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1.
A. E. Sveinbjornsdottir M. L. Coleman B. W. D. Yardley 《Contributions to Mineralogy and Petrology》1986,94(1):99-109
The Reykjanes and Krafla geothermal fields are both examples of active high temperature systems and show similar assemblages of alteration minerals, but the fluid at Reykjanes is dominantly sea water whereas that at Krafla is meteoric. Oxygen isotope analyses of surface rock and of drill chip samples from different depths are presented, together with results for the Krafla fluid, which is close to local precipitation (δ 18 O = ?11.9‰, δD= ?86.8‰). Calcite in both systems is apparently in equilibrium with the present deep fluid at the present field temperature, except for the upper 250 m at Reykjanes where the fluid may be more meteoric than at depth. Feldspar gives similar results. Quartz separates at Reykjanes are anomalously lighter than coexisting feldspar and give exceptionally high quartz-fluid temperatures. It is suggested that quartz originally grew when the fluid was more nearly meteoric (? glacial period) and has not re-equilibrated. Bulk-rock 18 O depletion supports this interpretation of the history of the Reykjanes system. Quartz in the Krafla system is mostly in equilibrium at the present field conditions but anomalies occur near the boundary between the upper and lower parts of the system, suggesting that this is not entirely stable. A high fluid:rock ratio (10–100 minimum) is indicated for the Krafla field. 相似文献
2.
H. Sakai E. Gunnlaugsson J. Tòmasson J.E. Rouse 《Geochimica et cosmochimica acta》1980,44(8):1223-1231
Pyrite from altered basalts from Nàmafjall and Krafla high-temperature fields and deep zones at Reykir, Leira and other low-temperature fields, and aqueous sulfides from Nàmafjall, have δ34S values of 0 to 2.6%. These values are close to those for postglacial basaltic lavas from the Reykjanes Peninsula. The major source of sulfur in these meteoric hydrothermal systems is the upper-mantle or basalt. At the low-temperature fields, however, the δ34S values of sulfide decrease with decreasing depth, suggesting the presence of a light sulfur source in the shallower aquifers.In contrast, in the Reykjanes and Svartsengi geothermal fields, where seawater contributes to the hydrothermal systems, sulfide sulfur is distinctly enriched in 34S at all depths except for one Reykjanes pyrite from 84 m depth. The enrichment is about 8%. at the deepest core (1734 m) of Reykjanes and decreases with decreasing depth. These enrichments are most likely due to seawater sulfate being involved in the hydrothermal systems. However, in the Reykjanes fluid, dissolved heavy sulfates are not in isotopic equilibrium with sulfide. Disequilibrium between sulfate and sulfide is also demonstrated in all other Icelandic geothermal systems studied. 相似文献
3.
Emily C. Pope Dennis K. Bird Thráinn Fridriksson Gudmundur Ó. Fridleifsson 《Geochimica et cosmochimica acta》2009,73(15):4468-190
The Reykjanes geothermal system is located on the landward extension of the Mid-Atlantic Ridge in southwest Iceland, and provides an on-land proxy to high-temperature hydrothermal systems of oceanic spreading centers. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, δD values of these fluids are as low as −23‰, which is indicative of a meteoric water component. Here we constrain the origin of Reykjanes hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of hydrothermal epidote from geothermal drillholes at depths between 1 and 3 km. δDEPIDOTE values from wells RN-8, -9, -10 and -17 collectively range from −60 to −78‰, and δ18OEPIDOTE in these wells are between −3.0 and 2.3‰. The δD values of epidote generally increase along a NE trend through the geothermal field, whereas δ18O values generally decrease, suggesting a southwest to northeast migration of the geothermal upflow zone with time that is consistent with present-day temperatures and observed hydrothermal mineral zones. For comparative analysis, the meteoric-water dominated Nesjavellir and Krafla geothermal systems, which have a δDFLUID of ∼ −79‰ and −89‰, respectively, show δDEPIDOTE values of −115‰ and −125‰. In contrast, δDEPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is −68‰; comparable to δDEPIDOTE from well RN-10 at Reykjanes.Stable isotope compositions of geothermal fluids in isotopic equilibrium with the epidotes at Reykjanes are computed using published temperature dependent hydrogen and oxygen isotope fractionation curves for epidote-water, measured isotope composition of the epidotes and temperatures approximated from the boiling point curve with depth. Calculated δD and δ18O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric or glacial origin are a significant component of the geothermal solutions. Additionally, δDFLUID values in equilibrium with geothermal epidote are lower than those of modern-day fluids, whereas calculated δ18OFLUID values are within range of the observed fluid isotope composition. We propose that modern δDEPIDOTE and δDFLUID values are the result of diffusional exchange between hydrous alteration minerals that precipitated from glacially-derived fluids early in the evolution of the Reykjanes system and modern seawater-derived geothermal fluids. A simplified model of isotope exchange in the Reykjanes geothermal system, in which the average starting δDROCK value is −125‰ and the water to rock mass ratio is 0.25, predicts a δDFLUID composition within 1‰ of average measured values. This model resolves the discrepancy between fluid salinity and isotope composition of Reykjanes geothermal fluids, explains the observed disequilibrium between modern fluids and hydrothermal epidote, and suggests that rock-fluid interaction is the dominant control over the evolution of fluid isotope composition in the hydrothermal system. 相似文献
4.
