Metal Deposits  The specification states that you should be able to:  a) Explain the low crustal abundances of metallic minerals; show an understanding of concentration factors to produce economic deposits; calculate concentration factors.  b) Describe the concentration of magnetite by gravity settling in igneous intrusions.  c) Explain the formation of porphyry copper deposits of chalcopyrite and the secondary enrichment of copper deposits.

Metal Deposits 2  d) Explain the hydrothermal processes associated with igneous intrusions, sedimentary basins and ocean ridges forming veins of galena, sphalerite and cassiterite.  e) Explain how residual deposits of bauxite are formed as the insoluble product of chemical weathering.  f) Explain the formation of placer deposits of cassiterite and gold in rivers and beaches and the characteristics of these minerals, which make them suitable.

 If a rock or mineral deposit can be mined economically for a particular metal element then it is called an ore body.  Within an ore body there are ore minerals and gangue minerals (waste).  The grade of an ore body is related to the total % of the metal element it contains.  To be economic a copper ore body needs 5- 10% Cu.  Sometimes if the rock is abundant and easily mined the grade may be as little as %.

 Most metals have low average crustal abundances and in order for them to be worth mining they must be concentrated in some way.   Look at the table on page 272 of McLeish to see: average crustal abundances cut off grades concentration factors.  The elements need to be concentrated and this is done either within the Earth or at the Earth surface.

MAGMATIC SEGREGATION:  Examples include ores of chrome and magnetite.  These minerals may form first during crystallisation of a basic magma and due to their high density they may sink to the base of the intrusion and be concentrated in layers.  The Bushveld complex in South Africa is a good example.

HYDROTHERMAL DEPOSITS:  At the end of crystallisation of acid magmas some elements that do not easily fit into rock forming minerals may be left in a water rich volatile fluid.  These are called incompatible elements.  These may mix with water rich fluids and may escape in a hydrothermal fluid.  These hot water rich fluids (about 600  C) move into fractures or through pore spaces.

HYDROTHERMAL DEPOSITS 2  Metals dissolved in these fluids are precipitated in the fractures or pore spaces.  These fluids have a number of origins:  1) From the final water rich fluids from an acid magma.  2) Burial of sediments will mean that any trapped water (connate water) will heat up and may flow through the rocks I convection currents scavenging out any metals within the sediments and concentrating the metal in the hydrothermal fluids.  3) Seawater at M. O. R.’s can circulate through the ocean crust in convection currents and scavenge metals from the crust. These fluids are then emitted from “black smokers” precipitating metal sulphides into the oceans as sediment and nodules.

HYDROTHERMAL DEPOSITS 3  Hydrothermal deposits are most common in veins (filling joints or faults) but if they are precipitated in the pore spaces they are called Porphyry Copper Type deposits.  The order that the ores are precipitated is in order of cooling T and so will vary along a vein away from the source.

PLACER DEPOSITS  On weathering the rocks are broken up and resistant ores can be transported in rivers as solids.  The density of the ores effects the distance they can be transported and as soon as the energy drops the dense ore will be dumped amongst river sands and gravels.  The ore will concentrate where a river moves from the mountains onto the plane or on the inside of meanders.  Cassiterite and gold can be concentrated in this way.  They are dense and inert. Concentration at the Surface

RESIDUAL DEPOSITS  Chemical weathering acting on rocks such as basalt can weather them deeply under the right conditions (humid and warm).  The weathering will remove material in solution but will leave a gradually accumulating layer of insoluble residue that may be metal rich.  The best example is bauxite (Al Ore) in places such as Jamaica and N. Australia, which forms from the weathering of basalt and is in effect a very deep fossil soil.  This is very easily mined.

SECONDARY ENRICHMENT (OF HYDROTHERMAL VEINS):  This process leads to a very high metal concentration.  This usually happens close to the water table as water plays an important role.  Above the water table oxidation of the ore takes place which changes insoluble sulphides into soluble sulphates.  These sulphates are carried downwards by percolating water down to the groundwater.  Below the water table the conditions are reducing and the sulphates are reprecipitated as sulphides.

SECONDARY ENRICHMENT (OF HYDROTHERMAL VEINS) 2  If we take chalcopyrite as an example then above the water table this happens: Chalcopyrite + O 2 + H 2 O Chalcocite + Iron Hydroxide Cu/Fe sulphide Cu sulphate  Therefore the soluble compounds are leached from the upper zones and re-deposited lower.  The upper zone is reduced in Cu although close to the water table where oxidising conditions are less effective CuCO 3 can be deposited = Malachite and Azurite.

SECONDARY ENRICHMENT (OF HYDROTHERMAL VEINS) 3  Most of the Cu is carried into the saturated zone where the conditions are reducing and it is precipitated as copper sulphide : chalcocite.  Frequently a brown solid residual deposit called gossan forms which is usually Fe sulphide left behind after the Cu is washed out and is a good indicator of metal ores below.