Four "pulses" of post-Paleozoic igneous activity are recognized in the Basin and Range Province of southern Arizona. The first pulse occurred during early-Cretaceous times. Volcanism during this period was incredibly savage. Cretaceous tuffs and volcanic breccia formations are extremely massive (some are 3 miles thick!) The second pulse, known as the Laramide Orogeny, produced most (but not all) of southern Arizona's mineral deposits. The third, mid-Tertiary pulse produced vast quantities of rhyolitic and basaltic flows, tuffs, breccias, and agglomerates. A fourth pulse occurred during the Quaternary and produced widespread basalt flows. The Trigo Mountains are a product of early-Cretaceous volcanism and consist mostly of andesite flows, tuffs, and agglomerates. The andesite intrudes and blankets slightly older Mesozoic-age metamorphic rocks; these older schists and gneisses occur as scattered outcrops throughout the range. The largest exposure is found at the southern tip of the range and has a total outcrop area of roughly 20 square miles. The Trigo Mountain andesite is intruded by small Laramide-age dikes and granite stocks; these younger intrusives are clustered in the southern portion of the range. Most of the mineralization in the Trigo Mountains is associated with these Laramide intrusions. This is especially true where faulting has provided a conduit for mineral-rich fluids. The Red Cloud mining district (which includes the Papago mine and Black Rock Mine) is located in just such an area.
The area north of the Clip Mine is also mineralized but less so than the mining districts in the southern part of the range. Several mines and prospect pits occur on the west flank of the range, about 8 1/2 miles north-northwest of the Red Cloud Mine.
Therefore, the rugged mountains between Clip Wash (to the south) and Hart Mine Wash (to the north) should be thoroughly prospected. The bedrock here is almost all andesite except for the aforementioned Laramide plug near Clip Wash and an exposure of Mesozoic schist at the Hart Mine. The contact between the andesite and the older country rock should be traced and a search for silver-bearing float conducted within the area of interest.