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USGS1973

Solid solutions in the system Cu-Fe-S, part I: The Cu-S and CuFe-S joins

T he data of Rau (1967) on the H 2 S/H 2 ratios of gas in equilibrium with high digenite solid solutions permit the calculation of the activity of Cu 2 S as a function of temperature and composition. The activity of Cu 2 S falls to 0.5 when the mole fraction (in the system Cu 2 S-S) drops only to 0.87. These calculations also permit the estimation of the activity of S 2 -temperature curve for the hexagonal chalcocite-high digenite reaction and a revised standard free energy for covellite.New data on the variation of the activity of S 2 in equilibrium with various metal/sulfur ratios for Cu = Fe in the intermediate solid solution permit the calculation of the activity of CuFeS 2 and show that it may drop as low as 0.2 at a mole fraction of 0.90 (system CuFeS 2 -Cu 2 Fe 2 S 2 ). The behavior of the copper-bearing solid solutions closely parallels that of pyrrhotite (Toulmin and Barton, 1964) in which the activity of FeS falls to 0.4 at a mole fraction of 0.9. A phase diagram is given for the CuFe-S join between 400 degrees and 700 degrees C.

Economic Geology↗
USGS1973

Thermochemical approximations for sulfosalts

Most sulfosalts may be regarded as intermediate phases on joins between simple sulfide components (e.g., all lead sulfbismuthinides lie on the PbS-Bi 2 S 3 join). Many of the structures are characterized by subunits whose individual structures are similar to those of the component simple sulfides (e.g., galena-like and stibnite-like layers in the lead sulfantimonides). Therefore, as a first approximation one may estimate the properties of many sulfosalts in terms of mixtures of the simple sulfides.Recent work has shown that the free energy of reaction from the end-member sulfides, delta G m , for more than 20 sulfosalts is usually less negative than the hypothetical ideal free energy of mixing and that the standard free energy of formation, delta G degrees , per gram atom of sulfur in the formula may be represented as:delta G degrees = (N a delta G a degrees + . . . N i delta G i [degree) + (1.2 + or - 0.8)(N a RT ln N a + . . . N i RT ln N i )where N i is the mole fraction of the i-th simple sulfide component, R is the gas constant, and T is temperature in kelvins. The first term is far larger than the second. Estimates made for compounds in which the structural environment for the metals is quite different from that in the end-member sulfides, e.g., enargite, are subject to the greatest uncertainty.The estimated free energies may permit prediction of solubilities to a precision sufficient for many purposes, e.g., for H. C. Helgeson's computer-modeled hydrothermal systems. One may introduce some predictive capability into experimental design and anticipate some aspects of phase diagrams. This is especially true for redox reactions such as the behavior of proustite in the oxidized zone or the partial reduction of jamesonite to antimony + galena + pyrrhotite. However, other aspects, such as the prediction of the configuration of joins, e.g., PbS-As 2 S 3 , requires greater precision than the present rough estimates.

Economic Geology↗
USGS1973

Stable isotope and chemical relations during mineralization in the Bodie mining district, Mono County, California

Stable isotope and chemical relations have been determined in a typical epithermal Au-Ag deposit located in the Bodie mining district of California. Analyses were made of altered host rocks, vein minerals, alteration clays, fluid inclusions, modern spring waters, and unaltered rocks of the area.The results indicate that a hydrothermal convection system was set up by the interaction of a cooling shallow intrusion and local meteoric water. The water traveled to depth where it picked up ore constituents and SiO 2 , K, and Rb, without significant shifts in the stable isotope ratios of the water. The altered rocks have equilibrated to various degrees with an ore fluid of constant K/Rb, O 18 /O 16 and D/H ratios. Deposition of ore took place over the approximate temperature range 215 degrees -245 degrees . From C 13 /C 12 ratios of minor calcite, a volcanic source of CO 2 is postulated. The chemical and isotope composition of the ore fluid is strikingly similar to that of modern spring waters in the area.All ore deposition at Bodie took place from essentially isotopically unaltered ground water with delta O 18 = -13ppm and delta D = -98ppm.

