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

posted on 2023-05-26, 00:55 authored by Reid, R
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 Volcanics. 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."


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