Over the course of a week, participants of this student field trip visited several epithermal and... more Over the course of a week, participants of this student field trip visited several epithermal and porphyry precious metal deposits in the Maricunga belt and copper-silver IOCG and manto deposits in northern Chile, with visits to active open-pit and underground operations, including Punta del Cobre, Atacama Kozan, Las Luces, El Guanaco, and Marimaca, as well as drill core reviews of exploration projects at Caspiche, Cerro Casale, and Salares Norte. The geology and mineralization of these deposits was studied in the context of regional and local geological settings, focusing on their similarities and differences, ore resources, deposit models, and exploration methods; the strong structural control on most of the deposits was highlighted. The guidebook includes detailed course itinerary and route map.
This SEG Foundation-sponsored student field trip visits fivemajor Cu-Mo porphyry deposits and two... more This SEG Foundation-sponsored student field trip visits fivemajor Cu-Mo porphyry deposits and two advanced exploration projects. A mill complex visit is also included. Examination of open pit exposures, drill cores from mines and exploration prospects, and outcrops will focus on recognizing the characteristics of porphyry and skarn mineral assemblages, including study of the distribution and zoning of ore-forming minerals, variations in alteration assemblages, multiple intrusion/mineralization events, hydrothermal processes and their geochemical signatures. Additionally, the Mission orebody and the Rosemont copper deposit are superb examples of Laramide orebodies that have been tilted and dismembered by major post-mineralization tectonic events characterized by low-angle faulting. Recognition of post-mineralization structural features is an important key in exploration for these types of deposits.
Quantitative estimates of peak metamorphic P-T-X have been obtained for the Manitouwadge district... more Quantitative estimates of peak metamorphic P-T-X have been obtained for the Manitouwadge district, Ontario, Canada, by combining the results of well-calibrated metamorphic reactions as well as other geothermometers and barometers. Aluminosilicate, alm and ine-rutile-ilmenite, anorthite-grossular, cordierite-garnet, staurolite-garnet, Kfeldspar-sillimanite equilibria and sphalerite geobarometry fix peak metamorphic P-T-X at 6 (+OR-) 1 kb and 650 (+OR-) 30(DEGREES)C with a(,H(,2)O) 0.8 (+OR-) 0.1. Two-feldspar and Fe-Ti oxide thermometry indicates low, re-equilibrated temperatures of 450-500(DEGREES)C and 450-610(DEGREES)C, respectively, while biotite-garnet K(,D) temperatures ranged from 570 to 670(DEGREES)C. Interactions of the massive sulfide ores with their enclosing wall-rocks produced an asymmetrical chemical and mineralogical zoning around the Geco massive sulfide deposit during the regional metamorphism. In a south-to-north traverse through the east-west striking vertically-dipping orebody the following assemblages are encountered: (A) Po (+OR-) Py, (B) Mt + Py, (C1) Po + Py, (C2) Po + Py + Mt, (C3) Po + Py, and (D) Mt + I lm. Anhydrite occurs close to the ore, aluminous ferromagnesian minerals occur to the north (stratigraphic foot-wall), and biotite occurs throughout the section. In buffered assemblages, biotite compositions are invariant, and in the ferromagnesian minerals X(,Fe)('Mineral) decreases systematically with proximity to the orebody. This mineralogical zoning and the variation in solid solution of the ferromagnesian minerals monitor a systematic increase of over one log unit in f(,O(,2)) and f(,S(,2)) towards ore. While documenting the phenomenon of metamorphic zoning, this study underscores the importance of detailed petrologic studies in new districts before practical applications of such research to mineral exploration can be made. Tin is highly enriched in the Geco deposit (500 ppm av.) and its foot-wall rocks (70-300 ppm). Cassiterite, which occurs in the massive sulfide ores, recrystallized during the regional metamorphism, and nigerite formed in the aluminous, Ca-deficient rocks adjacent to the ores. In the district-wide cordierite-gedrite gneisses (10-30 ppm Sn) Sn is accommodated in hogbomite. Thermodynamic calculations, as well as experimental and observed field relations, indicate that cassiterite is the stable tin mineral in the massive sulfides. Nigerite, a potential pathfinder for mineral exploration, is the stable Sn-bearing phase in the Ca-deficient, Al-rich foot-wall rocks. It is proposed here that Sn, Cu and Zn were co-deposited, and that all the base metals had the same ultimate source.Ph.D.GeochemistryUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160014/1/8412225.pd
Lunar and Planetary Science Conference, Mar 1, 2008
ABSTRACT The Navajo Sandstone concretions were evaluated to detect mineralogical changes and chem... more ABSTRACT The Navajo Sandstone concretions were evaluated to detect mineralogical changes and chemical gradients. Sequential relationships suggest an evolution of phases of cements. The Mars "blueberries" may have a similar evolution of cements.
