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Evolution of Hydrothermal Fluids in the 

Alta Stock, Central Wasatch Mountains, Utah 

By DAVID A. JOHN 

U.S. GEOLOGICAL SURVEY BULLETIN 1977 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1991 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 
CONTENTS 

Abstract 1 

Introduction 1 

Acknowledgments 2 

Geology of the Alta stock 2 

Hydrothermal alteration of the Alta stock 4 

Mineralization associated with the Alta stock 6 

Fractures and veins in the Alta stock 6 

Types and relative ages 7 

Distribution and orientations 7 

Comparison of fracture data in the Alta stock to other areas in the central Wasatch 

Mountains 8 

Crystallization sequence and estimation of ~0 contents of the magma 10 

Compositions of primary biotite and amphibole as indicators of magmatic fluid 11 

Biotite compositions 11 

Estimation of intensive parameters using biotite compositions 14 

Amphibole compositions 16 

Interpretation of compositional zoning in amphibole 17 

Hornblende geobarometry 20 

Stable-isotope studies 22 

Methods 22 

Results 22 

Discussion 24 

Fluid-inclusion studies 26 

Scope and types of materials studied 26 

Petrographic characteristics of fluid inclusions in the Alta stock 27 

Distribution of fluid-inclusion populations 28 

Heating and freezing experiments 29 

Western samples 29 

Pressure corrections and trapping temperatures 31 

Eastern samples 32 

Igneous quartz-homogenization temperatures and salinities 34 

Vein quartz-homogenization temperatures and salinities 35 

Pressure corrections and trapping temperatures 35 

Origin of fluid inclusions in the Alta stock 37 

Comparison of the evolution of hydrothermal fluids in the Alta stock to porphyry copper 

systems 42 

Summary and conclusions 45 

References cited 46 

Appendix 1. Descriptions of samples used in fluid-inclusion studies 50 

FIGURES 

1. Location map of central Wasatch Mountains and surrounding region, showing 

distribution of middle Tertiary igneous rocks and major structures 3 

2. Photograph of Alta stock, looking west 4 

3. Generalized geologic map of Alta stock and vicinity 5 

4. Photograph of intrusive contact between equigranular and porphyritic phases of Alta 

stock 6 

5. Ternary diagram of modal quartz, plagioclase, and K-feldspar contents recalculated 

to 100 percent for samples of Alta stock based on point counts of stained slabs 7 

6. Photograph of type 7 pyrite+chlorite veins in Alta stock 11 

Contents Ill 
IV Contents 

7. Maps showing distribution and orientations of veins in Alta stock 12 

8. Lower-hemisphere equal-area net projection of poles of structural data in Alta 

stock 14 

9. Diagram showing sequence of mineral crystallization in Alta stock 15 

10. Graph of temperature vs. weight percent ~0 at 2 kbars total pressure, showing 

phase relations for a synthetic granodiorite composition (modified from fig. 4 of 

Naney, 1983) 16 

11. Map showing locations of samples used in chemical analyses, and in microprobe, 

stable-isotope, and fluid-inclusion studies 17 

12. Plots of biotite compositions in Alta stock based on electron-microprobe 

analyses 20 

13. Graph of calculated temperature vs. estimated log oxygen fugacity for biotite from 

Alta stock 21 

14. Ternary plot ofFe+2-Fe+3-Mg (mole ratios) in biotite from Alta stock 22 

15. Backscattered SEM images of hornblende phenocrysts in Alta stock (sample 83-PC206), showing patchy compositional zoning 23 

16. Graph showing variations of Alvr, Mg, Fe, Ti, Ca, Mn, Na, and K with Si content in 

amphibole crystals from Alta stock 24 

17. Graph of <5D vs. water content of biotite and chlorite in Alta stock 26 

18. Diagram showing <5180 and <5D relations for Alta stock 26 

19. Photomicrographs of fluid inclusions in Alta stock 27 

20. Map showing distribution of fluid-inclusion types in igneous quartz in Alta 

stock 28 

21. Cross section of Alta stock showing distribution of fluid-inclusion populations in 

igneous quartz 29 

22. Histograms showing measured homogenization temperatures of fluid inclusions in 

western part of Alta stock 30 

23. Histograms showing calculated salinities of fluid inclusions in western part of Alta 

stock 32 

24. Photomicrograph of type 1 hornblende+K-feldspar vein in sample 83-PC-292, 

showing textures of hornblende, K-feldspar, and quartz 34 

25. Histograms showing measured homogenization temperatures of fluid inclusions in 

eastern part of Alta stock 36 

26. Histograms showing calculated salinities of fluid inclusions in eastern part of Alta 

stock 38 

27. Temperature-composition diagram for high-temperature part of NaCl-~0 system 

(from Chou, 1987), showing isobars for two-phase liquid+ vapor field and 

compositions of coexisting liquid and vapor phases 41 

28. Isoplethal pressure-temperature diagram for part of NaCl-H20 system (modified 

from Chou, 1987) 41 

29. Activity diagrams for fluids in equilibrium with various vein assemblages in Alta 

stock 44 

TABLES 

1. Chemical analyses, CIPW norms, and modal analyses of the Alta stock 8 

2. Vein assemblages in the Alta stock 10 

3. Representative microprobe analyses of biotite in the Alta stock 18 

4. Representative microprobe analyses of amphibole in the Alta stock 19 

5. Oxygen and deuterium analyses of minerals and rocks in the Alta stock 25 

6. Pressure corrections for type 1 fluid inclusions in the Alta stock 33 

7. Summary of pressure corrections for type 3 fluid inclusions in the eastern part of the 

Alta stock 40 
Evolution of Hydrothermal Fluids in the Alta Stock, 

Central Wasatch Mountains, Utah 

By David A. John 

ABSTRACT 

The Alta stock is a small, composite granodiorite to 

quartz monzodiorite stock that lies between the Park City 

and Cottonwood mining districts in the central Wasatch 

Mountains, Utah. The stock was tilted about 20° east subsequent to its emplacement, and present exposures of the 

stock represent paleodepths ranging from about 3.7 km on 

the east to 6.3 km on the west. Primary crystallization of 

biotite before hornblende suggests initial magmatic water 

contents of 3 to 4 weight percent. Calculations based on 

biotite compositions suggest that oxygen fugacity during 

crystallization was between nickel-nickel oxide and hematite-magnetite buffers and that the oxidation state may 

have slightly increased during crystallization and from 

shallow to deeper depths in the stock. Amphibole phenocrysts have conspicuous patchy compositional zoning 

similar to that reported in other shallow intrusions and 

characterized by systematic increases in Mg, Ca, and Mn 

and decreases in Fe, AI, Ti, Na, and K with increasing Si. 

Compositional zoning may indicate interaction of the amphibole with an aqueous fluid during crystallization. Fluidinclusion data, however, suggest that vapor saturation of 

the magma may not have occurred until temperatures approached the solidus (725-770 °C). In the upper part of 

the stock at depths S:5 km, magmatic fluids either boiled, 

forming high-salinity brine (35-70 weight percent NaCI 

equivalent} and low-salinity vapor phases, or else highsalinity brines were directly exsolved from the crystallizing 

magma. In deep parts of the stock at depths >5 km, magmatic fluids were supercritical and of low to moderate salinity (S:12 weight percent NaCI equivalent). 

The hydrothermal system is characterized by pervasive, generally weak, deuteric alteration. This alteration 

is characterized by partial sericitization of plagioclase, 

chloritization of biotite, and development of turbid perthite 

crystals. Stable-isotope relations and fluid-inclusion data 

suggest that alteration occurred at temperatures near 500 

°C from magmatically derived fluids. Weak fracture-controlled alteration also is present. Early veins contain calcic 

amphibole (hornblende), K-feldspar, and epidote. These 

Manuscript approved for publication, February 4, 1991. 

are followed by biotite+magnetite (±K-feldspar), quartz±Kfeldspar, quartz+pyrite±sericite, and pyrite+chlorite. All 

veins except pyrite+chlorite initially formed at temperatures near 470-550 °C from dilute (S:1 0 weight percent 

NaCI equivalent), nonboiling fluids that were probably derived from magmatic fluids. Veins and joints have strong 

preferred northeast orientations that parallel regional structures. Veins are much less abundant than in the nearby 

Mayflower stock and in the vicinity of typical porphyry 

copper-related intrusions elsewhere, suggesting that fracture permeability was much lower in the Alta stock than in 

mineralized intrusions. 

The hydrothermal system of the Alta stock developed in several ways distinct from intrusion-hosted porphyry copper systems. These include the following: (1) 

Early fluids had relatively high ac/+falH+ ratios that stabilized calcic amphibole; (2) late fluids deposited chlorite 

instead of sericite, probably indicating relatively high at12

+/ 

a2 +ratios; (3) there is only weak fracture-controlled alterat~n; (4} most veins formed from relatively dilute fluids, 

including early, high-temperature veins; (5) pressure remained near lithostatic throughout crystallization and cooling of the stock; and (6) there is no stable-isotope evidence for incursion of meteoric water. Most of these differences probably are the result of the Alta stock being 

emplaced at somewhat greater depths than most porphyry 

copper stocks and possibly having slightly lower initial 

water contents: both of these factors would lead to low 

fracture permeability. 

INTRODUCTION 

Recent papers describing an orthomagmatic model 

for the generation of porphyry copper and molybdenum 

deposits by Burnham (1979a, 1981, 1985) and Burnham 

and Ohmoto (1980) have emphasized the role of exsolution of magmatic fluids during crystallization of shallowly emplaced granitic magmas. In this model, early release of fluids may lead to concentration of ore-forming 

elements, hydrofracturing, and hydrothermal alteration 

characteristic of these deposits. These authors argue that 

relatively volatile-rich magmas and shallow levels of 

emplacement may be more important factors in the genIntroduction 
eration of porphyry copper-molybdenum deposits than 

specific sources for calc-alkaline magmas or unusually 

high concentrations of metals, sulfur, and chlorine in the 

magma. 

The Alta stock, a small, composite granodiorite to 

quartz monzodiorite intrusion in the central Wasatch 

Mountains, Utah (fig. 1), has many features similar to 

intrusions that host porphyry copper deposits. Other temporally equivalent intrusions in the central Wasatch 

Mountains (Park City and Cottonwood mining districts) 

formed significant precious- and base-metal mineralization, both within intrusive rocks and in the surrounding 

sedimentary wall rocks. The Alta stock, however, generally lacks stockwork veins, intense hydrothermal alteration, and disseminated copper sulfide mineralization 

typical of porphyry copper-related intrusions. 

This study was undertaken to examine the evolution of magmatic and hydrothermal fluids in a wellexposed, weakly altered and weakly mineralized system 

that is similar in many aspects to porphyry copper intrusions. By comparing results from the poorly mineralized 

Alta stock with those from porphyry copper systems, it 

was hoped that differences between barren and mineralized systems would become more apparent. This study 

emphasized the determination of the presence or absence 

of a fluid phase during crystallization and the spatial and 

temporal evolution of the hydrothermal system within 

the Alta stock. Small Fe-Cu skarns that formed along the 

margins of the Alta stock were not studied. Studies undertaken included systematic mapping of veins and fractures in the stock, electron-microprobe analyses of mafic 

minerals, fluid-inclusion studies of igneous rocks and 

hydrothermal veins, and reconnaissance stable-isotope 

analyses. 

Acknowledgments 

This paper is an outgrowth of part of a Ph.D. thesis 

completed at Stanford University. I wish to thank my 

thesis advisor, M.T. Einaudi, for much help and many 

ideas. W .J. Moore introduced me to the geology of the 

Wasatch Mountains, and D.P. Cox encouraged me to undertake this study. T.G. Theodore and T.F. Lawton introduced me to the world of fluid inclusions and helped 

with interpretation of the fluid-inclusion data. L.C. Calk 

helped with the microprobe analyses, and R.L. Oscarson 

assisted with the SEM work. Discussions with G.K. 

Czamanske and S.D. Ludington were helpful in interpreting the microprobe and SEM data. R.H. Brigham 

provided most of the stable-isotope data, and discussions 

with him and J.R. O'Neil helped with the interpretation 

of these data. Some of the samples used in this study 

were collected by W.J. Moore and M.L Sorensen. Reviews of early versions of the manuscript by M.T. 

Einaudi, T.G. Theodore, and S.D. Ludington measurably 

improved it. 

GEOLOGY OF THE ALTA STOCK 

The Alta stock is a small (approximately 10 km2

) 

composite intrusion that is one of 11 middle Tertiary 

(31-42 Ma) calc-alkaline stocks in the central Wasatch 

Mountains (fig. 1). The regional geologic setting of igneous rocks in the central Wasatch Mountains was recently 

summarized by John (1987, 1989). Previous studies of 

the Alta stock were made by Calkins and Butler (1943) 

and Wilson (1961). Studies of metamorphism related to 

emplacement of the stock have been made by Smith 

(1972), Cranor (1974), Moore and Kerrick (1976), and 

Bowman and Cook (1981). Kemp and Bowman (1984) 

studied skarns that locally formed along the margins of 

the stock. Mineralization in and around the stock was 

described by Calkins and Butler (1943) and James 

(1978), and Belt (1969) described reconnaissance geochemical sampling of the stock. Estimated depths of emplacement and fluid-inclusion populations of the Alta 

and other stocks in the central Wasatch Mountains were 

described by John (1987, 1989). 

The Alta stock consists of two major phases (equigranular and porphyritic), a discontinuous mafic border 

phase, and numerous pegmatite and aplite bodies that intrude Paleozoic sedimentary rocks (fig. 2; Calkins and 

Butler, 1943; Wilson, 1961). The two major phases are a 

medium-grained, more or less equigranular phase and a 

vaguely to strongly porphyritic phase with a fine-grained 

aplitic groundmass. The equigranular phase forms most 

of the western and outer parts of the stock, and the porphyritic phase forms the eastern and inner parts of the 

stock (fig. 3). Contacts between the phases vary from 

sharp and intrusive along the southern margin of the porphyritic phase near Lake Mary (fig. 4) to obscure and 

gradational along the northern margin of the porphyritic 

phase. Near Lake Mary the porphyritic phase truncates 

foliation, jointing, and aplite dikes in the equigranular 

phase. Textures of the eastern part of the stock are 

highly variable, and, owing to poor outcrop, contacts between different textural types are not exposed. Wilson 

(1961) suggested that the porphyritic phase resulted from 

injection of partially crystallized magma into the equigranular phase and subsequent pressure quenching of 

this magma (Jahns and Tuttle, 1964). 

The mineralogy of major phases of the stock is 

fairly uniform except for slight variations in modal contents of biotite and hornblende, which are the main 

mafic silicate minerals. Clinopyroxene was an early 

crystallizing phase but was almost entirely resorbed by 

the magma and is now only found as small inclusions in 

cores of plagioclase crystals. Magnetite is the dominant 

2 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
Great 

Salt 

Lake 

TRANSVERSE 

0 

0 

~ Volcanic rocks (Tertiary) 

r::::] Granitic rocks (Tertiary) 

Contact 

10 

I 

Area of map 

I 

L_v:_y~- UTAH 

UTAH 

uu-rfA 

10 20 MILES 

20 30 KILOMETERS 

EXPLANATION 

--'- · · Normal fault-Dotted where concealed. Bar and ball on downthrown side 

.&... • • Thrust fault-Dotted where concealed. Sawteeth on upper plate 

-+- Anticl.ine 

- - Approximate extent of Wasatch Mountains 

Figure 1. Central Wasatch Mountains and surrounding region, showing distribution of middle Tertiary igneous rocks and 

major structures. Geology simplified from Hintze (1980). Abbreviations: 8, Bingham mine; CN, Charleston-Nebo thrust 

fault; DC, Deer Creek fault; WF, Wasatch fault zone. 

Geology of the Alta Stock 3 
Fe-Ti oxide mineral and appears to be a liquidus phase; 

ilmenite is sparse to absent in most samples. Sphene 

forms euhedral phenocrysts throughout most of the 

stock. Modal and chemical analyses indicate that the 

composition of the stock varies from quartz monzodiorite to granodiorite with a slight increase in silica contents from the margin inward and toward the eastern part 

of the stock (fig. 5, table 1). Most of the stock is 

metaluminous, although the most felsic part of the stock 

is weakly corundum normative (table 1). 

John (1987, 1989) showed that the central Wasatch 

Mountains have been tilted east about 20° since the emplacement of the Alta stock. Paleodepths of present exposures of the Alta stock were estimated to range from 

about 6.3 km on the west to about 3.7 km on the east, 

corresponding to lithostatic pressures varying from about 

1. 7 kbars on the west to 1.0 kbar on the east. 

HYDROTHERMAL ALTERATION OF 

THE ALTA STOCK 

Hydrothermal alteration of the Alta stock generally 

is weak and mostly consists of pervasive deuteric alteration. Other types of alteration include fine-grained, 

shreddy biotite that locally replaces hornblende and is 

present in the groundmass of rocks in the eastern part of 

the stock; thin K-feldspar selvages around hornblende 

veins; and thin sericitic selvages on quartz and 

quartz+pyrite veins. Deuteric alteration is present 

throughout the stock and appears to be unrelated to 

megascopic or microscopic fractures. This alteration 

consists of variable amounts of fine-grained white mica 

(sericite) replacing plagioclase, chlorite+ sphene+ magnetite replacing biotite along crystal margins and cleavage planes, sphene locally rimming magnetite, and perthite forming in potassium feldspar and antiperthite in 

plagioclase. In more strongly altered rocks, epidote partially replaces plagioclase and biotite, prehnite locally 

replaces biotite, and potassium feldspar becomes turbid 

and more coarsely perthitic. 

Seven vein types were distinguished during field 

studies (table 2). Early veins (type 1) are hornblende + 

K-feldspar +sphene± quartz± magnetite± epidote. Plagioclase crystals along the margins of type 1 veins range 

from unaltered to partially replaced and overgrown by 

K-feldspar to sericitized. Many type 1 veins have thin 

(~1 em) white selvages of potassium feldspar (approximately Or91Ab9). Most hornblende crystals are euhedral 

and appear to have filled open spaces that were later 

filled by quartz. Epidote (±magnetite ±pyrite) veins (type 

2) are present sporadically in the eastern part of the 

stock and locally cut hornblende veins. Biotite+ magnetite veins (type 3) locally cut hornblende veins. Type 3 

Figure 2. Alta stock, looking west. Alta stock forms light-colored rocks seen in lower 

half and at skyline on left side of photograph. Peak on left side of the photograph is Mt. 

Millicent. High peak in background is Dromedary Peak, which is composed of Proterozoic and Paleozoic sedimentary rocks. Lake Mary is in center of photograph and Twin 

Lakes Reservoir is on far right side. Grassy clearcuts in lower part of photograph are ski 

runs. Brighton is just to right of photograph. 

4 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
veins also may contain quartz, K-feldspar, pyrite, chalcopyrite, and molybdenite. Wall-rock alteration of plagioclase varies from partial replacement by K-feldspar 

0 

0 

to fresh or weakly sericitized, similar to other plagioclase crystals outside the immediate vein selvage. Potassium feldspar crystals along vein margins are fresh. 

1 MILE 

2 KILOMETERS 

EXPLANATION 

~ .... I 

Quaternary surficial deposits 

Porphyry phase 

Equigranular phase 

} Alta stock 

Clayton Peak stock 

Mesozoic sedimentary rocks 

Paleozoic sedimentary rocks 

Precambrian sedimentary, metamorphic, and intrusive rocks, undivided 

Contact-Dashed where approximately located 

~--·· Normal fault-Dashed where approximately located; dotted where concealed. Bar and ball on 

downthrown side 

~ Thrust fault-Sawteeth on upper plate 

Figure 3. Alta stock and vicinity. Geology simplified from Baker and others (1966) and Crittenden (1965). Abbreviations: SFF, Silver Fork fault; AT, Alta Grizzly thrust. 

Hydrothermal Alteration of the Alta Stock 5 
Quartz veins (type 4 ), generally conta1rung small 

amounts of K-feldspar, cut the biotite+ magnetite veins. 

Plagioclase crystals along the margins of type 4 veins 

generally are sericitized, and biotite crystals are 

chloritized. Quartz + pyrite veins (type 5), generally 

with coarse-grained sericite selvages, locally cut biotite 

+ magnetite veins. Plagioclase, biotite, and hornblende 

are sericitized in vein selvages, whereas potassium feldspar varies from unaltered to sericitized. Vuggy quartz 

veins partially filled with fine-grained chlorite + pyrite 

(type 6) cut all the above types of veins. Plagioclase 

along vein margins is sericitized, and biotite is 

chloritized. Potassium feldspar appears fresh or weakly 

sericitized. The latest veins (type 7) are pyrite + chlorite 

that coat joint surfaces (fig. 6). These veins lack vein 

selvages. 

