Comparing rare earth element patterns in Central African ore deposits

COMPARING RARE EARTH ELEMENT PATTERNS IN CENTRAL AFRICAN COPPERBELT ORE DEPOSITS. David Debruyne1*, Lieve Balcaen2, Frank Vanhaecke2 and Philippe Muchez1 1Geodynamics and Geofluids Research Group, Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200E, B-3001 Leuven *david.debruyne@ees.kuleuven.be 2Department of Analytical Chemistry, Ghent University, Krijgslaan 281- S12, B-9000 Ghent, Belgium The Copperbelt is a prolific metallogenic province straddling the border between Zambia and the Democratic Republic of Congo. The region is characterized by numerous sediment-hosted Cu-Co and Cu-Zn-Pb deposits, dispersed over an area roughly twice the size of Belgium. The ore bodies are formed by episodic pulses of hot, saline fluids carrying metals leached from the deep subsurface. Although this economically important province is thoroughly studied, some issues remain unresolved, such as the nature of the metal sources and the processes operating along the hydrothermal pathways towards the deposits. This project investigates the latter by comparing the rare earth element (REE) composition of carbonate minerals alongside the ores in various deposits. The REE have a systematically decreasing ionic radius across the element series. This often results in different affinities for light and heavy REE in various minerals and complexing species. For this reason, REE signatures are among the most widely used proxies for studying hydrothermal processes. REE patterns from shale-hosted deposits (Konkola and Nkana) contents were compared with Congolese deposits (Luiswishi and Kamoto) hosted by carbonates and with polymetallic deposits (Kipushi and Dikulushi) that cross-cut the stratigraphy. The REE concentrations in the carbonate-hosted deposits are generally lower than those in the siliciclast hosted deposits. This could be attributed to dissolution of REE-poor host-rock carbonate by (initially) acidic mineralizing brines, decreasing the fluid’s REE/Ca ratio, which in turn decreases the REE-content in the carbonates that ultimately precipitate at the deposit site. The Kipushi veins show bimodal REE patterns with a different redox-sensitive element content. This most likely reflects interaction with different sources at distinct physicochemical conditions. This bimodality is absent at the Dikulushi vein-type deposit, despite the chemical differences between an early Cu-Pb-Zn-Fe mineralization and a late Cu-Ag phase. This indicates that the late phase remobilized the REE from the earlier phase. In general the REE trace element patterns cannot be traced back to their origin, because of selective REE leaching from the sources and since quantitative constraints for competing hydrothermal processes are not available.status: publishe