G.E.M. Hall

Selective leaches revisited, with emphasis on the amorphous Fe oxyhydroxide phase extraction☆

A sequential extraction scheme for application to soils, tills and surficial sediments is described that elucidates the form in which an element is held and provides information as to its provenance. The phases selected for extraction have been categorised as: adsorbed/exchangeable/carbonate (using CH3COONa as extradant); amorphous Fe oxyhydroxide (0.25 M NH2OH-HCl); crystalline Fe oxide (l M NH2OH-HCl); sulphides and organics (KClO3/HCl); and residual, mainly silicates (HF-HClO4-HNO3-HCl). Particular attention has been paid to the specificity of the reagent used for extraction of the amorphous Fe oxyhydroxide phase, well recognised for its scavenging properties of trace metals in the surficial environment. Atomic absorption spectrometry (AAS) was the main analytical technique employed to determine the elements Zn, Cu, Pb, Ni, Co, Mn and Fe. Inductively coupled plasma emission spectrometry (ICP-ES) and ICP mass spectrometry were used to extend this element suite. Results from standard reference materials, TILL 1–4 and LKSD-4, indicate long term relative standard deviations (RSD) for Zn, Cu, Pb, Ni, Co, Mn and Fe extracted from the five phases of 5–12%, at concentrations a decade above detection limits. Comparison of results for the TILL series with those obtained by direct digestion in HF-HClO4-HNO3-HCl shows that losses of element through the various manipulations of sequential extraction are minimal (< 10%). Replicate analyses of 1-g subsamples of soil, till and humus show excellent precision, at 3–10% RSD. A second extraction with fresh solution in each of the first three leaches was examined. An additional recovery of 35–40% for the CH3COONa extraction, 20–25% for 0.25 M NH2OH · HCl, and 15–20% for the 1 M NH2OH · HCl reagent was seen for Zn, Cu, Pb. Ni and Co. Manganese behaves differently: a less consistent pattern between the two leaches is shown by the 221 samples tested; and the second application generally dissolves proportionately less than is the case for the other elements. Omission of this second step could lead to erroneous interpretation of the results. The relatively high concentration of HCl, at 0.25 M, used in the extraction of the amorphous Fe oxyhydroxide phase leads to significant dissolution of sphalerite and galena. Less than 1 % of these sulphides is dissolved if the acid strength is reduced to 0.05 M. However, at either HCl concentration, this attack was shown to dissolve a considerable amount of “soluble” organic component. As much as 15000 ppm C was dissolved from the SRM LKSD-4 by this leach, an amount equal to 63% of that extracted by 0.1 M sodium pyrophosphate which is designed to dissolve metals bound to humate and fulvate components of soil.

Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in groundwaters associated with Zn–Pb massive sulphide deposits

Ground and surface waters collected from two undisturbed Zn–Pb massive sulphide deposits (the Halfmile Lake and Restigouche deposits) and active mines in the Bathurst Mining Camp (BMC), NB, Canada were analysed for the rare earth elements (REE). REE contents are highly variable in waters of the BMC, with higher contents typical of waters with higher Fe and lower pH. There are significant differences between ground- and surface waters and between groundwaters from different deposits. The REE contents of surface waters are broadly similar within and between deposit areas, although there are spatial variations reflecting differences in pH and redox conditions. Surface waters are characterised by strong negative Ce anomalies ([Ce/Ce*]NASC as low as 0.08), produced by oxidation of Ce3+ to Ce4+ and preferential removal of Ce4+ from solution upon leaving the shallow groundwater environment. Groundwaters and seeps typically lack significant Ce anomalies reflecting generally more reducing conditions in the subsurface environment and indicating that Ce oxidation is a rapid process in the surface waters. Deeper groundwaters at the Halfmile Lake deposit are characterised by REE patterns that are similar to the host lithologies, whereas most groundwaters at the Restigouche deposit have LREE-depleted patterns compared to NASC. Halfmile Lake deposit groundwaters have generally lower pH values, whereas Restigouche deposit groundwaters show greater heavy REE-complexation by carbonate ions. Shallow waters at the Halfmile Lake and Stratmat Main Zone deposits have unusual patterns which reflect either the adsorption of light REE onto colloids and fracture-zone minerals and/or precipitation of REE–phosphate minerals. Middle REE-enrichment is typical for ground- and surface waters and is highest for neutral pH waters. The labile portion of stream sediments are generally more middle REE-enriched than total sediment and surface waters indicating that the REE are removed from solution by adsorption to Fe- and Mn-oxyhydroxides in the order middle REE≥light REE>heavy REE.

