Precambrian Iron-Formations

Sedimentary rocks, including chemical precipitates formed by exhalative processes, often contain high enough iron to be considered ferruginous, or even iron deposits. Two major groups of iron-rich sedimentary rocks are recognized (James 1966): (a) ironstones, which are non-cherty, oölitic, poorly banded, and largely of Phanerozoic age; and (b) iron-formations, which are typically laminated with chert, generally non-oölitic, and largely (but not exclusively) of Precambrian age. Other groups of iron-rich rocks of lesser economic importance not considered here (but discussed, for example, by Borchiert 1960, Stanton 1972) include: (a) the blackband and clayband ores, most of which are diagenetic and post-diagenetic deposits of siderite found in coal measures and in some clays; (b) the bog iron ores found in many bogs and small lakes in higher latitudes; and (c) latentes derived by weathering of Fe-rich rocks (see Ch. 2). In this chapter we restrict our attention to the so called iron-formations, which constitute by far the most abundant and economically the most important iron-rich sediments.