Elsevier

Precambrian Research

Volume 335, 1 December 2019, 105480
Precambrian Research

Hematite geochemistry and geochronology resolve genetic and temporal links among iron-oxide copper gold systems, Olympic Dam district, South Australia

https://doi.org/10.1016/j.precamres.2019.105480Get rights and content

Highlights

  • Characterisation of U-bearing hematite and early magnetite from the Wirrda Well and Acropolis prospects, Olympic Dam district.

  • U-W-Mo-Sn-rich hematite is ubiquitous in IOCG systems of the Olympic Cu-Au province.

  • Hematite geochemistry and geochronology can be used to compare IOCG genesis.

  • IOCG prospects and deposits in the Olympic Dam district formed at ~1590 Ma.

Abstract

The Wirrda Well and Acropolis prospects, ~25 km south from the Olympic Dam deposit, are among several dozen examples of iron-oxide copper gold (IOCG) mineralisation forming a 600 km-long province in the eastern Gawler Craton, South Australia. IOCG systems across the province differ in terms of host lithology, iron-oxide mineral associations, intensity of brecciation, and grade of Cu-(U)-Au mineralisation. Previous efforts to provide a geochronological framework for mineralisation, and correlate this with igneous activity, were inhibited by a lack of clear crosscutting features (particularly in breccia-hosted systems) and a scarcity of reliable hydrothermal mineral geochronometers. The earliest generation of hematite, characterised by enrichment in U-W-Sn-Mo and previously reported from Olympic Dam, provides confident U-Pb dates and insights into hydrothermal fluid signatures. Similar hematite is recognised from Wirrda Well and Acropolis, allowing direct comparison between the three IOCG systems. Relationships between early, silician (Wirrda Well) and titaniferous (Acropolis) magnetite and U-bearing hematite at the two prospects differ. Inferred martitisation of magnetite at Acropolis has generated zoned hematite with U-W-Mo-enrichment at constant Sn concentration, but with a marked loss of REE + Y (REY) and Ti, whereas at Wirrda Well, single-crystal zoned hematite, resembling that from Olympic Dam, retains high REY and Sn concentrations. Although morphologies, textures and compositions of U-bearing hematite at Olympic Dam, Wirrda Well and Acropolis vary significantly, a common U-Pb (laser ablation-inductively coupled plasma-mass spectrometry) age of ~1590 Ma is obtained from the most reliable data (207Pb/206Pb: 1598.9 ± 6.3 Ma at Wirrda Well and 1590.6 ± 6.5 Ma at Acropolis). The geochemical signatures of iron-oxides from the two prospects share common trends for U, W, Sn, Mo, high field strength and siderophile elements, comparable with signatures of Fe-oxides from the ‘outer shell’ of the Olympic Dam deposit. Given that datable iron-oxides would seem to be ubiquitous phases throughout the eastern Gawler Craton, the commonality in geochemistry and age of U-bearing hematite from the Olympic Dam district is significant, as it provides a single, traceable mineral to compare IOCG systems across the province.

Introduction

Mesoproterozoic magmatic-hydrothermal ore deposits and prospects occur within a Cu-Au ± U-rich metallogenic province, extending for 600 km along the eastern margin of the Gawler Craton, South Australia (Fig. 1A). Climaxing at ~1.6 Ga, a craton-scale tectonothermal event resulted in emplacement of voluminous, bi-modal intrusive and volcanic rocks of the co-magmatic Gawler Range Volcanics (GRV) and Hiltaba Suite (Drexel et al., 1993, and references therein). Together, these rocks form an intracontinental Large Igneous Province (LIP), which unlike most mantle plume derived LIP’s (Ernst, 2014), is dominated by felsic rather than mafic rocks (Allen et al., 2008). The magmatism, and associated metallogenesis in the region, has been temporally correlated with the episodic breakup of the Columbia/Nuna supercontinent (Goldfarb et al., 2010, Groves et al., 2010).

Based on geophysical and geochemical data, lithospheric models for the region acknowledge the important role of mantle-derived components and, moreover, recognise delamination of metasomatised subcontinental lithospheric mantle (SCLM) accompanying the LIP event (Skirrow et al., 2018, Wade et al., 2019). Such a setting, in which SCLM was fertilized with volatiles and ore elements during earlier subduction, accompanied by back-arc development, accounts for bi-modal magmatism with a high component of metals and incompatible elements typical of IOCG systems (Groves et al., 2010).

