Article

Turbidity development and dissipation in paleoplacer gold deposits, southern New Zealand

Authors:
  • Vision Consulting Engineers Ltd
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Abstract

Placer gold mining inevitably produces highly turbid processing waters. Five historic paleoplacer mining sites in Central Otago, New Zealand, provided 24 samples which were used in laboratory-based settling experiments. Turbidity was examined in the context of stratigraphy of paleoplacer deposits, their environments of formation, and the groundwater processes that affected the deposits as well as the underlying bedrock. Settling rates were characterised by measuring turbidity levels over time (up to 40 days) using turbidimeter and Coulter counter methods. High clay mineral (mostly kaolinite) contents of the materials were confirmed by X-ray diffraction. Grain size distributions of suspended materials were very comparable across all samples with majority of particles falling between <1.2 and 2.5 μm. The levels of turbidity produced by the auriferous sediments were partly controlled by the level of sorting and winnowing that the sediments were subjected to during transport and deposition. Debris flow material generated high turbidity [initial levels of 120–420 nephelometric turbidity units (NTU)] which settled slowly, as did eolian siltstone (1,600 NTU). Fluvial sediments generally generated lower turbidity which settled more rapidly, on the scale of hours to days. It was found that cementation of the sediment can reduce turbidity generation by limiting disaggregation of the clay minerals. On the other hand, the presence of altered lithic clasts within the sediments contributes to higher turbidity production. There was poor correlation between the level of bedrock alteration, as indicated by Chemical Index of Alteration, and the resulting turbidity. Settling rates were more rapid in experiments conducted in saline solution, as opposed to stream water, due to floc formation.

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... All mine excavations and processing activities involve disturbance of variably mineralised rock; this can lead to elevated levels of suspended solids (cf Druzbicka and Craw 2012). Excessive suspended solids in mine discharges can cause significant environmental impacts, especially if amounts exceed those of natural floods in absolute concentrations and duration. ...
... Suspended and deposited fine sediment can reduce the diversity and ecosystem health of stream communities (Burdon et al. 2013). The amount of suspended solids in mine water is affected by the nature of the substrate that is being disturbed, with clay-rich materials proving most problematic (Druzbicka and Craw 2012). Clay-rich substrates are relatively rare in orogenic gold deposits, where hydrothermal alteration of wall rocks is limited (Fig. 2). ...
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Arsenic is released into the environment by decomposition of natural arsenopyrite in gold‐bearing veins in east Otago. Natural release is slow, but is accelerated by mining activity. Waters from old mine adits have pH of 6–7, compared to normal groundwater pH of 7–8, and adit waters have arsenic concentrations up to 4 ppm compared to background levels of c. 0.01 ppm. Tailings disposed about 80–90 years ago into what is now a wetland at Barewood have arsenic concentrations of up to 33 000 ppm, principally in the form of the secondary arsenic mineral scorodite. The maximum dissolved arsenic concentration in the Barewood tailings is 0.75 ppm at pH of c. 5.5. Dissolved arsenic release at that pH may be restricted to levels near 1 ppm by the formation of scorodite with solubility and/or kinetic controls on scorodite redissolution. Modern mining activity in the Macraes area has resulted in localised dissolved arsenic concentrations in excess of 200 ppm at pH near 10. Evaporation of this water produces scorodite precipitates. Dissolved arsenic released from arsenopyrite decomposition is attenuated by fine‐grained minerals, predominantly phyllosilicates, in soils around weathered veins, and in wetlands downstream from mine workings. Lithic soils can hold at least 50–100 ppm As on the fine fraction (c. 1 μ,m), and wetland soils can hold hundreds to thousands of parts per million on their fine fractions. Both soil adsorption and scorodite mediation slow the release of arsenic into the environment, allowing more effective dilution by downstream waters to safe arsenic levels.
