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Ravenswood, Mount Wright, Sarsfield, Buck Reef West, Nolans |
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Queensland, Qld, Australia |
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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 Ravenswood gold deposit is located ~85 km south of Townsville and 75 km east of Charters Towers in north Queensland, Australia (#Location: 20° 6' 39"S, 146° 53' 59"E).
The Mount Wright gold deposit is located ~10 km northwest of the Ravenswood mine (#Location: 20° 2' 28"S, 146° 49' 56"E).
The two deposits, which are in close proximity, both occur within the composite Ordovician to Permian Ravenswood Batholith and have similar Late Carboniferous mineralisation ages (Perkins and Kennedy, 1998). However, they each otherwise have distinct geological characteristics. Ravenswood is characterised by quartz-sulphide vein networks hosted by Silurian intermediate-to-mafic intrusive rocks. Mount Wright occurs as sulphide veins and as breccia infill within a rhyolite-dominated magmatic-hydrothermal breccia complex, cutting an Ordovician granite. Never the less, Lisowiec and Morrison (2017) interpret both deposits to be intrusion-related gold systems, similar to the Mount Leyshon and Kidston deposits, also in north-east Queensland.
Gold was discovered in the Ravenswood Goldfield in 1868, with mining initially focused on alluvial and shallow veins that were oxidised to a depth of ~20 m. Production declined in the 1870s as these resources were depleted and the refractory sulphide ores challenged satisfactory recoveries. With consolidation of the mines in the 1890s and the introduction of Wilfley tables, recovery improved and production expanded to average ~1.5 t per annum from 1900 to 1910, with workings to a depth of 250 m. Due to a number of factors, including labour shortages during World War I, mining ceased in 1917. Minor, erratic production took place through the early to mid-1900s, with historic production to 1967 estimated at ~28 t. Modern exploration commenced at ~1980, with a series of high-grade, low tonnage deposits ibeing delineated in major structures in the historic mining area. Mining commenced in 1987, mostly from open pits. Subsequent drilling outlined larger tonnages of lower grade resources between the major mineralised structures, and large-scale open pit mining commenced as the Nolans and Sarsfield pits in 1993 and 2001 respectively. This ore was processed through a carbon-in-pulp plant commissioned in 1993. Open pit mining ceased in 2009, with low-grade stockpiles processed until 2011.
Gold was discovered at Mt Wright in 1917, but only ~47 kg had been produced by 1942. Drilling in the 1980s outlined the small Mother Lode deposit which was mined in 1992-93, with ore trucked to Ravenswood for processing. The main deposit was intersected in 1992 as a result of deep conceptual drilling of a poorly mineralised, magmatic/hydrothermal breccia on the western slopes of Mt Wright. Significant mineralisation is primarily hosted in variably brecciated rhyolite from ~150 m below surface. Underground mining commenced in 2006, with first ore delivered to the Ravenswood plant in 2007. Development was completed to a depth of -850 m in 2016, with ore production to be completed in late 2017.
Since 2009, drilling has outlined and upgraded resources adjacent to and below the existing Ravenswood mine. In 2017 ~120 t of gold reserves plus resources have been identified at the Sarsfield/Nolans and Buck Reef West areas. In August 2016, open pit mining at Nolans East commenced, with open pit mining commencing at Buck Reef West in 2018.
The two deposits have been mined as a single operation since 2007, utilising the same treatment plant at Ravenswood. The endowment (historical production plus current resources) of the Ravenswood and Mount Wright deposits, as known at 2017, is estimated to be 230 and 34 t of gold respectively (Lisowiec and Morrison, 2017).
Regional Setting
The Ravenswood and Mount Wright deposits lie within the Neoproterozoic to Palaeozoic Charters Towers Mineral Province, in the north of the Thomson Orogen of the Tasman Fold Belt in eastern Australia.
The Ravenswood deposits are hosted within the Ravenswood Batholith which constitutes a major part of the Charters Towers Province, which also includes Neoproterozoic to Cambrian meta-sedimentary rocks of the Cape River Province and Cambrian-Ordovician volcano-sedimentary units of the Seventy Mile Range Croup. The Batholith is predominantly composed of Early to mid Ordovician hornblende and biotite bearing I-type granites of the Macrossan Igneous Association, and Late Silurian to Early Devonian I-type and lesser S-type granites of the Pama Igneous Association. Late Carboniferous to Early Permian Kennedy Igneous Association intrusions, sulvolcanic complexes and volcano-sedimentary units are also found throughout the Batholith. The Kennedy Igneous Association is interpreted to be the off-arc extension of the Carboniferous Connors Volcanic Association arc. Intrusive rocks of this latter age are interpreted to be directly related to gold mineralisation at Ravenswood, Mt Wright and Mt Leyshon, based on Ar-Ar dating of biotite and muscovite, as well as U-Pb dating of zircon (Perkins and Kennedy, 1998; Allan, Morrison and Yardley, 2011; Lisowiec and Morrison, 2017).
