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Portia, North Portia |
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South Australia, SA, Australia |
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Cu Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Portia and North Portia gold and copper-gold-molybdenite deposits are located ~125 km WNW of Broken Hill and ~425 km NNE of Adelaide in eastern South Australia. The economic mineralisation at both, which are only a few hundred metres apart, are the result of supergene processes upgrading or redistributing lower grade hypogene mineralisation (#Location: 31° 26'S, 140° 27'E).
Regional Setting
Both deposits are located in the core of the cratonic, 200 x 400 km, Palaeo- to Mesoproterozoic Curnamona Province. For an outline of the setting and geology of the Curnamona Province, see the detailed description in the Kalkaroo record.
Portia and North Portia Deposits
These deposits are found on the southern part of the Benagerie Ridge (and corresponding Mudguard Domain), a north-south trending, fault bounded Proterozoic basement structure, which is almost entirely concealed by younger cover. They occur on the eastern flank of the Benagerie Ridge Magnetic Complex (BRMC), one of number of such magnetic features within the Benagerie Ridge. The north-south elongated ~25 x 8 km BRMC represents the core of a complex, eroded, buried north-trending anticline, around which the mineralised sequence forms an annulus, essentially occupying the interface between the lower oxidised and upper reduced parts of the Willyama Supergroup. Portia and North Portia are the most significant of a suite of mineral occurrences that occur around this annulus. The core of this magnetic complex is occupied by albite-magnetite rocks. Some of the other similar magnetic complexes within the Curnamona Province host to massive magnetite ±hematite accumulations .
The mineralised sequence of the BRMC can be subdivided into a minimum of nine units, as follows (Teale and Fanning, 2000; Teale, 2006), over a thickness of >500 m, from the base, upwards:
Unit 1 - albitised, magnetic footwall, predominantly white to grey albitites which are locally darker grey due to the presence of carbonaceous matter and can be finely laminated to massive, commonly containing scapolite porphyroblasts that have been pseudomorphed by albite. In the vicinity of the the Portia-North Portia deposits, the albitites contain high modal magnetite, often locally replaced by finer grained hematite;
Unit 2 - an albitic carbonate unit containing calcite-biotite marble and pink albite (±calcite) lenses and ellipsoids; containing disseminated to locally massive pyrite, and is variably mineralised;
Unit 3 - scapolitic albitite, with albite ellipsoids; containing disseminated to locally massive pyrite, and is variably mineralised;
Unit 4 - evaporitic, banded to finely laminated, intercalated marble, albitic and K feldspar beds (up to 25 marble beds per metre, with minor pyrite - rhombohedral
and six sided albite crystals in the carbonates imply former evaporite phases, while gypsum/anhydrite pseudomorphs are also observed;
Unit 5 - a finely laminated pyritic albitite with local K feldspar nodules, which has an upper sub-unit containing carbonate ellipsoids and pyritic/pyrrhotitic nodules, and a lower sub-unit which marks the first appearance of biotite-rich carbonate beds;
Unit 6 - a laminated to thinly bedded Fe- and Mg-rich calc-silicate, with nodules of silica within carbonate, and monomineralic bands of actinolite, biotite, calcite, siderite and K feldspar;
Unit 7 - carbonaceous phyllites/pelites, becoming more sodic towards the base, and including a cross-cutting intense Upper Albitite zone;
Unit 8 - bedded carbonate;
Unit 9 - Upper Carbonaceous Phyllite - a strongly carbonaceous phyllite.
This sequence in the Portia-North Portia area dips shallowly to the ESE with dips varying from ~20 to 50°. The area is strongly faulted, with evidence for thrusting and bedding parallel shears, accompanied by kimberlitic sills.
Prior to albitisation, the host Portia Formation at North Portia and Portia comprised finely laminated to planar bedded carbonaceous and non-carbonaceous shales, evaporitic and carbonate-rich beds and other saline silts and shales, punctuated by possible local disconformities. Flaser cross beds indicate intertidal conditions for some of the rock-types and much of the sequence is considered to have been deposited in shallow water. Abundant kimberlitic sills are recognised at North Portia. Two mica and muscovite granites, as well as highly fractionated diorite intrusives, are also present in the vicinity of North Portia (Teale and Fanning, 2000).
Mineralisation at Portia-North Portia is developed predominantly in Units 2, 3 and 5, above the oxidised albite-magnetite-hematite-rich footwall (Unit 1) and below the highly reduced carbonaceous phyllites of the hanging wall units (Unit 7-9). The meta-sediments and tuffs of the sequence have been albitised, with dark grey to black carbonaceous and/or biotite-rich shales now bleached white grey and/or brick red. These mineralised units constitute the Portia Formation which is generally ~200 m thick and as at Kalkaroo, comprises an upper and lower marble units, separated by a siliciclastic-calc-silicate unit. The middle part of unit 5 contains a tuff-bearing layer that has been dated at 1703±7Ma (Teale and Fanning, 2000).
