A FLUID MIXING MODEL FOR KONKOLA NORTH, ZAMBIAN COPPERBELT

The origin of Cu mineralization in the Zambian Copperbelt has long been disputed, but is known to reflect underlying basement topography, with barren gaps overlying basement highs. The geological setting and distribution of mineralization suggest similarities between the Copperbelt and the Pb-Zn mineralized St. Francois Mountains, Missouri. Both are hosted in rift-related sediments, show marked basement paleotopography, which is overlain by clastics and then carbonates. Both also show K-metasomatism, more pronounced in the Copperbelt where bulk K2O approaches 15 wt. %, overprinting by hematite, and evidence of Ti mobility. That the St. Francois mineralization formed as a result of fluid mixing is widely accepted, and we propose Konkola North mineralization also resulted from paleotopography-driven fluid-mixing. Geochemical and mineralogical evidence suggest that a Cu-bearing lower fluid moved laterally through the footwall, but was forced upward into the hanging wall by basement topography. As the Cu-bearing fluid rose, it encountered an upper fluid travelling through higher strata. The lower fluid was oxidizing and enriched in Ba, as well as Cu, and possibly K and Cl. In contrast, the upper fluid was reducing and carried sulfide, and possibly Co, Mg, and minor Sr. Fluid mixing resulted in both Cu and Comineralization and spatially associated barite precipitation. Where the oxidizing lower fluid encountered sulfide, sulfate formed and, with the Ba, precipitated barite. These barite concentrations are immediately below, coincident with, or immediately above the Cu-mineralization and may demarcate the mixing interface. Where barite is coincident with mineralization, Cu is in malachite, not sulfide, which suggests the ore zone was locally oxidizing. Where the barite is above or below, much of the Cu is in sulfide, suggesting the fluid mixture in contact with the host shale was reducing. Massive K-metasomatism has also affected these rocks, and is spatially associated with Cu-mineralization. Where Ba-rich fluids failed to encounter sufficient sulfide/sulfate to exhaust the Ba via barite precipitation, Ba-rich K-feldspar cement is common. Because of the insolubility of barite, Ba was removed from the mineralizing fluid more efficiently than Cu and has correspondingly larger barren gaps.