Another PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
IOCG 2013
Australian Iron Oxide Copper-Gold Deposits
19 to 27 February, 2013
Porter GeoConsultancy Home Page | More on This Tour | Other Tours | New Tours | Contact us
Ernest Henry Ore
Image: Magnetite-chalcopyrite ore, Ernest Henry    

   Porter GeoConsultancy, continued its International Study Tour series of professional development courses during February 2013 by visiting a representative selection of the different styles, and the most important examples, of iron oxide copper-gold deposits of the Gawler-Curnamona craton in South Australia and the Eastern Fold Belt of the Mount Isa Inlier in NW Queensland.
   The mine and project visits were complemented by field & classroom workshops presented by experts from academia, government geological surveys and industry.
   The full tour commenced in Adelaide, South Australia on the evening of Monday 18 February, 2013 and ended in Brisbane, Queensland, on the evening of Wednesday 27 February.
   Participants were able to take any 3 or more days, up to the full tour, as suited their interests or availability, with participants joining and leaving the tour at appropriate locations along the route.

The main components of the planned itinerary were:
Scheduled dates for each visit or workshop is shown alongside the heading of the respective description below.

New & Recent International
Study Tours:
  Click on image for details.
Andean Porphyries
CopperBelts 2014
Click Here

Click Here
Adelaide Workshop - Hosted by DMITRE Minerals  -  SA Government   ........... Tuesday 19 February, 2013.

This workshop included expert lectures in a classroom setting, supported by drill core at the nearby  DMITRE Minerals core library  in Adelaide.   It included:
  • A global overview of the occurrence of Iron Oxide Copper-Gold and related Iron-oxide Associated, Alkali-altered mineralised systems by Mike Porter;
  • Overview presentations on the tectonics, geology and geophysical expression of the Gawler-Curnamona craton, and the distribution and controls of IOCG mineralisation in the craton as a whole, and within individual districts, delivered by DMITRE Minerals specialists Martin Fairclough (Chief Geoscientist), Adrian Fabris, Simon van der Weilan and Claire Wade, and expert consultant Dr Colin Conor;
  • An outline of the geology and mineralisation of significant IOCG and related occurrences from the Olympic IOCG Province, other than those to be visited. These included:
    - magnetite deposits devoid of Cu-Au (<500 ppm Cu) mineralisation (e.g., Murdie Murdie);
    - magnetite±hematite with 0.1 to 0.3% Cu throughout (e.g., Titan and Manxman A1); and
    - magnetite±hematite with sub-economic to economic byproduct Cu-Au (e.g., Emmie Bluff and Cairn Hill);
  • Inspection of drill core through the deposits described in the previous point that is held at the DMITRE core library;
  • Expert presentation and study of drill core from the Kalkaroo deposit, including its regional setting within the Curnamona Province (Dr Chris Giles of Havilah Resources). Kalkaroo (see description from the link above) is a significant deposit in advanced feasibility with a total resource of 124.51 Mt @ 0.50% Cu, 0.39 g/t Au. The presentation included a detailed demonstration of the deposit using the Vulcan 3D ore model with all data. The deposit is located on a featureless arid plain below cover, with no outcrop, and hence an actual site visit was not undertaken.
Return to top

Geological summaries   of the deposits visited are as follows:

Hillside Project, Yorke Pensinsula, South Australia  -  Rex Minerals   ............ Wedneday 20 February, 2013.

The Hillside copper-gold deposit is located on the east coast of Yorke Peninsula, approximately 15 km south of Ardrossan, 72 km NW of Adelaide, and 48 km SW of Port Wakefield which is situated at the head of Gulf St Vincent in South Australia. It is 455 km south of Olympic Dam.

The Hillside and other similar IOCG-style occurrences and deposits of the Moonta-Wallaroo district, and the Olympic Dam, Prominent Hill and Carrapateena copper-gold (uranium) deposits are hosted within Palaeo- to Mesoproterozoic rocks of the Olympic IOCG Province that extends along the eastern margin of the currently preserved Gawler Craton. Underlying and to the west is Mesoarchaean to Palaeoproterozoic basement, partly overlain by the thick Mesoproterozoic Gawler Range Volcanics (GRV). Unconformably overlying these deposits are the Neoproterozoic sediments of the Stuart Shelf marking the western platformal flank of the Adelaide Geosyncline, an intracontinental rift-complex developed as a response to the late Neoproterozoic break-up of the Rodinian Super-continent.

The predominantly sub-cropping host to the Moonta-Wallaroo region comprise the volcano-sedimentary succession of the Palaeoproterozoic Wallaroo Group, and early Mesoproterozoic granites and mafic rocks of the Hiltaba Suite. The underlying basement of the region is believed to include gneissic granitoids of the ~1850 Ma Donnington Suite, which are uplifted and exposed in the southern part of Yorke Peninsula by faulting that limits the southern extent of the Moonta sub-domain (Raymond, 2003). The Moonta-Wallaroo region, which is located on the western flank of the Neoproterozoic Adelaide Geosyncline rift, is unconformably overlain and almost totally blanketed by thin, incomplete, successions of Neoproterozoic, Cambrian, Permian and Tertiary sediments upon which the present day regolith and soils are developed.

The Hillside IOCG (LREE-U) deposit was discovered in early in 2008 by Rex Minerals Ltd as a result of exploration drilling of discrete magnetic and gravity features spatially associated with the regional NNE-trending Pine Point (Ardrossan) Fault, adjacent to the historic Hillside copper mine. It represents the southern extension of the mineralisation exploited by that mine, which was active prior to 1916 and between 1929 and 1932, produced around 50 tonnes of ore for 8 tonnes of recovered copper (Wade and Cochrane, 1954), with grades varying from 0.5% to 44% Cu. The historic ore was taken from two NE-striking, 70 to 80° W dipping lodes that varied from <0.5 to 3 m in thickness, and comprised chalcopyrite, bornite, malachite, chalcocite, atacamite and covellite, hosted in brecciated and sheared schist with quartz stringers.

The Hillside mineralisation is concealed by a sequence of Tertiary calcareous sediments ranging from <1 to 30 m in thickness, locally occuring as up to 30 m thick channel-fill alluvium in NNW-trending palaeochannels. Gravity and magnetic features confined to the Pine Point Fault structure most likely represent gabbroic intrusives emplaced within this structural corridor, while immediately to the east and parallel to the Pine Point Fault, there is a large magnitude, >40 km long, deep seated geophysical anomaly (the Ardrossan-Snowtown Magnetic-Gravity Feature) which may have some bearing on mineralisation along the Pine Point Fault trend.

The structural style of the Hillside mineralising system, like those at the Moonta-Wallaroo mines, the deeper Cloncurry-style IOCG(U) deposits (e.g., Starra; Williams et al., 2005; Mt. Elliott; Fortowski & McCracken, 1998; and Wang and Williams, 2001) and some of the IOCG-style deposits from the Curnamona Province (e.g., Kalkaroo; Teale, 2006) with mineralisation hosted within discrete, but apparently laterally and vertically continuous, structures. This structural style is in contrast to the Olympic Dam and Carrapateena-style IOCG(U) deposits that are characterised by large polygonal to circular hematite-dominant breccia bodies.

The Hillside deposit is hosted by highly deformed and folded metasediments of the Wallaroo Group, intruded by Mesoproterozoic igneous rocks which comprise numerous phases of granite, micro-gabbro, porphyritic gabbro and gabbro-diorite, presumed to be related to the Hiltaba Suite. The metasediments are invariably intensely altered within the Pine Point structural corridor, but have also commonly undergone late retrogression. All intrusions, including numerous pegmatites that have been emplaced along minor structures, have been intensely altered, including both endoskarns and exoskarns.

Gabbroic rocks in the Hillside area contain an early high temperature potassic alteration accompanied by the development of magnetite-biotite-K feldspar±bornite. The late replacement of plagioclase by K feldspar is common.

The copper-gold-(uranium) mineralisation is hosted within metasediments and meta-mafic rocks and can develop within and adjacent to gabbros and A-type felsic intrusives. The metasediments are folded by pre-intrusion open to tight, south-plunging folds, including both upright folds (local F2), and a series of recumbent to strongly inclined folds (local F1). The local F1 folds are also associated with possible early thrusts in some sections. Folds have a north to NE trend, with some evidence for later NW cross-folding in some areas. The folding varies from parallel-coincident, to acutely discordant to the north-south trending skarn and breccia bodies.

Significant mineralisation is focussed in numerous, north-trending, sub-vertical to steeply west dipping bodies intimately associated with prograde and retrograde skarn assemblages and associated steeply west-dipping 'breccia' structures. Mineralisation and associated skarn development is variable both laterally and vertically. The overall depth extent of the individual high-grade mineralised zones suggests mineralisation was emplaced over a vertical interval of >700 m. Stratabound replacement of metasediments occurs adjacent to the skarns, e.g., in the immediate footwall of the western branch of the Pine Point Fault structure.

Three major separate anastomosing, ~1.5 km long copper-mineralised structures have been defined, the Zanoni, 'Songvaar' and 'Parsee' structures. These structures are broadly defined by a magnetic anomaly that exists over an area that is 2 km long and 500 m wide. Together they have a combined strike length in excess of 4 km, although copper mineralisation remains open both along strike and at depth, and has been observed from as shallow as 5 m below surface to 700 m in depth.

Numerous high to low temperature skarns are developed within the Hillside deposit. The earliest, higher temperature phases are dominated by magnetite±quartz±pyrite±garnet and almost monomineralic garnet skarn. The earlier skarns are replaced by clinopyroxene, K feldspar, epidote, actinolite, allanite and biotite-rich assemblages with, for example, clinopyroxene-bearing skarn often developed on the margins of and replacing garnet skarn. Primary copper mineralisation developed within and adjacent to skarn lithotypes, comprises high grade, parallel, steeply-dipping domains which may be flanked by lower grade vein, blebby and lace-like chalcopyrite accumulations.

Primary copper mineralisation at Hillside is dominantly chalcopyrite with lesser bornite and chalcocite, with the latter two phases often intergrown with apparent common unmixing textures, although some parts also contain significant primary bornite and chalcocite. These sulphides coincide with extremely oxidised domains, and bornite is often found with carbonate and hematite and magnetite is replaced by hematite±chalcopyrite. Extreme increases in copper grades are accompanied by late carbonate and silica flooding, and in many areas are associated with the development of chlorite+chalcopyrite which replace clinopyroxene, actinolite and garnet. Pyrite is abundant in some domains but is usually replaced by chalcopyrite during skarn retrogression. Gold appears to be hosted in chalcopyrite. Rare galena, tennantite, bismuthinite and aikinite are present and uraninite and pitchblende are often associated with carbonate-rich zones. LREE are contained within allanite.

Post-mineralisation faulting is evident, particularly north-trending, steep to sub-vertical structures, and others that are moderate to shallow and NW-trending. Recent U-Pb isotopic dating of two titanite samples from alteration at Hillside indicate that the alteration is broadly coeval with granite emplacement (1570±8 Ma; Gregory in Reid, 2010), which relates to the latest stage of Hiltaba Suite magmatism in the wider Olympic Copper-Gold Province.

Secondary copper mineralisation is predominantly supergene chalcocite with lesser malachite, azurite, native copper and rare cuprite, atacamite and chrysocolla, overlying primary copper mineralisation along the eastern domains of the deposit ('Songvaar' and 'Parsee' structures and elsewhere in the deposit).

The inferred mineral resource at 27 July 2011 (Rex Minerals release to the ASX) was:
    217 Mt @ 0.7% Cu, 0.2g/t Au, 12.4% Fe

The inferred mineral resource at 30 July 2012 (Rex Minerals release to the ASX) to depths of 400 to 700 m, was:
    330 Mt @ 0.6% Cu, 0.16 g/t Au, 13.7% Fe, at a 0.2% Cu cutoff. -or-,
    226 Mt @ 0.7% Cu, 0.18 g/t Au, 14.1% Fe, at a 0.4% Cu cutoff. -or-,
    116 Mt @ 0.9% Cu, 0.20 g/t Au, 14.2% Fe, at a 0.6% Cu cutoff.
Within the 0.2% Cu cutoff resource, there are indicated + inferred resources of :
    oxide ore - 22 Mt @ 0.54% Cu, 0.22 g/t Au, 12.8% Fe, and
    secondary sulphide ore -13 Mt @ 0.59% Cu, 0.12 g/t Au, 13.5% Fe, and
    primary sulphide ore - 294 Mt @ 0.60% Cu, 0.12 g/t Au, 13.75% Fe.