S. R. Hart 《Geochimica et cosmochimica acta》1986,50(12):2751-2761
Cation exchange experiments (ammonium acetate and cation resin) on celadonite-smectite vein minerals from three DSDP holes demonstrate selective removal of common Sr relative to Rb and radiogenic Sr. This technique increases the Rb/Sr ratio by factors of 2.3 to 22 without significantly altering the age of the minerals, allowing easier and more precise dating of such vein minerals. The ages determined by this technique (site 261—121.4 ±1.6 m.y.; site 462A—105.1 ±2.8 m.y.; site 516F—69.9 ±2.4 m.y.) are 34, 54 and 18 m.y. younger, respectively, than the age of crust formation at the site; in the case of site 462A, the young age is clearly related to off-ridge emplacement of a massive sill/flow complex. At the other sites, either the hydrothermal circulation systems persisted longer than for normal crust (10–15 m.y.), or were reactivated by off-ridge igneous activity.
Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust.
Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water/rock ratios of 20–200. 相似文献
5.
湘西头坡脑汞锌矿成矿流体特征及地质意义 总被引：1，自引：0，他引：1
湘西头坡脑汞锌矿床位于扬子陆块东南缘,矿床受北西向构造带控制,赋矿围岩为寒武系中统敖溪组第三段细晶白云岩,矿石矿物主要为辰砂及闪锌矿,脉石矿物主要为石英、白云石及方解石。头坡脑汞锌矿床矿石中的流体包裹体主要为纯液相包裹体,部分气液两相包裹体及少量纯气相包裹体。成矿流体具有中低温(84.3℃~183.9℃)、中高盐度(6.16 wt%~22.24 wt% NaCl)及中高密度(0.97~1.10 g/cm~3)等特征。气相成分测定显示包裹体中气相成分主要为CH_4,少量N_2及CO_2,液相成分为H_2O,成矿流体富烃。氢氧同位素组成分别为δD(-105×10~(-3)~-46×10~(-3))和δ~(18)O_(H_2O)(-7.1×10~(-3)~11.4×10~(-3)),指示成矿流体主要为建造水,也有大气降水的加入。根据头坡脑汞锌矿床成矿地质背景、流体特征、流体来源及区域成矿特点,认为本区汞成矿事件应当为上扬子台褶带构造运动长期性和持续性的产物。 相似文献
6.
Sm---Nd isotopic systems have been studied in: (1) a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite; (2) coronitic leuco-gabbro norite; and (3) country-rock eclogite, all from the Nordøyane-Brattvåg area, western Norway. Isochrons defined by combinations of whole-rock samples and relict igneous phases give ages of 1198 ± 56 and 1289 ± 48 Ma (olivine gabbro) and 926 ± 70 Ma (leuco-gabbro norite) and have been interpreted in terms of two different episodes of igneous intrusion.
In gabbro with advanced corona formation, relict augite tends to retain its original Nd isotopic composition, while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet. In completely eclogitized samples, there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium. Whole-rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases, and preserve original igneous Nd isotopic composition. Results for eclogite minerals suggest Caledonian ages, but these phases have not achieved complete isotopic equilibrium. In contrast, Nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole-rock scale. Minerals from an external eclogite define a Sm---Nd isochron with an age of 400 ± 16 Ma which is interpreted to date synkinematic eclogite equilibration at high-P and -T conditions. 相似文献
7.
Ellen Gitlin 《Geochimica et cosmochimica acta》1985,49(7):1567-1579
The behavior of sulfide minerals during the physical and chemical changes accompanying seafloor alteration was studied in three basalt flows from the bottom of D.S.D.P. Hole 418A, Leg 53. The rocks are mildly altered, and contain primary, authigenic, and vein sulfide minerals. Sulfide habit, mineralogy, and trace element content are inter-related and are correlated with the extent and type of silicate and oxide alteration. Incipient alteration at > 90°–100°C was accompanied by low temperature reequilibration of pyrrhotite, and locally, by the oxidation of pyrrhotite to pyrite plus magnetite. The dominant stage of alteration, at ≤90°C, is characterized by dissolution and local redistribution of pyrite and chalcopyrite, whose precipitation appears to be controlled by the water/rock ratio and the extent to which the water has been modified by reaction with the basalt. Chalcopyrite was concentrated relative to pyrite by slight changes in fluid composition caused by reaction with other minerals. Concurrent precipitation of smectite causes a net increase in rock volume, tending to restrict seawater access. Calculations of rock cooling rate through time suggest that the most prolonged hydrothermal circulation occurs at low temperatures, giving rise to pervasive low temperature alteration assemblages. 相似文献
8.
Aqueous solutions that contain volatile (gas) components are one of the most important types of fluid in the Earth's crust. The record that such fluids have left in the form of fluid inclusions in minerals provides a wealth of insight into the geochemical and petrologic processes in which the fluids participated. This article reviews the systematics of CO2–H2O fluid inclusions as a starting point for interpreting the chemically more complex systems. The phase relations of the binary are described with respect to a qualitative P–T–X model, and isoplethic–isochoric paths through this model are used to explain the equilibrium and non-equilibrium behaviour of fluid inclusions during microthermometric heating and cooling. The P–T–X framework is then used to discuss the various modes of fluid inclusion entrapment, and how the resulting assemblage textures can be used to interpret the P–T conditions, phase states, and evolution paths of the parent solutions. Finally, quantitative methods are reviewed by which bulk molar volume and composition of CO2–H2O fluid inclusions can be determined from microthermometric observations of phase transitions. 相似文献
9.
铁同位素体系及其在矿床学中的应用 总被引：7，自引：4，他引：3
本文报道了世界范围内不同含铁矿物的Fe同位素组成,进一步了解了铁同位素在不同含铁矿物中的基本分布特征;系统总结了铁同位素在不同储库和不同类型矿床中的分布特征,构筑了铁同位素体系的基本框架;结合最新的研究成果,较全面地总结了铁同位素在矿床学领域的应用,得出了铁同位素可以用来示踪流体出溶、流体演化、表生蚀变作用和成矿物质来源的基本认识。在流体出溶过程中,相对于岩体,出溶的流体富集铁的轻同位素;成矿流体体系的演化过程中,矿物的结晶沉淀会导致铁同位素发生分馏,随着Fe(III)矿物的结晶沉淀,流体逐渐富集铁的轻同位素,随着Fe(II)矿物的结晶沉淀,流体逐渐富集铁的重同位素,随着矿物的结晶沉淀,流体的Fe同位素组成随时间发生演化;在成矿后的表生蚀变作用过程,高温蚀变作用形成的产物相对于原矿物富集铁重同位素,低温蚀变作用形成的产物基本保留了原矿物的铁同位素组成;Fe同位素在示踪成矿物质来源具有应用潜力,流体出溶、流体演化等重要成矿作用过程中Fe同位素组成的变化规律是利用Fe同位素示踪Fe来源的关键所在。 相似文献
10.
