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1.
The aim of this study is to discriminate the geochemical anomalies in the Zarshuran district, NW Iran, using different geochemical methods and present a more useful method where anomalous areas better coincide with the geological features. For this methods of delineation, geochemical anomalies were compared using geological features, occupied area of anomalies respect to the total study area, and field observations. Frequency based analysis such as mean + 2SDEV and median + 2MAD and concentration–area (C–A) multifractal methods were adopted for estimating thresholds and separating geochemical anomalies in uni-element data, as well as multi-element ones. Threshold values obtained from mean + 2SDEV and median + 2MAD, from original point geochemical data, are smaller than those of the pixel values; this may be due to the stronger variance of pixel values. In addition, the C–A multifractal method, as a useful tool to identify weak geochemical anomalies, was applied for defining the threshold values. Robust principal component analysis (RPCA) methods coupled with isometric log-ratio (ilr) transformations were utilized to open the geochemical data in order to reduce the effects of the data closure problem. The 20-quantile intervals decomposed anomaly maps from PC1 were obtained from the classical PCA, robust PCA showed that the upper quintile (>80 quintile) of classical PCA covers a larger area (32.54%) than the robust PCA (18.16%), and as a result, the robust PCA displayed smaller areas and has good spatial associations with outcrops of hydrothermal Au–As mineralization in this area; coincident with the known Zarshuran former mining area (ore field), Zarshuran unit, Ghaldagh silicified limestone occurrence and newly explored works confirmed by field observation. Although the C–A model shows a smaller area (8.06%), this anomaly location is limited to the Zarshuran old mining area with no new exploration targets. Comparison of the models indicates that the RPCA model is not only beneficial to further Au exploration in the study area, but also provides a meaningful geological study to the community of the compositional data analysis.  相似文献   

2.
Porphyry and skarn Cu–Fe–Au–Mo deposits are widespread in the Middle and Lower Yangtze River metallogenic belt (MLYMB), eastern China. The Matou deposit has long been regarded as a typical Cu–Mo porphyry deposit within Lower Yangtze part of the belt. Recently, we identified scheelite and wolframite in quartz veins in the Matou deposit, which is uncommon in other porphyry and skarn deposits in the MLYMB. We carried out detailed zircon U–Pb dating and geochemical and Sr–Nd–Hf isotopic studies of the granodiorite porphyry at Matou to define any differences from other ore-related granitoids. The porphyry shows a SiO2 content ranging from 61.85 wt.% to 65.74 wt.%, K2O from 1.99 wt.% to 3.74 wt.%, and MgO from 1.74 wt.% to 2.19 wt.% (Mg# value ranging from 45 to 55). It is enriched in light rare earth elements and large ion lithophile elements, but relatively depleted in Nb, Ta, Y, Yb and compatible trace elements (such as Cr, Ni, and V), with slight negative Eu anomalies (Eu/Eu* = 0.88–0.98) and almost no negative Sr anomalies. Results of electron microprobe analysis of rock-forming silicate minerals indicate that the Matou porphyry has been altered by an oxidized fluid that is rich in Mg, Cl, and K. The samples show relatively low εNd(t) values from −7.4 to −7.1, slightly high initial 87Sr/86Sr values from 0.708223 to 0.709088, and low εHf(t) values of zircon from −9.0 to −6.5, when compared with the other Cu–Mo porphyry deposits in the MLYMB. Zircon U–Pb dating suggests the Matou granodiorite porphyry was emplaced at 139.5 ± 1.5 Ma (MSWD = 1.8, n = 15), which is within the age range of the other porphyries in the MLYMB. Although geochemical characteristics of the Matou and other porphyries in the MLYMB are similar and all adakitic, the detrital zircons in the samples from Matou suggest that Archean lower crust (2543 ± 29 Ma, MSWD = 0.25, n = 5) was involved with the generation of Matou magma, which is different from the other porphyries in the belt. Our study suggests that the Matou granodiorite porphyry originated from partial melting of thickened lower crust that was delaminated into the mantle, similar to the other porphyries in the MLYMB, but it has a higher proportion of lower crustal material, including Archean rocks, which contributed to the formation of the porphyry and related W-rich magmatic-hydrothermal system.  相似文献   