For more detail see: Conor, C. Raymond, O., Baker, T., Teale, G, Say, P. and Lowe, G., , 2010 - Alteration and Mineralisation in the Moonta-Wallaroo Copper-Gold Mining Field Region, Olympic Domain, South Australia; in Porter, T.M., (ed.), Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, v. 3 - Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide, pp. 147-170.

Return to top

Carrapateena  -  East of Woomera, Northern South Australia  -  OZ Minerals   ...................... Thursday 21 February, 2013.

The Carrapateena iron oxide copper-gold deposit, which is located within the Olympic IOCG Province on the eastern rim of the preserved Gawler craton in northern South Australia, is ~100 km SSE of Olympic Dam and ~130 km north of Port Augusta in South Australia, immediately to the SW of the Carrapateena embayment on the central-western shore of Lake Torrens.

Regional Setting

Carrapateena, Olympic Dam, Prominent Hill, Moonta-Wallaroo and Hillside and all of the other significant known IOCG mineralised systems of the Gawler craton are hosted within Palaeo- to Mesoproterozoic rocks and are distributed along the eastern edge of the currently preserved craton to define the Olympic IOCG Province.

The oldest rocks of the Gawler craton comprise Mesoarchaean to early Palaeoproterozoic metamorphics and igneous suites that form a core to the craton, immediately to the west of the Olympic IOCG Province. On its eastern margin, and within the Olympic IOCG Province, this older nucleus was overlain after ~2000 Ma by the Hutchison Group, a sequence of subaerial to shallow marine clastic and chemical metasedimentary rocks, with minor felsic and mafic volcanic rocks, that were deposited on a continental passive margin (Parker, 1993; Daly et al., 1998). Along the eastern margin of the craton, including the Carrapateena district, the cratonic core and the Hutchison Group were both intruded by ~1850 Ma granitoids of the Donington Suite during the Cornian orogeny (Hand et al., 2007; Reid et al., 2008). This suite is dominated by granodiorite gneiss, with subordinate metamorphosed alkali-feldspar granite, gabbronorite, tonalite and quartz monzonite intrusions with mafic dykes (Ferris et al., 2002). Between ~1770 and 1740 Ma, subsequent to the emplacement of the Donington Suite, extension along the eastern, northern and western margins of the craton, resulted in the development of a series of extensive basins, some of which contain bimodal volcanic rocks, including the Wallaroo Group, which is extensively developed within the Olympic IOCG Province and is an important host to IOCG alteration and mineralisation. During the Palaeoproterozoic, the Curnamona Province is believed to have collided with and been accreted to the Gawler craton in the east. A major linear discontinuity in magnetic and gravity data beneath the Neoproterozoic cover of the Adelaide Geosyncline intracratonic rift complex is interpreted to mark the suture zone (Hayward and Skirrow, 2010, and sources quoted therein). The interval between ~1730 and ~1630 Ma encompasses the Kimban orogeny (1730 to 1690 Ma), Ooldean Event (1660 to 1630 Ma), and the widespread emplacement of various felsic igneous rocks, and formation of several small intracontinental sedimentary basins (Hand et al., 2007).

Towards the close of the Palaeoproterozoic, from ~1630 to 1615 Ma, the Nuyts Volcanics and St Peter Suite bimodal magmas were emplaced in the southwestern part of the Gawler craton, which although poorly exposed, cover an extensive area (Fanning et al., 2007). Between ~1600 and ~1575 Ma the centre of magmatism shifted eastward with the development of the high-volume intrusive Hiltaba Suite and extensive co-magmatic bimodal Gawler Range Volcanics (GRV) to form a large felsic igneous province (covering a preserved area of >25 000 km
2) over the central and eastern parts of the Gawler craton, including the Olympic IOCG Province. Between 1580 to 1550 Ma, magmatism progressed eastward to form the Benagerie volcanics and Bimbowrie Suite I- and S-types in the Curnamona Province, the easternmost development of the diachronous eastnortheast-trending corridor of continental I- and A-type magmatism that extends across the Gawler and Curnamona cratons from the St Peter Suite in the southwestern part of the craton (Hayward and Skirrow, 2010).

Known significant IOCG districts/deposits within the Olympic IOCG Province, including Carrapateena, are found where oxidised (magnetite-series), A-type granitoid plutons of the 1595 to 1575 Ma Hiltaba Suite which were emplaced into an accreted Palaeoproterozoic terrane, and where mafic volcanic rocks of the lower GRV are most abundant. These rocks were emplaced during a short-lived episode of NNW-SSE extension that approximately coincided with eruption of the GRV (~1595 to 1590 Ma), preceded and followed by more protracted NW-SE to NNW-SSE contraction (Hayward and Skirrow, 2010).

Tectonism subsequently appears to have migrated northwards and westward, with the ~1570 to 1540 Ma Kararan and 1470 to 1440 Ma Coorabie orogenies respectively. The Archaean to Mesoproterozoic crystalline basement rocks of the Gawler craton were not subjected to any substantial deformation after ~1450 Ma until the early Palaeozoic Delamerian orogeny (Parker, 1993). Much of the Olympic IOCG Province is overlain by flat lying Neoproterozoic to Lower Palaeozoic sedimentary rocks of the Stuart Shelf, equivalents of the sedimentary succession of the Adelaide Geosyncline intracratonic rift complex which separates the Gawler craton and Curnamona Province and was the result of extension preceding and during the rifting and break-up of the Rodinia supercontinent from immediately to the east of the Curnamona Province.

Carrapateena Geology and Mineralisation

The Carrapateena deposit is hosted by strongly brecciated granitoids (variably foliated and/or sheared gneissic quartz-granite and quartz-diorite) which have been dated at 1857±6 Ma and are assigned to the Palaeoproterozoic Donington Suite. It occurs within the core of a north-south oriented, 30 x 100 km mass of that suite, that is overlain 10 to 15 km to the west by ~1590 Ma mafic and felsic volcanic rocks of the Gawler Range Volcanics, which are comagmatic with the Hiltaba Suite granitoids that host the Olympic Dam deposit.

The ore deposit lies beneath a ~470 m thickness of flat lying Neoproterozoic sedimentary rocks, and occupies a north-south elongated area of approximately 800 x 600 m at the unconformity surface with the underlying Palaeoproterozoic host rocks. It is reflected by a broad, weak and diffuse 200 nT magnetic peak and a slightly offset, ellipsoidal, 3.5 km diameter, 2 mGal gravity anomaly.

Mineralisation is confined to a steeply plunging, pipe-like body of hematite and hematite-granite breccia, the Carrapateena Breccia Complex (CBC), which is interpreted to be cut at its centre by an east-west- to eastnortheast-trending complex zone of faulting. To the north of this inferred zone of faulting, the mineralised mass is wedge-shaped, tapering rapidly downward into the fault zone and may conceivably follow that structure to depth.

The Carrapateena Breccia Complex (CBC) varies from heterolithic clast- to matrix-supported hematite-rich breccias. Many of the clasts are milled and rounded such that the 'breccia' may have the appearance of a 'conglomerate' when samples are viewed in isolation. The clasts are predominantly of medium grained, gneissic diorite, with granite gneiss and vein quartz, variably altered to chlorite, sericite and hematite, as well as hematite-dominated clasts of earlier breccia phases within a matrix with a variety of textures that has also been altered to an assemblage of hematite, quartz and sericite. Higher grade copper intersections are typically associated with a grey hematite matrix within strongly brecciated granite.

To the south, the CBC comprises an irregular, ~300 to 400 m diameter, ellipsoidal-cylindrical mineralised body that has been traced by drilling from the unconformity to a depth in excess of 1 km below that surface, from where it continues unclosed.

Mineralisation is zoned laterally outward, and to the north, vertically downward, from bornite to chalcopyrite-bornite to chalcopyrite to chalcopyrite-pyrite. Three kernels of bornite-rich mineralisation have been delineated, one wedge-shaped zone to the northeast that tapers southward into the inferred central zone of faulting, and two steeply plunging elongate zones, one above the other, in the upper and lower parts of the core to the main mineralised pipe-like mass of the CBC in the south.

The principal alteration minerals are hematite, chlorite and sericite, with locally abundant quartz and carbonate (siderite and/or ankerite), and secondary barite, monazite, anatase, magnetite, apatite, fluorite and zircon.

Reserves and Resources

An audited inferred (OZ Minerals, 2011) resource for the main deposit, in the southern half of the deposit area, at a cut-off of 0.7% Cu, totals:
    203 Mt @ 1.31% Cu, 0.56 g/t Au, 0.27 kg/t U
308, 6 g/t Ag;
The northern half, has a potential to contain a further:
    25 to 45 Mt @ 1.0 to 1.1% Cu, 0.4 g/t Au, 0.14 kg U

The estimated Mineral Resource at 31 October 2012 (Oz Minerals ASX Release, 21 January, 2013) has been upgraded to:
    Indicated Resource at
      0.3% Cu cut-off - 392 Mt @ 0.97% Cu, 0.39 g/t Au, 165 ppm U, 4.2 g/t Ag;
      0.5% Cu cut-off - 282 Mt @ 1.20% Cu, 0.48 g/t Au, 197 ppm U, 5.2 g/t Ag;
      0.7% Cu cut-off - 202 Mt @ 1.43% Cu, 0.56 g/t Au, 227 ppm U, 6.2 g/t Ag;
    Inferred Resource at
      0.3% Cu cut-off - 368 Mt @ 0.58% Cu, 0.21 g/t Au, 120 ppm U, 2.3 g/t Ag;
      0.5% Cu cut-off - 193 Mt @ 0.76% Cu, 0.26 g/t Au, 144 ppm U, 2.8 g/t Ag;
      0.7% Cu cut-off -   90 Mt @ 0.96% Cu, 0.30 g/t Au, 162 ppm U, 3.6 g/t Ag;
    Total Resource at
      0.3% Cu cut-off - 760 Mt @ 0.78% Cu, 0.30 g/t Au, 143 ppm U, 3.3 g/t Ag;
      0.5% Cu cut-off - 475 Mt @ 1.02% Cu, 0.39 g/t Au, 175 ppm U, 4.2 g/t Ag;
      0.7% Cu cut-off - 292 Mt @ 1.29% Cu, 0.48 g/t Au, 207 ppm U, 5.4 g/t Ag;

Ore reserves at 18 August, 2014 and mineral resources at 31 November 2013 (OZ Minerals ASX releases), were:
  Indicated + inferred resources at 0.3% Cu cutoff - 800 Mt @ 0.8% Cu, 0.3 g/t Au, 3.3 g/t Ag, 0.155 kg/t U;
  probable reserves, lift 1 from 470 to 970 m below surface - 110 Mt @ 0.9% Cu, 0.5 g/t Au, 5.3 g/t Ag;
  probable reserves, lift 2 from 970 to 1470 m below surface - 160 Mt @ 1.0% Cu, 0.4 g/t Au, 4.3 g/t Ag;
  TOTAL probable reserves - 270 Mt @ 0.9% Cu, 0.4 g/t Au, 4.5 g/t Ag.

JORC compliant Mineral resources as at 25 September 2015 (OZ Minerals press release to the ASX) were as follows, based on a AUD 120/t NSR (AUD=USD 0.78) cut-off:
  Indicated resource - 55 Mt @ 2.4% Cu, 0.9 g/t Au, 11.7 g/t Ag, 335 ppm U;
  Inferred resource 6 Mt @ 2.5% Cu, 0.7 g/t Au, 11.6 g/t Ag, 257 ppm U;
  TOTAL resource 61 Mt @ 2.4% Cu, 0.9 g/t Au, 11.7 g/t Ag, 328 ppm U.

For more detail see: Porter, T.M., 2010 - The Carrapateena Iron Oxide Copper Gold Deposit, Gawler Craton, South Australia: a Review; in Porter, T.M., (ed.), Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, v. 3 - Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide, pp. 191-200.

Khamsin Deposit

The satellite Khamsin copper-gold deposit, ~10 km to the northwest of Carrapateena, is also an iron oxide copper-gold (IOCG) occurrence. Mineralisation is hosted within the Khamsin Breccia Complex, a polymictic granite-hematite-carbonate breccia which is surrounded by altered Donington Suite granite and some dykes, and is overlain by approximately 460 to 680 m of mostly Neoproterozoic sedimentary cover. Mineralisation occurs as fine- to medium-grained blebby disseminations of chalcopyrite, rare bornite and minor chalcocite, and mostly occur as either disseminations in the breccia matrix or within clasts (OZ Minerals ASX release, 2014).
  The first drill hole into the deposit intersected 440.6 m @ 0.43% Cu, 0.08 g/t Au from 1005.4 m depth, including 26.7 m @ 1.48% Cu, 0.13 Au g/t from 1005.4 m, within a broader zone of 569.6 m @ 0.39% Cu, 0.08 g/t Au from 1003 m (Oz Minerals ASX Release, 21 January, 2013). The hole was drilled at a dip of -55° and azimuth of 173°. This mineralised intersection comprises a strong, grey hematite and chlorite altered, clast and matrix supported, heterolithic granite breccia.
  The alteration and mineralisation style encountered is comparable to that intersected on the margins of the main Carrapateena deposit. This mineralisation coincides with a significant gravity feature that is both larger in size and has the same intense residual gravity response as that related to the Carrapateena deposit. It displays a prominent co-incident magnetic feature located within the central portion of the gravity anomaly. Depths to basement for Khamsin vary between 480 and 630 m below surface (Oz Minerals, 2013).
  The Khamsin prospect was previously the Salt Creek anomaly that was tested by a number of vertical drill holes in the late 1970s and early 1980s following the discovery of Olympic Dam, targeting the gravity and/or aeromagnetic highs. One of these holes, SASC04, was completed at 1250 m and intersected hematite-sericite altered granitoids of the Mesoproterozoic Donington Suite from a depth of 540 m to the bottom of the hole, but without significant sulphide or associated copper mineralisation.

Mineral resources at 26 May 2014 (OZ Minerals ASX release), were:
  Indicated + inferred resources at 0.3% Cu cutoff - 202 Mt @ 0.6% Cu, 0.1 g/t Au, 1.7 g/t Ag, 0.086 kg/t U.

Return to top

Olympic Dam  -  Roxby Downs, Northern South Australia  -  BHP Billiton Limited   ...................... Friday 22 February, 2013.

The Olympic Dam copper-gold-uranium-REE ore deposit is located some 550 km NNW of Adelaide and 275 km NNW of Port Augusta, in northern South Australia (#Location: 30° 26' 24"S, 136° 53' 22"E).

Olympic Dam and all of the other significant known IOCG mineralised systems of the Mesoarchaean to Mesoproterozoic Gawler Craton are hosted within Palaeo- to Mesoproterozoic rocks, and are distributed along the eastern rim of the currently preserved craton to define the Olympic IOCG Province (Skirrow et al., 2007).   Olympic Dam lies below the Neoproterozoic Stuart Shelf, where >300 m of flat lying, barren, Neoproterozoic to lower Palaeozoic sedimentary rocks unconformably overlie both the craton and the ore deposit. Some 75 km to the east, this cover sequence expands over the major NNW trending Torrens Hinge Zone at the edge of the craton, into the thick succession of the north-south aligned Neoproterozoic Adelaide Geosyncline rift basin, that masks the mid- to late-Palaeoproterozoic suture between the Gawler craton and Palaeo- to Mesoproterozoic Curnamona Province to the east.

The oldest basement rocks in the Gawler craton are Meso- to Neoarchaean gneisses (to the west) and metasedimentary and meta-volcanosedimentary rocks, and deformed granites correlated with the Palaeoproterozoic 1.96 to 1.85 Ga Hutchison Group, the 1.79 to 1.74 Ga Wallaroo Group, and the 1.85 to 1.69 Ga Lincoln Complex (Donington Suite) granitoids, respectively. These rocks are intruded by the widespread Mesoproterozoic A- and I-type granitoids of the ~1.59 Ga Hiltaba Suite (with the former dominating in the Olympic IOCG Province) and are overlain by comagmatic bimodal volcanic rocks of the areally extensive Gawler Range Volcanics (GRV).

Mineralisation at Olympic Dam is hosted by the 50 km
2 Olympic Dam Breccia Complex (ODBC) that is developed within the Mesoproterozoic (1600 to 1585 Ma) Roxby Downs Granite. The Roxby Downs Granite is a pink to red coloured, undeformed, unmetamorphosed, coarse to medium grained, quartz-poor syenogranite with A-type affinities that is petrologically and petrochemically similar to granitoids of the Hiltaba Suite. Other lithologies within the ODBC comprise a variety of granite- to hematite-rich breccias, sedimentary facies, felsic/mafic/ultramafic dykes, volcaniclastic units, basalts and their altered/mineralised equivalents. The ODBC and the surrounding Roxby Downs Granite form a local basement high on a broader regional basement uplift.

Within the overall alteration envelope, the distribution of mineralisation and alteration exhibits a downward and outward zonation, while the ODBC correspondingly comprises a downward narrowing, funnel-shaped body of fractured, brecciated and hydrothermally altered granite which has resulted in a great variety of granitic, hematitic and siliceous breccias. The complex has a conical, downward tapering, central "core" of barren, but intensely altered hematite-quartz-breccia, passing outwards through concentrically zoned breccia types, including heterolithic hematite breccias (with clasts dominantly of granite and recycled hematite breccias, and domains where abundant sedimentary and volcaniclastics rocks predominate locally), to monoclastic granite breccias with a magnetite/hematite matrix, to weak incipient microfracturing of the RDG on the outer margins. A halo of weakly altered and brecciated granite extends out approximately 5 to 7 km from the core in all directions to an indistinct and gradational margin with the host granite. This progression represents an outward decrease in the degree of brecciation and intensity of iron metasomatism away from the core of the complex. The quantity of recycled hematite breccia, GRV and sedimentary rock clasts within the heterolithic hematite breccias decreases from shallow to deep levels (Ehrig, 2010; McPhie et al., 2010). The areal extent of more intensely hematite altered breccias within the complex is >5 km in a NW-SE direction, up to 3 km across, and is known to extend to a depth of at least 1400 m.

The development of the ODBC, which shows textural evidence of polycyclic alteration and brecciation events, can be considered as having formed by the progressive hydrothermal brecciation and iron metasomatism of the host granite. In detail, alteration assemblages are highly variable with complex mineral distribution patterns resulting from the polycyclic nature of the hydrothermal activity. Never-the-less, there are systematic patterns of alteration that are recognised across the deposit as a whole, and at the scale of individual breccia zones, with the degree of alteration intensity being directly related to the amount of brecciation.

The bulk of the mineralisation within the Olympic Dam deposit is associated with an assemblage of hematite-sericite-fluorite-barite-chalcopyrite-bornite-chalcocite (djurleite), the outer margin of which largely corresponds to the limits of the ODBC, where a deeper magnetite-carbonate-chlorite-pyrite±chalcopyrite zone marks the transition to the more regional magnetite-K feldspar±actinolite±carbonate assemblage (Ehrig, 2010). No associated sodic metasomatism has been observed.

The better mineralisation and strongest alteration outside of the barren core corresponds to the best-developed hematite-granite breccias. The concentric, moderate to steeply inward dipping breccia zones of the ODBC are cut by a convoluted, but overall roughly horizontal, ~50 m thick layer characterised by chalcocite and bornite, ~100 to 200 m below the unconformity with the overlying Neoproterozoic cover sequence. Both the upper and lower margins of this zone are mappable. Above the upper margin, sulphides are rare and little copper mineralisation is found in the same hematitic breccias. The lower margin marks a rapid transition to chalcopyrite, which decreases in copper grade downwards, corresponding to an increase in the pyrite:chalcopyrite ratio. While this zone is largely horizontal, as it approaches the central barren core it steepens markedly, but is still evident at depths of >1 km below the Neoproterozoic unconformity (Reeve et al., 1990; Reynolds, 2000; Ehrig, 2010). The geometry of this mineral zonation, strongly suggest interaction between upwelling and downward percolating fluids. For all fluids related to hematite alteration, fluid inclusion homogenisation temperatures are mostly between 150 and 300°C and salinities range from ~1 to ~23% NaCl equiv. (Knutson et al., 1992; Oreskes and Einaudi, 1992; Bastrakov et al., 2007).

The higher grade underground resource occurs as up to 150 separate bodies distributed within an annular zone up to 4 km in diameter surrounding the central barren hematite-quartz breccia. These bodies correspond to the overlap of the flat-lying chalcocite-bornite layer and the steeper, inwardly dipping ring of hematite-granite breccias.

The principal copper-bearing minerals are chalcopyrite, bornite, chalcocite (djurleite-digenite), which on the basis of Nd isotopic data, textural and geochemical features appear to have precipitated cogenetically. Minor native copper and other copper-bearing minerals are locally observed. The main uranium mineral is uraninite (pitchblende), with lesser coffinite and brannerite. Minor gold and silver is intimately associated with the copper sulphides. The main REE-bearing mineral is bastnaesite. Copper ore minerals occur as disseminated grains, veinlets and fragments within the breccia zones. Massive ore is rare.

At the end of 1989, after commencing mining operations in mid-1988, reported resources and reserves (Reeve et al., 1990) amounted to:
    Measured + indicated resource = 450 Mt @ 2.5% Cu, 0.6 g/t Au, 6.0 g/t Ag, 0.8 kg/tonne U
    Inferred resource = 2000 Mt @ 1.6% Cu, 0.6 g/t Au, 3.5 g/t Ag, 0.6 kg/tonne U
    Proved reserve = 13 Mt @ 3.0% Cu, 0.3 g/t Au, 10.2 g/t Ag, 1.1 kg/tonne U
    Proved gold reserve = 2.3 Mt @ 1.6% Cu, 3.6 g/t Au, 2.9 g/t Ag, 0.3 kg/tonne U

At December 2004, published (BHP Billiton, 2005) reserves and resources were:
    Proved+probable reserves totalled 761 Mt @ 1.5% Cu, 0.5 g/t Au, 0.5 kg/tonne U
    within a total resource of   3810 Mt @ 1.1% Cu, 0.5 g/t Au, 0.4 kg/tonne U

At 30 June 2012, the published resources (BHP Billiton, September, 2012) amounted to:
    Measured resource = 1474 Mt @ 1.03% Cu, 0.35 g/t Au, 1.95 g/t Ag, 0.30 kg/tonne U
    Indicated resource = 4843 Mt @ 0.84% Cu, 0.34 g/t Au, 1.50 g/t Ag, 0.27 kg/tonne U
    Inferred resource = 3259 Mt @ 0.70% Cu, 0.25 g/t Au, 0.98 g/t Ag, 0.23 kg/tonne U
    Total resource = 9576 Mt @ 0.82% Cu, 0.31 g/t Au, 1.39 g/t Ag, 0.26 kg/tonne U
This resource includes a total proved + probable reserve of:
    629 Mt @ 1.76% Cu, 0.73 g/t Au, 3.36 g/t Ag, 0.57 kg/tonne U
At the same date, the separate non-sulphide gold resource was 364 Mt @ 0.75 g/t Au, comprising:
    Measured resource = 73 Mt @ 0.85 g/t Au;   Indicated resource = 255 Mt @ 0.73 g/t Au;   Inferred resource = 36 Mt @ 0.70 g/t Au.

At 30 June 2015, the published resources (BHP Billiton Annual Report, 2015) amounted to:
    Measured resource = 1.330 Gt @ 0.96% Cu, 0.40 g/t Au, 2.0 g/t Ag, 0.29 kg/tonne U
    Indicated resource = 4.610 Gt @ 0.79% Cu, 0.32 g/t Au, 1.0 g/t Ag, 0.24 kg/tonne U
    Inferred resource = 4.120 Gt @ 0.71% Cu, 0.24 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U
    Total resource = 10.060 Gt @ 0.78% Cu, 0.30 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U
This resource includes a total proved + probable reserve of:
    484 Mt @ 1.95% Cu, 0.74 g/t Au, 4.0 g/t Ag, 0.59 kg/tonne U
    Stockpile - 44 Mt @ 0.99% Cu, 0.51 g/t Au, 2.0 g/t Ag, 0.37 kg/tonne U
At 30 June 2015, a separate non-sulphide gold resource was 283 Mt @ 0.84 g/t Au, which was not reported in 2015.