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Water quality at four coal mines in southern New Zealand can be related directly or indirectly to the geology and mineralogy of the stratigraphic sequence in which the coal mines occur. The Late Cretaceous Taratu Formation of the Kaitangata coalfield formed in a marginal marine setting during regional marine transgression, and marine incursions punctuated coal formation. Abundant authigenic and remobilised pyrite at the Wangaloa mine (typically 4 wt%S in coal) in the Kaitangata coalfield has oxidised and caused acidification of mine waste rocks (to pH 1) and mine waters (to pH 3). Similar or even greater amounts of pyrite occur locally at the nearby Kai Point mine, although the mined seam typically has 1 μm), detrital kaolinite at Kai Point, Wangaloa, and Newvale mines settles readily (within days). Chlorite from disaggregated labile clasts in overlying Pliocene gravels dominates longer term turbidity at Newvale, but the turbidity decreases rapidly on a time‐scale of months. Fine‐grained (including sub‐micrometre) poorly sorted authigenic kaolinite from labile clasts in the coal‐bearing strata at Ohai causes long‐term turbidity, with negligible setting over 9 months. The stratigraphically controlled processes quantified in this study can be used to predict the nature and scale of potential water quality changes in new coal mines in southern New Zealand.
Article
The surface electrical charge on suspended particles in four estuaries of the U.K. has been measured as a function of salinity by the technique of particle microelectrophoresis. Two characteristic types of behavior were found. In rivers low in dissolved cations, especially Caz+ (Conwy, Beaulieu), the electrophoretic mobility uE was negative in sign at all salinities, increasing slightly in magnitude from the seawater end member to lower salinities of 5-10%0, with a more pronounced increase toward the river water end member. In rivers draining calcareous terrain and having relatively high concentrations of Ca2' (Alde, Orwell) uE showed a similar dependence on salinity above 5-l% but no marked increase in magnitude at lower salinities. Ionic composition of the water appears to be the major factor controlling changes in ug with salinity. Positively charged particles were entirely absent. The charge distribution of all samples was highly unifoml, in spite of the mixed nature of the suspended matter, indicating a dominant control of surface properties by adsorbed organic matter, metallic ox- ides, or both. This implies that differential flocculation of different suspended minerals is largely suppressed in the estuarine zone. Measurements of dissolved organic carbon (DOC) and surface-active substances (by suppression of polarographic maximum) in the same estu- aries indicate a sufficient supply of organic matter for the adsorption process. No evidence for nonconservative removal of DOC or surface-active substances was found. Sewage inputs into some of the estuaries are clearly seen by the measurements of surface-active substances. Significant quantities of surface-active materials are injected into the Alde estuary through tidal flushing of a salt marsh area.
Article
The giant gold placer system on the Otago Schist of southern New Zealand was derived from Mesozoic orogenic gold deposits in the underlying schist basement. The core of the schist basement was exhumed in the middle Cretaceous, coeval with the accumulation of the oldest preserved nonmarine sedimentary rocks in the area (ca 112Ma). Those sedimentary rocks contain quartz clasts, with distinctive ductile deformation textures, that were derived from structural zones in, or adjacent to, major orogenic gold deposits. Quartz textures in these structural zones are readily distinguishable from the rest of the schist belt, and hence provide a fingerprint for erosion of gold. The earliest sedimentary rocks on the margins of the gold-bearing schist belt are immature, and were derived from unoxidised outcrops in areas of high relief. Gold was not liberated from unoxidised basement rocks during erosion, and was removed from the system without placer concentration. Placer concentration did not begin until about 20million years later, when oxidative alteration of gold deposits had facilitated gold grain size enhancement from micron scale (primary) to millimetre scale (secondary). Subsequent erosion and recycling of gold in the early Cenozoic, and again in the late Cenozoic, caused additional concentration of gold in progressively younger deposits. The Klondike giant placer goldfield of Canada had a similar geological history to the Otago placer field, and Klondike placer accumulation occurred in the late Cenozoic, at least 70million years after Mesozoic exhumation of orogenic gold. The giant placer deposit on the western slopes of the Sierra Nevada in California occurs in Eocene and younger sedimentary rocks, at least 40million years younger than the timing of major exhumation of the source rocks. Circum-Pacific giant gold placers formed under entirely different tectonic regimes from the emplacement of their source orogenic deposits, and these giant placer deposits do not form in foreland basins associated with convergent orogens. Formation of giant placers requires less active erosion and more subdued topography than the collisional orogenic activity that accompanied emplacement of source gold deposits in basement rocks, as well as oxidative alteration of the primary deposits to liberate gold from sulfide minerals and enhance secondary gold grain size.
