Abstract
Rhenium and Osmium abundances were determined on molybdenites from eighteen ore deposits in Japan; fifteen vein-type, a skarn-type, a greisen-type, and a pegmatite-type deposit. Re Os ages obtained for molybdenites from northeastern, central, and southwestern Japan are 100–130 Ma, 50–75 Ma, and ca. 10 Ma, respectively, reflecting the ages of regional igneous activities in individual areas. For vein-type ore deposits, Re Os ages for molybdenites agree with Rb Sr whole-rock ages of host granitoids, while they are 3–12 Ma older than K Ar mineral ages of the host rocks. On the other hand, Re Os ages of molybdenites from skarn-, pegmatite-, and greisen-type ore deposits agree with K Ar ages of the host rocks. The comparison of Re Os ages for molybdenites with Rb Sr and K Ar ages of host rocks in vein-type deposits suggests that Re Os closure temperature for vein molybdenite is close to whole-rock Rb Sr closure temperature for host granitic rocks, i.e., roughly estimated to be around 500°C, and higher than K Ar closure temperature for host granodiorites, i.e., 230–370°C, if the thermal history of vein molybdenites is essentially equivalent to that of host rocks. One possible explanation for age discrepancies of 3–12 Ma between Re Os ages for molybdenites and K Ar ages for the wall rocks, observed in vein-type molybdenum deposits, is that later thermal events after formation of molybdenum ores may reset K Ar ages. Hydrothermal alteration might have occurred in such a temperature condition as to disturb K Ar mineral systems in wall rocks and made them younger but not the Re Os system in molybdenite. The other explanation might be that the age discrepancies correspond to a cooling interval from ca. 500 to 300°C of host granitoid. On the other hand, the agreement of Re Os ages of molybdenites and K Ar ages of host rocks in skarn-, pegmatite-, and greisen-type deposits in this study indicates no later disturbance for K Ar system in the wall rock or the rapid cooling of the ore and wall rocks after the formation of molybdenite deposits.
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