Shear zone origin of quartzite mylonite and mylonitic pegmatite in the Coyote Mountains metamorphic core complex,Arizona

Mylonites derived largely from granite, pegmatite and sedimentary quartzite occupy a 500 m thick, gently N-dipping zone along the northern flank of the Coyote Mountains, west of Tucson, in southeastern Arizona. The quartzite mylonites are exceptionally well developed and occur as discrete layers and lenses, 2–5 m thick, within yet thicker, boudinaged, sill-like lenses of mylonitic pegmatite. Mylonitization took place in the Tertiary within a normal-slip ductile shear zone. The shear zones formed in response to regional extension of continental crust. Extension is along a north-south line, and N-directed sense of shear is revealed by mica fish, oblique foliations in dynamically recrystallized quartz aggregates, and asymmetric quartz c-axis fabrics. The microstructures and c-axis fabrics, taken together, disclose that ductile and brittle deformation was achieved by intense, penetrative, non-coaxial laminar flow dominated by progressive simple shear.     

Mud dispersal across a Cretaceous prodelta: Storm‐generated,wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies

Determining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.     

Tectonometamorphic evolution of the Koralm Complex (Eastern Alps): constraints from microstructures and textures of the ‘Plattengneis’ shear zone

Lattice preferred orientations (LPO) of quartz have been investigated along a south–north oriented section across the Plattengneis of the Koralm Complex (Eastern Alps). The Plattengneis forms an important shear zone within the Austroalpine nappe complex of the Eastern Alps, which has developed during the Cretaceous evolution of the Alpine orogen. The quartz c-axes form small circle distributions in the southernmost parts of the Koralm Complex, which represent the uppermost structural level of the Plattengneis. Further to the North two maxima between the Y and Z directions of the finite strain can be interpreted in terms of preferred slip on the rhomb planes. These fabrics continuously grade into (type I and type II crossed) girdle distributions in a northward direction. A strong maximum near the Y-axis with the tendency to be distributed along a single girdle, with three corresponding maxima of a-axes near the margin of the pole figure, can be observed in the central and northern parts. Such LPO are characteristic for both high grade metamorphic conditions and high finite strain. The microstructures display that the deformation within the Plattengneis shear zone was synmetamorphic. A continuous increase of peak temperatures (and pressure) from approximately 550 °C to approximately 750 °C from the South to the central parts can be inferred from geothermometric calculations. The temperatures then decrease to approximately 650 °C from the central parts to the North. The related pressures increase from 8 to 16 kbar, and then decrease to 10 kbar. The LPO changes that have been observed in the study area are best interpreted in terms of temperature dependence of the activation of glide systems within quartz aggregates. The temperature and pressure evolution may indicate that the central parts of the Koralm Complex have been exhumed by larger amounts than the northern and southern parts. This is also documented by the LPO evolution. Therefore, we assume that the Plattengneis shear zone formed during the exhumation of the Koralm Complex, and is related to the exhumation of high-pressure units in the footwall of this shear zone. Accordingly, the kinematics of the Plattengneis shear zone is rather extensional than thrust-related. The implications for the structural evolution of the Eastern Alps are shortly discussed.     

Fabric development during shear deformation in the Main Central Thrust Zone, NW-Himalaya, India

Quartz microfabrics and associated microstructures have been studied on a crustal shear zone—the Main Central Thrust (MCT) of the Himalaya. Sampling has been done along six traverses across the MCT zone in the Kumaun and Garhwal sectors of the Indian Himalaya. The MCT is a moderately north-dipping shear zone formed as a result of the southward emplacement of a part of the deeply rooted crust (that now constitutes the Central Crystalline Zone of the Higher Himalaya) over the less metamorphosed sedimentary belt of the Lesser Himalaya. On the basis of quartz c- and a-axis fabric patterns, supported by the relevant microstructures within the MCT zone, two major kinematic domains have been distinguished. A noncoaxial deformation domain is indicated by the intensely deformed rocks in the vicinity of the MCT plane. This domain includes ductilely deformed and fine-grained mylonitic rocks which contain a strong stretching lineation and are composed of low-grade mineral assemblages (muscovite, chlorite and quartz). These rocks are characterized by highly asymmetric structures/microstructures and quartz c- and a-axis fabrics that indicate a top-to-the-south sense that is compatible with south-directed thrusting for the MCT zone. An apparently coaxial deformation domain, on the other hand, is indicated by the rocks occurring in a rather narrow belt fringing, and structurally above, the noncoaxial deformation domain. The rocks are highly feldspathic and coarse-grained gneisses and do not possess any common lineation trend and the effects of simple shear deformation are weak. The quartz c-axis fabrics are symmetrical with respect to foliation and lineation. Moreover, these rocks contain conjugate and mutually interfering shear bands, feldspar/quartz porphyroclasts with long axes parallel to the macrosopic foliation and the related structures/microstructures, suggesting deformation under an approximate coaxial strain path.On moving towards the MCT, the quartz c- and a-axis fabrics become progressively stronger. The c-axis fabric gradually changes from random to orthorhombic and then to monoclinic. In addition, the coaxial strain path gradually changes to the noncoaxial strain path. All this progressive evolution of quartz fabrics suggests more activation of the basal, rhomb and a slip systems at all structural levels across the MCT.     

