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Hillside

South Australia, SA, Australia

Main commodities: Cu Au
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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.

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.


  References & Additional Information
   Selected References:
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), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3 pp. 147-170
Ismail, R., Ciobanu, C.L., Cook, N.J., Teale, G.S., Giles, D., Schmidt Mumm, A. and Wade, B.,  2014 - Rare earths and other trace elements in minerals from skarn assemblages, Hillside iron oxide-copper-gold deposit, Yorke Peninsula, South Australia: in    Lithos   v.184-187 pp. 456-477
Ismail, R., Lin, Y., Ciobanu, C.L. and Cook, N.J.,  2014 - The Hillside Cu-Au Deposit, South Australia: A Preliminary Fluid Inclusion Study: in    Acta Geologica Sinica   v.88, Supp.2, pp. 1454-1456.
Kontonikas-Charos, A., Ciobanu, C.L., Cook, N.J., Ehrig, K., Ismail, R., Krneta, S. and Basak, A.,  2018 - Feldspar mineralogy and rare-earth element (re)mobilization in iron-oxide copper gold systems from South Australia: a nanoscale study: in    Mineralogical Magazine   v.82 (S1), pp. S173-S197.


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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