Full title:
New developments in laser Rb–Sr dating and their application in exploring Proterozoic sedimentary basins — an example from the northern Stuart Shelf, South Australia
Alan S Collins1, Morgan L Blades1, Carmen BE Krapf2, Sarah Gilbert3, Wolfgang Preiss1,2 and Adrian Fabris2
1 Tectonics and Earth Systems Group and MinEx CRC, School of Physics, Chemistry and Earth Sciences, The University of Adelaide
2 Geological Survey of South Australia, Department for Energy and Mining
3 Adelaide Microscopy, The University of Adelaide
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Published May 2024
Introduction
Exploration for Proterozoic sedimentary-hosted mineral and energy resources can greatly benefit from geochronological methods that date the depositional age of sedimentary rocks that predate the Early Cambrian evolution of biomineralizing organisms. Previously, the lack of such methods made intra-basin (let alone extra-basin) correlation difficult and hindered the development of sequence-stratigraphic models that form a major parameter in any mineral system analysis. In addition, the lack of accurate and reasonably precise age constraints makes it hard to reconstruct the evolution of the earth-surface systems (biosphere, hydrosphere and atmosphere) that make our planet habitable to large metazoans such as ourselves.
New developments in laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICPMS), using a reaction-cell embedded within the instrument that is bracketed by two separate mass spectrometers (LA-ICPMS/MS), has allowed a resurgence in geochronology that uses beta-decay radiometric systems, such as Rb–Sr (Armistead et al. 2020; Redaa et al. 2021; Redaa et al. 2022; Subarkah et al. 2022a; Subarkah et al. 2022b; Zack and Hogmalm 2016), Lu–Hf (Simpson et al. 2021; Tamblyn et al. 2021), Re–Os (Hogmalm et al. 2019) and even K–Ca (Hogmalm et al. 2017).
Recently, the Tectonics and Earth Systems Group at The University of Adelaide successfully dated shales from the McArthur Basin in the Northern Territory, and also hydrothermally-altered mafic intrusions that were altered at, or soon after, the time of intrusion (Subarkah et al. 2022a). Here we present the results of a reconnaissance study where we used these techniques to date units of uncertain stratigraphic position in the northern Stuart Shelf of South Australia, intersected in drillhole WC05D001 (29.42933°S, 135.81344°E). WC05D001 is one of 25 drillholes that was relogged as part of the Geological Survey of South Australia’s ‘Sedimentary Copper Mineral Systems of the Stuart Shelf’ project (Krapf et al. 2023). Drill core was sampled for Rb–Sr dating by this method with the aim of constraining the age of these rocks and hence to correlate them within the stratigraphic framework of the Stuart Shelf and the wider Adelaide Superbasin (Lloyd et al. 2020). Samples included the base of a volcanic unit, as well as an altered and amygdaloidal part of the same unit. In addition, we dated two sedimentary units directly underlying the volcanic unit: a shale and a stromatolitic dolomite.
Regional geology of the northern Stuart Shelf
The Stuart Shelf is a wedge of Neoproterozoic to Cambrian platform cover that overlies the eastern Gawler Craton. The dominantly flat-lying siliciclastic sediments are distinguishable from their lithostratigraphic equivalents in the Adelaide Rift Complex by markedly condensed thickness and no significant metamorphic overprint. Its eastern margin is defined by the Torrens Hinge Zone which is marked by thickening and mild folding of stratigraphic units, transitional into the strongly folded and faulted rocks of the Adelaide Rift Complex to the east (Fig. 1). While the Stuart Shelf sediments are known to onlap the eastern part of the Gawler Craton, its northern and southern depositional margins are not preserved.
Figure 1 Geology and copper deposits and occurrences of the Stuart Shelf in the region of drillhole WC05D001.
The deposits of the Stuart Shelf overlie a variety of Archaean to Mesoproterozoic basement units including the Gawler Range Volcanics and the Pandurra Formation. The oldest Neoproterozoic/Adelaidean units are the Beda Basalt and Backy Point Formation, which are part of the Callanna Group and relate to the earliest rift phase of the Adelaide Superbasin (Lloyd et al. 2022). These units have previously been identified mainly in the southern part of the Stuart Shelf and are less well known in its northern part. The dominant formations on the Stuart Shelf belong to the Umberatana Group, including Sturtian and Marinoan glacials and prospective carbonate-bearing siltstone of the Tapley Hill Formation, and the lower part of the Ediacaran Wilpena Group.
