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Prominent Hill — the discovery that proved “under cover” doesn’t mean “out of reach” - An IOCG Discovery.

The discovery of a major mineral deposit is one of the most coveted achievements for any geoscientist. Anyone who has spent time searching for mineral systems will understand that being involved in a significant discovery—of any commodity—sits firmly at the top of the professional bucket list.

Figure 1: Bau Location plan. (source Bau Feasibility Study - 2013)

Figure 1: Bau Location plan. (source: Besra Gold inc)

For me, as a geoscientist since 1992, and with a family history that traces back to my grandfather discovering a gold deposit, (in Bau, Sarawak, Malaysia - Figure 1 and Figure 2) in the 1940s, that ambition has always been deeply personal. Being part of a major discovery, in any form, remains my number one aspiration.

Figure 2: My grandfather had the Saburan (I believe that the spelling here should be Siburan)and the Taiton gold mine. There is a third which is not listed in the figure. (source Bau Feasibility Study - 2013)

Figure 2: My grandfather had the Saburan (I believe that the spelling here should be Siburan)and the Taiton gold mine. There is a third which is not listed in the figure. (source Bau Feasibility Study - 2013)

I was fortunate to have had experience that moment early in my career. In 1992, I was part of the team that discovered the Bronzewing Gold Deposit (Figure 3), and a year later, I was on the drill rig when we intersected the Ector Diamond Pipe—an event that ultimately led to the discovery of the Merlin Diamond Province (Figure 4), Northern Territory in 1993.

Figure 3: Regolith section along Line 9200 mN showing buried lateritic residuum in the Discovery Pit. The discovery RAB hole is projected onto the section. The discovery hole intersected up to 0.3 ppm Au in lateritic residuum and 12 m at 1 ppm Au in saprolite. (source: R.R. Anand, C. Phang and Z.S. Varga - BRONZEWING GOLD DEPOSIT, WESTERN AUSTRALIA - CRC LEME- 2005)

Figure 3: Regolith section along Line 9200 mN showing buried lateritic residuum in the Discovery Pit. The discovery RAB hole is projected onto the section. The discovery hole intersected up to 0.3 ppm Au in lateritic residuum and 12 m at 1 ppm Au in saprolite. (source: R.R. Anand, C. Phang and Z.S. Varga - BRONZEWING GOLD DEPOSIT, WESTERN AUSTRALIA - CRC LEME- 2005)

Those experiences were formative, but like many in this industry, the pursuit of deeper involvement in a truly transformative discovery remains an elusive and driving force in my life.

Figure 4: Merlin Diamond Project. (source: Lucapa Diamond Company)

Figure 4: Merlin Diamond Project. (source: Lucapa Diamond Company)

That persistent fascination with how discoveries are made, and the people behind them, has led to this latest Samso Insight. This article serves as a prelude to an upcoming Coffee with Samso conversation focused on the discovery of Prominent Hill—one of Australia’s most significant IOCG (Iron Oxide Copper-Gold) discoveries, made in South Australia in 2001.

It is important to understand the significance of Iron Oxide Copper-Gold deposits (IOCGs) as an important source of Australia’s most valuable mineral exports, including: iron, gold, copper and uranium. Australia hosts two major IOCG provinces, the Mount Isa Block and Gawler Craton (Figure 5).

Figure 5: Location map showing the main IOCGs and the two best known IOCG provinces in Australia. (source: James Austin and Clive Foss, CSIRO - CESRE, North Ryde: Rich, attractive and extremely dense:A geophysical review of Australian IOCGs, 22nd International Geophysical Conference and Exhibition, 26-29 February 2012- Brisbane, Australia.

Figure 5: Location map showing the main IOCGs and the two best known IOCG provinces in Australia. (source: James Austin and Clive Foss, CSIRO - CESRE, North Ryde: Rich, attractive and extremely dense:A geophysical review of Australian IOCGs, 22nd International Geophysical Conference and Exhibition, 26-29 February 2012- Brisbane, Australia.

Ahead of that discussion, I felt it was important to introduce the deposit itself and provide some geological and historical context, helping the Samso community better appreciate the scale of the achievement and the individuals who made it possible.

Introduction — why Prominent Hill still matters

The Prominent Hill IOCG deposit was discovered through diamond drilling of a gravity anomaly in 2001. Prominent Hill sits on the southern margin of the Mt Woods Inlier in South Australia’s Gawler Craton, within the broader Olympic IOCG Province. It is widely recognised as an iron oxide copper–gold (IOCG) system and, critically, it was discovered beneath cover rather than by obvious surface expression.

