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1.
《Chemical Geology》2006,225(1-2):40-60
Fluorite is the most common fluoride mineral in magmatic silicic systems and its crystallization can moderate or buffer fluorine concentrations in these settings. We have experimentally determined fluorite solubility and speciation mechanisms in haplogranitic melts at 800–950 °C, 100 MPa and aqueous-fluid saturation. The starting haplogranite compositions: peraluminous (alumina saturation index, ASI = 1.2), subaluminous (ASI = 1.0) and peralkaline (ASI = 0.8) were variably doped with CaO or F2O−1 in the form of stoichiometric mineral or glass mixtures. The solubility of fluorite along the fluorite–hydrous haplogranite binaries is low: 1.054 ± 0.085 wt.% CaF2 (peralkaline), 0.822 ± 0.076 wt.% (subaluminous) and 1.92 ± 0.15 wt.% (peraluminous) at 800 °C, 100 MPa and 10 wt.% H2O, and exhibits a minimum at ASI ∼ 1. Fluorite saturation isotherms are strongly hyperbolic in the CaO–F2O−1 space, suggesting that fluorite saturation is controlled by the activity product of CaO and F2O−1, i.e., these components are partially decoupled in the melt structure. The form of fluorite liquidus isotherms implies distinct roles of fluorite crystallization: in Ca-dominant systems, fluorite crystallization is controlled by the fluorine concentration in the melt only and remains nearly independent of calcium contents; in F-rich systems, the crystallization of fluorite is determined by CaO contents and it does not buffer fluorine concentration in the melt. The apparent equilibrium constant, K, for the equilibrium CaO + cF2O−1 = CaF2 (+ associates) is log K= − (2.449 ± 0.085)·Al2O3exc + (4.902 ± 0.066); the reaction-stoichiometry parameter varies as follows: c= − (0.92 ± 0.11)·Al2O3exc + (1.042 ± 0.084) at 800 °C, 100 MPa and fluid saturation where Al2O3exc are molar percent alumina in excess over alkali oxides. The reaction stoichiometry, c, changes at subaluminous composition: in peralkaline melts, competition of other network modifiers for excess fluorine anions leads to the preferential alkali–F short-range order, whereas in peraluminous compositions, excess alumina associates with calcium cations to form calcioaluminate tetrahedra. The temperature dependence of fluorite solubility is described by the binary symmetric Margules parameter, W = 36.0 ± 1.4 kJ (peralkaline), 39.7 ± 0.5 kJ (subaluminous) and 32.8 ± 0.7 kJ (peraluminous). The strong positive deviations from ideal mixing imply the occurrence of CaF2–granite liquid–liquid immiscibility at temperatures above 1258 °C, which is consistent with previous experimental data. These experimental results suggest very low solubilities of fluorite in Ca-rich melts, consistent with the lack of fluorine enrichment in peralkaline rhyolites and calc-alkaline batholiths. On the other hand, high CaO concentrations necessary to crystallize fluorite in F-rich peraluminous melts are not observed in nature and thus magmatic crystallization of fluorite in topaz-bearing silicic suites is suppressed. A procedure for calculating fluorite solubility and the liquidus isotherms for a whole-rock composition and temperature of interest is provided. 相似文献
2.
An experimental study on the origin of ferric and ferrous carbonate-silicate melts, which can be considered as the potential metasomatic oxidizing agents and diamond forming media, was performed in the (Ca,Mg)CO3-SiO2-Al2O3-(Mg,Fe)(Cr,Fe,Ti)O3 system, at 6.3 GPa and 1350–1650 °C. At 1350–1450 °C and ?O2 of FMQ + 2 log units, carbonate–silicate melt, coexisting with Fe3 +-bearing ilmenite, pyrope-almandine and rutile, contained up to 13 wt.% of Fe2O3. An increase in the degree of partial melting was accompanied by decarbonation and melt enrichment with CO2, up to 21 wt.%. At 1550–1650 °C excess CO2 segregated as a separate fluid phase. The restricted solubility of CO2 in the melt indicated that investigated system did not achieve the second critical point at 6.3 GPa. At 1350–1450 °C and ?O2 close to CCO buffer, Fe2 +-bearing carbonate–silicate melt was formed in association with pyrope-almandine and Fe3 +-bearing rutile. It was experimentally shown that CO2-rich ferrous carbonate-silicate melt can be an effective waterless medium for the diamond crystallization. It provides relatively high diamond growth rates (3–5 μm/h) at P,T-conditions, corresponding to the formation of most natural diamonds. 相似文献
3.
