Detrital gold morphology and recycling around the early Miocene St Bathans paleovalley, Central Otago, New Zealand

ABSTRACT

The St Bathans paleovalley developed as a major southwest-draining fluvial system during early Miocene rejuvenation of the Central Otago landscape. Rounded quartz pebbles and detrital gold were recycled from Eocene quartz pebble conglomerates into the headwaters of this Miocene paleodrainage system on the northern margin of the Otago Schist belt. Gold particle morphology is mostly irregular and rough, locally with relict gold crystal shapes, plus a minor component of well-defined flakes. Incremental rounding and surface deformation at the 10 µm scale has occurred through several recycling events in Central Otago, hindering linkage of Miocene and younger placer gold to ultimate basement sources. More intense deformation of the gold, forming thin flakes, occurred in the lower reaches of the Miocene paleodrainage system as gold was transported >100 km to eastern Southland. Hence, the St Bathans paleovalley has been an important source of gold for historic mining in Central Otago and Southland.

KEYWORDS:

• Paleodrainage
• fluvial
• quartz
• conglomerate
• gold
• placer
• flake
• Southland

Introduction

The Otago area of southern New Zealand has produced >8 million ounces (240 tonnes) of placer gold since the 1861 gold rush, with mining occurring almost continuously throughout that time (Williams 1974; Henley and Adams 1979; Craw 2013; Christie 2016; Henderson et al. 2016). Placer gold was concentrated in fluvial sedimentary units that range in age from Cretaceous to Holocene, with progressive recycling of detrital gold into younger units as older units were uplifted and eroded (Henley and Adams 1979; Youngson et al. 2006; Craw 2013). These processes resulted in creation and recycling of quartz pebble conglomerates (QPC) that host placer gold in many of the sedimentary units formed through time (Williams 1974; Youngson et al. 2006; Craw 2013).

In the Central Otago area (Figure 1B), the most important auriferous QPC-bearing sedimentary units are the Eocene Hogburn Formation and the Miocene Dunstan Formation (Figure 1C,D; Douglas 1986; Youngson and Craw 1996; Youngson et al. 1998). The palaeogeography of these units is difficult to reconstruct because of subsequent uplift and erosion that has dismembered these old fluvial systems. However, part of the Hogburn Formation sedimentary system involved transport of channelised auriferous QPC towards the northeast to North Otago (Figure 1B; Williams 1974; Youngson et al. 2006). In contrast, the Miocene paleodrainage of the Dunstan Formation was towards the southwest, discharging to Eastern Southland (Figure 1B; Stein et al. 2011; Craw 2013; Craw et al. 2015). The profound change in paleodrainage directions (Figure 1B) occurred during and after Oligocene partial or complete marine inundation of the Otago area (Figure 1D; Landis et al. 2008).

Figure 1. Location and stratigraphic setting for the Miocene quartz pebble conglomerates (QPC) of this study. A, Studied deposits are on Otago Schist basement. B, Regional hillshade digital elevation model (DEM; geographx.co.nz) of Central Otago, showing principal mining areas and contrasting paleodrainage directions in Eocene and Miocene. C, DEM of the upper Manuherikia basin, with locations of principal historic mines in Miocene QPC in the general St Bathans (Blue Lake) area. D, Sketch (not to scale) showing the general stratigraphic relationships of Cenozoic sediments in Central Otago, including the principal Eocene and Miocene QPCs mentioned in this study.

This study focuses on placer gold in QPC in the basal portion of the Dunstan Formation, and in particular the Early Miocene St Bathans paleovalley (Douglas 1986; Pole and Douglas 1998; Pole 2019). The St Bathans paleovalley QPC is well exposed in historic mining areas in the northeastern part of the Manuherikia basin, in the vicinity of the St Bathans historic town (Figure 1B,C; Figure 2A–C). In addition, recent gold exploration activities have provided new insights from still-buried portions of the paleovalley (Henderson et al. 2016). These sediments represent the headwaters of the Early Miocene Dunstan Formation paleodrainage. In this area, essentially all the QPC in the St Bathans paleovalley has been recycled from Eocene Hogburn Formation deposits, and possibly remnants of even older QPC, that were formerly present in these Miocene fluvial headwaters (Mildenhall 1989; Youngson and Craw 1996; Youngson et al. 2006; Craw 2013; Stewart et al. 2017). Hence, the Miocene St Bathans paleovalley was a major recipient of gold recycled from older sediments, and the eroded QPCs from the St Bathans paleovalley have been important sources for gold in younger sediments. We document the nature of Miocene placer gold and enclosing sediments for the following four specific situations: (a) as gold received from the Hogburn Formation; (b) as precursors to the gold in more distal and younger Miocene fluvial deposits farther southwest, including Eastern Southland; (c) for comparison to gold entering the Miocene fluvial system from tributaries farther downstream and (d) for comparison to gold that occurs in younger sediments in the area and was at least partly derived from the Miocene QPCs.

