An ‘old-school gold rush’ kicked off in the Pilbara goldfields in 2017 with the discovery of hundreds of gold nuggets shaped like watermelon seeds.
Since then, the mystery of the source of these uniquely shaped rocks has grown and the CSIRO has hypothesised a response.
Two competing theories were proposed to explain the distribution and appearance of the infamous ‘Pilbara nuggets.’
While some looked like the seeds of a watermelon, the nuggets ranged in size and shape, suggesting they may have been caused by a ‘rough and tumble’ life in ancient riverbeds in a sedimentary process.
This involves weathering and erosion by wind, water and waves, breaking up the rock into smaller sediments, which are then compacted and squashed into another form of rock, called conglomerate.
An alternative theory is the gold conglomerate may have been transformed and modified through hydrothermal events.
These occur when hot fluids seep through faults and cracks in the rock causing the metal to deform under high temperature and pressure.
Using advanced characterisation equipment, CSIRO scientists have examined the geology and geochemistry of samples taken from the region, with imaging results of the 2.8 billion-year-old rocks revealing information about the history of the nuggets.
CSIRO data suggested a clear hydrothermal influence at some point in their history, however, whether the nuggets were originally formed through sedimentary processes is harder to confirm.
This is because hallmarks of their sedimentary origin may have been destroyed by later occurring hydrothermal processes.
The next step of the mystery is trying to date the hydrothermal events, which have left their mark on the conglomerates, with CSIRO scientists attempting to determine the scale of hydrothermal activity throughout the Pilbara.
Understanding how these deposits form is crucial to the discovery of new areas for gold exploration both in the Pilbara and throughout Australia.