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Woodcutters

Northern Territory, NT, Australia

Main commodities: Zn Pb Ag
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The Woodcutters Zn-Pb-Ag deposit is located ~80 km south of Darwin, Northern Territory, Australia (#Location: 12° 58' 42"S, 131° 6' 19"E).

The deposit lies within the Archaean to Palaeoproterozoic Pine Creek Orogen of the North Australian Craton. This craton also includes the neighbouring Tanami region, the Murphy, Tennant and Arnhem Inliers and the Northern Arunta Province. These provinces underwent extensive orogenic movement and regional metamorphism during the 1.88 and 1.85 Ga Barramundi Orogeny, followed by transitional tectonics and igneous activity from 1.85 Ga, in some area continually, to 1.80 Ga (Ahmad et al., 2001).

The Pine Creek Orogen is composed of a series of late Archaean granite‚Äźgneiss basement domes, which are overlain by a fluvial to marine sedimentary sequence, representing as one of a series of successive rift, sag and orogenic intracratonic phases (Needham and De Ross, 1990). The regional sequence is a follows, from the base:
Basement gneiss, schist and granite, the youngest phase of which is 2525±5 Ma. The Rum Jungle and Waterhouse Complexes form the basement locally.
Angular unconformity
Namoona Group, composed of
  - Beestons Formation, ~300 m of quartz conglomerate, feldspathic quartz sandstone, minor banded ironstone.
  - Celia Dolostone, less than 300 m of stromatolitic magnesite, dolostone, some of which is silicified, with rare para-amphibolite and mudstone.
Unconformity
Mount Partridge Group, which was deposited between 2.5 and 2.19 Ga, and is composed of
  - Crater Formation, up to 900 m of feldspathic quartz sandstone, quartz-pebble conglomerate, poorly sorted pebble to boulder conglomerate (with BIF-quartz clasts), siltstone, mudstone. In the Woodcutters are is comprises a basal haematite boulder conglomerate containing basement clasts of banded ferruginous rock or quartz sandstone. The conglomerate passes upwards into quartz greywackes, sandstones, shale and hematitic siltstone (French, 1969), and is characterised by an intermittent magnetic and a strong radiometric response (Fleming et al., 1994).
  - Coomalie Dolomite, up to 600 m of stromatolitic, magnesite and marble, which is in places chloritic and tremolitic, and is commonly silicified or lateritised at the surface. The unit also contains metalutite, which is commonly graphitic.
  - White's Formation, up to 1000 m of calcareous and carbonaceous pyritic mudstone/siltstone, dolomitic mudstone and rare quartzite.
  - Wildman Siltstone, which is up to 2000 m thick and is regionally subdivided into the
   Acacia Gap Quartzite Member - which is commonly pyritic, with interbedded shale and phyllite that is frequently carbonaceous.
   Mount Deane Volcanic Member - altered basic volcanic rocks, which are in places vesicular or brecciated and dated at 2026 Ma (U/Pb-Pb/Pb ion probe);
   Yarrawonga Volcanic Member - comprising a basal, highly altered, sericitic, ignimbritic acid volcanic rock with a fragmental texture; a middle, weakly chloritised dacite; and an upper rhyolite and perlite.
Minor uplift and erosion
South Alligator Group - up to 5000 m of quartz-sericite and quartz-chlorite-sericite schist, siltstone, phyllite, chert, pitchstone, andesite and saccharoidal quartzite, subdivided into,
  - Koolpin Formation, a sulphidic and carbonaceous argillite, with ferruginous chert, ironstone, silicified dolomites and phyllitic mudstones which were deposited in a low energy environment. It varies in thickness from <300 to > 1,000 m, confused by the inclusion of of several sills of the ~1.87 Ga Zamu Dolerite. The contact between the Wildman Siltstone and the overlying Koolpin Formation is partially conformable and partially an angular unconformity.
  - Gerowie Formation, up to 400 m thick and consists of tuff, tuffaceous chert and tuffaceous siltstone, with subordinate amounts of laminated cherts and carbonaceous siltstones.
  - Mount Bonnie Formation, a 150 m and 400 m thick transitional facies between the Koolpin and Burrell Creek Formations, comprising greywacke, carbonaceous siltstone, chert, tuff and ironstone.
Finnis River Group - up to 5000 m of quartz-sericite and quartz-chlorite-sericite schist, siltstone, phyllite, chert, pitchstone, andesite and saccharoidal quartzite, subdivided into,
  - Burrell Creek Formation, a 1500 m thick sequence of turbiditic sediments including greywackes, siltstones and mudstones.
Deformation, granite intrusion and metamorphism - from 18.8 to 17.6 Ga.

