Geological interpretation of geophysical results, Glenormiston special airborne survey, Queensland, 1977

G.A. Young

iu of Mineral Resources, Geology and Geophysics

1 ] F O R !< C P ^ C I A L

BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS 0 m i , Y

REPORT 253

BMR MICROFORM MF209

GEOLOGICAL INTERPRETATION OF GEOPHYSICAL RESULTS,

GLENORMISTON SPECIAL AIRBORNE SURVEY,

QUEENSLAND, 1977

by

G.A. YOUNG

AUSTRALIAN GOVERNMENT PUBLISHING SERVICE CANBERRA 1986

DEPARTMENT OF RESOURCES AND ENERGY

Minister: Senator the Hon. Gareth Evans, QC

Secretary: A.J. Woods, AO

BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS

Director: R.W.R. Rutland

ISSN 0084-7100

© C o m m o n w e a l t h of Australia 1986

Published for the Bureau of Mineral Resources, Geology and Geophysics by the Australian Government Publishing Service

Printed in Australia by Canberra Publishing and Printing Co. , Fyshwick, A.C.T .

CONTENTS

ABSTRACT v

INTRODUCTION 1

GEOLOGY 1

PREVIOUS GEOPHYSICAL INVESTIGATIONS 6

GEOLOGICAL INTERPRETATION DERIVED FROM ANALYSIS OF GEOPHYSICAL DATA 13

Basement composition 13

Basement structure 20

Basement geometry 20

Bouguer anomaly attributable to basement composition and geometry 26

CONCLUSIONS 26

REFERENCES 30

APPENDIX 1 : MAGNETIC BASEMENT DEPTH DETERMINATION USING WERNER DECONVOLUTION 31

Fig. Al. Depth estimates for a rectangular body at 60° magnetic latitude 32

Fig. A2. Depth estimates for Aero Service Corporation

example - interface model 34

FIGURES

1. Location map and flight-line system of the Glenormiston

Special airborne survey area 2

2. Total magnetic intensity contours 3

3. Total magnetic intensity profiles 4

4. Total magnetic intensity surface profiles 5

5. Geology 7

6. Bouguer gravity contours 8 7. Total magnetic intensity contours of a region around the

Glenormiston special survey area 9 8. Geophysical domains of the basement underlying the Georgina

Basin 10

iii

Page

Page

9. Magnetic basement stratigraphy interpreted by extrapolation from Precambrian outcrops fringing the Georgina Basin 11

10. Distribution of Proterozoic units of the Mount Isa Inlier 12

11. Interpreted composition and structure of Proterozoic basement 14

12. Assemblage of magnetic rock units which constitute types A and B basement, as determined by computer modelling, along flight-line L1400 16

13. Interpreted stratigraphy of magnetic basement in the southeast of the Ardmore 1:100 000 Sheet area 17

14. Interpreted stratigraphy of magnetic basement in the southeast of the Ardmore 1:100 000 Sheet area north of the area shown in Figure 13. 19

15. Magnetic basement depth interpretation 22

16. Magnetic basement depth estimates determined from the Werner deconvolution method for flight-line L1010 23

17. Magnetic basement depth estimates determined from the Werner deconvolution method for flight-line L1400 24

18. Magnetic basement depth estimates determined from the Werner deconvolution method for flight-line L1600 25

19. Comparison between calculated and interpolated Bouguer gravity anomalies 2 7

iv

ABSTRACT

A semi-detailed airborne magnetic and gamma-ray spectrometer survey was flown

over part of the GLENORMISTON 1:250 000 Sheet area in 1977 by the Bureau of

Mineral Resources. The principal aims of the survey were: to define relief,

structure and composition of the basement which may have controlled early

Palaeozoic sedimentation; to define the distribution of non-outcropping

granite; and to define the western margin of the Mount Isa Orogen.

Interpretation of the magnetic data indicates that the Proterozoic-

Palaeozoic unconformity forms a broad ridge which extends roughly north-south

through the survey area. This ridge deepens from about 100 m below the

surface at the survey's northern boundary to over 400 m below the surface at

the southern boundary. The thickest Palaeozoic sections occur in the

southwest and southeast corners of the survey area, where section thicknesses

of 800 m and 700 m respectively are interpreted.

Analysis of both magnetic and Bouguer gravity data indicate that the

Proterozoic basement can be divided into four principal rock units. Granite

has been interpreted to form a fifth basement rock unit in the extreme

southwest of the survey area, and to underlie the axial region of the broad

basement ridge.