《Chemical Geology》2007,236(1-2):1-12
The nucleation of H2O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 800 °C and either 50 or 125 MPa, and then decompressed by 20–91 MPa at temperatures that ranged between 550 and 700 °C, and held at the lower pressures for 10–720 s before being quenched. Bubbles nucleated in number densities (NB) that vary between 3 × 107 and 2 × 108 cm− 3. Blocky shaped magnetite or the ends of needle-shaped hematite acted as sites for nucleation, but only if a minimum super-saturation was exceeded, which increases with increasing melt viscosity. Bubbles did not nucleate along the lengths of hematite needles nor on plagioclase. Both the beginning and ending times of the nucleation event increases with increasing melt viscosity. Using nucleation theory predictions, neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature, water diffusivity, or viscosity. Instead, the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor, resulting from the increasing polymerization of the melt at lower temperatures and water contents. Because only ∼ 108 bubbles cm− 3 nucleate on magnetite in the rhyolite melt used, almost regardless of experimental conditions, results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions. 相似文献
4.
Chromitite bodies of various sizes associated with dunite envelopes have been found in the Dehsheikh ultramafic massif, in the southeastern part of the outer Zagros ophiolite belt. The chromitites occur as layered and lenticular bodies, and show both magmatic and deformational textures, including massive, disseminated, banded and nodular types. The Dehsheikh chromitites display a variation in Cr# [100 × Cr / (Cr + Al)] from 69 to 78, which is typical of high-Cr chromitites. The Al2O3 and TiO2 contents of chromites range from 10.3 wt.% to 16.9 wt.% and 0.12 wt.% to 0.35 wt.%, respectively. The Al2O3, TiO2, and FeO/MgO values calculated for parental melts of Dehsheikh chromitites are within the range of boninitic melts. Chondrite-normalized distribution patterns of platinum-group elements show relative enrichments in Ru, Ir, and Os, and depletions in Rh, Pd, and Pt that are typical of chromitites associated with ophiolites formed by high degrees of mantle partial melting. The presence of Na-rich amphibole inclusions in chromite grains, together with the mineralogical and chemical composition of the chromitites and estimates of their parental melt compositions are used to help establish the tectono-magmatic setting. It is shown that the Dehsheikh massif is an ophiolite formed in a suprasubduction zone setting. We suggest that the composition of the rocks in this section was influenced by hydrous partial melts which might be formed in the subduction zone. Variable melt/rock interaction produced melt channel networks in the dunite which allowed the parental melt of the chromitite to percolate through them. Similar characteristics have been observed in other ophiolite complexes from the outer Zagros Iranian ophiolitic belt; these are believed to be the product of magmatism in a fore-arc environment. 相似文献
5.
The Qiangma gold deposit is hosted in the > 1.9 Ga Taihua Supergroup metamorphic rocks in the Xiaoqinling terrane, Qinling Orogen, on the southern margin of the North China Craton. The mineralization can be divided as follows: quartz-pyrite veins early, quartz-polymetallic sulfide veinlets middle, and carbonate-quartz veinlets late stages, with gold being mainly introduced in the middle stage. Three types of fluid inclusions were identified based on petrography and laser Raman spectroscopy, i.e., pure carbonic, carbonic-aqueous (CO2–H2O) and aqueous inclusions.The early-stage quartz contains pure carbonic and CO2–H2O inclusions with salinities up to 12.7 wt.% NaCl equiv., bulk densities of 0.67 to 0.86 g/cm3, and homogenization temperatures of 280−365 °C. The early-stage is related to H2O–CO2 ± N2 ± CH4 fluids with isotopic signatures consistent with a metamorphic origin (δ18Owater = 3.1 to 5.2‰, δD = − 37 to − 73‰). The middle-stage quartz contains all three types of fluid inclusions, of which the CO2–H2O and aqueous inclusions yield homogenization temperatures of 249−346 °C and 230−345 °C, respectively. The CO2–H2O inclusions have salinities up to 10.9 wt.% NaCl equiv. and bulk densities of 0.70 to 0.98 g/cm3, with vapor bubbles composed of CO2 and N2. The isotopic ratios (δ18Owater = 2.2 to 3.6‰, δD = − 47 to − 79‰) suggest that the middle-stage fluids were mixed by metamorphic and meteoric fluids. In the late-stage quartz only the aqueous inclusions are observed, which have low salinities (0.9−9.9 wt.% NaCl equiv.) and low homogenization temperatures (145−223 °C). The isotopic composition (δ18Owater = − 1.9 to 0.5‰, δD = − 55 to − 66‰) indicates the late-stage fluids were mainly meteoric water.Trapping pressures estimated from CO2–H2O inclusions are 100−285 MPa for the middle stage, suggesting that gold mineralization mainly occurred at depths of 10 km. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO2-rich to CO2-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ18Owater values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Qiangma gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic−Early Cretaceous continental collision between the North China and Yangtze cratons. 相似文献
6.
