Laminated metal deposits through time

Eleven samples were selected (Table 1) that cover a broad range of metalliferous settings and metal combinations. These examples are presented in general order of age, from laboratory experiments (months), through historic mine sites (decades), to supergene weathering and deep-ocean processes (millions to tens of millions of years). Older deposits have fewer constraints on their formation processes and retain less evidence for biological activity. Hence, we combine that limited evidence with inferences from the younger examples to obtain an integrated view of the biogeochemical processes and their metallic residues through time.

Table 1. Summary of examples of laminated metal deposition from a range of aqueous environments over a range of time scales.

IOB = Fe-oxidising bacteria; IRB = Fe-reducing bacteria; SOB = S-oxidising bacteria; SRB = S-reducing bacteria; MOB= Mn-oxidising bacteria; *Inferred. General references: 1. Henne et al. (Reference Henne, Craw, Gagen and Southam2019a); 2. Kerr and Craw (Reference Kerr and Craw2020); 3. Kerr et al. (Reference Kerr, Craw, Pope and Trumm2018); 4. Alpers et al. (Reference Alpers, Nordstrom, Spitzley, Jambor, Blowes and Ritchies2003); 5. Henne et al. (Reference Henne, Craw, Gagen and Southam2021); 6. Craw and Kerr (Reference Craw and Kerr2017); 7. Reith et al. (Reference Reith, Stewart and Wakelin2012b); Craw et al. (Reference Craw, MacKenzie and Grieve2015); Craw and Lilly (Reference Craw and Lilly2016); 8. Henne et al. (Reference Henne, Craw, Gagen and Southam2019a, Reference Henne, Craw, Gagen and Southam2020); 9. Manceau et al. (Reference Manceau, Lanson and Takahashi2014); Shiraishi et al. (Reference Shiraishi, Mitsunobu, Suzuki, Hoshino, Morono and Inagaki2016); 10. Levett et al. (Reference Levett, Vasconcelos, Gagen, Rintoul, Spier, Guagliardo and Southam2020); Paz et al. (Reference Paz, Gagen, Levett, Zhao, Kopittke and Southam2020); 11. Salama et al. (Reference Salama, Gazley and Bonnett2016).

Samples in this study were selected from material that we obtained in a range of previous studies for entirely different purposes, as outlined in the references in Table 1. These samples were collected from a wide variety of settings associated with mining, exploration and rehabilitation activities and include samples from historic mine waters in the South Island of New Zealand, acid mine drainage in California, USA, anthropogenically neutralised mine drainage in Queensland, Australia, along with samples from oxidised and supergene zones overlaying ore bodies in the South Island of New Zealand, in Salobo, Brazil and Queensland, Australia. In addition, one sample originated from a laterite/soil interface associated with plants over iron ore bodies in Serra Norte, Brazil and one from the North Pacific Ocean floor (manganese nodule). We also include a sample derived from long-term laboratory experiments designed to model the biogeochemical weathering of Cu-bearing lithologies at Salobo, Brazil. These samples, and the near-surface settings from which they were taken, were chosen because they display evidence of fluctuating biogeochemical conditions that have led to laminated deposits. The details of biological and geochemical contributions to formation of these laminated deposits are covered more specifically in references cited herein and other related studies, and some biological aspects of the geologically older material can only be inferred on the basis of the known near-surface settings.

Our chosen examples all show evidence for fluctuating biogeochemical conditions during their formation. We focus on the generalised variations and gradients in redox and pH conditions that have contributed to the formation of these disparate precipitates, and the mineralogical results of these processes. Most of these examples have dominant or primary metals (especially Fe and/or Mn), in which geochemical variations largely control the precipitation processes. These dominant metals are accompanied by other metals that become incorporated into laminated precipitates as a result of secondary or collateral effects.

