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Angas |
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South Australia, SA, Australia |
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Zn Pb Ag Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Angas zinc deposit is located 2 km east of the town of Strathalbyn, and 60 km south-east of Adelaide in South Australia (#Location: 35° 15' 5"S, 138° 55' 5"E).
It is a semi-concordant, coarse grained, zinc-lead-silver-gold deposit hosted by a monotonous sequence of Cambrian biotite-feldspar-quartz schists after flysch-style interbedded arkosic sandstone and mudstone. The deposit occurs as a number of steep shoots that are surrounded by a proximal halo of the zinc spinel gahnite, within a broader tabular envelope of garnet-staurolite-sillimanite-chlorite-biotite-muscovite-andalusite-quartz. This envelope is characterised by the manganese garnet spessartine.
Mineralisation has been known in the immediate vicinity of Angas from soon after European settlement. Approximately 2000 t of supergene ore @ 12% Zn, 8% Pb, 220 g/t Ag and a 'few' g/t Au were mined between 1848 and 1858 from the Strathalbyn Mine, 800 m to the north. The current Angas resource was discovered by Aberfoyle Exploration in 1991 and was mined by Terramin Australia Limited at a rate of ~0.4 Mt of ore per annum from 2008, until placed on care and maintenance in late 2013.
Regional Setting
The Angas deposit is hosted within the Kanmantoo Group, a 7 to 8 km thick pile of quartz-feldspar and quartz-feldspar-mica schist, which on the basis of abundant preserved sedimentary textures, are interpreted to represent a sequence of metamorphosed immature and predominantly turbiditic sandstone and mudstone. The sequence was deposited within an extensional continental-margin delta and submarine-fan complex following the break-up of the supercontinent Rodinia. It overlies and interfingers with the limestones and shales of the Early Cambrian (540 to 510 Ma) Normanville Group of the broad Ardrossan Shelf to the west. These rocks, in turn, unconformably overlie the south-eastern margin of the Neoproterozoic Adelaide Fold Belt. The onset of Kanmantoo rift activity is heralded by the 526 Ma Truro Volcanics, a suite of altered basaltic and intermediate submarine lavas and pyroclastics, with an in-plate geochemical signature. These volcanics, which are at the top of the Normanville Group, are faulted out to the west of the Kanmantoo-Strathalbyn district. However, they recur to the east under the Murray Basin, bracketing the Kanmantoo Trough sequence, where they include an upper suite with MORB-like compositions (Belperio et al., 1998).
Kanmantoo Group sedimentation commenced with a thick (up to 1100 m) sequence of quartz-feldspar schist after arkosic, partly crossbedded, sandstone, the Backstairs Passage Formation, followed by a succession of quartz-feldspar-mica schists. Preserved sedimentary textures and structures indicate the latter are dominated by a repetitive sequence of thick, massive, structureless to poorly graded sandstone beds, passing up into hemipelagic siltstones. The major turbidite packages within this succeeding succession (e.g., the Tapanappa and Balquihdder formations; Fig. 1) correspond to periods of prograding low stand fans, separated by high stand events marked by thinner, deeper basinal shales (e.g., the Talisker and Tunkilla formations). Sedimentation was terminated when the basin was inverted from ~516 Ma (Belperio et al., 1998).
The rocks of the Kanmantoo Trough were subjected to three main deformation events. D1 is reflected by a bedding-parallel schistosity and possibly resulted in overthrusting of the Kanmantoo Group onto the Adelaide Fold Belt to the NW. D2 produced more upright, tight to isoclinal folds and a steep reverse and sinistral strike-slip regime. During this event, extensive syn-tectonic ~510 to 500 Ma I- and S-type granitic plutons with S2 fabrics were emplaced, mostly concealed by the Cenozoic Murray Basin to the east. D2 also coincides with peak metamorphism under low pressure, high temperature conditions of 5.5 to 2.2 Kb and 630 to 530°C. Metamorphism obliquely crosscuts and post-dates the dominant structural trends, and comprises a NNW-SSE aligned belt of amphibolite facies with a migmatite core, and is flanked by biotite to greenschist facies zones to the east and west. The amphibolite facies are divided into higher-grade sillimanite (650 to 600°C) and outer andalusite-staurolite (550 to 600°C) zones (Fig. 1). The latter encompasses the Angas deposit. This zonation is centred on the granitic intrusions within the migmatitic core, and is essentially thermal metamorphism, reflecting advective heat transfer in the lithosphere via granitic magma. D3 produced a series of west to WNW thrusts and overturned west vergent folds, and was followed by emplacement of voluminous, undeformed, post-tectonic A-type granites and mafic to ultramafic intrusions from 490 to 480 Ma, during ensuing relaxation and renewed extension (Sandiford et al., 1995; Belperio et al., 1998).
The known Cu and Zn-Pb mineralisation of the Kanmantoo Trough is restricted to the metamorphosed calc-siltstone, pyritic siltstone and graphitic shale of the Talisker Formation and a stratigraphic thickness of several hundred metres within the conformably overlying Tapanappa Formation. The Talisker Formation hosts the Brukunga pyrite deposit, a 200 m thick unit carrying 32 Mt @ 12% pyrite and 6% pyrrhotite (10.5% S), and the Mount Torrens prospect with 0.7 Mt @ 6.4% Pb. 1.6% Zn, 41 g/t Ag (Fig. 1).
