SESSION 15

GEMRMAL RESEARCH IN NEVADA

PRESENTED TO :

WASHINGTON, D, C B UNITED STATES DEPARTMENT OF ENERGY

PRESENTED BY:

DENNIS TI TREXLER, DIRECTOR THOMAS FLYNN, RESEARCH ASSOCIATE/GEOLOGIST

DIVISION OF EARTH SCIENCES UNIVERSITY OF NEVADA, LAS VEGAS

ENVIRONMENTAL RESEARCH CENTER 255 BELL ST, SUITE 200

RE NO^ NEVADA 89503 (702) 784-6151

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INTRODUCTION

Geothermal research i n Nevada during t h e pas t year included two programs,

d ive r se i n na ture , ye t t imely i n t h e i r consequences.

** Geochemistry, Age and Strontium I s o t o p i c Cbmposition of Late Ter t i a ry and Quaternary Basalts and Andesites in Western Nevada

has shown t h a t geothermal systems assoc ia ted with young b a s a l t i c t o a n d e s i t i c

vo lcanic rocks exhib i t unique c h a r a c t e r i s t i c s compared t o volcanic rocks

similar i n composition and age which are not assoc ia ted wi th geothermal

resources.

Techniques derived from t h i s study provide reconnaissance t o o l s that

can be used t o i d e n t i f y those volcanic rocks which have been derived from

near su r face magma chambers that maintain longevi ty through per iodic

input of heat from b a s a l t i c magmas.

Techniques developed during t h i s program can be employed i n a

reconnaissance phase of explorat ion of volcanic t e r r a n e s t o determine which

volcanic rocks were derived from near-surface, resurgent magmatic a c t i v i t y

that could provide the heat necessary f o r a v i a b l e geothermal system.

UNION Geothermal has expressed i n t e r e s t i n techniques developed from t h i s

research i n evaluat ion of t h e i r holdings i n t h e Phi l ipp ines and Malaysia.

A second research program performed during t h e pas t year was:

** Geologic and Hydrologic Research on t h e Moana Geothermal System

which provided much needed information on a heavi ly used geothermal resource.

A minimum of 140 geothermal w e l l s provide heat and.hot water t o many

residences, severa l churches and two l a r g e motels. Although development has

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been successfu l , it has a l s o been l a r g e l y unregulated and un res t r i c t ed .

Within t h e pas t few years , r epor t s on water l e v e l dec l ine , loss of artesian

pressures and widespread sur face d i sposa l of pumped geothermal f l u i d s

have r a i sed quest ions about t h e v u l n e r a b i l i t y of t h e resource t o imprudent

development and non-conservat i v e prac t ices .

With t h e support of t h e U.S. Department of Energy, a r e s e r v o i r management

program w a s i n i t i a t e d based on t h r e e e s s e n t i a l tasks:

--Resemior d e f i n i t i o n based on geologic, geophysical and geochemical data .

--Observation and monitoring of r e se rvo i r response t o h a l t demand.

- - Interact ion with appropriate regula tory agencies providing needed da ta f o r prudent resource management.

The following technica l papers provide d e t a i l s on these research

programs.

Current and an t i c ipa t ed programs include a geothermal resource def ini-

t i o n of t h e U.S. Marine Corps Base, Twentynine Palms , Cal i forn ia , as p a r t of

t h e U.S. DOE - Navy, Interagency Cooperative Agreement. The Division of

Earth Sciences, UNLV, w i l l coordinate, supervise , and i n t e r p r e t da t a derived

f r o m d r i l l i n g seven, 1,000 foot temperature gradient holes on t h e Marine B a s e .

Based on previous geophysical work by Navy Geothermal U t i l i z a t i o n Division

personnel, and review by DOE San Francisco Operations and DES r ep resen ta t ives ,

d r i l l sites have been se lec ted . I n addi t ion to confirming a low- t o modcrate-

temperature geothermal resource a t t h e base adminis t ra t ive compound, a 1,000

foot temperature gradient hole w i l l be d r i l l e d at Pisga Craters which is an

area of volcanic a c t i v i t y less than 10,000 y e a r s o ld .

Phase Two of t h i s program w i l l e n t a i l resource confirmation by d r i l l i n g

and flow t e s t i n g a large diameter geothermal production w e l l .

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A research program which has obtained the support of concerned l e g i s l a t o r s

is a cooperat ive e f f o r t between t h e Cal i forn ia Division of Mines and Geology

and t h e Universi ty of Nevada, L a s Vegas, t o evaluate r e se rvo i r c h a r a c t e r i s t i c s

and geothermal f l u i d genesis assoc ia ted wi th magmatic a c t i v i t y i n Eastern

Cal i forn ia and Western Nevada. This program would u t i l i z e t h e enormous

amount of d a t a generated as pa r t of t h e U.S. DOE - U.S.G.S. program f o r

volcanic hazards mi t iga t ion at Mammoth Lakes, Cal i fornia . The de ta i l ed

geologic, geophysical and geochemical inves t iga t ions in t h e Long Valley

Caldera w i l l be used as base l ine da t a f o r a reg iona l study of t h e geothermal

p o t e n t i a l of t h e Mono, Bodie-Aurora and L i t t l e Walker Calderas.

p r i n c i p l e emphasis of cu r ren t i nves t iga t ion focuses on volcanic hazards

mi t iga t ion , t h e r e s u l t s of t h i s work are not being in t e rp re t ed in r e l a t i o n

t o t h e p o t e n t i a l fo r high temperature geothermal resources.

t h e cooperat ive program is t o eva lua te t h e po ten t i a l f o r high temperature

steam resources i n an area of recent and p o t e n t i a l l y a c t i v e volcanism.

Since t h e

The purpose of

320

GEOLOGIC AND HYDROLOGIC RESEARCH ON THE MOANA GEOTHERMAL SYSTEM WASHOE COUNTY, NEVADA

THOMAS FLYNN AND GEORGE GHUSN. JR.

UNIVERSITY OF NEVADA, LAS VEGAS 255 BELL ST., SUITE 200

RENO, NEVADA 89503

ABSTRACT

The Moana geo the rma l a r e a is t h e l a r g e s t s i n - /

g l e low- t o modera te - tempera ture geothermal re- source i n t h e S t a t e of Nevada p r e s e n t l y employed f o r d i r e c t - u s e a p p l i c a t i o n s . Approximate ly 150 i n d i v i d u a l w e l l s , r e p r e s e n t i n g a t o t a l e s t i m a t e d inves tment of $5 t o $7 m i l l i o n , are p r e s e n t l y used t o p r o v i d e h e a t and h o t w a t e r t o more tha'n 130 p r i v a t e r e s i d e n c e s , s e v e r a l churches and two l a r g e mote l s . Al though most of t h e w e l l s are c o n s t r u c t e d t o meet t h e h e a t i n g needs of i n d i v i d u a l homes, a l a r g e - s c a l e d i s t r i c t s p a c e h e a t i n g sys t em, des igned t o supp ly h e a t t o 60 houses from a s i n g l e w e l l , is now be ing developed . p a b l e t empera tu res r a n g e from 50$ t o 99OC (120 , t o 210'F); w e l l d e p t h s r a n g e from 60 t o 400 m (100 t o 1300 f t . ) . The number of new wells coming o n - l i n e i n Moana is two t o t h r e e per month. Development of t h e r e s o u r c e has been l a r g e l y un regu la t ed and q u e s t i o n s d e a l i n g w i t h

' r e p o r t e d r e s e r v o i r t e m p e r a t u r e and water l e v e l de- c l i n e s , l o s s of a r t e s i a n f low, and f l u i d d i s p o s a l have r e c e n t l y s u r f a c e d .

' - In O c a e d a g e o l o g i c and hydro log ic r e s e a r c h program began that w a s des igned t o pro- v i d e d e t a i l e d geothermal r e s e r v o i r d a t a t o p r e s e n t or p r o s p e c t i v e d e v e l o p e r s . The program combines geophys ica l , geochemica l , and g e o l o g i c a l s u r v e y s of t h e Moana r e s o u r c e area w i t h a d r i l l i n g program f o r 5 m o n i t o r / o b s e r v a t i o n w e l l s . Data from t h i s program are s u p p l i e d d i r e c t l y to d e v e l o p e r s as w e l l a s s t a t e and l o c a l government a g e n c i e s t o p rov ide f o r p ruden t r e s o u r c e development. T h i s paper summarizes t h e program e lemen t s and de- s c r i b e s t h e p r e s e n t s t a t u s . I

/ INTRODUCTION

Bateman and Sche lbach (1975) r e p o r t e d a t o t a l o f "35 geothermal h e a t i n g sys t ems i n u s e through- ou t t h e Truckee Meadows." Ghusn ( i n T r e x l e r and o t h e r s , 1982) l i s t e d n e a r l y 120 i n d i v i d u a l w e l l s i n t h e Moana area a l o n e ( f i g . 1). Today t h e r e are approximate ly 150 i n d i v i d u a l geothermal w e l l s and t h a t number is i n c r e a s i n g by 2 t o 3 w e l l s pe r month.

