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Ardlethan |
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New South Wales, NSW, Australia |
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Sn
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Super Porphyry Cu and Au
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The Ardlethan tin mine was located within central New South Wales, 5 km NW of the town of Ardlethan, 55 km SW of West Wyalong and some 570 km to the WSW of Sydney (#Location: 34° 19' 32"S, 146° 51' 21"E).
Tin was initially discovered at Ardlethan in 1912 near the old Carpathia Mine. Subsequent prospecting located a number of small high grade cassiterite-sulphide-tourmaline pipes and veins which were worked by underground methods. A number of alluvial accumulations were also worked, and in 1953 the main Yithian Lead, which originated from the now mined out Wild Cherry orebody, was discovered. Sporadic production from alluvial and underground workings, followed later by two small open cuts, resulted in some 9500 tonnes of tin concentrate. The hard rock workings had head grades varying between 1% and 4% Sn during this period.
Aberfoyle Tin NL commenced drilling at the property in 1961 and subsequently delineated the White Crystal and Wild Cherry orebodies. Open cut mining commenced in 1964 following the incorporation of the Aberfoyle controlled Ardlethan Tin NL to operate the mine. Since then, to the mine closure in 1986, some 9 Mt @ 0.46% Sn were mined. Remaining resources include 10.7 Mt @ 0.2% Sn in tailings (Inferred + Indicated Resources; Reynard Australia Pty Ltd Report, October 2011) and an underground resource of 5.5 Mt @ 0.45% Sn (Inferred + Indicated Resources; Ardlethan Information Brochure, Molina & Doran, May 1989).
Geological Setting
The Ardlethan tin deposit occurs on the margin of a late Silurian granitic body which cuts Ordovician sediments in the core of the Wagga Girilambone Arch. Siluro-Devonian acid volcanics and intrusives, which may be partially related to the rhyolitic dykes found in the core of the ore zone, are intruded by the granite to the north, while Silurian to late Devonian red beds form a low ridge to the east.
The Ordovician sediments are predominantly quartzites, impure sandstones, black carbonaceous slate and siltstone, occurring locally as phyllites and schists. They have been tightly folded in a general north to north north westerly trend.
The granites at Ardlethan are part of a 320 x 80 km belt of late Silurian granitoids, extending from Wagga to Condobolin, that were formed in the core of the Wagga Arch during the Bowning Orogeny. These granites yield 400-410 Ma age dates and are classified as post kinematic shallow porphyritic intrusives. They are strongly crosscutting with sharp margins and are well differentiated. Most of the granites in this section of the Wagga Arch carry around 10 ppm Sn in contrast to those within and to the east of the Bogan Gate Platform which generally have <3 ppm Sn.
The Siluro-Devonian acid porphyries of the Ardlethan area comprise one of a number of developments of shallow intrusives and associated extrusives found sporadically along the length of the Wagga-Girilambone Arch.
The main igneous rock types within the Ardlethan area are as follows:
* Garnetiferous quartz-feldspar porphyry - This rock type is believed to be principally an intrusive in the immediate Ardlethan area, although a number of outcrops in the vicinity have flow textures. It appears to predate the granites, and constitutes the earliest igneous activity at Ardlethan. It comprises phenocrysts of quartz, plagioclase, K-feldspar and subordinate biotite in a fine grained quartzo-feldspathic matrix. Accessory minerals include garnet, sphene and apatite. The average tin content is 7 to 8 ppm Sn, double the average for acid igneous rocks.
* Mine Granite - The Mine Granite was intruded shortly after the garnet porphyry. It is very uniform in composition throughout and comprises a grey to cream-grey, medium to coarse grained, weakly foliated, biotite granite with 25% quartz, 35% K-feldspar, 25% plagioclase and 15% biotite and traces of apatite, magnetite, zircon and chlorite. A few small co-magmatic micro-granite dykes intrude it, while numerous biotite-schist, chloritic-siltstone and quartzite xenoliths are found, particularly in the mine area where roof pendants occur. Away from the main alteration zone it carries from 6 to 15 ppm Sn averaging 10 ppm.