Production in 2011-12 totalled 192 600 tonnes of Cu, 3.66 t Au, 28.21 t Ag, 3885 tonnes U
Production in 2014-15 totalled 124 500 tonnes of Cu, 3.26 t Au, 22.52 t Ag, 3144 tonnes U

The mine is owned and operated by a subsidiary of BHP Billiton Ltd.

Return to top

Prominent Hill  -  SE of Coober Pedy, Northern South Australia  -  OZ Minerals   ...................... Saturday 23 February, 2013.

The Prominent Hill iron oxide copper-gold (IOCG) deposit is located approximately 150 km north-west of Olympic Dam and 650 km NNW of Adelaide in northern South Australia (#Location: 29° 42' 45"S, 135° 34' 48"E).

Prominent Hill, Carrapateena, Olympic Dam, Moonta-Wallaroo and Hillside, and all of the other significant known IOCG mineralised systems of the Gawler craton, are hosted within Palaeo- to Mesoproterozoic rocks and are distributed along the eastern edge of the currently preserved craton, to define the Olympic IOCG Province.

Cratonic to Domain-scale Setting

See the Carrapateena record for a summary of the regional, cratonic setting of the Olympic IOCG Province.

Prominent Hill lies on the southern margin of the Mount Woods Domain (MWD), which is located within the Olympic IOCG Province in the northeast of the preserved Archaean to Mesoproterozoic Gawler Craton. The MWD comprises Palaeo- and Mesoproterozoic metamorphic and igneous rocks that have a well defined aeromagnetic signature. It is characterised by high magnetic and gravity signatures, caused by multiple iron-oxide and mafic rock sources, and its boundaries are sharp and structurally controlled. The MWD encloses a major regional ~75 x 50 km magnetic complex with an overall 'U'-shape, elongated in a NW-SE direction and open to the SE. Outcrop is sparse, with flat lying Phanerozoic cover reaching thicknesses of up to 400 m in places, although it is generally <200 m.

The MWD comprises at least two separate sedimentary successions that have been subjected to one or more amphibolite to granulite facies metamorphic events, three periods of deformation, and two (probably three) episodes of magmatism, as well as a pulse of Neoproterozoic mafic dyke intrusion. Nine basement sub-domains have been defined, within the southern half of the domain within a 40 km radius of Prominent Hill.

The core of the southern half of the MWD is occupied by an 'eye-shaped', east-west elongated, 25 x 10 km kernel of magnetically anomalous rocks, reflecting the lopolithic White Hill Igneous Complex, which is characterised by extremely high amplitude magnetic lineaments on its margins, more subtle concentric magnetic zoning in its centre, and a very pronounced and complex gravity signature. The complex (where drilled) comprises pyroxenite, norite and gabbro, with pronounced layering defined by plagioclase and pyroxene-rich layers, with interleaved disseminated to massive magnetite-ilmenite bands. The gabbros and pyroxenites are hydrous, alkaline and enriched in volatile components, and carry up to 8% modal apatite. The complex is divided by a north-south fault into the White Hill and Joe's Dam sub-domains to the east and west respectively, based on differences in the magnetic pattern. The White Hill Complex is not as well developed in the Joe's Dam Sub-domain, which mainly comprises intercalated quartzo-feldspathic and magnetite-rich gneiss, and lesser mafic intrusive rocks.

The lenses-shaped 20 x 5 km Kennedy's Dam Sub-domain forms much of the northern margin of the White Hill and Joe's Dam sub-domains, and is characterised by relatively low magnetic susceptibility enclosing a few moderate to high amplitude, linear magnetic anomalies. Where drilled, it is composed of a sequence of quartz-feldspar-biotite-magnetite gneisses and “granulites” with very minor amphibolite and calc-silicate units, intruded locally by coarse grained granite and pegmatite.

The Skylark Sub-domain bounds the Kennedy's Dam Sub-domain to the NW, and is characterised by large intrusions with low to moderate magnetic responses, surrounded by narrow aureoles of higher magnetic intensity related to either hornfelsing or magnetite metasomatism. Granite with actinolite±magnetite veins and pyroxenites, norites and diorites dated at 1587±4 Ma (U-Pb; Jagodzinski, 2005) have been encountered in drill holes, intruding a variety of quartz-feldspar-biotite gneisses, all with accessory magnetite. Metasediments away from the intrusions include quartz-rich meta-sandstone with plagioclase, opaque oxide (magnetite and hematite), schistose biotite, fluorite and tourmaline. Samples of cordierite-garnet-bearing pelite and magnetite psammite have maximum depositional ages of ~1750 Ma respectively ( U-Pb zircon; Jagodzinski et al. 2007), and were intruded by the syn-metamorphic Engenina Adamellite (~1691±25 Ma; Finlay 1993; Daly et al. 1998). Geophysical data suggest the domain may be underlain by a continuation of the White Hill Igneous Complex.

The 50 x 5 km Taurus Sub-domain forms the NE margin the Kennedy's Dam Sub-domain, but persists further to the east, delineating the northern margins of the White Hill and then the Blue Duck sub-domains, and the southern margin of the Ware's Peak Sub-domain to the NE. This terrane, characterised by a series of short strike length curvilinear magnetic features by major discontinuities, is interpreted to represent rocks caught up between a major strike-slip shear couple.

The extensive Ware's Peak Sub-domain occupies a large area in the NE of the MWD and is characterised by a few high amplitude, strike continuous, but folded magnetic linears, and by large areas of diffuse magnetic signature suggesting moderately magnetic and eastward deepening younger cover. Drill holes testing prospects encountered a highly variable suite of magnetite, plagioclase and garnet bearing paragneisses (including graphite rich units), as well as feldspathic and quartz-poor to quartz-rich meta-igneous lithologies (including quartz diorite, monzodiorite, syenite, granite and pegmatite). Other lithologies include metasandstone, iron formation, calc-silicate, skarn, dolomite and marble units. Undeformed granite and gabbroic intrusions have also been identified in drilling. Metamorphic grades range from mid-amphibolite to granulite facies.

The curvilinear, lensoid 2 to 6 km wide and >60 km long Blue Duck Sub-domain extends along the southern margin of the Joe's Dam, White Hill, Taurus and Ware's Peak sub-domains. In the west, it forms the southernmost sub-domain of the MWD. It is thickest in the centre, where it is immediately to the north of the Neptune Sub-domain that contains the Prominent Hill deposit near the contact between the two terranes, and thins on either extremity. In the east, its southern margin is marked by the regional ENE-trending Bulgunnia Fault. It is characterised by narrow, medium amplitude magnetic linears which are continuous for several kilometres or more. Drilling at a number of locations (mostly aimed at magnetic targets), has encountered metamorphosed calcsilicate units with associated magnetite, schistose, recrystallised K feldspar-biotite-chlorite-quartz-scapolite clastic metasediments, iron-rich, pelitic carbonate rock, marble, calc-silicate, magnetite-pyroxene-quartz rock and possible meta-evaporites. These rocks have been tentatively correlated with the 1760 to 1740 Ma Wallaroo Group found elsewhere in the Olympic IOCG Province (Freeman and Tomkinson, 2010).

The elongate Neptune Sub-domain forms the southern boundary of the MWD over an interval of ~30 km, where it fringes part of the southern Blue Duck Sub-domain. It has an overall 'V' shape, turning on its eastern extremity from an east-west to WSW trend, along the northern side of the regional Bulgunnia Fault. It includes the Neptune Volcanics (which have been tentatively correlated on lithological grounds with the ~1590 Ma Gawler Range Volcanics) and the Prominent Hill Mine Sequence. Compared to terrains further to the north, it is characterised by relatively low amplitude linear aeromagnetic anomalies and contains a sequence of lower greenschist facies, relatively undeformed, mafic to felsic volcanic rocks (basalt-andesite-dacite-rhyolite), hematite-cemented quartz conglomerate, sandstone, argillite and dolostone. This sequence coarsens southwards from argillaceous and calcareous rocks into coarse grained siliciclastic rocks. The volcanic component in the structural footwall to the Prominent Hill deposit are basaltic to andesitic in composition, commonly porphyritic and amygdaloidal. Several kilometres to the west, felsic volcanic rocks (dacite to rhyolite) become much more voluminous with ‘redrock’ hematite dusting of alkali feldspar, compared to the predominantly sericite-chlorite-earthy hematite-leucoxene-carbonate alteration within the basalts and andesites of the mine sequence. Fragmental lithologies are intercalated with the coherent footwall volcanic rocks at Prominent Hill, and include agglomerate, felsic tuff or ignimbrite (Belperio et al., 2006) and volcanic clast conglomerate. They are also intercalated with mafic to intermediate volcanic rocks and hematitic, quartz-feldspar conglomerate and interbedded coarse grained sandstone. The east-west-trending Hangingwall Fault at Prominent Hill has been inferred to separate the hematite-stable sedimentary rocks of the copper-gold mineralised host sequence of the Neptune Sub-domain in the south, from a magnetite-stable hanging wall sequence of chloritic pelite and pelitic carbonate rock of the Blue Duck Sub-domain to the north. Proterozoic basement is overlain by 90 to 150 m of flat-lying Permo-Carboniferous sandstone and diamictite and Cretaceous sandstone and black claystone.

The Danae Hill Sub-domain forms the southern margin of the MWD to the southeast of the regional Bulgunnia Fault. It is characterised by a similar magnetic signature to the Neptune Volcanics, with low to moderate amplitude aeromagnetic linears. Drilling has identified a suite of altered, low-grade metamorphosed, sheared and brecciated basalts, with lesser metasediments and acid volcanic rocks, which collectively suggest a bimodal volcanic suite. These volcanics have a markedly different trace element signature similar to that of the Neptune Volcanics and are inferred to be of late Neoarchaean to Palaeoproterozoic in age.

The Christie Domain, is found to the immediate south of the of the MWD Neptune/Blue Duck and Danae sub-domains, and is dominated by latest Archaean to earliest Palaeoproterozoic metasedimentary protoliths, metamorphosed in the earliest Palaeoproterozoic at ~2450 Ma during the Sleaford Orogeny to become the Christie Gneiss of the Mulgathing Complex. The MWD and has been overthrust onto the Christies Domain from the NE, with a major north dipping thrust marking the boundary (Betts et al., 2003).

In summary, the MWD is predominantly composed of ~1760 to ~1740 Ma Palaeoproterozoic meta-sedimentary and meta-volcanic rocks, tentatively correlated with the Wallaro Group seen elsewhere in the Olympic IOCG Province, including banded iron formations (possibly equivalent to the Middleback Range BIFs further to the south). These successions, which may include inliers of older late Neoarchaean to lower Palaeoproterozoic metamorphic rocks, underwent peak metamorphism at ~1736±14 Ma to amphibolite and granulite facies. They are bounded on the southern margin of the domain by a narrow band of younger, low- to mid-greenschist facies, ~1590 Gawler Range Volcanics equivalents and intercalated volcanic and sedimentay breccias of the Neptune Sub-domain, hosting the Prominent Hill ore deposit. All of these rocks have been intruded by the ~1691±25 Ma Engenina Adamellite, the extensive Hiltaba Suite 1584±18 Ma Balta Granite and the 1587±4 Ma mafic to ultramafic White Hill Igneous Complex.

The presence of Hiltaba-aged zircon interpreted as metamorphic (Holm, OZCHRON) in quartzites and felsic gneisses suggests that metamorphic grade in the MWD at ~1590 to ~1580 Ma was significantly higher than is typical elsewhere in the Gawler Craton during that period. This, in turn, suggests that the MWD was at deeper crustal levels than the adjacent Olympic, Christie and Wilgena Domains.

