Base-metal deposits of the Cordillera Negra, department of Ancash, Peru

Abstract

The Cordillera Negra, the westernmost range in the Departamento de Ancash, is just north of the central mineral province of Peru. Within several kilometers of its crest and along a length of 140 kllometers are more than 60 base-metal deposits that total several hundred veins. These deposits were studied in 1947 by the Umted States Geological Survey m cooperation with the Instituto Geológico del Perú. Ores in the deposits contain principally the sulfides of lead and zinc and, in some places, copper, one mine produces antimony ore and another, silver. Basemetal production is wholly dependent on silver content of ore, hence only argentiferous sulfides of lead or copper are sought and sphalerite, which contains little or no silver, is discarded. At the time of our examination, three mines were operated on a comparatively large scale, but one of these subsequently closed, in 1947 the three produced about 37,000 tons of ore, which was concentrated in flotation plants. In the same year fifteen small mines produced probably less than 1,000 tons of ore, which was concentrated by hand cobbing or hand jigging. The deposits in the Cordillera Negra have been worked for many years, so the majority of both large and small active mines now contain small reserves. The reserves of most of the abandoned mines could not be estimated, owing to inaccessibility of their workings. The oldest rocks that can be dated in the Cordillera are Cretaceous in age An estimated 2,200-meter-thick sequence of sedimentary rocks of Early Cretaceous age consists predominantly of nonmarine sandstone and shale but includes several coal beds, a thin unit of limestone, and, toward the top of the sequence, a unit of tuff. These rocks are overlain by Albian to Turonian limestone, estimated to be from 200 to 400 meters thick. This sedimentary sequence is overlain by a sequence of layered volcanic rocks, possibly in part of Late Cretaceous age but mostly of Tertiary age The volcanic rocks are estimated to be at least 1,000 meters thick, lava and agglomerate are the most common rocks, but some tuff and a few beds of nonvolcanic sediments are included in the sequence. Most of the -lava and the agglomerate is porphyritic andesite and most of the tuff is rhyolite. The volcanic sequence can be divided into two groups a lower group includes layers that are moderately closely folded, an upper group includes those that are only slightly folded. The sedimentary and volcanic rocks are intruded by granodwntic and granitic batholiths and stocks, by porphyritic andesite and rhyolite stocks and plugs, and by sills and dikes ranging from pegmatite to andesite in composition. Some sedimentary rock has been converted to andalusite schist near contacts with the batholiths or the larger gramtic stocks. The layered rocks have been deformed during two periods of major orogeny, the first of possible Late Cretaceous or very early Tertiary age, and the second of possible early Tertiary age. Each is marked by a conspicuous unconformity. The lower unconformity is between the sedimentary sequence and the folded lower volcanic group, and the upper is between the folded lower volcanic group and the slightly folded upper volcanic group. Plutonic intrusion of granodiorite and granite and shallow-seated intrusion of porphyry seems to have occurred in the Tertiary Strata of the sedimentary sequence are closely folded and those of the lower volcanic group are moderately closely folded, and strata of both are broken by thrust and reverse faults. Axial planes of folds dip steeply and strike generally N 30° W, parallel to the structural trend of the Andean Cordillera in this part of Peru. Open folds and warps characterize deformation in the upper volcanic group, and are accompanied by steeply dipping normal and reverse faults of small displacement. Only limited regional metamorphism of the older rocks resulted during orogenesis, slaty cleavage is found in some shale. Most deposits are minerahzed fractures containig pyrite, galena, and sphalerite in a gangue of quartz and carbonate, other minerals that may be present are arsenopyrite, chalcopyrite, enargite, or tetrahedrite. Other deposits are quartzpyrite veins, and in some the sulfides commonly are marmatite, pyrrhotite, and pyrite. One vein contains stibnite and pyrite in quartz. Either primary or secondary silver minerals occur in all deposits, in some in sufficient quantities to repay fairly high mining costs, gold also is present in many deposits but generally in insignificant amounts. Nearly all minerals were deposited as fissure-fillings, but some replaced wall-rock on a small scale in several veins. Alteration in most deposits was slight and altered zones are confined to adjacent wall-rock. Several deposits, however, occur in widespread altered zones. Textures and the prevalence of low-temperature minerals suggest that the deposits were formed at shallow depths, and that the vertical range of ore deposition was small. Minining discloses the maximum range was almost 600 meters At several mines zoning is well developed in both vertical and horizontal dimensions, in distances as small as 200 meters. The type of minerals, degree of crystalhnity, and vein structures indicate that the veins formed at shallow depths under conditions of moderate to low temperature and pressure, and thus can be classed as ranging from mesothermal to epithermal. Although 600 meters is about the maximum vertical range of strongly mineralized deposits, most veins do not reach this depth Veins range in length from several meters to 2.5 kilometers, and range in width from several centimeters to many meters. Lenticular ore shoots of irregular sizes and shapes occupy only small parts of most veins, this precludes any great increase in the rate at which ore is mined.