There are a considerable number of minerals which are formed in all three of these zones, although in differing proportions. There are comparatively few which are uniformly characteristic of a single zone. On the whole, it is possible to contrast satisfactorily mineral deposits representing very intense metamorphic conditions, usually associated with formation at great depth, with those formed at or near the surface; but there are many deposits with intermediate characteristics which it is difficult to place satisfactorily.
The accessible deposits of the deep zone are associated with plutonic igneous rocks which have been deeply eroded, and not with surface lavas. They are characterized by minerals of gold, tin, iron, titanium, zinc, and copper, and sometimes of tungsten and molybdenum, in a gangue of quartz, which contains also minerals such as garnet, corundum, amphibole, pyroxene, tourmaline, spinel, and mica. The deep-zone minerals are not unlike the pegmatite minerals in their grouping and associations.
Deposits formed at shallow depths are related to extrusive rocks and to intrusives near the surface. Erosion has not been deep. Mercury, silver and gold (tellurides, native metals, and silver sulphides), antimony, lead, and zinc minerals are characteristic, together with alunite, adularia, and barite. Metallic copper also is not infrequent. Very often the gangue material is more largely calcite than quartz, whereas calcite is not present in the deep zone.[5]
The trend of evidence in recent years has favored the conclusion that the principal ores associated with igneous rocks have not developed at very great depths. Even within our narrow range of observation there is a difference in favor of the shallower depths, and the greatest depths we can observe are after all but trivial on the scale of the earth.
A survey of the ore deposits of Utah has suggested the generalization that ores are more commonly related to intrusive stocks than to the forms known as laccoliths, and that within and about intrusive stocks the ores are much more abundant near the top or apex of a stock than lower down.[6] In parts of the region where erosion has removed all but the deeper portions of the stocks, ore bodies are less abundant. It will be of interest to follow the testing of this generalization in other parts of the world.
The scientist is constantly groping for underlying simple truth. Such glimpses of order and symmetry in the distribution of ore around igneous rocks as are afforded by the facts above stated, tempt the imagination to a conception of a simple type or pattern of ore distribution around intrusions. For this reason we should not lose sight of the fact that, in the present state of knowledge, the common and obvious case is one of irregular and heterogeneous distribution, and that there are many variations and contradictions even to the simplest generalization that can be made. The observer is repeatedly struck by the freakish distribution of ores about igneous masses, as compared with their regularity of arrangement under sedimentary processes to be discussed later. It is yet unexplained how an intrusive like the Butte granite can produce so many different types of ores at different places along its periphery or within its mass, and yet all apparently under much the same general conditions and range of time. It is difficult also to discern the laws under which successive migrations of magma, from what seems to be a single deep-seated source or melting-pot, may carry widely contrasting mineral solutions. Far below the surface, beyond our range of observation, it is clear that there is a wonderful laboratory for the compounding and refinement of ores, but as to its precise location and the nature of its processes we can only guess.
Other features of distribution of minerals associated with igneous rocks are indicated by their grouping in metallogenic provinces and epochs (see pp. 308-309).
The Relation of Contact Metamorphism to Ore Bodies of the Foregoing Class.
The deposition of ores of igneous source in the country rock into which the igneous rocks are intruded is a phase of contact metamorphism. Ordinarily where this deposition occurs there are further extensive replacements and alterations of the country rock, resulting in the development of great masses of quartz, garnet, pyroxene, amphibole, and other silicates, and in some cases of calcite, dolomite, siderite, barite, alunite, and other minerals. Looked at broadly, the deposition of ores at igneous contacts under contact metamorphism is a mere incident in the much more widespread and extensive alterations of this kind. Hence it is that the subject of contact metamorphism is of interest to economic geologists. The minerals here formed which do not constitute ores throw much light on the nature of the ore-bearing solutions, the conditions of temperature and pressure, and the processes which locally and incidentally develop the ore bodies. The subject, however, is a complex one, the full discussion of which belongs in treatises on metamorphism.[7] We may note only a few salient features.
For many hundreds of yards the rocks adjacent to the intrusions may be metamorphosed almost beyond recognition. This is especially true of the limestone, which may be changed completely to solid masses of quartz and silicates. The shales and sandstones are ordinarily less vitally affected. The shales become dense, highly crystalline rocks of a "hornstone" type, with porphyritic developments of silicate minerals. The sands and sandstones become highly crystalline quartzites, spotted with porphyritic developments of silicates. Occasionally even these rocks may be extensively replaced by other minerals, as in the Coeur d'Alene district, where quartzites adjacent to the ore veins may be completely replaced by iron carbonate.