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Opalas em sedimentos

by: S. Paradis1, J. Townsend2 and G J. Simandl3


Ref: opala, sedimentos

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.


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).


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.


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.



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.


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.


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.

DEPÓSITOS - 30/04/2004 19:12:00

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