The famous La Posa Plain stretches nearly 60 miles in a north-south direction in the arid lower Colorado River country of southwest Arizona. It is home to the historic town of Quartzite which lies roughly in the center of the plain. The La Posa Plain is surrounded by mountain ranges. The Dome Rock Mountains form the western boundary of the plain while the Plumosa Mountains make up its eastern boundary. Both of these ranges trend north-south. The Buckskin Mountains bound the plain to the north while at least three ranges form its southern boundary. These include the Kofa, Castle Dome, and Chocolate Mountains.
The surrounding mountain ranges are home to many mines. The Dome Rock Mountains in particular are richly mineralized. They are host to a number of mines including the Dome Basin Mine, Darling Mine, and Bear Mine. The Plomosa Mountains also contain many mines and prospect pits. These include the Mudersbach Mine, Southern Cross Mine, and Jac-o-Lou Mine. The Buckskin Mountains are home to the Pride Mine and the famous Planet Mine.
The flat topography and gentle gradient of the La Posa Plain offers generally easy going for the travelers who pass through the valley. In 1884, the owner of the Planet Mine made a journey that he had probably made many times before. He was traveling northward across the La Posa Plain on his way to the Planet Mine, located in the Buckskin Mountains. Somewhere in the northern part of the La Posa Plain he began to encounter extensive sand dunes. Eventually he came to a small, exposed black hill surrounded by sand. He dismounted and examined the heavy black rocks scattered at his feet. The owner of the Planet Mine recognized rich ore when he saw it! He collected a number of specimens and logged the discovery in his notebook. He resumed his journey but unfortunately encountered one of the frequent sandstorms that sweep this part of the La Posa Plain. He never made it back to the mine - only his horse showed up carrying saddlebags filled with rich ore. A search party found the mine owner near the old Bouse-Parker wagon road. In his pocket was the notebook describing his discovery. The ore was later assayed at $750 per ton in gold, silver, and copper.
The history of mining in the lower Colorado River country of southwest Arizona must certainly extend back to the times of the early Spaniards. When the Americans first came to this country they found many old mines and prospect pits. Evidence of much earlier mining activity was found in the Plomosa Mountains, the Castle Dome area, and near present-day Ehrenberg. But the first real gold rush in Arizona occurred in 1858. It was in that year that a party of prospectors led by Colonel Jacob Snively discovered rich gold deposits some 20 miles up the Gila from its junction with the Colorado River. The mining camp that sprang up nearby came to be called Gila City, but it didn't last long. The rich placer deposits soon played out.
Much richer gravels were discovered in 1862 near the old Spanish diggings at Ehrenberg. These new gold fields, known as the La Paz District, produced nearly $8 million worth of gold before they were depleted. Much smaller placer deposits were also discovered in this part of Arizona during the 1860's. The La Cholla placers are located on the east side of the Dome Rock Mountains, about 7 miles west of Quartzite, in an east-west trending belt that stretches nearly 5 miles. These placers have been worked intermittently since the 1860's. Gold-bearing quartz veins in the nearby mountains served as the source of this placer gold.
The Plomosa placers occur on the west side of the Plomosa Mountains in and near Plomosa Wash. This area is located south of Scaddon Mountain. Gold-bearing gravels occur in a belt 3 to 4 miles long, along the western edge of the mountains. The Plomosa gold fields were discovered shortly after the La Paz rush. The source rock for this placer gold was a massive formation which ran for many miles along the western flank of the Plomosas. After the turn of the century, minor amounts of placer gold were recovered from the so-called Cienga District, located in the Buckskin Mountains. The Buckskin Mountains form part of the mountain front that bounds the La Posa Plain on its northern end. The Cienga District contains minor copper and gold deposits consisting of small pockets of ore minerals found in shear zones in sedimentary rocks. Some of these pockets were rich in free gold. In 1912, placer gold was recovered from the old Planet (or Santa Maria) District, also located in the Buckskin Mountains. Although the district was best known for its copper deposits, some gold ores did occur.