Trigo Mountains are an uplifted block of volcanic rocks intruded
by small Laramide-age dikes, plugs, and granitic stocks. These
younger dikes and intrusions are clustered in the southern portion
of the Trigo Mountains, especially in the Red Cloud Mine area.
Here, the contact between younger Laramide granite and slightly
older Cretaceous andesite is sometimes fault-bounded; mineralization
frequently occurs along these fault boundaries. The Trigo Mountains
are mostly composed of Cretaceous andesite which occurs as flows,
dikes, plugs, tuffs, and agglomerates. This andesite intrudes
and blankets slightly older Mesozoic metamorphic rocks. These
Mesozoic gneisses and schists are the oldest rocks in the Trigo
Mountains and occur as isolated exposures throughout the range.
The largest exposure is found on the southernmost spur of the
range and has an outcrop area of roughly 20 square miles. The
mountains lying between Norton's Landing and the Red Cloud Mine
are comprised of Cretaceous andesite and younger Laramide granites.
The younger Laramide granites and related intrusives occur near
the Red Cloud mining district (which includes the Papago Mine
and the Black Rock Mine). This mining district is also the locus
for all the faulting that has occurred in the Trigo Range.
The Trigo Mountains are part of the Basin and Range Province
of North America. The mountain ranges in this province are
characteristically elongated and trend north-south to northwest-southeast.
Many times, the oldest rocks in southern Arizona can be found
in the cores of these mountain ranges. Interestingly, most
of these "Precambrian metamorphic core complex" ranges trend
northeast-southwest, which marks them as somewhat unique in
the Basin and Range Province. Examples include the Buckskin
Mountains, Harcuvar Mountains, and Harquahala Mountains of
western Arizona. Most of the other mountain ranges in the
Basin and Range Province have volcanic origins; some of Cretaceous
age, some of slightly younger Laramide age, and some of younger
still mid-Tertiary age. No volcanism at all occurred during
Paleozoic time, nor were any new intrusions formed. Most,
but not all of southern Arizona's mineral deposits are products
of Laramide mountain-building.
The Cretaceous andesites that make up the bulk of the Trigo Mountains also form most of the Chocolate Mountains, Middle Mountains, and Kofa Mountains, which lie to the east of the Trigos.
southern portion of the Trigo Mountains is richly endowed with
mineral deposits. This part of the Trigo range is home to many
mines and prospect pits and probably has potential for future
mineral finds. Starting from Norton's Landing, prospectors should
probably head northeast up the washes and arroyos of the Trigos
toward the Papago/Red Cloud Mine area. The contact between the
andesite and older schist merits some attention, but the fault
zones and igneous intrusions near the mines have greater potential
Most of the ore bodies in the district are fault controlled
so particular attention should be paid to the local shear
zones and fault systems. In any case, the mountainous country
between Norton's Landing and the Red Cloud Mine should be
the focus of any search.