The Lost Horse Mine and Mill site is located in Joshua Tree National Monument, Riverside County, California, three miles southeast of Ryan Campground and Lost Horse Well.
Situated in the central part of Lost Horse Mountain, which reaches 5,178’above sea level, the mining and milling operation was erected directly over the main vertical shaft at 5,000′ on the southwest side of Lost Horse Mountain, which runs northwest to southeast. The mill site complex straddles a shallow valley–the mill on one side and several support and habitation sites on the other side. Several habitation features sit some distance from the milling site along the road to the mill and along a wash connecting the mill site with the access road.
The Lost Horse Mine gave its name to some of the surrounding areas such as the Lost Horse Valley and Lost Horse Mountains. They are part of the eastern Transverse Ranges that lie between the San Bernardino Mountains to the south and the Pinto Mountains to the north, a region characterized by numerous faults: the Pinto Mountain, Blue Cut, Porcupine Wash-Substation, Smoke Tree Wash-Victory pass, and Chriaco.
The Lost Horse Mountains, extending north from the San Bernardino Mountains, are a topographic expression of gneiss between two bodies of the White Tank quartz monzonite. The area is characterized by the presence of quartz, biotite, andulusite, and/or sillimanite, and cordirite with most mining activity taking place on shear zones with quartz veins containing remobilized gold and silver. Contacts between the area’s quartz monzonite and gneiss are either very abrupt or broadly gradational, with rarely complete mineralogical and textural transition between the two types of rocks over a distance greater than a few feet.
The workings of the mine explore a quartz vein exposed on the surface at several points for about 800 feet. The vein strikes east, dips 85° N., and ranges from 6 inches to 5 feet in width. Approximately 20 feet east of the main shaft, a 4 inch vertical quartz vein strikes N. 20° E. East-striking vertical quartz veins in shear zones in gneiss are explored by two shallow shafts on the ridge (300 feet and 500 feet to the east) as well as by several pits. The best defined shears in the upper shaft strike N. 15° W., and dip 70° southwest. About 100 feet west of the main shaft an adit extends N. 80° E. along a 5 foot wide shear zone. The adit is 80 feet long with a 50 foot winze and a 50 foot drift east driven from the bottom of the winze. The vertical 500 foot main shaft shows a small amount of drifting of the vein on the 100 foot , 200 foot , 300 foot, and 400 foot levels. By 1896, the main vertical shaft was already at 235 feet and was being worked with a horse-whim.
Gold production at Lost Horse suggests the richness of the vein. During the period 1895-1901, the company processed just over 10,000 ounces of gold and 16,000 ounces of silver. By the end of the century, $350,000 had been extracted from the Lost Horse. This six year period produced over 95 percent of the wealth made in the first seventeen years of the mine’s recorded production.