Starra-Selwyn (Mt Dore, Merlin)
Queensland, Qld, Australia
Cu Au Mo
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The Selwyn/Starra gold-copper mines and the associated Mount Dore copper-gold, Merlin molybdenum and SWAN Mt Elliott deposits are located ~110 km south of Cloncurry in the Eastern Succession of the Mount Isa Inlier in North-west Queensland, Australia (#Location: Starra 257 - 21° 40' 42"S, 140° 28' 28"E).
For geological background on the setting, see the Cloncurry IOCG Province record.
The oldest rocks within the immediate Selwyn/Starra district are the Double Crossing Metamorphics which comprise felsic gneiss and migmatites, and are Intruded by the 1741±7 Ma, strongly foliated Gin Creek biotite granite.
These are followed by the Staveley Formation, which comprises a >2000 m thick belt of shallow water, well-bedded to brecciated, variably calcareous, ferruginous, feldspathic, micaceous and siliceous sandstone, siltstone and phyllite, impure limestone (marble) and lenses of breccia, together with schist and banded calc-silicate rocks (mainly near granite), with minor basalt lava, conglomerate and banded quartz + hematite±magnetite rock.
The Kuridala Formation is a tightly folded package of moderately deep-water turbiditic sediments (schistose greywacke, siltstone and shale) with quartzite, carbonaceous and pyritic slate and calc-silicate rocks.
The youngest Proterozic rock unit in the district is the non-foliated medium to coarse grained Mount Dore Granite, part of the Williams Naraku Batholith suite, which has been thrust over the Kuridala Formation.
The district scale structure is dominated by the north-south striking regional-scale Starra, Selwyn and Mount Dore shear and thrust zones, which run through the core of the district. These may be summarised as follows (after Chinova, 2014):
• Mount Dore Fault Zone - A north-south striking deep seated structure following the limb of two major regional folds, producing a sheared sub-parallel series of faults which host a number of prospects including Marilyn, Mount Dore, Merlin, Flora, Busker and Metal Ridge.
• Mount Dore Silicified Zone - A north-south striking silicified ridge that forms the footwall to the Merlin and Mount Dore mineralisation, and extends for at least 10 km both north and south of Mount Dore, broadly marking the western limit of the Mount Dore Fault Zone.
• The Starra Shear Zone, which is located 2 km west of, and parallel to, the Mount Dore Fault Zone, comprising a melange of various protoliths, including amphibolites, set in a strongly-sheared matrix. It is interpreted to extend at least several km to the north of the Starra 276 orebody, and tens of km to the south of the Starra 222.
• The Selwyn Line, an intensely alkali-iron-silica-carbonate altered section of the Starra Shear Zone, which hosts multiple high grade gold-copper shoots. These shoots are developed in structural loci within a larger tabular copper-gold mineralised system.
• The Selwyn Shear Zone, which parallels to the Starra Shear Zone and the Starra deposits, and essentially marks the Eastern Hematites, a line of ironstones. It may also continue from Mount Elliott to Tip Top where it appears to mark the boundary between the Staveley Formation and the Kuridala Formation.
The gold-rich mineralised ironstone shoots at Starra are hosted by metamorphosed siliciclastic-carbonate rocks of the Staveley Formation. The hanging wall comprises strongly altered, interbedded, calcareous sandstone and siltstone, and the weakly mineralized footwall is dominated by chloritised quartz-feldspar-biotite-magnetite schist. The sequence also contains numerous amphibolite bodies.
The principal lithologies, from the hanging wall in the east, towards the footwall in the west, are (Sleigh, 2002):
• Metasediments - a package of interlayered carbonate bearing meta-psammites and metapelites of the Staveley Formation, located in the hanging wall of the main mineralised zones. They exhibit variable quartz-albite-calcite-magnetite/hematite ±scapolite, pyrite and biotite alteration, overprinting an earlier greenschist facies metamorphic assemblage. These metasediments are increasingly strained to the west, as demonstrated by local intensification of shear fabric and breccia development.
• Massive Ironstones - occurring as crudely tabular or lensoid bodies, typically composed of variable proportions of medium to coarse grained hematite and magnetite, with quartz, albite, calcite, sulphides and chlorite in their matrix. Granular hematite dominates in both massive and brecciated ironstones, but is variably replaced by coarse grained concordant and discordant magnetite/martite that is commonly associated with chalcopyrite or bornite. The magnetite replacement follows both S1, S2 and S4 foliation and crosscutting shear structures. Locally within the Selwyn Line, the hematite ironstones follow 'tramline' hangingwall and footwall bounding shears in the Starra Shear zone, enclosing internal structures that are dislocated, or develop splays or flexures, with magnetite replacement becoming dominant. Magnetite ironstones are typically developed along obliquely discordant, sigmoidal structures, linking hangingwall to footwall structures, both along strike and down dip, typically striking at 10 to 15°east of the principal hematitic shears. This replacement can be observed on scales from millimetres to tens of metres in width, and over hundreds of metres down dip and along strike. (Adshead-Bell, 2000).
