516 tyler. SEDIMENTARY IRON DEPOSITS [Ch. 28 



The deposits consist of hematite or limonite, which may occur either 

 in the form of oolites, as replaced fragments of fossils, or as a fine 

 powder. Calcium carbonate, silica, phosphorus, and clay are usually 

 present as impurities. The calcium carbonate occurs both as chemi- 

 cally precipitated calcite and as fragments of fossils. Siderite is usu- 

 ally absent, and the sulphur content is normally low. The iron content 

 ranges from 10 to 50 percent. 



Some of the larger marine oxide deposits include the Wabana hema- 

 tites of Bell Island, Newfoundland (lower Ordovician), the Neda 

 hematites of Wisconsin (upper Ordovician), the Brassfield hematites 

 of Kentucky (lower Silurian) , the Clinton hematites of the Appala- 

 chian region (Silurian), the hematites of eastern Kentucky (middle 

 Devonian) , the Minette ores of Alsace-Lorraine (middle Jurassic) , 

 and the limonites and hematites of Salzgitter (Cretaceous) and the 

 Lahn-Dill (Devonian) deposits of Germany. 



The Clinton ores of the Appalachian region rank second in im- 

 portance to the Lake Superior deposits. The ores are interbedded with 

 shales, limestones, and sandstones and occur as thin, but rather ex- 

 tensive, lenses at slightly different stratigraphic horizons in the Clinton. 

 In the Birmingham district of Alabama, where the ores are extensively 

 mined, there are four beds which thicken and thin from 30 feet to a 

 few inches. The Big Seam at Birmingham outcrops for a distance in 

 excess of 20 miles and ranges from 16 to 40 feet thick, of which 7 to 

 12 feet is minable. There are two main types of ore in the Clinton: 



(1) fossil ore, composed of fragments of fossils partially or completely 

 replaced by hematite, set in a matrix of powdery and oolitic hematite; 



(2) oolitic ore consisting of spherules of hematite 1 to 2 millimeters in 

 diameter with a rounded sand grain or fossil fragment as a nucleus, 

 set in a matrix of powdery hematite, fossil fragments, and calcite. The 

 oolites are often flattened and resemble flaxseed. This type is re- 

 ferred to as the flaxseed ore. The ore, when encountered at depth, 

 ranges from 20 to 40 percent iron, is hard, and averages about 15 per- 

 cent silica. Near the surface (0 to 400 feet) the carbonate has been 

 leached to produce an enriched "soft ore," which may contain 50 to 

 60 percent iron and 12 percent silica. The ores are high in lime, alu- 

 mina, and phosphorus. 



Although the Clinton ores have long been recognized to be of marine 

 origin, the environment of deposition of the iron has received consider- 

 able discussion. Ailing (1947, p. 1012) concludes that the iron replaced 

 carbonates shortly after deposition, as part of the diagenesis of the 

 sediment. Others have concluded that the hematite was deposited 