Figure 2. Geological setting for the St Bathans paleovalley auriferous QPC deposits in the St Bathans area, and mined localities farther downstream in the context of Miocene paleodrainage. A, Geological map of the Manuherikia basin (after Turnbull 2000; Forsyth 2001). B, Generalised structural cross section from Blue Lake to Grey Lake QPC mining areas (locations in Figure 1C). C, QPC-rich St Bathans paleovalley sediments at Grey Lake historic mine site.

General setting

The Eocene and Miocene paleodrainage systems were extensively disrupted by uplift of northeast-trending antiformal ranges of schist basement that now separate basins that contain remnants of these fluvial sediments (Figure 1B,C; Figure 2A; Jackson et al. 1996; Turnbull 2000; Forsyth 2001). These ranges and intervening synformal basins have risen during the Pleistocene and are still tectonically active (Jackson et al. 1996; Bennett et al. 2005; Craw et al. 2013). In addition, the northwest-trending Kakanui and Hawkdun Ranges have been rising since the Miocene, with most uplift during the Pliocene, resulting in separation of Central Otago from North Otago (Figure 1B; Youngson et al. 1998; Forsyth 2001; Craw 2013). Likewise, the Old Man Range has been rising since the Miocene and has separated Central Otago from Eastern Southland since the late Miocene or Pliocene (Turnbull 2000; Craw 2013; Craw et al. 2015).

The Miocene QPCs that are the topic of this study are part of the Dunstan Formation in the Manuherikia Group, and underlie the lacustrine siltstones and mudstones of the Bannockburn Formation (Figure 1D; Figure 2A; Douglas 1986; Pole and Douglas 1998; Pole 2019). These Miocene sedimentary rocks have been folded and faulted with the underlying basement as the range and basin topography evolved, and mostly stripped off the higher parts of the ranges (Figure 2A,B; Craw 2013). The remaining Miocene sediments are largely overlain by, and have been variably recycled into, Pliocene and Pleistocene fluvial deposits on basin floors and margins (Figure 1D; Figure 2A; Turnbull 2000; Forsyth 2001). However, extensive mining activity, especially around basin margins, has removed some of the overlying sediments, leaving exposures of the Miocene sequence (Figure 2C; Figure 3A–D). Eocene fluvial deposits and their gold placers have been affected by these same tectonic and erosional processes (Figure 1B; Forsyth 2001; Kerr et al. 2017; Stewart et al. 2017). Remnants of Eocene QPC deposits have been mined in the Maniototo basin, on a ridge crest at Mt Buster, and on the lower northeastern slopes of the Kakanui Range in North Otago (Figure 1B; Williams 1974; Youngson et al. 2006; Kerr et al. 2017; Stewart et al. 2017).

Figure 3. Outcrop features of basal Miocene QPCs and underlying basement. A, General view of historically sluiced basement unconformity zone beneath the St Bathans paleovalley at Blue Lake. The unconformity dips towards the lake (lower left, as in Figure 2B) and remnants of QPC remain on the clay-rich unconformity zone. B, Outcrop of friable clay-altered metagreywacke, as in a, with minor cross-cutting quartz veins (orientations indicated with dashed lines). C, Basal QPC with abundant rounded quartz pebbles recycled from Eocene Hogburn Formation. D, Outcrop of basal Miocene QPC at Garibaldi historic mine site (Figure 1B; recycled from Eocene QPC), with recycled overlying Pleistocene QPC.

The basement beneath the Miocene sediments in the St Bathans area lies within the faulted transition from prehnite-pumpellyite facies greywacke to greenschist facies Otago Schist at the northern margin of the schist belt (Figure 1A, Figure 2A, Figure 3A; Forsyth 2001; Turnbull et al. 2001; Henne et al. 2011). For the purposes of this study, we refer to this basement as sub-greenschist facies metagreywacke (Figure 2A,B). The rocks retain many greywacke features, including some bedding, but are deformed and variably foliated (Forsyth 2001; Henne et al. 2011). However, many such features are obscured near the Miocene unconformity by intense clay alteration that has been imposed by groundwater interaction beneath the sediments (Figure 3A,B; Chamberlain et al. 1999). Similar clay-alteration of basement has occurred beneath QPCs from Cretaceous to Miocene in the area (Chamberlain et al. 1999), and results in decomposition of much of the rock apart from resistant metamorphic quartz fragments (Figure 3B). Some of these quartz fragments are added to the next generation of sediments during uplift, erosion and recycling (Youngson et al. 2006). Most of the quartz for QPCs and associated sandstones and siltstones was derived from the underlying basement (Figure 1A), especially the greenschist facies rocks that host extensive quartz-rich metamorphic segregations (Turnbull et al. 2001; Youngson et al. 2006; Craw et al. 2015). The schist basement was also the source for the gold in these sediments, in the form of numerous post-metamorphic auriferous quartz veins (Mortensen et al. 2010).

