970 Subsurface Geologic Methods 



tionship also exists between mineral deposits and the source pluton. 

 Butler ^ and Emmons ® have pointed out that the great bulk of epigenetic 

 deposits are found either in the top of stocks or above the stocks in the 

 country rock. Thus, if erosion has truncated the stocks and the main mass 

 of the batholith is exposed, most deposits genetically related to that ig- 

 neous mass have been removed. Many deposits occur in igneous rocks 

 that are older than the source rocks, in which case the enclosing rock is 

 the host and not the source rock. 



Rising, reactive liquids or gases will follow and be controlled by 

 pre-existing structures. They react more readily with certain rock types 

 than with others and will leave behind evidences of their passage. The 

 general geologic approach to the study of epigenetic deposits is based on 

 the controls affecting the passage of liquids and gases and effects of these 

 liquids and gases on wall rocks. 



Structural control "^ ^^ refers to the controls on rising liquids by pre- 

 existing (premineral) structures. These controls may act as guides or as 

 barriers. A consideration of structural control involves a study of the 

 regional pattern as well as a detailed study of an individual mining dis- 

 trict or mine. Billingsley and Locke ^° have suggested that the location of 

 important mining districts is, in part, determined by erogenic belts which 

 are continental in scope. They claim that orogenies have strengthened or 

 made the crust sufficiently competent to support the deep-seated faults 

 which are necessary to tap concentrations of metal-bearing material. But- 

 ler ^^ has pointed out that a zone of structural disturbance and igneous 

 activity surrounds much of the Colorado Plateau and that many major 

 ore deposits of the Rocky Mountain region are found in this disturbed 

 zone. In a more restricted area, Butler ^- has shown that much of the 

 Utah production is from mining districts located near the intersection of 

 the westerly-trending Uinta axis and the northerly-trending Wasatch belt. 

 Specifically, in individual deposits, premineral structures are of extreme 

 importance. Faults are good premineral structural controls in that some 

 form zones of greater permeability, which act as channelways or conduits 

 aiding and controlling the passage of ore-bearing solutions. In a similar 

 manner, the more open zones at the crests of certain folds, at contacts of 

 dissimilar rocks, and along bedding planes, act as guides for rising solu- 

 tions. Slates, shales, clays, and faults with considerable gouge act as 



^Butler, B. S., Relation of Ore Deposits to Different Types of Intrusive Bodies in Utah: Ecotn. 

 Geol., vol. 10, pp. 101-122, 1915. 



* Emmous, W. H., Principles of Economic Geology, pp. 185-190, New York, McGraw-Hill Book 

 Co., 1940. 



' Newhouse, W. H., editor. Ore Deposits as Related to Structural Features, Princeton University 

 Press, 1942. 



8 McKinstry, H. E., Mining Geology, pp. 277-342, N»w York, Prentice-Hall, Inc., 1948. 



* Butler, B. S., Ore Deposits of the Western States, (Lindgren volume) , pp. 198-326, Am. Inst. Min. 

 Met. Eng., 1933. 



^^ Billingsley, Paul, and Locke, Augustus, Structure of Ore Districts in the Continental Framework: 

 Am. Inst. Min. Met. Eng. Trans., vol. 114, pp. 9-64, 1941. 



" Butler, B. S., Ore Deposits of the Western States, (Lindgren volume) , pp. 215-222, Am. Inst. 

 Min. Met. Eng., 1933. 



"Butler, B. S., et al.. Ore Deposits of Utah: U. S. Geol. Survey, Prof. Paper 111, 1920. 