青海可可西里北缘韧性剪切带的显微构造特征

在可可西里北缘发现的糜棱岩化带,经显微构造研究确定为韧性剪切带。此带发育流劈理及拉伸线理。糜棱岩化花岗岩和糜棱岩化石英脉中的石英发育亚晶粒构造、位错构造和动态重结晶,长石主要发育机械双晶。石英c轴组构属韧性剪切带中的典型形式,石英变形以位错蠕变机制和位错滑移机制共存为特征,石英的动态重结晶作用是由亚晶粒旋转机制形成。长石的变形主要是通过机械双晶实现的。此韧性剪切带形成时的温度约400℃,差异应力约30MPa,应变速率约1.9×10^-13s^-1.     

坦桑尼亚恩泽加绿岩带玛汀杰金矿地质特征及成因初析

通过对坦桑尼亚恩泽加绿岩带玛汀杰金成矿区的4个金矿（化）体的成矿背景和地质特征进行分析,初步了解该区金矿体地质特征及矿床成因。玛汀杰金矿为产自韧性剪切带的石英脉型金矿,赋矿层位为太古代尼安兹超群绿片岩,矿体由NW向和EW向两组构造控制,矿石类型包括石英脉型、蚀变岩型和构造角砾岩型。普遍发育黄铁绢英云化。矿石w（Au）=0.62×10^-6~3.04×10^-6,部分矿石w（Au）值大于100×10^-6。玛汀杰地区金矿为典型的产自绿岩带韧性剪切带的石英脉型金矿,矿床成因为热液成因。     

Constraints on phase diagram topology for the system CaO−MgO−SiO2−CO2−H2O

Published phase diagrams for the siliceous carbonate system CaO–MgO–SiO₂–CO₂–H₂O are contradictory because of different estimates of the relative stability of magnesite. Experimental data on magnesite are too ambiguous to determine the validity of these estimates. Therefore, field evidence is used to select the correct phase diagram topology for siliceous carbonate and carbonate ultramafic rocks at pressures of about 2–5 kbar. The primary selection criterion is provided by the existence of the stable assemblage talc+dolomite+forsterite+tremolite+antigorite, which occurs in the Bergell contact aureole and Swiss Central Alps. Field evidence also is used to argue that the reaction magnesite+quartz=enstatite must occur at lower temperature than the reaction dolomite+quartz=diopside. T-X _{CO 2} and P _{CO 2}-T phase diagrams consistent with these observations are calculated from experimental and thermo-dynamic data. For antigorite ophicarbonate rocks, remarkable agreement is obtained between the spatial distribution of low variance mineral assemblages and the calculated diagrams.     

Benthic Foraminifera in Siliceous Black Ore from the Ezuri Kuroko Deposit, Hokuroku District, Japan

