Fluid inclusion evidence for rapid formation of the vapor-dominated zone at Sulphur Springs, Valles caldera, New Mexico, USA

Fluid inclusion evidence for rapid formation of the vapor-dominated zone at Sulphur Springs, Valles caldera, New Mexico, USA

Joumalof volcanology and geotbennal research ELSEVIER Journal of Volcanology and Geothermal Research 67 (1995) 161-169 Fluid inclusion evidence for...

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Joumalof volcanology and geotbennal research

ELSEVIER

Journal of Volcanology and Geothermal Research 67 (1995) 161-169

Fluid inclusion evidence for rapid formation of the vapor-dominated zone at Sulphur Springs, Valles caldera, New Mexico, USA Masakatsu Sasada a,*, Fraser Goff b h

a Geothermal Research Department, Geological Survey of Japan, Tsukuba 305,Japan Earth and Environmental Sciences Division, ESS-J, MS D462, Los Alamos National Laboratory, Las Alamos, NM 87545, USA

Received 12 October 1992; accepted 29 September 1994

Abstract Microthennometric measurements were obtained for 618 fluid inclusions in hydrothermal quartz, fluorite and calcite and magmatic quartz phenocrysts in intracaldera tuffs from the VC-2A core hole in order to study evolutionary processes of the Sulphur Springs hydrothennal system in the Valles caldera. Relatively high Th values in samples from shallow depths indicate erosion of about 200 m of caldera fill since deposition of hydrothermal minerals at shallow depths in the Sulphur Springs hydrothennal system, accompanied by a descent in the water table of the liquid-dominated reservoir. For samples collected below the current water level of the well, the minimum values of homogenization temperature (Th ) fit the present thennal profile, whereas minimum Th values of samples from above the water level are several tens of degrees higher than the present thennal profile and fit a paleo-thermal profile following the boiling point curve for pure water, as adjusted to 92 e C at 20 m below the present land surface. This is attributed to development of an evolving vapor zone that fonned subsequent to a sudden drop in the water table of the liquid-dominated reservoir. We suggest that these events were caused by the drainage of an intracaldera lake when the southwestern wall of the caldera was breached about 0.5 Ma. This model indicates that vapor zones above major liquid-dominated geothermal reservoirs can be formed due to dramatic changes in geohydrology and not just from simple boiling.

1. Introduction Core hole VC-2A at Sulphur Springs is the second in a series of scientific holes designed to study the Valles caldera magma-hydrothermal system (Fig. 1; Goff and Nielson, 1986; Goff et aI., 1992). The objectives of the VC-2A project were: (1) to investigate the vapor zone above, and its interface with, an active hightemperature (> 200°C), liquid-dominated geothermal system; (2) to in vestigate subsurface structure and stratigraphy near the boundary of the resurgent dome and

* Coorresponding author 0377-0273/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0377-0273(94)00101-4

the western ring-fracture zone; and (3) to investigate possible mechanisms of ore deposition in a large, Quaternary silicic caldera (Goff et aI., 1987). One of the major scientific discoveries of VC-2A was a sub-oregrade molybdenite deposit at 25-125 m depth in brecciated (Hulen et aI., 1987) , post-caldera tuffs and sedimentary rocks whose ages are about 1 Ma. K-Ar dates on hydrothermal illite associated with the molybdenite indicates the deposit was formed ;;;. 0.66 Ma (WoldeGabriel and Goff, 1989). The secondary mineral assemblage and preliminary fluid inclusion data indicate that the deposit was precipitated from liquid water at temperatures of about 200°C (Sasada, 1987;

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2. Geology

Gonzalez and McKibben, 1987), although the deposit now occurs in a region of the hydrothermal system where vapor is the pressure-controlling phase (Hulen et ai., 1987). Thus, the present vapor zone at Sulphur Springs was formed sometime after about 0.66 Ma. In this paper we will discuss evolutionary processes of the Sulphur Springs hydrothermal system involving the formation of vapor zone, based on the microthermometric data of fluid inclusions in minerals from core hole VC-2A. Relatively high Th values at very shallow depths (200°C at 25 m) indicate that the paleo-water table was some 200 m higher than the present land surface. Sometime after the shallow molybdenum deposit was formed from liquid water at about 0.66 Ma, the water table of the 'hydrothermal system dropped. The lack of Th values between the present thermal profile and the boiling point curve adjusted to 92°C at 20 m below the ground surface (2494 m a.s.i.) suggests rapid creation of the vapor zone after a sudden drop in the water table. We conclude that the rapid descent of the top of the liquid-dominated reservoir in the Sulphur Springs area was caused by breaching of the southwestern wall of the Valles caldera and sudden draining of an intracaldera lake at about 0.5 Ma (Doell et ai., 1968; Goff and Shevenell, 1987). This model indicates that dramatic changes in geohydrology can create vapor zones above major liquid-dominated geothermal systems. ~~

