Geology of the Coalstoun porphyry copper prospect, southeast Queensland, Australia

Abstract

The Coalstoun porphyry copper prospect in southeast Queensland lies adjacent to the north-northwest-trending Perry fault zone near the intersection with an east-northeast-trending structural lineament. Late Permian intrusions at the prospect are dominantly porphyritic biotite + or - hornblende microtonalite and quartz microdiorite emplaced into Devonian ?-Carboniferous quartz-rich fine-grained metasediments. Three intrusive centers, each containing several distinct phases, are recognized in an area of 6 X 2.5 km. The intrusions appear to have attained shallow crustal levels as evidenced by porphyritic texture, abundance of metasedimentary xenoliths, rafts and screens, moderately developed fracturing, and the presence of numerous surrounding breccia pipes.A core of hydrothermal biotite alteration containing copper grades of 2,000 to 4,000 ppm and up to 150 ppm Mo is present in the central intrusion. The biotite zone possibly overlies a deep, relatively unaltered core and grades outward into a chlorite-carbonate alteration zone which extends out into surrounding metasediments. Small pockets of quartz-sericite-pyrite and hypogene sericite-clay alteration occur surrounding, and within, the biotite zone, and the complete hypogene alteration system is partially blanketed by supergene clay (-seriate) alteration containing enriched copper grades. Associated breccia pipes are dominantly composed of angular metasedimentary fragments and exhibit quartz-sericite-pyrite and sericite-clay alteration with sporadic tourmaline. Copper grades decrease outward from the biotite zone, whereas pyrite: chalcopyriteratios increase.Alteration in the biotite and chlorite-carbonate zones is nearly isochemical, with a minor decrease in Na 2 O/K 2 O, an increase in Rb/Sr, and increases in Cu and K 2 O in the former. Volumetrically minor quartz-sericite-pyrite and sericite-clay zones generally show decreases in MgO, CaO, Na 2 O, Ba, Cr, Ga, Sr, V, Y, Zn, and Na 2 O/K 2 O and increases in H 2 O, S, K 2 O, K/Rb, and Rb/Sr.Hydrothermal biotites commonly pseudomorph magmatic biotite and hornblende pheno-crysts, are finer grained, and are pale brown to greenish-brown compared to magmatic biotites which are orange-brown. Hydrothermal biotites contain lower TiO 2 , total Fe and FeO, Fe/Fe + Mg + Mn, and Na 2 O, and higher (A1) VI and possibly higher Fe (super 3+) /Fe (super 2+) + Fe (super 3+) than the magmatic biotites. The differences between the two biotite types are interpreted as indicating a decrease in pressure and/or temperature and increases in oxygen and sulfur fugacities in the formation of hydrothermal biotite.Initial emplacement of porphyry intrusions (possibly generated in a continental margin tectonic environment) to high crustal levels was controlled by regional fault intersections. Columns of shattered rock above the intrusions were mobilized as breccia plugs ahead of the rising stocks. Crystallization and cooling of the stocks was accompanied by retrograde boiling, fluid expulsion through the breccias, additional collapse structures, and stockworking. A high-temperature biotite zone formed with pervasive and fracture-controlled mineral deposition from high-salinity, possibly oxidizing, magmatically derived hydrothermal fluids, which introduced Cu, Mo, S, and K 2 O and initially extended toward the margins of the intrusives.Expulsion of magmatic fluids and interaction with convecting meteoric waters possibly caused deposition of quartz-sericite-pyrite and hypogene sericite-clay alteration assemblages peripheral to, and within, the biotite zone. Gradual cooling of the system initiated an inward collapse of the meteoric fluid convective cells, forming pervasive retrograde chlorite-carbonate alteration in the metasediments and outer portions of the stocks.Possibly from the Tertiary to the present, topographically controlled supergene clay (-sericite) alteration has occurred, with significant secondary copper sulfide development.