A critical comparison between the Fungurume 8 and 88 Cu-Co deposits, Central African Copperbelt

Ore Geology Reviews
Volume 140: 104627

Abstract. The Fungurume 88 deposit located in the Tenke-Fungurume Mining (TFM) district is situated in the northern extent of the Central African Copperbelt (CACB). It is an unusual sedimentary stratiform copper (SSC) deposit of this area, since it contains high-grade hypogene carrollite mineralization in the SD-1b subunit of the lower Shales Dolomitique (SD) Formation of the Mines Subgroup. These lithologies are normally regarded as being barren to poorly mineralized, here however, they boast cobalt concentrations as high as 3 wt% along with Cu concentrations in excess of 4 wt%. Detailed logging of drill core revealed that the sediments of the Fungurume 88 deposit which host the anomalous SD-1b mineralisation differ in both grainsize and mineralogy to the well-recognised Long Facies sediments exhibited by the majority of deposits in the TFM district - including the overlying Fungurume 8 deposit. Petrographic studies have revealed several distinct sulphide textures including the dominant disseminated style, stratiform style, bedding parallel jack veins and crosscutting veins. These sulphides also exhibit distinct trace element compositions suggesting several episodes of mineralization and support a multiphase hypogene Cu-Co model. The specific timings of the mineralisation events are constrained to an early phase (probably contemporaneous with diagenesis and early lithification) as well as a later phase associated with quartz-carbonate-sulphide veining and possibly linked to the regional brecciation event, and thus, in support of existing literature, indicate the presence of at least two mineralizing fluids. Host rock geochemistry obtained from SEM analyses together with LA-ICP-MS analyses on sulphide phases have helped to constrain the mineralizing process in the Fungurume 88 deposit by corroborating fluid-rock interaction, sulphide mineral paragenesis and trace element variation. Drill core assay data has also revealed prominent zonation and segregation between Cu and Co and the latter’s local enrichment observed in the SD-1b subunit. Metal zonation of this kind has also been described in some deposits of the Zambian copperbelt and are therefore used as a proxy here. The detailed petrographic analyses on the host rock shows that the sediments of the Fungurume 88 deposit potentially belong to the Kalumbwe, Menda or a geochemically similar deep marine sedimentary facies rather than the usual Long Facies. This lithological variation was likely the key ingredient, providing an optimal reducing environment or “trap site” for the precipitation of carrollite and other copper sulphides in the SD-1b subunit. These geological and geochemical observations provide insights into the genesis for the Fungurume 88 deposit, understood in a context of the regional geology and Cu-Co mineralisation throughout the greater Copperbelt. Moreover, it highlights that the northern apex of the Lufilian arc and the broader CACB hold vast exploration potential for similar deeper-water sub-basins which may host significant, and yet-undiscovered metal (notably Co) resources.

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