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1.
Major, trace and rare earth elements were measured in 27 samples of the Middle to Late Permian limestones from the Tieqiao section located on the marginal zone of an isolated platform (Laibin, South China). Shale-normalized REE+Y patterns of all samples show notably negative Ce anomalies (0.21–0.66, average 0.33), slightly positive Gd anomalies (1.08–1.30, average 1.20), and positive Y anomalies with superchondritic Y/Ho ratios (36–91, average 55), which are consistent with those of modern shallow seawater. Their relative LREEs enrichment with higher NdSN/YbSN ratios (0.58–1.80) than those of modern shallow seawater (0.21–0.50) suggests complicated sources of REEs for all samples. Compared with geochemical features of sediments influenced by terrigenous particles and hydrothermal fluids, it is concluded that ambient shallow seawater was the primary source of REEs in these limestones. Comparing the indicators of REE+Y elements (ΣREE, NdSN/YbSN, Ce/Ce*, Gd/Gd*, Eu/Eu* and Y/Ho) in limestones with those in bedded cherts from the Tieqiao section, we consider that limestone and bedded chert have similar sources of REE+Y elements: ambient shallow seawater with more or less hydrothermal fluids. In addition, there is a completely negative correlation between CaCO3 and SiO2 contents in limestones and bedded cherts. These results imply that precipitation of CaCO3 was inhibited by that of SiO2 which was derived mainly from hydrothermal fluid, especially in bedded cherts from the Tieqiao section.  相似文献   

2.
High-frequency metre-scale cycles are present within the Lower-Middle Ordovician carbonate successions in northern Tarim Basin, NW China. These metre-scale cycles were variably dolomitised from top to bottom. Three types of replacive dolomites were recognised, including dololaminite(very finely to finely crystalline, planar-s to nonplanar-a dolomite;type-1), patterned dolomite(finely crystalline, planar-s dolomite; type-2), and mottled dolomite(finely to medium crystalline,nonplanar-a(s) dolomite; type-3). Petrographic evidence indicate these dolomites were primarily deposited in supratidal to restricted subtidal environments, and formed in near-surface to shallow burial realms. Geochemically, all types of dolomites have similar δ13C and 87Sr/86 Sr ratios comparable to calcite precipitated in equilibrium with the Early-Middle Ordovician seawater. These geochemical attributes indicate that these dolomites were genetically associated and likely formed from connate seawater-derived brines. Of these, type-1 dolomite has δ18O values(.4.97‰ to.4.04‰ VPDB) slightly higher than those of normal seawater dolomite of the Early-Middle Ordovician age. Considering the absence of associated evaporites within type-1 dolomite, its parental fluids were likely represented by slightly evaporated(i.e., mesosaline to penesaline) seawater with salinity below that of gypsum precipitation. More depleted δ18O values(.7.74‰ to.5.20‰ VPDB) of type-2 dolomite and its stratigraphic position below type-1 dolomite indicate the generation of this dolomite from mesosaline to penesaline brines at higher temperatures in near-surface to shallow burial domains. Type-3 dolomite yields the most depleted δ18O values(–9.30‰to –7.28‰ VPDB), pointing to that it was most likely formed from coeval seawater-derived brines at highest temperatures in a shallow burial setting. There is a downward decreasing trend in δ18O values from type-1 through type-2 to type-3 dolomites, and in abundance of dolomites, indicating that the dolomitising fluids probably migrated downward from above and persisted into shallow burial conditions.  相似文献   

3.
Here we report a detailed trace element study of the cherts from Liuchapo Formation, which is a terminal Ediacaran (551-542 Ma) succession in South China deposited in deep-water basinal setting. The REE of Liuchapo cherts shows similar features as observed for anoxic modern seawater (but not for hydrothermal fluids), characterized by positive La anomaly (LaN/CeN = 0.83–1.91, average 1.37), moderately negative Ce anomaly (0.53–1.1, average 0.73), positive Gd anomaly (average 1.08), positive Y anomaly (average 1.21), and depleted LREE and MREE. In addition, the Liuchapo cherts have low ΣREE (3.36–56.13 ppm, average 20.6 ppm), low Al2O3, Ti, Th and Zr concentrations, and high Y/Ho ratios (up to 43.9). The redox-sensitive trace elements concentrations in the cherts do not correlate with detrital input proxies. All of these features suggest that the redox-sensitive trace elements in the cherts were authigenically concentrated in water column and their concentrations thus are excellent indicators of ancient redox conditions. Very low Th/U ratios, high V/(V+Ni) and Fe⊤/Al ratios, enrichments of redox-sensitive trace elements (U, V, Mo), and low concentration of Mn in the cherts imply anoxia in the deep seawater. Our data reveal that the terminal Ediacaran ocean was not completely oxidized and the deep ocean was still anoxic, at least in South China. We propose that although the oxidative events existed in the terminal Ediacaran oceans, decomposition of organic matter prolonged anoxia in the deep ocean. Supported by National Natural Science Foundation of China (Grants Nos. 40532012, 40873007, 40603021) and Chinese Academy of Sciences (Grant No. KZCX3-SW-141)  相似文献   

4.
Lithological and geochemical features of platformal carbonates record the signatures of the global climates and the regional environmental settings and also reconstruct the diagenetic history and porosity evolution, which are essential to evaluate the potential of hydrocarbon reservoirs. This study investigates the platformal carbonates of the Oligo‐Miocene Krunji Formation of North East Java Basin, which are potentially significant hydrocarbon reservoirs. The carbonate sequence in a 283 m thick section at Kranji in East Java is subdivided into three lithological units: limestone unit 1, dolostone unit 2, and limestone unit 3, in ascending order. The strontium‐isotope ratios of well‐preserved calcite samples indicate the depositional period from Chattian (late Oligocene) to Burdigalian (early Miocene), which is consistent with ages of the foraminifer assemblages. Unit 1 consists of low‐porosity limestone, in which two horizons of subaerial exposure are recognized by the occurrence of red‐colored matrix and lower δ13C values. Unit 2 consists of dolomitic rock and exhibits coarse‐grained calcitic grains and cross‐stratified structure. Considering that this unit has been subject to dolomitization, the sediment of unit 2 was initially permeable and was likely deposited in a shoal setting. The overlying unit 3 of Aquitanian–Burdigalian age is characterized by a highly granular texture. High porosity and uniformly low δ13C and δ18O values indicate that Unit 3 was subjected to more intense meteoric diagenesis than the Chattian unit 1. This was likely a consequence of the Antarctic ice‐sheet expansion during the Oligocene/Miocene transition, which amplified sea level change. The unit 2 dolomite has high δ13C and δ18O values and a high 87Sr/86Sr ratio which resulted from the reflux of seawater into permeable the sediment body in middle‐late Miocene (Burdigalian Tortonian) following the deposition of unit 3. The porosity and permeability of the Kujung Formation were initially controlled by sedimentological processes, but largely modified by later diagenetic processes.  相似文献   

5.
Ferromanganese nodules from the Bauer Basin of the south equatorial Pacific are unlike virtually all oceanic nodules so far analyzed in showing negative Ce anomalies in their REE abundance patterns. In comparison with similarly Cu-Ni-enriched nodules from the north equatorial Pacific they are depleted in REE by 50–80% and are heavy REE enriched relative to intermediate REE. The REE patterns can be accounted for by the input of hydrothermal iron oxyhydroxides and associated REE to the Bauer Basin and the transfer of the REE to the nodules because of diagenetic reactions in the sediment. The excess iron input also is reflected in lower Cu/Ni ratios in the nodules as compared with nodules from the north equatorial zone, apparently because of the larger proportions of a residual Fe phase in the nodules relative to todorokite. Cerium anomalies of the Bauer Basin nodules range from ?0.17 to ?0.29 as compared with +0.33 to +0.07 in the north equatorial Pacific but show a parallel sensitivity of Ce anomaly to Mn/Fe ratios of the nodules. Nodules with the more positive anomalies within each group have the smallest Mn/Fe ratios and have been subjected to the greatest seawater influence whereas nodules with the more negative anomalies have the largest Mn/Fe ratios and have been subjected to the greatest diagenetic influence.  相似文献   

