Os-rich nuggets from Au-PGE placers of the Maimecha-Kotui Province, Russia

a multi-disciplinary study

K. N. Malitch*, T. Auge, I. Yu Badanina, M. M. Goncharov, S. A. Junk, E. Pernicka

*Corresponding author for this work

Research output: Contribution to journalArticle

26 Citations (Scopus)

Abstract

Geological, mineralogical and Os isotopic data for detrital PGE-mineralization derived from the Guli and Bor-Uryah ultramafic massifs, within the Maimecha-Kotui Province (the northern part of the Siberian Platform, Russia), are presented for the first time. The detrital platinum-group minerals (PGM) are dominated by Os-Ir-(Ru) species, which is typical for ophiolites or Alpine-type complexes. However, the PGM assemblage in the placers investigated is similar to that derived from zoned platiniferous clinopyroxenite-dunite massifs (also known as Uralian-, Alaskan-type and Aldan-type massifs). The unique features of the Au-PGE placers at Guli are (1) the dominance of Os-rich alloys over other PGM and Au, and (2) the considerable predicted resources of noble metals, particularly osmium. Dominant chromite, olivine and clinopyroxene inclusions recorded in Os-Ir-(Ru) alloys imply that they were derived from ultramafic sources (e.g., chromitite, dunite and clinopyroxenite). The first in situ osmium-isotope measurements by laser ablation -multiple collector - inductively coupled plasma mass spectrometry of different, intimately intergrown, PGM (e.g., laurite and Os-rich alloys) in various nuggets from Guli have revealed low 187Os/188Os and γOs values. They yield a very narrow range of 187Os/188Os (0.12432 to 0.12472) and γOs (-2.39 to -2.07). These values are indicative of a common chondritic or subchondritic mantle source of PGE. 187Os/188Os and γOs values of Os-rich alloys, derived from the Bor-Uryah massif, are different (i.e., γOs ranges from -2.67 to -1.30). The mineral-isotopic data obtained are consistent with the conclusion that the PGM were derived from parent ultramafic source rocks. Os-isotope model ages in the range of 495 to 240 Ma constrain the age of ultramafic protoliths in the northern part of the Siberian Craton. The variation in 187Os/188Os values for detrital PGM, where the provenance source is unknown, is considered to be a useful technique for distinguishing parent bedrock sources.

Original languageEnglish
Pages (from-to)121-148
Number of pages28
JournalMineralogy and Petrology
Volume76
Issue number1-2
DOIs
Publication statusPublished - 2002
Externally publishedYes

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