February 2009 LIP of the Month

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A Mesoproterozoic silicic LIP in South Australia: the Gawler Range Volcanics and Hiltaba Suite

S. Allen, J. McPhie, C. Simpson, V. Kamenetsky, I. Chambefort, A. Agangi, A. Bath, A. Garner, N. Morrow

School of Earth Sciences and CODES, University of Tasmania, Private Bag79, Hobart,
Tasmania 7001, Australia
E-mail address: Sharon.Allen@utas.edu.au


The Gawler Range Volcanics (GRV) and Hiltaba Suite (HS) granites of South Australia are a record of an intracontinental, Mesoproterozoic, dominantly silicic volcanic province that was generated during the Laurentian supercontinent assembly (Blissett et al., 1993; Creaser, 1995; Allen and McPhie, 2002). Relatively few examples of silicic volcanic LIPs (SLIPs) have been documented worldwide. Of those, three are Proterozoic and were formed during supercontinent assembly (ie. ~2100 Ma rhyolites and dacites of the Rooiberg Felsite, South Africa, Twist and French, 1983; Skilling and Chalot-Prat, 2007; 1592 Ma Gawler Range Volcanics, South Australia, Creaser, 1995; Allen and McPhie, 2002; and the 1100 Ma rhyolites in the North Shore Volcanic Group, Minnesota, Green and Fitz, 1993).

The GRV have been the subject of a series of volcanological and petrological research projects undertaken by staff and students at the School of Earth Sciences and CODES (ARC Centre of Excellence in Ore Deposits) at the University of Tasmania over the past 15 years. Our research has focussed on:

  • Stratigraphy and structure
  • Evolution and architecture of the province
  • The origin of voluminous (>500 km3), widespread felsic volcanic units
  • The location of eruptive centres
  • Magma compositions, sources and volatile budgets

Most of the results relating to these topics have been published. Current projects include a detailed volcanological and petrological study of the Glyde Hill and Chitanilga volcanic complexes (Andrea Agangi, Jocelyn McPhie, Sharon Allen, Vadim Kamenetsky), investigation of the abundance and significance of fluorine in GRV-HS magmas (Vadim Kamenetsky, Isabelle Chambefort, Adam Bath, Sharon Allen) and a collaborative project with BHP-Billiton on the host succession to the giant Olympic Dam Cu-U-Au deposit (Isabelle Chambefort, Vadim Kamenetsky, Jocelyn McPhie).

Here we present (1) a summary of the character and evolution of the GRV, focussing on the topics given above, (2) an outline of new research on fluorine in the GRV and HS, and (3) a brief review of the setting and character of the Olympic Dam Cu-U-Au deposit.


Regional geological setting

The GRV and co-magmatic HS granites cover more than 25000 km2 of the central part of the Gawler Craton (Fig. 1), and extend beneath younger Proterozoic and Phanerozoic sedimentary formations of the Stuart Shelf in South Australia. The total magma volume represented by the province is in the order of ~100,000 km3. U-Pb zircon isotopic dating of the GRV has yielded indistinguishable ages of 1591±3 and 1592±3 Ma (Fanning et al., 1988; Creaser, 1995). However, the magmatism relating to the HS granites spans a much longer time interval, 1583±7 to 1598±2 Ma (U-Pb in zircon isotopic dates; Flint, 1993).

Figure 1: Regional geological map. Interpreted sub­surface geology of the Gawler Craton, after Daly et al., (1998) and Reynolds (2001).

The GRV overlie deformed Archaean and Palaeoproterozoic granitoid suites, metamorphic complexes and metasedimentary formations. The volcanic units were emplaced in a subaerial, intracontinental setting on a landmass that included the Mawson Continent (Gawler Craton and East Antarctic Shield, Fanning et al., 1996) and the proto-Yilgarn Craton. Continental reconstructions (e.g., Dalziel, 1991; Borg and DePaolo, 1994; Burrett and Berry, 2000) propose that this landmass was part of western Laurentia.

