650 



UNITED STATES MINERAL RESOURCES 



in parts per million: Be=41, Sn=l,100, W=70, 

 Cu=150, Pb=150, Zn=300, As=3,000, Li=3,000. 

 Soils developed over weakly mineralized limestones 

 near the ore bodies gave maximum values, in parts 

 per million, of Be=270, Cu=610, Pb=2,100, Zn= 

 5,000, and Sn=l,500, representing enrichment fac- 

 tors respectively for the above elements of >180, 

 76, >210, >50, and 500 as compared to nearby soils 

 over unmineralized bedrock (Sainsbury and others, 

 1968). Because many base-metal sulfide deposits con- 

 tain some tin in sulfide minerals such as chalcopyrite, 

 or discrete tin-sulfide minerals, tin values in the 

 range 10-40 ppm may be found in stream sediments 

 nearby, but they do not indicate tin lodes. In assess- 

 ing small tin anomalies, it is valuable to consider 

 the associated elements lithium, boron, and bis- 

 muth, which generally are associated with tin de- 

 posits. 



For a more complete discussion of pitfalls to 

 avoid in prospecting for tin lodes, see Hosking 

 (1965), as well as the numerous papers presented 

 at the First and Second International Technical 

 Conferences on Tin (Internat. Tin Council, 1968, 

 1969). These volumes give excellent data on search- 

 ing for both lode and placer tin deposits in various 

 parts of the world. 



Most sulfide-cassiterite tin deposits, except pos- 

 sibly those in Bolivia, are associated with large 

 amounts of minerals containing fluoi'ine and boron, 

 especially fluorite and tourmaline. Therefore, all 

 granites of potentially stanniferous areas which are 

 fluoritized or tourmalinized should be carefully in- 

 vestigated. In fact, the worldwide association of 

 cassiterite with fiuorine or boron is sufficiently close 

 that most workers are agreed that tin leaves the 

 parent magma as tin complexes of fluorine and 

 boron. It is immaterial whether we know exactly 

 how these complexes form, or how they leave the 

 magma ; the association of tin and fluorine or boron 

 still is a useful prospecting tool. 



PROBLEMS FOR RESEARCH 



The geology of lode tin deposits is well under- 

 stood; the genesis of tin deposits and tin-granite 

 remains a fertile field of debate, speculation, and 

 little fact. Much of the recent information on the 

 origin of tin-granites is of questionable value be- 

 cause of sampling and analytical diflSculties, and the 

 senior author questions whether fundamental laws 

 will ever be shown to exist to explain the so-called 

 tin-granites. With respect to the actual search for 

 tin deposits, the efforts required to collect large 

 suites of rocks, treat them in the laboratory, analyze 

 them, and manipulate the results by complex mathe- 



matical mechanisms might well- be less effective in 

 finding ore than a similar amount of time, money, 

 and effort invested in actual prospecting of known 

 stanniferous areas. But no doubt geologists and geo- 

 chemists will continue such investigations. 



With respect to scientific studies, several areas of 

 total ignorance are at once apparent. For instance, 

 no information is available on the isotopic composi- 

 tion of tin in tin deposits. Such data are needed, 

 though they may never lead to discovery of an ore 

 body. Laboratory experiments are needed to dupli- 

 cate the transfer of the numerous elements found 

 together in tin deposits. Because the deposits in 

 rhyolite are clearly formed during the crystalliza- 

 tion of the magma, studies of fluid inclusions in 

 these deposits would be invaluable. 



The authors are unaware of any studies currently 

 in progress that would apply isotopic studies of 

 oxygen and sulfur to the problem of genesis of 

 diflferent types of tin deposits. The very exhaustive 

 and important study of the Bolivian tin deposits 

 by Kelly and Turneaure (1970) suggests that of 

 all the numerous tools available to study the tem- 

 perature of formation of tin deposits, as well as 

 the composition of the ore-forming fluids, fluid- 

 inclusion studies are by far the most usable. Their 

 study should provide a model for much more work 

 like theirs — detailed, factual, and with arguments 

 based upon the observed facts. That the ore-forming 

 solutions were high-temperature (to 560°C) highly 

 saline solutions that probably boiled at times will 

 not but please hydrothermalists. And the added 

 conclusions of Kelly and Turneaure that 



"The Bolivian province has provided an ideal test- 

 ing ground for numerous laboratory geothermom- 

 eters that have been proposed in recent years. The 

 results obtained with the various sulfide thermom- 

 eters have been generally disappointing and hardly 

 justify the time invested." 



will certainly disturb the "laboratory economic 

 geologist" and delight the field economic geologist. 

 If their conclusion is true, cannot one validly ask 

 ■'If such rigidly controlled laboratory experiments 

 cannot provide dependable data on even the tem- 

 perature of deposition of tin deposits, how shall we 

 ever discover (and prove) the method of generation 

 of a tin-rich magma?" 



One last and troubling problem in petrogenesis 

 needs mention. The alkalic rocks in many parts of 

 the world contain tin in amounts equal to or greater 

 than the so-called tin-granites, yet no commercial 

 tin lodes are ever associated with them. In a way, 

 this fact refutes the possibility that the tin granites 

 derive their tin from the surrounding rocks invaded 