3.
In recent years mineral exploration has concentrated on concealed deposits in regolith-covered terrains. In China, the regolith-covered landscapes mainly include desert windblown sand basins, desert peneplains, semi-arid grassland, loess plateaus, forestry land, alluvial plains and laterite terrains. These diverse regolith-covered areas represent geochemical challenges for mineral exploration in China. This paper provides an overview of recent progress on mechanisms of metal dispersion from the buried ore deposits through the transported cover to the surface and penetrating geochemical methods to detect the anomalies. Case studies show that, in arid and semi-arid desert sand-covered terrains, sampling of fine-fraction (− 120 mesh, < 0.125 mm) clay-rich horizon soil is cost-effective for regional geochemical surveys for sandstone-type uranium, gold, and base metal deposits. Fine-fraction sampling, selective-leaching and overburden drilling geochemical methods can effectively indicate the 210 gold ore body at Jinwozi goldfield. In alluvium-covered terrains, fine-grained soil sampling (− 200 mesh, < 0.074 mm) combined with selective leaching geochemistry shows clear ring-shaped anomalies of Cu and Ni over the Zhouan concealed Cu–Ni deposit. In laterite-covered terrains, the anomalies determined by the fine-fraction soils and selective leaching of absorbed metals on coatings of Fe–Mn oxides coincide well with the concealed deposit over the Yueyang ore deposits at the Zijin Au–Cu–Ag field. Nanoparticles of hexagonal crystals mainly native copper, gold and alloys of Cu–Fe, Cu–Fe–Mn, Cu–Ti, and Cu–Au were observed in gases, soils and ores using a transmission electron microscope (TEM). The findings imply that nanoparticles of gold and copper may migrate through the transported cover to the surface. Uranium is converted to uranyl ions [UO22 +] under oxidizing conditions when migrating from ore bodies to the surface. The uranyl ions are absorbed on clay minerals, because clay layers have a net negative charge, which needs to be balanced by interlayer cations. Nanoparticles of Au and Cu and ion complexes of U are more readily absorbed onto fine fractions of soils containing clays, colloids, oxides and organic matters. Thus, fine-grained soils enriched with clays, oxides and colloids are useful media for regional geochemical surveys in regolith-covered terrains and in sedimentary basins. Fine-fraction soil sampling combined with selective leaching geochemistry is effective for finding concealed ore bodies in detailed surveys. Penetrating geochemistry at surface sampling provides cost-effective mineral exploration methods for delineation of regional and local targets in transported cover terrains.  相似文献   

4.
Results are presented of a surficial geomicrobiological investigation of glacial cover overlying buried mineralization at the Talbot prospect, Manitoba, Canada, where previous surficial geochemistry surveys indicated anomalous concentrations of elements above the buried mineralization. The Cu–Zn volcanogenic massive sulfide (VMS) occurrence is overlain by 100 m of Paleozoic dolomites and Quaternary glacial cover. The geomicrobiological investigation demonstrates that there is a distinct microbial ecology at the anomalous sampling locations, especially directly overlying buried mineralization. The combined geochemical and geomicrobiological analyses reveal the presence of an anomaly directly over mineralization due to oxidation of the buried ore. Specifically, geomicrobiological analyses yield an inverse correlation between Zn in the clay-size (<2 μm) fraction and total microbial biomass and a direct correlation between Cu in the clay-size (<2 μm) fraction and abundance of methanotrophic bacteria. These results demonstrate that microbiological analyses can be a useful addition to geochemical exploration by revealing metal transport and sequestration processes and enhancing surficial anomalies.  相似文献   

5.
The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni–Mo–PGE–Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy δ98/95Mo values of 0.94 to 1.38‰ (avg. 1.13 ± 0.14‰, 1σ, n = 11), and the host black shale and phosphorite have slightly more variable δ98/95Mo values of 0.81‰ to 1.70‰ (n = 14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 107 compared to present-day seawater. PAAS-normalized REE + Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni–Mo–PGE–Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts.  相似文献   

