Fluorite as indicator mineral in iron oxide-copper-gold systems: explaining the IOCG deposit diversity

doi:10.1016/j.chemgeo.2020.119674

Chemical Geology

Volume 548, 20 August 2020, 119674

https://doi.org/10.1016/j.chemgeo.2020.119674 Get rights and content

Highlights

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Fluorite-REE signatures are specific to the fluid types involved in IOCG mineralization.
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REE signatures can be used as proxy for the fluid origins in IOCG deposits.
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A volcanic lake water-derived fluid is present in Cu ores from Olympic Dam.
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Global IOCG deposits do not form by a single sequence of processes.

Abstract

Hydrothermal iron oxide-copper-gold (IOCG) ore deposits are globally important sources of Cu and Au. IOCG systems show many common features, but also considerable diversity in terms of geological setting, mineralization style and ore fluid characteristics. The key factors that control ore formation are controversially debated and no general ore deposit model has been able to explain the diversity of IOCG deposits on the global scale. Building on previous work that characterized four distinct fluid types at the Prominent Hill IOCG deposit, South Australia, we have analyzed the rare earth element (REE) composition of fluorite from the Prominent Hill, Olympic Dam and Ernest Henry IOCG deposits and from two prospects close to Prominent Hill. The REE data of fluorite from Prominent Hill show four distinct chondrite-normalized patterns, which reflect the four fluid types identified as metalliferous volcanic lake water derived fluid, magmatic-hydrothermal fluid, sedimentary basin brine, and wall rock-buffered basement brine. Two strikingly similar REE pattern types were found in fluorite from the Olympic Dam deposit and one in an exploratory data from the Ernest Henry deposit, demonstrating the involvement of the same principal fluid types. We therefore conclude that world-class IOCG deposits can form by interaction of host rocks with different characteristic types of ore fluids and their combinations, thus IOCG deposits do not necessarily form by a single sequence of processes.

Graphical abstract

Introduction

Iron oxide-copper-gold (IOCG) deposits are a globally important but diverse group of hydrothermal ore deposits showing similarities to porphyry Cu-Au, sedimentary Cu and skarn deposits, reflecting variable fluid chemistry, links to intrusions and tectonic settings. Despite considerable research efforts in the last decades, there is no consensus yet on a consistent ore formation model explaining this diversity. Nevertheless, it has become clear that IOCG deposits are always characterized by abundant iron oxides, and that structurally controlled economic Cu-Au ores are hosted by rocks affected by zoned iron oxide-alkali alteration (Fig. 1; Hitzman et al., 1992; Williams et al., 2005; Richards and Mumin, 2013; Barton, 2014; Corriveau et al., 2016).

Current formation models for IOCG deposits suggest that iron oxide-apatite (IOA) deposits may represent the deeper roots of IOCG systems and highlight the importance of ascending magmatic-hydrothermal fluids in creating vertically zoned mineralization (Fig. 1; Sillitoe, 2003; Williams et al., 2005; Knipping et al., 2015a, Knipping et al., 2015b; Corriveau et al., 2016; Reich et al., 2016; Simon et al., 2018). Many studies emphasize the significance of fluid-rock interaction involving at least two fluids of either magmatic, surficial, sedimentary, or metamorphic origin in the formation of world-class IOCG deposits such as Olympic Dam, Prominent Hill and Ernest Henry, whereas this may not be required for forming smaller deposits (Fig. 2; e.g. Oreskes and Einaudi, 1992; Mark et al., 2000; Bastrakov et al., 2007; Williams et al., 2015; Schlegel et al., 2018). Despite the presence of a range of contrasting fluid types, the ultimate sources of fluids, metals and sulfur in many deposits of the IOCG spectrum remain contentious. Therefore, identification of robust tracers of the fluid sources would make it possible to understand which processes and ingredients are critical for formation of world-class IOCG deposits in different provinces, compared to smaller deposits and subeconomic occurrences.

