Geology, mineralogy, geochemistry and alteration patterns of Banana zone copper-silver mineralisation , Ghanzi-Chobe belt, northwest-Botswana

The Banana Zone Cu-Ag Prospect is within the SW-NE trending Ghanzi-Chobe Belt

(GCB), which extends from the Namibia-Botswana border in the south to Shinamba Hills

in the north near the Botswana-Zambia border. The Banana Zone is also part of the Ghanzi

Project owned by Khoemacau Copper Mining and situated between the Ghanzi and Maun

townships, in Botswana. The Banana Zone has inferred resource of over 225.4 million

metric tonnes (Mt) grading 0.64% Cu and 8.1 g/t Ag. The GCB consists of deformed and

metamorphosed (greenschist) metasedimentary rocks of the Ghanzi Group, that is

commonly subdivided into four formations: the Kuke Formation, Ngwako Pan Formation,

D’Kar Formation, and Mamuno Formation, in ascending stratigraphic order. In this study

only two formations including the Ngwako Pan and the D’Kar Formations were

investigated. The Ngwako Pan Formation is made up of continental siliciclastic sediments,

mainly immature to submature sandstone with siltstone and shale intercalations, while the

D'Kar Formation contains shallow marine to lacustrine grey-green and locally carbonaceous mudrocks (siltstone and mudstone/shale) with fine-grained sandstone and

minor limestone to marlstone. The Cu-Ag mineralisation is hosted at the contact between

hematite-rich Ngwako Pan and reduced organic matter-bearing D'Kar Formations. The

presence of reduced organic matter is likely to have controlled Cu-Ag precipitation by

providing a reducing environment required for the precipitation of sulphides.

Petrographic and geochemical data were combined in order to decipher the petrogenesis of

the Neoproterozoic clastic sedimentary succession of the Ngwako Pan and the D’Kar

Formations associated with the Banana Zone Cu-Ag mineralisation. Sandstone of the

Ngwako Pan and the D’Kar Formations is arkose and subarkose, composed of quartz (Q),feldspars (F), and lithic fragments (L). Moreover, geochemically the sandstone of the

Ngwako Pan and the D’Kar Formations is considered as potassic and classified as arkose.

Therefore, sandstone of both the Ngwako Pan and the D’Kar Formations is chemically and

compositionally immature. On the other hand, mudrocks of the D’Kar Formation are

finely laminated and are dominated by muscovite, sericite, chlorite, and quartz. The

carbonate rocks of the D’Kar Formation are composed of pinkish-white crystalline

limestone and yellowish grey or greyish black marlstone characterised by laminations that are marked by undulating marly and silty layers that give the rock a “zebra-skin”

appearance. The marlstone exhibits variable degree of deformation from shearing to

folding and recrystallisation.

The modified chemical index of weathering (CIW’) values indicated an intense chemical

weathering of the source rock. The dominance of detrital quartz and feldspar grains

coupled with Al2O3/TiO2 ratios (average 29.67 and 24.52 for the sandstone of the Ngwako

Pan and D’Kar Formations, respectively) and Ni and Cr depletion in this sandstone,

suggests a dominant felsic source. However, high concentrations of Ni and Cr and a low

Al2O3/TiO2 ratio (