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ARIZONA

The Lost Flannigan Mine

GEOLOGY OF THE AREA

The Gila Bend Mountains, and surrounding ranges, are part of the Basin and Range Province of North America. The Basin and Range Province is characterized by linear mountain ranges separated by downthrown, alluvium-filled basins. Most of the ranges trend north-south or northwest-southeast. The Basin and Range Province comprises the southern third of the state of Arizona, plus its western edge. Here, most of the mountain ranges follow the general trend, but a few deviate from the pattern.

These deviant ranges trend northeast-southwest and contain cores of ancient Precambrian metamorphic rocks. Known as Precambrian "metamorphic core complex" ranges, they include the Buckskin Mountains, Harcuvar Mountains, and Harquahala Mountains. These metamorphic core complex ranges occur in a "belt" that stretches diagonally across the state. This indistinct "belt" more or less parallels and borders the Central Highlands Province, which lies just north of the Basin and Range Province.

The majority of the desert mountains in southern Arizona trend north-south or northwest-southeast and contain cores predominantly composed of younger volcanic rocks. Four "pulses" of post-Paleozoic igneous activity are recognized in southern Arizona. The earliest occurred during early-Cretaceous times and produced vast quantities of andesitic lava flows, tuffs, and agglomerates. The second pulse occurred during Laramide times and produced silicic to intermediate volcanic rocks and granitic plutons. Most of the mineralization in southern Arizona is associated with Laramide igneous activity. The third pulse occurred during mid-Tertiary times and produced abundant basalts, rhyolites, and andesites. The final pulse occurred during Quaternary times and produced vast amounts of basalt.

The Gila Bend Mountains contain a wide variety of rock types ranging in age from Precambrian to Quaternary. The central third of the range is overlain by Cretaceous andesites, mostly lava flows and tuffs. The western third of the range is predominantly composed of ancient Precambrian basement rock - granites, gneisses, and schists. The eastern third of the range is mostly overlain by a large flow of Quaternary basalt, but some Precambrian granite and gneiss is exposed on the eastern edge of the range.

The Eagletail Mountains are also volcanic in nature. A distinctive Laramide-age dike runs lengthwise along the spine of the range. The flanks of the Eagletail Mountains consist mostly of early-Cretaceous andesites and younger Quaternary basalts. A small wedge of Precambrian gneiss crops out on the eastern flank of the range.

PROSPECTING POTENTIAL

The Gila Bend Mountains are composed of three distinct rock types that crop out in the western, central, and eastern thirds of the range. As previously mentioned, the western third of the range is mostly composed of ancient Precambrian basement rock while the central portion is overlain by younger Cretaceous andesites. The eastern third is mostly overlain by very young Quaternary basalts, although some Precambrian granite and gneiss occurs in the area. As previously mentioned, most of the mineralization in southern Arizona is associated with Laramide igneous activity. Younger, Quaternary volcanics are almost invariably lacking in mineral deposits. Prospectors may want to concentrate on the western two-thirds of the range based on the above observations.

The southern portion of the Eagletail Mountains is very close to the area of interest and also merits attention. Particular attention should be paid to all contacts and faults. Based on the description of the deposit given by Flannigan, the water hole has formed the only catchbasin for weathered gold particles. That placer deposit must be small but incredibly rich.