History and Geology

The Fingerboards mineral sands deposit in the Glenaladale area lies in a localised 2.6 to 4 million year old, marine silty sand geological unit called the Coongulmerung Formation on the northern margin of the Gippsland Basin in Eastern Gippsland, Victoria. It is an offshore marine placer deposit.

The heavy mineral (zircon, monazite, xenotime, rutile and ilmenite) and quartz grains that make up the deposit are quite specific in origin and were derived from the weathering and erosion of 250 to 540 million year old Palaeozoic age sedimentary, volcanic, metamorphic and igneous rocks lying within the Tambo and Mitchell Rivers drainage basins of the Victorian portion of the Australian Alps abutting the younger southern Gippsland Basin.

The heavy mineral content in particular, were originally crystals which grew when Silurian to Devonian age (358 to 443 million year old) intrusive granitic magma cooled to form the granitic plutons of the high country and associated intrusive pegmatites in metamorphosed sediments.

Heavy mineral and quartz are very resistant to chemical and mechanical weathering. Over millions of years, the igneous rocks where exposed, weathered, and slowly eroded and the hard, resilient grains of sand left (heavy mineral and quartz) found their way into creeks in the Nicholson-Tambo and Mitchell Rivers drainage basins to be carried by those rivers to the then Gippsland coast particularly during floods and heavy rainfall.

The grains of sand were then dispersed offshore of the river mouths and carried westwards by longshore drift parallel to the coastline by the then prevailing ocean current. Heavy mineral was principally concentrated by a slow repetitive gravity separation process, winnowing of the lighter quartz grains from denser heavy mineral by bottom currents and longshore drift, to form the economic concentration of heavy minerals at Glenaladale.

During the past 2.6 million years, the Gippsland Basin has been subject to tectonic activity and eustatic sea level change. As a result, the deposit was covered and hidden by gravel and conglomerate (Haunted Hills Formation) fans sourced from the Australian Alps, stranded well away from the Gippsland lake coastline that we know today.