The geology of part of the Southern foreland of the Damara orogenic belt in South West Africa and Botswana

A characteristic feature of the Damara geosyncline in northern South West Africa is the gradual transgression from the relatively undeformed sediments of the miogeosynclinal facies to the metamorphosed eugeosynclinical facies. A crude mirror-image of the arc separating the two facies is present along the southern foreland of the Damara geosyncline and conforms quite clearly with the northern limit of the Kalahari craton. However, owing to the presence of major thrust-faulting which eliminates the transition-beds between the two facies a direct correlation is not possible. The pre-Damara rocks on the southern side of the Damara geosyncline have been divided into the Marienhof Formation and the Dordabis Group in central South West Africa, and the Kgwebe Group in Botswana. These rocks form the base on which the strata constituting the Damara geosyncline have been deposited, and have been subjected to metamorphism both before and during the Damara orogenesis. The term Southern Marginal Facies is proposed for the unmetamorphosed strata deposited unconformably on the pre-Damara rocks along the southern flanks of the Damara geosyncline. These unmetamorphosed strata are composed of the Klein Aub, Witvlei, Ghanzi and Tsumis Formations which are regarded as the equivalent of the Nosib Group. They are unconformably overlain by the Nama Group which is considered to be, at least in part, the equivalent of the Hakos and Khomas Formations. Copper mineralization associated with calcareous shale, limestone and argillite occurs within the Klein Aub, Witvlei and Lower Ghanzi Formations. The mineralized strata closely resemble that of the Zambian Copper-belt with which they may well be correlated.     

Groundmass oxide minerals in the Koidu kimberlite dikes,Sierra Leone,West Africa

Petrology and oxide mineral chemistry are presented on 5 kimberlite dikes that are classified into three groups: (1) one dike is highly carbonated and highly oxidized (> MH) and is characterised by chlorite+Mn-titanomagnetite+rutile+hematite (after chlorite)+maghemite (after titanomagnetite), with ilmenite and perovskite being absent; (2) three dikes are typified by atoll-textured spinels +phlogopite+euhedral Mn-picroilmenite, of intermediate oxidation state (WM-FMQ) with coexisting deuterically serpentinized olivine+Ni-Fe alloys and magnetite; (3) the remaining dike records an early crystallization event under very low ( WM) redox conditions that precipitated anionic-deficient spinels and a Mg-Ti-Cr-wüstite-type phase, followed by late stage more oxidizing (=FMQ) Mnpicroilmenite.Spinels are complexly zoned and crystallization trends among the dikes are diverse, underscoring the fact that no single compositional trend, or evolutionary sequence is typical of kimberlites. Ilmenites are euhedral, and criteria for groundmass crystallization are established. Extraordinarily high MnO (max 17 wt%) contents and high geikielite (62 mole%) concentrations expand the ilmenite field typically assigned to that of kimberlites. Zirconium, Nb and Cr are present in concentrations of 0.5–3 wt% (as oxides) in ilmenite. These highly incompatible elements, along with Mn, are concentrated in late stage melt fractions. The high pyrophanite contents, which are more typical of silicic alkali suites, are accompanied by phlogopite in the Koidu dikes.Objective evaluations of kimberlite-carbonatite relations, as outlined in the literature, cannot be made based on the oxide mineral group. Much of the compositional data for oxides in kimberlites are on mantle-derived xenolith suites and are not from oxides derived from the crystallization of kimberlitic melts.Assessments of the fO₂'s of kimberlites have considerable potential in evaluating diamond survival through redox reactions. Manganese-rich (+Nb, Zr, Cr) ilmenites are typical of many kimberlites and should be considered in the suite of index minerals employed in prospecting.     

