Aug 23, 2007 - Cover Design: Woodcut showing methods of deepening mines in the 16th Century. ... Kamoto and Musonoi) and high-grade Cu-Pb-. Zn-Fe-Ag ...
PROCEEDINGS OF THE NINTH BIENNIAL MEETING OF THE SOCIETY FOR GEOLOGY APPLIED TO MINERAL DEPOSITS DUBLIN, IRELAND 20TH-23RD AUGUST 2007
DIGGING DEEPER Edited by Colin J. Andrew et al Cambridge Mineral Resources plc, Navan, Ireland.
VOLUME 1
The 9th Biennial SGA Meeting is organized by the Irish Association for Economic Geology with assistance from the Society of Economic Geologists
Cover Design: Woodcut showing methods of deepening mines in the 16th Century.
From Georgius Agricola (1556) "De Re Metallica"
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Published and distributed by the Irish Association for Economic Geology
ISBN [0-950989-4-4] set of two volumes ISBN [0-950989-4-4] Volume 1 ISBN [0-950-989-4-4] Volume 2
© 2007 IAEG
Contributions from structural analysis and remote sensing to the timing of mineralization in the Lufilian arc and its foreland. M. Haest, H. El Desouky & Ph. Muchez
Geodynamics & Geofluids Research Group, K.U. Leuven, Celestijnenlaan 200E, bus: 2408, B-3001 Heverlee, Belgium
S. Dewaele
Department of Geology and Mineralogy, Royal Museum for Central Africa, Leuvensesteenweg13, B-3080 Tervuren, Belgium
ABSTRACT: The Lufilian arc and its foreland host numerous stratiform and vein-type Cu mineralizations that formed during different stages. The main phase of the stratiform Cu-Co mineralization formed during diagenesis prior to the Lufilian orogeny. The Lufilian orogeny was characterised by folding and thrusting that extended north into the foreland. The folds were crosscut by faults at an angle to the fold axis. These oblique faults were preferential sites for vein-type polysulfide mineralization in the Lufilian arc and its foreland. NE-oriented faults developed subsequently and these were favourable fluid conduits for remobilisation and enrichment of the former mineralization. KEYWORDS: Copperbelt, structural analysis, remote sensing, geodynamics
1
INTRODUCTION
The Lufilian arc in D.R.Congo and Zambia forms one of the largest Cu provinces known on earth. It includes stratiform Cu-Co (e.g. Kamoto and Musonoi) and high-grade Cu-PbZn-Fe-Ag vein-type deposits (e.g. Kipushi, Lombe and Kengere). The Lufilian foreland, north of the Lufilian arc, is characterised by minor stratiform Cu-Ag mineralization (e.g.. Lufukwe) and some high-grade vein-type CuAg deposits (e.g. Dikulushi) (fig. 1). The aim of the current study is to reconstruct a sequence of formation for the different types of mineralization in the Lufilian fold-and-thrust belt and its foreland. This sequence is based on the structural setting of some selected mineralization, which is obtained from remote sensing analysis at Dikulushi and Lufukwe, from detailed structural analysis on the field at Dikulushi, and from the available literature. 2 GEOLOGIC SETTING The Lufilian arc and its foreland are composed of sediments belonging to the Katanga Supergroup. The Katanga Supergroup is a ~7km thick sequence that is subdivided in three
Fig. 1. Geological map of the southern part of Katanga (Ke: Kengere; Lo: Lombe; K: Kipushi; Lu: Lufukwe; D: Dikulushi; LT: Lake Tschangalele; LM: Lake Mweru) (Modified after Lepersonne, 1974).
