Regional geology and mineral systems of the Staverly Arc, western Victoria

225 Pages·2017·33.51 MB·English

by  Geoscience Australia

Record 2018/02 | eCat 115622
Regional geology and mineral systems
of the Stavely Arc, western Victoria
Edited by A. Schofield1
With contributions by A. Bailey1, E. Bastrakov1, C. Cairns2, R. Cayley2, R. Duncan2, D. Huston1, C. Lewis1, S. McAlpine1,
A. Schofield1, P. Skladzien2, D. Taylor2 and M. Thomas1
APPLYING GEOSCIENCE TO AUSTRALIA’S MOST IMPORTANT CHALLENGES www.ga.gov.au
Regional geology and mineral systems of
the Stavely Arc, western Victoria
GEOSCIENCE AUSTRALIA
RECORD 2018/02
Edited by A. Schofield1
With contributions by A. Bailey1, E. Bastrakov1, C. Cairns2, R. Cayley2, R. Duncan2, D. Huston1, C.
Lewis1, S. McAlpine1, A. Schofield1, P. Skladzien2, D. Taylor2 and M. Thomas1

1. Geoscience Australia.
2. Geological Survey of Victoria.
Department of Industry, Innovation and Science
Minister for Resources and Northern Australia: Senator the Hon Matthew Canavan
Secretary: Dr Heather Smith PSM
Geoscience Australia
Chief Executive Officer: Dr James Johnson
Department of Economic Development, Jobs, Transport and Resources
Minister for Resources: Hon. Tim Pallas MP
Secretary: Mr Richard Bolt
Executive Director Earth Resources Policy and Programs: Mr Anthony Hurst
Geological Survey of Victoria
Director: Mr Paul McDonald
This paper is published with the permission of the CEO, Geoscience Australia and the Director,
Geological Survey of Victoria.
© Commonwealth of Australia (Geoscience Australia) 2018
With the exception of the Commonwealth Coat of Arms and where otherwise noted, this product is
provided under a Creative Commons Attribution 4.0 International Licence.
(http://creativecommons.org/licenses/by/4.0/legalcode)
Geoscience Australia has tried to make the information in this product as accurate as possible.
However, it does not guarantee that the information is totally accurate or complete. Therefore, you
should not solely rely on this information when making a commercial decision.
Geoscience Australia is committed to providing web accessible content wherever possible. If you are
having difficulties with accessing this document please email [email protected].
ISSN 2201-702X (PDF)
ISBN 978-1-925297-67-6 (PDF)
eCat 115622
Bibliographic reference: Schofield, A. (ed.) 2018. Regional geology and mineral systems of the
Stavely Arc, western Victoria. Record 2018/02. Geoscience Australia, Canberra.
http://dx.doi.org/10.11636/Record.2018.002
Individual Sections of this report may be cited using the following example:
Cayley, R. and Skladzien, P. 2018. Structure. In: Schofield, A. (ed.). Regional geology and mineral
systems of the Stavely Arc, western Victoria. Record 2018/02. Geoscience Australia, Canberra.
Contents
Executive summary .................................................................................................................................. 1
1 Introduction ............................................................................................................................................ 4
1.1 Overview of the geology and mineral systems of the Stavely Arc ................................................... 6
1.2 The Stavely Project .......................................................................................................................... 7
1.3 Previous Stavely Project releases .................................................................................................15
1.4 This report ......................................................................................................................................15
1.5 Key findings ...................................................................................................................................16
2 Geology ...............................................................................................................................................18
2.1 Basement geology .........................................................................................................................18
2.1.1 Glenelg Zone (and South Australian stratigraphic correlates) .................................................18
2.1.2 Grampians-Stavely Zone ..........................................................................................................23
2.1.3 Stawell Zone .............................................................................................................................35
2.2 New geochronology constraints on the development and duration of the Stavely Arc .................39
2.2.1 Implications for timing and duration of Stavely Arc magmatism ..............................................39
2.3 Cover ..............................................................................................................................................44
2.3.1 Grampians Group .....................................................................................................................45
2.3.2 Rocklands Volcanic Group .......................................................................................................47
2.3.3 Urana and Bacchus Marsh Formations ....................................................................................49
2.3.4 Otway Basin sediments ............................................................................................................50
2.3.5 Murray Basin sediments ...........................................................................................................51
2.3.6 Newer Volcanic Group .............................................................................................................55
2.3.7 Recent alluvial and colluvial cover ...........................................................................................56
2.4 Landscape evolution and post-mineralisation preservation ...........................................................57