The Moana area i s t h e l a r g e s t s i n g l e geo the r - m a l r e s e r v o i r i n t h e S t a t e of Nevada t h a t s u p p o r t s d i r e c t h e a t a p p l i c a t i o n s . An es t ima ted $3 t o $5 m i l l i o n has a l r e a d y been inves t ed i n r e s i d e n t i a l h e a t i n g sys tems. An a d d i t i o n a l $2 t o $3 m i l l i o n i n

I

! !

I. MAP AREA ! \ '\ L

, .*

F i g u r e 1: Loca t ion map of t h e Moana geo the rma l a r e a .

commercial money b r i n g s t h e e s t i m a t e d t o t a l i n v e s t - ment t o $5 t o $7 m i l l i o n . ]? resent development i n c l u d e s a d d i t i o n a l new i n d i v i d u a l w e l l s and a t l e a s t one proposed d i s t r i c t s p a c e h e a t i n g sys tem.

Development of t h e r e s e r v o i r h a s been l a r g e l y unmanaged and u n r e g u l a t e d . Moana is t h e s i t e of Moana Hot S p r i n g s which r e p o r t e d l y ceased t o f low 2 0 t o 25 y e a r s ago. More r e c e n t l y , an a r t e s i a n the rma l w e l l t h a t d i s c h a r g e d 5-7 GPM a l s o ceased t o flow. I n a d d i t i o n , Ghusri ( i n T r e x l e r and o thers , 1982) has 5h0WT1 t h a t r e s e r v o i r con tamina t ion can occur i n p o o r l y comple ted w e l l s . A d d i t i o n a l pro- blems i n c l u d e premature d e t e r i o r a t i o n of copper h e a t exchange r s o r s t ee l c a s i n g o r bo th as a r e s u l t o f anode-cathode r e a c t i o n s . D i sposa l of f l u i d s from geothermal w e l l s i s a n o t h e r problem t h a t has been l a r g e l y ignored .

T h i s pape r d e s c r i b e s work i n p r o g r e s s on a r e s e a r c h program t h a t f o c u s e s . on t h e g e o l o g i c and hydro log ic a s p e c t s of t h e Moana r e s e r v o i r . Much

. .- 32 1

Flynn and Ghusn of t h e d a t a i s d e r i v e d from p r e v i o u s s t u d i e s (Bate- man and Scheibach , 1975; Ghusn, i n T r e x l e r and o t h e r s , 1982; Car l son , 1982). Th i s i n fo rma t ion is supplemented by g e o l o g i c r econna i s sance of t h e sus- pec ted r e s e r v o i r r o c k s t h a t o u t c r o p t o t h e n o r t h and w e s t of Moana. A g r a v i t y su rvey t h a t extended from Moana t o Steamboat (10 miles t o t h e s o u t h ) was completed t o p o s s i b l y i d e n t i f y s t r u c t u r e s i n t h e v o l c a n i c basement ( n c r t h - t r e n d i n g normal f a u l t s ) which a r e be l i eved t o p rov ide t h e h o t water t o t h e sha l low r e s e r v o i r . Samples of t he rma l and non- the rma l f l u i d s w e r e c o l l e c t e d and ana lyzed f o r major , minor and t r a c e e l emen t s , s t a b l e i s o t o p e s of hydrogen and oxygen ( i n c l u d i n g t r i t i u m ) , and carbon-14. Many of t h e d e v e l o p e r s have coopera ted by p rov id ing d r i l l c h i p samples from t h e many w e l l s t h a t a r e p r e s e n t l y under c o n s t r u c t i o n . These d a t a , coupled w i t h t empera tu re -dep th p r o f i l e s de r ived from t h e same w e l l s have been used t o i d e n t i f y t h e r e s e r v o i r r o c k .

The f i n a l phase of t h e program c o n s i s t s of long t e r m o b s e r v a t i o n s and measurements i n s t r a - t e g i c a l l y p laced moni tor w e l l s . d r i l l e d t o a t o t a l d e p t h of 400 f e e t (one was d r i l - l e d t o 800 f e e t ) and cased w i t h 2$ i nch d i ame te r s teel p i p e . I n a d d i t i o n t o t h e l i t h o l o g i c i n f o r - mation de r ived from each h o l e , t h e water l e v e l s and t empera tu res w i l l be moni tored f o r 6 t o 1 2 months p r i o r t o , d u r i n g , and a f t e r t h e h e a t i n g season .

These w e l l s were

GEOLOGY

The Moana a r e a is l o c a t e d a l o n g t h e wes te rn edge of t h e Truckee Meadows. It is a s t r u c t u r a l b a s i n bounded on t h e east by t h e V i r g i n i a Range and on t h e w e s t by t h e Carson Range, a spu r of t h e S i e r r a Nevada. f a u l t i n g o f f s e t s T e r t i a r y v o l c a n i c s and v o l c a n i - c l a s t i c s ed imen t s and Quaternary Blluvium and ou t - wash. Late Q u a t e r n a r y a l luv ium and g l a c i a l outwash cover many Qua te rna ry and T e r t i a r y s t r u c t u r e s (Bing- ler , 1975; M i z e l l , 1975) .

L a t e T e r t i a r y and Quaternary

The l i t h o l o g i c u n i t s found w i t h i n t h e Moana area i n c l u d e a l l u v i a l d e p o s i t s of t h e Truckee Meadows, g l a c i a l outwash from Quaternary g l a c i a l p e r i o d s , T e r t i a r y l a c u s t r i n e and f l u v i a l sediments, and T e r t i a r y v o l c a n i c l a s t i c and v o l c a n i c u n i t s ( B i n g l e r , 1975) . Dri l ler ' s l o g s p rov ide some sub- s u r f a c e l i t h o l o g i c in fo rma t ion , bu t t h e q u a l i t y of t h e s e d a t a are g e n e r a l l y poor and i n c o n s i s t e n t .

The o l d e s t fo rma t ions i n t h e Moana area a r e T e r t i a r y v o l c a n i c and v o l c a n i c l a s t i c r o c k s . are g e n e r a l l y a n d e s i t i c f l ows , agg lomera te s and b r e c c i a s w i t h in t e rbedded t u f f s , and l a c u s t r i n e and d ia tomaceous sed iments . These u n i t s a r e a l l cons ide red t o be p a r t of t h e Kate Peak Formation (Biingler, 1975; Cohen and L o e l t z , 1964; Thompson and White, 1964) .

These

Unconformably o v e r l y i n g t h e Kate Peak Forma- t i o n are T e r t i a r y l a c u s t r i n e and f l u v i a l sed iments . Thompson and White (1964) a s s igned t h e s e u n i t s t o t h e Truckee Formation, f i r s t d e s c r i b e d by King (1878) . These u n i t s c o n s i s t of f ine-gra ined l a c u s t r i n e sand and s i l t d e p o s i t s w i th i n t e r c a l a t - ed g r a v e l s , v o l c a n i c l a s t i c s , d ia tomaceous s i l t - s t o n e , and d i a t o m i t e . Estimates of t h e t h i c k n e s s

of t h e Truckee Formation r ange from 2100 f e e t (Anderson, 1908) t o g r e a t e r t han 3000 f e e t (Cohen and L o e l t z , 1964) .

B i n g l e r (1975) i d e n t i f i e d t h o s e u n i t s a s so - c i a t e d w i t h t h e Truckee Formation i n t h e Truckee Meadows as t h e Sands tone of Hunter Creek. L i tho - l o g i c d e s c r i p t i o n s of t h i s format ion i n c l u d e some c l a s t i c f l u v i a l and v o l c a n i c l a s t i c members pre- v i o u s l y a s s o c i a t e d w i t h t h e Kate Peak Formation. Es t ima tes of t h i c k n e s s of t h e Sands tone of Hunter Creek r ange from 3000 t o 4000 f e e t . The lowes t member c o n s i s t s o f a sequence of fining-upwards c o a r s e g r a v e l s and c l a s t i c sed iments . Over ly ing t h i s member is a t h i c k (1000 t o 3000 f e e t ) s e c t i o n of d i a t o m i t e , d ia tomaceous s i l t s t o n e and sands tone . I n o u t c r o p s w e s t of Moana, t h e s e d ia tomaceous u n i t s are whi t e t o medium g ray i n c o l o r . The uppermost u n i t s are c o a r s e t o f i n e sands and s i l ts i n t e r - bedded w i t h d ia tomaceous sed imen t s .

