Presentation on theme: "WORLD’S OPAL DEPOSITS Two basic geological types : Sedimentary and Volcanic opals Mode of Formation : Deep weathering theory e.g. Australian opals Opal."— Presentation transcript:
WORLD’S OPAL DEPOSITS Two basic geological types : Sedimentary and Volcanic opals Mode of Formation : Deep weathering theory e.g. Australian opals Opal classification : 1. Precious opal (shows play-of colour) Type 1: entirely homogenous chemical composition Type 2: attached to host rock of different composition (boulder opals) Type 3: open-space filling in host rock (matrix opals) 2. Potch or common opal
Most Precious Opal is mined in Australia, the U.S., and Mexico. In Mexico, Precious Opals and Fire Opals come from several deposits. The most important are near Queretaro, in Queretaro state, and near Magdalena, in Jalisco state. The U.S. has some of the most outstanding Opal occurrences. Virgin Valley, Humboldt Co., Nevada is rich in Opal mines producing all types of Precious Opal. Also worthy of mention are the Spencer area Opal mines in Clark Co., Idaho; Opal Butte, Morrow Co., Oregon; and the Last Chance Opal Mine, Kern Co., California. In Canada, a notable deposit exists in Vernon, British Columbia. Other significant worldwide Precious Opal deposits are in Ethiopia, the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil, Honduras, Guatemala, and Nicaragua.
OPAL VARIETIES Black opal Wh ite Op al Jelly Opal White Opal Fire Opal Cherry Opal Lemon Opal
PRECIOUS OPAL DEPOSITS OF AUSTRALIA Worlds most important source (discovered 1872). New South Wales - Lightning Ridge District, since 1905; exceptional Black Opal.exceptional Black Opal South Australia - Coober Pedy, White Opals only, since Worlds largest and richest producing area. - Mintabie - since 1921; significant producer only since 1976 for balck and white opal. - Coober Pedy and Mintabie account for over 80% of the worlds precious opal production. - Andamooka - since 1930, White Opal. Not much production today. Queensland - Opal is found within siliceous ironstone concretions or boulders, which range in size from less than a few centimetres to a boulder size of greater than 20 centimetres. - Yowah is one of the most famous opalfields.
Yawah Lightning Ridge White Cliff Andamooka Coober Pedy Mintabie Great Artesian Basin and distribution of opalfields
"Light Opal" is the most common type of opal found and is mined primarily in South Australia (SA). 'Light' opal refers to both the crystal type opal (translucent) and the milky opal (opaque). As it is the most common variety, it is generally the least valuable, however, gem quality Light Opal can exceed the value of some Black and Boulder Opals.South Australia "Boulder Opal" occurs spasmodically within a belt of Cretaceous sedimentary rocks from the New South Wales' border to about 1000 km north-west in Queensland (the "Boulder belt"). It consists of opal on a dark ironstone (the host rock). The opal occurs as a thin uneven layer naturally adhering to the ironstone. Queensland "Black Opal" is primarily mined in and around the Lightning Ridge region of Northern New South Wales. It is the rarest and therefore the most valuable variety of opal. Fine quality black opals can sell for more per carat than diamonds. The term Black Opal is somewhat misleading as the opal is not 'black' in colour, but has a very dark base body tone, which enhances the brilliant colours.New South Wales Opal is Australia's national gemstone. Ninety-six percent of the world's gem quality opal is mined in Australia! Most opal is million years old, dating back to the Cretaceous period when dinosaurs roamed the earth. Opal formed as silica from decomposing rocks mixed with ground water to form a kind of silica gel, which collected and hardened in underground cavities and fissures.
Geology of Opal All precious opal in Australia occurs in rocks affected by weathering during the Tertiary Period ( million years ago). The weathering process broke down minerals of the country rock to produce kaolin (a clay) and soluble silica. It also created cavities in the rock by dissolving soluble minerals and fossil shells. These cavities, together with faults and fractures in the rock, provided pathways for underground water containing the soluble silica. Periodic lowering of the watertable, possibly caused by changes in climate, carried silica-rich solutions downwards to deposit opal in the rock cavities.
The opal is found in sedimentary rocks that were deposited in a freshwater environment during the Cretaceous period, about 110 million years ago.
