STUDIES IN HYDROTHERMAL ALTERATION 



PART I. THE ACTION OF CERTAIN ALKALINE SOLUTIONS ON 

 FELDSPARS AND HORNBLENDE 



E. A. STEPHENSON 

 University of Chicago 



In the study of ore deposits from a genetic standpoint the 

 subject of attendant wall-rock alteration has received deserved at- 

 tention from geologists. Profound changes of a chemical and min- 

 eralogical character have been recorded at many places, and the 

 relation existing between the various types of wall-rock alteration 

 accompanying ore deposition led to the suggestion that a knowledge 

 of the conditions which bring about such alteration would throw 

 great light on the problem of ore genesis. The data obtained in 

 regard to the temperatures, pressures, and nature of solutions would 

 also be of value in interpreting the geologic history of such occur- 

 rences. 



The most important of these alteration minerals are kaolin, 

 sericite, and chlorite, and the knowledge concerning their origin is 

 chiefly confined at present to speculations based on their modes of 

 occurrence and their associated minerals. There is especially great 

 difference of opinion 1 as to the origin of kaolin. By some writers 

 kaolinitic alteration is attributed to the action of meteoric waters 

 rich in carbonic acid, by others to meteoric waters which have made 

 a cycle of underground courses, and by still others to the emanations 

 from a cooling magma while these are possibly yet in a gaseous 

 state. In regard to sericite and chlorite associated with ore bodies 

 there is nearly general agreement that these have been formed by 

 the action of the solutions which deposited the primary minerals, 

 whether these solutions be magmatic or meteoric, upon the feld- 



1 Bibliographies of the literature on kaolin are given by Rosier, Neues Jahrb., 

 Beil. Bd. XV (1902), 231, and by Lazarevic, Zelt. prakt. Gcol., XXI (1913), 345. An 

 introduction to a discussion of the origin of kaolin was initiated by Lindgren, Econ. 

 Geol., January, 191 5. 



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