Re-Os geochronometry is a promising technique to determine the age of sulfide minerals directly. We have hitherto applied this method to molybdenite (MoS2) and successfully obtained Re-Os ages which show good agreement with related wall rock ages, an indirect indicator of the real age (Suzuki et al., 1993; Suzuki and Masuda, 1993). In the early stages of Re-Os chronometry, obtained ages were not consistent enough with the wall rock ages (Luck and Allègre, 1982); this discrepancy was assigned to Re loss after primary mineralization. McCandless et al. (1993), who investigated molybdenite through near infrared (N-IR) microscopy, electron microprobe and SEM, reported that high transparency of molybdenite to N-IR or existence of secondary minerals in molybdenite minerals indicated alteration and also Re loss. In our study, experiments have been carried out to clarify the effect of alteration on Re-Os age of molybdenite.
Sample molybdenite was heated at 180°C for 20 days in various solutions, such as NaClaq, NaHCO3 aq, CaCl2 aq and
AlCl3 aq and H2O, in sealed quartz tubes. Re-Os ages of the molybdenite treated with NaClaq, NaHCO3 aq and H2O deviate from that of the original molybdenite, suggesting that Re and Os is likely to be fractionated during alteration in nature under such condition. Meanwhile, Re-Os ages of the molybdenite heated in CaCl2 aq and AlCl3 aq were in good agreement with that of the untreated molybdenite. N-IR transparency of the treated sulfides increased in most cases. On the other hand, the molybdenite altered in NaClaq and NaHCO3 aq remained opaque despite its Os loss. This indicates that an increase in N-IR transparency of molybdenite in hydrothermal heating does not necessarily mean alteration and a loss of Re, although McCandless et al. (1993) claimed that high transparency of molybdenite to N-IR may indicate alteration and Re loss. As expected from the results of Re and Os analysis, Re-Os fractionation would be detected by replicate analyses of various parts of the same sample. In addition, it was observed in our experiments that intensity
in micro-X-ray diffraction (XRD) pattern of the treated molybdenite was reduced, suggesting possible reduction of crystalinity of molybdenite. The combination of Re-Os analyses and micro- XRD could be useful to detect a possible mobilization of Re-Os in molybdenite after mineralization.
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