CHAPTER 6. GEOPHYSICAI RESPONSES OT KNOWN NOCK TYPES 75

CHAPTER 6, GEOPHYSICAL RESPONSES OF I(NOWN ROCI( TYPES 66

CHAPTER 6. CHAPTER 6. GEOPHYSICAI RESPONSES OT KNOWN NOCK TYPES 75

Granite lVlagnetic anomaly

nT

Radiometric anomaly

Oxidation ratio (average)

susceptibility

X 1O-5 SI (average)

Opaque oxides

Genetic type

Magnetìc classifi- cation SYN-TECTONIC GRANITES

PG ³⁵⁰ High 54.t ¹⁰⁰⁰ mt

I

^mt-series

RG r20 High 37.6 ¹²⁰⁰ mt mt-series

TCG 400 High ^43.9 mt mt-series

MI(G

⁴⁰⁰ Low mt mt-selies

RCG =500 ^{37.7 1000} mt

I

^mt-series

MNG ^{31.5 5}

ilm

^? ilm-series

P OST-TECTONIC GRANITES

MMG r1500

^{54.8 3000 mt}

A

^mt-series

MBG ₌₅₀₀ ^{34.3 600} mt

I

^mt-series

SG p500 600 mt

I

^mt-series

WG Low ilm-series

Table

6.1:

Magnetic classification

of granites.

Abbreviations

- ^PG: Palmer Granite,

RG:

Rathjen Gneiss, TCG: Tanunda Creek Gneiss, MKG:

Mt.

Kitchener Granite, RCG: Reedy Creek Granodiorite, MNG: Monarto Granite, MMG: Mannum Granite, MBG: Murray Bridge Granites, SG: Sedan and Long Ridge Granites, WG: Wellington

"Granitet'.

Magnetite is abbreviated to mt and ilmenite to

ilm.

Oxidation ratios and susceptibiüty measurements ^are listed in Appendices F,

B

and C.

CHAPTER 6. GEOPHYSICAL

RESPONSES OF

I(NOWN ROCI(

TYPES ⁷⁶ classified

by

Foden

et

al.

(op. cit.)

as an

A-type

granite (high-level, siliceous,

"dry"

^gra,nite)

but

Ishihara's (op. cfú.) study deals only

with

the original classiflcation

by

Chappell and White (I974) of granites ^as

I

or S

type.

The Mannum Granite had the highest measured susceptibilities of all the granites. The magnetic properties of the Monarto Granite are consistent

with it

being a member of the ilmenite series.

Ishihara (1981) points out

that

recognition of the two ^series of granitoids is an important frrst step

in

mineral

exploration.

The magnetite-series are related

to major

sulphide mineralization

and the

ilmenite-series

to

cassiterite

and wolframite mineralization.

Since

the

classilication depends

on the

magnetic properties

of

^granites,

it

^can

^be

accomplished

by interpretation

of aeromagnetic

data

and

by

outcrop susceptibility measurements. Even

in

weathered outcrops, complete

martitization of

magnetite

to

haematite

rarely

reaches 50%, unless

the

granite has been hydrothermally altered.

On

the

basis of magnetic properties, dimension, and location,

four

groups

of

granites have been recognized:

1.

magnetite-series, generally small (spatial dimensions of the order of a few

km)

granites in

the CMZ

^which have been deformed during

the

Delamerian Orogeny. These include the Palmer Granite, Rathjen Gneiss, Tanunda Creek Gneiss and

Mt.

Kitchener Granite and the much larger Reedy Creek Granodiorite.

2.

ilmenite-series Monarto Granite which has intruded Kanmantoo Group metasedirnents in the CMZ.

3.

magnetite-series,large granites (spatial dimensions of the order of many kilometres) which intrude the EMZ and form ^a belt of intense magnetic anomalies. ^These include the granites

at Murray

Bridge, Mannum, Sedan, Swanport and the

Truro

Creek

"Granite".

4.

^probably ilmenite-series granite (though identification based only on magnetic interpreta-

tion)

intrusive

in

the EMZ

-

the Wellington

"Granite".

According

to

the magmatic

history

outlined

by

Foden et aI.

(in

press), the

first

two groups

of

granitoids were

intruded during the

Delamerian Orogeny and

the

granites

in the last

two groups were post-tectonic.

During the Early

Ordovician, syn-tectonic,

I-type granitic

magÍLas

intruded

Cambrian metasediments.

Most

post-tectonic granites discussed above

are

several

times

more magnetic

than

syn-tectonic granites

probably

due

to the extra magnetite.

The

intrusion of the

post-tectonic granites

is

related

to

an extensional phase following

the

close of

the

Delamerian Orogeny. These granites (and

the

gabbros, basalt, etc. discussed below) form the

Murray

Magnetic High

(MMH).

