514 tyler. SEDIMENTARY IRON DEPOSITS [Ch. 28 



The brown ores of Alabama, Georgia, and Virginia are residual de- 

 posits of limonite and goethite formed during the Tertiary on Cam- 

 brian-Ordovician and Silurian limestones and dolomites. The deposits 

 occur as small, rather thin, irregular lenses surrounded by clay, or as 

 lumps or nodules of limonite in the clay. The deposits range in iron 

 content up to 55 percent, are high in alumina, contain 5 to 30 percent 

 silica, 0.1 to 2.0 percent phosphorus, 0.3 to 10.0 percent manganese, and 

 are low in sulphur. 



The Tennessee River brown ores occur as pockets in residual clay 

 and as nearly horizontal mantles on lower Carboniferous limestones. 



The deposits of Bilbao, Spain, differ from the normal residual iron 

 deposits on limestone in that they are high-grade and low in impurities. 

 This is due to an unusual situation. A 250-foot bed of Cretaceous 

 limestone has been hydrothermally replaced by masses of siderite 

 'with minor ankerite and sulphides. Weathering of the iron-bearing 

 minerals has produced surficial blankets of residual iron oxide, the 

 largest of which is about 100 feet thick and about 2 miles long and 

 three quarters of a mile wide. The oxidized ore ranges from 50 to 

 57 percent in iron and is of bessemer grade. 



The brown ores of eastern Texas are residual deposits formed by 

 ordinary weathering processes on the Weches glauconite of Eocene age. 

 The ore occurs as a horizontal blanket 3 to 4 feet thick resting on white 

 clay, which grades downward into weathered glauconite ; or it occurs as 

 nodules or thin lenses in a 5- to 30-foot zone of greensand. The ore 

 contains 42 to 48 percent iron, 10 to 12 percent silica, 8 to 12 percent 

 alumina, and less than 0.24 percent phosphorus. 



The great iron deposits of the Lake Superior region are the leached 

 portions of former sedimentary iron beds. Silica and other substances 

 have been removed by circulating ground waters from the iron-bearing 

 formations resulting in a residual concentration of iron oxide. These 

 deposits, which include the Mesabi, Vermillion, and Cuyuna in Min- 

 nesota, the Gogebic in Wisconsin and Michigan, and the Marquette 

 and Menominee ranges in Michigan, have produced over 2 billion 

 tons of iron ore. The original pre-Cambrian iron formations consist 

 of siderite, greenalite, hematite, and chert, and average about 28 

 percent in iron. Van Hise and Leith (1911, pp. 539-540) conclude 

 that the ore was formed by pre-Cambrian surface waters which leached 

 the silica of the chert, leaving a residual mass of porous hematite and 

 limonite. Gruner (1937, pp. 121-130), on the other hand, has sug- 

 gested that the leaching of the silica was mainly effected by hydro- 

 thermal solutions. The ore, which extends to depths of 3,000 to 4,000 

 feet locally, occurs as large blanket deposits on the Mesabi, above 



