pandurra Formation. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 139-142.
638 639 640 641
Cross, K.C., Daly, S.J., Flint, R.B., 1993. Mineralisation associated with the GRV and Hiltaba Suite granitoids. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 132-138.
642 643 644
Direen, N.G., Cadd, A.G., Lyons, P., Teasdale, J.P., 2005. Architecture of Proterozoic shear zones in the Christie Domain, western Gawler Craton, Australia: Geophysical appraisal of a poorly exposed orogenic terrane. Precambrian Research, v. 142, p. 28-44.
645 646 647 648
Ehrig, K., McPhie, J., Kamenetsky, V., 2012. Geology and mineralogical zonation of the Olympic Dam Iron Oxide Cu-U-Au-Ag Deposit, South Australia. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.), Society of Economic Geologists, Inc., Special Publication 16, p. 237–267.
Ac ce p
te
d
M
an
us
cr
ip t
615
27 Page 27 of 47
Elliott, T., 1986. Siliciclastic shorelines. In: Reading, H.J. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, pp. 155-188.
651 652
Fanning, C.M., Flint, R.B., Preiss, W.V., 1983. Geochronology of the Pandurra Formation. Geological Survey of South Australia, Quarterly Geological Notes, Number 88, p. 11-16.
653 654 655
Fanning, C.M., Blissett, R.B., Parker, A.J., Ludwig, K.R., Blissett, A.H., 1988. Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb geochronology. Precambrian Research, v. 40-41, p. 363-386.
656 657
Fanning, C.M., Reid, A., Teale, G., 2007. A geochronological framework for the Gawler Craton, South Australia. South Australia, Geological Survey, Bulletin 55, pp. 258.
658 659
Fisher, R.V., Schmincke, H-U., 1984. Pyroclastic Rocks. Springer-Verlag Berlin Heidelberg, 472 pp.
660 661 662
Fraser, G., McAvaney, S., Neumann, N., Szpunar, M., Reid, A., 2010. Discovery of early Mesoarchean crust in the eastern Gawler Craton, South Australia. Precambrian Research, v. 179, p. 1-21.
663 664
Giles, C.W., 1988. Petrogenesis of the Proterozoic Gawler Range Volcanics, South Australia. Precambrian Research, v. 40-41, p. 407-427.
665 666 667
Groves, D.I., Bierlein, F.P., Meinert, L.D., Hitzman, M.W., 2010. Iron oxide copper-gold (IOCG) deposits through Earth history: Implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits. Economic Geology, v. 105, p. 641-654.
668 669
Hand, M., Reid, A., Jagodzinski, E., 2007. Tectonic framework and evolution of the Gawler Craton, southern Australia. Economic Geology, v. 102, p. 1377-1395.
670 671
Haynes, D.W., Cross, K.C., Bills, R.T., Reed, M.H., 1995. Olympic Dam ore genesis: A fluid mixing model. Economic Geology, v. 90, p. 281-307.
672 673 674 675 676 677 678 679
Hayward, N., Skirrow, R.G., 2010. Geodynamic setting and controls on iron oxide Cu-Au (±U) ore in the Gawler Craton, South Australia. In Porter, T.M. (Ed.), Hydrothermal Iron Oxide copper-Gold and Related Deposits: A Global Perspective, Volume 3. PGC Publishing, Adelaide, pp. 119-146.
680 681 682 683
Huang, Q., Kamenetsky, V.S., McPhie, J., Ehrig, E., Meffre, S., Maas, R., Thompson, J., Kamenetsky, M., Chambefort, I., Apukhtina, O., Yongbin Hu, 2015. Neoproterozoic (ca. 820-830 Ma) mafic dykes at Olympic Dam, South Australia: Links with the Gairdner Large Igneous Province. Precambrian Research, v. 271, p. 160-172.
Ac ce p
te
d
M
an
us
cr
ip t
649 650
Hitzman, M.W., Oreskes, N., Einaudi, M.T., 1992. Geological characteristics and tectonic setting of Proterozoic iron-oxide (Cu-U-Au-REE) deposits. Precambrian Research, v. 58, p. 241-287.
684
28 Page 28 of 47
Huang, Q., Kamenetsky, V.S., Ehrig, K., McPhie, J., Kamenetsky, M., Cross, K., Meffre, M., Agangi, A., Chambefort, I., Direen, N.G., Maas, R., Apukhtina, O., 2015 (submitted to Precambrian Research) Olivine-phyric basalt at Kokatha, Mount Gunson, Wirrda Well and Olympic Dam: insights into the petrogenesis of the Gawler silicic large igneous province, South Australia.
690 691 692
Jagodzinski, E., 2014. The age of magmatic and hydrothermal zircon at Olympic Dam. Geological Society of Australia, Abstracts No. 110, p. 260, Australian Earth Sciences Convention 2014, Newcastle, Australia.
693 694 695
Johnson, J.P., Cross, K.C., 1995. U-Pb geochronological constraints on the genesis of the Olympic Dam Cu-U-Au-Ag deposit, South Australia. Economic Geology, v. 90, p. 10461063.
696 697
Kamenetsky, V., Clocchiatti, R., 1996. Primitive magmatism of Mt Etna: Insights from mineralogy and melt inclusions. Earth and Planetary Science Letters, v. 142, p. 553-572.
698 699 700
Kamenetsky, V.S., Crawford, A.J., Meffre, S., 2001. Factors controlling chemistry of magmatic spinel: An empirical study of associated olivine, Cr-spinel and melt inclusions from primitive rocks. Journal of Petrology, v. 42, p. 655-671.
701 702 703
Lowe, D.R., 1982. Sediment gravity flows: II Depositional models with special reference to the deposits of high-density turbidity currents. Journal of Sedimentary Petrology, v. 52, p. 279- 297.
704 705
Johnson, J.P., McCulloch, M.T., 1995. Sources of mineralising fluids for the Olympic Dam deposit (South Australia): Sm-Nd isotopic constraints. Chemical Geology, v.121, p. 177-199.
706 707 708
McAvaney, S.O., Wade, C.E., 2015. Stratigraphy of the Lower Gawler Range Volcanics in the Roopena area, northeastern Eyre Peninsula. South Australia, Department of State Development, Report book 2015/00021.
709 710 711
McPhie, J., Kamenetsky, V.S., Chambefort, I., Ehrig, K., Green, N., 2011. Origin of the supergiant Olympic Dam Cu-U-Au-Ag deposit, South Australia: Was a sedimentary basin involved? Geology, v. 39, p. 795-798.
712 713
O'Driscoll, E.S.T., 1986. Observations of the lineament--ore relations. Philosophical Transactions of the Royal Society of London, Series A, v. 317, p. 195-218.
714 715 716
Oreskes, N., Einaudi, M.T., 1990. Origin of rare earth element-enriched hematite breccias at the Olympic Dam Cu-U-Au-Ag deposit, Roxby Downs, South Australia. Economic Geology, v. 85, p. 1-28.
717 718 719
Preiss, W.V., 1993. Neoproterozoic. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 171-203.
Ac ce p
te
d
M
an
us
cr
ip t
685 686 687 688 689
29 Page 29 of 47
Reeve, J.S., Cross, K.C., Smith, R.N., Oreskes, N., 1990. Olympic Dam copper-uraniumgold-silver deposit. In: Hughes, F.E., (Ed.), Geology of the Mineral Deposits of Australia and Papua New Guinea. The Australian Institute of Mining and Metallurgy, Melbourne, pp. 10091035.
724 725
Roberts, D.E., Hudson, G.R.T., 1983. The Olympic Dam copper-uranium-gold deposit, Roxby Downs, South Australia. Economic Geology, v. 78, p. 799-822.
726 727 728 729 730
Skirrow, R.G., Bastrakov, E., Raymond, O.L., Davidson, G., Heithersay, P., 2002. The geological framework, distribution and controls of Fe-Oxide Cu-Au mineralisation in the Gawler Craton, South Australia. Part II – Alteration and mineralisation. In Porter, T.M. (Ed.), Hydrothermal Iron Oxide copper-Gold and Related Deposits: A Global Perspective, Volume 2. PGC Publishing, Adelaide, pp. 33-47.
731 732 733 734
Skirrow, R.G., Bastrakov, E.N., Barovich, K., Fraser, G.L., Creaser, R.A., Fanning, C.M., Raymond, O.L., Davidson, G.J., 2007. Timing of iron oxide Cu-Au-(U) hydrothermal activity and Nd isotope constraints on metal sources in the Gawler Craton, South Australia. Economic Geology, v. 102, p. 1441-1470.
735 736
Stow, D.A.V., 1986. Deep clastic seas. In: Reading, H.J. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, pp. 399-444.
737 738 739
Swain, G., Hand, H., Teasdale, J.P., Rutherford, L., Clark, C., 2005. Age constraints on terrain-scale shear zones in the Gawler Craton, southern Australia. Precambrian Research, v. 139, p. 164–180.
740 741 742
Wingate, M. T. D., Campbell, I. H., Compston, W., Gibson, G. M., 1998. Ion microprobe U– Pb ages for Neoproterozoic basaltic magmatism in south-central Australia and implications for the breakup of Rodinia. Precambrian Research, v. 87, p. 135-159.
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30 Page 30 of 47
743 Figure 1. The regional geological setting of Olympic Dam in the Gawler Craton, South Australia,
745
showing the distribution of the major rock units according to ages. The top left inset shows the
746
location of the Gawler Craton in Australia. The top right inset shows the geological setting of
747
Olympic Dam in more detail. On the main map: N, Nawa Domain, W, Wallaroo Group, M,
748
Mugathing Complex, S, Sleaford Complex. On all maps: star, Olympic Dam. After Hand et al.
749
(2007).
750
Figure 2. Distribution and structure of the bedded clastic facies in the southern mine area (SMA) at
751
Olympic Dam. A. Map of the bedded clastic facies at the level of the unconformity level with the
752
Neoproterozoic sedimentary formations (~350 m below the surface). Drill holes examined in this
753
study are marked. Inset map shows the extent of the granite breccia and the mineralised hematite
754
breccia of the Olympic Dam Breccia Complex within the Roxby Downs Granite (“granite”), after
755
Reeve et al. (1990); SMA, southern mine area. The five maar-diatreme centers of Reeve et al. (1990)
756
and Johnson and Cross (1995) are in black on the inset map, and labelled V1 to V5. The positions of
757
V1, V3, V4 and V5 in relation to the newly defined distribution of the bedded clastic facies are shown
758
as grey circles on the main map. The northern and southern bedded clastic facies (BCF) are hosted in
759
quartz-hematite breccia within the mineralised hematite breccia. The location of the north-south
760
section through the southern bedded clastic facies is indicated by the grey line. B. North-south cross
761
section of the western part of the southern bedded clastic facies. The upper surface is the
762
unconformity surface between the Mesoproterozoic units and the overlying Neoproterozoic
763
sedimentary formations. C. Simplified log of underground drill hole RU38-2626 showing the
764
distribution of the different bedded clastic facies between hematite breccia to the east (top) and granite
765
breccia to the west (base). Recorded younging directions (bold triangles) and faults (bold lines) are
766
also shown.
767
Figure 3. Interbedded sandstone and red mudstone facies association. A. Interbedded sandstone and
768
red mudstone; RD2767, ~778 -782 m; drill core is 5 cm across. B. Load casts and flame (f) structures
769
at the bases of tabular sandstone beds, amalgamated sandstone beds and low-angle truncation of
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31 Page 31 of 47
mudstone beds (RD2767, ~777.5 m); s, sandstone, m, mudstone. The curved mudstone flames
771
indicate that loading and dewatering accompanied deposition of the sandstone. C. Cross-bedded
772
sandstone (top, cross-bed sets ~2 cm high, parallel to dashed lines) and coarse massive sandstone
773
(base); RU38-2626, ~337.7-339 m. D. Lenticular sandstone beds interbedded with laterally
774
continuous mudstone laminae (RD1625, 685.4 m). E. Photomicrograph of sandstone composed of
775
granite clasts (g) and abundant angular quartz grains (q); crossed nicols (RD1624, 626.1 m).
776
Figure 4. Interbedded sandstone and red mudstone facies association. A-D, RD3287, 375.6 m. A.
777
Dark cross laminae (e.g., dashed line) defined by fine hematite, titanium oxide and zircon (RD3287,
778
375.6 m; thin-section, plane-polarised light). Hematite-free domains (hfd) cut across these laminae. B.
