55

F eo0.-JS ItMR PUBLICATIONS COMPACTUS (LENDING SECTION)

000869

DEPARTMENT OF NATIONAL RESOURCES

BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS Record 1977/55

MAGNETIC INDUCED POLARISATION (MIP) SURVEY WOODLAWN, NEW SOUTH WALES, 1975

by

D.F. Robson

The information contained in this report has been obtained by the Department of National Resources as part of the policy of the Australian Government to assist in the exploration and development of 91 resources. It may not be published in any form or used in a company prospectus or statement t the permission in writing of the Director. Bureau of Mineral Resources, Geology and Geophysics.

BMR Record 1977/55 c.3 I I^ "

^

11 1 1 1 '^1111,1r^

II

Record 1977/55

MAGNETIC INDUCED POLARISATION (MIP) SURVEY WOODLAWN, NEW SOUTH WALES, 1975

by

D.F. Robson

CONTENTS Pap N o. ^SUMMARY^ 1. INTRODUCTION^ 2. 3. 4.

GEOLOGY^ BACKGROUND GEOPHYSICS^ THE MAGNETIC INDUCED POLARISATION METHOD^ - Principles of operation^ - Equipment^ - Parameters recorded^

5.^SURVEY DETAILS^ - Woodlawn survey ^ - Black slate area survey 6.^RESULTS^

^ - Woodlawn ^ - Black slate area 7.^DISCUSSION^ - Interpretation^ - Comparison with EIP results^ - Operational characteristics ^ 8.^CONCLUSIONS^ 9.^REFERENCES^

1 2 2 2 3 3 4 4 5 7 7 7 8 8 8 9 10 10 11

TABLES 1.

Physical property measurements

2.

Location of current dipoles and traverses PLATES

1. 2.

Locality map. Geology and traverse locations.

3.

MIP results and geological cross-sections, traverses A, G and H.

4.

MIP results and geological cross-sections, traverses .7 and L,

5.

MIP results and geological cross-sections, traverses S and U.

6.

Chargeability contours.

7.

Normalised horizontal magnetic field contours.

8.

Secondary horizontal magnetic field contours.

9.

Chargeability decay ratio contours.

10. HIP results, black slate area. 11. LIP gradient-array apparent resistivity contours. 12. LIP gradient-array percent frequency effect contours. 13. EIP dipole-dipole array and MIP profiles, traverse G.

SUMMARY During March and April 1975 the Bureau of Mineral Resources conducted a magnetic induced polarisation survey over and around the Woodlawn orebody. Normalised magnetic field results clearly delineate the surface expression of the massive sulphides, but negative chargeabilities recorded over the orebody are similar to chargeability anomalies recorded over black shales and weakly mineralised dolerites and volcanics. Results of secondary magnetic field measurements are similar to the chargeability results, while chargeability decay ratios provide no information on the nature of the chargeable sources.

-2-

1. INTRODUCTION During March and April 1975, the Bureau of Mineral Resources GDIRI conducted a magnetic induced polarisation 04IP1 survey over the Wbodlawn ore deposit, N.S.W., which is located in the Canberra 1:250 000 Sheet area, 40 km south of Goulburn (Plate 11. The survey was made to record the response of a volcanogenic sulphide deposit to the MEP method. For comparative purposes, a traverse was also made over a pyritic black slate about 2 km southwest of Woodlawn. The survey was undertaken with the co-operation of Jododex Australia Pty Ltd and Scintrex Pty Ltd. 2. GEOLOGY The Woodlawn orebody is a concordant lens of bedded copper-leadzinc sulphides lying within a sequence of Silurian acid volcanics of the Lachlan Fold Belt. Mineralisation is essentially massive, and the main primary minerals are sphalerite, pyrite, galena, and chalcopyrite (Malone and others, 1975). The orebody has a north-south strike, and is about 200 m in length. The extent of the orebody in depth is greater than 300 m. In

°

the north it is massive, sharply bounded, dips to the west at about 45 , is up to 45 m thick and is capped by a 12 m thick clay gossan. In the south the orebody is smaller, and is surrounded by disseminated sulphides. The black slate southwest of the orebody contains numerous stringers and veins of quartz-calcite containing up to 50 percent pyrite. 3. BACKGROUND GEOPHYSICS The Woodlawn orebody is an excellent site for test surveys as its shape has been accurately defined by over one hundred drill holes, and numerous geophysical tests have provided a guide to the physical properties of the deposit. Table 1 shows the results of some laboratory physical property determinations by MAR CYoung, 1976) which demonstrate the marked contrasts in specific gravity, resistivity and IP response between the massive sulphides and the host rocks.

