BMR Journal of Australian Geology & Geophysics, 2 (1977) 241-251 241

The geology and magnetic characteristics of precious opal deposits, southwest Queensland

B. R. 'Senior, D. H. McColl, B. E. Long*, & R.I. Whiteley*

Precious opal in southwest Queensland is found within a weathered proffie developed In sedimentary rocks of the Cretaceous Winton Formation. This kaolinitic proffie is the product of two periods of deep chemical weathering. The first weathering event Is Maastrichtian to Early Eocene, and formed a tri-Iayered proffie (Morney proffie) In excess of 90 m In thickness. At this time Ironstone was chemically precipitated within the basal layer. A period of sedimentation along river systems, with fragmentation and minor erosion of this proffie along InterOuves, was followed by a second weathering event about the Late Oligocene. A morphologically distinct proffie formed (Canaway proffie), consisting of an Indurated crust and mottled zone, which grades down Into a varying thickness of the residual older proffie. During the second weathering event silica migrated as an aqueous sol, and was precipitated within voids In ironstone host rocks. The gemmological properties of Queensland boulder opal are described In relation to the weathering history and beddIng characteristics of the opal deposits.

Rocks which contain scattered opal deposits are poorly exposed In scarp-bounded mesas and Oat-topped landforms, or are concealed below a pedimented land surface. Ironstone host rocks are remanently magnetised, and an exploration method based on ground magnetic surveying, using portable and vehicle•borne magnetometers, is discussed.

In:trod uction Precious opal deposits in Queensland are scattered

throughout a 300 km-wide belt, which extends northwest from the Queensland/New South Wales State border for about 900 km to the vicinity of Winton township (Fig. 1). Approximately sixty opal fields, mines and prospects have been worked, and some production is currently in progress , particularly in the Eromanga and Quilpie districts. Not all of the mines and prospects are named in Figure 1 owing to the scale used, and also because the locations of many open•cut workings post-date the most recent geological maps of the region.

All opal deposits in western Queensland lie within the central part of the Eromanga Basin. The deposits are in chemically weathered sedimentary rocks of the Late Cretaceous Winton Formation, which is the youngest unit of the Eromanga Basin sequence. The distribution of the Winton Formation, and opal-bearing weathered rocks, is given in Figure 1.

Previous geological mapping in this part of the Eromanga Basin established that opal was a product of deep chemical weathering of the Winton Formation, and its occurrence was related by Ingram (1969) to the distribution of kaolinitic weathered profiles. However, subsequent geological and geomorphological investigations, which involved detailed mapping, geochemistry and clay mineralogy of the Winton Formation, have shown that the Cainozoic weathering history of the region is more compli•cated than was previously thought. Three weathered pro•files are recognised, and the one which contains sporadic occurrences of precious opal is believed to be the product of more than one weathering event.

The objectives of this paper are to elucidate the weathering history and geological occurrence of the south•west Queensland opal deposits. In addition the distinctive gemmological properties of opal from this region is discussed in relation to sedimentary structures in which opal deposits are found. Finally, a review is made of potential exploration techniques, particularly ground magnetic surveying, which could be of assistance in the search for new opal deposits.

Mining activity The history of the opal mining activity in western

Queensland was reviewed by Connah (1966). Early activity commenced about 1880 and virtually ceased by 1900, when a severe drought and shortage of food for horses resulted in the abandoning of the workings. Jackson (1902) visited numerous workings in the Eromanga Mineral Field in 1901 and found them almost deserted. Mining activity almost ceased in the region-except for the Hayricks mine and nearby localities, where production continued until about 1963. Elsewhere, production was a result of the activities of fossickers who worked known deposits only during favour•able seasons (Connah, 1966).

Open-cast mining commenced in the mid 1960's. Success with this method led to the mining of many formerly abandoned workings using bulldozers, and activity reached a peak in about 1973, when about 20 bulldozers were working in the northwest part of the Eromanga Mineral Field. Most of the operations were engaged in extending the deposits discovered in the last century.

Jackson (1902) estimated that the total value of produc•tion from 1891 to 1901 was approximately £130 000 ($260000). At that time Queensland opal was regarded as inferior gemstone to the opal produced in the Lightning Ridge and White Giffs opal fields of New South Wales. However, the distinctive bright green, blue and red colours set in the dark background of Queensland 'boulder'- opal have in recent years become greatly prized and values have increased dramatically. Total recorded value of opal pro•duced in Queensland between 1966 and 1974 is estimated at 1.3 million dollars (Gourlay and others, 1976). This figure is a conservative estimate as there has been an increasing trend of direct sales to overseas buyers , on which statistical information is incomplete or non-existent. Production in Queensland probably peaked in 1974 and the industry is now declining, owing to the depletion of opal from old workings which were re-opened by earthmoving machinery. Very few, if any, new deposits have been discovered despite the apparent geological potential ofthe region. Possibly this is because of the conservative attitudes of the miners who hesitate to explore unknown areas, and the miners' limited financial resources-which usually do not permit more than

• School of Applied Geology, University of New South Wales, P.O. Box 1, Kensington, N.S.W. 2033.

242 B. R. SENIOR AND OTHERS

K o 50 150km LI ____ -L� ____ ~ ____ ~,~I --c\

I K 1....""'