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Volcanic activity of the Jemez Mountains region in New Mexico began in the Miocene (16.5 Ma), but culminated during two big ignimbrite-forming eruptions at 1.50 and 1.14 Ma; the younger eruption produced the Valles caldera (Smith and Bailey, 1966, 1968; Doell et ai., 1968; Gardner et ai., 1986; Self et ai., 1986; Spell et ai., 1990). Lacustrine deposits are interbedded with ring fracture rhyolites in the northern and western caldera moat, indicating that (a) paleolake (s) filled the caldera depression intermittently until roughly 0.5 Ma (Smith et ai., 1970). The volcanic and volcaniclastic rocks within a large part of the caldera depression display intense hydrothermal alteration much of which apparently occurred beneath the caldera lake (Doell et ai., 1968; Dondanville, 1978). Age dates evaluated by the 234U / 238 U method on older travertine deposits above Soda Dam in San Diego Canyon (southwest of the Valles caldera) (Fig. 1) indicate that the paleo-lake drained sometime between 0.5 and 0.43 Ma (Goff and Shevenell, 1987). The Sulphur Springs area is presently a zone of acidsulfate hot springs, mud pots, and fumaroles that discharge from the intersection of the Sulphur Creek Fault and several cross faults (Goff et ai., 1985). The immediate area around the springs is devoid of vegetation and bleached white due to argillic to advanced argillic alteration (Charles et aI., 1986). In other areas, such

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M. Sasada. F. Goff/Journal of Volcanology and Geothermal Research 67 (1995) 161-169

features are associated with vapor-dominated conditions above deeper liquid-dominated geothermal reservoirs (White et ai., 1971).

3. The VC·2A core hole Core hole VC-2A penetrates landslide debris, volcaniclastic sediments, the Upper Tuffs of Nielson and Hulen ( 1984), the Tshirege Member of the Bandelier Tuff (1.14 Ma), the Otowi Member of Bandelier Tuff (1.50 Ma) and the Lower Tuffs of Nielson and Hulen (1.78 Ma; Spell et aI., 1990) to a total depth of 527.6 m. Subordinate volcaniclastic sediments occur between the tuff units (Hulen et aI., 1987, 1988; Hulen and Nielson, 1991). The uppermost volcaniclastic sequence contains rhyolitic pumice and 1-5 mm diameter accretionary lapilli that appear to have a phreatomagmatic origin (probably involving a shallow lake) and whose age is estimated at 1 Ma. The core recovered from VC-2A shows intense but variable alteration, and the following vein-forming minerals are observed: quartz and fluorite mainly above 160 m depth; sericite throughout the hole; and chlorite and calcite below 160 m depth (Hulen et aI., 1987, 1988). Borehole temperatures were measured several times after drilling, and the bottom hole temperature measured 33 days after completion of the drillhole was 212°C (Goff et aI., 1987).

4. Occurrence of fluid inclusions Fluid inclusions are present in hydrothermal quartz, fluorite and calcite from veins and vugs, and in magmatic quartz phenocrysts and their hydrothermal overgrowth rims in the rhyolitic tuffs. The hydrothermal minerals contain primary and secondary aqueous inclusions, and the phenocrysts secondary aqueous and primary silicate-melt inclusions. Populations of both liquid-rich and vapor-rich aqueous inclusions coexist in the core samples collected from twelve levels (Fig. 2-3), except in those from the 522 m depth, suggesting boiling of the geothermal fluids. Evidence that the geothermal fluids were boiling during their trapping is also found in the presence of primary vaporrich inclusions occurring singly or distributed three-

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dimensionally in hydrothermal minerals (Fig. 2-1 and 2-2). The aqueous inclusions generally contain two phases at room temperature, vapor and liquid. However, some of the fluid inclusions in fluorite from 165 m depth contain a transparent birefringent mineral, which is not dissolved at temperatures lower than the liquid-vapor homogenization temperature. Halite crystals are only observed in vapor-rich and liquid-rich secondary inclusions in the same healed-fracture plane in quartz phenocrysts from 363 m depth (Fig. 2-4).