Ravenswood
The 424 Ma Late Silurian Jessop Creek Tonalite pluton of the Pama Igneous Association hosts the Ravenswood deposits. The pluton also includes up to 500 m across bodies of gabbro, interpreted to be either roof pendants or large xenoliths. There is an outward zonation from the gabbro bodies into the main phase tonalite, from gabbro → diorite → quartz-diorite → tonalite and granodiorite which also occurs as bodies within the tonalite. Cross-cutting felsic dykes of microgranite, aplite and pegmatite are dated as Late Silurian (U-Pb zircon), suggesting they are the latest phase of the crystallisation sequence. Minor andesite dykes of undetermined age cut the Jessop Creek Tonalite and occur within some of the major structures that were reactivated during the mineralising event (Lisowiec and Morrison, 2017).
Minor albite marks the earliest recognised pre-mineralisation alteration at Ravenswood, followed by more extensive biotite-actinolite-magnetite which predominates in the mafic units and major structures around Sarsfield, the northern half of the NNW-SSE trending main open pit. Epidote-albite-chlorite alteration with minor epidote veining is also widespread throughout the district. Other pre-mineralisation features include steeply dipping to subvertical faults that are often several kilometres long and which typically have a complex history of activity, mostly strike-slip kinematics, as indicated by subhorizontal slickensides and S-C fabrics. These structures have associated chlorite and biotite infill and alteration, and include the east-west Buck Reef Fault that has been traced along the northern margin of the Ravenswood mining area between the Sarsfield and Buck Reef West mining areas; and the Area 4 and Nolans Faults, in the Sarsfield and Nolans areas respectively.
A complex vein network characterises the occurrence of gold mineralisation at Ravenswood. This network is controlled by a combination of reactivated pre-existing structures and a coincident, broadly NW-striking conjugate quartz-sulphide vein array related to an initial WSW-ENE to SW-NE shortening followed by localised extension. Subvertical structures, e.g., the pre-existing east-west Buck Reef and NNW-SSE Area 4 faults, have been subjected to by chlorite and biotite alteration, overprinted by amorphous silica and pyrrhotite. These faults are are often brecciated, suggesting continued fault activity, and are typically mineralised near the intersection with cross-cutting, gently-dipping structures. Mineralisation comprises an assemblage characterised by relict early pyrrhotite with pyrite and marcasite-sphalerite-chalcopyrite. These zones, which can be locally 20 m or more wide, are surrounded by a halo of intense chlorite and biotite alteration, persisting for up to 10 m or more from the structure. Biotite alteration dominates at Sarsfield, with chlorite after biotite more common elsewhere.
Moderate to shallow dipping quartz-sulphide veins, which host the majority of gold in the mineralised system, are found adjacent to and locally overprint the steep, mineralised faults. The structures hosting these veins have often undergone reverse shearing, and are regularly clustered in zones or sets up to 10 to 20 m wide with variable orientations that form lodes. At Sarsfield, the bulk of the veins dip ENE, although several major lodes dip WSW. At Nolans, the southeastern continuation of the Sarsfield pit, the dominant vein sets dip NW or SW, resulting in a NW-SE trending stockwork. At Buck Reef West, 500 m west of the northern section of the Sarsfield pit, the majority dip ENE to ESE, although broadly W-dipping veins are also encountered.
Three main types of mineralised quartz-sulphide vein have been recognised in the network:
i. Pyrite-dominant with only minor to no quartz;
ii. Quartz-dominant with pyrite ±sphalerite-pyrrhotite-chalcopyrite. These veins typically have selvages of fined-grained white mica and chlorite or white mica-carbonate alteration envelopes, which can vary from several millimetres up to 50 cm in width. The white micas immediately adjacent to mineralised veins typically have elevated Fe+Mg contents. Similarly, chlorite is indicated to have higher Fe contents adjacent to major mineralised structures, with intermediate Fe-Mg contents elsewhere;
iii. Carbonate-base metal which is siderite dominant ±quartz-pyritesphalerite-chalcopyrite-arsenopyrite-galena. These paragenetically late carbonate-base metal veins often include clasts of earlier veining, suggesting they may occupy reactivated earlier mineralised structures. These latter veins were the source of much of the historic production, with strike lengths of over 100 m, widths of up to 1 m and grades of >30 m g/t Au.
