Placer Garnet of the West Coast, New Zealand

Abstract

Garnet is primarily used as an abrasive for sandblasting and waterjet cutting, with global demand having increased dramatically in the last few decades. Garnet suitability as an abrasive is determined by its grain size, shape, composition, inclusion and fracture content. Prior to 2015, the mineral resource potential of detrital garnet in Westland, New Zealand, had been overlooked and its regional abundance, composition, provenance and grain morphology poorly understood. This thesis gives a context for exploration and development of Westland’s garnet resources. 1132 quantitative garnet compositions were compiled from the literature for all major garnet-bearing basement rocks of the west coast of the South Island and a comprehensive detrital garnet chemical classification scheme devised for determining provenance. This is primarily based on Ca cation content versus 100*Mg/(Mg+Fe). Thirteen beach sand samples were collected along the Westland coastline from Neils Beach (in the south) to Westport (in the north). Heavy minerals were isolated and inspected under a scanning electron microscope, and an image analysis technique developed to determine the modal mineralogy, grain size and grain morphology for each sample. 70-100 quantitative garnet compositions were collected per sample. The results show that a vast majority of garnet on the Westland beaches is derived from the adjacent greenschist to amphibolite facies Haast Schist. Furthermore, there is a clear subdivision between Alpine Schist and Otago Schist-derived garnet, and changes in the metamorphic isograds can be seen in the dataset. At Rapahoe and Westport in the north of the study area, Western Province garnet increases to 12-18% of the heavy mineral population. Significant variation in the garnet-ilmenite (G/I) ratio occurs throughout Westland, with garnet in the south and north being preferentially supplied over ilmenite (G/I > 2) but being supplied in sub-equal ratios in central Westland where the Alpine Schist is exposed at the surface. Abundant graphitic inclusions result in a high proportion of garnet grains appearing black and it is suggested that previous visual estimates of garnet and ilmenite content may have been inaccurate and the graphitic garnet counted as ilmenite. Westland garnet has a mean grain size of ~230 μm (fine sand) and grain size distribution remains relatively consistent in south and central Westland. The marked decrease in grainsize (to 150μm) north of the Grey River, however, significantly reduces the potential crude value of industrial along the Paparoa Coast. The overall results of this study indicate that the deposits centred at Hokitika have the greatest economic potential. Furthermore, the most promising location near Hokitika is at Ruatapu where the large volume of easily accessible garnet sand has an average grade three times higher than the standard cut-off mineable grade.