5. Th' Tm and crushing results

Homogenization temperatures (Th ) of 618 primary and secondary liquid-rich inclusions, and the final melting point of ice (Tm) of 183 inclusions, in which ice was visible in freezing runs, were measured using a USGS-design heating/freezing stage with accuracy of ± 2°C on heating runs and ± 0.1 °C on freezing runs (Fig. 3). Measurements of the Th of vapor-rich inclusions were also attempted, but none were suitable for an accurate determination (Fig. 2-1). Bubble behavior of 35 fluid inclusions on crushing was observed under the microscope to semiquantitatively estimate the CO 2 content (Sasada et aI., 1986). Th data range generally several tens of degrees in samples from each depth, and those in the samples from shallow levels range over a hundred degrees. In samples from above the present water level of the well, approximately 120 m depth (Musgrave et ai., 1989), minimum Th values are several tens of degrees higher than bore hole temperatures, whereas in samples from below the water table, the minimum Th values plot very close to the present temperature profile (Fig. 4). Tm values of the fluid inclusions in the hydrothermal minerals are slightly negative, whereas those of secondary inclusions in quartz phenocrysts range from 0.2 to - 12.8°C. Several secondary inclusions in hydrothermal minerals have low Th values ( 135°C) and positive Tm values (+3.0 to + 3.3°C) , because of superheating of ice (Roedder, 1967). Behavior of vapor bubbles in fluid inclusions during crushing is very sensitive to gas content, especially to CO 2 , With respect to VC-2A samples, most of the bubbles in liquid-rich inclusions expanded on crushing and often completely filled the inclusions. Only one fluid

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M. Sasada, F. Goff/ Journal of Volcanology and Geothermal Research 67 (1995) 161-169

Fig. 2. Occurrence of the fluid inclusions from VC-2A. (1) Primary liquid-rich (L) and vapor-rich (V) inclusions in the growth zone of hydrothermal quartz from 107 m depth. Th values of liquid-rich inclusions range between 225 and 227°C, and those of vapor-rich inclusions are 216, 217 and 231°C. Th of the vapor-rich inclusions were measured at disappearance of liquid in sharply concaved margin, but ambiguity of measurements cannot be dismissed. (2) Primary vapor-rich inclusions with negative crystal form in the center of a hydrothermal quartz crystal from 107 m depth. (3) Secondary liquid-rich inclusions (L) and secondary vapor-rich inclusions (V) in fluorite from 56 m. (4) Secondary, halite(X)-bearing vapor-rich and liquid-rich inclusions, and vapor-rich inclusions without halite in a quartz phenocryst from 363 m depth. Th(vI) ranges from 436°C to over SOO°C, and Th (NaCI-I) from 287 to 410°C. (They are not shown on Figs. 3 and 4). They are distributed on the same healed fracture plane. These fluid inclusions probably trapped heterogeneous halite-bearing boiling fluid.

inclusion (in fluorite from 56 to 58 m depth) showed bubble collapse on crushing. The crushing results indicate that the hydrothermal fluids trapped in the liquidrich inclusions mostly contain > 0.3 wt. % CO 2 , based on the diagram of Sasada et al. (1986), if we assume that the measured inclusions contain only H2 0 and CO 2 , Because CO 2 content lowers the freezing point depression of fluid inclusions (Hedenquist and Henley, 1985), a CO 2 correction of + 0.1 °C based on the crushing results was made on the calculated salinities reported as NaCI equivalent solid solute (Sasada et aI., 1986).

