Although sediment-hosted Cu-Co deposits are significant sources of Cu and the most important sources of Co, very little work has been done on them using laser ablation inductively coupled mass spectrometry (LA-ICP-MS). LA-ICP-MS has proven to be a useful tool in deciphering formation processes for a range of sulfides and their ore-forming processes, especially when complemented with additional analytical techniques. In the pursuit of better the understanding of sediment-hosted Cu-Co deposits, this work investigates the recently discovered Cu-Co-Zn Dolostone Ore Formation (DOF) deposit in northwestern Namibia, by a multi-method geochemical approach. The DOF deposit is situated within the Kaoko Belt and is hosted in calcareous siltstones of the Ombombo Subgroup, which is part of the Neoproterozoic Damara Supergroup, and is the first recognized Co deposit in Namibia. Although hosted in an analogous tectonic setting and stratigraphic position to the sediment-hosted Cu-Co deposits of the Central African Copperbelt, little is known on the genesis of the DOF deposit. This study applies optical microscopy, electron beam techniques, LA-ICP-MS, and atom probe tomography to gain insights into the ore-forming processes responsible for the DOF deposit on a macro-, to micron-, and nano-scale. Additionally, monazite dating and sulfide sulfur isotopes were carried out to attain additional information on the genesis of the DOF deposit.
The DOF deposit’s metal enrichment shows a bell curve-like distribution in e.g. Cu, Co, and Zn. The peak concentration of this metal enrichment is referred to as the Main DOF horizon, whilst the lower concentrations are known as the Wider DOF horizon. The Main DOF horizon is also characterized by an alteration mineral assemblage of stilpnomelane, siderite, and ankerite. The sulfide mineralogy of the DOF deposit is relatively simple, with dominant sulfides being pyrite, pyrrhotite, cobaltpentlandite, chalcopyrite, sphalerite, and linnaeite. Linnaeite and cobaltpentlandite are the main Co-bearing mineral in the DOF, although pyrite was shown to contain oscillatory zonation wherein high concentrations of Co can be found (up to 10 wt%). Linnaeite occurs as euhedral crystals in nodules and clusters, partially broken down, whilst cobaltpentlandite occurs as exsolution-like inclusions in both pyrite and pyrrhotite. Galena, cobaltite, and pentlandite occur as accessory phases. Sulfides occur in six types of mineralization styles in the DOF deposit: (1) disseminated; (2) polysulfide cluster aggregates; (3) polysulfide nodules; (4) veins, both sulfide- and gangue-dominated; (5) mineralized pressure shadows; and (6) “Events”, which is a locally applied term for mineralization that occurs within vein- and/or slump-like structures that portray both ductile and brittle textures. Additionally, framboidal pyrite occurs throughout the stratigraphy, but is almost completely absent from the Main DOF horizon.
Pyrite, pyrrhotite, sphalerite, and chalcopyrite from these six mineralization styles were analyzed by LA-ICP-MS for their trace element composition. The trace elements show two main populations of sulfides between the mineralization style, in particular in regards to sphalerite and chalcopyrite, where disseminated, nodule, cluster, and “Event” sulfides group together (Group 1), whilst vein and pressure shadow sulfides group (Group 2). This is interpreted as evidence for at least two main stages of ore-formation. The fact that sphalerite and chalcopyrite both overgrow the iron-sulfides and linnaeite in the Group 1 mineralization styles indicates that the Cu-Zn mineralization post-dates the iron-sulfide formation. The linnaeite, which is only found within Group 1 mineralization styles, was suggested to have formed through pyrite decomposition and Co remobilization from the pyrite, through changes in oxygen and/or sulfur fugacity. The cobaltpentlandite inclusions within the iron-sulfides of Group 1, were an intermediate product of this process. The Ga-Ge-In-Mn-Fe in sphalerite geothermometer indicated formation temperatures of >310 ± 50 °C, for both sulfide groups, linking the ore formation to the Damara Orogeny.
Additional sulfide trace element data was acquired from sediment-hosted Cu(-Co) deposits from the Central African Copperbelt and the Polish Kupferschiefer, to enable a deposit-type comparison of the DOF. Random Forest analyses were used to investigate similarities and differences between the metallogenic Cu(-Co) districts, which showed that the sulfide trace elements vary greatly between the districts, which is most probably due to different metal sources in the basement or the detrital material form each basin’s basement. This was also evident as the African districts shared more similarities to each other than to the Polish Kupferschiefer, which is reasonable as they are hosted in analogous tectonic settings and stratigraphies.