Rhenium behavior in molybdenite in hypogene and near-surface environments: Implications for Re-Os geochronometry

Rhenium is concentrated mainly in molybdenite (MoS₂) and occurs as a major cation in only a few rare minerals. This affinity makes molybdenite an ideal mineral for Re-Os geochronometry, but research on the behavior of Re in molybdenite is limited. Infrared microscope, XRD, back-scattered electron (BSE), and microprobe techniques have been used in this study to document Re behavior in molybdenite affected by hypogene and near-surface processes. In the hypogene environment, both 3R and 2H molybdenite can experience Re loss during hydrothermal alteration, which also causes increased infrared transparency (IR). Alteration at temperatures as low as ~ 150°C can cause Re loss in the presence of advecting fluids, and will affect Re-Os dating if it occurs long after primary mineralization. Re loss in 3R and 2H molybdenite under supergene conditions does not increase IR transparency. Rhenium is not incorporated into supergene ferrimolybdite, but is enriched in K-Al-silicate intergrowths which may be illite(?). These minute intergrowths are present in several samples and would not be detected in a simple optical examination. In the supergene environment elemental Os is stable, whereas Re is not. Rhenium in molybdenite may be removed by supergene fluids after some has decayed to ¹⁸⁷Os, causing erroneously old ages, or it may be adsorbed into the illite intergrowths, creating ages which are too young. In the weathering environment, Proterozoic molybdenites have altered to Re-enriched powellite, which can be detected using back-scattered electron imagery. Combined microprobe, XRD, BSE, and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating. These techniques are non-destructive and should be performed before any molybdenite is dated by the Re-Os system.