Effects of hydrothermal alteration on mafic lithologies at the Olympic Dam Cu-U-Au-Ag deposit

Accepted Manuscript Effects of hydrothermal alteration on mafic lithologies at the Olympic Dam CuU-Au-Ag deposit Qiuyue Huang, Vadim S. Kamenetsky, Kathy Ehrig, Jocelyn McPhie, Maya Kamenetsky, Olga Apukhtina, Isabelle Chambefort PII: DOI: Reference:

S0301-9268(16)30295-9 http://dx.doi.org/10.1016/j.precamres.2017.02.013 PRECAM 4678

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Precambrian Research

Received Date: Revised Date: Accepted Date:

31 July 2016 17 February 2017 20 February 2017

Please cite this article as: Q. Huang, V.S. Kamenetsky, K. Ehrig, J. McPhie, M. Kamenetsky, O. Apukhtina, I. Chambefort, Effects of hydrothermal alteration on mafic lithologies at the Olympic Dam Cu-U-Au-Ag deposit, Precambrian Research (2017), doi: http://dx.doi.org/10.1016/j.precamres.2017.02.013

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Effects of hydrothermal alteration on mafic lithologies at the Olympic Dam

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Cu-U-Au-Ag deposit

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Qiuyue Huang1*, Vadim S. Kamenetsky1, Kathy Ehrig2, Jocelyn McPhie1, Maya Kamenetsky1, Olga Apukhtina1 , Isabelle

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Chambefort1,3

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1

School of Physical Sciences, University of Tasmania, Private Bag 79, Hobart, Tasmania, Australia 7001

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2

BHP Billiton Olympic Dam, Adelaide, South Australia 5001, Australia

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3

GNS Science, Wairakei Research Centre, Taupo 3377, New Zealand

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*Corresponding author. Tel.: +61 413159276; Email: [email protected]

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Abstract

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Two groups of mafic to ultramafic lithologies occur at the Olympic Dam copper-uranium-

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gold-silver deposit, South Australia. The first group consists of olivine-phyric basalt and mafic dykes

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belonging to the ca. 1590 Ma Gawler Range Volcanics; the second group is the Olympic Dam dolerite

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affiliated with the ca. 820 Ma Gairdner Dyke Swarm. Both groups of mafic rocks occur within and

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adjacent to the Olympic Dam Breccia Complex, the immediate host to the deposit, and have been

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variably altered. They contain a number of secondary minerals, mainly including chlorite, sericite,

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iron oxides and carbonates. Drill core assays of the ca. 1590 Ma olivine-phyric basalt reflect the

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variable abundance of secondary minerals such as carbonates and iron oxides (e.g. up to 26 wt. %

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CO2 and 50 wt.% Fe2O3 ). Positive correlations between compatible Cr and other incompatible

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elements (e.g. Ti and Zr) in the olivine-phyric basalt suggest whole-rock mass and/or volume loss due

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to hydrothermal alteration, in accordance with the previously proposed role of the olivine-phyric

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basalt as an important source of Cu using Sm-Nd isotope constraints. However, rigorous mass balance

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calculation is not feasible due to a lack of fresh equivalents. Geochemical comparisons between the

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least-altered ca. 820 Ma Olympic Dam dolerite and the more-altered equivalents reported in this study

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reveal that the concentrations of some elements have been affected by hydrothermal alteration. In

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particular, Mg, K, Cs, Ba, Rb, Pb, U, and heavy rare earth elements have mostly been enriched; Ca, Sr

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and light rare earth elements have mostly been depleted; and Si, Fe, and Na have been affected both

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ways. Elevated Zn and Pb but depleted Cu concentrations in some Olympic Dam dolerite samples

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suggest that these metals were also mobilized. Petrographic observation and drill core assays are

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consistent with sodic alteration having been responsible for the Cu depletion in some dolerite samples.

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However, there is currently no evidence that this had any significant impact on the Cu content and

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distribution of the Olympic Dam deposit. Further, alteration of both groups of mafic rocks implies

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multiple hydrothermal events at the Olympic Dam deposit.

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Key words

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Olympic Dam; Olivine-phyric basalt; Dolerite; Hydrothermal alteration; Copper source

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1. Introduction

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The Olympic Dam copper-uranium-gold-silver deposit contains the world’s largest uranium,

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fifth largest copper and third largest gold resource (Ehrig et al., 2012). It was discovered by Western

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Mining Corporation in 1975 based on a sediment-hosted copper deposit model developed in Haynes

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(1972). This model proposed that sodic-altered mafic lithologies (e.g. basalts) were a potent source of

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Cu, and that sedimentary formations on top of such altered mafic lithologies were trap rocks, hence

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exploration targets (Haynes, 1992; 2006). The sodic-altered and Cu-depleted signature of Proterozoic

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basalt in South Australia drew attention to the Stuart Shelf which contains flat-lying Neoproterozoic

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sedimentary formations (Haynes, 2006). Geophysical investigation of the Stuart Shelf defined five

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gravity and aeromagnetic anomalies of interest, including the anomaly at Olympic Dam which had the

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shallowest interpreted depth to the source (Haynes, 2006). The anomaly at Olympic Dam was drilled,

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but no Cu-sulfides were found in the Neoproterozoic Stuart Shelf sedimentary formations. However,

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in the underlying Gawler Craton basement, Cu-sulfides were discovered, in what was later referred to

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as the Olympic Dam Breccia Complex (ODBC) (Reeve et al., 1990). The ODBC is the immediate

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host to the Olympic Dam deposit and occurs at the intersections of a number of faults (Reeve et al.,

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1990; Ehrig et al., 2012).

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Mafic to ultramafic lithologies have been intersected by subsequent drilling at Olympic Dam.

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There are two groups present. The first group is correlated with the ca. 1590 Ma Gawler Range

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Volcanics (Huang et al., 2016), and consists of intensely altered olivine-phyric basalt and mafic dykes

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with variable textures (aphanitic, porphyritic, and doleritic) (Ehrig et al., 2012). The second group

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mainly comprises doleritic dykes (named the Olympic Dam dolerite), belonging to the ca. 820 Ma

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Gairdner Dyke Swarm in the Gawler Craton (Huang et al., 2015).

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The first group of mafic rocks has been proposed to be an important Cu source to the deposit

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using Sm-Nd isotopic constraints (Johnson, 1993; Johnson and McCulloch, 1995). In this study,

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recently available diamond drill core assays and whole-rock analyses of the ca. 1590 Ma olivine-

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phyric basalt (Huang et al., 2016) and the ca. 820 Ma Olympic Dam dolerite, combined with

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petrography, allow us to examine the textural and compositional effects of hydrothermal alteration on

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both groups of mafic rocks, and to critically assess evidence for links between hydrothermal alteration

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and Cu addition or depletion. Also, considering that the ca. 820 Ma Olympic Dam dolerite has been

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variably altered (Huang et al., 2015), we investigate whether metals (i.e. Pb, Zn and Cu) in the

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younger dolerite have been mobilized (enriched or depleted) as a result of hydrothermal alteration.

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2. Geological background

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The Olympic Dam deposit is located in the northeastern Gawler Craton. The deposit occurs in

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Mesoproterozoic rocks below undeformed Neoproterozoic and Cambrian sedimentary formations of

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the Stuart Shelf. A synthesis of the geology of the Gawler Craton can be found in Hand et al. (2007)

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and Reid and Hand (2012). The Olympic Dam deposit is situated where two large igneous provinces

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(LIP) overlap, i.e. the ca. 1590 Ma Gawler silicic LIP (SLIP; Allen et al., 2008) and the ca. 820 Ma

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Gairdner LIP (Claoué-Long and Hoatson, 2009).

