Geochemical Fingerprint and Iron Ore Potential of the Siliceous Itabirite from Palaeoproterozoic Nyong Series,Zambi Area,Southwestern Cameroon

The study area forms part of an emerging iron ore province of southern Cameroon. Geochemistry analyses reveal that the siliceous itabirite has a very simple chemical composition, with Fe₂O₃ and SiO₂ representing more than 96 wt.% of the average composition; suggesting chemical precipitates of silica and iron. Low Al₂O₃ and TiO₂ concentrations and a weak positive correlation between them point to a minor detrital component in the precipitated marine sediments. The Si/Al ratio (average 52.7) indicates the hydrothermal origin of the studied itabirite. The Al–Si discrimination diagram supports this interpretation through the plot of all data in the hydrothermal field. The studied samples have low iron content (about 39.32% Fe), high gangue content (40.97% SiO₂ and 1.3 % Al₂O₃) and low concentration of the deleterious elements (0.16 % P and < 0.01% S). The main gangue mineral is silica which can be efficiently removed from iron ores during preparation of raw materials for the blast furnace process. According to commercial standards for crude iron ores, it may be concluded that the Zambi iron ores are a low‐grade magnetic ore that can be profitably exploited for the production of iron for steel production.     

Viscosity of liquid silica,silicates and alumino-silicates

Viscosity measurements are reported for amorphous silica and liquids belonging to the systems SiO₂-M, SiO₂-Al₂O₃-M, where M is an alkali-earth metal oxide, MnO, or alumina, and the systems SiO₂-“FeO”, SiO₂-FeO-Fe₂O₃-CaO, and SiO₂-Al₂O₃-N, where N = Na₂O or K₂O. The implications of these measurements concerning the coordination of Al and the structure of these liquids are briefly discussed. Stable liquids in the systems SiO₂-Al₂O₂-M show a non-Arrhenian temperature dependence of their viscosity, in general. Results obtained with empirical methods to calculate the viscosity of silicate liquids are compared with our observations.     

A study on pre- and post-tsunami shallow deposits off SE coast of India from the 2004 Indian Ocean tsunami: a geochemical approach

A study was made on samples from one core collected immediately after the December 2004 Asian tsunami to know the geochemical nature of the offshore tsunami sediments. The core sample was analyzed for sediment grain size, CaCO₃, organic carbon (OC) and major elements (SiO₂, TiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Na₂O, K₂O, P₂O₅, MnO). The results indicate that the core sample can be divided into two parts: (1) upper 0–25 cm, interpreted to be deposited after tsunami (AT), and (2) lower 25–45 cm, interpreted as before tsunami (BT) as evidenced by the sandy nature AT with fluctuating CaCO₃ contents. The AT part is devoid of OC suggesting that the sediment could have been transported to deeper regions along with the finer particles. Major elements such as SiO₂, TiO₂, CaO indicate high values than the other elements in AT part than in the BT part. The BT part contains Al rich alluvium mud associated with finer mud and organic particles. An analysis of the correlation matrix indicates the possible source of elements and transport of heavy minerals in the AT part than the BT part. The overall results suggest that the sediments could possibly have two different origins.     

Climate implications of major geochemical elements in the Holocene sediments of the North and East China monsoonal regions

Two Holocene sediment cores were retrieved respectively from the enclosed Lake Daihai in the monsoon/arid transition zone of North China and the Taihu Lake coast in the monsoonal area of the Yangtze delta, Eastern China. Distribution of major geochemical elements and their ratios were employed to reveal the characteristics of Holocene climate and associated environmental implications in the two regions. It is suggested that the temporal distribution of major elements serve as a useful indicator to denote the variations of monsoon effective precipitation for the enclosed lake area. High values of resistant elements such as Al₂O₃, SiO₂, TiO₂, (FeO + Fe₂O₃), MnO in the lake sediments correspond to the depressed chemical weathering and weakened mon-soon effective precipitation, while the highs of mobile and easy soluble elements such as MgO, CaO, Na₂O reflect the enhanced chemical weathering and increased monsoon effective precipitation in the lake basin. In comparison, the behaviors of the major elements in sediments of the Taihu Lake coast were largely controlled by the changes both in sea transgression in the different Holocene time periods and the monsoon precipitation. The relatively highs of Al₂O₃, TiO₂, (FeO + Fe₂O₃), in marine-influenced sediments suggest relatively strong coastal hydrodynamics and chemical weathering, and vice versa. Meanwhile, the lows of SiO₂, Na₂O and CaO in the non-marine-influenced sediments also denote relatively strong hydrodynamics and chemical weathering due to enhanced monsoon precipitation, and vice versa. Sedimentary environment should be taken into account when achieving a full understanding of their climate implications.     

