Evaluation of the molybdenite rhenium-osmium geochronometer, and its application to base-metal porphyry mineralization.

Abstract

The potential application of molybdenite as a geochronometer in the Re-Os isotopic system has been recognized for nearly 40 years, but dating attempts have been sporadic and produced mixed results. This study documents the behavior of Re in molybdenite by applying microprobe/back-scattered electron imaging, x-ray diffraction, and infrared microscopy to molybdenite samples from base metal porphyry deposits, pegmatites, and skarns, that experienced processes active in hypogene, supergene, and weathering environments. Hypogene 3R and 2H molybdenite experience Re loss during hydrothermal alteration, which also causes increased infrared transparency (IR). Under supergene conditions, apparent Re loss in 3R and 2H molybdenite does not increase IR transparency. Re is not incorporated into supergene phases such as ferrimolybdite, but is enriched in an unidentified K-Al-silicate (illite ?) intergrown with molybdenite. These minute intergrowths would not be detected in a simple optical examination. Rhenium in molybdenite may be removed by later processes after some has decayed to ¹⁸⁷Os, causing erroneously old ages, or it may be adsorbed into the illite intergrowths, creating ages that are too young. Molybdenites exposed to weathering alter to Re-enriched powellite which can be detected using back-scattered electron imagery. Hypogene and near surface processes produce discernible effects in molybdenite that can be detected prior to dating. Re-Os dates for base metal porphyry deposits associated with the Laramide orogeny (∼90-50 million years ago) reveal that within individual deposits, mineralization occurs near the final stages of magmatic activity irrespective of the time of inception, magnitude, or duration of magmatism. The deposits differ widely in location and in extent and timing of magmatism, but have nearly identical ages for mineralization. Re-Os ages suggest that mineralization occurred during two distinct intervals, from ∼74 to 70 million years ago and 60 to 55 million years ago. Deposits that formed in the oldest interval are in > 1.7 Ga basement of northwestern Arizona, whereas the younger deposits are in < 1.7 Ga basement in southern Arizona and northern Mexico. Synchronous, widespread mineralization implies that similar crust-mantle interaction occurred on a regional scale, for ore deposits previously thought to be the product of localized processes.