Gravity stratification in the Freetown Basic Igneous Layered Complex,Sierra Leone,West Africa

An area of 78 sq. km in the vicinity of Freetown is mapped for the first time. By systematic mapping and study of about 1,500 thin sections, nine macro layered units have been recognized in this lowest exposed part of the intrusion. Each unit grades upward from magnetite or olivine-rich layers, through pyroxene-rich to plagioclase-rich rock. Field data of fine-scale layering, ‘false-bedded’ layers and slumping are given. These and other sedimentary structures and features indicate that the Freetown magma was convective. It is suggested that rhythmic layering in the Freetown basic intrusion originated from slow accumulation of density-sorted crystals which precipitated simultaneously from the magna-pulses.     

The geology of the Taparko gold deposit, Birimian greenstone belt, Burkina Faso, West Africa

The Taparko gold deposit, located in the eastern branch of the Proterozoic Birimian Bouroum-Yalogo greenstone belt (Burkina Faso) consists of a network of quartz veins developed in a N 170° trending shear zone (250 m wide, 4 km long) superimposed on the regional Birimian structural pattern. The quartz vein network is composed of: (a) a dominant array of quartz veins (type 1), parallel to the shear zone and comprising strongly deformed dark quartz exhibiting foliation, layering, ribbon, tension gashes, etc.; (b) oblique and subparallel related veins (type 2) of gray to white weakly deformed quartz crosscutting the dominant quartz veins resulting in breccia structures; and (c) shallow dipping veins (type 3), cross-cutting veins types 1 and 2 and filled by undeformed white buck structure quartz. Cross-cutting relationships and different quartz types in different veins and within individual veins imply a concomitant filling of the veins during the progressive deformation. Initial sinistral transcurrent shearing evolved with time to sinistral reverse shearing. Metallic minerals occur only in type 1 and 2 veins and were deposited in two stages, with native gold being related to second stage sulfides. Gold (and chalcopyrite) precipitated preferentially upon the surfaces of fractured pyrite grains in low-pressure sites (pressure shadow zones) around and/or within the sulfide grains (along subsequently annealed fractures). The formation of the South Taparko deposit can be divided into a succession of events: (a) during the first event, N 170°-directed sinistral transcurrent shearing resulted in a N 20° mylonitic foliation and fractured rock which allowed H₂O-, CO₂- and SiO₂-rich fluids to circulate and deposit quartz with buck texture; (b) during the second event, type 1 quartz was strongly deformed and type 2 veins formed with sigmoidal shapes as viewed on a horizontal plane; and (c) during the third event, the sinistral transcurrent shearing evolved to sinistral reverse shearing and the deformation style evolved correspondingly from ductile to brittle-ductile. During the last phase of deformation gold nucleated and deposited in low-pressure zones. Received: 9 July 1997 / Accepted: 23 March 1998     

对胶南造山带基础地质问题的新认识

:胶南造山带主体是由晚元古代同造山双花岗岩组成的造山带 ,有少量造山前幔源深成岩及造山后碱性花岗岩类。零星分布的地层残片可分为晚太古代胶东岩群 ,早元古代荆山群、粉子山群及震旦纪朋河石岩组。造山带经历了高压相系—中低压相系多期、多相变质作用 ,主要构造形迹为韧性剪切带 ,其次为穹隆构造、弧形构造。胶南造山带可分为北段、中段和南段 3部分 ,北段、中段属于华北板块南部边缘带 ,南段属扬子板块北部边缘带。     

中非铜矿带造山带特征演化模式

本文对中非铜矿带造山带基本地质构造特征进行了分析,阐述了其大地构造演化模式,认为造山带的加丹加超群标准地层剖面由4个群组成,分别为罗安群、木瓦夏群、下昆代隆古群和上昆代隆古群,底部沉积岩时代为8.8亿年,其中以5.5~5.6亿年的卢菲利造山运动产生大规模北东向逆推作用为主要造山事件,并认为裂谷拉张与陆陆碰撞为本区大地构造演化的基本模式.     

The Kasila Group,Sierra Leone,an interpretation of new data

Recent geological mapping and U/Pb age determinations show that the Kasila Group has many of the characteristics shown by the Limpopo Belt and other high-grade linear metamorphic belts. The Kasila Group appears to form the southwestern periphery of the dominantly low-grade West African Archaean Craton.     