6.
Geochemical analyses of granitic rocks from southwest England reveal that the rare earth elements (REE) were potentially mobile during hydrothermal and supergene alteration. In particular, trivalent REE were removed from the system during K-silicate alteration, Eu was lost during sericitic alteration; all REE were lost during tourmalinization, and light REE were lost during chloritization and argillic alteration. The fluids themselves had low concentrations of REE; in only one case (chloritization) were heavy REE introduced during alteration. Analysis of separated minerals indicated that the behaviour of the REE could be partly explained in terms of their different affinities for the primary and secondary assemblages. Thermodynamic calculations indicated that REE mobility is enhanced by the presence of fluorine in the alteration fluids partly because REE form more stable complexes with F than with Cl and partly because elements such as Ti, Zr and P that form REE-bearing minor phases are themselves potentially mobile.  相似文献   

7.
The mineralogical and geochemical characteristics of Fe-oxyhydroxide samples from one dredge station (long. 103°54.48′W, lat. 12°42.30′N, water depth 2655 m) on the East Pacific Rise near lat 13°N were analyzed by XRD, ICP-AES, and ICP-MS. Most Fe-oxyhydroxides are amorphous, with a few sphalerite microlites. In comparison with Fe-oxyhydroxides from other fields, the variable ranges in the chemical composition of Fe-oxyhydroxide samples are very narrow; their Fe, Si, and Mn contents were 39.90%, 8.92%, and 1.59%, respectively; they have high Cu (0.88%–1.85%) and Co (65×10−6−704×10−6) contents, and contain Co+Cu+Zn+Ni> 1.01%. The trace-element (As, Co, Ni, Cu, Zn, Ba, Sr) and major-element (Fe, Ca, Al, Mg) contents of these samples are in the range of hydrothermal sulfide from the East Pacific Rise near 13°N, reflecting that this type of Fe-oxyhydroxide constitutes a secondary oxidation product of hydrothermal sulfide. The Fe-oxyhydroxide samples from one dredge station on the East Pacific Rise near 13°N are lower in ΣREE (5.44×10−6–17.01×10−6), with a distinct negative Ce anomaly (0.12–0.28). The Fe-oxyhydroxide samples have similar chondrite-normalized rare-earth-element (REE) patterns to that of seawater, and they are very different from the REE composition characteristics of hydrothermal plume particles and hydrothermal fluids, showing that the REEs of Fe-oxyhydroxide are a major constituent of seawater and that the Fe-oxyhydroxides can become a sink of REE from seawater. The quick settling of hydrothermal plume particles resulted in the lower REE content and higher Mn content of these Fe-oxyhydroxides, which are captured in part of the V and P from seawater by adsorption. The Fe-oxyhydroxides from one dredge station on the East Pacific Rise near 13°N were formed by secondary oxidation in a low temperature, oxygenated environment. In comparison with the elemental (Zn, Cd, Pb, Fe, Co, Cu) average content of hydrothermal sulfide samples from the East Pacific Rise near 13°N, the Zn, Cd, and Pb contents of the Fe-oxyhydroxides are lower, and their Fe, Co, and Cu contents are higher. Supported in part by the Pilot Project of Knowledge Innovation Project, Chinese Academy of Sciences (Grant No. KZCX3-SW-223), and the Special Foundation for the Tenth Five Plan of COMRA (Grant No. DY105-01-03-1)  相似文献   

