Comun.lnst. Geol. e Mineiro, 2003, 1. 90, pp. 189-212

Synopsis of Lunda geology, NE : Implications for diamond exploration

EURICO PEREIRA*; JOSE RODRIGUES* & BERNARDO REIs**

Key-words: Congo Craton; structural control of the I lamproites emplacement; South Atlantic opening; break-up of Congo Craton; evolution ofCongo Basin; Kwango Group; Kalahari Group; Quaternary detritus deposits; diamondiferous resources.

Abstract: In the present synthesis, the geology of Lunda (NE Angola) is presented and discussed with the objective of providing a better understanding of the regional occurrence of primary and secondary diamond deposits. The huge regional diamondiferous potential is viewed in a global perspective, firstly based on the edification ofthe Congo Craton from an Archaean nucleus, till several mobile belts accreted successively to that core. In this cratonic block the geothermal gradient variation and the stability curve ofdiamond are determinant both in the genesis and the transportation ofthis precious mineral up to the surface. Following this, we analyse the tectonosedimentary evolution ofthe Congo Basin, an enormous depression in central Africa; where the depo­ sition of thick sedimentary sequences is a consequence ofan endorheic drainage, that begins in the upper Palaeozoic and has its maximum expression in the Meso-Cenozoic. The sedimentological processes, essentially terrigenous and continental, is controlled by the pre-Atlantic deformation of the Congo Craton and reaches a critical point with the opening ofthe South Atlantic since the . The oceanic rifting produces important tectonic consequences in the African plate, reactivating structures from previous orogenic cycles. These structures exerted the control ofkimberlite emplace­ ment and associated alkaline ring structures. The sedimentological analysis and the lithostratigraphic definition of post-kimberlites sedimentary units are emphasised: Kwango Group, Kalahari Group and Quaternary deposits, formed in successive erosion/deposition cycles with consequent reworking ofits diamond content. These are viewed with an exploration and economical potential. Finally, a critical balance is made and the economical consequences of these vast diamondiferous resources are discussed based on future economic perspectives.

Palavras-chave: Cratao do Congo; controlo estrutural da implantacao de kimberlitos Ilamproitos; abertura do Atlantico Sui; fragmentacao do Cratao do Congo; evolucao da Bacia do Congo; Grupo do Kwango; Grupo do Kalahari; depositos quaternaries; reservas diamantiferas.

Resumo: No presente estudo, apresenta-se uma sintese da geologia da Lunda (NE de Angola) com vista a urn melhor entendimento das ocorrencias de jazigos primaries e secundarios de diamante . As enormes potencialidades diamantiferas da regiao sao encaradas de forma abrangente, apoiadas em primeiro grau na definicao do Cratiio do Congo, a partir de urn nucleo do Arcaico, ate aos varies cinturoes moveis acre­ tados, sucessivamente, a este nucleo . No conjunto, diio corpo ao edificio cratonico onde a variacao do gradiente geotermico e a curva de estabili­ dade do diamante sao determinantes na genese e transporte ate it superficie deste precioso mineral. Giza-se de seguida a evolucao da Bacia Central do Congo, grande regiao situada na zona central de Africa onde, merce de drenagem endorreica, se acumulam espessas sequencias sedimentares, iniciadas no Paleozoico superior, e que assumem expressao maxima no Meso­ -Cenozoico. Tambern se faz referencia it deformaeao pre-atlantica do Cratao do Congo que comanda os processos sedimentologicos, essencial­ mente terrigenos e continentais, deformacao essa que atinge pontos criticos com a abertura do Atlantico SuI. Referem-se as grandes repercussoes do "rift" ocednico no interior do cratao, reactivando estruturas tectonicas herdadas de ciclos anteriores com direccoes dominantes que controlam, em absoluto, as vindas kimberliticas e estruturas alcalinas associadas. Dedica-se ainda urn largo espaco its unidades sedimentares portadoras de diamante, com especial destaque para a Formacao Calonda do Grupo Kwango, Grupo Kalahari e depositos eluvio-aluviais do Quaternario que, em fases sucessivas, colectam e redistribuem 0 diamante. Discute-se, caso a caso, 0 potencial destas unidades na inventariacao e prospeccao de jazigos secundarios detriticos. A finalizar, faz-se 0 balance critico e alvitram-se as consequencias economicas que impendem sobre os jazigos de diamante de Angola, primaries e secundarios, nas vertentes da sua potencialidade e perspectivas para 0 futuro.

* Instituto Geologico e Mineiro de , Apartado 1089, Rua da Arnieira; 4466-956 - S. Mamede Infesta, Portugal. ** Geologist Consultant, Former Director ofDiamang Exploration Department, Praceta Andre Soares, 34; 4710-220 - Braga, Portugal. 190

INTRODUCTION Regarding the primary diamond sources, several hundred bodies are known in Angola. Less Lunda Province, in NE Angola, is known as a than 50 % of these kimberlites were studied and, from diamondiferous region since the beginning of the XX these, less than 5 % are economically exploitable. Among century. Despite the fact that this region is extremely rich these bodies, there are three that are in the 10 biggest in diamondiferous gravels and in mineralised kimber­ diatremes in the world. In the past, only the upper yellow lites, the diamond mineralisation practically extends to ground levels were exploited and a large amount of the entire area of Angola. However the Lunda geology proven reserves are still unexploitable. Recently, new encompasses all kinds of diamond ores and thus, its findings of kimberlites in Lunda have been reported. In knowledge is very important in exploration programmes. the present state of knowledge the diamond reserves in Taking this into account, this work deals mainly with primary sources still unknown, but a huge potential is the geology ofthe Lunda area. However, it is also neces­ devised in view ofthe alluvial ore grades. sary to focus on some aspects of the general geology of Because of this, there is a growing interest in the Angola, namely the Congo Craton evolution and some primary diamond sources and some contracts have been continental sedimentary units pre and post-kimberlite established with international companies for exploration emplacement, that are important to the overall compre­ and mining ofprimary sources. hension ofthe diamond problem in Angola. The prolonged mining history of Lunda diamonds, The diamond exploration in the Lunda province is the data relative to past and present mining activities, the reported to have started in 1912 with the finding of 7 vast areas with huge mining potential both for primary or diamonds in the Mussalala River, an eastern margin secondary diamond deposits, and the end of civil war tributary of the Chiumbe River. With the adequate technical with progressive stabilization of social and political life support, diamond exploration began in 1913, which in the country justifies the renewed interest for diamond resulted in the opening of the first open cast mines in exploration in Angola, and consequently in the geology 1919 - Cavuco, Camimanga, Cassanguidi and Luaco of the Lunda Provinc e. Table I shows a synopsis of mines. Since then until the year of 1999, about 63,6 x 106 Lunda geology, with the geological units grouped 6 carats were produced, in addition to the estimated 9 x 10 considering the main tectonic cycles recognised in carats illegally exploited between the years 1984-1999. Angola. The ongoing exploitation works are conducted both in kimberlites and in eluvio-aluvial gravels. In the past, the main exploitation activities were The Congo Craton developed essentially in alluvial deposits, either in recent alluvial gravels related with the present drainage system, The occurrence of a Precambrian cratons, especially or in other sedimentary units which post-date the kimber­ those ofArchaean age, have long been recognised as a lite intrusions. The most significant is the Calonda major guideline for the exploration ofprimary diamond Formation, a Cretaceous continental unit considered as deposits - Clifford's rule (e. g. CLIFFORD, 1966; the first sedimentary collector of diamonds after the JENNINGS, 1995 and references therein). This empirical supergenic destruction of the kimberlites. The sedimen­ rule has been explained with the geological conditions tary diamondiferous deposits that are linked to the for the preservation of diamonds, which occur domi­ present river system drainage activity are associated with nantly under these Archaean cratons, until a volcanic the erosion of the Calonda Formation basal conglome­ episode brings them to the surface (e. g. HELMSTAED & rates. GURNEY, 1995). The continual depletion of reserves in recent alluvial The influence ofthe geological nature ofthe cratonic gravels rich in diamonds demands that new efforts be basement, in the location of the kimberlites, is paradig­ placed on diamond exploration, which in spite ofcurrent matic in the case ofAngola. It is notorious the dominance smuggling, new findings are possible. However, the of kimberlite occurrences in Archaean terranes that are future main target in alluvial exploration and mining characterized by amphibolites, amphibolitic gneisses, ' should be the Calonda Formation basal gravel, even with mafic granulites and charnockites. The kimberlite pro­ the inconvenience of the high overburden thickness that vinces, defined by RBIS (1972), are coincident with the reaches frequently 50 to 80 meters or more. zone of influence of the Archaean tectonometamorphic 191