Late Proterozoic to Early Palaeozoic supracrustal succession of Sierra Leone: an aulacogen at the western margin of the West African Craton

Three parallel zones of Upper Proterozoic to Lower Palaeozoic sedimentary-volcanic cover rocks have been recognized in western Sierra Leone. The Kasila Group m the SW ist metamorphosed to the granulite facies and grades into the Marampa Schist Group, and this in turn grades into the unmetamorphosed Rokel River Group in the NE. Prior to this investigation, these rocks were thought to belong to different ages; the most highly metamorphosed was thought to be the oldest and the unmetamorphosed, the youngest. This view is unrealistic since field mapping has failed to reveal any sharp contacts between them and all available age data indicate their involvement in the Pan-African (ca. 550 Ma) Orogeny A model is proposed involving an intracontinental graben (aulacogen) extending from western Sierra Leone into the adjacent territories developed about 1000 Ma ago. During the Pan-African Orogeny a portion of the aulacogen fill overlying the remobilized basement was subjected to a regional metamorphism and deformation. The grade of metamorphism and the degree of deformation decreased steadily from the centre of the Pan-African domain to the contact with the West African craton. The unmetamorphosed and mildly deformed to undeformed equivalent rests on the craton.

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Zusammenfassung Aus Sierra Leone werden drei Untereinheiten von oberproterozoischen bis unterpaläozoischen vulkano-sedimentären Schichten beschrieben, die nebeneinander liegen und ineinander übergehen.Die Kasila-Gruppe im SW, die bis zur Granulitfazies metamorphisiert ist, geht in die Marampa-Schiefer-Gruppe über, die wiederum in die unmetamorphisierte Rokel-River-Gruppe im NE überleitet. Vor dieser Untersuchung wurden diesen Gesteinen unterschiedliche Alter zugeordnet; die am stärksten metamorphisierten wurden als älteste, die unmetamorphisierten als jüngste Gesteine angesehen. Diese Betrachtung ist unrealistisch, da Geländekartierungen keine scharfen Kontakte erkennen lassen und alle erhältlichen Altersdaten die Einbeziehung in die Panafrikanische Orogenese (ca. 550 Ma) anzeigen. Es wird daher angenommen, da\ sich vor ungefähr 1000 Ma ein intrakontinentaler Graben (Aulakogen) entwickelte, der vom westlichen Sierra Leone bis in die benachbarten Territorien reichte. Während der Panafrikanischen Orogenese wurde ein Teil der dem remobilisierten Basement auflagernden Aulakogenfüllung einer Regionalmetamorphose und Deformation unterworfen. Die Stärke der Metamorphose und der Grad der Deformation nimmt vom Zentrum der panafrikanischen Region bis zum Kontakt mit dem westafrikanischen Kraton hin gleichmä\ig ab. Auf dem Kraton selbst liegen die nicht metamorphisierten, schwach- bis undeformierten Schichten (Rokel-River Gruppe).

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Résumé On a reconnu, en Sierra Leone occidentale, trois zones parallèles de roches volcano-sédimentaires, d'âge protérozoÏque supérieur à paléozoÏque inférieur. Le groupe de Kasila, dans la région sud-ouest, est métamorphisé dans le faciès des granulites; il passe progressivement au groupe des Schistes de Marampa, qui passe lui-mÊme au groupe non métamorphique de Rokel River, dans la région nord-est. Jusqu'ici, ces roches ont été considérées comme d'âges différents, les plus métamorphiques étant les plus vieilles, et les non-métamorphiques les plus jeunes. Ce point de vue s'est avéré incorrect; en effet, la cartographie n'a pas révélé de contacts nets entre ces roches et, par ailleurs, les données disponibles relatives à leur age relient leur évolution à l'oro genèse pan-africaine (ca 550 Ma). Le modèle proposé fait appel à un graben intra-continental (aulacogène) qui s'étend de la Sierra Leone occidentale aux territoires contigus et s'est développé, il y a 1000 Ma environ; lors de l'orogenèse pan-africaine, la partie du remplissage de l'aulacogène qui recouvre le socle remobilisé a été soumise à une déformation et à un métamorphisme régionaux. Le degré du métamorphisme ainsi que l'intensité de la déformation décroissent régulièrement du centre du domaine pan-africain jusqu'au contact avec le craton ouest-africain. Sur le craton lui-mÊme reposent les couches équivalentes non métamorphiques, peu ou pas déformées, du groupe de Rokel-River.