California↗
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

Annealing history limits for inhomogeneous, native gold grains as determined from Au-Ag diffusion rates

Quantitative study of intrinsic inhomogeneities in native gold grains from three deposits in the western United States has revealed concentration profiles that represent the integrated sum of natural diffusion plus original chemical heterogeneity. By assuming that measured natural concentration gradients result solely from diffusion, upper limits may be placed on the temperature-time annealing history of the gold nuggets. This assumption focuses on the end member case in which an initial step-discontinuity is assumed between measured extremes of concentration.Concentration changes of up to 30 weight percent Ag indicate probable deposition temperatures of less than 300 degrees C for electrum from Copper Basin, Arizona, and Alder Gulch, Montana. The gold in the Homestake, South Dakota, deposit probably was formed at temperatures well under 400 degrees C.In support of this study, new data for interdiffusion in the Au-Ag system were obtained from a series of annealing experiments followed by electron microprobe analysis. The interdiffusion coefficient, D, in the range 10 (super -10) to 10 (super -17) cm 2 sec (super -1) was determined from measured profiles across synthetic alloy pairs held at eight fixed temperatures from 297 degrees to 799 degrees C, for periods ranging from 32 hours to 730 days.

Arizona, Montana, South Dakota↗
USGS1974

Carbon, hydrogen, oxygen, and sulfur isotope study of the Darwin lead-silver-zinc deposit, Southern California

The ores at Darwin occur as massive replacement bodies in silicated limestones of Pennsylvanian and Permian age adjacent to a Jurassic quartz monzonite stock. Three types of ore have a definite spatial relationship to the quartz monzonite: (1) pyrite-sphalerite-galena ores, (2) pyrite-pyrrhotite-magnetite-sphalerite-galena ores, and (3) galena-Ag-Bi-Se ores.The delta 34 S values of all sulfide minerals range from +4.4 to --5.7 permil. The delta 34 S values for individual minerals tend to decrease with respect to both space and paragenetic time. The delta (sub sl-gn) values range from 1.5 to 2.5 permil and correspond to a temperature range of 325 degrees + or - 55 degrees C. The sphalerite-galena sulfur isotope temperatures at a given locality are reproducible to + or -30 degrees C and are consistent with temperatures determined by other means.Analyses of water in three samples of fluid inclusions in sphalerite indicate that the ore fluids had delta D values of --66 + or - permil, a total salinity that reached at least 20 percent, and K/Na atomic ratios of 0.23 + or - 0.03. Isotopic and thermochemical data indicate that the ore fluids had average values of delta 34 S (sub Sigma S) = 3 permil, delta 13 C (sub Sigma O) = -- 3.5 permil, Sigma S = 0.01 mole/Kg H 2 O, f (sub CO 2 ) = 12 + or - 8 bars, and Sigma C = 0.15 + or - 0.06 mole/Kg H 2 O.The delta 18 O values of postsulfide calcites have a range of 12.8 + or - 1.5 permil in the pyrite ores and 17 + or - 2 permil in the pyrrhotite ores. Similar values for relict limestone host rock in these areas indicate that oxygen isotopic equilibrium was established between the calcite precipitating fluids and the wall rock. The delta 13 C values of the calcites, however, range from --5.8 to --3.6 permil and are considerably more negative than the limestone in the host rock, indicating that carbon isotopic equilibrium was not established between the fluids and the wall rock.Thermochemical data indicate that the pH of the ore fluids was about 4.8 as they traveled through the quartz monzonite at T nearly equal 350 degrees C. When the fluids completely equilibrated with the relict limestone and calc-silicate host rock, the pH increased to 6.7. This increase in pH as the fluids traveled upward and away from the quartz monzonite was responsible for the spatial distribution of iron sulfide assemblages, for variations in delta 34 S values of sulfides, and probably caused precipitation of the ore.

California↗
USGS1974

Alteration and fluid inclusion studies of the porphyry copper ore body at Bingham, Utah

