MINING GEOLOGY/Magmatic Ores 637 Hedenquist JW and Lowenstern JB (1994) The role of magmas in the formation of hydrothermal ore deposits Nature 370: 519 527 Humphris SE, Zierenberg RA, Mullineaux LS, and Thompson RE (1995) Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geochemical Inter actions AGU Geophysical Monograph 91, Washington, DC: American Geophysical Union McKibben MA and Hardie LA (1997) Ore forming brines in active continental rifts In: Barnes HL (ed.) Geochemistry of Hydrothermal Ore Deposits, 3rd edn., pp 875 933 New York: Wiley Interscience Pirajno F (1992) Hydrothermal Mineral Deposits: Prin ciples and Fundamental Concepts for the Exploration Geologist Heidelberg: Springer Verlag Rye RO (1993) The evolution of magmatic fluids in the epithermal environment: the stable isotope perspective Economic Geology 88: 733 753 Thompson JFH (1995) Magmas, Fluids and Ore Deposits Mineralaogical Association of Canada, Short Course Series 23 White DE (1981) Active Geothermal Systems and Hydro thermal Ore Deposits, pp 392 423 Economic Geology Seventy Fifth Anniversary Volume El Paso, Texas: Eco nomic Geology Publishing Company White NC and Herrington RJ (2000) Mineral deposits as sociated with volcanism In: Sigurdsson H (ed.) Encyclo pedia of Volcanoes, pp 897 912 San Diego: Academic Press Magmatic Ores J E Mungall, University of Toronto, Toronto, ON, Canada ß 2005, Elsevier Ltd All Rights Reserved Introduction Magmatic ore deposits may be defined as rocks of igneous origin (see Igneous Processes), which can profitably be mined for their constituent chemical elements For example, the worth of annual global production from magmatic ore deposits exceeded $10 billion in 2001, and dominates or contributes significantly to world supplies of elements representing more than a third of the periodic table This article summarises the petrogenetic controls on the genesis of magmatic ore deposits, with an emphasis on the application of geochemical models Readers are encouraged to consult the Further Reading Section at the end of this article for detailed descriptions of the multitude of types of magmatic ore deposits (Figure 1) Fundamental Controls The generation of a magmatic ore deposit depends upon the successful operation of four fundamental processes Elements that are normally widely distributed at low concentrations must be extracted from a large volume of rock by a natural melting event The resulting magma must be collected into or channelled through a relatively small volume of the Earth’s crust, such as a conduit or magma chamber While within the conduit or magma chamber the magma must become saturated with a phase within which the element of interest is highly concentrated The phase containing high concentrations of the target element must then be mechanically sorted from the remainder of the magma, and collected in sufficient quantity and at sufficient grade to constitute an economically attractive ore deposit (see Economic Geology) The study of magmatic ore deposits is, therefore, inextricably tied to studies of the partitioning of elements between coexisting phases in magmas, and of the processes of intrusion and differentiation of magmatic systems (Figure 2) Element Partitioning The key to a quantitative approach to the description of the evolution of a magmatic system lies in the distribution of chemical components among the coexisting phases The distribution of an element between two phases at equilibrium may be described through the use of a partition coefficient D, which is defined as the ratio of the concentration of the element in one phase (e.g., mineral or sulphide melt) to its concentration in another (e.g., silicate melt) (see Minerals: Sulphides); for example, the partition coefficient for Cu between sulphide and silicate melts can be expressed as sulphide sulphide=silicate DCu ¼ CCu Csilicate Cu ½1 where C denotes concentration of the subscripted element in the superscripted phase Partition coefficients can be used in quantitative thermodynamic calculations if the element in question is a trace element whose concentration varies through ranges too small to affect D (Henry’s Law is obeyed) If the element is a major or stoichiometric constituent of one of the phases, then D loses its utility as a constant Values of