N S W D P I Quarterly Notes Geological Survey of New South Wales

April 2005 No 118 Murray–Riverina region: an interpretation of bedrock Palaeozoic geology based on geophysical data ABSTRACT Interpretation 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. INTRODUCTION CONTRIBUTORS The Murray–Riverina airborne magnetic/radiometric/digital elevation survey (Figure 1) was fl own in early 2003 as a major Michael Hallett New South Wales Government Exploration NSW initiative. Geological Survey of New South Wales, The aim of the survey is to encourage mineral exploration in NSW Department of Primary Industries southwestern New South Wales which is covered by Cainozoic [email protected] sediments. The survey data was also acquired to assist a whole- Jeff rey Vassallo of-government study in the Riverina Bioregion and was co- Geological Survey of New South Wales, funded by the Resource and Conservation Assessment Council. NSW Department of Primary Industries Data acquisition jeff [email protected] Richard Glen The new survey covered approximately 63 000 square km and involved the acquisition of over 160 000 line km of data Geological Survey of New South Wales, (Fugro Airborne Surveys 2005). It covered all the Booligal NSW Department of Primary Industries 1:250 000 map sheet area and parts of the Hay, Deniliquin, [email protected] Bendigo, Swan Hill, Balranald, Pooncarie, Manara and Steve Webster Ivanhoe 1:250 000 map sheet areas. The airborne survey lines Steve Webster Pty. Ltd were flown east–west at an interline spacing of 400 m and [email protected] 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 Papers in Quarterly Notes are subject to external review. additional data provided in the central part of the region by External reviewer for this issue was Ian Hone. His assistance detailed road traverses (1.0 km station spacing) and in the is appreciated. northwest by a nominal 4 km x 4 km grid. In Victoria, the Quarterly Notes is published to give wide circulation to gravity stations are at a nominal 1.5 km x 1.5 km grid. 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 GEOLO GICAL FRAMEWORK Th e survey area (Figure 1) is larg el y covered by th e R i ver ina Plain o f th e eastern Murray Basin . Desp i te poo r o u tcro p an d CONTENT S spar s e dr illh o l e dat a, p rev io us wo r ker s have est ab lish ed th e fo ll ow in g b roa d st ratigrap hy. C ain ozoic a eo lian, flu v ial an d la cust r in e d eposi ts (B row n & Step h ens o n 19 91; C am ero n 19 96, 19 97 ). ABSTRACT 1 Murray Basin s edim ents. Kin gham (19 9 8) summar is ed th e INTRODUCTION 1 g eo l ogy o f th e Murray Basin, w hich co nt ains C ain ozoic Data acquisition 1 s edim ents related to three d eposi tio nal s equ en ces in th e Tertiary. D r illh o l e dat a d efin e up to a few hun dred m et res GEOLOGICAL FRAMEWORK 2 o f Tertiary an d Quaternary san dsto n es an d co n gl o m erates (Kell ett 19 89). GEOPHYSICAL INTERPRETATION 2 Permian–C ar bo ni fero us infrabasins (Pear s o n 2003). Th e Bendigo Zone 5 Oaklan ds Basin (graben) w hich li es directl y s o u th east o f th e survey area (Figure 1) co nt ains Permian st rat a up to 1300 m Tabberabbera Zone 5 thick. Hay–Booligal Zone 5 Limi ted o u tcro ps o f E ar l y an d Late Devo nian st rat a Stawell Zone 7 rest r icted in th e n o rth o f th e survey area (C am ero n 19 97 ). Devo nian infrabasins (Sni ffin 19 85) inferred larg el y o n th e GEOPHYSICAL MODELLING 7 interp ret atio n o f grav i ty l ows as s edim ent ary t ro u ghs. Granitic bodies and contact metamorphic aureoles 7 Silur ian to C ar bo ni fero us grani tes just n o rth o f th e Murray NEW INTERPRETATION OF BASEMENT R i ver (Van d enBerg et al 2000) an d grani te at M oss gi el w i th STRUCTURAL FRAMEWORK 9 an inferred emp la cem ent a g e o f ap p rox imatel y 425 Ma (M offitt 2001). Stawell Zone 9 Ord ov ician m et a- s edim ent ary rocks ~ 80 km east- s o u th east Bendigo Zone 9 o f Denili quin (Pogs o n 1972) an d thus east o f th e stu dy area. Hay–Booligal Zone 10 GEOPHYSICAL INTERPRETATION Tabberabbera Zone 14 Th e bas em ent interp ret atio n d es cr ibed in this paper reli es Structural Zones — geophysical data profiles 14 h eav il y o n n ew air bo rn e ma gn etic dat a m erg ed w i th th e air bo rn e ma gn etic dat a fro m Victo r ia ( Geo l ogic al Survey o f CONCLUSIONS AND IMPLICATIONS 15 Victo r ia 19 9 9) (Figures 2 & 6) , in co njun ctio n w i th ex istin g ACKNOWLEDGMENTS 15 grav i ty dat a (Wynn e & Ba cchin 2004) (Figures 3 & 4) an d ext rapo latio n o f kn ow n g eo l ogy fro m Victo r ia. Usin g REFERENCES 15 di fferen ces in patterns an d lin ear i ty o f ma gn etic an d grav i ty features , th e Murray –R i ver ina regio n has been di v id ed into six st ru ctural zo n es (Figures 2, 3 & 4) that are interp reted to refl ect di fferen ces in Pala eozoic g eo l ogy. Th es e in clu d e a number o f un ex pos ed grani te bodi es , s edim ent ary basin an d cluster s o f int rusi ve p ipes. Interp ret atio n o f th e ma gn etic fab r ic enab l ed th e extensio n o f th e St awell, Ben dig o an d Tab berab bera zo n es recognis ed in Victo r ia, n o rthw ards into N ew S o u th Wal es. This in turn has all owed a n ew tecto nic interp ret atio n o f this b ur i ed part o f th e Tasmanid es. A g eo p hy sic al interp ret atio n map o f th e R i ver ina Bio regio n Pre-Permian g eo l ogy is availab l e at a map s c al e o f 1:500 000 (Hall ett et al 2004).