TABLE 1 Tectonic events and lithostratigraphic units in Lunda Province (NE Angola)

CHRONO- LITHOSTRATiGRAPHY TECTONICS STRATiGRAPHY KALAHARI GROUP EROSION PLIOCENE Ochre Sands and Clays Fm EROSION-HIATUS EOCENE- Gres Polimorfos Fm MIOCENE EROSION-HIATUS KWANGO GROUP Calonda Fm: CENOMANIAN Arenites and silcretes; mudstones; conglomerate intercalations; [g Arenites and arkoses of different colours; reddish muddy arenites; EROSION C Coarse basal conglomerate . KIMBERLITIC EMPLACEMENT t:l ALBIAN CONTINENTAL RIFTING ~ CONTiNENTAL INTERCALAR GROUP ~ Mudstones and arenites with conglomerate intercalations; White and reddish sandstone s (arkosic with kaolinization) ; Brown mudstones intercalations; Mudstones and sandstones; fine-grained conglomerate intercalations . WoEEXTENSION TECTONICS EROSION-HIATUS KARROOSUPERGROUP ...... _------_.------_.. ...- ....._- -_ ...... _------_...... _------_...... _------_.... Continental tholeiites CASSANGE GROUP NNE-SSW FRACTURING TECTONICS Beds with Phyllopodia Fm Beds with Plant Fossils Fm Beds with Fish Fossils Fm LUTOEGROUP NNW-SSE FRACTURING Fluvio-Glacial Conglomerate Fm TECTONICS Muddy-Psammitic Fm Yellow Mudstone Fm Violet Sandstone Fm Tillite Fm WoEEXTENSION TECTONICS EROSION-HIATUS Hyper-alkaline granites ofLunda WSW-ENE FRACTURING Basic rocks ofLunda TECTONICS .w.~~.T...<;.Q!~{GQJ:!!!Q.m~______Luana Fm: Cartuch i-Camaunao Fm PAN-AFRICAN Quartzites and red phyllites; Meta-arkoses and phyllites; OROGENY NEOPROTEROZOIC Brown quartzites; Meta-greywackes, quartzites; Conglomerates. Conglomerates .

METAMORPHIC UPPER GROUP ~ (LULUA GROUP) Superior Unit: silicified meta-limestones; phyllites and quartz-phyllites; phyllites and meta-sandstones with conglomerate intercalations. PALEO- Inferior Unit: coloured schists and phyllites; fine-grained quartzites ; black-shales, PROTEROZOIC quartz-feldspathic schists, amphibo litic schists and gneisses. EBURNEAN/ UBENDIAN Porphyry granites of Lunda OROGENY METAMORFPHIC LOWER GROUP / LOVUA GROUP ~ (LUIZA G.) Mica schists, quartzites and itabirites; Amphibolites, gneisses and gneissic granites. _IJ..~~~!-SQ~~~~~J~~~_~!!f~!.:~_tj!l.~t.=~2 ?__~:?g.~ ______...... __.______...._._.. ______~ (DIBAYA GROUP) Gneisses, migmatites and gneissic granitoids. ARCHEAN CHARNOCKITiC COMPLEX LIMPOPO-LIBERIAN Chamockites; OROGENY(?) Quartzites; Amphibolitic gneisses, amphibolites and meta-gabros. 192 cycle (Fig. 1), despite the fact that the outcropping ofthe aluminous metasediments and little recycling of previ­ Archaean is very discontinued due to granitoid intrusions ously formed crust. The granitoid domain of this in latter orogenic cycles, mainly in the Ebumean. Ebumean crust seems to reveal incompatibility with The nature of the Archaean crust, composed by kimberlite emplacement. metasedimentary rocks with a great abundance of mafic With this in mind, a description is made about the rocks, later metamorphosed to granulitic facies, shows main tectono-stratigraphic units that constitute the craton great mantle activity. The Eburnean crust, on the and the corresponding orogenic events, with the aim of contrary, composed by gneisses, migmatites and large dissociating the cratonic core from the various frag­ scale granites, is mainly acid, due to the fusion of the ments, successively accreted through Precambrian times.

14° 16° 18° 20° 22° 5°- ANGOLA

7°__

9

15° ---

17°

40 80 120 160 200 Ld Ld I

Fig. I - Precambrian basement of Angola - Geological Sketch Map. Adapted after International Geological Map of Africa, 1:5000 000 scale, (UNESCO, 1986); CARVALHO (1983); ARAuJO & GUIMARAES (1992). 193