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The Burkinian orogenic cycle,precursor of the Eburnian orogeny in West Africa

In the best known areas of the West African craton structural, petrographic, and geochronological data provide a distinction between two separate units of the Lower Proterozoic in West Africa. A lower unit was metamorphosed from low to medium grade around 2170 Ma by horizontal shearing. This unit is mainly composed of dominantly basic bimodal magmatic rocks, and some trondhjemitic to tonalitic anatectic gneisses, and locally mesozonal mica schists known in Ivory Coast as the ‘Kounoukou formation’ which has been dated as 2183 Ma old. An upper unit, which frequently begins with polygenic conglomerates, also shows important bimodal vulcanism, but in contrast to the lower unit is dominantly silicic. This unit is affected by lower grade conditions of metamorphism and is weakly deformed. These two units correspond to the classic Eburnian I/Eburnian II succession. However, a clearer distinction is possible, with the recognition of two successive orogenic cycles. In the basal part of each of the two units a major episode of tholeiitic magmatism is evidence for separate periods of lithospheric thinning and fracturing. This was followed by the deposition of various lithological sequences, then by one or more tectonometamorphic events. Later uplift led to the emplacement of anorogenic subvolcanic granitic massifs which are displayed occasionally as ring complexes. This suggests that ‘Eburnian I’ is actually an independent orogenic cycle appearing in the West African formations between 2400 and 2150 Ma, for which we propose the name ‘Burkinian cycle’. This cycle has affected the lower magmatic and sedimentary formations, which we designate Dabakalian. This implies a restricted time span for the Eburnian cycle from 2100–2150 to 1800 Ma. The stratigraphic term ‘Birimian’ is applied to the sedimentary and magmatic formations of the upper unit. This scheme proposed for the Lower Proterozoic in West Africa is probably applicable to other Lower Proterozoic terranes in Africa.     

北祁连造山带西段车路沟岩体U-Pb年代学、地球化学特征及岩石成因

车路沟岩体位于北祁连造山带最西端,岩石类型以英云闪长玢岩和闪长玢岩为主。本次对英云闪长玢岩进行U-Pb年代学研究,获得加权平均年龄为(462.1±3.8)Ma,表明岩体形成于中奥陶世。岩石地球化学分析结果显示,英云闪长玢岩中SiO_2含量60.37%～66.73%,Al_2O_3含量15.65%～16.45%,Na_2O/K_2O值7.47～9.96,A/CNK为0.90～1.00,属中酸性岩系列,显示相对富钠、低钾的特征, A/CNK=0.90～1.00,属于准铝质低钾钙碱性系列;稀土总量较低(33.40×10~(-6)～39.27×10~(-6)),LREE/HREE值为3.95～6.51,在稀土元素配分图解上呈现出右倾特征;δEu值1.23～1.49之间,呈现Eu呈正异常特征;在微量元素原始地幔标准化图解上,显示富集Ba、Sr,亏损Nb、Ti、Yb高场强元素的特征。对车路沟岩体地质、地球化学特征与产出背景的全面分析,认为其为奥陶纪祁连洋持续向华北板块俯冲,诱使俯冲洋壳部分熔融形成的埃达克岩。     

