AN Re-Os DATE FOR MOLYBDENITE-BEARING QUARTZ VEIN MINERALIZATION WITHIN THE KANGERLUSSUAQ ALKALINE COMPLEX, EAST GREENLAND: IMPLICATIONS FOR THE TIMING OF REGIONAL METALLOGENESIS
The Kangerlussuaq Alkaline Complex, East Greenland, is one of the largest alkaline complexes in the world. It is known to host a number of occurrences of base and precious metal mineralization, including the Flammefjeld porphyry Mo deposits and a suite of epithermal style base and precious metal veins in an area around the Sondre Syenitgletscher. Here we describe and date a previously unknown occurrence of molybdenite-pyrite-scheelite–bearing quartz veins associated with alkali basalt dikes that intrude the main syenites of the complex. The veins appear to be cogenetic with the dikes, which crosscut and brecciate granodiorites of the Cirque 1320 complex. Sulfur isotope signatures of molybdenite and pyrite in the veins give a tight range with mean δ 34 S of 2.2 ± 0.7‰, consistent with a magmatic S source related to the intrusion of the dikes. Molybdenite from the veins gave an Re-Os age of 52.74 ± 0.26 Ma, some 13 m.y. older than molybdenite at the Flammefjeld porphyry deposit, thus distinguishing this metallogenic episode as a distinct event, temporally unrelated to the known Mo mineralization at Flammefjeld. Significantly, in terms of the timing of regional metallogenesis and magmatism, our date suggests that the age of the Kangerlussuaq Alkaline Complex itself may be slightly older than the 50 Ma currently accepted, based on 40 Ar- 39 Ar dating from biotites in the main syenites. Given the common discrepancy in ore systems of 40 Ar- 39 Ar, which produce slightly younger dates than those by Re-Os and U-Pb, we suggest the actual age of the Kangerlussuaq Alkaline Complex may be closer to 53 to 52 Ma.