-3-

TABLE 1 - Physical property measurements

Sample

^

Specific^Resistivity (ohm-m)^IP response CP.E.%) gravity^(1000 Hz)^CO.3 and 5 Hz)

Massive sulphides^4.6 - 3.8^3.3 - 0.4^ 50 - 10 Host rocks^2.9 - 2.6^26 000 - 1000^ 0

4. THE MAGNETIC INDUCED POLARISATION METHOD Principles of operation The MIP method uses a magnetic sensor to measure the magnetic fields associated with current flow in the ground. Current flow is generated in a conventional manner using an IP transmitter and grounded electrodes. The electrodes are generally placed along strike, and measurements of horizontal magnetic field are made using a sensitive magnetometer at stations along traverses perpendicular to strike. When the primary current is being transmitted the equipment records the primary horizontal magnetic field. After the primary current ceases, the secondary field created by discharging sources is measured. Seigel (1974) has developed a model to explain the MIP response. This model recognises that the current flow due to polarised sources is composed of an internal current flow in the source, and an oppositely directed external current flow around the source. Seigel's model predicts that for highly conductive bodies the internal current density will be greater than the current density around the source and negative chargeabilities may result. Howland-Rose (1976) has reported negative MIP chargeabilities over sulphide deposits in Australia.

-4-

Equipment The survey was carried out using a Scintrex MFM-3 fluxgate magnetometer coupled to a Scintrex IPR-S IP receiver. The transmitter used was a 2.5 kU time-domain unit employing a standard 3-second cycle. The specifications of the MFM-3 magnetometer indicate a sensitivity of about 100 mV per nT and a noise level of about 10 pT. The IPR-8 receiver measures the decaying IP voltage as the average value over a single, 3, or 6 time-intervals in the period 130 ms to 1690 ms after current switch off. Chargeability values recorded by the IPR-8 are normalised with respect to a standard induced polarisation curve (Dolan & McLaughlin, 1967), and variations in the value of normalised chargeabilities at different time intervals indicate departure of the decay transient from the standard induced polarisation decay curve. The time interval over which the chargeabilities are measured are commonly called slices and, depending on the mode of measurement used, chargeability slices are sequentially labelled M 1 , M 2 ^ to M 6. Under most survey conditions six chargeability slices are recorded. Parameters recorded Apparent chargeability (4). This dimensionless parameter provides an indication of the chargeability of subsurface sources, and is commonly expressed at ml per T. Primary horizontal magnetic field (Hp). This parameter is the measured primary magnetic field and is expressed in units of nT. Normalised horizontal magnetic field (Hn). This is a dimensionless, derived parameter which indicates conductivity changes in the ground. The parameter is derived by normalising Hp with respect to current and geometry as shown in equation 1. I is the current expressed in amps and K is a geometric factor with units of nT per amp. Hn% = Hp^x 100 Kx I

..• (1)

-5-

High normalised fields indicate zones of high conductance, and low normalised fields indicate zones of low conductance. If the ground is homogeneous with respect to conductivity the value of Hn will be 100 percent at all locations. Secondary magnetic field (Hs). This derived parameter has units of pT per amp, and is calculated from Hp, M and I, as shown in equation 2. Hs = Hp x M^

C2)

If Hm is the value of the magnetic field caused by polarisation currents, then M = Hm and Hs = Jim. Hence Hm is proportional to Hp and accordingly Hp varies with array geometry, as well as the chargeability of subsurface

sources. Chargeability decay ratio. If chargeability slices have the same sign, the shape of the IP decay curve is indicated by the ratio of early and late chargeability slices. When six chargeability slices are recorded it is normal to use the parameter NM 1 as the chargeability decay ratio. 5. SURVEY DETAILS Woodlawn survey Grid co-ordinates. The survey was made on an imperial grid laid out by Jododex Australia Pty Ltd. The grid was oriented approximately NS-EW, and was pegged at 100 ft intervals. The relationship between traverses surveyed and the Jododex metric grid is shown in Plate 2. Array locations. To investigate the HIP response of the Woodlawn deposit, various current-dipole arrays ranging from 240 m to 300 m were placed parallel to the orebody. The main body of mineralisation, and areas to the north and south, were investigated by seven traverses across the strike of the orebody. The location of current dipoles and traverses are indicated in Table 2. All IP measurements were made using 6 chargeability slices.

