40Ar/39Ar step-heating geochronology on drill core samples reveals multiple alteration events in the Anabama Hill Cu-Mo prospect, South Australia

Goswami, Naina1,2, Forster, Marnie1,2, Reid, Anthony3

1 Research School of Earth Sciences, Australian National University, Canberra ACT 2601, 2 MinEx CRC, Canberra ACT 2601, 3 Geological Survey of South Australia, Adelaide SA 5001

40Ar/39Ar ultra-high-vacuum (UHV) step-heating experiments can routinely provide key data using small samples taken from drill core, in this case revealing the timing of the evolution of alteration zones, in the Anabama Granite. 40Ar/39Ar geochronology was conducted in conjunction with 39Ar diffusion experiments on K-feldspar, white-mica and biotite from different depths from seven core samples, and provided evidence for alteration in what appeared to be unaltered host rocks. This is because potassium feldspar can grow and regrow during periods of alteration and metasomatism, with the timing of these events constrained precisely by the ages of the high retentivity-core domains. White-mica and biotite behave differently to K-feldspar due to their hydrous nature, crystal structure and chemical composition, but nevertheless can still preserve important age information and retentivity data that helps constrains the evolution of temperature with time, as extracted by conjoint inversion of the data from 39Ar UHV diffusion experiments and 40Ar/39Ar geochronology.

The Anabama Granite is hosted within weakly metamorphosed metasediments in the Adelaide fold belt. The proposed early Ordovician age of ~468Ma is based on a Rb/Sr isochron, implying emplacement at the climax of the Delamerian Orogeny. The granite shows progressive stages of alteration grading from fresh granitic rocks to greisen envelopes hosted in variety of lithologies (granite, granodiorite and adamellite). The region is host to variety of alteration styles introducing secondary minerals such as quartz-sericite-pyrite (phyllic alteration), epidote-chlorite-albite (propylitic alteration), fleshy-pink K-feldspar (potassic alteration), kaolinite-illite (argillic alteration) and precious metals such as Cu, Mo in various sulfide-bearing phases (sulfide-mineralisation) which are indicative of existence of different environments and range of temperature conditions during these process(s). A Cu-Mo prospect has been indentified, but its geological history is yet to be completely deciphered.

Our results show younger ages of ~375-371 Ma preserved within the retentive cores of K-feldspar domains that otherwise show slow cooling from a minimum age of ~380Ma with growth of highly retentive new K-feldspar at ~360Ma. The co-existing white-micas when analysed, reflect an older age of ~457Ma, thus in agreement with previously determined K-Ar ages for muscovite in greisen from alteration zones, at ~455Ma. However, since microstructural analysis demonstrates the presence of K-feldspar overgrowing previously crystallised white-mica, we suggest the presence of an as yet unrecognised alteration event. This later-stage potassic alteration did not exceed temperatures required (or was not of sufficient duration) to reset argon-systematics in mica. These ages can be correlated with ages of thermal perturbations in the Lachlan Fold Belt (LFB) to the east, in which case Anabama Granite represents a western extent of the metamorphic and deformation imprint.

Acknowledgements

This work has been supported by the Mineral Exploration Cooperative Research Centre (MinEx CRC) whose activities are funded by the Australian Government’s Cooperative Research Centre Program.


Biography

Naina Goswami is a second year PhD candidate at The Australian National University (ANU). She finished her Masters (advanced) from ANU in 2018 and her B.Sc (Hons) Chemistry from University of Delhi 2016. She specialises in 40Ar/39Ar geochronology and geochemistry. Currently she is undertaking her PhD at ANU working on a MinEx-CRC project based in South Australia.

