conceivable that under favorable conditions the concentration might take place in a main fracture so as to produce a vein owing its origin largely if not entirely to the action of surface waters.

Turning from the outline of the theories of ore deposition to the application of them, we enter a very interesting field. In the first place there are certain general considerations which are treated in the chapter on prospecting. Some of the more detailed considerations involved in the study of ore deposits may be treated as a series of rather generalized cases.

1. The ore body, in an igneous rock, consists of sulphides, having as a gangue the typical rock minerals. In basic rocks, look for segregations of sufficient importance to make workable ore bodies. In the acidic rocks, granites, and granite porphyries look for impregnation general enough to make a large low-grade ore body. Such an ore body would probably be of no economic importance at present unless subjected to secondary enrichment (see page 139).

2. A vein showing the minerals characteristic of pegmatitic conditions, giving assays for gold; from general theoretical con

siderations it seems unlikely that such a vein will show any increase of values with depth due to changing primary mineral content, or to enrichment. It should be regarded with great conservatism, bearing in mind the formation of placers from lowgrade gold veins. Pegmatitic veins are, however, the source of cassiterite or tinstone, and may become important either for the original content or the placers which have been formed from them.

3. A contact metamorphic deposit between limestone and diorite porphyry. The limestone is silicified and shows minerals of zone 3 of the table. Characteristically are found hornblende, garnet, vesuvianite, chalcopyrite, and galena. Since this is recognized as one of the lower vein zones it would not be expected to change greatly in depth. Without good evidence of enrichment such a deposit should be prospected carefully with close sampling. Deposits of this class sometimes offer possibilities as large low-grade smelting propositions, in view of the limey character of the gangue. The question of contact metamorphism is treated in more detail in Chapter XI.

Before going further with the discussion of the general cases it may be well to emphasize the fact that various stages of deposition are encountered often within a limited portion of an ore body; and that on the other hand not all of the stages may be encountered in any particular deposit. If it be supposed that the heat derived from an igneous mass has been a large factor in lending activity to mineral-bearing solutions, the ideal theoretical condition would be that of quick cooling with the hot solutions emanating during a relatively short period. Then we would expect to find the vertical range of different stages more or less well defined. But as a matter of fact the temperature of intrusives in large masses falls slowly, and as a result the stages overlap, and any given portion of a vein may lie first in the zone where the solutions are hot and under great pressure later as the heat center retreats, the upper zones tend to migrate downward. Thus the vein walls may show pyrite and chalcopyrite, and perhaps blende, associated with quartz, fluorite and garnet.

Next will be a band of galena chiefly, and blende, with siderite, barite, and quartz; a third stage may be represented by gold, quartz, and calcite; and finally there will be a barren calcite. stage. These conditions might be represented diagramatically as shown in Fig. 60. The tendency is for the upper stages to work downward overlapping and cutting the earlier ones.

This process would be reversed if the

heat center were rising instead of sinking, as might be the case if an intrusive mass were slowly penetrating overlying strata. This is believed to have been the case at Aspen, Colorado, where the normally earlier vein stages cut those which are usually characteristic of later periods. (Refer.: J. E. Spurr, Monograph 31, U. S. Geological Survey, and Economic Geology, Vol. IV, No. 4-"Ore Deposition at Aspen, Colorado.")

4. In this case the ore is argentiferous galena and gold quartz gangue chiefly; on the lower levels of the mine gold values are smaller and more scattered, there are galena and blende, with some pyrite and chalcopyrite. The inference is that the deposit is passing from the middle vein zones to the lower zone and will probably be less valuable.

3

2

FIG. 60.-Diagrammatic illustration of overlapping vein stages.

5. The richer ore is chiefly tetrahedrite, pyrargyrite, and polybasite; lower grade material is galena and blende, gangue is quartz, barite, and calcite. It is probable that the deposit is in the upper middle vein zone, and that with depth the rich sulphides will give way more and more to galena and blende or chalcopyrite and bornite.

The various cases outlined above are simply intended to give the general idea of the application of the theory to possible In practice, of course, an infinite number of variations may arise and it is impossible to lay down a set of definite rules to govern the judgement of ore bodies. However, enough has

cases.

been said to emphasize the importance of studying carefully the vein history, the zones represented, their relations, and relative importance.

One point that should be noted in this connection is that the presence of the upper or barren vein zone should not be taken as indicating that the other zones will necessarily be encountered with depth. The possibility of overlap of the zones is so great that it would be rare not to discover some traces of the productive types. It is also probable that the gangue minerals of this horizon could be derived from a magma originally poor in its content of the metals; therefore, unless there is some good reason for it such outcrops should be let alone. Where the productive veins of a district are known to show the barren stage strongly developed in their upper portions it is possible that apparently barren veins occurring in the higher regions of a range of hills may be the surface representatives of lower productive stages, but it is usually possible to find confirmatory evidence. It should be remembered in this connection that such veins, even if they develop values below, may not offer favorable conditions for secondary enrichment.

CHAPTER XI

APPLICATION OF GEOLOGICAL THEORY-SECONDARY ENRICHMENT

Everyone is more or less familiar with the ideas of oxidation and enrichment. An ore body consisting of sulphide ores has been exposed to the action of atmospheric agents and the oxidizing and transporting action of waters carrying carbon dioxide, organic acids, and acids derived from the oxidation of the sulphides themselves, such as sulphuric acid: The oxidized products go into solution and are carried downward to be precipitated under the influence of the "primary," or unchanged sulphides, increase of pressure, etc. It may be that the outcrop will show oxidized ore minerals, or that it may be entirely leached. The descending solutions carrying a heavy metallic content, encounter the primary sulphides and from the reactions ensuing results the formation of other sulphides richer in the metals. At the ground water level, where the surface waters accumulate and remain comparatively stationary, the enrichment products are concentrated; since there is little circulation below the ground water level the sulphides of that region are not so much subject to change and will probably be low-grade.

The term "primary sulphides" refers to those which exist in the condition in which they were first deposited, presumably from hot solutions emanating from some region of igneous activity.

"Secondary sulphides" are those which have resulted from changes in the primary sulphides due to the enriching action of descending surface waters.

"Secondary sulphide enrichment" is the change from primary to secondary sulphides.

It is apparent that several factors may have a bearing upon secondary enrichment. These are (1) topography of the region;