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E.H. McKee

Publications and source records attributed to E.H. McKee.

At least 19 recordsLinked to original sources
USGS2012

Peralkaline ash flow tuffs and calderas of the McDermitt Volcanic Field, southeast Oregon and north central Nevada

The McDermitt volcanic field covers an area of 20,000 km 2 in southeastern Oregon and northwestern Nevada and consists of seven large-volume ash flow sheets that vented from 16.1 to 15 Ma ago. The volcanic field is characterized by peralkaline, high-silica rhyolite, and all but one of the sheets are comendites. The ash flow field overlies a 1.5-km-thick section of Miocene Steens volcanic series and Steens Basalt, indicating a minimum age of 16.1 Ma for the Steens Basalt. The Orevada rift, a 200-km-long NNW-striking structure, developed just prior to eruption of the mafic lavas. Each ash flow sheet resulted in the formation of a large collapse caldera along the Orevada rift. The center of the volcanic field is dominated by four overlapping and nested calderas; satellitic to these are three additional calderas. The areas adjacent to the incipient calderas subsided during ash flow eruption. Thickening of the ash flow sheets, monoclinal warping outside the caldera ring fault, and tilting in toward the caldera of blocks bounded by curvilinear faults all indicate regional subsidence prior to caldera collapse. The McDermitt caldera complex is highly mineralized; it contains ore deposits of Hg, Sb, Cs, Li, and U. The peralkaline tuffs have high contents of these elements and were the source rocks from which metals were leached by hydrothermal systems developed during the last stage of caldera-related volcanism.

Journal of Geophysical Research Solid Earth↗
USGS2000

Geochemistry and geodynamics of a Late Cretaceous bimodal volcanic association from the southern part of the Pannonian Basin in Slavonija (Northern Croatia)

In this paper we present petrological and geochemical information on a bimodal basalt-rhyolite suite associated with A-type granites of Late Cretaceous age from the South Pannonian Basin in Slavonija (Croatia). Basalts and alkali-feldspar rhyolites, associated in some places with ignimbrites, occur in volcanic bodies that are interlayered with pyroclastic and fossiliferous Upper Cretaceus sedimentary rocks. The petrology and geochemistry of the basalts and alkali-feldspar rhyolites are constrained by microprobe analyses, major and trace element analyses including REE, and radiogenic and stable isotope data. Basalts that are mostly transformed into metabasalts (mainly spilites), are alkalic to subalkalic and their geochemical signatures, particularly trace element and REE patterns, are similar to recent back-arc basalts. Alkali-feldspar rhyolites have similar geochemical features to the associated cogenetic A-type granites, as shown by their large variation of Na 2 O and K 2 O (total 8–9%), very low MgO and CaO, and very high Zr contents ranging between 710 and 149 ppm. Geochemical data indicate an amphibole lherzolite source within a metasomatized upper mantle wedge, with the influence of upper mantle diapir with MORB signatures and continental crust contamination. Sr incorporated in the primary basalt melt had an initial 87 Sr/ 86 Sr ratio of 0.7039 indicating an upper mantle origin, whereas the 87 Sr/ 86 Sr ratio for the alkali-feldspar rhyolites and associated A-type granites is 0.7073 indicating an apparent continental crust origin. However, some other geochemical data favour the idea that they might have mainly originated by fractionation of primary mafic melt coupled with contamination of continental crust. Only one rhyolite sample appears to be the product of melting of continental crust. Geological and geodynamic data indicate that the basalt-rhyolite association was probably related to Alpine subduction processes in the Dinaridic Tethys which can be correlated with recent back-arc basins. The difference in geological and isotope ages between the bimodal basalt-rhyolite volcanism with A-type granite plutonism (72 Ma) and the final synkinematic S-type granite plutonism (48 Ma) can be taken as a lifetime of the presumed BARB system of the Dinaridic Tethys. Remnants of this presumed subduction zone can be traced for 300 km along the surrounding northernmost Dinarides.