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Dunstan Formation lignite bearing strata within the 1,000m thick Miocene Manuherikia Group of Central Otago represents an important coal resource. Three principal coalfields, Blackstone, Roxburgh and Nevis are identified. Detailed stratigraphic and sedimentologic data from outcrop and over 120 boreholes were assessed to evaluate Central Otago coalfield stratigraphy and the paleoenvironmental controls governing peat deposition on a broad alluvial plain surrounding a large freshwater basin (Lake Manuherikia). The study emphasizes the significance of detailed observations and analyses of coal-bearing strata, demonstrating the potential role of paleoenvironmental modeling in coal exploration and mining activities.
Article
Simultaneous measurements of currents, salinities, and suspended sediment concentrations were made at locations upstream, inside and downstream of an area of rapid shoaling in Savannah Harbor throughout a tidal cycle during each of three tide ranges to learn the importance of flocculation processes to the formation of shoals. The sampling stations were located in the zone of mixing of river and ocean waters. As shown in the data, chemical and hydraulic conditions prevail that together with an abundant supply of suspended particles provide the cohesion, frequency of collision, and time for formation necessary to form aggregations of large numbers of mineral particles. Changes of concentration at slack and concentration profiles at the strength of flow showed that particles have settling velocities much greater than those of the individual particles that comprise the shoal. Flocculation determines the settling velocities of suspended material at Savannah.
Article
The colloidal stability of particles transported in suspension by rivers is controlled by surface electrical properties1 which are conventionally regarded as undergoing rapid change during estuarine mixing1–3 by interaction with polyvalent ions, especially Ca2+ and Mg2+. Although wide differences in rates of coagulation of clay minerals as a function of salinity have been demonstrated in the laboratory4,5, Gibbs6 and others7,8 have concluded that field studies provide no real evidence for the importance of such differential flocculation. Instead, changes in clay mineral distribution with distance from the source river can be accounted for by differences in the particle size spectra and the resulting sedimentation velocities. The effects of differing electrical and surface properties on coagulation rates are apparently suppressed by the formation of uniform films of metal oxides and/or organic matter around the particles6. We present here direct measurements of the electrokinetic charge on river and estuarine particles which confirm this hypothesis.
Article
Antimony and arsenic are commonly mobilised into the environment from mesothermal mineral deposits. Both these metalloids are potentially toxic in the environment when dissolved in water at low levels (<0.01 mgll). Mobility of antimony, in comparison to that of arsenic, is documented at mine sites in four different mesothermal systems in low grade Palaeozoic-Mesozoic metamorphic terranes of New Zealand (Globe Hill, Reefton; Macraes, Otago; and Endeavour Inlet, Marlborough) and Australia (Hillgrove, New South Wales). Dissolved antimony can reach ∼50 mg/kg in mine waters where evaporative concentration occurs in oxidised near-neutral pH mine waters in contact with stibnite. Such waters are chemically saturated with respect to antimony oxides, and antimony oxide precipitation occurs locally. Most mine waters have lower dissolved Sb concentrations, especially where high rainfall causes dilution. However, high rainfall areas have widespread diffuse mobilisation of both Sb and As to give elevated background levels (0.1-0.01 mg/kg) in downstream waters. Dissolved Sb is decreased by adsorption of Sb to hydrated iron oxide (HFO) precipitates in streams. Bulk distribution coefficient, Kd for this Sb adsorption ranges up to at least 105, and is similar to that of As adsorption in the same settings. Attenuation of dissolved Sb by HFO results in an order of magnitude decrease in Sb concentrations on a scale of metres. In the absence of HFO, dissolved Sb can be transported in streams fior many kilometres. Elevated dissolved Sb concentrations can arise distant (tens of kilometres) from a mine site because of dissolution of antimony-bearing minerals physically transported downstream. Hence, without HFO attenuation, environmentally toxic levels of Sb can be readily leached from mine sites and distributed widely.
