This is an extract from ‘The Dudley Bug’ section in the Black Country Geological Society newsletter 209: October 2011.
Today we use metal for practically everything, over the last 50 years uses and demand for metals has increased due to their properties. The most common metals we use in today’s modern society include copper, aluminium, iron and steel. Less abundant metals include gold, titanium, platinum and silver.
Metals are found naturally occurring within rocks but are only economically viable when the geological processes which formed the rock have caused the metals to concentrate. These concentrations are known as ore deposits. They can be found within the earth formed through magmatic processes, or on the surface formed through weathering and deposition.
In order to work out the processes which have formed the ore deposit we use SADE: Source Agent Deposition Energy. The source of metal is where it is found on a long term basis such as the crust or the oceans. The agents of transport indicate how they are removed such as in magma or water. This can depend on the solubility of the metal which can be altered by temperature, pH and other substances around. In order for a metal to be deposited a suitable site and the correct conditions are required. Finally the energy to move the substance is required, such as convection or radioactive decay. The three main ways in which ores are deposited are through igneous, surface and hydrothermal processes.
Magmatic processes form three types of deposits: magmatic segregation deposits, pegmatite deposits and porphyry copper deposits. Magmatic segregation deposits can form through fractional crystallisation within a magma chamber. This occurs when different minerals cool and crystallise at different times. As it cools this separates, and may sink to the bottom of the magma chamber and form layers at the base. The layers concentrated the different minerals in order of freezing point. Another method is liquid immiscibility, which occurs when two magma fractions separate from a homogeneous magma. They may be chemically similar or one of the fractions may be enriched in a particular mineral. This means that this remaining magma could be saturated with ore minerals such as sulphides.
Pegmatite deposits are formed as pegmatite rock which is a very coarse igneous rock associated with granite plutons. They form with the final part of the magma chamber that is still un-crystallised. Because this part of the magma usually has high water content, the crystals which form as it cools are very large, for example the Bikita Pegmatite deposit in Zimbabwe.
Porphyry copper deposits are associated with porphyritic felsic intrusions which form large but very low grade deposits. These deposits are formed as rising magma degasses and the fluid is left carrying soluble metals known as chlorite complexes. As the magma continues to rise it boils causing the surrounding rocks to fracture. The fluid quickly fills the gaps created by the fracturing and begins to alter the rocks. As it cools the metals within the fluid cool and form vein deposits which can be large enough to form ore deposits. Because of the location of these deposits within rock, it isn’t economically viable to extract the ore deposit on a small scale. The main metals which are sourced from these deposits include Cu (copper), Sn (tin), Au (silver) and Ag (gold). One of the largest and best known examples of a Porphyry Copper Deposit is the Bingham Deposit in Utah, USA. This deposit measures 2.5km x 1.7km and is 1.0km deep.
Surface process deposits form from processes which include chemical leeching where a substance is removed from a rock by fluids which run through the rock. Others include secondary enrichment deposits and chemical transport. The latter method forms banded iron ore deposits.
The stability of a metal ore depends on how reactive the metal is. Very reactive metals such as aluminium react to form stable oxides. Less reactive metals such as iron react to form less stable oxides. So the reactivity of the metal will dictate its extraction method. Different methods are used to extract metals.