The GRV comprise a lower sequence of texturally and compositionally varied volcanic units that amount to 0.5-3 km in thickness, and are gently to moderately dipping (10o-30o). These units are best exposed on the western and southern margin of the GRV (Fig. 2). These lower GRV units range in composition from basalt to rhyolite and comprise lavas, ignimbrites and other minor volcaniclastic facies. Tholeiitic basalt and andesite are subordinate to dacite and rhyolite (~90 vol%). In comparison, the upper GRV consist of very gently dipping (<~5o), widespread, thick (250-300 m), crystal-rich (15-40%), rhyolitic and dacitic units considered to be lavas (Garner and McPhie, 1999; Morrow and McPhie, 2000; Allen and McPhie, 2002; Allen et al., 2003). The GRV have been intruded by porphyritic rhyolitic dykes and HS granites. Juxtaposition of granites with the GRV suggests that at least 1 km of the overlying rocks (GRV and/or sedimentary formations) have been eroded.

The GRV are essentially undeformed and unmetamorphosed. There is also no indication of deep burial and the province has formed a long-lived region of positive relief. The GRV have well preserved primary textures and depositional structures (e.g., spherulites, amygdales, compacted shards, columnar joints, flow bands, perlite, bedding and other depositional structures). Some units retain near-glassy (cryptocrystalline) black domains and pristine glassy melt inclusions are present in quartz phenocrysts throughout (Kamenetsky et al., 2000). Phenocrysts other than quartz are variably altered to chlorite (ferromagnesian phases) and sericite (feldspar), but retain their original crystal shapes and twinning.

Creaser and White (1991) and Stewart (1994) established that the magmas that fed the widespread felsic lavas had a high temperature (900-1100°C) and were relatively dry (<2 wt % H2O). Estimates of viscosity based on whole-rock composition and crystal contents using the methods of Shaw (1972) and Pinkerton and Stevenson (1992) are in the range of 2.7-3.5 x 106 Pa s (Stewart, 1994). In addition, Stewart (1994) recognised that the dissolved volatiles in these magmas included significant halogens, particularly F, which would further reduce the viscosity by depolymerising the melt (Manning and Pichavant, 1983).

The lower GRV

The lower GRV was the first expression of this intracontinental LIP. This early volcanism exhibited varied compositions and lithofacies and was erupted from numerous separate volcanic centres (Blissett et al., 1993). The Chitanilga Volcanic Complex at Kokatha and Glyde Hill Volcanic Complex at Lake Everard are the two best exposed volcanic centres and occur on the western margin of the GRV. In addition, the Menninnie Dam volcanic centre at Nonning in the southern GRV was intersected in drill core, and a volcanic succession at Myall Creek on the eastern margin was also intersected during drilling. Products of the lower GRV volcanic centres are also exposed at Tarcoola (northwest), and within inliers beneath the upper GRV at Lake Acraman.

Chitanilga Volcanic Complex

Figure 2: Simplified geological map of the Gawler Range Volcanics in South Australia based on published regional maps (Blissett, 1977; Blissett et al., 1988; Allen et al., 2003). Extensive dacitic and rhyolitic ‘lava-like’ units dominate the upper GRV in the southern and central map area. Inset: the location and distribution of the GRV in the Gawler Craton, South Australia.

The Chitanilga Volcanic Complex is exposed in a ~ 3 km-thick section. The large number of mappable stratigraphic units present range from basalt to rhyolite (Blissett, 1975, 1977a; Branch, 1978; Giles,1980; Robertson, 1989; Stewart, 1994). Felsic units are dominant, comprising 70% of the volcanic pile (Fig. 3). The first (~1.5 km-thick) part of the sequence has the steepest dips at 10-30o  representing primary depositional dips of ~5-20o. The second part is 800 m thick and faulted, and the third part is more gently dipping, comprises only two units and has a disconformable lower contact. This complex shows highly variable compositions. With increasing SiO2, major and trace elements show a somewhat scattered distribution and offsets in the fractionation trend coincide with the pyroclastic units and felsic lavas in the second part. Such variations could be in part due to crystal enrichment processes and incorporation of country-rock or basement clasts, but may also indicate the involvement of separate magmas.

