The Capitan deposit is a steep cylindrical body which is about 1,300 feet in diameter. It lies in gently westward-dipping San Andres lime-stone of Permian age, and is nearly on the axis of a westward-plunging nose formed by a laccolithic tongue from the large Capitan Mountains aplite. The igneous rock underlies the deposit at depths ranging from 250 to 450 feet. The structure which localized the deposit is interpreted as a slowly developed collapse breccia caused by ground-water leaching in Triassic time. The intrusive and mineral deposit are middle Tertiary (?) in age. Much pyrometasomatic silicate is associated with the magnetite iron ore. Epidote, phlogopite, and tremolite comprise the silicate mass, and they are distributed within the cylindrical body as concentric zones which occur from the center outward in the order given. Origin of the collapse structure is discussed first and is followed by consideration of the effects of time and temperature in the formation of the mineral zones. The ore fluids probably came from the Capitan intrusive where they were concentrated during its crystallization. A temperature arrangement set up by heated ore fluids appears to have been more effective in the creation of mineral zones than do successive introductions of magmatic fluid extracts. Open ground in the collapse structure greatly aided mineral zoning.