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ARIZONA

The Lost Trigo Mountains Silver Ledge

GEOLOGY OF THE AREA

The Trigo Mountains are part of the Basin and Range Province of North America. This physiographic province is made up of elongated mountain ranges separated by alluvium-filled basins. Most of the mountain ranges trend either north-south or northwest-southeast. The ranges are uplifted blocks known as "horsts" while the intervening downthrown basins are known as "grabens". The oldest rocks in southern Arizona are occasionally exposed in the cores of these mountain ranges. Interestingly, most of these Precambrian "metamorphic core complex" ranges trend northeast-southwest, opposite that of most mountain ranges in the Basin and Range Province. Examples of Precambrian metamorphic core complex ranges include the Buckskin Mountains, Harcuvar Mountains, White Tank Mountains, and the Harquahala Mountains. Most of the other mountain ranges in southern Arizona are comprised of much younger igneous rock, usually volcanics. The cores of these mountain chains may be composed of Cretaceous-age intermediate volcanics (andesites) or granite plutons, somewhat younger Laramide-age rhyolite dikes and granite stocks, or younger still mid-Tertiary volcanics.

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.

PROSPECTING POTENTIAL

A number of prospectors and cattlemen have reported the existence of a silver vein somewhere north of Clip Mountain. The area seems to offer some potential for future mineral finds. Mineralization in southern Arizona is almost always associated with Laramide igneous activity. A number of Laramide intrusions occur in the southern portion of the Trigos. The central part of the range also harbors a few of these Laramide plutons. For example, a small Laramide plug is exposed along the middle reaches of Clip Wash, about 5 miles north-northwest of the Red Cloud Mine. In addition, all accounts of the story agree on one thing - the lost silver ledge crops out less than one day's journey north of the Clip 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.