Abstract

Iron ores of the Kiruna type are unusual formations restricted both in time and space. They are principally found in the Precambrian of Sweden and Missouri. The purpose of the present paper is to summarize the features that characterize the ores and to discuss their origin. It is concluded that the ores are intrusive-magmatic. The exhalative-sedimentary hypothesis is rejected. The following observations support the magmatic hypothesis of formation:The nature of the host rocks--exclusively acid to intermediate volcanics of alkali-rich character; the contact relations between ore and host rocks--the ore behaved as a younger igneous body and often occurs as a complex network of veins and dikes ("ore breccia") in the host rocks; alteration of the host rocks--formation of skarn minerals (amphibole, occasionally pyroxene or garnet) in connection with the ore and local silicification and sericitization at a late stage of ore formation; internal relationship between different ore phases--apatite-rich varieties are always younger and intrude the apatite-poor varieties; chemical composition of apatite--it differs from that in sedimentary apatites both in content and distribution of fluorine-chlorine-hydroxyl and rare earth elements; and co-existence in the same region of hydrothermal iron ores (hematite impregnations) and apatite-bearing iron ores--the intimate relationship between these types of deposit is indicated by an enrichment of barium, while the late-stage hydrothermal ores are impoverished in trace elements.

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