Theoretical basis for biogeochemical variations

Although biological processes can catalyse chemical reactions that operate within thermodynamic principles, there is an underlying thermodynamic control on the formation of laminated metallic deposits. In the surficial environment, the Gibbs free energy change of a reaction, ΔG _reaction, has two components, the standard Gibbs free energy of the reaction, ΔG°_reaction, and the reaction quotient, Q:

(1)$$\Delta G_{{\rm reaction}} = \Delta G^\circ _{{\rm reaction}} + RT\ln Q$$

where R is the gas constant and T is the temperature (~298 K was chosen as an average temperature for the surficial environments modelled in this study). The reaction quotient Q is the ratio of multiplied activities of reaction products and multiplied activities of reactants. At equilibrium, Q is equal to the equilibrium constant, K _eq. As ΔG _reaction is zero at equilibrium,

(2)$$\Delta G^\circ _{{\rm reaction}} = \ndash RT\ln K_{{\rm eq}}$$

Combining reactions 1 and 2 yields:

(3)$$\Delta G_{{\rm reaction}} = RT\lpar {\ln Q\ndash lnK_{{\rm eq}}} \rpar $$

Hence, when Q < K _eq the reaction goes forward, and when Q > K _eq the reaction is reversed. The special condition of equilibrium is relatively rare in dynamic surficial environments. Bacteria, in particular, can use enzymes to gain energy for metabolic processes that catalyse reactions from a disequilibrium state where Q < K _eq and thereby can accelerate the rate at which reactions progress towards equilibrium (Q = K _eq) (Deamer and Weber, Reference Deamer and Weber2010).

As an example, one of the most common reactions in the surficial environment is the relationship between dissolved ferrous iron and ferrihydrite:

(4)$${\rm F}{\rm e}^{2 + } + 0.25{\rm O}_2 + 2.5{\rm H}_2{\rm O} \to {\rm Fe}\lpar {{\rm OH}} \rpar _3 + 2{\rm H}^ + $$

The reaction quotient is:

(5)$$Q_{{\rm ferrihydrite}} = \lsqb {\rm H}^ + \rsqb ^2/\lpar \lsqb {\rm F}{\rm e}^{2 + }\rsqb \lpar {{\rm p}{\rm O}_2} \rpar ^{0.25}\rpar $$

where pO₂ is the partial pressure of oxygen. Therefore, the ferrihydrite reaction (4) is dependent on pH, oxygen partial pressure or redox conditions, and the amount of dissolved ferrous iron. While temperature can be an important variable within surficial environments, here we focus on fluctuations of the three variables above, as they are common in surficial environments for a wide range of biogeochemical reasons, as outlined below. Consequently, disequilibrium dissolution and/or precipitation of iron (and other metals) is a widespread biogeochemical phenomenon, as in many of the examples in Table 1 that are described below.

Laminated precipitates in laboratory weathering experiments

In laboratory-scale weathering experiments of fresh rocks from the Salobo iron-oxide copper–gold (IOCG) deposit in Brazil, finely laminated metalliferous precipitates were produced within ~400 days in a biogeochemical system dominated by Acidithiobacillus ferrooxidans, an acidophilic, iron-oxidising bacterium (Henne et al., Reference Henne, Craw, Vasconcelos and Southam2018, Reference Henne, Craw, Gagen and Southam2019a, Reference Henne, Craw, Gagen and Southam2020). These experiments were conducted in small-scale (10 cm³) polypropylene columns, partially filled with crushed, weakly mineralised rock. The rocks were dominated by iron-bearing silicates with minor disseminated chalcopyrite and bornite. A strain of A. ferrooxidans was cultured from the Salobo mine site and used for the experiments, as described by Henne et al. (Reference Henne, Craw, Vasconcelos and Southam2018, Reference Henne, Craw, Gagen and Southam2019a,Reference Henne, Craw, Gagen and Southamb, Reference Henne, Craw, Gagen and Southam2020). Samples of the column solutions (2 mL) were obtained every two days and replaced with fresh growth medium to sustain active metabolic activity of bacteria. After completion of the experiments, some columns were opened to view the precipitates that had formed on rock clasts (Fig. 1a,b). Some columns were embedded in situ, sectioned, and polished for examination by light microscopy, SEM and XFM (Fig. 1c,d,e; Henne et al., Reference Henne, Craw, Gagen and Southam2019a, Reference Henne, Craw, Gagen and Southam2020).

Fig. 1. Laminated precipitates formed in laboratory-based biogeochemical weathering experiments. (a) General SEM view of ferrihydrite precipitate with dehydration cracks from sample preparation on a rock clast. (b) Cells of A. ferrooxidans on ferrihydrite precipitate. (c) Back-scattered electron image of a section through ferrihydrite precipitate coating the wall of a leaching column with trace fossils of A. ferrooxidans (yellow circles). (d) Back-scattered electron image of a section through laminated ferrihydrite precipitate with trace fossils of A. ferrooxidans (yellow circles and arrows) on a chalcopyrite clast. (e) XFM element-distribution maps for Fe and Cu of laminated ferrihydrite precipitate on the wall of the experimental vessel. Enhanced Cu deposition at bacterial trace fossils are indicated (white circles).