Chemical and Sr isotopic analyses have been made of waters from 16 geothermal sites in Iceland with particular reference to the systems at Reykjanes and Svartsengi for which compositions of geothermal sea water and fresh and hydrothermally-altered rocks have been compared. The alkalies display mixing relationships indicating a hydrothermal input of Rb and K to local meteoric and sea waters as do results for Sr and Ca involving high-temperature fluids. ratios of the geothermal waters of meteoric origin parallel those of associated rocks but are higher. Ratios for geothermal sea waters are 0.7042 (Reykjanes) and 0.7040 (Svartsengi), lower than for normal sea water (0.7092) because of leaching of Sr from rocks followed by partial removal into alteration minerals, of which epidote and chlorite may be most important. Consequently, associated hydrothermally-altered rocks have been subject to significant Sr isotopic contamination by sea water Sr raising ratios from 0.7032 for fresh rock to 0.7038–0.7042 (Reykjanes) and to 0.7039–0.7041 (Svartsengi). Altered basalt is only ~50% equilibrated isotopically with geothermal sea water, at a water/rock ratio of ~2, but is internally equilibrated whereas palagonitized rocks (water/rock ratio of 3 to 4) are close to Sr isotopic equilibrium with associated sea water but show significant internal Sr disequilibrium. Hydrothermal input is unlikely to be important in the oceanic mass balance of Sr but is likely to be highly significant in controlling the strontium isotopic composition of sea water. 相似文献
11.
Cores and cuttings of lamproite sills and host sedimentary country rocks in southeastern Kansas from up to 312 m depth were analyzed for major elements in whole rocks and minerals, certain trace elements in whole rocks (including the REE) and Sr isotopic composition of the whole rocks. The lamproites are ultrapotassic (K2O/Na2O = 2.0–19.9), alkalic [molecular (K2O/Na2O)/Al2O3 = 1.3-2.8], enriched in mantle-incompatible elements (light REE, Ba, Rb, Sr, Th, Hf, Ta) and have nearly homogeneous initial Sr isotopic compositions (0.707764-0.708114).
These lamproites could have formed by variable degrees of partial melting of harzburgite country rock and cross-cutting veins composed of phlogopite, K-Ti richterite, titanite, diopside, K-Ti silicates, or K-Ba-phosphate under high H2O/CO2 ratios and reducing conditions. Variability in melting of veins and wall rock and variable composition of the metasomatized veins could explain the significantly different composition of the Kansas lamproites.
Least squares fractionation models preclude the derivation of the Kansas lamproites by fractional crystallization from magmas similar in composition to higher silica phlogopite-sanidine lamproites some believe to be primary lamproite melts found elsewhere. In all but one case, least squares fractionation models also preclude the derivation of magmas similar in composition to any of the Kansas lamproites from one another. A magma similar in composition to the average composition of the higher SiO2 Ecco Ranch lamproite (237.5–247.5 m depth) could, however, have marginally crystallized about 12% richterite, 12% sanidine, 7% diopside and 6% phlogopite to produce the average composition of the Guess lamproite (305–312 m depth).
Lamproite from the Ecco Ranch core is internally fractionated in K2O, Al2O3, Ba, MgO, Fe2O3, Co and Cr most likely by crystal accumulation-removal of ferromagnesian minerals and sanidine. In contrast, the Guess core (305–312 m depth) has little fractionation throughout most of the sill except in several narrow zones. Lamproite in the Guess core has large enrichments in TiO2, Ba, REE, Th, Ta and Sc and depletions in MgO, Cr, Co and Rb possibly concentrated in these narrow zones during the last dregs of crystallization of this magma.
The Ecco Ranch sill did not show any evidence of loss of volatiles or soluble elements into the country rock. This contrasts to the previously studied, shallow Silver City lamproite which did apparently lose H2O-rich fluid to the country rock. Perhaps a greater confining pressure and lesser amount of H2O-rich fluid prevented it from escaping. 相似文献
12.
13.
