Gold is where you find it. Isn’t it? – David Cliff

David’s talk comprised, principally, the geology of gold deposits on a world-wide scale by linking these historically with more recent knowledge of plate tectonics and the role of hydrothermal processes in concentrating gold from its trace element distribution in the earth’s crust into economically viable mineral deposits. Gold’s near-unique properties include its relative chemical inertness, combined with its high specific gravity and malleability to make the metal sought-after as a store of wealth. Some 50 per cent of all gold mined is utilised for that purpose, with 40% for jewellery and 10% for industrial uses such as, computers and mobile telephony. The current imbalance between newly-mined supply and physical demand is accounted for by recycling through scrappage. All the gold ever mined historically would occupy a space of 21 metres cubed. China is the world’s biggest producer and its second biggest consumer, after India. South African production has declined inexorably over the last four decades and there will be little production after 2030.

Most historic production has come from placer gold deposits, which occur in river beds or river terraces where gold has been concentrated after transportation and, frequently, a form of coagulation into nuggets due to mechanical and (arguably) bacterial processes. Some 150 years ago, the famous gold rushes of the Klondyke, California and Australia (Victoria) were to make fortunes from large placer accumulations. The source of the Klondyke gold has never been found.

The biggest placer accumulation by far, accounting for more than 50% of the gold ever mined, comes from the Witwatersrand Basin in South Africa, discovered by George Harrison on the farm Langlaagte such outside what is now Johannesburg. The basin, now mined over a 230km radius from Evander to Theunissen through a wide arc, is dated at 2.5 to 3 billion years. It would not have been exposed at the basin edges at all, but for the meteorite impact centred on the Vredefort Dome in the northern Free State, which caused a crustal ripple. The gold was concentrated in braided river systems that have a different geological history in various parts of the basin. The origin of the gold itself is still a matter of conjecture, but possibly originated from what are now understood to be ‘greenstone belt’ origin in the development of Archaean cratons (the original micro-plates of lighter crust that evolved early in the earth’s history. The South African mines are in decline due to increasing depth, high costs and difficulty of extraction compared with other deposit types.

Currently-mined important deposit types are;

  • ‘Greenstone Belt’, which occur in Archaean and, less commonly, Proterozoic, terranes. These are frequently high grade and are found throughout the world notably in Quebec and Ontario, Ghana, Western Australia and Tanzania. The deposits formed at depth along major shear structures at the junction between ductile and brittle deformation zones.

  • Porphyry and epithermal. Porphyry gold occurs consanguineously with copper and, often, molybdenum in major copper deposits that formed above subduction zones in areas such as; the western cordillera of the Americas and Indonesia. Epithermal gold often occurs in close association with porphyries, but at shallower depth. These are classed as high and low sulphidation, according to the acidity of the transporting fluids.

  • ‘Carlin-type’ gold is almost dominantly found in the basin and range terranes of Nevada, where similarly ascending fluids became trapped by impermeable rocks and partially replaced the host carbonate sequences.

  • There are a mix of other, less economically-important, gold deposit such as massive sulphides associated with seafloor spreading zones and ‘black smokers’. It can be argued that these are the manifestation of the leaching and redeposition mechanisms that focus and concentrate gold in mineral deposits.

The unifying concept is considered as;

  • Plate tectonics is the underlying cause of all gold deposits, from seafloor spreading through to subduction, mountain building and intrusive activity.

  • The transporting mechanism is hydrothermal fluid. Typical reactions are;



FeCO3+Au(HS)2=FeS2 pyrite+CO2+H2O+Au

  • Deposition is the consequence of temperature and pressure, often focused by faults and fractures.

  • Crustal evolution has played a part in the formation and preservation of permissive geology.

  • Placer deposits are due to the erosion of and re-deposition of any of the above.

  • original

To answer the original question, gold can be found in all the right places, but don’t expect to be successful!