Pannonian Basin↗
USGS1999

Inversion of gravity data to define the pre-Tertiary surface and regional structures possibly influencing ground-water flow in the Pahute Mesa-Oasis Valley region, Nye County, Nevada

A three-dimensional inversion of gravity data from the Pahute Mesa–Oasis Valley region reveals a topographically complex pre-Tertiary basement surface. Beneath Pahute Mesa, the thickness of the Tertiary volcanic deposits may exceed 5 km within the Silent Canyon caldera complex. South of Pahute Mesa in Oasis Valley, basement is shallower (< 1 km) but between this valley and the Timber Mountain caldera complex is a basin that probably represents, in part, a moat related to the Timber Mountain caldera complex. Of particular interest is a NE-trending lineament, named here the Thirsty Canyon lineament (TCL), separating terranes at significantly different elevations. Southeast of the TCL, a highly undulating basement surface descends deeply into several calderas, whereas NW of the TCL basement is relatively flat and shallow. Because as many as four calderas seem to abruptly terminate at the TCL, the TCL may reflect a major buried fault zone, which influenced caldera growth. This inferred Thirsty Canyon fault zone and several EW basement ridges in the derived 3-dimensional basin thickness model may influence the flow of ground water from the Pahute Mesa region to Oasis Valley.

Nevada↗
USGS1997

Sedimentary carbonate-hosted giant Bayan Obo REE-Fe-Nb ore deposit of Inner Mongolia, China; a cornerstone example for giant polymetallic ore deposits of hydrothermal origin

Detailed, integrative field and laboratory studies of the textures, structures, chemical characteristics, and isotopically determined ages and signatures of mineralization of the Bayan Obo deposit provided evidence for the origin and characteristics favorable for its formation and parameters necessary for defining giant polymetallic deposits of hydrothermal origin. Bayan Obo is an epigenetic, metasomatic, hydrothermal rare earth element (REE)-Fe-Nb ore deposit that is hosted in the metasedimentary H8 dolostone marble of the Middle Proterozoic Bayan Obo Group. The metasedimentary sequence was deposited on the northern continental slope of the North China craton. The mine area is about 100 km south of the suture marking Caledonian subduction of the Mongolian oceanic plate from the north beneath the North China craton. The mineralogy of the deposit is very complex, consisting of more than 120 different minerals, some of which are epigenetic minerals introduced by hydrothermal solutions, and some of which are primary and secondary metamorphic minerals. The major REE minerals are monazite and bastnaesite, whereas magnetite and hematite are the dominant Fe-ore minerals, and columbite is the most abundant Nb mineral. Dolomite, alkali amphibole, fluorite, barite, aegirine augite, apatite, phlogopite, albite, and microcline are the most widespread gangue minerals. Three general types of ores occur at Bayan Obo: disseminated, banded, and massive ores. Broad zoning of these ore types occurs in the Main and East Orebodies. Disseminated ores are in the outermost zone, banded ores are in the intermediate zone, and massive ores are in the cores of the orebodies. On the basis of field relations, host rocks, textures, structures, and mineral assemblages, many varieties of these three types of ores have been recognized and mapped. Isotopic dating of monazite, bastnaesite, aeschynite, and metamorphic and metasomatic alkali amphiboles associated with the deposit provides constraints on the ages of mineralization and the history of the deposit. Textural relations, differences in chemical composition, and 232Th/208Pb internal isochron ages of monazite and bastnaesite samples indicate that many episodes of REE mineralization occurred at Bayan Obo, ranging from about 555 Ma to about 398 Ma. Initial 208Pb/204Pb ratios suggest different sources of REE's for different generations of REE minerals. Relative ages of Fe mineralization were deduced from textural relationships of Fe minerals with other, dated mineral phases in the deposit. Most Nb mineralization was in the area of the West Orebodies and resulted in disseminated ore. Aeschynite, an early stage of Nb mineralization (438+-25.1 Ma), occurs with huanghoite and alkali amphiboles in veins. The 40Ar/39Ar ages of amphiboles, as well as petrographic textures, were used to distinguish three periods of regional metamorphism in the Bayan Obo mine area: (1) Late Proterozoic, about 890 Ma, which recrystallized H8 carbonate to marble and crystallized lineated alkali amphiboles along foliation planes in the marble; (2) Caledonian, about 425-395 Ma, which resulted in metamorphic and metasomatic-metamorphic alkali amphiboles; and (3) Hercynian, about 300 Ma, based on biotite 40Ar/39Ar analyses from biotite schist and folded banded ores. The 40Ar/39Ar ages of metasomatic alkali amphiboles also place time constraints on the hydrothermal history of the ore deposit. Metasomatic amphiboles represent periods of intense hydrothermal activity, which began as early as 1.26 Ga; that date is based on the age of amphibole from a vein that crosscuts the H6 quartzite that underlies the H8 dolostone marble. Although much of the metasomatic amphibole formed during periods that overlapped the peak period of REE mineralization of banded ores, REE and alkali amphibole phases generally occur in different mineral assemblages or are of very different ages in the same assemblage and, therefore, may have been derived from

