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ARIZONA

The Lost Gold Placers of the Laguna Mountains

GEOLOGY OF THE AREA

The heavily weathered and dissected Laguna Mountains are located at the northern terminus of the Gila Mountains and are indeed an outlier of the Gila range. Both are composed of Mesozoic-age rocks, mostly metamorphics, but some igneous and sedimentary rocks too. The Laguna and Gila ranges occur in the Basin and Range Province of North America. Like most of the mountain chains in this province, the Laguna/Gila chain trends northwest-southeast.

Three physiographic provinces comprise the state of Arizona: the Colorado Plateau, the Central Highlands Province (or "disturbed belt"), and the Basin and Range Province. The Colorado Plateau consists of extremely thick sequences of nearly horizontal sedimentary rocks and encompasses most of northern and northeastern Arizona. The central band of rugged mountains lying just south of the Colorado Plateau comprises the second of Arizona's three provinces. The Central Highlands Province or "disturbed belt" consists of folded and faulted igneous, sedimentary, and metamorphic rocks that form a transition zone between the Colorado Plateau and the Basin and Range Province of southern and western Arizona. Parts of the Central Highlands Province are richly mineralized, particularly the areas near Clifton/Morenci, Globe/Miami, and Jerome. The famous Mogollon Rim marks the boundary between the Colorado Plateau and the Central Highlands Province to the south.

 

The Basin and Range Province comprises the southern third of Arizona plus its western edge. Most of the mountain chains in the Basin and Range Province of Arizona trend north-south or northwest-southeast; these mountains generally have cores consisting of igneous rock. In general, four "pulses" of post-Paleozoic igneous activity are recognized in the desert ranges of southern Arizona. The earliest occurred during early-Cretaceous times and produced vast amounts of andesite flows, tuffs, breccias, and agglomerates. The second pulse occurred during Laramide times and produced silicic to intermediate dikes and granitic stocks and plutons. Most of southern Arizona's mineral deposits are products of Laramide igneous activity. The third pulse occurred during mid-Tertiary times and produced vast amounts of basaltic and rhyolitic lavas, breccias, ash-flow tuffs, and pyroclastics. The final pulse occurred during Quaternary times and produced widespread basalt flows.

Some mountain ranges in the Basin and Range Province of southern Arizona deviate from the general trend. These ranges trend northeast-southwest and have cores predominantly composed of ancient Precambrian metamorphic (and sometimes igneous) rocks. These Precambrian "metamorphic core complex" ranges include the Buckskin Mountains, Harcuvar Mountains, Harquahala Mountains, White Tank Mountains, and the Buckeye Hills.

The Laguna Mountains are a highly weathered block of Mesozoic-age schist and gneiss draped by a thick apron of Quaternary-Tertiary sands, silts, gravels, and conglomerates. A small exposure of Cretaceous andesite occurs on the western edge of the Lagunas, near Sugarloaf Peak. This is the only occurrence of igneous rock in the Laguna Mountains. A small wedge of Laramide (Cretaceous-Tertiary) sediments occurs on the southeast flank of the Lagunas, along the Gila River. These clastic sedimentary rocks dip slightly to the west and have an outcrop area of just over one square mile. But again, the most abundant rocks in the Lagunas are Mesozoic schists and gneisses; schists make up the eastern half of the range, gneisses the western half.

The Gila Mountains, lying just south of the Lagunas, consist mostly of Mesozoic schists, gneisses and granites. The northern half of the range is mostly schist and gneiss with one small granitic stock (outcrop area: about 10 square miles) in the northernmost portion of the range. The southern half is all Mesozoic granite.

PROSPECTING POTENTIAL

The Laguna Mountains have been prospected since the 1700's. The western and southern flanks of the range have been the most productive, yielding fine gold and nuggets up to 2 ounces in size. The gold occurs in placer deposits near the river's edge and in the canyons and gullies of the mountains. Gold is

also found in potholes and crevices in the bedrock up to 100 feet above the valley floor. Present-day prospectors should probably concentrate on the southern and western slopes of the range. In this area, a metal-detector may prove to be useful.