Queensland, Qld, Australia
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The Mount Carlton high-sulphidation epithermal gold deposit located ~150 km south of Townsville, in the northern segment of the Bowen Basin, in northeast Queensland, Australia. In 2018, mining is by open-pit at the Au-rich V2 pit in the NE and the Ag-rich A39 pit in the SW.
The NNW elongated Bowen Basin is an asymmetric sedimentary basin covering an area of ~0.2 million km2, formed as part of the Early Permian to Middle Triassic New England Orogen, and is the northern part of a larger basin system that also includes the Gunnedah and Sydney basins to the south in New South Wales (Donchak et al., 2013). It was initiated in the Early Permian by extension and rifting of the back-arc continental crust inland of the Connors arc near the current Pacific coast (Esterle et al., 2002; Korsch et al., 2009), resulting in a series of isolated NNW trending graben and half-graben basins that were infilled by volcanic and sedimentary rocks (Murray, 1990; Hutton et al., 1999; Esterle et al., 2002; Korsch et al., 2009). This extensional stage was accompanied by deposition of the Lizzie Creek Volcanic Group, a succession of calc-alkaline, andesitic to rhyolitic volcanic rocks and minor terrestrial sediments, which host most of the precious and base metal mineralisation of the northern Bowen Basin (Paine et al., 1974). Mineralisation at Mt. Carlton occurred during this event, partly contemporaneously with the deposition of Lizzie Creek Volcanic Group in localised half-graben and graben basins.
The back-arc extension and rifting was succeeded by an episode of Mid Permian thermal relaxation and subsidence which led to flooding of the Bowen Basin and deposition of marine and coastal plain sedimentary units (Malone et al., 1969; Esterle et al., 2002; Allen and Fielding, 2007; Korsch and Totterdell, 2009).
In the Late Permian, the thermal relaxation stage was abruptly terminated by the onset of the ~265 to 235 Ma Hunter-Bowen Orogeny (Donchak et al., 2013), leading to foreland loading, tectonic inversion and development of a foreland basin which was filled by terrestrial sediment rocks that make up the bulk of the Bowen Basin infill, and include extensive coal deposits (Fielding et al., 1990; Fergusson, 1991; Holcombe et al., 1997; Esterle et al., 2002).
The basement at Mount Carlton is a fine-to medium-grained monzogranite belonging to the ~302 to 296 Ma Urannah batholith which contains several high-temperature, I-type granites (Allen et al., 1998; Donchak et al., 2013). This is overlain by a volcanosedimentary sequence of the ~288 to 275 Ma Lizzie Creek Volcanic Group, designated units 2 to 8 inclusive, as follows:
Unit 2 - the basal unit of the Lizzie Creek sequence, which is an ~300 m thick andesite that comprises fine-grained plagioclase-pyroxene ±hornblende phyric andesite, with minor monomict autoclastic andesite breccia.
Unit 3 - up to 200 m of massive and locally flow banded quartz-feldspar phyric rhyodacite, with minor monomict autoclastic quartz-feldspar phyric rhyodacite breccia. This unit was affected by silicic and quartz-alunite alteration and hosts mineralisation in the Mount Carlton V2 pit.
Unit 4 - up to 100 m of dacitic to rhyodacitic tuffs and sedimentary rocks that has has been subdivided into a lower unit 4A, which comprises well-bedded, fragmental rhyodacite lapilli tuffs with interbedded carbonaceous lacustrine sediments, locally containing fossilised wood; and an upper unit 4B which is composed of massively bedded dacitic tuffs.
Unit 5 - up to 150 m of dacitic and andesitic volcaniclastic rocks which have a variety of facies, which include dacitic ignimbrite of unit 5A, coarse volcanic conglomerate of unit 5B and fragmental dacite breccia of unit 5C.
Unit 6 - which in the open pit area is an up to 50 m thick unit of fragmental andesite that locally contains rounded boulders.
