Geology topics

Research about Granite Mountains

Source-linked reports with geographic coverage including Granite Mountains.

4 recordsLinked to original sources

USGS1978

Radiometric results and areal distribution for granitic samples from the Granite Mountains, Wyoming

Analyses of surface samples from the Granite Mountains, Wyoming, generally agree with the findings from drilling at two different localities. The dominant Precambrian granitic rock type is the biotitic phase of the granite of Lankin Dome. This rock type is characterized by high potassium and anomalously high thorium concentrations, and is the most favorable granitic source for the uranium deposits in the surrounding area. Areal distribution patterns for radioelement concentrations show that the most favorable source regions are located closest to known uranium deposits. Most of the granitic rocks in the Granite Mountains region are characterized by high thorium-uranium ratios. These high ratios are interpreted to be the result of uranium loss. Non-gaussian distribution of radioelement contents and poor correlation between radioelement pairs are interpreted to be the result of some redistribution of radioelements during the last phase of granite crystallization and to recent uranium loss. A good correlation between thorium and iron is attributed to the mobilization of thorium into microcrystalline iron oxides during a late portion of the magmatic history. A comparison of sulfur data for surface and shallow drill-hole samples suggests that sulfur has been removed from the granite to at least a depth of 20 m. The loss of uranium and sulfur from these samples suggests that other trace elements found in association with uranium deposits might have been removed from the granite.

Wyoming↗

USGS1977

Reconnaissance geology and geochronology of the Precambrian of the Granite Mountains, Wyoming

The Precambrian of the western part of the Granite Mountains contains a metamorphic complex of gneisses, schists and amphibolites that were derived through amphibolite-grade metamorphism from a sedimentary-volcanic sequence perhaps similar to that exposed in the southeastern Wind River Mountains. Whole-rock Rb-Sr dating places the time of metamorphism at 2925 ± 81 m.y. A high initial 87 Sr/ 86 Sr ratio of 0.7048 suggests that either the protoliths or the source terrane of the sedimentary component is several hundred million years older than the time of metamorphism. Following an interval of 300 ± 100 m.y. for which the geologic record is lacking or still undeciphered, the metamorphic complex was intruded by a batholith and satellite bodies of medium to coarse-grained, generally massive biotite granite and related pegmatite and aplite. The main body of granite is dated at 2602 ± 60 m.y. by the Rb-Sr method. Limited data suggest that diabase dikes were emplaced and nephrite veins were formed only shortly after intrusion of the granite. Emplacement of the granite at about 2600 m.y. ago appears to be related to a major period of regional granitic plutonism in the Precambrian of southern and western Wyoming. Granites sensu stricto that are dated between 2500 and 2600 m.y. occur in the Teton Range, the Sierra Madre, the Medicine Bow Mountains and the Laramie Range. This episode of granitic plutonism occurred some 50 to 100 m.y. later than the major tonalitic to granitic plutonism in the Superior Province of northern Minnesota and adjacent Ontario--the nearest exposed Precambrian W terrane that is analogous to the Wyoming Province. Initial 87 Sr/ 86 Sr ratios of some of the Wyoming granites are higher than expected if the rocks had been derived from juvenile magmas and it is likely that older crustal rocks were involved to some degree in the generation of these granites. Slightly to highly disturbed Rb-Sr and K-Ar mineral ages are obtained on rocks of the metamorphic complex arid on the granite. These ages range from about 2450 to 1450 m.y. and are part of a regional pattern of lowered mineral ages of Precambrian W rocks of southern Wyoming. A major discontinuity in these mineral ages occurs along a line extending from the northern Laramie Range, through the northern part of the Granite Mountains, to the southeastern Wind River Mountains. North of this line, Rb-Sr and K-Ar biotite ages are 2350 m.y. or greater whereas to the south, the biotite ages decrease drastically over a short distance to a common range of 1400-1600 m.y. We suggest that these lowered ages represent regional cooling below the 300° C isotherm as a consequence of uplift and erosion of the large crustal block occurring south of the age discontinuity. In this interpretation, the westerly-trending age discontinuity would be a zone of major crustal dislocation that resulted from vertical tectonics in late Precambrian X or early Precambrian Y time.

