518 tyler. SEDIMENTARY IRON DEPOSITS [Ch. 28 



Griffen (1892, p. 985) states that 15 feet of concretionary ore was 

 mined from the bottom of Lac-a-la-Tortue near Rodnor Forges, Que- 

 bec, and that 10 years later sufficient limonite had accumulated to 

 make it profitable to mine again. 



IRON CARBONATE 



Iron carbonate occurs only as the mineral siderite (FeC0 3 ) . Siderite 

 is found in sedimentary deposits as concretions, lenses, and beds asso- 

 ciated with shales, sandstones, limestones, and chert. Iron carbonate 

 deposits are in general low in grade and rarely contain more than 40 

 percent iron. This is due to the relatively low metallic iron content of 

 siderite (48.2 percent) and to the presence of impurities, such as or- 

 ganic matter, clay, sand, chert, and calcite. 



Thin beds of iron carbonate which have great lateral extent are com- 

 monly associated with coal measures. Deposition apparently took 

 place in brackish marine waters or in marine swamps where abundant 

 decaying vegetation inhibited oxidation of the iron and promoted the 

 formation of siderite. 



The concretionary ironstones, sometimes referred to as "kidney ores," 

 are spherical to disk-shaped, laminated concretions composed of sider- 

 ite, calcite, and clay. They are abundant at certain horizons in the 

 Permian shales and sandstones of Pennsylvania but are not of eco- 

 nomic significance. 



The blackband ores consist of thin beds and lenses (usually less 

 than 1 foot thick) of siderite interbedded with clays and shales. The 

 black color is due to organic matter. Blackband ores are especially 

 abundant in the Mississippian and Pennsylvanian rocks of the Appa- 

 lachian coal fields. They also occur in England and Germany. There 

 are 75 horizons of siderite in the lower coal measures of Wales. Black- 

 band deposits are not of economic importance today because of their 

 low grade (25 to 40 percent iron). They are high in sulphur and 

 phosphorus, and they occur in thin beds. 



Siderite occurs as oolites associated with chamosite in shales and 

 sandstones of Jurassic age in the Cleveland Hills of England. The ore 

 bodies that are mined range from 6 to 25 feet in thickness and average 

 about 28 percent iron. They contain about 15 percent silica, 11 percent 

 alumina, and 1.0 to 2.0 percent phosphorus. 



Van Hise and Leith (1911, p. 500) state that the pre-Cambrian iron 

 formations of the Lake Superior region were originally composed of 



