At that time, the region that we now know as western Tasmania was probably rather similar to the modern south-west Pacific, with volcanic islands and deep ocean trenches. There would have been lots of earthquakes and violent volcanic activity, erupting lava and volcanic ash onto the islands and the surrounding seafloor.
In the ocean crust, seawater that was trapped within the volcanic deposits became heated by magmas deep below the volcanoes. The seawater was heated to extreme temperatures, probably in excess of 300°C, and began to circulate or convect through the volcanic oceanic crust. As it did so, it dissolved metals, including gold, silver, copper, lead and zinc, present in trace amounts in the volcanic rocks. In places, the hot water erupted through cracks and fissures in the ocean floor, and mixed with much cooler seawater in the ocean. The cooling caused the metals to precipitate as solid mineral particles of chalcopyrite, galena, sphalerite, native (metallic) gold and native silver. These minerals accumulated in layers in depressions on the seafloor, and eventually formed the ore bodies that are now being mined at Henty and in other parts of Tasmania's mineral-rich west coast.
Scientists observe similar processes happening on the floor of the Earth's modern oceans. Plumes of super-hot water emanate from fissures in the oceanic crust, precipitate metal-bearing sulfide minerals onto the ocean floor. These plumes are known as "black smokers" and over millions of years, may form new ore bodies on the seafloor.
A black-smoker on the ocean floor. The smoke-like plumes are eruptions of superheated water from beneath the seafloor, cloudy because of the formation of metal-bearing sulfide minerals when the hot water mixes with cold seawater. The mineral grains will eventually settle on the sea-floor and over millions of years, may build up a future ore deposit (photo from US Geological Survey).
