Dissecting the Re-Os molybdenite geochronometer

Rhenium and osmium isotopes have been used for decades to date the formation of molybdenite (MoS₂), a common mineral in ore deposits and the world's main source of molybdenum and rhenium. Understanding the distribution of parent ¹⁸⁷Re and radiogenic daughter ¹⁸⁷Os isotopes in molybdenite is critical in interpreting isotopic measurements because it can compromise the accurate determination and interpretation of mineralization ages. In order to resolve the controls on the distribution of these elements, chemical and isotope mapping of MoS₂ grains from representative porphyry copper-molybdenum deposits were performed using electron microprobe and nano-scale secondary ion mass spectrometry. Our results show a heterogeneous distribution of ^185,187Re and ¹⁹²Os isotopes in MoS₂, and that both ¹⁸⁷Re and ¹⁸⁷Os isotopes are not decoupled as previously thought. We conclude that Re and Os are structurally bound or present as nanoparticles in or next to molybdenite grains, recording a complex formation history and hindering the use of microbeam techniques for Re-Os molybdenite dating. Our study opens new avenues to explore the effects of isotope nuggeting in geochronometers.