Duration of Hydrothermal Activity at 

Steamboat Springs, Nevada, From Ages of 

Spatially Associated Volcanic Rocks

GEOLOGIC AiL SURVEY PROFESSIONAL PAPER 458-D
Duration of Hydrothermal Activity at 

Steamboat Springs, Nevada, From Ages of 

Spatially Associated Volcanic Rocks

By M. L. SILBERMAN, D. E. WHITE, T. E. C. KEITH, and R. D. DOCKTER 

GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA

GEOLOGICAL SURVEY PROFESSIONAL PAPER 458-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data 

Main entry under title:

Duration of hydrothermal activity at Steamboat Springs, Nevada, from ages of 

spatially associated volcanic rocks.

(Geology and geochemistry of the Steamboat Springs area, Nevada)

(United States. Geological Survey. Professional paper ; 45 8-D)

Bibliography: p. D13-D14.

1. Geothermal resources-Nevada-Steamboat Springs. 2. GeologyNevada Steamboat Springs. 3. Potassium-argon dating. 

I. Silberman, Miles L. II. Series. III. Series: United States. 

Geological Survey. Professional paper ; 45 8-D.

QE75.P9no. 458-D [GB1199.7.N3] 557.3'08s [553] 79-16870

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03215-5
CONTENTS

	Page

Abstract __________ _______.._____________ Dl

Introduction ______________________________________________ 1

Regional geologic relations ________________________________ 1

Geology of the area near Steamboat Springs ___________ 3

Structural control of hot-spring activity ________________ 3

Alteration and ore minerals___________ _______________ 5

Ore minerals, trace-metals, and isotopic compositions____ 5

Geologic limits on the time span of hydrothermal activity _ 6

Potassium-argon ages______________________________________ 7

Samples ______________________________________________ 7

Basaltic andesite _____________________________________ 8

Potassium-argon ages Continued

Rhyolite domes______________________ _ D8

Altered basaltic andesite _____________ ______________ 10

Interpretation ____________________________________________ 10

Implications of the age of the Steamboat Springs thermal

system __________________________________________________ 10

Steamboat Springs as related to other thermal systems and

hydrothermal ore deposits ____________________________ 11

Methods of sample preparation ____________________________ 12

References cited ___________________________________ 13

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of Storey and southern Washoe Counties, Nevada ___________ ____________________________ D2

2. Generalized geologic map of the region near Steamboat Springs, Nevada __________ _____ ________-----__ 4

3. Diagram showing composite stratigraphic relations at Steamboat Springs, Nevada ______________ 6

TABLES

TABLE 1. Analytical data and radiometric ages for volcanic rocks from the Steamboat Springs region 

2. Timespans of geothermal and ore-forming epithermal systems _________________________ .

Page

Dl

2

III

GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA

DURATION OF HYDROTHERMAL ACTIVITY AT STEAMBOAT

SPRINGS, NEVADA, 

FROM AGES OF SPATIALLY ASSOCIATED VOLCANIC ROCKS

By M. L. SILBERMAN, D. E. WHITE, T. E. C. KEITH, and R. D. DOCKTER

ABSTRACT

Steamboat Springs is a presently active equivalent of epithermal 

gold-silver ore-forming systems. Hot-spring sinter deposits contain 

small amounts of gold, silver, mercury, antimony, and arsenic. 

Hot-spring activity probably started before extrusion of the basaltic 

andesite of Steamboat Springs. Old sinter from the Steamboat 

Springs system occurs in gravels above and below the basaltic 

andesite. Intense hydrothermal alteration, including almost complete replacement by hydrothermal potassium-feldspar, has affected 

the basaltic andesite. Three plagioclase separates of differing potassium content from fresh basaltic andesite yielded potassium-argon 

ages of 2.52-2.55 m.y. Basaltic andesite almost completely replaced 

by potassium-feldspar yielded an age of 1.1 m.y.

The thermal area lies approximately on a line connecting four 

rhyolite domes, the largest of which is 3 km southwest of the 

springs and the others are to the northeast. Several domes occupy 

vents from which tephra of pre- and post-basaltic andesite age was 

erupted, as indicated by pumice blocks up to 5 cm diameter in 

gravel underlying the andesite, and isolated pumice fragments 

lying on the andesite. The source of energy for the thermal convection system is probably a large rhyolitic magma chamber that 

supplied the pumice and from which the rhyolite domes were 

emplaced. Sanidine and obsidian from four of the rhyolite domes 

yielded potassium-argon ages of 1.15-1.52 m.y. and obsidian from 

one of the northeastern domes yielded apparent ages of 2.97 and 

3.03 m.y.

The data indicate that hydrothermal activity has occurred at 

Steamboat Springs, possibly intermittently, for more than 2Vz m.y. 

These data agree with other radiogenic age studies indicating 1- 

and 2-m.y. lifetimes for the hydrothermal systems that generate 

epithermal gold-silver deposits.

INTRODUCTION

The timespan and continuity of hydrothermal systems are important to establish for two reasons. First, 

the geothermal energy that may be recoverable from 

active systems has been related to the age of hotspring activity and its volcanic heat source, through 

Smith and Shaw's (1975) magmatic model. Second, a 

major group of active hydrothermal systems, closely 

associated with large rhyolitic systems, is similar in 

water composition, metal content, subsurface temperature, and hydrothermal alteration to epithermal 

gold-silver-depositing systems (White, 1955, 1974). 

Because some active hydrothermal systems, including 

Steamboat Springs, seem to be the present-day equivalents of the ore-forming systems, they help to enhance our understanding of ore-forming processes. 

Few direct data on the limits of age, duration, and 

continuity of activity are yet available either for active hot-spring systems or for epithermal ore-forming 

systems of the past.

Our study provides much age data for a notable active hot-spring system. Steamboat Springs and the 

surrounding region have been thoroughly studied 

by geologic, geochemical, and hydrologic methods 

(Thompson and White, 1964; White and others, 1964; 

White, 1968). The hot-spring activity has occurred 

over a relatively long period of time. Uplift and erosion of the thermal area alternated with burial by volcanic rocks, alluvium, and hot-spring deposits, thus 

preserving much geologic evidence that permits reconstruction of its history. Potassium-argon ages, now 

available for many of the fresh and altered young volcanic rocks in and near the thermal area, permit precise estimates to be made for the total time span of 

hydrothermal activity. However, we do not yet know 

whether the activity has been continuous or intermittent through this timespan.

REGIONAL GEOLOGIC RELATIONS

The Steamboat Springs area is located near the 

northeast end of Steamboat Hills, a small northeasttrending range that lies between the north-trending 

Carson Range on the west and the Virginia Range on 

the east (figs. 1 and 2). The springs are noted for 

transport and deposition of mercury, antimony, gold, 

silver, and other metals and gangue minerals (White 

and others, 1964).