6. Variation of salinity in hydrothermal vein minerals and in quartz phenocrysts of wall rocks

Salinity of the primary and secondary inclusions in the hydrothermal minerals ranges from 0.2 to 2.5 wt. % NaCl eq., whereas that of the secondary inclusions in quartz phenocrysts ranges from 0.2 wt.% NaCI eq. to greater than halite saturation (Fig. 3). This suggests that the boiling mechanism for fluids circulating in the open cracks (veins) where hydrothermal minerals were precipitated is different from processes control-

M. Sasada. F. Goff/Journal of Volcanology and Geothermal Research 67 (1995) 161-169

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ling the fluids trapped in the microcracks of quartz phenocrysts in the host rocks. The salinity of boiling fluids trapped in the hydrothermal minerals is relatively low throughout the core hole. The small variation in the salinity could have resulted from extensive boiling or mixing of the fluids circulating in veins of the Sulphur Springs hydrothermal system. The low-salinity fluids trapped in these hydrothermal minerals are comparable to the salinities of hydrothermal fluids in the Sulphur Springs reservoir at 490 and 1750 m (Meeker and Goff, 1988; Goff et aI., 1992), and also to low-salinity inclusions in hydrothermally altered and mineralized Paleozoic rocks from the VC-l core hole located in the southwestern ring fracture zone of the Valles caldera (Sasada, 1988).

In contrast, the salinity of fluids trapped in healed fracture planes of quartz phenocrysts in the host rocks, in many cases, is much greater than both that of the fluid inclusions in hydrothermal vein minerals and that of the current fluids in the open cracks (Fig. 3). A different evolutionary process should have produced the higher concentration of salts in the fluids trapped in the quartz phenocrysts. Descending pressures may have caused intensive boiling of the small volume of fluids trapped in the microcracks, as described below. An alternati ve interpretation of the high-salinity inclusions is that they trapped a separate hydrothermal fluid at a different time, but no other evidence for such a fluid exists in the Valles caldera.

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M. Sasada, F. Goff/ Journal o/Volcanology and Geothermal Research 67 (1995) 161-169 VC-2A STRATIGRAPHY

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7. Drop of water table suggested from Th values The maximum temperatures of a geothermal reservoir under hydrostatic pressure are controlled by the boiling point with depth curve for water (BPCW) (White, 1968). Many Th values of fluid inclusions in minerals from the VC-2A core hole, however, plot on the higher-temperature side of the BPCW adjusted to 92°C at the current water level of 120 m depth. This result means that the paleo-water tables of the hydrothermal system are much higher than the present one. However, it is difficult to obtain the unique solution for the highest water table of the paleo-hydrothermal sys-

tern from the Th data because Th of fluid inclusions are affected by excess vapor trapped in fluid inclusions from boiling fluids and by necking down on cooling processes. Thus, deviations from trapping temperatures may occur, even if fluid inclusions trapped hydrothermal fluids on BPCW. The local presence of hydrothermal breccia in the VC-2A (Hulen et aI., 1987) also indicates that thermal fluids were at least intermittently overpressured. The high Th inclusions might have been produced under such conditions because overpressure makes BPCW shift to the higher-temperature side. If very rough matching of BPCW is applied so that 90% of the Th data are contained along its lower tem-

M. Sasada. F. Goff/ Journal o/Volcanology and Geothermal Research 67 (1995) 161-169

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perature side, the paleo-water table would be determined at some 200 m above the present ground surface (Fig. 4). Many Tb values of primary and secondary inclusions trapping boiling fluids, including those in the growth zone of hydrothermal quartz from 107 m depth (Fig. 2-1), plot along the low-temperature side of the BPCW (200 m). A paleo-water table at about 200 m above the present ground surface simply means that a drop of the water table is estimated to be roughly 320 m, and that the Sulphur Springs area has been eroded at least 200 m, probably after the hydrothermal molybdenite deposition was precipitated at 25-125 m depth at ;;;. 0.66 Ma (Hulen et aI., 1987; WoldeGabriel and Goff, 1989).

water/rock ratio were still at higher temperatures. This hypothesis on a sudden drop of water table seems to successfully explain the Tb variations at shallow depths of the VC-2A core hole. If we apply this hypothesis consistently to all the Tb values, we should also expect small Tb gaps below the present water table, but no gaps are found there. Mixing of geothermal fluids may have produced small perturbations in temperature profile. We should also take excess vapor trapping, necking and over pressure effects into consideration. These effects cause greater deviation of Tb from the trapping temperature. These reasons are probably why we cannot detect an apparent Th gap at 165 m and at lower levels in VC-2A.

