Cairn Hill

South Australia, SA, Australia

Main commodities: Fe Cu Au
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The Cairn Hill magnetite-copper-gold deposit is located 700 km NW of Adelaide, 55 km south of Coober Pedy and ~65 km NW of the Prominent Hill IOCG deposit in South Australia (#Location: 29° 18' 13"S, 135° 07' 22"E).

It is located in the northern half of the Mount Woods Domain (MWD) on the northeastern margin of the preserved Gawler craton, and also lies within the Olympic IOCG Province that encloses all of the significant IOCG and related deposits of that craton.

For background on the Olympic IOCG Province see the detailed description in the separate Carrapateena record.

The MWD is composed of Palaeo- and Mesoproterozoic metasedimentary rocks and intrusions of syn- to post-tectonic granitoids, which have a variable intensity aeromagnetic signature, located in the NE of the Gawler craton, lying to the east and SE of the Coober Pedy Ridge and the Mabel Creek Inlier. These rocks abut an interpreted Archaean age cratonic core to their south and west. The block contains major regional structures (including the east-west-trending Karari Fault Zone which marks the northern boundary between the MWD and the Coober Pedy Ridge) and is traversed by several prominent NW-trending structures along which significant thicknesses of Permian sediments have been deposited. In the Cairn Hill area these older rocks are overlain by Tertiary and Quaternary sedimentary cover of variable thickness overlying Cretaceous Bulldog Shale with minor Mount Anna Sandstone of the Lower Cretaceous Cadna-owie Formation. The Bulldog Shale, which comprises marine and silty shales, with minor sandstone lenses is exposed in breakaways surrounding the mine area. The Mount Anna Sandstone outcrops to the south of the main pit.

The Cairn Hill deposit is a structurally controlled magnetite-rich IOCG deposit hosted by mid-amphibolite to granulite facies quartzo-feldspathic gneisses, belonging to the Palaeoproterozoic (1750 Ma) Mount Woods Complex and Mesoproterozoic (1600 to 1575 Ma) Hiltaba-equivalent Balta-suite granites and granodiorites. It is reflected by a high intensity magnetic anomaly that appears to be located along a major east-west to ENE trending fault or shear zone, defining a major structural zone that forms part of the complex boundary between the Mt. Woods Domain and the Coober Pedy Ridge. On a regional scale this fault/shear zone corresponds to an intense magnetic anomaly, although in the deposit area it is less extreme, except on its very western end.

The iron, copper and gold mineralisation on the mine site occurs within two main E-W striking sub parallel sulphidic quartz-magnetite-apatite units, the North- and South- Lodes, that have been tested over a 1.3 km strike to a depth of ~170 m, within a defined strike length of ~17 km. These lodes are coincident with local structures within the Cairn Hill Shear Zone, and are concordant with the strike of the encompassing magnetic anomaly. The larger of the mineralised iron bands is 10 to 40 m thick, the smaller, which is up to 200 m into the hanging wall is only ~5 to 10 m thick in the pit. Within the northern of the lenses, copper mineralisation only persists over a strike length of ~750 m, although the magnetite continues beyond the pinch out of the copper zone. The southern zone in mineralised over a strike length of ~475 m (Clarke et al., 2014).

These lodes may be broad equivalents of the 1950 to 1850 Ma (or older - see the Middleback Ranges record) iron formations of the Middleback Ranges near Whyalla some 400 km to the SSE, also in the Gawler craton. At Cairn Hill, the lodes are characterised by stacked massive magnetite lenses with variable amounts of vein and blebby pyrite, chalcopyrite±pyrrhotite±bornite and sphalerite. Irregular to massive quartz veining occurs in places through the ore zone and are referred to as "quartz magnetite" zones. The lodes are intercalated within a suite of Palaeoproterozoic quartz-feldspar-biotite-magnetite gneisses which form both the hanging wall and footwall to the mineralised interval within the mine area. These gneisses comprise variable amounts of banded felsic, mafic and magnetite rich zones which texturally vary from fine-grained through to very coarse-grained, with occasional granite-pegmatoid bands. The MWD to the south of these two more intense magnetic ridges, is occupied by further anastomosing, but less intense magnetic ridges. Approximately 100 to 250 m to the north of the northern sulphidic lode, the sequence changes across a generally east-west boundary into non-magnetic gneiss and granite, although a further ~500 m north, more magnetic ridges are apparent, until a linear feature ~1.6 km to the north of the deposit marks a rapid transition to a magnetically bland terrane.

Hydrothermal activity was likely localised by E-W striking structures belonging to the Cairn Hill Shear Zone, defining the northern edge of the Mount Woods Inlier, in a transpressional (dilational) jog environment. Alteration involves the following stages, from oldest to youngest (Clarke et al., 2014):
• Early Na-Ca alteration, affecting the host rocks, characterised by scapolite-diopside-albite┬▒andradite-grossular garnet±actinolite±titanite.
• Extensive K-metasomatism which has overprinted the sodic phase in the host rocks, overprinted by localised zones of intense, texturally-destructive high-temperature magnetite-biotite alteration that is typical of a transitional-style IOCG system. Associated hypogene iron mineralisation predominantly consists of magnetite.
• Skarn development - which is poorly constrained, only occurring as narrow zones of <10 m wide, identified at depth in drill holes. This lithotype is characterised by a texturally complex andradite-grossular garnet-scapolite-magnetite-diopside skarn, immediately adjacent to a hornblendite. Overprinting retrograde alteration is characterised by epidote-group minerals, actinolite/tremolite±titanite-quartz-oligoclase-biotite, talc, flourapatite, magnetite, titanite and chlorite.
• Main mineralising event, occurring as a pyrite-pyrrhotite-chalcopyrite-bornite-chalcocite-hematite-sphalerite-galena-LREE-Y assemblage. Native gold primarily occurs as inclusions within chalcopyrite-pyrite, as well as minor electrum. Mineralisation is associated with zones of quartz brecciation and occurs in veins overprinting massive hypogene magnetite within shear zones.
• Late stage alteration - abundant, possibly Fe-rich, chlorite and orthoamphibole, which pervasively replace all of the previous mineralised zones.

Although copper and gold mineralisation is closely associated with the magnetite horizons, it also occurs in the adjacent gneissic host rocks, clearly post-dating the magnetite ironstone lithologies and highlighting its secondary nature. The mineralisation has been interpreted to represent a recrystallised BIF, overprinted in parts by with copper/gold.

Cairn Hill lies within a recognised IOCG province and is characterised by an ore assemblage that includes iron oxides (magnetite), copper and gold. However, it differs from the IOCG deposits of the province in that the bulk of the pre-existing iron oxide upon which the Cu and Au is overprinted is a banded iron formation, rather than iron oxide related to magmatic hydrothermal processes.

JORC compliant inferred + indicated resources within the 1.3 km tested to a depth of 170 m (the initial pit resource), total 11.4 Mt @ 49.5% Fe, 0.4% Cu, 0.1g/t Au, plus an inferred resource of 3.8 Mt @ 43.8% Fe (IMX Resources website).

Processing of the ore involves magnetic separation to produce a magnetite concentrate for pelletisation, and flotation to produce a copper-gold concentrate. Mining operations commenced in December, 2010. The ore is trucked 58 km to the Rankin Dam rail siding on the Darwin-Adelaide railway line, where it is loaded onto a train for the 879 km journey to Port Adelaide.

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 8/9/2012
This description is a summary from published sources, the chief of which are listed below.
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