I \ \ \ J I \ \ 0 \

~ I

K

Quaternary alluvium Tertiary sediments Whitula Formation Eyre Formation

WInton Formation Rolling Downs Group

Weathered prof iles main ly on Winton Format ion

Morney ~ Conaway profile ~ profi le

Kynuna 2 name unknawn 3 Opaltan 4 Horse Creek

Mt Fairv ie w \.r\ ~_\

../ \

5 6 Magic

\

Kw " \

l , ') ( , \ (

7 Junda h 8 Opalville 9 Ger man a Bronks 10 Hayricks II Valdare 12 Marble Arch 13 Bull Creek 14 Northwest Eromanga area * 15 Gooseberry a Quartpot 16 Mulcahy1s 17 Coona valla 18 Pinnacles 19 Yellow Nell a Litt le 20 Lamons a Potts 2 1 Ru sse ls · 22 nome unknawn

Lush ingtons Pride of the Hills Goodmans Flat

26 Duck Creek 27 Sheep Stat ion Creek 28 Emu Cree k 29 nome unknown 30 Fiery Cross 31 Koroit a Hollaways 32 Yowah 33 Block Gate 34 Elbow

Wonde r

* Includes: Breakfa st, Stanley, Friday, Fish Ponds, Hausingtons, Exhibition, Stoney Creek, Bung Bung, Scotchman, Al add in, Mots

-- Formation or weathered profile bdy - - Margin of the WInton Formation

Hard, Hammonds , Pepp in and Webbers, Gem, Mascotte and Gl adstone

- Fault 0 16 Precious opal occurrence with reference no.

Figure 1. Distribution of weathered promes, IIIId Mesozoic IIIId Cainozoic rocks, in relation to precious opal occurrences In lIOuthwest Queensllllld.

cursory excavation where opal is not immediately encountered.

The geological occurrence of opal in southwest Queensland

Weathered rocks of the Winton Formation constitute two weathered profiles. Measured reference sections are estab•lished (Fig. 2), and are here infonnally named the Morney and Canaway profiles. Both profiles are composed of kaolinised sedimentary rocks , and consist of a sequence of

weathered layers which grade from the land surface down to unweathered parent rock. The arrangement of the compon•ent layers serves to distinguish each profile. For example, the older Morney profile (Fig. 2) consists of three layers of roughly equal thickness forming a profile up to 90 m thick. The younger Canaway profile is somewhat thinner, and consists offour layers of unequal thickness in a profile up to 40m thick.

In many areas the weathered profiles are concealed below younger quartzose sedimentary rocks (Cainozoic Eyre Fonnation and unnamed equivalents), and these in turn have been strongly silicified to form silcrete. The

PRECIOUS OPAL DEPOSITS, SOUTHWEST QUEENSLAND 243

W

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o

10

20

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50

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100 METRES

Lat. 25°16'20" Long . 141° 50 ' 20"

Si

Si

Si

Om Om Om <= Om Si Si

Si

Om

<= "f Om Om

e Fe

~ 01 Om

°

e Om

°

Fe

Fe

Fe

Fe

so mpli ng points '1,1 / v h columnar Silcrete it:. ~

s • nodular Silcrete c fe stnd qtz

. .. .. .. .

wh m t c grnd kaol, c minor intbdd

wh kaol, c conchoidal fracture 8 rd-brn mottled zones . Minor intbdd m grnd kaol, 8 Mdst

wh kaol wh kaol, c conchoidal fracture

v f kaol, 8 wh silic

wh sllic kaol

wh kaol f grnd, c intbdd lamd Mdst 8 Sltst Ironstone lens

wh sllic f grnd kaol hd yel stJic, c -vertical fe pipes yel - brn f grnd brn, c vertical fe tubules

dk brn ochreous f grnd, c ironstone wh f grnd kaol, c minor intbdd purp Sltst. Ironstone concretions wh intbdd wh f grnd kaol brn fe stnd

wh f grnd kaol, c intbdd pI purp intbdd lamb, 8 f grnd kaol brn f grnd kaol purp 8 wh m grnd kaol , c fe lam

purp 8 y el f grnd kaol

purp 8 yel, minor intbdd Mdst (wh) to v p I purp f grnd kaol m grnd hd kaol, above yel brn lam

concealed

brn m grnd kaol wh 8 fe stnd, c minor intbdd Mdst

purp m grnd kaol, c basal wh

concealed intbdd purp Sltst 8 m grnd kaol f grnd kaol scree

Figure 2. Reference sections of the Morney and Canaway proffies.

distribution of the kaolinitic Morney and Canaway profiles and the silcrete profile is shown in Figure 1. Deposits of precious opal relate to the distribution of the Canaway profile.

Later Cainozoic erosion has truncated or completely des•troyed these profiles in places , particularly along the axial zones of folds and in the vicinity of faults . Elsewhere the profiles form flat-topped landforms which are in part scarp•bounded, or crop out as low rubble-covered rises on an extensively pedimented surface.

Geological investigations have provided a maximum and minimum age for weathering in this region . The Winton Formation, the youngest formation in the Eromanga Basin, is Cenomanian based on palynology (Burger in Senior, 1977). The Morney and Canaway profiles extend as an

Lot. 26°00'40" S Long. 143° 53' 50" E

o /sompli ng points fe st sll/(; breccia c vertical cracks, p ipes 8 turbulence zones. Om 8 fe pisol i tes w

...J

u.. 10

o a: Cl.

