赞比亚地质报告英文版.doc

3 Geology地质3. 1 Introduction说明Zambia comprises a number of geologically diverse terrains ranging from stable Archaean and early Proterozoic cratons to structurally complex “mobile belts” and younger cover rocks, and it is this diversity which creates the considerable exploration potential of the country. The geological complexities and multiple tectono-thermal events evident in Zambia are due, in large pan, to the countrys unique geographic location between three major cratons, the massive Kasai Craton to the west and the Zimbabwe-Kaapvaal (Kalahari) and Tanzania cratons to the south and north respectively (Duane and Saggerson, l993; Unrug, 1996) (Figure 6). Inter-cratonic dislocations and the buttressing effects of these stable blocks have exerted considerable control on the geological evolution of the country. As a result, the stratigraphic record is far from complete but it is reviewed briefly below and subsequently major tectono-thermal events and structural domains are considered. Historically, and perhaps not surprisingly, most geological studies have focused on the Copperbelt, and much information from these studies has appeared in scientific journals as well as the geological reports produced by the Geological Survey Department.赞比亚是一个地质多样化国家，从稳定的太古代和早期的Proterozoic克拉通到结构复杂的“移动带”以及新近的覆盖岩都有分布，正是这种多样性使得这个国家具有相当大的开发潜力。地质复杂性和多重地壳热运动事件造成了这个国家在三个主要克拉通（到西部的大Kasai克拉通，南部的津巴布韦-kalahari克拉通，北部的坦桑尼亚克拉通）的独特地理位置（图6）。这三个稳定地块之间的内部作用变形和支撑影响对这个国家的地质变化起到了相当大的控制作用。因此，目前的地层记录还远未完成，但是下部以及其后的主要地壳热运动事件和构造范围已被粗略的认识到。从历史上来看，也许毫不出人意料的，大部分的地质工作主要是集中在铜带，而且很多的这些成果也已经出现在了科学期刊以及地质调查局的地质报告当中。Considerable advances have been made in the last decade or so towards unravelling the geological complexities of the various terrains in Zambia but a comprehensive understanding of the overall geological evolution is an objective yet to be achieved. Nevertheless, it is these very complexities and uncertainties, which offer the potential for new conceptual exploration models that may lead to the discovery of previously unpredicted mineral and energy resources. The large number of known prospects and mineral occurrences throughout the country also highlight the potential of known deposit-types.考虑到过去十年所取得的成就，或者朝着阐明有着多变地形的赞比亚的地质复杂结构的目的出发，综合理解这个国家的整体地质变化这个目标已经实现。然而，地质有太多的复杂性和不可确定性，它也为新的概念上的开发模型提供了潜力，这种开发模型可能会发现珍贵的没有被预测到的矿产和能源资料。整个国家的大量的已知的前景以及矿产点也突出了这种潜力。Recent years have also seen considerable advances in understanding the stratigraphy of the Copperbelt both in Zambia and in the Congo and the International Geological Correlation Programme 302 has published a recent volume with contributions from both countries (Wendorff anti Tack, l995), Theoverall lithostratigraphy of Zambia has not changed fundamentally since the publication of thestandardised succession by Ray (1983) and this has been followed by most later authors. A broad fivefold division into Basement Complex, Muva Supergroup, Katanga Supergroup, Karoo Supergroup andKalahari Group has been used, with the relatively less important Kataba Group and the Lueti Formationseparating the latter three divisions (Figure 7).近几年对赞比亚及刚果境内的铜矿带地质地层的认识也取得了相当大的进步，由于这两个国家的贡献，国际地质相关规划也出版了一些卷册。自从Ray出版了标准之后，赞比亚整体的岩相层序并没有根本行的改变，而且也被大部分的学者沿用。整体来说可以分为5大块，基底杂岩，Muva大组，Katanga大组，Karoo大组以及Kalahari组。 Figure 6 Major cratons and mobile belts in Central and Southern Africa (after ECL,1999)3.2 Stratigraphy3.2.1 Basement ComplexThe term Basement Complex is used to distinguish a range of often strongly deformed metamorphic and igneous rocks, which predate the metasedimentary quartzite and metapelite assemblage of the Muva Supergroup. It is not all of one single age but contains protoliths ranging from Archaean to Palaeoproterozoic.Kasai CratonOnly one part of Zambia has been shown to include rocks of definite Archaean age and this is part ofthe Kasai craton, which extends into the extreme northwest comer of the country, where the borders ofZambie, Angola and D.R. Congo meet. Recent mapping by the ERIPTA project on the |Mwiwinilungasheet (Key et al., 2001) has shown that four units can be distinguished:A; an older sequence of migmatites, variably gneissic granites, amphibolites and maficgranodioritesB: an equigranular medium grained, biotite bearing leucograniteC: K-feldspar phyric leucogranite with biotitc granite xenolithsD: diorite stocks, locally pyritic and ranging from qnartz-diorite to dolerite Figure 7 