INTERPRETATION DOWN THE TRACK · More extensive Winduck Group ... Interpretation Down the Track, Investigating Geophysical Anomalies highlighted in Regional Interpretations for Hay-Balranald

INTERPRETATION DOWN THE TRACKInvestigating Geophysical Anomalies highlighted in

Regional Interpretations for Hay-Balranald and Cobham Lake map sheets

Rosemary Hegarty and Astrid Carlton

COBHAM LAKE 1:250 000 MAP SHEET• platform of shallow basement the “Tindara Zone”• modelling curious magnetic anomalies • visit to previously-unmapped basement outcrops.

LOUTH 1: 250 000 MAP SHEET• reconnaissance mapping in the Louth - Bourke area to understand the stratigraphy and extent of Cobar Supergroup rocks.

REGIONAL TECTONIC ELEMENTS

Geophysical-Geological Interpretation Map for Cobham Lake mapped this subsurface basement platform by extending the distribution of particular units

exposed on the Broken Hill sheet to the south.

TMI image shows major long wavelength features of high magnetic intensity along western side (deep intrusions?) and short wavelength features throughout most of the zone.

TINDARA ZONE• Revised depth to basement using BHP drilling, water bores and seismic data • Discovery of new bedrock exposures by Steve Hill (CRCLEME) in Teilta area

Tindara Zone

Smithville 1:100 000 Map Sheet

Teilta 1:100 000 Map Sheet

1VD TMI RTP Image

• exposures of gently warped, clastic metasediments

• low metamorphic grade with tension quartz veining

On TEILTA 1:100 000 MAP SHEET: basement outcrops were visited

• presence of drop-stones: < 1cm up to 50cm

• interpreted as an extension of theNeoproterozoic Yancowinna Subgroup

A single phase of exploration by BHP Minerals in 1992-1993 (unsuccessfully) sought Willyama Supergroup rocks (GS1993/164)

SMITHVILLE 1:100 000 map sheet:• has unusual magnetised features and major

structures (under cover)• may have exploration potential subject to

basement depth

Several sparse drillholes tested some magnetic highs and encountered mainly Adelaidean sedimentary units, with magnetic highs relating to basalt or dolerite.

1VD TMI RTP Image

BHP survey had flight line spacing of 300m 1VD TMI RTP Image

Gravity multiscale edge detection gradient contours overlain pseudocolour gravity image indicate deep density contrasts which are interpreted as the margins of large intrusions.

Unusual magnetic anomalies on SMITHVILLE 1:100 000 map sheet

PURPLE: as above, but extending out from magnetised intrusions

GREEN: discontinuous narrow horizons of moderate to high magnetic intensity, portions of which show strong reverse polarity due to remanent magnetisation (not similar in character to basalts further south): structure appears folded/complex

YELLOW: lower amplitude trends, discontinuous along structures and occurring within broad zones of low magnetic intensity

BLUE: small, discrete, moderate amplitude features: scattered in distribution (subsidiary fractures or maybe random?)

Guides to basement depth on Smithville 1:100 000 map sheet

• BHP exploration drillhole

• Bedrock logs from water bores

• Winnathee seismic survey 1970

• ModelVision depths

1VD TMI RTP Image

Remanent magnetisation

TMI (non-RTP)

TEN MILE BORE

GSNSW gathered a core tail at the base of Ten Mile water bore on Border Downs (GS Report 2010/0173)

It intersected moderately cleaved dark grey shale, interbedded with medium to coarse grained sandstone. Core contained quartz and carbonate veining, with galena and pyrite identified in fractures.

The bedrock platform under the Smithville 1:100 000 sheet deepens from

• approx 170 m at southern boundary, to

• approx 400m at the north- eastern edges of the Tindara Zone.

The highlighted features could be expected to lie around the 250 to 350 m depth below surface.

1VD TMI RTP Image

LOUTH MAP SHEET• reconnaissance mapping in the Louth - Bourke area to understand the stratigraphy and extent of Cobar Supergroup rocks.

Structural Framework of NSWScheibner and Basden 1996

THOMSON INTERPRETATION Building up a history…

402 Ma U-Pb SHRIMP

420 Ma U-Pb SHRIMP

More extensive Winduck Group • Wiltagoona Sandstone• underlying Mt Gunderbooka• underlying Mount Deerina• north of Darling River

Investigating outcrops and drill core/chipsfrom the Louth area (viewed looking north-northwest)

LOUTH

MT DEERINA

IONALout

h vo

lcanic

s unde

r cov

er

DEVONIANDETECTION DEVICE

Differentiating non-magnetic metasediments• Ordovician from Devonian units• Complex fold and thrust structures & onlap

