648 



UNITED STATES MINERAL RESOURCES 



that large new tin districts will be found. 



Resources which may exist in Antarctica and 

 Greenland are not considered because data on which 

 to base realistic evaluations are not available. 



The grand total of all the reserves and resources 

 listed above is about 37 million long tons of tin. 

 This is sufficient to sustain a world consumption of 

 200,000 long tons of tin per year for about 185 years. 



PROSPECTING TECHNIQUES 



Before discussing the methods used in searching 

 for tin deposits, the authors would like to repeat 

 two statements made by K. F. G. Hosking at the 

 First International Symposium on Tin, in London, 

 in 1967 (Hosking, 1967, p. 269) . 



Sound prospecting programmes designed to locate new 

 primai-y tin deposits are usually based, to no small degree, 

 on the designers' views concerning the relationship between 

 such deposits and granitic rocks. 



As far as possible the writer has refrained from discuss- 

 ing those questions of relationship which require considera- 

 tion of the sources of granite magmas, granitising agents 

 and ore-forming ones, and theories of the nature of these 

 agents and of the chemistry of ore genesis. He has adopted 

 this line of action because he believes that a good explora- 

 tion program must be based essentially on facts, and that 

 a program founded largely on theoretical concepts, how- 

 ever sophisticated the latter may appear to be, is little 

 better than one which requires only that a blind-folded per- 

 son should stick a pin in a map in order to find a tin de- 

 posit. 



Following somewhat along the lines above quoted, 

 the authors are here confining their discussion to 

 the accepted exploration techniques. These tech- 

 niques are based in part on application of concepts 

 that were originally theoretical but have been "sta- 

 bilized" by a long history of facts. 



In regional exploration, analyses of major tec- 

 tonic and petrologic features are of great value. 

 Because of the obvious relation of all known tin 

 deposits to acid granitic rocks, tectonic elements 

 that localize such granitic rocks are considered; 

 these elements vary from major mountain ranges 

 with granitic cores, subjacent volcanic rocks, and 

 extrusive rhyolites to isolated stocks and bosses of 

 proper composition. Certain factors may then be 

 evaluated for localizing favorable ground within the 

 larger elements; these factors may be grouped 

 under two broad categories: tectonic and geo- 

 chemical. 



Numerous authors have noted that the apical 

 parts of granitic masses, as well as the margins, are 

 especially favorable for the occurrence of tin de- 

 posits. Hosking (1967, 1969, 1972) holds that these 

 apical parts, which he calls "cusps," are of great. 



almost overriding, importance in localizing tin de- 

 posits. Other authors (Stemprok, 1967 ; Sainsbury, 

 1969; Dines, 1956) are inclined to consider that 

 postgranite fractures of both regional and local 

 extent are of great importance in localizing the 

 larger and richer lode ore shoots of some of the 

 world's tin districts. When one studies the descrip- 

 tions of the great lode mining districts of the world, 

 one cannot escape the conclusion that most rich veins 

 are localized along fractures which formed long after 

 the consolidation of the enclosing granite. Where 

 great tin lodes are mined within great expanses of 

 granite, such as in Cornwall, in veins that (1) con- 

 form to regional fracture patterns which cut the 

 granites, and (2) may lie entirely blind thousands 

 of feet below the present surface (which itself is an 

 unknown distance below the original apical part of 

 the granites) as at South Crofty mine in Cornwall, 

 then it is extremely difficult to relate the tin to 

 "cusps" (cupolas?) on the original surface of the 

 granite. Even Hosking himself pointed out that re- 

 gional fracture patterns have localized deposits in 

 Cornwall (Hosking, 1967, p. 297). 



The margins of granites, however, have been 

 proved to be especially favorable places for the oc- 

 currence of placer deposits, especially where streams 

 have followed long distances along such margins. 

 Stemprok (1967) showed that margins of granitic 

 rocks in the Erzgebirge in Czechoslovakia are en- 

 riched in tin, and he related this enrichment to the 

 escape of volatiles from deeper parts of the granite 

 mass along joints and fractures in the shell of the 

 granite. Sainsbury (1964b) has shown that even 

 though the main tin-tungsten-beryllium-fluorite lode 

 at Lost River, Alaska, lies generally above the apex 

 of the granite (thus supporting Hosking's views), 

 the main tin lodes are along faulted dikes intruded 

 along faults of a regional set which cut the granites 

 and postdate them by an appreciable time. An en- 

 richment of tin in the marginal shell of the granite 

 is related to small veinlets localized in the cooling 

 joints of the granite. Again, Stemprok and Sulcek 

 (1969) have shown that tin-bearing granites in the 

 KruSne hory Mountains of Czechoslovakia have 

 marked alteration along flat-lying zones at great 

 depths in the granite. The Bolivian deposits are 

 localized near subjacent porphyry intrusions, but 

 they lie along veins that pass downward out of and 

 beyond some of the porphyry intrusives. 



A reasonable conclusion is that tin deposits of 

 large tonnage and low grade may be directly related 

 to the "bleeding off" of volatiles above cupolas 

 ("cusps") of granites or along the fractured regions 

 of such intrusives but that the large and rich veins 