A series of lineaments oriented N40°E is evident in the magnetic

data. These lineaments are interpreted as faults, some of which show

displacements of up to 3 km in a dextral sense. A major fault mapped a few

kilometres north of the survey area, in the southern part of the Ardmore

1:100 000 Sheet area, appears to extend southwards through the survey area.

The western boundary of the Mount Isa Inlier is interpreted to lie

outside the survey area west of the Georgina River.

INTRODUCTION

In September 1977 the Bureau of Mineral Resources (BMR) flew a semi-detailed

airborne magnetic and gamma-ray spectrometer survey over part of

GLENORMISTON* (Fig. 1 ) . Data were acquired along a 1 km x 6 km grid of

flight lines at a height of 100 m above ground level over an area of 70 km x

70 km. The aims of the survey were: to define relief, structure, and

composition of the basement which may have controlled early Palaeozoic

sedimentation; to assist the search for favourable environments for the

formation of stratabound Pb-Zn deposits; to define the distribution of non-

outcropping granite; and to define the western margin of the Mount Isa

Orogen.

Maps showing the magnetic data in contour, profile, and surface profile

form, at a scale of 1:250 000, were released to the public in July 1981.

Copies of these maps are available from:

Copy Service Government Printer (Production) GPO Box 84 CANBERRA ACT 2600

Figures 2,3, and 4 are small-scale reproductions of these maps.

Radiometric results are not included in this Report as they did not

provide useful information in terms of meeting the aims of the survey.

GEOLOGY

Stratigraphy. The oldest rocks exposed in the survey area are interbedded

metasediments and metavolcanics of the Proterozoic Eastern Creek Volcanics,

which crop out near the northern boundary of the survey area (Fig. 5 ) .

Similar rocks previously mapped as Eastern Creek Volcanics north of the

Names of 1:250 000 Sheet areas are printed in capitals.

2 2 / F 5 4 - 9 / I I

Southern limit of outcropping Proterozoic igneous and metamorphic rocks

Reference to 1:250000 mop ser ies

SANDOVER RIVER

URANDANGI DUCHESS

TOBERMORY GLENORM -ISTON

BOUUA

HAY RIVER MOUNT

WHELAN SPRING VALE

50 km I

Fig. I. Locat ion map and f l ight- l ine system of the Glenormiston Special airborne survey a rea .

2

Magnetic contour range ( n T )

4500 4650 4750 4850 5000 5250 5500

0 10 20 30 km 1 1 1 I

F i g . 2 . Total magnet ic intensity contours.

3

F ig .4 . Total magnetic intensity sur face profiles.

5

survey area have been remapped as separate units - the Oroopo Metabasalt,

Jayah Creek Metabasalt, Eastern Creek Volcanics, and Sulieman Gneiss

(Bultitude, 1982). Younger units, probably totalling about 500 m thick near

Roxburgh Downs, are flat-lying, and consist mainly of calcareous and

quartzose sediments.

Structure. According to Reynolds (1965), Precambrian structural trends

are commonly reflected in the Palaeozoic rocks to the south and southwest of

the Precambrian rock outcrop, and the most prominent lineaments may also be

apparent in the overlying Mesozoic rocks. Minor structures include prominent

east-northeasterly lineaments (Fig. 5) shown by lower Palaeozoic rocks and by

streams, and minor faults.

PREVIOUS GEOPHYSICAL INVESTIGATIONS

BMR has completed extensive regional gravity and aeromagnetic surveys over

the late Adelaidean to Palaeozoic Georgina Basin and the Precambrian Mount

Isa Inlier, which included coverage of GLENORMISTON. This work was done

mainly in the early 1960s, and has been reported by Gibb (1967) and Wells &

others (1966). Bouguer anomaly contours for the survey area (Fig. 6 ) , and

aeromagnetic contours for a region including the survey area (Fig. 7, which

shows the most prominent lineaments in the magnetic data relating to the

survey are a ) , are derived from the regional survey data.

Tucker & others (1979) completed a reinterpretation of all available

geophysical and geological information, to provide a better understanding of

the basement structure and history of the Georgina Basin (see Figs. 8 and 9 ) .

