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J. F. Whelan

Publications and source records attributed to J. F. Whelan.

23 records · Page 2Linked to original sources
USGS1988

Stable isotope geochemistry of sphalerite and other mineral matter in coal beds of the Illinois and Forest City basins

Cleat and clastic dikes of Middle Pennsylvanian-age coal beds of the Illinois and Forest City basins of Illinois, Iowa, Missouri, and Kansas locally contain appreciable amounts of sphalerite within a kaolinite-pyrite-sphalerite (+ or - pyrite)-calcite paragenetic sequence. The sphalerite and associated minerals are of interest as a partial record of the history of fluids in the sedimentary basin and as possible indicators of Mississippi Valley-type mineralization. Moreover, zinc from the sphalerite may represent an exploitable by-product of coal mining and combustion.Pyrite is abundant in these coals both as fine-grained disseminated framboids that formed during early diagenesis and as coarser grained cleat fillings, etc., that formed much later. The delta 34 S values of this later pyrite, the sphalerite, and the coal organic matter range from -12 to 19 per mil. Within individual samples, however, pyrite, sphalerite, and organic matter sulfur delta 34 S values are nearly identical. This similarity strongly suggests related origins. The delta 34 S values of pyrite, sphalerite, and organic matter from coals of the Illinois basin and the central and southern Forest City basin range from -12 to 0 per mil. This range is not unique to the sphalerite-bearing coals; pyrite and organic matter from sphalerite-free high sulfur coals have similar delta 34 S values (Price and Shieh, 1979). We believe that sulfur produced by the breakdown of organosulfur compounds in the coal is incorporated into the epigenetic sulfides. In the northern Forest City basin, however, many samples have delta 34 S values between 8 and 19 per mil. These are similar to the delta 34 S values of sphalerite and pyrite from the Upper Mississippi Valley Zn-Pb district which lies northeast of the Forest City basin and north of the Illinois basin. This similarity suggests input from a Mississippi Valley-type hydrothermal system.Cleat kaolinite has delta D values of -36 to -24 per mil and delta 18 O values of 19.2 to 20.2 per mil. Assuming that clearing postdated the lignite rank of coalification implies that kaolinite was deposited between 40 degrees and 100 degrees C. At the lower temperatures, the kaolinite fluid would have had an isotopic composition similar to seawater; at the higher temperatures, it would have had a composition similar to the sphalerite-depositing fluid. Sphalerite was deposited from a fluid that was chemically and isotopically similar to the fluids responsible for the main-stage ores of the Upper Mississippi Valley Zn-Pb district (75 degrees -113 degrees C, approximately 21 equiv wt % NaCl, delta D = -65 to -3ppm, and delta 18 O = -0.9-9.9ppm). The isotopic compositions (delta 13 C = -12.4 to -2.7ppm and delta 18 O = 21.1-25.8ppm) and paragenetic positions of the cleat calcite samples are similar to those of the late-stage calcite of the Upper Mississippi Valley Zn-Pb and Illinois-Kentucky fluorspar districts. Calcite inclusion fluids had delta D (sub H 2 O) values of -80 and -58 per mil, implying a meteoric water source. At probable temperatures of 15 degrees to 75 degrees C, this calcite formed from fluids having calculated delta 18 O (sub H 2 O) values of -9 to 1 per mil, implying slight to moderate exchange with sedimentary rocks. The diagenetic-epigenetic history of fluids recorded by the cleat minerals probably began with a slightly evolved seawater, followed by a metal-bearing deep basin brine, and concluded with a cooler, slightly evolved, meteoric-water fluid.

Economic Geology↗
USGS1986

The Schwarzwalder uranium deposit, III: Alteration, vein mineralization, light stable isotopes, and genesis of the deposit

