Scheelite in the Boulder District, Colorado

Scheelite accompanies ferberite as an ore of tungsten at many localities in the Boulder tungsten district. It is only of accessory importance in most veins, but in a few mines it affects grade of the ore substantially. It occurs in small veins, in vugs, disseminated in sericitized rock, and in gouge and minute fractures in shear zones. The vein and shear-zone deposits are the most important economically, but all scheelite-bearing ores owe their value chiefly to ferberite. The ratio of scheelite to ferberite is as high as 1:1 in some deposits containing one per cent WO (sub 3) , but in scheelite-bearing ore containing 3 to 5 per cent WO (sub 3) , the ratio ranges from 1:5 to 1:10.Scheelite is later than all minerals except calcite and some late dolomite.The mineralogy and paragenesis of the tungsten veins indicate a change from acid to alkaline solutions during mineralization. This change may be attributed partly to reaction with the alkaline wall rocks, but the distribution of alteration zones is in some respects incompatible with this hypothesis, and it is suggested in this paper that the solutions depositing tungsten changed at the source as well as by reaction.Ferberite was deposited from slightly acid, and scheelite from alkaline solutions. Early acid solutions leached lime from plagioclase in the wall rocks. Late alkaline solutions deposited a little lime as carbonate but carried little or no tungsten. As neither the solutions nor the altered walls could furnish much lime at the time when tungsten appears to have been most abundant in early alkaline solutions, scheelite formed only in relatively minor quantities.