Lead-zinc deposits of Cordillera Blanca and northern Cordillera Huayhuash, Peru

Abstract

Base metal deposits in the central and southern Cordillera Blanca and the northern Cordillera Huayhuash comprise 6 mineral districts, 5 in the Departamento cle Ancash and 1 in the neighboring Departamento de Huanuco. Although many of the deposits were prospected and some mined on a small scale during the Spanish Colonial period, most activity has taken place in the last hundred years. Concentrates were produced in 194S and 1949 at the rate of about 1,000 tons annually, and the rate of production during ceveral previous years was evidently about the same. Silver-bearing galena concentrate is the chief product. Sphalerite either is not mined or is discarded because its value lias been too low to cover costs of production and transportation. The sedimentary rock units of the region are Jurassic (?) marine quartzites and phyllites, Lower Neocomian iluviatile and brackish-water sandstones, shales, and coals, Barremian marine and nonmarine limestones, marls, shales, and tuffs, and Middle Cretaceous marine limestones. During a period of orogeny beginning at the end of the Cretaceous the sedimentary sequence was compressed, commonly forming upright folds, but also some recumbent folds, many small bedding-plane faults, and some large reverse faults. At or near the end of deformation the sedimentary rocks were intruded by a batholith of granodiorite. The sedimentary rocks are slightly metamorphosed on a regional scale as a result of folding and locally are more highly metamorphosed in zones bordering igneous bodies. During subsequent uplift of the Andean block the sedimentary rocks were dislocated along normal faults and intruded by igneous stocks, dikes, and sills. Deposition of base metals is thought to have accompanied or immediately followed this stage of igneous activity. The most common sulflde minerals in the deposits are galena, sphalerite, and pyrite; chalcopyrite, tetrahedrite-teunantite, arsenopyrite, and stibuite may also be present. Silver occurs in galena, in the copper sulfides, or as free sulfides. The sulflde minerals are oxidized to shallow depths, and the upper parts of many veins have been secondarily enriched in silver. Common non-metallic gangue minerals in most deposits are quartz and carbonates; fluorite and barite are found in some deposits, and silicate minerals are found in contact deposits in calcareous rocks. Most deposits are fissure filling veins along small fault or shear zones, but a number are replacements of wall rock along small faults, certain sedimentary beds, or contact zones. The fissure filling veins occur in any type of country rock, sedimentary, metaseclimentary, or igneous. Most replacement deposits are in calcareous rocks, but a few are in other rock types. Mineralogy and textures of the deposits indicate that nearly all are within the range of inesotherrual-epithermal origin.Mineralized faults and shear zones are from several hundred meters to more than a kilometer long and have vertical extents of at most a few hundred meters; they are generally from 0.0 to 1 meter wide. Ore shoots containing argentiferous galena occupy only a fraction of such structures, have sporadic distribution, and in most deposits are small and lenticular, with galena ranging in tenor from 5 to SO percent. Replacement deposits have a wide range in size and are characterized by a low tenor of galena. At most mines ore is mined selectively, crushed, and concentrated by hand. For this reason fissure-filling veins containing ore shoots with high tenor of galena are preferred by miners. Small concentrators in the Quebrada Honda-Vesuvio and the Pachapaqui districts treat ore from only a few deposits. The size of ore bodies in most deposits precludes larger scale mining or mechanizations of operations, and it is concluded that only minor expansion of production is possible under existing conditions. The greatest deterrent to increased production is the high cost of transportation of concentrates by pack animals to roadheads. It is recommended that access roads be constructed to, or at least part way to, four of the mineral districts. With reduced transportation costs, miners could extract lower grade ore. Construction of small mills in other districts would also permit mining of lower grade ore. Replacement deposits of lead-zinc sulfides in the Pacllon-Llamac and Pachapaqui districts and copper sulfldes in the Antamina-Contonga district contain large reserves of possibly minable material. However, development of these deposits would require large investments in roads, concentrating plants, and mining machinery.