V11A-1995
Hydrochemical fluxes from El Chichon volcano, Mexico
The most probable scenario for the evolution of El Chichon volcano after the 1982 catastrophic dome- destroying eruption is a growth of a new extrusive dome. In that case the active volcano-hydrothermal system of El Chichon certainly will be affected, changing the chemical and heat output. Chemical monitoring of almost inaccessible hot springs on the volcano slopes and crater fumaroles can be replaced by a monthly monitoring of Rio (River) Magdalena, the main drainage of the volcano-hydrothermal system. Flow rates and chemical compositions of the river as well as of all inflow streams coming from the volcano were measured in March and June, 2008. The main aims of this study were (i) to develop a baseline of data for Rio Magdalena for the further monitoring the El Chichon volcano activity, (ii) to estimate the total discharge of hydrothermal solutes from this volcano-hydrothermal system, and (iii) to find unknown thermal manifestations. A periodic (monthly) sampling of the river and streams could be the best way to monitor the volcano, because they represent all types of thermal waters and the integrated composition of the total discharge from the El Chichon volcano-hydrothermal system. Rio Magdalena, at the end of its course along Chichon flanks, had in March 2008 a total flow rate of 11000 l/s and transported 150 tons/day of cations. This flux results from the rock dissolution of the volcano edifice and from the Rio Magdalena riverbed. The total Cl flux was estimated to be around 35 tons/day. Among all tributaries flowing from El Chichon slopes to Rio Magdalena, six were identified to have a thermal signature with a maximum chloride discharge reaching in one of the streams 15 tons/day and 18 tons/day of cations. Two new thermal streams were found, what allows suspect the presence of at least two more unknown groups of thermal springs in still not explored canyons. Others 5 sampled streams and the upper part of the main river do not transport thermal derivates (Cl, B) but have significant concentrations of Ca, HCO3 and SO4 due to water-rock interaction with the 1982 ash deposits and limestone basement. A renewal in activity of El Chichon could be expressed by changes in chemical ratios and magmatic Cl and S fluxes. The geochemical monitoring of the Rio Magdalena network is thus the best way to detect pre-eruptive periods.
V11A-1996
Sulfide mineral paragenesis at the Hugo Dummett porphyry Cu-Au deposit, Oyu Tolgoi, South Mongolia
Mineralogical studies of ore minerals have been conducted for the Hugo Dummett porphyry copper deposit. The Hugo Dummett porphyry copper gold deposit is located in the South Gobi region, Mongolia and currently being explored. This deposit divided into the Cu-rich South Hugo Dummett and the Cu-Au-rich North Hugo Dummett deposits. The Hugo Dummett deposits contain 1.08% copper (1.16 billion tonnes in total) and 0.23 g/t gold. Copper-gold mineralizations at this deposit are centered on a high-grade copper (typically > 2.5%) and gold (0.5–2 g/t) zone of intense quartz stockwork veining. The high grade copper and gold zone is mainly within the Late Devonian quartz monzodiorite intrusions and augite basalt, also locally occurs in dacitic rocks. Intense quartz veining forms a lens up to 100 m wide hosted by augite basalt and partly by quartz monzodiorite. Although many explorations have been carried out, but only a few scientific works were done in the Oyu Tolgoi mining area. Therefore the nature of copper-gold mineralization and orgin of the deposit is not fully understood. North Hugo Dummett and South Hugo Dummett porphyry copper-gold deposits are characterized by three mineralized stages based on our study: (1) early stage (2) middle stage and (3) late stage. The main copper- gold mineralization occurs in the early and middle stages, which is related to the quartz monzodiorite and dacitic rocks. Pyrite, chalcopyrite and bornite were continuously crystallized from early to late stage. The early stage of pyrite, chalcopyrite, bornite, molybdenite and sphalerite were replaced by middle stage of minerals. The middle stage minerals are sphalerite, tennantite, tetrahedrite, chalcocite, covellite, eugenite, galena, electrum, and gold, those are dominantly occur in the quartz monzodiorite. Additional pyrite, bornite and chalcopyrite were also deposited during this stage. In the late stage, pyrite, chalcopyrite and bornite are dominantly occurs as veins, veinlets and fracture filling in the quartz. The bornite, chalcopyrite, pyrite, sphalerite, tetrahedrite, tennantite, enargite, chalcocite, covellite, molybdenite, gold and eugenite occur as main ore minerals. All ore minerals were deposited in the temperature increasing condition with time. Eugenite is determined for the first time in the North Hugo Dummett deposit.
V11A-1997
Fossil Fumarolic Pipes in the Tshirege Member of the Bandelier Tuff
The geology exposed on the walls (3000 m2) of a large pit in the Bandelier Tuff gives unparalled 3-D exposures of many structures that develop in thick deposits of pyroclastic flows. Subunits of the Tshirege Member of the Bandelier Tuff, erupted at 1.25 Ma, exhibit distinct rubble-filled fissures, or pipes, that range in width from centimeters to meters. The fissures exhibit zones of fines depletion, indurated wall structures, upward flaring geometry to the top of the host unit, and fissure-filling blocks of the host unit as well as rubble derived from overlying pyroclastic units. In the units directly overlying the fissures are intensely fractured in- place rubble zones. Additionally, the fissures appear to be regularly spaced at about 4.5 or 7.5 meters apart. All these field characteristics are indicative of fumarolic activity. Petrographic, XRD, and XRF studies of distinct pipes were done to investigate the physical changes imparted to the tuff by the fumarolic activity. Petrography indicates that the pipe wall and pipe centers are enriched in tridymite and potassium feldspar. These minerals fill the void spaces in pumice and groundmass void spaces of the pipe wall rocks, imparting the indurated nature. Other mineralogic indicators of late stage fumarolic gas phase deposition are optically continuous, feathery overgrowths on sanidine phenocrysts and scapolite in pipe centers. Also, clinopyroxenes in the overlying rubble zones have oxidized rims indicative of highly oxidizing gases emanating from the fumarolic pipes below. XRD analyses of rubble zones above the pipes show decreased cristobalite (4 wt. %) and feldspar (9 wt. %), and increased clay contents (12 to 26 wt. %). This change from feldspar and cristobalite to clay suggests an acidic nature of fumarolic gases. XRF analyses show trends in bulk chemistry consistent with fumarolic data from other tuffs. The data indicate that there have been systematic changes in the geochemistry of the fissures readily attributable to the action of fumarolic gases. Field relations (e.g., funnel-shaped pipes, overlying rubble zones, apparent regular spacing), petrography (e.g., tridymite/sanidine enrichment, oxidized clinopyroxene rims, scapolite), XRD (cristobalite and feldspar loss, clay production) and XRF major and trace element analyses all indicate that the filled fissures are fossil fumarolic pipes. Fumarolic activity, established after deposition, must have continued after emplacement and induration of overlying units.