Simplified stratigraphic column and orogenic events (after Ray,1003)Ion microprobe U-Pb dating of zircons have yielded an age of 2543+5Ma for granulites from unit A,aSimilar age of 2561+10Ma for granite B (both Neoarchaean ) and an age of 2058+7Ma for unit C (Palaeoproterozoic). Unit D was not dated and may be Mesoproterozoic or Kibaran (=Irumide) in age.Metamorphic rocks ,including schists and migmatites ,are found in the Kabompo Dome in the east of the Mwinilunga area ,but the metamorphic age of these rocks is unclear.One porphyritic granite cutting the migmatites has a U-Pb age on zircons of 2058+7Ma(Key et al.2001),which is Ubendian.Bengweulu BlockThe Bengweulu Block is a cratonic unit underlying most of northern Zambia and extends into TanzaniaAnd the D.R.Congo.It consists of a crystalline basement and a weakly deformed sedimentary cover ofProterozoic age belonging to the Muva Supergroup.Deans in 1938 first established the three-fold division of the basement into schists ,porphyries and granitoids and this has been confirmed by Andersen and Unrug(1984).Three parallel schist belts in the eastern part of the Bengweulu blkck trend ESE to E and are separated by 50-60km of granitoid terrain. Their dominant lithology is a micaceous or chloritic quartz-feldspar schist and semi- politic to psammitic sediments.Quartzites form a relatively minor proportion of the schist belts .The metavolcanics (or porphyries of the older accounts ) outcrop around the margin of the Mporokoso basin, which forms the ccntrc of the Bengweulu block. In the north and west they form an almost continuous band, but to the south it is reduced to thin bands within the granitoids. No relationships canbe seen with the schist belts. The metavolcanics are predominantly pyroclastics ranging fromagglomerate and breccias to fine grained tuffs and the dominant lithology is ignimbrite. Some flows ofandesite, dacite and rhyolite are also seen as well as small hyperbyssal intrusions. The volcanics have ahigh-K calc-alkaline chemistry compared by Andersen and Unrug (1984) with US Cordillera, Andeanand Yilgarn volcanics. Metavolcanics from the southwestern part of the Bengweulu block have yieldeda whole rock Rb-Sr isochron of 18l6i22Ma (Brewer et al., 1979), which is probably a Ubendianmetamorphic age.The granitoid terrain, which forms the major part of the basement, is composed of a number ofcomposite batholiths. The batholiths are zoned inwards from granodiorites to foliated porphyriticgranite cores. Locally they show intrusive contacts and evidence of lit-par-lit stopping of the Volcanics.They are chemically similar to the metavolcanics and an origin similar to the Andean batholiths, whichintrude consanguineous lavas and pyroclastics is proposed. Radiometric Rb-Sr ages of the granitesrange from 1S69i40Ma for the Mambwe granodiorite, 1824i124Mzl for the Luchewe granite andl833+18Ma for the Mansa granitoids (Brewer et al., 1979). Some later discordant intrusions are alsoseen in the northeast part of the block and along the margins of the Ubendian mobile belt. One of thesethe Kate granite has a Whole rock Rb-Sr isochron of 1K38i86Ma.The granitoid plutonism is, therefore, dated around 1S5Od:35Ma and is thus part of the Ubendiantectono-thermal event (1800-2000Ma). Pans of the Bengweulu block may be elder than this, because ofthe intrusive relationship of the granites to the schist belts and the continental nature of the Volcanism,and thus could be Palaeoproterozoic. However, there is little evidence at the surface for Archaenprotoliths The Tanzanian and Kasai cratons to the north and west are elder and the Bengweulu blockwas probably stabilised and accreted to the former during Ubendian NW-SE trending tectonism .LaterKibaran and Irumide tectono-thermal events took place to the northwest and southeast of the combinedTanzanian craton and Bengweulu block.Basement Rocks in Southern and Eastern ZambiaWithin the belt of reeks deformed by the Irumide Orogeny are high grade metamorphic reeks, whichappear to be elder than the slightly lower grade metamorphosed psammites and petites of the MuvaSupergroup. These reeks have been grouped as the Mkushi and Mvnvye Gneiss and their age isprobably Ubendian and a U-Pb zircon age of 2049+6Ma has been obtained from the Mkushi Gneiss(Ngambi et al., l986.The similar Mpande Gneiss, which terms part of the basement within theZambezi