Subsurface Methods as Applied in Mining Geology 975 



less than an inch of pyrite may alter the wall rock for a number of feet 

 on both sides of the fracture. Conversely, a vein containing several feet 

 of copper ore may show less wall-rock alteration than a one-inch pyrite 

 seam. 



The foregoing type of alteration is caused by rising hypogene solu- 

 tions. Descending supergene solutions act on the hypogene deposits and 

 the enclosing country rock and also bring about typical changes.^^ ^^ The 

 bases of supergene action are several: pyrite in the presence of air and 

 water will oxidize to form sulphuric acid; dilute sulphuric acid will react 

 with most minerals in the hypogene zone, take them into solution, and carry 

 them down toward the water table; pyrite below the water table will pre- 

 cipitate copper and silver. If sufficient pyrite is present, all common 

 metals with the possible exceptions of gold and lead may be entirely 

 leached from the outcrop. As a result of the leaching action, character- 

 istic but complex phenomena can develop and conclusions may be reached 

 as to the character of the original deposit.^^ ^^ Below the leached zone, the 

 metal-bearing supergene solutions may react with a carbonate to form 

 azurite, malachite, or smithsonite. If the solution reaches the water table 

 and comes in contact with pyrite, then chalcocite, covellite, or argentite 

 may be precipitated. 



Geochemical and Biogeochemical Guides — The geochemist is con- 

 cerned with minute traces of metals that work their way into the soil, either 

 as a result of hypogene mineralization or as a result of supergene action. 

 These solutions may cause characteristic alteration or may deposit minute 

 amounts of the metals in the overlying and surrounding rocks. Supergene 

 action affects the primary deposits, and the metals are moved from their 

 original position. Gold, tin, and lead may travel mechanically, and cop- 

 per, zinc, and silver may be removed in solution. By systematic sampling 

 of the soil and ground water, traces of metal may be found. ^^ ^^ ^^ A study 

 of the movement of the soil and ground water may indicate the source of 

 the metals and lead to an ore body. 



The biogeochemist, who is interested in the relation between vegeta- 

 tion and ores, reasons geologically in the same manner as the geologist. 

 The distribution of specific types of vegetation has been studied with ref- 

 erence to ore deposits; and, although the work is not conclusive, it appears 

 that in a few districts, there is a distinct relationship between vegetation 

 and ore deposits. As a variation of the distribution of specific types 



2" Emmons, W. H., The Enrichment of Ore Deposits: U. S. Geol. Survey Bui!., 625, 1917. 



" Lindgren, Waldemar, op. cit., pp. 813-877, 1933. 



^* Locke, Augustus, Leached Outcrops as Guides to Copper Ores, Baltimore, Williams and Wilkins, 

 1926. 



^' Blanchard, R., and Boswell, P. F., A Group of Papers on Various Types of Limonite Boxworks : 

 Econ. Geol., vols. 22, 25. 29, 30, 1925-1935. 



'" Sergeev, E. A., Geochemical Method of Prospecting for Ore Deposits: from Materials of the 

 U.S.R.R. Geol. Inst., Geophysics, Fasc. 9-10, pp. 3-55, 1941. 



^ Hawkes, H. E., Annotated Bibliography of Papers on Geochemical Prospecting for Ores: U. S. 

 Geol. Survey, Giro. 28, Aug. 1948. 



^ Hawkes, H. E., Geochemical Prospecting for Ores: A Progress Report: Econ. Geo!., vol. 44, no. 

 8, pp. 706-712, 1949. 



