Defining the style of mineralisation at the Cairn Hill magnetite-

sulphide deposit; Mount Woods Inlier, Gawler Craton, South

Australia

Jesse M. Clark1, Nigel J. Cook1, Cristiana L. Ciobanu1,Anthony J. Reid1, 2, Peter Hill3

1Centre for Tectonics, Resources and Exploration, (TRAX), University of Adelaide, North Terrace, SA 5005, Australia

2Department for State Development GPO Box 1264, Adelaide, SA, 5001, Australia.

3IMX Resources, 47 Tynte Street, North Adelaide SA 5006 Australia

IMX Resources Author Contact:

jessemattclark@gmail.com

Introduction

Petrography and Mineralogy

Conclusions

Geological Setting

1. Forty grab samples were collected from the Cairn Hill open pits to undertake a petrographic-mineralogical

study. The sample suite covers a 1.3 km-long E-W interval, includes primary ore zones, structures and altera-

tion assemblages and is thus considered representative.

2. Geological mapping of the deposit was undertaken to understand and interpret regional and local structural and

lithological diversity. Accompanied with paragenetic diamond drill-core logging of three drill-holes (~1.5 km)

targeting both ore-lodes and adjacent magnetic anomalies.

3. Microscopic observations were conducted using a Nikon LV100 polarizing petrographic microscope in trans-

mitted and reflected light, and using a Quanta 450 Scanning Electron Microscope (SEM) equipped with an en-

ergy dispersive X-ray spectrometer (EDS) to provide semi-quantitative information

Methodology

Regional Geology

The Mount Woods Inlier (MWI) is a geologically complex

region representing remnants of an accreted Palaeoprote-

rozoic terrane comprising sparsely outcropping, poly-

deformed high temperature amphibolite to granulite facies

metamorphosed supracrustal rocks[4]. The geological evo-

lution of the MWI is poorly constrained. Three major de-

formational events have shaped the inlier: D1: Regional,

high temperature metamorphism at ~1736 Ma coinciding

with early stages of Craton-wide Kimban Orogeny (ca.

1730-1690 Ma); D2: Regional deformation and exhuma-

tion coinciding with early stages of Karanan Orogeny (ca.

1690-1670 Ma), and; D3: Deformation associated with

emplacement of the Hiltaba Suite granitoids. Coincides

with late stages of Karanan Orogeny (ca. 1565-1540 Ma).

Figure 1: Location map of the Gawler Craton, including the N-NE

trending Olympic Cu-Au (IOCG) Province and Mount Woods Inlier.

Selected IOCG prospects are shown, including the Cairn Hill deposit.

References

Figure 2: Interpreted geology and major structures of the Mount Woods Inlier,

showing the location of the Cairn Hill deposit and Cairn Hill Shear Zone.

The 1.6 Ga Olympic Cu-Au Province is a metallogenic

belt extending over 700km along the eastern margin of

the Proterozoic Gawler Craton, South Australia. The

Province hosts a diverse range of iron-oxide-copper-

gold (IOCG) systems from the giant Olympic Dam Cu

-Au-U deposit, to numerous barren or weakly mineral-

ised systems[1]. Definition of the IOCG deposit clan re-

mains a contentious issue, primarily due to mis-

classification and poor understanding of many individ-

ual deposits[2]. This study tests the hypothesis that the

Cairn Hill Fe(-Cu-Au) deposit represents a deeper end-

member, magnetite-rich IOCG deposit. If confirmed,

characterisation of the deposit and its geological set-

ting provides major implications for regional metallog-

eny and crustal architecture of the Mount Woods Inlier,

and also the Gawler Craton.

The Cairn Hill Fe(-Cu-Au) deposit is located 55km SE

of Coober Pedy, South Australia, and was discovered

in 2005. Indicated resources as of March 2012 are

8.1Mt at 51.6% Fe, 0.37% Cu and 0.12 g/t Au[3].

The results of this study allows the following conclusions to be made:

The Cairn Hill deposit possesses common features of a deeper, magnetite-rich end-member IOCG

deposit, based upon:

(1) Strong structural control (ore hosted within brittle-ductile shear zones)

(2) Constrained ages of mineralisation and alteration are coeval with Hiltaba Suite magmatism

(3) Replacement of magnetite by hematite accompanied copper mineralisation

(4) Proximity to a major magnetic-gravity structure

Overprinting of proximal chlorite-amphibole alteration and silicification resulted from significant

influx of diluted fluids within the E-W shear zones, which represent effective fluid pathways.

K-Fe alteration suggests a transitional magnetite-to-hematite deposit

Minor brecciation and late veins with open-space infill textures potentially indicate progressive ex-

humation of the hydrothermal system.

Deposit evolution points to: (i) regional circulation of hypersaline metalliferous fluids; intense fluid

-rock interactions resulting in highly modified lithotypes; (iii) evolution of physico-chemical pa-

rameters indicate early, high-temperature (500-400ºC) (scapolite-albite alteration) to lower-

temperature (300ºC) retrogressed (late mineralised veins), accompanied by decrease in salinity and

pH and increase of fO2.