Secondary precipitates that formed around some particles and on the walls of the columns were dominated by ferrihydrite (Fe[OH]₃; although this material was largely amorphous). Some of the precipitates exhibited fine (micrometre-scale) laminations reflecting differing degrees of hydration, chemical composition, and different nanoparticulate particle sizes. These laminations followed the relief of the substrate as well as abundant micrometre-scale circular holes that were trace fossils of individual A. ferrooxidans cells, permineralised in situ during deposition of acicular ferrihydrite (Fig. 1c,d). In general, the distribution of Cu followed the deposition of ferrihydrite laminations, which contained up to 7 wt.% Cu. Interestingly, Cu content was elevated locally within bacterial trace fossils (Fig. 1e).

Laminated precipitates in historic mine wastes

Laminated precipitates found in historic mine wastes, which were abandoned without rehabilitation, provide an opportunity to observe results of biogeochemical processes over human time scales. These laminations record changes in biogeochemical conditions during surficial weathering processes, which influence metal mobility. In two examples from orogenic gold deposits in New Zealand (Waiuta mine and Macraes mine; Fig. 2a–d; Table 1), metals have been dissolved and redeposited by shallow groundwater, forming precipitates that cement the waste materials (Fig. 2a–d). These cementation processes are affected by the weather and seasonal variations that are imposed on inhomogeneous materials that will inevitably include biological components. The constantly changing groundwater compositions result in laminated precipitates reflecting this wide range of biogeochemical conditions. The ore, and subsequent tailings, from these mines contained abundant arsenic (As). The ambient pH at both sites was circumneutral because of ample carbonate in the rocks, but processing of the ore and subsequent weathering has resulted in localised acidification within the tailings deposits (Table 1).

Fig. 2. Laminated precipitates formed at historic mines. (a) SEM view of cemented tailings on a wood fragment; Waiuta orogenic gold mine, New Zealand. Laminated As-bearing Fe-oxyhydroxide precipitates (mid-grey) coat some of the clasts, with an outer layer of realgar (AsS; bright white) precipitate. (b) Magnified view of the same deposit as in (a), showing arsenolite crystals (As₂O₃) with overgrowths of hair-like realgar. (c) SEM view of As–Hg-bearing ferrihydrite cement in historic tailings from Macraes orogenic gold mine, New Zealand, in which Hg was used for gold extraction. The cement is laminated with differing proportions of As and different Hg contents are shown as brighter and darker layers. (d) Magnified view of the same deposit as in (c), showing precipitate lamina rich in schuetteite (Hg₃[SO₄]O₂) and possibly preserved bacterial cells partially per-mineralised by schuetteite. Some of the smallest white dots are nanoparticulate liquid Hg droplets. (e) Photograph of laminated precipitates forming stalactites on the roof of an underground tunnel, Iron Mountain mine, California. (f) Photograph of laminated precipitates on the wall of the mine in (e).

Tailings from the Waiuta mine have been cemented variably by As minerals with a wide range of oxidation states, and these different minerals occur in close proximity at the micrometre-scale (Fig. 2a,b). These tailings were deposited in a shallow (~10 m) depression that has been water-saturated for >80 years and subjected to regular heavy rain events in a wet climate (rainfall >2000 mm/year). The deposit hosts abundant organic matter, including mosses, algae and bacteria, and some woody material from the surrounding rainforest (Kerr et al., Reference Kerr, Craw, Pope and Trumm2018). The most chemically-reduced cementing mineral, realgar, occurred principally on decomposing wood fragments within the tailings (Fig. 2a,b). This setting has resulted in extreme and fluctuating biogeochemical redox gradients within the tailings, leading to laminated precipitates that range from highly oxidised to highly reduced (Fig. 2a,b).

