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Mineralogical
Society of Tasmania
Past Newsletter Items |
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A selection of articles from past newsletters
published by the Mineralogical Society of Tasmania.
- Adapted and updated from an Article published in
Mineralogical Society of Tasmania Newsletter #8, 1994,
by Steve Sorrell & Ralph Bottrill.
Introduction
- A number of interesting skarn assemblages exist in altered
Ordovician Gordon Limestones at the Kara Mine, near Hampshire, Northwest Tasmania. The
skarns occupy a trough-like pendant within late Devonian red granite known as the Housetop
Granite. At the Kara No.1, ore-grade scheelite mineralisation forms an irregularly-shaped
blanket draped 15-25m above the granite. Between the skarn and the granite is a
tungsten-poor, quartz-epidote reaction zone (Turner, 1990).
Open pit mining of scheelite has been in progress since 1977 (Whitehead,
1990).but magnetite is presently of more major economic importance. Limestone Creek, north
of Kara No. 1, has been investigated as a wollastonite resource.
Minerals
- Actinolite
Felted green mats, some replacing diopside crystals, grading into tremolite
and possibly hornblende or other amphiboles.
Allanite
Minor amounts are recorded in the associated granite
(Turner, 1990).
Andradite
Excellent lustrous crystals, to ~70 mm (but
mostly <1 cm) in calcite or in vughs in massive andradite skarn. Colours range from very pale yellow, green, red, brown and black. Forms are a combination of rhombic dodecahedron and a trapezohedron.
Anorthite
This plagioclase feldspar occurs as the
almost pure end-member (Microprobe analyses give An70-100: Bacon & Bottrill, 1988), as
small grains with wollastonite and calcite in the Limestone Creek skarn.
Anthoinite
A rare tungstate mineral, occuring as a white powdery mixture with mpororoite,
pseudomorphing scheelite in weathered skarn (Matsubara & Kato, 1984)
Bavenite
This rare calcium beryllium silicate occurs as small sprays of transparent
colourless crystals with a thomsonite-like habit, in cavities associated with fluorite,
pyrite and epidote.
Biotite
Recorded in the associated granite (Turner, 1990).
Calcite
As crystals in a variety of habits including: short hexagonal prisms; long
hexagonal prisms with flat terminations; long hexagonal prisms with pyramidal
terminations. Also as vein infillings in magnetite: white to clear, some with bright pink
long-wave fluorescence; golden openings to mamillary surfaces in cavities.
Chalcopyrite
In fine grains in vesuvianite and pyrite bearing skarn.
Danalite
Uncommon as cherry red octahedral crystals and grains in
magnetite-actinolite-fluorite skarn, rarely to a few cm across.
Diopside
Recorded by Turner (1990). Largely altered to actinolite-tremolite. Grades into
hedenbergite?.
Epidote
Transparent, small lustrous green crystals with andradite
Fluorapatite
Minor amounts recorded in the associated granite (Turner, 1990).
Fluorapophyllite?
May occur as small glassy crystals in cavities - needs confirmation.
Fluorite
Clear to deep purple octahedrons to 5mm occur in cavities with epidote, pyrite,
?actinolite, and bavenite. Also as druzy secondary colourless octahedral microcrystals in
the same cavities. As pink to purple octahedrons enclosed in calcite. Also recorded as
minor amounts in the associated granite (Turner, 1990).
Halloysite
Light brown and earthy, in weathered skarn (Matsubara & Kato, 1984)
Hedenbergite
Probably grades into diopside (qv).
Hornblende?
Black fibrous amphibole crystals, actual species not yet determined
(Hastingsite?), in granite and skarn.
Hydrozincite?
Fibrous whitish with ?rosasite
Magnetite
Black massive, bright crystals including octahedra and dodecahedra, mostly dull
crystals to 20mm. Also recorded as minor amounts in the associated granite (Turner, 1990).
Malachite
As blue-green to bright green botryoidal aggregates, spheroids and rosettes.
