TIN 



649 



are localized in faults and fractures that postdate 

 the intrusives and may or may not have any rela- 

 tionship to "cusps." 



Hence, in regional exploration of granitic ter- 

 ranes, a study of the major fracture patterns is 

 invaluable. It is especially valuable to determine 

 whether regional faults or fractures followed the 

 intrusion of the granites closely enough in time to 

 allow for the cooling of the shells of the intrusives, 

 entrapment of volatile elements in deeper parts of 

 the granites, and then rupture of these deep parts 

 by major fractures while volatiles were still able 

 to escape at high temperature. 



Once a favorable granitic terrane is recognized, 

 can one use the petrogenic characteristics of the 

 granites to give a dependable clue as to whether 

 tin deposits are likely to be associated? The litera- 

 ture upon this subject is voluminous and in large 

 part contradictory. For a good discussion, see Hos- 

 king (1967), who rather effectively showed that to 

 date the only infallible indication of a granite that 

 is favorable for tin deposits is the known presence 

 of tin deposits! On the other hand, Beus (1969, p. 

 70) has shown, by a statistical treatment of data 

 obtained by analyses of both bulk granite and bio- 

 tite, that statistically significant variations are 

 found between granites of nonstanniferous regions 

 and granites of stanniferous regions, as well as 

 between biotites from such granites, in certain areas 

 in Russia. Beus' figures show that 98 percent of 

 samples from granites of nontin-bearing provinces 

 must contain less than 10 ppm tin, whereas only 

 12 percent of samples from granites of stanniferous 

 areas contain less than 10 ppm tin. However, we 

 must still face the fact that the determination that 

 a granite (or granites) contains a high amount of 

 tin (more than 100 ppm) is still no guarantee that 

 a single workable tin lode will be found in or near 

 that granite. 



A further problem encountered in the search for 

 tin deposits using the geochemical and petrologic 

 features of granites is that the analytical methods 

 used by different workers are not directly compara- 

 ble and give wide variations in tin values for a 

 single rock, and that the degree of freshness, possi- 

 ble deuteric alteration, and classification of the rocks 

 investigated are seldom standardized by scouting of 

 several workers. 



Following a suggestion at the First International 

 Technical Conference on Tin in London in 1967, and 

 after additional discussion at the Second Interna- 

 tional Technical Conference on Tin in Bangkok in 

 1969, a cooperative program was set up to have 

 analyses and petrologic descriptions made of the 



same granites (in fact, sawed slabs of a single large 

 piece of granite were to be used in the laboratories) 

 to determine just how many variations would be 

 found. Results obtained to date show that analyses 

 and study by three different geological surveys have 

 resulted in wide variations in tin content, as well as 

 different classifications, for identical pieces of two 

 Alaskan tin-granites. Results of continuing studies 

 will be reported in an International Symposium to 

 be held in Europe in 1974 (Dr. Miroslav Stemprok, 

 Ustredni Ustav Geologicky, Malostranske Nam 19, 

 Praha, Czechoslovakia) . 



In the geochemical prospecting of an area pre- 

 sumed to contain tin deposits, the standard tech- 

 niques of panning of stream gravels and collection 

 of stream sediments are used — and this cannot be 

 overemphasized — but they do not always give com- 

 parable results ! Because cassiterite, the main ore 

 mineral of tin, is both heavy and chemically stable 

 in the surface environment, panned concentrates 

 from streams are a necessity in searching for tin 

 deposits. Near the nonsulfide types of tin deposits 

 no geochemical anomalies will be found in stream 

 sediments, especially in the finer grained fraction. 

 Varlamoff (1969) has shown that in the Congo only 

 panned concentrates will reveal tin anomalies. Brun- 

 din (1969) has shown that in geochemical prospect- 

 ing in Sweden, in glaciated areas, heavy-mineral 

 concentrates were necessary in searching for de- 

 posits containing scheelite, cassiterite, and wolfram- 

 ite, and also served in finding ores of sulfide min- 

 erals containing base metals. Sainsbury, Hudson, 

 Kachadoorian, and Richards (1970) documented an 

 area in Alaska where a stream-sediment survey 

 using — 80 mesh or — 40 mesh fractions of total 

 stream sediments failed entirely to disclose tin 

 placers (and associated base-metal lode deposits), 

 whereas panned concentrates, irrespective of the 

 concentration ratio achieved by panning, were in- 

 fallible in tracking down the source area for the 

 tin in placers. 



Where tin deposits of the sulfide-cassiterite type 

 occur, stream sediments are useful indicators of 

 such ore deposits. In the surface environment, stan- 

 nite breaks down to varlamoffite, an earthy friable 

 tin mineral which is easily incorporated into fine- 

 grained sediments. Because many other ore elements 

 are found in sulfide-cassiterite deposits, tin may be 

 accompanied in stream sediments by such elements 

 as copper, lead, zinc, silver, tungsten, beryllium, bis- 

 muth, arsenic, and antimony, even in the clay-size 

 or — 80 mesh fraction. Typical values obtained in 

 — 40 mesh stream sediments downstream from a 

 sulfide-cassiterite lode in northwestern Alaska are. 