Article
This study investigated the settling rates and sediment volumes of aqueous, flocculated kaolin suspensions as functions of kaolin concentration, container dimensions, and chemical composition of the aqueous phase. Equations which correlated the data were derived, based on a structural model which assumed that in a flocculated suspension, the basic flow units are small clusters of particles (plus enclosed water) called flocs. These flocs retain their identity under the mild forces experienced in gravity settling. At low shear rates, the flocs group into clusters of flocs, called aggregates. The aggregates may form networks which extend to the walls of the container and give the suspension its plastic and structural properties.
Article
Suspended sediment in coastal environments with high inorganic content have characteristic broad size distributions and are composed of both single grains and flocculated aggregates. These flocculated suspensions have stable size distributions the modal size of which is dependent on the modal size of the deflocculated single grain distributions. Comparison between theoretical settling speeds of quartz grains and the settling speed of particles in natural suspensions indicates that most grains smaller than the deflocculated single grain mode settle as part of flocs, whereas the particles larger than the mode settle as single grains. As a result the size distribution curves of sediment populations which settle out during consecutive intervals are composed of a modal peak of larger grains and a low flat portion of smaller grains and resemble the asymmetrical non-normal curves common for muddy sediments.
Article
1. ‘Placer’ mining for alluvial deposits of gold in a number of stream systems in interior Alaska represents a major disturbance to the stream bed and affects habitat for biotic communities. 2. The potential of analysing aerial photographs to map changes in channel habitat and morphology within gravel-bed rivers is outlined with reference to the impact and recovery of Faith Creek, a second-order stream with a history of placer mining. 3. A strong correlation between the reflectance of the channel bed and water depth is necessary to use the technique succesfully, together with a knowledge of the effects of ‘broken’water on the spectral characteristics of rivers. 4. Image analysis demonstrated that a wide range of water depths and instream mesoscale habitats existed prior to mining. During mining, the stream was confined to a channellized reach with negligible deep water or habitat diversity. 5. Since mining ceased the stream has abandoned its channellized course and formed a new channel with few deep pools. It is suggested that geomorphological recovery and associated habitat recovery takes a number of large flood events and is likely to require more than 10 years.
Article
During the summers of 1982, 1983, and 1985, we assessed the effects of placer mining sedimentation on Arctic grayling, Thymallus arcticus, in the headwaters of Birch Creek, northeast of Fairbanks, Alaska. We compared differences between two streams (one that was undisturbed and one with mining activity upstream) near the confluence. Studies of caged fish demonstrated that, if grayling could not escape from streams carrying mining sediments, they would either die at high rates (sac fry) or suffer gill damage, starvation, and slowed maturation (age-O fingerlings and age-2 juveniles). Indirect effects of sedimentation, through loss of summer habitat for feeding and reproduction, may more severely affect grayling populations than the direct effects of sedimentation on the health and survival of individual fish.