Hone & others (1982) completed a reinterpretation of the geophysical

data over the Mount Isa Orogen, to provide a better understanding of the

principal geological factors which control the size and distribution of

gravity, magnetic, and radiometric anomalies. Their interpretation (Fig. 10)

6

I38°38 139° 17

22°50'

22°03

I38°50 '

C A I N O Z O I C -

M E S O Z O I C -

P A L A E O Z O I C -

22°39 '

7 I39°29'

20 L_

30 km I

U N D I F E R E N T I A T E D

T E R T I A R Y

L O W E R C R E T A C E O U S

C A M B R I A N TO ORDOVICIAN

MIDDLE CAMBRIAN TO U P P E R CAMBRIAN

MIDDLE C A M B R I A N

{ -£ Austral Downs Limestone

Wilgunya Formation

Longsight Sandstone

-f Ninmaroo Formation

Mungerebar Limestone

-£ Steamboat Sandstone

Alluvium, soil and sand

Limestone, chalcedony

Mud stone, sandy to pebbly

Sandstone, conglomerate, siltstone

Limestone, dolomite, sandstone interbeds Calcilutite and calcorenite, soft marly interbeds, chert Sandstone, calcareous sandstone and siltstone, limestone

P R E C A M B R I A N { M I D D L E P R 0 T E R 0 Z 0 l c { Eastern Creek Volcanics [^f] '^/Utic d

s™t°t

s9dimMta

Geological boundary

—$- Anticlinal axis

H(f— Synclinal axis

— Fault

Lineament

Where location of boundaries, faults and folds is approximate, line is broken; where inferred, queried ; where concealed, boundaries and folds ore dotted, faults are shown by short dashes 2 2 / F 5 4 - 9 / I 5

Fig.5. Geology (modified from Reynolds , 1965).

7

F i g . 6 . Bouguer gravity contours.

8

—Genormiston Special survey boundary

^—— Area referredto in Figure 13

— — — Area referred to in Figure 14

Lineaments in magnetic data

Fig.7. Total magnetic intensity contours of a region around the Glenormiston Special survey a r e a .

9

i°30' 139° 30' I 1 1 1 • 1 17° 30'

Domain 4 2 2 / 5 4 - 9 / 1 8

Limit of Georgina Basin (pre -late Adelaidean) outcrop

— Glenormiston Special survey boundory

Fig.8. Geophysical domains of basement underlying the Georgina Bas in (from Tucker & others , 1979).

10

Fig. 9. Magnetic basement stratigraphy interpreted by extrapolation from Precambrian outcrops fringing the Georgina Bas in (from Tucker 8; others, 1979).

11

I38°00' IJ9°30 ' 4I°00'

F^] Basement

| | Western sequence

[ . -| Eastern sequence

L^ j ] Substantially granite

150km

2 2 / F 5 4 - 9 / 2 0 Limit of geophysical expression of Mount Isa inlier rocks

Western limit of shallow Eastern Creek Volcanics type magnetic basaltic rocks

Boundary of Mount Isa Inlier

Glenormiston Special survey boundary

Fig. 10. Distribution of Proterozoic units of the Mount Isa Inlier (after Hone 8 o t h e r s , 1982).

12

places the western boundary of the Orogen near 138°30'E, whereas Tucker &

others (1979) indicated this boundary to be closer to 139°00'E in

GLENORMISTON (Fig. 8 ) . Another difference between these two interpretations

is the distribution of granitic bodies.

Rees (1981) commented briefly upon the significance of the results of

the detailed aeromagnetic survey which is the subject of this Report.

GEOLOGICAL INTERPRETATION DERIVED FROM ANALYSIS

OF GEOPHYSICAL DATA

Basement composition

Rock units which can be recognised as comprising basement are interpreted

primarily from the distribution and characteristics of associated magnetic

anomalies, and to a lesser extent from Bouguer gravity anomalies. Structure

is interpreted from patterns evident in the magnetic data. These patterns

are in part formed by a combination of anomaly truncations and offsets, and

by significant changes in anomaly amplitude along strike. In general such

patterns are related to faulting. Folds are indicated by patterns involving

curvature or closure of elongated anomalies.

From an analysis of the detailed magnetic and regional gravity data,

basement has been divided into four principal categories - A, B, C, and

D; a fifth basement type, E, is interpreted to underlie parts of B, C, and D

type basements (Fig. 1 1 ) .

