Embed Size (px)
Citation preview
N S W D P I
Quarterly NotesGeologic al Sur vey of New South Wales
April 2005 No 118
Murray–Riverina region: an interpretation of bedrock Palaeozoic geology based on geophysical dataABSTRACTInterpretation of data from the Murray–Riverina airborne magnetic and radiometric survey, acquired in early 2003, has revealed many underlying bedrock features in a region covered by a blanket of Cainozoic sediments.
The new airborne magnetic data, in combination with regional gravity data, contribute to a greater understanding of the geometry and evolution of basement geology through the defi nition of structural zones. The data are interpreted to defi ne major faults, granitic bodies, basins and igneous centres. Interpretation of the magnetic and gravity data suggests that both the Stawell, Bendigo and Tabberabbera structural zones defi ned in Victoria continue north into New South Wales. A new zone, named the Hay–Booligal Zone, covers the central section of the survey area and is wholly within New South Wales.
These new data suggest enhanced economic potential in the Murray–Riverina region for extensions of the gold-rich Bendigo and Stawell Zones from Victoria under the Murray Basin in New South Wales.
Keywords: aeromagnetic, gravity, Bendigo Zone, Stawell Zone, Tasmanides, Lachlan Orogen, Delamerian Orogen, Hay–Booligal Zone.
INTRODUCTIONThe Murray–Riverina airborne magnetic/radiometric/digital elevation survey (Figure 1) was fl own in early 2003 as a major New South Wales Government Exploration NSW initiative. The aim of the survey is to encourage mineral exploration in southwestern New South Wales which is covered by Cainozoic sediments. The survey data was also acquired to assist a whole-of-government study in the Riverina Bioregion and was co-funded by the Resource and Conservation Assessment Council.
Data acquisitionThe new survey covered approximately 63 000 square km and involved the acquisition of over 160 000 line km of data (Fugro Airborne Surveys 2005). It covered all the Booligal 1:250 000 map sheet area and parts of the Hay, Deniliquin, Bendigo, Swan Hill, Balranald, Pooncarie, Manara and Ivanhoe 1:250 000 map sheet areas. The airborne survey lines were flown east–west at an interline spacing of 400 m and at a nominal ground clearance of 60 m (Geological Survey of New South Wales 2003). The airborne contractor was Fugro Airborne Surveys Pty Ltd. Gravity data examined as part of this interpretation are from the Australian National Gravity Database (Wynne & Bacchin 2004). In New South Wales, the gravity stations are at a nominal 11 km x 11 km grid with additional data provided in the central part of the region by detailed road traverses (1.0 km station spacing) and in the northwest by a nominal 4 km x 4 km grid. In Victoria, the gravity stations are at a nominal 1.5 km x 1.5 km grid.
Michael Hallett
Geological Survey of New South Wales, NSW Department of Primary Industries
Jeff rey Vassallo
Geological Survey of New South Wales, NSW Department of Primary Industries
jeff [email protected]
Richard Glen
Geological Survey of New South Wales, NSW Department of Primary Industries
Steve Webster
Steve Webster Pty. [email protected]
Papers in Quarterly Notes are subject to external review. External reviewer for this issue was Ian Hone. His assistance is appreciated.
Quarterly Notes is published to give wide circulation to results of studies in the Geological Survey of New South Wales. Papers are also welcome that arise from team studies with external researchers. Contact: [email protected]
ISSN 0155-3410
CONTRIBUTORS
June 2005 Quarterly Notes
GEOLOGICAL FRAMEWORKThe survey area (Figure 1) is largely covered by the Riverina Plain of the eastern Murray Basin. Despite poor outcrop and sparse drillhole data, previous workers have established the following broad stratigraphy.
Cainozoic aeolian, fluvial and lacustrine deposits (Brown & Stephenson 1991; Cameron 1996, 1997).
Murray Basin sediments. Kingham (1998) summarised the geology of the Murray Basin, which contains Cainozoic sediments related to three depositional sequences in the Tertiary. Drillhole data define up to a few hundred metres of Tertiary and Quaternary sandstones and conglomerates (Kellett 1989).
Permian–Carboniferous infrabasins (Pearson 2003). The Oaklands Basin (graben) which lies directly southeast of the survey area (Figure 1) contains Permian strata up to 1300 m thick.
Limited outcrops of Early and Late Devonian strata restricted in the north of the survey area (Cameron 1997).
Devonian infrabasins (Sniffin 1985) inferred largely on the interpretation of gravity lows as sedimentary troughs.
Silurian to Carboniferous granites just north of the Murray River (VandenBerg et al 2000) and granite at Mossgiel with an inferred emplacement age of approximately 425 Ma (Moffitt 2001).
Ordovician meta-sedimentary rocks ~80 km east-southeast of Deniliquin (Pogson 1972) and thus east of the study area.
GEOPHYSICAL INTERPRETATIONThe basement interpretation described in this paper relies heavily on new airborne magnetic data merged with the airborne magnetic data from Victoria (Geological Survey of Victoria 1999) (Figures 2 & 6), in conjunction with existing gravity data (Wynne & Bacchin 2004) (Figures 3 & 4) and extrapolation of known geology from Victoria. Using differences in patterns and linearity of magnetic and gravity features, the Murray–Riverina region has been divided into six structural zones (Figures 2, 3 & 4) that are interpreted to reflect differences in Palaeozoic geology. These include a number of unexposed granite bodies, sedimentary basin and clusters of intrusive pipes. Interpretation of the magnetic fabric enabled the extension of the Stawell, Bendigo and Tabberabbera zones recognised in Victoria, northwards into New South Wales. This in turn has allowed a new tectonic interpretation of this buried part of the Tasmanides. A geophysical interpretation map of the Riverina Bioregion Pre-Permian geology is available at a map scale of 1:500 000 (Hallett et al 2004).