辽西中生代板内造山带构造基本特征及构造演化

辽西中生代构造运动可划分为印支早期(早、中三叠世)、印支晚期(晚三叠世)、燕山早期(早侏罗世)、燕山中期(中、晚侏罗世)、燕山晚期(早白垩世)、燕山末期(晚白垩世)6个构造幕。中生代造山带有别于板缘或板间造山带的一种特殊类型的造山带,也不是板缘或板间造山带的一个发展阶段。因此,具有独特的大地构造背景、造山期前演化历史,以及造山带构造变形变质、岩浆活动、沉积作用等特点。中生代板内造山过程是复杂的、多阶段的、非单一的过程,三叠纪以来,共经历了多次裂陷与伸展、挤压与收缩作用和多阶段的盆地发展历史。在每一次盆地演化过程中,在早期表现为裂陷与伸展作用,并有中基性—中酸性火山岩浆喷发和侵入,具有从早期向晚期岩浆由偏基性向偏酸性演化的特点,同时形成断陷盆地,沉积陆源粗碎屑建造;中期,断陷盆地向坳陷盆地转化,沉积陆源细碎屑和含煤及红色建造;晚期表现为挤压和收缩的造山作用,使地层褶皱,并发育逆冲断层,盆地抬升遭受剥蚀,从此构成了一个火山喷发—沉积盆地从形成→发展→萎缩→消亡的完整过程。这样多旋回的变化,塑造了辽西地区的中生代板内造山过程。     

Geochemistry and mineralogy of laterites in the Sula Mountains greenstone belt, Lake Sonfon gold district, Sierra Leone

Geochemical and mineralogical investigations have been carried out on laterite profiles developed in the Lake Sonfon Au district of northern Sierra Leone. The area is underlain by Archean metavolcanics and constitutes part of the Sula Mountains greenstone belt, which is mineralized in Au. Extensive lateritization has affected the rocks of this region, resulting in a profile which from bottom to top consists typically of a decomposed bedrock zone, a pisolitic laterite layer and a duricrust layer. Both the pisolitic and duricrust layers of the laterite are sometimes punctuated by lenses of ironstones containing high amounts of Cu, Zn, Ni, Co and Ce. Gold occurs as small grains within the heavy mineral fraction recovered from the decomposed rock zones and pisolitic layers of the profiles and also in gravels of streams draining the area. The mineralogy of the duricrust and pisolitic layers is dominated by goethite, gibbsite and quartz, with minor amounts (<5% by volume) of ilmenite, magnetite, haematite, rutile and kaolinite. The kaolinite content increases towards the decomposed rock zone, where talc, vermiculite and other layer lattice silicates become abundant. The heavy-mineral fraction of stream sediments is composed essentially of ilmenite, magnetite, haematite, and traces of rutile, zircon, tourmaline and Au. The Au grains are often characterized by a 10–200-μm-wide rim having a much lower content of Ag (0.3 wt.% or lower) than the grain interior (about 5 wt.% on average). Dissolution effects are also observed on the grain surfaces. It is considered that Au derived from the amphibolite parent rock is dissolved, transported, and redeposited during laterization.The duricrust cover of the laterite profiles is characterized by high contents of Fe₂O₃ (ca. 60 wt.%) and Al₂O₃ (ca. 32wt.%) and low content of SiO₂ (ca. 9 wt.%). In comparison, the pisolitic layer is higher in SiO₂ (ca. 18 wt.%) as well as a slightly higher in Al₂O₃ (ca. 34 wt.%). Lateritic weathering has resulted in the removal of CaO, Na₂O, MgO and SiO₂, with relative enrichment of Fe₂O₃ and Al₂O₃. The geochemical distribution of the trace elements in the laterite profiles can be related to the occurrence of the auriferous mineralization. The significance of these observations is discussed in relation to the origin of the lateritic Au and the role of the associated trace elements as indicators of the mineralization.     