Figure 3: Stratigraphic logs of the GRV in the Chitanilga Volcanic Complex, Glyde Hill Volcanic Complex, Menninnie Dam volcanic centre and in drill core at Myall Creek.

Glyde Hill Volcanic Complex

The Glyde Hill Volcanic Complex is a 1 km-thick sequence on the western margin of the GRV and is the subject of a detailed volcanological and petrological study by PhD student, Andrea Agangi (UTas). Felsic units dominate (90%) and only very minor andesite and basalt are present (Blissett, 1975, 1977a, 1977b; Giles, 1980, 1988; Ferris, 2003). Evenly porphyritic felsic lavas are interbedded with polymictic breccia and pumiceous and crystal-rich volcaniclastic facies (Fig. 3). The lowest unit (Childera Dacite) has been intruded by dykes of porphyritic andesite and dacite. Reverse faults are inferred from stratigraphic repetition. The compositions of units in the Glyde Hill Volcanic Complex define a broad fractionation trend although small deviations coincide with the Childera and Bunburn Dacites.

Southern lower GRV

In the south, the lower GRV are sporadically exposed over an east-west distance of 200 km. They are dominated by two moderately widespread felsic units (Waganny Dacite and Bitalli Rhyolite) separated by ~2m-thick interval of mudstone. The Waganny Dacite is the lowermost unit and extends for >50 km. It is several hundred metres thick and composite, consisting of single lavas separated by monomictic breccia (autobreccia) and crystal-rich, volcaniclastic facies that are cross cut by vertical to subvertical polymictic breccia dykes.

The Bitalli Rhyolite is ~200 m thick and extends over 150 km east-west. It is a composite of several different rhyolitic units that are laterally and vertically juxtaposed. The distinguishing feature of the Bitalli Rhyolite is the high abundance of 2 mm, round quartz crystals. At Hiltaba, the lava is compositionally heterogeneous and composed of mingled feldspar-phyric and quartz-phyric domains that include partially melted, co-magmatic granitoid clasts, similar to those in the felsic lava in the Chitanilga Volcanic Complex.

Menninnie Dam volcanic centre

The unit mapped as the Bitalli Rhyolite around Menninnie Dam is <200 m thick and directly overlies basement. It has been comprehensively drilled and consists of the products of two rhyolitic eruption episodes: (1) a small (<600 m diameter) rhyolite lava dome and associated pyroclastic deposits, and (2) a lithic-rich ignimbrite (Fig. 3) (Roache, 1996; Roache et al., 2001). The first eruption episode was initially hydromagmatic and explosive, producing lithic-rich welded fallout and later effusive, producing a rhyolitic lava dome. The lithic-rich ignimbrite is massive, strongly welded and at least 265 m thick. Roache (1996) suggested that the lithic-rich nature and substantial thickness of this ignimbrite were consistent with it being an intra-caldera facies.

Myall Creek

Drill core from the eastern extremity of the main mass of the GRV at Myall Creek revealed a >500-m-thick sequence dominated by felsic lava and ignimbrite (90%); minor basaltic lava (8%) and fine-grained volcaniclastic facies are also present (Blissett, 1981).

The upper GRV

The upper GRV are exposed in the southern and central parts of the province and are dominated by thick (250-300 m), widespread (>160-225 km), felsic lavas (Eucarro Rhyolite, Pondanna Dacite, Moonaree Dacite) that combined represent at least 4000 km3 of magma (Creaser and White, 1991; Allen and McPhie, 2002). North-south extents are more limited (5-100 km) due to northerly dips and erosion. The Pondanna and Moonaree Dacites are members of the Yardea Dacite, and were mapped as separate lavas by Allen et al. (2003). The Carnding Rhyodacite (Tarcoola area) and Chandabooka Dacite (Kokatha area) were also recognised by Blissett et al. (1993) to be laterally extensive units of the upper GRV and field investigations suggest that at least the Chandbooka Dacite is a lava. Contacts between the lavas and the presence of near-vertical columnar joints indicate that they are horizontal to gently northward dipping. Slightly steeper northward dips occur at the bases of the units, and adjacent to major faults or to Hiltaba Suite intrusions. Bases of the units are exposed along their southern margins, and the upper or outer parts are exposed along the northern margins.