-6-

Current Dipoles*^MIP Stations* II

North

South

From

To

9620N/9490E

9370N/9490E

9320E

9700E

9620N/9790E

9370N/9790E

9670E

9970E

9310N

9370N/9910E

9070N/9910E

9720E

10120E

9230N

9370N/9640E

9070N/9640E

9460E

9820E

9370N/9910E

9070N/9910E

9820E

9990E

9370N/9640E

9070N/9640E

9450E

9760E

9370N/9910E

9070N/9910E

9715E

10080E

9370N/9640E

9070N/9640E

9460E

9760E

9370N/9910E

9070N/9910E

9760E

10120E

Traverse^Metric grid locations* A^

J^

L^

9500N

9220N

9160N

II II

8950N

8890N

"Black Slate"^6600N

9070N/9640E

8830N/9640E

9640E

9820E

9070N/9910E

8830N/9910E

9730E

10100E

9070N/9640E

8830N/9640E

9480E

9760E

9070N/9910E

8830N/9910E

9730E

10060E

6720N/9670E

6400N/9670E

9590E

9850E

* locations are shown as approximate positions on Jododex metric grid TABLE 2: Location of current dipoles and traverses

11

-7-

Station spacing was generally 100 ft C30 m), with a 50 ft C15 m1 spacing over anomalous zones. Black slate area survey A traverse was made over a body of pyritic black slate located about 2 km southwest of the orebody. The traverse was 360 in long, and exployed a 240 in current array parallel to the strike. A station spacing of 30 in was used along the traverse. 6. RESULTS Woodlawn Profiles of the chargeability and normalised horizontal magnetic field results are shown with geological cross-sections in Plates 3, 4 and 5. Contour plans of the chargeability, normalised horizontal magnetic field, secondary horizontal magnetic field, and chargeability decay ratio are shown in Plates 6, 7, S, and 9 respectively. Note that only the M 3 chargeability is shown, this being a measure of the average chargeability between 650 ms and 910 ms after current cut-off. The MIP chargeability results (Pl. 6) show six distinct negative chargeability zones, of which the largest is zone A. Each of the zones has a characteristic chargeability and normalised magnetic field. The chargeability decay ratios do not correlate with the chargeable zones or with any geological units. Zone A. This zone occurs over the massive ore immediately west of its subcrop. Chargeabilities of greater than -8mT/T are recorded in this zone. Updip of this chargeable zone, and directly over the conductive subcropping mineralisation, normalised magnetic fields CPlate 7) are in excess of 400 percent. Secondary magnetic fields (Plate 8) have a similar form to the chargeability results.

-.8-

Zone B. A zone of strong negative chargeabilities occurs over mineralised volcanics at the southern end of the orebody. Coincident with the negative chargeabilities are secondary magnetic fields in excess of -40 pT/A. Unlike zone A, which is over the main body, zone B is not associated with an increase in the normalised magnetic field. Zones C, D and E. Chargeability zones C, D and E have similar magnitudes of approximately -4mT/T, and show only minor variations in the normalised magnetic field. Zone C occurs over dolerite; however the response could be caused by a buried rubbish pit at 9815E, traverse A. The secondary magnetic field results for zone C are similar to the chargeability results. Zone D lies mainly over black shale and may be caused by disseminated pyrite in the shale. Small negative secondary magnetic fields are recorded over this zone. Zone E lies over coarse-grained acid volcanics which include a prominent quartz vein; either of these units may be mineralised. Secondary magnetic fields greater than -10 pT/A occur over this zone. Zone F. This zone is located directly over a small gossan west of the main orebody. Negative chargeabilities of -3mT/T occur with secondary fields of -2pT/A. There is no change in the normalised magnetic field. Black slate area Plate 10 shows the MIP results and geological cross-section across the pyritic black slate. A peak chargeable response of -12mT/T was observed within a broad chargeable zone of about -5mT/T. Broad normalised magnetic fields in excess of 170 percent are associated with the chargeable response. 7. DISCUSSION Interpretation The magnetic induced polarisation results show a strong response over the orebody. However only the normalised magnetic field results CPlate 7) clearly discriminate the orebody from other sources. The strong normalised

