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Woodlawn |
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New South Wales, NSW, Australia |
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Zn Pb Cu Ag
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Super Porphyry Cu and Au
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The Woodlawn zinc-lead-copper deposit is located in the Lachlan Fold Belt, 7 km east of Tarago and 45 km NE of Canberra in south-eastern New South Wales, Australia (#Location: 35° 3' 45"S, 149° 34' 14"E).
Regional Setting
Woodlawn is located near the eastern margin of the Lachlan Fold Belt which extends from northeastern Tasmania, into Victoria and through much of eastern NSW. The northern, western and eastern boundaries are covered by younger sedimentary basin cover. The Lachlan Fold Belt is divided into a number of
stratigraphic-tectonic zones defined by anticlinorial and synclinorial belts, separated by regional faults, thrusts, igneous bodies and unconformities, as well as lithological, metallogenic and tectonic contrasts. The synclinorial stratigraphic-tectonic zones mostly represent marine intra-cratonic rift to back-arc basin that opened across the Lachlan Orogen in the middle to late Silurian under an extensional tectonic regime (Thomas & Pogson 2012). They were developed on the continental side of a west-dipping subduction zone and an intraoceanic arc and subduction complex, both preserved in the southern section of what is now the New England Fold Belt in northeastern NSW and eastern Queensland. This Siluro-Devonian extension led to dismembering of the Ordovician intra-oceanic Macquarie Arc into several structural belts, formation of sedimentary rift and transtensional basins and emplacement of both I and S-type granitic batholiths (Glen, 2005).
The Woodlawn deposit lies within the NNE trending Captains Flat-Goulburn Synclinorial zone, which is in excess of 300 km in length and <30 km wide, and largely comprises Siluro-Devonian volcanic and sedimentary rocks, and hosts a range of base metal and gold occurrences, e.g., Captains Flat.
This synclinorial zone is bounded by the Molong and South Coast anticlinorial zones to the west and east respectively, both of which are composed of deep marine Ordovician sedimentary rocks that are generally intruded by Siluro-Devonian granites.
The sequences of the Captains Flat-Goulburn Synclinorial zone was developed on a basement of Ordovician to earliest Silurian siliciclastic turbidite rocks and black shale that was deformed and thickened during the late Ordovician Benambran Orogeny (Glen, 2005). Initial deposition of shallow conglomerates and limestones was followed by the deposition of deeper siltstone dominated sequences, accompanied by deep-water submarine volcanism at discrete, commonly bimodal, volcanic centres. The resultant volcanic rocks typically interfinger with the siltstone-rich packages. These volcanic sequences host polymetallic volcanic hosted massive sulphide (VHMS) mineralisation at Woodlawn, Currawang and Captains Flat. This mineralising event is contemporaneous with the intrusion of bimodal plutonic rocks (the Thurralilly Suite) beneath the basin sequences. Continued opening of the Goulburn Basin was accompanied by the deposition of >2 km of post-rift turbidite to mass flow sequences during the late Silurian.
Uplift during the latest Silurian Bowning Orogeny most likely was responsible for the termination of rifting and filling of the basin, followed by a transition to subaerial conditions and renewed magmatism in the Early Devonian. The Glenborg and Candelo suite plutons were intruded along the eastern margin of the deep-water basin, close to the base of the Silurian successions, whilst extensive ignimbrite piles were erupted along the eastern margin of the basin.
During the Middle Devonian and early Carboniferous, the entire area underwent compression related to the Tabberabberan Orogeny, resulting in inversion of sedimentary and volcanic rich rifts, deformation of granitoids and re-deformation and renewed imbrication of older rocks reflected in regional-scale folding and complex fault systems (Glen, 2005). The faulting was responsible for the uplift of the late Silurian felsic to mafic plutons that are related to the mineralised bimodal volcanic centres (Deyssing and Fitzherbert, 2014).
In the immediate Woodlawn area, the late Silurian Mt Fairy Group, predominantly acid to basic volcano-sedimentary sequences, unconformably overlie an Ordovician basement of quartz-rich flysch sediments (Birkenburn Beds). Early Devonian shallow to deep water sediments unconformably overlie the Mt Fairy Group. The sequence is folded and regionally metamorphosed to lower greenschist facies, and intruded by Early Devonian granites. Early Devonian dolerites intrude the entire sequence.
