236 



PAIGE AND STEIGER: SERICITIZATION 



ment" — the nature of the formation of kaolin below the chal- 

 cocite ore bodies seemed to require more than the ordinary 

 ■explanation, for the evidence is fairly conclusive that quartz 

 in considerable amount, together with the other constituents of 

 the porphyry, has been metasomatically replaced by kaolin. 



In figure 1 a .series of veins is illustrated, cutting a very brec- 

 ciated porphyry. The arrangement of the fragments is such that 

 it is evident that there could have been no movement anlog the' 

 veins in any direction. The veins are composed of kaohn. In 

 figure 2 another kaolin vein is illustrated. Here there is a perfect 

 gradation from the common sericitized porphyry at one end of 

 the vein to kaolin at the other end. About midway, quartz 

 phenocrysts, residual from the porphyry still remain inclosed 



Fig. 2. Illustrating the progressive replacement of porphyry by kaolin. 

 Quartz is the last mineral to disappear. 



in the kaolin, whereas the pure masses of kaolin contain none. 

 This is apparently a clear case of the progressive removal of 

 quartz by metasomatic replacement. 



In discussing with Dr. Adolph Knopf, of the Geological Survey, 

 this occurrence, which appeared to demand more than the 

 ordinary explanation, Dr. Knopf suggested that perhaps fluorine 

 had been active. In the Tyrone district fluorine occurs in fluor- 

 ite in veins, at one place in a vien several feet thick and at other 

 places in small veins a few inches thick. It is presumed there- 

 fore that it also occurs more generally in small veinlets. That 

 it has not been more generally recognized in small veinlets, how- 

 ever, is natural, for processes of secondary enrichment by waters 

 carrying sulphuric acid are precisely those that would readily 

 attack fluorite and remove it. Nevertheless, it was felt that while 

 fluorine viight have been present in fluorite in numberless veinlets, 