In contrast, the historic tailings from the Macraes mine have been weathering in a semiarid climate (rainfall ~600 mm/year) for ~80 years with associated evaporation facilitating formation of dry, hard, As-bearing ferrihydrite cement (Fig. 2c,d). Carbonaceous debris and relict sulfide minerals within the tailings have facilitated formation of small scale (micrometre to millimetre) redox gradients within the thin (<1 m) cemented deposit that is fully exposed to seasonal weather variations ranging from hot summers (>30°C days) to cold winters (<10°C nights). Historically, mercury amalgamation was used for gold extraction, and some remnants of this Hg remain in the tailings as liquid Hg droplets and as the more oxidised mineral schuetteite (Fig. 2c,d). Mercury has been incorporated variably into the ferrihydrite cement, forming distinct laminations (Fig. 2c,d). Liquid mercury droplets coexist with, and have locally formed on, schuetteite precipitates, and vice versa (Fig. 2d), reflecting the variable and fluctuating redox gradients in the tailings. Partially permineralised bacterial cells (Fig. 2d) attest to a biological component of the cementation processes.

Laminated precipitates from acid mine drainage

Aqueous environments affected by acid mine drainage (AMD) commonly host laminated deposits (Shuster et al., Reference Shuster, Reith, Izawa, Flemming, Banerjee and Southam2017, Reference Shuster, Rea, Etschmann, Brugger and Reith2018). Acid mine drainage results from discharge of shallow groundwater that has been affected by oxidation of sulfide minerals (especially pyrite) that have been exposed by mine excavations. From a biogeochemical perspective, sulfide oxidation is facilitated invariably by a wide range of bacterial species involving various reactions (Baker and Banfield, Reference Baker and Banfield2003; Johnson and Hallberg, Reference Johnson and Hallberg2005; Dold, Reference Dold2014; Quatrani and Johnson, Reference Quatrini and Johnson2018). The historic Iron Mountain underground mine in California, USA (Fig. 2e,f; Table 1) is one of the most spectacular examples of AMD in the world where laminated metalliferous precipitates have formed. At the Iron Mountain underground mine, bacterially mediated oxidation reactions generated underground waters with a pH that ranges down to negative values (Alpers et al., Reference Alpers, Nordstrom, Spitzley, Jambor, Blowes and Ritchies2003). The resulting AMD compositions change on time scales of seasons to years (Alpers et al., Reference Alpers, Nordstrom, Spitzley, Jambor, Blowes and Ritchies2003) and precipitates are laminated accordingly (Fig. 2e,f). Most of the precipitates are dominated by ferrous and ferric sulfate minerals (Fig. 2e,f), reflecting the very high dissolved sulfate concentrations in the AMD and fluctuating redox conditions (Alpers et al., Reference Alpers, Nordstrom, Spitzley, Jambor, Blowes and Ritchies2003). In addition, variable amounts of Cu and Zn sulfate minerals contributed to the laminated precipitates (Fig. 2e).

In an example of pH neutral surface waters at the Cu–Au mine at Mt Chalmers, Queensland, Australia (Table 1), AMD derived from tailings and relict mine walls have been neutralised with alkaline waste rock. Treatment of AMD discharges from historic mines is becoming common practice at many sites to ameliorate the downstream environmental effects. Treatment typically involves raising the pH using an acid-consuming material. The neutralisation of discharging waters at Mt Chalmers, which initially had a low pH (<4) and very high Cu and Zn concentrations, caused Cu and Zn sulfate precipitation on downstream cobbles. Precipitates are locally intergrown with biofilms, creating spectacularly laminated metalliferous deposits (Fig. 3a,b). The biofilms contain a wide variety of bacteria and archaea, some of which are photosynthetic, as well as moss, algae and macroinvertebrates. Some parts of the biofilms also include fossilised fragments of eukaryotes that have enhanced localised Cu and Zn deposition (Henne et al., Reference Henne, Craw, Gagen and Southam2021).

Fig. 3. XFM element-distribution maps of a thin section through laminated mineral precipitates on a biofilm growing on a cobble downstream of the treated AMD at the Mt Chalmers Cu–Au mine, Australia. (a) General view of Cu contents in the laminated deposit. (b) Magnified view of the portion indicated in (a), showing relative concentrations of Cu, Zn, Fe, Mn, S and As.