Mining Opal Opal is one of the few minerals which can still be extracted economically by a miner working alone. The simplest form of mining is by shaft sinking with a pick and shovel. Driving along the level is then carried out with picks and explosives. A handpick or screwdriver is used to extract any opal found. Calweld-type drills are used to sink vertical holes about one metre in diameter using an auger bucket. Waste material, or mullock, from the shafts and drives was originally lifted in buckets by hand windlass, but power winches or automatic bucket tippers are now used. There has been a rapid increase in the use of mining machines since the 1970s. Tunnelling machines with revolving cutting heads and small underground front-end loaders (boggers) have been introduced. Bulldozers are employed to remove overburden and expose the level where it is shallow. Spotters follow behind watching for traces of opal, any seam found is then worked over by handpick.
Noodling Many locals make a living out of searching through heaps of discarded mullock for pieces of precious opal. Noodling machines in which mullock is passed by conveyor under an ultroviolet light in a darkened enclosure are also used. If you're going to noodle, you will first need a current Fossickers' Licence, issued under the Fossicking Act Licences are available for individuals, families, clubs, educational organisations and commercial tourist operators for terms varying from one month to 12 months. Noodling Contacts Winton Shire Council, PO Box 288, Winton, QLD 4735, Australia. Tel Fax
General: New South Wales: Queensland: South Australia : Suggested Websites
OPAL DEPOSITS OF QUEENSLAND Queensland is noted for "Boulder Opal" and "Yowah nuts"; hollow, walnut-sized ironstone concretions.Yowahironstone concretions Some of the most productive fields are Bull Creek, Hungerford, Opalton, Opalville, Quilpie and Yowah. YOWAH NUT
MAP OF QUEENSLAND
The Yowah opal field is well known for its Yowah Nuts, an ironstone rock with opal matrix and/or hopefully a crystal centre. The Opal from this area is very different from the "ordinary" opal most people know. The opal can be mixed in with black or red ironstone, vitrified clay or silicified sandstone to form the most beautiful patterns and pictures.
Opal field at Yowah in Outback Australia
In the 1890s, up to 600 miners and their families lived around Opalton, supported by a pub, a bakery and a police station. A drop in opal prices combined with the ever- present water shortage to cause the field to be abandoned in After a long period of absence, miners trickled back into the area in the 1960s and Opalton is now the hub of fossicking in the Winton district. fossicking
OPAL DEPOSITS OF NEW SOUTH WALES Lightning Ridge Opalfield
Opal is currently being mined from depths between 1 metre (3 feet) and 30 metres (90 feet) below the surface.
There are in excess of 200 blowers in Lightning Ridge. Althrough there may have been some blowers prior to the first commercially built in Lightning Ridge which was built by Knud Nexo in December 1973
The wet puddler is used to separate the Opal clay from the actual Opal and harder particles.The Puddler is basically a basket made out of 1/4 inch mesh with a rotating plate at the bottom of the basket driven by an external power source through a modified diff from a truck.
Lightning Ridge Black Opals
OPAL DEPOSITS OF SOUTH AUSTRALIA
Precious Opal Localities of South Australia
Coober Pedy The opal field of Coober Pedy lies in the Stuart Range, 750 km north of Adelaide.Many of the locals prefer to live underground in dugouts where it is cool in summer and warm in winter. Opal was discovered in 1915 by a 14-year old boy who was camping with his father's gold prospecting party and, by 1916, Big Flat was established as the principal field. After the First World War, an influx of miners caused a major increase in production. The dugout style of living introduced by these ex-soldiers gave the field its name which is derived from the Aboriginal ’okupa piti' meaning ’white man's burrow'. Opal at Coober Pedy is found in deeply weathered white to mauve Bulldog Shale of Cretaceous age. Miners call the light, porous host rock 'sandstone'. Within the sandstone, opal may be found as veins in horizontal' levels' or in steeply dipping 'verticals' down to 25 m below the surface. Distribution is unpredictable and opal may not persist from one claim to the next. Coober Pedy is the world's largest producer of precious opal.
Andamooka Situated 520 km north of Adelaide, Andamooka miners work over an area of about 52 square kilometres on the Arcoona plateau in shafts up to 18 metres deep, large bore-holes,open-cut excavations and small tunnels. Mintabie During the First World War, Aborigines sold black opal at Coober Pedy which probably came from Mintabie, 350 km to the northwest. White opal miners discovered the field in the 1920's. Mintabie was deserted for many years until 1976, when heavy earthmoving machinery moved in and since then there have been many large finds.
Opal mining is a key industry in the town. The underground kitchen shown above is a regular part of life in Coober Pedy The image above is of a blower, which is used to extract the waste dirt from the
Life underground at Coober Pedy
A magnificent parcel of Mintabie Rough Opal Opal Mining in Mintabie