V11A-1998
Shallow vs. Deep Fluid Sources In Hydrothermal Systems: New Insights From VOC Composition In Fumarolic Discharges And Soil Gases Of Yellowstone National Park (USA)
The origin of non-methane volatile organic compounds (VOCs) in hydrothermal fluids is related to two distinct mechanisms regulated by different thermodynamic conditions (e.g. Des Marais et al., 1981; Mango, 2000; Capaccioni and Mangani, 2001): i) thermogenic reactions, such as catalytic reforming and/or thermal cracking, which proceed within the main reservoir at medium-to-high temperature (150-350°C) and reduced conditions; ii) biodegradation processes, occurring at relatively shallow depth, where uprising fluids have <150°C and suffer oxidizing conditions. According to these considerations, the main aim of the present investigation is to discriminate the different fluid sources feeding the hydrothermal system on the basis of the C2-C15 organic compounds in fumarolic discharges and soil gases collected at the Yellowstone National Park (USA). A total of 64 and 66 different species were identified in the gas discharges and in the soil gas samples, respectively. The composition of the organic gas fraction in the fumarolic fluids is relatively homogeneous, being dominated by C2-C6 alkanes (81 %) and showing relatively high concentrations of alkenes (13 %), aromatics (3.7 %) and cyclics (1.4 %). Differently, the relative percentages of alkanes and alkenes in the soil gas, where VOC abundances are about two orders of magnitude less abundant than those in the gas discharges, are significantly lower (64 and 6.8 %, respectively) and cyclics are absent. On the other hand, oxygenated species (17.8 %), aromatics (5.6 %) and Cl-bearing compounds (4.5 %) results to be enriched with respect to those measured in the gas vents. Such compositional differences are likely to be due to the bacterial activity in the soil that causes the production of ketones, esters, alcohols, aldehydes and organic acids from the C-H species (hydrocarbons sensu strictu). Organic acids, mainly constituted by ossalic acid and traces of tartaric, malonic citric and succinic ones, were also determined in the fumarolic condensates. This seems to indicate that biodegradation likely occurs even within the hydrothermal systems, since the production of low molecular mass organic acids is to be related to bacterial activity (Arnetoli et al., 2008 and references therein). S-bearing compounds are strongly controlled by the fS values and this would explain the relatively high concentrations of these species in the H2S-rich fumarolic discharges. As far as it concerns the relatively high abundance of halogenated compounds in the soil gases, it can be suggested that the origin of these species is likely to be related to atmospheric contribution. Actually, formation of Cl- bearing species from reactions between VOCs and Cl-rich fluids, such as those of the deep hydrothermal reservoir, is still matter of debate. Therefore, we may speculate that these compounds, characterized by chemical inertness, are added to the hydrothermal fluids from meteoric water recharging the system. References: Arnetoli, M., Montegrossi, G., Buccianti, A., Gonnelli, C., 2008. J. Agricol. Food Chem., 56, 789- 795. Capaccioni, B., Mangani, F., 2001. Earth Planet. Sci. Lett., 188, 543-555. Des Marais, D.J., Donchin, J.H., Truesdell, A.H., Nehring, N.L., 1981. Nature, 292, 826-828. Mango, F.D., 2000. Geochim. Cosmochim. Acta, 64, 1265-1277.
V11A-1999
Identification of secondary minerals crystallized by low and high temperature alteration in the Northern Kyushu-Palau Ridge volcanic rocks
Seafloor rocks were affected by hydrothermal alteration and low temperature seawater weathering display various elemental behaviors, necessitating detailed investigations to evaluate primary bulk rock compositions without the effect of elemental behaviors during alteration. Seafloor alteration entails primary minerals being changed into hydrous minerals. Bulk chemical compositions of seafloor igneous rocks are changed by high- temperature hydrothermal alteration and low-temperature seafloor weathering. In this study, I report the secondary mineral identifications by XRD analyses in the rocks from the northern Kyushu-Palau Ridge, and consider to condition of alteration processes. Volcanic rocks dredged from the Northern Kyushu Palau Ridge during cruise by Tansei-maru, ORI, University of Tokyo show petrological and geochemical characteristics of low and high temperature alterations. These rocks are classified into bulk water content, that is, low H2O- and LOI samples at the Miyazaki Seamount, high H2O- samples at the Nichinan Seamount, and high LOI samples at the Komahashi-Daini Seamount. The Nichinan Seamount samples show flesh phenocrysts, low altered groundmass minerals, and high degree alteration of groundmass glass, assumed to replace into clay minerals. These altered phenocrysts are identified by XRD to be serpentine, saponite, and talc. And these altered groundmasses are identified by XRD to be saponite with primary plagioclase and clinopyroxene. These results are assumed to replacement of glass into clay minerals under low temperature seafloor weathering. Nichinan Seamount rocks show high alkali-elements contents. The remarkable movement of bulk composition is not occur under the low temperature seafloor weathering except for K and Rb, and these enrichments reflect secondary deposition of celadonite, K-rich smectite (e.g. Nakamura, 2001). Saponite is typical identified, but celadonite is not identified in the Nichinan Seamount rocks. Therefore, the characteristics of bulk composition of the Nichinan Seamount rocks are assumed similarity to primary signature. On the other hand, the Komahashi-Daini Seamount samples show completely re-crystallization, and igneous textures are observed to pseudomorph. These are identified by XRD to be quartz, clinochlore (one of chlorite), and albite. Secondary mineral assemblage is homogeneous in these rocks. The temperature of replacement by chlorite accompanied by enrichment in MgO is estimated to be more than 150°C on the basis of experimental studies (e.g. Mottle 1983). And interpreted two types of albitization, low temperature (< 50°C) and high temperature (> 100°C), are identified on the basis of study of ODP Leg 123 Site 765 igneous rocks (Gillis et al. 1992). Therefore, it is considered that volcanic rocks from the Komahashi-Daini Seamount were under effect of hydrothermal alteration more than 150°C. Many elements show significant movement under high temperature hydrothermal alteration (e.g. Laverne et al. 1996). That is, re-crystallization of chlorite under high temperature hydrothermal alteration accompanied addition of magnesium from seawater and remarkable bulk MgO enrichment (e.g. Nakamura, 2001). Therefore, it is considered that the Komahashi-Daini Seamount rocks show significant MgO-enrichment because of secondary mineralization of chlorite, and assumed to significant movement of other elements. These observations suggest that geochemical investigation of highly altered rocks must be made with caution.
