Composition-volume changes during hydrothermal alteration of andesite at Buttercup Hill, Noranda District, Quebec

Hydrothermally altered andesites in the upper member of the Amulet formation at Buttercup Hill, Noranda, Quebec represent part of the aquifer and cap of a self-sealing geothermal system that focussed the discharge of hydrothermal fluids during the formation of massive Cu-Zn sulfide deposits. Five alteration facies are recognized

1. 1) pervasive greenschist faciés regional metamorphism (least-altered andesite)

2. 2) epidotization-silicification

3. 3) albitization-silicification

4. 4) chloritization

5. 5) sericitization-silicification. Alteration is localized on permeable zones such as amygdules, fractures, flow tops, discordant breccia dikes, and conformable breccia horizons.

Epidotized-silicified andesite is enriched in Ca-Sr-Eu and depleted in Mg and first transition series metals (FTSM) relative to least-altered andesite. Albitized-silicified andesite is significantly enriched in Na and depleted in most FTSM relative to least-altered andesite. The abundances and inter-element ratios of the rare-earth elements (REE) and most high field-strength elements (HFS: Y, Zr, Th, U, Hf, Ta) are similar in least-altered, epidotized-silicified and albitized-silicified andesites. The most silicified andesites are strongly enriched in Na-Si, strongly depleted in Mg and divalent FTSM and slightly but systematically depleted in REE and most HFS elements. Serialized andesites were previously silicified; they are very strongly enriched in K-Rb-Cs-Ba, very strongly depleted in Na-Ca-Sr-Eu and slightly depleted in light REE relative to silicified andesite. Chloritized andesitic rocks exhibit heavy REE and HFS element ratios similar to those of leastaltered andesite, but are relatively strongly enriched in Mg and divalent FTSM, strongly depleted in Si and large ion lithophile (LIL) elements and slightly depleted in light REE.

The coupled behavior of the heavy REE and most HFS elements during epidotization, albitization, silicification, chloritization and serialization suggests that they were inert during hydrothermal modification of the andesite. Mass balance calculations suggest that volume was conserved during epidotization-silicification and albitization-silicification, but that intense silicification was accompanied by volume increases up to 30 percent.     

Water-level changes in the Hammock River marsh palaeovalley, Connecticut between 13,000 and 6000 (C) yrs B.P.

Detailed analysis of basal organic deposits underlying Hammock River marsh, Connecticut allowed documentation of water-level changes that occurred between 13,000 and 6000 yrs B.P. Four main periods of groundwater- and lake-level movements and related environmental changes can be identified.

1. (1) 12,500-10,200 yrs B.P. (lake stage): very rapid rise of the groundwater table of about 2 to 3 m, resulting in a shallow lake, followed by a more gradual rise of about 2.5 to 1.5 m.

2. (2) 10,200-7000 yrs B.P. (freshwater marsh, stage 1): slow overall rise of the water table interrupted by a drop of at least 1 m between about 9500 and 9000 yrs B.P. and of at least 0.8 m between about 8000 and 7500 yrs B.P., each event leading to oxidation and maceration of organic material.

3. (3) 7000-6400 yrs B.P. (complete desiccation of the swamp): rapid fall of the water table of at least 3.9 m. causing large-scale down-wasting of the accumulated peat.

4. (4) After 6400 yrs B.P. (freshwater marsh, stage 2): rapid rise of the water table.

The water-table rise of period 1 and the lowering of period 3 are attributed to predominantly local causes, while the groundwater fluctuations during period 2 are probably climate-related. The final water-level increase reflects the influence of Holocene relative sea-level rise.     

On the origin of the western Mediterranean Sea basins

This is a report of a symposium organized by the Netherlands Commission for the Upper Mantle Project. The data relative to the generation of the western Mediterranean Sea basins, presented during this symposium, are summarized in the Appendix.

