Samso Insights: Prominent Hill vs Carrapateena — Two Discoveries That May Signal That The Gawler Craton Is Hiding Hybrid-IOCG like a Palabora - Saloboi or Candelaria.
- Noel Ong
- 2 days ago
- 11 min read
Introduction – Why These Two Discoveries Still Matter - Is the Cover Hiding Other Different Types of IOCGs?
When we talk about modern mineral discoveries in Australia, two names inevitably surface in the same breath: Prominent Hill and Carrapateena. Both sit in South Australia’s Gawler Craton, both are world-class Iron Oxide Copper-Gold (IOCG) systems, and both ended up becoming cornerstone assets for OZ Minerals, now part of BHP.
But lumping them together as “IOCG discoveries” misses the deeper story.
Prominent Hill and Carrapateena were not just discoveries of ore bodies — they were discoveries of ideas. Each one forced the industry to confront uncomfortable assumptions about where mineral systems can exist, how they express themselves, and how much conviction is required to find them when there is nothing obvious at surface.
In this Samso Insights piece, I want to step back from grades and tonnages and focus on something far more important: the discovery models themselves — why Prominent Hill was found the way it was, why Carrapateena required an entirely different mindset, and what these two discoveries continue to teach explorers and investors today.
Let's Discuss The IOCG Factor
IOCG deposits (Figure 1) are found globally and my current thought is what if they are all occurring in the same mineral system but are really defined by the amount or reactivation of geology and timing. Investors in the ASX who are chasing these stories or are told to chase these stories forget that it is very difficult in terms of the capital required and the time required to establish their value.
The discovery hole is always sexy, but the establishment of a deposit like Prominent Hill or Carrapateena takes more drilling, and in recent times, the excitement has fizzled out to despair for investors. Those who rode the Minotaur Resources journey in 2001 did fare better, but this search for the next IOCG in South Australia has not been that great. There is no doubt that when the value is established by discovering something like a Tier-1 deposit, there is no mistake in realising that the value to shareholders is immediate and abundant.
Figure1: Geographical distribution of IOCG provinces with selected deposits worldwide. (source: Zhu, Zhimin. (2016). Gold in iron oxide copper–gold deposits. Ore Geology Reviews. 72. 37-42. 10.1016/j.oregeorev.2015.07.001.)
Setting the Scene – The Gawler Craton Before Prominent Hill - Carrapateena - The IOCG Race
It is popular thinking that before Prominent Hill was drilled in 2001, the prevailing narrative around the Gawler Craton was simple: Olympic Dam was unique.
Discovered in 1975, Olympic Dam was so large, so metal-rich, and so geologically complex that it was viewed more as an anomaly than a template. For years, the industry struggled to decide whether Olympic Dam represented a repeatable mineral system or a once-in-a-generation geological accident.
For me, I think the Olympic Dam story is a geological event that may still not be repeated. Evaluating the genesis of a geological phenomenon like Olympic Dam will most likely draw out more questions than answers.
I tell people that if you made a discovery like Olympic Dam, why bother about how it formed? Your attention to developing and maintaining the production to its full potential will take up multiple generations as it is.
Figure 2 below shows why the Gawler IOCGs are always being talked about in terms of grade and size. Olympic Dam is easily ahead of its competitors compared to other global IOCG deposits, and when you take the three biggest singular deposits together, Grasberg, Bingham, and Olympic Dam, these three are in their own league.
Figure 2: A diagrammatic representation of the Gawler IOCG comparison to other known global IOCG deposits regarding copper endowment in 2006. Grade-tonnage data showing copper grades for selected IOCG deposits and other deposits (magnetite-apatite and skarn), after Williams et al. (2005: fig.3). Note the extreme range (three orders of magnitude) of copper grades when deposits are combined in this way and even one order of magnitude variation for the Cu-bearing IOCG deposits. Note also the generally higher copper (and gold, not shown) grades than those of most porphyries, at the same size.
(source: Groves, David Ian et al. “Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits.” Economic Geology 105 (2010): 641-654. | Williams, P.J., Barton, M.D., Johnson, D.A., Fontboté, 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: ECONOMIC GEOLOGY 100TH ANNIVERSARY VOLUME, p. 371–405.)
In South Australia, the compounding issue is that the whole IOCG scene is under cover. Much of the Gawler Craton is buried beneath younger sedimentary basins. There were few outcrops, limited geochemical clues, and plenty of reasons to believe that meaningful exploration success would be slow, expensive, and uncertain.
