Abstract

With longshore processes enabling sediment littoral migration along coastal shelves for tens to hundreds of kilometers, beach placers form important locations of heavy mineral accumulation and mining. We report a method for rapid composition and morphology characterization of placer sediment mineralogy using scanning electron microscope element mapping and image analysis, and we apply this technique to investigate garnet from 13 strandline samples from beaches along 427 km of the Westland coast of New Zealand’s South Island with the purpose of establishing patterns of mineral composition, morphology, and distribution in modern and associated raised beaches. Mineral modes show garnet to be least abundant on the South Westland beaches, intermediate in abundance along the Paparoa coast, and most abundant in Central Westland and North Westland. The garnets are typically almandine rich and are mostly derived from the Alpine Schist, even when this unit is distal such as along the Paparoa coastline. Garnet average grain size, which is an important parameter for a garnet resource, is coarsest in Central and North Westland (>200 μm) and finest along the Paparoa coast. Once it is abraded to ≤40 μm, it is largely removed from the sediment load. Inclusions in garnet, which reduce the viability of a resource, decrease northward. As extensive historical prospecting has been undertaken on ilmenite on narrow but elongate stranded beach sand terraces formed during higher sea levels in Westland, the calculated garnet-ilmenite area ratio for the beach placer samples coupled with the historical ilmenite bulk values indicate that several raised beach terraces likely have >5 million tonnes (Mt) of detrital garnet. Our study therefore shows that (1) garnet and other heavy minerals (including ilmenite and gold) are moved northward along the Westland coastline except where submarine canyons intercept the transport direction, (2) garnet composition can be used to fingerprint the heavy mineral sources, and there are systematic morphology variations with distance from source, (3) many beach placers and stranded terraces that formed during higher sea levels in Westland are extremely rich in garnet, and (4) the Central Westland beaches contain garnet of suitable composition, morphology, and lack of inclusions to be worthy of further mineral exploration. The scanning electron microscope and image analysis techniques employed could be readily applied to other sedimentary mineral deposits.

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