Eucarro Rhyolite

The Eucarro Rhyolite extends more than 225 km from west to east. The lava is approximately 300 m thick and for the most part, consists of texturally uniform, evenly porphyritic plagioclase-phyric rhyolite, with 15-21% phenocrysts of feldspar, ferromagesian phases and rare quartz (Allen and McPhie, 2002; Allen et al., 2003).  The basal facies is black or brown whereas most of the middle part of the unit is red or pink and columnar jointed. The outer northern margin is mingled with the Paney Rhyolite, a compositionally distinct quartz-phyric flow-banded rhyolite (73.8-75.9 wt.% SiO2; Paney Rhyolite) (Morrow and McPhie, 2000; Kamenetsky et al., 2000; Allen et al., 2003). The Paney Rhyolite is more heterogeneous than the rest of the Eucarro Rhyolite in terms of its phenocryst abundance (11–21%) and texture, as it is compositionally flow banded.

Apart from mingled domains and those with granitoid clasts, the Eucarro Rhyolite is compositionally uniform throughout its lateral extent (70.3-73.7 wt.% SiO2) although the base is slightly more silica-rich. Flow directions inferred from anisotropy of magnetic susceptibility (AMS) and petrofabric analysis indicate a source to the south-southwest of the preserved distribution (McPhie et al., 2008).

Pondanna Dacite

The Pondanna Dacite spans more than 160 km from west to east across the southern Gawler Ranges. Rare occurrences of this lava overlie the Eucarro Rhyolite at Paney indicating that its southern extent was much greater. It is ~250 m thick. Minor basal black or grey facies occurs along the southernmost contacts and amygdaloidal facies occurs close to northern contacts, but the majority of the lava is red. The basal black or grey facies is typically flow banded. and slightly finer and less crystal-rich (20-25%) than the red facies (23-37% phenocrysts), and shows mingled and gradational relationships with the red facies. The red facies is generally massive and columnar jointed, although browner occurrences towards the top of the unit in the westernmost portion are steeply flow banded. The amygdaloidal northern part is mottled green and includes abundant mm-cm vesicles infilled by quartz, calcite or fluorite, similar to the Eucarro Rhyolite. The Pondanna Dacite is compositionally uniform throughout (68.5-70.3 wt.% SiO2) and lacks granitoid clasts. The source of the lava is unknown.

Moonaree Dacite

The Moonaree Dacite is the uppermost preserved unit in the GRV. It covers the central part of the GRV, spanning more than 160 km east-west and 100 km north-south and is ~250 m thick.  Two compositions are dominant: (1) a red facies with a granophyric groundmass, and (2) a brown, more silica-rich, quartz-bearing facies (Allen et al., 2003). The red facies and quartz-bearing facies occupy separate large areas and also occur complexly intermingled. Both are columnar jointed.

The red facies is generally massive, although steeply dipping flow bands are very rarely present towards the basal contact with the black facies around Yardea. It is crystal-rich (up to 40% crystals) with a predominantly spherulitic or granophyric groundmass. Megacrysts of K-feldspar, less common plagioclase and rare quartz are most abundant (up to 5 vol%) and largest (7-25 mm) near the base. Megablocks of partially melted, co-magmatic granitoid clasts and basement lithic clasts (up to 50 m in diameter) are locally abundant at Hiltaba and Yardea (Garner and McPhie, 1999), but are uncommon elsewhere. Fine- to medium-grained, mafic and intermediate igneous clasts are most abundant (up to 5 vol%) and largest (up to 5 cm) at Hiltaba.

The quartz-bearing facies is less crystal-rich (25-32%) than the red facies and also distinguished by the presence of small (0.5-1 mm diameter), round and embayed quartz phenocrysts. The groundmass is also distinctive in comprising a finely microcrystalline quartz-feldspar mosaic that contains abundant, randomly oriented, 50-70 µm, feldspar laths and essentially no granophyre or spherulites. Crude flow bands, where present, are defined by variations in quartz abundance, or in the size of feldspar crystals, or by groundmass textures (microcrystalline versus spherulitic). Feldspar megacrysts (up to 12 mm) and mafic inclusions are sparsely scattered throughout (<2 vol%).