CONTENTS
ABSTRACT 1
INTRODUCTION 1
Data acquisition 1
GEOLOGICAL FRAMEWORK 2
GEOPHYSICAL INTERPRETATION 2
Bendigo Zone 5
Tabberabbera Zone 5
Hay–Booligal Zone 5
Stawell Zone 7
GEOPHYSICAL MODELLING 7
Granitic bodies and contact metamorphic aureoles 7
NEW INTERPRETATION OF BASEMENT STRUCTURAL FRAMEWORK 9
Stawell Zone 9
Bendigo Zone 9
Hay–Booligal Zone 10
Tabberabbera Zone 14
Structural Zones — geophysical data profiles 14
CONCLUSIONS AND IMPLICATIONS 15
ACKNOWLEDGMENTS 15
REFERENCES 15
The information contained in this publication is based on knowledge and understanding at the time of writing (June 2005). However, because of advances in knowledge, users are reminded of the need to ensure that information upon which they rely is up to date and to check currency of the information with the appropriate officer of New South Wales Department of Primary Industries or the user’s independent adviser.
3
Figure 1 Location diagram of the Murray–Riverina airborne geophysical survey (brown) and the adjacent New South Wales airborne geophysical surveys (olive).
2
Figure 1 Location diagram of the Murray–Riverina airborne geophysical survey (brown) and the adjacent New South Wales airborne
IVANHOEMANARA NYMAGEE
POONCARIEBOOLIGAL CARGELLIGO
BALRANALD HAY NARRANDERA
DENILIQUIN JERILDERIESWAN HILL
BENDIGO WANGARATTAST ARNAUD
0 100 km
2005-05-0082
Older data
Murray-Riverina, 2003
To be flown in 2005
AIRBORNE GEOPHYSICAL COVERAGE
River and lake
Road
State border
Locality
REFERENCE
Hay
BrokenHill
Canberra
Sydney
STUDYAREA
Murray River
MURRAY BASIN
DARLING BASIN
OAKLANDSBASIN
Balranald
Deniliquin
Lake Cargelligo
Hay
Booligal
Ivanhoe
Hatfield
Swan Hill
N E W S O U T H W A L E S
V I C TOR I A
V I C .
N . S .W.
June 2005 Quarterly Notes
Figure 2 Palaeozoic basement interpretation of the Murray–Riverina region overlain on a sunshaded-pseudocolour image of the new
Total Magnetic Intensity data, merged with the previously acquired New South Wales and Victorian data.
0 100 km
River and lake Road
Cross sectionZone boundary
Locality
Granites
REFERENCE
2005-05-0083
xxxState border
x
x
x x
x
xx x
x
x
xx
x
x xx x
x
x
xx
x
x
x
x x
x
x
xxx
xxx
x
x
x
x
x◊
x
x
x
xxx
x
x
xx
x
x
xx
xxx
xx
xx
x
x xxx
x
x
x
xx
x
x
x
x
x xxx
x
x
x
x
x
xxx
x
Balranald
Deniliquin
Hay
Booligal
Ivanhoe
Hatfield
LakeCargelligo
HAY-BOOLIGALZONE
TABBERABBERAZONE
BENDIGOZONE
MELBOURNEZONE
STAWELLZONE
OMEOZONE
Lake BogaGranite
BoundaryBend
Granite
Swan Hill
N E W S O U T H W A L E S
V I C T O R I A
MossgielGranite
Bo
oth
eragan
drra Fault
Avo
ca Fau
lt
Heathcote Fault Governor Fault
(See Figure 5)
5
Bendigo ZoneMagnetic data
The main features in the magnetic image (Figures 2) of the Bendigo Zone can be summarised into two types.
1. Curvilinear north-northwest trends are interpreted to broadly reflect the strike-parallel higher and lower magnetite content of mudstone-sandstone rich packets of Ordovician turbidites (respectively). They most likely reflect lithological fabric. This zone is the northern extension of the Bendigo Zone in Victoria that is dominated by Early–Middle Ordovician turbidites (VandenBerg et al 2000). The magnetic signature of these units is overprinted at their northern extent by the magnetic signatures of a number of interpreted granite bodies.
2. Elliptical to circular magnetic lows, interpreted as S-type granites fringed by highly magnetised contact metamorphic aureole rocks. The interpreted granite bodies are elongated in a north–south direction. The very distinctive magnetic signature clearly identifies seven granite intrusions in this part of the survey area, with several granite bodies intruding the metasediments (Figure 2).
Gravity data
Gravity lows overlie some of the granite bodies outlined from the aeromagnetic data discussed above (Figures 3 & 4). The shape and amplitude of these anomalies is similar to the magnetic and gravity lows over the Lake Boga Granite which lies immediately south of Lake Boga and Swan Hill in northern Victoria (Figures 3 & 4), previously mapped by Simons and Moore (1999).
Tabberabbera ZoneMagnetic data
The southeastern part of the survey area covers the northern continuation of the Tabberabbera Zone in Victoria which is dominated by Early Silurian turbidites in the west and Ordovician turbidites in the east (VandenBerg et al 2000). A broad magnetic high trending west-southwest to east-northeast in the centre of this zone indicates magnetic units, possibly I-type magnetic granites, at depth.
