Abstract Although known since at least 1897, Uchucchacua was first explored on a major scale by Compaúía de Minas Buenaventura since 1960. Narrow vein mining started in 1975, but orebodies discovered at depth enabled expansion to today’s 2,000-t/d operation, transforming “Chacua” into the largest primary silver producer in South America. The ores occur in fractures and faults, as well as in pipes, irregular replacement bodies, and mantos hosted by Late Cretaceous limestone. Porphyritic dacite bodies are probably pre-, syn-, and postore. Most of the ore occurs in distal manganiferous exoskarn and limestone and is mineralogically diverse, consisting mostly of the following. rhodonite rhodochrosite sphalerite pyrargyrite- quartz bustamite kutnahorite wurtzite proustite pyrite alabandite galena argentite The grade of the ore mined varies between 16 and 20 oz/t Ag combined with about 10 percent Mn, 1.5 percent Zn, and 0.9 percent Pb. Between 75 and 80 percent of the reserves are high in silver and manganese, whereas about 7 percent contain high zinc and lead grades with only moderate silver and low manganese. Logarithmic-grade graphs show very good positive linear correlations for zinc versus lead, moderate correlations for silver versus manganese, and arcuate correlation bands for silver or manganese versus zinc or lead. These relationships indicate that the outward zoning sequence is from lead-zinc to silver-manganese or vice versa. The corresponding longitudinal vein sections can generally be contoured unambiguously, showing that the bands of highest grades of lead and zinc coincide very well. The highest silver grades can be contoured convincingly as a band that is zoned outward and/or at a higher elevation than the lead and zinc bands. However, the manganese grades often require two high-grade bands: a main band that mostly coincides with the highest silver grades and a thinner upper band that may represent near-surface manganese enrichment. Ore intervals in individual veins, pipes, and replacement bodies are up to 200 m in vertical extent. However, the elevations of these intervals change progressively, reflecting the overall geometry of the hydrothermal cell (or cells) responsible for the mineralization. In addition, postore faulting has displaced the ore intervals. As a result, ore has been found to date over a vertical interval of 600 m, between 4,730 and 4,040 m. At surface, manganese oxide stains in the host limestone and limonite in fractures and faults indicate proximity to ore. Underground, multiple calcite veinlets constitute a guide to nearby orebodies. Geochemical anomalies of 60 to 80 ppm Ag have been documented up to 15 m from an orebody. By extrapolation, 10 ppm Ag anomalies may extend 25 m from ore, and 1 ppm Ag anomalies may attain 40 to 45 m. Ore continues to be found at depth as well as laterally and between known ore zones.

Abstract The <10,000-yr-old volcanoes of the Central Volcanic zone (15°-27° S) form a 50-km-wide belt that widens locally to 100 to 150 km and has three outliers 100 to 200 km east of it. The locations of over 1,800 radiometrically dated igneous rocks and hydrothermal ore/alteration minerals between 6° S and 33° S are plotted for 25 time intervals (varying between 2 and 65 m.y.), from the Precambrian to the Holocene. For short time intervals, these locations define 25- to 75-km-wide belts that widen occasionally to 75 to 125 km and have local outliers of volcanic tuffs or ignimbrites. Nonmagmatic stretches, such as the current Northern (2°-15° S) and Southern (27°-34° S) Nonvolcanic zones, probably occurred at various times and locations in the past, but were distinctly subordinate in strike length and duration to the magmatic zones. There are two roughly parallel belts that are 200 to 400 km apart (locally separated by only 125 km or up to 500 km). Over the chosen time intervals, both magmatic belts were often active. However, judging from presently active magmatic belts, they were probably seldom coeval over geologically very short time spans. The western belt corresponds to the conventionally envisaged magma generation by a subducting oceanic plate at 100- to 125-km depth. The eastern belt is akin to a back arc in an oceanic setting, except that it occurs in a continental plate. The apparent parallelism of both belts suggests that they were generated by linked mechanisms. Pegmatites, granites, rhyolites, and rhyodacites occur in both belts, but as a group are more common in the eastern belt. Calc-alkaline igneous rock compositions also occur in both belts, but as a group predominate in the western belt. Although basaltic rocks occur in both belts, as a group the mafic igneous rocks appear to be largely restricted to the western belt. The two phonolites and the nepheline syenite dated are in the eastern belt. In many areas, the location of the magmatic belt did not change significantly over a long period of time. The location of the magmatic belt gives the appearance of essentially continuous magmatism accompanied by occasional hydrothermal activity that resulted in the formation of ore deposits. Significant changes in the activity and locations of magmatic belts can occur in about 5 million years. As magmatic belts shift eastward or westward, individual magmatic centers may be dragged along. Integrated over a long time, this process may give rise to transverse magmatic alignments or transbatholiths with associated hydrothermal ore deposits of different ages that appear to be controlled tectonically. The relatively straight magmatic belts have local deflections. These deflections can be interpreted as smooth changes in the dip of the subducting plate or as faulting of either the oceanic or the continental plate. Oceanic plate subduction below the central Andes has occurred since the Cambrian. Folding and overthrusting in the continental plate did not significantly disturb the geometry of the magmatic-hydrothermal belts.

