FURTHER INSIGHTS INTO MO ISOTOPE VARIATIONS IN MOLYBDENITES FROM DIFFERENT ORE DEPOSITS

The Mo isotope composition of molybdenite is a potentially useful tool for understanding ore-forming processes, but has not been studied as extensively as other metal isotope systems (e.g., Cu). This study provides a large dataset to test a recent hypothesis that Mo isotopes can distinguish between different ore deposits. We determine how much variability exists within a single hand sample as well as within a single molybdenite grain. We also explore Mo isotope variations in epithermal and disseminated gold deposits, which have not previously been studied in detail. Molybdenite samples from 8 porphyry deposits, 2 porphyry epithermal deposits, 2 greisen deposits, 1 tungsten pipe deposit, and 1 disseminated gold deposit were collected and analyzed for Mo isotope compositions by MC-ICP-MS. The porphyry deposits varied with respect to size, grade, paleoclimate, alteration style, and complexity and number of mineralization events.

A wide range of Mo isotope compositions was observed (δ⁹⁸Mo = –0.59‰ to +2.05‰ relative to NIST SRM 3134 = 0.25‰; on this scale, seawater δ⁹⁸Mo = 2.3‰). A significantly wider range of δ⁹⁸Mo was observed for porphyry deposits (–0.24‰ to +2.05‰) compared to previous studies. Our results point to substantial overlap in δ⁹⁸Mo among different deposit types, indicating that Mo isotopes cannot be used to fingerprint deposit types. We found little Mo isotope variation (up to ~0.1‰) both within single coarse grains (fractions cut along as well as across cleavage planes) as well as within single hand samples containing fine-grained molybdenite from porphyry and epithermal deposits. This study also explored Mo isotope variations in molybdenites from different areas of the Eocene Berg porphyry epithermal deposit in British Columbia and the Late Archean Hemlo disseminated gold deposit in Ontario. Mo isotope fractionation within the Berg deposit was found to span 0.94‰, but most samples define an average δ⁹⁸Mo of –0.05 ± 0.19 ‰ (2SD; except for a value of +0.77‰ for one sample). Initial results from a single hand sample from the Hemlo deposit revealed δ⁹⁸Mo values of ~2.1‰. Ongoing work seeks to determine the contributing factor(s) to the high δ⁹⁸Mo, including Rayleigh distillation, fluid boiling, redox variations, and later metamorphism.