Copper-Tin Veins of the Ala'nobé District. 5^ 



The veins of this district are unique in the fact tliat the ore- 

 bringer for them is a dioritic magma. Considering, however, that 

 acid rocks can be derived from a basic magma by processes of 

 differentiation chiefly due to fractional crystalhzation and setthng^> 

 or by the expulsion of the residual fluid magma,^) and tliat an 

 acid differentiation-prod act with a composition of quartz-monzonite- 

 pegmatite or aplite, akenobeite, is actually found as a small boss 

 in this district, the genetic connection of the veins with the 

 dioritic rocks is highly probable. 



Summarizing all that has been stated, the copper-tin veins of 

 the Akénobé district were deposited from hydrothermal solutions,^) 

 still containing fair quantities of mineralizers, at gradually decreasing 

 temperatures, chiefly considerably below 360^ C. The solutions had 

 naturally a temperature far above the critical point of water 

 (364° C.) and were gaseous in character, after emanation from the 

 consolidating diorite magma. As they ascended through the sur- 

 rounding slate complex, the rate of the fall of temperature was- 

 very rapid, and they soon changed to superheated hydrothermal 

 solutions. 



Lastly, the writer frankly states that he is quite in the dark 

 as to whether the stannic oxide first separated from the solutions 

 in the colloid state and subsequently became crystalline, or whether 

 it crystalhzed directly as cassiterite by chemical reactions between 

 stannic fluoride and other compounds. 



1) N. L. Bowen, "The Later Stages of the Evokition of the Igneous Eocks," Journ. Geol , 

 Vol. XXIII., 1915, Supplement to No. 8, pp. 1-91. 



2) A. Harker, " Natural History of Igneous Eocks," p. 323. 



3) It should be borne in mincl that tin oxide is, according to C. Doelter (Min. petrog. Mitt., 

 Vol. 11, 1890, p. 325), perceptibly soluble in water at 80' C, and more so in the presence of 

 sodium fluoride. This solubility is also indicated by several natural occurrences of tin-stone, as 

 in an opaline deposit from a thermal spring in Selangor, etc. (S. Meunier, Compt. Bend., Vol. 

 110, 1890, p. 1083; J. H. Collins, Min. Mag., VoL 4, 1880, pp. 1, 103, and Vol. 5, 1883, p. 121 , 

 etc.) 



