Iron-formation in South America

Except for recent studies by certain South American governmental and quasi-governmental companies and agencies, little effort has been devoted to study of the iron-formations from which the great iron ore deposits of South America formed. Great gaps in basic information exist. Iron-formation is found in the Guayana and Brazilian Precambrian Shields as a common rock type and also occurs in Chile and astride the Bolivian-Brazilian border. Only the carbonate and oxide facies are known, the former being quite rare. The dominant oxide facies occurs in major units averaging more than 100 m in thickness and extending over hundreds of square kilometers, generally in a miogeosynclinal or intra-cratonic basin environment. The relation of such deposits with volcanism is tenuous and obscure, if indeed there is any direct relation. Smaller units of oxide facies iron-formation occur in many minor beds from widely varying geologic environments. The carbonate facies is found in a eugeosynclinal suite in Minas Gerais, Brazil, and is of the Algoma type. The deposits range in age from about 3,200 m.y. to late Precambrian or early Paleozoic; although the major epoch of deposition is debatable, it probably was about 2,000 m.y. ago. The South American oxide facies iron-formations are richer than many in the Northern Hemisphere, those of early and middle Precambrian age averaging about 40 percent in Fe and the same in SiO ₂. Younger iron-formations are still richer, averaging perhaps 50 percent Fe. Scanty trace element data do not indicate volcanic affiliations. The iron is present as magnetite, hematite, and martite; most rocks have been metamorphosed, and accordingly it is not known how much of the magnetite is metamorphic and how much is diagenetic or depositional in origin. Hematite and martite are dominant in most iron-formations. South American iron-formations are quite similar in lithology and occurrence to the major deposits in Africa and India and possibly formed when these continents were contiguous. These formations differ from those in the Northern Hemisphere in having a narrower range in lithologic facies and a generally higher iron content. In few areas can any direct relation with volcanism be demonstrated. © 1973 Society of Economic Geologists, Inc.