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The Superficial Alteration of Ore Deposits.^[1]

General Features.

Scope of the subject.—The modern idea of ore deposits teaches that formations of this kind represent a process of concentration of mineral matter, either by chemical or physical means; in other words, that they are unusual localizations of certain minerals which are often found disseminated in smaller quantities in many common rocks, and that they differ from the same minerals situated in other conditions, only in their degree of concentration. These concentrations may take place at different times in the history of the rocks in which the deposits occur. If they occur in sedimentary rocks, they may sometimes be formed during the deposition of the rocks with which they are associated, as in the cases of placer gold, stream tin, and sometimes of other ores;

↑ The superficial alteration of ore deposits is a recognized principle of geology, in the same way as is the superficial alteration of any of the common rocks. Its importance in some classes of ore deposits is also well understood, as in many precious metal deposits; while in other classes, its importance has been proved in individual cases, as in the Lake Superior iron deposits. The causes and effects of superficial alteration in many classes of deposits, however, are not so generally understood, and it is the object of the present paper to show that such changes almost invariably give rise to exceedingly important chemical and physical phenomena, while in many deposits, the question as to whether they can or cannot be profitably worked, depends largely on the extent and character of this alteration.

The various treatises on ore deposits published in the United States and Europe make frequent mention of superficial alteration, but have not treated the subject fully. As early as 1854, however, before which time but little accurate information was had on the geologic nature of ore deposits, Professor J. D. Whitney in his classic volume, The Metallic Wealth of the United States, describes the alteration products, or gossans, in certain deposits and mentions others. On the more purely chemical side of the question, the work of Bischof, Daubrée, Roth, Rose, Hunt, Breithaupt, Blum, Julien, Deville, Debray, Volger, Moissan, Fremy, Lévy, Fouqué and others have afforded much valuable information and many useful suggestions. The chemical principles brought out by these various authors have been applied, to a certain extent, to the solution of the phenomena of the origin of ore deposits, but have not as yet been applied to anything like their possible extent to the solution of the phenomena of the alteration of ore deposits.

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[1] The superficial alteration of ore deposits is a recognized principle of geology, in the same way as is the superficial alteration of any of the common rocks. Its importance in some classes of ore deposits is also well understood, as in many precious metal deposits; while in other classes, its importance has been proved in individual cases, as in the Lake Superior iron deposits. The causes and effects of superficial alteration in many classes of deposits, however, are not so generally understood, and it is the object of the present paper to show that such changes almost invariably give rise to exceedingly important chemical and physical phenomena, while in many deposits, the question as to whether they can or cannot be profitably worked, depends largely on the extent and character of this alteration.

The various treatises on ore deposits published in the United States and Europe make frequent mention of superficial alteration, but have not treated the subject fully. As early as 1854, however, before which time but little accurate information was had on the geologic nature of ore deposits, Professor J. D. Whitney in his classic volume, The Metallic Wealth of the United States, describes the alteration products, or gossans, in certain deposits and mentions others. On the more purely chemical side of the question, the work of Bischof, Daubrée, Roth, Rose, Hunt, Breithaupt, Blum, Julien, Deville, Debray, Volger, Moissan, Fremy, Lévy, Fouqué and others have afforded much valuable information and many useful suggestions. The chemical principles brought out by these various authors have been applied, to a certain extent, to the solution of the phenomena of the origin of ore deposits, but have not as yet been applied to anything like their possible extent to the solution of the phenomena of the alteration of ore deposits.

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