Mafic and ultramafic rocks, and platinum mineralisation potential, in the Longwood Range, Southland, New Zealand

Abstract

Intrusive rocks in the Longwood Range represent a component of the Permian Brook Street Terrane. They include diffusely layered, cumulate-textured olivine gabbro, troctolite, and gabbro, and gradations into non-cumulate gabbro and gabbronorite. Volumetrically small ultramafic layers occur (plagioclase wehrlite), and thin veins of felsic rocks ranging from quartz diorite to trondhjemite. Primary olivine, plagioclase, clinopyroxene, and subordinate orthopyroxene and hornblende are commonly altered or metamorphosed to amphiboles, minor spinel, magnetite, chlorite, biotite and clinozoisite, and serpentine in olivinerich rocks. Accessory primary Ti-bearing magnetite and ilmenite occur, and trace Cr-magnetite is characteristic of olivine-rich rocks. Trace pyrrhotite, chalcopyrite, pentlandite, and pyrite could reflect equilibrated late magmatic, and alteration-derived phases. Key petrochemical characteristics of the rock suite are high Mg, Al, Ca, and Sr contents, and low alkali, LILE, and sulfur contents. Platinum and Pd are locally enriched in drill-hole intercepts, but zones appear unrelated to rock type, magnetic properties, or to S, Cu, Ni, Cr, or Au values. Local platinum group element (PGE) enrichment in altered rocks implies metamorphic and/or hydrothermal redistribution. Pervasive PGE enrichment in Longwood rocks is an indicator of potential 'fertility', but evidence is currently lacking for the precipitation of primary stratiform PGE accumulations from a sulfide liquid saturated magma.

Pleistocene paleodrainage and placer gold redistribution, western Southland, New Zealand

“…11C). The Ourawera Stream sediments, derived partly from mafic sources in the Longwood Range, may have contained ankeritic carbonates as part of their metamorphic or hydrothermal alteration assemblages (Ashley et al 2012). The observed Na/Cl ratios suggest that some of the dissolved load has been derived from marine aerosols, and that is the probable source of at least some of the sulfate that contributed to authigenic pyrite deposition .…”

“…The Takitimu Mountains consist entirely of these rock types, whereas the Longwood Range, extending to the south coast (Figs 1-2) consists mainly of gabbroic and felsic plutons that were intruded in to these dominantly sedimentary host rocks during the Permian ( Fig. 1; Mortimer et al 1999;Turnbull & Allibone 2003;Ashley et al 2012).…”

Revised three-dimensional geometry of the platiniferous Hekeia Gabbro, Longwood Range, Southland

“…Thus it is uncertain how many of the c. 40 measurements of igneous layering and lamination by Rombouts (1994) are in situ. More reliable moderate westerly dips to igneous layering are inferred from nearby borehole intersections (Bluck, 1998;Ford, 1999;Ashley et al, 2012).…”

“…Measured NRM directions and intensities are very variable for sites and samples supposedly from the same pluton. Mortimer et al (2012) attributed most of this variation to either a secondary magnetisation, possibly serpentinisation (Ashley et al, 2012), or to induced remanent magnetisation caused by lightning strikes on the crest of the range. For the southern half of the Longwood Range the inferred primary magnetisation has an approximate inclination of -70° and a declination of 200°.…”

“…It is underlain by Late Permian, Triassic and Jurassic plutons of the Median Batholith that intrude Early Permian volcanic and sedimentary rocks of the Takitimu Group of the Brook Street Terrane (Mortimer et al, 1999;Turnbull and Allibone, 2003;McCoy-West, Mortimer and Ireland, 2014;Craw, MacKenzie and Ashley, this volume). The gabbros of the Longwood Range have long been considered to be prospective for platinum group element (PGE) mineralisation because of reported rare PGE minerals in surface samples, anomalously high PGE assays of basement drillcore, and because of detrital platinum at Orepuki (Challis and Lauder, 1977;Cowden et al, 1990;Bluck, 1998;Ford, 1999;Ashley et al, 2012). The principal target for PGE mineralisation in the Longwood Range is the Late Permian Hekeia Gabbro (Mortimer et al, 1999;McCoy-West, Mortimer and Ireland, 2014) which is restricted to inland outcrops.…”