Type A basement comprises magnetic rock units which form a belt 5 to 10

km wide along the eastern margin of the survey area. Modelling of the

magnetic data indicates that rock units are commonly 2-3 km wide, and that

apparent bulk susceptibilities are in the range 0.04 to 0.12 SI units. All

anomalies have a marked northerly elongation. Forward modelling of these

anomalies for a number of profiles indicates that type A basement has a

simple internal structure involving commonly three magnetic

13

22° |4' I 3 8 ° 3 8

22°50'

3 9 ° I 7 ' 2 2 ° 0 3 '

138° 50

L I 4 0 0

22 /F54-9 /2 I 22°39

I39°29'

B + +

Magnetic rock unit with susceptibility mainly in the range 0015 to 0 03 SI units

Complex magnetic rock unit with susceptibility mainly in the range 004 to 0/2 SI units

Basement type classification Basement type boundary Boundary of granite interpreted from gravity data Lineament interpreted from magnetic data Magnetic anomaly closure

10

i 20km

F i g . II Interpreted composition and structure of Proterozoic basement.

14

rock units - Al, A2, and A3 (Fig. 1 2 ) .

Previously recorded magnetic anomalies over URANDANGI (BMR, 1966) leave

no doubt that these basement rocks are the strike extension of the Jayah

Creek Metabasalt mapped by Bultitude (1982) in the Ardmore 1:100 000 Sheet

area. Relatively non-magnetic elements (A2,3 in Fig. 12) would be

interpreted as sandstone similar to the Timothy Creek Sandstone Member

within the Jayah Creek Metabasalt.

Type B basement magnetic units parallel those of type A basement,

exhibiting the same pronounced northerly anomaly elongation, which tends to

swing east of grid north in the northeast corner of the survey area.

Modelling indicates that these rock units have bulk susceptibilities in the

range 0.01 to 0.04 SI units; most are in the range 0.015 to 0.03 SI units.

The shape and amplitude of the anomalies suggest their source to be a rock

unit consisting of metabasalt and interlayered sandstone beds with a lesser

overall content of metabasalt than type A basement magnetic rock units.

The strike of type B basement extends northwards into the southeastern

part of Ardmore 1:100 000 Sheet area. The stratigraphy of magnetic basement

in this locality (Fig. 13) has been interpreted by extrapolating from

outcrop, which implies that either the Sulieman Gneiss or Jayah Creek

Metabasalt is the source of type B basement. As some anomaly trends

associated with the Sulieman Gneiss are oblique to those of type B basement,

the preferred interpretation is that the Jayah Creek Metabasalt is the source

of type B basement, which is further interpreted to contain less metabasalt

than type A basement.

Type C basement contains two sets of cross-cutting magnetic units -

oriented northerly and northwesterly - with anomaly amplitudes tending to

increase at magnetic unit intersections. Bulk susceptibilities for both sets

15.

51000

49750

D / E type basement-

C R 0 S S - S E C T I 0 N B type basement A type basement-

20

A 2

A 2 , 3 :

Susceptibilty -0025 -015 018

Depth to upper -4 -4 -4 surface (km)

•013

4

• 0025 •025 031 0 3 8 078

7 7 -8

10 km _ J

• 0025 •059 0 6 5 0 0 2 5

2 2 / F 5 4 - 9 / 2 2

Refer to Fig. II for legend

Fig. 12. Assemblage of magnetic rock units which constitute types A and B basement , as determined by computer model l ing , along f l ight- l ine L I 4 0 0 .

16

2 I ° 5 0 '

TOTAL MAGNETIC INTENSITY CONTOURS

] Less thon 2500 nT

| | 2500 - 2750 nT

2 750 - 3500nT

GEOLOGY BASED ON DUCHESS S P E C I A L

1-250 0 0 0 S H E E T

] e g s Sybeiio Granite

| Pjv J o y a h Creek Metobasall

B ° v Oroopo Metabasalt

Bsg Suliemon Gneiss

Fault

I 39° I0 ' 2 2 ° 0 0 '

139° 20'

2 l ° 5 0

MAGNETIC B A S E M E N T S T R A T I G R A P H Y INTERPRETED FROM MAGNETICS AND

MAPPED GEOLOGY

~ — Mapped fault Interpreted boundary of geological unit Trend of mognetic anomaly

2 2 ° 0 0 ' I 3 9 ° I 0 Z 2 / F 5 4 - 9 / 2 3 | 3 g o 2 C , '

Fig.13. Interpreted stratigraphy of magnetic basement in the southeast of the Ardmore 1.100 0 0 0 Sheet area. Location of this area relative to the Glenormiston Specia l survey area is shown in Figure 7.

