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H. H. Mehnert

Publications and source records attributed to H. H. Mehnert.

At least 19 recordsLinked to original sources
USGS2012

Oligocene basaltic volcanism of the northern Rio Grande Rift: San Luis Hills, Colorado

The inception of the Rio Grande rift in northern New Mexico and southern Colorado was accompanied by voluminous mafic volcanism preserved in part as erosional remnants on an intrarift horst within the current axial rift graben of the San Luis Valley. Oligocene (∼26 Ma) volcanic rocks of the Hinsdale Formation at San Luis Hills range from 49 to 57 wt % SiO 2 and include nepheline and hypersthene normative lavas. A mildly alkalic series consisting of trachybasalt, basaltic trachyandesite, and trachyandesite is volumetrically dominant, olivine tholeiites are subordinate, and xenocrystic trachyandesites containing abundant quartz and plagioclase xenocrysts occur only locally. Relative to the San Luis Hills olivine tholeiites which have La/Sm n ∼ 2, the more alkaline series are enriched in light rare earth elements (LREE) and have La/Sm ratios that increase in the trachybasalt-basaltic trachyandesite suite (La/Sm n ∼ 3) to xenocrystic trachyandesites that are the most LREE enriched (La/Sm n ∼ 4). Chondrite-normalized, trace element patterns for the lavas in the San Luis Hills are similar in shape within the mildly alkaline to transitional series; they have characteristic Nb and Ta depletions and high K and Th relative to Ta, Nb, and LREE. Major and trace element constraints support a petrogenetic model of fractionation plus lower crustal assimilation for petrologic suites within the San Luis Hills rocks, although the model cannot relate lavas for the entire series to a common parent. Most mafic lavas of the San Luis Hills were evolved (Mg # <60) and contaminated by LREE-enriched silicic partial melts of granulitic lower crust depleted in Rb, Th, and U. Pb isotopes are the most sensitive indicators of crustal contamination, whereas shifts in Nd and Sr isotope ratios are associated with large amounts of assimilation. However, relatively noncontaminated lavas can be identified and indicate at least two mantle source regions were involved.

Journal of Geophysical Research↗
USGS1987

Changes in the H O Ar isotope composition of clays during retrograde alteration

K-Ar ages of illite alteration associated with Middle Proterozoic Athabasca unconformity-type U deposits in Saskatchewan range from 414 to 1493 Ma. The K-Ar ages correlate with water contents and δD values such that illites with young K-Ar ages have δD values as low as −169 and water contents as high as 7.7 wt.% whereas illites with older ages have δD values near −70 and water contents near 4 wt.%. Water extracted at 400°C from illites with low δD values and high water contents has low δD and δ 18 O values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration. The alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the K-Ar ages and introduction of excess structural water in the form of interlamellar water. The interlamellar water is enriched in 18 O by about 7 per mil relative to the water that physically surrounded the clay particle. Further alteration decreases the δD value and increases the δ 18 O value of the illite by isotopic exchange between the mineral and the interlamellar water. Although the chemical compositions and XRD patterns of the altered illites indicate that no detectable smectite component is present in the samples, the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed-layer illite/smectite by retrograde alteration of pure illite. The wide variation of δD values of chlorite and kaolinite from these U deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial. The general association of altered clays with areas containing the highest concentrations of U is probably related to localized permeability within the ore zone.

Geochimica et Cosmochimica Acta↗
USGS1986

Ages of igneous and hydrothermal events in the Round Mountain and Manhattan gold districts, Nye County, Nevada

About 60 new isotopic age determinations help clarify the geologic history of the Round Mountain and Manhattan gold districts, Nevada. Cretaceous granite of Shoshone Mountain, southeast of Round Mountain, was emplaced about 95 m.y. ago and was domed, metamorphosed, and mineralized with tungsten-bearing quartz veins 80 m.y. ago. The granite of Pipe Spring, south of Manhattan, was emplaced probably at about 80 m.y. ago. Mineralization associated with the Pipe Spring body occurred about 75 m.y. ago. A granodiorite stock and associated rhyolite and andesitc dikes intruded the granite southeast of Round Mountain 36 m.y. ago; apparently base and precious metals mineralization accompanied their intrusion, but mineralization has not been dated isotopically. Ash-flow tuff at Round Mountain was emplaced about 26 m.y. ago, the Manhattan caldera formed about 25 m.y. ago, and gold mineralization at Round Mountain occurred at about 25 m.y. ago. Gold mineralization at Manhattan took place 16 m.y. ago.The isotopic data allow interpretation of the cooling histories of some of the igneous rocks and suggest possible thermal events that reset mineral ages.