Geochemistry of Mansar Lake sediments, Jammu, India: Implication for source-area weathering, provenance, and tectonic setting

The sediment geochemistry, including REE, of surface and core samples from Mansar Lake, along with mineralogical investigations, have been carried out in order to understand the provenance, source area weathering, hydrolic sorting and tectonic setting of the basin. The geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters.The major element log values (Fe₂O₃/K₂O) vs (SiO₂/Al₂O₃) and (Na₂O/K₂O) vs (SiO₂/Al₂O₃) demarcate a lithology remarkably similar to that exposed in the catchment area. The chondrite normalized REE patterns of lake samples are similar to Post Archaean Australian Shale (PAAS) with LREE enrichment, a negative Eu anomaly and almost flat HREE pattern similar to a felsic and/or cratonic sedimentary source. However, the La–Th–Sc plot of samples fall in a mixed sedimentary domain, close to Upper Continental Crust (UCC) and PAAS, suggesting sedimentary source rocks for the Mansar detritus. It also indicates that these elements remained immobile during weathering and transportation. The mineralogical characteristic, REEs, and high field strength elements (HFSE), together with the high percentage of metamorphic rock fragments in the Siwalik sandstone, support a metamorphic source for lower Siwalik sediments. A very weak positive correlation between Zr and SiO₂, poor negative correlation with Al₂O₃, negative correlation of (La/Yb)_N and (Gd/Yb)_N ratios with SiO₂ and positive correlation with Al₂O₃, suggest that Zr does not dominantly control the REE distribution in Mansar sediments. The petrographic character and textural immaturity indicate a short distance transport for the detritus. The distribution of elements in core samples reflect fractionation. The higher Zr/Th and Zr/Yb ratios in coarse sediments and PAAS compared to finer grained detritus indicate sedimentary sorting. Plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower Siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting. It therefore reveals lower Siwalik depositional history.     

Are spatial distributions of major elements in soil influenced by human landscapes?

The present study attempted to evaluate the influence of human activity on major elements (Na₂O, MgO, Al₂O₃, SiO₂, K₂O, CaO, Fe₂O₃), and to find a method to explore correlations between major elements and human disturbances, according to geospatial theories and methods. The study results indicate that landscapes influence major elements in diverse ways: Al₂O₃ is closely related to road and mine landscapes; strong relationships exist between MgO, Fe₂O₃, CaO, and SiO₂ and roads; Na₂O, SiO₂, and Fe₂O₃ are unrelated to city landscapes; and Na₂O is unrelated to road and mine landscapes.     

Chemical composition,volatile components,and trace elements in the melts of the Gorely volcanic center,southern Kamchatka: Evidence from inclusions in minerals