• Starra Shear Schists the chlorite/biotite-magnetite/hematite-albite-quartz-carbonate schists that comprise the central portions of the Starra Shear zone, representing the altered and sheared Staveley Formation metasediments. These schists are the main host to mineralisation. The proportions of chlorite, feldspar, magnetite and hematite in this unit vary significantly along the Selwyn Line. The texture and alteration mineral composition in this unit is closely linked to the type of mineralisation, and to the development of structures (Sleigh, 2002).
• Feldspar-Quartz-Magnetite Schists - which are typically between 50 and 100 m thick, and are distinguished from the Starra Shear Schists by their distinctive mineralogy, characterised by only minor biotite/chlorite. They are typically found in the footwall of the main shear zone but also occur as extensive lenses and zones within the body of the shear. The unit is variably brecciated, and the degree of strain is crudely related to the amount of chalcopyrite present. They typically contain zones of abundant, coarse, deformed, pyrite ±scapolite in the vicinity of the footwall bounding shears. The thickness of this lithology, combined with its magnetite contact results in their close coincidence with the regional magnetic anomaly.
• Amphibolites - are medium to fine grained actinolite (after pyroxene) - plagioclase - magnetite bearing rocks that are both concordant and discordant within the metasediments on both the eastern and western sides of the Selwyn Line, but are not observed to crosscut the mineralised units.
Mineralisation at the original Starra group of mines occurred as 5 discrete high grade, structurally controlled clusters of shoots of gold-copper ore associated with magnetite-hematite-quartz "ironstones", from Starra 222 in the south, through 224, 251 and 257 to Starra 276 in the north. All plunge steeply to the north or south, and were distributed over a ~5.5 km strike length of an extensive (~15 km long) magnetite-hematite ironstone ridge that coincides with the Starra Shear high strain zone, which has a history of ductile-brittle, brittle-ductile and brittle deformation.
At Starra 222 for example, a number of shoots are located within multiple zones of alteration, which form broad envelopes. The high grade zones mined are associate with tabular ironstones which extend down dip for >500 m, along strike for ~250 m and which are 2 to 20 m wide. These plunge at 60 to 80° to the northeast. The deposit contions abundant chlorite ±biotite ±sericite alteration which define broad breccia zones and shear fabrics demarcating north to NE trending shears. These zones of alteration are accompanied by coarse magnetite/martite disseminations which are commonly accompanied by chalcopyrite with minor gold. Gold grades increase in the presence of bornite, high concentrations of chalcopyrite and in reworked, non-sulphidic, massive hematite breccias which are both sub-parallel with, or subtly discordant to, the principal foliations. Starra 222 is located at the intersection of the main north to NNE trending (S2/S4) Starra Shear structures, and a NE-SW trending (S1?) cross shear (Sleigh, 2002).
Further north at Starra 244, schists are chlorite dominated and developed in elongate duplexes and lenses within the main shear, but are typically still located in the footwall of the massive magnetite ironstones that mark portions of the principal hangingwall bounding shears. These chlorite dominated Starra Shear Schists are mineralised to ore grades in parts, and overlie the dominantly hematitic massive ironstones and the albite-quartz-magnetite ±pyrite ±scapolite schists which form the western or footwall stratigraphy of the Selwyn Line. In many areas these schists become increasingly deformed westward until they form a mylonitic straightening zone which, together with the hematitic footwall ironstones, forms the western bounding shears (Sleigh, 2002).
Further north again, at Starra 276, chloritic schists are less well developed, compared to the magnetite bearing metasediments and footwall feldspar-quartz-magnetite schists (Sleigh, 2002).
The shear that hosts the mineralisation in this string of shoots at Starra, is located on the western margin of the major regional corridor of deformation, the Mount Dore fault zone. It is the western of two main, southward converging ironstone ridges evident at surface, and comprises a quartz-magnetite-hematite ironstone that hosts the known copper-gold mineralisation. The second, subparallel, eastern ridge, is a quartz-hematite ironstone that appears to only be mineralised near the Starra 222 deposit, close to where the two ridges converge (Williams et al., 2001).
