R. H. Rastall — The Genesis of Tungsten Ores. 243 



are mineralized patches in a dolomitic limestone of Cambrian age. 

 They form flat horizontal masses, highly siliceous in composition, 

 and containing pyrite, fluorite, barytes, and occasionally gypsum. 

 The wolframite is specially associated with the barytes. There is 

 also a small amount of vanadium minerals. This deposit appears to 

 have been formed by gradual replacement of the calcareous country 

 rock by highly siliceous solutions ascending from below, and the 

 concentration at this particular horizon may have been determined 

 by the presence of impervious strata above. The tungsten may be 

 derived from the underlying Algonkian Series, where wolframite 

 occurs in pegmatites with cassiterite. If this is so, this must be 

 regarded as a case of secondary metasomatism, but the resemblance 

 to the wolfram-gold ores of Colorado and Nevada must also be taken 

 into account, and the tungsten-bearing solutions may really be of 

 direct magmatic origin, of post-Cambrian date, and independent of 

 the Algonkian tin-wolfram deposits below. 



A very interesting and remarkable occurrence of wolframite at 

 Trumbull, in Connecticut, is described by Hobbs.^ An oval hill, 

 some 1,000 feet long and 200 feet high, is composed of coarsely 

 crystalline marble, with sills of epidiorite above and below. The 

 ore-bodies occur along the plane of contact between the lower 

 epidiorite sill and the marble, and are concentrated in the epidiorite 

 just below the contact. The ore consists of both wolframite and 

 scheelite intimately mixed, with a little pyrite. The marble near 

 the contact contains many metamorphic minerals, especially scapolite 

 and garnet. The contact deposit seems to have been fed hj veins in 

 the underlying rock, which contain quartz, felspar, fluorite, and 

 topaz. There are also some pure quartz veins. All of these are 

 evidently of the usual granite-pegmatite type, and their relation to 

 the basic sills is not clear. It seems probable that the ore-bodies are 

 really due to derivation from a granitic magma and that their 

 association with the basic intrusions is purely fortuitous. The latter 

 are evidently of the normal chlorine-bearing type, as shown by the 

 development of scapolite in the metamorphosed limestone. The 

 presence of much scheelite is easily accounted for by derivation of 

 lime from the calcareous rock. This, then, is not a contact deposit 

 in the ordinary sense of the word, since the metallic constituents, 

 and especially the tungsten, do not seem to have been derived from 

 tlie rock in which they actually occur. It seems much more likely 

 to be an example of granitic metamorphism which has happened to 

 act on a basic rock and a limestone, and has segregated some con- 

 stituents from each, depositing them in combination at or near their 

 plane of junction. 



The tungsten ores of Canada have been exhaustively described by 

 "Walker- in a special report. They do not seem, so far as yet known, 

 to be of much economic importance, thougli some of the lodes are or 



^ Hobbs, Bull. 213, U.S. Geol. Surv., 1903, p. 98; and Twenty-second 

 Ann. Kep. U.S. Geol. Surv., 1901, p. 7. 



- Walker, Report on the Tungsten Ores of Canada, Department of Mines, 

 Ottawa, 1909. Also frequent references in the Annual Keports of the same 

 department. 



