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Mineral resources 2 Gold Read pages 80-82 to find out more about gold Gold • Your handouts on gold would come in really handy right now • You must know the following: – How gold is formed and the geological setting – Where it is mined from in WA – https://www.youtube.com/watch?v=bAKu2YuE N3w Gold • Hydrothermal process for gold formation • Hot saline fluids passing through country rock • Gold picked up in fluids which pass through cracks, fractures and soluble rocks • As the fluids move away from heat source, fluids crystalise (mostly silica -> quartz, therefore quartz veins) and gold forms • First miners chased quartz veins – now mining last of veins and soluble rocks like the Golden Mile Dolerite • Geological setting is an area of rock that has been hydrothermally altered and a likely host of disseminated gold Hydrothermal processes • Most hydrothermal solutions are hot aqueous brines, containing salts including those derived from circulating sea water • Metals e.g. gold, copper, lead, iron, zinc, arsenic and molybdenum leached out of rocks and solutions circulate and travel within the brines • As solutions cool, individual metals precipitate out at different temperatures, forming zoned deposits Hydrothermal alteration • Hot and contain variety of metals and other elements • Hot fluids head rock walls to temp of solution reaction causes change in mineral composition • Elements added or redistributed • Mineralogy of rock changed – hydrothermal alteration • Hydrothermal alteration forms zone known as alteration halo around intruded fluid Hydrothermal alteration • Hydrothermal solutions lose heat as energy is passed to surrounding rock • Varying temperatures result in zonation in the alteration mineralisation assemblages • Alteration halo can be used as a guide in the field to find smaller orebodies (large mass of ore) Hydrothermal ore deposits • Formed by hot aqueous solutions • Hydrothermal gold vein deposits are a good example • Task: read page 82 and make notes on case study 2 – important for exam! Superpit, Kalgoorlie High tonnage, low grade, 2-3ppm! Price of gold makes it worth it ;) Exam practice • Some ore deposits are formed by the redistribution of metals associated with hydrothermal activity, while others are associated with magmatic processes, such as fractional crystallisation, gravitational settling or immiscible liquid separation. • Choose one of the above processes of ore formation and then use a labeled diagram or flow chart, to explain clearly how the minerals are concentrated. Include the source of metals and fluids in the system you illustrate and the physical conditions or properties that are required for the ore deposit to form. (6 marks)