598 



UNITED STATES MINERAL RESOURCES 



Rico, probably contains more silver-bearing deposits 

 than almost any other nation in the world, and is one 

 of the most favorable of nations for potential dis- 

 covery of new deposits. The search for new deposits 

 should be nationwide and not just restricted to the 

 West. Perhaps one of the greatest silver anomalies 

 in the United States is in southeast Missouri, and 

 the Appalachian Mountain belt contains many silver 

 occurrences of many geologic varieties. Large parts 

 of Alaska are favorable for silver deposits, but the 

 limited accessibility has discouraged intensive ex- 

 ploration. The Western United States has so many 

 districts that it is still considered prime "prospect- 

 ing ground." 



Perhaps one of the most promising of the specu- 

 lative resources which includes silver is the possi- 

 bility that massive sulfides of the Japanese Kuroko 

 type may occur in the young volcanic belt of the 

 Aleutian Island chain of western Alaska, and along 

 the adjacent mainland. Geologically, this volcanic 

 region is almost ideal for Kuroko-type volcanogenic 

 massive sulfide deposits, and such deposits should be 

 present somewhere in this long chain of islands. 



The geology of the German and Polish copper 

 shales should be studied as examples in the search 

 for similar shales in the United States. The Euro- 

 pean shales lie in a Permian saline basin with per- 

 meable sandstones above and below. Beneath them in 

 the basins are thick salt, potash, and gypsum-anhy- 

 drite beds. During deposition the basins were shal- 

 low and were surrounded by subdued metamorphic 

 uplands that contained many mineral deposits. The 

 copper shales are in thin beds (6-8 in. thick) of 

 black carbonaceous shale that are mineralogically 

 zoned and that grade laterally into lead-, zinc-, and 

 silver-rich strata. Copper shales containing silver 

 are now mined in southwest Oklahoma. Similar cop- 

 per shales may occur unrecognized in many other 

 places in the United States where shallow, nearly 

 closed basins previously existed. 



More than half the Basin and Range province is 

 covered by alluvial deposits ; many of the basins are 

 largely pediment surfaces with thin gravel covers, 

 beneath which much of the geology is similar to that 

 of the adjacent mountain ranges. Many diff'erent 

 kinds of silver-bearing deposits could exist in these 

 basins at shallow depths. Exploration problems to be 

 solved are much like those of glacial drift-covered 

 shield areas. At present, geochemical prospecting is 

 one of the best tools for finding them. 



Large Precambrian shield regions are covered by 

 thin glacial deposits and thin Paleozoic sedimentary 

 covers in northern New York, central and northern 

 Wisconsin, upper Michigan, northerii Minnesota, 



and eastern South Dakota. For years the geology of 

 central and northern Wisconsin was shown as a 

 great green undiflferentiated area on the State geo- 

 logic map because of the thick glacial cover and the 

 sparseness of outcrops. This map (now being re- 

 placed) did not show the detailed and meticulous 

 unpublished surveys of the geology completed prior 

 to 1920. Dutton and Bradley (1970), in cooperation 

 with the Wisconsin Geological and Natural History 

 Survey, compiled maps and field-checked the out- 

 crops indicated by the old unpublished notes. Several 

 metavolcanic belts were delineated, and in them, 

 large massive sulfide copper deposits have been 

 found. Similarly, detailed mapping by Brown (1970) 

 located a small deposit of sphalerite in a marble 

 belt west of Balmat, N.Y., in the same rock as the 

 major Balmat-Edwards zinc-lead deposits. Outcrops 

 of the Balmat and Edwards ore bodies were quite 

 small and not very promising (J. S. Brown, oral 

 commun. to Heyl, 1967). Discoveries of similar small 

 outcrops of mineral deposits in the region could lead 

 to the discovery of major new ore bodies. Similar 

 studies are warranted in the other Precambrian 

 shield areas of the United States. 



The Carlin-type epithermal gold deposits are near- 

 ly unrecognizable in the outcrop, as no gossans 

 exist in the slightly altered limestones. At Mercur, 

 Utah, a similar deposit produced much gold ore, but 

 also some silver ore. Carlin-type epithermal silver 

 ores may occur elsewhere in Utah, Nevada, Arizona, 

 and New Mexico. They can be found only by careful 

 geochemical analysis of slightly altered limestones 

 and shales. 



Because of the major importance of porphyry 

 copper deposits as a source of silver, the reader is 

 referred to the excellent discussion of speculative 

 resources of porphyry copper in the "Copper" chap- 

 ter of this volume. Almost all outcropping deposits 

 in the Western United States have probably been 

 tested, but the recent discovery at Catheart Moun- 

 tain, Maine, suggests that a whole new eastern belt 

 of porphyry copper deposits is possible in the Appa- 

 lachian Mountain belt from Newfoundland to Ala- 

 bama; and recent discoveries in Puerto Rico and 

 Panama point to new regions of possible discoveries 

 in the Caribbean. 



The andesitic belts of the United States should be 

 geologically mapped and reexamined for silver- 

 bearing deposits. Ancient Precambrian belts of the 

 West and Midwest should be located beneath glacial 

 drift and the thin capping of Paleozoic sedimentary 

 rocks. The use of statistical geochemical prospecting, 

 geologic drilling, and the tracing of glacial boulder 

 trains are also suggested. Where this has recently 



