Petrogenesis of the copper-bearing breccia pipes, Redbank, Northern Territory, Australia

Abstract

Copper-bearing breccia pipes in the Redbank area intersect an interbedded sequence of igneous and dolomitic sedimentary rocks which have undergone various degrees of metasomatism.The steeply inclined breccia pipes are of small size and cylindrical form and typically show in situ brecciation. The breccia matrix and associated veins consist essentially of various proportions of microbreccia, dolomite, quartz, chlorite, celadonite, hematite, K-feldspar, apatite, and chalcopyrite, with minor barite, rutile, galena, and pyrobitumen. K-metasomatism is most intense in the vicinity of the breccia pipes and associated veining, and there is mineralogical and textural evidence indicating that fluids enriched in K, Cl, P, Mg, Ce, La, CO 2 , and H 2 O were introduced at the time of breccia-pipe formation. Carbonate and sulfide minerals from brecciated and metasomatized rocks at lower stratigraphic levels have isotopic compositions consistent with magmatic hydrothermal derivation. However, the delta 13 C values of the great bulk of the dolomite in the breccia pipes indicate remobilization of sedimentary carbonate. Furthermore, the sulfur isotope ratios of the main sulfide mineralization, which occurs near the top of the brecciated sequence, are variable and generally enriched in 34 S relative to the minor amounts of sulfide at lower levels.It is concluded that the breccia pipes formed by explosive release of fluids following the buildup of significant over-pressure in a postulated carbonated, K-rich trachytic magma at depths of roughly 2 to 3 km beneath the surface. This was accompanied by intense metasomatism and precipitation of some carbonate and sulfide minerals which partly infilled the open spaces. The thermal gradients and fracturing caused extensive circulation of connate brine (and possibly descending sea water). The brine remobilized sedimentary and magmatic hydrothermal components in and around the pipes and copper mineralization in the Redbank area mainly precipitated from this brine.