an example from Western Victoria

Exploration Geophysics (1997) 28, 270-275

New Data, New Insights: an Example from Western Victoria

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D.H. Moore Geological Survey of Victoria, PO Box 2145 MDC, Fitzroy Vic. 3065. Phone (03) 9412 5687

ABSTRACT The aeromagnetic surveys flown under the Victorian Initiative for Minerals and Petroleum and by others give a major regional dataset over western Victoria. This paper presents a new interpretation of the structural and tectonic history for the early Palaeozoic rocks of the region derived from integrating the aeromagnetic, regional mapping and other datasets. It provides a new geological model for exploration beneath the thin Murray Basin cover. Two distinct packages of rocks are present in the western Victorian basement; the eastern Stawell Zone and the western Glenelg Zone. The Stawell Zone contains turbidites and oceanic basalts typical of much of the Victorian Lachlan Fold Belt and which host major gold deposits. They are weakly magnetic with the basalts and pyrrhotitic slates providing most magnetic expression. Recent Ar-Ar dating gives a deformation age of about 435 Ma. Their western boundary is the northern extension of the Moyston Fault. A new tectonic framework is proposed for the Glenelg Zone. Island arc volcanic rocks and metasediments of the Zone have been variably metamorphosed and simply deformed in the 500 Ma Delamerian Orogeny. The rocks lie in a similar structural and tectonic position to those of the Mount Read Volcanics and have potential for VHMS deposits. Correlations can also be made with other Cambrian VHMS deposits and prospective regions in eastern Australia. Along the South Australian border part of the Glenelg Zone, the Ozenkadnook Subzone, contains amphibolite grade rocks that have been complexly folded in a deformation that seems to be earlier than the Delamerian. These rocks are of uncertain affinity, but may be a northern correlative of the upper Proterozoic rocks seen in northwestern Tasmania. Keywords: magnetic interpretation, gravity interpretation, volcanic hosted massive sulfides, gold, tectonic framework

INTRODUCTION This paper reports the results of an interpretation of geophysical data collected in western Victoria as part of the Victorian Initiative for Minerals and Petroleum (VIMP). In late 1994, the Geological Survey of Victoria contracted out over 140 000 line kilometres of magnetic and radiometric data acquisition at 400 m and 200 m line spacing and with 80 m mean terrain clearance. This supplemented surveys flown at 250 m spacing in 1980 for CRA Exploration Pty. Ltd. It gave high quality coverage over all areas in northwestern Victoria likely to have potential for significant gold or base metal occurrences and covered by the variably lithified sediments of the Cainozoic Murray Basin. The early Palaeozoic geology of western Victoria has been poorly understood. Further west, in South Australia, the Tasman Line marks the eastern limits of outcropping Middle Proterozoic rocks (Figure 1). Adjacent to the Tasman Line, the middle Proterozoic craton is overlain by

Figure 1. Basement elements of western Victoria and surrounds. M is at the Moyston Fault, the presently preferred position of the eastern edge of Delamerian folding in Victoria. A marks the Avoca Fault, the eastern edge of dominantly Cambrian sediments.

the Cambrian Kanmantoo Group (see for example Gravestock et al., 1995). This includes shallow water deposits that become progressively deeper water up the stratigraphic column, with an upward shallowing sequence at the top. The Kanmantoo Group was deformed in the 480 Ma to 515 Ma Delamerian Orogeny, some 40 Ma older than the oldest deformation known in the Lachlan Fold Belt.



In western Victoria, the limited, isolated exposures are generally seen where creeks cut down through the edges of Murray and Otway Basins. The outcrops show complex rock suites, including turbidite and related sedimentary rocks, ocean floor basalts, medium- to high-potassium calcalkaline andesites and dacites with adakitic affinities, and serpentinites (see for example Ramsay and VandenBerg, 1990; Crawford et al., 1996). Metamorphic grades vary from lower than greenschist facies to migmatites. Only rarely can major lithological or metamorphic boundaries be correlated between exposures. The turbidites and greenstones of the Lachlan Fold Belt rocks lie to the east. Until recently, the boundary between the Delamerian and Lachlan Fold Belts has not been well understood. However, mapping by Cayley and Taylor (in prep.) and age dating by Black and Stuart-Smith (pers. comm., 1996) has now placed the eastern boundary of rocks affected by the Delamerian Orogeny at the Moyston Fault. Again, mapping the isolated poor outcrops gives an incomplete picture that cannot be confidently extrapolated to covered areas. Attempts to correlate Victorian and Tasmanian geology have also proved difficult. Powell (1992) and Woodward et al. (1993) showed thin-skinned tectonic models, with west-directed thrusting in both the Late Cambrian and the Devonian. Whereas Gray and Willman's (1991) Victorian interpretation showed a similar model, there was no direct evidence of equivalents to the Tasmanian Precambrian rocks