V11A-2000
A Study on Solidification of Abandoned Mine Tailings with Hydrated Lime
Solidification is one of the stabilization processes for wastes and their components to reduce their toxicity and migration rates to surroundings. Hydrated limes were applied as cementing materials to solidify heavy metal contaminated tailings from the Geumjang mine and the solidified tailing specimens were tested for their appropriateness in accordance with the suggested test methods. In the preliminary tests for the solidified tailing specimens, all the specimens have higher uniaxial compressive strengths than 3.5kgf/cm2, the standard recommended for land reclamation solids by EPA(Environmental Protection Agency). Even in leaching tests for the solidified tailing specimens, concentrations of heavy metals such as As, Cd, Cu, Pb, and Zn were decreased significantly below the environmental warning standards in comparison with those of raw tailing samples. The optimum mixing ratio of tailings, hydrated lime, and water was determined through the preliminary tests. The solidified mixtures of mine tailings and hydrated lime through pozzolanic reaction were tested for their durability against repeated freezing and thawing processes. After repeated freezing and thawing, the uniaxial compressive strengths of all the solidified mixture specimens decreased in comparison with those before test but still higher than 3.5kgf/cm2, and concentrations of heavy metals such as As, Cd, Cu, Pb, and Zn were below the standards. Effluents in the repetitive artificial tests show pH's of 7.4 to 9.1 and concentrations of heavy metals such as As, Cd, Cu, Pb, and Zn of below 0.05ppm. Conclusively this study shows potential applicability of hydrated limes to in-situ stabilization of abandoned mine tailings.
V11A-2001
Submarine Hydrothermal Activity on the Aeolian Arc: New Evidence from Helium Isotopes
In November 2007 we conducted a water-column and seafloor mapping study of the submarine volcanoes of
the Aeolian Arc in the southern Tyrrhenian Sea aboard the R/V Urania. A total of 26 CTD casts were
completed, 13 vertical casts and 13 tows. In addition to in situ measurements of temperature, conductivity,
pressure and suspended particles, we also collected discrete samples for helium isotopes, methane, and
trace metals. The helium isotope ratio, which is known to be an unambiguous indicator of hydrothermal input,
showed a clear excess above background at 5 out of the 10 submarine volcanoes surveyed. We found the
strongest helium anomaly over Marsili seamount, where the 3He/4He ratio reached maximum
values of δ3He = 23% at 610 m depth compared with background values of ~7%. We
also found smaller but distinct δ3He anomalies over Enerato, Eolo, Palinuro, and Secca del Capo.
We interpret these results as indicating the presence of hydrothermal activity on these 5 seamounts.
Hydrothermal venting has been documented at subsea vents offshore of the islands of Panarea, Stromboli,
and Vulcano (Dando et al., 1999; Di Roberto et al., 2008), and hydrothermal deposits have been sampled on
many of the submarine volcanoes of the Aeolian Arc (Dekov and Savelli, 2004). However, as far as we know
this is the first evidence of present day hydrothermal activity on Marsili, Enerato, and Eolo. Samples
collected over Filicudi, Glabro, Lamentini, Sisifo, and Alcioni had δ3He very close to the regional
background values, suggesting either absence of or very weak hydrothermal activity on these seamounts.
Helium isotope measurements from the background hydrocasts positioned between the volcanoes
revealed the presence of an excess in 3He throughout the SE Tyrrhenian Sea. These background
profiles reach a consistent maximum of about δ3He = 11% at 2300 m depth. Historical helium
profiles collected in the central and northern Tyrrhenian Sea in 1987 and 1997 do not show this deep
3He maximum (W. Roether and B. Klein, private comm.). Furthermore, the maximum is too deep to be
attributed to the volcanoes of the Aeolian Arc, which are active at <1000 m depth. We are currently
conducting additional measurements to determine whether this deep 3He maximum is from a local
hydrothermal source or is somehow related to the deep water mass transient which occurred in the eastern
Mediterranean in the
1990's.
V11A-2002
Older Hydrothermal Activity along the Northern Yellowstone Caldera Margin at Sulphur Creek, Yellowstone Park, Wyoming
The Tuff of Sulphur Creek (480 ka) is well exposed in the Seven Mile Hole area of the Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming. The rhyolitic tuff erupted after the collapse of the Yellowstone Caldera (640 ka) and hosts more than 350 vertical meters of hydrothermal alteration. Two epithermal alteration assemblages with different mineral associations have been identified in the area: an illite-silica-pyrite phase and a kaolinite-alunite-silica-pyrite phase. Kaolinite and opal occur along the canyon rim, montmorillonite and other smectites are found at intermediate depths, and illite and sulfides (pyrite) are found deepest in the section. Our work on the north side of the Sevenmile Hole altered area has found a complex system of veining. The veins are concentrated in the eastern portion of the canyon and are less frequent to the west. Brecciated cross-cutting veins ranging from 2 to 30cm wide are found at the base of the canyon. Moving vertically up the canyons walls, the veining style becomes less complex. These veins are about 1 to 1.5cm wide and are not brecciated, occurring less frequently than the brecciated veins. The canyon walls and the canyon rim mainly contain millimeter-scale cross-cutting silica veinlets. These stockwork-like veinlets are the most abundant fracture filling that we find throughout the canyon walls. Veins at the base of the system, found in the stream bed, contain abundant sulfides (mainly pyrite). Sulfides are present in three forms: disseminated in a silica matrix, as massive pyrite in healed fractures, and encrusting clays and silica. The latter is the least common. Disseminated and massive sulfides are typically associated with the matrix in the brecciated veins. Breccias include angular clasts of altered tuff with argillized feldspar phenocrysts and fragments of earlier vein-filling opal. Sulfides are most abundant in the bottom of the canyon and in the western part of the field area. Hydrothermal silica, dominantly opal, is pervasive in the veins throughout the entire system. At the base of the system it is very dark grey and found in the matrix of the veins. The silica veins found on the canyon walls are most often white to cream in color. These veins rarely have cross-cutting relationship with other veins. The stockwork veinlets are found throughout the system. Though the veins are narrow, the wall rock surrounding them is pervasively silicified. The silica in these veins is usually bluish in color, or is rarely clear, and contains very minor sulfides. Wall rock alteration associated with all of the vein types does not vary significantly across the field area. The tuff deeper in the system is completely silicified, so much so that outcrops are well exposed and extensive. The canyon walls are mostly silicified though occasionally there are unsilicified pockets. The top of the system is not silicified though there is one section, on the ridge, that has been thoroughly altered such that the original lithology can not be determined in a hand sample.
