This study deals with the experimental determination of the rate of weathering and As release from Au-bearing sulfide ores of the Mokrsko-West deposit (Czech Republic). The elevated As contents in the ore and the possible As mobilisation must be taken into account due to the close proximity of the deposit to the Vltava River, a source of drinking water for Prague. Crushed and homogenized ore material was divided into four grain-size fractions showing uniform chemical and mineralogical composition and subjected to batch and column leaching experiments. The most important reactions controlling metal leaching include (i) oxidation of sulfides, (ii) carbonate equilibrium and (iii) precipitation of hydrated ferric oxides (HFO). Hydrated ferric oxides were found to be important sorbents of mobilized As. The highest concentration of As in the leachates was obtained from the 64–1000 μm fraction. The oxidation rate in the agitated batch experiment calculated on the basis of sulfate concentrations is an order of magnitude higher (3.8×10−9 to 4.4×10−8 mol SO42− h−1 g−1) than in the static column experiment (9.6×10−10 to 7.4×10−9 mol SO42− h−1 g−1). The higher oxidation rate of batch experiments is explained by abrasion of oxidation products (mainly HFO) and by further exposition of reaction surfaces of the sulfide grains. 相似文献
Volcan Popocatépetl is a Quaternary stratovolcano located 60 km southeast of Mexico City. The summit crater is the site of
recent ash eruptions, excess degassing, and dacite dome growth. The modern cone comprises mainly pyroclastic flow deposits,
airfall tephras, debris flows, and reworked deposits of andesitic composition; it is flanked by more mafic monogenetic vents.
In least-degassed fallout tuffs and mafic scoria, transition metals are concentrated in phases formed before eruption, during
eruption, and after eruption. Preeruptive minerals occur in both lavas and tephra, and include oxides and sulfides in glass
and phenocrysts. The magmatic oxides consist of magnetite, ilmenite, and chromite; the sulfides consist of both (Fe,Ni)1-xS (MSS) and Cu–Fe sulfide (ISS). Syn- and posteruptive phases occur in vesicles in both lavas and tephra, and on surfaces
of ash and along fractures. The mineral assemblages in lavas include Cu–Fe sulfide and Fe–Ti oxide in vesicles, and Fe sulfide
and Cu–Fe sulfide in segregation vesicles. Assemblages in vesicles in scoria include Fe–Ti oxide and rare Fe–Cu–Sn sulfide.
Vesicle fillings of Fe–Ti oxide, Ni-rich chromite, Fe sulfide, Cu sulfide, and barite are common to two pumice samples. The
most coarse-grained of the vesicle fillings are Cu–Fe sulfide and Cu sulfide, which are as large as 50 μ in diameter. The
youngest Plinian pumice also contains Zn(Fe) sulfide, as well as rare Ag–Cu sulfide, Ag–Fe sulfide, Ag bromide, Ag chloride,
and Au–Cu telluride. The assemblage is similar to those typically observed in high-sulfidation epithermal mineralization.
The fine-grained nature and abundance of syn- and/or posteruptive phases in porous rocks makes metals susceptible to mobilization
by percolating fluids. The abundance of metal compounds in vesicles indicates that volatile exsolution prior to and/or during
eruption played an important role in releasing metals to the atmosphere.
Received: March 1997 · Accepted: 27 May 1997 相似文献
A nomogram is developed to show that pH, redox potentials (EhNHE) and measures of dissolved sulfides (H2S + HS− + S2−)(total free S2−) can be used to classify organic enrichment impacts in marine sediments. The biogeochemical cycle of sulfur in marine sediments is described to show that changes in macrobenthic infauna community structure associated with high levels of organic matter supply result from stress due to oxygen deficiency (hypoxia and anoxia) and toxic effects of S2−. The changes reflect enhancement of microbial sulfate reduction under conditions of high organic matter sedimentation and the progressive formation of hypoxic–anoxic conditions measured by decreased EhNHE and increased concentrations of S2−. The nomogram provides a basis for classification of the oxic status of marine sediments based on changes in inter-related biological and biogeochemical variables along an organic enrichment gradient. 相似文献
Road construction through sulfidic materials in Virginia has resulted in localized acid rock drainage (ARD) that threatens water quality, sedimentation, integrity of building materials, and vegetation management. Geologic formations associated with acid roadcuts were characterized by potential peroxide acidity (PPA), expressed as calcium carbonate equivalence (CCE), and total sulfur (total-S) in order to develop a statewide sulfide hazard rating map. The geologic formations were grouped into four categories based on potential acid-producing severity: i) Tabb formation (PPA<6 Mg CCE/1000 Mg; S<0.2%); ii) Ashe formation (PPA<18 Mg CCE/1000 Mg; S<2.0%); iii) Chesapeake Group, Lower Tertiary deposits, Millboro shale, Marcellus shale, and Needmore Formation (PPA<60 Mg CCE/1000 Mg; S<2.6%), and; iv) Chattanooga shale and Quantico slate (PPA<99 Mg CCE/1000 Mg; S<3.9%). Sulfide hazard analysis should be an essential step in the pre-design phase of highway construction and other earth-disturbing activities. 相似文献