MINERALIZATION ASSOCIATED WITH 

THE ALTA STOCK 

Weak, fracture-controlled base-metal mineralization is present within the Alta stock. Thin (generally 

~2 mm), widely spaced biotite+ magnetite ±quartz ±pyrite veins, locally containing small amounts of chalcopyrite and (or) molybdenite, are present throughout the 

stock but are nowhere abundant. These veins are similar 

to early veins in porphyry copper deposits (Titley, 1982). 

Thicker, milky white quartz veins containing chalcopyrite and pyrite are locally present near Mt. Wolverine 

(James, 1978). 

Two types of mineralization are present in wall 

rocks around the Alta stock. These are small Fe-Cu 

skarns and Ag-Pb-Zn fissure and replacement ores. 

Skarns locally contain pods of massive magnetite ± 

bornite ± chalcopyrite ± pyrrhotite in a complex gangue 

of silicate minerals (for example, Big Cottonwood mine; 

fig. 3). Ag-Pb-Zn sulfide ores are present in fault zones 

(particularly thrust faults) and replace carbonate rocks 

where these rocks intersect mineralized faults (Calkins 

and Butler, 1943; James, 1978). Most of the Ag-Pb-Zn 

ores were found west of the Silver Fork fault (fig. 3) in 

the Cottonwood mining district. 

FRACTURES AND VEINS IN THE ALTA STOCK 

Joints and mineral-coated joints (veins) are present 

throughout the Alta stock, and reconnaissance studies of 

the distribution, orientation, and abundance of joints and 

veins are described below. Traverses were made across 

all parts of the stock, and data on the types, orientations, 

and abundances of veins and fractures were measured at 

approximately 250 locations spaced at fairly uniform 

intervals (approximately 60-100 m) along these traverses 

(fig. 7A). 

Figure 4. Intrusive contact between equigranular and porphyritic phases of Alta 

stock. Foliation defined by parallel alignment of mafic minerals and flattened mafic 

inclusions in equigranular phase (upper half of photograph) is truncated by porphyritic phase (lower half of photograph). Photograph taken on western shore of Lake 

Mary. 

6 Evolution of Hydrothermal Auids in the Alta Stock, Central Wasatch Mountains, Utah 
Types and Relative Ages 

Joints are well developed throughout most of the 

stock and tend to form one or two well-defined sets. 

Spacing between joints varies from about 2 em to >1m. 

Most joints have approximately planar surfaces and appear to be continuous along strike for several to tens of 

meters. No strike separation was found on joints, which 

suggests that movement was dominantly normal to the 

surface of the joint. Wilson (1961, pl. 2) found that joint 

sets in the equigranular and porphyritic phases have different orientations and spacings, but this relationship 

was not confirmed in the present study. 

A small percentage of joints are coated with hydrothermal minerals and are shown as veins (fig. 7). Most 

veins are thin, with average widths of 1 to 2 mm; few 

veins exceed 1 em in width. Aplite and pegmatite dikes 

also are common and predate all vein types. Pyrite + 

chlorite veins (type 7) are the most abundant veins; they 

are present at nearly every outcrop and cut all other 

types of veins. Veins are not offset by the contact between the porphyritic and equigranular phases, indicating 

that the vein-forming hydrothermal system postdates emplacement of the porphyritic phase. 

Distribution and Orientations 

The distribution and orientations of veins are 

shown in figure 7. Hornblende+K-feldspar veins are notably sparse in the eastern (upper) part of the stock, 

whereas epidote veins are concentrated in this part of the 

stock (fig. 7A). Hornblende+K-feldspar veins are concentrated along the western, northern, and southern marQuartz 

Granite Tonalite 

• 

Quartz syenite Quartz monzonite Quartz monzodiorite 

Syenite Monzonite Monzodiorite 

K-feldspar 10 35 65 90 Plagioclase 

Figure 5. Modal quartz, plagioclase, and K-feldspar contents recalculated to 100 percent for samples of Alta stock on the 

basis of point counts of stained slabs. Solid circles, samples of equigranular phase; open circles, samples of porphyritic 

phase. Compositional fields from Streckeisen (1976). 

Fractures and Veins in the Alta Stock 7 
Table 1. Chemical analyses, CIPW norms, and modal analyses of the Alta stock 

[Major-element analyses except FeO made by wavelength-dispersive X-ray fluorescence spectroscopy (analyst, A. Bartel). FeO, F, and Cl determined by wet chemical methods (analyst, K. 

Stewart). Minor-element analyses made by energy-dispersive X-ray fluorescence (analysts, J. 

Taggert and T. Vercoutere)] 

Sample 82-PC-17 83-PC-206 83-PC-272 83-PC-276 

Si02(wt %) 62.9 63.5 63.9 67.2 

Al20 3 16.4 16.6 16.0 15.5 

Fe20 3 2.48 2.53 2.30 1.56 

FeO 2.43 2.13 2.15 1.71 

MgO 2.25 1.81 2.19 1.61 

CaO 4.84 4.39 4.21 2.73 

Na20 3.66 3.88 3.70 3.38 

K20 2.99 3.14 3.08 4.22 

Ti02 .58 .58 .58 .50 

P20s .32 .32 .29 .26 

MnO .08 .08 .05 .03 

LOI --:21 --:.ll __,§]_ ~ 

Total 99.16 99.27 99.12 99.28 

F .05 

Cl .018 

Ba(ppm) 1,300 1,750 

Ce 102 110 

Cr 20 13 

Cu <20 146 

La 58 67 

Nb 8 14 

Ni 6 10 

Rb 91 116 

Sr 680 696 

y 20 20 

Zn 59 60 

Zr 177 176 

gins of the stock (fig. 7A). Both biotite+magnetite veins 

and quartz+pyrite+chlorite veins are concentrated in the 

western (deeper) parts of the stock (fig. 1B, C). 

Stereographic plots suggest that joints and veins 

have regular orientations (fig. g). Joints have two dominant orientations: approximately N. 35° E. dipping goo 

NW. and N. g5o E. dipping goo NW. (fig. gA). All veins 

except type 7 pyrite+chlorite veins have similar orientations to the joints (fig. gB). Pyrite+chlorite veins are 

more systematically oriented than other types of veins, 

with a dominant trend of N. 3go E., 76° NW. subparallel 

to one set of joints (fig. gc). 

The approximate total number of megascopic 

fractures per meter (joints + veins) varies from <1/m to 

>20/m, although the number of veins generally is less 

than 1-2/m. The integrated fracture density (total length 

of veins/area of outcrop; Haynes and Titley, 19gO), 

measured in three locations where veins are most abun-

.05 .06 .05 

.010 .025 .013 

2,000 1,350 

125 80 

26 18 

<20 310 

80 52 

13 13 

15 4 

109 184 

745 450 

25 20 

56 27 

227 161 

dant, ranges from O.og;cm to 0.16/cm. There is no 

obvious correlation between abundance of joints and 

veins with depth in the stock or between abundance of 

fractures with location relative to the margins of the 

stock. 

Comparison of Fracture Data in the Alta Stock to 

Other Areas in the Central Wasatch Mountains 

Villas and Norton (1977) showed that there 

are two dominant trends of veins in the nearby 

Mayflower stock: N. 50° E., goo NW. and N. 50° W., 

goo SW. They noted that the northeast-trending set is 

parallel to major fissures in the Park City mining district, but that there was no regional counterpart to the 

northwest-trending set. They suggested that these fracture sets were conjugate shear fractures formed at high 

8 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
Table 1. Chemical analyses, CIPW norms, and modal analyses of the Alta stockContinued 

Sample 82-PC-17 83-PC-206 83-PC-272 83-PC-276 

Normative minerals (weight percent) 

Quartz 17.33 17.67 19.11 23.69 

Corundum 1.04 

Orthoclase 17.86 18.75 18.49 25.27 

Albite 31.30 33.17 31.80 28.98 

Anorthite 19.70 18.80 18.24 12.00 

Diopside 1.96 .87 .84 

Hypersthene 6.35 5.16 6.34 5.16 

Magnetite 3.64 3.71 3.39 2.29 

Ilmenite 1.11 1.11 1.12 .96 

Apatite .75 .75 .68 .61 

Modal analyses (volume percent) 

Quartz 13.6 18.7 20.8 21.5 

Plagioclase 52.6 44.5 46.9 44.6 

K-feldspar 15.9 22.0 16.8 22.0 

Biotite 4.6 6.4 7.5 9.9 

Hornblende 10.8 6.0 6.4 .8 

Opaque oxides 1.8 1.5 1.5 .6 

Sphene .5 1.0 .3 .5 

Apatite .1 .1 tr tr 

Other z All,Z Cpx,Z All,Z 

Abbreviations: LOI, loss on ignition at 900 °C; tr, trace; All, allanite; Cpx, clinopyroxene; 

z. zircon. 

Sample descriptions (see fig. 11 for sample locations): 

82-PC-17-marginal part of Alta stock, equigranular phase 

83-PC-206-interior part of Alta stock, equigranular phase 

83-PC-272-upper part of Alta stock, equigranular phase; plagioclase moderately sericitized 

and biotite moderately chloritized, 

83-PC-276-upper part of Alta stock, porphyritic phase, hornblende partially replaced by 

biotite 

temperatures during emplacement and cooling of the 

Mayflower stock. They also showed that the abundance 

of mineralized fractures ranges from about 6 to 21 per 

meter in the upper part of the Mayflower stock and decreases downward. 

Numerous silicic, lamprophyric, and andesitic 

dikes, which intrude the Alta and Little Cottonwood 

stocks and the wall rocks between them, are shown on 

maps of the Dromedary Peak and Brighton quadrangles 

(Crittenden, 1965; Baker and others, 1966). The dikes 

tend to have three major trends---N. 5° W. to N. 10° E., 

N. 40-60° E., and N. 80° E. to E.-W.-and most appear 

to have steep dips. Major joint sets in the Little Cottonwood stock mapped by Crittenden (1965) have orientations similar to the dikes. 

Fractures in the Alta stock have fairly systematic 

orientations subparallel to regional fracture patterns and 

lack a strong set of conjugate shear fractures similar to 

those present in the Mayflower stock. These data suggest 

that most of the fractures in the Alta stock formed by 

tensile stresses and were localized in directions of regional weaknesses. Burnham ( 1985) has suggested that 

fracture patterns in typical porphyry copper systems are 

preferentially concentrated where wall rocks initially 

have relatively uniform tensile strengths. In contrast, for 

wall rocks that initially possess major zones of weaknesses, Burnham suggests that localization of fractures in 

directions of regional weaknesses and release of energy 

during second (resurgent) boiling lead to early failure 

and concentration of decompressional energy into these 

Fractures and Veins in the Alta Stock 9 
Table 2. Vein assemblages in the Alta stock 

Type Mineralogy Typical width Relative abundance Selvage 

(millimeters) 

1 Hornblende + K-feldspar +sphene± quartz± 1-10 (generally Moderate White K-feldspar 

magnetite ± epidote 

2 Epidote ± magnetite ± pyrite 

3 Biotite +magnetite± quartz± K-feldspar ± 

pyrite± chalcopyrite ±molybdenite 

4 Quartz± K-feldspar 

5 Quartz + pyrite ± sericite selvages 

6 Quartz + chlorite + pyrite 

7 Pyrite + chlorite 

zones, stopping development of extensive "porphyrytype fracture systems." This latter process may have 

occurred in the Alta stock. 

The abundance of mineralized fractures (veins) 

in the vicinity of the Mayflower vein system in the 

Mayflower stock is about 5 to 20 times greater than in 

the Alta stock. Fossil fracture permeability of the Alta 

stock cannot be estimated accurately because the apertures of the fractures are unknown (Snow, 1970; Norton 

and Knapp, 1977). However, if aperture widths in the 

Alta stock are similar to those estimated by Villas and 

Norton (1977) for the Mayflower stock and if the 

abundance of mineralized fractures is an accurate representation of the number of fractures that were open 

during circulation of hydrothermal fluids, then fracture 

permeability of the Alta stock was about an order 

of magnitude less than that in the Mayflower stock. 

The integrated fracture densities measured in the Alta 

stock are considerably lower than those measured by 

Haynes and Titley (1980) and Titley and others (1986) 

in the Sierrita, Arizona, porphyry copper deposit and are 

similar to values that these authors measured several 

kilometers outside of the mineralized zone, suggesting 

that the fracture permeability in the Alta stock is considerably lower than in porphyry copper systems. 

CRYSTALLIZATION SEQUENCE 

AND ESTIMATION OF H20 CONTENTS 

OF THE MAGMA 

The mineralogy and crystallization sequence of the 

Alta stock suggest that the magma of the Alta stock had 

initial H20 contents around 3 to 4 weight percent, which 

may be less than typical porphyry copper-related 

intrusions. In the Alta stock, textural evidence indicates 

that both biotite and hornblende crystallized relatively 

S1-2) (0-5 mm) 

1-2 Sparse except at east end 

<1-2 Moderate Local K-feldspar 

(<1-2mm) 

1-10 Sparse 

1-50 Generally sparse Sericite 

(1-10mm) 

1-10 Generally sparse Narrow sericite 

<1-3 Abundant 

early, but that biotite began to crystallize before 

hornblende (fig. 9; John, 1987). The presence of hornblende as an early phenocrystic phase indicates magmatic H20 contents ~3 weight percent for bulk compositions 

similar to the Alta stock (Burnham, 1979a). Comparison 

of the mineral crystallization sequence of the Alta stock 

(fig. 9) with Naney's (1983) experimental studies of a 

synthetic granodiorite with a composition similar to the 

most felsic parts of the Alta stock suggests water 

contents of about 3.5 to 4.25 weight percent (fig. 10). 

Comparison of the crystallization sequence of the Alta 

stock to experimental crystallization studies of the 

Oligocene Fish Canyon Tuff at 2 kbars (Johnson and 

Rutherford, 1989a), which has a whole-rock composition 

similar to most of the Alta stock, also suggests low 

H20 contents in the magma (approximately 3 weight 

percent) and vapor undersaturation during most of the 

crystallization (mole fraction of H20 in the fluid phase 

approximately 0.25; Johnson and Rutherford, 1989a, 

fig. 10). 

Mineral-crystallization sequences and estimates of 

magmatic H20 contents of porphyry copper intrusions 

are seldom reported. Dilles (1984, 1987) noted that in 

the Yerington batholith, biotite crystallized before hornblende in the premineralization granodiorite phase, but 

that biotite and hornblende crystallized simultaneously in 

the quartz monzonite porphyry and porphyritic quartz 

monzonite phases, indicating somewhat higher H20 contents in the mineralizing intrusions. He suggested that 

H20 contents of the quartz monzonite porphyry and porphyritic quartz monzonite were >4 weight percent. 

Burnham's (1979a, 1981, 1985; Burnham and Ohmoto, 

1980) model for porphyry copper systems suggests crystallization of hornblende before biotite. These data suggest that the initial H20 contents of the Alta stock might 

have been slightly less than in typical porphyry copper 

intrusions. 

10 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
COMPOSITIONS OF PRIMARY BIOTITE 

AND AMPHIBOLE AS INDICATORS OF 

MAGMATIC FLUID 

The compositions of biotite and amphibole can be 

sensitive indicators of the temperature and of the oxygen 

and water fugacities of crystallization (Wones, 1981), although the effects of subsolidus reequilibration and alteration must be understood before these parameters can be 

estimated. Several studies of porphyry copper systems 

and of shallow plutonic complexes suggest that the oxidation state, and in some cases the oxygen fugacity as 

well, increased during crystallization and cooling (for 

example, see Murakami, 1969; Czamanske and Wones, 

1973; Mason, 1978; Chivas, 1981; Hendry and others, 

1985). In contrast to these oxidizing trends, oxygen fugacity decreases with falling temperature, a trend that is 

typical of closed systems where the oxidation state maintains values close to oxygen buffer curves (for example, 

see Dodge and others, 1969; Lipman, 1971; Hildreth, 

1979). An increasing oxidation state during crystallization is reflected in higher Mg/(Mg+Fe) contents of biotite and hornblende as crystallization progresses. 

Czamanske and Wones (1973), Mason (1978), Chivas 

(1981), and Hendry and others (1985) attributed exsolution of a vapor phase from the magma and subsequent 

diffusive loss of hydrogen as the most likely cause of 

oxidation during crystallization. However, a numerical 

model of the effects of vapor exsolution on magmatic 

oxygen fugacity by Candela ( 1986) suggests that this 

process is an ineffective method for increasing oxygen 

fugacity in normal granodioritic magmas, and that some 

other process, such as introduction of meteoric water, 

must account for increasing oxidation state. 

The compositions of biotite and hornblende in 26 

samples of the Alta stock were determined by electron 

microprobe (fig. 11). Analyses were made of phenocrysts, groundmass crystals, small inclusions in the interiors of plagioclase crystals, and hydrothermal crystals. 

Ferrous/ferric iron ratios were determined for four biotite 

separates by wet chemical analysis of ferrous iron and 

recalculation of microprobe analyses. Representative 

analyses of biotite and hornblende are given in tables 3 

and 4, respectively. 

Biotite Compositions 

Biotite in the Alta stock has relatively low Fe/ 

(Fe+Mg) ratios (0.31 to 0.51) compared with values 

from biotite in the Sierra Nevada batholith (Dodge and 

others, 1969; Ague and Brimhall, 1988), the California 

Coast Ranges (Dodge and Ross, 1971), and Japanese 

granitoids (Czamanske and others, 1981). Biotite phenocryst compositions are very similar to those of biotite in 

other intrusions in the Wasatch Mountains (Nash, 1982; 

Aiken, 1982; D.A. John, unpub. data, 1990); in the least 

altered biotite in the Santa Rita (Jacobs and Parry, 

Figure 6. Type 7 pyrite+chlorite veins in Alta stock. Subparallel pyrite+chlorite veins 

are well developed along one joint set. Note felt-tip marking pen for scale. 

Compositions of Primary Biotite and Amphibole as Indicators of Magmatic Fluid 11 
0 1 ,000 METERS 

0 3,000 FEET 

~ N 

I 

I.~ 

/ 

- .... 

A 

0 1 ,000 METERS 

0 3,000 FEET 

~ N 

I 

8 

Brighton 

• 

. . . . . . . . \ . .. : \ \. 

. ······ .. ' \ 

•')( 

Brighton 

• 

EXPLANATION 

Hornblende+K-feldspar 

Epidote 

EXPLANATION 

Biotite+ Magnetite 

Quartz±K-feldspar 

Figure 7. Distribution and orientations of veins in Alta stock. See figure 3 for location. A, Type 1 hornblende-bearing veins 

and type 2 epidote veins. Dots show other locations where fracture data were collected. 8, Type 3 biotite+magnetite and 

12 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
0 1 ,000 METERS 

0 3,000 FEET 

~ N 

I 

0 1 ,000 METERS 

I I 

0 3,000 FEET 

~ N 

I 

Brighton 

• 

Brighton 

• 

EXPLANATION 

Quartz+Pyrite±Sericite 

Quartz+Pyrite+Chlorite 

EXPLANATION 

Pyrite+Chlorite 

type 4 quartz±K-feldspar veins. C, Type 5 quartz+pyrite±sericite and type 6 quartz+pyrite+chlorite veins. D, Type 7 

pyrite+chlorite veins. 

Compositions of Primary Biotite and Amphibole as Indicators of Magmatic Fluid 13 
N 

A 

N 

B 

N 

c 

1979), Bingham (Czamanske and Moore, 1973; Lanier 

and others, 1978), and Yerington (Dilles, 1984, 1987) 

porphyry copper deposits; and in the Fish Canyon Tuff 

(Whitney and Stormer, 1985). 

Most biotite phenocrysts show only weak Fe-Mg 

zoning, with either nearly constant Fe/{Fe+Mg) ratios or 

Fe/{Fe+Mg) ratios that are slightly lower for rims compared with cores (table 3). Four samples along the margin of the stock show the opposite trend, however, with 

two samples showing strong Fe-enrichment from core to 

rim [for example, Fe/{Fe+Mg) increases from 0.39 to 

0.48 from core to rim in sample 83-PC-253, table 3]. In 

crystals that show significant core to rim zoning, most of 

the crystal is fairly homogeneous, with only the rims 

having large Fe-Mg variation from the rest of the crystal. 

