macla nº 16. junio ‘12 210 revista de la sociedad española de mineralogía
Insights on the Effects of the Hydrothermal Alteration in the El Laco Magnetite Deposit (Chile) / FRANCISCO VELASCO (1. ), FERNANDO TORNOS (2)
(1) Dpto. de Mineralogía y Petrología.Universidad del País Vasco UPV/EHU, Sarriena s/n, 0 Leioa, Spain. (2) Instituto Geológico y Minero de España, Madrid, Spain.
INTRODUCCIÓN
The understanding of the origin of the recent (ca. 2 Ma) El Laco deposit (Fig. 1), with near 1 Gt of almost pure magnetite/hematite, is considered critical for the interpretation of the Kiruna type magnetite-apatite style of mineralization, an end-member of the IOCG group of deposits. Despite the abundant studies conducted in the last decades on El Laco, with little erosion, well preserved volcanic features and excellent conditions of exposure, there is no agreement between models that support a genesis related to the hydrothermal replacement of preexisting andesitic rocks (Rhodes & Oreskes, 1999; Rhodes et al., 1999) and those which interpret the deposit as magmatic flows and dikes product of the crystallization of an iron oxide melt (Frutos & Oyarzun, 1975; Nyström & Henríquez, 1994; Naslund et al., 2002; Henríquez et al., 2003; Tornos et al., fig. 1 Schematic geological map of the magnetite orebodies (black) at the El Laco district hosted in the Plio- 2011). To solve this fascinating Pleistocene andesitic volcanic arc, northern Chile (modified from Frutos Oyarzun, 1 ). controversy is crucial to understand the problem from a global point of view, host rocks. Except for some dikes, most crystals (size mm to several cm) integrating geological and geochemical of the magnetite orebodies (Laco Sur, intergrown with prismatic-acicular data of the magmatic and hydrothermal Laco Norte, S. Vicente Alto, S. Vicente diopside growing perpendicularly to the rocks present in the area. Bajo, Rodados Negros, Laquito) occur as walls. Here, chemical etching shows the stratiform bodies interbedded with presence of well defined zoned crystal The hydrothermal alteration is andesite flows. Selective etching shows growth (Fig. 2b), with continuous widespread in El Laco deposit and that the massive magnetite is made up individual zones some µm in thickness; covers an area of several km2; despite of 0.1 mm sized grains that show they are interpreted as representing having a relationship with the magnetite common 120º dihedral boundaries and pulses of precipitation from a ore, not always the magnetite is hosted development of subgrains, something supersaturated low density fluid with within the alteration halo. interpreted as related with annealing very limited capability of transport of Fe during cooling of a melt. This mosaic that exsolved from the crystallizing melt THE TEXTURAL FEATURES OF THE texture (Fig. 2a) confirms widespread and also indicates that their growth MAGNETITE recovery and recrystallization after the postdate the previous annealing. deposition thereby destroying much of A key question in the interpretation of the evidences of primary deposition. ALTERATION SEQUENCE the genesis of these deposits is the characterization of the mineralogical The stratiform bodies are crosscut by Regional mapping and geochemistry changes undergone by the iron oxide ore numerous pipes or degassing tubes, allow to distinguish four stages of after crystallization and the recognition several meters long and up to 40 cm hydrothermal activity at El Laco: of the extent of the hydrothermal diameter. The walls of these pipes are alteration on both the orebody and the carpeted by octahedral magnetite
palabras clave: El Laco, Yacimiento magnetita, Alteración key words: El Laco, Magnetite deposit, Hydrothermal alteration. hidrotermal. resumen SEM/SEA 2012 * corresponding author: [email protected]
macla nº 16. junio ‘12 revista de la sociedad española de mineralogía 211
host rock. The observed extreme acid AC NOWLEDGEMENTS alteration caused the complete leaching of all cations (except the Si We thank the Department of Education, and Ti) from the andesite. Minor Universities and Research of the Basque amounts of pyrite precipitate during Government (Ref. IT340-10) and the this stage; the low proportion of Spanish project CGL2011-23207 for sulfides here is probably related with economic support and the Compañía the highly oxidized nature of the Minera del Pacífico for granting access melts, with low H2S/SO4 ratios. This to the mine. acid alteration affected the alkaline
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