A review initiated as part of the Discover Gold program has revised the gold mineral systems of the Central Gawler Craton region. Over one hundred deposits and occurrences were described according to their host rocks, sulphide assemblages, element suites, veins styles, alteration, isotopic character and deformation (pre-, syn-, post-).

Three major Mineral Systems corresponding to the deformation/magmatic events were recognised with the Kararan System being further subdivided into intrusive and extrusive related sub-systems (Table 1). Clusters of mineral occurrences showed local similarities which allowed grouping into Mineral Fields.

An important factor not considered in most previous studies was the overprinting or incorporation of earlier mineralisation within subsequent mineralising events. Major, crustal-scale, fluid conduits have been reactivated repeatedly during deformation of the area and the passage of multiple mineralising events has the potential to explain much of the conflicting data seen in the Gawler Craton gold mineralisation.

Further work is required to refine the Mineral Field definitions by collecting a more consistent, wider data set. This could easily be done with spectral scanning of a wider selection of deposits to objectively define alteration styles and collection of more consistent geochemistry using pXRF methods. The methodology of this study should be used to define the gold mineral systems across the rest of the State initially, as well as the other economic minerals and commodities.

Table 1 List of Gawler Craton mineral fields

Sleafordian Mineral System

The Sleafordian gold mineral system is defined by the region of preserved Archean to early Paleoproterozoic age lithologies (~2555–2460 Ma, Reid et al. 2014) which wraps around the northern and south-eastern side of the Gawler Craton (Figure 1). Belts of mineralised sediment/greenstone/banded iron formation (BIF) lithologies within this arc can be sub-divided into five mineral fields: Christie, Hilga, Harris Ultramafic, Kyancutta and Hall Bay. These broadly follow stratigraphic belts of mafic/ultramafic volcanics/intrusives and associated BIF/Carbonate/clastic sediments. These belts may have been part of an originally contiguous zone which has been subsequently folded and dissected. Although no solid evidence exists for this connection, as no direct studies have been undertaken, geophysical, geochronological and stratigraphic similarities between the northern and south-eastern of these belts suggests that similar rock packages extend under the Gawler Range Volcanics.

This mineral system is defined as the entire region of relatively shallow Archean rock as all this area has high potential to host Archean orogenic mineralisation. The region underwent deformation during the 2480-2420 Ma Sleafordian Orogeny and the majority of mineralisation is likely to be of this age although the only dated deposit is the Challenger Mine (~2520 Ma, e.g. McFarlane et al. 2007). There is strong potential for new mineral fields to be discovered in the poorly explored region west of the Challenger Mineral Field.

The region around the Middleback Ranges (Figure 1) has been highlighted although no confirmed Archean gold mineralisation has been located here. This region is particularly complex with structural interlayering of Archean (Sleaford Complex) and Palaeoproterozoic (Hutchison Group) lithologies. This region has then been very strongly overprinted by the intrusion of the Mesoproterozoic Hiltaba Suite which has masked the potential presence of Archean orogenic mineralisation.

Kimban Mineral System

The Kimban gold mineral system is mainly defined by the extent of the Palaeoproterozoic basins which developed on the eastern side of the Gawler Craton (Figure 2). These basins consist of quartzite, dolomite, iron formation, schist and amphibolite which was deposited in a passive margin setting. It is unclear whether this was one single basin margin or a series of basins as they have been folded and thrust between inliers of Archean basement, particularly on the Eyre Peninsula. There appears to be potentially three separate periods of deposition, however the similarity of the sequences and the incomplete geochronological dataset makes correlations difficult.

They were subsequently deformed by the Kimban Orogeny (~1730–1690 Ma). Because of the level of deformation, it is often visually and geochemically hard to distinguish the Archean from the Palaeoproterozoic rocks unless geochronology in undertaken. This has also lead to confusion in distinguishing between the different mineral systems in the area.

In addition, the majority of the Paleoproterozoic basins have been over-printed to a greater or lesser extent by the Hiltaba magmatic event. This has made it particularly difficult in places to distinguish between primary sedimentary iron formation and overprinting skarn mineralisation. The Weednanna deposit is a prime example. Currently it is classified in the Kimban mineral system, but evidence suggests it may be Sleaford aged sediments, mineralised in the Kimban and then over printed with the intrusion of Hiltaba granites. This has led to a very complex distribution of ore zones within the deposit.

Although no Au-only mineralisation has been identified in the Middleback Ranges region, there is high potential for BIF hosted occurrences to be present. This region has historically been the focus of FeO exploration and a small amount of IOCG exploration. It has not seen a significant amount of Au exploration.