Titanium and barium contents also decrease from core to 

rim in crystals showing Fe-Mg zoning (table 3), which is 

consistent with falling temperature and progressive crystallization (Barriere and Cotten, 1979) and with the 

change from magmatic to hydrothermal conditions 

(Jacobs and Parry, 1979). 

Two textural types of "hydrothermal'' biotite are 

present: (I) thin veins associated with magnetite, quartz, 

K-feldspar, and, locally, some sulfide minerals, and (2) 

fine-grained clots of shreddy biotite that are present as 

replacements of hornblende phenocrysts and as small 

crystals in the groundmass. Shreddy biotite was analyzed 

in one sample and has a composition very similar to the 

rim compositions of biotite phenocrysts (sample 83-PC276, table 3). Vein biotite of three samples has significantly lower Ti and Ba and higher Si contents than do 

biotite phenocrysts in the same sample, and lower Ti 

contents than the shreddy biotite in sample 83-PC-276 

(fig. 12, table 3). Fe/{Fe+Mg) ratios of vein biotite are 

variable and overlap phenocryst compositions (fig. 12, 

table 3). 

Estimation of Intensive Parameters 

Using Biotite Compositions 

Temperatures of biotite crystallization were 

estimated to help evaluate the oxidation state during 

crystallization of the Alta stock. These estimates used 

...... Figure 8. lower-hemisP.here equal-area net projection 

of pores of structural data in Alta stock. A, Joints m Alta 

stock, 336 measurements. Contour intervals at 2 (lined), 3 

(stippled), and 4 (black) percent per 1-percent area. 8, 

Types 1 through 6 veins in the Alta stock, 206 measurements. Contours at 3 (lined), 4 (stippled), and 5 (black) 

percent per 1-percent area. C, Type 7 pyrite+chlorite 

veins, 255 measurements. Contours at 5 (lined), 6.5 

(stippled), and 8 (black) percent per 1-percent area. 

14 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
the Ti and Fe contents of biotite and empirical equations 

derived by Dilles (1984, 1987) and Luhr and others 

(1984) for biotite coexisting with quartz and sphene or 

ilmenite. These geothermometers are based on regression 

of Fe-Ti oxide temperatures from volcanic rocks and 

oxygen-isotope temperatures in plutonic and metamorphic rocks against the ratio of mole fractions of Ti and 

Fe in octahedral sites in biotite. Temperatures were estimated using the Dilles (1984) geothermometer because it 

is based on a larger data set and includes lower temperature samples than does the geothermometer of Luhr and 

others (1984). The accuracy of the Dilles geothermometer is unknown, but Dilles (1984) suggests an error of 

+50 oc over the temperature range of 450 octo >900 °C. 

Temperature estimates (figs. 12, 13; table 3) are 

fairly consistent with known biotite phase relations 

(Wones and Eugster, 1965; Naney, 1983; Johnson and 

Rutherford, 1989a) and with solidus temperatures 

estimated from the experimental studies of Piwinskii 

(1968) and Naney (1983). Several temperature estimates 

in the eastern part of the stock, however, may be too 

high (>900 °C), and several estimates in the western part 

of the stock are slightly subsolidus (650-700 °C). The 

anomalously high temperatures may be due to the lack 

of a Ti-bearing phase during early crystallization; in particular, these samples do not contain phenocrystic sphene 

and contain little, if any, ilmenite. All biotite phenocrysts showing Fe-Mg zoning have higher calculated 

core temperatures than rim temperatures (fig. 13). These 

data suggest that most biotites may have retained their 

high-temperature compositions, and that compositional 

zoning is the result of progressive crystallization. 

MINERAL 

Plagioclase 

Clinopyroxene 

Biotite 

Hornblende 

Potassium feldspar 

Quartz 

Magnetite 

Ilmenite "r--? -?-

Estimates of oxygen fugacity were made using 

temperatures calculated from the Dilles (1984, 1987) 

calibration, biotite compositions, and biotite stability 

relations from Wones and Eugster (1965) and Wones 

(1972) using the expression log fH2o= 7409{I' + 4.25 + 

0.5 log f D2 + 3 log aannite - log asanidine - log amagnetite for 

biotite coexisting with quartz, magnetite, and potassium 

feldspar. The activity of annite was calculated as 

aannite=X3p/2X2oH, where Xp/2 is the mole fraction of 

Fe+2 in the octahedral site and X0 His the mole fraction of 

OH in the hydroxyl site. For these calculations, Fe+2 was 

assumed equal to 0.75 total Fe (average of 

Fe+2-Fe+3 determinations; see fig. 14), asanidine equal to 

0.8, and amagnetite equal to 0.8. H20 pressure was assumed 

equal to total pressure, and fH.p values were taken from 

Burnham and others (1969). Calculated oxygen fugacities range from about 10-10·

9 to 10-15·

7 bars (fig. 13). Calculated oxygen fugacities are between the Ni-NiO 

(NNO) and hematite-magnetite (HM) buffers and are 

about 0.5 to 2 log units above the titanite-magnetitequartz (TMQ) buffer (Wones, 1989), in general agreement with estimates from Fe+3

-Fe+2 contents of biotite 

(fig. 14) and with the lack of stable clinopyroxene in the 

later stages of crystallization and the near-absence of ilmenite. Biotite crystals showing strong compositional 

zoning generally suggest crystallization along oxygenbuffered paths, with rims having compositions indicating 

lower temperatures and lower oxygen fugacities (fig. 

13). In general, the western samples and hydrothermal 

biotite have calculated temperature~ and oxygen fugacities that suggest crystallization at slightly higher oxidation states than for the eastern phenocrysts (fig. 13). 

Sphene -?- ? - ---------------1 

Apatite 

Early Late 

TIME 

Figure 9. Sequence of mineral crystallization in Alta stock. Relative ages of crystallization 

based on textural relations among minerals. Queried segments indicate uncertain timing. 

Compositions of Primary Biotite and Amphibole as Indicators of Magmatic Fluid 15 
These calculations are subject to large errors resulting 

from uncertainties in estimates of temperatures, pressures, and activities of biotite and potassium feldspar. 

Temperature uncertainties of ±50 °C result in oxygenfugacity uncertainties of ±0.7 log units. Uncertainties in 

other parameters lead to smaller uncertainties, but 

oxygen-fugacity estimates are probably no better than ± 

1log unit. 

The effects of PH2o<P total (water-undersaturated 

magma) were estimated using Burnham's (1979b) watersilicate melt model assuming 3 percent H20 in the melt. 

Water undersaturation reduces calculated f'h values by 

0.5 to 0.7 log units. If core compositions of biotite represent crystallization under PH2

o<P total conditions, and if 

rim compositions represent crystallization at PH2o=P total' 

then compositionally zoned crystals suggest slight increases in the oxidation state during crystallization. The 

increases in oxidation state are similar in magnitude to 

1100 

L+V 

en 

::> 1000 

Ci5 

....J 

w Liquidus (.) 

en 

w 

w 

a: 

C) 900 

w Orthopyroxene in 

() 

~ Hornblende in 

u.i 

a: 

::> 

I- 800 < a: 

w 

a.. 

::! 

w ----------- I- Hornblende out 

700 

Solidus 

600;--r~~--r-~,--r~~~r-~,-. 

0 2 4 6 8 10 12 

WEIGHT PERCENT H20 

Figure 10. Temperature plotted vs. weight percent H20 at 2 

kbars total pressure, showing phase relations for a synthetic 

granodiorite composition (modified from fig. 4 of Naney, 

1983). Stippled field shows range of H20 contents where 

biotite will crystallize before hornblende and possible 

range of H20 contents for Alta stock. Points A, B, and C 

portray possible crystallization path described in text. 

Abbreviations; L, liquid; V, vapor. 

those calculated by Candela (1986) but are considerably 

smaller than the increases suggested by Czamanske and 

Wones (1973). There is no indication of an increase in 

oxygen fugacity during crystallization, even in strongly 

zoned crystals. 

Amphibole Compositions 

Amphibole in the Alta stock shows a wide range of 

textures, compositions, and compositional zoning. Compositions span the range from magnesio-homblende to 

actinolite (table 4; Leake, 1978). Most amphibole in the 

Alta stock is compositionally similar to amphibole in 

other intrusions in the Wasatch Mountains (Aiken, 1982; 

Nash, 1982; D.A. John, unpub. data, 1990). Amphibole 

phenocrysts in the Alta stock are not replaced by fibrous, 

fine-grained actinolitic amphibole and do not contain pyroxene cores as they do in the Clayton Peak stock 

(Aiken, 1982). The amphibole phenocrysts have low Fe/ 

(Fe+Mg) ratios and low total AI, Ti, Na, and K concentrations compared with primary amphibole in the Sierra 

Nevada batholith (Dodge and others, 1968; Ague and 

Brimhall, 1988), the California Coast Ranges (Dodge 

and Ross, 1971), Japanese granitoids (Czamanske and 

others, 1981 ), the Fish Canyon Tuff (Whitney and 

Stormer, 1985; Johnson and Rutherford, 1989a), and 

most other "igneous" amphibole (Leake, 1968; Helz, 

1982). 

Most analyzed amphibole crystals in the Alta stock 

contain two types of compositional zoning: (1) overall 

core to rim zoning, and (2) irregular patchy zoning. In 

general, neither type of zoning is optically discernible 

except as vague changes in pleochroism. However, 

patchy compositional zoning is readily apparent in 

backscattered SEM images (fig. 15). 

Patchy compositional zoning is present in most 

samples and is similar to zoning in amphibole described 

by Czamanske and Wones (1973), Chivas (1981), and 

Hendry and others (1985). This zoning consists of small 

domains that are highly enriched in AI, Fe, Ti, Na, and 

K and depleted in Si, Mg, Mn, and Ca relative to adjacent domains. These domains appear to be discontinuous 

and are not obviously related to concentric growth 

zones, mineral inclusions, or any alteration visible under 

the microscope (fig. 15). Domains within single crystals 

may vary by as much as 0.7 Si atoms per unit cell (23 

oxygens). Despite the complex distribution of compositional domains, overall compositional trends are well defined, as shown in figure 16, where variations of major 

elements are plotted against silicon atoms per unit cell. 

Regular increases in Mg, Mn, and Ca and decreases in 

Fe, AIVI, Ti, Na, and K are apparent with increasing Si. 

The compositional trends are similar to trends observed 

by Chivas (1981) for the Inamumu zoned pluton, 

16 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
Guadalcanal, Solomon Islands, and by Hendry and others (1985) for the Christmas intrusive complex, Arizona. 

The apparent compositional gap between 7.2 and 7.3 Si 

atoms per unit cell reported by Chi vas ( 1981) as a possible break between igneous and subsolidus amphibole is 

not present in the Alta stock, and vein amphibole falls 

on both sides of this previously reported "break." 

Most analyzed crystals also show overall core to 

rim zoning, with AliV, Fe/(Fe+Mg), Ti, Na, and K decreasing and Si, Mg, Mn, and Ca increasing from core to 

rim (table 4). However, rims in four samples show opposite trends, although the variation in Fe/(Fe+Mg) is not 

nearly as extreme as in biotite that shows this reverse 

type of zonation (table 3). Iron-enrichment trends are 

only found in samples located along the margin of the 

stock, although only one of these samples contains biotite showing the same trend (sample 83-PC-253). 

Hydrothermal amphibole from early hornblende+ 

K-feldspar+quartz+sphene veins was analyzed in two 

samples. The amphibole ranges from actinolitic hornblende to actinolite (fig. 16) and is compositionally inhomogeneous. These samples span the range of compositions found in amphibole phenocrysts from the same 

sample, and in sample 83-PC-253 they show the same 

Fe-enrichment trend from core to rim as shown by amphibole phenocrysts (table 4). 

0 1 ,000 METERS 

0 3,000 FEET 

~ N 

I 

• 82-PC-20 (I) 

83-PC-152 (F,M) 

83-PC-258 (I) 

Interpretation of Compositional Zoning 

in Amphibole 

Complex patchy zoning in amphibole similar to 

zoning found in the Alta stock has been interpreted by 

Czamanske and Wones (1973), Chivas (1981), and 

Hendry and others ( 1985) as the result of fluctuating oxygen fugacity during both igneous and hydrothermal crystallization. Chivas (1981) notes that the smooth 

compositional variations and overall variations between 

textural types in the Inamumu zoned pluton are consistent 

with fluctuating but generally increasing oxygen fugacity 

from early magmatic through late magmatic to early 

hydrothermal crystallization. He also correlates high f~ 

domains (Si-, Mg-, and Ca-rich) with periods of fluid exsolution and suggests that Fe, Ti, Na, K, and Cl are partitioned into the fluid phase in preference to the amphibole. 

In contrast, for amphibole in "barren" intrusions, Chivas 

notes that there is little evidence for the former presence 

of a fluid phase, and that the amphibole either shows Feenrichment with increasing Si or shows very irregular 

compositional trends. Mason (1978) also suggests that 

Mg-enrichment of amphibole rims is the result of increasing oxygen fugacity during crystallization. 

Overall compositional trends of amphibole in the 

Alta stock suggest both falling temperature and increas84-Pc-40 (F,O 

8-38 (F,I,M) 

83-PC-274 (F,M) 

8~C'.~c~r5,276 82-PC-79 <I> 

' ~ 83-PC-37A (F) 

Brighton '--Jaa-PC-277 O> 

83-PC:-151 (F) 

82-PC-25 (I)• 

A198(1) 

Figure 11. Locations of Alta stock samples used in chemical analyses (C) and in microprobe (M), stable-isotope (1), and 

fluid-inclusion (F) studies. See figure 3 for location. 

Compositions of Primary Biotite and Amphibole as Indicators of Magmatic Fluid 17 
... Table 3. Representative microprobe analyses of biotite in the Alta stock 

= 

[n.a, not analyzed] 

~ 2" Sample 83-PC-276 83-PC-276 83-PC-276 83-PC-276 83-PC-253 83-PC-253 83-PC-253 S-30 S-30 83-PC-148 83-PC-148 6· 

= Spot1 6A (core) 7A (rim) 3 (shred) 2A (vein) 4 (inc) 1 (core) 2 (rim) 1A (core) 2A(rim) 4 (pheno) 1 (vein) 

Q 

- Si02 36.78 37.96 38.35 38.48 36.45 37.10 36.98 36.74 37.24 37.17 38.26 :t 

"< c. Al20:J 14.08 13.79 13.58 14.57 15.57 14.48 14.52 14.04 14.17 14.63 14.44 .. 

Q 

... FeO* 15.83 15.30 15.60 15.30 17.70 16.07 18.89 17.11 16.25 16.90 16.96 

l 

.. MgO 13.99 14.79 15.10 14.11 11.72 14.04 11.60 13.46 13.71 13.49 14.16 

3 MnO .23 .22 .31 .29 .38 .30 .34 .32 .24 .40 .44 a!. Ti~ 4.19 3.50 3.20 2.93 4.09 3.80 3.86 4.16 3.66 3.89 2.36 Jl 

c K20 8.97 9.29 9.41 9.38 9.52 9.42 9.28 8.95 8.93 9.62 9.10 a: N~O .11 .07 .07 .07 .07 .11 .08 .11 .10 .11 .10 

:r CaO .00 .01 .01 .02 .11 .01 .02 .01 .03 .02 .04 ~ BaO 1.00 .07 .08 .03 .72 .26 .19 1.04 .82 n.a. n.a. II 

~ F .71 1.05 .85 .91 .45 .60 .43 .47 .55 .59 .63 

S' 

Cf) 

Cl _J.i ~ ---:.!1 _M ~ ~ ~ ____.11. _J2 __,ll ~ 

Total 96.04 96.11 96.71 96.14 96.92 96.27 96.25 96.58 95.86 96.93 96.57 S' 

n Total 95.71 95.65 96.32 95.75 96.70 96.00 96.06 96.34 95.59 96.66 96.29 F (-0 =F+Cl) n 

!! 

~ Structural formula based on 11 oxygens !!.. 

~ Si 2.760 2.815 2.833 2.845 2.735 2.769 2.793 2.761 2.797 2.763 2.838 n AllY 1.240 1.185 1.167 1.155 1.265 1.231 1.207 1.239 1.203 1.237 1.162 :r 

~ AIVI Q 

c .006 .021 .016 .115 .112 .043 .086 .005 .052 .045 .101 

= Fe .994 .949 .964 .946 1.111 1.003 1.193 1.075 1.021 1.051 1.052 S' j' Mg 1.565 1.634 1.662 1.555 1.311 1.562 1.306 1.507 1.535 1.495 1.565 

c: Mn .015 .014 .019 .018 .024 .019 .022 .020 .015 .025 .028 

g Ti .237 .195 .178 .163 .231 .213 .219 .235 .207 .218 .132 

Sum VI 2.817 2.813 2.839 2.797 2.789 2.840 2.826 2.842 2.830 2.834 2.878 

K .859 .879 .887 .885 .911 .897 .894 .858 .856 .912 .861 

Na .016 .010 .010 .010 .010 .016 .012 .016 .015 .016 .014 

Ca .000 .001 .001 .002 .009 .001 .002 .001 .002 .002 .003 

Ba .033 .002 .003 .001 .024 .008 .006 .034 .027 

F .169 .246 .199 .213 .107 .142 .103 .112 .131 .139 .148 

Cl .019 .008 .019 .006 .018 .010 .008 .022 .020 .014 .010 

Fe/(Fe+Mg) .388 .367 .367 .378 .459 .391 .477 .416 .399 .413 .402 

Biotite 

temp(°C)2 791 731 692 669 736 744 690 755 726 735 574 

* Total Fe as FeO 

1Core=core of biotite phenocryst; rim=rim of biotite phenocryst; pheno=average of entire phenocryst; inc=biotite inclusion in plagioclase phenocryst; shred=shreddy, hydrothermal biotite; 

vein=hydrothennal vein-filling biotite 

2Biotite temperature calculated from Dilles (1987) 
Table 4. Representative microprobe analyses of amphibole in the Alta stock 

Sample 83-PC-272 83-PC-272 84-PC-86 84-PC-86 84-PC-86 83-PC-253 83-PC-253 83-PC-253 83-PC-253 82-PC-27 82-PC-27 82-PC-27 82-PC-27 

Spotl 3 (core) 4(rim) 1 (core) 2 (rim) 3 (inc) 4 (core) 5 (rim) 2 (vein) 3 (vein-rim) 1 (inc) 2 (inc) 3 (core) 4 (rim) 

Si02 49.77 50.31 48.00 50.00 46.57 50.50 50.29 51.25 49.89 43.45 50.07 50.72 51.48 

Al20:3 5.36 5.09 6.89 5.41 7.73 4.98 5.04 4.58 5.44 9.47 5.10 5.07 4.64 

FeO* 11.00 10.75 13.53 13.01 14.30 11.73 11.95 11.84 12.96 16.36 13.67 11.99 11.67 

MgO 16.52 16.74 14.40 15.09 13.58 15.90 15.77 16.08 15.19 11.73 14.60 16.07 16.38 

MnO .53 .51 .60 .60 .59 .57 .54 .32 .40 .55 .64 .70 .68 

Ti~ 1.23 1.16 1.28 .87 1.35 .72 .72 .54 .69 1.68 .83 .78 .59 

CaO 12.01 11.85 12.04 12.21 12.01 12.25 12.23 12.49 12.28 12.00 12.61 12.34 12.40 

Na20 1.24 1.21 1.18 .90 1.25 .97 .93 .86 .96 1.37 .69 .91 .82 

K20 .53 .50 .64 .47 .77 .39 .42 .38 .45 1.15 .41 .44 .37 

F .46 .52 .25 .25 .28 .31 .32 .35 .33 .18 .21 .21 .21 

Cl ~ _1I1. ~ __M _..Q8 _____.m _____.m _.Q2 _____.m __.li _.JM __M __M 

Total 98.73 98.71 98.87 98.86 98.51 98.35 98.24 98.71 98.62 98.09 98.87 99.28 99.29 

Total 98.52 98.48 98.75 98.74 98.37 98.21 98.10 98.56 98.47 97.98 98.77 99.18 99.19 

n (-O=F+O) Q 

3 Structural formula based on 23 oxygens "'CC 

Q 

!t!. 

g. 

ii Si 7.139 7.196 6.973 7.212 6.839 7.272 7.259 7.341 7.207 6.552 7.245 7.249 7.332 Q AIIV .861 .804 1.027 .788 1.161 .728 .741 .659 .793 1.448 .755 .751 .668 -., 

.. 3. AIVI .045 .054 .153 .132 .177 .117 .117 .114 .134 .235 .115 .103 .111 ~ Fe 1.320 1.286 1.644 1.570 1.756 1.413 1.443 1.418 1.566 2.063 1.654 1.433 1.390 '< 

= Mg 3.531 3.568 3.118 3.244 2.972 3.412 3.392 3.432 3.270 2.636 3.149 3.423 3.477 [ Mn .064 .062 .074 .073 .073 .070 .066 .039 .049 .070 .078 .085 .082 ;: Ti .133 .125 .140 .094 .149 .078 .078 .058 .075 .191 .090 .084 .063 

!I 

c.. Sum VI 5.093 5.095 5.129 5.113 5.127 5.090 5.096 5.061 5.094 5.195 5.086 5.128 5.123 

> 3 Ca 1.846 1.816 1.874 1.887 1.890 1.890 1.892 1.917 1.901 .939 1.955 1.890 1.892 "'CC 

:::r' Na .345 .336 .332 .252 .356 .271 .260 .239 .269 .401 .194 .252 .226 a: 

Q K .097 .091 .119 .086 .144 .072 .077 .069 .083 .220 .076 .080 .067 

ii" 

at F .209 .235 .115 .114 .130 .130 .146 .159 .151 .085 .096 .095 .095 S" 

c.. Cl .019 .017 .015 .020 .012 .007 .007 .005 .007 .038 .010 .012 .012 

;::r 

~ Q Fe/(Fe+Mg) .345 .265 .345 .326 .371 .293 .298 .292 .324 .439 .345 .295 .286 (il 

Q 

- Pressure .42 .22 1.54 .47 2.18 .17 .22 -.13 .50 3.59 .27 .20 -.10 ~ (kbars) 2 Jcl 

3 

~ 

;:::;- FeO*=All Fe as FeO ., 1Core=core of hornblende phenocryst; rim=rim of hornblende phenocryst; inc=small hornblende inclusion in plagioclase phenocryst; vein=vein amphibole; vein-rim=rim of amphibole crystal in vein c 

c.: 2Pressure calculated from total aluminum content using equation of Johnson and Rutherfotd (1989b) 

..... 