对中国西部造山带地质研究若干问题的思考

从大陆研究的视角,认为大陆造山带地质研究是广义上的陆缘地质研究,阐述了陆缘多岛弧盆构造系统及其演化过程,提出造山带内时代相近的一组蛇绿混杂岩带是“古陆缘多岛弧盆格局”的残留,“古陆缘多岛弧盆构造系统”的基本残留形式在平面和剖面上都可能是“网结状”等认识。     

关于造山带花岗岩类填图方法的讨论——以西秦岭天水地区为例

花岗岩类的分类理论决定其填图方法.现行的单元-超单元填图方法是依据花岗岩类的I型、S型分类理论和同源岩浆演化理论而建立的.由于同源岩浆演化理论的不完善性,造山带地壳结构及地质构造特征的复杂性、地质作用过程的多样性,导致了形成花岗岩类源岩组分的复杂性,使得I型、S型分类在造山带具有不确定性,因而单元-超单元填图方法在造山带实践中存在较多问题和矛盾.造山带花岗岩类填图方法应以基本岩石分类方案为依据,以野外可识别、易掌握的客观岩石学、矿物学、组构学为准则, 客观、真实地反映造山带花岗岩类的特色.     

Petrology and geochemistry of metamorphosed komatiites and basalts from the Sula Mountains greenstone belt, Sierra Leone

The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises a thick (5 km) lower volcanic formation and a thinner (2 km) upper metasedimentary formation. Komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti komatiites – depleted in light REE; low-Ti komatiites – with flat REE patterns; high-Ti komatiitic basalts – with flat REE; low-Ti basalts – depleted in light REE; high-Ti basalts – with flat REE patterns. Much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence of a thick pre-existing crust which would have acted as a “filter” to komatiite eruption. Received: 10 February 1998 / Accepted: 28 July 1998     

The Adam Talha epizonal sequence (North Mauritania) and a general scheme for the Eburnean orogenic belt of West Africa

The newly discovered Adam Talha epizonal sequence extends to the west the epizonal sequences defined by Sougy (1960) in the Reguibat Dorsale. In the Algerian part of the Reguibat Dorsale, Sabate and Lameyre (1973) have shown the existence of two Eburnean orogenic cycles. These two cycles: the Yetti cycle (older than 2,000 m.y.) and the Eglab cycle (near 2,000 m.y.) have been observed in the Mauritanian part of the Reguibat Dorsale. In the Ivory Coast, the two structural stages defined by Tagini (1971) seem to be the equivalents of the Yetti and Eglab cycles. However, some differences exist in the chemical compositions of the volcanic rocks (spilitic in the Ivory Coast, acid in the Reguibat Dorsale) and in the absence of volcanic emissions in the upper stage of the Ivory Coast. In Mali, according to Reichelt (1972), the Gourma Basin formations (Hombori and Ydouban groups) belong to the pan-African and not to the Eburnean orogeny. The greater part of West Africa seems to have been affected by the two cycles of the Eburnean orogeny.     

西天山造山带晚古生代构造演化和成矿作用

西天山是我国重要的内生金属矿床集中区,其矿种包括有铜、镍、钼、铁、金、锌等,这些矿产在前寒武纪和显生宙的地质历史中分别形成了大量的成矿体系。通过对西天山成矿地质背景和成矿类型的研究,厘定了5种内生金属矿床类型：1）岩浆型铜镍矿床;2）斑岩铜钼矿床;3）火山岩型铁矿床;4）浅成低温热液型金矿床;5）造山型金铜矿床。典型矿床有菁布拉克铜镍矿床、达巴特铜钼矿床、喇嘛苏铜矿床、敦德铁锌矿床、备战铁矿床、阿希金矿、伊尔曼得金矿床和卡特巴阿苏金矿床等。从构造上看,这些矿床的发育与西天山造山带的增生和汇聚作用密切相关。矿床的形成经历了3个主要阶：早志留世,南天山洋北向俯冲于伊犁—中天山地块之下,形成与铜镍矿床有关的菁布拉克岩体;早-晚石炭世,由于北向俯冲作用,形成了斑岩型铜金矿床、低温热液型金矿床和火山岩型铁矿床;晚石炭世早期,南天山洋壳可能被消耗殆尽,导致了塔里木克拉通与南天山造山带的碰撞。大花岗岩体广泛分布于北天山增生楔和伊犁—中天山地块,并伴随造山型的金矿床形成。     