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The consanguineous Gawler Range Volcanics (GRV) and Hiltaba Suite granitoids have ages

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between ca. 1595 and 1575 Ma (Blissett et al., 1993; Hand et al., 2007; Jagodzinski, 2014), and

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constitute the Gawler SLIP. The GRV have a maximum preserved thickness of ~1.5 km; they are

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exposed over more than 25,000 km2 in the central Gawler Craton and also extend farther north into 3

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the Mt Woods domain (Harris et al., 2013) and east beneath the Stuart Shelf (Blissett et al., 1993;

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Allen et al., 2003). Mafic units occur only in the lower GRV and are subordinate (~10 vol.%) to felsic

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units (Blissett et al., 1993); the upper GRV consist of widespread, thick (250-300 m), rhyolitic and

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dacitic lavas (Garner and McPhie, 1999; Morrow and McPhie, 2000; Allen and McPhie, 2002; Allen

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et al., 2003). At Olympic Dam, the ODBC is situated within the Roxby Downs Granite, a member of

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the Hiltaba Suite (Fig. 1, Creaser, 1989). Mafic units at Olympic Dam that are correlated with the

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GRV consist of intensely altered olivine-phyric basalt, and aphanitic, doleritic and porphyritic mafic

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dykes (Ehrig et al., 2012; Huang et al., 2016). The intensely altered olivine-phyric basalt occurs in

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two situations: 1) an apparent flat-lying unit which has been intersected in six drill holes (intersections

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ranging in thickness from 15 to 45 m) directly beneath the unconformity between the ODBC and the

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Neoproterozoic to Cambrian sedimentary cover (drill hole locations marked in Fig. 1, see Fig. 2 for a

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simplified log of drill hole RD1756-1756W1); 2) olivine-phyric basalt dykes (a few centimetres to ~1

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m wide) which are NW-striking and steeply E-dipping.

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The Gairdner Dyke Swarm in the Gawler Craton was first recognized on aeromagnetic

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images as a NW-trending array of linear magnetic anomalies (Boyd in Goode, 1970). The dykes

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intruded the Gawler Craton at ca. 820 Ma (Wingate et al., 1998), and together with other igneous

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suites such as the Wooltana Volcanics and Beda Volcanics, constitute the Gairdner LIP (also known as

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the Willouran Basic Province, Crawford and Hilyard, 1990; Claoué-Long and Hoatson, 2009; Wang et

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al., 2010; Huang et al., 2015). This LIP formed contemporaneously with the break-up of the

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supercontinent Rodinia (Li et al., 2008). At Olympic Dam, the Gairdner Dyke Swarm has been

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recognized as mainly doleritic dykes intruding the Roxby Downs Granite and the ODBC (Fig. 1).

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These subvertical dykes are commonly several metres to approximately 100 m in width and can be

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traced for up to 3 km along a NW-strike. The presence of sharp chilled margins within dykes suggests

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multiple generations of dykes. Compared to the ca. 1590 Ma mafic GRV at Olympic Dam, the

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Olympic Dam dolerite is generally fresher and has a higher magnetic susceptibility. The Olympic

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Dam dolerite can also be distinguished from mafic GRV based on their compositions: the ca. 820 Ma

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dolerite is characterized by a lower Zr/TiO2 ratio of ~50 compared to that of mafic GRV (~150 to 250,

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Huang et al., 2016).

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3. Analytical techniques

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3.1 Mineral compositions and element mapping

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Preliminary semi-quantitative analyses of minerals (i.e. Cr-Spinel and magnetite) were

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conducted using an FEI MLA650 environmental scanning electron microscope (ESEM) fitted with

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two Bruker XFlash 5030 energy dispersive spectrum (EDS) analysers at the Central Science

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Laboratory, University of Tasmania. High magnification images, including secondary electron (SE)

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images and back-scattered electron (BSE) images, were also acquired using the same instrument.

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Element (i.e. Si, Al, Fe, Mg, Ca, Na, Cu) mapping of a former plagioclase crystal was conducted

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using a Hitachi SU-70 field emission scanning electron microscope (FESEM) in the same laboratory.

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X-rays were detected by an Oxford XMax80 energy dispersive spectrometer. The operating conditions

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were accelerating voltage of 15 keV and beam current of 8nA. The pixel size for each map image is

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491*288. Quantitative analyses of chlorite and sericite were conducted using the same instrument.

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Hornblende Kakanui (Jarosewich et al., 1980) was used as the secondary standard. The operating

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conditions were accelerating voltage of 15 keV and beam current of 3nA.

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3.2 Whole-rock compositions

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Whole-rock samples of the Olympic Dam dolerite were crushed in a WC (Tungsten Carbide)

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mill for X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS)

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analyses at the University of Tasmania. Major and some trace elements (V, Cr, Ni, Cu, Zn, Rb, Sr, Y,

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Zr, Nb, Ba, and La) were measured by XRF; other trace elements were analysed by ICP-MS. Samples

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were digested in HF/H2SO4 with the PicoTrace high-pressure digestion equipment and analysed with

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an Agilent 4500 ICP-MS. XRF analyses were made on a Philips PW1480 X-ray Fluorescence

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Spectrometer. Detection limits for trace elements in ICP-MS are ≤0.01 ppm (rare earth elements) and

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≤0.5 ppm for other elements, except As (5 ppm). Comparison of XRF and ICP-MS trace element data

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indicates a good correlation between the two methods.

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Drill core samples of the ca. 1590 Ma olivine-phyric basalt (flat-lying basalt unit) taken at 1-

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m intervals and the ca. 820 Ma Olympic Dam dolerite in drill hole RU65-8333 taken at 2.5-m

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intervals were assayed by Genalysis/Intertek Laboratory (Adelaide). The basalt samples were assayed

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using a combination of four-acid digestion and inductively coupled plasma-optical emission

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spectroscopy (ICP-OES) for Cu, Ag, As, Co, Ni, Pb, and Zn; lithium metaborate/tetraborate fusion

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ICP-OES for Ba, Al, Ca, Ce, Fe, K, La, Mg, Mn, Na, P, Si, Sr, Ti, Y, and Zr; inductively coupled

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plasma-mass spectroscopy (ICP-MS) for U3O8 , Bi, Sb, and Mo; induction furnace-infrared

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spectrometry (IF-IS) for CO2 and S; fire assay-flame atomic absorption spectrometry (FA-AAS) for

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Au. Other elements including Be, Cl, Cr, Cs, Dy, Er, Eu, F, Ga, Gd, Hf, Ho, Li, Lu, Nb, Nd, Pr, Rb, Sc,

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Se Sm, Sn, Ta, Tb, Te, Th, Tl, Tm, V, W and Yb have been measured via Mineral Liberation Analyzer

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(MLA; Gu, 2003). Further details are in Ehrig et al. (2012). The dolerite samples were assayed using

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the same method but for a smaller group of elements, including Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, S,

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Sc, V, Cr, Co, Ni, Cu, Zn, As, Sr, Y, Zr, Mo, Ag, Sb, Ba, La, Ce, Au, Pb, Bi, and U.

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4. Hydrothermal assemblages in mafic lithologies at Olympic Dam

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Detailed petrographic descriptions of primary igneous mineralogy and texture of the ca. 1590

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Ma olivine-phyric basalt and the ca. 820 Ma Olympic Dam dolerite are given in Huang et al. (2016)

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and Huang et al. (2015). This section focuses on the petrographic features attributed to secondary

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processes (i.e. hydrothermal alteration).

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4.1 Ca. 1590 Ma olivine-phyric basalt

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The effects of hydrothermal alteration are particularly intense within the ca. 1590 Ma olivine-

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phyric basalt (flat-lying unit and dykes), primary igneous mineralogy and texture having been largely

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obliterated. Typical olivine-phyric basalt at Olympic Dam contains ~5 to 20 vol.% pseudomorphed

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olivine phenocrysts in a groundmass that now mainly consists of fine-grained sericite, carbonates and

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hematite and quartz (Fig. 3a). Cr-spinel mostly occurs as inclusions within olivine pseudomorphs and

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is rarely present in the groundmass. The compositions of Cr-spinel (~400 analyses) have been

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previously reported in Huang et al. (2016). Some samples that show a strong alignment of elongate

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olivine pseudomorphs (Fig. 3b and c) contain a higher volume percentage (up to ~30 vol. %) of

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olivine pseudomorphs. In these cases, Cr-spinel is also widespread in the groundmass (Fig. 3b and d).

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Apart from some Cr-spinel, primary igneous minerals in the olivine-phyric basalt have been

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completely replaced. The majority of the Cr-spinel crystals have been partly or completely replaced

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by chlorite + magnetite, or carbonates. Chlorite (chamosite-clinochlore series, Mg/Fe molar ratios of

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0.4 to 1.9, average 0.9, Inline Supplementary Table S1), sericite (muscovite, K/Al molar ratios of 0.3

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to 0.5, average 0.4, Inline Supplementary Table S1), carbonates (mostly ankerite-dolomite solid-

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solution, and minor siderite-calcite solid-solution), iron oxides (magnetite and hematite) and quartz

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are the dominant secondary phases. Minor fluorite, rare earth element (REE)-bearing minerals (e.g.