I.D 
CJ) 

z 

UJ 

~ 0 

CJ) 

z 

UJ 

~ 

CJ) 

z 

UJ 

~ 0 

0.4 r----......--""T""""----...----r---......-----. 

0.3 

oo 

0.2 

o.1 A 

0.3 

0 

0 

• 

0.4 

• 0.5 

IRON/(IRON+MAGNESIUM) 

-

0.6 

0.04 r---......--...,..-----...----r---......-----. 

0.1 

0.03 

0.02 

0.01 

0.2 0.3 

TITANIUM ATOMS PER 11 OXYGENS 

•o 

• 

0 0 

0 

0 

0 

0 

0 0 0 

0 

0 

0.4 

-

C ........ 0 

0.00 .....__.a....= .... .,.;;._.L..-_,____L....-_,___---li-___..____J 

600 700 800 900 1000 

BIOTITE TEMPERATURE, IN DEGREES CELSIUS 

EXPLANATION 

• Hydrothermal samples 

0 Eastern samples 

• Western samples 

ing oxidation state during progressive crystallization. 

The strong positive correlation between AIIV, Ti, Na, 

and K contents and their depletion from core to rim and 

from inclusions to phenocrysts is consistent with decreasing temperature during crystallization (Helz, 1982). 

However, decreasing Fe/(Fe+Mg) ratio with decreasing 

temperature is the opposite trend expected from experimental studies and suggests an increase in the oxidation 

state during crystallization (Helz, 1982). Decreasing Ti 

also is consistent with an increase in the oxidation state 

(Helz, 1982). The magnitude of the change in the oxidation state is indeterminable, however . 

The compositional overlap between hydrothermal 

amphibole and amphibole phenocrysts and the lack of 

obvious signs of recrystallization or replacement of early 

amphibole by more siliceous, late amphibole suggest 

that at least some of the more siliceous amphibole 

phenocrysts crystallized in the presence of a hydrothermal, possibly magmatic(?) fluid. The presence of K-feldspar, sphene, and, locally, magnetite in hornblende veins 

suggests that early hydrothermal fluids had high K, Na, 

Ti, AI, and Fe contents, and that these elements could 

have been partitioned into minerals other than amphibole. Alternatively, amphibole with higher Fe, AI, Ti, 

Na, and K contents may not be stable under subsolidus 

conditions. 

In summary, amphibole compositional trends are 

complex but generally suggest increasing oxidation state 

with decreasing temperature. It is not obvious, however, 

when the interaction of the Alta stock with an aqueous 

phase began, and the magnitude of the change in oxidation state is unknown. The origin of patchy compositional zoning in amphibole remains poorly understood. 

Hornblende Geobarometry 

Recent empirical (Hammarstrom and Zen, 1986; 

Hollister and others, 1987) and experimental (Johnson 

and Rutherford, 1989b) studies show that the aluminum 

content of hornblende in granitic magma varies linearly 

with pressure of crystallization. Thus, the aluminum 

content of hornblende can be used as a geobarometer in 

~ Figure 12. Biotite compositions in Alta stock based on electron-microprobe analyses. Eastern and western samples 

separated by approximate depth of crystallization: eastern 

samples, ~5.2 km; western samples, >5.2 km. A, Ti atoms 

per 11 oxygens vs. Fe/(Fe+Mg). Note that hydrothermal 

biotite has considerably lower Ti contents than igneous 

biotite. 8, Ba atoms per 11 oxygens vs. Ti atoms per 11 

oxygens. C, Ba atoms per 11 oxygens vs. calculated biotite 

equilibration temperature. Note that Ba content decreases 

with decreasing temperature. See text for description of 

calculation of biotite temperature. 

20 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
9 

10 

11 

12 

C\1 13 .e 

C) 

0 

_J 

I 14 

15 

16 

17 

600 

EXPLANATION 

• Western samples 

• Eastern samples 

x Hydrothermal samples 

700 800 900 1000 

TEMPERATURE, IN DEGREES CELSIUS 

Figure 13. Calculated temperature plotted vs. estimated Jog oxygen fugacity (f02) 

for biotite from Alta stock. See text for methods of calculation. Tie lines connect 

zoned crystals; arrows point to rims. Eastern and western samples separated by 

approximate depth of crystallization: eastern samples, :S5.2 km; western samples, 

>5.2 km. HM and FMQ are, respectively, the hematite-magnetite and fayalitemagnetite-quartz oxygen buffer curves from Chou (1978), NNO is the nickelnickel oxide buffer curve of Huebner and Sato (1970), and TMQ is titanitemagnetite-quartz buffer curve of Wones (1989) calculated at 1,400 bars. TMQ 

equilibria are adjusted for clinopyroxene compositions of Alta stock. 

rocks where hornblende crystallizes with quartz, plagioclase, K-feldspar, biotite, magnetite, and sphene or 

ilmenite, as is common during the later stages of crystallization of granodiorite and tonalite magma. Data used in 

calibration of the hornblende geobarometers are for pressures between 2 to 8 kbars, and Johnson and Rutherford 

(1989b) and Hollister and others (1987) suggest errors in 

pressure estimates of ±0.5 and ±1.0 kbar, respectively, 

for pressures between 2 and 8 kbars. The three equations 

of the hornblende geobarometer diverge at high pressures (>5 kbars) but yield nearly identical results at low 

pressures (2 kbars). 

The hornblende geobarometers were applied to 

hornblende crystals in the Alta stock using aluminum 

contents estimated from electron-microprobe analysis 

(table 4). Pressures estimated using the experimentally 

derived equation of Johnson and Rutherford (1989b) 

range from 4.1 kbars to negative values; most pressure 

estimates are Sl.O kbar (table 4). Many samples yield >2 

kbars ranges in pressure estimates between adjacent domains within single crystals. Pressures estimated using 

hornblende rim compositions, which should yield the 

most accurate estimates of pressure, range from 3.3 

kbars to negative values; most are S0.5 kbar. Pressures 

estimated using the equations of Hammarstrom and Zen 

(1986) and Hollister and others (1987) are similarly low 

and variable. 

The hornblende geobarometer does not appear to 

yield reasonable estimates of the pressure of crystallization of the Alta stock. Based on pressures estimated 

from regional field relations, contact-metamorphic mineral assemblages in wall rocks of the Alta stock, and 

fluid-inclusion data, final pressures of crystallization of 

the Alta stock varied from about 1. 7 kbars on the west 

Compositions of Primary Biotite and Amphibole as Indicators of Magmatic Fluid 21 
side of the stock to 1.0 kbar on the east side of the stock 

(John, 1987, 1989). These values are thought to be accurate within ±0.5 kbar and are more reasonable than the 

highly variable or negative pressures estimated for many 

samples using the hornblende geobarometer. 

There are several possible reasons for the failure of 

the hornblende geobarometer in the Alta stock, including 

(1) lack of calibration of the geobarometer at low pressures ( <2 kbars); (2) partial reequilibration of hornblende 

phenocrysts at low pressures and (or) low temperatures; 

or (3) the effects of other variables, such as vapor saturation, on composition of hornblende crystallizing at low 

pressures. Most hornblende phenocrysts in the Alta stock 

contain numerous systematic compositional variations 

that appear coupled to variations in aluminum content 

(that is, increasing Fe, Ti, Na, and K contents, and decreasing Si, Ca, and Mn contents with increasing AI; see 

fig. 16). Thus, it is unlikely that low-temperature or lowpressure reequilibration of aluminum content has taken 

place and is the cause of the low and highly variable 

pressure estimates for the Alta stock (see also 

Hammarstrom and Zen, 1986). It is more likely that a 

combination of low-pressure crystallization of the 

magma, at least partially during vapor-saturated conditions, and the lack of calibration of the hornblende geobarometer at low pressures leads to the poor results 

given by the hornblende geobarometer in the Alta stock. 

STABLE-ISOTOPE STUDIES 

Reconnaissance studies of hydrogen- and oxygenisotope compositions of minerals in the Alta stock were 

Fe+2 

Figure 14. Fe+2-Fe+3-Mg (mole ratios) in biotite from Alta 

stock. HM, NNO, and FMQ oxygen buffer curves (defined 

in fig. 13 legend) from Wones and Eugster (1965). X, 

analysis from Wilson (1961 ). 

made to assess the possible role of meteoric water in the 

evolution of the hydrothermal system. Mineral separates 

from 16 samples representing several types of alteration 

and spread throughout the stock were analyzed for oxygen and (or) hydrogen isotopes. Sample locations are 

given in figure 11. 

Methods 

Isotopic analyses of hydrogen and (or) oxygen 

were made on mineral separates using standard techniques for extraction of hydrogen and oxygen 

(Bigeleisen and others, 1952; Clayton and Mayeda, 

1963; Borthwick and Harmon, 1982). Water contents of 

hydrous minerals were measured by a manometer during 

extraction of hydrogen gas. Isotopic compositions of prepared gasses were measured using gas ratio mass spectrometers and are reported in standard delta notation 

relative to standard mean ocean water (SMOW). Analytical precision is 0.1--0.2 permil for a180 and approximately 1 permil for an. 

Results 

Results of the isotopic analyses gathered in this 

study are presented in table 5, along with previous 

analyses reported by Taylor (1968). Oxygen-isotope 

compositions of primary quartz and primary K-feldspar 

show rather restricted ranges of a180 from +8.3 to +9.7 

and + 7.3 to +8.0 permil, respectively. Fractionation between quartz and K-feldspar is normal (>1 permil) in six 

of the seven samples, suggesting isotopic equilibrium. 

Whole-rock a180 values measured by Taylor (1968) are 

between the quartz and K-feldspar values (+8.0 and +8.4 

permil) and somewhat higher than the +6.5 to + 7.5 permil values reported by Kemp and Bowman (1984). 

These data suggest that the primary whole-rock oxygenisotope composition of the Alta stock is about + 7 to +8 

permil. Two samples of hydrothermal vein quartz 

yielded B180 values of +9 .5 and + 11.5 permil (samples 

83-PC-203 and 84-PC-40, table 5). 

Quartz-K-feldspar pairs having normal oxygen-isotope fractionation are used with the equations of 

Bottinga and Javoy (1973, 1975) to calculate temperatures of isotopic equilibration (table 5). These temperatures range from 407 °C to about 560 °C. Oxygenisotope fractionations involving other minerals in one 

sample (A95, table 5) also suggest equilibration temperatures near 500 oc (Bottinga and Javoy, 1975). Quartz in 

one sample (82-PC-79) is relatively depleted ( +8.3 permil), whereas K-feldspar is isotopically similar to other 

samples ( + 7. 7). The reason for the depletion in this 

quartz is unknown. 

22 Evolution of Hydrothermal Auids in the Alta Stock, Central Wasatch Mountains, Utah 
A 0 400 MICROMETERS 

B 0 100 MICROMETERS 

Figure 15. Backscattered SEM images of hornblende phenocrysts in Alta stock 

(sample 83-PC-206), showing patchy compositional zoning. A, Subhedral hornblende phenocryst containing small, partially chloritized biotite inclusion (bt) 

and magnetite inclusions (mt). Patchy compositional zoning shown by slight 

variations in brightness; lighter colored areas contain higher concentrations of 

Fe, AI, Ti, K, and Nathan darker areas that contain higher concentrations of Si, 

Mg, and Ca. B, Enlargement of A showing irregular compositional domains. 

Stable-Isotope Studies 23 
Hydrogen-isotope analyses of biotite and mixtures 

of biotite and chlorite have 8D values ranging from -86 

to -109 permil. These values are relatively light compared with those of most igneous biotite (for example, 

en 

z 

w 

(!) 

> X 

0 

('I) 

N 

a: 

w 

Q.. 

en 

::E 

0 

~ 

Na 

Ti 

AI(VI) 

0.5 

0.4 

0.3 

0.2 

0 . 1 

0.0 

0.3 

0.2 

0 . 1 

0 .0 

0.4 

0.2 

0 .0 

4.0 

Mg 3 .5 

3 . 0 

X 

X 

l\. X 

x 'x • .1 •• 

X X X X 

X X X 

X.x 

XX X X X X • • : ·. 

X X X 

: "x 

• X 

.x 

X X 

_x 

~-.At 

.. · .. :·.:~ ....... 

J:•···" ~. 

~· 

X V 

ic •• '( X 

,.·,.X ··:~:;:::.: •x • 

~'V 

V X 

6 .4 6.6 6 .8 7 . 0 7 . 2 7.4 7 .6 7 .8 

SILICA ATOMS PER 23 OXYGENS 

0.2 

0.1 K 

0 .0 

2.0 

1.9 Ca 

1.8 

0 . 1 

Mn 

0.0 

2 .0 

1.5 Fe 

1.0 

Figure 16. Variations of Alv1

, Mg, Fe, Ti, Ca, Mn, Na, and 

K with Si content (in atoms per 23 oxygens) in amphibole 

crystals from Alta stock. There are a total of 78 analyses 

from 25 samples. X, small amphibole inclusions in plagioclase phenocrysts; V, vein amphiboles. 

see Nabelek and others, 1983; Brigham, 1984). There are 

no obvious relationships between 8D and position in the 

stock, but 8D generally increases with increasing 

amounts of structurally bound water in partially 

chloritized samples (fig. 17). 

Hydrogen-isotope analyses of three chlorite samples from two different parageneses (two samples of 

chloritized biotite phenocrysts and one sample of a type 

6 vein) are nearly identical, with 8D values of about -75 

permil. These values are notably enriched in deuterium 

by 10 to 30 permil relative to fresh biotite. The increase 

in 8D during chloritization is similar to the trend found 

in the Notch Peak stock by Nabelek and others (1983) 

and is opposite to the trends found in systems where meteoric-water interaction has been demonstrated (for example, Criss and Taylor, 1983) and to the trend reported 

for chloritization of biotite in the Little Cottonwood 

stock along the Wasatch fault zone (Bromley and others, 

1985). Nabelek and others (1983) and Taylor (1988) 

have suggested that deuterium enrichment during 

chloritization indicates alteration resulting from subsolidus interaction of magmatic fluids in an essentially 

closed system. 

Isotopic compositions of fluids in equilibrium with 

measured 8180 and 8D values of quartz, K-feldspar, and 

biotite at 500 oc and 700 oc were calculated using equations for water-mineral fractionations from Bottinga and 

Javoy (1973) and Suzuoki and Epstein (1976) (table 5). 

Calculated water compositions lie within the "magmaticwater" box of Taylor (1974) over the temperature range 

of 500-700 °C, although deteurium values straddle the 

low end of this range (fig. 18). The isotopic composition 

of hydrothermal fluids in equilibrium with sample 83-

PC-203 at 500 °C, calculated from measured 8180 and 

8D values for vein quartz and vein chlorite using equations of Bottinga and Javoy (1973) and Graham and others (1987), also lies within the magmatic-water box (fig. 

18). Thus, calculated water compositions are consistent 

with the idea that isotopic reequilibration at 500 °C only 

involves waters that are compatible with magmatic 

origins. 

Discussion 

Oxygen-isotope fractionations between quartz and 

K-feldspar have values indicative of isotopic reequilibration at subsolidus temperatures (approximately 500 °C). 

Isotopic reequilibration at subsolidus temperatures is 

consistent with petrographic observations that suggest 

subsolidus processes (for example, unmixing of potassium feldspar and plagioclase, formation of myrmekite, 

and other forms of "deuteric alteration"), with feldspar 

equilibration temperatures of <500 oc estimated from 

the two-feldspar geothermometer (Whitney and Stormer, 

24 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
Table 5. Oxygen and deuterium analyses of minerals and rocks in the Alta stock 

Sample Mineral 01a0 1 oo1 %H2o2 Temperature 3 

81-PC-11 Quartz 9.4,9.2 494 

K-feldspar 7.7,7.6 

Biotite -89 3.82 

S-38 Quartz 9.1,9.1 472 

K-feldspar 7.3,7.4 

Biotite -95 3.28 

82-PC-20 Quartz 9.2,9.1 559 

K-feldspar 7.9,7.6 

Biotite -102 3.28 

82-PC-17 Quartz 9.3 518 

K-feldspar 7.8,7.7 

Biotite -109 2.93 

82-PC-79 Quartz 8.3,8.3 

K-feldspar 7.8,7.7 

Biotite (+chlorite) 4 4.1,4.1 -101 4.10 

S-30 Biotite -89 3.82 

83-PC-177 Biotite (+chlorite) -98 4.39 

83-PC-203 Quartz (vein) 9.5 

Chlorite (vein) -71 11.13 

83-PC-277 Chlorite (after biotite) -76 10.57 

83-PC-178 Biotite (+chlorite) -87 4.59 

82-PC-25 Biotite (+chlorite) -100 3.91 

83-PC-258 Chlorite (after biotite) -75 9.23 

82-PC-51 Biotite (+chlorite) -86 5.05 

84-PC-81 Biotite -100 2.81 

84-PC-54 Biotite -105 3.19 

84-PC-40 Vein quartz 11.5 

5A95 Quartz 9.7 482 

K-feldspar 8.0 

Andesine 7.3 

Hornblende 5.5 

Biotite 4.3 

5A69 Groundmass 7.0 

5A58 Quartz 8.9 

Whole rock 8.0 

5 A198AP1 Quartz 9.7 

Whole rock 8.4 

5A198AP2 Quartz 9.6 407 

K-feldspar 7.5 

1Parts permil (

0 / 00) relative to standard mean ocean water (SMOW) 2w ater contents of biotite and chlorite directly measured during extraction of water for deuterium analysis 30xygen-isotope equilibration temperatures, in degree Celsius (0

C), for quartz-K-feldspar pairs calculated from Bottinga and Javoy 

(1973) 4Impure mineral separates containing variable but generally small amounts of chlorite as reflected by the water yields 5Samples from Taylor (1968) 

Stable-Isotope Studies 25 
1977; Brown and Parsons, 1981), and with fluid-inclusion data that indicate the widespread presence of hydrothermal fluids at these temperatures (see the section on 

fluid-inclusion studies below). 

Calculated water compositions and measured oD 

values for chlorite and 0180 values for hydrothermal 

quartz suggest that there has been little interaction of 

meteoric water with the Alta stock, despite pervasive 

subsolidus alteration and isotopic reequilibration. Present-day meteoric waters in the Wasatch Mountains are 

quite light (0180=-17.5, OD=-130 permil; Bromley and 

others, 1985) and probably also were quite light in the 

early Oligocene [J.R. O'Neil, oral commun., 1985; see 

also Bowman and others (1987) for meteoric-water values from the nearby Bingham porphyry copper deposit]. 