Large Pliocene alluvial fans developed at the foot of the rising St Bathans and Hawkdun Ranges and spread southwards into the Manuherikia basin (Figure 1C,D). These fans covered the St Bathans area where they interfinger with, and overlie, the lacustrine Bannockburn Formation (Youngson et al. 1998). Some QPCs of the St Bathans paleovalley headwaters were recycled into the Pliocene Wedderburn Formation at the base of the alluvial fans at the early stages of uplift (Figure 4A–C; Youngson et al. 1998; Craw 2013). The Wedderburn Formation is locally gold-bearing, especially in the Naseby area of the Maniototo basin where it was mostly derived from Hogburn Formation QPCs, rather than Dunstan Formation (Youngson et al. 1998). Above the Wedderburn Formation, most of the Pliocene fans consist of greywacke-derived gravels derived from the basement in the rising mountains (Youngson et al. 1998). Greywacke-derived gravels are joined by schist-derived gravels in the Pleistocene over large areas of the Manuherikia basin (Figure 2A), but most Pleistocene gravels in the St Bathans area are still dominated by greywacke cobbles (Figure 4A).

Figure 4. Recycled St Bathans paleovalley QPCs in the Manuherikia basin. A, General view of the unconformity between Miocene Bannockburn Formation (bottom) and Pliocene Wedderburn Formation QPC and quartz sandstone (white, centre), at the base of a Pliocene fan, Manuherikia River (Figure 1C). Pleistocene terrace gravels consist of greywacke cobbles. B, Outcrop of Wedderburn Formation with rounded quartz pebbles in QPC. C, Basal QPC in Wedderburn Formation with clasts of oxidised mudstone. D, General view (3 km, looking south) of historic mine gold exposures of Pleistocene Tinkers Formation at Drybread (Figure 2A), with QPC beds (white) recycled from uplifted St Bathans paleovalley (Craw et al. 2013). These QPCs were further recycled into Late Pleistocene fans in distance. E, Outcrop of recycled rounded quartz pebbles in Tinkers Formation, with some angular quartz and schist debris.

The most prominent examples of QPCs recycled from St Bathans paleovalley in the Manuherikia basin occur at Drybread (Figure 2A; Craw et al. 2013). In this area, the Pleistocene Tinkers Formation contains abundant recycled rounded quartz and gold, and this unit has been uplifted and tilted, and in turn recycled into late Pleistocene alluvial fans that are dominated by schist debris (Figure 4D,E; Craw et al. 2013, 2016; Henderson et al. 2016). Smaller scale recycling of Miocene QPC into Pleistocene sediments has occurred at Garibaldi (Figure 3D) during uplift of Rough Ridge (Figure 1B; Bennett et al. 2005; Craw 2013; Stewart et al. 2017).

Methods

Several of the samples used for this study were collected as part of a gold exploration project in the Manuherikia basin (Henderson et al. 2016). The samples were collected from drillholes and processed mechanically to concentrate any contained gold. For assays, the gold concentrates were weighed with ∼0.1 mg precision and recalculated back to original sample volume to give results in mg/m³ (details in Henderson et al. 2016). Most exploration samples examined herein were from the Shepherds Flat area, downstream of the prominent Vinegar Hill historic mining area (Figure 1C). Drillholes penetrated through historic mine tailings to Miocene Dunstan Formation, including some basal QPCs, and ended when they contacted schist basement (Figure 5A–F). Some additional exploration samples were obtained from Pleistocene alluvial fans in the Drybread area (Figure 2A; Figure 4D,E) where Miocene QPCs have been extensively recycled. The gold concentration equipment was carefully cleaned between samples. However, one flake of platinum was found that was probably a contaminant derived from processing samples obtained near the south coast of South Island (e.g. Palmer and Craw 2023). Additional gold samples were obtained by traditional gold panning from QPCs in the wider St Bathans area.