The T e r t i a r y u n i t s a l l d i p t? t h e east forming a homocl ina l s t r u c t u r e from t h e Carson Range i n t o t h e Truckee Meadows. Anderson (1908) i d e n t i f i e d t h e Carson Range as a broad a n t i c l i n e and Truckee Meadows as a broad s y n c l i n e .

The Sands tone of Hunter Creek is o v e r l a i n by outwash g r a v e l s from Quaternary g l a c i a l runof f a s w e l l as a l l u v i a l f a n and pediment u n i t s from t h e su r round ing r anges . B i n g l e r (1975) d e s c r i b e s t h e s e units i n d e t a i l i n h i s s tudy on Quaternary geology o f t h e Truckee Meadows.

Pre-Holocene f a u l t i n g r e s u l t e d i n a series of h o r s t s and g rabens i n t h e Moana area and t h e w e s t - e r n Truckee Meadows. These f a u l t s j u x t a p o s e l a t e Quaternary and T e r t i a r y u n i t s i n s e v e r a l a r e a s . Younger f a u l t i n g o f f s e t s bo th Quaternary and Terti- a r y u n i t s . The Holocene a l l u v i a l u n i t s obscu re many of t h e f a u l t s i n t h e Moana area, bu t t hey are known t o e x i s t by t h e s t r a t i g r a p h i c v a r i a t i o n s and o f f s e t s s een i n w e l l l o g s o f t h e area.

Of s p e c i a l i n t e r e s t i n t h e Moana area is t h e geo log ic r e l a t i o n s h i p between t h e Sands tone of Hunter Creek and t h e Kate Peak Formation. The Kate Peak has been mapped as t h e fo rma t ion under- l y i n g t h e Hunter Creek, and ev idence of t h i s r e l a t i o n s h i p i s c l e a r l y seen i n t h e h i l l s w e s t of Moana. The Kate Peak a n d e s i t e recovered from wells d r i l l e d i n Moana c o n s i s t s of e i t h e r f i n e - g ra ined d e v i t r i f i e d r h y o l i t e / r h y o d a c i t e o r h i g h l y a l t e r e d "b lue c l a y . " Both t h e a l t e r e d and una l - t e r e d Kate Peak c o n s t i t u t e t h e r e s e r v o i r r o c k f o r t he rma l f l u i d s . Quaternary a l l u v i a l f a n d e p o s i t s of pebb les , brown c l a y , sand , and s i l t , and t h e d ia tomaceous si l t- s t o n e member o f t h e Sands tone of Hunter Creek.

The o v e r l y i n g u n i t s c o n s i s t of

GRAVITY SURVEY

T h i s su rvey w a s used t o d e l i n e a t e t h e base- ment s t r u c t u r e from Steamboat n o r t h through Moana. Three hundred and f i f t e e n s t a t i o n s were occupied , i nc lud ing 8 5 s t a t i o n s f o r which e l e v a t i o n s were a l r e a d y known and 230 s t a t i o n s whose e l e v a t i o n s w e r e ob ta ined by su rvey ing . were ob ta ined w i t h a LaCoste-Romberg Gravimeter .

G r a v i t y measurements

322

Flynn and Ghusn Reduction of t h e s e d a t a inc luded c a l c u l a t i o n

of t h e s imple Bouguer anomaly acco rd ing t o t h e 1939 I n t e r n a t i o n a l G r a v i t y Formula and t e r r a i n c o r r e c t i o n s c a l c u l a t e d by H a m m e r (1939) f o r s e l e c - ted p o i n t s and a p p l i e d t o a l l p o i n t s . From t h e r e s u l t i n g comple te Bouguer g r a v i t y v a l u e s , a n even ly spaced g r i d of v a l u e s was i n t e r p o l a t e d by computer a p p l i c a t i o n of Lap lac i an cub ic s p l i n i n g ( f i g . 2 ) . The s u r f a c e h e r e is i n v e r t e d ; a g r a v i t y h igh a p p e a r s as a d e p r e s s i o n .

F igu re 2 : Three-dimensional complete Bouguer G r a v i t y Anomaly Grid from Steamboat Hot S p r i n g s t o t h e Moana geothermal a r e a .

F igu re 2 shows a g r a v i t y h igh through t h e c e n t e r of t h e Moana area. The g r a v i t y low ( r a i s e d i n t h e d iagram) co r re sponds t o t h e low d e n s i t y d i a t o m i t e of t h e Sands tone of Hunter Creek. There is some i n d i c a t i o n t h a t t h e n o r t h - e a s t - t r e n d i n g s t r u c t u r e i n Moana co r re sponds t o f a u l t t r e n d s mapped i n t h e area. T h i s t r end may a l s o r e p r e s e n t t h e c o n t a c t between t h e Kate Peak a n d e s i t e and t h e less dense sed imentary r o c k s . The l a r g e g r a v i t y h igh ( d e p r e s s i o n ) i n t h e c e n t e r of t h e d iagram co r re sponds t o o u t c r o p s of t h e Kate Peak a n d e s i t e . F a u l t s c a r p s h e r e are d i f f i c u l t t o i d e n t i f y because o f f s e t a l o n g t h e f a u l t s is s m a l l .

THERMAL FLUIDS GEOCHEMISTRY

The Moana geothermal f l u i d s are sodium-sul fa te t ype waters t h a t show l i t t l e o r no a b s o l u t e c o r r e - l a t i o n w i t h t h e sodium-chlor ide type water from Steamboat Hot Spr ings . Although r e l a t i v e pe rcen t - ages of c a t i o n s are i d e n t i c a l f o r f l u i d s from Moana and Steamboat ( f i g . 3 ) , t h e a b s o l u t e c o n c e n t r a t i o n s of c a t i o n s and a n i o n s are d i f f e r e n t by a f a c t o r of a t least two. The thermal f l u i d s from Moana are chemica l ly s i m i l a r t o o t h e r low- t o moderate-temp- e r a t u r e geothermal f l u i d s t h a t a r e widespread throughout wes te rn Nevada.

V a r i a t i o n s i n t h e composi t ion of Moana thermal f l u i d s a r e d i r e c t l y r e l a t e d t o t h e deg ree of n i x i n g

F igu re 3: Chemical c h a r a c t e r i s t i c s of thermal and non-thermal f l u i d s from Steamboat a r e a ( 1 , 2) and Moana (3-10).

of t he rma l and non-thermal f l u i d s . These v a r i a - t i o n s a r e a l s o observed i n t h e g r a d u a l d e c r e a s e i n t empera tu re from w e s t t o e a s t a c r o s s Moana. I n g e n e r a l , a long a w e s t t o e a s t t r a v e r s e , b i c a r b o n a t e and magnesium i n c r e a s e and 3oron. s i l i c a , s u l f a t e , and ca lc ium dec rease . Mixing of thermal and non- thermal f l u i d s can a l s o be seen i n t h e Steamboat a r e a ( f i g . 3 ) . Zo lezz i S p r i n g s ( # 2 . 34OC) f l u i d composi t ion appea r s t o be t i e r e s u l t of mixing Steamboat f l u i d s (81 , 96OC) w i t h t h e s u r f a c e waters from Thomas Creek ( # l o , 10°C).

Major chemica l a n a l y s e s a r e supplemented by a n a l y s e s of t r i t i u m and car'2on-14. The carbon-14 i s o t o p i c a g e of Steamboat f l u i d s i s i n e x c e s s of 2 5 K a . The carbon-14 i s o t o p i c a g e of Moana f l u i d s r anges from 1 Ka t o 32 Ka. These a r e uncor rec t ed va lues , bu t t h e o l d e r f lu id : ; a r e h igh- tempera ture (>85OC) and are c l o s e l y a s s o c i a t e d w i t h mapped f a u l t s in t h e wes t - cen t r a l T a r t of t h e r e s e r v o i r . r h e s e f a u l t s a r e be l i eved t o be t h e u l t i m a t e s o u r c e of ho t wa te r i n Moana. T r i t i u m is v i r t u a l - l y a b s e n t i n t h e h igh- tempera ture wa te r s i n Moana (and Steamboat ) , bu t i n c r e a s e s i n t r i t i u m a r e ob- se rved i n t h e c o o l e r w a t e r s i n t h e e a s t e r n p a r t of Yoana .