V11A-2003
Formation Sequences of Iron Minerals in the Acidic Alteration Products and Variation of Hydrothermal Fluid Conditions
Iron minerals have important role in environmental issues not only on the Earth but also other terrestrial planets. Iron mineral species related to alteration products of primary minerals with surface or subsurface fluids are characterized by temperature, acidity and redox conditions of the fluids. We can see various iron- bearing alteration products in alteration products around fumaroles in geothermal/volcanic areas. In this study, zonal structures of iron minerals in alteration products of the geothermal area are observed to elucidate temporal and spatial variation of hydrothermal fluids. Alteration of the pyroxene-amphibole andesite of Garan-dake volcano, Oita, Japan occurs by the acidic hydrothermal fluid to form cristobalite leaching out elements other than Si. Hand specimens with unaltered or weakly altered core and cristobalite crust show various sequences of layers. XRD analysis revealed that the alteration degree is represented by abundance of cristobalite. Intermediately altered layers are characterized by occurrence including alunite, pyrite, kaolinite, goethite and hematite. A specimen with reddish brown core surrounded by cristobalite-rich white crust has brown colored layers at the boundary of core and the crust. Reddish core is characterized by occurrence of crystalline hematite by XRD. Another hand specimen has light gray core, which represents reduced conditions, and white cristobalite crust with light brown and reddish brown layers of ferric iron minerals between the core and the crust. On the other hand, hornblende crystals, typical ferrous iron-bearing mineral of the host rock, are well preserved in some samples with strongly decolorized cristobalite-rich groundmass. Hydrothermal alteration experiments of iron-rich basaltic material shows iron mineral species depend on acidity and temperature of the fluid. Oxidation states of the iron-bearing mineral species are strongly influenced by the acidity and redox conditions. Variations of alteration products sequence observed in the geothermal area suggest that the hydrothermal fluids change their geochemical conditions temporally and spatially.
V11A-2004
Mongolian Coal-bearing Basins and Their Characteristics
In Mongolia, more than 200 coal occurrences and deposits have been known and they are classified into 2 coal-bearing provinces, 12 basins, and 3 areas. The ages of coals tend to become younger from west to east. The deposits in the western Mongolia are Pennsylvanian, in the southern Mongolia they are Upper Permian, whereas those in central and northern Mongolia are mainly Lower-Middle Jurassic. In the Eastern Mongolia, almost entire coals are Lower Cretaceous. Concomitant with the eastward younging of coal- bearing sedimentary sequences, their coal display a decrease in rank. High rank bituminous coals, probably semianthracite, occur within Carboniferous rocks. Medium- to low rank bituminous coals are related to Upper Permian rocks. Subbituminous coals are in Jurassic sedimentary rocks, whereas lignite is hosted in Lower Cretaceous rocks. The coalification trend can be determined based on the ultimate, proximate analysis, calorific value, and vitrinite reflectance data. Carboniferous coals have 82.7% carbon, 4.5% hydrogen, 0.5% total sulfur contents, and 20.9% ash yield. Calorific value ranges from 27.5 to 35.2 MJ/kg. Permian coals have 22.1% ash yield, 0.6% total sulfur contents, and 33.4 MJ/kg calorific values. Jurassic coals have 75.3% carbon, 4.6% hydrogen, 1.0% sulfur, 16.7% ash yield, and 28.4 MJ/kg calorific values. For Cretaceous coals, average carbon and hydrogen contents are 66.2% and 4.5%, respectively. Total sulfur content and ash yields are 1.2% and 16.6%, respectively. Average calorific value is 26.4 MJ/kg. The mean vitrinite reflectance values of Carboniferous and Permian coals are 1.7% and 1.0% respetively, whereas those of Jurassic and Cretaceous coals are 0.5% and 0.3%, respectively. Petrographical studies confirmed that vitrinite-huminite groups of Carboniferous, Permian, and Creatceous coal range from 44.9 to 82.9% on mineral matter free basis. Inertinite group varies between 15.0 and 53.3%, and liptinite group does not exceed more than 7%. Jurassic coals are characterized by high concentration of vitrinite (87.3-96.6%) and liptinite (up to 11.7%) groups, and by low concentration of inertinite groups (up to 6.0%). The identified reserve of Mongolian coals is calculated at 10.1 billion ton, of which predominant portions are lignite in Eastern Mongolia (67.7% of total reserves) and coking coals in Southern Mongolia (29.2%).
V11A-2005
Geochemistry of Geothermal Springs In Northern Dominica, Lesser Antilles
The island of Dominica, Lesser Antilles contains eight potentially active volcanoes, many of which are associated with geothermal springs. During the period 2003-2007 most of these springs were sampled and analyzed geochemically. The data presented here are for three groups of geothermal springs located in the northern part of the island. One group, the Penville Cold Soufrière, is located within the summit area of Morne Aux Diables volcano. The second group, Picard Warm Springs, is located on the northwestern flank of Morne Diablotins volcano. The third group is located in the Portsmouth area, including the adjacent Prince Rupert Bay, and consists of both subaerial and submarine springs. It is not known with which volcano these springs may be associated. The chemistry of each sample from the geothermal springs was compared to “reference standards” including three seawater samples (standard seawater and two surface samples from Prince Rupert Bay) and three fresh water samples (Emerald Pool and two rainwater samples). Of the more than 38 elements analyzed, 21 elements were consistently two or more orders of magnitude higher when compared to the “reference” standards. When these values were plotted on chemical variation diagrams, two trend lines were consistently developed. One included seawater and the submarine hot springs; the other, the fresh water samples and all the subaerial springs. The intersection of these trend lines is interpreted to represent the composition of a possible magmatic component prior to dilution with seawater and/or meteoric water. Oxygen and hydrogen isotopic analysis of the springs for the whole island have also been undertaken. VSMOW graphs of δ 18O and δD indicate that all of the geothermal springs on Dominica lie on a trend between the meteoric water line (MWL) and a magmatic source. The compositions of the geothermal springs from northern Dominica are interpreted to represent mixing of variable amounts of freshwater or seawater with a magmatic source that appears to be constant for all these northern springs. The similarity of the isotopic results for all the geothermal springs on Dominica suggests that this magmatic source may be constant throughout the island.
