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ARIZONA

The Lost Mine of the Little Horn Mountains

GEOLOGY OF THE AREA

The Little Horn Mountains are part of a series of desert mountain ranges in southwestern Arizona whose cores are predominantly volcanic in nature. Most of these mountain chains trend north-south or northwest-southeast. A few of the mountain ranges in the Basin and Range Province of southern Arizona deviate from this pattern - their cores are composed of ancient Precambrian metamorphic and igneous rocks and they trend northeast-southwest. These Precambrian "metamorphic core complex" ranges occur in a belt that more or less borders the Central Highlands Province lying to the north. South of this "belt" of Precambrian metamorphic core complex ranges lies the majority of the Basin and Range mountain chains, those whose cores are predominantly volcanic in nature. The Central Highlands Province is the second of three physiographic provinces that occur in Arizona; it consists of folded and faulted igneous, sedimentary, and metamorphic rocks. The Central Highlands Province forms a transition zone between the Basin and Range deserts of southern and western Arizona and the high mesas of the Colorado Plateau. Again, an indistinct "belt" of Precambrian metamorphic core complex ranges borders and parallels the Central Highlands Province along its southern edge. These metamorphic core complex ranges include the Buckskin Mountains, Harcuvar Mountains, Harquahala Mountains, White Tank Mountains, Buckeye Hills, Tortolito Mountains, Rincon Mountains, and Pinaleno Mountains.

Four "pulses" of post-Paleozoic volcanism are recognized in the Basin and Range Province of southern Arizona. The earliest occurred during early-Cretaceous times and produced vast amounts of andesite flows, tuffs, and agglomerates. The second pulse occurred during Laramide times and produced abundant silicic to intermediate flows, dikes, and plugs. Most of the mineralization in southern Arizona is associated with this Laramide igneous activity. The third pulse occurred during mid-Tertiary times and produced vast amounts of rhyolite, basalt, and andesite. The final pulse occurred during Quaternary times and produced widespread basalt flows.

The Little Horn Mountains are a highly weathered block of volcanic rock consisting of Cretaceous-age andesites and rhyolites and much younger Quaternary-age basalts. The Cretaceous andesites and rhyolites are known as the Kofa volcanics - these rocks comprise much of the Kofa Mountains, Tank Mountains, and Castle Dome Mountains. As it turns out, the Little Horn Mountains are mostly overlain by Quaternary basalts (which are generally barren of mineralization). Large exposures of Kofa volcanics occur along the western flank of the Little Horns and along the northern and southeastern slopes of the range. Any mineralization in the Little Horn Mountains would probably be associated with the Kofa volcanics, rather than the basalts.

PROSPECTING POTENTIAL

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