THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE VI. VOL. V. 



No. VI.— JUNE, 1918, 



0:ElXC3rXlsr.A.JL. j^JEirrxpTLj 



^mGsomrn 



I. — The Genesis of TuNGSTsk^^ES. 



By E. H. Eastall, M.A., JP-G.SsijJt#^,^^ -^^l 



{Continued frovi the May Ntcmber, p. 203.) 



PaET II : WOLFKAM LoDES WITHOUT CaSSITEKITE. 



AS already stated in the first part of this paper, a regular 

 gradation may be traced from the cassiterite-wolframite lodes 

 to wolframite-quartz veins without cassiterite, a type which appears 

 on the whole to be more common in America than elsewhere. In 

 many cases this difference is clearly due to a more complete 

 differentiation of the magma, but in other instances a purely igneous 

 origin is less conclusively established. In Cornwall and other 

 granitic areas a tin- wolfram lode can sometimes be traced con- 

 tinuously into a wolfram lode, and this again into a pure quartz vein. 

 Here the pegmatitic origin of the lodes is demonstrated, and as a rule 

 the wolfram is accompanied by fluorite and other recognized pneu- 

 matolytic minerals, as well as by sulphides similar to those found in 

 the tin lodes. Thus the genetic connexion with the tin-bearing 

 types is beyond doubt. In a few instances only wolfram is found in 

 association with gold ores ; the significance of this will be discussed 

 in a later section. 



One of the most interesting cases of quartz-wolfram veins without 

 tinstone is seen in the Sierra de Cordoba in Argentina.^ Here the 

 veins occur in gneiss, and are actually traceable into a large granite 

 mass of unknown age which is intruded into the gneiss. They are 

 clearly pegmatitic differentiates of the granite. The wolfram occurs 

 in large crystals in the quartz and in nests up to half a cubic metre 

 in size. The chief minerals found in association with the wolfram 

 are mica, apatite, fluorite, molybdenite, and chalcopyrite, and in the 

 oxidation zone there are various oxidized copper minerals derived 

 from the chalcopyrite. The abundance of apatite is notable and 

 somewhat unusual in wolfram lodes. This is an instance of 

 differentiation from a magma rich in volatile constituents, especially 

 fluorine, but apparently without tin and boron. 



Somewhat similar to the foregoing are some wolfram lodes found 

 near Lircay in the province of Angaraes in Peru.- Two dykes or 



^ Bodenbender, "Die Wolfram-Minen der Sierra von Cordoba in der 

 Argentinischen Eepublik " : Zeits. fiir prakt. Geol. , 1894, p. 409. 



- De Habich, " Informe sobre los Jacimientos de Tungsteno de la Provincia 

 de Angaraes " : Boll. Cuerpo Ingen. de Minas del Peru, No. 11, p. 31, 1904. 

 DECADE VI. — VOL. V. — NO. VI. 16 



