科研之友
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摘要
We analysed the Mo isotope composition of a comprehensive series of molybdenite samples from the porphyrytype Questa deposit (NM, USA), as well as one rhyolite and one granite sample, directly associated with the Mo mineralization. The delta Mo-98 of the molybdenites ranges between -0.48 parts per thousand and + 0.40 parts per thousand, with a median at -0.05 parts per thousand. The median Mo isotope composition increases from early magmatic (-0.29 parts per thousand) to hydrothermal (-0.05 parts per thousand) breccia mineralization (median bulk breccia = -0.17 parts per thousand) to late stockwork veining (+ 0.22 parts per thousand). Moreover, variations of up to 0.34 parts per thousand are found between different molybdenite crystals within an individual hand specimen. The rhyolite sample with 0.12 mu g g(-1), Mo has delta Mo-98 = 0.57 parts per thousand and is lighter than all molybdenites from the Questa deposit, interpreted to represent the igneous leftover after aqueous ore fluid exsolution. We recognize three Mo isotope fractionation processes that occur between about 700 and 350 degrees C, affecting the Mo isotope composition of magmatic-hydrothermal molybdenites. Delta Mo-1: Minerals preferentially incorporate light Mo isotopes during progressive fractional crystallization in subvolcanic magma reservoirs, leaving behind a melt enriched in heavy Mo isotopes. Delta Mo-2: Magmatic-hydrothermal fluids preferentially incorporate heavy Mo isotopes upon fluid exsolution. Delta Mo-3: Light Mo isotopes get preferentially incorporated in molybdenite during crystallization from an aqueous fluid, leaving behind a hydrothermal fluid that gets heavier with progressive molybdenite crystallization. The sum of all three fractionation processes produces molybdenites that record heavier delta Mo-98 compositions than their source magmas. This implies that the mean delta Mo-98 of molybdenites published so far (similar to 0.4 parts per thousand) likely represents a maximum value for the Mo isotope composition of Phanerozoic igneous upper crust.
- 出版日期2014-3