779
Enhanced MLA image of A showing the association of hematite and zircon within the dark laminae.
780
Zircon grains have diverse shapes that possibly reflect different provenance. C and D, BSE images of
781
dark bands in A displaying abundant hematite and zircon. In D, hematite grains are blue and zircon
782
grains are red. The close association of hematite and zircon in mm-thick layers suggests that they have
783
been concentrated during sedimentation. E. Minor fold (arrow) in interbedded sandstone and red
784
mudstone (RD1625, 331.5 m). F. Photomicrograph (plane-polarised light) of the interbedded
785
sandstone and red mudstone (RD1989, 416 m). No cleavage is evident in the mudstone or the
786
sandstone.
787
Figure 5. Well-sorted quartz-rich sandstone facies. A. Simplified log of drill hole RD2751 showing
788
the context of the brecciated well-sorted quartz-rich sandstone (~848.7 to 928 m) in the ODBC. B.
789
Well-sorted quartz-rich sandstone from the topmost ~2 m of the section in RD2751. This interval is
790
either intact or a large clast in breccia. Note the diffuse cross bedding (RD2751, 852 m). C.
791
Photomicrograph (crossed nicols) of the well-sorted quartz-rich sandstone, showing abundant quartz
792
with undulose extinction; m, muscovite, f, foliated clast. D. Matrix-supported breccia composed of
793
clasts of the well-sorted quartz-rich sandstone facies (RD2751, 917-919 m). Note the cross-beds
794
(parallel to dashed line) within one clast and the mottled pale versus red matrix. E. Jigsaw-fit texture
795
in the brecciated well-sorted quartz-rich sandstone facies (RD2751, 881 m).
796
Figure 6. Green sandstone and mudstone facies association. A. Thinly bedded and laminated green
797
sandstone and mudstone (RD2821, 654.1 m). B. Thick bed of sandstone within the green sandstone
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32 Page 32 of 47
and mudstone facies association (RU38-2626, 285 – 290 m), displaying abundant green mudstone
799
intraclasts (arrow). C. Photomicrograph (plane-polarised light) of a basalt clast (bas) in the green
800
sandstone (RD1625, 389.5 m. Pseudomorphs of olivine phenocrysts (arrow) contain black chromite
801
grains. D. Close-up BSE image of chromite (Cr-Sp) and hematite grains in fine-grained lamina
802
(RD916, 410.5m). E. Reflected light image of pyrite (py) intergrown with euhedral quartz and minor
803
chalcopyrite (arrows) between clasts (RD1625, 636.8 m). Note laths of hematite in the left hand
804
corner and irregular hematite in the top right.
805
Figure 7. Polymictic volcanic clast-conglomerate from the northern bedded clastic facies (A; RD3449,
806
414 m), and the southern bedded clastic facies (B; RU38-2626, 373 m). Both display rounded
807
feldspar-phyric felsic volcanic (fv) and mafic volcanic (mv) clasts. Clasts are variably altered and
808
some have irregular margins. The conglomerate is clast- to matrix-supported. The matrix consists of
809
granule conglomerate to coarse sandstone mainly composed of altered felsic and mafic volcanic
810
clasts, together with minor granitic quartz and feldspar; olivine pseudomorphs, zircon, tourmaline and
811
rutile are also present in the matrix.
812
Figure 8. Thinly bedded green and red mudstone facies association. A. Laminated red, purple and
813
grey mudstone from the northern bedded clastic facies (RD3449, 462 m). B. Microfaults in the thinly
814
bedded green and red mudstone from the northern bedded clastic facies (RD3449, 431.4 m).
815
Hematite-rich chert laminae are bright red (arrows). C. Reflected light image of red chert lamina
816
which consists mainly of quartz (blue) and very fine hematite (bright) (RD3449, 431.4 m); pores are
817
red. The red colour of these laminae is imparted by the very fine hematite. D, E. Bubble-wall shards
818
in felsic tuffaceous mudstone; combination of plane polarised and reflected light image; RD3449,
819
429.8 m. Quartz crystal fragments (D) suggest the felsic tuffaceous mudstone has a rhyolitic
820
composition.
821
Figure 9. Structural data for the bedded clastic facies areas. A. Stereoplot of the bedding data from the
822
northern bedded clastic facies (RD3450, inverted triangles; RD2765, solid dots; drill holes
823
orientations, solid squares). The poles to bedding define a fold with an overall plunge of 24° towards
824
033°; the star is the pole to the best-fit great circle. B. Stereoplot of the bedding data from the
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798
33 Page 33 of 47
southern bedded clastic facies; data from RD2766, RD2771, RD3287, RD3285 and RD 3426. The
826
poles to bedding roughly define a fold with an overall plunge of 28° towards 040°(star). C. Stereoplot
827
of the fault data from the southern bedded clastic facies (RD2766). Faults (triangles) are commonly
828
parallel to bedding (solid dots). Most have steep to subvertical dips and strike NE-SW. Some
829
orthogonal faults have NW-SE strikes. Squares are the drillhole orientation. Stereoplots were
830
completed using CoreSolutions and Openstereo software.
831
Figure 10. Fault morphologies in Olympic Dam drill core. A. Abrupt changes in bedding orientation,
832
fractures (arrow) and closely spaced joints associated with hematite- and sericite-altered zones
833
(RD1989, 377-381 m). B. Fault zone intruded by mafic dyke (dol) (RD2821, 603-607 m); baked
834
contact zone, arrow. C. Narrowly spaced fractures (<5 cm) displacing beds by millimetres to
835
centimetres and defining rotated angular clasts; fractures are healed with minerals such as hematite,
836
barite and siderite (RD2765, 462.3 m).
837
Figure 11. Selected major and trace element abundances for samples of the bedded clastic facies,
838
against TiO2 wt%. Compositional data are given in Supplementary Data Table 1.
839
Figure 12. A. Relative probability plot of zircon age data for 320 analyses of the well-sorted quartz-
840
rich sandstone facies that are less than 10 % discordant. This facies has the 1600 Ma population found
841
in other bedded clastic facies, in addition to populations at ~1740 Ma and 2500 Ma. B, C. Relative
842
probability plot of zircon age data for (B) the interbedded sandstone and red mudstone and green
843
sandstone and mudstone and (C) felsic clasts in the volcanic-clast conglomerate. Zircons in these
844
bedded clastic facies have ages around 1600 Ma. Age peaks and associated uncertainties have been
845
calculated using weighted mean age calculation.
846
Figure 13. Model for the fault-bounded, sedimentary basin in which the bedded clastic facies may
847
have accumulated. The master fault could belong to the northwest-striking regional fault set but its
848
location and orientation are not known. The pre-lithification folds in the bedded clastic facies
849
probably developed in response to remobilization down slopes orthogonal to the strike of the master
850
fault. Both the size and the orientation of the depocenter are uncertain.
Ac ce p
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825
34 Page 34 of 47
Figure1
134 °E
Ga
132 E
138 °E
Crato n
ler w
30 °S
c. 1590 Ma Gawler Range Volcanics c. 1590 Ma Hiltaba Suite
ip t
c. 1850-1750 Ma metasedimentary units
Po rt A ug ust a
1850 Ma Donington Suite pre-1850 Ma A ust ralia
Fault 0
Olympic Dam
100 km 36 °S
cr
A delaide
us
50 km
te
d
M
an
30 °S
Neoproterozoic to recent basins
Ac ce p
1595–1575 Ma Hiltaba Suite
1590 Ma Gawler Range Volcanics 1625–1615 Ma St Peter Suite 34 °S
1690–1670 Ma Tunkillia Suite 2000–1730 Ma metasedimentary units 1850 Ma Donington Suite
Late Archean (c. 2550 Ma) and Paleoproterozoic (1850–1650 Ma) 134 E°
138 E°
Page 35 of 47
Figure 2
RD 106 RU3 1 60 18 RU3 1 6020
ip t
RD2765 RD3449 RD3450
RD2768 RU38 2626 RU38 2625
RD2749
an
RD2767
RD3426 RD9 16
M
RD 1624 RD2766
RD 1989
RD3287 RD3285
te
d
RD282 1
RD 1625
RD 1627
RD896
Ac ce p
S
RD2767
RD2766
us
cr
RD 1680
30 °27 ’S 136 °54 ’E
N
RD2768
RU38_2626 270 m
RD896
,- ./01brecc ia Inte rbedded sa ndsto ne a nd red mudsto ne
RU38#2625 RU38#2626
HI JK LMNON vo lca nic c last co nglo me rate
G ree n sa ndsto ne a nd mudsto ne
m 0 10
G ra nite brecc ia
100 m 550 m
Page 36 of 47
Figure
A
us
cr
ip t
5 cm
m
E
s
E
ce Ac
D
1 cm
pt
1 cm
ed
M
f
C
an
B
H
q
g 0.5 mm
1 cm
Figure 3 – 2-column
Page 37 of 47
Figure 3
A
Hem
hfd
cr
Zr
ip t
B
Zr
us
0.5 mm
D
ed
M
an
C
100 mm
pt
200 mm
F
Ac
ce
E
5 cm
1 mm
Figure 4 – 2-column
Page 38 of 47
A
B
Hematite breccia
ip t
Thinly bedded red and green mudstone
cr
Polymictic breccia
an
us
Brecciated wellsorted quartz-rich sandstone
Granite breccia
M
Polymictic breccia
ed
100 m
C
m
f
Ac
ce
D
pt
200 mm
E
Figure 5 – 2-column
Page 39 of 47
B
ip t
A
cr
1 cm
an
us
C
M
bas
5 cm
ed
0.2 mm
Fe-ox
Barite
ce
Ac
py
pt
E
D
Cr-Sp
0.1 mm
100 mm
Figure 6 - 2-column
Cr q
Page 40 of 47
A
fv
cr
cm
ip t
mv
an
us
fv
M
B
mv
Ac
ce
pt
1cm
ed
fv
Figure 7 – 1.5-column
Page 41 of 47
A A
ip t
D
cr
5 cm
100 mm
ed
M
an
us
E
B
Bubble-wall shard
Quartz
100 mm
cm
Ac
ce
pt
C
Figure 8 - 2-column 100 mm
Page 42 of 47
an
us
cr
ip t
A
ce
pt
ed
M
B
Ac
C
Figure 9 – 1-column Page 43 of 47
ip t
A
B
us
cr
FZ
FZ
5 cm
Ac
ce
pt
ed
C
M
an
Dol
5 cm
1 cm
Figure 10 – 2-column
Page 44 of 47
Na2O wt%
MgO wt%
Cr ppm
V ppm
ce
pt
ed
P2O5 wt%
M
an
us
cr
ip t
Al2O3 wt%
Ac
TiO2 wt%
TiO2 wt%
Green sandstone and mudstone
Thinly bedded green and red mudstone
Interbedded sandstone and red mudstone
Sandstone matrix of polymictic volcanic-clast conglomerate
Figure 11 - 2-column
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Well-sorted quartz-rich sandstone
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N=139 analyses
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Sandstone and red mudstone, and green sandstone and mudstone
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Figure 12 - 1-column B
Felsic clasts N=24 analyses
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NW
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Figure 13
Bedded clastic facies
Mudstone and sandstone
Gawler Range Volcanics
Well-sorted quartz-rich sandstone Polymictic volcanic-clast conglomerate
Hiltaba Suite Granite Paleoproterozoic and older
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S0301-9268(16)00050-4 http://dx.doi.org/doi:10.1016/j.precamres.2016.01.029 PRECAM 4448
To appear in:
Precambrian Research
Received date: Revised date: Accepted date:
16-7-2015 27-1-2016 27-1-2016
Please cite this article as: McPhie, J., Orth, K., Kamenetsky, V., Kamenetsky, M., Ehrig, K.,Characteristics, origin and significance of Mesoproterozoic bedded clastic facies at the Olympic Dam Cu-U-Au-Ag deposit, South Australia, Precambrian Research (2016), http://dx.doi.org/10.1016/j.precamres.2016.01.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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3 to 5 bullet points (maximum 85 characters, including spaces, per bullet point) Five bedded clastic facies associations have distinct composition and grain size.
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Bedded clastic facies contain zircons dated at 1600 Ma, 1740 Ma, 1900 Ma and 2500 Ma.