T he information con taine d in this publication is ba s e d on knowle dge and understanding at the time of wr iting (June 2005). How ev er, becau s e of adv ances in knowle dge, u s ers are reminde d of the nee d to en sure that information upon which they rely is up to date and to check c urrency of the information w ith the appropr iate officer of New South Wales Departmen t of P r imary Indu str ies or the u s er’ s independen t adv is er. June 2005 Quarterly Notes MANARA IVANHOE NYMAGEE DARLING BASIN

Ivanhoe Lake Cargelligo

BOOLIGAL CARGELLIGO POONCARIE

Hatfield Booligal

NEW SOUTH W ALE S

Hay BALRANALD HAY NARRANDERA

Balranald MURRAY BASIN

OAKLANDS BASIN

Swan Hill SWAN HILL DENILIQUIN JERILDERIE

Murray RiverDeniliquin VICT ORI A

0 100 km

ST ARNAUD BENDIGO WANGARATTA

AIRBORNE GEOPHYSICAL COVERAGE REFERENCE Broken N.S. W. Hill Murray-Riverina, 2003 River and lake STUDY AREA Road Hay Sydney Older data Canberra Locality To be flown in 2005 State border VIC. 2005-05-0082

FigureFigure 1  LLocationocation ddiagramiagram ofof thethe Murray–RiverinaMurray–Riverina airborneairborne geophysicalgeophysical surveysurvey (brown)(brown) andand thethe adjacentadjacent NewNew SouthSouth WalesWales airborneairborne geophysical surveys (olive).

2 3 Lake Ivanhoe x x x Cargelligo x x x x x x Mossgiel x x Granite x x x OMEO NEW SOUTH WA LE S ZONE

Hatfield x Booligal B

x o o

t

h

e

r a

x HAY-BOOLIGAL g

a ZONE n x dr ra

F a u l t Hay

Boundary Bend x Balranald Granite x x x x x x x x x x x x x x x x x x x A v x o x x

c a x x x

F x x a u l t Swan Hill TABBERABBERA STAWELL x ZONE x Deniliquin ZONE Lake Boga x x Granite VICT ORI A x x x x H x x x x x x x ea x x x th x x x x x x x co x BENDIGO te x x x x x x

ZONE x x x x Faul x t nor F Gover a x x x u lt MELBOURNE ◊ x ZONE

0 100 km REFERENCE River and lake Locality Road

x State border Zone boundary x x Granites Cross section (See Figure 5)

2005-05-0083

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.

