During the formation of some gold deposits was produced the segregation of magmas. It has been noted that the solidification of magmas under conditions that do not permit the escape of their metallic content, there is a tendency for some particles to form segregations probably through mass action, or the mutual attraction of the particles. Magmatic segregations are commonly made up of compounds of relatively low mobility. Basic rocks appear to be the most suitable for the formation of segregations, the characteristic minerals of which are magnetite, ilmenite, chromite, pyrrhotite, pyrite, and pentlandite. The gangue minerals are those of the containing rock.
Acid, or quartzose segregations, however, are known. Gold and platinum have been established as constituent minerals of rock masses, chiefly as sparse disseminations, with variable grade, but important as the sources of placer deposits. Magmatic segregations are rarely of economic importance as ores, and may usually be recognized in thin section under the microscope by the manner of intergrowth of the ore with the rock minerals. It is not unusual to find that rocks containing such deposits have become further differentiated into two or more parts of different mineralogical composition. Clearly established examples of magmatic segregations are more rare than was at one time supposed. Deposits may be associated with basic intrusives in such a way as to indicate that the ore minerals were segregated from the containing rock as it cooled. Gold ore may not be formed along the contacts of these intrusives with the enclosing rocks, and the gold grades into the containing rock through a transition zone that shows a gradual change in the quantities but not in the kinds of the constituent minerals. In the old deposit of Silver Peak, Nevada, auriferous quartz appears to have segregated from enclosing alaskite, which is a phase of the accompanying granite; the ore bears the same relation to the alaskite that the latter bears to the granite. The alaskite is a granite without biotite; the quartz is an alaskite without feldspar. . The segregated quartz is in this case of the same age and generation as the granules of quartz that make up a large proportion of the alaskite and granite.
Contact deposits are deposits formed along the contacts between intrusives and their enclosing rocks, or in these rocks in immediate proximity to the intrusives. They are the result of direct emanation of minerals-bearing solutions from the intruding magma. Contact deposits are usually limited to rocks that exert strong precipitation action. The characteristic minerals of contact deposits are specularite, magnetite, bornite, chalcopyrite, pyrite, pyrrhotite, and more rarely, galena and blende, associated with garnet, wollastonite, epidote, ilvaite, amphibole, pyroxene, quartz and calcite, and rarely, fluorite and barite. The unique feature is the association of the oxides of iron with sulphides. The sulphides frequently carry gold and silver, but usually in small quantities only.
Contact deposits are rarely of commercial importance, although there are certain notable exceptions. Many deposits formerly included in this class are now more correctly considered as replacement deposits. Contact deposits are usually quite irregular in form, due largely to the common irregularity of the igneous intrusion, and when the ore is lost, it is generally recovered with difficulty; while there is apparently no genetic reason why contact deposits, which are of deep-seated origin, should not be persistent, it is well known that few contact deposits are mined at more than shallow depths. In most cases certain beds of the intruded rocks exert a greater precipitation action, or are more easily replaced, than other beds of the series, and contact minerals develop much more abundantly in them than elsewhere. It is necessary in the examination of contact deposits to trace these favorable beds and to determine their probable thickness beneath the better exposures; not infrequently, the important mineralization is confined to one or more such beds, to which, therefore, exploration should be limited, and below which the mineralization should not be expected to extend.