Queensland, Qld, Australia
Super Porphyry Cu and Au|
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The Mount Dore copper-gold deposits are located ~110 km south of Cloncurry in the Eastern Succession of the Mount Isa Inlier in North-west Queensland, Australia (#Location: 21° 39' 43"S, 140° 29' 32"E).
The Mount Dore deposit is part of the Starra/Selwyn cluster of deposits, while the high grade Merlin Mo-Re deposit comprises some of the footwall lenses of the Mt Dore resources. For details of the regional and local setting, see the Cloncurry IOCG Province, the composite Starra-Selwyn (Mt Dore, Merlin) and the Merlin records.
The copper and molybdenum-rhenium mineralisation at Mount Dore is hosted within a reduced sequence of variably faulted black shales/carbonaceous meta-pelites, grey meta-siltstones with thicker beds of phyllite and schist of the Palaeoproterozoic Kuridala Formation in the Eastern Fold Belt of the Mount Isa Inlier. This suite of rocks occurs within a 250 m thick reverse fault zone known as the Mount Dore Fault Zone (Beardsmore 1992), likely to be a reactivated D1 thrust.
This succession lies to the west of the over-thrust Mount Dore Granite, and occurs over north-south strike length of several kilometres, and dips eastward beneath the granite. The footwall to the deposits comprises a massive, easterly-dipping, intensely-silicified quartzite ridge on the western side of the area.
Copper and Mo-Re mineralisation is hosted within a variable suite of interfingering black carbonaceous and grey micaceous metasiltstone and grey metashales, including with thicker beds of phyllite and schist. These metasedimentary facies have recrystallisation textures, although sedimentary features such as bedding and folding are evident in outcrop. The principal lithologies may be summarise as follows:
• Hanging Wall Granite - which is the Mount Dore Granite, part of the extensive Williams-Naraku Batholith. It dominates the eastern side of the deposit area, forming the barren hanging wall to the Cu and Mo mineralisation, conceals the bulk of the ore body.
• Metasiltstone is composed of recrystallised quartz grains with a micro mosaic texture and occasional incipient K feldspar grain of hydrothermal origin. A variety of the metasiltstone has a 'slightly silky sheen' due to the presence of fine white mica, and is probably derived from a clayey or muddy protolith. The metasiltstone is the dominant unit within in the metamorphic sequence.
• Black Shale, a carbonaceous metashales consisting of extremely fine-grained oriented white mica, with the dark colour attributed to the presence of abundant (but generally <5%) ultrafine graphite along laminations. This unit is predominantly found within the hanging wall of the mineralisation, structurally below the Mount Dore Granite. However, it also locally a host rock to mineralisation, as well as occurring in the footwall. It bifurcates into at least three discrete lithological units above the mineralisation to the north, and is interpreted to be offset down dip by vertical faults.
• Phyllite predominantly occurs as a single unit that broadly follows the geometry of the bounding granite and quartzite. Mineralisation can be found within the phyllite, although it also occurs in the footwall in the south of the deposit.
• Quartzite (SQT) occurs to the west of the deposit, and is the footwall unit of the Kuridala Formation. It forms a narrow (<40 m true thickness), linear, north-south trending, east-dipping, massive, intensely silicified 'quartzite' ridge with little internal texture. It may define the Mount Dore Fault, serving as the boundary between the host metasedimentary package of the Kuridala Formation and the underlying siltstone and shale units of the Staveley Formation.
A typical section shows a footwall sequence of siltstone of the Staveley Formation, followed by the ~40 m thick quartzite band. This is followed by a siltstone unit that thickens markedly down dip, from <40 m near surface, to >150 m at 500 m down dip, with an increasing number of interbedded carbonaceous metashale lenses. The main siltstone is overlain by ~20 m of carbonaceous metashale, followed by ~160 m of phyllite to schist, enclosing 10 to 20 m thick lenses of carbonaceous metashale, separated by a reverse fault from the hanging wall of Mount Dore Granite.
At a depth of ~100 m, the uppermost Mo-Re ore zone appears at the contact between the main carbonaceous metashale and the phyllite, with new lenses forming down dip at progressively lower stratigraphic positions, to form three tabular bodies from a depth of ~280 to 540 m.
The suite of mainly pelitic rocks hosting the Mount Dore Deposit has been subjected to intense deformation confined between the competent hanging wall granite and quartzite footwall, thus generating a zone of confined high strain. This has resulted in uneven strain partitioning, enhancement of brittle brecciation and fracturing within the black shales/carbonaceous meta-pelites. In contrast, the meta-pelites and phyllites bounding the black shales have accommodated strain in a more ductile fashion resulting in less structurally generated permeable space for mineralising fluids. Consequently, the black shales/carbonaceous meta-pelites host the bulk of the mineralisation. Primary sulphide mineralisation was largely deposited within thrust-induced, shallow dipping sections of the normally moderately dipping black shales.