Several modes of origin have been discussed:

1. (1) the basins are remnants of a former larger ocean;

2. (2) they are formed in the wake of drifting continental blocks;

3. (3) by an erosion and denudation of a continental crust;

4. (4) by an upheaval and later subsidence of an ocean floor; or

5. (5) by sub crustal erosion of a continental crust.

It is concluded that, although many data are in agreement with the drift model, this process cannot have been the sole agent in the generation of the basins.     

Paleozoic and Lower Mesozoic magmas from the eastern Klamath Mountains (North California) and the geodynamic evolution of northwestern America

The Paleozoic to Early Mesozoic geology of the eastern Klamath Mountains (N California) is characterized by three major magmatic events of Ordovician, Late Ordovician to Early Devonian, and Permo-Triassic ages. The Ordovician event is represented by a calc-alkalic island-arc sequence (Lovers Leap Butte sequence) developed in the vicinity of a continental margin. The Late Ordovician to Early Devonian event consists of the 430–480 Ma old Trinity ophiolite formed during the early development of a marginal basin, and a series of low-K tholeiitic volcanic suites (Lovers Leap Basalt—Keratophyre unit, Copley and Balaklala Formations) belonging to intraoceanic island-arcs. Finally, the Permo-Triassic event gave rise to three successives phases of volcanic activity (Nosoni, Dekkas and Bully Hill) represented by the highly differentiated basalt-to-rhyolite low-K tholeiitic series of mature island-arcs. The Permo-Triassic sediments are indicative of shallow to moderate depth in an open, warm sea. The geodynamic evolution of the eastern Klamath Mountains during Paleozoic to Early Mesozoic times is therefore constrained by the geological, petrological and geochemical features of its island-arcs and related marginal basin.

A consistent plate-tectonic model is proposed for the area, consisting of six main stages:

1. (1) development during Ordovician times of a calc-alkalic island-arc in the vicinity of a continental margin;

2. (2) extrusion during Late Ordovician to Silurian times of a primitive basalt-andesite intraoceanic island-arc suite, which terminated with boninites, the latter suggest rifting in the fore-arc, followed by the breakup of the arc;

3. (3) opening and development of the Trinity back-arc basin around 430–480 Ma ago;

4. (4) eruption of the Balaklala Rhyolite either in the arc or in the fore-arc, ending in Early Devonian time with intrusion of the 400 Ma Mule Mountain stock;

5. (5) break in volcanic activity from the Early Devonian to the Early Permian; and

6. (6) development of a mature island-arc from the Early Permian to the Late Triassic.

The eastern Klamath Mountains island-arc formations and ophiolitic suite are part of the “Cordilleran suspect terranes”, considered to be Gondwana margin fragments, that have undergone large northward translations before final collision with the North American craton during Late Mesozoic or Cenozoic times. These eastern Klamath Mountains island-arcs could be associated with the paleo-Pacific oceanic plate that led to accretion of these allochthonous terranes to the American margin.     

The applicability and limitations of thermodynamic geochemical models to simulate trace element behaviour in natural waters. Lessons learned from natural analogue studies

Natural analogue investigations aim to understand key phenomena and processes in natural systems related to those expected to occur in radioactive waste repositories. One of the key applications of natural analogue studies has been the possibility to test the geochemical models to be used to describe the migration of radionuclides in a future radioactive waste repository system. To this end, several geochemical modelling testing exercises (commonly denoted as blind predictive modelling, BPM) have formed an integral part of these studies over the last decade.

We have reviewed, discussed and compared the results obtained from geochemical modelling BPM exercises carried out within six natural analogue studies: Poços de Caldas, Cigar Lake, Maqarin, El Berrocal, Oklo and Palmottu. To make this comparison meaningful, we present the main geochemical characteristics of each site in order to highlight the most relevant mineralogical and hydrochemical differences. The elements selected for discussion are: Sr, Ba, Sn, Pb, Se, Ni, Zn, REEs, Th and U. We have based our discussion on the results obtained from the calculated aqueous speciation as well as by comparing solubility calculations with the actually observed concentrations.