The northern part of the Stuart Shelf is not well studied owing to a lack of deep drilling. Drillhole WC05D001, one of only a few drillholes that intersect the Neoproterozoic units, was drilled by Minotaur Exploration Ltd to a depth of 849.70 m in 2005 and is located approximately 140 km southeast of Cooper Pedy (Fig. 1). The drillhole targeted a significant basement gravity and aeromagnetic anomaly. The initial interpretation of the pre-Mesozoic strata was: Neoproterozoic (220.70–510 m), Mesoproterozoic (510–649.2 m) and Palaeoproterozoic (649.20 m to end of hole at 849.70 m). However, there was considerable uncertainty about the ages of the stratigraphic intervals intersected, with interpretations including equivalents of the Mesoproterozoic Pandurra Formation, basal Adelaidean (Neoproterozoic) sediments and volcanics of the Callanna Group, and even Cambrian sediments (Flint et al. 2005).
Applying the newly developed laser Rb–Sr dating technique to drill core samples from WC05D001, we tested the usefulness of the technique to obtain meaningful data from previously hard to date rock samples. From these data, we applied age constraints to assessing the stratigraphy position and correlation of the rocks intersected in this drillhole. The results of this work provide a baseline for future investigations in this part of the Stuart Shelf.
Summary of lithologies in drillhole WC05D001 and their potential correlation
Drill core was retrieved from WC05D001 from 221 m to the end of the hole at a depth of 849.75 m. Crystalline basement of assumed Palaeoproterozoic age intersected at 649 m consists predominantly of garnetiferous quartz-feldspar-biotite paragneiss and quartz-feldspar pegmatite with lesser, foliated granitoid, chlorite-amphibole skarn and biotite-cordierite schist (Flint et al. 2005). The lithologies of units overlying Palaeoproterozoic basement rocks intersected in drillhole WC05D001 are displayed in Figure 2 and include predominantly sandstone of the Mesoproterozoic Pandurra Formation (depth 649.07–526.92 m), shale (in parts dolomitic), dolomite (partly stromatolitic; depth 526.92–266.20 m) and basalt (depth 266.20–214 m). Overlying Carboniferous–Permian, Mesozoic and Quaternary sediments (depth 214–0 m) were not cored.
The interval of interest for this study is between 214 and 526.92 m depth intersecting multiple beds of siltstone, sandstone, shale, dolomite, stromatolitic dolomite and vesicular basalt (Figs 2 and 3). The occurrence of multiple lava flows provided an opportunity to attempt to date this part of the Stuart Shelf succession.
Figure 2 Summary drillhole log of WC05D001 with locations of samples for Rb–Sr dating.
In assessing correlation of these units a number of potential equivalents were considered, but this is impeded by the lack of outcrop and other deep drillholes in the vicinity. The red shale unit with shallow-water sedimentary structures bears some resemblance to the mud-flat facies of the Angepena Formation in the western and northeastern parts of the Adelaide Superbasin, however, the overlying stromatolitic dolomite has no known equivalent on the Stuart Shelf. Potential equivalents of the mafic volcanics at the top of the succession include the Beda Basalt in the southern Stuart Shelf, the Wooltana Volcanics of the northern Flinders Ranges, the unnamed volcanics at Depot Creek, and the Wantapella Volcanics in the Officer Basin. Further afield are the Kulyong Volcanics of Cambro-Ordovician age. However, the closest lithological match is with the Arkaroola Subgroup section in the nearby Peake and Denison Ranges, which comprises the basal Younghusband Conglomerate, overlain by the stromatolitic Coominaree Dolomite and the mafic Cadlareena Volcanics (Ambrose et al. 1981). The stromatolites in the Coominaree Dolomite were described by Preiss (1973, 1987), and closely resemble the columnar and digitate forms in drillhole WC05D001. The stromatolitic and mafic volcanic interval also closely resembles the Neoproterozoic succession intersected in drillhole Manya 5 in the northeastern Officer Basin, which has been correlated with the Coominaree Dolomite and Cadlareena Volcanics (Preiss 1993).
Figure 3 Drill core image of contact between sediments (now correlated with the Arkaroola Subgroup) and overlying basalts (now correlated with the Wooltana Volcanics) in WC05D001, with three of the sample locations indicated by a star.
Rb-Sr geochronology of shale and volcanic rocks in WC05D001
Four samples from WC05D001 between the depths 247.95 and 268.18 m were collected for dating (Figs 2 and 3), including the base of the volcanic unit, an altered and amygdaloidal part of the same unit, and from the two sedimentary units directly underlying the volcanic unit: a shale and a stromatolitic dolomite. The samples were analysed to constrain the age of the Neoproterozoic succession in this part of the northern Stuart Shelf (Fig. 4).
Figure 4 Photograph of cut blocks of the four samples collected from drill core WC05D001. (a) Sample 3404227 – carbonate with columnar stromatolite. (b) Sample 3404228 – shale. (c) Sample 3404229 – altered amygdaloidal basalt. (d) Sample 3404230 – altered basalt.