This distinction is of particular importance, because the that can be explained by a process analogous to the for­ relation between the geothermal gradient and the mation ofthe present-day magmatic arcs and subsequent diamond equilibrium curve defines stability zones for mechanisms of subduction / A-type collision (KRONER, this mineral under the cratonic cores and instability zones 1981; 1983). under the mobile belts (HAGGERTY, 1986). The various mobile belts, Luizian-Ubendian (Ebur­ nean), Kibarian and Pan-African that are successively accreted to the Congo cratonic core have a weak expres­ Archean cratonic nucleus sion in the Lunda region. However, they are worth considering, due to the fact that some of the structures The diamondiferous area of Lunda is situated in the that preserve the metasedimentary sequences representa­ Congo Craton, which comprises an Archaean mafic tive ofthose tectonic events correspond to grabens whose charnockitic complex. Around this Archaean core several faulted limits control the kimberlite emplacement. gneissic-migmatitic series and Palaeoproterozoic grani­ toids were accreted whereas Meso- or Neoproterozoic metasedimentary belts were disposed around the external Eburnean-Ubendian Cycle limits ofthe Congo Craton. The oldest isotopic data (Rb/Sr and U/Pb) indicates The behaviour of the Archaean crust, present in an age between 2.9 - 2.85 Ga for the chamockitisation of Lunda and other zones ofthe centre and west ofAngola, the mafic complex (norites, tonalites and amphibolites) relative to the development of the successive orogenic (DELHAL & LEDENT, 1971; DELHAL et al., 1976). In cycles (CARVALHO &ALVES , 1993) is further analysed . general, this complex is composed of acid and mafic During the period of Archaean-Proterozoic transition granulites , quartzites, amphibolitic gneisses and amphi­ it suffers extension, mantle pounding, rupture and, in bolites. some cases, intrusion of mafic-ultramafic masses, for Near the border with the Congo Democratic Republic example, the gabbro-anorthositic complex ofSW Angola a suite of gneisses, migmatites and granitoids crops out with a N-S direction along an extension of more than along the valleys of Chicapa and Luachimo and are 200 km, and, certainly, some of the mafic intrusions of included in the so called Dibaya Group of the Kassai Lunda. These great mantle masses induce partial fusion region (Congo), and associated with the mafic char­ of the Archaean crust and formation of granitoids and nockitic complex. Here, the U/Pb concordia, obtained granodioritic porphyries that, in the westem part of from zircons and titanites, suggest an age of 2.65 Ga Angola, clearly marks the fragmentation of that same (DELHAL et al., 1975). crust. In the Lunda sector, that fragmentation has a In addition to the mafic chamokitic complex and the WSW-ENE general orientation and gives way to impor­ Dibaya rocks, some authors (MONFORTE, 1988) consider tant subsiding zones, where sedimentary, generally silici­ the existence, in the Lunda area, of an older Basal clastic, sequences accumulate, and are now outcropping Complex composed of amphibolitic schists, gneisses, as medium to high-grade metamorphic rocks accreted to migmatites and granitoids, which is not, in our opinion, the Archaean nucleus during the Ebumean-Ubendian oro­ separate from the Dibaya Group in terms ofits lithology, geny. In Lunda, these are divided, from the base upwards metamorphism and structure . In fact, in the referred in the Lower Metamorphic Group (L6vua/Luiza Group) units, the foliation has a NE-SW to ENE-WSW direction and the Upper Metamorphic Group (Lulua Group): and is affected by various deformational phases with The L6vua Group, equivalent to the Luiza Group of hinge folds oriented NW-SE and NNE-SSW, probably Kassai, is also present in Mufo, Luembe River, in the due to superposition of later orogenic cycles. Therefore, prolongation of the Kassai Group (Delhal and Ledent, the ages ofthe basic magmatism and the original crust of 1973; Andre, 1993). It is composed of a thick sequence the Basal Complex and/or the Dibaya Group may be of quartzites, itabirites, micaceous schists and, in some older, but that crust was certainly regenerated, structured places, amphibolites and gneisses. The Rb/Sr metamor­ and metamorphosed in a large period oftime (2.9 ± 0.2 Ga). phism age ofthese sequences is ca. 2.2 ± 0.1 Ga (DELHAL The metamorphism is amphibolitic or granulitic facies & LEDENT, 1973). with the presence of acid granulites (chamockites) and Also in Congo, close to the NE Angola border, a gneissic intermediate-basic rocks (tonalites and norites) graben with the same WSW-ENE fracture direction, 194 separated from the Dibaya Group by the Malafudi fault, mapping by RODRIGUES & PEREIRA (1973) raised suspi­ preserves a low-grade metasedimentary sequence, named cion that there were two distinct groups, the lowest being the Lulua Group . It is composed ofschists , quartzites and the oldest. On the other hand, the classical works , basic volcanic rocks (continental tholeiites) considered although including the sequence in the Kibarian cycle, as Kibarian (DELHAL & LEGRAND, 1957; FIEREMANS, always matched it to the Lulua Group (POLINARD, 1934; 1958; 1986) and always considered connected to the FIEREMANS, 1958; DELHAL, 1973), that is now considered metasediments that occur in the Luembe River, in Angola older than previously admitted (ANDRE, 1993). In the (ANDRADE, 1953a; MONFORTE, 1988). However, the same sequence, basic (doleritic and gabbro-dioritic) Rb/Sr dating of the metamorphism affecting the basic intrusions with K/Ar isotopic ages of 1320 ± 36 Ma and volcanic rocks yielded an age of 1901 ± 41 Ma (ANDRE, 1490 ± 40 Ma (CARVALHO et al., 1983) strengthen this 1993). This author admits that the deposition ofthe low­ hypothesis. According to these authors, these ages may grade metasedimentary sequence may express, locally, correspond to an Ar loss during the Kibarian cycle and the global extensive phase that displaced the Archaean the rocks would therefore correspond to the Eburnean lithospheric plates and permitted the confined deposition (CAHEN et al., 1984). of the metasedimentary sequences here analysed. Concluding, these sequences, affected by successive Pan-African Cycle tectono-metamorphic episodes, are designated generi­ cally in Angola as the Lower Metamorphic Group of Regarding the sedimentogenesis of the Pan-African Lunda, and distinguished from the Basal Complex. They cycle (KRONER & CORREIA, 1980; BLACK, 1984; may be the expression, in the interior ofthe Congo Craton, PORADA, 1989), in Lunda there are only the outcrops of of the tectono-metamorphic episode, accompanied by the region , on the Chicapa River, to the south; intense granitization, that developed to the west of the the ensemble Luangando-Luana, on the Luachimo and craton and, also, in the surroundings of the Tanzania Luana rivers; and those ofthe Cassanguidi region , on the Craton , being the equivalent to the more vast Ubendian­ Cartuchi-Camaungo streams, tributaries of the Luembe Eburnean cycle (2.0 ± 0.2 Ga). The syn-tectonic porphy­ River, to the north. They allowed to define: the Luana ritic granites ofLunda may belong to the same cycle. Formation, composed of conglomerates, salmon-colou­ In the -Cassanguidi region, on both margins red quartzites, immature red sandstones, phyllites and red of the Luembe River, outcrops the most complete or black schists; and the Cartuchi-Camaungo Formation, sequence of the Upper Metamorphic Group of Lunda also consisting from the base to top of conglomerates, (CARVALHO, 1984), previously named "Metamorphic greywackes, quartzites, ferruginous feldspathic sand­ Series of NE Angola" (RODRIGUES & PEREIRA, 1973; stones and schists with purple colour. Both units are MONFORTE, 1988). The present group includes two series, perfectly equivalent to each other, exhibit low-grade metamorphism and preserve primary structures such as a lower and an upper, each of which consists of three graded bedding and cross-stratification, which indicate formations . Thus, the lower series consists of: A 1a - gneis­ shallow platform deposition. They were preserved in ses and amphibolitic schists, quartz-feldspathic schists and extensive grabens with WSW-ENE general orientation phyllites, carbonaceous schists and phyllites; Alb - fine­ and, more rarely, NE-SW orientation. The deformation is grained quartzites; Alc - purple, red or grey schists and insignificant without the development of schistosity that phyllites. The upper series is discordant over the lower only becomes penetrative in the pelitic layers. series and consists of: A2a - phyllites and sandy schists Based on the analysis of the lithostratigraphic with intercalations of conglomerates; A2b - phyllites, sequence, these units may be considered equivalent to talc-schists and quartz-phyllites; A2c - local silicified the Xisto-Gresoso Group of the W Congo Super-Group limestones. These rocks were folded and schistosed in a (SCHERM ERHORN, 1981). The age ofthe Luana Formation main deformational event with NE-SW to NNE-SSW was temporarily considered by ANDRADE (1953a) as post­ direction, although later fragile folding and kinks with "Metamorphic Series of NE Angola" and pre-Karroo, NNW-SSE and WNW-ESE directions are observable. concerning the fact that the clasts present in the Luana . From the earlier works on Lunda (ANDRADE, 1953a; conglomerate revealed fine-grained quartzites, typical of MONFORTE, 1960) resulted a tendency to associate the those series, and also, due to the fact that the Lower Upper Metamorphic Group of Lunda to the Kibarian Karroo tillite (Lutoe Group) contains fragments of cycle , designating it "Kibaras System". Geological quartzites and schists ofthe Luana Formation. 195

Congo Basin Since this period, the depositional situation changed remarkably and the Congo Basin becomes a strongly Pre-Atlantic evolution subsiding area, due to the large-scale extensive tectonics, induced by distant continental collisions, such as the The Congo Basin (VEATCH, 1935) has its origin in collision of Laurentia-Baltica with Gondwana in the Gondwana's interior. This basin is surrounded by the -Carboniferous and the collision of Patagonia Francevillien, Sembien, Liki-Bembe and Urundi-Kiba­ with the Gondwana in the Triassic. The completely conti­ rien mobile belts from the Kibarian orogenic cycle nental sedimentological evolution and the endorheic drainage originated a thick sedimentary sequence, which (1.2 ± 0.2 Ga), and by the WestCongolian, Oubangui-Lin­ in some sections attained approximately 9 km in thick­ dien, Bushimaien and Katangien mobile belts from the ness (DALY et al., 1992). Pan-African orogenic cycle (850 - 450 Ma). Posterior to Duringthe Carboniferous and Permianthe cratonicbase­ the last deformational phase ofthis latter cycle, the basin ment was covered by the continental terrigenous sequences admits little deposition in the middle with a of the Karroo and Continental Intercalar Group. In the sandy marine transgressive sequence overlain by pelagic particular case of Angola, and in the diamondiferous area mud sediments and, at the top, by arkoses (Banalia of the Lunda province, it is possible to find some frag­ Arkoses) that prograde in the basin from the E and W. ments of a glacio-fluvial and lacustrine-deltaic sequences, The age of these arkosic sandstones is essentially correlated with the Karroo (LEPERSONNE, 1951), preserved -Devonian (DALY et al., 1992). in tectonic fault-bounded structures (Table 2). This

TABLE 2 Lithostratigraphic units ofthe Karroo Supergroup (NE of Angola)

