Jayde Winsley, of Ardgowan School, asks :-
Harri Deacon and Tegan Wenlock, of Balclutha Primary School, asked:-
How is gold formed in the ground?
Garry Davidson, a geologist at the Centre for Ore Deposits and Exploration Studies at the University of Tasmania, responded.
Gold is forming below the ground in special places today, even though we mostly discuss the formation of gold deposits in the past tense. The Champagne Pool in the Waiotapu geothermal field is one example. Gold is even being mined underground from Phillipines vein systems that are still coursing with hot waters, places in which gold deposition is arguably still occurring. The Pacific 'Rim of Fire', known mainly for its large earthquakes and volcanoes, is also the location of very active modern gold deposition.
Gold formation is really just one big chemical reaction. Firstly, a gold-rich source is needed, and this may be a gold-rich molten magma, or buried gold-bearing rocks which are leached of their gold by infiltrating hot waters, 5 to 40 km below surface. Secondly, a way must exist to carry the gold from its source to its final resting place, and this is usually achieved by hot water. An enrichment in sulphur or chlorine helps the water dissolve and carry gold. Thirdly there is the trap. The trap is the circumstance which causes dissolved gold to solidify from its parent water, by mixing with colder waters, or after changes in underground pressure. Both of these circumstances reduce the amount of gold that can remain dissolved in water.
Gold deposits don't form in an instant. Even small deposits take tens to thousands of years to form, resulting from the repetition of the same process at the same place, many many times.