42 



ECONOMIC GEOLOGY 



In carbonaceous shales the iron and manganese minerals are 

 precipitated as carbonates by the loss of carbon dioxide from 

 their solutions and the reducing action of organic matter. The 

 ore deposits thus precipitated occur either as continuous sheets 

 or as bands of concretionary minerals. (See Fig. 31.) 



The sulphides of these metals occur as precipitates in bedded 

 deposits either through the action of H 2 S upon their solutions 

 or by the reduction of their soluble sulphates. In the Harz 

 Mountains, the sulphide of copper occurs in bedded deposits that 

 appear to have been reduced from sulphate solutions by the action 

 of decomposing organic matter. 



Gold is precipitated in siliceous sinter in Queensland, Australia, 



FIG. 31. Limonite concretions in the Kittany Valley, Pennsylvania. 

 (Photograph by T. C. Hopkins.} 



where the sinter has been deposited from highly alkaline siliceous 

 waters. This ore has yielded over $350 per ton in gold. 



A few precipitates as silicates are well known. Illustrations 

 may be cited in the greenalite of the Mesabi iron district, Min- 

 nesota, and the glauconite or green sand marl, of New Jersey. 



Metamorphism. Metamorphism is the process by which a 

 complete or nearly complete chemical change has been effected 

 in an ore body. These changes may take place either in the upper 

 part of lodes and bedded masses exposed to percolating meteoric 

 waters or at considerable depths below the surface by thermal 

 and dynamic agencies. The first implies the downward trans- 

 ference of minerals in solution for the subsequent enrichment of 

 metalliferous deposits; the second, the reconstruction of an ore 