> <t

20

3: 30

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BE DD ING STRUCTUR ES

bioturbation

0 1 ferr uginous concretions

\\ slickensides

° mudclast ( rounded)

t:. mudclast (angular)

<= parallel lamination

fe stnd, wh to red - brn breccia c macrocolumnar structure.

fe Om c cell patterns, breccia 8 remnont Sltst pk intbdd Sltst 8 Mdst Mudclast conglomerate pk silk:, fe stng, dessication cracks

intbdd wh kaol, c yel 8 pk, minor brn kaol m Sst

pk m grnd silty

wh f grnd kaol

wh to pk m grnd kaol, c ironstone

Scree

II cross stratification (group)

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S i siliceous

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BED THI CK NESS

rtJ very thick > 120 cms

D thick 60 - 120

c medium 5 - 60

LITHO LOGY

o sandstone G stlfstone

thin 1-5

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a mudstone or claystone

unbroken mantle from the opal-bearing region eastwards into the Surat Basin. In the Roma-Amby area of this basin there are a number of small basalt flows which overlie these weathered rocks (the identity of the particular profile is unknown). Exon and others (1970) dated these basalts radiometrically, using the KI Ar method, at 23 m.y., and equated the underlying weathered rocks with those of the Winton Formation of the Eromanga Basin . Consequently the maximum and minimum constraints for weathering fall approximately within 7S m.y. (Cenomanian to Early Miocene).

The recognition of two kaolinitic profiles in the Eromanga Basin (Eg. 1) indicates that more than one weathering event may have occurred. It can be demon•strated that the simple trizonal form of the Morney profile

244 B. R. SENIOR AND OTHERS

merges laterally into the somewhat thinner four-layer Can away profile. The crust and mottled zones of the Can away profile may grade directly into unweathered rock, or more usually as illustrated in Figure 1 a variable thick•ness of kaolinitic weathered rock underlies and is a relict of the former Morney profile. Thus the crust and mottled zones have formed from and cut across the former tri•layered arrangement ofthe parent profile. This relationship was produced by variable erosion and fragmentation of the Morney profile, followed by cementation of the surficial fragmented layers. The convergence of the basal ferru•ginous zone with the indurated crust appears to be an important factor in opal formation . In places where the crust and basal ferruginous zones are in close proximity, the host ironstone bodies were favourably located within a fluctuating groundwater table which permitted the deposition and dehydration of siliceous material. Other factors are also important, however, and will be discussed later.

Idnurm & Senior (in press) studied the palaeomagnetic directions of the ferruginous components of these profiles and compared their results with the Late Cretaceous and Cainozoic apparent polar-wander curve for Australia (McElhinny and others, 1974). This investigation demon•strated conclusively that an age difference exists for the two profiles. The Morney profile was found to be Maastrichtian to Early Eocene, and the Canaway profile to be approx•imately Late Oligocene. The Canaway profile apparently formed along interfluves, and may have developed simul•taneously with the surface silcrete across adjacent plains mantled with quartzose clastics. Silicification was wide•spread, and was probably contemporaneous with the pre•cipitation of silica within voids present in ironstone rock bodies in the Canaway profile. The absence of precious opal in silicified quartzose rocks is a matter for conjecture, but may be related to the lack of suitable bedding structures , coupled with the high porosity and permeability of the sandstones. However, if the association between opal and silcrete development is correct, and if the opal is a product of rock weathering which formed the Canaway profile, it is likely that the opal is 15 to 32 m.y. old.

During earlier geologic investigations (Ingram, 1969, 1971a & b; Senior, 1971) it was thought that all kaolinitic weathered rocks of the Winton Formation were potentially opal-bearing. The Morney profile (Fig. 2) with its three layers, contains suitable bedding structures, and in the basal zone the essential ironstone host rocks. The ironstone bodies, however, are non-opaline although they contain voids with characteristic concentric, radial or septarian patterns. Emplacement of opal within these voids was the result of partial erosion of the Morney profile and develop•ment of a silica and iron oxide-rich crust. The resultant Can away profile has four zones. Truncation of the older profile brought potential ironstone host rocks closer to the weathering landsurface, and hence into a geochemical environment where silica was mobilised .and precipitated.

The size and geometric arrangement of the voids within the ironstone appear to have affected the microstructure, and hence the intensity of the colour, of the opal. Within individual ironstone concretions, fissures tend to vertical rather than horizontal orientation, with some concentric and subradial arrangements. Large voids and fissures (> 1 cm wide) tend to contain non-precious varieties of white, grey, or blue opal. Most of the precious opal is within narrow, or attenuated, voids. Very fine hair-like cracks frequently contain high quality, though unusable, varieties of precious opal.

Commonly the opal displays rhythmic or cyclic layering, which appears to have formed where successive increments of opaline silica have settled under essentially hydrostatic

Flgure 3. Oplll partially filling a void within an ironstone concretion.

conditions to produce a pronounced horizontal layering and colour banding. Commonly the void is incompletely filled and a free , level, opaline surface with a meniscus-like curve is present; as in Figure 3.

