Contrasting evolution of beach gold on two sides of an active orogen, Southern Alps, New Zealand

Figures & data

Figure 1. Location hillshade DEM maps (geographx.co.nz) for aspects of this study, with transport directions for detrital gold. A, Topography of the Southern Alps in South Island of New Zealand. B, Oblique view from northwest of a portion of the Southern Alps goldfield, showing the topographic setting of beach placers. C, Map view of the northern portion of Southern Alps goldfield with the location of principal samples examined in this study in Late Pleistocene-Holocene beach placers near Lake Mahinapua. D, Map view of southern South Island showing topography on the southeast side of Southern Alps in the Otago and Southland goldfields and Cenozoic transport of gold to south coast beach placers.

Figure 2. Typical gold compositions in Otago and Southland goldfields compared to Southern Alps. A, Plot of Ag versus Hg for orogenic gold in Otago (data replotted from MacKenzie and Craw 2005), with range for Southern Alps (red dashed box; Hg <1 wt%). B, SEM backscatter image of Southland Pleistocene fluvial gold (etched with aqua regia) overlain with Au map (left) and combined Au and Ag maps (right), showing red Ag-rich etching residues. Modified from Craw et al. (2017). C, Silver content of bulk gold bars produced at Waikaia mine, Southland goldfield (Figure 1D), in a Pleistocene paleochannel. Data replotted from Craw et al. (2017). Ag content decreased over 2 km down the paleochannel. D, Range of Ag and Hg in placer gold from Nokomai mining area in Southland placer goldfield (Figure 1D). Data summarised from Youngson et al. (2002). E, Ag contents of beach gold particle cores on the south coast of Southland placer goldfield (Round Hill mining area; Figure 1D). Data replotted from Mitchell (1996).

Figure 3. Location and geological setting for Southern Alps Late Pleistocene-Holocene beach gold sample sites near Lake Mahinapua (Figure 1C). A, Map of landscape features around the sample sites, with moraine distribution from Nathan et al. (2002). B, Modern storm-eroded cliff section (site shown in a) through beach sediments including several heavy mineral layers. C, Total magnetic intensity (TMI) survey map (relative scale) of inland beach sands in the area for which samples were taken for this study. Heavy mineral concentrates with magnetite (and gold) are indicated in red-orange colours.

Figure 4. Geological features of the active components of Southern Alps goldfield. A, An example of source area (Callery valley; Figure 1B) for erosion of Plio-Pleistocene orogenic gold from uplifted schist basement. B, Example of eroded orogenic gold in a quartz vein cobble. C, Coarse fluvial gold from a small mine located at the break in slope immediately upstream of Alpine Fault (Whataroa River; Figure 1A). Some particles retain remnants of host vein quartz (Qtz). D, Active beach placer concentrations of gold-bearing heavy mineral sands (dark streaks). E, Section through modern beach sand, showing several subsurface heavy mineral layers.

Figure 5. Late Pleistocene-Holocene beach sediments were excavated ∼1 km inland from the modern shoreline for extraction of gold samples. A, Excavation wall in beach sands near Figure 6 sample site, showing truncated heavy mineral layers. B, Close view of an excavation wall in beach sands (as in a) showing heavy mineral layers. C, An excavation site in beach sands at foot of moraine deposit (Figure 3A,C). D, Close view of basal heavy mineral layers in c, from which gold in Figure 7 was extracted.

Figure 6. SEM backscatter images of beach gold extracted from exploration bulk sample near Lake Mahinapua (Figure 1C; Figure 3A,C; Figure 5A,B). A, General view of gold shapes. B–E, Particles with remnants of orogenic crystalline gold forms. F–I, Irregular particles with variably deformed rough surfaces. J–M, Flakes with smoothed and deformed surfaces. The particle in l has embedded fine silicate clasts. N, Close view of a cavity in m, with a cluster of vermiform gold.