The Moonaree Dacite is composed of two compositions represented by the red facies (66.8-68.3 wt.% SiO2) and quartz-bearing facies (68.3-69.2 wt.% SiO2). The source for the lava is unknown. The localised abundance of megablocks around Hiltaba and Yardea may indicate proximity to source, however, calculations by Garner and McPhie (1999) suggest that the megablocks were near-neutrally buoyant in the molten dacite and could have been rafted well away from the source vent.

Magma sources

The GRV are dominated (~90 vol%) by dacitic to rhyolitic units. Petrogenetic studies of the GRV have suggested that the mafic magmas, though subordinate in volume, were derived from the mantle and that the silicic magmas were generated by fractional crystallization of the mafic parent, and in some cases, melting and assimilation of Archaean or Palaeoproterozoic crust (Giles, 1988; Stewart, 1994; Creaser, 1995). Stewart (1994) showed that the lower GRV units (including the moderately widespread Chandabooka Dacite) have highly variable εNd values (-7.4 to -1.6 Chitanilga; -3.6 to +1.2 Glyde Hill volcanic complexes). These wide ranges in isotopic signatures, together with the great variety in bulk-rock compositions in the lower GRV, suggest multiple sources of magma even within single volcanic centres. Each magma batch involved varying degrees of assimilation of the surrounding crust. Each lower GRV eruption involved <1-~50 km3 of magma, so the rate for small eruptions could have been in the order of one per century.

The upper GRV involved the production of at large-volume, felsic lavas. Three are well preserved and each amounts to 1000-3000 km3 of felsic magma. Only minimal other felsic volcanism (small-volume lavas and ignimbrites) and no mafic volcanism occurred during this stage. The only evidence for mafic magma being involved is the small (<5 cm) mafic igneous clasts that are scattered within the lavas. One particular feature of the widespread felsic lavas is that in each case, the dominant facies has single, narrowly defined composition. For example, the main mass of the Eucarro Rhyolite, the Pondanna Dacite and two mingled facies of the Moonaree Dacite, each plot in well-defined fields on Harker diagrams. The three widespread felsic lavas define a single fractionation trend, and have relatively constant εNd values (εNd values between -3.8 and -4.5; Stewart, 1994).

Two lavas in the upper GRV (Eucarro Rhyolite and Moonaree Dacite) involved mingling between at least two compositionally distinct felsic magmas. For the Eucarro Rhyolite, one magma was overwhelmingly dominant, whereas in the Moonaree Dacite, the two magmas had subequal volumes. Local concentrations of granitoid and other basement lithic clasts in the Eucarro Rhyolite and Moonaree Dacite coincide with the appearance of these compositionally distinct mingled magmas. The partially melted state of some of the granitoid clasts strongly suggests that they formed wall rock around the magmas responsible for the upper GRV. Allen and McPhie (2002) speculated that, in each case, the influx of wall-rock lithic clasts resulted from syn-eruptive rupture of barriers separating closely adjacent, compositionally distinct magmas.

Facies architecture

The lower GRV are dominated by small- and moderate- volume volcanic units (<1-50 km3) that accumulated in nested, and/or overlapping volcanic centres (Fig. 4). The felsic lavas and lava domes are 100-300 m thick and have a basal autobreccia or peperitic contact, overlain by the main coherent part with steeply dipping flow banding, and an upper compositionally mingled or amygdaloidal part. Several texturally similar lavas can be vertically stacked and separated by volcaniclastic facies a few metres thick, producing successions in excess of 300 m thick. The small-volume lavas are inferred to be sitting on, or near to their source vents. Some felsic lavas were more moderate volume, extending several tens of kilometres from source. The sources for the moderately widespread lavas are less well defined, but were presumably located within or close to their present extents. Mafic and intermediate lavas are thinner (1-30 m thick) and amygdaloidal, and vesicle size and abundance increase upwards within single units. Mafic and intermediate lavas, although minor overall, are thickest (up to 500 m thick) and most abundant in the lower GRV in the Chitanilga Volcanic Complex.

