496 mckelvey. SEDIMENTARY MINERAL DEPOSITS [Ch. 27 



which placers have been derived is also important in prospecting for 

 many alluvial placers (many lodes have been discovered, incidentally, 

 by tracing placer minerals to their sources, and, conversely, many 

 placers have been discovered after the lode) . 



Geologic guidance in the search for placers and similar deposits has 

 gone far beyond the mere location of the favorable host material, and 

 many other geologic principles have been successfully applied. For 

 example, the relation between the size and density of materials and the 

 stream velocity has been used to predict the location of placer con- 

 centrations with respect to changes in gradient, channel configuration, 

 bars, tributaries, etc. Shingling and pebble orientation have been used 

 to determine direction of flow of streams in which now-buried placers 

 ("drift" placers) were deposited. The pattern of ancient drainage 

 (the products of which are now buried under lava flows, later alluvium, 

 etc.) has been worked out locally by reconstructing the old surface 

 from a combination of sedimentologic, physiographic, and other geo- 

 logic data (including the structural pattern as well as the distribution 

 of the bedrock formations) , and a knowledge of the general structural 

 and physiographic history has often been applied successfully to the 

 search for concealed deposits. See Jenkins (1946) for a good sum- 

 mary of the geologic principles utilized in the search for placers. 



APPRAISAL 



The methods used in appraising deposits are considerably more ad- 

 vanced than those used in finding them. This is understandable for, 

 whereas most deposits have been found at little cash outlay (even 

 though at great cost in terms of prospector man-hours), they cannot 

 be mined without investment of considerable capital. Investors can- 

 not afford to develop a mine, build a mill or plant, and perhaps con- 

 struct an access road or railroad spur without advance assurance that 

 the deposit contains sufficient minable material to amortize their 

 capital investment. 



Several factors receive prime consideration in the appraisal of a 

 mineral deposit of any kind, regardless of its genesis. The first is 

 location of the deposit, not only with respect to railhead and access 

 roads, but also with respect to the market. (This is especially true 

 with many of the low-value non-metallic materials. For example, a 

 gravel deposit more than a few miles from the highway or dam in 

 which the gravel is to be used has no value whatsoever; a brick manu- 

 facturer distributing his products over a geographically restricted area 

 has no interest at all in the clay deposits in another state.) The 