V11A-2006
Multicollector High Precision Resolution of Primordial Kr and Xe in Mantle CO2 Well Gases
Noble gas isotopes in magmatic CO2 well gases provide a unique insight into mantle volatile origin and dynamics [1-3]. Previous work has resolved mantle 20Ne/22Ne ratios consistent with a solar wind irradiated meteoritic source for mantle He and Ne [1]. This is distinct from Solar Wind values that might be expected if the primary mechanism of terrestrial mantle volatile acquisition was through the gravitational capture of solar nebula gases [see 4]. Within the CO2 well gases a primordial component has also been resolved in the non- radiogenic Xe isotopic composition [2,3]. Using multicollector mass spectrometry we have observed a 124Xe/130Xe excess of 1.85 percent over air plus/minus 0.17 percent for the least air contaminated samples. At this level of precision we are for the first time able to differentiate between a trapped meteoritic origin (average carbonaceous chondrite or Q Xe) rather than Solar Wind origin as the primordial Xe component. The well gases also contain Kr which, in the least air contaminated sample, have a correlated 86Kr/82Kr excess of 0.55 percent over air plus/minus 0.04 percent. Whilst mass dependent fractionation can theoretically produce correlated excesses in 124Xe-128Xe and 82Kr-86Kr isotopes, no fractionation from air is observed in 38Ar/36Ar [3] and the Kr excesses are in the opposite sense to that of Xe. From 136Xe excesses, Kr fission yield from Pu and U can be calculated and subtracted from the Kr isotopic signature. This fission-corrected signature is most reasonably explained as a primordial component. This is the first time that primordial Kr has ever been resolved in a terrestrial sample. The primordial Kr isotopic signature is distinct from Solar Wind Kr and is consistent with the primordial Kr also originating as a trapped component within meteorites. We are now able to demonstrate that both the light (He and Ne) and Heavy (Kr and Xe) noble gas origin in the terrestrial mantle is consistent with a trapped component during the accretionary process and can discount gravitational capture as the principle mechanism of interior planetary volatile acquisition. References: [1] Ballentine, C.J. et al., Nature, 433. 33-38, (2005) [2] Caffee, M.W. et al., Science, 285, 2115- 2118, (1999) [3] Holland G. and Ballentine C. J., Nature, 441, 186-191 (2006) [4] Pepin R.O. and Porcelli D. Rev. Min. Geochem. 47, 191-246 (2002)
V11A-2007
Hydrogen Gas from Serpentinite, Ophiolites and the Modern Ocean Floor as a Source of Green Energy
Hydrogen gas is emitted by springs associated with serpentinites and extensive carbonate deposits in Oman, The Philippines, the USA and other continental locations. The hydrogen springs contain unusually alkaline fluids with pH values between 11 and 12.5. Other workers have described off-ridge submarine springs with comparably alkaline fluid compositions, serpentinite, abundant free hydrogen gas, and associated carbonate edifices such as Lost City on the Atlantis Massif 15 km west of the Mid-Atlantic Ridge (D.S. Kelley and associates, Science 2005). The association of hydrogen gas with ultramafites is a consistent one that has been attributed to a redox couple involving oxidation of divalent iron to the trivalent state during serpentinization, although other possibilities exist. Some of the hydrogen springs on land are widespread. For example in Oman dozens of alkaline springs (Neal and Stanger, EPSL 1983) can be found over thousands of sq km of outcropping ophiolite. While the deposits in Oman and the Philippines are well-known to much of the geochemical community, little interest seems to have been displayed toward either the ophiolitic occurrences or the submarine deposits for energy production. This may be a mistake as the showings because they could lead to an important source of green energy. Widespread skepticism currently exists about hydrogen as a primary energy source. It is commonly said that free hydrogen does not occur on earth and that it is therefore necessary to use other sources of energy to produce hydrogen, obviating the general environmental benefit. However the existence of numerous occurrences of hydrogen gas associated with ophiolites and submarine occurrences of hydrogen suggests the likelihood that natural hydrogen gas may be an important source of clean energy for modern society remaining to be tapped. Calculations in progress should establish whether or not this is likely to be the case.
V11A-2008
Measuring fluorite solubility in situ by using a hydrothermal diamond anvil cell
Fluorite is often found in fissure veins and is considered a replacement deposits in many hydrothermal systems, often closely associated with igneous activity. Understanding fluorite solubility in H2O and in NaCl- and HCl-bearing aqueous solutions over a large range of temperatures and pressure will increase our understanding of fluorite deposited in the natural environment. This study sought to estimate the solubility of fluorite (CaF2) in aqueous solutions by using a hydrothermal diamond anvil cell (HDAC) and an in situ weight loss technique up to 648 Kelvin and 35.1 MPa. This allowed direct observation of the sample though the diamonds, eliminating any experimental errors associated with quench. Optically clear and unaltered fluorite chips were chosen based on shape and size. The ideal shape is a cube so as to more accurately determine the volume and, thus, the mass of the chip, however, equidimensional tetrahedron were also used. A single fluorite chip was placed into the sample chamber with an aqueous solution. The load pressure was manipulated until a vapor bubble and liquid were present within the sample chamber. The sample was heated until the fluid homogenized to either a vapor or liquid. High-resolution images were taken of the sample chamber and the fluorite sample throughout the duration of the experiment. The volume of the sample chamber and the fluorite chip were estimated through visual measurements and by using Visual Inspector image capture and annotation software. The calculated mass of the chip during the experiment was compared to measurements of the chip taken prior to heating to determine the mass of fluorite lost into the solution. The mass of the chip was also measured before and after the experiment. The experiments conducted thus far in a pure water system show very little mass loss of fluorite, indicative of a low solubility. For example, the range of calculated solubilities of fluorite in pure water along the 0.60 g/cm3 isochore up to 648 K and 35.1 MPa are on the order of 200 ppm. Experiments in NaCl-H2O and HCl-H2O systems will also be discussed.
