Faulting, fracturing and in situ stress prediction in the Ahnet Basin, Algeria — a finite element approach

Many low-efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures. Accurate and low-cost information on basic fault and fracture properties, orientation in particular, is critical in reducing well costs and increasing well recoveries. This paper describes how we used an advanced numerical modelling technique, the finite element method (FEM), to compute site-specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties, structural position and tectonic stress. Presented are the numerical results of two-dimensional, plane-strain end-member FEM models of a hydrocarbon-bearing fault-propagation-fold structure. Interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling; however, it still allows comparisons with (the little available) geological and geophysical data.

In all models, the weak mechanical strength and flow properties of a thick shale layer (the main seal) leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement, and results in flexural slip across the shale layer. All models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression. The stress regime for the formation of new fractures changes from compressional to shear with depth. If pre-existing fractures exist, only (sub)horizontal fractures are predicted to open, thus defining the principal orientation of effective reservoir permeability. In models that do not include a blind thrust fault in the basement, flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks. The folding-induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading. Models incorporating a blind thrust fault correctly predict the formation of secondary syn- and anti-thetic mesoscale faults in the basement and sediments of the hanging wall. Some of these faults cut reservoir and/or seal layers, and thus may influence effective reservoir permeability and affect seal integrity. The predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure (and proximity to failure). These numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs.     

克拉通演化的超大陆背景与克拉通盆地的成因机制

研究表明克拉通的形成与超大陆的汇聚和裂解有着重要关系。本文对近年来超大陆重建的研究进行了分析对比,对克拉通发展与超大陆事件的关系做出了总结。前人对超大陆的研究表明,其形成与地幔动力有直接联系,地幔柱重组的旋回导致了超大陆的旋回。Phillips and Bunge（2007）在前人三维球体地幔对流模型的基础上加入大陆进行了模拟实验,结果显示周期性的超大陆旋回只发生在理想模型中,而Senshu et al.（2009）对代表陆壳的英云闪长岩-奥长花岗岩-花岗岩（TTG）地壳进行了研究,提出随着俯冲的TTG地壳产热速率的下降,超大陆旋回的周期随之变长;更有许多学者指出,历史上哥伦比亚超大陆存在时间明显较长,因此超大陆的旋回并不具有周期性。对近年来不同学者提出的哥伦比亚、 罗迪尼亚、 冈瓦纳、 潘基亚4个超大陆新的重建证据进行分析,大致确定出上述4个超大陆的形成时间、 格局及演化过程。此外,对华北、 东欧、 西伯利亚、 亚马孙、 刚果、 西非6个克拉通各自的演化进行分析,也显示出克拉通演化与超大陆汇聚及裂解在时间与空间上有对应关系。通过分析得出克拉通演化与超大陆旋回有关,且确定出克拉通演化的4个超大陆旋回。本文最后讨论了克拉通盆地的成因机制以及3种端元类型,并将盆地的发育与超大陆演化的巨旋回相联系。     

Metallogenesis of the late Pan-African gold-bearing East Ouzzal shear zone (Hoggar, Algeria)

In the W Hoggar (Algeria), the major transcurrent N–S East Ouzzal shear zone (EOSZ) hosts several world-class gold deposits over a 100-km length. The late Pan-African EOSZ separates two contrasting Precambrian domains: the Archaean In Ouzzal block to the west (orthogneisses with subordinate metasediments, reworked and granulitized in the c. 2 Ga Eburnean event) and a Middle Proterozoic block to the east (again orthogneisses and metasediments, involved in the c. 600 Ma Pan-African event). The EOSZ is a mylonite belt, 1–3 km wide, with a 50-m-wide ultramylonite belt hosting numerous quartz veins and lenses (giant hydrothermal quartz system) associated with a quartz-sericite-pyrite-carbonate (beresite) alteration. These hydrothermal events occurred under ductile (evolving towards brittle) conditions, between 500 and 300 MPa, at 500–300°C, with aqueous-carbonic fluids derived both from underlying devolatilized metamorphic rocks and a mantle source, as recorded by stable (C, O) isotope data. No gold mineralization was associated with these typical mesothermal events. Following a pressure drop (to 130 MPa), related to the inception of extensional tectonics, the EOSZ was later percolated by a new set of hydrothermal fluids, evolved from basinal waters that deeply penetrated into the In Ouzzal basement. These fluids were Ca-bearing brines (up to 25% wt. eq. NaCl), characterized by high δD (-9 to + 18‰ range), mobilized by the thermal energy released by the late Pan-African granite magmatism (Taourirt granites). As demonstrated by Pb isotope data, the brines leached Au from the In Ouzzal granulites (which contain 3 ppb Au). Fluid inclusion studies indicate that gold was deposited from these brines in the EOSZ at a depth of c. 5 km, due to mixing and cooling with descending diluted fluids.     