Molybdenite
Small rosettes and flakes in skarn.
Montmorillonite
Yellowish green and earthy in "the old trench" (Matsubara & Kato)
and other weathered skarns.
Mpororoite
Another rare tungstate, this occurs as a white powdery mixture with Anthoinite
pseudomorphing scheelite in skarn (Matsubara & Kato, 1984)
Orthoclase
This feldspar is abundant in the associated granite (Turner, 1990).
Plagioclase group
Andesine-Oligoclase (~An30) was recorded in the associated granite (Turner,
1990). Small grains in wollastonite skarns in Limestone Creek.vary from albite (An0) to
anorthite (An100)
Prehnite??
May occur as pale green botryoidal coatings - to be confirmed.
Pyrite
As small cubes & cube-octahedra in skarn
Quartz
Recorded in the associated granite (Turner, 1990). Present in radiating white
crystal aggregates with calcite and diopside(?).
Rosasite??
Blue-green spheroids and rosettes - to be confirmed
Scheelite
Mainly as white blebs in magnetite to a few cm across. Rare pseudoctahdral
crystals occur.
Stolzite
Rare tabular to blocky orange-yellow crystals replacing scheelite in andradite
skarn.
Titanite
Crystals not dissolved when calcite etched, suspected to be titanite (F.
Doedens, pers. comm.). Also recorded as minor amounts in the associated granite (Turner,
1990).
Tremolite
See actinolite.
Vesuvianite
Recorded in the skarns by Turner (1990) and confirmed by XRD.
Wollastonite
Abundant in Limestone Creek skarns (Bacon & Bottrill, 1988).
Zircon
Minor amounts recorded in the associated granite (Turner, 1990).
References
- Bacon, C.A., & Bottrill, R.S., 1988. Industrial
Minerals in Tasmania: Wollastonite. Tas. Dept. Mines Unpub. Rept. 1988/29
Bottrill, R.S., in prep. The Kara Mine, Tasmania
Matsubara, S. & Kato, A., 1984, Mpororoite &
Anthoinite from the Kara Mine, Tasmania, Mineralogical Magazine, 48, 397-400
Turner, N.J., 1990, Kara Scheelite-Magnetite Deposit -
Geology, in: Turner, N.J. & Taheri, J. (eds.) 10th Australian Geological Convention,
Hobart 1990, Excursion Guide E2, "Tin & Tungsten Deposits & Related Devonian
Granitoids", Geological Survey of Australia, p. 25-29
Whitehead, C.,1990, Kara Scheelite-Magnetite Deposit -
Production and deposits, , in: Turner, N.J. & Taheri, J. (eds.) 10th Australian
Geological Convention, Hobart 1990, Excursion Guide E2, "Tin & Tungsten Deposits
& Related Devonian Granitoids", Geological Survey of Australia, p. 24-25
Back to Contents
- Adapted from an Article published in Mineralogical Society
of Tasmania Newsletter #10, 1994, by Ralph Bottrill.
>
Introduction
- Two principal types of corundum occurrence are known in
Tasmania: gem sapphires in various alluvial deposits, and finer grained rock-forming
corundum in situ in granite-related deposits in the Mt Read Volcanics. These occurrences
are described further below.
Sapphires
- Sapphires are a gem variety of corundum (Al2O3). The name
was originally intended only for stones with a blue colour, but is now used for stones of
any colour except red (usually designated as the variety ruby), and including green,
yellow and purple. The colours are mostly due to varying proportions of iron and titanium
or, in the case of ruby, traces of chromium, in the crystal lattice. The most common
colour, dark blue, is rich in titanium and iron; with decreasing titanium the colours tend
towards green and yellow (Deer et al., 1962). During cooling, excess amounts of titanium
may exsolve as very fine, oriented needles of rutile, producing an attractive asterism
known as "star sapphire", usually in dark blue stones.
