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 (1.1), 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 M Oyarzun, 1PQR).! 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 ACdNOWLEDGEMENTS 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 alteration with dissolution or REFERENCES replacement of the anhydrite by gypsum, of plagioclase by alunite, Broman, C., Nystrm, J.L., Henríquez, F., formation of Fe-rich smectite clays Elfman, M., (1): Fluid inclusions in (nontronite) and the replacement of magnetite-apatite ore from a cooling magmatic system at El Laco, Chile. GFF the diopside by saponite and 121, 2-26. magnetite by hematite. Frutos J., J., Oyarzun M., J., (1): Tectonic ! d) late and local silicification, with and geochemical evidence concerning the formation of tridymite, amorphous genesis of El Laco magnetite lava flow silica, and rutile-anatase, replacement deposits, Chile: Economic Geology, Q0, 0. f ig. 2 (a, upper) Recrystallized magnetite annealed of the alunite by jarosite and growth Henríquez, F., Naslund, H.R., Nystrm, J.O., during cooling (b, lower) Detail of euhedral of zones with gypsum (rubbly Vivallo, W., Aguirre, R., Dobbs, F.M., Lledó, magnetite with zonal growth pattern barely gypsum), silica and sulfur with minor discernable after etching. Width of fields is 2.0 mm. H., (200): New field evidence bearing on P olished sections etched with HCl. variscite. This late process was related the origin of the El Laco magnetite deposit, with channelized fumarolic activity. northern Chilea discussion: Economic ! a) an early high-temperature Geology P^, 1100. hydrothermal lining of the degassing The formation of the above Naslund, H.R., Henríquez, F., Nystrm, J.O., Vivallo, W., Dobbs, F.M., (2002): Magmatic tubes by euhedral magnetite and hydrothermal minerals graded to iron ores and associated mineralization: diopside with local scapolite, sanidine, common supergene products such as the replacement of magnetite to Examples from the Chilean High Andes and albite, and anhydrite; Coastal Cordillera, in Hydrothermal iron ! b) likely at the same time ocurred hematite and pyrite to goethite. oxide copper-gold and related deposits: A the widespread high temperature and global perspective v. 2, Porter, T.M., ed.,:, early alkaline pervasive hydrothermal CONCLUDING REMARS PGC Publishing, Adelaide. 20226. alteration of the andesite underlying Nystrm, J.O., Henríquez, F., (1): the magnetite orebody, which The geology of El Laco deposit shows Magmatic features of iron ores of the diruna type in Chile and Sweden: Ore includes the replacement by albite, the existence of a large zone of textures and magnetite geochemistry: diopside, magnetite, biotite- stratabound steam heated hydrothermal alteration which is part of a continuum Economic Geology ^P, 20. phlogopite, quartz and marialite and Rhodes, A.L., Oreskes, N., Sheets, S.A., abundant veins with the same with the extrusion and crystallization of (1PPP): Geology and rare earth element assemblage. The melt and fluid the iron melts, a deeper alkaline geochemistry of magnetite deposits at El inclusions in the diopside crystals alteration and vigorous degassing of the Laco, Chile, in nGeology and Ore Deposits suggest the relationship with melts. This acid alteration is interpreted of the Central Andes B.J. Skinner, ed. hypersaline brines at circulating at as related with the replacement of the Special Publication Society of Economic Geologist Q, 2PP-ii2. 710-840ºC (Broman et al., 1999). andesite by acid fluids resulting from the mixing of vapour degassed from the Rhodes, A.L., Oreskes, N., (1): Oxygen ! c) a pervasive low temperature water-rich iron oxide melts during their isotope composition of magnetite deposits (<250ºC) stratabound but fault- at El Laco, Chile: evidence for formation crystallization and mixing with surficial controlled acid alteration that caused from isotopically heavy fluids, in nGeology waters, likely of nival derivation. Such a the replacement of the andesite, and Ore Deposits of the Central Andeso B.J. large and unusual alteration zone, Skinner, ed. Special Publication Society of including the previous alkaline almost exclusively made up of alunite, Economic Geologist , -1. alteration by crystalline and can only be interpreted as related with Sillitoe, R.H., Burrows, D.R., (2002): New amorphous alunite with minor extremely aggressive and unusually Field Evidence Bearing on the Origin of the amounts of cristobalite and anatase advanced alteration. The underlying El Laco magnetite Deposit, Northern Chile. synchronously with the widespread alkaline alteration is here interpreted as Economic Geology PQ, 1101-110. oxidation of magnetite to hematite Tornos F, Velasco F, Morata D, Barra F, Rojo related with the circulation of the (nuts); there are no evidences of iron M (2011): The magmatic hydrothermal accompanying condensed brines that oxide precipitation during this stage. evolution of the El Laco deposit as tracked separated from the vapour. In this zone, there are very little clays by melt inclusions and isotope data In: representative of the argillic or Lets Talk Ore deposits. Barra F, Reich M, Field evidences and petrographic data Campos,E., Tornos F eds. Antofagasta, propyllitic alteration zones. The suggest that these three types of -. mineralogy and the morphology of the hydrothermal activity represent a this acid alteration zones suggest that continuous alteration process that took they were produced by steam-heated place from high to low temperatures and processes (Sillitoe & Burrows, 2002), alkaline to acid conditions related to the related with reaction of a SO2, P and liquid-gas separation of fluids degassed F-rich gas with surficial waters and the from crystallizing iron oxide melts. .