Prominent Hill Deposit

The Prominent Hill deposit was discovered under approximately 100 m of cover, and is reflected by a discrete gravity anomaly, the target of the discovery drillhole (Carter et al. 2003), corresponding to the hydrothermal iron altered (magnetite-deficient) hematite matrix breccias that host the copper-gold-silver-uranium-cerium-lanthanum ore deposit within the Neptune Sub-domain and to corresponding palaeotopographic highs. The peak of the gravity anomaly coincides with a mass of massive, barren "steely" hematite-silica flooded volcanics (on the eastern end of the main ore zone), flanked to the west by a ~2 km long mineralised hematite matrix-supported breccia. The overall gravity anomaly has an east pointing "V" form, with a 1 km long northern arm trending WNW-ESE, and the southern 2 km long limb trending WSW-ENE. The southern limb reflects the main mineralised hematite breccia, with the peak of the anomaly near the hinge, and has no magnetic expression (Belperio et al., 2006).

The associated 750 m long magnetic anomaly coincides with the northern limb of the gravity feature. It is centred ~500 m to the north of the orebody, across the Hangingwall Fault in the Blue Duck Sub-domain (Hart and Freeman 2003), and reflects a package of magnetite-chlorite-tremolite-phlogopite altered metasomatic 'skarn-like' altered calc-silicate/carbonate-rich metamorphics intercalated with highly altered, intermediate, porphyritic intrusives and chlorite matrix tectonic breccias (Belperio et al., 2006).

The deposit comprises several phases of hematite alteration with associated sericite, clay minerals and chlorite, hosted by a northerly dipping series of interbedded lithic sandstone and greywacke, siltstones, coarse grained sedimentary and volcanic breccias, and dolostone. These rocks are found in the immediate footwall to the transition from the Neptune to Blue Duck sub-domains (marked by a relatively brittle, steep northward dipping fault, known as the Hangingwall Fault Zone in the mine). Magnetite is absent in the breccias which host the mineralisation but to the north of the deposit, and separated from it by components of the Hangingwall Fault, is a body of massive magnetite with associated, pyrite, actinolite, phlogopite, chlorite, serpentinite, carbonate and talc (magnetite "skarn"). This body is very poorly mineralised in copper and gold and its relationship to the Prominent Hill orebody immediately to the south is not understood. Copper, gold, uranium and REE mineralisation is relatively late stage and overprints at least one phase of massive hematite replacement of the breccias.

The intense hematite alteration within the coarse grained breccias is locally texturally destructive and this has inevitably led to varying interpretations as to their origin. A recent re-evaluation (Freeman and Tomkinson, 2010) of the breccias has shown that they originated as a sequence of very coarse to fine-grained, laminated, clastic sedimentary rocks, that have been subject to later hydrothermal replacement with only minor additional brecciation during metasomatism. Distinctly bedded breccias, with bedding defined by the alignment of clasts and by grading, occurs over intervals to ~5 to 10 m within more massive, non-layered, poorly sorted hematite-matrix breccia.

In the open pit, individual bodies of breccia occur as a series of strata-bound, steeply dipping, tabular east-west trending sheets and westerly plunging shoots, collectively bound to the north by dolostone/Hangingwall Fault Zone and to the south by volcanic rocks. At the Western Copper deposit the stratigraphy, which includes mineralised hematite breccia and carbonaceous rocks, appears to be complexly folded.

The host sequence rocks are intensely altered by hematite-sericite-chlorite-carbonate (±quartz±barite±fluorite±REE phosphates). Copper mineralisation occurs as fine grained disseminations of chalcocite, bornite and chalcopyrite in the breccia matrices and (to a lesser extent) within clasts of hematite-rich breccias. The copper sulphides display a variety of intergowth, replacement and infill textures including chalcocite-bornite and replacement of early formed pyrite.

The current weight of evidence indicates that the Prominent Hill mineralisation originated through relatively passive infiltration of hydrothermal fluids and metasomatism localised by porosity within a sequence of coarse grained sedimentary breccias. The amount of further brecciation that can be attributed to hydrothermal processes is unclear. The copper and gold mineralisation can be shown to be paragenetically very late in the sequence and a direct genetic relationship between the copper and gold and the hematite is not proven. Geochemical trends suggest a strong relationship between gold mineralisation (in the copper-rich zones of the deposit) and REE phosphates.

Global reserves and resources at the deposit as of May 2010 stood at: 278.8 Mt @ 0.98% Cu, 0.75 g/t Au and 2.5 g/t Ag.

The declared reserves and resources at Prominent Hill in mid 2008, prior to the commencement of production in 2009, were:

Copper resource - Measured + indicated + inferred resources at 0.5% Cu cut-off: 174.20 Mt @ 1.39% Cu, 0.56 g/t Au, 3.4 g/t Ag.
Gold resource - Measured + indicated + inferred resources at 0.5 g/t Au cut-off & <0.5% Cu: 109.2 Mt @ 0.09% Cu, 1.21 g/t Au, 1.0 g/t Ag.
TOTAL Resource  -  Measured + indicated + inferred resource: 283.4 Mt @ 0.89% Cu, 0.81 g/t Au, 2.48 g/t Ag.
Western Copper resource (additional)  -  Inferred resources at a 0.5% Cu cut-off:  14.5 Mt @ 1.69% Cu, 0.28 g/t Au, 3.7 g/t Ag.

The declared reserves and resources at Prominent Hill in June 2011 were (OZ Minerals, 2012):

Proved + probable reserves:   72.3 Mt @ 1.13% Cu, 0.64 g/t Au, 3.03 g/t Ag. (included within resources)
Copper resource - Measured + indicated + inferred resources:  214.9 Mt @ 1.23% Cu, 0.5 g/t Au, 2.8 g/t Ag.
Gold resource - Measured + indicated + inferred resources:  57.8 Mt @ 0.07% Cu, 1.5 g/t Au, 1.1 g/t Ag.

Remaining reserves and resources at Prominent Hill in June 2015 were (OZ Minerals, 2015):

Proved + probable reserves:   73 Mt @ 1.0% Cu, 0.6 g/t Au, 2.9 g/t Ag.
Copper-gold resource - Measured + indicated + inferred resources:  152 Mt @ 1.2% Cu, 0.6 g/t Au, 2.8 g/t Ag.
Gold resource - Measured + indicated + inferred resources:  27 Mt @ 0.1% Cu, 1.3 g/t Au, 1.5 g/t Ag.

This description is largely based on Freeman and Tomkinson, 2010.

Return to top

Cloncurry Workshop  -  Led by Dr Nick Oliver (of HolcombeCoughlinOliver)   ...................... Sunday 24 February, 2013.

This workshop is to be held in Cloncurry, NW Queensland, and will involve both classroom and field components.   It will include:
  • An overview of the key characteristics and origins of IOCG deposits,
  • The geology and IOCG deposits of the Cloncurry district - including:
    - regional to local setting,
    - key geological and geochemical aspects relative to IOCG mineralisation,
    - distribution and controls on alteration and mineralisation in the Cloncurry terrane,
    - context to the deposits we will see and key details of those we wont be visiting, (e.g., Osborne, Eloise)
    - characteristics pertinent to exploration,
  • Interspersed field visits to nearby geological features illustrating the geology and regional alteration related to IOCG events.
Return to top

Ernest Henry and E1 Mines  -  NE of Cloncurry  -  Xstrata Copper, Ernest Henry Mines   ...................... Monday 25 February, 2013.

The Ernest Henry IOCG style Cu-Au deposit is located 35 km NE of Cloncurry, 150 km east of Mt Isa and 750 km west of Townsville in north-west Queensland (#Location: 20° 26' 40"S, 140° 42' 21"E).

The deposit lies to the east of the Cloncurry Overthrust, within the Cloncurry-Selwyn zone of the Cloncurry Terrane, which comprises the eastern exposed margin of the Mount Isa Inlier of North-west Queensland. It contains IOCG deposits that are hosted by Palaeoproterozoic (1760-1660 Ma) silici-clastic metasedimentary and metavolcanic rocks that were deposited during periods of ensialic rifting.

The Ernest Henry deposit is hosted within the Eastern Succession of the Mount Isa Inlier, that consists of a poly-deformed Palaeo- and Mesoproterozoic volcano-sedimentary succession which is largely composed of evaporite-rich Cover Sequence 2 and silici-clastic-rich Cover Sequence 3 rocks (CS2 and 3). CS2 and 3 were deposited between 1790 and 1690 Ma and from 1680 to 1610 Ma respectively. To the west, these sequences overlie an older crystalline basement and a core of predominantly Cover Sequence 1 felsic volcanic and related intrusive rocks that correspond to the 1870 to 1850 Ma Barramundi Orogeny of northern Australia. Basement is not exposed in the Cloncurry district. Both CS2 and 3 were deposited in intracontinental rift settings, although the relationship between some parts of the sequence is obscured by the deformation history. Both sequences were also accompanied by the emplacement of various intrusive and volcanic rocks.

The first significant deformation to affect CS2 (but not CS3) was the 1750 to 1735 Ma Wonga extensional event. CS2 was extensively intruded by the 1750 to 1730 Ma Wonga Granite to the west, while the coeval Mount Fort Constantine volcanics are found to the NE. Minor tonalites, granitoids and diorite emplaced between CS2 and 3 have been dated at 1686 to 1660‚ÄāMa (including the Ernest Henry Diorite).

Thin skinned deformation of the ~1600 to 1520 Ma Isan Orogeny terminated deposition of Cover Sequence 3, and resulted in gross eastward tectonic transport, interleaving of major lithostratigraphic units, and a dominant north-south tectonic grain. This deformation has been divided into: a D1 event, which involved overall north-south compression, and is characterised by large-scale thrusts and isoclinal folds, thrust reactivation of large, km-scale, basin bounding extensional faults with CS3 rocks thrust over CS2, resulting in overturned limbs and a penetrative rock mass foliation; a D2 event, involving horizontal east-west compression producing major north-south upright to isoclinal folding of CS2 and 3 rocks, and a penetrative cleavage, which peaked at 1595 to 1580 Ma with a regional greenschist to upper amphibolite facies metamorphism and the development of anatectic pegmatites; and a D3 event which includes NW- and NE-trending brittle-ductile corridors of faulting, kinking and folding with steep plunges to the NW and NE, and dominantly north-south trending shear and fault zones and associated breccia formation.

Both CS2 and 3 were intruded by the voluminous Williams and Naraku granite batholiths at 1540 to 1500 Ma (including the 1530 Ma Mt Margaret Granite immediately to the east of the E1 deposits; Marshall and Oliver, 2007; Page and Sun, 1998). These represent the youngest felsic intrusions in the inlier, and have an outcrop exposure of >1500 km
2. They were emplaced in an intracratonic environment, and have a pre-, syn- and post-D3 timing, and are largely composed of alkaline to sub-alkaline, K-rich, A-type, magnetite-bearing granitoids. They range from diorite to syenogranite in composition and are typically more oxidised than similar older (~1670 Ma) granitoids in the Western Fold Belt of the Mount Isa Inlier. Sodic intrusions of similar age are rare.

A regionally extensive Na-Ca hydrothermal system in the Cloncurry district (>1000 km
2) affected all rock types, especially the resultant calc-silicate-rich lithologies of cover sequence 2. This alteration appears to have been formed by multiple periods of hydrothermal activity that locally overlapped and is most intense in breccia zones along large structural conduits and within calc-silicate-rich units. The bulk of the sodic-calcic alteration, dominantly regional albite and scapolite, was associated with fluids that were initially mostly sedimentary formation waters with lesser magmatic components, prior to and during peak metamorphism (Kendrick et al., 2008; Oliver et al., 2008; Baker et al., 2008). Subsequent more structurally controlled albite-actinolite-magnetite-titanite±clinopyroxene assemblages, were synchronous with major granite (e.g., Williams-Naraku batholiths) emplacement (Baker et al., 2008), involving a larger magmatic fluid component, and coincided with formation of the majority, but not all of the significant oxide Cu-Au deposits. These deposits may have some stratigraphic control, but are usually associated with brittle and brittle-ductile shear and fault structures which acted as conduits for the transport of high temperature (300 to 500°C) saline fluids into the host rocks (Williams, 1998).