, in recognition of his contributions to the geology and mineralogy of ore deposits. Type materia... more , in recognition of his contributions to the geology and mineralogy of ore deposits. Type material is preserved at the Smithsonian Institution.
Economic geology and the bulletin of the Society of Economic Geologists, Apr 1, 1986
Tin and tin-bearing minerals are prominent in the metamorphosed (amphibolite-granulite facies) Ge... more Tin and tin-bearing minerals are prominent in the metamorphosed (amphibolite-granulite facies) Geco Cu-Zn volcanogenic massive sulfide deposit as well as in some of the smaller massive sulfides in the Manitouwadge district. Cassiterite and minor Sn-bearing andradite, epidote, rutile, and phlogopite occur within the massive sulfide ores. Nigerite-24R of composition (Sn, Ti) x (Zn, Mg, Fe, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 2, and mixed polytypes of hoegbomite with the general formula (Ti, Sn) x (Fe, Mg, Zn, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 1, occur in the footwall rocks which comprise the feeder zone. Hoegbomite is the primary tin-bearing mineral in the districtwide cordierite-gedrite gneisses which represent footwall conformable alteration zones.A relatively high f (sub O 2 ) during metamorphism and also during the formation of the ores accounts for the presence of cassiterite rather than stannite or the simple tin sulfides in the ores. Nigerite and hoegbomite occur only in Ca-Na-deficient Mg-Fe-rich aluminous rocks and are associated with Zn staurolite, corundum, cordierite, sillimanite, (Zn, Fe) spinels, and Fe-Ti oxides.Bulk-rock tin assays systematically increase as the Geco orebody is approached: districtwide cordierite-gedrite gneisses (10-30 ppm), feeder zone (50-330 ppm), and orebody (100-3,000 ppm). Hanging-wall felsic gneisses and nearby mafic gneisses and granites contain less than 3 ppm tin. Small nearby pegmatites contain 2 to 10 ppm tin.From textural evidence, phase equilibria calculations, and the lithologic distribution of tin, a model for the Geco deposit is developed in which the districtwide cordierite-gedrite gneisses represent one segment of the fossil hydrothermal system and in which Cu-Zn and Sn mineralizations are closely associated. Nigerite, hoegbomite, and spinel from Geco occupy distinct and characteristic (Zn, Fe, Mg, Mn) solid solution fields. Hence, these species, which are known to occur as detrital heavy minerals may be useful pathfinders in geochemical exploration for eroded volcanogenic massive sulfide deposits.
