PROCESSES OF ALTERATION. 
193 
When mineralized the basic dikes become filled with minute crystals of pyrite 
and traversed by veinlets of quartz and calaverite. Sometimes calaverite forms 
metasomatically in the altered basalt. Specks and also larger bunches of galena and 
zinc blende develop in the basic dike of the Conundrum mine. 
A peculiar variety of metasomatic alteration has affected portions of the princi¬ 
pal basic dike in the Elkton mine, south of the main shaft. The altered rock is light 
gray, porous, and tliickly sprinkled with minute crystals of pyrite. The porosity is 
due to numerous little cavities of dissolution which are lined with small white crystals. 
Similar crystals coat the walls of the irregular seams traversing the rock. The 
material of the dike shows no trace of its original structure and is evidently much 
altered. 
The microscope shows that the dike has been locally changed to an aggregate of 
adularia, apatite, pyrite, sericite, and probably some calaverite. The apatite, 
which is remarkably abundant, is in slender greenish-white prisms of an entirely 
different habit from the stout smoky crystals characteristic of thfe latite-phonolite. 
The drusy crystals coating the pores and seams in the rock arc adularia. No chem¬ 
ical analysis has been made of this material, but there has clearly been a considerable 
addition of potash sulphur and phosphoric acid to the original dike rock. 
METASOMATIC CHANGES IN GRANITE. 
The Pikes Peak granite, along the breccia contact from the Elkton to Stratton’s 
Independence mine, is a reddish coarse-grained rock, noticeably deficient in dark 
constituents and frequently poor in quartz. It consists chiefly of microcline, some 
individuals of which are noticeably larger than the rest and give the rock a semipor- 
phyritic appearance; besides microcline, some quartz, oligoclase, microperthite, 
orthoclase, and biotite are present, the latter often chloritized and containing a little 
magnetite and epidote. A typical analysis of the Pikes Peak granite contains SiO,, 
77.03 per cent; MgO, 0.04 per cent; CaO, 0.80 per cent; Na 2 0, 3.21 per cent; K 2 0, 
4.92 per cent, but in the locality mentioned the amount of quartz is often smaller 
than would be indicated by the figures given, and the rock is rather a quartz-syenite, 
ranging over to a normal syenite, as shown by analysis A in the table on page 194. 
Near lodes this granite is often subject to marked metasomatic change; the 
reddish color changes to a light gray; the rock contains disseminated pyrite, sphal¬ 
erite, fluorite, and tellurides and becomes very noticeably drusy and honeycombed, 
the vugs being coated with the minerals just indicated. This is the so-called granite 
ore, which is of great economic importance in the part of the district outlined above. 
Penrose* noticed this ore, especially from Stratton’s Independence mine, but 
assumed that the change involved consisted of the dissolution of quartz, leaving a 
porous mass of feldspar. The characteristic feature of this mode of alteration b is, 
however, the abundant solution of orthoclase and microcline and reprecipitation as 
typical adularia. The quartz grains are also attacked and newly formed quartz 
deposited together with adularia. The biotite is the first mineral to be replaced by 
various other minerals, among them pyrite, calcite or dolomite, fluorite, adularia, 
a Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 201. 
b First described by Lindgren, W., Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., vol. 30, 1901, 
p. 650. 
* 