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Pre-lithification folds have gently northeasterly plunging fold axes.
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Structure and facies suggest deposition in a fault-controlled sedimentary basin.
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Characteristics, origin and significance of Mesoproterozoic bedded clastic facies at the
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Olympic Dam Cu-U-Au-Ag deposit, South Australia
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Jocelyn McPhiea, Karin Ortha, Vadim Kamenetskya, Maya Kamenetskya, Kathy Ehrigb
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7001, Australia
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School of Physical Sciences and CODES, University of Tasmania, Private Bag 79, Tasmania
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BHP Billiton Olympic Dam, Adelaide, South Australia 5001, Australia
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Jocelyn McPhie
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Karin Orth
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Vadim Kamenetsky < [email protected]>
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Maya Kamenestky
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Kathy Ehrig
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Corresponding author
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Jocelyn McPhie
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Abstract
Mesoproterozoic bedded clastic facies occur in the Olympic Dam Breccia Complex
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which hosts the Olympic Dam Cu-U-Au-Ag deposit, South Australia. Contacts of the bedded
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clastic facies with breccia of the Olympic Dam Breccia Complex are faulted and/or
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brecciated; fragments of the bedded clastic facies occur in the breccia complex. The bedded
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clastic facies comprise five main facies associations, four of which are organised into
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mappable units. These associations have distinct textures and components and have not been
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mixed. Major sources of detritus were felsic and mafic volcanic units and granitoids of the
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~1590 Ma Gawler Silicic Large Igneous Province. Archean and Paleoproterozoic zircons in
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well-sorted quartz-rich sandstone indicate that older Gawler Craton basement successions
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also contributed sediment to the bedded clastic facies. Accumulation of the bedded clastic
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facies was at least partly contemporaneous with rhyolitic explosive eruptions from vent(s)
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probably tens of km away because bubble-wall shards are present in tuffaceous mudstone
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facies. Deformation of the bedded clastic facies prior to final lithification produced folds that
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have diverse shapes and sizes and lack cleavage. The folds have a reasonably consistent
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gentle plunge to the northeast. These soft-sediment folds could be related to slumping on a
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northeast-striking paleoslope that dipped either to the northwest or southeast. The depocenter
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in which these facies accumulated may have been bounded by a combination of northeast-
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striking and northwest-striking faults. Because the bedded clastic facies occur at the top of
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the Olympic Dam Breccia Complex and contributed clasts to it, the fault-controlled
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sedimentary basin was in place when the breccia complex and the Olympic Dam ore deposit
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formed.
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Key words: Olympic Dam, bedded clastic facies, provenance, pre-lithification fold,
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sedimentary basin, Mesoproterozoic
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1. Introduction
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The supergiant Olympic Dam Cu-U-Au-Ag deposit is famous for its size and
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polymetallic character (e.g., Ehrig et al., 2012), and for being one of the deposits upon which
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the iron oxide-copper-gold (IOCG) ore deposit class is based (Hitzman et al., 1992). When
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first discovered but prior to major underground development, the breccia-dominated host
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succession to the deposit was interpreted to be largely sedimentary and was described in
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terms of two formations that were further divided into members (Roberts and Hudson, 1983).
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Extensive underground exposure and further drilling led to the recognition that the clastic
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host rocks are part of a breccia complex formed mainly by a combination of hydrothermal
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and tectonic processes (Olympic Dam Breccia Complex, ODBC; Reeve et al., 1990). Reeve
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et al. (1990) also reported “volcaniclastic rocks in diatreme structures” associated with mafic
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dykes and showed five such diatremes on their map (Reeve et al., 1990, Figure 2(a)). The
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diatremes were interpreted as the roots of maar volcanoes where hydrothermal and
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phreatomagmatic eruptions had taken place. Even though the mapped diatremes occupy less
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than 2% of the total area of the ODBC (~17 km2), Reeve et al. (1990) attributed much of the
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brecciation to eruption-related decompression. Domains of well-bedded, mostly fine-grained
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lithologies in the ODBC were thought to be “crater facies epiclastics”. The maar-diatreme
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interpretation has been developed and/or repeated in several subsequent papers on Olympic
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Dam and has been incorporated in genetic models for Olympic Dam and for IOCG deposits
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in general (e.g., Cross et al., 1993; Haynes et al., 1995; Johnson and Cross, 1995; Skirrow et
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al., 2002; Groves et al., 2010; Hayward and Skirrow, 2010).
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McPhie et al. (2011) proposed that the well-bedded, mostly fine-grained clastic
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lithologies in the ODBC were the remnants of a Mesoproterozoic sedimentary basin, using
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data on facies characteristics and relationships, and on the clastic components. Here we
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provide new data on the facies, stratigraphy, age and structure of the bedded clastic facies at
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Olympic Dam. Our data come from a selection of the ~1000 drill holes drilled between 2003
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and 2008 in the eastern two-thirds of the deposit; these drill holes are the most recent
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available. They represent ~60% of the total number of holes, and of the total meters drilled, at
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Olympic Dam, and have added substantially to knowledge and understanding of the deposit
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and its setting (Ehrig et al., 2012). These data are used to argue that the bedded clastic facies
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were deposited in a fault- bounded sedimentary basin. The presence of mappable internal
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stratigraphy cannot be accommodated by deposition of the bedded clastic facies in the
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isolated maar craters interpreted by Reeve et al. (1990). Because the fault-bounded basin was
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present when the Olympic Dam ore deposit formed, this setting ought to be considered in
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models of ore genesis.
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2. Regional Geological Setting of Olympic Dam Olympic Dam is on the eastern edge of the Gawler Craton in South Australia (Fig. 1).
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The Gawler Craton comprises Meso- to Neoarchean complexes surrounded by
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Paleoproterozoic to Mesoproterozoic successions (Swain et al., 2005; Fanning et al., 2007;
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Fraser et al., 2010). In the southern and eastern Gawler Craton, the Paleoproterozoic
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sedimentary and igneous units were deformed during the Cornian Orogeny (around 1850 Ma,
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Hand et al., 2007), and the Kimban Orogeny (1730 – 1690 Ma, Hand et al., 2007).
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Voluminous igneous units dominate the Mesoproterozoic successions of the Gawler Craton. In the central and eastern Gawler Craton, the principal units are the Hiltaba Suite
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(1595-1575 Ma; Hand et al., 2007) and the Gawler Range Volcanics (~1591 Ma, Fanning et
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al., 1988) which together constitute the Gawler Silicic Large Igneous Province (SLIP; Allen
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et al., 2008). The lower Gawler Range Volcanics include mafic and felsic lavas, and felsic
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ignimbrites and other pyroclastic facies (Giles, 1988; Blissett et al., 1993; Allen et al., 2008;
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Agangi et al., 2012). The upper Gawler Range Volcanics are dominated by three large-
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volume rhyolite lavas (Allen and McPhie, 2002; Allen et al., 2008).
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The Gawler Range Volcanics and younger units are flat-lying or gently dipping and
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have not been metamorphosed. Regional deformation roughly synchronous with the Gawler
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SLIP (1590-1570 Ma, Hand et al., 2007) was limited to movement on faults and shear zones
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in response to northwest-southeast directed shortening. Similarly, younger deformation
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events (Kararan Orogeny, 1570-1540 Ma, Hand et al., 2007; Coorabie Orogeny, 1470-1450
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Ma, Direen et al., 2005; Hand et al., 2007) involved reactivation of fault and shear zones,
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mainly in the northern and western parts of the Gawler Craton.
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The eastern Gawler Craton in the region of Olympic Dam is overlain by the Pandurra Formation which is composed of continental sedimentary rocks (Cowley, 1993) that are
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neither deformed nor metamorphosed. The age estimate of the Pandurra Formation is
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1424±51 Ma (Rb-Sr date, Fanning et al., 1983). The Pandurra Formation is overlain by
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Neoproterozoic to Cambrian sedimentary rocks of the Stuart Shelf (Preiss, 1993). East of the
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Torrens Hinge Zone, the sedimentary formations belong to the Adelaide Geosyncline and
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they are markedly thicker. The Gairdner Dyke Swarm comprises a 180-250 km-wide,
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northwest-striking group of mafic dykes which intruded the eastern and northern Gawler
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Craton around 825 Ma (Wingate et al., 1998).
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3. Geology of Olympic Dam
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The Olympic Dam Cu-U-Au-Ag ore deposit occurs in the Olympic Dam Breccia
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Complex (ODBC) which is surrounded by and has gradational contacts with the Roxby
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Downs Granite (Reeve et al., 1990). The Roxby Downs Granite has an age of 1594+/- 5 Ma
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(U-Pb in zircon, Jagodzinski, 2014) and is part of the Burgoyne Batholith, a member of the
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Hiltaba Suite; it is surrounded by the Paleoproterozoic Hutchison Group, Donington Suite
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and the Wallaroo Group. Both the Roxby Downs Granite and the ODBC are unconformably
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overlain by 300-350 m of Neoproterozoic and Cambrian Stuart Shelf sedimentary formations.
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The Gawler Range Volcanics have been logged in drillholes at nearby prospects (Wirrda
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Well, Snake Gully, Acropolis; all <25 km from Olympic Dam) but only a single unit of
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almost intact Gawler Range Volcanics has been identified at Olympic Dam (strongly altered
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olivine-phyric picrite; Ehrig et al., 2012). The Pandurra Formation is not present at Olympic
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Dam, but it occurs to the west, north and south.
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The main protolith of the ODBC was the Roxby Downs Granite and granite clasts are weakly to intensely hematite-altered clasts (Reeve et al., 1990). In the granite-rich breccia,
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granite clasts are weakly to moderately altered whereas in the hematite-rich breccia, the
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granite clasts are strongly to intensely altered and a hematite-rich, fine-grained matrix is
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present (Reeve et al., 1990). The ore minerals (Cu sulfides and uraninite, coffinite and
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brannerite) are typically fine-grained and disseminated in the hematite-rich breccia of the
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ODBC. There is a gross zonation in the Cu sulfides from chalcopyrite at depth through
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bornite to chalcocite at the shallowest levels (Reeve et al., 1990).
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In the southern mine area, the ODBC also includes large domains of polymictic breccia
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composed of feldspar-phyric clasts of the Gawler Range Volcanics, and two areas of bedded
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clastic facies (Fig. 2A). The distribution of the bedded clastic facies shown in Figure 2A is
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constrained by intersections in 302 diamond drill holes. The northern area of bedded clastic
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facies is 600 m long, 370-380 m wide and 210 m in vertical extent below the unconformity
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with the Stuart Shelf sedimentary formations. It reaches the deepest point below the
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unconformity in the northwest where the basal contact slopes shallowly to the northwest. The
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southern area of bedded clastic facies is 750 m south of the northern bedded clastic facies. It
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comprises one main domain and nearby outliers to the northeast (northern outlier) and
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southwest (western outlier). The main domain of the southern bedded clastic facies is
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elongate east-west and is 1300 m long, up to 370-380 m wide and 700 m in vertical extent
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below the unconformity with the Stuart Shelf sedimentary formations. The contact of the
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southern bedded clastic facies with breccia of the ODBC is deepest at the western end and
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shallower to the east. Intervals of the bedded clastic facies have been internally deformed and
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they have both faulted and gradational contacts with the ODBC. At the gradational contacts,
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the bedded clastic facies are commonly brecciated and separated from hematite-rich breccia
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or granite-rich breccia by intervals composed of fragments of the bedded clastic facies in
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hematite-rich breccia or granite-rich breccia.
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Mafic dykes intrude the ODBC and the Roxby Downs Granite (Reeve et al., 1990). Relatively fresh plagioclase-pyroxene-phyric basalt and dolerite dykes belong to the ~825 Ma
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Gairdner Dyke Swarm (Huang et al., 2015). Variably altered olivine-phyric, aphyric and
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doleritic dykes are considered to be of GRV age (~1590 Ma; Johnson and McCulloch, 1995;
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Huang et al., in review). In the northern mine area, these dykes are intensely altered and
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commonly mineralised.
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data relating to five main sets of steeply dipping faults at OD, four of which (north-,
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northwest-, west-northwest-, and northeast-striking) were present before and/or during
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formation of the Olympic Dam deposit. Some of the northwest- and northeast-striking fault
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sets are regionally continuous, and have been mapped beyond the Olympic Dam area (e.g.
