One of the Richest Gold Copper Discoveries in Australian History: Ernest Henry

The 1991 Discovery – A “Magnetic” Find in the Outback

Imagine it’s 1991 in the remote Queensland outback, near Cloncurry. A team of geologists from Western Mining Corporation or (WMC) are flying a small plane, scanning the ground with instruments like a giant metal detector. Suddenly, the sensors go haywire over a featureless patch of ground – a strong magnetic anomaly. There’s nothing visible on the surface (just 30-60 meters of barren soil and rock covering whatever’s below), but the instruments suggest something big and dense is hidden underground. Curiosity piqued, the geos stake a claim and drill a test hole. Boom – they hit rich copper and gold deep below the surface. That’s the dramatic discovery of the Ernest Henry deposit in October 1991, found not by old-fashioned prospectors with pickaxes, but by savvy scientists reading the subtle whispers of the Earth’s magnetic field.

News of the discovery spread fast in the mining world – this was a major Iron Oxide Copper Gold (IOCG) deposit. WMC and its partners knew they’d found something special – the kind of deposit that can turn a patch of desert into a bustling mine. By 1996, they had begun “pre-stripping” the site (basically scooping off the barren surface layers to get ready for mining). The project quickly gained momentum: first ore was processed by 1997, and by March 1998 Ernest Henry was officially open for business as a large open-pit mine.

Geology of a Giant – Inside an IOCG System

Now, what exactly had they found under there? Let’s dive into the geology of Ernest Henry – and don’t worry, we’ll keep it digestible. Ernest Henry is what geologists call an IOCG deposit, which stands for Iron Oxide Copper Gold. In plain language, that means the ore is a mix of copper, gold, and lots of iron oxide minerals (like magnetite) in a particular geological setting. If you’ve heard of Olympic Dam (the famous monster deposit in SA), Ernest Henry is in the same family – formed by hot, metal-rich fluids in the Earth’s crust around 1.5 billion years ago during the Proterozoic era. Back then, Australia’s crust was ripping apart and colliding in places, and huge granite bodies were pushing up deep underground. Those processes helped “cook up” deposits like Ernest Henry.

So, picture the orebody as a giant pipe-like shape of broken rock (geologists say breccia). It’s about 250 m thick and 300 m wide – literally the size of several football stadiums – and it extends downward at a 40° angle, plunging to the southeast for well over a kilometre. Think of it like a leaning cylinder buried in the ground. This cylinder isn’t made of solid metal, of course – it’s made of shattered pieces of the host rocks, cemented together by a dark matrix of magnetite (iron oxide) and sulphides like chalcopyrite (the main copper mineral). The whole thing formed when explosions of hot fluid burst through the rocks, brecciating them (like shattering glass) and depositing copper, iron, gold and other minerals in the cracks and voids. It likely happened in multiple pulses – evidence for that lies in the ore itself, where deeper portions contain chunks of earlier mineralized rock that got broken up again and re-cemented by later mineralization. In other words, Ernest Henry went through several “lives” of mineralization, each one building on the last.

The gold at Ernest Henry is mostly found mixed in tiny amounts within the chalcopyrite; you don’t see gold nuggets or anything, it’s microscopic but valuable when you process tonnes of ore.

*Image shows IOCG ore (not from Ernest Henry) with a large amount of chalcopyrite.

An easy way to visualize the orebody is to think of a fruitcake or a rocky plum pudding. The broken bits of the original rocks are the “cake” (chunks of dark volcanic rock that Ernest Henry lived in), and the sweet gooey part binding it all is the ore – magnetite, copper, gold, and a bunch of other minerals (there’s also quartz, carbonate, a bit of fluorite, even traces of uranium and rare earth elements in tiny quantities). The more “filling” or matrix in a given part of the breccia, the higher the copper grade – in fact, at Ernest Henry the copper grade is directly proportional to how much matrix vs. rock chunks there is. 

To sum up the geology in plain speak: Ernest Henry is a big, inclined pipe of broken rocks that got flooded with iron, copper, and gold-rich fluids deep underground, around 1.5 billion years ago. It was completely hidden under ~50 m of younger sediments – no clues on the surface, which is why its discovery was such a triumph of geophysics and science. It’s the kind of deposit that makes geologists’ eyes light up: large, high-grade, and telling a complex geologic story of an ancient IOCG hydrothermal system. As one of the largest copper-gold resource bases in Australia (about 167 million tonnes of ore in total resource, containing around 1.1% Cu and a few million ounces of gold), Ernest Henry is not just geologically fascinating – it’s extremely valuable.

*Image shows the Ernest Henry orebody (From https://evolutionmining.com.au/ernest-henry/)

From Open Pit to Underground – Engineering the Mine

By 1997–98, WMC had built the initial pit and processing plant. The open pit quickly ramped up production – by the early 2000s they were mining around 10–11 million tonnes of ore per year. At full tilt, giant haul trucks carrying 200+ tonnes of ore were crawling out of a pit that grew to 1.3 km long, 1.5 km wide and about 530 m deep. If you flew over it, the pit looked like a colossal amphitheatre or crater. For scale, 530 m deep is like stacking nearly two Eiffel Towers end-to-end into the pit – a long way down! And by 2011, they’d reached the bottom of that pit, having mined out all the ore reachable from the surface.