6.
Geochemical anomaly separation using the concentration–area (C–A) method at Kahang (Gor Gor) porphyry system in Central Iran is studied in this work. Lithogeochemical data sets were used in this geochemical survey which was conducted for the exploration for Cu mineralization in dioritic and andesitic units at Kahang Cu–Mo porphyry system. Similar surveys were also carried out for Mo and Au exploration in these rock units. The obtained results have been interpreted using rather extensive set of information available for each mineralized area, consists of detailed geological mapping, structural interpretation and alteration data. Anomalous threshold values for the mineralized zone were computed and compared with the statistical methods based on the data obtained from chemical analysis of samples for the lithological units. Several anomalies at a local scale were identified for Cu (224 ppm), Mo (63 ppm), and Au (31 ppb), and the obtained results suggests existence of local Cu anomalies whose magnitude generally is above 1000 ppm. The correlation between these threshold values and ore grades is clearly interpreted in this investigation. Also, the log–log plots show existence of three stages of Cu enrichment, and two enrichment stages for Mo and Au. The third and most important mineralization event is responsible for the presence of Cu at grades above 1995 ppm. The identified anomalies in Kahang porphyry system, and distribution of the rock types, are mainly monzodiorite and andesitic units, do have special correlation with Cu and monzonitic and dioritic rocks, especially monzodioritic type, which is of considerable emphasis. The threshold values obtained for each element are always lower than their mean content in the rocks. The study shows threshold values for Cu is clearly above the mean rock content, being a consequence of the occurrence of anomalous accumulations of phyllic, argillic and propyllitic alterations within the monzonitic and dioritic rocks especially in monzodioritic type. The obtained results were compared with fault distribution patterns which reveal a positive direct correlation between mineralization in anomalous areas and the faults present in the mineralized system.  相似文献   

7.
The newly discovered Fuxing porphyry Cu deposit is located in the Dananhu–Tousuquan arc, adjacent to the Tuwu–Yandong Cu deposits of Eastern Tianshan, in the southern Central Asian Orogenic Belt. The Fuxing deposit is hosted by volcanic rocks (basalt and dacite) in the Early Carboniferous Qi'eshan Group and Carboniferous felsic intrusions (plagiogranite porphyry, monzogranite, and quartz diorite). New SIMS zircon U–Pb dating indicates that the plagiogranite porphyry and monzogranite emplaced at 332.1 ± 2.2 Ma and 328.4 ± 3.4 Ma, respectively. The basalts are characterized by low SiO2 contents (47.47–54.90 wt.%), a lack of Eu anomalies, strong depletion of Na, Ta, and Ti elements but positive Sr, U, and Pb anomalies, high Y (20.8–28.2 ppm) and HREE concentrations (Yb = 2.23–3.06 ppm), and relatively low (La/Yb)N (2.20–3.92) values; the dacite samples have high SiO2 contents (66.13–76.93 wt.%), clearly negative Eu anomalies, high Mg# values (36–51), and high Y (41.8–54.9 ppm) and Yb (5.76–8.98 ppm) concentrations. The basalts and dacites exhibit similar signatures as normal arc rocks, and were considered to be derived from partial melting of mantle-wedge peridotite that was previously metasomatized by slab melts. In contrast, the plagiogranite porphyry, monzogranite, and quartz diorite show the same geochemical affinity with modern adakites, which are characterized by high SiO2 contents (67.55–79.00 wt.%), minor negative to positive Eu anomalies, strong depletion of heavy rare earth elements (Yb = 0.17–1.19 ppm) and Y (1.86–10.1 ppm), positive K, Rb, Sr, and Ba but negative Nb, Ta, Th, and Ti anomalies, and high (La/Yb)N ratios and Mg# values. Moreover, these adakitic felsic intrusions display relatively high positive zircon εHf(t) values (+ 11.4 to + 18.3), low 87Sr/86Sr (0.706080–0.711239), high 143Nd/144Nd (0.512692–0.512922) ratios, and consistent zircon δ18O values (4.41‰–5.48‰), suggesting that their parental magma were most likely derived from partial melting of the subducted oceanic crust followed by mantle peridotite interaction. Based on the whole-rock geochemical and Sr–Nd–Hf–O isotopic data, as well as detailed petrographic analyses, we further suggest that the Fuxing igneous rocks and associated porphyry Cu mineralization were generated by the northward subduction of the paleo-Tianshan oceanic plate beneath the Dananhu–Tousuquan island arc during the Early Carboniferous.  相似文献   

