The Chocolate Mountains and the adjoining Trigo Mountains of southwest Arizona are part of the Basin and Range Province of North America. This physiographic province comprises the southern third of the state plus its western edge. Most of the mountain ranges in the Basin and Range Province trend north-south or northwest- southeast and consist of uplifted, fault-bounded blocks (known as horsts) separated from each other by downthrown, alluvium-filled basins (known as grabens). In southern Arizona, ancient Precambrian basement rock is sometimes exposed in the cores of mountain ranges. Interestingly, many of these Precambrian "metamorphic core complex" ranges trend northeast-southwest, marking them as somewhat unique in Basin and Range geology. Examples in southern Arizona include the Buckskin Mountains, Harcuvar Mountains, Harquahala Mountains, Buckeye Hills and the White Tank Mountains. All of these mountain ranges trend northeast-southwest and are made up almost entirely of ancient Precambrian granite gneiss. This is the oldest rock in Arizona.
Most of the ranges in southern Arizona have a younger igneous origin. Four major volcanic "pulses", beginning in the middle of the Mesozoic, are recognized. These episodes of volcanism produced massive amounts of lava, hypabyssal rocks, tuffs, and agglomerates. The majority of the mountain ranges in southern Arizona are therefore made up of early-Cretaceous intermediate volcanics (andesites), younger Laramide-age volcanic dikes and granite plutons, younger still mid-Tertiary volcanics, or fairly recent Quaternary basalts. This is true of the Chocolate Mountains and the adjoining Trigo Range. Most (but not all) of southern Arizona's mineral deposits are products of Laramide mountain-building processes.
The highly-weathered Chocolate Mountains (and the adjoining Trigo Mountains) are predominantly composed of Cretaceous-age andesite flows, tuffs, and agglomerates. These older volcanics make up most of several mountain ranges in southwestern Arizona, including the Middle Mountains, the Kofa Mountains, and the southern half of the Plomosa Mountains. In this respect, the Chocolate Mountains are similar to the majority of the desert ranges of southern Arizona. However, they differ in that they trend roughly northeast-southwest.
The Chocolate Mountains merge with the north-south trending Trigo Mountains at their southernmost points. Here, Yuma Wash separates the southern spurs of the two ranges. Rock type in this area is mostly andesite. A small exposure (less than 1 square mile in outcrop area) of Laramide volcanics occurs about 8 miles northeast of the mouth of Yuma Wash, in the Chocolate Mountains. The Trigo Mountains are home to the Red Cloud mining district (the Red Cloud Mine Road enters the mountains via Yuma Wash). The mineralized zone at the Red Cloud Mine lies only about 5 to 6 miles northwest of the mouth of Yuma Wash.
Although most of the Chocolate Mountains and Trigo Mountains are composed of Cretaceous andesites, some exposures of slightly older Mesozoic schists and gneisses occur in the area. Indeed, the largest body of Mesozoic schist makes up the southern spur of the Trigo Range and therefore forms the western boundary for much of the lower sections of Yuma Wash.
Mine, Papago Mine, Black Rock Mine, and Clip Mine. Prospectors should probably concentrate on the canyons in and around Yuma Wash. This area includes the southernmost flanks of the Trigo and Chocolate Mountains.