Ch. 28] IRON OXIDES AND HYDROXIDES 515 



the intersection of basic dikes with impervious horizons on the Gogebic 

 and Marquette, above large basic sills on the Marquette, and in syn- 

 clinal troughs on the Menominee range. The ore bodies generally lie 

 on impervious rocks, such as slate, that guided and controlled the 

 ground-water circulation. The permeability and the iron content of 

 specific sedimentary horizons seem to have played an important role 

 in localizing the ore. In general, the more permeable horizons per- 

 mitted more extensive leaching of silica, and the horizons with a higher 

 original iron content required less leaching to form a high-grade ore. 



The composition of the ore varies considerably even within the limits 

 of one mine. The iron content of the direct-shipping ore ranges from 

 38 to 66 percent. Sulphur is generally low but ranges from a trace to 

 2.0 percent; phosphorus ranges from 0.008 to better than 1.0 percent 

 and averages about 0.09 percent; silica ranges from 2 fo 40 percent and 

 averages about 8.5 percent; alumina ranges from 0.16 to 6.0 percent; 

 and moisture averages about 11 percent. The ores range from 7 to 20 

 and average about 12 cubic feet per ton. Manganese is important 

 only on the Cuyuna range, where it ranges from 5 to 20 percent. 



Residual deposits of limonite also form as surface oxidation products 

 on sulphide deposits. These are called gossans. The sulphide deposits 

 (pyrrhotite, pyrite, chalcopyrite, sphalerite, bornite) at Ducktown, 

 Tennessee, illustrate extreme surface weathering. Oxidation and 

 leaching have changed the sulphide ores near the surface into a porous, 

 cellular gossan consisting essentially of limonite with a little silica, 

 kaolin, and a fraction of a percent of copper and sulphur. Limonite of 

 this type has been mined at Rio Tinto, Spain, Shasta County, Cal- 

 ifornia, and at various localities in Missouri. 



Makine Deposits 



Marine deposits of iron oxide are widely distributed geographically, 

 as well as throughout the geologic column. Some of the largest and 

 most extensive iron deposits of the world are of this type. These de- 

 posits seem to have been formed in shallow epeiric seas surrounded by 

 low-lying land masses, which furnished little if any clastic sediments to 

 the areas of iron sedimentation. The presence of ripple and current 

 markings and mud cracks indicate that the waters were very shallow. 

 The alternation of shale and sandstone beds with the iron-bearing 

 sediments indicates that from time to time the conditions were such 

 that clastic sediments could be transported in relatively large quan- 

 tities to the sites of deposition. This in effect terminated the periods 

 of iron deposition. 



