The Unsolved Mystery of The Gold & Silver Rich Hera Deposit

16 November, 2025
Oz Geology

Hera: The Rule-Breaking Deposit That Shattered the Cobar Basin Playbook

If you ever wanted a deposit that breaks the rules—in a good way—Hera it is. Hera is the geological equivalent of someone showing up at a costume party in a completely different outfit and then winning best dressed anyway. For years, the Cobar Basin was thought to be one of the most predictable mineral provinces in Australia. Everyone believed they understood it. Everyone thought the deposits were all “Cobar-type”—a category of ore system believed to form from cooler, structurally focused fluids, producing long, pipe-like lodes rich in base metals and silica.

Then Hera came along and casually changed the entire narrative. Not only did it turn out to be extremely valuable—producing somewhere in the neighbourhood of a billion dollars’ worth of gold, silver, lead and zinc—but it revealed a secret hiding underneath the supposedly well-understood Cobar Basin: the presence of a full-blown skarn system. And not just any skarn, but a beautifully zoned, thermally intense, multi-stage, intrusion-related skarn tucked neatly inside a sedimentary basin where it absolutely wasn’t supposed to exist. A skarn is a hard, chemically altered rock formed when hot, metal-rich fluids from a nearby intrusion react with carbonate-rich rocks—typically limestone or dolomite—creating a distinctive suite of calc-silicate minerals.

Hera is one of those deposits that doesn’t just contain valuable metals—it contains a mystery. A contradiction. A twist ending that no one saw coming. And that’s why, even though the mine has operated and moved beyond its peak years, geologists still talk about it with excitement. It wasn’t just another orebody. It was a quiet revolution.

*Image depicts the Cobar Basin

Rewinding the Clock: How the Cobar Basin Set the Stage

To understand why Hera is special, we need to step back in time—way back, hundreds of millions of years ago—when the rocks beneath central NSW were being deposited, buried, squeezed, heated, and reshuffled like layers in a giant, messy geological lasagne.

The Cobar Basin, where Hera sits, formed during the Silurian and Devonian periods, long before dinosaurs, long before mammals, long before trees even became common. Back then, eastern Australia was a collage of volcanic arcs, shallow seas, rift basins, and giant fault systems grinding past each other. Sediments poured into deep water, forming thick stacks of sandstone, siltstone, and mudstone. Over time, these layers were buried and folded and uplifted—nothing unusual for a landscape shaped by shifting tectonic plates.

The northern part of the basin eventually became famous for its metal deposits. These northern deposits, like CSA and Peak, are classic “Cobar-style” lodes. They formed when warm, mineral-laden fluids moved up major faults during periods of tectonic activity. These fluids left behind veins and pipes of metal-rich rock—gold, copper, lead, zinc, silver. That’s the standard model. That’s how the basin was “supposed” to behave.

And for decades, everyone assumed that this model applied everywhere in the basin.

Then the drills at Hera hit something no one expected.

*Image depicts CSA, Peak and Hera mines (From North to South)

Heat, Minerals, and Mystery: The Making of an Unusual Orebody

The rocks showed signs of intense heating—far more than the gentle warming that the northern deposits experienced. Instead of the mild greenschist-level temperatures you see at Peak or CSA, the rocks at Hera had been pushed into hornfels territory. Hornfels is a type of rock that forms when the surrounding rock is cooked—literally baked—by the heat of molten magma sitting nearby.

This isn’t “warm bathwater” geology. This is “stove-top burner” geology.

The strange thing is that no one ever found the magma. No granite. No volcanic pipes. No intrusive body poking through the layers. Nothing. It was like someone had fired a heat gun into the Earth and then cleaned away the gun itself, leaving only the roasted rocks behind.

But that heat left a very distinctive mark.

Wherever the hot fluids moved, they reacted with the surrounding rocks. Normally, you need limestone or marble for a proper skarn to form—rocks rich in carbonate minerals. Hera didn’t have much of that. The host rocks were mostly sandstones and siltstones deposited in deep water. But sprinkled through these sediments were occasional thin beds or clasts containing carbonate minerals. Even though these carbonate bits were few and far between, they were just enough.

When the boiling-hot fluids from the hidden intrusion hit these carbonates, the chemical reaction was immediate and dramatic. The carbonates dissolved. New minerals formed in their place. The entire chemistry of the rock changed. It was like watching a recipe transform mid-cooking—the flour becomes cake batter, the cake batter becomes a sponge, the sponge rises into something entirely new.

This transformation created what geologists call a “skarn.”

*Image depicts a skarn (Not from Hera)

At Hera, this process created a suite of colourful, unusual minerals that had rarely, if ever, been seen in the Cobar Basin. Garnet appeared first—large, blocky, sometimes beautifully zoned crystals that formed during the earliest, hottest stage. Then came diopside, a green pyroxene that forms at very high temperatures. And then, as the system cooled, amphiboles like tremolite and actinolite appeared. These minerals grow in fibrous, silky, or needle-like forms and signal that the fluids were cooling down.

