Hiltaba Suite Research Articles

Regional Crustal Setting of Iron Oxide Cu-Au Mineral Systems of the Olympic Dam Region, South Australia: Insights from Potential-Field Modeling

Detailed analysis and modeling of regional gravity and magnetic data from the Stuart Shelf around the Olympic Dam iron oxide copper-gold (IOCG) deposit, constrained by geologic observations obtained from deep drill cores, show that the eastern Gawler craton basement at 1.61 to 1.59 Ga consisted of an Archean core with two sequences of successively younger supracrustal rocks stepping out eastward from it. These were overprinted by the tectonothermal Hiltaba event. Forward models of potential-field data show no convincing evidence for the presence of widespread mafic rocks or extensional basin systems developed immediately prior to, or during, IOCG mineralization. The high intrusive level and sill-like geometry of Hiltaba Suite plutons emplaced immediately prior to IOCG mineralization is difficult to explain in terms of a genetic association with a dominantly extensional tectonic setting. Instead, the tectonic setting inferred from the basement architecture during mineralization is one of low-strain shortening, consistent with regional field observations. A case can be made for intrusion of the Hiltaba Suite in localized accommodation zones in an overall orogenic setting. This style of low-strain shortening (<10%) is consistent with maximum rates and volumes of fluid flow in the upper crust, and access of these fluids to a variety of basement rock types to buffer metal-scavenging fluids. This fluid-driving mechanism does not necessitate a role for regional mafic magmatism and volcanism in the formation of IOCG deposits. Modeling of regional gravity and magnetic data is generally applicable to exploration for IOCG systems, and other mineral systems, in covered terranes that have distinctive geophysical signatures resulting from fluid–wall-rock redox geochemical reactions.

Constraining models of the tectonic setting of the giant Olympic Dam iron oxide–copper–gold deposit, South Australia, using deep seismic reflection data

In the Gawler Craton, the completeness of cover concealing the crystalline basement in the region of the giant Olympic Dam Cu–Au deposit has impeded any sufficient understanding of the crustal architecture and tectonic setting of its IOCG mineral-system. To circumvent this problem, deep seismic reflection data were recently acquired from ∼ 250 line-km of two intersecting traverses, centered on the Olympic Dam deposit. The data were recorded to 18 s TWT (∼ 55 km). The crust consists of Neoproterozoic cover, in places more than 5 km thick, over crystalline basement with the Moho at depths of 13–14 s TWT (∼ 40–42 km). The Olympic Dam deposit lies on the boundary between two distinct pieces of crust, one interpreted as the Archean–Paleoproterozoic core to the craton, the other as a Meso–Neoproterozoic mobile belt. The host to the deposit, a member of the ∼ 1590 Ma Hiltaba Suite of granites, is situated above a zone of reduced impedance contrast in the lower crust, which we interpret to be source-region for its ∼ 1000 °C magma. The crystalline basement is dominated by thrusts. This contrasts with widely held models for the tectonic setting of Olympic Dam, which predict extension associated with heat from the mantle producing the high temperatures required to generate the Hiltaba Suite granites implicated in mineralization. We use the seismic data to test four hypotheses for this heat-source: mantle underplating, a mantle-plume, lithospheric extension, and radioactive heating in the lower crust. We reject the first three hypotheses. The data cannot be used to reject or confirm the fourth hypothesis.

Age constraints on terrane-scale shear zones in the Gawler Craton, southern Australia

