Agua Dulce Mountains and surrounding ranges all lay within the
Basin and Range Province of North America. The Basin and Range
Province comprises the southern third of the state of Arizona,
plus its western edge. It is characterized by linear mountain
ranges that mostly trend north-south or northwest-southeast.
The mountain chains are separated from each other by downthrown,
alluvium-filled basins. Starting from the Tinaja Altas Pass
and running eastward to the Agua Dulce Mountains, the Camino
del Diablo more or less parallels the Mexican border through
nearly 50 miles of mountains and deserts. The mountain ranges
in this area all trend northwest-southeast and are predominantly
composed of Mesozoic igneous and metamorphic rocks. The metamorphic
rocks consist of gneisses, schists, and phyllites. The schists
and phyllites are derived from Mesozoic-age sedimentary rocks.
The igneous intrusions are silicic, ranging in composition from
granite to quartz diorite.
The rock types described above dominate this part of the
Basin and Range Province, an area that includes the Gila Mountains,
Tinaja Altas Mountains, Cabeza Prieta Mountains, Tule Mountains,
Copper Mountains, Sierra Pinta Mountains, Mohawk Mountains,
Granite Mountains, and Agua Dulce Mountains. A few Quaternary
lava flows occur in the area; the two largest occur in the
Cabeza Prieta Mountains and in the Tule Desert. The Tule Desert
flow is known as the Pinacate Lava Field; it harbors several
cinder cones and is a classic "malpais". All of these Quaternary
lava flows are basaltic in composition.
The Agua Dulce Mountains themselves consist almost entirely of Mesozoic-age gneisses. A small exposure of Quaternary basalt lies just south of Papago Well; a few smaller exposures lie scattered to the northwest of Papago Well. Nevertheless, schists, phyllites, and gneisses form the bedrock in the area east of the Pinacate Lava Field and comprise the rugged hills and mountains for nearly 10 miles in any direction.
previously stated, this part of the Basin and Range Province
is weakly mineralized; therefore very few major strikes have
been made in the area. But the Fortuna Mine serves as an example
of how a major deposit can be overlooked for years. Prospectors
should probably concentrate on the rugged country east of Cabeza
Prieta (Black Head Mountain). At least one account of the story
has the vein partially covered by windblown sand on the southern
flank of the peak. Metal-detectors may therefore be of some
use in the search. Rich gold-bearing float has been found in
this part of southwest Arizona for many years. Careful mapping
of any similar discoveries of iron-stained quartz may reveal
the source of the gold-bearing float. In any case, it appears
that the famous landmark known as Cabeza Prieta will be clearly
visible from the lost vein.
Prospectors should realize that much of the Camino del Diablo
lies within the Barry M. Goldwater Air Force Range and the
Cabeza Prieta National Wildlife Refuge. A permit is required
to enter both areas. Permits for entry to the Barry M. Goldwater
Range are available from the Marine Corps Air Station, Range
Management Department, Yuma, Arizona. Permits for entry to
the Cabeza Prieta National Wildlife Refuge are available from
the U. S. Fish and Wildlife Service, Ajo, Arizona.