2.4.1 Landscape evolution in STAVELY ...........................................................................................57
2.4.2 Preservation potential of arc-related mineral systems and supergene enrichment .................61
2.4.3 Summary ..................................................................................................................................62
2.5 Structure.........................................................................................................................................63
2.5.1 Regional tectonic setting and deformation events ...................................................................63
2.5.2 Methods for understanding the structural history of STAVELY................................................70
2.5.3 Delamerian Orogeny: D1a deformation ...................................................................................71
2.5.4 Delamerian Orogeny: D1b deformation and associated magmatism ......................................76
2.5.5 D2: Benambran Orogeny and formation of the Grampians Basin ...........................................77
2.5.6 Bindian Orogeny: D3 and D4 deformation ...............................................................................78
2.5.7 Tabberabberan Orogeny and other post-D4 deformation ........................................................85
2.5.8 Implications for the architecture of the Stavely Arc through time .............................................85
2.5.9 Other key insights arising from new data and interpretation ....................................................95
2.5.10 A scalable structural template for STAVELY through time ....................................................98
2.5.11 Summary ..............................................................................................................................100
2.6 Geochemistry of the Stavely Arc .................................................................................................101
2.6.1 Introduction .............................................................................................................................101
2.6.2 Data used in this investigation................................................................................................103
2.6.3 Geochemistry .........................................................................................................................106
2.6.4 Discussion ..............................................................................................................................120
Regional geology and mineral systems of the Stavely Arc, western Victoria iii
2.6.5 Summary ................................................................................................................................130
3 Mineral systems .................................................................................................................................135
3.1 Known mineral occurrences in STAVELY ...................................................................................135
3.1.1 Cambrian-aged mineral occurrences within the Stavely Arc .................................................135
3.1.2 Ordovician- to Silurian-aged mineral occurrences .................................................................147
3.1.3 Early Devonian-aged occurrences .........................................................................................148
3.1.4 Heavy mineral sands ..............................................................................................................149
3.1.5 Conceptual mineral systems in STAVELY .............................................................................150
3.2 Isotopic characteristics of known mineral occurrences in STAVELY ..........................................151
3.2.1 Sulphur isotope geochemistry of prospects from the Stavely Arc..........................................151
3.2.2 Lead isotope characteristics of prospects from the Stavely Arc ............................................154
3.3 Footprints of mineral systems in STAVELY .................................................................................160
3.3.1 Pyrite geochemistry of selected Stavely Arc prospects .........................................................160
3.3.2 Surface geochemical expressions of selected Stavely Arc prospects ...................................161
4 Synthesis ...........................................................................................................................................165
4.1 Geodynamic synthesis and implications for the geological evolution of STAVELY ....................165
4.1.1 Previously proposed geodynamic models for western Victoria ..............................................167
4.1.2 Constraints on geodynamic models .......................................................................................170
4.1.3 Implications for the evolution of the Stavely Arc in western Victoria ......................................175
4.2 Synthesis of mineral system styles and potential of STAVELY ...................................................185
4.2.1 Comparison of the Grampians-Stavely Zone with other metallogenic provinces of the
Tasman Element .............................................................................................................................185
4.2.2 Potential for mineral systems in STAVELY ............................................................................190
5 Conclusions .......................................................................................................................................201
Acknowledgements ..............................................................................................................................205
References ...........................................................................................................................................206

