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PLATINUM AND ALLIED METALS. 345
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TASMANIA.
Osmiridium
has been found in the following places in Tasmania: Savage River,
Heazlewood River, Whyte River near the junction with the Pieman, Badger
gold diggings west of Savage River, Castray River, Huskisson River,
Wilson River, Salisbury goldfield near Bea-consfield, tributary of
Fourteen Mile Creek near Tyenna, Boyes River (flowing into the Gordon
below the Great Bend), and Bald Hills.
The
production of iridosmine from the Bald Hills district, near Waratah in
the northwestern part of Tasmania, began about 1900, and has shown a
steady increase. The district has been described recently in an
official publication,1 a summary of which is given below.
The
Bald Hill district is situated in the northwestern part of the island
north of the road from AVaratah to Corinna, about 19 miles west of the
former town. To reach the district the main road to Corinna is followed
to the saddle known as " Nineteen Mile," from which place a foot track
is followed northward for a mile to the district.
Bald
Hill forms the divide between Nineteen Mile Creek and Heazlewood River
and is at the northeastern end of the Long Plains, which are believed
to be part of an extensive peneplain, in which the streams have cut
their present courses in comparatively recent times. The general level
of the higher parts of the peneplains and of Bald Hill is 1,600 feet
above the sea level. The greater relief of Bald Hill is due to the hard
siliceous lodes which outcrop on its summit.
This
hill is at the north end of a belt of serpentinized rocks, which is 2
to 4 miles wide and 30 miles long north and south and lies on the east
side of the Meredith Mountains. The peridotites and pyroxene rocks have
been intruded by granite, but both types are thought to be of Devonian
age. The Basic rocks have intruded pre-Silurian and Silurian slates and
sandstones.
The
peridotites and pyroxenites are said to represent basic developments
of the gabbroid magma, and where least serpentinized are composed of
bronzite, enstatite, and olivine. Serpentinization has progressed far,
however, in most places. The metallic minerals of the serpentines,
probably primary minerals of the basic rocks, are magnetite, pyrite,
pyrrhotite, nickel, gold, and osmiridium.
Osmiridium
has been mined from placer deposits in Nineteen Mile Creek and its
tributaries, Linger-and-Die, McGintys, and Barren creeks, and from
Savage River for some distance below the mouth of Nineteen Mile Creek.
This metal has also been found in the rocks of Bald Hill, particularly
in certain belts of altered serpentine, and also in association with
chalcedony and opaline silica in lode-like forms.
Concerning the origin of these veins, Twelvetrees makes the following statement:2
The
whole question of the origin of these veins of chalcedony and opaline
silica is full of interest. It is probable that they have been formed
in the rock joints as the result of solutions started in the serpentine
by the expiring action of the magma. On this hypothesis the cooling and
attenuated aqueo-siliceous solutions filtered into existing joint
fissures, and might possibly involve mechanically small quantities of
osmiridium and other minerals during their passage.
1 Twelvetrees, W. H., The Bald Hill osmiridium field : Tasmanian Dept. Mines, Geol. Survey Bull. 17, 1914.
2 Op. cit., p. 19.
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