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 clinopyroxenephyric 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 islandarc systems and allows detailed study of subvolcanic arc
processes.