The selective extraction of Pb and Zn in selected mineral and soil samples,application in geochemical exploration (Portugal)

The study described herein concerns the application of selective chemical extractions on metal-bearing minerals and soils to geochemical exploration. Specifically, the study addresses itself to the detection of anomalous soils in the vicinity of a mineralized zone.A kinetic study of selectivity of partial chemical extractions-applied to minerals of Pb and Zn was undertaken in order to establish a systematic procedure (reagents, time of extraction, etc.) which could be recommended for the soils of the study area in Portugal. It is shown that: (1) NH₄ acetate dissolves smithsonite, willemite, cerussite, anglesite, pyromorphite completely and coronadite, galena, sphalerite only partially; (2) hydroxylamine hydrochloride dissolves coronadite only partially; (3) H₂O₂ dissolves sphalerite and galena only partially; (4) hot mixed acids dissolve residual galena, sphalerite and coronadite.About 850 soil and gossan samples were collected from two mineralized districts in Portugal, one in a carbonate environment (Portel) and the other in an aluminosilicate zone (Sanguinheiro).For the kinetic study in soil samples from Portel and Sanguinheiro zones, the minus 80 mesh fraction (−180 μm) was submitted to an extraction procedure using the following reagents, in sequence: NH₄ acetate, hydroxylamine hydrochloride, NH₄ oxalate in darkness, NH₄ oxalate under U.V. irradiation and finally strong acids.The mineralogy of the soils from the two areas is different. In the aluminosilicate zone, the products of weathering of sulphide mineralization are mainly anglesite, plumbogummite, amorphous iron oxides, goethite and hematite rich in Pb and Zn. In the carbonate area, the Pb- and Zn-bearing phases generally found in the soil and gossan are Mn oxides, lepidocrocite and hematite. The gossan, in this case, also contains smithsonite and cerussite.In soils from the aluminosilicate zone the use of NH₄ oxalate (dark) given a better geochemical response for Pb than NH₄ acetate, NH₄ oxalate (U.V.) and conventional acid digestion. On the other hand, NH₄ oxalate (U.V.) gives better results for Pb and Zn than conventional acid digestion in samples from carbonate area. The analysis of NH₄ oxalate (U.V.) extractions, instead of conventional acid digestion, can nevertheless be recommended for both environments.     

The dissolution kinetics of atacamite in the acid range and the stability of atacamite containing soils from Namaqualand,South Africa

The Cu hydroxy mineral, atacamite, is commonly associated with saline environments and is generally thought to dissolve rapidly in the presence of fresh water. A Cu contaminated soil from the arid Namaqualand region, South Africa, shows atacamite as the dominant Cu containing mineral. The stability of the Cu phase in this soil was determined through equilibrium and leaching studies using both deionised water (DI) and a concentrated (0.5 M) NaCl solution. Initially a high concentration of exchangeable Cu was released from the soils leached with NaCl. Continued leaching with NaCl resulted in a substantial decrease in Cu release as atacamite equilibria started to control dissolved Cu. This suggests that an initial spike of Cu laden water will leach from the soils at the onset of a large rainfall event. Further additions of water will result in a lower but sustained release of Cu from the soil. The Cu contaminated soils are exposed to acidic sulphate leachate thus the dissolution kinetics of synthetic atacamite in the acidic range (pH 5.5–4.0) was determined in both NaCl and DI solutions. The kinetic data showed that atacamite dissolution rates are significantly higher in DI than in NaCl but the rates converge at pH 4. In comparison to common acid soluble minerals, atacamite displays a moderate dissolution rate (10^−9.55–10^−7.14 mol m⁻² s⁻¹) within the acid range (pH 5.5–4.0). The atacamite dissolution reaction order with respect to pH is 1.3 and 1.6 in DI and NaCl solutions, respectively, suggesting that dissolution rates of atacamite are highly pH dependent in the acid range. The type of acid used to lower the pH had no effect on the reaction kinetics, with HNO₃ and H₂SO₄ resulting in comparable dissolution rates of atacamite at pH 4.5.     

Pleistocene recycling of copper at a porphyry system,Atacama Desert,Chile: Cu isotope evidence

We present Cu isotope data of hypogene and supergene minerals from the Late Paleocene Spence Cu-Mo porphyry in the Atacama Desert of northern Chile. Chalcopyrite displays a restricted range of δ⁶⁵Cu values within the values reported for primary porphyry Cu sulfides (+ 0.28‰ to + 0.34‰, n = 6). Supergene chalcocite samples show heavier and remarkably homogeneous δ⁶⁵Cu values, between + 3.91‰ and + 3.95‰ (n = 6), consistent with previous models of Cu leaching and enrichment in porphyry systems. Secondary Cu minerals from the oxide zone show a wider range of composition, varying from + 1.28‰ and + 1.37‰ for chrysocolla (n = 6) to very light Cu isotope signatures reported for atacamite between -5.72‰ to -6.77‰ (n = 17). These data suggest redox cycling of Cu during supergene enrichment of the Spence Cu deposit, characterized by a first stage of supergene chalcocite formation from acidic, isotopically-heavy leach fluids of meteoric origin down-flowing in a semi-arid climate (44 to ~ 15-9 Ma). Reworking of the initial supergene copper assemblage, during the Pleistocene, by rising neutral and chlorine-rich deep formation waters under well-established hyper-arid climate conditions lead to the formation of atacamite with extremely fractionated Cu compositions. Essentially coeval chrysocolla formed by dissolution of atacamite during short episodes of wetter climatic conditions occurring in the latest Pleistocene.     

Angélica Copper Deposit: Exotic Type Mineralization in the Tocopilla Plutonic Complex of the Coastal Cordillera, Northern Chile

The Angélica copper deposit is situated at the southernmost sector of the Jurassic Tocopilla plutonic complex in the North Chilean Coastal Cordillera. This deposit occurs in monzonitic to monzodioritic rocks, and has platelike orebodies with no appreciable hydrothermal alteration nor sulfide mineralization. The mineralized zones are located in the western side of the two main normal faults with NE and NW orientations, and are characterized principally by impregnation of supergene copper products of atacamite and minor amounts of chrysocolla, lavendulan and “black copper”. Generally, chrysocolla is more abundant at a distal NE sector of the deposit. The black copper is Cu‐Fe‐Mn‐Si‐Cl‐rich multimineral aggregates composed of atacamite with minor amounts of quartz, pseudomalachite, dioptase, neotocite, gypsum, paratacamite and melanothallite, and its surface exhibits nanometer‐sized cylindrical morphologies. All these characteristics suggest an exotic origin for the Angélica copper deposit. A few vein‐type copper deposits situated at the southwestern sector along the NE‐oriented fault are inferred as the possible source of the Angélica copper deposit.     

Selective sequential dissolution of organic-rich stream sediments from Talvivaara, Finland

Organic-rich stream sediments, a widely used material in geochemical prospecting in Finland, were investigated by analyzing Mn, Fe, Co, Ni, Cu and Zn after a series of selective extractions.Forty-seven organic-rich stream-sediment samples were collected in the vicinity of a sulphide mineralization in Talvivaara, eastern Finland. Loss on ignition of the samples at 500°C was 20–95%. pH of the samples, measured in the water pressed from the samples, was 3.4–6.2 with a mean of 4.2.The < 0.5 mm grain-size fraction was subjected to a sequential extraction as follows: NH₄-acetate for exchangeable cations; cold NH₂OH·HCl for dissolution of Mn oxides; H₂O₂ for destruction of organic material; hot NH₂OH·HCl for dissolution of Fe oxides; HF/HClO₄/aqua regia for dissolving the residue. Each extraction was analyzed by flame AAS for Mn, Fe, Co, Ni, Cu and Zn.Manganese and Co are mostly in exchangeable form and in the residue, while the proportion complexed by the organic components is minimal. Iron and Ni own a lot of exchangeable character although a significant proportion of these elements is complexed by the organic components. Most of Cu is in organic form. Zinc is the element most evenly distributed between extractions.     

Copper mineralization in sedimentary cover associated with tectonic elements and volcanism in Israel

Copper mineralization occurs within the thick Cretaceous to Neogene sedimentary cover of central and northern Israel. Two distinct types of copper-rich anomalies are encountered. One form of copper mineralization is characterized by the presence of copper minerals such as malachite, atacamite, paratacamite, chrysocolla, and/or chalcopyrite forming veins and concretions within the sedimentary host rocks. The second type of copper enrichment is found dispersed within iron oxide veins associated with enrichments of other trace metals such as Ni, V, Zn, Co, Mo, and As. Both forms of copper anomalies lie along and are confined to the vicinity of tectonic elements. These features may be the surface expression of the longitudinal faults of the Dead Sea Rift Valley, of the transverse shear zone of southern and central Negev, or of the EW-trending horsts and grabens of central Israel. In many cases the copper anomalies are found near to or at the contact of volcanic bodies of intermediate to basic composition; in others, they are found in the vicinity of thermal springs. The fault zones may have provided conduits along which solutions, in places possibly briny, could have risen and leached subsurface mineralized volcanic or sedimentary bodies.     

Comparison of extractability of Cd, Cu, Pb and Zn with sequential extraction in contaminated and non-contaminated soils

Various extraction procedures were employed for measuring extractable concentrations of potential toxic elements in soil. The extractability of Cd, Cu, Pb and Zn in four contaminated and four non-contaminated soils of Japan, was compared by single extraction (CaCl₂, DTPA, NH₄Cl, 0.1 M HCl and 1 M HCl ) and sequential extraction procedures [(six operationally defined chemical phases, viz. water soluble (Fl), exchangeable (F2), carbonate (F3), oxide (F4), organic (F5) and residual (F6) fractions)]. Extractability of metals from soils samples varied depending on metals and/or extradants used. Among the extradants, 1 M HCl extracted the largest proportion of Cd (79 to 96% of total), Cu (61 to 83%), Pb (51 to 99%) and Zn (23 to 52%) from soils followed by 0.1 M HCl, NH₄Cl, DTPA and CaCl₂. In all the extradants, the proportion of extractability of metals was higher in the contaminated soils than the non-contaminated soils. Regardless of soils and extradants, relative extractability was higher for Cd as compared to other three metals. The use of 1 M HCl may be recommended for first-level screening of soil contamination with heavy metals. The other four weak extradants are believed to provide a better assessment of bioavailable/mobile metals content in soils than 1 M HCl extradant. However, 0.1 M HCl mobilized all four metals irrespective of soil types, therefore, might be the best choice if only one extradant is to be used. The sequential extraction procedures showed 22 to 64% of total Cd was in the mobile fraction (sum of Fl to F3), while the corresponding values for Cu, Pb and Zn in this fractions were 2 to 23% suggesting higher mobility of Cd than other three metals. The single extraction procedures are simple and easy to perform and obtained results are comparable with sequential extraction procedure.     

Effect of chemical fertilizers on the fractionation of Cu,Cr and Ni in contaminated soil

Effect of chemical fertilizers (urea, NH₄Cl, Ca(NO₃)₂, KCl and KH₂PO₄) on the fractionation of Cu, Cr and Ni was studied by a 4-month incubation experiment. Using sequential extraction procedure, it was found that the application of fertilizers could change the distribution of Cu, Cr and Ni in the fractions of soil. Applying urea (CO(NH₂)₂) significantly decreased the concentrations of Cu, Cr and Ni in water soluble plus exchangeable (WE) fraction, but increased those in Fe–Mn oxides bound (FM) fraction (p < 0.01). However, application of NH₄Cl caused an increase in the WE fraction by 27.7% for Cu, 111.5% for Cr and 20.4% for Ni. The CO(NH₂)₂ raised the soil pH from 4.51 to 4.96, whereas NH₄Cl lowered the pH of soil by 0.44 units. The WE fraction of the three heavy metals was significantly increased, while the FM fraction was significantly decreased by adding KCl (p < 0.01). Moreover, the supply of KH₂PO₄ reduced the WE and carbonate bound (CB) fractions of Cu, Cr and Ni in the soil, however, it raised Cu and Ni in the residual (RS) fraction and Cr in the FM fraction. In addition, the mobility index indicated that KCl and NH₄Cl increased the mobility of Cu, Cr and Ni in the soil, whereas urea and KH₂PO₄ decreased the mobility of the three metals in the soil. These results suggest that applying chemical fertilizers does not only provide plant nutrients, but may also change the speciation and mobility of heavy metals in the soil.     

Partitioning of transition metals in oxidised and reduced zones of sulphide-bearing fine-grained sediments

Distribution and forms of transition metals (Ti, Zn, Ni, Co, Mn, Fe, Cu, V and Cr) were investigated in oxidised, partly oxidised and reduced zones of sulphide-bearing fine-grained sediments located in the coastal areas of western Finland. Samples for the analysis and study of vertical distribution of elements were taken from each vertical 10 cm section in pits ranging in depth between 2 and 3 metres, while bulk samples for characterisation of species and forms of metals were taken from 3 zones in each pit: the acid sulphate soil (characterised by acid and oxidising conditions), transition zone (characterised by a steep pH gradient and partly oxidising conditions) and the reduced zone (pH >6). The former samples were digested in aqua regia (3:1:2 HCl:HNO₃:H₂O), while the latter were digested in aqua regia and hot concentrated acids (HClO₄–HNO₃–HCl–HF) and were subjected to extractions with acid ammonium acetate, H₂O₂ and acid ammonium oxalate. Each leachate was analysed for metals with ICP–AES.The vertical variation in the concentrations of Ti were small at all the studied sites indicating that the sediments are homogeneous and that the total losses of other elements from the soil profiles (acid sulphate soil+transition zone) are not extensive. Field observations, extractions with ammonium oxalate, and concentration–variation patterns indicated that Fe-oxide is largely precipitated and retained also in these acid soils. There are, however, indications of redistribution of Fe within the soil profiles. The results also demonstrated that Mn, Ni, Zn, Co and Cu have been lost in considerable amounts from the acid sulphate soils. However, whereas Mn in general has been lost throughout the soil profile, part of the Zn, Ni and Co released in the acid sulphate soils have migrated downward and been reimmobilised in the transition zone immediately above the reduced zone. Also Cu has been lost from the acid sulphate soil, but generally in smaller proportions than Mn, Zn, Ni and Co. Dissolved metal sulphides seem to be major sources of the mobilised metal fractions. A main part of the V and Cr in the sediments are associated with weathering-resistant minerals. These metals are therefore, like Ti, only to a limited extent mobilised by the oxidation of the sulphide-bearing sediments.     

Distribution of trace elements in soils surrounding the El Teniente porphyry copper deposit,Chile: the influence of smelter emissions and a tailings deposit

In the area surrounding the El Teniente giant porphyry copper deposit, eight soil sites were sampled at three depth levels in the summer 2004. The sites were selected for their theoretical potential of being influenced by past SO₂ emissions from the smelter and/or seepage from a now idle tailings impoundment. The soil mineralogy, grain size distribution, total organic matter contents, major element composition, cation exchange capacity, and Cu, Mo, Pb, Zn, As and SO₄ ²⁻ concentrations were determined for all samples after nitric acid extraction and separate leaches by ammonium acetate (pH 7) and sodium acetate (pH 5). For water rinses, only Cu could be determined with the analytical set-up used. Cu and SO₄ ²⁻ enrichment in topsoils was found at six sites either downwind from the smelter or within the combined influence of the smelter and the tailings impoundment. Both elements were released partially by ammonium and sodium acetate extractions. Due to the scarce background trace element concentrations of soil and rock outside the immediate mine area, assessment of trace element mobility for Mo, Zn, Pb and As was difficult. Arsenic was found to be concentrated in soil horizons with high smectite and/or organic matter contents. Mo appears to be linked to the presence of windblown tailings sediment in the soils. Mobilization of Mo, Zn, and As for the acetate extractions was minimal or below the detection limits for the AAS technique used. The presence of windblown tailings is considered to be an additional impact on the soils in the foothills of the El Teniente compound, together with the potential of acidity surges and Cu mobilization in topsoils after rainfalls. Two sites located at the western limit of the former SO₂ saturated zone with strongly zeolitized soils and underlying rock did not show any Cu or SO₄ ²⁻ enrichment in the topsoils, and remaining total trace element concentrations were below the known regional background levels.     

Atacamite formation by deep saline waters in copper deposits from the Atacama Desert,Chile: evidence from fluid inclusions,groundwater geochemistry,TEM, and <Superscript>36</Superscript>Cl data

The presence of large amounts of atacamite in oxide zones from ore deposits in the Atacama Desert of northern Chile requires saline solutions for its formation and hyperarid climate conditions for its preservation. We investigated the nature and origin of atacamite-forming solutions by means of coupling groundwater geochemical analyses with fluid inclusion data, high-resolution mineralogical observations, and chlorine-36 (³⁶Cl) data in atacamite from the Mantos Blancos and Spence Cu deposits. In both deposits, the salinities of fluid inclusions in atacamite are comparable to those measured in saline groundwaters sampled from drill holes. The average salinity of fluid inclusions in atacamite for the Mantos Blancos and Spence deposits (~7–9 and 2–3 wt.% NaCl_eq, respectively) are strongly correlated to the salinities at which gypsum supersaturates from groundwaters in both deposits (total dissolved solids ~5–9 and 1–3 wt.% NaCl_eq, respectively). This correlation is confirmed by transmission electron microscopy observations of atacamite-bearing samples, revealing an intimate association between atacamite and gypsum that can be traced down to the nanometer scale. ³⁶Cl data in atacamite provide new lines of evidence concerning the origin and age of the saline waters that formed atacamite in various stratabound and porphyry Cu deposits from the Atacama Desert. All atacamite samples show very low ³⁶Cl-to-Cl ratios (11 × 10⁻¹⁵ to 28 × 10⁻¹⁵ at at⁻¹), comparable to previously reported ³⁶Cl-to-Cl ratios of deep formation waters and old groundwaters. In addition, ³⁶Cl-to-Cl ratios in atacamite correlate with U and Th concentration in the host rocks but are independent from distance to the ocean. This trend supports an interpretation of the low ³⁶Cl-to-Cl ratios in atacamite as representing subsurface production of fissiogenic ³⁶Cl in secular equilibrium with the solutions involved in atacamite formation. Therefore, ³⁶Cl in atacamite strongly suggest that the chlorine in saline waters related to atacamite formation is old (>1.5 Ma) but that atacamite formation occurred more recently (<1.5 Ma) than suggested in previous interpretations. Our data provide new constraints on the origin of atacamite in Cu deposits from the Atacama Desert and support the recent notion that the formation of atacamite in hyperarid climates such as the Atacama Desert is an ongoing process that has occurred intermittently since the onset of hyperaridity.     

Ages and petrogenesis of the late Triassic andesitic rocks at the Luerma porphyry Cu deposit,western Gangdese,and implications for regional metallogeny

The Tibetan Plateau is one of the most significant Cu poly-metallic mineralization regions in the world and preserves important information related to subductional and collisional porphyry Cu mineralization. This study investigates a new occurrence of Cu mineralization-related andesitic porphyries in the western domain of the Gangdese magmatic belt and assesses its petrologic, zircon U-Pb geochronology, whole-rock chemistry, and Sr-Nd-Hf-Pb isotope data. Zircon U-Pb dating of three ore-related porphyries yields crystallization ages of 212–211 Ma. These ages are consistent with previous molybdenite Re-Os dating, indicating a late Triassic magmatic and Cu mineralization event in the western Gangdese magmatic belt. Nb, Ta, and Ti depletion, Th and LREE enrichment, and high La/Yb and Th/Yb ratios in addition to high U/Yb ratios from zircons suggest that the magma was generated in an active continental arc setting. The porphyries have radiogenic isotopic compositions with (⁸⁷Sr/⁸⁶Sr)_i 0.70431–0.70473, ε_Nd(t) +1.1 to +3.8, (²⁰⁷Pb^/204Pb)_i 15.601–15.622, and (²⁰⁸Pb/²⁰⁴Pb)_i 38.450–38.693, as well as high positive zircon ε_Hf(t) values from +6.2 to +10.6 (mean value 8.3), corresponding to model ages (T_DM) ranging from 509 Ma to 819 Ma (mean 646 Ma). This suggests that the andesitic magmatism was dominantly sourced from depleted mantle materials that were modified by subducted oceanic sediment-derived melts during the subduction of the Neo-Tethys Ocean. The mineralization-related porphyries contain amphibole and epidote, as well as high whole-rock Fe₂O₃/FeO and zircon Ce⁴⁺/Ce³⁺ ratios, suggesting hydrous and highly oxidized parent magmas. Considering the existing Cu mineralization and highly oxidized magma of the well-preserved Triassic andesitic igneous rocks in the western Gangdese belt, the subduction-related continental arc magma system is favorable for subduction-related porphyry Cu deposits. The existence of Luerma porphyry mineralization demonstrates that there are at least five generations of porphyry Cu-(Mo-Au) mineralization in the Gangdese magmatic belt, which advances the timeframe of porphyry mineralization to the late Triassic.     

Supergene enrichment of copper deposits since the onset of modern hyperaridity in the Atacama Desert,Chile

Supergene enrichment of Cu deposits in the Atacama Desert has played a critical role in making this the prime Cu-producing province of the world. Previously, this has been believed to have occurred exclusively over a long period from the middle Eocene to the late Miocene, which ended when climatic conditions changed from arid to hyperarid. Here, we report U-series disequilibrium ages in atacamite-bearing supergene assemblages that provide a new conceptualization on both the supergene enrichment process and the onset of extreme hyperaridity in the Atacama Desert. ²³⁰Th–²³⁴U ages of gypsum intergrown with atacamite in supergene veins from Cu deposits cluster at ~240 ka (Chuquicamata), 130 ka (Mantos Blancos, Spence), and 80 ka (Mantos de la Luna, Michilla). When coupled with previous data, these results indicate that supergene enrichment of Cu deposits did not cease after the onset of hyperaridity. We propose that supergene enrichment in the Atacama region developed in two main stages. The main phase, caused by downward circulation of meteoric waters in a semi-arid setting, was active from 45 until ~9 Ma, with a last pulse ca. 5 Ma in the southern Atacama Desert. During this phase, atacamite-bearing supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when contacted by meteoric water. This was followed by a second stage starting at ~2–1.5 Ma and continuing until at least the late Pleistocene, when deep formation waters derived from the basement passed up through and modified the pre-existing supergene Cu oxide minerals. Atacamite has then been preserved in the prevailing hyperarid climate.     

Distribution and speciation of arsenic around roots in a contaminated riparian floodplain soil: Micro-XRF element mapping and EXAFS spectroscopy

Riparian soils are periodically flooded, leading to temporarily reducing conditions. Diffusion of O₂ through plants into the rhizosphere maintains oxic conditions around roots, thereby promoting trace element fractionation along a redox gradient from the reduced soil matrix towards the oxic rhizosphere. The aim of this study was to determine the distribution and speciation of arsenic around plant roots in a contaminated (170-280 mg/kg As) riparian floodplain soil (gleyic Fluvisol). The analysis of soil thin sections by synchrotron micro-X-ray fluorescence (μ-XRF) spectrometry showed that As and Fe were enriched around roots and that As was closely correlated with Fe. Arsenic contents of three manually separated rhizosphere soil samples from the subsoil were 5-9 times higher than respective bulk As contents. This corresponds to the accumulation of about half of the total As in the subsoil in Fe-enrichments around roots. The speciation of As in the soil was assessed by oxalate extractions at pH 3.0 as well as by X-ray absorption near edge structure (XANES) and extended X-ray fine structure (EXAFS) spectroscopy. More than 77% of the total As was oxalate extractable in all samples. XANES and EXAFS spectra demonstrated that As was predominantly As(V). For the accurate analysis of the EXAFS data with respect to the bonding of As(V) to the Fe- or Al-octahedra of (hydr)oxides and clays, all 3-leg and 4-leg multiple scattering paths within the As^(V)O₄-tetrahedron were considered in a fully constrained fitting scheme. We found that As(V) was predominantly associated with Fe-(hydr)oxides, and that sorption to Al- and Mn-hydroxides was negligible. The accumulation of As in the rhizosphere may affect As uptake by plants. Regarding the mobility of As, our results suggest that by oxygenation of the rhizosphere, plants attenuate the leaching of As from riparian floodplain soils during periods of high groundwater levels or flooding.     

Reversibility of radiocaesium sorption on illite

Adsorption of trace amounts of radiocaesium on NH₄-, K-, and Na-saturated Fithian illite and subsequent desorption by 1 M NH₄ showed that a substantial amount of radiocaesium (44%, 46%, and 91% for NH₄-, K-, and Na-illite, respectively) cannot be desorbed after only 5 min of adsorption. Our results suggest that this instantaneous fixation is caused by the collapse of the frayed edges of the clay mineral and the relatively high concentration of radiocaesium building up in solution in the batch desorption experiments. Consequently, commonly applied high-NH₄ extractions underestimate truly exchangeable amounts of radiocaesium in soils and sediments containing illitic clay minerals. The rate of desorption of trace amounts of radiocaesium from the solids using high NH₄ or Cs concentrations has a half-life of about 2 yr, reflecting radiocaesium desorption from (partially) collapsed interlayers. Extraction of radiocaesium from illite after 5 min of contact time with a Cs-selective adsorbent or a 1 × 10^-6 M CsCl solution shows that 100% of the bound radiocaesium is readily available. The desorption rate in the presence of a Cs-selective adsorbent has a half-life of about 0.2 yr. Desorption of radiocaesium from illite using different ammonium concentrations shows that radiocaesium partitioning follows reversible ion-exchange theory if the NH₄ concentration is below 1 × 10^-4 M, and sufficient time (weeks) is allowed for the reaction to proceed. Thus, radiocaesium sorption reversibility in the natural environment is much higher than generally assumed, and equilibrium solid/liquid partitioning may be assumed for the long-term modelling of radiocaesium mobility in the natural environment. In the particular case of anoxic freshwater sediments with very high NH₄ concentrations in the pore waters (up to several mmol.L^-1), collapse of the frayed edges of illite may occur, influencing radiocaesium partitioning. If collapse occurs before radiocaesium adsorbs to illite, high caesium sorption reversibility as measured by high-NH₄ extraction can be expected because further collapse of the frayed edges during the extraction procedure will be limited. This effect has indeed been observed earlier in the extraction of radiocaesium from anoxic freshwater sediments with high-NH₄ solutions and was as yet unexplained.     

The role of evaporation on the formation of secondary Cu-hydroxy minerals in the arid Namaqualand soil system of South Africa

The soils surrounding the Spektakel mine in the arid Okiep copper district of the Northern Cape Province, South Africa have been exposed to Cu containing acid mine drainage (AMD) for well over a century. This has led to acute soil contamination with Cu concentrations present as high as 10 wt%. Despite the sulphate-rich environment, associated with AMD, the Cu-hydroxy chloride mineral, atacamite [Cu₂(OH)₃Cl] is the exclusive secondary Cu mineral identified in the soils. Brochantite [Cu₄(OH)₆SO₄], the sulphate equivalent was not detected. Evaporation is a major hydrologic process in the arid soil system, thus the chemical evolution of the saline, metal containing brine was determined both experimentally and using PHREEQC modelling in the presence and absence of calcite. In samples that evaporated in the absence of calcite, the pH of the evaporating solution progressively decreased to below 3.5, whereas the solution in contact with calcite retained a circumneutral pH throughout the evaporation process. Thus in the absence of calcite, evaporation alone will not favour the formation of Cu-hydroxy minerals. Chemical activities of the evaporating solutions were modelled using the SIT.dat database of PHREEQC. This allowed the inclusion of aqueous complexes into the activity calculations of the evaporating solutions. Contrary to the conservative molality evolution of sulphate, the evolution of the sulphate activity was substantially suppressed due to the formation of MgSO₄⁰ aqueous complexes. We propose that these MgSO₄⁰ aqueous complexes are responsible for the absence of brochantite whereas the conservative chloride ion is more available for secondary Cu mineral formation. This suggests that ion pairs and aqueous complexes play an important role in determining the speciation of evaporite minerals and should be included in the modelling of such systems.     

High oxidation magmatic evolution in the Naruo porphyry Cu deposit,Tibet, China

The Naruo porphyry Cu deposit is the third largest deposit discovered in the Duolong metallogenic district. Previous research has focused mainly on the geochemistry of the ore-bearing granodiorite porphyry; the metallogenesis remains poorly understood. In the present work, on the basis of outcrops and drilling core geological mapping, phases of early mineralization diorite, two inter-mineralization granodiorite porphyries, and late-mineralization granodiorite porphyry have been distinguished. Furthermore, the alteration zones were outlined, and the vein sequence was identified. The diorite and three porphyry phases were subjected to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (La–ICP–MS) zircon U–Pb dating and in situ Hf isotope analyses as well as bulk major element, trace element, and Sr–Nd isotopic analyses. Molybdenite Re–Os dating was also conducted.The zircon U–Pb dating results show that the diorite and porphyry intrusions were emplaced at about 120 Ma, and the molybdenite Re–Os isochron age is 118.8 ± 1.9 Ma; this indicates that the Naruo porphyry Cu deposit was formed during a continuous magmatic–hydrothermal process. All of the diorite and granodiorite porphyry samples showed arc magmatic characteristics. Moreover, the moderate (⁸⁷Sr/⁸⁶Sr)_i ratios and low ε_Nd(t) and ε_Hf(t) values of the diorite and porphyry intrusions suggest the source region of the juvenile lower crust. The lower (⁸⁷Sr/⁸⁶Sr)_i and (¹⁴³Nd/¹⁴⁴Nd)_i ratios and higher ε_Nd(t) values and incompatible element concentrations than those in the granodiorite porphyry samples indicate a two-stage magmatic generation process for the intrusions. The early mineralization diorite has a high Cu concentration, implying that the source is enriched in Cu. However, the slightly lower Cu content of the late-mineralization granodiorite porphyry samples might imply Cu release from magmas and deposition within the metallogenic stage. The multiple stages of intrusions and subsequent volcanism within the Duolong metallogenic district, together with high Sr/Y features, indicate persistent magmatism during the metallogenic epoch, which is necessary for maintaining the activity of magmatic–hydrothermal and mineralization processes. Thus, the high Cu content in the source region, mantle-derived melt upwelling, and multiple stages of persistent magmatism were favorable for the formation of the Naruo porphyry Cu deposit.The high Fe₂O₃/FeO ratios of the diorite and granodiorite porphyry intrusions show very high oxidation features, which is coincident with estimated magmatic oxidation state calculated by the zircon trace element compositions. The high oxidation facilitates sulfur and chalcophile metals to be scavenged into the magmatic–hydrothermal systems, which is crucial for the metallogenesis of the Naruo porphyry Cu deposit.     

Sulphidizing reactions in the flotation of oxidized copper minerals,II. Role of the adsorption and oxidation of sodium sulphide in the flotation of chrysocolla and malachite

The rate of consumption of sulphide in the sulphidizing reactions of malachite and chrysocolla has been measured. The oxidation of sulphide ions at the surface of sulphidized chrysocolla was shown to take place. The influence of thiosulphate anions on the xanthate flotation of sulphidized malachite and chrysocolla was investigated and it was shown to depress the flotation of chrysocolla strongly.The result suggest, that the presence of thiosulphate as a product of simultaneous oxidation can be one of the reasons for the more difficult flotation of sulphidized chrysocolla.     

Nitrogen,phosphorus and sulfur mineralization as affected by soil depth in rangeland ecosystems

The potential mineralization and immobilization of soil nitrogen (N), phosphorus (P) and sulfur (S) are relatively high in natural ecosystems. This study was conducted to investigate the changes in essential plant macronutrients; N, P, and S status in response to different soil depth in rangeland ecosystems in vitro. The net nutrient mineralization was measured during 90 days at different depths (0–15, 15–30, 30–45 and 45–60 cm), using kinetic models to estimate the release rate. The net ammonification and mineralization of P and S were described using parabolic diffusion equation, while the power function equation was used to describe the net nitrification. The results indicated that the amount of released ammonium (NH₄ ⁺) decreased with time and depth and the rates of net ammonification were negative in all samples. Conversely, nitrification increased with time and depth and the rates were all positive. The net mineralization for both P and S reduced with time. The concentration of mineralized SO₄ ^2? increased with depth like nitrate (NO₃ ^?). Accumulation of SO₄ ^2? and NO₃ ^? in subsurface soils and NH₄ ⁺ and P at surface horizons can increase the potential of their loss by leaching or volatilization.