Figure 5. Summary gold non-zero (gold present) assay data for exploration drillholes in the Vinegar Hill-Shepherds Flat area (Figure 1C; mapped in Henderson et al. 2016). Locations (NZMG) for holes in B–F are indicated. A, Combined assay data plotted against sample depth. B–F, Representative individual holes drilled through surficial tailings deposits to the underlying Dunstan Formation containing variable amounts of auriferous QPC, with holes stopping at basement.

Gold particles were mounted onto aluminium stubs using double-sided carbon tape for observations of particle morphology using a scanning electron microscope (SEM). The Zeiss Sigma variable-pressure SEM instrument at the Otago Micro and Nanoscale Imaging laboratory (OMNI; University of Otago, New Zealand) was used for all aspects of this study. Secondary-electron images (SE) and backscatter-electron images (BEI) were obtained using an accelerating voltage of 15 kV and an aperture of 120 μm without carbon coating.

Subsamples of particles were embedded in 25-mm epoxy resin discs that were then ground to expose sections through the particles. The gold was finely polished (1 µm diamond paste) and then etched with aqua regia to remove the smeared surficial gold polishing layer, and a carbon coat was added to the etched surfaces. Silver contents of gold particles were determined via energy dispersive spectrometry (EDX) using an Oxford Instruments X-Max 20 mm silicon drift detector. Spot analyses were carried out using an accelerating voltage of 15 kV, aperture of 120 μm and a working distance of 8.5 mm and had 1 SD measurement uncertainties of 0.3 wt% for Ag and 0.35 wt% for Au. Internal grain structures of etched gold particles were determined by electron backscatter diffraction (EBSD) maps constructed from crystallographic data collected with an Oxford Instruments Nordlys F EBSD camera, with methods described in detail by Palmer and Craw (2023). The imaged EBSD patterns were automatically indexed by Oxford Instruments Aztec version 4.1 SP1software. The resulting EBSD data were exported to HKL Channel 5 where noise reduction was carried out according to the method detailed in Palmer and Craw (2023). EBSD map pixels were digitally coloured using the HKL Channel 5 software and are presented as inverse pole figure (IPF-Y) maps. Grain boundaries were added where misorientation angles were >10° and subgrain boundaries added where misorientation angles were >2°.

Results

Miocene gold-bearing QPCs in St Bathans area

The metagreywacke basement at Blue Lake and Grey Lake (Figure 1C; Figure 3A,B) has only minor quartz veining and has no known Au-bearing veins to contribute to auriferous QPC. Basement beneath St Bathans paleovalley sediments in the Shepherds Flat area is sub-greenschist facies schist (Figure 2A), with more abundant metamorphic quartz veins but no known auriferous veins. Likewise, nearby channelised QPCs at Fiddlers Flat and Pennyweight Hill were incised into veined sub-greenschist facies schist with no known auriferous veins (Figure 1C; Figure 2A).

Quartz pebbles in the St Bathans paleovalley are predominantly well-rounded, with only minor angular components (Figure 3C). This dominant well-rounded quartz is the clast component that was recycled from the Eocene Hogburn Formation, with only minor local additions of more angular locally-derived quartz (Figure 3B,C). Eocene pollen has also been recycled into the Miocene paleovalley (Mildenhall 1989). QPCs at all localities in the St Bathans area have been locally cemented with authigenic quartz by the same groundwater processes responsible for the clay alteration of the underlying basement (Chamberlain et al. 1999). Cementation followed specific beds in the QPCs at the 0.5–1 m scale to form silcrete layers that are especially common in the Fiddlers Flat area (Henne et al. 2011). Erosion of these silcrete layers produces prominent silcrete boulders in younger surficial sediments.

Historic mining focused on QPCs immediately above the basement unconformity at all localities in the St Bathans area (Figure 2B; Figure 3A). However, exploration drilling in the Shepherds Flat area shows that placer gold also occurs at various horizons up to 20 m above the basement (Figure 5A–F). Typical assay values for these horizons (at metre scale) range from 100 to 1000 mg/m³, although these assays can be highly variable down drillholes (Figure 5A–F).

Miocene St Bathans gold particle external morphology

Gold from the St Bathans paleovalley has a wide range of shapes, from rough irregular particles to well-flattened flakes (Figure 6A–I). Rare particles with delicate protrusions show little sign of sedimentary deformation, and one such particle is intergrown with quartz (Figure 6B). However, most irregular particles have undergone some deformation and rounding on their outer margins (Figure 6A,C–F). Relict primary gold crystal shapes and some crystal structures are locally preserved within less-deformed parts of irregular particles (Figure 6C–E). Many of the irregular gold particles have small (1–20 µm scale) rounded quartz particles embedded in surficial cavities (Figure 6C–F).