Approximately 30 km southeast of Hay (Figure 2), a broad (40 km wide) northeast–southwest trending magnetic low (100 km long), is interpreted to be caused by a large granite body. The boundary between the Tabberabbera Zone and the Hay–Booligal Zone to the north is obscured by this body. The low magnetic signature is identical to the Middle Devonian granites mapped on the border with Victoria (Simons & Moore 1999). Gunn (2004) suggested that the northeastern margin of this magnetic low was due to granite at depth.
Immediately south of Deniliquin, a large east–west trending magnetic low stretching over 100 km, is interpreted to be a granite body (Simons & Moore 1999).
Gravity data
The Tabberabbera Zone in New South Wales is underlain by a broad gravity high trending approximately east–west (Figures 3 & 4). A northeast-trending gravity low south of Hay, corresponds to the magnetic low discussed above and is interpreted to be caused by a granite. Immediately south of Deniliquin is a deep gravity low that corresponds to a large, mapped granite body and also has a corresponding low magnetic signature (discussed above). A narrow gravity low extending north from Deniliquin (Figure 3) may represent a narrow trough of Tertiary sediments.
Hay–Booligal ZoneThe Hay–Booligal Zone is a newly defined structural zone in the central part of the survey area. There is some uncertainty about the actual position of the boundaries of this zone. The northwestern boundary is drawn along the southern side of a northwest trending line of magnetic lows that are inferred to represent Silurian-Devonian S-type granites (Figures 2 & 4). Alternatively, the boundary could be drawn along the northern side of these intrusive bodies, or even through them. The southern boundary against the Bendigo Zone is drawn at the northern edge of the prominent NNE-trending magnetic grain of that zone. A southerly pointing salient of the Hay-Booligal Zone between the Bendigo and Tabberabbera zones contains igneous rocks that could represent more deformed versions of granites intruding the Bendigo Zone. The southern boundary against the Tabberabbera Zone is drawn along the northern edge of a large gravity low inferred to represent a major granitic body.
Magnetic data
The magnetic character of the Hay–Booligal Zone reflects deeply buried basement sources that are interpreted to be igneous rocks due to their irregular and elevated magnetic signatures. Overlying the basement is up to 1700 m of Early Devonian sedimentary rocks (Matti & Moffitt 2001). The nature of the boundary with the Bendigo Zone to the south is not clear, because the granite bodies and their aureoles overprint the northern extents of the turbidites of the Bendigo Zone. The stratigraphic layering evident in the Bendigo Zone, observed in the magnetic imagery (Figure 2), is not evident in the Hay–Booligal Zone.
Two magnetic lows (with coincident gravity lows) immediately east and west of Hay, trend north-northwest for approximately 20 km (anomaly axes marked as black dashed lines on Figures 2, 3 & 4). These are interpreted to be sedimentary troughs or grabens and may be filled with Permian and younger strata.
4
June 2005 Quarterly Notes
Figure 3 Image of Bouguer gravity data over the Murray–Riverina survey area and surrounding regions in New South Wales and Victoria. (For the location of this image, see Figure 1.)
0 100 km
2005-05-0085
River and lake Road
Cross sectionZone boundary
Locality
Granites
REFERENCE
xxxState border
x
x
x ◊
x
xx x
◊
x
xx
x
x xx x
x
x
xx
x
x
x
x x
x
x
xxx
xxx
x
x
x
◊
x◊
x
◊
x
xxx
x
x
xx
x
x
xx
xxx
xx
xx
x
x xxx
x
x
x
xx
x
x
x
x
x xxx
x
x
x
x
x
xxx
x
Balranald
Deniliquin
Hay
Booligal
Ivanhoe
Hatfield
LakeCargelligo
HAY-BOOLIGALZONE
TABBERABBERAZONE
BENDIGOZONE
STAWELLZONE
OMEOZONE
Swan Hill
Lake BogaGranite
BoundaryBend
Granite
MossgielGranite
N E W S O U T H W A L E S
V I C T O R I A
MELBOURNEZONE
Wentworthgravity
low
Wilkurra DDH–1
Mossgiel DDH–1
Bo
oth
eragan
dra Fault
Avo
ca Fau
lt
Heathcote Fault Governor Fault
(See Figure 5)
7 6
Magnetic highs (approximately 200 nT above a nominal background of 54 800 nT) in the northern and eastern part of the Hay–Booligal Zone (Figure 2) are interpreted to represent deeply buried igneous complexes. An irregularly shaped magnetic complex with high frequency signature located approximately 50 km northeast of Booligal, coincides in part with the mapped Early Devonian volcanic units of the Hillston Volcanic Complex (Cameron 1997). The extent of the magnetic anomaly implies that the volcanic complex extends under cover to the south and to the west of the mapped outcrop.
Gravity data
The Hay–Booligal Zone includes a broad gravity high, fl anked to the northwest by the Booligal gravity low and a shallow north-northwest-trending gravity low through Hay (anomaly axes marked as black dashed lines on Figure 3). This low saddle in the high coincides with two magnetic troughs and may represent sedimentary troughs or grabens, as suggested above.
The Booligal gravity low, located between Ivanhoe and Booligal, straddles the boundary between the Hay–Booligal Zone and the Stawell Zone (Figures 3 & 4). The northern section of this oval shaped gravity low lies within the Stawell Zone, immediately southwest of Ivanhoe (Figures 3 & 4) and is interpreted to be granite at depth. Drilling during hydrocarbon exploration (Mossgiel DDH–1) confi rmed that the coincident gravity and magnetic lows are due to a large granite body — the Mossgiel Granite (Matti & Moffi tt 1999) as shown on Figures 2, 3 and 4 which was dated 427Ma ±7 (Wilde 2000). Overlying the Mossgiel Granite is the Booligal Trough with up to 1500 m of sedimentary rocks inferred to consist of Devonian Winduck Group and overlying Mulga Downs Group equivalent (Matti & Moffi tt 1999).
