N2-Ar-He compositions in fluid inclusions: Indicators of fluid source

Geochimica et Cosmochimica Acta, Vol. 58, No. 3, pp. I I I9- I I3 1, 1994 Copyright 0 1994 Elsevier Science Ltd

Pergamon

Printed in the

USA. All rights reserved 0016-7037/94$6.00 + .OO

N2-Ar-He compositions

in fluid inclusions: Indicators of fluid source*

DAVID I. NORMAN ’ and JOHN A. MUSGRAVE’ ‘Department of Geoscience, New Mexico Tech, Socorro, NM 87801, USA *INC-9, MS J5 14, Los Alamos National Laboratory, Los Alamos, NM 87545, USA (Received January 8, 1992; accepted in revised form May 20, 1993)

Abstract-A quadrupole mass spectrometer was used to measure bulk samples of conservative gas species NZ, Ar, and He in fluid inclusions from a variety of hydrothermal systems. Analyses of these tracer elements help determine ( 1) if gases extracted by bulk inclusion analyses can provide accurate measurement of N,-Ar-He in active and fossil geothermal systems, (2) if hydrothermal fluids associated with paleogeothermal systems in a continental setting follow N,-Ar-He systematics similar to those in the western Pacific Rim active geothermal systems, specifically New Zealand, and (3) whether different deposit types systematically vary with regard to Nz-Ar-He. The Nz-Ar-He ratios of fluid inclusion volatiles released from recently deposited minerals from the Valles system are similar to those of present day Valles thermal waters. Those inclusion samples from deep within the Valles system, below a regional aquitard, increase in N2. Compositions for inclusions from the Questa and Copper Flat-porphyry deposits are Nz-rich, similar to those of arc-related volcanic gases, whereas those from Taylor Creek Sn deposit appear to be mixtures of magmatic and crustal components. N1-Ar-He ratios of the Precambrian Tribag deposit suggest a basalt source, but significant levels of self-generated He from U and Th in the inclusion fluids are also possible. Inclusions from two epithermal deposits with low-salinity inclusions have N2-Ar-He ratios trending towards air-saturated meteoric waters (ASW), and those inclusions with higher salinities indicate minor to no ASW component. The N2-ArHe ratios in Fresnillo and Cochiti inclusions, which have magmatic helium isotopic ratios, indicate additions of magmatic gases to meteoric fluids. Inclusions from sediment-hosted deposits that contain hydrocarbon-bearing brines are He-rich, as are meteoric waters with a long residence time in the crust. At relevant pressure-temperature-composition conditions, Henry’s Law constants of N2, Ar, and He are similar, and thus, the relative amounts of these species trapped under boiling conditions will not vary appreciably from solubility-controlled amounts in coexisting liquid. Furthermore, there is no evidence of He loss or gain in inclusions by diffusion process. Data from the various systems that were examined indicate that fluid inclusion Nz-Ar-He compositions may be related to magmatic fluid, sedimentary brine, deep circulating meteoric fluid, and shallow circulating meteoric fluid sources, which we modified from those proposed by GICGENBACH ( 1986). Relationships between the NZ-Ar-He tracer gases and other measurable quantities in fluid inclusions are indicated to be helpful in understanding fluid mixing processes in paleogeothermal systems. INTRODUCTION

tions between those of the ASW and the crustal endmember. and meteoric waters that acquired volatiles from an intrusive would have compositions that plot between those of the ASW and the magmatic fields in Fig. 1. Similarly, when magmaderived volatiles mix with evolved waters, the results should be evident. The novel aspect of the GIGGENBACH (1986) proposal is that higher N2 indicates magmatic gas components in geothermal fluids, except in the case of volatiles from basalts associated with divergent plate boundaries that are He rich ( GIGGENBACH and MATSUO, 199 1; GIGGENBACH and GLOVER, 1992). This paper presents data on N,-Ar-He ratios measured in fluid inclusions to determine if ( 1) fluid inclusion gas analyses can provide an accurate measurement of NZ, Ar, and He in paleogeothermal systems; (2) geothermal fluids associated with intrusives in the Western United States are as N2 rich as those on the Western Pacific rim; and (3) if N,-Ar-He ratios in paleogeothermal systems systematically vary as GIGGENBACH ( 1986) proposed. Samples analyzed are from one active geothermal system and nine mineral deposits representing different deposit types: the Valles geothermal area represents an active volcanic hydrothermal system in a continental setting; the Copper Flat porphyry copper and Questa

A LARGE BODY OF ISOTOPIC evidence indicates that water in active geothermal systems is principally of meteoric origin (TRUESDELL and HULSTON, 1980). However, the gaseous chemistry of these solutions differs considerably from that of air-saturated surface waters (ASW). The changes in gas chemistry of meteoric waters that occur during recharge of geothermal systems can only be the result of water-rock interactions and additions of magmatic fluids. Hence, gaseous components ofgeothermal fluids may have magmatic, crustal, and atmospheric sources. GIGGENBACH ( 1986) introduced the concept that N2-ArHe ratios can reveal the magmatic, crustal, and meteoric gaseous constituents in geothermal fluids. These species are good tracers because they are conservative and each of the three sources has distinctive Nz-Ar-He ratios when plotted on an NAH ternary diagram (Fig. 1). Meteoric fluids that have circulated through heated rock would exhibit gas composi-

* Presented at the fourth biennial Pan-American Conference on Research on Fluid Inclusions (PACROFI IV), held May 22-24, 1992, at the UCLA Conference Center, Lake Arrowhead, California, USA. 1119

D. I. NORMAN and J. A. M~JSGRAVE

I 120

N/l

00

crustal

10

Ar

He

FIG. 1. An NAH diagram after GIGGENBACH( 1986). Stars mark compositions of N2-Ar-He associated with the principal sources of these species in geothermal fluids. Dashed line with arrows indicates the change in composition of meteoric fluids as they accumulate radiogenic He in the subsurface and become evolved fluids. Climax-type molybdenum deposits, Taylor Creek tin district, and the Tribag breccia-type deposit are examples of magmatic-hydrothermal ore deposits; the St. Cloud-U.S. Treasury, Cochiti, and Fresnillo are representative of epithermal vein deposits; and Hansonburg and Bushman are examples of sedimentary ore deposits.