8.
This paper considers the geochemistry of volcanogenic mineralization in the northeastern segment of the Pacific Ore Belt (Northeast Russia). We give new evidence for the compositions and concentrations of trace and rare-earth elements (REE) in the ores of volcanogenic fields: Au-Ag epithermal (of various types and ages), Cu-Mo-Au porphyritic, Au-Bi related to granitoidal intrusions, Sn-Ag subvolcanic and kies polymetallic enriched in Au and Ag, as well as REEs in alkaline volcanic rocks. Geochemical signatures have been compiled for 17 formation types of volcanogenic fields. It was found that the ore-forming fluids in most fields belonged to an NaCl-H2O hydrothermal system that was enriched in Cl relative to F; the values of Y/Ho in the ores of nearly all types correspond with the interval of ratios characteristic for present-day hydrothermal fluids in backarc basins; most of the ores that we studied had near-chondrite spectra with configurations similar to those of the REE spectra in volcanic rock sequences of the andesite-diorite series. Comparative analysis of REE spectra in the distribution of trace elements over classes of gold concentration shows synchronous enrichment of ores in similar sets of trace elements. The high Co/Ni ratio in volcanogenic ores probably reflects the superposition of a later magmatic fluid upon an earlier mineralization. Samples from ores of volcanogenic fields, except for Kuroko, show δCe and δEu varying from negative to mildly positive values, thus indicating low-oxidizing conditions during deposition. It was found for Au-Ag epithermal ores that they are enriched in a wide range of trace elements; they have low concentrations of REEs, the light REEs are more abundant than the heavy ones, and the Eu anomalies vary considerably from small negative to low and high positive values. The results provide evidence of an exhalation hydrothermal origin of the Khotoidokh field. It has been shown that the REEs in the ores of the Bol’shoe field are of the type that is most valuable to industry. The results can be used to deal with practical problems: determining the formation type and evaluating the industrial value of a field; detecting accessory components in ores; and discriminating between the types of geochemical anomalies (in rocks or in soil) and stray fluxes as to the potential of a field.  相似文献   

9.
Marine limestones from southern Germany were analysed by the non-destructive neutron activation/Ge(Li) spectrometric method for La, Ce, Sm, Eu, Tb, Yb, Lu, Sr, Ta, U, Sc, Th, Cr, Hf, Cs, Fe and Rb. On the basis of carbonate and non-carbonate portions of the rock, the elements could be divided into (1) those associated with clastic material (Sc, Cr, Hf, etc.), (2) those which were partly associated with authigenic material (REE), and (3) those with pronounced diagenetic history (Sr, U). The calculated effective REE distribution coefficient (Di,eff) between CaCO3 and seawater varied from about 1400 for La to about 460 for Lu suggesting fractionation of the lighter and the heavier REE's in favour of the former.  相似文献   

10.
The Eoarchean Nulliak supracrustal rocks in the Saglek Block of northeastern Labrador, Canada, contain some of the world's oldest carbonate rocks. This work attempted to reveal the origin of the carbonate rocks and estimate the surface environmental conditions of the early Earth based on their occurrence and geochemistry. They occur together with mafic and ultramafic rocks in Pangertok Inlet and St. John's Harbour South, whereas they are interlayered with pelitic rock layers with quartzofeldspathic mineral assemblages in St. John's Harbour East and Big Island. The geological occurrence suggests that the formers were formed around hydrothermal fields, whereas the latters were deposited near a continental margin. Some carbonate rocks have high SiO2, Al2O3, and Zr contents, indicating that the silicification and involvement of detrital materials influenced their composition; thus, pure carbonate rocks were selected using a combined filter of the SiO2, TiO2, Al2O3, Zr, and Ba contents. The selected carbonate rocks have positive La, Eu, Gd, Y, U, Pb, and Sr anomalies, negative Nb, Zr, and Hf anomalies, and relatively small enrichment in heavy rare earth elements (HREEs). The La and Y anomalies suggest that they originated from chemical sediments precipitated from seawater. On the other hand, the small HREE-enrichment suggests that REEs were mainly dissolved as REE-carbonate complexes in seawater or that the riverine influxes were dominated by the detritus of Eoarchean continental crusts, presumably composed of HREE-depleted TTG. The U anomaly suggests that uranium was more dissolved than Th as U-bearing carbonate complexes in seawater. The Nulliak carbonate rocks also show a positive correlation between Y and Eu anomaly values, suggesting that the precipitation of iron-oxyhydroxide causing the Y anomaly was more significant near the hydrothermal fields than the continental margin, consistent with an alkaline hydrothermal model.  相似文献   