V11A-2009
The impact of new data on an expert elicitation of the probability of volcanic intersection of the nuclear waste repository at Yucca Mountain
An expert elicitation completed in 1996 estimated the frequency of intersection by a dike of the proposed high-level radioactive waste repository at Yucca Mountain. That assessment resulted in a mean annual frequency of intersection of 1.7e -8, or about 1 chance in 60 million per year. The expert panel primarily considered the past 5 million years of volcanic history when estimating the probability of intersection, and included in their assessment the possibility that at least a few volcanic centers were buried in alluvial basins near Yucca Mountain. The results of a regional aeromagnetic survey completed in 1999 suggested the potential for a higher number of buried volcanic centers than previously considered in the 1996 elicitation. Therefore, a new expert elicitation was convened in 2004 with the primary purpose of assessing the impact of buried volcanic centers on probability estimates. A major data-gathering effort completed in 2006 provided high-resolution aeromagnetic, drilling, and geochronology results that provided information on the number, location and age of buried basaltic centers near Yucca Mountain. Most buried basalt was determined to be of Miocene age. The youngest group of buried volcanic centers, 20-25 km south of Yucca Mountain, has an age of about 3.9 Ma. These results indicate that post-Miocene basaltic volcanism only occurred to the south and west of Yucca Mountain and not to the east, an important constraint in models that forecast the potential location of future volcanism. Other new data considered included the characteristics of dike swarms based on analog volcanoes, variations in crustal extension across the region, mantle tomography, and differences in calculated lithostatic pressure between basins and ranges. The new distribution for the annual frequency of intersection (5th and 95th percentiles of ~6e -10 and 1e -7) overlaps the 1996 distribution but is broader with more weight at both higher and lower values. This results in a slightly higher mean value and a slightly lower median value. The results are consistent with consideration of a broader range of conceptual models for the spatial and temporal behavior of volcanism, as well as more complex models of the geometry of volcanic events, all influenced in part by availability of new data.
V11A-2010
High temperature potassic alteration in magmatic hydrothermal systems
During the ascent of magma to shallow portions of the crust, decompression of the melt or the crystallization of anhydrous phases may cause one or more magmatic volatile phases (MVP) to be exsolved. The MVPs are most commonly chlorine-rich waters that are hypothesized to be responsible for some areas of hydrothermal alteration and the transfer of metals to sites of ore deposition. The importance of the subcritical, vapor undersaturated high-salinity liquid phase (brine) is widely known, however, a dearth of data exists on the properties of that phase at elevated temperatures and pressures. This study is intended to fill in the gaps in the experimental database, specifically, those associated with the acidity of the brine. We examined phase stability and noted equilibrium fluid compositions in the K-feldspar-muscovite-andalusite- quartz system from 550 to 750°C and 50 to 80 MPa. Determinations of mineral phase stabilities were conducted by varying the pressure, temperature, and log10(KCl/HCl) values of the fluids. The mineral run products were analyzed by using a JEOL JSM-5610LV Scanning Electron Microscope with Energy Dispersive Spectrometry (SEM-EDS). The stable mineral phase was determined by surface morphology and by the K/Al and Al/Si ratios. The nucleation and growth of new phases were observed easily: K-feldspar would form blocky crystals, muscovite would form platy crystals, and andalusite was commonly observed as long needle-like crystals. Once the phases were identified, the phase stability fields were plotted as a function of temperature and log10(KCl/HCl) ratios of the fluids. The results of this study show that the equilibrium KCl/HCl values are much lower than in brine-undersaturated vapor and supercritical fluid systems. It appears that as salinity increases, the aKCl/aHCl will not remain fixed because γKCl and γHCl are not equivalent for KCl and HCl in high-salinity fluids. Our data should be used to update thermodynamic mineral datasets to improve models of high-temperature mineral alterations in the presence of high-salinity fluids. Currently, according to our data, present thermodynamic models underestimate the HCl concentration of the fluid by up to two orders of magnitude.
V11A-2011
Geochemical Modeling of Zinc Silicate Ore Formation from Sedimentary Hydrothermal Fluids
Sediment-hosted zinc deposits dominated by willemite (Zn2SiO4) instead of sphalerite (ZnS) are known from several prominent occurrences worldwide, including Vazante, Brazil, the Aroona Trend, Australia, Kabwe, Zambia, Berg Aukas, Namibia, and Abu Samar, Sudan. Although willemite-dominant zinc deposits appear to be much less common and are on average smaller than sphalerite-dominant zinc deposits, they nonetheless represent major enrichments of zinc in the Earth's crust, reaching sizes on the order of 1's to 10's of millions of tons and grades commonly between 20 and 40%. Sediment-hosted willemite- and sphalerite-dominant deposits share many similarities including their predominantly carbonate host rocks, gangue mineralogy, presumed derivation from sedimentary basinal brines, and spatial proximity. However, the conditions and processes that led to one style of mineralization versus the other have only recently begun to be investigated. The current study presents solubility, reaction path, and reactive transport modeling results that attempt to define more clearly the conditions that favor willemite ore formation in sedimentary basins, with a focus on the Vazante deposit. Solubility calculations for willemite and sphalerite as a function of temperature, pH, salinity, and oxidation potential were carried out using a simple 3 molal NaCl solution saturated with respect to quartz. The results show that (1) willemite solubility is relatively insensitive to changes in temperature and oxidation potential whereas sphalerite solubility decreases sharply with decreasing temperature and oxidation potential, (2) willemite solubility decreases more strongly than sphalerite with increasing pH, (3) willemite and sphalerite have a similar strong decrease in solubility with decreasing salinity. The results support a previously proposed genetic model for a willemite-dominant, sphalerite-subordinate ore body like Vazante in which a hot, acidic, metal-rich ore fluid mixed with a cooler, more oxidizing, dilute, and basic fluid. This scenario was investigated further with reaction path and reactive transport modeling. In these models, a more complex ore fluid was used that was assumed to have a major element composition similar to the global average for Mississippi Valley-type (MVT) deposits determined from the literature, modified by heating from 150 to 300° C, saturated with respect to dolomite and quartz, moderately acidic, and an oxidation potential near the value defined by magnetite-hematite equilibrium. The ore fluid was allowed to mix with a second, possibly meteoric fluid with about three orders of magnitude lower salinity, neutral pH, and a temperature of 50° C. The modeling results showed general agreement with the mineral assemblage observed at Vazante, and confirmed the need for a strong pH increase to induce willemite precipitation, and no more than a moderate increase in oxidation potential to allow some sphalerite to precipitate. The localization of mineralization within a shear zone was found to depend strongly on the shear zone having acted as a high permeability conduit for the ore fluid from deeper parts of the sedimentary basin.