Geology of the Early Devonian oolitic iron ore of the Gara Djebilet Field, Saharan Platform, Algeria

The oolitic iron ore of the Gara Djebilet field occurs within the Early Devonian sediments of the Tindouf Basin (Algerian Sahara), particularly in the Upper Djebilet Formation of Pragian age. Three large lenses form three individual deposits, extending E-W for about 60 km, namely Gara West, Gara Center and Gara East.The mineralization is interbedded with argillaceous to sandy sediments and it can be related to a barrier island palaeoenvironment, bordered by an inner lagoon or shallow embayment and an epicontinental sea. Trapped by Palaeozoic shoals, the oolitic sediments show a mineralogy marked mainly by magnetite, hematite, goethite, maghemite, chamosite (bavalite), siderite, apatite and quartz. Three paragenetic associations present a vertical distribution with a Lower non-magnetitic ore, a magnetitic ore and an Upper non-magnetitic ore.Three petrographical facies types have been defined: a cemented facies (FOC); a detrital facies (FOD); and a non-detrital facies (FOND).Chemical data for the whole field show a difference between the Lower non-magnetitic ore (Fe=54.6%), the Magnetitic ore (Fe=57.8%) and the Upper non-magnetitic ore (Fe=53%). The Magnetitic ore, which corresponds mainly to the workable ore (cutoff grade at 57%), has the following composition: SiO₂=4.9%, Al₂O₃=4.2%, Fe₂O₃=61.43%, FeO=19.2%, and P₂O₅=1.8%. The corresponding calculated economical ore reserves are 985×10⁶t, with 57.8% Fe.Regarding the genesis of the oolitic iron ore, a southern source is suggested for the iron, with deposition taking place in a quiet environment. There, the ooids developed by an intrasedimentary accretion mechanism around detrital grain within an iron-rich mud.The Gara Djebilet field is an important occurrence of the “North African Palaeozoic Ironstone Belt” extending from the Zemmour to Libya which also includes ironstones of Ordovician, Silurian and Devonian age.     

Amphibole-rich xenoliths and host alkali basalts: petrogenetic constraints and implications on the recent evolution of the upper mantle beneath Ahaggar (Central Sahara,Southern Algeria)

Quaternary basanitic to nephelinitic volcanoes from Tahalra (western Ahaggar, southern Algeria) contain numerous Mg-ilmenite and amphibole-rich inclusions (±olivine, ±salite) and spinel lherzolite (±pargasite) inclusions associated with kaersutite megacrysts. On the basis of petrological, geochemical and Sr isotopic study of representative xenoliths (including a composite nodule defined as a vein cross-cutting peridotite) and lavas, we attribute the series of amphibole-rich xenoliths and megacrysts to segregation under upper mantle conditions from a hydrous high Ti and LREE melt geochemically similar to the Quaternary basanite but isotopically different. Amphibole-rich rocks and megacrysts are the results of magmatic events (less than 40 Ma) probably contemporaneous with the various pre-Quaternary volcanic phases recognized in Ahaggar. The amphibole-rich veins and the Quaternary lavas have a garnet lherzolitic source enriched in REE (7 to 9 times chondritic in LREE, 2 times in HREE). This enrichment probably results from former metasomatic events unrelated to the recent magmatic history. Melts from which these veins precipitated within upper mantle peridotite also account for mantle enrichment processes; they induced a local partial melting and contact metasomatism (pargasitization). The upper mantle beneath the volcanic areas of Ahaggar is veined and hydrous, and consequently lightened: thus, the uplift of basement may be the isostatic response to magmatism and related metasomatism and therefore the result of the Cenozoïc igneous activity.     