Dl
D2 GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA

120°00'

39°30' -

39°15'

EXPLANATION

Gravel, alluvium, sinter deposits

120 00'

15 MILES

Basalt, andesite, and dacite

Includes McCellan Peak Olivine Basalt 

(Pleistocene), Mustang Andesite (Pleistocene?), 

Lousetown Formation (Pleistocene to Miocene), 

and basaltic andesite of Steamboat Springs (Pliocene)

Rhyolite

Includes Steamboat Hills Rhyolite 

(Pleistocene and late Pliocene?) 

and Washington Hill Rhyolite 

(Pliocene)

Volcanic rocks,, mostly intermediate

Includes Alta and Kate Peak

Formations, and Hartford Hill

Rhyolite (Miocene)

Includes Davidson Granodiorite (Miocene)

and other Tertiary granitic rocks

Metamorphic rocks

Contact

s

oo

00

2

H

Approximate location of K-Ar sample. 

Numbers refer to numbers in table 1

FIGURE 1. Generalized geologic map of Storey and southern Washoe Counties, Nevada. Geology modified from Bonham (1969).

In the region surrounding Steamboat Springs (fig. 

2), pre-Tertiary Triassic (?) metamorphic rocks and 

Cretaceous granitic rocks that form the basement are 

overlain by a sequence of middle Tertiary to Pleistocene volcanic rocks that has been discussed in detail 

by Thompson and White (1964) and Bonham (1969). 

Rock units considered in this report and generalized

on figure 2 include the Alta and the Kate Peak Formations of Miocene age, the Lousetown formation, including local basaltic andesite of Steamboat Springs of 

Pliocene age, the Steamboat Hills Rhyolite of late 

Pliocene (?) and Pleistocene age, and alluvium of late 

Pliocene to Holocene age. The geologic relations in the 

vicinity of Steamboat Springs are summarized here
DURATION OF HYDROTHERMAL ACTIVITY FROM AGES OF VOLCANIC ROCKS D3

from earlier papers in this series (Thompson and 

White, 1964; White and others, 1964). Ages of the volcanic rocks in the region are based on potassiumargon dates, largely summarized by Silberman and 

McKee (1972). Two rhyolite domes northeast of the 

area of figure 2 are Miocene in age, and one rhyolitic 

dome east of the area of figure 2 is correlated with the 

Steamboat Hills Rhyolite and is Pleistocene in age.

GEOLOGY OF THE AREA NEAR 

STEAMBOAT SPRINGS

Pre-Tertiary metamorphic and granitic rocks are 

exposed in and near the thermal area (fig. 2). Soda trachyte flows of the Alta Formation and andesite flows 

of the Kate Peak formation are the most widespread 

volcanic rocks near the thermal area. Dikes and extrusive tuff-breccia of the Kate Peak Formation occur 

at depth (as shown in drill core) beneath younger alluvium and sinter. A basaltic andesite flow of the 

Lousetown Formation up to 24 m thick, informally designated the Steamboat basaltic andesite by White, 

Thompson, and Sandberg (1964), and herein called 

basaltic andesite, caps much of the area to the west, 

southwest, and south of the hot spring sinter deposits 

(fig. 2). Four small pumiceous rhyolite domes lie on a 

line that extends from about 3 km southwest to 5 km 

northeast of the hot springs (Thompson and White, 

1964, pis. 1, 2). A related rhyolite, perhaps a shallow 

intrusion, is suspected from structural evidence to 

underlie sinter deposits of Sinter Hill along the same 

linear trend (White and others, 1964, pi. 2, p. B51).

Rhyolitic pyroclastic material was ejected from several of the vents now occupied by domes of Steamboat 

Hills Rhyolite, shown on figure 2. Other rhyolitic 

domes occur in the region and are shown on fig. 1 and 

discussed by Thompson and White (1964). These include the Mustang dome and other smaller intrusions 

in the Pah-Rah Range north of the Truckee River canyon (loc. 7, fig. 1) and the Sutro and Washington Hill 

domes in the Virginia Range (Iocs. 6 and 8, fig. 1). 

These domes range in age from about 1.2 to 10.9 m.y., 

according to the radiometric data, indicating a long 

history of rhyolite magma generation in the region.

Alluvial deposits of pre-Lousetown and postLousetown age (pre-Lake Lahontan) are exposed near 

the springs. The pre-Lousetown alluvium lies on a 

pediment surface cut before eruption of the basaltic 

andesite, and is included with this andesite in figure 

2. Chalcedonic and opaline hot-spring sinter deposits 

occur over large parts of the thermal area, particularly in its northern and eastern parts. At the borders 

of the sinter terraces, sinter is interbedded with alluvial deposits, and some sinter is still forming from the 

active springs.

Alteration has affected nearly all of the rock units 

exposed in the thermal area. Veins and veinlets of 

opal, chalcedonic quartz, quartz, and calcite cut the 

granitic and volcanic rocks and sediments, as shown 

in drill core. The opal and chalcedony veinlets are 

largely restricted to depths of less than 25 m. Veins 

up to 2 l/2 m thick of interlayered quartz, chalcedony, 

and calcite are relatively common at greater depths 

below the hot spring terraces, with calcite being especially abundant at depths greater than 50 m.

Present spring discharge is localized along the Low 

and Main Terraces at altitudes below 1,530 m (4,670 

ft) in the eastern part of the thermal area (fig. 2). The 

top of the convecting thermal water under the western 

part of the thermal area at altitudes ranging from 

1,540 to 1,660 m (4,700 to 5,050 ft) is now from 15 to 

more than 35 m below the surface. Discharge at the 

surface in these western areas is now restricted to 

warm and hot vapors, but these western areas were 

formerly major loci of hot-water discharge and sinter 

deposition, related to higher ground-water levels in 

the past.

STRUCTURAL CONTROL OF HOT-SPRING ACTIVITY

Extensive normal faulting has occurred in and near 

the Steamboat Springs area (generalized in fig. 2). 

Most major fault movement occurred after Kate Peak 

volcanism and before cutting of the pre-Lousetown 

pediment (White and others, 1964). No displacement 

is demonstrably younger than the Lake Lahontan and 

Holocene sinter deposits and sediments (White and 

others, 1964), although some younger faulting seems 

likely because local earthquakes are frequent in the 

area (White and others, 1964, p. B53-B54). Fissures 

controlling the thermal activity may be maintained in 

part as open channels by periodic slight movement. 

However, no vertical displacement is evident on 

fissures in near-surface sinter. The fracture systems of 

the sinter terraces are clearly tectonic in origin, although chemical attack by sulfuric acid and disintegration adjacent to these fissures maintain them as 

open channels of varying width near the surface. The 

Steamboat Springs fault system, which bounds the 

eastern margin of Steamboat Hills and underlies the 

Low and Main Terraces (shown as faults, fig. 2) is a 

major fault system with displacements of at least 300 

m, mostly older than the pre-Lousetown pediment 

gravels (White and others, 1964).

