58 ECONOMIC GEOLOGY 



often rich in silver content or highly argentiferous. The char- 

 acter of the rock which the vein traverses is often variable, for 

 the gold-bearing veins appear in either igneous or sedimentary 

 rocks, sometimes at the contact zone between igneous and sedi- 

 mentary rocks, little influenced by the character of the rock which 

 the vein traverses save in the case of replacement (Fig. 42). 

 Again, true fissure veins, in the more recent lava flows, often 

 appear intimately associated with tellurium, and these often give 

 rise to pockets of great richness. In the telluride group the ores 

 occur either as definite tellurides of gold with silver, lead and 

 antimony, or as native gold accompanied with various tellurides. 

 The gangue minerals are quartz and fluorite; calcite may be spar- 

 ingly present. In Boulder County, Colorado, roscoelite, a vana- 

 dium mica, is associated with the tellurides. 



Classification. Gold deposits are often classified according 

 to their association. The first of these may be catalogued as gravel 

 deposits. It includes all classes, whether as river or beach gravels 

 or covered with volcanic material. The gold appears as a result 

 of the disintegration of superincumbent strata where the gold by 

 its insolubility and the higher specific gravity has been trans- 

 ported to lower altitudes in the adjacent valleys. Pay gold is not 

 transported far from the site of its parent rock. The lower por- 

 tions of the gravel bed are generally richer in gold than the upper 

 layers because of the higher specific gravity of the metal. Where 

 the rocks are fractured as by joint planes, fissures, and the action 

 of the frost, these places of breakage are often found well studded 

 with gold so that 2 or 3 ft. of the decomposed and altered bed 

 rock is removed for the extraction of the gold it contains. It 

 seems that some gold-bearing gravels decrease in richness toward 

 the surface, and then there will appear a rich layer of greater or 

 smaller width than the zone of extreme richness at the bottom of 

 the placer. This implies that there was a cessation in the de- 

 livery of auriferous gravel, and then a return of the original stream 

 to the valley, carrying a second contribution of auriferous gravel 

 over the same course as the former. Ordinarily there is only 

 one zone in the gravel and that directly at the base, but where 

 others occur it must imply that there was some disturbance or 

 cessation in the delivery. 



The second class of gold ores (Fig. 43) may be catalogued as 

 quartzose. This implies that the gangue mineral is acid, that is, 

 quartz, and that fluorite may abound, or even the other gangue 



