700 MINERAL RESOURCES, 1913—PART II.
The
group of hills in which the Jarilla mining district is situated rises
several hundred feet above the plains country on the east. In the
region around the turquoise mines the hills are steep, with basinlike
valleys among them. The northern group of turquoise deposits is in such
a basin with outlet to the east, and the southern group is in a smaller
basin with outlet also on the east through a draw draining into the
main gulch through Brice. The turquoise deposits lie chiefly around the
edges of the basins near the foot of the steepei slopes, at an
elevation of approximately 4,600 feet above sea level. Tfie country is
arid, and vegetation is slight and typical of the desert.
According
to Waldemar Lindgren ' the Jarilla Hills consist of carboniferous
limestone strata domed up by an irregular intrusive mass of
fine-grained monzonite porphyry. Interesting deposits of andradite
garnet, diopside, quartz, epidote, hematite, and pyrite have been
formed along the contacts between the limestone and porphyry.
Observations by the writer show sill-like sheets of porphyry
inter-bedded with the limestone with contact mineral zones between
each. There has been faulting, so that the outcrops of the strata do
not retain symmetrical positions around the sides of the basins. The
turquoise deposits are in the lowest sill or possibly in the top of the
main part of the laccolitn intrusion exposed in the basins.
Tne
De Meules mine consists of two sets of workings near the western part
of the basin, one on the north side of a draw and the other about 175
yards to the south across the draw. Both are in gentle sloping ground
at the foot of the hills. At the north workings an open cut 40 to 50
feet across and 20 feet deep has been filled up, but some of the
branching tunnels extending northeast from it are still open. These
tunnels probably aggiegate over 300 feet in length and open into
iiregulai rooms and small stopes, some of which extend to the surface.
They are probably nowhere more than 25 feet deep. Other prospects were
opened around the workings. At the south workings a dozen or more
irregular cuts, pits, and shallow shafts with ground-hog tunnels were
made. One old shaft was about 40 feet deep. The dumps of waste rock
from both sets of workings are large, and it is probable there have
been more extended underground workings than could be seen at the time
of examination. . The rock at the De Meules mine is decomposed
monzonite porphyry. The stage of decomposition varies but is more
advanced near the turquoise deposits than at some distance from them.
The less altered porphyry is a dark-gray rock, showing white feldspar
and biotite phenocrysts. The altered phases are light-gray to light
brownish-gray with numerous white spots. In the turquoise-bearing areas
the porphyry has been broken into small blocks by numerous joints. The
jointing has aided in the decomposition of the rock by furnishing
channels for the passage of water. The joints cut the rock in several
directions, but in the north workings the tunnels followed two sets of
joints carrying turquoise and striking northeast and northwest with
high to vertical dips. Here one prominent joint or fissure striking
northwest across the end of the tunnels carried both quartz and
chrysocolla, with a small amount of turquoise. Prospects to the north
and to the west of the underground workings exposed much chrysocolla in
seams and veins with but little turquoise. The joints
1 T£e ore deposits of New Mexico: U. S. Geol. Survey Prof. Paper 68, p. 185,1910.
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