Re-Os ages for Archean molybdenite and pyrite, Kuittila-Kivisuo, Finland and Proterozoic molybdenite, Kabeliai, Lithuania: testing the chronometer in a metamorphic and metasomatic setting

Seven ^sup 187^Re-^sup 187^Os ages were determined for molybdenite and pyrite samples from two well-dated Precambrian intrusions in Fennoscandia to examine the sustainability of the Re-Os chronometer in a metamorphic and metasomatic setting. Using a new ^sup 187^Re decay constant (1.666×10^sup -11^y^sup -1^) with a much improved uncertainty (±0.31%), we determined replicate Re-Os ages for molybdenite and pyrite from the Kuittila and Kivisuo prospects in easternmost Finland and for molybdenite from the Kabeliai prospect in southernmost Lithuania. These two localities contain some of the oldest and youngest plutonic activity in Fennoscandia and are associated with newly discovered economic Au mineralization (Ilomantsi, Finland) and a Cu-Mo prospect (Kabeliai, Lithuania). Two Re-Os ages for vein-hosted Kabeliai molybdenite average 1486±5 Ma, in excellent agreement with a 1505±11 Ma U-Pb zircon age for the hosting Kabeliai granite pluton. The slightly younger age suggests the introduction of Cu-Mo mineralization by a later phase of the Kabeliai magmatic system. Mean Re-Os ages of 2778±8 Ma and 2781±8 Ma for Kuittila and Kivisuo molybdenites, respectively, are in reasonable agreement with a 2753±5 Ma weighted mean U-Pb zircon age for hosting Kuittila tonalite. These Re-Os ages agree well with less precise ages of 2789±290 Ma for a Rb-Sr whole-rock isochron and 2771±75 Ma for the average of six Sm-Nd T^sub DM^ model ages for Kuittila tonalite. Three Re-Os analyses of a single pyrite mineral separate, from the same sample of Kuittila pluton that yielded a molybdenite separate, provide individual model ages of 2710±27, 2777±28, and 2830±28 Ma (Re=17.4, 12.1, and 8.4 ppb, respectively), with a mean value of 2770±120 Ma in agreement with the Kuittila molybdenite age. The Re and ^sup 187^Os abundances in these three pyrite splits are highly correlated (r=0.9994), and provide a ^sup 187^Re-^sup 187^Os isochron age of 2607±47 Ma with an intercept of 21 ppt ^sup 187^Os (MSWD=1.1). It appears that the Re-Os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt ^sup 187^Os intercept reflects the in situ decay of ^sup 187^Re during the 160 to 170 m.y. interval from 2778 Ma (time of molybdenite±pyrite deposition) to 2607 Ma (time of pyrite resetting). When the Re-Os data for molybdenites from the nearby Kivisuo prospect are plotted together with the Kuittila molybdenite and pyrite data, a well-constrained five-point isochron with an age of 278