This is the environment in which Prominent Hill was conceived.
Prominent Hill – A Discovery of Conceptual Courage
The declared Ore Reserves and Mineral Resources at Prominent Hill in mid 2008, prior to the commencement of production in 2009, were:
Copper resource - Measured + Indicated + Inferred Resources: 174.20 Mt @ 1.39% Cu, 0.56 g/t Au, 3.4 g/t Ag (0.5% Cu cut-off).
Gold resource - Measured + Indicated + Inferred Resources: 109.2 Mt @ 0.09% Cu, 1.21 g/t Au, 1.0 g/t Ag (0.5 g/t Au cut-off & <0.5% Cu).
TOTAL Resource: 283.4 Mt @ 0.89% Cu, 0.81 g/t Au, 2.48 g/t Ag.
Western Copper resource (additional) - Inferred Resources: 14.5 Mt @ 1.69% Cu, 0.28 g/t Au, 3.7 g/t Ag (0.5% Cu cut-off).
(source: PorterGeo - Prominent Hill)
Challenging the “Olympic Dam Is Unique” Assumption
Prominent Hill was discovered by Minotaur Resources in 2001, at a time when the dominant exploration risk was not depth or drilling cost, but model risk.
Minotaur’s central idea was deceptively simple: If the Gawler Craton could host Olympic Dam, it should be capable of hosting other IOCG systems.
That idea may sound obvious today, but at the time, it ran directly against industry thinking. The risk was not whether a target could be drilled, but whether the underlying geological thesis was even valid. The unthinkable and the lack of history are two of the biggest quandaries that mineral explorers have faced in the past and definitely in the present.
The Prominent Hill Discovery Model
Prominent Hill’s discovery relied on a combination of:
Airborne magnetics, identifying iron-rich alteration consistent with IOCG systems
Gravity surveys, confirming a dense subsurface body
Moderate depth drilling, testing basement targets without extreme cost
Importantly, Prominent Hill was not screaming from surface. There was no obvious copper staining or gold anomalism. The signal was subtle, and the interpretation required confidence in geophysics and mineral systems thinking.
When drilling finally intersected hematite-rich breccias with copper-gold mineralisation, the implications were immediate. Olympic Dam was no longer an outlier. The IOCG model had legs.
Why Prominent Hill Changed Everything
Prominent Hill proved three critical points:
IOCG systems are repeatable
Cover does not eliminate discovery potential
Conceptual thinking matters more than surface noise
For the Australian exploration industry, this was a turning point. Prominent Hill didn’t just add a mine to the map — it opened an entire province.
Carrapateena – Taking the Model to Its Logical Extreme
JORC compliant Mineral resources as at 30 June 2020 (OZ Minerals 2020 Mineral Resources and Ore Reserves Statement ) were as follows:
Measured Resource - 130 Mt @ 0.96% Cu, 0.42 g/t Au, 3.6 g/t Ag;
Indicated Resource - 500 Mt @ 0.62% Cu, 0.26 g/t Au, 2.9 g/t Ag;
Inferred Resource - 330 Mt @ 0.32% Cu, 0.16 g/t Au, 2.0 g/t Ag;
TOTAL Resource - 950 Mt @ 0.57% Cu, 0.25 g/t Au, 2.7 g/t Ag.
(source: PorterGeo - Carrapateena)
From Concept to Conviction
If Prominent Hill answered the question “Can IOCGs exist beyond Olympic Dam?”, then Carrapateena asked something far more confronting:
“How deep are we prepared to go to find them?”
Discovered by Oxiana in 2005, Carrapateena was a deliberate escalation of the Prominent Hill model. This was not conceptual bravery — this was conviction exploration.
By the time Carrapateena was drilled, the industry broadly accepted that IOCGs existed across the Gawler Craton. The challenge was no longer belief; it was commitment.
The Carrapateena Discovery Model
Carrapateena sits beneath more than 400 metres of sedimentary cover, making it one of the most deeply buried major copper-gold discoveries in Australia.
Key elements of the Carrapateena model included:
Gravity as the primary targeting tool, rather than magnetics
Acceptance that magnetics might be subdued or ambiguous
Deep, expensive drilling, with long lead times and limited early feedback
Early drilling did not deliver instant gratification. There were no spectacular first holes that screamed “discovery.” Instead, Carrapateena required patience, internal conviction, and a willingness to absorb cost without immediate validation.
