Shallow oxide gold formation: The role of redox reactions and groundwater

by Dr Jim Yaxley, Owner of Grasstree Resources

The beauty, rarity and unique properties of gold make it one of the most sought after metals. One of the significant processes contributing to the genesis of gold nugget formation is the interplay of redox (reduction-oxidation) reactions driven by the ebb and flow of groundwaters in ancient surface systems. These reactions shape the mineralogy and availability of gold in unique geological settings.

Groundwater occurrence can be simplified by grouping into three aquifer categories: surficial, sedimentary, and fractured rock aquifers. Locally, groundwater provides most domestic and stock water requirements from small supplies of fresh to brackish groundwater sourced from colluvium, valley-fill alluvium, and laterite aquifers. Away from the drainage lines, low-yielding supplies can be sourced from colluvial hillslope wash, weathered bedrock and from fractures and shear zones within the bedrock. Bore yields from fractured rock aquifers are variable and water quality is mostly related to bedrock type, with fluctuations of water levels driving redox reactions below the surface often related to dissolved oxygen and the movement of ions as can be seen in Figure 1.

Figure 1 - An example of a seasonal fracture aquifer bringing copper (malachite and azurite) to precipitate on the surface (oxide zone) — **Figure 1** – An example of a seasonal fracture aquifer bringing copper (malachite and azurite) to precipitate on the surface (oxide zone)

A geological explanation

Australia is characterized by a rich geological history that includes the deposition of sediments in ancient palaeodrainage systems and lakes. These lakes provided an ideal environment for various geochemical processes to occur, particularly during periods of fluctuating water levels that drive redox reactions. The interaction between organic matter, minerals, and changing environmental conditions facilitated the growth of gold nuggets.

Figure 2 - Oxidized sulfide vein — **Figure 2** – Oxidized sulfide vein

Large gold nuggets which have formed within a supergene alteration zone associated with hydrothermal mineralized deposits. These nuggets are intergrown with quartz and iron oxyhydroxide that have replaced sulfide minerals, and they can also be found along fractures in both quartz and the surrounding host rocks. The supergene in this case oxidation processes driven by groundwater interactions with rock (Figure 2), occur with the oscillation of reduction and oxidation driven by groundwaters. This process has been described in Craw et al. (2016) as involving clay alteration and oxidation extending up to tens of meters below the surface.

In sulfide deposits, the oxidation of pyrite and arsenopyrite produced temporary thiosulfate ligands that mobilized microparticulate gold previously encapsulated in sulfide minerals (Machesky et al., 1991).

Figure 3 - 59 oz (1.67 kg) nugget with manganese oxide coating — **Figure 3** – 59 oz (1.67 kg) nugget with manganese oxide coating

Nugget morphology can vary markedly, ranging from large massive pieces (Figure 3) to those that display some crystalline structure, composed of anhedral grains, elongated gold plates, and intricate intergrowths of gold and iron oxyhydroxide. Their surfaces feature additional micron-scale overgrowths of microparticulate gold, gold plates, and gold crystals.

Redox reactions explained

Redox, or reduction-oxidation reactions, are chemical processes involving the transfer of electrons between substances. In the context of shallow oxide gold formation, redox reactions play a pivotal role in mobilizing and precipitating gold from its primary sulfide sources. The cycling of redox states in sediments influences the solubility of gold-bearing minerals, affecting their concentration and localization.

Oxidation of sulfides: The oxidation of sulfide minerals, often rich in gold, occurs when they are exposed to oxygen, particularly in shallow, oxygenated (aerobic) waters. This can lead to the breakdown of sulfide minerals, releasing gold into the solution with subsequent deposition in the gossan (Figure 4). This process is pivotal in weathering environments, where oxidizing conditions prevail (see Boyle, 1979; Webster, 1984; and Mann, 1984).

**Figure 4** – Gold formation within gossanous rock mass resulting from the oxidation of sulfides (images courtesy of Stephen Woods)

Figure 5 - Thiosulfate (image courtesy PubChem) — **Figure 5** – Thiosulfate (image courtesy PubChem)

Formation of gold complexes:
As sulfides oxidize in the rock mass and become gossanous, gold may form complexes with various ligands, such as chloride or thiosulfate (Figure 5), which enhance its solubility in water and mobility. This solubility is affected by pH, temperature, and the presence of organic matter, which can act as stabilizing agents. The presence of organic matter can also play a crucial role, as it may contribute to the complexation of gold ions (Vasconcelos, 1991 and Kyle, 1991; Usher et al., 2009).

Precipitation and concentration:
As conditions — such as changes in pH or redox potential — fluctuate, gold complexes can precipitate, often forming secondary gold minerals. This precipitation is more likely in environments with lower solubility, such as during dry periods when water table levels drop.

The role of groundwater

The interaction between groundwater and oxidizing conditions enhances the dissolution and subsequent reprecipitation of gold, creating nuggets characterized by distinctive textures and compositions (see Reith et.al. 2007; 2010; 2012). Oxidizing groundwaters also play a critical role in the geochemical precipitation and gold formation within voids and fractures of the quartz (Figure 6) often accompanied by limonite (oxyhydroxide). As groundwater flows through reduced mineral-rich environments, it facilitates the breakdown of sulfide minerals, such as pyrite and arsenopyrite, releasing sulfuric acid and enhancing the acidity of the water. This acidic environment promotes the dissolution of gold and other metals releasing them, forming soluble thiosulfate complexes that can transport gold over considerable distances. The interplay between the oxidizing conditions and the reactive carbonaceous material creates a favorable setting for gold mobilization and concentration. As the groundwater continues to circulate and equilibrate, changes in pH and redox conditions lead to the precipitation of native gold in nearby structures, contributing to the formation of significant gold deposits within these quartz reefs. This intricate geochemical process underscores the importance of understanding groundwater chemistry in exploration.