Extensive studies of diverse hydrothermal mineral deposits have shown that the REE patterns of fluorite reflect the composition of the fluid from which it precipitated and permit characterization of the source rocks (Möller et al., 1976; Bau et al., 2003; Schwinn and Markl, 2005). On this basis we have investigated the REE signatures of fluorite in ores from the large Prominent Hill, Olympic Dam, and Ernest Henry IOCG deposits, as well as from the small Triton and Neptune prospects by LA-ICP-MS (Fig. 2). Building on recent advances in characterizing IOCG fluids, the four distinct fluid types identified in the Prominent Hill deposit (Schlegel et al., 2018) were used to calibrate four characteristic REE signatures of fluorite. Each signature is specific to one fluid type involved in IOCG mineralization and can be used as a proxy for the fluid origins in other IOCG deposits. Comparison with published data confirms that interactions between a magmatic-hydrothermal and at least one additional fluid lead to formation of world-class IOCG deposits. Potential fluids involved in IOCG deposit formation include volcanic lake water derived fluids in areas of active volcanism, magmatic-hydrothermal fluids, sedimentary basin brines and rock buffered basement brines including metamorphic fluids.

Section snippets

IOCG fluids and mineralization in the Prominent Hill area

We report only the distinctive characteristics of the Prominent Hill deposit and the Triton and Neptune prospects as a basis to interpret the REE data; further background information for the Prominent Hill, Olympic Dam and Ernest Henry deposits is summarized in Fig. 1 and in the literature (e.g. Oliver et al., 2008; Ehrig et al., 2012; Schlegel and Heinrich, 2015; Schlegel et al., 2017, Schlegel et al., 2018).

The Prominent Hill deposit is located in the footwall of a fault system, which has

Sampling

Forty-seven fluorite bearing samples were collected mostly from drill core and they originate from the Prominent Hill, Olympic Dam and Ernest Henry IOCG deposits as well as the Neptune and Triton prospects near Prominent Hill. The location of each sample including a summary description is given in Table A1 in the Electronic supplementary material, which also contains specific references to Figures from previous studies.

Fifteen of the 19 samples from the Prominent Hill deposit were collected

Petrographic relations in samples across different deposits

Representative mineral textures of samples from each locality are documented in Fig. 3, Fig. 4 and A2–5 (Electronic supplementary material) and show that fluorite is intergrown with Cu-(Fe)-sulfides as an integral part of the mineral assemblage. Careful petrography made it possible to clearly relate fluorite from Prominent Hill to FIA containing one of the four fluid types (Fig. 3A–D). We consider some of the textural differences among the hematite breccias from Olympic Dam and Prominent Hill

Classification of fluorite

The classification of fluorite in this study is strictly based on a combination of their REE patterns and their position in the Tb/La–Tb/Ca diagram (Fig. 5; c.f. Möller et al., 1976). The classification diagram builds up on the earlier works of Schneider et al. (1975), Jacob (1974) and Fleischer (1969) who analyzed the REE data of fluorite from numerous deposits located in Asia, Bulgaria and the United States. The following paragraph summarizes its background information originally published in

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project was made possible by funding from the German Research Foundation (DFG), grants number WA1526/7-1 and INST222/1235-1. We thank Christoph A. Heinrich for providing samples from the Olympic Dam and Ernest Henry ore deposits and we thank Peter Pollard for his insightful comments and constructive review. We also acknowledge the comments of Adam Simon and Andrew Tomkins on an early version of this paper.