Results can be differentiated into two categories of elemental behaviour:

1. those elements like Th and U under reducing conditions that can be fairly well described by assuming solubility control exerted by pure solid phases as their oxyhydroxides;

2. elements such as Sr, Zn, REEs and U under oxidising conditions for which the association to major geochemical components of the system must be considered in order to explain their concentrations in groundwaters.

Additionally, we discuss the main improvements made to the thermodynamic databases and the geochemical calculation methodologies due to the BPM exercises. Furthermore, the most important characterisation geochemical data needed to complete predictive solubility and speciation calculations are identified.     

Rotation and strain of linear and planar structures in three-dimensional progressive deformation

Rotation and progressive strain have been studied for a sheet embedded in a matrix which undergoes rotational three-dimensional strain under constant volume conditions. The mathematics gives explicit information on the following features:

1. (1) The length and position (relative to a defined coordinate system) of the principal axes of the strain ellipsoid at any stage of the progressive deformation.

2. (2) The position and length of the principal axes in any plane intersecting the strain ellipsoid, also at any stage of the deformation.

3. (3) The position and length of passive markers which initially coincided with the principal axes in an intersecting plane. This is of consequence for the distinction between passively rotating structures and actively forming structures.

4. (4) The shear strain parallel to an intersecting plane or sheet, as indicated by the angular difference between the normal to an intersecting plane at any time and the marker at the same time which initially, however, was parallel to the normal. This layer-parallel shear causes boudins to rotate and the axial plane of buckles to tilt.

The relationships have been expressed quantitatively in the bulk of the paper and illustrated in diagrams. The analysis presented is basic for the study of the deformational behavior of competent sheets of rocks embedded in less competent ones.     

Coulees volcaniques dans le paleozoique superieur des zones internes helleniques (Grece continentale) — environnement sedimentaire et caracterisation petrographique

Volcanic flows of Late Paleozoic age in the Pelagonian zone belonging to the internal Hellenic zones, have been studied, for the first time, from petrographical and geochemical viewpoints.

Relevant petrographical data and the geochemical analysis lead us to consider that:

1. (a) the basic lavas can be linked to tholeitic basalts.

2. (b) the acid lavas are associated to metarhyolites.

3. (c) the two magmatic flows do not originate from the same parental magma.

The geochemical results obtained, compared to those from other deposits of nearly the same age outcropping around the Mediterranean, indicate that the latest Hercynian volcanism has been affected by intra-plate distension phenomena preceding the great Lower Mesozoic break-up which caused the formation of the Tethys Ocean.

It is remarkable that in the Mesogean Basin the sedimentary environment is nearly similar to that described in this paper. The volcanic flows end up in a sialic sedimentary basin containing coarse detritic formations that resulted from erosion of the Variscan chain along the southern margin of the European continent.

Abstract

Dans la zone pélagonienne appartenant aux zones internes helléniques, des coulées volcaniques d'âge paléozoïque supérieur sont étudiées, pour la première fois, pétrographiquement et géochimiquement. Les différents agencements pétrographiques et les analyses géochimiques nous apprennent que:

1. (a) les laves basiques peuvent être rattachées á la famille des basaltes tholéitiques.

2. (b) les laves acides s'apparentent aux métarhyolites.

La quantification des différents éléments analyses nous amène á considérer que les deux lignées magmatiques ne sont pas issues d'un même magma parental.

La comparaison des résultats géochimiques avec d'autres gisements, sensiblement contemporains affleurant autour de la Méditerranée actuelle, nous indique que le volcanisme fini-hercynien est assujetti á des phénomènes de distension intraplaques, préparatoires á la grande fracturation mésozoïque inférieure ayant donné naissance á l'océan téthysien.

