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 Re-187-Os-187 ages were determined for molybdenite and pyrite sa mples from two well-dated Precambrian intrusions in Fennoscandia to ex amine the sustainability of the Re-Os chronometer in a metamorphic and metasomatic setting. Using a new Re-187 decay constant (1.666 x 10(-1 1)y(-1)) with a much improved uncertainty (+/- 0.31%), we determined r eplicate Re-Os ages for molybdenite and pyrite from the Kuittila and K ivisuo prospects in easternmost Finland and for molybdenite from the K abeliai prospect in southernmost Lithuania. These two localities conta in some of the oldest and youngest plutonic activity in Fennoscandia a nd are associated with newly discovered economic Au mineralization (Il omantsi, Finland) and a Cu-Mo prospect (Kabeliai, Lithuania). Two Re-O s ages for vein-hosted Kabeliai molybdenite average 1486 +/- 5 Ma, in excellent agreement with a 1505 +/- 11 Ma U-Pb zircon age for the host ing Kabeliai granite pluton. The slightly younger age suggests the int roduction of Cu-Mo mineralization by a later phase of the Kabeliai mag matic system. Mean Re-Os ages of 2778 +/- 8 Ma and 2781 +/- 8 Ma for K uittila and Kivisuo molybdenites, respectively, are in reasonable agre ement with a 2753 +/- 5 Ma weighted mean U-Pb zircon age for hosting K uittila tonalite. These Re-Os ages agree well with less precise ages o f 2789 +/- 290 Ma for a Rb-Sr whole-rock isochron and 2771 +/- 75 Ma f or the average of six Sm-Nd TDM model ages for Kuittila tonalite. Thre e Re-Os analyses of a single pyrite mineral separate, from the same sa mple of Kuittila pluton that yielded a molybdenite separate, provide i ndividual 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 Os-187 abundances in these three pyrite splits are highly correlated (r = 0.9994): and provide a Re-187-Os-187 isochron age of 2607 +/- 47 Ma with an intercept of 21 ppt Os-187 (MSWD = 1.1). It appears that th e Re-Os isotopic system in pyrite has been reset on the millimeter sca le and that the 21 ppt Os-187 intercept reflects the in situ decay of Re-187 during the similar to 160 to 170 m.y. interval from similar to 2778 Ma (time of molybdenite +/- pyrite deposition) to similar to 2607 Ma (time of pyrite resetting). When the Re-Os data for molybdenites f rom the nearby Kivisuo prospect are plotted together with the Kuittila molybdenite and pyrite data, a well-constrained five-point isochron w ith an age of 2780 +/- 8 Ma and a Os-187 intercept (-2.4 +/- 3.8 ppt) of essentially zero results (MSWD = 1.5), We suggest that the pyrite i sochron age records a regional metamorphic and/or hydrothermal event, possibly the time of Au mineralization. A proposed Re-Os age of simila r to 2607 Ma for Au mineralization is in good agreement with radiometr ic ages by other methods that address the timing of Archean Au mineral ization in deposits worldwide (so-called ''late Au model''). Molybdeni te, in contrast, provides a robust Re-Os chronometer, retaining its or iginal formation age of similar to 2780 Ma, despite subsequent metamor phic disturbances in Archean and Proterozoic time.