The Schwartzwalder uranium deposit formed at 69.3 + or - 1.1 m.y. in a complex fracture system during the inception of Laramide uplift of the Front Range in Colorado. Geologic and isotopic evidence demonstrates that the ore-forming fluids were in chemical equilibrium with the Proterozoic metavolcanic and metasedimentary host-rock terrane at depth and that the metals, sulfur, and carbonate deposited in the fractures were derived from the metamorphic rocks. The data are not consistent with chemical contributions from an unrecognized magma or from the overlying Phanerozoic sedimentary rocks. The protoliths for the metamorphic rocks were submarine volcanic rocks and related volcanogenic exhalative iron-formations and chert. Water trapped along the basement faults and in the regolith between the basement and the overlying Paleozoic sedimentary rocks interacted with the metavolcanic rocks to produce isotopically heavy fluids containing high concentrations of dissolved metals and carbonate. Calculated delta 18 O values for this fluid range from 4.3 to 8.2 per mil, indicating a low water/rock ratio in the source terrane.Two stages of alteration and three stages of vein mineralization are recorded in the Schwartzwalder deposit. At the onset of Laramide faulting, fluids migrated along the fracture systems to zones of low hydraulic potential. These fluids contained CO 2 and had a metastably large K/Na ratio; they altered the gneissic wall rocks to a carbonate-sericite assemblage, adding K (super +) and CO 2 and removing SiO 2 with little or no change in volume. As the fractures continued to open, CO 2 was evolved from the fluids, increasing the pH and superimposing a hematite-adularia alteration assemblage on the earlier alteration.The veins record three stages of mineralization, the second of which generated the high-grade uranium veins. Evidence for the stage I sulfide-carbonate mineralization is poorly preserved, but isotopic and temperature data from this stage are consistent with a trend in fluid composition culminating in stage II pitchblende deposition. Sudden, large movements along the faults caused episodic evolution of CO 2 from the fluid. This loss of CO 2 decreased the solubilities of carbonates and adularia and the stabilities of sulfur species in solution. Uranyl carbonate complexes dissociated and sulfur species in solution likely reduced the uranyl ions to produce stage II pitchblende. Carbonate, adularia, and sulfides dominated the vein mineralogy after deposition of pitchblende. Progressively lower delta 18 O values in vein carbonates suggest the mixing of cooler, less evolved, perhaps meteoric, waters during the later stages of mineralization. However, the fluid pressure remained high, as indicated by explosion breccias and inward collapse features which formed as fault movements produced sudden decreases in the confining pressure. Only the stage III carbonate-iron disulfide assemblage in the major postore segment of the Illinois fault may be the product of meteoric water alone.

Economic Geology↗
USGS1984

The Balmat-Edwards zinc-lead deposits; synsedimentary ore from Mississippi Valley-type fluids

Reconstruction of some aspects of the initial environment of ore deposition through detailed S, C, and O isotope studies of the Fowler orebody, and through reconnaissance trace element and S isotope studies of sphalerite concentrates and composite samples of ore from 22 orebodies; and (2) evaluation of the effects of high-grade metamorphism on the stable isotope geochemistry of both the ore and gangue minerals of a massive sulfide orebody. The delta 34 S values of pyrite, sphalerite, and galena in the Fowler orebody range from 11.5 to 17.5 per mil; anhydrite associated with ore ranges from 25.6 to 27.0 per mil. Average equilibration temperatures calculated from the delta 34 S differences between coexisting sulfur-bearing phases are 388 degrees C for pyrite-sphalerite, 338 degrees C for sphalerite-galena, 329 degrees C for pyrite-galena, and 602 degrees C for anhydrite-pyrite; all pairs display narrow temperature distributions. The delta 13 C and delta 18 O values of carbonates from the Fowler orebody range from -1.6 to 1.4 per mil and from 24.8 to 27.0 per mil, respectively; quartz delta 18 O values range from 23.0 to 27.0 per mil. The delta 34 S values of sphalerite concentrated from composite samples of the orebodies range from 12.7 to 14.7 per mil. This narrow range contrasts sharply with the wide variability of associated evaporitic anhydrite lenses, which range from 8 to 28 per mil. A stratigraphic increase in the delta 34 S value of the evaporitic anhydrite at Balmat, and large sulfide iron-formation occurrences near Balmat, imply that bacterial sulfate reduction occurred on an unusually large scale during Balmat sedimentation. The presence of such an increase in a sedimentary section may constitute a valuable exploration guide.

Economic Geology↗
USGS1979

Preliminary isotopic studies of fluids from the Cerro Prieto geothermal field

Preliminary isotopic studies of Cerro Prieto geothermal fluids and earlier studies of Mexicali Valley ground waters suggest local recharge of the geothermal system from the area immediately to the west. Oxygen isotope exchange of water with reservoir rock minerals at temperatures increasing with depth has produced fluids with oxygen-18 contents increasing with depth, and pressure drawdown in the southeastern part of the field has allowed lower oxygen-18 fluids to invade the production aquifer from above. The contents of tritium and carbon-14 in the fluid suggest only that the age of the fluid is between 50 and 10,000 years. The isotopic compositions of carbon and sulfur are consistent with a magmatic origin of these elements but a mixed sedimentary-organic origin appears more likely for carbon and is also possible for sulfur. Investigations of the isotopic compositions of geothermal and cold ground waters continue and are being expanded as fluids become available and as separation and analysis methods are improved. ?? 1979.

Geothermics↗