A series of north plunging, overturned anticline/syncline pairs with west dipping axial planes has resulted from east-west compression. The Currawang Anticlinorium and Mulwaree Synclinorium in the west and east respectively, are the major fold structures in the area, with major subsidiary folds being the Woodlawn Syncline and Pylara Anticline. The primary Woodlawn deposit lies in a structurally defined zone of lineaments within the Lachlan Fold Belt, believed to represent a major northwest trending palaeo-geographic feature, known as the Woodlawn Corridor.
Local Geology
The Woodlawn deposit is located on the eastern limb of the NNW plunging, asymmetric Woodlawn Syncline. The axial plane dips at ~60°W, parallel to a strong slaty cleavage, or locally a schistosity, through the mine sequence, paralleling the regional ~NNW structural trend.
The Woodlawn deposit is hosted within the Upper Silurian (423.3±2.6 to 419±3.2 Ma) Woodlawn Volcanics, a sequence of regionally metamorphosed, greenschist facies, fine- to coarse-grained felsic to intermediate volcanic rocks, volcano-sedimentary rocks and minor carbonaceous shale.
The VHMS mineralisation occurs as a series of sub-parallel lenses dipping moderately to steeply westward and occupying a series of generally fault bounded stratabound packages. Late dolerite sills now located above and below the Woodlawn deposit, intrude the host rocks, and comprise 30 to 40% of the hanging wall succession. The lower south and west sections of the mine area are dominantly felsic and intermediate volcanic rocks, while the upper north and east are predominantly felsic volcaniclastic mudstones. The volcanic rocks at Woodlawn have been subjected to strong hydrothermally alteration adjacent to the VHMS lenses, characterised by sericite, chlorite, silica, pyrite and minor carbonate, largely obliterating primary rock fabrics (Ebbels et al., 2015).
According to McPhie (2015), the host succession of the Woodlawn VHMS lenses includes coherent rhyolite, bedded monomictic rhyolite breccia and quartz-crystal-rich sandstone that are typical of the proximal facies of submarine rhyolitic lavas or domes. Thinly bedded volcanic mudstone and fine felsic fiamme breccia, representing finer and more thinly bedded felsic volcaniclastic facies, are also observed, possibly representing either more distal settings or different felsic volcanic centres. Amygdaloidal basalt/dolerite and basaltic fluidal clast-breccia are also mapped, with the latter being an extrusive facies, suggesting proximity to a volcanic vent. Tabular beds of mudstone and sandstone, and graded sandstone beds are consistent with a relatively deep (below-wavebase) depositional setting.
Mineralisation
Two main styles of mineralisation have been recognised (McKay and Hazeldene, 1987):
• footwall chalcopyrite-rich "copper stringer ore", essentially chalcopyrite and pyrite, occurring as a network of copper-rich stringers, composed of fracture filling assemblages of pyrite and chalcopyrite with subordinate to minor sphalerite, galena, pyrrhotite, quartz, calcite, chlorite and barite. This ore is hosted within a chlorite schist, and is capped by,
• poly-metallic "complex ore", occurring as sheet-like lenses of massive sulphides that include pyrite with variable sphalerite, galena and chalcopyrite, and minor arsenopyrite, tetrahedrite-tennantite, pyrrhotite, marcasite and electrum in a gangue of chlorite, talc, phlogopite, quartz and sericite. Compositionally, the "complex ore" is banded on a scale of millimetres to centimetres, with banding defined by variations in the amounts of sulphides or in the sulphide to gangue ratio (Ayres, 1979; McKay and Hazeldene, 1987). Gangue chlorite, muscovite and talc are strongly schistose (Ayres, 1979). In contrast, there is little apparent deformation of sulphides (Ayres, 1979; McKay and Hazeldene, 1987). Pyrite, the dominant sulphide, is anhedral to subhedral to euhedral in shape, commonly with zonal patterns. Sphalerite is anhedral and commonly preserves fine intergrowths with galena and cuspate contacts against chalcopyrite (Ayres, 1979; McKay and Hazeldene, 1987).
McKay (1989) recognised a four-stage paragenesis: Stage 1 - growth of framboidal and colloform pyrite and sphalerite; Stage 2 - growth of anhedral to euhedral pyrite, sphalerite, galena and chalcopyrite; Stage 3 - recrystallisation of sphalerite, chalcopyrite and galena, and replacement of stage 2 pyrite and arsenopyrite; and Stage 4 - secondary pyrite.