Figure 4: Schematic cartoon of the facies architecture of the GRV based on the successions in the Chitanilga Volcanic Complex(CVC), Glyde Hill Volcanic Complex (GHVC) and the Menninnie Dam volcanic centre (MDVC). The lower part of the GRV is dominated by small, felsic lava domes and moderate-volume felsic composite lava flows. A thick succession of basaltic and andesitic lavas representing a volcanic cone occurs at the base of the CVC. Ignimbrites and other volcaniclastic facies are subordinate and scattered throughout the lower GRV. The upper GRV is dominated by three widespread felsic lavas. See Figure 3 for symbols.

Welded ignimbrites with conventional vitroclastic textures, and pumiceous and other volcaniclastic facies occur sporadically throughout the lower GRV. The ignimbrites are up to 260 m thick and can be relatively widespread (Lake Gairdner Rhyolite). Only one ignimbrite (at Menninnie Dam; Roache, 1996) appears to be an intracaldera facies. The presence of numerous pumiceous, crystal-rich or fine-grained volcaniclastic beds (both primary and reworked) in the Chitanilga and Glyde Hill Volcanic Complexes suggests that small-volume pyroclastic eruptions occurred throughout the history of the lower GRV. Reworked volcaniclastic facies and the presence of accretionary lapilli in the Glyde Hill Volcanic Complex, together with possible water-settled fallout (Chitanilga Volcanic Complex), indicate that surface water, perhaps rivers and lakes, was present.

The upper GRV are dominated by voluminous (~1000-3000 km3) lavas, each one 100-300 m thick (Fig. 4). Successive lavas partly overlap and young progressively northwards. Rare small-volume welded ignimbrites and lava domes also occur in the upper GRV. Vent sources for the widespread felsic lavas have yet to be identified. The lavas show no textural or lithofacies variations that might serve as reliable vectors to source vents. However, at least one of them, the Eucarro Rhyolite, retains well defined and consistent AMS characteristics, and the AMS data suggest outflow from sources located to the southwest of the present GRV outcrop area (McPhie et al., 2008).


Fluorine in the Gawler Range Volcanics and Hiltaba Suite granites

Figure 5: Euhedral crystal of fluorite as inclusion in a quartz phenocryst in the Eucarro Rhyolite.

The GRV are a Mesoproterozoic subaerial silicic large igneous province (Allen et al., 2008; Blissett et al., 1993) associated in time with the HS granites. The upper GRV comprises three very widespread and voluminous felsic units (Allen et al., 2008; McPhie et al., 2008). The lowest unit, the Eucarro Rhyolite, mainly comprises plagioclase-phyric rhyolite; quartz-phyric rhyolite occurs locally. It is weakly altered to sericite, chlorite, hematite and carbonate. The groundmass is cryptocrystalline. Accessory phases include apatite, opaques and zircon. Fluorite is a ubiquitous accessory mineral. It is anhedral, ranges from 0.1 to 0.3 mm in size, and is colourless to purple. Fluorite occurs as a late phase in the groundmass, in amygdales associated with other accessory minerals such as zircon and/or and as inclusions in the quartz (Fig. 5).

Figure 6: Melt inclusion hosted in a quartz phenocryst of the Eucarro Rhyolite. Daughter crystals are fluorite and a hydrous silicate phase, sample GC8.

The quartz phenocrysts in the Eucarro Rhyolite host melt inclusions. The inclusions are unaltered and represent the melt trapped during quartz growth. The inclusions can be subdivided into glassy, volatile-rich and composite inclusions types. Glassy inclusions range from 10 to 60 mm in size and are colourless to brown. The shrinkage bubble(s) represent 1 to 6 volume % of the inclusion. Glassy inclusions can contain daughter crystals. These crystal-rich inclusions range in diameter from 10 to 110 mm and have shrinkage bubbles, which represent 1 to 10 volume % of the inclusions. Daughter phases are acicular apatite, some undetermined opaque (magnetite?), potassic chloro-hastingsite and bladed colourless fluorite (Fig.6). Some melt inclusions have a granular texture and some silicate melt inclusions are associated with volatile-rich CO2- and H2O-bearing fluid inclusions.