长白山地区地热系统的研究目前还处于初级阶段,热储温度仍然是具有争议的问题。为进一步明确其高温地热成因机理,本文对该区域的4个温泉点与2口地热井进行了离子及气体组分测定与分析,并应用地球化学温标估算了热储温度。Na-K-Mg三角图和部分矿物I_S值指示长白山地区地热水与围岩未达到水岩平衡状态,稀释作用明显,仅石英、玉髓和部分含Ca2+矿物达到饱和并发生沉淀。根据本文及前人的研究,研究区同时存在高温喷气孔、高_ρ(Cl^-)水和高_ρ(SO_42-)水,这符合White汽-液分离模式提出的地热地表显示组合,因此推断长白山地区下部流体发生汽-液分离作用(沸腾)且地热系统为双相地热系统。由于双相地热系统的存在制约了水化学温标与部分气化学温标在研究区热储温度估算中的应用,因此本文结合研究区气组分特征,选取CO_2/H_2温标作为可靠温标,估算出热储温度在234.5～284.7℃之间。将长白山天池地区地质特征与地热流体特征结合,建立了长白山地区地热成因模式。 相似文献
14.
Ilya V. Veksler 《Chemical Geology》2004,210(1-4):7-31
Several types of fluid immiscibility may affect the evolution of volatile-rich magmatic systems at the magmatic–hydrothermal transition. The topology of silicate–salt–H2O systems implies that three-fluid immiscibility (silicate melt+hydrosaline melt+vapour) should be stable in a broad range of compositions and P–T conditions. The most important factor controlling the immiscibility appears to be the Coulombic properties (electric charges Z and ionic radii r) of the main network-modifying cations and the capacity for immiscibility appears to decrease in the following sequence: Mg>Ca>Sr>Ba>Li>Na>K. Liquid immiscibility is enhanced in peralkaline compositions and in the presence of nonsilicate anions such as F−, Cl−, CO32− and BO33−. In volatile-rich magmatic systems, the H2O is likely to react with the chloride, fluoride, borate and carbonate species and the chemical effects of high-temperature hydrolysis may be greatly enhanced by phase separation in systems with multiple immiscible fluid phases. Natural granitic magmas can thus exsolve a range of chemically and physically diverse hydrosaline liquids and the role of these fluid phases is likely to be especially significant in pegmatites and Li–F rare-metal granites. 相似文献
15.
The Darrehzar porphyry Cu-Mo deposit is located in Southwestern Iran (～70 km southwest of Kerman City). The porphyries occur as Tertiary quartz-monzonite stocks and dikes, ranging in composition from microdiorite to diorite and granodiorite. The Darrehzar stock is highly altered, and even in the outermost part of the intrusion, it is not possible to find completely fresh rock. Surface weathering was developing ferrous Fe-rich lithologic units in leached zone and concentrated copper minerals in supergene zone. Unlike eastern areas which do not account for deep faults, the supergene zone is well developed in western areas with maximum of 118 m thickness. Hydrothermal alteration and mineralization at Darrehzar are centered on the stock and were broadly synchronous with its emplacement. Early hydrothermal alteration was dominantly potassic and propylitic, and was followed by later phyllic and argillic alteration. The hydrothermal system involved both magmatic and meteoric water and boiled extensively. Copper mineralization was accompanied by both potassic and phyllic alteration. Four main vein groups have been identified: (I) quartz?+?pyrite?±?molybdenite?±?anhydrite?±?K-feldspar?±?chalcopyrite?±?bornite?±?Cu and Fe oxidic minerals (peripheral); (II) quartz?+?chalcopyrite?+?pyrite?+?molybdenite; (III) quartz?+?pyrite?±?calcite?±?chalcopyrite?±?anhydrite (gypsum); and (IV) quartz or calcite, gypsum or ± pyrite. Based on abundance, nature, and phases number observed at room temperature, three types of fluid inclusions are typically observed in these veins: (1) vapor-rich, (2) liquid-rich, and (3) multi-phase. Early hydrothermal alteration was caused by high temperature, high salinity orthomagmatic fluid and produced a potassic assemblage. Phyllitic alteration was caused by high salinity and lower temperature orthomagmatic fluid. Magmatic and meteoric water mixture was developed in the peripheral part of the stock and caused propylitic alteration which is attributed to a liquid-rich, lower temperature. 相似文献
16.
