A few notes on the geology of the region are given before the deposits are described.
Johnson'
describes the Cerrillos Hills as remnants of laccolithic intrusions
into Cretaceous strata. The sedimentary strata covering the intrusions
have been eroded away, and the present hills are the more resistant
portions of the igneous rocks. The principal rocks are described as
augite andesite, hornblende andesite, and mica andesite. Augite
andesite is mapped as most abundant and with hornblende andesite forms
the matrix for the turquoise. Lindgren 2 in discussing the
same rocks points to the fact that orthoclase is present in many of
them and that the texture is decidedly holocrys-talline and
porphyritic. Such rocks belong in the monzonite group.
Examination
of thin sections of rock from several places in the areas mapped as
augite andesite by Johnson confirms Lindgren's determination.
Alteration has made difficult the identification of the feldspars in
many sections. In some the striations of plagioclase are largely
concealed by secondary minerals and add uncertainty to the
determination of altered orthoclase present in the same sections.
Locally the texture of the monzonites is so fine grained that they
might be called latite, of which andesite is a variety. A specimen from
an area mapped as hornblende andesite had a porphyritic texture and
was composed of such minerals as are characteristic of diorite or
andesite.
The
rocks have been strongly fractured, especially in those areas where
turquoise is now found. Both large and small fissures were formed, and
numerous minor joints cutting the rock in all directions. The rocks
have been considerably altered in many places, and around the
turquoise-bearing areas alteration has been extensive. Alteration was
favored by the easy channels open to solutions in the badly fractured
areas. The process outlined by Lindgren offers a good explanation of
the present associations at the turquoise deposits. The original
monzonite porphyry was greatly altered by mineralizing solutions in
fracture zones and abundant secondary sericite developed. Later
weathering by surface waters produced kaolin and other hydrous minerals
characteristic of the oxidized zone and among them turquoise.
In describing the occurrence at Mount Chalchihuitl, Johnson 1 says.
The
rock is yellow or white in color, sometimes mottled or streaked with
iron stains. * * * The turquoise occurs as seams throughout the rock,
filling crevices formed by crushing and shearing, and as little nodules
in streaks or patches of kaolin. The color varies from green through
greenish-blue to pure sky-blue. Many of the specimens are marred by
streaks of limonite, kaolin, etc.
Practically
all the rock on Mount Chalchihuitl is altered. The general appearance
is white to gray and yellow, but some is stained brown and purplish in
small masses. Most of it has a somewhat porous earthy texture but is
not broken with a hammer as easily as its appearance would lead one to
expect.
At
Michael O'Neil's turquoise mine turquoise has been found over a
distance of about 300 yards in a direction N. 30° W. Ancient workings
and prospecting for silver have left numerous openings along this
i Johnson, D. W., Geology of the Cerrillos Hills: School of Mines Quart., vol. 24, 1902-3, pp. 455-477. * Lindgren, Waldemar, The ore deposits of New Mexico: Prof. Paper TJ. S. Geol. Survey No. 68, 1910, pp. 165-166.
