The geology and geochemistry of granitoids in the Childara region, West Gawler craton, South Australia : implications for the Proterozoic tectonic history of the Western Gawler craton, and development of lode-style gold mineralisation at Tunkilla

The western Gawler Craton comprises poorly outcropping basement units which has hampered the tectonic understanding of the area. Recent high-resolution aeromagnetic data has provided an insight into the basement composition and allowed preliminary tectonic interpretation. The Gawler Craton is here divided into tectonic subdomains based on structural/metamorphic and aeromagnetic signature, and records a dominantly Proterozoic structural history. Regional mapping on CHILDARA 1:250 000 and reconnaissance sampling on FOWLER, STREAKY BAY, and NUYTS, was designed to determine the geochemistry of the basement lithologies, and attempt to construct a geological history of the western Gawler Craton, building on recent geochronology and structural/metamorphic work within the Fowler Subdomain to the west. This understanding is further used to constrain the age and controls on gold and base-metal mineralising events. Previous solid geology interpretations of the western Gawler Craton showed an Archaean basement to much of the area, however, this study has shown no Archaean exists within the Nuyts Subdomain. Four main periods of Proterozoic granitoid plutonism over a 130 My period (-1690 ‚ÄövÑvÆ 1560 Ma) are recognised on CHILDARA. Recent geochronology has shown that major structures post-date the Kimban Orogeny (1845 ‚ÄövÑvÆ 1695 Ma), hence deformation and associated magmatism on the western and northern Gawler Craton are now attributed to the Kararan Orogeny (1690 ‚ÄövÑvÆ 1540 Ma). The 1690-1670 Ma Tunkillia Suite is restricted to the eastern Nuyts Subdomain and scattered plutons within the Christie, Fowler and Cleve Subdomains. The Tunkillia Suite is felsic (67 ‚ÄövÑvÆ 76.5 wt% Si02 values) with Na/K <1, has high Rb/Sr, variable HREE enrichment and moderate to strong negative Eu anomalies. On CHILDARA, the Tunkillia Suite crops out within the Yarlbrinda Shear Zone, and hosts the Tunkillia and Nuckulla Hill gold prospects. The 1630-1608 Ma St Peter Suite is restricted to the Nuyts Subdomain. Within the Kalanbi and Rocky Point-Cape Beaufort areas, these rocks generally resemble magmatic arc-related batholiths. The St Peter Suite is characterised by an expanded SiO2 range (47-76 wt%), is sodic (Na/K >1), shows high Sr, K/Rb and Sr/Y and relatively low K20, Rb, Rb/Sr, Th, U (ie: low heat producing granites), REE, and Nb. The St Peter Suite is LREE enriched, HREE and Y-depleted, with little or no Eu anomaly. These geochemical characteristics are similar to modern magmatic arc rocks and are consistent with formation within a subduction related environment, with magmas produced from partial melting of hydrous basalt at depths within the garnet stability field. The St Peter Suite is considered to have formed by fractionation of arc-type magmas and/or partial melting of arc-related intrusions, or mafic under-plating within a subduction related continental margin setting. The Hiltaba Suite (1595-1575 Ma) is an extensive suite of felsic granites (>70 wt % Si02) characterised by low Na/K (<1), high Rb/Sr and variable FREE enrichment, with moderate to strong negative Eu anomalies. The comagmatic Glyde Hill Volcanics correlate with the Gawler Range Volcanics, and comprise a series of dominantly felsic lavas and ignimbrites. The Hiltaba Suite/Gawler Range Volcanics generally do not show an arc affinity. They record a major shift in tectonic environment from a dominantly compressional environment during the St Peter Suite magmatic event, to an intracontinental extensional environment, but the initial source of heat may be related to earlier subduction (ie: back-arc extension origin). The 1560 Ma S-type Munjeela Granite represents a late stage melt and is related to a period of extension and basin formation to the west of the Koonibba Fault Zone. The most likely tectonic model that accounts for these geochemical features is as follows. Continental collision occurred between the Fowler Subdomain and the proto-Yilgarn Craton to the west, initiating southward dipping subduction to produce the arc related St Peter Suite magmatism. However, this change appears to have been unsustainable and subsequent intraplate extension, possibly together with basaltic underplating of the continental crust, produced the voluminous Hiltaba Suite/Gawler Ranges Volcanics. These geological events produced lode gold-style mineralisation within the Yarlbrinda Shear Zone. The Yarlbrinda Shear Zone is a major north-south trending shear zone located on eastern CHILDARA, which hosts the Tunkillia and Nuckulla Hill gold prospects. The host granites are extensively altered, with mineralisation associated with zones of intense sericite alteration. K-Ar dating of sericite from Tunkillia recorded a cooling age of ‚ÄövÑvÆ1600 Ma, suggesting a relationship to the Hiltaba Suite/Gawler Range Volcanic magmatic event. Two main vein generations are defined at Tunkillia, with mineralisation located within narrow quartz veins (V1). V1 veins comprise quartz and sulphides and combine to form steeply dipping lenses ranging in thickness from cm-scale up to 1 m, which are sub-continuous along strike. Pyrite is the main sulphide with accessory galena and chalcopyrite. V2 veins comprise late, crosscutting calcite veins, which are barren. Primary fluid inclusions hosted by V1 veins are typically two-phase at room temperature containing an aqueous phase and vapour. Preliminary fluid inclusion data suggests that gold deposition took place over a temperature range of 200 0 - 300¬¨‚àû C from a low salinity fluid (‚Äöv¢¬ß4 wt% NaCl equivalent). Sulphur isotope analysis shows the source of sulphur is magmatic (-2.23 to 3.19), and lead isotope data indicate that mineralisation is consistent with an origin by fluid release from Hiltaba Suite granites, however, the Pb-isotopic composition of other granitoid suites has not been characterised. The Yarlbrinda Shear Zone shows evidence for an early phase of dextral strike-slip deformation, which is expressed by a regional foliation and shallow plunging stretching lineations. Kinematic indicators including S-C fabrics and rotated porphyroclasts show a dextral sense of shear. Later deformation comprised dip-slip movement, which produced steeply plunging stretching lineations and rotated porphyroclasts, showing a west-side-up sense of movement. Mineralisation at Tunkillia appears to be localised at the junction of the regional foliation and steeply plunging lineations, and is interpreted as late in the structural history. Late stage brittle structures related to the locking up of the Yarlbrinda and Yerda Shear Zones crosscut the shear zone and are unrelated to mineralisation. The Yarlbrinda Shear Zone is part of a large-scale west-directed transpressional duplex that developed on a regional restraining bend of the shear system. U-Pb zircon geochronology shows that deformation within the Yerda Shear Zone was taking place at -1592 Ma, but deformation within the Yarlbrinda Shear Zone must have ceased by -1580 Ma when the Kondoolka Batholith was intruded. The extent of sericite alteration requires a voluminous influx of low pH fluids, with the Hiltaba Suite the most likely source. This fluid mixed with low salinity metamorphic fluids which leached Pb and S from the granites at depth.