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The magmatic to magmatic-hydrothermal evolution of the El Laco Deposit (Chile) and its implications for the genesis of magnetite-apatite deposits

Fernando Tornos, Francisco Velasco and John M. Hanchar
The magmatic to magmatic-hydrothermal evolution of the El Laco Deposit (Chile) and its implications for the genesis of magnetite-apatite deposits
Economic Geology and the Bulletin of the Society of Economic Geologists (November 2017) 112 (7): 1595-1628

Abstract

The geology and geochemistry of the El Laco iron oxide deposit (Central Andes, Chile) support a genesis related to the ascent, degassing, and subvolcanic emplacement of an unusual oxidized silica-poor but water-and iron-rich melt that took place during the growth of the host Pliocene-Holocene andesitic volcano. The model proposed in this paper for the evolution of the deposit involves the formation of a shallow telescoped magmatichydrothermal system with complex melt-fluid unmixing in a vertical column of less than 1 km. The dominant mineralization occurs as large stratabound apatite-poor magnetite bodies interfingered with an andesite host and rooted in vertical dikes of magnetite with minor apatite. The stratabound mineralization is crosscut by abundant coeval diatreme-like structures indicative of vigorous degassing. The andesite underlying the mineralization is pervasively replaced by a high-temperature alkali-calcic alteration assemblage (K feldspar-diopside-magnetite-scapolite) that includes coarse-grained diopside-magnetite-anhydrite veins and large subvertical bodies of magmatic-hydrothermal breccias. The host andesite also shows a large strata-bound steam-heated acid alteration that is devoid of any magnetite but has produced the replacement of a significant proportion of the early magnetite by hematite. The El Laco system is rich in anhydrite but poor in sulfides, suggesting that there were persistent oxidizing conditions that inhibited the formation of a sulfide-bearing mineralization.Field evidence, oxygen isotope geothermometry, and thermodynamic constraints suggest that the magnetite mineralization formed close to the surface at temperatures above 800 degrees C. The magnetite textures, similar to those of subaerial low-viscosity basalts, and the presence of melt inclusions in the host andesite recording the presence of immiscible Fe-Mg-Ca-(Si-Ti-P-S) and Si-K-Na-Al melts, suggest that the magnetite ore formed by direct crystallization from an iron-rich melt; its chemistry inhibited the formation of most other magmatic phases except minor apatite, anhydrite, and diopside. The crystallization of the iron-rich melt at shallow depths promoted the separation of large amounts of two immiscible aqueous fluids: a dominant low-density vapor phase and a small volume of hypersaline fluid. Diopside-magnetite-anhydrite veins are interpreted as the product of the crystallization of the residual melts, whereas the interaction of the brine with the host andesite formed the deep alkali-calcic hydrothermal assemblage. The condensation and mixing of the low-density magmatic vapor with meteoric water produced the steam-heated alteration.Isotope data from the host andesite ( (super 87) Sr/ (super 86) Sr: 0.7066-0.7074; epsilon Nd: -5.5 to -4.1; delta (super 18) O (sub whole rock) : 7.2-9.6 ppm; delta (super 18) O (sub magnetite) : 5.1-6.2 ppm) and an underlying andesite porphyry ( (super 87) Sr/ (super 86) Sr: 0.7075-0.7082; epsilon Nd: -5.9 to -4.6) reflect the interaction of a primitive mantle melt with Andean crustal rocks. The isotope geochemistry of the magnetite ore ( (super 87) Sr/ (super 86) Sr: 0.7083; epsilon Nd: -5.4 to -5.1; delta (super 18) O 3.5-5.5 ppm) and the alkali-calcic alteration and related diopside-magnetite-anhydrite veins ( (super 87) Sr/ (super 86) Sr: 0.7080-0.7083; epsilon Nd: -5.1 to -4.6; delta (super 18) O (sub diopside) : 7.2-8.2%c; delta (super 18) O (sub magnetite) 4.4-6.3 ppm) show that the mineralization has a more crustal signature than the host andesite and all the volcanic rocks of the Central Andes. Therefore, ore-forming fluids/melts were not equilibrated with the host volcanic rocks and are interpreted as related to a deep yet undiscovered batch of highly contaminated igneous rocks. Crustal contamination is interpreted as due to major interaction of a juvenile melt with the underlying Late Mesozoic-Tertiary Salta Group, located 1 to 6 km beneath the volcano and which has high (super 87) Sr/ (super 86) Sr values (0.7140-0.7141).


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 112
Serial Issue: 7
Title: The magmatic to magmatic-hydrothermal evolution of the El Laco Deposit (Chile) and its implications for the genesis of magnetite-apatite deposits
Affiliation: Consejo Superior de Investigaciones Cientificas, Centro de Astrobiologia, Madrid, Spain
Pages: 1595-1628
Published: 201711
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 169
Accession Number: 2018-025702
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 2 tables, geol. sketch map
S56°00'00" - S17°45'00", W76°00'00" - W67°00'00"
Secondary Affiliation: Universidad del Pais Vasco, ESP, SpainMemorial University of Newfoundland, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 2018
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