Only the meteoric nitrogen component in groundwater may be specified. The N2-Ar ratio in ASW is about 36, but may be as high as 50 as a result of entrained air ( N2/ Ar = 84) ( HEATON and VOGEL, 198 1) Crustal rocks contain low but significant concentrations of Nz that average 30 ppm in basalts. 2 1 ppm in granites, 73 ppm in carbonates, 120 ppm in sandstone, and 602 ppm in shales ( WLOTZKA, 1972). Biogenie gases include N2, which may comprise up to 90% of natural gas ( WLOTZKA, 1972). However, groundwaters have N2 and Ar concentrations near those of ASW (BENSON and PARKER, 196 1; HEATON and VOGEL, 198 1 ), indicating little input of N2 from crustal sources and little loss to nitrogenconsuming bacteria. Fluids in geothermal systems associated with recent magmatic activity have N2/Ar > 100. This is the case for many of the geothermal systems in New Zealand ( GIGGENBACH, 1986) as well as some in Japan ( SEKI, 1990; HEDENQUIST and AOKI, 1991) and Ethiopia (GIGGENBACH and LE GUERN, 1976). One can infer a magmatic source for the added N2 in New Zealand from the magmatic isotopic values for He, H2S, and CO2 and from the distribution of Nz-Ar-He compositions in thermal waters, which are between those for ASW and those measured in New Zealand volcanoes (GIGGENBACH, 1986). The strong correlation between N,/Ar ratios and magmatic stable isotope values of coeval water indicates a magmatic source for N, in the geothermal systems associated with the Esan volcano, Japan ( HEDENQUIST and AOKI, 1991). Analyses

PREVIOUS STUDIES

(MATSUO

There is much information on meteoric and crust-derived rare gas components in groundwaters and geothermal fluids. Groundwaters contain concentrations of Ne, Ar, Kr, and Xe near those in saturated water at the mean annual temperature of the recharge area ( MAZOR, 1972; PHILLIPS, 1981; KENNEDY et al., 1990, 1991; OZIMA and PODOSEK, 1983). Geothermal waters may have additions of radiogenic Ar, but the amounts dissolved

are generally Ar

minor

constituting

less than

half the

of volcanic et al.,

1978).

gases indicate It is difficult

N2/Ar

from

to say how

5 to 2000 common

Nz-rich volcanic gases are because most analyses have reported NZ/Ar ratios near that of air ( TAZIEFF and SABROUX, 1983). Improved methods of sampling volcanic gases have eliminated most air contamination ( HIRABAYASHI, 1986: GIGGENBACH and MATSUO, 199 1). and analyses using these new techniques for gases from felsic volcanoes, including Vulcan0 in Italy, indicate N2/Ar between about 200 and 900 ( CORAZZA, 1986: GIGGENBACH and MATSUO, 199 1).

(KENNEDY et al., 1985; MAZOR, 1976, 1988;

WELHAN et al., 1988). Helium, on the other hand, is a minor constituent of air (5.2 ppm v/v) but He concentrations in groundwaters and geothermal fluids are orders of magnitude above those in ASW, principally from radiogenic sources (HEATON, 1984; MAZOR, 1987, 1976; MARRIN, 1987; TORGERSEN and CLARKE, 1985; TORCERSEN et al. 1982; TORGERSON and IVEY, 1985). The amount of He in groundwaters increases at such a regular rate that He concentrations are used to calculate groundwater residence times ( GIGGENBACH, et al., 1983; ANDREWS and WILSON, 1987; ANDREW& 1985). The primordial He that magmas supply to the crust has 3He/4He ratios greater than six times that of the atmosphere (R/R,,) (CRAIG et al. 1978: LUPTON, 1983). Magmatic He is present in many geothermal systems associated with recent volcanic and intrusive activity, in particular those that have reservoir temperatures > 200 C ( WEHLAN et al., 1988; TORGERSEN et al., 1982; MAZOR, 1976). The amount of magmatic He in geothermal waters (about 0.5 to 5 ppm v/v) is four to five orders of magnitude greater than in ASW. Helium concentrations above that amount in thermal waters generally are from admixed radiogenic helium ( WEHLAN et al., 1988 ).

METHODS Using methods described in NORMAN and SAWKINS( 1987), we performed bulk analyses of fluid inclusions by heating or crushing 0.1-S gm of inclusion-bearing minerals in vacuum. Surface contamination of samples was removed by cleaning with either hot acids or NaOH. boiling for 24 h in distilled-demineralized water, and heating in vacuum until a pressure 4 X lo-’ mbar was attained. In all cases the released volatiles were separated with liquid NZ. The noncondensing fraction composed of H2, He. CH4, Ne, CO. N2, and Ar was measured by a quadrupole mass spectrometer. The condensed fraction that includes CO,, HIS, SOZ, and CZ_~hydrocarbon compounds was separated from HZ0 with a dry ice-alcohol trap and measured in a like manner. Reaction of H2S and SO2 with the extraction line made of glass is minimal because there is little exposed metal, and further. these species remain frozen until the analysis is preformed. A quantitative analysis was obtained by measuring the pressure of each gas fraction contained in known volumes of the extraction line. Water was measured by pressure measurement or by freezing and weighing in a capillary tube. Calibration of the mass spectrometer was done with commercial gas mixtures and an in-house fluid inclusion standard HF-I. Water measurements were calibrated by thermal decrepitation of hydrous minerals such as gypsum and muscovite, and by means of capillary tubes containing weighed amounts ofwater. Present day gases from Valles were determined by gas chromatography. Analyses were performed on minerals from a single generation

1121

N2-Ar-He compositions in fluid inclusions removed from a hand-size sample. In many instances, for example Taylor Creek samples, analyses were made on individual mineral grains obtained from vugs. Only some Fresnillo samples contained two minerals. The complexly intergrown calcite and quartz in some samples was impossible to separate without destroying the contained fluid inclusions. Sulphur Springs samples were selected from various depth intervals throughout the entire length of the core. They were handpicked, crushed to ~0.5 mm, cleaned in warm 3.0 M NaOH for 20-30 minutes (followed by another 20-30 min treatment with 10% Hcl for quartz and fluorite), and washed in hot distilled-deionized water for 12-24 h. The cleaned sample was dried in an oven at about 60°C for several hours to several days (For additional discussion of analytical methods, see NORMAN and SAWKINS, 1987; MUSGRAVEand NORMAN, 1993b). There are few analytical problems in the measurement of Nz, Ar, and He. These gas species are not absorbed to any degree within the vacuum line and are not subject to reactions during thermal decrepitation except for the breakdown of NH3 to N2 (NORMAN et al., 1991a; NORMAN, 1978; NORMANand SAWKINS,1987). The coincidence of the principal mass peaks for CO and N2 does not seriously affect the accuracy of the measurements because matrix solution methods can determine the relative proportion of the two (NORMAN and SAWKINS,1987). Also, CO is not detected in all analyses and the CO concentration is generally less than that of Nz, which typically is the principle specie in the noncondensable fraction. The principal analytical difficulty is that Ar and He typically occur at concentrations of less than 10 ppm v/v in inclusion fluids; hence, it is difficult to detect them if the amount of released inclusion volatiles is small. The quadrupole mass spectrometer used for these measurements has a measurement limit of I ppm v/v and a detection limit of lo-l5 moles. This ppm limit affects the detection limit of He. Fluid inclusions typically contain about 1000 ppm v/v noncondensable gas species: hence at best, we can measure concentrations of He = I ppb TABLE1.