Figure 6. Microscopic views of gold particle morphology in samples from Vinegar Hill-Shepherds Flat drillholes. A,B, are light microscope views; rest are SEM backscatter electron images. A, Typical combination of flakes and irregular particles. B, A rare complex particle with delicate protrusions grown over quartz. C, Rough and irregular particle with embedded fine quartz particles (white electron-charging). D, Rough particle with remnants of primary gold crystals. E, Close view of D, showing relict crystal outlines. F, Dumbell-shaped particle with heavily modified ends. G–I, Examples of increasing fold development on progressively thinner flakes. J, Two generations of ductile smearing of surficial gold on the surface of folded flake in I. K, Oblique impact crater in the surface of the flake in I.

The largest gold particles are flakes that have irregular flattened surfaces up to 1 mm across, and these flakes range in thickness from ∼100 to <20 µm (Figure 6A,G–I). Thick flakes have only incipient rounding and folding of edges (Figure 6G), but thinner flakes are commonly folded as well as rounded (Figure 6H,I). Flake surfaces all retain some irregular cavities and rounded protrusions on the 10–50 µm scales (Figure 6G–J). However, flake surfaces also show abundant evidence for sedimentary deformation, such as smearing and impact gouging, at the 10 µm scale (Figure 6I–K). In particular, repeated ductile smearing of portions of particle surfaces has been responsible for at least some of the flattening of flake surfaces (Figure 6J).

Micron-scale vermiform overgrowths of authigenic gold are widespread on Eocene gold particle surfaces, especially in close association with clay coatings (Figure 7A; Stewart et al. 2017). Similar authigenic gold occurs in association with clay coatings on gold particles panned from basal Miocene QPCs at Pennyweight Hill (Figure 7B) and in St Bathans paleovalley QPC at Blue Lake (Figure 7C). However, gold extracted for the exploration project at Shepherds Flat has little remaining clay coating, as this was removed during the mechanical processing to ensure accurate assaying. Hence, no well-defined authigenic clay-gold intergrowths were observed on these particles. Nevertheless, particles from Shepherds Flat have abundant remnants of authigenic gold adhering to particle surfaces without associated clay (Figure 7D–G). Most of this authigenic gold has been deformed, so that the vermiform shapes are now flattened (Figure 7E,F), and this gold is presumably inherited from the Eocene precursor (e.g. Figure 7A). Some undeformed authigenic gold occurs in surficial cavities on Shepherds Flat particles (Figure 7G). It is not clear whether this undeformed surficial gold was inherited from Eocene authigenesis and has been protected from recycling-related deformation, or this gold is a result of post-Miocene authigenesis as is inferred for the panned Miocene samples (Figure 7B,C).

Figure 7. Backscatter electron images of authigenic gold overgrowths (yellow Au labels) on detrital gold particles. A, Typical view of overgrowths on a particle in Eocene QPC. Black background includes clay. B, Semi-transparent clay coating on a gold particle is impregnated with sub-µm authigenic gold in Miocene basal QPC at Pennyweight Hill (Figure 1C). Detrital gold from Miocene QPC at Blue Lake (Figure 3A,B), showing relict primary crystalline structure (left), and minor µm authigenic gold in clay patch (right). D, Flake from St Bathans paleovalley QPC with remnants of deformed authigenic gold overgrowths. E, Close view of deformed overgrowths in D. F, Authigenic overgrowth gold is preserved in an embayment of a gold flake from St Bathans paleovalley QPC. F, Close view of relict authigenic overgrowths in a cavity within a rough particle from St Bathans paleovalley QPC.

Garibaldi Miocene QPC

The paleodrainage links are unknown between the Garibaldi QPC channel (Figure 3D) and the St Bathans paleovalley because of subsequent uplift and erosion, but both channel systems are presumed to have been linked in the Miocene southwestward drainage (Figure 1B). The Garibaldi QPCs are unconformably incised into clay-altered greenschist facies schist basement. Quartz pebbles are predominantly rounded (Figure 3D), reflecting their derivation from nearby Eocene QPCs (Figure 1B). However, the Garibaldi QPCs also contain numerous angular clasts, especially in beds immediately above the basement. The QPCs have been locally cemented to form silcrete layers, similar to those in the St Bathans area.

Gold in the Miocene QPC at Garibaldi (Figure 8A–F) has a similar range in morphology to that described above for the St Bathans area. In particular, many of the particles are irregular in shape with many protrusions and cavities but have variably rounded external surfaces (Figure 8A–C). Well-developed thin flakes form a minor component (Figure 8D). Micron-scale vermiform authigenic gold is common on many particle surfaces, especially within cavities with clay (Figure 8E). Flattened patches of vermiform authigenic gold occur on some deformed particle surfaces (Figure 8F).