iv Regional geology and mineral systems of the Stavely Arc, western Victoria
Executive summary
The Stavely Arc is a Cambrian magmatic arc system developed on the eastern margin of the
Delamerian Orogen within the Grampians-Stavely Zone in western Victoria. Radiometric dating of the
Stavely Arc yields ages of approximately 511-500 Ma, although many lithologies are unable to be
dated and the full duration of the arc is unknown. Regional geological constraints, however, suggest
that the Stavely Arc initiated around 525 Ma, and was active for approximately 25 million years.
Stavely Arc rocks are poorly exposed at the surface and occur in narrow, fault-bound belt segments of
predominately andesitic to dacitic volcanic rocks that are collectively assigned to the Mount Stavely
Volcanic Complex. The low to medium K, calc-alkaline affinity of the most evolved volcanic rocks is
most similar to present-day arc settings involving (thinned) continental crust. Such a setting is
favourable for a range of arc-related mineral systems. Other igneous rocks with mostly basaltic,
tholeiitic compositions, intersected in the stratigraphic drill holes, are more characteristic of a rift-
related or extensional (back-arc?) tectonic setting. The belt segments of volcanic rocks are separated
by panels of Cambrian sedimentary rocks of the Nargoon Group and Glenthompson Sandstone.
Intermediate to felsic intrusive rocks (genetically and temporarily associated with the Stavely Arc),
including mineralised porphyries, intrude the entire Cambrian succession.
A number of known mineral occurrences with porphyry, epithermal and volcanic-hosted massive
sulphide characteristics are exposed and are spatially associated with igneous rocks of the Stavely
Arc. Constraints on the uplift history of the Grampians-Stavely Zone indicate that Cambrian mineral
systems are likely to be preserved, and long-lived tectonic quiescence and associated deep
weathering across the region has also resulted in local supergene enrichment of copper
mineralisation. In addition to Cambrian arc-related mineral systems, Ordovician- to Silurian-aged
orogenic gold, Early Devonian intrusion-related gold, and base metal mineral occurrences are also
present in the project area. Cover sequences, generally <100 m thick within the project area, conceal
much of the prospective Cambrian basement and act to obscure its mineral potential. Cover is
deepest (≥300 m) in the far north and far south of the project area, and beneath the Grampians
Ranges. World-class heavy mineral sand occurrences are hosted within Miocene cover sequences.
Although the outcrop extent of prospective Stavely Arc rocks is limited to approximately 0.5% of the
20 000 km2 project area, other belt segments of inferred arc-related rocks have been interpreted within
the Grampians-Stavely Zone from regional geophysical data and localised drilling. The collaborative
Geoscience Australia-Geological Survey of Victoria Stavely Project has investigated the buried parts
of the Stavely Arc, which has helped to advance our understanding of the geological evolution and
mineral potential of this region. New lithological observations and geochemical data from pre-
competitive stratigraphic drill testing of several of the buried belt segments demonstrate that there are
correlatives to the prospective Mount Stavely Volcanic Complex throughout the project area. They are,
therefore, considered to be parts of a single, mineralised, arc succession. Seismic reflection data
show that the fault belt segments extend to depth, and to the north, link into large bodies that
represent parts of a buried, intact Stavely Arc edifice.
The present-day distribution of the 18 belt segments of Stavely Arc rocks is the product of multiple
deformation events. Inter-belt relationships are complex and have previously been difficult to
understand. Interpretation of the regional architecture and structural history of the Grampians-Stavely
Zone allows dismembered Stavely Arc volcanic belt segments to be restored to a Cambrian
configuration of four strike-persistent, subparallel fault-bounded belts with an estimated collective
Regional geology and mineral systems of the Stavely Arc, western Victoria 1
strike length of 1 160 km. Only two of the four restored belts are significantly exposed, albeit poorly,
yet they contain multiple base and precious metal mineral occurrences. A third is extremely poorly
exposed, while the large Dimboola Belt in the north of the project area has no surface expression.
Although there are no known mineral occurrences associated with the remaining two belts, largely due
to their poor exposure, they are correlated with the mineralised belts, and are expected to have the
same mineral potential.
The porphyry, epithermal and volcanic-hosted massive sulphide mineral occurrences all appear to
occur in camps clustered along syn-arc faults, attributed to successive phases of the Delamerian
Orogeny (D1a and D1b). Subsequent deformation events (particularly D4) have served to dismember
and re-orient the Cambrian belts and mineralisation. A scalable structural template, developed as part
of this project, which provides the framework for reassembling the Cambrian Stavely Arc geometry
prior to D4 will be of assistance for targeting Cambrian mineral systems in the region. Once restored, a
series of magmatic centres spaced approximately 75-80 km along the length of the Grampians-
Stavely Zone can be recognised. This spacing is typical of modern magmatic arc systems.
A new geodynamic interpretation of the Grampians-Stavely Zone provides the context for mineral
exploration in the Stavely Arc. New stratigraphic data, together with re-examination of existing
mapping data, geochronological, geochemical, and structural constraints, suggests that Cambrian
stratigraphy in parts of South Australia can be extended into western Victoria. This stratigraphic
continuity indicates that no major terrane-bounding structures occur in, or west of, the Grampians-
Stavely Zone.
The preferred geodynamic model for the Stavely Arc involves evolution from a Neoproterozoic passive
margin through to an extension-dominated Japan-style arc at around 525 Ma, evolving to a more
thrust-dominated Andean-style continental system from around 505 Ma. Arc magmatism took place in
a west-dipping subduction regime throughout, with the subduction zone located just to the east of the