Fluorphlogopites (up to XF = 0.96) and fluortremolites (up to XF = 0.82) have been found in Grenv... more Fluorphlogopites (up to XF = 0.96) and fluortremolites (up to XF = 0.82) have been found in Grenville marbles near Balmat, New York. In these minerals high fluorine contents correlate inversely with iron solid solution, but do not appear to affect the stable isotope fractionation of hydrogen. The substitution of fluorine for hydroxyl in micas and amphiboles is partly responsible for stabilizing hydrous minerals in the granulite facies marbles of the Adirondacks. The importance of this etrect can be calculated from thermochemical data and a knowledge of F/ OH distribution in natural assemblages. Phlogopite concentrates fluorine relative to tremolite. The F/OH distribution coefficient does not vary with metamorphic grade but differs significantly in the most F-rich sample from the other samples. Application of volatilization equilibria in marbles without consideration of possible fluorine substitution can lead to large errors in estimated pressures, temperatures and fluid compositions. The fugacities of eight C-O-H-F fluid components are restricted by the assemblage fluorphlogopite * calcite + quarlz + graphite. At 650'C, 6 kbar, the estimated range in these values is: log fH2O = 2.47 to 3.37 ,log f CO2 = 4.52 to 4.22,Iog.fCtt. = 0.25 to 2.35, loC fCO : 2.07 to l.92,log f}{z = 0.34 to l.39,log /Oz = -18.01 to -18.31, log fHF = -1.27 to -0.37 and log /Fz = -34.19 to -33.43. H2O and CO2 were the dominant fluid components in these graphitic marbles and CHn was minor. Calculations of many oxidation-fluoridation equilibria (fFz vs. fOz) for common calcsilicate minerals support these values of fFz and /Oz. The inferred /Oz is within 0.5 log unit of the QFM buffer, near that commonly inferred for other Adirondacks marbles. Minimum values of log /F2 are -35.0 for end-member fluorphlogopite, -34.4 for end-member fluortremolite and -34.1 for norbergite plus fluortremolite in calcite marbles at these P and T. Such values of /F2 may not be unusual suggesting that fluorphlogopite and fluortremolite could be relatively common rock-forming minerals in iron-poor marbles. The mineral assemblage fluortremolite + fluorphlogopite + diopside + calcite + quaftz + graphite and fluortremolite + calcite + graphite + norbergite restrict A@4 (fluortremo- lite) to -10,378t11 kjoules relative to available free energy datafor the otherphases.
Economic geology and the bulletin of the Society of Economic Geologists, Apr 1, 1989
Page 1. Economic Geology Vol. 84, 1989, pp. 444-449 SCIENTIFIC COMMUNICATIONS FLUID INCLUSION EVI... more Page 1. Economic Geology Vol. 84, 1989, pp. 444-449 SCIENTIFIC COMMUNICATIONS FLUID INCLUSION EVIDENCE FOR FLUID MIXING IN THE OXEC CYPRUS-TYPE COPPER DEPOSIT, GUATEMALA PAULA N. WILSON ...
Proceedings of the Ocean Drilling Program, Apr 1, 1997
Oceanic gabbro from Leg 153 records retrograde metamorphism caused by progressive unroofing in th... more Oceanic gabbro from Leg 153 records retrograde metamorphism caused by progressive unroofing in the footwall of a brittle-ductile normal fault. Decreasing temperature is reflected by amphiboles that evolved from pargasitic to actinolitic compositions. Highest degrees of water-rock interaction are associated with greenschist facies hydrothermal alteration that produced actinolite + chlorite ± talc ± epidote ± prehnite ± pyrite ± quartz ± titanite. Alteration is strongest near microfractures and veins that we interpret to be the main fluid conduits. Chlorite and actinolite make up 50% of highly altered samples. Chlorite typically occurs as zoned coronas around felted cores of actinolite, and reaction textures show that chlorite predominately replaces Plagioclase and actinolite replaces olivine. Reaction textures and strong linear correlations in the enrichment of secondary chlorite and actinolite and depletion of primary Plagioclase and olivine indicate that the two reactions operated simultaneously. Based on mass-balance calculations and lack of significant variations in whole-rock composition, we infer that calcium required to transform olivine to actinolite was derived from the alteration of Plagioclase to chlorite; whereas iron and magnesium required to transform Plagioclase to chlorite were derived from the amphibolitization of olivine. Mass balance of hydrothermal alteration of troctolitic gabbro from Hole 922A shows that the dominant metasomatic change was the addition of water, approximately 5110 mol/m 3. The enrichment patterns of other elements are largely antithetic to patterns observed in submarine hydrothermal water relative to seawater. Therefore, alteration could have occurred in the root zone of such hydrothermal systems.