Ce qui est remarquable dans le bassin mésogéen, c'est que l'environnement sédimentaire est toujours, á peu près, comparable á celui que nous décrivons ici. En effet, les coulées volcaniques s'épanchent dans un bassin sédimentaire á fond sialique au sein de formations détritiques grossières nées de la destruction de la chaîne varisque sur la bordure méridionale du continent europeen.     

The distribution of certain elements among coexisting calcic pyroxenes, calcic amphiboles and biotites in skarns

Skarns composed predominantly of Ca pyroxene, Ca amphibole, and biotite occur locally in the Grenville metamorphic terrain of the Canadian pre-Cambrian Shield. The enclosing rocks are Grenville gneisses and limestones, gabbroic rocks, veined gneisses, and granitic rocks. The metamorphic grade of the skarns is identical to that of the enclosing rocks, namely the upper region of the amphibolite facies.

Twelve skarn specimens were selected for chemical study. An investigation is made of the concentrations of Al, Fe, Mg, Mn, Ti, Ca, Na, K, Ba, V, Cr, Zr, Y, and Sc in eleven Ca pyroxenes, ten Ca amphiboles, and eleven biotites.

Consideration is given to the distribution of elements among coexisting minerals. Regular relationships appear when

1. (1) the concentration of an element in a mineral is related to the concentration of the element in a coexisting mineral by a linear or non-linear function

2. (2) the linear or non-linear function is itself a function of the concentration of another element in one or both of the coexisting minerals. These relationships indicate that, for the most part, minerals within each skarn specimen closely approached a state of chemical equilibrium.

A preliminary attempt is made to explain the distribution relationships in terms of crystal chemistry.     

滇东南晚二叠世铝土矿地球化学特征及物源分析

滇东南地区晚二叠世铝土矿属典型的喀斯特型铝土矿,矿体赋存于上二叠统吴家坪组(龙潭组),分析其地球化学特征对研究其物质来源具有重要意义。全岩分析显示铝土矿石成分以Al₂O₃、Fe₂O₃、SiO₂和TiO₂为主,其中Al₂O₃与Fe₂O₃ 、SiO₂具有较好的负相关关系。微量元素Cr、Zr、 Hf、Ta、Th、U和稀土元素在铝土矿矿化过程中不断富集,元素Zr-Hf、Nb-Ta之间具有明显的正相关关系。lgCr-lgNi图解、稳定元素比值(Zr/Hf)及稀土元素配分模式等地球化学特征说明峨眉山玄武岩为铝土矿的形成提供了主要的物质来源,同时下伏碳酸盐岩也提供了部分成矿物质。     

Extreme enrichment of Sb, Tl and other trace elements in altered MORB

We have analyzed 25 trace elements (e.g., Sb, Tl, Sn, rare earth elements (REE), Th, U, Nb, Pb, Zr, Hf, and Y) in altered mid-ocean ridge basalts (MORB) from locations near the mouth of the Gulf of California. Our results imply that the heavy REE and Y are not seriously affected by seawater alteration, in agreement with previous studies. The elements Zr, Hf, Nb, light REE and Sr are enriched up to a factor of 2 in some extremely altered samples. However, element ratios between Zr, Hf, and Nb (e.g., Zr/Hf, and Zr/Nb) are not greatly affected, presumably due to the chemical similarity of these elements during any exchange process. The enrichment of Th and Sn is even higher. Antimony, Tl, Cs, Rb, Rb, and Ba are most easily altered by water-rock interaction and are therefore the best indicators for seawater alteration. The enrichment factor of the most mobile element Sb is up to 2000.

There is a weak correlation between the concentration in seawater and the enrichment factors. On the other hand, the worldwide pelagic clay pattern matches the enrichment pattern much more closely, and the limited data available for local oceanic sediments give an even better correlation. A plausible model to explain the enrichment pattern may be an elemental exchange between basalt and seawater that had interacted earlier with overlying sediments.     