Figure 8. Backscatter electron images of Garibaldi gold morphology. Z = zircon. A–F, Gold from basal Miocene QPC, recycled from Eocene Hogburn Formation. G–I, Gold from Pleistocene gravels, recycled from Miocene QPC.

Internal structure of Miocene gold

Relative crystallographic orientations of internal grains in the gold particles are indicated with contrasting colours in the electron backscatter diffraction (EBSD) images in Figure 9A–F, and these images quantify contrasting grain sizes. Most gold particles have a relatively coarse grained (>50 µm grains) core surrounded by a rim that is typically 10–50 µm wide and consists of finer grained (5–20 µm) gold (Figure 9A–F). The large core grains are typically internally deformed, with abundant crystallographic distortion and development of subgrains on the 10 µm scale (Figure 9B–F). The core grains contain silver (Ag), typically 5–10 wt%, whereas the fine rim grains have Ag <1 wt% (Figure 9A–F). Vermiform gold coatings on the exterior surfaces have <1 wt% Ag as well. Highly flattened flakes are dominated by fine grained gold throughout, and that gold has <1 wt% Ag (Figure 9G,H). There are some remnants of coarser grains in such flakes, but they are highly deformed with extensive subgrains (Figure 9G,H).

Figure 9. SEM views of the interior structure of Miocene Shepherds Flat QPC gold (A–H) and Garibaldi Pleistocene QPC gold recycled from Miocene (I). A, Backscatter electron image of an irregular particle, showing Ag-bearing core and low Ag rim, highlighted by aqua regia etching. B, Electron backscatter diffraction (EBSD) inverse-pole (IP) image of particle in a, showing crystallographic and grain size variations. C,D, Enlarged areas in B, showing detail of subgrain structure in coarse grained core, and fine grained rims (latter is poorly indexed). E, Irregular particle image, as in A. F, EBSD IP view of part of coarse grained core in E. G, EBSD IP view of a complexly folded flake (as in inset BEI). H, Enlarged portion of G. I, EBSD image (Euler colours; details in Stewart et al. 2017) of a rounded grain and same view as analytical map to show contrast between Ag-bearing core and Ag-poor rim.

Recycled Miocene gold

Erosion of Miocene QPC at Garibaldi during Pleistocene uplift of Rough Ridge (Figure 1B) has resulted in formation of new young QPC lying on top of the Miocene deposits and immediately downslope of the Miocene deposits (Figure 3D). These Pleistocene QPCs consist of rounded quartz pebbles recycled from the Miocene QPC, with an additional component of angular quartz pebbles from the clay-altered schist basement, and a minor component of schist debris. In addition, basal portions of the Pleistocene QPC contain boulders of silcrete eroded from nearby Miocene QPC (Figure 3D). Gold was recycled from Miocene to Pleistocene QPCs as well, and the Pleistocene gold is essentially indistinguishable from that in the underlying Miocene QPC (Figure 8A–I). The generally irregular but surficially rounded gold population is morphologically similar in both Miocene and Pleistocene QPCs (Figure 8A–C,G,H). Similarly, vermiform authigenic gold in cavities in the Pleistocene gold (Figure 8I) is essentially the same as that seen in Miocene gold (Figure 8E) and also in nearby Eocene gold (Figure 7A). Similarly, the internal structure of gold particles from the Pleistocene QPC at Garibaldi shows the same coarse-grained Ag-bearing cores and finer grained low-Ag rims (Figure 9I).

The spatial scales of recycling of Miocene QPC into Pleistocene sediments at Drybread are much larger than at Garibaldi (km scale rather than m scale; Figure 4D). The recycled gold at Drybread is generally more rounded than gold in the St Bathans area Miocene QPC, but many particles still retain the irregular morphology with numerous protrusions and cavities (Figure 10A–I). Some of the more angular particles retain hints of original straight gold crystal outlines (Figure 10B,C,E). However, there is also abundant evidence of surface abrasion to form smoother exterior surfaces (Figure 10A,D). Cavities commonly contain µm-scale vermiform and dusty authigenic gold, and much of this authigenic gold is intimately intergrown with clay coatings (Figure 10F–I).

Figure 10. Gold from Pleistocene Drybread sediments, recycled from St Bathans paleovalley QPC (Figure 4). A, Typical flake. B–F, Typical rounded irregular particles. G, Close view of authigenic gold and clay in (F). H, Typical rough irregular particle. I, Close view of particle in H, with smeared surface (left) and cavity with clay and authigenic gold.

