Nothing but actual work can positively determine the question whether we have merely the top or the tail end of an ore body, as if we look upon the ore body as a roughly shaped lens, two parallel lines drawn through it, one just below the top and the other just above the bottom, would each cut off. a portion showing the same width and length; but if the ore is softer than the inclosing rock, and the vein crops in the bottom of a deep gorge, there is every probability that the gorge has been formed by the wearing down and removal of the ore body, that the outcrop found, if of small dimensions, is merely the lower end where the increased portion of country rock resisted the action of the stream to a larger extent and stopped further erosion. In a similar way, if the outcrop is on a steep hillside and of only limited length, it may in many cases indicate the termination of the ore body, as the erosion of the ravine which has exposed it has also disclosed some of the secrets of its penetration in depth, as just described in discussing the quality. But if the outcrop is of considerable length and thickness, this question need not trouble us, as there is sufficient justification for the expenditure of considerable money in development.
The condition of the mineral in the outcrop may also furnish a slight guide as to probable permanence in depth. Some of the common minerals in ore, especially the several varieties of pyrite, are easily decomposed under exposure to air and water, and where there is evidence in the rustiness of the outcrop, or the presence of a spongy looking mass from which the crystals have been perfectly removed, we may certainly infer that the drainage of the vein extends to some depth and involves a continuous fissure, as it is only by the presence of such conditions that the decomposition could have been effected. But if the pyrites remain in the croppings entirely undecomposed, we may infer that the walls of the fissure are so tightly in contact below as to prevent the percolation or seepage of water downward, and this may be taken as an unfavorable sign.
In soft and easily decomposed ore bodies the removal of the outcrop running along the face of a hill, instead of across it may result in shallow depressions, instead of a conspicuous ridge, in which the ore can only be found by digging; and when the ore is of this character, the search for it becomes a very laborious task, especially in moist and wooded countries, where the vegetation may be exceedingly rank and tangled.
Float. From what has been said of the origin of veins and the subsequent carving out of the ravines and valleys as they exist to-day, it is evident that there must be on the hillsides and in the ravines of a country rich in minerals, many fragments of mineral detached from the ore bodies. These are called "float," and it is equally evident that the harder the material forming the gangue of the ore, the larger should be the number of fragments which have escaped the destructive action of air, frost, water and sunshine. Such minerals as decompose very readily, or are excessively brittle, may disappear altogether, or nearly so, as coal or some forms of galena; but quartzose varieties may survive a long journey and be found at considerable distances from their original source. Large fragments indicate this source to be much nearer than small ones. Gravity has constantly carried these fragments to a level lower than their source, so that when tracing float, as so often done, we look for this source above the place at which the float has been found. It may have reached this place either by rolling down the hillside in the immediate neighborhood, or it may have been carried down the bed of the ravine by water. The position of the float will generally indicate which has been the method of transport, but in either case we must trace it upward, and if we find the fragments increasing both in number and in size, we may conclude that we are nearing the source of supply. Sometimes these may lead to the discovery of well-defined lodes, and again, in the case of quartz, we may finally lose all trace of it, without encountering anything of value.
This is especially apt to be the case in slate countries, which have been crushed and crumpled and subsequently filled with innumerable thin quartz seams. All the float may have been derived from these seams, and the float ceases because we have reached the limit of the slate formation, and enter a new series of rocks without quartz. This structure explains numbers of cases of "lost leads," but while there may be no well-defined vein or lode at the point where the ravine cuts the formation, if metal has been found in the float it shows that the region is worth prospecting and the search should be continued along the line of contact of the rock which has cut out the metal-bearing series.
Topography and Water System. The reverse is just as likely to be the case where the ore body is soft, as nature always carves out a country on the lines of least resistance, which may be lines of faulting, or contact of a soft and a hard rock, or through soft ore bodies or between consolidated hard ones. For this reason the study of the water system of any particular mountain region affords a good insight into its physical structure. If all rocks were of uniform and equal hardness, the face of a country would be planed down to uniform smooth slopes. It is the varying resistance of rocks which diversifies the mountain regions, in combination with the dislocations they have undergone.
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Gold Prospecting Basics, Part 5
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Gold And Silver Basic Prospecting Methods