Laminated precipitates from supergene weathering and oxidation

Laminated metalliferous precipitates, formed during incipient weathering of sulfide-bearing deposits, record the steep redox gradient between the primary sulfides and the most oxidised exterior minerals exposed to the atmosphere and/or oxygenated groundwater. Our example from an orogenic gold deposit from New Zealand (Table 1) of this type of redox gradient is on primary stibnite (Fig. 4a–d). The sulfides of the deposit have been exposed at the surface by erosion on a steep mountainside, and a thin (<1 m) zone of variably oxidised metallic secondary minerals has formed, with minimal metal mobility as transformations occurred largely in situ. The secondary minerals have a complex paragenesis of mutual overgrowths in crude layering within the redox gradient (Fig. 4a–d). The most oxidised veneer consists of Sb-bearing ferrihydrite and tripuhyite, and variably distributed senarmontite and secondary stibnite have formed in between the oxidised veneer and the primary stibnite (Fig. 4a). The senarmontite locally overgrows secondary stibnite and is locally overgrown by the secondary stibnite (Fig 4b–d). The secondary stibnite has a distinctive tubular texture with a high surface area reminiscent of permineralised filamentous bacteria, indicating a biological influence on formation (Fig. 4d).

Fig. 4. SEM images of laminated secondary Sb minerals developed in an oxidised Au–As–Sb orogenic deposit, Otago Schist, New Zealand. (a) General view of contrasting redox minerals developed on primary stibnite (bottom), with secondary stibnite beneath Sb-bearing ferrihydrite and tripuhyite (FeAsO₄). (b, c) Magnified view of secondary minerals in the redox gradient in (a), with secondary stibnite overgrowing secondary senarmontite crystals (Sb₂O₃). (d) Magnified view of secondary stibnite, showing a tubular morphology produced potentially by filamentous bacteria.

More advanced weathering and oxidation was observed in our example from the saprolitic zone developed on the Precambrian Salobo IOCG deposit of Brazil (Fig. 5a–c; Table 1). This led to the development of a subsurface saprolitic zone in which the rocks have been oxidised extensively and variably altered to clays. This oxidation and alteration was driven partly by a wide range of bacteria, including Fe-, Mn- and S-oxidisers and Fe-reducers, that occur within water-saturated fractures exposed by the mine excavation (Henne et al., Reference Henne, Craw, Gagen and Southam2020). In this saprolitic zone, typically more extensive metal mobility has occurred than in the previous incipient weathering example, and metal-bearing veins can form in structurally controlled sites. The example from the saprolitic zone developed at Salobo consists of a delicately laminated Cu- and Mn-rich vein that has formed on a fracture surface in a rock dominated by weathered Fe-silicates (Fig. 5). Although there is abundant Fe in the general environment, there is only minor Fe in the vein (<1 wt.%; Fig. 5a,c). However, the vein includes a wide range of other elements in addition to the dominant Cu and Mn (Fig. 5c). Synchrotron-based XANES (Fig. 6a) revealed that Cu within the vein material, and the relatively low levels within the ferrihydrite substrate, occurred as Cu(II).

Fig. 5. XFM element-distribution maps of a thin section through a laminated vein formed during weathering of the Salobo IOCG deposit, Brazil. (a) Fe and Mn maps show the location of the vein (Mn rich) on the margin of oxidised rock (Fe rich). (b) Magnified view of the vein (as indicated in (a) showing laminae enriched in Mn and Cu. (c) Juxtaposed elemental sections through the vein, as indicated in (b).

Fig. 6. Regions of interest from which Cu XANES spectra are extracted are numbered and highlighted in green, and are shown against a background map of Cu K-edge fluorescence intensity at 9.3 keV. A range of Cu(0), Cu(I) and Cu(II) standards are shown for comparison. (a) Spectra from the laminated vein and its substrate in saprolite from the Salobo mine (Fig. 5). (b) Spectra from the interior, inner and outer rim zones, and outer edge of the native copper sample from Mt Isa (Fig. 7).