Figure 7. SEM backscatter images of beach gold panned from a heavy mineral concentrate near moraines at Lake Mahinapua (Figure 1C; Figure 3A,C; Figure 5C,D). A, General view of gold shapes. B, Rounded particle with remnants of crystalline forms. C,D, Smoothed flakes. E–H,. Close views of flake surfaces with vermiform gold in depressions. I, J, Smooth flake with embedded silicate clasts (S), partially covered with smeared surface gold.

Figure 8. SEM views of the internal structure of an irregular Southern Alps beach gold particle. A, Backscatter electron image of surface etched with aqua regia but showing no contrasts in internal structure. B, Part of an image in B, overlain with Au and Ag element maps to show dominant Ag-bearing gold (green) and Ag-depleted gold on the upper margin (increasing red colouring). A portion of a coating of Ag-free vermicular gold occurs on the bottom side (red). C, EBSD IP view of the least-deformed, coarse-grained portion of the particle. D, EBSD crystallographic misorientation map of part of a large grain on particle margin in C. E, EBSD crystallographic misorientation map of moderately deformed part of a particle. F, EBSD IP view of the most-deformed and extended part of the particle. G, EBSD crystallographic misorientation map of part of the deformed extension in f.

Figure 9. SEM views of the internal structure of a complexly folded Southern Alps beach gold flake. Legends are the same as in Figure 8. A, Backscatter electron image of surface etched with aqua regia but showing no contrasts in internal structure. B, EBSD IP view across the particle. C, Close view of ribbon textured gold, as indicated in B. D, EBSD crystallographic misorientation map of part of the ribbon textured gold, as indicated in b. E,F, Parts of an image in A, overlain with Au and Ag element maps to show dominant Ag-bearing gold (green) and Ag-depleted marginal gold (increasing red colouring). A portion of a coating of Ag-free vermicular gold occurs on the top side of e (red).

Figure 10. Evolution of fluvial placer gold particles in Otago and Southland goldfields. A, SEM backscatter image of an etched polished section through the edge of a particle of Otago Cretaceous fluvial gold, showing Ag variations. B, SEM EDS element map for Ag on a flat portion of the exterior surface of a Cretaceous particle. C, SEM backscatter image of the surface of Otago Eocene fluvial gold particle, with vermiform authigenic gold (left) and smeared gold (right). D, EBSD Euler colour map (modified from Kerr et al. 2017) of fine grains of the smeared gold surface, as in d. E, EBSD Euler colour map of a particle of Otago Miocene fluvial gold, with a yellow dashed boundary between Ag-bearing coarse-grained core and low-Ag fine-grained rim. F, Incident light view of etched gold from Pleistocene deposit (Waikaia mine; Figure 1D) in Southland goldfield. Large particle has a deeply leached Ag-bearing core and prominent rim of low-Ag gold. G, EBSD IP view of grain structure in small particle in F, showing contrasts between core and rim. H, Incident light view of thin Ag-poor Southland flake with a small remnant of Ag-bearing core. I, Backscatter electron image of thin folded Southland flake with very small remnant Ag-bearing core.

Figure 11. Microscopic views of the internal structure of Otago and Southland fluvial gold (etched with aqua regia), showing details of Ag leaching to leave Au-rich portions. A, Backscatter electron image of core grains of a particle, overlain with Au and Ag element maps to highlight Ag-poor (red; high relief) grains that have formed by Ag-leaching along coarse grain boundaries. B, Backscatter electron image of a particle in a, showing larger scale zones of Ag leaching (paler, Ag-poor zones) along grain boundaries. C, EBSD IP view across a gold particle from Southland goldfield showing contrasts in grain sizes and Ag contents. D, Incident light view of a particle in C. E,F, Close views of areas of Ag leaching and grain size reduction, as indicated in C. G, Incident light view of a Southland particle with a prominent Ag-poor rim (darker yellow) and extensive irregular and scattered internal Ag-leached patches and fine grains.