The thermal area lies along a line of Quaternary 

pumiceous rhyolite domes (fig. 2) that extend 3 km 

southwest of Steamboat Springs and 5 km to the 

northeast. Structural tilting away from the crest of 

Sinter Hill (loc. 2, fig. 2) suggests the presence of 

another rhyolite dome or shallow intrusion beneath
D4 GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA

119° 45'

39°25'

39° 20'

3 1 2 

I I I 1

1 1

3 1

3 

I

I

2

4 

I

c

I

I

3 f

5 KILOMETERS

FIGURE 2. Generalized geologic map of the region near Steamboat Springs, Nevada. Geology modified from Thompson and White (1964).
DURATION OF HYDROTHERMAL ACTIVITY FROM AGES OF VOLCANIC ROCKS D5

EXPLANATION

(Holocene and Pleistocene)

Hot spring sinter

(Holocene to late Pliocene)

(Pleistocene to late Pliocene)

Steamboat Hills Rhyolite

(Pleistocene and late Pliocene?)

Basaltic andesite of Steamboat Springs

Kate Peak and Alta Formations

Intermediate rocks (Miocene)

- QUATERNARY

TERTIARY

AND 

QUATERNARY

- TERTIARY

- CRETACEOUS

- TRIASSIC(?)

Metamorphic rocks

Contact

Fault 

Ball and bar on downthrown side

K-Ar sample locality

Numbers refer to numbers

in table 1

FIGURE 2. Continued.

this hill (White and others, 1964, pi. 2). The line of 

rhyolite domes also parallels a major set of faults in 

the Steamboat Hills (not shown on fig. 2; see White 

and others, 1964, pi. 1). The hills have a northeasttrending anticlinal form produced by a combination of 

warping and tilting of fault blocks (Thompson and

White, 1964). The alinement of domes and the trend of 

the hills probably have a common origin as parts of a 

major structural zone of weakness.

ALTERATION AND ORE MINERALS

Hydrothermal alteration has affected most of the 

igneous rocks and sediments in the thermal area. The 

characteristics of hydrothermal alteration of the 

Steamboat Springs system and the extent of nearsurface leaching are summarized by White, Thompson, and Sandberg (1964) and Schoen, White, and 

Hemley (1974). Acid sulfate alteration is generally 

dominant near the surface and above the water table 

where hydrogen sulfide rises and oxidizes to sulfuric 

acid, chemically attacking the rocks (Schoen and 

others, 1974). Where generation of acid has been 

abundant, the end result is replacement of the silicate 

minerals by opal or cristobalite, except for relict original quartz. Below the water table of an acid-leached 

area, the silicate minerals are generally replaced by 

alunite and kaolinite (Sigvaldason and White, 1962). 

Drill cores show that this advanced argillic alteration 

dies out with depth. In one drill hole in the Thermal 

area, kaolinite extends to -83 m. At greater depths in 

this hole and in other drill holes at shallower depths, 

albite and the clay minerals montmorillonite, illite, 

and chlorite dominate most alteration assemblages. 

Pyrite is usually disseminated in the altered rocks. 

Calcite and rarely manganiferous calcite occur as 

veins. Hydrothermal monoclinic potassium feldspar 

(adularia) is an abundant replacement of plagioclase 

in several drill holes but does not occur in veins. 

Adularized rocks change at greater depth to a more 

complex assemblages of illite, other clay minerals, 

adularia, calcite, chlorite, and pyrite. A sample of 

basaltic andesite that was almost completely replaced 

by adularia, quartz, and minor celadonite was dated 

in this study.

ORE MINERALS, TRACE-METALS, AND ISOTOPIC 

COMPOSITIONS

Sinter at Steamboat Springs generally contains detectable quantities of gold and silver, and as much as 

10 ppm of gold and nearly 40 ppm of silver have been 

reported in siliceous muds deposited from the springs 

(Brannock and others, 1948). These muds also contain 

up to 0.02 percent mercury and 3.9 percent antimony. 

Fine-grained cinnabar is disseminated in old chalcedonic sinter of Sinter Hill, and elemental mercuiy 

has been identified in vapors from several drill holes 

and hot-spring vents. Stibnite has been deposited as 

heedlelike crystals in hot-spring pools, and in veinlets
D6 GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA

and cavities to depths as great as 45 m. Veins of chal 

cedony, quartz, and calcite up to 2V2 m thick contain i 

few rich stringers of pyrargyrite (Ag3SbS3 ).

White (1955, 1974) has emphasized the similarity in 

trace-metal association and alteration mineralogy between hot-spring systems such as Steamboat Springs 

and epithermal gold-silver deposits. The veins and 

alteration assemblages at Steamboat Springs are in 

distinguishable from those of many vein gold-silver 

deposits, but as yet no large mass of ore metals o 

economic grade has been found by deep drilling at 

Steamboat Springs or in any other active hot-spring 

system.

Stable isotopes of spring waters indicate an almost 

exclusively meteoric source for the thermal waters 

but the data are such that as much as about 10 per 

cent of magmatic water would not be detectable 

(White, 1974). The isotope relations show an oxygen 

shift, or increase in 8ISO, of 2 to 3 per mil, which is 

common in high-temperature geothermal waters. The 

shift results largely from interaction of meteoric 

water and rock silicates during alteration and, commonly in small part, from boiling (Truesdell and 

others, 1977). The system is probably recharged from 

both the Virginia and Carson Ranges, with only 

minor near-surface dilution from local precipitation. 

Isotopic studies of the waters and rocks from active 

hot-spring systems and epithermal ore-depositing systems (Taylor, 1974; O'Neil and Silberman, 1974; and 

White, 1974) demonstrate that many of these systems 

have been strongly influenced by water of meteoric 

origin. Steamboat Springs has isotopic similarities to 

many high-temperature hydrothermal systems, both 

active and fossil, elsewhere in the Great Basin and 

the world. Hence, information on the duration of hydrothermal activity in this presently active system 

yields valuable limits on the time spans of fossil oredepositing systems.

GEOLOGIC LIMITS ON THtf TIME SPAN 

OF HYDROTHERMAL ACTIVITY

The limits on the time span of activity in the 

Steamboat Springs systems come largely from the 

mapped geologic relations of the volcanic and sedimentary rock units (fig. 3) and from radiogenic ages 

of fresh and altered rocks in and near the thermal 

area. The youngest sinter deposits are porous opaline 

silica that is amorphous by X-ray. With increasing 

depth of burial, consequent higher-than-surface temperatures, and time, opaline sinter is first converted 

to /3-cristobalite, then frequently to a-cristobalite, 

and finally to chalcedonic sinter (White and others, 

1964). The interrelations of the sinters, the alluvial

Alluvium

Lake Lahontan 

alluvium

-Erosional disconformity

Pre- Lake. Lahontan 

alluvium

Rhyolite pumice

f ^

( Major break in the deposition of 

sinter and alluvium)

Cristobalite 

and calcedonic 

sinter (largely 

chalcedonic)

(Much undeciphered history)

Rhyolite 1.2 m.y. 

(with local dome or

Basaliticandesite (2.5 m.y.)