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Elizabeth Creek fault zone and Todd Dam fault zone; Hayward and Skirrow, 2010).
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Displacement directions in the vicinity of Olympic Dam are difficult to determine but normal
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displacement predominates. Hayward and Skirrow (2010) presented evidence for normal
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displacement on two regional northwest-striking faults that occur to the southwest (Elizabeth
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Creek fault zone, ~40 km from OD) and northeast (Gregory fault zone, ~60 km from OD).
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4. Facies Associations in the Bedded Clastic Facies
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The bedded clastic facies stand out markedly from the rest of the ODBC because they
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are consistently well bedded and dominated by fine grain sizes (mainly <2 mm). The rest of
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the ODBC lacks consistent bedding and is dominated by coarser grain sizes (fragments
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commonly >1 cm). Five facies associations defined on the basis of color, components, grain
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size and bedding characteristics are present in the bedded clastic facies: interbedded
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sandstone and red mudstone, well-sorted quartz-rich sandstone, green sandstone and
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mudstone, polymictic volcanic-clast conglomerate, and thinly bedded green and red
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mudstone.
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4.1 Interbedded sandstone and red mudstone The interbedded sandstone and red mudstone facies association comprises up to 50% of
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the bedded clastic facies and is present in both the northern and the southern bedded clastic
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facies areas. This association mainly consists of red mudstone intercalated with coarse to
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fine-grained, thinly bedded (3-10 cm thick) and laminated white, yellow and pink sandstone
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(Fig. 3A,B). Some thicker (30 cm) sandstone beds and 50-cm-thick polymictic, granule to
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pebble conglomerate and breccia beds are also present.
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The interbedded sandstone and red mudstone facies association is thickest at the northern edge of the southern bedded clastic facies, where it reaches 90 m (true thickness,
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measured in drill core) near the unconformity and tapers to 50 m at depth. It is significantly
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thinner (10-20 m thick) at the southern edge of the southern bedded clastic facies where it is
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in contact with granite-rich breccia of the ODBC (Fig. 2B). In the east, the interbedded
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sandstone and red mudstone facies association forms an outlier north of the southern bedded
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clastic facies. In the west, it comprises 25% of the western outlier of the southern bedded
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clastic facies. In the northern bedded clastic facies, the interbedded sandstone and red
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mudstone facies association makes up 5-20% of the bedded clastic facies, commonly as <5-
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m-thick intervals intercalated with the polymictic volcanic-clast conglomerate facies. It may
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be more abundant in the northern bedded clastic facies, but this area is intensely hematite-
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altered and discriminating among the various fine-grained thinly bedded facies associations is
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problematic.
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The thicker (30 cm) sandstone beds in the interbedded sandstone and red mudstone
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facies association are typically internally massive, parallel laminated or graded, and tabular
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though some are lenticular (Fig. 3C, D). Sandstone beds also show flame structures (Fig. 3B),
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ball-and-pillow structures, and cross-beds and cross-laminae defined by dark bands of grey
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hematite (Fig. 3C, Fig. 4A-D). The red mudstone intervals are planar laminated to very thinly
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bedded. Local disharmonic folds are common (Fig. 4E), as are mm- to cm-scale bed offsets
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along faults. The polymictic conglomerate and breccia beds in this facies association
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generally display normal grading upwards into parallel laminated sandstone.
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Granitoid-derived quartz, granitoid clasts and feldspar are the dominant recognisable
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components in the sandstone (Fig. 3E); hematite, zircon, tourmaline, titanite and rutile are
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also present. Clasts are angular to subangular. Some laminae are defined by concentrations of
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detrital hematite and zircon (Fig. 4A-D). Quartz, fine-grained white mica and hematite are
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common between the sand grains; interstitial sulfides (pyrite, chalcopyrite, bornite and
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chalcocite), florencite and Pb telluride are present locally. The red mudstone is almost
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entirely composed of red hematite, grey hematite and minor magnetite; scattered fine quartz,
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zircon, white mica and Fe-chlorite are also present. Clasts in the polymictic conglomerate and
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breccia in this association include grey and red fine-grained hematite, feldspar-phyric and
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mafic volcanic fragments, granite and granophyric clasts.
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4.2 Well-sorted quartz-rich sandstone
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Well-sorted quartz-rich sandstone has been identified in one drill hole (RD2751) in the
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northern bedded clastic facies area and one drill hole in the southern bedded clastic facies
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area (RD1628). In both drill holes, the sandstone is present as a single ~80-m-thick interval
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(RD2751, 848.7 m to 928.3 m; RD1628, 340.6 to 416.7 m) and is almost entirely brecciated
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(Fig. 5A). Only the topmost ~2.3 m (Fig. 5B) and another ~2-m-thick interval at ~898 m in
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RD2751 appear to be intact (although both could be large clasts). The clasts of well-sorted
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quartz-rich sandstone are internally massive or diffusely stratified and some are cross-
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stratified (Fig. 5D, E). The principal component of the sandstone is sub-rounded quartz; mm-
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size muscovite grains, tourmaline and fine-grained red hematitic fragments are also present
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(Fig. 5C). The quartz grains have undulose extinction, subgrains, and abundant fluid
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inclusions, and some are poly-grain aggregates with annealed, triple-junction boundaries. In RD2751, the brecciated well-sorted quartz-rich sandstone has a sharp upper and a
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more gradational lower contact with polymictic breccia of the ODBC (Fig. 5A). Higher in
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the same hole, the thinly bedded green and red mudstone facies association occupies an
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interval that is ~165 m thick. None of the nearby holes on the same section (~50 m southwest,
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or ~250 m northeast) includes the well-sorted quartz-rich sandstone facies.
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4.3 Green sandstone and mudstone
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clastic facies in the southern mine area at Olympic Dam. It is present in the center of the
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southern bedded clastic facies and the western outlier, is up to 100 m thick and can be traced
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over 1.3 km along strike (Fig. 2). This association is also present in the northern bedded
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clastic facies but is much thinner (<20 m).
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This association consists of interlaminated green sandstone and mudstone (Fig. 6A), 1-
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m-thick green sandstone beds, and a few 10-m-thick beds of green sandstone (e.g. RU38-
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2626, 400-430 m). Most of the thick sandstone beds contain green mudstone intraclasts (Fig.
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6B). The sandstone and mudstone have sharp contacts, although some sandstone beds grade
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to mudstone. Laminae and beds are laterally continuous and of even thickness.
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Altered mafic igneous clasts are the main component of the green sandstone. They have
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porphyritic, doleritic and vesicular textures (Fig. 6C). A mafic composition is suggested by
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the presence of chlorite-pseudomorphed olivine crystals that contain chromite inclusions
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(Fig. 6C) and free chromite crystals (Fig. 6D). Other clast types include feldspar-phyric felsic
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volcanic clasts, granitoid fragments and granitoid quartz, minor volcanic quartz, tourmaline
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and zircon. The cement between the clasts in sandstone includes quartz, carbonate, fine-
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grained white mica and chlorite. Sulfide minerals, including pyrite, chalcopyrite, bornite and
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chalcocite, occur together with siderite, barite, fluorite and apatite between the grains (Fig.
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6E).
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The green colour of the mudstone is caused by phyllosilicates, mainly chlorite, though fine-grained white mica is also present. Angular grains of quartz and feldspar are dispersed
256
through the mudstone. Accessory minerals include zircon and titanite.
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4.4 Polymictic volcanic-clast conglomerate
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The polymictic volcanic-clast conglomerate is present in the northern bedded clastic
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facies, the western portion of the southern bedded clastic facies and in the western outlier
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(Fig. 2). In the northern bedded clastic facies, it is 70 m thick and has fault contacts with both
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the thinly bedded green and red mudstone facies association and the ODBC. At the western
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end of the southern bedded clastic facies and in the western outlier, it forms 3-m-thick beds
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within intervals of the green sandstone and mudstone. In the southern bedded clastic facies, it
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is 10-20 m thick and separates the interbedded sandstone and red mudstone facies association
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(above) and the green sandstone and mudstone facies association (below).
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The association consists mostly of polymictic volcanic-clast conglomerate and subordinate polymictic volcanic sandstone and mudstone. The conglomerate is characterized
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by a polymictic assemblage of pink-red hematite-altered or green chlorite-altered volcanic
269
cobbles and pebbles (Fig. 7). Most of the clasts are feldspar-phyric and felsic in composition,
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and very similar to feldspar-phyric rhyolites in the Gawler Range Volcanics. A minor
271
proportion (<1%) of the felsic volcanic clasts are sparsely quartz-phyric. Some clasts have a
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mafic composition reflected by the presence of pseudomorphs of skeletal olivine phenocrysts
273
that contain chromite. Granite and minor hematite clasts are also present. Most clasts are well
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rounded, although some clasts are angular and others have irregular margins (Fig. 7). The
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conglomerate is typically clast-supported, but locally matrix-supported. Between the clasts is
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sandstone composed of polymictic volcanic fragments (Fig. 7), and less abundant feldspar,
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chromite, and rare quartz and pseudomorphs of olivine. The matrix has a patchy green or
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purple colour due to being chlorite- or hematite-altered, respectively.
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In the northern bedded clastic facies, the polymictic volcanic-clast conglomerate is interbedded with 1-2-m-thick intervals of polymictic volcanic sandstone and mudstone. The
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polymictic volcanic sandstone is very similar to the sandstone matrix between cobbles and
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pebbles in the polymictic volcanic-clast conglomerate. The sandstone matrix is composed of
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0.5-2 mm felsic and mafic volcanic fragments and subordinate feldspar, chromite,
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pseudomorphs of olivine and rare quartz. In the mafic clasts, pseudomorphs of strongly
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chlorite-altered, skeletal olivine contain chromite.
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4.5 Thinly bedded green and red mudstone
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The thinly bedded green and red mudstone facies association comprises up to ~20% of
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the northern bedded clastic facies. It is also a minor component at the eastern end of the
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southern bedded clastic facies.
This association is characteristically very fine grained (<1 mm) and delicately
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laminated. It consists predominantly of laminated red mudstone (Fig. 8A) and interlaminated
292
green and red mudstone (Fig. 8B), and minor laminae and thick beds of pale grey, pale brown
293
and purple fine-grained sandstone. Red mudstone consists of fine-grained hematite and white
294
mica and the fine-grained sandstone beds contain quartz, feldspar, and felsic volcanic clasts.
295
Some fine-grained sandstone beds are normally graded. Clasts in the sandstone are angular or
296
subangular. Laminae and thicker beds are laterally continuous and of even thickness.
297
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In the northern bedded clastic facies, this association includes thin beds and laminae of
298
siliceous mudstone, ironstone, barite and felsic tuffaceous mudstone. Some of the siliceous
299
mudstone laminae are red due to the presence of fine hematite (Fig. 8C). Cracks
300
perpendicular to bedding, and local cm- and m-scale folds and faults are present (Fig. 8B).
13 Page 13 of 47
301
Some laminae and thin beds of green mudstone contain sparse chromite grains, and others
302
contain magnetite rimmed by hematite. The felsic tuffaceous mudstone is pale green, and occurs in internally massive laminae
304
and thin to thick beds. The tuffaceous origin is demonstrated by the presence of bubble-wall
305
shards, the shapes of which are very well preserved even though the original volcanic glass is
306
entirely altered (Fig. 8D). The tuffaceous mudstone probably has a felsic composition
307
because it contains fine, angular volcanic quartz (Fig. 8E) and zircon.
308
5. Stratigraphic Relationships in the Bedded Clastic Facies
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All five facies associations are present in both the northern and southern bedded clastic
310
facies areas. Many contacts between the facies associations are faults. In the northern bedded
311
clastic facies, the main interval of the polymictic volcanic-clast conglomerate appears to
312
overlie the thinly bedded green and red mudstone. Stratigraphic relationships are somewhat
313
better preserved in the southern bedded clastic facies area (Fig. 2). Interbedded sandstone and
314
red mudstone is overlain by green sandstone and mudstone or thinly bedded green and red
315
mudstone. In the eastern part of the southern bedded clastic facies, thinly bedded green and
316
red mudstone occurs gradationally between interbedded sandstone and red mudstone below
317
and green sandstone and mudstone above. In the western part of the southern bedded clastic
318
facies, the green sandstone and mudstone is overlain by polymictic volcanic-clast
319
conglomerate (RU38-2626, 400 to 355 m, Fig. 2C; RU38-2625, 470 to 380 m). Two
320
intervals of interbedded sandstone and red mudstone occur along the northern and southern
321
margins of the southern bedded clastic facies and an interval of green sandstone and
322
mudstone occurs in between (Fig. 2A, B). It is unclear if there are two units of interbedded
323
sandstone and red mudstone, one below and one above the green sandstone and mudstone, or
324
if the two interbedded sandstone and red mudstone intervals are at the same stratigraphic
325
level and repeated by faults and/or folds.
