Lithofacies and geochemistry of the host sequence to the Currawong massive sulphide deposit, Benambra, Victoria

The Currawong massive sulphide deposit is one of two known deposits at Benambra, eastern Victoria. The deposit occurs near the base of the Gibsons Folly Formation which is a deep water, basin centre facies association of the Late Silurian Cowombat Rift (Allen, 1992). Coherent volcanic units of the Gibsons Folly Formation comprise andesite and plagioclase-phyric rhyodacite. These are predominantly shallow sills with margins of sediment-matrix hyaloclastite. The rhyodacite also forms cryptodome-like intrusions which have penecontemporaneously deformed the sequence. These and the sills were emplaced into a relatively unlithified sequence of mudstone (interbedded with thin, fine sandstone turbidites) and andesitic volcaniclastic units prior to the mineralising event. Units of strongly flow-banded and/or brecciated rhyodacite in the footwall sequence may be lavas but textures are equivocal. Quartz-plagioclase-phyric rhyolite (the Currawong Porphyry) is a sill which intruded relatively lithified rocks at the base of the Gibsons Folly Formation. Geochemical evidence indicates that it represents on-going silicic volcanism of the Middle-Upper Silurian Thorkidaan Volcanics. A sequence of andesitic scoriaceous breccia and plagioclase-quartz-bearing altered rocks comprises several depositional units separated by thin mudstone units. These are ambiguous rocks but several features suggest that they are lava-derived mass-flow deposits. They are lithologically and geochemically distinctive and host the mineralisation at Currawong. Ti, Zr, Nb and Y have behaved essentially in an immobile manner during hydrothermal alteration and subsequent metamorphism of the volcanic rocks at Currawong. Volcanic lithologies are best distinguished using the plots Zr/TiO2 vs Nb/Y (after Winchester and Floyd, 1977) and Nb vs Zr. The coherent volcanic units of this sequence form a fairly continuous geochemical magmatic evolution trend but coherent andesite and andesitic volcaniclastic rocks show a broad range of compositions. Some of the andesitic units contain xenocrystic-quartz and volcaniclastic rocks of andesitic composition also contain silicic volcanic clasts. Together these suggest that the andesitic rocks are the result of magmatic differentiation combined with mixing of andesitic and quartz-phyric silicic magmas. The Currawong deposit is interpreted as a subsea-floor replacement style volcanic hosted massive sulphide deposit. Massive pyritic mineralisation is intercalated with, and laterally equivalent to, strongly altered volcanic units which carry variable disseminated or vein mineralisation. Alteration and mineralisation show a strong stratigraphic control related to primary permeability of the host sequence. Quartzxenocrystic, andesitic scoriaceous volcaniclastic rocks were the locus of the strongest mineralisation at Currawong, and possibly at the nearby Wilga deposit. These should be a primary target for future exploration at Benambra.