Methodology
Laser analyses on samples and standards were carried out at Adelaide Microscopy, University of Adelaide using a laser ablation (RESOlution-LR ArF 193 nm excimer laser) inductively couple-mass spectrometer (Agilent 8900x ICP-MS/MS) with the analytical parameters and tuning conditions following Redaa et al. (2021) and Subarkah et al. (2022a).
One spot analysis consisted of 20 seconds of gas background with 40 seconds of ablated signal. Strontium isotopes were measured in the oxidised molecule SrO. Strontium is oxidised with N2O gas in the reaction chamber (e.g. 87Sr16O formed from 87Sr at 103 amu), whilst the unreactive 85Rb was measured on-mass. Dwell times for each SrO isotope were 50 ms, 10 ms for Rb and 5 ms for all other masses totalling to 0.31 seconds. Average pit depths for each spot analysis are approximately 25 µm. As such, each spot will comprise of an ‘average’ value from all the minerals within the crater. For this reconnaissance study, we haven’t investigated the mineralogy or textural relationships of minerals within the analyses pits (c.f. Subarkah et al. 2022a). We note that a number of studies now are indicating the lack of detrital clay minerals in pre ca. 600 Ma shales that is likely due to the lack of biologically-generated terrestrial soils before this time (Rafiei and Kennedy 2019; Rafiei et al. 2020; Subarkah et al. 2022a).
In situ Rb–Sr dating of CRPG reference material Mica-MG (Govindaraju 1979) prepared as a nano-powder pellet and a phlogopite mineral sample MDC, sourced from the same quarry in Bekily, Madagascar, were assumed to be of the same age (Redaa et al. 2021; Subarkah et al. 2022a) and were used as primary and secondary standards respectively (Appendix 1). An independent secondary standard glauconite grain mount sample GL-O from Bagnols-sur-Cèze, France, was also analysed during the same run (Rousset et al. 2004). Glass standards NIST-610 and BCR-2G (Jochum et al. 2016) were also analysed as standards for element quantification. Isochron ages were calculated using IsoplotR software (Paton et al. 2011; Vermeesch 2018) and a Rb decay constant of 1.393 x10-11 (Nebel et al. 2011) was used.
The standards Mica-MG and MDC were anchored to its reported 87Sr/86Sr value of 0.72607 ± 0.00363 (Hogmalm et al. 2017). These gave an age of 520 ± 4 and 537 ± 13 Ma, respectively, within uncertainty of Mica-MG dated at 519 ± 7 Ma (Hogmalm et al. 2017; Appendix 1). The secondary standard GL-O of known yielded a weighted average mean age of 96 ± 10 Ma, within uncertainty of published K–Ar geochronology of 95 ± 1.5 Ma (Derkowski et al. 2009). Data for the secondary standard and all unknowns can be found in Appendix 2.
Samples 3404228 (266.72–266.79 m) and 3404227 (268.13–268.18 m)
Sample 3404227, a stromatolitic dolomite, and 3404228, a shale, from below the intersected basalt, were analysed (Figs 4a, b; Appendix 2). Carbonates are often low in Rb and therefore record the initial Sr ratios, which can be used to help constrain a lower intercept of an isochron. As a result, these two samples (3404227 and 3404228) are plotted on the same isochron (Fig. 5). Together, these gave 87Sr/86Sr initial ratios (denoted as 87Sr/86Sr0) of 0.7180 ± 0.002, higher than the expected 87Sr/86Sr signature of Proterozoic seawater (Chen et al. 2022; Kuznetsov et al. 2018; Shields and Veizer 2002). The shale and carbonate sample yielded an isochron age of 904 ± 46 Ma (n=86, MSWD=0.62). To determine the level of detrital flux within these samples, cross-plots between titanium, which is commonly of detrital origin and usually immobile during diagenesis, and other elements were used. Statistically significant correlation (P-values less than 0.05) with higher R2 values are evident between Fe (R2=0.48), Sr88 (R2=0.34) and Zr (R2=0.29). This suggests that there is a reasonable detrital component within this sample, which is likely to have resulted in an ‘age’ that is older than the true depositional age.
Figure 5 Isochron for samples 3404228 and 3404229 combined.
Samples 3404229 (247.95–248 m) and 3404330 (265.9–265.95 m)
Samples 3404229 and 3404330 are taken from the middle and base of the altered basalt, respectively (Figs 4c, d; Appendix 2). These gave ages and 87Sr/86Sr initial ratios within uncertainty of each other. Sample 3404229, taken from the middle part of the basalt unit, has many calcite-filled amygdales that were also analysed to constrain the initial 87Sr/86Sr ratio. Similarly, sample 3404330 is brecciated with calcite veining and therefore the age derived from these samples provides a minimum constraint for the emplacement of the basalt. A total of 67 spot analyses were done on sample 3404330, four of which were excluded in the final calculation as they did not sit along the isochron. Sixty-nine analyses were done on sample 3404229, with none excluded in the calculated isochron. The initial 87Sr/86Sr ratios sit between 0.7082 and 0.7101 with isochron ages of 766 ± 26 Ma (3404229, Fig. 6a) and 754 ± 44 Ma (3404330, Fig. 6b).