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of the various massifs. Gondwana reconstructions, such as those by de Wit et al. (1988), traced them into Antarctica. Are there equivalents in Victoria, not 100 km from their last outcrop on King Island? Such questions are relevant to mineral exploration. The whereabouts on the mainland of tectonic equivalents to the Mount Read Volcanics is important to base metal explorers, and the western limits to the 80-million ounce Victorian goldfields are also economically highly significant. HORSHAM 1:250 000 MAP SHEET AREA The Horsham 1:250 000 map sheet area (HORSHAM) (Figure 2) has been covered by the CRAE 250 m data and the adjoining VIMP 200 m and 400 m magnetic and radiometric surveys. The gravity data are mainly at 11 km station spacing, although in the southeastern corner there have been petroleum exploration surveys at about 3 km spacing. Other data available included about 100 drill holes that penetrated to "basement" and mapping and geophysical surveys over Palaeozoic outcrops to the south and east of the region. Palaeozoic rocks are restricted to a few isolated outcrops of Silurian Grampians Group rocks. No older outcrops are known. The rest of HORSHAM is covered by the Murray Basin sediments. Drillhole results and Naudy depth to basement calculations show cover thicknesses which typically range from less than 20 m thick along the southern edge to 300 m in the north. The southeastern half of the map

Figure 2. Geological interpretation of HORSHAM overlaid on a residual Altered magnetic image. The zones, subzones and domain boundaries are shown. Note the strong parallelism of the 330° basement magnetic grain in the Dimboola and Miga Subzones. The Ozenkadnook Subzone shows a more complex pattern. In some areas magnetic responses from near-surface features are from interdune or palaeochannel maghemite in the Murray Basin.


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sheet is mostly less than 120 m to basement, and so should be amenable to modern exploration techniques. All the data were interpreted (Moore, 1996a) to define structural and geophysical domains. These domains were grouped into two zones (Figure 2), an eastern Stawell Zone and a western Glenelg Zone. The boundary between the two is the northward extension of the Moyston Fault — the northern continuation of the western boundary of the Lachlan Fold Belt. The Stawell Zone rocks are turbidites, black slates, volcaniclastic rocks and ocean floor basalts typical of the Lachlan Fold Belt in central and western Victoria. Sedimentdominated domains are characterised by rocks with lower magnetic susceptibilities and lower densities than more volcanic domains. Within the sedimentary domains, linear packages giving magnetic highs up to 10 nT may be caused by pyrrhotitic black slates. The volcanic domains contain magnetic highs up to 50 nT and also contain rare reversely magnetised layers. Both sedimentary and volcanic domains trend between 330° and 360°. To the southeast, on the Ballarat 1:250 000 map sheet area, the rocks are metamorphosed to amphibolite facies along the Moyston Fault. It is likely that similar metamorphic grades are present on HORSHAM, since the regional gravity response rises as the boundary between the two zones is approached. Ar-Ar age dating by Bucher et al. (1996) showed the age of the deformation to be about 435 Ma. Known base metal deposits are confined to the ocean floor basalts. The largest known is the Ararat deposit southeast of HORSHAM, containing about 1 million tonnes of 2.7% copper and 0.6 g/t gold, with minor zinc and silver. The Glenelg Zone has been divided into three subzones. In the Dimboola Subzone, the dense basement rocks are highly magnetic and the magnetic layering generally strikes about 330°. Most of the Subzone has a higher gravity response than die Lachlan Fold Belt rocks to the east. The western boundary of the Subzone cuts across all magnetic features further west, and can be traced for over 250 km on images of the magnetic data. The basement rocks correlate along strike south to the Jallukar and Dryden volcanic rocks, which include calc-alkaline basalt to rhyolite as well as tuffs, immature sedimentary rocks and serpentinite (Crawford, 1988). Traces of zinc are present in a gabbro near the centre of HORSHAM. Metamorphic grades are lower than greenschist facies. The Grampians Group cover sequence is thickest over the central part of the Subzone. Gold occurs in the Grampians Group at Pohlmers Workings, just south of HORSHAM. In the Miga Subzone, the rocks are weakly to moderately magnetic and dense. The dominant strike in the magnetic layering is about 330°. The rocks correlate with trachyandesites to dacites in the Black Range, the Stavely Volcanic Complex and greenschist to amphibolite grade metasediments south of HORSHAM. The Stavely Complex has been dated (SHRIMP) at 495±5 Ma and 501±9 Ma by Stuart-Smith and Black (1995). Crawford et al. (1996) proposed that the Mount Stavely Complex correlated with the Mount Read Volcanics, based on the exact age equivalence and dominance of medium- to high-potassium calc-alkaline volcanic rocks. The western boundary of the Subzone shows complex timing relationships. Figure 2 shows that all of the Miga Subzone trends are truncated, but there appears to be a later event which has thrust part of the Miga Subzone west across the original boundary. Minor copper mineralisation is known from the Mount Stavely Volcanic Complex and weak copper mineralisation from reconnaissance drilling in the Black Range area. Most of the rocks of the Ozenkadnook Subzone show complex magnetic patterns. They also have densities greater

