geology

Some Geography data from the area taken from a paper
I found recently. Reading for Quiet time. LATE PAN AFRICAN FAULT-RELATED, SEDIMENT-HOSTED, BASE METAL MINERALIZATION IN THE DEMOCRATIC REPUBLIC OF CONGO (DRC): EMPLACEMENT AND GENETIC SIMILARITIES AND DIFFERENCES. THE EXAMPLES OF BAMBA KILENDA (BAS-CONGO), DIKULUSHI, KAPULO and KIPUSHI (KATANGA)
V. KANDA NKULA, G. FRANCESCHI and L. TACK

Summary.


In recent years, many geoscientists have paid attention to the understanding of pre-Palaeozoic plate motions, reconstruction of Neoproterozoic palaeogeography and distribution of potential mineralizations, e.g. in the Neoproterozoic - Early Palaeozoic West Congo belt and in the Katanga belt (also often called Lufilian belt or Lufilian Arc). Moreover, the Late Neoproterozoic marks an important transition in Earth history because of 1) extreme climatic signatures as proposed in the "Snowball Earth" hypothesis, 2) increased oceanic production of organic C as a result of complex multicellular organization (in particular through the Precambrian - Cambrian boundary) and 3) major changes in seawater (and atmosphere) chemistry. The Neoproterozoic was also a period of major plate re-organizations including the breakup of the Rodinia supercontinent and the subsequent dispersal and later reassembly during the Pan African orogeny of its constituent fragments into the Palaeozoic major Gondwana continent.
The Neoproterozoic of Central Africa hosts a large number of world-class base metal deposits of both stratiform (stratabound) and discordant/epigenetic types (see e.g. the 2002 International Metallogenic Map of Africa; 1/5.000.000). The "Copperbelt" deposits of Katanga (Democratic Republic of Congo; DRC) and Zambia form one of the greatest stratiform copper-cobalt provinces in the world, containing deposits of 10Mt and more of copper metal. Rightfully, these have been the subject of abundant investigations but with the exception of a few specific cases like Kipushi (see below), this resulted in a lesser interest for the usually smaller, fault-related and more irregular mineralizations occurring in the same area.

While the latter lens-shaped deposits are usually overclassed in size by their stratiform counterparts, a number of them yield attractive high-grade mineralization and/or (re)assemble a number of metals of special interest, including Au, Ag, Ge, Ni, U, etc.

This paper focuses on some of these fault-related deposits occurring in two different segments of the Neoproterozoic Pan African Orogenic System around the Congo craton:
in the West Congo belt: the deposits of Bamba-Kilenda, Kussu, Toni, etc… in the Bas-Congo Province of the DRC, of Mindouli - M'Passa - M'Fouati in the Popular Republic of Congo (Brazzaville) and of Mavoio - Bembe in Angola;
in the Katanga belt: the deposits of Dikulushi, Kapulo, Kipushi, Lombe, Kengere, etc… in the Katanga Province and of Kabwe (Broken Hill) e.g. in Zambia.
From the structural point of view, these deposits are located in a similar tectonic setting, encompassing an arcuate external thrust and fold belt and/or an adjacent subtabular in situ externalmost foreland domain of the Pan African Orogenic System. In both cases, tectonic polarity is directed towards the foreland, corresponding with the present-day Congo basin ("Cuvette Centrale du Congo"). Regional metamorphism is decreasing with the tectonic polarity, from amphibolite to greenschist facies rocks to unmetamorphosed sedimentary and subordinate (meta)volcanic rocks. Discordant faulting subperpendicular to the arcuate folding trend is obviously in both segments "late", i.e. (at least slightly) younger than the age of the tectonogenesis at the origin of the general trend of the segments.

Chronologically, the Pan African compressional deformation for the West Congo belt is best constrained by a recent Ar-Ar age of 566 ± 42 Ma obtained by Boven (Tack et al., 2001). In the Katanga belt, the climax of "a major orogenic event involving northeast directed thrusting (Lufilian Orogeny)" is comprised between 560-550 Ma (Porada and Berhorst, 2000; Key et al., 2001). Reliable age constraints for the discordant faulting event(s) are scarce, the more that (repeated) reactivation discordant to the Pan African trend may have occurred in both segments during the Phanerozoic. Consequently, interpretation of the available isotopic data in terms of emplacement age of a mineralization or formation age of its source often proves hazardous.

