978 Subsurface Geologic Methods 



microns (screen size 100-120) and immersed in oils of varying indices ^^ '**' 

 is a very satisfactory means of identifying nonopaque minerals. In thin 

 sections, minerals also can be identified,"*^ and in addition, grain-relation 

 studies can be made. All wall-rock-alteration products should be in- 

 vestigated under the petrographic microscope to determine changes that 

 nave taken place within the rock and properly to classify the alteration 

 as to kind and intensity. Structural-petrology problems entail orienta- 

 tion studies of the individual minerals, which necessitate the use of the 

 universal stage under the petrographic microscope. 



To prepare an ore for study under the reflecting microscope one 

 surface of the opaque mineral must be given a high polish. Because of 

 the ease of handling and storage, many laboratories mount the ore speci- 

 mens in bakelite or sealing wax before polishing, but the ores also can 

 be studied if the specimen is mounted in clay when polished. After a 

 flat surface is obtained, either by grinding on a coarse lap or with a rock 

 saw, the specimen is further treated on metal laps using powders of in- 

 creasing fineness to remove most of the pits. Polishing may be done in a 

 number of ways but cloth-covered laps or lead laps are generally em- 

 ployed. ^^ Identification under the reflecting microscope is based on a 

 number of factors, mainly color, hardness, polarized-light eff"ects, standard 

 etch tests, and microchemical tests.^^ The metallurgical microscope, after 

 calibration by use of stage and ocular micrometers, can be used to de- 

 termine grain sizes. Grain size is of specific importance to the metallurgist, 

 who must know the grinding size necessary to free the various ore minerals. 

 Mineralographic studies "^^ ^° may help determine the origin of a deposit 

 and materially aid the mining geologist in his search for ore; hypogene or 

 supergene mineralization may be indicated, or age-relation studies may 

 show a definite relationship between ore and ages of fracturing. 



Other laboratory techniques such as fire and wet assaying, chemical 

 analysis, spectrographic analysis, X-ray-diff"raction analysis, diff"erential- 

 thermal analysis, and heavy-mineral determinations are used in conjunc- 

 tion with petrographic and ore microscopes in determining the kind and 

 amount of minerals and metals present in the ore, gangue, and wall rock. 



Exploration and Sampling Methods "^^ ^^ 



No matter how complete the geologic data and how accurate the 

 interpretation, mining properties cannot be proved on geology alone. 



" Rogers, A. F., and Kerr, P. F., Optical Mineralogy, pp. 135136, New York, McGraw-Hill Book 

 Co., Inc., 1942. 



*' Larsen, E. S., and Barman, Harry, The Microscopic Determination of the Non-Opaque Minerals: 

 U. S. Geol. Survey Bull. 848, 1934. 



*" Rogers, A. F., and Kerr. P. F., op. cit., pp. 3-7, 1942. 



■^^ Short, M. N., Microscopic Determination of the Ore Minerals: U. S. Geol. Survey Bull. 914, 

 pp. 4-44, (1948 reprint), 1940. 



*5 Short, M. N., op. cit., pp. 59-292. 



** McKinstry, H. E., op. cit., pp. 141-155, 1948. 



^ Bastin, E. S., et at.. Criteria of Age Relations of Minerals with Special Reference to Polished 

 Sections of Ores: Econ. Geol., vol. 26, no. 6, pp. 561-610, 1931. 



" McKinstry, H. E., op. cit., pp. 35-114, 1948. 



*' Forrester, J. D., op. cit., pp. 349-441, 1946. 