Distribution patterns for biotitic alteration, sericitic alteration, and distinctive fluid-inclusion types in igneous host rocks of the porphyry copper ore body at Bingham, Utah, have been determined by petrographic examination of about 300 samples. These patterns are related to differences in original rock composition, variations in physical-chemical conditions during periods of intrusion and mineralization, and spatial position within the ore body. The distribution of biotitic (potassium-silicate) alteration assemblages and high-salinity fluid inclusions generally follows the crudely triangular form of the disseminated copper ore zone. Variations in abundance of hydrothermal biotite are attributed to differences in original mafic mineral content of the igneous host rocks. Biotitic alteration and initial copper mineralization were accomplished by high-salinity fluids concentrated during final crystallization of the monzonitic parent magma; genetic continuity between magmatic and hydrothermal stages is indicated . Pervasive sericitic alteration of plagioclase is confined to rocks in the northern one-third of the Bingham stock; a subzone of argillic alteration in the north-central part of the ore body occurs within the broader area of sericitic alteration. Fluids responsible for sericitic and argillic alteration were channeled by a broad zone of northeast-trending fractures. Hydrothermal minerals and high-salinity fluid inclusions occur within a large volume of shattered rock. Boiling of fluids during crystallization of the aplitic porphyry may account for the shattering. Sericitic (and argillic) alteration were apparently super-imposed on the earlier biotitic assemblage as the hydrothermal system cooled. Cooling and hydrolytic alteration were promoted by progressive introduction of meteoric waters. The many generations of inclusions trapped from boiling fluids in the temperature range 400 degrees to 600 degrees C suggest that the system was recharged repeatedly during the period of mineralization. Estimated fluid pressures of about 800 bars in the early stages of mineralization correspond to a lithostatic load of about 3 km; pressures were even lower (less than 200 bars) in the later stages and were probably controlled by hydrostatic conditions. Distribution patterns for biotitic alteration, sericitic alteration, and distinctive fluid-inclusion types in igneous host rocks of the porphyry copper ore body at Bingham, Utah, have been determined by petrographic examination of about 300 samples. These patterns are related to differences in original rock composition, variations in physical-chemical conditions during periods of intrusion and mineralization, and spatial position within the ore body.The distribution of biotitic (potassium-silicate) alteration assemblages and high-salinity fluid inclusions generally follows the crudely triangular form of the disseminated copper ore zone. Variations in abundance of hydrothermal biotite are attributed to differences in original mafic mineral content of the igneous host rocks. Biotitic alteration and initial copper mineralization were accomplished by high-salinity fluids concentrated during final crystallization of the monzonitic parent magma; genetic continuity between magmatic and hydrothermal stages is indicated.Pervasive sericitic alteration of plagioclase is confined to rocks in the northern one-third of the Bingham stock; a subzone of argillic alteration in the north-central part of the ore body occurs within the broader area of sericitic alteration. Fluids responsible for sericitic and argillic alteration were channeled by a broad zone of northeast-trending fractures.Hydrothermal minerals and high-salinity fluid inclusions occur within a large volume of shattered rock. Boiling of fluids during crystallization of the aplitic porphyry may account for the shattering. Sericitic (and argillic) alteration were apparently super-imposed on the earlier biotitic assemblage as the hydrothermal system cooled. Cooling and hydrolytic alteration were promoted by progressive introduction of meteoric waters. The many generations of inclusions trapped from boiling fluids in the temperature range 400 degrees to 600 degrees C suggest that the system was recharged repeatedly during the period of mineralization. Estimated fluid pressures of about 800 bars in the early stages of mineralization correspond to a lithostatic load of about 3 km; pressures were even lower (less than 200 bars) in the later stages and were probably controlled by hydrostatic conditions.

Utah↗
USGS1974

K-Ar Age Relations of Granodiorite Emplacement and Tungsten and Gold Mineralization near the Getchell Mine, Humboldt County, Nevada

A granodiorite stock intrudes complexly folded and thrust-faulted Paleozoic sedimentary rocks in the Osgood Mountains of eastern Humboldt County, Nevada. Within the metamorphic aureole surrounding the pluton, the sedimentary rocks are converted to cordierite hornfels and marble; tungsten-bearing tactites developed along the contacts of the granodiorite. Cutting the granodiorite and sedimentary rocks is the Getchell fault, along which the disseminated gold ore bodies of the Getchell mine are localized. K-Ar ages of the granodiorite, andesite porphyry, tungsten-bearing tactites, and altered granodiorite from the Getchell mine indicate that emplacement, alteration, and mineralization are all part of a magmatic-thermal episode which took place approximately 90 m.y. ago.

Nevada↗
USGS1974

Fluid inclusion and light stable isotope study of the climax molybdenum deposits, Colorado

The Climax mine contains three molybdenite ore bodies and widespread late quartz-pyrite-sericite barren mineralization, each related to separate intrusive phases of the Climax stock. Alteration zones spatially related to each molybdenite ore body include a silica zone below, a potassium-silicate zone that approximately coincides with ore, and overlying quartz-sericite-pyrite-topaz, argillic, and propylite zones . Delta 18 O values of quartz range from +8.4 to 10.9 per mil; delta 18 O values of muscovite and sericite range from --0.8 to +7.5 per mil. Eight of ten sericite values are between +5.8 and +7.5. All of the quartz and most of the sericite is in isotopic equilibrium with a calculated water at 350 degrees C with delta 18 O values of +3.0 to 5.5 per mil. Delta 18 O values of K-feldspars range from --4.5 to +7.5 per mil. This wide range indicates partial reequilibration of the feldspar with later, isotopically light, waters. Two muscovites have delta D values of --146 and --91 per mil; sericite ranges from --168 to --116 per mil. The calculated delta D values of water at 275 degrees C in equilibrium with sericite are --144 to --92. This range is the same as that of fluid inclusions in ore samples.Kaolinite in the argillic zone has delta 18 O values of --0.9 to --2.2 and delta D values of --162 per mil. Calculated water at 250 degrees C in equilibrium with kaolinite delta 18 O values of --4.7 to -6 and delta D of about -130 per mil. The isotopic and fluid inclusion data best fit a model whereby the Climax ore bodies were formed from a hydrothermal fluid that originated by mixing light and heavy waters. The light water is isotopically similar to present-day meteoric water; the heavy water has the isotopic composition postulated for magmatic water, although it could be meteoric water that was heavier than present-day meteoric water.