Melt inclusions in olivine and plagioclase phenocrysts from rocks (magnesian basalt, basaltic andesite, andesite, ignimbrite, and dacite) of various age from the Gorely volcanic center, southern Kamchatka, were studying by means of their homogenization and by analyzing the glasses in 100 melt inclusions on an electron microprobe and 24 inclusions on an ion probe. The SiO₂ concentrations of the melts vary within a broad range of 45–74 wt %, as also are the concentrations of other major components. According to their SiO₂, Na₂O, K₂O, TiO₂, and P₂O₅ concentrations, the melts are classified into seven groups. The mafic melts (45–53 wt % SiO₂) comprise the following varieties: potassic (on average 4.2 wt % K₂O, 1.7 wt % Na₂O, 1.0 wt % TiO₂, and 0.20 wt % P₂O₅), sodic (3.2% Na₂O, 1.1% K₂O, 1.1% TiO₂, and 0.40% P₂O₅), and titaniferous with high P₂O₅ concentrations (2.2% TiO₂, 1.1% P₂O₅, 3.8% Na₂O, and 3.0% K₂O). The melts of intermediate composition (53–64% SiO₂) also include potassic (5.6% K₂O, 3.4% Na₂O, 1.0% TiO₂, and 0.4% P₂O₅) and sodic (4.3% Na₂O, 2.8% K₂O, 1.3% TiO₂, and 0.4% P₂O₅) varieties. The acid melts (64–74% SiO₂) are either potassic (4.5% K₂O, 3.6% Na₂O, 0.7% TiO₂, and 0.15% P₂O₅) or sodic (4.5% Na₂O, 3.1% K₂O, 0.7% TiO₂, and 0.13% P₂O₅). A distinctive feature of the Gorely volcanic center is the pervasive occurrence of K-rich compositions throughout the whole compositional range (silicity) of the melts. Melt inclusions of various types were sometimes found not only in a single sample but also in the same phenocrysts. The sodic and potassic types of the melts contain different Cl and F concentrations: the sodic melts are richer in Cl, whereas the potassic melts are enriched in F. We are the first to discover potassic melts with very high F concentrations (up to 2.7 wt %, 1.19 wt % on average, 17 analyses) in the Kuriles and Kamchatka. The average F concentration in the sodic melts is 0.16 wt % (37 analyses). The melts are distinguished for their richness in various groups of trace elements: LILE, REE (particularly HREE), and HFSE (except Nb). All of the melts share certain geochemical features. The concentrations of elements systematically increase from the mafic to acid melts (except only for the Sr and Eu concentrations, because of active plagioclase fractionation, and Ti, an element contained in ore minerals). The paper presents a review of literature data on volcanic rocks in the Kurile-Kamchatka area in which melt inclusions with high K₂O concentrations (K₂O/Na₂O > 1) were found. K-rich melts are proved to be extremely widespread in the area and were found on such volcanoes as Avachinskii, Bezymyannyi, Bol’shoi Semyachek, Dikii Greben’, Karymskii, Kekuknaiskii, Kudryavyi, and Shiveluch and in the Valaginskii and Tumrok Ranges.     

羌北-昌都地块那益雄组火山岩锆石U-Pb年代学及地球化学特征

本文对青藏高原羌北-昌都地块阿布日阿加措地区的晚二叠世那益雄组火山岩进行了年代学和地球化学研究。该火山岩主要由玄武岩、安山玄武岩、安山岩、流纹岩、凝灰岩组成,具有碱性玄武岩到酸性熔岩的特征。锆石U-Pb年代学研究表明该火山岩的形成时代为251. 1±4. 8～249. 6±1. 3 Ma之间。地球化学分析结果表明,该地区的流纹岩具有高的Si O2(74. 85%～77. 55%)和Na2O+K2O(5. 40%～6. 61%)含量,较低的MgO、K2O和Ca O含量,Al2O3含量低且稳定,里特曼指数平均为1. 15,小于3. 3。安山岩Si O2含量55. 13%～56. 28%,Na2O+K2O含量4. 13%～6. 15%,里特曼指数平均为2. 20,小于3. 3,属于钙碱性安山岩。碱性玄武岩Si O2含量51. 49%,Na2O+K2O含量6. 34%,里特曼指数为4. 73,属于碱性系列。稀土元素配分曲线为富集LREE的右倾型。另外,富集大离子亲石元素(LILE) Th、U,亏损高场强元素(HFSE) Nb、Ta等特征,均说明羌北-昌都地块阿布日阿加措地区的火山岩形成于陆缘岛弧环境。     

