Hillside Copper-Gold Deposit, SA

Hillside Copper-Gold Deposit, South Australia

Location and Setting

The Hillside Cu-Au deposit is located on the eastern coast of the Yorke Peninsula in South Australia. It sits approximately 15 km south of the township of Ardrossan. Geographically, it's reasonably close to Adelaide, about 72 km northwest, across the Gulf of Saint Vincent.

Crucially, Hillside is positioned within the Olympic Cu-Au Province, also known as the Olympic Domain, located along the eastern margin of the Gawler Craton. This province is renowned as the world's most richly-endowed IOCG (Iron Oxide-Copper-Gold) ore province, hosting massive deposits like Olympic Dam (455 km to the north), Prominent Hill, Carrapateena, and the historic Moonta-Wallaroo district. Hillside is near the historic mining towns of Moonta and Wallaroo.

The regional setting involves Palaeo- to Mesoproterozoic rocks along the eastern margin of the Gawler Craton. These rocks are partly overlain by the thick Mesoproterozoic Gawler Range Volcanics and unconformably by Neoproterozoic sediments of the Stuart Shelf. The deposit itself lies within the regional NNE-trending Pine Point Fault Zone (PPFZ), a major structural corridor. Exploration targeted discrete magnetic and gravity features associated with this fault zone. A significant cover sequence of Tertiary sediments, ranging from less than 1m to 30m thick, and local channel-fill alluvium up to 30m thick, conceals the mineralization. This cover is a defining challenge for exploration in the region.

Hillside Copper-Gold Deposit Location Map.

Deposit Geology

The Hillside deposit was discovered in 2008. It is classified as an Iron Oxide Copper Gold deposit.

The mineralization is hosted by a highly-deformed sequence of metasedimentary rocks of the ~1750 Ma Moonta-Wallaroo (MW) Group and Mesoproterozoic Hiltaba Suite granites and gabbros. These rocks are situated within the Pine Point Fault Zone (PPFZ). The granites and gabbros are described as interfingered along the deposit's N-S strike, with gabbroic bodies inferred along the PPFZ based on geophysics.

Hillside is characterized by a skarn-style alteration zone. The skarn is patchy and appears tied to immediate contacts between igneous rocks (granite and gabbro) and a carbonate protolith, suggesting a deep skarn setting.

The deposit is primarily controlled by structure, occurring along four distinct faults within the Pine Point Fault. Mineralization typically strikes north-south and is hosted by steeply west-dipping, intensely altered structures. These structures form sub-vertical to steeply west-dipping bodies. The deposit strikes N-S over ~2.5 km or 2.3 km and is ~700 m to 900 m wide. At least four main structures (Dart, Zanoni, Parsee, Songvaar) coincide with parallel faults in the PPFZ, with a combined copper-mineralized strike length exceeding 4 km. Mineralization has been confirmed from shallow depths (5-30 m below surface) to over 700 m (or 710 m) depth and remains open at depth.

Hillside Copper-Gold Deposit Geology Map.

Mineralisation

The primary copper mineralization at Hillside is dominated by chalcopyrite. Lesser amounts of bornite and chalcocite are also present, often intergrown. Some zones specifically contain significant primary bornite and chalcocite.

Gold is an important co-product and appears to be hosted as inclusions within chalcopyrite. Pyrite is abundant but is typically replaced by chalcopyrite during skarn retrogression.

Other minerals associated with the ore include rare galena, tennantite, bismuthinite, and aikinite. Uraninite and pitchblende are often associated with carbonate-rich zones. Light Rare Earth Elements (LREE) are contained within allanite.

High-grade copper zones occur as parallel, steeply-dipping domains. Lower-grade mineralization includes vein, blebby, and lace-like chalcopyrite accumulations flanking these zones. Notably, late carbonate and silica flooding can lead to extreme increases in copper grade in many areas.

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

Alteration and Mineral Paragenesis

Hillside exhibits a skarn-style alteration signature. The alteration reflects a spatial and temporal development of ore zones rooted within the skarn. Petrographic study identifies a transition from an early magmatic to skarn stage (pyroxene-albite) to main calcic skarn formation and finally to a late-hydrothermal (ore) stage.

Early/Prograde Skarn: Higher temperature skarns are dominated by assemblages like magnetite ± quartz ± pyrite ± garnet and almost monomineralic garnet skarn. Gabbroic rocks show early high-temperature potassic alteration (magnetite-biotite-K feldspar ± bornite).
Retrograde Skarn / Main Ore Stage: This stage overprints the early prograde skarn. Replacement assemblages include clinopyroxene, K feldspar, epidote, actinolite, allanite, and biotite-rich skarns. Chalcopyrite, the main ore mineral, is deposited during this stage.
Late Ore Stage: Copper mineralization is specifically associated with highly oxidizing fluids where magnetite is replaced by hematite ± chalcopyrite. Late-stage alteration is characterized by quartz-calcite ± chlorite, occurring interstitially and as veins. Chlorite and chalcopyrite commonly replace earlier minerals like clinopyroxene, actinolite, and garnet. Late carbonate and silica flooding are also part of this stage, increasing copper grades. Late calcite–quartz veinlets crosscut all earlier assemblages, including sulfides.

Mineralogical-petrographic work shows a high degree of skarn retrogression from prograde garnet ± pyroxene + magnetite to clinozoisite ± actinolite + hematite assemblages.