A common component of supergene weathering zones on copper-rich mineral deposits is native copper, which typically forms in the lower (more reduced) portions of an overall redox gradient in these zones. The example we depict in Fig. 7a–d is from the saprolitic weathering zone on the Precambrian rocks of the Mt Isa inlier in Queensland, Australia (Table 1). Weathering and progressive oxidation have resulted in the formation of a laminated rim zone around the mass of native copper (Fig. 7a–d). The redox gradient and associated mineralogy are similar to that of the supergene zone over the Salobo deposit in which we have abundant evidence for bacterial involvement (previously described above), and the Fe-oxyhydroxide minerals associated with native Cu grains in the Mt Isa samples display the acicular texture typical of Fe-oxyhydroxide precipitates facilitated by Fe-oxidising bacteria (Loiselle et al., Reference Loiselle, McCraig, Dyar, Léveillé, Shieh and Southam2018) and bacterially sized moulds within these precipitates (Fig. 7a,b). The outermost part of the native copper rim zone consists of crystalline malachite (Fig. 7c–f) that is readily identifiable in hand specimen and is clearly a Cu(II) mineral in XANES analysis (Fig. 6b). The innermost portion of the rim zone shows up in incident light microscopy and XFM mapping as a diffuse zone that is different texturally from both the outermost crystals and the native Cu interior (Fig. 7d,e). This inner rim is shown by XANES analysis (Fig. 6b) to contain Cu(I) and is presumed to be cuprite. Iron is confined to the outer layers (Fig. 7e,f), and some relatively Fe-rich parts of this layer contain arsenic, possibly as scorodite (Fig. 7f).

Fig. 7. SEM images and XFM element-distribution maps of laminations observed in supergene samples from Mt Isa, Queensland, Australia. (a) Back-scattered electron image of a cross-section through Fe-oxyhydroxides in native-copper-bearing rocks organised in colloform textures. (b) Potential bacteria fossil remnant in Fe-oxyhydroxide with acicular texture. (c) Copper XFM map of laminated rims on a supergene native copper nugget. (d) Magnified view of the rim zone in (a), showing principal minerals. (e) Paired Cu and Fe XFM maps of rim layers. (f) Copper, Fe and As XFM maps of rim layers. The As map has high background from the mounting glass, which obscures a zone of As enrichment that follows the outer edge.

At the interface of saprolitic rock and lateritic soil, more advanced oxidation during weathering and progressive leaching of mobile metals can leave a goethite-cemented zone known variously as ferricrete, duricrust, or canga (Gagen et al., Reference Gagen, Levett, Paz, Gastauer, Caldeira, Valadares, Bitencourt, Alves, Oliveira, Siqueira, Vasconcelos and Southam2019; Levett et al., Reference Levett, Vasconcelos, Gagen, Rintoul, Spier, Guagliardo and Southam2020; Paz et al., Reference Paz, Gagen, Levett, Zhao, Kopittke and Southam2020). In our samples of canga over a supergene Fe deposit near the Serra Norte mine in Brazil (Table 1), deposits of Fe-oxyhydroxides dominated by goethite form a hard surface crust to the landscape. Native sedges (Cyperaceae) grow in rupestrian grasslands on the canga and extend their roots into the Fe-rich substrate (Fig. 8a). Biogeochemical processes have led to the formation of laminations with varying proportions of Fe-oxyhydroxides at a range of scales: around centimetre root channels (Fig. 8a,b), and in smaller root-related cavities (tens of μm; Fig. 8c). Microorganisms are preserved extensively as permineralised fossils in goethite-rich bands around root channels in our samples (Fig. 8c,d). The biogeochemical processes also mobilised and reprecipitated some aluminium as part of the microlaminated deposits in the rhizosphere (Fig. 8a–d), typically as gibbsite (e.g. Levett et al., Reference Levett, Gagen, Diao, Guagliardo, Rintoul, Paz, Vasconcelos and Southam2019).

Fig. 8. Lamination in an empty root channel in canga from Serra Norte, Brazil. (a) Photograph of a horizontal cross-section with laminations inside a root channel, with (b) corresponding iron distribution from XFM. (c) Back-scattered electron image of various Fe-oxyhydroxide bands in cavities and infilling cracks. (d) Back-scattered electron image of laminations composed of per-mineralised fossils and encrusted cells enveloped by partially Al-substituted goethite.

