The geology and magnetic characteristics of precious opal deposits

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
Mining activity
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.
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).
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.
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
oLI____
50
-L
Quaternary alluvium
150km
--
Tertiary sediments
Whitula Formation
Eyre Formation
____~____~
,
~I
c\
K
I
WInton Formation
Rolli ng Downs Group
1....""'
I
\
\
\
Weathered
prof iles main ly on Winton Format ion
Morney
profile
J
I
~
~
Conaway
profi le
Kynuna
\
\
0
\
2 name
~
K
I
\.r\
Kw
~_\
../
\
\
"\
unknawn
3 Opaltan
4 Horse Cree k
5 Mt Fairv ie w
6 Magic
7 Junda h
,
8
9
10
II
12
13
l
')
,
(
\
Opalville
Ger man
a
Bronks
Hayricks
Valdare
Marble Arch
Bull Creek
14 Northwest Eromanga area
15 Gooseberr y a Quartpot
16 Mulcahy1s
(
*
17 Coo na valla
18 Pinnacles
19 Yellow Nell
a
20 Lamons
2 1 Ru sse ls ·
Potts
a
Litt le Wonde r
22 nome unknawn
Lush ingtons
Pride of the Hills
Goodmans Flat
26 Duck Creek
27 Sheep St at ion Creek
28 Emu Cree k
29 nome unknown
30 Fiery Cross
31 Koroit
Hollaways
a
32 Yowah
33 Block
Gate
34 Elbow
*
- - Formation or weathered profile bdy
-
Figure 1.
Fau lt
0 16
- - Margin of the WInton Formation
Includes: Breakfa st, Stanley,
Friday, Fish Ponds, Hausingtons,
Exhibition, Stoney Cr eek, Bung
Bung, Scotchman, Al add in, Mots
Hard, Hamm onds , Pepp in and
Webbers, Ge m, Mascotte and
Gl adstone
Precious opal occurrence with reference no.
Distribution of weathered promes, IIIId Mesozoic IIIId Cainozoic rocks, in relation to precious opal occurrences In lIOuthwest
Queensllllld.
cursory excavation
encountered.
where
opal
is
not
immediately
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
yo unger 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
o
Lat.
25°16'20"
Long . 141° 50 ' 20"
so mpli ng points
Si
'1,1
/
v h columnar Silcrete
"'
...J
...J
"
cr
Si
:::>
u
it:.
s•
~
nodular Silcrete
c fe
Lot.
26°00'40" S
Lo ng. 143° 53' 50" E
stnd qtz
Si
wh m t c grnd kaol,
10
"'oz
W
U>
20
:::>
o
"'
u
...J
::J
;;;
u..
o
30
a:
Cl.
Om
Om
Om
<=
Om
Si
Si
"'oz
Om
wh sllic kaol
<=
wh kaol f grnd, c intbdd lamd
Mdst 8 Sltst Ironstone lens
Om
Om
o
50
~
o
u
cr
">
>
z
a:
~
01
Om
°
e
Om
°
70
o
~
e
Fe
60
w
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 irons tone
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
concealed
:::>
o
90
"cr
Fe
"'"-
Fe
cr
Fe
Fe
100
METRES
Figure 2.
fe stnd, wh to red - brn breccia
c macrocolumnar structure.
...J
u..
fe Om c cell patterns, breccia
8 remnont Sltst
pk intbdd Sltst 8 Mdst
Mudclast conglomerate
pk silk:, fe stng, dessication cracks
10
o
A
a:
w
z
o
Cl.
intbdd wh kaol, c yel 8 pk,
minor brn kaol m Sst
N
o
"'cr:::>
>
<t
3:
pk m grnd silty
cr
c:
wh f grnd kaol
" I
"'oz
Z
U
c:
">
30
<t
<t
o
...J
o
U
II
N
U>
o
z
40
wh to pk m grnd kaol,
c iron stone
01
:::>
c;
II
cr
cr
"I
:::>
..."'
".
BE DD ING
01
brn m grnd kaol
wh 8 fe stnd, c minor intbdd Mdst
p urp m grnd kaol,
c basal
wh
concealed
intbdd purp Sltst 8 m grnd kaol
f grnd kaol
scree
.......
.
