Mining in the Marble Mountains of southeastern California was active in the earliest 1900s and gradually declined to very few active mines by 1959. Most mining consisted of hard-rock prospects and mines, with a few soft-rock prospects and one mine. The Marble Mountains are a 10 km by 30 km, gently NE-dipping dipping structural block composed of Proterozoic plutonic and metamorphic rocks, Paleozoic sedimentary rocks, and Jurassic granitoids exposed along the western anti-dip slopes, and Miocene volcanic and sedimentary rocks exposed along the crest of the range and eastern dip slopes. Mineralization occurred in metamorphic aureoles of intrusions, along dikes, as contact metasomatic replacement bodies in carbonate rocks, or adjacent to or along thrust faults. Mineralization locally formed gold, copper, malachite, azurite, bornite, chalcopyrite, magnetite, specularite, limonite, quartz, epidote, actinolite, and garnet. Hard-rock prospects and mines are clustered into five locations. The mines are small open pits, a few consist of a shaft or two with a few adits, most are just a single shaft or adit. There are a few small open-pit marble mines, and one is an open pit fossil mine. Eight prospects were developed in the Miocene tuffaceous deposits; three in the northwest and five prospects, and a single adit, in the south. Most of the Marble Mountains are now in the Trilobite Wilderness or adjacent Areas of Critical Environmental Concern, and the area of the Golden Cycle district (Castle Mine area) and prospect areas in the southern part of the range are in the Mojave Trails National Monument. The Iron Hat mine, several nearby areas, and the Trilobite mine area are privately owned. No rock or mineral mines are currently active, but the Trilobite mine in the south end of the range is still open to the public.