Year

2011

Degree Name

Bachelor of Science (Honours)

Department

School of Earth & Environmental Sciences

Abstract

The Nymagee and Hera deposits in western New South Wales are located on the eastern margin of the Cobar Basin. The Cobar Basin is host to several sediment-hosted polymetallic precious and base-metal orebodies, and forms part of the highly metallogenic Lachlan Orogen. Unlike the well described mineralisation around the Cobar region, the Nymagee and Hera deposits are less well known. The documentation of the geological nature, as well as a description of the ore associations, the temperature of formation, and the source of mineralising fluids, will help constrain the physical and chemical conditions at the time of mineralisation, as well as provide the processes responsible for ore formation. The two deposits occur in a strongly altered and deformed sequence of shelf and turbiditic sedimentary rocks and are host to two different styles of mineralisation. The Nymagee deposit being Cu-rich and the Hera deposit, located ~5km southeast of the Nymagee deposit being Pb-, Zn- and Au-rich. This metal zonation between the two deposits is thought to be related to temperature gradients of the mineralising hydrothermal fluids that were focussed along the shear zone that connects the two deposits. Petrographic and mineralogical analyses using XRD and HyLogger reveal that the quartz-rich turbidites have been metamorphosed to upper greenschist to lower amphibolite faces as evident by the common occurrence of metamorphic muscovite, biotite, chlorite and amphibole that define a strong foliation. Liquidrich fluid inclusions were found hosted in massive quartz from the Nymagee deposit and contained low salinities. The vapour phases contained major methane and minor nitrogen. Fluid inclusion analysis resulted in a minimum formation temperature of 240ºC, which agrees with the presence of cubanite found in chalcopyrite at the Nymagee deposit and chlorite geothermometry. However sulfur isotope geothermometry and the presence of amphiboles in the samples indicate much higher temperatures of around 400 to 800ºC. The accuracy of these temperatures is questioned. Sulfur isotopes indicate that the Nymagee deposit has sulfur sourced from reduced seawater sulfate from the host metasedimentary sequences of the Cobar Basin. The Hera deposit has sulfur values that are lighter than the Nymagee deposit and indicates a similar sulfur source from to that of Nymagee but with the possible addition of magmatic sulfur. Lead isotopes results suggest that the two deposits have a crustal source that was derived from a Silurian to Early Devonian lead reservoir. The lead isotope signatures for the two deposits align with Silurian VHMS and Devonian granite signatures for the Lachlan Orogen.

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