Whilst the Starra group of deposits are developed in the Western Ironstone, the Mount Dore copper-gold deposit and the high-grade Merlin molybdenum-rhenium deposit, which is located in the footwall of the Mount Dore mineralised zone, occur in the Eastern Ironstone. These deposits are in the same structural and stratigraphic position as the SWAN-Mount Elliott, on the eastern margin of the broader Mount Dore fault zone. These deposits are 1250 m east of the northeastern margin of the Selwyn IOCG system. The Merlin mineralisation is a late phase, associated with silica-albite alteration and interstitial clay, and was emplaced along reactivated fractures and shear zones, replacing the matrix of structurally controlled breccias that occur in carbonaceous shale and metasiltstone, and host the copper-zinc sulphides at Mount Dore. At Mount Dore, early regional scale sodic-calcic alteration is cut by K feldspar and quartz veining, succeeded by brecciation which hosts the earliest episode of primary copper mineralisation. A second phase of brecciation was followed by a hydrothermal event that deposited mainly dolomite with chalcopyrite, pyrite, sphalerite, cobaltite and bornite, with trace galena, arsenopyrite and molybdenite (Lazo and Pal, 2009). In the upper 180 m thickness of the ore zone, primary sulphides have been converted to chalcocite, which has subsequently been oxidised to chrysocolla, native copper, cuprite and pseudomalachite, resulting in the an upper copper-oxide zone overlying a narrow chalcocite dominated transition to primary sulphides without significant iron oxides (Ivanhoe Australia website, 2010).
Extensive alteration is evident at both the district and deposit scale. The key alteration types and episodes noted include (Chinova, 2014):
• Early iron-oxide alteration to produce shear hosted magnetite or hematite zones, e.g., at Starra;
• Later iron oxide alteration as veined replacive magnetite, e.g., at Mt Elliott;
• Widespread sodic alteration, as replacive, veined, stockworking and brecciation, predominantly as hematite dusted albite;
• Sodic-calcic alteration occurring as banded, veined, massive and breccia replacive and infill actinolite, diopside, calcite, albite (from the earlier sodic alteration), epidote, chlorite, biotite, scapolite and apatite. This style appears to have formed during at least two events, the first associated with early ironstone development (e.g., Starra) and a later phase (precursor to magnetite and sulphide deposition) seen at other deposits e.g., at SWAN and Mount Elliott;
• Widespread potassic alteration, as veins and breccia clast replacement, and as matrix infill K feldspar, carbonate and biotite veining e.g., at Mount Dore, Merlin Lady Ella and Starra.
The Starra Shear is altered over widths of between 100 and 500 m, and for more than 10 km along strike. A wide zone of chalcopyrite-pyrite mineralisation occurs within the shear, although the highest grades are confined to the ironstones package, usually within its eastern-most layers. High gold grades are also typically restricted to the ironstones, particularly on the margins of individual ironstone bodies, but unlike copper, do not extend into the surrounding shear zone (Ivanhoe Australia website).
Structural criteria, and the paragenetic relationships between hematite and magnetite, indicate that the alteration and mineralisation at Starra overprint metamorphic scapolite and biotite. They also suggest very early hematite metasomatism was associated with the regional sodic-calcic alteration (quartz-albite-actinolite-scapolite-titanite) that post-dated peak metamorphism, but was syn-D4, in a brittle-ductile setting (Rotherham, 1997). This was overprinted by potassic-iron alteration (biotite-magnetite-hematite-quartz-pyrite) and then by the gold-copper mineralisation. The gold-copper mineralisation was emplaced in a brittle, although otherwise similar, deformation regime and consists of an early quartz-anhydrite-barite-calcite-hematite-gold assemblage (with extensive hematisation of magnetite) that progressed to pyrite-bornite-chalcopyrite-chlorite-muscovite and then to magnetite mineralisation (Adshead-Bell, 1998; Rotherham et al., 1998).
Fluid inclusion homogenisation temperatures and oxygen isotope geothermometry suggest that ironstone formation and mineralisation occurred at 550 to 400°C and 360 to 220°C respectively. Fluid δ18O compositions for ironstone formation (9.2 to 6.0‰) and gold-copper mineralisation (10.9 to 8.4‰) fall within the range of magmatic and metamorphic fluids, although a magmatic metasomatic origin has been preferred on the basis of stable isotopic, fluid inclusion, and thermodynamic considerations (Rotherham et al., 1998).
Resources and Production
The original mining reserve of the Starra project in 1988 (Kary and Harley, 1990) was:
5.3 Mt @ 1.98% Cu, 5.0 g/t Au;
The total ore mined from the five Selwyn mines (275, 257, 251, 244 and 222) to 1999 was:
6.84 Mt @ 4.6 g/t Au and 2.1% Cu.
In 2002, Selwyn Operations Ltd estimated a larger low grade total resource of:
253 Mt @ 0.34% Cu, 0.48 g/t Au, at a 0.2% Cu equiv. cutoff (Sleigh, 2002). This figure included part of the SWAN-Mount Elliott deposit,
which has been subsequently defined in detail (see the SWAN, Mt Elliott record)
Duncan et al. (2011; after Ivanhoe Mines 2008) quote:
past production + current resources at Starra-Selwyn as totalling 37.4 Mt @ 1.2% Cu, 1.6 g/t Au.
