Mo isotope composition in Mo-rich high- and low-T hydrothermal systems from the Swiss Alps

We analyzed the molybdenum (Mo) isotope compositions (IC) of 59 samples from two molybdenite mineralizations (Alpjahorn and Grimsel) and from a Mo-rich hydrothermal breccia (Grimsel) from the Aar Massif, Switzerland. The formation temperature of the Late Paleozoic Mo mineralizations (300-600 degrees C) is much higher than that of the Pliocene breccia (100-160 degrees C). The Mo IC of the molybdenites varies over 1.35 parts per thousand. Even in a single hand specimen it spans 0.45 parts per thousand, indicating that fractionation processes during molybdenite precipitation can vary on a cm scale. The Mo IC of most molybdenites analyzed here are significantly heavier than that of the host rock (delta(98/95)Mo = (0.05 +/- 0.1)parts per thousand) and show a bimodal distribution centered around delta(98/95)Mo approximate to 1.1 parts per thousand and 0.2 parts per thousand. This result rules out single stage Rayleigh fractionation as the relevant formation mechanism and instead, redox variations are suggested to be a main factor controlling the Mo IC of the studied high-temperature Mo deposits. The range of the Mo IC in one single deposit, the Alpjahorn, overlaps with the variation range of almost all other published values for Mo IC in Mo deposits. Compared to the molybdenites, the breccia shows an even wider variation of 3.0 parts per thousand (delta(98/95)Mo between -1.6 parts per thousand and +1.4 parts per thousand). In contrast to the high-T molybdenite deposits, here the Mo was transported via oxidized surface waters into the breccia system, where it was reduced and precipitated. This indicates that oxidation and reduction of Mo complexes may lead to highly variable Mo IC in hydrothermal systems. (C) 2011 Elsevier Ltd. All rights reserved.