Discussion

Morphological evolution of Miocene gold

Previous studies (Kerr et al. 2017; Stewart et al. 2017) have shown that gold in Eocene QPCs in the Maniototo basin area has a wide range of shapes, including gold that is rough, irregular and locally crystalline (Figure 11A). This gold is near to probable sources in the underlying schist basement (∼5 km; Kerr et al. 2017), and has undergone only minor transport-related deformation. The proximal Eocene gold is accompanied by a minor component of well-developed flakes that have been extensively deformed during transport, and these have presumably been recycled from older QPCs (Youngson and Craw 1996; Youngson et al. 2006). It is this gold, and associated rounded quartz pebbles that was recycled into the St Bathans paleovalley in the early Miocene (Figure 11A; Youngson et al. 2006). Addition of some angular quartz from clay-altered basement beneath the Eocene sediments (Figure 11A; Chamberlain et al. 1999) also occurred (Figure 11B).

Figure 11. Summary diagram that shows variations in gold textures and associated QPC from Eocene QPC (top), through recycling into the St Bathans paleovalley, and addition of more primitive material from a downstream Miocene tributary. Recycling to Pleistocene Drybread gravels occurred progressively during uplift of the St Bathans paleovalley. Ultimate paleodrainage discharge was to Eastern Southland (bottom).

Despite the history of recycling of gold in the Miocene St Bathans paleovalley and nearby Garibaldi QPC, most of the gold remains irregular in shape with abundant rough surfaces, or even relict crystal shapes (Figure 6A–H; Figure 8A–C; Figure 11B). Well-defined flakes form only a minor component of the gold population, and even the most flaky particles retain some irregularities and rough surfaces (Figure 6A,I; Figure 8D; Figure 11B). There has been some obvious rounding of protrusions on particles and hammering of exterior surfaces of irregular particles at the 10 µm scale (Figure 6A–F; Figure 8A–C), so that sharp crystal outlines that are common in Eocene gold (Figure 11A) have become rounded except in protected cavities in the Miocene gold (Figure 6D,E). In detail, many Miocene particle surfaces have ductile smears and gouges (Figure 6J,K), but similar features were common on Eocene gold as well (Kerr et al. 2017), so that the extent of additional deformation at this scale is not resolvable. Vermiform authigenic gold that was added to the surfaces of Eocene gold (e.g. Figure 7A) has been superficially deformed and flattened on Miocene gold, but is still recognisable at the µm scale (Figure 7D–F; Figure 8F).

Farther downstream (southwestwards) in the Miocene paleodrainage, gold-bearing quartz veins were exposed at the Miocene erosional unconformity on schist basement (Figure 2A; Craw et al. 2013, 2016; Stephens et al. 2015). In particular, exposed vein systems in the Raggedy Range (Craw and Lilly 2016) and Conroys area near Alexandra (Stephens et al. 2015; Craw et al. 2017) were shedding proximal gold into Miocene streams (Figure 11C). The gold from these sources was rough, irregular and locally crystalline, and resembled the proximal gold that contributed to the Eocene QPCs (Figure 11A,C; Stephens et al. 2015; Craw et al. 2017). This proximal gold was accompanied by abundant angular quartz fragments from the underlying clay-altered greenschist facies schist, and locally some angular gold-bearing vein quartz (Figure 11C). Hence, the Miocene QPC-bearing paleodrainage in this area received a combination of recycled gold and new proximal gold, although most of the gold was rough and irregular in shape (Figure 11B,C).

Recycling into younger sediments

Local recycling of Miocene gold, such as at Drybread and Garibaldi, has caused additional rounding and associated surficial deformation, but most of the gold remained rough and irregular (Figure 8G,H; Figure 10A–H; Figure 11D). The proportion of flakes (e.g. Figure 10A) appears to increase in the younger sediments, but most of these flakes are thick and still have rough surfaces. Many particles of gold from all stages in Central Otago recycling, from Eocene to Pleistocene, have undeformed µm-scale vermiform authigenic gold in cavities (Figure 7A–C,G; Figure 8E,I; Figure 10F–I), and it is not possible to infer the timing of emplacement of this gold. At least some of this authigenic gold appears to survive local recycling, as indicated by the presence of deformed equivalents on some surfaces (Figure 7E,F; Figure 8F; Figure 10G).

The most profound transformations of gold morphology have occurred during longer-distance transport and subsequent recycling into the Eastern Southland area (Figure 1B; Figure 11E). Gold-bearing QPCs in Eastern Southland are predominantly Pliocene, and have been recycled from the Central Otago Miocene paleodrainage system that included the St Bathans paleovalley (Figure 1B; Craw et al. 2015). These QPCs and their gold were further recycled into younger, more lithic, fluvial sediments (McLachlan et al. 2018). After ∼100 km of travel from the Central Otago source area, Southland gold consists almost entirely of thin flakes that are extensively folded and flattened (Figure 11E; Craw et al. 2015). Some of the flakes are so thin they have abrasion-driven perforations through them (McLachlan et al. 2018).

