Copper(-Iron) Mineralization and Superposition of Alteration Events in the Punta Del Cobre Belt, Northern Chile
Robert Marschik, Lluis FontbotÉ, 1998. "Copper(-Iron) Mineralization and Superposition of Alteration Events in the Punta Del Cobre Belt, Northern Chile", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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Two superimposed alteration events control the regional distribution of five main alteration types in the Punta del Cobre belt, south of Copiapó, Chile. An early event of alkali metasomatism caused extensive albitization and was overprinted locally by potassic alteration. Albite-quartz-chlorite or K-feldspar-quartz-chloritelbiotite assemblages characterize the metasomatized rocks. The alteration resulted in marked changes in chemical composition, with extremes of 10 wt percent Na20 or 11 wt percent K2O. The second alteration event was associated with emplacement of a middle Cretaceous batholith, which is exposed in the western part of the area. The intrusion produced contact metamorphism which overprinted, with variable intensity, the areas affected by the alkali metasomatism. The contact effects are expressed as north-northeast-trending, largely overlapping zones which, from west to east, are characterized by: Caamphibole± biotite ± sericite, biotite ± chlorite ±sericite ± epidote, and epidote-chlorite ± quartz ± calcite. The Ca-amphibole is mainly actinolite, actinolitic hornblende, and magnesio-hornblende. The three alteration zones show only minor chemical modifications compared to the inferred average compositions of unaltered precursors.
Chalcopyrite, pyrite, magnetite, and hematite are the principal ore minerals. Alkali metasomatism, in particular potassic alteration, is related spatially to mineralization both east and west of the Copiapó River. In contrast, calcsilicate assemblages, unrelated genetically to the Cu mineralization, occur only west of the river, close to the batholith contact. In the Punta del Cobre and Ladrillos districts, the mineralization appears to be controlled by north-northwest- to northwest-trending faults, which were active since the middle Cretaceous. A middle Cretaceous age for the mineralization is supported by a 40Ar/39Ar inverse isochron age of 114.9 ±0.5 Ma for hydrothermal biotite. The alteration pattern and ore formation temperatures (up to 400°-500°C) support an association of the Cu mineralization with deep-seated magmatic intrusion(s). Fluid inclusion data for postore calcite indicate involvement of saline fluids. A magmatic source of the sulfur is indicated by isotope ratios determined for chalcopyrite and pyrite (834S between -0.7 and +1.1 %o).The deposits in the Punta del Cobre belt are somewhat similar and possibly transitional to both the Chilean magnetite (-apatite) deposits and porphyry copper deposits.
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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,