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F. C. W. Dodge

Publications and source records attributed to F. C. W. Dodge.

16 recordsLinked to original sources
USGS2012

Magnetic susceptibility and relation to initial 87Sr/86Sr for granitoids of the central Sierra Nevada, California

Measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the Mariposa 1° by 2° quadrangle, which crosses the central part of the Sierra Nevada batholith between 37° and 38°N latitude, shows that magnetic susceptibility values are above 10 −2 SI units in the east and central parts of the batholith and drop abruptly to less than 10 −3 SI units in the western foothills. In a narrow transitional zone, intermediate values (10 −3 to 10 −2 ) prevail. Magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values. Magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite, which depends, in turn, on the total iron content of the rocks and their oxidation ratio. Lower magnetic susceptibilities of felsic members of Sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content, but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks. The relatively unaltered condition of the samples and restriction of δ 18 O to the range of +7 to +10.3‰ indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios. Correlations of magnetic susceptibility with initial 87 Sr/ 86 Sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized. Reduction of Fe 3+ to Fe 2+ by organic carbon or other reducing substances may also have affected magnetic susceptibility.

Journal of Geophysical Research↗
USGS2012

Some additional observations on inclusions in the granitic rocks of the Sierra Nevada

Microgranular quartz diorite and diorite inclusions are widespread in central Sierra Nevada granitoid rocks and are almost exclusively restricted to hornblende-bearing rocks, most commonly felsic tonalites and mafic granodiorites. Strontium 87/strontium 86 values of the inclusions and host granitoids from individual plutons generally plot on single isochrons that agree closely with previously determined ages. Neodymium 143/neodymium 144 values of five inclusions ranged from 0.51229 to 0.51248 and yield ε Nd values that correlate closely with values of their respective hosts. The Nd-Sm and Rb-Sr systematics indicate that most inclusions were in isotopic equilibrium with enclosing materials at the time of formation. Silica contents of inclusions and granitoids are contiguous, but inclusions generally contain less than, and granitoids more than, 60% SiO 2 . Linear continuity of the femic oxide and H 2 O + trends relative to SiO 2 suggests many inclusions formed as concentrations of hydrous mafic minerals. Variation of other major element oxides and trace elements support this inference. The aforementioned data are generally inconsistent with a direct genetic link between the mafic inclusions and the mantle-derived basalt. However, many initial Sierra Nevada magmas were produced by mixing of mafic, mantle-derived melts and felsic crustal materials, resulting in isotopically varied tonalitic and granodioritic magmas. Most inclusions thus represent fragmented crystal accumulations of early-formed, near-liquidus minerals generated from these previously mixed magmas.

Journal of Geophysical Research Solid Earth↗
USGS1987

Oblique map of the northern Sierra Nevada, California, showing location of gold-bearing areas

More than a third of the gold produced by the United States was mined in California. The bulk of this gold was recovered from the western slope of the northern half of the Sierra Nevada between the Merced River in the south and the Feather River to the north, a distance of about 170 mi. Gold was first discovered, in this region, on the American River at Coloma in 1848, triggering the famed California gold rush. Mining was continuously active, somewhere in the area, from the discovery of gold until World War II when mining was legally prohibited. Dramatic increases in gold prices in the past decade coupled with recent advances in extractive techniques have revitalized prospecting, and major deposits are currently being explored and developed. Gold has been found in a variety of geologic environments in the region. In addition to production from the complex vein systems of the historically famous Mother Lode and associated East Gold Belt and West Gold Belt, large amounts of gold have also been recovered from the Grass Valley-Nevada City and other isolated lode districts and from Tertiary river channels and Quaternary alluvium. This oblique map illustrates the relation of the different gold-bearing environments to each other and to the general terrain of the northern Sierra Nevada. The map was derived from the 1970 U.S. Geological Survey 1:500,000-scale topographic map of California and the 1976 U.S. Geological Survey 1:500,000-scale topographic map of Nevada, using an isometrograph, a mechanical instrument that produces an oblique framework by tracing individual contours. Form lines sketched over this framework graphically portray the physiographic configuration of the region. Relief on the oblique map has a 3:1 vertical exaggeration and appears as if viewed from a 30° angle above the horizon. Locations of lode gold prospects and mines shown on the map were obtained from the U.S. Geological Survey's Mineral Resource Data System (MRDS), a computerized mineral-resource information file, and plotted in their respective locations (D.F. Huber, written commun., 1986). Some locations from two northern counties, missing from the MRDS retrival, were added. The twenty lode mines believed to be the most productive are cited in table 1. A total of nearly 4,000 sites, including both prospects and mines, were initially plotted, but about a third of those were obscured by topography on the oblique map. Locations of Tertiary river channels and gold-dredging fields were taken from published general references modified by examining specific sources and by cursory field examination. Seven of the major dredge fields are identified in table 2.

