GSV Report 110 - St Arnaud 1:100 000 map area geological report

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Abstract This report describes the geology of the area covered by the St Arnaud 1:100 000 map area, 7524. It gives a comprehensive account of the geomorphology, geological history, Palaeozoic rock units, structural and economic geology, and a comprehensive list of references. Particular emphasis is placed on the Cambo-Ordovician Saint Arnaud Group metasediments, which have been subdivided into the Warrak, Beaufort and Pyrenees Formations. A high T-low P regional metamorphic complex has been mapped in the eastern part of the St Arnaud 1:100 000 map area and the petrography and structure of rocks in this region is described and discussed in detail. Several Early Devonian post-tectonic I-type plutons and smaller granitic intrusions and various dykes intrude the metasedimentary sequence, and most of these, and the surrounding contact aureole rocks have been petrographically described. A complex group of granites occurs as an elongate belt in the eastern half of the St Arnaud 1:100 000 map area, and these have been emplaced into pull-apart structures originated during dextral transtension in a broad zone adjacent to the Bendigo-Ballarat and Stawell Zone boundary.

The Palaeozoic rocks have been affected by a few major deformation events. Each event is characterised by a different fold and fault style and character, which are described in detail. The St Arnaud 1:100 000 map area can be divided into two major structural provinces: the Fentons Creek domain east of the St Arnaud Fault is characterised by a N-S to NNW-SSE main structural direction, while the domain to the west of the St Arnaud Fault is characterised by a predominantly NW-SE striking structural grain. The earliest, and main regional deformation event is the Silurian (Benambran?) deformation, which is characterised by widespread F2 folds which are open to tight, isoclinal, asymmetric, and upright to slightly overturned, with predominantly eastern vergence and probably a sinistral en-echelon pattern. The regional deformation is of a thin-skinned style, and has caused shortening of up to 78%.

A younger, overprinting set of D3 structures are mainly related to the St Arnaud Fault. This fault is a major regional structure, possibly an R1 Riedel derivative splay from the mid-crustal root zone of the Avoca Fault, which outcrops to the east. The D3 deformation resulted in ductile to brittle subvertical F3 folds and thrust to reverse faults with a superimposed sinistral strike-slip component in the hangingwall of the St Arnaud Fault. Overprinting brittle reverse faults (D4) are widespread across the St Arnaud 1:100 000 map area, and have dextral strike-slip, as well as low-angle conjugate compressional components. A further conjugate set of NE-NW and ESE-WNW striking faults (D5) and normal faults (D6) have also been recognised. Two main generations of joints within the metasedimentary succession and the granites have been recognised.

Gold-quartz mineralisation occurs as narrow vein deposits within brittle reverse faults. The siting and geometry of the veins is strongly controlled by bedding, F2/F3 folds and faults. The economic geology has been compiled and discussed in sections covering exploration, mapping and mining history, mineralisation style, economic potential and prospectivity.
A digital database of the mines on the St Arnaud 1:100 000 map area accompanies this report.

Bibliographic reference Krokowski de Vickerod, J., Moore, D.H. and Cayley, R.A., 1997. St Arnaud 1:100 000 map area geological report. Geological Survey of Victoria Report 110.