THE SUPERFICIAL ALTERATION OF ORE DEPOSITS.^ 



GENERAL FEATURES. 



Scope of the subject. — The modern idea of ore deposits teaches 

 that formations of this kind represent a process of concentration 

 of mineral matter, either by chemical or physical means; in other 

 words, that they are unusual localizations of certain minerals 

 which are often found disseminated in smaller quantities in many 

 common rocks, and that they differ from the same minerals situ- 

 ated in other conditions, only in their degree of concentration. 

 These concentrations may take place at different times in the 

 history of the rocks in which the deposits occur. If they occur 

 in sedimentary rocks, they may sometimes be formed during the 

 deposition of the rocks with which they are associated, as in the 

 cases of placer gold, stream tin, and sometimes of other ores; 



' The superficial alteration of ore deposits is a recognized principle of geology, in 

 the same way as is the superficial alteration of any of the common rocks. Its impor- 

 tance in some classes of ore deposits is also well understood, as in many precious 

 metal deposits ; while in other classes, its importance has been proved in individual 

 cases, as in the Lake Superior iron deposits. The causes and effects of superficial 

 alteration in many classes of deposits, however, are not so generally understood, and 

 it is the object of the present paper to show that such changes almost invariably give 

 rise to exceedingly important chemical and physical phenomena, while in many 

 deposits, the question as to whether they can or cannot be profitably worked, depends 

 largely on the extent and character of this alteration. 



The various treatises on ore deposits publislied in the United States and Europe 

 make frequent mention of superficial alteration, but have not treated the subject 

 fully. As early as 1854, however, before which time but little accurate informa- 

 tion was had on the geologic nature of ore deposits, Professor J. D. Whitney in his 

 classic volume. The Metallic Wealth of the United States, describes the alteration 

 products, or gossans, in certain deposits and mentions others. On the more purely 

 chemical side of the question, the work of Bischof, Daubree, Roth, Rose, Hunt, 

 Breithaupt, Blum, Julien, Deville, Debray,. Volger, Moissan, Fremy, Ldvy, Fouqud 

 and others have afforded much valuable information and many useful suggestions. 

 The chemical principles brought out by these various authors have been applied, to a 

 certain extent, to the solution of the phenomena of the origin of ore deposits, but 

 have not as yet been applied to anything like tlieir possible extent to the solution of 

 the phenomena of the alteration of ore deposits. 



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