June 2005 Quarterly Notes Be n d i g o Zo ne Immediat ely south o f Denili quin, a larg e east– west t ren din g ma gn eti c l ow st retchin g over 100 km, is interp reted to be a Ma gnetic da ta grani te body (Sim o ns & M oo re 19 9 9). Th e main feat ures in th e ma gn eti c ima g e (Fi gures 2) o f th e Gr avity da ta Ben di g o Zo n e c an be summar is ed into two types. Th e Tab berab bera Zo n e in N ew S o u th Wal es is un d er lain 1. Curv ilin ear n o rth-n o rthwest t ren ds are interp reted by a b roa d grav i ty hi gh t ren din g ap p rox imatel y east– west to b roa dl y refl ect th e st r ike-parall el hi gh er an d l ower (Fi gures 3 & 4). A n o rth east- t ren din g grav i ty l ow s o u th o f Hay, ma gn eti te co ntent o f mu dsto n e- san dsto n e r i ch pa ckets o f co rrespo n ds to th e ma gn eti c l ow dis cuss ed above an d is Ord ov i cian t ur b i di tes (respecti vel y). Th ey m ost likel y refl ect interp reted to be c aus ed by a grani te. Imm ediatel y s o u th o f li th o l ogi c al fab r i c . This zo n e is th e n o rth ern extensi o n o f Denili quin is a d eep grav i ty l ow that co rrespo n ds to a larg e, th e Ben di g o Zo n e in Vi cto r ia that is d o minated by E ar l y – map ped grani te body an d als o has a co rrespo n din g l ow M i d dl e Ord ov i cian t ur b i di tes (Van d enBerg et al 2000). Th e ma gn eti c si gnat ure (dis cuss ed above). A narrow grav i ty l ow ma gn eti c si gnat ure o f th es e uni ts is overp r inted at th eir exten din g n o rth fro m Denili quin (Fi gure 3) may rep res ent a n o rth ern extent by th e ma gn eti c si gnat ures o f a number o f narrow t ro u gh o f Tertiary s edim ents. interp reted grani te bodi es. 2. Ellipti c al to circular ma gn eti c l ow s , interp reted as S- H ay –Bo o li g al Zo ne type grani tes fr in g ed by hi ghl y ma gn etis ed co nt a ct Th e Hay –Boo li gal Zo n e is a n ew l y d efin ed st ru ct ural zo n e in m et am o rp hi c aureo l e rocks. Th e interp reted grani te th e cent ral part o f th e survey area. Th ere is s o m e un cert ainty bodi es are el o n gated in a n o rth–s o u th directi o n . Th e very abo u t th e a ct ual posi ti o n o f th e bo un dar i es o f this zo n e. Th e distin cti ve ma gn eti c si gnat ure cl ear l y i d enti fi es s even n o rthwestern bo un dary is draw n al o n g th e s o u th ern si d e o f grani te int rusi o ns in this part o f th e survey area, w i th s everal a n o rthwest t ren din g lin e o f ma gn eti c l ow s that are inferred grani te bodi es int ru din g th e m et as edim ents (Fi gure 2). to rep res ent Silur ian-Devo nian S- type grani tes (Fi gures 2 & 4). Gr avity da ta Al ternati vel y, th e bo un dary co ul d be draw n al o n g th e n o rth ern si d e o f th es e int rusi ve bodi es , o r even thro u gh th em . Th e G rav i ty l ow s over li e s o m e o f th e grani te bodi es o u tlin ed s o u th ern bo un dary a gainst th e Ben di g o Zo n e is draw n at th e fro m th e a ero ma gn eti c dat a dis cuss ed above (Fi gures 3 & 4). n o rth ern ed g e o f th e p ro min ent NNE- t ren din g ma gn eti c grain Th e shape an d amp li t u d e o f th es e an o mali es is similar to th e o f that zo n e. A s o u th er l y po intin g sali ent o f th e Hay -Boo li gal ma gn eti c an d grav i ty l ow s over th e Lake Boga G rani te w hi ch Zo n e between th e Ben di g o an d Tab berab bera zo n es co nt ains li es imm ediatel y s o u th o f Lake Boga an d Sw an Hill in n o rth ern i gn eo us rocks that co ul d rep res ent m o re d efo rm ed ver si o ns o f Vi cto r ia (Fi gures 3 & 4) , p rev i o usl y map ped by Sim o ns an d grani tes int ru din g th e Ben di g o Zo n e. Th e s o u th ern bo un dary M oo re (19 9 9). a gainst th e Tab berab bera Zo n e is draw n al o n g th e n o rth er n Tab be rab be ra Zo ne ed g e o f a larg e grav i ty l ow inferred to rep res ent a maj o r grani ti c body. Ma gnetic da ta Ma gnetic da ta The southeast ern part of the survey area c overs the northern Th e ma gn eti c chara cter o f th e Hay –Boo li gal Zo n e refl ects c ontinuation of the Tabberabbera Z one in Vict oria which d eep l y b ur i ed bas em ent s o urces that are interp reted to be is dominat ed by Early Silurian turbidit es in the west and Ordovician turbidit es in the east (V andenBerg et al 2000). i gn eo us rocks du e to th eir irregular an d el evated ma gn eti c A broad magnetic high trending west-southwest t o east- si gnat ures. Over l yin g th e bas em ent is up to 1700 m o f E ar l y northeast in the c entre of this zone indicat es magnetic units, Devo nian s edim ent ary rocks (Matti & M o ffi tt 2001). Th e nat ure possibly I-type magnetic granit es, at depth. o f th e bo un dary w i th th e Ben di g o Zo n e to th e s o u th is n ot cl ear, bec aus e th e grani te bodi es an d th eir aureo l es overp r int Approximat ely 30 km southeast of Hay (Figure 2), a broad th e n o rth ern extents o f th e t ur b i di tes o f th e Ben di g o Zo n e. Th e ( 40 km wide) northeast–southwest trending magnetic low st rati grap hi c layer in g ev i d ent in th e Ben di g o Zo n e, o bs erved (100 km long), is int erpret ed t o be caused by a large granit e in th e ma gn eti c ima g ery (Fi gure 2) , is n ot ev i d ent in th e Hay – body. The boundary between the Tabberabbera Z one and Boo li gal Zo n e. the Hay–Booligal Z one t o the north is obscured by this body. The low magnetic signature is identical t o the Middle Two ma gn eti c l ow s (w i th co in ci d ent grav i ty l ow s) imm ediatel y Devonian granit es mapped on the border with Vict oria east an d west o f Hay, t ren d n o rth-n o rthwest fo r ap p rox imatel y (Simons & Moore 1999). Gunn ( 2004) suggest ed that the 20 km (an o mal y axes mar ked as b la ck dash ed lin es o n northeast ern margin of this magnetic low was due t o granit e Fi gures 2, 3 & 4). Th es e are interp reted to be s edim ent ary at depth. t ro u ghs o r grabens an d may be fill ed w i th Permian an d yo un g er st rat a.