As mentioned

in

Chapter 5, the

MMH

is a major magnetic feature

which

demarcates

the

eastern

limit of

Precambrian

outcrop. The

extensional ^phase which resulted

in

the intrusion of the granites

is

a significant event

in

the history of the region.

6.2.2 ^Gabbros, amphibolites and dolerites

Small scattered outcrops

of

amphibolites and dolerites

(Liu

and Fleming, 1989) have varialile magnetic

properties.

However

they

are generally

too

small

to

be picked up on more

than

^one or two

flight

lines and

their

magnetic ^response is not known. The Woodside dyke swatm (Pain, 1968) trends

NNW.

Ilmenite has been identified (Alderman, 1931)

but not magnetite.

NNW anomalies

in the vicinity of Tinpot

may be caused

by

magnetic dolerite dykes. The magnetic

trend is

opposite

to the trend of the

arkoses

of the

Backstairs Passage Formation as rnapped

from

aerial photographs

by

Mancktelow (1979).

CHAPTER 6.

GEOPHYSICAL RESPONSES OF K¡üOWN

ROCI{

TYPES 77

Gabbro intrusions give rise

to

distinctive magnetic anomalies.

A

circular magnetic anomaly,

5km in

diameter, and around 1000

nT in

amplitude

(top right

corner

of

Figure 5.4)

is

found

in the MMH.

Lewis, P. (1985) records

that

a

drill

hole

into

the anomaly intersected gabbro at 244m.

The Black Hill Norite

gives rise

to a

negative

anomaly. The

negative

intensity

implies a

strong negative remanent

polarization.

There are several such anomalies

in the vicinity

of the original Black

Hill

Norite which have been interpreted ^as being caused by similar gabbros. Some

of these "Black

Hill"

style magnetic anomalies have been drilled and confirmed

to

be Black

l{ill Norite

equivalents (Wegmann, 1980). Wake-Dyster (fOZa) suggests a

field

reversal during the Ordovician

to

account

for

the negative polarization. According

to

Foden et al.

(in

press), these mafic intrusions are post-tectonic. The variable magnetic properties of the post-tectonic glanites and gabbros may ref,ect several ^phases of post-tectonic intrusion.

6.3 Radiometric ^response

For reasons outlined

in

the

Introduction, it

has

not

been possible

to

estimate

the

radio isotope content

of the soil and

hence,

the

concentration

of

potassium

and

equivalent

uranium

and

thorium in

the rocks. Nevertheless, the use of images of the

total

count channel have been ^used

to

confirm and

in

some cases suggest models for the interpretation of the magnetic

data.

Colour integration of aeromagnetic and radiometric data helped

in

the mapping of faults, contacts, and

in

the correlation of anomalies

with

similar radiometric and magnetic properties.

Establishing

"ground truth" or the

correlation between known geology

with

geophysical response is

afirst

step before extrapolating beyond the areas ofgeological control. The ^grey-scale image

of total

radiometric count and

the

colour composite image (Figure 3.5) were compared

with

geological maps.

The main sources of radiometric anomalies include transported and

in

siúu soils, and exposed metasediments, migmatites and granites. Other rock units

in

the area, e.g. pegmatites, dolerites and amphibolites, are generally too small in spatial extent to ^be readily identified on a

total

count image, assuming they had a characteristic, anomalous

radioactivity. In

the higher metamorphic grade aÌeas, zones of pronounced enrichment and depletion

in

the radioactive elements (usually potassium) are associated

with

metasomatism,

faulting

and

migmatization.

There were also

many anomalous areas which could

not

be readily associated

with

mapped geology.

In

situ soils and bedrock can be generally grouped

into

zones of pronounced enrichment or depletion

in

radioactive elements and ^zones reflecting

stratiform

anomalies. The main lithora- diometric

units

are the metasediments and

granitic

rocks. Possible sources of

radioactivity

are

the

K-minerals

(particularly

orthoclase, microcline and micas)

,

^{and Th}

^and

^[/ bearing rninerals (zircon, sphene, monazite,

allanite). After the

/(-minerals, zircons are probably

the

most irn-

portant

carrier

of

radioactive

minerals. They

^h,ave been identifled

in

biotites

by

means of the pleochroic haloes generated. The relation between anomalies and geology is listed belolv.

1.

Transported soils identified

in the

area include

limited

outcrops of non-radioactive ^sands and moderately radioactive

alluvial of Tertiary or

Quaternary

age.

Stream channel ^and drainage patterns show

up

as radioactive highs

or

lows depending

on the

sediment they carry.

2. Stratiform

anomalies due

to the

metasediments show

up

as curvilinear highs (black and

pyritic

shales

and

siltstones), as curvilinear lows (limestones, almost

pure

marble and