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J. Thomas Nash

Publications and source records attributed to J. Thomas Nash.

44 records · Page 3Linked to original sources
USGS1973

Fluid-inclusion studies of the fluorspar and gold deposits, Jamestown district, Colorado

The Jamestown district, Boulder County, Colorado, is a major producer of fluorspar; prior to 1940, gold, gold-telluride, and lead-silver ores were mined. Fluorite occurs as a primary mineral in phases of the composite sodic granite stock at Jamestown and in breccia zones, stockworks, and pipe-shaped bodies in and adjacent to the stock. Gold and telluride mineralization occurs with and without fluorite in veins peripheral to the stock. A great variety of fluid-inclusion types is present in the district, reflecting chemically and physically diverse fluids. Fluid inclusions in early-stage quartz from the fluorspar deposits have filling temperatures of 250 degrees to 375 degrees C, salinities of 20 to 30%, and boiling is indicated at many locations. Most inclusions associated with the main fluorite stage have salinities of at least 26% to more than 50% and filling temperatures in the range of 250 degrees to 350 degrees C. Probable cognate fluorite on the inclusion walls and as many as ten daughter minerals precipitated from some primary inclusions in fluorite attest to the presence of salt and CaF 2 -rich polycomponent fluorspar-depositing fluids. Carbon dioxide-rich liquids and vapors were present during and after the main period of fluorspar deposition. Gold-bearing veins contain fluid inclusions indicating that two fundamentally different types of fluids were present. Inclusions in pregold quartz and fluorite have filling temperatures which range up to 375 degrees C, and most have very high salinities comparable to those from the fluorspar deposits, suggesting that early quartz and fluorite of the gold-bearing veins was roughly contemporaneous with the main period of fluorspar deposition. Fluid inclusion and other thermometric data (Kelly and Goddard, 1969) suggest that gold and telluride deposition occurred at temperatures below 300 degrees C. Fluid inclusions most directly associated with gold deposition have filling temperatures in the range 205 degrees to 270 degrees C and have about 4% salinity. These data suggest that the gold-bearing fluids either evolved from the fluorite-bearing fluids or were later unrelated hydrothermal pulses introduced on reopened structures. Boiling occurred at many locations that were at relatively high elevations or adjacent to the sodic granite stock. From the interpretation that boiling occurred and from the heterogeneity of CO 2 contents, we deduce that fluid pressures were low and variable, commonly near 150 bars, but ranging to as much as approximately 500 bars when CO 2 pressures were high; these determinations are compatible with the depth of cover estimated from geomorphic reconstruction using the Flattop peneplain. The absence of strong thermal or salinity zonation of fluids relative to the outcropping stock suggests that fluids were emanating from a larger intrusive body at depth.

Colorado↗
USGS1973

Geochemical studies in the Park City district; I, ore fluids in the Mayflower mine

Ore bodies in the Mayflower mine, Park City district, Utah, are localized along a normal fault zone which cuts Mississippian sedimentary and Tertiary intrusive rocks. Fissure filling and replacement Pb-Zn-Cu-Ag-Au mineralization occurs in both sedimentary and intrusive host rocks over a known vertical distance of 3,000 feet. The paragenesis of three major recognized veins in the mine is nearly the same, consisting of early quartz, anhydrite, hematite, and pyrite + or - chalcopyrite, followed by sphalerite and galena, grading into pyrite + chalcopyrite + hematite, in turn followed by quartz + carbonate and minor anhydrite, followed by sphalerite and chalcopyrite. Deep, early veins outside of the Mayflower ore zone are characterized by quartz, K-feldspar, biotite, pyrite, and anhydrite, and some also contain magnetite, amphibole or chalcopyrite; these veins contain halite-bearing and gas-rich inclusions which are not known from the ore zone. The deep, early fluids had 34 to 44% salinity, homogenization temperatures of 315 degrees to 430 degrees C, and at times were boiling. Fluid inclusions in samples from the three ore-bearing veins, which contain only simple two-phase inclusions with consistent phase proportions, indicate a marked change in the fluids prior to ore deposition. Homogenization temperatures range from 220 degrees to 300 degrees C, and probably require a pressure correction of approximately + 10 degrees C. Freezing tests indicate salinities in the range 0.3 to 11 wt %NaCl equivalent; no CO 2 -bearing phases were detected at reduced temperatures. Near-surface veins, presumably contemporaneous with the Mayflower ore zone, show evidence for boiling and suggest that there was approximately 90 bars pressure at the present Mayflower vein outcrop. The distribution of fluid densities, temperatures, boiling, and key minerals in time and space indicates a dramatic change from very hot dense post-magmatic fluids to cooler, relatively low salinity fluids at the onset of economic basemetal deposition, probably concurrent with normal faulting. The changes in the physical properties of the hydrothermal fluids are believed to reflect the structural and magmatic evolution of the area.