Merlin (Ivanhoe Australia website, 2010)
measured + indicated + inferred resource - 6.7 Mt @ 1.33% Mo, 23.1 g/t Re;
Mount Dore - a pre-mining JORC compliant resource estimate reported by Chinova in September 2014 @ 0.25% Cu Cut off was:
indicated + inferred resource - 111.2 Mt @ 0.53% Cu, 0.09 g/t Au, 0.06% Pb, 0.31% Zn.
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.
References to this deposit in the PGC Literature Collection:
Adshead-Bell N S 1998 - Evolution of the Starra and Selwyn high-strain zones, Eastern Fold Belt, Mount Isa Inlier: implications for Au-Cu mineralization: in Econ. Geol. v93 pp 1450-1462|
Adshead-Bell N S 1998 - Evolution of the Starra and Selwyn high-strain zones, Eastern Fold Belt, Mount Isa Inlier: Implications for Cu-Au mineralisation: in Econ. Geol. v93 pp 1450-1462|
Brown M and Porter T M, 2010 - The Mount Elliott IOCG System, Eastern Fold Belt, Mount Isa Inlier, Northwest Queensland: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide v.3 pp. 219-232|
Duncan R J, Hitzman M W, Nelson E P and Togtokhbayar O, 2014 - Structural and Lithological Controls on Iron Oxide Copper-Gold Deposits of the Southern Selwyn-Mount Dore Corridor, Eastern Fold Belt, Queensland, Australia : in Econ. Geol. v.109 pp. 419-456|
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|
Fortowski D B, McCracken S J A 1998 - Mount Elliott copper-gold deposit: in Berkman D A, Mackenzie D H (Eds), Geology of Australian and Papua New Guinean Mineral Deposits The AusIMM, Melbourne pp 775-782|
Foster D R W and Austin J R, 2008 - The 1800-1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes: in Precambrian Research v.163 pp. 7–30|
Kary G L, Harley R A 1990 - Selwyn copper-gold deposits: in Hughes F E (Ed), Geology of the Mineral Deposits of Australia and Papua New Guinea The AusIMM, Melbourne v1 pp 955-960|
Rotherham J F, 1997 - A metasomatic origin for the iron-oxide Au-Cu Starra orebodies, Eastern Fold Belt, Mount Isa Inlier : in Mineralium Deposita v32, pp 205-218|
Rotherham J F, Blake K L, Cartwright I, Williams P J 1998 - Stable isotope evidence for the origin of the Mesoproterozoic Starra Au-Cu deposit, Cloncurry district, northwest Queensland: in Econ. Geol. v93 pp 1435-1449.|
Rotherham J, Blake K L 1998 - Stable isotope evidence for the origin of the Mesoproterozoic Starra Au-Cu deposit, Cloncurry district, Northwest Queensland: in Econ. Geol. v93 pp 1435-1449|
Shiqi Wang and Williams P J 2001 - Geochemistry and origin of Proterozoic skarns at the Mount Elliott Cu-Au(-Co-Ni) deposit, Cloncurry district, NW Queensland, Australia: in Mineralium Deposita v36 pp 109-124|
Shiqi Wang, Williams P J 1996 - The alteration and mineralisation styles of a skarn hosted Mount Elliott Cu-Au deposit and adjacent SWAN prospect, Cloncurry district: in Baker T, et. al. (Eds), MIC 96, New Developments in Metallogenic Research, The McArthur-Mt Isa-Cloncurry Minerals Province EGRU Contribution 55 (Townsville, Qld) pp 139-142|
Sleigh D W W, 2002 - The Selwyn Line tabular iron-copper-gold system, Mount Isa Inlier, NW Queensland, Australia: in Porter T M (Ed), 2002 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, PGC Publishing, v2, pp. 77-93|
Williams P J, Guoyi Dong, Ryan C G, Pollard P J, Rotherham J F, Mernagh T P, Chapman L H 2001 - Geochemistry of hypersaline fluid inclusions from the Starra (Fe oxide)-Au-Cu deposit, Cloncurry district, Queensland: in Econ. Geol. v96 pp 875-883|
Williams, P.J. and Pollard, P.J., 2003 - Australian Proterozoic Iron Oxide-Cu-Au Deposits: An Overview with New Metallogenic and Exploration Data from the Cloncurry District, Northwest Queensland: in Exploration & Mining Geology, CIM v.10, No. 3, pp. 191-213.|
| References to this deposit in PGC Publications:||
Sleigh D W W, 2002 - The Selwyn Line Tabular Iron-Copper-Gold Mineralised System, Mount Isa Inlier, NW Queensland, Australia, in Porter T M, (Ed.), Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, v2 pp 77-93|
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