OCCURRENCE. The larger number of the silver minerals given in the above table occur together in many deposits. so that the ores received at the smelting, leaching, or milling works usually consist of a mixture of several silver minerals. Generally native silver and the halogen com pounds (chlorides, bromides, or iodides) occur in the upper portions of the deposits, while the sulphides, arsenides, and antimonides are found in the lower portions. Tetrahedrite in most cases occurs by itself. The minerals containing silver as an accidental ingredient are galenite (galena), sphalerite, chalcopyrite, pyrrhotite, pyrite, bournonite, chalcocite. bornite, native arsenic, arsenopyrite, and certain nickel, cobalt, and bismuth ores. Galena often contains silver up to 1 per cent. in quantity (291 ounces to the ton), so that at times the value of the silver in the ore is greater than that of the lead. In Europe the greater portion of the silver output is derived from galena ores, and in the United States at least 85 per cent. of the annual pro duction of lead is obtained from argentiferous lead ores, which necessitate the separation of the silver from the lead bullion tornicd, not only to extract the value of the silver, but also to render the lead of proper purity for commercial purposes. Copper ores frequently contain a
considerable percentage of silver, notably in the Butte district, Montana, where every pound of copper extracted contains on the average an ounce of silver. The famous copper schist of Mansfeld, Germany, also carries silver.
Silver ores occur in the rocks of various geo logic ages: in gneiss and allied rocks, in por phyry, trap, sandstone, limestone, and shales. The veins often intersect eruptive rocks, as trachyte or porphyry, or the sedimentary forma tions in the vicinity of such rocks, and have owed their existence in many eases to the dynamic processes and vapors from below at tending the eruptions. As mentioned above, sil ver ores are often associated with those of lead, zinc. copper, cobalt, and antimony, and the usual gangue is calcite or quartz with, frequently, dolo mite or barite.