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CALIFORNIA

Lost Butler Mine

GEOLOGY OF THE AREA

The Santa Rosa Mountains and the adjoining San Jacintos are fault-bounded, uplifted blocks of early Mesozoic granites and older metamorphic rocks. Several large exposures of slightly younger, sheared Pre-Cretaceous metamorphic rocks occur in the north and north-central portions of the Santa Rosas. A small exposure occurs in the southwestern end of the range, approximately 3 miles east-southeast of Clark Well. This core of early Mesozoic country rock is intruded by slightly younger Mesozoic granitic rocks ranging in composition from granite to tonalite. The contact between the younger granites and older country rock is sometimes sharp, sometimes gradational, and occasionally fault-bounded. The Santa Rosa Mountains and adjoining fault zones all trend northwest-southeast. Two major fault zones bound the range. On the western side of the mountains, the San Jacinto Fault zone runs up the middle of

Clark Valley, Butler Canyon, and Dry Wash. On the eastern side of the mountains, the famous San Andreas Fault zone runs along the edge of the Coachella Valley.

The Borrego Badlands lie just south of the southernmost spur of the Santa Rosa Mountains. Rattlesnake Canyon and Palo Verde Canyon both drain the southern end of the range; this area is underlain by sheared metamorphic rock.

Rockhouse Canyon is located on the western edge of the range, near Buck Ridge. The area is underlain by early Mesozoic granites and older metamorphic country rock. Some small exposures of slightly younger Mesozoic granite occur on both sides of the canyon.

PROSPECTING POTENTIAL

Tales of Santa Rosa gold notwithstanding, the mountains have officially produced very little gold. They contain no major mineral deposits or mines. The limited and localized nature of all "blowout" deposits precludes their exploitation by large mining companies. It is left to the individual prospector to locate and harvest the gold-bearing pocket.

Prospectors may want to concentrate on the western edge of the range, near Rockhouse Canyon. At least one account of the story places a rich "blowout" somewhere in this area. Interestingly, some of the famous black-coated "Pegleg" Smith gold nuggets have been found just south of the Santa Rosa Mountains, in the Borrego Badlands. Further south lie the Fish Creek Mountains with Superstition Mountain just to the

east. Black-coated gold nuggets have also been found in these areas. If indeed the Santa Rosas are the source of at least some of the gold nuggets in the Borrego Badlands, then the canyons draining the southern slopes of the mountains would be possible sites for metal-detecting.

Some portions of the Santa Rosa range are off-limits to prospecting. Much of the northern part of the mountains lies within the Santa Rosa Indian Reservation, the Santa Rosa Mountains State Game Refuge, or the Anza Borrego Desert State Park. Some sections of the southern part of the range also lie within the Anza Borrego Desert State Park. More recently, the entire range has been declared a National Monument.