Grampians-Stavely Zone. Understanding the geodynamic evolution helps to predict the likely
presence, location and host rocks of mineral systems through time improving mineral exploration
targeting and reducing mineral exploration risk.
These new data have helped to advance our understanding of the geological evolution and mineral
potential of the Stavely Arc and has significantly expanded the mineral exploration search space to
include all of the Cambrian rocks of the Grampians-Stavely Zone.
Key findings
• The Cambrian Stavely Arc is located in the Grampians-Stavely Zone and developed during west-
directed subduction along the eastern margin of Gondwana in the Early to Mid-Cambrian. Such a
setting is favourable for a range of arc-related mineral systems (e.g. porphyry, epithermal and
volcanic-hosted massive sulphide systems)
• The Stavely Arc is now preserved as 18 structurally-dismembered, narrow, fault-bound, poorly
exposed volcanic belt segments that contain features characteristic of well-endowed volcanic arc
terranes worldwide. These belt segments are easily identifiable in regional geophysical datasets
and have been interpreted to persist under cover
• Removal of the effects of Silurian to Devonian deformation events from the volcanic belt segments
restores them to four strike-persistent, sub-parallel volcanic belts that extended for a collective
strike length of approximately 1 160 km in the Cambrian
• Recognition of the original along-strike volcanic belt continuity, along with geochemical data
(largely derived from material collected during a stratigraphic drilling program) support the
2 Regional geology and mineral systems of the Stavely Arc, western Victoria
correlation of the volcanic belts under cover. Previously there was confusion about these
correlations
• Geochronological data and geological evidence suggests that the volcanic arc was active between
at least 511 Ma to 500 Ma, but perhaps extending back as far as 525 Ma. It appears that porphyry
intrusions and associated mineralisation occurred relatively late during this event, between
approximately 506 and 500 Ma. This timing post-dates the main phase of the Delamerian Orogeny
which tilted and faulted the volcanic belts. Any mineral systems associated with late orogenic
intrusions would, therefore, likely be preserved upright and may also occur outside of the volcanic
belts themselves in the intervening panels of Cambrian sedimentary rock. This greatly expands the
mineral exploration search space in the Stavely Arc
• Geochemical data for volcanic and intrusive igneous rocks of the Stavely Arc have close affinities
to present-day continent-fringing island arcs with a core of thinned continental crust. The new
geodynamic model proposed for the Stavely Arc involves evolution from a Neoproterozoic passive
margin through to an extension-dominated Japan-style arc and evolving to a more Andean-style
system during the Delamerian Orogeny
• Re-examination of the geological significance of mineral occurrences located in the small areas of
exposure in the Stavely Arc (e.g. Stavely and Black Range belt segments, and in adjacent
Cambrian sedimentary rocks), together with new S isotope data, show features consistent with
porphyry, epithermal, and volcanic-hosted massive sulphide styles of mineralisation. ‘Hybrid’-style
systems, with characteristics similar to both porphyry and volcanic-hosted massive sulphide
systems, have also been recognised. It is predicted that additional mineral occurrences occur
under cover
• Lead isotope data derived from mineral occurrences in the Stavely Arc are dominated by a crustal
source. Lead isotope data derived from mineral deposits to the east of the Stavely Arc in the
Stawell Zone are juvenile, indicating that the boundary between the Stavely Arc and the Stawell
Zone (the Moyston Fault) is a major terrane boundary structure. The Pb isotope data for the
Stavely Arc mineral occurrences also differs from those derived from the Mount Read Volcanics of
western Tasmania, suggesting that the Stavely Arc does not represent the northern extension of
the Mount Read Volcanics, but is rather a separate metallogenic province that may have some
similarities to the Mount Read Volcanics, but also important differences. Relative to the Mount
Read Volcanics, the Stavely Arc is considered to have greater potential for porphyry and
epithermal mineral systems
• Since the Devonian, the Stavely Arc has undergone relatively little uplift, which means that
potential arc-related mineral systems are likely to have a high preservation potential. Some of the
mineral occurrences show evidence for near-surface supergene Cu enrichment.
Regional geology and mineral systems of the Stavely Arc, western Victoria 3
1 Introduction
The Stavely Project area of investigation is located in western Victoria, south-eastern Australia, and is
approximately 230-300 km west and northwest of Melbourne (Figure 1.1). The area incorporates the
southern two-thirds of the Grampians-Stavely Zone in Victoria and parts of the adjacent Glenelg and
Stawell zones (Figure 1.1). Throughout this report, the term STAVELY is used to refer to this area,
which is approximately 70 km wide and 290 km long (Figure 1.1). An approximate cut off of ~300 m
depth of cover was used to define the north and southern limits of the project area, while the eastern
and western limits are approximately five kilometres either side of the major zone-bounding structures.
The regional centres of Hamilton and Horsham are located in the south and within the centre of
STAVELY, respectively (Figure 1.2). STAVELY has generally very low relief, with the only exceptions
being the Grampians Ranges and Mount Arapiles, which rise above the plains and effectively mark the
western end of the Great Dividing Range. STAVELY hosts a diverse range of rock types, including
Cambrian arc-related volcanics and intrusives, some of which are associated with known
mineralisation, and which are the main focus of this report.

Figure 1.1 Location of STAVELY and structural geological zones of Victoria. Figure adapted from VandenBerg et
al. (2000).
4 Regional geology and mineral systems of the Stavely Arc, western Victoria
Figure 1.2 Location of major geographic features and localities mentioned in this report.

Regional geology and mineral systems of the Stavely Arc, western Victoria 5