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~ICRO FILMED J

RESULTS OF GEOPHYSICAL SURVEYS

IN THE BALFOUR AREA (NW TAS.)

D.S.M.[:r

Reg"""I1--­

E & II.:,

Authors

Date

Submitted to

Accepted

Copie

A.D. McKay andM.F. Flis

November 1980

L. & L. SyndicateL. & B. SyndicateGeopeko - DevonportGeopeko - SydneyTasmanian Mines Dept.CRAB Melbourne LibraryBurnie OfficePreston Office

10467

CONTENTS

010002

1. INTRODUCTION 1

2. SUMMARY 1

3. CONCLUSIONS AND RECOMMENDATIONS 1

'-4. GRAVITY SURVEY 1

5. MAGNETIC SURVEYS 2

6. IP SURVEYS 2

7. DIGHEM II SURVEYS 3

,-

KEYWORDS

LIST OF PLANS

4

4

010003

1. INTRODUCTION

A review of the geophysical work completed in the Balfourarea was undertaken. All the available data on magnetic,IP and Dighem are compiled and the results interpreted.

The details of geology and geochemistry will be reportedseparately.

2. SUMMARY

Detailed ground magnetics carried out over the Balfour areahave defined three major and one minor anomaly. These occurin a linear pattern trending approximately 3200 mag. Oneof these anomalies occurs above Specimen Hill.

Two linear IP anomalies have been identified as shown onPlan No. Tv 335. The IP anomaly on the eastern portion ofthe grid extends along strike for some 800 m and co-incideswith a clearly defined Sn and Zn geochem anomaly. Also thiszone lies along the lithological boundary separating thegreen chloritic shale sequence and the grey shale/quartzitesequence as identified in the regional mapping.

During 1980, a Oighem survey was flown over the Balfour area.Three distinct linear EM anomalies were defined on the westernportion of the grid. Lithologies in this area are dominantlyblack and grey shales. The Dighem survey failed to give anyresponse over the Murray's Reward Cu mineralisation.

3. CONCLUSIONS AND RECOMMENDATIONS

The Specimen Hill area is a significant magnetic anomaly.Results from the BHP drilling suggest that this magnetismis caused by disseminated pyrrhotite in the grey shales.The shape of the anomaly would suggest that there may alsobe some deeper seated "magnetic basement" feature.

IP results show two clearly defined anomalies. Furtherfill-in IP work has recently been completed over the easternanomaly to better define this zone. This anomaly co-inc ideswith a Sn and Zn geochem anomaly and represents an excellentdrill target.

4. GRAVITY SURVEY

Carried out in the early • 60 's, the survey appears to be of little useas no terrain corrections were applied. Anomalies can bedirectly correlated to topographic relief. Original data isunavailable.

010004

- 2 -

5. MAGNETIC SURVEYS

In 1965/66 BHP carried out the first ground magnetic surveyover the Specimen Hill area, to investigate an anomalydetected by the 1956 Rio Tinto aeromag survey of NW Tasmania.The survey isolated three distinct anomalies, two of whichwere drilled with vertical holes with no success. InFebruary of 1979 CRAE covered the area with ground magneticsonce again to update the older (torsion balance) version andfurther define the anomalies.

A low level aeromag survey was carried out in March of 1979over Specimen Hill and Frankland River prospects to obtaina more regional feel of the significance of the Specimen Hillanomaly and its relation to a second anomaly at FranklandRiver. Apart from this regional information, the surveyshowed little else.

An extension of the grid in May of 1980 saw the groundmagnetic coverage extended on both the eastern and westernflanks of the original grid. This was to allow more completecoverage of the southmost magnetic anomaly (Plan No. Tv 86)and to investigate IP responses to the east.

Three major anomalies are known, two of which have beenmodelled. Line 108N has been modelled to show a westwarddipping "dyke" at a depth of 70 m whilst line 89N shows asteeply eastward dipping "dyke" at a depth in excess of 300 m(Plan No. Tv69) •

BHP drilled a hole into the southernmost magnetic anomalycentred on line 88N. This hole(DD B5) intersected disseminatedpyrrhotite in a sequence of interbedded shales and quartziteswith minor white quartz veining. The core was relogged byGeopeko/CRAE and it was concluded that the anomaly could notbe explained by the results of the drill hole because theamount of disseminated pyrrhotite was usually less than 5%(vol.) of the shale. To clarify this it was decided to submitcore samples of DD B5 to the CSIRO with a request to carry outmagnetic susceptibility measurements. The results (Appendix 1)show that the remnant susceptibility (Q) is about 20 timeslarger than the mean susceptibility. It is likely that theorientation of this remnant susceptibility is parallel withthe earth's field, thus giving a positive anomaly. Theresults show that the remnant magnetism of the pyrrhotite issufficient to explain the size of the magnetic anomaly.