SEDIMENTOLOGY AGE GROUP FORMATION and TECTONICS

ANGOLA: (Mouta , 1954) LUNDA: (Real, 1959) Continental tholeiitic magmatism Continental tholeiitic magmatism Volcanism U... and volcanism and volcanism Hypabyssal magmatism rn W-E extension tectonics ~ 6- Beds with Phyllopodia 8- Muddy Sandy Complex ~ (white and red) with: ..., ENE-WSW Fracturing Esteriella cassambens is Teix. Pala eolimnadiopsis reali Teix. U DELTAIC r;; 5- Beds with Plant Fossils rn (Epicontinental) < CASSANGE til 7- Marly and muddy shales with fossils LACUSTRINE '"' (fishes, insects and Esth eria) (Post-Glacial) Z 4- Beds with Fish Fossils < 6- Muddy sandstones with: Compressive Tectonics (?) Estheria anchietai Teix. ~ NNW-SSE Fracturing =-~ 5- Fluvio-Glacial Conglomerate GLACIO-FLUVIAL 3- Black Shales (Lunda) (Dark clays with dolomite and gypsum) v: ~ 4- Muddy Sandstone 0 LUTOE c:.:: 3- Yellow Mudstones : ~ f:l Sandy shales and yellow mudstones with Z plant fossils 0 2- Inferior Sandstone GLACIO-LACUSTRINE =cr: 2- Violet Sandstone: < U Sandstone with conglomerate levels; I- Basal Conglomerate Violet Muddy sandstones TILLITE 1- TILLITE MORAINE 196 sequence ends with hypabissal and volcanic episodes of and Bokungu Groups (CAHEN et al., 1960; CAHEN, 1981; continental thole iites . In the places without volcanism, a 1983). Later, DALY et al. (1991) proposed the strati­ new fluvio -lacustrine sequence appears, which was graphic rank of Formation to those units, with the mainte­ named, in Angola, the Continental Intercalar Group. nance of the unit names (Table 3). In the Kassai region, This lithostratigraphic unit was informally defined by the Loia Group overlies the Stanleyville Group by a very Diamang geologists (MONFORTE, 1960; 1988), compris­ low angle unconformity. The Bokungu Group overlies both ing terrigenous deposits, bounded by a tectonic uncon­ by an unconformity (CAHEN, 1983). The chronostratigra­ formity with the underlying Karroo Supergroup, phy of those units are based on Phyllopodes biozones, preserved in graben structures, and with the overlying which allow the redefin ition ofthe Lualaba Series. Kwango Group. Previously, the Continental Intercalar We think that, in Angola, man y of the continental Group (Table 3) was con sidered as pertaining to Karroo, terrigenous deposits included in the Cas sange III as upper Cassange or mismatched with the so called (MOUTA, 1954) or included in the "Camadas com "Lunda Stage" (ANDRADE, 1953b). The important work Filopodes" of several authors that have worked in the made by REAL (1959) gave a decisive contribution to the Cassange or Lunda regions, could and must be revised, knowledge ofthose continental units and, particularly, to similarly to what happened in the Congo Central Basin. the definition of the ango lan Karroo deposits. In the There is a lack of detail ed stratigraphic and biostrati­ established lithostratigraphic column, he does not graphic work, refinement of tectonic interpretation and, include, however, any sequence that may be correlated to finall y, formal definition of stratigraphic units. the old Lualaba Series (old designation for the set of The Continental Intercalar Group outcrops in exten ­ Stanleyville and Loia Groups). sive areas of the Cassange region. In Lunda, it can also In the Congo Democratic Republic (former Zaire), be seen in several localities along the Chicapa, Luachimo drilling works allow the definition ofthe Stanleyville, Loia and Lovua Rivers, rang ing several tens of meters (REAL,

TABLE 3 Continental terrigenous units of Congo Basin (Pos-Karroo and Pre-Kalahari)

SEDIMENTOLOGY AGE GROUP and TECTONICS CONGO ANGOLA ANGOLA DEMOCRATIC (Baixa de Cassange) (Lunda) REPUBLIC FLUVIO-LACUSTRINE CENOMANIAN KWANGO KWANGO KWANGO (CALONDA-Fm) (CALONDA Fm) TORRENTIAL

[IJ UPPER ALBIAN BOKUNGU KIMBERLITIC ;:l 0 INTRUSIONS fol U UNCONFORMITY ~ fol FLUVIO-LACUSTRINE ~ CONTINENTAL CONTINENTA L U INTERCALAR INTERCALAR LOWER ALBIAN LOlA

REGIONAL HIATUS APTIAN

U

1959; MONFORTE, 1988). Despite the referred ambiguity, The main inherited structures and fractures (COWARD the units sequence, observable on the left margin of the & DALY, 1984) are represented in Fig. 2. It is with the Cassamba River - east margin tributary ofChicapa River analyses of the reactivation of the fractures that the near Calonda village - are, from base to top (PEREIRA et control of the kimberlite emplacement is made. al., 2000b) (Table 3): i) - Inherited structures • Sandy mudstones with conglomerates, breccias and The oldest structures and, certainly, those of major red claystones with alternating layers of whitish importance in the structural control of kimberlite and reddish sandstones (= 10 m); diamond sources, and consequently in the distribution of • White kaolinised arkoses and red sandstones with secondary deposits, are those that define the WSW-ENE levels ofbrown mudstones (= 6 m); direction of the Graben (DELVILLE, 1961 ; MONFORTE, 1970). However, this structure is not • Reddish clays and sands with microconglomeratic confined to the Lunda area, and extends from the mouth intercalations (= 4 m). of the Cuanza River, in Angola, to the Kassai, in the These sequences and the respective basement were Congo Republic (WASILEWSKY, 1950). The age of this strongly segmented by extensive tectonics , coeval with the structure is, at least, related to the Pan-African Cycle, but opening ofthe South Atlantic . Several fault-bounded struc­ could be older than this, because the Charnockitic tures (grabens and semi-grabens) trending mostly Complex and the Dybaia Group (= 2.9 ± 0.2 Ga) are preserved inside the Lucapa Graben. During the Pan­ WSW-ENE and NNW-SSE were originated, which African, the tension field responsible for the W Congo preserve the pre-Cretaceous sediments. These structural and Damara mobile belts, implies a maximum tension alignments, mostly inferred from the occurrence of the orientated WSW-ENE. These forces must have produced Karroo and Continental1ntercalar deposits, have extreme extension movements in the Lucapa Graben, that lead to importance for they favour kimberlite emplacement and, the preservation of the Charnockitic Complex and, also, consequently, are excellent guides for kimberlite explo­ the Luana / Cartuchi-Camaungo Formation. ration. Several ofthese and other main structural lineaments ii) - Consequences ofthe Laurentia-Baltica collision and structures correspond to inherited structures from with the Gondwana during the Devonian-Carboniferous previous tectonic episodes.

This collisional process originated the Variscan chain , essentially developed in central and west Europe, and Congo Craton pre-Atlantic deformation also the Appalachian chain and the Morocco-Mauritania Atlas Mountains, evolving, respectively, the eastern part Inevitably, the Congo Craton exhibits the main struc­ ofNorth America and the northern part ofAfrica. tural trends that characterized the orogenic cycles referred In present-day geography, the placement of the Afri­ to before : WSW-ENE directions from the Archaean; NW­ can Hercynian chains, with dominant W-E to ENE-WSW SE trend from the Eburnean structures and N-S to NNW­ structural trends for the major structures, implies a maxi­ SSE directions from the Pan-African cycle. These are the mum compression with a NNW-SSE direction. As a directions of the tectonic flow, which imply specific consequence of continental uplift, foreland basins or tension fields for each cycle that will not be mentioned simple continental depressions are formed in the here. Due to the rigid rheology of the main cratonized orogenic front. Examples ofthis situation are the epicon­ blocks intervening in the Pan-African cycle, the fragile tinental basins that occupy the whole northern part of structures of this cycle play an important role in the frag­ Africa, namely, the Tindouf, Algeria and Taoudeni basins mentation ofthe Congo Craton . It will be reactivated in the (CAPUTO & CROWELL, 1985). tectonic episodes acting over the Gondwana, after It is plausible that those NNW-SSE tensions could have Cambrian times, which culminate with the opening ofthe some consequences on the Congo Basin, reactivating the south Atlantic. Amongst these episodes it is worth empha­ inherited Pan-African structures as tensile fractures sizing the continental collisions that occurred in the along the main faults with that direction. In fact, the Devonian-Carboniferous and in the Triassic. occurrence of the tillites and fluvio-glacial deposits of 198 ~ --00 I (

200S

LEGEND: , y/ FAULT / (WITH SHEAR SENSE )

