Acercamientos metodológicos al campo universitario

The Lumwana deposits consist of unmineralized quartz-feldspar ± phlogopite basement gneiss to Cu±Co mineralized quartz-phlogopite-muscovite-kyanite- sulphide schist. Transitional contacts are observed that result from the increased removal of feldspar associated with increased fabric development and deformation. Although the host rocks to the Lumwana copper sulphide mineralization have been correlated with the Lower Roan by previous authors (Benham et al. 1976; McGregor 1965), the observation of progressive

alteration from hanging wall to ore schist suggests that this is untenable.

6.2.1 Comparison of the Lumwana deposits with pre-Katangan basement

mineralization of the Zambian Copperbelt

Minor mineralized basement is widespread in the Kafue Anticline but has been subject to little scientific analysis. Whilst commonly of low grade when compared to the mineralization of the Zambian Copperbelt, higher grade mineralization has been identified associated with basement shear zones (Mendelsohn 1961; Pienaar 1961; Wakefield 1978).

Mineralogical and textural similarities are apparent between the Lumwana deposits and the basement mineralization of the Kafue Anticline. The Fimpimpa mineralization is vein hosted but occurs within a basement shear zone and feldspathized quartz diorite. Garnetiferous quartz-mica schist and biotite gneiss host the 8 metre wide shear zone that can be traced for more than a kilometre. The shear zone is sporadically mineralized with malachite, chalcopyrite and bornite with the entire width containing less than 0.4% copper (Pienaar 1961). The Lumwana deposits are also hosted by shear zones indicated by c-s fabrics, mylonites and porphyroblast rotation.

The Samba deposit of the Lufubu Metamorphic Complex (Rainaud et al.

2005) is the largest concentration of copper mineralization in the pre- Katangan basement of the Kafue Anticline. The sulphide mineralization of the Samba deposit is similar to Lumwana, consisting of disseminations, stringers and veinlets of pyrite, chalcopyrite and bornite in deformed quartz-sericite

CHAPTER 6: DISCUSSION

Textural and mineralogical similarities are apparent between the Lumwana and Samba deposits, including the observations that sulphides are aligned with the foliation and that bornite and pyrite have an antithetic relationship (Wakefield 1978) (observed at Chimiwungo). The deformed rocks at Samba are typically L-S tectonites that are also prevalent in the quartzites at Lumwana.

Wakefield (1978) also describes a pervasive sericitization of plagioclase at the Samba prospect that increases towards the mineralized zone. The degree of sericitization correlates with the intensity of the D1 deformation. However,

local zones of undeformed and totally sericitized rock indicate that alteration predates the D1 deformation. Although the Samba deposit has been subject to

greenschist PT conditions, the early sericitization is highly analogous to the alteration observed at Lumwana. This is responsible for the transition from basement gneiss to ore schist.

The origin of the protolith to the mineralization is ambiguous at Lumwana with transitional contacts from unmineralized quartz-feldspar ± phlogopite basement gneiss to Cu±Co mineralized quartz-phlogopite-muscovite-kyanite- sulphide schist. The transitional contacts and structural controls on mineralization have led to the hypothesis that these deposits represent metasomatically altered, mineralized and sheared basement. This challenges the interpretation that the ore schist horizons correlate with units of the Lower Roan, representing mineralized neo-Proterozoic sediments with amphibolite grade metamorphism (Benham et al. 1976, McGregor 1965).

6.2.2 Migration of hydrothermal fluids

At Lumwana the ore schist has undergone 3 dominant phases of alteration, an early potassic stage that results in the formation of phlogopite and the destruction of feldspar. The alteration envelops the ore and increases in intensity into the ore zone with increased fabric development and shearing. The transitional contacts observed at Lumwana are the result of an alteration event associated with mineralization that removed feldspar from ore horizons resulting in depleted Na and Ca and relatively higher Al components.

CHAPTER 6: DISCUSSION

However, the host rocks to the Lumwana deposits have undergone intense wall-rock alteration with various styles of mineralization resulting from the changing chemistry of the fluids and rocks before, during and after the Lufilian Orogeny. Unfortunately, K-Ar and Rb-Sr dating of mica provide a cooling age of 500 Ma, corresponding to when the temperature dropped below 350-400°C rather than the timing of the alteration (Cosi et al. 1992).

In the Zambian Copperbelt the host rocks to the mineralization exhibit various styles of alteration although the timing and origin of the alteration is controversial. Three main styles of alteration are observed including calcium- magnesium, potassic and sodic alteration (Mendelsohn 1961; Fleischer et al.

1976; Sweeney et al. 1991; Binda 1995).

6.2.3 Deposition of copper and cobalt

The syngenetic “Copperbelt Style” origin of the mineralization proposed by previous authors (McGregor 1965; Benham et al. 1976) is not possible as the

mineralization is hosted by units of the Mwombezhi Dome basement.

Sulphides are deformed by the S1 fabric and overprinted by kyanite which

formed at peak metamorphism. This indicates that copper was introduced to the basement either syn or pre-peak metamorphism. S2 metamorphism with

associated quartz-muscovite alteration has remobilized sulphides into low strain micro-structural dilatant sites including pressure shadows around porphyroblasts.

Several different styles of mineralization can commonly be observed in the Copperbelt that either overlap or form spatially distinct zones and include disseminated, pre-folding vein-hosted, post-folding vein-hosted, shear zone hosted and oxidation-supergene mineralization (Selley et al. 2005). The

primary Copperbelt mineralization is overprinted with secondary chalcocite, copper carbonate, sulphate, cuprite and native copper (after Selley et al.

2005) (table 2.2). A progressive change from primary sulphide to secondary sulphide assemblages commonly occurs within 30 to 70 metres of the surface

CHAPTER 6: DISCUSSION

depths of more than 1km at Konkola and greater than 600 metres at Nchanga (McKinnon and Smit 1961). Post folding vein-hosted mineralization includes chalcopyrite, accessory bornite, pitchblende, brannerite and molybdenite. The uranium mineralization is typically sub economic in the Zambian Copperbelt with the exception of the Mindola uranium orebody and typically occurs in the late veins and as local disseminations (Selley et al. 2005).

6.2.4 Deposition of uranium

Cosi et al. (1992) propose a uranium deposit model where the oxidising

conditions of the Lower Roan Group hematite rich quartzite results in uraninite mineralization, whereas the reducing sulphide ore schist horizons are mineralized with disseminated brannerite. Textural relationships indicate that reduced brannerite pre-dates the remobilized sulphide and that the uranium bearing fluids left large amounts of Cl and CO2 (Cosi et al. 1992). Only vein

hosted uranium within the ore schist horizons were observed as part of this study.

6.2.5 Metamorphic remobilization of sulphide

Textures of remobilized sulphide minerals are controlled by the fabric development, typically forming coarse grains elongate within the S2 quartz-

muscovite fabric. It is unlikely that copper was introduced during the muscovite event as areas of intense quartz-muscovite alteration are commonly unmineralized. However, where quartz-muscovite alteration overprints the S1 fabric, copper sulphides have been remobilised into pressure

shadows around porphyroblasts, and into low strain dilatant sites.

Sulphides also locally replace silicate phases (phlogopite, muscovite, quartz and feldspar) and are crosscut by large muscovite and chlorite laths indicating that any metamorphic remobilisation of sulphides had finished before the late muscovite retrogressive event.