Jiang Feng Chen Yong-Fei Zheng Zi-Fu Zhao Bo Li Zhi Xie Bing Gong Hui Qian 《Contributions to Mineralogy and Petrology》2007,153(3):251-271
A combined study of mineral O and Rb–Sr isotopes was carried out for a number of Mesozoic granitoids in China in order to
compare the degree of O isotope equilibrium between coexisting minerals, with the validity of mineral Rb–Sr isochrons for
granitoids. A scrutiny of both O isotope geothermometry and Rb–Sr internal isochron dating for corresponding minerals indicates
that equilibrium O isotope fractionation between Rb–Sr isochron minerals corresponds to geologically meaningful isochron ages
if the variation in 87Rb/86Sr ratio is big enough to provide reasonably small uncertainties in age. Significant deviation of the Rb–Sr isochron age from
the actual age appears to depend on the difference in Sr isotopic composition between an external fluid and the igneous minerals.
As a result, O isotope disequilibrium is often caused by interaction between the rock and the external fluid that results
in mineral alteration. Post-magmatic alteration can cause isotope exchange between the minerals and an internally buffered
fluid that is isotopically identical to the host rock. The O isotope composition of coexisting minerals in studied samples
changed principally due to a decrease in temperature. Both Rb and Sr concentrations and the Sr isotope ratios of isochron
minerals also changed due to the mixing of different Sr reservoirs. Nevertheless, the isochron age can remain unchanged if
the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals
formed. This provides an insight into the effect of internal and external fluids on the validity of mineral Rb–Sr chronometry.
In addition, an alternative approach is proposed to construct the cooling curve by a combined use of O isotope temperature
and mineral isotope age for the granitoids of interest. Comparing with the traditional method using the empirical closure
temperature for Rb–Sr chronometry, the proposed approach utilizes fewer variables with smaller uncertainties than the traditional
way. 相似文献
17.
火星次生含水蚀变矿物是火星地质历史时期水环境和气候演变历史的真实记录,一直以来都是火星探测、火星陨石研究的重点,是认识火星环境特征和气候演化的重要研究对象。文中对比研究了表土角砾岩NWA7034、火成堆晶岩MIL03346等两块最富蚀变矿物火星陨石,以及Gale撞击坑出露的Sheepbed泥岩3种岩石类型的蚀变程度及其蚀变矿物类型和组合,分析了层状硅酸盐、铁氧化物/氢氧化物、钙硫酸盐等蚀变矿物的成因及环境指示意义。发现这3类岩石的蚀变作用各不相同。火星陨石NWA7034的蚀变作用以氧化和加热作用为主,无蒸发盐类矿物。火星陨石MIL03346的蚀变程度最低,为后期水溶液进入缝隙而引发的,蚀变作用以橄榄石的伊利石化、裂隙和缝隙中填充次生矿物细脉为主。而火星Sheepbed泥岩经历了后期的等化学风化过程(isochemical weathering),次生过程包括成岩蚀变和成岩后蚀变两个阶段。其中,成岩过程中的蚀变以橄榄石蚀变为铁氧化物和蒙皂石矿物为主,成岩后以形成蒸发盐类矿物硫酸钙为主。以上3种岩石蚀变矿物组成差异反映了火星上不同地质背景中、不同气候条件下蚀变过程的复杂性。文中对火星含水矿物及部分典型矿物的形成条件和过程进行系统总结,这对于理解未来火星探测任务、识别含水矿物的形成、揭示火星水环境和地质历史具有重要指导意义。 相似文献
18.
Abstract. Vein type tungsten mineralization at Degana is genetically and spatially associated with the Degana Granite. The deposit is characterized by pervasive wall rock alteration around the mineralized quartz veins. Laterally three different alteration zones, greisen, silicification and potassic zones, are marked based on the field features, mineral assemblages and geo-chemical characteristics. In the present paper, systematic mineralogical and chemical variation in these alteration zones is reported. Thick mono-mineralic (zinnwaldite) selvages around the veins characterize the deposit. Plagioclase and alkali feldspar are low in the greisen zones while K-feldspar shows more increase than plagioclase in the potassic zone. Quartz is uniformly high in all the alteration zones, but it shows an anomalous value in the silicification zone. Al2 O3 concentration shows initial depletion in greisen zone with gradual increase away from the contact. MgO and FeO are higher in greisen zone than silicification and potassic zones. The potassic zone is characterized by the depletion of Na2 O and higher value of K2 O.
