Abstract

Scheelite of Precambrian age has been found at several localities in Colorado and Wyoming in recent years. Most of it is disseminated in regionally metamorphosed rocks, principally in calc-silicate gneiss, but also in amphibolite. Small amounts are found in pegmatites and in Au- or Cu-quartz veins of Precambrian age. The scheelite in most of the deposits contains Mo, and much of it is accompanied by discrete powellite (CaMoO 4 ). The scheelite deposits vary in age with the rocks that contain them and show no consistent genetic associations. Many of the deposits are products of some phase of the major plutonic stage that included regional metamorphism, migmatization, and emplacement of syntectonic granites. A few deposits may be related to younger Precambrian granites, and some are related to late diorite and pegmatite. W originally present in minor concentration in the sedimentary rocks that gave rise to the gneisses was evidently redistributed and recrystallized through successive Precambrian plutonic episodes. Reworked Precambrian deposits may have been a source of the W in the economically important Tertiary deposits, and possibly also of Mo. The Precambrian scheelite deposits found thus far have proved to be of minor economic importance, but others are still being found, and many more can be presumed to exist; among these may be some of importance.

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