Base-metal veins of the Peruvian Andes
Peru is world-renowned as a major silver producer, and much of this metal comes from rich mineral veins that are polymetallic in character that have credits for Pb, Zn, Au, and Cu. During my research in Peruvian geology I had the opportunity to map the quadrangles covering the Castrovirreyna, Huachocolpa, and Julcani base-metal vein districts in central Peru. Each of these programs included conducting structural studies of the veins from both surface and underground exposures. Furthermore, over the years, I have had the opportunity to visit many of the major base-metal camps in Peru, and have worked compiling in GIS the vein patterns for all of the mineral districts. This summary on the base-metal veins takes a look at the patterns held in common between all of these camps, and places it into the tectonic framework that controlled making similarly oriented polymetallic veins.
The below map shows at the same scale the vein patterns in the districts of Quiruvilca, Ticapampa, Morococha, Castrovirreyna, Huachocolpa, and Julcani. It will become immediately evident to even the non-geologist that shapes and directions of the veins between these different camps have some repeating elements. These are not accidental, the orientation of the veins have to do with the Miocene crustal stresses related to the processes of plate subduction along the western margin of South America. All of these camps formed under a similar situation of paleo-stress, having in common EW-oriented horizontal principal direction of compression, the intermediate stress axis was vertical, and the least direction of stress was north-south. These principal directions of stress in structural geology are referred to as sigma1, sigma2, and sigma3, respectively. During major phases of regional folding, the sigma1 direction was similarly EW-directed, but the sigma2 or intermediate stress axis was horizontal, running parallel to the fold hinges. One should note that during maximum compression phases vertical to moderate dipping veins should not form; this direction of geological forces would result in sub horizontal veins, a feature typically not seen in Peru. An in-depth mechanical analysis of paleo-stress versus the vein patterns can be found in my report called “Evaluation of conjugate vein formation in the Huachocolpa base-metal district of central Perú“. The vein patterns formed during sigma1 EW horizontal, sigma2 vertical, and sigma3 horizontal towards the NS comes with veins lying within the sigma1-sigma2 plane, a type of fracture known as Mode 1 formation. The second set of directions that develop are conjugate veins, veins that will either be NW or NE oriented. Under the Peru paleo-stress configuration the conjugate veins will typically have lateral fault slip along them, which will emphasize ore shoots formed in dilational bends along the veins.
An additional complication that comes during vein formation is the interference in the paleo-stress field by the growth of the veins. This topic was addressed in my study “Undulatory silver-rich polymetallic veins of the Castrovirreyna district, central Peru: fault growth and mineralization in a perturbed local stress field“. The Castrovirreyna district has very well developed examples of multiple-ordered curved veins that come from feedback between adjacent veins during their growth. Stress interference patterns, by the veins, and also in heterogeneous aspects of the host rock, result in curving veins that deviate from the main controls of EW, NE, and NW sets.
Many lesser but nonetheless significant mining districts in Peru (Casapalca, San Cristobal, Colqui, Huraon-Aminon, Raura, Iscaycruz, Colquijirca, and Hualgayocc, to name a few) that formed during the late Cenozoic Era have similar structural controls as the major camps outlined in this summary. Sometimes these will only have a couple of veins, but the pattern can immediately be placed in terms of the vein formation. There are periods of time in Peru’s paleo-stress when arc-perpendicular tension resulted during gravitational collapse of the over thickened Andean orogen. Mineralization during these periods may result with veins being oriented parallel with the major longitudinal faults, but because the Peru margin has been rotated counter-clockwise, many of these tensional events yielded NS-oriented dikes and veins, keeping consistent with the still EW-directed convergence. A fine example of NS-striking earliest Pliocene dikes is found in the Huancavelica mercury district.
The Julcani district has an additional control on the structural pattern resulting from the point-source loading configuration of dacite dome intrusion. The earliest formed structures, tourmaline cemented pebble dikes, developed with a radial pattern (Shelnut and Noble, 1985). With time the paleo-stress field at Julcani reverted to the far-field directions and a superposition of conjugate vein patterns for the most metal mineralized veins in the district.