11.
Rare earth element (REE) abundances determined by activation analysis in rocks, plagioclase and mafic separates from the Fiskenaesset Complex are presented together with data on major and trace elements in the minerals. The REE data for the rocks and plagioclases are distinct from those of many other anorthositic complexes and the abundances are some of the lowest recorded for plagioclase from terrestrial anorthosites. The bulk and trace element compositions of the Fiskenaesset plagioclases show a number of similarities to those of lunar plagioclases. The plagioclases show a positive Eu anomaly of about 10 and a depletion in the heavy REE relative to the light ones. The mafic separates are enriched in the heavy REE relative to the light ones, and show no Eu anomaly except in one sample with a positive anomaly not attributable to plagioclase contamination. It is estimated, from experimental partition coefficient data, that the REE pattern in the magma at an early stage of fractionation was La (17×) to Lu (0.7× chondrites) with a possible positive Eu anomaly. This highly fractionated REE pattern may be attributed to partial melting of a garnet-bearing source.  相似文献   

12.
Twelve apatite samples from igneous and metamorphic rocks from the Black Forest and igneous rocks from the Kaiserstuhl were analysed for their REE content. The ΣREE range from 0.116 to 1.69 wt.%; the lowest values were found in the metamorphic rocks. All apatites from the various parent rocks show a general enrichment of the lighter rare earths over the heavier and their chondrite-normalized rare earth patterns exhibit a more or less pronounced negative Ce anomaly. This Ce depletion is accompanied by relatively low La and Pr values. In addition, the apatites from igneous rocks from the Black Forest show a marked negative Eu anomaly. An explanation is offered for the simultaneous occurrence of a negative Ce and a negative Eu anomaly in one investigated rock system. The negative Ce anomaly is attributed to the occurrence of phases enriched in Ce (e.g., monazite, allanite) which crystallized prior to or simultaneously with apatite. The negative Eu anomaly in these magmatic apatites most probably is caused by discrimination of Eu2+ from the apatite lattice. The result of this discrimination is a selective Eu enrichment in the later crystallizing feldspars, plagioclase and orthoclase. At least in this case, the positive Eu anomaly in feldspars is not a reliable indicator of low oxygen fugacity during their crystallization; the Eu depletion of the earlier crystallized apatites is preferable for this purpose.  相似文献   

13.
Based on the concentration and distribution pattern analysis of the rare earth elements (REEs) at the Shell Bar section from Qaidam Basin, we studied the geochemical fractionations of REE in paleolake deposits and their paleo-environmental significance. Our results show that the REE concentration in AS (acid soluble) and AR (acid residual) fractions are 20.9 μg/g and 95.4 μg/g (except element Y) individually, showing a strong REE differentiation between AS and AR fractions. However, the two types of fractions (AS & AR) have similar REE distribution patterns, which are slightly rich in light earth rare elements (LREEs), with slightly right-tilting and negative Eu anomaly. The LREE of AR is richer than that of AS. There were no significant correlations between the REE in AS, AR and other proxies. It indicated that the lacustrine deposition had different material sources and experienced varying geochemical procedures. Correlation analysis between the REE and the content of fine grain-size (<4 μm) of the sediments, Rb/Sr ratio and Mn concentration showed high correlation coefficients. Our results demonstrated that the REE in acid soluble and residual fractions bear different environmental significances, which are strongly dependent on local environment. The REE of acid soluble fraction is closely related to the paleoclimatic changes in the lake catchment and the evolutionary processes of lake itself. Our results suggest that δCe and (La/Yb)n could be used as proxies of the reduction-oxidation conditions and furthermore the temperature change and the lake level fluctuations. Using these proxies, we reconstructed the paleoclimate and water level fluctuation history during the high lake level period lasting between 43.5 and 22.4 cal. ka BP.  相似文献   