8. Formation of vapor zone

9. Geological processes for sndden drop of the water table

As described above, minimum Tb of secondary inclusions from several depths above the current water level coincide with a boiling point curve adjusted to 92°C at 20 m below the present ground surface. Temperatures defining this curve are several tens of degrees higher than those of the present thermal profile. That is, a considerable gap in Tb values between the minimum Tb and the present bore hole temperature profile occurs above the current water level. This gap may have been produced when the veins were completely filled with secondary minerals or when the upper levels of the Sulphur Springs reservoir changed to vapor-dominated conditions as the liquiddominated reservoir descended. The former process is not reasonable because many open cavities, faults and fractures are still observed in core samples from shallow zones. On the other hand, the location of the present water level at 120 m depth is consistent with the latter process. The gap in Th values between the BPCW ( - 20 m) and the present thermal profile was probably caused by a sudden drop in the water table of the hydrothermal system. A similar model of boiling has been proposed by McKibben and Eldridge (1990) who correlated anomalous gold concentrations in core from the present vapor zone with radical 34S zonations in coexisting pyrite. The saline inclusions in quartz phenocrysts in the wall rocks could have resulted from intensive boiling under decreasing pressure accompanied by the sudden drop of the water table, because the wall rocks with low

After the deposition of molybdenum from liquid water at about 0.66 Ma, the water table of the hydrothermal system dropped. The sudden drop of the water table is recorded in the gap of Th in the shallow part above the current water level. The water table was suddenly lowered when it was at - 20 m from the present surface. Because an intracaldera lake was (intermittently?) present from about 1.14 to 0.5 Ma, the water table in the Sulphur Springs area was once much higher than the present level (Doell etal., 1968; Smith etal., 1970). When the southwestern wall of Valles caldera was breached at about 0.5 Ma, the caldera lake was drained (Goff and Shevenell, 1987; Hulen and Nielson, 1988) and rapid intracaldera erosion began along streams like Sulphur Creek. The Sulphur Springs area presently occurs within a shallow canyon along Sulphur Creek. Draining of the lake probably caused a sudden drop in the water table of the hydrothermal system (Trainer, 1984), preventing vein minerals from effectively trapping liquids during the rapid decent of the water table. The gap of Tb produced by the vapor zone cooling processes is a manifestation of the present vapor zone.

10. Conclusions Fluid inclusion studies of hydrothermal minerals and quartz phenocrysts from the VC-2A core hole have

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M. Sasada, F. Goff/Journal o/Volcanology and Geothermal Research 67 (1995) 161-169

suggested the following evolutionary process for the Sulphur Springs hydrothermal system involving sudden drops in the water table. ( 1) Boiling low-salinity fluids precipitated the hydrothermal minerals observed in veins and vugs. High-salinity fluids, possibly produced by intensive boiling accompanied by the drop of the water table, were trapped in quartz phenocrysts of the host rhyolitic tuffs. (2) Relatively high Th values at very shallow depths suggest that the paleo-water table was about 200 m higher than the present land surface. Sometime after the shallow molybdenum deposit was formed from liquid water at about 0.66 Ma, the water table of the hydrothermal system dropped to 120 m below the present land surface. (3) Erosion of the land surface is estimated to be about 200 m. Much of this erosion is localized along canyons cutting the resurgent dome and the southwestern moat of the caldera. ( 4) The lack of Th values between the boiling point curve adjusted to 92°C at 20 m below the ground surface and the present thermal profile above the current water level of about - 120 m was caused by rapid creation of the vapor zone after sudden drop in the water table. (5) We propose that the rapid descent of the top of the liquid-dominated reservoir in the Sulphur Springs area was caused by breaching of the southwestern Valles caldera wall and sudden draining of an intracaldera lake at about 0.5 Ma (Doell et al., 1968). Loss of hydrostatic head on the geothermal system after draining the lake caused the water table of the reservoir to fall and produced the present vapor zone during descent (Trainer, 1984; Goff and Shevenell, 1987). This model indicates that dramatic changes in geohydrology can create vapor zones above major liquiddominated geothermal systems. Although such a model has not been directly suggested as viable mechanism in active geothermal systems, Simmons (1991) has indicated that such a process occurred during formation of the Fresnillo (Mexico) ore deposit.

Acknowledgements

The first author was supported by Sunshine Project of Ministry ofInternational Trade and Industry, Japan.