Abstract: Abundant benthic foraminifera have been identified in thin sections of the siliceous black ore in the Ezuri Kuroko deposit, Hokuroku, Japan. By treating samples with conventional hydrofluoric acid digestion techniques, sponge spicules and radiolaria have also been recognized in the residue. Under microscopic observations, 94 individual foraminiferal specimens have been detected. However, as it is difficult to identify species or genera by means of microscope observations alone, only a small number of genera have been identified based on morphology. The foraminiferal assemblage is composed predominantly of agglutinated species (83%) with subordinate calcareous species (17%), and is assigned to the Cyclammina Assemblage based on the preponderance of Cyclammina (57%). The foraminifera are generally well preserved within micro‐crystalline to cryptocrystalline quartz, and exhibit no obvious features related to compaction or secondary deformation. Textural observations suggest that the siliceous component of this rock was not derived from an allochthonous block but instead constitutes autochthonous proto‐Kuroko sediment. The Cyclammina Assemblage in the ore is different from recently described foraminiferal assemblages in the vicinity of present deep‐sea hydrothermal vents, but is identical to those found in black shales of the Onnagawa to Funakawa stages in the Green Tuff region. The proto‐Kuroko sediment is assumed to have been deposited in an oxygen‐deficient environment within a closed, deep‐seated basin. The existence of siliceous microfossils suggests that the silica in the siliceous ore did not originate from silica sinter deposits produced by submarine hydrothermal activities, but from a biogenic siliceous ooze, probably composed of diatoms. Sulfide mineralization in the interstices of some of the microfossils is inferred to relate primarily to bacterial sulfate reduction associated with the decomposition of organic matter. The later sulfide mineralization associated with larger crystals (which contain fluid inclusions with homogenization temperatures of approximately 250C) cuts across the siliceous masses and foraminiferal septa, and may have been formed after consolidation of the siliceous ooze, accompanying the formation of acidic intrusive rocks during the late Onnagawa stage.     

Geochemical Tracing of Ore-forming Material Sources of Carlin-type Gold Deposits in the Yunnan-Guizhou-Guangxi Triangle Area --A Case Study of the Application of the Combined Silicon Isotopes Geochemistry and Siliceous Cathodoluminescence Analysis

This paper deals with characteristics of silicon isotope compositions and siliceous cathodolumines-cence of host rocks, ores and hydrothermal silicified quartz of the Carlin-type ore deposits in the Yunnan-Guizhou-Guangxi triangle area. The study shows that primary silicified quartz is nonluminescent but quartz in host rocks andsecondary silicified quartz are luminescent by the action of cathode rays. Correspondingly, silicon isotope composi-tions of host rocks, ores and hydrothermal quartz veins are clearly distinguished. In strata from the Middle Triassic tothe "Dachang" host bed, δ~(30)Si of the host rocks ranges from 0.0‰-0.3‰, while that of primary ore-forming silici-fied fluids from -0.1‰ to -0.4‰, in the Upper Permian and Lower Carboniferous strata and Indosinian diabasehost beds, δ~(30)Si of the host rocks is from -0.1‰ to -0.2‰ and that of the primary silicified quartz veins from 0.3‰-0.5‰. This pattern demonstrates the following geochemical mineralization process: primary ore-forming sili-ceous fluids migrated upwards quickly along the main passages of deep-seated faults from mantle to crust and en-tered secondary faults where gold deposits were eventually formed as a result of permeation and replacement of thesiliceous ore-forming fluids into different ore-bearing strata. This gives important evidence for the fact that ore-forming fluids of this type of gold deposits were mainly derived from upper mantle differentiation and shows goodprospects for deep gold deposits and geochemical background for large and superlarge gold deposits.     

江西武功山东区大型韧性剪切带的显微构造特征

武功山东区存在一条大型韧性剪切带。鞘褶皱倒向以及旋转变形构造(如S-C面理组构、旋转碎斑系、雪球构造和粒内显微破裂构造等)显示此剪切带为由南向北逆冲推覆性质。砾石、黄铁矿还原斑和石英斑晶的有限应变分析表明剪切带西段和东段岩石分别以收缩型椭球和压扁型椭球变形为特征。剪切带的主要变形时代是早古生代,可能与早古生代华夏陆块和扬子陆块之间的碰撞造山作用有关。     

Quartz <Emphasis Type="Italic">c</Emphasis>-axis texture mapping of mylonitic metapelite with rod structures (Calabria,southern Italy): Clues for hidden shear flow direction