\ PAN AFRICAN '1 THRUST 30° S PAN AFR ICAN FOLD BELTS o 500 Km I E3 E3 I

10° E 30° E 40° E

Fig. 2 - Main cratonic blocks in Southern Africa. Basement fractures: I - structure; 2 - Cuanza-Kassai structure; 3 - Mwembeshi structure.

the Lutoe Group, outcropping along the Chicapa River in In fact, in the southern limit ofthe basin, in the Angolan Lunda, are preserved in fault-bounded structures with territory within the basin, several structures could be NNW-SSE directions. seen and are perhaps more important than those formerly On the other hand, faults with NW-SE and NNE-SSE described (REAL, 1959). directions could be reactivated with wrench movements, The most recent ofthese structures, with NNE-SSW while the WSW-ENE faults act as compression structures. direction and which exert a tectonic control over the Cassange Group ofPermian-Triassic age, could be inter­ iii) - Consequences of the Patagonia - Gondwana preted as tensile fractures related with the distal collision in the Triassic Patagonia-Gondwana collision. This collisional event originated the Cape Town Subordinate to the present tension field, the NNW-SSE orogen with a WNW-ESE structural trend (DALY et al., and WSW-ENE fractures, may still be activated as 1991), and also had consequences on the Congo Basin. wrench faults, dextral and sinistral, respectively. 199

The Triassic age for this reactivation followed by a brian ages, a succession ofPalaeozoic ages and several reverse sense extension, near the W-E direction, preser­ Meso-Cenozoic ages with a main Cretaceous period of ves the Cassange Group deposits, and favours the ascent emplacement. On the other hand, the isotopic ages of magmatism and volcanism of the upper Karroo, and obtained from diamond inclusions clearly reveal that controls the Mesozoic sedimentogenesis. These struc­ the diamond ages are generally comprised between tures are also fundamental for the control of kimberlite 3300-990 Ma . In some cases those ages are much emplacement that mainly affects along the NNW-SSE older than the kimberlites that transport the diamond and WSW-ENE conjugated directions (MONFORTE, 1970; up to the surface, with ages more frequently between REIS, 1972). 100-90 Ma (RICHARDSON et al., 1984; RICHARDSON et Thus, continental rifting begins in the Jurassic. The al., 1990). Congo Basin is under confined traction, crustal rupture It is possible to synthesise the consequences of the and continental tholeiitic magmatism and volcanism difference between the ages ofthe diamonds and the ages occurs and sedimentation is interrupted. The opening of of their transport to the surface: i) the diamond is origi­ the northern Atlantic is coeval with these geological nated and stored in the mantle during long periods of events. The continental rifting in the south Atlantic and in time; ii) it is transported to the surface in specific periods Angola begins in the Cretaceous, pre-Aptian (BROGNON (inter-orogenic) of Earth's history; iii) diamonds of & VERRIER, 1965; PEREIRA, 1971). different ages occur in the same pipe, as in the case ofthe In the Albian , the opening of the south Atlantic Finch mine (RICHARDSON et al., 1990), which implicates continues and sedimentation ofthe Continental Intercalar the existence of several mantle sources crossed by the Group occurs simultaneously. After the Albian, in the same kimberlite during ascencion; iv) and, finally, the passive Atlantic margin, and particularly in the Cuanza diamonds are not genetically related with the kimberlites, Basin, alkaline and hyperalkaline intrusions and lava that serve only as a transportation medium up to the flows (PEREIRA, 1969; PEREIRA & MOREIRA, 1978) are found intercalated in the sedimentary units. These are surface. The diamond sampling in the mantle is done by reliably dated by post-Albian and possibly Cenomanian a mechanism of geothermal gradients and thermody­ fossil fauna (LAPAO & GALVAO, 1971). In the same namic conditions for diamond stability (KENNEDY & period, in the Congo basin, kimberlite and carbonatite KENNEDY, 1976; HAGGERTY, 1986). volcanic activity starts, associated with the "volcanic belt In the distribution map of kimberlites and alkaline of Angola" (MACHADO, 1959). This age is corroborated ring structures in Angola (Fig. 3), it is possible to see the by Rb/Sr radiometric dating in several places in Angola, seven kimberlite provinces that are presently known such as 87 ± 11 Ma, in phonolites (SILVA, 1973) and 92 ± 7 (REIS, 1972; REIS & BARROS, 1981). Ma, in tinguaites from the carbonatite ring structure of The four most important of those provinces are: Catanda (SILVA & PEREIRA, 1971). Lunda (I); Cucumbi, Cacuilo and Cuango Rivers region Subsequently to this period of strong tectono­ (II); Cuanza Basin (Ill) and the Cunene, Queve and volcanic and sedimentary activity at the end of the River springs region (IV). These provinces Cretaceous, followed a strong cycle of erosion during are aligned along one NE-SW macrostructure which the regional relief was eroded and the region was ( L u n d a - Walvis Bay lineament or Volcanic Belt of covered by thick sedimentary mud-sand-conglomerate Angola). The Lunda province is controlled by fault­ sequences of the Kwango and Kalahari Groups. As a bounded compartments with WSW-ENE direction in result, only the erosional activity of the present drainage an echelon pattern, dislocated by NNW-SSE faults . pattern allows the crystalline basement, the alkaline and The other three main provinces (II, III and IV) are kimberlite volcanic structures, and also the first sedi­ tectonically controlled by NE-SW and NW-SE frac­ mentary collector of diamonds, the Calonda Formation, tures. to outcrop (Table 3). The remaining provinces: Longa River springs (V), Cassinga region (VI) and Cubango River springs (Vll), Kimberlite / Lamproite emplacement in Angola are tectonically controlled by NW-SE fractures . Together with provinces III and IV, they form a structural linea­ The emplacement ages ofthe kimberlites in the world ment with this same orientation. are well known (KIRKLEY et al., 1992; FIPKE et al., 1995; In the crossing of these two systems of major frac­ GURNEY, 1989; MEYER, 1985; 1987) with three Precam- tures several kimberlite pipes were found using airborne 200

LEGEND: @ Village

D Dlamondlterous placers and conglomerates o Kirnberlites

/

ATLANTIC OCEAN

ZAMBIA MAV~~,0~ ~:'!; i / ,

NGIVA y .~ @ -- CUb8~no . NAMIBIA ~ " ...."...."..".. ·'·· 0 50 1001S0 200km ._.-",

Fig. 3 - Occurrence map ofkimberlites, alluvial deposits and ring alkaline structures (Kimberlitic ), after (RBIS, 1972).

