236 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



outer edge of the altered zone, as in No. 1, the case was different. Here calcite 

 was abundantly formed and, with abundant chlorite, makes up a good part of 

 the rock which now exhibits the typical "propylitic" alteration. 



PROI'YLITIC ALTERATION OF EARLY ANDESITE. 



Propylite was a name applied in 1867 by von Richthofen to certain early 

 Tertiary volcanic rocks of Nevada and California, especially to rocks observed near 

 the Comstock lode in Nevada. It was defined as being always porphyritic, and 

 very similar to porphyritic diorite, with oligoclase feldspars and dark-green fibrous 

 hornblendes, in a green groundmass which owes its color to small particles of fibrous 

 hornblende; as being very rich in mineral veins, and the earliest of the Tertiary 

 volcanic rocks. These definitions were accepted and new areas of propylite were 

 discovered by many prominent geologists. But Dr. G. F. Becker's work, published 

 in 1882, showed that the "propylites" near the Comstock were altered rocks 

 originally identical with fresh diorites, andesites, etc., from the same region; that 

 the characteristic supposed green fibrous hornblende was chlorite, a decomposition 

 product; and that this rock phase owed its association with mineral veins to the 

 altering mineral waters which produced the veins and this rock at the same time. 

 Other investigators have come to the same opinion, and the name propylite, as 

 signifying a rock type, has been dropped. It has, however, been sometimes used to 

 signify this especial form of alteration, and is in this sense characterized by Rosen- 

 busch as follows: 6 



''The characteristic feature of the propylitic facies consists in the loss of the 

 glassy habit of the feldspars; in the chloritic alteration of the hornblende, biotite, 

 and pyroxene (often with an intermediate stage of uralite), with simultaneous 

 development of epidote; further, in alteration of the normal groundmass into 

 holocrystalline granular aggregates of feldspar, quartz, chlorite, epidote, and calcite, 

 and in a considerable development of sulphides (usually pyrite)." 



Epidote has not been detected in the earlier andesite at Tonopah, and is rare in 

 the district in general; otherwise the rocks like 1 and 2 correspond to the "propy- 

 litic" phase. At the Comstock Becker c found epidote uncommon underground, 

 while abundant at the surface. 



Mr. Waldemar Lindgren'' has considered gold and silver veins accompanied by 

 a "propylitic" alteration of the wall rock as a group, and has separated them from 

 another class (the sericitic and kaolimtic gold and silver veins) whose wall rocks 

 show characteristic alteration to sencite and kaolin. In a subsequent note he 

 remarks that "it is perhaps not advisable * * to retain the name propylitic 

 for the whole group, a.s some of them do not show alteration in typical form.'' 



a Mon. U. S. Geol. Survey, vol. , p 88, etc. ' Trans. Am. lust Mln. Eng., vol. 30. pp. 645-664. 668-666. 



fcElemenlederGcstelnslehre, Stuttgart. 1898, p. 302. 'Ibid , vol. 33. p 798 



o Mon. V. S. Geol. Survey, vol. 4, p 212. 



