The Dulzura Mining District is 30 miles east and a little south of the
city of San Diego, in a range of rather rugged mountains. Metamorphic
rocks occur frequently, but masses of igneous rock have been intruded
and constitute large hills throughout the district. Among these rocks
are a light, greenish-gray feldspar porphyry, a dark-green, fine-grained
diorite, a black aphanitic diorite, sometimes porphyritic, and a light
cream-colored or greenish-white felsitic rock. In the immediate vicinity
of Dulzura the latter rock occurs in the form of immense dikes striking
northwest and southeast, crossing Cottonwood Creek to the south into
Mexico. In one of these great dikes, which is several hundred feet in
width, the mines of the district are found. They are principally
chambered veins occurring on the line of a fault plane which has
fractured the felsite, the hanging wall side of the slip going down, the
movement causing the rock to become crushed and broken. Percolating
waters have carried into the crushed mass mineral solutions, which have
deposited the ores, chiefly iron sulphurets, where the most favorable
conditions were found along the line of this fault plane. The subsequent
oxidizing of the masses of ore thus formed has stained the rock a bright
or dark red and sometimes yellow.
The rock contains gold ranging from a trace to $20 or over per ton. It
was said the average was about $8. The oxidation of the ore bodies does
not extend to any very great depth, the result being the occurrence of
sulphuretted ores comparatively near the surface, though it was claimed
by the mine owners that the sulphurets contained sufficient gold to make
concentration and treatment by chlorination profitable. The mines are
quite interesting from a geological point of view, and may, in time,
prove equally so from a financial standpoint. No machinery had been
built at the time of my visit. The felsite throughout the district,
whenever observed, contained a considerable amount of pyrite, the
surface rock nearly always presenting a highly mineralized appearance.