Abstract

Paleocene and Eocene rocks in central and eastern Wyoming contain four distinct assemblages of nonopaque heavy minerals. Two assemblages are largely plutonic in origin and two are largely volcanic in origin. One plutonic assemblage is characterized by an abundance of tourmaline and zircon derived largely from reworking of Paleozoic and Mesozoic rocks; it generally constitutes more than 60 percent of the nonopaque heavy minerals in Paleocene rocks (Fort Union Formation and equivalents). The other plutonic assemblage is characterized by the abundance of first-cycle blue-green hornblende, epidote, and garnet derived largely from Precambrian metamorphic and igneous rocks; it generally constitutes more than 60 percent of the nonopaque heavy minerals in lower* Eocene rocks (Wasatch, Wind River, Battle Spring, and Green River Formations). There appear to be no major differences between the heavy mineral assemblages of the lacustrine phases of the Green River Formation and those from the intertonguing paludal and fluviatile phases of the Wasatch and Battle Spring Formations. Although variable from place to place, both assemblages are detrital and were derived predominantly from the Precambrian cores of the mountains.

Two volcanic assemblages of nonopaque heavy minerals characterize the upper* Eocene rocks. One is a green-brown hornblende and augite assemblage that constitutes 40 percent or more of the nonopaque heavy minerals in the Bridger, Uinta, and Continental Peak (of Nace, 1939) Formations in the Green River, Washakie, and Great Divide Basins. The other is a zircon assemblage that generally constitutes 50 percent or more of the nonopaque heavy minerals in the Wagon Bed Formation of the Granite Mountains-Shirley Basin area. The green-brown hornblende and augite assemblage may have come from the vicinity of the Absaroka Range in northwestern Wyoming, whereas the zircon assemblage is believed to have been derived largely from volcanoes in the vicinity of the Rattlesnake Hills in central Wyoming.

Heavy minerals aid in establishing age relationships of lower Tertiary rocks, and thus may help in establishing structural and stratigraphic controls affecting the deposition of uranium.