A

' 

Fed' ent

<RhyofitS^'SirTter ^iCpbbleso?

-Hchatcedoniclp> chalcedonic

sinter

FIGURE 3. Composite stratigraphic relations at Steamboat Springs, 

Nevada. Constructed from geologic data of White, Thompson, and 

Sandberg (1964) and Thompson and White (1964).

deposits, and the dated volcanic rocks provide geologic limits on the inferred time span of hydrothermal 

activity. However, specific ages are not nearly abundant enough to determine whether or not activity 

was continuous in the area, or if the recognized intervals of nondeposition of sinter at specific sites represent short intervals or major parts of the total time 

span. Our present interpretations are represented by 

figure 3, but times of sinter deposition may be greatly 

exaggerated. 

A complex history of erosion alternating with depoition of alluvium and hot-spring sinter throughout 

the late Tertiary and Quaternary is revealed from

urface mapping and subsurface drill-hole data (White 

and others, 1964). In general, the hot-spring deposits 

are interpreted as local facies of the sedimentary formations during periods of alluviation; interfingering 

of alluvium and sinter is largely inferred rather than 

observed because the few drill holes were sited near 

;he centers of hot-spring activity and sinter deposition, which are usually positive landforms rising 

above surrounding areas of alluviation. Interpreted 

 elations are best illustrated in sections A-A' and 

B-B' of plate 2 of White, Thompson, and Sandberg

1964).

Indirect evidence indicates that the oldest hotipring deposits antedate the local basaltic andesite 

flows of the Lousetown Formation (reviewed by White
DURATION OF HYDROTHERMAL ACTIVITY FROM AGES OF VOLCANIC ROCKS D7

and others, 1964, p. B34). Chalcedonized cinnabarbearing sinter in the western part of the thermal area 

was incorporated in pediment gravels that underlie 

the basaltic andesite. Thus, basaltic andesite is the 

most important single stratigraphic unit in establishing the total time span of hydrothermal activity.

Post-Lousetown sinter deposits are thick and extensive and have been mapped by White, Thompson, and 

Sandberg (1964) as two major units; the older is 

largely chalcedonic and cristobalitic, and the younger 

consists of largely of amorphous opal. Each major unit 

probably consists of an older and a younger part that 

can be distinguished locally, especially in drill holes, 

but not throughout the area. The minimum age of the 

older part of the chalcedonic sinter is established by 

the radiometric age of hydrothermal potassium 

feldspar that replaced the underlying basaltic andesite, as discussed in a later section.

The drill-hole data were first interpreted (White 

and others, 1964) as indicating continuous or nearly 

continuous activity during much of the Quaternary, 

especially including the last three glacial periods 

when alluviation and water tables were at high levels. 

During periods of deep erosion and low water table, 

according to this explanation, springs did not always 

emerge at the surface, but thermal waters continued 

to circulate below the surface. A second alternative 

assumes that surface and near-surface activity was 

not continuous. Perhaps for relatively long periods of 

time, no water was discharged from the system, even 

below the surface into surrounding ground. The first 

alternative was preferred by White, Thompson, and 

Sandberg (1964), but the span of time indicated by 

present data is so long and the required quantities of 

total heat and rare soluble constituents are so large 

that the hypothesis of continuous activity must be 

reevaluated.

The rhyolite domes nearby (fig. 2) are next in importance to the basaltic andesite in establishing the 

time span of activity in the thermal system. A 

pumiceous rhyolite dome was extruded about 5 km 

southwest of the thermal area. Three pumiceous 

domes are from 2 to 5 km to the northeast (Iocs. 3, 4, 

and 5), fig. 2, and another pumiceous dome is 20 km 

to the east-southeast (loc. 6, fig. 1). All of these domes 

are similar in mineralogy, bulk chemistry, abundance 

of phenocrysts, and degree of erosion, and all were included by Thompson and White (1964) in their 

Steamboat Hills Rhyolite.

Other domes to the north and northeast (Iocs. 7 and 

8, fig. 1) consist of devitrified rhyolite and, locally, 

perlite. Although they are chemically similar to 

domes of the Steamboat Hills Rhyolite, their degree of 

devitrification and erosion and their relations to the

Kate Peak and Coal Valley ("Truckee") Formations 

indicate that these northern domes are older 

(Thompson, 1956; Bonham, 1969).

The dome in Steamboat Hills (fig. 2) was extruded 

into a crater from which pumice and older rock fragments had been ejected; this dome is also capped by 

the eroded remnants of a small crater from which some 

pumice was erupted. Pumice fragments tentatively 

correlated with the earlier pyroclastic eruption are 

locally abundant in pediment gravels beneath flows of 

the basaltic andesite (Thompson and White, 1964; 

White and others, 1964), and pumice lapilli correlated 

with the last explosion are widely but sparsely scattered over the surface of the same flows. Thus, the 

basaltic andesite flows and the rhyolite domes had 

been considered almost contemporaneous in age. This 

conclusion now requires modification on the basis of 

potassium-argon dates.

POTASSIUM-ARGON AGES

SAMPLES

Samples from most of the pumiceous rhyolite domes 

in the region were collected for dating. The rocks consist of small crystals of sanidine, sodic plagioclase, 

quartz, sparse biotite flakes, and spherulitic crystallites in a pumiceous glassy matrix. The rhyolites of 

Mustang dome (No. 7, fig. 1) and the Washington Hill 

dome (No. 8, fig. 1) are much denser than the pumiceous rhyolites (probably because the pumiceous mantle was eroded) and consist of small phenocrysts of 

plagioclase and spherulites in a devitrified groundmass. Sparse small biotite crystals occur in the Washington Hill dome, and vesicles in the rhyolite of Mustang dome are partly filled with tridymite.

Feldspar mineral separates were prepared from all 

the rhyolite samples. A biotite separate was also obtained from the Washington Hill dome, and obsidian 

samples were collected from the domes on the flanks 

of the Virginia Range (Nos. 5 and 6, figs. 1 and 2).

The basaltic andesite contains abundant plagioclase, common olivine and ilmenite, and very sparse 

pyroxene phenocrysts, and a few partly resorbed 

quartz crystals. The largest plagioclase phenocrysts 

commonly contain cores or intermediate zones showing rounded resorption boundaries and a distinctive 

grained texture (White and others, 1964, p. B36- 

B37) that is recognizable even in rocks completely replaced by opal or by hydrothermal potassium feldspar. 

Groundmass feldspar is cloudy with inclusions and 

consists of at least two species, one of which is anorthoclase overgrowths on plagioclase crystals. The anorthoclase may grade into sanidine in the groundmass (Schoen and White, 1967). Feldspar separates
D8 GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA

of different densities were prepared from the basaltic andesite by techniques described at the end of 

this report, and a whole-rock sample was also prepared for dating. A sample of altered basaltic andesite 

from a depth of 30 m in GS-6 drillhole (loc. 2, fig. 2), 

almost completely replaced by potassium feldspar 

with minor celadonite and iron oxide after olivine, 

was also prepared for dating. The altered basaltic 

andesite, recognizable from its characteristic texture 

and high TiO2 content, was intersected between 28 m 

and 40 m (cross section /-/', pi. 2, White and others, 

1964).

