Investigating Geophysical Anomalies highlighted in Regional ...

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Investigating Geophysical Anomalies highlighted in Regional ...

INTERPRETATION DOWN THE TRACK

Investigating 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.

REGIONAL TECTONIC ELEMENTS

LOUTH 1: 250 000 MAP SHEET

• reconnaissance mapping in the Louth - Bourke

area to understand the stratigraphy and extent

of Cobar Supergroup rocks.


TINDARA

Geophysical-Geological TMI image ZONE shows major Interpretation long wavelength Map features for Cobham of high Lake magnetic mapped intensity this sub-

• Revised along surface western depth basement to side basement platform (deep intrusions?) using by extending BHP drilling, and the short distribution water wavelength bores of and particular features seismic units data

• Discovery throughout of most new exposed bedrock of the zone.

exposures the Broken by Hill Steve sheet Hill to (CRCLEME) the south. in Teilta area

Smithville

1:100 000

Map Sheet

Teilta

1:100 000

Map Sheet

Tindara Zone


On TEILTA 1:100 000 MAP SHEET:

basement outcrops were visited

• exposures of gently warped,

clastic metasediments

• low metamorphic grade with tension

quartz veining

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

interpreted as an extension of the

1VD Neoproterozoic TMI RTP ImageYancowinna Subgroup


A single phase of exploration by

BHP Minerals in 1992-1993

(unsuccessfully) sought Willyama

Supergroup rocks (GS1993/164)

Several sparse drillholes tested some magnetic

highs and encountered mainly Adelaidean

sedimentary units, with magnetic highs relating

to basalt or dolerite.

SMITHVILLE 1:100 000 map sheet:

• has unusual magnetised features and major

structures (under cover)

• may have exploration potential subject to

basement depth

1VD TMI RTP Image


Unusual magnetic anomalies on

SMITHVILLE 1:100 000 map sheet

BLUE: small, discrete, moderate

amplitude features: scattered in

distribution (subsidiary fractures or

maybe random?)

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

Gravity multiscale edge detection gradient

BHP survey had flight line spacing of 300m

contours overlain pseudocolour gravity image

1VD TMI RTP Image

indicate deep density contrasts which are

interpreted as the margins of large intrusions.


Guides to basement depth on

Smithville 1:100 000 map sheet

• BHP exploration drillhole

Remanent magnetisation

• Bedrock logs from water bores

• Winnathee seismic survey 1970

• ModelVision depths

TMI (non-RTP)

1VD TMI RTP Image


TEN MILE BORE

GSNSW The bedrock gathered platform a core under tail at the the

base Smithville of Ten 1:100 Mile water 000 sheet bore on

Border deepens Downs from (GS Report

2010/0173)

• approx 170 m at southern

boundary, to

It intersected moderately cleaved

dark • approx grey shale, 400m at interbedded the north- with

medium eastern to edges coarse of the grained Tindara

sandstone. Zone. Core contained quartz

and carbonate veining, with

The highlighted features could

galena and pyrite identified in

be expected to lie around the 250

fractures.

to 350 m depth below surface.

1VD TMI RTP Image


THOMSON INTERPRETATION

Building up a history…

LOUTH MAP SHEET

• reconnaissance mapping in the Louth - Bourke

area to understand the stratigraphy and extent

of Cobar Supergroup rocks.

Structural Framework of NSW

Scheibner and Basden 1996


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/chips

from the Louth area (viewed looking north-northwest)

LOUTH

IONA

MT DEERINA

Louth volcanics under cover


DEVONIAN

DETECTION

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 NSW

Results from ground surveys, near Euston,

over an anomaly interpreted as a diatreme

Results of modelling from airborne TMI data for

two high-amplitude short-wavelength TMI

anomalies southeast of Hay


Southwest NSW

Silurian-Devonian granodiorites

Stawell Zone:

Cambrian-Ordovician

turbidites and volcanic tuffs

Hay-Booligal Zone:

Silurian-Devonian sediments

over a Proterozoic?

crystalline basement

Greyscale tilt-filtered TMI image

Silurian-Devonian granites

Silurian-Devonian granodiorites


Anomalies of Interest

South-west NSW

Isolated volcanic intrusions

A swath of possible diatremes


Possible Diatremes

over 500 anomalies

swath extents over 230 km


Possible Diatremes

•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

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


Possible Diatremes

Magnetic susceptibility Vs depth-to-top

Magnetic susceptibility (SI)

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

50 150 250 350 450 550 650 750

Depth-to-top (m)

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 Diatremes

Natural remanent magnetisation

values produced by the

modelling were used to

calculate the palaeopole, from

which a Late Permian date was

inferred.

Reversely polarised sources

Normally 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.

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


Total Magnetic Intensity

Bouguer Gravity


Euler Deconvolution

Euler deconvolution was

used to determine the

depth to the source for

the gravity anomaly

Depth-to-top 110-260 m


Magnetic Inversion

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.

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


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. 320 nT

Anomaly diameter approx. 2-4 km

Peak approx. 660 nT

Anomaly diameter approx. 2-5 km

a

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.

b

Leeton Igneous Complex

Drillhole intersecting granodiorite at 335 m

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


Volcanic Intrusions (a)

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


Volcanic Intrusion (a)

0.082 SI

261 m depth-to-top

0.087 SI

230 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

0.100 SI

230 m depth-to-top

0.09 SI

160 m depth-to-top

0.09 SI

200 m depth-to-top

0.1 SI

144 m depth-to-top

0.09 SI

150 m depth-to-top

0.11 SI

140 m depth-to-top

0.09 SI

160 m depth-to-top

0.09 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 depthto-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!