安徽庐枞盆地酸性蚀变岩帽地质地球化学特征研究

酸性蚀变岩帽是浅成低温热液系统演化的产物,形成于酸性高氧化性流体的化学条件下;在高硫化型浅成低温热液金矿床中广泛发育,是该类矿床的显著识别特征。通过对酸性蚀变岩帽的野外地质特征、矿物共生组合和地球化学特征研究,能较好阐明浅成低温成矿热液系统的特征、性质、发生和发展演化及成矿作用过程。庐枞矿集区是长江中下游成矿带重要的矿集区之一,盆地内广泛发育以明矾石为特征蚀变矿物的酸性蚀变岩帽,面积超过30km~2,指示盆地内高硫化浅成低温热液系统的存在。目前为止,前期工作主要针对明矾石矿床地质特征和明矾石资源储量进行,该酸性蚀变岩帽的地质地球化学特征研究尚未开展。本次工作通过对酸性蚀变岩帽系统的野外采样、全岩地球化学分析和短波红外光谱测试分析技术(PNIRS测试)分析,确定其主要赋存在砖桥组火山岩中,组成矿物为石英、明矾石、高岭石、地开石,此外有少量绢云母、伊利石、珍珠陶土、叶蜡石、褐铁矿,极少数的叶腊石和黄钾铁矾等,在钻孔深部存在浸染状和半自形粒状黄铁矿。由于受到地表风化剥蚀和不同热热中心的影响,水平方向从矾山明矾石矿床向外围发育石英+明矾石带、石英+高岭石/地开石+明矾石带、石英+高岭石/地开石带、硅化带以及最外围的泥质带即高岭石±绢云母±伊利石带。根据酸性蚀变岩帽的矿物组合和主量元素特征,可将其分为三类:硅质蚀变岩、明矾石蚀变岩和粘土蚀变岩。硅质蚀变岩中SiO_2含量发生明显的富集作用,其余主量元素(K_2O、Na_2O、Al_2O_3、Fe_2O_3、P_2O5)含量显著降低;明矾石蚀变岩和粘土蚀变岩具有相似的地球化学特征,SiO_2、Al_2O_3、Fe_2O_3、P_2O_5元素含量范围变大,K_2O和Na_2O含量降低,且Na_2O降低更加明显;而钛为不活泼元素,在岩石发生蚀变过程中TiO_2含量变化很小。矾山地区的酸性蚀变岩帽的产状、蚀变类型、地球化学特征受构造和地层的双重控制。     

Geochemistry of sediments from Lake Grand,Northeast Russia

Major and trace element distribution in the bottom sediments from Hole 13 drilled in Lake Grand, Magadan district, was studied using the method of principal components. It was established that geochemical characteristics are correlated with environmental changes. The sediments of cold MIS2 and MIS4 are characterized by the enriched TiO₂, MgO, Al₂O₃, Fe₂O₃, and Cr and low Na₂O, K₂O contents, which is related to the grain-size composition of sediments. Sediments of warm stages show an opposite tendency. High concentration peaks of iron, phosphorus, and manganese correspond to the accumulation levels of vivianite and ferromanganese rocks. Silica is represented by biogenic and abiogenic varieties. Maximum SiO₂ contents were found in the Late Holocene sediments and mark the high biological productivity of the basin. Revealed variations of some elements are correlated with the Heinrich events.     

Geochemistry of Proterozoic Radioactive Arkoses in Khammam District,Andhra Pradesh,India: Evidence for K-rich Evolved Granitoid Upper Crust in the Late Archaean

Geochemical studies on radioactive arkoses (43–153 ppm U and 387–862 ppm Th) of the Proterozoic Pakhal Supergroup from Bangaruchilka, Khammam district, Andhra Pradesh, India, indicate that their gross major and trace element chemistry reflect their mineral composition. Chemically, arkoses are rich in silica (83% to 88% SiO₂) and potassium (3% to 5% K₂O), with consistently high Al₂O₃/Na₂O (36 to 50) and K₂O/Na₂O (18 to 25) ratios, which indicate that they are chemically mature sediments. The arkoses also show higher concentrations of Ti, V, Cr, Ni, Cu, Y, Zr, Nb, La and Pb.The values (60% to 68%) of chemical alteration index (CIA) of studied arkoses are moderate, and indicate that the source rocks have undergone lesser degree of chemical weathering. Tectonic setting discriminate plots of Fe₂O₃ (total)/MgO vs. TiO₂ and Al₂O₃/SiO₂, and K₂O/Na₂O vs. SiO₂ and SiO₂/Al₂O₃ indicate that the Bangaruchilka arkoses represent the sediments that were deposited in passive continental margin (PM), which is further supported by association of platformal type of sediments (quartzites and phyllites) with them. Unlike middle Archaean sedimentary rocks, the studied arkoses are depleted in Na₂O, MgO and CaO, and distinctly enriched in SiO₂ and K₂O. These geochemical features match with post-Archaean clastic sediments, which argues for the involvement of late Archaean granitic crust in supply of detritus of studied arkoses. Enrichment of potassium alongwith abundant microcline and elevated concentrations of Y, Zr, Nb, U, Th, La, etc. in arkoses indicate K-rich evolved granitoid upper crust in the provenance during the late Archaean-early Proterozoic period.Dominance of mechanical weathering over chemical weathering favoured arkose formation, and also transfer of radio-elements with clastic sediments into the Pakhal basin. After sedimentation, uranium seems to have been remobilised from the rocks of Archaean-Pakhal system, consequent to post-sedimentation structural disruptions, and concentrated along suitable structural loci. Therefore, it is likely to encounter significant concentration of uranium in close proximity of unconformable Archaean and Pakhal contacts and tectonic zones, thereby making Pakhal basin and its environs a suitable terrain to search for concealed uranium mineralisation.     

