Ernest Henry

Queensland, Qld, Australia

Main commodities: Cu Au
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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 Ma (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 km2. 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 plc.

The most recent source geological information used to prepare this summary was dated: 2011.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

Ernest Henry

  References & Additional Information
   Selected References:
Betts, P.G., Giles, D., Mark, G., Lister, G.S., Goleby, B.R. and Ailleres, L.,  2006 - Synthesis of the Proterozoic evolution of the Mt Isa Inlier: in    Australian J. of Earth Sciences   v.53, pp. 187-211.
Collier P A and Bryant J D,  2003 - Successful Mineral Resource Development at the Ernest Henry Copper-Gold Mine, NW Queensland: in   5th International Mining Geology Conference, Bendigo, Vic, 17-19 November, 2003 The AusIMM, Melbourne,    Conference Proceedings,  pp 73-88
Craske T E  1995 - Geological aspects of the discovery of the Ernest Henry Cu-Au deposit, northwest Queensland: in    Australian Institute of Geoscientists   Bull. 16 pp 95-109
Derrick G M  1996 - The geophysical approach to metallogeny of the Mt Isa Inlier - what sort of orebody do you want: in    Proc The AusIMM Annual Conference, Perth, 24-28 March, 1996 The AusIMM, Melbourne    pp 349-366
Foster, D.R.W. and Austin, J.R.,  2008 - The 1800-1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes: in    Precambrian Research   v.163, pp. 7-30.
Haynes D W,  2000 - Iron Oxide Copper (-Gold) Deposits: Their Position in the Ore Deposit Spectrum and Modes of Origin: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.1 pp. 71-90
Kendrick M A, Mark G and Phillips D,  2007 - Mid-crustal fluid mixing in a Proterozoic Fe oxide–Cu–Au deposit, Ernest Henry, Australia: Evidence from Ar, Kr, Xe, Cl, Br, and I: in    Earth and Planetary Science Letters   v256 pp 328-343
Mark G, Wilde A, Oliver N H S, Williams P J and Ryan C G,  2005 - Modeling outflow from the Ernest Henry Fe oxide Cu–Au deposit: implications for ore genesis and exploration : in    J. of Geochemical Exploration   v85 pp 31-46
Mark, G., Oliver, H.S., Williams, P.J., Valenta, R.K. and Crookes, R.,  2000 - The hydrothermal evolution of the Ernest Henry Fe-Oxide (Cu-Au) system: in Porter T M (Ed), 2000 Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.1, pp. 123-136.
Mark, G., Oliver, N.H.S. and Williams, P.J.,  2006 - Mineralogical and chemical evolution of the Ernest Henry Fe oxide-Cu-Au ore system, Cloncurry district, northwest Queensland, Australia: in    Mineralium Deposita   v.40, pp. 769-801.
Oliver N H S, Rubenach M J, Fu B, Baker T, Blenkinsop T G, Cleverley J S, Marshall L J and Ridd P J,  2006 - Granite-related overpressure and volatile release in the mid crust: fluidized breccias from the Cloncurry District, Australia: in    Geofluids   v.6 pp. 346-358
Rusk B, Oliver N, Blenkinsop T, Zhang D, Williams P, Cleverley J and Habermann P,  2010 - Physical and Chemical Characteristics of the Ernest Henry Iron Oxide Copper Gold Deposit, Australia; Implications for IOCG Genesis;: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3 pp. 201-218
Ryan A J  1998 - Ernest Henry copper-gold deposit: in Berkman D A, Mackenzie D H (Eds),  Geology of Australian and Papua New Guinean Mineral Deposits The AusIMM, Melbourne    pp 759-768.
Smith R J,  2002 - Geophysics of Iron Oxide Copper-Gold Deposits: in Porter T M (Ed.), 2002 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.2 pp. 357-367
Webb M, Rowston P, Donohue J  1996 - The geophysics of the Ernest Henry Cu-Au deposit (NW) Qld: in    James Cook Univ. of N Qld, Econ. Geol. Res. Unit, Contributions   no. 55 pp 143-145
Williams P J  1998 - An introduction to the metallogeny of the McArthur River-Mt Isa-Cloncurry Minerals Province: in    Econ. Geol.   v93 pp 1120-1131
Williams P J, Barton M D, Johnson D A, Fontbote L, de Haller A, Mark G, Oliver N H S and Marschik R,  2005 - Iron oxide copper-gold deposits: Geology, space-time distribution and possible modes of origin: in Hedenquist J W, Thompson J F H, Goldfarb R J and Richards J P (Eds.), 2005 Economic Geology 100th Anniversary Volume, Society of Economic Geologists, Denver,    pp 371-405
Williams, P. J., Kendrick, M.A. and Xavier, R.P.,  2010 - Sources of Ore Fluid Components in IOCG Deposits: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3, pp. 107-116.
Williams, P.J. and Pollard, P.J.,  2003 - Australian Proterozoic Iron Oxide-Cu-Au Deposits: An Overview with New Metallogenic and Exploration Data from the Cloncurry District, Northwest Queensland: in    Exploration & Mining Geology, CIM   v.10, No. 3, pp. 191-213.
Zhu, Z.,  2016 - Gold in iron oxide copper-gold deposits: in    Ore Geology Reviews   v.72, pp. 37-42.

   References in PGC Publishing Books: Want any of our books ? Pricelist
Mark G, Oliver N H S, Williams P J, 2000 - The Evolution of the Ernest Henry Fe-Oxide-(Cu-Au) Hydrothermal System,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 123-136
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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