Figure 6 Isochrons for (a) sample 3504229 and (b) sample 3404330.
Implications for using laser Rb–Sr ICPMS/MS dating for sedimentary-hosted mineral exploration
The late Tonian ages derived from the altered volcanics in WC05D001 are interpreted to provide a minimum age of volcanism in this drillhole as the incorporation of altered basalt as well as direct hydrothermal minerals (calcite amygdales and veins) have been used to produce a well-constrained isochron by ensuring a large spread of rubidium in the analyses. We suggest that this is valid as the basalt appears pervasively altered in hand specimen, which we interpret to be coeval with hydrothermal veining and amygdale filling. These ages, therefore, help constrain the age of both the volcanics and the sedimentary rocks within WC05D001. The age of alteration rules out a Cambrian or later Neoproterozoic magmatic age and the volcanics most likely correlate with other Adelaide Superbasin Tonian volcanic successions, such as the ca. 827 Ma Gairdner Dolerite and its extrusive equivalents (Beda Basalt, Wooltana Volcanics and Cadlareena Volcanics of the Arkaroola Subgroup; Preiss 2000), or the ca. 790 Ma Boucaut Volcanics (Armistead et al. 2021) that mark the base of the Burra Group.
Analysis of the sedimentary rocks below the volcanics yielded apparent ages that are interpreted to reflect a detrital component, which is supported by the trace element analysis. The apparent age of 904 ± 46 Ma is therefore interpreted as a maximum depositional age for this sequence. This constrains this sequence to being Tonian and refutes the possibility of them being a part of the underlying Pandurra Formation (Flint et al. 2005). These constraints are consistent with the lithological and stromatolite correlation of the dated succession with the Arkaroola Subgroup in the Peake and Denison Ranges and in drillhole Manya 5 in the Officer Basin. We therefore suggest that the Neoproterozoic succession in WC05D001 belongs to the Arkaroola Subgroup of the Callanna Group. Lithologically, it differs from the Arkaroola Subgroup in the southern Stuart Shelf, where the Beda Volcanics interfinger with coarse-grained basement-derived clastics of the Backy Point Formation.
Conclusions
Here we demonstrate the utility of the new in-situ laser Rb–Sr geochronological technique to constrain ages of altered volcanic rocks and Proterozoic sedimentary rocks. A succession of sedimentary and mafic volcanic rocks, intersected in borehole WC05D001 in the northern Stuart Shelf has been constrained to being of Tonian age by a minimum age for the overlying volcanics of 766 ± 26 Ma and a maximum age for deposition of the lower sedimentary rocks of 904 ± 46 Ma, consistent with lithological correlation with the Arkaroola Subgroup of the Callanna Group. This has implications for sedimentary-hosted copper exploration in this region of South Australia as the Callanna Group contains known copper-cobalt mineralisation and is age-equivalent to the highly prospective Roan Group in the Central African Copperbelt (Cailteux et al. 2005). Applying laser Rb–Sr dating to other drillholes from the northern part of the Stuart Shelf where undifferentiated Neoproterozoic or unassigned sedimentary rocks and volcanics are reported should build better understanding of the presence and extent of the Callanna Group and therefore will also refine the sedimentary-hosted copper exploration search space in this region.
Appendix 1
Laser analyses on standards carried out at Adelaide Microscopy, University of Adelaide using a laser ablation (RESOlution-LR ArF 193nm excimer laser) inductively couple-mass spectrometer (Agilent 8900x ICP-MS/MS).
Appendix 2
Rb–Sr and REE data from standards acquired at Adelaide Microscopy, University of Adelaide using a laser ablation (RESOlution-LR ArF 193nm excimer laser) inductively couple-mass spectrometer (Agilent 8900x ICP-MS/MS).
Download data spreadsheet (XLSX 499 KB)
Acknowledgements
We acknowledge that drillhole WC05D001 comes from the land of the Arabana people. This study resulted from an extensive drill core review as part of the GSSA-CSIRO ‘Sedimentary Copper Mineral Systems of the Stuart Shelf’ project. It is also MinEx CRC contribution #2023/01. ASC and MLB acknowledge funding from the Australian Research Council Linkage Grant LP200301457 with Santos, Empire Oil and Gas, NT Geological Survey, BHP and Teck Resources as partners.
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