Scree
S TRUCTUR ES
cross stratification (group)
bioturbation
II
ferr uginous concretions
Om
mottled
Si
siliceous
ferruginous
\\ slickensides
°
mudclast ( rounded)
Fe
t:.
mudclast (angular)
_ _ remnant bedding trace
<= parallel lamination
z
:::>
w
purp 8 y el f grnd kaol
N
U>
c
20
purp 8 y el, minor intbdd Mdst
(wh) to v p I purp f g rnd kaol
m grnd hd kaol, above yel brn lam
"'oz
80
fracture
v f kaol, 8 wh silic
N
"':::>cr
c conchoidal
Si
"f
40
c minor intbdd
wh kaol, c conchoidal fracture
8 rd-brn mottled zones . Minor
intbdd m grnd kaol, 8 Mdst
wh kaol
wh kaol,
/sompli ng points
fe st sll/(; breccia
vertical
cracks, p ipes 8 turbulence zones.
Om 8 fe pisoli tes
o
..il
cross stratification (solitary)
BED
':In
Joints or cracks
oJVvo.
unconformtfy
THI CK NESS
rtJ very thick > 120 cms
D
thick
c
medium
1-5
laminate
<1
5 - 60
LITHO LOGY
o
thin
_
60 - 120
sandstone
G
stlfstone
a
mudstone or claystone
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 yo ungest formation in the Eromanga Basin ,
is Cenomanian based on palynology (Burger in Senior,
1977). The Morney and Canaway profiles extend as a n
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
Erom anga 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
...
- 0
....... .
. .. . .... . :- .... ....... .... .. .. . :::;:::::::;:;:::::;:;:: ......... .
,
..... . .... . . .. "
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....... . .. . ..... .
. ..... . ... •.. ..•.. . . -:. :- Pink
·........
..............
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-2
.:.:.:.:: : :::::::::::::::::::::::::::::::::::~:~:~:~:~;:;
-
Ironslon~ concretions
willi precious opo/
EKcO-:aI;;' - - --------------------------=--•
-
-
-
-
-
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ir~O;/~ific ' 'Soll(tsfo~e ::::: : :::::: : ::: : : '
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......
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. .. ........
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. . . . . . ....
.. ............. ......
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:::: P/~k
ir~,;I,:n ; f;-c · ~a;'dsi';n'e':: : : : : : .. .. .•. . ..• . .• . .. • . .
..........
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..................
.
............
.
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........ . . ....
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,
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--=----=±=--t ==~==--- =-=- ---- =
-
-
-
-
-
-
-
-
Dense ironstone with
seom opol
>
tir'
.
..
.
..... :> ..2 :::::::··· )8··
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..
..:':': Pink. koolinifi c
~Ondsfone : )
.:
... ............
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.......
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........ .. .
......
· . . . . . .....
. .... .
........
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...
..... .. .. ...... ..... .. . .
. .. .. . . . . . . . . ...
. . . ..........
. . . . . . ... .. ... . .
It
: .. ... :... . :' ...... ..
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.
~ii!.a.~-",,-.:..:.:..j~-
. . . Wilde ' . . . ' . ' . '.. ' . . . ' . ' . . . . .
~
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.... ..'''''''"" .""",,_~""~'M
•
.
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--
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1
. ~ .•. :.:::
./
..
,.,.,/
/'
'
:.'.:. . . .
.
'
-
Dense ironstone wilh
/
/
/
/
/
./
/'
I
- 2
seam opol
./
............... ...... . .
..
'
---==~~~-~
----
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. .' .
-------~------
- --- --- - ---- - ---
E
k :::
/\.,
...
./
/
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Cross bedded kooli nifi c sondsfone
,
,/' /
./
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··:··:·······:·:::
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--=-,
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..... .
....... .
- - - - - .. ...... .. . ... .
Dense ironstone with
seam opal
Concretionary ironstone with veins
- 2
of precious opol of file lower end
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
d.
Figure 6.
Tl'IIIIsvene section through a ferruginous concretion
with precious opal septaria.
'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 Yowah 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
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:
'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 undul ations.
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.
,.
a.
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
Ingram (1969), might be employed to establish the presence
of subsurface ferruginised layers , and to calculate the depth
of any proposed open cut.
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
Ground magnetic surveying
employed to expose any ironstone host rocks present.
Frequently these methods are unsuccessful, and the large
The association of opal with distinct ironstone segrega•
number of barren open cuts detracts from the profitability tions, which for the. most part conform to the bedding
of the operation.
geometry of the parent profile, offers a means of pros•
It may be possible to improve this situation. The gross
pecting for opal deposits. In general the limitations
distribution of the opal-bearing Canaway profile can be imposed by very flat terrain and poor outcrops severely
determined by aerial photograph interpretation; it is restrict the direct geological appraisal of sedimentary struc•
distinguished by flat-topped landforms which have distinct ture. However, the relatively high remanent magnetisation
banding of vegetation across the crust-forming layer. of the ironstone (Idnurm & Senior, in press) indicated that
Unfortunately the existing RC9 aerial photographs at field trials with equipment that measures local changes in
approximately 1 : 80 ()()() scale have insufficient resolution to the magnetic field might be useful in locating boulder opal.