Stawell ZoneMagnetic data
The Stawell Zone covers the western and northern parts of the Murray–Riverina region. The zone extends from west of Balranald, northeast through Hatfi eld and north and east of Ivanhoe, wrapping around the northern margin of the Hay-Booligal Zone. The boundary between the Stawell Zone and the Hay–Booligal Zone lies on the eastern side of a line of oval-shaped magnetic lows that are interpreted to be Silurian–Devonian S-type granites (Figures 2 & 4).
There are three major magnetic features of the Stawell Zone in the Murray–Riverina region.
1. A magnetically quiet area in the south, with magnetic features extending north of Hatfi eld showing magnetic fabric striking northeast (Figure 2) and then east, north of Ivanhoe.
2. Discrete circular to elliptical, magnetic lows embedded in the background fabric of the feature described above, best seen approximately 50 km northwest of Balranald.
One of these lows, 30 km west of Balranald, correlates with the “Boundary Bend Granite” (Figures 2, 3 & 4), an S-type Silurian–Devonian granite (Simons & Moore 1999). The eastern edge of this granite appears to be faulted. The other magnetic lows in this area are interpreted to be more S-type Silurian–Devonian granites.
3. East–west magnetic trends with truncations and folds in the northeastern part of the Stawell Zone correspond to inferred extensions of outcrops of the Early Devonian Cobar Supergroup and of the late Early–Late Devonian Mulga Downs Group mapped to the north by Cameron (1997).
Gravity data
The Stawell Zone is characterised by a northeast-trending gravity high, lying directly northwest of a string of gravity lows interpreted as granite bodies (Figure 3). The circular gravity low 30 km west of Balranald coincides with the mapped “Boundary Bend Granite” (Figures 2, 3 & 4), which is also defi ned as a magnetic low.
A large gravity low in the extreme northwest of the region is interpreted to correspond to the eastern part of the deep (approximately 4 km) Wentworth Trough (Figures 3 & 4). However, there is accumulating evidence that the gravity low over the Wentworth Trough may, at least in part, refl ect basement granite at depth rather than Devonian sedimentary rocks. Just east of the Wentworth Trough, Planet drill hole Nambucurra 1 (142° 50’ E / 33° 20’ S) bottomed in 16.5 m of granite, dated at c. 415 Ma (Moffi tt 2001). The Department of Mineral Resources (now DPI) hole DM Wilkurra DDH1 on the eastern margin of this trough (142° 54’ E / 33° 04’ S) penetrated almost 400 m of the Wilkurra Granite. It was estimated to have an emplacement age of 425-420 Ma (Moffi tt 2001), beneath rocks correlated with the Early Devonian Cobar Supergroup and the late Early- Late Devonian Mulga Downs Group.
GEOPHYSICAL MODELLING
Granitic bodies and contact metamorphic aureoles Magnetic and gravity modelling was carried out to confi rm the interpretation that major gravity and magnetic lows in the region represent granite bodies (Hallett & Webster 2004). The grid data modelling along the east–west (red) line in Figures 2, 3 and 4, indicate that the oval magnetic and gravity lows were consistent with the presence of large, low-magnetic, low-density (2.6 g/cm³) central plutons with higher density, magnetised contact metamorphic aureoles in the surrounding metasediments. These geophysical responses and oval geometries are consistent with bodies modelled by Webster (2004) as S-type granites at depth (1 to 2 km).
Additional modelling of these anomalies using magnetic line data (fl ight line 10630 located at approximately 6 114 000N) was completed using the inversion processes of Encom
June 2005 Quarterly Notes
Figure 4 Combined image of Bouguer gravity (pseudocolour) overlain on sunshaded image (85° elevation at 90° azimuth) of Total Magnetic Intensity. Interpreted granites are fi lled with crosses. Axes of magnetic lows and Bouguer gravity lows possibly refl ecting troughs or grabens are identifi ed by dotted lines. The east–west line (red) shows the location of a magnetic profi le used for modelling, presented later in this paper. (For location of this image, see Figure 1.)
Figure 4 Combined image of Bouguer gravity (pseudocolour) overlain on sunshaded image (85° elevation at 90° azimuth) of Total
0 100 km
2005-05-0084
River and lake Road
Cross sectionZone boundaryState border
Locality
Granites
REFERENCE
xxx
x
x
x x
x
xx x
x
x
xx
x
x xx x
x
x
xx
x
x
x
x x
x
x
xxx
xxx
x
x
x
◊
x◊
x
◊
x
xxx
x
x
xx
x
x
xx
xxx
xx
xx
x
x xxx
x
x
x
xx
x
x
x
x
x xxx
x
x
x
x
x
xxx
x
Balranald
Deniliquin
Hay
Booligal
Ivanhoe
Hatfield
LakeCargelligo
HAY-BOOLIGALZONE
TABBERABBERAZONE
BENDIGOZONE
STAWELLZONE
OMEOZONE
Swan Hill
N E W S O U T H W A L E S
V I C T O R I ALake Boga
Granite
BoundaryBend
Granite
MELBOURNEZONE
MossgielGranite
Wentworthgravity
low
Bo
oth
eragan
dra Fault
Avo
ca Fau
lt
Heathcote Fault Governor Fault
(See Figure 5)
Wilkurra DDH–1
Mossgiel DDH–1
9
Technology’s ModelVision Pro and Automag (Figure 5) modelling programs. Figure 5a shows the Total Magnetic Intensity (TMI) profi le (black) with the model profi le (red). Figure 5b shows a cross-sectional scan of the interpreted model blocks using Automag. The pink and dark green clusters of blocks generated by Automag indicate the location, depth and attitude of a source. The cluster density, indicate narrow dyke sources or the edges of a broad source with no clear distinction between sources.