-9-

magnetic field response is probably caused by the highly conductive subcropping mineralisation. Although the chargeability and secondary magnetic field results do not uniquely identify the orebody, a large negative chargeability and negative secondary magnetic fields appear to be caused by black shales and mineralised volcanics and dolerites. A comparison of the MIP results and the geological crosssections shows that the chargeability anomalies over the orebody are down dip of the normalised magnetic field anomalies and suggests that the source of chargeable anomalies is deeper than the source of normalised-field anomalies. Contours of chargeability decay ratio M /II 6

1

(Plate 9) provide no

information on the orebody or the geology of the area. Comparison with EIP results

Results of an EIP gradient array and a dipole-dipole array survey have been provided by Jododex Australia Pty Ltd, [Plates 11, 12 and 13) and allow a comparison of EIP and MIP results. Gradient array. The results of an EIP gradient array survey centred on the

Woodlawn orebody are shown in Plates 11 and 12. This array used an eastwest current dipole of 1830 m, and a 30 m potential dipole. EIP resistivity results Plate 11) outline the subcropping mineralisation in a manner similar to the MIP Hn results. However EIP results indicate a second low resistivity zone centred at 9000N, 9770E. EIP frequency effect results [Plate 12) show IP effects of greater than 10 percent over the mineralised volcanics in the southern part of the orebody. Surrounding this anomalous feature is a broader zone of greater than 5 percent, which clearly outlines the black shale east of the orebody. This result compares unfavourably with the strong MIP chargeability anomaly, which was directly over the massive ore in the north. Dipole-dipole array. ^MIP and LIP dipole-dipole array results and the

geological cross-section along traverse G are shown in Plate 13. The dipole-dipole array used a dipole spacing of 30 m, and a dipole separation of 60 m, 120 in and 180 m [n = 1, 3 and 5).

The dipole-dipole array results show a frequency effect centred at about 9900E, traverse G over black shale, whereas MIP negative chargeabilities lie over the region of mineralisation. EIP apparent resistivities highlight the conductive orezone directly below 9830E, whereas HIP normalised magnetic fields indicate a broad conductive high over the orezone. Operational characteristics Speed. At Woodlawn, an average of 50 NIP readings were recorded daily. Noise. The primary magnetic field between stations at Woodlawn varied from

1 to 15 nT, and chargeability readings were usually repeatable to within 3 mT/T. MIP results are generally repeatable to within 0.5 mV/V, Cost. With an average of SO readings a day using a 30 m station spacing, the cost for an HIP survey in country such as at Woodlawn would be about $1000 per line kilometre. 8. CONCLUSIONS The magnetic induced polarisation method successfully delineated the massive, pyritic, zinc-lead-copper orebody at Woodlawn. The high conductance of the subcropping mineralisation was particularly highlighted by the normalised magnetic field. Although the orebody produced an MIP chargeability response, it is difficult to discriminate the chargeability anomaly over the orebody from anomalies over black shales, and mineralised dolerites and volcanics. Secondary horizontal magnetic fields were similar to the chargeability responses, and did not provide any additional information. The chargeability decay ratio parameter was not characteristic of any geological units. Comparison of EIP and HIP results over Woodlawn indicates that MIP chargeabilities resolved the main massive sulphide mineralisation better than EIP chargeabilities.

9. REFERENCES DOLAN, W.M. & McLAUGHLIN, G.1-I., 1967 - Considerations Concerning Measurement Standards and Design of I.P. Equipment. Proceedings of the Symposium on Induced Electrical Polarization, Berkeley, University of California, p. 2-31. HOWLAND-ROSE, A.W., 1976 - The magnetic induced polarization method - a simple method of interpretation of typical anomaly forms. 25th Interna-

tional Geological Congress, Sydney, Australia, Abstracts, Volume 2, Section 9B p. 392. MALONE, E.J., OLGERS, F., CUCCHI, F.G., NICHOLAS, T., and McKAY, W.J„ 1975 - Woodlawn copper-lead-zinc deposit. In KNIGHT, C.L. CEditorI

J

ECONOMIC GEOLOGY OF AUSTRALIA AND PAPUA NEW GUINEA; I METALS. Australasian Institute of Mining and Metallurgy, Melbourne, p. 701-710. SEIGEL, H.O., 1974 - The magnetic induced polarization (NIP) method.