Provenance of iron,bauxite and clay deposits of the Carboniferous Benxi Formation in southern margin of North China Craton: An example from Da'an bauxite and clay deposit of Sanmenxia area,Henan Province

During the Middle Ordovician to Late Carboniferous period,the North China Craton(NCC)was exposed and experienced prolonged weathering that resulted in the formation of large-scale iron,clay and bauxite deposits. The source of ore-forming material has always been a research focus,in particular,whether the sources of the iron ore and the Fe-bearing clay at the bottom of Benxi Formation are the same as the upper bauxite and clay deposit is still unclear. In this study,the Da'an bauxite and clay deposit at the southern margin of the NCC was chosen to carry out a detailed analysis of the micro-region mineral composition and elemental geochemical characteristics for further exploring the sources of iron,bauxite and clay deposits. The composition of the ore-bearing rocks in the Da'an bauxite deposit from the bottom to top includes Fe-bearing clay(locally iron ore),bauxite,and bauxitic clay;locally,in karstic uplift,bauxitic clay layer is directly overlying on the Fe-bearing clay. The Fe-bearing clay is dominated by siderite,pyrite,and illite in the karstic depression,and hematite,illite,and kaolinite in the uplift. Bauxite is mainly composed of diaspore,illite,and anatase,while bauxitic clay is mainly composed of illite. Mineral microanalysis revealed the development of large amounts of moissanite and small amounts of natural silica,silicalite,and chromite at the bottom of bauxitic clay layer. Regional comparison and correlation reveal that the ophiolite in the Shangdan suture zone and Erlangping Group in the North Qinling orogenic belt(NQOB)likely provides source materials for bauxite and clay deposits. The obvious differences in immobile element ratios (e.g., Zr/TiO₂,Hf/TiO₂,Nb/TiO₂,Ta/TiO₂)between the bottom Fe-bearing clay layer and the upper bauxite and clay layer in Da'an deposit,revealing that they are from different sources. The bottom Fe-bearing clay and iron ore layers are the products of in-situ weathering of underlying carbonates,while the top bauxite and clay are allochthonous. The regional uplift of the NQOB during the Late Carboniferous period provided important ore-forming materials for the formation of the NCC bauxite and clay deposits.     

华北克拉通南缘石炭系本溪组铁-铝黏土矿物质来源: 以河南三门峡大安铝黏土矿床为例*

华北克拉通在中奥陶世至晚石炭世期间一直出露地表,经历了长期的风化作用,形成大规模的铁-铝黏土矿,其成矿物源一直是研究的热点,尤其是本溪组底部铁矿和铁质黏土矿与上部铝黏土矿是否为同一来源尚未查清。本研究选取克拉通南缘大安铝黏土矿床作为研究对象,展开微区矿物及元素地球化学组成分析,进一步探讨铁-铝黏土矿物质来源。大安矿床内含矿岩系自下而上包括铁质黏土岩、铝土矿、铝质黏土矿;局部喀斯特高地缺失铝土矿,铝质黏土矿直接覆盖于铁质黏土岩之上。铁质黏土岩在洼地以菱铁矿、黄铁矿和伊利石为主,在隆起区以赤铁矿、伊利石和高岭石为主。铝土矿以硬水铝石、伊利石和锐钛矿为主;铝质黏土矿主要矿物为伊利石。矿物微区分析在黏土矿底部发现大量的碳化硅和少量自然硅、硅铁矿、铬铁矿;区域对比揭示北秦岭造山带内商丹缝合带和二郎坪群中的蛇绿岩为铝黏土矿形成提供了成矿物质。本溪组底部铁质黏土与上部铝黏土矿稳定元素比率(例如Zr/TiO₂、Hf/TiO₂、Nb/TiO₂、Ta/TiO₂)存在明显差异,揭示二者为不同来源: 底部铁质黏土岩和铁矿层为底板碳酸盐岩原地风化的产物;而上部铝黏土矿是异地搬运物,北秦岭造山带在晚石炭世的整体抬升为华北铝黏土矿形成提供了重要的成矿物质。     