非洲中南部赞比西造山带地质演化及矿产资源现状

赞比西造山带位于非洲中南缘,是新元古代-早古生代泛非造山运动的重要组成部分,形成于冈瓦纳古陆中心陆块缝合期间。赞比西带大地构造位置位于刚果克拉通和卡拉哈里克拉通之间,东连莫桑比克带,北接卢弗里安弧,西部和纳米比亚的达马拉构造带相呼应。带内主要地层单元为基底杂岩和一套沉积在硅铝质基底上的浅变质沉积序列。岩石学及构造学证据证明沉积作用发生在陆内裂谷盆地中,大量同位素年龄限制了盆地演化时代大致为880～820Ma;赞比西带在600～450Ma的泛非造山事件中再次活化,韧性剪切和大范围糜棱岩化导致了角闪岩相变质作用和同构造期花岗质岩体侵入事件同时发生。赞比西带目前已发现的矿产包括岩浆通道型镍硫化物矿床和浅成低温热液型硅锌矿等。笔者通过对赞比西带的地质演化和矿床成矿作用进行系统总结,并将赞比西带内的主要矿床和国内类似矿床进行对比分析,以期为赞比西带找矿实践提供依据或线索。     

Towards an understanding of man-firewood relations in Freetown (Sierra Leone)

This paper examines ‘folk’ evaluation and classification of firewood as a means towards understanding man-firewood relations in Freetown. It demonstrates how access to ‘wood views’ renders comprehensible identifiable spatial and temporal patterns in firewood exploitation and utilisation. It is argued that the extension of the ethno-scientific approach beyond mere identification of wood categories to the examination of the relationship between cognition and action generates information capable of informing efforts towards short-term gains and/or long-term development policy aimed at a more efficient management and utilisation of the firewood resource. The article concludes by emphasising the need for an understanding of the view both from ‘above’ (technological forestry) and ‘below’ (folk ecology) of man-firewood relationships in the Freetown area in order that meaningful co-operative modifications based on a blend of sodo-cultural and ecological considerations, on the one hand, and economic and technological factors on the other, can be effected.     

Geological setting and tectonic subdivision of the Neoproterozoic orogenic belt of Tuludimtu, western Ethiopia

The N–S trending Tuludimtu Belt in the extreme west of Ethiopia has been subdivided into five lithotectonic domains, from east to west, the Didesa, Kemashi, Dengi, Sirkole and Daka domains. The Kemashi, Dengi and Sirkole Domains, forming the core of the belt, contain volcano-sedimentary successions, whilst the Didesa and Daka Domains are gneiss terranes, interpreted to represent the eastern and western forelands of the Tuludimtu Belt. The Kemashi Domain, which consists of an ophiolitic sequence of ultramafic and mafic volcanic and plutonic rocks together with sedimentary rocks of oceanic affinity, is interpreted as oceanic crust and is considered to represent an arc-continent suture zone. The Dengi Domain, composed of mafic to felsic volcanic and plutonic rocks, and a sequence of volcanoclastic, volcanogenic, and carbonate sediments, is interpreted as a volcanic arc. The Sirkole Domain consists of alternating gneiss and volcano-sedimentary sequences, interpreted as an imbricated basement-cover thrust-nappe complex. All the domains are intruded by syn- and post-kinematic Neoproterozoic granitoids. Structural analysis within the Didesa and Daka Domains indicate the presence of pre-Pan African structures, upon which Neoproterozoic deformation has been superimposed. The gneissic rocks of these two domains are regarded as pre-Pan African continental fragments amalgamated to West Gondwana during Neoproterozoic collision events. Unconformably overlying all of the above are a series of tilted but internally undeformed conglomerate–sandstone–shale sequences, regarded as post-accretionary molasse-type deposits, formed during gravitational collapse of the Tuludimtu Belt. The Tuludimtu Belt is interpreted as a collision orogenic belt formed during the assembly of West Gondwana prior to final closure of the Mozambique Ocean.