BASALTIC ANDESITE

The indicated ages of the three feldspar separates of 

the basaltic andesite, which differ in K2O content by 

a factor of 4, agree well at approximately 2.5 m.y. (table 1, loc. 1). The whole-rock age is slightly lower, 

possibly because argon was lost from the fine-grained 

interstitial potassium-rich material, a fraction of 

which is undevitrified glass.

RHYOLITE DOMES

Ages of feldspars from the pumiceous rhyolite 

domes near the thermal area all agree within experimental uncertainty at approximately 1.2 m.y. The 

Sutro dome (loc. 6, fig. 1) has concordant obsidian and 

plagioclase ages of 1.5 m.y. and thus is slightly older 

than the other three age-dated domes of the Steamboat Hills Rhyolite (table 1, Iocs. 3-6). Two separate 

samples of obsidian were collected from the dome at 

locality 5 (fig. 2). One was an obsidian marble 

(marekanite) from the top of the dome (sample 

SR-5A, table 1), and the other was a nonperlitized residual of similar shape collected from a trench cut in 

perlitic obsidian (941-B). Both yielded ages of 3.0 

m.y., in marked disagreement with the sanidine age 

of 1.2 m.y. from pumiceous rhyolite from the top of the 

same dome. It is possible that the obsidian marble was 

ejected during a previous eruption and was carried up 

to its present location by extrusion of the dome. The 

age of the other sample, however, cannot be explained 

by this process. Close agreement of the ages of 

feldspars from all of the domes in the vicinity of the 

thermal area suggests that those domes were extruded at approximately the same time.

Three possible explanations could account for the 

discordance. The first is that the obsidian age represents the true age of extrusion (and cooling) of this 

dome, and that the feldspar age has been reset 

through argon loss during a later thermal episode. A 

thermal event of this magnitude that could affect the 

feldspar phenocrysts of undevitrified obsidian erupted

at the surface, without affecting the argon content of 

the obsidian, seems highly improbable.

A second possible explanation entails the presence 

of extraneous argon-40 in the obsidian sample. If 

argon incorporated from the environment was present 

in the rhyolite magma (Damon and others, 1967), it is 

possible that rapid cooling, indicated by the presence 

of the obsidian, did not permit outgassing or loss of 

this argon.

Experimental studies by Fyfe, Lanphere, and Dalrymple (1969) have demonstrated that argon is soluble in hydrous granitic melts and that a significant 

quantity of it is retained in the glass resulting from 

quenching these melts. Anomalously old ages, attributed to the presence of extraneous argon-40, have 

been reported from nonhydrated volcanic glass from a 

vitrophyre in welded ash-flow tuff in southern Nevada 

(Marvin and others, 1970); the amount of excess 

argon-40 necessary to explain the discrepancy of this 

example is on the order of 0.5 to lxlO~ n mole/g. If 

extraneous argon-40 in the glass accounts for the discordance between the sanidine and obsidian ages of 

the Truckee Meadows dome, approximately 1.3xlO~n 

mole/g of extraneous argon must be present, an 

amount approximately twice that reported by Marvin, 

Mehnert, and Noble (1970). The environment of generation of the magmas was probably similar as well. 

Both were rhyolitic and presumably resided in, and 

were erupted from, shallow levels in the crust.

A problem with this explanation is the low volatile 

content, expressed by H2O in the obsidians from this 

dome. An obsidian marble (marekanite) collected from 

the dome had a content of only 0.25 percent H2O (D. E. 

White, unpub. data). Friedman and Smith (1960) indicate that the best explanation for the low H2 O contents 

of nonperlitic glassy obsidian is loss of water during 

and before extrusion, which presumably would be accompanied by exsolution of other volatiles, including 

argon. However, in the experimental study by Fyfe, 

Lanphere, and Dalrymple (1969), in which granitic 

compositions with added water contents between 0 

and 37 percent were melted in a closed system and 

then quenched, the amount of argon remaining in the 

melt after quenching was roughly in inverse proportion to the water content, being controlled by its partial pressure relation to EbO pressure. However, the 

emplacement of rhyolite domes at surface pressure is 

an open system (as evidenced by the generation of 

pumiceous mantles), so that the experimental work is 

not directly applicable to the natural extrusion of the 

rhyolite dome. There is evidence, however, of extraneous argon in nonhydrated glass from both field and 

experimental work, and this explanation for the apparently discordant ages should be considered.
DURATION OF HYDROTHERMAL ACTIVITY FROM AGES OF VOLCANIC ROCKS D9

TABLE 1. Analytical data and radiometric ages for volcanic rocks from the Steamboat Springs area

Location 

No.

1

2

3

4

5

6

7

8

Sample

No.

128-0 A

128-0 B 

128-0 D

128-0 E

GS-6-100

SR-1

SR-4

SR-5

SR-5A

941-B

SR-3 

SR-3A

MR-2

C-3

C-3A

Rock and 

locality

Basaltic andesite, Steamboat Hills.

Basaltic andesite altered to

adularia, Steamboat

Hills.

Rhyolite dome, Steamboat 

Hills.

Rhyolite dome, Truckee 

Meadows.

__do __________________

Sutro rhyolite dome, 

Virginia Range, 

do

Mustang rhyolite dome, 

northeast of Reno.

Rhyolite dome, Washington 

Hill.

__do ____________________

Mineral

Whole rock _

Plagioclase, 

An38_H2 . Alkali

feldspar and 

plagioclase. 

Alkali

feldspar. 

Whole rock __

Sanidine ____

__do ________

do

Obsidian ____

Whole rock __

__do ________

Alkali 

feldspar. 

Obsidian ____

Plagioclase __

Biotite

Plagioclase

(percent) 1

2.25

2.27

.845 

.837 

2.68

2.70 

3.65

3.67 

10.07

10.07

9.93

9.92

9.06 

9.02

10.53 

10.53

10.53 

10.54

4.40

4.38

4.96

4.97

4.85

4.80

4.63

4.63

2.27 

2.24 

4.43 

4.29

4.58

4.38

1.039 

1.039

7.69

7.72

.812 

.813

'Potassium was analyzed using a lithium metaborate fusion flame photometry technique 

standard (Ingamells, 1970); mineral standards used for calibration. Analytical uncertainty is

Ar"'(radl Ar4 

(mole/g Ar4' 

x 10 -'r) J

0.717 0

.314 

1.014

1.369

1.453

1.792

1.525

1.893

1.801

2.094

2.074

2.078

.502 

.987

1.261

12.75 

12.22

1.168

Vad)/ 

'(total)

.30

.17 

.27

.27

.08

.06

.48

.33

.51

.78

.45

.61

.14 

.27

.34

.34 

.43

.42

Age3 

(million 

years)

2.15±0.10

2.52±0.19 

2.55±0.11

2.53±0.10

l.ltttO.ll

1.14±0.05

1.21±0.06

1.16±0.05

3.03±0.12

2.97±0.09

1.51±0.22 

1.51±0.06

8.2 ±0.3

10.9 ±0.3

9.7 ±0.3

with the lithium serving as an internal 

approximately ± 1 percent of content reported. Analysts, L. Schlocker and J. A. Christie.