than the Stawell Zone, but generally less than the volcanic rocks of rocks in the region. Some of the lithologies present correlate with higher grade parts of the Glenelg Metamorphic Complex to the south, where Gibson and Nihill (1992) recorded clastic sedimentary rocks, carbonaceous shales, carbonates and volcanic rocks that have been metamorphosed to andalusite facies. The magnetic character suggests that some of the volcanic rocks may be ocean floor basalts similar to those in the Stawell Zone. In the southwestern part of HORSHAM, the Subzone includes part of a major granite batholith. East of the granite and just north of the southern edge of HORSHAM, the magnetic patterns indicate at least three phases of ductile deformation, with the youngest oriented north-south. Another, later, brittle faulting event is parallel to the 330° Delamerian trends seen further east. This suggests that the ductile deformations belong to an earlier folding event not recorded elsewhere in Victoria. The metasedimentary component of these rocks must predate any earlier deformation and may be Precambrian in age, but this possibility can only be confirmed by further age dating. No indications of mineralisation have been found in the Subzone or further south, but exploration in the region has been minimal. At least two ages of granites are present. Early granites intrude rocks deformed in the Delamerian Orogeny and are cut by ?Delamerian faults. SHRIMP ages in Gravestock et al. (1995) from the Padthaway Ridge in southeastern South Australia indicate crystallisation ages around 500 Ma to 520 Ma. K-Ar ages by Turner et al. (1993) and Rb-Sr ages by Gray (1990) from western Victoria give dates of around 480 Ma to 500 Ma. The magnetic signatures of later granites are obvious where they intrude Stawell Zone rocks or Grampians Group sedimentary rocks. Fanning (1991) gave a 410 Ma SHRIMP age for the Rocklands Volcanics, which overlie the Grampians Group and are probably coeval with the western Victorian intrusions Rb-Sr dated by Gray (1990) at 390 Ma to 400 Ma. It is likely that some of the granites in the Glenelg Zone are also of this age. Within the Stawell Zone, some granites seem to have a regional spatial relationship to gold mineralisation. The geophysical expression of areas favourable for these deposits is beyond the scope of this paper, but was presented in Moore (1996b). REGIONAL OBSERVATIONS The subdivisions and tectonic framework established on HORSHAM can be broadly extrapolated along strike to other areas in western Victoria. The boundary between the Glenelg and Stawell Zones can be traced to the north, at least as far as 35°S (Figure 1). The magnetic and gravity high of the Dimboola Subzone seems to truncate all other units. North of 35°S, the Dimboola Subzone is not present. Within the Stawell Zone, the strikes of the magnetic units gradually swing from 340° to 020° at the New South Wales border (Figure 1). The basement magnetic trends also converge towards paralleling the Tasman Line. West of about 40 km west of HORSHAM, the strikes of the magnetic zones in the rocks trend between 330° and 000°. Near the South Australian border strikes are closer to the Victorian trends (330°), whilst further west the trends parallel Adelaide Fold Belt trends. All trends are cut by major faults trending between 330° and 340°. Gravestock et al. (1995) included all these rocks within the Kanmantoo Group and the region of Delamerian deformation. There is no evidence in the magnetic data of the complex polydeformation seen in the Ozenkadnook Subzone. In South Australia, as in New South Wales, the regional strikes of magnetic and gravity features tend to converge