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groups, based on the regional occurrence of two diamictites. Sedimentation of the Roan Group started after intrusion and erosion of the Nchanga granite (877±11 Ma) (Armstrong et al. 2005). The Roan Group consists of dolomites and dolomitic siltstones, with carbonaceous shales and fine pyroclastic layers in the upper part. The Nguba, with the “Grand Conglomérat” at its base, was deposited on top of the Roan after ~750 Ma. The Nguba and the overlying Kundelungu Groups are separated from each other by the “Petit Conglomérat”, which was deposited around ~620 Ma. Both groups contain dominantly siliciclastic rocks, with few carbonate layers, above the two diamictites. The top of the Kundelungu consists of coarse-grained sandstones (~Plateaux Subgroup) that were deposited after 573±5 Ma. The Lufilian orogeny was characterised by a northeast-oriented transport direction, with folding and thrusting in the Lufilian arc and its foreland (Trefois & Fernandez 2000). At the end of the Lufilian orogeny the folds were crosscut by faults at an angle to the axial surfaces (François 1974). 3 MINERAL DEPOSITS 3.1 Lufilian foreland 3.1.1 Stratiform mineralization Stratiform Cu-Ag mineralization at Lufukwe is contained in the Monwezi sandstone of the Nguba Group. This sandstone underwent first a
Fig. 2. Geological map of the Dikulushi quarry, with the 5% Cu-envelope, based on borehole data and the alteration (MF: Main fault corridor; NF: Northern fault; SF: Southern fault).
phase of compaction and silica cementation, followed by feldspar dissolution that created a secondary porosity. In this secondary porosity 218
developed the mineralization. Satellite image interpretation shows that the NW-oriented Lufukwe anticline is crosscut by WNWoriented strike-slip faults that displace the layers from the Katanga Supergroup, except those of the Plateaux subgroup. Only strike-slip faults, with an overall NE- to ENE-orientation crosscut the Plateaux subgroup. The highest grade bore hole intersections are close to NEto ENE-oriented faults, which suggests an association of the mineralization with these strikeslip faults. 3.1.2 Vein-type mineralization The Dikulushi Cu-Ag deposit is situated at the north-eastern end of the Dikulushi anticline. Four lithologically distinct units are observed in the quarry (Dikulushi unit, Mixed unit, Plastecine shales Unit, and Kiaka Unit, fig. 2), which belong to the Kundelungu Group. The bedding in the Dikulushi unit has a mean orientation of ~098/48. It becomes irregular, in a strongly faulted zone around the mineralization (fig. 2). Three separate groups of layer discordant faults are observed. The first group of sub vertical NNW-oriented faults are covered with slickenlines pointing to a dip-slip movement. The second group of steeply south-dipping (~70°) EW-oriented faults build up the Main fault corridor (MF in fig. 2) and part of these faults underwent a reverse oblique-slip movement. The third group of discordant faults are steeply south-dipping (~70°) NE-oriented faults. These faults form a NE-oriented fault corridor that is bordered to the north by the Northern fault (NF in fig. 2) and to the south by the Southern fault (SF in fig. 2). The Southern fault underwent an important phase of dextral strike-slip movement. The Dikulushi ore body is positioned in the zone between the Main fault corridor (MF) and the Northern fault (NF). It has an ENEorientation and dips 70° to the southeast. It remains open to a depth of 400m and originated from two distinct mineralization phases. The first polysulphide Cu-Fe-Pb-Zn-As mineralization phase precipitated in a complex set of EWand NE-oriented faults and is dominantly observed in the western part of the quarry. At depth this mineralization extends further to the east. The polysulphide mineralization contains quartz with an undulose extinction, saddle dolomite, and calcite with bent twins as gangue minerals. The second phase of mineralization is dominantly located near the surface, a1ong NE"Digging Deeper" C.J. Andrew et al (editors)