Fold-axis parallel extension in an arcuate fold- and thrust belt: the case of the Helvetic nappes

The Helvetic nappes of western Switzerland are discussed as an example of an arcuate foreland fold- and thrust belt in which active fold-axis parallel stretching occurred. Fold-axis parallel extension is recorded by:

1. (1) Incremental strain data from pressure shadow fibres. The significance of pressure shadow fibres for the determination of the deformation history of a region is discussed. Pressure shadows are used to quantify the amount of, and to describe the distribution of fold-axis parallel extension occurring in the Helvetic nappes.

2. (2) The extension directions of conjugate systems of en échelon veins. It is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern. The relative age of the conjugate en échelon vein systems in the Helvetic deformation history can be assessed, the geometry of the conjugate sets relative to the local anisotropy plane is described, and the significance of the preferred orientation of their extension directions is discussed.

3. (3) Fold-axis parallel sections. A comparison of the regional distribution of the fold-axis parallel strain with the shape of the Helvetic nappes in fold-axis parallel sections shows that the fold-axis parallel strain cannot be related to the footwall topography of the nappes.

It is concluded that the fold-axis parallel extension in the Helvetic nappes was induced by a change of direction of overthrust shear. This change occurred late in the deformation history and was superposed on the already formed nappes. The changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting Alpine nappe pile, relative to the European plate, a rotation which lead to the arcuate shape of the Western Alps.     

Structure of the crust and upper mantle in Canada

The seismic probing of the crust and upper mantle in Canada started in 1938 and since then has involved many government and university groups using a wide variety of techniques. These have included simple profiling with both wide and narrow station spacing, areal time-term surveys, detailed deep reflection experiments, very long-range refraction studies and the analysis of surface wave dispersion between stations of the Canadian Standard Network.

A review of the published interpretation leads to the general conclusion that:

1. (1) P_n-velocities vary from a value possibly as low as 7.7 km/sec under Vancouver Island to 8.6 km/sec and higher in the extreme eastern part of the shield and some parts of the Atlantic coast.

2. (2) Large areas of Canada have a crustal thickness of 30–40 km, with Vancouver Island, the southwestern Prairies, the Lake Superior basin and parts of the eastern shield of Quebec being thicker. No continental area in Canada is known to have a crust thinner than 29 km.

3. (3) The Riel discontinuity — a deep intra-crustal reflector and sometime refractor, is widely reported in the Prairies and Manitoba. It is not seen to the north in the vicinity of Great Slave Lake, nor in the Hudson Bay, Lake Superior and Maritime regions, nor in the interior of British Columbia. It may be present in some areas of the eastern shield.

4. (4) As experiments have become more detailed, crustal structures of greater complexity have been revealed. The concept that crustal structure becomes simpler with increasing depth is apparently unfounded.

Long-range refraction studies suggest that the Gutenberg P-wave low-velocity channel is poorly developed under the Canadian Shield. The analysis of the dispersion of surface waves, however, suggests that the channel is better developed for S-waves, and is present throughout the country. The lid of the channel is deepest under the central shield and shallowest under the Cordillera.     

Geochemistry and Radioactive Potentiality of Um Naggat Apogranite, Central Eastern Desert, Egypt