LITHOLOGIC AND TEMPERATURE-DEPTH MEASUREMENTS

Drill c h i p s are c o l l e c t e d r e g u l a r l y from w e l l d r i l l e r s and deve lope r s th roughout Moana. I n a d d i t i o n , t empera tu re -dep th p r o f i l e s of geothermal w e l l s a r e completed soon a f t e r d r i l l i n g , bu t p r i o r t o hardware i n s t a l l a t i o n . F igure 4 i s a t y p i c a l l i t h o l o g i c l o g f o r t h e c e n t - r a l p o r t i o n of Moana. The accompanying t empera tu re -dep th p r o f i l e shows t h a t t h e h i g h e s t t empera tu res are achieved and main ta ined in t h e d a c i t e - r h i o l i t e (Kate Peak an-

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32 3

Flynn and Ghusn

AQ- I feet 10

50

100

I50

200

250

TEMPERATURE 'C

Figure 4: L i t h o l o g i c l o g and temperature-depth p r o f i l e of geothermal w e l l i n c e n t e r of Moana.

d e s i t e ) . To t h e w e s t and n o r t h , t h e K a t e Peak an- d e s i t e is o v e r l a i n by a t h i c k s e c t i o n of diatoma- ceous s i l t s t o n e ( t h e middle member of t h e Sand- s t o n e of Hunter Creek) , and w e l l s i n t h i s area must be d r i l l e d t o d e p t h s of 800 t o 1300 f t . t o r e a c h high-t emperatur e f l u i d s .

S u r f a c e exposures of t h e Kate Peak a n d e s i t e c rop o u t s o u t h and w e s t of t h e Moana geothermal area, and c o n s i s t of p e r l i t i c ( d e v i t r i f i e d volcan- i c g l a s s ) r h y o l i t e - d a c i t e . I n t h e subsu r face , t h e Kate Peak a n d e s i t e is a hydrothermally a l t e r e d product of t h e s u r f a c e r o c k s , and o f t e n has t h e appearance of a sandy "blue c l ay . " X-ray d i f - f r a c t i o n p a t t e r n s of t h i s c l a y show t h a t i t is com- posed almost e n t i r e l y of s m e c t i t e and no o t h e r c l a y m i n e r a l s (Dave B i sh , M L , per sona l communi- c a t i o n ) , I n a d d i t i o n , minor amounts of q u a r t z , c r i s t o b a l i t e , a l k a l i f e l d s p a r , c a l c i t e and gypsum were also d e t e c t e d i n t h e c l a y . Smec t i t e s a r e o f - t e n formed by hydrothermal a l t e r a t i o n of v o l c a n i c g l a s s . Whole rock chemical a n a l y s e s of t h e "blue c l ay" reveal anomalous c o n c e n t r a t i o n s of a r s e n i c , antimony, t h a l l i u m , gold and s i l v e r . ments probably r e p r e s e n t hydrothermal a l t e r a t i o n of t h e v o l c a n i c g l a s s i n t h e Kate Peak a n d e s i t e by m i n e r a l i z i n g f h i d s .

These ele-

Although water t empera tu res w i t h i n t h e "blue c l ay" are h igh (80 t o 95OC), a q u i f e r t r a n s m i s s i - v i t y is ve ry low. Many geothermal wel l s a r e com- p l e t e d w i t h draw-off pumps t h a t remove cooled f l u i d s from t h e w e l l bore . The pumped f l u i d s a r e then r ep laced by h o t t e r formation f l u i d s . spent f l u i d s are t r a d i t i o n a l l y disposed of i n near-

i r r i g a t i o n d i t c h e s and storm sewers.

These

OBSERVATlONlMONlTOR WELLS

I n o rde r t o a c c u r a t e l y determine t h e e f f e c t s of thermal f l u i d withdrawal du r ing t h e hea t ing season (October through March), f i v e obse rva t ion / monitor wells were d r i l l e d i n s t r a t e g i c l o c a t i o n s throughout t h e Moana geothermal area ( f i g . 5) . Four wel ls (1 , 2 , 3 , 4) were d r i l l e d t o a t o t a l dep th of 400 f e e t . The remaining w e l l (5) was d r i l l e d t o a dep th of 800 f e e t . All f i v e w e l l s were cased wi th 2% i n c h diameter steel p i p e t o t o t a l dep th . The bottom 20 f e e t of t h e c a s i n g has torch-cut p e r f o r a t i o n s and is gravel-packed. The top 50 f e e t of t h e w e l l s have cement s a n i t a r y seals. The remaining annulus i n t e r v a l is back- f i l l e d wi th d r i l l c u t t i n g s .

F igu re 5: Locat ion map of Moana area and observa- t i on lmon i to r w e l l s .

The l i t h o l o g i c u n i t s pene t r a t ed by t h e s e f i v e h o l e s i l l u s t r a t e t h e complex i t i e s of t h e Moana geothermal r e s e r v o i r . ho t water; bottom h o l e t empera tu res r ange from 35OC ( w e l l 1/2) t o an e s t ima ted 85OC (we l l 65).

A l l f i v e wells encountered

I n wells 1 and 2 , on ly two l i t h o l o g i c u n i t s were encountered, a l luvium u n d e r l a i n by diatomace- ous s i l t s t o n e (middle member of t h e Sandstone of Hunter Creek). I n wel l s 3 and 4 , t h e al luvium was u n d e r l a i n by hydrothermally a l t e r e d Kate Peak an- d e s i t e ; no d i a t o m i t e was encountered. Hole 5 was s t a r t e d i n a l luvium, pene t r a t ed t h e Sandstone of Hunter Creek ( inc lud ing a p rev ious ly unknown or- gan ic - r i ch u n i t nea r t h e base) and w a s completed i n t h e a l t e r e d a n d e s i t e .

Monitoring t echn iques f o r t h e s e wells i n c l u d e measurements of s t a t i c water l e v e l and temperatufe- d e p t h p r o f i l e s . Measurements are performed p r i o r t o t h e h e a t i n g season t o de t e rmine n a t u r a l f l u c - t u a t i o n s that r e s u l t from barometr ic changes.

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PRESENT DEVELOPMENT

Flynn and Ghusn

REFERENCES

Geothermal w e l l d r i l l i n g and comple t ion f o r i n d i v i d u a l r e s i d e n c e s is p roceed ing th roughou t Moana a t a rate of 2 t o 3 new w e l l s pe r month. Ca r l son (1982) d e s c r i b e d a geo the rma l s p a c e hea t - i ng sys tem t h a t is des igned t o d e l i v e r ho t water t o a s p a c e h e a t i n g d i s t r i c t c o n s i s t i n g of as many as 60 homes. S e v e r a l new churches and b u s i n e s s e s have developed p l a n s f o r s p a c e h e a t i n g . One e x i s t i n g mote l t h a t a l r e a d y u s e s geo the rma l waters f o r hea t - i ng has expanded and w i l l deve lop a n o t h e r geo the r - m a l w e l l t o supp ly a d d i t i o n a l h e a t t o t h e new s t r u c t u r e .

Cor ros ion of metal p a r t s i n h e a t exchange r s o c c u r s s p o r a t i c a l l y th roughou t Moana. completed w e l l s now u s e f i b e r g l a s s t u b i n g as a h e a t exchanger i n s t e a d of copper o r steel. A l - though t h e the rma l c o n d u c t i v i t y of f i b e r g l a s s i s much less t h a n copper o r s t ee l , it is an e x c e l l e n t i n s u l a t o r ; t h e water is less l i k e l y t o c o o l from t h e w e l l b o r e t o t h e house . I n a d d i t i o n , t h e f i b e r g l a s s c a r r i e s a 50 y e a r gua ran tee .

Most newly

D i s p o s a l of f l u i d s from geothermal w e l l s r ema ins a problem i n Moana. S t a t e r e g u l a t o r s u r g e d e v e l o p e r s t o r e i n j e c t s p e n t f l u i d s i n t o t h e a q u i - f e r from which t h e y are produced. The c o s t of a r e i n j e c t i o n w e l l is p r o h i b i t i v e f o r small-scale d e v e l o p e r s ; l a r g e - s c a l e d e v e l o p e r s must i n c o r p o r a t e a r e i n j e c t i o n system.