4 5 Wentworth gravity low

Wilkurra DDH–1 Lake Ivanhoe x x x Cargelligo x x x x x x Mossgiel DDH–1 Mossgiel x x Granite x x x OMEO NEW SOUTH W ALE S ZONE

Hatfield x Booligal B

x o o

t

h

e

r a

x HAY-BOOLIGAL g

a ZONE n x d ra

F a u l t Hay

Boundary Bend x Balranald Granitex x x x x x x x x x x x x x x ◊ x x x A v ◊ o x x

c a x x x

F x x a u l t Swan Hill TABBERABBERA STAWELL x ZONE x Deniliquin ZONE Lake Boga x x Granite VICT ORI A x x x ◊ H x x x x x x x ea x x ◊ th x x x x x x x co x BENDIGO te x x x x x x

ZONE x x x x Faul x t nor F Gover a x x x u lt MELBOURNE ◊ x ZONE

0 100 km REFERENCE River and lake Locality Road

x State border Zone boundary x x Granites Cross section (See Figure 5)

2005-05-0085

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.)

June 2005 Quarterly Notes Magnetic highs (approximately 200 nT above a nominal One of these lows, 30 km west of Balranald, correlates with background of 54 800 nT) in the northern and eastern part of the “Boundary Bend Granite” (Figures 2, 3 & 4), an S-type the Hay–Booligal Zone (Figure 2) are interpreted to represent Silurian–Devonian granite (Simons & Moore 1999). The deeply buried igneous complexes. An irregularly shaped eastern edge of this granite appears to be faulted. The other magnetic complex with high frequency signature located magnetic lows in this area are interpreted to be more S-type approximately 50 km northeast of Booligal, coincides in part Silurian–Devonian granites. with the mapped Early Devonian volcanic units of the Hillston 3. East–west magnetic trends with truncations and folds in Volcanic Complex (Cameron 1997). The extent of the magnetic the northeastern part of the Stawell Zone correspond to anomaly implies that the volcanic complex extends under inferred extensions of outcrops of the Early Devonian Cobar cover to the south and to the west of the mapped outcrop. Supergroup and of the late Early–Late Devonian Mulga Gravity data Downs Group mapped to the north by Cameron (1997). The Hay–Booligal Zone includes a broad gravity high, fl anked Gravity data to the northwest by the Booligal gravity low and a shallow The Stawell Zone is characterised by a northeast-trending north-northwest-trending gravity low through Hay (anomaly gravity high, lying directly northwest of a string of gravity lows axes marked as black dashed lines on Figure 3). This low saddle interpreted as granite bodies (Figure 3). The circular gravity low in the high coincides with two magnetic troughs and may 30 km west of Balranald coincides with the mapped “Boundary represent sedimentary troughs or grabens, as suggested above. Bend Granite” (Figures 2, 3 & 4), which is also defi ned as a The Booligal gravity low, located between Ivanhoe and magnetic low. Booligal, straddles the boundary between the Hay–Booligal A large gravity low in the extreme northwest of the region Zone and the Stawell Zone (Figures 3 & 4). The northern is interpreted to correspond to the eastern part of the deep section of this oval shaped gravity low lies within the Stawell (approximately 4 km) Wentworth Trough (Figures 3 & 4). Zone, immediately southwest of Ivanhoe (Figures 3 & 4) and is However, there is accumulating evidence that the gravity interpreted to be granite at depth. Drilling during hydrocarbon low over the Wentworth Trough may, at least in part, refl ect exploration (Mossgiel DDH–1) confi rmed that the coincident basement granite at depth rather than Devonian sedimentary gravity and magnetic lows are due to a large granite body rocks. Just east of the Wentworth Trough, Planet drill hole — the Mossgiel Granite (Matti & Moffi tt 1999) as shown on Nambucurra 1 (142° 50’ E / 33° 20’ S) bottomed in 16.5 m of Figures 2, 3 and 4 which was dated 427Ma ±7 (Wilde 2000). granite, dated at c. 415 Ma (Moffi tt 2001). The Department of Overlying the Mossgiel Granite is the Booligal Trough with up Mineral Resources (now DPI) hole DM Wilkurra DDH1 on the to 1500 m of sedimentary rocks inferred to consist of Devonian eastern margin of this trough (142° 54’ E / 33° 04’ S) penetrated Winduck Group and overlying Mulga Downs Group equivalent almost 400 m of the Wilkurra Granite. It was estimated to have (Matti & Moffi tt 1999). an emplacement age of 425-420 Ma (Moffi tt 2001), beneath Stawell Zone rocks correlated with the Early Devonian Cobar Supergroup and the late Early- Late Devonian Mulga Downs Group. Magnetic data GEOPHYSICAL MODELLING The Stawell Zone covers the western and northern parts of the Murray–Riverina region. The zone extends from west of Granitic bodies and contact metamorphic Balranald, northeast through Hatfi eld and north and east of aureoles Ivanhoe, wrapping around the northern margin of the Hay- Booligal Zone. The boundary between the Stawell Zone and Magnetic and gravity modelling was carried out to confi rm the Hay–Booligal Zone lies on the eastern side of a line of the interpretation that major gravity and magnetic lows in oval-shaped magnetic lows that are interpreted to be Silurian– the region represent granite bodies (Hallett & Webster 2004). Devonian S-type granites (Figures 2 & 4). The grid data modelling along the east–west (red) line in Figures 2, 3 and 4, indicate that the oval magnetic and gravity There are three major magnetic features of the Stawell Zone in lows were consistent with the presence of large, low-magnetic, the Murray–Riverina region. low-density (2.6 g/cm³) central plutons with higher density, 1. A magnetically quiet area in the south, with magnetic magnetised contact metamorphic aureoles in the surrounding features extending north of Hatfi eld showing magnetic metasediments. These geophysical responses and oval fabric striking northeast (Figure 2) and then east, north of geometries are consistent with bodies modelled by Webster Ivanhoe. (2004) as S-type granites at depth (1 to 2 km). 2. Discrete circular to elliptical, magnetic lows embedded Additional modelling of these anomalies using magnetic line in the background fabric of the feature described above, data (fl ight line 10630 located at approximately 6 114 000N) best seen approximately 50 km northwest of Balranald. was completed using the inversion processes of Encom