The reason this discovery continues to echo through exploration history is simple: Prominent Hill was found by trusting the physics and the model, not by chasing what was visible at surface. It became a repeatable example of how a covered craton can still deliver large mineral systems when gravity, magnetics, and disciplined drilling are integrated properly.

The under-cover problem — and why the Gawler Craton demanded a different playbook

A large part of the Olympic Province is blanketed by younger sediments. That makes classic “walk-up” exploration unreliable and pushes explorers toward geophysics-led targeting. Prominent Hill is a clean example of a target that was defined by a near-coincident gravity and magnetic feature, and the deposit itself is noted as being discovered under roughly ~100 m of cover sediment in the SA Geodata record (Figure 6).

Figure 6: Simplified geological map of the Prominent Hill deposit area below the 90 to 150 m thick Phanerozoic cover overlying a Permian unconformity, and is based on drilling. The main copper-gold bearing clastic and shelf carbonate sequence is located between metamorphic calcsilicate and metasomatic skarn altered rocks to the north, and unmetamorphosed but strongly altered, volcanic rocks to the south. The main copper-gold ore is hosted by the hematite altered polymictic breccias, while the hematite quartz breccias contain variable gold. The location of the principal resource blocks (Main Prominent Hill Cu-Au, Eastern Cu-Au, Western Au and Western Cu) which lie within these breccias are indicated. (source: PorterGeo - Modified after: Williams et al. (2017) and Freeman and Tomkinson (2010).

Figure 6: Simplified geological map of the Prominent Hill deposit area below the 90 to 150 m thick Phanerozoic cover overlying a Permian unconformity, and is based on drilling. The main copper-gold bearing clastic and shelf carbonate sequence is located between metamorphic calcsilicate and metasomatic skarn altered rocks to the north, and unmetamorphosed but strongly altered, volcanic rocks to the south. The main copper-gold ore is hosted by the hematite altered polymictic breccias, while the hematite quartz breccias contain variable gold. The location of the principal resource blocks (Main Prominent Hill Cu-Au, Eastern Cu-Au, Western Au and Western Cu) which lie within these breccias are indicated. (source: PorterGeo - Modified after: Williams et al. (2017) and Freeman and Tomkinson (2010).


The discovery moment — URN 1 and the gravity target

The discovery story is unusually well documented in South Australia’s exploration reporting.

The Target

The first decisive test was diamond drillhole URN 1 (often referenced as Uranus/Prominent Hill discovery drilling). In the MESA Journal write-up, URN 1 is described as the first hole drilled to test a discrete gravity anomaly measuring ~1500 m x 500 m, partly coincident with a magnetic anomaly of similar size, and explicitly framed as a common targeting criterion for Olympic Dam–style mineralisation.

The intercepts that changed the conversation

Two independent sources that summarise the discovery intercepts are:

  • South Australia’s gold commodity page, which states the prospect was discovered in November 2001 by Minotaur and reports URN 1 results including 20.2 m @ 3.03 g/t Au, 107 m @ 1.94% Cu and 0.65 g/t Au, and 152 m @ 1.10% Cu and 0.61 g/t Au. (energymining.sa.gov.au)

  • An OZ Minerals ASX investor presentation (2011) which repeats the same style of intercept reporting for URAN1 and adds an important detail about the target selection (Figure 7).

Figure 7: The URN 1 discovery intercept. (source: Oz Minerals presentation April 2011)

Figure 7: The URN 1 discovery intercept. (source: Oz Minerals presentation April 2011)


What Minotaur actually did right — discovery as a sequence of decisions

Prominent Hill is often simplified into a single sentence: “drilled a gravity anomaly and hit copper-gold.” The reality is more instructive.

  • They prioritised a discrete anomaly, not a vague regional trend

The MESA write-up describes the anomaly as discrete and sizeable (~1500 m x 500 m). Discrete targets generally allow clearer yes/no testing.

  • They used coincident datasets as a filter

The same article notes the anomaly was partly coincident with magnetics and frames gravity and magnetics coincidence as a targeting criterion for Olympic Dam–style mineralisation.

  • They understood that “magnetic” isn’t always where the ore is

Here’s a detail I like because it’s practical. The OZ Minerals deck describes discovery drilling as targeting “a high frequency, non-magnetic portion of [a] gravity anomaly” and then presents the URAN1 results

That wording is a reminder that IOCG systems can be messy in magnetic space: magnetite can dominate some parts; hematite can dominate others; alteration can destroy or create magnetism. Prominent Hill being commonly described as hematite-dominant IOCG sits neatly with that nuance.

What is an “IOCG” ?

IOCG deposits form within a broader family of Cu–Au–Fe mineral systems and commonly show large alteration footprints, often involving iron oxides (hematite and/or magnetite). One reason gravity and magnetics are so frequently discussed is because iron oxides and associated alteration can produce measurable physical property contrasts (Figure 8).