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Figure 3. Regional cross section from the Tyennan Massif to the Tamar Graben (from Woodward et al., 1993). This shows the thin skinned style of deformation, with Precambrian basement sliced into younger rocks. Cambrian thrusts appear to have been overprinted by Devonian thrusts, since some Devonian packages of rocks have been faulted, whilst others terminate faults. Both fault sets have been overprinted by later normal faults.

towards parallelling the Tasman Line. Overall, this arcuate, concave-east pattern strongly suggests west-directed thrusting towards die older craton. Tarlowski et al. (1996) have recently presented a magnetic image which includes Bass Strait. It shows no indication of an along-strike connection between western Tasmania and western Victoria. The magnetic highs correlated with the mafic to ultramafic rocks along the Arthur Lineament in western Tasmania may possibly cross Bass Strait as far as the Victorian coastline. Few other features in western Tasmania can be correlated along strike with any in Victoria. DISCUSSION Despite the apparent lack of along-strike correlations, the regional setting of western Victoria in the Late Cambrian has many similarities in Tasmania. According to Crawford et al. (1996) and Crawford and Berry (1992), in Tasmania arc-continent collision took place at 515 Ma to 510 Ma. At 500 Ma, the intermediate to acid Mount Read Volcanics were formed from the melting of continental crust and then extruded in a marine graben setting adjacent to, and probably through, thin Precambrian crust. In northern Tasmania, Cambrian rocks have then been thrust west onto Precambrian basement. Figure 3, from Woodward et al. (1993), shows Cambrian turbidites and intermediate to acid volcanic rocks being thrust in the Late Cambrian over the strongly metamorphosed Precambrian Tyennan Massif. In western Victoria, the Cambrian rocks of the Miga and Dimboola Subzones seem to have been deposited at about 500 Ma, after the metamorphism of the Kanmantoo Group at about 515 Ma (Gravestock et al., 1996). Some of the felsic volcanic rocks were formed from melting lower continental crust (Crawford et al., 1996). Both subzones were thrust onto the Ozenkadnook Subzone and then intruded by granite at about 490 Ma. The tectonic evolution for the Late Cambrian of western Victoria is illustrated in Figure 4. The tectonic similarities with Tasmania support lithological comparisons by Crawford et al. (1996). As well, the similar lithologies and tectonic evolution confirm the prospectivity of western Victoria for massive sulfide deposits similar to Mount Lyell, Hellyer and Rosebery. Other Late Cambrian to Early Ordovician volcanic-hosted massive sulfide deposits may also fit this tectonic position, adjacent to the Tasman Line on thin continental crust. Figure 5 shows a plot of the position of the Tasman Line as determined from the magnetic map of Australia (Tarlowski et al., 1996), together with the significant base metal mines and prospective areas. Henderson (1986, p. 358) described the Mount Windsor Subprovince, host to the Thalanga deposit, and considered its tectonic setting as "a back-arc

