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Abstracts of the 28th annual FM-MSA-TGMS Tucson Mineralogical Symposium, February 10, 2007: gems and minerals from down under.

Pegmatite mineralogy of Western Australia

Mark I. Jacobson

108 Farmington Drive, Lafayette, LA 70503, USA

John Reeve

13 Buchan Place, Hillarys, WA 6025, Australia

Sue Koepke

46 Gemmell Way, Hillarys, WA 6025, Australia

Nimal Perera

13 Elderberry Drive, Parkwood, WA 6147, Australia

Pegmatites are widespread in the Archean and Proterozoic terranes of Western Australia, a state that covers one-third of the Australian continent. Attractive as well as rare mineral specimens have been recovered from several areas.

Archean LCT (Li-Cs-Ta) complex pegmatites are abundant in the East Pilbara, particularly in the areas south of Port Hedland. Some of these pegmatites have produced excellent specimens of rare and common minerals. Excellent euhedral simpsonite crystals up to 1 cm in diameter are known from its type locality, the Tabba Tabba Main tantalite pegmatite. Lustrous, white, equant, terminated beryl crystals in matrix are also common from this pegmatite. The depleted Wodgina Main tantalite dike is known for wodginite, its co-type locality, as well as having produced cesium-rich beryl, lithium phosphates and sharply formed tantalite crystals. Massive primary and secondary phosphates are known from several pegmatites, including Lewis's Rock hole, the Wodgina Main tantalite, the Strelley pegmatite and Congo ML at Mount Francisco. The Cooglegong area, possibly the Trigg Hill pegmatites, is the type locality for formanite and tanteuxenite and is also known for its yttrotantalite, fergusonite and gadolinite. The Pilgangoora pegmatite field is the type locality for ferrocolumbite.

The Proterozoic Gascoyne Province is well-known by collectors for its terminated dravite crystals in muscovite schist from Yinnie-tharra. The adjacent LCT beryl-columbite pegmatites at Morrissey Hill, Beryl Hill and Pyramid Hill only rarely contain lithium minerals such as lepidolite, elbaite and triphylite. Columbite, beryl, schorl and uranpyrochlore are the more common minerals. The Kempton brothers beryl pegmatite, south of Pyramid Hill, is the type locality for clinobisvanite, where it was found with beryl, ferrocolumbite, triphylite, rockbridgeite and uranpyrochlore.

The Archean Yilgarn Craton contains an array of LCT complex pegmatites and at least one NYF (Nb-Y-F) pegmatite field. The Murchison terranes at the north end of the craton contain numerous pegmatite fields. The pegmatites at Poona have only produced opaque green beryl with cassiterite and rarely lepidolite. The adjacent schists and quartz boudins are well known for their attractive but mostly non-gem-quality emeralds. Silvery brown bladed zinnwaldite in spherical clusters and botryoidal masses is particularly common in the Dalgaranga-Mount Farmer pegmatite field. Microlite, manganocolumbite and opaque but euhedral white to blue topaz are also known. The Goodingnow feldspar pegmatites south of Paynes Find have produced attractive opaque yellow to yellow-green beryl and columbite crystals. Other cleavelandite-rich pegmatites in the field have produced 2 to 4-cm tantalite crystals and very fine-grained, dense masses of purple lepidolite.

The NYF Mukinbudin pegmatite field has been known for rare minerals including allanite, euxenite, fergusonite, ilmenorutile, monazite, xenotime and zircon since 2000. Excellent smoky quartz crystals to 30 cm long and microcline have been found for several years in vugs in the Mukinbudin feldspar and Calcaling pegmatites.

Numerous specimens of spodumene, elbaite (opaque pink, green, blue and watermelon), montebrasite, sphalerite, manganocolumbite and beryl are commonly found at the Cattlin Creek pegmatite, located just 2 km north of Ravensthorpe. Just 15 km to the south-east, silvery brown zinnwaldite in bladed spheres exceeding 5 cm is particularly common in the quarry pegmatite, one of many in the Cocanarup pegmatite field.

The 1986 discovery of gemmy pink elbaite, tantite and kimrobin-sonite from the strongly weathered Forrestania Rubellite pegmatite near Mount Holland is further proof that vast unexplored areas in Western Australia still remain. The Forrestania Rubellite pegmatite is the type locality of kimrobinsonite, as well as being known for the rare minerals cesstibtantite and hafnon. Vugs were found in the central area of the pegmatite containing pink, green and blue elbaite, clear quartz crystals, pink beryls in association with albite (variety cleavelandite) and lepidolite. The largest previously illustrated elbaite was 2 by 2.5 cm.