If chloritization were the result of the introduction of 

meteroic water, chlorite should be strongly depleted in 

deuterium (fig. 17). Also, quartz veins should be 

strongly depleted in 0180 (calculated 0180quartz=-14.3 

permil at 500 °C; Bottinga and Javoy, 1973). Thus, the 

stable-isotope data are compatible with an essentially 

closed-system deuteric alteration that has not been modified by the influx of large volumes of meteoric ground 

water. 

w 12r-----~----~----~----~----~----~ 

1-

0: 

0 

_J 5 10 

c 

z 

<( 

~ 8 

i= 

0 

co 

z 6 

0 

£ 

1-

z 4 w 

u 

0::: 

w 

0.. 

1- 2 

I 

(!) 

w 

. . 

.. 

I 

I 

I 

soooc 450 oc 

• • 

I 

I 

/ 

~ 0 ~--~----~----~----~--~~--~ -60 -80 -100 -120 -140 -160 -180 

80 (BIOTITE AND CHLORITE), 

IN PARTS PERMIL RELATIVE TO SMOW 

Figure 17. 80 plotted vs. water contents of biotite and 

chlorite in Alta stock. High water contents of most biotite 

are result of partial chloritization of biotite. Trend expected from progressive chloritization with meteoric waters (for example, see Criss and Taylor, 1983) is shown as 

dashed line. Stars, approximate compositions of chlorite 

formed from meteoric water calculated at 450 °C and 500 

oc using present-day meteoric-water composition (Bromley 

and others, 1985) and equation of Suzuoki and Epstein 

(1976). SMOW, standard mean ocean water. 

FLUID-INCLUSION STUDIES 

Scope and Types of Materials Studied 

Fluid inclusions were studied both to look for direct evidence of fluids trapped at magmatic temperatures 

and to characterize distribution and temporal development of hydrothermal fluids. Fluid inclusions were studied petrographically in about 150 samples of the stock 

and of veins cutting the stock, and heating and freezing 

experiments were conducted on approximately 1,735 

fluid inclusions in 22 samples (appendix 1). Reconnaissance crushing experiments were made on samples used 

in the heating and freezing studies. Daughter minerals in 

opened inclusions in igneous quartz were examined by 

SEM methods in sample S-38. Because the ages of fluid 

~ 

0 

20 

0 

~ -20 

en 

0 

1-

w 

> 

~ w 

a: 

....J 

~ 

a: 

w 

(L 

-40 

-60 

-80 

en I- -120 

a: 

<( 

(L 

z -140 

ci 

c.o -160 

Magmatic water 

Calculated 

water 

700 oc 

Measured 

values 

-180~------~------~--------~------~ 

-20 -10 0 10 20 

o18Q, IN PARTS PER MIL RELATIVE TO SMOW 

Figure 18. 8180 and 80 relations for Alta stock. Measured 

and estimated whole-rock values (using estimated wholerock 8180 values and biotite 80 values) are shown along 

with calculated compositions of waters in equilibrium with 

whole-rock values at 500 oc and 700 °C. Water compositions are calculated from equations in Bottinga and Javoy 

(1973) and Suzuoki and Epstein (1976). Composition of 

water in equilibrium with sample 83-PC-203 at 500 °C is 

calculated from measured 8180 and 80 values for vein 

quartz and chlorite, respectively, and equations in 

Bottinga and Javoy (1973) and Graham and others (1987). 

Magmatic-water box is from Taylor (1 97 4). Meteoric-water 

line from Craig (1961). Values for present-day meteoric 

water in Wasatch Mountains from Bromley and others 

(1985). SMOW, standard mean ocean water. 

26 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
inclusions relative to one another were ambiguous in 

most samples, fluid inclusions were studied in 14 

samples containing both igneous and hydrothermal 

quartz. The seven other samples were of igneous rock 

that did not have veins near the sample. Heating 

and freezing data were collected on all major types of 

veins except types 2 and 7, but owing to the paucity 

of veins, generally only one vein type and its host rock 

were studied from a single outcrop. Sample localities are 

shown in figure 11. Experimental methods for heating 

and freezing experiments were given by John (1989). 

Petrographic Characteristics of Fluid Inclusions in 

the Alta Stock 

Based on room-temperature phase relations, four 

types of fluid inclusions are distinguished in the Alta 

stock: type 1, liquid rich; type 2, vapor rich; type 3, high 

salinity; and type 4, mixed H20-C02 (fig. 19). 

Type 1 fluid inclusions contain liquid plus vapor at 

room temperature (fig. 19A). Vapor contents range from 

<5 to about 30 volume percent. Crushing experiments indicate that small amounts of C02 are present in some of 

the vapor bubbles. Small birefringent and (or) opaque 

daughter(?) crystals occasionally are present in type 1 

fluid inclusions. 

Type 2 fluid inclusions contain a large vapor 

bubble and smaller amounts of liquid (fig. 19A, B). Vapor contents generally are >60 volume percent. Crushing 

experiments suggest that C02 probably is the main component in many vapor bubbles. Type 2 fluid inclusions 

are relatively uncommon. 

Type 3 fluid inclusions contain liquid plus vapor 

plus halite at room temperature (fig. 19B, C). Vapor contents tend to be small ( <20 volume percent). Other birefringent and opaque daughter minerals, including hematite and calcite, are present locally, and sylvite is 

identified in one sample. Type 3 fluid inclusions are subdivided into three types (3a, 3b, and 3c) based on their 

homogenization behavior and the presence or absence of 

sylvite. In type 3a fluid inclusions halite dissolves at 

temperatures lower than liquid-vapor homogenization, 

whereas in type 3b fluid inclusions halite dissolution 

takes place at temperatures greater than liquid-vapor homogenization. Type 3c fluid inclusions contain daughter 

crystals of sylvite in addition to halite. 

Type 4 fluid inclusions are uncommon and have 

only been identified in two samples. At room temperature, these inclusions have three phases: C02 vapor, C02 

liquid, and H20-rich liquid. The volume percent of C02 

varies from about 20 to 35 percent, corresponding to 

about 5 to 10 mole percent C02 (Burruss, 1981). 

Attempts were made to classify fluid inclusions in 

a temporal sense as primary, secondary, or pseudosecA 20 MICROMETERS 

B 40 MICROMETERS 

c 0 20 MICROMETERS 

Figure 19. Photomicrographs of fluid inclusions in Alta 

stock. A, Type 1 (liquid-rich) and type 2 (vapor-rich) fluid 

inclusions. Igneous quartz in sample S-38. 8, Types 1, 2, 

and 3 (high-salinity) fluid inclusions. Igneous quartz in 

sample S-38. C, Enlargement of 8 showing type 3 fluid inclusion containing halite cube (H) and vapor bubble (V). 

Fluid-Inclusion Studies 27 
ondary using the criteria of Roedder (1984). In most 

samples, however, the relative ages of various types of 

fluid inclusions could not be unequivocally determined 

owing to (1) either very sparse or extremely abundant 

numbers of fluid inclusions and (2) the lack of crystal 

growth features. Despite attempts to avoid measuring 

secondary fluid inclusions, pressure-corrected homogenization temperatures suggest that most fluid inclusions in 

igneous quartz crystals were trapped at subsolidus temperatures and are therefore secondary. 

Distribution of Fluid-Inclusion Populations 

Liquid-rich (type 1) fluid inclusions are present in 

igneous quartz throughout the Alta stock, whereas vaporrich and high-salinity fluid inclusions (types 2 and 3) are 

restricted to the eastern, upper portions of the stock (fig. 

20). Some exceptions to this overall distribution scheme 

are present in a downfaulted block west of the Silver 

Fork fault and at several topographic highs. Fluid-inclusion populations in igneous quartz were divided into two 

types based on the presence or absence of high-salinity 

(type 3) fluid inclusions. The approximate threedimensional distribution of these two sample populations 

is shown in figure 21, where samples have been project-

~ N 

I 

0 1,000 METERS 

0 3,000 FEET 

Equigranular • • phase ·/·,"-.--; ___ _ 

// -.., ________ __...,. ....... ___ _ 

A 

ed onto a N. 80° E. cross section. An extremely regular 

distribution of fluid-inclusion populations in igneous 

quartz is present, with high-salinity (type 3) fluid inclusions restricted to the upper parts of the Alta stock. An 

approximately planar surface dipping about 15° E. separates igneous quartz crystals that contain high-salinity 

fluid inclusions (+liquid-rich fluid inclusions) (eastern 

samples) from samples that only contain liquid-rich fluid 

inclusions (western samples). Vapor-rich (type 2) fluid 

inclusions also are limited to the upper (eastern) part of 

the stock, although they are not present in all samples 

that contain high-salinity fluid inclusions. The one 

sample lying above this surface that does not contain 

type 3 fluid inclusions (sample 83-PC-163) apparently 

does not contain any fluid inclusions trapped prior to 

vein formation (see the section on fluid inclusions 

in eastern samples below). One sample along the 

northern margin of the Alta stock contains type 3 fluid 

inclusions and lies slightly below this surface. The 

surface is offset and apparently repeated by the Silver 

Fork fault in the westernmost part of the stock (fig. 21). 

Apparent offset of the surface is comparable to the 

amount of movement on the Silver Fork fault estimated 

by Calkins and Butler (1943) using stratigraphic offset. 

The surface separating the two fluid-inclusion populations in the Alta stock is subparallel to the inferred 

Brighton 

• 

0 

+ 

X 

---.-- ... 

A A' 

EXPLANATION 

Types of fluid inclusions 

Type 1 

Types 1 and 4 

Types 1 and 3 

Types 1, 2, and 3 

Intrusive contact--Approximate! y located. 

See figure 3 

Normal fault--Dotted where concealed. Bar 

and ball on downthrown side 

Line of section 

A' 

Figure 20. Distribution of fluid-inclusion types in igneous quartz in Alta stock. See figure 3 for location. Section 

A-A' shown in figure 21. 

28 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
preintrusion surface (fig. 21) and is subparallel to a similar, but slightly offset, surface separating samples of the 

Clayton Peak stock that contain high-salinity fluid inclusions from samples that lack high-salinity fluid inclusions (John, 1989, fig. 6). 

Heating and Freezing Experiments 

Salinities of types 1 and 2 fluid inclusions were 

calculated from the freezing point depression using the 

equation of Potter and others (1978). Salinities of types 

3a and 3b fluid inclusions were calculated from halite 

dissolution temperatures using the equations of Potter 

and Brown (1977) and Chou (1987). Salinities of type 3c 

fluid inclusions were calculated using halite and sylvite 

dissolution temperatures and data from Sterner and others (1988). Salinities of type 4 fluid inclusions were calculated from melting temperatures of COrhydrate using 

the relations of Bozzo and others (1975). 

A 

SOUTHWEST 

FEET 

12,000 

10,000 

0 

I 

0 

. _/ 

2,000 FEET 

500 METERS 

0\/ <i,o/ 

/ Projected 

"paleoboiling" 

~ horizon 

/ -----_/ / ._ 

Western Samples 

Fluid inclusions in igneous quartz were studied in 

10 samples from the western part of the stock (fig. 22). 

Eight of these samples also contain quartz veins, whose 

fluid inclusions were also studied. These veins represent 

all major types of veins except type 2 (epidote) and type 

7 (pyrite+chlorite) veins. Nearly all fluid inclusions observed in both igneous and vein quartz are liquid-rich 

(type 1) inclusions containing liquid + vapor at room 

temperature and homogenizing to the liquid phase. 

Sparse type 4 (liquid COrbearing) fluid inclusions are 

present in two samples but are not discussed further because of insufficient data. 

Homogenization temperatures range from about 

180 octo 445 °C in igneous quartz and from 200 °C to 

410 oc in the quartz veins (fig. 22). Calculated salinities 

of type 1 inclusions range from 0 to 16 weight percent 

NaCl equivalent, although nearly all are less than 12 percent (fig. 23). With few exceptions, only minor differEXPLANATION 

Types of fluid inclusions 

Type 1 

0 Types 1 and4 

+ Types 1 and 3 

X Types 1, 2, and 3 

A' 

NORTHEAST 

FEET 

12,000 

10,000 

X 

8,000 ---l.--L--L.. 4'" ________________________ _.::::.. __________ .J.._ 

-- 8,000 

Figure 21. Cross section of Alta stock showing distribution of fluid-inclusion populations in igneous quartz. Samples 

shown are projected onto plane A-A' (fig. 20). Inferred high-temperature "paleoboiling" surface also is shown. Amount of 

offset of this surface by Silver Fork fault is similar to amount of stratigraphic offset estimated by Calkins and Butler (1943). 

Also shown is inferred preintrusion surface from John (1989). Pz, Paleozoic rocks. Arrows show direction of relative 

movement. 

Fluid-Inclusion Studies 29 
ences exist between homogenization temperatures and 

calculated salinities of fluid inclusions in igneous quartz 

and those in quartz veins (compare figs. 22, 23). 

Igneous quartz in two of the western samples contains sparse fluid inclusions that homogenize at significantly higher temperatures than fluid inclusions in vein 

20 

10 

81-PC-11 (phenocryst) 

17=49 

100 200 300 400 500 600 

10

0 

J-+---~-=-~_C-~27-(-ph-en-oc~rys+tu)~--~ r1 __ ~~~-----.----~ ~ npD~n 0 

20 

10 

0 

100 200 

82-PC-58 (phenocryst) 

17=50 

100 200 

300 400 500 600 

300 400 500 600 

10] --1----~-:s_C,-14-8-(p-he-n~oc~''_~~)-----.~~~-----.-----. ~ n~Cl,ng 0 100 200 300 400 500 600 

10] --1----~-~-C.-15-2-(p-he_n_oc~ry~~)~--~--~~----~----~ ~ ,dJcA n 

0 100 200 300 400 500 600 

10 J 83-PC-253 

n=46 (phenocryst) J1 ~----~--~0~9~~0~~----~.0~--~----~ 

0 100 200 300 400 500 600 

quartz (samples 83-PC-253 and 83-PC-279, fig. 22). 

These high measured homogenization temperatures and 

high apparent trapping temperatures after pressure corrections (table 6) suggest that some fluid inclusions may 

have been trapped prior to vein formation but at solidus 

or slightly subsolidus temperatures (~720 °C). 

EXPLANATION 

Types of fluid inclusions 

D Type 1 

l!il Type 1 (critical) 

IDI Type 4 

10 

--1~----~-~-C.-2-79-(p-he_n_oc,~~t)-----.--~~~n~-.-----. J ,&9 0 

30 

20 

10 

0 

100 200 

83-PC-292 (phenocryst) 

11=73 

100 200 

300 400 500 600 

300 400 500 600 

10 --IJ ___ 8_~_a~--.31-2-(p-he_n_ocr~y-st)~~~flaa~~-----.-----. ~ om~nm 

30 

20 

10 

0 

100 200 

83-PC-315 (phenocryst) 

17=95 

100 200 

300 400 500 600 

300 400 500 600 

A 

HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS 

Figure 22. Measured homogenization temperatures of fluid inclusions in western part of Alta stock. n, total number of 

fluid inclusions measured in each sample. A, Igneous quartz samples. 8, Vein quartz samples. 

30 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
Pressure Corrections and Trapping Temperatures 

Type 1 fluid inclusions were trapped at pressures 

greater than liquid-vapor equilibrium, and measured homogenization temperatures must be pressure corrected to 

estimate trapping temperatures. Pressure corrections for 

type 1 fluid inclusions were made using the data of 

Zhang and Frantz (1987) for the NaCl-H20 system. 

Lithostatic pressure was assumed, and the effects of salts 

other than NaCl and the small amounts of dissolved C02 

were ignored. Lithostatic pressures were estimated from 

figure 21 under the assumption that 1 kbar = 3.7 km. 

Pressure-corrected homogenization temperatures of 

type 1 fluid inclusions in igneous quartz samples range 

from about 445 °C to 720 oc (table 6). These temperatures represent maximum possible trapping temperatures 

because of the assumptions of lithostatic pressure and 

negligible effects of C02 and salts other than NaCl. 

Pressures probably approached lithostatic during cooling 

of the Alta stock because of low fracture permeability, 

lack of major throughgoing fractures, and relatively great 

depths (3.7 to 6.3 km). Small amounts of dissolved C02 

raise the vapor pressure of the fluid inclusion, which 

slightly decreases the pressure correction. The effects of 

10 l 83-PC-148 (vein) 

n--33 

0 I ~0 I 

100 200 300 400 500 600 

10 l 83-PC-152 (vein) 

fl=35 {j (/) 

~ 

z 

w {l, 0 0 :::::?1 I w 

a: 0 100 200 300 400 500 600 => 

(/) 

<C 

w 

:::::?1 

u.. 20 0 

a: 

w 83-PC-253 (vein) 

co fl=29 

:::::?1 

=> 10 z 

0 

100 200 300 400 500 600 

10 l 83-PC-279 (vein) 

~D fl=40 

0 I 

100 ?00 300 400 500 600 

salts other than NaCl are minor except at high Ca/Na, K/ 

Na and Mg/Na ratios (Potter and Clynne, 1978; Zhang 

and Frantz, 1987). 

Comparison of maximum pressure-corrected homogenization temperatures and salinities of fluid inclusions from various types of veins reveals few differences 

(table 6). Maximum trapping temperatures suggest formation of all of the veins studied between about 470 °C 

and 550 oc from moderately saline (:o:;10 weight percent 

NaCl equivalent) fluids. The notably high temperatures 

recorded in sample 83-PC-315 (654 °C) may be theresult of measuring fluid inclusions trapped in a thin aplite 

dikelet that was later reopened and filled with quartz, 

biotite, epidote, magnetite, chalcopyrite, and molybdenite (appendix 1). Early hornblende veins yield the lowest maximum trapping temperatures (approximately 465-

475 °C), yet they are consistently the oldest type of vein. 

The discrepancy between field relations and fluid-inclusion data could result from several processes, including 

(1) absence of primary fluid inclusions, (2) deposition of 

quartz after other minerals were deposited in the veins, 

(3) multiple periods of fluid flow and entrapment of 

fluid inclusions in the veins, ( 4) variations in the pressure regime, and (5) formation of all these veins from 

20 83-PC-292 (vein) 

fl=26 

10 

0 100 200 300 400 500 600 

20 83-PC-312 (vein) 

fl=56 

10 

0 

0 100 200 300 400 500 600 

20 83-PC-315 (vein (aplite)) 

fl=44 

10 

0 100 200 300 400 500 600 

B 

HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS 

Figure 22. Continued. 

Fluid-Inclusion Studies 31 
fluids that had similar temperatures but slightly different 

compositions that are not reflected in the bulk estimates 

of salinity. Multiple periods of fluid circulation and entrapment of fluid inclusions are suggested by the wide 

range of homogenization temperatures (generally ~200 

°C) and partial alteration of early formed minerals, such 

as chloritization of biotite. The lack of contemporaneity 

of quartz and hornblende in type 1 veins is suggested by 

the lack of quartz in many type 1 veins and the textural 

relations between quartz and hornblende in the two type 

1 veins studied (fig. 24). Variations in the pressure regime from early, near-lithostatic conditions to late, more 

nearly hydrostatic values also could lead to erroneously 

20 

10 

20 

10 

(/) 

1-

z 

w 

::E 

w 

a: 

::::> 

(/) 

c( 

w 

::E 10 u. 

0 

a: 

w 

Ill 

::E 

::::> 

z 

10 

0 

10 

A 

0 

81-PC-11 (phenocryst) 

11=32 

10 20 

82-PC-27 (phenocryst) 

11=30 

10 20 

1 7 0 ~~ ~~ooc~IJ 

0 10 20 

83-PC-152 (phenocryst) 

11=15 

0 10 20 

0 10 20 

20 

10 

0 

20 

10 

0 

10 

0 

0 

10 

83-PC-148 (phenocryst) 

11=22 

10 20 

83-PC-292 (phenocryst) 

11=36 

10 20 

83-PC-279 (phenocryst) 

11=15 

10 20 

83-PC-312 (phenocryst) 

11=13 

10 20 

10 L ;;:·~~~ 0 10 20 

SALINITY, IN WEIGHT PERCENT NaCI EQUIVALENT 

high pressure corrections for the late veins. The lack of 

evidence for boiling in late veins, however, suggests that 

large pressure decreases did not occur. The discrepancy 

between field relations and the fluid-inclusion data probably results from a combination of multiple generations 

of fluids, lack of contemporaneity of quartz and hornblende in type 1 veins, and slight variations in fluid 

chemistry. 

