Subsurface Methods as Applied in Mining Geology 971 



barriers to rising solutions damning the ores below premineral features. 

 In referring to the barriers, Mackay ^^ contends that impounding struc- 

 tures or rocks do not act as perfect barriers. If that were the case, stagnant 

 solutions would be trapped and very little ore deposited. Mackay argues 

 that the solvent passes through the relatively impermeable barrier after 

 the metals have been deposited. 



Sedimentary rocks act as host to numerous deposits of igneous origin, 

 but the control exerted by the sediments on localization of the deposit is 

 not fully understood.^^ In many replacement deposits, the control is 

 structure modified by stratigraphy. Selective replaceability is not confined 

 to sedimentary rocks. The Smuggler vein, Telluride, Colorado, passes 

 through both andesite, which is productive, and an overlying rhyolite, 

 which is essentially barren. ^^ 



Rising solutions, in addition to the depositing of ore and gangue 

 minerals, penetrate into the wall rock and form wall-rock-alteration min- 

 erals.^^ ^^ ■^^ ^^ ^° Detailed work has shown that in certain districts a 

 direct relationship exists between alteration and metalliferous deposition. 

 Numerous types of epigenetic deposits are mined and in general the 

 exploration techniques differ for each. 



The more important types of epigenetic deposits may be defined as 

 follows: 



A vein is a tabular or sheetlike body of minerals occupying 

 or following a fracture or a group of fractures. 



A disseminated deposit is an irregularly shaped body in 

 which the ore minerals occur as small individual grains and in 

 small seams or veinlets. 



A pipe is a vertically elongated deposit of circular or ellip- 

 tical cross section. 



A replacement deposit is one in which the host rock has 

 been partly or completely replaced by the ore and gangue min- 

 erals. The resulting deposit may be tabular, sheetlike, pipelike, 

 or irregular and is more or less restricted to one horizon, bed, 

 formation, or structure. 



Metamorphic Ore Deposits 



Some ore minerals are formed by metamorphic processes, and if the 

 deposits are of sufficient size and grade they can be termed "metamorphic 



^^ Mackay, R. A., The Control of Impounding Structures on Ore Deposition: Econ. Geo}., vol. 41, 

 no. 1, pp. 13-46, 1946. 



"Rove, O. A., Some Physical Characteristics of Certain Favorable and Unfavorable Ore Horizons: 

 Econ. Geol., voL 42, pp. 57-77, 161-193, 1947. 



^° Emmons, W. H., op. cit., p. 217. 



" Butler, B. S., Influence of the Replaced Rock on Replacement Minerals Associated with Ore 

 Deposits, Econ. Geol., vol. 27, no. 1, pp. 1-24, 1932. 



" Lindgren, Waldemar, op, cit., pp. 444-694, 1933. 



"Levering, T. S., Rock Alteration as a Guide to Ore: East Tintic District, Utah: Econ. Geol. 

 Mono. 1. 



i^McKinstry, H. E., op. cit., pp. 233-242, 1948. 



'^"Applied Geology: Colorado School of Mines Quart., vol. 45 (75th Anniversary volume), no. IB, 

 Jan. 1950. 



