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Paradis, S., Townsend, J. and Simandl,
G.J. (1999): Sedimentary Rock-hosted Opal; in Selected British Columbia
Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl,
Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy
and Mines.
IDENTIFICATION
SYNONYMS: Australian opal
deposits.
COMMODITY: Gem quality
opal (precious and common).
EXAMPLES (British Columbia -
Canadian/International): Lightning Ridge and White Cliffs
(New South Wales, Australia) , Mintabie, Coober Pedy, Lambina and
Andamooka (South Australia) Yowah, New Angledool (Queensland, Australia).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Most
of the Australian opal occurs in cracks, partings, along bedding planes,
pore spaces and other cavities in strongly weathered sandstones generally
underlain by a subhorizontal barrier of reduced permeability. The barriers
consist mainly of claystones, siltstones and ironstone strata.
TECTONIC SETTINGS: The
tectonic setting at the time of deposition and lithification of the
opal-bearing lithologies is not indicative of favourable environment for
opal. However, the presence of a terrestrial (non-marine)
environment at the time of intense weathering is essential.
DEPOSITIONAL ENVIRONMENT /
GEOLOGICAL SETTING: Clastic sediments were deposited in the
shallow inland basins. Subsequently, these areas were affected by climatic/paleo-climatic
changes (transformation into desert environment) that have resulted in
rapid fluctuation in water table levels and entrapment of silica-rich
waters.
AGE OF MINERALIZATION: In
Queensland, Australia the host rocks are Cretaceous or Paleozoic and have
been affected by deep weathering during the Early Eocene and Late
Oligocene. The latter period is believed by some to be related to opal
precipitation. Similar conditions favourable for opal deposition could
have prevailed in different time periods in other parts of the world.
HOST/ASSOCIATED ROCKS:
Sandstones, conglomerate, claystone and silty claystone. Associated
lithologies are feldspathic rocks weathered to kaolinite, silcrete and
siliceous duricrust, shales and shaley mudstones, limestones, dolostones
and ironstones. Exceptionally, precious opal may be found in weathered
crystalline basements stratigraphically underlying the lithologies
described above.
DEPOSIT FORM: Opal
occurrences are stratabound. Favorable subhorizontal, precious
opal-bearing intervals can exceed 10 m in thickness, and are known to
persist for distances of one to over 100 km. The distribution of
individual precious opal occurrences within favorable areas is erratic.
Veins are subhorizontal to subvertical and locally up to 10 cm thick. They
pinch and swell, branch or terminate abruptly. A single vein can contain
chalky to bony to blue, gray or milky common opal and precious opal.
TEXTURE/STRUCTURE: Opal
occurs as veinlets, thin seams in vertical and horizontal joints,
desiccation cracks in ironstone layers, lenses and concretions, and
replacing fossils (shell and skeletal) and wood fragments. Opal also forms
pseudomorphs after glauberite4. In places opal seems to follow
cross bedding. In unusual cases opal pieces eroded from the original host
are incorporated into younger sediments. In silicified sandstones precious
opal may form the cement around detrital quartz grains, in other areas,
the opal may be cut by gypsum or alunite-filled fractures. The lithologies
above the opal may contain characteristic red-brown, gypsiferous
silt-filled tubules.
4 Glauberite: 4[Na2
Ca(SO4 )2 ], widespread as a saline deposit formed
as a precipitate in salt lake environments, also occurs under arid
conditions as isolated crystals embedded in clastic sediments.
ORE MINERALOGY: Precious
opal.
GANGUE MINERALOGY [Principal and
subordinate]: Host rock, common opal, gypsum and gypsum-shot
opal, alunite, hematite, limonite/goethite.
ALTERATION MINERALOGY:
N/A.
WEATHERING: Feldspathic
rocks strongly altered to kaolinite typically overly the Australian
precious opal-bearing deposits. Opal exposed to arid weathering
environments may desiccate, crack and lose its value; however, gem quality
opal may be found at depth.
ORE CONTROLS: 1) Regional
configuration of impermeable layers permitting groundwater pooling. 2)
Local traps within regional sedimentary structure, such as bedding
irregularities, floored by impermeable layers, porous material (e.g.
fossils) or voids where opal can precipitate.
GENETIC MODELS:
Australian opal is hosted mainly by strongly weathered sandstones which
are underlain by claystone, siltstone and ironstone that form relatively
impermeable barriers. Periods of intense weathering are evidenced by
indurated crust horizons. Silica-transporting solutions derived from
intense weathering of feldspar within sandstones percolated downward to
the contact between the porous sandstone and the underlying impermeable
layers. During a subsequent dehydration (dry) period silica was
progressively concentrated by evaporation. The last, most concentrated
solutions or colloidal suspensions were retained within bedding
irregularities at the permeable/impermeable rock interface, in joints and
in other traps. Gem-quality opal was formed by ordered settling and
hardening of silica microspheres of uniform dimensions. Disordered
arrangement of silica microspheres or variability in microsphere size
results in formation of common opal.