A mineralogical and geochemical study of polymict eucrite discovered in Sahara of southwest Algeria

NWA2268 is a polymict eucrite discovered in the Sahara, at southwest Algeria, close to the region of Tindouf. This meteorite weighs 65 g and presents a thin black fusion crust. The rock is fine- to medium-grained breccia and contains mineral fragments of plagioclases, pyroxenes, spinel, olivine and silica. The rock contains some basaltic fragments with sub-ophitic or cumulative textures, constituted by plagioclases and exsolved pigeonite. Pyroxferroite grains are present and locally destabilised in an association of hedenbergite, fayalite and silica. It also presents unequilibrated eucritic clast with heterogeneous pyroxenes and plagioclases compositions. Pyroxenes in the all of the other clasts have equilibrated composition, with exolved pigeonites with augite lamellaes. This polymict eucrite contains also partially devitrified glass that represents impact melts linked to impact event. None recrystallization of this glass confirms a lack of post-brecciation metamorphism. Diogenitic fragments are less abundant than 10 %. The oxygen isotopic composition of NWA2268 is Δ¹⁷O (?0.43). This meteorite is interpreted as belonging to the HED group attributed to the 4-Vesta asteroid.     

南华盆地新元古代成冰纪成锰作用及其成矿背景

南华盆地成冰系大塘坡组锰矿是我国最重要的锰矿产出层位之一,它形成于成冰纪Sturtian冰川事件之后,其成矿背景及形成机理一直是研究的重点。在系统总结Sturtian冰川事件起始与结束时间、南华裂谷盆地结构演化及古气候演变等重大地质事件的最新研究进展的基础上,综合分析了南华盆地大型沉积型锰矿成矿作用过程与这些重大地质事件之间的联系。揭示了南华盆地Sturtian冰期的启动和结束与全球其他地区基本一致,分别发生在～717 Ma和～660 Ma之前。同时,对南华系大塘坡锰矿成矿时代进行了约束,大约形成于～660 Ma之前。在新元古代中期Rodinia超大陆裂解作用的影响下,南华裂谷盆地内部发育一系列由同沉积断层控制的地垒—地堑次级盆地。沿同沉积断层运移的热液流体为大塘坡锰矿的形成提供了大量的成矿物质,并控制着大塘坡锰矿的发育分布。化学蚀变指数(CIA)、锂同位素(δ~7Li)及锇同位素组成[n(~(187)Os)/n(~(188)Os)]等风化指标显示,南华盆地Sturtian冰期晚期至间冰期大塘坡期早期的气候为寒冷干燥,随后转为温暖湿润并很快变为寒冷干燥。至大塘坡中晚期,气候逐渐由寒冷干燥恢复至温暖湿润,并一直保持至大塘坡晚期。整体来看,Sturtian冰期结束后,南华盆地表层海水逐渐氧化,深部沉积水体出现局部间歇式氧化环境,裂陷阶段热液和陆源输入的Mn~(2+)被氧化为MnO_2发生沉淀,并在底部伴随着有机质的埋藏及早期成岩作用而最终形成菱锰矿。     

