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Abra |
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Western Australia, WA, Australia |
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Pb Ag Cu Ba Au
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Super Porphyry Cu and Au
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The Abra lead-silver-copper-gold occurrence is located within the Palaeo- to Mesoproterozoic Bangemall or Edmund basin of the Capricorn Orogen, hosted by sedimentary rocks, some 870 km NNE of Perth in Western Australia. It occurs within a 60 x 1 to 9 km wide belt of base metal occurrences and showings known as the Jillawarra Belt.
The Edmund Basin lies within the east-west elongated, 1000 x 500 km Capricorn Orogen which separates the Archaean Yilgarn and Pilbara cratons. The orogen is composed of variably deformed and metamorphosed igneous and sedimentary rocks. It includes the deformed margins of the two cratons and the incorporated exotic Glenburgh Terrane of the Gascoyne Province, and the associated continental margins that constitute the Hamersley and Turee Creek basins in the north. Then the medium to high grade metamorphic rocks of the Gascoyne Province and various low grade metasedimentary rocks deposited in the Ashburton, Blair, Padbury, Bryah, Yerrida, Earaheedy, Edmund and Collier basins that overlie these tectonic units and encroach onto the Yilgarn Craton. The Edmund Basin lies along the core of the orogen.
Mineralisation at Abra is hosted by carbonaceous and siliciclastic sedimentary rocks of the Irregully and the Kiangi Creek Formations that belong to the Edmund Group, which in turn is part of the Bangemall Supergroup. The up to 2000 m thick Irregully Formation is composed of stromatolitic dolostone, massive dolostone, parallel-laminated dolomitic siltstone, siltstone, silty sandstone, quartz sandstone, and conglomerate. Carbonate-rich lithofacies of this formation contain detrital muscovite flakes parallel to lamination, whilst sandstones facies consist of medium- to coarse-grained, poorly sorted quartz and dolomudstone grains in a matrix of recrystallized dolomite and quartz (Martin et al., 2005). The Kiangi Creek Formation comprises siltstone, fine- to very coarse-grained sandstone, dolostone; minor conglomerate, chert, felsic volcanic rock. Siltstones are composed of quartz, white mica±chlorite (Martin et al., 2005).
Minor reactivation of major faults within the Edmund Group during the Mutherbukin tectonic event (Johnson et al., 2011, 2016; Korhonen et al., 2015; Cutten et al., 2016) has allowed local intense hydrothermal fluid flow (Zi et al., 2015) at 1321 to 1171 Ma. The 1030 to 950 Ma Edmundian orogeny (Martin et al., 2005; Sheppard et al., 2007; Occhipinti and Reddy, 2009) metamorphosed the Edmund and Collier Groups to prehnite-pumpellyite and subgreenschist grade (Occhipinti et al., 2004; Arkai et al., 2007; Sheppard et al., 2007) and the 570 Ma Mulka tectonic event is responsible for anastomosing shear zones concentrated within discrete corridors (Johnson et al., 2013). The Abra deposit and other mineral occurrences in the Edmund basin are located along the east-west striking Quartzite Well fault that is thought to be an eastern extension of the Lyons River fault. This latter structure forms the centre-line of the Orogen and is south dipping, interpreted to extend to the Moho (Johnson et al., 2011, 2013).
Mineralisation occurs in the upper part of a sequence of alternating thinly bedded lutite, interbedded with lithic and feldspathic quartz arenites and minor rudite and carbonate which overlie granitoid basement. The host sequence is, in turn, overlain by 200 to 500 m of clastics which conceal the completely blind occurrence.
The Abra deposit occurs along the crest of a small anticline and takes the form of a funnel-shaped brecciated zone, overlain by vertically zoned strata-bound mineralisation (Pirajno et al., 2016). The downward tapering brecciated cone has a 400 m radius and 330 m height. The upper limit of the stratabound mineralised beds is defined by a conglomerate succession at a depth of ~300 m below the surface (Vogt and Stumpfl, 1987; Vogt, 1995; Pirajno et al., 2009, 2016). This conglomerate has a maximum thickness of ~40 m at Abra (Vogt, 1995) and may mark the unconformity at the base of the Kiangi Creek Formation. The stratabound mineralisation immediately below the conglomerate comprises, from the top downwards, the:
• Red Zone which is 40 to 110 m thick, and comprises a sub-horizontal unit of chalcedonic quartz, locally with associated chlorite, jasperoidal material, carbonate, siderite, barite and hematite forming banded and colloform aggregates (Pirajno et al., 2016).