Analyses

Analysis

of N,, Ar,

and

He

in

Sulphur

v/v without further cryogenic separation of the gas sample, which we did not do. The concentration of He in ASW at 20°C is 0.036 ppb v/v. We can detect this concentration in ASW that has about IO ppm v/v dissolved N2-02-Ar because the concentration of He in the separated gas phase is 36 ppm v/v. However, helium at concentrations of 0.036 ppb v/v is not detectable in most inclusion fluids by our standard method of fluid inclusion volatile analysis. Argon in ASW occurs at concentrations about 250 ppb v/v, and, therefore, we can readily measure it in most inclusion liquids.

RESULTS Valles System Inclusion volatiles were measured in quartz, calcite, and fluorite from Continental Scientific Drilling Program (CSDP) core holes VC-2A and VC-2B at Sulphur Springs (Table 1). Some analyses indicate concentrations of gaseous species far greater than present-day waters. These analyses are thought to have excess gas that was contained in vapor-dominant inclusions. Volatiles extracted from VC2b 5507 were contaminated by air and the analysis could not be repeated because of limited sample. Only two of the mineral samples indicated recent formation. These occur late in the paragenesis and their Th (homogenization temperature) and salinity (calculated from the freezing point depression) are about the same as those of present geothermal system. The Nz-Ar-He ratios of these two samples plot on the same trend as analyses of Valles geothermal fluids from production wells in the Redondo Creek area (Baca) and the two CSDP core holes at Springs

Sample No. VCZA

VCZA

VCZA

VCZA

VCZA

129

160

339

541

1137

o.co14 0.0012 1.41

0.005 0.00045 0.507

0.002 0.00001 0.01

o.OOil2 n.d. 0.008

Quartz

QWW

QUWtz

Sample No. VCZB 882

VCZB 895

0.0084 O.WX6 0.409

Fluid

inclusions

and

present

day

waters.

VCZA 1485QTZ

VCZA 1485CC

VCZA 1712

VC28 760

o.ooo11 O.OOWl 0.441

0.0016 O.OOOQ5 0.458

0.002 o.OLlO2 0.127

0.002 O.ooOOl 0.026

0.0137 0.0023 0.652

Fhmrite

Calcite

QUUtZ

Calcite

Calcite

Quartz

VCZB 933

VCZB 1254

VC2B 1322FLR

VCZB 3017

VCZB 4565

VCZB 4755

VCZB 5507

0.003 0.00051 1.768

0.00033 o.OaOO2 0.095

0.0024 OS008 6.27

n.d. n.d. n.d.

0.00097 O.CQO69 1.44

o.ooo7 0.00003 0.28

0.11 0.0033 45.63

OUUtZ

Fluorite

Fluorite

Calcite

Calcite

OUUtZ

BACA 15#

BACA 19#

BACA 241

Gas species (mole %) Ar He N, Mineral Phase

Analysis

Gas species (mole 46) AI He N,

0.0008 0.0004 0.92

Mineral Phase

Quartz Sample No. VCZB 5533

Analysis

OUWtZ

VCZA’

VCZB90-

BACA 41

BACA 13#

O.OOGQ8 o.OOOo7

0.0001 7.68-06

O.OK08 0.00003

0.0003 9.48-06

0.0023

0.0028

0.0029

0.0039

Gas Species (mole %) Ar He

0.015 O.OCG9

0.00019 4.OE-06

l.GE-06 1.9E-08

o.OOcm7 0.00005

N,

1.91

0.011

0.0001

0.0028

Mineral Phase

QUUtZ

analyzed

r LB., not P1.d.. not detected I ‘Average I

of unpublished analyses of VCZA gases collected on S/27 and S/28/87 by C. Janik ‘Modified from Gaff et al., 1990 and Janik. 1986

li &From True&II

OUaItZ

D. I. NORMAN

1122

and J. A.

MUSGRAVE

Sulphur Springs (Fig. 2a). Gas compositions of Valles geothermal waters plot as one would expect of meteoric waters that had accumulated varying amounts of crustal He (Fig. 1). However, isotopic composition of helium in Sulphur Springs waters is RfRA = 5.06(WELHAN et al., 1988) and the R/ RA of the Redondo Creek wells is 4.8 (SMITH and KENNEDY, 1985); hence, the accumulated He is, in large part, from a magmatic source. Valles hydrothermal system has been active for the past 1 Ma (GOFF and SHEVENELL,1987; GOFF et al., 1988), and on the basis of vein mineral paragenesis and fluid inclusion data, we can divide fluid inclusions into those that represent

Nz/l

00

He

1

n

N2/1 q

00

Tribag

b) A

air saturated

magmatic

l

H A air /

00 SULPHUR SPRINGS FLUID INCLUSIONS

/4q magmatic a”

l

em 0 0

e /

:

{ %

m m

\

0

10

He

Ar

FIG. 3. Analyses of fluid inclusions from deposits associated with intrusions: (a) The Copper Flat copper-porphyry deposit, New Mexico; Questa MO-porphyry deposit, New Mexico; and Taylor Creek tin-deposit, New Mexico. Data respectively: unpublished; SMITH (1983);ECCLESTON (1987).(b) Analyses of fifty-seven samples from the Tribag deposit, Ontario (NORMANand SAWKINS, 1985). Inclusions generally have high levels of He and most analyses overlap in the 10 He corner of the diagram.

air saturated

8

.