Geochemical behavior during mineralization and alteration events in the Baiyinchang volcanic-hosted massive sulfide deposits,Gansu Province,China

The Zheyaoshan deposit is the largest within the Baiyinchang (BYC) volcanic-hosted massive sulfide (VHMS) district, located in the northern Qilian orogenic belt of North China. The deposit is hosted by quartz keratophyre tuffs, with wall-rock alteration mainly comprising chlorite, sericite, quartz, pyrite and epidote. Mineral assemblages within the altered host rocks can be divided into a sericite-quartz-dominant assemblage (sericite-silicified zone), a chlorite-dominant assemblage (chlorite-dominant zone) and a pyrite-dominant assemblage (mineralized zone) based on geochemical analysis and alteration characteristics. We have conducted detailed processing and critical analysis of the geochemical data of both the altered and least-altered host rocks in order to investigate the problem of closure in the geochemical dataset to eliminate the influence that each component has on the other in terms of mass change, and have applied the standardized method of the mass change calculation to analyze this data. The results show that: (1) the sericite-silicified zone formed along fissures due to the ingress of hydrothermal fluids, with MnO₂, Na₂O and CaO being mobilized into the hydrothermal fluids leached and MgO, Fe₂O₃, SiO₂, K₂O, BaO deposited. Additionally, Ag, Cu and chalcophile elements (Ag, As and Bi) were enriched while Pb, Zn and large ion lithophile elements (LILEs) (Cs, Sr, Eu, Be) were mobilized into hydrothermal fluids; (2) the physiochemical conditions and pH levels of the hydrothermal fluids changed during sericitization, with MgO, Fe₂O₃, BaO being further enriched and MnO, Na₂O, CaO further depleted, leading to formation of chlorite and the initial precipitation of metallogenic the (Cu, Zn, Pb) and chalcophile elements (Ag, As, Bi); (3) the negative Eu anomaly was mainly due to its strong activity when Eu is mobilized into the hydrothermal fluids during since plagioclase break-down during the sericite-silicification process; (4) AI and CCPI values gradually increase towards the orebody. The chlorite-dominant assemblage and sericite-quartz-dominant assemblage on the periphery of the chlorite-dominant zone can all be used as vectors towards the volcanic massive sulfide orebody and for regional-scale mineral exploration. Either leached elements or enriched elements can be considered as significant indicator elements and as prospective indicators for geochemical exploration within the BYC district. The Eu anomaly may be especially useful as an indicator for distinguishing the least-altered rocks which has great significance for exploration on the regional scale.     

Provenance,tectonic setting and source-area weathering of the lower Cambrian sediments of the Parahio valley in the Spiti basin,India