The Ernest Henry deposit is concealed by 35 to 60 m of extensive Phanerozoic cover and does not outcrop. While the exact stratigraphic position of the host rocks is not known, they have been tentatively correlated with the 1730 ±10 Ma Mount Fort Constantine Meta-volcanics towards the top of Cover Sequence 2. The Mount Fort Constantine metavolcanics comprise dacite and andesite with subordinate metabasalts and calc-silicate metasedimentary rocks. The only other outcrop in the district is the 1480 Ma Mount Margaret granite some 12 km to the east. Within Cover Sequence 2, volcanism is common between 1790 and 1780 Ma, and 1760 to 1720 Ma, with later 1540 to 1450 Ma granitoids.

Within the immediate orebody area the principle lithologies encountered are: i). altered plagioclase phyric andesitic volcanic/hypabyssal rocks (ca 1740 Ma) which host the orebody where they are brecciated; ii). various siliciclastic, calc-silicate-rich and graphitic metasedimentary rocks that occur as <10 m thick intercalations within the metavolcanic rocks; and, iii). medium-grained metadiorite (ca 1660 Ma).

Structural analysis suggests that ore deposition accompanied reverse-fault movement between two northeast trending bounding shear zones and formed a pipe-like zone of dilation in the K-feldspathised metavolcanic rocks. The breccia pipe, plunges at ~45° to the SSESSE, nested between the ductile shear zones (Rusk et al., 2010). The orientation of this dilational zone is consistent with the shape and dip of the Ernest Henry ore breccia.

Four stages of alteration are recognised at Ernest Henry:
i). Regional pre-ore Na-Ca alteration, occurring mainly as albitic plagioclase-, magnetite-, clinopyroxene- and amphibole-rich veining and fault-related breccia-fill.
ii). Pre-mineralisation potassic-(manganese-barium) alteration which only contains minor sulphides, and is typified by multiple stages of K feldspar-, biotite-, amphibole-, magnetite-, garnet- and carbonate-bearing veins, and by fault-related breccia and alteration.
iii). Mineralisation associated alteration, characterised by K feldspar veining and alteration. K feldspar alteration is most intense in the vicinity of copper-gold mineralisation, but forms a halo extending from several hundred meters up to 2 km beyond the ore body (Mark et al., 2006), although this outer halo may represent part of pre-ore regional alteration zone. Mineralisation is divided into two main stages, characterised by similar mineral assemblages. The first stage of economic Cu-Au mineralisation was the main ore-forming event, associated with a matrix-supported hydrothermal breccia that is enveloped by crackle veined K feldspar altered meta-volcanic rocks. The second stage of mineralisation occurs as a network of veins cutting earlier infill-supported ore-breccias, and contains a largely identical mineralogy to earlier stage. The ore-bearing assemblage dominantly comprises magnetite, pyrite, chalcopyrite, carbonate and quartz, with lesser apatite, barite, titanite, actinolite, biotite and fluorite. In the upper levels of the deposit, the bulk of the ore is present as hypogene chalcopyrite infilling between K feldspar-altered breccia clasts, while at greater depths, it both infills between, and replaces clasts. Electrum and native gold are closely associated with pyrite and chalcopyrite (Foster et al., 2007).
iv). Post-ore, volumetrically minor, multiple stage calcite-dolomite- and/or quartz-rich veining and alteration which lacks magnetite, and only carries a little gold. Deeper in the deposit, breccias include rounded clasts of previously mineralised breccias containing magnetite, pyrite and chalcopyrite, indicating multiple superimposed brecciation events (Rusk et al., 2010).

Rusk et al. (2010) interpret the data from Ernest Henry to be consistent with the following genetic trend:
i). Rapid devolatilisation (of possibly both chloride-rich brines and CO
2-rich fluids) within the source magma chamber;
ii). Fluid over-pressuring in the roof of the magma chamber as a result of volatile exsolution and vapour expansion, assisted by a seal created by magma solidification, sodic-calcic alteration and/or contact metamorphism in the carapace of the igneous complex;
iii). Possible leakage of over-pressured magmatic fluid along structures controlling the location of the later breccia pipe, producing a pre-ore potassic alteration halo;
iv). The eventual failure of the seal and sudden release of fluid pressure, resulting in a high-energy fluid flow event driving brecciation and upward transported and milled clasts. The resultant breccia mass permitted the mixing and/or subsequent ingress of basinal brines circulating within fractured rocks several kilometres above the magma chamber. Fluid mixing, rapid depressurisation and resultant cooling led to ore precipitation within the matrix porosity between breccia clasts at the top of the orebody, where, as the fluid flow, temperature and pressure declined the breccia was sealed;
v). At depth, closer to the heat source, the temperature and pressure gradient degraded more slowly, allowing for fluid-rock reaction to be more protracted, such that prolonged chemical interaction between K feldspar-rich host rocks and ore fluids led to replacement style mineralisation within clasts, with the same mineral assemblage as observed in the shallower parts of the deposit.
vi). At the deepest levels, repetition of the cycle may have resulted in the release of a new pulse of fluids which brecciated and tapped earlier formed magnetite-chalcopyrite rich rocks, telescoping mineralised clasts upwards into the orebody along narrow channels, thereby upgrading ore.

The brecciated volcanic mass that hosts the ore forms a plunging elongate body, some 250 m thick, 300 m average length and extending at least 1000 m down plunge to the SSE. The breccia ranges from the unbrecciated volcanics, to crackle fracture veining to clast supported and matrix supported breccia to total clast digestion (massive matrix). The breccias typically contain 5-20 mm subrounded to rounded meta-volcanic and rare biotite altered meta-sedimentary clasts. The matrix is largely composed of magnetite, calcite, pyrite, biotite, chalcopyrite, K feldspar titanite and quartz. Accessory minerals include garnet, barite, molybdenite, fluorite, amphibole, apatite, monazite, arsenopyrite, a LREE fluorcarbonate, galena, cobaltite, sphalerite, scheelite, uraninite and tourmaline. The bulk of the economic mineralisation is restricted to breccia zones with more than 10% matrix.

The total reserve + resource prior to the commencement of mining in 1998 was 166 Mt @ 1.1% Cu, 0.54 g/t Au.
As of June 2003 the remaining resource totalled 117.9 Mt @ 1.13% Cu, 0.52 g/t Au.
At 30 June 2006, the reserves and resources were (Xstrata, 2007):
    Open cut proved reserves - 41 Mt @ 0.9% Cu, 0.5 g/t Au + probable reserves of 20 Mt @ 0.8% Cu, 0.4 g/t Au,
    Open cut measured + indicated resources were the same as, and included the proved and probable reserves,
    Open cut inferred resources - 1 Mt @ 0.4% Cu, 0.2 g/t Au,
    Underground indicated resources - 21 Mt @ 1.5% Cu, 0.7 g/t Au + inferred resources of 23 Mt @ 1.4% Cu, 0.7 g/t Au,

Open pit as at December, 2011 (Xstrata, 2012):
    Total resource and reserve - depleted during 2011 from 17 Mt @ 1.0% Cu, 0.5 g/t Au, 23% magnetite at December 31, 2010
Underground as at December, 2011 (Xstrata, 2012):
    Measured resource - 4 Mt @ 1.3% Cu, 0.7 g/t Au, 32% magnetite
    Indicated resource - 71 Mt @ 1.3% Cu, 0.7 g/t Au, 28% magnetite
    Inferred resource - 13 Mt @ 1.2% Cu, 0.6 g/t Au, 26% magnetite
    Total resource - 88 Mt @ 1.3% Cu, 0.7 g/t Au, 28% magnetite
    Total ore reserve (all probable) - 74 Mt @ 0.95% Cu, 0.5 g/t Au, 23% magnetite.

The operation is controlled by Ernest Henry Mining Pty Ltd, a subsidiary of Glencore Xstrata Ltd.

The Mount Margaret mining operation exploits the Monakoff and E1 group of IOCG-style copper-gold deposits in the Cloncurry District of NW Queensland, Australia.   Monakoff is located ~15 km ENE of Cloncurry, and is 20 km south of the Ernest Henry deposit. The E1 group of deposits are 22.5 km to the NNE of Monakoff and 8 km east of Ernest Henry.   The Mount Margaret ore is treated at the Ernest Henry mine (#Location: E1 20° 26' 36"S, 140° 47' 10"E; Monakoff 20° 37' 30"S, 140° 41' 20"E).

All of these deposits are hosted within the Eastern Succession of the Mount Isa Inlier, which consists of poly-deformed Palaeo- and Mesoproterozoic volcano-sedimentary Cover Sequences 2 and 3 (CS2 and 3), deposited between 1790 and 1690 Ma and from 1680 to 1610 Ma respectively.

For details of the regional setting and geology of the northeastern Cloncurry district, see the Ernest Henry record.

E1 Group

The four main E1 group deposits (E1 North, South, East and Central) are distributed over a NNW-SSE elongated area of approximately 1750 x 650 m. They are overlain by between 15 and 40 m of unconsolidated Mesozoic to Cenozoic cover, underlying an essentially flat pastoral landscape, and are hosted in a series of steeply dipping, folded and brecciated meta-sediments and meta-volcanic lenses that belong to the Soldiers Cap Group of CS3.

The host sequence in the deposit area comprises a lower unit of variously brecciated, silicified and 'red-rock' altered, meta-sedimentary and calc-silicate rocks, that include polymictic breccias. These pass up into a 100 to 200 m thick basalt, overlain by a trachyte unit with a similar thickness, followed in turn by black shale. It is likely that the 'red-rock' altered, meta-sedimentary and calc-silicate rocks belong to the Doherty/Corella Formation of CS2, and the basalt and trachyte units represent the Toole Creek Volcanics of the CS3 Soldiers Cap group.

In the immediate deposit area, this sequence has been tightly folded to form an 'S' shaped structure, representing two limbs of a NNW-SSE-trending syncline, with a tight, complexly faulted anticlinal axis to the NNW, and a faulted, but slightly less faulted synclinal axis to the SSE. The structure is truncated by two faults, the regional, north-south-trending Mount Margaret to the east, and another semi-parallel north-south to NNE-SSW-trending fault 1.5 km to the west.

The full sequence is represented on the western limb of the main syncline, with the black shale occupying the core of the structure. However, a fault parallel to, and just to the east of the fold axis, juxtaposes the black shale of the synclinal core, with brecciated metasedimentary rocks to the east, with both volcanic units being structurally removed. These rocks are then cut to the east by the Mount Margaret granite. Sill-like lenses of dolerite cut the brecciated metasedimentary rocks on the eastern limb of the syncline. Black shales are found to the east across the Mount Margaret Fault from the synclinal closure to the SSE.

Continuity of the host sequence to the west of the anticlinal closure in the NNW is unclear from the data available.

A folded, NNW-SSE-elongated, ~550 x 150 m structural inlier of undifferentiated mafic volcanic rocks, with intercalated black shales, is enclosed within the brecciated metasedimentary rocks in the northern part of the eastern limb, hosting the E1 East orebody. This enclave includes two steeply dipping (structurally repeated ?) 20 to 75 m thick banded-massive ironstone lenses that host the sulphide ore over strike length of up to 300 m, terminated at ~280 m below the Mesozoic unconformity by calc-silicate/polymict breccias.

The E1 North deposit is hosted by a series of steeply dipping lenses of magnetite-rich metasedimentary and metavolcanic rocks in the main faulted anticlinal closure to the NNW. The immediate deposit is bounded by two NNE-trending faults to the east and west, each dipping inwards and intersecting at depth, although mineralisation is repeated across these faults to the southeast. The main body plunges steeply over a down-plunge distance of >300 m and widths of 20 to 100 m. A high grade, flat lying blanket of supergene mineralisation straddles the base of oxidation, laterally outwards for up to 100 m from the steep hypogene ore zone, over thicknesses of 15 to 65 m, with intersections such as 64.6 m @ 2.87% Cu, 0.32 g/t Au, 153 ppm U
3O8 from 31.4 to 96 m, including 16.6 m @ 9.04% Cu, 0.72 g/t Au & 280 ppm U3O8 from 31.4 to 48 m.

