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.