Abstract: The northern part of Um Naggat granite massif (UNGM) has suffered extensive post-magmatic metasomatic reworking which results into the development of (Zr, Hf, Nb, Ta, U, Th, F)– and albite-enriched and greisenized apogranite body (UNAP) of 600 m thick, and more than 3 km in the strike length.
Albitization produced an enrichment in Zr (av. 2384 ppm), Hf (61), Nb (419), and U (43). The Th/U ratio ranges between 1. 33 and 1. 90. Extreme albitization (i. e. the albitite rock) is characterized by sharp decrease in the rare metals contents. However, extreme greisenization (i. e. endogreisen bodies) is characterized by a considerable enrichment in Zr (av. 5464 ppm), Hf (143), Nb (2329), Ta (152), U (66) and Th (178). The Th/U ratio ranges between 1. 57 and 3. 60. In contrast to extreme greisenization, it seems that extreme albitization does not apparently change the fluid pH and therefore poor amounts of rare metals are localized in the albitites.
It is suggested that the presence of Na⁺, H⁺ and F^- in the ore fluids was essential to stablize complexes of Zr, Hf, Nb, Ta, U, Th, and HREE during extraction and transportation. In contrast, contemporaneous decrease of temperature and increasing pH due to decreasing pressure are considered the essential factors for localization of disseminated mineralization of Zr and Nb in the apical parts of the UNAP. The enhanced uranium content in the alteration facies of UNAP coupled with the absence of significant uranium mineralization may indicate the metalliferous rather than mineralized nature for the UNAP. The high uranium contents are stabilized in refractory accessory minerals. However, with repect to Zr and Nb, the UNAP especially the albitized and greisen facies, can be categorized as a mineralized productive granite.     

Monitoring of movement of potential earthquake areas with precise distance measuring and leveling systems

Whether monitoring crustal movements in localized volcanic areas along known fault lines, or over large crustal-movement areas, the geodesist has been restricted by the measurement accuracy of the instruments used, the accumulation of errors, the lack of reliable air refraction information and the problem of finding proper measurement procedures and mathematical solutions to assure that the inherent errors of the measurement-mathematical procedures do not exceed any conceivable ground movement.

Recent technological advances have placed new instruments and systems at the disposal of the geodesist, so that is now feasible to measure and analyze these micro and macro crustal movements within the accuracies required.

The paper describes three such systems:

1. (1) The Wild Electronic Theodolite T-2000 with a highly precise distance-measurement instrument, the DI-4S, together with a data collector, the GRE-3, which are connected to a computer and a plotter to measure and analyze both micro and macro crustal movements.

2. (2) The Wild NAK-2 level with an antimagnetic compensator which increases the accuracy in the height/velocity monitoring of vertical crustal movements by virtual elimination of the influence of natural or man-made magnetic fields on the automatic level.

3. (3) The use of analytical photogrammetry employing both terrestrial and aerial photography to monitor crustal movements.

By taking advantage of these new instruments and systems, the scientists capability to provide crustal movement data for use in the analysis and prediction of micro or macro crustal movement is greatly enhanced.     

小兴安岭西北部与永新金矿有关岩浆岩的年代学和地球化学及成矿构造环境

永新金矿是近年发现的大型蚀变岩型脉状金矿床,闪长玢岩脉等与金矿脉相互穿切并侵入于上盘龙江组火山岩中。锆石LA ICP MS U-Pb测年显示,龙江组安山岩结晶年龄为(1137 ± 18)Ma,闪长玢岩结晶年龄为(1148 ± 19)Ma,均形成于早白垩世晚期。二者钠钾含量变化相对较大,相对富铝富铁、钙镁含量中等,从火山岩到脉岩表现为钙碱性向碱性系列过渡特征。火山岩、脉岩与矿石的稀土元素配分曲线有很好的一致性,矿石稀土含量低于岩石。岩石微量元素总体显示Zr、Hf、Nd、U等明显富集,而Nb、Ta、Th、P、Ti等明显亏损。在Sr/Y-Y和(La/Yb)N-(Yb)N图解中,所有样品均落入经典岛弧岩石范围;在Nb-Y、Ta-Yb的构造环境判别图中均处于火山弧岩浆岩区,在(La/Yb)N-δEu变异图上落在壳-幔型岩浆范围。结合矿床稳定同位素及区域金矿时空分布规律等综合特征,认为永新金矿的形成与早白垩世古太平洋板块俯冲所引发的(火山)岩浆活动有关。     