17

of units lie within the range 0.01 to 0.04 SI units (most are in the range

0.015 to 0.03 SI u n i t s ) ; the northerly set occupies the upper end of the

range, and the northwesterly set has susceptibilities at the lower end of the

range.

Magnetic contours and geology in the Ardmore Sheet area (Fig. 13)

indicate that the northwesterly trending magnetic units correlate with either

the Oroopo Metabasalt or Sulieman Gneiss. The northerly trending magnetic

units are considered to indicate the presence of type B basement rock units.

Type D basement underlies much of the remainder of the survey area to

the west of types B and C basement. Magnetic rock units are characterised by

elongated anomalies which in many localities have such a pronounced curvature

as to complete anomaly closures, indicative of basins or domes.

Figure 14 illustrates the contour pattern created by what is interpreted

to be the doming of magnetic rock units by intrusion of Sybella Granite in

the Ardmore Sheet area. The magnetic rock units are interpreted to be

metadolerite intrusions, possibly sills, within Saint Ronans Metamorphics.

The Saint Ronans Metamorphics are interpreted as the source of type D

basement with magnetic anomalies produced by amphibolite or metadolerite

horizons at the basement surface. The ratio of non-magnetic to magnetic-

related strata is much higher than in types B or C basement. Bulk

susceptibilities of magnetic units are, however, similar to those for types B

and C basement.

Type E basement, or perhaps intrabasement, is interpreted as intrusive

Sybella Granite, primarily from an analysis of the Bouguer gravity data.

Inspection of gravity results and geology in URANDANGI reveals the

distinctive anomaly gradient associated with this granite/metabasalt contact

between latitudes 139°15' and 139°25' throughout the Sheet area but best

displayed in the north.

18.

I 3 9 ° I 0 ' I 3 9 ° I 8 '

Fig.14. Interpreted stratigraphy of magnetic basment in the southeast of the Ardmore 1:100 0 0 0 Sheet area north of the area shown in Figure 13. Locat ion of this area relat ive to the Glenormiston Special survey area is shown in Figure 7.

19

It appears that only a thin veneer of metamorphics overlies the

granite, and that some magnetic anomalies may have a significant component of

remanent magnetisation resulting from metamorphic aureole effects. Granite

is most likely to be present at the basement surface at the centre of

elliptically shaped magnetic anomalies.

Basement structure

A series of lineaments, Ll to Lll, which strike northeasterly (roughly N

40°E true) is interpreted from the magnetic data (Fig. 1 1 ) .

Lineaments Ll, L2 and L6, spaced at intervals of 15 km, are most evident

in regions of types A and B basement where magnetic rock units are offset by

up to 3 km in a dextral sense.

Lineaments L5, L7, and L10, spaced at intervals of 10 km, are most

obvious in the region of type D basement. These lineaments mark significant

changes in anomaly amplitude along strike, including anomaly truncation, and

distinctive anomaly lows. L10 is particularly notable as an alignment of

anomaly lows, most apparent where it intersects the northern end of the domal

type anomaly closure dl.

Anomaly closures or near-closures, dl to d4 (Fig. 1 1 ) , are interpreted

as being produced by the doming of magnetic rock units in type D basement by

intrusion of Sybella Granite as mentioned previously. In the western part of

the survey area, flexures in anomaly trends suggest that the principle axis

of folding is oriented a few degrees west of north (about N 15°W tr u e ) .

Alternatively the sources of anomalies dl to d4 could be associated with

metamorphic aureoles.

Basement geometry

There is no evidence to suggest that any significant magnetic anomalies

are produced by the Palaeozoic or Mesozoic cover rocks in the survey area.

20.

These rocks are primarily of calcareous, quartzose composition. Accordingly

magnetic basement in general is expected to define the unconformity between

the Proterozoic metamorphic and igneous rocks, and the flat-lying lower

Palaeozoic sedimentary rocks.

Analysis of depth to basement (Fig. 15) was carried out using a

combination of interpretative techniques, which included:

1) computer-based interpretation of data along selected flight lines, using

the Werner deconvolution method (see Appendix 1) ;

2) computer-based interpretation of distinct anomalies, using inversion

modelling procedures; and

3) manual methods of interpretation, using analogue data records applying

such methods as Peters's half-slope (Peters, 1949).