Exploration implications:

Deposit characteristics can be compared to IOCG systems within the Cloncurry district and several

prospects within the Olympic Province (e.g. Hillside; Manxman; Oak Dam)

Transitional-style may suggest preservation of shallower hematite-rich IOCG systems within the

MWI.

[1] Skirrow et al., 2002. The Geological Framework, Distribution and Control of Fe-Oxide Cu-Au Mineralisation in the Gawler Craton, South Australia – Part 2. PGC Publishing - Hydrothermal Iron Oxide

Copper-Gold & Related Deposits: A Global Perspective. 2. 33-47.

[2] Barton, M.D., 2014. Iron Oxide(-Cu-Au-REE-P-Ag-U-Co) Systems. Treatise on Geochemistry 2nd Edition. 515-541.

Groves., 2010. Iron Oxide Copper-Gold (IOCG) Deposits though Earth History: Implications for Origin, Lithospheric Setting, and Distinction from other Epigenetic Iron Oxide Deposits. Economic Geol-

ogy.105.641-654.

Hitzman et al., 1992 Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au -REE) deposits. Precambrian Research 58, 241-287.

[3] IMX Resources, 2012, Annual Report 2012: http://www.imxresources.com.au/_content/documents/1243.pdf

[4] Betts et al., 2003. Evolution of the Mount Woods Inlier, northern Gawler Craton: an integrated structural and aeromagnetic analysis. Tectonophysics, 366, 83-111.

Chalmers, N.C., 2007a. Mount Woods Domain: Proterozoic metasediments and intrusives. South Australia Department of Primary Industries and Resources. Report Book 2007/20.

Deposit Geology

The Cairn Hill deposit is located within the hanging-wall

of the long-lived, E-W trending transpressional Cairn Hill

Shear Zone (CHSZ); within a dilational jog geometry. The

deposit strikes E-W over a distance of approximately 1.3

km up to 40 m wide. Two mineralized zones (N- and S-

lodes) are coincident with a set of parallel faults and shear

zones within the CHSZ and are concordant with the strike

of the encompassing magnetic anomaly. Mineralisation is

hosted within Hiltaba-equivalent Balta Suite granitoids,

quartz-K-feldspar-biotite-magnetite gneiss and narrow

skarn-amphibolite zones that are variably mylonitized and

hydrothermally altered. Timing of mineralisation is con-

strained between ~1587 Ma and ~1525 Ma by U-Pb

(zircon) SHRIMP geochronology of host rocks and cross-

cutting dykes.

Stage I: Na-Ca: Albitisation has greatly affected the host rocks

and is commonly recognised away from the main shear zones.

Accompanying albite are: scapolite-rutile±magnetite-Fe-chlorite.

Calcic alteration is characterized by pervasive diopside-

oligoclase-actinolite/tremolite-hornblende±titanite.

Stage II: K-Fe: Early magnetite-flourapatite-actinolite-biotite-K

feldspar ± quartz-pyrite-pyrrhotite commonly replaces early al-

bite and associate minerals. Fe-alteration is concentrated within

shear zones, where as K-alteration is pervasive throughout the en-

tire deposit.

Skarn Development (Stage III-IV): The timing of skarn formation

is poorly constrained, as only narrow zones (<10m) are identified

at depth as a result of drill-core logging. This lithotype is charac-

terized by a texturally-complex andradite-grossular garnet-

scapolite-magnetite±diopside skarn immediately adjacent a horn-

blendite. Overprinting retrograde alteration characterized by epi-

dote-actinolite/tremolite±titanite-quartz-oligoclase-biotite is evi-

dent.

Stage V: Main Mineralising Event: Characterised by a pyrite-

pyrrhotite-chalcopyrite±bornite-chalcocitehematite-sphalerite-

galena-LREE-Y assemblage. (Native) gold occurs primarily as

inclusions within chalcopyrite-pyrite, as well as minor electrum.

Mineralisation is associated with zones of quartz brecciation and

occurs as veins overprinting massive hypogene magenetite within

shear zones.

Stage VI: Late-stage Alteration: Abundant, proximal Fe-rich

chlorite and orthoamphibole characterizes this stage and perva-

sively replaces all stages, including mineralised zones.

200 m

Figure 3: Geological map of the Cairn Hill deposit, including the North

and South mineralised lodes and associated host lithologies.

Figure 5: Back-scatter-electron (A-D), reflected-light (E) and plane-polarized light (F) photomicrographs showing common minerals and their textural relationships representing the paragenetic stages of CHD. Bt=biotite; Di=diopside; Zrn=Zircon; Mag=magnetite; Ap=apatite; Scp=scapolite; Ab=albite; Or=orthoclase (K-feldspar); Hem=hematite; Ccp=chalcopyrite; Ged=gedrite (orthoamphibole); Chl=chlorite; Py=pyrite; Bn=bornite; Cc=chalcocite; Gt=garnet

The Cairn Hill deposit represents a multi-stage hydrothermal

system characterized by a diverse range of mineralogy. Early,

high-T assemblages occur distally from the deposit. In proxi-

mal zones, overprinting of pervasive lower-T, retrogressed

phases prevail.

Figure 4: Detailed paragenesis of the Cairn Hill deposit identifying primary stages and asso-

ciated mineralogy.

Mag

Scp

Gt

A B C D E F