Cowell Jade Province Outcrops 1 - 110, Cowell, Eyre Peninsula, South Australia, Australia

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(From J.Townsend, 2005)
Discovery and History
In 1965, Harry Schiller, a local farmer prospecting in the Cowell area of the Eyre Peninsular (South Australia), collected a 3-4 kilogram boulder of dense hard rock near an outcrop of white, dolomitic marble. In early 1966, this specimen was identified as nephrite at Adelaide University. This identification was confirmed subsequently at the South Australian Museum and Australian Mineral Development Laboratories in Adelaide.

During the following years many attempts were made to develop the deposits, but with limited success. The period from 1966 to 1973 saw many disputes and changes in tenure. Involvement of the South Australian State Government commenced in February 1974 when the potential of the area for commercial production was fully evaluated. The 1974 geological appraisal was followed, in 1976, by Government funding of a trial mining program and an evaluation of the colours, texture and carving potential of the nephrite at O'Halloran Hill College of Advanced Education.

Ninety-one (91) separate jade outcrops were identified in 1974, but subsequent mapping has now increased this to well over a hundred. All outcrops of nephrite are located within an area of about 10 km2, referred to as the Cowell Jade Province.

After its formation in 1976, Cowell Jade Pty Ltd became the dominant lease holder and marketeer of Cowell jade. In January 1987, Cowell Jade Pty Ltd became a public company and changed its name to Gemstone Corporation of Australia Ltd. After more than a decade of mixed fortune, the Gemstone Corporation entered an agreement with the Malaysian-based Royal Selangor Group to design, produce and market carvings, and items of jewellery, and tiles from Cowell (nephrite) jade.

Geology
The (nephrite) jade bodies at Cowell typically have elongate, lensoid shapes in outcrop. Host rocks for Cowell nephrite are dolomitic marble and banded calc-silicate of the Early to Middle Proterozoic Minbrie Gneiss Complex. These high-grade metamorphic rocks were produced during the first and second deformational events of the Kimban Orogeny, about 1840 and 1780 million years (Ma) ago respectively. Subsequent retrogression occurred about 1700 Ma with later cross-warping and alteration about 1590-1600 Ma. Nephrite only formed within the alteration/retrogression assemblages.

Alteration zones, consisting of tremolite, chlorite, epidote, clinozoisite/zoisite and talc, occur particularly along the margins of, or in close proximity to intrusive chloritised feldspar rock that has intruded dolomitic marble.

Cowell nephrite formed:

Within alteration zones as large lenses up to 40 m long by 3 m wide, conformable with lithological layering.
Within cross fractures up to a metre wide parallel to the axial plane of late-stage cross-warping. Nephrite in cross fractures is typically high quality, fine grained, massive, and dark green to black - but sometimes it does occur in rarer translucent light green colours.
By irregular alteration of coarse grained, brecciated diopside¾ producing schistose nephrite that often has diopside inclusions.
The Cowell geological setting of Middle Proterozoic dolomitic marble host rocks contrasts with other major occurrences of nephrite in New Zealand, Canada and Taiwan; where nephrite lenses occur within or along the faulted margins of serpentinised ultramafic rocks such as peridotites.


SARIG data records note;
"Jade was first recognised in 1965, with further investigations revealing over 110 outcrops of nephrite scattered over an area of >9 km2. Jade and nephrite are similar in appearance, with the COWELL JADE PROVINCE a nephrite-only field. The mineral was formed in an alteration/retrogression metasomatic event as poddy/lensoid alteration zones up to 40m long by 3m wide, and conformable with lithological layering. Also within cross fractures up to 1m wide parallel to the axial plane of late stage cross-warping. Irregular uralitisation of coarse-grained diopside produced a schistose nephrite, often with diopside inclusions. It also formed along the margins of a chloritised feldspar rock where it intruded dolomitic marble. Nephrite occurred as a microcrystalline intergrowth of tremolite-actinolite, and because of its interlocking nature was extremely tough. The weathered nephrite is fractured or shows patchy variation in grainsize, with a variety of inclusions. It often grades into green tremolite or marble. Cowell Jadeis translucent to opaque, and ranges in colour from yellow-green, through dark green to black. Dark olive-green makes up the bulk of the gem quality jade, with black the highest grade material. It was estimated that ~2% of the jade was of prime quality suitable for jewellery. Production from 1974-2009 mainly from better gem quality outcrops was estimated at 248,100kg. There has been minor production of dolomite, and talc products. A resource of 45,700 tonnes to 5m depth was estimated in 1977."


Although this site is sometimes refered to as The Mount Geharty Jade Mines (even though this name was never officailly used), it should not be confused with the nearby Mount Geharty Talc Mines.

Mineral List

Actinolite

'Ferro-Actinolite-Tremolite Series var: Nephrite'

'Jade'

3 entries listed. 1 valid mineral.

References

External Links