Geochemistry and mineralogy of contrasting supergene gold alteration zones, southern New Zealand

Craw, Dave; Kerr, Gemma

doi:10.1016/j.apgeochem.2017.08.005

Cited by 23 publications

(14 citation statements)

References 68 publications

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“…(b) Vegetation macronutrient contents of typical Otago Schist [34]. (c) Sodium and chloride concentrations in waters from two of the historic mine sites examined in this study, in relation to sea water and water in an inland evaporative salt lake ( Figure 1a) [25]. Chapman Road data [27] indicate the dominance of NaCl salts, compared to the sulphate-rich schist shallow groundwater in a that yields mainly Ca-carbonate precipitates (Figure 2c…”

Section: Background Schist-dominated Substrates Water and Vegetationmentioning

confidence: 87%

“…These data have been augmented with new data that were specifically collected to quantify geological and geochemical thresholds between different geoecological zones that have been defined by the presence of distinctive vegetation species (Table 1). The background data for typical Otago Schist compositions, mineralogy, and water-rock geochemical interactions have been compiled from previous workers [22,25,30]. General methods for quantifying salinity and site-wide salinity variations at Chapman Rd and Springvale placer gold mines were outlined by previous workers at these sites [23,27].…”

Section: Methodsmentioning

confidence: 99%

“…These residues can accumulate in the more arid parts of the rain shadow, the easternmost example of which occurs as an ephemeral salt lake ( Figure 1b) [24]. Rainfall in Coastal Otago is too high to permit the accumulation of salts, but a geochemical signal for these salts is detectable in shallow groundwaters [24,25]. Figure 1a), with late-stage encrustations of epsomite.…”

Section: General Setting 21 Underlying Geologymentioning

confidence: 99%

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Geoecological Zonation of Revegetation Enhances Biodiversity at Historic Mine Sites, Southern New Zealand

Craw

Rufaut

2021

Minerals

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Rocks exposed by mining can form physically, mineralogically, and geochemically diverse surface substrates. Engineered mine rehabilitation typically involves covering these rocks with a uniform layer of soil and vegetation. An alternative approach is to encourage the establishment of plant species that are tolerant of challenging geochemical settings. The zonation of geochemical parameters can therefore lead to geoecological zonation and enhanced biodiversity. Abandoned gold mines in southern New Zealand have developed such geoecological zonations that resulted from establishment of salt-tolerant ecosystems on substrates with evaporative NaCl. A salinity threshold equivalent to substrate electrical conductivity of 1000 µS separates this ecosystem from less salt-tolerant plant ecosystems. Acid mine drainage from pyrite-bearing waste rocks at an abandoned coal mine has caused variations in surface pH between 1 and 7. The resultant substrate pH gradients have led to differential plant colonisation and the establishment of distinctive ecological zones. Substrate pH <3 remained bare ground, whereas pH 3–4 substrates host two acid-tolerant shrubs. These shrubs are joined by a tree species between pH 4 and 5. At higher pH, all local species can become established. The geoecological zonation, and the intervening geochemical thresholds, in these examples involve New Zealand native plant species. However, the principle of enhancing biodiversity by the selection or encouragement of plant species tolerant of diverse geochemical conditions on exposed mine rocks is applicable for site rehabilitation anywhere in the world.

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Section: Background Schist-dominated Substrates Water and Vegetationmentioning

confidence: 87%

Section: Methodsmentioning

confidence: 99%

Section: General Setting 21 Underlying Geologymentioning

confidence: 99%

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Geoecological Zonation of Revegetation Enhances Biodiversity at Historic Mine Sites, Southern New Zealand

Craw

Rufaut

2021

Minerals

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“…Metalloids Sb and As are typically abundant around orogenic gold deposits (Craw and Kerr, 2017). Both these metalloids are variably soluble in the surficial environment and they are especially susceptible to variations in redox and pH conditions (Figs 12d; 13a).…”

Section: Discussionmentioning

confidence: 99%

“…Metalloids Sb and As are typically abundant around orogenic gold deposits (Craw and Kerr, 2017). Both these metalloids are variably Fig.…”

