The Cretaceous magnetite (apatite-actinolite) ore deposits of the Chilean territory are grouped in a N-S narrow band, extended between latitudes 25° and 31°S. This paper relates the principal stage of iron mineralization to a period of crustal extension and mafic magmatism (125-110 m.y.), associated to a low and uniform convergence speed of the lithospheric plates (FRUTOS, 1981).
Several hypothesis proposed for the genesis of these deposits are discussed. As a general conclusion, the Cretaceous iron mineralization is considered to be a result of the coincidence of particular tectonic and magmatic conditions. Hydration of dry mafic magmas by deep ground water probably had a major role in the genesis of the deposits. As a general principle, it is postulated that dry mafic magmas, with low Fe clinopyroxene as dominant ferromagnesian phase, have a higher iron mineralization potential than the hydrous hornblende-rich ones, where a larger part of iron is retained by the silicate phase.