CONCLUSIONS

Repor t s of overdevelopment and r e s e r v o i r d e p l e t i o n of t h e Moana geothermal area are p re - s e n t l y be ing e v a l u a t e d w i t h a combina t ion of geo- l o g i c , g e o p h y s i c a l , and geochemical s u r v e y s coupled w i t h down-hole measurements i n o b s e r v a t i o n w e l l s . Geologic s u r v e y s r e v e a l t h a t a complex sequence of T e r t i a r y sed imen ta ry r o c k s and hydro the rma l ly a l t - e red a n d e s i t e c o n s t i t u t e s t h e r e s e r v o i r r o c k . Spa- t i a l v a r i a t i o n s i n s u b s u r f a c e t empera tu res a r e re- l a t e d t o t h e d i s t r i b u t i o n of hydro the rma l ly a l t e r e d Kate Peak a n d e s i t e . A g r a v i t y su rvey completed i n this area a p p e a r s t o have d e l i n e a t e d t h e eastern- most boundary of t h e less dense sed imen ta ry r o c k s . Geochemical a n a l y s e s of Moana the rma l f l u i d s sug- g e s t l i t t l e o r no c o r r e l a t i o n w i t h nea rby Steam- b o a t Hot S p r i n g s . compos i t ion from w e s t t o east c o r r e l a t e w i t h d e c r e a s i n g t empera tu res . Radiocarbon and t r i t i u m a n a l y s e s sugges t t h a t t h e Moana the rma l f l u i d s o r i g i n a t e from a series of no r th - t r end ing f a u l t s i n t h e w e s t - c e n t r a l p a r t of t h e r e s e r v o i r . These f l u i d s t h e n sp read o u t l a t e r a l l y t o t h e east , n o r t h , and sou th . A series of o b s e r v a t i o n w e l l s w i l l be moni tored p r i o r t o , d u r i n g , and a f t e r t h e h e a t i n g season t o d e t e r m i n e t h e e x t e n t of reser- v o i r d e p l e t i o n as a r e s u l t of f l u i d wi thdrawal by t h e 150 geo the rma l w e l l s i n t h e Moana a r e a . Re- commendations f o r p ruden t r e s o u r c e development are be ing fo rmula t ed and s u p p l i e d t o government agen- c i e s and p r o s p e c t i v e u s e r s .

Work performed under U.S. Dept. of Energy Con t rac t no. AC03-82-RA50075.

S l i g h t v a r i a t i o n s i n f l u i d

Anderson, R . , 1908, Geology and o i l p r o s p e c t s of t h e Reno r e g i o n , Nevada: U.S. Geol. Surv . B u l l . 381, p. 475.

Bateman, R . L . , and Scheibach , B . R . , 1975, Evalua- t i o n s of geothermal a c t i v i t y i n t h e Truckee Meadow, Washoe County, Nevada: Nevada Bur. Mines and Geol . , Rep t . 25, 38 p.

B i n g l e r , E . C . , 1975, Guidebook t o t h e Qua te rna ry geology a long t h e y e s t e r n f l a n k of t h e Truc- kee Meadows, Washoe County, Nevada: Nevada Bur. Mines and Geo l . , Rept . 22, 14 p.

Ca r l son , David E., 1982, Dis t r ic t s p a c e h e a t i n g from a s i n g l e g e o t h e r n a l w e l l , Warren Es ta tes , Reno, Nevada: Geothermal R e s . Coun., T rans . , V . 6, Oct. 1982, p. 429-431.

Cohen, P h i l i p , and L o e l t z , O . J . , 1964, E v a l u a t i o n of hydrogeology and hydrogeochemis t ry of Truckee Meadows area, Washoe County, Nevada: U.S. Geol. Surv. Water-Supply Pape r , 1779-S.

Ghusn, George, Jr., 1982, 13ase l ine d a t a f o r Moana geothermal area: a t u r e geothermal r e s o u r c e a s ses smen t f o r Nevada: Area s p e c i f i c s t u d i e s , Pumpernicke l Va l l ey , C a r l i n , and Moana: T r e x l e r , F lynn , Koenig, B e l l , and Ghusn: Work performed under c o n t r a c t No. DE-AC08-81NV10220 t o U.S. Dept. of Energy, by D i v i s i o n of E a r t h S c i e n c e s , UNLV.

& LOW- t o moderate-temper-

Hammer, S . , 1939, T e r r a i n c o r r e c t i o n t a b l e s f o r g rav ime te r s t a t i o n s : Geophys ics , v . 4 , p. 184-

King, C la rence , 1878, Sys t ema t i c geology: U.S. Geol. Exp lo r . , 4 0 t h P a r a l l e l (K ing) , v . 1 ,

M i z e l l , N.B.H. , 1975, Q u a t i r n a r y geology of t h e c e n t r a l Truckee Meadows, Nevada: Univ. of Nev., Reno, Master's 'Thesis, 68 p.

Thompson, G.A. , and White, D.E. , 1964, Regional geology of t h e Steamboat S p r i n g s area, Washoe

458-A, 52 p . County, Nevada: U.S. Geol. Surv . P r o f . Paper

White, D.E. , Thompson, G.A., and Sandberg , C . A . , 1964, Rocks, s t r u c t u r e , and g e o l o g i c h i s t o r y of Steamboat S p r i n g s thermal area, Washoe County, Nevada: U.S. Geol. Surv . P r o f . Paper 458-B, 63 p.

ACKNOWLEDGMENTS

The a u t h o r s would l i k e t o thank Bob Loux of t h e Nevada Department of Energy and t h e U.S. Dept. of Energy, San F ranc i sco Opera t ions O f f i c e f o r s u p p o r t i n g t h i s r e s e a r c h . Many thanks t o t h e resi- d e n t s and w e l l d r i l l e r s i n Moana. The hard working p r o f e s s i o n a l s and s t a f f a t t h e D i v i s i o n of E a r t h Sc iences , i n c l u d i n g Dennis T r e x l e r , E l a i n e B e l l , Susan P a r k h u r s t , and Cam Covington, helped w i t h p r e p a r a t i o n of t h e manusc r ip t . S t even Weiss w r o t e t h e computer program t h a t gene ra t ed t h e 3-D g r i d .

325

VOLCANIC ROCK PETROCHEMISTRY A S AN EXPLORATION TECHNIQUE FOR GEOTHERMAL ENERGY

L. A. FULTZ, E. J. BELL AND D. T. TREXLER

DIVISION OF EARTH SCIENCES

UNIVERSITY OF NEVADA, LAS VEGAS

- RENO, NEVADA 89503

ABSTRACT

Large high-level s i l i c i c magma chambers o f f e r a high p o t e n t i a l f o r economically v i a b l e geother- mal systems. While pu re ly b a s i c v o l c a n i c systems

t h e necessary long-term hea t i npu t t o s i l i c i c systems, by unde rp la t ing , t o s u s t a i n a high-temp- e r a t u r e geothermal system. croprobe, geochemical, geochronologic , and i so - t o p i c d a t a on young vo lcan ic rocks i n wes t - cen t r a l Nevada i n d i c a t e compositions that may r e s u l t from magmatic d i f f e r e n t i a t i o n , c r y s t a l f r a c t i o n a t i o n , v a r i a t i o n i n magmatic sou rce r e g i o n s and i n p a r t i - c u l a r , magma mixin-

%is of t h e pe t rochemis t ry and t h e recog- n i t i o n of magma mixing t e x t u r e s of e x t r u s i v e r o c k s may i n d i c a t e i n t e r a c t i n g mafic magma wi th buried shal low s i l i c i c magma systems. provide a shal low h e a t sou rce f o r development of

r- r a r e l y form thermal anomalies, they may provide

Pe t rog raph ic and m i -

These systems may

suggest t h a t pu re ly b a s i c vo lcan ic systems r a r e l y form thermal anomalies; however, under c e r t a i n cond i t ions , such as t h e presence of a b a s a l t i c shadow, young b a s i c vo lcan ic systems a r e p o t e n t i a l i n d i c a t o r s of bu r i ed s i l i c i c high l e v e l magma chambers that may o the rwise have no obvious s u r - f a c e m a n i f e s t a t i o n s .

geothermal r e sources .

INTRODUCTION

Subsequent workers i n c r e a s i n g l y emphasized t h e importance of b a s a l t i c volcanism i n t h e devel- opment of l a r g e s i l i c i c magma chambers. berger and Gooley ( 1 978) suggested t h a t a d d i t i o n a l hea t i npu t t o a s i l i c i c magma chamber by under- p l a t i n g of b a s a l t i c l a v a s o f f e r s a p l a u s i b l e ex- p l a n a t i o n t o account f o r t h e observed l i f e spans of some s i l i c i c magma systems. D i sc repanc ie s between the observed l i f e span of geothermal sys-

Eichel-

The Divis ion of E a r t h Sc iences (UNLV), under a r e s e a r c h g r a n t from t h e Department of Energy, has i n v e s t i g a t e d t h e r e l a t i o n s h i p of l a t e T e r t i - a r y t o Quaternary b a s a l t s and a n d e s i t e s t o t h e a c t i v e geothermal systems i n wes t - cen t r a l Nevada.