Glen et al. (1995) demonstrated that in addition to the early "copper stringer" and "complex ore", there is also an important ore-type related to and probably emplaced as part of the Devonian deformation events, e.g., in parts of the A Lens, where the ore is structurally controlled and overprints layered sphalerite-chert rock which predates folding and thrusting. This younger mineralisation is copper rich at the base, but changes upward to become complex ore containing sphalerite, galena, chalcopyrite and pyrite. The footwall of the ore zone is defined by a steeply west-dipping mineralised chloritic shear zone that widens upward. The hanging-wall contact is defined by a combination of chloritic and mineralised faults. This syn-tectonic mineralisation is associated with a west-dipping, east-vergent contractional fault system overprinted by steep normal faults.
The same authors concluded that the ore at Woodlawn is a composite of pre-, syn- and post-deformational type. McPhie (2015) have also shown the original Woodlawn massive sulphides, at least in part, probably formed sub-ocean floor with broad zones of hydrothermal chlorite, sericite and pyrite alteration extending into both the hangingwall and footwall sides of the massive sulphide lenses.
The original open pit C Lens resource comprised a single polymetallic Cu-Zn-Pb-Ag massive banded sulphide body of "complex ore" which occurred as a 5 to 45 m thick, 420 m long, folded conformable, NNW striking body, that extended for ~300 m down a 40° dip. This mineralisation was underlain by the footwall "copper stringer ore" which was best developed down dip. Both the "complex ore" sheet and the "copper stringer ore" thinned up dip. Subsequent exploration traced C Lens further down-dip over an interval of 800 m before lensing out (Ebbels et al., 2015).
Two further lenses, A and B are stacked ~150 and ~200 m in the hangingwall of C Lens, with down dip extents of ~400 and ~800 m respectively.
At least 8 other blind lenses have been identified down-dip from the open pit, over a further 240 m thickness in the hangingwall of A Lens. Most of these lenses are elongated in a NW-SE direction, parallel to the NW plunge direction and have thicknesses of from 4 to 14 m. Together, all of these lenses define a broad envelope that is ~600 m thick, >1000 m down dip and >800 m in a NNW-SSE direction (from Ebbels et al., 2015).
At surface, the complex ore is expressed by a 6 to 15 m layer parallel to surface, made up of 1 to 3 m of ferruginous surface soil underlain by heterogeneous ferruginous clayey gossan composed of talc, goethite, amorphous iron hydrate, complex base metal sulphates and arsenates, with common small veins and nodules of barite, as well as cerrusite, minor malachite, azurite and rare cuprite and chrysocolla (Malone et al., 1975).
The gossan is underlain by a thin, 1 to 2 m thick zone of supergene enrichment composed of friable, fine-grained pyrite, chalcocite, covellite and digenite, overlying primary massive sulphides containing <5% supergene sulphides which decrease in abundance to become negligible 10 m below the friable supergene interval.
The felsic volcanics of the Woodlawn sequence were followed by the Currawong Basalts which host the smaller Currawong base metal deposit some 10 km to the north of Woodlawn.
Open cut mining commenced in 1978, followed by underground operations to recover deeper ore and satellite orebodies. The mine was closed in March 1998 due to prevailing low metal prices and external corporate issues.
Reserves prior to mining were:
Massive sulphides - 6.3 Mt @ 1.7% Cu, 5.5% Pb, 14.4% Zn, 89 g/t Ag
Footwall ore - 3.7 Mt @ 1.9% Cu.
Total production between 1978 and 1998 from the Woodlawn open pit, underground and satellite deposits was:
13.8 Mt @ 9.1% Zn, 1.6% Cu; 3.6% Pb, 0.5 g/t Au; 74 g/t Ag,
comprising:
Woodlawn open pit between 1978 and 1987 - 8.0 Mt @ 8.3% Zn, 3.1% Pb, 1.6% Cu, 62 g/t Ag,
Woodlawn Underground between 1987 and 1998 - 5.8 Mt @ 10.1% Zn, 1.6% Cu, 4.1% Pb, 0.5 g/t Au, 90 g/t Ag.
Currawang mine between 1991 and 1995 - 0.53 Mt @ 13.0% Zn, 2.2% Pb, 1.6% Cu, 33 g/t Ag.
Remaining mineral resources as of June 2015 at a 7% Zn Equiv. cutoff (Ebbels et al., 2015) include:
Indicated resource - 1.6 Mt @ 10.7% Zn, 1.5% Cu, 4.0% Pb, 0.46 g/t Au, 78 g/t Ag,
Inferred resource - 3.0 Mt @ 8.1% Zn, 1.6% Cu, 3.2% Pb, 0.88 g/t Au, 70 g/t Ag,
Copper ore,
Indicated resource - 0.8 Mt @ 1.0% Zn, 2.8% Cu, 0.3% Pb, 0.07 g/t Au, 16 g/t Ag,
Inferred resource - 1.1 Mt @ 1.1% Zn, 2.8% Cu, 0.2% Pb, 0.14 g/t Au, 15 g/t Ag,
TOTAL RESOURCE - 6.5 Mt @ 6.7% Zn, 1.9% Cu, 2.5% Pb, 0.5 g/t Au, 56 g/t Ag.