Figure 12. Beach gold on the south coast of the Southland goldfield, in the Round Hill mining area (Figure 1D). A,B, Gold-bearing heavy mineral concentrates (dark) on an active beach. C,D, SEM backscatter images of detrital gold on an active beach at the mouth of a gold-bearing river. E, Recycled toroidal beach gold in a Pleistocene marginal marine fluvial channel. F, Polished section view of toroids (yellow) with relict Ag-rich cores arrowed. White flakes are platinum. G, Backscatter electron image (modified from Palmer and Craw 2023) of a spheroidal toroid with relict Ag-bearing core (dark green, with EDX analytical spots). The image is overlain with Au and Ag element maps to highlight Ag-poor (lighter green) rim and attenuated rim protrusions. H, EBSD crystallographic misorientation map of a toroid, showing variations in grain size (white grain boundaries) and amounts of internal crystallographic misorientations. I, Enlarged view of toroidal rim protrusion in h, showing fine grain size and misorientations.

Figure 13. Geochemical models for solubility of Ag and Au. A, Geochemical model (from Geochemists Workbench®) of Ag solubility in chloride solutions, with typical environmental water compositions (green boxes). The main diagram is for moderately oxidised waters; the effects of lower redox conditions are shown at the lower left. B, Backscatter electron image of authigenic silver (Ag) on clay at Round Hill placer mine, south coast (Figure 1D). C, Solubility of gold in near-surface environments (modified from Craw and Lilly 2016).

Figure 14. A, Differential chloride leaching of Au and Ag by aqua regia (e.g. Figure 2B). B, Young Southern Alps beach gold has resisted Ag leaching and is frequently buried out of the wind by surf and sand. C, Otago-Southland fluvial gold has progressive Ag leaching along grain boundaries (e.g. Figure 10). D, Southland beach toroid with enhanced Ag leaching during wind-driven deformation.

MacKenzie DJ, Craw D. 2005. The mercury and silver contents of gold in quartz vein deposits, Otago Schist, New Zealand. New Zealand Journal of Geology and Geophysics. 48:265–278. doi:10.1080/00288306.2005.9515114.

 Web of Science ®Google Scholar

Craw D, McLachlan C, Negrini M, Becker N. 2017. Quantification and prediction of bulk gold fineness at placer gold mines: a New Zealand example. Minerals. 7:226. doi:10.3390/min7110226.

 Web of Science ®Google Scholar

Youngson JH, Wopereis P, Kerr LC, Craw D. 2002. Au-Ag-Hg and Au-Ag alloys in Nokomai and Nevis valley placers, northern Southland and Central Otago, New Zealand, and their implications for placer-source relationships. New Zealand Journal of Geology and Geophysics. 45:53–69. doi:10.1080/00288306.2002.9514959

 Web of Science ®Google Scholar

Mitchell M. 1996. Alluvial platinum group minerals from southern New Zealand. Unpublished PhD thesis, University of Otago, Dunedin, New Zealand. 277 pp.

 Google Scholar

Nathan S, Rattenbury MS, Suggate RP. 2002. Geology of the Greymouth area: scale 1:250 000 geological map 12. 1 sheet + 58 pp. Lower Hutt, New Zealand: Institute of Geological and Nuclear Sciences.

 Google Scholar

Kerr G, Falconer D, Reith F, Craw D. 2017. Transport-related mylonitic ductile deformation and shape change of alluvial gold, southern New Zealand. Sedimentary Geology. 361:52–63. doi:10.1016/j.sedgeo.2017.09.019.

 Web of Science ®Google Scholar

Palmer M, Craw D. 2023. Morphology and structural evolution of fine beach gold in comparison to detrital platinum, southern New Zealand. Mineralium Deposita. 59:69–83. doi:10.1007/s00126-023-01200-z.

 Web of Science ®Google Scholar

Craw D, Lilly K. 2016. Gold nugget morphology and geochemical environments of nugget formation, southern New Zealand. Ore Geology Reviews. 79:301–315. doi:10.1016/j.oregeorev.2016.06.001.

 Web of Science ®Google Scholar

Data availability statement

All data are included in this paper and cited references.