To ascertain the provenance of water reaching wetlands in an area sustaining a population of Plei... more To ascertain the provenance of water reaching wetlands in an area sustaining a population of Pleistocene–Holocene foragers, 87-strontium/86-strontium isotopic ratios (87Sr/86Sr) of mollusks from channels of the Old River Bed inland delta of central Utah were measured. Potential provenances examined included overflow from Pleistocene–Holocene Lake Gunnison, ground water flow from the Sevier basin, ground water discharge from piedmont aquifers infiltrated by Lake Bonneville, and ground waters from local regional aquifers. Old River Bed inland delta channels active from ~13.2 cal ka BP until ~11.2 cal ka BP have87Sr/86Sr values of 0.70930–0.71049 that are consistent with water sourced from Lake Gunnison in the Sevier basin. Inland delta channels active from ~11.2 cal ka BP until shortly after ~9.3 cal ka BP have87Sr/86Sr values of 0.70977–0.71033, suggesting ground water flowed from the Sevier basin during the early Holocene. Ratios of87Sr/86Sr did not match known values for Lake Bonneville, but the youngest Old River Bed inland delta channel system has an87Sr/86Sr ratio consistent with a local ground water source, perhaps Government Creek. Consistent ground water discharge may explain the persistence of foragers in the region despite the increasingly arid climate of the Great Basin.
A new approach to EM forward modeling from the grain-scale to deposit-scale is presented using a ... more A new approach to EM forward modeling from the grain-scale to deposit-scale is presented using a porphyry copper system. The Generalized Effective Medium Theory of the induced polarization (GEMTIP) effect and electromagnetic (EM) field propagation in heterogenous polarizable media presented by Zhdanov (2006) allows the incorporation of rock-scale parameters such as mineralization and/or fluid content, matrix composition, porosity, anisotropy, and the polarizability of the formations. GEMTIP is used for rock-scale forward modeling from grain-scale parameters. Empirical data from rock-scale measurements (Ostrander and Zonge, 1978) are in good agreement with GEMTIP forward modeling output for pyrite and chalcopyrite bearing rocks. To further our understanding of IP on a larger scale, deposit-scale modeling is conducted for a porphyry system. A simplified porphyry model is created for future detectability and mineral discrimination studies.
Over the course of a week, participants of this student field trip visited several epithermal and... more Over the course of a week, participants of this student field trip visited several epithermal and porphyry precious metal deposits in the Maricunga belt and copper-silver IOCG and manto deposits in northern Chile, with visits to active open-pit and underground operations, including Punta del Cobre, Atacama Kozan, Las Luces, El Guanaco, and Marimaca, as well as drill core reviews of exploration projects at Caspiche, Cerro Casale, and Salares Norte. The geology and mineralization of these deposits was studied in the context of regional and local geological settings, focusing on their similarities and differences, ore resources, deposit models, and exploration methods; the strong structural control on most of the deposits was highlighted. The guidebook includes detailed course itinerary and route map.
This SEG Foundation-sponsored student field trip visits fivemajor Cu-Mo porphyry deposits and two... more This SEG Foundation-sponsored student field trip visits fivemajor Cu-Mo porphyry deposits and two advanced exploration projects. A mill complex visit is also included. Examination of open pit exposures, drill cores from mines and exploration prospects, and outcrops will focus on recognizing the characteristics of porphyry and skarn mineral assemblages, including study of the distribution and zoning of ore-forming minerals, variations in alteration assemblages, multiple intrusion/mineralization events, hydrothermal processes and their geochemical signatures. Additionally, the Mission orebody and the Rosemont copper deposit are superb examples of Laramide orebodies that have been tilted and dismembered by major post-mineralization tectonic events characterized by low-angle faulting. Recognition of post-mineralization structural features is an important key in exploration for these types of deposits.
