Geologic setting and epithermal silver veins of the Arcata District, southern Peru

Abstract

Arcata, situated 180 km north-northwest of Arequipa at an elevation of 4,600 to 4,900 m, is the youngest known adularia-sericite-type epithermal precious metal district in the broad Miocene volcanic belt of southern Peru. Total modern production from 1964 through 1989 was 1.902 X 10 6 kg Ag and 4.489 X 10 3 kg Au at an average grade of 17.5 oz/metric ton Ag and 1.3 g/metric ton Au.The veins are hosted by intermediate lavas and breccias dated at 6.1 + or - 0.2 Ma. These rocks overlie silicic ash-flow tuffs about 6.3 Ma and are intruded by domes of phenocryst-poor rhyolite dated at 5.9 + or - 0.2 Ma. The district may lie near the margin of a small collapse caldera related to the ash-flow tuffs. The veins are hosted by faults of a west-northwest-trending graben that possibly overlies a buried intrusion. Six K-Ar ages on vein and wall-rock adularia and alunite cluster closely around 5.4 Ma, indicating a short but definite interval between volcanic activity and hydrothermal mineralization.Precious metal ores are typically fine grained and banded, with grain size tending to increase downward. Hydrothermal breccia is present and several episodes of fault movement during ore deposition can commonly be discerned. Principal ore minerals include pyrargyrite, other Ag sulfosalts, argentian tetrahedrite, acanthite, and electrum. These minerals are accompanied by pyrite, sphalerite, galena, and chalcopyrite, local early arsenopyrite and pyrrhotite, and marcasite and stibnite in the upper levels. Gangue minerals in the ore zone are quartz, adularia, calcite, rhodochrosite, Mn silicates, and chlorite with minor fluorite and late siderite. Adularia, radial aggregates of white fibrous calcite, and chlorite are preferentially associated with ore. Most veins were reopened repeatedly by structural movement during vein deposition and show evidence of both tectonic and hydrothermal brecciation.Ore shoots are much more continuous laterally than in many epithermal veins. Subhorizontal to undulating upper and lower surfaces enclosing the shoots are 250 to 350 m apart and are located at similar absolute elevations throughout the district. Average vein widths range from 1.2 to 2.5 m. Upper boundaries of the ore shoots are sharper and more regular than lower limits, and barren prismatic calcite + or - quartz, in part chalcedonic, extends above the productive parts of veins. In the Marion vein bodies of rich polymetallic ore project downward to greater depths; these roots may reflect wider zones of greater fluid flow. Although deposited within a related system of structures during a relatively short period of geologic time, the ore shoots of the several veins display differences in mineralogy, texture, chemistry and vertical zoning that suggest deposition from several hydrothermal subsystems.Precious metal ores contain low concentrations of Cu (generally 0.025-0.15 wt %), Pb (0.1-0.5 wt %), and Zn (0.15-1.0 wt %). The veins are assignable to the silver and base metal-rich epithermal subtype of Heald et al. (1987). Precious metal paragenetic stages in the Luisa, Marciano, and Marion veins show modest increases in the ratio of base to precious metals with depth. In the Marion vein, an early base metal stage extends from 150 m or more below to about 50 m above the economic base of the precious metal stage. Ores dominated by this stage may contain more than 20 wt percent lead plus zinc accompanied in many cases by high contents of Ag and Au. Because of increasing production of these base metal-rich ores, the mine began to produce Pb and Zn concentrates in November 1989. The Ag/Au ratios in precious metal ores range from [asymp] 200 to > or = 1,000 and base metal ores also show significant variations. Overall, the Ag/Au ratio shows little variation with depth but rather appears to reflect temporal variations in solution composition. Wall rocks in the ore zone are K metasomatized; they contain adularia, sericite, chlorite, and calcite and grade outward to propylitized dacite. Along the upper parts of the Tres Reyes vein and the western part of the Baja vein wall rocks are altered to kaolinite, locally with alunite, grading outward to montmorillonite. These selvages of advanced argillically altered rock grade laterally into, and overprint at depth, assemblages containing adularia and/or sericite.No significant orebodies have been found in the highly visible Tres Reyes structure at depths of 500 m or more below the original land surface. This may reflect unfavorable structural conditions and/or higher CO 2 contents of the fluids, resulting in ore deposition at greater depths than in the producing veins of the district.Fluid inclusion studies give T h between 175 degrees and 275 degrees C, with T h for sphalerite between 230 degrees and 270 degrees C. Salinities are higher for inclusions in sphalerite, pyrargyrite, and some quartz and fluorite (> or =3 equiv wt % NaCl) than for most quartz (< or =2.5 wt %).