Bachelor of Science (Honours)
School of Earth & Environmental Sciences
Dr Solomon Buckman
McGregor, Angus J., Mineralisation of Pipeline Ridge, Cobar, NSW - Style and genesis, Bachelor of Science (Honours), School of Earth & Environmental Sciences, University of Wollongong, 2013.
The Pipeline Ridge Au-Cu-Pb-Zn deposit of central New South Wales is contained within the Sarona Downes tuff member of the Kopyje Shelf, 20 km southeast of the town of Canbelego, adjacent to the Coonara Fault. It is a poorly understood, currently sub-economic, deposit on the eastern extent of the Cobar Basin; a part of the north-northwest striking region separated out as the Canbelego-Mineral Hill volcanic belt. The Cobar Superbasin is an extensional basin characteristic of the Central Lachlan Orogen and as of 2006 is the richest polymetallic basin within the Lachlan Orogen.
Petrography, whole rock geochemistry (XRF), geochronology (U-Pb SHRIMP zircon dating) and sulphur isotope analysis have been used to describe the geology, mineralisation and produce a petrogenetic model of the Pipeline Ridge system. Two felsic volcanic rock samples above and below mineralisation were sampled for zircon extraction, and 44 zircons were dated using the SHRIMP at ANU. Both samples fall within error of each other giving a combined age of 426.0 ± 5.3 Ma. This indicates that the volcanic pile at Pipeline Ridge consists of at least two separate interlayered acidic tuff units deposited on the shelf around the Silurian-Devonian boundary (426.0 ± 5.3 Ma) and probably represents the early stages of rifting within the basin. Petrographic and geochemical evidence suggest these tuff units represent two separate sources from evolved magmatic sources produced from decompressional melting of continental crust and underplated basalts. Epithermal mineralisation stocks were emplaced into the steeply eastward dipping volcanic pile, sub-parallel to an interpreted fault zone probably during deformation and inversion of the basin during the Mid Devonian Tabberabberan Orogeny. Petrographic evidence of the strongest mineralisation zones indicates three phases of hydrothermal influx to be responsible for the mineralisation of the system. Phase 2 alteration is responsible for the main pulse of economic mineralisation introducing base metals, Au and carbonates in hydrothermal fluids. Sulphur isotope analysis was obtained for pyrite (10.3 δ34 S ‰), chalcopyrite (11.6 δ34 S ‰), and galena (9.0 δ34 S ‰). Pyrite-galena pairs are used cautiously as geothermometres, indicating hydrothermal fluid deposition temperatures to be at and above the upper limits for epithermal deposits (approximately 350oC).
Extensive hydrothermal alteration in the form of sericitisation and chlorite alteration is evident in thin-sections and has been taken into to account when using immobile element geochemical classification and discriminant plots, which indicate these volcanics are derived from an alkali-rich (A-type) source. This deposit may represent an initial Kuroko VMS style deposit associated with the initial eruption of felsic volcanics in a marine environment but the strong hydrothermal alteration and recrystallisation makes it difficult to interpret. The chalcedonic, coliform and vuggy quartz veining as well as mineralisation and alteration assemblages suggests the final mineralisation best fits an intermediate sulphidation epithermal deposit model. The regional tectonic setting of the Cobar Basin within the central Lachlan Orogen during the latest Silurian to Devonian is thought not to be reflected by one single modern day tectonic analogue by a combination of tectonic factors characterising a number of modern day, subduction related extensional settings.