The Tapanappa Formation protolith was composed of fine to coarse-grained interbedded meta-sandstone, siltstone and mudstone. The finer facies are often rich in pyrite and pyrrhotite with anomalous base metals over thicknesses of 1 to 10 m. Other deposits within the Tapanappa Formation are distributed over an interval of ~40 km within the andalusite-staurolite metamorphic zone. These include mineralisation that is either discordant (e.g., Kanmantoo and Bremer Cu-Au), or stratabound (e.g., Aclare, Wheal Margaret and Wheal Ellen Zn-Pb-Ag deposits). Most of the latter have garnet halos and pass into manganiferous and garnetiferous chert bands along strike (Belperio et al., 1998).
Geology and Mineralisation
At the Angas deposit, the host Tapanappa Formation is composed of a repetitive sequence of quartzite and biotite-feldspar-quartz schist after flysch-style interbedded arkosic sandstone and mudstone in which sedimentary structures such as load casts and graded bedding are preserved. The Angas deposit is located on the shallowly east dipping limb of the gently south-plunging asymmetric Strathalbyn Anticline. Graded bedding and narrow zones of intense parasitic folding suggest the sequence on this limb has been folded into a series of tight isoclinal folds. The deposit is broadly stratabound, sub-parallel to both bedding and the dominant S2 schistosity. Mineralisation lies within the 50 to 200 m thick host unit. This unit is defined by the presence of minor (1 to 2%) to common (>10%) fine-grained euhedral garnet, and occurs as a continuous tabular body subparallel to stratigraphy, with a strike length of at least 2 km and a down-dip extent of >500 m. The garnet, which is dominantly the Mn-rich spessartine variety, is regarded to be a hydrothermal alteration product, and encloses economic mineralisation over a strike length of ~700 m and 500 m down dip, with a thickness varying from <0.5 to 20 m.
The sulphide mineralisation is sandwiched between upper and lower outer halos of garnet-staurolite-andalusite-sillimanite-chlorite-biotite-muscovite-quartz schists containing minor cordierite, pyrite, pyrrhotite and chalcopyrite. These grade into an inner, increasingly sulphidic proximal halo adjacent to the ore, characterised by the additional presence of gahnite. Sparsely disseminated garnet extends into the footwall below the lower outer halo. The combined thickness of the package of halos and ore is ~70 m, with a strike length of 1.2 km.
Mineralisation occurs in two main forms:
Type 1, are massive sulphides that are generally coarse grained (up to 3 mm), composed of sphalerite, galena, pyrite, pyrrhotite and trace chalcopyrite with 0 to 40% gangue of quartz, gahnite, garnet, chlorite, biotite and muscovite. Compositional, 5 to 20 mm thick, S2 parallel banding of pyrite and sphalerite is common. Rarely, the banding is folded.
Type 2 has textures that vary from massive to brecciated, disseminated and veined, and is defined by the dominance of pyrrhotite. The grain size is also widely variable and the gangue includes quartz and chlorite. Where quartz is abundant, chalcopyrite becomes an important component of type 2 mineralisation. The two types are usually interspersed, although type 2 tends to occur on the margins of the main mineralised zone, and along strike and down dip.
Three distinct north-south oriented ore shoots have been recognised within the Angas deposit, namely the Rankine, Hanging wall and Garwood lodes, each extending from near surface to ~400 m depth, with down plunge extensions indicated by the Rankine Extension and Western Zone. Only the Rankine zone, which accounted for 75% of resources and has a strike length of ~250 m, outcropped. The Rankine (or Main) Lode consists of a south-plunging linear body of massive and disseminated Pb-Zn-Fe-Cu sulphides, about 500 m in length, 150 m to 200 m wide down dip, and from 1 m to over 10 m in thickness (Ogierman, 2021).
Terramin geologists consider the mineralisation to be syn-tectonic, but emplaced prior to peak metamorphism, with the layering in type 1 mineralisation being tectonic. Mineralisation is discordant to bedding and appears to have been focussed in dilation zones along shearing associated with regional tectonism.
The pre-mining Indicated Mineral Resource in 2007 (Terramin website, 2017; SARIG 2017) was estimated as:
3.04 Mt @ 8.0% Zn, 3.1% Pb, 0.3% Cu, 34 g/t Ag, 0.5 g/t Au, including a
Probable Ore Reserve of
2.34 Mt @ 8.1% Zn, 3.1% Pb, 0.3% Cu, 33 g/t Ag, 0.5 g/t Au.
The mine operated at ~0.4 Mt of ore per annum from 2008, and was placed on care and maintenance in late 2013, with a remaining JORC compliant:
Indicated + Inferred Mineral Resource of 0.91 Mt @ 4.2% Zn, 1.7% Pb 0.1% Cu, 19 g/t Ag, 0.3 g/t Au .
Few dedicated geological paper has been identified describing Angas, and the above is drawn from a variety of sources including the Terramin and Geological Survey of South Australia websites, but particularly a BSc Honours thesis on the deposit by McElhinney (1994).
The most recent source geological information used to prepare this decription was dated: 2007.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Angas
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Selected References:
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Ogierman, J., 2021 - Discovery of the Angas deposit: missed and dismissed, a tale 150 years in the making: in Mesa Journal v.95, pp. 49-59.
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Porter, T.M., 2017 - Angas Zn-Pb-Ag-Au deposit: in Phillips, G.N., 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy, Mono 32, pp. 665-666.
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Tott, K.A., Spry, P.G., Pollock, M.V., Koenig, A., Both, R.A. and Ogierman, J., 2019 - Ferromagnesian silicates and oxides as vectors to metamorphosed sedimenthosted Pb-Zn-Ag-(Cu-Au) deposits in the Cambrian Kanmantoo Group, South Australia: in J. of Geochemical Exploration v.200, pp. 112-138.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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