Previous s t u d i e s of igneous r e l a t e d geother- m a l systems by Smith and Shaw (1975) i n d i c a t e that t h e presence of l a r g e high-level s i l i c i c magma chambers o f f e r s a h igh p o t e n t i a l f o r economically i n t e r e s t i n g geothermal systems. Eva lua t ion of geographical areas having geothermal p o t e n t i a l c u r r e n t l y invo lves s t u d i e s of young vo lcan ic sys- tems dominated by t h e p re sence of s i l i c i c f lows, t u f f s and domes bel ieved i n d i c a t i v e of high l e v e l magma chambers (e.g. , Coso, Long Val ley and Clear Lake (Geyers) i n C a l i f o r n i a , and Yellowstone i n Wyoming).

Conversely, rocks of b a s a l t i c and a n d e s i t i c

These magmas rise r a p i d l y t o t h e s u r f a c e composition are de r ived from t h e mantle and lower c r u s t . through narrow p i p e s and f i s s u r e s and do no t con- t r i b u t e s i g n i f i c a n t amounts of s t o r e d hea t t o t h e c r u s t . For t h i s r eason , Smith and Shaw (1975)

tems and t h e s h o r t e r l i f e spans c a l c u l a t e d from conven t iona l models may be explained by sus t a ined hea t i npu t t o t h e system by b a s a l t i c magmas (Lach- enbruch and o t h e r s , 1976). As a r e s u l t , t h e po- t e n t i a l s i g n i f i c a n c e of b a s a l t i c and a n d e s i t i c volcanism i n t h e development of a c t i v e geothermal systems i s g r e a t l y inc reased .

Post-Miocene volcanism i n wes t - cen t r a l Nevada has been p r i m a r i l y b a s a l t i c t o a n d e s i t i c i n n a t u r e (Stewart and Carlson, 1976; Silberman and o t h e r s , 1975). Numerous geothermal s p r i n g s and w e l l s i n t h e a r e a inc lude some h igh temperature systems wi th power product ion p o t e n t i a l ( temperatures >350°F w i t h i n 4000 f e e t of s u r f a c e ) : Steamboat Hot Spr ings geothermal area, Desert Peak geother- m a l f i e l d and t h e Upsal Hogback (Carson Sink) geo- thermal anomaly. Two of t h e s e a r e a s (Steamboat and Carson Sink) a r e s p a t i a l l y a s s o c i a t e d wi th young vo lcan ic rocks of b a s a l t i c t o a n d e s i t i c a f f i n i t i e s .

The s p a t i a l a s s o c i a t i o n i n wes t - cen t r a l Nevada of h igh temperature geothermal systems with young v o l c a n i c s o f f e r s a unique oppor tun i ty f o r s tudy ing t h e r e l a t i o n s h i p of b a s a l t i c t o a n d e s i t i c t ype volcanism t o a c t i v e geothermal systems. The Divis ion of E a r t h Sciences u t i l i z e d petrographic, , geochemical, geochronologic and s t ron t ium i s o t o p i c composi t ions f o r a comparison of v o l c a n i c s asso.- c i a t e d wi th high temperature geothermal systems wi th v o l c a n i c s from non-geothermal a r e a s .

326

F u l t z e t al. GENERAL GEOLOGY

The s t u d y area, w i t h i n t h e Reno 1 x . 2 O quad- r a n g l e ( f i g . l ) , e x t e n d s from t h e S i e r r a Nevada i n t o t h e w e s t e r n Great B a s i n . Volcanism d u r i n g t h e m i d - T e r t i a r y (pre-Miocene) w a s p r e d o m i n a n t l y r h y o l i t i c and r h y o d a c i t i c i n c o m p o s i t i o n and oc- c u r r e d a s a s h f l o w s h e e t s . Post-Miocene v o l c a n i s m w a s d o m i n a n t l y c a l c - a l k a l i n e a n d e s i t i c w i t h t r a n - s i t i o n t o p r e d o n i r i a n t l y b a s a l t i c v o l c a n i s m w i t h minor amounts of r h y o l i t e s i n c e 5-7 m.y. a g o ( S i l - berman and o t h e r s , 1975) .

Rock u n i t s of s p e c i f i c i n t e r e s t t o t h i s s t u d y i n c l u d e P l i o c e n e and P l e i s t o c e n e a g e v o l c a n i c s of t h e Lousetown Format ion , S teamboat H i l l s R h y o l i t e , McCle l lan Peak O l i v i n e Basalt, and t h e Basalt ic A n d e s i t e of Steamboat S p r i n g s .

The Steamboat Hot S p r i n g sys tem is t h e o l d e s t and most s t u d i e d g e o t h e r m a l a r e a i n Nevada ( S i l - berman and o t h e r s , 1979) . The g e o t h e r m a l s y s t e m i s a s s o c i a t e d w i t h t h e S teamboat H i l l s b a s a l t i c a n d e s i t e d a t e d by S i l b e r m a n and o t h e r s (1979) a t 2 .53 . l l m.y. The t h e r m a l area a l s o l i es a l o n g a t r e n d c o n n e c t i n g 4 r h y o l i t e domes r a n g i n g from 1.14 t o 3.0 m.y. o l d . A g e o t h e r m a l tes t w e l l d r i l l e d i n 1979 by t h e P h i l l i p s P e t r o l e u m Company i n t h e b a s a l t i c a n d e s i t e v e n t area y i e l d e d temper- a t u r e s g r e a t e r t h a n 400°F.

GEOCHRONOLOGY

Age d a t e s of t h e v o l c a n i c s r a n g e from 0.1 m y . a t Upsal Hogback t o 11 m.y. f o r t h e S i l v e r S p r i n g s f l o w . The d i s t r i b u t i o n of v o l c a n i c r o c k s s u g g e s t s t h a t a much l a r g e r volume of l a v a e r u p t e d d u r i n g t h e p e r i o d o f 4-8 m.y., w i t h i n c r e a s e d r h y o l i t i c dome emplacement s i n c e 4 m.y.

Age d a t e s ( t h i s s t u d y ) f o r C h u r c h i l l B u t t e (4 .5 L 0.6 m.y.) , T a b l e Mountain ( 6 . 3 and C l e a v e r P e a k ( 8 . 5 2 0.6 m.y.) are c o m p a t i b l e w i t h Lousetown a g e v o l c a n i c s and c o n c o r d a n t w i t h f i e l d r e l a t i o n s h i p s s u g g e s t e d by Moore (1969). The S i l v e r S p r i n g s b a s a l t f l o w a g e d e t e r m i n a t i o n (11.0 - + 0.7 m.y.; t h i s s t u d y ) is o l d e r t h a n t h e Louse- town a g e s u g g e s t e d by Thompson (1956) .

0.7 m.y.) ,

The Desert Peak a s h f l o w has y i e l d e d f o u r d i s c o r d a n t K-Ar a g e d a t e s on p l a g i o c l a s e s e p a r a t e s of 13.3 2 1.1 m.y. ( t h i s s t u d y ) , and 11.2, 4 . 3 , and 2 . 3 m.y. ( H i n e r , 1979) which l e a v e t h e a c t u a l a g e of t h i s u n i t i n q u e s t i o n . Hiner (1979) sug- g e s t s that t h e 4 . 3 and 2 .3 m.y. a g e d a t e s are more r e l i a b l e and c o m p a t i b l e w i t h f o s s i l e v i d e n c e ob- t a i n e d f rom t h e u n d e r l y i n g T r u c k e e Format ion .

GEOCHEMISTRY

Major e lement c o m p o s i t i o n s o f t h e r c c k s sam- p l e d v a r y f rom t r u e a l k a l i b a s a l t s r e p r e s e n t a t i v e of McCle l lan Peak and Upsa l Hogback-Soda Lake vol- c a n i c s t o r h y o d a c i t e and r h y o l i t e from Desert Peak and t h e S teamboat H i l l s . Basaltic a n d e s i t e s from Steamboat H i l l s a re more e n r i c h e d i n t o t a l alka- l i e s t h a n o t h e r v o l c a n i c r o c k s of Lousetown a g e t h a t were sampled.