California↗
USGS1983

Isotopic studies of mariposite-bearing rocks from the south- central Mother Lode, California.

Gold-bearing vein formation in the Mother Lode belt of the study area apparently occurred during the Early Cretaceous between 127 and 108 m.y. B.P. The hydrothermal fluids that carried the gold precipitated quartz and mariposite at approx 320oC, similar to the T of precipitation of gold-bearing quartz veins in the Allegheny district. The O- and H-isotopic composition calculated for the fluid indicate that it was similar to formation water or was metamorphic in origin. If the carbonate in the veins was in isotopic equilibrium with this same fluid, it apparently precipitated at a higher T of approx 400oC. The Sr in the carbonate is much less radiogenic than that in any known marine carbonate, but is similar in isotopic composition to that in metamorphosed mafic volcanic rocks of the general region. These mafic rocks could have been the source for the Sr in the hydrothermal veins. This observation supports the contention that the gold-mariposite-quartz-carbonate rocks were formed as an alteration product of serpentinite and other mafic igneous rocks.-A.P.

California Geology↗
USGS1980

Late Cenozoic volcanic rocks of the southern Sierra Nevada, California: I. Geology and petrology: Summary

The geology and petrology of the Cenozoic volcanic rocks of the region of California between 38° and 35°45'N latitude and 117°30' and 120°W longitude, including the rocks of a major potassic magmatic province on the west slope of the Sierra Nevada, have been described in Part I (Moore and Dodge, 1980) of this study. The geochemical features of these rocks may provide clues to their origin and aid in comparing and contrasting the Sierra Nevada potassic province with other localized potassic provinces elsewhere in the world. Basaltic rocks occur in numerous small lava-flow remnants, dikes, and plugs that intrude the predominantly granitic terrain of the western slope of the southern Sierra Nevada. Leucite-bearing rocks are present at several localities in this western region. More voluminous basalt, commonly associated with rhyolite, is present east of the range in the Basin and Range province. However, leucite-bearing rocks have been found-at only one locality in this region. Most of the late Cenozoic volcanic rocks in the southern Sierra Nevada eastward through Owens Valley and in the extreme western Basin-Range occur in five rather distinctive areas—the San Joaquin-Kings, Kern, Big Pine, and Coso volcanic fields, and the Mono-Long Valley volcanic center (Fig. 1).

Geological Society of America Bulletin, Part I↗
USGS1978

Fusion of granodiorite by basalt, central Sierra Nevada

A trachybasalt plug, 100 m in diameter, has partially fused inclusions and wallrock of porphyritic granodiorite near Tuolumne Meadows, Yosemite National Park, Calif. Granodiorite surrounding the plug was altered within distances of about 3.5 m. Within this distance, (1) trace amounts of glass occur along fractures and grain boundaries, (2) biotite has been altered from dark olive to dark reddish brown and contains bands of fine reddish iron oxide grains, and (3) optic axial angles of potassium feldspar decrease toward the contact. As much as 20 volume percent of glass occurs in the granodiorite in a reentrant and in inclusions within the plug. Detailed study of two partially fused samples shows that, relative to unfused granodiorite, SiO 2 is clearly depleted in both samples, whereas K 2 O is depleted in one but not the other. Total iron, Al 2 O 3 , MgO, Na 2 O, and H 2 O show apparent increases in both samples; other constituents show no significant changes. Chemical analyses suggest that much original material was lost from the partially fused rocks, probably by mass migration of melt rather than by chemical diffusion. The composition of glasses now present in the partially fused rocks was dominated by melting of quartz and feldspars, whereas the composition of the early-formed "lost" melt was strongly influenced by subsolidus reaction and subsequent melting of biotite.