V11A-2012
New Evidence for the Origin and Sources of Metals for the Giant Mississippi-Valley Type Pine Point Zn-Pb District
The Pine Point mining district is located on the southern shore of the Great Slave Lake in the North West
Territories, Canada. Two million tons of lead and seven million tons of zinc were extracted from the sphalerite
and galena-rich ore produced from the open pit mines in the district from 1965 to 1987. The ore
mineralization is confined to secondary dolomites of Middle Devonian age near the northern edge of the
Alberta section of the Western Canada Sedimentary Basin (WCSB).
Despite the significant number of
studies, both the nature of the event(s) that have lead to the ore formation and the age of the deposits are
still in debate. Ore deposition in the district is attributed to either a large-scale topographically driven fluid
flow or to a localized influx of geothermal fluids from the Precambrian basement. Also, despite the general
agreement that mineralization was caused by fluid mixing involving a concentrated CaCl2 brine, there is
no agreement on the composition and origin on the other end-member fluid(s), nor is there an agreement on
which fluid(s) carried the base metals.
Ore bodies of the Pine Point district have Pb/Zn ratios higher than
1.0, whereas most Alberta Basin fluids have Pb/Zn ratios lower than that. Therefore, these have been
considered poor candidates for Pine Point ore forming fluids. The perceived absence of suitable present-day
analogues of the Pine Point ore forming fluids among the Alberta Basin formation-waters has been also used
by some to propose that ore formation in the district was associated with the expulsion of hypothetic hot, Zn-
rich brines along a shear zone in the underlying Precambrian basement. However, new information we
present here demonstrates that a recently discovered highly concentrated CaCl2 brine in Middle
Devonian sedimentary rocks of north-central Alberta is Zn-rich and Pb-poor and, hence, a suitable modern-
day analogue to the principal Pine Point ore forming fluid(s). The dissolved element and isotope
compositions of that newly discovered CaCl2 brine indicate that it is well preserved highly evaporated
Ca-rich seawater of Devonian age. The Alberta Basin brine samples are very similar to CaCl2 brines
found in potash mines in the Williston part of the WCSB demonstrating that these fluids are common through
the WCSB.
In this presentation we will discuss the implications of the presence of highly evaporated Ca-rich
Devonian seawater in the Alberta Basin and its role in the formation of the Pine Point Pb-Zn deposits and
similar ore mineralizations basin- and world-wide.
V11A-2013
Strontium Isotopes in Secondary Silicate Minerals Produced During Paleogroundwater Flow, Socorro, New Mexico
Physical microsampling of silicate alteration phases produced during past episodes of groundwater flow, combined with Sr isotopic data, can potentially be used to determine the age of the alteration phases and to define paleogroundwater flow paths. Recent studies of the K-metasomatized, mid-Tertiary Lemitar Tuff near Socorro, central New Mexico (Fritz and Farmer, Chem, Geol., 2006) suggested that secondary adularia produced during metasomatism may record the original Sr isotopic compositions of altering fluids, the later providing information regarding the source of the solutes in the fluids. Here we expand our microsampling of secondary minerals in the altered Lemitar Tuff to a wider geographic area with the intent of determining if spatial variations in fluid Sr isotopic compositions are recorded in these minerals and if these variations can be used prescribe the direction of paleogroundwater flow. Samples were obtained of K-metasomatized tuff and overlying Popotosa Conglomerate along along an ~40 km transect through the entire region of alteration from potential recharge area in Precambrian granitic rocks of the Magdalena Mountains to the west, and discharge region in similar rocks some 4 km to the east. Most of the new adularia microsamples plot on a steeper errorchron (~12 m.y.) with lower apparent initial ratios (~0.711) than determined for the adularia samples analyzed by Fritz and Farmer (2006) (~7 m.y., ~0.721), regardless of geographic position. Unlike the samples obtained in the early study, our new microsamples contain varying proportions of secondary clay minerals, which are difficult to avoid during sampling. Clay mineral-rich adularia microsamples have higher Sr contents (100 ppm) and lower measured 87Sr/86Sr (~0.711) than their clay poor counterpart, suggesting that Sr –errorchron" may be a mixing line between a high Rb/Sr, high 87Sr/86Sr adularia and low Rb/Sr, low 87Sr/86Sr clay minerals. If so, then the clay minerals must either have formed at later time than adularia and incorporated Sr from a fluid containing solely tuff-derived Sr, or the Sr incorporated into clay minerals must have come solely from altering, low 87Sr/86Sr, igneous plagioclase, unlike the adularia. In either case, the intimate association between clay minerals and adularia, and differences in either the timing or mode of their crystallization, renders it difficult to extract with our current methodology unambiguous information regarding the Sr isotopic compositions of the K-metasomatizing fluids.
V11A-2014
Formation of massive chromitite by assimilation of iron formation in the Blackbird Deposit, Ontario, Canada
The conventional classification of chromitite into two principal varieties, stratiform and podiform, fails to account for an important third type which is exemplified by the Kemi Deposit of Finland, the Ipueira-Medrados deposit in Brazil, and the Sukinda deposits of India. Stratiform deposits are widely recognized as regionally extensive layers up to one or two metres thick occuring as part of the ultramafic or mafic stratigraphy in large layered intrusions whereas the podiform chromitites appear as irregular masses hosted by discordant dunite pods or dikes in the mantle section of ophiolite complexes. Here we report a new occurence of the Kemi type at the Blackbird deposit in the Superior Province of Ontario, Canada. Like the other Kemi type deposits, the Blackbird deposit consists of lensoid bodies of layered chromitite and dunite up to several tens of metres thick and hundreds of m long. The host rocks have been thoroughly altered to talc and carbonate but are interpreted to be meta-harzburgites and meta-dunites forming an ultramafic sill several km long and several hundred m thick. The sill was emplaced at the base of a supracrustal succession of iron formation and mafic to intermediate lavas and tuffs overlying granitoid basement rocks. There is textural evidence for the assimilation of iron formation by the parent magma. Chromite modes in the intrusion vary from below 1% to over 90%; Cr2O3 concentrations in chromite range from 45 to 60 wt%. We have modeled the petrogenesis of the chromitite bodies by considering the effects of assimilation of water-bearing iron formation on the evolution of a primary picritic magma using MELTS thermodynamic modeling software (Ghiorso and Sack 1995; Contrib Mineral Petrol 119, 197-212). Contrary to our expectations, the addition of H2O alone to the picrite magma delays chromite crystallization slightly relative to olivine, compared with the results for the picrite magma alone. Addition of 20 wt% dry iron formation causes an additional 1.4% chromite to crystallize by the time that 30% of the contaminated picrite has crystallized as olivine or orthopyroxene. Addition of a mixture of 20% iron formation and 1% H2O causes the crystallization of 4% chromite, producing a modal abundance of chromite in the resuting cumulates of about 12 modal %. We conclude that the contamination of a primary picritic magma by water-rich iron formation in an active magam conduit, followed by mechanical sorting into chromite-rich and chromite-poor depositional bedforms, can account for the genesis of the Kemi- type massive chromitite deposit as it occurs at the Blackbird deposit in Ontario.
