Alluvial gold placers in Otago and northern Southland occur at several stratigraphic horizons within the Late Cretaceous - Recent sedimentary sequence. The gold is derived ultimately from primary sources in the Otago Schist. Poor correlation between the distribution of placers and that of the known primary deposits reflects repeated recycling of gold in the present drainage network and two precursor networks, each with substantially different architecture. The previous drainage networks were inundated and buried during marine or lacustrine transgression. There has been local addition of first cycle gold and immature detritus during each recycling phase.
Most of the placer deposits are fluvial in origin but colluvial placers occur locally along the margins of several Central Otago ranges. Aeolian placers and marine placers are rare. Most of the gold in placers north of the Caples/Torlesse Terrane boundary is Au-Ag alloy, except in vicinity of the Hyde-Macraes Shear Zone, where α-Au-Ag-Hg alloy is also present. Conversely, α-Au-Ag-Hg alloy dominates in placers south of the Caples/Torlesse Terrane boundary, except those whose headwaters lie, or lay, in the Torlesse Terrane. These systems that cross the terrane boundary contain Au-Ag and Au-Ag-Hg alloys in the reach downstream of the boundary, and placers with their source in Aspiring Lithologic Association also contain both alloys. Textural and compositional maturity of the placer host generally increase with decreasing age, reach maxima at the Waipounamu Erosion Surface, and generally decrease in tandem with age above this surface. Exceptions occur in northern Southland, where quartz pebble conglomerate placers are actively forming.
Fluvial quartz pebble conglomerate placers have not formed in a single sedimentary cycle. Instead, they have formed from precursor sediment where a high water table drives alteration of the labile component, and when uplift and erosion rates, topography and stream gradients are all sufficiently low to drive sedimentary recycling without significant input of low-grade basement detritus. The maturity of pre-marine examples (Taratu, Papakaio and Hogburn Formations) was enhanced by wave-reworking before final inundation during marine transgression. Colluvial placers in alluvial fans at the margins of Quaternary schist antiforms are repeatedly recycled into younger fan sediments during range growth. These ranges grow in width, as well as length and height, at the expense of the intervening basins, which become progressively narrower. The colluvial placers are ultimately reworked into a fluvial placer in an axial river between two ranges, which concentrates all of the colluvial gold into an incised channel once the widening ranges meet. Aeolian placers have formed from fluvial precursors in the semi-arid parts of the rain shadow east of the Southern Alps, particularly on the lower slope of ranges exposed to westerly winds. Silcrete and less common greywacke ventifacts are commonly associated with these placers.
Progressive changes in gold particle shape by flattening during transport in fluvial systems has been the most important process in the concentration of gold in placers. Flattening changes the hydrodynamic behaviour of gold particles by increasing their surface area to volume ratio, thereby making them easier to entrain and enabling transport to lower energy parts of the fluvial system. Gold particle flatness determines whether transport or concentration occurs and there is a predictable relationship between particle flatness and transport distance. This relationship explains the typical occurrence of placers immediately downstream of terminal moraines, the confluence with steeper tributary streams and the mouth of incised gorges. In each case, gold with sub-critical flatness is deposited from a higher energy system or reach into a lower energy system or reach, and must be flattened to a critical state before further transport can occur.
Chemical mobility of gold in groundwater occurs during uplift, commonly in association with sedimentary recycling. Secondary gold overgrowths are common in some placers and stitch or overgrow transport-induced features such as folds and abrasion marks. Chemical mobilisation and re-precipitation of gold is of minor importance, however, and results in volumetrically insignificant amounts of secondary gold. Increases in gold grain size upward through the section on both the local and the regional scale does not result from gold 'growth', but instead from preservation of progressively more proximal reaches of the host placers with decreasing age.
| Identifer | oai:union.ndltd.org:ADTP/217782 |
| Date | January 2007 |
| Creators | Youngson, John Hughan, n/a |
| Publisher | University of Otago. Department of Geology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright John Hughan Youngson |
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