Whilst these three vein types are distributed throughout the Ravenswood deposit area, pyrite-dominant veins are concentrated around Sarsfield, with quartz-pyrite veins the most abundant and hosting most of the gold. Gold, which is free milling with a fineness ~820 to 920, is typically found along grain boundaries or in fractures in sulphides, and locally has associated native bismuth and bismuth tellurides. Ore-stage fluid inclusions indicates salinities of 7 to 10 wt.% NaCl equivalent, and homogenisation temperature of 200 to 300°C at Buck Reef West, and 300 to 400°C for Sarsfield (Bertelli et al., 2019). Stable isotope compositions suggests a mostly magmatic origin for these mineralising fluids, whilst Ar-Ar analysis of alteration-related muscovite from mineralised veins gave an age of -310 Ma (Perkins and Kennedy, 1998).
A series of post-mineralisation faults are common in the deposit area (e.g., the Jessop Creek Fault), although the degree of displacement is unclear due to the absence of marker units. These structures have a reverse or strike-slip kinematic history, where displacement can be measured, and vary from 0.3 to 5 m in width. They are variably healed, either with poorly consolidated clay-carbonate gouge, or calcite cement and alteration. In the Buck Reef West area, such structures have locally offset the mineralised Buck Reef Fault by as much as 80 m.
Mount Wright
Mineralisation at Mount Wright is hosted by a breccia complex developed in the 480 Ma Ordovician Millaroo Granite of the Macrossan Igneous Association. The granite is cut by NE-trending dolerite dykes that are common throughout the district, interpreted to be of comparable age to the similar 446 Ma Silurian dykes dated elsewhere in the Ravenswood Batholith (Beams, 2017).
The host breccia complex is 250 to 300 m in diameter and persists to a depth of >1200 m. It is the product of successive intrusive and hydrothermal events at ~305 Ma in the Upper Carboniferous (Perkins and Kennedy, 1998). Andesite dykes, which occur around the complex, have been mapped intruding the granite breccia, suggesting they may also be of Carboniferous age (Johnson, Morrison and Lisowiec, 2013).
The first and most extensive phase of the breccia complex is a fine variety composed of disaggregated granite with minor to rare rhyolite clasts. The next phase comprised at least two separate generations of sub-volcanic rhyolite intrusion, the first of which was predominantly as dykes that are preserved on the eastern and northern sides of the complex. The second rhyolite of this second phase, the 'main rhyolite pipe', which became the major host to ore, was intruded with an elongated shape on the western side of the complex. The rhyolite in this pipe is texturally zoned from massive, fine-grained rhyolite with minor quartz phenocrysts in the core → contorted and flow banded varieties with variable zones of fragmental auto-breccia, → more coherent flow-banded rhyolite at the margins. The upper section of the 'main rhyolite pipe' grade into, and is surrounded by, a pipe-shaped body of rhyolite-rich, fine fragmental rock referred to as tuffisite (Johnson, Morrison and Lisowicc, 2013), interpreted to be a magmato-hydrothermal breccia. Further brecciation accompanied the intrusions, producing areas of polymictic, clast-supported breccias composed of subrounded rhyolite and granite clasts, mostly occurring between the rhyolite contact and pre-existing granite breccia. A second, deeper tuffisite, which includes clasts of mineralised rhyolite is interpreted to be late to post-mineralisation.
Mineralisation is interpreted to have been deposited immediately following emplacement of the 'main rhyolite pipe'. The bulk of the gold mineralisation occurs within sulphide veins and as breccia infill within the rhyolite, and to a lesser extent in adjacent zones of tuffisite, polymict granite breccia and competent granite. Economic gold grades of the Main Lode only occur within a zone that is from 150 to 850 m below surface, although sulphide mineralisation extends to the surface. The shallow section of the rhyolite and tuffisite, between the surface to a depth of 350 m, is characterised by carbonate-base metal sulphide mineralisation, comprising sphalerite-galena-pyrite-siderite and quartz. Base metal sulphide grades decrease with depth, with the primary gold bearing ore phases of marcasite-pyrite and pyrite-only becoming the dominant assemblages. The marcasite replaces an earlier pyrrhotite, interpreted to represent a widely distributed, early, pre-mineralisation hydrothermal pulse. The pyrite-only gold ore assemblage is interpreted to reflect mineralisation in areas with no such pre-existing pyrrhotite. The amount of un-replaced pyrrhotite which accompanies the marcasite-pyrite increases with depth, until it becomes more significant at 800 to 850 m below surface, which broadly correlates with diminished gold grades over the same interval.
