Abstract Placer gold particles have traditionally been considered as either detrital products of weathering or authigenic minerals growing within placers. Recent advances in understanding of gold chemistry/bio-geochemistry demonstrate that gold growth in specific environments is plausible, but opinions differ on the importance of ‘new’ gold in the overall placer inventory. Here we draw upon visual inspection over 40 000 polished gold particle sections from locations worldwide to evaluate the implications of gold alloy composition and particle heterogeneity in determining the contributions of detrital and authigenic gold to fluvial placers. We conclude that: (1) the detrital model of placer gold formation is widespread and demonstrable; (2) supergene gold may be a locally important constituent of fluvial placers; (3) gold-rich rims on placer gold particles comprise two distinct components – a surface micron-scale addition of pure Au and a tens-of-microns-scale inner rim formed by Ag depletion; and (4) the importance to placer inventories of gold particle formation and modification by biogenic processes is considerably overstated.

Abstract Graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization. We examine processes of graphitic carbon emplacement and deformation in the active Alpine Fault Zone, New Zealand by analysing samples obtained from Deep Fault Drilling Project (DFDP) boreholes. Optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain. Raman spectroscopy allowed interpretation of the degree of graphite crystallinity, which reflects both thermal and mechanical processes. In the amphibolite-facies Alpine Schist, highly crystalline graphite, indicating peak metamorphic temperatures up to 640°C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions (500–600°C), resulting in clustered (in protomylonites) and foliation-aligned graphite (in mylonites). In cataclasites, derived from the mylonitized schists, graphite is most abundant (<50% as opposed to <10% elsewhere), and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon. Tectonic–hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences. The increasing abundance of graphite towards the fault zone core may be a significant source of strain localization, allowing fault weakening. Supplementary material: Raman spectra of graphite from the Alpine Fault rocks is available at https://doi.org/10.6084/m9.figshare.c.3911797

Abstract Numerical models have been run to evaluate the key parameters that affect fluid flow and gold mineralization at a range of scales, from the full thickness of the crust to the mineral grain scale. These models are constrained with real examples of orogenic gold in southern New Zealand. Large scale modelling shows that differences in crustal strength and thickness affect the locus, scale, and rate of crustal fluid flow and hence influence gold deposition. The most vigorous hydrothermal activity and gold mineralization occurs in narrow zones with maximum uplift close to a major crustal boundary. Relatively strong middle crust develops deformation-driven permeability and fluid flow in a broad diffuse zone. Deformation of randomly inhomogeneous rocks under mid-crustal conditions results in horizontal fabric and horizontal shear zones, and these can control fluid flow locally. The addition of graphite to flat shear zones causes an increase in deformation-induced permeability, and this can encourage further graphite deposition in a feedback effect. Fluid flow in these flat shears is very slow (mm/year) compared to fluid flow in fracture-controlled permeability at shallow crustal levels. Rock strength inhomogeneities in mid-crustal shear zones can result in localized decrease in differential stress, facilitating the switching of orientations of principal stress axes and the formation of steeply-dipping quartz veins.

Abstract Scanning electron microscopy was used to observe a variety of gold remobilization textures and authigenic gold overgrowths on detrital gold particles associated with quartz pebble conglomerates (QPC) from two localities in eastern Southland, New Zealand. Gold spheroids, spheroids with budlike protrusions, polyspheroidal aggregates, budded mass structures, and pseudohexagonal plates are common on detrital gold particles at these two sites. There has been no anthropogenic introduction of mercury or amalgam at either site. Some gold precipitation textures can be distinguished from some gold dissolution textures. Precipitation textures are characterized by surface textures and pure gold rims that occur on the outermost margin of sharply defined silver (± mercury)-depleted rims in most detrital particles. In contrast, dissolution textures are characterized by varying degrees of dissolution occurring preferentially along subgrain boundaries, thereby revealing the polycrystalline makeup of placer gold. Surface textures reflect a close association between gold precipitation and gold dissolution which may lead to the ambiguity inherent in interpreting gold-remobilization textures from placer environments.