Subduction of heterogeneous lithologies (sediments and altered basalts) carries a mixture of volatile components (H2O ± CO2) into the mantle, which are later mobilized during episodes of devolatilization and flux melting. Several petrologic and thermodynamic studies investigated CO2 decarbonation to better understand carbon cycling at convergent margins. A paradox arose when investigations showed little to no decarbonation along present day subduction geotherms at subarc depths despite field based observations. Sediment diapirism is invoked as one of several methods for carbon transfer from the subducting slab. We employ high-resolution 2D petrological–thermomechanical modeling to elucidate the role subduction dynamics has with respect to slab decarbonation and the sediment diapirism hypothesis. Our thermodynamic database is modified to account for H2O–CO2 binary fluids via the following lithologies: GLOSS average sediments (H2O: 7.29 wt.% & CO2: 3.01 wt.%), carbonated altered basalts (H2O: 2.63 wt.% & CO2: 2.90 wt.%), and carbonated peridotites (H2O: 1.98 wt.% & CO2: 1.50 wt.%). We include a CO2 solubility P–x[H2O wt.%] parameterization for sediment melts. We parameterize our model by varying two components: slab age (20, 40, 60, 80 Ma) and convergence velocity (1, 2, 3, 4, 5, 6 cm year− 1). 59 numerical models were run and show excellent agreement with the original code base. Three geodynamic regimes showed significant decarbonation. 1) Sedimentary diapirism acts as an efficient physical mechanism for CO2 removal from the slab as it advects into the hotter mantle wedge. 2) If subduction rates are slow, frictional coupling between the subducting and overriding plate occurs. Mafic crust is mechanically incorporated into a section of the lower crust and undergoes decarbonation. 3) During extension and slab rollback, interaction between hot asthenosphere and sediments at shallow depths result in a small window (~ 12.5 Ma) of high integrated CO2 fluxes (205 kg m− 3 Ma− 1). 相似文献
7.
The Ulu Sokor gold deposit is one of the most famous and largest gold deposits in Malaysia and is located in the Central Gold Belt. This deposit consists of three major orebodies that are related to NS- and NE-striking fractures within fault zones in Permian-Triassic meta-sedimentary and volcanic rocks of the East Malaya Block. The faulting events represent different episodes that are related to each orebody and are correlated well with the mineralogy and paragenesis. The gold mineralization consists of quartz-dominant vein systems with sulfides and carbonates. The hydrothermal alteration and mineralization occurred during three stages that were characterized by (I) silicification and brecciation; (II) carbonatization, sericitization, and chloritization; and (III) quartz–carbonate veins.Fluid inclusions in the hydrothermal quartz and calcite of the three stages were studied. The primary CO2–CH4–H2O–NaCl fluid inclusions in stage I are mostly related to gold mineralization and display homogenization temperatures of 269–389 °C, salinities of 2.77–11.89 wt.% NaCl equivalent, variable CO2 contents (typically 5–29 mol%), and up to 15 mol% CH4. In stage II, gold was deposited at 235–398 °C from a CO2 ± CH4–H2O–NaCl fluid with a salinity of 0.83–9.28 wt.% NaCl equivalent, variable CO2 contents (typically 5–63 mol%), and up to 4 mol% CH4. The δ18OH2O and δD values of the ore-forming fluids from the stage II quartz veins are 4.5 to 4.8‰ and − 44 to − 42‰, respectively, and indicate a metamorphic–hydrothermal origin. Oxygen fugacities calculated for the entire range of T-P-XCO2 conditions yielded log fO2 values between − 28.95 and − 36.73 for stage I and between − 28.32 and − 39.18 for stage II. These values indicate reduced conditions for these fluids, which are consistent with the mineral paragenesis, fluid inclusion compositions, and isotope values.The presence of daughter mineral-bearing aqueous inclusions is interpreted to be a magmatic signature of stage IIIa. Combined with the oxygen and hydrogen isotopic compositions (δ18OH2O = 6.8 to 11.9‰, δD = − 77 to − 62‰), these inclusions indicate that the initial fluid was likely derived from a magmatic source. In stage IIIb, the gold was deposited at 263° to 347 °C from a CO2–CH4–H2O–NaCl fluid with a salinity of 5.33 to 11.05 wt.% NaCl equivalent, variable CO2 contents (typically 9–15 mol%), and little CH4. The oxygen and hydrogen isotopic compositions of this fluid (δ18OH2O = 8.1 to 8.8‰, δD = − 44 to − 32‰) indicate that it was mainly derived from a metamorphic–hydrothermal source. The CO2–H2O ± CH4–NaCl fluids that were responsible for gold deposition in the stage IIIc veins had a wide range of temperatures (214–483 °C), salinities of 1.02 to 21.34 wt.% NaCl equivalent, variable CO2 contents (typically 4–53 mol%), and up to 7 mol% CH4. The oxygen and hydrogen isotopic compositions (δ18OH2O = 8.5 to 9.8‰, δD = − 70 to − 58‰) were probably acquired at the site of deposition by mixing of the metamorphic–hydrothermal fluid with deep-seated magmatic water and then evolved by degassing at the site of deposition during mineralization. The log fO2 values from − 28.26 to − 35.51 also indicate reduced conditions for this fluid in stage IIIc. Moreover, this fluid had a near-neutral pH and δ34S values of H2S of − 2.32 to 0.83‰, which may reflect the derivation of sulfur from the subducted oceanic lithospheric materials.The three orebodies represent different gold transportation and precipitation models, and the conditions of ore formation are related to distinct events of hydrothermal alteration and gold mineralization. The gold mineralization of the Ulu Sokor deposit occurred in response to complex and concurrent processes involving fluid immiscibility, fluid–rock reactions, and fluid mixing. However, fluid immiscibility was the most important mechanism for gold deposition and occurred in these orebodies, which have corresponding fluid properties, structural controls, geologic characteristics, tectonic settings, and origins of the ore-forming matter. These characteristics of the Ulu Sokor deposit are consistent with its classification as an orogenic gold deposit, while some of the veins are genetically related to intrusions. 相似文献
8.