Research by the second author and core hole VC-2A were funded by the U.S. Department of Energy, Office of Basic Energy Sciences. This paper was much improved by the reviews of Phil Bethke and Keith Bargar, USGS, and Jeff Hulen, UURI.

References Charles, R.W., Vidale-Bulen, RJ. and Goff, F., 1986. An interpretation of the alteration assemblages at Sulphur Springs, Valles caldera, New Mexico. J. Geophys. Res., 91: 1887-1898. Doell, RR, Dalrymple, G.B., Smith, RL. and Bailey, RA, 1968. Paleomagetism, potassium-argon ages, and geology of rhyolites and associated rocks of the Valles caldera, New Mexico. In: R.R Coats, RL. Hay and C.A. Anderson (Editors), Studies in Volcanology. Geol. Soc. Am. Mem., 116: 211-248. Dondanville, R.F., 1978. Geologic characteristics of the Valles caldera geothermal system, New Mexico. Geotherm. Resour. Counc. Trans., 2: 157-160. Gardner, J.N., Goff, F., Garcia, S. and Hagan, RC., 1986. Stratigraphic relations and lithologic variations in the Jemez volcanic field, New Mexico. J. Geophys. Res., 91: 1763-1778. Goff, F. and Nielson, D.L. (Editors), 1986. Caldera processes and magma-hydrothermal systems, Continental Scientific Drilling Program -thermal regimes; Valles caldera research, scientific and management plan. Los Alamos Natl. Lab. Rep. LA-10737OBES, 163 pp. Goff, F. and Shevenell, L., 1987. Travertine deposits of Soda Dam, New Mexico, and their implications for the age and evolution of the Valles caldera hydrothermal system. Geol. Soc. Am. Bull., 99: 292-302. Goff, F., Gardner, J., Vidale, R and Charles, R., 1985. Geochemistry and isotopes of fluids from Sulphur Springs, Valles Caldera, New Mexico. l Volcanol. Geotherm. Res., 23: 273-297. Goff, F., Nielson, D.L., Gardner, J.N., Hulen, J.B., Lysne, P., Shevenell, L. and Rowley, J.C., 1987. Scientific Drilling at Sulphur Springs, Valles caldera, New Mexico -core hole VC-2A. Eos, Trans. Am. Geophys. Union, 68: 649, 661-662. Goff, F., Gardner, J.N., Hulen, lB., Nielson, D.L., Charles, R, WoldeGabriel, G., Vuataz, F.-D., Musgrave, lA., Shevenell, L. and Kennedy, B.M., 1992. The Valles caldera hydrothermal system, past and present, New Mexico, USA. Sci. Drill., 3: 181204 Gonzalez, C.M. and McKibben, MA, 1987. Thermal history of vein mineralization in CSDP core hole VC-2A, Sulphur Springs, Valles caldera, New Mexico. Geol. Soc. Am., Abstr. Progr., 19: 679. Hedenquist, J.W. and Henley, R.W., 1985. The importance of CO2 on freezing point measurements of fluid inclusions: Evidence from active geothermal systems and implications for epithermal ore deposition. Econ. Geol., 80: 1379-1406. Hulen, J.B. and Nielson, D.L., 1988. Clay mineralogy and zoning in CSDP core hole VC-2A -Further evidence for collapse of isotherms in the Valles caldera, New Mexico. Geotherm. Resour. Counc. Trans., 12: 291-298.