6 7 Wentworth gravity low

Wilkurra DDH–1 Lake Ivanhoe x x x Cargelligo x x x x x x Mossgiel DDH–1 Mossgiel x x Granite x x x OMEO NEW SOUTH W ALE S ZONE

Hatfield x Booligal B

x o o

t

h

e

r a

x HAY-BOOLIGAL g

a ZONE n x d ra

F a u l t Hay

Boundary Bend x Balranald Granite x x x x x x x x x x x x x x x x x x x A v x o x x

c a x x x

F x x a u l t Swan Hill TABBERABBERA STAWELL x ZONE x Deniliquin ZONE Lake Boga x x Granite VICT ORI A x x x ◊ H x x x x x x x ea x x ◊ th x x x x x x x co x BENDIGO te x x x x x x

ZONE x x x x Faul x t nor F Gover a x x x u lt MELBOURNE ◊ x ZONE

0 100 km REFERENCE River and lake Locality Road

x State border Zone boundary x x Granites Cross section (See Figure 5) 2005-05-0084

FigureFigure 4 CombinedCombined imageimage ofof BouguerBouguer gravitygravity (pseudocolour)(pseudocolour) overlainoverlain onon sunshadedsunshaded imageimage (85°(85° elevationelevation atat 90°90° azimuth)azimuth) ofof TotalTotal 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.)

June 2005 Quarterly Notes Technology’s ModelVision Pro and Automag (Figure 5) modelling Stawell Zone programs. Figure 5a shows the Total Magnetic Intensity (TMI) profi le (black) with the model profi le (red). Figure 5b shows In north–western Victoria, the Avoca Fault separates the a cross-sectional scan of the interpreted model blocks using Stawell Zone to the west from the Bendigo Zone to the east Automag. The pink and dark green clusters of blocks generated (Gray 1988). Regional magnetic trends in the Stawell Zone are by Automag indicate the location, depth and attitude of a truncated by this fault (VandenBerg et al 2000). The Stawell source. The cluster density, indicate narrow dyke sources or Zone is dominated by turbidites of the St Arnaud Group of the edges of a broad source with no clear distinction between probable Late Cambrian age (VandenBerg et al 2000). These sources. are intruded by Silurian-Devonian granites and pass into Cambrian volcanic rocks along the western edge of the zone In section, the clusters of blocks at approximately 208 000 E (VandenBerg et al 2000). and 220 000 E imply contact metamorphic aureole magnetic anomalies around the edges of an interpreted The recently acquired New South Wales airborne magnetic granite (yellow block) and the gap between the blocks data suggests that the turbidites of the Cambrian St Arnaud defi nes the non-magnetic granite. A similar pattern is Group extend north into New South Wales. They, or their lateral observed just east of the interpreted granite (blue block) equivalents, swing to the east near Ivanhoe, wrap around centred at 228 000 E; the northern margin of the Hay–Booligal Zone (Figure 7) and extend east to the Bootheragandra Fault. The Bootheragandra The cluster of blocks in the vicinity of 265 000 E to 272 000 E Fault links northwards into the Paddington Line (Glen et extend to depth and refl ect dyke-like sources (dark purple al 1996) and southwards into the Kancoona Fault, that separates bodies) for the two sharp (approximately 100 nT) anomalies the Omeo and Tabberabbera zones in Victoria (VandenBerg et shown in the TMI profi le (Figure 5a); al 2000). The northern part of the Stawell Zone contains rocks Between 230 000 E and 242 000 E the cluster of blocks are of the Early Devonian Cobar Supergroup and Early to Late fl at dipping. They are modelled as two broad magnetic Devonian Mulga Downs Group deposited over the St Arnaud sources (dark purple blocks) centred at 232 000 E and Group equivalents. This suggests that the Stawell Zone was 236 000 E, and are interpreted as mafi c volcanic units; reactivated in the Devonian. The cluster of blocks at approximately 180 000 E, refl ect The tectonic affi nity of the Stawell Zone is not clear. Mapping in the edges of the broad sources modelled (two light green western Victoria by Cayley and Taylor (1996), suggested that the bodies) and confi rms the earlier interpretation of large low boundary between the Lachlan Orogen and the Delamerian magnetic source that may be due to a granite source. Orogen to the west was the Moyston Fault. As a result, the Stawell Zone was regarded as part of the Lachlan Orogen. Both Automag and block model interpretations give However, recent Ar–Ar age dating (Miller et al 2003) showed similar results. Depths to source of the order of 300+ m that cleavage micas in the Mooroondal Metamorphic Complex from interpretations of fl ight line data are 200 m shallower (the high-grade part of the St Arnaud Group in the hangingwall than obtained from the earlier modelling of the grid data of the Moyston Fault east of Stawell) had been deformed by (Webster 2004). This might be expected, as the fi ltering process the 500 Ma Delamerian Orogeny. As a result, Miller et al (2004) inherent in gridding lines at 400 m spacing, would smooth the regarded the Stawell Zone as being transitional between the data and result in deeper estimates of source depths. Delamerian and Lachlan Orogens. Alternatively, the Stawell NEW INTERPRETATION OF BASEMENT Zone could be considered part of the Delamerian Orogen unless there is a substantial break in the St Arnaud Group or STRUCTURAL FRAMEWORK within the Stawell Zone (Glen 2005). In this interpretation, the Reinterpretation of the Palaeozoic geology underlying the Avoca Fault and its curved extension continues northwards into Murray–Riverina region has prompted a re-examination of New South Wales along a line of granite bodies and a gravity the basement structural framework of this area and linkage low zone. This fault is considered to be the boundary between to the Palaeozoic geology of central Victoria. The new the Delamerian Orogen to the west and north, and the Lachlan geophysical interpretation implies that the Stawell, Bendigo Orogen to the east and south. The nature and location of the and Tabberabbera zones extend north from Victoria into the western and northern boundaries of the Stawell Zone are Murray–Riverina region of New South Wales. This has major outside the scope of this paper. implications for the mineral prospectivity in the southwestern part of New South Wales as the Stawell Zone hosts the rich Bendigo Zone Stawell gold deposits and the Bendigo Zone hosts the world Originally named the Bendigo–Ballarat Zone by Gray (1988), the class orogenic gold deposits of Bendigo, Ballarat, Castlemaine, Bendigo Zone consists of Early to Middle Ordovician turbidites among many others. The Stawell Zone produced >167 tonnes of the Castlemaine Group intruded by granites (VandenBerg et of gold by the year 2000 and over 1870 tonnes of gold has been al 2000). Deformation occurred mainly around the Ordovician– recovered from the Bendigo Zone (VandenBerg et al 2000). Silurian boundary (Foster et al 1999) and produced chevron

