Metamorphism and Skarn Mineralisation in the Cobar Basin: Implications for Exploration

Abstract

A metamorphic map of the Siluro-Devonian Cobar Basin highlights zones of high heat (hornblende hornfels facies) overprinting cool (sub-greenschist facies) southern basin sequences. Contrary to current models for mineralisation in the Cobar Basin, which involve metals/fluids derived from basement during regional metamorphism and basin inversion, these zones of localised, large thermal contrast imply proximal, albeit blind, magmatic heat sources. Mineralisation in the main northern Cobar mineral field is associated with greenschist facies high-strain zones and is linked with fluids that exploited major fault systems and regional lithological contacts. Correlative mineralisation along these same fault systems and lithological contacts at Hera and Nymagee orebodies to the south, is associated with hornblende hornfels facies skarn alteration and Au–Ag–Cu–Zn–Pb mineralisation. Petrographic studies at Hera reveal orebody-scale mineralogical zonation from southern garnet-rich, central pyroxene-rich and northern anorthite–tremolite-rich skarn (with remnant carbonate blocks/clasts). Chemical composition of skarn minerals, including sub-calcic garnet (Mn–Ca-rich), low-Mo scheelite, Mn-enriched diopside/tremolite, zoisite and anorthite are consistent with the Hera–Nymagee orebodies representing reduced, low XCO2 distal Zn(W–Cu–Au)-skarns. Stable isotope (H–O–S) data from the Hera and Nymagee orebodies are consistent with magmatic water/sulfur sources, with formational water/sulfur also represented. A distal skarn origin for mineralisation in the southeastern Cobar Basin and the distinction of zones of elevated hydrothermal heat reflect a relatively intrusion proximal southern mineral system, while correlative deposits in the main Cobar mineral field to the north are hosted in lower temperature hydrothermal zones and likely reflect relatively intrusion-distal parts of the same magmatically driven system. Zn-rich skarns akin to the Hera–Nymagee orebodies generally occur in the distal portions of major magmatic/hydrothermal systems. Thus, the absence of near-orebody intrusions implies the mineral systems of the southeastern Cobar Basin may not have been traced to their ends and the basin is immature with respect to exploration for skarn and intrusion-related mineral systems.