A new global database of δ98Mo in molybdenites: A literature review and new data

Abstract

Isotopic compositions of Mo in molybdenites were used for deciphering a possible genetic link between isotopic variations and mineralizing processes, based on a worldwide molybdenite databank. We compared the δ98/95Mo (hereafter referred as δ98MoNIST) of 391 molybdenite samples (193 from the literature, 198 for this study) from different localities, different types of occurrences and different ages. The 198 molybdenite samples we analysed represent various types of mineralization in 6 granites, 11 pegmatites, 6 perigranitic veins, 2 greisen, 28 porphyry deposits, 5 skarns, 1 IOCG, and 9 Alpine-type fissure veins, with ages varying from 5Ma to 2.7Ga. The Mo isotopic composition was determined with an MC-ICP-MS Neptune after aqua regia dissolution and adjustment to [Mo]=1μg·ml−1. Mass bias was corrected by using Zr as dopant and standard-sample-standard bracketing. The δ98MoNIST ratios were normalized to NIST3134. External reproducibility is 0.07‰ (2σ). The overall range of the δ98MoNIST ratio in the 391 molybdenite samples varied from −1.62 to 2.27‰, being higher for molybdenite formed in Alpine-type veins, greisen, perigranitic veins and IOCG, than for that in granite, pegmatite, porphyry deposits and skarns. The crystallization temperature can explain some of these differences, as polymetallic Alpine-type fissure veins broadly crystallize at lower temperatures than granite, pegmatite and porphyry deposits. For some occurrences the δ98MoNIST was determined on several molybdenite samples, showing variability at occurrence scale. For example, in the Azegour skarn (Morocco) the δ98MoNIST varies from −0.60 to 0.42‰ (n=29), and in “Ravin de la Ruine” Alpine-type fissure veins (France) the variation is from −0.08 to 0.77‰ (n=3). No correlation is seen between δ98MoNIST and the age of the deposits.