The best-colored and best-formed emeralds from the Wonder Well emerald mine, Riverina Station, west of Menzies and the Bulla Bulling emerald mine, west of Coolgardie are all found in schist or quartz boudins and only rarely along the edges of quartz-feldspar pegmatites. Usually, the beryl in the pegmatites is only a moderate to dark green and not true emerald.

In Western Australia, the Coolgardie pegmatite field is probably the best known. Within this field, world-class specimens of ferrocolumbite have been mined for many decades from the Giles beryl-columbite pegmatite at Spargoville. Gemmy green and blue elbaite, rarely preserved as matrix specimens on quartz, have been found at the Giles elbaite pegmatite, just south of the beryl-columbite pegmatite. The Londonderry feldspar pegmatite has been a well known source for gem petalite, excellent ixiolite and columbite crystals, and the rare minerals eucryptite, bavenite, bityite and moraesite. The thin Barbara gold mine pegmatites are known for their sharply-formed brown danalite crystals. The Lepidolite Hill pegmatite is the only known locality for pollucite in Australia, discovered in a 1960's exploratory core hole prior to mining.

The famous Greenbushes pegmatite, although a key world source of tantalum and lithium and the type locality for holtite and stibiotantalite, is not a good producer of mineral specimens. Only lustrous terminated schorl crystals in kaolinized pegmatite and holmquistite in pegmatite contact schists have been abundant enough to be found in many collections.

Western Australia's size and diversity of pegmatites assures that new discoveries will be made as collectors continue to inspect the outcrops. Most of the new finds will remain in Australian collections as self-collected specimens. Only a few Australian pegmatites have produced a handful of world-class mineral specimens.

Unusual secondary copper assemblages from deposits in eastern Australia

Peter A. Williams

School of Natural Sciences

University of Western Sydney

Locked Bag 1797

Penrith South DC NSW 1797, Australia

Recent open-cut mining operations associated with Cu and Cu-Au orebodies in eastern Australia have revealed a number of exotic assemblages of secondary minerals. The cases examined here all feature secondary phosphates or arsenates and other rather rare species; each of them is noteworthy in terms of the abundance of material that has been recovered.

In the Northparkes E22 and E27 deposits at Goonumbla, New South Wales, upper oxidation zones are dominated by libethenite and pseudomalachite, with azurite and malachite emplacement at greater depths. In the nearby E26 deposit, oxidation associated with much more saline (NaCl) ground waters has led to extensive development of sampleite in the phosphate assemblage. Lower in the profile much atacamite was present. A similar pattern of zoning is associated with the oxidized zone of the Girilambone mine, at Girilambone, New South Wales. Secondary copper phosphates were encountered at all levels of the oxidized zone but were particularly pronounced in the upper benches. Deeper in the profile, abundant crystalline native copper was encountered, together with magnificent groups of azurite crystals. Related secondary copper arsenates were conspicuous in a paragenetically early sequence of the oxidized zone of the New Cobar mine, Cobar, New South Wales. Associated phases were bayldonite, duftite, gartrellite, agardite, mimetite and various arsenic-bearing members of the jarosite supergroup. A later carbonate sequence (azurite-malachite) was superimposed on this assemblage.

The Great Australia mine, at Cloncurry, Queensland, displays a related phosphate sequence that carries rarer phases including hentschelite and cloncurryite, a basic copper vanadyl phosphate fluoride closely related to nevadaite. However, a remarkable assemblage of rare species was found in highly siliceous cupritenative copper ore in both the main and B Tangye lodes from near the surface to the water table. This was protected from further reaction by encapsulation and bears a chemical relationship to certain material recovered from deposits at Bisbee, Arizona. Many tonnes of nantokite associated with cuprite and native copper were mined and processed. Associated with this material were atacamite, brochantite, claringbullite, gerhardtite and considerable amounts of connellite.

Geochemical processes responsible for the generation of these assemblages have been explored. Unusual groundwater compositions are important to a large extent, as is the lack of carbonate gangue in several instances. Rare secondary copper nitrate mineralization in the Great Australia deposit has its origin in a bizarre relationship involving termite activity.