Electron Probe Micro Analysis (EPMA) chemical dating of monazite from crustal scale shear zones in the late Archaean to Mesoproterozoic Gawler Craton, southern Australia, highlights a long history of shear zone development associated with assembly and reworking of the craton. In the southeast Gawler Craton, dextral transpression was focussed into the >250 km long Kalinjala Shear Zone, which dominates the structural character of that part of the craton. Monazite in upper amphibolite facies shear fabrics within the Kalinjala Shear Zone give a mean age of 1682 ± 10 Ma. This is indistinguishable from an age of 1693 ± 16 Ma obtained from monazite inclusions within garnet porphyroclasts within the shear fabric. In the western Gawler Craton, monazite populations from two separate locations along the >400 km long Tallacootra Shear Zone give ages of 1680 ± 37 and 1679 ± 39 Ma, respectively, suggesting that both the Kalinjala and Tallacootra Shear Zone systems record regional-scale strain partitioning toward the end of the ca. 1730–1700 Ma Kimban Orogeny. In the northwest Gawler Craton, the Karari Fault Zone can be traced for >700 km, and separates late Archaean rocks from an assembly of tectonic terranes that comprise the northern and northwest Gawler Craton. Monazites from amphibolite-grade mylonites associated with the Karari Fault Zone have a mean age of 1631 ± 12 Ma. This age is identical to the timing of regional arc-like magmatism represented by the St. Peter Suite in the central Gawler Craton, and suggests that exhumation of the ca. 1650–1640 Ma ultra high-grade granulites of the Nawa Domain in the northwest Gawler Craton was linked to the development of the St. Peter Suite magmatic margin. In the central Gawler Craton, monazite in the >200 km long Yerda Shear Zone, which sheared granites belonging to the ca. 1580 Ma Hiltaba Suite, have a mean age of 1503 ± 16 Ma. Similar ages of 1516 ± 18 and 1508 ± 14 Ma are obtained from monazite inclusions in garnet porphyroclasts within mylonites along the >300 km long Coorabie Fault Zone in the western Gawler Craton. Younger ages of 1468 ± 12 and 1457 ± 22 Ma from the amphibolite-grade mylonitic fabrics in the Coorabie Fault Zone are similar to 40Ar– 39Ar ages from the western Gawler Craton, and suggest that reactivation of the Coorabie Shear Zone in part was associated with regional cooling and stabilisation of the Gawler Craton at ca. 1450 Ma. The history of deformation on the terrane-scale shear zones in the Gawler Craton therefore does not reflect a single period of terrane assembly or reworking, rather it points to a complex pattern of strain partitioning during tectonic events in the late Palaeoproterozoic to early Mesoproterozoic.

Granite gneiss basement on Flinders Island, South Australia

A U–Pb zircon age of 1762 ± 11 Ma is reported for granite gneiss located on Flinders Island, South Australia. This age is identical, within analytical uncertainty, to a previously reported age for schists of the Price Metasediments located 100 km to the southeast on the southwestern coast of the Eyre Peninsula. The outcrop represents the only known country rock to the Early Mesoproterozoic Calca Granite (Hiltaba Suite) of Flinders Island, the largest island of the Investigator Group of islands, in the southwestern Gawler Craton. The stratigraphic name Investigator Granite Gneiss is proposed for this rock unit. The discovery of the Investigator Granite Gneiss now considerably increases the extent of known Late Palaeoproterozoic rocks on the eastern side of the peninsula. The outcrop was previously included with the considerably younger St Peter Suite granite‐monzogranite, and grouped together with other islands in the Investigator Group. This new dating suggests that the geology on the other islands may require revision. For the first time, detailed major and trace‐element geochemistry is supplied for the granite gneiss on Flinders Island.

Geophysical characteristics of shear zone-hosted Proterozoic gold, Nuckulla Hill, South Australia

Gold mineralisation was discovered at the Sheoak and Myall prospects in the Nuckulla Hill region of the central Gawler Craton in December 1995 and at the Bimba Prospect in early 1997, following regional calcrete sampling and subsequent RAB/aircore drilling. The region was selected by Equinox Resources NL for copper-gold exploration in 1993 based on structural interpretation of South Australian Exploration Initiative aeromagnetic data. That interpretation identified the Yarlbrinda shear zone as a major craton-scale shear zone active at the time of emplacement of Hiltaba Suite granites ca. 1595-1585 Ma and hence potentially mineralised. Subsequent detailed aeromagnetic and ground magnetic surveys have accurately defined the nature and location of the Yarlbrinda shear zone. Calcrete gold anomalism in the Sheoak-Myall area is coincident with an intense demagnetised zone and there is a subtle ground magnetic anomaly associated with bedrock gold mineralisation at the Sheoak prospect. Gradient-array IP data reflect the depth of weathering and nature of the ~40 m thick regolith developed over the shear zone.