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bastnasite, monazite, florencite, synchysite, and xenotime), apatite, barite and sulfides (bornite,

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chalcopyrite and pyrite) are also present. Cr-bearing magnetite in the groundmass has round to sub-

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round shapes and is compositionally zoned. The cores contain higher concentrations of Cr, whereas

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the rims consist of magnetite (low or no Cr) and chlorite. Magnetite also occurs (Fig. 3e) in

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aggregates of magnetite-apatite ± chlorite ± quartz. In this assemblage, magnetite contains low or no

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Cr. Apatite occurs both within magnetite crystals and in the interstices between magnetite (Fig. 3f).

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This assemblage has been partly or completely replaced by hematite ± fluorite. Veinlets of chlorite,

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quartz, carbonates and iron oxides cut across the olivine-phyric basalt.

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4.2 Ca. 820 Ma Olympic Dam dolerite

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The ca. 820 Ma Olympic Dam dolerite is variably altered (Huang et al., 2015). Hydrothermal

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alteration accompanied the emplacement of the ca. 820 Ma Olympic Dam dolerite, verified by the U-

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Pb dating of secondary apatite and titanite (Huang et al., 2015; Apukhtina et al., 2016). Euhedral to

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sub-euhedral clinopyroxene and plagioclase are the dominant mineral phases in the least-altered

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dolerite. Minor Ti-magnetite and accessory apatite also occur. The petrographically more altered

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and/or brecciated Olympic Dam dolerite is located mostly along the immediate intrusive contacts with

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the Roxby Downs Granite and the ODBC, where the dyke margins have been replaced mainly by

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chlorite

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Supplementary Table S1), quartz, sericite (muscovite, K/Al molar ratios of 0.3 to 0.4, average 0.4,

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Inline Supplementary Table S1), albite, and carbonates and minor pumpellyite, apatite, titanite, iron

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oxides (i.e. magnetite and hematite) and sulfides (e.g. pyrite, chalcopyrite, galena and sphalerite).

(chamosite-clinochlore series, Mg/Fe molar ratios of 0.8 to 2.7, average 2.1, Inline

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In many cases, intrusive contacts with the Roxby Downs Granite are marked by brecciated

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granite and a chilled margin on the dolerite (Fig. 4). The brecciated granite is represented by granite-

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derived quartz cemented by chlorite + sericite + magnetite ± apatite. The dykes locally show a

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spherulitic texture within the chilled margin (Fig. 4b). The spherulites commonly contain crystal

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fibres of chlorite, sericite and iron oxides, together with chalcopyrite (Fig. 4b). Thin hydrothermal

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veinlets (Fig. 4a and c) that cut the dolerite consist of chlorite, sericite, magnetite and minor

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carbonates and apatite. Magnetite in the veinlets is characterized by oscillatory zones (Fig. 4d) and

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contains chalcopyrite and Au-Ag-bearing tellurides and/or sulfides (Fig. 4e). The hydrothermal

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veinlets are also surrounded by disseminated chalcopyrite.

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Dolerite that intruded the ODBC is brecciated along the intrusive contacts. In some cases, the

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primary minerals in the dolerite have been replaced by an assemblage of magnetite, apatite, chlorite,

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and quartz with minor monazite, uraninite, coffinite, pyrite and chalcopyrite (Fig. 5). Magnetite and

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apatite crystals are sub-round in shape; apatite crystals commonly show compositional zones (Fig. 5b).

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Uraninite, coffinite and/or chalcopyrite occur in the interstices between magnetite and quartz crystals

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(Fig. 5b, c, and d), within magnetite crystals (Fig. 5d), or rarely within monazite crystals (Fig. 5e).

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Elemental mapping of an altered plagioclase crystal in one dolerite sample has been

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conducted; the plagioclase lath has been partly replaced by albite, chlorite and chalcopyrite (Fig. 6).

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This mapping will be further discussed in the last discussion section.

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In terms of secondary mineral compositions, compared to the sericite in the ca. 1590 Ma

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olivine-phyric basalt, sericite in the younger dolerite shows higher K and Al contents. As for the

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compositions of chlorite, the dolerite mainly contains clinochlore with higher Mg/Fe molar ratios and

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higher Mg but lower Fe contents, compared to the olivine-phyric basalt (Fig. 7).

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5. Geochemistry

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5.1 Geochemistry of the ca. 1590 Ma olivine-phyric basalt

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Olivine-phyric basalt (flat-lying unit) contains less than 11.1 wt.% of MgO (Fig. 8, Inline

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Supplementary Table S2), and shows a wide range of variations in other major components, for

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example, Al2 O3 (5.4 to 25.0 wt.%), Fe2O3 (3.8 to 50.2 wt.%, total Fe as Fe2O3) and K2O (0.6 to 8.1

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wt. %). MgO (0.6 to 11.1 wt.%), CaO (0.6 to 17.1 wt.%) and MnO (0 to 2.5 wt.%) are positively

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correlated with CO2 (0 to 26 wt.%), whereas SiO2 (21 to 64 wt.%) is negatively correlated with CO2

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(Fig. 9).

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Near linear positive correlations exist among a number of components including Al2O3, K2O,

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Cr and high field strength elements (HFSE) such as Ti, Nb, Ta, Zr, Hf and Th (Figs. 8 and 10). Light

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rare earth elements (LREE, i.e. La, Ce, Nd, and Sm) show positive linear correlations, and so do

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heavy rare earth elements (HREE, i.e. Dy, Ho, Er, Tm, Yb, and Lu). However, LREE do not correlate

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with HREE. On the chondrite-normalized REE variation diagram (Fig. 11), all the samples show

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relatively consistent patterns, characterized by LREE enrichment compared to HREE and slight

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negative Eu anomalies, irrespective of a large variation in REE concentrations. Further, some analyses

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have extremely high concentrations of certain elements, most strikingly ~6,000 ppm of Cr, ~280 ppm

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of Nb, ～14 ppm of Th and ~370 ppm of Zr (Fig. 10, Inline Supplementary Table S2). As for other

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trace elements (e.g. large-ion lithophile elements, LILE) which are commonly more hydrothermally

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mobile compared to Cr and HFSE, there are no well-constrained linear co-variations, except for Sr

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and Ba. Components which are commonly positively correlated in mafic to ultramafic igneous suites,

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such as MgO, Ni and Cr, do not show linear correlations.

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5.2 Geochemistry of the ca. 820 Ma Olympic Dam dolerite

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Data on the least-altered Olympic Dam dolerite (clinopyroxene and plagioclase mostly

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preserved) and Gairdner Dyke Swarm (outside Olympic Dam) show primary compositional trends

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(Huang et al., 2015) and have been plotted for comparison (Fig. 12). Petrographically more-altered

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Olympic Dam dolerite samples (i.e. clinopyroxene and plagioclase mostly replaced) follow the same

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primary magmatic trend defined by the least-altered Olympic Dam dolerite and Gairdner Dyke Swarm,

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but show a wider spread of major element abundances, in particular SiO2, FeOt (total Fe as FeO),

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MgO, CaO, Na2O, and K2O. Compared to the least-altered dolerite, the more-altered dolerite samples

236

display elevated MgO (up to 12 wt.%, loss on ignition-, LOI-, free) and K2O (up to 4.4 wt.%, LOI-

237

free), but generally lower CaO (down to 0.4 wt.%, LOI-free) contents at similar TiO2 contents (Fig. 12,

238

Table 1).

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With increasing TiO2 contents, compatible elements (e.g. Cr and Ni) in the Olympic Dam

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dolerite and Gairdner Dyke Swarm decrease, whereas incompatible elements (e.g. HFSE) increase

241

(Fig. 13), as expected for magmatic fractionation. Among all the trace elements, Cr, Ni, and HFSE

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(including Y, Zr, Nb, Hf, Ta, and Th) exhibit near linear correlations with TiO2 , though slight scatter is

243

commonly evident in binary plots. However, for other trace elements, for example, LILE (Cs, Ba, Rb,

244

and Sr) and REE, the more-altered Olympic Dam dolerite samples deviate from the primary magmatic

245

trend defined by the least-altered counterparts (Fig. 14). Assays (Fig. 15, Inline Supplementary Table

246

S3) of samples from a dolerite dyke intersected by drill hole RU65-8333 are within the range of

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whole-rock compositions.

