GEOLOGY OF MOONMERA-PORPHYRY-DEPOSIT, QUEENSLAND, AUSTRALIA

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. Sulfide mineralization took place during three principal episodes that are time equivalents of the three members of the Moonmera intrusive complex. The geologic data presented here does not indicate the origin of the fluids responsible for alteration or mineralization inasmuch as they could be either ground-water derived or orthomagmatic.; Some features of porphyry deposits in the southwestern Pacific and Australia are briefly considered. Though differences correlative with the island-arc setting of many of these deposits are evident (more mafic rocks, less K and Mo, more Au), these deposits display a diversity of characteristics which are essentially similar to Cordilleran deposits. Common descriptive elements are combined in a wide variety of spatial and temporal combinations. The complex patterns we see are the integrated effect of a number of processes controlled by a set of at least partly independent geologic factors. Unfortunately a qualitative and partly intuitive definition of the essential common elements of these deposits and the factors which control their variation is the best we can do at the present. Common elements include porphyritic rock associations, structural style, and zoning and sequence of mineral assemblages. The geologic factors each evolve with time during the formation of a deposit and include: structure; depth, size, and timing of intrusion; ground-water incursion; volatile evolution from the magma reservoir; and wall-rock composition. This time factor is not adequately taken into account in most descriptions or idealized models of porphyry copper deposits. On a larger scale, precise definition of tectonic factors remains elusive even where deposits of very young age occur.