basin formed by continental extension" with thin continental crust as the basement to the sequence. Other prospective areas are less well known, but lie hard against the Tasman Line and may have had a similar tectonic setting. The Balcooma group of deposits (Figure 5) lies in a package described by Coney et al. (1990, p. 532) as "often mylonitised meta-felsic volcanics, quartzite, pelite, semipelitic gneiss and mafic amphibolite which lies fault bounded against the (Proterozoic) Georgetown block". Mills (1992) described the rocks of the Wonominta Block, which include highly deformed metasediments, acid and basic volcanic rocks, and shelf type lithologies such as dolomitic limestone, quartzite and polymictic conglomerates. It could be in a comparable position east of the Broken Hill Block, although its tectonic relationships are even less well known than the western Victorian Cambrian. CONCLUSIONS The magnetic basement of HORSHAM has been divided into two zones, the western Glenelg Zone and the eastern Stawell Zone. The boundary is the northern extension of the Moyston Fault, the eastern limit of rocks affected by the Delamerian Orogeny. The Stawell Zone rocks are turbidites and ocean floor basalts typical of the Lachlan Fold Belt in central and western Victoria. They have been variably metamorphosed to as high as amphibolite facies and faulted along north- to north west-trending faults. Elsewhere in western and central Victoria the Lachlan Fold Belt rocks host significant gold deposits. Similar deposits may be present on HORSHAM. Using magnetic, gravity and drillhole data, the Glenelg Zone has been subdivided into the Dimboola, Miga and Ozenkadnook Subzones. All have been affected by the Delamerian Orogeny. The Ozenkadnook Subzone rocks appear to have been deformed by older events, and may include Precambrian rocks. The Dimboola Subzone is dominated by Cambrian basaltic to rhyolitic volcanic rocks that have been only weakly metamorphosed. The faults which bound the Dimboola Subzone can be traced on magnetic images for up to 250 km. The Miga Subzone includes both sedimentary rocks and acid to intermediate volcanic rocks that have been more highly metamorphosed than the Dimboola Subzone. The volcanic rocks include 500 Ma medium- to high-potassium andesites that are also found within the Mount Read Volcanics. Age dating to the south, in the Mount Stavely Volcanic Complex, indicates that the sequences are also coeval with the hosts to the Mount Lyell, Rosebery and Hellyer deposits. Although only traces of mineralisation have been found to date on HORSHAM, the rocks must be considered prospective.


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Figure 4. The Cambrian evolution of the western Victoria and southeastern South Australia. In Victoria, middle Palaeozoic events overprint these events. All timings are approximate. The model presented relies heavily on that of Pelletier and Stephan (1986) for Taiwan. South Australian data mostly from Gravestock et al. (1995).

Other prospective Cambrian volcanic sequences in eastern Australia may have similar tectonic positions along the Tasman Line. Integration of the best understanding of the geology with both regional and detailed studies of the magnetic and gravity data along and to the east of the Tasman Line may highlight other areas for more detailed exploration.

ACKNOWLEDGMENTS This study has drawn extensively on ideas generated in discussions with other members of the Geological Survey of Victoria, particularly R. Cayley. A J . Crawford (University of Tasmania) was most generous in discussing his ideas on western Victoria, in providing key references that helped my


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REFERENCES

Figure 5. Cambrian volcanic-hosted massive sulfide areas in relation to the Tasman Line and probable subsurface Precambrian basement.

understanding of the tectonics of the region and in refereeing the paper. A. Belperio (Mines and Energy, South Australia) provided the magnetic data from the South Australian Exploration Initiative. Without it, the connections to South Australian outcrops would not have been attempted.

