Geology of the Manto Verde Copper Deposit, Northern Chile: A Specularite-Rich, Hydrothermal-Tectonic Breccia Related to the Atacama Fault Zone
TomÁS Vila, Zamora Richard, Nicholas Lindsay, 1998. "Geology of the Manto Verde Copper Deposit, Northern Chile: A Specularite-Rich, Hydrothermal-Tectonic Breccia Related to the Atacama Fault Zone", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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The Manto Verde copper deposit is associated with specularite-rich breccias emplaced during extensional tectonism along brittle faults of the Atacama fault zone (AFZ). The AFZ is a complex sinistral strike-slip/dip-slip system that originated during oblique subduction in a Jurassic to Early Cretaceous magmatic arc. Activity of the AFZ is documented from the Middle Jurassic, with intermittent movements occurring at least into the late Miocene. South of Manto Verde, the AFZ is a major control on the emplacement of apatite-bearing magnetite deposits in the Cretaceous Iron Belt (CIB).
The Manto Verde deposit is hosted by cataclased andesitic volcanics and coeval (?) diorite porphyry stocks of Early Cretaceous age affected by low-grade regional metamorphism. The deposit consists of three breccia units paralleling the Manto Verde fault (MVF) for at least 1500 m. The MVF is a 12 km long, north-northwest — striking- 40°-50°E — dipping structure that joins two major branches of the AFZ, and shows evidence of premineralization strike-slip motion followed by postmineralization, east-side-down, dip-slip movement. The hanging - wall Manto Atacama breccia is a ±100 m wide, specularite-rich, matrix-supported hydrothermal breccia that follows the MVF along strike and down dip, although it becomes narrower with depth. A Transition Zone is defined between the Manto Atacama breccia and the andesitic country rocks as brecciation intensity and associated copper mineralization decrease progressively away from the MVF. In contrast, the footwall Manto Verde breccia is represented by a ±20m wide cataclasite displaying a relatively sharp mineralization boundary.
These units are all deeply oxidized to 200 m, so that the present mineralogy consists of a suite of copper carbonates, sulfates, and silicates with minor copper chlorides, which occur as fracture fillings, patches, and disseminations. Only a thin, poorly developed supergene enrichment zone exists. Hypogene sulfide mineralization is poorly explored and corresponds to chalcopyrite ± pyrite disseminated in a specularite matrix.
District-wide chloritization and K-silicate alteration, characterized by microcline replacement and veining of the andesitic country rock, is recognized. Superimposed sericitic alteration is localized by the MVF and neighboring fracture zones. This late alteration event accompanied the main deposition of specularite and associated copper(-gold) mineralization dated by the KlAr method at 121 ± 3 and 117± 3 Ma.
Preliminary studies indicate a predominance of three-phase fluid inclusions as well as the coexistence of vapor-rich and liquid-rich, two-phase inclusions evidencing boiling. Homogenization temperatures for two and three-phase inclusions in translucent quartz veinlets and fragments accompanying early iron - copper - (gold) mineralization range between 180° and 320° C, although they are mainly between 180° and 250° C. Salinities for the three-phase inclusions vary from 30 to 47 wt percent NaCl equivalent, although a decrease in fluid salinity (14-21 wt % NaCl equiv.J is detected in two-phase inclusions from late-mineralization, chalcopyrite-bear-ing calcite veins.
The geologic features of the Manto Verde deposit suggest a common magmatic-hydrothermal origin with the apatite-bearing magnetite deposits of the CIB. Manto Verde probably represents the copper-rich, specularite-dominated, and possibly younger end member of a continuum of deposits that extends to copper-poor, magnetite-dominated deposits at the other extreme.
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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.
The following information on the history of copper mining in Perú has been condensed largely from Samame (1979), Petersen et al.(1990) and Benavides (1990).
In Perú, gold and silver were apparently used before copper. The latter was first mined and processed by the pre-Inca Chimú culture along the northern coast and by the Tiahuanaco civilization in the Lake Titicaca region.
Copper became an important metal during the Inca period,