Liberated gold particles

Delicate gold particles with highly irregular shapes are a minor but distinctive component of many gold samples collected in Central Otago and Southland (Craw et al. 2015, 2016; McLachlan et al. 2018), including this study (e.g. Figure 6B; Figure 11B,E). These particles have complex branching morphology and their surfaces show little sign of abrasion and rounding compared to the majority of accompanying gold (Figure 6B versus C–K). Some such gold particles have intergrown angular quartz fragments (Figure 6B) that are different from the rounded quartz particles commonly embedded in other more-deformed gold particles (Figure 6C–F; Figure 7F). These particles resemble gold that has been shed directly from hydrothermal zones in the schist basement in Central Otago, especially gold that has undergone some supergene modification (Stephens et al. 2015; Craw et al. 2017).

Gold-mining folklore commonly interprets these particles as indicators of nearby undiscovered basement gold sources. However, these particles occur in some places where basement is apparently unmineralised, such as the metagreywacke in the St Bathans area in this study, and the greywacke terranes of Eastern Southland (Figure 11B,E; Craw et al. 2015). An alternative explanation is that these particles were liberated, by breakage, from gold-bearing quartz pebbles during sedimentary transport and recycling of QPCs, in a similar manner to the initial liberation of gold near sources (Youngson and Craw 1999; Craw et al. 2017). Gold-bearing quartz pebbles do occur in QPCs (Figure 11C,E), although they are rare compared to the large volumes of metamorphic quartz pebbles (Craw et al. 2015). Hence, we suggest that this downstream liberation of gold from quartz pebbles in QPCs is a more plausible explanation for the source of anomalous delicate gold particles (Figure 11C,E).

Conclusions

The southwestward-draining Miocene St Bathans paleochannel in Central Otago received quartz pebbles and gold particles via recycling from Eocene quartz pebble conglomerates. These Eocene QPCs were originally transported towards the northeast from gold-bearing greenschist facies basement to barren metagreywacke basement. Gold and quartz pebbles were recycled in turn from Miocene sediments into younger deposits during Pliocene and Pleistocene tectonic evolution of Central Otago and Eastern Southland. Despite these various stages of recycling in Central Otago, initially irregular gold particle morphology merely became slightly more rounded with increasing surficial deformation at the 10 µm scale, while some new irregular gold was added from basement sources downstream (Figure 11A–D). Minor µm-scale authigenic gold, some deformed during recycling, occurs on gold particles in QPCs of all ages.

The total distances of travel of gold particles at the northern end of Central Otago during formation of Eocene QPCs into Miocene QPCs, particularly the St Bathans paleochannel, is not known. However, this transport is probably on a scale of tens of km cumulatively, during several stages of transport and recycling. This included transport from gold-bearing greenschist facies basement to low grade metagreywacke basement on the northern edge of the schist belt. Our observations above suggest that because these relatively short distances of travel have only minor incremental effects on gold morphology, linking particle shape to travel distance is difficult in this context. The recurrent recycling history further complicates any attempts at linking gold particle shapes and potential sources. However, on the scale of >100 km, the production of thin folded flakes during long-distance transport in river systems is an inevitable end morphology, as demonstrated by Youngson and Craw (1999) over ∼250 km in the modern Clutha River system (Figure 1B). This conclusion is reiterated in the present study with the strong contrast between Central Otago and Eastern Southland Cenozoic gold morphologies (Figure 11A–E). In contrast, rare anomalous delicate irregular gold particles occur within most of the gold-bearing sediments of Central Otago and Eastern Southland, and these particles were liberated from gold-bearing quartz pebbles that were broken near to the site of deposition and are not indicative of nearby basement gold sources.

Acknowledgements

This study would not have been possible without the enthusiasm and generosity of Simon Henderson, who provided site information and key gold samples. Stimulating discussions with Donna Falconer and John Youngson over many years were fruitful in developing ideas on gold mobility and recycling. Dave Prior gave useful guidance on EBSD observations. James Stewart provided useful information on the Garibaldi deposit. Gemma Kerr greatly assisted with SEM observations on gold at Otago Micro and Nanoscale Imaging (OMNI) at University of Otago. Helpful comments from Kari Bassett and an anonymous reviewer improved the presentation of the ms.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

All data relevant to this study are included within this paper or within cited references.

Additional information

Funding

This research was funded by University of Otago.