ALBERT

Description: Major and minor geochemistry information and measured EBSD (electron backscatter diffraction), accompanied with MTEX toolbox scripts example.

Description: 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.

Description: The ability of hydrothermal scheelite (CaWO4) and calcite (CaCO3) to bind Sr in their crystal lattices makes them useful minerals for tracing fluid-rock interaction in mineralizing systems. Strontium isotopes in scheelite, calcite, and epidote were measured in situ by laser ablation-multicollector-inductively coupled plasma-mass spectrometry to assess the extent of fluid-rock interaction for Early Cretaceous orogenic W-Au mineralization occurring in the Otago Schist in New Zealand. Scheelite Sr isotope populations, regardless of their geographical location or stage of mineralization, are mostly homogeneous and cluster between 87Sr/86Sr = 0.7068 and 87Sr/86Sr = 0.7087, with the lack of Rb indicating that these are the initial ratios of the mineralizing fluids. However, while scheelite 87Sr/86Sr values in the Hyde-Macraes shear zone and the Barewood and Bendigo deposits overlap with the age-corrected 87Sr/86Sr values of the host Rakaia terrane, scheelite 87Sr/86Sr values in the Glenorchy and Waipori deposits are commonly more radiogenic than the age-corrected values of the host Caples terrane rocks. Because the Caples terrane was thrust over more radiogenic rocks before mineralization, the Caples terrane scheelite deposits are interpreted to result from fluids that were derived from deep crustal dehydration reactions within the underlying radiogenic and dominantly metaturbiditic (grayschist) Rakaia and Aspiring terranes, and/or that acquired their Sr isotope signatures from extensive fluid-rock reaction with Rakaia and Aspiring terrane grayschists during ascent. It is also evident that the scheelite 87Sr/86Sr values in the Caples terrane become more similar to the host-rock values with increasing incorporation of wall rock at the deposit scale as well as with increasing distance above the Caples-Rakaia/Aspiring terrane boundaries. Late-stage calcite that has replaced scheelite takes on the scheelite Sr isotope composition, whereas tabular calcite grains in the same samples have less radiogenic compositions that are equivalent to the wall rock. Application of in situ 87Sr/86Sr analysis therefore reveals that (1) there was extensive fluid-rock exchange of Sr during the formation of orogenic W-Au deposits, and (2) the Cretaceous W-Au mineralizing fluids in the Otago Schist were mobile over at least km-scale and probably greater vertical distances.