In section, the clusters of blocks at approximately 208 000 E and 220 000 E imply contact metamorphic aureole magnetic anomalies around the edges of an interpreted granite (yellow block) and the gap between the blocks defi nes the non-magnetic granite. A similar pattern is observed just east of the interpreted granite (blue block) centred at 228 000 E;
The cluster of blocks in the vicinity of 265 000 E to 272 000 E extend to depth and refl ect dyke-like sources (dark purple bodies) for the two sharp (approximately 100 nT) anomalies shown in the TMI profi le (Figure 5a);
Between 230 000 E and 242 000 E the cluster of blocks are fl at dipping. They are modelled as two broad magnetic sources (dark purple blocks) centred at 232 000 E and 236 000 E, and are interpreted as mafi c volcanic units;
The cluster of blocks at approximately 180 000 E, refl ect the edges of the broad sources modelled (two light green bodies) and confi rms the earlier interpretation of large low magnetic source that may be due to a granite source.
Both Automag and block model interpretations give similar results. Depths to source of the order of 300+ m from interpretations of fl ight line data are 200 m shallower than obtained from the earlier modelling of the grid data (Webster 2004). This might be expected, as the fi ltering process inherent in gridding lines at 400 m spacing, would smooth the data and result in deeper estimates of source depths.
NEW INTERPRETATION OF BASEMENT STRUCTURAL FRAMEWORK Reinterpretation of the Palaeozoic geology underlying the Murray–Riverina region has prompted a re-examination of the basement structural framework of this area and linkage to the Palaeozoic geology of central Victoria. The new geophysical interpretation implies that the Stawell, Bendigo and Tabberabbera zones extend north from Victoria into the Murray–Riverina region of New South Wales. This has major implications for the mineral prospectivity in the southwestern part of New South Wales as the Stawell Zone hosts the rich Stawell gold deposits and the Bendigo Zone hosts the world class orogenic gold deposits of Bendigo, Ballarat, Castlemaine, among many others. The Stawell Zone produced >167 tonnes of gold by the year 2000 and over 1870 tonnes of gold has been recovered from the Bendigo Zone (VandenBerg et al 2000).
Stawell ZoneIn north–western Victoria, the Avoca Fault separates the Stawell Zone to the west from the Bendigo Zone to the east (Gray 1988). Regional magnetic trends in the Stawell Zone are truncated by this fault (VandenBerg et al 2000). The Stawell Zone is dominated by turbidites of the St Arnaud Group of probable Late Cambrian age (VandenBerg et al 2000). These are intruded by Silurian-Devonian granites and pass into Cambrian volcanic rocks along the western edge of the zone (VandenBerg et al 2000).
The recently acquired New South Wales airborne magnetic data suggests that the turbidites of the Cambrian St Arnaud Group extend north into New South Wales. They, or their lateral equivalents, swing to the east near Ivanhoe, wrap around the northern margin of the Hay–Booligal Zone (Figure 7) and extend east to the Bootheragandra Fault. The Bootheragandra Fault links northwards into the Paddington Line (Glen et al 1996) and southwards into the Kancoona Fault, that separates the Omeo and Tabberabbera zones in Victoria (VandenBerg et al 2000). The northern part of the Stawell Zone contains rocks of the Early Devonian Cobar Supergroup and Early to Late Devonian Mulga Downs Group deposited over the St Arnaud Group equivalents. This suggests that the Stawell Zone was reactivated in the Devonian.
The tectonic affi nity of the Stawell Zone is not clear. Mapping in western Victoria by Cayley and Taylor (1996), suggested that the boundary between the Lachlan Orogen and the Delamerian Orogen to the west was the Moyston Fault. As a result, the Stawell Zone was regarded as part of the Lachlan Orogen. However, recent Ar–Ar age dating (Miller et al 2003) showed that cleavage micas in the Mooroondal Metamorphic Complex (the high-grade part of the St Arnaud Group in the hangingwall of the Moyston Fault east of Stawell) had been deformed by the 500 Ma Delamerian Orogeny. As a result, Miller et al (2004) regarded the Stawell Zone as being transitional between the Delamerian and Lachlan Orogens. Alternatively, the Stawell Zone could be considered part of the Delamerian Orogen unless there is a substantial break in the St Arnaud Group or within the Stawell Zone (Glen 2005). In this interpretation, the Avoca Fault and its curved extension continues northwards into New South Wales along a line of granite bodies and a gravity low zone. This fault is considered to be the boundary between the Delamerian Orogen to the west and north, and the Lachlan Orogen to the east and south. The nature and location of the western and northern boundaries of the Stawell Zone are outside the scope of this paper.
Bendigo ZoneOriginally named the Bendigo–Ballarat Zone by Gray (1988), the Bendigo Zone consists of Early to Middle Ordovician turbidites of the Castlemaine Group intruded by granites (VandenBerg et al 2000). Deformation occurred mainly around the Ordovician–Silurian boundary (Foster et al 1999) and produced chevron
8
June 2005 Quarterly Notes
folds, steeply west-dipping cleavage and mainly east-vergent thrust faults that range in displacement from a few metres to kilometres (Gray & Willman 1991a, 1991b). The world-class gold deposits of Bendigo, Ballarat and other fi elds formed mainly during this Benambran Orogeny.