南华盆地新元古代成冰纪成锰作用及其成矿背景

南华盆地成冰系大塘坡组锰矿是我国最重要的锰矿产地之一,它形成于成冰纪Sturtian冰川事件之后,其成矿背景及形成机理一直是研究的重点。在系统总结Sturtian冰川事件起始与结束时间、南华裂谷盆地结构演化及古气候演变等重大地质事件的最新研究进展的基础上,综合分析了南华盆地大型沉积型锰矿成矿作用过程与这些重大地质事件之间的联系。揭示了南华盆地Sturtian冰期的启动和结束与全球其他地区基本一致,分别发生在~717 Ma和~660 Ma之前。同时,对南华系大塘坡锰矿成矿时代进行了约束,大约形成于~660 Ma之前。在新元古代中期Rodinia超大陆裂解作用的影响下,南华裂谷盆地内部发育一系列由同沉积断层控制的地垒—地堑次级盆地。沿同沉积断层运移的热液流体为大塘坡锰矿的形成提供了大量的成矿物质,并对大塘坡锰矿的发育具有明显的控制作用。化学蚀变指数（CIA）、锂同位素（δ7Li）及锇同位素组成（187Os/188Os）等风化指标显示,南华盆地Sturtian冰期晚期至间冰期大塘坡期早期的气候为寒冷干燥,随后转为温暖湿润并很快变为寒冷干燥。至大塘坡中晚期,气候逐渐由寒冷干燥恢复至温暖湿润,并一直保持至大塘坡晚期。整体来看,Sturtian冰期结束后,南华盆地表层海水逐渐氧化,深部沉积水体出现局部间歇式氧化环境,裂陷阶段热液和陆源输入的Mn2+被氧化为MnO2发生沉淀,并在底部伴随着有机质的埋藏及早期成岩作用而最终形成菱锰矿。     

Shale basins,sulfur-deficient ore brines and the formation of exhalative base metal deposits

The massive sulfide deposits of the Iberian Pyrite Belt are interbedded with felsic volcanic rocks and shale, and underlain by several thousand meters of siliciclastic sedimentary rocks known as the PQ Group. Isotope geochemistry and regional geology are both consistent with equilibration of the ore-forming fluids with the PQ Group, prior to ore deposition near the former seafloor. The average Cu:Zn:Pb ratio of the PQ Group rocks (ca. 26:55:19) is similar to the weighted average of all the massive sulfide orebodies combined (ca. 25:52:23).The genetic relationship between massive sulfide deposits and a siliciclastic sedimentary metal source is explained here by a thermodynamic model, proposing that mildly reducing redox conditions imposed by equilibration with the sedimentary rocks are most critical for the formation of an effective ore-forming fluid. Relatively metal-rich but organic-poor pyrite-bearing shale undergoing dewatering of saline pore fluids is an effective source for the generation of sulfur-deficient but relatively iron and base metal-rich brines. Thus, we propose that the giant deposits of the Iberian Pyrite Belt owe their existence not to exceptionally metal-enriched (e.g., magmatic) fluids, but to the existence of a fairly ordinary but large metal source in reactive siliciclastic sediments, combined with an underlying igneous heat source and a particularly efficient mechanism of sulfide precipitation by mixing with H₂S-rich fluids at or near the seafloor.Essentially similar mineral equilibria are imposed when saline fluids are buffered by typical continental basement rocks. Leaching of retrograde minerals and possibly residual salts from their magmatic or metamorphic prehistory is expected to generate similar, variably metal-rich but relatively sulfide-deficient fluids. Thus, the existence of mildly reducing rocks can be the dominant chemical control in the source of fluids generating many volcanogenic, Irish-type or sedex deposits, many of which are known to precipitate their metal load in response to biogenic sulfide addition at the ore deposition site.     

菲律宾迪纳加特岛30#矿权区红土型镍矿床地质特征及成矿作用探讨

30~#矿权区位于菲律宾迪纳加特岛北部,区内发育有红土型镍矿床。本次勘查圈定4处矿(化)体,其中以Ⅰ、Ⅱ、Ⅲ矿(化)体的矿化范围较大,厚度变化系数为45%~50%(相对稳定),经济价值较高。红土型镍矿体平面上呈地毯状展布于地表,厚度受地形控制,富镍矿体分布于矿权区东部的5、6、11、12小区;剖面上从地表向深部矿体为铁质红土层、松散红土层及高岭土化蛇纹石化橄榄岩层。30~#矿权区红土镍矿矿床的形成经历了从普通矿到富矿两个阶段,红土镍矿石属于酸性矿石。     

The late Pan-African intracontinental linear fold belt of the Eastern Hoggar (Central Sahara,Algeria): geology,structural development,U/Pb geochronology,tectonic implications for the Hoggar shield