Kararan Gawler Range Volcanic (GRV) Mineral System

This mineral system is defined by the original extent of the sub-volcanic system of the Gawler Range Volcanic event (Figure 3). Five mineral fields are defined within this system (Tarcoola MF, Labyrinth, MF, Glenloth, Tunkillia MF and Southern GRV MF) and these are based mainly on local criteria affecting the intrusion and deposition of the mineral systems in the area. Other mineral fields are likely to be able to be defined with further work.

The mineral system is postulated to extend across the original extent of the GRV outcrop and associated sub-volcanic hydrothermal systems. While this is hard to define with subsequent erosion, the distribution of GRV felsic dykes gives some indication and has been used to define the extent of the system in this study.

This mineral system is interpreted to be a typical Low sulphidation, Subalkalic epithermal system associated with oxidised intrusions. Research by the PMD CRC (Potma and Bastrakov 2006), has shown that a thick blanket of Gawler Range Volcanics would act as an aquitard, promoting hydrothermal alteration of the underlying lithologies and focussing fluid flow through any fault/fracture zones penetrating the GRV. This is supported in the Tarcoola district with evidence from sulphur isotopes (Hein et al. 1994) showing sulphur and most likely metals being derived from the underlying Tarcoola Formation sediments.

This type of system is likely to be strongly affected by the local lithologies underlying the individual hydrothermal cells. The fluid flow is also likely to migrate along long lived fracture systems which are likely to have been mineralised by previous orogenic events. Thus confusion arises with hybrid systems showing characteristics of multiple styles of mineralisation (Boomerang, Weednanna).

Kararan Hiltaba Mineral System

The Hiltaba Intrusive Suite mineral system is defined as all the mineral systems related to Hiltaba Suite intrusions. The previously defined Olympic MF (Figure 4) has been included although this has been described as a separate mineral system in its own right (Skirrow et al. 2019).

The Western Gawler MF is very broadly defined and covers the known outcrop/subcrop of the Hiltaba magmatic event intrusives to the west of the Olympic MF. Only a handful of occurrences have been attributed to this system and most are very poorly defined. Many of the attributes linking these occurrences to Hiltaba intrusives may be wishful thinking on the part of the over enthusiastic geologist logging the chips on the rig. Much more analysis is required to confirm the extent and nature of this system.

The Wudinna and Tunkillia MFs have tentatively been included as the mineralisation has much more in common with Intrusive-related gold systems than epithermal gold systems, but these deposits may represent a continuum with the epithermal systems.

The St Peter Suite (SPS) Mineral system is the most poorly defined, relying on a single confirmed occurrence to date (Nankivel Intrusive Complex). It has thus been included under the Kararan (Hiltaba) Mineral System. However, because of the potential extent and volume of SPS intrusives in the very poorly explored portions of the western Gawler Craton, this may represent a significant exploration target.

References

Hein KAA, Both RA and Bone Y 1994. The geology and genesis of the Tarcoola gold deposits, South Australia. Mineralium Deposita 29:224–236.

McFarlane CRM, Mavrogenes JA and Tomkins AG 2007. Recognizing hydrothermal alteration through a granulite facies metamorphic overprint at the Challenger Au deposit, South Australia. Chemical Geology 243:64–89.

Morrissey L, LaFlamme C, Payne J and Raimondo T 2018. Archean sulphur identified in Mesoproterozoic skarn deposits associated with the Olympic Dam iron oxide copper gold (IOGC) event. Australian Geoscience Council Convention. Big Issues and Ideas in Geoscience. Adelaide, October 2018.

Potma W and Bastrakov E 2006. Tarcoola Modelling: Predictive Targeting Outcomes. Central Gawler Gold Province Modelling Project Update. Predictive Mineral Discovery CRC.

Reid AJ, Jagodzinski EA, Fraser GL and Pawley MJ 2014. SHRIMP U–Pb zircon age constraints on the tectonics of the Neoarchean to early Paleoproterozoic transition within the Mulgathing Complex, Gawler Craton, South Australia. Precambrian Research 250:27–49.

Skirrow RG, van der Wielen SE, Champion DC, Czarnota K and Thiel S 2018. Lithospheric Architecture and Mantle Metasomatism Linked to Iron Oxide Cu-Au Ore Formation: Multidisciplinary Evidence from the Olympic Dam Region, South Australia. Geochemistry, Geophysics, Geosystems 19:1–33.

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