Broken Hill Exploration Initiative web mapping system Ameroo Hill, SA

• Introduction and Regional Context
• Stratigraphy
  • Table 1. Major lithostratigraphic divisions and igneous suites in the Olary Domain and correlatives in the Broken Hill Domain
• Structural History
  • Table 2. Alternative structural histories and nomenclatures adopted by various investigators for different deformational/folding events in Olary Domain.
• References

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• View a detailed Ameroo Hill map legend
• The Broken Hill Exploration Initiative main page

Introduction and Regional Context

As part of the Broken Hill Exploration Initiative (BHEI), Geoscience Australia and the Department of Primary Industries South Australia (PIRSA) recently embarked on a two-year joint project to better understand the deformational history and tectonic evolution of Palaeoproterozoic rocks in the Outalpa Inlier, South Australia. This project is one of several structural studies undertaken on these rocks although to date no consensus has emerged regarding the number, orientation and geometry of the main deformational events. To address this problem, detailed structural and stratigraphic studies were undertaken around Ameroo Hill and Tommie Wattie Bore region in the central part of the inlier where a well defined stratigraphic succession is exposed (Table 1).

Comparable studies undertaken in other parts of the Olary Domain indicate that the same stratigraphy and structural history are developed across the entire Curnamona Province and that the structures developed around Ameroo Hill are not atypical of the province as a whole. Included in the Olary stratigraphy is the regionally extensive and highly prospective "Bimba Formation", a marble and calc-silicate dominated unit known for its high base-metal content (Pb, Zn, Mn, Cu, Co). Geochronological studies have recently confirmed this unit as a correlative of the Ettlewood Calc-silicate in the Broken Hill Group and there is growing geochronological evidence (Page et al., 2000) that other parts of the Olary sequence can be similarly matched to equivalent units in the Broken Hill Domain (Table 1).

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Stratigraphy

Lithostratigraphic nomenclature in the legend follows Conor (2000; 2001) who subdivided the Willyama Supergroup into two major units of contrasting magnetic susceptibility and composition: the Curnamona and Strathearn Groups (Table 1). The former comprises mainly variably oxidised, magnetite-bearing quartzofeldspathic gneisses and psammopelitic schists whereas the overlying Strathearn Group is dominated by metapelitic compositions that are chemically more reduced and contain varying amounts of graphite and sulphide. Their mutual contact is an important redox boundary, marked by a prominent aeromagnetic anomaly that elsewhere in the Olary Domain has served as the locus for several recently discovered mineral deposits (Kalkaroo, White Dam, Portia). The Bimba Formation lies at, or close to, this boundary and for this reason much exploration activity has been focussed on this particular unit and the rocks lying on either side of it. This boundary is particularly well exposed in the Ameroo Hill area and in outcrop is manifest as either an abrupt change from shallow to deep water sedimentation or as a rapid thinning and omission of stratigraphy within the older Curnamona Group (e. g. area around Tommie Wattie Bore). Felsic material from Plumbago Formation making up the lowermost part of the Strathearn Group has been dated at 1693 ± 3 Ma (SHRIMP zircon age) (Page et al., 2000) and provides a minimum age for the stratigraphically older Bimba and Ethiudna sequences. The older Ethiudna and Wiperaminga Subgroups are host to mafic and silicic intrusions whose ages range from 1710-1680 Ma (Ashley et al., 1996; Page et al., 2000; Conor and Fanning, 2001).

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Structural History

Up to four major phases of pre-Adelaidean deformation (D1-D4) have been identified with opinion divided about the geometry, orientation, and metamorphic conditions under which each event formed. A summary of these different structural models is presented in Table 2, along with the interpretation preferred here. Especially controversial is interpretation of the earliest recognisable structure: a layer-parallel foliation or schistosity (S1) that formed under low pressure-high temperature conditions and which is locally overprinted by andalusite crystals. The most common interpretation of this foliation is that it is axial planar to regional-scale recumbent folds or nappes with northeast-trending axes (e.g. Clarke et al., 1986). However, D1 fold closures are rarely, if ever, observed. In contrast, post-D1 folds have been reported from all parts of the Outalpa Inlier and are usually described as upright structures with a moderate to steeply dipping crenulation cleavage as the dominant axial plane foliation (Table 2).