Horsfall & Senior (1976) carried out ground magnetic
permit the geological mapping of the component layers of
the profiles. Colour aerial photography, if available, would surveys at a number of opal prospects and mines in the
be of immense assistance-potential localities where the Eromanga and Quilpie districts, including the Maynesfde
crust and ferruginous layers are in close vertical proximity leases at Yeppara (Fig. 8), the Bull Creek Mine, and Bulgroo
could be identified for field examination.
. and Nickavilla prospects near Quilpie. Magnetic suscepti•
As noted previously, most opal deposits lie in favourable bility and remanence measurements were also made on ironsedimentary structures in the ferruginous layer. It is prob• stone and co un try rock sam pies from these and other areas.
A Geometric G816 total-field magnetometer, with the
able that palaeochannels and fault-bounded traps (Fig. 5)
could be identified on large-scale colour aerial photo• sensor mounted on a 2.5 m long pole, was used to evaluate
graphs, and might take the form of linear or curvilinear the sites. Readings were generally taken on square grids of
features, or sinous lines of dark-toned ferruginous gravel. 6.1m spacing, with follow-up traverses across anomalies of
Photographs at 1 : 25 ()()() scale would be required to identify interest at 1 m to 1.5 m spacings. Corrections for diurnal
linear features , which in all probability are less than 0.3 km variations were made at 15 minute intervals. These were
long and 10 m wide. Intersecting linear features might prove generally small, and seldom exceeded 2nT.
Significant magnetic anomalies were detected at all the
to be potential sites worthy of field investigation because of
the possibility of enhanced vertical permeability which sites surveyed. At Mayneside anomaly amplitudes ranged
could facilitate the ingress of siliceous groundwater. up to 6OnT, with most anomalies of the order of 35nT.
Shallow core-hole drilling, using the method described by Anomalies were generally larger at the Bull Creek
' -____--------____--435
?so
"
i--:-~~
430
I Pink sandstone. Some I
IL Ironstone
concretionS!
__________
-'
Pink sandstone.
Some ironstone
concretions
~
r--~
v..ro
~ Wllite mudstone.
I
No concretions
:
I
I
I
--440- Total mor;netic intensity contour
,-----1J
______
Existinr; excavation
2??5 "
I
I
; --- I
I
---L
I
I
I
I
I
Wllite mudstone
no concretions
~_ ~_ _
~
___
!
o
1
Excavation sited by r;round
mor;netic surveyinr;
Figure 8.
..---.:.." ""0
Total magnetic intensity contours of an lU"ea at Mayneside's leases, YepplU"a·district. ·
10
1
Ironstone layer
I m depth
.
30 m
20
I.
Contollr inter vol : 5 nT
Q/ A/657
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
PRECIOUS OP AL DEPOSITS, SOUTHWEST QUEENSLAND. 249
o
o
o
\
H
Magnetic high
l
Magnetic low
A31
Figure 10.
Anom 0/J1/ reference number
area.
Magnetic anomalies A31 to A34, west Bull Creek
.
5
0 _ _ _.1....-_
Contour interval 5 n T
10 m
I
250
B. R. SENIOR AND OT HERS
/
/
10 Opal
\
\
boul ders
I
N
\
Costean "~~.2.../
3
Flat
topped
Mesa
o
Opal i n surface float
H
Magnetic high
L
Magnetic
low
AI7 Anomaly refer ence number
o
I
10
I
20
I
30 m
I
--
Contour inter vol 5 nT
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
mAl metre
No.
761264
761265
761266
761267
761268
76/ 269
5100
S600
5500
4300
3600
3900
Declination
deg.
Inc/ination
deg.
Susceptibility
SI units x IfF'
306.5
306.6
306.2
305.5
307.4
306.9
+*10.2
+ 9.9
+ 11.8
+ 11.0
+ 11.6
+ 10.8
6600
7100
6900
5100
5300
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
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Queensland. Queensland Governm ent Mining Journal. 67,23-39.
DARRAGH . P. J_. GASKIN. A. J.• TERRELL. B. c.. & SANDERS. J. V..
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INGRAM,1. A., 1969-Drilling for opal in Queensland. Queensland
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INGRAM; J. A., 1971a-Eromanga, Qld-l :2SO 000 Geological
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