248 249

A summary of the petrographical and geochemical features of the ca. 1590 Ma olivine-phyric basalt and the ca. 820 Ma Olympic Dam dolerite is given in Inline Supplementary Table S4.

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6. Discussion

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6.1 Effects of alteration on the ca. 1590 Ma olivine-phyric basalt at Olympic Dam

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The ca. 1590 Ma olivine-phyric basalt contains ~6.2 wt.% MgO on average (Fig. 8) which is

254

below the 16 wt.% expected given the volume (~20 vol.% for undeformed samples) and compositions

255

(inferred from compositions of a large number of Cr-spinel inclusions) of former olivine phenocrysts

256

(Huang et al., 2016). The positive correlations among MgO, CaO, MnO and CO2 indicate that these

257

components reside in secondary carbonates (mostly ankerite-dolomite solid-solution, and minor

258

siderite-calcite solid-solution) in the basalt (Fig. 9). Strongly sericite-altered samples are enriched in

259

K2O, Al2O3 (Fig. 8) and Rb. Drill core intervals that contain abundant iron oxides display high Fe2O3

260

contents (up to 50 wt.%).

261

The extreme trace element abundances (e.g. up to ~6,000 ppm Cr, ~280 ppm Nb, ~370 ppm

262

Zr, Fig. 10) of some assays are unlikely to be primary igneous values for basalts or picrites. They must,

263

therefore, result either from the hydrothermal introduction of Cr, Nb and Zr, and/or from the

264

concentration of these elements in the residual left as other elements were removed. In this regard, we

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note the near linear co-variations between Cr and HFSE (Fig. 10). In the evolution of mafic magmas

266

(i.e. partial melting, cooling and crystallization), Cr is a compatible element, and its concentration

267

decreases with increasing concentrations of incompatible elements such as HFSE (cf. Rollinson,

268

1993). Therefore, the positive correlations between Cr and HFSE (Fig. 10) are most likely a result of

269

variable degrees of whole-rock mass and/or volume loss (i.e. density variation) due to hydrothermal

270

modification of primary compositions. During the alteration, we infer that Cr and HFSE were retained

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in secondary iron oxides (i.e. Cr-bearing magnetite and hematite) in the basalt whereas other

272

components were leached into the hydrothermal fluids. More intense whole-rock leaching (mass loss)

273

produced higher concentrations of Cr and HFSE. Mass balance calculations based on Cr and Nb

274

concentrations using a less-altered analogue (olivine-phyric dyke at Wirrda Well, Huang et al., 2016)

275

as a reference for the initial compositions of the olivine-phyric basalt at Olympic Dam, suggest up to

276

70 to 80 % mass losses of the bulk of the latter in the course of hydrothermal alteration. Such large

11

277

mass losses are consistent with the higher-than-normal concentrations of olivine phenocrysts (Figs 3b

278

and c) and strongly degraded state of most drill core intersections. Much lower mass losses can be

279

expected in rare samples in which original igneous textures are preserved (Fig. 3a). However, since

280

there is no fresh ca. 1590 Ma olivine-phyric basalt sample recovered within and/or near the deposit,

281

rigorous mass balance calculations to quantify the effects of alteration are not possible at this stage.

282

We conclude that the compositions of the ca. 1590 Ma olivine-phyric basalt have been so

283

intensely modified by hydrothermal alteration that they largely reflect the various secondary minerals

284

(dominantly chlorite, sericite, carbonates, quartz and iron oxides) and secondary processes (mass

285

and/or volume loss) rather than primary geochemical features. Also, the rare occurrence of albite and

286

low Na2O contents (~0.13 wt.% on average, Inline Supplementary Table S2) in the basalt is

287

contradictory to the addition of Na predicted by the sediment-hosted Cu deposit exploration model

288

(Haynes, 1992; 2006). It is possible that there was an episode of sodic alteration of the basalt and that

289

the products of this sodic alteration were subsequently entirely replaced by the current dominant

290

secondary assemblage (i.e. chlorite, sericite, hematite, carbonates and quartz), however, we have

291

found no petrographic or geochemical evidence that sodic alteration has affected the ca. 1590 Ma

292

olivine-phyric basalt at Olympic Dam.

293

6.2 Effects of alteration on the ca. 820 Ma Olympic Dam dolerite

294

Hydrothermal modification of the Olympic Dam dolerite has been assessed by comparing

295

petrographically more-altered samples analysed in this study against the least-altered equivalents

296

(Huang et al., 2015).

297

Major components including Al2O3 and P2O5, and trace elements such as Cr, Ni, Y and HFSE

298

(i.e. Zr, Nb, Hf, Ti and Ta) are among the least-altered constituents in the dolerite, as their abundances

299

in the more-altered samples comply with the primary compositional range (Figs. 12 and 13). The

300

negative correlation between Cr and TiO2 in the dolerite (Fig. 13) most likely represents a primary

301

magmatic trend, suggesting that no significant whole-rock mass and/or volume losses resulted from

302

alteration, in contrast to the older olivine-phyric basalt (Fig. 10).

12

303

Elevated MgO and K2O, and depleted CaO contents in the more-altered samples probably

304

reflect hydrothermal addition and removal, respectively (Fig. 12). Inconsistent variations in SiO2,

305

FeOt and Na2O contents imply that these components have been added or depleted in the more-altered

306

samples. Concentrations of hydrothermally mobile trace elements (e.g. LILE) have been modified to

307

variable degrees, and such modifications can be observed on element vs. TiO2 binary diagrams: Cs,

308

Ba, and Rb were mostly enriched in the dolerite whereas Sr was largely depleted (Fig. 14). LREE (e.g.

309

La) may have behaved differently from HREE (e.g. Lu) as a result of alteration (Fig. 14). The LREE

310

are depleted in the more-altered dolerite, whereas the HREE are enriched in the same rocks. Uranium

311

and Pb have similar compatibility to Nb and P, respectively, during mafic magmatism (i.e. partial

312

melting, cooling and crystallization) (Sun and McDonough, 1989). Niobium and P are among the

313

least-modified components in the dolerite, as aforementioned. Therefore, U/Nb and Pb/P ratios will

314

remain the same in the dolerite unless U and/or Pb have been affected by hydrothermal alteration. The

315

least-altered dolerite shows generally narrower and lower U/Nb ratios compared to the more-altered

316

counterparts, implying that U was introduced to the dolerite, in particular the more-altered dolerite, as

317

a result of hydrothermal alteration (Fig. 14). The wide range of Pb/P ratios (Fig. 14) and the elevated

318

Pb concentrations (~2 orders of magnitude, Fig. 14) indicate that Pb contents in both the least- and

319

more-altered samples have been modified.

320

6.3 Ca. 820 Ma Olympic Dam dolerite: were metals mobilized?

321

Though not as intensely altered as the ca. 1590 Ma basalt, the ca. 820 Ma dolerite at Olympic

322

Dam has also been hydrothermally altered. Here we consider whether metals (i.e. Cu, Pb and Zn) in

323

the younger dolerite have been mobilized.

324

An investigation of some trace elements concentrations (i.e. Zr, Zn, Cu and Pb, Fig. 16) in the

325

Olympic Dam dolerite and a comparison of the dolerite with well-studied unmineralized dolerite

326

suites, may resolve this question. Zirconium is an incompatible element during magmatic evolution

327

and presumably immobile during hydrothermal alteration (Rollinson, 1993). Its concentration

328

increases with increasing TiO2 contents, representing a primary magmatic signature, evident from

13

329

dolerite suites including the Gairdner Dyke Swarm, the Olympic Dam dolerite, Tasmania Dolerite,

330

Karoo and Deccan dolerite (Fig. 16). Zinc may be geochemically similar to Zr during magmatic

331

evolution but it is more sensitive to hydrothermal alteration (Fig. 16): the variably altered Olympic

332

Dam dolerite shows irregular and commonly elevated Zn concentrations in contrast to other

333

unmineralized mafic suites which display near linear Zn-TiO2 correlations. Copper is also

334

incompatible during dolerite evolution, as inferred from the positive near-linear Cu-TiO2 correlation

335

shown by the Gairdner Dyke Swarm, most of the least-altered Olympic Dam dolerite, Tasmania

336

Dolerite, Karoo and Deccan dolerite (Fig. 16). From the same diagram, we infer that some Olympic

337

Dam dolerite samples, i.e. most more-altered dolerite samples (~17 out of 23, ~74%), and a small

338

number of the least-altered equivalents (~16 out of 91, ~18%) are depleted in Cu. We may speculate

339

that the Olympic Dam dolerite was a subordinate Cu source to the Olympic Dam deposit, though there

340

is no evidence for the net movement of Cu from the dykes and their parental magma to the deposit.