Many Late Paleozoic Cu–Au–Mo deposits occur in the Central Asian Orogenic Belt (CAOB). However, their tectonic settings and associated geodynamic processes have been disputed. This study provides age, petrologic and geochemical data for andesites and granitic porphyries of the Taerbieke gold deposit from the Tulasu Basin, in the northwestern Tianshan Orogenic Belt (western China). LA-ICP-MS zircon U–Pb dating indicates that the granitic porphyries have an Early Carboniferous crystallization age (349 ± 2 Ma) that is broadly contemporaneous with the eruption age (347 ± 2 Ma) of the andesites. The andesites have a restricted range of SiO2 (58.94–63.85 wt.%) contents, but relatively high Al2O3 (15.39–16.65 wt.%) and MgO (2.51–6.59 wt.%) contents, coupled with high Mg# (57–69) values. Geochemically, they are comparable to Cenozoic sanukites in the Setouchi Volcanic Belt, SW Japan. Compared with the andesites, the granitic porphyries have relatively high SiO2 (72.68–75.32 wt.%) contents, but lower Al2O3 (12.94–13.84 wt.%) and MgO (0.10–0.33 wt.%) contents, coupled with lower Mg# (9–21) values. The andesites and granitic porphyries are enriched in both large ion lithophile and light rare earth elements, but depleted in high field strength elements, similar to those of typical arc magmatic rocks. They also have similar Nd–Hf–Pb isotope compositions: εNd(t) (+0.48 to +4.06 and −0.27 to +2.97) and zircons εHf(t) (+3.4 to +8.0 and −1.7 to +8.2) values and high (206Pb/204Pb)i (18.066–18.158 and 17.998–18.055). We suggest that the Taerbieke high-Mg andesitic magmas were generated by the interaction between mantle wedge peridotites and subducted oceanic sediment-derived melts with minor basaltic oceanic crust-derived melts, and that the magmas then fractionated to produce the more felsic members (i.e., the Taerbieke granitic porphyries) during late-stage evolution. Taking into account the Carboniferous magmatic record from the western Tianshan Orogenic Belt, we suggest that the formation of the Early Carboniferous andesites and granitic porphyries in the Taerbieke area were related to the Paleo-Junggar Oceanic plate southward subduction under the Yili–Central Tianshan plate. The close association of the Early Carboniferous magmatic rocks and Au mineralization in the Taerbieke area suggests that the arc magmatic rocks in the Tulasu basin may have a high potential for Au mineralization. 相似文献
9.
《Chemie der Erde / Geochemistry》2014,74(3):311-329
Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO2 (∼36–39 wt.%), high in MgO (∼27–30 wt.%) and notably high in FeO (∼26–37 wt.%), and low in Al2O3 (∼0.2–2.5 wt.%) and combined alkalis Na2O and K2O (∼0–0.7 wt.%). They consist mostly of olivine (∼71–96 vol.%; ∼Fo64–73), major clinopyroxene (minor to ∼15 vol.%; ∼En40–63Wo36–48), with variable small amounts of plagioclase (0 to ∼10 vol.%; ∼An15–33), and minor to trace amounts of orthopyroxene (none to ∼20 vol.%; En69–73Wo2–4), Fe-sulfides (trace to ∼7 vol.%), chromite (none to ∼5 vol.%), phosphates (none to ∼3 vol.%) and metallic Fe,Ni (trace to ∼2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (∼0.1–1.5 mm), with olivine commonly displaying triple junctions. Brachina has near-chondritic lithophile element abundances, whereas other brachinites show variable depletions in Al, Ca, Rb, K, Na, and LREE. Siderophile element abundance patterns vary and range from ∼0.01 to ∼0.9 CI. Oxygen isotope composition (Δ17O) ranges from ∼−0.09 to −0.39‰, with the mean = −0.23 ± 0.14‰. Brachinites are ancient rocks, as was recognized early by the detection, in some brachinites, of excess 129Xe from the decay of short-lived 129I (half-life 17 Ma) and of fission tracks from the decay of 244Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using 53Mn–53Cr systematics, relative to the Pb–Pb age of the angrite LEW 86010, and yielded an age of 4563.7 ± 0.9 Ma. Thus, Brachina is at most ∼4 Ma younger that the CAIs whose age is 4567.2 ± 0.6 Ma. There is no consensus on the origin of brachinites, but they most likely are primitive achondrites, i.e., ultra-mafic residues from various low degrees of partial melting. Partial melting experiments suggest that they possibly formed from a parent lithology chemically similar but not identical to the Rumuruti (R) chondrites, although the different oxygen isotopic compositions of the R chondrites and the brachinites put a serious constraint on this hypothesis. The apparent lack of abundant rocks representing the partial melts suggests that brachinites may have formed on a parent body <∼100 km in radius, where early partial melts were removed from the parent body by explosive pyroclastic volcanism. Graves Nunataks 06128 and 06129 are felsic, andesitic basalts which have properties that suggest a relationship to brachinites and thus, may be samples of the elusive partial melts. 相似文献
10.
