![]() They appear to be genetically and spatially linked to deep seated porphyry copper-gold deposits. An epithermal gold deposit in the making! High Sulphide DepositsĪs the name suggests, high sulphide deposits tend to be more sulphide-rich, containing pyrite (iron sulphide) and enargite (copper-arsenic sulphide) as well as gold and silver. Geochemical analysis of these scales indicates the presence of appreciable quantities of gold and silver. In the active geothermal fields of New Zealand, the formation of silica scales on the inside of the pipes that feed steam into power station turbines is a maintenance issue. This layering is called colloform-crustiform banding and is often associated with high-grade ore, including the highly prized bonanza grade. These minerals form layers alternating with layers of native gold or electrum (a mixture of gold and silver). The escape of the super-heated fluids results in a sudden reduction in pressure along the fault causing boiling and cooling of the fluid with subsequent precipitation of very fine-grained quartz, calcite and adularia. Figure 2: The Wairakei geothermal field near Taupo, New Zealand. Around Wairakei in the North Island of New Zealand, steam from these fluids is harnessed for electricity generation (Figure 2). Geysers and hot springs are created when these hydrothermal fluids erupt at the earth’s surface. Earthquakes can result in an abrupt injection of hot and highly pressurised hydrothermal fluids into newly created spaces along the fault. New Zealand has several commercial epithermal gold deposits and is also a country where you can observe the creation of epithermal gold deposits.Īs the cooling hydrothermal liquids seal up cracks, they create plugs which trap pressure. The metals carried in the super-heated groundwater become concentrated within the quartz veins. As the fluids rise towards the surface, they cool and precipitate mainly quartz, some calcite and adularia (a type of potassium feldspar). These fluids move through cracks, joints and faults in the rock. Large kilometre-scale circulation cells are set up by the movement of these heated groundwaters, which are also known as hydrothermal fluids. When groundwater comes into contact with hot molten rock deep under volcanoes, silicate minerals within the rock are dissolved along with metals like gold, silver, arsenic, cadmium, lead, zinc, antimony and mercury. Figure 1: Diagram showing low and high sulphidation epithermal deposits Low Sulphide Deposits There are two main types of epithermal gold deposits: low sulphide and high sulphide (Figure 1). ![]() It comes from the Greek word “epi” meaning shallow, while “thermal” refers to the heated fluid necessary for the deposit’s formation. The term epithermal was coined to describe a relatively shallow deposit. Rarely, closely spaced veins or vein swarms can occur, allowing open cut mining methods to become economically feasible. They are best mined by underground narrow vein methods. In association with epithermal veins are large alteration haloes where minerals within the country rock have been converted into clay minerals by the circulating hydrothermal fluids.Įpithermal deposits are typically high grade, small tonnage and relatively shallow deposits. Gold mineralisation within epithermal veins can occur over vertical intervals of a few hundred metres, vein thicknesses of one to two metres and lengths of up to several hundred metres. The tonnage of ore in epithermal veins is typically smaller than other types of low-grade gold deposits. Epithermal gold deposits are among the richest gold deposits in the world, with some bonanza-grade ore shoots containing more than 1000 g/t gold-equivalent or a kilogram of gold for everyone tonne of rock mined.
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