Abstract
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.
Highlights
Ore deposits are the main source of metals for society, and their efficient and sustainable exploration requires a precise understanding of the factors that control their distribution within the upper crust
Molybdenite (MoS2) the most common molybdenum ore mineral constitutes a particular case within sulphide minerals because it contains high Re and 187Os, but almost no initial or common 187Os, all 187Os in molybdenite is of radiogenic origin[1,2,5]
Distribution is critical in interpreting the accuracy of isotopic measurements, and explain spurious Re-Os ages obtained by microbeam techniques
Summary
Ore deposits are the main source of metals for society, and their efficient and sustainable exploration requires a precise understanding of the factors that control their distribution within the upper crust. Molybdenite (MoS2) the most common molybdenum ore mineral constitutes a particular case within sulphide minerals because it contains high Re (in the ppm range) and 187Os (at ppb levels), but almost no initial or common 187Os, all 187Os in molybdenite is of radiogenic origin (i.e. produced from decay of 187Re)[1,2,5] These unique characteristics explain why Re-Os molybdenite dating using the whole mineral approach is currently the most widely used single mineral geochronometer in ore deposits, where reliable crystallization ages have been obtained by the direct measurement of 187Re and 187Os concentrations in the mineral. The samples were previously analyzed for Re and Os using N-TIMS2,14 and were selected because of their high Re and Os content (Supplementary Table 1), which facilitate their detection by EMPA and NanoSIMS
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