The geochemical study of siliciclastic rocks from the Lower Cambrian of Parahio Valley has been studied to describe the provenance, chemical weathering and tectonic setting. The K₂O/Al₂O₃ ratio and positive correlation of Co (r=0.85), Ni (r=0.86), Zn (r=0.82), Rb (r=0.98) with K₂O reflects that the presence of clay minerals control the abundances of these elements and suggests a warm and humid climate for this region. The chondrite normalized REE pattern of the samples is equivalent to upper continental crust, which reflects enriched LREE and flat HREE with negative Eu anomaly. The tectonic setting discriminant diagram log[K₂O/Na₂O] vs. SiO₂; [SiO₂/Al₂O₃] vs. log[K₂O/Na₂O]; [SiO₂/20] – [K₂O+Na₂O] – [TiO₂+Fe₂O₃+MgO] indicates transitional tectonic setting from an active continental margin to a passive margin. The discriminant function plot indicates quartzose sedimentary provenance, and to some extent, the felsic igneous provenance, derived from weathered granite, gneissic terrain and/or from pre-existing sedimentary terrain. The CIA value indicates low to moderate degree of chemical weathering and the average ICV values suggests immature sediments deposited in tectonically active settings. The A–CN–K diagram indicates that these sediments were generated from source rocks of the upper continental crust.     

Geochemical Constraints of Sediments on the Provenance, Depositional Environment and Tectonic Setting of the Songliao Prototype Basin

The geochemistry of sediments is primarily controlled by their provenances, and different tectonic settings have distinctive provenance characteristics and sedimentary processes. So, it is possible to discriminate provenances, depositional environments and tectonic settings in the development of a sedimentary basin with the geochemistry of the clastic rocks. The analytical results of the present paper demonstrate that sediments in the Songliao prototype basin are enriched in silica (SiO2=66.48-80.51 %), and their ΣREE are 30-130 dmes of that of chondrite with remarkable Eu anomalies. In discriminating diagrams of Eu/Eu vs eeeeeREE and (La/Yb)N vs ΣREE, most samples locate above the line Eu/ Eu=l, on the right of the line Eu/Eu/ΣREE=1 and under the line La/Yb)N/eeeeeREE=1/8, which indicates that the depositional environment of sediments in the basin was oxidizing. In addition, variations of MgO, TiO2, A12O3, FeO+Fe2O3, Na2O and CaO vs SiO2 reflect a tendency of increasing mineral maturity of sediments     

内蒙古嘎仙镍钴矿区岩浆作用与成矿

内蒙古嘎仙矿床为大兴安岭北段与岩浆作用有关的大型低品位镍钴硫化物矿床,成矿作用主要与花岗质岩浆作用有关。文章主要对矿区内矿体下盘的花岗岩类(花岗斑岩、长石斑岩、伟晶状花岗岩、黑云母花岗岩)进行了锆石LA-ICP-MS U-Pb定年,获得花岗斑岩的谐和线年龄(125.3±1.1)Ma～(127.5±4.5)Ma,长石斑岩的谐和线年龄为(128.1±2.2)Ma,伟晶状花岗岩的谐和线年龄为(127.9±2.3)Ma,黑云母花岗岩的谐和线年龄为(127.9±1.4)Ma,说明这些花岗岩类主要形成于中生代早白垩世。通过对矿化超镁铁岩、科马提岩、镁铁岩(辉绿岩、玄武岩)、长英质岩(闪长岩、长石斑岩、斜长花岗岩、花岗斑岩、伟晶状花岗岩、黑云母花岗岩)及围岩(大理岩)的主量、微量元素地球化学测试分析,结果表明,与吉峰科马提岩成分相比较,矿化超镁铁岩具有较高的w(SiO_2)(40.53%～54.96%)、w(TiO_2)(0.24%～0.86%)、w(Al_2O_3)(3.58%～10.47%)、w(FeO)(5.30%～8.80%)、w(CaO)(7.35%～13.66%)、w(Na_2O)(0.01%～0.76%)、w(K_2O)(0.02%～0.66%)和w(P_2O_5)(0.06%～0.61%);镁铁岩(包括辉绿辉长岩、玄武岩)铝含量较高,w(Al_2O_3)=16.34%～17.74%;长英质岩类也富铝质(Al_2O_3/(CaO+Na_2O+K_2O)=1.34～1.63),多数岩石属于钙碱性系列。闪长岩与镁铁岩相比,具有较高的硅、铝、钾、钠,较低的铁、镁和钙,微量元素具有大离子亲石元素富集,高场强元素相对亏损的右倾模式;稀土元素具有轻稀土元素富集,重稀土元素相对亏损特征,超镁铁岩类成分点位于N-MORB与OIB范围之间,而镁铁岩和长英质岩类成分点位于E-MORB和OIB之间。镁铁岩落入火山弧玄武岩范围,长英质岩落入火山弧花岗岩+同碰撞花岗岩范围,同属于造山后花岗岩的范围,因此镁铁质岩的形成应属于俯冲-碰撞环境,而长英质岩的形成应属于造山后伸展环境。根据各岩类所含成矿元素和亲流体元素分析,认为含矿热液来自矿区西部的深部,并且构建了嘎仙矿床的成矿模型,即超镁铁岩先期侵位,后期经历了区域的变质变形,最后发生燕山期大规模花岗质岩浆活动及成矿流体的蚀变矿化。     

