U N I TED STATES DEPARTMENT OF THE I N T E R I O R

GEOLOGICAL SURVEY

METALLOGENIC AND TECTONIC S I G N I F I C A N C E OF OXYGEN ISOTOPE DATA AND WHOLE-ROCK POTASSIUM-ARGON AGES OF THE N I K O L A I GREENSTONE, MCCARTHY QUADRANGLE, ALASKA

OPEN-FILE REPORT 80-2019

By M. L. Silberman, E. M. MacKevett, Jr., C. L. Connor, and Alan Matthews

This repor t i s pre l iminary and has not been reviewed f o r conformity

wi th U.S. Geological Survey editorial standards and s t r a t i g r a p h i c nomenclature.

CONTENTS

Abstract---------------------------------------------------------- Introduction-------------------------------------d--w------------- Wrangel l ia and geology o f t h e Wrangell Mountains------------------

C h a r a c t e r i s t i c s o f the N i k o l a i Greenstone---------------------- M i n e r a l i z a t i o n i n t h e N i k o l a i Greenstone-----------------------

S tab le i so tope re~~lts-------------------------------------------- K-Ar age determinations------------------------------------------- Discussion-------------------------------------------------------- References--------------------------------------------------------

Page 1

ILLUSTRATIONS

F igu re 1. Map showing l o c a t i o n o f McCarthy quadrangle, t h e Wrangell Mountainsand the d i s t r i b u t i o n o f the N i k o l a i Greenstone---------------------------------- 3

2. General ized geology o f p a r t o f the McCarthy quadrangle, Alaska----------------------------------------------- 5

3 . S t r a t i g r a p h i c column fo r the Wrangell Mountains i n the McCarthy quadrangle---------------------------------- 7

4. I n i t i a l argron diagram f o r N i k o l a i Greenstone whole rock K-Ar data--------------------------------------- 19

5. Map showing d i s t r i b u t i o n o f se lected Mesozoic te r ranes i n southern Alaska-------------------------..------- 22

TABLES

Table 1. Chemical composit ion and CIPW norms f o r "average" N i k o l a i Greenstone-------------------------------- 9

2. Oxygen iso tope composit ion o f quar tz and ep ido te f rom copper-bearing ve ins i n N i k o l a i Greenstone---------- 12

3. P a r t i a l chemical analyses f o r t r a c e elements i n quar tz - ep idote ve ins o f the N i k o l a i Greenstone------------- 12

4. Oxygen iso tope composit ion o f t y p i c a l whole-rock N i k o l a i Greenstone samples-------------------------- 13

5. Calcu lated temperature and f l u i d oxygen iso tope composi t i o n f o r copper bear ing quar tz-epi dote ve ins i n N i k o l a i Greenstone------------------------------- 14

6. K-Ar ages o f whole rock N i k o l a i Greenstone samples----- 17

METALLOGENIC AND TECTONIC SIGNIFICANCE OF WHOLE-ROCK POTASSIUM-ARGON AGES OF THE N IKOLAI GR EENSTONE, MCCARTHY QUADRANGLE, ALASKA

M. L. Silberman*, E. M. MacKevett Jr.*, C. L. Connor*, Alan Matthews**

ABSTRACT

The Middle and ( o r ) Late T r i a s s i c N i k o l a i Greenstone, p a r t of t he

al lochthonous te r rane of Wrangell i a, i s t y p i c a l l y a l t e r e d and l o c a l l y

metamorphosed t o p rehn i te -pumpel ly i te f a c i e s w i t h c h l o r i t e and epidote as the

most comnon secondary minerals. I n t r i n s i c copper content averages 155 ppm,

and two types o f concentrat ions o f copper i n t he N i k o l a i are comnon: (1 )

n a t i v e c o p p e r f i l l i n g s o f amygdu lesandrubb lezones t y p i c a l l y n e a r f l o w

tops, and (2) veins and t h i n replacement zones t h a t conta in n a t i v e copper and

copper- i ron s u l f i d e s i n quar tz-epidote or c a l c i t e gangue i n f a u l t s and

f rac tu res . Oxygen isotope data f rom quar tz and epidote from three copper-

bear ing veins y i e l d ca lcu la ted ore f l u i d temperatures o f approximately 2 0 0 ~ ~

and 6180 of approximately +1 per m i l i n agreement w i t h a metamorphic-

segregat ion o r i g i n o f these deposits, as suggested by S i n c l a i r (1977).

Seven K-Ar ages o f c h l o r i t i z e d greenstone, i n c l u d i n g those adjacent t o

ve ins f a l l on an i n i t i a l argon diagram w i t h a zero i n t e r c e p t and a s lope which

y i e l d s an isochron age of 112 - + 11 m.y. The ages def ine a Cretaceous thermal-

metamorphic episode which i s respons ib le f o r a1 t e r a t i o n and m ine ra l i za t i on .

The episode i s younger than a major Jurassic orogeny, accompanied by g r a n i t i c

i n t r u s i o n , i n the area, and appears t o be unaffected by minor g r a n i t i c

i n t r u s i o n i n the middle t o l a t e T e r t i a r y . We be l i eve the Cretaceous event i s

r e l a t e d t o accre t ion of Wrangel l ia t o i t s present r e l a t i v e p o s i t i o n i n Nor th

America. This age of accre t ion agrees w i t h s t r a t i g r a p h i c and s t r u c t u r a l

*U.S. Geological Survey, Menlo Park, CA 94025. **Hebrew Un ivers i ty , Jerusalem, I s r a e l .

evidence c i ted by other workers.

INTRODUCTION

The Nikolai Greenstone, a thick sequence o f t h o l e i i t i c , dominantly

subaerial basal t flows exposed i n the Wrangell Mountains and nearby areas of

southern Alaska and the Yukon Terr i tory ( f i g . 1) forms an important part of

the allochthonous terrane of Wrangellia. On the basis of paleomagnetic data

(Hillhouse, 1977), Wrangellia i s believed t o have formed a t low la t i tudes ,

within 15 degrees of the equator, and to have been tectonical ly transported t o

i t s present position (Jones and others, 1977). A1 lochthonous terranes w i t h

lower Mesozoic s t ra t igraphy s imilar t o tha t of Wrangellia i n the McCarthy

quadrangle, are juxtaposed against d i f ferent lower Mesozoic and older rocks

from southern Alaska t o Vancouver Island and possibly in the Hel l ' s Canyon

area of Oregon-Idaho, and are believed t o be the disrupted remnants of a once

coherent sub-continental block (Jones and others, 1977). In the McCarthy

area, emplacement of Wrangel l i a appears to have been accompanied by f r i c t i o n a l

heating, which caused a l t e ra t ion and (o r ) low grade metamorphism of t he

Nikolai Greenstone, and the generation of copper-bearing vein deposits re la ted

to f l u id s o f metamorphic-segregation origin. These veins are localized along

pre-existing s t ructures . This report includes preliminary s tab le isotope data

which bear on the conditions of origin of the deposits and on the

alteration/metamorphism of the greenstone, and K-Ar whole rock ages which we

believe date the time of arr ival of Wrangellia t o i t s present position

r e l a t i v e to adjacent ter ranes .

WRANGELLIA AND THE GEOLOGY OF THE WRANGELL MOUNTAINS

The oldest rocks known in Wrangellia in the Wrangell Mountains and

e l sewhere consist of s l ight ly metamorphosed upper Paleozoic sedimentary and

volcanic rocks ( f ig . 2 ) , believed to represent an upper Paleozoic Island arc,

formed 1 argely on oceanic crust (MacKevett , and others, 1977; MacKevett ,

1978). The upper Paleozoic rocks are uncomformably overlain by more than

3000 m of Triassic subaerial t h o l e i i t i c flows and minor subaqueous pillow

lavas of the Nikolai Greenstone. Fossils in underlying and overlying rocks

indicate that the Nikolai Greenstone flows were extruded in the Middle and

(or ) Late Triassic (MacKevett, 1978). The Nikolai i s disconformably overlain

by thin, inner platform carbonate rocks, the Chitistone and Nizina Limestones

of Late Triassic age. Thick sequences of Triassic basalts and overlying

platform carbonate are a dis t inct ive character is t ic of a l l terranes believed

to be part of Wrangellia (Jones and others, 1977). These carbonate rocks were

deposited in a marginal sea which developed in the Late Triassic and persisted

into the Late Jurassic. Late Triassic and Jurassic sedimentary rocks

deposited in th i s basin include shales and impure cherts o f the McCarthy

Formation of Late Triassic and Early Jurassic age. A major orogeny began in

the Wrangell Mountains in Late Jurassic time and probably culminated in the

Early Cretaceous. Orogenic act ivi ty included thrust faulting, folding,

formation of conglomerates and intrusion of grani t ic rocks of the Chitina

Valley Batholith and related bodies (mostly in the western and southern parts

of the quadrangle, not shown on figure 2 ) which give la te Jurassic K-Ar ages

(MacKevett, 1978).

Lower and Upper Cretaceous sedimentary rocks unconforrnably over l i e the

Jurassic and older sedimentary and volcanic rocks and the Jurassic grani t ic

Figure 2.--Generalized geology o f part o f the McCarthy quadrangle, Alaska.

rocks. The Cenozoic h i s t o r y o f the area i s dominated by the ex t rus ion o f

vo lcan ic rocks o f t he Wrangell Lava which i s w ide l y d i s t r i b u t e d i n t he

nor thern p a r t of the quadrangle and g ives K-Ar ages o f between about 3 and 10

m.y. (MacKevett, 1978).

Hypabyssal g r a n i t i c and in termediate i n t r u s i o n s re1 ated t o the Wrangell

vo lcan ic a c t i v i t y occur throughout t he quadrangle and g i ve K-Ar ages between

about 14 and 7 m.y. (MacKevett, 1978; E. M. MacKevett and M. L. Silberrnan,

unpub. data, 1980). No major deformation accompanied t h i s T e r t i a r y igneous

a c t i v i t y .

The lower Mesozoic rocks o f the McCarthy area sumnarized i n the

s t r a t i g r a p h i c column o f f i g u r e 3 are c h a r a c t e r i s t i c throughout Wrangell i a

(Jones and others, 1977), and cont ras t s t r o n g l y w i t h those of adjacent

terranes. La ter Mesozoic and younger superjacent s t r a t a d i f f e r f rom area t o

area, suggesting t h a t Wrangell i a shared i t s pos t -ear ly Mesozoic h i s t o r y w i t h

adjacent terranes (Jones and others, 1977).