Quantitative estimates of peak metamorphic P-T-X have been obtained for the Manitouwadge district... more Quantitative estimates of peak metamorphic P-T-X have been obtained for the Manitouwadge district, Ontario, Canada, by combining the results of well-calibrated metamorphic reactions as well as other geothermometers and barometers. Aluminosilicate, alm and ine-rutile-ilmenite, anorthite-grossular, cordierite-garnet, staurolite-garnet, Kfeldspar-sillimanite equilibria and sphalerite geobarometry fix peak metamorphic P-T-X at 6 (+OR-) 1 kb and 650 (+OR-) 30(DEGREES)C with a(,H(,2)O) 0.8 (+OR-) 0.1. Two-feldspar and Fe-Ti oxide thermometry indicates low, re-equilibrated temperatures of 450-500(DEGREES)C and 450-610(DEGREES)C, respectively, while biotite-garnet K(,D) temperatures ranged from 570 to 670(DEGREES)C. Interactions of the massive sulfide ores with their enclosing wall-rocks produced an asymmetrical chemical and mineralogical zoning around the Geco massive sulfide deposit during the regional metamorphism. In a south-to-north traverse through the east-west striking vertically-dipping orebody the following assemblages are encountered: (A) Po (+OR-) Py, (B) Mt + Py, (C1) Po + Py, (C2) Po + Py + Mt, (C3) Po + Py, and (D) Mt + I lm. Anhydrite occurs close to the ore, aluminous ferromagnesian minerals occur to the north (stratigraphic foot-wall), and biotite occurs throughout the section. In buffered assemblages, biotite compositions are invariant, and in the ferromagnesian minerals X(,Fe)('Mineral) decreases systematically with proximity to the orebody. This mineralogical zoning and the variation in solid solution of the ferromagnesian minerals monitor a systematic increase of over one log unit in f(,O(,2)) and f(,S(,2)) towards ore. While documenting the phenomenon of metamorphic zoning, this study underscores the importance of detailed petrologic studies in new districts before practical applications of such research to mineral exploration can be made. Tin is highly enriched in the Geco deposit (500 ppm av.) and its foot-wall rocks (70-300 ppm). Cassiterite, which occurs in the massive sulfide ores, recrystallized during the regional metamorphism, and nigerite formed in the aluminous, Ca-deficient rocks adjacent to the ores. In the district-wide cordierite-gedrite gneisses (10-30 ppm Sn) Sn is accommodated in hogbomite. Thermodynamic calculations, as well as experimental and observed field relations, indicate that cassiterite is the stable tin mineral in the massive sulfides. Nigerite, a potential pathfinder for mineral exploration, is the stable Sn-bearing phase in the Ca-deficient, Al-rich foot-wall rocks. It is proposed here that Sn, Cu and Zn were co-deposited, and that all the base metals had the same ultimate source.Ph.D.GeochemistryUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/160014/1/8412225.pd
Lunar and Planetary Science Conference, Mar 1, 2008
ABSTRACT The Navajo Sandstone concretions were evaluated to detect mineralogical changes and chem... more ABSTRACT The Navajo Sandstone concretions were evaluated to detect mineralogical changes and chemical gradients. Sequential relationships suggest an evolution of phases of cements. The Mars "blueberries" may have a similar evolution of cements.
, in recognition of his contributions to the geology and mineralogy of ore deposits. Type materia... more , in recognition of his contributions to the geology and mineralogy of ore deposits. Type material is preserved at the Smithsonian Institution.
