Re-Os dating of molybdenites from ore deposits in Japan: Implication for the closure temperature of the Re-Os system for molybdenite and the cooling history of molybdenum ore deposits

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 degrees C, and higher than K-Ar closure temperature for host granodiorites, i.e., 230-370 degrees 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 degrees 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.