Geophysical constraints of shear zones and geometry of the Hiltaba Suite granites in the western Gawler Craton, Australia

The emplacement of the ca 1590–1575 Ma Hiltaba Suite granites records a large magmatic event throughout the Gawler Craton, South Australia. The Hiltaba Suite granites intrude the highly deformed Archaean‐Palaeoproterozoic rocks throughout the craton nuclei. Geophysical interpretation of the poorly exposed central western Gawler Craton suggests that the region can be divided into several distinct domains that are bounded by major shear zones, exhibiting a sequence of overprinting relationships. The north‐trending Yarlbrinda Shear Zone merges into the east‐trending Yerda Shear Zone that, in turn, merges into the northeast‐trending Coorabie Shear Zone. Several poorly exposed Hiltaba Suite granite plutons occur within a wide zone of crustal shearing that is bounded to the north by the Yerda Shear Zone and to the south by the Oolabinnia Shear Zone. This wide zone of crustal shearing is interpreted as a major zone of synmagmatic dextral strike‐slip movement that facilitated the ascent of Hiltaba Suite granite intrusions to the upper crust. The aeromagnetic and gravity data reveal that the intrusions are ∼15–25 km in diameter. Forward modelling of the geophysical data shows that the intrusions have a tabular geometry and are less than 6 km deep.

A fluid inclusion and stable isotope study of the Moonta copper–gold deposits, South Australia: evidence for fluid immiscibility in a magmatic hydrothermal system

The copper–gold ores of the Moonta mineral field, in the eastern part of the Archaean–Mesoproterozoic Gawler Craton, South Australia, are vein deposits occupying fractures and shear zones within the Palaeoproterozoic Moonta Porphyry. The veins show a spatial relationship with Mesoproterozoic granites, equivalents of the Hiltaba Suite granites that are widespread in the Gawler Craton. The ores were deposited in four hydrothermal stages separated by episodes of fracturing; the first two stages were dominated by Fe oxides, the third by Fe sulfides and the final stage by a Cu–Fe–Co–Au–Zn–Pb–S assemblage. Three types of fluid inclusions are present in quartz associated with the oxide and sulfide minerals. Type 1 fluid inclusions are two-phase vapour-rich, with homogenization temperatures up to 473 °C and low to moderate salinity (2 to 22 wt.% NaCl equivalent). Type 2 are two-phase liquid-rich with lower homogenization temperatures (up to 269 °C) and low to moderate salinities (0.7 to 22 wt.% NaCl equivalent). Type 3 are liquid-rich, solid(s)-bearing with homogenization temperatures up to 467 °C and high salinities (28 to 55 wt.% NaCl equivalent). The coexistence of types 1 and 3 suggests that these fluid inclusions resulted either from trapping of boiling fluids or represent two immiscible fluids, most likely derived from a magma. The fluid history, reflected in fluid inclusion characteristics, was complex, involving variable amounts of boiling, cooling and mixing in the vein system. Salinity–homogenization temperature relationships indicate mixing of the hypersaline fluid and the vapour-rich fluid with a surface-derived fluid of low temperature and low to moderate salinity. δD H 2O values (−22‰ to −31‰) and δ 18O H 2O values (2.9‰ to 5.1‰) calculated from analyses of chlorite and δ 18O H 2O values calculated from analyses of quartz (0.6‰ to 8.8‰) on the basis of mean to maximum fluid inclusion homogenization temperatures are intermediate between magmatic water and surface-derived fluids (i.e. seawater or meteoric water). The narrow range of δ 18O values of quartz (10.8‰ to 12.7‰) may be explained by mixing of a magmatic fluid and either seawater or a low latitude, moderately evolved meteoric water, with a coupled decrease in fluid temperature and 18O content. Similar ranges for δ 34S values of ore sulfides (−2.3‰ to 6.4‰) and disseminated sulfides in the Moonta Porphyry and interdigitated Doora Schist (−1.5‰ to 4.6‰) suggest assimilation of crustal sulfur by the granitic magma or ore fluids. The combined fluid inclusion and stable isotope data support previous proposals, based on field and mineralogical evidence, for a genetic relationship between the Moonta ores and the Mesoproterozoic granitoids.

Partially melted lithic megablocks in the Yardea Dacite, Gawler Range Volcanics, Australia: implications for eruption and emplacement mechanisms

