Notices of Memoirs — 'Tungsten Deposits of Essexvale. 377 



Minerals of the Greisen. — The minerals detected in the greisens 

 comprise quartz, soft yellow mica, felspar, dark-green chlorite in 

 rosettes, hlack tourmaline, pyrite (altered to cubes of limonite at the 

 surface), fluorspar (blue, mauve, green, white, and colourless), topaz 

 (pale brown and colourless), galena (rather rarely), pyrrhotite, 

 wolframite, and scheelite. 



Small quantities of each of these occur in the quartz. Here and 

 there a bunch or streak of any one of them, including the tungsten 

 minerals, lies in the quartz. The distribution of the minerals in the 

 quartz or in the altered aplite is in fact generally patchy, as is always 

 the case in greisens. Coarse aggregates of any one mineral are 

 occasionally noted ; for example, single aggregates of very large 

 wolframite crystals weighing 235 and 157 lb are said to have been 

 found at the stockwork deposit, and similar groups of crystals have 

 been obtained at the Lunar Block (the specimen in the Rhodesia 

 Museum weighing 172 lb. came from here). Pieces of wolframite 

 weighing up to 8 lb. are not uncommon, and groups of pale pinkish 

 scheelite crystals measuring 3 or 4 inches are to be found. The two 

 tungsten minerals are commonly intergrown ; but in spite of this and 

 of the fact that scheelite, containing as it frequently does several per 

 cent more tungstic oxide than wolframite, may be worth several 

 pounds sterling per ton more than the wolframite, it was found that 

 the scheelite was neglected by the workers ; in fact, considerable 

 trouble was taken by them to separate it from the wolframite and 

 reject it. 



Scheelite is a mineral very easily recognized, and the natives 

 engaged in panning the concentrate should be taught to know it. 

 Although it is not unlike quartz so far as colour is concerned — being 

 white, pinkish, or yellowish — its characteristic greasy lustre, softness 

 (it is easily scratched by the knife or by quartz), and heaviness are 

 properties which differentiate it sufficiently from any of the minerals 

 with which it is associated. If boiled in dilute hydrochloric acid it 

 . becomes coated with bright yellow powder soluble in alkali. 



Among the dark minerals got in the concentrate, magnetite may be 

 recognized (and separated) by the magnet, and limonite by being in 

 brown cubes. Coarse and moderately fine wolframite is easily 

 distinguished from the other black minerals by its greater specific 

 gravity and chocolate-brown streak ; it breaks into flat slabby pieces 

 with lamellar structure owing to the presence of a single perfect 

 cleavage ; the flat surfaces are bright and shiny (submetallic to 

 resinous lustre), whilst the cross fractures are dull. Ilmenite, which 

 is rather abundant in very fine round grains in the concentrate of the 

 rubble, is difficult to distinguish from fine wolframite by simple tests, 

 and this fact had led to the rejection of the finest concentrate. 



Mineralization. — In addition to the minerals common to greisen, 

 the presence both in the stockwork and in the veins, of galena, 

 pyrite, pyrrhotite, and presumably gold, together with the large 

 amount and constant presence of a kind of quartz which is indis- 

 tinguishable from the ordinary vein quartz of gold deposits, suggests 

 that the Essexvale tungsten deposits are not normal greisens, but to 

 some degree assume the characters of the gold-quartz vein type of 



