Abstract

Four major periods of postorogenic porphyry-type Paleozoic and Mesozoic metallogenesis are recognized in eastern Queensland. The four periods, Siluro-Devonian, Permo-Carboniferous, Permo-Triassic, and Early Cretaceous, are widely distributed within, or immediately west of, the Paleozoic-early Mesozoic Tasman Orogenic Zone. The age of mineralization within the three younger periods is confined to discrete "pulses" with the ages of the deposits within each period decreasing with progression toward the east.Overall, the deposits are characterized by weakly developed potassic alteration assemblages, although widespread phyllic and propylitic assemblages are almost always present. Sulfide and alteration mineralogy is predominantly fracture controlled and usually exhibits rough zonation, commonly about a central core. Supergene enrichment is weakly developed or lacking and all deposits are at present economically submarginal. Depth of exposure increases with deposit age and from west to east.From east to west, the deposits exhibit characteristics of those found in island-arc, continental margin, and ensialic environments, with the majority exhibiting island-arc characteristics. However, with the exception of several deposits to the west, they were probably emplaced in a continental margin environment. This apparent inconsistency is thought to result from deposit emplacement into thin continental crust on the eastern edge of the developing Australian Plate.Within each period of metallogenesis, the deposits are confined to narrow, sometimes parallel, linear belts up to 25 km in width. These are either longitudinal or transverse in relation to the regional strike of the Tasman Orogenic Zone. Porphyry-type alteration, brecciation, and mineralization of calc-alkaline intrusives are restricted to the belts, rarely extending outside them. Diagonal intrabelt trends of mineralization and alteration are common, particularly at the intersection of transverse lineament zones and longitudinal belts. Mineralization within longitudinal porphyry-type belts is considered to be genetically related to either postulated subduction of westerly dipping oceanic plates during the Paleozoic and Mesozoic or to tension breaks on the flanks of structural highs, or to both. The location of deposits within these belts is influenced by transverse elements such as faults and major lineaments which are oriented east-northeast and east-west in southern and northern Queensland, respectively. Most of these elements are thought to be zones of structural weakness and appear to contain porphyry-type mineralization, especially in areas where previous copper mineralization has taken place. The intrabelt mineralization trends may result from shear stress in these zones during porphyry-type emplacement.

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