TIN 



641 



to other common metallic ions, tin may proxy for 

 these ions in common minerals or may form tin ana- 

 logs of common minerals. Thus tin may be dispersed 

 in minerals containing iron, titanium, or calcium — 

 for example, in biotite (up to several hundred parts 

 per million), rutile (900 ppm), or andradite garnet 

 (up to 1.7 percent). A good example of a tin ana- 

 log is the mineral malayaite (CaSnSiOj), in which 

 tin replaces titanium in the mineral sphene. Malaya- 

 ite is found in contact-metamorphic skarns (El 

 Sharkawi, 1966) . It has long been known that mag- 

 netite from skarns near tin deposits commonly 

 contains tin in anomalous amounts, and recently 

 Desborough and Sainsbury (1970) showed that cas- 

 siterite from the Lost River tin deposits, Alaska, 

 has exsolved from magnetite. Thus the iron-tin 

 association is well established. 



The crustal abundance of tin is in the range of 

 2-3 ppm (parts per million) . Work by several inves- 

 tigators on meteorites (Goldschmidt and Peters, 

 1933; Onishi and Sandell, 1957; Borchert and Dy- 

 bek, 1960) has shown that the nickel-iron phase 

 contains an average of 100 ppm tin, whereas the 

 chondritic phase contains about 5 ppm and the 

 troilite phase about 15 ppm. This suggests that a 

 primary enrichment occurred in the nickel-iron phase 

 in the earth's core. During the magmatic differentia- 

 tion of the crust, tin is progressively concentrated 

 in the more acid differentiates, as shown in table 133. 



Table 133. — Range and average contents of tin in various 

 materials 



[Values in parts per million except where noted] 



Within sedimentary rocks, the amount and dis- 

 tribution of tin is imperfectly known, although 

 some shales may contain as much tin as tin-rich 

 granites (table 133) . However, no sedimentary rocks 



are known that are enriched in tin to such a degree 

 as to approach commercial ore. 



Of all the metals found in epigenetic deposits, tin 

 shows perhaps the most clear-cut relation to a single 

 rock type — almost all primary tin lodes are asso- 

 ciated with biotite or biotite-muscovite granites, or 

 their extrusive equivalents. This imposes certain 

 parameters on rampant speculation about the gene- 

 sis of tin deposits. Whatever the genesis, it has 

 something to do with geochemical, physicochemical, 

 or petrochemical processes that are peculiar to gran- 

 ites. This is discussed briefly in the later section on 

 genesis. 



GEOCHEMICAL CYCLE 



The knowledge of the geochemistry of tin is based 

 largely on comparison of different types of analyses 

 of different rocks and different types of deposits, 

 from widely dispersed areas of the world. Before 

 the geochemical cycle of tin can be understood fully, 

 many more studies on complete rock suites, ores, and 

 minerals from single districts are needed. The work 

 of Butler (1953) in Cornwall is valuable in tracing 

 tin through the weathering cycle. Data have been 

 assembled, but only in preliminary form, from the 

 Lost River area, Alaska (Sainsbury and others, 

 1968), and are summarized in figure 71. (For de- 

 tailed descriptions of the tin deposits and regional 

 geologic setting, see Knopf, 1908, and Sainsbury, 

 1964b and 1969.) This tin-bearing area is typical of 

 the sulfide-cassiterite type of high-temperature de- 

 posit, but unusual in that it includes large deposits 

 of fluorite, containing beryllium in economic 

 amounts which are zoned about the tin deposits. 



Several significant observations should be made 

 about figure 71: (1) Tin is greatly concentrated in 

 the granites relative to the limestone and shale 

 through which the granites penetrated. (2) The 

 heavy-mineral fraction of the granite unquestion- 

 ably is the clearest indicator of associated tin de- 

 posits, although the biotite also contains a high 

 amount of tin. (3) Tin is slightly concentrated in 

 the hornfels zones around the granite, and is highly 

 concentrated in all the individual minerals separated 

 from the tin and beryllium lodes, clearly recording 

 a common genesis. (4) Soils (over limestone) are 

 greatly enriched in tin over the ore deposits, and the 

 tundra plants over ore bodies clearly are enriched 

 in tin. (5) Bulk stream sediments are enriched in 

 tin downstream from tin deposits. (Water was not 

 analyzed.) 



More studies, and in greater detail, of this type 

 are needed to establish criteria required for making 

 valid geochemical comparisons in diverse areas of 