Sapphires are widely distributed in eastern Australia and South East Asia,
in a belt extending from Tasmania through central and eastern Victoria, northeastern NSW,
eastern Queensland, Thailand, Kampuchea, Vietnam and southern China (Tasmania Department
of Mines, 1970; Stone, 1976; Coenraads et al., 1990; Robertson & Sutherland, 1992).
Most of the stones have been recovered from alluvials, usually associated with zircon,
pleonaste spinel and ilmenite. The sapphires are usually closely associated with Tertiary
alkaline basalt (basanite, nephelinite, etc.) which occur as flows, plugs and
pyroclastics. The sapphire has been found as corroded megacrysts in these basalts, along
with numerous xenoliths (including metasediments, granulite, granite, anorthosite,
pyroxenite, lherzolite, etc.) and other xenocrysts (including zircon, spinel and
anorthoclase) (Coenraads et al., 1990, Robertson & Sutherland, 1992). The major
primary host rock in central Queensland is thought to be the associated pyroclastics,
bringing corundum and other phases rapidly to the surface from a possible nepheline
syenite source rock at pressures >10 kbars (>35 km depth; Robertson &
Sutherland, 1992).
Diamond is a rarer associate and may have the same
source, but be somewhat earlier in age than the sapphires (Robertson & Sutherland,
1992).
Sapphires in Tasmania
- Sapphires in Tasmania occur in many areas, particularly
the northeastern tinfields, usually associated with relatively coarse grained red-brown
zircon and black pleonaste spinel. Other areas include the Boat Harbour - Sisters Creek
area, Blythe River, Table Cape, Bell Mount, Lisle, Stanley River, Adamsfield, Coles Bay
and Launceston (Tasmania Department of Mines, 1970; W.L. Mathews, pers. comm.; Fig. 1).
The stones are typically a dark blue, but range from yellow to green and purple, and some
are part-coloured. Star sapphires have been recorded in several areas. The largest
recorded stone is a 52.8 g (264 carats) parti-coloured stone from the Weld River area, but
most are much smaller (Tasmania Department of Mines, 1970; W.L. Mathews, pers. comm.).
Most of the stones were found in sediments that originating in areas of Tertiary basalt,
with the exception of the Adamsfield and Stanley river occurrences. In these areas much of
the country is highly eroded and dissected, and basalts may not have survived.
Alternatively, small plugs may not have been recognised in these relatively inaccessible
areas.
In the northeastern tinfields the main
occurrences are at Branxholm, Derby, Gladstone, Lottah, Main Creek, Moorina, Mt Stronach,
Mt Cameron, Thomas Plain and in particular the Weldborough - Weld River area (Tasmania
Department of Mines, 1970; W.L. Mathews, pers. comm., Fig. 2). Cassiterite and topaz are
typically associated, but are unlikely to have the same source. Tertiary basalt is present
in the Ringarooma River valley and as isolated remnants at higher level, such as the
Weldborough Pass, at the headwaters of the Weld River. In this basaltic remnant can be
seen a pumiceous breccia dome, suggestive of an eruptive centre, overlain by massive
basalt flows containing xenoliths of anorthoclasite and sanidinite. These rocks may relate
to those at the sapphire occurrences in Queensland (Robertson & Sutherland, 1992).
Corundum-bearing Rocks
- Unusual corundum bearing rocks have been identified in
several small outcrops, one km west of the Lea River/Fall River Junction, near Moina,
Tasmania. The dyke-like bodies occur within the Bond Range porphyry and Back Creek Beds,
of the Cambrian Mt Read Volcanics, and appear to cross-cut the stratigraphy (Pemberton
& Vicary, 1989).
The rocks contain fine to
medium grained blue corundum with quartz, muscovite, kaolinite, pyrophyllite, feldspar and
andalusite. Textures indicate disequilibrium and a possible two stage process: alteration
of quartz porphyry by low temperature, acidic granitic fluids to kaolinite - diaspore -
quartz, and rapid heating by high temperature, boiling granitic fluids to a metastable
corundum - andalusite assemblage (Bottrill & Pemberton, in press). The corundum
observed by the author does not appear to be of gem quality.