Unit 7 - up to 150 m of volcanosedimentary rocks that comprise andesitic to dacitic volcaniclastic breccias and volcanic sediments. The latter include water-laid, graded sandstone-siltstone beds, coal-bearing laminated mudstones with tuffaceous interbeds, and moderately to well-sorted, monomict to polymict fluvial conglomerates that locally contain clasts of hydrothermally altered granite and volcanic rocks. Porphyritic andesitic to dacitic lavas are locally found within this unit in the northwest wall of the A39 pit.
Unit 8 - which comprises flow-banded rhyolites that overlie units 1 to 7 throughout the district (Coughlin, 1995). These rocks contain aligned phenocrysts of lath-shaped plagioclase and alkali feldspar with rare quartz, within an aphanitic, burgundy-red groundmass. It is exposed as a semihorizontal sheet along the hilltops to the south of the open pits.
Unit 9 - is a younger, 25 m wide dacitic to rhyodacitic volcanic vent that occurs locally and crosscuts unit 3. It contains at least two distinct vent facies, including porphyritic, rhyodacitic lava with well-developed columnar jointing in the centre and weakly layered, boulder-rich, tuffaceous, dacitic rocks along the margins.
The Mount Carlton deposit has undergone seven stages of extensional deformation and dyke emplacement, with no evidence for compression having been observed. The spatial distribution of the stratigraphy, hydrothermal alteration and mineralisation is intimately linked to this deformation sequence, which may be summarised as follows:
D1 - which involved rifting and high-angle normal faulting in response to both east-west and north-south extension. The rifting was initiated during deposition of unit 2, was most intense during deposition of units 3 and 4, and waned during deposition of units 5 to 8. The D1 normal faults resulted in displacements of tens of metres and include synsedimentary growth faults in localised half-graben and graben basins. Locally, these half-grabens were filled with Lizzie Creek volcanic rocks that are bounded by D1 faults that also host mineralisation. Hydrothermal enargite, pyrite and dickite grew in these faults have mineral lineations that record a normal component of shear. Hydrothermal alteration and epithermal mineralisation are therefore interpreted to have been partly contemporaneously with rifting and deposition of volcanic sediments during earlier stages of D1, with rifting and sedimentation lasting longer than mineralisation.
D2 - corresponds to a continuation of east-west extension, and resulted in development of 1 to 5 m wide, low-angle, locally layer-parallel, fault zones and associated, high-angle antithetic normal faults. Within the pit area, the low-angle faults accommodated eastward vergent displacement along a broadly east-west axis, that has truncated the stratigraphy, the hydrothermal alteration halo, and the ore zones. Major through going D2 structures, with displacements potentially of hundreds of metres, appear to be restricted to unit 6 and above that blanket the regional D1 horst and graben topography. Where low-angle D2 faults cut the lower parts of the stratigraphy, including the mineralised units 3 and 4A, the faults are relatively narrow (i.e., <0.8 m), and displacement is smaller and more localised. Unit 9, crosscuts D2 faults.
D3 - involved high-angle normal faulting in response to north-south extension, with partial reactivation of D1 and D2 faults. Based on facies distribution, the overall sense of D3 displacement was probably south-down, resulting in a shallow, southerly tilt of the layering.
D4 - caused block rotation of kilometre-scale lithological domains across steep, NNW trending normal faults and ENE trending cross faults. One of these NNW trending D4 normal fault cuts the entire stratigraphic pile and passes between the main V2 and A39 pits, segmenting the stratigraphy and the ore zones within the deposit, as well as a reorienting primary layering and mineralisation. Whilst bedding in the northeast fault block, which includes the V2 pit, have remained near horizontal after the D4 event, in the southwest fault block, which includes the A39 pit, bedding has been rotated in a WSW direction by ~32°.
D5 - involves emplacement of basaltic dykes along high-angle D1, D3 and D4 faults and, to a lesser degree, along low angle D2 structures. These D5 dykes contain fine-grained plagioclase, pyroxene and hornblende phenocrysts in a tan to dark green groundmass, showing a distinct 'salt and pepper' textures.
D6 - which involved dominantly dextral strike-slip faulting along the margins of D5 dykes, although some sinistral movement has also been observed. Displacement in the pit area was minor, generally of <20 m.