This quantitative microstructural study deals with textures of quartz domains within a mylonitized metapelite collected near a thrust surface corresponding to the tectonic contact between two metamorphic units, which crop out in the Aspromonte Massif, southern Calabria (Italy). The sample investigated lacks a mesoscopic stretching lineation. Therefore, quartz c-axis fabrics were investigated in two mutually orthogonal thin sections (a) parallel to the quartz rod lineation and perpendicular to the foliation (YZ plane) and (b) perpendicular to the quartz rods and perpendicular to the foliation (XZ plane); the data were generated using classical (manual measurements of quartz c-axis using U-stage) and modern methods (Computer Integrated Polarization microscopy). Both these sections show oblique foliations at ca. 40° from the main shear plane, implying that the actual X direction (stretching lineation that is absent on the mesoscopic scale) must lie between these two sections. Quartz c-axis data from the YZ section when rotated by 90° are similar with those from the XZ section. Hence, the data from the two sections are merged. These data when rotated by an angle of 50° from the direction of quartz rod lineation, gives an asymmetrical pattern indicating top-to-the-North sense of shear. This was confirmed by investigating quartz c-axis patterns in a section striking NS and perpendicular to the foliation. Based on the study it is thus concluded that this method can be used to do kinematic analysis in rocks that are devoid of stretching lineations. Apart from the above, the advantages and disadvantages of the classical and modern methods of quartz c-axis analysis are discussed.     

Activity of Silica-Rich Hydrothermal Fluid and Its Impact on Deep Dolomite Reservoirs in the Sichuan Basin, Southern China

Well-developed dissolution pores occur in the dolomites of the Sinian Dengying Formation, which is an important oil and gas reservoir layer in the Sichuan Basin and adjacent areas in southern China. The pores are often filled with quartz, and some dolomites have been metasomatically altered to siliceous chert. Few studies have documented the characteristics, source or origin of silica-rich fluids and their effects on the dolomite reservoir. The peak homogenisation temperatures(T_h) of fluid inclusions in pore-filling quartz are between 150℃ and 190℃, with an average of 173.7℃. Gases in the inclusions are mainly composed of CO_2, CH_4 and N_2. Compared with host dolomite, pore-filling quartz and metasomatic chert contain higher amounts of Cr, Co, Mo, W and Fe, with average concentrations of 461.58, 3.99, 5.05, 31.43 and 6666.83 ppm in quartz and 308.98, 0.99, 1.04, 13.81 and 4703.50 ppm in chert, respectively. Strontium levels are lower than that in the host dolomite, with average concentrations in quartz and chert of 4.81 and 11.06 ppm, respectively. Rare earth element compositions in quartz and chert display positive Eu anomalies with a maximum δEu of 5.72. The δD_(SMOW) values of hydrogen isotopes in water from quartz inclusions vary from-85.1‰ to-53.1‰ with an average of-64.3‰, whereas the δ~(18)O_(SMOW) values range from 7.2‰ to 8.5‰ with an average of 8.2‰. The average ~(87)Sr/~(86)Sr ratios in quartz and chert are 0.711586 and 0.709917, respectively, which are higher than that in the host dolomite. The fluid inclusions, elemental and isotopic compositions demonstrate that the formation of quartz and chert was related to silica-rich hydrothermal fluid and that the fluid was the deep circulation of meteoric water along basement faults. Interactions with silica-rich hydrothermal fluids resulted in densification of dolomite reservoirs in the Dengying Formation through quartz precipitation and siliceous metasomatism. However, it increased the resistance of the host dolomite to compaction, improving the ability to maintain reservoir spaces during deep burial. Evidence for silica-rich hydrothermal activity is common in the Yangtze Platform and Tarim Basin and its influence on deep dolomite reservoirs should be thoroughly considered.     

Mineralogy and geochemistry of a Late Permian coal in the Dafang Coalfield, Guizhou, China: influence from siliceous and iron-rich calcic hydrothermal fluids

This paper describes the influence of siliceous and iron-rich calcic low-temperature hydrothermal fluids (LTHF) on the mineralogy and geochemistry of the Late Permian No. 11 Coal (anthracitic, R_r=2.85%) in the Dafang Coalfield in northwestern Guizhou Province, China. The No. 11 Coal has high contents of vein ankerite (10.2 vol.%) and vein quartz (11.4 vol.%), with formation temperatures of 85 and 180 °C, respectively, indicating that vein ankerite and vein quartz were derived from low-temperature calcic and siliceous hydrothermal fluids in two epigenetic episodes. The vein quartz appears to have formed earlier than vein ankerite did, and at least three distinct stages of ankerite formation with different Ca/Sr and Fe/Mn ratios were observed.The two types of mineral veins are sources of different suites of major and trace metals. Scanning electron microscope and sequential extraction studies show that, in addition to Fe, Mg, and Ca, vein ankerite is the dominant source of Mn, Cu, Ni, Pb, and Zn in the coal, and the contents of these five elements are as high as 0.09% and 74.0, 33.6, 185, and 289 μg/g, respectively. In contrast, vein quartz is the main carrier mineral for platinum-group elements (PGEs) Pd, Pt, and Ir in the coal, and the contents of Pd, Pt, and Ir are 1.57, 0.15, and 0.007 μg/g, respectively. Sequential extraction showed a high PGE content in the silicate fraction, up to 10.4 μg/g Pd, 1.23 μg/g Pt, and 0.05 μg/g Ir, respectively. It is concluded that the formation of ankerite and quartz and the anomalous enrichment of trace elements in the No. 11 Coal in the Dafang Coalfield, Guizhou, result from the influx of calcic and siliceous low-temperature hydrothermal fluids.     

