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Cambrian intrusion-related copper mineralisation at the Thomas Creek prospect, Southwestern Tasmania


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Reid, R 2001 , 'Cambrian intrusion-related copper mineralisation at the Thomas Creek prospect, Southwestern Tasmania', Coursework Master thesis, University of Tasmania.

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The Thomas Creek Prospect lies at the southern end of the Sorell Peninsula, western
Tasmania and consists of intrusions believed to be coeval with copper mineralisation
within a dioritic intrusive complex. The prospect is hosted by the Noddy Creek
Volcanics, which are geochemically correlated to the calc-alkaline Mt Read
Widespread (600 by 400 m) but low grade copper mineralisation is hosted by diorites
and feldspar~augite porphyritic andesite intrusions, which have been intruded by
chalcopyrite-bearing porphyritic micromonzodiorites. Early disseminated magnetite,
pyrite and chalcopyrite formed prior to and synchronous with pervasive feldsparsilicate
alteration. Various chalcopyrite-bearing vein generations (actinolite,
tounnaline, K-feldspar- smectite, epidote and carbonate) have overprinted the earlyformed
magnetite and feldspar-silicate alteration assemblages.
The "immobile" elements (Ti and P) were mostly mobilised during the fonnation of
pervasive K-feldspar hydrothermal alteration at Thomas Creek. A subset of the
geochemical data set exhibits coherent behaviour, indicating that the diorites and
porphyritic micromonzodiorites form a co-magmatic fractionation series. Coherent
behaviour ofBa, Rb, Sr and K20 has been recognised for the igneous suite. Both
primary and secondary K-feldspar appear to be intrusion-related. Fe, S, P, Zn and Cu
appear to have been lost during fractionation due to devolatilisation of the
micromonzodiorite magmas.
Sulphur isotope values of sulphides at Thomas Creek range from -4.9 to 11.5%o.
Sulphides in the early formed magnetite and feldspar-silicate alteration assemblages
have similar s34s ranges (5.3 to 9.3%o). The isotopic range in the porphyritic
micromonzodiorites (8.2 to 11.5%o) is believed to have formed in response to
fractionation and devolatilisation of 32s-rich hydrothermal fluids, resulting in
isotopically lighter signatures (-4.9 to 7.7%o) in veins related to the intrusions. Sulphur isotope geothennometry, utilising co-existing pyrite-chalcopyrite pairs, indicates
mineralisation temperatures in veins ranged from 254 to 611 °C.
Four stages of mineralisation have been recognised at Thomas Creek: 1:- early
magnetite and feldspar-silicate alteration; 2:- emplacement of Cu-bearing
micromonzodiorite intrusions and precipitation of coeval actinolite and tourmaline
veins; 3:- K-feldspar- smectite vein formation; and 4:- epidote and carbonate veining_
Phases 1 and 2 represent periods of magma emplacement with some mixing of
magmatic-hydrothermal water with seawater-derived fluid. Phase 3 veins appear to be
of magmatic character, with minimal seawater influence. Phase 4 probably represents
final incursion of seawater-derived fluids as the magmatic system waned.
Thomas Creek can be correlated to Mt Lyell copper-gold mineralisation, based on
similarity of sulphur isotope distribution and ore mineralogy. Mineralisation at the
Thomas Creek Prospect is believed to represent the root zone of a Mt Lyell-type
hydrothermal system_ There are also some similarities with the alkaline porphyry CuAu
deposits of British Columbia, with Thomas Creek possibly being the submarine
analogue of a porphyry system formed in a back arc environment.

Item Type: Thesis - Coursework Master
Authors/Creators:Reid, R
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