magnetic surveys (REIs, 1971). The Atlantic traction the Carboniferous-Permian and the Cretaceous (PEREIRA forces cause the formation ofgraben-type structures that et al., 2000a). were formerly described, that control and preserve the In Lunda, there are several petrographic facies of kim­ secondary deposits of diamonds: Kwango Group, river berlite: porphyritic type, breccias and tuff-breccias. The terraces and present alluvial deposits. mode ofoccurrence ofkimberlites is mainly in diatremes From the exposed, we emphasise the strong structural or pipes, but dikes and sills are also known. The pipes are control for the kimberlite emplacement. The main weak­ elliptical or circular, irregular, with variable dimension ness fracture zones where the kimberlites are found were and are generally small in size. However, the Catoca, inherited from Precambrian orogens and from the tensile Camafuca-Camazambo, Camutue, Camatchia-Camagico fracture systems actuating in the Congo craton between pipes form the exception in this area as these are much 201 larger in size ; uneroded and consist of groups of alkaline magmatism of Walvis Bay is of Cenoma­ diatremes. Their composite nature gives them the larger nian-Turonian age. If we consider an expansion rate of dimension. The dikes and sills are thin find can attain 10 ern/year for the South Atlantic and/or equal velocity in lengths up to one hundred meters , showing a porphyritic the NE movement of the African Plate, the distance of structure and few xenoliths. Generally these are related approximately 2000 km between Lunda and Walvis Bay with fracture lineaments or have a radial disposition in is overcome in 10 Ma. These numbers point out the relation to the larger pipes , as in the case ofCamutue, validity of the hypothesis of an enormous magmatic As far as we know there is no isotopic data on kim­ diapir in expansion that must establish the control of the berlites from Angola. The kimberlites are probably post­ diamond in Angola. Paleozoic since in the Chicapa River, the Camafuca-Cama­ All these geological episodes are related to the open­ zambo kimberlite, the Karroo and the Continental ing ofthe Atlantic. In fact the breaking up of Pangea and Intercalar deposits are cross cut by the diatreme (REAL, the opening of the Atlantic caused intense fracturing in 1959). We therefore presume that the Lunda kimberlites the African continent, with a triple joint near Lake Chad and, in general, the Angolan kimberlites are emplaced in and three radial megastructures: one in the direction of the Cretaceous period with the opening of the South Tunis, another till the Fernando PO islands and the third, Atlantic. more irregular, that passes through Mombassa (Fig. 4a). Further age indication comes from the relation ofthe This megafracturing originated three more or less kimberlite occurrences within the "Volcanic Belt of cratonic megablocks: Occidental (0. B.); Arabian-Nubic Angola", a major structure in Angolan geology, which (A. N. B.) and Austral (A. B.). In the Neocomian-Aptian plays an important role in the kimberlite magmatism. occurred the separation between the Arabian-Nubian and The "Hotspot"/ "Thinspot" of Walvis Bay - Rio Austral blocks , causing a left wrench movement in the Grande do Sui, has been known for a long time as the Chad-Tunis structure; in the upper Aptian-lower "Volcanic Belt ofAngola"(MACHADO, 1959;EDWARDS, Albian, by NE-SW extension, the separation between the 1971; CORREIA, 1988). The evolution of this hot spot is Occidental and the Arabic-Nubi an blocks occurs, imply­ reflected in the strong differentiation that can be seen ing the development of the dextral transform fault that along the Volcanic Belt from NE to Sw. In the north­ separates the Occidental and Arabian-Nubic blocks from eastern kimberlite provinces, there are rich exploitable the Austral block (Fig. 4b) (FAIRHEAD, 1988; UNTERNEHR kimberlites in Lunda (I) and in Bakwanga (Congo etal. , 1988; GIRA UD & MAURIN, 1991). Democratic Republic) (WASILEWSKY, 1950); poor kimber­ Since the Albian, the Atlantic opening process induces lites with diamond and pyrope in Cucumbi, Cacuilo and intensive W-E tensile activity in the Austral crustal block Cuango (II); kimberlites with pyrope and subordinate (MARTIN, 1973; OJEDA, 1982; REYRE, 1984), which, in diamond in the Cuanza basin (III); practically sterile tum, is also divided in at least three other crustal cratonic kimberlites in the Cunene; Queve and Catumbela River blocks: Congo, Tanzania and Kalahari (Fig. 2). The Congo springs (IV); and, finally, concentric structures with and Tanzania cratonic blocks are also separated from the nepheline syenites and carbonatites in the south-western by the Mwembeshi left transform fault, part of the Lunda - Walvis Bay lineament, such as: particularly active in Pan-African times and responsible Zenza, Nonga, Elonga, , Longojo, Bonga, for the formation ofthe Damara belt (KRONER, 1982), and Tchivira, Serra da Neve, Virulundo (LOUREIRO, 1967). the Lufilian Arc (COWARD & DALY, 1984). The tensile These occurrences seem to confirm MILASHEV(1965) and forces reactivate weakness zones inherited from the BARDET (1973-1977) ideas. The variation in geothermal Precambrian orogenic cycles and from the continental gradient from the centre to the external part ofthe craton collisions with Gondwana, formerly referred. and the fraction ation of the alkaline magma, associated Concerning the Congo craton, and in the specific case with the conditions of diamond stability with the phase ofAngola, the main weakness zones defined are: WSW­ transformation of diamond in graphite, is totally ENE, NW-SE and NNE-SSW. Under the W-E tensile confirmed in space and time in Angola (Fig. 3). forces, coeval with the Atlantic opening, the WSW-ENE There is a question related with the time of this fractures will react mainly with wrench movements, magma evolution between the Lunda and the Namibe while the other become mainly tensile. region (PEREIRA, 1969). An Albian age (110 Ma) is Considering all these facts and models it is worth admitted for the Lunda kimberlite intrusions while the emphasising some ideas about the most successful methods 202

TUNIS _--R, TUNIS o -----~

Lj) O. B. 1JA. N. B. / ~"\,.- j ~ FERNANDOpd /1 // i ' (g/./ /\I 1MOMBASA ~ 1)( I / I \: I -/ /1' /1/1 A. B. ®

Fig. 4 - Rift episodes and fractures reactivated in the cratonic basement ofthe African continent during the lower Cretaceous. Adapted after FAIRHEAD (1988) and UNTERNEHR et al. (1988): a) Neocomian-lower Aptian; b) Upper Aptian-lower Alb ian.

ofprimary diamond source exploration in Lunda. In view subjected to different diastrophic phases that have modi­ of the previous ideas and assumptions on the major fied the physical properties ofthe various lithologicalunits. geological controls ofkimberlite emplacement, the most These changes have been brought out by metasomatic favourable areas are those that have a cratonic basement, phenomena and magmatic differentiation process (RBIS, and alkaline volcanism associated with deep transcurrent 1966). On the other hand, the basic and ultrabasic comple­ or extensive tectonics. In Lunda, very thick post-kimber­ xes, as shown by their circular alkaline-carbonatite struc­ lite terrigenous sediments, deep weathering profiles and tures, have had a history of tectonic control along the main dense vegetal cover obscure these geological conditions. lines of structural weakness, together with magmatic dif­ This inevitably leads to choosing geophysical methods of ferentiation (RBIS, 1971; RBIS & BARROS, 1981; REfS & kimberlite exploration over other conventional methods. MONFORTE, 1981). The main suite of volcanic rocks consist predomi­ Importance ofairborne magnetic surveys nantly of andesitic laves, rhyolites and gabbros which are related to the alkaline complexes and the nepheline This method ofexploration revealed itselfvery useful syenites, ijolites, trachytes, phonolites, breccias, tuffs, in the location of the major lineaments that control the carbonatites and kimberlites. The carbonatites and ascent and emplacement ofkimberlites. Thus, this method kimberlites associations under similar tectonic condi­ was decisive to the discovery ofvolcanic alkaline struc­ tions has been discussed by several authors (DAWSON, tures where those rocks are found. In fact, the crystalline 1980; BARDET, 1973; NIXON, 1987; KORNPROBST, complexes have, throughout geological time, been 1984). 203

A good example of the aeromagnetic application for The structural map shows all the geological and kimberlite detection is shown in Fig. 5, which gives a geophysical data and delimits the three main WSW-ENE, zoom sketch map from the Uambo region, in central NW-SE and NE-SW fault systems. Minor structures are Angola. also marked. The oldest tectonic structures are perhaps 5 '1'--0------151 ~~~Nt,tT- ~-0 -----1T.

10'1

ANGOLA REGION

11

12

@ Village

o Kimberlites 21 Carbonatites Alakaline Complexes

100 150 200Km ! I

Fig. 5 - Magnetic and tectonic struc tures, in the Uambo region, centra l Ango la; they establish the control ofcarbonatite-kimberlite occ urrences, after (REIS, 197 1). 204

C the W-E ones, which were reactivated several times with (I) E WSW-ENE deflections. E ~ (I) u, The magnetic anomalies related to these structures "5 (I) '"0 "5 E u, E E .0 :l ~ u, are, generally, very elongated, parallel to subparallel, "5 E E.Q , C U'" 0 u, 0 u c ,~ Ll.g (l)E being interpreted as the reflex of deep fractures that c 0 0 00 gLl. B b 0", (/) ~ U U() E control the ascent of explosive kimberlites and alkaline --~ c c- o' Z ~ 00 ",(I) 0 I.U 0'" .c .Q ""0 () magmas. In Angola, two main structural bands of high '58- () ~'" 0 -'"00 0 .... ~ <9 ]