Bulletin↗
USGS1996

Age of supergene oxidation and enrichment in the Chilean porphyry copper province

Twenty-five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees S in northern Chile yield K/Ar ages ranging from about 34 to 14 Ma. Therefore supergene oxidation and enrichment processes were active from the early Oligocene to the middle Miocene, a minimum of 20 m.y. Supergene activity at individual deposits lasted for at least 0.4 to 6.2 m.y. The early Oligocene supergene activity affected deposits in the Paleocene porphyry copper belt, whereas early and middle Miocene supergene processes are documented in the Early Cretaceous, Paleocene, and late Eocene to early Oligocene porphyry, copper belts. Middle Miocene oxidation also affected the oldest epithermal gold-silver deposits in the Maricunga belt farther east. Supergene activity commenced no less than 11 m.y. after generation of each porphyry copper deposit because of the time required to unroof the copper-bearing parts of the system. Supergene activity throughout northern Chile ceased at approximately 14 Ma. The geologic features of deposits and prospects and their morphotectonic positions, present latitudes, and present elevations display no obvious correlations with the supergene chronology. Exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels (+ or - ignimbrites) of Oligocene through middle Miocene age unless their accumulation is demonstrably late in the documented history of supergene activity, or in porphyry copper provinces, such as those of central Chile and northwestern Argentina, which formed after approximately 14 Ma. The uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern Chile and contiguous areas unless their effects were much more prolonged. Intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern Chile, and tectonic uplift was its most probable cause. However, more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role.

Economic Geology↗
USGS1996

The age and thermal history of Cerro Rico de Potosi, Bolivia

Cerro Rico de Potosi, Bolivia, is the world's largest silver deposit and has been mined since the sixteenth century for silver, and for tin and zinc during the twentieth century, together with by-product copper and lead. The deposit consists primarily of veins that cut an altered igneous body that we interpret to be a dacitic volcanic dome and its underlying tuff ring and explosion breccia. The deposit is compositionally and thermally zoned, having a core of cassiterite, wolframite, bismuthinite, and arsenopyrite surrounded by a peripheral, lower-temperature mineral assemblage consisting principally of sphalerite, galena, lead sulfosalt, and silver minerals. The low-temperature assemblage also was superimposed on the high-temperature assemblage in response to cooling of the main hydrothermal system. Both the dacite dome and the ore fluids were derived from a larger magmatic/hydrothermal source at depth. The dome was repeatedly fractured by recurrent movement on the fault system that guided its initial emplacement. The dome was extruded at 13.8 ± 0.2 Ma (2σ), based on U-Th-Pb dating of zircon. Mineralization and alteration occurred within about 0.3 my of dome emplacement, as indicated by a 40 Ar/ 39 Ar date of 13.76 ± 0.10Ma (1σ) for sericite from the pervasive quartz-sericite-pyrite alteration associated with the main-stage, high-temperature, mineralization. The last thermal event able to reset zircon fission tracks occurred no later than 12.5 ± 1.1 Ma (1σ), as indicated by fission-tract dating. Minor sericite, and magmatic-steam alunite veins, were episodically formed around 11 Ma and between 8.3 and 5.7 Ma; the younger episodes occurring at the time of extensional fracturing at Cerro Rico and wide-spread volcanism in the adjacent Los Frailes volcanic field. None of these younger events appear to be significant thermal/mineralizing events; the exceptionally flat thermal release pattern of 39 Ar from sericite and the results of the fission-tract dating of zircon show that none of the younger events was hot enough, and lasted long enough, to cause significant loss of Ar or annealing of zircon fission tracks. U-Th-Pb dating of zircon cores indicates a Precambrian progenitor for some zircons, and REE analyses of dated samples of hydrothermally altered dacite show the presence of a prominent positive Eu anomaly, which constrains interpretations of the origin and evolution of the magmatic/hydrothermal system.

