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Mineral paragenesis, geochemistry and fluid characteristics of the Kara scheelite-magnetite skarn deposit, Northwestern Tasmania


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Singoyi, Blackwell 1995 , 'Mineral paragenesis, geochemistry and fluid characteristics of the Kara scheelite-magnetite skarn deposit, Northwestern Tasmania', Research Master thesis, University of Tasmania.

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The Kara scheelite-magnetite deposit is located in northwestem Tasmania 40km
south of Burnie at latitude 41°18' S and longitude 145° 48'E. The deposit consists of several
skam bodies with total remaining mineable ore reserves estimated at 1. 7 Mt magnetite @
>30% Fe, and 0.3 Mt scheelite @ 0.52 % W03 by 1992. The major orebodies at Kara are
hosted by limestone or calcareous sandstones of the Ordovician Gordon Limestone
Subgroup which are in contact with the Devonian Housetop Granite or separated from it by
the Ordovician Moina Sandstone or Owen Conglomerate.
The predominant skarn minerals display a zonation that ranges from epidote-quartz
to garnet, magnetite-amphibole, vesuvianite and clinopyroxene zones away from the granite
contact. Mineral paragenetic studies reveal at least four stages of skarn formation and ore
1) Stage I --clinopyroxene-gamet-vesuvianite±wollastonite±quartz±scheelite,
2) Stage II -gamet-vesuvianite-magnetite±scheelite±apatite±quartz,
3) Stage III -magnetite-amphibole-epidote-fluorite±quartz±gamet±vesuvianite
±chlorite±scheelite±sphene±hematite±pyrite±clinopyroxene, and
4) Stage IV -hematite±fluorite±calcite±quartz.
Stages I and II mineral assemblages represent early skarn formation and are dominated by
anhydrous minerals of clinopyroxene and garnet. Stages ill and IV minerals represent late
skam-forming phases and pervasively replace early mineral assemblages. Scheelite occurs
in Stages I-III and generally shows a close spatial association with hydrous minerals
(vesuvianite in Stage I/II and amphibole in Stage III). Abundant scheelite is found in Stage
ill where it forms very coarse grains up to >5cm. Magnetite occurs in Stages II and m but
is also more abundant in Stage III.
Microprobe analyses indicate that the majority of the skam minerals are calcic, and
have high Fe3+fFe2+. Clinopyroxene is diopside-rich generally having a mole composition of >80% diopside and <25% hedenbergite, while garnet composition from Stages I to III are
andradite-rich, displaying mole proportions of> 70% andradite and <30% grossular. Garnet
appears to have the highest levels of tin (up to 0.37 wt. % Sn02) and may be the major
carrier of tin in the skarn. Amphiboles are largely of hastingsite and magnesian hastingsite
composition. Scheelite contains moderate amounts of molybdenum ( <3 wt. % Mo03) which
give powellite mole proportions of up to 6% and is unaccompanied by molybdenite.
Fluid inclusion microthermometric measurements of primary fluid inclusions for
skam minerals reveal systematic declining temperatures from early mineral assemblages to
later stages, but salinity values are variable. Stage I clinopyroxene yielded a
homogenisation temperature range of 460° to 620°C (modes at 520°C and 600°C). The
Stage II minerals gave a homogenisation temperature range of 350° to 570°C (mode at
500°C). The Stage ill mineral assemblage yielded homogenisation temperatures ranging
from 240° to 360°C (mode at 300°C). Stage I clinopyroxene gave salinity values from 2.0
to 10.0 equiv. wt.% Naa. The Stage II mineral assemblages showed salinities of 12.0 to
16.0 equiv. wt.% NaCL The Stage ill mineral assemblage yielded salinity values of0.2 to
19.8 equiv. wt. % Naa (modes at 5 and 15 equiv. wt .. % Naa).
Isotopic measurements of the protolith Gordon Limestone reveal o13c composition
from -1.6 to -4.4 %0 (PDB) and o18o composition of 10 to 23%0 (SMOW) which are both
depleted with respect to the unmetamorphosed Gordon Limestone values (o13c = -1.5 to
1.8%0, PDB; 0180 = 22 to 27 %0, SMOW). The isotopic compositions of the Stages ill and
IV skarn calcite range from -1.7 to -7.3%0 (PDB) (mean -4.5%0) for carbon and 3.4 to
14.0%0 (SMOW) (mean 11 %0) for oxygen. Calculated isotopic compositions yield S13Cco2
values of 0.0 to -5.0%0 (PDB) (mean -3%o) and B180H2o values of 0.0 to 10.0%0 (SMOW)
(mean 7%o). Depletion in isotopic values for_the protolith marble are interpreted to be
dominantly due to interaction with magmatic fluids from the nearby granite which are also
responsible for the formation of skam calcite. The values below the magmatic range in the
skam calcite are interpreted to be due to minor contribution from meteoric fluids. Oxygen
isotope values of Stages II to ill magnetite vary from 2.7 to 4.7%o (SMOW) and yield
o18oH20 values of 12.3 to 13.9%0, whereas oxygen isotopic compositions of Stages II to ill scheelite vary from 5.2 to 6.4%0 (SMOW) and yield o180H20 values of 6.0 to 9.0%0. These
ol80H20 values are generally in agreement with the isotope values from skam calcite and
confirm a magmatic source for the fluids.
The geological, mineral paragenesis, mineral chemistry, fluid inclusion and stable
isotope studies indicate that the Kara deposit formed in stages as a proximal skam
assemblage in carbonate host rocks following the emplacement of the Devonian Housetop
Granite and was characterised by early, high temperature (up to >600°C) mineral
assemblages dominated by anhydrous minerals, and late low temperature (300°C)
assemblages with abundant mineralisation (scheelite, magnetite) and hydrous minerals.
Fluid inclusion and stable isotope data indicate that early skam formation was probably
solely due to magmatic fluids, while later skam formation and ore deposition was
developed from magmatic fluids mixed with minor meteoric fluids. Chemistries of
clinopyroxene, garnet and scheelite (unaccompanied by molybdenite) suggest that the Kara
skam deposit was formed under highly oxidised conditions. The deposit differs significantly
from other scheelite deposits such as CanTung in the abundance of magnetite (up to >90
vol. % ) and lack of sulphides ( eg. pyrrhotite ).

Item Type: Thesis - Research Master
Authors/Creators:Singoyi, Blackwell
Keywords: Paragenesis, Geochemistry, Scheelite, Magnetite
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Thesis (M.Econ.Geol.)--University of Tasmania, 1997

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