Re-Os Dating of Boddington Molybdenite, SW Yilgarn: Two Au Mineralization Events

he Boddington Au-Cu mine is located in the southwestern corner of the Yilgarn craton in the Western Gneiss terrane in Western Australia. The mineralization is hosted in the northern part of the Saddleback greenstone belt which includes an early sequence of basaltic to dacitic volcanic rocks and associated shallow level porphyry intrusions emplaced between ~2714-2696 Ma. After two periods of deformation, a subsequent volcanic-intrusive event occurred ~2675 Ma. A third period of deformation and shearing and a fourth period of brittle faulting followed. The intrusion of unmineralized granitoid plutons at ~2611 Ma marks the end of the magmatic history related to Boddington. Two timings for the Au mineralization at Boddington have been proposed. The first advocates that Boddington is an example of an Archean porphyry copper deposit associated with convergent plate tectonics at 2714-2696 Ma (e.g. Roth 1992, Barley et al. 1992). The second suggests that Boddington formed 30-80 m.y. after the emplacement of arc sequences, and that the age of Au mineralization is bracketed by movement on late brittle faulting and the intrusion of pyroxenite dikes (less than ~2675 Ma) and the intrusion of ~2611 Ma unmineralized granite plutons (Allibone et al. 1998). The results of this Re-Os study clearly show that Au mineralization occurred at both times, creating a complex superposition of veining and alteration, and an extraordinary Au ore deposit. This is the first Re-Os study of molybdenites where the analytical work was carried out at two different laboratories. In our analysis of the Re-Os data, we use the 187 Re-187 Os isochron method (Stein et al. 1998), which provides the most rigorous and conservative treatment of the analytical data. Interlaboratory agreement coupled with geologically accurate results clearly demonstrates that the Re-Os chronometer in molybdenite is robust and enduring in Archean rocks that have experienced subsequent thermal events and intense shearing. Four analyses from three different molybdenites from the older generation yield a 2-sigma Re-Os age of 2726 ± 24 Ma or 2707 ± 17 Ma (Ludwig 1999), depending on inclusion of the assumed 187 Os initial zero point (Fig. 1). The zero point is implicit in any molybdenite model age calculation, and because the older molybdenites are Re-rich (328, 344, 435, 832 ppm), the points lie far to the right on the isochron diagram, making for a long extrapolation to the origin. In this case, the 2707 ± 17 Ma age is the best estimate for …