Geophysics, 39, p. 321-339. YOUNG, G.A., 1976 - Drill-hole logging and transient electromagnetic test surveys, Woodlawn deposit, New South Wales, 1973. Bur. Miner. Res. Aus. Rec. 1976/52 Cunpubl.).

Plate 149 0^149°30'

GOULBURN

Collector

Bungendore

OUEANBEYON

Bra idwood

Captains Fiat

155/B7-1854

Record No 1977/55

Locality map

^

Plate 2 •

. .^. .^•

O

• . •^•^'

.^•^• •^• • ' •^• •

O \ 0

2

1-

'^•^• .^. •

9500N

9500'N— •

•^.

V — —

4

—

VV— V— V .—

-

—V—V —

—

V —

V— V—V— V-

V — V V— V—V

-

- V— A,- V — V— VV—V— V— V—V— V— V-

—V

—

v — V— V-r, —V—V

—V— V— V— V -

V—V— V

—

VO

V —V-

V— V-- see4V

—

V

-

9000 m N

s

7

r /1

V A

C

—V-

A

V—V— V —

-1

— V

V —

-

—

V

—

V—V

V

—

V

—

V

V—V -

—

v — V—V—V

V —V —V—V— V —V — —V— V — V

4

V—V —V— V—V—V — V —V

—

V

—

V

—

--V—V—V

V

—

V

—

V

—

V

V —V— V — V—V-

-A 1

z

V —V— V--V—V —

V —V—V—V-

V— V— V— V—V V— V —V- V

—

V

—V 8500 m N

•- 850C mN

0-

8000 n N

8000 mN

155/87-190A

MIDDLE

—

-V —V-, V—V—N

UPPER ORDOVICIAN

To UPPER SILURIAN

Dolerite

Sediments

Shale, line-groined acid volcanics

Black shale

Coarse-groined acid volcanos

Record No /977/55

(0)

A A A

Black shale ••1^'

Massive sulphides .•••—••-01 base of gosson ^ Gosson

Approximate gage of orebody Foss , ' loccilly

Vein Quad/

UnCoolOrmIty

Mineralised volcanic's 0 base of oosson

Dip and strike of strata

Geology and traverse locations

Plate 3 940C..f^

960C^

9800mF

000,,E

'R.IvERSE a

-

HN

_F 'C

V

PP0

SuRIAN

L„

200 -

Jo e , ,r'e

00 -

Spp ze f,ne•gra , necl oc , c7 vo/con,c5

SiocA shale

al

F

A V Z-7 , ,

7 V P,

A

:oc - se-;rooneo oc,d o;;-cen,cs

5ossc , Mo55,e soo/ph/des

100^ 200,

mi 0 — T .10

/VAL' rs^VAT‘if"<31^11^7'1/ \ ( 7 /, /1 I / I/

• 1 • s, ,1

v i,v n" VA

A A 4 ^ V fr ^ h Is .1

A )\ /

I^--/ • t \

NP ^ 4 "74 4 ,I ts11t.

Record No. /977/55

155/137-191A

Ml P results and geological cross -sections , traverses A ,G and H

Plate 4

9,100,E

9800 mE

tC 000...E

r 20C

cc

So

MIDDLE TO UPPER SILURIAN

v —v-11^f,„e.9„,ned Block shah.

!, ^4.1 v r NVL

A

'J AAV

Coolse-grO , ned cold volcanic's

Gosson Moss,ve Su/ph/des

100^200., r 200

,00%

L0

Record No 1977/55

MI

P results and geological cross-sections ,traverses J and L

15S/87 - 192 A

^ ^

Plate 5 9800mE^

9600E^

9400mE^

000mE

TRAVERSE S^m3

mr - 0 T

HN

A 4 V A L. •1 -v-V-V-V-V VVV-V-VV-V- -^- A V A 'j v -1 ,1 ^ v . A, V v -I^ ___v___,___v-y___ v -V-V-V-V-V-V-V-V "A f \ IV A '-4^v v r A > N^ 1, V > -V-V - V - V^V- V- V-V-V-V-V-V 4 V A .4 V, L. ^V -V - V -^-V -V -V -V -V- \ : -V -1 /I " 1\ t_V - V - v-v -^4Ie-Y- v -v-v -v -V -V --V-- 0 v ,,^■1 - V- v-V-V- V-V- V o A -V --- V -V-..d-V - -V-V- V - V - V

o oki eglo ,,,ii^-V-

-V----V----V-V----V-VV-

V- V-V- V

^-

V

-

V

..

