Queensland, Qld, Australia

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The Eloise copper-silver deposit is located ~60 km southeast of Cloncurry in North-west Queensland, Australia, and is interpreted to represent a sulphide rich variety of IOCG style mineralisation (#Location: 20° 57' 18"S, 140° 58' 44"E).

It occurs within the Mesoproterozoic Eastern Fold Belt - the Cloncurry Terrane - of the Mt Isa Inlier. The Eastern Fold Belt (EFB) comprise variably metamorphosed, altered, and deformed sedimentary and igneous rocks of Palaeo- to Mesoproterozoic age deposited in rift basins, the depocentres of which moved progressively eastward with time, as did the centres of magmatism. These rocks can be broadly divided into two sequences, Cover Sequences 2 and 3 (CS2 and CS3), based on lithofacies and age.   CS2 and CS3, were deposited between 1790 and 1690 Ma and from 1680 to 1610 Ma respectively. CS2 includes a rift fill succession commencing with predominantly clastic sediments, overlain by both felsic and basaltic volcanics with siltstones, sandstones and quartzites, which are all succeeded by the laterally extensive platformal evaporitic carbonates (with minor volcanic, clastic and jaspilitic rocks) of the Corella and Doherty formations. The sequence was extensively intruded by the 1750 to 1730 Ma Wonga Granite and the coeval Mount Fort Constantine volcanics. CS3, which extends much further to the east than does CS2, is composed of a thick, extensive succession of quartzites, pelites, volcanic rocks and carbonates. In the Cloncurry District, it is divided into a thick eastern clastic and thinner western sequence characterised by carbonates, separated by a major north-south structure. Deposition of CS3 in the EFB was terminated by the onset of the Isan Orogeny at ~1600 Ma, which was dominated by east-west compression and persisted until ~1500 Ma.

In the region surrounding Eloise, the host sequence has been divided into metamorphosed siliciclastic and basic volcanic rocks of the Soldiers Cap Group and Fullerton River Group (which comprise the informally named Maronan supergroup; Beardsmore et al., 1988). The Soldiers Cap Group, which hosts the Eloise deposit, comprises three units, namely the Toole Creek volcanics (amphibolite and schist), the Mount Norna Quartzite (meta-arkose, schist, quartzite, and amphibolite), and the Llewellyn Creek Formation (psammite and schist).

Granites occupy a large area of the Eastern fold belt, the majority of which were emplaced during the Isan orogeny (ca. 1540 to 1500 Ma; Page, 1994; Perkins and Wyborn, 1998). Granite emplacement was coeval with the widespread metasomatism which was concentrated along major evolving ductile-brittle to brittle structures, including the Cloncurry and Mount Dore faults, to produce regional early pervasive Na-Fe (albite-magnetite) and Na-Ca±Mg (albite-actinolite-magnetite-titanite±clinopyroxene) metasomatism, overprinted by later K-feldspar-quartz±chalcedony localised in brittle structures.

The host sequence is concealed below some 50 to 70 m of flat lying mudstone and unconsolidated Mesozoic sediments. Mineralisation is hosted within a steeply dipping, strongly foliated Proterozoic meta-sedimentary sequence of interlayered meta-arkose and quartz-biotite-schist, which also contains a coarse-grained amphibolite body possibly representing an early intrusion of gabbroic composition, and is cut by faults of various ages. The meta-sediments are typical of the Soldiers Cap Group. At the southern end of the deposit, the host rocks are characterised by the presence of magnetite schists and magnetite carbonate rock.

The Eloise deposit comprises a number of steeply plunging, structurally complex mineralised zones and is characterised by very high grade chalcopyrite-pyrrhotite rich mineralisation hosted by mafic silicate alteration. Alteration and mineralisation are found within and adjacent to the major Levuka shear zone and a series of secondary shears, including the Southern, Scrubby Creek and Eloise shears. The deposit is characterised by a lack of magnetite, and an abundance instead of pyrrhotite. Baker and Ling (1998) interpreted the main metasomatism and mineralisation to have been coincident with the D3 ductile-brittle deformation. Ar/Ar dating suggest mineralisation took place between 1530 and 1514 Ma (Baker et al., 2001). Fluid inclusion data from Eloise is taken to suggest that high salinity mineralising fluids from a magmatic source, tapped by deep structures, evolved from high-temperature brines through to cooler, lower salinity fluids associated with the main stage of copper mineralisation, and that ore formation was predominantly controlled by the cooling and sulphidation of early Fe oxide-rich alteration (Baker, 1998).

The mineralised zone occurs as two main sub-parallel, steeply dipping, tabular ore zones about 15 to 20 metres apart that are more or less conformable with the schistosity, strike north-south and dip steeply to the east. The two main ore zones occur over a strike length of about 650 m and have been drilled to depths in excess of 1200 m (Baker, 1998).

The NNE trending, near vertical, A Lode (Elrose), the smaller of the two main orebodies, plunges at 60 to 75°S, with a maximum strike length of 180 m, an average width of 6 m, (maximum 20 m) and extends over a vertical range of >700 m. The B Lode (Levuka), the largest and most persistent, lies parallel to and east of A Lode with a similar plunge and dip. It is separated from the A lode by weakly mineralised host arenite. This lode has an average width of 18 m, reaching a maximum of 25 m, a strike length up to 240 m at the upper levels, although it is more commonly 100 to 150 m long, and extends down plunge for over 1200 m (Hodkinson et al., 2003).

