Abstract

The Moonmera deposit in central coastal Queensland is thus far the oldest documented porphyry deposit on the margin of the Pacific Ocean. Age of mineralization based on this investigation is 245 m.y. The deposit is associated with late-stage porphyritic differentiates of the Bouldercombe Complex, a late Permian granodioritic body of batholithic dimensions. This complex was emplaced into an east-west dilatant zone developed across the Yarrol Basin at the close of the Permo-Triassic Hunter-Bowen orogeny. The members of the intrusive complex comprise an early granodiorite porphyry, followed by an andesitic tuffisite pipe, and finally the youngest granodiorite porphyry, with associated radial and ring-dike sets. Equigranular quartz diorite of the Boulder-combe Complex forms about 95 percent of the deposit's wall rocks.Sulfide mineralization took place during three principal episodes that are time equivalents of the three members of the Moonmera intrusive complex. Stage 1 mineralization is pyrite-chalcopyrite-molybdenite that preceded the oldest granodiorite porphyry and is volumetrically the most important mineralization at Moonmera. This mineralization which is principally in steeply dipping fractures and veins in quartz diorite has a well-developed zonation surrounding the intrusive complex. The zones are a low-grade core, a chalcopyrite-molybdenite inner halo, a chalcopyrite-pyrite intermediate halo, and a pyrite outer halo. Stage 2 mineralization occurs as very finely disseminated chalcopyrite in the matrix of the tuffisite pipe. Stage 3 mineralization is disseminated chalcopyrite either in the core of a young granodiorite porphyry pluton or in its uppermost roof zone. It also includes massive chalcopyrite and pyrite fillings of voids in breccias that mantle the roof zones of individual young intrusions.Alteration has taken place during three equivalent episodes. Alteration accompanying stage 1 mineralization is composed of a weakly developed zone of pervasive biotite which overlies a deeper zone of moderate biotite and anhydrite. Secondary orthoclase in veins, and selvages and envelopes to veins, occurs in both the shallow and deep biotite zones. Strong biotitization of the older granodiorite porphyry groundmass also occurred during this episode. Alteration accompanying stage 2 mineralization was predominantly strong biotitization of the tuffisite matrix. The episode accompanying stage 3 mineralization includes moderate to intense sericitization of the cores of the younger granodiorite porphyry intrusions and their capping breccias and propylitization of the outer margins of these intrusions.The parental magma of the Bouldercombe Complex and the Moonmera intrusive complex is believed to be derived by partial melting of oceanic lithosphere that was subducted beneath the eastern margin of Queensland during the late Permian.

First Page Preview

First page PDF preview
You do not currently have access to this article.