The source of the gangue minerals deposited near the gold vein is found in the country rocks of the region, mainly the eruptives, which contain nearly all, and probably would be shown to contain quite all, of those known; or even from the secondary or stratified rocks derived from older igneous rocks, into which the constituents of the latter must unquestionably have been carried during their deposition as sediment. Also, the lavas have been deprived of their valuable mineral constituents.
In some deposits, the hanging wall of the vein is diabase, known by some gold prospectors as blue porphyry, a term which well expresses its appearance when freshly broken. The environment has exposed the structure of the county, which is on the hanging wall side, for a long distance and developed a series of bands showing plainly the results of the heated fluids. Outside of, and next to the gold vein, the roc is hard and suggestive of no change, except in some cases may be noted the presence of pyrite, which may be a product of decomposition, but under microscope it becomes evident that extensive changes have taken place in it. Its structure has been modified, the hornblende and augite (silicates) have been altered, pyrite having made out of their iron, and the silver which is present in them further away from the vein will disappear.
It is important to mention that all rocks present in gold deposits show lines of fracture of different structure. Some lines present a series of parallel planes along which they split with great facility, sometimes along the lines of original gold deposits. The layers may vary greatly in thickness, depending on the amount of sediment brought down by the stream and the length of the flood periods. All rocks have acquired two or three sets of joints, more or less at right angles to each other, which divide the mass into large blocks and greatly facilitate the labor of gold miner. These may be altogether independent of stratification, being the results of the upheaval and compression of the earth’s crust, though in some cases they may follow some of the more or less horizontal lines. During the formation period, rocks tend to change their character. For example, the mud banks of today will be the shales and slates of the far future and the sand banks, the sandstones of a coming era. In the same way and by the same agencies of time, moisture, pressure and heat, many of the older sedimentary rocks have lost much of their original character. The sandstones have become massive quartzites, in which small grains of quartz are not detected easily. In many cases there is not true gangue, the ore occurring in the original country rock, decomposed it is true, in bands or stringers more or less parallel to the walls or strike of the rocks which have undergone decomposition, but fading out into the unaltered country rock.