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.
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