ORIGIN. The fact that the deposits occur as masses of greater or less concentration may be explained in two ways: that they have been formed contemporaneously with the inclosing rock. or that they have been formed by a process id concentration at a later date. The former theory involves a consideration of ore occurrences in both igneous and sedimentary rocks. If the ore in an igneous rock were formed at the same time as the rock, it would indicate a crystalliza tion of metallic minerals from the igneous magma bduring cooling, and this in some eases is true.' if the ores of sedimentary rocks were of contempo raneous origin, then the deposit must be a bedded one, conforming to the strata of the rock: this supposition requires the presence of metallifer ous minerals in and their deposition from sea I water. While certain metallic elements are found in the water= of the ocean, their quantity is ex tremely small and not to be compared with the amount which may be found in disseminated or concentrated form in sedimentary and igneous rocks. some economic geologists have assigned a contemporaneous origin to certain ores found in sedimentary strata. but the majority at the day believe that most ore deposits have been formed by the process of concentration. That metallic minerals are widely distributed, al though in small quantities, in both igneous and sedimentary rocks, has been shown by the re searches of Sandberger and others. and the quan tity of them found in igneous rocks is slightly greater than that occurring in sediments. Since. however, the sediments were originally derived from the igneous rocks, it follows that the latter must be the original source of the minerals. Where ores have resulted by a natural process of concentration, their accumulation requires the presence of disseminated metals in the earth's crust, the existence of a solvent and carrier, and the presence in most cases of cavities in which precipitation of the ore occurs. The first point has already heen referred to. As regards the second. it is found that the analyses of many spring and mine waters have shown the presence of metallic elements in solution, including gold, silver, copper, zinc. lead, and mercury. Indeed. some of these metals are actually being deposited in some hot springs at the present day: Weed has described a spring in Montana which carries gold and has deposited its burden of auriferous quartz on the plants near its mouth. That there is a wide circulation of meteoric water in the rocks of the earth's crust has been quite clearly shown by the work of Van Hise. The return of this water to the surface may occur along fissures or other openings which it can easily follow. That these circulating meteoric waters may play an important role in the concentration of many 1 , ores is no doubt true, and some geologists even consider that most ores have been formed in this manner. On the other hand, it has been
pointed out that most ore deposits are closely associated with igneous rocks, and in some cases with hot springs, and that therefore the former have served to open the way for heated waters and vapors whose solvent pourer is much greater than that of cold water. The advocates of the meteoric circulation theory believe that the waters have penetrated to the lower depths of the crust or barysphere, where metallic particles are considered to he abundant, and brought them to ward the surface. where they were deposited. Those who consider the presence of igneous rocks to be an important factor in ore deposition also believe that the minerals may in some cases have been brought from a great depth in solution in the waters given off by the igneous magma, hut that in some iustances the igneous intrusion itself may have been the source of the minerals.
While all minerals are slightly soluble in cold water, this solvent power may he greatly in creased by heat, pressure, and the presence of al kaline salts or other compounds. The metals may thus he leached out of the rock at some depth and out of contact with the air. When the solutions approach the surface or enter a cavity, the load of dissolved minerals is deposited either wholly or in part, as a consequence of cooling of the ore-bearing solution. decrease in pressure. and in some cases of the oxidizing effect of the atmosphere which converts certain soluble salts into an insoluble form. Iron eompounds, for example, may gn into solution in the form of carbonate, hut on exposure to the air the latter is rapidly changed to limonite, the hydrous oxide, which is insoluble. Since many (AT deposits are formed iu cavities, the question arises to what depth in the earth's crust cavities may extend. Investigations of Vall 111,e hate 'Minted out there can he recognized in the earth's crust two physico-chemical zones. In the upper one, which is close to the surface, the temperature and pres sure in the rock are not great. In the lower zone, which is at some depth below the surface, both temperature and pressure are great, and consequently chemical reactions take place. This lower zone Van 'Ilse divides into three parts, namely: an upper zone of fracture, in which the rocks are broken up by movements along the stir file(' of the zones, but no immanent in the zone itself; an intermediate zone of fracture and flow age; and a lower zone of flowage. in which a mashing or kneading action takes place. which involves every particle of rock. Within the last mentioned 7011e it would not he possible. there fore. for any cavities to exist, and Van Ilise has figured out that approximately the maximum depths at which tan ities can exist varies from I6'_'5 feet in shales to 33,500 feet in firm granites.