34 



ECONOMIC GEOLOGY 



of the irruptive rock from which the minerals were derived; 

 second, to the nature of the minerals in the lode, and third, to 

 the character of the non-metalliferous minerals both in the 

 lode and the metamorphic aureole. The magma may have 

 been either basic or acid, deep-seated or superficial, irruptive 

 or eruptive. 



In the classification of hydatogenetic deposits, it is customary 

 to select the most important economic mineral. Many deposits 

 are catalogued as gold ores that are strictly pyritic lodes charac- 

 terized by a small amount of gold. 



Primary Gold Veins. Hydatogenetic gold veins are associated 



FIG. 23. Section to illustrate the formation of auriferous quartz lenses in 

 alaskite. (After Thomas and MacAlister's Geology of Ore Deposits.) 



with granite, porphyry, andesite, trachyte and rhyolite. (See 

 Fig. 23.) The chief gangue is quartz, but calcite, siderite and 

 magnesite may be present with the quartz. Barite and fluorite 

 may form the gangue; tourmaline and orthoclase may also be 

 present. In what form the gold was extracted from the rocks 

 and transported is perhaps uncertain. The solutions were di- 

 lute and the gold reduced to the elemental state by the accom- 

 panying sulphides. The presence of carbon or hydrocarbons 

 in the walls of the ore body aids in its enrichment. In the 

 secondary concentration near the surface melanterite has been 

 the precipitant. Gold may occur as a telluride of gold, or 



