Alluvial platinum-group minerals from southern New Zealand

Mitchell, Mark John, 1969-

This thesis provides a detailed account of the mineralogy of alluvial platinum-group minerals (PGM) recovered from the Waiau River, Western Southland, and the south coast of the South Island, New Zealand. The study is based on observations made from thousands of PGM particles collected from six sample localities, three of which were collected as part of alluvial gold mining operations and represent concentrations from 10-1000 m3 of sediment.

Pt-Fe alloy is the most common PGM recovered at all sample localities. Iridium, osmium, cooperite, braggite, sperrylite, Pt-Cu alloys, isomertieite, cuprorhodsite, and laurite are also present as detrital phases. PGM recovered from the Waiau River are the coarsest in the region (up to 1 mm). PGM recovered from other localities are significantly finer-grained (200-300 μm).

Pt-Fe alloy particles exhibit a range of habits from euhedral cubic crystals to well rounded flakes. Two populations of Pt-Fe alloy are recognised on the basis of I. Fe+ Cu + Ni (at%) contents; low Fe alloy (average of I 6 at%) and high Fe alloy (average of 24 at%). X-ray diffraction studies indicate both populations have a crystal structure corresponding to native platinum (and the variety ferroan platinum). Compositional zonation related to changes in the concentration of minor elements (Type 1) or Fe (Type 2) is observed in a small number of Pt-Fe alloy particles.

Euhedral, cubic or octahedral, Pt-bearing iridium is the most common IPGE alloy, followed by minor to trace amounts of hexagonal osmium. Ruthenium alloys are rare. A small number of IPGE alloy particles contain coexisting osmium and iridium(± Pt-Fe alloy). Chemical zonation is observed in one composite particle. Cooperite and braggite is characterised by extremely low Ni contents (maximum of 1.2 wt%) and an apparent compositional gap extending from 73-64 mol% PtS. Sperrylite is present as euhedral crystals with a nearstoichiometric composition.

Mineral inclusions within detrital PGM range in size from <2 ~-tm - 100 ~-tm and exhibit a variety of forms ranging from euhedral, single phase crystals to rounded, polyphase aggregates intergrown''On a sub-micron scale. Two types are recognised; inclusions formed via the entrapment of crystals and/or liquids at the time of host formation, and inclusions resulting from exsolution. Entrapped inclusions predominate and contain an extensive mineral suite consisting of; laurite, erlichmanite, cooperite, braggite, bowieite, cuprorhodsite, sperrylite, Pt-Cu alloy, Pt-Fe alloy, kotulskite, gold, base-metal sulphides (BMS), silicates, oxides, and a number of unknown phases. Exsolution of osmium is primarily restricted to Pt-Fe alloy of the low Fe population, whereas iridium exsolves from the high Fe population. Exsolved inclusions of hollingworthite are also present in Pt-Fe alloy. Iridium hosts exsolution of Pt-Fe alloy from the high Fe population.

Exsolution textures between IPGE and Pt-Fe alloys, the composition of cooperite and braggite, and the presence of primary mafic phases within silicate inclusions, suggest the majority of PGM formed at temperatures > 800 °C, and are consistent with derivation from a magmatic source via mechanical weathering. Textural associations suggest a magmatic crystallisation sequence of; osmium, iridium, laurite, Pt-Fe alloy, cooperite, braggite, cuprorhodsite, hollingworthite. The majority of sperry lite crystals and their inclusion suite may have formed in response to late stage magmatic or hydrothermal fluids. The presence of thin rims containing Pt-Cu alloy, PGE sulpharsenide, and unknown PGM developed about small numbers of Pt-Fe alloy and iridium particles record minor modification of primary mineralogy at lower temperatures, possibly during metamorphic or serpentinisation events. Apart from transport-induced changes in the morphology of Pt-Fe alloy there is no evidence to suggest modification of PGM mineralogy within the surficial environment.

The present distribution of alluvial PGM suggests introduction to the middle to lower reaches of the Waiau River and dispersal by longshore currents upon reaching the coast. Differences in both the morphology and proportion of PGM recovered from fluvial and coastal localities are a result of hydraulic constraints imposed by longshore transport. The geological history suggests PGM in the Waiau River could be derived directly from a magmatic source or be recycled from Tertiary sediments or Quaternary glacial deposits.

The dominance of alloy phases suggests PGE mineralisation was predominantly controlled by variation in oxygen fugacity. Extremely low Ni contents within cooperite and braggite indicate that an immiscible sulphide liquid did not play a role in mineralisation. The mineralogy of silicate and oxide inclusions suggests crystallisation from a hydrous, relatively oxidised, possibly calc-alkaline, magma. The entire PGM suite is consistent with derivation from an Alaskan-type complex, a type of ultramafic intrusion recognised from convergent plate margin settings. A similar tectonic environment is represented in southern New Zealand by the Median Tectonic Zone (MTZ). None of the numerous mafic-ultramafic bodies within the MTZ and adjacent tectonostratigraphic terranes have strong similarities to Alaskan-type intrusions. The source of alluvial PGM
from southern New Zealand remains unknown.

1 v. (various pagings) : ill. (some col.), maps ; 30 cm.

1996Mitchell

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