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Techniques and methodology used in a mineralogical and osmium isotope study of platinum group minerals from alluvial deposits in Colombia, California, Oregon and Alaska

Published online by Cambridge University Press:  05 July 2018

I. C. Lyon ,
H. Tamana ,
D. J. Vaughan ,
A. J. Criddle ,
J. M. Saxton  and
P. van Lierde
I. C. Lyon
Affiliation:
Department of Earth Sciences, The University of Manchester, Manchester, MI3 9PL, UK
H. Tamana
Affiliation:
Department of Earth Sciences, The University of Manchester, Manchester, MI3 9PL, UK Department of Mineralogy, The Natural History Museum, London, SW7 5BD, UK
D. J. Vaughan
Affiliation:
Department of Earth Sciences, The University of Manchester, Manchester, MI3 9PL, UK
A. J. Criddle
Affiliation:
Department of Mineralogy, The Natural History Museum, London, SW7 5BD, UK
J. M. Saxton
Affiliation:
Department of Earth Sciences, The University of Manchester, Manchester, MI3 9PL, UK
P. van Lierde
Affiliation:
VG Isotech, Fisons Instruments, Aston Way, Middlewich, CWl0 0HT, UK
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Abstract

Platinum-group minerals (PGM) from placer deposits in Colombia, California, Oregon and Alaska were investigated with the electron microprobe, proton microprobe (μ-PIXE) and ion probe to analyse their major and trace element contents and 187Os/186Os isotopic ratios. Most of the grains in the samples investigated proved to be essentially homogeneous alloys of Pt-Fe and Os-Ir-Ru although a few of them contained inclusions of other PGM such as cooperite and laurite. Detailed analyses were undertaken on the Os-Ir-Ru alloy phases.

The 187Os/186Os isotope ratios fell into a range from 1.005 to 1.156 and are consistent with data published on PGM from other placer deposits from these regions. The ratios, together with the trace element data (and in particular the low rhenium content) determined by ion probe and μ-PIXE, indicate that crustal osmium was not incorporated in the grains and that no significant evolution of the 187Os/186Os ratios occurred during their history. These data, along with mineralogical and textural evidence, are consistent with a mantle origin for the grains through ultramafic intrusions, although the data do not entirely rule out alternative interpretations.

Keywords

platinum group mineralsosmium isotopesalluvial depositsColombiaOregonAlaska
Type
Mineralogy
Information
Mineralogical Magazine , Volume 61 , Issue 406 , June 1997 , pp. 367 - 375
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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Footnotes

*

Present address: Charles Evans Associates, 301 Chesapeake Drive, Redwood City. CA 94063, USA

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