8 9 (a)

(b)

Figure 5  Magnetic profi les and models across proposed S-type granite bodies. (For the location of this traverse, see Figures 2, 3 & 4.)

folds, steeply west-dipping cleavage and mainly east-vergent acquired in 1980 at a survey altitude of 80 m (Geological Survey thrust faults that range in displacement from a few metres to of Victoria 1999) while the 2003 Murray–Riverina survey data kilometres (Gray & Willman 1991a, 1991b). The world-class gold were collected at an altitude of 60 m. deposits of Bendigo, Ballarat and other fi elds formed mainly This study now suggests that the Governor Fault does not during this Benambran Orogeny. extend west of the Melbourne Zone and is interpreted to be Most interpretations of the Bendigo Zone show it restricted truncated along the northwestern margin of that zone by the within Victoria, being bounded to the north by the west– Heathcote Fault. The Heathcote Fault can then be followed northwest-trending Murray River Lineament which broadly northwards into New South Wales as the western boundary of follows the trend of the Murray River (Scheibner & Basden 1996, the Tabberabbera Zone to increasingly greater cover thickness, fi gure 9.1), or by an extension of the Governor Fault west of the until it becomes indistinct between 50 km and 100 km north of northern tip of the Heathcote Fault (Figure 6). The northwest- the Victorian border (Figures 6 & 8). trending Governor Fault separates the Melbourne Zone from the Tabberabbera Zone to the east and contains Cambrian Hay–Booligal Zone ultramafi c–mafi c units in its hangingwall (VandenBerg The Hay–Booligal Zone (Figure 8) is characterised by interpreted et al 2000). The Governor Fault swings to the west near Silurian–Devonian felsic volcanic rocks and granites buried Dookie in Victoria, at the northern margin of the Melbourne beneath thick cover (including Devonian Cobar Supergroup Zone, and VandenBerg et al (2000) and Willman et al (2002), rocks) except for the northeastern corner, where the Early suggested that the fault can be extrapolated further to the Devonian Hillston Volcanic Complex were mapped by Cameron west–northwest under cover, to pass just south of the Murray (1997). These volcanic rocks have a high-frequency magnetic River, cutting off the Heathcote Fault and the Avoca Fault and response, which is obscured (probably due to deeper burial) passing under the Lake Boga Granite (Figure 6). The 2003 data to the south and to the west. The western edge of the Hay– shows no single through-going fault, or fault zone, that delimits Booligal Zone terminates near Balranald at the north-trending the Bendigo Zone along the path of the previously proposed magnetic grain of the turbidites and deformed granitoids of the Governor Fault. Instead, the airborne magnetic data show that Bendigo Zone to the south. The Bootheragandra Fault marks magnetic lineaments in the Ordovician turbidites continue from the eastern boundary of the Hay–Booligal Zone against the Victoria northwards into New South Wales (immediately east of Omeo Zone. This fault links into the Paddington Line to the the Lake Boga Granite) and cut across the previously proposed north and the Kancoona Fault to the south and are stitched location of the Governor Fault (Figure 6). Although the regional by a northeast-trending granite in the southeast corner of the magnetic fabric is not as evident in the Victoria data as in the zone. 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

June 2005 Quarterly Notes 35˚00'S

NE WS OUTH WA LES

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a

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Swan Hill t e

F

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u

l t 35˚30’S

A

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a TABBERABBERA Granite

F ZONE

a

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143˚30'E 144˚00'E 144˚30'E

REFERENCE

River and lake 0 20 km Locality

2005-05-0126 State border

Previously inferred location of the Governor Fault

Magnetic lineations

Figure 6Greyscale 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.)