This mineral shift tells a story. It’s basically a fossilised temperature map. In the south, where the garnet is most abundant, the system was at its hottest. In the centre, diopside took over. In the north, tremolite and actinolite dominate, showing that the outer parts of the system cooled faster.

This is exactly what you would expect if the heat source—the hidden intrusion—sat somewhere beneath the southern or central part of the deposit, pumping heat upward like a slow burner.

But minerals aren’t the only clue. Hera also contains metals—and a lot of them.

The fluids that formed the skarn didn’t just bring heat; they brought gold, silver, zinc, lead, copper, and tungsten. These metals were carried into the system during multiple waves of fluid flow. Each pulse brought a different mix of metals, and each reacted differently with the rocks they encountered.

That’s why Hera isn’t uniform. It’s a mosaic.

The southern lenses contain gold and garnet-rich skarn. The central lenses—especially the Far West lens—host some of the highest-grade zinc and lead, along with scheelite, the main tungsten mineral. And the northern lenses, like the North Pod, contain some of the strangest mineral combinations in the whole district, including rare silver and antimony minerals that barely show up anywhere else in Australia.

This variety makes Hera fascinating to geologists. It’s like a buffet of mineral styles all served on the same plate, each one telling a slightly different part of the story.

*Image depicts a cross section of the southern Cobar Basin

The Billion-Dollar Discovery That Raised Bigger Questions Than It Answered

And then there’s the tectonic history—the earthquakes, the faulting, the folding.

The Cobar Basin went through major tectonic upheaval during the Tabberabberan Orogeny, around 390 to 380 million years ago. This event squeezed and crunched the rocks across a huge swath of eastern Australia. At Hera, you can still see the signs. The ore is folded, stretched, and broken into separate lenses. The rocks contain a foliation—a kind of geological grain or texture—caused by the pressure. Some minerals appear sheared or smeared out along these planes. Gold in particular shows signs of being remobilised—moved from its original position and redeposited along fractures, cracks, and cleavage planes.

*Image depicts known faults in the Cobar Basin.

In some cases, the deformation happened while the ore was still forming. In other cases, the deformation happened after. The end result is a deposit that is not frozen in time but shaped and reshaped repeatedly—more like layers of paint added to a mural over millions of years than a single brushstroke.

Multiple radiometric ages—tiny clocks preserved inside minerals—tell us that Hera didn’t form in one go. It formed in stages, starting around 403 million years ago and continuing episodically until around 381 million years ago. Over these twenty million years, heat pulsed through the system, fluids flowed in waves, faults shifted, rocks cracked, metals accumulated, and the deposit took shape bit by bit.

Eventually, many millions of years later, humans arrived with drills and excavators and began to unlock the treasure hidden beneath the quiet landscape. And what a treasure it was.

At its peak, Hera produced thousands of ounces of gold and millions of ounces of silver. It shipped out zinc and lead concentrates worth hundreds of millions of dollars. When you tally the total metal value across its mine life, Hera comfortably lands somewhere between 900 million and 1.3 billion Australian dollars in gross contained metal. It wasn’t just a side gig for the company operating it—Aurelia Metals—it was a cornerstone. Hera fed ore to the Peak mill, stabilised production, and kept the regional mining network thriving.

But Hera’s biggest gift wasn’t the money.

It was the knowledge.

The Hidden Intrusion Beneath Hera: The Mystery That Refuses to Die

The realisation that the southern Cobar Basin has an intrusive, high-temperature root system fundamentally reshaped how geologists explore the region. Instead of limiting their search to the shallow, cooler, fault-controlled systems in the north, exploration teams now look for heat signatures, metamorphic gradients, and mineral assemblages that hint at deeper magmatic activity.

In other words, Hera revealed that the basin has a dual personality. The north is the classic Cobar style—cooler, structurally dominated, and well-known. But the south? The south may hide a much larger, much hotter, much more complex system driven by a buried intrusion big enough to influence kilometres of rock.

A system that might still contain undiscovered deposits.

A system that Hera only hinted at.

But Hera is still shrouded in mystery. There is no clean conclusion to this story. All the geological evidence screams “intrusion!” — but the intrusion itself has never been physically found.

The heat at Hera was far too high to come from normal burial, simple faulting, or basin-scale fluids. The rocks were cooked to hornfels facies — meaning 400–500°C in places. That level of heating requires a local, powerful heat source. The only thing that can do that is:

A magmatic body (an intrusion) sitting fairly close to the deposit.

But even though the heat signature is obvious…
the rock that caused it is still missing.

And that’s why, even after the mine has done its job, the story of Hera is far from over. It still invites questions. It still inspires exploration. And it still reminds us that geology is not a finished book, but a series of ongoing chapters—many of them hidden beneath our feet, waiting for the right person to turn the page.

Hera wasn’t just a deposit. It was a discovery that forced everyone to rethink what they thought they knew.

And that is its true treasure.