Ab initio quantum-chemical calculations of the spatial and electronic structures of sphalerite (ZnS), pyrite (FeS2) and galena (PbS), using the density functional theory (DFT) local density approximation (LDA) and generalized gradient approximation (GGA), the Hartree–Fock (HF) method and the hybrid functional B3LYP, have been carried out. For galena, the DFT LDA and GGA functionals provided the best estimate of the band gap, from within –0.1 eV to +0.4 eV of the measured value. B3LYP and RHF gave rise to errors of +1.3 and +5.4 eV, respectively. The unit cell parameter error varied from between –1.1% and +2.3% for all the functionals examined. For sphalerite the B3LYP functional provided the best estimate of the band gap (error +0.3 eV). The unit cell parameter error varied between –2.1% and +2.0% for the various DFT functionals and B3LYP. RHF gave rise to an error of +3.8%. For FeS2, the DFT-GGA approach provides the best results for both the unit cell and the band gap. This may be due to mutual cancellation of the crystal field splitting and band separation force, which are of equal but opposite magnitudes. The calculated density of states (DOS) for the conduction band is used to interpret the experimental features of the S 1s XANES (X-ray absorption near-edge structure) spectra obtained using synchrotron radiation. Because of the l = ±1 selection rule for electron excitation, the S K-edge XANES spectra represent a transition of the S 1s electron to conduction band S p-like orbitals. The near-edge region, up to 15 eV past the edge is approximated well by the DOS. Individual peaks in the DOS correlate with peaks in the XANES spectra. In addition, the imaginary part of the dielectric function, which reflects the transitions from occupied to unoccupied levels, is used to model the near-edge region of the XANES, using the DFT-GGA formalism. Individual peaks in the XANES spectrum are moderately well resolved using the dielectric function, especially for ZnS and FeS2, while the DOS for the conduction band is more successful in predicting the shape of the XANES spectra for all three minerals. 相似文献
We have examined Re, Platinum-Group Element (PGE) and Os-isotope variations in suites of variably fractionated lavas from Kohala Volcano, Hawaii, in order to evaluate the effects of melt/crust interaction on the mantle isotopic signature of these lavas. This study reveals that the behavior of Os and other PGEs changes during magma differentiation. The concentrations of all PGEs strongly decrease with increasing fractionation for melts with MgO < 8 wt.%. Fractionation trends indicate significantly higher bulk partition coefficients for PGEs in lavas with less than 8 wt.% MgO (DPGE = 35–60) when compared to values for more primitive lavas with MgO > 8 wt.% (DPGE ≤ 6). This sudden change in PGE behavior most likely reflects the onset of sulfur saturation and sulfide fractionation in Hawaiian magmas at about 8 wt.% MgO.
The Os-rich primitive lavas (≥ 8 wt.% MgO, > 0.1 ppb Os) display a narrow range of 187Os/188Os values (0.130–0.133), which are similar to values in high-MgO lavas from Mauna Kea and Haleakala Volcanoes and likely represent the mantle signature of Kohala lavas. However, Os-isotopic ratios become more radiogenic with decreasing MgO and Os content in evolved lavas, ranging from 0.130 to 0.196 in the shield-stage Pololu basalts and from 0.131 to 0.223 in the post-shield Hawi lavas. This reflects assimilation of local oceanic crust material during fractional crystallization of the magma at shallow level (AFC processes). AFC modeling suggests that assimilation of up to 10% upper oceanic crust could produce the most radiogenic Os-isotope ratios recorded in the Pololu lavas. This amount of upper crust assimilation has a negligible effect on the Sr and Nd-isotopic compositions of Kohala lavas. Thus, these isotopic compositions likely represent the composition of the mantle source of Kohala lavas. 相似文献
A study of deep penetrating geochemistry-the surface exploration techniques for finding concealed mineral deposits-was carried out over the Spence porphyry copper deposit in Chile by Australian,Canadian and Chinese laboratories using selective weak leach methods and is briefly reviewed here.Erratic and weak copper anomalies were obtained above the Spence deposit,which is concealed below a thick piedmont gravel cover.In the literature,such patterns are common in most of the weak selective leach studies carried out over known buried deposits.During exploration in unknown area,such weak leach techniques lead to some success and many dry holes.In this paper,stronger selective leaching methods are developed for oxides and sulfides,which may be the stable end products transformed and accumulated by a mobile precursor phase carried upward through the overburden by gas or water through long spans of geological time.The data obtained by the methods developed in this paper produced more significant and consistent anomalous data than previous efforts.Consequently,the leach approaches described here may increase the effectiveness of deep penetrating geochemistry in exploration for concealed ore deposits. 相似文献