\ air saturated water

N/l

l

crustal

He

At-

He

wells Hz0 Sulphur Springs fluid inclusions in late minerals

\

b)

10

10

\

A Baca

*

=

10

air saturated water

crusta 0 Sulphur Springs Hz0

magmatlc

a)

Ar

FIG. 2. Analyses of Sulphur Springs fluid inclusions and Valles geothermal waters, New Mexico (Table 1): (a) Comparison of recent Sulphur Springs fluid inclusions in fluorite to analyses of Valles thermal waters obtained from bore holes. Fluids from the deeper Sulphur Springs bore hole VC-2B have significantly more He than fluids from VC-2A. Baca waters are from deep bore holes drilled for exploration and production from the Valles geothermal system. (b) Analyses of all fluid inclusions measured from bore holes VC-2A and VC-2B at Sulphur Springs, Jemez Mountains, New Mexico. Inclusions are divided by occurrence above or below 300 m. Microthermometry of older fluid inclusions below the Permian shale aquitard at 900 m ( MUSGRAVE, 1991) indicates they contain higher salinity fluids and were trapped at higher temperatures than the present geothermal system.

fluids present early in the history of the system and those formed in the recent past ( MUSGRAVEand NORMAN,1993a). Inclusion gas from minerals below 300 m, which are all older inclusions, plot mostly as Nz-rich fluids on a NAH diagram (Fig. 2b). Furthermore, analysis of some early formed minerals from depths above 300 m also are N,-rich (Fig. 2b) (depth of samples in feet is listed below each sample number in Table 1). However, in general, those samples from shallow depths, regardless of their paragenetic stage, plot either along the evolved water trend line or between this line and values from inclusions at greater depths. Two samples exhibit excess Ar. Magmatic

Volatiles

Inclusions from four mineral deposits that apparently were mineralized in part by magmatic solutions exhibit gas ratios

1123

Nz-Ar-He compositions in fluid inclusions TABLE 2.

Representative analyses of fluid inclusion volatiles. Most analyses are by thermal decrepitation, hence may exhibit concentrations of H2 and CO that

represent gas equilibrium near the decrepitation temperatures of 4OO'C to 500°C (Norman et al., 1991).

34

n.d., not detected;

n.d.

0.96

14

35

0.013 0.013

tz, topaz; fl, flourite; ca, calcite; qt, quartz

'analysis by crushing

markedly different then ASW. Analyses from two porphyrytype deposits in New Mexico plot near the Nz apex of the N,-Ar-He ternary diagram (Fig. 3a), except for the one Questa sample that plots near the He apex (representative analyses are in Table 2). Stable isotopic data indicate the 23 Ma (ISHIHARA, 1967) Questa MO-porphyry deposit was formed by solutions containing 40-70% magmatic waters (SMITH, 1983). There is no stable isotopic data on the 76 Ma. Copper Flat Cu-porphyry deposit (NORMAN et al., 1989); however, the solutions associated with the potassic alteration and primary copper sulfide stage of mineralization in this type of deposit are principally of magmatic origin ( GUSTAF~ON and HUNT,

1975).

Taylor Creek is a fumarolic deposit of cassiterite and wood tin ( EGGLESTON, 1987 ) . Analyzed inclusions are from topaz and cassiterite that have Th values in excess of 650°C and late veinlets comprising quartz, calcite, and fluorite that have Th values of 380-130°C. Stable isotopic data indicate early fluids were magmatic, whereas later fluids were a mixture of magmatic and meteoric fluids ( EGGLESTON, 1987). However, measurements of complex hydrocarbon gas species in all analyses of Taylor Creek inclusions (Table 2) suggest a crustal component in the mineralizing fluids. Gas data exhibit a broad range of Nz/ He ratios (Fig. 3a). There is no relationship between the NJHe ratio of inclusion gases and the type of host mineral, Th, concentration of organic compounds, or paragenesis. Remarkably, there are no indications of gas ratios typical of ASW or air in spite of the strong geological evidence that deposits formed near the surface of rhyolite domes ( EGGLESTON, 1987).

Tribag is a 1.06 Ga breccia-pipe deposit (NORMAN and 1985) that limited oxygen, hydrogen, and sulfur stable isotopic data indicate formed from magmatic solutions. It is associated with a small volume felsic porphyry stock intruded during the later stages of the Keweenawan continental rifting event. Basalt intrusion both preceded and followed formation of the ore bodies. Fluid inclusions have T, SAWKINS,

values between 248 and 469 C and calculated salinities of 1.5 to 48.5 eq. wt% NaCl. Inclusion volatiles are generally He-rich (Fig. 3b). Helium values are as high as 580 ppm v/v and about half the fifty-seven analyses indicated greater than 10 ppm v/v. Helium concentrations positively correlate (R* = 0.75) with fluid salinity determined by leachate analysis (NORMAN and SAWKINS, 1985). There are no uranium- or thorium-bearing minerals in the deposit, and quartz is clear with no sign of radiation effects. This suggests the measured He occurs within Tribag fluid inclusions. Epithermal-Type

Deposits

The St. Cloud-U.S. Treasury Cu-Pb-Zn-Ag deposit in New Mexico formed during the early stages of the Rio Grande rifting (30 to 28 Ma) and occurs on faults related to that event (NORMAN et al., 199 1b). Fluid inclusion microthermometry indicates fluid temperatures of 2 lo-294°C and salinities ofO-3.4 eq. wt% NaCl. Inclusions from the ore bodies have N2-Ar-He ratios that plot along the evolved water line; Stage 2 generally exhibits slightly higher amounts of Nz than do Stage 1 inclusion fluids (Fig. 4a). Inclusions in the massive barren quartz occurring above the ore bodies have different gas compositions typified by N2/Ar ratios well above ASW and very low to nondetectable amounts of He and H2S. A high proportion of these inclusions are vapor-dominant indicating boiling. Helium isotope analysis of inclusions in two samples of ore-quartz indicated an R/R* < 1 (BEHR, 1988). The 5.6 to 6.5 Ma Cochiti gold mineralization is an adularia-&cite-type epithermal deposit located on the southern flank of Valles caldera, New Mexico ( WRONKIEWICZ et al., 1984; APODOCA, 1987; WOLDEGABRIELand GOFF, 1989). Fluid inclusions are a mixture of vapor- and liquid dominated types that have Th generally between 240 and 300°C and salinities between 0 and 4.5 eq. wt% NaCl. Nondetectable He typifies most inclusion samples from Cochiti, and these samples exhibit a large range in N2/Ar ratios, similar to those

D. 1. NORMAN and J. A. MUSCRAVE

1124 NJ1

00

1.Q

St. Cloud - U.S. Treasury Deposit. New Mexico: 0 Stage 1 l stage 2 A Non-ore

0)

A

a air

/

A

0.5

2 + 2

0.0

‘;;Z

-0.5 c3 s

n

-1.0

N2/1 00 0 Cochiti Deposit, New Mexico

-1.5

Fresnillo Ag-Cu-Zn Mine, Mexico

1

c

-5

I

I

-4

I

-3

LOG

H&

I

1

-1

0

FIG. 5. Plot of H2S vs. N,/( He + Ar) for the same analyses illustrated in Fig. 4c. A positive relationship between concentrations of Nz and H2S in Santa Nirio fluid inclusions is evident.

