Special Cases of Gold-Bearing Veins
Gold-bearing veins have some variations and some of them are special cases. Spurs may be present when a vein has more than one outcrop, the main lode presenting itself at the surface with several minor lateral branches. These are generally more numerous on the hanging wall side of the lode and present in the greatest numbers when the dip of the vein is flat, as was the case in the Comstock lode. These spurs may run nearly parallel to the main lode and standing more vertically unite with it in depth, or they may run out into the general bedding of the rock. In the former case it is easy to understand how the cracks in which they are formed were made by the weight of the overhanging wedge breaking it across from time to time, or the sliding of the hanging wall on the footwall pulling it apart, a cause which might also give rise to the spurs which branch out into the bedding.
In other cases, especially in very large gold veins, the spurs after leaving the vein may reunite with it in all directions, resulting in a mass of barren rocks entirely surrounded by vein matter, to which the term horse is usually applied. These are frequently called masses of rock which have fallen into the fissure, but they have usually moved, but a very slight distance, if at all, from their original position. When the vein is vertical the walls may be distinguished by the points of the compass, as the north, south, east or west wall, but when the vein has a slanting dip, the lower wall is designated as the foot wall and the upper as the hanging wall. In England the term Cheeks is sometimes applied to the walls.
Not infrequently occurs between the vein matter and the country rock, a seam of clayey matter called gouge or selvage, which may be of extreme thinness or reach a thickness of as much as eight meters as occurred in the Potosi mine on the Cromstock lode. This is apparently the results of a grinding or crushing movement of the walls of the vein upon each other, under enormous pressure, and where a portion of the vein has been mixed with material from the walls, the gouge is rich enough in gold bearing minerals.
Where the motion has been considerable and the walls or veins are hard enough to resist the movement action and reduction to clay, both the wall and the vein matter become market with parallel lines or strice called slickensides or slickens, which indicate the direction of the motion, and frequently the quartz or walls has taken a polish equal to anything, which can be produced artificially. In other cases, it may happen that there is only one well defined wall, the vein matter being firmly attached to the other, and gradually fading out into the low gold grade rock. The condition of the walls is a matter of much interest to gold prospector. If the ore is frozen to its alls on both sides, it is almost a positive indication of uncertainty of continuance in depth, but heavy clay seams or gauges are taken as favorable indications from the evidence they supply of a deep-seated fissure.
The material with which the gold vein is filled is known as gangue or matrix. It is not necessarily quartz. It may be calcite, fluorspar, barites or even the decomposed remains of a porphyry dike. The term matrix alludes to the filling being the mother of the ore. It does not follow that all this gangue carries ore, or that it is of uniform thickness through the entire length of the lode. The movement of the walls may have brought two swells opposite each other, and the vein is then said to pinch or peter out; or the ore may be confined to slabs on the walls.