Eastern Samples 

Heating and freezing tests were made on fluid inclusions in 10 samples of igneous quartz from 9 outcrops 

10 6:: 7 

0 EXPLANATION 

0 10 20 Types of fluid inclusions 

D Type 1 

Ell Type 1 (critical) 

liD Type 4 

20 83-PC-152 (vein) 

11=18 

83-PC-292 (vein) 

(/) 

1-

z 

w 

::E 

w 

10 10 

0 LL-. a: 

::::> 0 10 20 10 20 

(/) 

c( 

w 

::E 

u. 

0 

a: 

w 

Ill 

::E 

::::> 

z 

10 83-PC-253 (vein) 10 rL 1&: 7 

0 0 

10 20 0 10 20 

10 83-PC-279 (vein) 10 83-PC-315 (vein (aplite)) 

0 

~ 10 20 

0 

0 

~ 10 20 

SALINITY, IN WEIGHT PERCENT NaCI EQUIVALENT 

B 

Figure 23. Calculated salinities of fluid inclusions in western part of Alta stock. n, total number of fluid inclusions measured in each sample. A, Igneous quartz samples. 8, Vein quartz samples. 

32 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
Table 6. Pressure corrections for type 1 fluid inclusions in the Alta stock 

Sample Pressure Homogenization Salinity (wt % Pressure Trapping 

(bars) temperature (°C) NaCI equiv.) correction (OC) temperature (°C) 

Western Samples 

83-PC-148 Phenocryst 1,450 200--395 0-5 120-180 320-575 

Vein (type 3) 1,450 220-365 0-5 95-160 315-525 

82-PC-27 Phenocryst 1,400 200-375 4-7 83-177 283-552 

82-PC-58 Phenocryst 1,700 290-390 10 134-209 424-599 

83-PC-312 Phenocryst 1,450 290-351 2-8 120-160 410-511 

Vein (type 5?) 1,450 290-353 2-12 120-160 410-513 

83-PC-315 Phenocryst 1,700 255-414 0-15 121-195 376--609 

Vein (+aplite) 1,700 294-407 4-10 141-247 435-654 

81-PC-11 Phenocryst 1,400 212-429 4-9 87-253 299-682 

83-PC-253 Phenocryst 1,700 263-415 4-7 115-303 378-718 

Vein (type 1) 1,700 272-316 5-6 132-158 404-474 

83-PC-292 Phenocryst 1,600 256-305 3-6 118-143 374-448 

Vein (type 1) 1,600 245-315 3-6 114-149 359-464 

83-PC-152 Phenocryst 1,450 213-354 1-8 92-154 305-508 

Vein (type 6) 1,450 272-357 4-6 113-162 385-519 

83-PC-279 Phenocryst 1,400 221-444 0-9 93-249 314-693 

Vein (type 6) 1,400 277-373 3-8 110-163 387-536 

Eastern Samples 

83-PC-37A Phenocryst 1,050 217-487 0-22 67-179 284-646 

83-PC-276 Phenocryst 1,250 218-338 3-5 80-127 298-465 

Vein (type 4) 1,250 229-380 3-6 83-155 312-535 

83-PC-275 Phenocryst 1,250 220-381 2-10 77-160 297-541 

Vein (type 5) 1,250 280-378 5-6 98-153 378-531 

S-38 Phenocryst 1,000 160-540 variable see text 

84-PC-40 Vein (type 5) 1,000 265-399 5-10 64-124 329-523 

83-PC-274 Vein (type 1) 1,100 257-349 0-8 83-123 340-472 

83-PC-151 Phenocryst 1,400 194-420 5-20 79-130 273-550 

S-30 Phenocryst 1,200 200-316 3-24 69-92 269-408 

82-PC-51 Phenocryst 1,450 210-468 (409) 1 5-20 90-156 300-2:565 1 

84-PC-36 Phenocryst 1,150 210-384 7-23 67-104 277-488 

Vein (type 6) 1,150 230-391 3-16 75-108 305-499 

83-PC-163 Phenocryst 1,250 200-353 4-7 75-134 275-487 

Vein (type 5) 1,250 218-376 3-7 80-151 298-527 

83-PC-177 Phenocryst 1,300 187-348 3-8 75-142 262-490 

Vein (type 3) 1,300 200-336 2-4 79-134 279-470 

1

Salinity of fluid inclusions with highest homogenization temperatures unknown; calculations use fluid inclusion with lower homogenization temperature (in parentheses) 

Fluid-Inclusion Studies 33 
and in 7 quartz veins (fig. 11). Two of the samples were 

late igneous differentiates (pegmatite and aplite dikes). 

There were no obvious differences in fluid-inclusion 

populations between quartz in late igneous dikes and 

quartz phenocrysts in granitic wall rocks of the late 

dikes. Igneous quartz crystals contain three major types 

of fluid inclusions: liquid rich (type 1), high salinity 

(types 3a, 3b, and 3c), and, locally, vapor rich (type 2). 

Type 3a fluid inclusions are relatively sparse or are not 

present in several samples. Type 3c fluid inclusions are 

only found in one sample (83-PC-151). Type 4 fluid inclusions apparently are not present in the eastern part of 

the Alta stock. 

Igneous Quartz-Homogenization Temperatures and Salinities 

· Type 1 fluid inclusions have a wide range in homogenization temperatures and estimated salinities. Homogenization temperatures range from <200 oc to 540 

°C, although most type 1 fluid inclusions homogenize to 

the liquid phase between 250 oc and 370 oc (fig. 25). 

Calculated salinities span the continuum from 0 to 26 

weight percent NaCl equivalent (fig. 26). Many haliteundersaturated fluid inclusions have first melting points 

at less than -30 oc (as low as -53 °C) and final melting 

points at less than -24 °C, indicating the presence of 

salts other than NaCl and KCL These fluid inclusions 

probably contain appreciable amounts of CaC12 and (or) 

FeC12 as is suggested by their low melting temperatures 

(Crawford and others, 1979; Kwak and Tan, 1981) and 

0 1 MILLIMETER 

Figure 24. Photomicrograph of type 1 hornblende+Kfeldspar vein in sample 83-PC-292, showing textures of 

hornblende, K-feldspar, and quartz. Euhedral hornblende 

(hb) is overgrown by subhedral, turbid perthitic K-feldspar 

(Ksp) and subhedral-to-anhedral quartz (qtz). Note abundant type 1 fluid inclusions in quartz. Biotite (bt) is in wall 

rock along edge of vein. Plane-polarized light. 

by the presence of calcite and hematite daughter(?) crystals in some type 3 fluid inclusions in these samples 

(particularly in samples 83-PC-37 A and S-38). No evidence for the presence of C02 was seen in fluid inclusions with low melting points, even when temperatures 

were lowered to less than -100 °C during many of the 

freezing runs. 

Type 2 fluid inclusions vary considerably in abundance and are sparse or absent in several samples (fig. 

20). Most type 2 fluid inclusions have vapor contents 

nearly filling the inclusion cavity (fig. 19A, B), and 

freezing-point depression and homogenization temperatures cannot be obtained for these fluid inclusions. However, homogenization temperatures were measured for 10 

fluid inclusions, ranging from 300 oc to 542 °C. Bodnar 

and others (1985) have shown that apparent homogenization temperatures of vapor-rich fluid inclusions may be 

as much as 200 oc to 400 oc less than the actual homogenization temperatures. Thus, the measured homogenization temperatures should be regarded as minimum 

values. Freezing points were only measured for four type 

2 fluid inclusions with calculated salinities ranging from 

1.7 to 16 weight percent NaCl equivalent. 

Most type 3a fluid inclusions have homogenization 

temperatures between 180 oc and 360 oc (fig. 25). Salinities calculated from halite dissolution temperatures 

range from about 26.5 to 39 percent NaCl equivalent 

(fig. 26). One type 3a fluid inclusion in sample 84-PC36 has a much higher homogenization temperature (515 

°C) and a higher estimated salinity ( 45 weight percent 

NaCl equivalent). Daughter minerals other than halite 

are uncommon in type 3a inclusions, except in sample 

83-PC-37 A, where calcite(?) and opaque daughter minerals are present locally. 

Type 3b fluid inclusions have a much larger range 

in homogenization temperatures and calculated salinities 

than type 3a fluid inclusions (figs. 25, 26). Homogenization temperatures range from about 185 °C to 495 °C, 

corresponding to minimum salinities of 31 to 56 weight 

percent NaCl equivalent. Halite dissolution commonly 

occurs at temperatures 200 oc or more higher than liquid-vapor homogenization. Type 3b inclusions commonly contain several daughter minerals in addition to 

halite, including sylvite, hematite, calcite, anhydrite(?), 

and several other small birefringent and (or) opaque 

phases. The chemistry of the daughter minerals indicates 

that fluids in type 3b fluid inclusions are complex mixtures of Na, K, Ca, Fe, Cl, and so forth, and that the 

NaCl-H20 system may not adequately represent them 

(see Kwak and Tan, 1981). 

NaCl-KCl compositions were estimated from the 

dissolution temperatures of halite and sylvite in four 

type 3c fluid inclusions from sample 83-PC-151 (which 

did not contain calcite) using NaCl-KCl-H20 relations 

from Sterner and others (1988). These fluid inclusions 

34 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
have estimated total salinities of 41 to 4 7 weight percent 

NaCl+KCl equivalent and K/Na molar ratios of 0.22 to 

0.27. First melting temperatures were not measured for 

these fluid inclusions; thus, the possible effects of Ca 

cannot be evaluated. 

Vein Quartz-Homogenization Temperatures and Salinities 

Heating and freezing measurements of fluid inclusions in seven quartz veins from six outcrops were 

made. In six of the seven veins, only type 1 liquid-rich 

fluid inclusions are present. In the other vein (sample 

83-PC-274), vapor-rich (type 2) and high-salinity (types 

3a and 3b) fluid inclusions are present in addition to liquid-rich fluid inclusions. 

Type 1 fluid inclusions have measured homogenization temperatures ranging from 130 °C to 392 oc (fig. 

25). With the exception of sample 83-PC-274, calculated 

salinities are relatively low, with most fluid inclusions 

having salinities of ~7 weight percent NaCl equivalent 

(fig. 26). Eutectic temperatures significantly lower than 

-23 oc were not observed in these fluid inclusions. 

Quartz crystals in a quartz+K-feldspar+amphibole 

vein (type I) in sample 83-PC-274 contain types 1, 2, 

3a, and 3b inclusions. Igneous quartz crystals in this 

sample also contain these types of fluid inclusions, but 

owing to optical problems with fluid-inclusion plates, 

heating and freezing measurements could not be made 

on fluid inclusions in igneous quartz. Homogenization 

temperatures of type 1 inclusions in vein quartz range 

from 257 °C to 349 °C, with most of the determinations 

in the range 320-350 °C (fig. 25). Calculated salinities 

range between 0 and 19 weight percent NaCl equivalent 

(fig. 26). Measured homogenization temperatures of type 

3a fluid inclusions are between 269 °C and 313 °C and 

have salinities ranging from 29 to 34 weight percent 

NaCl equivalent. Type 3b inclusions have homogenization temperatures ranging from 260 oc to 560 °C, corresponding to estimated salinities of 35 to 70 weight percent NaCl equivalent. No homogenization or freezing 

data were obtained on the sparse type 2 fluid inclusions 

in the vein quartz. 

Pressure Corrections and Trapping Temperatures 

Vapor-rich (type 2) fluid inclusions are absent in 

several igneous quartz samples and in the quartz veins 

(except sample 83-PC-274). This suggests that most type 

1 and possibly type 3a fluid inclusions were not trapped 

from boiling solutions and that homogenization temperatures for these inclusions must be pressure corrected to 

estimate trapping temperatures. Type 3b fluid inclusions 

homogenize by halite dissolution at temperatures greater 

than liquid-vapor homogenization and could not have 

been trapped from boiling solutions. 

Maximum pressure-corrected homogenization temperatures for type 1 fluid inclusions in igneous quartz 

samples range from about 408 °C to 646 °C, whereas 

type 1 fluid inclusions in vein quartz range from about 

470 oc to 535 °C (table 6). Salinities and pressure-corrected homogenization temperatures for type 1 fluid inclusions in paired samples of igneous and vein quartz are 

nearly identical, suggesting that most type 1 fluid inclusions in igneous quartz were trapped from fluids that 

formed the quartz veins. Fluid inclusions are very similar in all veins, except for sample 83-PC-274, with maximum salinities and trapping temperatures similar to 

those for corresponding types of veins in the western 

parts of the stock (table 6). 

Several type 1 fluid inclusions in sample S-38 have 

measured homogenization temperatures higher than 500 

°C. These fluid inclusions have estimated compositions 

and measured homogenization temperatures similar to 

those of other fluid inclusions in this sample that homogenize either to the vapor phase or homogenize by 

critical behavior. These high-temperature fluid inclusions 

may have been trapped near the critical point (540 °C, 

750 bars, 16 weight percent NaCl; Sourirajan and 

Kennedy, 1962), and thus they do not require pressure 

correction. 

Type 3a (high-salinity) fluid inclusions may have 

been trapped at pressures greater than vapor pressure. 

This is suggested by the lack of vapor-rich fluid inclusions in several samples containing type 3a fluid inclusions (fig. 20), by the unreasonably low pressures (<170 

bars) implied by these fluid inclusions if they were 

trapped from boiling solutions (Sourirajan and Kennedy, 

1962), and by the presence of type 1 fluid inclusions that 

have similar or higher homogenization temperatures and 

should have boiled if type 3a fluid inclusions were directly trapped from boiling solutions. Vapor pressures 

and densities of type 3a fluid inclusions at measured homogenization temperatures were estimated from the data 

of Urusova (1975) and Haas (1976). Approximate pressure corrections were then made assuming lithostatic 

pressure and using isochores interpolated from data in 

Urusova (1975). Estimated pressure corrections range 

from 75 °C to 150 oc and raise maximum trapping temperatures to 310 oc to 590 °C (table 7). These temperatures probably represent maximum temperatures possible 

at the time of trapping. 

Type 3b fluid inclusions could not have been 

trapped under vapor-saturated conditions because halite 

dissolves after liquid-vapor homogenization, and homogenization temperatures must be pressure corrected. Densities of type 3b fluid inclusions calculated using the 

method of Roedder and Bodnar (1980, p. 285-286) assuming pure NaCl-H20 solutions are quite high (1.08 to 

1.41 g/cm3

). Most of the calculated densities are well 

outside the range of available P-V-T-X data and imply 

Fluid-Inclusion Studies 35 
high internal pressures that probably exceed estimated 

lithostatic pressures. The presence of other salts (KCl, 

CaC12, etc.) in the fluid-inclusion fluids, however, probably significantly lowers the pressures (Stewart and Potter, 1979; Roedder and Bodnar, 1980) and could lead to 

significant pressure corrections for these fluid inclusions. 

10 l S-30 (phenocryst) 

11=56 

0 IJ j':)J. 0 100 200 300 400 500 600 

20 S-38 (phenocryst) 

11=114 

10 

0 

0 100 200 300 400 500 600 

(J) 

Iz 

w 20 :::2 82-PC-51 (phenocryst) 

w 11=79 a: 

:::> 

(J) 

<( 

w 10 

:::2 

LL 

0 

a: 

w 

Ill 0 

:::2 

:::> 0 100 200 300 400 500 600 z 

20 83-PC-37A (aplite) 

11=63 

10 

0 

100 200 300 400 500 • 600 

20 83-PC-151 (phenocryst) 

11=83 

10 

0 

0 100 200 300 400 500 600 

Unfortunately, the compositions of the solutions in these 

fluid inclusions are poorly known, and P-V-T-X data are 

not available for complex solutions at high pressures and 

temperatures. Thus, halite-dissolution temperatures must 

be regarded as minimum estimates of trapping temperatures of type 3b inclusions. 

EXPLANATION 

Types of fluid inclusions 

0 Type 1 

Iii Type 1 (critical) 

• Type 2 

~ Type 3a 

1,; Type 3b 

[J Type 3c 

10 l 83-PC-163 (phenocryst) 

11=31 

0 g~tLM 0 100 200 300 400 500 600 

10 l 83-PC-177 (phenocryst) 

n---33 

0 Bll~.rkr;o~ 100 200 300 400 ~00 600 

20 83-PC-275 (phenocryst) 

11=79 

10 

0 100 200 300 400 500 600 

10 l 83-PC-276 (phenocryst) 

11=22 

0 c::::::JIJ~~ .., I I 

0 100 200 300 400 500 600 

10 l 84-PC-36 (phenocryst) 

n=54 

r£=1 ~~~ 1\'1 j2! 

I 

100 200 300 400 500 600 

A HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS 

Figure 25. Measured homogenization temperatures of fluid inclusions in eastern part of Alta stock. n, total number of fluid 

inclusions measured in each sample. A, Igneous quartz samples. B, Vein quartz samples. 

36 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
Origin of Fluid Inclusions in the Alta Stock 

The origin of fluids and fluid inclusions in the Alta 

stock is constrained by several factors, as discussed 

above. First, stable-isotope data indicate that meteoric 

water has played an insignificant role in development of 

the hydrothermal system, and that fluids trapped in the 

stock were derived from magmatic fluids or fluids that 

equilibrated with the magma at high temperatures. Second, there is little evidence that C02 played a major role 

in the hydrothermal system. Third, high-salinity fluids 

appear to have formed early and, in most cases, predate 

low-salinity fluids that circulated through the fracturecontrolled hydrothermal system. Fourth, there is a regular geometric distribution of high-salinity fluid inclusions (type 3) subparallel to the inferred preintrusion 

surface, with high-salinity fluid inclusions limited to 

depths of less than about 5 km. Finally, most, if not all, 

fluid inclusions were trapped at subsolidus temperatures, 

and most fluid inclusions were not trapped directly from 

boiling fluids. These observations suggest that highsalinity fluids formed at high temperatures from magmatic fluids, and that pressure probably exerted a major 

influence on the generation of high-salinity fluids. 

10 +l--~-~~-1-63(-vei-n)~t1~~~~~~~--~--~ 100 200 300 400 500 600 

10 

l 

83-PC-177 (vein) 

fl=14 

(/') 

1-

9 nr=!l CJ z 

w 

::::!!: 

w 

a: 100 200 300 400 500 600 :::> 

(/') 

<( 

w 

::::!!: 

u_ 20 0 

a: 

w 83-PC-274 (vein) ID fl=79 ::::!!: 

:::> 10 z 

0 

100 200 300 400 500 600 

10 

l 

83-PC-275 (vein) 

fl=37 

D D I Db~ I 

100 200 300 400 500 600 

The most likely explanation for the formation of 

high-salinity fluid inclusions in the Alta stock is that 

magmatic fluids, or fluids that isotopically equilibrated 

with a magma at high temperatures, either boiled in the 

upper parts of the Alta stock (paleodepths less than 

4,600 to 5,200 m), forming high-salinity brines, or that 

high-salinity brines were directly exsolved from the 

crystallizing magma. The paleo-high-salinity horizon in 

the Alta stock, separating samples containing high-salinity fluid inclusions from samples lacking high-salinity 

fluid inclusions, may be thought of as a high-temperature "paleoboiling" horizon. 

Direct exsolution of an immiscible high-salinity 

brine from crystallizing granitic magmas has been demonstrated in plutonic blocks from Ascensi6n Island 

(Roedder and Coombs, 1967; Harris, 1986) and has been 

suggested as the mechanism leading to formation of 

high-salinity brines lying on "halite trends" (Cloke and 

Kesler, 1979; Wilson and others, 1980). In the Alta 

stock, most high-temperature, high-salinity fluid inclusions homogenize by halite dissolution after liquid-vapor 

homogenization (type 3b), indicating that the fluids were 

not vapor saturated and thus were not boiling at the time 

of trapping. However, vapor-rich (type 2) fluid inclu20 83-PC-276 (vein) 

fl=62 

10 

0 100 200 300 400 500 600 

10 

l 

84-PC-36 (vein) 

n--36 

D ru£J, 9 

0 100 200 300 400 500 600 

20 84-PC-40 (vein) 

fl=48 

10 

100 200 300 400 500 600 

HOMOGENIZATION TEMPERATURE, IN DEGREES CELSIUS 

B 

Figure 25. Continued. 

Fluid-Inclusion Studies 3 7 
sions are present in most samples that contain high-salinity fluid inclusions (fig. 21}--a fact that strongly suggests that boiling occurred at some point in time. The 

general lack of vapor-rich fluid inclusions in quartz 

veins in the Alta stock suggests that boiling occurred 

prior to formation of the veins and may have been contemporaneous with formation of high-salinity brines. 