10 11 Figure 7Greyscale 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.)

June 2005 Quarterly Notes Figure 8Proposed orogen boundaries (green lines) and new zone boundaries (black lines) in the Murray–Riverina region overlain on a Figuregreyscale 7Greyscale image of image First Vertical of the First Derivative Vertical of Derivative Total Magnetic of Total Intensity. 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 13 (a)

(b)

Figure 9Profi 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 Zone Structural Zones — geophysical data The Tabberabbera Zone extends from Victoria northwestwards profi les into New South Wales (VandenBerg et al 2000), where it occupies the southeastern part of the Murray–Riverina survey The location of the magnetic and gravity profi les across the area (Figure 8). Interpretation of the new airborne magnetic interpreted zone boundaries are shown in Figure 8 (a–a1 & b– data in New South Wales, suggests that the Tabberabbera b1). Figure 9a shows a change in magnetic character from low Zone can be followed as far north as a large, interpreted middle to high between the Stawell Zone and the Hay–Booligal Zone. Devonian granite (Figures 2, 3 & 4) 30 km south of Hay. The It coincides with a gravity low and indicates a granite body. The Tabberabbera Zone is characterised by Ordovician and Silurian boundary between the Stawell Zone and the northern part sedimentary rocks with low magnetic amplitude, consistent of the Bendigo Zone in New South Wales (Figure 9b) is clearer with fi ndings in Victoria (VandenBerg et al 2000). The boundary on the magnetic profi le than on the gravity profi le. Geological with the Melbourne Zone to the south is based on the units of the Tabberabbera Zone are evident as a distinctly VandenBerg et al (2000) interpretation of the eastern part of the lower-amplitude magnetic response compared to the magnetic Governor Fault Zone. The western boundary that lies against intrusive units of the Hay–Booligal Zone (Figure 9b). The the Bendigo Zone is less clear. In contrast to the interpretations boundary between the Omeo Zone and the Hay–Booligal Zone of VandenBerg et al (2000) and Willman et al (2002), this study (Figures 9a & 9b) is overlain by the higher magnetic peaks of an suggests that the Heathcote Fault, which marks the eastern interpreted large I-type granite that also has a corresponding boundary of the Bendigo Zone in Victoria extends north, gravity low. The distinctly higher magnetic responses of the truncates the Governor Fault and extends into New South Hay–Booligal Zone in Figure 9b refl ect the presence of a granite Wales, as best shown in Figure 6. The eastern boundary of the (east) and an igneous centre (west). The western extent of Tabberabbera Zone, the Bootheragandra Fault, is an extension the Omeo Zone in both Figure 9a and Figure 9b marks the of the Paddington Line to the north (Glen et al 1996) and the approximate location of the Bootheragandra Fault. Kancoona Fault (VandenBerg et al 2000) to the south.

June 2005 Quarterly Notes CONCLUSIONS AND IMPLICATIONS REFERENCES Interpretation of the new Murray–Riverina airborne geophysical Brown C.M. & Stephenson A.E. 1991. Geology of the Murray survey data provides two new insights into a key part of the Basin southeastern Australia. Bureau of Mineral Resources, Tasmanides of eastern Australia. Firstly, it suggests that the Bulletin 235, 430 pp. Stawell Zone, part of the Delamerian Orogen, extends north and east into western New South Wales and as far east as the Cameron R.G. 1996. Pooncarie 1:250 000 geological map, SI/54-8. Bootheragandra Fault. Secondly, it suggests that the Bendigo Geological Survey of New South Wales, Sydney. Zone extends from Victoria northwards into New South Wales. As a consequence, there seems to be little evidence of the Cameron R.G. 1997. Booligal 1:250 000 geological map, SI/55-5. Governor Fault extending west-northwest south of Swan Geological Survey of New South Wales, Sydney. Hill. This has implications for the position of the Baragwanath Transform of VandenBerg et al (2000) and Willman et al (2002), Cayley R.A. & Taylor D.H. 1996. Geological evolution and which was inferred to be a Silurian–Devonian continental scale economic potential of the Grampians area, Victoria. fault along which the eastern and central parts of the Lachlan In Hughes M.J., Ho S.E. & Hughes C.E. eds. Recent Orogen in New South Wales underwent >500 km southwards Developments in Victorian Geology and Mineralisation. transport with respect to the Delamerian Orogen and the Australian Institute of Geoscientists, Bulletin 6, 11–18. Stawell and Bendigo Zones in western Victoria. Foster D.A., Gray D.R. & Bucher M. 1999. Chronology of From an economic point of view, extension of the gold-rich deformation within the turbidite-dominated Lachlan Stawell and Bendigo Zones into New South Wales raises the orogen: Implications for the tectonic evolution of eastern mineral prospectivity of this part of the State. Australia and Gondwana. Tectonics 18, 452–485. ACKNOWLEDGMENTS Fugro Airborne Surveys. 2005. Murray–Riverina magnetic / Thanks to John Whitehouse, Llew Cain, Rob Barnes and John radioelement geophysical survey for NSW DMR, acquisition Watkins for their contributions during and after a workshop and processing report. Geological Survey of New South conducted to gain an initial interpretation of the data. Thanks to Wales, File GS2005/323 (unpublished). Roger Cameron for his contributions to a better understanding of the geology of the region and to technical editor Richard Geological Survey of New South Wales. 2003. Murray / Facer and geological referee Ian Hone who significantly Riverina geophysical data, grids & imagery – survey area improved this paper. Y-025. Department of Mineral Resources, Sydney. Published with the approval of the Deputy Director-General, (More information at http://www.minerals.nsw.gov.au/ New South Wales Department of Primary Industries. 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.

