MAGMATIC FEATURES OF IRON-ORES OF THE KIRUNA TYPE IN CHILE AND SWEDEN- ORE TEXTURES AND MAGNETITE GEOCHEMISTRY

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 consp icuous form of magnetite at El Laco with occasional dendritic branchin g, 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. Prese nce of columnar magnetite in an iron ore is suggested to be diagnostic of a magmatic origin. Platy magnetite, another dendritic form widespr ead at Kiruna, also occurs at El Laco. Moreover, many ores of the thre e provinces contain pyroxene or pseudomorphs after it with dendritic m orphology. The occurrence of similar rapid-growth textures in the inve stigated apatite iron ores demonstrates a similar origin with emplacem ent of ore magmas at or near the surface. In fact, existence of vesicu lar ore lava and pyroclastic ore at Kiirunavaara shows that this depos it is volcanic. A common origin of the ores is supported by similar co mpositions of their magnetites. Analysis of ca. 50 concentrates from 1 7 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 typical ly low (ca. 100-1,000 ppm, with occasional values up to 5,000 ppm). Co mmon ranges (in ppm) for other elements are Al = 200 to 1,500, Mg = 50 0 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 fiv e times higher at El Laco (4,000-8,000 ppm). Magnetite in sedimentary ores appears to be significantly lower in V.