8.
Biomarker and n-alkane compound specific stable carbon isotope analyses (CSIA) were carried out on 58 crude oil samples from shallow water and deepwater fields of the Niger Delta in order to predict the depositional environment and organic matter characteristics of their potential source rocks. Using a source organofacies prediction approach from oil geochemistry, the presence in the western deepwater oils relatively abundant C27 steranes, C30 24-n-propyl cholestane, low oleanane index, relatively low pr/ph ratios, gammacerane, and positive to nearly flat C12–C30 n-alkane compound specific stable carbon isotope profiles, suggests that the source facies that expelled these oils contain significant marine derived organic matter deposited under sub-oxic and stratified water column conditions. This contrasts with the terrigenous organic matter dominated source rocks accepted for shallow water Niger Delta oils. Oils in the shallow water accumulations can be separated into terrigenous and mixed marine-terrigenous families. The terrigenous family indicates expulsion from source rock(s) containing overwhelmingly higher plant source organic matter (average oleanane index = 0.48, high C29 steranes) as well as having negative sloping n-alkane isotope profiles. Oxic source depositional conditions (pr/ph > 2.5) and non-stratified conditions (absence to low gammacerane content) are inferred for the terrigenous family. The mixed marine-terrigenous family has biomarker properties that are a combination of the deepwater and terrigenous shallow water oils. Bitumen extracts of the sub-delta Late Cretaceous Araromi Formation shale in the Dahomey Basin are comparable both molecularly and isotopically to the studied western deepwater oil set, but with an over all poor geochemical correlation. This poor geochemical match between Araromi shale and the western deepwater oils does not downgrade the potential of sub-delta Cretaceous source rock contribution to the regional oil charge in the deepwater Niger Delta.  相似文献   

9.
Porphyry Cu deposits occurred in the southern West Junggar of Xinjiang, NW China and are represented by the Baogutu and newly-discovered Jiamantieliek porphyry Cu deposits. Petrographical and geochemical studies show that both Jiamantieliek and Baogutu ore-bearing intrusions comprise main-stage diorite stock and minor late-stage diorite porphyry dikes and are the calc-alkaline intermediate intrusions. Based on U–Pb zircon SHRIMP analyses, the Jiamantieliek intrusion formed in 313 ± 4 Ma and 310 ± 5 Ma, while, based on U–Pb zircon SIMS analyses, the Baogutu intrusion formed in 313 ± 2 Ma and 312 ± 2 Ma. Rocks in the Jiamantieliek intrusion are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) with negative Nb anomaly. Their isotopic compositions (εNd(t) = +1.6 to +3.4, (87Sr/86Sr)i = 0.70369–0.70401, (207Pb/204Pb)i = 15.31–5.41) suggest a mixing origin from depleted to enriched mantle sources. In the Baogutu intrusion, the rocks are similar to those of the Jiamantieliek intrusion. Their Sr-Nd-Pb isotopic composition (εNd(t) = +4.4 to +6.0, (87Sr/86Sr)i = 0.70368–0.70385, (207Pb/204Pb)i = 15.34–5.42) shows a more depleted mantle source. These features suggest generation in an island arc. The Jiamantieliek and Baogutu intrusions have similar characteristics, indicating that a relatively uniform and integrated source region has existed in the southern West Junggar since the Palaeozoic. A larger contribution of calc-alkaline magma would be required to generate the Jiamantieliek intrusion, which may reflect the development of magma arc maturation towards the western section of the southern West Junggar.  相似文献   

10.
A highly efficient, low-density sampling strategy was employed to study the geochemical expression of geological bodies and the mineral potential on the county scale in Central Norway. Soil O and C horizon samples (N = 752) were collected in Nord-Trøndelag and parts of Sør-Trøndelag, and analysed for 53 chemical elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Pd, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr) and Pb isotopes in an aqua regia extraction. At the sample density of one site/36 km2 the four metal deposits, which have been mined in the area within the last 50 years were all detected as geochemical anomalies. In addition, a number of new anomalies that may warrant follow-up surveys were found. In terms of geology the Grong–Olden Culmination is marked by a distinct 206Pb/207Pb isotope anomaly. Geochemical differences distinguish the most important belts of mafic metavolcanic lithologies in the area. Though the Fosdalen iron ore deposit is only marked in the soil O horizon, the C horizon outlines the more prominent anomalies of possibly economic interest. Climatic factors like the input of marine aerosols along the coast are clearly visible in the soil O horizon. Low-density geochemical mapping of two sample materials provides important complementing information for the interpretation of the geochemical variation in Nord-Trøndelag county.  相似文献   

11.
《Applied Geochemistry》2005,20(11):2017-2037
The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH4, thermogenic C2+ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH4 is bacteriogenic (Calculated δ13CC1–C = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH4. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ13CC1–C = −35.80‰; Hamitabat Field) and Group-3 (δ13C1–C = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C2+ gases. The Group-2 C2+ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ13C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C2+ gases, produced from one field, were generated from isotopically light marine kerogen (δ13C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C2+ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ13C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (Req = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.  相似文献   

