MINING GEOLOGY/Magmatic Ores 639 fertile source Metals which begin to crystallise in a concentrating phase, when the bulk concentration of the element in the magma itself is has not yet attained ore grade, must undergo a final concentrating stage in which the mineral phase carrying the element is physically separated from the magma and collected in one place to form an ore body The effect of separation of sulphide melt from silicate melt is usually described using the assumption of batch equilibration of the two phases in a closed system: ẳ Csulphide i Cbulk R ỵ 1ị i ẵ4 sulphide=silicate R ỵ Di where R is the ratio of the mass of silicate melt divided by that of sulphide melt, and C and D are defined as above Equation [4] can be derived from eqn [2] by redefining some of the terms, and is effectively identical to the equilibrium crystallization equation Based upon their relative affinity for the three principal types of phase present in magmatic systems, the elements shown in Figure may be classed into three main groups (see Table 1) The chalcophile elements are partitioned strongly into sulphide melt at equilibrium with oxides or silicates, whereas the lithophile elements are those which reside in silicate or oxide phases in preference to metal or sulphide liquids The lithophile elements may be subdivided into those which are partitioned into oxide or silicate minerals (the compatible lithophile elements) and those which are preferentially retained in silicate melt (the incompatible lithophile elements) There is considerable overlap between the three groups of elements listed above; for example, V shows variable compatibility because it is an incompatible lithophile element during melting of the mantle, but becomes highly compatible when a magma is saturated with ilmenite or magnetite Pd is strongly chalcophile, but in the absence of sulphide melt it behaves as an incompatible element Incompatible Lithophile Elements Some combination of very small degrees of partial melting and fractional crystallization to very small residual melt fraction is required to elevate the concentrations of highly incompatible lithophile elements from their trace abundances in the bulk silicate Earth to the weight percent levels necessary for economic extraction The principal host silicate magmas are granitic pegmatites or highly evolved alkaline or peralkaline felsic plutons Carbonatites are a related class of rocks derived from carbonate magmas, either through liquid immiscibility with felsic alkaline rocks, or directly by extremely small degrees of partial melting of carbonate-rich mantle Incompatible lithophile element deposits may be subdivided into the Li-Cs-Ta (LCT) and the Nb-Y-F (NYF) classes Deposits of the LCT class are almost exclusively granitic pegmatites associated with highly fractionated intrusions of reduced ilmenite-series (S-type) granites, whereas the NYF class tend to be derived from alkaline or peralkaline felsic intrusions LCT pegmatites are thought to originate by fluidabsent melting of pelitic or semipelitic metasedimentary schists in Abukuma-type regional metamorphic belts Pelitic sediments are generally rich in alkali elements including Li and Cs, and also contain large amounts of F and B Incongruent melting of white micas and complete melting of tourmaline produces water-poor peraluminous magmas rich in the fluxing components P, F, and B The compositional undercooling caused by degassing of H2O as waterrich magmas rise through the crust, causes them to Table Summary of partitioning behaviour of metals Element group Examples DÃi Incompatible lithophile