Cerro Rico de Potosi↗
USGS1996

A tale of 10 plutons - Revisited: Age of granitic rocks in the White Mountains, California and Nevada

40Ar/39Ar incremental heating analysis and conventional K-Ar age determinations on plutonic rocks of the White Mountains define two stages of magmatic emplacement: Late Cretaceous, between ca. 90 Ma and 75 Ma, and Middle-Late Jurassic, between ca. 180 and 140 Ma. The Jurassic stage can be divided into two substages, 180-165 Ma and 150-140 Ma. Thermal effects of the younger plutons on the older granitoids partially to completely reset ages, making it difficult to determine the age of emplacement and cooling of several of the plutons even by 40Ar/39Ar incremental heating analyses. New data together with published ages and regional geochronological synthesis of the Sierra Nevada batholith indicate that regions within the batholith have coherent periods or episodes of magmatic activity. In the White Mountains and Sierra Nevada directly to the west there was little or no activity in Early Jurassic and Early Cretaceous time; magmatism took place during relatively short intervals of 15 m.y. or less in the Middle and Late Jurassic and Late Cretaceous periods. The new K-Ar and 40Ar/39Ar analyses of granitoids from the White Mountains help, but do not completely clarify the complex history of emplacement, cooling, and reheating of the batholith.

California, Nevada↗
USGS1994

Possible Mesozoic age of Ellenville Zn-Pb-Cu(Ag) deposit, Shawangunk Mountains, New York

Ore textures, epithermal open-space filling of Permian structures of the Alleghanian orogeny, and largely postorogenic mineralization of the Ellenville, New York, composite Zn-Pb-Cu(Ag) vein system, provide permissive evidence for post-Permian mineralization. Isochron ages determined by 40Ar/39Ar laser-fusion techniques for K-bearing liquid inclusions in main-stage quartz from the Ellenville deposit additionally suggest a Mesozoic time of mineralization, associated with extensional formation of the Newark basin. The best 40Ar/39Ar total-fusion age range is 165 ?? 30 to 193 ?? 35 Ma. The Mesozoic 40Ar/39Ar age agrees with that of many other dated northern Appalachian Zn-Pb-Cu(Ag) deposits with near-matching lead isotope ratios, and adds new evidence of Jurassic tectonism and mineralization as an overprint to Late Paleozoic tectonism at least as far north as Ellenville (lat. 41??43???N). ?? 1994 Springer-Verlag.

Mineralium Deposita↗
USGS1994

Age and Sr isotopic composition of volcanic rocks in the Maricunga Belt, Chile: Implications for magma sources

K-Ar and 40 Ar/ 39 Ar dating of volcanic rocks from the Maricunga belt of north-central Chile indicate that igneous activity took place throughout most of Miocene time at various places in the 150 by 30 km belt. No migration patterns of volcanism appear in the Miocene rocks of the belt. Volcanic activity ceased by the end of the Miocene. All the Miocene volcanic rocks studied are calcic andesites to dacites with about 62% SiO 2 , 18% A1 2 O 3 , 4% Fe 2 O 3 (total), and 2% K 2 O. Initial 87 Sr/ 86 Sr (Sr i ) values fall into two groups, one of lower values around 0.7050 to the west and the other of higher values around 0.7060 to the east. It is postulated that the two Sr i groups reflect two adjoining coherent lower-crustal magma sources of possibly different age and subtly different composition that form part of the western edge of the South American craton.

Journal of South American Earth Sciences↗
USGS1994

The northern Nevada rift: Regional tectono-magmatic relations and Middle Miocene stress direction

As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500 km from southern Nevada to the Oregon Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that, based on new radiometric dating, erupted between 17 and 14 Ma and probably during an even shorter time interval. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space (at its northern terminus) with the oldest silicic caldera complex along the Yellowstone hot-spot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault.

Geological Society of America Bulletin↗
USGS1993

Geochronology of the Sleeper deposit, Humboldt County, Nevada: Epithermal gold-silver mineralization following emplacement of a silicic flow-dome complex

The high-grade gold-silver deposits at the Sleeper mine are low sulfidation, quartz-adularia-type epithermal deposits, formed during the final stages of igneous hydrothermal activity of a small middle Miocene silicic flow-dome complex in north-central Nevada. Potassiumargon and 40 Ar/ 39 Ar ages of alkali feldspar indicate that the rhyolitic flow-dome complex was emplaced before about 16.5 Ma; hydrothermal alteration and mineralization followed, lasting from about 16.2 to 14+ Ma based on ages of adularia associated with electrum. There were multiple pulses of alteration and mineralization but all occurred within a period of less than 2 m.y. Later supergene alteration formed opal and alunite about 5.4 Ma but produced no Au or Ag mineralization other than some remobilization to produce locally rich pockets of secondary Au and Ag enrichment and is unrelated to the older magmatic hydrothermal system. The Sleeper deposit in the northern part of the Great Basin is genetically related to bimodal volcanism that followed a long period of arc-related andesitic volcanism in the same general region.

Economic Geology↗