California↗
USGS1977

The Jabal Guyan ancient gold mine, Wadi Malahah quadrangle (sheet 18/43D), Kingdom of Saudi Arabia

Gold occurs in quartz veins and associated altered country rocks at the Jabal Guyan ancient mine in southern Saudi Arabia. The gold-bearing veins follow northwest-trending fault zones which cut the Precambrian metavolcanic rocks of the area. The veins are brecciated, and gold may have been remobilized during cataclasis. A second generation of unbrecciated, barren quartz cements the earlier quartz and also occurs in north-northeast-trending veins. Gold values are greater in waste in the mine area than in vein quartz remaining in the ancient workings, and it is believed that considerable gold was extracted from high-grade ore by the ancient miners. Exploration drilling to determine if veins and alteration zones persist at depth at the Jabal Guyan mine and examination of other ancient mines in the region are recommended.

Wadi Malahah quadrangle↗
USGS1975

Mineral deposits in western Saudi Arabia; a preliminary report

Mineral deposits in Saudi Arabia include a variety of deposits which were formed in many geologic environments. These include magmatic and late magmatic deposits in igneous masses, contact metamorphic deposits along the margins of igneous bodies, and stratiform sulfide deposits and veins. Notable deposits of sedimentary origin include deposits of iron oxides and phosphate. Strata-bound massive sulfide deposits containing copper, zinc, and nickel sulfides associated with pyrite and pyrrhotite in complexly deformed volcanic rocks in Saudi Arabia have been the subject of controversy. They are considered by some geologists to be of syngenetic or volcanogenic origin, and by others to be epigenetic. The principal mineral deposits in western Saudi Arabia are localized in mineral belts within tectonic zones that trend northerly, northeasterly, and northwesterly. These belts contain the exploration target areas most promising for future exploration. Noteworthy among these are: 1) the northerly-trending Bidah copper-zinc belt; 2) the Sayid copper-zinc zone; 3) the northwest-trending Nuqrah copper zinc-silver belt; and 4) the north-northwesterly-trending Al Amar belt. Gold and silver deposits are mainly localized in north-trending structural features of the Hijaz geotectonic cycle and northwest Najd trends. Saudi Arabian metal deposits known thus far contain reserves in the 2 billion dollar range. Further exploration should result in discoveries that could significantly increase this amount; Saudi Arabia may well have workable metallic and nonmetallic mineral resources for industrial developments in many parts of the Kingdom. In order to effectively carry on a search for new mineral deposits, the belts should be mapped in detail, with emphasis on the delineation of stratigraphic and structural features that control metallization. In addition, geochemical and geophysical studies should be made of promising areas to outline exploration targets. These targets could then be systematically explored.

Open-File Report↗
USGS1970

Radiogenic heat production of contrasting magma series: Bearing on interpretation of heat flow

Variation in radiogenic heat production of rocks of diverse magma series (representative of calcic, calc-alkalic, alkali-calcic, and alkalic petrographic provinces on a worldwide basis) is better correlated with some form of magmatic differentiation index, rather than simply with potassium content alone as commonly supposed. The “lime-alkali” (Peacock) index generally gives the best correlation of the various indices tested. Plots of heat production for igneous suites versus indices of differentiation (“heat production trends”) demonstrate systematic correlations of heat productivity as functions of fractionation trends and/or spatial-temporal associations. In general, heat production trends of calcic suites tend to indicate lower radiogenic heat productivity than those of more alkalic suites for a given silica or potassium content. Because many recent measurements of heat flow in plutons can be linearly related to heat production of surface rocks, these “heat production trends” have important bearing on the interpretation of crustal heat flow. Our data suggest possible alternative interpretations of the heat production-heat flow pattern for the Sierra Nevada which differ from that in Lachen-bruch's (1968) preliminary geothermal model.