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326 327
6. Structure of the Bedded Clastic Facies Bedding data for the bedded clastic facies presented here come exclusively from oriented drill core. Folds are evident in many logged intervals of the bedded clastic facies
329
(e.g. Fig. 4E) though no fold axes were directly measured. Folds do not have associated
330
cleavage or other foliations, even in mudstone (Fig. 4F). The folds are variable in size.
331
Wavelengths range from 1-2 cm to several metres, and are evident from changes in bedding
332
orientation, and in some cases, changes in younging direction. Concentric folds occur in some
333
sandstone beds that are thicker than 10 cm (Fig. 4E). Disharmonic folds are most abundant in
334
fine-grained laminated facies.
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The internal structure of the northern bedded clastic facies is consistent with the
336
presence of roughly cylindrical folds with gentle northeasterly plunges (Fig. 9A). The
337
exceptions are beds in the upper part of RD3450 (not plotted in Figure 9A) in the northern
338
bedded clastic facies, where all beds are subvertical and there is no consistent strike direction.
339
A similar but less well defined northeasterly fold plunge is present in the bedding data for the
340
southern bedded clastic facies (Fig. 9B). In detail, the internal structure is complex and beds
341
are not necessarily parallel to the boundaries of the facies associations. For example, in the
342
western part of the southern bedded clastic facies, the boundaries of the facies associations
343
dip 70⁰ to the north whereas the beds strike northeasterly, are subvertical near the top and less
344
steeply dipping with depth, dipping to the southeast, and the beds young to the south.
345
In the bedded clastic facies, faults can be marked by juxtaposition of unrelated
346
lithologies (e.g. granite against hematite-rich bedded clastic facies in RD3449 at 590 m, in
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the northern bedded clastic facies), abrupt changes in bedding orientation (Fig. 10A), and/or
348
one- to several-metres of fractured and closely jointed core associated with strongly hematite-
349
and sericite-altered zones. Some faults have been intruded by mafic dykes (Fig. 10B).
350
Fracture zones on cm-scale may be marked by rotated angular clasts healed with minerals
351
such as hematite, barite and siderite (Fig. 10C). Narrow fractures (<5 cm) also displace beds
352
by mm to cm (Figs 8B, 10C).
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Many internal faults are parallel to bedding or at 90⁰ to bedding (Fig. 9C). Early
354
northeast-striking faults separate the facies associations in the southern bedded clastic facies
355
and are possibly offset along northwest-striking transfer faults. Northeast-striking faults
356
define the boundaries of the green sandstone and mudstone in the western outlier. Northwest-
357
striking faults truncate the northern bedded clastic facies in the southwest and the northeast
358
(Fig. 2). Faults striking northwest and west also displace the facies associations within the
359
northern bedded clastic facies. Steep east-west striking faults separate the southern bedded
360
clastic facies from the ODBC (Fig. 2).
361
7. Composition of the Bedded Clastic Facies
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As predictable from the componentry, the green sandstone and mudstone facies
363
association differs in composition from the interbedded sandstone and red mudstone facies
364
association. The green sandstone and mudstone has higher Ti, Mg, Mn, Li, Cr, Sc, Ni and V
365
(Fig. 11, Supplementary Data). Most green sandstone and mudstone samples also contain
366
more abundant P2O5 (>0.2 wt.%) and have a higher volatile content (LOI) than samples of the
367
interbedded sandstone and red mudstone. Most of the green sandstone and mudstone samples
16 Page 16 of 47
have TiO2 >0.68 wt.% and >300 ppm Cr. Sandstone from the polymictic volcanic-clast
369
conglomerate, sampled from the western part of the southern bedded clastic facies, has a
370
similar composition to the green sandstone and mudstone.
371
8. Zircon Geochronology
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The well-sorted quartz-rich sandstone facies contains four populations of zircons. The
best defined populations have ages of ~1600 Ma and ~1740 Ma; less pronounced peaks occur
374
at ~1900 Ma and 2500 Ma (Fig. 12A).
cr
373
All zircons sampled from felsic volcanic clasts in the polymictic volcanic conglomerate
376
and from sandstone of both the green sandstone and mudstone and interbedded sandstone and
377
red mudstone display a cluster of ages around 1600 Ma (Fig. 12B, C). Jagodzinski (2014)
378
reported a SHRIMP U-Pb zircon age of 1592 +/- 4 Ma for sandstone from the green
379
sandstone and mudstone facies association in the southern bedded clastic facies. A few
380
younger and older ages are present in these samples but cannot be considered reliable.
381
Zircons from granite clasts in the polymictic volcanic-clast conglomerate have the same age.
382
9. Discussion
383
9.1 Depositional processes
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Planar laminated to thinly bedded mudstone is an important and locally dominant facies
385
in the bedded clastic facies. It accounts for ~35% of the interbedded sandstone and red
386
mudstone, ~55% of the green sandstone and mudstone, and ~75% of the thinly bedded green
387
and red mudstone. The fine grain size and planar beds indicate subaqueous deposition from
388
suspension and dilute muddy turbidity currents (e.g., Stow, 1986). The presence of pristine
389
bubble-wall shards in the felsic tuffaceous mudstone is best accounted for by settling from
390
suspension in a quiet subaqueous setting; shards deposited on land are abraded and mixed
391
with other components, or entirely destroyed (e.g., Fisher and Schmincke, 1984).
17 Page 17 of 47
392
Many of the sandstone beds that are present in all five facies associations are massive or normally graded, and dominantly tabular. These characteristics indicate deposition from
394
relatively dilute sandy turbidity currents (e.g., Lowe, 1982). Some sandstone beds in the
395
interbedded sandstone and red mudstone facies association are cross bedded (cross-bed sets
396
<5 cm thick; Fig. 3C, 4A), consistent with traction deposits related to waning turbidity
397
currents. The very thick, poorly sorted beds of polymictic volcanic-clast conglomerate imply
398
rapid deposition from high-particle concentration gravity currents such as debris flows (e.g.,
399
Lowe, 1982) and could indicate within-channel settings.
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The bed characteristics of the brecciated well-sorted quartz-rich sandstone facies are
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not preserved though the presence within clasts of cross-bed sets in the order of 8-10 cm
402
thick, the good sorting and rounding, and the absence of interbedded mudstone all imply
403
deposition of this facies involved sandy traction currents; such currents operate most
404
commonly in shallow subaqueous (above wave base; e.g., Elliott, 1986) or fluvial-alluvial
405
settings (e.g., Collinson, 1986). The relationship between this shallow or subaerial setting
406
and the below-wave base setting indicated by the rest of the bedded clastic facies has not
407
been resolved.
408
9.2 Provenance of the bedded clastic facies
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The five facies associations have distinctive components that reflect derivation from
410
different sources. A mafic igneous provenance is apparent for the green sandstone and
411
mudstone facies association. It contains Cr-spinel, pseudomorphs of olivine and abundant
412
mafic volcanic clasts. The high abundances of Sc, Cr, V and Ti are also consistent with a
413
mafic igneous provenance. The elevated Cr is caused by the presence of chrome spinel.
414
Analyses of chrome spinel (McPhie et al., 2011) separated from the green sandstone and
415
mudstone facies association at Olympic Dam indicated affinity with a wide variety of mafic
18 Page 18 of 47
416
igneous magmas. The small proportion of volcanic quartz and zircon indicates that a felsic
417
volcanic source also contributed to this association.
418
Sandstone of the interbedded sandstone and red mudstone has a dominant granitoid provenance. It contains abundant granitoid-sourced quartz, bands of heavy minerals including
420
hematite, rutile and zircon, and granite clasts (Fig. 3E, 4A-D). This association has higher Si,
421
Fe, As, Sb and Ba than the green sandstone and mudstone, but lower Ti, Sc, V, and Cr,
422
reflecting the stronger influence of felsic rocks and Fe-oxides in the source region. A felsic
423
volcanic source provided a small proportion of volcanic quartz to this association. The
424
zircons separated from samples of sandstone have an age of ~ 1600 Ma (Fig. 12B). Both the
425
Hiltaba Suite granites and/or the felsic volcanic units of the Gawler Range Volcanics, are
426
consistent with this age and are probable source rocks for zircon, but the overwhelming
427
abundance of granitoid-derived quartz implies that Hiltaba Suite granites (rather than
428
volcanic rocks) were the major source. Although much of the hematite in this association is
429
likely to be hydrothermal, some of it is detrital (Fig. 4A-D), and indicates that the source area
430
included ironstone.
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The feldspar-phyric volcanic clasts in the polymictic volcanic-clast conglomerate have
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432
textures, compositions and ages that indicate derivation from the felsic Gawler Range
433
Volcanics. The mafic volcanic clasts probably also came from the Gawler Range Volcanics,
434
but there are no age data available for the mafic clasts. The very minor proportion of granitic
435
clasts in the conglomerate is likely to come from Hiltaba Suite granites.
436
The well-sorted quartz-rich sandstone has zircon populations (Fig. 13A) that reflect
437
derivation from Gawler SLIP sources as well as older basement sources. The ~1740 Ma
438
population is the same age as metamorphic and granitoid units exposed in the northern and
439
southern Gawler Craton (e.g., Mount Woods Domain, Nawa Gneiss, Peak and Denison
440
Domain; Hand et al., 2007) and the ~2500 Ma population matches the age of the Sleaford and
19 Page 19 of 47
Mulgathing complexes (Hand et al., 2007). The high abundance of metamorphic- and
442
granitoid-derived quartz in this facies is consistent with the provenance indicated by the
443
zircon populations. The well-sorted quartz-rich sandstone is part of the ODBC but because it
444
has been found in two drill holes only and it is not interbedded with any of the other facies
445
associations, its significance has yet to be fully explained.
The provenance of very fine grains that dominate the thinly bedded green and red
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441
mudstone is diverse. The felsic tuffaceous mudstone would have been generated by
448
contemporaneous eruptions because delicate glass shards typically have very poor
449
preservation potential in surface environments. The shard shapes and association with quartz
450
fragments (Fig. 8C, D) indicate that the eruption was explosive, magmatic-volatile driven
451
(rather than phreatomagmatic), and probably rhyolitic (e.g., Fisher and Schmincke, 1984).
452
The uniformly fine grain size and good sorting imply that the shards and crystal fragments
453
settled from an ash cloud and that the Olympic Dam depositional site was far (tens, possibly
454
hundreds km) away from the source vent(s). In contrast, detrital chromite and magnetite in
455
some mudstone beds have a mafic igneous source. Some facies in this association are
456
unlikely to be detrital, and instead possibly consist of chemical and/or hydrothermal
457
components, especially the siliceous mudstone, ironstone and barite laminae; Oreskes and
458
Einaudi (1990) also suggested a hydrothermal origin for these components.
459
9.3 Depositional setting – a fault-bounded sedimentary basin at Olympic Dam
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The associations in the southern bedded clastic facies are mappable and continuous
461
through the area delineated as three separate maar-diatreme centers by previous workers (Fig.
462
2A, V3, V4, V5; Reeve et al., 1990; Johnson and Cross, 1995). One other interpreted maar-
463
diatreme center (Fig. 2 inset,V2) comprises faulted intervals of the green sandstone and
464
mudstone facies and the polymictic volcanic-clast conglomerate that may originally have
465
been continuous with the same facies associations in the rest of the southern bedded clastic
20 Page 20 of 47
facies ~200 m to the east. The V1 interpreted maar-diatreme center (Fig. 2A) consists of
467
intervals of polymictic volcanic-clast conglomerate and thinly bedded green and red
468
mudstone facies association that have faulted contacts with the ODBC, and appears to be a
469
down-dropped fault block. Although disrupted, it is clear that at least four of the associations
470
are interbedded; facies boundaries are gradational but distinct, and the associations are not
471
mixed as might be expected from deposition in one or more small closed craters.