Other corundum occurrences
- A mafic, mica and hornblende bearing dyke cutting
granodiorite in the Lisle valley was described as corundum-bearing by Thureau (1882) and
probably represents one of the widespread Devonian and Cretaceous lamprophyres in
Tasmania. Corundum is also a probable constituent of skarn (magnesite - serpentine -
pyrrhotite) assemblages in altered dolomites intruded by Devonian tin-bearing porphyries
and greisens in the Mt Bischoff Tin Mines (Groves et al., 1973; Kwak,1987). Ford (1984)
described corundum inclusions in porphyries at Cygnet. The gem variety ruby has also been
reported rarely from the tin workings in the northeastern Tasmania, but has not been
confirmed (Tasmania Department of Mines, 1970).
Summary
- Corundum is very widespread in Tasmania, mostly in the
form of sapphire grains in alluvial deposits. This is probably related to the occurrence
of similar material throughout eastern Australia and South East Asia. The corundum is
thought to have been formed in nepheline syenites or similar rocks at great depth, and be
brought to the surface rapidly in pyroclastic eruptions of alkaline basalts, which are
usually closely associated. Another type of occurrence in Tasmania may be relevant as a
possible source rock: the formation of corundum in argillised silicic volcanics, probably
by rapid dehydration accompanying boiling granitic fluids.
Bibliography: available on request
Back to Contents
- Adapted from an Article published in Mineralogical Society
of Tasmania Newsletter #5, 1993,
by John Richmond & Ralph Bottrill.
Geological Notes
- The major rock type present here is a basalt, a fine
grained plagioclase-pyroxene-olivine bearing volcanic rock common in Tasmania, here of
late Tertiary age (~ 10-20 million years). There is also an old stream channel exposed,
filled with clayey sand and gravel. The basalt contains abundant columnar jointing in
diverse orientations suggesting that the original surface was very irregular and the
volvanic vents were probably close by. The lava probably flowed down the old Jordan River
into the Derwent Valley, to about Claremont. The basalt is vesicular in part (particularly
near the top), and these vesicles (frozen gas bubbles) commonly contain interesting
minerals. Zeolites are notably lacking, perhaps due to insufficient depth of burial and/or
low groundwater temperatures and/or unsuitable fluid compositions; they are commonly found
at deeper levels than exposed here.
Minerals
- AragoniteThis occurs as small to large, attractive, lustrous prismatic
crystals, colourless or white to pale violet coloured; also fibrous-massive, colourless to
pale brown veinlets.
Barite
Occurs as attractive groups of colourless, bladed crystals to about 5 mm, with
calcite.
Calcite
Attractive spheroids and botryoidal aggregates to about 15 mm diameter, white or
banded to pale green and brown.
Ilmenite
Small black crystals lining vesicles.
Montmorillonite-nontronite
Massive, blue, green and black.
Opal
Black, massive.
Plagioclase Feldspar
Fine white crystals lining vesicles.
Quartz
Small drusy surfaces and chalcedony.
Siderite
Botryoidal, brown aggregates.
Back to Contents
- An Article by Carl Bjorklund - Published 09/08/96
Mineralogical Society of Tasmania Newsletter
Silver
is a native metallic element with the chemical symbol Ag from the Latin argentum.
Physical Properties
- Colour: silver/white on fresh surfaces, tarnishing to grey
and black.
- Hardness: 2.5-3
- Specific Gravity: 9.6-12 (10.5 when pure).
- Lustre: metallic
- Transparency: opaque
- Cleavage: none
- Fracture: hackly
- Streak: glossy silver-white
- Malleable, ductile and sectile
Crystal System
- Cubic, but rarely distinct
- Forms: crystals, dendrites, wires, plates, sheets and
compact masses.
- Crystals occur as indistinct cubes, hexahedra and
octahedra.
Genesis
In oxidised zones of hydrothermal sulphide veins.