C h a r a c t e r i s t i c s of the N i k o l a i Greenstone

The N i k o l a i Greenstone, as exposed i n the McCarthy quadrangle ( f i g . I ) ,

i s most ly subaerial , p o r p h y r i t i c , t h o l e i i t i c basa l t , c o n s i s t i n g dominantly o f

in te rmixed pahoehoe and aa f lows which are c h a r a c t e r i s t i c a l l y arnygdaloidal,

and between 0.2 and 15 m i n th ickness. L o c a l l y i t exceeds 3900 m i n

cumulat ive th ickness (MacKevett, 1970; 1978). The b a s a l t i s most ly f i n e -

grained, con ta in ing phenocrysts o f l a b r a d o r i t e and aug i te and sparse o l i v i n e

i n an i n t e r g r a n u l a r groundmass composed c h i e f l y o f p lag ioc lase and augite.

Pr imary minera ls i n order of decreasing abundance are p lagioc lase, augite,

r e 1 i c t o l i v i n e , opaque minerals, sphene and a p a t i t e (MacKevett, 1971).

SUPERJACENT ROCK8

. . . . .+.*. . . .

Y A m V D A C m

w

PLIOCENE -'unwrrr

MIOCENE DIORCtU

UNCONF

C SEDIMENTARY

DISCONFM.

TRIASSIC

MIDDLE TRIASSIC

I I

MIDDLE A N D ( 0 R ) I .

LATE

DISCONFM. U i NlKOLAl

GREENSTOME

QUATERNARY

TERTIARY

CRETACEOUS

JURASSIC

TRIASSIC

UPPER WLEOZOIC

Figure 3 . - -S t ra t ig raph ic column f o r t he Wrangell Mountains i n the McCarthy quadrangle.

Nikolai basal ts are generally slightly quartz normative tholeiites, with some

having olivine in the norm (MacKevett and Richter, 1974). The chemical

analysis (table 1) is an average of 39 samples of the Nikolai Greenstone

collected in the region.

Most of the Nikolai basalts have been altered or metamorphosed locally to

prehni te-pumpel leyite f acies assemblages. The most comnon secondary minerals

are, in order of decreasing abundance: chlorite, iron oxides, epidote, clay

minerals, sericite, prehnite, serpentine minerals, pumpel leyite, quartz and

zeolites. Most of the amygdules in the basalt now contain chlorite - + calcite,

the rest are rich in chalcedonic quartz and epidote. A few contain zeolites,

prehnite or native copper (MacKevett, 1971; 1978). Primary volcanic textures

are preserved despite the pervasive nature of the a1 teration/metamorphism.

Mineralization in the Nikolai Greenstone

The Nikolai Greenstone is intrinsically rich in copper. The mean copper

content in 140 Nikolai Greenstone samples from the McCarthy quadrangle is 157

ppm (MacKevett and Richter, 1974). Small copper concentrations of economic

and subeconomic grade are comnon in the Nikolai and are of two main types:

(1) thin veins and narrow rep1 acement tones and genetical ly a f f i 1 i ated

deposits such as small pods and local disseminations which are

characteristically localized along faults and fractures. The veins are small,

from a few centimeters to approximately a meter in width and are rarely

traceable for more than 200 m along strike. Mineralogy of these deposits

consists largely of bornite, chalcopyrite, minor chalcocite and native copper,

with quartz, calcite and epidote as the chief gangue minerals (Bateman and

McLaughlin, 1920; MacKevett, 1976). A few veins contain some sphalerite and

galena or less comnonly stibnite and realgar or molybdenite. Minor amounts of

Table 1 .--Cherni c a l composit ion and CIPW norms f o r "average" N i k o l a i Greenstone

[From MacKevett and R ich ter , 1974 .]

Oxi de Weight percent CIPW norms

Si02 47.90 Q t z 0.91

2O3 14.50 Or 2.84

Fe203 5.20 Ab 26.25

FeO 6.40 An 24.25

N a20 3.10 Fs 5.48

K2° .48 M t 7.55

H20T 3.76 I 1 2.66

T i02 1.40 A P .38

P2°5 -16 Cc 1.23 MnO .18

S um 99.92 Sum 96.25

D i f f e r e n t i a t i o n index 30.0. I r v i n e and Barager c l a s s i f i c a t i o n (1971), t h o l e i i t i c basa l t .

s i l v e r and some go ld are a l s o present i n some deposi ts (MacKevett, 1976). ( 2 )

The second major type o f occurrence cons is ts o f n a t i v e copper as f i l l i n g s i n

amygdules o r i n b recc ia ted o r r u b b l y upper zones o f c e r t a i n f lows. A few

f l ows con ta in broad b u t e r r a t i c disseminat ions o f f i n e l y p a r t i c u l a t e n a t i v e

copper (Bateman and McLaughlin, 1920; MacKevett, 1976). The veins and

associated deposits o f the type (1) occurrence were be l ieved by MacKevett

(1976) t o be products o f hydrothermal processes r e l a t e d t o Late Jurassic o r

T e r t i a r y p l u t o n i c a c t i v i t y t h a t af fected the area. MacKevett ( w r i t t e n

commun., 1980) i n p a r t i c u l a r be l ieves t h a t veins conta in ing s t i b n i te, rea lgar ,

molybdenite and most of the go ld are g e o l o g i c a l l y r e l a t e d t o the T e r t i a r y

i n t rus ions .