Hydrothermal alteration and mineralisation characteristics at Anabama Hill: a porphyry Cu-Mo prospect in the Delamerian Orogen, South Australia

Wei Hong1,2,3, Adrian Fabris2,3, Stacey Curtis2,3, Rian A. Dutch2,3

1 Department of Earth Sciences, School of Physical Sciences, The University of Adelaide, Adelaide, SA, Australia, 5005, 2 Geological Survey of South Australia, Department for Energy and Mining, 11 Waymouth Street, Adelaide, SA, Australia, 5001, 3 Mineral Exploration Cooperative Research Centre (MinEx CRC)

The Delamerian Orogen is the oldest component of the Phanerozoic Tasmanides and is argued to have constituted a Proterozoic continental rift margin overprinted by convergent, west-dipping subduction since the Cambrian (Foden et al., 2020). It has experienced a complex geological history that includes deformation, metamorphism and magmatism from the middle Cambrian (ca. 520 Ma) to early Ordovician (ca. 480 Ma), involving of the emplacement of a series of I-type, S-type and A-type granitoids. Magmatic-hydrothermal Cu-Mo mineralisation in the Delamerian Orogen was first identified in the 1970s, and related Cu-Mo occurrences include the Anabama Hill, Blue Rose, Netley Hill, Cronje Dam-Oak Dam, and Bendigo, situated along the eastern margin of the Adelaide Fold Belt. This belt forms part of a Cambro-Ordovician arc that extends southeast into Victoria (e.g., Thursday’s Gossan Cu deposit) and northeast into New South Wales (e.g., Loch Lilly-Kars Cu prospect), respectively.

The Anabama Hill Cu-Mo prospect is located in the northeast part of the Delamerian Orogen and associated with the Anabama Granite that has limited surface outcrop. Recent geophysical investigations shows that it is a NE-trending large batholith, with an estimated subsurface area > 50 km2. A granodiorite pluton is the major component of this batholith, which was intruded by quartz diorite and monozogranite. Minor components include microgranodiorite, dacite porphyry, and lamprophyre dikes as identified in seven legacy diamond drill cores. These magmatic facies were emplaced into a Neoproterozoic sedimentary sequence composed of mica schists, tillites, quartzites, and siltstones. Only a Pb-Pb plateau zircon age of 485 ± 4 Ma is currently available for the granodiorite (Nasev, 1998). Extensive pyrite-muscovite-quartz alteration prevails in the granodiorite, diorite and monzogranite, which is capped by a 50m-thick weathering zone consisting of kaolinite, montmorillonite, jarosite and goethite. The pyrite-muscovite-quartz zone extends intermittently downwards for more than 700 meters, and occurs commonly as multiple veins and veinlets. Epidote-quartz-magnetite-pyrite veinlets contain chalcopyrite and/or molybdenite disseminations and are overprinted by the pyrite-muscovite-quartz veins. Euhedral, coarse-grained molybdenite, pyrite and muscovite typically occur along fractures or as disseminated patches within massive quartz (> 1m wide). Narrow K-feldspar-quartz veinlets (several cm in width) cut the diorite porphyry and equigranular granodiorite and generally occur below the intense muscovite-dominated alteration zone. Copper grade in the muscovite-rich altered zone range from 0.17% to 0.38%, whereas Mo contents increase from 2-10 ppm in the granodiorite up to 620 ppm in the muscovite-rich assemblages. Epidote-chlorite alteration generally develops in a peripheral domain, extending a few hundred metres laterally from the Cu and Mo anomalies. The granodiorite, monzogranite and diorite have mostly undergone selective epidote-chlorite replacement. Locally, epidote, chlorite, pyrite and minor quartz and magnetite assemblage develop intensely and occur as thick veins (1 to 30 cm wide) that truncate the intrusive facies. Epidote and chlorite mineral chemistry are used to further characterise alteration patterns and assess mineralisation fertility of the Anabama Hill prospect and broader Cu-Mo porphyry-style mineralisation potential of the Delamerian Orogen, which forms part of ongoing research of the Mineral Exploration Cooperative Research Centre and Geological Survey of South Australia.


Biography

Wei Hong is currently a postdoc researcher in the University of Adelaide and embedded researcher in GSSA and MinEx CRC, conducting research on assessing mineralisation potential in the Delamerian Orogen. He completed his PhD in 2017 on Tasmanian granite and related Sn-W mineralisation and following two-year postdoc at CODES. 

