CHAPTER IX— PROCESSES OF ALTERATION. 
METASOMATISM IX CONNECTION WITH VEIN FORMATION. 
EXTENT OF ALTERATION. 
The volcanic rocks of Cripple Creek have been extensively altered, so much so 
that in places it is impossible to obtain entirely fresh specimens. This alteration 
may be traced to three causes—vein formation, ordinary hydrometamorphism, 
and direct oxidation. The first of these is-not now operative, while the two latter 
processes are still actively engaged and are apt to superimpose their effects on the 
alteration due to vein formation. The more important of these two is oxidation, 
which is described in detail on pages 196 to 204. Ordinary hydrometamorphism 
producing chlorite, serpentine, sericite, etc., is caused by surface waters below the 
zone of oxidation. Its changes are not very far-reaching and only in very few 
cases has it radically altered the mineralogical composition of the rocks. 
To processes of vein formation may unhesitatingly be referred the greater part 
of the alteration below the reach of oxidizing waters. All rocks have been affected, 
the breccia most of all in consequence of its porous and permeable character. There 
are, in fact, comparatively few places where the breccia retains its original appearance. 
Latite-phonolite and syenite have yielded less readily to alteration, and fresh spec¬ 
imens maybe obtained from all of the bodies mapped. Phonolite and basalt dikes, 
especially the latter, are usually more or less affected, while the larger outlying 
phonolite masses are, in most cases, practical^ fresh. The Beacon Hill phonolite 
has suffered much more than the ring of phonolite intrusions which, contained in 
granite, surround the central breccia. Granite, schist, and gneiss are only locally 
altered near the breccia contact or near veins traversing them. 
CHARACTER OF ALTERATION. 
On the whole, and compared to similar changes in other districts, the alteration 
of the Cripple Creek rocks is remarkably slight. It is elsewhere not uncommon 
near gold-quartz veins to find diorites, amphibolites, or diabases completely changed 
to white, soft rocks chiefly composed of sericite, quartz, and carbonates. Here, 
on the contrary, alteration very rarely obliterates the original character of the rock 
as seen under the microscope or in specimen. The minerals formed are pyrite, asso¬ 
ciated occasionally, but not commonly, with a little zinc blende or galena, further 
sericite, adularia, dolomite, fluorite, and quartz; the latter is wholly subordinate and 
forms only under certain exceptional conditions, resulting in complete replacement 
of the rock by quartz and pyrite. Sometimes apatite also forms. Besides sericite, 
roscoelite (a yellowish-green vanadium mica) and a light-green mica of doubtful 
composition are sometimes present, and rutile, with some calcite, develops from 
titanite. 
184 