Melt inclusion compositions

The Upper GRV and the Hiltaba Suite granites span a wide range of silica content (67-80 wt%). Fluorine content of the whole rock has been analyzed by ion specific electrode with a detection limit of 50 ppm. In the Eucarro Rhyolite, F ranges from 1250 to 1700 ppm. Creaser and White (1991) reported values of 2150 to 2640 ppm for the granitic rocks “coeval” with Olympic Dam deposit, and a content of 1800 ppm for the Moonaree Dacite. It seems there is a slight increase in F associated with the increase in silica content.

Melt inclusions in the Eucarro Rhyolite were analyzed for major and trace elements using an electron microprobe and LA-ICPMS. Glassy, glassy with daughter phases and granular melt inclusions have been homogenized at 850°C for 20 and 70 hours. The silica content of melt inclusions varies from 68 to 79 wt% (Fig. 7A). Major elements, such as Al, Ca, Ti, Mg, present a classic fractionation trend with the different whole rocks and the groundmass. The F content in the heated melt inclusions shows a negative correlation with the silica content (Fig. 7A). Glassy inclusions with crystals are more enriched in F than the whole rock; purely glassy inclusions show the same concentration. Microprobe analysis totals on unheated inclusions range from 96 to 100 wt%, implying a very low water content in the melt, in agreement with previous petrology studies on GRV magmas (Stewart, 1994). Chlorine content varies from 0.12 to 0.30 wt%.

Figure 7: Analyses of melt inclusion hosted in quartz phenocrysts of the Eucarro Rhyolite. Daughter crystals are fluorite and a hydrous silicate phase, sample GC8.

Melt inclusions appear to be systematically enriched in trace elements compared to the whole rock and groundmass except in Ba, Eu and Sr (Fig. 7B). Fractionation of apatite at the end of the crystallization may have controlled the REE concentration decrease in the groundmass. However, melt inclusions are more enriched in W, Th, U, Pb, Mo, Sn than groundmass and than the whole rock, and the low content of these elements in the rock cannot be explained by fractionation of these element by a mineral such as apatite.

Significance of the GRV-HS fluorine anomaly

Melt inclusions in the GRV show a significant content of fluorine (up to 1.3wt%), suggesting that fluorine was present in the melt phase prior to and during eruption. Fluorine may occur at noticeable concentrations in some magmas, and can affect melt properties, phase relations, and ligands (Carroll and Webster, 1994; Dingwell and Mysen, 1985). Of particular importance is the role of fluorine in lowering melt viscosity, a circumstance that could allow normally high-viscosity rhyolite magmas to erupt effusively and form very extensive lavas.

The GRV and HS are the main host rocks to the giant Olympic Dam Cu-Au-U-REE deposit. This deposit is also enriched in fluorine: fluorine-bearing phases are ubiquitous through the deposit. The presence of F in magma causes many high field strength elements to become highly incompatible (Keppler, 1993; Keppler and Wyllie, 1991). REE-bearing minerals are associated with fluorite and it seems that fluorine-rich fluid can easily transport REE (Bau and Dulski, 1995). We speculate that the regional GRV-HS anomaly in fluorine influenced the composition of the hydrothermal system responsible for formation of the Olympic Dam ore deposit.


Geological setting of Olympic Dam

The Olympic Dam (OD) iron-oxide-associated Cu-Au-U-REE deposit is located in the Stuart Shelf region, South Australia, 520 km NNW of Adelaide (Fig. 1; Reynolds, 2001). The deposit is on the eastern margin of the Gawler Craton, and unconformably overlain by ~ 300 m of Neoproterozoic to Cambrian age, horizontal sedimentary rocks. The basement comprises metasedimentary and deformed granitoid successions (Hutchison Group and Lincoln Complex; Parker, 1990). These rocks are intruded by Mesoproterozoic HS granitoids and locally overlain by the GRV (Creaser, 1987; Oreskes and Einaudi, 1990; Johnson and Cross, 1995; Johnson, 2001).