Accumulation of precious opal was a slow process in which successive cycles of saturation and dehydration are recorded by the varied colour laminations indicating sharp changes of particle sizes. This cyclic layering might be the product of seasonal or annual effects. Less commonly voids may receive a massive increment of uniformly sized silica at a particular stage and develop a thick layer of constant colour and pattern with an enhanced gemmological value.

Hardening ofthe opaline silica within a void was at times incomplete before a successive increment of opaline silica was introduced. Groundwater movements of this type distort the partially hardened gel and produced textures analogous to those seen in sediments. These structures include fold , fault and flow contortions on a micro-scale, and are usually discernible in parts of almost every specimen (Fig. 4). Breccia textures are less common, and

Flgnre 4. Boulder oplll showing layer and Bow textures.

consist of a suspension of contrasting coloured fragments of opal within later deposited material. These structures indicate that the precipitation of opaline silica was cyclic, and may have occurred over a longer period of time than the non-banded opal typical of the New South Wales and South Australian opal fields. Opal from southwest Queensland is thought to have a lower water content, because it is very stable after removal from the ground and is virtually free from shrinking and cracking caused by dehydration. This process is known as 'crazing'; it affects a proportion of the opal from all the fields in South Australia and New South Wales. Queensland opal has the disadvantage that it is

PRECIOUS OPAL DEPOSITS, SOUTHWEST QUEENSLAND 24S

A ... , ....... .

.. " . .. . .... . :- .... .... ... .... .. .. . :::;:::::::;:;:::::;:;:: ......... . . . . ... . . . . ........ ........ ... . . ..... . .... . . . .. .. .. . .. .

" .......... ... . . . . ......... . . ', ' . . ... . . . . .. . . . . . .. .. . ....... . .. . ..... . . ..... . ... •.. ..•.. . . -:. :- Pink · . . . . . . . . . . . . . . . . . . . . . .

.:.:.:.:: : :::::::::::::::::::::::::::::::::::~:~:~:~:~;:;

Ironslon~ concretions willi precious opo/

- - - - - - - - - - - - Koo/inific sllfsfone - -EKcO-:aI;;' - - --------------------------=--•bOse

B ~~~~~.~. ~ .. ~ .. ~.~ . . ~--~~~~

:.: .: .: Pink ir~O;/~ific ' 'Soll(tsfo~e ::::: : :::::: : ::: : : '

, . . ... . . . .. . . . . . ...... . ........ . . . . . . .. .. .

.. ... . . .. . . ... .. . .. . . . . , . . .

. . .. .. . .

~ii!.a.~-",,-.:..:.:..j~- - - - - - - - -

--=----=±=- = =~==--- =-=- ---- = -t Dense ironstone with seom opol

D >tir' .. ... ..... :> .. 2 :::::::··· )8·· .. ..:':': Pink. koolinifi c ~Ondsfone : ) .

. ~ .•. :.::: ............... ...... . .

. .:~ ........ .. . .. ... ......... .. k ::: . . . . . . . . . ....... . ...... . ... . · . . . . . . . ........ . .. . . . . · . . . . . . . . . . . ..... .. .. ...... ..... .. . .

. . . . . . . . . . . . . . . . It . . . . . . . . . . . . .......... . .. . .. .

: ~:s··:··:·······:·::: ..... . . ...... . .. ... . . . ----- .. . .. . .

/\., . .

Dense ironstone with seam opal

c ...

" ... , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ .. . . ... .. ............. ..... . :::: P/~k ir~,;I,:n ; f;-c · ~a;'dsi';n'e':: : : : : : .. .. .•. . ..• . .• . .. • . .

.......... . .......... . ........ . .................. . ............ . . . . ... . . . . . . . . ~

..: ... . Wilde ' .. . ' . ' . ' .. ' . . . ' . ' .... .

. .... .. '''''''"" .""",,_~""~'M • ~ ... ' . : .. ... : ... . :' ...... .. . . ' . ' .. :.'.: . . . . ' . . --...:...:

1-- -------~------

---==~~~-~ - ---- -- --- --- -- ------- ------

Dense ironstone wilh seam opol

E /

./

/

./ /

/ /

/

/' ,/

./ ./

/

/ /

/

/ /

/ /

/ /

/ /

/' Cross bedded koolinifi c sondsfone /

/ ,.,.,/ , ,/' / ,//

/' ./ ./ /' ./

./ ./

./

./ ./

Concretionary ironstone with veins of precious opol of file lower end

/ /

/

/

/ /

/

/

/ /

/

- 0

-2

-4

- 0

- I

- 2

- 0

--=-,

./

- 2

O/ A/ UI

FigureS. Sedimentary structures and precious opal occurrences observed at Russel's Mine (A·D) and little Wonder Mine (E).

difficult to separate from the ironstone host, and its commercial value depends upon acceptability of ironstone inclusions or backings in the finished gems.

Sedimentary structure as a primary control in the depo•sition of precious opal has long been known. Jackson (1902) for example described 'sandstone opal' at the interface

246 B. R. SENIOR AND OTHERS

Figure 6. Tl'IIIIsvene section through a ferruginous concretion with precious opal septaria.