V11A-2015
Weathering Fluxes From Volcanic and Ultramafic Terranes on Luzon Island, The Philippines - Preliminary Observations
Chemical weathering of silicates consumes atmospheric CO2. This process is the main regulator of global climate on million-year timescales. Basalt weathers faster than other silicates and chemical weathering of basalt is therefore a particularly important control on long-term global climate. Observed physical erosion rates on high-standing volcanic islands in the humid tropics are very high and we hypothesize that chemical weathering rates and the concomitant consumption rates of atmospheric CO2 on these islands are very high. To test this, we are conducting a survey of representative rivers and streams in volcanic and ophiolitic areas in the Philippines, with the aim of illuminating the rates at which chemical weathering is proceeding in high-standing volcanic and ultramafic areas in the Philippines. The data set we present here is the first extensive data set of chemical composition of rivers draining volcanic and ultramafic areas in the Philippines. Samples were collected during the dry season (January and February) of 2007 and 2008 and a project is underway to conduct additional sampling monthly until the summer of 2009. Stream discharge was measured during sampling. We will present weathering fluxes calculated using our chemistry and discharge datasets. Preliminary results indicate that lithology exerts a first-order control on water chemistry as illustrated with a distinct difference in pH between water draining the Zambales ophiolite (high pH) and volcanic regions (slightly acidic to circumneutral pH). Nevertheless, the river and ground water sampled, regardless of bedrock, are saturated w. respect to primary silicate minerals. Many samples, most notably from the Southern Luzon Volcanic Zone, plot near the saturation line for amorphous silica. This may be a function of low flow conditions during time of sampling. Patterns of solute provenance and magnitude of weathering fluxes as a function of lithology and other spatial parameters will be further discussed in this presentation.
V11A-2016
Toward More Efficient Heat Mining: A Planned Enhanced Geothermal System Demonstration Project
We describe plans for a project of about 12-months duration to demonstrate new technology aimed at lowering the cost of an Engineered Geothermal System (EGS). An EGS creates or enhances a geothermal reservoir by stimulating fractures in hot rock, and then cycles water through the fractures to mine the in-place heat. The goal of the project is to create multiple independent fracture zones from a single well in order to increase the aggregate flow rate without increasing the pressure drop through the system. Because well drilling and completion together represent the biggest single cost of an EGS, the ability to increase flow rates without resorting to additional wells would significantly reduce the cost of this power-generation technology. A microseismic array will allow real-time monitoring of fracturing progress and post-stimulation development. New methods will be tested to evaluate reservoir heat-exchange area, flow-paths, and fracture connectivity. Fluid chemistry will be monitored for chemical changes. The stimulation target is a hot, intrusive body in Northern California underlying an existing geothermal resource.
V11A-2017
Hydrothermal Quartz Oxygen Isotope Ratios in Altered Post-Collapse Rhyolite at Sevenmile Hole, Grand Canyon of the Yellowstone River, Yellowstone National Park, WY
The Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming, displays regions of pervasively hydrothermally altered rock formed in the shallow, epithermal portions of a hydrothermal system. Hydrothermal fluid circulation causing the alteration is driven by magmatism related to the Yellowstone Caldera thermal anomaly. The protolith, the Tuff of Sulfur Creek, is a 480 ka high silica, low δ18O rhyolitic tuff that erupted after the Yellowstone caldera collapse at 640 ka. Incision of the canyon has exposed 350 vertical meters in the Sevenmile Hole vicinity. Hydrothermal mineralogy determined by standard XRD powder techniques and PIMA on over 90 samples shows both vertical and lateral variation. A vertical transition occurs from kaolinite at depths less than about 100 meters below the present day canyon rim, to illite in deeper exposures. This transition may correspond to a temperature of 150°C, based on a similar transition in the active Yellowstone hydrothermal system. A lateral variation of mineral assemblages in the altered tuff suggests temperatures that may range up to 330°C. Alteration was most likely caused by a liquid due to the presence of pyrite throughout. Local zones of suspected hydrothermal fluid upwelling correspond to the most intense silicification and highest temperature mineral assemblages. This alteration includes quartz + illite ± hyalophane, slawsonite, and buddingtonite. At similar depths outside inferred fluid upwelling zones, lower temperature assemblages are quartz + illite/smectite ± alunite and buddingtonite. At shallow depths, the lowest temperatures are suggested by the presence of quartz + kaolinite ± alunite and opal. Dickite, a kaolinite polymorph, may indicate locally higher temperatures in the shallow kaolinite zones. Oxygen isotope ratios of silica phases were measured for approximately 50 samples using laser fluorination techniques with an error of ±0.2‰. Hydrothermal quartz displays δ18O signatures more negative (ranging from -5.1 to -0.7 ‰) than quartz phenocrysts in the unaltered tuff (typically 1.7 ‰). Relict quartz phenocrysts in altered tuff have an average value of 0.5 ‰ (n=23, σ=0.7), suggesting minimal oxygen exchange with the hydrothermal fluid. Paragenetically later prismatic vug-filling quartz yielded the lowest average values of -4.5 ‰ (n=5, σ=0.4) while earlier massive quartz in the same vugs range up to -1.0 ‰. The variation shows that the fluid ratio and/or temperature evolved as the vug crystallized inward. Matrix silicification produced quartz δ18O values averaging -4.1 ‰ (n=11, σ =0.8). The low hydrothermal silica signatures suggest that they formed in equilibrium with a fluid that was dominantly meteoric water. There is obvious disequilibrium between open space filling hydrothermal quartz and matrix silicification that may be due to the timing of interaction with circulating waters, progressing water-rock ratios, and/or variable temperatures of alteration. Higher temperature mineral assemblages appear to correspond laterally with the more negative quartz δ18O values. Water-rock interaction was likely greater in these areas and they may mark the location of intense hydrothermal fluid upflow and/or higher temperatures.