14.
The study on marine hydrothermal sedimentationtends to be perfect at present, and a suit of hydrother-mal sedimentary distinguishing criterions (includingstructure, conformation, geochemistry, etc.) has beenfound[1—12]. While it is unsubstantial on the study ofhydrothermal sedimentation in continental deposi-tional environment, and little was known about therelation between continental hydrothermal sedimenta-tion and metal mineralization. The Lincang Ge deposit,hosted in coal with independent…  相似文献   

15.
Yue-Feng Sun 《Island Arc》2004,13(1):227-241
Abstract The origin of dolomite and its diagenesis are still not fully understood. The timing and occurrence of dolomite beds remain key problems. Many thin‐bed layers of dolomite in marine sediments were discovered at drill sites in the forearc basin west of the Japan Trench during the Ocean Drilling Program Leg 186. The thickness of the dolomite layers ranges from 0.5 to 2 m, estimated from the well log. These thin‐bed dolomite layers were able to be identified on seismic data, through quantitative analysis and comparison of the data with synthetic seismograms. Amplitude‐preserved seismic data processing with broadband high‐resolution enhancement helps to estimate the reflectivity series dominated by the large impedance contrast between the dolomite layers and the host sediments. In this report, the first geophysical evidence of large‐scale dolomitization is provided. The occurrence of thin dolomitic layers embedded in diatomaceous sediments is on a basin scale rather than being an isolated event, which constrains the type of dolomitization and the timing of layered dolomitization. It was found that thin‐bed dolomite layers occur where there is an abrupt change in sedimentation rate. Layered dolomitization starts when sedimentation rate changes (from high to low or vice versa). Two dolomite layers coincide with the Miocene–Pliocene and Pliocene–Pleistocene age boundaries. Formation of dolomite beds could be a timing signal of a dramatic change in paleoceanographic and paleoclimatic environments.  相似文献   

16.
Four layers of cherts were found for the first time in the Yangla copper deposit, western Yunnan Province. The cherts possessed the following geochemical characteristics: 1 Low TiO2 and Al2O3 contents, but high ore-forming element (e.g. Cu, Au, Ag) contents; 2 low total REE contents and clear negative Eu anomalies when normalized to chondrite similar to the REE contents and distribution patterns of associated massive sulfide ores; 3 silicon isotopic compositions of cherts in the Yangla deposit being the same as cherts and geyserite of hot-water sedimentary origin; 4 lead and sulfur isotopic compositions of cherts in the Yangla deposit being similar to those of the massive sulfide ores in the Yangla deposit; 5 Rb-Sr isochron age of cherts from the Yangla deposit being identical with that of host strata. Hence, we conclude that the cherts in the Yangla deposit are of hot-water sedimentary origin, which have a close relationship with the massive sulfide ores. The discovery of hydrothermal cherts from the Yangla copper deposit provides further evidence for the hydrothermal exhalative origin of the massive sulfide deposits.  相似文献   

