LINDGREN AND 



RANSOME. 



] TYPES OP DEPOSITS. 25 



Peak type of granite, may remain unaltered, the rest of the rock, con- 

 sisting originally of microcline, oligoclase, quartz, and biotite, may be 

 completely recrystallized as a porous, vuggy aggregate of secondary 

 orthoclase(valencianite), quartz, fluorite, pyrite, calaverite or sylvanite, 

 and, in exceptional cases, sphalerite and galena. The ore minerals are 

 partly inclosed in the other secondary minerals, but occur most abun- 

 dantty with little projecting crystals of fluorite, quartz, and valencianite 

 on the walls of the irregular pores so characteristic of the altered rock. 

 The biotite of the original granite yields most readity to alteration, and, 

 in rock otherwise almost entirely unaltered, may be changed to an 

 aggregate of fluorite, quartz, and ore minerals. Some of the ore of 

 the Ajax mine exhibits well this initial stage of alteration. With fur- 

 ther alteration the original quartz and oligoclase of the granite are 

 attacked. The quartz, originally in large homogeneous and irregular 

 grains, recrystallizes as aggregates. Secondaiy orthoclase or valenci- 

 anite forms often in clear, sharply idiomorphic crystals, which either 

 project into open cavities or form aggregates with the secondary quartz. 

 In many cases, however, the secondary valencianite results from the 

 recrystallization of the older microcline practically in place. The two 

 generations are sometimes distinguishable by the greater clearness and 

 more or less idiomorphic form of the younger mineral and the absence 

 of the characteristic microcline twinning. But it is often impossible 

 to determine the line between feldspar which, from its association with 

 quartz and fluorite, is clearly secondary and the original microcline of 

 the granite. Occasionally a little calcite may be detected in the 

 altered granite, but this is rare. The original apatite and zircon of 

 the granite are not, so far as observed, affected by the alteration 

 described. 



While the replacement deposits in granite are important because of 

 their size and the readiness with which the ore may be mined free 

 from waste, the ore itself is usually of lower grade than that formed 

 in the fissures of the sheeted zones. 



Mineralized "basalt" dikes. The ore bodies formed by the minerali- 

 zation of basic dikes are in some ways closely related to the sheeted zones 

 already described. Like the phonolite dikes, the "basalt" exhibits 

 a pronounced tendency to split into thin sheets parallel with the dike 

 walls. Normally, the minute fissures so formed are filled with veinlets 

 of calcite and contain no ore. When, however, a zone of fissuring 

 coincides with the dike the latter may be traversed by veinlets of 

 quartz and fluorite carrying sylvanite or calaverite, while the body of 

 the dike may be impregnated with pyrite. Such ore differs from that 

 of the usual sheeted zones in breccia or phonolite in that the tellurides 

 are not so clearly confined to the actual fissures, but appear to some 

 extent to permeate the rock with the pyrite. The richest portion of 

 the ore, however, undoubtedly occurs in the small veinlets in the diko, 