The c a l c i t e - quar tz - ep ido te mineralogy o f many o f the veins and i n t he

adjacent wa l l rocks along w i t h c h l o r i t e and o ther minera ls c h a r c t e r i s t i c o f

low grade metamorphism suggest t o us t h a t the o r i g i n o f the veins might be

re1 ated t o metamorphic-hydrothermal process t h a t caused segregat ion o f t he

i n t r i n s i c a l l y h igh copper content o f the basa l t i n t o f r a c t u r e s and shear

zones. The veins, where we have seen them, lack selvages o r zones t h a t d i f f e r

i n mineralogy from the greenstone i t s e l f suggesting t o us t h a t l i t t l e o r no

temperature d i f f e rences e x i s t e d between the veins and t h e i r wall rocks. The

shear zones were more permeable than the surrounding unbroken rock and

probably represented areas of c o l l e c t i o n o f heated waters which had d isso lved

copper from the wa l l rocks. The n a t i v e copper concentrat ions o f the type (2 )

occurrences are be l i eved t o be r e l a t e d e i t h e r t o te rmina l stages of t he

o r i g i n a l magmatic a c t i v i t y - deu te r i c a l t e r a t i o n and m ine ra l i za t i on , or t o

metamorphism where copper was concentrated i n hydrated p a r t s of t he lava p i l e

(MacKevett , 1976).

S inc l a i r (1977) described a copper deposi t hosted by the N i k o l a i

Greenstone i n the White R iver area o f t he southwest Yukon T e r r i t o r y t h a t has

many s i m i l a r i t i e s t o those i n the N i k o l a i i n the McCarthy quadrangle. The

deposi t cons is ts o f copper su lph i de minera ls and n a t i v e copper i n amygdules,

r u b b l e zones, cross c u t t i n g f r a c t u r e s and l o c a l disseminat ions i n t he N i k o l a i ,

which i s metamorphosed t o p rehn i te -pumpel ley i te f a c i e s minera l assemblages.

He suggests t h a t the ore f l u i d s were der ived by metamorphic dehydrat ion w i t h

some component o r i g i n a t i n g f rom connate waters der ived f rom the under ly ing

upper Paleozoic sedimentary rocks. We will attempt to show a similar origin

for the small copper-containing vein deposits in the Nikolai Greenstone in

McCarthy area.

STABLE I SOTOPE RESULTS

We performed standard oxygen isotope analyses on samples of the Nikolai

Greenstone and on veins contained within it. The data (table 2) include

oxygen isotopic composition of quartz and epidote separated from grab samples

of three mineralized veins that cut the upper part of the Nikolai Greenstone

near the Kennecott mines. Trace element chemical data for these veins are

listed in table 3. The veins are composed mostly of quartz and.epidote with

minor amounts of disseminated bornite, chalcopyrite and native copper. Oxygen

isotopic composition of 5 whole rock samples of the Nikolai Greenstone,

including two samples taken adjacent to quartz epidote veins are listed in

t a b l e 4 . Our objectives i n this study are to determine the origins of the ore

fluids and the temperatures of vein formation, as well as the origin of fluids

in equilibrium with the alteration/metamorphic mineral assemblage of the

Nikolai Greenstone and its temperature of metamorphism. Fluid inclusions are

present in the quartz of the veins, but their small size precluded

quantitative measurement of filling temperatures. Isotopic fractionation of

oxygen between quartz and epidote as a function of temperature has not yet

been determined experimentally or calculated from theoretical considerations,

although experimental studies in the system zoisite-water are presently in

progress (Alan Matthews, unpub. data, 1980).

Table 2 .--Oxygen iso tope composit ion o f quar tz and ep i dote from copper bear ing veins i n N i k o l a i Greenstone

[ D e l t a values are repor ted i n pa r t s per mil.]

Sample number Loca t i on

6 180 s 180 A 1

fo r quar tz f o r epidote -. - - - . - - - -- -- - . - - - - --

E3 E r i e Mine +16.5 +7.2 +9.3

9 Bonanza Mine +16.1 +6 .O +lo. 1

18 Bonanza Ridge +15.7 +7.4 +8.3

' ~ v e r a ~ e f r a c t i o n a t i o n ( 6 9 - 6Ep) = +9.2 per m i l .

Table 3 . - -Par t ia l chemical analyses f o r t r a c e elements i n quar tz-epidote veins of t he N i k o l a i Greenstone

[Reported i n ppm.]

Width o f v e i n C O ~ c r l number Cul ~ i l pbl !5r1 zn2 AS^ sb2 ( i n cm)

l ~ e m i q u a n t i t a t i v e spectrographic analyses, r e s u l t s repor ted i n t he se r ies 1, 1.5, 2, 3, 5, 7, 10 and so on. N = Not detected a t l i m i t s o f de tec t ion given i n parenthesis, L= detected, bu t below l i m i t o f determinat ion, a t o r below value shown. Analyst: E. L. Mosi er.

' ~ n and Sb by atomic absorbt ion a n a l y t i c a l methods. Analysts: R . M. O'Leary and J. A. C r i swe l l .

3 ~ s by c o l o r i m e t r i ~ a n a l y t i c a l method. Analyst: R. M. OILeary.

Table 4 .--Oxygen iso tope composit ion o f t y p i c a l whole-rock N i k o l a i Greenstone samples

[De l ta values are repor ted i n pa r t s per m i l .]

Sampl e number Locat ion Average 6180

N 1 N i k o l a i Creek +10.7

1 8 ~ ~ Bonanza Ridge +9.6

8 Bonanza Mine +8.1 \ + 9.5 - + 0 . 4 ~

9~~ Bonanza Mine +9.1 1 11C Bonanza Mine +10.1 J

l ~ d j acent t o quar tz-epidote ve in.