The polymetallic Mykonos vein system in the Cyclades, Greece, consists of 15 tension-gashes filled with barite, quartz, pyrite, sphalerite, chalcopyrite and galena in ca. 13.5 Ma, I-type, Mykonos monzogranite. Zones of silica and chlorite–muscovite alteration are associated with the veins and overprint pervasive silicification, phyllic and argillic alteration that affected large parts of the monzogranite. The mineralization cements breccias and consists of an early barite–silica–pyrite–sphalerite–chalcopyrite assemblage followed by later argentiferous galena. A combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17 wt.% NaCl equivalent, at temperatures of ~ 225° to 370 °C, under a hydrostatic pressure of ≤ 100 bars. The mineralizing fluids boiled and were saturated in H2S and SO2.Calculated δ18OH2O and δDH2O, initial 87Sr/86Sr isotope compositions and the trace and REEs elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the Mykonos intrusion and subsequently mixed with Miocene seawater. Heterogeneities in the calculated δ34SSO4− 2 and δ34SH2S compositions of the ore fluids indicate two distinct sources for sulfur, namely of magmatic and seawater origin, and precipitation due to reduction of the SO4− 2 during fluid mixing. The physicochemical conditions of the fluids were pH = 5.0 to 6.2, logfS2 = − 13.8 to − 12.5, logfO2 = − 31.9 to − 30.9, logfH2S(g) = − 1.9 to − 1.7, logfTe2 = − 7.9 and logα(SO4− 2(aq)/H2S(aq)) = + 2.6 to + 5.5. We propose that retrograde mesothermal hydrothermal alteration of the Mykonos monzogranite released barium and silica from the alkali feldspars. Barite was precipitated due to mixing of SO4− 2-rich Miocene seawater with the ascending Ba-rich magmatic fluid venting upwards in the pluton. 相似文献
11.
《Applied Geochemistry》2004,19(11):1655-1686
Water samples from short-screen monitoring wells installed along a 90-km transect in southwestern Kansas were analyzed for major ions, trace elements, isotopes (H, B, C, N, O, S, Sr), and dissolved gases (He, Ne, N2, Ar, O2, CH4) to evaluate the geochemistry, radiocarbon ages, and paleorecharge conditions in the unconfined central High Plains aquifer. The primary reactions controlling water chemistry were dedolomitization, cation exchange, feldspar weathering, and O2 reduction and denitrification. Radiocarbon ages adjusted for C mass transfers ranged from <2.6 ka (14C) B.P. near the water table to 12.8 ± 0.9 ka (14C) B.P. at the base of the aquifer, indicating the unconfined central High Plains aquifer contained a stratified sequence of ground water spanning Holocene time. A cross-sectional model of steady-state ground-water flow, calibrated using radiocarbon ages, is consistent with recharge rates ranging from 0.8 mm/a in areas overlain by loess to 8 mm/a in areas overlain by dune sand. Paleorecharge temperatures ranged from an average of 15.2 ± 0.7 °C for the most recently recharged waters to 11.6 ± 0.4 °C for the oldest waters. The temperature difference between Early and Late Holocene recharge was estimated to be 2.4 ± 0.7 °C, after taking into account variable recharge elevations. Nitrogen isotope data indicate NO3 in paleorecharge (average concentration=193 μM) was derived from a relatively uniform source such as soil N, whereas NO3 in recent recharge (average concentration=885 μM) contained N from varying proportions of fertilizer, manure, and soil N. Deep water samples contained components of N2 derived from atmospheric, denitrification, and deep natural gas sources. Denitrification rates in the aquifer were slow (5 ± 2× 10−3 μmol N L−1 a−1), indicating this process would require >10 ka to reduce the average NO3 concentration in recent recharge to the Holocene background concentration. 相似文献
12.
Felix V. Kaminsky Sergei M. Sablukov Elena A. Belousova Paulo Andreazza Mousseau Tremblay William L. Griffin 《Lithos》2010,114(1-2):16-29
The Juina diamond field, in the 1970–80s, was producing up to 5–6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006–2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2–1.8 ct/m3. Chrome spinel has 30–61 wt.% Cr2O3. Picroilmenite contains 6–14 wt.% MgO and 0.2–4 wt.% Cr2O3. Manganoan ilmenite has less than 3 wt.% MgO and 0.38–1.41 wt.% MnO. The 176Hf/177Hf ratio in kimberlitic zircons is 0.028288–0.28295 with εHf = 5.9–8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79–80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean 206Pb/238U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma. 相似文献
13.