CIPW Norms show b a s a l t i c a n d e s i t e s a s q u a r t z n o r m a t i v e w i t h h y p e r s t h e n e w h i l e b a s a l t s from Upsal Hogback-Soda Lake and McCle l lan Peak a r e q u a r t z d e f i c i e n t h a v i n g n o r m a t i v e n e p h e l i n e w i t h ol i v i n e .

C o m p o s i t i o n a l l y , t h e s u i t e is bimodal w i t h most of t h e samples h a v i n g l e s s t h a n 59% S i 0 2 . Only one sample (DP-5) i s a t r u e a n d e s i t e . T r a c e e lement p l o t s s u g g e s t t h a t v o l c a n i c s from t h e Carson S i n k are c h e m i c a l l y d i f f e r e n t from samples o b t a i n e d from o t h e r l o c a t i o n s i n t h e s t u d y a r e a . Rb/Sr and Sr/Y ( f i g . 2) p l o t s show r e g i o n a l d e p l e - t i o n i n S r and h i g h v a r i a b i l i t y of Rb i n t h e s e l a v a s . Lower Sr c o n c e n t r a t i o n may r e f l e c t d i f f e r - e n t s o u r c e r e g i o n s f o r t h e Carson S i n k v o l c a n i c r o c k s .

C o r r e l a t i o n of t h e Desert Peak a s h f l o w sam- p l e s w i t h t h e Upsal Hogback-Soda Lake t r e n d sug- g e s t s t h e s e t y p e b a s a l t s may b e p a r e n t a l s o u r c e s of t h e a s h f l o w u n i t . L e a s t s q u a r e s mixing c a l c u - l a t i o n s (Wright and Doher ty , 1970) and R a y l e i g h f r a c t i o n a t i o n (Shaw, 1970) were used t o t es t com- p a t a b i l i t y of c r y s t a l l i z a t i o n d i f f e r e n t i a t i o n of r e p r e s e n t a t i v e Upsa l Hogback c o m p o s i t i o n s t o pro- d u c e t h e Desert Peak a s h f l o w ( E R 2 < 2 . 0 ) . Frac- t i o n a t i o n model ing d e m o n s t r a t e s t h a t f r a c t i o n a t i n g 12% p l a g i o c l a s e and 30% amphibole of a Upsa l Hog- back b a s a l t i s c a p a b l e of p r o d u c i n g t h e low s i l i ca pumice found a t Desert Peak . Cont inued f r a c t i o n a - t i o n of 10% amphibole and p l a g i o c l a s e would pro- d u c e c o m p o s i t i o n s s i m i l a r t o t h a t o f t h e main a s h f l o w s e q u e n c e . i n g t h e above p h a s e p e r c e n t a g e s show c l o s e a g r e e - ment w i t h observed v a l u e s ( e x c e p t f o r S r concen- t r a t i o n ) . Moreover, amphibole f r a c t i o n a t i o n is s u p p o r t e d by d e p l e t i o n of Sc and C r w i t h i n t h e Carson S i n k v o l c a n i c s .

Trace e lement c o m p o s i t i o n s employ-

B a s a l t i c a n d e s i t e s from Steamboat H i l l s r e v e a l r e l a t i v e e n r i c h m e n t i n Cc/l..i a n d S r / Y . Enrichment of C e and La, which are l i g h t Rare E a r t h Elements (LREE), may b e e x p l a i n e d t h r o u g h c r y s t a l f r a c t i o n a t i o n .

L e a s t s q u a r e s mixing model ing (Wright and Doher ty , 1970) and R a y l e i g h f r a c t i o n a t i o n (Shaw, 1970) were per formed u s i n g a h i g h K b e a r i n g b a s a l t such a s McCle l lan Peak as a p a r e n t a l magma t o d e r i v e S teamboat H i l l s b a s a l t i c a n d e s i t e . Calcu- l a t i o n s u s i n g 16% p l a g i o c l a s e , 13% o l i v i n e and 17% c l i n o p y r o x e n e g i v e p r e d i c t e d c o m p o s i t i o n s similar t o Steamboat H i l l s b a s a l t i c a n d e s i t e s . tents of t h e proposed p a r e n t a l magma would be less than that o f McCle l lan Peak. I n a d d i t i o n , p r e - d i c t e d trace e lement c o m p o s i t i o n s d i f f e r by a n o r d e r o f magni tude w i t h o b s e r v e d v a l u e s . t h e i r g e n e r a l t r e n d s are c o m p a t i b l e . Enrichment of L E E ' S i n t h e S teamboat H i l l s b a s a l t i c ande- s i t e may t h e r e f o r e b e c o m p a t i b l e w i t h c r y s t a l f r a c t i o n a t i o n from a p a r e n t a l b a s a l t i c magma s i m i - lar t o McCle l lan Peak b u t h a v i n g lower K , Rb, La and Ce.

K 2 0 con-

However,

S teamboat H i l l s r h y o l i t e shows d e p l e t i o n i n B a , Z r , S r , C e , and Zn w i t h enr ichment i n Rb and Nd which are c h a r a c t e r i s t i c of h i g h l y d i f f e r e n t i -

32 7

.._

0 5 0 10 w -

miles km I

I I

1 CARSON CITY j

''-7

I

\ \

CLEAVER PEAK

ESERT PEAK

UPSAL HOGBACK

CARSON DESERT

SODA LAKE 6 RATTLESNAKE

HILL

\@O v \

/

Figure 1. Location map of s tudy a r e a showing . l a t e T e r t i a r y and Quaternary vo lcan ic rocks.

1202 .-Table Mtn

E

c 2 457 .c * 308

McClel lan Pk, Steamboat 'e ly8'e\,- Bas-And

Cleaver Pk--. \ 9 I. \/

Churchill Butte 0

Carson City-g-silver Springs I 0'2

0 /0-. * \ Carson Sink / 0 0

0 ' volcanics 1 / 0

/*. >/ : /' -4 Steamboat

159

- o- Rhyol i te 10 I 1 I I I I 1 I I

'2 5 8 12 15 18 22 25 28 31

Yttrium (ppm)

Figure 2. Stront ium v s . y t t r i um p l o t f o r w e s t - c e n t r a l Nevada.

a t ed magmas (Hildredth, 1979). The wide v a r i a t i o n i n C e , La, Sc and S r between t h e r h y o l i t e and t h e Steamboat b a s a l t i c a n d e s i t e s suggest t h a t t h e s e magmas r e p r e s e n t s e p a r a t e magmatic pu l se s .

Microprobe of Phenocryst Phases

Microprobe ana lyses w e r e performed on plagio- c l a s e phenocryst phases from Steamboat H i l l s basal- t i c a n d e s i t e and Upsal Hogback-Soda Lake b a s a l t . Anor th i t e (An) histogram of a microprobe t r a v e r s e a c r o s s a c r y s t a l is shown on f i g u r e 3.

C e l l u l a r p l a g i o c l a s e phenocrysts found- in t h e Steamboat b a s a l t i c a n d e s i t e and Upsal Hogback- Soda Lake b a s a l t s are r e v e r s e l y zoned va ry ing from An 20-30 t o An 50-60 on t h e o u t e r margins. o u t e r zone approaches compositions of non-cel lular l a t h shaped p l a g i o c l a s e found wi th in t h e ground- m a s s . A r e p r e s e n t a t i v e p l a g i o c l a s e t r a v e r s e ( f i g . 3) d i s p l a y s a normal zoning p a t t e r n wi th in a s o l i d c o r e An 40 t o An 25. Anor th i t e compositions with- i n t h e c e l l u l a r zone i n c r e a s e t o An 52 wi th s o l i d c a l c i c overgrowths similar t o groundmass plagio- c l a s e compositions.

The

328

Fultz e t a l .

A Figure 3.

(b) Anorthi te histogram of nicroprobe t r a v e r s e (a) Photomicrograph of reverse ly zoned p lag ioc lase fe ldspar from Steamboat H i l l s basaltic-ande- s i t e , showing s o l i d core , c e l l u l a r zone and

A-A' shown on photomicrograph ( a ) .

so l i d overgrowth ,

The microprobe d a t a demonstrates t h e cores of the c e l l u l a r p lag ioc lase did not c r y s t a l l i z e i n t h e i r b a s a l t i c a n d e s i t e host . c l a s e probably represents foreign microphenocrysts c r y s t a l l i z e d i n a more s i l i c i c magma. phenocrysts represent e i t h e r highly ass imi la ted wal l rock fragments o r mixing with more s i l i c i c magma upon ascent t o t h e surface.

