Clustered, Gold-Bearing Oligocene Porphyry Copper and Associated Epithermal Mineralization at La Fortuna, Vallenar Region, Northern Chile
JosÉ PerellÓ, Felipe UrzÚA, JosÉ Cabello, Francisco Ortiz, 1998. "Clustered, Gold-Bearing Oligocene Porphyry Copper and Associated Epithermal Mineralization at La Fortuna, Vallenar Region, Northern Chile", Andean Copper Deposits: New Discoveries, Mineralization, Styles and Metallogeny, Francisco Camus, Richard M. Sillitoe, Richard Petersen
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The La Fortuna porphyry cluster is centered on a series of porphyritic stocks of Tertiary age that intruded a sequence of Paleozoic rhyolites and Jurassic red beds. Much of the area is covered by ignimbrites and gravels of Miocene age. Structure is dominated by regional, north-trending reverse faults crosscut by local, northwest-trending faults which are intimately associated with the porphyry-style mineralization. Two main types of mineralization are present in the area: porphyry copper-gold at La Fortuna itself and El Negro, and high-sulfidation epithermal gold at Cantarito.
At La Fortuna, copper-gold mineralization is associated with pyrophyllite-rich advanced argillic alteration hosted by dacitic porphyries and hydrothermal breccias. Intermediate argillic alteration (sericite-chlorite) is preserved in deeper portions of the system. The mineralization occurs as supergene sooty chalcocite and hypogene assemblages of chalcocite, chalcopyrite, and bornite. Pyrite, specularite, and tourmaline are also important constituents. A gold-rich leached capping comprises abundant hematite and minor copper oxides.
The El Negro system consists of a quartz diorite porphyry that intruded a sequence of andesitic volcanic rocks. Hypogene copper-gold mineralization is contained mainly in quartz-chalcopyrite-(bornite) stockworks associated with K-silicate assemblages composed of biotite, K-feldspar, and magnetite (averaging 6 vol. %). Copper and gold values display a sympathetic relationship.
The high-sulfidation epithermal gold mineralization at Cantarito, consists of a core of vuggy and massive silica with abundant alunite, which is bordered outward by zones dominated sequentially by kaolinite, sericite, smectite, and chlorite. The mineralization is contained in the siliceous core, where native gold grains are associated with quartz-alunite in hairline fractures, veinlets, and breccias. Pyrite, barite, and scorodite are present locally.
Available radiometric ages suggest that the hypogene copper and gold mineralization in the cluster was generated over a period of 3 m.y., between 35 Ma (K-silicate alteration at El Negro) and 32 Ma (advanced argillic alteration at Cantarito). Synmineralization uplift and erosion are interpreted to have been responsible for the unroofing of the hydrothermal systems and the superposition of epithermal associations over higher-temperature, deeper-seated assemblages.
The style of the mineralization in the La Fortuna cluster displays many of the features that characterize the gold-rich porphyry copper deposits of the Philippines and Indonesia, as well as the porphyry gold deposits of northern Chile.
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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,