The Woodcutters mineralisation is localised within a series of steep, markedly crosscutting faults within the Whites Formation, occurring as at least seven pyrite, sphalerite and galena bearing veins distributed over a strike length of at least 1.1 k m within steeply dipping axial plane faults within a sequence of black slates with interbedded carbonate rocks and minor ash tuffs of the Lower Proterozoic Whites Formation. The ore extends for as much as 900 m above the contact with the underlying Coomalie Dolomite. No mineralisation other than minor disseminations has been intersected in the Coomalie Dolomite (Fleming et al., 1995).

The stratigraphy in the mine area comprises from the base (from Goulevitch, 1997):
Coomalie Dolomite - >100 m of pale cream to white, massive dolomite, that is thinly bedded in places, grading downward into grey stromatolitic dolomite at depth. It appears to grade into the overlying unit, although a reliable boundary marker is not observed.
White's Formation - subdivided into,
  Massive Dololutite to Dolomite Unit, 5 to 20 m of interlayered medium to light grey and creamy dololutite, which is thinly bedded in places.
Lower "Scraggy" Dololutite Unit, 45 to 60 m of medium to pale grey dololutite with thin 1 to 10 cm thick pale grey-white distinctive dololutite bands. The lower boundary appears to be transitional.
  Lower Slate-Dololutite Unit, 55 to 60 m of generally thinly interbedded grey black slate and slatey dololutite grading to lighter grey dololutite units several metres thick but not laterally consistent. It appears to be generally darker grey and less dolomitic overall than the Upper Slate-Dololutite Unit.
  Dololutite Marker, 0.3 to 1 m of pale grey-white laminated dololutite that is generally complexly folded such that it may occupy up to several metres of core.
  Upper Slate-Dololutite Unit, 14 to 18 m of generally thinly interbedded grey-black slate and slatey dololutite, grading to lighter grey dololutite units several metres thick but not laterally consistent.
  Upper "Scraggy" Dololutite Unit, >25 m of grey-black to medium grey slatey dololutite/dolomitic slate with thin pale grey to white distinctive dololutite bands that are 1 to 10 cm thick. Spaced transverse fractures characterise most bands. The slate-dololutite bands are generally thinly bedded. Some bands have been subjected to intraformational brecciation, although brecciation is not stratigraphically consistent laterally. The unit is intensely internally folded within 'smoother' enveloping surface.

In the mine area, the Zamu Dolerite occurs as folded sills ranging from several to 200 m thick within the White's Formation (Nicholson and Ormsby, 1993). These sills are composite bodies ranging from metadolerite to metagranophyre, and are variably chlorite and biotite altered. The chlorite and biotite aligned by the S2 cleavage, suggesting the alteration was pre- or syn- the main period of regional deformation.

The orebodies generally occur in north-south transpressional faults and along dyke boundaries as replacement veins, in part associated with carbonatised Zamu Dolerite lamprophyre dykes. The known orebodies continue at depth, while bonanza grade ore has been mined in the main body in an open pit over a thickness of 30 m. A number of features influence the location and shape of these bodies, namely (Fleming, Ormsby and Nicholson, 1994):
• North-south transpressional faults host the bulk of the mineralisation, particularly in dilational zones at fault flexures associated with sinistral strike-slip movement;
• Intersection of the north-south transpressional faults with the axial plane of the NNE trending Woodcutters Anticline, producing an en-echelon pattern for the orebodies in plan;
• Intersection of the north-south transpressional faults with the NW-SE cross-faults, producing dilation zones;
• Intersection of dololutite rich units with the north-south transpressional faults where local replacement of carbonate occurs, generally resulting in thicker mineralised zones. Locally, the mineralisation can pass from one structure to the next along a "dololutite bridge", particularly along anticlinal fold axes;
• Thickening of the stratigraphy in the vicinity of the Woodcutters Anticlinal axis; and
• The anticline doubly plunging to both the north and south.