14 15 Gray D.R. 1988. Structure and tectonics. pp 1–36. In Douglas J.G. Miller J.M., Phillips D., Wilson C.J.L. & Dugdale L.J. 2004. A new & Fergusson J.A. eds. Geology of Victoria. Geological Society tectonic model for the Delamerian and western Lachlan of Australia, Melbourne, 663 pp. orogens. Geological Society of Australia, Abstracts 73, 174.

Gray D.R. & Willman C.E. 1991a. Deformation in the Ballarat Slate Moffi tt R.S. 2001. Well completion report DM Wilkurra DDH1. Belt, central Victoria, and implications for crustal structure Geological Survey of New South Wales, Report GS2001/202 across southeast Australia. Australian Journal of Earth Sciences (unpublished). 38, 171–201. Pearson P. 2003. Darling Basin SEEBASE project, 2003. SRK Gray D.R. & Willman C.E. 1991b. Thrust-related strain gradients Project Code MR701. and thrusting mechanisms in a chevron-folded sequence, southeastern Australia. Journal of Structural Geology 13, Pogson D.J. 1972. Geological map of New South Wales, scale 691–710. 1:1 000 000. Geological Survey of New South Wales, Sydney.

Gunn P. 2004. Interpretation of gravity and magnetic data over Scheibner E. & Basden H.E. 1996. Geology of New South Wales– the Oaklands Basin, NSW. Geological Survey of New South Synthesis. Volume 1 Structural Framework, Geological Survey Wales, Report GS2004/134 (unpublished). of New South Wales, Sydney, 295 pp.

Hallett M.S., Vassallo J.J. & Glen R.A. 2004. Riverina Bioregion Pre- Simons B.A. & Moore D.H. 1999. Victoria 1:100 000 pre-Permian Permian Geology–Geophysical interpretation 1:500 000 map. geology. Geological Survey of Victoria, Melbourne. Geological Survey of New South Wales, Sydney. Sniffi n M.J. 1985. Petroleum data package Murray Basin New Hallett M.S. & Webster S. 2004. Overview interpretation of the South Wales. Geological Survey of New South Wales, Report Murray–Riverina Exploration NSW airborne magnetic/ GS1985/008 (unpublished). radiometric survey and regional Bouguer gravity. Australian Society of Exploration Geophysicists & Petroleum Exploration VandenBerg A.H.M., Willman C.E., Maher S., Simons B.A., Cayley Society of Australia (NSW), 17th Geophysical Conference and R.A., Taylor D.H., Morand V.J., Moore D.H. & Radojkovic A. Exhibition, Sydney. Extended abstracts (published on CD- 2000. The Tasman Fold Belt System in Victoria. Geological ROM). Survey of Victoria, Special Publication, 462 pp.

Kellett J.R. 1989. The Ivanhoe Block—its structure, hydrogeology Webster S. 2004. Riverina Bioregion Study. Report on analysis of and eff ect on groundwaters of the Riverine Plain of New regional geophysical data. Geological Survey of New South South Wales. BMR Journal of Geology & Geophysics 11, 333– Wales, File GS2004/135 (unpublished). 353. Wilde S.A. 2000. SHRIMP U–Pb dating of four “granite” samples Kingham R.A. 1998. Geology of the Murray–Darling Basin- from the Wilkurra No 1 and Mossgiel No 1 boreholes, NSW. simplifi ed lithostratigraphic groupings. Australian Geological Isotope Studies Group, Curtin University of Technology. Survey Organisation, Record 1998/21 (unpublished). Perth, Western Australia. Geological Survey of New South Wales, File GS2001/207 (unpublished). Matti N. & Moffi tt R.S. 2001. Well completion report DM Mossgiel DDH-1. Geological Survey of New South Wales, Willman C.E., VandenBerg A.H.M. & Morand V.J. 2002. Evolution Report GS2002/849 (unpublished). of the southeastern Lachlan Fold Belt in Victoria. Australian Journal of Earth Sciences 49, 271–289. Miller J., Phillips D., Wilson C. & Dugdale J. 2003. 40Ar/39Ar dating in western Victoria: implications for the evolution of Wynne P. & Bacchin M. 2004. Gravity point located data for the the Lachlan and Delamerian orogens. Geological Society of Australian region. Australia, Abstracts 72, 73. 5911 04/05 5911

NSW Department of Primary Industries — Mineral Resources Division 516 High Street, Maitland NSW 2320 PO Box 344 Hunter Region Mail Centre NSW 2310. T: 1300 736 122 T: (02) 4931 6666 June 2005 Quarterly Notes www.dpi.nsw.gov.au