2Argon was extracted from purified mineral separates or crushed and sized whole-rock samples (see text for details of sample preparation) in a Pyrex high-vacuum system using external R. F. induction heating. During fusion a calibrated spike of purified argon-38 is 

introduced. Reactive gases were removed by an artificial molecular sieve, copper-copper-oxide and titanium furnaces. Mass analyses of 

the purified argon were made with a Neir-type 60° sector 15.2-cm-radius mass spectrometer operated in the static mode. Analytical uncertainty of argon measurement is approximately 2 to 10 percent, depending on amount of atmospheric argon (Dalrymple and Lanphere, 

1969). Analyst, M. L. Silberman.

3Analytical uncertainty estimated at one standard deviation; includes uncertainties in measurement of argon and potassium.

Another problem with this explanation is the concordance in age of alkali feldspar and obsidian from 

the Sutro dome (loc. 6, fig. 1). The obsidian in this 

sample was collected from rhyolite breccia, including 

obsidian fragments, from the west flank of the dome. 

The obsidian at this locality has no extraneous argon 

relative to the feldspar. The obsidian does contain a 

higher proportion of crystallites and may have lost 

most or all or its volatiles, perhaps owing to slightly 

slower cooling than the sample from locality 5, which 

has few crystallites and consists mostly of glass.

The third explanation, favored by White, is that the 

dome at locality 5 actually includes rhyolite of two different ages, with the analytically determined ages 

being approximately correct. Although we have not 

recognized petrographic or other evidence for distinguishing two mappable units in this dome, the age

data are consistent with the geologic evidence for two 

distinct extrusive events, thus accounting for rhyolitic 

pumice both above and below the 2.5-m.y. basaltic 

andesite. Within the geologic framework, this explanation is the most easily acceptable one.

The Mustang dome (loc. 7, fig.l) yielded an 8.2-m.y. 

age from plagioclase. This dome was considered partly 

correlative in age with the Coal Valley ("Truckee") 

Formation of late Miocene to middle Pliocene age, because it and similar rhyolites intrude the lower part of 

the formation, and debris from the domes is incorporated in clastic sediments of the upper part (Bonham, 

1969). The 8.2-m.y. age places this rhyolite in the late 

Miocene according to the time scale of Berggren 

(1972), although the age is Pliocene according to the 

time scale used by Bonham (1969).

The rhyolite dome at Washington Hill (loc. 8, fig. 1)
D10 GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA

yielded ages of 10.9±0.3 m.y. (biotite) and 9.7±0.3 

m.y. (plagioclase). These ages suggest that this dome 

is also late Miocene. Thompson and White (1964) 

interpreted this rhyolite as contemporaneous with 

the upper part of their Truckee Formation (Coal Valley Formation of Bonham, 1969). The Coal Valley 

("Truckee") overlies and intertongues with the upper 

part of the Kate Peak Formation (Thompson and 

White, 1964), from which a potassium-argon age of 

12.4±0.2 m.y. was reported (Silberman and McKee, 

1972). The radiometric ages of these domes are in 

agreement with their stratigraphic relations.

ALTERED BASALTIC ANDESITE

The altered basaltic andesite from drill hole GS-6 

(loc. 2, fig. 2; Schoen and White, 1967) was dated as a 

whole-rock sample. The rock is almost completely replaced by potassium feldspar with some quartz, and 

minor celadonite, illite, montmorillonite, iron oxides, 

and sulfides. Temperatures measured as drilling progressed indicate a present temperature of about 53°C at 

the sample depth, which is too low to drive argon from 

the feldspar. However, temperatures are certain to 

have been much higher in the past, when Sinter Hill 

was an active sinter-depositing part of the whole system. If we assume that about 15 m of sinter has been 

eroded and that former temperatures and pressures 

had adjusted to the then-existing reference boilingtemperature curve, sample GS-6-100 was altered at a 

temperature near 150°C. Still higher temperatures 

may have been caused by a shallow rhyolitic intrusion under Sinter Hill (White and others, 1964). These 

possibilities suggest that the indicated age of 1.1 m.y. 

for the hydrothermal alteration of this sample may be 

ajminimum figure.

INTERPRETATION

The radiometric ages and the geologic evidence indicate that thermal spring activity has occurred over 

a time span of about 3 m.y., in part predating the 2.5 

m.y. basaltic andesite (fig. 3). Rhyolitic tephra also 

predates the basaltic andesite, perhaps correlating 

with the enigmatic potassium-argon ages of about 3.0 

m.y. obtained from the two obsidian samples of the 

dome at locality 5.

A number of small pumiceous rhyolite domes were 

extruded in the region from 1.2 to 1.5 m.y. ago, and 

these domes show clear relations to the thermal activity. The thermal area occurs on a northeast-trending 

line that connects the four domes of Steamboat Hills 

and Truckee Meadows; vertical uplift under Sinter 

Hill is likely to have been caused by a shallow intrusion correlative with these domes; and the age of hydrothermal alteration of basaltic andesite (1.1 m.y.) 

also suggests approximate contemporaneity.

A second cluster of domes near the Truckee River 

12-20 km northeast of Steamboat Springs yields considerably greater ages, from 8 to 10 m.y. These domes 

are not related to the Steamboat thermal system and, 

instead, are probably late stages of the volcanism associated with generation of the Kate Peak Formation 

and the epithermal silver-gold deposits of the Cornstock lode.

IMPLICATIONS OF THE AGE OF THE 

STEAMBOAT SPRINGS THERMAL SYSTEM

White (1968, p. C102) estimated that a magma 

chamber of at least 100 km3 underlying the Steamboat Springs area must have cooled and crystallized to 

maintain the hydrothermal system at its present rate 

of activity for 100,000 years. The total time span from 

earliest activity to the present, in the light of our new 

age data, is about 30 times more than had been assumed. If heat was supplied at a constant rate over 

this span of time (a doubtful assumption), a batholithic volume of 3,000 km3 is required.

Smith and Shaw (1975) estimated the volume of the 

magma chamber below the Steamboat Springs thermal system as between 45 and 180 km3 , with a best 

estimate of 90 km3 . Their estimate is based on a 

model that relates volume of chamber to surface distribution of exposed silicic volcanic rocks. R. L. Smith 

(oral commun., 1975) recognizes the inconsistencies 

between the two sets of estimates and suggests that a 

large magma chamber may underlie the area but at 

great depth, with abnormally low representation by 

extruded rhyolite. The Steamboat Springs system 

plots in the "low geothermal potential" field of their 

age-volume diagram (Smith and Shaw, 1975, p. 74). 