Observations on Specimen Gold in Southeastern Australian Museums

Robert B. Cook

Department of Geology and Geography

Auburn University, Alabama 36849

Carl A. Francis

Harvard Mineralogical Museum

24 Oxford Street

Cambridge, Massachusetts 02138

Specimen gold was recently examined in three major Australian Museums--the Australian Museum in Sydney; Museum Victoria, Melbourne; and the Gold Museum, Ballarat. At the Australian Museum gold specimens both on display and in storage were inspected. Here display gold is dominated by exceptional nuggets including the 9.8-ounce "Sesquicentary Nugget" from Goanna Patch, Leonnora, Western Australia, and a peculiar and attractive flattened sheet-like 19.5-ounce nugget from the Ovens River, Victoria, and by good matrix specimens from Welha, Victoria and a 32-ounce piece from Hill End, New South Wales. Also on display are two historically significant gold specimens from Great Britain and casts of the enormous Welcome Stranger, Platypus, and Hand of Faith nuggets. A wide array of locality and second-tier display gold specimens are stored in the systematic collection. Specimens of note include sharp complex gold crystals on matrix from Tooloom Drake, New South Wales; deep yellow gold in the central part of quartz veins from Rutherglen, Victoria; gold in and on quartz from several localities at Lauriston, Victoria; peculiar "fibrous" gold in serpentine from Gundagai, New South Wales; gold and gold tellurides in calcite from Major's Creek, New South Wales; and somewhat crude gold crystals on quartz matrix from the Murchison Gold Field, Western Australia.

Virtually all of the excellent gold collection of Museum Victoria is in storage. Of particular note are well-crystallized golds from Matlock, Castlemain and Wedderburn, Victoria; Beaconfield, Tasmania; and the Murchison Gold Field, Western Australia. Attractive matrix specimens include a variety of localities such as Mongalata, South Australia and the Golden Crown mine, Victoria. Good nuggets are represented from Kingower and Bridgewater, Victoria. Other Australian specimens of note include those from Bendigo, Woods Point, Ballarat, and Beechworth, Victoria; and the Carnation mine at Payne's Find, and Kalgoorlie, Western Australia. The collection also contains an array of good to excellent specimens from many of the world's other gold specimen-producing localities.

Exceptional nuggets and matrix or crystalline gold specimens are on display in the Gold Museum, which is located conveniently across the street from the Soverign Hill Museum, a living history museum depicting life on the Ballarat goldfields in the 1850's. Included are the 21-ounce Terry and 13-ounce Knight nuggets, and the attractive 40-ounce Eureka nugget found on the outskirts of Ballarat in 1992 with a metal detector. Central to the collection is a group of 13 huge nuggets belonging to a single collector. Good specimens of gold crystals from Tasmania and crystalline gold from Echunga, South Australia are also on display.

Although the best of Australia's gold specimens have either been exported or melted, these three museums collectively preserve a uniquely important selection of Australia's gold heritage.

Minerals of the Cobar District, New South Wales

John Chapman

Geological Survey of New South Wales

New South Wales Department of Primary Industries

PO Box 344, Hunter Region Mail Centre

Maitland, New South Wales, 2310, Australia

The Cobar district in central western New South Wales, Australia, has a mining history that dates back over 130 years, with significant production of copper, lead, zinc, silver and gold. The total mineral endowment for the district exceeds $14 billion in value at current metal prices and mining is ongoing with five mines currently in production. Although the combination of deep weathering and the typical pipe-like configuration make Cobar-type deposits difficult exploration targets, a number of new discoveries have been made in recent decades.

Weathering over an extended period, at least back to the Tertiary, has resulted in deep oxidation of the primary sulfide bodies to typically 80-100 meters below the surface. Migration of metals downward formed rich supergene deposits of oxide and secondary sulfide minerals at the redox boundary.

Early underground mining encountered fine specimens of native copper, malachite and azurite but few of these are preserved in collections. More recent mining of the oxide zones at the Girilambone, New Cobar and Elura (Endeavor) deposits has exposed a rich diversity of secondary minerals including copper carbonates and phosphates, copper and lead arsenates, and silver minerals. Mining has also enabled the recovery of some spectacular specimens, particularly of native silver and mimetite from the Elura (Endeavor) deposit and azurite from the Girilambone deposit. Mimetite specimens from Elura are regarded as amongst the finest of this species in the world.

[Published with the permission of the Deputy Director General, DPI Mineral Resources, NSW.]
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Publication:The Mineralogical Record
Article Type:Abstract
Geographic Code:8AUST
Date:Jul 1, 2007
Words:2097
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