But here’s the thing: the orebody didn’t stop there. Remember, it continues down at 40° for at least another 600 m below the pit’s bottom – and in fact remains open (not fully drilled out) at depth. So, the miners had a new challenge: how to extract those deeper riches? The solution was to transition to an underground mining operation – essentially, to continue following the orebody down into the earth. And they didn’t wait until the pit was finished to start this transition. In 2008, while the open pit was still in full swing above, the team started driving an exploration decline (an inclined tunnel) down from the pit wall into the earth. This was quite an engineering feat: they had to establish underground infrastructure beneath an active open pit. They managed challenges like navigating around the final pit blasts, controlling water inflows (imagine groundwater seeping into your brand-new tunnel), and coordinating surface and underground operations safely.

A major milestone was in late 2011: open-pit mining ended, and the operation shifted fully to the underground sub-level cave. From that point, Ernest Henry became an entirely underground operation, hoisting ore up a 1,000 m deep shaft to surface. Yes, they sunk a kilometer-deep vertical shaft – that’s like three Eiffel Towers deep – which now lifts ore to the surface in big skips (imagine giant elevators for rocks). Miners and equipment, however, go in and out via the decline tunnel (the one that starts in the pit and spirals down), while the shaft is mainly for quickly hauling ore and waste rock.

*Image shows the kilometre deep shaft that hoists up ore.

Let’s talk numbers – because this mine has churned out some serious metal (and money) over the decades. Ernest Henry is often spoken of in superlatives: it has been one of Queensland’s top-producing copper mines and a significant gold producer as well. During its peak open-pit years in the 2000s, the operation was a beast. For example, in the 2001–2002 financial year, about 10.2 million tonnes of ore were processed at an average grade of ~1.12% copper and 0.56 g/t gold. That yielded roughly 100,000+ tonnes of copper and well over 150,000 ounces of gold in a single year – a volume of copper that could build literally thousands of kilometres of electrical wiring. In fact, by 2005 the mine produced around 129,000 tonnes of copper in concentrate for the year (along with a hefty amount of gold) – that’s just in one year’s production. To put 129,000 tonnes in perspective: that much copper metal could make nearly 8 million electric vehicle motors (since an EV uses about 15-20 kg of copper just for the motor and battery wiring). It’s a staggering output for one mine.

Over its lifetime so far, Ernest Henry has likely produced on the order of 1.2–1.5 million tonnes of copper metal and upwards of 2 million ounces of gold (exact figures keep climbing with ongoing production). Year in, year out, the mine has been a solid earner. Even as the operation transitioned to the smaller-volume underground phase, it still pumps out impressive quantities of metal. In the 2021 financial year, for instance, Ernest Henry produced ~64,000 tonnes of copper and ~92,000 ounces of gold from the sub-level cave – and that was considered a somewhat “normal” year for the new setup. At commodity prices of that year (2021 saw copper hit record highs around US$9,400/tonne), we’re talking nearly A$900 million worth of copper in a single year. In other words, at high copper prices the mine is “printing cash”. Even in lower price environments, Ernest Henry has been profitable, thanks to its decent grades and the gold by-product credits.

It’s also worth noting the mine’s economic impact on the region: It’s not just the sales of copper and gold; it’s also jobs, contracts, royalties and taxes. Ernest Henry has injected hundreds of millions of dollars into the Queensland economy through local procurement and wages. The Queensland government, for example, has collected royalties on all that copper and gold production – helping fund public services statewide. During construction of the mine and plant in the mid-90s, it was a massive jobs creator – about 3,500 people were involved in the construction phase (everything from engineers to truck drivers to camp cooks). Once operations started, the ongoing workforce settled to a few hundred.

Financially, if we summed up all the metal produced since 1997, the total value would easily exceed A$10 billion at today’s prices. In some years, the mine’s revenue alone was on the order of A$500 million to $1 billion. And with recent high copper prices and strong production, it’s likely breaking revenue records again. No surprise then that Ernest Henry is often cited as one of the most economically significant mining operations in Queensland of the past quarter-century. It has also been a low-cost producer – meaning high margins – which is why companies have fought to own a piece of it.

Legacy and Impact – Ernest Henry’s Place in Queensland Mining History

Ernest Henry isn’t just another mine – it’s a bit of a legend in Queensland’s mining story. When it was discovered and developed in the 90s, it became the third-largest copper deposit in Australia at the time (after Olympic Dam and Mt Isa’s giant ore bodies). Its successful discovery showed that the Cloncurry district had world-class IOCG potential, which gave a huge boost to mineral exploration in the region. In the years following, many companies flocked to northwest Queensland searching for “the next Ernest Henry.” The deposit basically opened up a new frontier in QLD exploration: people started to seriously explore under cover (beneath barren surface rocks) using geophysics, realizing that big deposits can hide completely out of sight – as Ernest Henry had for eons.

Ernest Henry’s saga shows how a dramatic discovery can lead to a legacy of innovation, wealth, and knowledge. And with operations slated to continue for years to come, this grand old copper-gold deposit’s story is far from over – truly a fascinating chapter in the book of Australian mining.

Here's the video we made on this on the OzGeology YouTube channel: