Abstract EN:

Located between 25°30’5and 31°S latitudes, the Chilean Iron Belt constitutes the first important mineralization episode of the Andean orogeny. It is made up of more than 40 deposits of economic interest, aligned along the axis of the Peruvian-Chilean Trough. El Algarrabo area has been chosen for a case study. The magmatic formations, emplaced in a subduction environment, are characterized by: A bimodal volcanism, Neocomian in age, emplaced in an island­ arc, with large volumes of andesitic s. 1. Lava flows, lately metamorphosed by cooling plutons. The major deposits of the Belt are located at their contacts. A plutonic suite, markedly calc-alkaline, with a development of intermediate rock types. The oldest and most abundant, at the present erosion level, are cpx-opx bearing diorites, which have crystallized under increasing f. 02, and which could ultimately liberate their iron during mineralization processes. The iron deposit is composed of magnetite, locally associated with tremolite, apatite and sulphides (pyrite-chalcopyrite). During the cooling, after the consolidation of the plutons (850-900°C), the super-critical fluid phase is exsolved and contaminated by external waters. This would result in a H+ and Cl- enrichment and in the sequential leaching of Fe (at less than 700°C), then Ca and Mg (between 500 and 600°C) from minerais of the primary magmatic assemblage (facies 1): iron-titanium oxides, clinopyroxenes, orthopyroxenes and calcic plagioclases. The metallic elements are carried in a cationic form, associated with C1- anions, toward the cooler wall-rocks where they are concentrated and precipitated. As a consequence of the centrifugal Sense of cationic leaching, residua parageneses in the plutons (epidiorites) are concentrically zoned, resulting from a sequence of retromorphic assemblages (facies 2 to 5). Correlatively, wall-rocks have evolved as skarns under conditions of the greenschist amphibolite facies boundary (450°C). Plutonic magmas, emplaced at shallow levels (4 km), yielding a basic to intermediate composition, crystallizing from a dry melt and controlled by regional structures, constitute the major factor of the iron deposition in a subduction domain. On the opposite, potassic alteration of hydrous magmas, evolving as closed systems, would promote porphyry-copper deposition processes.