ORE-HOSTING KAMOA-KAKULA DIAMICTITE MATRIX (DEMOCRATIC REPUBLIC OF CONGO) EXHIBITS TWO-STAGE CU MINERALISATION

Ore in the Kamoa-Kakula deposits (1387 Mt 2.74% Cu; Democratic Republic of Congo) in the central African copperbelt consists primarily of fine-grained Cu minerals disseminated in matrix of the Cryogenian Mwale formation diamictite, commonly with subordinate sulphide-gangue ‘caps’, ‘beards’, and rims on larger diamictite clasts. Previous work focussed primarily on caps and beards and yielded contrasting diagenetic and tectonic interpretations for the timing of mineralisation. Because the bulk of the Cu is in diamictite matrix, this study addresses matrix paragenesis, using optical and SEM petrography. Away from the ore-zone, least-altered diamictite matrix consists of sedimentary and early diagenetic minerals and lacks burial or tectonic fabric, except where weak foliation, microscopically present beside some ‘larger’ clasts, is conferred by vertically aligned chlorite and orientation of the longest axes of subequant particles. Ore-zone matrix exhibits a superficially similar heterogeneous strain: in non-aligned areas it consists of randomly disposed, subequant detrital particles, randomly oriented phyllosilicate minerals including abundant muscovite, and equant Cu-sulphide minerals, hematite, and rutile that occupy matrix pores; there is no fabric. Aligned ore-zone matrix, however, is dominated by high proportions of vertically aligned muscovite, and elongated, aligned detrital grains; the Cu content of aligned matrix is systematically lower than that of non-aligned matrix. Micro-scale redistribution of material by preferential pressure-solution in the non-phyllosilicate minerals of aligned ore-zone matrix and syntaxial reprecipitation produced grain alignment and elongation, and Cu depletion. Solutes liberated by pressure solution could also reprecipitate as the previously documented clast-caps- and –beards, which developed in pressure-shadows of diamictite clasts during the Lufilian orogeny. Diamictite matrix therefore records ore characteristics that were produced by two separate events: the bulk of the ore formed as textureless ‘early’ disseminated sulphides emplaced prior to the Lufilian orogeny, whereas ‘late’ remobilisation of Cu and gangue minerals formed clast-rims during Lufilian compression.