341

Finally, the Olympic Dam dolerite shows a wider range of Pb concentrations than other comparable

342

mafic suites; the elevated Pb concentrations in the majority of the dolerite indicate Pb mobilization

343

and probable addition of Pb from hydrothermal fluids into the variably altered dolerite.

344

6.4 Sodic alteration of the ca. 820 Ma Olympic Dam dolerite

345

In the exploration model leading to the discovery of the Olympic Dam deposit (Haynes, 1972;

346

2006), the ideal copper source, i.e. altered mafic lithologies, gain Na and are depleted in Cu, as well

347

as other components (e.g. Sr and S), as a result of hydrothermal alteration.

348

The negative correlation between Cu concentration and Na2 O/CaO ratio (Fig. 17) of the ca.

349

820 Ma Olympic Dam dolerite, possibly reflects Cu depletion coupled with sodic alteration (i.e.

350

albitization of plagioclase). Drill core assays of the Olympic Dam dolerite may provide additional

351

evidence for sodic alteration accompanied by Cu depletion. Figure 15 shows selected major and trace

352

element compositions of a dolerite dyke that has intruded the Roxby Downs Granite. This drill hole

353

(RU65-8333) intersects the ODBC, the Roxby Downs Granite and the dolerite sub-horizontally.

354

Abundances of some major element oxides and trace elements (e.g. SiO2, TiO2, Fe2O3(t), total Fe as

14

355

Fe2O3 , MgO and Ni) show slight variations within the range of the compositional fields of the dolerite

356

(Figs. 12 and 13). Since Ni is a compatible element in the evolution of mafic magma (cf. Rollinson,

357

1993), the concave pattern in Ni content agrees with the inward cooling and crystallization of the dyke.

358

The increasing Na2O content in the first 20-m interval (462.5 to 482.5 m) of the dolerite is

359

accompanied by decreasing CaO and Cu concentrations. These trends probably also result from sodic

360

alteration coupled with Cu depletion.

361

Though sodic alteration coupled with Cu depletion, in accordance with the exploration model

362

(Haynes, 1992; 2006), has been revealed in the younger Olympic Dam dolerite, this pattern is not

363

consistently observed and/or preserved in every sample. One possible explanation is that sodic

364

alteration was not pervasive and only locally affected the Olympic Dam dolerite. One additional

365

complication is that albitized plagioclase crystals in a few dolerite samples contain very fine crystals

366

of chalcopyrite (Fig. 6). Element mapping of one former plagioclase crystal using FE-SEM has

367

revealed that the albite has almost replaced the whole plagioclase lath, except for a small portion

368

preserved in the middle of the former plagioclase crystal (Fig. 6f and g). Chlorite is present within the

369

albitized plagioclase and is mainly distributed along cleavage surfaces within the plagioclase (Fig. 6d

370

and e); chalcopyrite is widely dispersed within the former plagioclase lath (Fig. 6h and i). One

371

possible way to interpret this phenomenon is that Cu in the dolerite was mobilized and re-precipitated

372

as chalcopyrite in the altered plagioclase, i.e. Cu within the dolerite was locally redistributed.

373

Subsequent leaching of Cu from the chalcopyrite in the altered plagioclase by hydrothermal fluids

374

would cause overall Cu depletion of the dolerite, as previously inferred from the whole-rock

375

compositions (Fig. 16). However, whether the two processes (i.e. Cu depletion and sodic alteration)

376

were coupled, as required by the exploration model, or independent, is unknown in this case, and

377

further study of this phenomenon is required. The impact of Cu mobilization (e.g. depletion), revealed

378

in some dolerite samples, on the distribution and total resource of Cu in the Olympic Dam deposit

379

remains to be determined.

15

380

7. Conclusions

381

Mafic rocks have long been proposed as important metal sources, especially Cu, for the

382

Olympic Dam deposit. The ca. 1590 Ma olivine-phyric basalt at Olympic Dam has experienced

383

intense alteration, and the present compositions reflect the compositions of various secondary

384

minerals (chlorite, carbonates, quartz and iron oxides) and secondary processes (e.g. mass and/or

385

volume loss). The ca. 1590 Ma olivine-phyric basalt lacks the secondary minerals typically associated

386

with sodic alteration (e.g. albite) and whole-rock Na contents are very low (~0.13 wt.% on average).

387

Metal (e.g. Cu, Pb and Zn) mobilization has been detected in the ca. 820 Ma Olympic Dam dolerite

388

and some dolerite samples are depleted in Cu. The Cu depletion is occasionally associated with sodic

389

alteration recorded by albitization of plagioclase. It remains to be determined whether the Cu

390

mobilization in the dolerite had any significant impact on the Cu content and distribution of the

391

Olympic Dam deposit. Both generations of mafic rocks are hydrothermally altered, indicating that at

392

least two separate episodes of hydrothermal activity have occurred at the Olympic Dam deposit.

393

Acknowledgments

394

This research was supported by BHP Billiton Olympic Dam and the Australian Research

395

Council (ARC Linkage Grant ‘The supergiant Olympic Dam U-Cu-Au-REE ore deposit: towards a

396

new genetic model’). We thank Dr. Karsten Goemann and Dr. Sandrin Feig (CSL, University of

397

Tasmania) for helping with SEM analyses. We benefited from discussions with Ken Cross (Terramin

398

Australia Ltd), Dr. Sarah Gilbert, Dr. Ivan Belousov, Jay Thompson, and Dr. Sebastien Meffre

399

(CODES, University of Tasmania). Dr. Patrick Williams, one anonymous reviewer, and editor

400

Guochun Zhao provided valuable comments which helped improve the manuscript, and they are also

401

acknowledged here.

402

Figure Captions

403

Fig. 1 Simplified geological map at –350 mRL, showing the distribution of unaltered Roxby Downs

404

Granite, outer limits of significant brecciation and iron metasomatism (5 wt.% Fe contour), granite16

405

rich breccia (5–20 wt.% Fe), hematite-rich breccia (>20 wt.% Fe), green sandstone and mudstone,

406

interbedded sandstone and red mudstone, thinly bedded green and red mudstone, polymictic volcanic-

407

clast conglomerate, hematite-rich breccia consisting of dominantly porphyritic felsic volcanic clasts

408

(VBx) and the ca. 820 Ma Olympic Dam dolerite. The locations of drill holes intersecting the ca. 1590

409

Ma flat-lying olivine-phyric basalt (RD6, RD1756, RD635, RD917A, RD3008) and the ca. 820 Ma

410

Olympic Dam dolerite (RD2773, RD271, RU39-5426, RU65-8333, RU65-8337, RD222) are marked

411

by solid red circles. Further details on the geology of the deposit can be found in McPhie et al. (2011,

412

2016) and Ehrig et al. (2012). Modified after Figure 2 in Ehrig et al. (2012). The inset map of

413

Australia shows the location of the Gawler Craton (black shade) and Olympic Dam (white star).

414

Fig. 2 Simplified log of the near vertical surface drill hole of RD1756-1756W1 showing the

415

distribution of olivine-phyric basalt, bedded clastic facies, and hematite-rich and granite-rich breccias.

416

The first 335 m of this hole intersects the cover sequence (Neoproterozoic to Cambrian sedimentary

417

cover) and is not shown here. Faults (bold lines) are also shown.

418

Fig. 3 The ca. 1590 Ma olivine-phyric basalt. (a) The undeformed basalt (RX8488, RD3008, 402.1m)

419

contains ~5 to 20 vol.% olivine (Ol) pseudomorphs, in a cryptocrystalline or formerly glassy

420

groundmass. (b-d) The slightly deformed basalt samples contain higher volume percentage of

421

elongate and aligned olivine pseudomorphs than samples shown in (a), and Cr-spinel (Cr-sp) is

422

widespread in the groundmass. (e-f) A magnetite (Mt)-apatite (white arrow)-chlorite (Chl)-quartz (Qtz)

423

assemblage is common in the olivine-phyric basalt (OD24, RD3008, 406.2m). (a-d) plane polarized

424

light photomicrographs; (e-f) BSE images.