Mineralisation is largely located within the short limb of an open F3 antiform-synform pair. This fold plunges at 20 to 30°NNE and is well exposed in outcrop (Marjoribanks 2008). The Mount Dore folds form part of a complex series of north-south faults, juxtaposing and truncating much of the Kuridala Formation and a portion of the Staveley Formation close to the Mount Dore quartzite. It is likely that mineralising fluids migrated up these faults and the footwall thrust below the Mount Dore Granite.
Pal et al. (2010) divided the deposit scale structures 4 main components. The mineralised zone is apparently part of a thrust duplex formed during the late D2 to syn D3 stage of deformation which moved the Mount Dore granite to the WNW to overlie the Kuridala Formation. The four main components of this duplex are:
• A roof thrust, generally at dipping 45°E, marking the base of the granite.
• A sole thrust, that has been ramped upward to be located within the 60°E-dipping schistose and phyllitic rocks of the older Mount Dore ductile shear.
• An imbricate zone of steeply dipping reverse faults, sandwiched between the bounding thrusts. These reverse faults flatten into the bounding thrusts above and below, giving them a sigmoidal shape in cross section. Locally some of the steep reverse faults marginally displace the granite contact, altjhough this is regarded as probably being the result of a later reactivation.
• Northwest verging overturned folds confined between the bounding thrusts.
The Mount Dore Copper deposit comprises the first vertical 200 to 250 m of the open pitable and leachable Cu mineralisation.
The mineralisation at Mount Dore on which this deposit is base, is polymetallic, hosted within within the Kuridala Formation, containing Cu, Zn, Ag, Au, Pb and Co, and Mo with rhenium in the Merlin zone. The near surface mineralisation has been extensively oxidised, with the copper rich zones outcropping as copper oxides above a thick zone of supergene chalcocite mineralisation. The hypogene mineralisation comprises chalcopyrite and sphalerite with minor to trace pyrite, galena, bornite, molybdenite and arsenopyrite at depth (Kirkby, 2009). Chalcocite extends into the primary zone and may also be of hypogene origin.
The mineralisation occurs as an up to 180 m true thick body which dips to the east beneath the structurally emplace Mount Dore Granite body.
The supergene copper sulphide and copper oxide minerals, as well as native copper, overlaps the hypogene sulphide assemblage to form a mixed ore zone. Supergene processes have converted the primary copper and other sulphides to an assemblage of dominantly chalcocite followed by further oxidation to produce chrysocolla, native copper, cuprite and pseudomalachite.
The bulk of the known Cu mineralisation comprises supergene Cu oxides and carbonates (chrysocolla, cuprite, chalcotrichite, pseudomalachite, minor to trace azurite and malachite) and native Cu after chalcocite, developed to a depth of ~100 m. Secondaries Cu minerals are often deposited in anastomosing fracture networks by the meteoric fluids.
This oxide zone is underlain by a 40 to 50 m thick transition zone dominated by chalcocite (replacing pyrite, chalcopyrite, and sphalerite) and trace covellite (Lazo and Pal, 2009). Both the oxides and native Cu penetrate deeper into the transition zone within major shears and fault zones. Hypogene Cu mineralisation is hosted within breccias and fractures that were best developed in the meta-siltstones and black shales and are only weakly developed in the schists and phyllites.
The Supergene zone extends to a depth of 400m with considerable variation due to weathering along deeply incised faults.
Two main episodes of hypogene Cu mineralisation have been outlined: i). an earlier chalcopyritepyrite-sphalerite-bornite assemblage emplaced into brecciated metasiltstone and black shale with associated K feldspar±quartz, and ii). a later dolomite-hosted breccia with chalcopyrite-pyrite sphalerite. Trace to minor galena, cobaltite, arsenopyrite and molybdenite occur in the primary sulphide zone. Both types of hypogene sulphide are the source of the supergene enrichment assemblages, following the unroofing of the granite cover by erosion. Little gossan was developed.
The most common hypogene mineralisation style is breccia matrix fill. The sulphides, carbonates and silicates commonly occur in the matrix of the breccias and in some cases in the clasts. In many cases sulphide-rich network of veins result in an apparent crackle-brecciated texture. Secondary Cu minerals tend to form as fracture fill and veins, while primary sulphides typically occur as:
• Matrix fill breccias which locally crosscut the meta-sediments at variable depths, more commonly found towards the eastern part of the deposit area. Pyrite and sphalerite, as well as weak to moderate gold accompany the copper mineralisation. The breccia is usually monomictic and is supported in a matrix that comprises mainly light pink coloured dolomite. Sulphides are either disseminated in the matrix or sometimes occur as clasts within the breccia.