Olympic Dam ore deposit

Figure 8: Simplified geological plan of the ODBC showing the general distri­bution of major breccia types, after Reynolds (2001).

The OD ore deposit is one of the biggest deposits in the world (3810 Mt at 1.0 wt% Cu, 0.5 g/t Au, 0.04 U3O8, and 3.6 g/t Ag; Williams et al., 2005). A zoned breccia complex seated within the Roxby Downs Granite (HS) hosts the Cu-U-Au mineralization (Figs 8,9). The barren hematite-quartz breccia core passes outward to a polymictic hematitic breccia and hematitic breccia, then monomictic granite-derived breccia and finally the Roxby Downs Granite in the peripheral zones. Well-bedded volcaniclastic mudstone, sandstone and conglomerate occur locally in the southern mine area (Fig. 8), and have fault contacts with the hematitic breccias. The Cu, U, Au, Ag (± base metals) and REE ore is widespread through the deposit, associated with intense iron oxide mineralization.

The breccias of the OD breccia complex consist of varying proportions of clasts of Roxby Downs Granite, felsic and mafic GRV, veins (fluorite, barite, siderite, quartz) and hematite. They are typically non-stratified, poorly sorted, and chaotic; both clast- and matrix-supported fabrics are present, and clast dimensions range from mm to several metres. Other lithologies occurring as minor clast types include highly altered mafic igneous rocks and bedded sedimentary lithologies.

Figure 9: Schematic east-west cross-section of the OD breccia complex, after Reeve et al. (1990).

Numerous generations of variably altered mafic dykes have intruded the Roxby Downs Granite, and the OD breccia complex. They were probably emplaced during late stages of mineralization and are themselves strongly chlorite- and/or sericite-altered. Most of the dykes are subvertical though locally irregular. They have widths from several centimeters up to few metres. Both coherent and brecciated facies are present. They are aphanitic or micro-porphyritic, containing relic olivine and chrome-rich spinel.

Equigranular medium-grained dolerite occurs in crosscutting intrusions. The dolerite can be relatively fresh and consists of unaltered plagioclase and pyroxene. Pervasive hematite-, chlorite-, and sericite- alteration is common. Small skeletal crystals of magnetite are preserved. The dolerite intrusions have been assumed to be significantly younger than the OD deposit (Creaser, 1989; Reeve et al., 1990). However, some recent drillcores show intimate relationships between dolerite, hematitic breccia and mafic dykes that imply the temporal relationships are more complicated.

The characteristic hydrothermal alteration mineralogy at OD is sericite-hematite, with less abundant chlorite, silica, siderite, and magnetite. Sericite alteration occurs throughout the deposit whereas hematite alteration is more abundant near the centre of the deposit. Most of the hematite is thought to have replaced pre-existing minerals including those of granitic and secondary hydrothermal origin. Veins are common both as fragments and as crosscutting features. The major minerals in macroscopic veins are fluorite, barite, siderite, hematite, and sulfides (Oreskes and Einaudi, 1990). Veins typically are 1 to 10 cm wide, moderately to steeply dipping, rarely more abundant than one vein per metre, and they lack visible alteration envelopes.


The origin of OD ore deposit is still debated. Early models of the deposit being sediment-hosted were strongly influenced by interpretation of the hematitic breccias as altered sedimentary facies (Roberts and Hudson, 1983). More recent models regard the OD breccia complex as hydrothermal in origin. Mafic igneous units are thought to have been particularly important in generating the Cu mineralization (Johnson and McCulloch, 1995; Haynes et al., 1995). The model of Olympic Dam proposed by Reeve et al. (1990) and Haynes et al., (1995), used data on ore mineralogy and fluid-rock thermodynamic modelling. They concluded that the ore deposition involved a mixing of hot saline water and cooler meteoric water that interacted with basaltic and granitic wall rock (Haynes et al., 1995).


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