Supergene Nonsulfide Zinc-Lead Deposits: The Examples of Jabali (Yemen) and Yanque (Peru)

It is important to note that the hydrothermal cement, which impregnates these lithotypes, has the same composition of sulfides. Similar values have been detected in the hydrothermally affected igneous rock. This could indicate that the same hydrothermal fluids, which determined the phyllic/sericitic halo around the Cu-porphyry, deposited also the Yanque sulfides. However, the observed significant spread of the Pb isotopic compositions is not compatible with a simple genetic model involving just one fluid circulation episode. The primary ores have been totally oxidized, and the economic concentrations at Yanque are dominated by supergene Zn-Pb nonsulfides. The remnants of the sulfide minerals are represented by moderate amounts of galena and pyrite, and only by traces of sphalerite. The nonsulfide ore association consists of sauconite, hemimorphite, smithsonite and cerussite. Zinc is allocated mainly in sauconite (Zn-smectite), rather than in carbonates: a factor strictly related to the prevailing siliciclastic character of the Yanque host rock. Sauconite is derived from a typical process of supergene wall-rock replacement and alteration, represented by the reaction: K-feldspar (detrital) ? sericite/illite and kaolinite (hydrothermal) ? sauconite (supergene); sericite/illite and kaolinite are enriched in zinc at the reaction boundaries with the replacing sauconite, this pointing to an almost progressive modification process of clays. Secondary Pb-minerals are mostly linked to direct replacement of galena. The Zn-Pb Yanque deposit can be compa