• Veins, mainly K feldspar and carbonate-rich, which usually contain sulphides, and are commonly found in the meta-sediments. These veins commonly have associated selvages in which the host metasediments are altered to chloritic/albitic to K feldspar. Carbonate veins with sulphides are sometimes brecciated with angular clasts of the host rocks along with the sulphides. Veins are usually on the cm scale.
• Massive sulphide mineralisation, essentially comprising pyrite and chalcopyrite. These zones are rare at Mt Dore, and are more often found at depth.
• Disseminated sulphides, commonly pyrite and rarely chalcopyrite, can occur in the host phyllites and black shales, but do not significantly contribute to the grade.
Mineralisation is broadly confined to six sub parallel lenses that strike ~north-south with stratigraphy. Lens 5 hosts the bulk of mineralisation that forms the leachable resource and is dominated by matrix fill secondary copper. Copper lenses have a strong spatial association with the footwall metasediments such as calc silicates and blacks shales. The basal highly silicified meta-shales, calc-silicates and quartzite appear to have acted as a barrier for mineralisation. This has resulted in an accumulation of metals in favourable structures just above the SQT in close proximity to the Staveley-Kuridala contact.
The overall Mount Dore mineralised zone is developed over a 2.5 km long north-south zone (from 7 606 500 m N to 7 604 000 m N) that is generally 500 m wide in plan. Hypogene polymetallic mineralisation occurs over much of this area.
High grade molybdenum, which includes the Merlin zone and Little Wizard, extends over a strike length of just under 1 km, (from 7 606 000 mN to 7 605 100 mN) and width of ~350 m in the central to northern section of the mineralised zone.
Mount Dore South comprises a zone of deeply weathered copper dominated mineralisation has been targeted for near surface copper oxide material suitable for leach extraction (south of 7 605 100 m N to 7 604 000 mN), and constitutes the Mount Dore leachable copper resource.
Mount Dore North mineralisation (north of 7 605 000 mN) is less deeply weathered but overlain by a depleted zone, such that the copper does not come near surface and has only been defined by more recent extension drilling. Polymetallic mineralisation with notable Zn mineralisation occurs in the hypogene zone.
A pre-mining JORC compliant resource estimate reported by Chinova in September 2014 @ 0.25% Cu Cut off was:
Mount Dore South
Indicated resource - 43.3 Mt @ 0.69% Cu, 0.08 g/t Au, 0.01% Pb, 0.08% Zn;
Inferred resource - 10.7 Mt @ 0.51% Cu, 0.10 g/t Au, 0.02% Pb, 0.17% Zn;
Sub-total - 54.0 Mt @ 0.66% Cu, 0.08 g/t Au, 0.01% Pb, 0.10% Zn;
Mount Dore North Upper
Indicated resource - 25.1 Mt @ 0.43% Cu, 0.10 g/t Au, 0.09% Pb, 0.47% Zn;
Inferred resource - 32.0 Mt @ 0.41% Cu, 0.11 g/t Au, 0.11% Pb, 0.56% Zn;
Sub-total - 57.1 Mt @ 0.42% Cu, 0.11 g/t Au, 0.10% Pb, 0.52% Zn;
TOTAL - 111.2 Mt @ 0.53% Cu, 0.09 g/t Au, 0.06% Pb, 0.31% Zn;
This summary is drawn from "McCarthy, P., Horton, J. and Miller, G., 2011 - Mount Dore copper heap leach project preliminary economic assessment north west Queensland, Australia; an NI 43-101 Technical report, prepared by AMC Consultants Pty Ltd and Golder Associates for Ivanhoe Australia Limited, 172p." and
"Chinova Resources, 2014 - Mount Dore Copper Deposit, Resource Estimate as at September, 2014; A Technical Report prepared by Chinova Resources, 110p."
The most recent source geological information used to prepare this summary was dated: 2014.
Record last updated: 29/8/2016
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.
Duncan RJ, Stein HJ, Evans KA, Hitzman MW, Nelson EP and Kirwin DJ, 2011 - A New Geochronological Framework for Mineralization and Alteration in the Selwyn-Mount Dore Corridor, Eastern Fold Belt, Mount Isa Inlier, Australia: Genetic Implications for Iron Oxide Copper-Gold Deposits : in Econ. Geol. v106 pp. 169-192|
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