Harrow and Hillingdon Geological Society
Gold from Volcanoes
Gold from Volcanoes
Prof. Howard Colley
The talk began with a few slides to illustrate some of the better known volcanoes such as Erebus in Antarctica, Mount Etna, a molten lave flow on Hawaii, Mount St Helier in the West Indies and Mount St Helens whose recent eruption was a lateral explosion and still shows a large area of devastation. Gas emissions from volcanoes are often monitored regularly as an increase in intensity and composition can indicate a possible impending eruption.
Volcanoes area associated with subduction zones and the mid-ocean ridges. Much of the world’s gold comes from volcanic areas. The mechanism is due to the formation of an intrusion associated with circulation of water. The water must be of low salinity and is essentially rain or ground water which sinks deep underground into the hot core of the volcano. Here, at 200-250 oC, it leaches out metals such as Cu, Zn, Pb, Ag, As, Sb, Hg, Te and Au. The water travels back by a combination of temperature and pressure, but as the latter begins to fall the gold is precipitated near the surface of the volcano. This process is called epithermal mineralization but if the gold is deposited on the surface of the caldera it will be eroded very rapidly.
The presence or the hot fluids will convert the volcanic rocks into clays. On Fiji there is a single volcano on the north side of the island from which two types of gold are mined. The first type is associated with steep vertical fault zones but there are also horizontal faults and dykes as the result of the inward collapse of the caldera and this second type of gold was deposited where the fractures ran into the dyke and can be mined from the surface.
Gold is also associated with tellurite minerals in volcanoes formed four million years ago but in N. Chile within the Atacama Dessert are volcanoes formed nearer twenty million years ago and here Au is associated with sulphur and at 17,000 ft is a flow of molten sulphur. This terrain is barren but there are actual veins of gold unlike the normal micro quantities of 10g/ton of rock, these veins yield 250 ounces/ton.
On the North Island of New Zealand is the volcanic region of Taupo. It is an area of high activity typically, geothermal where the hot springs have a circulation to a depth of about 8 kilometres. M.Tarawera in the Waimangu Valley is another area of geothermal power. Here, the mud volcanoes are associated with clay alteration. The gold is carried around as a bisulphide complex. Au(Hs)2 then boils and liberates H2S which combines with the circulating water. At Lihir in Papua New Guinea is a gold deposit formed in a young caldera <106years ago. This is probably the biggest and richest gold mine in the world outside S. Africa.
Although many of the clays shown are light in colour these rocks can also be pyroclastic deposits with no mineralization. However, these can often be differentiated by computer maps using satellite imaging and this has worked very well in Chile. Within tropical rain forests it does not work well as the vegetation masks the ground.