The mineralisation has vein-like features and epigenetic textures. Veins generally range up to 10 m thick, although thicknesses to 25 m have been encountered. The vein boundaries are sharp, with thicknesses varying rapidly down dip and along strike. No alteration selvedge is observed, although stringer veins of quartz-carbonate-pyrite and sphalerite/galena occur in close proximity.
  Mineralisation comprises >60% sulphide, with textures that range from massive to banded to brecciated, reflecting the replacement and vein-like epigenetic nature of the mineralisation. Banding usually parallels the boundaries of the host structure or reflects bedding in the replaced dololutite beds (Fleming et al., 1995).
  A number of sulphide mineral assemblages have been recognised, each separated by periods of deformation and partially replacing the earlier assemblages. Vertically continuous zonation patterns highlight the presence of rich galena-sphalerite-silver in zones of maximum dilation and pyrite-arsenopyrite-gold concentrated at the along strike peripheries.
  In excess of 20 different sulphide minerals have been identified, the main minerals being sphalerite, pyrite, galena, boulangerite, falkmanite and arsenopyrite, with minor chalcopyrite, geocronite, jamesonite, tetrahedrite, greenockite and stannite. Five different metal oxide and phosphate minerals have also been recognised including cassiterite. Scattered high levels of gold (up to 5 g/t), nickel (up to 1%), cobalt (up to 1%), indium (up to 170 ppm), and tin (up to 0.1%) have been recorded. Gangue minerals include quartz, dolomite, tourmaline, calcite and apatite (Fleming et al., 1995).
  Just (1990) recognised a number of different metallic mineral assemblages and determined the following paragenetic sequence, in order of decreasing age:
i). Pyrite, occurring as corroded crystal and crystal aggregates, from 200 µm to 2 mm in size.
ii). Pyrite-Arsenopyrite. Arsenopyrite occurs as needles and crystals ranging from 200 to 1200 µm in size, interlocked with pyrite. Moderate gold grades occur. This phase appears gradational with the earlier pyrite assemblage.
iii). Sphalerite. Grains of <40 µm (locally up to 10 mm) occur as disseminations in mineralised wall rock, inclusions in pyrite, interstitial to pyrite-arsenopyrite aggregates and as monomineralic granular masses.
iv). Lead/Antimony Sulphosalts-Galena. Possibly representing a continuous gradation from an earlier sulphosalt-rich assemblage to a later galena-rich assemblage. Lead minerals occur as small (µm to tens of µm) inclusions, veinlets, metasomatic bodies and interstitially to pyrite, arsenopyrite, sphalerite, quartz and dolomite. Locally, monomineralic patches and veins up to centimetre size can occur.
  Silver sulphosalts, cobalt/nickel sulphides and copper/tin minerals are all minor assemblages often occurring together. Their paragenesis is uncertain, although some occurrences are late. Silver-bearing minerals occur as grains up to 100 µm in size where diffuse veinlets of lead minerals dissipate in areas rich in pyrite, sphalerite, quartz and dolomite. Traces of cassiterite are ubiquitous as fine grained disseminations and veins (Fleming et al., 1995).

The total production from the mine, from 1985 to 1999 (NT Geological Survey) was:
    4.65 Mt @ 12.3% Zn, 5.6% Pb, 87 g/t Ag.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 21/1/2015
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.


Woodcutters

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Fleming M J, Ormsby W R and Nicholson P M,  1994 - The Geology of the Woodcutters Lead-Zine-Silver Mine: in   1994 AuslMM Annual Conference, Darwin, August 1994  The AusIMM, Melbourne,    pp. 21-28
Roberts W M B  1975 - Woodcutters L 5 Lead-zinc prospect, Rum Jungle: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 277-281
Smolonogov S and Marshall B  1993 - A genetic model for the Woodcutters Pb-Zn-Ag orebodies, Northern Territory, Australia: in    Ore Geology Reviews   v8 pp 65-88


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