Die obere Stabilitätsgrenze von Tremolit bei der Metamorphose von kieseligen Karbonaten

During the metamorphism of siliceous carbonates, decomposition of tremolite yields diopside, enstatite, quartz and H₂O according to the following reaction: 1 tremolite 2 diopside + 3 enstatite + 1 quartz + 1 H₂O.For the application to natural processes, it is of special interest to evaluate the equilibrium temperature of this reaction, as a function of the CO₂-content of the H₂O-CO₂ fluid phase for several total pressures. These values were calculated for the total pressures of 1000 and 2000 bars, usingBoyd's experimentally determined univariant equilibrium data [Fig. 1 (Boyd, 1954 and 1959)]. Curves (a) and (b) of Fig. 3 give the results in a temperature-X _{CO 2}-diagram.The decomposition curves of tremolite intersect the equilibrium curves of other reactions which also take place during metamorphism of siliceous carbonates. If the total pressure can be estimated, these points of intersection together with the appropriate field observations will give information on the temperature and composition of the fluid phase during metamorphism.     

Proterozoic crustal scale shearing in the Bundelkhand massif with special reference to quartz reefs

Quartz reefs that constitute the most spectacular feature of the Bundelkhand massif of Central India are as yet riddled with controversy. We consider the NE-SW trending quartz reefs and the NE-SW trending brittle-ductile shear system developed outside the quartz reefs to constitute a single tectonic fabric of the massif. The study of structural features of the quartz reefs and the associated NE-SW trending shear zones coupled with the mineralogical and structural features suggests that the quartz reefs represent strike-slip dominated vertical to subvertical shear zones with dominantly sinistral sense of shear. The loci of the quartz reefs constitute the large scale fractures developed as a consequence of extensional processes in this part of Central India. These fractures were the sites for later emplacement of siliceous magmatic melt, commonly known as quartz reef that was also responsible for the mineralization of some base metals as well as pyrophyllite and diaspore. The lithotectonic system of these quartz reefs thus developed was later on subjected to strike-slip movements. A generalized model of large scale extensional tectonics during the Proterozoic times has thus been suggested for the development of the NE-SW tectonic fabric of the Bundelkhand massif.     

Experimental shear zones and magnetic fabrics

Magnetic fabric analysis has been used as a non-destructive means of detecting petrofabric development during experimentally produced multi-stage, transpressive deformations in ‘shear zones’. Artificial, magnetic-bearing silicate sands and calcite sands, bonded with Portland cement, were deformed at room temperature and at 100 and 150 MPa confining pressure. The slip-rate for the shear zone walls was 0.73 × 10⁻⁴ mm s⁻¹ and the maximum shear strains were about 0.38, across zones that were initially about 5 mm thick. The magnetic fabric ellipsoid rapidly spins so that the maximum and intermediate susceptibilities tend to become parallel to the shear zone walls throughout the sheared zone. The ellipsoid becomes increasingly oblate with progressive deformation. However, in all cases, the anisotropy is strongly influenced by the pre-deformation magnetic fabric. During deformation the cement gel collapses so that cataclasis of the mineral grains is suppressed. In the quartz-feldspar aggregates the magnetite's alignment is accommodated by particulate flow (intergranular displacements) of the grains. In the calcite aggregates stronger magnetic fabrics develop due to plastic deformation of calcite grains as well as particulate flow. However, the calcite grain fabrics are somewhat linear (L ≥ S) whereas the magnetic fabrics are planar (S >L). The preferred dimensional orientations of magnetite are weak and it is possible that the magnetic fabrics are due to intragranular rearrangements of magnetic domains.The transpressive shear zones are much more efficient than axial-symmetric shortening in the increase of anisotropy of the magnetic fabrics, especially in the case of the calcite aggregates. This suggests that flow laws derived for axial-symmetric shortening experiments may not be appropriate for non-coaxial strain histories such as those of shear zones.     

