The Copper Is Hiding in the Minerals, Not the Assays — CSIRO's Tobias Schlegel on IOCG Vectoring

Coffee with Samso — with Dr Tobias Schlegel, Senior Research Scientist, CSIRO

We're back at Samso HQ at the University Club, on the UWA campus in Crawley, and this is a conversation that has been a few years in the making.

I first came across Tobias Schlegel at an IOCG workshop in Adelaide. He was up the front talking about how these deposits actually form, and I remember thinking, I need to talk to that guy. It has taken two or three years, but here we are.

Tobias is a Senior Research Scientist at CSIRO, where he has spent close to seven years, and he has been working on Iron Oxide Copper Gold (IOCG) systems since around 2009 — including his doctoral work on Prominent Hill. If you want someone who has spent well over a decade staring at the rocks behind Australia's most important copper story, he is the man.

"Geochemistry Isn't the Silver Bullet"

Why this conversation matters

Copper is on everyone's lips. Investors want to know where the next tonne is coming from. And in Australia, the answer keeps pointing back to IOCGs.

Globally, porphyries are the elephant in the room — Tobias puts them at roughly 60% of world copper supply. But IOCGs are where Australia lives. In South Australia and Queensland, they are the dominant copper producer. And they carry a quiet advantage: where porphyries typically run somewhere around 0.4–0.6% copper, IOCGs tend to scrape around 1%, particularly the hematite-dominated ones. When you are exploring undercover — as we increasingly are in this country — that grade difference matters.

The thing that stopped me in my tracks

Here is the part of the conversation that I keep coming back to.

We spend our entire exploration lives assaying for metals. Copper. Gold. Rare earths. That's what we want to find, so that's what we measure.

Tobias's point is that the metals are only half the story.

Metals travel in a fluid as cations — copper plus, iron two plus. But a fluid cannot just dump cations into a rock and walk away. Something has to balance the charge. So the anions come along too: chlorine, hydroxide, sulfur, phosphate. And when that fluid hits the rock, it doesn't only deposit metal — it also rebuilds the mineralogy, turning feldspars into hydrous minerals like chlorite and muscovite.

You may have thirty metals sloshing around in the system. But they are charge-balanced by a handful of anions. And as Tobias puts it, the signature of the anions is sometimes far clearer than the signature of the metals.

Which means the alteration mineralogy can be telling you something your assay sheet simply cannot.

From geochemistry to mineralogy

This is why, over the last five or six years, Tobias has drifted away from whole-rock geochemistry and towards mineralogy — measuring surface chemistry with automated SEM-based systems and translating it into minerals, sample by sample, down a drill hole.

What emerges is not the presence or absence of some magic indicator mineral in a single sample. It is the systematic change in mineral abundance through space. And it tends to behave predictably: broadly speaking, the bigger and more intense the alteration zone, the better the copper intersection at the end of it.

He is careful — and I respect him for it — not to sell this as a silver bullet. Geochemistry is still the method of choice, still the thing companies use to publish a resource, and it works especially well in homogeneous host rocks like granites. But when the host rock gets messy, and when the first hole finds smoke rather than fire, this is where the toolbox needs to get deeper.

What else we covered

• Why IOCGs are complex — but not as chaotic as they look. Olympic Dam, Prominent Hill and Oak Dam West sit in completely different host rocks, of different ages, yet the alteration zoning tells a strikingly similar story: a hematite-quartz core, with the copper wrapped around it.

• Hematite versus magnetite. Why the hematite-dominated systems in South Australia carry the chalcocite, bornite and chalcopyrite everyone wants, while magnetite-dominated systems tend to run lower grade with more pyrite.

• The breccia debate. Were these bodies blown apart, or are they structural jogs that later became plumbing for mineralising fluids? At Prominent Hill, Tobias argues the breccia was there first.

• Rare earths as a clue. The unusual lanthanum and cerium signature in South Australian IOCGs, and cerium's role in the magnetite-to-hematite transformation.

• The carbonatite question. With the Arunta throwing up hybrids, is there a real link between carbonatites and IOCGs? The idea has been around for about 25 years. Work at Carrapateena with the Geological Survey of South Australia shows carbonates replacing chlorite at depth — and the copper grade going up where they do. A hint, Tobias says, not a proven link.

• AI in exploration. A refreshingly honest answer, and a warning worth writing on the wall: if we feed the system what we already know, we will tend to find what we already know. Exploration is not a data-rich space.

• Advice for the next generation. Stay curious. The learning doesn't stop at graduation.

The takeaway

Geochemistry is not just about the element itself. It's about how that element moves through the system, and what it leaves behind on the way. The assay tells you what arrived. The mineralogy tells you how it got there — and, if you read it properly, which way to point the next hole.

And if you're stuck? Tobias's parting thought was simple: reach out. CSIRO, the universities, the geological surveys — they are there to help industry solve exactly these problems.

Grab a coffee. This one is worth your time.