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The dominant facies of the bedded clastic facies indicate a predominantly low-energy, below wave base subaqueous depositional setting. The polymictic volcanic-clast
474
conglomerate is consistent with such a setting but reflects much higher energy depositional
475
event(s). This setting extended across the area now occupied by the bedded clastic facies in
476
the southern mine area at Olympic Dam and probably extended farther because the contacts
477
of the bedded clastic facies with the ODBC are faults and the top is the unconformity with the
478
overlying Neoproterozoic sedimentary formations. It is also clear that the bedded clastic
479
facies includes components from outside the Olympic Dam area. Confirming the presence of
480
remotely sourced components is an important argument against the former presence of maar
481
craters within which mainly locally sourced sediment would be likely to accumulate. By
482
“local”, we mean within the area now occupied by the ODBC.
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483
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McPhie et al. (2011) concluded that two components in the bedded clastic facies,
484
chromite and volcanic quartz, have no recognised local sources. The wide range in
485
composition of chromite grains in the green sandstone and mudstone facies association
486
implies that either (1) multiple sources in different tectonic settings contributed the chromite
487
grains to the Olympic Dam bedded clastic facies, or (2) the source was local but the tectonic
488
setting changed in a complex fashion over a short period of time (e.g., chromites in Mount
489
Etna lavas reflect a complicated succession of tectonic settings operating in less than 1
490
million years; Kamenetsky and Clocchiatti, 1996), or (3) chromite compositions are not
21 Page 21 of 47
useful as provenance indicators in ancient cratonic settings. The second alternative would
492
appear untenable given current understanding of the tectonic events affecting the Gawler
493
Craton in the Mesoproterozoic (e.g., Hand et al., 2007). The third alternative is also rejected
494
because there is no reason why the well-understood controls on chromite compositions in
495
modern settings (e.g., Kamenetsky et al., 2001) would not operate in ancient settings. Hence,
496
the first alternative remains the most plausible explanation of the chromite compositions
497
found in the bedded clastic facies at Olympic Dam. Furthermore, although the green
498
sandstone and mudstone facies association has a strong mafic volcanic provenance, and
499
records contemporaneous mafic volcanism, the overall fine grain size and the well-developed
500
tabular bedforms are not consistent with a vent-proximal depositional setting.
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491
The presence of volcanic quartz in the bedded clastic facies at Olympic Dam (McPhie
502
et al., 2011) can now be understood in the light of the recognition of quartz-bearing rhyolitic
503
ash (felsic tuffaceous mudstone facies, Fig. 8D, C) in the thinly bedded green and red
504
mudstone facies association. The ash was probably erupted far from Olympic Dam and
505
transported there in a regional ash cloud. Such ash clouds deposit fine pyroclasts in all
506
surface environments over very large areas (tens to hundreds of km2; e.g., Fisher and
507
Schmincke, 1984). Although the felsic tuffaceous mudstone is present as discrete beds only
508
in the thinly bedded green and red mudstone association, the small proportion of fine
509
volcanic quartz grains found in all the other associations is also likely to be pyroclastic in
510
origin and from a remote source.
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511
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Intact felsic Gawler Range Volcanics units do not occur within the ODBC though such
512
units were probably originally present above the Roxby Downs Granite. The polymictic
513
volcanic-clast conglomerate is dominated by well-rounded clasts of felsic Gawler Range
514
Volcanics that must have been rounded outside the present area of occurrence of the
515
conglomerate because this and other bedded clastic facies mainly record below-wave-base
22 Page 22 of 47
516
subaqueous settings (where rounding does not occur). We therefore conclude that the source
517
of the well-rounded clasts of felsic Gawler Range Volcanics was outside the local area.
518
The Paleoproterozoic and Archean zircons in the well-sorted quartz-rich sandstone (Fig. 12A) also suggest that the provenance of the bedded clastic facies extended beyond the
520
local Olympic Dam area. Paleoproterozoic units (mainly Wallaroo Group; Skirrow et al.,
521
2007) are widespread to the north, east and south of Olympic Dam (Fig. 1) and the nearest
522
occurrences are less than 10 km away. However, the nearest Archean rocks are about ~30 km
523
west of Olympic Dam (Mulgathing Complex; Fraser et al., 2010).
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519
Further constraints on the depositional setting are provided by structural analysis of the
525
bedded clastic facies. The beds of the bedded clastic facies were deformed prior to complete
526
lithification because there is no associated cleavage, and the shapes and sizes of mesoscopic
527
folds are highly variable. In addition, no fold-forming deformation events are known to have
528
affected this part of the Gawler Craton since the Paleoproterozoic. Bed orientations in the
529
northern bedded clastic facies are consistent with the presence of gently northeast-plunging
530
folds (Fig. 9A); a similar though less well-defined relationship exists in the southern bedded
531
clastic facies (Fig. 9B). Given the evidence for the deformation being pre-lithification, the
532
fold axis orientation could be defining the strike of the original depositional slope, down
533
which unstable portions of the bedded clastic facies eventually slid. Down-slope soft-
534
sediment slumping does not fully account for the deformation of the bedded clastic facies, in
535
particular, the sections of sub-vertical beds and the ~20-30o plunge on the axis of the slump
536
folds; tectonic deformation focussed on faults probably contributed, also prior to complete
537
lithification. Nevertheless, the fold axis orientation could be indicating that the bedded clastic
538
facies accumulated on a paleoslope with a northeasterly strike; the dip direction (northwest or
539
southeast) is unknown. This strike direction is parallel to the mapped northeast-striking faults
540
known to have been present when the Olympic Dam hydrothermal system was active
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23 Page 23 of 47
(Hayward and Skirrow, 2010; Ehrig et al., 2012). Several authors have inferred the presence
542
of large northwest- or west-northwest-striking faults at Olympic Dam (e.g., O’Driscoll, 1986;
543
Roberts and Hudson, 1983; Reeve et al. 1990; Cross et al., 1993; Hayward and Skirrow,
544
2010). We propose that the depocenter within which the bedded clastic facies accumulated
545
was fault-controlled and bounded by a combination of northeast-striking and northwest-
546
striking faults (Fig. 13). The northwest-striking master fault orientation shown in Figure 13 is
547
not constrained by our data; this strike direction is consistent with the orientation of both
548
local and regional faults (Cross et al., 1993; Hayward and Skirrow, 2010) and of a
549
continental-scale lineament upon which the Olympic Dam deposit is located (O’Driscoll,
550
1986).
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Thick sections of the Gawler Range Volcanics occur at Acropolis and Wirrda Well,
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551
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541
~25 km southwest and ~25 km south of Olympic Dam, respectively, but no fully intact
553
sections of the Gawler Range Volcanics occur at Olympic Dam. Clasts derived from the
554
Gawler Range Volcanics are locally abundant in breccia of the ODBC, and are the major
555
component in the polymictic volcanic-clast conglomerate facies of the bedded clastic facies.
556
In general, the bedded clastic facies occur immediately below the unconformity with the
557
Neoproterozoic formations. These relationships suggest that the Gawler Range Volcanics
558
were originally present at Olympic Dam and underlay the bedded clastic facies. Thick bedded
559
clastic facies are known from only one other section of the exposed Gawler Range Volcanics
560
(McAvaney and Wade, 2015), so special circumstances must have existed in the vicinity of
561
Olympic Dam for such a thick section of bedded clastic facies to accumulate. A fault-
562
controlled depocenter as has been inferred here on the basis of the structure and facies
563
characteristics of the bedded clastic facies, would also account for the local presence of thick
564
clastic facies in a region otherwise dominated by widespread, topography-building felsic
565
volcanic units.
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566
We conclude that the maar-diatreme setting for the bedded clastic facies is untenable and does not adequately account for their facies characteristics, facies geometry, provenance
568
and internal stratigraphy. The inference that a fault-bounded sedimentary depocenter existed
569
at Olympic Dam is not new. Roberts and Hudson (1983) first recognised the bedded clastic
570
facies and relied on these facies in framing their interpretation of Olympic Dam deposit as “a
571
very unusual type of sediment-hosted mineralisation” (p. 815). Although Roberts and
572
Hudson’s (1983) sediment-hosted interpretation of Olympic Dam is no longer accepted, other
573
authors have concluded that a fault-bounded basin existed at Olympic Dam (e.g., Oreskes and
574
Einaudi, 1990; Hayward and Skirrow, 2010).
575
9.4 Relationships of the bedded clastic facies to the ODBC and the Olympic Dam ore deposit
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Although most contacts between the bedded clastic facies and the ODBC are faults,
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576
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567
some contacts are gradational; intact but folded intervals of the bedded clastic facies grade
578
into brecciated bedded clastic facies that grades into hematite-rich breccia containing clasts
579
derived from the bedded clastic facies. These relationships indicate that the bedded clastic
580
facies were present at the time of the formation of the ODBC. Furthermore, although not
581
considered “ore”, the bedded clastic facies contain the same Cu sulfide and other
582
hydrothermal minerals as the ore zones. It thus appears that the bedded clastic facies pre-date
583
the ODBC and the Cu mineralising event. Given that the ODBC contains macroscopic clasts
584
of the bedded clastic facies, it is plausible that some fine-grained (<1 mm) components of the
585
breccia were also derived from the bedded clastic facies. In particular, some of the fine
586
hematite in the ODBC could be recycled from hematite-rich sandstone and mudstone in the
587
bedded clastic facies, partly accounting for the marked iron anomaly at Olympic Dam.
588
10. Conclusions
589 590
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The bedded clastic facies comprises five main facies associations that reflect different sources - a granitoid-dominated source (interbedded sandstone and red mudstone), a
25 Page 25 of 47
Paleoproterozoic and older basement provenance (well-sorted quartz-rich sandstone facies), a
592
strongly mafic volcanic source (green sandstone and mudstone), a mixed felsic-mafic
593
volcanic source (polymictic volcanic-clast conglomerate), and a partly hydrothermal and
594
partly volcanic source (thinly bedded green and red mudstone). Zircons from the granitoid
595
and felsic volcanic-derived facies indicate a ~1600 Ma age for these sources.
ip t
591
The dominant facies are planar bedded and relatively fine grained, suggesting
597
deposition in a below-wave base subaqueous setting from suspension and dilute turbidity
598
currents. The five associations are distinct, four of them are interbedded, and at least three of
599
them are mappable across the southern mine area, implying that the original depocenter was
600
much larger than the area within which they are preserved. The internal stratigraphy and
601
characteristics of the bedded clastic facies are inconsistent with deposition in separate but
602
nearby maar craters.
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The two main areas of bedded clastic facies are strongly deformed but the absence of
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603
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596
cleavage and the disharmonic nature of mesoscopic folds suggest that the deformation
605
occurred prior to complete lithification. In these two areas, there is a consistent pattern of
606
gently northeast-plunging folds. The folds may have been produced by slumping of
607
unconsolidated sediment on a southeasterly or northwesterly dipping paleoslope in an
608
extensional basin bounded by northwesterly and northeasterly striking faults. The northern
609
and southern bedded clastic facies are down-faulted remnants of an originally more extensive
610
sedimentary succession. The bedded clastic facies were present when the ODBC formed and
611
Olympic Dam mineralising hydrothermal system was active. The fault-bounded sedimentary
612
basin that was present during mineralisation may have contributed to or otherwise influenced
613
the formation of the Olympic Dam ore deposit.
614
Acknowledgements
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26 Page 26 of 47
Chantelle Lower, Robert Scott, and Susan Belford provided assistance with the analysis of
616
drill core structural data. Discussions of our data and interpretations with Ron Berry, Ken
617
Cross and Sebastien Meffre are gratefully acknowledged. We thank Julie Hunt and an
618
anonymous reviewer for constructive comments that have helped improve the manuscript.
619
This research was funded by BHP-Billiton.
620
References
621 622 623
Agangi, A., Kamenetsky, V.S., McPhie, J., 2012. Evolution and emplacement of high-F rhyolites in the Mesoproterozoic Gawler silicic large igneous province, South Australia. Precambrian Research, v. 208-211, p. 124-144.
624 625 626
Allen, S.R., McPhie, J., 2002. The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A > 675 km3, compositionally zoned lava of Mesoproterozoic age. Geological Society America Bulletin, v. 114, p. 1592-1609.