Woodlawn tailings as of 31 December 2017 (Heron Resources, 2018):
Measured + Indicated resource - 9.8 Mt @ 2.3% Zn, 0.51% Cu, 1.3% Pb, 0.31 g/t Au, 32 g/t Ag (=6.2% Zn Equiv.),
Inferred resource - 1.1 Mt @ 2.3% Zn, 0.47% Cu, 1.2% Pb, 0.25 g/t Au, 27 g/t Ag (=5.8% Zn Equiv.),
TOTAL RESOURCE - 10.9 Mt @ 2.3% Zn, 0.51% Cu, 1.29% Pb, 0.3 g/t Au, 31 g/t Ag (=6.2% Zn Equiv.).
Remaining mineral resources as of 31 December 2017 at a 7% Zn Equiv. polymetallic and 1% Cu cutoff (Heron Resources, 2018) include:
Measured + Indicated resource,
Polymetallic ore - 2.7 Mt @ 10.0% Zn, 1.5% Cu, 4.0% Pb, 0.7 g/t Au, 79 g/t Ag (=21.6% Zn Equiv.),
Copper ore - 1.9 Mt @ 0.7% Zn, 2.6% Cu, 0.1% Pb, 0.2 g/t Au, 14 g/t Ag (=9.7% Zn Equiv.),
COMBINED - 4.6 Mt @ 6.7% Zn, 1.9% Cu, 2.4% Pb, 0.5 g/t Au, 52 g/t Ag (=16.7% Zn Equiv.),
Inferred resource,
Polymetallic ore - 1.9 Mt @ 7.3% Zn, 1.5% Cu, 3.0% Pb, 0.8 g/t Au, 61 g/t Ag (=16.9% Zn Equiv.),
Copper ore - 0.7 Mt @ 0.7% Zn, 2.5% Cu, 0.1% Pb, 0.2 g/t Au, 12 g/t Ag (=9.2% Zn Equiv.),
COMBINED - 2.6 Mt @ 5.6% Zn, 1.8% Cu, 2.2% Pb, 0.6 g/t Au, 48 g/t Ag (=14.9% Zn Equiv.),
TOTAL RESOURCE - 7.2 Mt @ 6.3% Zn, 1.9% Cu, 2.3% Pb, 0.5 g/t Au, 51 g/t Ag (=16.1% Zn Equiv.).
This summary has largely been drawn from: "Ebbels, A.-M., Brown, R., Guibal, D. and Allen, P., 2015 - Preliminary economic assessment of the Woodlawn Project, New South Wales, Australia; An NI 43-101 Technical report, prepared for Heron Resources Limited by SRK Consulting."
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1996.
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.
Woodlawn
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Selected References:
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Bishop J R, Lewis R J G 1992 - Geophysical signatures of Australian volcanic-hosted massive sulfide deposits: in Econ. Geol. v87 pp 913-930
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Davis L W 1990 - Silver-Lead-Zinc-Copper mineralisation in the Captains Flat-Goulburn synclinorial zone and the Hill End synclinorial zone: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1375-1384
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Glen R A, Walshe J L, Bouffler M, Ho, T, Dean J A 1995 - Syn- and post-tectonic mineralization in the Woodlawn deposit, New South Wales, Australia: in Econ. Geol. v90 pp 1857-1864
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Large R R 1992 - Australian volcanic-hosted massive sulfide deposits: features, styles, and genetic models: in Econ. Geol. v87 pp 471-510
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Malone E J, Olgers F, Cucchi F G, Nicholas T, McKay W J 1975 - Woodlawn copper-lead-zinc deposit: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne Mono 5 pp 701-710
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McKay W J, Hazeldene R K 1987 - Woodlawn Zn-Pb-Cu Sulfide deposit, New South Wales, Australia: an interpretation of ore formation from field observations and metal zoning: in Econ. Geol. v82 pp 141-164
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Ryall W R, 1979 - Mercury distribution in the Woodlawn massive sulfide deposit, New South Wales : in Econ. Geol. v74 pp 1471-1484
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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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