Smith and Shaw assumed an age of 1.2 m.y. rather 

than 3 m.y. as the time span of the system, but this 

makes little difference in the position of the point on 

their plot. An assumed volume of 3,000 km3 makes 

the Steamboat Springs system much more consistent 

with other magma-sustained geothermal systems.

Many large volcanic systems are now known to 

have complex histories that may span as much as 8 to 

10 m.y., with the silicic events generally occurring 

late in this history and involving two or more cycles of 

ash-flow tuff eruptions and caldera collapse. The Yellowstone Park volcanic system of Wyoming has 

undergone three such cycles, with culminations occurring 2.0, 1.2, and 0.6 m.y. ago (Christiansen and 

Blank, 1972). The Valles Caldera of New Mexico had 

two major events 1.4 and 1.0 m.y. ago (Doell and 

others, 1968). These clear indications of major pulses 

of rhyolitic magma suggest that the magma source for
DURATION OF HYDROTHERMAL ACTIVITY FROM AGES OF VOLCANIC ROCKS Dll

the Steamboat thermal system has also had major 

pulses. An assumption of a steady state for as much as 

3 m.y. should now be questioned seriously.

Smith and Shaw (1975) developed another principle 

that implies variability, the "shadow-zone" concept for 

utilizing basaltic volcanism to indicate the physical 

state and crude dimensions of an underlying silicic 

chamber. Briefly stated, this concept assumes that 

basalt from the mantle can be erupted outside of and 

contemporaneously with a molten silicic magma 

chamber, but that basalt cannot be erupted through 

silicic molten magma without mixing. A very important corollary is that if basalt is erupted through a 

silicic magma chamber, the chamber must have been 

sufficiently crystallized and cooled to sustain fracturing and formation of a conduit for the basalt. These 

concepts, as applied to the Steamboat system and in 

view of our age data, indicate the presence of a molten 

rhyolitic magma chamber of unknown volume and 

distribution about 3 m.y. ago. This silicic chamber 

could not have maintained the same dimensions in a 

molten state 2.5 m.y. ago, when flows of the basaltic 

andesite were erupted only IVk km southwest of the 

thermal area (Thompson and White, 1964). In contrast, a single molten silicic chamber probably did develop under Steamboat Hills and adjacent parts of 

Truckee Meadows and central Virginia Range roughly 

1 to 1.5 m.y. ago, engulfing the channels of the earlier 

basaltic andesite.

Approximately steady-state discharge of heat and 

dissolved constituents was favored by White (1968, p. 

CD for 10,000 or even 100,000 years: "During periods 

of deep erosion and low water table, springs did not 

always emerge at the surface, but for at least parts of 

these periods thermal waters continued to circulate 

below the surface***." Now, in view of the indicated 

long time span of perhaps 3 m.y., we must suspect 

that periods of surface discharge and sinter deposition 

alternated with periods of no activity, as indicated in 

figure 3.

STEAMBOAT SPRINGS AS RELATED TO OTHER

THERMAL SYSTEMS AND HYDROTHERMAL

ORE DEPOSITS

It is useful to compare the age of hydrothermal activity of the Steamboat Springs thermal system with 

other hydrothermal systems, both active and fossil. 

The similarities of ore-forming systems and active 

thermal spring systems indicate that their timespans 

may also be comparable. Table 2 lists the estimated 

timespans of thermal spring systems (perhaps including major interruptions) and epithermal ore-depositing systems. It is difficult to determine accurately 

the timespan of hydrothermal systems unless isotopic dating studies are made for that specific purpose. The best information on both fossil and active 

systems comes from the isotopic ages of alteration and 

vein minerals, as interpreted by geologic relations. In 

most places, such minerals are either not dated or dating is inadequate to estimate the timespan and continuity. Generally, geochronological studies in thermal areas emphasize the volcanic history. It is then 

necessary to estimate the age of hydrothermal activity 

from the available data. These ages were estimated 

for several of the systems listed in table 2 using 

geologic and stratigraphic relations of sinter deposits, 

or sediments and sedimentary rocks that have sinter 

associated with them, that are bracketed by or interbedded with volcanic rocks of known age. In many 

cases, the ages can be estimated with some precision.

According to Renner, White, and Williams (1975), 

most deeply eroded volcanic systems are associated 

with extensive hydrothermal alteration, and most of 

this alteration involves high proportions of meteoric 

water. Smith and Bailey (1968, p. 642) state that hot 

springs and solfataras are probably active throughout 

most cauldron cycles, especially during the late stages 

of each cycle. The hydrothermal stage may overlap all 

others, but it becomes uniquely characteristic only 

after the major eruptions have ceased, when it constitutes the terminal phase of waning volcanic activity. 

The few available potassium-argon ages from epithermal ore deposits (table 2) show that hydrothermal alteration and mineralization start near 

but not quite at the end stages of volcanic activity and 

generally continue for some time after the end of the 

volcanism (Silberman and others, 1972; Ashley and 

Silberman, 1976).

In many calderas, a caldera lake forms after collapse. During subsequent resurgent doming and ringfracture volcanism, the late volcanic rocks are commonly hydrothermally altered, with the lake water as 

an obvious immediate source of the alteration fluid. If 

other evidence is lacking, the ages of volcanic rocks 

associated with resurgent doming or ring-fracture volcanism are used to estimate the lifespan of the hydrothermal alteration reported in table 2. This method 

of estimating the age of hydrothermal activity is not 

considered as reliable as the others discussed above.

These data show that timespans on the order of 0.1 

to 1.5 m.y. have been recorded for thermal spring activity. Steamboat Springs, with an indicated timespan 

of about 3 m.y., may represent an upper limit for 

thermal springs that are still active. For the fossil 

systems, the ages are probably minimum estimates 

because it is not always possible to determine the earliest and latest phases of alteration or veining.

The results of the long-lived hydrothermal activity 

at Steamboat Springs, and the shorter but still relatively long activity indicated for some other active
D12 GEOLOGY AND GEOCHEMISTRY OF THE STEAMBOAT SPRINGS AREA, NEVADA 

TABLE 2. Timespans of geothermal and ore-forming epithermal systems

Locality

(active systems)

Timespan Evidence References

Valles Caldera, 

N. Mex.

Long Valley, Calif

Steamboat Springs,

Nev.

Yellowstone Canyon 

hydrothermal system, Wyoming.

The Geysers, Calif __

Sulphur Bank mine, 

Clear Lake, Calif.

Wairakei, New 

Zealand.

Orakeikorako, New 

Zealand.

Ore deposits 

Bodie, Calif __.

1.2 m.y. to present 

(probably not continuous, with present 

geothermal system 

active perhaps 

100,000 yr).

Maximum intensity 

 0.3 m.y. ago, perhaps intermittent to 

present.

~3 m.y. to present 

(probably intermittent).

Perhaps 0.6 m.y. to 

present.

>57,000 yr to 

present.

-27,000 yr to 

present.