-^-Atv- vie/ - V - V - V - V - V -V- V---?i,'-V.e-V-V - V- V- V-----V -', - V - V -,0-V---,- V- V _ , _ \,■

-

-

10

tv1 3

TRAVERSE U

mT T

200 -

-AdalkiAlk._

°/c, )00 -

0-

ANIL Av\i LA V A \I \ I-^

IA^,^A^_

v_ v_v-v-^v_ v- v-lv A4 V< .1 A

^ J V .--v-.-v-v-v-vv \J 4' /1 \J v^-v-v-v-v-v - v-v-v-v-vv--V- V-V-V^V- V- V-V-V-

-\\ A

V -- V --V^-V- •-V-V-V- V- V

-v^

MIDDLE TO UPPER SILURIAN

-v—v--V—V— V -V

Dolerite - V - V- V/-V-V-

V- V -V - V- V---v

Shale, line grained acid vo/conics

-V-V-V-V -V- v- V - V- V- V -V- V^- V- VV-V-V-V-V-- V - V-

Black shale AvA

V< V c-,4 , V1 v L

Coarse grained acid volcanics

A AL

Vein quartz

Gosson Massive sulphides

^

0^ 100^200 m

1^ I^ 1

MIP results and geological cross-sections, traverses S and U Record No. /977/55

155/87- I93A

Plate 6

000mE

950CmE

A ; • • it
OIX!...t.tt.

9500mN -

9000 mN

<- 8

Approx. edge of orebody

1:-0 --8 4 41 -

>

^

Anomaly

rrT,T

A1

^Traverse //,-7e

0 155/B7-194A

Chargeability contours Record No /977/55

Plate 7 9500 mE

10 000mE

A

9500 mN -

9000 mN -

Approx. edge of oreboo'y

15 5/ 97- 1 95 A

Normalised horizontal magnetic field contours Record No /977/55

Plate 8 10 000mE

9500mE

9500m N

9000 mN

Approx edge of orebody A

0

I Traverse line

155/87 -196A

Secondary horizontal magnetic field contours Record No.1977/55

Plate 9 9500 mE

^

10 000mE

9500 m N

j

900 0 m N -

>2 15102

Chargeability decay ratio contours Record No /977/55

155/B7-197A

Plate 10

9800mE

96010mE 6600mN ^

300 -

200 HN

(0/0) 100 -

0

-

-40

-

-20

- +20

-

W43

Black ski fe

97,sl^V

acid volcanics

1_

1,< \IV V A

Record No /977/55

166/87-198A

Ml P results, black slate area

+40

Plate II

10000mE

9600mE 9500m N

E 50

9000mN —

Approx edge of orebody E I P results courtesy of Jododex Aust Pty Ltd ) 15 5/ 87- 1 99A

E I P gradient-array apparent resistivity contours (ohm-ft/2n) Record No 1977./55

P(ote 12 960OrnE 9500 m N

(El P results courtesy of Jodotlex Aust Pty Ltd ) 155/B7-200A

E I P gradient-array percent frequency effect contours Record Ale 1977/55

Plate 13

P

200

Ioo (c'/0) 0 P DIP'JLE - DIPOLE Cd1p;:le Length 10()ft)

REOL.,1 ENC', EFFECT (3 0 8 100-1z'

20 (%)

n

,

3

0: 5

RESISTivar 100 8.1

10 E .c 0

n= 3

n:5

9800mE

10 00OrnE

Dolerile Shale, f,ne - orolned (E I P resuits courtesy of Jododex Aust Pty Ltd )

acid volcanic's Block shofe Casson Massive L/iPhideS

Record /Vo.1977/55

155/B7-2014

E IP dipole-dipole array and MI P profiles ,traverse G