Olivine-phyric basalt in the Mesoproterozoic Gawler silicic large igneous province, South Australia: Examples at the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit and other localities
Introduction
Large igneous provinces (LIP) involve emplacement of voluminous magma into and/or onto the Earth’s crust in events of relatively short duration (Coffin and Eldholm, 1994, Ernst et al., 2005, Bryan and Ernst, 2008). Most LIP are mafic-dominated, consisting of flood basalt lavas, dyke swarms and layered intrusions (Bryan and Ferrari, 2013), composed of mantle-derived magmas. However, a wider range of magma compositions from basaltic to rhyolitic may be present in LIP, and silicic large igneous provinces (SLIP) have been defined, where silicic igneous rocks (>65 wt.% SiO2) predominate (Bryan et al., 2002). The paucity of outcropping mafic and ultramafic igneous rocks in SLIP may result either from poor exposure of these units due to concealment beneath felsic units, or from buoyant stalling of mafic magmas at lower crustal levels or at the mantle-crust boundary; that is, SLIP may be associated with ‘hidden mafic large igneous provinces’ (Ernst, 2014). Whichever is true, the thermal (± mass) input from underplating mafic and ultramafic magmas (Bryan et al., 2002) required to drive large-scale crustal partial melting and to generate large volumes of silicic magmas makes the investigation of the mafic igneous units in any SLIP of scientific interest.
The formation of the ca. 1590 Ma Gawler SLIP (Allen et al., 2008), consisting of the Gawler Range Volcanics (GRV) and the Hiltaba Suite (HS), in South Australia (Fig. 1) was probably coeval with the assembly of the Laurentian supercontinent (Blissett et al., 1993, Creaser, 1995, Allen and McPhie, 2002, Payne et al., 2009). However, rare exposures of mafic units in the Gawler SLIP and the generally evolved character (clinopyroxene–plagioclase-phyric) of these units have hampered follow-up research into the petrogenesis (importantly, the characteristics of the mantle source), and thus any implications for the tectonic setting of the overlying crust at ca. 1590 Ma.
In this study, we focus on the occurrence of olivine-bearing mafic igneous rocks at four locations in the Gawler SLIP: Kokatha (Giles, 1988, Blissett et al., 1993), the Mount Gunson copper deposit (Knutson et al., 1992), the Wirrda Well Cu–Au prospect and the Olympic Dam Iron Oxide Cu–U–Au–Ag deposit. These rocks have comparable petrographic features. U–Pb apatite dating results support correlation with the GRV and the Gawler SLIP. Compositions of Cr-spinel inclusions in the olivine pseudomorphs and whole-rock samples provide constraints on the parental melt compositions and magma sources. We consider the significance of the magma sources in relation to interpretations of the tectonic setting of the Gawler SLIP.
Section snippets
Regional geology
The Gawler Craton records several cycles of magmatism, sedimentation, and orogenesis in its protracted evolution from the Meso-Archean to Mesoproterozoic. A brief introduction is presented here and further details can be found in Hand et al., 2007, Reid and Hand, 2012.
Meso-Archean to early Paleoproterozoic rocks occur centrally in the Gawler Craton, and comprise highly deformed metasedimentary sequences including aluminous metasedimentary units interlayered with banded iron formations,
Mafic GRV at Kokatha, Mount Gunson, Wirrda Well, and Olympic Dam
Former-olivine-bearing mafic GRV units have been reported at Kokatha (Giles, 1988, Blissett et al., 1993), Mount Gunson (Knutson et al., 1992), Wirrda Well (Ehrig, 2013) and Olympic Dam (Johnson, 1993). Outcrop (Kokatha) and drill core (Mount Gunson, Wirrda Well and Olympic Dam) samples collected for this study are described below.
Cr-spinel compositions
Cr-spinel mainly occurs as inclusions in the olivine pseudomorphs in all studied olivine-phyric rocks described above. Less commonly, it is enclosed in the clinopyroxene phenocrysts and present in the groundmass. Cr-spinel inclusions within olivine pseudomorphs have been analyzed (A.1. Methods).
Considering that all the olivine-phyric rocks in this study are altered and that primary olivine has been invariably replaced by a variety of secondary minerals, it is possible that the compositions of
Apatite U–Pb dating of the olivine-phyric rocks
Apatite occurs as a primary accessory phase in the olivine-phyric basalt at Mount Gunson, and Wirrda Well, and in the NW-striking doleritic to porphyritic olivine-bearing dykes at Olympic Dam. In the basalt at Olympic Dam, spongy apatite occurs in the groundmass and less commonly in the olivine pseudomorphs. The Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) U–Pb dating of apatite (Chew et al., 2011, Chew et al., 2014) in these rocks provides the first opportunity to
Geochemistry of the mafic GRV
The compositions of the olivine-phyric basalts at all locations have been modified by secondary processes to variable degrees, as indicated by their petrographic features (Fig. 2, Fig. 3, Fig. 4, Fig. 5). In the case of the most intensely altered olivine-phyric basalt at Olympic Dam, the major element compositions largely reflect different proportions of hydrothermal chlorite, quartz, carbonate, sericite and hematite, rather than primary magmatic features. Drill core assays (BHP Billiton,
Recognition of high-Mg rocks in the Gawler SLIP
Weakly deformed to undeformed olivine-phyric basalt at Olympic Dam has similar petrographic textures (e.g. porphyritic and amygdaloidal) and comparable mineralogy (e.g. olivine, Cr-spinel, and possible phlogopite) to other olivine-phyric basalts at Kokatha, Mount Gunson and Wirrda Well, but generally contains a higher abundance of olivine pseudomorphs (commonly ∼20 vol.%).
Compositional relationships between Cr-spinel inclusions and host olivine phenocrysts in mafic volcanic rocks in different
Conclusions
U-Pb dating of primary accessory apatite in basalts and dykes at Mount Gunson, Wirrda Well and Olympic Dam, and the occurrence of strongly olivine-phyric basalt at Olympic Dam, demonstrate that primitive high-Mg magmas were produced as an integral component of the Gawler SLIP in the Gawler Craton at ca. 1590 Ma. The compositions of Cr-spinel inclusions in the olivine-phyric basalts and whole-rock samples allow us to evaluate the tectonic setting of the mafic GRV from the perspective of
Acknowledgements
This research was supported by BHP Billiton and the Australian Research Council (ARC Linkage Grant “The supergiant Olympic Dam U-Cu-Au-REE ore deposit: towards a new genetic model”). We thank Teena Rusak and Stacey McAvaney (DMITRE) for introduction to the SARIG Database. We also thank Karsten Goemann and Sandrin Feig (CSL, University of Tasmania) for helping with microprobe analyses, and Jay Thompson (CODES, University of Tasmania) for help with apatite dating. We thank Hugh Rollinson for his
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