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Charters Towers |
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Queensland, Qld, Australia |
| Main commodities:
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 Charters Towers Gold Field lies some 1100 km NNW of Brisbane and 100 km SW of Townsville in north Queensland, Australia. To 1987, it was the third largest gold producer in Australia with a total cumulative output of 211 t Au at an average grade of 34 g/t Au.
The gold field was discovered in 1871 and sustained a production rate of over 6 tonnes (200 000 ounces) per year for 15 years from 1892 to 1906. The main operating mines were closed by 1918.
More recent work has outlined an additional Inferred Mineral Resource of 331 tonnes of contained gold as 23 Mt @ 14.7 g/t Au, of which 220 tonnes is currently (2007) to be mined over a 30 year period. The first of the planned new mines commenced operation in 2004. The resource is contained within 22 separate shoots (CitiGold Resources, 2007).
The Charters Towers Gold Field lies within the older granodioritic phases of the 455-385 Ma Ravenswood Batholith. The auriferous lodes are cut by Carboniferous dykes and are absent from the adjacent Middle Devonian sediments.
The goldfield is part of a major 60 km long, east-west trending mineralised sytem composed of multiple lode zones. It lies along the Mosgardies Shear Zone and is believed to be related to the 400 Ma Lolworth Igneous Complex or the related thermal metamorphic event. Mineralisation is interpretted to be localised in a structural trap zone representing an extensional roof above a non-exposed granitic intrusive of the Lolworth Complex which subsequently underwent cooling and shrinkage. Mineralisation is believed to have been derived from fluids involved in deep late metamorphism, rather than from magmatic or metamorphic events higher in the crust.
There are some 80 major reefs in and around Charters Towers, of which 26 are included in the current resource estimate. The main reef systems strike east-west and include the Brilliant, Day Dawn, Mexican, Queen and Sunburst lode zones, which extend over a strike length of five kilometres and are cut by NNW trending cross veins. A second east-west system, 800 m to the south, comprises the Golden Sunrise, Mary and Clark's Moonstone line of reefs, while a third system 500 m further south, includes the Ruby and Gladstone line of reefs. The most productive ore-bearing lodes (the Day Dawn, Brilliant and Queen) dip to the north beneath the city of Charters Towers.
The majority of the historic production was concentrated within a set of fractures extending over a 5 km long east-west strike length, and 500 to 1600 m down dip interval in a north-south direction. The mineralised lodes lie in two predominant directions, namely east-west, dipping at moderate to shallow angles to the north (the main production), and the NNW cross-veins which dip ENE.
The mineralised reefs are typically 0.3 to 1.5 m, locally up to 6 m thick, and in isolated cases up to 15 m. The average width has been estimated to have been less than 0.9 m over the majority of the field. Displacements on the mineralised fissures are mostly from 1 to 12 m.
The mineralised veins are both simple and composite hydrothermal quartz-gold systems with a gangue comprising 10% sulphides (in decreasing order of abundance - pyrite, galena, sphalerite, chalcopyrite and arsenopyrite, with traces of tetrahedrite and tellurides), carbonate, chlorite, gypsum, barite and clays, and locally containing crushed granodiorite. They are flanked by extensive sheet-like alteration zones comprising green muscovite and ankerite selvages, which pass laterally into propylitic (montmorillonite-illite) alteration.
There is a relatively consistent and simple paragenesis over the entire region, commencing with the deposition of white quartz. The early quartz was subsequently fractured, and grey silica and pyrite deposited. Further refracturing was accompanied by the deposition, principally within the pyrite, of galena, gold, sphalerite ±quartz, and trace chalcopyrite. Late carbonate was the final phase of the main mineralising event. In places however, the mineralised fissures were reactivated and carbonate, carbonate/laumontite and other assemblages deposited. The economic gold is restricted to the quartz reefs (particularly within pyrite) and associated shearing. Significant gold is not normally present in disseminated pyrite within the adjacent sericitic alteration.
Ore shoots within the fissure system occur with a periodicity typically of the order of 200 to 300 m within the lodes, and have dimensions of mostly 200 to 700 m in the down plunge direction, and 70 to 300 m normal to the plunge direction. The structural control of the Charters Towers ore shoots is variable. Some reefs thicken and thin over short distances with no obvious controlling feature. There is also no consistent enrichment or thickening where the east-west trending lodes and the NNW trending cross veins intersect.
The ore shoots are predominantly hosted by the sulphidic sections of the fissures, mostly comprising sheet-like masses sub-parallel to, and within the quartz reefs, but may occur as disseminations and irregular concentrations within the quartz. The continuity of the pyrite bodies is considerably less than that of the quartz reefs. Although usually coarse-grained, fine-grained sulphides are locally present in some shear zones.
The heterogeneous distribution of sulphides within the quartz, and the often erratic nature of the gold concentrations within the pyrite, result in a spotty distribution of ore grades and a high to extreme nugget effect. It is no uncommon for poorly mineralised zones of the fissures to pass rapidly along strike into high-grade ore, and vice versa. The ore is locally very rich, with several ore shoots known to average over 60 g/t Au (e.g. in the New Queen Cross, Talisman, parts of the Brilliant lode).
Trial mining was carried out by Citigold Corporation Ltd between 1998 and 2000 in several parts of the lode systems, producing 1.2 tonnes of gold from two open pit and three underground operations to establish grade distribution and the relationship between drilled and actual mineralisation. In addition some 141 000 m of RC and diamond drilling have been utilised to support ore resource estimates.
Citigold Corporation Ltd plans to open four new open pit and underground mines in the gold field over a five year period. The first of these, the Warrior mine, commenced operating with its initial blast in March 2004. Four main production areas are to be exploited, three within the main central Charters Towers area (on the Brilliant, Day Dawn and Sunburst lode systems), and the Warrior area approximately 4 km to the south of the city. All ore will be treated at the existing gold processing plant 6 km south of the city.
The above summary was based largely on information provided by Citigold Corporation Limited.
The most recent source geological information used to prepare this decription was dated: 2004.
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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Kreuzer O P 2006 - Textures, paragenesis and wall-rock alteration of lode-gold deposits in the Charters Towers district, north Queensland: implications for the conditions of ore formation: in Mineralium Deposita v40 pp 639-663
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Kreuzer O P, 2005 - Intrusion-Hosted Mineralization in the Charters Towers Goldfield, North Queensland: New Isotopic and Fluid Inclusion Constraints on the Timing and Origin of the Auriferous Veins: in Econ. Geol. v100 pp 1583-1603
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Kreuzer O P, Blenkinsop T G, Morrison R J and Peters S G, 2007 - Ore controls in the Charters Towers goldfield, NE Australia: Constraints from geological, geophysical and numerical analyses: in Ore Geology Reviews v32 pp 37-80
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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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