The Late Miocene to Early Pliocene RÍO Blanco-Los Bronces Copper Deposit, Central Chilean Andes
L. Serrano, R. Vargas, V. Stambuk, C. Aguilar, M. Galeb, C. Holmgren, A. Contreras, S. Godoy, I. Vela, M. A. Skewes, C. R. Stern, 1998. "The Late Miocene to Early Pliocene RÍO Blanco-Los Bronces Copper Deposit, Central Chilean Andes", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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Rio Blanco-Los Bronces, one of three giant late Miocene to early Pliocene copper deposits in the Andes of central Chile, formed as a result of emplacement of both multiple mineralized breccias and porphyry intrusions into early and middle Miocene plutonic rocks and Cenozoic lavas. The deposit is distinctive in that a significant proportion of the >50x106 metric tons of Cu it contains occurs within the matrix of mineralized breccia pipes or disseminated in the host rocks around the breccias. Approximately 50 percent of the Cu ore in the deposit occurs as breccia-matrix, stockwork, and disseminated mineralization in a zone of potassic alteration which formed during the emplacement of biotite-rich breccias of the Rio Blanco breccia complex and quartz monzonite porphyry intrusions. Following a period of uplift and erosion, younger tourmaline-rich breccia pipes, containing the other 50 percent of the Cu in the deposit, and weakly mineralized early Pliocene porphyries were emplaced both within and peripherally to the earlier zone of biotite breccias and potassic alteration. Clasts within the tourmaline breccias are sericitized. This sericitic alteration developed during the emplacement of these breccias, later than and independent of the earlier potassic alteration.
Fluid inclusion and 0-, S-, and H-isotopic data indicate that the metal-rich fluids that generated both the older biotite-rich and younger tourmaline-rich breccias, and caused the potassic and sericitic alteration associated with these two breccia generations, were magmatic in origin. Sr- and Nd-isotopic data imply that the magmas that exsolved the breccia-forming fluids, as well as those that formed the late porphyries, were distinct isotopically from the older host rocks of the deposit. The breccia-forming fluids are believed to have exsolved from magmas that crystallized to form plutons that are still not exposed at the surface, consistent with the deep, as yet unknown roots of these breccias.
The emplacement of the mineralized breccias and porphyries at Rio Blanco-Los Bronces occurred in conjunction with late Miocene changes in Andean magma chemistry and crustal thickness, within a period of<3 m.y. during the final stages of>15 m.y. ofMiocene magmatic activity in central Chile. The temporal changes in magma chemistry, the crustal thickening, uplift, and erosion which caused the younger mineralized tourmaline breccias to be superimposed on the earlier and deeper potassic alteration zone, and the decline of igneous activity in the Miocene magmatic belt all resulted from decreasing subduction angle beneath central Chile beginning in the middle to late Miocene.
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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,