Based on detailed structural and lithostratigraphic mapping around Ameroo Hill and Tommie Wattie Bore a revised interpretation of the regional structure is presented here:

• Variably plunging D4 kink folds and micaceous shear zones whose kinematic history is dominated by dip-slip displacements (south side up). These structures have an east to eastnortheast trend and formed under retrograde metamorphic conditions. A pre-Adelaidean age is assumed because the east-west-trending Ameroo Hill shear zone truncates at the Adelaidean unconformity (Table 2).
• D3 folds post-date peak metamorphism, occur at all scales, trend NE, and are characterised by a moderate to steeply dipping crenulation cleavage (S3). S3 typically dips southeast and obliquely transects the limbs of earlier formed D2 folds. Superposition of D3 on an already folded lithostratigraphic sequence has produced variably plunging D3 fold axes and S0/S3 intersection lineations.
• D2 folds formed under amphibolite facies conditions and have a high-grade crenulation cleavage as the dominant axial surface fabric. This fabric generally dips more gently than S3 but distinction between these two fabrics in not always easy, particularly where S2 has been reoriented into steeper attitudes as a result of the D3 deformation. Southeast-plunging folds predominate although it is not uncommon for D2 fold axes to show significant deviations from this trend (e.g. Tommie Wattie Bore) that are difficult to explain solely in terms of D3 overprinting. Such deviations are better explained in terms of a non-cylindrical D2 fold geometry.
• D1 folds and related nappe structures have not been recognized. The earliest recognisable structure is a layer-parallel schistosity usually defined by muscovite and biotite but which may also include fibrolitic sillimanite. An extensional origin has been proposed for a comparable fabric in the Broken Hill Domain (Gibson, 2000).

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References

Ashley, P. M., Cook, N. D. J., and Fanning, C. M., 1996. Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary Block, South Australia, and implications for mineral exploration. Lithos, 38: 167-184.

Berry, R. F., Flint, R. B., and Grady, A. E., 1978. Deformation history of the Outalpa area and its application to the Olary province, South Australia. Transactions of the Royal Society of South Australia, 102: 43-53

Clarke, G. W., Burg, J. P., and Wilson, C. J. L., 1986. Stratigraphic and structural constraints of the Proterozoic tectonic history of the Olary Block, South Australia. Precambrian Research, 34: 107-137.

Conor, C. H. H., 2000. Definition of major sedimentary and igneous units of the Olary Domain, Curnamona Province, South Australia. MESA Journal, 19: 51-56.

Conor, C. H. H., 2001. Geology of Olary Domain, Curnamona Province, South Australia: Field Guide book. PIRSA, Adelaide.

Conor, C. H. H., and Fanning, C. M., 2001. Geochronology of the Woman-in-White amphibolite, Olary Domain. MESA Journal: 20: 41-43.

Flint, R. B., 2002. Geological mapping of the Outalpa Inliers, Curnamona Province, South Australia. MESA Journal 26: 34-41.

Flint, R. B., and Parker, A. J., 1993. Willyama Inliers, In: Drexel, J. F., Preiss, W. V. and Parker, A. J. (eds.) The geology of South Australia, Volume 1: The Precambrian. Geological Survey of Australia, Bulletin 54: 82-89.

Gibson, G. M., 2000. Tectonic evolution of the Palaeoproterozoic Willyama Supergroup: the early years, In: Peljo, M (compiler) Broken Hill Exploration Initiative Abstracts Volume, AGSO Record 2000/10: 45-47.

Laing, W. P., 1995. Interpreted solid geology lithostratigraphic map, Olary Domain, Curnamona Province 1: 100 000 scale. Geological Survey of South Australia.

Page, R. W., Stevens, B. P. J., Gibson, G. M., and Conor, C. H. H., 2000. Geochronology of Willyama Supergroup rocks between Olary and Broken Hill, and a comparison to northern Australia. In , In: Peljo, M (compiler) Broken Hill Exploration Initiative Abstracts Volume, AGSO Record 2000/10: 72-75.