Laminated precipitates developed during surficial gold mobility and redeposition

Laminated rims around gold particles, such as in our example from the Otago Schist terrane, New Zealand (Table 1; Fig. 9) are common in alluvial gold particles. Gold is locally mobile within supergene alteration zones and in near-surface sedimentary environments (Fig. 9a–e). Though some local Au enrichment does occur, most Au mobility results in increases in some particle sizes at the expense of other smaller particles nearby. As a consequence, gold particles can show evidence for this particle size increase as laminated rims (Fig. 9a). Supergene nuggets are commonly crystalline, and stacked layers of crystalline gold in the rim zones attest to the progressive addition of gold in the near-surface environment (Fig. 9b,c). In addition, more irregular, commonly vermiform gold overgrowths occur on both supergene nuggets, and on detrital gold particles. There is abundant evidence that gold overgrowths at the 1 to 50 μm scale have a biological component to their origins (Reith et al., Reference Reith, Lengke, Falconer, Craw and Southam2007, Reference Reith, Falconer, Van Nostrand, Craw, Shuster and Wakelin2020; Kerr and Craw, Reference Kerr and Craw2017). The role of direct biological processes in the growth of larger nuggets is more controversial. However, biological activity such as bacterial decomposition of primary sulfide minerals in supergene zones has an indirect effect on the biogeochemical processes of Au mobility by generating metastable thiosulfate ions that can complex with Au (Shuster et al., Reference Shuster, Bolin, MacLean and Southam2014).

Fig. 9. Layering on the margins of gold particles from the Otago Schist terrane, New Zealand. (a) Incident light view of laminated rim of a supergene gold nugget. (b) SEM view of layered crystalline gold on the margin of a supergene nugget. (c) Magnified view of crystalline gold laminations in (b). (d) Vermiform gold overgrowths on an alluvial gold particle close to the bedrock supergene source. (e) Magnified view of vermiform gold overgrowth in (d).

Laminated precipitates forming manganese nodules

Manganese nodules occur on the deep ocean floor where very low sedimentation rates allow biogeochemical processes to contribute to localised metal accumulation. These nodules precipitate from metals that are mobilised from underlying abyssal sediments and metals that precipitate directly from the surrounding water, and have spectacular laminated structures (Fig. 10). Nodules have been investigated extensively as potential sources of metal resources, so geochemical, mineralogical and biological features have been well established (Hein et al., Reference Hein, Mizell, Koschinsky and Conrad2013; Hein and Kaschinsky, Reference Hein and Koschinsky2014; Manceau et al., Reference Manceau, Lanson and Takahashi2014; Shiraishi et al., Reference Shiraishi, Mitsunobu, Suzuki, Hoshino, Morono and Inagaki2016; Benites et al., Reference Benites, Millo, Hein, Nath, Murton, Galante and Jovane2018). Here we summarise conclusions from these authors and references therein to provide a context for our general observations on the well-developed metallic laminations in our sample from the North Pacific Ocean that we depict in this paper (Fig. 10).

Fig. 10. XFM element-distribution maps showing laminations in a manganese nodule from the North Pacific Ocean floor. (a) General view of Mn distribution in a polished section through the nodule. (b) Magnified view of a strip through (a), showing Mn, Cu, Fe and Ni distributions in a combination of primary laminations and secondary (diagenetic) layers that truncate the primary laminae and locally obscure them. (c) XFM element-distribution maps showing a detailed view of primary metallic laminations of Mn, Cu, Fe, Co, Ni, Ti and Zn in an individual protrusion.

Manganese nodules are dominated by Mn oxides and Fe oxides, with incorporation of a range of other transition metals, particularly Cu, Co and Ni. Some primary depositional parts of the nodules undergo diagenetic transformations of mineralogy and texture, accompanied by redistribution of metals. While recent research indicates that the mobility of metals is probably linked to the structural properties of the hosting Mn oxide (Wu et al., Reference Wu, Peacock, Lanson, Yin, Zheng, Chen, Tan, Qiu, Liu and Feng2019), the specific minerals and metals that occur in various laminations in the nodules are also controlled by localised redox gradients that occur near to the nodule-sediment interface and the resulting diagenetic processes. Redox gradients and associated chemical activity are partially controlled by biological agents, particularly bacteria and archaea in the sediments and on the nodule surfaces. Biological processes can also facilitate localised changes in pH to more acidic conditions that differ from the ambient pH ≈ 8 of surrounding seawater. Importantly, the redox and pH gradients change with time as a result of variations in environmental factors, such as currents and changing supply of decomposing organic material. Consequently, the delicately laminated nodules record accumulation and transformation in situ in an environment of slow biogeochemical cycling. In our sample, there was a general separation of Mn and Fe deposition during these processes, although they were not entirely mutually exclusive (Fig. 10b,c). Likewise, other metals are generally enriched with Mn rather than Fe, but some metal-enriched Fe laminae also occur (Fig. 10b,c).