Kwango Group - Calonda Formation • Coarse fanglomerate / conglomerate, with a sandy­ -clay matrix showing several different sedimento­ The Calonda Formation (CF) is considered the first logical types whose definition and classification sedimentary collector ofdiamonds, and is correlative with was proposed by several authors (e. g. FIEREMANS, the continental diastrophism and supergenic destruction of 1955; MONFORTE, 1960; SARAIVA, 1973). In most kimberlite diamond sources. As a consequence, the CF cases the basal conglomerate produces diamonds colIects the products of surface destruction of kimberlites and some attempts were made in order to relate the and lamproites (DELVILLE, 1973; RODRIGUES, 1993). sedimentological type of each conglomerate with The Calonda Fm sedimentogenesis is, thus, con­ its diamond content. trolIed by the filling-up of fault-bounded large depres­ • Sequences ofimmature arenites with rare mudstone sions that originated by extensional tectonics coeval with intercalations; the sandstones have cross stratifica­ the opening of the South Atlantic (REIS et al., 2000). tion and several colours that vary from place to place: The Calonda stratigraphic sequence is composed violet coloured muddy arenites; white arkosic-kao­ mainly of torrential deposits, correlated and proximal to linitic arenites and reddish limonitic arenites; the elevations, that express high energy and transport capacity in a dense and viscous medium, where angular • Fine-grained conglomerate recurrences, with an coarse and fine clasts are transported by a dense argilla­ argillaceous matrix; ceous mass, in water suspension. It show stages of great recurrence, with conglomeratic and argilIaceous interca­ • Argillaceous component increasing upwards with lations, as the reliefbecomes less pronounced. Gradually, sandy-argillaceous levels with limonitic horizons, the sedimentogenesis gains lagoonal characteristics silcretes and calcretes indicating dry periods. downstream, as a result of the grouping and coalescence of weak currents coming from lower lands. • Predominance of argillaceous and sandy-argilla­ In a mature stage of pediplanation, in the Upper ceous levels with brownish colours at the top ofthe Cretaceous, this type ofdeposit implies an abundance of sequence. Sometimes with dispersed pebbles. regolith and a great rainfall volume, as was interpreted from the textural maturity ofthe arenaceous sequences and from the sedimentary structures, namely cross-bedding. Kalahari Group At the top of the CF sequence, the limonitic, silcrete and calcrete levels, indicates the larger frequency of dry The lithostratigraphy, sedimentology and tectonics of seasons and the complete disappearance of surface the formations included in the Kalahari Group must be waters. Generally, the sedimentological column ends studied in the light of the global climate changes that with terrigenous sandy-argillaceous materials with sparse occurred during its sedimentation. In fact, the Kalahari angular pebbles. These uppermost levels correspond to a sedimentogenesis is coeval with the main erosive periods low energy laminar flow transport with aeolian transport that have sculptured the relief, resulting in vast planes. It episodes. . is the knowledge of the chronology of these planes The Calonda Formation was informally defined in surfaces, coeval with depositional hiatus and together Lunda (NE Angola) (ANDRADE, 1953a, b; 1954). In the with climatic changes that must be present in the study of Democratic Republic of Congo, together with other the Kalahari Group. The formal definition of the units of the so-called lithostratigraphic units, it is placed in the frame of the Kalahari System, was made in the Conference of Geo­ Congo Basin and it has been included in the Kwango logists of the Congo Basin in 1945 (LEPERSONNE , 1945) Group (MONFORTE et al., 1979; CAHEN, 1983), with an and completed later by CAHEN et al. (1946). These authors age considered as Cenomanian, on the basis of fish proposed three main divisions for the Kalahari, which macrofossils and palynomorphs. In the latter years ofthe resulted in the three formations considered until today : DIAMANG mining activities, several detailed studies were carried out from a lithostratigraphic and economic • Kalahari C (superieur) - Kalahari Sands point of view (SARAIVA, 1973). • Kalahari B (intermedium) - "Gres Polimorphyiques" The Calonda Fm lithostratigraphy, with an average thickness ofabout 40-60 meters , is the base upwards: • Kalahari A (inferieur) - Kamina Formation 206

The lower Kalahari is not present in Lunda or, what Basin. This event is post the Ochre Sands Formation, seems no probable, it is confused with the top of the partly coeval with the late Pliocene pediplanation and Calonda Formation. This lack of definition could be due cuts the Miocene and the late Cretaceous peneplains. to the facies similarities when the conglomerate is not Most of the Angolan geomorphology is dominated by present (MONFORTE, 1960). such planation surfaces, with a stepping succession of Based on the works of several authors (POLINARD, planation surfaces from the interior ofthe country to the I948a, b; MOUTA & DARTEVELLE, 1952; JANMART, 1953; coast. The surfaces are also present in the endorheic CARVALHO, 1955; GREKOFF, 1958; DE PLOEY et aI., morphology ofthe Congo Basin. 1968), it was possible to define a lithostratigraphy for the The Pliocene peneplain is, thus, the best reference Kalahari ofLunda: surface for the Quaternary eluvio-alluvial deposits, with • Upper Series or Ochre Sands Formation, formed or without diamonds. This surface covered by: by gravel layers at the base with ochre sands and • Very dispersed plateau gravels composed by fer­ yellow sands at the top; ralite elements and clasts of "Gres Polimorfos"; • Lower Series or "Ores Polimorfos" Formation, these deposits express the last stage of planation composed, from base to top, of conglomerate, once the finer elements have been removed by the purple sandstones locally silicified, chalcedony and wind and the surface-waters; sandy chalcedony, silicified quartzitic sandstones, • Sands covering the former plateau gravels, some­ silcretes (FERREIRA, 1958), and white or red friable times with several meters ofthickness. These sands sandstones. occur between altitudes of 800-900 m, and are of aeolian origin, resulting from the redistribution of An Eocene to Oligo-Miocene age has been consi­ Kalahari sands in an arid climate. dered for the "Gres Polimorfos" Formation based on fossil findings, namely Ostracods, Gasteropods, and The maximum altitude ofthe Pliocene surface marks oogonic fragments of Chara genera. the dividing line between the main Lunda rivers flowing From our personal experience in NE Lunda, the tran­ northwards . This surface is cut by the sub-actual and sition between Calonda Fm and the Kalahari Group is actual river drainage, with the correspondent planation made by a sedimentological unconformity with a weak levels. Some of these planation levels are marked by erosive event: terrace deposits, some of which are economically impor­ tant. MONFORTE (1988), based on previous studies - Wherever the contact is observable, it is moderately (POLINARD, 1949; BREUIL & JANMART, 1950; JANMART, undulate; 1953), defined these erosion cycles for the Lunda region. - There could be several contact lithologies: in the As mentioned previously, humid tropical climates are C. F.sandstones it is possible to find mudstone levels, favourable for intense alteration while sub-arid climates cross-bedding, parallel bedding and the presence of generate great masses of regolith. The quaternary feldspar; in the "Gres Polimorfos", apart from the climate, in the region of the Congo Basin, of low lati­ strong silicification, these units do not include tudes, is characterised by an alternation of these condi­ mudstones, the feldspars are not visible in hand spe­ tions. Therefore, the development ofplacers and residual cimen and the bedding planes are not perceptible. deposits depends on the combination of tectonic move­ - In the basal levels of the "Gres Polimorfos", there ments and climatic conditions (HALL et aI., 1985). is occasionally a well-rounded gravel made up of Tectonism takes place in the Holocene. In fact, a tilt clasts of quartz and quartzite with angular chal­ movement to the W, affecting a sector of the Congo cedony that contains scarce diamonds. Basin induced the drainage of the great African river to flow to the Atlantic. From this point on the sedimentation is definitely altered in the basin and it becomes no longer Post-Pliocene diamondiferous alluvial-eluvial endorheic, the level base is lowered and the encasement deposits of the great tributaries of the starts. Wide valleys with flat floors are generated and rapidly the In a study ofthese kinds ofdeposits, it is very impor­ riverbed is excavated on the friable units of the, also tant to consider the last planation event in the Congo designated, Central African Basin. 207