2 ~ t a n d a r d e r r o r .

We have est imated the temperatures o f fo rmat ion o f the quar tz -ep ido te ve ins i n

t h e N i k o l a i f rom comparison w i t h publ ished data on oxygen iso tope

f r a c t i o n a t i o n between quar tz and epidote i n n a t u r a l environments where the

temperature o f e q u i l i b r a t i o n i s known from independent evidence (Tay lo r and

O'Nei l , 1977; Heaton and Sheppard, 1977). The r e s u l t s o f our temperature

est imates, and c a l c u l a t i o n s based on these temperatures of t h e ore f l u i d

oxygen i s o t o p i c composit ion are l i s t e d i n t a b l e 5. Our c a l c u l a t i o n s suggest

t h a t t he ve ins were deposited a t approx imate ly 200°C from a f l u i d o f 6180 = 1

per m i l . P r e l i m i n a r y r e s u l t s o f the zo i s i t e -wa te r oxygen iso tope

f r a c t i o n a t i o n experiments suggest t h a t t h i s temperature est imate i s accurate

( A 1 an Matthews, unpub. data, 1980).

Table 5 .--Calculated temperature and f l u i d oxygen isotope composition fo r copper bearing quartz-epidote veins in Nikolai Greenstone

[ ~ e l t a values are reported i n parts per mi 1 .]

Approximate temperature a 180 6 180 of f ract ionat ion (quartz) (epi dote) dq - dep

Assume l inear re la t ionship between dq - d e ~

and 1 / ~ * (Urey, 1947; Bigeleisen and Mayer, 1947) and use publis ed data on quartz and epidote oxygen isotopic f ract ionat ion from natural environments where temperature is known. Extrapolate l inear re1 at ionship to measured Nikolai quartz-epidote f ract ionat ions t o calcula te temperature, and use Bottinga and Javoy (1973) quartz-water f ract ionat ion t o calcula te ore f l u id 6180,

480° t o 5 5 0 ~ ~ +13.6 +8.1 +5.5 (Osgood M t s . , Nevada, +14.0 +9.3 +4.7 scarn data of Taylor and O'Neil, 1977)

Average +5.1

340° t o 4 0 0 ~ ~ (Quartz-epi dote veins in Troodos Complex, Cyprus, data of Heaton and Sheppard, 1977 )

- - - - - - - - - - - - 1 - 1 - - - -

Average +6.3 - - - - * - - - - - - - - - - - - - -

Sample number Cal cu 1 atedo temperature ( C ) Calculated 6180 ore f l u i d

Average +1.2

The +8 t o +11 per m i l , oxygen isotope composit ion of the greenstone

samples ( t a b l e 4 ) , l i e s w i t h i n the range o f 6180 repor ted f o r the upper p a r t s

o f ophi 01 i t e sequences t h a t were metamorphosed a t temperatures between about

50°c and 3 5 0 ~ ~ by the ac t i on o f heated sea water, (Spooner, and others, 1974;

1977; Heaton and Sheppard, 1977). We lack 6180 data f o r i n d i v i d u a l minera ls

f rom t h e greenstones, so i t i s no t poss ib le t o c a l c u l a t e metamorphic

e q u i l i b r a t i o n temperatures. Because the minera l assemblages o f our greenstone

samples c o l l e c t e d i n t he v i c i n i t y o f the Kennecott mines area ( f i g . 2 ) do no t

vary w i t h distance from quartz-epidote veins, we be l ieve t h a t no l a rge

temperature d i f f e rences ex i s ted between the veins and t h e i r wa l l rocks when

the veins were emplaced. The temperature l i m i t s f o r z e o l i t e f a c i e s

metamorphism are about 1 0 0 ~ ~ t o 3 0 0 ~ ~ (Miyashiro, 1973; Winkler, 1976); we

consider t h i s a reasonable range o f temperatures f o r format ion o f the quar tz-

ep ido te veins as we l l as f o r t he greenstone minera l assemblage.

P lag ioc lase and c h l o r i t e are the dominant minera ls i n the N i k o l a i

Greenstone samples. Spooner and o thers (1977) suggest t h a t the muscovite-

water oxygen iso tope f r a c t i o n a t i o n r e l a t i o n s h i p w i t h temperature could be used

t o approximate oxygen i s o t o p i c a l t e r a t i o n o f basa l t s and greenstones s ince

oxygen i s o t o p i c f r a c t i o n a t i o n between muscovite and water i s in termediate

between t h a t o f ch lo r i t e -wa te r , and feldspar-water (Taylor , 1974). Using the

oxygen iso tope f r a c t i o n a t i o n vs. temperature r e 1 a t i onsh ip f o r muscovite-water

f rom Bo t t i nga and Javoy (1973) and the temperature ca l cu la ted f o r t he

formation o f the quar tz-epidote veins, we est imate t h a t the oxygen i s o t o p i c

cornpositi on o f the f l u i d i n equi 1 i brium w i t h t h e metamorphosed greenstones