ASSOCIATED DEPOSIT TYPES:
Possibly clay deposits (B05).
COMMENTS: There is good
reason to believe that a similar mode of opal formation could also take
place in porous terrestrial and waterlain pyroclastic rocks, assuming
favorable geological and paleo-climatic setting.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: N/A
GEOPHYSICAL SIGNATURE:
Most opal fluoresces brightly if exposed to ultraviolet light. Limited
success was achieved using magnetic field and resistivity to find
ironstone and ironstone concretions that commonly contain precious opal in
Queensland.
OTHER EXPLORATION GUIDES:
Unmetamorphosed or weakly metamorphosed areas known for:
1) prolonged periods of deep chemical
paleoweathering characterized by rock saturation and
dehydration cycles;
2) broad sedimentary structures permitting shallow underground solution
pooling;
3) local traps where opal could precipitate from nearly static,
silica-bearing ground waters; and
4) presence of common opal.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE:
No reliable estimates of grade or tonnage are available for individual
deposits. Until 1970 the only records of production were annual returns
submitted by opal buyers. Miners fear that reporting the true production
would be used for taxation purposes. As with other gemstones, reporting
the grades in terms of grams or carats per tonne may be strongly
misleading. Large and exceptional quality stones command very high prices.
Precious opal may be transparent, white, milky-blue, yellow or black. It
is characterized by the internal play of colors, typically red, orange,
green or blue. The best opal from Lightning Ridge was worth as much as
$Aus. 10 000.00 per carat in cut form and Mintabie opal varied from $Aus.
50.00 to 10 000.00 per ounce of rough. Most of the white to milky colored
opal from Coober Pedy was worth $Aus. 10.00 to 100.00 per ounce of rough,
but the prices of top quality precious black and crystal opals exceeded
$Aus. 5 000.00 per ounce. The value-added aspect of the gem industry is
fundamental. An opal miner receives 1 to 50% of the value of cut and
polished stone.
ECONOMIC LIMITATIONS: In
Australia mining is largely mechanized, either underground or on surface.
Opal-bearing seams are generally found at shallow depths (< 30 metres).
Opal is still recovered from old tailings by hand sorting over conveyer
belts using ultraviolet light. Large and exceptional quality stones
command very high prices and the unexpected recovery of such stones may
change an operation from losing money to highly profitable. Stones from
these deposits are believed to have better stability under atmospheric
conditions than opal from most volcanic-hosted deposits.
END USES: A highly priced
gemstone that is commonly cut into solid hemispherical or en cabochon
shapes. Doublets are produced where the precious opal is too thin,
needs reinforcement or enhancement; plastic cement, a slice of common opal
or other support is added to the back of the opal.
IMPORTANCE: Australian
sedimentary-hosted opal deposits account for most of the opal produced
today. The situation is likely to continue since these deposits recently
attracted important Japanese investment. In 1990, the Coober Pedy,
Andamooka and Mintabie produced opal worth over $Aus. 47 million. Total
production estimates for Australia are in the order of $Aus. 100 million
annually.
REFERENCES
Barnes, L.C. Towsend, I.J.,
Robertson, R.S. and Scott, D.C. (1992): Opal, South Australia’s
Gemstone; Handbook No.5 (revised edition), Department of Mines and
Energy, Geological Survey of South Australia, 176 pages.
Cipriani, C. and Borelli, A.
(1986): Simon & Schuster’s Guide to Gems and Precious Stones; K.
Lyman, Editor, Simon & Schuster Inc., New York, 384 pages.
Daragh, P.I., Gaskin, A.J. and
Sanders, J.V. (1976): Opals; Scientific American, Volume
234, pages 84-95.
Downing, P.B. (1992): Opal
Identification and Value; Majestic Press, 210 pages.
Hiern, M.N. (1976):
Precious Opal-South Australia; in Economic Geology of Australia and
Papua New Guinea, Volume 4, Industrial Minerals and Rocks, C.L. Knight,
Editor, Australian Institute of Mining and Metallurgy, Monograph
Series, Volume II, pages 322-323.
Jones, J.B. and Segnit, E.R.
(1971): The Nature of Opal. Nomenclature and Constituent Phases;
Geological Society of Australia Journal, Volume 18, pages 57- 68.
Keeling, J.L. and Farrand, M.G.
(1984): Origin and Formation of Matrix Opal from Andamooka;
South Australia Geological Survey, Quarterly Geological Notes, Volume
90, pages 3-10.
Nichol, D. (1975): Opal
Occurrences near Granite Downs Homestead; Mineral Resources Review,
South Australia, Volume 135, pages 164-168.
Towsend, I.J., Wildy, R.L., Barnes,
L.C. and Crettenden, P.P. (1988): The Opal Industry in South
Australia 1984-1986; Mineral Resources Review, South Australia,
Volume 156, pages 106-107. |