Most interpretations of the Bendigo Zone show it restricted within Victoria, being bounded to the north by the west–northwest-trending Murray River Lineament which broadly follows the trend of the Murray River (Scheibner & Basden 1996, fi gure 9.1), or by an extension of the Governor Fault west of the northern tip of the Heathcote Fault (Figure 6). The northwest-trending Governor Fault separates the Melbourne Zone from the Tabberabbera Zone to the east and contains Cambrian ultramafi c–mafi c units in its hangingwall (VandenBerg et al 2000). The Governor Fault swings to the west near Dookie in Victoria, at the northern margin of the Melbourne Zone, and VandenBerg et al (2000) and Willman et al (2002), suggested that the fault can be extrapolated further to the west–northwest under cover, to pass just south of the Murray River, cutting off the Heathcote Fault and the Avoca Fault and passing under the Lake Boga Granite (Figure 6). The 2003 data shows no single through-going fault, or fault zone, that delimits the Bendigo Zone along the path of the previously proposed Governor Fault. Instead, the airborne magnetic data show that magnetic lineaments in the Ordovician turbidites continue from Victoria northwards into New South Wales (immediately east of the Lake Boga Granite) and cut across the previously proposed location of the Governor Fault (Figure 6). Although the regional magnetic fabric is not as evident in the Victoria data as in the New South Wales data, it is clearly seen on the southern side of the of the Governor Fault (Figure 6) as proposed by Simons and Moore (1999). Part of the disparity in detail refl ects the diff erent acquisition parameters of diff erent airborne geophysical surveys. The airborne magnetic data from Victoria were
acquired in 1980 at a survey altitude of 80 m (Geological Survey of Victoria 1999) while the 2003 Murray–Riverina survey data were collected at an altitude of 60 m.
This study now suggests that the Governor Fault does not extend west of the Melbourne Zone and is interpreted to be truncated along the northwestern margin of that zone by the Heathcote Fault. The Heathcote Fault can then be followed northwards into New South Wales as the western boundary of the Tabberabbera Zone to increasingly greater cover thickness, until it becomes indistinct between 50 km and 100 km north of the Victorian border (Figures 6 & 8).
Hay–Booligal ZoneThe Hay–Booligal Zone (Figure 8) is characterised by interpreted Silurian–Devonian felsic volcanic rocks and granites buried beneath thick cover (including Devonian Cobar Supergroup rocks) except for the northeastern corner, where the Early Devonian Hillston Volcanic Complex were mapped by Cameron (1997). These volcanic rocks have a high-frequency magnetic response, which is obscured (probably due to deeper burial) to the south and to the west. The western edge of the Hay–Booligal Zone terminates near Balranald at the north-trending magnetic grain of the turbidites and deformed granitoids of the Bendigo Zone to the south. The Bootheragandra Fault marks the eastern boundary of the Hay–Booligal Zone against the Omeo Zone. This fault links into the Paddington Line to the north and the Kancoona Fault to the south and are stitched by a northeast-trending granite in the southeast corner of the zone.
Figure 5 Magnetic profi les and models across proposed S-type granite bodies. (For the location of this traverse, see Figures 2, 3 & 4.)
(a)
(b)
11
Figure 6Greyscale image of Total Magnetic Intensity covering the region around the Victoria–New South Wales border (white dashed line). The location of the Governor Fault (Simons & Moore 1999) is marked (dashed yellow line). A tentative new location of the Heathcote Fault is indicated, terminating the Governor Fault and continuing north into New South Wales. The continuation of magnetic lineations across the State border and across the previously inferred Governor Fault (Simons & Moore 1999) are indicated by dotted white lines. (For location of this image, see Figure 1.)
144˚00'E
35˚00'S
144˚30'E143˚30'E
35˚30’S
36˚00'S
Swan Hill
H
a
eathcoteF
utl
Av
oc
aF
au
lt
STAW ELLZONE
O
TABBERABBERAZONE
Heath
cote
Fa
ult
N E W S O U T H W A L E S
V I C T O R I A
20 km
2005-05-0126
0River and lake
Locality
REFERENCE
Previously inferred locationof the Governor Fault
Magnetic lineations
State border
BENDIZONE
G
Lake Boga Granite
10
June 2005 Quarterly Notes
Figure 7Greyscale image of the First Vertical Derivative of Total Magnetic Intensity for the northern part of the Murray-Riverina region. Turbidites of the St Arnaud Group (Stawell Zone) extend northeastwards from the southwestern corner of the image, eventually trending east-west and abutting the Bootheragandra Fault at the northeast edge of the image. (For location of this image, see Figure 1.)
13
Figure 7Greyscale image of the First Vertical Derivative of Total Magnetic Intensity for the Murray–Riverina region. Turbidites of the St Arnaud Group (Stawell Zone) extend northeastwards from the southwestern corner of the image, eventually trending east–west and abutting the Omeo Zone at the northeast edge of the image. (For location of this image, see Figure 1.)
12
Figure 8Proposed orogen boundaries (green lines) and new zone boundaries (black lines) in the Murray–Riverina region overlain on a greyscale image of First Vertical Derivative of Total Magnetic Intensity.
June 2005 Quarterly Notes
Figure 9Profi les of Total Magnetic Intensity data (nT) in black, and Bouguer gravity data (µms-²) in red for Traverses A-A1 (Figure 9a) and B-B1 (Figure 9b). (Traverse locations shown as red lines on Figure 8.)