Quartz fabrics in shear-zone mylonites: Evidence for a major imprint due to late strain increments

Geometrical relations between quartz C-axis fabrics, textures, microstructures and macroscopic structural elements (foliation, lineation, folds…) in mylonitic shear zones suggest that the C-axis fabric mostly reflects the late-stage deformation history. Three examples of mylonitic thrust zones are presented: the Eastern Alps, where the direction of shearing inferred from the quartz fabric results from a late deformation oblique to the overall thrusting; the Caledonides nappes and the Himalayan Main Central Thrust zone, where, through a similar reasoning, the fabrics would also reflect late strain increments though the direction of shearing deduced from quartz fabric remains parallel to the overall thrusting direction. Hence, the sense of shear and the shear strain component deduced from the orientation of C-axis girdles relative to the finite strain ellipsoid axes are not simply related nor representative of the entire deformation history.     

Titanium and rare metal mineralization in arenaceous reservoir rocks of South Timan oil fields

Riphean metamorphic schists dipping into the Pechora trough contain leucoxenized porphyroblasts of rare earth sphene and ilmenite. Rutile leucoxene occurring in Devonian sediments overlying the schists was derived from the rare earth sphene and ilmenite by secondary crystallization of randomly oriented rutile microcrystals intergrown with quartz microcrystals. A light gray variety of rutile leucoxene contains 56.3 to 60.8% titanium dioxide; dark gray rutile leucoxene, the other variety, contains 57.1 to 67% titanium dioxide. Niobium, tantalum, and rare earths, present in fractions of a percentage, are thought to be of economic importance. It is considered that concentrations of zircon and rutile leucoxene represent offshore deposition in the deeper parts of a Devonian marine basin. Data were derived from examination of drill cores from the Devonian section of an oil field located on the southeast slope of the Timan range.     

滇东北会泽地区南红玛瑙产出特征及成因分析

会泽地区的南红玛瑙矿主要产于上三叠统飞仙关组的中上部砾岩层、上二叠统宣威组底部砾岩层以及晚二叠世峨眉山组杏仁状玄武岩中。玛瑙新鲜面多为红色、浅红色及灰白色;呈椭球状或扁球状分布于砾岩之中,分布不均匀,局部富集;玛瑙砾石大小不一,玛瑙粒径多集中在0.5~5 cm,偶见大于6 cm。会泽地区的南红玛瑙矿形成可分为三个阶段：1）晚二叠世时期,玄武质火山岩（P₃e）中的含硅质热液在晚期充填于玄武岩气孔或裂隙中,经过（快速）冷却形成隐晶质石英或晶质石英集合体——玛瑙,即原生玛瑙;2）晚二叠世晚期的华力西运动和印支运动早期的地壳继续隆升,使得含玛瑙玄武岩遭受风化-剥蚀作用,两次剥离出来的玛瑙单体经流水搬运作用及河湖相沉积作用,形成宣威组（P₃x）、飞仙关组（T₁f）含玛瑙砾石层——沉积砾岩型玛瑙矿产;3）近-现代地质作用,使得二叠纪玄武岩气孔中原生玛瑙和宣威组、飞仙关组沉积砾岩层中的次生玛瑙砾石,再次受到风化-剥蚀作用,玛瑙砾石经流水搬运至现代河流或湖泊中,形成次生玛瑙砂矿。     

Geology, Geochemistry and Genesis of the Hongshijing Gold Deposit in Ruoqiang,Xinjiang

1IntroductionTheHongshijinggolddepositislocatedinthenorthofLuobupouLakeofRuoqiang ,about 30 0kmsouthwestofHamiCity ,Xinjiang .ItwasdiscoveredbytheSixthGeologicalTeamofXinjiangduringgeo chemicalexploration .TheHongshijinggolddeposit,whichoccursinthegold bearingformationcomposedofMiddleandLateCarboniferousvolcanicandpyroclasticrocks ,isabrittle ductileshearzonetypegolddepositcontrolledbyariftbelt.TheHongshijinggolddepositislocatedinthesouthwestoftheHongshi jing -Maotoushanmineralizationb…