D7 - comprised emplacement of WNW trending basaltic dykes, comprising a black aphanitic groundmass containing medium-grained plagioclase phenocrysts and quartz amygdales.
Five distinct alteration zones have been recognised at Mount Carlton:
• Silicic alteration zone - which occurs as multiple 10 to 100 m wide cores that were formed in and around D1 high-angle structures within units 3 and 4A. The cores are characterised by virtually complete replacement of the protolith by microcrystalline quartz, producing a very hard residual rock in which primary volcanic and sedimentary textures have been obliterated, other than primary quartz phenocrysts. The texture is predominantly massive, with some local vuggy textures and silicic hydrothermal breccias. The silicic zones contain minor alunite, pyrite, dickite, kaolinite, aluminum-phosphate-sulphate (APS) minerals and pyrophyllite. Pyrite is disseminated in the quartz groundmass, while sulphate and clay minerals are mainly found in vugs or as hydrothermal breccia infill. Aggregates of tabular anhydrite are developed within small fractures, although most anhydrite has been dissolved, leaving a prominent cast texture in the altered rock. Local alunite veins, which are up to ~20 cm wide, contain banded, plumose alunite without sulphides. Some are synchronous with silicic alteration, whereas others postdate it. Many were reopened by later mineralised veins. Alunite veins are largely concentrated in the V2 pit, although they have locally also been observed in the A39 pit.
• Quartz-alunite alteration zone - which defines a ~100 to 300 m wide gradational envelope to the silicic alteration zones in units 3 and 4A. This alteration is not as intensely developed as the silicic alteration, with primary rock textures generally preserved. A groundmass of microcrystalline quartz contains disseminated alunite with locally developed pyrite, dickite and kaolinite, and, more rarely, barite and rutile. Alunite and clay minerals have replaced preexisting feldspar crystals.
• Quartz-dickite-kaolinite alteration zone - occurs as a laterally extensive (>1 km wide) halo to the silicic and quartz-alunite zones within units 3 and 4A. It is a whitish- to grey-coloured rock that typically has primary volcanic and sedimentary textures preserved. It comprises a groundmass of microcrystalline quartz containing disseminated dickite, kaolinite and locally pyrite, with the former two minerals also replacing preexisting feldspar crystals. The quantity of quartz declines away from the controlling structures, with distal dickite-kaolinite altered rocks being relatively soft and friable. Kaolinite-pyrite alteration also locally occurs within unit 9.
• Illite-montmorillonite alteration zone - which forms a zoned alteration halo within unit 3 and unit 4A, grading from silicic → quartz-alunite → quartz-dickite-kaolinite, with sharp transitions across D2 sheared contacts to the overlying and underlying units 2, 5, and 6 and section of unit 4. This alteration is pervasive in rocks above and below the fault-bounded ore zones. The swelling properties of the clay minerals in these peripheral rocks make them very friable and poorly preserved. In addition to illite and montmorillonite, they locally have red hematite dusting. Gypsum veins that are syntectonic, with well-developed shear fabrics are found near the major D2 faults within this alteration zone.
• Chlorite-illite alteration zone - which is found below a sharp downward transition from the illite-montmorillonite alteration zone, controlled by a D2 fault along the contact between unit 2 and the granite basement. It occurs as green veinlets that crosscut the primary granitic texture, with chlorite and illite having also replaced preexisting feldspar, hornblende and biotite crystals.
The outward zoned alteration halo found in units 3 and 4A that involves silicic → quartz-alunite → quartz-dickite-kaolinite is interpreted to be directly linked to the mineralising hydrothermal event at Mt. Carlton (e.g., Steven and Ratté, 1960; Stoffregen, 1987; Arribas, 1995; Hedenquist et al., 2000). However, the illite-montmorillonite alteration is partially developed in units deposited after the introduction of mineralisation and has no obvious zonal relationship to the other assemblages. Sahlström et al. (2018) interpret this alteration assemblage to have developed regionally during D2 deformation. The presence of pyrite and kaolinite in unit 9 implies a hypogene origin for kaolinite alteration, but because unit 9 crosscuts both the primary hydrothermal alteration halo and D2 faults in unit 3, a later hydrothermal alteration event is inferred, confined to within and proximal to unit 9.