12.
The stable carbon isotopic compositions of light hydrocarbon gases adsorbed in near-surface soil and sediments from the Saurashtra basin were characterized for their origin and maturity. Saurashtra is considered geologically prospective for oil and gas reserves; however, a major part of the basin is covered by the Deccan Traps, hindering the exploration of Mesozoic hydrocarbon targets. Surface geochemical prospecting, based on micro-seepage of hydrocarbons from subsurface accumulations, could be advantageous in such areas. In light of this, 150 soil samples were collected from the northwestern part of Saurashtra, around the Jamnagar area, where a thick sedimentary sequence of about 2–3 km exists under 1–1.5 km of Deccan basalt. The concentration of acid desorbed alkane gases from soil samples was found to vary (in ppb) as: methane (C1) = 3–518; ethane (C2) = 0–430; propane (C3) = 0–331; i-butane (iC4) = 0–297; n-butane (nC4) = 2–116; i-pentane (iC5) = 0–31 and n-pentane (nC5) = 0–23, respectively.Fifteen samples with high concentrations of alkane gases were measured for their δ13C1; δ13C2 and δ13C3 compositions using gas chromatography–combustion-isotope ratio mass spectrometry (GC–C-IRMS). The values for methane varied from ? 27 to ? 45.4‰, ethane from ? 20.9 to ? 27.6‰, and propane from ? 20.4 to ? 29.1‰ versus the Vienna PeeDee Belemnite (VPDB). The carbon isotope ratio distribution pattern represents isotopic characteristics pertaining to hydrocarbon gases derived from thermogenic sources. Comparisons of carbon isotopic signatures and compositional variations with the standard carbon isotopic models suggest that hydrocarbon gases found in the shallow depths of the study area are not of bacterial origin but are formed thermally from deeply buried organic matter, likely to be mainly a terrestrial source rock with a partial contribution from a marine source. These gases may have migrated to the near-surface environment, where they represent an admixture of thermally generated hydrocarbon gases from mixed sources and maturity. The maturity scale (δ13C versus Log Ro %) applied to the surface sediment samples of the Jamnagar area indicated the source material to be capable of generating oil and gas. The detection of thermogenic alkane gases in near-surface sediments offers the possibility of hydrocarbons at depth in Saurashtra.  相似文献   

13.
Decomposing mixed geochemical patterns is a challenge in geochemical exploration and environmental assessment. In this paper, the spectrum–area technique (S–A) is used to decompose a mixed pattern of arsenic in Gangdese belt based on stream sediment data. S–A is a multifractal model based on power–law relationships between area of the set consisting of wave numbers with spectral energy density above S[A(>S)] on the 2D frequency domain. The original spatial distribution map of arsenic obtained by inverse distance weighted (IDW) shows a mixed pattern due to superposition of different geological processes or events and is converted into the frequency domain by means of Fourier transformation. Two components, including power spectrum density and phases, are obtained. The spectrum energy density (S) and the area (A) enclosed by the above-threshold spectrum energy density is plotted on a log–log scale. Two cutoff values determined by three straight lines define three filters which decompose the original map of arsenic into background, anomalous, and high frequency (noise) components. The areas with high anomaly of arsenic mainly are located surrounding known Cu deposits, indicating that arsenic anomalies may be related to Cu mineralization.  相似文献   

