Abstract

Magnetite lavas and feeder dikes on the flanks of the volcano El Laco in the Chilean Andes are characterized by textures demonstrating rapid crystal growth from supersaturated melts. Columnar magnetite, a conspicuous form of magnetite at El Laco with occasional dendritic branching, has been found in two other apatite iron provinces: the Cretaceous iron belt in Chile, a 600-km-long zone along the Pacific with about 40 deposits, and the Early Proterozoic Kiruna ore field in Sweden. Presence of columnar magnetite in an iron ore is suggested to be diagnostic of a magmatic origin. Platy magnetite, another dendritic form widespread at Kiruna, also occurs at El Laco. Moreover, many ores of the three provinces contain pyroxene or pseudomorphs after it with dendritic morphology. The occurrence of similar rapid-growth textures in the investigated apatite iron ores demonstrates a similar origin with emplacement of ore magmas at or near the surface. In fact, existence of vesicular ore lava and pyroclastic ore at Kiirunavaara shows that this deposit is volcanic.A common origin of the ores is supported by similar compositions of their magnetites. Analysis of ca. 50 concentrates from 17 deposits shows that the magnetites are very poor in Cr (<10 ppm) and relatively rich in V (ca. 1,000-2,000 ppm); the Ti content is typically low (ca. 100-1,000 ppm, with occasional values up to 5,000 ppm). Common ranges (in ppm) for other elements are Al = 200 to 1,500, Mg = 500 to 2,000, Mn = 200 to 900, Ni = 100 to 250, Co = 20 to 140, Zn = 50 to 120, and Cu = 10 to 50. The magnetites from El Laco and Kiruna are remarkably similar with the exception of Mg values which are about five times higher at El Laco (4,000-8,000 ppm). Magnetite in sedimentary ores appears to be significantly lower in V.

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