Highly Siderophile and Strongly Chalcophile Elements in Magmatic Ore Deposits

Abstract

An ore deposit by definition must be economically viable, that is to say it must contain sufficient material at high enough grade to make it possible to mine and process it at a profit (Bates and Jackson 1987). This requires the elements to be collected and concentrated by some phase and for them to be deposited close to the surface of the earth. At the oxygen fugacities found in the crust, native Fe is not normally stable and thus the highly siderophile elements (defined as Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) cannot behave as siderophile elements except in rare cases such as on Disko Island (Klock et al. 1986) where the magma is sufficiently reduced for native Fe to be present. However, if mafic magmas become saturated in a base-metal-sulfide liquid, the highly siderophile elements behave as highly chalcophile elements (Table 1). Thus these elements are generally found in association with base-metal-sulfide minerals which crystallized from a magmatic sulfide liquid, namely pyrrhotite, pentlandite, chalcopyrite, cubanite ± pyrite. An exception to this is Au. Although Au is strongly chalcophile and is produced as a by-product from many platinum-group element (PGE) deposits (Table 2), most primary Au deposits consist of native Au (Groves et al. 1998). These will not be discussed in this chapter. There are many PGE-deposits (i.e., accumulations of PGE minerals and base metal sulfides containing PGE; Bates and Jackson 1987) around the world, but most of these do not constitute PGE ore deposits, because they are either too small or their grade is too low, or other political or infrastructure factors prevent the economic exploitation of the deposit (Bates and Jackson 1987) For the purpose of this work we have defined PGE ore deposits as those which have significant production (> 2% of the annual world …