Tasmanian geologists explain new Bendigo gold reef theory

Two University of Tasmania geologists have achieved international recognition with a new theory about the formation of gold deposits in Bendigo.

For seven years Dr Stuart Bull and Professor Ross Large studied the geology and chemistry around the Bendigo gold reefs in Victoria, one of the sites of Australia’s first gold rush, and have bucked conventional theories about the deposit.

Published by the Geological Society of London, ‘Setting the Stage for the Genesis of the giant Bendigo ore system’ explains that the gold deposits were not made by gold rising as a hot fluid through the earth’s crust, which is the conventionally accepted view of how most large deposits we formed, such as those around Kalgoorlie or the Victorian goldfields.

Instead, Bull and Large have proposed that the gold was sourced from adjacent mountain ranges under the ocean, through erosion and sedimentation into the rock sequence.

The sedimentary rocks around Bendigo are part the ocean floor that existed there 450 million years ago, a body which extended from southern NSW to south eastern Tasmania.

In his studies of the ancient ocean floor, Dr Bull found that gold was eroded from mountain walls under the ocean, then deposited as sediment in a giant channel-levee complex, an under-sea river, in the area of the mines at Bendigo.

The end result was that fine grained silt carried the gold to be deposited in the margins of the levee system, leaving behind mustone enriched with gold up to 10 to 100 times more than ordinary sedimentary rocks.

Later tectonic and structural events released the gold from the mudstone and concentrated it into the rich gold reefs that fuelled the first Australian gold rush in 1851.