CHAPTER 2. ROCK MAGNETISM AND RADIOACTIVITY 23

BIOTITE

CHAPTER 2. CHAPTER 2. ROCK MAGNETISM AND RADIOACTIVITY 23

and

I-types.

Ishihara's ^scheme is

of

great use

in

aeromagnetic

interpretation

as glanitoids can now be classified on the basis of

their

aeromagnetic signatures. Magnetite-series are ^associated

with major

sulphide mineralization whereas ilmenite-series granitoids are related

to

cassiterite and wolframite mineralization (Ishihara, ^1981).

2.3 Rock ^magnetism and opaque mineralogy of rocks in the studY area

The

preceding sections have established

that rock

properties are necessary

to

^{constrain and}

imprãve magnetic

interpretation.

Though many researchers have worked on Kanmantoo Group

,o.kr,

very few whole rock analyses have been carried

out.

^The opaque mineralogy of the rocks has

only

rarely been studied

(Offier,

1966;

Brotherton,

1967; Fleming, 1971; Spry, 1976) as

it is

more usual

to

make

thin

sections

of

rocks

rather than the

^polished

thin

section which are required for the study of opaque minerals. Most descriptions of rock types

in

the literature refer

to

^*opaque minerals"

or "iron

^ores"

without

any

further differentiation.

^Oxidation

ratio

^mea- surements have not generally been carried

out

(for exceptions ^see Brotherton, op.

cif.;

Fleming, op.

cit.;

Toteff, I977).

Rock samples and

thin

sections referred

to by

previous researchers were

not

always

still

available

in

the rock collection of the Department of Geology and Geophysics, and those which were could

not

always be located on ^a map and placed

in

the appropriate stratigraphic interval' For these reasons,

the

amount of available rock property data

to

draw on is severely restricted.

The

time

available

for

field work was

limited

and

the

area

to

^{be covered}

(-

⁸⁰⁰⁰ krn2) ^was

large. It

was decided

to

carry

out

the following steps so as

to

acquire a leasonable amount of rock property data.

1.

Summarize available data

in

the

form

of whole rock analyses and oxidation ra,tios deter- mined by previous researchers

at

the University of Adelaide and

at

the South Australian

Institute

of Technology. Measure the susceptibility of all properly identified ^{hand samples} available from these institutes.

2.

From the magnetic interpretation, identify ^ateas where magnetic units may be expected to outcrop, search for the outcrop and measure the magnetic susceptibility over the outcrop.

Collect samples

for

opaque mineral studies, whole rock and ferrous

iron

analyses.

3. Identify

magnetic units suspected of carrying a strong

NRM,

and collect oriented ^sarnples from the outcrop.

To obtain representative susceptibility values, sevetal measurements ^were made on each hand sample, and between 50

to

100 measurements on every

outcrop.

Usually most outcrops could

quite

easily be classed

into a

small range

but for the

few outcrops

for

which several different

Àog".

were observed, the outcrop was divided into the appropriate number of smaller outcrops.

123 localities were

visited.

Magnetic susceptibility ^was measured ón situ and on ^samples using a Geoinstruments JH-8 susceptibility meter. Polished

thin

sections and polished blocks frorn 39

rocks were analyzed

for

opaque mineralogy. Facilities

for

whole rock analysis were restricted so

only

¹⁰ measurements were made, all on rocks from the Backstairs ^Passage Formation. Felrous

iron

determinations were carried.

out

on 21 samples (Backstairs ^Passage

Formation:

14; Ulupa

q

Formation Magnetic susceptibility x10-5 SI

0-50 50-100 100-500 500-1000 1000-2000 2000-5000

>

5000

PRE 124 3

t2

BSG 62 39 35 12 4

NG 42

CH

t25

2

BP 333 20 66 19 18 4 1

TC 62 6 108 27 1 8

TP 358 6 2 1

GR ⁴⁰ 1 21 31 6

I

AMP

45 T7 1 1 1

DIO

I

^{6 1}

MYL

7

Table

2.5:

Susceptibility measurements on samples. Abbreviations are as follows

-

^{PRE: Pre-}

cambrian rocks below Brachina Subgroup, BSG: Brachina Subgroup, NG: Normanville Group,

CH:

Carrickalinga Head

Fm., BP:

Backstairs Passage

Fm., TC:

Talisker Calc-siltstone, TP:

Tapanappa

Fm., GR:

Granite,

AMP: Amphibolite

and meta-dolerite,

DIO: Diorite

and

MYL:

Mylonite.

Fm. stands for Formation. FuIl details are given

in

Appendix

c.

CHAPTER 2. ROCK MAGNETISM

AND

RADIOACTIVITY

²⁴ Siltstone: 1,

Middleton

Sandstone: 1, Tapanappa Formation: 2, Carrickalinga Head Formation:

1).

To study the signifrcance of

NRM,

15 oriented samples were selected. The

dip

and strike of the top surface of each sample was marked.

It

was often difñcult to orient samples either because of the weathered nature or the schistosity of the sample. As a result, measuted directions of the

inclination

and declination

of the

remanent

fleld

are

not

reliable though

the

measurements of

intensity

are

reliable.

Several cores were

drilled into

each sample

in

order

to obtain

consistent results. NRM intensity and direction were determined using a Schonstedt spinner magnetometer

at

the Department of

Earth

Sciences, Flinders

Universityl.

For major

^element

and iron

analyses,

the

samples were

ground to fine

powder using ^a Siebtechnic tungsten carbide

mill.

Weathered surfaces had previously been removed. Powder for whole

rock

analysis was

ignited at

960oC

to

determine the percentage loss of

volatiles.

280mg of

ignited

sample, 20 mg of sodium

nitrate

and ^1.5 g of

flux

were mixed and fused

in

a platinum crucible

to form buttons. The

whole

rock

analyses wete

then

determined

on

a programmable Siemens

XRF

machine

by

John Stanley.

Total iron

analyses wele carlied

out on

15 samples (4 overlapped

with

whole rock analyses measurements

-

^{see above).}

^All

sampies were dried

at

110oC, then weighed

into

teflon beakers.

Samples (100 mg) were digested

with IICl,

H

F

and

HCIOa

acids, and then made up to volume (100m1)

in the

presence

of

Lanthanum Zirconate (0.2% wf

v La,0.l% wlv Zr)

and

I(

^(0.12%

w/v)

and l0% ^v

lv ^HCI.

The solutions were then measured by Atomic Absorption Spectrometry

for total

FeTo

at

372.0rrr¡¡'

f'e

line using a Varian

AA-6D Atomic

Absorption Spectrometer.

To determine ferrous

iron

content,400mg of powdered sample was mixed

with 5ml

of water

in

a

platinum crucible. To this mixture, 5ml of

50%

vlv

sulphuric acid and

5ml

50%

w/w

hydrofluoric acid was added and

the

crucible heated

at

350o C

for

approximately 20 minutes.

This was added

to

³⁰⁰ ml of distilled water, 10

ml

of 50To v

lv

sulphuric acid and 30

ml

saturated boric acid before

titration with

a standardized solution of 0.02 normal CeSOa and the indicator, N-Phenyl

anthranilic

acid

to

determine ferrous

iron

content.

2.3,L Magnetic susceptibility and NRM

Measurements of magnetic susceptibility were made on outcrops (Appendix

B)

and tabulated in Table 2.4. Based on quantitative

interpretation,

a number of shallow magnetic anomalies were chosen. During

field

work,

the

surface expressions

of the

anomalous sources were searched for, and susceptibility measurements made on outcrops

in their

vicinity.

Measurements were also rnade

on hand

samples collected

by

previous researchers (Ap-

pendix C).

These results are tabulated

in

Table

2.5. They

were

not

integrated

with

outcrop measurements because

both

distributions are biased

for

different teasons. Outcrops were ^chosen based on the possibility of finding rocks capable of giving rise

to

a detectable magnetic anomaly.

Previous researchers

probably

chose

their

samples according

to the state of

weathering and silicate lithology.

SusceptibiJity values

tend to vary from low (<

100

x

10-5

SI), to

moderate (100-1000

x

10-5

SI).

Some

high

measuïements

(-

¹⁰⁰⁰

x

^10-5

SI)

were made and a few very high values

(>

¹⁰⁰⁰

x

10-5 Sl)were recorded.

1Dr. F.Chamalaun provided access to the ecluipment and advised on its usage

Unit Sample Core Declination degrees

Inclination

degrees

Intensity

( x tO-u emu/cc)

Susceptibility

(x10-5

SI)

a

TC

Et4

814 D14 814 E14 814 E14 816

A1 A2 A3 B1 B2 B3 C1 A1

223 200 190 224 226

2r6

228 328

-8 6 -4 -28 -23 -19 -15 30

474t 3262 2975 6332 3818 3259 3687 16265

450

800

72

27 BP

BP

S1 S1 S1 S1 S1 S1

TR5 TR5 TR5 TR5 TR5 TR5 TR5 TR5 TR5 TR5

A1

^2

A3 B1 B2 C1 A1 A2 B1 C1 C2 D1 D1 D2 E1 E2

292 297 296

32r

282 277 762

t62

770 174 166 179 180

r44

185 180

-bt) -46 -46 -61 -49 -44

_3^

-28

-t7

-24 -26 -22 -22 -29 -18 -18

898

t243 1740 1504

It02

847 4 4 nI

5 5 8 7 5 nI

5

1000

210

1.6

.03

US

x2

A1 220 -4 2922 1700 2.3

RG S2

S2 S2 S2

A1 A1 B1 B2

301 298 303 307

-8 -2.5 -36

-54

107 104 80 68

1500 04

Table

2.6: NRM

rneasuremenl,s.

Abbreviations: TC:

Talisker Calc-siltstone,

BP:

Backstairs Passage -bbrmation, US: Ulupa Siltstone, RG: Rathjen Gneiss. The average remanent intensity was used

to

compute the Q ratio.