oriented faults in the east of the quarry that cut through the set of EW- and NE-oriented faults and contain an orthorhombic chalcocite, with a high Ag-content. At depth, this orthorhombic chalcocite surrounds remnant blebs of bornite. Calcite without bent twins, quartz, and barite are associated with the orthorhombic chalcocite. 3.2 Lufilian fold- and thrust-belt 3.2.1 Stratiform mineralization The stratiform Cu-Co mineralization at Kamoto and Musonoï mainly occurs in two separate levels in the Mines Subgroup of the Roan Group. The main phase of the stratiform mineralization is interpreted to have formed during diagenesis, before the Lufilian orogeny (Bartholomé et al. 1972, Dewaele et al. 2006). 3.2.2 Vein-type mineralization The Zn-Cu-Pb-Ag mineralization at Kipushi is located near the western end of the Kipushi anticline. This mineralization is associated with the steeply west-dipping (~70°) NNE-oriented Kipushi fault that cuts across the WNW-ESE trend of the anticline. The hanging wall west of the fault consists of shales and sandstones, which belong to the Kundelungu Group. The footwall consists of the dolomitised Kakontwe limestone and the “Série Récurrente”, belonging to the Nguba Group. The Kipushi ore body is located along the Kipushi fault for a length of 200 to 500m and extends to a depth below 1800m (De Magnée & François 1988). The ore body developed during different mineralization stages which broadly change from early Zn-dominated to later Cudominated. The Cu-bearing mineralization remobilised certain parts of the Zn-bearing mineralization and formed during two separate phases. A Co-rich chalcopyrite with molybdenite precipitated first and a Ag-rich bornite precipitated later. At the surface malachite and chalcocite with a high Ag content are present. At depth bornite and chalcopyrite dominate (Intiomale & Oosterbosch 1974). The Pb-Zn mineralization at Kengere and the Pb-Zn-Cu-Ag mineralization at Lombe are situated along NS- and NNE-oriented faults respectively. Both faults crosscut folds, with an axial breccia (Unrug 1988).
4 MINERALIZATION IN THE LUFILIAN ARC AND ITS FORELAND The main phase of Cu-Co mineralization in the stratiform deposits of Kamoto and Musonoi developed during diagenesis (Bartholomé et al. 1972. The Cu-Co mineralization has, however, been overprint by later phases (Bartholomé et al. 1972, Dewaele et al. 2006). The Lufilian orogeny began after deposition of the major part of the Kundelungu sediments and led to folding and thrusting in the Lufilian arc. It also resulted in the formation of anticline structures in the Lufilian foreland, which are interpreted as the surface expressions of detachments at depth (Trefois & Fernandez 2000). Injection of the Plastecine shales at the contact between the Kiaka and the Dikulushi units brecciated and folded the overlying units. This injection lies subparallel to the axial surface of the Dikulushi anticline. As such, the injection of the Plastecine shales could be derived from an underlying detachement that formed the Dikulushi anticline. Similar detachments developed in the Lufilian belt. Dechow & Jensen (1965) proposed that the southern limb of the Kipushi anticline was thrust on the northern limb, based on the observed axial breccia. The vein-type deposits of Lombe and Kengere are also associated with axial breccias that could be the surface expressions of such detachment faults. The folds were crosscut by faults at an angle to their axial surfaces at the end of the Lufilian orogeny (François 1974) or possibly postdating the Lufilian orogeny. The mineralizations of Kipushi, Lombe and Kengere in the Lufilian belt and of Dikulushi in the Lufilian foreland are all positioned along such crosscutting faults. The orthorhombic, Ag-rich chalcocite of the second mineralization phase at Dikulushi occurs associated with NE-oriented faults. The polysulphides of the first mineralization phase probably became remobilised and enriched during the second phase. The remobilisation and enrichment are indicated by the remnant bornite blebs surrounded by chalcocite at depth and the high Ag-content of the orthorhombic chalcocite respectively. The stratiform mineralization at the Lufukwe anticline is associated with subparallel NE- to ENE-oriented faults. This mineralization also contains Ag, in addition to Cu. Thus, the structural setting and the mineralogy of the mineralization at the Lufukwe anticline and the second mineralization at Dikulushi are
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similar. The upper parts of the mineralization at Kipushi also are reported to be rich in Ag (Intiomale & Oosterbosch, 1974). 5 CONCLUSIONS Stratiform Cu-Co mineralization in the Lufilian belt mainly occurs in two distinct levels in the Mines Subgroup. The main phase of the Cu-Co mineralization formed during diagenesis before the Lufilian orogeny. The Lufilian orogeny developed folds and thrusts that were afterwards crosscut by faults at an angle to the axial surfaces. Along these faults developed polymetallic (Zn-Cu-Pb) veintype mineralization (Kipushi – Lombe – Kengere) in the Lufilian arc. Another possibly related (?) phase of dominantly Cu-Zn-Pb mineralization at Dikulushi in the Lufilian foreland is similarly associated with NE- and EW-oriented faults that crosscut a fold at an angle to its axial surface. Subsequent remobilisation produced a CuAg rich mineralization along NE-oriented faults at Dikulushi and in the Lufukwe anticline in the foreland and at Kipushi in the Lufilian arc.
