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Deposit Synonyms
Big Flat
Mineral District
Coober Pedy Opalfield
Deposit Summary
opal as seams and in levels in host weathered Cretaceous Bulldog Shale. Discovered 1915, now original field largely destroyed by dozer trenching. Subsequently the Coober Pedy Precious Stones Field was declared & became largest producer of precious opal in the world.
Deposit Description
COOBER PEDY, with an original name Big Flat Diggings was the discovery area of the world renowned Coober Pedy Opal Field. Opal was discovered in 1915, with diggings well established by 1916. Today this original area has been largely destroyed by dozer trenching. What followed in the subsequent decades was the development of the largest producer of precious opal in the world from the larger area of the Cooper Pedy declared opal field.
Opal is a hydrated form of quartz, and dispersed as very rare small masses and veins throughout host marine Cretaceous Bulldog Shale. Its formation is thought to be related to weathering processes, in particular the fluctuations of the water table. Local geology is host Bulldog Shale overlain by variably ferruginised, and calcreted Pliocene to Quaternary Russo Beds.
The irregular distribution of opal is demonstrated by the pattern of diggings, which tend to clump around significant finds. Most excavations occur along the east-facing escarpment of the Stuart Range and the adjacent gently west sloping tableland to the west. For many decades mining has been largely by mechanized methods, both as trenches, and underground. Host siltstone of the Bulldog Shale is deeply weathered and kaolinised, tending to very pale purple low in the weathering profile, to unoxidised black siltstone at depth.
Some known guides to finding opal include the identification of "level," in places, fault type structures (`slides) are known controls.
40Ar/39Ar geochronology on alunite samples (Newberry), yield Miocene ages. Laser step heating plateau ages for alunite grains range from 19.34 to 6.45 Ma. Mineralogical and geochemical studies by Newberry relate alunite formation to the metasomatism of kaolinite to produce monosilic acid, which may be a plausible source for opal formation. Some observed textural relationships suggest opal formation occuring after alunite precipitation. A close relationship is observed between hollandite (Ba(Mn) oxide) and opal suggesting precipitation of the 2 phases is closely related. Newberry identified a distinct trend of age dates with depth, with older ages at higher elevations. This is interpreted as a record of deepening groundwater table over timeband at a rate of 3.48m/My.
Morris (2015) confirms opal formation at the base of the weathering profile and associated with supergene minerals alunite, kaolinite, gypsum, and iron oxides. Age dates of alunite suggest a range of dates from 15.71 to 12.37 Ma, and showing a younging of ages with depth. Precious opal is considered to have formed towards the end of the Miocene from the remobilisation of disseminated amorphous opal that may be as old as 60Ma.
Discovery Year
? 1915
Commodities
Opal, Alunite
Ore Minerals
Opal
Gangue Minerals
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