The Nonsulfide Zinc Deposit at Accha (Southern Peru): Geological and Mineralogical Characterization

The Accha-Yanque zinc belt is located in the southern Altiplano of Peru, a major zinc-rich metallogenic province hosting a number of economic mineral deposits (porphyry copper and skarn ores). Several nonsulfide-type occurrences, showings, and mineral deposits are situated ill a belt, peripheral to the northern, northeastern, and northwestern edge of the Oligocene-(Miocene?) Yauri-Apurimac batholith. Mineralization is hosted in breccias of both sedimentary and tectonic origin in the limestones of the Middle to Upper Cretaccous Ferrobamba Formation. Primary ores belong to the carbonate replacement deposit type and are at least ill part structurally controlled. Currently the Zn mineralization is almost fully oxidized: the Accha deposit can be assigned to both direct replacement and wall-rock replacement types. The mineralized zone (indicated resources 5.1. Mt @ 8.2% Zn and 0.9% Pb) occupies the hinge of an anticlinal dome that has been exposed by erosion. The southern, limb of the structure dips about 55 degrees to the south-southwest, whereas its northern limb is truncated by faults. The nonsulfide concentrations, consisting of a mineralized zone 5 to 20 m thick, are continuous along strike to the west for at least 700 m. The mineralogy of the Accha deposit shares many characteristics with that of the typical carbonate-hosted calamine-type nonsulfide Zn ores. The nonsulfide mineral association consists mainly of smithsonite and hemimorphite replacing both primary ore minerals and carbonate host rocks. Hydrozincite has been detected only in samples near the surface. Smithsonite occurs in zoned concretions with goethite, Mn (hydr)oxides and Zn clays, as well as replacive cement ill the limestone intervals. One of the most peculiar nonsulfide Zn minerals at Accha is a sauconite-like, zincian smectite, variably concentrated throughout the deposit. Locally sanconite occurs as replacement of detrital feldspars and/or detrital fragments occurring ill marly sediments or in infills of karst cavities. It also replaces both hemimorphite and smithsonite deposited during earlier stages. The age of the supergene products in the whole belt is poorly constrained, although there is geomorphologic evidence that the formation of supergene minerals postdates by more than 10 m.y. the last large-scale secondary enrichment event that terminated,with central Andean climatic desiccation at similar to 15 Ma. The age of the Accha deposit may be consistent with a Pliocene K-Ar date of 3.3 +/- 0.2 Ma obtained for supergene alunite from the top part of the leached cap ill the nearby Cotabambas Cu deposit.