Mineral Chemistry

A reconnaissance study focused on the rare earth elements (REE) and other trace elements in minerals from skarn assemblages and accessories. Analyses were performed on minerals like feldspar, calcite, garnet, pyroxene, clinozoisite, actinolite, titanite, apatite, and allanite.

Key findings from mineral chemistry:

Garnet is a major repository for Heavy Rare Earth Elements (HREE), particularly in the prograde skarn stage.
Clinozoisite is the principal host for Light Rare Earth Elements (LREE) in the retrograde skarn. Allanite also hosts LREE.
REY (REE + Y) patterns show pronounced partitioning among coexisting minerals.
Compositional variations within minerals and assemblages define the evolution from early feldspar–pyroxene skarn through main calcic skarn to the ore-stage.
A significant observation is the switch from a prograde, HREE-dominant signature to a LREE-enriched signature observed in both retrograde and distal skarn.
Trace element signatures are considered applicable to exploration. Minerals like titanite and apatite show particular promise for developing exploration vectoring tools because of their distinct REY patterns in magmatic and hydrothermal stages and their responses to redox changes.

Hillside Copper-Gold Deposit, Box and Whisker Plot.

Hillside Copper-Gold Deposit, Radar Plot.

Geochronology

Dating studies using U-Pb isotopes on specific minerals help constrain the timing of events at Hillside.

SHRIMP U-Pb dating of hydrothermal titanite and allanite constrains the age of skarn mineralization at Hillside to 1601 ± 16 Ma to 1584 ± 7 Ma.
SHRIMP U-Pb zircon dating of altered granites gives ages of 1602 ± 13 Ma and 1588 ± 10 Ma.
More recently, U-Pb isotopic dating of titanite from alteration indicates the alteration is broadly coeval with granite emplacement (1570 ± 8 Ma), which relates to the latest stage of Hiltaba Suite magmatism in the wider Olympic Copper-Gold Province.

This places the timing of alteration and mineralization squarely within the timeframe of the major thermal event (~1600-1500 Ma) and the emplacement of the 1595–1575 Ma Hiltaba Suite granitoids and coeval Gawler Range Volcanics.

Comparison to Other IOCG Deposits

Hillside is situated within the Olympic IOCG Province, home to some of the world's largest IOCG deposits. While part of this major province, Hillside exhibits characteristics that both align with and differ from other notable deposits.

Hillside is characterized by a skarn-style alteration zone, which is suggested to represent a deeper style of mineralization compared to the giant sericite-altered, hematite-dominant systems like Olympic Dam and Prominent Hill.
The structural style at Hillside, where mineralization is hosted within discrete, continuous structures, is similar to the historic Moonta-Wallaroo mines, deeper Cloncurry-style IOCG(U) deposits, and some Curnamona Province deposits. This is markedly different from the hematite-dominant, breccia-hosted styles seen at Olympic Dam and Carrapateena.
Despite structural differences, Hillside shares common features with other Gawler Craton IOCG(U) deposits:

The age of mineralization and alteration is coeval with Hiltaba Suite magmatism and the mineralization timing of the other major deposits in the province.
Replacement of magnetite by hematite accompanied copper mineralization. This aligns with the idea that the gradient between magnetite and hematite stability is favorable for copper deposition.
Proximity to a major magnetic-gravity structure.
Strong structural control on mineralization.

Fluid inclusion data from the broader Moonta-Wallaroo region, including Hillside, indicate the involvement of both weakly and highly saline, multi-cation fluids, similar to other IOCG(U) regions like the northern Olympic Province, Cloncurry, and Wernecke Breccias. Evidence for boiling and mixing with lower salinity fluids is also noted.

Exploration Implications

Exploring in the Olympic Province is challenging because the bedrock has negligible surface expression and lies under a thick cover sequence. This necessitates the identification and understanding of geochemical footprints and the development of vectoring tools that can work beneath this cover.

Understanding REE distributions in minerals is considered a potential guide to mineralization. The distinct trace element signatures of skarn-type IOCG deposits are applicable to exploration.
Minerals like calc-silicates or apatite with distinct REY patterns or enrichment in key elements could potentially indicate proximity to Cu-Au bearing zones. Titanite and apatite are specifically highlighted as promising for exploration vectors due to their characteristic REE patterns and trace element responses.
The Hillside discovery, located based on geophysical features (magnetic and gravity anomalies) associated with the Pine Point Fault, highlights the importance of these tools.
Targeting structurally-controlled demagnetised zones and the oxidized margins of magnetic anomalies, a strategy successful elsewhere (e.g., Prominent Hill), is also applicable to the Moonta-Wallaroo region and potentially the extensions of Hillside. The replacement of magnetite by hematite during copper mineralization is a key process creating these demagnetized zones.
The discovery of Hillside itself is a major positive implication, demonstrating the continued prospectivity of the southern portion of the Olympic Province on Yorke Peninsula. Even after considerable unsuccessful exploration, potential for significant economic mineralization remains.
An improved understanding of the Hillside mineral system, recognizing its structural controls and how they appear in updated regional geophysical datasets, is expected to help define new, previously untested targets in the region.

In conclusion, Hillside is a structurally-controlled, skarn-modified IOCG deposit within a world-class province. It provides valuable insights into the formation temperatures, fluid evolution, and mineral chemistry of this style of mineralization, offering crucial clues and vectoring tools for future exploration efforts beneath the concealing cover of South Australia.

Sources:

Australian Mineral Deposits

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