The geology and geochemistry of tungsten mineralisation at Doctor Hill and Falls Creek, Central Westland, New Zealand

Abstract

The Doctor Hill and Falls Creek tungsten deposits are located inland from Ross and 25km south of Hokitika, incentral Westland. The basement geology of the study area consists of a graben of S-type biotite granite of the Karamea Batholith, truncated to the south by the Alpine Fault zone,and flanked by Greenland Group metasediments. The study area granitoids form a related evolutionary series from volumetrically dominant porphyritic biotite to more fractionated equigranular biotite granite to biotite-muscovite granite (aplite) dykes. These granitoids host two greisen related, scheelite bearing, sheeted quartz vein systems. One, located near the Doctor Hill trig station, adjacent to an intrusive contact, was discovered by investigation of anomalous scheelite grain counts in panned concentrates. The other, located further north, near Falls Creek, contains cassiterite in addition to scheelite. Both deposits were investigated at depth by a drilling programme in 1982. Greisenisation of the granite host rocks is spatially related to both vein systems. Earlygreisenisation, prior to vein formation is characterised by a secondary assemblage of quartz, muscovite and often albite, resulting from the introduction of silica and alkalis, and wallrock leaching of femic and calcic components by the hydrothermal fluid. Vein formation and associated greisenisation is characterised by openspace filling of dilatant joints and wallrock replacement by quartz, tourmaline, scheelite and muscovite with cassiterite and rare topaz at Falls Creek. Silica, boron and tungsten were introduced in the hydrothermal fluid at both localities, with tin and fluorine at Falls Creek, however a wallrock source for the calcium in scheelite is indicated by observed replacement textures. Lategreisenisation is characterised by secondary silicates, sulphides and carbonates, resulting from the introduction of chalcophile metals, sulphur and carbon dioxide in the hydrothermal fluid. A model for the genesis of tungsten mineralisation at Doctor Hill and Falls Creek is proposed involving progressive enrichment of volatiles and lithophile elements such as tungsten and tin in the residual melt, by inward fractional crystallisation of the study area granitoids. Hydrothermal activity was initiated by vapour saturation of the residual melt,and dilation of joints to allow upward movement of the hydrothermal fluid when hydraulic pressure exceeded confining pressures. Tungsten, tin, silica, alkali and volatile components were partitioned from the rest melt into the hydrothermal fluid. Deposition of scheelite (and cassiterite at Falls Creek) was probably in response to temperature loss to wallrocks, increasing pH, and/ or increasing oxygen fugacity. The differences in mineralogy, and intensity of veining and greisenisation between Doctor Hill and Falls Creek are probably related to the greater height during mineralisation of the vein system on Doctor Hill.