regional and district geology - university of tasmania · lufbu schist are widespread and part of...
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2.1 INTRODUCTION
This chapter outlines the geodynamic history of southern and central Africa and the regional and deposit scale geology at NKM. The chapter focuses on orogenesis and crustal growth, basin formation, deformation and intrusives events during the late Proterozoic to early Palaeozoic period. District scale stratigraphic and structural relationships are discussed using previously published data and unpublished historical data. The distribution of copper and cobalt across the region suggests that significant regionally extensive basin and structural controls were important during the mineralisation process.
The Neoproterozoic to earliest Phanerzoic Lufilian Fold Belt (LFB) is host to the Zambian and Congolese Copperbelts (ZCB and CCB). The LFB forms part of a series of linked Pan-African orogenic belts fringing the Congo and Kaapvaal-Zimbabwe cratons of southern Africa (Fig. 2.1a) (Porada, 1989; Porada and Berhorst, 2000; Selley et al., 2005). The tectonic evolution of southern Africa has been the focus of numerous studies (e.g. Bateman, 1930; Miller, 1983; Cahen et al., 1984; Daly et al., 1984; Daly, 1986; Cosi et al., 1992; Porada and Berhorst, 2000; Hanson, 2003; Johnson et al., 2005; Selley et al., 2005). The interpretation of the tectonic evolution is still controversial, however during the past decade advances in geochronology and field based studies have provided important new information (e.g. Porada and Berhorst, 2000; Hanson, 2003; Johnson et al., 2005). A comprehensive discussion of this research is beyond the scope of this study and readers are directed to the reviews of Porada and Berhorst (2000), Hanson (2003), Johnson et al., (2005) and Selley et al.,(2005).
The Neoproterozoic sedimentary and volcanic sequences that form the supra-crustal component of the fold belts record a history of crustal extension, subsidence and intraplate magmatism between 1000 and 600 Ma that are conventionally interpreted to relate to the dispersal of the Rodinia Supercontinent ( Wilson et al., 1997; Porada and Berhorst, 2000). The term Pan-African herein will only be used for the Palaeozoic collisional event forming Gondwana and the post-orogenic magmatism, shearing and uplift.
2.2 ARCHEAN AND MESOPROTEROZOIC BASEMENT IN THE LUFILIAN FOLD BELT
The regional geology of southern Africa is subdivided into three main Proterozoic orogenic mobile belts which enclose Archean and Palaeoproterozoic crustal fragments. The development of these belts was controlled by six Archean cratonic nuclei: the Kaapvaal, Zimbabwe, Tanzania, Bangweulu, Congo and Angola-Kasai Cratons (Fig. 2.1). These stable fragments form the core of the tectonic assemblage in southern Africa and include Archean fragments which were amalgamated with Palaeo- and Mesoproterozoic fragments to form two stable
Chapter 2
regional and distriCt geology
16
CongoCraton
KalahariCraton
LufilianFoldBelt
MwembeshiShear Zone
West CongoBelt
ZambeziBelt
GariepBelt
MozambiqueBelt
Fig 2.1(B).
DamaraBelt
fold belt foreland basin
Kibar
anBel
t
Irum
ide
Belt
KalahariCraton
BangweuluBlock
1.3-1.0Ga orogen
Archaean shield
Copper deposit
2.05-1.8Ga orogen
0.56-0.53Ga felsic province
0.76-0.73Ga mafic province
Foreland basin
External Fold and Thrust Belt
Domes Region
Synclinorial Belt
Katanga High
Zambezi Belt
Basement
Central African Copperbelt
Lufilian Fold Belt
MwembeshiShear Zone
CCBA’
A
ZCB
Fig 2.5.
250
150
ZIMBABWE
ZAMBIA
DRC
(B)(A)
Mozambique Belt
Zambezi Belt
20KM
1000KM
Fig 2.4.
Figure 2.1. a). The crustal architecture of southern Africa. Simplified geology map of the Pan-African system of central and southern Africa. The Lufilian Fold Belt (LFB) is situated between the Congo and Zimbabwe-Kaapvaal Cratons to the north and south respectively. The LFB hosts the Zambian and Congloese Copperbelts (modified from Kampunzu and Cailteux, 1999; Porada and Berhorst, 2000).b). The tectonic zoning in the LFB (from Selley et al., 2005). The LFB is divided into four separate tectonic zones and the copper deposits are distributed mainly within the External Fold and Thrust Belt, the Domes region and the Synclinorial belt. Selley et al. (2005) included the basement inliers of the Zambian Copperbelt within the Domes Region, rather than the External Fold and Thrust Belt, as have previous subdivisions of the LFB (e.g. Kampunzu and Cailteux, 1999).
blocks. In the north the Congo block includes the Angola-Kasai and the Tanzania cratons and the Bangweulu block, while in the south the Kalahari block includes the Zimbabwe and Kaapvaal cratons. The assembly of Archean and Paleoproterozoic cratons during the Mesoproterozoic (1300 to 1000 Ma) formed the Rodinia supercontinent (Hoffman, 1999; Rainaud et al., 2002).
Archean rocks are not exposed in the LFB, however a ~3.1 Ga detrital zircon population in the Neoproterozoic supracrustal assemblage implies that Archean rocks either comprise part of basement or contributed material during Proterozoic evolution (Hanson, 2003). The oldest rocks (i.e. ‘basement’) of the LFB consist of Palaeoproterozoic and volumetrically subordinate Mesoproterozoic meta-granites, migmatites, meta-volcanic and meta-sedimentary units (Hanson, 2003). They are predominantly exposed in the north-western (Kibaran Belt) and south-eastern (Irumide Belt) peripheries of the LFB, and as smaller inliers between these areas, in a belt known as the Domes Region (Fig. 2.1). Basements rocks in the Domes region and Irumide Belt consist of two lithologies and temporal distinct assemblages. The oldest group of rocks known as the Lufbu Schist are widespread and part of metamorphosed Paleoproterozoic (~1994 to 1873Ma) magmatic arc sequence of sedimentary and volcanic rocks (Master et al., 2002; Mendelsohn, 1961). Unconformably overlying these rocks is the ~1300 to 1100 Ma Muva Group, a meta-sedimentary succession of conglomerates, orthoquartzites and meta-pelites (Rainaud et al., 2002).
2.3 NEOPROTEROZOIC EXTENSION
The break-up of the Rodinia supercontinent occurred between ~1000 and 700 Ma (Miller, 1983; Munyanyiwa et al., 1997; Unrug, 1997). Rifting was extensive between the Kalahari and Congo cratons and formed a series
17
of eastward younging rift basins, now distinguished as separate orogenic belts including the Gariep, Damara, Zambezi, Lufilian and Mozambique belts (Hanson et al., 1993, 1994; Unrug, 1997). This sequence of meta-sedimentary and meta-volcanic rocks is known as the Katangan Supergroup. The supracrustal sequences of the Damara-Lufilian-Zambezi belts were previously amalgamated with the Brasiliano belt of South America (Hanson et al., 1993). The LFB is a record of intracontinental rift basins containing coarse-grained terrigenous and marine units. These were deposited within fluvial and alluvial fan environments and are overlain by carbonate and evaporitic strata, indicating a restricted marine or lacustrine environment with minor volcanic rocks (Porada and Berhorst, 2000; Selley et al., 2005). The relationships and architecture of the Neoproterozoic basins, originally deposited along the trend of Damara-Lufilian-Zambezi Orogen, are subjected to ongoing debate.
The rifting history of the Zambezi belt is recorded by multiple magmatic phases spanning from ~880 to ~740 Ma (Hanson, 2003; Hargrove et al., 2003). At the western end of the rift trend line in the Damara Belt Miller (1983), Porada (1989) and Hanson (2003) suggest that the deposition of the marine sequence of carbonates and turbidites were synchronous with continental rift sedimentation in the east. The Damara Belt records a complete Late Proterozoic Wilson cycle from intracontinental rifting and the opening of oceanic basins, to the deposition of >10 km thick package of fluviatile to turbidite siliciclastics, carbonates and igneous rocks, through to collision and development of a foreland basin (Miller, 1983; Stanistreet et al. 1991; Munyanyiwa et al., 1997; Hoffman, 1999;). Volcanism accompanied extension in the Damara Belt, with the deposition of thick alkaline rhyolitic ignimbrite sequences along the northern rift margin. Continental tholeiitic and alkaline basalts were deposited higher in the sequence (Miller, 1983; Hanson, 2003). The eastern limit of the Damara belt is marked by a major transform fault inherited from pre-existing crustal weak zone, which separated the Damara and Katangan rift basins.
Magmatism in the LFB is limited to sparse bimodal volcanic rocks and mafic to intermediate igneous bodies in the middle part of the Katangan Supergroup (Kampunzu et al., 2000). Basalts, pyroclastic and metagabbroic rocks are identified higher in the sequence and restricted to the western and northern sections of the LFB (Kampunzu et al., 2000; Key et al., 2001). Armstrong et al. (1999) reported a single U-Pb zircon age of 877 ± 11Ma for an extension-related A-type granite which intruded Palaeoproterozoic basement rocks in the ZCB prior to deposition of the Katangan Sequence (Kampunzu et al., 2000; Porada and Berhorst, 2000; Tembo et al., 2000; Hanson, 2003). Within the ZCB, gabbroic and dioritic intrusions predominate (Selley et al., 2005). The mafic and intermediate lavas and intrusive complexes in the western and central parts of the LFB are associated with a relatively short-lived magmatic event dated between ~765 and ~735 Ma (Key et al., 2001; Barron, 2003) (Fig. 2.3). The geochemical evolution of the mafic units ranges from earliest continental tholeiite, to alkaline and tholeiitic magmas, and finally to lavas with E-MORB affinities, suggest progressive continental rifting which resulted in an embryonic oceanic rift in the western LFB (Kampunzu et al., 2000). The northern border of rift basin was marked by a carbonate platform with a lagoonal basin developing to the south of the platform (Porada and Berhorst, 2000).
Interestingly, there are significant differences between the Neoproterozoic Katangan Supergroup of the LFB and the Zambezi and Damaran belts. The Katangan Supergroup has a far greater metal endowment, is significantly thinner and contains limited igneous units within the basal portion of the succession (Selley et al., 2005). Selley et al. (2005) suggest that the absence of volcanism during deposition of syn-rift rocks indicates subdued crustal heat flow and low rate of crustal attenuation.
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2.4 STRATIGRAPHY OF THE KATANGAN SUPERGROUP IN THE ZAMBIAN COPPERBELT
The interpretation of the basin evolution resulting in the deposition of the rocks of the Katangan Supergroup is still controversial (e.g. Binda, 1994; Porada and Berhorst, 2000; Selley et al., 2005). The ~1.5 to 3 km thick (pre-erosional thickness of ~5 to 7 km; Annels, 1989) Neoproterozoic Katangan Supergroup in the ZCB is a package of deformed and metamorphosed sedimentary rocks unconformably overlying the basement (Fig. 2.2). North of the ZCB exists the laterally equivalent and equally extensive Congolese Copperbelt (CCB) (Table 2.1).
Up
pe
rR
oa
nG
rou
pL
ow
er
Ro
an
Gro
up
Kit
we
Fm
.
CopperbeltOrebody Member
Ba
se
me
nt
Mw
ash
iaG
rou
pL
ow
er
Ku
nd
elu
ng
uG
rou
p
500m
Min
dola
Cla
stic
sFm
.
877+/-11Ma
~740Ma
Emplacementof mafic lavasand intrusivesin central andwestern DomesRegion ~765 Ma- 735 Ma
main ore-bearinginterval
GrandConglomerate
basement gneiss
Nchanga red granite
conglomerate
sandstone
mixed sandstone - siltstone-carbonate
gritty siltstone
mixed carbonate - siliciclastic
carbonate
breccia
diamictite
siltstone-shale
intrusive gabbro
eva
pori
tic
deposi
tion
alenvir
on
men
ts
Figure 2.2. Lithostratigraphy of the Katangan Supergroup, Zambian Copperbelt showing approximate average unit thickness (from Selley et al., 2005). The Nkana-Mindola Deposit (NKM) is hosted at the base of the Kitwe Formation. This chapter provides a description of the stratigraphy and basin architecture of the Lower Roan Group at the NKM deposit. The Katangan Supergroup is divided into a five-fold subdivision which includes (1) Lower Roan Group (siliciclastic rock dominated succession); (2) Upper Roan Group (platformal carbonates, chaotic breccia and subordinate siliciclastics rocks); (3) Mwashia Group (carbonates and generally fine grained siliciclastics rocks); (4) Lower Kundulungu Group (glacial diamictite overlain by carbonate and carbonate-bearing clastic rocks); and (5) Upper Kundulungu Group (basal diamictite overlain by carbonate). Gabbroic rocks mainly occur in Upper Roan and Mwashia Groups. The maximum age of sedimentation is constrained by the Nchanga Red Granite and the upper sedimentation is constrained by the Sturtian glaciation (Grand conglomerate diamictites), and mafic lavas and intrusive rocks in the Domes Region (from Selley et al., 2005).
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Group Formation Member Clemmey (1976)
Nkana-Mindola
(mine termi-nology)
Chambishi Nchanga (Binda and Mulgrew,
1974)
Konkola Musoshi (Lefe-bvre, 1989)
Lubembe(Lefebvre,
1989)
Kundelungu(former Upper Kun-delungu)
Plateau (Ku 3)
Klubo (Ku 2)
Kalule Ku 1.3
Ku 1.2
Ku 1.1
Nguba(former-Lower Kun-delungu)
Monwezi Ng 1.2
Likasi Ng 1.3
Kakontwe (Ng 1.2)
Grand Conglom-erate (Ng 1.1)
Mwashia Upper Mwashia Mwashia Upper Mwashia
Middle Upper Roan
Lower Ultra Far Water Fm.
Lower Mwashia Mines Group (R.A.T.)
Upper Roan Bancroft Fm. Dolomite Fm. Upper Roan Kanwangungu Fm.
Kibalonfo Fm. Musoshi Fm.Lower Roan Kitwe For-
mationAntelope Antelope
Clastic Mbr.Far Water Sediments
Sandy talc schist
Shale with grit
Shale with grit
Kibalongo Fm.
Chambishi Chambishi Dolomite Mbr.
Far water DolomitesDolomite-Argillite Seq.
Cherty Dolo-miteInterbedded Argillite and Dolomite
Chingola DolomiteDolomitic schistUpper Banded shale
Hangingwall Aquifer
Chingola For-mation
Kitotwe Mbr.Kabemba Mbr.
Nchanga Nchanga quartzite Mbr.
Upper quartzite
Upper quartzite Feldspathic Quartzite
Hangingwall Quartzite
Pelitic-Arkosic Formation
Rokana Rokana Evaporites Member
Near Water Sediments
Interbedded Argillite and Dolomite
Banded Sand-stoneUpper Pink QuartziteShale markerBanded Sand-stoneLower chart marker
Copperbelt Copperbelt orebody Member
Ore Shale and Hang-ingwall Argillite
Ore Shale Lower Banded Shale
Ore Shale Ore Shale F.Q.
Mindola Kafue Kafue Aren-ites
Footwall Conglom-erate; Arkose and argillite; Lower Conglom-erate
Footwall Con-glomerate; Arkose and argillite; Lower Conglomerate
Transition ArkoseArkoseConglomerate
Footwall conglom-erateFootwall SandstonePorous sandstone
Mutonda formationKafufya Forma-tionChimfunsi Formation
Simbi
Lubembe
Konkola Basal Quartz-ite
Footwall Quartz-ite; Basal Conglom-erate
Footwall Quartzite; Basal Conglom-erate
Footwall QuartzitePebble conglom-erateBasal conglom-erate
Table 2.1. Stratigraphic nomenclature of the Katangan Supergroup on the Zambian Copperbelt and relationship between nomenclature used at different deposits. The Mindola Clastic Formation and the Kitwe Formations are the focus of this study at the NKM deposit (modified from Cailteux et al., 2005; Selley et al., 2005; Batumike et al., 2007).
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900 Ma 800 Ma 700 Ma 600 Ma 500 Ma
877+/-11Ma
Lufilian Orogeny
Maxi
mum
age o
f Kata
ngan s
edim
enta
tion
Stu
rtia
n G
laci
ation ~
740 M
a
T1 T2 T3 T4 T5
Nchanga Red Granite
12
16
7
1
7
54,6
3 2
4
8,10,119
9
12
1212
9
916
14,15,1613
1,16
ZambianCopperbelt
U-Pb uraninite
U-Pb monazite
U-Pb rutile
Pb-Pb Cu sulfide
Re-Os molybdenite
Re-Os Cu-Co sulfide
(B)
1
2 3 45
6
7
89
10
11
12
1314
15
16
11°
27°
Congolese Copperbelt
m
Do es Region
DRC
ZAMBIA
(A)
Kamoto
Monwezi
Mindigi
Shinkolobwe
Kambove
Luisha
Luiswishi
Kawanga
Kansanshi
Kimale
Dumbwa
Musoshi
Konkola
Nchanga
Chibuluma West
Nkana-Mindola
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Basement
Katangan Sgpand youngerstrata
Hook Massif
The interpretation of basin evolution and deposition of the Katangan Supergroup remains controversial (e.g. Binda, 1994; Porada and Berhosrt, 2000; Selley et al., 2005). Stratigraphic correlation between ZCB and CCB is a controversial issue based on the stratigraphic sub-division of the Katangan Supergroup. Within the ZCB, the Katangan Supergroup is preserved in several predominantly NW trending structural “basins” (a series of west-northwest- to north-northwest- trending synclines). These are separated by, or in some cases are entirely enclosed by, reworked Palaeoproterozoic crystalline basement including Mushi, Bwana-Ndola, Roan-Muliashi, Chambishi-Nkana, Nchanga and the Luwuishi Basins (Mendelsohn, 1961; Selley et al., 2005) (Fig. 2.4). However a regionally robust five-fold stratigraphic division has been defined for the Katangan Supergroup
Figure 2.3. Summary of geochronological data associated with Cu and U mineralization in the Lufilian Fold Belt (modified from Selley et al., 2005). This figure shows the relationship of mineralizing events to significant stages of basin development. The location of the samples shown in insert A. Thermal events include: T1 (765-735 Ma) extension-related magmatism (Key et al., 2001), T2 (614-570 Ma) eclogite facies metamorphism in the Zambezi belt (John et al., 2003) and greenschist facies metamorphism in the Zambian Copperbelt (Rainaud et al., 2002), T3 (577-532 Ma) felsic magmatism in the Katanga High (Hanson et al., 1993), T4 (534-521 Ma) whiteschist facies metamorphism in the central part of the Domes Region (John et al., 2004), T5 (510-460 Ma) post-collisional uplift and cooling throughout the Domes Region (Cosi et al., 1992; Rainaud et al., 2002; John et al., 2004).
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(e.g. Mendelsohn, 1961; Clemmey, 1976; Annels, 1984; Cailteux, 1994; Kampunzu and Cailteux, 1999; Porada and Berhorst, 2000, Selley et al., 2005). At the deposit scale, a range of differing stratigraphic nomenclature exists for each Cu deposit on the ZCB (Table 2.1).
The major lithostratigraphic units, from base to top are the (Fig. 2.2):• Lower Roan Group (siliciclastic-carbonate package); • Upper Roan Group (platform carbonates, siliciclastics and chaotic breccias); • Mwashia Group (carbonates and generally fine-grained siliciclastics);• Nguba Group (formerly the Lower Kundelungu - glacial diamictite, carbonates and minor siliciclastic); and• Kundelungu Group (basal diamictite overlain by mixed carbonate and clastic rocks) is poorly defined in the
Democratic Republic of Congo (DRC) and only partially preserved, due to erosion in the ZCB.The Katangan Supergroup preserved in Zambia has previously been described by Jordaan (1961),
Clemmey (1976) and Binda (1994) and is summarised in the next section, including observations from this study.
2.4.1 Lower Roan GroupThe Lower Roan Group is subdivided into the basal Mindola Clastic Formation (MCF), consisting mainly of arenaceous strata, and the overlying siltstone-dolomite-shales of the Kitwe Formation (KF) (Clemmey, 1976) (Fig. 2.2).
2.4.2 Mindola Clastic Formation (MCF)The MCF is characterised by significant lateral and vertical facies variations involving texturally immature breccias, conglomerates and sub-arkosic sandstones, deposited in fluvial, alluvial fan, eolian and fan-delta environments. The MCF at NKM has an average thickness of 200-300 m and ranges from being absent in some areas to a maximum of 1 km at Konkola, northern ZCB. The MCF accumulated within sub-basins of limited strike extent and bounded by basement-cored topographic highs. The fault controlled origin of these basins is evidenced by their systematic west-northwest to north-northwest orientations, and the local preservation of cross-section half-graben cross-section geometries and the inverted axes of sub-basins coincide with synformal closures (Selley et al., 2005). Clemmey (1976) sub-divided the MCF into 2 members; however the classification is difficult to apply consistently, particularly where the MCF is deformed. At NKM, the Basal Sandstone Member (BSM) is distinguished from the overlying Kafue Arenite Member (KAM) by a widespread 5 to 15 m thick conglomeratic unit (Fig. 2.5).
2.4.3 Kitwe Formation (KF)The Kitwe Formation contrasts with the MCF and displays a well defined internal layer cake stratigraphic architecture. The Kitwe Formation consists of an approximately 200 m thick sequence of sandstone, marginal marine-evaporitic argillaceous sandstone, dolomitic siltstone-sandstone and massive dolomite. The formation occurs within a 125 km long and 13–25 km wide west-northwest trending belt, frequently referred to as the ‘Shale-belt’ (Fig. 2.6) and has been sub-divided in to five members (Fig. 2.5) (Binda and Mulgrew, 1974; Binda, 1994). The eastern edge of the ‘Shale Belt’ is poorly defined because lithostratigraphic equivalents exist on the eastern side of the Kafue Anticline (Binda, 1994). Binda and Mulgrew (1974) and Porada and Berhorst (2000) indicate that the western margin is defined by the pinch-out of the Copper Orebody Member (COM) with gabbro, dolomitic talc-schist and brecciated dolomites directly overlying the MCF. The MCF and KF occurring at NKM are the focus of detailed descriptions and discussion in Chapter 3.
The basal COM is the principal host to Cu-(Co) mineralisation (Clemmey, 1976). This unit is commonly referred to as the ‘Ore Shale’ west of the Kafue Anticline, and east of the Kafue Anticline, in the vicinity of
22
KatangaHigh
SynclinorialBelt
DomesRegion
External Fold &Thrust Belt
A A’
100km BasementKatangan Supergroup
upper
lower & middle
undifferentiated
overriding platePan-Africanage granite
overthrust plate
the Mufulira deposit, as the ‘Mudseam’ (Binda, 1994). Directly overlying the COM is the marginal marine sandstone-siltstone of the Rokana Evaporite Member (REM) (Clemmey, 1978). Higher in the sequence are the Nchanga Quartzite (NQM) and Chambishi Dolomite (Fig. 2.5 and 2.7).
2.4.4 Upper Roan GroupThe contact between the Lower Roan Group and the Upper Roan Group is poorly defined however it is historically distinguished by the predominance of carbonate strata (Mendelsohn, 1961) (Fig. 2.2). Selley et al. (2005) define the base of the Upper Roan Group by the appearance of >1m thick, regional extensive dolomite beds. The Upper Roan Group consists of laterally extensive decimetre to metre scale, upward fining cycles of sandstone, siltstone, dolomite, algal dolomite and patches of anhydrite. The preserved thickness of the sequence is variable, ranging from ~30 m to >800 m (Selley et al., 2005).
The Upper Roan Group at NKM consists of ~300 m thick sequence of interbedded argillites, dolomitic argillites, dolomites, dolomitic sandstone and argillaceous dolomites. All units are laterally continuous across the NKM area, however, no studies of the Upper Roan Group were undertaken as part of this research due to the poor nature of the exposures.
The Upper Roan Group may contain stratabound and discordant breccia units. These range from centimetres to hundreds of metres in scale, and appear to have accommodated much of the thickness variations in the Upper Roan Group (Wendroff, 2000; 2003; Selley et al., 2005). The breccias are composed of intraformational fragments within a matrix of carbonate, albite, quartz, anhydrite and/or chlorite (Annels, 1984). Selley et al. (2005) report that these breccias cross cut down stratigraphy to the south and west in the Mufulira, Konkola, Luanshya Basin and Chambishi Basin areas. The breccia bodies had developed along former evaporitic horizons, similar to the breccias occurring in the DRC described by Francois (1973) and Jackson et al (2003), however, Wendroff (1997, 2003) suggested the breccia formed as a molasse deposit.
2.4.5 Mwashia GroupThe Mwashia Group is a shale dominated sequence conformably overlying the Upper Roan Group (Fig. 2.2). Unlike the underlying Roan Group, relatively few Cu-Co deposits have been identified within the Mwashia Group rocks and consequently there are few published studies of the Mwashia Group and the overlying Kundelungu Group. Within the ZCB, Selley et al. (2005) describe the approximately 400 m thick Mwashia Group as consisting of a lower dolomite package overlain by a dolomite-siltstone-mudstone sequence capped
Figure 2.4. Schematic cross section (section A-A’) of the Lufilian fold belt showing the variation in the structural style between the tectonic zones (modified from Porada, 1989; Selley et al., 2005).
23
basement gneiss
Nchanga granite
conglomerate
sandstone
mixed sandstone - siltstone - carbonate
gritty siltstone
siltstone - shale
mixed carbonate - siliciclastic
carbonate
breccia
Min
do
laC
lasti
cF
orm
ati
on
Basement Complex
Kit
we
Fo
rma
tio
n
Bancroft DolomiteFormation
mainore-
bearinginterval
Kafue Arenites Member
Copperbelt Orebody Member(see associated diagram)
Rokana Evaporites Member
Nchanga Quartzite Member
Chambishi Dolomites Member
Antelope Clastics Member
Main
stra
tigra
ph
icpack
age
un
der
invest
igati
on
Stratigraphic Nomenclature(Modified from Clemmey, 1976)
Basal Sandstone Member(new terminology)
Basal Conglomerate/Breccia
Basal Quartzite
Basal Sandstone
Lower Conglomerate
Footwall Sandstone
Footwall Conglomerate
Mine Terminology atNkana-Mindola
Basement
(See associated diagram)
Far Water Formation
Far Water Sediments
Near Water sediments
Up
pe
rR
oa
nG
rou
p
Hangingwall Quartzite
Upper Quartzite
Ultra Far Water Sediments
10m
Lo
we
rR
oa
nG
rou
p
Ore Shale
Lo
we
rR
oa
nG
rou
pU
pp
er
Ro
an
Gro
up
Undulating unconformitysurface
by a siltstone-mudstone-carbonaceous mudstone sequence (Fig. 2.7). The base of the Mwashia Group is defined by the polylithic breccia in the DRC and the upper portion of the group is dominated by a clastic sequence (Cailteux, 1994). As with the Upper Roan Group, no exposures of the Mwashia Group were examined during this study at NKM.
2.4.6 Nguba and Kundelungu GroupsThe base of the Nguba Group (Ng 1.1 of Nguba - Cailteux and Kampunzu, 2002) is marked by the ~10–600 m thick Grand Conglomerate (Fig. 2.2). This unit is a regionally extensive sequence of debris flows and
Figure 2.5. Detailed lithostratigraphy sub-division of the Lower Roan Group at Nkana-Mindola. The local mine terminology is included for reference. The focus of this study is on the Mindola Clastic Formation and the basal portion of the Kitwe Formation, which hosts the majority of economic copper mineralisation.
24
diamictites, intercalated with minor, thin interbeds of siltstone and sandstone (Binda and Van Eden, 1972). Classical interpretations suggest the group is a chronostratigraphic equivalent of the oldest globally recognized Neoproterozoic Snowball Earth glaciation event and is correlated with the Sturtian diamictites deposited at ~740Ma (Hoffman, 1999). In the Democratic Republic of Congo (DRC), the Grand Conglomerate is up to 600m thick and has a broad temporal association with ~760 to 750 Ma mafic extensional igneous activity including gabbroic sills and mafic volcanic flows and tuffs (Armstrong et al., 1999; Key et al., 2001; Rainaud et al., 2002). Limited data is available for the Kundulungu Group at NKM. An intersection of black to grey limestones in the Mindola Central Dam Wall has been assigned to the Kakontwe Limestone Formation and is overlain by purple to grey-black argillites. The series occupies the synclinal hinge of the Nkana Syncline in the northern and central areas at NKM.
2.4.7 Intrusive Rocks Mafic and ultramafic intrusive rocks of the Upper Roan are of continental tholeiitic, alkaline and EMORB basaltic composition and comprise a minor proportion of the Katangan sequence. These intrusive bodies are in close spatially association with breccia units The bodies are generally discontinuous and are typically strongly altered, suggesting they were emplaced as sills, however, Porada and Berhorst (2000) suggest the bodies have been tectonically emplaced. Variations in the composition of the mafic rocks record different stages of continental break-up, from pre-continental rift to a continental rift system and then to an oceanic rift system (Kampunzu et al., 2000).
There are few documented intrusive rocks within the Nkana-Mindola area. Meta-gabbroic rocks have been identified east of the Mindola pit near the Mindola Dam and intrude the Upper Roan, Mwashia and Lower Kundelungu Groups (Mendelsohn, 1961). These rocks are medium to coarse-grained, dark-grey to greenish in colour with a high biotite composition (Jordaan 1961) and contain slivers of metasediments. The surface expression of the intrusive bodies broadly approximates the shape of the Nkana Syncline. Previous surface mapping of the NKM mining lease identified several gabbroic bodies. Outcrop pattern suggests the gabbroic rocks have been folded. According to Whyte and Green (1971), syenitic to gabbroic rocks at the Chibuluma Deposit have been extensively altered and metamorphosed to scapolitized amphibolitic rocks.
The most significant intrusive rock at Mindola are lamprophyre dykes. At approximately 2000 N at the Mindola Shaft, a 10m to 30m wide east southeast steeply dipping fine-grained, biotite rich dyke cross-cuts basement schists and the Lower Roan Group rocks. On the upper levels at Mindola Shaft the same dyke dips west-north-west at shallow angles (Jordaan, 1961). Jordaan (1961) classified the lamprophyre dyke as a kersantite, based on the petrographic and chemical analyses of eight samples. The dyke has poikilitic plagioclase laths set in a fine ground mass of quartz, mica and epidote. Biotite and chlorite grains impart an overall weak schistosity to the rock. Accessory minerals include skeleton crystals and spiral form ilmenite. The margins of the dyke are diffuse and the contact with the basement schist is difficult to recognise due to thermal metamorphism of the wallrock and the interfingering of dyke rock with argillaceous rocks (Jordaan, 1961). Minor copper sulphides, in the form of bornite and chalcocite, are associated with the margins of dyke when it cross cuts through basement lithologies and commonly where it is interfingered with the argillites of the COM. Other cross cutting dykes in the ZCB occur at River Lode, Nchanga and at the North Orebody Konkola (Mendelshon, 1961).
2.4.8 Correlation with the Congolese Copperbelt (CCB)To north of the ZCB exists the lateral equivalent and equally extensive CCB. Stratigraphic correlation between the CCB and ZCB are based on the stratigraphic sub-division of the Katangan Supergroup. Correlation of the
25
Basement
Lower Roan Group
Upper Roan and Mwashia Groups
Gabbro
Lower and UpperKundelungu Groups
Shale BeltM
okambo D
ome
KonkolaDome
ChililabombweDome
Kafue Anticline
Chambishi Basin
Luanshya Basin
28°
13°
Kitwe
Chingola
Ndola
20KM
B
B’
upper Mwashia and Kundelungu Groups have been documented (e.g. Binda, 1994; Cailteux et al., 1994; Porada and Berhorst, 2000), however correlation of the underlying Upper Roan and Lower Roan Groups between the CCB and ZCB remains partially unresolved. The most widely accepted correlation suggests the Kitwe Formation of the Lower Roan Group is equivalent to the Congolese Mines Subgroup (Table 2.1) (Cailteux et al., 1994; Binda and Porada, 1995; Kampunzu and Cailteux, 1999). The ‘Roches Argilo-Talqueuses’ (R.A.T) is the oldest unit and correlates to the siliciclastic rocks of the Lower Roan Group in the ZCB, however, tectonic displacement between the Mines Subgroup and R.A.T. generally blurs the relationship between the units. Porada and Berhorst (2000) suggest that many of the units are laterally equivalent facies stacked by northeast directed thrusting.
Figure 2.6. The distribution of the ‘Ore Shale’ belt on the Zambian Copperbelt (modified from Selley et al., 2005). See figure 2.7b for stratigraphic correlation along section line B-B’.
26
diamictite
siltstone and shale
polylithic and crackle breccia
sandstone dominant
carbonate dominant
200
m
A A’
Lower and Upper Kundelungu groups
Roan & Mwashia groups
basement
Upper Roan Gp
Mwashia Gp
LowerKundelungu Gp
Kitwe
Mufulira
A
A’
KLB
145
KW
24
KW
26
KW
22
LB
18
L62
L79
L80
MW
107
DH
218
DH
219
IT27
IT25
Konkola Nchanga Chambishi Nkana Luanshya Mufulira
Kafu
eA
nti
clin
e
Upper
Roan
Carb
on
ate
Kit
we
Form
ati
on
Low
er
Roan
Min
dola
Cla
stic
Base
men
t
75m
NW SE
Upper Roan
Roan AntelopeMember
Chambishi DolomiteMember
Nchanga QuarziteMember
Rokana EvaporitesMember
Shale
Copperbelt OrebodyMemberFootwall arenite andarkose
Footwall conglomerate
Mava Schist & quartzite
Basement granite
Basement LufubuSchist
Unconformity
B’B
B’
B
A
B.
27
2.5 PAN-AFRICAN OROGENESIS – DEFORMATION OF THE LUFILIAN FOLD BELT (LFB)
The Lufilian Fold Belt (LFB) is a north verging fold-thrust belt which formed during the closure of the Neoproterozoic basins. The LFB is defined on the northern margin by relatively undeformed uppermost Katangan Supergroup while the southern margin is marked by the sinistral, east-northeast trending Mwembeshi shear zone. Movement in the shear zone has resulted in the present juxtaposition of low-grade LFB rocks to high-grade rocks of the Zambezi Belt (Unrug, 1989). Recent research specifically documents the deformational events associated with the Pan-African orogenesis. The Pan-African orogenesis is manifested as major thrusting, backfolding and backthrusting in response to convergent tectonics (e.g. Daly et al., 1984; Daly, 1986; Unrug, 1987; Kampunzu and Cailteux, 1999; Hanson, 2003). Despite numerous significant studies (e.g. Daly, 1986; Unrug, 1989; Kampunzu and Cailteux, 1999; Porada and Berhorst; 2000; Hanson, 2003), the tectonic evolution remains unresolved and Table 2.2 is a summary of several different tectonic models for the LFB.
The Lufilian orogenesis is thought to span ~100 m.y. The oldest metamorphic ages of greenschist facies rocks in the ZCB are U-Pb monazite (592 ± 22 Ma) and Ar-Ar biotite (585.8 ± 0.8 Ma) (Rainaud et al., 2002). Hanson et al. (1993) constrained the main phase of orogenesis to between ~560 and ~530 Ma using U-Pb zircon dating of syn- to post-tectonic granites and rhyolites in the Katangan high. John et al. (2004) report U-Pb monazite ages of ~530 Ma for peak metamorphism for white schist facies rocks in the central and western Domes region. Postorogeneic cooling is recorded by the widespread 510 to 465 Ma Ar-Ar biotite, Rb-Sr muscovite ages (Cosi et al., 1992; Torrealday et al., 2000; Rainaud et al., 2002; John et al., 2004) (Fig. 2.3).
The LFB consists of four north verging tectonic domains. De Swardt and Drysdall (1964) suggested the LFB could be divided into four orogenic zones - the ‘External Fold and Thrust Belt’, the ‘Domes region’ the ‘Synclinorial’ belt and the ‘Katangan High’ (Fig 2.1b and 2.5). The ZCB occurs adjacent to the easternmost basement inlier of the Domes region while the CCB occurs within the External Fold and Thrust belt. The boundary between the two coincides with an abrupt southward increase in metamorphic grade and structural style (Ramsay and Rigdeway, 1977; Francois and Cailteux, 1981; Key et al., 2001; Selley et al., 2005).
Rocks in the ‘Domes region’ were metamorphosed at upper greenschist to amphibolite metamorphic grade. Upper greenschist facies rocks occur in the eastern part of the region, where high amplitude folding dominates the structural style (Daly, 1986). Basement involved deformation throughout the Domes Region implies thick-skinned style of deformation and the position of the basement inliers to reflect structural culminates developed above ramps that splayed off a deep crustal detachment (Daly et al., 1984; Daly, 1986). In the western and central Domes areas, Cosi et al. (1992) and Key et al. (2001) provide evidence of large-scale thrusting by the existence of basement-cored nappes emplaced within the Katangan strata and juxtaposition of units of different metamorphic grade.
OPPOSITE: Figure 2.7. a). Stratigraphic correlation of the Upper Roan Group on the eastern flank of the Kafue Anticline (modified from Selley et al., 2005). b). Stratigraphic correlation of the Lower Roan Group of the Katangan Supergroup on the western flank of the Kafue Anticline, including a comparison with the Mufulira deposit on the eastern flank (modified from Binda and Mulgrew, 1974; Binda, 1994).
28
De Swardt and Drysdall (1964) defined the External Fold and Thrust Belt portion of the LFB as a foreland facing outer zone and record fragmentation, repetition and inversion of the Katangan stratigraphy (Fig. 2.8). There is little evidence of basement involvement in the exposed structural pile suggesting a more thin-skinned structural style compared to the Domes Region to the south (Porada and Berhorst, 2000; Selley et al., 2005). Decoupling along evaporitic strata, positioned in the middle Katangan stratigraphy, accounts for the lack of basement involvement in this portion of the LFB (Porada and Berhorst, 2000; Jackson et al., 2003; Selley et al., 2005). The western portion of the LFB has complex structures that trend obliquely to the regional folds and thrusts. The Roan and Kundelungu sequences in this area are allochthonous and Jackson et al. (2003) proposed salt tectonics as the dominant mechanism for the formation of Roan gigabreccias, which are characteristic of the External Fold and Thrust belt.
The relationship of the Domes Region to the Synclinorial Belt is uncertain. One model suggests a major change in the basin architecture was coincident with thrust dislocation between the Domes region and Synclinorial belt (e.g. Cosi et al., 1992; Porada and Berhorst, 2000). This boundary is interpreted as an abrupt break on a southward attenuating passive margin (Porada and Berhorst, 2000). To the north, a succession of marginal marine platformal-lagoonal rocks were deposited, while on the southern side of the break, deeper water facies dominated, however, Selley et al. (2005) suggested there is no evidence for a deeper marine sequence in the Synclinorial belt.
2.6 METAMORPHISM
Detailed accounts of the metamorphic facies are presented in Ramsay and Ridgway (1977), Francois and Cailteux (1981), Cosi et al. (1992) and Tembo (1994). The Proterozoic to early Palaeozoic rocks of the LFB vary from low-grade to high-grade metamorphic mineral assemblages, in part reflecting the complex tectonic development of the region. Ramsay and Ridgway (1977) recognised two non-parallel metamorphic belts. Recrystallization of the rocks is extensive with the main metamorphic minerals observed being biotite and sericite, and lesser amounts of scapolite, tourmaline, chlorite, tremolite-actinolite, epidote and apatite. The Lufilian metamorphic belt curves along the southern margin of the broader scale LFB and the Luangwa-Kariba metamorphic belt covers the eastern and south-eastern region of the Zambia. Ramsay & Ridgeway (1977) concluded that all the rocks younger than the basement complex in the LFB were metamorphosed in a single metamorphic cycle of Pan-African age. The metamorphic isograds parallel the broad structural framework, and the degree of metamorphism increases from the outer zones of the External Fold and Thrust Belt and into the Domes region. Prehnite-pumpellyite facies assemblages are recorded in the outer areas of the External Fold and Thrust Belt, shifting to greenschist facies metamorphism in the inner portions of the External Fold and Thrust Belt and further grading into amphibolite facies in the Domes Region (Kampunzu et al., 2000). Whiteschist and high pressure eclogites occur to the south of the Domes Region (Cosi et al., 1992). The lowest grade of metamorphism on the ZCB occurs at Konkola and Mulfulira, and increases to upper greenschist to amphibolite facies towards the southeast. Within the Roan-Muliashi “basin” epidote-amphibolite facies grade is reached, while carbonate rich rocks at Nkana consist of tremolite and talc (Mendelsohn, 1961).
2.7 Cu-Co MINERALISATION OF THE ZCB
The major deposits in the ZCB are distinctly aligned parallel to the Kafue Anticline (Fig. 2.9) and have been interpreted as indicating the presence of a deep structural feature (e.g. Annels, 1989) or a palaeoshoreline (e.g. Clemmey, 1976). Two broad classes of sediment-hosted Cu deposits occur within the ZCB (e.g. Fleischer, 1976;
29
Up
pe
rR
oa
nG
rou
pL
ow
er
Ro
an
Gro
up
Kitw
e
Fm
.
ba
se
me
nt
Mw
ash
iaG
rou
pL
ow
er
Ku
nd
elu
ng
uG
rou
p
Copperbelt Orebody Member (and equivalents)
50
0m
Min
do
la
Cla
stics
Fm
.
Min
do
la
Nk
an
a
Ch
an
bis
hiS
E
Ch
an
bis
hi
Ch
ibu
lum
a
Ch
ibu
lum
aW
est
Ch
ibu
lum
aS
ou
th
Mw
am
ba
sh
iB
Mw
am
ba
sh
iA
Pita
nd
a
Sa
mb
a
Fitu
la
Mim
bu
la
Ch
ing
ola
A&
C
Ch
ing
ola
B
Ch
ing
ola
D
Ch
ing
ola
E
Ch
ing
ola
F
Nch
an
ga
Ko
nko
laN
ort
h
Ba
lub
a
Lu
an
sh
ya
Bw
an
aM
ku
bw
a
Nd
ola
We
st
Mu
fulir
a
Lu
an
so
be
Lu
be
mb
e
Lo
nsh
i
Fro
ntie
r
?
Mu
so
sh
i
Chambishi
Basin
Luanshya
Basin
Nchanga-
Chingola
district
Konkola
district
Eastern
Kafue
Anticline
"ha
ng
ing
wa
ll"d
ep
osits
"fo
otw
all"
de
po
sits
carbonate/breccia-hosted
diamictite/carbonate-hosted
argillite-hosted
arenite-hosted
mixed argillite-arenite
basement-hosted
Cu Co Mineralization±
granite, gneiss, schist
conglomerate
sandstone
sandstone-siltstone-carbonate
gritty siltstone
mixed carbonate-siliciclastic
carbonate
breccia
diamictite
siltstone-shale
Figure 2.8. Two broad categories of the sedimentary-hosted copper mineralisation on the ZCB can be defined. Mineralisation is either the ‘arenite hosted’ or ‘argillite hosted’ style, however within a single deposit both styles of mineralisation can occur (modified from Selley et al. 2005). The NKM deposit is primarily an ‘argillite’ hosted deposit, however mineralisation transgresses the contact between the Mindola Clastic Formation and the Copperbelt Orebody Member.
30
Selley et al., 2005): the “arenite-hosted” mineralised systems and the classical “argillite-hosted” deposits (Fig. 2.8) and mineralisation commonly is not confined to one specific stratigraphic horizon. However, the argillite-hosted deposits are generally confined to the lower portions of the COM (e.g. Nkana –Mindola and Nchanga orebodies) (Fig. 2.8), while the arenite-hosted systems are mainly recognised within the MCF (e.g. Chibuluma, Chibuluma West, Mwambashi B, and Chingola B). Argillite hosted Cu mineralisation also occurs at the higher levels of the Kitwe Formation and within the Mwashia and lower Kundelungu Groups. Table 2.3 summarises the key features of the major deposits of the ZCB.
The major orebodies are preserved in synforms within basement inliers (Unrug, 1989). These structures have been interpreted as former basins which controlled the deposition of the rocks of the Lower Roan Group and influenced the localisation of sulphide mineralisation (Mendelsohn, 1961; Selley et al. 2005). Cu mineralisation transgresses stratigraphy, however, each orebody has a grossly stratabound geometry. Within a single deposit several types of mineralisation maybe recognised, including disseminated, pre-folding vein-hosted, post-folding vein hosted, shear-zone-hosted and oxidation-supergene mineralisation. Significant vein hosted Cu mineralisation also occurs in the western ZCB at the Kansanshi Deposit, which is hosted at a higher level in the stratigraphic succession (Broughton et al. 2002). In addition to Cu mineralisation hosted within the Katangan Supergroup, significant disseminated Cu mineralisation occurs in basement lithologies (e.g. Lumwana, Samba deposits).
12
76
4
5
3
8
9
10
11
13
12
14
15
16
1718
20
21
22
23
24
25
26
2930
31
32
27
28
19
Basement
Lower Roan Group
Upper Roan and Mwashia Groups
Gabbro
Lower and UpperKundelungu Groups
Recent Discovery
Shale Belt
Cu-(Co) Deposit(surface projection)
Mokam
bo Dom
e
KonkolaDome
ChililabombweDome
Kafue Anticline
Chambishi Basin
Luanshya Basin
28°
13°
Kitwe
Chingola
Ndola
20KM
Figure 2.9. Geological map of the Zambian Copperbelt and the location of the major Cu-Co deposits hosted by the Katangan Supergroup (modified from Jordaan, 1961; Fleischer et al., 1976; Selley et al., 2005). 1 Luanshya, 2 Roan Extension, 3 Baluba, 4 Lufubu South, 5 Chibuluma South, 6 Chibuluma West, 7 Chibuluma, 8 Nkana-Mindola, 9 Chambishi SE,10 Chambishi, 11 Pitanda, 12 Mwambashi A,13 Mwambashi B, 14 Samba, 15 Fitula, 16 Mimbula, 17 Chingola A-F, 18 Nchanga, 19 Fitwaola,20 Konkola, 21 Konkola North, 22 Musoshi, 23 Lubembe, 24 Luansobe, 25 Kasaria, 26 Mufulira,27 Frontier (Lufua), 28 Mwekera, 29 Ndola West, 30 Itawa, 31 Bwana Mkubwa, 32 Lonshi,33 Mokambo.. (Sourced from Darnley (1960), Mendelsohn (1961), Annels (1984), Fleischer (1984), Sweeney and Binda (1989) and Selley et al., 2005).
31
International lithostrati graphic subdivision and orogenic cycle
Interpretations from Francois (1993); Cahen et al. (1984)
Interpretation from Kampunzu and Cailteux (1999) and Hanson (2003)
Event and Age Regional effect Event and Age Regional EffectPaleozoic Transverse folding -
~503 MaTransverse undulations to the main trend of the Lufilian arc
Chilatembo Late transverse folding
Neo
Pro
tero
zoic
(Lufi
lian
Oro
geny
)
Monwezian~ 602 Ma
E-W faulting
Monwezian
Strik-slip and escape bloack tectonicsLateral extrusion with cumula-tive displacement ~ 130kmClockwise rotation of crustal blocks and related development of convex structure of the Lufil-ian arc.
Kundelunguian~ 656 Ma
Folds with axial planes vertical or dipping N in the external folds of the Lufil-ian Arc
EpeirogenesisAge ?
Uplift in and near the Kundelungu Plateau
Kolwezian
Northward fold and thrust tec-tonics present day orientation of the Lufilian arc: E-W trend in the western sector and NW-SE in the eastern part.Major vergence to the N, back folding to S
Kolwezian656 Ma
Folds with axial planes dip-ping S. Nappes displaced several tens of km from S to N in southern DRC.
Lusakan FoldingLomamian Orogeny
Table 2.2. Summary of the recognized key structural events in Zambia during the Lufilian Orogeny (~600Ma to 500 Ma).
Supergene and oxidation ore minerals commonly overprint hypogene sulphides in the near surface environment. At NKM, a mixed oxide-sulphide assemblages occur within 100m of the surface Jordaan (1961). Malachite and chalcocite have been recorded at depths of > 600m in the Nchanga Lower Orebody and >1km at the Konkola North Deposit (McKinnon and Smit, 1961; Pollington and Bull, 2002).
2.7.1 Argillite-hosted Cu depositsThe argillite-hosted Cu deposits of the COM vary lithologically from dolomitic siltstones, siltstones and minor sandstones (e.g. Mindola and Roan Antelope) to black, carbonaceous shale orebodies (e.g. Nkana South). Unmineralised intervals occur at sites where the facies changes to either a massive arenaceous or carbonate facies (e.g. Annels, 1984).
2.7.2 Arenite-hosted Cu depositsThe arenite-hosted deposits account for approximately 30% of known Cu-Co mineralisation while argillite-hosted deposits comprise the remainder. The small high grade footwall arenite-hosted mineralisation occurs within condensed sections of the Mindola Clastic Formation such as at Mwambashi B (Selley and Bull, 2002; Selley et al. 2005) (Fig. 2.10). However, arenite-hosted Cu orebodies situated stratigraphically above the COM also occurs (e.g. Nchanga-Chingola, Mufulira) (McGowan et al., 2003; Selley et al., 2005). The mineralogical composition of the arenite-type deposit is complex and predominantly controlled by variations in feldspars, mica and organic carbon. The full spectrum of the two lithological end-members, hosting the different styles of mineralisation, can occur within a single copper deposit, forming a complex, transgressive mineralised system.
32
2.7.3 Chambishi ‘Basin’The Kafue Anticline dominates the regional structure in the central and southern corner of the ZCB (Fig. 2.11). To the east is the Mufulira Syncline while on the western flanks are the Chambishi and Roan-Mulashi Basins (synclines). Within the Chambishi Basin, the metasedimentary rocks of the Katangan Supergroup are enclosed by basement granites, the Lufubu Schists and the Muva metasediments, and intrusive, sill-like gabbroic bodies. Existing geological maps of the Chambishi Basin were used to define macroscopic structural domains. The southeastern corner of the basin is dominated by the NW striking Nkana Syncline, while in the northern area of the ‘basin’ WNW trending folds are common. The fold patterns exhibit evidence of ‘inheritance’ from pre-existing basement structures, including partitioning of strain, nucleation of folds parallel to inverted rift margins, and deflected orientations of the Lower Roan-Basement contact above inverted growth faults (Croaker and Selley, 2003; Selley et al., 2003, 2005). In the northern and western areas of the basin, the broad WNW striking synclines are separated by fault bounded basement inliers.
2.8 Cu-Co DEPOSITS OF THE CHAMBISHI ‘BASIN’
The Chambishi Basin has several significant copper deposits hosted within arenite and argillite sequences and comprises a significant proportion of the total copper resource on the ZCB. The Chambishi Basin is host to six significant Cu deposits with the largest being the Nkana-Mindola Mine. Four significant deposits occur
100 m
Copperbelt Orebody Mbr
sub-arkose
conglomerate
carbonaceous shale
argillaceous sandstone and siltstone
talus breccia
granitebasement granite
middle Kitwe Fm.
Upper Roan Gp
Mindola
Clastics Fm.
dolomite, sandstone and gabbro
W E
mineralized interval
MW 5 BN7 BN26 BN10 BN14
BN24
Figure 2.10. The stratigraphic-structural position of the Cu mineralization at the ‘arenite’ hosted Mwambashi prospect. Cu mineralization at the Mwambashi prospect is hosted in the MCF. There is a close relationship between basement geometry and the distribution of mineralization at both deposits (modified from Selley and Bull, 2003).
33
i eR v r
RIV
ER
bMwam
ashi
KAFUE
KITWE
CHAMBISHI WEST
PITANDA
MWAMBASHIB
CHIBULUMA WEST
CHIBULUMA EAST
CHAMBISHI SOUTHEAST PROSPECT
CHAMBISHI MAIN
CHIBULUMA SOUTH
MINDOLA
CENTRAL
SOUTH OREBODY
Lower and Middle Kundelungu
Basal Kundelungu - Kakontwe Limestone and Basal Tillite
Mwashia Group
Upper Roan Group
Lower Roan Group
Muva
Lufubu
Mines and known Ore Deposits
Granite
Gabbro
10KM
������
MWAMBASHI A
Lusaka
������
Figure 2.11. Geological map of the Chambishi basin. The NKM deposit is situated in the south eastern corner of the Chambishi Basin (modified from Garlick, 1961 and Jordaan, 1961).
34
DEP
OSI
TEs
timat
ed
Rese
rves
-Re
sour
ces
(~ 1
989
Mt
Gra
de~
Cu
%,
~ C
o %
~ A
g g/
t
Dep
osit
Type
Min
eral
isat
ion
Hos
t Li
thol
ogy
Foot
wal
lLi
thol
ogy
Alte
ratio
n-Ev
apor
ites
Zona
tion
Refe
renc
es
Konk
ola
500
4 %
Cu
2.7
g/t
AgAr
gilli
teSt
rata
boun
dBn
, Cpy
, Cc,
Py
and
Ag
Lam
inat
ed s
iltst
one
and
carb
onat
eH
emat
ite a
reni
tes
and
sand
ston
e, c
ongl
omer
ate
Dol
omite
and
anh
y-dr
ite p
seud
omor
phs,
K-
feld
spar
Brou
ghto
n, 2
003
Kirk
ham
, 198
9Sw
eene
y an
d Bi
nda,
198
4
Nch
anga
550
3.8
% C
u2.
7 g/
t Ag
Argi
llite
and
ar
enite
Stra
tabo
und
Bn, C
py, C
c, P
y an
d Ag
Silts
tone
; Fe
ldsp
athi
c qu
artz
iteSa
ndst
one,
qua
rtzi
tes
and
shal
esK-
fleds
par
and
seric
teCp
y+Bn
>Cp
y>Py
Voet
and
Fre
eman
, 197
6Ki
rkha
m, 1
989
Mck
inno
n an
d Sm
it, 1
961
McG
owan
et
al. 2
003
Muf
ulira
300
3 %
Cu
2.7
g/t
AgAr
enite
Stra
tabo
und
Bn, C
py, C
c, P
y an
d Ag
Feld
spat
hic
sand
ston
e an
d si
ltsto
neCo
nglo
mer
ate,
qua
rtzi
tes
and
sand
ston
e-si
ltsto
neAn
hydr
ite, K
-fel
dspa
r an
d se
ricite
Cc-B
n-Cp
yKi
rkha
m, 1
989
Scot
t, 2
003
Bran
dt e
t al
., 19
61
Cham
bish
i10
02.
7 %
Cu
15 g
/t A
gAr
gilli
teSt
rata
boun
dBn
, Cpy
, Cc,
Py
and
Ag
Biot
itic
argi
llite
, do
lom
iteAr
kose
s to
con
glom
erat
eAn
hydr
ite, K
-fled
spar
Cc>
Bn>
Cpy>
PyKi
rkha
m, 1
989
Anne
ls, 1
989
Cham
bish
i So
uthe
ast
502.
4 %
Cu
Argi
llite
Stra
tabo
und
Bn, C
py, C
c, P
yCa
rbon
aceo
us
Shal
e an
d ar
gilli
teQ
uart
zite
s, c
ongl
omer
ate,
sa
ndst
one-
silts
tone
Anhy
drite
, K-fl
edps
ar
and
dolo
mite
Kirk
ham
, 198
9An
nels
, 198
9Bu
ll an
d Se
lley,
200
3
Chib
ulum
a So
uth
124.
3 %
Cu
Aren
iteSt
rata
boun
dBn
, Cpy
, Cc,
Py
Seric
itic
sand
ston
es
and
quar
tzite
sSe
riciti
c ar
enite
s an
d qu
artz
ites
Seric
ite, a
lbite
Cc>
Bn>
Cpy>
PyKi
rkha
m, 1
989
Win
field
, 196
1
Chib
ulum
a Ea
st &
Wes
t25
4.9
% C
u0.
21 %
Co
2.7
g/t
Ag
Aren
iteSt
rata
boun
dBn
, Cpy
, Cc,
Py
Seric
itic
sand
ston
esSe
riciti
c ar
enite
s an
d qu
artz
ites
Seric
ite, a
lbite
Cc>
Bn>
Cpy>
PyKi
rkha
m, 1
989
Selle
y et
al,
2002
Nka
na65
02.
8 %
Cu
0.17
% C
oAr
gilli
teSt
rata
boun
dBn
, Cpy
, Cc,
Py
Dol
omite
, dol
omiti
c ar
gilli
te a
nd c
arbo
na-
ceou
s sh
ale
Cong
lom
erat
e, a
rkos
es,
feld
spat
hic
sand
ston
eAn
hydr
ite, d
olom
ite,
K-fe
ldsp
ar, a
lbite
Bn-C
py>
Ca>
Cpy
>Ca
>Py
Kirk
ham
, 198
9Jo
rdaa
n, 1
961
Balu
ba90
2.5
% C
u0.
15 %
Co
Argi
llite
Stra
tabo
und
Bn, C
py, C
c, P
yCa
rbon
aceo
us a
rgill
ite
and
shal
eCo
nglo
mer
ate,
are
nite
s an
d ar
gilli
teSe
ricite
Cc>
Bn>
Cpy>
Py>
CcKi
rkha
m, 1
989
Anne
ls e
t al
. 198
3Si
mm
onds
, 198
3
Luan
shya
230
2.7
% C
u2.
7 g/
tAr
gilli
teSt
rata
boun
dBn
, Cpy
, Cc,
Py
Carb
onac
eous
arg
illite
an
d sh
ale
Cong
lom
erat
e, q
uart
zite
an
d ar
gilli
tes
Py>
Cpy>
Bn>
CcKi
rkha
m, 1
989
Men
dels
ohn,
196
1
Lum
wan
a12
000.
7 %
Cu
0.05
5 C
oBa
sem
ent?
Stra
tabo
und
Bn, C
py, C
c, P
yG
ensi
s an
d sc
hist
Schi
st a
nd q
uart
zite
Seric
ite, q
uart
z,
phlo
gopi
teKi
rkha
m, 1
989;
Ber
nau,
20
07
Kins
ansh
i30
01.
17 %
Co
0.17
g/t
Au
Vein
hos
ted
Kund
elun
guD
isco
ncor
dant
&
con
cord
ant
Cpy,
Py
Schi
st a
nd c
arbo
nate
ve
ins
Schi
st a
nd q
uart
zite
Albi
teBr
ougt
on e
t al
. 200
3To
rrel
day
et a
l. 20
02
Mw
amba
shi
BAr
enite
Stra
tbou
ndCp
y-Bn
Sand
ston
eCo
nglo
mer
ate,
san
dsto
ne-
silts
tone
K-fe
ldsp
arSe
lley
et a
l. 20
03
Tabl
e 2.
3. T
he k
ey g
eolo
gica
l and
min
eral
isat
ion
char
acte
ristic
s of
the
rec
ogni
zed
copp
er d
epos
its o
ccur
ring
in t
he Z
ambi
an C
oppe
rbel
t.
35
within the northern sector of the Chambishi Basin (Fig. 2.11), however mining activity has only taken place at the Chambishi Deposit. Copper mineralisation at the Chambishi and Chambishi SE deposits is hosted within littoral argillite-siltstone facies of the COM. A small, high-grade Cu deposit is hosted within the footwall and separated from the COM by a barren argillaceous quartzite unit (Annels, 1984; Bull and Selley, 2003). Further west at the Pitanda and Mwambashi ‘B’ prospects and the Chibuluma deposit, copper mineralisation occurs within the arenite rocks of the MCF.
2.8.1 Chambishi DepositThe Chambishi Deposit is described in detail by Annels (1974, 1984) and Binda and Mulgrew, (1974). The deposit consists of two mineralised horizons separated by a low-grade zone (<1% Cu) directly overlying basement. The Ore Shale itself is a laminated biotitic argillite and calcareous argillite varying between 15 and 30 m thick and is coincident with changes in depth-to-basement. The Ore Shale contains carbonate-anhydrite-sulphide lenticles and changes from the argillite facies to slightly more arenaceous or carbonate facies over the basement highs. Most economic mineralization is found in the lower half of the Ore Shale, and locally within the upper footwall conglomerates and sandstones of the MCF. The orebodies pinch-out down dip corresponds to a change in the gradient of the footwall thickness. Chalcopyrite is the dominant Cu sulphide with minor bornite. Pyrite and pyrrhotite are typical co-existing gangue phases. Co mineralisation is unevenly distributed within the Cu orebody and present as Co-pyrite and carrollite.
2.8.2 Chambishi SE ProspectThe Chambishi SE prospect is a lateral continuation of the Chambishi Deposit. The two deposits are separated by a non-mineralised carbonate facies in the stratigraphic equivalent position to the mineralised host sequence. The position of the massive carbonate facies coincides with a basement high and it has been demonstrated that mineralisation occurs on the fringes of the basement high (Annels, 1984; Garlick, 1961).
2.8.3 Mwambashi B ProspectThe Mwambashi B arenite-hosted Cu prospect is located on the western margin of the Chambishi Basin (Fig. 2.9), northward of the WNW-trending Ore Shale pinch zone with low levels of mineralisation occurring within the upper MCF. Recent studies by Selley et al. (2002) suggested that disseminated chalcopyrite is grossly stratabound and generally partitioned into clean and heavy-mineral bearing arenaceous units and mineralisation is directly overlain by argillaceous rocks. The highest grades of mineralisation are restricted to more condensed intervals of MCF with a sharp well defined hangingwall being the COM. Pyrite is disseminated throughout the COM and minor chalcopyrite and bornite maybe present. The distribution of mineralisation is strongly controlled basin geometry, being localised to smaller, restricted sub-basins (Fig. 2.10) (Selley et al., 2002)
2.8.4 Chibuluma DepositTowards the SW margins of the Chambishi Basin, the high grade Cu and Co Chibuluma Deposit is hosted in the Basal Sandstone Member (BSM) of the MCF, immediately below by the KAM (Fig. 2.11). The Chibuluma orebodies are lower in the MCF sedimentary sequence when compared to Mwambashi B, however, the ore is hosted in small restricted basins similar to Mwambashi B. Garlick (1961), Whyte and Green (1971), Selley and Bull (2001, 2002) and Selley and Cooke (2001, 2002) have conducted detailed studies on this deposit. Copper mineralisation is mainly as bornite and chalcopyrite and the Cu orebody is grossly stratabound being confined to the margins of intrabasinal highs where strata pinch out against the basement highs. Mineralisation is partitioned into uppermost portions of the clean arkosic sandstones, while Co mineralisation is confined poorly sorted sub arkosic units consisting predominately of pyrite.
36
2.8.5 Nkana-MindolaThe NKM deposit is located in the SE corner of the Chambishi Basin and is predominately hosted in the lower portion of the COM (Fig. 2.11). The Chibuluma Mine lies on the south-west limb of the Nkana syncline and the Chambishi SE prospect is situated to the northwest of the NKM mine, along the eastern side of the Kafue Anticline (Fig. 2.11). The following section will briefly introduce the geological setting of the NKM deposit. Detailed surface mapping by ZCCM company geologists from 1950 to 1990 provides sufficient data on outcrop pattern and stratigraphic units occurring at NKM.
Nkana-Mindola mining area is situated on the eastern limb of the Kafue Anticline (Fig. 2.11). The mining area is dominated by the north-west plunging, asymmetric Nkana syncline with curved axial planes inclined steeply to the northwest in the hinge zone, but upright to westward dipping on higher structural levels (Fig. 2.12). The NKM deposit and dips at ~30o to the west in the northwestern most portion of the deposit, while in the southeastern corner of the deposit it is a complexly deformed orebody in the hinge zone area of the Nkana Syncline. The axis of the Nkana Syncline has an undulatory profile, with localised steepening and shallowing of fold plunges. Subtle changes in the fold geometry result from interference between northwest and west northwest fold geometries. Within the hinge zone of the Nkana Syncline folding is tight to isoclinal, while to the northwest the fold becomes more open and asymmetrical.
The NKM deposit is an argillite-hosted, primarily copper sulphide orebody consisting of chalcopyrite and bornite hosted in the upper portion of the MCF and the basal portion of the COM of the Kitwe Formation. Known economic mineralisation is confined to the northeast limb and the hinge zone area of the Nkana Syncline. It has a surface strike length of ~15 km and extends to a depth of ~1500 m beneath the current land surface. Sub-economic copper mineralization at the level of the COM is continuous for a further 17 km on the western limb of the Nkana Syncline (Fig. 2.12). ‘Barren Gaps’ truncate the mineralised system into two discrete zones; the northern dolomite-argillite hosted and the southern carbonaceous-carbonate argillite hosted areas (Fig. 2.13) Minor secondary oxide ore bodies of malachite, azurite, chalcocite and native copper occur at surface.
Basement Complex at the Nkana-Mindola DepositThe first classification of the basement was made by Gray (1932) with the division of the Lufubu System, now commonly referred to as the Lufubu Schist from the younger Muva System. The Lufubu rocks are gneissic to schistose in texture. At NKM the Lufubu rocks are quartz-biotite schists with minor interbedded sugary grey micaceous quartzites, graphitic schists, phyllites, semipelitic schist and migmatitic to banded gneisses. All rocks have a prominent northeast-southwest striking foliation. The granitic bodies occurring at NKM are interpreted as direct age and correlate with the felsic Nchanga Red Granite (877 ± 11 Ma; Armstrong et al., 1999). They are unconformably overlain by the basal Katangan strata (Garlick and Brummer, 1951; Armstrong et al., 1999; Master et al., 2002). At the SOB Shaft, granitic bodies have previously been described in association with ‘palaeohills’ (Mendelson, 1961). The granitic rocks (e.g. 1250 L SOB Shaft) are grey to pink with holocrystalline to porphyritic quartz-biotite. The edges of the intrusive bodies on the 1250L at SOB trend towards chlorite-biotite-sericite schist, which is related to post-intrusion shearing along the contact between the Lufubu Schist and the intrusive bodies.
Boundinaged quartz veins, confined to the Lufubu Schists, are parallel to the prominent foliation. Vein compositions are varied, with differing amounts of quartz, pink euhedral microcline, sericite, anhydrite and calcite. Minor ilmenite and tourmaline appear to have developed by metamorphic segregation. No visible sulphides have been identified in any of these veins. The contact between rocks of the basement and rocks of the Lower Roan Group is an unconformity, however considerable shearing and increased strain is focused at the this contact and in some areas the contact is faulted.. High strain zones within the gneisses are identified
37
MindolaPit
Todo
Nla
CITYOF
KITWE
MINDOLADAM
2KM
26 0
00m
E
34 000mN
26 000mN
42 000mN
30 000mN
38 000mN
30 0
00m
E
34 0
00m
E
38 0
00m
E
Basement Complex
Mindola Clastic Formation
Kitwe Formation
Upper Roan Group
Mwashia Group
Kundelungu Group
LowerRoanGroup
Mindola North Shaft(500L, 610L, 780L, 1050L)
Mindola Shaft(4180L, 4440L, 5510L)
Central Shaft(2320L, 2370L, 3130L)
SOB Shaft(790L, 1250L, 1810L, 2370L, 2880L, 3140L, 3360L)
Figure 2.12. Geological map of the Chambishi-Nkana area. The NKM deposit occurs in the northwesterly plunging Nkana Syncline. All economic copper mineralization occurs along the north eastern limb of the Nkana Syncline.
by the development of mica-boarded lenticles and separated by schistose layers consisting of biotite, sericite, chlorite, polycrystalline quartz and minor quartz. No outcrops of Muva Quartzite have been recognised at NKM, however to the south-west of NKM, mapped rocks of the Muva Quartzite exhibit fold axes parallel to the west-northwest striking folds within the overlying Katangan Supergroup (Mendelsohn, 1961).
38
IchimpeBarren Gap
KitweBarren
Gap
?
?
dolomitic siltstone facies carbonaceous facies
bn
No.3 shaftBarren Gap
Nkana Syncline
No. 4 shaftBarren Gap
bn
bn + cpy
cpy
cpy + py
bn
+cp
y
South OrebodyBarren Gap
foot
wall c
onta
ct
info
ldhi
nge
Kundelungu Gp
Mwashia Gp
Upper Roan
Kitwe Fm.
Mindola Clastics Fm.
Basement
arenaceous
dolomitic
Barren Gaps
2000 m
500
m
shaft
N
Gp
Figure 2.13. Plane of ore projection, modified from Jordaan (1961), and district scale map of the NKM deposit, showing broad down-dip and lateral (southward) sulfide mineral zonation from bornite (bn), to bornite and chalcopyrite (cpy), to chalcopyrite and pyrite (py). Orebodies are separated by arenaceous and dolomitic barren gaps. Dolomitic siltstone facies of the Copperbelt Orebody Member occurs only in the region of sediment input points (arenaceous barren gaps). In each case, barren gaps coincide with strike changes in the basement-Katangan Supergroup contact. These are most pronounced at the northern end of the system. The Kitwe barren gap overlies a subtle basement high and projects towards a major inflection affecting the trace of the Nkana Syncline. Coincidence of lithofacies variation and perturbations in fold geometry reflects inheritance of sub-basin geometry in Lufilian strain patterns (modified from Jordaan, 1961 and Selley et al., 2005).
2.8.6 BrecciasApproximately 1 km west of the Mindola Pit and waste dumps, surface mapping by NCCM geologists between 1960 and the mid-1980s identified the Mwashia and Kundelungu Groups in the hinge zone of the Nkana Syncline (Fig. 2.14). Importantly this mapping has identified an irregular, though recognisable synformal outcrop pattern of a breccia unit at the base of the Mwashia Group. Within this breccia unit gabbro, dolomite, sandstone, shale and skarn units have been identified during the mapping programme. No outcrop or drillcore of this portion of the stratigraphy was observed during the course of this study.
It is interpreted that this breccia is similar to the units that have been recognised at the Chambishi SE prospect to the NW and near the Chibuluma Mine on the western side of the Chambishi Basin (Fig. 2.15) (Annels, 1984; Binda, 1994; Selley and Bull, 2003; Wendroff, 2003). The breccia unit near the Chibuluma Mine is interpreted as cross-cutting down stratigraphy by Binda (1994) and coinciding with the pinch out of the COM. These units have been described as so-called ‘hydridised rock’ consisting of gabbro bodies and angular clasts of shale, sandstone and dolomite within a matrix of albite-dolomite cemented microbreccia (Annels, 1984; Tembo, 1994).
39
Figure 2.15. The geometry and relationship of breccia and gabbroic units at the Chibuluma Deposit (from Annels, 1984; 1989).
e
o
Bas
of Kitw
eFr
mation
c e pe I h m
Ba r n pr e Ga
NORTH SHAFT
MINDOLA PIT
Ga obbr
iaSalt brecc
S rka n
Ba m nse e taniteGr owe R an Gr upL r o o
Upper Roan Group
Mwashia Group
1 kmc
na Qu
Nha
g
artzite
ebe
Mm
r
Third orderscale F2 folds
To Ndola
CITYOF
KITWE
2KM
26 0
00m
E
34 000mN
26 000mN
30 000mN
38 000mN
30 0
00m
E
34 0
00m
E
38 0
00m
E
Mindola Shaft
S
l
Nkana ync ine
Basement Complex
Mindola Clastic Formation
Kitwe Formation
Upper Roan Group
Mwashia Group
Kundelungu Group
LowerRoanGroup
SOB Shaft
Mindola North Shaft
Central Shaft
42 000mN
NkanaPit
MINDOLADAM
Mining Licence Area
Stoped Out Area
MindolaPit
AREA OF ENLARGEMENT
26
0m
00
E
Mindola dam
Golf Club dambo
i
open t
Mndola
pi
NORTH SHAFT
MINDOLA SHAFT
Anticlinal Structure - basement high?
2 km
Ichempe Barren Gap
Bottom of Mwashia Group
Nkana Syncline
Bottom of Mwashia Group
aul
Zo
Ft
ne ?
Figure 2.14. Several significant breccia units have been previously mapped at NKM. They occur towards the top of the Upper Roan and appear from analysis of historic map datasets to be semi-conformable to stratigraphy. These units were not observed during this study and maps compiled from historic data at Mopani Copper Mines. The broader distribution of the gabbro rocks elsewhere in the Chambishi Basin is shown in Figure 2.11.
CHAMBISHI MINE
SSW NNE
AMPHIBOLITESILL
BASEMENT GRANITE
ORE-SHALE
ChertyDolomite
Hybrid Rocks/Breccias
Mine Levels×100m×100metres
0 2 4
2
4
6
8
10