* Ardlethan Granite - The Ardlethan Granite, which is a more differentiated phase than the Mine Granite that it intrudes, is found to the west of the mine area. It is a pink to white, medium to coarse grained granite, with 40% quartz, 30% K-feldspar, 20% plagioclase, 5% biotite, 2% muscovite and a trace to 3% tourmaline, with accessory apatite, magnetite, topaz, zircon and sphene. The texture is variable, particularly towards the margin, with both fine and coarsely porphyritic phases being present. The chemical composition however, shows little variation. Numerous tabular cassiterite bearing alteration zones, comprising sugary quartz-tourmaline with interspersed masses of medium to coarse sericite, occur along the eastern contact with both the garnet porphyry and Ordovician sediments. These often carry limited pods of 0.1 to 0.2% Sn, enclosing rod-like shoots up to 200 x 20 x 20 m carrying 0.3 to 1.0% Sn as at Little Bygoo. These occurrences are markedly different to the Ardlethan orebodies in size, shape and style of alteration and mineralisation. In general, the outer contact of the Ardlethan Granite carries between 10 and 60 ppm Sn, averaging near 30 ppm (with around 0.31% F), while the central sections of the granite average around 10 ppm Sn.
Mine Geology & Mineralisation
The main group of orebodies at Ardlethan is located close to the southern contact of the Mine Granite with the Ordovician sediments. All of the known orebodies are contained within two broadly cylindrical, vertical, pipe-like, alteration zones. The smaller of these has lateral dimensions of approximately 200 x 250 m and hosts the White Crystal orebody. The second, which hosts the Wild Cherry, Ardwest, South Cherry, Carpathia and Stackpool ore pods, has horizontal dimensions of 900 x 350 m and has been shown to persist undiminished to below 600 m from the surface. This alteration is feldspar destructive and the overall alteration geometry is not controlled by fractures or veins. Quartz is the only original mineral retaining its identity, although it too is partially modified. The progressive alteration of the Mine Granite is believed to have proceeded as follows: biotite is chloritised; plagioclase is sericitised; K-feldspar is sericitised and the sericite is chloritised; the altered rock is tourmalinised, silicified and topazised. Chemically the alteration has resulted in leaching of Na2O, CaO, K2O, and Al2O3, accompanied by the addition of Fe, S, B, F, SiO2, H2O, CO2, Sn, Cu, As, Zn, and Pb. Disseminated cassiterite is found sporadically throughout the entire alteration zone, varying from a minimum of around 0.05 to 0.2% Sn or more. The orebodies occur as discrete higher grade pods within this low grade halo.
The margins of the alteration zones are relatively sharp, progressing from unaltered to poorly altered to intensely altered in a distance of 20 m.
The two main alteration zones are essentially different and will be described separately.
* CHERRY ALTERATION ZONE - This is the largest zone, hosting the majority of the orebodies. Localised within this zone are a series of quartz porphyry dykes. Unaltered, these dykes are composed of glassy quartz phenocrysts (up to 3 mm) and occasional plagioclase, in a khaki to dark grey aphanitic ground mass of K-feldspar and quartz. Unaltered specimens are extremely rare. Rosettes of acicular tourmaline replace mica and feldspar, while cross cutting veinlets of quartz-tourmaline and topaz are common. The quartz porphyry dykes were intruded as swarms and appear to have been explosively introduced producing brecciated contacts. The only known occurrence of these porphyry dykes within the whole Ardlethan district is within the Cherry alteration zone.
The alteration within this zone has affected both the quartz porphyry and the Mine Granite. However, although the Mine Granite is evenly and intensely altered throughout, the quartz porphyry is only intensely altered on its outer margin and in more fractured zones. The central portions of the quartz porphyry are only moderately altered.
The alteration of the Mine Granite results in a product comprising - 4 mm round quartz phenocrysts in a mesh of sericite-chlorite, with variable black tourmaline (dravite-schorlite), fluorite, carbonate, arsenopyrite, pyrite and the ore minerals, cassiterite, chalcopyrite and subordinate sphalerite and galena. Both the ore and gangue minerals occur as very fine disseminations, patches and multi directional veinlets. Open space features are rare to absent. The alteration product surrounding the remnant quartz crystals is dense and fine grained with no foliation. Brecciation is obvious on weathered surfaces, enhanced by differential weathering. This 'breccia' is most intense in the ore pods which are distributed around the margins of the porphyry dykes. This 'breccia' comprises scattered pebbles and cobbles from 1 cm to 30 cm across of rounded to subrounded fragments, predominantly quartz porphyry, set in a matrix of the alteration product. The sequence of events deduced from this is as follows:
* Introduction and cooling of the Mine Granite
* Explosive introduction of the quartz porphyry leads to a zone of intense brecciation on the 'dyke' margins which contains mechanically redistributed blocks of both semi-consolidated porphyry and Mine Granite, surrounded by a broader shatter zone;
* Introduction of mineralising solutions through the brecciated Mine Granite resulting in alteration and mineral deposition.
