CHAPTER 12 



SUBSURFACE METHODS AS APPLIED IN 

 MINING GEOLOGY 



TRUMAN H. KUHN 



Origin, Characteristics, and Controls 



Many geologic factors enter into the concentration of ore minerals 

 within the earth's crust. In fact, so many conditions must be met to pro- 

 duce an economic mineral accumulation that an ore deposit has been 

 termed an "accident of geology," and such accidents underlie consider- 

 ably less than one percent of the earth's surface.^ To prospect a region 

 most efficiently it is important that origins, characteristics, and controls 

 be understood. 



Sedimentary Ore Deposits 



Sedimentary ore deposits are formed by normal sedimentary proc- 

 esses and possess all of the characteristics of sedimentary rocks. They 

 may be products of mechanical concentration such as placers, or they 

 may be products of chemical concentration. The chemical products can 

 either be soluble salts such as halite and the borates or insoluble products 

 such as some iron, aluminum, and clay deposits. 



Igneous Ore Deposits 



The origin of deposits associated with igneous activity is not defi- 

 nitely understood and agreed upon, but most geologists consider that ores 

 result from differentiation " of a magma. One product of differentiation 

 is the usual igneous-rock series, and another is the ore-bearing solutions 

 that accumulate in the top of stocks. During solidification the ore-bearing 

 solutions are forced into the chilled hood of the batholith or into the sur- 

 rounding country rock. The bulk of introduced deposits (epigenetic) is 

 collectively termed "hydrothermal deposits," and these have been further 

 divided into "hypothermal" (high temperature and pressure) , "mesother- 

 mal" (intermediate temperature and pressure), and "epithermal" (low 

 temperature and pressure) classes.^ 



According to Buddington,^ a relationship exists between kinds of 

 metallization and kinds of igneous rocks. Thus, platinum, chromium, and 

 diamond are genetically associated with ultrabasic igneous rocks. Tin 

 and tungsten are genetically related to granite. Copper, lead, and zinc 

 are almost always related to intermediate rock groups. A spatial rela- 



* Lovering, T. S., Minerals and World Affairs, p. 5, New York, Prentice Hall, Inc., 1943. 



^ Differentiation is the splitting up of an originally homogeneous magma into different or unlike 

 fractions. 



' Lindgren, Waldemar, Mineral Deposits, New York, McGraw-Hill Book Co., Inc., pp. 444-694, 1933. 



* Buddington, A. F,, Ore Deposits of the Western States, (Lindgren volume) , pp. 350-385, Am. Inst. 

 Min. Met. Eng., 1933. 