belt, has been dated using the U-Pb zircon ripper-intercept age to 1106+19Ma (Hanson et al.,loss). Gneisses, further north in the Irurnide belt, could be derived from migmatised Muva sedimentsor metamorphosed syntectonic granites intruded during the Irumide orogeny. At present there is noevidence for a Ubendian age for these high grade gneisses but firm conclusions mast await furtherdating, The Lutembwe River granulite to the west of Chipata has been dated at c.3.0Ga (Liyungu andVinvn, 1996 indicating that some of these high grade reeks are considerably older.Basement Rocks in the CopperbeltThe age of the rocks, which occur in the Basement Domes of the Copperbelt, is unclear and thegranites, migmatites and schists have been assigned a variety of ages. The Nchanga Granite has anintrusion age of 877il 1Ma as shown by U-Pb dating of magmatic zircons (Armstrong et al., 1999). Theelder reeks in the basement denies, the Lufubu schists, may belong to the Muva Supergroup havingbeen metamorphosed during the Irumide-Kibaran orogeny (dated at 1310i25Ma, Armstrong, op. cit,),but they could be older and similar to the schists in the Bengweulu block. 1n the D.R. Congo theporphyritic granite of the Luina Dome (east of Konkola) has a U-Pb Zircon age of 18S0+20Ma(Dejonghe, 1995), similar to the granites in the Bengweulu block, which could have extended across theCongo pedicle into the Copperbelt and northwestern Zambia. Detrital zircons at the base of theKatangan sequence have two distinct age populations, at 880 and 1800-2000Ma, showing that thesediments were derived from sources containing reeks of similar ages to the Nchanga granite and theUbendian basement (Armstrong, op. sit).3.2.2 Muva SupergroupThe term “Muva System was used by Guernsey (1941) for a NE-striking sequence of foldedmetasediments in the Irumide Belt to the south of the Bangweulu block, This sequence is up to 10kmthick and comprises alternating layers of quartzites and pelites, each often hundreds of metres thick.These pass eastwards into granite-migmatite terrain where only remnants of metasediment are present.The relationship between these rocks and the sedimentary sequence of the “Plateau Series” on theBangwenlu block we unclear in the original original (Guernsey, 1941) but he noted their similarityMore recent went by Daly and Unrug (1982) has defined the Move Supergroup he comprising the post-Ubendian (2100-l 800Ma) and pre-Irumide (1300-l l00Ma) sedimentary and metasedimentary rocks ofNorthern, Eastern and Southern Zambia. They outlined the following lithostratographic units:Manshyu River GroupManganga River FormationKasama Formation / Mitoba River GroupMporokoso Group (oldest)The Mporokoso Group form a roughly trianguler area extending form Mansa in the south to LakeMweru Wantipa in the north and Mbase in the west. The component beds are arranged in e broadsynclinal structure with the oldest around the margin and the younger rocks in the centre of the basin. ithas been subdivided into four formations as shown in Table 3.Table 3 Mporokoso Group formations, thicknesses and depositional environmentsThe Kasarna Formation occurs as an E-W belt in the central part of the Bangweulu block, east of theMporokoso basin. It is composed of a sequence of mature quartzites, haematitic sandstones and redmudstones deposited as fluvial channel and floodplain sediments. To the southeast the formation passesinto the Mitoba River Group increasing dramatically in thickness from S0-300m in the west to 3500min the Irumide belt to the east (Figures 8 and 10). The latter Group contains several repetitions of coarsegrained congomeratic sandstones fining upwards to micaceous siltstones and mudstones. The MangangaRiver Formation commences with a pelitic sequence of laminated mudstones and siltstones which passupwards to a moderately sorted, bedded quartzite with heavy mineral bands. The topmost ManshyaRiver Group includes three pelitic members and two major quartzites and a number of fining upwardscycles can be seen along with a variety of sedimentary structures in the type section of the ManshyaRiver. The topmost bed is a calcitic marble and its presence is unusual in rocks assigned to the MuvaSupergroup.The Mporokoso Group Wes deposited on e uplifted block at the end of the Ubendian Orogeny end thefirst formation consists of very immature sediments deposited in alluvial fees