M. Sasada, F. Goff/Journal of Volcanology and Geothermal Research 67 (1995) 161-169 Hulen, J.B. and Nielson, D .L., 1991. Evolution of the western Valles caldera complex, New Mexico: Evidence from intracaldera sandstones, breccias, and surge deposits. J. Geophys. Res., 96: 81278142. Hulen, J.B., Nielson, D.L., Goff, F., Gardner, J.N. and Charles, R.W., 1987. Molybdenum mineralization in an active geothermal system, Valles caldera, New Mexico. Geology, 15: 748-752. Hulen, J.B., Gardner, J.N., Nielson, D.L. and Goff, F., 1988. Stratigraphy, structure, hydrothermal alteration and ore mineralization encountered in CSDP corehole VC-2A, Sulphur Springs area, Valles caldera, New Mexico. A detailed overview. Univ. Utah Res. Inst. Rep. ELS-8800 1-TR, 44 pp. McKibben, M.A. and Eldridge, C.S., 1990. Radical sulfur isotope zonation of pyrite accompanying boiling and epithermal gold deposition: A SHRIMP study of the Valles caldera, New Mexico. Econ. Geol., 85: 1917-1925. Meeker, K. and Goff, F., 1988. Geochemistry of the 490-m 210°C aquifer in corehole VC-2A, Sulphur Springs, and comparison with other hydrothermal fluids in Valles caldera, New Mexico. Eos, Trans. Am. Geophys. Union, 69: 1049. Musgrave, J.A., Goff, F., Shevenell, L., Trujillo Jr., P.E., Counce, D., Luedemann, G., Garcia, S., Dennis, B., Hulen, J., Janik, C. and Tomei, F.A., 1989. Selected data from Continental Scientific Drilling core holes VC-l and VC-2a, Valles caldera, New Mexico. Los Alamos Natl. Lab., Rep. LA-11496-0BES, 71 pp. Nielson, D.L. and Hulen, J.B., 1984. Internal geology and evolution ofthe Redondo Dome, Valles caldera, New Mexico. J. Geophys. Res., 89: 8695-8711. Roedder, E., 1967. Metastable superheated ice in liquid-water inclusions under high negative pressure. Science, 155: 1413-1417. Sasada, M., 1987. Fluid inclusions from VC-2A core hole in Valles caldera, New Mexico, U.S.A.: Evidence for a transition from hot water dominated system to vapor-dominated system. Geotherm. Res. Soc. Jpn., Abstr. Progr. 103. Sasada, M., 1988. Microthermometry of fluid inclusions from the VC-l core hole in Valles caldera, New Mexico. J. Geophys. Res., 93: 6091-6096.

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Sasada, M., Roedder, E. and Belkin, H.E., 1986. Fluid inclusions from drill hole DW -5, Hohi geothermal area, Japan: Evidence of boiling and procedure for estimating CO2 content. J. Volcano!. Geotherm. Res., 30: 231-251. Self, S., Goff, F., Gardner, J.N., Wright, J.V. and Kite, W.M., 1986. Explosive rhyolitic volcanism in the Jemez Mountains -Vent locations, caldera development and relation to regional structure. J. Geophys. Res., 91: 1779-1798. Simmons, S.F., 1991. Hydrologic implications of alteration and fluid inclusion studies in the Fresnillo district, Mexico: Evidence for a brine reservoir and a descending water table during the formation of hydrothermal Ag-Pb-Zn orebodies. Econ. Geol., 86: 1579-1601. Smith, R.L. and Bailey, RA., 1966. The Bandelier tuff: A study of ash flow eruption cycles from zoned magma chambers. Bull. Volcanol., 29: 83-103. Smith, RL. and Bailey, R.A., 1968. Resurgent cauldrons. In: R.R Coats, R.L. Hay and C.A. Anderson (Editors), Studies in Volcanology. Geol. Soc. Am. Mem., 116: 613-662. Smith, R.L., Bailey, R.A. and Ross, C.S., 1970. Geological Map of Jemez mountains, New Mexico. U.S. Geol. Surv., Map 1-571. Spell, T.L., Harrison, T.M. and Wolff, J.A., 1990. 4°Arj39Ar dating of the Bandelier Tuff and San Diego Canyon Ignimbrites, Jemez Mountains, New Mexico: Temporal constraints on magmatic evolution. J. Volcanol. Geotherm. Res., 43: 175-193. Trainer, F.W., 1984. Thermal mineral springs, in Canon de San Diego as a window into Valles caldera, New Mexico. 35th Field Conf. Guideb. New Mexico Geol. Soc., pp. 249-255. White, D.E., 1968. Hydrology, activity, and heat flow of the Steamboat Springs thermal system, Washoe County Nevada. U.S. Geol. Surv., Prof. Pap. 458-C, 109 pp. White, D.E., Muffler, LJ.P. and Truesdell, A.H., 1971. Vapor-dominated hydrothermal systems compared with hot-water systems. Econ. Geol., 66: 75-97. WoldeGabriel, G. and Goff, F., 1989. Temporal relations of volcanism and hydrothermal systems in two areas of the Jemez volcanic field, New Mexico. Geology, 17: 986-989.