Abstract: Preliminary Geochemical and Petrologic Studies of Core Samples from Granite Mountains, Wyoming

J. S. Stuckless, C. M. Bunker, C. A. Bush, W. P. Doering, J. H. Scott

Drill-core samples obtained in the fall of 1974 from the U.S. Geological Survey drill hole GM-1 in the Granite Mountains, Wyoming, exhibit considerable variability in geochemical, petrographic, and geophysical characteristics. The upper 215 m are petrographically similar to typical Precambrian granites. These samples display hypidiomorphic-granular textures and minor alteration of plagioclase to clays and white mica. Chemically this upper zone is anomalously high in U, Th, and Pb, averaging 11.3, 52.7, and 51.7 ppm, respectively. The average of the Th/U ratios for 40 samples is 6.26; this is also high relative to most granitic rocks. Average K of 4.42 percent is only slightly higher than most granites.

Samples from 215 to 350 m are generally xenomorphic-granular and contain rounded quartz grains poikilitically enclosed by microcline. Within this zone several intervals contain rounded and retrograded garnets, a few places have minor pyrite concentrations, and a few other intervals contain large (2 cm) magnetite crystals. Average concentrations of U and Th decrease to 7.07 and 6.73 ppm, respectively, and the average of 18 Th/U ratios decreases to 1.17. Limited data for K and Pb are not markedly different from the upper zone.

Magnetic anomalies are common throughout the bore hole, reaching two orders of magnitude above the background intensities. These commonly are associated closely with minor gamma-radioactivity anomalies. Larger gamma anomalies are present at three small fractured zones (0.5-3 m thick) and suggest equivalent U contents of 100 to 500 ppm.

AAPG Search and Discovery Article #90979©1975 AAPG – SEPM Rocky Mountain Sections Annual Meeting, Albuquerque, New Mexico