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 |
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| Authors: | H J Stein K Sundblad R J Markey J W Morgan G Motuza |
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| Institution: | (1) AIRIE Group, Department of Earth Resources, Colorado State University, Fort Collins, CO 80523–1482 USA (hstein@cnr.colostate.edu), US;(2) US Geological Survey, 910 National Center, Reston, VA 20192 USA , US;(3) Department of Geology and Geochemistry, Stockholm University, S-106 91 Stockholm, Sweden, SE;(4) Geological Survey of Lithuania, Konarskio 35, Vilnius, Lithuania, LT |
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| Abstract: | Seven 187Re-187Os 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 187Re decay constant (1.666 × 10−11y−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
TDM 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 187Os abundances in these three pyrite splits are highly correlated (r = 0.9994), and provide a 187Re-187Os isochron age of 2607 ± 47 Ma with an intercept of 21 ppt 187Os (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 187Os intercept reflects the in situ decay of 187Re 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 2780 ± 8 Ma and a 187Os intercept (−2.4 ± 3.8 ppt) of essentially zero results (MSWD = 1.5). We suggest that the pyrite isochron age records a
regional metamorphic and/or hydrothermal event, possibly the time of Au mineralization. A proposed Re-Os age of ∼2607 Ma for
Au mineralization is in good agreement with radiometric ages by other methods that address the timing of Archean Au mineralization
in deposits worldwide (so-called “late Au model”). Molybdenite, in contrast, provides a robust Re-Os chronometer, retaining
its original formation age of ∼2780 Ma, despite subsequent metamorphic disturbances in Archean and Proterozoic time.
Received: 25 September 1996 / Accepted: 27 August 1997 |
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