Abstract

A narrow zone of sericitic alteration encases the Boulder County ferberite veins, but gives way abruptly to a wide outer envelope of argillized rock. The mineralogy indicates sustained attack by early acid solutions that finally changed to neutral and alkaline solutions. Sericitic alteration is conspicuous next to the gold telluride veins and the argillic envelope is rare. The mineralogy of the tungsten and telluride vein filling indicates a progressive change from acid to alkaline solutions and harmonizes with the evidence given by the wall rock alteration. Experiments and the paragenesis suggest that ferberite was precipitated in slightly acid solutions. Galena, tetrahedrite and the gold tellurides are later and presumably precipitated in alkaline solutions. Relatively late pre-ore dikes of biotite latite and latitic explosion breccia are associated with the tungsten and gold-teluride veins. Their composition and field relations suggest that a biotite latite magma heavily charged with volatiles was the source of the mineralizing solutions. It is believed that acid emanations from this magma rose through hot porous explosion breccia with little change in character,, but soon dropped their load above the upward termination of the breccia in the overlying fissures where they reacted with the wall rock and became neutralized and finally alkaline through the acquisition of bases from the quartz monzonite that makes up the country rock of the deposit.

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