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Magmatic Iron Ores and Associated Mineralisation: Examples from the Chilean High Andes and Coastal Cordillera
by
H Richard Naslund, Dept. of Geological Sciences, SUNY, Binghamton, NY, USA, Fernando Henríquez, Departamento de Ingeniería en Minas, Universidad de Santiago, Chile, Jan O Nyström, Swedish Museum of Natural History, Stockholm, Sweden, Waldo Vivallo, Servicio Nacional de Geologia y Mineria, Santiago, Chile, F Michael Dobbs, Departamento de Ingeniería en Minas, Universidad de Santiago, Chile,

in - Porter, T.M. (Ed), 2002 - Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC        Publishing, Adelaide, v. 2, pp 207-226.

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ABSTRACT

The El Laco magnetite-apatite ore deposits in the Andean Cordillera of northern Chile occur as massive, tabular bodies, as stratified, pyroclastic ores, and as crosscutting dykes and vein complexes. The ore deposits and surrounding volcanic rocks, mainly andesites, are Plio-Pleistocene in age and preserve many of their original volcanic textures and structures. All the field and laboratory data are consistent with an origin by eruption and shallow intrusion of a high-temperature, volatile-rich, iron-oxide magma. A number of other iron-oxide-apatite deposits of Cenozoic age in the Andean Cordillera, and a belt of Cretaceous iron deposits in the Coastal Cordillera of Chile also have features that suggest a magmatic origin. Associated with these magmatic ore deposits are economic and sub-economic concentrations of Cu, Au, U, and REE.

CONCLUSIONS & IMPLICATIONS

The field relationships and the well-preserved volcanic structures and textures at El Laco indicate that the main, massive orebodies were crystallised from volatile-rich, iron-oxide magmas, and that the nearby magnetite-pyroxene and magnetite-apatite veins were formed at high temperature from magmatic fluids released from the crystallising magmas. All of the available analytical data are consistent with a magmatic origin for these deposits. There is no evidence to suggest that any of the iron orebodies at El Laco were produced by hydrothermal fluids. The clear evidence for iron-oxide magmas at El Laco suggests that a magmatic origin should be considered for other similar, less-well-preserved, deposits. The nature of such volatile-rich magmas is that they alter their host rock during emplacement and crystallisation. As such, even orebodies with alteration halos and gradational contacts may have formed from iron-oxide magmas. In other localities, hydrothermal breccias and vein deposits may be related to the degassing of an iron-oxide magma at depth.

Economic and subeconomic concentrations of Cu, Au, U, and REE are often associated with iron-oxide-apatite ores, either within the iron-oxide orebodies, or occurring nearby as breccias, veins, or stockworks, or in areas without known magnetite-apatite deposits. From an exploration standpoint it is important to determine if a Cu-(Au-U-REE) deposit is associated with the intrusion or eruption of iron-oxide magma, or unrelated to such magma and formed from hydrothermal processes alone. Iron-oxide magmas that have vented to the surface as ignimbrites, airfall ash, or lavas are less likely to have associated Cu or Au mineralisation because these elements are likely to be partially lost during degassing. Exploration targets for these "vented" systems will be at depth below the iron ores. U and REE are less likely to escape in the vented gases and may be present within the iron orebodies. Iron-oxide magmas that have intruded as sills or dykes are more likely to have associated Cu, U, Au, or REE mineralisations within, adjacent to, or stratigraphically above the iron ores, although additional mineralisation at depth can not be ruled out. Systems, which have formed from hydrothermal solutions, may have associated Cu, U, Au, or REE mineralisations as well. In these deposits, Cu, U, Au, and REE mineralisation will be most likely controlled by fluid circulation pathways, temperature gradients, host-rock permeabilities, and fluid mixing patterns within the hydrothermal system derived from an iron-oxide magma.

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