Over much of the survey area, depth contours are expected to be reliable

within an accuracy of about ±15 per cent. The main exception to this is in

the southeast corner where closely spaced sources produce a particularly

complex anomaly. Depth estimates on this compound anomaly using any method

will tend to produce values too large. Nevertheless, the shape of the

anomaly does indicate a deepening of the unconformity.

Source depth analysis by the Werner deconvolution method, applying a

simple susceptibility interface model, for survey lines L1010, L1400, and

L1600 (Figs. 16, 17, and 18) shows that the shallowest sources resolved are

weakly magnetised; these are most probably associated with laterite. The

analysis for L1600 (Fig. 18) is of particular interest as it indicates two

magnetic horizons: the more clearly resolved horizon is at a depth of 700 m;

above this, at about 300 m, is a less distinctly resolved magnetic source.

Neighbouring lines provide better evidence for the shallower source at about

400 m and less evidence for the deeper source. The shallower source is more

readily resolved using manual interpretation methods.

21.

F i g . 15. Magnetic basement depth interpretation.

22

Fig.16. Magnetic basment depth estimates determined from the Werner deconvolut ion method for f l i g h t - l i n e L I O I O .

Magnetic basement surface derived from upper l imits of causat ive s o u r c e s

Depth estimate of causative body, symbol size reflects intensity of magnet isat ion; magnetisation direction indicated by s t ra ight - l ine segment within symbol

0

Magnetic basement surface derived from upper l imits of causat ive s o u r c e s

Depth estimate of causative body, symbol size reflects intensity of magnetisation ; magnetisation direction indicated by straight- l ine segment within symbol.

The deeper magnetic horizon, if real, would have to be interpreted as an

intrabasement feature, most probably associated with the Sybella Granite.

Bouguer anomaly attributable to basement composition and geometry

A calculated Bouguer gravity anomaly - modelled from the basement

composition presented in Figure 11, from the basement geometry presented in

Figure 15, and from density values consistent with those measured by Hone &

others (1982) - correlates well with the observed Bouguer anomaly (Fig. 19),

thus lending support to the analysis of basement composition. However, the

following points must be noted:

1. The gravity data themselves have been used to extrapolate the extent and

lateral boundaries of granitic bodies.

2. A lower-density cell (-0.03 t/m^) has had to be introduced to develop

fully the gravity low west of the most magnetic rock unit. A dome-

shaped granitic body with the lower-density value would be one of many

possible models to fit the data.

3. The lower surfaces of all major units have been set arbitrarily at 5 km,

which therefore controls density contrasts as indicated. Should the

lower surfaces be either shallowed or deepened, then density contrasts

would need to be increased or decreased respectively to retain data

correlation.

4. The Bouguer contour map presented in Figure 6, from which profile XY was

constructed, is based on a 7 x 15 km gravity observation grid. This

necessarily limits the accuracy of profile XY, particularly with respect

to anomaly shape and to a lesser extent anomaly amplitude.

CONCLUSIONS

Analysis of the semi-detailed magnetic and regional gravity data has enabled

an interpretation to be made of the relief, structure, and composition of

26

Bouguer onomoly calculated (datum -120pm s'2)

Bouguer onomoly interpolated

Densities p in units t . m 3

X

1

0 L

iO

p 2 68

I O O O —\

2000

x 3000

0. ui

° 4000 —I

5000

p2 78

+ + + + +

+ + + + + +

+ + + + p2 68

-r f + + + +

+ + +

p2 78 \p2 65 p2 75

B A S E M E N T T Y P E E

20km

P2 82

22 /F54-9 /29

,+ +; Granite basement 4

F i g . 19. Compar ison of Bouguer grav i ty anomaly in terpola ted a l o n g sect ion X Y

( F i g u r e 6) with that m o d e l l e d from interpreted b a s e m e n t c o m p o s i t i o n and g e o m e t r y p r e s e n t e d in F i g u r e s II and 15.

27

Proterozoic basement which underlies the lower Palaeozoic sedimentary rocks

in the Glenormiston survey area.

The Proterozoic-Palaeozoic unconformity is interpreted from the magnetic

data to form a broad ridge which extends through the survey area from

22°05'S, 139°10°E to 22°43'S, 139°15'E. This ridge pitches to

the south from less than 100m below the surface at the northern survey

boundary to over 400 m below the surface at the southern survey boundary.