627 628 629
Allen, S.R., McPhie, J., Ferris, G., Simpson, C., 2008. Evolution and architecture of a large felsic igneous province in western Laurentia: The 1.6 Ga Gawler Range Volcanics, South Australia. Journal of Volcanology and Geothermal Research, v. 172, p. 132-147.
630 631 632
Blissett, A.H., Creaser, R.A., Daly, S.J., Flint, R.B., Parker, A.J., 1993. Gawler Range Volcanics. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 106-124.
633 634
Collinson, J.D., 1986. Alluvial sediments. In: Reading, H.J. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, pp. 20-62.
635 636 637
Cowley, W.M., 1993. Cariewerloo Basin, Pandurra Formation. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 139-142.
638 639 640 641
Cross, K.C., Daly, S.J., Flint, R.B., 1993. Mineralisation associated with the GRV and Hiltaba Suite granitoids. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 132-138.
642 643 644
Direen, N.G., Cadd, A.G., Lyons, P., Teasdale, J.P., 2005. Architecture of Proterozoic shear zones in the Christie Domain, western Gawler Craton, Australia: Geophysical appraisal of a poorly exposed orogenic terrane. Precambrian Research, v. 142, p. 28-44.
645 646 647 648
Ehrig, K., McPhie, J., Kamenetsky, V., 2012. Geology and mineralogical zonation of the Olympic Dam Iron Oxide Cu-U-Au-Ag Deposit, South Australia. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.), Society of Economic Geologists, Inc., Special Publication 16, p. 237–267.
Ac ce p
te
d
M
an
us
cr
ip t
615
27 Page 27 of 47
Elliott, T., 1986. Siliciclastic shorelines. In: Reading, H.J. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, pp. 155-188.
651 652
Fanning, C.M., Flint, R.B., Preiss, W.V., 1983. Geochronology of the Pandurra Formation. Geological Survey of South Australia, Quarterly Geological Notes, Number 88, p. 11-16.
653 654 655
Fanning, C.M., Blissett, R.B., Parker, A.J., Ludwig, K.R., Blissett, A.H., 1988. Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb geochronology. Precambrian Research, v. 40-41, p. 363-386.
656 657
Fanning, C.M., Reid, A., Teale, G., 2007. A geochronological framework for the Gawler Craton, South Australia. South Australia, Geological Survey, Bulletin 55, pp. 258.
658 659
Fisher, R.V., Schmincke, H-U., 1984. Pyroclastic Rocks. Springer-Verlag Berlin Heidelberg, 472 pp.
660 661 662
Fraser, G., McAvaney, S., Neumann, N., Szpunar, M., Reid, A., 2010. Discovery of early Mesoarchean crust in the eastern Gawler Craton, South Australia. Precambrian Research, v. 179, p. 1-21.
663 664
Giles, C.W., 1988. Petrogenesis of the Proterozoic Gawler Range Volcanics, South Australia. Precambrian Research, v. 40-41, p. 407-427.
665 666 667
Groves, D.I., Bierlein, F.P., Meinert, L.D., Hitzman, M.W., 2010. Iron oxide copper-gold (IOCG) deposits through Earth history: Implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits. Economic Geology, v. 105, p. 641-654.
668 669
Hand, M., Reid, A., Jagodzinski, E., 2007. Tectonic framework and evolution of the Gawler Craton, southern Australia. Economic Geology, v. 102, p. 1377-1395.
670 671
Haynes, D.W., Cross, K.C., Bills, R.T., Reed, M.H., 1995. Olympic Dam ore genesis: A fluid mixing model. Economic Geology, v. 90, p. 281-307.
672 673 674 675 676 677 678 679
Hayward, N., Skirrow, R.G., 2010. Geodynamic setting and controls on iron oxide Cu-Au (±U) ore in the Gawler Craton, South Australia. In Porter, T.M. (Ed.), Hydrothermal Iron Oxide copper-Gold and Related Deposits: A Global Perspective, Volume 3. PGC Publishing, Adelaide, pp. 119-146.
680 681 682 683
Huang, Q., Kamenetsky, V.S., McPhie, J., Ehrig, E., Meffre, S., Maas, R., Thompson, J., Kamenetsky, M., Chambefort, I., Apukhtina, O., Yongbin Hu, 2015. Neoproterozoic (ca. 820-830 Ma) mafic dykes at Olympic Dam, South Australia: Links with the Gairdner Large Igneous Province. Precambrian Research, v. 271, p. 160-172.
Ac ce p
te
d
M
an
us
cr
ip t
649 650
Hitzman, M.W., Oreskes, N., Einaudi, M.T., 1992. Geological characteristics and tectonic setting of Proterozoic iron-oxide (Cu-U-Au-REE) deposits. Precambrian Research, v. 58, p. 241-287.
684
28 Page 28 of 47
Huang, Q., Kamenetsky, V.S., Ehrig, K., McPhie, J., Kamenetsky, M., Cross, K., Meffre, M., Agangi, A., Chambefort, I., Direen, N.G., Maas, R., Apukhtina, O., 2015 (submitted to Precambrian Research) Olivine-phyric basalt at Kokatha, Mount Gunson, Wirrda Well and Olympic Dam: insights into the petrogenesis of the Gawler silicic large igneous province, South Australia.
690 691 692
Jagodzinski, E., 2014. The age of magmatic and hydrothermal zircon at Olympic Dam. Geological Society of Australia, Abstracts No. 110, p. 260, Australian Earth Sciences Convention 2014, Newcastle, Australia.
693 694 695
Johnson, J.P., Cross, K.C., 1995. U-Pb geochronological constraints on the genesis of the Olympic Dam Cu-U-Au-Ag deposit, South Australia. Economic Geology, v. 90, p. 10461063.
696 697
Kamenetsky, V., Clocchiatti, R., 1996. Primitive magmatism of Mt Etna: Insights from mineralogy and melt inclusions. Earth and Planetary Science Letters, v. 142, p. 553-572.
698 699 700
Kamenetsky, V.S., Crawford, A.J., Meffre, S., 2001. Factors controlling chemistry of magmatic spinel: An empirical study of associated olivine, Cr-spinel and melt inclusions from primitive rocks. Journal of Petrology, v. 42, p. 655-671.
701 702 703
Lowe, D.R., 1982. Sediment gravity flows: II Depositional models with special reference to the deposits of high-density turbidity currents. Journal of Sedimentary Petrology, v. 52, p. 279- 297.
704 705
Johnson, J.P., McCulloch, M.T., 1995. Sources of mineralising fluids for the Olympic Dam deposit (South Australia): Sm-Nd isotopic constraints. Chemical Geology, v.121, p. 177-199.
706 707 708
McAvaney, S.O., Wade, C.E., 2015. Stratigraphy of the Lower Gawler Range Volcanics in the Roopena area, northeastern Eyre Peninsula. South Australia, Department of State Development, Report book 2015/00021.
709 710 711
McPhie, J., Kamenetsky, V.S., Chambefort, I., Ehrig, K., Green, N., 2011. Origin of the supergiant Olympic Dam Cu-U-Au-Ag deposit, South Australia: Was a sedimentary basin involved? Geology, v. 39, p. 795-798.
712 713
O'Driscoll, E.S.T., 1986. Observations of the lineament--ore relations. Philosophical Transactions of the Royal Society of London, Series A, v. 317, p. 195-218.
714 715 716
Oreskes, N., Einaudi, M.T., 1990. Origin of rare earth element-enriched hematite breccias at the Olympic Dam Cu-U-Au-Ag deposit, Roxby Downs, South Australia. Economic Geology, v. 85, p. 1-28.
717 718 719
Preiss, W.V., 1993. Neoproterozoic. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South Australia, Volume 1: The Precambrian. South Australia, Geological Survey, Bulletin 54, pp. 171-203.
Ac ce p
te
d
M
an
us
cr
ip t
685 686 687 688 689
29 Page 29 of 47
Reeve, J.S., Cross, K.C., Smith, R.N., Oreskes, N., 1990. Olympic Dam copper-uraniumgold-silver deposit. In: Hughes, F.E., (Ed.), Geology of the Mineral Deposits of Australia and Papua New Guinea. The Australian Institute of Mining and Metallurgy, Melbourne, pp. 10091035.
724 725
Roberts, D.E., Hudson, G.R.T., 1983. The Olympic Dam copper-uranium-gold deposit, Roxby Downs, South Australia. Economic Geology, v. 78, p. 799-822.
726 727 728 729 730
Skirrow, R.G., Bastrakov, E., Raymond, O.L., Davidson, G., Heithersay, P., 2002. The geological framework, distribution and controls of Fe-Oxide Cu-Au mineralisation in the Gawler Craton, South Australia. Part II – Alteration and mineralisation. In Porter, T.M. (Ed.), Hydrothermal Iron Oxide copper-Gold and Related Deposits: A Global Perspective, Volume 2. PGC Publishing, Adelaide, pp. 33-47.
731 732 733 734
Skirrow, R.G., Bastrakov, E.N., Barovich, K., Fraser, G.L., Creaser, R.A., Fanning, C.M., Raymond, O.L., Davidson, G.J., 2007. Timing of iron oxide Cu-Au-(U) hydrothermal activity and Nd isotope constraints on metal sources in the Gawler Craton, South Australia. Economic Geology, v. 102, p. 1441-1470.
735 736
Stow, D.A.V., 1986. Deep clastic seas. In: Reading, H.J. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, pp. 399-444.
737 738 739
Swain, G., Hand, H., Teasdale, J.P., Rutherford, L., Clark, C., 2005. Age constraints on terrain-scale shear zones in the Gawler Craton, southern Australia. Precambrian Research, v. 139, p. 164–180.
740 741 742
Wingate, M. T. D., Campbell, I. H., Compston, W., Gibson, G. M., 1998. Ion microprobe U– Pb ages for Neoproterozoic basaltic magmatism in south-central Australia and implications for the breakup of Rodinia. Precambrian Research, v. 87, p. 135-159.
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30 Page 30 of 47
743 Figure 1. The regional geological setting of Olympic Dam in the Gawler Craton, South Australia,
745
showing the distribution of the major rock units according to ages. The top left inset shows the
746
location of the Gawler Craton in Australia. The top right inset shows the geological setting of
747
Olympic Dam in more detail. On the main map: N, Nawa Domain, W, Wallaroo Group, M,
748
Mugathing Complex, S, Sleaford Complex. On all maps: star, Olympic Dam. After Hand et al.
749
(2007).
750
Figure 2. Distribution and structure of the bedded clastic facies in the southern mine area (SMA) at
751
Olympic Dam. A. Map of the bedded clastic facies at the level of the unconformity level with the
752
Neoproterozoic sedimentary formations (~350 m below the surface). Drill holes examined in this
753
study are marked. Inset map shows the extent of the granite breccia and the mineralised hematite
754
breccia of the Olympic Dam Breccia Complex within the Roxby Downs Granite (“granite”), after
755
Reeve et al. (1990); SMA, southern mine area. The five maar-diatreme centers of Reeve et al. (1990)
756
and Johnson and Cross (1995) are in black on the inset map, and labelled V1 to V5. The positions of
757
V1, V3, V4 and V5 in relation to the newly defined distribution of the bedded clastic facies are shown
758
as grey circles on the main map. The northern and southern bedded clastic facies (BCF) are hosted in
759
quartz-hematite breccia within the mineralised hematite breccia. The location of the north-south
760
section through the southern bedded clastic facies is indicated by the grey line. B. North-south cross
761
section of the western part of the southern bedded clastic facies. The upper surface is the
762
unconformity surface between the Mesoproterozoic units and the overlying Neoproterozoic
763
sedimentary formations. C. Simplified log of underground drill hole RU38-2626 showing the
764
distribution of the different bedded clastic facies between hematite breccia to the east (top) and granite
765
breccia to the west (base). Recorded younging directions (bold triangles) and faults (bold lines) are
766
also shown.
767
Figure 3. Interbedded sandstone and red mudstone facies association. A. Interbedded sandstone and
768
red mudstone; RD2767, ~778 -782 m; drill core is 5 cm across. B. Load casts and flame (f) structures
769
at the bases of tabular sandstone beds, amalgamated sandstone beds and low-angle truncation of
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744
31 Page 31 of 47
mudstone beds (RD2767, ~777.5 m); s, sandstone, m, mudstone. The curved mudstone flames
771
indicate that loading and dewatering accompanied deposition of the sandstone. C. Cross-bedded
772
sandstone (top, cross-bed sets ~2 cm high, parallel to dashed lines) and coarse massive sandstone
773
(base); RU38-2626, ~337.7-339 m. D. Lenticular sandstone beds interbedded with laterally
774
continuous mudstone laminae (RD1625, 685.4 m). E. Photomicrograph of sandstone composed of
775
granite clasts (g) and abundant angular quartz grains (q); crossed nicols (RD1624, 626.1 m).
