Mineralogy and mineral chemistry of detrital heavy

Authors: Thomas Oberthür Frank Melcher Simon Goldmann Hermann Wotruba Axel Gerdes Arjan Dijkstra Christopher W Dale

Publish Date: 2015/05/08

Volume: 105, Issue: 2, Pages: 637-657

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Abstract

In the course of studying the goldbearing heavy mineral spectrum of sediments from the upper Rhine River a distinct suite of detrital grains comprising platinumgroup minerals PGM cassiterite columbitegroup minerals and uraninite was identified and investigated using conventional and modern analytical methods This study aimed to characterize the selected mineral groups mineralogically and geochemically in order to identify possible source areas and to reconstruct different aspects of the complex sedimentary history of the Rhine River sediments The PGM assemblage is dominated by grains of Ru–Os–Ir alloys ~70  followed by Pt–Fe alloys sperrylite and rare other PGM Accordingly this PGM assemblage represents highly mature physically and chemically extremely resistant compounds which may have experienced and survived repeated reworking during their sedimentary history Pt–Fe alloys and sperrylite may originate from various sources however the predominant Ru–Os–Ir alloy grains point to an origin from ophiolite sequences of unknown age but likely preAlpine Variscan or older The exact locations of the primary sources and the complex prolonged sedimentary history of the detrital PGM with possibly multiple intermittent storages remain unknown Detrital cassiterite grains were dated by the U–Pb method using LAICPMS The age dates of cassiterite largely overlap with zircon age distributions by peaking distinctly at ca 325 Ma majority of ages thereby implying a predominantly Variscan age of the cassiterite grains and possible derivation from mineralization in the Black Forest area Columbitegroup minerals are dominantly tapiolite originating from pegmatites Rare uraninite grains attest that this mineral experienced rapid erosion transport and deposition in a reducing environmentMany thanks go to HansJoachim Sturm BGR for drafting Fig 1 Continuous and professional work at the EPMA and the SEM was ably performed by Jerzy Lodziak Christian Wöhrl and Detlev Klosa BGR and is gratefully acknowledged We are indebted for the thoughtful and critical contributions of two reviewers anonymous and H Bahlburg which significantly improved both the flow of the text and the clarity of presentation

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