Figure 6 - Gold seeded on the face of quartz, likely part of cavity fill within a vein (Pine Creek, Northern Territory) — **Figure 6** – Gold seeded on the face of quartz, likely part of cavity fill within a vein (Pine Creek, Northern Territory)

Placer gold

Erosion subsequently transports these nuggets into sediments sometimes called placers, where they concentrate in placers located near the basal unconformity. Further recycling introduced gold into Pleistocene fluvial channels. During this process, gold dissolution and redeposition occur on the exterior surfaces of placer gold particles, resulting in the formation of plates and crystals with minimal change in mass.

A technical summary: How it all fits into the current natural environment

Redox reactions drive the mobility of gold within shallow oxide zones, involving the transfer of electrons in the chemical environment, particularly the oxidation state of various minerals. The key processes include:

Gold complex formation: The released gold can form soluble complexes with inorganic ligands (e.g. chloride, thiosulfate) that enhance its mobility. This solubility is affected by pH, temperature, and the presence of organic matter, which can act as stabilizing agents (Vasconcelos and Kyle, 1991; Usher et al., 2009).

Precipitation mechanisms: As environmental conditions fluctuate—such as during periods of drying or changes in salinity— gold complexes may precipitate, leading to localized enrichment. This phenomenon is often observed in supergene environments, where weathering processes concentrate gold from underlying hydrothermal sources (Bowell et al., 1993; Hough et. al., 2007; 2009).

The role of ancient paleo environments

Figure 7 - This nugget is full of cavities and was quite ‘spongey’ due to the growth around alluvial sediments on a paleo lake shore in the Northern Territory — **Figure 7** – This nugget is full of cavities and was quite ‘spongey’ due to the growth around alluvial sediments on a paleo lake shore in the Northern Territory

Ancient water tables in northern Australia were dynamic ecosystems where the chemistry of the water influenced gold solubility and precipitation. Factors that contributed to gold remobilization and concentration include:

Evaporative processes: Seasonal evaporation can increase salinity, promoting the precipitation of gold as environmental conditions become less favorable for its solubility (Hough et al., 2007; 2009).

Biological mediators: Microbial activity can alter the local redox state, facilitating the oxidation of sulfide minerals and the subsequent release of gold. These biological processes are essential in understanding the gold dissolution pathways within these ecosystems (Reith et. al., 2007; 2010; 2012).

Hydrological changes: Fluctuations in water levels and chemistry due to climatic changes impacted the concentration of gold in these ancient environments. As climate shifted towards more arid conditions, the cycling of water and sedimentation patterns further influenced gold distribution (Youngson and Craw, 1995). This can lead to formation of large nuggets, commonly with distinctive morphology (Figure 7).

References

Bowell, R.J., Gize, A.P. and Foster, R.P., 1993. The role of fulvic acid in the supergene migration of gold in tropical rain forest soils. Geochimica et Cosmochimica Acta, 57(17), pp.4179-4190.

Boyle, R. W. (1979). The geochemistry of gold and its deposits. Geological Survey of Canada Bulletin, 280, 1-584.

Craw, D., & Lilly, K. (2016). Gold nugget morphology and geochemical environments of nugget formation, southern New Zealand. Ore Geology Reviews, 79, 301-315.

Hough, R. M., Noble, R. R. P., & Reich, M. (2007). Natural gold nanoparticles. Ore Geology Reviews, 42(1), 55-61.

Hough, R. M., Noble, R. R. P., Hitchen, G. J., Hart, R., Reddy, S. M., Saunders, M., … & Anand, R. (2009). Naturally occurring gold nanoparticles and nanoplates. Geology, 37(5), 443-446.

Machesky, M. L., Andrade, W. O., & Rose, A. W. (1991). Interactions of gold (III) chloride and elemental gold with peat-derived humic substances. Chemical Geology, 102(1-4), 53-71.

Reith, F., Lengke, M. F., Falconer, D., Craw, D., & Southam, G. (2010). The geomicrobiology of gold. The ISME Journal, 4(9), 1133-1147.

Reith, F., Rogers, S. L., McPhail, D. C., & Webb, D. (2007). Biomineralization of gold: biofilms on bacterioform gold. Science, 313(5784), 233-236.

Reith, F., Stewart, L., & Wakelin, S. A. (2012). Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand. Chemical Geology, 320-321, 32-45.

Usher, A., McPhail, D. C., & Brugger, J. (2009). A spectrophotometric study of aqueous Au(III) halide–hydroxide complexes at 25–80°C. Geochimica et Cosmochimica Acta, 73(11), 3359-3380.

Vasconcelos, P. M., & Kyle, J. R. (1991). Supergene geochemistry and crystal morphology of gold in a semiarid weathering environment: Application to gold exploration. Journal of Geochemical Exploration, 40(1-3), 115-132.

Webster, J. G., & Mann, A. W. (1984). The influence of climate, geomorphology and primary geology on the supergene migration of gold and silver. Journal of Geochemical Exploration, 22(1-3), 21-42.

Youngson, J. H., & Craw, D. (1995). Evolution of placer gold deposits during regional uplift, Central Otago, New Zealand. Economic Geology, 90(4), 731-745.

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