Abstract
The development and evolution of supergene gold and sulphide mineralisation that occurs in quartz pebble conglomerates (QPCs) from two contrasting environments is investigated. The contrasting sites in this study represent a progression in the development of supergene conditions that occur in relatively high energy fluvial settings. Mineralisation at two separate localities from a Miocene-Pliocene aged QPC in southern New Zealand is examined in detail. In contrast to this, gold mineralisation from an Eocene aged palaeochannel from the deeply weathered lateritic regolith of the Yilgarn Craton, West Australia, is also investigated. Although from vastly different geological settings, both occurrences of supergene mineralisation are in fluvial environments characterised by anomalous gold (Au-Ag alloy) and fine-grained diagenetic sulphides. Given the analogous fluvial environments, they have undergone similar processes and consequently a more complete picture of gold and sulphide diagenesis has been built up.
A number of gold precipitation and dissolution textures are distinguishable in the active supergene environment at Belle-Brook (New Zealand). Precipitation textures include a variety of semi-spheroidal gold forms that are variously aggregated, budded and occasionally crystalline. Dissolution textures are associated with the loss of Ag from the Au-Ag alloy and are characterised by preferential grain boundary dissolution along subgrain boundaries. Fresh dissolution textures are observed as a well-defined line occurring between subgrains, however with increasing evolution and modification to the surface early textures are often obliterated. The diagenetic sulphide suite at Belle-Brook shows a progression of sulphide diagenesis from detrital sulphides through to fine-grained framboidal pyrite and spheroidal marcasite that may also develop into a pervasive cement or large (cm’s) lumps. The low pH surface waters at Belle-Brook preclude the oxidation of these sulphides to goethite. The sulphide suite is predominantly marcasite in which extensive precipitation periodically occurs most likely in response to fluctuating redox conditions associated with the alluvial mining operation that occurs at the site.
At Sunrise Dam coarse-grained chunky palaeochannel gold is interpreted to be supergene. A high-grade hypogene source intersects the palaeochannel and hypogene gold is observed but is readily identifiable based on morphology. Supergene gold in the palaeochannel occurs as distinctive highly irregular grains characterised by embayments associated with detrital quartz grains, or filled with clays that host fine-grained microcrystalline gold. Episodic gold precipitation is inferred from microcrystalline gold that forms lineations or bands. Fluctuating redox conditions are most likely responsible for what may be episodic gold precipitation.
In the hypersaline lateritic regolith at Sunrise Dam, gold is strongly redox controlled as indicated by enrichments associated with palaeo-redox fronts. With evolving aridity associated with the change in climate from wet and tropical in the Eocene, to the present arid climate, the resultant drying out of the profile and lowering of redox fronts has resulted in a redistribution of gold enrichment and depletion zones. With the exception of fine-grained marcasite and pyrite preserved in the reduced basal lags of the palaeochannel, these changing redox conditions have resulted in the oxidation of the original diagenetic sulphide suite to goethite and iron-rich clays.
Whilst entirely supergene gold particles are rare in the QPCs in New Zealand, they are common in the palaeochannel at Sunrise Dam where there is a nearby high-grade hypogene gold source. Despite a lack of hypogene source, examples of entirely supergene gold do occur in the QPCs indicating that in very specific environments supergene conditions are conducive for mobilising and precipitating significant amounts of gold. In contrast to the lateritic regolith where supergene processes are responsible for and control gold enrichment, physical processes are the most significant for gold enrichment in QPCs from New Zealand.
The main site at Belle-Brook is characterised by actively occurring supergene processes and as such provides valuable insight into early diagenetic processes that are often obscured or not preserved in other more modified settings. This study finds that gold in the supergene profile behaves in a similar fashion, regardless of whether it occurs in an arid weathered regolith profile in Western Australia or a quartz pebble conglomerate gold placer in southern New Zealand.
Morphological features and geochemical textures on gold from all sites indicate that electrochemical refinement of the Au-Ag alloy is a major processes that drives the coupled dissolution and precipitation of gold that occurs in the supergene environment.