Within the V2 pit, Au-Cu mineralisation occurs in moderately to steeply, ~60 to 90° dipping, predominantly NE to NNE trending D1 fracture networks cutting within the rhyodacite porphyry of unit 3, with NW to east-west trending fractures being secondary. Hydrothermal breccias are locally developed along intersection between mineralised fractures and are infilled with ore minerals. These features are evident on the 1 to 10 m scale.
On a scale of hundreds of metres, there are three distinct ore zones in the V2 pit, the Western, Eastern and Link ore zones that are aligned in an en echelon pattern along a generally east-west trending corridor within SW trending grade envelopes. The Eastern and Link ore zones are entirely within the V2 pit, whilst the Western ore zone extends to the SW and is traceable across a D4 fault into the A39 pit. Along its ~600 m mineralised strike length, the Western ore zone has a distinct metal zonation, from proximal to distal, of Au-Cu → Cu-Zn-Pb-Ag → Ag-Pb-(Cu) → Ag. The distal Ag mineralisation lies within the A39 pit, where it is hosted by the volcanolacustrine sedimentary rocks of unit 4A that overlie the rhyodacite porphyry. Mineralisation in A39 occurs in a stratabound position oriented parallel to primary sedimentary layering and locally exhibits synsedimentary ore textures. Mineralisation in A39 is distributed parallel to the lineation defined by the intersection of the sedimentary bedding planes of unit 4A and the mineralised feeder structures in the underlying unit 3.
A NNW trending D4 normal fault positioned between the two open pits dismembers both the high-grade ore and its hydrothermal alteration halo of the Western ore zone. The rotation of the SW fault block containing the A39 mineralisation across this fault results in the originally subhorizontal Ag ore in the latter plunging to the SW, whilst the feeder system in V2 in the little rotated NE block on the opposite side of the fault is essentially in its original orientation.
The zoned hydrothermal alteration assemblages in units 3 and 4A, Stage 1, are crosscut by three stages of epithermal mineralisation, Stages 2A, 2B and 2C. The resultant ores were, in turn, overprinted by a late stage of dickite and pyrite void fill, Stage 3. Following D2 deformation and the associated Stage 4 alteration, a final Stage 5 supergene oxidation locally overprinted the mineralised rocks. The paragenetic sequence in V2 is identical to that in A39, and hence Sahlström et al. (2018) infer the subsequently segmented Western ore zone originally formed as a single hydrothermal system. Both stage 2A and stage 2B mineralisation contain high grade Au, locally assaying >600 g/t Au and Ag, as well as variable amounts of base metals (e.g., Cu, Zn, and Pb) and rare metals (e.g., Ge, Ga, In, Te, Se, Sn). The deposit-scale metal zonation is the result, in part, of the temporal zonation of metals through stages 2A to C inclusive, and in part by the spatial zonation of metals within each individual stage, as follows:
• Stage 2A - Cu-Au-Ag mineralisation - the initial and most voluminous mineralising event, which was developed in all three ore zones, and produced a high sulphidation mineral assemblage dominated by massive enargite. Mineralisation was associated with silicic alteration of the wall rocks. Crystallisation of the Stage 2A assemblage commenced with the tetrahedrite-group minerals which occur as up to ~100 µm subhedral crystals, overprinted by silver minerals, which include members of the pearceite-polybasite group. These were, in turn, overprinted by the main enargite assemblage, which includes pyrite and luzonite and lesser electrum, sphalerite, galena, chalcopyrite, bornite and argyrodite. Electrum is found as up to ~100 µm anhedral to subhedral grains intergrown with massive enargite and quartz. Barite is a common gangue, and it is predominantly concentrated in the distal parts of the deposit in the A39 pit.