Figure 7. A 'Yowah nut' partly Olled with opal.

between an upper sandstone and lower lutite bed . Connah (1966) illustrates examples of this control in occurrences at the Yowah Opal Field. However, owing to the restrictive view of the surrounding geology in small drives and shafts , the full spectrum of sedimentary traps was unknown until the advent of larger scale open·cutting. Six principal sedi•mentary structures which control the location of precious opal deposits within the Canaway profile are illustrated in Figure 5. With the exception of Section 'E' , all appear to relate to bedding irregularities which are floored by a relatively impermeable layer of kaolinitic siltstone. mud•stone or claystone.

Queensland opal is invariably associated with the kao•linitic weathered rocks of the Winton Formation. Almost all of it is found within ironstone-enriched layers, lenses and concretions. The miners distinguish follr forms:

a. 'Seam' or 'Sandstone' opal, is found within layered ironstone along the contact between a kaolinitic arenite and underlying lutite. Such opal is not usually extensive, but may occur in pockets along the base of bedding undulations.

b. 'Boulder' opal occurs within radial, irregular or concentric veins in ironstone ' lenses and concretions (Fig. 6). If the veins are very fine and irregular the opal may form an intermeshed micro-network and the term 'opal matrix'"; or just 'matrix' , is often used.

c. 'Pipe' opal is an infilling of solid opal in branching pipe-like accumulations of ironstone (former burrows?), which form pendants extending fromJhe lower ·surfaces of some ironstone layers. ,.

d. 'Yowah nuts' are small, usually hollow, almost spherical ironstone concretions up to 10 cm in diameter, which are filled or partly filled with solid opal. These concretions occur discontinuously along distinct stratigraphic levels, and contain the bulk of all opal found at the Y owah opal field (Fig. 7).

The host ironstone is dominantly of goethite limonite and hematite which, either by itself or as a cement, ap•parently imparts the degree of strength essential to sup•port the voids during the fairly lengthy period in which the opal is formed . Most other cavities produced by weathering or tectonism in the Can away profile are apparently lost by compaction and settling of its relatively poorly indurated components. Some opal may occur independently of the ironstone host; examples include infillings of voids, produced by leaching of organic material such as fossil wood. There are rarer examples, of opal •veinlets within secondary minerals such as gypsum and alunite. These occurrences are found infrequently at almost every location examined. However, the bulk of commercial opal in southwest Queensland is associated with various forms of ironstone, which in itself is a unique and distinguishing feature.

Geological controls of opalisation Geological factors believed to have influenced opal

formation may be summarised as follows: (a) A prolonged period of deep chemical weathering is

required to degrade the host rock mass, and initiate hydrolysis ofthe silicate components. Amorphous silica is a resulting product, and occurs either as a solution or a colloidal suspension in groundwater.

(b) Appropriate sedimentary structures are needed to provide partial lateral closures into.which silica-laden water will be directed and trapped by porosity and permeability barriers.

(c) Within the sediitiehlary structures suitable voids are necessary to serve as reservoirs in which the silica can precipitate and/or flocculate from virtu'ally static siliceous groundwater.

(d) Climatic conditions must provide periods of satura•tion and dehydration of the rock, with considerable fluctua•tions in groundwater levels. Within the sedimentary traps dehydration would be brought about by a slow combination of downward filtration and upward evaporation. Colloidal spherical silica particles would be enlarged by both aggregation and further deposition from solution. Gelat•inisation and final hardening ofthe opaline silica take place over a fairly lengthy period, working upwards from the base of the voids.

(e) Precious opal, as distinct from the non-precious varieties without a play of colours ('potch '), formed within cavities where uniformly sized silica spheres were pre•cipitated and accumulated in a regularly close-packed, three-dimensional symmetrical array, which resembles an ultra coarse-crystal lattice. The mechanism controlling the size of the silica spheres is almost certainly the intensity and duration of the saturation and dehydration periods. Uniformity of size could be a function of settling rates; the means by which the close-packed structure is imposed is still conjectural, although it has been accomplished by various synthetic means in laboratory experiments (Darragh & Perdrix, 1973).

The three-dimensional structure of precious opal was dis•cussed by Jones and others (1964), and Sanders (1964), using electron microscopy. Their work quickly led to the identification of diffraction from the regular silica layers as the cause of the play of spectrum colours seen in precious opal (Darragh and others, 1966; Darragh & Gaskin, 1966).

PRECIOUS OPAL DEPOSITS, SOUTHWEST QUEENSLAND 247

Geological prospecting At present all prospecting for opal is based either on

intuition, or careful search for traces of opal in patches of ironstone lag gravel. Earthmoving machinery is then employed to expose any ironstone host rocks present. Frequently these methods are unsuccessful, and the large number of barren open cuts detracts from the profitability of the operation.

It may be possible to improve this situation. The gross distribution of the opal-bearing Canaway profile can be determined by aerial photograph interpretation; it is distinguished by flat-topped landforms which have distinct banding of vegetation across the crust-forming layer. Unfortunately the existing RC9 aerial photographs at approximately 1 : 80 ()()() scale have insufficient resolution to permit the geological mapping of the component layers of the profiles. Colour aerial photography, if available, would be of immense assistance-potential localities where the crust and ferruginous layers are in close vertical proximity could be identified for field examination.