Article
The impact of clay discharges on benthic invertebrates was investigated by comparison of communities upstream and downstream of alluvial gold mining on 6 streams on the West Coast of the South Island, New Zealand. Mean turbidity was increased by 7–154 NTU above background (mean 1.3–8.2 NTU) by the mine discharges during the 2 months before sampling. Patterns of increase in suspended solids (strongly correlated with turbidity, r=0.95) were similar. Invertebrates densities were significantly lower at all downstream sites, ranging from 9 to 45% (median 26%) of densities at matched upstream sites. Downstream densities as a proportion of those upstream were negatively correlated with the logarithm of the turbidity loading (r=–0.82, P
Article
Blue Lake is an abandoned, water-filled alluvial gold-mine pit in Central Otago, New Zealand. Alluvial gold mining is generally considered to be chemically benign, unless mercury is added to assist gold separation. The major element, trace metal and isotopic composition of the pit lake was compared to nearby, unaffected streams. Blue Lake was found to be enriched in the major cations, with levels that were 2 - 5 times higher than in unaffected streams. Furthermore, Cu, Ni and Zn concentrations exceeded 10 nmol L-1 in Blue Lake; these levels were 2 - 30 times higher than those in nearby, unaffected streams. Processes affecting the lake's characteristics include evaporative concentration, and the oxidation and dissolution of locally derived sulfide and sulfate minerals. Localised acidification in surface and ground waters around the lake leads to the mobilisation of Zn and Ni, resulting in lake waters being strongly enriched in these trace metals ( concentrations greater than 40 nmol L-1), whereas surrounding stream waters have much lower Ni and Zn concentrations (less than 5 nmol L-1). Ongoing evaporative concentration, and the continuing mobilisation of trace metals, implies that metal enrichment in lake waters will continue to occur. The present study demonstrated that the 'benign' process of alluvial gold mining can have significant chemical consequences in resulting water bodies.
Article
New Zealand is an active orogenic belt which varies along its length from continental collision to continent–ocean subduction tectonics. Mesothermal gold deposits, with rare mercury, have formed in collisional settings from late Mesozoic to Pleistocene, with youngest deposits along the axis of the actively rising Southern Alps mountain chain. Epithermal gold and mercury deposits have formed, and are still forming associated with calcalkaline volcanism above the subduction zone, and with basaltic volcanism associated with extensional faulting. The environmental impact of mineral deposits is related to tectonic setting, mineralogy, and climate, all of which are governed by the geometry of the orogen. Mesothermal mineralisation added carbonates to calcite-bearing host rocks in what is now cool semiarid or extreme Alpine settings, and oxidation of sulphides does not result in significant acidification of the environment. Arsenic is the principal metal of environmental significance, and is readily mobilised from these deposits at neutral to alkaline pH. Mercury may be leached from cinnabar and/or gold on geological time scales. In contrast, epithermal mineral deposits have hydrothermal clay alteration, locally further clay-altered by deep temperate to subtropical weathering, and calcite is generally subordinate to sulphides. Acidification accompanies oxidation of these deposits, and copper, cadmium, lead and zinc are readily mobilised into the environment. Mercury can be mobilised by these acid solutions as well, where mercury occurs in minerals other than cinnabar.
Article
The Southern Alps are developing as a consequence of oblique collision between the Pacific and Australian plates. The Southern Alps lie on the west side of the South Island of New Zealand and create a massive rain shadow where greater than 12 m/year of rain falls on the west coast and semiarid conditions exist to the east. The rain-out effect across the mountains causes precipitation west of the Southern Alps to have δD and δ values averaging −30‰ and −5.5‰, whereas precipitation in the rain shadow to the east is isotopically lighter (δD=−72‰ and δ=−9.8‰). Such large differences in the isotopic composition of precipitation would not have existed prior to the development of significant topography. We have examined the topographic evolution of the Southern Alps using oxygen isotope analyses of authigenic kaolinites formed in the rain shadow to the east of the mountains between the Cretaceous (low topography) and the Pleistocene. Changes in the isotopic composition of authigenic clay minerals forming in equilibrium with meteoric water in the stratigraphic sequence record the development of Southern Alps topography and the resultant rain shadow. Our oxygen isotope analyses of authigenic kaolinites show a 5–6‰ decrease in the early Pliocene, from ∼18.2‰ in older rocks, to ∼12.3‰ in younger rocks. In addition, smectite is abundant in all samples from the Late Miocene to Recent, but is conspicuously absent in most older rocks, suggesting a change to a generally drier climate roughly coincident with the isotopic shift in kaolinites. This method may be useful in unraveling timing of development of mountain belts elsewhere in the world.
