Exotic Deposits - Products of Lateral Migration of Supergene Solutions from Porphyry Copper Deposits
Carlos MÜNchmeyer, 1998. "Exotic Deposits - Products of Lateral Migration of Supergene Solutions from Porphyry Copper Deposits", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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Exotic deposits form in conjunction with the supergene oxidation and enrichment of porphyry copper deposits. Supergene alteration primarily involves vertical solution movement, but percolation often involves a horizontal component whereby copper-rich acidic solutions migrate laterally within the vadose zone. Depending on Eh and pH conditions, copper may be transported through paleodrainage systems for distances of up to 8 km from source to produce continuous copper mineralization. The copper is deposited primarily in oxide form, but sulfide copper also may be present close to source.
Favorable conditions for formation of exotic deposits existed between latitudes 12° and 27° Sin northern Chile and southern Perú. In Chile, twelve deposits are recognized, ranging from large deposits like Exotica, El Tesoro, and the recently discovered Damiana (1.2 to 3.5 million metric tons of Cu), through medium-sized ones like Huinquintipa and Sagasca (160,000 to 400,000 metric tons of Cu), to small ones like Quebrada Blanca and La Planada (100 to 10,000 metric tons of Cu).
Geomorphologic conditions of the paleochannels and the chemical reactivity of the rocks and sediments in them influence the shapes of the exotic deposits confined to paleochannels (e.g., Sagasca). Other deposits are fan-shaped and cover large areas on the slopes of topographic highs underlain by the source deposits (e.g., Damiana). A few small deposits are controlled by faulting, the orientation of which controls their form (e.g., Ichuno).
As a result oflateral flow of copper-rich solutions, exotic deposits display zoned alteration and mineralization patterns controlled by the reactivity of the host rocks, the CulFelS ratios of the source deposits, and the evolution of pH and redox state of solutions during deposit formation. Mineralization characteristics and mineral associations enable subdivision of deposits into proximal (0 - 2 km), intermediate (2- 4 km), and distal (4-6 km) zones. The changes in solution pH over time, from acidic to relatively alkaline, may give rise to vertical zoning.
The origin of exotic deposits is linked to the transpressive tectonism and rapid uplift that produced the Andean chain in the late Oligocene-early Miocene interval. The generation of paleorelief caused water tables to fall, favoring formation of supergene enrichment and exotic copper deposits. The process was terminated by the change from semiarid to extremely arid conditions and, locally, by the deposition of extensive ignimbrite flows that sealed off access of supergene solutions.
An empirical model for exotic deposits emphasizes the lateral and vertical zonation of mineralization and alteration. This zoning is interpreted in terms of successive solution fronts acting repeatedly during the period of mineralization. Low-pH solutions percolated through fractured rocks and rendered them impermeable and unreactive. Subsequently, more alkaline groundwater advanced over the impermeable kaolinized horizons, reaching higher levels and greater distances from the source and precipitatingoxide copper minerals in either unaltered gravels or basement rocks.
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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,