Wyoming↗

USGS1977

Geochemical and petrological studies of a uraniferous granite from the Granite Mountains, Wyoming

Granite rocks from the Granite Mountains, Wyo. have been proposed as the source of uranium deposits in the Crooks Gap, Gas Hills and Shirley Basin uranium districts, Wyoming. We have divided these granitic rocks into four units: (1) a biotitic phase which forms the dominant unit at the western end of the Granite Mountains, (2) a leucocratic phase which was found from 215 to 405 metres in drill hole GM-1, (3) silicified zones which crosscut the granitic rocks and form topographic highs, and (4) fractured zones, in drill hole GM-1, which seem to have been hydrothermally altered. The biotitic phase is hypidiomorphic-granular to xenomorphic-granular alkali granite with anomalously high contents of U (10 parts per million), Th (50 ppm), and Pb (50 ppm). Fission-track studies show that uranium is located in zircon, sphene, apatite, monzite, xenotime, biotite, chlorite, epidote, and magnetite; no intergranular uranium was found. The leucocratic phase is mineralogically similar to the biotitic phase, but contains less than half as much iron. It is xenomorphic granular and commonly contains rounded and retrograded garnets, which suggests that this phase is either metamorphic or contaminated with metamorphic materials. The leucocratic phase has anomalously high contents of U (8 ppm) and Pb (55 ppm), but has a low Th content (10 ppm). The silicified phase and fracture zones exhibit cataclastic and crystalloblastic textures and are highly variable in mineralogy. Potassium-bearing minerals are generally absent. Microcline is replaced by albite and (or) quartz, and biotite is replaced by clinozoisite. Uranium values may be anomalously high in the fracture zones. One sample contains 1100 ppm radium-equivalent uranium. In this and other uranium-rich samples from the fracture zones, the uranium is associated with iron oxides which commonly fill microfractures. According to our model and currently available data, an alkali granite is the best crystalline source rock for uranium, especially if it is unmetamorphosed and rapidly exposed to near-surface conditions for the first time when a favorable basin existed nearby.

Wyoming↗

USGS1973

Lead isotope systematics and uranium depletion in the Granite Mountains, Wyoming

Isotopic composition and concentration of lead in whole rock and microcline and concentration of uranium and thorium in whole-rock samples of granite from the Granite Mountains, Wyoming, have been determined. The lead isotopic composition in the whole rocks was found to be highly radiogenic with a range in Pb 206 /Pb 204 of 19.58 to 42.27; the corresponding range in microclines is 15.39 to 22.44. A Pb 206 /Pb 204 versus Pb 207 /Pb 204 plot of the whole-rock data yields an apparent isochron age of 2,790 ± 80 m.y. as the time of crystallization of the granite. Chemically determined values of U 238 /Pb 204 in the whole rocks lie between 3.3 and 18.4 and are too low to account for the amount of radiogenic lead observed. A material balance of lead, thorium, and uranium components indicates that an average of approximately 75 percent of the amount of uranium required to produce the radiogenic lead was removed from the rocks, whereas, on the average, there was no apparent loss of thorium. Loss of uranium from the granite is demonstrated to extend at least to a depth of 165 ft in a drill core. The average uranium loss from the samples analyzed represents about 20 g uranium per 1,000 kg of rock that apparently was removed during the Cenozoic and that probably constitutes the major source of uranium now in ore deposits in central Wyoming basins. The lead isotopic composition of the microclines indicates that lead was mobilized within the granite and was isolated in the feldspar during a thermal event about 1,640 + 120 m.y. ago. However, there is no evidence that the whole rocks themselves became open systems at that time. Whole-rock and microcline isochrons intersect at Pb 206 /Pb 204 and Pb 207 /Pb 204 of 13.77 and 14.86, respectively, indicating a characteristic U 238 /Pb 204 of 8.96 in the source region of the granite magma.

Wyoming↗