Colorado↗
USGS1975

Geochemical studies in the Park City district; II, Sulfide mineralogy and minor-element chemistry, Mayflower mine

Fissure-filling and replacement Pb-Zn-Cu-Au-Ag ores of the Mayflower mine occur in calcareous sedimentary and intrusive wall rocks over a vertical interval of 3,000 feet. The ores are exceptional in the district for their chalcopyrite and gold content and for the unusual associated gangue minerals anhydrite, chlorite, and hematite. High oxygen fugacities are indicated for many stages of mineralization. Sphalerite compositions are highly variable in the range 0.09 to 5.9 weight percent Fe, an indication of large fluctuations in sulfur fugacity. Silver is carried in tetrahedrite-tennantite (0.2 to 16 wt %), enargite (0.5 to 0.8 wt %), in coupled substitution with Bi + Sb in galena (0.02 to 2.9 wt %), and in the minerals argentite, matildite, chalcocite, and bornite. Content of As, Se, Te, and other minor elements is small. Fractionation of Cd between sphalerite and coexisting galena is relatively consistent whereas the fractionation of Mn between sphalerite and galena is variable; calculated temperatures of formation are high and unreliable.

Economic Geology↗
USGS1975

Fluid inclusion studies of vein, pipe, and replacement deposits, northwestern San Juan Mountains, Colorado

Base and precious metal ores valued in excess of $300 million have been mined from vein, pipe, and replacement deposits located on the northwest side of, and structurally related to, the middle Tertiary Silverton caldera. A belt of normal faults radial to the caldera contains veins which have been mined for as much as 15,000 feet (4,600 m) along strike and 3,700 feet (1,100 m) downdip. Structural relations, metal distributions, and fluid inclusion data suggest that many of the veins were open at the same general time and received the same mineralization. Fluid inclusions in quartz and sphalerite of the main ore stage of the Argentine and adjoining veins have filling temperatures ranging from 315 degrees to 249 degrees C and salinities from 7.9 to 0.1 wt percent NaCl equivalent. However, the majority of inclusions from base metal and gold stages are in a relatively narrow range near 280 degrees to 290 degrees C and salinities are unusually low (less than 2 wt %). Filling temperatures in postsulfide quartz and fluorite are as low as 153 degrees C and salinities are generally less than 0.5 wt percent. A pressure correction of about +25 degrees C should be added to the filling temperatures. Ore fluids were physically homogeneous liquids over the 3,700-foot (1,100 m) vertical and 11,200-foot (3,400 m) lateral range studied and no systematic variations in filling temperatures or salinity in this space are noted, but significant fluctuations are evident locally. Recently developed base metal replacement deposits occur in the Eocene Telluride Conglomerate adjacent to many metal-bearing veins. Calc-silicate alteration of the conglomerate host was probably by moderately saline fluids that ranged in temperature from 320 degrees to about 400 degrees C. Quartz-bearing clasts in altered zones contain secondary halitebearing inclusions with filling temperatures of about 250 degrees C and salinities about 34 wt percent; such inclusions are not known from the base metal stage and are interpreted to have been preore. Hydrothermal quartz, sphalerite, and calcite from replacement pods contain inclusions with filling temperatures ranging from 204 degrees to 309 degrees C and salinities from 11.2 to 0.2 wt percent. That the replacement zones and throughgoing veins are genetically related is suggested by the similarity of filling temperatures and compositions of base metal-stage fluid inclusions and by structural evidence. Small high-grade pipe deposits of base and precious metals occur along the ring-fault zone of the Silverton caldera. Filling temperatures for inclusions in quartz, sphalerite, and barite of the main ore stage of these deposits range from 216 degrees to 308 degrees C and salinities range from 1.6 to 0.2 wt percent NaCl. Quartz in altered wall rocks and silicified capping contains moderately saline secondary inclusions with relatively uniform liquid: vapor ratios that suggest filling temperatures of about 225 degrees to 300 degrees C. From the available samples there is no fluid inclusion evidence for boiling. Throttling, which had been hypothesized to explain the telescoped nature of the pipe ores, is neither substantiated nor discredited by the inclusion data.

Colorado↗