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ARIZONA

The Lost Nummel Silver

GEOLOGY OF THE AREA

The 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.

PROSPECTING POTENTIAL

The 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 for mineralization.

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.