The Linghou deposit, located near Hangzhou City of Zhejiang Province, eastern China, is a medium-sized polymetallic sulfide deposit associated with granitic intrusion. This deposit is structurally and lithologically controlled and commonly characterized by ore veins or irregular ore lenses. In this deposit, two mineralization events were identified, of which the former produced the Cu–Au–Ag orebodies, while the latter formed Pb–Zn–Cu orebodies. Silicification and calc-silicate (skarn type), phyllic, and carbonate alternation are four principal types of hydrothermal alteration. The early Cu–Au–Ag and late Pb–Zn–Cu mineralizations are characterized by quartz ± sericite + pyrite + chalcopyrite + bornite ± Au–Ag minerals ± magnetite ± molybdenite and calcite + dolomite + sphalerite + pyrite + chalcopyrite + galena, respectively. Calcite clusters and calcite ± quartz vein are formed during the late hydrothermal stage.The NaCl–H2O–CO2 system fluid, coexisting with NaCl–H2O system fluid and showing the similar homogenization temperatures (385 °C and 356 °C, respectively) and different salinities (16.89–21.68 wt.% NaCl eqv. and 7.70–15.53 wt.% NaCl eqv.), suggests that fluid immiscibility occurred during the Cu–Au–Ag mineralization stage and might have given rise to the ore-metal precipitation. The ore-forming fluid of the Pb–Zn–Cu mineralization mainly belongs to the NaCl–H2O–CO2 system of high temperature (~ 401 °C) and mid-high salinity (10.79 wt.% NaCl eqv.).Fluids trapped in the quartz-chalcopyrite vein, Cu–Au–Ag ores, Pb–Zn–Cu ores and calcite clusters yielded δ18OH2O and δD values varying from 5.54‰ to 13.11‰ and from − 71.8‰ to − 105.1‰, respectively, indicating that magmatic fluids may have played an important role in two mineralization events. The δ13CPDB values of the calcite change from − 2.78‰ to − 4.63‰, indicating that the CO32 − or CO2 in the ore-forming fluid of the Pb–Zn–Cu mineralization was mainly sourced from the magmatic system, although dissolution of minor marine carbonate may have also occurred during the ore-forming processes. The sulfide minerals have homogeneous lead isotopic compositions with 206Pb/204Pb ranging from 17.958 to 18.587, 207Pb/204Pb ranging from 15.549 to 15.701, and 208Pb/204Pb ranging from 37.976 to 39.052, indicating that metallic elements of the Linghou deposit came from a mixed source involving mantle and crustal components.Based on geological evidence, fluid inclusions, and H–O–C–S–Pb isotopic data, the Linghou polymetallic deposit is interpreted as a high-temperature, skarn-carbonate replacement type. Two types of mineralization are both related to the magmatic–hydrothermal system, with the Cu–Au–Ag mineralization having a close relationship with granodiorite. 相似文献
14.
The Loulo–Gounkoto complex in the Kédougou–Kéniéba Inlier hosts three multi-million ounce orogenic gold deposits, situated along the Senegal–Mali Shear Zone. This west Malian gold belt represents the largest West African orogenic gold district outside Ghana. The Gounkoto deposit is hosted to the south of the Gara and Yalea gold mines in the Kofi Series metasedimentary rocks. The ore body is structurally controlled and is characterised by sodic and phyllic alteration, As- and Fe-rich ore assemblages, with abundant magnetite, and overall enrichment in Fe–As–Cu–Au–Ag–W–Ni–Co–REE + minor Te–Pb–Se–Cd. Fluid inclusion analysis indicates that the deposit formed at P–T conditions of approximately 1.4 kbar and 340 °C and that two end member fluids were involved in mineralisation: (1) a moderate temperature (315–340 °C), low salinity (< 10 wt.% NaCl equiv.), low density (≤ 1 g·cm− 3), H2O–CO2–NaCl–H2S ± N2–CH4 fluid; (2) a high temperature (up to 445 °C), hypersaline (~ 40 wt.% NaCl equiv.), high density (~ 1.3 g·cm− 3), H2O–CO2–NaCl ± FeCl2 fluid. Partial mixing of these fluids within the Jog Zone at Gounkoto enhanced phase separation in the aqueo-carbonic fluid and acted as a precipitation mechanism for Au. These findings demonstrate the widespread, if heterogeneously distributed, nature of fluid mixing as an ore forming process in the Loulo–Gounkoto complex, operating over at least a 30 km strike length of the shear zone. Stable isotope analyses of ore components at Gounkoto indicate a dominant metamorphic source for H2O, H2S and CO2, and by extension Au. It thus can be reasoned that both the aqueo-carbonic and the hypersaline fluid at Gounkoto are of metamorphic origin and that the high levels of salinity in the brine are likely derived from evaporite dissolution. 相似文献
15.