14.
The abandoned Sn-W Ribeira mine, northeast of Portugal, contained quartz veins with cassiterite, wolframite, scheelite, pyrite, arsenopyrite, sphalerite, chalcopyrite, manganocolumbite, bismuthinite, native bismuth, phosphates and carbonates. The exploration took place on the northern slope of the Viveiros stream, which is an affluent of the Sabor River. The waste-rock dumps and tailings were deposited on the hillside, close to the mine and are nowadays exposed to significant weathering and erosion, as they are not vegetated. The eroded material is transported by the Viveiros stream toward the Sabor River. A seasonal stream drains the tailings. The stream sediments samples were collected along the Viveiros stream, in the seasonal stream, in a seasonal spring at the bottom of the tailings, in the Sabor River and in other streams not affected by mine workings, following the mine influence along the Viveiros stream and in the Sabor River (1.2 km away from the mine workings). The data show that the degree of pollution increases along the Viveiros stream, especially in winter. The highest degree of pollution is for As, In, W, Sn and Bi. The sediments from the drainage of the main tailings are particularly polluted during winter, by Bi, In and Sn. The sedimentary precipitate from the spring is polluted in Cu, As, In, Sn, Ta, W, Bi, Zn, Nb, Ag, Sb and Ta. The sediments from the Sabor River are significantly polluted by As, Ag, In, Sn, W and Bi. The sediments from the regional streams, Viveiros stream and Sabor River have similar REE (NASC normalized) patterns (ΣREE = 131.7–185.9 mg/kg, LaN/LuN = 1.23–1.42 and Eu/Eu* = 1.02), while those from the seasonal stream, crossing the main tailings, are enriched in REE (ΣREE = 250.3–283.6 mg/kg, LaN/LuN = 1.6–2.09 and Eu/Eu* = 0.96). The general decrease in LaN/LuN values with increase in total Fe2O3 can be explained by the partitioning of HREE to the solid Fe-oxides phase. The sedimentary precipitate and coatings, which are mainly formed by Fe-oxy-hydroxides, but also contain jarosite, are impoverished in all REE. The impoverishment can be explained by the release of REE from the surface of the Fe-oxy-hydroxides, which occurs due to a local lowering of pH, caused by jarosite dissolution. During successive alternate cycles of wet and dry conditions, takes place the formation of Fe-oxy-hydroxides and jarosite in the sedimentary precipitate and coatings. The subsequent dissolution of jarosite releases acidity, thus promoting de-sorption of REE from the Fe-oxy-hydroxides mineral phases.  相似文献   

15.
To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(87Sr/86Sr)i = 0.70722–0.71145], slightly negative εNd, positive εHf [(εNd)i = −2.7 to 0.0; (εHf)i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 18.20–19.19; (207Pb/204Pb)i = 15.60–15.77; (208Pb/204Pb)i = 38.38–39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(87Sr/86Sr)i = 0.70576–0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(εNd)i = −14.0 to −1.4; (εHf)i = −17.9 to +3.7; (206Pb/204Pb)i = 17.83–18.25; (207Pb/204Pb)i = 15.57–15.63; (208Pb/204Pb)i = 38.20–38.70]. The “crust-like” signatures, such as negative Nb–Ta anomalies, elevated Sr isotopic compositions, and negative εNd(t) and εHf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered eclogite. Basaltic volcanism in the Yeongyang sub-basin is coeval with the basaltic trachyandesite magmatism, but it exhibits an elevated 87Sr/86Sr ratio at a given 143Nd/144Nd and highly radiogenic Pb isotopic compositions, which imply an origin from an enriched but heterogeneous lithospheric mantle source. Melts from subducted altered oceanic basalt and pelagic sediments are considered to be the most likely source for the metasomatism. An extensional tectonic regime induced by highly oblique subduction of the Izanagi Plate beneath the eastern Asian margin during the Early Cretaceous might have triggered the opening of the Gyeongsang Basin. Lithospheric thinning and the resultant thermal effect of asthenospheric upwelling could have caused melting of the metasomatized lithospheric mantle, producing the Early Cretaceous basaltic volcanism in the Gyeongsang Basin.  相似文献   

16.
Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite of alkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising their petrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon 206Pb/238U age of 88.1 ± 1.1 Ma (n = 8), whereas two phlogopites studied by 40Ar/39Ar dating methods give emplacement ages of 85.25 ± 0.57 Ma and 87.51 ± 0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these Late Mesozoic ultramafic dykes belong to the alkaline magma series due to their high K2O (3.31–5.04 wt.%) contents. The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements (LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative to primitive mantle, low initial 87Sr/86Sr ratios (0.7060–0.7063) and positive εNd(t) values (0.3–0.4). Such features suggest derivation from low degree (< 1%) partial melting of depleted asthenospheric mantle (garnet-lherzolite), and contamination to various degrees (～ 10%) by interaction with upper crustal materials.  相似文献   

17.
《Chemical Geology》2007,236(1-2):112-133
The Cida A-type granitic stock (∼ 4 km2) and Ailanghe I-type granite batholith (∼ 100 km2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (εNd(t) = − 0.25 to + 1.35 and (87Sr/86Sr)i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic εNd(t) ratios (− 6.34 to − 6.26) and high (87Sr/86Sr)i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks.In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.  相似文献   

