The Boleo copper deposits of Lower California occur in volcanic tuffs and associated conglomerates of Tertiary age. They have certain peculiar mineralogic associations--the ores containing large quantities of all the common copper oxide minerals, and a number of rare oxide minerals of copper, lead, silver, and cobalt, together with gypsum, sulphur, and much iron and manganese oxide. The copper oxides and carbonates are in places gathered into rounded concretions called "boleos" (balls). Sulphides are present in the lowest beds and may represent the form in which the copper was originally deposited. The copper-bearing beds have been much silicified, and it has been suggested that mineralization was accomplished by hot-spring waters, probably of igneous origin. These deposits have a few marked similarities to the Lake Superior copper ores.
=Copper veins in igneous rocks.= A second group of copper ores in igneous rocks is made up of deposits in distinct fissure veins and as replacements along such veins. The chief deposits of this type are at Butte, Montana--which is, from the standpoint of both past and present production, the greatest single copper district in the world. Here a large batholith of Tertiary granite was intruded by porphyry dikes; and faulting, accompanying and following the intrusions of the dikes, developed numerous fissures. The fissures were mineralized with copper sulphides and arsenides, iron sulphides, and locally with zinc sulphide and manganese carbonate,--all in a matrix of quartz. At the same time the wall rocks were extensively mineralized and altered; the fissure veins grade off into the wall rock, and in fact the larger part of the ore is simply altered granite with disseminated sulphides. The solutions which deposited the ores are inferred to have been hot from the nature of the wall-rock alterations, from the presence of hot-water minerals like fluorite, cassiterite, and others, and from the general association of the ores in time and place with the porphyry intrusions. The solutions are believed to have originated from the porphyry and possibly from other intrusives.
In the Butte district, and in the great majority of copper sulphide vein ores throughout the world, secondary concentration by surface waters has played a considerable part in developing ores of commercial value. Near the surface the copper is leached out and carried down by waters containing various solvents, particularly sulphuric acid from the oxidation of pyrite. A leached zone is formed containing the ordinary products of rock weathering,--rusty quartz and clay, sometimes black with manganese oxides. A small part of the copper remains in this zone as oxides, carbonates, and silicates. Below the oxidized and leached zone there is evidence of deposition of a large amount of secondary copper sulphide in the form of chalcocite. This is supposed to have been formed by the leaching of copper from above as soluble copper sulphate, and its precipitation below by iron and other sulphide minerals which the solutions meet on their downward course--a reaction which has been demonstrated experimentally. It was formerly supposed that most of the chalcocite was of this origin; but as chalcocite is found in important amounts with enargite and chalcopyrite to great depths (now 3,500 feet), where the veins are still rich and strong, it begins to appear that much of the chalcocite is of primary origin.