974 



Subsurface Geologic Methods 



unravel structure in igneous and metamorphic rocks and thus aid in the 

 search for new ore. By a study of planar and linear structures and spatial 

 relations of the rock minerals, the fabric of a rock can be analyzed and 

 the more or less hidden structures determined. These igneous and meta- 

 morphic structures can play about the same role in ore deposits as do the 

 more obvious deformations in sedimentary rocks. Such work involves 

 very detailed field studies followed by microscopic studies. (See "Petro- 

 fabric Analysis," this volume.) 



Contacts of dissimilar rocks may be considered under structural con- 

 trol. In general, contacts are more permeable than the rocks on either side 

 and in the absence of fractures, solutions more readily pass along the 

 contacts than across them. Deposits occur at contacts of dissimilar sedi- 



Zone of wollrock alteration 



Zone of wollrocK alteration 



Figure 523. Sketch showing relation of wall-rock alteration to a vein. 



mentary rocks, dissimilar igneous rocks, or any combination of sedimen- 

 tary, igneous, and metamorphic rocks (fig. 522). 



Mineralogic Guides — Solutions rising from below (hypogene) or 

 descending from the surface (supergene) react with the wall rock and leave 

 evidence of their passage. In many cases, these mineralogic changes prove 

 to be useful tools and when applied in conjunction with other guides or 

 controls are helpful in locating ore. 



Wall-rock alteration surrounds the ore body as a halo or shell, and at 

 places variations within the ore body are reflected by detectable differ- 

 ences in the alteration zone. The most useful and the most obvious man- 

 ner in which alteration is used as a guide is to consider the alteration halo 

 as an enlargement of the target with more intense alteration near the met- 

 allized zone and with decreasing intensity away from the zone. Thus, the 

 presence of ore not actually encountered in prospecting may be indicated 

 by alteration (fig. 523). It should not be inferred that alteration neces- 

 sarily means ore. Solutions passing through a fracture and depositing 



