Geology of the Enterprise Hydrothermal Nickel Deposit, North-Western Province, Zambia

The Enterprise nickel deposit (40 Mt of 1.07% Ni) is located on the eastern edge of the Kabompo dome in the North-Western Province of Zambia. The deposit area contains basement schists overlain by Neoproterozoic metasedimentary rocks. Nickel sulfides are hosted within a sequence of quartz-, carbonate-, and carbon-rich metasedimentary rocks that interfinger with and overlie siliciclastic metasedimentary rocks. The host rocks contain significant kyanite, talc, and magnesian chlorite. Silicification and magnesian metasomatism occurred prior to or concurrent with a regional metamorphic event (590–500 Ma). Mineralization resulted in the precipitation of nickel and iron-nickel sulfides in veins and as semimassive replacements of the host rocks. Nickel sulfides precipitated in two main stages: a millerite-vaesite-pyrite assemblage that formed disseminations and semimassive replacements in vuggy textured rocks, and a later millerite-bravoite-(molybdenite) assemblage in quartz-kyanite veins and local semimassive replacements. The deposit contains minor copper and trace amounts of cobalt and platinum group elements (PGEs). A discrete zone of copper sulfides underlies a portion of the nickel sulfide zone. Re-Os geochronology on molybdenite yields a 540.6 ± 1.8 Ma age for mineralization at Enterprise, the approximate age of metamorphism. Sulfur isotope results indicate that the sulfur at Enterprise was derived largely from Neoproterozoic marine sulfate by thermochemical sulfate reduction. Significant volumes of mafic igneous rocks are not present in the immediate area of the Enterprise deposit. No evidence of prealteration concentrations of nickel exists within the sedimentary rock sequence at the deposit. The alteration and mineralization style of the Enterprise deposit is similar to the much less metamorphosed nickel-bearing Shinkolobwe uranium deposit in the Democratic Republic of Congo (DRC), though the Enterprise deposit does not contain significant uranium. The sediment-hosted nickel-rich deposits of Central African Copperbelt exemplified by Enterprise appear to represent a new style of hydrothermal nickel mineralization.