51 Platinum-bearing magmatic rocks of the Southern Brook Street Terrane, South Island, New Zealand R.J. Arculus, C.J. Spandler, K. Worden, S. M. Eggins and J. Mavrogenes Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200, Australia Abstract Major occurrences of platinum group minerals (PGM) are in crustally-contaminated, huge, layered igneous complexes (Bushveld and Stillwater) and gabbroic intrusions (Norilsk); relatively minor occurrences include arc-related “zoned Alaskan ultramafics”. In terms of arc-related provenances, the recognition of alluvial PGM in river and beach deposits of southern South Island (Mitchell, 1995), in-situ in the Longwoods (Cowden et al., 1990), and the ultramafic-mafic layered Greenhills Complex at Bluff (Spandler et al., 2000) raise the possibility that some economically-significant PGM deposits may exist in New Zealand. The Longwoods and Greenhills are in the Brook Street Terrane which is a remnant of a primitive intra-oceanic arc system of Late Permian-Early Triassic age. The Terrane comprises volcanogenic sequences containing plagioclase- and clinopyroxene- phyric basalts, high-MgO ankaramite and dolerite dikes, trondhjemite plutons, and basaltic to andesitic volcaniclastic and sedimentary rocks. The cumulate complexes contain early-formed olivine- and clinopyroxene-rich ultramafic cumulates overlain by anorthite and hornblende-bearing gabbros. Primary (isoferroplatinum) and secondary (sperrylite) platinum group minerals are preserved in chrome-rich spinel pods in basal dunite of the Greenhills Complex. Saturation during early fractionation of primitive high-MgO magmas rather than magma mixing seems likely for the primary PGM. Together with correlative terranes in Queensland and New Caledonia, the Brooks Street Terrane is an exposed cross-section of an extensive Permo-Triassic island-arc system. Most parental magmas were primitive island-arc tholeiites, but other primary magma-types include high–MgO ankaramites and trondhjemites – the latter formed by partial melting of lower crustal clinopyroxene-rich cumulates and gabbros, respectively. The parental ankaramites fractionated to form mafic-ultramafic cumulates and primitive to evolved melts of high-Al basalt to andesite compositions (Takitimus). The Brook Street Terrane is an analogue for modern intra-oceanic island- arc systems and allows detailed study of subvolcanic arc processes. Introduction Due to high relative insolubilities of Pt and Pd in basaltic magmas, saturation with PGM is known experimentally to occur early in the crystallisation sequence. Other mechanisms for saturation however, especially in some of the world’s largest and economically significant deposits, (Merensky – Bushveld; JM Reef – Stillwater) include magma mixing wherein one of the magmas bears the PG elements and the other sulfur. Development of large PGM deposits in either case requires of course large volumes of magma and comparatively rare events such as the impact of a mantle plume head with the continental crust. A relatively minor occurrence of PGM is within island arc- related plutonic bodies such as the “zoned Alaskan ultramafics”. While the potential for discovery of huge layered plutons in New Zealand is minimal, the possibility of relatively small volume PGM deposits developed in island arc-associated plutons seems more reasonable. However, if there is a requirement that such deposits only become developed in primitive cumulate fractions, then the general paucity of exposure of such materials in arc terranes makes exploration difficult.