(average 6180 = +9.5 per m i l ) was +4 per m i 1. A t 200°c, the temperature

considered most l i k e l y f o r t he formation of t he veins, a s h i f t o f the f l u i d

oxygen iso tope composit ion t o heavier 6180 would be requ i red i f the same

f l u i d s were involved i n metamorphism o f the basa l t and format ion o f the

veins. I f the f l u i d s were us ing the f r a c t u r e s t h a t now l o c a l i z e the veins as

channelways, and then d i f fused ou t i n t o the wa l l rocks du r ing the metamorphic

process, and the water t o rock r a t i o was r e l a t i v e l y low (<I ) , an oxygen s h i f t

t o h igher values would be expected (O'Nei l and Silberman, 1974; Spooner and

others, 1977) . Although our ca l cu la ted i s o t o p i c compositons o f the f l u i d s are based on

several assumptions, we be l i eve t h a t i t i s poss ib le f o r the same f l u i d t h a t

generated the quar tz-epidote veins t o be invo lved i n metamorphism o f t he

basal ts . The low temperature o f ve in fo rmat ion r e s u l t s i n a ca l cu la ted ore

f l u i d composit ion t h a t would e s s e n t i a l l y be dominated by "modi f ied connate"

water (Taylor , 1974). Thus, on the bas is o f ava i lab le , a l b e i t incomplete,

s t a b l e iso tope data, our i n t e r p r e t a t i o n i s t h a t t he copper bear ing v e i n

deposits i n t h e N i k o l a i Greenstone could have formed by the process suggested

by S i n c l a i r (1977). Fur ther support f o r t h a t process i s given by the r e s u l t s

o f t he K-Ar age determi nat ions.

K-Ar AGE DETERMINATIONS

Seven samples o f t h e N i k o l a i Greenstone were chosen f o r whole rock K-Ar

age determinat ion f rom near the v i c i n i t y o f the Kennecott Mines ( f i g . 2; t a b l e

6 ) . The samples were ground t o -60 +lo0 mesh, bu t were otherwise untreated.

The minera l assemblage o f the samples i s t y p i c a l o f the metamorphosed N i k o l a i

Greenstone. The dominant minera ls i n a l l samples are c h l o r i t e and a l t e r e d

p l a g i o c l ase w i t h subordinate amounts o f the o ther comnon minera ls o f the

greenstone. The K-Ar ages range f rom 91 t o 131 m.y., and al though the ages

Tab le 6.--K-Ar ages o f whole rock N i k o l a i Greenstone samples

Loca t i on Field

number number Loca t i on (percent )1 K2° Age (may. ( f i g . 2 )

1 Swan Swan Lake 0.448 90.9 - + 4.5

2 N 1 N i k o l a i Creek .531 105 - + 5

3 ~ 2 ' N i k o l a i Creek ,285 111 - + 6

4 N 4 N i k o l a i Creek .814 109 + 6

5 1 8 ~ ~ Bonanza Ridge .268 120 + 6

6 8 Bonanza Ridge 1.29 131 - + 7

7 11C Bonanza R i dge .932 113 - + 6

l~ll K20 analyses by Paul Klock, U.S. Geolog ica l Survey, Menlo Park, C A .

'sample N2, argon ana lys is r u n a t U.S. Geo log ica l Survey, Menlo Park, CA. Analysts : M. L. Si lberman and C. L. Connor. Other samples r u n by Teledyne Isotopes, Westwood, NJ. Analyst : Georgiana Ka lech i t z .

3 ~ d j a c e n t t o quar tz -ep ido te ve in .

tend t o c l u s t e r somewhat i n any g iven area, t hey are apparen t l y una f f ec ted by

p r o x i m i t y t o the l a t e T e r t i a r y hypabyssal, Wrangell Lava-re1 ated, p l u tons i n

t h e reg ion, which g i v e K-Ar ages o f 7 t o 15 may. (M. L. Si lberman and E. M.

MacKevett Jr., unpub . data, 1980). Th is episode o f T e r t i a r y magmat ism

appa ren t l y was unaccompanied by o the r than v e r y l o c a l thermal e f f e c t s . One

sample, no. 18A ( t a b l e 6 ) was c o l l e c t e d immediate ly ad jacent t o one o f t he

quar tz -ep ido te ve ins o f t a b l e 2 (sample no. 18).

Of p a r t i c u l a r importance i s the l ack o f any s i g n i f i c a n t age d i f f e r e n c e

between the sample c o l l e c t e d ad jacent t o t h e quar tz -ep ido te v e i n and those

c o l l e c t e d long d is tances away from any veins. I f the veins were emplaced

s i g n i f i c a n t l y l a t e r than metamorphism o f the basa l t , f o r example i n t he l a t e

T e r t i a r y as a hydrothermal e f fec t of i n t r u s i o n o f the Wrangell p lu tons as

suggested by Bateman and McLaughlin (1920), then we should have obta ined a

younger age from the ve in wa l l rock sample. K-Ar age s tud ies have documented

t h a t vo lcan ic wa l l rocks adjacent t o hydrothermal ve in deposi ts formed a t

temperatures s i m i l a r t o those ca l cu la ted f o r the N i k o l a i veins y i e l d wa l l rock

ages t h a t are rese t t o the age o f m i n e r a l i z a t i o n if t h i s process i s

s i g n i f i c a n t l y younger than the age o f the host rocks (Silberman and others,

1972; Ashley and Silberman, 1976; Morton and others, 1977). We i n t e r p r e t our

age r e s u l t s t o i n d i c a t e t h a t metamorphism and quar tz-epidote ve in ing were

nea r l y simultaneous. These age data support our conclusions based on our

s t a b l e iso tope data, which suggest t o us t h a t the same f l u i d s respons ib le f o r

depos i t ion o f t he quar tz-epi dote veins were a1 so invo lved i n metamorphic

r e c r y s t a l l i z a t i o n o f the N i k o l a i Greenstone.