Sylvite daughter crystals are rare in high-salinity 

fluid inclusions in the Alta stock, and fluid inclusions 

~ z 

w 

~ 

w 

a: 

:::> 

(/) 

<( 

w 

~ 

u. 

0 

a: 

w 

(]) 

~ 

:::> 

z 

1 0 l S-30 (phenocryst) 11=45 

0~ on 

10 

0 

10 20 

82-PC-51 (phenocryst) 

11=31 

10 20 

10 l 83-PC-37A(aplite) 

11=39 

0 M n 1

r=1 [l,q nniJ!! 

10 

20 

10 

10 

10 20 

83-PC-177 (phenocryst) 

11=17 

10 20 

83-PC-275 (phenocryst) 

11=70 

10 20 

83-PC-276 (phenocryst) 

n=11 

10 20 

30 

~-40 50 

30 40 50 

30 40 50 

30 40 50 

30 40 50 

30 40 50 

20 

10 

10 

20 

10 

10 

defining a "halite trend" are not present. This fact suggests that high-salinity brines had low K/Na molar ratios 

(generally S0.2) that would not be in equilibrium with 

two feldspars (or granitic magmas) at magmatic or nearmagmatic temperatures (Lagache and Weisbrod, 1977; 

Burnham, 1979a; Cloke and Kesler, 1979). However, the 

K/Na molar ratio of high-salinity fluid inclusions may 

not be an accurate indicator of the bulk composition of 

magmatic fluids, because if boiling has occurred near 

EXPLANATION 

Types of fluid inclusions 

0 Type 1 

Ell Type 1 (critical) 

IZ Type 3a 

lSI Type 3b 

o Type 3c 

83-PC-163 (phenocryst) 

11=17 

0 10 20 

l 84-PC-36 (phenocryst) 

11=50 

npD ~no 

0 10 20 

S-38 (phenocryst) 

11=86 

10 20 

83-PC-151 (phenocryst) 

11=49 

l AcQ I [l,p 0 10 20 

nlpL p 1.'\1 

I 

30 40 50 60 

30 40 50 60 70 

I ~ ~ ~10 CEI I 

30 40 50 

A SALINITY., IN WEIGHT PERCENT NaCI EQUIVALENT 

Figure 26. Calculated salinities of fluid inclusions in eastern part of Alta stock. n, total number of fluid inclusions measured in each sample. A, Igneous quartz samples. Values for type 3c fluid inclusions in sample 83-PC-151 are given in 

weight percent NaCI+KCI equivalent. B, Vein quartz samples. 

38 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
700 °C, K is preferentially partitioned into the vapor 

phase, lowering the K/Na ratio of the brine phase 

(Sterner and Bodnar, 1986). Silicate-melt inclusions and 

mixed inclusions of high-salinity fluids and silicate melts 

similar to inclusions at Ascension Island (Roedder and 

Coombs, 1967) also are absent in the Alta stock. These 

data suggest that at least minimal boiling occurred early 

in the hydrothermal history of the Alta stock. It is not 

possible, however, to determine if high-salinity brines 

formed by direct exsolution of an immiscible high-salinity brine from the crystallizing magmas with later boiling 

or by boiling of low- to moderate-salinity fluids released 

during crystallization of magmas. 

The magma of the Alta stock probably contained 

about 3 to 4 weight percent H20, most of which was released as an aqueous vapor phase during crystallization 

and cooling of the stock, because less than 0.7 weight 

percent H20 is contained in hydrous phases in the Alta 

stock (table 1). Burnham (1979a) suggested that the initial salinity of aqueous fluids separating from a granodioritic magma will be in the range of a few to 20 weight 

percent NaCl equivalent. Type 1 fluid inclusions in the 

western part of the stock with the highest estimated 

trapping temperatures suggest that such fluids may have 

had salinities of 5 to 12 weight percent NaCl equivalent. 

The NaCl-H20 system can be used as a simplified 

model for the fluids in the Alta stock. Chou (1987) has 

~ z 

w 

::E 

w 

c: 

:::> 

CJ) 

<( 

w 

::E 

u_ 

0 

c: 

w 

aJ 

::E 

:::> 

z 

83-PC-163 (vein) 

']i~ 

10 

0 

10 20 

83-PC-177 (vein) 

11=5 

10 

10 

20 

20 

83-PC-274 (vein) 

11=56 

0 9 

20 30 

presented a temperature-composition diagram for this 

system showing two-phase liquid+vapor curves and the 

compositions of coexisting fluid phases at various pressures (fig. 27). At 1,300 bars pressure and temperatures 

higher than 750 °C, 10 weight percent NaCl solutions 

are unstable and will boil, separating into high-salinity 

liquid and lower salinity vapor phases. At 1 ,000 bars, 10 

weight percent NaCl solutions will separate into two 

phases at temperatures greater than about 640 °C, 

whereas at 1,500 bars, 10 weight percent NaCl solutions 

will remain in the one-phase field at all temperatures 

lower than 850 °C. 

Possible temperatures of boiling and probable temperatures of vapor saturation of the magma were estimated using NaC1-H20 phase relations shown by Chou 

(1987), maximum salinities of type 3b inclusions estimated in each sample, and lithostatic pressures estimated 

from paleodepths in figure 21. The estimated temperatures range from about 725 °C to 860 °C, with most between 725 oc and 770 oc (table 7). Provided that the 

NaCl-H20 system is applicable to the complex fluids of 

the Alta stock, the temperature estimates suggest that 

vapor saturation and boiling did not occur until 

relatively late during crystallization, possibly at only a 

few degrees above the solidus. These temperature 

estimates are probably maximum temperatures because 

lower salinities and (or) lower pressures both lead to 

10 ~84-PC-36(vein) 11=24 

0 

0 

10 

0 

40 

10 

83-PC-275 (vein) 

11=10 

20 

10 20 

84-PC-40 (vein) 

n=20 

10 

50 60 

20 

70 80 

B SALINITY, IN WEIGHT PERCENT NaCI EQUIVALENT 

Figure 26. Continued. 

Fluid-Inclusion Studies 39 
~ 

0 

g' 

c 

5· 

= Q 

-::::t: 

"< 

c. 

0 

:;:. 

11> 

3 

a!. 

Jl 

c 

~ 

=· 

-

~ 

i!:: 

S' 

VI 

S' 

n 

~ 

(") 

11> 

= [ 

~ ~ ::T 

3:: 

Q 

c 

= i. 

~ 

c: 

iTable 7. Summary of pressure corrections for type 3 fluid inclusions in the eastern part of the Alta stock 

[T H• homogenization temperature] 

Sample Pressure Type 3a fluid inclusions 

(bars) 

TH (oC) Maximum Pressure Trapping TH vapor (°C) 

salinity (wt% correction (°C) temperature 

NaCI equiv .) (oC) 

83-PC-151 1,400 360 38 150 510 311 

S-30 1,200 196 

82-PC-51 1,450 206 32 120 325 221 

S-38 1,000 349 39 110 460 252 

84-PC-36 1,150 515 45 75 590 238 

83-PC-177 1,300 205 31 105 310 229 

83-PC-37A 1,050 319 35 80 400 275 

83-PC-276 1,250 160 

83-PC-275 1,250 294 37 145 440 325 

83-PC-274 1,100 312 34 120 430 213 

Type 3b fluid inclusions 

TH halite (°C) Maximum Densi~ Maximum 

salinity (wt% (g/cm) temperature of 

NaCI equiv .) boiling (°C) 

322 39.5 1.08 770 

370 43.4 1.21 730 

412 49.1 1.23 810 

495 60.6 1.21 770 

431 51.3 1.23 735 

249 34.5 1.11 725 

413 48.8 1.16 725 

381 44.3 1.26 735 

354 45.1 1.08 740 

550 70.1 1.41 860 
lower temperature estimates for boiling (fig. 27). In 

general, the maximum salinity of type 3 fluid inclusions 

decreases with increasing pressure (table 7), which is 

consistent with vapor saturation and boiling at similar 

temperatures throughout the stock (fig. 27) and may 

explain the sharp, nearly planar break between rocks 

containing high-salinity fluid inclusions and those 

lacking high-salinity fluid inclusions that is interpreted 

as a high temperature paleoboiling horizon (fig. 21). The 

one sample that lies below this horizon and contains 

high-salinity fluid inclusions may have become vapor 

saturated at slightly higher temperatures or initially may 

have had a slightly more saline composition, allowing it 

to boil at lower temperatures and form high-salinity 

fluids (fig. 27). 

High-salinity fluid inclusions were not trapped directly from these "boiling" fluids, and they may have 

been trapped at temperatures 100 oc to 300 oc lower 

than those at which the high-salinity fluids formed (table 

7). An isoplethal pressure-temperature projection of the 

high-salinity portion of the NaCl-H20 system (fig. 28) 

from Chou (1987), Bodnar and others (1985), and 

Gunter and others (1983) shows that a 10 weight percent 

1000 

I 

I 

900 I 

en 

::) 

en 

....I 

w 

(.) 800 en 

w 

w 

a: 

\ 

\ I 

\ I 

\ I 

' .......... - -f- /~f;;:) ,, 

I 

(!) 

w 

0 700 

~ 

w 

a: 

::) 

~ 600 

a: 

w 

Q.. 

::E 

w 

t- 500 

400 ~~-----~-----~~-----~~~~-----~~ 

0 1 0 20 30 40 50 60 70 80 90 

WEIGHT PERCENT NaCI 

Figure 27. Temperature-composition diagram for high-temperature part of NaCI-H20 system (from Chou, 1987), 

showing isobars for the two-phase liquid+vapor field and 

compositions of coexisting liquid and vapor phases. 

Dashed curves are extrapolated. Critical curve is loci of 

critical points. 

NaCl solution exsolved at 725 oc and 1,000 bars (point 

A) will unmix into a small amount of brine with a composition of about 53 weight percent NaCl and a volumetrically dominant vapor phase with a salinity of about 

3 weight percent NaCl (Sourirajan and Kennedy 1962; 

Chou, 1987). If the high-salinity brine is not trapped in 

fluid inclusions at the temperature of boiling (725 °C) 

but is allowed to cool along a nearly isobaric path similar to A-B without remixing with the low-salinity vapor, 

its density will increase as it moves through the field of 

liquid stability, and it will become vapor undersaturated. 

Trapping of this fluid somewhere along this path at temperatures above the liquidus (point C) will result in the 

formation of a fluid inclusion that homogenizes by halite 

dissolution and has a bulk composition of 53 weight percent NaCI. If the brine is allowed to cool to still lower 

temperatures without dilution, it will cross the liquidus 

surface at point c (about 450 °C) and begin precipitating 

halite. Further cooling will result in fluids that have salinities less than 53 weight percent NaCl equivalent, and 

because of the fixation of NaCl into precipitating halite, 

any fluid inclusions trapped from the residual solutions 

will also homogenize by halite dissolution (type 3b fluid 

inclusions). 

1.5 

~itf 

~~ ~~ ~~ 

(/) 

a: c 

<( 1.0 m 

0 

.....1 

S2 

~ 

w 

a: 

:::> 

(/) 

(/) 

w 0.5 a: 

a.. 

0 

300 400 500 600 700 800 900 1 000 

TEMPERATURE, IN DEGREES CELSIUS 

Figure 28. lsoplethal pressure-temperature diagram for part 

of NaCI-H20 system (modified from Chou, 1987). Liquidus 

surfaces from Gunter and others (1983). Critical isochore 

for 5 weight percent NaCI estimated from Roedder (1984, 

fig. 17-17). Path A-B-C is described in text. Lines labeled 

40, 53.5, 62.4, and 70 are isopleths of liquid composition 

in weight percent NaCI. Dashed curves are interpolated. 

Abbreviations: L, I iquid; V, vapor; C.P. (H20), critical 

point of H20. 

Fluid-Inclusion Studies 41 
Type 3a fluid inclusions may have formed by partial mixing of the high-salinity brines with lower salinity, less dense fluids. Type 3a fluid inclusions generally 

have lower salinities, lower densities, and lower homogenization temperatures than type 3b fluid inclusions 

(figs. 25, 26; table 7). With the exception of one fluid 

inclusion in sample 84-PC-36, maximum pressurecorrected homogenization temperatures of type 1 fluid 

inclusions are equal to or greater than pressure-corrected 

homogenization temperatures for type 3a fluid inclusions. These data suggest that type 3a fluid inclusions 

may have formed by partial mixing of the high-salinity 

brines with the lower salinity fluids that were trapped as 

type 1 fluid inclusions. Fluid inclusions trapping mixtures of these fluids will have intermediate salinities and 

densities and could homogenize either by halite dissolution (type 3b) or by vapor disappearance (type 3a), depending on the bulk density of the fluid inclusion. 

Type 1 fluid inclusions in the eastern samples 

could have formed in several ways. They may represent 

low-salinity fluids that flowed into this part of the stock 

either from the wall rocks (meteoric or metamorphic waters) or from deeper parts of the stock, or they could 

represent small portions of the low-salinity vapor phase 

that formed during high-temperature boiling. Stable-isotope relations suggest that large volumes of meteoric or 

metamorphic water did not circulate through the stock. 

The sharp interface between rocks containing high-salinity fluid inclusions and those that lack high-salinity fluid 

inclusions suggests that there probably was not widespread circulation of fluids upward in the stock. Gravitational sinking of the high-density brines would be expected if lower density fluids were flowing upward, as 

discussed by Henley and McNabb (1978) and Fournier 

(1983). Apparent trapping temperatures of types 1, 3a, 

and 3b fluid inclusions suggest that they were all trapped 

at similar temperatures. These data further suggest that 

type 1 fluid inclusions represent either small portions of 

the vapor phase formed by boiling at high temperatures 

and not trapped until temperatures ~620 °C or low-salinity fluids released during crystallization of the deeper 

parts of the stock. 

The composition of the vapor phase formed during 

boiling of a 10 weight percent NaCl solution at 725-770 

°C and 1,000-1,400 bars will vary between about 3 and 

8 weight percent NaCl (Sourirajan and Kennedy, 1962; 

Bodnar and others, 1985; Chou, 1987). Most type 2 vapor-rich fluid inclusions may represent a small fraction 

of this fluid, although salinity and homogenization data 

are too limited, and actual homogenization temperatures 

too uncertain, to test this hypothesis. If the vapor cools 

isobarically and does not mix with the high-salinity 

brine, its density will increase, and eventually it will 

cross its critical isochore between 600 oc and 700 °C 

(fig. 28). Fluid inclusions trapped at temperatures lower 

than the critical isochore will homogenize to the liquid 

phase. Partial mixing of the high-salinity brines with the 

low-salinity fluids may have led to the higher salinities 

locally found in type 1 fluid inclusions. This suggestion 

is supported by the strong correlation between type 3 

fluid inclusions that probably have high Ca contents, as 

exemplified by the presence of calcite daughter crystals, 

and the presence of relatively high salinity type 1 fluid 

inclusions. 

Type 1 fluid inclusions in the western part of the 

Alta stock probably represent residual intergranular fluids exsolved during the late stages of igneous crystallization. These fluids locally circulated through small fractures forming the relatively sparse veins. High-salinity 

fluids apparently were not formed at depths corresponding to lithostatic pressures greater than about 1,400 bars 

(approximately 5.2 km). For fluids containing about 10 

weight percent NaCl equivalent, this suggests that vapor 

saturation of the magma did not occur until temperatures 

fell below about 800 °C (Chou, 1987), which is consistent with the temperatures estimated above for vapor 

saturation in the eastern parts of the stock. 

Most overall fluid-inclusion populations in igneous 

quartz in the western samples are similar regardless of 

their proximity to megascopic veins (table 6). Measurements of fluid-inclusion properties in three or more igneous quartz crystals in most samples indicate that there 

are large differences in the salinities and homogenization 

temperatures of fluid inclusions contained in different 

crystals within single samples. These data suggest that 

most fluid inclusions formed during repeated 

microfracturing and healing of quartz crystals, and that 

an intergranular pore fluid was present throughout subsolidus cooling of the stock. A pore fluid also is suggested by the pervasive weak deuteric alteration of the 

stock. 

COMPARISON OF THE EVOLUTION OF 

HYDROTHERMAL FLUIDS IN THE ALTA STOCK 

TO PORPHYRY COPPER SYSTEMS 

Porphyry copper systems associated with calcalkaline magmatism are characterized by a number of 

features that may vary in detail but are distinct enough 

to have been described as "variations on a common 

theme" (Gustafson and Hunt, 1975). These features include (1) close association with stocklike, epizonal or 

subvolcanic granitic bodies that commonly are porphyritic; (2) extensive development of steeply dipping fractures centered on the apical portions of intrusions; and 

(3) strong hydrothermal alteration centered on the fractured part of the intrusion, including early potassic alteration (biotite and potassium feldspar stable) containing 

sulfide minerals with high Cu:Fe ratios (chalcopyrite ± 

42 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
bornite) and later biotite and potassium feldspar-destructive alteration (generally sericitic) containing sulfide 

minerals with low Cu:Fe ratios (pyrite ± chalcopyrite). 

Pressure also may vary from early, near-lithostatic values to late, near-hydrostatic values corresponding to a 

change from dominantly magmatic to dominantly meteoric processes. The hydrothermal system of the Alta 

stock appears to have evolved in several ways distinct 

from hydrothermal systems in typical porphyry copper 

intrusions. These differences include apparent timing of 

vapor saturation of the crystallizing magma; types and 

extents of hydrothermal alteration; salinities of veinforming fluids; interaction of meteoric fluids with the 

stock; fracturing and fracture permeability of the intrusions; and, most importantly, lack of significant copper 

mineralization within the stock. 

Vapor saturation and fluid exsolution from the 

crystallizing magma occurred relatively late in the Alta 

stock, probably at near-solidus temperatures. Late vapor 

saturation is suggested by the mineral crystallization sequence, the fracture pattern, and the compositions and 

densities of early fluid inclusions. In most porphyry copper stocks, timing of vapor saturation is poorly known, 

although generally it is thought to be relatively early (for 

example, see Burnham, 1979a; Dilles, 1984, 1987). 

Relatively late vapor exsolution in the Alta stock compared with porphyry copper systems also is consistent 

with both H20 solubility in silicate melts, which increases with increasing pressure (Burnham, 1979a, 

1985), and the somewhat greater depths of crystallization 

of the Alta stock (approximately 3.7 to 6.3 km) compared with most porphyry copper systems [for example, 

Bingham, 1-3 km (Einaudi, 1982); Yerington, 1-2.5 km 

(Dilles, 1987); Ann Mason (Yerington district), 3-4.5 

km (Dilles, 1987); Ely, 2-3 km (James, 1976); El Salvador, 2 km (Gustafson and Hunt, 1975)]. Late vapor saturation and (or) lower initial H20 contents of the magma 

of the Alta stock may have led to the low fracture permeability, which resulted in weak fracture-related alteration, inhibited incursion of large volumes of meteoric 

water into the stock, and allowed pressure to remain near 

lithostatic values throughout cooling of the stock. Late 

vapor saturation also may have prevented removal of 

copper from the melt and formation of copper-rich 

hydrothermal fluids, as shown by the somewhat high 

copper contents of biotite, hornblende, and magnetite 

phenocrysts in the Alta stock compared with copper contents of these minerals in porphyry copper intrusions 

(Hendry and others, 1985). 

Five features of hydrothermal alteration in the Alta 

stock are notably different from hydrothermal alteration 

in most intrusion-hosted porphyry copper systems (for 

example, see Lowell and Guilbert, 1970; Gustafson, 

1978). First, hornblende is stable in early alteration assemblages, and large areas of pervasive biotitization are 

not present. Second, alteration zones, which typically 

form in porphyry copper systems in areas where one 

type of fracture-controlled alteration becomes dominant 

and commonly are more or less concentrically distributed around one or more intrusive phases, are absent in 

the Alta stock. Most hydrothermal alteration is weak, 

pervasive deuteric alteration. Fracture-controlled alteration is relatively sparse and nowhere strongly developed, and the same vein assemblages are present 

throughout the Alta stock (fig. 7). Third, the youngest 

and most common vein assemblage in the Alta stock is 

pyrite+chlorite, in contrast to the quartz+sericite+pyrite 

vein assemblage that occurs in nearly all continental porphyry copper systems. Quartz+sericite+pyrite alteration 

locally is present in the Alta stock, but it is cut by 

pyrite+chlorite veins that are much more abundant. 