The thickest Palaeozoic sections occur in the southwest and southeast corners

of the survey area, where section thicknesses of 800 m and 700 m respectively

are interpreted. The reliability of the latter estimate is not high owing to

the complex character of large magnetic anomalies used for depth analysis.

The Proterozoic basement within the survey area has been divided into

four principal rock units. These are equated with a southerly extension of

the Jayah Creek Metabasalt adjacent to the eastern survey boundary; to the

immediate west, a near-parallel belt which could constitute a southerly

extension of the Sulieman Gneiss or alternatively, and more likely, a rock

unit showing some similarity to the Jayah Creek Metabasalt unit but with

a lower metabasalt content; a small zone which flanks the southern outcrop of

the Sybella Granite and which has anomaly characteristics of cross-cutting

metabasalt units; and finally, over the western half of the survey area, an

extensive basement unit which contains arcuate magnetic anomalies of a

similar character to those occurring over the Saint Ronans Metamorphics

exposed to the north. The Sybella Granite is interpreted to underlie the

eastern extremity of the westernmost basement unit.

The distribution of non-outcropping granite has been interpreted

primarily from gravity data. Results obtained are consistent with those

presented by Hone & others (1982) in their regional study of the Mount Isa

Inlier. Furthermore the results of this more detailed study support Hone &

28.

o t h e r s 1 interpretation of the western boundary of the Mount Isa Orogen to the

west of the Georgina River, and not east of it as suggested by Tucker &

others (1979).

The most obvious basement structure evident in the magnetic data is a

suite of lineaments oriented approximately N40°E. These lineaments are

interpreted as faults, three of which show displacement of magnetic rock

units by up to 3 km in a dextral sense.

Arcuate magnetic anomalies in the most westerly basement unit are

interpreted as evidence for the presence of domed magnetic rock units. The

contrast between this anomaly form and the prominent parallel linear

anomalies over the eastern basement units may delineate a major fault which

could be a continuation of the major fault mapped 9 km to the east of the Rufus

Fault Zone in the southern part of the Ardmore 1:100 000 Sheet area.

The interpretation presented in this Report should enable hypotheses to

be developed to outline the more favourable areas for the possible location

of stratabound Pb-Zn mineral occurrences. The intersections of N40°E

interpreted faults with the possible major fault marking the eastern boundary

of the Sybella Granite must be attractive targets, particularly as the loci

of these intersections lie along the interpreted Proterozoic basement high.

29

REFERENCES

AERO SERVICE CORPORATION, 1974 - Understanding and use of Werner deconvolution in aeromagnetic interpretation. Aero Service Corporation Publication

BMR, 1966 - Urandangi, Queensland - 1:250 000 total magnetic intensity contours. Bureau of Mineral Resources, Australia, F54/B1-17.

BULTITUDE, R.J., 1982 - Ardmore, Queensland - 1:100 000 Geological Map Commentary. Bureau of Mineral Resources, Australia.

GIBB, R.A., 1967 - Western Queensland reconnaissance gravity survey 1957-1961. Bureau of Mineral Resources, Australia, Report 129.

HONE, I.G., CARBERRY, V., REITH, H., & WARNES, A., 1982 - Mount Isa Inlier geophysical study; in Geophysical Branch summary of activities 1981. Bureau of Mineral Resources, Australia, Report 238,18-20.

HSU, H.D., & TILBURY, L.A. 1977 - A magnetic interpretation program based on Werner deconvolution. Bureau of Mineral Resources, Australia, Record 1977/50.

PETERS, L.J., 1949 - The direct approach to magnetic interpretation and its practical application. Geophysics., 14( 3 ), 290-320.

REES, J.W., 1981 - Glenormiston aeromagnetic interpretation; in Geophysical Branch summary of activities 1980. Bureau of Mineral Resources, Australia, Report 231, 207-208.

REYNOLDS, M.A., 1965 Glenormiston, Queensland - 1:250 000 Geological Series. Bureau of Mineral Resources, Australia, Explanatory Notes SF/54-9.

TUCKER, D.H., WYATT, B.W., DRUCE, E . C , MATHUR, S.P., & HARRISON, P.L., 1979 - The upper crustal geology of the Georgina Basin region. BMR Journal of Australian Geology & Geophysics, 4, 209-2 26.

WELLS, R., MILSOM, J.S., & TIPPER D.B., 1966 - Georgina Basin aeromagnetic survey, Queensland and Northern Territory, 1963-1964. Bureau of Mineral Resources, Australia, Record 1966/142.