The genetic concept of a single metallogenic province along the eastern margin of the Gawler Craton has evolved through complimentary studies of structural architecture, mineralogy and geochronology across the region, leading to definition of an iron-oxide copper–gold (IOCG) belt termed the “Olympic Cu-Au province” (Reid, 2019, Skirrow et al., 2007). The province hosts the world-class Olympic Dam Cu-U-Au-Ag deposit (Ehrig et al., 2012), as well as numerous other deposits and prospects, displaying a variety of mineralisation styles (e.g., breccia, skarn, vein) and hosted within lithologies including granite, metasedimentary and volcanic rocks.

A spectrum of geochronological data has confirmed the relationship between IOCG mineralisation and alteration with the ~ 1.6 Ga magmatic event (e.g. Skirrow et al., 2007, Reid et al., 2013, Ciobanu et al., 2013, Bowden et al., 2017). The synchronicity of timing is, however, considered broad, with even the most reliable U-Pb dates for the primary Cu-Au mineralisation event spanning an interval as large as ~30 Ma (Table 1). Several factors have contributed to this given timespan: (1) there is a lithological and temporal range in the rocks hosting the IOCG systems; (2) geochronology has been based on a wide variety of minerals, capturing different stages of the same geological event, or even events that are temporally quite distinct; (3) radio-isotopic systems and instrumentation used for dating are often not directly comparable; (4) high-precision geochronological data for major breccia-hosted deposits other than Olympic Dam (e.g., Prominent Hill, Carrapateena) remains relatively limited; and (5) there is a lack of geochronological data from minerals which can only be hydrothermal in origin and therefore directly related to Cu-Au-(U) mineralisation.

Hematite, which is ubiquitous to generic schemes for IOCG formation, can co-exist with magnetite in different proportions throughout alteration assemblages forming the broader footprints of a given mineralising system (Barton, 2013, and references therein). Advances in hematite U-Pb geochronology have shown that reproducible, <1 Ma precision is achievable by isotope dilution thermal ionisation mass spectrometry (ID-TIMS) (Courtney-Davies et al., 2019a), as demonstrated using hydrothermal hematite from Olympic Dam, dated at 1589.94 ± 0.91 Ma (207Pb/206Pb). Datable hematite is compositionally zoned with respect to U (+radiogenic Pb), W, Sn, and Mo, (representing a primary association hereafter termed ‘granitophile elements’), reaching up to wt.% concentrations. Transmission electron microscopy on thinned foils of U-richest zoned hematite at Olympic Dam, prepared in-situ by focused ion beam SEM methods, confirm that U and radiogenic Pb is lattice-bound and thus not present as nanometer- to micrometer-scale inclusions (Ciobanu et al., 2013). At the micron scale, LA-ICP-MS spot analysis consistently provides stable downhole ablation profiles for U and Pb, while LA-ICP-MS mapping demonstrates that concentrations of U and Pb correlate with grain-scale zoning (Ciobanu et al., 2013, Verdugo-Ihl et al., 2017). Moreover, experimental studies have successfully doped synthetic hematite with U, demonstrating that up to several wt. % can be incorporated (Duff et al., 2002, Courtney-Davies et al., 2019b).

The granitophile hematite signature is present throughout the ~6 km-strike and ~2 km-depth of the breccia complex hosting the Olympic Dam orebody (Dmitrijeva et al., 2019a, Verdugo-Ihl et al., 2017). Coupled with chemical abrasion-ID-TIMS ages of magmatic zircon from granite hosting the deposit, dated at 1593.87 ± 0.21 Ma (Cherry et al., 2018a), a transitional timeframe between the magmatic and hydrothermal systems can be measured (~2–4 Ma). The accuracy of U-Pb microbeam data acquisition is also confirmed, albeit with somewhat lower analytical precision, facilitating fast and reliable hematite dating of additional representative samples via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS; Ciobanu et al., 2013, Courtney-Davies et al., 2016, Courtney-Davies et al., 2019a).

High-precision magmatic-hydrothermal geochronology explicitly links the mineralising fluids forming Olympic Dam to a single, major ore-forming event, tied to the granite hosting the deposit. This conclusion is supported by other studies, e.g., (1) isotope studies and modelling, that have proposed a single magmatic sulphur source for IOCG deposits in the Cu-Au Olympic Province (Schlegel et al., 2017); (2) the overlap between, and low range of, δ238U values in the Olympic Dam Breccia Complex and host granite (Kirchenbaur et al., 2016), indicating that δ238U mineralisation values reflect a source from a granitoid rock; (3) chloro-hydroxy-zircon-nanoprecipitates have been imaged within magmatic zircon from the host granite and are interpreted to have formed through pervasive coupled dissolution reprecipitation reaction driven metasomatism shortly after granite crystallisation (Courtney-Davies et al., 2019c); and (4) the granitophile element signature defining IOCG mineralisation (Dmitrijeva et al., 2019a) is particularly well expressed by early-stage, oscillatory (growth)-zoned hematite (Verdugo-Ihl et al., 2017), the dominant Fe-oxide in the deposit (Ehrig et al., 2012).

