492 MINERAL DEPOSITS AND THEIR GENESIS the Yilgarn block of Western Australia (another discovery of the 1960s) and to very large masses of metallurgical grade chromite at Selukwe (now named Shurugwi) in Zimbabwe The Selukwe occurrences contrast with the genetically similar, but much smaller podiform chromite segregations in ophiolites of the Tertiary Alpine belt in eastern Europe A source of ilmenite alternative to beach sands is found in alkaline igneous complexes such as the anorthosites at Egersund in Norway, Allard Lake in Canada and Stillwater in the USA The distinctive pipe-like intrusive carbonatites are closely associated with peralkaline intrusive complexes and are mined for an unusual range of minerals The highly mineralized Palabora complex, South Africa (Figure 6), produces copper (average ore grade 0, 54% Cu), apatite, vermiculite, magnetite, baddeleyite (a zirconium oxide), uranium and by-product sulphuric acid Kimberlite pipes, diatremes and some dykes are famous as the primary source of diamonds Economic grades are typically only a few carats to the tonne (1 carat ¼ 0.2 g), but most kimberlites not contain diamonds in sufficient quantity and/or quality to meet economic criteria These deep-seated intrusives bring xenoliths of mantle material to surface, and diamond mining has spurred geological research This has resulted in genetic understanding that has in turn lent impetus to the current exploration boom and new discoveries – a sequence of events typical of progress in economic geology Figure Palabora mine open pit, averages 1750 M across the rim and has reached its economic working depth of 822 M in 38 years Beneath the open pit underground mass mining by block caving methods is now producing 30 000 tonnes per day of copper ore from the core of this very productive carbonatite pipe Photo graph courtesy of Rio Tinto plc Associations with Felsic Igneous Activity Granitic magmas consolidate in a dynamic and highenergy environment dominated by superheated fluids at high temperatures and pressures As rising magma slowly consolidates the portion still melted becomes progressively more enriched in volatiles Release of pressure, by volcanic eruption, for example, may result in the mechanical release of gases capable of extensive fracture damage to both the intrusive and also the intruded country rocks At high crustal levels the heat around the magma may create convection cells of surrounding groundwaters and augment the fluid content, so that episodic igneous and hydrothermal activity may persist over tens of millions of years The fluids incorporated into the magmatic system may thus include deep connate waters and meteoric waters, both of which could be mineralized through leaching of the rocks that they traverse These mineralized fluids may then transport and precipitate their dissolved load in rocks at higher levels, at lower temperatures and pressure, or escape to the surface The terms hypothermal, mesothermal and epithermal are used to describe mineral assemblages attributed to hydrothermal fluids at relatively high, moderate and low temperatures and pressures (Figure 7) The vast range of mineral deposits associated with felsic intrusions include pyrometasomatic replacement such as skarns (tactites) forming scheelite at contacts with limestones, and corundum at contacts with basic rocks Complex coarse grained pegmatites are a product of late-stage fluids, and are mined for a variety of minerals including cassiterite, wolframite, beryl, mica and lithium minerals Extensive vein systems may form in and around intrusive granite cupolas, well displayed by the Cornubian batholith of southwest England, mined since pre-Roman times, but no longer economic Rich metallic mineralization of Sn-Cu-Zn-Pb displays a marked zonal pattern, and hydrothermal alteration of some granite areas (supplemented by later surface weathering) formed extensive kaolin deposits Zonal patterns of wall-rock alteration and of metallic-sulphide mineralization also characterize the immensely productive, but lowgrade porphyry copper and molybdenum deposits that contribute so much to current world production of base metals and by-product gold These huge ore bodies are formed as impregnations at depth within and around hydrofractured calc-alkaline intrusives above subduction zones at destructive plate margins Many gold- and silver-rich epithermal ore deposits formed nearer the surface are attributed to late-stage magmatic or mixed magmatic-meteoric fluid systems linked to the porphyry bodies One interesting