IO

Ar

He N2/1 00

8 Fresnillo mine, Zacotecos, Mexico

of minerals and inclusion waters indicate ore solutions were mixtures of highly exchanged waters and solutions that could be magmatic (BENTON, 1991; SIMMONSet al., 1988). Fluid inclusion salinities range from 0- 12 eq. wt% NaCl (SIMMONS, 199 1). Analyses of inclusions from the Santa Niiio vein suggest that the gaseous components were mixtures derived from magmatic and evolved fluid sources (Fig. 4c), Hydrogen sulfide compositions of fluid inclusions and N2/( Ar + He) positively correlate (Fig. 5), which indicates a common source for both. Sedimentary Ore Deposits

air saturated water

10 He

Ar

Fluid inclusion gases in minerals from the Tertiary fluoritegalena-barite Hansonburg deposit New Mexico (NORMANet al., 1985; PUTNAMand NORMAN, 1984; NORMANand PUTNAM, 1983) and Cretaceous Bushman deposit, Botswana,

FIG. 4. Analyses of inclusions from epithermal deposits: (a) The St. Cloud-U.S. Treasury deposit, New Mexico. Data from BEHR

N/l

00

( 1988). (b) Analyses of inclusions from the Cochiti Au deposit that occurs on the flank of the Valles caldera. Data from AFQLIOCA ( 1987).

(c) Analyses of inclusions from the Santa Nirio vein, Fresnillo mine, Zacatecas. Mexico. Data are from BENTON( 199 I ).

for Sulphur Springs inclusions (Fig. 4b). Analyses of He isotope ratios in fluid inclusion fluids from both He-rich and He-poor inclusions indicate R/R* > 2 to 6 ( MUSGRAVEet al., 199 1; STUART SIMMONSunpubl. data). A considerable body of data indicate that mineralization in the Fresnillo mining district, Zacatecas, Mexico ( RUVALCABA-RUIZand THOMPSON,1988) occurred when two fluids that contained magmatic components mixed (SIMMONS, 199 1). Helium, sulfur, and lead isotopic studies indicate magmatic sources for these constituents (SIMMONS et al., 1988; GONZALEZet al. 1984; GUMMINGet al., 1979; MACDONALD et al. 1986). Oxygen and hydrogen isotopic analyses

0 Hansanbur

A Bushman,

%atNsM,ana

oir soturoted water

10

He

Ar

FIG. 6. Analyses of inclusions in fluorite and quartz from the Hansonburg, New Mexico, Mississippi Valley-type deposit (NORMANet al., 1985) and analyses offluid inclusion in fluorite, calcite, and quartz from the Bushman Mine, Botswana (data from LONG, 1987).

N,-Ar-He compositions in fluid inclusions Pb-Zn-Cu-fluorite-barite deposit ( LONG, 1987; BALDOCK et al., 1977 ) are quite similar (Fig. 6). Both deposits formed in organic-rich sedimentary rocks. Fluid inclusions are brines up to 22 eq. wt% NaCl that have Th values mostly between 100 and 200°C. Both deposits have hydrocarbons in fluid inclusions and neither is spatially associated with an intrusive. Hansonburg leads have a large radiogenic component attributed to a Precambrian basement source ( SLAWSON and AUSTIN,1962; EWING, 1979), and sulfur isotopic studies indicate that sulfur was derived from Permian sediments ( ALLMANDINGER,1974). Bromine-chlorine ratios of inclusion fluids indicate an evaporite source for the halides and Ar isotope ratios suggest atmospheric and crustal sources ( B~HLKE and IRWIN,1992). The Nz-Ar-He analyses indicate He-rich fluids with N2/Ar ratios above ASW in most inclusions (Fig. 6). ACCURACY OF FLUID INCLUSION GAS ANALYSES When interpreting bulk analysis of fluid inclusion volatiles, it is necessary to consider the heterogeneity of fluids in primary inclusions, the heterogeneity of inclusion types, any secondary inclusions, leakage since the time of trapping, diffusion of gaseous species into and out of inclusions, and selfgenerated gases. Generally, inspection of mineral samples

1125

before analysis can eliminate minerals that have been deformed or have large numbers of secondary inclusions and are thus unsuitable for analysis, Of greatest concern is whether the gas chemistry in inclusions is the same now as when the fluids were trapped and if analysis of assemblages of vaporand liquid-dominated inclusions will yield spurious results. Effects of Trapping under Boiling Conditions Measurement of mixtures of primary vapor-dominated and liquid-dominated inclusions trapped under boiling conditions will not exactly indicate the gas composition of the parent liquid. Fluid boiling fractionates N2, Ar, and He between liquid and vapor because each gaseous species has different partitioning behavior. Therefore, bulk analyses of minerals containing mixtures of vapor- and liquid-filled inclusions will indicate a different proportion of gas species than those occurring in the liquid phase. The fractionation of N2, Ar, and He was calculated for temperatures of 200-300°C in order to determine the expected variation from true values in analysis of mixtures of vapor- and gas-filled inclusions. We performed these calculations for several scenarios, including closed-system continuous boiling, open-system single-step boiling, and open-system boiling with continuous vapor loss

N/l 00 -

vapor

air saturated

10

He

Ar

FIG. 7. Calculated Rayleigh fractionation of N2, Ar, and He between liquid and vapor phases during open system boiling, assuming a fluid temperature of 300°C and magmatic composition. Each symbol on the curves represents a temperature decrease of I “C. Note the steam fraction (y) values of 0.008 and 0.02 on the curves. We terminated the calculation at 292°C when y = 0.03 because the fluid was depleted of more than 95% of each volatile specie. Analysis of gas species in a mineral that contains only vapor-filled inclusions trapped during initial stages of boiling (y < 0.01) would indicate the Nz content is higher than in the parent fluid. A bulk analysis of fluid inclusions that comprise both trapped liquid (liquid-dominant inclusions at room temperature) and vapor (vapor-dominant inclusions at room temperature) when y = 0.02 would yield a composition somewhere on a line connecting the y = 0.02 points on the two curves. (See text for sources of data and method of calculation.)

