Malachite, an indicator of major Pliocene Cu remobilization in a karstic environment (Katanga, Democratic Republic of Congo)

Malachite [Cu3(OH)2(CO3)] is a common mineral in the Central African Copperbelt, where it constitutes a high-grade (~ 57% Cu) copper ore and is exploited industrially and by artisanal miners. The genesis of this supergene ore in Katanga has yet received little attention. Here we focus on the genetic processes giving rise to this regionally important mineral. Malachite is secondary to primary copper sulfides, which indicates that it ultimately formed by oxidation of the latter. In fine-grained siliciclastic host-rocks, it is often partly impregnative, commonly with variations in malachite content parallel to the bedding; this type may form either by absolute enrichment or by in situ replacement of sulfides. Within voids of various types, including dissolution cavities in carbonate host-rocks, it precipitates in distinctive forms, such as thick laminated crusts and speleothemes (up to several tens of centimeters in length). REE geochemistry suggests that malachite formed in groundwater environments, through in situ recombination of carbonate ions originating from dissolving host-rock carbonates and downward percolating Cu2+ (leached from copper sulfides). Thermodynamic modeling confirms this genetic environment, and shows that chrysocolla [(Cu,Al)2H2Si2O5(OH)4·nH2O] coexists with malachite when Si content increases in the groundwater fluids. The importance of karstic features in Katanga is also re-evaluated. Karstic features appear to be rather frequent and to constitute ideal site/traps for the formation of secondary base metal ores.