Geometries and genesis of breccias within the halokinetic Kalukundi diapir at the Mashitu mine, Katanga, Democratic Republic of Congo

Abstract

The Mashitu Cu-Co deposit is located within the western part of the Kalukundi breccia complex in the Democratic Republic of Congo (DRC). The Kalukundi breccia complex, similar to others in the Congolese Copperbelt, consists of clasts and megaclasts of Neoproterozoic sedimentary rocks ranging from millimeters to hundreds of meters in diameter contained in a quartz-chlorite-dolomite matrix. The Congolese Copperbelt breccia complexes remain poorly understood because mining has traditionally focused on mineralized megaclasts within the breccias and the matrix to the breccias has undergone recessive weathering. The Mashitu open pit mine exploited typical Cu-Co mineralized megaclasts as well as supergene mineralized breccia matrix material. The breccias exposed at the Mashitu mine, together with data from exploration diamond drill cores beneath the open pit, presented an exceptional opportunity to generate detailed geological maps and a three-dimensional model of the internal geometry of a Congolese Copperbelt breccia complex and allowed for improved understanding of its genesis. Textures of the breccias in outcrop and in drill core, together with a variety of petrographic data on a suite of representative samples, formed the basis for characterization of different clast, matrix, and breccia types. Clasts are less than 1 meter in diameter while megaclasts or blocks, range in size from >1 meter to 100’s of meters in diameter; megaclasts commonly display stratigraphic coherence but are commonly broken. Clast and megaclast size is broadly related to the strength of the dominant rock types; sand-rich, silicified, and carbonate-rich units display the most coherence while shale- or clay-rich rock types are commonly disrupted and disaggregated. Spatial analysis of megaclasts within the breccia complex exposed at Mashitu indicates stacking of megaclasts (blocks). Clast geometry in the breccia complex displays a fractal pattern. Currently the clast to matrix or ‘block’ to breccia ratio exposed at Mashitu is approximately 5:1. The distinctive geometry, presence of clasts of stratigraphy hundreds to thousands of meters above their apparent depositional position, and mineralogy of the breccia matrix supports hypotheses presented by earlier workers that the Congolese breccia complexes represent the remnants of salt diapirs probably formed in the late Neoproterozoic; later replacement by carbonate minerals and dissolution of halite and anhydrite within these diapiric structures led to the complex breccia bodies observed today in the Congolese Copperbelt. Mashitu contains mineralized megaclasts typical of the Congolese Copperbelt. It also contains lithologies and breccia matrix containing a supergene assemblage of supergene black Cu-Co-Mn-Fe-oxide and malachite that have not been reported from elsewhere in the district. Much of this supergene assemblage replaced former carbonate minerals in carbonate veins cutting clasts or in breccia matrix. Supergene mineralized breccia material is best developed beneath mineralized megaclasts. The distribution of mineralized zones in breccia indicates that metals were derived from such megaclasts. The supergene mineralized breccia zones at Mashitu represent a new style of orebody in the Congolese portion of the Central African Copperbelt.