《Chemie der Erde / Geochemistry》2016,76(2):299-308
The ultramafic member of the Variscan Ślęża Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1—olivine (Fo 90.2–91.0%, NiO 0.38–0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2—olivine (Fo 90.5–91.5, NiO 0.32–0.44 wt.%), olivine with magnetite inclusions (Fo 87.1–92.5, NiO 0.01–0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1–93.2, Al2O3 3.00–4.00 wt.%, Cr2O3 1.00–1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2–94.1, Al2O3 2.20–3.20 wt.% and Cr2O3 0.80–1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8–92.7, NiO 0.20–0.38 wt.%) and clinopyroxene (Mg# = 96.0–98.1, Al2O3 0.00–1.00 wt.% and Cr2O3 0.20–0.60 wt.%).The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Ślęża ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre. 相似文献
16.
We analyzed major and trace elements, Sr and Nd isotopes in ultramafic xenoliths in Miocenic age Hyblean diatremes, along with noble gases of CO2-rich fluid inclusions hosted in the same products. The xenoliths consist of peridotites and pyroxenites, which are considered to be derived from the upper mantle. Although the mineral assemblage of peridotites and their whole-rock abundance of major elements (e.g., Al2O3 = 0.8–1.5 wt.%, TiO2 = 0.03–0.08 wt.%) suggest a residual character of the mantle, a moderate enrichment in some incompatible elements (e.g., LaN/YbN = 9–14) highlights the presence of cryptic metasomatic events. In this context a deep silicate liquid is considered the metasomatizing agent, which is consistent with the occurrence of pyroxenites as veins in peridotites. Both the Zr/Nb and 143Nd/144Nd ratios of the investigated samples reveal two distinct compositional groups: (1) peridotites with Zr/Nb ≈ 4 and 143Nd/144Nd ≈ 0.5129, and (2) pyroxenites with Zr/Nb ≈ 20 and 143Nd/144Nd ≈ 0.5130. The results of noble-gas analyses also highlight the difference between the peridotite and pyroxenite domains. Indeed, the 3He/4He and 4He/40Ar* ratios measured in the fluid inclusions of peridotites (respectively 7.0–7.4 ± 0.1 Ra and 0.5–8.2, where Ra is the atmospheric 3He/4He ratio of 1.38 × 10? 6) were on average lower than those for the pyroxenites (respectively 7.2–7.6 Ra and 0.62–15). This mantle heterogeneity is interpreted as resulting from a mixing between two end-members: (1) a peridotitic layer with 3He/4He ≈ 7 Ra and 4He/40Ar* ≈ 0.4, which is lower than the typical mantle ratio (~ 1–4) probably due to melt extraction events, and (2) metasomatizing mafic silicate melts that gave rise to pyroxenites characterized by 3He/4He ≈ 7.6 Ra, with a variable 4He/40Ar* due to degassing processes connected with the ascent of magma at different levels in the peridotite wall rock. The complete geochemical data set also suggests two distinct mantle sources for the xenolithic groups highlighted above: (1) a HIMU (high-μ)-type source for the peridotites and (2) a DM (depleted mantle)-type source for the pyroxenites. 相似文献
17.
Numerous intrusive rocks of varying ages and compositions exist in the Paleozoic to Tertiary periods in the Eastern Pontides. Carboniferous intrusive rocks are commonly observed in the southern part of the Eastern Pontides. The nature of the rocks in the northern part of the region has not been determined because of Upper Cretaceous and Tertiary volcano-sedimentary units. Whole-rock geochemical, isotopic and geochronological data obtained from five different mapped granitoid bodies located in the northern part of the Eastern Pontides allow for the proper reconstruction of Carboniferous magmatism and the geodynamic evolution of the region.According to laser ablation ICP-MS U–Pb zircon dating, the Özdil, Soğuksu, Seslikaya, Kızılağaç and Şahmetlik plutons have similar 206Pb/238U vs. 207Pb/235U concordia ages of 340.7 ± 1.8 Ma and 323.1 ± 1.5 Ma, 348.4 ± 1.6 Ma, 335.4 ± 1.4 Ma, 337.2 ± 0.6 Ma and 334.5 ± 1.4 Ma, respectively. The aluminium saturation index (ASI) values of all of the samples from the plutons are between 1.0 and 1.32, which indicate peraluminous melt compositions. The plutons have SiO2 contents between 59 and 79 wt.% and show low- to high-K calc-alkaline characteristics. The plutons are enriched in large-ion lithophile and light rare earth elements and are depleted in high-field strength elements. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies, with LaCN/YbCN = 1.9–46.8 and EuCN/Eu* = 0.19–1.76. The studied plutons show considerable variations in 87Sr/86Sr(i) (0.70255 to 0.71006) and εNd(i) values (− 4.8 to − 7.1), as well as Nd model ages (1.15 to 2.47 Ga). The Pb-isotopic ratios are 206Pb/204Pb = 17.11–18.60, 207Pb/204Pb = 15.58–15.64 and 208Pb/204Pb = 36.95–38.62. The crystallization temperatures of the melts range from 676 to 993 °C, as determined by zircon and apatite saturation thermometry.These data suggest that the Carboniferous granitic magmas were produced by the partial melting of meta-mafic to meta-felsic lower crustal source rocks, with minor contributions from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the northern part of the Eastern Pontides. 相似文献
18.