Dholkata Bauxite Deposit on Metavolcanics, Keonjhar District, Orissa, India

Abstract. The Dholkata bauxite deposit of Keonjhar district, Orissa, has developed on the metavolcanics of tholeiitic basalt composition. The weathered profile reveals five distinct altered zones, such as topsoil, laterite, bauxite, lithomarge and altered metavolcanics. The mineralogy of different zones studied in a representative pit shows the association of major mineral constituents like gibbsite, goethite, hematite, kaolinite, limonite and quartz. Gibbsite is the most dominant one followed by goethite and hematite in the bauxite zone. The geochemical study of all weathering zones indicates the geochemical affinity of the elements Ni, Th and U for laterites and Cr, Zr and Hf for bauxites to occur in high quantities. Trend surface maps predict the bauxite zones in the different levels of the deposit. If the zones having A1₂O₃ 35–40 % are blended with high grade ores, the deposit may prove to be a potential one.     

The Oslo Rift: P-T relations and lithospheric structure

Extrusion temperatures for basaltic lavas in the Permo-Carboniferous Oslo Rift, estimated from whole rock major element compositions, are estimated to be 1270 to 1340°C. This means that magmatism during the Oslo rifting event was not associated with a large temperature anomaly in the underlying upper mantle. Partial melting is believed to be caused by a combination of crustal extension, a weak temperature anomaly in the underlying asthenosphere, and/or high fluid-contents in the mantle source region (“wet-spot”). Petrological and gcochemical data imply that large masses of cumulate rocks were deposited in the deep crust during the Oslo rifting event. The densities and seismic velocities (V_p) of these cumulate rocks are estimated to be 2.8–3.5 g/cm³ and 7.5–8.0 km/s. A rough estimate suggests that cumulus minerals alone account for a net transfer of at least 2 × 10¹⁷ kg of magmatic material from the mantle into the deep crust. In addition comes material representing

1. (a) cumulate minerals corresponding to eroded magmatic surface and subsurface rocks

2. (b) intercumulus material, and

3. (c) magmas crystallized to completion in the deep crust.

Estimates based exclusively on geophysical data tend to underestimate the true transfer of mass into the lower crust as gabbroic cumulate rocks, and melts crystallizing to completion in the lower crust have densities and seismic velocities similar to those of lower crustal wallrocks.     

二连盆地土壤微细粒铀地球化学分布特征及找矿方向

二连盆地是我国砂岩型铀矿的重要产地之一,已相继发现了多个砂岩型铀矿床. 盆地中部赛汉高毕-苏尼特左旗地区南北缘发育大量古-中生代的花岗岩体,铀源丰富;且该区发育北东向展布的巴彦乌拉、赛汉高毕等古河道,为铀矿体的形成和保存提供了有利的构造条件,具有较好的找矿前景. 目前针对该地区砂岩型铀矿的勘查主要是通过地球物理勘探（电法、地震、重力、航磁）和钻探,常规的地球化学勘查方法在该区效果不理想. 为了满足大范围找矿勘查的需求,基于土壤微细粒分离测量技术在赛汉高毕-苏尼特左旗地区开展1∶25万区域化探扫面工作. 2 821件土壤样品中Hg、As、Mo、F、Zr、Sr、Sb、Li、Th、U等元素既有相对较高的富集系数,又有相对较大的变异系数. 通过元素相关分析和R型聚类分析将其分为Zr?Th?U?Mo组合和As?Sb?F?Li?Sr?Hg组合,优选U?Mo为铀矿的成矿指示元素. 依据U?Mo衬值地球化学综合异常圈定了3个铀矿找矿有利地段,其中地段I和II位于盆地次级坳陷内部,异常与古河道砂体有关,寻找砂岩型铀矿的潜力较大.