GSA Bulletin↗
USGS1970

Variations of major chemical constituents across the central Sierra Nevada batholith

A study of 193 chemical analyses of plutonic rocks from 132 localities in the central Sierra Nevada shows convincingly that K 2 O decreases systematically westward and suggests that Fe 2 O 3 and TiO 2 may also decrease westward and that FeO, MgO, and CaO may increase. The ratio K 2 O/SiO 2 obviously decreases westward across six of eight provisionally established sequences of granitic rocks. Plots of analyses of rocks from each sequence form discrete fields that are strongly elongate toward zero K 2 O at 40 to 45 percent SiO 2 . The boundaries between fields on these plots and between fields on plots of normative minerals on triangular diagrams are sharp. Compositional trends within sequences are different than the compositional changes that take place across the batholith—rocks in the western Sierra Nevada probably are not compositionally identical with rocks that are present at depth beneath the eastern Sierra Nevada. Progressive decrease of K 2 O in the Paleozoic and Mesozoic country rocks westward across the batholith is consistent with the anatectic model for its origin. However, it also is consistent with the hypothesis developed to explain chemical patterns in volcanic island arcs—that K 2 O increases toward continental land masses because of increasing depth of magma generation along landward-dipping seismic (Benioff) zones. The seismic-zone hypothesis encounters several difficulties, but it cannot be ruled out.

California↗
USGS1969

Biotites from granitic rocks of the central Sierra Nevada batholith, California

Biotites from plutonic rocks of the central Sierra Nevada and Inyo Mountains, California, have been examined and characterized by powder X-ray diffraction and optical and chemical methods. Compositions of the biotites define a trend in the compositional triangle Fe+3 Fe+2Mg. When related to the experimentally studied ternary system KFe 3 +3AlSisO 12 H- 1 -KFe 3 +2 AlSi 3 O 10 (OH) 2 -KMg 3 AlSi 3 O 10 (OH) 2 and coupled with the estimated positions of biotite solid solutions for different oxygen buffers, the trend suggests that oxygen fugacities in magmas during biotite crystallization were slightly higher than those defined by the Ni-NiO buffer. The compositional data also suggest that magmas were ‘buffered’ with respect to oxygen by oxides existing within the magmas themselves. Correlation between the Fe/(Fe+Mg) ratio, an inferred temperature indicator, and other elements is generally poor, which suggests that factors other than temperature at the time of crystallization exerted an important influence on compositions.

California↗
USGS1969

Fission-track ages of accessory minerals from granitic rocks of the central Sierra Nevada batholith, California

Ages of apatite, sphene, allanite, epidote, and garnet from plutonic rocks of the central Sierra Nevada and Inyo Mountains have been determined by the fission-track method. Ages of 44 specimens of apatite range from 54 to 128 m.y. Oldest apatites generally occur in rocks from the western portion of the batholith; youngest are from granitic rocks along the eastern slope of the Sierra Nevada. Thirty-four specimens of sphene have been dated and range in age from 71 to 118 m.y. Within experimental error, all sphenes are as old as, or older than, coexisting apatites. The oldest sphene is from granitic rock of the Inyo Mountains; however, sampled rocks from the western Sierra Nevada do not contain sphene. The youngest sphenes are generally from rocks slightly west of the Sierran crest.

California↗
USGS1968

Hornblendes from granitic rocks of the central Sierra Nevada batholith, California

Twenty samples of hornblendes from rocks of 14 plutonic units in the central Sierra Nevada and Inyo Mountains, California, have been studied in detail. Optical, density, single-crystal and powder X-ray diffraction, and major and minor element chemical data are reported.The compositions of the hornblendes show only limited correlation with the chemistry of the rocks in which they occurred. Hornblendes from granitic rocks of the eastern Sierra Nevada and Inyo Mountains have a wide range of tetrahedral aluminum content which is often as low as three-quarters of an atom per formula unit, whereas hornblendes from younger granitic rocks elsewhere in the Sierra Nevada batholith contain more than one atom of tetrahedral aluminum per formula unit. Because an increase of aluminum in tetrahedral co-ordination is considered indicative of higher temperatures of crystallization, the observed differences in the hornblendes suggest that older plutonic rocks of the batholith may have been metamorphosed regionally or may have been affected by widespread hydrothermal action prior to consolidation of later granitic rocks.

California↗