The structural development of the Tiririne Belt, located in the eastern Hoggar, central Sahara, Algeria has been entirely controlled by a high-strain, now vertical, N—S-trending zone, along which metamorphism and deformation gradually appear to the north in the Tiririne Formation, a clastic unit up to 8000 m thick whose sedimentological environment is described. U/Pb geochronology on zircons from a pretectonic sill within the Tiririne Formation indicates a pre-660 Ma depositional age. This intracontinental belt has been formed during the ‘late Pan-African’, as shown by the 604 and 585 Ma U/Pb zircon ages obtained on syn- to late-kinematic granitic phases from a circular pluton emplaced in the belt (using λ₈ = 0.155125 · 10^?9 y^?1 and λ₅ = 0.98485 · 10^?9 y^?1). The tectonic implications are discussed in the light of the structural and geochronological data. They suggest an eastwards younging of the peak of Pan-African deformation towards the East-Saharan craton, which has been stable for more than 660 Ma.     

Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria)

In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplacement of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite–fayalite–quartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the migmatitic metapelites and metabasites make it possible to reconstruct the P–T–aH₂O path undergone by these formations. This path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5 kbar at 825 °C to 6 kbar at 700 °C; Tidjenouine: 7.5 kbar at 875 °C; to 3.5 kbar at 700 °C; Tin Begane: 13.5 kbar at 850 °C; to 5 kbar at 720 °C), followed by amphibolitization that corresponds to a fall of temperature (from 700 to 600 °C) and an increase in water activity (from 0.2–0.4 to almost 1).The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamorphism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally due to Pan-African events.     

Isothermal decompression of garnet metabasites from Laouni terrane in the LATEA,Central Hoggar,Algeria

We study the first occurrence of clinopyroxene-free garnet bearing metabasites encased in migmatitic gneisses and metapelites, in the area of Tin Begane belonging to the Laouni terrane in the Pan-African Trans-Saharan belt (Hoggar, Algeria). They present two successive parageneses after a primary assemblage characterized by the presence of high titanium amphiboles equilibrated with coarse-grained garnet, plagioclase, biotite, quartz, and rutile. The two retrograde parageneses show decompression textures with the developing of coronas- and worm-like symplectites consisting of orthopyroxene + plagioclase ± amphibole for the secondary paragenesis and plagioclase + amphibole for the late stage paragenesis. These garnet metabasites do not show high pressure paragenesese compared to those previously studied from the Laouni terrane and which are known to present at least one high pressure paragenesis involving primary pyroxenes. We investigate the exhumation of these metabasites by combining detailed petrographic and thermobarometric studies in order to constrain the P-T conditions for each metamorphic stage. The results suggest an isothermal decompression from peak granulitic conditions (850 ± 50 ^° C and 0.85 ± 0.15 GPa) down to transitional granulitic conditions (830 ±50 ^° C and 0.5 ±0.1 GPa) followed by a cooling episode to the amphibolite-greenschist facies transition at 480 ± 80 ^° C and 0.4– 0.5 GPa. These conditions are consistent with a tectonic exhumation process most likely provoked by a lithospheric thinning accompanied by a magmatic episode and partial melting of the lower continental crust.     

Petrology and U/Pb geochronology of the Telohat migmatites,Aleksod, Central Hoggar,Algeria

The Aleksod region is composed of metasedimentary rocks and large areas of biotite and hornblende-bearing migmatites. Anatexis associated with the main deformation stages, occurred under high pressure and temperature conditions estimated at 13±2 Kbar and 750±50° C. The bulk mineralogical composition of the Telohat migmatites shows that their protolith was granodioritic. Internal structures of zircons and U-Pb data suggest a polyphased evolution, with a 2131±12 Ma age for the protolith and a 609±17 Ma age for the Pan-African tectono-metamorphic evolution, thus precluding any Kibaran event in the Aleksod area. Leucosomes are richer in Sr and display lower Rb, Zr, Nb, Y, Th, U and REE contents than melanosomes wherein accessory phases are stored. Eu contents are also lower in the leucosomes but in lesser proportion than the other rare earth's, leading to a significant positive anomaly. Petrogenetic modelling accounting for accessory mineral phases clearly shows that the trace element contents of leucosomes and melanosomes follow a distribution law consistent neither with equilibrium nor fractional melting. Their trace element patterns are best explained by the model of disequilibrium melting, with mixing of a few residual phases. The present results and previous Sr isotopic data as well raise the question of disequilibrium melting in anatexis of crustal material CRPG Contribution no 782     