Abstract The El Indio district, Chile, contains two types of high-sulfidation, precious metal deposits hosted in intensely altered Tertiary rhyodacitic volcanic rocks: El Indio, with enargite-pyrite and gold-quartz mineralization in complex vein systems, and Tambo, with alunite-barite-gold, mainly in tectonic breccia pipes. This single, world-class district contains more than 10 Moz of gold, 100 Moz of silver and 1 Mt of copper. At El Indio, the banded alunite and enargite + pyrite veins, peripheral to the main copper and gold veins, suggest alternating fluid conditions prior to the spectacular high-grade gold mineralization accompanied by sericitic-argillic alteration. The δ 34 S, δ 18 O, and δD ratios indicate the 250° to 300°C, moderate to low salinity (<5 wt % NaCl equiv), weakly acidic (pH = 3.5–4.5), reduced mineralizing fluids for both El Indio ore types had a dominantly magmatic water component (>60%), with no evidence of boiling. Copper deposition is attributed to decreasing temperature while precipitation of large quantities of gold is ascribed to mixing with an acid-oxidized fluid. At Tambo, gold was deposited with early barite and again after intermediate-stage alunite, from 200° to 250°C, low-salinity, intermittently boiling fluids. The δ 34 S ratios at Tambo indicate that the barite fluids were mildly reduced (sulfide/sulfate ratio of 10–25) and contained disproportionated magmatic SO 2 . The δ 18 O and δD ratios indicate that alunite formed from condensed, δD-depleted magmatic vapor between gold stages. Calculations show that, within a limited range of dissolved sulfur and pH conditions, a single magmatic fluid could have evolved to produce the multiple mineral assemblages seen at both El Indio and Tambo, with the former in a deeper, more reduced, hydrothermal environment and the latter in a near-surface setting.

Abstract The copper deposits of Perú consist of porphyry Cu±Mo, Au, Ag, breccia pipe Cu-Mo, enargite vein and replacement Cu±Au, Ag, Zn, Pb, calcic skarn Cu±Fe, Au, Zn, amphibolitic skarn Cu±Fe, volcanogenic massive sulfide Cu-Zn, vein and manto Cu±Ag, Pb, Zn, Sn, W, and sandstone (“red bed”) Cu types. The vast majority of these deposits formed during the Andean Orogeny and are geographically and chronologically distributed in well-defined metallogenic domains. These domains correlate with geochemically distinct magmatic episodes. The magmatic and metallogenic domains appear to be controlled in part by transverse growth-faults in the Mesozoic and older basement rocks underlying the intensely folded and thrust-faulted Mesozoic and Tertiary rocks of the higher structural levels of the Cordillera. During the Andean Orogeny the extent of magmatism and the corresponding metallogenic provinces were influenced by subducted plate segmentation and by continental margin basement tectonics. In addition, the lithologic nature of the host rocks played an important role in determining the types of copper deposits formed. Porphyry Cu, breccia pipe Cu-Mo and calcic skarn Cu deposits are related to the Pomahuaca, Coastal and Caldera batholiths, as well as to felsic Cordilleran volcanism between 8° and 12°S. However, the largest and richest porphyry Cu deposits are related to the Caldera batholith. The Cobriza Cu-bearing skarn is the only significant copper deposit of pre-Mesozoic age. Perú has many ore deposits associated with the Miocene felsic extrusive and intrusive rocks along the Cordillera, forming veins and disseminations in igneous rocks and noncarbonate sedimentary rocks, and replacement mantos, pipes and veins in limestones. Several are large and high-grade enargite-type deposits containing mainly Cu, Ag, Au, Pb and Zn, accompanied by significant amounts of Cd, Te, Se, In, Bi and Tl. Others are veins and mantos containing Cu±Ag, Pb, Zn, Sn, W . The Mesozoic volcanosedimentary sequences along the coast host volcanogenic massive sulfide Cu-Zn and vein/manto-type amphibolitic skarn Cu±Fe deposits. Red bed Cu deposits are relatively unimportant in Perú.