• Stage 2B - Zn-Pb-Au-Ag mineralisation - mainly occurs in veins that cross cut stage 2A mineralisation, predominantly within the Western ore zone. Textures of these veins vary from massive to colloform, whilst the assemblages are dominated by Fe-poor sphalerite. Subordinate minerals include galena and pyrite and, to a lesser degree, electrum, tetrahedrite-group, chalcopyrite, bornite, a Zn-In mineral and barite. Electrum occurs as up to ~75 µm anhedral grains intergrown with massive sphalerite, pyrite and bornite. Tetrahedrite group minerals are present in two forms, i). up to ~10 µm segregations within galena; and ii). up to ~100 µm subhedral crystals intergrown with pyrite, sphalerite and galena.
• Stage 2C - Cu-Au-Ag mineralisation - which was a minor event overprinting Stage 2B mineralisation, and has only been seen at a microscopic scale within the Western ore zone. The assemblage is dominated by massive tennantite, which hosts a variety of subordinate minerals, including luzonite, chalcopyrite and galena, and, to a lesser degree, electrum and aggregates of intergrown hessite and petzite.
• Stage 3 - Hydrothermal void fill - which comprises a widely distributed late stage of dickite and pyrite that has been observed within all three ore zones. These minerals generally occur in massive veins or as void fill that overprinted all of the previous mineralisation.
• Stage 5 - Supergene oxidation - which is only irregular distributed and is not strongly developed, only affecting the upper ~50 m of the preserved deposit. The discovery outcrop of the Mount Carlton deposit (at Silver Hill) was a gossan developed on Stage 2A veins of sulphosalt mineralisation containing over 1000 g/t Ag. Secondary covellite, chalcocite and malachite are locally found on minerals such as enargite, luzonite and bornite, particularly along fractures within the hypogene minerals. Supergene oxidation is limited, only affecting ~1 to 5 % of the total ore.
40Ar/39Ar dating of hydrothermal alunite yielded an age range of 284±7 to 277±7 Ma, which links the formation of the Mount Carlton deposit to the Early Permian back-arc rift stage in the Bowen Basin.
Reserves and Resources
Published Ore Reserves and Mineral Resources at 31 December, 2017 (Evolution Mining website, viewed April, 2019) were:
Measured resource - 0.59 Mt @ 3.65 g/t Au;
Indicated resource - 10.57 Mt @ 2.60 g/t Au;
Inferred resource - 0.73 Mt @ 4.90 g/t Au;
TOTAL resource - 11.89@ 2.76 g/t Au for a total of 32.85 t of contained gold.
Proved + Probable reserve - 4.50 Mt @ 4.92 g/t Au;
NOTE: Minerals Resources are inclusive of Ore Reserves.
The information in this summary is drawn from Sahlström et al., 2018 parts I and II.
The most recent source geological information used to prepare this summary was dated: 2018.
This description is a summary from published sources, the chief of which are listed below.
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Sahlstrom, F., Arribas, A., Dirks, P., Corral, I. and Chang, Z., 2017 - Mineralogical Distribution of Germanium, Gallium and Indium at the Mt Carlton High-Sulfidation Epithermal Deposit, NE Australia, and Comparison with Similar DepositsWorldwide: in Minerals (MDPI) v.7, 28p. doi.org/10.3390/min7110213 .|
Sahlstrom, F., Chang, Z., Arribas, A., Dirks, P., Johnson, C.A., Huizenga, J.M. and Corral, I., 2020 - Reconstruction of an Early Permian, Sublacustrine Magmatic-Hydrothermal System: Mount Carlton Epithermal Au-Ag-Cu Deposit, Northeastern Australia: in Econ. Geol. v.115, pp. 129-152. |
Sahlstrom, F., Dirks, P., Chang, Z., Arribas, A., Corral, I., Obiri-Yeboah, M. and Hall, C., 2018 - The Paleozoic Mount Carlton Deposit, Bowen Basin, Northeast Australia: Shallow High-Sulfidation Epithermal Au-Ag-Cu Mineralization Formed During Rifting: in Econ. Geol. v.113, pp. 1733-1767.|
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