The Magmatic to Magmatic-Hydrothermal Evolution of the El Laco Deposit (Chile) and Its Implications for the Genesis of Magnetite-Apatite Deposits

• DOI: 10.5382/econgeo.2017.4523
• Publication Date: 2019-07-25T17:09:12Z
• Authors: Fernando Tornos; Francisco Velasco; John M. Hanchar
• Keywords: Breccia; Ore genesis; Wolframite

Abstract The geology and geochemistry of the El Laco iron oxide deposit (Central Andes, Chile) support a genesis related to ascent, degassing, subvolcanic emplacement an unusual oxidized silica-poor but water-and iron-rich melt that took place during growth host Pliocene-Holocene andesitic volcano. model proposed in this paper for evolution involves formation shallow telescoped magmatichydrothermal system with complex melt-fluid unmixing vertical column less than 1 km. dominant mineralization occurs as large stratabound apatite-poor magnetite bodies interfingered andesite rooted dikes minor apatite. is crosscut by abundant coeval diatreme-like structures indicative vigorous degassing. underlying pervasively replaced high-temperature alkali-calcic alteration assemblage (K feldspar-diopside-magnetite-scapolite) includes coarse-grained diopside-magnetite-anhydrite veins subvertical magmatic-hydrothermal breccias. also shows strata-bound steam-heated acid devoid any has produced replacement significant proportion early hematite. rich anhydrite poor sulfides, suggesting there were persistent oxidizing conditions inhibited sulfide-bearing mineralization. Field evidence, oxygen isotope geothermometry, thermodynamic constraints suggest formed close surface at temperatures above 800°C. textures, similar those subaerial low-viscosity basalts, presence inclusions recording immiscible Fe-Mg-Ca-(Si-Ti-P-S) Si-K-Na-Al melts, ore direct crystallization from melt; its chemistry most other magmatic phases except apatite, anhydrite, diopside. depths promoted separation amounts two aqueous fluids: low-density vapor phase small volume hypersaline fluid. Diopside-magnetite-anhydrite are interpreted product residual whereas interaction brine deep hydrothermal assemblage. condensation mixing meteoric water alteration. Isotope data (87Sr/86Sr: 0.7066–0.7074; εNd: −5.5 −4.1; δ18Owhole rock: 7.2–9.6‰; δ18Omagnetite: 5.1–6.2‰) porphyry 0.7075–0.7082; −5.9 −4.6) reflect primitive mantle Andean crustal rocks. 0.7083; −5.4 −5.1; δ18O 3.5–5.5‰) 0.7080–0.7083; −5.1 −4.6; δ18Odiopside: 7.2–8.2%c; δ18Omagnetite 4.4–6.3‰) show more signature all volcanic rocks Central Andes. Therefore, ore-forming fluids/melts not equilibrated yet undiscovered batch highly contaminated igneous Crustal contamination due major juvenile Late Mesozoic-Tertiary Salta Group, located 6 km beneath volcano which high 87Sr/86Sr values (0.7140–0.7141).