Page 434 - Global Tectonics

416 CHAPTER 13

brines of the Red Sea or by precipitation from hydro- Exploration models that integrate the characteristics of
thermal solutions which became enriched in metals by these deposits with the formation of LIPs rely on
circulating within the volcanic rocks. The deeper, plu- detailed information about the sources of the magma
tonic portions of ophiolites contain economic deposits and the deep plumbing systems that transport it through
of chromite, which occur as podiform bodies and the crust (Pirajno, 2004).
tabular masses within the harzburgites and dunites of Several forms of mineralization are present in sub-
the upper lithospheric mantle. These deposits may have duction zone environments, their types depending
formed by partial melting of primitive mantle material upon whether the overriding lithosphere is continental
or by crystal fractionation within the magma chamber or oceanic. Hedenquist & Lowenstern (1994) have
underlying ocean ridges (Section 6.10). Similarly associ- reviewed the role of magmas in the formation of hydro-
ated with the magma chamber are nickel and platinum thermal ores in such environments. The most impor-

sulfides. The model for mineralization of the oceanic tant mineralizations are the porphyry coppers. These
lithosphere derived from ophiolite studies is shown in are relatively rare, low grade deposits that are gold-rich
Fig. 13.12 (Rona, 1984). and molybdenum-poor when associated with island
The formation of ore deposits composed of nickel, arcs and gold-poor and molybdenum-rich in Andean-
copper and platinum group elements also can be linked type mountain ranges. They are found, for example, in

to the mafic and ultramafic magmatism that results in the Andes themselves, as well as in the Philippines,

the formation of LIPs (Section 7.4.1). Examples of these Taiwan, Puerto Rico, and the Ryuku and Burman arcs.
types of deposit include the 250 Ma Noril’sk deposit in The broad uniformity of these deposits worldwide sug-
Siberia, which currently produces 70% of the world’s gests that the controls on their location are related to
palladium, and the Proterozoic Bushveld intrusion the primary subduction-related magmatism and do not

(Section 11.4.1) of South Africa, which produces large require the presence of any specific crustal or magma
quantities of platinum and chrome (Naldrett, 1999). type. Magma may be emplaced and porphyry coppers

Fig. 13.11 Mineralization in the Troodos Ophiolite (redrawn from Searle & Panayiotou, 1980, with permission from the
Ministry of Agriculture and Natural Resources, Cyprus).