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Cordilleran geochronology deduced from hydrothermal leads Small, William David 1968

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CORDILLERAN GEOCHRONOLOGY DEDUCED FROM HYDROTHERMAL LEADS by WILLIAM DAVID SMALL B.S., Rensselaer Polytechnic Institute, 1951 M.S., Harvard University, 1954 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of GEOPHYSICS We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1968 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced d e g r e e a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department or by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f _ Geophysics The U n i v e r s i t y o f B r i t i s h Co lumbia V a n c o u v e r 8, Canada Date December 6, 1968 ABSTRACT A t o t a l of 34 lead ore samples from selected hydrothermal deposits i n Wyoming, Montana, and Idaho have been i s o t o p i c a l l y analyzed and geochronological interpretations made from the re s u l t s . Leads from along the southeastern flank of the Idaho Batholith appear to have a primary component 2500 my old. Leads from Butte and Cassia counties, Idaho, may be interpreted as having this same primary component with an added component that is estimated to be 1400 to 1600 my old. The radiogenic component of leads along the southeastern flank of the Idaho Batholith commenced development in a closed system 2500 my ago. Radiogenic components of the leads from Butte and Cassia counties commenced development 1900 and 2700 my ago respec-t i v e l y . Preliminary results of analyses from the south end of the Wind River Mtns, Wyoming, and the L i t t l e Belt Mtns, Montana, show primary lead ages of about 3200 and 2200 my respectively. Common lead geochronology indicates that the basement rocks of Southern Idaho may be assigned to the Superior Province of North America as defined by Kanasewich (1965). A second Precambrian event was recorded by a change i n the lead iso-tope abundances during the Penokian era. Thus, Southern Idaho had been.subjected to several u p l i f t s during parts of Early and Middle Precambrian time. The ages of the anomalous leads from Butte and Cassia counties could represent the times of formation of sedimentary layers which remained closed systems u n t i l the time of formation of the ore bodies. i i i A model f o r c o n t i n e n t a l a c c r e t i o n and growth i s d i s -cussed. The c o n t r i b u t i o n of the present r e p o r t to t e c t o n i c development models i s i n the suggestion of a g e o l o g i c a l sequence which may give r i s e to anomalous l e a d s u i t e s . This g e o l o g i c a l sequence i s concerned with r e g i o n a l t e c t o n i c events which take p l a c e i n the lower c r u s t and are manifested by igneous a c t i v i t y . Examples of leads with apparent enrichment i n the 208 i s o t o p e were found during t h i s study and other i n s t a n c e s are mentioned. The enrichment i s t e n t a t i v e l y c o n s i d e r e d to occur as a r e s u l t of c o n c e n t r a t i o n of the thorium decay product i n sedimentary b a s i n s . This c o u l d occur i f the thorium i s i n more e a s i l y weathered minerals than are the uranium i s o t o p e s . Evidence s u p p o r t i n g n a t u r a l l y o c c u r r i n g l e a d i sotope e n r i c h -ment phenomena i s c i t e d . i v ACKNOWLEDGEMENTS We are pleased to have t h i s o p p o r t u n i t y to express our g r a t i t u d e to P r o f e s s o r and Mrs. Slawson f o r the kindness and f r i e n d s h