17.
The distribution and anomalies of rare earth elements(REEs) of granitic regolith were studied in Inner Mongolia and Hainan Island, China. One profile showed slight REE enrichment of an upper layer and no obvious light REE/heavy REE(LREE/HREE) fractionation(La_N/Yb_N of 0.9). The second profile was significantly enriched in REEs and enriched in LREEs in the upper portion(La_N/Yb_N1.8). Eu, Ce, and Gd anomalies of the two profiles are different. Slightly negative Eu, Ce, and Gd anomalies in NMG-3-1 indicate slow dissolution of primary minerals and little secondary products; in contrast, a positive Eu anomaly in HN-2 suggests the vegetation cycle may contribute to soil. The Ce anomaly of HN-2 reflects oxidation of Ce and coprecipitation by Fe-and Mn-oxides and organic matter. Correlation between Ce and Gd anomalies in HN-2 suggests Ce and Gd are both influenced by redoxreduction.  相似文献   

18.
The Zhuxi ore deposit is a super-large scheelite(copper) polymetallic deposit discovered in recent years. It grew above copper/tungsten-rich Neoproterozoic argilloarenaceous basement rocks and was formed in the contact zone between Yanshanian granites and Carboniferous-Permian limestone. Granites related to this mineralization mainly include equigranular, middle- to coarse-grained granites and granitic porphyries. There are two mineralization types: skarn scheelite(copper) and granite scheelite mineralization. The former is large scale and has a high content of scheelite, whereas the latter is small scale and has a low content of scheelite. In the Taqian-Fuchun Basin, its NW boundary is a thrust fault, and the SE boundary is an angular unconformity with Proterozoic basement. In Carboniferous-Permian rock assemblages, the tungsten and copper contents in the limestone are both very high. The contents of major elements in granitoids do not differ largely between the periphery and the inside of the Zhuxi ore deposit. In both areas, the values of the aluminum saturation index are A/CNK1.1, and the rocks are classified as potassium-rich strongly peraluminous granites. In terms of trace elements, compared to granites on the periphery of the Zhuxi ore deposit, the granites inside the Zhuxi ore deposit have smaller d Eu values, exhibit a significantly more negative Eu anomaly, are richer in Rb, U, Ta, Pb and Hf, and are more depleted in Ba, Ce, Sr, La and Ti, which indicates that they are highly differentiated S-type granites with a high degree of evolution. Under the influence of fluids, mineralization of sulfides is evident within massive rock formations inside the Zhuxi ore deposit, and the mean SO_3 content is 0.2%. Compared to peripheral rocks, the d Eu and total rare earth element(REE) content of granites inside the Zhuxi ore deposit are both lower, indicating a certain evolutionary inheritance relationship between the granites on the periphery and the granites inside the Zhuxi ore deposit. For peripheral and ore district plutons, U-Pb zircon dating shows an age range of 152–148 Ma. In situ Lu-Hf isotope analysis of zircon in the granites reveals that the calculated e_(Hf)(t) values are all negative, and the majority range from -6 to -9. The T_(DM2) values are concentrated in the range of 1.50–1.88 Ga(peak at 1.75 Ga), suggesting that the granitic magmas are derived from partial melting of ancient crust. This paper also discusses the metallogenic conditions and ore-controlling conditions of the ore district from the perspectives of mineral contents, hydrothermal alteration, and ore-controlling structures in the strata and the ore-bearing rocks. It is proposed that the Zhuxi ore deposit went through a multistage evolution, including oblique intrusion of granitic magmas, skarn mineralization, cooling and alteration, and precipitation of metal sulfides. The mineralization pattern can be summarized as "copper in the east and tungsten in the west, copper at shallow-middle depths and tungsten at deep depths, tungsten in the early stage and copper in the late stage".  相似文献   