The lithostratigraphic sequences of the West Congolian Group (Tack et al., 2001 and references there in) and of the Katanga Supergroup in Congo and Zambia (Porada and Berhorst, 2000 and references there in) are quite reminiscent from each other and display striking similarities. 1) In both, these correspond to passive margin deposits under extensional regime with some relatively early mafic volcanic intercalations. Siliciclastic and carbonate rocks predominate. 2) Basal conglomerates resting on an eroded, ca 900 Ma old granite substratum have been observed, testifying the continental rifting of Rodinia (Tack et al., 2001; Rainaud et al., 2002; Master et al., 2002. 3) Two separate diamictite horizons have been recognized for a long time. 4) Generally, the occurrence of algal mats, stromatolites and cyanobacteria increases with time in the sequences. 5) The uppermost part of the lithostratigraphic succession corresponds with "Red Beds" facies rocks. Their precise age is disputable, but is probably at least for some of the series post-Neoproterozoic and pre-Karoo (i.e. "Lower Palaeozoic").

In the basal Roan Group of the Katanga Supergroup, temporarily anoxic conditions in a lagoonal to mudflat environment prevailed, giving rise to intercalations of evaporitic rocks in the siliciclastic-carbonate successions. The very peculiar physico-chemical conditions (pH, eH, % organic C, etc …) of such a palaeo-environment are considered to have played an essential role in the trapping of the metals. It must have been at the origin of the "primary" synsedimentary/syndiagenetic stratiform deposits ("ore bodies") located in the Roan Group (Porada and Berhorst, 2000; Cailteux et al., 2002; Robb et al., 2002).

In opposition to this, for the fault-related deposits a hydrothermal origin has generally been proposed with - simplified - leaching and remobilisation of a number of elements from the stratiform metal concentrations, followed by secondary epigenetic sulphide redistribution(s) and deposition within fault conduits. This hypothesis, although acceptable, remains in a number of cases controversial and so far, not enough scientifically supported. Furthermore, composition of the migrating fluids, age and mechanism of deposition, relationship with the Panafrican orogenic events and source rock(s) have not been adequately investigated.

In this context a group of researchers has recently decided to focus their efforts on a selected (non-exhaustive) number of DRC fault-related deposits, which are briefly reviewed hereafter. They include Bamba-Kilenda (Bas-Congo Province), Dikulushi, Kapulo and Kipushi (Katanga Province).

The Bamba-Kilenda deposit (70km south of Kinshasa) occurs along the E-W oriented Masungu fault. The (on average) 70°N-dipping fault is situated on the contact between the Schisto-Calcaire, Mpioka and Inkisi Subgroups of the West Congolian Group. The mineralization occurs as irregular fillings in the fault, as sulphide impregnations and sometimes lenses in the arenites, and as impregnated massive veins in the limestone or at the contact between the limestones and the arenites. The mineralization has been recognized over the entire length of the fault over >3.5km in the form of traces, gossans, disseminated sulphides and/or more or less massive lenses. It takes the shape of a “pinch-and-swell” vein or elongated lens in the fault structure. The deposit shows a clear zonation: the western part of the mineralization contains predominantly Cu, while Pb, Zn, V, etc … appear in the eastern part of the fault, characterized by lower Cu grades. Up to 150.000t of Cu+Zn+Pb with associated Ag have been estimated on the basis of a relatively dense (100m) drilling campaign, performed in the fifties by the Syndicat de Recherches minières des Bas- et Moyen-Congo (BAMOCO) (Archives CRGM, Kinshasa; Lavreau, 1977). Apart from Ge contained in sphalerite and Ag in galena, erratic random assays indicate the presence of Au, sometimes in considerable quantities and possibly over impressive widths, but little or no attention has been paid to these elements.