425

Fig. 4 The ca. 820 Ma Olympic Dam dolerite dyke sample OD1064 (RD222, 781.5m). (a) A scan of a

426

thin section of this sample. Two white arrows mark hydrothermal veinlets cutting the dyke. (b) The

427

intrusive contact consists of brecciated granite and a chilled margin. The dyke shows a spherulitic

428

texture in the chilled margin. Some spherulites contain chalcopyrite (Cpy). (c) Hydrothermal veinlets

429

comprise chlorite, sericite, magnetite (Mt) and minor carbonate and apatite. Plagioclase (Pl) is a

430

dominant primary component of the dyke. (d-e) Magnetite (Mt) in veinlets shows oscillatory zones

431

and contains chalcopyrite and Au-Ag-bearing telluride and/or sulfide (Au-Ag-Te-S).

17

432

Fig. 5 Magnetite-apatite-altered Olympic Dam dolerite (sample OD948, RU65-8337, 495.6m). (a)

433

Photoscan of the polished drill core. (b-e) Photomicrographs of the same sample. Abbreviations: Qtz,

434

quartz; Mt, magnetite; Urn, uraninite; Cof, coffinite; Mnz, monazite; Cpy, chalcopyrite; Ap, apatite.

435

(b,c) BSE images; (d,e) SEM images.

436

Fig. 6 Element mapping of one former plagioclase crystal in the ca. 820 Ma Olympic Dam dolerite

437

(sample RX8195, RU65-8337, 541m). (a) The dolerite is composed of clinopyroxene (Cpx), former

438

plagioclase (altered to albite, Ab) and Ti-magnetite (Mt), and other secondary phases. Dashed white

439

frame indicates the mapping area. BSE image. (b-h) The concentration of each element is correlated

440

with the brightness of the image. Brighter color indicates higher concentration. White arrows in (d)

441

and (e) mark the occurrence of chlorite, and in (f) and (g) indicate residual plagioclase. (i) Zoom-in

442

view of the area marked in (a). Residual plagioclase (Pl) coexists with albite and chalcopyrite (white

443

arrow). BSE image.

444

Fig. 7 Upper, K vs. Al contents (wt.%) of sericite and lower, Mg vs. Fe contents (wt.%) of chlorite in

445

the ca. 1590 Ma olivine-phyric basalt and the ca. 820 Ma dolerite.

446

Fig. 8 Major element compositions (wt.%) of the ca. 1590 Ma olivine-phyric basalt. Compositional

447

fields (grey) for SiO2 and Fe2O3 represent those of primary high-Mg basalts (MgO>12 wt.%)

448

collected from the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georoc/).

449

Fig. 9 MgO, MnO, CaO, SiO2 vs. CO2 (wt.%) of the ca.1590 Ma olivine-phyric basalt. Same dataset

450

as Fig. 8.

451

Fig. 10 Selected trace element compositions (ppm) vs. TiO2 (wt.%) of the ca. 1590 Ma olivine-phyric

452

basalt. Same dataset as Fig. 8. The compositional field (grey) for Cr represents primary high-Mg

453

basalts (MgO>12 wt.%) collected from the GEOROC database.

454

Fig. 11 Chondrite-normalized REE variation diagram for the ca. 1590 Ma olivine-phyric basalt. Same

455

dataset as Fig. 8. Chondrite estimates have been taken from Sun and McDonough (1989).

456

Fig. 12 Major element compositions (wt.%) of the ca. 820 Ma Olympic Dam (OD) dolerite. Gairdner

457

Dyke Swarm data have also been plotted for comparison (Huang et al., 2015). Data on the least-

18

458

altered Olympic Dam dolerite and Gairdner Dyke Swarm have been combined from Huang et al.

459

(2015). Data on the more-altered Olympic Dam dolerite are from this study.

460

Fig. 13 Selected relatively immobile trace elements compositions (ppm) vs. TiO2 (wt.%) of the ca.

461

820 Ma Olympic Dam dolerite. Same dataset as Fig. 12.

462

Fig. 14 Selected relatively mobile trace element compositions (ppm) and ratios vs. TiO2 (wt.%) of the

463

ca. 820 Ma Olympic Dam dolerite. Same dataset and legend as Figs. 12 and 13.

464

Fig. 15 Drill core assays of the Olympic Dam dolerite intersected in drill hole RU65-8333. The

465

horizontal axis indicates distance from the drilling base. This underground E-trending sub-horizontal

466

drill hole intersects the Roxby Downs Granite in the 0-10-m interval, the ODBC in the 10-76-m

467

interval, the Roxby Downs Granite in the 76-462.5-m interval, the Olympic Dam dolerite in the

468

462.5-522.5-m interval (marked on the SiO2 vs. Distance diagram, same for the rest), and extends

469

farther into the Roxby Downs Granite until the end of hole at 600 m. Compositions of the dolerite

470

interval (462.5-522.5 m) have been plotted. Oxides are in wt.%, and trace elements (Cu and Ni) are in

471

ppm.

472

Fig. 16 Zr, Zn, Cu and Pb (ppm) vs. TiO2 (wt.%) of the ca. 820 Ma Olympic Dam dolerite compared

473

to a number of dolerite suites including Karoo (Galerne et al., 2008; Neumann et al., 2011) and

474

Deccan dolerite (Peng et al., 1998; Vanderkluysen et al., 2011), and Tasmania dolerite (Hergt, 1987) .

475

See text for further discussion. Data on the Gairdner Dyke Swarm and the Olympic Dam dolerite are

476

the same as Fig. 12. The dashed arrow on the Cu vs. TiO2 plot indicates copper depletion in a number

477

of the Olympic Dam dolerite samples.

478

Fig. 17 Cu vs. Na2O/CaO diagram of the ca. 820 Ma Olympic Dam dolerite. Same dataset as Fig. 12.

479

The dashed line with an arrow possibly indicates a trend arising from sodic alteration, i.e. albitization

480

of plagioclase.

481

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20

539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589

at Olympic Dam, South Australia: Links with the Gairdner Large Igneous Province. Precambrian Research 271, 160-172. Jagodzinski, E., 2014. The age of magmatic and hydrothermal zircon at Olympic Dam, Australian Earth Sciences Convention, AESC-Abstract-Proceedings, Newcastle. Jarosewich, E., Nelen, J.A., Norberg, J.A., 1980. Reference Samples for Electron Microprobe Analysis. Geostandards Newsletter 4, 43-47. Johnson, J.P., 1993. The geochronology and radiogenic isotope syetematics of the Olympic Dam copper-uranium-gold-silver deposit, South Australia. The Australian National University. Unpublished Ph.D thesis. 251 pp., unpublished. Johnson, J.P., McCulloch, M.T., 1995. Sources of mineralising fluids for the Olympic Dam Deposit (South Australia) - Sm-Nd isotopic constraints. Chem Geol 121, 177-199. 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 39, 795-798. Li, Z.X., Bogdanova, S.V., Collins, A.S., Davidson, A., De Waele, B., Ernst, R.E., Fitzsimons, I.C.W., Fuck, R.A., Gladkochub, D.P., Jacobs, J., Karlstrom, K.E., Lu, S., Natapov, L.M., Pease, V., Pisarevsky, S.A., Thrane, K., Vernikovsky, V., 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research 160, 179-210. 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 39, 795-798. McPhie, J., Orth, K., Kamenetsky, V., Kamenetsky, M., Ehrig, K., 2016. Characteristics, origin and significance of Mesoproterozoic bedded clastic facies at the Olympic Dam Cu–U–Au–Ag deposit, South Australia. Precambrian Research 276, 85-100. Morrow, N., McPhie, J., 2000. Mingled silicic lavas in the Mesoproterozoic Gawler Range Volcanics, South Australia. J Volcanol Geoth Res 96, 1-13. Neumann, E.-R., Svensen, H., Galerne, C.Y., Planke, S., 2011. Multistage Evolution of Dolerites in the Karoo Large Igneous Province, Central South Africa. J Petrol 52, 959-984. Peng, Z.X., Mahoney, J.J., Hooper, P.R., Macdougall, J.D., Krishnamurthy, P., 1998. Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy. Journal of Geophysical Research: Solid Earth 103, 2984329865. Reeve, J.S., Cross, K.C., Smith, R.N., Oreskes, N., 1990. Olympic Dam copper-uranium-gold-silver deposit, in: Hughes, F.E. (Ed.), Geology of the mineral deposits of Australia and Papua New Guinea. Australasian Institute of Mining and Metallurgy, Melbourne, pp. 1009-1035. Reid, A.J., Hand, M., 2012. Mesoarchean to Mesoproterozoic evolution of the southern Gawler Craton, South Australia. Episodes-Newsmagazine of the InternationalUnion of Geological Sciences 35, 216. Rollinson, H.R., 1993. Using geochemical data: evaluation, presentation, interpretation. Longman Scientific & Technical Essex. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special Publications 42, 313-345. Vanderkluysen, L., Mahoney, J.J., Hooper, P.R., Sheth, H.C., Ray, R., 2011. The feeder system of the Deccan Traps (India): insights from dike geochemistry. J Petrol 52, 315-343. Wang, X., Li, X., Li, Z., Liu, Y., Yang, Y., 2010. The Willouran basic province of South Australia: Its relation to the Guibei large igneous province in South China and the breakup of Rodinia. Lithos 119, 569-584. 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 87, 135-159.