Tabberabbera ZoneThe Tabberabbera Zone extends from Victoria northwestwards into New South Wales (VandenBerg et al 2000), where it occupies the southeastern part of the Murray–Riverina survey area (Figure 8). Interpretation of the new airborne magnetic data in New South Wales, suggests that the Tabberabbera Zone can be followed as far north as a large, interpreted middle Devonian granite (Figures 2, 3 & 4) 30 km south of Hay. The Tabberabbera Zone is characterised by Ordovician and Silurian sedimentary rocks with low magnetic amplitude, consistent with fi ndings in Victoria (VandenBerg et al 2000). The boundary with the Melbourne Zone to the south is based on the VandenBerg et al (2000) interpretation of the eastern part of the Governor Fault Zone. The western boundary that lies against the Bendigo Zone is less clear. In contrast to the interpretations of VandenBerg et al (2000) and Willman et al (2002), this study suggests that the Heathcote Fault, which marks the eastern boundary of the Bendigo Zone in Victoria extends north, truncates the Governor Fault and extends into New South Wales, as best shown in Figure 6. The eastern boundary of the Tabberabbera Zone, the Bootheragandra Fault, is an extension of the Paddington Line to the north (Glen et al 1996) and the Kancoona Fault (VandenBerg et al 2000) to the south.
Structural Zones — geophysical data
profi lesThe location of the magnetic and gravity profi les across the interpreted zone boundaries are shown in Figure 8 (a–a1 & b–b1). Figure 9a shows a change in magnetic character from low to high between the Stawell Zone and the Hay–Booligal Zone. It coincides with a gravity low and indicates a granite body. The boundary between the Stawell Zone and the northern part of the Bendigo Zone in New South Wales (Figure 9b) is clearer on the magnetic profi le than on the gravity profi le. Geological units of the Tabberabbera Zone are evident as a distinctly lower-amplitude magnetic response compared to the magnetic intrusive units of the Hay–Booligal Zone (Figure 9b). The boundary between the Omeo Zone and the Hay–Booligal Zone (Figures 9a & 9b) is overlain by the higher magnetic peaks of an interpreted large I-type granite that also has a corresponding gravity low. The distinctly higher magnetic responses of the Hay–Booligal Zone in Figure 9b refl ect the presence of a granite (east) and an igneous centre (west). The western extent of the Omeo Zone in both Figure 9a and Figure 9b marks the approximate location of the Bootheragandra Fault.
(a)
(b)
15 14
CONCLUSIONS AND IMPLICATIONSInterpretation of the new Murray–Riverina airborne geophysical survey data provides two new insights into a key part of the Tasmanides of eastern Australia. Firstly, it suggests that the Stawell Zone, part of the Delamerian Orogen, extends north and east into western New South Wales and as far east as the Bootheragandra Fault. Secondly, it suggests that the Bendigo Zone extends from Victoria northwards into New South Wales. As a consequence, there seems to be little evidence of the Governor Fault extending west-northwest south of Swan Hill. This has implications for the position of the Baragwanath Transform of VandenBerg et al (2000) and Willman et al (2002), which was inferred to be a Silurian–Devonian continental scale fault along which the eastern and central parts of the Lachlan Orogen in New South Wales underwent >500 km southwards transport with respect to the Delamerian Orogen and the Stawell and Bendigo Zones in western Victoria.
From an economic point of view, extension of the gold-rich Stawell and Bendigo Zones into New South Wales raises the mineral prospectivity of this part of the State.
ACKNOWLEDGMENTSThanks to John Whitehouse, Llew Cain, Rob Barnes and John Watkins for their contributions during and after a workshop conducted to gain an initial interpretation of the data. Thanks to Roger Cameron for his contributions to a better understanding of the geology of the region and to technical editor Richard Facer and geological referee Ian Hone who significantly improved this paper.
Published with the approval of the Deputy Director-General, New South Wales Department of Primary Industries.
REFERENCES
Brown C.M. & Stephenson A.E. 1991. Geology of the Murray Basin southeastern Australia. Bureau of Mineral Resources, Bulletin 235, 430 pp.
Cameron R.G. 1996. Pooncarie 1:250 000 geological map, SI/54-8. Geological Survey of New South Wales, Sydney.
Cameron R.G. 1997. Booligal 1:250 000 geological map, SI/55-5. Geological Survey of New South Wales, Sydney.
Cayley R.A. & Taylor D.H. 1996. Geological evolution and economic potential of the Grampians area, Victoria. In Hughes M.J., Ho S.E. & Hughes C.E. eds. Recent Developments in Victorian Geology and Mineralisation. Australian Institute of Geoscientists, Bulletin 6, 11–18.
Foster D.A., Gray D.R. & Bucher M. 1999. Chronology of deformation within the turbidite-dominated Lachlan orogen: Implications for the tectonic evolution of eastern Australia and Gondwana. Tectonics 18, 452–485.
Fugro Airborne Surveys. 2005. Murray–Riverina magnetic / radioelement geophysical survey for NSW DMR, acquisition and processing report. Geological Survey of New South Wales, File GS2005/323 (unpublished).
Geological Survey of New South Wales. 2003. Murray / Riverina geophysical data, grids & imagery – survey area Y-025. Department of Mineral Resources, Sydney. (More information at http://www.minerals.nsw.gov.au/prodServices/mapsDigitalData/geophysData2).
