Sir: The El Laco ores occur as massive, horizontal, tabular bodies, as crosscutting dikes and/or vein networks, and as stratified, fragmental material. A controversy exists as to their origin. One group of investigators believes the orebodies formed from iron oxide magma that intruded the local volcanic sequence and in places erupted at the surface (see Naslund et al., 2002). Another group believes the deposits formed from hot, iron-rich fluids that completely replaced older silicate rocks (see Rhodes and Oreskes, 1999, and Rhodes, et al., 1999). Sillitoe and Burrows (2002) describe a number of outcrop features at El...

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