18.
The Huangshaping polymetallic deposit is located in southeastern Hunan Province, China. It is a world-class W–Mo–Pb–Zn–Cu skarn deposit in the Nanling Range Metallogenic Belt, with estimated reserves of 74.31 Mt of W–Mo ore at 0.28% WO3 and 0.07% Mo, 22.43 Mt of Pb–Zn ore at 3.6% Pb and 8.00% Zn, and 20.35 Mt of Cu ore at 1.12% Cu. The ore district is predominantly underlained by carbonate formations of the Lower Carboniferous period, with stocks of quartz porphyry, granite porphyry, and granophyre. Skarns occurred in contact zones between stocks and their carbonate wall rocks, which are spatially associated with the above-mentioned three types of ores (i.e., W–Mo, Pb–Zn, and Cu ores).Three types of fluid inclusions have been identified in the ores of the Huangshaping deposit: aqueous liquid–vapor inclusions (Type I), daughter-mineral-bearing aqueous inclusions (Type II), and H2O–CO2 inclusions (Type III). Systematic microthermometrical, laser Raman spectroscopic, and salinity analyses indicate that high-temperature and high-salinity immiscible magmatic fluid is responsible for the W–Mo mineralization, whereas low-temperature and low-salinity magmatic-meteoric mixed fluid is responsible for the subsequent Pb–Zn mineralization. Another magmatic fluid derived from deep-rooted magma is responsible for Cu mineralization.Chondrite-normalized rare earth element patterns and trace element features of calcites from W–Mo, Pb–Zn, and Cu ores are different from one another. Calcite from Cu ores is rich in heavy rare earth elements (187.4–190.5 ppm), Na (0.17%–0.19%), Bi (1.96–64.60 ppm), Y (113–135 ppm), and As (9.1–29.7 ppm), whereas calcite from W–Mo and Pb–Zn ores is rich in Mn (> 10.000 ppm) and Sr (178–248 ppm) with higher Sr/Y ratios (53.94–72.94). δ18O values also differ between W–Mo/Pb–Zn ores (δ18O = 8.10‰–8.41‰) and Cu ores (δ18O = 4.34‰–4.96‰), indicating that two sources of fluids were, respectively, involved in the W–Mo, Pb–Zn, and Cu mineralization.Sulfur isotopes from sulfides also reveal that the large variation (4‰–19‰) within the Huangshaping deposit is likely due to a magmatic sulfur source with a contribution of reduced sulfate sulfur host in the Carboniferous limestone/dolomite and more magmatic sulfur involved in the Cu mineralization than that in W–Mo and Pb–Zn mineralization. The lead isotopic data for sulfide (galena: 206Pb/204Pb = 18.48–19.19, 207/204Pb = 15.45–15.91, 208/204Pb = 38.95–39.78; sphalerite: 206Pb/204Pb = 18.54–19.03, 207/204Pb = 15.60–16.28, 208/204Pb = 38.62–40.27; molybdenite: 206Pb/204Pb = 18.45–19.21, 207/204Pb = 15.53–15.95, 208/204Pb = 38.77–39.58 chalcopyrite: 206Pb/204Pb = 18.67–19.38, 207/204Pb = 15.76–19.90, and 208/204Pb = 39.13–39.56) and oxide (scheelite: 206Pb/204Pb = 18.57–19.46, 207/204Pb = 15.71–15.77, 208/204Pb = 38.95–39.13) are different from those of the wall rock limestone (206Pb/204Pb = 18.34–18.60, 207/204Pb = 15.49–15.69, 208/204Pb = 38.57–38.88) and porphyries (206Pb/204Pb = 17.88–18.66, 207/204Pb = 15.59–15.69, 208/204Pb = 38.22–38.83), suggesting Pb206-, U238-, and Th 232-rich material are involved in the mineralization. The Sm–Nd isotopes of scheelite (εNd(t) = − 6.1 to − 2.9), garnet (εNd(t) = − 6.8 to − 6.1), and calcite (εNd(t) = − 6.3) from W–Mo ores as well as calcite (εNd(t) = − 5.4 to − 5.3) and scheelite (εNd(t) = − 2.9) from the Cu ores demonstrate suggest more mantle-derived materials involved in the Cu mineralization.In the present study we conclude that two sources of ore-forming fluids were involved in production of the Huangshaping W–Mo–Pb–Zn–Cu deposit. One is associated with the granite porphyry magmas responsible for the W–Mo and then Pb–Zn mineralization during which its fluid evolved from magmatic immiscible to a magmatic–meteoritic mixing, and the other is derived from deep-rooted magma, which is related to Cu-related mineralization.  相似文献   

