VOLUME 43 NUMBER 1
Genesis of fibrous calcite and emerald by amagmatic processes in the southwestern Uinta Mountains, Utah
Stephen T. Nelson1,*, Jeffrey D. Keith1 ,
Kurt N. Constenius2 , Jay Olcott1 ,
Erin Duerichen1 and David G. Tingey1
1 Department of Geological Sciences, S389 ESC, Brigham Young
University, Provo, Utah 84602, U.S.A.
2 Department of Geosciences, University of Arizona, Tucson, Arizona
85721-0077, U.S.A.
*Correspondence should be addressed to: steve_nelson@byu.edu .
Large bodies of fibrous calcite, up to 400 m long and up to 20 m wide, are hosted by Mississippian carbonate units in the Uinta Mountains, Utah. Subjacent to the carbonate rocks, emerald mineralization has been recognized recently in the Neoproterozoic Red Pine Shale. Two types of fibrous calcites are recognized on the basis of appearance and geochemical characteristics. Fibrous orange calcite is translucent, similar to "Mexican calcite" commonly sold at gem-and-mineral shows, and is being mined and used in carvings and for other decorative purposes. Recently recognized fibrous brown calcite is also present in the same general vicinity.
Calcite veins hosted with Mississippian carbonate rocks are located a few kilometers south of the South Flank fault zone (SFFZ) in the vicinity of a swarm of north-trending faults. Parts of the Red Pine Shale along the SFFZ contain abundant secondary pyrite. Altered shale also contains quartz and calcite veins as well as albitized feldspar in some arkosic units and is variably enriched in Cu and Zn. At least three emeralds have been discovered in altered Red Pine Shale.
18OVSMOW
values of calcite (–10 to –13
)
would be in equilibrium with basin brines (assumed to be –3 to
+5),
suggesting precipitation in the range of 100°–300° C. Our work
suggests that sulfate-bearing basinal brines from the Uinta Basin may
have migrated upward along the SFFZ and interacted with the
carbon-rich shale. As sulfate was reduced by organic carbon to form
sulfides, emerald was produced and a CO2-rich fluid
migrated upward and dissolved parts of the Mississippian carbonates.
The fluid experienced intermittent loss of CO2 gas, which
allowed fibrous calcite with cone-in-cone textures to precipitate
rapidly. This model and geologic setting have broad similarities to
both Mississippi Valley and Colombian emerald deposits. The genesis
of fibrous brown calcite, however, is less certain. These calcites
lack critical geochemical characteristics that tie orange calcite to
precipitation from hot brines; rather stable isotope data are
permissive of origin from low-temperature ground water.
Key Words: emerald • fibrous calcite • karst • stable isotopes • Uinta Mountains • Utah
Structural and tectonic evolution of the Cherokee Ridge arch, south-central Wyoming: Implications for recurring strike-slip along the Cheyenne Belt suture zone
Jeffrey W. Bader
URS Corporation, 8181 E. Tufts Avenue, Denver, CO 80237, U.S.A.
email: jeff_bader@urscorp.com
The Cherokee Ridge arch is an east–west-trending structure that separates the Washakie Basin of south-central Wyoming from the Sand Wash Basin of northwestern Colorado. It is characterized by a distinct east–west-trending lineament on Landsat images and a zone of northwest–southeast-striking, en echelon, high-angle faults that cut Upper Cretaceous to Miocene sedimentary rocks. The lineament is interpreted to be the surface expression of the Cheyenne belt, a buried suture zone that separates Proterozoic crust of the Colorado province from Archean crust of the Wyoming province. Results of this study indicate that the surface and near-surface structures along the Cherokee Ridge arch are the result of periodic strike-slip motion along the Cheyenne belt since Late Cretaceous time and possibly before. Previous structural and paleomagnetic studies support the conclusion that left-lateral transpressional motion occurred during the Late Cretaceous and early Tertiary as the area was under the contractional tectonic regime of the Laramide orogeny. The left-lateral shear zone is interpreted to be a lateral ramp accommodating west-directed thrusting that culminated in the formation of the Rock Springs uplift as the Colorado Plateau rotated in a clockwise direction at the end of the Laramide orogeny. A significant amount of vertical reverse displacement would also have occurred at this time along the south-dipping shear zone, giving the arch its present-day structural relief. This interpretation is also consistent with uplift of the Uinta Mountains during the Eocene. Right-lateral transtensional motion is interpreted to have taken place after the Miocene as the Colorado Plateau rotated to the west relative to the more stable Wyoming province during Tertiary extensional events. Northwest translation of the Colorado Plateau during Cenozoic extension is also supported by structural and paleomagnetic studies. Most surface structures observed along the arch are related to this most recent period of wrenching.
Key Words: Cherokee Ridge arch • Cheyenne belt • Colorado Plateau • Laramide orogeny • wrench fault • Washakie Basin • Wyoming
A new genus and species of Stenomylinae (Camelidae, Artiodactyla) from the
Moonstone Formation (late Barstovianearly Hemphillian) of central Wyoming
Michael Cassiliano
Department of Geology and Geophysics, The University of Wyoming, Laramie,
Wyoming 82071, U.S.A.
email: mcassil@uwyo.edu
Wyomylus whitei is a new genus and species of stenomyline camel from the Moonstone Formation (late Barstovian–earliest Hemphillian, middle–late Miocene) of central Wyoming. W. whitei, the youngest known species of the Stenomylinae, is a small, gracile camel that is primitive in many of its characters for such a late-appearing representative. Among the diagnostic characters of W. whitei are: extreme reduction of P1–P3; reduction of P4–M1; reduction of p4–m1; small posterior heel on M3; extreme nasal retraction; large and very deep, but unpocketed anterior maxillary fossa; large, very deep and slightly pocketed preorbital fossa; anteromaxillary and preorbital fossae confluent via an elongate depression directly ventral to the premaxilla–maxilla suture; posteriorly placed orbits; small, but well-developed wing-shaped internal pterygoid processes; very narrow dorsal moiety of the supraoccipital; very weak lambdoidal crest that does not overhang the occiput; and partially fused metatarsals III and IV.
Key Words: Barstovian • Camelidae • Clarendonian •
Hemphillian • internal pterygoid processes • maxillary fossae • metapodials •
Miocene • Moonstone Formation • Stenomylinae • Wyoming • Wyomylus whitei
Grove Karl Gilbert, master of laccoliths and lakes
M. Dane Picard
Department of Geology and Geophysics, University of Utah, Salt Lake City, UT
84112, U.S.A.
email: mdane@mines.Utah.edu
Key Words: Basin and Range • biography • Henry Mountains • history of geology • Lake Bonneville • Utah