Finlayson. — Scheelite of Otayo 



117 



Locality. G. W0 3 . Mo0 3 . CaO. 



Schwargenberg(a) .. 6-12 79-94 Tr. 19-57 



(a) .. 6-02 80-17 0-07 19-49 



Schlaekeuwald .. .. 6-13 79-76 Tr. 19-67 



Haslithal .. .. 614 80-16 Tr. 19-65 



Traversella(fc) .. .. 6-06 78-57 1-62 19-37 



(/;) .. .. 6-04 79-68 0-76 19-29 



CarrickFels. .. .. 601 79-97 0-35 19-27 



Pot Mine, South Africa(c) 5-96 70-57 8-09 20-05 



71-59 7-63 20-51 



Mount Kamsay, Tasmania 6-09 79-77 Tr. 19-65 



New Zealand . . . . 601 80-29 Tr. 19-44 

 (a) MgO, trace. (b) Ce 2 3 , trace. (6) CuO, 0-34. 



(4.) Deposition op the Scheelite. 



Microscopic examination of the ore, and chemical analyses of 

 the wall-rock of the veins, prove that the scheelite has been 

 formed by metasomatic processes — namely, by combination of 

 tungstic acid with lime-bearing minerals in the adjoining rock. 



Microscopic Evidence. 



The relations of scheelite and calcite, as seen under the 

 microscope, are very striking. Plate XVI, c, shows scheelite in 

 clear granules with fresh sharp boundaries enclosing a corroded 

 core of calcite, and indicating the replacement process by which 

 the ore has been formed. This phenomenon is best studied at 

 Macrae's, where the country rock contains a considerable amount 

 of calcite. In general, the scheelite is always fresh, the calcite 

 where seen is much attacked and corroded. Several of the 

 plates accompanying Mr. Lindgren's classic work on " Metaso- 

 matic Processes in Fissure-veins "* show very similar processes 

 to that illustrated in the above plate. 



A similar association of scheelite and calcite is occasionally 

 seen in sections cut from Glenorchy ore. A characteristic fea- 

 ture of the Glenorchy mineral is the manner in which strings 

 of pyrite occur along the border between scheelite and gangue 

 (Plate XVI, d). The pyrite thus appears to have segregated 

 along the line of most intense metasomatism. The process of 

 osmosis, regarded by many authorities as the central factor 

 in ore-deposition, f would evidently be equally favourable to 

 the formation both of scheelite and of pyrite, the latter being, 

 like the former, essentially a replacement product. 



* '• Genesis of Ore-deposits," Trans. Amer. Inst. Man. Eng., 1901, p. 498. 

 t H. P. Gillette, " Osmosis as a Factor in Ore-formation," Trans. 

 Amer. Inst. Min. Eng., vol. xxxiv (1903), p. 710. 