Supergene, in some sulphide deposits. Rarely in basalts and dolerites, except in the
notable occurrence in the Keeweenaw Peninsular, Michigan (see below). Also in some
sedimentary deposits.
Notable locations
- Schneeberg, Saxony, Germany, where wires up to 40 cm long
have been found. Also at Freiberg and Schwarzwald in Germany.
- Kongsberg in Norway has, since 1624 produced more than
1,350 tonnes of native silver from approximately 300 mines.
- At Jachymov in Czechoslovakia, wires up to 30 cm have been
found.
- In Southern Arizona a piece of native silver weighing 1350
kg was found.
- In Cobalt, Gowganda & O'Brien in Ontario, silver is
obtained in sheets, slabs and irregular masses many centimetres across (<612 kg).
- In Michigan on the
Keeweenaw Peninsular, beautiful specimens of crystalline silver have been found associated
with native coper.
- Silver Islet in Ontario presented one of the 19th
Century's most difficult mining challenges, due to rich veins extending below water level.
- Since the discovery by the Spanish of vein deposits at
Batopilas in Chihuahua, Mexico, seven times the total amount of silver produced at
Kongsberg has been mined. Most of this silver occurs as wires, crystals and herringbone
crystal groups.
- Silver wire specimens have been produced from mines at
Broken Hill and Elura (near Cobar) in NSW.
- In Tasmania native silver has been recorded from the
Magnet, Godkin, Whyte River and South Curtain Davis Mines. The state's finest examples
were from the Hercules Mine, Mt Read, where silver wires was associated with masses of
crystalline cerussite.
Uses
As a precious metal. Used in minting, jewellery,
medicine, chemistry, photography, electronics.
Back to Contents
- An article by Ralph Bottrill - published 21/11/96
Nature: Barite (Barium sulphate, BaSO4) is a relatively common
mineral, belonging to the barite group of orthorhombic sulphates (with anglesite and
celestite). It may contain large amounts of strontium grading into celestite) and minor
calcium and lead.
Name: Barite derives from the Greek "baros":
baros: heavy, in allusion to its high specific gravity. It was described (from an unknown
locality) as "baryt" by Karsten in 1800, and "baryte" by Hauy in 1801,
but was anglicised by Dana in 1868. Before 1800 it had been known at least as early as
1640 (as " Lapis Bononiensis", famous for its phosphorescence, and Hepatite and
other names).
Description & diagnostics: Barite may be colourless
to white, yellow, red, brown, blue, pink and other pale colours; transparent to
translucent. Hardness: 3-3.5; S.G: 4.5; lustre vitreous to resinous and pearly. It is
brittle with three cleavages, one very strong and two at right angles. It may be massive,
concretionary to crystalline. The crystals are variable in form, from platey to bladed,
blocky and prismatic, as shown below. The platey crystals may occur as groups known as
desert roses, which may be full of sand inclusions. The mineral is identified by its high
S.G., pale colour, distinctive cleavage and crystal forms, and the flame colour (green,
compared with red for the similar celestite).
Occurrence: Mostly in hydrothermal veins, also as
concretions in sediments, in hot spring deposits, in Pb/Zn/Cu deposits, vesicular basalts
and other rock types. Associates include calcite, quartz, fluorite, galena, sphalerite,
chalcopyrite, pyrite, and oxidised Pb and Cu minerals.
Uses: Mostly mined for use in drilling mud, also for
chemicals, medicinal uses, cosmetics, pigments and fillers. It is important for colouring
fireworks and was used by Bengalese Priests for colouring fire.
- Localities: (numerous)
- Europe: Germany, Romania, Czechoslovakia, France
- England: Cumberland, Derbyshire, Northumberland,
Westmoreland
- USA: Colorado, Connecticut, Illinios, South Dakota,
California
- Australia: South Australia (var.), Mt Isa, Broken Hill,
Victoria
- Tasmania: Cremorne, Bridgewater, Queenstown, Rosebery,
Rossarden.
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