The commonest gangue minerals in the deeper parts of the lodes are quartz and the sulphides of iron (pyrites, marcasite, arsenopy rite). Nearer the surface the hydrated forms of silica (opal and chalcedony), the carbonates of iron, calcium, barium and magne sium (siderite, calcite, aragonite, witherite, dolomite) and the sul phates of calcium and barium (gypsum, anhydrite, barytes) are common.
Important deductions as to the origin of an ore deposit can be drawn from the nature of its mineral associates, always provided that care is taken to distinguish between the different epochs of mineralization, i.e., to say whether the mineral in question belongs to the primary period of mineralization, or has been introduced at a later date from without, or derived from the original mineral by the chemical action of permeating solutions. Thus, for example, the presence of fluorine, boron and phosphorus-bearing minerals, such as fluorspar, apatite, tourmaline and axinite, indicates deposi tion at a temperature above the critical temperature of water, by the aid of the so-called "mineralizers." Heavy anhydrous silicates such as the pyroxenes, the hornblendes and the dark micas, also imply a deep-seated origin at high temperatures. On the other hand, the presence of hydrated minerals, such as chlorite, opal, chalcedony and members of the zeolite group, points to an origin near the surface at comparatively low temperatures. The asso ciation of minerals belonging to the same epoch of formation is connoted by the term paragenesis.
Concentration by magmatic differentiation gives rise to ore bodies associated mainly with plutonic rocks. These, because of the slowness with which they cool, offer the greatest facility for such concentration. A marginal segregation of the heavier min erals is brought about in the cooling magma either by fractional crystallization and gravitation of the crystals thus formed or by liquation, the latter process being a separation into two immiscible fluids, the heavier of which sinks to the bottom of the magma basin.
The density increases with the number of molecules of metallic oxides, such as those of iron, chromium, nickel, copper, calcium, magnesium. Hence these molecules will be more abundant in the lower than in the upper portion of a magma basin. In general,
therefore, the net result is the formation of a basic peripheral zone and an acid (that is siliceous) central portion. As a rule the ores are concentrated in the peripheral portion, although exceptionally they occur in the heart of the igneous mass itself.
The formation of tin and copper deposits as the result .of the consolidation of a granite magma is worthy of consideration in this connection. An undifferentiated granite magma may be con sidered as a solution containing, among other constituents, a cer tain amount of water and the chemically active elements boron, fluorine, chlorine, phosphorus, sulphur, tellurium, etc., which are spoken of as "mineralizers," since they possess the property of forming volatile compounds with the heavy metals. As the magma cools it separates into differentiates of varying composition ; and consolidation of the different fractions takes place in the order of decreasing viscosity, the final residuum containing an excess of silica, a portion of the alkalies, practically all of the water and compounds of the metals, tin, tungsten, molybdenum, uranium, lead, copper, iron and many of the rarer metals.
In the early stages of consolidation cracks and fissures are formed in the cooling crust, into which the liquid residuum of the magma is injected. The earliest injections give rise to quartz porphyries and felsites; later differentiates are pegmatites and aplites; while the most acid extract consolidates as almost pure quartz. While the temperature of the crust is still high and before it has fallen below the critical temperature of the acid vapours, a proportion of the latter escapes into the surrounding rocks. These gaseous emanations deposit their mineral burden to a minor extent in fissures in the solidified crust of the igneous mass itself ; but the main deposition is in the older rocks beyond, to which access is gained through bedding planes, joints, faults and crush-zones.