This is where many explorers fail.
Why Carrapateena Worked
Carrapateena succeeded because Oxiana was prepared to:
Trust gravity data in covered terrain
Accept delayed technical validation
Maintain focus on a single, high-quality target
The discovery ultimately revealed a large, vertically extensive, magnetite-rich IOCG system, distinct in character from Prominent Hill but no less significant in scale.
Carrapateena proved that world-class mineral systems could exist far deeper than previously assumed — and still be economically viable.
Prominent Hill vs Carrapateena – Two Very Different Risks
Although both deposits sit within the same province and belong to the same mineral system family, the risks involved in their discovery were fundamentally different.
Prominent Hill was dominated by model risk. Carrapateena was dominated by depth and capital risk.
Prominent Hill could be tested relatively quickly and cheaply once the concept was accepted. Carrapateena required long-term funding, corporate patience, and technical resilience.
This distinction is critical, particularly for investors watching today’s exploration juniors attempt to emulate these successes.
Lessons for Modern Exploration - Looking for the Elusive IOCG Prize.
Geological Lessons
IOCG systems are not uniform — hematite-dominant and magnetite-dominant systems express differently
No single geophysical tool is sufficient on its own
Gravity becomes increasingly important as the cover thickens
Strategic Lessons
Prominent Hill shows the power of conceptual clarity
Carrapateena demonstrates the necessity of financial endurance
Not all “Carrapateena-style” targets are appropriate for juniors
The Investor Lens – Why Does the Discovery of IOCG Deposits like Prominent Hill and Carrapateena Still Matter
From a Samso perspective, these discovery models are being tested against what I call corporate matters. It's a simple description, as I always see that the technical information is typically consistent, but they have to argue against limited cash and against the opportunity cost and the resulting corporate environment after an unsuccessful drilling campaign.
Prominent Hill-style exploration often suits well-run juniors: moderate depths, manageable budgets, and quicker technical feedback.
Carrapateena-style exploration is a different beast entirely. It demands:
Strong balance sheets
Long timelines
Acceptance of low news flow and high uncertainty
When juniors talk about “Carrapateena analogues,” investors should ask a simple question: Do they have Carrapateena-level patience and funding?
Samso Concluding Comments
Prominent Hill and Carrapateena remind us that discovery is not about luck — it is about thinking clearly, committing deeply, and staying the course when validation takes time.
One could say that Prominent Hill gave the industry permission to believe again in the Gawler Craton. Carrapateena forced it to grow up and accept that the next generation of discoveries would be harder, deeper, and more expensive. It is now abundantly clear that the mineral industry, especially in the Gawler, discovering IOCG deposits like Prominent Hill and Carapateena, is all about having a lot of courage, capital, and luck to seek the deep discoveries. All the easy discoveries are now a memory.
For the foundation of the mineral exploration foundation, the discovery of both IOCG deposits, Prominent Hill and Carrapateena, may have formed the blueprint for modern undercover exploration at that time, and a warning that shortcuts rarely work. To mineral explorers in 2026, it is accepted that in many of the well-explored regions in Australia, one must accept that undercover mineral exploration is the norm. Fortunately, Australian explorers have been used to the deep weathering profile for many decades.
To give some form of comparison, the Degrussa Copper project, which was a great copper deposit that has yet to be repeated in recent times, had a resource of 10.67 Mt @ 5.6% Cu, 1.9 g/t Au, 15 g/t Ag. The copper metal was about 597,530 tonnes of contained metal. Prominent Hill had a discovery resource of 101MT at 1.5%Cu, 0.55g/t Au for approximately 1,515,000 tonnes of contained copper metal.
As quoted by PorterGeo, the JORC compliant Mineral resources for Carrapateena as at 30 June 2020 (OZ Minerals 2020 Mineral Resources and Ore Reserves Statement ) were 950 Mt @ 0.57% Cu, 0.25 g/t Au, 2.7 g/t Ag.
For comparison, the Ernest Henry IOCG style CU-Au deposit in Queensland had a total reserve and resource prior to the commencement of mining in 1998 of 166 Mt @ 1.1% Cu, 0.54 g/t Au.
One could say that Prominent Hill, compared to Carapateen, may be considered small in terms of resources for an IOCG, but I will take the discovery of Prominent Hill as my discovery any time. No matter the size, the discoveries of both deposits ultimately showed that depth is hiding great wealth if you have the courage and the ability to chase it.
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