Finding deeply buried deposits using geochemistry

It has become increasingly common for geologists to drill through 100 m or more of cover in search for buried mineral deposits. Geochemistry is one tool applied to this search, using a variety of approaches, including selective leaching of soils to extract the mobile component of elements, and the measurement of inorganic and organic gases. This paper provides an overview of some of the work carried out by the project Deep-Penetrating Geochemistry, sponsored by the Canadian Mining Industry Research Organization (CAMIRO), and supported by 26 Canadian and international companies and by the Ontario Geological Survey and the Canadian Geological Survey. The objective was to provide the mining industry with information relating to processes that may form anomalies at surface over buried deposits and to provide comparative data on methods used to detect these anomalies. Phase I of the project considered the theoretical and experimental framework for the movement of material from deeply buried deposits to the surface; much of this information has come from research on the containment of buried nuclear waste. In arid or semi-arid terrain, with a thick vadose zone, advective transport, which is the mass transfer of groundwater or air along with their dissolved or gaseous constituents, is the only known viable means of moving elements to the surface; diffusion of ions in water or gases in air is orders of magnitude slower. Examples of advective transport are pumping of mineralized groundwater to the surface during seismic activity and the extraction of air plus gas by barometric pumping. Both mechanisms require fractured rock and the interpretation of the derived anomalies requires consideration of neotectonic structures. In wetter climates, where water lies close to the surface, a variety of mechanisms have been proposed for creating anomalies at the surface. Diffusion-based models again suffer from slow rates of migration. Electrochemical models show a cathodic zone at the top of a buried sulphide conductor. Cations are attracted to the cathode, rather than to the surface, yet metals that most commonly migrate as cations are found to form anomalies at the surface. Phase II of the CAMIRO study involved field studies at ten test sites. The test sites included buried porphyry deposits in northern Chile, a gold–copper deposit in the Carlin district of Nevada, and volcanogenic massive sulphide bodies covered by glacial sediments in the Abitibi greenstone belt of Ontario. In all cases anomalies were found in soils above buried mineralization. It is suggested that anomaly formation is an episodic and cyclic process, in which batches of metal in water-soluble form are introduced and the metal is then progressively incorporated with time into the secondary minerals of soil. Selective leaches have been developed to dissolve specific phases in the soil to detect these anomalies. We have compared the results for five selective leaches that are available from commercial laboratories: deionized water, ammonium acetate, hydroxylamine hydrochloride, Enzyme Leach and Mobile Metal Ion (MMI) plus one non-selective decomposition, aqua regia. In addition, the Institute of Geophysical and Geochemical Exploration laboratory in China has supplied data for four sequential selective leaches: water-extractable, adsorbed, organic-bound and iron- and manganese-bound. The weakest leaches dissolve mainly the most recently introduced metals that remain in water-soluble form. Other leaches dissolve specific secondary minerals, such as carbonates, or iron and manganese oxides, which contain the introduced metals. The usefulness of leaches that dissolve secondary minerals depends on the ratio of introduced (exogenic) metal that the minerals contain relative to that of endogenic origin derived from the primary minerals of soils. Our results indicate that this ratio is variable from site to site, so that there is no universal ‘best’ leach for dissolving secondary minerals in exploration surveys. For the test sites in Chile and Nevada, anomalies may have formed incrementally over a period of a million years or more, which permitted metals of exogenic origin to become incorporated into many secondary minerals. For these sites, some anomalies can be detected by aqua regia, although the anomaly/background contrast is less than for selective leaches. For the test sites in Ontario, only a few thousand years have elapsed since glacial sediments were deposited to conceal mineralization. Over this short period, metal of exogenic origin has been incorporated into only the most labile of secondary minerals and it is the leaches that dissolve these labile minerals that can successfully identify anomalies. At the two sites where the most detailed studies have been carried out, the Spence deposit in Chile and Cross Lake near Timmins, we have found that the optimum sampling depth in soils is critical to detecting anomalies.

Role of sediment composition in trace metal distribution in lake sediments

Sediment cores were collected from 20 lakes from the Muskoka region of Ontario, Canada, to study vertical changes in trace metal concentrations with depth and the distribution of metals amongst humic material, amorphous and crystalline Fe and Mn oxides, insoluble organics/sulphides, and silicates. Based on their total concentrations, trace elements displayed different degrees of affinity for the organic fraction (represented by organic C) and the mineral fraction (represented by Al). Certain elements (Hg, As, Sb, Pb, Cd, and Zn) displayed a positive correlation with organic C, a negative correlation with Al, and enrichment in surface sediments (with enrichment factors ranging from 2 to 24). Detailed speciation studies revealed that these elements were associated mainly with humic material and to a lesser extent with oxides in surface sediments. Other elements (Al, Cr, Co, Fe, and Mn) displayed a negative correlation with organic C, a positive correlation with Al, and no consistent enrichment in their total concentration at the surface. The speciation study revealed that metals of the latter group were mainly associated with the silicate fraction in both surface and deep sediments. This study shows that relative affinities for organic and mineral fractions play an important role in the distribution of trace metals during burial and diagenesis, and hence in the shape of their vertical profiles.

The chemical composition of shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea and their effect on ambient seawater

Abstract Submarine, hydrothermal venting occurs at Tutum Bay in shallow (5–10 m) water along the inner shelf that contains a patchy distribution of coral–algal reefs. Two types of venting are observed. (1) Focused discharge of a clear, two-phase fluid from discrete orifices, 10–15 cm in diameter. Discharge temperatures are between 89 and 98°C and estimated flow rates are as high as 300 to 400 l/min. (2) Dispersed or diffuse discharge that consists of streams of gas bubbles ubiquitous in the area. The composition of the gas is mainly CO 2 (92.6–97.9%) with minor amounts of N 2 (2.2–4.7%), O 2 (0.43–0.73%), CH 4 (0.6–2%) and He (∼0.01–0.02%). Based on their geographic position and chemical composition, the vents have been divided into two groups, A and B. Area B vents have higher K, Rb, Sb, Cs, Tl, and As and lower Ca, Li, Mn, Fe, and Sr concentrations. Their chemical difference is likely caused by subsurface mixing of a CO 2 -rich water with a deep reservoir neutral chloride fluid in varying proportions. A two- or possibly three-step process controls fluid evolution and final chemical composition: (1) phase separation in the deep reservoir beneath Ambitle Island produces a high temperature vapor that rises upward and subsequently reacts with cooler ground water to form a low pH, CO 2 -rich water of approximately 150–160°C. (2) The steep topography causes lateral movement of this CO 2 -rich fluid towards the margin of the hydrothermal system where it mixes with the marginal upflow from the deep reservoir. This produces a dilute chloride water of approximately 165°C. A third step may be the entrainment of minor amounts of ground or seawater during its final ascent. Based on a B–Rb/Cs mixing model, it has been estimated that approximately 10% of the deep reservoir fluid reaches the surface. Compared to seawater, the hydrothermal fluids are depleted in Cl, Br, SO 4 , Na, K, Ca, Mg, and Sr and enriched in HCO 3 , B, Si, Li, Mn, Fe, Rb, Cs, Sb, Tl and As. Although some elements are significantly enriched, they do not have a clear impact on ambient seawater composition because their concentration is buffered by mixing and uptake into secondary minerals. Only the surface water in Tutum Bay carries a clear imprint of the hydrothermal fluids.

Application of a sequential extraction scheme to ten geological certified reference materials for the determination of 20 elements

Methods are described for a sequential extraction scheme to dissolve selectively elements bound in soils and sediments in the following nominal forms: (1) adsorbed, exchangeable, carbonate (AEC); (2) amorphous iron oxyhydroxide (am Fe ox), including manganese oxides; (3) crystalline iron oxides (cry Fe ox); (4) organics and sulfides; and (5) residual (mainly silicates). This scheme has been applied in triplicate to a suite of ten international CRMs, i.e., soils SRM 2709–2711 and the SO-1–4 series, marine mud MAG-1, lake sediment LKSD-4 and the till sample TILL-2. Elements determined comprise: As (by HG–quartz tube AAS, HG–QTAAS); Be, Ca, Co, Cr, Cu, Ni, P, Ti and V (by ICP-AES); Mn, Fe and Zn (by FAAS); and Cd, Ce, La, Li, Tl, Pb and U (by ICP-MS). The precision obtained is excellent, generally in the range 2–10% RSD, at concentrations 10 × higher than detection limits. Most results for the element concentrations summed over the five phases agree, within statistical uncertainties, with the recommended total values for the CRMs. Those where recoveries are significantly below 90% are for elements such as Cr and V, which are known to be present in refractory minerals and would require fusion for complete dissolution. The results presented herein for samples SO-1–4 and MAG-1 do not agree well with those recently published using a scheme purported to dissolve similar phases. This highlights the need to be more definitive in describing the nature and extent of the phases actually extracted so that comparisons can be made between different laboratories and studies.

Isotopic and elemental hydrogeochemistry of a major river system: Fraser River, British Columbia, Canada

The Fraser drains one quarter of British Columbia, but within its watershed there occurs a disproportionately large share of the provinces economic activity. Agriculture, fishing, forestry, mining and tourism are all important, the Fraser being the worlds largest source of salmon. To preserve this range of activities, the quality of its waters must be ensured. This study documents the chemistry of the waters. Samples from seventeen sites along the Fraser and its principal tributaries were analyzed for major anions and cations, for 28 dissolved trace and ultratrace elements, and for the isotope ratios of O, H, S, C and Sr. The data show that the primary control on chemistry is the diverse geological terranes that are drained. For example, S04 in the headwaters comes from dissolution of sedimentary sulphate minerals. Towards the coast, weathering of sulphide minerals become progressively more important as a source of SO4, including igneous sulphides that contribute a substantial flux of elements such as Se and Sb. Superimposed on the natural fluxes are anthropogenic contributions. Some, such as dissolved organic carbon and NaCl from pulp mills, can be readily identified. The sources of others are less certain. Molybdenum is much higher in the Fraser than in the Ottawa River. But it is not clear if this high level of Mo is natural or is the result of mining for this metal. During the period of peak flow in July, 1993, Fraser waters had an unusually high level of CO2. The CO2 is derived from the decay of vegetation on land and may have been influenced by timber operations.

Platinum concentrations in urban road dust and soil, and in blood and urine in the United Kingdom †

Increasing Pt concentrations from vehicle catalysts have been reported from a number of countries. Analysis of Pt and Pd in soils and road dusts taken from areas of high and low traffic flows in SE England show concentrations of Pt in the range < 0.30-40.1 ng g-1 and Pd in the range < 2.1-57.9 ng g-1. Higher concentrations of Pt are associated with high traffic densities. Samples taken from streets of lower traffic flows were found to contain the lower concentrations of the ranges. Pilot studies of Pt concentrations in blood and urine using ICP-MS have been carried out. Platinum concentrations in whole blood were: precious metal workers, 780-2170, mean 1263 pmol l-1 (0.152-0.423, mean 0.246 microgram l-1); motorway maintenance workers, 645-810, mean 744 pmol l-1 (0.126-0.158, mean 0.145 microgram l-1); Imperial College staff, 590-713, mean 660 pmol l-1 (0.115-0.139, mean 0.129 microgram l-1). Platinum concentrations in urine in pmol Pt per mmol creatinine were: precious metal workers, 122-682, mean 273 [0.21-1.18, mean 0.47 microgram Pt (g creatinine)-1]; motorway maintenance workers, 13-78, mean 33.7 [0.022-0.135, mean 0.058 microgram Pt (g creatinine)-1]; Imperial College staff, 28-130, mean 65.6 [0.048-0.224, mean 0.113 microgram Pt (g creatinine)-1]. Detection limits were 0.03 microgram l-1 for both blood and urine. The possible health effects of increasing Pt in the environment are discussed. Platinum provides an excellent example of the significance of speciation in metal toxicity. Platinum allergy is confined to a small group of charged compounds that contain reactive ligand systems, the most effective of which are chloride ligand systems. Metallic Pt is considered to be biologically inert and non allergenic and since the emitted Pt is probably in the metallic or oxide form, the sensitising potential is probably very low. Platinum from road dusts, however, can be solubilised, and enter waters, sediments, soils and the food chain. There is at present no evidence for any adverse health effects from Pt in the general environment, particularly allergic reactions.

Stability of inorganic arsenic (III) and arsenic (V) in water samples

The objective of this study was to formulate a protocol for the collection and preservation of natural water samples (lakes and streams) to be analysed for the inorganic species As(III) and As(V). The analytical technique employed was HPLC-ICP-MS, using a proprietary anion exchange resin (ANX-1606-AS from Cetac) as the basis for separation of the two species. Initial experiments carried out at room temperature, where de-ionised water and Ottawa River water samples were spiked at low (0.5-20) µg l –1 concentrations of As(III) and As(V), demonstrated that As(V) is actually reduced to As(III) within a few days. This reduction is matrix and concentration dependent and was confirmed by independent analysis using HGAAS with hydride generation at pH 4.5 and <1. Such conversion of As(V) to As(III) does not occur in spiked deionised water maintained at 5 °C but is evident in the Ottawa River sample at this temperature. A time and temperature study of a filtered (0.45 µm) natural water sample, with a total As concentration of 21 µg l –1 , showed that immediate storage in a filled bottle at about 5 °C will preserve As (III) and As(V) concentrations for about 30 d. If kept at room temperature, changes for this particular sample occur after about 5 d, with As(V) becoming the dominant species via oxidation and gradually declining thereafter. Although acidification to 0.1% HNO 3 stabilises the As species for at least 15 d at 22 °C, its effect is immediately to alter the species distribution. With the natural water sample, this effect was to increase the concentration of As(III) substantially and, to a lesser extent, that of As(V), at the expense of other forms of the element. Acidification to 0.1% HCl also produced these results. The study of spiked de-ionised water and Ottawa River samples at 0.1 and 0.4% in HNO 3 and HCl demonstrated that both acids cause the oxidation of As(III) to As(V) but HNO 3 showed a higher degree of oxidation with greater acid strength, as expected.

Review of methods to determine gold, platinum and palladium in production-oriented geochemical laboratories, with application of a statistical procedure to test for bias

Abstract Current methods for determining Au, Pt and Pd in geological materials in Canadian commercial laboratories are reviewed. The relative merits of fire assay (Pb and NiS) and wet-chemical attack as methods of decomposition of rocks, soils and sediments are discussed. Amongst various analytical techniques, neutron activation is compared to atomic absorption spectrometry (AAS), and to three types of analysis employing the inductively coupled plasma as a source-emission spectrometry, mass spectrometry and atomic fluorescence. Clearly, the sensitivity and flexibility of ICP-mass spectrometry, introduced commercially in 1983, ensure a dominant role for this technique in the determination of Au and the platinum-group elements. A new statistical method for detecting bias, applied to the determination of Au in 157 rock samples, shows that instrumental neutron activation analysis (INAA) yields values which are 30% greater than those by graphite furnace AAS with an aqua regia attack; this same bias occurs in a second data set. Although INAA is an excellent technique for the determination of Au in vegetation, the low background levels of Pt and Pd and poor sensitivity of INAA for these elements make analysis by GF-AAS or ICP-MS more attractive. Further research is required in wet-chemical dissolution procedures for Pt and Pd in dried and ashed vegetation. Fire assay is an alternative choice of decomposition for ashed vegetation but, currently, low-level detection (to 1 ppb for 1 g samples) is hampered by contamination during fusion and by contributions to blank levels from flux constituents. Gold, Pt and Pd in waters can now be determined to levels of 1, 4 and 2 ppt, respectively, in a 1-L water sample using ICP-MS or graphite furnace AAS following preconcentration by adsorption on to activated charcoal. The detection limits for Pt and Pd must be lowered further by a factor of ten for application to exploration goechemistry.