华北克拉通南缘嵩箕地区喀斯特铝(粘)土矿成矿作用及其后生改造研究

华北克拉通铝(粘)土矿是我国喀斯特型铝(粘)土矿的典型代表, 前人针对其物质组成、物质来源、成因机制开展了大量研究, 但对其成矿后生改造尚未开展系统研究。本文对嵩箕地区铝(粘)土矿进行了研究, 野外剖面显示, 含矿岩系由底到顶包括风化壳层、铁质粘土岩、粘土质铝土矿、粘土岩、块状铝土矿层。通过矿物空间穿插关系、X射线衍射、扫描电镜、电子探针分析显示, 铝(粘)土矿成矿期主要形成矿物包括硬水铝石、锐钛矿、伊利石、绿泥石等矿物。黄钾铁矾、赤铁矿、针铁矿、高岭石代表了后期改造矿物。矿物垂相分布指示赤铁矿和针铁矿随着深度变浅逐渐减少, 硬水铝石在粘土质铝土矿层显著富集, 黄钾铁矾广泛分布于铁质粘土岩层和风化壳顶部, 穿插于粘土矿物或围绕在硬水铝石周围, 以脉状、颗粒状分布, 且与伊利石含量呈消长关系。同时, 元素地球化学垂相分带显著, 大离子亲石元素(Rb、Cs、Ba、Sr)与K₂O以及高场强元素(Zr、Hf、Nb、Ta、U、Th)与TiO₂显示正相关。铝土矿层中Al₂O₃明显富集, 而SiO₂则明显亏损。底板灰岩、风化壳、铁质粘土和粘土岩中, Al₂O₃和SiO₂具有一致的变化规律。铁质粘土岩中黄钾铁矾明显控制了K₂O、Fe₂O₃、S的含量, 但对微量和稀土元素的影响较弱。电子探针分析结果显示不同形态黄钾铁矾成分相似, 结合其空间分布特征, 确定黄钾铁矾是由黄铁矿氧化形成的酸性流体改造含矿岩系形成。因此, 嵩箕铝(粘)土矿成矿之后, 经历了构造抬升作用, 在地下水或大气降水影响下黄铁矿被分解、氧化为黄钾铁矾、针铁矿、赤铁矿等稳定矿物, 早期生成的粘土矿物分解为高岭石。

     

Litho- and biostratigraphy,facies patterns and depositional sequences of the Cenomanian-Turonian deposits in the Ksour Mountains (Saharan Atlas,Algeria)

The Cenomanian-Turonian deposits exposed in the Ksour Mountains, western part of the Saharan Atlas (Algeria), document marine shelf environments that had been thriving on the North African passive margin, connected northwards to the Tethys Ocean, and fringed southwards by the Saharan craton. Their lithological, palaeontological, and sedimentological characteristics have been investigated to provide new insights into the biostratigraphy, palaeo-environmental evolution and sea-level changes in this western part of the Saharan Atlas. Three formations are recognized, from base to top of the studied succession: 1. The El Rhelida Formation comprising two informal units: the mixed siliciclastic-carbonate unit deposited under different flow regime conditions, from shoreline to backshore environments, and the limestone–claystone unit including coastal mudflat deposits prone to storm events. The early Cenomanian age of the El Rhelida Formation is supported by vertebrate assemblages. 2. The Mdaouer Formation comprising two units: the evaporitic unit dominated by claystone and evaporite alternations deposited on a flat coastal sabkha with occasional storms, and the marlstone–limestone unit formed in a peritidal-lagoonal environment. The Mdaouer Formation is of early-middle Cenomanian age. 3. The Rhoundjaïa Formation comprising three units: the lower limestone unit consisting of relatively homogeneous fossiliferous limestones; the middle marly unit composed of marlstone and bioclastic limestones, and the upper limestone unit consisting of carbonates showing vertical variations in faunal content and stratonomy. The Rhoundjaïa Formation was deposited in homoclinal ramp setting. Ammonite data indicate an early late Cenomanian to early Turonian age for this interval. Within an overall transgressive trend, the Cenomanian-Turonian deposits of the Ksour Mountains record three third-order depositional sequences bounded each by regional discontinuities.     