Fourth, no argillic or advanced argillic alteration is present in the Alta stock. Finally, the results of reconnaissance stable-isotope studies indicate that meteoric water 

did not play a significant role in the late stages of the 

hydrothermal system of the Alta stock. These features 

suggest that fluid abundance, composition, and circulation patterns in the Alta stock were markedly different 

from those in "typical" porphyry copper systems. 

Early hydrothermal fluids circulating through 

megascopic fractures in the Alta stock were in equilibrium with an essentially magmatic assemblage (hornblende, potassium feldspar, biotite, magnetite, sphene, 

and, locally, oligoclase), and over time fluids evolved 

from compositions that were in equilibrium with biotite 

and magnetite, similar to early veins in porphyry copper 

systems, to compositions in equilibrium with combinations of sericite, chlorite, and pyrite. The distribution of 

vein types (fig. 7) suggests that fluids with similar compositions were present throughout most of the Alta stock 

at some point during its cooling. It is not possible, however, to determine if different parts of the stock saw 

identical types of fluids at similar stages of cooling. 

Also, as a result of the relative!* low abundance of fractures, the volume of rock that int~racted with these fluids 

at any location probably was small. 

Activity diagrams for the sy!stems Ca0-Mg0-K20-

Al203-SiOrH20-HCl and Mg0-K20-Al20 3-SiOrH20-

HCl at 1,000 bars and 500 oc and 300 °C, calculated 

from hydrolysis reactions in Bowers and others (1984), 

are displayed in figure 29. Early vein assemblages in the 

Alta stock contain calcic amphibole (hornblende) and 

(or) epidote, suggesting that they have higher ac/+fr}H+ 

ratios than do typical early vein assemblages in porphyry 

copper systems, where biotite replaces calcic amphibole 

(fig. 29A). Potassium feldspar+amphibole (actinolitic 

hornblende) is only stable over a narrow range of aMg2

+/ 

a 2 + and a 2+1a 2 + and increasing a 2+1a 2 

H ~I' H' Mg + ratios will /' H 

result in biotite saturation, whereas increasing ac.2+/a2 H + 

ratios will result in formation of clinopyroxene ( diopComparison of the Evolution of Hydrothermal Fluids in the Alta Stock to Porphyry Copper Systems 43 
7 

6 

+ 

~ 

CtJ 5 

-+ C\1 

ns 

0 

~ 4 

(!) 

0 

....J 

3 

2 

-2 

4 

0 

-1 

0 

7 

6 -

+ :r: 

C\1 5 - ttl 

+ 

C\1 

0> 

:E 4 f-

~ 

(!) 

0 

....J 3 f2 

0 

500 °C, 1,000 bars 

+Quartz, +K-feldspar 

Wollastonite saturation 

Plagioclase (An 20) 

Muscovite 

A 

Biotite 

saturation 

-1 0 1 2 3 

LOG (a Mg2+/a 2H+) 

LOG (aca2+/a 2w)= 500 °C, 1,000 bars 

- - - 5.o +Quartz 

5.3 

--- 5.8 

Tale saturation 

;--

Clinopyroxene saturation 

Andalusite 

B 

2 3 4 5 

LOG (a K+laJ-t) 

I I I I 

300 °C, 1,000 bars 

+Quartz 

- Tale saturation 

Chlorite ___...______Biotite 

-

Pyre phyllite Muscovite K-

(sericite) feldspar 

-

c I I I 

4 5 

side), which has not been found in the Alta stock (fig. 

29B). Stoichiometric clinozoisite is unstable at 500 °C, 

and the presence of epidote in early veins implies significant Fe+3 substitution (Bird and Helgeson, 1981). The 

lack of anhydrite is apparently due to relatively low activity of so2 in the fluids. 

In the upper part of the Alta stock, either early fluids boiled at hypersolidus temperatures, forming high-salinity brines, or else high-salinity brines were directly 

exsolved from the crystallizing magma. These brines, 

however, mostly remained as interstitial pore fluids and 

did not circulate through fractures. High-salinity fluids 

also did not lead to the formation of disseminated copper 

mineralization within the stock, although most silverlead-zinc fissure and replacement ores in the Cottonwood mining district and the small iron-copper skarns, 

such as the Big Cottonwood mine, are spatially associated with parts of the stock where high-salinity fluids 

were present. The behavior of the high-salinity fluids in 

the Alta stock is quite different from that of most porphyry copper systems, where early fluid inclusions in 

quartz veins forming potassic alteration generally are of 

high salinity (for example, see Nash, 1976; Chivas and 

Wilkins, 1977; Eastoe, 1978; Roedder, 1984), and commonly there is a strong correlation between copper grade 

and the abundance of high-salinity fluid inclusions (for 

example, see Moore and Nash, 1974). The lack of high-

...... Figure 29. Activity diagrams for fluids in equilibrium with various vein assemblages in Alta stock. Diagrams are 

calculated using hydrolysis reactions in Bowers and others 

(1984), ideal ionic solid-solution models for biotite and 

amphibole (Bohlen and others, 1980), activity relations 

from Orville (1972) for plagioclase and Waldbaum and 

Thompson (1969) for potassium feldspar, and mineral 

compositions determined by microprobe analyses. All diagrams are for 1,000 bars water pressure. A, Log (aca2

+/ 

irH+)-Iog (a"(('42+/a2H+) diagram for system Ca0-Mg0-K20-

AI203-Si02-H20-HCI at 500 °C with excess quartz and 

potassium feldspar and unit activity of water. Hachured 

field shows possible range of compositions of early fluids 

forming hornblende-stable veins in Alta stock; stippled 

field shows possible range of fluid compositions in early 

alteration in typical porphyry copl?er systems (biotite 

replaces hornblende). B, Log(aM 2+/a2H+)-Iog(aK+/aH+) 

diagram for system CaO-MgO-K20-Af20 3-SiOrH20-HCI at 

500 °C with excess quartz and unit activity of water. 

Biotite+K-feldspar assemblages in porphyry copper systems 

requires lower (ac/+/alH+) ratios than in Alta stock, where 

hornblende+K-feldspar is stable in early veins. C, Log 

(aM~+/a H+)-Iog(aK+/aH+) diagram for system MgO-K20-

AI2U3-SiOrH20-HCI at 300 oc with excess quartz and 

unit activity of water. Chlorite (+pyrite) assemblages in late 

veins in Alta stock suggest higher (aM/+!alH+) ratios and 

(or) lower (aK+/aH+) ratios than in typical porphyry copper 

systems, where sericite (+pyrite) is stable late assemblage. 

44 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
salinity fluid inclusions in quartz veins may result from 

the low fracture permeability and the near-lithostatic 

pressures, which did not allow hydrothermal fluids to 

boil. Unlike most porphyry copper systems, late hydrothermal fluids that formed quartz veins in the Alta stock 

did not have markedly different salinities from early, 

vein-forming hydrothermal fluids. Late veins in the Alta 

stock also deposited large amounts of chlorite instead of 

sericite, which is present in nearly all continental porphyry copper systems. This suggests that late fluids in 

the Alta stock may have had higher aMg2

+ /a2 H + ratios 

than in typical porphyry copper systems (fig. 23C). Late 

fluids in the Alta stock may have remained undiluted by 

Mg-poor meteoric water, leading to higher aMg2

+/a2H+ 

values than for late fluids in typical porphyry copper 

systems. 

SUMMARY AND CONCLUSIONS 

The evolution of the hydrothermal system associated with the Alta stock was a complex process that has 

been partially deciphered through field, petrographic, 

stable-isotope, fluid-inclusion, and electron-microprobe 

studies. The Alta stock is a small, composite granodioritic body that was emplaced in two main pulses closely 

spaced in time. Present exposures represent paleodepths 

ranging from about 3. 7 km on the east to 6.3 km on the 

west. Compositions of the two intrusions are similar, and 

the main difference between the two phases is a variable, 

but generally small, amount of aplitic groundmass that 

probably resulted from pressure quenching of residual 

magma of the younger (porphyritic) phase. The magma 

of both bodies was moderately H20 rich (probably 3 to 4 

weight percent H20), as shown by the presence of 

phenocrystic hornblende and biotite and the lack of pyroxene, but H20 contents may have been somewhat less 

than in typical porphyry copper intrusions because biotite crystallized earlier than hornblende. Relatively deep 

emplacement, strong zones of regional weaknesses, and 

relatively late vapor saturation of the magma led to low 

fracture abundance and low fracture permeability and 

probably prevented the formation of fracture patterns 

typical of porphyry copper systems (Haynes and Titley, 

1980; Burnham, 1985; Titley and others, 1986). 

Biotite and amphibole show a wide range of compositions and compositional zoning. Both minerals generally are characterized by decreasing Fe/(Fe+Mg), Ti, 

and AI during progressive crystallization from early inclusions in plagioclase to phenocrysts to hydrothermal 

crystals. Compositions and zoning patterns of both minerals are similar to those of minerals reported in porphyry copper deposits and other shallow intrusions (for example, see Jacobs and Parry, 1979; Chivas, 1981; Dilles, 

1984, 1987). Biotite compositions and temperatures estimated from its Fe-Ti contents suggest that crystallization 

occurred under moderately oxidizing conditions, probably slightly above the Ni-NiO oxygen buffer, and that 

the oxidation state may have slightly increased (less than 

one log unit) during crystallization. Biotite in the western, deeper part of the stock generally has a higher Fe/ 

(Fe+ Mg) ratio than does the eastern, shallower parts of 

the stock, but calculations suggest that this biotite may 

have crystallized at a slightly higher oxidation state than 

the more Mg-rich biotite in the eastern parts of the 

stock. Generally increasing Mg, Si, and Ca contents in 

amphibole with crystallization also suggest an increase 

in the oxidation state during crystallization. Overall 

amphibole compositional trends are nearly identical to 

those of the Inamumu zoned pluton, which Chivas 

(1981) suggested may have crystallized under vaporsaturated conditions. 

The compositions and densities of early fluids varied markedly within the Alta stock and were strongly dependent on pressure. At depths less than about 5.2 km, 

low- to moderate-salinity fluids (probably released during crystallization of magma) boiled, forming lowdensity vapor and high-density, high-salinity (35 to 70 

weight percent NaCl equivalent) brine phases; alternatively, a high-salinity brine was directly exsolved from 

the crystallizing magma. Maximum salinities of fluid inclusions in this part of the stock suggest that vapor saturation of the crystallizing magma may have occurred at 

temperatures between about 725 oc and 770 °C. At 

depths greater than about 5.2 km, early fluids were 

supercritical and of low to moderate salinity (<12 weight 

percent NaCl equivalent), and there is no fluid-inclusion 

evidence for the presence of fluids at temperatures 

greater than approximately 720 °C. 

Hydrothermal alteration is characterized by pervasive, generally weak deuteric alteration and only weak 

fracture-controlled alteration. Deuteric alteration generally consists of partial to complete chloritization of biotite, partial sericitization of plagioclase, and formation of 

turbid perthite crystals. Oxygen-isotope fractionation between quartz and potassium feldspar, an values for biotite and chlorite, fluid-inclusion data, and two-feldspar 

thermometry suggest that deuteric alteration occurred at 

temperatures around 500 oc probably from magmatically 

derived fluids. In particular, deuterium enrichment of 

chlorite by about 10 to 30 permil relative to fresh biotite 

suggests that little meteoric water circulated through the 

Alta stock during deuteric alteration. The lack of meteoric water probably is due to the relatively low fracture 

permeability of the Alta stock. Fracture abundance is 

about an order of magnitude lower than in the nearby 

Mayflower stock (Villas and Norton, 1977) and considerably lower than measured in the proximity of porphyry 

copper deposits (Haynes and Titley, 1980; Titley and 

others, 1986). 

Summary and Conclusions 45 
A wide variety of veins formed during cooling of 

the Alta stock. Early veins were calcic-amphibole 

(homblende)+K-feldspar+sphene±quartz±magnetite± epidote. These were followed by epidote ± magnetite ± pyrite, biotite+magnetite±K-feldspar ± quartz ± sulfide 

minerals, quartz±K-feldspar, quartz+pyrite±sericite, 

quartz+pyrite+chlorite, and pyrite+chlorite veins. Little 

wall-rock alteration is associated with the veins. 

Pyrite+chlorite veins are the most abundant type. All 

veins predominantly have northeast orientations that 

parallel regional structures. Fluid inclusions in these 

veins suggest that all veins except pyrite+chlorite may 

have formed initially at similar temperatures between 

about 465 oc and 535 oc from low- to moderate-salinity 

(~10 weight percent NaCl equivalent) nonboiling fluids. 

Early high-salinity fluids in the upper part of the stock 

apparently did not widely circulate through fractures and 

remained as intergranular fluids. The 18 0 values 

of quartz from quartz+pyrite+sericite and quartz+pyrite+ 

chlorite veins and the ~D value of chlorite 

in a quartz+pyrite+chlorite vein also suggest that fluids 

forming these veins were derived from magmatic 

sources. 

There are several major differences in the evolution of the hydrothermal system of the Alta stock compared with that of typical porphyry copper intrusions. 

These include (1) relatively low fracture abundance and 

low fracture permeability, resulting in relatively weak 

fracture-controlled alteration in the Alta stock; (2) early 

amphibole-stable veins and relatively little biotite replacement of hornblende, suggesting that early fluids in 

the Alta stock had relatively high ac.2+/a2

H+ ratios compared with early fluids in porphyry copper systems; (3) 

widespread late pyrite+chlorite veins in the Alta stock 

instead of quartz+pyrite±sericite veins, suggesting that 

late fluids had higher aM8

2

+/a2H+ ratios than in most porphyry copper systems; (4) a general lack of high-salinity 

fluid inclusions in quartz veins in the Alta stock despite 

abundant high-salinity fluid inclusions in igneous quartz 

in the upper parts of the stock; (5) similar salinities and 

pressure-corrected homogenization temperatures of fluid 

inclusions in all types of quartz veins, regardless of age, 

in the Alta stock; (6) the lack of isotopic evidence for 

the interaction of meteoric water during the late stages 

of alteration (quartz+pyrite±sericite and pyrite+chlorite 

veins) in the Alta stock, again probably resulting from 

the low fracture permeability; and (7) no evidence for a 

change in the pressure regime from early lithostatic pressures to late hydrostatic pressures. These differences 

mostly result from the slightly greater depths of emplacement and possibly the lower initial water contents 

of the Alta stock, which resulted in relatively late vapor 

saturation of the magma and did not allow development 

of an extensive fracture system typical of porphyry copper systems. Late vapor saturation of the magma may 

have prevented removal of copper from the melt and 

concentration of it in hydrothermal fluids 

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48 Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
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Orville, P.M., 1972, Plagioclase cation exchange equilibria 

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Potter, R.W., and Clynne, M.A., 1978, Solubility of highly 

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--- 1984, Fluid inclusions, v. 12 of Reviews in Mineralogy: Washington, D.C., Mineralogical Society of America, 

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APPENDIX 1. DESCRIPTIONS OF SAMPLES 

USED IN FLUID-INCLUSION STUDIES 

83-PC-37A: 1-cm aplite dikelet cutting equigranular 

phase of Alta stock, with weak deuteric alteration in 

stock; aplite has 4-5 mm aplitic quartz+K-feldspar center line with 3-mm graphic intergrowths of quartz and Kfeldspar on either side; fluid inclusions-moderately 

abundant types 1, 2, and 3 [type 3 have several birefringent daughter minerals including calcite(?)] in both aplite and host rock 

83-PC-148: Equigranular phase with generally weak 

deuteric alteration; cut by 1-mm-wide quartz+biotite+ 

magnetite+chalcopyrite vein; biotite largely chloritized; 

fluid inclusions-both igneous and vein quartz have 

moderately abundant type 1 

83-PC-151: Pod of quartz+K-feldspar pegmatite in equigranular phase; fluid inclusions-abundant type 1 and 

sparse type 3, with some type 3 having sylvite daughters 

in addition to halite 

83-PC-152: Equigranular phase, weak deuteric alteration; cut by 2 mm quartz+pyrite+chlorite vein (trace 

epidote and chalcopyrite) with stronger sericite and chlorite alteration along vein edge; fluid inclusions-igneous 

quartz has sparse type 1, and vein quartz moderately 

abundant type 1 

83-PC-163: Porphyritic phase, moderate deuteric alteration; cut by 7-mm vuggy quartz+K-feldspar vein with 

vug-filling pyrite; fluid inclusions-both igneous and 

vein quartz have moderately abundant type 1 

83-PC-177: Equigranular phase, weak to moderate deuteric alteration; cut by 1-mm quartz+biotite+magnetite 

vein, with biotite largely chloritized; fluid inclusionsigneous quartz has sparse types 1, 2, and 3, and vein 

quartz sparse type 1 

83-PC-253: Equigranular phase, moderate deuteric alteration; cut by 1-cm quartz+hornblende+Kfeldspar+sphene vein, with quartz interstitial to euhedral 

hornblende, K-feldspar strongly perthitic; plagioclase 

phenocrysts along vein margin overgrown with K-feldspar; fluid inclusions-both igneous and vein quartz 

have abundant type 1 

83-PC-274: Porphyritic phase, generally weak deuteric 

alteration; cut by 2-3 mm quartz+( actinolite) vein with 

thin K-feldspar center line; weak sericitic alteration 

along vein margin; fluid inclusions-both igneous and 

vein quartz have abundant types 1 and 3 and sparse 

type2 

83-PC-275: Equigranular phase, moderate deuteric alteration, biotite partially replaces hornblende; cut by 5-7 

mm vuggy quartz+pyrite vein with 1-cm sericitic selvage; fluid inclusions-igneous quartz has sparse types 1 

and 3, and vein quartz sparse type 1 

83-PC-276: Subporphyritic (same outcrop as 83-PC275), weak deuteric alteration; cut by 3-mm vuggy 

quartz vein with thin K-feldspar center line; biotite 

weakly chloritized along vein edge; fluid inclusions-igneous quartz has moderately abundant types 1 and 3, and 

vein quartz sparse type 1 

83-PC-279: Equigranular phase, weak deuteric alteration; cut by 5 mm, vuggy quartz+pyrite+chlorite vein 

(pyrite+chlorite may be late vug filling), trace calcite in 

vugs, moderate to strong sericitic alteration along vein 

SO Evolution of Hydrothermal Fluids in the Alta Stock, Central Wasatch Mountains, Utah 
edge; fluid inclusions-both igneous and vein quartz 

have sparse type 1 

83 -PC-292: Equigranular phase, weak to strong deuteric 

alteration; cut by 2-mm quartz+hornblende+Kfeldspar+sphene vein, with quartz interstitial to euhedral 

hornblende and sphene; fluid inclusions-igneous quartz 

has sparse type 1, and vein quartz moderately abundant 

type 1 

83-PC-312: Equigranular phase, weak to moderate deuteric alteration; cut by 1-cm granular quartz vein with 

weak sericitic envelope; fluid inclusions-igneous quartz 

has moderately abundant type 1 and sparse type 4, and 

vein quartz abundant type 1 and sparse type 4 

83-PC-315: Equigranular phase, weak to strong deuteric 

alteration; cut by two veins: (1) 5-mm quartz+K-feldspar 

aplite that was opened and filled by (2) 5-mm quartz+ 

biotite+epidote+magnetite+chalcopyrite+molybdenite 

vein; fluid inclusions-both igneous and vein quartz 

have moderately abundant type 1 and sparse type 4 

81-PC-11: Porphyritic phase, moderate deuteric alteration; fluid inclusions-sparse type 1 

82-PC-27: Porphyritic phase, moderate deuteric alteration; fluid inclusions--sparse type 1 

82-PC-51: Fine-grained, equigranular phase, weak to 

moderate deuteric alteration; fluid inclusions-moderately abundant types 1, 2, and 3 

82-PC-58: Equigranular phase, weak to moderate deuteric alteration; fluid inclusions-moderately abundant 

type 1 

84-PC-36: Equigranular phase, generally moderate deuteric alteration; cut by 3-mm quartz+pyrite+chlorite+ 

(epidote) vein with weak sericitic envelope; fluid 

inclusions-igneous quartz has abundant types 1, 2, and 

3, and vein quartz abundant type 1 

S-30: Equigranular phase, moderate deuteric alteration; 

fluid inclusions-abundant types 1 and 3, sparse type 2 

S-38: Equigranular phase, generally weak deuteric alteration; fluid inclusions-abundant types 1, 2, and 3 

84-PC-40: 4-5 em vuggy granular quartz+(pyrite) vein 

with broad sericitic envelope cutting sample S-38; fluid 

inclusions-sparse to abundant type 1 

Appendix 1. Description of Samples Used in Fluid-inclusion Studies 51 

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