Dewaele S, Muchez P, Vets J, Fernandez-Alonzo M, Tack L (2006) Multiphase origin of the Cu-Co ore deposits in the western part of the Lufilian foldand-thrust belt, Katanga (Democratic Republic of Congo). Journal of African Earth Sciences 46: 455-469. François A (1974) Stratigraphie, tectonique et minéralisations dans l'arc cuprifère du Shaba (République du Zaïre). In : Bartholomé P (ed) Gisements Stratiformes et Provinces Cuprifères. Liège, La Société Géologique de Belgique, pp 79-101. Intiomale MM, Oosterbosch R (1974) Géologie et Géochimie du gisement de Kipushi, Zaïre. In : Bartholomé P (ed), Gisements Stratiformes et Provinces Cuprifères. La Société Géologique de Belgique, Liège, pp 123-164. Lepersonne J (1974) Carte Géologique du Zaïre au 1:2.000.000 et notice explicative. In : Rapport annuels, 1975. Tervuren, Royal museum for Central Africa. Trefois P, Fernandez M (2000) Updating the geological map of Katanga (D.R. of Congo) with space imagery combined to archive data compilations. Fourteenth International Conference on Applied Geologic Remote Sensing: Las Vegas, Nevada, Veridian ERIM International. Unrug R (1988) Mineralization Controls and Source of Metals in the Lufilian Fold Belt, Shaba (Zaïre), Zambia, and Angola. Economic Geology 83: 1247-1258.
ACKNOWLEDGEMENTS Prof. Dr. Ali Aït Kaci, Mr. Roger Tyler, Mr. Nick Franey, Mrs. Amanda Boutwood and Mr. Terry Lemmon of Anvil Mining are thanked for their geologic expertise and the logistic support during the different field surveys to Dikulushi. REFERENCES Armstrong RA, Master S, Robb LJ (2005) Geochronology of the Nchanga Granite, and constraints on the maximum age of the Katanga Supergroup, Zambian Copperbelt. Journal of African Earth Sciences 42: 32-40. Bartholomé P, Evrard P, Katekesha F, Lopez-Ruiz J and Ngongo M (1972) Diagenetic ore-forming processes at Kamoto, Katanga, Republic of the Congo. In: Amstutz GC and Bernard AJ (eds), Ores in Sediments: Berlin, Springer-Verlag, p. 21-41. De Magnée I, François A (1988) The origin of Kipushi (Cu, Zn, Pb) deposit in direct relation with a Proterozoic salt diapir. Copper belt of Central Africa, Shaba, Republic of Zaïre. In: Friedrich GH, Herzig PM (eds), Base metal sulfide deposits. Springer-Verlag, Berlin, pp 74-93. Dechow E, Jensen ML (1965) Sulfur isotopes of some Central African sulfide deposits. Economic Geology 60: 894-941.
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