Chemical and isotopic studies of granitic Archean rocks, Owl Creek Mountains, Wyoming
No abstract available.
Wyoming↗
Geology topics
K. R. Simmons
Publications and source records attributed to K. R. Simmons.
The Schwartzwalder uranium deposit, II: Age of uranium mineralization and lead isotope constraints on genesis
U-Pb isotope analyses of ores from the Schwartzwalder uranium mine, Colorado, show that these ores have high amounts of initial (common) Pb and that the initial Pb was both variable and relatively radiogenic in its Pb isotope ratios ( 206 Pb/ 204 Pb = 26-30). As a result, the only useful approach to dating these ores is with U-Pb isochrons, and even so, some means of dealing with the variable initial Pb isotope ratios is required. Because the common Pb in these ores was apparently derived from sources of similar age and Th/U, the observed 208 Pb/ 204 Pb of these Th-free ores can be used either to identify sample suites with similar initial Pb isotope ratios or to normalize for the variable initial Pb isotope ratios. The resulting U-Pb isochrons indicate an age of mineralization for both Illinois vein and Titan vein ores of 69.3 + or - 1.1 m.y., suggesting that the deposit was formed during the earliest stages of Laramide uplift and under at least 3 km of Phanerozoic cover. The initial Pb isotope systematics of the ores show that the metals in the Schwartzwalder ores were derived from source(s) of 1,730 + or - 130-m.y.-age, with a Th/U of 2.2 + or - 0.2 and 238 U/ 204 Pb of 30 to 60. These restrictions on the source(s) of the metals rule out the possibility that the deposit could have been formed by remobilization of a Proterozoic uranium deposit, and both the 69.3-m.y.-age of the ores and their initial Pb isotope ratios preclude any contribution of metals to the deposit by younger volcanics such as those in the Denver Formation.
Economic Geology↗
Isotopic studies of the late Archean plutonic rocks of the Wind River Range, Wyoming
Isotopic studies of the Rb-Sr and U-Th-Pb systems in whole-rock samples and the U-Pb systematics for zircons document the existence of two late Arehean intrusive events in the Wind River Range. All of the systems examined indicate an age of ∼2,630 ± 20 m.y. for the Louis Lake batholith. Apparent ages for the Bears Ears pluton range from 2,504 ± 40 m.y. to 2,575 ± 50 m.y. The scatter in apparent ages for the Bears Ears pluton does not appear to be primarily the result of disturbance by postintrusive events, but it may be explained by an isotopically inhomogenous magma at the time of intrusion. Data for a few samples indicate that the Wind River Range was affected locally by a postmagmatic hydrothermal event that was approximately Tertiary in age. This event lowered δ 18 O values and disturbed parent-daughter relationships in most of the isotopic systems investigated, but it was recent enough that there is no demonstrable effect in the Pb-Pb system. The Bears Ears pluton has some chemical and petrologic features that are similar to those reported for the granites in the Granite Mountains to the east. These granites are spatially associated with low-temperature uranium deposits of Tertiary age and have been shown to have lost large amounts of uranium during the early to middle Tertiary. U-Pb systematics indicate, however, that the low to moderate uranium contents and highly variable Th/U values noted for the Bears Ears pluton are best interpreted as being primary features. If uranium was lost after magma generation, the loss most likely occurred at the time of intrusion. Such a loss could account for uraniferous Precambrian pegmatites southwest of the main part of the Range. The two intrusive units apparently were derived from different protoliths that were formed during early to middle Archean. Initial isotopic ratios and petrochemistry for the Louis Lake batholith are consistent with an early Archean trondhjemitic to tonalitic source. The protolith for the Bears Ears pluton must have been more evolved and somewhat younger. Inconsistencies as to the degree of evolution of this protolith, as inferred from isotopic and trace-element data, suggest that the protolith may have been subjected to high-grade meta-morphism that caused loss of Rb and U prior to generation of the magma.
Wyoming↗
U-Pb ages of uraniferous opals and implications for the history of beryllium, fluorine, and uranium mineralization at Spor Mountain, Utah
The U-Pb isotope systematics of uraniferous opals from Spor Mountain, Utah, were investigated to determine the suitability of such material for geochronologic purposes, and to estimate the timing of uranium and associated beryllium and fluorine mineralization. The results indicate that uraniferous opals can approximate a closed system for uranium and uranium daughters, so that dating samples as young as ∼1 m.y. should be possible. In addition, the expected lack of initial 230 Th and 231 Pa in opals permits valuable information on the initial 234 U/ 238 U to be obtained on suitable samples of ≲10 m.y. age. The oldest 207 Pb/ 235 U apparent age observed, 20.8 ± 1m.y., was that of the opal-fluorite core of a nodule from a beryllium deposit in the Spor Mountain Formation. This age is indistinguishable from that of fission-track and K-Ar ages from the host rhyolite, and links the mineralization to the first episode of alkali rhyolite magmatism and related hydrothermal activity at Spor Mountain. Successively younger ages of 13 m.y. and 8–9 m.y. on concentric outer zones of the same nodule indicate that opal formed either episodically or continuously for over 10 m.y. Several samples of both fracture-filling and massive-nodule opal associated with beryllium deposits gave 207 Pb/ 235 U apparent ages of 13–16 m.y., which may reflect a restricted period of mineralization or perhaps an averaging of 21−and<13−m.y. periods of opal growth. Several samples of fracture-filling opal in volcanic rocks as young as 6 m.y. gave 207 Pb/ 235 U ages of 3.4–4.8 m.y. These ages may reflect hot-spring activity after the last major eruption of alkali rhyolite.
Utah↗