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Citation Excerpt :
These findings imply that evaporitic fluids were likely unimportant for transporting Cu and Au at Ernest Henry. A fluid mixing model supported by our data and that of previous studies involves Cu-Au-bearing magmatic-derived fluids mixing with a basinal brine that may or may not include metamorphic breakdown/devolatilization products (Baker et al., 2008; Schlegel et al., 2020). The spatial variability of Δ’17O remains poorly constrained and further work is needed to fully understand the importance of evaporitic fluids at Ernest Henry.
Iron oxide copper gold (IOCG) deposits are globally important sources of Cu; however, their origins remain poorly understood with respect to the metal and fluid sources involved in their formation. In this work, we utilize integrated Fe, Ti, and O isotopic data for magnetite to trace major metal and fluid inputs for the Proterozoic-aged Ernest Henry IOCG deposit—the largest known IOCG deposit in the Cloncurry District, Queensland, Australia. Magnetite separates from ore stage and pre-ore biotite-magnetite (bt-mgt) and magnetite-actinolite (mgt-act) alteration assemblages from Ernest Henry were analyzed for their O, Fe, and Ti isotope abundances, reported as δ¹⁸O (VSMOW), δ⁵⁶Fe (IRMM-14), and δ⁴⁹Ti (OL-Ti). Select magnetite samples were also analyzed for ¹⁷O (reported as Δ’¹⁷O_0.5305) and range from −0.118 to −0.056 ‰—suggesting evaporitic input. The δ¹⁸O values from ore stage (+1.57 to +7.36 ‰), bt-mgt (+0.34 to +5.68 ‰), and mgt-act (+1.68 to +2.10 ‰) samples are consistent with a magmatic-hydrothermal origin for ore and pre-ore mineralizing fluids at Ernest Henry; non-magmatic δ¹⁸O values > c. 5 ‰ may be explained through localized carbonate wall rock assimilation. Despite this, δ⁵⁶Fe values for ore stage (−0.50 to +0.33 ‰), bt-mgt (−0.65 to +0.38 ‰), and mgt-act (−0.26 to +0.30 ‰) magnetite are generally isotopically lighter than the accepted range (c. +0.06 to +0.49 ‰) for igneous and magmatic-hydrothermal magnetite and exhibit a relatively large range of c. 1 ‰, suggesting Fe source mixing within the deposit. Ore stage magnetite δ⁴⁹Ti (−1.64 to + 3.79 ‰; avg: +1.49 ‰; 2σ = 2.63 ‰) compositions are generally higher and more variable than either the bt-mgt (-0.44 to + 1.49 ‰; avg: +0.62 ‰; 2σ = 1.13 ‰) or mgt-act (+0.27 to + 1.89 ‰; avg: +1.05 ‰; 2σ = 1.56 ‰) and suggest that Ti isotope fractionation occurred due to differential mobility in the fluid. The data are best explained by models invoking both magmatic and non-magmatic metal and fluid input. Fluid flow channeled between the footwall and hanging wall shear zones introduced non-magmatic Fe that may have been leached from local mafic units by magmatic-hydrothermal fluids, resulting in neoformed and regeneratively replaced magnetite with magmatic δ¹⁸O, but non-magmatic δ⁵⁶Fe values during pre-ore alteration. These fluids may have mixed with Fe-poor evaporitic fluids prior to magnetite formation. Later magmatic Fe and O contributions during ore stage mineralization resulted in magnetite with variable δ⁵⁶Fe and irresolvable δ¹⁸O overprinting. Ernest Henry is the first known example of an IOCG deposit with a major leached metal component identified through metal stable isotope geochemistry.

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Fluorite as indicator mineral in iron oxide-copper-gold systems: explaining the IOCG deposit diversity

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

IOCG fluids and mineralization in the Prominent Hill area

Sampling

Petrographic relations in samples across different deposits

Classification of fluorite

Declaration of competing interest

Acknowledgements

Lithos

Geochim. Cosmochim. Ac.

Ore Geol. Rev.

Chem. Erde

Geochim. Cosmochim. Ac.

Precambrian Res.

Geochim. Cosmochim. Ac.

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Ac.

Geochim. Cosmochim. Acta

Chem. Geol.

Precambrian Res.

Chem. Geol.

Ore Geol. Rev.

Chem. Geol.

Chem. Geol.

Chem. Geol.

J. Volcanol. Geoth. Res.

J. Volcanol. Geoth. Res.

Ore Geol. Rev.

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