Relationship between chemical composition and magnetic susceptibility in the alkaline volcanics from the Isparta area,SW Turkey

Potassium-rich volcanic rocks in the Isparta area (SW Turkey) consist mainly of older (Pliocene) volcanic rock suites (e.g., lamprophyre, basaltic trachyandesite, trachyandesite, trachyte) and younger (Quaternary) caldera forming lava dome/flows (e.g., tephriphonolite, trachyte) and pyroclastics (ash/pumice fall deposits and ignimbritic flows). The magnetic susceptibility (K) was performed for both groups. The magnetic susceptibility value of the less evolved rocks characterized by SiO₂ < 57 wt% (e.g., basaltic trachyandesite, tephriphonolite, lamprophyric rocks) and having mostly mafic phenocrysts such as pyroxene, amphibole, and biotite-phlogopite is over 10 (10⁻³ [SI]). Fine to medium-grained and subhedral to anhedral opaque minerals are scattered especially in the matrix phase of the less evolved volcanic rocks. However, the K value of the more evolved rocks (e.g., trachyte and trachyandesites) with SiO₂ over 57 wt% vary between 0.1 and 28, but most of them below 10. SI values are negatively correlated with SiO₂, Na₂O, but positively correlated with Fe₂O₃, CaO, MnO, P₂O₅ and MgO contents, suggesting inverse variation of SI with fractionation of potassic magma. That is to say that less evolved volcanic rocks have relatively higher magnetic susceptibility values in the volcanic suite. Fine to medium-grained and subhedral to anhedral Fe-Ti oxides are scattered mainly in the matrix phase of the less evolved volcanics, presumably cause the pronounced relatively higher magnetic susceptibility.     

新疆西天山莫托萨拉热水沉积型铁锰矿床矿物学与地球化学特征

莫托萨拉铁锰矿床位于西天山阿吾拉勒成矿带东端,研究程度相对薄弱,在矿床成因方面存在热水沉积、沉积-热液改造、胶体化学沉积等争论。本文详细研究了莫托萨拉最上层锰矿及其围岩的矿物组成、结构构造和地球化学特征,并综合前人资料对整个铁锰矿床的成因做了进一步探讨。本研究首次在矿区发现了热液长石岩,其主要由钠长石、钾长石以及少量重晶石、霓石、锌铁黄长石等矿物组成,类似于"白烟型"热水沉积岩。莫托萨拉最上层锰矿主要由锰橄榄石、褐锰矿、红硅锰矿、磁锰铁矿以及少量重晶石、方铁锰矿等矿物组成,发育有典型的热水内碎屑结构,指示其沉积于海底热液喷流口附近。该层锰矿的Al/(Al+Fe+Mn)值很低(0～0.02)、Si/Al值较高(7.9～10.9)、Fe/Ti值很高(428～1353),通过UCC标准化后发现明显富集Zn、Ba、Pb等元素,而Co、Ni、Cu等元素未见富集,以上地球化学特征与现代海底热液成因铁锰沉积物一致。在Fe/Ti-Al/(Al+Fe+Mn)、Si O2-Al2O3、10×(Co+Ni+Cu)-Fe-Mn、100×(Zr+Ce+Y)-15×(Cu+Ni)-(Fe+Mn)/4等判别图中,莫托萨拉的锰矿层和铁矿层样品均落在海底热液沉积区。锰矿层和铁矿层的稀土元素经PAAS标准化后具有明显的Ce负异常、Eu正异常和Y正异常,与现代海底热液成因铁锰沉积物的稀土配分模式非常相似。综合分析本次研究的矿物学、岩石学、地球化学特征以及前人资料,本文认为莫托萨拉铁锰矿床为海相热水沉积成因,成矿与同期海底火山的间歇性活动密切相关,海底热液的化学组分、温度高低和活动强弱都具有明显的脉动性。莫托萨拉矿区铁锰共存但各自独立成矿,且铁锰分离程度较高,这在显生宙沉积型锰矿中独具特色。鉴于前人曾报道莫托萨拉铁矿石中存在菌藻类微生物化石,我们推测,该矿床的铁锰分离过程除了受控于沉积环境的氧化还原条件变化外,微生物的选择性氧化沉淀可能也发挥了重要作用,值得开展深入研究。     

