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Mineralium Deposita

, Volume 48, Issue 1, pp 81–97 | Cite as

Compositional variations and morphological evolution in platinum beach placers, southern New Zealand

  • D. CrawEmail author
  • M. Mitchell
  • R. McCann
  • A. Reay
Article

Abstract

Morphologies of placer platinum group minerals (PGM) are more variable and resistant to modification during transport than placer gold grains. This study documents morphological evolution of PGM placer grains during up to 120 km of transport in beach placers after river transport from inferred sources up to 200 km inland. PGM morphological changes are calibrated with changes in morphology of associated placer gold. Most of the PGM are Pt-Fe alloy and have been fed into the beach placer system from a large river at the western end of the beaches on the south coast of New Zealand. The incoming fluvial PGM suite includes Os, Ir and Ru alloys which may have been derived from distal ophiolitic sources. More proximal sources have Ural-Alaskan affinities and these contributed cooperite and braggite, or sperrylite, locally, as well as Pt-Fe alloy grains. Some PGM may have been recycled through Cretaceous-Quaternary fluvial sediments before entering the modern placer system. Recycled placer PGM grains have also been derived from elevated Quaternary beaches near the coastline. PGM grains entering beach placers have rough surfaces, with remnants of crystal faces, and these evolve to smooth flakes with progressive long-shore transport. PGM flakes have slightly thickened rims caused by impacts by saltating sand on windy beaches, and the most distal beach placers contain flakes with incipient toroidal shapes. These PGM incipient toroids are poorly developed compared to accompanying well-formed toroidal gold that has developed in nearly all beach placers, including those on elevated Quaternary beaches. Typical PGM and gold placer grain size decreases with increasing distance of transport, from fluvial grain size of 400–1,000 to ∼200 microns on the most distal beaches, accompanied by eastward loss of equant PGM grains and associated increase in proportion of flakes. Although net transport distance is ∼120 km in the beach placer complex, frequent aeolian transport of grains from beach to dunes and subsequent recycling by storm surges substantially increased total transport distance in a dynamic windy tectonic environment.

Keywords

Aeolian Placer Gold Platinum Alloy Fluvial Grain size Toroid 

Notes

Acknowledgments

This study was financed by University of Otago and the NZ Ministry for Science and Innovation. We are grateful for the generous assistance from the various miners in the study area, who provided local knowledge and samples of mineral concentrate. Discussions with Paul Ashley, Tony Christie, Doug MacKenzie, Nick Mortimer and Peter Waterman helped us to refine of the ideas expressed herein. Expert technical assistance was provided by Yosuke Kawachi, Kat Lilly, John Pillidge, Mike Trinder and Don Weston, and preparation of diagrams was ably assisted by Anna and Lisa Craw. Helpful comments by Bernd Lehmann and an anonymous reviewer substantially improved the presentation.

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Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  1. 1.Geology DepartmentUniversity of OtagoDunedinNew Zealand

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