Ore Controls and Metallogenesis of Au-Ag Deposits at Atalla Mine, Central Eastern Desert of Egypt

Gold-silver deposits in the Atalla area occur as hydrothermal quartz veins in NE–SW pre-existing fractures within the Atalla granitic pluton. The orientation of such quartz veins has been attributed to extensional behavior related to the Atalla Shear Zone(ASZ). The Atalla area is covered by a variety of lithologies that are(from oldest to youngest): metasedimentary rocks, metavolcanic rocks, ophiolite assemblage(serpentinites/talc-carbonates), Atalla granite and Dokhan volcanic rocks. Microscopi...     

哈萨克斯坦环巴尔喀什斑岩铜矿地质与成矿背景研究

中哈萨克斯坦位于中亚造山带中部,是中亚型造山带及中亚斑岩铜矿成矿域的重要组成部分,已发现数十个大型和超大型矿床,成群成带分布。主要的斑岩铜矿类型有斑岩铜-金矿、斑岩铜矿、斑岩铜钼矿,大多具同期火山岩。已建立的热液蚀变分带模式具有碱性蚀变和酸性蚀变两个阶段,已有的硫铅同位素数据表明成矿物质来源于深部。该地区的斑岩铜矿形成与多阶段构造演化有关,早古生代的斑岩铜矿与岛弧演化的早阶段有关,而晚古生代的斑岩铜矿与泥盆纪火山—岩浆弧、石炭纪—二叠纪的火山—岩浆弧有关。从中哈萨克斯坦的北西向南东方向,斑岩铜矿的形成时代逐渐变年轻。虽然经过数十年的研究,但该地区的有关斑岩铜矿的精细时空结构仍未建立。因此,含矿斑岩体与蚀变矿化年龄的精确测定、区域成矿地球动力学背景及其演化、斑岩铜矿的精细时空结构、与中国邻区的构造—岩浆—成矿带的连接对比将是以后的研究方向。     

Traces of paleoearthquakes in the Quaternary deposits of intermontane basins in central Gorny Altai

Study of the sections of Neopleistocene–Holocene deposits filling the basins in central Gorny Altai has revealed earthquake-induced soft-sediment deformation (seismites). They formed as a result of the brittle deformation of deposits and liquefaction of loose water-saturated sediments under vibration seismic impact. The paleoearthquakes resulting in such seismites had the minimum intensity I = 6 and magnitude M = 5–6. Hence, the study region underwent strong earthquakes in the Neopleistocene–Holocene.     

Section of Permian deposits and fusulinids in the Halvan Mountains, Yazd province, Central Iran

The Permian section situated northwest of Tabas in the Halvan Mountains is studied and fusulinids occurring in the section are described. The Chili, Sartakht, and Hermez formations distinguished in the section are separated by horizons of bauxitic laterite and belong to the Khan Group formerly ranked as a synonymous formation. Fusulinids occur at two levels in the section. The lower one confined to the Chili Formation yields the so-called Kalaktash fusulinid assemblage of the late Sakmarian age. The second late Asselian assemblage has been discovered in pebbles from conglomerate-breccia in the basal laterite of the Sartakht Formation. A brief characterization of fusulinids is presented and three new species are described. The new Benshiella genus is discriminated from the Rugosofusulinidae family. As Skinner and Wilde (1965, 1966) changed the original diagnosis of the Pseudofusulina genus, we suggest, regarding all species, which have been attributed to this genus but do not satisfy the new diagnosis, as representing the new Nonpseudofusulina genus.