590 591 21

592 593 594

Table 1 Major and trace element compositions for the more altered Olympic Dam dolerite Sample

Drill Hole

Depth

No.

No.

(m)

SiO

TiO

Al2

Fe

Mn

Mg

Ca

Na2

K2

P2O

Ni

2

2

O3

Ot

O

O

O

O

O

5

O

43. OD117

RD271

657.4

1

13. 3.2

14.7

42. OD73

RD2773

1196.7

7

RD271

645.3

6

2.8

15.4

RD271

650

7

2.1

10.6

RD2773

1244.4

1

2.5

13.0

RU39-5426

208.7

0

2.5

14.7

RD2773

1239.3

7

2.7

14.3

RU39-5426

195.6

5

2.7

14.5

RD2773

1255.2

6

2.4

13.6

RU39-5426

211.4

0

2.7

13.1

RD271

649.2

0

4.9

7.8

0.9

1.7

0.3

4

0

0

5

0

0

6

2.7

15.2

9

0.6

6.3

3.9

1.4

0.9

0.2

14.0

0

1 0.0

0.3

5.7

4.9

3.4

0.1

0.3

1 0.0

0.4

7.0

3.2

0.1

3.7

0.2

1 0.0

0.3

9.7

4.9

3.8

0.2

0.2

4 0.0

0.4

7.6

0.6

0.1

1.3

0.3

2 0.0

0.2

7.5

4.7

2.4

0.8

0.2

1 0.0

0.4

6.8

6.0

1.2

2.5

0.3

0 0.0

0.2

12.3

1.6

3.0

0.6

0.3

13. 2.7

1 0.0

12.

57. OD112

0.7

14.

50. OD3-2

6

1 0.0

13.

50. OD78

0.3

24.

53. OD6

2.0

12.

45. OD74

1.7

20.

50. OD4

9.5

11.

46. OD194

9.4

10.

57. OD113

1.4

20.

62. OD110

3

0.0

2 0.0

0.2

6.8

1.6

2.7

0.4

0.2

1

22

48. OD76

RD2773

1244.5

18.

1

2.5

15.5

6

52. OD116

RD271

652.7

RD2773

0

1241.9

2.9

16.2

RD271

2.9

13.8

RU65-8333

2.6

13.4

RD271

1.8

14.0

RD2773

3.1

8.8

RD271

725.4

2.8

12.3

RD2773

2.5

17.1

RU39-5426

2.9

13.2

RD2773

2.4

12.5

RD2773

1333.2

0.4

0.1

2.0

0.3

6.3

1.3

2.9

0.2

0.2

2.3

14.0

5.5

2

2.1

0.2

0.2

0.3

7.4

2.6

1.1

0.4

0.5

2.6

0.4

4

1

5.7

7.4

1.9

1.6

0.3

1 0.0

0.2

3.8

3.9

4.3

1.4

0.2

0 0.0

0.2

6.3

7.5

2.4

1.4

0.3

1 0.0

0.1

3.4

3.7

2.5

0.7

0.2

1 0.0

4

13.0

2

0.0

0.3

6.0

7.6

3.1

2.6

0.2

13.

2

0

0.0

12.

2

1

0.0

0

49. OD83

7.4

19.

5

1325.4

0.3

7

50. OD81-1

0.2

14.

6

211.4

0.2

10.

2

53. OD3-1

3.4

14.

7

1264.7

0.1

9

49. OD198-1

1.0

14.

2

2

0.0

8

50. OD120

0.3

28.

8

1264.7

0.3

5

51. OD198-2

7.5

14.

7

688.5

4.4

0.0

7

43. OD119

0.1

11.

9

517.7

0.1

6

49. OD942

1.8

21.

8

650.5

6.4

0.0

8

59. OD114

0.3

14.

48. OD75

0.0

2 0.0

1

0.2

5.8

9.2

3.5

1.5

0.3

1

595 596

Table 1 continued Sample No.

LOI

Li

Be

Sc

Ti

V

Cr

Mn

Ni

Cu

Zn

Ga

As

Rb

23

OD117

15.0

45.2

2.4

78.4

15793

382

103

9084

40.3

18

128

18.7

4.5

89.7

OD73

9.4

28.9

2.3

41.2

14964

378

91

4568

50.1

14

143

23.3

5.8

79.1

OD110

8.4

60.6

2.6

34.7

11512

307

79

3969

45.9

22

125

14.8

6.5

41.0

OD113

7.3

50.6

1.4

37.0

13444

285

101

2056

54.3

109

147

16.9

3.5

1.9

OD194

6.7

34.8

1.8

23.0

13797

254

64

2978

47.4

130

165

23.9

2.3

94.7

OD4

6.6

52.7

0.8

43.2

14987

377

106

2067

69.5

12

133

19.7

12.7

6.8

OD74

6.4

36.3

2.6

31.3

15121

340

75

2818

48.9

374

199

23.0

2.9

25.9

OD6

6.1

60.3

1.1

39.8

13454

370

99

1671

59.5

46

204

19.7

11.1

18.3

OD78

5.9

30.5

1.3

41.2

15098

430

78

3109

60.0

281

284

21.9

7.3

105.8

OD3-2

5.8

76.7

1.0

38.9

15040

369

106

1144

99.5

17

269

25.4

6.5

12.5

OD112

5.6

67.6

2.1

28.9

14768

343

112

1381

64.0

16

179

19.3

3.1

20.1

OD76

5.6

30.9

1.8

23.8

13996

259

61

2281

41.3

147

157

24.3

3.4

134.6

OD116

5.4

72.7

2.0

31.1

16390

419

124

634

73.4

72

200

24.2

4.2

5.2

OD75

5.4

40.5

2.9

42.9

16240

450

77

1958

55.3

52

199

23.0

7.9

71.1

OD114

5.1

75.8

2.0

29.9

14260

292

109

727

66.3

38

188

19.7

2.2

6.3

OD942

4.5

39.0

1.9

39.2

8780

372

170

1920

179.5

302

256

20.0

2.7

10.3

OD119

4.5

34.2

1.2

34.7

17064

320

<5

2332

46.2

319

284

20.7

10.4

9.0

OD198-2

4.1

25.1

1.1

38.4

15863

392

45

2709

46.9

148

197

21.2

16.5

68.0

OD120

3.2

18.9

1.0

32.8

13394

334

46

1673

40.3

50

218

24.8

5.5

49.9

OD198-1

3.1

31.3

1.5

41.0

16563

407

59

1232

50.9

223

151

22.2

20.2

45.7

OD3-1

2.5

25.0

0.9

42.3

13758

429

95

564

39.3

203

86

17.7

12.5

16.5

OD81-1

1.6

12.4

0.7

38.5

13746

375

108

1927

62.7

73

94

20.7

2.4

117.1

OD83

1.3

10.8

1.1

38.6

15712

390

72

1661

55.4

136

86

21.7

3.3

62.2

597 598

Table 1 continued

24

Sample No.

