Australian Chrysoprase

Rough and polished Chrysoprase from
Marlborough, Central Queensland

Rough Chrysoprase from
Mt Davies, NW South Australias

Chrysoprase Mine (Gumigil Pty Ltd)
Marlborough, Queensland

Chrysoprase, the most valuable variety of crypto/microcrystalline quartz (chalcedony), composed chemically almost entirely of silicon dioxide (Si0₂). It is an important ornamental gemstone that serves as the alternative birthstone for those born during the month of May. The Greeks, Romans and Egyptians used this material for signets and seals as well as jewellery during early times. In the Victorian era it was again popular being used in jewellery pieces. Its green colour, which is due to traces of nickel, ranges over various shades of pale green, through apple green to a deep rich green. Recent research by Vasconcelos and Singh (1996) has revealed that the colour-causing nickel occurs in chrysoprase as platelets of the Ni-talc mineral willemseite. Chrysoprase of high quality has been produced commercially since the early 1960s from the Marlborough district of central Queensland, some 90 km to the northwest of Rockhampton. Small quantities of chrysoprase also have been produced intermittently from deposits at Mount Davies in the remote northwest of South Australia, and adjacent deposits at Wingelina in Western Australia. More recently, chrysoprase has been won from Yerilla in Western Australia.

Chrysoprase is usually translucent, but may become opaque in poorer quality material. A small proportion of stone is semi-transparent to transparent and, if of good colour, is highly prized and valued. Chrysoprase is fashioned principally into cabochons, beads, bangles, or is carved to yield jewellery and other ornamental objects. It is easily worked and takes a fine polish. The highest quality material is a rich apple green of even colour, without flaws, fractures, inclusions, cavities or other imperfections.

Gem quality chrysoprase displays the following identifying properties:

Gemmological properties of Marlborough chrysoprase.

Occurrence
Chrysoprase occurs within the deep weathering profile that results from lateritiseration¹ of nickeliferous serpentinites² or other ultrabasic rocks. Serpentinised ultrabasics³ occur in three main areas of eastern Queensland: the Marlborough district, the Widgee-Kilkivan district, and the region between Greenvale and Mount Garnet.

Weathering and alteration (oxidation) of silicate minerals such as serpentine during lateritisation generates iron oxides and releases silica and nickel which are transported down through the developing laterite profile to be re-deposited in veins and nodules (possibly replacing magnesite) where chemical conditions provide favourable sites. At Marlborough, the chrysoprase occurs in veins and nodules in the magnesite-rich saprolite zone underlying an iron-rich silica cap.

Mining, Production and Outlook
The main mine at Marlborough is owned and operated by Gumigil Pty Ltd, a Hong Kong-based company. This mine's entire production is sent to Hong Kong for processing and sale. A second smaller mine, previously operated by Kajar Pty Ltd is now owned by Viper Resources Pty Ltd, a subsidiary of Cobra Resources NL. Considerable quantities of chrysoprase have been produced from this mine in past years, but it has been inactive for some time now. However, the present owners have undertaken a program of testing and drilling to assess the deposit and production is likely to resume shortly.

Mine workings comprise large open-cuts, within which benches are excavated using heavy earth-moving machinery. The barren iron-rich cap of the deep weathering profile, which may be up to 40 metres thick, is removed by bulldozer. The underlying saprolite (soft, earthy, clay-rich) zone, in which the chrysoprase veins and nodules occur, is progressively stripped and the freshly exposed floor is checked for signs of chrysoprase after each bulldozer pass. When chrysoprase is exposed, the veins are removed by backhoe or excavator, and the chrysoprase is collected by hand. The recovered chrysoprase is later cleaned by hosing with a high-pressure water spray, separated by wet trommelling, sorted for quality, and finally packed in drums for dispatch to market.

Queensland production of chrysoprase, from the Marlborough deposit, was valued at $1,071,300 in 1997/98 compared to $481,550 in 1996-97 and $1.22 million in 1995-96. Other minor Australian production of chrysoprase in recent years has come from Western Australia and South Australia. Production in Western Australia has come mainly from the Yerilla mine operated by Chrysoprase Mines of Australia (Bellmount Holdings Pty Ltd). In South Australia, material mainly of carving grade has been won from the Mount Davies (Pipalyatjara) deposits. Most of this material has probably been directed to the US and Korean markets. Future production in Queensland from existing resources is likely to last for at least 20 years and possibly up to 40 years at current production rates. Further production can be expected from other deposits elsewhere in Australia; but the Queensland deposit at Marlborough is by far the largest and generally contains chrysoprase of superior quality.

Footnotes:

1. Laterisation: Process of forming laterite/bauxite under special climatic conditions in tropical regions. Laterite - a highly weathered residual deposit (red soil rich in hydrated oxides of iron and/or aluminium) formed from basic igneous rocks and other iron-rich rocks, or granitic rocks or other rocks low in iron to yield bauxite.
2. Serpentinite: Ultrabasic rock consisting almost wholly of serpentine minerals that have been derived by alteration of ferromagnesian minerals such as olivine and pyroxene.
3. Ultrabasics: Igneous rocks consisting essentially of ferromagnesian minerals.

References:

DEPARTMENT OF MINES AND ENERGY, 1999: Queensland Minerals and Energy Review 1998-1999. Department of Mines and Energy, Brisbane, 75-76.

KROSCH, N.J., 1990: Queensland Mineral Commodity Report (Chrysoprase). Queensland Government Mining Journal, 91, 165-169.

VASCONCELOS, P., AND SINGH, B., 1996: The crystal chemistry and genesis of chrysoprase. In Mesozoic Geology of the Eastern Australian Plate Conference. Geological Society of Australia Inc., Extended Abstracts No.43, 545.

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