As noted previously, most opal deposits lie in favourable sedimentary structures in the ferruginous layer. It is prob•able that palaeochannels and fault-bounded traps (Fig. 5) could be identified on large-scale colour aerial photo•graphs, and might take the form of linear or curvilinear features, or sinous lines of dark-toned ferruginous gravel. Photographs at 1 : 25 ()()() scale would be required to identify linear features , which in all probability are less than 0.3 km long and 10 m wide. Intersecting linear features might prove to be potential sites worthy of field investigation because of the possibility of enhanced vertical permeability which could facilitate the ingress of siliceous groundwater. Shallow core-hole drilling, using the method described by

Ingram (1969), might be employed to establish the presence of subsurface ferruginised layers , and to calculate the depth of any proposed open cut.

Ground magnetic surveying The association of opal with distinct ironstone segrega•

tions, which for the. most part conform to the bedding geometry of the parent profile, offers a means of pros•pecting for opal deposits. In general the limitations imposed by very flat terrain and poor outcrops severely restrict the direct geological appraisal of sedimentary struc•ture. However, the relatively high remanent magnetisation of the ironstone (Idnurm & Senior, in press) indicated that field trials with equipment that measures local changes in the magnetic field might be useful in locating boulder opal.

Horsfall & Senior (1976) carried out ground magnetic surveys at a number of opal prospects and mines in the Eromanga and Quilpie districts, including the Maynesfde leases at Yeppara (Fig. 8), the Bull Creek Mine, and Bulgroo

. and Nickavilla prospects near Quilpie. Magnetic suscepti•bility and remanence measurements were also made on iron-stone and co un try rock sam pies from these and other areas.

A Geometric G816 total-field magnetometer, with the sensor mounted on a 2.5 m long pole, was used to evaluate the sites. Readings were generally taken on square grids of 6.1m spacing, with follow-up traverses across anomalies of interest at 1 m to 1.5 m spacings. Corrections for diurnal variations were made at 15 minute intervals. These were generally small, and seldom exceeded 2nT.

Significant magnetic anomalies were detected at all the sites surveyed. At Mayneside anomaly amplitudes ranged up to 6OnT, with most anomalies of the order of 35nT. Anomalies were generally larger at the Bull Creek

'-____ --------____ --435

430

?so "

i--:-~~ 2??5 " I Pink sandstone. Some I I Ironstone concretionS! L __________ -'

..---.:.." ""0

~ r--~ v..ro

: ~ Wllite mudstone. I I No concretions I I

Pink sandstone. I Some ironstone ---L concretions ; ---

I I I I I

--440- Total mor;netic intensity contour

,-----1 Existinr; excavation ______ J

Excavation sited by r;round mor;netic surveyinr;

I I I Wllite mudstone ~ _ ~ _ _ no concretions

___ ~ Ironstone layer ! I m depth .

o 1

10 1

20 30 m I .

Contollr inter vol : 5 nT Q/ A/657

Figure 8. Total magnetic intensity contours of an lU"ea at Mayneside's leases, YepplU"a·district. ·

248 B. R. SENIOR AND OTHERS

Mine, where amplitudes sometimes exceeded lOOnT. At most of the sites anomaly widths ranged between Sand 20 m. Some anomalies have linear or ovate shapes which were interpreted as resulting from variations in concentra•tions of iron oxide within the weathered rocks. A trial excavation was sited at Mayneside (fig. 8), between a 4SOnT magnetic high and a 43SnT low on the easternmost portion of the grid. An existing excavation to the east of this was barren of limestone, and coincides with an area of low magnetic intensity.

Ironstone concretions were first encountered in the trial excavation at a depth about 1 m below the surface. At the maximum depth of 3 m about one hundred ironstone con•cretions had been unearthed. Most of the ironstone boulders contained empty voids, although some had veins of non-precious opal.

Magnetic susceptibility and remanence measurements were made on oriented samples from the weathering profile at a number of areas. These showed that many of the iron•stones had substantial remanent magnetisation in both normal and reversed directions, as well as a low magnetic susceptibility. The measurements further revealed a variable magnetic contrast between the ironstone and country rocks, which in some areas would be expected to be sufficiently marked for ironstone bodies to be detected.

Horsfall & Senior (1976) concluded that in some instances magnetic anomalies relate to subsurface ironstone occurrences, in others merely reflect zones of diffuse iron•oxide enrichment. Interpretation of anomalies was con•fused in places by (1) magnetic inhomogeneities within allu•vium deposited after formation of the Canaway profile, (2) variations in the magnetic properties of the country rock, (3) variations in the depth ofthe ferruginous portion within the weathered profile, and (4) the effects of lightning strikes on the remanence of the country rock. They also concluded that some form of continuous magnetic recording was desir•able for exploration of large, potentially opal-bearing regions.

Figure 9. Vehicle-borne magnetometer system.

As a result of these conclusions a further ground survey was carried out in the vicinity of the Bull Creek Mine, using a continuously recording vehicle-borne magnetometer system for reconnaissance, and a Geometrics G816 magnet•ometer for detailed follow-up surveys.