The sodic plagio-

The micro-

Albi te-anorthi te-or thoclase t r i a n g u l a r p l o t s have been used t o determine c r y s t a l l i z i n g condi- t i o n s i n which phenocrysts form (Carmichael and o thers , 1974). Orthoclase content has been found t o increase with N a i n p lag ioc lase coexis t ing with K-feldspar. Microprobe analyses of Steamboat s a m - p l e s show compositions of An 70 t o An 20 wi th l i t t l e divergence toward or thoc lase compositions. Low or thoc lase content of t h e Na-plagioclase are s imi la r t o p lag ioc lase from b a s a l t t o b a s a l t i c andes i tes (Carmichael and o thers , 1974) and d i f f e r i n composition from plag ioc lase from r h y o l i t i c host rock analyzed by Bacon and Metz (1983).

DISCUSSION OF RESULTS Data obtained during t h i s study p laces geo-

chemical c o n s t r a i n t s on evolut ion of magma p r i o r t o erupt ion. Comparison of volcanic rocks asso- c i a t e d with geothermal systems and volcanic rocks

A'

assoc ia ted with non-geot hermal a r e a s r e v e a l s t h e following geochemical c h a r a c t e r i s t i c s :

1. B a s a l t i c andes i tes from Steamboat H i l l s a r e more enriched i n K , Rb, S r , and Ce than o thcr Pl iocene age volcanics .

2 . Volcanic rocks of t h e Carson Sink have a d i f f e r e n t source area than volcanic rocks located f u r t h e r w e s t . Compositional v a r i - a t i o n within t h e volcanic s u i t e is compa- t i b l e with f r a c t i o n a l d i f f e r e n t i a t i o n t o d e r i v e t h e Desert Peak ash flow.

3. Reversely zoned c e l l u l a r f e l d s p a r s a r e present i n b a s a l t i c t o basa l t ic -andes i t ic lavas from geothermal l o c a l i t i e s of Upsal Hogback-Soda Lake and Steamboat H i l l s .

Trace Elements

Higher t r a c e element concentrat ions within t h e Steamboat H i l l s vo lcanics may represent con- tamination, d i f f e r e n c e s i n source a r e a , o r f rac- t i o n a t i o n e f f e c t s . Rela t ive ly low Rb contents (31 -40 ppm) i n these l a v a s (30 not support large-scale contamination by Rb enri-hed Cretaceous granodio- r i t e basement rock (300 ppm, Thompson and White, 1964). I n addi t ion , s t ront ium 87/86 r a t i o s of

329

m l t z et a l . s p e c u l a t i v e ; however, shal low l e v e l c r u s t a l hold- ing chambers would b e compatible . The re fo re , r e v e r s e l y zoned c e l l u l a r f e l d s p a r t e x t u r e s may suggest evidence f o r shal low c r u s t a l r e s i d e n c e time f o r t h e s e magmas.

CONCLUSIONS

The occurrence o e l l u l a r l y zoned f e l d s p a r s a r e evidence of c r u s t a l r e s i d e n c e t i m e r e v e a l i n g a complex h i s t o r y of magma mixing and/or high l e v e l f r a c t i o n a t i o n . These geochemical constraints suggest t h a t t h e s e v o l c a n i c s d id not r ise r a p i d l y t o t h e s u r f a c e . The b a s a l t & t o basal t ic-ande- sites a s s o c i a t e d wi th geothermal systems of west- c e n t r a l Nevada show .evidence of i n t e r a c t i o n wi th a magma chamber where s u r f a c e expres s ion has been minimal (minor s i l i r i r domes and f lows) . The presence of a buried magma chamber a t dep th o f f e r s a d d i t i o n a l evidence f o r a magmatic hea t sou rce beneath t h e Steamboat H i l l s geothermal a r e a sug- ges t ed by Silberman and o t h e r s (1979).

.7052 from Steamboat H i l l s b a s a l t i c - a n d e s i t e (Mor- ton and o t h e r s , 1980) f a l l w i th in t h e r ange report- ed f o r west-central Nevada v o l c a n i c s (.7043-.7056). Add i t iona l i s o t o p i c d a t a f o r t h e s e r o c k s a r e forthcoming.

D i f f e ren t composi t ions w i t h i n sou rce a r e a s can not be r u l e d ou t as an enrichment f a c t o r . A heterogeneous mantle has been proposed by Hedge and Noble (1971) t o l i e beneath t h e Basin and Range Province; however, h ighe r S r 87/86 r a t i o s (>.7065) and Rb c o n t e n t s (20-75 ppm) a r e no t observed.

L Consequently, c r y s t a l f r a c t i o n a t i o n is widely used t o exp la in composi t ional d i f f e r e n c e s among vo lcan ic rocks (Rose and o t h e r s , 1980). Modeling presented i n t h i s r e p o r t demonstrates a c l o s e compa tab i l i t y f o r enrichment of t r a c e elements w i th in t h e Steamboat H i l l s v o l c a n i c s u i t e .

Trace element composi t ions suggest t h a t t h e Desert Peak a s h flow is g e n e t i c a l l y r e l a t e d t o

C r y s t a l f r a c t i o n a t i o n of mafic t o s i l i c i c t ype magma has been suggested t o occur i n some T e r t i a r y vo lcan ic systems of t h e Basin and Range Province (Noble and o t h e r s , 1976). Consequently, shal low l e v e l hold- i ng chambers f a v o r a b l e f o r d i f f e r e n t i a t i o n of magma o f f e r t h e h i g h e s t p o t e n t i a l f o r development of geothermal systems (Smith and Shaw, 1975). Geo- chemical d a t a suggest t h a t t h i s mechanism may have occurred w i t h i n t h e Carson Deser t a r e a which offers high f a v o r a b i l i t y f o r f u t u r e geothermal discoveries .

Revers e ly Zoned Feldspars

- 4 p s a l Hogback-Soda Lake type b a s a l t s .

The occurrence of c e l l u l a r r e v e r s e l y zoned f e l d s p a r s is a c l u e t o t h e p re -e rup t ive h i s t o r i e s of t h e magmas from which t h e s e l a v a s were de r ived . C e l l u l a r f e l d s p a r t e x t u r e s may be t h e r e s u l t of c r u s t a l contaminat ion (d iges t ed wa l l rock) o r a magma mixing event . The geochemistry of t h e Steamboat H i l l s b a s a l t i c a n d e s i t e does n o t f avor s i g n i f i c a n t contaminat ion wi th basement rock. Zoned f e l d s p a r s a r e found i n d i f f e r e n t t e r r a n e s from Upsal Hogback-Soda Lake and Steamboat H i l l s v o l c a n i c s where p a r t i a l l y d i g e s t e d o r t h o c l a s e phenocrysts were not observed. Low o r t h o c l a s e con ten t of t h e Na-plagioclase a rgues a g a i n s t c r y s t a l l i z a t i o n c o e x i s t i n g wi th o r t h o c l a s e , ex- pected from g r a n i t i c f e l d s p a r phenocrysts .

. Hibbard (1981) suggested t h a t c e l l u l a r man- t l e d f e l d s p a r t e x t u r e s occur i n hybrid rocks of magma mixing o r i g i n s . t o r e p r e s e n t a quench zone du r ing c r y s t a l l i z a t i o n of t h e f e l d s p a r s , developed du r ing i n t e r a c t i o n of magmas having d i f f e r e n t tempera t u r e s .

C e l l u l a r zoning is bel ieved

C e l l u l a r f e l d s p a r s from Steamboat H i l l s hav- ing An 20 t o An 30 c o r e s c r y s t a l l i z e d i n a more d i f f e r e n t i a t e d magma than t h e i r c u r r e n t hos t . The f e l d s p a r s may have r e a c t e d wi th and r e e q u i l i b r a t e d i n a more c a l c i c hos t magma. The p resence of resorbed q u a r t z phenocrysts and t r a c e element d a t a support a model t h a t t h e Steamboat H i l l s b a s a l t i c a n d e s i t e s (and Upsal Hogback-Soda Lake b a s a l t s ) i n t e r a c t e d wi th a more evolved magma p r i o r t o e rup t ion . Depths f o r magma i n t e r a c t i o n a r e highly

I n t e r a c t i o n of b a s a l t i c magma wi th a h igh l e v e l s i l i c i c system evidenced by magma mixing t e x t u r e s may t h e r e f o r e be a n important c r i t e r i a i n r ecogn iz ing c r u s t a l hea t sou rces . Analyses of f e l d s p a r phenocryst morphology have p o t e n t i a l u s e i n i d e n t i f y i n g a r e a s where mafic l a v a s have come i n c o n t a c t w i th f r a c t i o n a t i n g sha l low magma'cham- be r s . .

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Work performed under U.S. Dept. of Energy Contract no. AC03-82-RA50075.

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