19.
The Storø greenstone belt, southern West Greenland, consists of thrust-imbricated slices of Mesoarchean (>3060 Ma) and Neoarchean (ca. 2800 Ma) mafic to ultramafic volcanic rocks, volcaniclastic sediments, and gabbro–anorthosite associations. The belt underwent polyphase metamorphism at upper amphibolite facies conditions between 2650 and 2600 Ma. The contacts between the Mesoarchean and Neoarchean volcanic rocks, and surrounding Eoarchean to Neoarchean tonalite–trondhjemite–granodiorite (TTG) gneisses are tectonic and typically bounded by high-grade mylonites. Regardless of age, the volcanic rocks are dominated by mafic amphibolites with a tholeiitic basalt composition, near-flat to slightly enriched light rare earth element (LREE) patterns (La/Smcn = 0.91–1.48), relatively flat to slightly depleted heavy-REE (HREE) (Gd/Ybcn = 1.0–1.28), and pronounced negative Nb–Ta anomalies (Nb/Nb* = 0.34–0.73) on chondrite- and primitive mantle-normalized diagrams. These geochemical characteristics are consistent with subduction zone geochemical signatures and partial melting of a shallow (<80 km) mantle source free of residual garnet. There is no geochemical evidence for contamination by older continental crust. The overall field and geochemical characteristics suggest that the thrust-imbricated basaltic rocks were erupted in intra-oceanic subduction zone settings. Sedimentary rocks are represented by garnet–biotite and quartzitic gneisses. They are characterized by relatively high contents of transition metal (Ni = 10–154 ppm; Cr = 7–166 ppm) and enriched LREE patterns (La/Smcn = 1.38–3.79). These geochemical characteristics suggest that the sedimentary rocks were derived from erosion of felsic to mafic igneous source rocks. Collectively, the structural and lithogeochemical characteristics of the Storø greenstone belt are consistent with collision (accretion) of unrelated Archean volcanic rocks formed in supra-subduction zone geodynamic settings. Accordingly, the Mesoarchean and Neoarchean rock record of the Storø greenstone belt may well be explained in terms of modern-style plate tectonic processes.  相似文献   

20.
The Dongguashan skarn-porphyry Cu-Au deposit, located in the Tongling district of the Middle-Lower Yangtze River Valley metallogenic belt (MLYB), consists of skarn ore bodies in the upper part and porphyry ore bodies in the lower part, both of which are hosted in quartz diorite and quartz monzodiorite. Zircon U-Pb age and geochemical studies show that the quartz diorite of the Dongguashan intrusion formed at 140.3 ± 2.0 Ma (MSWD = 0.19) and belongs to the high potassium calc-alkaline series. It is enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field-strength elements (HFSE) and heavy rare earth elements (HREE), and has a slightly negative Eu anomaly. 176Hf/177Hf values of the rims of zircons show a variable range (0.282087–0.282391), corresponding with calculated εHf(t) values of − 10.72 to − 21.46. Plagioclases in the quartz diorite have unbalanced structure characterized by bright andesine and labradorite (An = 37.0–65.5) cores with higher contents of Fe and Sr and are corroded by dark oligoclase (An = 13.8–27.6) rim. Major elements, trace elements, Hf isotope, and the composition of plagioclases indicate that the parental magma of the Dongguashan intrusion was produced by the mixing of underplating mafic magma and felsic magma formed by remelting of Paleoproterozoic and Neoarchean crustal rocks, Neoproterozoic crust may also provide some material to the felsic magma. Mafic magma played a key role and made the parental magma rich in water, sulfur, metals (Cu, Au) and gave it a high oxygen fugacity. During its magmatic evolution, the parental magma underwent fractional crystallization of hornblende, apatite, sphene and other mafic minerals. Some quartz diorite and quartz monzodiorite samples that show adakitic signatures, may result from injection of mafic magma. Some inherited zircons of the quartz diorite in the Dongguashan intrusion gave ages of 2.40–2.50 Ga, 1.95–2.05 Ga and 0.74–0.81 Ga, coming from ultramafic, mafic and andesitic igneous rocks, and this indicates that there may have been three periods (2.4, 2.0, and 0.8 Ga) of magmatic activity in the Tongling district.  相似文献   

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