-INTERPRETATION DOWN THE TRACK-

-An update on the geophysical/geological modelling in south-western NSW

SW NSW

Overview:Introduction to the geology/geophysics in

southwest NSWResults from ground surveys, near Euston,

over an anomaly interpreted as a diatremeResults of modelling from airborne TMI data for

two high-amplitude short-wavelength TMI anomalies southeast of Hay

Southwest NSW

Stawell Zone:Cambrian-Ordovician

turbidites and volcanic tuffs Hay-Booligal Zone:Silurian-Devonian sediments

over a Proterozoic? crystalline basement

Silurian-Devonian granodiorites

Silurian-Devonian granodioritesSilurian-Devonian granitesGreyscale tilt-filtered TMI image

Anomalies of InterestSouth-west NSW

A swath of possible diatremesIsolated volcanic intrusions

Possible Diatremes

over 500 anomaliesswath extents over 230 km

Possible Diatremes

Transparent pseudocolour TMI image over a greyscale Tilt-filtered TMI image

•normal and reversely magnetised sources•approximately 10% are reversely magnetised•peak TMI anomaly values range between 4-240 nT•TMI anomaly are circular/oval and approx. 1-3 km in diameter

Possible DiatremesMagnetic susceptibility Vs depth-to-top

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

50 150 250 350 450 550 650 750

Depth-to-top (m)

Mag

netic

sus

cept

ibili

ty (S

I)

Approximately 100 anomalies were magnetically modelled. Results indicate the source bodies are:•Elliptical (100-500 m in diameter)•Steeply dipping•Of intermediate volcanic susceptibility (0.001-0.01 SI)•200-425 m below the surface

Possible DiatremesNatural remanent magnetisation values produced by the modelling were used to calculate the palaeopole, from which a Late Permian date was inferred.

Reversely polarised sourcesNormally polarised sources

Polar wander path (Clark & Lackie 2003)

Ground Surveys


To improve the accuracy of the geophysical models, in April 2009 high resolution data was acquired, using a G-858 magnetometer and Scintrex CG-5 gravity meter, at four sites in the Murray Basin. Data from an anomaly west of Euston is presented below.

Total Magnetic Intensity Bouguer Gravity

Euler Deconvolution

Depth-to-top 110-260 m

Euler deconvolution was used to determine the depth to the source for the gravity anomaly

Magnetic Inversion

Models—Area 11 Magnetic Susceptibility (SI) Depth (m) Length A (m) Length B (m) Q ratio

Aeromagnetic data–ellipsoid 0.006230 251 338 149 1.453

Ground magnetic data–ellipsoid 0.008930 266 88 384 1.703

Ground magnetic - frustrum 0.009519 236 780 450 1.254

Three models were generated, one prior to the ground survey (using the NSW state-wide aeromagnetic data), whilst the other two models were from the ground data, using different shapes.

Conclusion: Possible Diatremes

The Euler devconvolution results indicate a lower density source approximately 110-260 m below the surface. The magnetic inversion results indicate a magnetic source with a depth-to-top of approximately 236-266 m. These models are interpreted to be indicative of a breccia pipe. The gravity anomaly is possibly caused by brecciated material closer to the surface, whilst the magnetic anomaly is likely to be due to an intermediate-mafic pipe below the breccia.

Volcanic Intrusions


Peak approx. 660 nTAnomaly diameter approx. 2-5 km

Magnetic modelling has been conducted on two high-amplitude anomalies. One anomaly intrudes a body interpreted as a granodiorite and the other intrudes a body interpreted as apart of the Leeton Igneous Complex.

Peak approx. 320 nTAnomaly diameter approx. 2-4 km

b

a

Drillhole intersecting granodiorite at 335 m

Leeton Igneous Complex

Volcanic Intrusions (a)

Magnetic inversion model produced in ModelVision using lines generated from a regional merged TMI grid

Volcanic Intrusion (a)

0.082 SI261 m depth-to-top


Volcanic Intrusion (b)

Magnetic inversion model produced in ModelVision using TMI flight lines

Volcanic Intrusion (b)0.090 SI

200 m depth-to-top









Volcanic Intrusions

Magnetic inversion modelling indicates that the anomalies have an intermediate to mafic volcanic susceptibility (approx. 0.08-0.11 SI), that they extend approximately 300 m into the subsurface and may intrude into the Cenozoic Murray Basin sediments with depth-to-tops between 140-260 m.

Conclusions•Southwest NSW has hundreds of possible diatremes of intermediate-volcanic susceptibility (0.001 to 0.01 SI) that have an inferred Late Permian age. The depth- to-top of the magnetic source is approximately 200-425m.

•There are volcanic intrusion with a complex pipe-like source with a higher magnetic susceptibility (0.08 to 0.11 SI) that possibly intrude the Cenozoic Murray Basin sediments. The depth-to-top of the magnetic source is approximately140-260 m.

The modelling and data presented here highlights that the southwest of NSW has a high quantity of pipe-like sources, with geophysical characteristics similar to other known diatremes. The prospectivity of southwest NSW looks brighter…or perhaps more sparkly... as a diatremes often host gems and diamonds!

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INTERPRETATION DOWN THE TRACK · More extensive Winduck Group ... Interpretation Down the Track, Investigating Geophysical Anomalies highlighted in Regional Interpretations for Hay-Balranald