1126

D. 1. NORMAN

(Rayleigh fractionation ). Gas solubility data were obtained from CROVETTO~~ al. (1982), POTTER~~~~LYNNE( 1978) and GIGGENBACH ( 1980) and gas fractionation coefficients were calculated as outlined by DRUMMOND ( 198 1). The calculations indicate that fractionation is most pronounced at the higher temperatures and by Rayleigh fractionation processes. However, even for the worst case, which would be an analysis exclusively of vapor-filled inclusions trapped during the initial stages of open-system boiling, calculations indicate analyses should approximate actual gas ratios in the mineralizing fluids (Fig. 7). As it turns out, the three gas species have quite similar solubilities in pure water; hence, the fractionation resulting from Rayleigh fractionation is not extreme. To shift Nz-Ar-He ratios from near one member composition on the NAH ternary diagram (Fig. 1) to another requires changes in gas ratios by factors of 3 to 4, and this simply is not possible by boiling. Calculations that assume closed-system and single-step boiling indicate less fractionation than by Rayleigh processes. These processes result in a negligible shift in N2-Ar-He ratios when plotted on an NAH diagram. Some of the scatter in our analytical data could result from analyzing mixtures of vapor- and liquid-dominated inclusions that are in different proportions in each sample. Analysis of minerals that contain inclusions that formed when vapor separated during the initial stages of boiling will yield compositions more N2- and Ar-rich than those of the fluid. Cochiti inclusions (Fig. 4b) comprise of both inclusion types; hence, the actual fluids could have contained more He than analyses have indicated. However, the large range in Nz-Ar-He ratios exhibited by Hansonburg and Bushman inclusions (Fig. 6) could not result from trapping N&h vapor produced by boiling, nor can the difference between inclusion gases in St. Cloud-U.S. Treasury (Fig. 4a) ore and barren quartz be the result of analyzing, respectively, liquid-dominant and vapor-dominant inclusions from a single fluid as NORMAN et al. ( 199 1b) proposed. Effects of Fluid Inclusion

Heterogeneity

Bulk analysis of fluid inclusions will ideally yield an average analysis of the geothermal fluid over the time the host mineral was deposited, weighted by times during which inclusions more readily formed. Geothermal systems are not constant with time, and microthermometry on the inclusions we studied indicate inclusion-to-inclusion variations in T,,, (melting temperature) and Th. The sample-to-sample variation in N2Ar-He ratios in fluid inclusions from a single deposit is similar to that reported for analyses of active geothermal systems (compare Fig. 2a and analyses of ore quartz in Fig. 4a to Fig. 7 in GIGGENBACH and CLOVER, 1992, and compare Fig. 4a with Fig. 1 in GIGGENBACH, 1986). Hence, the differences between analyses may, as in the case of active geothermal systems, reflect heterogeneity of the gas chemistry of the geothermal system from a variety of causes. However, we cannot discount some scatter in the data as a result of analysis of admixed volatiles from secondary inclusions. Effects of Self-Generated

Helium and Argon

Calculations indicate there could be significant amounts of self-generated He and Ar in Precambrian inclusions that

and J. A. MU~GRAVE

, , / , ‘;‘I > E

_ Si nificant Z saf9 ?

E-

,

,

amounts gcneratsd He , v, ’ 0.x

of, /

,

/’

’ / /

,.

Q$

’ ,

/’ I _

j_

/

y,

,

/’

\). QQFO

,’

,’

’ ’

Q$” /

/’

/

I a./

/’

,

.w, ’ ’

0.x

/’

,

/

/

-ib/ 0

’

OS,

/’

FIG. 8. Calculation of self-generated Ar and He as a function of time and fluid composition (decay constants from HEAMANand LUDDEN ( 199 I )): (a) Ar formed in inclusions in relation to K content. The amount of self-generated Ar is considered significant when it exceeds the minimum amounts of He reported in geothermal waters ( WEHLANet al., 1988). (b) Calculation of radiogenic He produced in fluid inclusions as a function of U and Th in inclusion fluids. Selfgenerated He is considered significant when the amount exceeds minimum amounts in active geothermal systems ( WELHANet al., 1988). We assumed equal amounts of U and Th for the calculation.

have K in excess of 1% and more than a few ppm of U and Th (Fig. 8a,b). Measurements of U and Th in inclusion fluids from two localities (NORMAN et al., 1989; WALDER, 1993) indicate the geothermal fluids may have Th and U concentrations of 10 ppm, but not 100 ppm. Only Tribag inclusions are old enough to have been affected by self-generated He and Ar. The measured K contents of the inclusions are about 1000-l 3000 ppm (NORMAN and SAWKINS, 1985). Hence, if Tribag inclusions had about 10 ppm each U and Th, selfgenerated He would be in excess of self-generated Ar/ 10 and the changed Nt-Ar-He ratios would plot at the He apex of the NAH diagram. The correlation between dissolved solids and He contents is consistent with self-generated Tribag inclusion He. However, it would require unreasonable levels of U and Th in Tribag inclusion liquids to generate He concentrations greater than 10 ppm v/v during 1.06 Ga, particularly since the deposit has no U- and Th-bearing minerals. Therefore, the N,-Ar-He ratios of some of the Tribag inclusions might reflect self-generated He, but it is not possible to

N,-Ar-He compositions in fluid inclusions explain the overall He-rich nature of the fluids by additions of radiogenic He from U and Th in the inclusions. The correlation of He with salinity suggests an association of He with hydrothermal fluids. Effects of Helium Diffusion Helium has a high diffusion constant; hence, it is more able to move in and out of inclusions than other gaseous species. However, there is no evidence in our analyses to support this idea. If He diffused into minerals, all inclusions from a deposit should have similar concentrations, also, there might be a negative correlation of He with salinity because of the decreased solubility of gases in liquids with dissolved solids, the salting-out effect. Concentrations of He would be uniformly low if He diffused out of inclusions. Measurements of 3He/4He > 1 in fluid inclusions in quartz, calcite, and fluorite (SIMMONSet al., 1988; MUSGRAVEet al., 1991) and the occurrence of He in inclusions within alluvial diamonds ( LAL, 1989) indicates that He concentrations in fluid inclusions are not altered to any great extent by diffusion processes. Diffusive loss of helium that has 3He/4He > 1 is not a reversible process in crustal rocks except under rare conditions, for example at times of hydrothermal activity. Although inclusion-bearing minerals are within a hydrothermal mineral deposit, each inclusion is surrounded by similar He-bearing inclusions in a large volume of rock that itself was altered by fluids with the same He content as the inclusion. Hence, there should be little diffusion of He unless the host rock is permeable and is continually flushed by meteoric waters. It is more probable that losses of He may happen in the laboratory. Our sample preparation involved grinding samples to -10 mesh or less, boiling the crushed sample for days in acids and water, oven drying and storaging the samples until the analysis; this process would promote diffusion of He out of inclusions. Since we treated all of the inclusion-bearing minerals discussed here in much the same fashion, the loss of He during sample preparation could not have been extreme. DISCUSSION Nz and Helium in Magmatic Gases Analyses of inclusion gases presented here agrees with GIGGENBACH( 1986) and GIGGENBACHand MATSUO( 1991) that elevated N2/( Ar* 100 + He* 1000) is an indicator of a magmatic gas component in geothermal fluids from calcalkaline, I-type intrusives. Inclusion data from the two New Mexico porphyry-type deposits, considered to be I-type intrusives ( WESTRA and KIETH, 198 1)) have high Nz ratios. Data for inclusion gases from the Cochiti, Fresnillo, and Valles geothermal system deposits are also consistent with the contention that magmatic gases have significantly higher N2-Ar ratios than those of ASW, and sulfur and helium isotope analyses also support this conclusion. Inclusions from the Tribag and Taylor Creek deposits have significantly higher He/N2 ratios than New Mexico porphyry-type deposits. These intrusives may both be crustal melts which could account for the He-rich volatiles. Tribag is associated with rift basaltic magmatism that could have supplied the He-rich inclusion volatiles. The N2-Ar-He ratios in Tribag and Taylor