Eloise West and Eloise Northwest Lodes are narrower (up to 8.5 and 5 m wide respectively), north trending, and hosted by the western arenite, accompanied by relatively intense mafic alteration with spatially associated carbonate-magnetite 'ironstone' units. Both are far less laterally extensive than the main A and B Lodes with strike lengths of 150 m and 90 m respectively and only carry economic mineralisation over a limited vertical range of ~100 m. They also seem to lack any significant preferential plunge direction or extension. They appear to be the largest of a number of similar structures hosted within a 200 m wide corridor of mafic altered arenite on the western side of the amphibolite body. Other sub-economic lode structures within this corridor include the 40 Lode and Eloise Far West zone. Although this mineralisation style is well developed in arenites to the west of the amphibolite, comparable zones to the east of the amphibolite body are weakly developed (Hodkinson et al., 2003).

Alteration principally pervasive silica-carbonate flooding, with a lesser mafic over-print of course grained biotite and amphibole. The contact between the mineralisation and the enclosing country-rock is gradational over several metres. Patches of course grained carbonate with layers and disseminations of magnetite are common within the alteration envelope.

The main mineralising episode at Eloise is considered to be contemporaneous with the third stage of a complex alteration sequence (Baker 1998). Early, pervasive, regional sodic (albite±quartz) alteration (Stage I). This first stage was largely obliterated by a subsequent intense Fe-Mg-Ca-K mafic alteration event (Stage II) produced a ±quartz±hornblende±biotite replacement assemblage within the lodes and wallrocks, and was itself partially overprinted by a later alteration assemblage accompanying the main copper-gold mineralisation event (Stage III). The more mafic, hornblende rich, Stage II alteration acted as the locus for the main Stage III mineralisation. The latter episode was accompanied by alteration of the earlier Stage II alteration suite with hornblende commonly being replaced by an alteration assemblage of ±chlorite±quartz±calcite±actinolite. A final alteration stage (Stage IV) is represented by post-ore veining and weak alteration associated with declining fluid temperatures, and is regarded as probably representing small-scale remobilisation of material into late stage tension vein arrays within and along the margins of the lodes. Baker (1998) also identifies a ±chlorite±K-feldspar±calcite±tourmaline alteration assemblage accompanying these remobilised chalcopyrite±pyrite veinlets (Hodkinson et al., 2003).

Cu-Fe sulphide mineralisation occurs as massive to semi-massive lenses of coarse grained intergrowths of pyrrhotite and chalcopyrite, generally in close association with quartz and minor carbonate stockwork and stringer veins and mafic alteration. All of the lodes have comparable textures and mineralisation styles which reflect a high degree of shearing, brecciation and remobilisation that has been imposed on the sulphide rich orebodies. The mineralisation styles were classified by Baker (1996) into:
• Foliation parallel vein suite of quartz+hornblende±biotite veins which are common throughout the ore zones and frequently host later infillings of the main sulphide ore phases.,
• Stockwork and tension vein/breccia suite, which crosscut the dominant foliation direction, and commonly lack the early wall-rock assemblage and are relatively late in the overall paragenesis.
• Massive sulphide suite - which form locally extensive foliation parallel sheets and lenses with a range of shear related textures. Large-scale breccia textures are common, with clasts of host arenite up to several metres across. The sulphide-rich matrix may contain abundant subangular to rounded clasts of the enclosing arenite and early alteration assemblage material at a range of scales. In extreme circumstances, a milled texture may br developed, with fine-grained sulphides hosting numerous small clasts of variably altered country rock, down to several mm across. Schistose, biotite rich wall-rocks may show spalling and locally well developed 'durchbewegung' textures (Vokes, 1969) in where the schist fragments have been broken and pulled apart, rotated and deformed within the ductile sulphide matrix (after Hodkinson et al., 2003).

The pre-mining resource was 3.1 Mt @ 5.5% Cu, 1.4 g/t Au, 16 g/t Ag (Baker, 1998).
Remaining indicated + inferred resources in 2008 were 3.5Mt @ 3.1% Cu, 10 g/t Ag, 0.8 g/t Au (Breakaway Resources Ltd press release, 2008).

The most recent source geological information used to prepare this summary was dated: 2003.     Record last updated: 9/12/2014
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.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Baker T  1998 - Alteration, mineralisation and fluid flow evolution at the Eloise Cu-Au deposit, Cloncurry district, Northwest Queensland, Australia: in    Econ. Geol.   v93 pp 1213-1236
Baker T, Perkins C, Blake K L, Williams P J  2001 - Radiogenic and stable isotope constraints on the genesis of the Eloise Cu-Au deposit, Cloncurry district, northwest Queensland: in    Econ. Geol.   v96 pp 723-742
Brescianini R F, Asten M W, McLean N  1992 - Geophysical characteristics of the Eloise Cu-Au deposit, Northwest Queensland: in    Exploration Geophysics   v23 pp 33-42
Hodkinson I, Grimsley E and Baensch A,  2003 - Eloise Copper Mine Geology - Back to Basics: in   Fifth International Mining Geology Conference, 17 - 19 November 2003, Bendigo, Victoria, The AusIMM, Melbourne,    pp. 139-146

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