Huancavelica Group.(mining district in Peru)

Publication: The Mineralogical Record

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COPYRIGHT 1997 The Mineralogical, Inc.

THE JULCANI DISTRICT

Angaraes Province Huancavelica Department

LOCATION and HISTORY

The Julcani district is located about 65 km by road southeast of the city of Huancavelica, at an elevation of 4,200 meters. The district has been mined discontinuously since Spanish colonial times. Serious mining has been conducted since about 1907, and large-scale mining commenced in 1936. The present operator is Alberto Benavides Q., who founded the Compania de Minas Buenaventura S.A. in 1953. Julcani is one of Peru's largest producers of silver and bismuth.

GEOLOGY

Goodell and Petersen (1974) and Petersen et al. (1977) have described the geology and mineralogy of the Julcani area. The mineralogy in these papers, as well as one published by Goodell (1974), is used as background for the following synopsis of the district geology and ore deposits.

The Julcani district is dominated by a series of rhyodacitic to dacitic dikes, volcanic domes with related eruptive rocks, and pyroclastic material described as the Julcani volcanic center. There were three phases of activity. The first or main stage activity is characterized by explosive pyroclastic eruptions. The second stage resulted in the formation of a series of domes, and the third and final stage consisted of recurrent doming and dike formation. Hydrothermal alteration and mineralization occurred more or less simultaneously with the intrusion of the third stage volcanic domes and dikes.

Ore mineralization is generally confined to fracture filling, with limited replacement of the country rock within the vein structures. The mineralized area, containing the major mines of the district, measures approximately 3 x 5 km. Ore solutions appear to have spread upward and outward from several centers. This resulted in clearly defined zoning patterns. These centers are also the location of the major mines today. Some of the more important mines are the Herminia (in the south) and Mimosa (in the north), followed by the Estela, Tentadora, Lucrecia, Rita, Sacramento (Sacramento Otoni), Acchilla, and Manto mines.

Generally, as the ore-bearing solutions moved upward and outward from the innermost centers of the Herminia vein system, they first deposited pyrite-wolframite-enargite-barite, then enargite-pyrite-tennantite/tetrahedrite, then a zone of complex silver and bismuth sulfosalts with bismuthinite (Robinson et al., 1997; Vajdak, 1995), and at a greater distance they deposited galena, followed by lead sulfosalts, orpiment, and realgar. Pyrite was deposited first throughout the vein system, particularly in association with enargite; smaller amounts of pyrite also crystallized later. Barite commonly crystallized with, or slightly after, enargite and extends beyond the area of enargite deposition. In the outermost fringes of the zoning sequence, barite precipitated with tennantite/tetrahedrite, followed by bismuth sulfides and sulfosalts, then galena, followed by orpiment, realgar, and lead sulfosalts. Galena is the main ore mineral in the distal parts of the zoning sequence.

Pyrite-gold-tungsten mineralization is the dominant mineralization in the Tentadora mine, which is thought to be in the innermost zone of the Herminia vein system,

The Mimosa vein system is in the northern part of the district. In this vein system, tennantite/tetrahedrite is the dominant mineral in the innermost assemblage, with galena being the dominant mineral in the outer zones. The Mimosa system is similar to, and somewhat simpler than, the Herminia system. Pyrite was the first mineral to crystallize, followed, in order, by tennantite/tetrahedrite, sphalerite with chalcopyrite, and siderite. This first stage of mineralization was followed by crystallization of bismuth-bearing minerals, followed by galena, then lead sulfosalt-bearing mineral assemblages. In both the Herminia and the Mimosa vein systems, the final mineralization phase was a crystallization of sphalerite and siderite.

Southwest of the Mimosa mine is the Estela vein system....

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