Article
A regional unconformity cut into the Otago Schist belt in southern New Zealand is overlain by auriferous terrestrial sediments. Formation waters from the overlying sediments have reacted with the schist basement, resulting in a thick (up to 20 m) zone of intensely kaolinitized basement immediately below the unconformity. Primary rock textures are preserved throughout this alteration zone. Two distinct types of alteration zone have developed. Non-oxidizing alteration resulted in kaolinitization of muscovite, but some albite and chlorite was preserved even in the most altered rocks. Chlorite locally altered to ferrous-iron-bearing smectite—vermiculite during non-oxidizing alteration, and this unusual mineral occurs in basement and overlying sediments. Oxidizing alteration has resulted in degradation of almost all schist minerals, leaving kaolinite and goethite, with relict muscovite and quartz. A distinctive 12 Å interlayered clay mineral occurs in altered schist and overlying sediment at one locality. Oxidizing alteration is responsible for localized gold mobility in basement gold deposits. Non-oxidizing fluids have mobilized detrital gold in auriferous terrestrial sediments. The formational waters were mobilized during late Cenozoic regional deformation of the unconformity and overlying sediments.
Article
Silt and clay particles are unstable when suspended in saline waters. They coagulate into sediment flocs having settling velocities orders of magnitude higher than the individual particles constituting them. In the Danish Wadden Sea in-situ analyses were carried out to obtain the natural textural composition of the suspended sediment. A settling tube, Braystoke SK 110, was used; and the analysis revealed that the suspended sediment at the investigation sites was composed of an unflocculated population coarser than about 4.5 φ and of a flocculated population finer than this grain size. Median settling velocities from 10−4 to 10−3 m s−1 were determined within the concentration range 50–1000 mg 1−1. It is concluded that suspended sediment concentration is a major determinant for the natural textural composition of flocculated sediment. The variation in concentration can account for about 80% of the variance in median settling velocity. Turbulent mixing within the water column is found to be the limiting factor for the growth of sediment flocs.
Article
The early Proterozoic Huronian Supergroup of the north shore of Lake Huron (Fig. 1) is a thick (up to 12,000 m) succession of sedimentary and volcanic rocks deposited between about 2,500 and 2,100 Myr ago1. Here we present a palaeoclimatic interpretation of the Huronian based on approximately 200 major elements analyses of lutites. Most of these are new analyses from the Gowganda and Serpent Formations (Fig. 2). The remainder are from published sources cited in Fig. 4. The composition of lutites from the Huronian Supergroup records an early period of intense, probably tropical, weathering followed by climatic deterioration that culminated in widespread deposition of glaciogenic sediments of the Gowganda Formation. Climatic amelioration followed during deposition of the succeeding Huronian formations. The Huronian succession can be interpreted using a uniformitarian approach in that present day seafloor spreading rates and latitude-related climatic variations are compatible with available geochronological and palaeomagnetic data.
Article
This paper reviews a series of strategies for improving environmental performance in the small-scale gold mining industry. Although conditions vary regionally, few regulations and policies exist specifically for small-scale gold mining activity. Furthermore, because environmental awareness is low in most developing countries, sites typically feature rudimentary technologies and poor management practices. A combination of policy-, managerial- and technology-related initiatives is needed to facilitate environmental improvement in the industry. Following a broad overview of these initiatives, a recommended strategy is put forth for governments keen on improving the environmental conditions of resident small-scale gold mines.
Article
Suspended sediments in the turbidity maximum of Chesapeake Bay include composite particles which contain platy mineral grains, arranged both in pellets (attributable to fecal pelletization) and in networks of angular configuration (attributable to electrochemical flocculation and coagulation).
Coagulation in estuaries Blue Spur Conglomerate: Auriferous Late Cretaceous fluvial channel deposits adjacent to normal fault scarps, southeast Otago, New Zealand
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Effects of placer mining discharge on health and food of Arctic grayling Mobility of arsenic in groundwater in the Obuasi gold-mining area of Ghana: some implications for human health
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