CONGOLESE-STYLE HYPOGENE AND SUPERGENE COPPER MINERALIZATION AT THE KALENGWA DEPOSIT, ZAMBIA

The Kalengwa deposit (mined 1967-78; 4 Mt @ 5.2% Cu, 80 g/t Ag) is located in west-central Zambia. The mineralized zone is hosted in low-grade metasedimentary rocks of the Neoproterozoic Katangan succession. The structural hangingwall of the deposit consists of unmineralized metasiltstones. This unit is separated from an underlying package of mineralized, calcareous siltstones, sandstones, and medium- to coarse-grained conglomerates with thin marble beds by a breccia zone. The breccia contains matrix-supported clasts of metasedimentary rocks and metagabbro in a calcite-ferroan dolomite-chlorite-epidote-biotite-chlorite matrix. The breccia is similar to those in the Congo Copperbelt, which are interpreted to have formed during evaporite diapirism. The underlying metasedimentary sequence and the breccia have been affected by moderate to intense sodic- (calcic) alteration that has produced an albite-epidote-chlorite-scapolite-hematite assemblage. Locally, this sodic assemblage is cut and replaced by a potassic alteration assemblage consisting of potassium feldspar-biotite- (hematite). Hypogene mineralization appears to post-date sodic and potassic alteration. Sulfides (chalcopyrite, bornite, lesser pyrite) replace hematite and carbonate minerals. Hypogene sulfides are most common in the structural footwall to the breccia body. Sulfides display a range of sulfur isotopic values (+3 to –9 permil; average –5 permil). The mechanisms of sulfide precipitation are unclear – though a redox origin is envisioned, there is no evidence of an obvious reductant. The Kalengwa deposit was formed by supergene oxidation of the hypogene sulfide assemblage. Supergene alteration resulted in the formation of massive chalcocite pods and layers that extend to the base of weathering. A zone of anomalously deep weathering coincides with the center of the deposit. Chalcocite has sulfur isotopic values ranging from +3 to –11 permil, with most clustering around –9 permil. The spatial association of the mineralized zone to a major breccia body, style of hypogene mineralization, and importance of supergene processes suggest Kalengwa is similar in style to many deposits in the Congolese Copperbelt rather than to iron oxide-copper-gold (IOCG) deposits with which it has recently been compared.