The vehicle-borne system was constructed at the University of NSW by installing a Varian 4937 A total-field magnetometer with a sample time of 1 sec and a chart recorder, within a short-wheel base Landrover. The magnet•ometer sensor'head was mounted at the rear of the vehicle on an aluminium alloy boom to minimise the magnetic influences of the Landrover (Fig. 9). The boom arrangement was stabilised with guy ropes, and could be raised for rough traversing. The sensor was thermally insulated , and placed

about 6 m behind the vehicle at a height of approximately 1.S m. In this position a maximum heading error of about 2SnT was recorded. This was considered sufficiently small for reconnaissance magnetic surveying.

Traversing with this system was accomplished at speeds of about S km/hour. Significant magnetic anomalies were flagged, coded, and positioned approximately on aerial photographs for later detailed follow-up magnetic surveys.

A total of about 200 line-kilometres of reconnaissance magnetic traversing was completed in this manner around the Bull Creek Mine. Some fifty significant magnetic anomalies , with amplitUdes generally in excess of SOnT, were delineated and flagged. About 10 of these anomalies were investigated by detailed ground traversing and excavations were carried out at the sites of two of the anomalies . These follow-up surveys were done on rectangular grids, using a station spacing of 2.S m, and a line separation of S m. Additional readings were taken at intermediate stations where necessary. Spot readings were also taken around the edges of grids, and if additional anomalies were found the grids were usually extended. Using this method, because of the closeness of the sensor to the magnetic sources, magnetic anomalies were about 20 percent greater than ordinary anomalies. Features of the significant magnetic anomalies in the Bull Creek area are:

(a) Anomaly amplitudes are generally in the region of 30nT to 100nT although some can attain amplitudes in excess of IS0nT.

(b) Both normal and reversed anomalies occur, often in close proximity. The direction of the remanent component . of the anomaly is significantly different from the direction of the induced anomaly, and much greater in intensity.

(c) Anomalies are mostly less than 30 m in width (some are less than S m in width), and have shapes consistent with anomalies produced by shallow tabular, ovate or dipolar magnetic sources.

(d) Anomalies tend to occur in clusters or in linear belts adjacent to existing workings , or around the edge of mesas which have little if any magnetic character.

(e) Precious opal traces were found in the vicinity of the magnetic anomalies.

Figure 10 shows a complex linear belt of magnetic anomalies which were discovered just to the west of the Bull Creek Mine. The four prominent anomalies (A31, A32, A33 and A34) are caused by magnetic sources with a strong remanent magnetisation. The largest anomalies (A31 and A32) have nearly identical amplitude, but are reversed in direction with respect to each other. Anomaly A34 has not been fully defined . Anomaly A33 to the north is similar in direction to anomaly A32, but smaller in size and about one-third the amplitude of the latter anomaly.

All of these anomalies are approximately in line with a series of very old pits and shafts some 30 m to the south of A31. Hard ground consisting of silicified kaolinitic rocks precluded testing ofthese anomalies by excavation.

Figure 11 shows a detailed survey on the eastern side of a mesa about S Ian east of the Bull Creek Mine. A number of intense anomalies were delineated at this site, some of which were tested by excavation. Several traces of precious opal were also discovered in this area. They are present in one of the few areas in which such indications have been found away from the Bull Creek Mine site area itself.

Magnetic anomaly A17 in the south of this area is a broad feature with an amplitude of about 30nT (Fig. 11). The source of this anomaly is essentially normally magnetised, and the anomaly was tested with a wide costean (costean 1). The excavation extended to a depth of about 1 m, and revealed a soft pink ferruginous sandstone beneath a thin dry, blocky surface layer. No boulders were encountered, although occasional sandstone concretions were noted. A

Figure 10.

o

H Magnetic high

l Magnetic low

number 0/1/ reference A31 Anom J

t Bull Creek area. A31 to A34, wes Magnetic anomalies .

UEENSLAND SOUTHWEST Q AL DEPOSITS, PRECIOUS OP .

o

5 0 ___ .1....-_

interval 5 n T Contour

10 m I

o

\

249

250 B. R. SENIOR AND OT HERS

/ / 10 Opal boulders \ \ \

Costean "~~.2.../ 3

Flat

topped

Mesa

o Opal i n surface float

H Magnetic high

L Magnetic low

AI7 Anomaly reference number

o I

10 I

20 I

Contour intervol 5 nT

30 m I

--

N

I

Figure 11. Total field magnetic intensity contours, east Bull Creek area.

magnetic traverse along the centre of this coste an at its maximum depth did not produce any anomalies , and anomaly A17 is believed to be due to variations in the iron content within the soft '~d sandstone layer.

Magnetic anomalies A22 and A23 to the northwest of the area and immediately adjacent to the mesa are of greater intensity than anomaly A17. They are remanently mag•netised in essentially opposite directions. Anomaly A22 was

PRECIOUS OPAL DEPOSITS. SOUTHWEST QUEENSLAND 251

testeq by excavation along costean 2 and a cross-cut along costean 3. The smaller apparent dipole magnetic anomaly A23 was not tested by excavation. A22 has an amplitude of about 1SOnT. Both remanent and induced components can be recognised in this anomaly.

The remanent component of the anomaly is considerably stronger than the induced component, and at about 90 degrees to the present geomagnetic field direction. The shape of the anomaly A22 is consistent with a shallow body with strong remanence.