Geological Survey of Victoria. 1999. Murray Basin gridded airborne geophysics August 1999. Department of Natural Resources and Environment, Melbourne. (More information at www.dpi.vic.gov.au/dpi/nremp.nsf).
Glen, R. A. 2005. The Tasmanides of Eastern Australia: 600 million years of interaction between the proto-Pacific plate and the Australian sector of Gondwana. In Vaughan, A.P.M., Leat P.T. & Pankhurst R.J. eds. Terrane Processes at the Margins of Gondwana. Geological Society Special Publication 246, 23–96.
Glen R.A., Clare A. & Spencer R. 1996. Extrapolating the Cobar Basin model to the regional scale: Devonian basin-formation and inversion in western New South Wales. pp 43–83. In W.G. Cook, A.J.H. Ford, J.J. McDermott, P.N. Standish, C.L. Stegman & T.M. Stegman eds. The Cobar mineral field–a 1996 perspective. Australasian Institute of Mining and Metallurgy. Spectrum Series 3/96, 446 pp.
June 2005 Quarterly Notes
Gray D.R. 1988. Structure and tectonics. pp 1–36. In Douglas J.G. & Fergusson J.A. eds. Geology of Victoria. Geological Society of Australia, Melbourne, 663 pp.
Gray D.R. & Willman C.E. 1991a. Deformation in the Ballarat Slate Belt, central Victoria, and implications for crustal structure across southeast Australia. Australian Journal of Earth Sciences 38, 171–201.
Gray D.R. & Willman C.E. 1991b. Thrust-related strain gradients and thrusting mechanisms in a chevron-folded sequence, southeastern Australia. Journal of Structural Geology 13, 691–710.
Gunn P. 2004. Interpretation of gravity and magnetic data over the Oaklands Basin, NSW. Geological Survey of New South Wales, Report GS2004/134 (unpublished).
Hallett M.S., Vassallo J.J. & Glen R.A. 2004. Riverina Bioregion Pre-Permian Geology–Geophysical interpretation 1:500 000 map. Geological Survey of New South Wales, Sydney.
Hallett M.S. & Webster S. 2004. Overview interpretation of the Murray–Riverina Exploration NSW airborne magnetic/radiometric survey and regional Bouguer gravity. Australian Society of Exploration Geophysicists & Petroleum Exploration Society of Australia (NSW), 17th Geophysical Conference and Exhibition, Sydney. Extended abstracts (published on CD-ROM).
Kellett J.R. 1989. The Ivanhoe Block—its structure, hydrogeology and eff ect on groundwaters of the Riverine Plain of New South Wales. BMR Journal of Geology & Geophysics 11, 333–353.
Kingham R.A. 1998. Geology of the Murray–Darling Basin-simplifi ed lithostratigraphic groupings. Australian Geological Survey Organisation, Record 1998/21 (unpublished).
Matti N. & Moffi tt R.S. 2001. Well completion report DM Mossgiel DDH-1. Geological Survey of New South Wales, Report GS2002/849 (unpublished).
Miller J., Phillips D., Wilson C. & Dugdale J. 2003. 40Ar/39Ar dating in western Victoria: implications for the evolution of the Lachlan and Delamerian orogens. Geological Society of Australia, Abstracts 72, 73.
Miller J.M., Phillips D., Wilson C.J.L. & Dugdale L.J. 2004. A new tectonic model for the Delamerian and western Lachlan orogens. Geological Society of Australia, Abstracts 73, 174.
Moffi tt R.S. 2001. Well completion report DM Wilkurra DDH1. Geological Survey of New South Wales, Report GS2001/202 (unpublished).
Pearson P. 2003. Darling Basin SEEBASE project, 2003. SRK Project Code MR701.
Pogson D.J. 1972. Geological map of New South Wales, scale 1:1 000 000. Geological Survey of New South Wales, Sydney.
Scheibner E. & Basden H.E. 1996. Geology of New South Wales–Synthesis. Volume 1 Structural Framework, Geological Survey of New South Wales, Sydney, 295 pp.
Simons B.A. & Moore D.H. 1999. Victoria 1:100 000 pre-Permian geology. Geological Survey of Victoria, Melbourne.
Sniffi n M.J. 1985. Petroleum data package Murray Basin New South Wales. Geological Survey of New South Wales, Report GS1985/008 (unpublished).
VandenBerg A.H.M., Willman C.E., Maher S., Simons B.A., Cayley R.A., Taylor D.H., Morand V.J., Moore D.H. & Radojkovic A. 2000. The Tasman Fold Belt System in Victoria. Geological Survey of Victoria, Special Publication, 462 pp.
Webster S. 2004. Riverina Bioregion Study. Report on analysis of regional geophysical data. Geological Survey of New South Wales, File GS2004/135 (unpublished).
Wilde S.A. 2000. SHRIMP U–Pb dating of four “granite” samples from the Wilkurra No 1 and Mossgiel No 1 boreholes, NSW. Isotope Studies Group, Curtin University of Technology. Perth, Western Australia. Geological Survey of New South Wales, File GS2001/207 (unpublished).
Willman C.E., VandenBerg A.H.M. & Morand V.J. 2002. Evolution of the southeastern Lachlan Fold Belt in Victoria. Australian Journal of Earth Sciences 49, 271–289.
Wynne P. & Bacchin M. 2004. Gravity point located data for the Australian region.
NSW Department of Primary Industries — Mineral Resources Division516 High Street, Maitland NSW 2320PO Box 344 Hunter Region Mail Centre NSW 2310. T: 1300 736 122 T: (02) 4931 6666
www.dpi.nsw.gov.au
5911
04/
05