Within the centre of the Cherry alteration zone a remnant roof pendant of Ordovician sediments is preserved, surrounded by altered Mine Granite. This roof pendant is largely present as a breccia of highly angular, chaotically arranged fragments of quartzite, and weakly metamorphosed black shale, sandstone and siltstone permeated by altered fine grained granite and tourmaline. The three main orebodies within the Cherry Zone are mined in a single open pit. In general they are very irregular in outline, as defined by a cut off contour, but in summary are tabular to elliptical and steeply dipping with dimensions of the order of 300 x 25 x 100 m at Ardwest, to 230 x 50 x 150 m at South Cherry. The three main orebodies - Wild Cherry, South Cherry and Ardwest are interconnected.
The Stackpool and Capathia deposits, which are smaller, are found in another section of the same alteration zone. All of the orebodies occur over a clearly defined, but restricted vertical range of 200 m. Although the alteration zone with 0.05 to 0.2% Sn continues to a depth of >600 m, no ore grade (ie. >0.3% Sn) has been encountered at a depth of >200 m.
All of the orebodies in the Cherry alteration zone have a similar mineralogy, namely disseminated very fine pyrite and cassiterite with subordinate chalcopyrite, sphalerite and arsenopyrite. Superimposed are numerous high grade, large, 'sooty', green-black massive tourmaline sulphide rich pipes and veins up to 1 m in diameter that appear to follow joint/fracture intersections. Some 70% of the known economic mineralisation of this type is concentrated close to the contact of the porphyry dykes. The average grade of these orebodies, using a 0.3% Sn cut off, is 0.50% Sn, 0.10% Cu, 0.08% Zn.
* WHITE CRYSTAL ALTERATION - The main difference between the White Crystal and Cherry alteration zones is that:
* The White Crystal alteration zone is much smaller;
* There are no porphyry dykes within the White Crystal zone;
* The White Crystal zone is composed of a 'Cherry-type' alteration product that has been subjected to a very obvious collapse (or crackle) brecciation, with a second superimposed stage of alteration.
The White Crystal breccia is pipe-shaped with surface dimensions of some 200 x 100 m within the 200 x 250 m alteration zone. The contact between the breccia and the Mine Granite is very sharp, being affected over an interval of <1 m. The breccia is made up of a series of blocks from a few cms to 20 m across. These blocks are rectangular to elliptical, and can be seen to have been partially rotated, with the space between them being filled with white quartz. The outer rims of the fragments have been intensely altered and now comprise coarse and fine grained quartz as phenocrysts and annealed patches in a fine grained mosaic of very fine green-blue tourmaline, topaz, muscovite, interstitial sericite, minor chlorite, fluorite, apatite, variable pyrite and arsenopyrite with cassiterite, sphalerite, subordinate galena and very minor chalcopyrite. The majority of the cassiterite is found within the inter-fragment siliceous material and the altered rims which are from 1 to 10 cm thick. The central sections of the fragments are identical with the alteration product of the 'cherry-type' alteration.
The White Crystal ore zone has 5 to 10% pyrite throughout, in contrast to the 1 to 3% sulphides of the Cherry orebodies. In addition, the White Crystal orebody carried more Pb and Zn, largely as disseminations, peripheral to the main tin zone.
The average grade of the White Crystal orebody was 0.45% Sn, 0.20% Zn 0.15% Pb and 0.03% Cu. The orebody had a vertical dimension of 100 m, being capped by a highly siliceous, 30 m thick, 1% Sn zone that tapers into a series of diffuse fingers at the base. The sulphide content of the alteration zone and the breccia is known to continue for at least 150 m below the economic ore zone, without any further ore grade mineralisation being encountered.
There is very little tungsten within either of the ore zones. The alteration zones carry 10 to 50 ppm WO3, while the unaltered Mine Granite has 3 ppm and the Ardlethan Granite averages 6 ppm WO3.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1997.
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.
Ardlethan
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Selected References:
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Paterson R G 1990 - Ardlethan Tin deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1357-1364
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Ren S K, Walshe J L, Paterson R G, Both R A, Andrew A 1995 - Magmatic and hydrothermal history of the Porphyry-style deposits of the Ardlethan Tin field, New South Wales, Australia: in Econ. Geol. v90 pp 1620-1645
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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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