-0.5 m.y. to present

>20,000 yr to present 

(perhaps not continuous).

8.6-7.1 m.y.

Altered postcaldera rhyolite Smith and Bailey 

domes, possibly erupted (1968), Doell and 

into a 1.2-m.y. caldera others (1968). 

lake. Later ring fracture 

domes erupted between 1.1 

and 0.4 m.y. are not altered; present active system probably younger.

Altered lacustrine sediments Bailey and others 

interbedded with dated (1976, p. 741). 

rhyolite.

Goldfield, Nev ____ 21.0-20.0 m.y.

Creede, Colo

Silver City, DeLamar, 

Idaho.

Tui mine, New 

Zealand.

24.6±0.3 m.y. in 

26.4±0.6-m.y. volcanic rocks; probably 

discontinuous.

15.6 to 15.7±0.3-m.y. 

rhyolite containing 

veins 14.8 to 

15.2±0.6 m.y.

<7 m.y. >2.6 m.y.; 2.6- 

4.0 m.y. considered 

most probable.

Sinter older and younger 

than dated basaltic andesite, 2.5 m.y.

Chalcedonic sinter associated with lake sediments 

older than 0.266 m.y. 

pumice and less than 0.6 

m.y. rhyolite flow.

Calculated minimum time 

required to account for 

stored heat, assuming 

present heat flow.

Nearly continuous sequence 

of Hg anomalies in 14Ccontrolled lacustrine sediments 3 km offshore from 

mine.

Hydrothermal eruption 

debris in Huka Falls Formation; approximate age, 

not reliably controlled.

14C dating of hydrothermal 

eruption and of 20,000-yrold tephra overlying old 

sinter, probably disconformably.

K-Ar ages of host volcanic 

rocks and wallrock alteration and vein hydrothermal feldspar in epithermal 

precious-metal deposit.

K-Ar ages of wallrock alteration minerals and hydrothermal vein alunite in 

epithermal gold deposit.

K-Ar dating of fresh and altered rocks in epithermal 

base-metal-silver deposit.

This work.

Richmond (1976, 

p. 8); R. L. Christiansen (written 

commun., 1975).

D. E. White (unpub. 

data, 1975).

John D. Sims (written 

commun., 1977).

Grindley (1965, p. 85- 

87, 0.5 m.y.); 0.3 

m.y. now considered 

more probable (written commun., 1976).

Lloyd (1972, p. 44); 

E. F. Lloyd and 

E. E. White, unpub. 

data.

Silberman and others 

(1972).

Ashley and Silberman 

(1976).

Bethke and others 

(1976).

Vein adularia in epithermal Pansze (1975). 

gold-silver; veins contained in rhyolite.

K-Ar ages of altered host 

rocks of epithermal basemetal-silver deposit contained in 16 m.y. 

andesite.

Adams and others

(1974).

hydrothermal systems, suggest that timespans on the 

order of 1 to perhaps 3 m.y. commonly may be involved in the formation of epithermal ore deposits.

METHODS OF SAMPLE PREPARATION

Samples of the Steamboat Hills Rhyolite were prepared by crushing, grinding, and sizing quantities of

rhyolite to between 0.25 to 0.1 mm. The sized rock 

material was washed, and feldspar concentrates were 

prepared by passing the material through a Frantz 

magnetic separator and heavy liquids. Samples were 

optically examined with a binocular microscope and 

then under a petrographic microscope as grain mounts 

in immersion oils. X-ray diffraction patterns were 

made of the separates for structural determination of
DURATION OF HYDROTHERMAL ACTIVITY FROM AGES OF VOLCANIC ROCKS D13

the sanidines. The samples were determined to have 

sanidine structure by the three-reflection method of 

Wright (1968).

The drill-core sample of altered basalt was prepared 

by crushing the core segment (about 5 cm long) and 

sizing to 2 to 4 mm. Splits of this size were crushed 

using a diamond crusher and ceramic pulverizer 

to 150 mesh (less than 0.1 mm) and analyzed for 

potassium. Two splits of the 2- to 4-mm fraction, each 

of about 10 g, were analyzed for argon.

The fresh basaltic andesite was the critical sample 

that provided geologic limits on the minium age of 

hot-spring activity. First, a whole-rock sample was 

prepared by hand crushing in a steel mortar and sieving to -170 mesh (88 yu,m). This sample was split and 

analyzed for argon and potassium without further 

treatment.

Petrographic and X-ray examination indicated several different feldspars: (1) plagioclase phenocrysts 

with cores of calcic oligoclase (An26-3o) with overgrowth of andesine to labradorite (An42-62), (2) smaller 

plagioclase phenocrysts that seem to grade into tiny 

groundmass laths (White and others, 1964, p. B36), 

and (3) overgrowths of anorthoclase that may grade 

into (4) fine-grained sanidine in the groundmass 

(Schoen and White, 1967). All of these varieties contained potassium, which is probably also abundant in 

the minor interstitial glass, and all of these may differ 

in their argon-retention characteristics. Thus, feldspar concentrates of different densities were prepared 

from the basalt.

Each sample was crushed by hand in a steel mortar 

and sieved to 88-44 fjun size. The sample was elutriated for 1 minute in a 20-cm-tall beaker of water. 

The process was repeated until the water was clear of 

small particles; the settled material was then dried.

Magnetite was removed with a hand magnet. The 

sample was then passed several times through a 

Frantz separator at 1.2 amps. The non-magnetic fraction consisted of alkali feldspars and plagioclase. This 

fraction was then centrifuged in bromoform, diluted to 

a density of 2.691, to separate the bulk of the plagioclase from the fraction rich in alkali feldspar. The 

plagioclase was washed and dried, constituting sample 128-0 B, which was dated.

The light fraction was centrifuged in bromoform, diluted with acetone to a density of 2.586, to separate 

lighter material (largely ex-cristobalite). The heavy 

concentrate was centrifuged in a series of mixtures of 

bromoform and acetone of densities 2.663, 2.640, and 

2.617, resulting in alkali-feldspar and plagioclase 

mixtures of density 2.663-2.691 (128-0 C), 2.640- 

2.663 (128-0 D), and 2.617-2.640 (128-0 E).

The concentrates represent a series of samples of increasing proportion of alkali feldspar relative to

plagioclase. Samples D and E, along with B (pure 

plagioclase) were split and analyzed for potassium and 

argon. The analytical data are included in table 1.

A final, nearly pure concentrate of potassiumfeldspar (>95 percent) with density between 2.586 

and 2.617 was also prepared but was insufficient for 

dating. The slightly younger indicated age of the 

whole rock is, in our opinion, likely to be due to argon 

loss from interstitial glass.

REFERENCES CITED

Adams, C. J. U., Wodzicki, A., and Weissberg, B. G., 1974, K-Ar 

dating of hydrothermal alteration at the Tui mine, Te Aroha, 

New Zealand: New Zeal. Jour. Sci., v. 17, p. 193-199.

Ashley, R. P., and Silberman, M. L., 1976, Direct dating of 

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