Page:The Journal of geology (IA journalofgeology21894univers).pdf/332

This page has been proofread, but needs to be validated.
316
THE JOURNAL OF GEOLOGY.

in many cases it probably also occurred after they were desiccated.[1]

Summary.

It will be seen from the above discussion that:

  • (1) After the deposition of ore deposits and their subsequent exposure to surface influences, such as air, water and the materials contained in it, changes of temperature, etc., chemical and physical alterations occur which cause a total change in the mineralogical condition, and generally in the economic value, of the ore deposit.
  • (2) The process of this alteration is primarily one of oxidation and generally of hydration, and both of these actions may go on alone, but generally both have their effect on the same material. The other materials in solution in surface waters also react on the substances in the ore deposit, either before or after the oxidation of the latter, though generally after at least partial oxidation, and form various compounds different from those originally in the deposit. The difference, however, with few exceptions, is not in the metal or other base which forms the important feature of the deposit, but in the acidic portion or material representing this portion of the mineral. Thus, sulphide of copper may be altered to carbonate of copper, but the base remains the same. The action of surface influences is in rare cases one of reduction, which, however, often follows a previous oxidation. The process of alteration also frequently causes a leaching of certain ingredients of the ore deposit, either with or without previous oxidation, as in the removal of iron pyrites, calcite, etc. It also sometimes renders a hitherto worthless material valuable by the introduction of a valuable constituent, as in the replacement of carbonate of lime by phosphate of lime. It also causes the concentration, by capillary action in soils, of certain deposits like nitre, etc. The compounds formed with different ore deposits vary with the ores affected and the sta-
  1. ↑ Chlorides of other materials than silver and copper may also have been formed by a similar process, but the solubility of many metallic chlorides would prevent their being accumulated in any but very dry regions.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.