Bucher, M., Foster, D.A. and Gray, D.R., 1996, Timing of cleavage in the western Lachlan Fold Belt: new constraints from 40Ar/39Ar geochronology: Geological Society of Australia Abstracts 41, 66. Cayley, R.A. and Taylor, D.H., (in prep.), Explanatory notes on the Ararat 1:100 000 geological map: Geological Survey of Victoria Report: Department of Resources and Environment, Victoria. Coney, RJ., Edwards, A., Hine, R., Morrison, F. and Windrim, D., 1990, The regional tectonics of the Tasman orogenic system, eastern Australia: Journal of Structural Geology 12, 519-543. Crawford, A.J., 1988, Cambrian. In: Douglas, J.G. and Ferguson, J.A., eds, Geology of Victoria: Victorian Division, Geological Society of Australia, 37-62. Crawford, A.J., Donaghy, A.G., Black, L.P. and Stuart-Smith, P.G., 1996, Enhancing the prospectivity of Victoria: identifying Mount Read correlatives in western Victoria: Geological Society of Australia Abstracts 41, 100. De Wit, M., Jeffery, M., Berg, H. and Nicholson, L., 1988, Geological map of sectors of Gondwana reconstructed to their disposition at 150 Ma, scale 1:10 000 000: American Association of Petroleum Geologists, Tulsa, Oklahoma, U.S.A. Fanning, CM. 1991, Single and multi-grain U-Pb zircon dating of the Rocklands Rhyolite: Geological Survey of Victoria Unpublished Report 1991/6. Gibson, G.M. and Nihill, D.N., 1992, Glenelg River Complex: western margin of the Lachlan Fold Belt or extension of the Delamerian Orogen into western Victoria? In: Fergusson, C.L. and Glen, R.A., eds, The Palaeozoic eastern margin of Gondwanaland: Tectonics of the Lachlan Fold Belt, southeastern Australia and related orogens: Tectonophysics 214, 69-91. Gravestock, D.J., Alley, N.F., Benbow, M.C., Cowley, W.M., Farrand, M.G., Flint, R.B., Gatehouse, C.G., Kreig, G.W. and Preiss, W.V., 1995, Early and Middle Palaeozoic. In Drexell, J. and Preiss, W.V., eds, The geology of South Australia Volume 2: Mines and Energy South Australia, Adelaide, 3-61. Gray, CM., 1990, A strontium isotopic traverse across the granitic rocks of southeastern Australia: petrogenetic and tectonic implications: Australian Journal of Earth Sciences 37, 331-350. Gray, D.R. and Willman, C.E., 1991, Deformation in the Ballarat Slate Belt, central Victoria, and implications for the crustal structure across southeast Australia: Australian Journal of Earth Sciences 38, 171-201. Henderson, R.A., 1986, Geology of the Mount Windsor Subprovince — a Lower Palaeozoic volcano-sedimentary terrane in the Northern Tasman Orogenic Zone: Australian Journal of Earth Sciences 33, 343-364. Mills, K.J., 1992, Geological evolution of the Wonomlnta Block. In Fergusson, C.L. and Glen, R.A., eds, The Palaeozoic eastern margin of Gondwanaland: tectonics of the Lachlan Fold Belt, southeastern Australia and related orogens: Tectonophysics 214, 57-68. Moore, D.H., 1996a, A geological interpretation of the geophysical data for the Horsham 1:250 000 map sheet area: Victorian Initiative for Minerals and Petroleum Report 24: Department of Agriculture, Energy and Minerals, Victoria. Moore, D.H., 1996b, Geophysical signatures of gold deposits in western Victoria. In Hughes, M.J., Ho, S.E. and Hughes, C.E., eds, Recent developments in Victorian geology and mineralisation: Australian Institute of Geoscientists Bulletin 20, 19-23. Pelletier, B. and Stephan, J.F., 1986, Middle Miocene obduction and Late Miocene beginning of collision registered in the Hengchun Peninsula: geodynamic implications for the evolution of Taiwan: Tectonophysics 125, 133-160. Powell, CMcA., 1992, New perspectives on Tasmanian geology: Geological Survey Tasmania, Bulletin 70,177-187. Ramsay, W.R.H. and VandenBerg, A.H.M., 1990, Lachlan Fold Belt in Victoria — Regional geology and mineralisation. In: Hughes, F.E., ed., Geology of the mineral deposits of Australia and Papua New Guinea: Australasian Institute of Mining and Metallurgy, Melbourne, 1269-1273. Stuart-Smith, P. and Black, L.P, 1994, The Mount Stavely Volcanic Complex, western Victoria: mainland equivalents of the Tasmanian Cambrian Mount Read Volcanics: Australian Geological Survey Organisation Research Newsletter 21, November 1994, 13-14. Tarlowski, C , Milligan, PR., and Mackey, T, 1996, Magnetic anomaly map of Australia (second ed.), scale 1:5,000,000: Australian Geological Survey Organisation, Canberra. Turner, S.P., Adams, C.J., Flbttmann, T. and Foden, J.D., 1993, Geochemical and geochronological constraints on the Glenelg River Complex, western Victoria: Australian Journal of Earth Sciences 40, 275-292. Woodward, N.B., Gray, D.R. and Elliott, C.G., 1993, Repeated Palaeozoic thrusting and allochthoneity of Precambrian basement, northern Tasmania: Australian Journal of Earth Sciences 40, 297-311.