WERNER, S., 1953 - Interpretation of magnetic anomalies as sheet-like bodies. Sveriges Geologiska Undersok. Ser. C.C. Arsbok 43, N: 06

30

APPENDIX 1

MAGNETIC BASEMENT DEPTH DETERMINATION USING WERNER DECONVOLUTION

The following text has been extracted from a BMR Record written by H. Hsu &

L. Tilbury (1977), who developed a computer program for the quantitative

interpretation of magnetic data for use within BMR. The process of

interpretation involves analyses of magnetic data to provide information

about the source of an anomaly. The program was written in FORTRAN IV, and

applied successfully to both theoretical and observed marine magnetic data

using the CYBER 76 computer at CSIRO's Division of Computing Research.

Subsequently the program was extended for use with aeromagnetic data.

The initiative and idea of developing such a program was derived from a

document published by Aero Service Corporation (1974) about the technique of

interpretation known as Werner deconvolution (Werner, 1953). The model

adopted in BMR's interpretation procedure assumes that the observed magnetic

field arises from two discrete sources and a quadratic magnetic background.

Theoretical models

The magnetic anomaly produced by a simple rectangular body of 0.038 SI

units susceptibility contrast 4 km wide, 2 km thick, and buried to a depth of

1 km is shown in Figure Al, together with its interpretation applying the

Werner deconvolution for an interface model. It is apparent that the

technique defines the top corners of the bodies extremely well, as shown by

the major clusters of estimates. However, the bottom corners are transparent

to this method (and to most magnetic inversion methods). Note that outliers

occur, even for a simple theoretical anomaly curve. In practice, with many

bodies and observed data, these outliers can disguise the true form of the

magnetic basement. It is this aspect, of distinguishing between 'good' and

'bad' estimates, that is most difficult in the interpretation.

3 1

400

2 2 / F 5 4 - 9 / 3 0

Horizontal distance ( k m )

F i g . A l . Depth es t imates for a r e c t a n g u l a r b o d y at 6 0 ° magnet ic la t i tude .

32

Aero Service Corporation example

A simulated geological cross-section (Fig. A2) - similar to the example

of Aero Service Corporation (1974) - was chosen by Hsu & Tilbury (1977) for

comparison when testing the Werner deconvolution program. It simulates -

from left to right - a reversely polarised vertical dyke, a normally

polarised vertical dyke, a normally polarised dipping dyke, a suprabasement

plate terminated by small normal faults, a graben with dipping faults (or

contacts) on each side, and two vertical interfaces (or contacts) forming a

'well' in the basement. The theoretical anomaly was computed using a

susceptibility of 0.013 SI units for the main basement block, susceptibility

contrasts of 0.013 SI units for the dykes, and field parameters of 7°E

declination, 60° inclination, and 60 000 nT total field.

The results using the interface model option in the Werner program are

shown. The top corners of the major interfaces on the graben and 'well' are

reasonably well-defined, but the depth extents of these features are poorly

expressed. Estimates over the minor features on the section, namely the

dykes and small faults, are under-estimated: that is, the depths to the

bodies are too shallow, and the clusters are more diffuse.

In general when the interface model can be applied, the surface of the

magnetic basement is fairly well represented by a line joining the near-

apices of clusters.

References

AERO SERVICE CORPORATION, 1974 - Understanding and use of Werner

deconvolution in aeromagnetic interpretation. Aero Service Corporation

Publication.

33

0.0 10.0 20.0 30.0 40.0 SO.O 60.0 70.0 80.0 90.0 100.0

100.0 1 1 1 1 1 1 1 1 1 1 1

0.0 10.0 20.0 30-0 40.0 60.0 60.0 70.0 80.0 90.0 100.0

HORIZONTAL D ISTANCE (km) 2 2 / F 5 4 - 9 / 3 I

THEORETICAL MODEL - RERO SERVICE EXAMPLE

Fig A 2 . Depth estimotes for Aero Service Corporat ion example - interface model .

34

HSU, H.D., & TILBURY, L.A., 1977 - A magnetic interpretation program based on

Werner deconvolution. Bureau of Mineral Resources, Australia, Record

1977/50.

WERNER, S., 1953 - Interpretation of magnetic anomalies as sheet-like bodies.

Sveriges Geologiska Undersok Ser. C.C. Arsbok 43, N:06.

35

B M R Report 253 Interpretation of geophysical results, Glenormiston special airborne survey. G.A. Young