Albitisation has taken place at a number of times within the Curnamona province, ranging from late diagenesis (Ashley, 2000; Skirrow et al., 2000), and a subsequent 'syn tectonic', ~1595 to 1583 Ma, Na+Ca+Fe (albite, quartz, clinopyroxene, actinolite, epidote, magnetite, hematite, grossular-andradite garnet and sphene) metasomatic event (Skirrow
et al., 2000). The invasive, cross-cutting albitisation event in the southern Benagerie Ridge area is characterised by a dominant assemblage of albite, quartz, calcite(/ankerite), magnetite-hematite and rutile, with accompanying fluorite, F-phlogopite, REE-fluocarbonates and monazite. The Benagerie Ridge albitisation has been superimposed on rocks with distinct fabrics imposed by two separate tectonothermal events. In addition, early formed spessartine garnet, in carbonaceous shale for example, is totally replaced by albite. This albitisation has therefore been superimposed on a sequence that has previously been deformed and metamorphosed, but has then been subsequently subjected to the major ~1600 to 1590 Ma Olarian Orogeny tectonothermal event. Hydrothermal monazite at the albitisation 'front' in the Benagerie Ridge area has been dated at 1628±20 Ma (Teale, 2006). Repeated albitisation probably reflects the sodic nature of the evaporite-rich carbonate units of the sequence (Teale and Fanning, 2000; Teale, 2006).
Magnetite accompanies hydrothermal albite alteration, both during the pre-mineralisation ~1628 Ma and 'syn tectonic', ~1595 to 1583 Ma pulses. It is particularly well developed in unit 1 in the footwall of the Cu-Au mineralisation, where albitites contain high modal magnetite, as abundant disseminated magnetite (up to 30%) and bands up to 1 cm thick, often with finer grained hematite replacing porphyroblastic magnetite. Magnetite can also be present as a pseudomorph after bladed hematite and as small rounded grains in footwall albitites and can be replaced by, or intergrown with hematite in textural equilibrium. It can also be replacive, occurring with pyrite and/or chalcopyrite within carbonate beds at their contact with albitic metasediments. Strong magnetite developments in unit 1 and the base of unit 2 gradually grades into an actinolitic marble (Teale and Fanning, 2000; Teale, 2006).
Teale (2006) regards the juxtaposition of early, pre-ore ~1628 Ma albitites and other brittle alteration/lithotypes, interbanded with more ductile carbonates, to be one of the important parameters influencing the emplacement of the Cu-Au-Mo sulphide deposition. Cross-cutting large fault structures (also acting as fluid conduits) have exploited the extreme of competency and chemical differences, and strongly fractured and brecciated the host sequence, with ultra-fine grained albitites 'shattering' between ductile carbonates.
The Cu-Au mineralisation post dates the ~1628 Ma albitisation, with further brick red mineralisation related albitisation superimposed on existing albitites and K feldspar mineralised vein selvedges replacing albite, while chalcopyrite also replaces carbonate associated with the albitisation. However, some Mo replacement adjacent to quartz-carbonate veins on the margins of the albitised domain may have occurred at ~1630 Ma and consequently accompanied that phase of albitisation, and pre-dated the main Cu-Mo mineralisation (Teale, 2006).
Detailed logging has shown that there are significant facies changes in the host sequence, with Cu-Au-Mo mineralisation located in areas that contain the thickest carbonate and calcareous units (Teale, 2006). Teale and Fanning (2000) have dated the copper-gold mineralisation at 1605±12 Ma, while a SHRIMP U-Pb date on titanite from the replacement mineralisation of ~1588 to 1583 Ma suggesting that at least a part of the mineralising event was of that age (Skirrow and Ashley, 2000).
Copper-gold mineralisation occurs as replacement, vein infill, 'skarn-altered' and breccia styles at Portia and North Portia, as well as, bonanza gold veins at the Portia (e.g., 9m @ 237g/t Au) and nearby Shylock prospects (e.g., 5m @ 356g/t Au).
The gold-rich Portia and Shylock represent the structural 'top' of the North Portia system that has been separated and off-set by several hundred metres by faulting (Teale, 2006). Within the bonanza veins at Portia and Shylock, gold occurs as primary grains (Ag-rich, with sulphide and anhydrite inclusions) that can be up to 1cm in length. In contrast, at North Portia, gold grains are in the sub-micron to 50µm range, usually contained within chalcopyrite and to a lesser extent pyrite, often intimately associated with tetrahedrite, tennantite and various tellurides. The fineness of gold at North Portia increases from the structural bottom to the structural top of the mineralised system (Teale, 2006).