Economic geology and the bulletin of the Society of Economic Geologists, Apr 1, 1986
Tin and tin-bearing minerals are prominent in the metamorphosed (amphibolite-granulite facies) Ge... more Tin and tin-bearing minerals are prominent in the metamorphosed (amphibolite-granulite facies) Geco Cu-Zn volcanogenic massive sulfide deposit as well as in some of the smaller massive sulfides in the Manitouwadge district. Cassiterite and minor Sn-bearing andradite, epidote, rutile, and phlogopite occur within the massive sulfide ores. Nigerite-24R of composition (Sn, Ti) x (Zn, Mg, Fe, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 2, and mixed polytypes of hoegbomite with the general formula (Ti, Sn) x (Fe, Mg, Zn, Mn) (sub 8-2x) (Al, Fe) 16 O 32 , with x [asymp] 1, occur in the footwall rocks which comprise the feeder zone. Hoegbomite is the primary tin-bearing mineral in the districtwide cordierite-gedrite gneisses which represent footwall conformable alteration zones.A relatively high f (sub O 2 ) during metamorphism and also during the formation of the ores accounts for the presence of cassiterite rather than stannite or the simple tin sulfides in the ores. Nigerite and hoegbomite occur only in Ca-Na-deficient Mg-Fe-rich aluminous rocks and are associated with Zn staurolite, corundum, cordierite, sillimanite, (Zn, Fe) spinels, and Fe-Ti oxides.Bulk-rock tin assays systematically increase as the Geco orebody is approached: districtwide cordierite-gedrite gneisses (10-30 ppm), feeder zone (50-330 ppm), and orebody (100-3,000 ppm). Hanging-wall felsic gneisses and nearby mafic gneisses and granites contain less than 3 ppm tin. Small nearby pegmatites contain 2 to 10 ppm tin.From textural evidence, phase equilibria calculations, and the lithologic distribution of tin, a model for the Geco deposit is developed in which the districtwide cordierite-gedrite gneisses represent one segment of the fossil hydrothermal system and in which Cu-Zn and Sn mineralizations are closely associated. Nigerite, hoegbomite, and spinel from Geco occupy distinct and characteristic (Zn, Fe, Mg, Mn) solid solution fields. Hence, these species, which are known to occur as detrital heavy minerals may be useful pathfinders in geochemical exploration for eroded volcanogenic massive sulfide deposits.
Fluorphlogopites (up to XF = 0.96) and fluortremolites (up to XF = 0.82) have been found in Grenv... more Fluorphlogopites (up to XF = 0.96) and fluortremolites (up to XF = 0.82) have been found in Grenville marbles near Balmat, New York. In these minerals high fluorine contents correlate inversely with iron solid solution, but do not appear to affect the stable isotope fractionation of hydrogen. The substitution of fluorine for hydroxyl in micas and amphiboles is partly responsible for stabilizing hydrous minerals in the granulite facies marbles of the Adirondacks. The importance of this etrect can be calculated from thermochemical data and a knowledge of F/ OH distribution in natural assemblages. Phlogopite concentrates fluorine relative to tremolite. The F/OH distribution coefficient does not vary with metamorphic grade but differs significantly in the most F-rich sample from the other samples. Application of volatilization equilibria in marbles without consideration of possible fluorine substitution can lead to large errors in estimated pressures, temperatures and fluid compositions. The fugacities of eight C-O-H-F fluid components are restricted by the assemblage fluorphlogopite * calcite + quarlz + graphite. At 650'C, 6 kbar, the estimated range in these values is: log fH2O = 2.47 to 3.37 ,log f CO2 = 4.52 to 4.22,Iog.fCtt. = 0.25 to 2.35, loC fCO : 2.07 to l.92,log f}{z = 0.34 to l.39,log /Oz = -18.01 to -18.31, log fHF = -1.27 to -0.37 and log /Fz = -34.19 to -33.43. H2O and CO2 were the dominant fluid components in these graphitic marbles and CHn was minor. Calculations of many oxidation-fluoridation equilibria (fFz vs. fOz) for common calcsilicate minerals support these values of fFz and /Oz. The inferred /Oz is within 0.5 log unit of the QFM buffer, near that commonly inferred for other Adirondacks marbles. Minimum values of log /F2 are -35.0 for end-member fluorphlogopite, -34.4 for end-member fluortremolite and -34.1 for norbergite plus fluortremolite in calcite marbles at these P and T. Such values of /F2 may not be unusual suggesting that fluorphlogopite and fluortremolite could be relatively common rock-forming minerals in iron-poor marbles. The mineral assemblage fluortremolite + fluorphlogopite + diopside + calcite + quaftz + graphite and fluortremolite + calcite + graphite + norbergite restrict A@4 (fluortremo- lite) to -10,378t11 kjoules relative to available free energy datafor the otherphases.
Economic geology and the bulletin of the Society of Economic Geologists, Apr 1, 1989
Page 1. Economic Geology Vol. 84, 1989, pp. 444-449 SCIENTIFIC COMMUNICATIONS FLUID INCLUSION EVI... more Page 1. Economic Geology Vol. 84, 1989, pp. 444-449 SCIENTIFIC COMMUNICATIONS FLUID INCLUSION EVIDENCE FOR FLUID MIXING IN THE OXEC CYPRUS-TYPE COPPER DEPOSIT, GUATEMALA PAULA N. WILSON ...