The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4 ± 0.94 and 323.47 ± 0.95 Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO2 (42.90–46.61 wt.%), P2O5 (0.08–0.12 wt.%), MnO (0.35–0.97 wt.%) and TiO2 (0.74–0.82 wt.%), and relatively large variations in CaO (6.93–15.13 wt.%), Al2O3 (14.71–19.93 wt.%), total Fe2O3 (8.14–12.66 wt.%) and MgO (4.96–8.52 wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type. 相似文献
19.
《Russian Geology and Geophysics》2015,56(12):1717-1737
The paper is concerned with study of melt inclusions in minerals of ijolite xenoliths at Oldoinyo Lengai Volcano. Melt inclusions with different phase compositions occur in forsterite macrocrysts and in diopside, nepheline, fluorapatite, Ti-andradite, and Ti-magnetite crystals. Nepheline contains primary melt inclusions (silicate glass + gas-carbonate globule ± submicron globules ± sulfide globule ± daughter/trapped phases, represented by diopside, fluorapatite, Ti-andradite, and alumoakermanite). The gas-carbonate globule consists of a gas bubble surrounded by a fine-grained aggregate of Na-Ca-carbonates (nyerereite and gregoryite). Fluorapatite contains primary carbonate-rich melt inclusions in the core, which consist of nyerereite, gregoryite, thenardite, witherite, fluorite, villiaumite, and other phases. Their mineral composition is similar to natrocarbonatites. Primary melt inclusions (glass + gas bubble ± daughter phases) are rare in diopside and Ti-andradite. Diopside and forsterite have trails of secondary carbonate-rich inclusions. Besides the above minerals, these inclusions contain halite, sylvite, neighborite, Na-Ca-phosphate, alkali sulfates, and other rare phases. In addition, diopside contains sulfide inclusions (pyrrhotite ± chalcopy- rite ± djerfisherite ± galena ± pentlandite). The chemical compositions of silicate glasses in the melt inclusions vary widely. The glasses are characterized by high Na, K, and Fe contents and low Al contents. They have high total alkali contents (16–23 wt.% Na2O + K2O) and peralkalinity index [(Na + K)/Al] ranging from 1.1 to 7.6. The carbonate-rich inclusions in the ijolite minerals are enriched in Na, P, S, and Cl. The data obtained indicate that the parental melt in the intermediate chamber was heterogeneous and contained silicate, natrocarbonate, and sulfide components during the ijolite crystallization. According to heating experiments with melt inclusions, silicate-carbonate liquid immiscibility occurred at temperature over 580 °C. 相似文献
20.
The Dongping gold deposit hosted in syenites is one of the largest hydrothermal gold deposits in China and composed of ore veins in the upper parts and altered zones in the lower parts of the ore bodies. Pervasive potassic alteration and silicification overprint the wall rocks of the ore deposit. The alteration minerals include orthoclase, microcline, perthite, quartz, sericite, epidote, calcite, hematite and pyrite, with the quartz, pyrite and hematite assemblages closely associated with gold mineralization. The phases of hydrothermal alteration include: (i) potassic alteration, (ii) potassic alteration - silicification, (iii) silicification - epidotization - hematitization, (iv) silicification - sericitization - pyritization and (v) carbonation. Mass-balance calculations in potassic altered and silicified rocks reveal the gain of K2O, Na2O, SiO2, HFSEs and transition elements (TEs) and the loss of REEs. Most major elements were affected by intense mineral reactions, and the REE patterns of the ore are consistent with those of the syenites. Gold, silver and tellurium show positive correlation and close association with silicification. Fluid inclusion homogenization temperatures in quartz veins range from 154 °C to 382 °C (peak at 275 °C–325 °C), with salinities of 4–9 wt.% NaCl equiv. At temperatures of 325 °C the fluid is estimated to have pH = 3.70–5.86, log fO2 = − 32.4 to − 28.1, with Au and Te transported as Au (HS)2− and Te22 − complexes. The ore forming fluids evolved from high pH and fO2 at moderate temperatures into moderate-low pH, low fO2 and low temperature conditions. The fineness of the precipitated native gold and the contents of the oxide minerals (e.g., magnetite and hematite) decreased, followed by precipitation of Au- and Ag-bearing tellurides. The hydrothermal system was derived from an alkaline magma and the deposit is defined as an alkaline rock-hosted hydrothermal gold deposit. 相似文献