Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand

摘要: Biogeochemical processes drive the transformation of gold (Au) in surface environments. In this study we assess link between morphologies Au grains and supergene processes, with a focus on formation nano-particulate temperate settings. Gold were collected from six localities across South Island New Zealand. Deposit styles vary eluvial-, alluvial-, beach placer deposits areas moderate to very high levels precipitation. assessed using optical microscopy (OM), field emission scanning electron (FEG-SEM), focused ion beam-scanning (FIB-SEM) coupled X-ray dispersive analysis (EDXA) microprobe analyses (EPMA). Morphologies indicative Au- Ag dissolution, e.g., grain boundary as well abundant neoformation- aggregation observed all grains. The latter include variety secondary morphotypes, particular nano-particulate- μ-crystalline forms bacteriomorphic Au, sheet-Au porous, branched networks. Pervasive dissolution features an outcropping quartz-vein system extensive nano-particle weathered quartz-vein- west coast Zealand's Island, which is subject heavy orographic precipitation, suggest that these climatic conditions enhance At sites, nano-particles are most polymorphic layer (i.e., coating biofilms, siliceous carbonaceous materials transforming grains), soil associated Nano-particulate also highly carbonaceous, likely exopolymeric, coatings Orepuki Beach, suggesting seawater microbial biomineralization important contributors alteration deposits. conclusion, Zealand result transformations occurring current was previously thought be evaporation-driven, high-rainfall environments due other mechanisms, such biomineralization.