Figure 8: Heterolithic breccia with sandstone, mafic volcanic and hematitic clasts in a strongly altered to steely hematite matrix, at a depth of 440 m, within an interval between 430 and 466 m that averaged 1.6% Cu, 0.75 g/t Au; (source: PorterGeo - Core from SA Geological Survey Core Library; Images by Mike Porter 2022.)

Figure 8: Heterolithic breccia with sandstone, mafic volcanic and hematitic clasts in a strongly altered to steely hematite matrix, at a depth of 440 m, within an interval between 430 and 466 m that averaged 1.6% Cu, 0.75 g/t Au; (source: PorterGeo - Core from SA Geological Survey Core Library; Images by Mike Porter 2022.)


Samso translation: If the mineral system builds a big enough “iron engine,” it can show up in geophysics even when nothing shows at surface.

The geology lens — why Prominent Hill is described as hematite-dominant IOCG

A key reference on Prominent Hill is the Economic Geology paper titled “Prominent Hill: A Hematite-Dominated, Iron Oxide Copper-Gold System” (Antonio Belperio, Richard Flint, Hamish Freeman; Prominent Hill: A Hematite-Dominated, Iron Oxide Copper-Gold System. Economic Geology 2007, 102 (8): 1499–1510- Link). The abstract explicitly links the discovery to Minotaur in November 2001 and frames the deposit as a hematite-dominant IOCG system (Figure 9).


Figure 9: Simplified geological map of the Prominent Hill deposit area below the cover sequence, modified from Freeman and Tomkinson (2010). Locations (B) to (D) indicate the position of the cross-sections 55650 mE, 55950 mE and 56050 mE. The color symbols indicate the different ore minerals analyzed for their sulfur isotope composition in each sample. (source: Schlegel, Tobias & Wagner, Thomas & Boyce, Adrian & Heinrich, Christoph. (2019). A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic Iron Oxide Copper-Gold (IOCG) province of South Australia. 10.1016/j.oregeorev.2016.)

Figure 9: Simplified geological map of the Prominent Hill deposit area below the cover sequence, modified from Freeman and Tomkinson (2010). Locations (B) to (D) indicate the position of the cross-sections 55650 mE, 55950 mE and 56050 mE. The color symbols indicate the different ore minerals analyzed for their sulfur isotope composition in each sample. (source: Schlegel, Tobias & Wagner, Thomas & Boyce, Adrian & Heinrich, Christoph. (2019). A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic Iron Oxide Copper-Gold (IOCG) province of South Australia. 10.1016/j.oregeorev.2016.)


Later technical work illustrates the deposit’s contrasting textural styles, including hematite breccias hosting copper sulphides and associated alteration/mineral textures (Figure 10).

Figure 10: Photographs of representative drill core samples showing textures of (A-C) the magnetite skarn and (D-F) the hematite breccia body of the Prominent Hill IOCG deposit. (A) Patchy magnetite + pyrite ± chalcopyrite alteration (inside dashed yellow area) in chlorite and actinolite rich metasediment, Malu pit; PH56, 10016 mRL ( B) Massive magnetite and dolomite intergrown with pyrite; sample PH56, DP017, 218.8 m. (C) Magnetite + pyrite + K-feldspar veins cross-cut pervasively magnetite + pyrite altered granitoid; PH52, DP017, 208.02 m. (D) Hematite breccia with high copper grade showing abundant coexisting bornite and chalcocite associated with hematite and sericite altered clasts; sample PH446, PH05D057, 189.3 m. ( E) Chalcopyrite, hematite and fluorite-rich hematite breccia matrix; sample PH222, PH05D169, 367.8 m. ( F) Massive chalcocite, digenite and bornite replace pyrite in altered fine-grained diorite; PH466, PH05D186, 272.6 m. Mineral abbreviations: Act: actinolite; Bn: bornite; Carb: carbonate; Cc: chalcocite; Ccp: chalcopyrite; Dg: digenite; Dol: dolomite;; Fl: fluorite; Hem: hematite; Id: idaite; K -fsp: K-feldspar; Mag: magnetite; Py: pyrite; Qtz: quartz; Ser: sericite.(source: Schlegel, Tobias & Wagner, Thomas & Boyce, Adrian & Heinrich, Christoph. (2019). A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic Iron Oxide Copper-Gold (IOCG) province of South Australia. 10.1016/j.oregeorev.2016.)

