TIN DEPOSITS. 43 
South Dakota. Because of the unusualness of this mode of occurrence, tin deposits of this 
type have not been so thoroughly studied as the vein deposits. At many places the pegma- 
tite dikes cany, besides the tin mineral, compounds of lithium and of phosphorus, such as 
spodumene, lepidolite, apatite, triphylite, and autunite. In these cases the cassiterite has 
crystallized along with or a little earlier than most of the other constituents. The pegma- 
tite, being a product of solidification of molten material, is sharply defined from its walls, 
and in no case does the cassiterite replace portions of the country rock. On the other hand, 
the pegmatite produces at its contact metamorphism of the same kind as that produced by 
granite. Garnet is a rather common product in the wall rock; locally tourmaline is intro- 
duced and a chestnut-brown mica is developed in many places. 
As in the case of the tin-bearing veins, these pegmatite dikes likewise are regarded as one 
of the final products from the granite magma, being succeeded only by vein formation. Like 
the veins, the dikes represent a concentration of those rare elements and the water which 
has been gradually excluded by the crystallization of the main body of the rock. In 
many respects, therefore, these two modes of occurrence are closely related. 
Turning now to a consideration of the Carolina tin deposits, we find it evident at once that 
they belong to the second class, with cassiterite as an original constituent of pegmatite. In 
a preceding section (p. 22) it has been shown that the pegmatite dikes of this region are 
almost unquestionably related in origin to the granite and granitic gneiss which occur with 
some interruption all along the tin belt. Below the zone of surface decomposition these 
masses of tin-bearing rock, exist in the same condition in which they originally solidified 
from the magma; they have suffered no alteration; they are not greisen. The effect of these 
dikes on the surrounding rock is characteristic not of veins but of igneous intrusions. Care- 
ful search for cassiterite in the wall rock has been made both by crushing and panning and 
by microscopic examination of thin sections, and in no case has tin been found, nor has it 
ever been reported as occurring outside of the dike proper. The absence of tourmaline, 
topaz, and wolframite and the extremely small amount of lithia mica in the tin-bearing rock, 
while, on the other hand, spodumene, lithiophilite, and apatite are present, go far in support 
of the view that the tin is of igneous origin. In one case a crystal of cassiterite was found 
adjoining a large crystal of orthoclase that is perfectly unaltered except for a slight amount 
of weathering. Such an occurrence would be extremely unlikely if the cassiterite had been 
deposited by replacement from solutions. 
The unequal distribution of the cassiterite in the dikes is a subject difficult of explanation. 
Some combination of conditions or some process of differentiation as yet unknown has doubt- 
less caused the segregation of tin in the ore shoots. A process of probably a similar nature 
has resulted in the formation of relatively large masses of quartz and of feldspar in the dikes, 
such as are seen in the Faires mine. 
The complete cycle, beginning with granitic intrusion, followed by pegmatite intrusion, 
and brought to a close by vein formation, has been followed out in the Carolina region. The 
veins are closely associated with the pegmatites and in some cases with dikes of granite or 
aplite. They are quartz veins carrying pyrite, ilmenite, and in many places abundant 
tourmaline. Some of them, at least, carry gold. (See pp. 69-70.) Their walls also are in 
many places impregnated with tourmaline. They are usually parallel with and a short dis- 
tance away from the pegmatite. In no place have they been found actually cutting the peg- 
matite, but at the Ross mine some of the pegmatite contains secondary tourmaline which 
was probably derived from one of these veins. 
As has been previously stated (p. 40) these veins give evidence of having been formed at 
great depth and high temperature. It is probable that if the pegmatite had not been 
expelled from the magma reservoir, or had not carried all the tin with it, these veins would 
have received the tin of the magma, forming tin veins like those of most other tin deposits. 
To summarize the origin of these tin deposits, pegmatite dikes genetically related to bodies 
of granite contain cassiterite as an original constituent unevenly distributed throughout 
their mass. The cassiterite was formed at practically the same time as the quartz, mica, 
feldspar, and other minerals of the dikes by crystallizing from the molten pegmatite magma. 