i p they extended to us during our residen c e at the U n i v e r s i t y o f B r i t i s h Columbia. P r o f e s s o r Slawson suggested the present p r o j e c t and h i s a c t i v e support and encouragement was apparent at each ste p . The enthusiasm o f P r o f e s s o r J.A. Jacobs, and the ch a l l e n g e to match h i s own high standards of pe r s o n a l accomplishment, c o n t r i b u t e d toward making the stay at U.B.C. an enjoyable and p r o f i t a b l e experience. The s u c c e s s f u l completion of the present work was m a t e r i a l l y aided by the t e c h n i c a l . a d v i c e o f , and general d i s c u s s i o n s e s s i o n s w i t h , P r o f e s s o r R.D. R u s s e l l . V a l u a b l e d i s c u s s i o n s were he l d with Dr. T.J. U l r y c h , Dr. J.V. Ross and Dr. A.J. S i n c l a i r . Dr. J . S . Stacey of the U.S. G e o l o g i c a l Survey k i n d l y s u p p l i e d h i s unpublished data and i n t e r p r e t a t i o n s of the Utah l e a d s . Dr. B.D. Kybett and Dr. W.A. Gordon, both at the U n i v e r s i t y of Saskatchewan Regina Campus, made suggestions f o r improving t h i s r e p o r t . I thank Miss Sharon Newman f o r her h e l p f u l a s s i s t a n c e throughout the p r o j e c t , and f o r t y p i n g most of the s e v e r a l d r a f t s and the f i n a l copy of t h i s t h e s i s . Any successes which t h i s p r o j e c t a t t a i n s are d i r e c t l y t r a c e a b l e to the general atmosphere of i n q u i r y and e x a c t i t u d e which p r e v a i l s i n the Department of Geophysics. I accept s o l e r e s p o n s i b i l i t y f o r a l l shortcomings i n the work. Major f i n a n c i a l support f o r t h i s p r o j e c t was provided through both the N a t i o n a l Research C o u n c i l of Canada and the N a t i o n a l V Science Foundation (Grant GA-737). The Prin c i p a l of the University of Saskatchewan, Regina Campus provided some of the f i n a n c i a l support during the l a t t e r stage. v i TABLE OF CONTENTS ABSTRACT ACKNOWLEDGEMENTS TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES INTRODUCTION Isotopic Development of Lead SAMPLE ANALYSES AND INTERPRETATIONS At l a n t i c City D i s t r i c t , Wyoming, Sample Montana Samples Central Idaho Samples Butte County, Idaho, Samples Cassia County, Idaho, Samples Summary of Idaho Analyses Nevada Samples CRUSTAL ACCRETION AND PRIMARY LEADS Ringwood's Hypotheses Comments' on Ringwood's Hypothesis CRUSTAL DEVELOPMENT AND ANOMALOUS LEADS Crustal Development Normalized Pb208 Linear Trends SUMMARY BIBLIOGRAPHY APPENDIX v i i L I S T OF F I G U R E S F i g u r e 1 L o c a t i o n map o f t h e r e g i o n s f r o m w h i c h l e a d s a m p l e s w e r e o b t a i n e d f o r t h e p r e s e n t s t u d y 2 F i g u r e 2 L o c a t i o n map sho\tfing e x p o s u r e s o f P r e c a m b r i a n r o c k s i n t h e n o r t h w e s t e r n U n i t e d S t a t e s 3 F i g u r e 3 The b a s i c s i n g l e s t a g e l e a d i s o t o p e g r o w t h e q u a t i o n s 7 F i g u r e 4 The common l e a d i s o t o p e g r o w t h c u r v e s 9 F i g u r e 5 The s i m p l e two s t a g e l e a d i s o t o p e g r o w t h e q u a t i o n s a s s u m i n g t h e O s t i c - R u s s e l l - S t a n t o n p r i m a r y l e a d m o d e l 10 F i g u r e 6 G r a p h i c a l d e m o n s t r a t i o n o f t h e i n t e r p r e t a t i o n o f t h e l e a d i s o t o p e s t u d i e s i n s e l e c t e d r e g i o n s o f I d a h o , M o n t a n a a n d Wyoming 16 F i g u r e 7 L o c a t i o