644 



UNITED STATES MINERAL RESOURCES 



mon and generally consist of cassiterite or malaya- 

 ite in tactites near granite (Cornwall). The miner- 

 als customarily associated are magnetite, garnet, 

 pyroxene, fluorite, tourmaline, various sulfide min- 

 erals, and, in the Lost River area, Alaska, beryllium 

 minerals (Sainsbury, 1964a). The grade is generally 

 less than 0.5 percent tin, but large tonnages and po- 

 tential byproduct recovery of base metals, beryl- 

 lium, and fluorite may give added importance to 

 such deposits when other tin supplies dw^indle. 



FUMAROLE DEPOSITS 



The term "fumarole deposits" was used by Ahl- 

 feld (1958) to describe small but widespread depos- 

 its that form fracture fillings in Tertiary lavas. The 

 fracture fillings contain cassiterite generally inter- 

 grown with specular hematite, and minute grains of 

 cassiterite are distributed in kaolinized wall rock 

 along veins. A few deposits are mined commercially, 

 on a small scale. Placers derived from the erosion 

 of such veins are mined in Mexico, New Mexico, and 

 Argentina. 



PLACER DEPOSITS 



Worldwide, placer deposits are more productive 

 and more cheaply mined than lode deposits; they 

 furnish, therefore, most of the commercial produc- 

 tion. Cassiterite, the main tin mineral of commerce, 

 is both heavy and chemically resistant; conse- 

 quently, it forms large placers or residual concen- 

 trations. Placers can be conveniently classed as 

 residual, eluvial (slope), alluvial (stream), marine, 

 and fossil placers. 



RESIDUAL PLACERS 



Residual placers form in place over a bedrock 

 source of cassiterite by the chemical decay and re- 

 moval of the rock minerals. At places, such as Kito- 

 lolo in the Democratic Republic of the Congo, resid- 

 ual placers grade downward into weathered lodes 

 and are mined either as placers or as open-pit lodes. 

 In a truly residual placer, the enriched zone contains 

 not only heavy minerals such as cassiterite and 

 columbite-tantalite but also chemically resistant 

 light minerals such as beryl. In Indonesia, residual 

 placers commonly are cemented tightly by hydrous 

 iron oxides, forming the "kaksa" of industry, which 

 must be crushed to free the cassiterite. 



ELUVIAL (slope) PLACERS 



Eluvial placers are formed by the chemical decay 

 of tin-bearing rocks and the gravity separation of 

 cassiterite and other heavy resistates as the decayed 

 mantle moves downslope under the influence of 

 sheetwash, gravity, and, locally, frost action. Such 



placers grade imperceptibly into residual placers up- 

 slope, and often into alluvial (stream) placers down- 

 slope. Because of active gravity sorting, some elu- 

 vial placers are richer than residual placers; many, 

 however, contain coarse rubble, and, in cold regions 

 (Alaska and Siberia), fragments of unoxidized de- 

 leterious constituents such as arsenopyrite. 



ALLUVIAL (stream) PLACERS 



Alluvial placers, by far the largest and richest 

 placers, furnish most commercial tin. In 1964, al- 

 most 60 percent of the world's tin production came 

 from alluvial placers in Southeast Asia (Shelton, 

 1965). Other major producing placer areas are in 

 Nigeria and in the newly developed tin fields in the 

 Territory of Rondonia, Brazil (Brazil Div. Fomento 

 Prod. Mineral, 1964). Alluvial placers occupy both 

 modern and fossil streambeds, and the distribution 

 of tin is dependent upon the location of the source 

 areas and the hydraulics of running water. The best 

 placers are formed near lodes in sections of streams 

 where the velocity is sufficient to result in good 

 gravity separation but not enough so that the chan- 

 nel is swept clean. A type of location proved to be 

 especially favorable for placer tin is a long stream 

 that flows parallel to the margin of a tin-bearing 

 granite. 



In Southeast Asia, stream placers formed sea- 

 ward of present shorelines when sea level was eu- 

 statically lowered periodically during the Pleisto- 

 cene. These stream placers now lie beneath sea 

 water ; they are mined by seagoing dredges. In Ron- 

 donia, Brazil, some placers lie in areas so low that 

 sea water covered them during certain interglacial 

 periods when sea level stood much higher than at 

 present. Elsewhere in the world, as in Cornwall and 

 Tasmania, both lode and placer deposits occur 

 within areas which have fluctuated between land and 

 sea bottom during Pleistocene climatic changes; 

 placers in such areas may have characteristics of 

 both marine and alluvial deposits. 



Owing to the long exposure to air and water and to 

 good gravity separation, tin concentrates produced 

 from alluvial placers are relatively free of deleter- 

 ious constituents and may contain as much as 70 

 percent tin ; as such, they constitute prime smelter 

 concentrates. 



MARINE PLACERS 



Most marine placers are beach placers or inun- 

 dated beach placers. They form where a marine 

 shoreline intersects or transgresses either a stream 

 valley containing alluvial cassiterite or a bedrock 

 source of tin. Although a true beach placer norm- 