19.
Rare earth element (REE) plus yttrium (Y) patterns of modern seawater have characteristic features that can be used as chemical fingerprints. Reliable proxies for marine REE+Y chemistry have been demonstrated from a large geological time span, including Archaean banded iron formation (BIF), stromatolitic limestone, Phanerozoic reef carbonate and Holocene microbialite.Here we present new REE+Y data for two distinct suites of early Archaean (ca. 3.7-3.8 Ga) metamorphosed rocks from southern West Greenland, whose interrelationships, if any, have been much debated in recent literature. The first suite comprises magnetite-quartz BIF, magnetite-carbonate BIF and banded magnetite-rich quartz rock, mostly from the Isua Greenstone Belt (IGB). The REE+Y patterns, particularly diagnostic anomalies (Ce/Ce*, Pr/Pr*), are closely related to those of published seawater proxies. The second suite includes banded quartz-pyroxene-amphibole±garnet rocks with minor magnetite from the so-called Akilia Association enclaves (in early Archaean granitoid gneisses) of the coastal region, some 150 km southwest of the IGB. Rocks of this type from one much publicised and highly debated locality (the island of Akilia) have been identified by some workers [Nature 384 (1996) 55; Geochim. Cosmochim. Acta 61 (1997) 2475] as BIF-facies, and their 13C-depleted signature in trace graphite interpreted as a proxy for earliest life on Earth. However, REE+Y patterns of the Akilia Association suite (except for one probably genuine magnetite-rich BIF from Ugpik) are inconsistent with a seawater origin. We agree with published geological and geochemical (including REE) work [Science 296 (2002) 1448] that most of the analysed Akilia rocks are not chemical sediments, and that C-isotopes in such rocks therefore cannot be used as biological proxies.Application of the REE+Y discriminant for the above two rock suites has been facilitated in this study by the use of MC-ICP technique which yields a more complete and precise REE+Y spectrum than was available in many previous studies.  相似文献   

20.
Cheong-Bin  Kim  V. J. Rajesh    M. Santosh 《Island Arc》2008,17(1):26-40
Abstract Geochemical and Sr–Nd–Pb isotope characteristics, as well as K–Ar geochronology of a massive pitchstone (volcanic glass) stock erupted into Late Cretaceous lapilli tuff and rhyolite in the Gohado area, southwestern Okcheon Belt, South Korea, are reported. The pitchstones are highly evolved with SiO2 contents ranging from ～72 to 73 wt%, K2O/Na2O ratios of 1.04–1.23 and low MgO/FeOt values (0.17–0.20). The pitchstones are weakly peraluminous and the ASI (molar Al2O3/Na2O + K2O + CaO) values are significantly lower than 1.1. The pitchstones also display a general calc‐alkaline nature with significant alkali contents. The rare earth elements (REE) compositions show moderately fractionated nature with (La/Yb)N ranging from 11 to 16. Chondrite normalized REE patterns show relative enrichment of light REE over heavy REE and moderate Eu anomaly (Eu/Eu* ratio varies from 0.53 to 0.57). A distinct negative Nb anomaly is observed for all pitchstones on a primitive mantle normalized trace element diagram, typical of subduction‐related magmatism and crustal‐derived granites. All these features are characteristic of I‐type granites derived from a continental arc. The pitchstones have Zr contents of 98.5–103.5 ppm with zircon thermometry yielding temperatures of 749–755°C (mean 752°C). The K–Ar analyses of representative pitchstone samples yielded ages of 58.7 ± 2.3 and 62.4 ± 2.1 Ma with a mean age of 61 Ma. The rocks show nearly uniform initial 87Sr/86Sr isotopic ratios of 0.7104–0.7106 and identical 143Nd/144Nd initial ratio of 0.5120. The rocks display negative εNd (61 Ma) values of ?12. The depleted mantle model ages (TDM) range from 1.54 Ga to 1.57 Ga. The Pb isotope ratios are 206Pb/204Pb = 18.522–18.552, 207Pb/204Pb = 15.642–15.680 and 208Pb/204Pb = 38.794–38.923. These ratios suggest that the Gohado pitchstones were formed in a continental arc environment by partial melting of a 1.54 Ga to 1.57 Ga parental sources of lower crustal rocks probably of mafic or intermediate compositions.  相似文献   

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