The Dikulushi deposit is a high-grade chalcocite vein deposit located some 400km NE of the main Katanga Copperbelt and 20km to the west of Lake Moero (also Moëro or Mweru). Host rocks are interbedded arenites, argilites and intraformational breccias of the Upper Kundelungu Group (Dumont and Hanon, 1993; Trefois and Fernandez-Alonso, 2000). The Dikulushi mineralization is fault-related, with a strike length of 250m and high-grade zones up to 30m wide. Nevertheless, the origin of the mineralization remains controversial: fault-related hydrothermal or sedimentary and reworked (re-erected) by subsequent tectonic phenomena (Marenthier and Vairon, 1980) or related to large-scale décollement features (Franceschi et al., 1999; Trefois and Fernandez-Alonso, 2000). The principal copper mineral is chalcocite (with silver in solid solution), which occurs in massive form within the main mineralized structure and as disseminated pore fill within some adjacent arenite beds. It is a hypogene, not supergene mineralization feature (Turner and Bradford, 1998). Gangue minerals include calcite, quartz, dolomite and barite. Minor chalcopyrite, pyrite, sphalerite and galena occur in the west of the deposit, possibly reflecting a separate, earlier mineralising event. The deposit outcrops at the surface, is metallurgically simple, produces a high-grade concentrate and can be mined initially by open pit methods. Anvil Mining NL carried out a diamond and RC drilling program in the latter half of 1997, which established a resource of 1.94 Mt at an average grade of 8.5% copper and 266 g/t silver (using a 2% Cu cut-off) but the orebody remains open at depth. Dikulushi was put in production in September 2002. Note that it is probably the highest grade copper-silver mine in the world.

The Kapulo copper mineralization (within the Anvil Mining NL concession) occurs some 120 NE of Dikulushi and 45km E of Pweto, also near Lake Moero. It consists of 3 distinct ore bodies, which are part of a series of N-S oriented Cu-occurrences traceable over a length of more than 50km. The area is predominantly underlain by two main geological units, namely in the west by sedimentary series of an assigned (unconfirmed) Upper Kundelungu age, and, in the east, by a large Palaeoproterozoic granitoid body. The mineralization consists of malachite and chalcocite with subsidiary bornite, chalcopyrite, cuprite, native copper and azurite. Copper mineralization occurs at the base of a sedimentary sequence of decomposed shales, arkosic sandstones and conglomerate about 40m true width above the contact with the underlying granitoids. Cu mineralization is sharply bounded on the hanging wall by a narrow, decomposed black shale, which forms a prominent marker through both deposits. The formation of the orebody remains controversial and unresolved: Brien (1928) described the orebodies as contact-metamorphic deposits, de Magnée (1934) interprets them as hydrothermal and metasomatic, Rorive (1957) relates the mineralization to the Kapulo fault and Bornuat (1974) speculatively proposed a sedimentary syngenetic-diagenetic origin, posterior and completely independent of the fault mechanism. Most recent resource estimates (Bornuat, 1974) are set at about 1.12Mt of ore grading 4.73%Cu (using a 1% cut-off), but the spatial extent of the deposits is not precisely constrained.

The famous Kipushi Zn-Pb-Cu deposit (30km west of Lubumbashi) is a discordant ore body at the contact between a breccia body, occupying the core of the Kipushi anticline, and adjoining hosts rocks of the Lower Kundelungu (or Nguba) Group. The origin of the Kipushi deposit is controversial. Intiomale and Oosterbosch (1974) considered ascending hydrothermal solutions of magmatic origin as progenitors of the mineralization. De Magnée and François (1988) related the Kipushi deposit to the dissolution of a salt diapir, which produced the breccia in the anticlinal core. Unrug (1989) ascribed the mineralization to the deposition from large-scale circulating fluids formed by regional metamorphism-driven dewatering of the folded basin. Walraven and Chabu (1994) indicated a Pb-Pb probable mineralization age of 454 ± 14 Ma, which is younger than the peak of the Lufilian tectonism and suggests that the mineralization is clearly epigenetic and post-dates the folding and brecciation at Kipushi. The primary sulphide mineralization, presumed to be of hypogene origin, is very complex, and the distribution of the different mineral assemblages in the deposit is interpreted as being essentially the result of the superposition of a copper-bearing mineralization on a zinc-bearing mineralization (Intiomale and Oosterbosch, 1974). The Kipushi mine (currently closed) has produced in its 67 years of life an estimated 60Mt of ore grading 6% Cu and 11% Zn, with associated large quantities of germanium and silver.

Each of these four fault-related deposits has been the subject of some relatively advanced investigation at some stage of its history. However, a large number of key-features still require further detailed study to discriminate similarities from differences in terms of emplacement and genesis. Research will focus on renewed field observations, core inspections and sampling as well as on the use of present-day petrographic, mineralogic and geochemical techniques, to investigate in particular the role of the mineralising fluids (composition, nature, age,…) and their circulation during the regional structural evolution. The geological setting on a broader sub-regional scale will be evaluated with the support of remote sensing techniques (and geophysics, so far as made accessible). .