Constraining alteration in the buried Benagerie ridge, Curnamona province South Australia

Simpson, Alex1, Glorie, Dr Stijn1, Hand, Prof Martin1, Reid, Dr Anthony2, Gilbert, Dr Sarah3

1The University Of AdelaideMineral Exploration Cooperative Research Centre, School of Earth & Environmental Sciences, University of Adelaide, Adelaide, Australia, 2Mineral Exploration Cooperative Research Centre, Geological Survey of South Australia, Adelaide, Australia, 3Adelaide Microscopy, University of Adelaide, Adelaide, Australia

The Curnamona province, situated at the border between SA and NSW is a piece of Paleoproterozoic crust that is separated from the Gawler craton by the Adelaide Rift Complex. This geological region is highly prospective for a variety of mineralisation types, including the Pb-Zn-Ag (e.g. the world class Broken Hill deposit), and iron-oxide copper gold (IOCG) (e.g. Kalkaroo and Portia deposits) [Conor and Preiss, 2008]. The Benagerie ridge volcanic suite (BVS) sits in the centre of the province and is a correlative of the Gawler Range Volcanics (GRV), host of the world class Olympic Dam IOCG deposit [Wade et al., 2012]. The province has undergone multiple episodes of deformation, with the most important events considered to be the ~1600 Olarian Orogeny and the ~500 Delamerian orogeny  [Conor and Preiss, 2008], with some evidence for ~830 Ma dyke emplacement [Wingate et al., 1998]. Additionally, multiple episodes of alteration and mineralisation have occurred, particularly regional scale albitisation [Skirrow et al., 1999]. Limited direct constraints exist on the timing of these episodes, with most thought to have occurred prior to the Olarian Orogeny.

LA ICP MS elemental and isotopic mapping, combined with LA ICP MS U-Pb geochronology, and geochemistry provide insight into multiple episodes of post-Olarian fluid alteration, including ~820 Ma albitisation and mineralisation in the BVS. We further demonstrate the utility of U-Pb geochronology applied to hydrothermal apatite, titanite, calcite and magnetite to constrain the timing of episodes of fluid alteration.

Conor, C. H. H., and W. V. Preiss (2008), Understanding the 1720–1640Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis, Precambrian Research, 166(1-4), 297-317.

Skirrow, R., R. Maas, and P. M. Ashley (1999), New age constraints for Cu-Au(-Mo) mineralisation and regional alteration in the Olary-Broken hill region, AGSO Research Newsletter, 31.

Wade, C. E., A. J. Reid, M. T. D. Wingate, E. A. Jagodzinski, and K. Barovich (2012), Geochemistry and geochronology of the c. 1585Ma Benagerie Volcanic Suite, southern Australia: Relationship to the Gawler Range Volcanics and implications for the petrogenesis of a Mesoproterozoic silicic large igneous province, Precambrian Research, 206-207, 17-35.

Wingate, M. T. D., I. H. Campbell, W. Compston, and G. M. Gibson (1998), Ion microprobe U–Pb ages for Neoproterozoic basalticmagmatism in south-central Australia and implications for thebreakup of Rodinia, Precambrian Research, 87, 135-159


Biography

Alex Simpson is a PhD student at the University of Adelaide with interests in radiometric dating utilising novel and emerging laser ablation methods applied to   geochronology on non-traditional minerals, with applications for constraining alteration and remobilisation events.

Greening up brownfields: Adding new timelines to mineral systems models for the Mesoproterozoic Gawler Craton

Morrissey, Laura1,2,4, Payne, Justin3,2,4, Hand, Martin4,2, Bockmann, Mitchell4,2, Yu, Jie4,2, Reid, Anthony5

1Future Industries Institute, University Of South Australia, Adelaide, Australia, 2Mineral Exploration Cooperative Research Centre, Adelaide, Australia, 3UniSA STEM, University of South Australia, Adelaide, Australia, 4Department of Earth Sciences, University of Adelaide, Adelaide, Australia, 5Department for Energy and Mining, Geological Survey of South Australia, Adelaide, Australia

The Gawler Craton is commonly considered to record a complex tectono-metamorphic and magmatic history from the Archean to the Mesoproterozoic, culminating in voluminous magmatism of the Hiltaba Suite and the Gawler Range Volcanics (GRV) between 1600–1570 Ma. Although there is widespread evidence for tectono-metamorphic and magmatic events younger than 1570 Ma, these commonly receive little attention because they are enigmatic, occur in poorly outcropping parts of the craton and are difficult to correlate regionally. In addition, because the Hiltaba–GRV event was associated with the development of large Iron-Oxide-Copper-Gold (IOCG) and Au deposits it holds the most interest for mineral exploration, with the younger events considered to be of little importance. However, these younger events have potential for both new addition of metals and remobilisation of existing deposits, and therefore understanding their character and expression is vital to create a holistic mineral systems framework for the Gawler Craton.