Utah↗
USGS1973

Microprobe analyses of sericite, chlorite, and epidote from Jerome, Arizona

Volcanic rocks in the vicinity of the massive sulfide deposits at the United Verde mine, Jerome, Ariz., have been modified in several periods of hydrothermal alteration and greenschist metamorphism. Chlorite, 2M, mica (sericite), and epidote are characteristic alteration products. Microprobe analyses for sericite, chlorite, and epidote are recalculated to structural formulas by the method employing oxygen anion equivalents. The sericite has the general composition of muscovite, but is moderately phengitic, and two samples have 6-12 percent paragonite in solid solution. Most of the chlorite is ripidolite with approximately one-third of the tetrahedral sites filled by aluminum; octahedral aluminum slightly exceeds tetrahedral. Fe:Fe+Mg+Mn ratios range from 0.34 to 0.66; low values are associated with sulfide minerals; higher values occur in a sample peripheral to the massive sulfide deposit. The epidote is a solid solution of 70 percent epidote, 30 percent clinozoisite.

Arizona↗
USGS1972

Geology of the massive sulfide deposits at Jerome, Arizona; A reinterpretation

The massive sulfide deposits at Jerome, consisting largely of pyrite, chalcopyrite, and sphalerite, are concordant stratabound lenses in massive quartz-bearing crystal tuffs of Precambrian age and overlying bedded tuffaceous rocks. The crystal tuffs were eraplaced as submarine pyroclastic flows. The evidence permits the interpretation that the massive sulfide lenses are essentially syngenetic and related to hydrothermal brines that discharged into a submarine basin. The host rocks and sulfide lenses were folded and metamorphosed. Two periods of folding can be recognized; during the second period, the older folds were deformed along vertical axes, and some of the chalcopyrite in the major lens migrated downward to form shoots of intersecting veins in the crystal tuff and chloritized tuff (black schist). Fluid inclusions in volcaniclastic and vein quartz appear to be related to deformation and metamorphism and provide no direct evidence on the nature of the primary fluids that presumably introduced the ore metals to the volcanic rocks accumulating on the sea floor. Liquid CO2-bearing inclusions are restricted to rocks and veins near the United Verde deposit and associated chalcopyrite ore shoots. Fluids at that time were CO2-rich, with temperatures of approximately 235 ø C and confining pressures of about 700 bars or less.

Arizona↗
USGS1971

Ore fluids in the porphyry copper deposit at Copper Canyon, Nevada

The large, low-grade copper and gold deposit at Copper Canyon, Lander County, Nevada, formed in the contact metasomatic environment adjacent to a shallow Tertiary intrusion. Vein and disseminated chalcopyrite-pyrite-pyrrhotite-arsenopyrite mineralization, with lesser amounts of gold, galena, sphalerite, marcasite, and siderite occur in the Upper Cambrian Harmony and Middle Pennsylvanian Battle Formations. Studies of fluid inclusions in stages of vein quartz and in healed fractures through quartz phenocrysts and pebbles reveal the presence of very saline, sometimes CO 2 -rich fluids in the early and middle stages of mineralization. Ore fluid salinities during the base metal mineralization were approximately 40 wt percent, and temperatures were near 375 degrees C. Circulation of this hot brine was apparently restricted to an elongate highly fractured zone within 3,000 feet laterally of the intrusive. Later fluids in this central zone were somewhat cooler, near 300 degrees C, and had significantly lower salinities in the range 12 to 1.2 wt percent. Smaller lode deposits approximately 1 to 5 miles from the intrusion appear to have formed from low to moderate salinity fluids at temperatures generally in the range 250 to 335 degrees C. Geologic and fluid considerations suggest formation at approximately 6,000 feet in depth.

Nevada↗
USGS1968

Uranium deposits in the Jackpile Sandstone, New Mexico

Ultimate sources of uranium are believed to be either fluids from Morrison volcanoes (not demonstrable) or labile constituents in the Jackpile Sandstone. Petrographic studies indicate considerable pre-Dakota diagenesis, which would have made uranium in feldspars, heavy minerals, and volcanic debris available. Deposition of uranium occurred while host sandstone was near the surface during the pre-Dakota hiatus. Sedimentary permeability was the principal factor in distribution of ore and of the organic decomposition products that precipitated it.

New Mexico↗