The E1 South deposit is contained in a parallel series of 6 nested/stacked, folded, parallel and concordant geologically and geochemically defined lenses of banded-massive ironstone. These lenses are folded as a NNW plunging synclinal structure, with the axial plunge shallowing to the NNW and with depth. Individual lenses are 10 to 30 m in thickness, and are largely restricted to the trachytic unit, from the contact with the underlying basalt at the base, to the margin with the overlying black shale. The most extensive of these lenses are truncated to the east by the axial fault (which removes the volcanic units of the eastern limb of the syncline, as described above), and extend around the fold nose to the western limb where most lens-out. These lenses are continuous over varying strike lengths of between 100 and 600 m, and to a maximum vertical depth of 320 m below the unconformity (Exco Resources, 2008).

The E1 Central deposit represents one (or more?) of the E1 South 'lenses' which is continuous over a strike length of more than 1 km along the NNW-trending western synclinal limb, to connect with the E1 North deposit. At the unconformity with the overlying Mesozoic to Cenozoic cover, this mineralised ironstone follows the contact between the basalt and trachyte units, and is underlain by a variably developed zone (up to a few metres thick) of carbonate alteration within the basalt.

The sulphide copper-gold mineralisation occurs predominantly within magnetite, pyrrhotite, and chalcopyrite-pyrite mineral assemblages. The principal components include magnetite (20 to 30%), micas (25 to 30%), quartz (12 to 15%), ankerite/calcite (5 to 15%), pyrite (3 to 12%), alkali feldspars (2 to 7%), fluorite (1.5 to 2.5%), apatite (0.5 to 2.5%), chalcopyrite (1.5 to 2.5%), bornite (0 to 0.1%), sphalerite (0 to 0.15%).

The source of information for the E1 group of deposits was drawn from reports by Exco Resources dated from 2007 to 2011, and sketch maps included therein.


Two major units predominate in the Monakoff area: the Soldiers Cap Group (1680 to 1660 Ma; Page and Sun, 1998) of the informally defined Maronan Supergroup, which is part of CS3, and the Corella Formation of CS2 (~1740 Ma; Page, 1988). Surface geology and seismic interpretation indicates the two to always be in tectonic contact. The two major contacts between these units are the Cloncurry Overthrust and the Pumpkin Gully Fault. The Soldiers Cap Group comprises a conformable carbonate-poor stratigraphic succession that passes upwards from the pelites and arenites of the Llewyllen Creek Formation, through the quartzo-feldspathic arenite, carbonaceous pelite, metagreywacke, metabasalt and iron formation of the Mount Norna Quartzite, to the metabasalt, minor carbonaceous metasedimentary rocks and iron formation of the Toole Creek Volcanics (Derrick et al., 1976). Sills and dykes of dolerite were widely intruded, predominantly prior to the main metamorphism, and in some cases before consolidation. The Corella Formation comprises well bedded scapolitic carbonate and quartzo-feldspathic layers, which is inferred to have been deposited within a shallow marine to evaporitic shelf environment (Reinhardt,1986). It has been subjected to strong intra-formationally brecciated and in places contains exotic clasts. Cu-Au mineralisation in the district is mostly hosted within the Soldiers Cap Group, with only minor occurrences adjacent to or within the Corella Formation breccias within 1 km of more significant accumulations.

Monakoff occurs within an unusual east-west trending belt of Soldiers Cap rocks, on the south-dipping limb of the Pumpkin Gully Syncline which is considered to be a regional east-west fold formed during D2 deformation. The deposit occurs along a tectonic contact between the Mount Norna Quartzite and the Toole Creek Volcanics (Ashley, 1983), and corresponds to a regionally extensive linear magnetic anomaly, with sporadic iron formations and ironstones as its outcrop expression. No granitic intrusives have been identified at Monakoff. The nearest granites, 2 km to the north, are outcropping hills of the Naraku Batholith, with the main metamorphic aureole to the batholith occurring a further 2 km NW.

The host Soldiers Cap rocks are separated from Corella breccia to the north and west by D1 thrust faults, while a splay of the northern thrust (the Monakoff Shear) hosts the Monakoff mineralisation (Davidson, et al., 2002). An east-west orientated fault separates the western and eastern zones of mineralisation. The Monakoff deposits comprise sulphide copper-gold mineralisation hosted by a well defined, east-west striking iron-rich alteration zone within steeply south dipping meta-sediments and amphibolites. The western zone of mineralisation forms a steeply east-dipping sheet 700 m long by 2 to 10 m thick, which is open at depth. The smaller eastern zone, which is 100 m northeast of the end of the western zone, forms a pipe-like breccia body that plunges very steeply to the west, with a 40 m strike length at the surface. Monakoff East is slightly deeper in the stratigraphy than the Monakoff West Zone (Davidson et al., 2002).

The upper 100 m of footwall lithologies comprise thinly (1 to 3 mm) intercalated magnetite-bearing muscovite pelites, psammopelites, and meta-dolerites of the uppermost Mount Norna Quartzite, overlain by massive porphyroblastic garnet-biotite schist (mostly of intrusive andesitic igneous origin, although the upper 3 to 5 m are vesicular), then a regionally persistent magnetite iron formation, and finally by strongly sheared metasediments immediately hosting the Monakoff West ore. The continuous, 1 to 2 m thick, prominently banded, quartz-magnetite±hematite iron formation can be traced through the whole prospect, occurring ~5 to 10 m above the top of the porphyroblastic garnet-biotite schist, hosted by pelitic metasediment. Banding varies from 1 to 2 mm 'wrinkly' lamination (possibly crenulations), to 0.5 cm interbands of quartz-barite, and spessartine-magnetite. The metadolerites are present as continuous sills, locally with angular contacts and variable epidote alteration, particularly adjacent to sediment contacts.

The hanging wall above the thin strip of sheared metasediment that hosts ore, commences with a 20 to 30 m thick interval of short strike-length metabasalts, minor iron formation, volcanic conglomerate, and breccia bearing clasts of limestone and meta-dolerites. A local pillow breccia outcrops close to the ore package near the centre of the deposit, while black carbonaceous limestone and stromatolitic cherts are also reported. More common rocks of the Toole Creek Volcanics occur above this complex, comprising massive medium- to coarse-grained meta-dolerite to gabbro in the west, and mixed meta-dolerite, meta-basalt, siliceous siltstone and minor iron formations in the east (Davidson et al., 2002).

The "ore package" is defined as the altered rocks adjacent to the ore, the ores themselves, and the overlying altered sediments and basaltic tuff, all of which young and dip to the south.   Near-ore alteration in the footwall commences with porphyroblastic spessartine-biotite-quartz-plagioclase-chloritoid-tourmaline-biotite development within the meta-andesite, with idioblastic garnet occuring as disseminations or irregular bands localised on peperite zones. Locally, pink spessartine is overgrown by coarse-grained almandine + quartz, indicating peak metamorphism exceeded the almandine isograd. The timing of almandine stability is uncertain (Davidson et al., 2002).

The major enriched components of the ore compared to the country rocks are: S (>20% S), Ba (>25% BaO), F (~2 to 10% F), Fe (>10% FeO) and Ca (~10% CaO), with significant enrichments of minor element including economic Co, Cu, Au and Ag, plus, Pb, Sb, As, W, U, La, Ce and Zn. P
2O5 abundances are low, with most ores containing only 0.2 to 0.4 wt.% P2O5. The highest values are systematically found in the iron formation, although no strong Fe-P correlation exists when all of the data is considered. La and Ce abundances are also very high, with values typically of >1000 ppm. Rather than apatite, the host phase for REE, U, and possibly P2O5, is monazite, which is common in ore intersections, and concentrates the LREE (Davidson et al., 2002).

Post-peak metamorphic alteration is abundant above the garnet-biotite schist, zoned around the ores, low-angle shears and tension gashes. The general alteration sequence distributed over widths of 0.5 to 10 m from mineralised zones is: i). an outer chlorite-spessartine; ii). biotite-magnetite±pyrite; to iii). siderite-magnetite±pyrite assemblages, as ore or sulphide-bearing veins/fractures are approached. When this alteration occurs within meta-siltstones it takes the form of pseudo-breccias where 'clasts' have distinct dark biotite-rich centres. Carbonate alteration also preferentially affects thin basaltic dykes or extrusives. Between the Monakoff West and East deposits, porphyroblastic garnet-biotite schist is replaced by unmineralised albite around a sinistral D3- to post-D3 fault, This assemblage is not seen elsewhere in the prospect, although regionally it is common in association with post-orogenic granites (Davidson et al., 2002).

The western Monakoff ore zone is enclosed by, and replaces, magnetite-bearing meta-siltstones, occurring at surface as a friable, but resistant, massive unit with variable black pyrolusite and malachite staining. At depth it occurs as a massive unit composed of barite, ponite (Fe-rhodochrosite), magnetite, chalcopyrite, pyrite, spessartine, fluorite ±K-feldspar, sphalerite, galena, arsenopyrite, mackinawite, molybdenite, brannerite/davidite, pentlandite and linnaeite (Ashley, 1983).

The eastern Monakoff ore zone appears to mainly comprise replacement of a medium- to coarse-grained amphibolite, within a tight D2 fold adjacent to an unexposed east-west fault, represented at surface by buff-coloured silica, studded with randomly oriented crystals of coarse magnetite, magnetite-hematite, and hematite, that are consistent with faithful pseudomorphing of meta-gabbro. The silica is most likely a surficial regolith replacement product of carbonate. At depth this rock-type is a package of comparatively thin massive meta-dolerites and intercalated peperitised sediments, with the contact between the two lithologies preferentially silicified, and an alteration zonation into the sediment of i). siderite-magnetite-pyrrhotite-chalcopyrite; ii).  magnetite-siderite-quartz, to iii). biotite-quartz-magnetite over 1 m or less, while the adjacent dolerite contains incipient siderite alteration. It is interpreted that in the core of the fold, severe alteration affected the interior of the folded metadolerite as well as the sediment margins. The fluids, which were tightly focussed by the fold structure, produced wholesale replacement of dolerite, and a narrow west-plunging pipe with grades of 1.4 to 3.0% Cu, consisting of siderite-barite-magnetite-chalcopyrite, which is likely to have considerable depth extent (Davidson et al., 2002 and sources quoted therein).

Resource and reserve figures for the Mount Margaret Operation deposits were:
E1 group of deposits (Exco Resources, 2010 - Xstrata, 2012 quotes the same tonnages and grades to lesser decimal poiints) :
    Measured resource - 9.17 Mt @ 0.87% Cu, 0.25 g/t Au
    Indicated resource - 24.7 Mt @ 0.71% Cu, 0.21 g/t Au
    Inferred resource - 14.2 Mt @ 0.64% Cu, 0.2 g/t Au
    Total resource - 48.1 Mt @ 0.72% Cu, 0.21 g/t Au
Monakoff + Monakoff East deposits (Exco Resources, 2010):
    Indicated resource - 2 Mt @ 1.39% Cu, 0.44 g/t Au
    Inferred resource - 2 Mt @ 1.3% Cu, 0.4 g/t Au
    Total resource - 4 Mt @ 1.32% Cu, 0.42 g/t Au
Monakoff + Monakoff East deposits (Xstrata, 2012):
    Indicated resource - 2 Mt @ 1.4% Cu, 0.4 g/t Au
    Inferred resource - 1 Mt @ 1.2% Cu, 0.4 g/t Au
    Total resource - 3 Mt @ 1.33% Cu, 0.4 g/t Au
Nearly 3500 t of U
3O8 is contained within these deposits. The average in-situ recoverable grade is ~112 ppm U3O8.

Return to top

Mount Elliott and Merlin  -  Selwyn District, NW Queensland  -  Ivanhoe Australia   ...................... Tuesday 26 February, 2013.

The SWAN and Mount Elliott deposits are located within the Eastern Fold Belt of the Mount Isa Inlier in northwest Queensland, approximately 90 km south of Cloncurry and 20 km north of the Selwyn deposits (#Location: 21° 32'S, 140° 30'E).

For geological background on the setting, see the Cloncurry IOCG Province record.

These two deposits are part of the same mineralised system in which host rock chemistry and rheology control mineralisation style within the same low grade envelope. The two deposits are separated by a major steep fault zone.