Remobilised Calonda, Kalahari andplateau deposits These changes originated an important ferralite level named "Pleistocene Laterite I", whose genesis was The active slope erosion, cutting the easily friable favoured by the alternating succession and dominance of Calonda and Kalahari deposits, remobilise and deposits the hot dry season over the rainy one. its materials directly over the sloping crystalline base­ These terraces, with a thickness between 0.30 and ment or in various level terraces. 1.50 meters, rest on top ofthe crystalline basement or on Frequently, the ore grade diminishes, due to the intro­ top ofthe referred laterite. duction of large volumes of barren Kalahari sands and gravels. However, if the erosion is directed on the Calonda basal conglomerate, a fine deposit with rich ore Main rivers alluvialflat and riverbed deposits grades could be formed on top of the basement. In Fig. 6, the fine layers originated economically important Under this title are grouped several depositional diamondiferous deposits. events in several studies (e.g. JANMART, 1946a; LEAKEY, 1948; MONFORTE, 1988). They are low altitude terraces, with ferralitic levels, Upper Terraces (40 to 20 m) changes in depositional regime of the river courses, terrigenous intercalations due to the .emersion of the i) - 40 meter terraces deposits, etc., essential, without doubt, to the research on It contains non-rounded Kafuense industry, pre­ climatology and lithostratigraphy of recent Quaternary Chelles-Acheulense (JANMART, 1946a, b). Some doubts (HALL et al., 1985). Any work in this domain may not do persist about the origin ofthese deposits (LEAKEY, 1948). without this analysis. In fact, there is a mixture of angular and rounded clasts. In a mining perspective, all of these deposits are These materials could be slope degradation products, the located on the river margins, alluvial flat and on the rounded materials having come from the pre-Pliocene present riverbed. units.

ii) - 20 meter terraces Economical situation analysis of Angolan diamondiferous deposits They contain non-rounded Kafuense industry, pre­ Chelles-Acheulense (JANMART, 1946a, b). The preceding text is a synthesis of the main aspects Contrary to the former terraces, relative to which related with a complex geological problem that is the doubts subsist about the deposition process, these diamond deposits ofAngola. We have made reference to deposits constitute layers with a thickness that could rise some scientific and technical matters that must be to three meters, and are undoubtedly deposited by rivers considered in the study of the genesis and location of during their erosion activity until the present level posi­ primary and secondary diamond sources. Below, the tion. economical consequences of this knowledge in view of These terraces are situated approximately 20 meters future trends and possibilities for diamond exploration above the present riverbed level, on top ofthe crystalline and exploitation in Angola is discussed. basement and they are very common in NE Angola. Their economical value is higher than that of the i) Diamondiferous potential 40 meter terraces. When Angola became an independent country, on the lIth of November of 1975, the ore reserves in diamond Main rivers low-level terraces (~ 10 m) were considerable (65 x 106 carats), and the exploitation rate reached 2.1 x 106 carats/year. These numbers show They contain Oldwense industry and some other the effectiveness of the DIAMANG mining and explo­ types of Chellense and Chelles-Acheulense industry. ration activities until the end ofits work. The succeeding Betweenthe high terraces and the low terraces depo­ Angolan Diamond Mining Company (ENDlAMA) took sition, an important time span occurred, during which large benefits of this heritage to the present-day, if we important climate changes happened (MONFORTE, 1988). take into account that no more intensive significant 208

exploration works have been carried out since 1975. discovery of 383 kimberlites, between 1952 and 1973 Despite some rare exceptions the present-day mining (176 in 1973). Besides these , 46 new kimberlites were works are still based on the DIAMANG ore reserves. found by CONDIAMA(a DE BEERS and DIAMANG asso­ Today there is no information about new proven reserves ciated Company), in a total of 429 identified kimberlites and the depletion ofreserves is poorly controlled because until July of 1973, which can be grouped in the following of smuggling. manner: 2.8 % economically exploitable; 1.2 % under evaluation studies; 46.6 % without diamonds and 49.4 % In 1975 the known proved reserves are, by ore type: not studied. • Primary sources (kimberlites/lamproites) : "" 45 x 106 At present time , a number about 600 kimberlite carats; bodies were identified.

• Sedimentary detrital deposits (recent alluvial-elu­ ii) Future perspectives vial gravels, and some area with Calonda Fm basal gravels): "" 20 x 106 carats. As a result of the generalised illegal miners all over the country, the secondary diamond deposits related with In relation to these numbers, it is important to notice the actual river drainage does not represent an economic that statistical studies carried out in several tens ofyears source to the country, and, in our opinion, they are in the have shown that the exploitation results are 22 % higher future completely lost for any kind of organised indus­ than the ones of exploration. On the other hand, an trial scale mining activity. In fact, since 1978 and with an increase in 30-40 % of ore proved reserves must be ever increasing rate, since 1985 that those kinds of ille­ considered, because in almost all the defined mining gal mining activities have taken place in those kind of blocks the exploitation continues beyond the limits of deposits. The consequent inexistence ofeffective control these blocks with ore treatment plant ore grade control. on high-grade I little-overburden thickness and well­ The estimation of probable diamond reserves, was in sized good-quality stones lead to a "diamondiferous" chaos in Angola. 1975: As a result of these illegal activitie s, the benefits for • Primary sources (kimberlites/lamproites?): "" 40 x 106 the country were none and even highly negati ve, due to carat s; the illegal diamond exportation to foreign international • Sedimentary detrital deposits in recent alluvial-elu­ markets (Antwerp, Israel, USA , etc) , but also because the vial gravels (undervalued reserves): "" 7,5 x 106 carats; diamonds serve as financial support for other activities. A few, but important alternatives are left for the 6 3 • Calonda Fm basal gravels: "" 26 x 10 m (a total of secondary deposits: i) the main river bed deposits, unex­ 6 10.4 x 10 carats is estimated), with an average ore ploitable without adequate technical support; ii) in rela­ 3 grade of0.4 ctl m and 0.8 m for the conglomerate tion to the referred lost reserves, an alternative could be thickness with an overburden thickness of about the establishment of an official buying system, with 30-60 m. attractive prices, that could conteract the illegal diamond The Calonda Fm basal conglomerate was investi­ exportation. gated and sampled by drilling. In face of the ubiquitous Therefore, the good use of the remaining economi­ occurrence of this lithostratigraphic horizon in Lunda, it cally viable kimberlite and Calonda Fm diamond is easy to point out the huge potential of Calonda Fm as resources that, due to their specific geological charac­ a future diamond reserve. The need for an adequate teristics, are untouchable by illicit miners , is imperative. exploration program is obvious (PEREIRA, 1996; RODRI­ In relation to the Calonda Fm, the usually thick over­ GUES et al., 2000a; RODRIGUES et al., 2000b). burden hinders the activities of illegal diamond prospec­ All of the previously referred data on the diamond tors. There are three main regions of high economical potential was collected and known only due to the work potential (REIS et al., 2000): NE Lunda , Cuango drainage and experience ofthe DIAMANG geologists and miners basin, and particularly the middle course of the Cuando in several scientific areas such as: remote sensing, River, that allows the establishment of several economi­ geophysics, geochemistry, loaming mineralometry and cal projects: i) Maludi region (NE Angola) with recog­ also biology (botany and termites). Together, several nised basal gravels approximating 22 x 106 m-', with techniques developed in these scientific areas lead to the 30"'60 m of overburden thickness and extremely good 209 gem-quality diamonds; ii) Cuango River basin, between BLACK, R. (198 4) - The Pan- African event in geo logical Framework of the Luremo and Cuango villages, with well-known Afri ca. CIFEG, Ann. Meet. ojthe Africa Group oj German values ofmineralisation and also good quality and size of Geoscientists, Pangea, Distinguished Lecture, pp . 6- 16. diamonds; iii) middle ofthe Cuando drainage basin as a BREUIL , H. & JANMART,J. (19 50) - Museu do - Subsidios p ara potential area for the occurrence of the Calonda basal a Historia, Arqueologia e Etnografia dos povos da Lunda. gravel, but in a state of incipient geological knowledge. "Les limons et graviers de l'Angola du Nord-Est et leur contenu archeo logi que". Compo Diam. Ango la (Diama ng) In relation to the primary sources - kimberlites and Pub . Cult n." 5, Lisboa. lamproites - we point out the main provinces of location (REIs, 1972), but it is possible that numerous others are yet BROGNON & VERRIER(1965) - Tecto nic et sedimentation dans le Bassin du Cuan za (Angola). Bol. ServoGeol. Minas de Angola, T. 11, unknown. This type of ore demands a vast and intensive pp.5-90 . exploration program aimed at initialarea selection and later with refined exploration methods on a case to case basis. CAHEN, L. (198 1) - Precision sur la stratigraphie et les correlations du Groupe de la Haute Lueki et des form ations comparables Our opinion is that when the country become s (Triassique iJ.? Lia ssique d'Afrique centrale). Mu s. Roy. Afr. socially stabilized, enormous unknown diamondiferous Centr., Tervuren (Bel g.), Dept. Geol, Min., Rapp . 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