Sr

Y

Zr

Nb

Mo

Sb

Cs

Ba

La

Ce

Pr

Nd

Sm

Eu

OD117

51

37.1

170

11.6

1.1

0.36

2.75

111

7.3

18.3

2.78

13.5

4.22

1.16

OD73

29

41.4

177

11.8

0.4

1.41

0.83

145

15.1

33.0

4.32

18.4

5.15

1.20

OD110

36

24.0

124

8.9

10.3

0.71

1.71

91

9.3

18.4

2.33

9.8

2.73

0.93

OD113

64

33.7

143

10.0

47.4

0.74

0.22

386

7.4

17.7

2.70

13.1

4.34

1.47

OD194

37

45.4

206

17.0

0.8

0.53

0.92

400

16.8

34.3

4.20

16.6

4.40

0.98

OD4

164

34.1

156

10.4

0.6

0.49

1.96

270

10.3

26.1

3.84

18.6

5.28

1.62

OD74

25

30.2

180

12.4

0.4

1.00

0.40

305

5.4

13.2

1.76

7.9

2.48

0.65

OD6

144

27.8

143

9.5

0.9

1.31

1.85

469

9.7

22.8

3.30

16.0

4.74

1.72

OD78

122

38.7

186

12.4

0.9

0.70

4.00

567

21.7

49.5

6.80

31.5

7.70

2.30

OD3-2

228

30.0

158

10.5

0.4

0.49

2.46

643

16.3

44.8

6.56

30.9

6.86

1.88

OD112

32

28.6

156

10.8

2.3

0.59

0.91

287

34.3

64.3

8.47

37.2

8.71

2.54

OD76

34

44.1

237

18.3

0.8

0.70

1.27

478

23.1

46.1

5.40

20.9

5.11

1.14

OD116

17

30.5

173

12.2

1.4

0.57

0.44

24

4.8

12.9

1.96

9.4

3.13

1.03

OD75

11

36.9

203

13.9

0.8

1.10

1.70

220

8.7

24.5

3.20

14.2

3.90

0.80

OD114

21

33.6

152

10.5

1.5

0.46

0.57

71

4.5

11.9

1.82

8.9

3.08

0.99

OD942

201

26.0

113

7.1

1.1

0.35

1.00

72

9.5

23.2

3.37

15.2

4.14

1.53

OD119

86

55.9

284

20.2

147.8

0.87

0.68

65

20.1

49.6

7.13

33.0

8.77

2.13

OD198-2

149

38.0

187

13.0

0.6

0.35

2.10

374

18.5

37.3

5.04

23.3

6.26

1.68

OD120

222

30.9

169

10.7

2.2

0.10

0.30

253

20.0

45.5

6.00

27.7

6.70

2.20

OD198-1

133

43.6

195

13.5

1.0

0.55

2.34

263

14.4

37.7

5.64

26.5

7.33

2.60

OD3-1

185

29.8

147

9.7

1.2

0.86

0.89

395

16.1

37.2

5.26

24.3

6.52

2.40

OD81-1

172

33.6

156

10.5

0.5

0.27

0.52

478

14.5

34.1

4.74

21.4

5.74

1.78

OD83

151

40.0

186

12.6

0.7

0.52

0.64

217

18.1

42.4

6.03

26.9

7.11

2.03

599 600

Table 1 continued 25

Sample No.

Gd

Tb

Dy

1.0 OD117

5.43

5

6.72

1.2 OD73

6.25

8

8.04

0.7 OD110

3.49

3

4.86

1.0 OD113

5.52

0

6.05

1.1 OD194

5.51

1

7.27

1.0 OD4

6.24

9

6.59

0.7 OD74

3.46

5

5.08

0.9 OD6

5.76

8

5.74

1.5 OD78

8.70

0

8.00

0.9 OD3-2

6.44

5

5.62

1.0 OD112

8.13

6

5.49

1.1

Ho

Er

Tm

Yb

Lu

Hf

Ta

Tl

1.4

4.1

0.6

3.9

0.5

4.7

0.3

0.2

2

9

2

3

9

4

6

3

1.6

4.9

0.7

4.4

0.6

4.8

0.7

0.1

7

0

1

2

5

0

5

7

1.0

3.0

0.4

2.5

0.3

3.4

0.4

0.1

4

0

2

1

6

5

0

3

1.2

3.5

0.5

3.2

0.4

4.0

0.4

0.2

4

2

2

6

9

3

3

2

1.6

5.3

0.8

6.1

0.9

5.6

0.4

0.2

4

4

8

1

7

4

2

9

1.3

3.8

0.5

3.4

0.5

4.2

0.5

0.0

3

0

4

1

0

8

6

3

1.1

4.1

0.7

5.2

0.8

4.8

0.7

0.1

9

4

2

3

6

2

3

1

1.1

3.1

0.4

2.7

0.4

3.9

0.4

0.0

2

8

5

7

1

9

6

9

1.6

4.1

0.6

3.6

0.5

4.9

1.2

0.6

0

0

0

0

0

0

0

0

1.1

3.4

0.5

3.1

0.4

4.4

0.6

0.0

5

3

0

7

9

2

4

5

1.0

3.0

0.4

2.8

0.4

4.3

0.6

0.0

7

8

6

6

2

5

1

5

1.5

5.2

0.8

5.9

0.9

6.3

1.2

0.3

Pb

Th

U

8.6

2.16

2.18

7.2

2.04

1.14

7.6

2.34

2.89

17.0

2.03

1.69

4.1

8.78

5.61

17.2

1.88

0.40

10.9

2.27

4.86

17.8

1.74

0.52

13.5

2.40

0.70

10.0

1.79

0.36

8.7

2.02

1.60

17.3

OD76

5.82

3

7.30

9

3

8

7

4

8

7

7

4.9

0

8.25

OD116

4.27

0.8

5.35

1.1

3.4

0.5

3.1

0.4

4.8

0.8

0.0

5.8

2.25

10.8

26

OD75

5.00

4

6

0

1

9

7

3

0

3

1.1

1.6

4.9

0.9

5.8

0.9

5.4

1.1

0.3

0

0

0

0

0

0

0

0

1.2

3.7

0.5

3.4

0.5

4.2

0.5

0.0

5

5

6

6

2

3

8

4

0.9

2.6

0.3

2.5

0.3

3.0

0.6

0.0

0

6.80

0.8 OD114

4.21

6

5.73

0.7 OD942

3.52

14.6

1.40

0.36

8

8

9

0

0

4

10.1

1.7

10.8

2.1

6.3

0.9

5.9

0.9

7.8

1.1

0.0

0

9

1

9

2

3

1

0

8

5

7

13.8

1.4

4.0

0.5

3.5

0.5

5.2

0.6

0.3

200.

4

4

8

6

3

0

7

1

1.3

3.5

0.5

3.2

0.5

4.6

1.2

0.2

0

0

0

0

0

0

0

0

1.6

4.5

0.6

3.9

0.5

5.4

0.8

0.1

3

5

5

5

8

1

0

8

1.3

3.4

0.4

2.6

0.3

4.0

0.5

0.0

3

9

5

2

7

9

1

7

1.2

3.5

0.5

3.0

0.4

4.1

0.7

0.3

5

3

0

2

4

3

5

2

1.5

4.2

0.6

3.6

0.5

5.0

0.9

0.2

1

3

0

6

4

2

6

1

7.09

2

7.22

7.30

0

6.60

8.30

3

8.23

7.36

5

7.22

1.0 6.37

9

6.40

1.3 OD83

1.81

7

1.2

OD81-1

4.2

7

1.4

OD3-1

4.70

4.63

1.1

OD198-1

3.20

4

1.2

OD120

9.6

4.80

OD119

OD198-2

4

7.77

2

7.65

14.7 4.80

9

9

2.25

0.49

2.6

2.60

0.60

22.5

2.33

0.54

18.8

1.82

0.46

4.3

1.91

0.71

5.4

2.25

1.02

601

Major element (oxides) compositions are LOI-free, in wt.%, trace element in ppm. LOI-Loss

602

On Ignition.

603

27

604 605 606

Two generations of mafic rocks at the Olympic Dam deposit have been altered.

607

Compositions of mafic rocks have been modified to variable degrees due to alteration.

608

Both generations of mafic rocks could provide copper to the deposit.

609

At least two time-punctuated hydrothermal events have occurred at Olympic Dam.

610

28