Abstract:
Tungsten and gold mineralisation at Kirwan's Hill, located on the western margin of the Victoria Range and 10 km east of Reefton, occurs in quartz veins emplaced within Greenland Group metasediments, a turbidite sequence of alternating metapelites and metagreywackes of Early Ordovician age. A major NW-NWN trending lineament, informally named the Shaw-Drysdale Fracture Zone (SDFZ), has controlled the localisation and emplacement of mineralised quartz veins. The most significant mineralisation occurs at intersections between the SDFZ and ENE-WSW trending faults/shears, with the maximum W, Pb, Zn, Ag, Cu, B, and F geochemical anomalies occurring in the headwaters of Drysdale Creek and high in Kirwan's Creek.
Mineralisation in the quartz-fissure veins is characterised by scheelite, with minor cassiterite and associated base metal sulphides. Scheelite also occurs as fine disseminations in greisenized intervals of metagreywacke, which were intersected by an exploratory diamond bore hole sited in the southern part of the study area. Gold and associated arsenopyrite mineralisation occur in hydrothermal quartz-stockwork veins, topographically higher and distinctly lateral to the tungsten mineralisation.
Alteration of the metasediments, in association with the scheelite mineralisation, is characterised by biotite-tourmaline-muscovite-pyrrhotite assemblages in quartz-fissure vein alteration haloes, and phlogopite-clinozoisite-scheelite assemblages in the greisenized intervals. These assemblages are interpreted as being indicative of an exogreisen environment. Gold mineralisation is accompanied by silicification of the metasediments.
Exogreisen alteration around quartz-fissure veins is geochemically characterised by depletion in SiO2, Na2O, with increases in Al2O3 (due to SiO2 depletion), Fe2O3, K2O, Rb, B, F, W, and Sn. In greisenized intervals increases in CaO and W are the most significant changes.
Mineralisation is considered temporally and spatially linked with a magmatic source associated with the intrusion of Devonian-Carboniferous granites. Related tungsten mineralisation is dated around 300 m.y. B.P., and lateral gold deposition is likely to have occurred during a late phase of this event.
