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and various other minerals. The native silver is formed, probably, only after a preceding oxidation of the sulphide. Deposits carrying other unaltered silver-bearing minerals, such as the various silver sulphides, arsenides, antimonides, tellurides, etc., are, when exposed to surface influences, affected in much the same way as the silver in argentiferous galena.
Alteration of zinc deposits —In the case of zinc, the most common ore is the sulphide known as blende. This mineral, like galena, is generally oxidized on the surface, and forms by other chemical changes the carbonate (smithsonite), the basic carbonate (hydrozincite), and the basic silicate (calamine), in a manner similar to that described in copper and lead ores.
In the cases of both lead and zinc, oxidized ores are very desirable for metallurgical purposes, and are much sought after. To be sure, the carbonates, sulphates, etc., of lead and zinc contain less of these metals than the pure sulphides, but they occur in a more concentrated form than the sulphides, and, therefore, the ores containing them frequently carry as much or more of the metals than the ores containing the sulphides. Moreover, the oxidized ores are much more easy to treat and, therefore, have an additional value over the sulphide ores.
Alteration in gold deposits.—In the case of gold deposits, surface alteration has a most marked effect, and probably in no class of deposits is the change of more geologic and economic importance. The typical unaltered condition of gold in nature is in association with iron pyrites in quartz, the gold being sometimes in such association with the pyrites that it cannot be separated by mechanical means, while in rarer cases, it can be so separated. The effect of surface oxidation on such a deposit, is first to convert the iron pyrites into a hydrated sesquioxide of iron, which premeates the white quartz, with which the pyrites is usually associated, and turns it into a rusty brown mass. The next stage is the gradual leaching out of the hydrous sesquioxide by the action of surface waters. The iron is, in this way, finally removed altogether, and the remaining product is a pure white quartz, containing the gold which was originally in the