A Theory of Exhalative-Sedimentary Ores

Abstract

Welded tuffs and similar acid pyroclastics get an increasing geologic importance in the later years. With this as a background the author suggests that one specific geologic process for ore formation is of higher importance than formerly realized. This process is precipitation of metals in volcanic gases, escaping from a crystallizing granitic magma chamber into the sea. From igneous rock provinces like the Oslo region it is learned that gases containing a great number of metals (Fe, Cu, Zn, Pb, Sb, Bi, etc.) are expelled from granite plutons to form contact pneumatolytic (or pyrometasomatic) deposits. It is suggested that such gases, when emitted at the sea bottom, may be the source of the orogenic pyrite and magnetite ore beds in the Caledonides of Norway and Sweden as well as the Rio Tinto type of deposit. The Fosen type of magnetite ore and the Central Swedish iron ores are explained in the same way, because they also are concordant ore beds in a supracrustal rock series, closely associated with acid pyroclastics. Another district with iron ore of generally assumed exhalative-sedimentary origin is the Lahn-Dill district of Germany. It is further suggested that the iron formations, which often, but not always, are genetically connected with volcanism, have their iron source in volcanic exhalations. It is argued that the only probable explanation of the quite suddenly appearing iron in the sea, precipitating as sedimentary iron ore beds, is that the iron is exhalative in origin. By the gas emission it goes into solution and is precipitated when permitted by the chemical environment of the ocean. The factors determining this precipitation are discussed. Sedimentary features are shown by some types of ore beds, carrying zinc, lead, and copper, and it is tentatively suggested that also such ores may represent precipitations, just like the sedimentary iron ores.