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SUPERFICIAL ALTERATION OF ORE DEPOSITS.
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various stages of oxidation of materials from organic matter, but they all eventually, if allowed to become completely oxidized, pass into carbonic acid; while if they are in combination with different bases, these salts are eventually converted to carbonates.

Method and chemical effects of alteration.—Surface waters thus charged with various chemical ingredients percolate down into ore deposits, and there meet various materials which are even less stable under their influence than most of the common rocks. The alteration, therefore, is comparatively rapid, and, though only superficial, generally extends to much greater depths than in the surrounding country rock. From a chemical standpoint, the first effect of this superficial influence is usually the oxidation or hydration, or both, of certain ingredients, followed generally by the formation of other chemical combinations and by the leaching of certain materials. In the formation of these other chemical combinations, however, the base usually remains the same, and the alteration consists generally in a change of the materials associated with the base, that is, in the acidic portion of the mineral or the part that represents the acidic portion. Thus, iron sulphides are oxidized to iron sulphate, and then this is converted by further oxidation and by hydration to the hydrous sesquioxide. Copper sulphides may be oxidized to copper sulphate; and from the sulphate, by the agency of materials in surface waters, may be formed copper carbonates, haloid compounds, silicates, oxides, and even metallic copper; while from some of these, still other compounds may be produced. Similar reactions occur in many lead, zinc, silver, gold, and other deposits.

Occasionally, chemical changes may occur without previous oxidation, and sometimes, though rarely, surface influences under peculiar conditions may have a reducing effect, as in the formation of iron pyrites and copper pyrites from the sulphates of iron and copper, or in the formation of native copper by the action of a ferrous salt on certain copper salts, or in the formation of native silver in surface outcrops. In many of such cases, however, the chemical action is primarily one of partial oxida-

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