Figure 10: Photographs of representative drill core samples showing textures of (A-C) the magnetite skarn and (D-F) the hematite breccia body of the Prominent Hill IOCG deposit. (A) Patchy magnetite + pyrite ± chalcopyrite alteration (inside dashed yellow area) in chlorite and actinolite rich metasediment, Malu pit; PH56, 10016 mRL ( B) Massive magnetite and dolomite intergrown with pyrite; sample PH56, DP017, 218.8 m. (C) Magnetite + pyrite + K-feldspar veins cross-cut pervasively magnetite + pyrite altered granitoid; PH52, DP017, 208.02 m. (D) Hematite breccia with high copper grade showing abundant coexisting bornite and chalcocite associated with hematite and sericite altered clasts; sample PH446, PH05D057, 189.3 m. ( E) Chalcopyrite, hematite and fluorite-rich hematite breccia matrix; sample PH222, PH05D169, 367.8 m. ( F) Massive chalcocite, digenite and bornite replace pyrite in altered fine-grained diorite; PH466, PH05D186, 272.6 m. Mineral abbreviations: Act: actinolite; Bn: bornite; Carb: carbonate; Cc: chalcocite; Ccp: chalcopyrite; Dg: digenite; Dol: dolomite;; Fl: fluorite; Hem: hematite; Id: idaite; K -fsp: K-feldspar; Mag: magnetite; Py: pyrite; Qtz: quartz; Ser: sericite.(source: Schlegel, Tobias & Wagner, Thomas & Boyce, Adrian & Heinrich, Christoph. (2019). A magmatic source of hydrothermal sulfur for the Prominent Hill deposit and associated prospects in the Olympic Iron Oxide Copper-Gold (IOCG) province of South Australia. 10.1016/j.oregeorev.2016.)

Why this matters for discovery: A hematite-dominant IOCG can behave differently from a magnetite-dominant system in magnetic datasets. The Prominent Hill story—gravity first, magnetics as context, and a non-magnetic portion of the gravity target being meaningful—fits that reality.

The “near-miss” lesson — the covered province penalty

One of the harsh truths in covered terrains is that being close can still be a failure. The OZ Minerals deck notes earlier drilling in the broader area that effectively missed Prominent Hill by a few hundred metres (referencing early 1990s activity), which is a blunt lesson in vectoring and target refinement.

Samso takeaway:Covered provinces don’t forgive “almost.” If your targeting logic isn’t tight, you can drill right past the story and never know it.

What happened next — from discovery to asset

From a discovery-to-mine pathway perspective, Prominent Hill moved quickly into the corporate and development cycle. The discovery itself is widely attributed to Minotaur Resources (and their Mt Woods JV ground), with later ownership and development ultimately sitting with Oxiana/OZ Minerals in company histories and presentations.

For this article, the key point is not the corporate detail—it’s what the corporate path signals:

  • Discovery was sufficiently robust (grade + thickness + system scale) to justify rapid follow-up and major-company style advancement.

  • The model was strong enough to become a template for broader IOCG exploration across the province.1

Why did gravity mattered so much at Prominent Hill

The discovery target was a discrete gravity anomaly. Gravity responds to density contrasts—and IOCG systems can create dense bodies through iron oxides and alteration. South Australia’s own geoscience commentary on IOCG targeting emphasises the use of coincident anomalies to reduce the search space for IOCG prospects.

Samso translation: In the Gawler, gravity is often your first honest signal. It doesn’t care if the surface is quiet.

Samso Concluding Comments — what Prominent Hill teaches explorers and investors

A mineral deposit discovery is not a task that uses the words "simple, "easy" "often" and anyone who have been involved in one will say that it is a rare series of events in the duration of the career. The discovery of a mineral deposit like Prominent HIll will most likely sit in the once in a lifetime category. I don't think I know of anyone that has had a recurrence of such an event.

When I speak to the people who make discoveries like Prominent Hill, it can be forgiven to think that it is one of those discoveries that looks obvious only after it happens. The discovery hole URN 1 was not a random punt—it is a structured test of a discrete gravity target with a supporting magnetic context, explicitly aligned to Olympic Dam–style targeting logic. The problem with that statement is that these anomalies are often identified, the problem is the percentage of "discoveries" from these anomalies are indeed rare. It may be obvious in hindsight, but the number of theses occurrences is very rare.

For explorers, the lesson is integration and discipline: gravity defined the opportunity, magnetics shaped the interpretation, and drilling delivered the truth—however, I would go further and say that the truth is the believe and the tenacity of those behind the decision makers, the management making the decisions delivered the discovery.

For investors, Prominent Hill is a reminder that the highest-leverage discoveries often come from teams willing to commit to a model before consensus forms. Under-cover exploration isn’t a lottery ticket—it’s a test of whether management understands how to convert imperfect signals into a drill decision and then follow through with technical persistence.

Fortune Favours The Fearless

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