• Dolomite Zone, which is discontinuous, underlying the Red Zone and is composed of siderite, barite and galena (Austen, 2007).
• White or Altered Zone, which interfingers with the top of the underlying Black zone and comprises quartz veins up to 2 m thick with associated chalcopyrite in the host rock, specular hematite, galena and magnetite (Austen, 2007).
• Black Zone, which is between 30 and 100 m thick, and underlies and locally overlaps the White and Red zones (Pirajno et al., 2016). It is dominantly characterised by laminated or banded hematite and magnetite with associated pyrite, galena, chalcopyrite and quartz + barite layers and veins (Pirajno et al., 2016). Sericite alteration in the Black zone is associated with pyrite. Pirajno et al. (2016) suggested a possible paragenetic sequence, from older to younger, related to alteration, involving albite-barite-galena-sericite. Fe-rich chlorite also occurs in the Black zone, occurring as a late alteration phase replacing galena and magnetite, or with albite enclosing sulphides (Pirajno et al., 2016).
The underlying funnel-shaped brecciated zone is dominated by Cu, Au and Bi mineralisation, overprinting the brecciation. It is veined, chloritised and silicified cutting siltstone and arenite, and may extend for more than 400 m below the Black zone (Pirajno et al., 2016). Brecciated and carbonaceous siliciclastics of the Irregully Formation in the brecciated zone also contain hematite-quartz-white mica-siderite veinlets, flanked by brecciated quartz-chlorite assemblages that contain late-stage crystallisation of white mica (Pirajno et al., 2016).
The mineral system represents multiple overprinting phases of hydrothermal activity that have produced several stages of brecciation and fluid input (Pirajno et al., 2009, 2010). Oxygen and sulphur geothermometers suggest mean mineralisation temperatures of 300±30°C (Austen, 2007). Multiple generations of hydrothermal monazite and xenotime dated at ~1375, 1220 and 995 Ma, represent a complex regional-scale history of hydrothermal activity and reworking of the Abra deposit that has been correlated with known tectonic events, such as the Mutherbukin tectonic event and the Edmundian orogeny (Zi et al., 2015). The younger hydrothermal events are also associated with the injection of thin ore zone veins (barite-sphalerite-chalcopyrite-dolomite-galena ±pyrite) above the unconformity into the overlying lithologic units (Zi et al., 2015).
Regionally, hydrothermal alteration affected the host rocks of both the Abra deposit and other mineral occurrences within the Jillawarra sub-basin. This includes extensive silicification, albitisation and biotitisation which influenced all the rocks of the Jillawarra sub-basin, whilst chloritisation and sericitisation are more localised (Vogt, 1995). Silicification, albitisation, chloritisation, and sericitisation are strongest in the Abra area (Vogt and Stumpfl, 1987; Vogt, 1995), enclosed by a chlorite-siderite±white mica alteration halo (Collins and McDonald, 1994; Zi et al., 2015; Pirajno et al., 2016).
The deposit was originally detected as an aeromagnetic 400 nT "bulls eye" anomaly.
The geological resource amounts to 200 Mt @ 1.8% Pb, 6 g/t Ag, 0.18% Cu and 6% Ba, including 150 Mt @ 0.13 g/t Au (Boddington, 1990).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2017.
Record last updated: 1/12/2017
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.
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Selected References:
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Boddington T D M 1990 - Abra Lead-Silver-Copper-Gold deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v1 pp 659-664
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Lampinen, H.M., Laukamp, C., Occhipinti, S.A., Metelka, V. and Spinks, S.C., 2017 - Delineating Alteration Footprints from Field and ASTER SWIR Spectra, Geochemistry, and Gamma-Ray Spectrometry above Regolith-Covered Base Metal Deposits - An Example from Abra, Western Australia: in Econ. Geol. v.112, pp. 1977-2003.
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Pirajno, F., Mernagh, T.P., Huston, D., Creaser, R.A., and Seltmann, R., 2016 - The Mesoproterozoic Abra polymetallic sedimentary rock-hosted mineral deposit, Edmund basin, Western Australia: Ore Geology Reviews,: in Ore Geology Reviews v.76, pp. 442-462.
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Vogt J H, Stumpfl E F 1987 - Abra: a strata-bound Pb-Cu-Ba mineralization in the Bangemall Basin, Western Australia: in Econ. Geol. v82 pp 805-825
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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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