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ARIZONA

The Lost Lode of the La Posa Plain

GEOLOGY OF THE AREA

The Basin and Range Province comprises the southern third of the state of Arizona. It is characterized by linear mountain ranges separated by downthrown, alluvium-filled basins. In southern Arizona, a "belt" of Precambrian "metamorphic core complex" ranges forms a sort of transition zone between the younger, predominantly volcanic desert mountains of the south and the folded and faulted highlands of central Arizona. The Central Highland Province is a "disturbed belt" of igneous, sedimentary, and metamorphic rocks that extends northwest-southeast across the central part of the state. The Colorado Plateau is the third (and final) physiographic province of Arizona; it bounds the Central Highlands Province on the north and consists of extremely thick sequences of nearly horizontal sedimentary rocks. The distinctive and famous Mogollon Rim marks the boundary between the Colorado Plateau and the Central Highlands Province.

The northernmost portion of the La Posa Plain (also known as the Cactus Plain in this area) is largely covered by Quaternary sand dunes. This desert wilderness is bounded on the north by the basalt-covered crags of the Gibraltar Mountain/Black Peak area. The Buckskin Mountains loom to the northeast. This mountain range differs from the standard Basin and Range mountain chain in that it trends northeast-southwest and contains a core of ancient Precambrian metamorphic rocks. Most Basin and Range mountain chains trend north-south or northwest-southeast and contain younger igneous cores. Sometimes the cores are composed of intrusive rocks, but many are volcanic in nature. This is especially true in southern and western Arizona.

The heavily weathered Mesozoic-age granites and andesites of the Bouse Hills lie to the southeast of the dune-covered area of interest. The western portion of the Bouse Hills is overlain by Cretaceous andesite and harbors several mines and prospect pits. The Plomosa Mountains rise up just west of the Bouse Hills and march away southward. The Plomosas form the eastern wall of the La Posa Plain for nearly its entire length. The northern half of the Plomosas is predominantly composed of Mesozoic sandstones, shales, conglomerates, and limestones. Slightly younger Cretaceous to early-Tertiary sediments crop out along the northernmost point of the range. Small outcrops of ancient Precambrian gneiss also occur in the area. Finally, the sand dunes of the Cactus Plain are bounded on the west by the low-lying, heavily weathered hills near the mouth of Bouse Wash. These hills are composed of Mesozoic-age gneisses. Several very small exposures of Cretaceous andesite occur on the Cactus Plain itself; these lie about 8 to 9 miles northwest of Bouse. Could similar exposures lie buried in the sand dunes?

This section of the La Posa Plain lies at the contact between the narrow "belt" of Precambrian "metamorphic core complex" ranges (which includes the Buckskin Mountains, Harcuvar Mountains, and Harquahala Mountains) and the younger, predominantly igneous mountain ranges to the south.

PROSPECTING POTENTIAL

The rugged deserts and mountains of Arizona's lower Colorado River country are still producing gold nuggets today. A number of placer gold locations are located in the south-central portion of the La Posa Plain. These have produced many gold nuggets for modern-day prospectors. The La Cholla District is located on the eastern slopes of the Dome Rock Mountains. Here the gold is found in a bed of loose quartz rubble that occurs at a depth of about 80 feet. The gold-bearing gravels were carried to the surface by the early miners - it is this quartz rubble that present-day metal detectors find gold in. La Cholla is one of the few places in the west where crystallized gold has been found. The Plomosa District has also been a recent producer. The original gold-bearing formation ran for miles. The western flank of the Plomosas is deeply cut by ravines and washes and there may be small exposures of gold-bearing ore that have been overlooked. But more recently, and further north in the area near Lake Havasu City, a 47-ounce specimen of native gold in matrix was discovered in March 2001. The sample contained an estimated 29 ounces of gold and was located with the use of a metal detector. This area lies only 25 miles northwest of the Planet Mine. Obviously, the lower Colorado River country of southwest Arizona has good potential for future mineral discoveries.

The body of the Planet Mine owner was found near the old Bouse-Parker wagon road. This northern section of the La Posa Plain contains extensive sand dune deposits and is ringed by several mineralized areas. These include the Bouse area, the Buckskin Mountains, and the Dome Rock Mountains.

In this case, the metal-detector would seem to be a most useful tool in the search for the lost lode. It may be that the vein or deposit is entirely covered by sand. In any case, the northern half of the La Posa Plain and surrounding foothills appear to be the most promising area to search.