Excavation of this anomaly along costeans 2 and 3 inter•sected a dark pink kaolinitic ferruginous sandstone from a depth of about 1 m below the surface, and continued to 3 m. From the sections revealed in the two costeans the sand•stone body appeared ovate. Oriented samples of this sand•stone were later tested in the laboratory and found to have high remanence values (Table 1).

R emanence Specimen Intensity Declination Inc/ination Susceptibility

No. mAl metre deg. deg. SI units x IfF'

761264 5100 306.5 +*10.2 6600 761265 S600 306.6 + 9.9 7100 761266 5500 306.2 + 11.8 6900 761267 4300 305.5 + 11.0 5100 761268 3600 307.4 + 11.6 5300 76/ 269 3900 306.9 + 10.8 SOOO

Table 1. Ferruginous sandstone samples &om near the Bull Creek Mine.

* + indicates reverse magnetisation .

No opal of commercial grade was recovered from costeans sited by ground-magnetic methods. However, the number of costeans are few when compared with the total range of anomaly patterns which were delineated. Therefore the usefulness of this technique cannot be fully assessed at this stage, but the fact remains that the host ironstone has a discontinuous distribution and invariably a relatively high magnetic remanence. Laboratory measurements of excavated rock show that the contrast between .the ironstone and country rocks is very variable, though in some areas the contrast is sufficiently strong to detect ironstone bodies with little difficulty. Further magnetic field surveying and excavations are warranted, to examine the complex anomaly patterns in more detail and their relationships with precious opal deposits. The association of opal with fault and bedding structures lends itself to detailed aerial photo•graph lineament interpretation, and to experimentation with geoelectric and shallow seismic techniques.

Acknowledgements The authors wish to thank Mayneside Industries Pty Ltd

and Mr D. Burton for their valued time and assistance with transportation, excavation of some magnetically anomalous ground, and with generous supply of opal-bearing ironstone for study. Dr M. Idnurm is acknowledged for the magnetic remanence and susceptibility measurements of the iron•stone samples. Dr A. R. Jensen and Dr G. E. Wilford are acknowledged for their constructive criticisms of the original draft. The figures were drawn by Sue Davidson and A. Retter, Geological Drawing Office, BMR.

References CONNAH . T . H .. 1966-A prospectors guide to opal in western

Queensland. Queensland Government Mining Journal. 67,23-39. DARRAGH . P. J_. GASKIN. A. J .• TERRELL. B. c. . & SANDERS. J. V ..

1 966-0rigin of precious opal. Nature. 209, 13-16.

D ARRAGH. P. 1. . & GASKIN, A. J., 1966-The Nature and Origin of Opal. A ustralian Gemmologist. 66, S-9.

DARRAGH. P. 1., & PERDRIX, J. L.. 1973-Precious Opal-Develop•ments Towards Synthesis. Australian Gemmologist. 73, 17-21.

EXON, N. F. , LANGFORD-SMITH, T _, & McDOUGALL, I. , 1970-The age and geomorphic correlations of deep-weathering profiles, silcrete and basalt in the Roma-Amby region, Queensland. Journal of the Geological Society of Australia. 17,21-30.

GOURLAY, A. J., MCCOLL, D. H., & SENIOR, B. R.,1976-Review of the opal and sapphire industries. Australian Mineral Industry Quarterly Review, 28, 1-12.

HORSFALL, C. L., & SENIOR, B. R. , 1976-An evaluation of ground magnetic surveys as an aid to prospecting for opal in southwest Queensland. Bureau of Mineral Resources. Australia-Record 1976/6 (unpubl.).

IDNURM , M., & SENIOR, B. R., in press-Palaeomagnetic ages of weathered profiles in the Eromanga Basin, Queensland. Palaeo•geography. Palaeoclimatology. Palaeoecology.

INGRAM,1. A., 1969-Drilling for opal in Queensland. Queensland Government Mining Journal. 70,451-4.

INGRAM; J. A., 1971a-Eromanga, Qld-l :2SO 000 Geological Series. Bureau of Mineral Resources. Australia-Explanatory Notes. SG 54-12.

INGRAM, J. A. , 1971b--Toompine, Qld-l:250 000 Geological Series. Bureau of Mineral Resources. Australia-Explanatory Notes SG 55-13.

JACKSON, C. F. V., 1902-The opal mining industry and the distribution of opal deposits in Queensland. Geological Survey of QueensiandPublicationl77; reprinted as publication 291,1958.

JONES , 1. B. , SANDERS, J. V., & SEGNIT, E. R. , 1964-Structure of Opal. Nature. 204, 990-1.

McELHINNY, M. W_, EMBLETON, B. J. J., & WELLMAN, P., 1974-A synthesis. of Australian Cenozoic palaeomagnetic results. Geo•physical Journal. 36, 141-51.

SANDERS, J. V., 1964-Colour of precious opal. Nature. 204, 1151-3. SENIOR. B. R., 1971-Eulo, Qld 1:250000 Geological Series.

Bureau of Mineral Resources. Australia-Explanatory Notes SH 55-1.

SENIOR. B. R. , 1977-Landform development. weathered profiles and Cainozoic tectonics in southwest Queensland. Unpublished PhD thesis, University of New South Wales . Sydney.