Metasomatismo y alteracion hidrotermal en el Distrito Ferrifero Cerro Negro Norte, Copiapo, Chile

Abstract

ABSTRACT. Metasomatism and hydrothermal alteration in the Cerro Negro Norte iron ore district, Copiapo, Chile. The magnetite-apatite ores in the Cerro Negro Norte iron district, Copiapo, are hosted by hydrothermally altered andesitic volcanic rocks. As a result of the hydrothermal activity, the primary mineralogy of the volcanic rocks was changed, the original textures obliterated and hydrothermal breccias were formed close to the orebodies. The hydrothermally altered country rocks display a mineralogical and geochemical zonation around the orebodies, with an internal halo of actinolitized rocks followed outward by silicified ± tourmalinized rocks. Patches of albite-rich rocks are enclosed within the silicified ones. Tourmaline, quartz-tourmaline, and actinolite veins cut the iron ore and its altered wallrocks. The actinolitized rocks have been strongly enriched, metasomatically, in Ca, Mg, Fe, P, F, CI, V, Au, and B, and depleted in La, Ce, Nd, Sm, Na, K, Ti, Sr, Zr, Zn y Hf. The albitized rocks are characterized by a high Na/K ratio and with the exception of Si and Na, they are depleted in all major elements. Zr, S, V, Zn and Au display a distinct enrichment. Turmalinized rocks have a strong B enrichment, and a moderate increase of the volatile content, Zr, S, V and As. The cross-cutting relationships of the different mineral assemblages, representative of different hydrothermal events, along with the chemical composition of the altered rocks suggest a hydrothermal solution initially characterized by high contents of Ca, Mg, Fe, Na, P, F, CI, and CO2, which produced actinolitization and albitization around the orebodies. Coexisting plagioclase with actinolite in the internal halo of altered rocks and presence of albite-rich rocks are compatible with a neutral to slightly alkaline fluid. The hydrothermal fluid evolved with time to one rich in Si, B, and slightly acid pH, which produced the silicification and tourmalinization of the volcanic rocks occurring on the external alteration halo around the ores. The mineralogical and geochemical zones around the orebodies reflect thermal and compositional gradients in the hydrothermal fluids produced during the emplacement of the massive iron orebodies.