The vein patterns shown in the same-scale maps in this summary were not just copied from the published geology. In each case the locations of surface workings observed in Google Earth aerial photographs were used to exactly locate them in map space. The vein patterns at Quiruvilca and Morococha were done with additions from the below cited papers. Not all veins in a district are completely exposed at the surface, so some of the traces result for projecting them upwards to surface from the underground mine workings. The veins at Castrovirreyna, Huachocolpa, and Julcani were all mapped in the field along with use of black and white stereo photograph pairs. The veins at Ticapampa are all from air photo interpretation, plus included some field reconnaissance.
Understanding the process that formed the Peruvian base-metal veins is important. These are very significant districts for the Peruvian economy, and have been over several centuries. Some of the earliest mined veins in the Castrovirreyna district during Spanish exploitation had silver grades locally running up to 2,000 ounces per tonne of rock. Production numbers from all of these mining camps are limited because not everything was documented during the earlier Spanish extraction. Here are some rough estimates in millions of ounces (M Ag) of silver produced: Castrovirreyna ~200M Ag, Julcani ~85M Ag, Morococha ~250M Ag, and Quiruvilca ~50M Ag. Lacking proper records is one of the biggest shortfalls in economic geology and mining, having records on historic production from the most significant mines in many countries is crucial. Over the last twenty years I have hardly discovered any values on silver production for most of the camps in Peru. Current operators promote resources left in the ground to be mined without taking any sort of pride in the endowments of the districts in which they produce. National governments in South America have not done any better in tracking the mineral assets of their respective countries. Of course a certain part of this situation in not documenting the total output of precious metals comes from tax evasion and corruption. The rest of not tracking the total production of districts may be attributable to ignorance and lack of respect for the resource history.
The Peruvian base-metal veins continue to employ thousands of miners. Peru remains the world’s second largest producer of silver today (Mexico holds first place). During many years Peru held the top position for world silver production. U.S. Geological Survey silver production statistics for Peru amounts to 4,500 Mt during 2017 and during the last three decades the country produced 3.1 billion ounces of silver. Over the last two decades significant silver production has also come from large open-pit mines such as that of Antamina. Silver is part of the Peruvian culture from artifacts made by the Incans/pre-Inca civilizations to Spanish Catholic church adornments and silver-plated elaborate altars. Silver is widespread in locally crafted jewelry in Peru. With over 450 years of base-metal vein mining in Peru it is certain that this mining tradition will continue for many additional decades to perhaps centuries.
James M. Wise, July 2018
Annual silver production date from the U.S. Geological Survey
References
Bartos, P.J., 1987, Quiruvilca, Peru: Mineral zoning and timing of wall-rock alteration relative to Cu-Pb-Zn-Ag vein-fill deposition: Economic Geology, v. 82, p. 1431-1452.
Catchpole, H., Kouzmanov, Putlitz, B., Hun Seo, J., and Fontboté, L., 2015, Zoned base metal mineralization in a porphyry system: Origin and evolution of mineralizing fluids in the Morococha district, Peru: Economic Geology, v. 110, p. 39-71.
Lewis, R.W., 1956, The geology and ore deposits of the Quiruvilca district, Peru: Economic Geology, v. 51, p. 41-63.
Nagell, R.H., 1960, Ore controls in the Morococha district, Peru: Society of Economic Geologists Bulletin, v. 55, p. 962-984.
Shelnutt, J.P., and Noble, D.C., 1985, Premineralization radial dikes of tourmalized fluidization breccia, Julcani district Peru: Economic Geology, v. 80, p. 1622-1632.
Wise, J.M., 2010, Evaluation of conjugate vein formation in the Huachocolpa base-metal district of central Perú: Boletín de la Sociedad Geológica del Perú, v. 104, p. 59-80.
Wise, J.M., 2005, Undulatory silver-rich polymetallic veins of the Castrovirreyna district, central Peru: fault growth and mineralization in a perturbed local stress field: Economic Geology, v. 100/4, p. 689-705.
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