1127

Creek inclusions are similar to those measured in the Bushman and Hansonburg deposits, which suggests they could have been formed by fluids with a significantly evolved fluid component. The occurrence of hydrocarbon compounds in Taylor Creek inclusion fluids and petrographic evidence that the intrusives were volatile-poor ( DUFFIELD and DU BRAY, 1990) suggests this may also be true for the Taylor Creek deposits. N2-Ar-He as Indicators of Hydrothermal Processes Our data indicate that analyses of Nz-Ar-He in fluid inclusions may help identify sources of gaseous components in geothermal systems. Clearly, N2-Ar-He ratios in inclusion fluids may indicate meteoric waters that have not significantly interacted with crustal rocks because they should have ratios near that of ASW. Nz-Ar-He values that are distributed between those for ASW and the He or Nz apex on the NAH ternary diagram should indicate either mixtures of meteoric volatiles with gaseous species from a second source or, in the case of addition of He, waters that had varying residence time in the subsurface. The Nz-Ar-He components of fluid inclusions from the Fresnillo, Cochiti, and early-deep Valles deposits are consistent with results from isotopic analyses that indicate fluids are meteoric waters with admixed volatiles of magmatic origin. Fluid inclusion gas analyses may also indicate the lack of gas components from near-surface circulating meteoric waters. Such fluids do not have values plotting near those for ASW indicating they were dominated by volatile species from evolved waters or a magma. Inclusion analyses from the four intrusive-related deposits (Fig. 3a,b), Fresnillo (Fig. 4c), and two sediment-hosted deposits (Fig. 6) that plot far from ASW values on a NAH diagram all have salinity values > 5 eq. wt% NaCl as well as isotopic data that indicate that the geothermal systems were not dominated by near-surface circulating fluids. The hydrocarbon-bearing saline solutions in Hansonburg and Bushman inclusions are typical of oil field brines ( HANOR, 1979; SVERJENSKY,1984), as is the high He content of the fluid inclusions. Inclusions from both deposits contain more N2 than can be explained by simple addition of radiogenie He to ASW. The most logical explanation is that some of the added N2 is biogenic. It is not clear if N2 in volcanic gas is a magmatic volatile that originates in the mantle, or if it is a magma-associated gaseous specie. Nitrogen isotopic studies have not indicated a definite mantle value (FAURE, 1986); hence, it is not possible to determine the origin of N2 in volcanic gases with certainty. MATSUOet al., ( 1978) suggest that the Nz isotopic composition of volcanic gases sampled in Japan represents subducted biogenic nitrogen. Nitrogen could also exsolve from crustal rock as it is heated during intrusion. This would explain the lack of excess N2 in present Valles fluids, which isotopic data indicate contain magmaderived He and S (SMITH and KENNEDY, 1985; MCIBBEN and ELDRIDGE, 1990)) because such a process would evolve most N2 during the early stages of intrusion when wall rocks were first heated. Ratios of Nz-Ar-He in fluid inclusions can indicate if one gaseous component dominates or if there are several fluid sources. Analyses of inclusion volatiles and data on the com-

D. 1. NORMAN

1128

and

N&00

b)

\

\

A?

IO He

AI

IO He

N&100

Nt?/fOO

Ar

IO He

IO He

nr

N,/lOO

ef

Rt;. 9. Hatched areas are the compositions of: (a) magmatic fluids, (b) deep circulating meteoric fluids, (c) sedimentary brines. (d) shallow circulating meteoric rhyoiite. Fields represent

fluids. and (e)

fluids from rift basalt and

data from this paper, GEGENBACH( 1986). and MATSUO( I99 I ), and GIGGENBACH and CLOVER

GICGENBKH

(1993). positions of volcanic and geothermal gases indicate singlesource volatiles plot as illustrated in Fig. 9. Inclusion fluids

from Copper Flat and Questa indicate magmatic volatiles may be more N&ch (Fig. 9a) than GKXENBACH’S magmatic value. The field for deep-circulating meteoric fluids is quite broad (Fig. 9b) and is based on analyses of VALLES and ROTORUA

(

GIGGENBACH and GLOVER, 1992) geothermal

J. A. MUSGRAVE

they indicate how N7-Ar-He may be used to determine sources of gaseous components in geothermal fluids. The most ambiguous distribution of data would be that in Fig. 10~. We hope that more analyses of volcanic gases and sedimentary brines will refine the fields in Figs. 9 and 10. Fluid inclusion N2-Ar-He compositions may be used to indicate the source ofthese species, but it may not be possible to extend such interpretations to the aqueous component. Phase separation allows gaseous components to decouple from a fluid source and move inde~ndently. The sources of other gases such as HzS and CO2 may be indicated by a relationship between that gaseous species and one of the endmember gases, as was done in Fig. 9 and in G~GGENBACH ( 1986 ) Fig. 2. We have successfully used plots of excess Nz ( Nz - 50* Ar) vs. other gaseous species. This type of plot, for example, indicates that HZS concentrations in St. Cloud-U.S. Treasury fluids ( Fig. 4a) strongly correlates with excess NT. The data presented here from a variety of hydrothermal systems in a continental setting, as well as the data of GIGGENBACH ( 1986) for hydrothermal systems in an island-arc setting (New Zealand), point to a potentially fruitful approach for determining fluid source based solely on NZt Ar, and He abundances; however, the method does have limitations. In particular, there is a problem when using He and Nz to distinguish between crustal (He-rich) and magmatic ( Nz-rich) sources when both He and Nz can and do occur in both. Neither this study nor work by others examined the N2-ArHe ratios from metamorphic environments: these fluids may also plot in the N2 portion of an NAH ternary. Without other supporting data (e.g., light stable and/or noble gas isotopes, geologic setting), it may be difficult to state definitively the sources of fluids in hydrothermal systems. The Valles hydrothermal system is a good example: He-rich, present-day fluids plot in the crustal apex of the ternary (Fig. Za), suggesting that these fluids interacted and/or are derived from He-rich rocks (presumably rich in 4He, hence crustal). However,