Reanalysis of legacy drill holes using modern geochronologic and metamorphic techniques has led to the identification of younger metamorphic events between c. 1570–1550 Ma, c. 1530–1520 Ma and 1500–1400 Ma in the northern Gawler Craton. In the Mount Woods region, monazite age populations of c. 1570 Ma and c. 1550 Ma are interpreted to record a phase of high thermal gradient metamorphism, deformation and mafic magmatism that post-dates Hiltaba-aged granitic magmatism in this region. Similar metamorphic ages between c. 1570–1550 Ma elsewhere in the northern Gawler Craton, Yorke Peninsula and Barossa Complex suggests that this event is widespread across the Gawler Craton. In the Peake and Denison Inlier, calcic alteration has been dated at 1530 Ma, and is approximately coeval with migmatisation and metamorphism in the Nawa Domain at c. 1520 Ma. Magmatic rocks with inferred ages of c. 1520 Ma in the northern Gawler contain marialitic cavities, suggesting that they intruded at relatively shallow depths. Post 1500 Ma, the Gawler Craton is thought to record only minor sedimentation such as the intra-continental Pandurra Formation. However, A-type magmatism and high thermal gradient metamorphism occur at c. 1450 Ma in the Nawa Domain, and reactivation of lithospheric-scale shear zones occurs between 1470–1450 Ma. The crustal-scale Karari Shear zone and Cairn Hill Fe-Cu deposit also record monazite growth at c. 1490–1480 Ma.

The high thermal gradients recorded in the metamorphic rocks, combined with a lack of evidence for significant exhumation or dissection of the GRV in the central Gawler Craton, suggests that all these younger events record periods of extension. The locus of extension and shear zone reactivation may have migrated through time, leading to apparently discrete zones of reworking. These extensional events have the potential to drive fluid flow events along reactivated shear zones (derived from overlying sediments such as the Pandurra Formation, or from magmas intruded at depth), as well as add new metals to the crust. This, with the evidence for young metamorphism/alteration at Cairn Hill, suggests that the Hiltaba Event may not be the final mineralising event in the Gawler Craton.


Biography

Laura Morrissey completed a PhD in metamorphic petrology and tectonics at the University of Adelaide in 2016. She is now a research fellow at the University of South Australia and is working in the MinEX CRC.

Between the Stavely and Koonenberry: a structure with no arc, or an arc hidden in structure?

Wise, Tom1,2., Curtis, Stacey1,2,3 Robertoson, Kate1,2,4

1Geological Survey of South Australia, Adelaide. 2. Mineral Exploration Cooperative Research Centre. 3. University of South Australia. 4. University of Adelaide

Late Cambrian volcanic arc segments are recognised in the Stavely and Konnenberry belts, whilst the presence or absence of a similar segment in the South Australian section of the Delamerian Orogen has been debated. Regional scale geophysical imagery has been re-interpreted to suggest that the Mount Wright Volcanics, part of the calc-alkaline volcanic arc in the Koonenberry Belt, likely extends into South Australia beneath the central Murray Basin. This continuation into South Australia may link to fore-arc style magmatism near Keith (Foden et al., 2020), rendering a temporal-spatial link between the Stavely and Koonenberry belts unlikely due to the parallel nature of the potential arc segment near Keith with the Stavely Belt.

A potential volcanic arc/subduction system positioned along the eastern margin of South Australia has direct implications for the porphyry-epithermal potential of the Delamerian basement to the Murray Basin, but also prompts a re-evaluation of the regional effects of the Delamerian Orogeny. Province-wide magnetotelluric models provide information about the nature of the middle crust down to the lithospheric mantle, and reveal highly resistive zones associated with rift axes of the middle Cambrian Kanmantoo Group, whilst further east, where potential arc crust may reside, the crust and lithospheric mantle is less resistive.