Numerous breccia types are recorded, although not all are mineralised. Sulphidic and non-sulphidic milled, fluidised injection, tectonic and crackle breccias and possibly decompressive shock breccias are recognised. In stratigraphic and structurally lower domains, conformable, bedding parallel shear related breccias are evident, usually hosted by meta-carbonate and albitites containing clasts of these rock-types. However, structurally higher in the mineralised system, breccias are usually cross-cutting and less sulphidic. At North Portia, some of the best mineralisation occurs in sulphide breccias (Teale, 2006).
Cross-cutting, infill veins and 'fault' veins are more common in the upper levels of the North Portia system, while in structurally lower positions, early calcic veins containing the assemblage tremolite-calcite-sphene-allanite can be partially replaced by quartz-REE fluocarbonates-rutile-sulphides. The dominant carriers of REE are various REE fluocarbonates, rutile and monazite. The calcic veins become more sulphide-rich and biotite dramatically decreases higher in the system, with quartz-calcite-K feldspar-biotite-sulphide veins passing upwards into quartz-calcite-hematite-sulphides veins in the same upper parts of the system (Teale, 2006).
Stratabound replacement-style mineralisation in the Portia Formation replaces carbonate, and pyrite/pyrrhotite, which was in turn developed during diagenesis, replacing chemically reactive carbonate and evaporitic sulphate bands and nodules in the host Portia Formation sequence. Replacement fronts frequently emanate from veins that are either calcic or potassic. Molybdenite often replaces, fine-grained, delicate, early bedding parallel pyrite in albitised domains adjacent to quartz-carbonate±biotite±sulphide veins, but can also be developed in biotite selvedges adjacent to carbonate-biotite-quartz veins where it is often intergrown with biotite. These veins pass upwards into Mo-rich breccias. This style of mineralisation is characterised by moderate Cu and Au grades and often quite appreciable Mo. Chalcopyrite replaces carbonate and carbonate-albite beds, lenses and ellipsoids in the structurally and stratigraphically lower areas of the North Portia deposit. Chalcopyrite and albite form an unusual textured mosaic in the carbonate-albite beds, where the carbonate has been replaced and the albite retained (Teale, 2006).
Bedding parallel sphalerite and galena are found adjacent to the North Portia, with sulphides replacing carbonate ellipsoids and in bedding parallel 'veins' which are always coarser grained than the adjacent meta-sediment. These veins are associated with albite-quartz-tourmaline-carbonate±garnet±epidote±pyrite. Pb-isotopic studies indicate the galena in these veins has a similar isotopic composition to altaite (PbTe) in the Cu-Au mineralisation, and that the Zn-Pb forms a halo to the Cu-Au (Teale, 2006).
Major ENE structures are mapped at the Portia-North Portia deposits and are considered to represent conduits for high temperature magnetite-K feldspar±biotite alteration. Bedding parallel faults and shears are common and are usually mineralised within the Portia Formation. North Portia is also severely disrupted by NNW trending vertical to steep west dipping faults which displace mineralisation by up to 30 m (Teale, 2006).
The mineralisation at Portia comprises both a rich eluvial/detrital gold layer developed over an area of 600 x 100 m, ranging from 1 to 4 m in thickness, that rests on weathered bedrock and the primary gold source in the underlying bedrock. The bedrock resource is a broad zone, that is up to 100 m thick, straddling the shallowly dipping sheared contact between underlying pyritic albitite and overlying graphitic pelites, that is above the depressed base of oxidation. The base of oxidation, which is normally at a depth of ~100 m, increases to be >200 m in the deposit area. The mineralisation is overlain by 60 to 70 m of Tertiary clay (Havilah Resources).
Published resource figures (Havilah Resources, 2012) are:
Portia:
Inferred Resource = 0.72 Mt @ 2.9 g/t Au, using a high grade cut of 60 g/t Au
North Portia oxide supergene blanket:
Indicated + inferred resource = 11.36 Mt @ 0.89% Cu, 0.64 g/t Au, 0.05% Mo.
The most recent source geological information used to prepare this decription was dated: 2010.
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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Teale G S and Fanning C M, 2000 - The Portia-North Portia Cu-Au (-Mo) prospect, South Australia: Timing of mineralisation, albitisation and origin of ore fluid: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, PGC Publishing, v1 pp. 137-147
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Teale G S, 2006 - Structural and Stratigraphic Controls on the Zoned North Portia and Kalkaroo Cu-Au-Mo Deposits: in Korsch R J and Barnes R G, 2006 Broken Hill Exploration Initiative: Abstracts for the September 2006 Conference GeoScience Australia Record 2006/21 pp. 178-181
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Zang W L and Conor C H H, 2006 - Stratigraphic and depositional aspects of the Portia and Kalkaroo prospects, Mulyungarie Domain, SA: in Korsch R J and Barnes R G, 2006 Broken Hill Exploration Initiative: Abstracts for the September 2006 Conference GeoScience Australia Record 2006/21 pp. 202-206
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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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