If all mineralisation is genetically tied to fluids derived from deep-rooted magmas of common origin and evolution, hematite should contain a traceable geochemical signature, common to all IOCG deposits in the Olympic Cu-Au province, irrespective of its proportion relative to magnetite, or host lithology. In this contribution, we study hematite to ascertain whether the same geochemical signature defined by enrichment in granitophile elements that was identified in the Olympic Dam deposit also occurs ~25 km to the south, at the Wirrda Well and Acropolis IOCG prospects (Fig. 1B). The prospects differ from Olympic Dam, and one another, in terms of: (1) Fe-oxide association (paragenetically less evolved e.g., a dominance of magnetite over hematite); (2) major host lithologies (Donington Suite granite at Wirrda Well and GRV at Acropolis versus Hiltaba Suite granite at Olympic Dam); and (3) less intense Cu-Au mineralisation, alteration and brecciation. Through textural, geochemical and U-Pb analysis, hematite can be used to compare the IOCG systems in terms of both the timing of ore formation and the geochemical signatures of mineralising fluids. U-W-Sn-Mo-rich hematite can be used to underpin a robust U-Pb framework of mineralisation ages throughout the Olympic Cu-Au province, resolving the relative chronologies of IOCG systems, regardless of mineralisation style, host lithology or location.

Section snippets

Geological background

Basement rocks of the Olympic Dam District comprise the 2.5 Ga Mulgathing Metamorphic Complex, 1.85 Ga Donington Suite granitoid rocks, and 1.76–1.74 Ga Wallaroo Group (meta)sedimentary rocks, which are intruded by, and unconformably overlain by the ~1.6 Ga Hiltaba Suite and GRV, respectively (Fig. 1). Olympic Dam and two satellite prospects, Wirrda Well and Acropolis, occur close to intersections between regional NW-ENE trending structures, in association with second-order NE-ENE-striking

Methodology

One-inch polished blocks were examined for zoned hematite grains via reflected light microscopy and scanning electron microscopy (SEM) using a FEI Quanta 450 instrument equipped with a back-scattered electron (BSE) detector and an energy-dispersive X-ray spectrometer at Adelaide Microscopy, The University of Adelaide.

LA-ICP-MS U-Pb spot data were collected at Adelaide Microscopy using a 193 nm RESOlution-LR excimer laser microprobe (Applied Spectra) coupled to an Agilent 7900x Quadrupole

Sampling

The Fe-oxides studied here are collected from drillholes marked on cross-sections chosen to illustrate the main host lithologies, Cu grades and relationships with Hiltaba Suite granite for each prospect (Fig. 2). These samples are representative of magnetite and hematite from different parts of the mineralised domains. Magnetite is from the deepest parts of both prospects, whereas oscillatory-zoned hematite, enriched in U and granitophile elements (hereafter referred to as U-bearing hematite)

Formation and evolution of iron-oxides at Wirrda Well and Acropolis

The present study, although not a comprehensive assessment of all generations of Fe-oxides, highlights textural-geochemical characteristics that are suitable for comparison between the two prospects and with the Olympic Dam deposit.

Silician magnetite from Wirrda Well is directly comparable with the earliest hydrothermal magnetite found in the outer shell at Olympic Dam (based on the presence of Si-Fe nanoprecipitates and nanoparticles of silicates; Verdugo-Ihl et al., 2019, Ciobanu et al., 2019

Conclusions and outlook

  • 1.

    Silician and titaniferous magnetite are typical for the early-stage mineralisation at Wirrda Well and Acropolis, respectively. Late-stage, U-W-Sn-Mo-bearing, oscillatory-zoned hematite is characterised from both prospects, but with discrepant relationships to early magnetite. At Acropolis, martitisation of magnetite results in U-W-Mo-enrichment at constant Sn concentration, and loss of REY, whereas at Wirrda Well single-crystal zoned hematite, resembling those from Olympic Dam occurs.

  • 2.

Acknowledgements

This work is a contribution to the ‘FOX’ project (Trace elements in iron oxides: deportment, distribution and application in ore genesis, geochronology, exploration and mineral processing), supported by BHP Olympic Dam and the South Australian Government Mining and Petroleum Services Centre of Excellence. N.J.C. acknowledges additional support from the ARC Research Hub for Australian Copper-Uranium (Grant IH130200033). We acknowledge the valuable comments of John Hanchar, an anonymous reviewer,

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