Section: Laminated Precipitates Related To Gold Depositsmentioning

confidence: 99%

Biogeochemical formation of metalliferous laminations in surficial environments

Henne

Craw

Hamilton

et al. 2021

MinMag

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Finely laminated (cm–μm scale) metalliferous precipitates are widespread in the surficial environment, especially around mineral deposits and reflect biogeochemical processes that can pervade near-surface environments on a larger scale. Examples in this paper involve precipitates of the transition metals Fe, Cu and Mn with minor Co, Ni, V and Zn; the metalloids As and Sb; and authigenic Au. Mobility and re-precipitation are driven primarily by geochemical disequilibrium, especially with respect to pH and redox states, that arises from complex interactions between biological processes, geological processes, and variations in the surrounding environment. Different degrees of chemical disequilibrium arise on small spatial scales on time scales of days to millennia. Interactions between biota, waters and rocks in these small near-surface settings affect the biogeochemical environments. Sulfur- and iron-oxidising bacteria are common biogeochemical agents associated with sulfide-bearing lithologies, but localised reductive environments can also develop, leading to gradients in pH and redox state and differential metal mobility. In general, there is commonly a spatial separation of Fe-rich precipitates from those with Cu and Mn, and other transition metals also follow Cu and Mn rather than Fe. Metalloids As and Sb have a strong affinity for Fe under oxidising conditions, but not under more reducing conditions. However, complex biogeochemical parageneses of laminated metalliferous deposits preclude prediction of finer formation details. The textures, mineral species, and metal associations within these deposits are likely to be encountered in all facets of mineral deposit development: initial exploration activity of near-surface locations, mining of shallow portions of orebodies, especially supergene zones, and downstream environmental management with respect to discharging metalliferous waters.

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Evolution of Alkaline Mine Drainage and Unique Biodiversity on Soil-Free Mine Substrates, Southern New Zealand

2023

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Relatively impermeable soil-free substrates (clay-rich outcrops and sediments) exposed at eight abandoned placer gold mines generate alkaline mine drainage through evaporation and minor interaction between water and rock in a semi-arid rain shadow climate. Original mine sluicing a century ago created drainage channels over the land surface that still control the flow of ephemeral surface waters, with localised construction of erosional outwash pans. Ephemeral surface waters and associated evaporitic salts are dominated by halite derived from marine aerosols in rain, yielding circumneutral pH and electrical conductivity (EC) values locally exceeding 50 mS/cm. Weakly altered schist basement rocks and Miocene mudstone exposed at the surface contain abundant calcite, and surface waters are supersaturated with respect to Ca-carbonate minerals with a pH of ≈ 8 and an EC of ≈ 1 mS/cm. Water interaction with albite increases the dissolved Na/Cl molar ratio to > 1, and evaporative formation of Na-carbonate precipitates can raise the pH to > 10. Pyrite oxidation does not offset the alkaline pH in any of these processes, although dissolved sulfate can rise to high levels (> 2000 mg/L; EC ≈ 5 mS/cm), forming evaporative sulfate minerals. Ephemeral waters drain down outcrops and the relatively impermeable erosional pans, leaving salt encrustations with variable mineralogy and associated waters with pH and EC values that are partially controlled by rates of dissolution and reprecipitation of the minerals in the salts. The saline alkaline chemistry of the pans excludes most vegetation and has allowed development of salt-tolerant ecosystems with rare endemic halophytic plants. Maintenance of soil-free halophytic surfaces has potential applications in dryland mine sites around the world, especially as climate change enhances and extends aridity.

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Geochemistry and mineralogy of contrasting supergene gold alteration zones, southern New Zealand

Cited by 23 publications

References 68 publications

Geoecological Zonation of Revegetation Enhances Biodiversity at Historic Mine Sites, Southern New Zealand

Geoecological Zonation of Revegetation Enhances Biodiversity at Historic Mine Sites, Southern New Zealand

Biogeochemical formation of metalliferous laminations in surficial environments

Evolution of Alkaline Mine Drainage and Unique Biodiversity on Soil-Free Mine Substrates, Southern New Zealand

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