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

Abstract

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 in 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 Cretaceous Ferrobamba Formation. Primary ores belong to the carbonate replacement deposit type and are at least in 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° 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 in 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 sauconite occurs as replacement of detrital feldspars and/or detrital fragments occurring in 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 ~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 in the nearby Cotabambas Cu deposit.