Why Gold and Chromium Coexist Beneath Heathcote: A Geological Deep Dive

Solving A Geological Mystery in Victoria, Australia.

There's something interesting when you open up Geovic, apply the gold and base metal layer, and look at Heathcote. Amongst a sea of yellow-colored areas that signify known gold deposits, we have this tan color. An enrichment of Chromium and gold, that is only found in this area. So what happened here that differentiates it from the rest of Victoria?

To unravel this mystery, we must journey back through time, to an era when the Earth's crust was restless, and great geological upheavals shaped the land we now stand upon. The story of Heathcote’s chromium and gold enrichment is not a simple one—it is a tale of volcanic eruptions, oceanic rifting, and the relentless forces that shaped the landscape over hundreds of millions of years.

The Heathcote region, home to the Heathcote Greenstone Belt, holds the key to this enigma. This formation is primarily composed of pillow basalts, andesites, boninites, dolerites, volcanic sandstones, rhyolites, tuffs, black shales, and cherts. These rocks tell the story of a marginal sea, an ancient basin formed through rifting at the leading edge of the Australian continent during the Cambrian period (541–485 million years ago). This marginal sea formed in a backarc basin setting, where oceanic crust was actively spreading due to subduction further east, beneath a now-buried volcanic arc.

Boninite, a rare volcanic rock type found in Heathcote, is particularly significant. These high-magnesium, silica-rich lavas are associated with forearc and backarc basin settings and are known for their chromium and nickel enrichment. As boninitic magma erupted onto the ocean floor, it carried with it chromium, a metal that crystallized within its matrix. Over time, these volcanic rocks were altered by tectonic movements and hydrothermal fluids, which later played a crucial role in introducing gold.

The metamorphism of the Heathcote Greenstone Belt primarily took place during the Cambrian to Early Ordovician (510–480 million years ago), when the belt underwent low-grade burial metamorphism, transitioning into prehnite-pumpellyite facies in the northern and southern segments and lower greenschist facies in the central segment. The latter produced mineral assemblages including actinolite, chlorite, albite, quartz, and epidote, further altering the volcanic lithologies. Later, during the Devonian (~370–360 million years ago), regional granitic intrusions led to localized contact metamorphism, producing albite-epidote hornfels and hornblende hornfels facies, especially near the intrusions of the Cobaw and Crosbie Granodiorites.

The volcanic arc responsible for this subduction-driven spreading system was likely located further east of the Heathcote Greenstone Belt, possibly now buried beneath younger sedimentary cover in southeastern Australia. Some remnants of this arc may exist beneath the eastern Lachlan Fold Belt, including the Melbourne Trough and Snowy Mountains region, where deeply buried and metamorphosed volcanic sequences are present.

The collision of Tasmania with Victoria during the Paleozoic (~450–440 Ma, Benambran Orogeny) may have played a crucial role in separating the volcanic arc from the backarc basin. As Tasmania was accreted to the Australian continent, compressional forces could have displaced the arc eastward, effectively splitting it from the backarc system that hosted the Heathcote Greenstone Belt. This event likely contributed to the intense deformation, faulting, and burial of the original volcanic arc, making it difficult to locate today. The arc may now be structurally buried beneath the Lachlan Fold Belt, incorporated into deeply metamorphosed units beneath eastern Victoria.

The collision of Tasmania with Victoria did not only separate the arc and backarc but also played a fundamental role in mountain-building processes across southeastern Australia. The intense compressional forces thickened the crust, causing widespread folding and thrust faulting. This led to the formation of steeply west-dipping reverse faults, such as the Mount Ida-McIvor Fault and Mount William Fault, which accommodated the crustal shortening. The uplift of these fault-bounded regions contributed to the formation of mountainous terrains, resembling the modern-day South Island of New Zealand, where active compression is currently forming steep mountain ranges.

As the crust thickened, the deep burial of rocks resulted in further metamorphism and granitic intrusions, such as the Cobaw and Crosbie Granodiorites (~370–360 Ma). These granites, which intruded into the already deformed landscape, not only contributed to the metamorphism of the surrounding greenstones but also added further uplift and structural complexity to the region. Though these mountains have since eroded over millions of years, their remnants remain preserved in the highly folded and faulted rock sequences seen today in the Lachlan Fold Belt.

But the story does not end with volcanism. The Earth is a restless artist, constantly reshaping its creations. During the tumultuous periods of orogenesis, when vast mountain ranges were thrust skyward, fractures and faults splintered the landscape. It was along these wounds in the Earth that hydrothermal fluids, infused with gold from deep within the crust, surged and swirled. These fluids, heated by the infernal depths of the planet, reacted with the existing boninitic and basaltic host rocks, precipitating gold in unlikely companionship with the chromium that had slumbered there for eons.

Gold and chromite are unlikely bedfellows. The one, a noble metal, is known for its malleability and radiance; the other, an oxide of chromium, is an industrial titan, a mineral of steel and strength. Yet here in Heathcote, they exist side by side, a testament to the sheer complexity of geological alchemy. The processes that brought them together are both ancient and ongoing, a saga written in fault lines, mineral veins, and the unseen pressures of the Earth’s interior.

The Heathcote Greenstone Belt is structurally divided into three segments: the northern and southern portions exhibit characteristics of disrupted ophiolite sequences, while the central segment, hosting the Mount Camel Copper Show, is thought to be of rifted oceanic origin. A disrupted ophiolite sequence refers to a fragmented and structurally deformed remnant of ancient oceanic lithosphere, typically thrust onto a continental margin during tectonic collisions. Ophiolites consist of deep-sea sediment, basaltic pillow lavas, sheeted dikes, and ultramafic rocks derived from the Earth's mantle. When these sequences are disrupted, they are often broken up by faulting, shearing, and metamorphism, making them difficult to recognize in their original stratigraphic order. The presence of these disrupted sequences in Heathcote suggests that parts of the region once belonged to an oceanic crustal fragment that was later deformed and incorporated into the evolving Lachlan Fold Belt.

The marginal sea setting best explains the primary formation of the Heathcote Greenstone Belt, rather than a direct subduction-related volcanic arc. Geochemical evidence indicates that the metabasalts and metadolerites of the Heathcote Belt align more closely with mid-ocean ridge basalt (MORB) compositions rather than those of island arc tholeiites. This suggests that the belt initially developed in a spreading center within a backarc or marginal sea environment, accumulating volcanic material in an extensional setting before undergoing later tectonic deformation.

Here's the video we made on the Heathcote Chrome Gold Enrichment: