Maltrato físico directo

The general structure of the Mwombezhi Dome has been interpreted by previous authors as either complex recumbent folding of Katangan sediments overlying older basement (Mulela and Seifert 1998) or as an allochthonous nappe complex composed of both basement and cover (Cosi et al. 1992) (figure 2.17 and figure 2.3).

McGregor (1965) recognised one pre-Lufilian and three Lufilian deformation events whereas Equinox geologists have recognised a total of 5 Lufilian deformation events (Table 3.1). No pre-Lufilian structures have been identified in the basement gneiss units of the Mwombezhi Dome.

CHAPTER 3: GEOLOGY AND MINERALIZATION OF

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Event Timing Features

D1 Early

Lufilian

N-S compression resulted in nappe formation, pervasive layer parallel foliation (S1) and D1 shear zones with stretching lineation L1

D2 Mid

Lufilian

E-W compression produced recumbent isoclinal folds with localised fold hinge schistosity (S2). This event was also responsible for D2

shear zones and the thrusted tectono-stratigraphy

D3 Mid

Lufilian

NW-SE compression was responsible for listric thin skin thrusting and folding of the Katangan meta-sedimentary cover rocks. D3 deformation

was also responsible for the retrograde metamorphism and the final juxtaposition of the tectono-stratigraphy

D4 Late

Lufilian

NNW-SSE compression produced conical upright open folds that folded the L1 stretching lineation. D4 compression was responsible for

the development of a local crenulation and fracture cleavage and for the formation of the regional dome antiforms e.g. Mwombezhi Dome

D5 Post

Lufilian

Relaxation and post compression unroofing was responsible for NW- SE & N-S steep brittle normal faults and late felsic anorogenic stocks

Table 3.1 Structural geology of the Mwombezhi Dome (after unpublished Equinox Report)

North - south early Lufilian compression (D1) in the Mwombezhi Dome

produced extensive thrusting, nappe formation, and a pervasive layer parallel foliation (S1), which generally obliterates bedding. Thrusts trend in a north –

south direction at Malundwe but swing to the east south of Lumwana (McGregor 1965).

F1 fold axial planes were orientated at 160°/ 15° SW and fold hinge lines

plunge at 11°/212° (McGregor 1965). D1 shear zones are host to an intense

stretching lineation L1 (figure 3.3). The stretching lineation (L1) plunges from

6° towards 180°, which is the approximate dip of the bedding and slightly shallower than the S1 schistosity. Locally quartz veins form north-south

orientated rods.

During the mid-Lufilian, ongoing north - south Lufilian compression was resolved into local east – west compression (D2) producing recumbent

isoclinal folds with localised fold hinge schistosity (S2). F2 fold axial planes

were overturned and orientated at 170°/44°W. F2 fold hinge lines plunge at

12°/192° with limbs exhibiting an average dihedral angle of 36° (McGregor 1965). This event is also responsible for D2 shear zones and the thrusted

CHAPTER 3: GEOLOGY AND MINERALIZATION OF

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Figure 3.3 Photograph facing west. North-south mineral stretching lineation (L1) in banded

gneiss unit (371791E 8646340N)

Mid-Lufilian NW-SE compression (D3) is responsible for listric thin skin

thrusting and folding of the Katangan meta-sedimentary cover rocks. D3

deformation is also responsible for the retrograde metamorphism and the final juxtaposition of the tectono-stratigraphy of the basement complex of the Mwombezhi Dome.

Late Lufilian compression rotates from northwest – southeast to north northwest – south southeast (D4) producing local crenulation and fracture

cleavages. D4 also produced conical upright open folds (F4) and folded the L1

stretching lineation and was the last major ductile deformation event of Lufilian Orogeny. This low pressure deformation event has little associated new mineral growth and was responsible for the formation of the regional dome antiforms e.g. Mwombezhi Dome. F4 fold axial planes are frequently

curved orientated between 40° to 120° with a shallow dip to the southwest (figure 3.4) (McGregor 1965).

Post-Lufilian relaxation and post compression unroofing was responsible for NW-SE & N-S steep brittle normal faults and late felsic anorogenic stocks.

CHAPTER 3: GEOLOGY AND MINERALIZATION OF

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Figure 3.4 Photograph facing north-west. Complex sheath folds in quartzite demonstrating poly-phase deformation (371294E 8653091N)

In the Mwombezhi dome area, the deformation events D1 & D2 produced the

regional nappe, with recumbent folding and eastward directed thrusting, due to north – south regional compression being resolved locally into east – west compression. This was interpreted by McGregor (1965) as the result of the central location of the Mwombezhi dome within the Lufilian Arc (McGregor 1965).

The granite basement and Lower Roan Quartzite acted as one competent unit during the deformation phases with a décollement developing at the Lower Roan quartzite - Upper Roan carbonate contact. This situation is also observed in both the Copperbelt and Kabompo Dome (McGregor 1965).

A structural transect across the Malundwe deposit from hanging wall to footwall as well as numerous field excursions validated previous structural data collected by McGregor (1965) and Equinox (figure 3.5 & appendix A4).

CHAPTER 3: GEOLOGY AND MINERALIZATION OF

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Figure 3.5 Stereographic projections of structural data and rose diagram. A) Malundwe S1

poles to planes (n=858) B) Chimiwungo S1 poles to planes (n=632) and C) rose diagram of

mineral stretching lineations L1 recorded from the Mwombezhi Dome (n=206)

Kinematic indicators can be observed in diamond drill core in the host rocks to the mineralization. Shear sense can only be determined in orientated core and where orientated core was examined, kinematic indicators consistently suggest a north-south displacement, typically with top to the north deformation. Examples of kinematic indicators were examined in orientated diamond drill core from Eq-Chi-065 (appendix A2: 377475E 8640150N) including pressure shadows on porphyroblasts, C/S fabrics and asymmetrical extensional shear bands.

From 208.29 to 208.44 meters, Eq-Chi-065 comprises a muscovite- phlogopite-quartz-kyanite-sulphide schist with a 5 cm thick intervening unit with garnet porphyroblast grains. C/S fabrics are observed with sulphides distributed along both C and S planes and garnet porphyroblast grains exhibit sinistral quartz-muscovite-sulphide pressure shadows. In any C/S fabric, S planes represent a penetratively developed strain-sensitive flattening fabric whereas C planes are discrete narrow shear zones which are taken to lie parallel to the flow plane of the progressive deformation (figure 3.6 A) (Hanmer and Passchier 1991).

In addition to garnet, pressure shadows on kyanite porphyroblasts also provide kinematic indicators in the Lumwana deposits and are observed at 233.63 meters in Eq-Chi-065. Poikilolitic kyanite porphyroblast grains in muscovite-phlogopite-quartz-kyanite-sulphide schist exhibit sinistral quartz- muscovite-phlogopite pressure shadows (figure 3.6 B).

CHAPTER 3: GEOLOGY AND MINERALIZATION OF

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B A

c

Figure 3.6 Schematic diagrams demonstrating A) C/S fabrics. Cisaillement (C) refers to the shear planes whereas Schistosité (S) refers to the cleavage planes B) Pressure shadows around rigid inclusions C) Asymmetrical extensional shear bands (after Hanmer and Passchier 1991)

Pressure shadows are volumes of material deposited at the contact between the stiff inclusion and its less competent matrix by precipitation from solution. Material may be transported to the site of precipitation by diffuse and/or advective mass transfer (Hanmer and Passchier 1991). Asymmetrical extensional shear bands are common in both mineralized and unmineralized schists at Lumwana and are locally associated with retrograde metamorphic reactions with garnet porphyroblast grains altering to chlorite (figure 3.6 C).

C/S fabrics can be difficult to distinguish from asymmetrical extensional shear bands as geometrically; a mylonitic foliation cut by asymmetrical extensional shear bands bears some resemblance to a C/S fabric (Hanmer and Passchier 1991). All kinematic indicators in Eq-Chi-065 suggest a top to the north deformation direction and are representative of both the Chimiwungo and the Malundwe deposits.