6. IP SURVEYS

An IP survey was commissioned in May of 1979 to cover selectedportions of the Specimen Hill grid. These areas included themagnetic anomaly on line 108N and tin anomalies on the easternportions of 90N and 96N. In general, none of the anomalous IPzones were associated with magnetic anomalies; they were, however,correlatable to the elevated tin values.

•

010005

- 3 -

A second survey was designed to test the continuity of zonesdelineated in the previous survey and to test the southernmagnetic high. The anomalous zones were shown to be linearlycontinuous. A weak IP response was detected over the southernmagnetic anomaly (Plan No. Tv 335).

Self-potential (or spontaneous potential) readings takenconcurrently with this survey were inconclusive.

Further IP work was carried out in December 1980 to completelycover the IP anomaly defined on the eastern portion of the grid.The following lines were completed between 10000E and 10600E:

Lines - 92N, 93N, 94N, 98N, 99N, lOON.

As well, line 94N was completed from 10000E to 9400E. Thepurpose of this line is to test the IP responses in the areaof the Dighem anomalies on the western black shale - grey shalesequence.

Also, IP measurements were made over the Zn geochem anomalyon the south-western portion of the grid.

Lines 86N, 88N and 89N were completed from the base line west.

The results of this recent survey are being assessed and willbe reported in a later memo.

7. DIGHEM II SURVEY

During March of 1980 an airborne electromagnetic surveyutilizing the Dighem Multicoil II system was carried out.The survey consisted of ten two-kilometre lines spaced at150 metres. The EM bird height was a nominal 30 m whilst themagnetometer bird height was 50 m. The lines were orientedeast-west and designed to detect and test conductors in theSpecimen Hill vicinity. An EM response map (Plan No. Tv 75),resistivity map (Plan No. Tv 74) and aeromag map (Tv 72)were produced.

The survey detected a number of seemingly continuous conductorsto the west of and parallel to the magnetic anomalies(Plan No. Tv 86). These conductors strike at approximately1550 MN and appear to dip steeply to the west. Black, micaceous,carbonaceous siltstones have been mapped in the area.

The northern end of these conductors have been covered with IPon lines lOON and lOIN. Anomalous IP responses are coincidentwith the EM anomalies, whilst a general resistivity low isassociated with the whole length of the conductors.

010006

- 4 -

Poorer quality EM anomalies occur over the magnetic anomalyon lines 96N to 103N and are probably indicative of deeperseated sources. Stronger anomalies associated with thesouthmost magnetic anomaly may, however, deserve greaterattention as their correlation to a possible IP anomalyand their proximity to the magnetic anomaly is significant.

Finally, attention is drawn to the possible EM anomaly southof line 94N in the east. The anomaly is coincident with hightingeochem values, is along strike of an IP response andresides in an area of generally depressed resistivity values.As altitude fluctuations interfered with the EM readingsat this point (over Peter's Ridge) follow-up ground work willhave to be done to determine its potential.

KEYWORDS

Geophysics, magnetics, IP, EM, tin, pyrrhotite.

Locality:

Plan No.

86 ·TV

TV 335 ·Tv 75 •

Tv 74.

Tv 72 .

Tv 69 ·

Burnie 1:250 000 Sheet SK 55-3

LIST OF PLANS

Title

SPL 781 - Specimen Hill GridGround Magnetics Survey

Balfour Gridsummary of Geophysics

Dighem SurveyBalfour Test Area Electromagnetics

Dighem SurveyBalfour Test Area Resistivity

Dighem SurveyBalfour Test Area Magnetics

Dighem Line 9 Magnetic Profile

~ A.D. Me KAY

Scale

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010007

- 5 -

APPENDIX I

MAGNETIC PROPERTIES OF LAMINATED GREY SHALES

WITH DISSEMINATED FINE GRAINED PYRRHOTITE

INTERSECTED IN BHP HOLE DD B5

Depth No. of k J Q (±SE)Specimens (Range) (Range)

360 ft 4 520 804024 :!:l(210-820) (3090-13310)

452 ft 3 600 7340(130-840) (1750-10540) 20 ±l

k = mean emu susceptibil i ty x 106

(= cgs)

J = mean NRM intensity in mircogauss

Q = Koenigsberger ratio = J/kH (H = 0.63 Oersteds)

The results, although based on the very limited number ofsamples, indicate that remanence dominates induction in thesepyrrhotite-bearing rocks.

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DIG HEM mops are deSIgned to prOVIde a correct Impression atconductor quality by means of the conductance grade symbols Thesymbols can stand alone w th geology when planning a tollowupprogram The actual mho values are plotted for those who Wishquanhtatlve data The anomaly ppm (lnd deptn are indicated byinconspicuous dots WhlCn shaull'! not distract from the conductorpatterns while being helpful to those who Wish thiS mfbrmatlonthe mop prOVides on interpretation of all conductors In terms oflength strike duectlon conductance and depth The accuracys comparable to an interpretatIon from a ground EM survey

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