Techniques and Methodology Used in a Mineralogical and Osmium Isotope Study of Platinum Group Minerals from Alluvial Deposits in Colombia, California, Oregon and Alaska

I. C. Lyon1, H. Tamana1, 2, D. J. Vaughan1, A. J. Criddle2, J. M. Saxton1 and P. van Lierde1, 3, *
1 Department of Earth Sciences, The University of Manchester, Manchester, M13 9PL, UK
2 Department of Mineralogy, The Natural History Museum, London, SW7 5BD, UK
3 VG Isotech, Fisons Instruments, Aston Way, Middlewich, CW10 0HT, UK
*Present address: Charles Evans Associates, 301 Chesapeake Drive, Redwood City, CA 94063, USA

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 minerals • osmium isotopes • alluvial deposits • Colombia • Oregon • Alaska

Mineralogical Magazine; June 1997 v. 61; no. 406; p. 367-375; DOI: 10.1180/minmag.1997.061.406.04
© 1997, The Mineralogical Society
Mineralogical Society (