Proceedings of the Ocean Drilling Program, Apr 1, 1997
Oceanic gabbro from Leg 153 records retrograde metamorphism caused by progressive unroofing in th... more Oceanic gabbro from Leg 153 records retrograde metamorphism caused by progressive unroofing in the footwall of a brittle-ductile normal fault. Decreasing temperature is reflected by amphiboles that evolved from pargasitic to actinolitic compositions. Highest degrees of water-rock interaction are associated with greenschist facies hydrothermal alteration that produced actinolite + chlorite ± talc ± epidote ± prehnite ± pyrite ± quartz ± titanite. Alteration is strongest near microfractures and veins that we interpret to be the main fluid conduits. Chlorite and actinolite make up 50% of highly altered samples. Chlorite typically occurs as zoned coronas around felted cores of actinolite, and reaction textures show that chlorite predominately replaces Plagioclase and actinolite replaces olivine. Reaction textures and strong linear correlations in the enrichment of secondary chlorite and actinolite and depletion of primary Plagioclase and olivine indicate that the two reactions operated simultaneously. Based on mass-balance calculations and lack of significant variations in whole-rock composition, we infer that calcium required to transform olivine to actinolite was derived from the alteration of Plagioclase to chlorite; whereas iron and magnesium required to transform Plagioclase to chlorite were derived from the amphibolitization of olivine. Mass balance of hydrothermal alteration of troctolitic gabbro from Hole 922A shows that the dominant metasomatic change was the addition of water, approximately 5110 mol/m 3. The enrichment patterns of other elements are largely antithetic to patterns observed in submarine hydrothermal water relative to seawater. Therefore, alteration could have occurred in the root zone of such hydrothermal systems.
To ascertain the provenance of water reaching wetlands in an area sustaining a population of Plei... more To ascertain the provenance of water reaching wetlands in an area sustaining a population of Pleistocene–Holocene foragers, 87-strontium/86-strontium isotopic ratios (87Sr/86Sr) of mollusks from channels of the Old River Bed inland delta of central Utah were measured. Potential provenances examined included overflow from Pleistocene–Holocene Lake Gunnison, ground water flow from the Sevier basin, ground water discharge from piedmont aquifers infiltrated by Lake Bonneville, and ground waters from local regional aquifers. Old River Bed inland delta channels active from ~13.2 cal ka BP until ~11.2 cal ka BP have87Sr/86Sr values of 0.70930–0.71049 that are consistent with water sourced from Lake Gunnison in the Sevier basin. Inland delta channels active from ~11.2 cal ka BP until shortly after ~9.3 cal ka BP have87Sr/86Sr values of 0.70977–0.71033, suggesting ground water flowed from the Sevier basin during the early Holocene. Ratios of87Sr/86Sr did not match known values for Lake Bonneville, but the youngest Old River Bed inland delta channel system has an87Sr/86Sr ratio consistent with a local ground water source, perhaps Government Creek. Consistent ground water discharge may explain the persistence of foragers in the region despite the increasingly arid climate of the Great Basin.
A new approach to EM forward modeling from the grain-scale to deposit-scale is presented using a ... more A new approach to EM forward modeling from the grain-scale to deposit-scale is presented using a porphyry copper system. The Generalized Effective Medium Theory of the induced polarization (GEMTIP) effect and electromagnetic (EM) field propagation in heterogenous polarizable media presented by Zhdanov (2006) allows the incorporation of rock-scale parameters such as mineralization and/or fluid content, matrix composition, porosity, anisotropy, and the polarizability of the formations. GEMTIP is used for rock-scale forward modeling from grain-scale parameters. Empirical data from rock-scale measurements (Ostrander and Zonge, 1978) are in good agreement with GEMTIP forward modeling output for pyrite and chalcopyrite bearing rocks. To further our understanding of IP on a larger scale, deposit-scale modeling is conducted for a porphyry system. A simplified porphyry model is created for future detectability and mineral discrimination studies.
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Papers by Erich Petersen