The common presence of topaz and fluorite as both primary and secondary minerals and fluorine-bearing micas suggest fluorine partitioning in substantial amount between granitic melt and coexisting aqueous fluid phase and higher HF activity during the evolution of hydrothermal fluid. The mutual relationship of the fluorine minerals (topaz and fluorite) in the different alteration zones suggests an increase in the Ca2+ activity and decrease of H+ activity during the fluid evolution from greisenization towards alkali-metasomatised granite and the fluid is assumed to change from low to high activity ratio of Ca2+ /H+. 相似文献
The common presence of topaz and fluorite as both primary and secondary minerals and fluorine-bearing micas suggest fluorine partitioning in substantial amount between granitic melt and coexisting aqueous fluid phase and higher HF activity during the evolution of hydrothermal fluid. The mutual relationship of the fluorine minerals (topaz and fluorite) in the different alteration zones suggests an increase in the Ca
19.
吉伯特铁矿是新疆阿勒泰地区产于泥盆纪海相火山岩中的小型矿床。本文对吉伯特铁矿床的包裹体开展了研究,识别了熔体包裹体、熔体-流体包裹体以及富晶体的流体包裹体,并对其进行了初步的显微测温、激光拉曼光谱和电子探针等研究。熔体包裹体中含有富Si玻璃质、贫Si富Fe熔体、石英、萤石、方解石、磁铁矿等多种成分,它们分别组成不同的包裹体组合。熔体包裹体、熔体-流体包裹体和流体包裹体的存在表明它们被捕获时是一种熔体与流体共存的不混溶状态,这充分说明了吉伯特铁矿床的形成与岩浆熔体、岩浆-热液过渡性流体有直接的成因联系。吉伯特铁矿床中Fe的矿化是一个熔体相逐渐减少,流体相逐渐增加的连续演化过程,它受岩浆作用、岩浆-热液过渡性流体以及矽卡岩作用的共同制约。 相似文献
20.
Strontium as a tracer of weathering processes in a silicate catchment polluted by acid atmospheric inputs, Strengbach, France 总被引：4，自引：0，他引：4
This paper determines the weathering and atmospheric contributions of Ca in surface water from a small spruce forested silicate catchment (N–E France) receiving acid atmospheric inputs. The bedrock is a granite with K-feldspar and albite as dominant phases. The calcium content in plagioclase is low and the Ca/Na ratio in surface water is high, reflecting other sources of calcium from those expected from the weathering of major mineral phases. The biotite content is low. Only traces of apatite were detected while no calcite was found in spite of a major hydrothermal event having affected the granite. The strontium isotopic ratio 87Sr/86Sr and Sr content was used as a tracer of weathering and was determined in minerals and bulk bedrock, open field precipitation, throughfall, soil solution, spring and stream water. The Sr isotopic ratio of the reacting weathering end-member was predicted by simulating the alteration of the granite minerals by incorporating strontium into the water–rock interaction kinetic code KINDIS. In the early stages of water–rock interaction, K-feldspar and biotite strongly influence the isotopic composition of the weathering solution whereas, the Na-rich plagioclase appears to be the main long-term reactive weathering end-member. Approximately 50% of dissolved Sr in streamwater are atmospherically derived. The 87Sr/86Sr ratios of exchangeable Sr in the fine fraction at 1-m depth from a soil profile indicate that the amount of exchangeable Sr seems essentially controlled by atmospheric inputs. The exception is the deep saprolite where weathering processes could supply the Sr (and Ca). Na-Plagioclase weathering obviously control the chemistry and the isotopic composition of surface waters. The weathering of trace mineral plays a secondary role, the exception is for apatite when plagioclase is absent. Our hydrochemical, mineralogical and isotopic investigations show that a major part of the strong Ca losses detected in catchment hydrochemical budgets that result from the neutralization of acid precipitation has an atmospheric origin. Consequently, in the long term, in such areas, the availability of such an exchangeable base cation might be strongly limited and surface waters consequently acidified. 相似文献