776
Figure 4. Interbedded sandstone and red mudstone facies association. A-D, RD3287, 375.6 m. A.
777
Dark cross laminae (e.g., dashed line) defined by fine hematite, titanium oxide and zircon (RD3287,
778
375.6 m; thin-section, plane-polarised light). Hematite-free domains (hfd) cut across these laminae. B.
779
Enhanced MLA image of A showing the association of hematite and zircon within the dark laminae.
780
Zircon grains have diverse shapes that possibly reflect different provenance. C and D, BSE images of
781
dark bands in A displaying abundant hematite and zircon. In D, hematite grains are blue and zircon
782
grains are red. The close association of hematite and zircon in mm-thick layers suggests that they have
783
been concentrated during sedimentation. E. Minor fold (arrow) in interbedded sandstone and red
784
mudstone (RD1625, 331.5 m). F. Photomicrograph (plane-polarised light) of the interbedded
785
sandstone and red mudstone (RD1989, 416 m). No cleavage is evident in the mudstone or the
786
sandstone.
787
Figure 5. Well-sorted quartz-rich sandstone facies. A. Simplified log of drill hole RD2751 showing
788
the context of the brecciated well-sorted quartz-rich sandstone (~848.7 to 928 m) in the ODBC. B.
789
Well-sorted quartz-rich sandstone from the topmost ~2 m of the section in RD2751. This interval is
790
either intact or a large clast in breccia. Note the diffuse cross bedding (RD2751, 852 m). C.
791
Photomicrograph (crossed nicols) of the well-sorted quartz-rich sandstone, showing abundant quartz
792
with undulose extinction; m, muscovite, f, foliated clast. D. Matrix-supported breccia composed of
793
clasts of the well-sorted quartz-rich sandstone facies (RD2751, 917-919 m). Note the cross-beds
794
(parallel to dashed line) within one clast and the mottled pale versus red matrix. E. Jigsaw-fit texture
795
in the brecciated well-sorted quartz-rich sandstone facies (RD2751, 881 m).
796
Figure 6. Green sandstone and mudstone facies association. A. Thinly bedded and laminated green
797
sandstone and mudstone (RD2821, 654.1 m). B. Thick bed of sandstone within the green sandstone
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32 Page 32 of 47
and mudstone facies association (RU38-2626, 285 – 290 m), displaying abundant green mudstone
799
intraclasts (arrow). C. Photomicrograph (plane-polarised light) of a basalt clast (bas) in the green
800
sandstone (RD1625, 389.5 m. Pseudomorphs of olivine phenocrysts (arrow) contain black chromite
801
grains. D. Close-up BSE image of chromite (Cr-Sp) and hematite grains in fine-grained lamina
802
(RD916, 410.5m). E. Reflected light image of pyrite (py) intergrown with euhedral quartz and minor
803
chalcopyrite (arrows) between clasts (RD1625, 636.8 m). Note laths of hematite in the left hand
804
corner and irregular hematite in the top right.
805
Figure 7. Polymictic volcanic clast-conglomerate from the northern bedded clastic facies (A; RD3449,
806
414 m), and the southern bedded clastic facies (B; RU38-2626, 373 m). Both display rounded
807
feldspar-phyric felsic volcanic (fv) and mafic volcanic (mv) clasts. Clasts are variably altered and
808
some have irregular margins. The conglomerate is clast- to matrix-supported. The matrix consists of
809
granule conglomerate to coarse sandstone mainly composed of altered felsic and mafic volcanic
810
clasts, together with minor granitic quartz and feldspar; olivine pseudomorphs, zircon, tourmaline and
811
rutile are also present in the matrix.
812
Figure 8. Thinly bedded green and red mudstone facies association. A. Laminated red, purple and
813
grey mudstone from the northern bedded clastic facies (RD3449, 462 m). B. Microfaults in the thinly
814
bedded green and red mudstone from the northern bedded clastic facies (RD3449, 431.4 m).
815
Hematite-rich chert laminae are bright red (arrows). C. Reflected light image of red chert lamina
816
which consists mainly of quartz (blue) and very fine hematite (bright) (RD3449, 431.4 m); pores are
817
red. The red colour of these laminae is imparted by the very fine hematite. D, E. Bubble-wall shards
818
in felsic tuffaceous mudstone; combination of plane polarised and reflected light image; RD3449,
819
429.8 m. Quartz crystal fragments (D) suggest the felsic tuffaceous mudstone has a rhyolitic
820
composition.
821
Figure 9. Structural data for the bedded clastic facies areas. A. Stereoplot of the bedding data from the
822
northern bedded clastic facies (RD3450, inverted triangles; RD2765, solid dots; drill holes
823
orientations, solid squares). The poles to bedding define a fold with an overall plunge of 24° towards
824
033°; the star is the pole to the best-fit great circle. B. Stereoplot of the bedding data from the
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798
33 Page 33 of 47
southern bedded clastic facies; data from RD2766, RD2771, RD3287, RD3285 and RD 3426. The
826
poles to bedding roughly define a fold with an overall plunge of 28° towards 040°(star). C. Stereoplot
827
of the fault data from the southern bedded clastic facies (RD2766). Faults (triangles) are commonly
828
parallel to bedding (solid dots). Most have steep to subvertical dips and strike NE-SW. Some
829
orthogonal faults have NW-SE strikes. Squares are the drillhole orientation. Stereoplots were
830
completed using CoreSolutions and Openstereo software.
831
Figure 10. Fault morphologies in Olympic Dam drill core. A. Abrupt changes in bedding orientation,
832
fractures (arrow) and closely spaced joints associated with hematite- and sericite-altered zones
833
(RD1989, 377-381 m). B. Fault zone intruded by mafic dyke (dol) (RD2821, 603-607 m); baked
834
contact zone, arrow. C. Narrowly spaced fractures (<5 cm) displacing beds by millimetres to
835
centimetres and defining rotated angular clasts; fractures are healed with minerals such as hematite,
836
barite and siderite (RD2765, 462.3 m).
837
Figure 11. Selected major and trace element abundances for samples of the bedded clastic facies,
838
against TiO2 wt%. Compositional data are given in Supplementary Data Table 1.
839
Figure 12. A. Relative probability plot of zircon age data for 320 analyses of the well-sorted quartz-
840
rich sandstone facies that are less than 10 % discordant. This facies has the 1600 Ma population found
841
in other bedded clastic facies, in addition to populations at ~1740 Ma and 2500 Ma. B, C. Relative
842
probability plot of zircon age data for (B) the interbedded sandstone and red mudstone and green
843
sandstone and mudstone and (C) felsic clasts in the volcanic-clast conglomerate. Zircons in these
844
bedded clastic facies have ages around 1600 Ma. Age peaks and associated uncertainties have been
845
calculated using weighted mean age calculation.
846
Figure 13. Model for the fault-bounded, sedimentary basin in which the bedded clastic facies may
847
have accumulated. The master fault could belong to the northwest-striking regional fault set but its
848
location and orientation are not known. The pre-lithification folds in the bedded clastic facies
849
probably developed in response to remobilization down slopes orthogonal to the strike of the master
850
fault. Both the size and the orientation of the depocenter are uncertain.
Ac ce p
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825
34 Page 34 of 47
Figure1
134 °E
Ga
132 E
138 °E
Crato n
ler w
30 °S
c. 1590 Ma Gawler Range Volcanics c. 1590 Ma Hiltaba Suite
ip t
c. 1850-1750 Ma metasedimentary units
Po rt A ug ust a
1850 Ma Donington Suite pre-1850 Ma A ust ralia
Fault 0
Olympic Dam
100 km 36 °S
cr
A delaide
us
50 km
te
d
M
an
30 °S
Neoproterozoic to recent basins
Ac ce p
1595–1575 Ma Hiltaba Suite
1590 Ma Gawler Range Volcanics 1625–1615 Ma St Peter Suite 34 °S
1690–1670 Ma Tunkillia Suite 2000–1730 Ma metasedimentary units 1850 Ma Donington Suite
Late Archean (c. 2550 Ma) and Paleoproterozoic (1850–1650 Ma) 134 E°
138 E°
Page 35 of 47
Figure 2
RD 106 RU3 1 60 18 RU3 1 6020
ip t
RD2765 RD3449 RD3450
RD2768 RU38 2626 RU38 2625
RD2749
an
RD2767
RD3426 RD9 16
M
RD 1624 RD2766
RD 1989
RD3287 RD3285
te
d
RD282 1
RD 1625
RD 1627
RD896
Ac ce p
S
RD2767
RD2766
us
cr
RD 1680
30 °27 ’S 136 °54 ’E
N
RD2768
RU38_2626 270 m
RD896
,- ./01brecc ia Inte rbedded sa ndsto ne a nd red mudsto ne
RU38#2625 RU38#2626
HI JK LMNON vo lca nic c last co nglo me rate
G ree n sa ndsto ne a nd mudsto ne
m 0 10
G ra nite brecc ia
100 m 550 m
Page 36 of 47
Figure
A
us
cr
ip t
5 cm
m
E
s
E
ce Ac
D
1 cm
pt
1 cm
ed
M
f
C
an
B
H
q
g 0.5 mm
1 cm
Figure 3 – 2-column
Page 37 of 47
Figure 3
A
Hem
hfd
cr
Zr
ip t
B
Zr
us
0.5 mm
D
ed
M
an
C
100 mm
pt
200 mm
F
Ac
ce
E
5 cm
1 mm
Figure 4 – 2-column
Page 38 of 47
A
B
Hematite breccia
ip t
Thinly bedded red and green mudstone
cr
Polymictic breccia
an
us
Brecciated wellsorted quartz-rich sandstone
Granite breccia
M
Polymictic breccia
ed
100 m
C
m
f
Ac
ce
D
pt
200 mm
E
Figure 5 – 2-column
Page 39 of 47
B
ip t
A
cr
1 cm
an
us
C
M
bas
5 cm
ed
0.2 mm
Fe-ox
Barite
ce
Ac
py
pt
E
D
Cr-Sp
0.1 mm
100 mm
Figure 6 - 2-column
Cr q
Page 40 of 47
A
fv
cr
cm
ip t
mv
an
us
fv
M
B
mv
Ac
ce
pt
1cm
ed
fv
Figure 7 – 1.5-column
Page 41 of 47
A A
ip t
D
cr
5 cm
100 mm
ed
M
an
us
E
B
Bubble-wall shard
Quartz
100 mm
cm
Ac
ce
pt
C
Figure 8 - 2-column 100 mm
Page 42 of 47
an
us
cr
ip t
A
ce
pt
ed
M
B
Ac
C
Figure 9 – 1-column Page 43 of 47
ip t
A
B
us
cr
FZ
FZ
5 cm
Ac
ce
pt
ed
C
M
an
Dol
5 cm
1 cm
Figure 10 – 2-column
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Na2O wt%
MgO wt%
Cr ppm
V ppm
ce
pt
ed
P2O5 wt%
M
an
us
cr
ip t
Al2O3 wt%
Ac
TiO2 wt%
TiO2 wt%
Green sandstone and mudstone
Thinly bedded green and red mudstone
Interbedded sandstone and red mudstone
Sandstone matrix of polymictic volcanic-clast conglomerate
Figure 11 - 2-column
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Well-sorted quartz-rich sandstone
us
an
A
cr
ip t
N=158 analyses
M
N=139 analyses
ed
Sandstone and red mudstone, and green sandstone and mudstone
Ac
ce
pt
Figure 12 - 1-column B
Felsic clasts N=24 analyses
C Page 46 of 47
NW
Ac ce p
te
d
M
an
us
cr
ip t
Figure 13
Bedded clastic facies
Mudstone and sandstone
Gawler Range Volcanics
Well-sorted quartz-rich sandstone Polymictic volcanic-clast conglomerate
Hiltaba Suite Granite Paleoproterozoic and older
Page 47 of 47