Water／Rock Interactions and Changes in Chemical Composition During Zeolite Mineralization

Systematic analysis and comparative study of the chemical compositons of rocks and ores from the main types of zeolite deposits in the surroundings of the Songliao Basic have shown that the process of formation of zeolite from volcanic and pyroclastic rocks is generally characterized by the relative purification of SiO2,i.e.,SiO2/Al2O3 ratios tend to increase,alkali eart elements (CaO MgO)and H2O are relatively enriched,and the alkali metals(K2O Na2O)are depleted in their total amount.The alkali metals K and Na follow different rules of migration and enrichment during the formation of mordenite and clinoptilolite.In the process of formation of mordenite more Na^ will be imported and K^ will be lost remarkably.On the contrary,in the process of formation clinoptilolite more K^ will be incorporated and Na^ will become obviously depleted.     

Petrochemical constraints on the models of Cretaceous-Eocene tectonic evolution of the Eastern Pontic chain (Turkey)

Sixteen selected samples from the Upper Cretaceous volcanic belt of the Eastern Pontids have been analysed for major elements, Rb, Sr and Zr. On the basis of the K₂O versus SiO₂ distribution, two groups of rocks have been distinguished, one with calc-alkaline affinity and a second group with shoshonitic character. The calc-alkaline rocks have porphyritic texture with clinopyroxene, plagioclase and orthopyroxene as phenocryst and in the groundmass. The orthopyroxene is lacking in the shoshonites where plagioclase, clinopyroxene and, in the more evolved terms, amphibole and biotite are the main phenocryst minerals. The shoshonitic rocks have higher $\text{K}{}_2\text{O}\text{Na}{}_2\text{O}$ ratio, K₂O, P₂O₅ and Rb, contents with respect to the calc-alkaline samples. The TiO₂ content is invariably low, never exceeding approximately 1%. The occurrence of volcanic rocks ranging in composition from calc-alkaline to shoshonitic in the Upper Cretaceous volcanic belt of the Eastern Pontids suggests that the Upper Cretaceous volcanic cycle reached its mature stage before the onset of the Eocene calc-alkaline volcanism which is believed to be neither genetically nor tectonically related with the Upper Cretaceous volcanism.     

Aluminum coordination in metaaluminous and peralkaline silicate melts

Incremental amounts of Na₂O and K₂O added to immiscible melts in the MgO-CaO-TiO₂-Al₂O₃ SiO₂ system cause a decrease in critical temperature, phase separation and change in the pattern of Al₂O₃ partitioning. Al₂O₃, which is concentrated in the low SiO₂ immiscible melts in the alkali-free system, is increasingly partitioned into the high-SiO₂ immiscible melt as the alkali/aluminium ratio is increased. However, K₂O is more effective than Na₂O in stabilizing Al₂O₂ in the SiO₂-rich melt. The coordination changes occurring in the aluminosilicate melts upon the addition of the alkali oxides are described by CaAl₂O₄+2SiOK=2KAlO₂+SiOCaOSi where K (or Na) displaces Ca as the charge-balancing cation for the networkforming AlO₄ tetrahedra. The increased stability of the AlO₄ species in the highly polymerized SiO₂-rich melt and the consequent shrinkage of the miscibility gap is ascribed to positive configurational entropy and negative enthalpy changes associated with the formation of K, Na-AlO₄ species. Element partition systematics indicate that (Na, K)AlO₂ species favor the more polymerized, CaAl₂O₄ and TiO₂ species, the less polymerized silicate structure in the melt.