Range i n the i n d i v i d u a l ages i s r e l a t i v e l y large, perhaps due t o l o c a l

d i f f e rences i n metamorphic temperature h i s to ry , b u t i s unre la ted t o potassi urn

content o f t he samples. We p l o t t e d t h e K and Ar a n a l y t i c a l r e s u l t s on an

i n i t i a1 argon diagram (Sha f iqu l l ah and Damon, 1974) t o examine the systematics

of t he data ( f i g . 4) . On t h i s type o f diagram the slope o f a regression l i n e

through the po in t s i s p ropo r t i ona l t o the age o f c r y s t a l l i z a t i o n o f the system

and the i n t e r c e p t on the argon a x i s i nd i ca tes whether t he re i s extraneous

argon, or argon loss i n the samples. The use o f isochron analys is has the

imp1 i c i t assumption t h a t a l l o f the p l o t t e d samples conta in non-radiogenic

argon o f the same composit ion a t the t ime o f metamorphic r e c r y s t a l l i z a t i o n

HlKOLAl GREENSTONE I N I T I A L ARGON ISOCHRON

Figure 4 . - - I n i t i a l argon diagram for Nikolai Greenstone whole rock K-Ar d a t a .

(Sha f i qu l l ah and Damon, 1974; Turner and others, 1979). Because a l l o f our

samples come from a r e s t r i c t e d area, and are o f s i m i l a r mineralogy, t h i s

assumption, f o r t h i s p a r t i c u l a r case, i s probably v a l i d .

The s lope of t h e regress ion l i n e through t h e p o i n t s and i t s s t a t i s t i c a l

u n c e r t a i n t y y i e l d an isochron age of 112 - + 11 m.y., and an i n t e r c e p t o f

zero. These r e l a t i o n s i n d i c a t e t h a t t he re has been no l oss o f argon f rom t h e

samples s ince c r y s t a l l i z a t i o n o f the present minera l assemblage i n the midd le

Cretaceous. Agreement of t h e "s lope" age and the average age of t he samples

i s another i n d i c a t i o n t h a t a t r u e c r y s t a l l i z a t i o n event has been dated

(Shaf i q u l l ah and Damon, 1974). We conclude t h a t t h e N i k o l a i Greenstone

c r y s t a l l i z e d t o i t s present minera l assemblage dur ing a thermal episode i n t he

midd le Cretaceous and has been essent i a1 l y unaffected by s i g n i f i c a n t argon

l oss s ince t h a t t ime.

The age o f format ion o f the N i k o l a i Greenstone, Middle and ( o r ) Late

T r i a s s i c , would by t h e t ime sca le i n use by U.S. Geologica l Survey (Geologic

Names Committee, 1980) be about 210 t o 220 m.y. Two o ther reg iona l thermal

events, besides the o r i g i n a l vo lcan ic ex t rus ion , could have a f f e c t e d t h e

N i k o l a i Greenstone. The f i r s t was caused by i n t r u s i o n o f the g r a n i t i c rocks

o f t h e C h i t i n a Va l l ey b a t h o l i t h i n t o t h e Wrangell t e r rane i n t h e l a t e Jurass ic

(MacKevett, 1978). The metamorphism o f the N i k o l a i Greenstone i s c l e a r l y

younger than t h a t . The second thermal event, t he Late T e r t i a r y i n t r u s i o n o f

t h e Wrangell Lava r e l a t e d p lu tons appears t o have had no e f f e c t on the N i k o l a i

K-Ar ages.

G r a n i t i c rocks a lso occur i n the nor thern p a r t o f the Wrangell t e r rane i n

t h e eastern Alaska Range. Two l a rge composite p lu tons o f g rea te r than 100 km 2

outcrop area, w i t h associ ated hydrothermal a1 t e r a t i o n and porphyry copper and

molybdenum occurrences, and smal l e r sate1 1 i t i c p lu tons have g iven ages o f 80

to 120 m.y. (Richter and others, 1975; Silberman and others, 1977). These

gran i t i c rocks occur in the Nabesna quadrangle and the northeastern part of

the McCarthy quadrangle, over 80 km distant from the nearest dated Nikolai

Greenstone sample. No grani t ic rocks of th i s age have been found elsewhere in

the McCarthy quadrangle, and i t i s our opinion that thermal effects of these

plutons could not have possible have affected our Nikolai K-Ar resul ts .

However, plutons of th is age do have bearing on the limits for age of

accretion of the Wrangell terrane in southern Alaska.

