NEW FIELD EVIDENCE BEARING ON THE ORIGIN OF THE EL LACO MAGNETITE DEPOSIT, NORTHERN CHILE--A DISCUSSION

Abstract

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 Laco which they suggest support a replacement origin. All of the features they describe, however, are consistent with the magmatic hypothesis, and many are inconsistent with a replacement origin. Blocks and fragments of andesite within magnetite ore at Laco Sur range in size from centimeters to >5 m and comprise roughly 5 vol percent of the orebody (Sillitoe and Burrows, 2002). In places, subhorizontal trains of blocks occur, and veins of magnetite cut some blocks. These observations, however, do not uniquely support a replacement origin for the ore. We interpret the magnetite orebody at Laco Sur to be a near-vent facies volcanic deposit with a mixture of viscous Fe oxide lavas, interbedded Fe oxide pyroclastic material, and xenoliths, as would be expected in this facies. Andesite blocks in friable magnetite tend to lie along particular horizons within the deposit as would be expected for air-fall deposits. All of the blocks have razor-sharp contacts with the enclosing magnetite. Many are altered to pyroxene and other silicates, but the only magnetite in these blocks is in the form of minor amounts of small, crosscutting veins. Where blocks are enclosed within massive ore, which we interpret to be iron oxide lavas, the magnetite is very …