The Eastern Fold Belt comprises a sequence of 1890 to 1610 Ma metasediments and volcanics that have been intruded by three main phases of igneous rocks. On a local scale, the SWAN and Mount Elliott deposits are situated close to the contact of the Kuridala and Stavely Formations. Mount Elliott occurs within the phyllite, schist and black shale of the Kuridala Formation at the base of the Soldiers Cap Group, which in the deposit area comprises a package of intensely skarn-altered (clinopyroxene ±actinolite, magnetite, scapolite and apatite) phyllites, schists and metadolerites. SWAN is hosted within a breccia that has been emplaced into a package of banded and brecciated calc-silicates and calcareous sediments of the Stavely Formation, a younger unit which has been correlated with the Mount Norna Quartzite. The structural (overturned) footwall lithology to mineralisation at SWAN is a massive calc-silicate (coarse-grained calcite, tremolite, albite, scapolite, actinolite, muscovite and chlorite), layered calc-silicates (1 to 10 cm interlayered and boudinaged bands of very fine-grained siliceous material, commonly hematite-stained and albite-altered, alternating with coarse-grained calcite, tremolite, albite, scapolite, actinolite, muscovite and chlorite) and intensely altered and sheared metasediments (Brown, Lazo, Kirwin and Corlett, 2009).

A 200 to 400 m wide body of metadiorite/metagabbro lies within the southern portion of SWAN, comprising medium- to fine-grained actinolite, plagioclase, biotite, quartz, tremolite, magnetite, epidote and calcite. It has a relatively unaltered core, although the margin adjacent to the SWAN breccia is strongly hematite-stained, albite altered and brecciated (Brown, Lazo, Kirwin and Corlett, 2009).

The majority of the SWAN mineralisation is hosted by the SWAN breccia, which is crackle- to matrix-supported, with angular to rounded, strongly albite-altered calc-silicate and metadolerite fragments set in a fine- to coarse-grained matrix of hematite-stained albite, clinopyroxene, actinolite, magnetite, calcite, pyrite and chalcopyrite. The fragments are from centimetres to metres across. A 30 to 100 m thick banded calc-silicate unit forms the eastern margin of both the SWAN breccia and the massive calc-silicates, and comprises regular 0.5 to 3 cm thick bands of hematite-stained albite, magnetite, clinopyroxene, actinolite, epidote and calcite, which are commonly mineralised on margin of the breccia (Brown, Lazo, Kirwin and Corlett, 2009).

The banded calc-silicates grade, over a 10 to 30 m interval, into a quartz, muscovite, and chlorite schist to the east.

The SWAN deposit is cross-cut by several narrow, 1 to 30 m thick, ~30° SE dipping, late-stage, pink to grey felsic (plagioclase, K-feldspar, quartz, chlorite after biotite, titanite, magnetite, pyrite and chalcopyrite) dykes with chilled margins. These dykes cut the mineralised breccias and pre-date late-stage mineralised veins. A 5 to 30 m wide, commonly mineralised, steeply dipping, dominantly sinistral strike-slip fault separates the SWAN and Mount Elliott deposits. Numerous smaller anastomosing faults cut the main mineralised breccia and appear to have channelled late-stage fluids, dissolving the carbonate gangue of the breccia leaving friable highly porous ore (Brown, Lazo, Kirwin and Corlett, 2009).

Four main styles of primary Cu-Au mineralisation are recognised within the Mount Elliott and SWAN deposits: i). The Mount Elliot breccia, ii). the SWAN breccia, iii). banded/replacement mineralisation and iv). late vein hosted mineralisation. These styles are manifested as follows. At Mount Elliott, mineralisation is dominantly within the Mount Elliot breccia, with 0.1 to 20 m wide clasts set in a very coarse-grained, open spaced matrix containing voids up to tens of metres wide. The fragment within the SWAN breccia are much smaller than in the Mt Elliot breccia, and mineralisation at the former is considered to have formed some time after the development of the breccia which created a large porous and chemically suitable trap. Mineralisation permeated, replaced and altered the banded calc-silicate on the eastern margin of the SWAN breccia, forming banded mineralisation of magnetite, hematite-stained albite, chalcopyrite, pyrite, clinopyroxene, actinolite and epidote. Late-stage, 1 to > 200 cm thick veins of coarse-grained calcite, chalcopyrite, pyrite and molybdenite represent an event that crosscuts all of these mineralisation styles (Brown, Lazo, Kirwin and Corlett, 2009).

Mt Elliot mine was earlier (Shiqi Wang and Williams, 2001) described as containing 2.9 Mt @ 3.33% Cu, 1.47 g/t Au, hosted by carbonaceous meta-pelites and amphibolites, occurring in two zones controlled by NW trending, NE dipping brittle faults in a 200 m wide zone of intense post peak metamorphic alteration. It was interpreted as comprising an older, outer, system of albitised meta-sediments (bleached by the loss of biotite and carbonaceous material), over-printed by a "skarn" zone of diopside-hedenbergite veins and replacement features with abundant scapolite, apatite and calcite. Sulphides and magnetite (from outer pyrrhotite-pyrite to chalcopyrite-pyrrhotite-pyrite to chalcopyrite-pyrite-magnetite to magnetite-pyriteĪandradite in the core) occur within the skarn, veining and replacing clinopyroxene and intergrowing with calcite.

Brown, Lazo, Kirwin and Corlett (2009) quote a resource at SWAN and Mt Elliot of:

    475 Mt @ 0.5% Cu, 0.38 g/t Au at the cut-off grade of 0.25% eCu, still open at depth, including a high grade resource of:
      62 Mt @ 1.01% Cu, 0.4 g/t Au at a cut-off of 1.0% eCu.

Return to top

Roseby Project  -  NW of Cloncurry  -  Altona Mining   ...................... Wednesday 27 February, 2013.

The Roseby Project supergene oxide copper deposits, including Blackard, Scanlan, Longamundi, Legend, Great Southern, Caroline and Charlie Brown and the related Little Eva and associated primary IOCG deposits, are located within the Mt Roseby Corridor, approximately 65 km NNW of Cloncurry in north-west Queensland, Australia.

For geological background on the setting, see the Cloncurry IOCG Province record.

The Mt Roseby Corridor is occupied by weakly metamorphosed calc-silicates, siliciclastics and minor intermediate igneous rocks of the Mesoproterozoic Cover Sequence 2 Corella Formation in the Eastern Succession of the Mt Isa Inlier. In addition to the Little Eva and the Blackard deposits, the corridor also embraces the Dugald River discordant sediment-hosted (47.9 Mt at 12.1% Zn, 2.1% Pb, 44 g/t Ag) and Lady Clayre silica-dolomite hosted deposits (14 Mt at 0.56% Cu, 0.19 g/t Au).

Little Eva is the largest primary IOCG-style sulphide copper-gold deposit within the Roseby Project, and is located ~6 km north from the Blackard deposit. Fresh rock is overlain by a 5 to 25 m thick weathered zone of copper oxide mineralisation, in which copper occurs in both iron oxide and secondary 'oxide-zone' minerals (e.g., malachite). Mineralisation extends over a strike length of 1200 m, and is hosted within an intermediate feldspar porphyry unit of probable volcanic or shallow intrusive origin, which strikes north and dips 60° E. The porphyry ranges in width from 30 m in the north to over 300 m in the central zone. In the north, the porphyry is mineralised over its entire width, although in the central zone, while copper is ubiquitous, grades are lower. This mineralisiton occur as a series of linear, parallel sheets of higher grade separated by lower grade material. Within these zones, copper occurs on average as 0.1 to 2% coarse gained chalcopyrite as disseminations and veinlets, while only minor amounts of other sulphide minerals are present. The host rock is veined and altered to a hematite-albite-carbonate-quartz assemblage. Mineralisation is present to depths of >350 m. Metallurgical testing established an expected copper recovery of 95.8% to a concentrate grading 27.3% Cu and gold recovery of 94% for a concentrate grade of 5 g/t Au.

The Blackard-style mineralisation occurs as a group of supergene native copper deposits within the Mt Roseby Corridor. They are distributed along a north-south oriented, 5 to 60 m wide, folded and faulted copper-bearing unit within the Corella Formation, referred to as the Roseby Cupriferous Horizon. This horizon is exposed semi-continuously over a strike-length of around 30 km and is typified in outcrop by malachite-stained scapolite-biotite schist.

In the zone of oxidation and supergene enrichment, an approximately 30 m thick oxide cap which hosts some malachite resources, overlies a thick zone, locally extending to depths of 240 m, of supergene copper enrichment which comprises native copper with minor chalcocite. The supergene mineralisation is hosted by soft, clayey, scapolite schists derived from a dolomitic quartz siltstone and calc-silicate sequence. Native copper occurs as blebs, wafers and wire forms within the soft host.

The main Blackard deposit has a length of around 4 km and width of generally 100 m, but over sections up to as much as 400 m. The 0.5% cut-off mineralisation averages 40 to 80 m in vertical thickness, the top of which is at approximately 40 m below the surface.

The main Scanlan deposit is 10 km south of Blackard and has a length of around 1.5 km and width of generally 100 m, but over sections up to as much as 400 m. The 0.5% cut-off mineralisation averages 40 to 80 m in vertical thickness, the top of which is at a depth of approximately 40 m.

Below the base of weathering and the copper-only supergene deposits, the primary mineralisation comprises disseminated copper sulphides within carbonate-altered meta-siltstone and scapolitic calc-silicate. The sulphides are commonly zoned within the horizon, with chalcopyrite being found at the top, grading into bornite and finally to chalcocite at the base. Other sulphides are rarely seen, and the chalcocite and bornite have the characteristics of being primary in origin.

Generally, the cupriferous horizon within this interval does not have sufficient width or grade to be economic, although, in zones of increased deformation, particularly zones of tight to isoclinal folding, the width and grade is significantly enhanced.

At the Blackard deposit, lenses of hypogene copper mineralisation are found within the hinge zones of a syncline-anticline pair where the mineralisation occurs as fine-grained disseminated chalcopyrite, bornite and chalcocite within calcite alteration, breccia matrix and veining. Mineralisation is zoned, with low-grade, chalcopyrite-dominated assemblages surrounded by a broad halo of weak calcite alteration around these lenses, grading into bornite-chalcocite in the core of the lenses. This mineralisation occurs as disseminations, blebs and veinlets associated with hydrothermal alteration.

The global resource for deposits of the Mt Roseby Corridor in 2012 (Altona Mining Ltd ASX release, August, 2012) was:
  Measured resource - 63.2 Mt @ 0.65% Cu, 0.05 g/t Au,
  Indicated resource - 76.7 Mt @ 0.55% Cu, 0.06 g/t Au,
  Inferred resource - 120.1 Mt @ 0.56% Cu, 0.04 g/t Au,
  Total resource - 260.1 Mt @ 0.58% Cu, 0.05 g/t Au,
      comprising 123.4 Mt @ 0.55% Cu, 0.10 g/t Au (Cu-Au deposits) + 136.7 Mt @ 0.61% Cu (Cu only deposits).
The largest copper-gold deposit is:
    Little Eva - 100.3 Mt @ 0.54% Cu 0.09 g/t Au
The larger copper only deposits within this resource are:
    Blackard - 76.4 Mt @ 0.62% Cu,
    Scanlan - 22.2 Mt @ 0.65% Cu,
    Longamundi - 10.4 Mt @ 0.66% Cu.

Return to top

The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published.   Most of these sources are listed on the "Tour Literature Collection" soon to be available from the IOCG 2013 Tour options page.

Porter GeoConsultancy Home | More on This Tour | Other Tours | New Tours

For more information contact:   T M (Mike) Porter, of Porter GeoConsultancy   (

This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.

Porter GeoConsultancy Pty Ltd
6 Beatty Street
South Australia
Telephone: +61 8 8379 7397
Mobile: +61 422 791 776

PGC Logo
Porter GeoConsultancy Pty Ltd
 International Study Tours
     Tour photo albums
 Ore deposit database
PGC Publishing
 Our books  &  bookshop
     Iron oxide copper-gold series
     Super-porphyry series
     Porhyry & Hydrothermal Cu-Au
 Ore deposit literature
 What's new
 Site map