N,/lOO

wa-

ters. It is not clear if the Valles analyses represent fluids that have accumulated various amounts of He or if mixing is occurring between deep circulating (higher He) and shallower circulating (less He) fluids. Data from the two sedimentary hosted deposits indicate a broader field of compositions than that proposed by GIGGEN~ACH ( 1986) for fluids that had extensively interacted with the crust. The coincidence of data from HANSONBURC?and BUSHMAN and the known occur-

NzliOO

d)

rence of biogenic N2 in natural gases indicate that crustal brines may have higher N*/Ar than does ASW (Fig. 6). Vol-

atiles from S-granites may have Nz-Ar-He ratios similar to those of sedimentary brines; however, the origin of the Taylor Creek fluids is too tenuous to say this with any certainty. Our analyses of Tribag inclusions agree with GEGENBACH and MATSOU’S ( 199 I ) conclusion that fluids associated with continental rifts are He-rich (Fig. 9e).

Ideal binary mixtures of volatiles from fluid sources in Fig. 9 show surprisingly little overlap with those in Fig. 10; hence,

A?

IO He

Compositions areas), assuming

FIG. IO. (hatched

N,-Ar-He Ratios as a Tracer

A

of volatile mixtures from two sources endmember compositions as in Fig. 9:

(a) magmatic and shallow circulating meteoric fluids, (b) magmatic and deep circulating meteoric fluids, (c) magmatic fluids and sedimentarv brines or rift basalt and rhyolite fluids, and (d) shallow circulatkg

meteoric

fluids and sedimentary

brines.

1129

N2-Ar-He compositions in fluid inclusions present-day Valles waters have 3He/4He ratios that average 4.8RA (SMITH and KENNEDY, 1985). Clearly, the large excesses of total He are accompanied by excesses in 3He, which indicates a mantle, not crustal, source for He. The minerals we analyzed from early in the paragenetic sequence at Sulphur Springs contain N1-rich fluid inclusions that plot in the Nz apex of the ternary (Fig. 2b). These fluids may have derived nitrogen from the organics-rich Paleozoic section through which these fluids circulated, an obvious potential nonmagmatic source.

ALLMANDINGERR. J. ( 1974) Source of ore forming fluids at the Hansonburg mining district, central New Mexico. Gee!. Sot. Amer. Abstr. Prog. 6, 633. (abstract) ANDREWSJ. N. ( 1985) The isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers. Chem. Geol. 49,339-35 1. ANDREWSJ. N. and WILSONG. B. ( 1987) The composition of dis-

solved gases in deep groundwaters and groundwater degassing. In Saline Waters and Gases in Crystalline Rock; Special Paper 33, Geol. Assoc. Canada, pp. 245-252.

APODACAL. E. (1987) Geochemical study of the Cochiti Mining District, Sandoval County, New Mexico. MS Thesis, New Mexico Tech. BALDOCKM. A., HEPWORTHJ. W., and MARENGWAB. S. (1977)

CONCLUSIONS After comparing analyses of fluid inclusion volatiles those of geothermal fluids and comparing interpretations fluid sources from Nz-Ar-He ratios to those that made other geochemical data on inclusions and host minerals, drew the following conclusions:

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to of on we

1) Analysis of Nz-Ar-He in fluid inclusions can yield reasonably accurate analyses of these species in the depositing fluids. Care during sample preparation will forestall diffusion of He. 2) A clear distinction between geothermal systems that have a shallow-circulating meteoric fluid component and those that do not is readily evident from Nz-Ar-He ratios. 3) N,-Ar-He ratios in fluid inclusions can indicate the origin of these volatiles in terms of meteoric, crustal, and magmatic sources, as well as mixing of volatiles from two sources. More data is needed on inclusions deposited by magmatic brines to be certain of the range in values they may exhibit. These fluids appear to have the same ratios expected from mixtures of magmatic and evolved waters. Data from metamorphic rocks/ore deposits are required to establish the range in values from these environments/ systems. 4) It is possible to determine sources of gaseous species other than Nz, Ar, and He in two source mixtures by establishing correlations between them and one of the tracer gas species. One can also use correlations between Nz, Ar, or He and other fluid inclusion parameters such as T,,, salinity, and isotopic compositions to establish the source of inclusion constituents, but this method may not give conclusive results if the gaseous species have decoupled from parent fluids after phase separation. 5) N2-Ar-He ratios, while potentially useful for expanding our understanding of fluid origins, may not be definitive without other supporting data.

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MAZOR E. ( 1988) Noble gases as tracers identifying geothermal components in regions devoid of surface geothermal manifestations: A case study in the Baden Springs area. Chem. Geol. 72,47-6 1. MCKIBBENM. A. and ELDRIDGEC. S. ( 1990) Radical sulfur isotope zonation of pyrite accompanying boiling and epithermal gold deposition: a SHRIMP study of the Valles calderas. Econ. Geol. 85, 1917-1925. MUSGRAVEJ. A. ( 1991) Chemical evolution and mineralization, Sulphur Springs CSDP site, Valles caldera, New Mexico. Ph.D. thesis, New Mexico Tech. MUSGRAVEJ. A. and NORMAND. 1. (1994a) Chemical evolution and mineralization of the Sulphur Springs hydrothermal system, Valles caldera, New Mexico. Part I. Fluid inclusion evidence. Econ. Geol. (in review). MUSCRAVEJ. A. and NORMAND. I. (1994b) Chemical evolution and mineralization of the Sulphur Springs hydrothermal system, Valles caldera, New Mexico. Part II. Fluid inclusion gas chemistry (in review). MUSGRAVEJ. A., POTHSJ., and NORMAND. I. ( 1991) Noble gases in fluid inclusions from Valles caldera and the St. Cloud Mining district, New Mexico. In Alfred 0. Nier Symposium on Inorganic Mass Spectrometry. May 7-9. 1991. 40. (abstract) NORMAND. 1. ( 1978) Geology and geochemistry ofthe Tribag Breccias, Batchawana Bay, Ontario. Ph.D. thesis, Univ. Minnesota. NORMAND. 1. and PUTNAMB. ( 1983) Mississippi-Valley-Type leadfluorite-barite deposits of the Hansonburg District. In Guide Book .for the 34th New Me.xico Geological Society Field Conference (ed. C. E. CHAPIN), pp. 203-209. New Mexico Geol. Sot. NORMAND. I. and SAWKINSF. J. ( 1985) The Tribag breccia pipes: Precambrian copper-molybdenum deposits, Ontario. Econ. Geol. 80, l593- I62 I.

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