and in acolian sand dunes.The sequence becomes lacustrine towards the top and lower energy, silty mudstones terminate thesuccession seen in the Mporokoso syncline. The sequence above the Mporokoso Group is interpreted asbeginning with e fluvial environment, where mature sediments were deposited in river channels end asfloodplain end flood basin deposits, end terminating in e beach to offshore, shallow Water, marineenvironment. The dramatic increase an thickness of the Kasama Formation end its passage into theMitoba River Group marks the greater subsidence in the developing basin, which was the precursor tothe Irumide mobile hell. Further southeast across the Irumide belt bands of meta-carbonates ereincluded in the reeks assigned to the Muva Supergroup and they probably represent a more extensivedevelopment of offshore carbonate sediments in the marine basin (Andersen end Unrug, l9s4_ Withinthis sequence there are extensive areas of metacarbonates and graphitic pelites, near Petauke, which weinterpret as a fully marine sequence deposited in the central pan of the Muva basin. These reeks havebeen thrust northwards over reeks of the Sasare Group, which is composed of more typical MuveSupergroup psammitic lithologies, The limestones cannot be traced far along strike to the northeastbecause of intense deformation along the Pan-African shear zones but appear to thin and pass into thenear shore depositional environment seen on the north west side of the Luangwa valley.East of Lusaka and Kabwe, two E? to ENE-trending narrow (c. 10-20km) belts of meta-pelite, quartzite,psammite, and lesser meta-volcanic and meta-carbonate rocks extend for 200-300km and these havealso been tentatively identified as belonging to the Muva Supergroup. They are partially infolded withthe possible Basement Complex, and are rotated and imbricated within and adjacent to the MwembeshiShear Zone in the Chisamba area 40-50km north of Lusaka. This sequence has similarities with that atPetauke and the meta-volcanics may indicate rifting with submarine volcanism and fringing banks ofcarbonate sedimentation.3.2.3 Katanga SupergroupThe Katanga sequence is almost exclusively sedimentary but minor igneous rocks, including basalticflows, have been reported. Spanning an approximate time period of 7850-600Ma ,the sequence overliesthe Basement and Muve reeks with marked angular unconformity, s feature particularly evident in theNorthern and Luapula Provinces and through the Copperbelt westwards across the series of Basementdomes to the Angolan border.Following the accepted stratigraphic code in Zambia (Ray, 1983), two major divisions are recognized:the Mine Series Group and the Kundclungu Group. The former includes the Lower and Upper divisionsof the former Roan Group and the Mwashia Formation.Recent U-Pb age dating of zircons from andesitic basalts, correlated with part of the MwashiaFormation in NW Zambia, gives ages of around 765Ma and an age of 735Ma for porphyritic basaltshigher up the succession in the Kundelungu Group (Key et al., 2001). This is in agreement with the ageof the global Srurtian glaciation (around 748Ma), which is thought to be responsible for the depositionof the “Grand Conglomrat tillite.Recent age determinations from the Copperbelt (Armdstrong el al., 1999) have helped to constrain theage of the Katanga sedimentation. Ages from the local basement gives ages ranging from Ubendian forthe Mkushi Gness at ZO49i6Ma, Irumide for metamorphism of the Lufubu schist at 1310+25Ma, to anage of 877illMa for the Nchanga granite. Detrital zircons recovered from an arkose near the base ofthe Roan Group define two populations, one at 88OMa and the other between lSO0Ma and 2000Ma.Katangan sedimentation, and the Roan Group in particular, must, therefore have started after 880Maand incorporated detrital material from the Nchanga granite as well as older Ubendian basement,These units, particularly the Mine Series, are considered in slightly more detail below in view of theirimportance as hosts to the Copperbelt mineralisation. Within the Copperbelt, the Katanga rocks havebeen metamorphosed to greenschist facies but this increases to amphibolite facies further to the westalong the arc of the basement domes. Eastwards, beyond the Congo Pedicle, the largelyundifferentiated Katanga sequences have been subjected to only very low grade metamorphism.Undiffere