Gold typically occurs in fractures, as inclusions within marcasite and pyrite, and to a lesser extent within chalcopyrite and tennantite-tetrahedrite. The deep tuffisite also carries economic gold grades, although this is interpreted to be contained within mineralised clasts, consistent with the absence of sulphide veins or sulphide replacement of the tuffisite matrix, while gold grades decrease with increasing distance from the margin of the mineralised rhyolite.
A number of other small pods of mineralisation have been outlined. One of these, the small 'Mother Lode' which was mined between 1992-93, outcrops on the eastern margin of the complex is hosted within granite dominant breccia with carbonate-base-metal sulphide breccia fill. Ore-grade gold mineralisation was confined to an ~30 x 30 m plan area that extended to 80 to 100 m depth.
At depth, adjacent to the 'main rhyolite pipe', other blocks of economic mineralisation have been delineated within localised, possibly structurally controlled shoots, interpreted to be the result of localised extension and fluid decompression, or within the upper carapace of rhyolite dykes, taken to be be the result of entrapment and subsequent precipitation of metals from fluids. The relationship between gold and the marcasite-pyrite-dominant sulphide assemblage is mostly similar to that seen in the 'main rhyolite pipe' at the equivalent depth. Sparse quartz-siderite-basemetal veins also extend up to a kilometre from the complex, and often host high gold grades of >5 g/t Au, although vein widths are typically <20 cm.
Strong white mica-illite-quartz and carbonate alteration is developed throughout the rhyolite of the complex, whilst the granite and granite breccia is typically altered to and assemblage of white mica and chlorite, with minor kaolinite often found along the contact between the rhyolite and granite. At depths of >800 m, minor biotite and magnetite alteration has also been observed adjacent to the margins of the rhyolite. Fluid inclusions indicate variable salinities of 2.9 to 14.8 wt.% equivalent and homogenisation temperatures mostly 400 to 550°C (Bertelli et al., 2009). Alteration muscovite from the main rhyolite intrusion has been dated at 305 Ma (Ar-Ar) in the Upper Carboniferous, suggesting mineralisation and magmatism were essentially coeval (Perkins and Kennedv 1998; Johnson, Morrison and Lisowiec, 2013).
Reserves and Resources
Ore Reserves and Mineral Resources at 30 June, 2016, as quoted by Lisowiec and Morrison (2017) were:
Ravenswood
Measured Mineral Resource - 64.3 Mt @ 0.9 g/t Au;
Indicated Mineral Resource - 69.2 Mt @ 0.5 g/t Au;
Inferred Mineral Resource - 34.6 Mt @ 0.8 g/t Au;
TOTAL Mineral Resources - 168.1 Mt @ 0.7 g/t Au, for 117 t of contained gold;
Proved Ore Reserve - 42.7 Mt @ 0.8 g/t Au;
Probable Ore Reserve - 26.0 Mt @ 07 g/t Au;
TOTAL Ore Reserves - 68.7 Mt @ 0.8 g/t Au, included in resources
Mount Wright
Measured Mineral Resource - 0.8 Mt @ 2.9 g/t Au;
Indicated Mineral Resource - 0.4 Mt @ 3.3 g/t Au;
Inferred Mineral Resource - 1.1 Mt @ 3.1 g/t Au;
TOTAL Mineral Resources - 2.3 Mt @ 3.1 g/t Au, for 7 t of contained gold;
Proved Ore Reserve - 0.7 Mt @ 2.7 g/t Au;
Probable Ore Reserve - 0.2 Mt @ 2.7 g/t Au;
TOTAL Ore Reserves - 0.9 Mt @ 2.7 g/t Au, included in resources.
The information and interpretations in this summary are drawn from Lisowiec and Morrison (2017).
The most recent source geological information used to prepare this decription was dated: 2017.
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.
Ravenswood Mount Wright
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Derham, D.J., Chang, Z. and Lisowiec, N., 2014 - Geology of the Buck Reef West Au deposit, Ravenswood district, Queensland, Australia: in SEG 2014, Building Exploration Capability for the 21st Century, Keystone, Colorado, 28 to 30 September, 2014, Proceedings, 2p.
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Lisowiec, N. and Morrison, G., 2017 - Ravenswood and Mount Wright gold deposits: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy, Mono 32, pp. 705-710.
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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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