If volcanic arc rocks in South Australia represent the earliest subduction products along the east Gondwana margin (Foden et al., 2020) prior to roll-back and subsequent subduction in the Stavely zone, post-tectonic intrusives in a belt from the Padthaway Ridge to Anabama can be re-examined in the context of regional extensional episodes. Foden et al., (2020) attribute post-tectonic magmatic suites to delamination and asthenospheric upwelling. However, the geometry of the post-tectonic belt may imply that a crustal tear was developed earlier during rollback-induced extension, and exploited favourable original Kanmantoo rift structures. Potential field interpretation/modelling tests this hypothesis, whilst drilling as part of the MinEx CRC National Drilling Initiative will provide greater constraint for the proposed geologic framework of the region.


Biography

Tom Wise is a Senior Geologist at the Geological Survey of South Australia, and is the Technical Lead for the Delamerian project of the MinEx CRC National Drilling Initiative.

Uncovering the Cobar Basin – new results from the NSW MinEx NDI areas

Folkes, Chris1; Deyssing, Liann2; Trigg, Steven2; Carlton, Astrid1; Schifano, Joe3

1Mineral Exploration Cooperative Research Centre, Geological Survey of New South Wales, Department of Regional NSW, Maitland, NSW, Australia, 2Mineral Exploration Cooperative Research Centre, Geological Survey of New South Wales, Department of Regional NSW, Orange, NSW, Australia, 3Mineral Exploration Cooperative Research Centre, University of New South Wales, Sydney, NSW, Australia

The Cobar region of central-western New South Wales hosts numerous precious and base metal mineral systems, many exploited by active and historical mine workings. These are mostly located around rock outcrops and under shallow cover sequences of the Cobar Basin. The North Cobar and South Cobar MinEx CRC National Drilling Initiative (NDI) areas were selected by the Geological Survey of New South Wales (GSNSW) to improve the understanding of the geology, mineral systems and groundwater in basement and regolith profiles that extend away from the margins of these known mineralised areas.

Understanding the prospective basement geology in the Cobar region is complicated by variations in the depth and nature of the weathering profiles through the region. A mix of transported and in situ weathering reflects a complex palaeo-landscape.

Recent activities by the GSNSW are designed to map the thickness and character of the weathering profile and increase our knowledge of basement geology. This work will inform and complement proposed drilling in the NDI areas to further investigate the geochemical and petrophysical signatures of basement geology and mineral systems. The activities also have implications in understanding groundwater resources.

Comprehensive audit and gaps reports were published in 2020 for the North Cobar and South Cobar NDI areas. These reports reviewed existing data, provided recommendations for new sampling and data capture, and prioritised the scientific questions that should be addressed. Recent mineral potential mapping and mineral-systems studies identified new prospective areas and provided a better understanding of the main controls on, and timing of mineralisation for the Cobar region.

An airborne electromagnetic (AEM) survey was flown in the region in 2019, comprising 116 east‒west lines spaced at 2.5 km and 5 km, totalling approximately 11,000 line km. Ongoing modelling and interpretation of the data have enhanced knowledge of depth and nature of the cover, groundwater systems, structural geology (e.g. fault locations and geometry), mineral systems and stratigraphy. The AEM data, with other datasets such as fault attribution, NSW Seamless Geology, and crustal-scale fault models, are being integrated into a new 3D model of the region that will benefit mineral explorers and hydrogeologists.

The AEM data have also been integrated with drilling, hydrogeochemical, biogeochemical and spectral surveys. Downhole lithology and assay information from legacy drilling has been imported into 3D viewable datasets. Hydrogeochemical analyses from waterbores in the Cobar region were completed by CSIRO and GSNSW and highlighted new areas for potential exploration. Researchers from UNSW analysed Cypress Pine needles to provide a regional biogeochemical dataset with focus areas over known mineralisation. Spectral scanning of legacy drillcore using the HyLoggerTM has provided useful constraints on the depth and nature of weathering, and alteration of basement rocks.

These activities build on recent mineral system studies to advance the understanding of the geology, and mineral and groundwater resources in the North Cobar and South Cobar NDI areas. This has greatly helped to inform the site selection and scientific questions to be answered with future MinEx CRC drilling in these NDI areas.


Biography

Chris Folkes is a senior geoscientist in the Regional Mapping team at the Geological Survey of NSW. He completed his undergraduate studies at the University of Bristol in the UK, and a PhD at Monash University in volcanology, igneous geochemistry and geochronology.

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