Lithic megablocks ranging from 200 m) felsic volcanic unit in the Mesoproterozoic Gawler Range Volcanic Province (GRV) of South Australia. Throughout its vast extent, the Yardea Dacite shows typical lava-like features, in that it is massive, columnar jointed and evenly porphyritic with 30-40% crystals in a spherulitic and granophyric groundmass. In addition, flow banding is present at many locations. The megablocks are abundant at two sites 50 km apart, but isolated megablocks and smaller (<6 cm) lithic clasts are also scattered throughout the unit. At both sites the megablocks are matrix supported, non-graded, randomly oriented and show no evidence of being confined to a particular stratigraphic level in the dacite. The most abundant and largest megablocks are granitoids derived from older basement and from early-crystallised plutons of the Hiltaba Suite, which is broadly coeval and comagmatic with the GRV. The granitoid megablocks have been partially melted, most likely prior to eruption when resident in the thermal aureole of the Yardea Dacite magma chamber. The lithic megablock occurrences are unlike coarse pyroclastic breccias but are similar in distribution and abundance to xenoliths in lavas, consistent with the lava-like character of the host dacite. Using reasonable estimates of megablock density, magma density and magma viscosity, we show that the rise rate of the dacitic magma exceeded the settling velocity of the megablocks, implying that they could have been entrained and erupted effusively. All but the largest and least-melted megablocks would have remained suspended or else settled very slowly in the dacitic lava during outflow. The rapid rate of magma withdrawal required to produce such an extensive felsic sheet could have also triggered disintegration of the thermally stressed wallrock surrounding the magma chamber, dislodging megablocks that were later entrained and effusively erupted.

Neodymium isotopic constraints for the origin of Mesoproterozoic felsic magmatism, Gawler Craton, South Australia

Mesoproterozoic felsic magmatism of the Gawler Range Volcanics and Hiltaba Suite granites occurred at 1585–1595 Ma across much of the Gawler Craton, South Australia. Nd isotopic analysis of this felsic magmatism, combined with petrological and geochemical arguments, suggest derivation by partial melting of both Paleoproterozoic and Archean crust. The majority of samples analyzed have Nd isotopic and geochemical characteristics compatible with the involvement of Paleoproterozoic crust stabilized during the 1.85–1.71 Ga Kimban orogeny as sources for the Mesoproterozoic magmatism; others require derivation from sources dominated by Archean rocks. This cycle of Paleoproterozoic crustal stabilization followed by involvement of this crust Mesoproterozoic felsic magmatism is one previously documented from many parts of Mesoproterozoic Laurentia. On the basis of models proposing East Australia–Antarctica to be the conjugate landmass at the rifted margin of western North America, it appears that the voluminous magmatism of South Australia is another example of a typically Mesoproterozoic style of magmatism linked to Laurentia. This Mesoproterozoic magmatism appears temporally linked to regional high-temperature, low-pressure metamorphism of the region, and together with the presence of mantle-derived magmas, implicates the operation of large-scale tectono-thermal processes in the origin of felsic magmatism at 1590 Ma.

The geology and genesis of the Tarcoola gold deposits, South Australia

The Tarcoola goldfield is located in the Gawler Craton in northwestern Eyre Peninsula, South Australia. The gold deposits are hosted in the Middle Proterozoic Tarcoola Formation, comprised of the fluviatile Peela Coglomerate Member, the shallow marine Fabian Quartzite Member, and the marine Sullivan Shale Member. Mineralization in the goldfield consists of north-northeast to north-northwest trending gold-bearing quartz veins with associated hematite, pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, electrum and gold. Adamellite in contact with the Tarcoola Formation has previously been included in the Middle Proterozoic Hiltaba Suite granitoids, on the basis of an apparent intrusive relationship with the Tarcoola Formation, and the gold-quartz veins were interpreted as being genetically related to the cooling pluton. However, detailed field and petrographic studies have demonstrated that the contact between the Tarcoola Formation and adamellite is a nonconformity. Hence, there is no genetic relationship between the mineralization and the adamellite. Oxygen isotope data indicate that an oreforming fluid, derived from convective circulation of meteoric or seawater, or from formation water, underwent isotope exchange with sediments of the Tarcoola Formation. A magmatic heat source for the hydrothermal system is suggested by the presence of intrusive igneous rocks, including dykes of aplite, quartz monzonite and microdiorite. Sulphur isotope characteristics of the mineralization can be explained by reduction of seawater sulphate or dissolution of disseminated sulphides in the sedimentary sequence. Metals were probably derived from rocks of the Tarcoola Formation. A complex paragenetic sequence involved deposition of minerals in several stages separated by episodes of fracturing. Fluid inclusions in quartz and fluorite show that deposition took place over a temperature range of about 340° to 110°C from a low salinity fluid. Analyses of chlorite coexisting with sulphide minerals suggest deposition of sulphides from about 300° to 170°C. Gold was transported as Au(HS) 2 − and deposition appears to have coincided with a sudden decrease in fO2 at around 260° to 250 °C.