Economic Geology↗
USGS1984

Origins and exploration significance of replacement and vein-type alunite deposits in the Marysvale volcanic field, west central Utah

Alunite deposits formed 23 m.y. ago in near-surface, highly oxidizing conditions at the tops of hydrothermal plumes that were spaced at 3- to 4-km intervals around a monzonite stock. The delta 34 S values of 11.5 to 15.4 per mil for replacement alunite along with geologic constraints indicate that sulfate sulfur was derived from underlying Mesozoic evaporites. The delta 34 S values of -15.3 to +5.1 per mil of underlying pyrite, however, indicate that reduced sulfur, necessary to produce low pH during oxidation, either was produced by partial reduction of the evaporite sulfate or came from another source. Vein-type alunite deposits formed 14 m.y. ago as open-space fillings in extension fractures above a concealed stock. Crystals of alunite grew inward from the walls, forming veins of nearly pure alunite as much as 20 m thick. The delta 34 S values near zero per mil indicate that the vein-type alunite sulfur probably had a magmatic source, and it appears that the sulfur may have been present as SO 2 shortly after degassing from the magma. The probable magmatic origin for sulfur in the 14-m.y.-old vein-type alunite and its deposition from vapor-rich fluids, as well as the probable magmatic origin for sulfur in the surrounding base and precious metal deposits, suggest that the underlying stock may host porphyry-type deposits.

Economic Geology↗
USGS1981

Origin and structural implications of upper Miocene rhyolites in Kingston Canyon, Piute County, Utah

Kingston Canyon is one of the deepest antecedent canyons in the High Plateaus subprovince of the Colorado Plateaus. Here the East Fork of the Sevier River flows westward transversely across the gently east tilted Sevier Plateau, which is developed on a basin-range fault block uplifted more than 1,500 m along the Sevier fault zone on the west. Upper Tertiary rhyolites, uncommon in southwestern Utah, occur both on the northern rim and in the bottom of Kingston Canyon. Those on the northern rim consist of lava flows and volcanic domes of the rhyolite of Forshea Mountain, dated by K-Ar methods at 7.6 m.y. old. Those in the bottom of Kingston Canyon, the rhyolite of Phonolite Hill, are especially well exposed and provide spectacular examples of a pyroclastic cone whose base is about at river level and a steep-sided volcanic dome emplaced into and through these deposits. The pyroclastic deposits, formerly 500 or more metres thick, consist of airfall, mudflow, and ash-flow(?) material of rhyolite and foreign lithic fragments, especially olivine basalt. The dome consists of flow-banded, mostly devitrified rhyolite as much as 500 m thick; it has been dated by K-Ar methods at 5.4 m.y. In addition to the rhyolites, a dome and lava-flow complex, the rhyodacite of Dry Lake, occurs near the northern rim and is considered to postdate the rhyolite of Forshea Mountain and predate the rhyolite of Phonolite Hill. The rhyolite of Forshea Mountain was deposited near basin-range faults, before the uplift of the Sevier Plateau and before the cutting of Kingston Canyon. Before uplift, a river flowed across the site of the present Sevier Plateau toward the east-southeast and perhaps also across the Awapa and Aquarius Plateaus to the east. The rhyodacite of Dry Lake was deposited during uplift and perhaps before canyon cutting. During uplift, the river maintained itself and cut Kingston Canyon. The rhyolite of Phonolite Hill was deposited in this canyon, blocking the river flow, which probably formed new outlets to the east. The Awapa and Aquarius Plateaus later were uplifted along faults, disrupting the eastern part of the river segment. The topography then took on its present appearance, and drainage was re-established through Kingston Canyon. There has been little deepening since the reopening of Kingston Canyon.

Utah↗
USGS1978

Pleistocene rhyolite of the Mineral Mountains, Utah: Geothermal and archeological significance

Little-eroded rhyolitic tuffs, flows, and domes extend over about 25 km 2 along the western side of the Mineral Mountains, southwestern Utah, which is along the eastern edge of the Roosevelt KGBA (Known Geothermal Resource Area). Initial eruptions resulted in two low-viscosity lava flows of nonporphyritic rhyolite. These were followed by bedded pumice falls and nonwelded ash flows. The youngest activity produced at least nine viscous domes and small lava flows of rhyolite that contain 1-5 percent phenocrysts of quartz, plagioclase, sodic sanidine, and biotite; distinction between domes and eroded flow segments locally is difficult. Potassium-argon ages indicate that all the rhyolite of the Mineral Mountains was erupted between 0.8 and 0.5 m.y. ago. The rhyolite rests on dissected granite of the Mineral Mountains pluton, the largest intrusion in Utah, which has yielded published K-Ar ages of 9 and 15 m.y. A small older dissected rhyolite dome, about 8 m.y. old, occurs just west of the range front. Whether the young ages of the pluton represent time of intrusion or of later reheating, they, in conjunction with the Pleistocene rhyolite in the Mineral Mountains, do indicate a major late Cenozoic thermal anomaly, the size and age of which is significant to evaluation of the Roosevelt KGRA. The rhyolite is also the only known source of implement-grade obsidian in the southwest between eastern California and northern New Mexico.

Utah↗