HIGHLY PRECISE AND ACCURATE RE-OS AGES FOR MOLYBDENITE FROM THE EAST QINLING MOLYBDENUM BELT, SHAANXI PROVINCE, CHINA

Isotope dilution with a modified alkali fusion procedure and negative thermal ion mass spectrometry yields highly precise and accurate Re-Os ages for molybdenite from two well-studied molybdenite deposits in th e East Qinling molybdenum belt, China. Individual Re-Os ages carry a 2 sigma precision of +/-0.40 to 0.57 percent which includes a 0.31 perc ent uncertainty in the Re-187 decay constant. For the unusual carbonat ite-hosted Mo-Pb deposit at Huanglongpu, the weighted average of seven analyses yields an age of 221.5 +/- 0.3 (0.15%) Ma. The weighted aver age of two analyses of molybaenite from a porphyry Mo deposit at Jindu icheng, about 10 km to the southwest, yields an age of 138.4 +/- 0.5 ( 0.39%) Ma. These data provide uncertainties an order of magnitude less than previous Re-Os ages. Molybdenite Re-Os ages are slightly older t han ages obtained by other isotopic methods for genetically related ho st-rock and vein material. It appears that the direct dating of sulfid e, rather than altered host and vein material, may be critical to acqu iring the correct age for mineralization. The East Qinling molybdenum belt is part of a larger east-west-trending zone that marks the suture between two major cratonic blocks. Consequently, the belt was a site for Early-Middle Triassic compression (Indosinian orogeny) followed by Jurassic-Cretaceous extension (Yenshanian orogeny). We suggest that t he Huanglongpu and Jinduicheng deposits provide an analogue for proces ses that may have been important in generating major molybdenum deposi ts in the Colorado mineral belt. In Colorado, Late Cretaceous (Laramid e) compression-related, alkalic magmatism was followed by Tertiary (Ri o Grande) extension-related, granitic magmatism and the development of major Climax-type porphyry Mo deposits. In particular, the Jinduichen g deposit appears to be a nearly perfect match for Climax-type mineral ization in Colorado. In contrast, the older Huanglongpu deposit may re cord a mechanism whereby molybdenum is concentrated in the lower crust . In both the Qinling molybdenum belt and the Colorado mineral belt, a time gap of about 50 to 80 m.y. separates alkalic magmatism and excep tionally evolved granitic magmatism.