DI SCUSS ION

Jones and Silberling (1979) believe t h a t the bulk of tectonic ac t iv i ty

that formed the accretionary mosaic of disparate terranes in southern Alaska,

including Wrangellia ( f ig . 5), occurred in the middle to la te Cretaceous. The

arr ival of Wrangellia to i t s present relat ive position in southern Alaska

represents only part of a very complex and poorly understood history of

deposition, tectonic transport, accretion, and large-scale structural

juxtaposition. Figure 5 i l l u s t r a t e s just a few of the over 25 discrete

tectonostratigraphic terranes that make up southern and central Alaska (Jones

and Silberling, 1979). Approximate l imits on the time of arrival of

Wrangelli a to i t s present re1 ative position are based on widespread and

locally intense deformation of Upper Jurassic and Lower Cretaceous flysch

deposits that are exposed througout southern Alaska ( f ig . 5 ) . Stratigraphic

and structural studies by Csejtey and S t . Aubin (1980) in the Talkeetna

Mountains demonstrate that upper Paleozoic and Triassic rocks of the Wrangell

terrane are thrust over severely deformed argi 11 i t e and graywacke. Granitic

Figure 5.--Map showing Alaska, i n c

YOOIFIED FROM > O N E S AND

S I L B E R L I ~ Q (1q19)

distribution of selected Mesozoic terranes i n southern uding Wrangel l i a.

p l u tons i n t rude the deformed sedimentary rocks, are undeformed themselves, and

g i ve Late Cretaceous t o Paleocene K-Ar ages (Csej tey and others, 1978; Csej tey

and S t . Aubin, 1980).

Add i t i ona l cons t ra in t s on the t i m i n g o f j u x t a p o s i t i o n o f Wrangel l ia and

adjacent terranes comes from the g r a n i t i c p lu tons i n the Nabesna area. These

middle and upper Cretaceous p lutons are p a r t o f the Nutzotin-Chichagof b e l t o f

p l u t o n i c rocks, one o f f i v e such be1 t s i n southern and southeastern Alaska

def ined by Hudson (1979a), who be l ieves t h a t they may represent pa r t s o f

magmatic arcs, al though -- " the data are inconc lus ive o r simply too scarce f o r

a d e f i n i t i v e i n t e r p r e t a t i o n " (Hudson, 1979a, p. 231). The p lu tons t h a t form

t h e Nutzotin-Chichagof b e l t i n t r u d e rocks o f th ree d i f f e ren t terranes,

i n c l u d i n g Wrangel l ia, the Alexander te r rane and the Gravina-Nutzot in

sedimentary and vo lcan ic b e l t (Berg, 1972; Hudson, 1979b). I n the nor thern

p a r t o f the Nutzotin-Chichagof p l u t o n i c b e l t , the g r a n i t i c rocks, i n c l u d i n g

the p lu tons near Nabesna, are l a rge complex epizonal bodies t h a t i n t r u d e

Wrangell te r rane rocks, rocks of Precambrian(?) t o upper Paleozoic and lower

Mesozoic age assigned t o the Alexander terrane, and sedimentary and vo lcan ic

rocks of the middle Jurassic t o lower Cretaceous Gravina-Nutzot in b e l t , which

d e p o s i t i o n a l l y o v e r l i e rocks o f the two o lde r terranes (Berg, 1972).

I f these p lu tons represent a magmatic arc, then the arc developed on a

basement composed o f d ispara te terranes t h a t were juxtaposed, o r were

reasonably c lose t o each o ther by a t l e a s t E a r l y Cretaceous time, and were

i n t ruded together by Middle Cretaceous t ime (Berg, 1972).

Regional s t r u c t u r a l , s t r a t i g r a p h i c , and p l u t o n i c h i s t o r y i n southern

Alaska thus suggests t h a t j u x t a p o s i t i o n o f several of the tec tonos t ra t i g raph ic

terranes, i n c l u d i n g Wrangell i a, occurred by Late Jurassic -- E a r l y Cretaceous

o r Late Cretaceous-Paleocene time. We be l i eve t h a t t he 112 m.y.

r e c r y s t a l l i za t i on age f o r the Niko7ai Greenstone i n the McCarthy area r e s u l t e d

f rom heat ing o f the te r rane caused by accre t ion o f Wrangel l ia t o i t s present

r e l a t i v e pos i t i on . The McCarthy area i s some dis tance away f rom the

boundaries of Wrangel l ia, and i s u n l i k e l y t o have been a f fec ted d i r e c t l y by

thermal e f fec ts of Cretaceous plutonism. A1 teration/metamorphism o f the

N iko la i , and copper m i n e r a l i z a t i o n t h a t appears t o be o f metamorphic

segregat ion o r i g i n , from ore f l u i d der ived by upward migra t ion o f evolved

connate waters from the under ly ing upper Paleozoic sedimentary rocks, appear

t o have occurred dur ing the process o f accret ion.

This r e p o r t documents two important po in ts . The f i r s t , discussed by Berg

(1979) i s t h a t the process of accre t ion i t s e l f may g ive r i s e t o c e r t a i n types

o f minera l deposits, o r r e s u l t i n modi f i c a t i o n of p re -ex i s t i ng minera l

deposits. The second i s t h a t it may be poss ib le t o determine the age o f

accre t ion o f al locthanous terranes by a p p l i c a t i o n o f convent ional K-Ar

geochronology t o low grade metamorphic, or a l t e r e d rocks. By extending t h i s

reasoning, i t may be poss ib le t o determine the age o f accre t ion by dat ing the

gangue minera ls o f c e r t a i n types o f minera l deposits, such as f rac ture-and

shear -cont ro l led base metal r u l p h i d e deposits, if by geologic mapping and

i s o t o p i c work they can be r e 1 ated t o metamorphic remobi l i z a t i o n and

concentrat ion o f ore metals. We p lan on t e s t i n g t h i s technique f u r t h e r i n

areas where independent evidence cons t ra ins the t i m i n g o f accre t ion i n

greenstone-bear i ng al lochthonous terranes, such as the Chugach and

Pr ince W i 11 i am terranes.

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