n map o f M o n t a n a s a m p l e s 19 F i g u r e 8 B e a r t o o t h M t n s , M o n t a n a . N o r m a l i z e d l e a d 207 v s l e a d 206 i s o t o p e r a t i o s 20 F i g u r e 9 B e a r t o o t h M t n s , M o n t a n a . N o r m a l i z e d l e a d 208 v s l e a d 206 i s o t o p e r a t i o s 21 F i g u r e 10 L i t t l e B e l t M t n s , M o n t a n a . N o r m a l i z e d l e a d 207 v s l e a d 206 i s o t o p e r a t i o s 22 F i g u r e 11 L i t t l e B e l t M t n s , M o n t a n a , N o r m a l i z e d l e a d 208 v s l e a d 206 i s o t o p e r a t i o s 23 F i g u r e 12 L o c a t i o n map o f c e n t r a l I d a h o s a m p l e s 25 F i g u r e 13 C e n t r a l I d a h o . N o r m a l i z e d l e a d 207 v s l e a d 206 i s o t o p e r a t i o s 26 F i g u r e 14 C e n t r a l I d a h o , N o r m a l i z e d l e a d 208 v s l e a d 206 i s o t o p e r a t i o s 27 F i g u r e 15 L o c a t i o n map o f B u t t e C o u n t y , I d a h o s a m p l e s 30 F i g u r e 16 B u t t e C o u n t y , I d a h o . N o r m a l i z e d l e a d 207 v s l e a d 206 i s o t o p e r a t i o s 31 F i g u r e 17 B u t t e C o u n t y , I d a h o , N o r m a l i z e d l e a d 208 v s l e a d 206 i s o t o p e r a t i o s 32 F i g u r e 18 L o c a t i o n map o f C a s s i a C o u n t y , I d a h o s a m p l e s 35 v i i i Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 Figure 25 Figure 26 Figure 27 Figure A-1 Cassia County, Idaho. Normalized lead 207 vs lead 206 isotope ratios 36 Cassia County, Idaho. Normalized lead 208 vs lead 206 isotope ratios 37 Geological provinces of North America 40 The v a r i a t i o n of calculated seismic wave ve l o c i t y with composition 45 Assumed chemical model for the upper mantle 56 Possible mineral assemblages of p y r o l i t e 56 Petrological model and c h a r a c t e r i s t i c geotherms for the upper mantle 57' Sudbury, Ontario, Canada. Normalized lead 207 and 206 isotopic ratios 76 Sudbury, Ontario, Canada. Normalized lead 208 and 206 isotopic ratios 77 Frequency d i s t r i b u t i o n of percent difference between duplicate measurements of 32 di f f e r e n t samples 9 3 Figure A-2 Chronological values obtained for twelve measure-ments of U.B.C. sample 1 96 i x LIST OF TABLES Table I The lead i s o t o p e sources 4 Table II Tabulated r e s u l t s of the lead i sotope s t u d i e s i n s e l e c t e d regions of Idaho, Montana, and Wyoming 15 Table A - I L i s t o f lead i s o t o p e analyses r e s u l t s 90 Table A-II S t a t i s t i c a l a n a l y s i s of twelve measurements of UBC sample 1 95 1 INTRODUCTION The present study commenced with an experimental attempt to use lead i s o t o p e s to determine the age of basement rocks i n a r e g i o n that has c o n s i d e r a b l e o v e r p r i n t i n g by repeated orogenic events and subsequent sedimentary cover. Success of such a study i s l i m i t e d by the v a l i d i t y of the assumptions r e g a r d i n g genesis of hydrothermal ore d e p o s i t s and models of lead i s otope development. Several r e l a t e d items became apparent as the study progressed. These items were the d e t e r m i n a t i o n of d i s c r e t e regions with a 2 0 8 common geochronology, the e x i s t e n c e of l i n e a r trends i n the Pb 2 0 6 i s o t o p e r e l a t i v e to the Pb i s o t o p e , and the i n d i c a t i o n of 2 0 8 i s o t o p i c d i f f e r e n t i a t i o n r e s u l t i n g i n abnormally high Pb abundances. A d d i t i o n a l l y , i t i s necessary to d e f i n e what type of event i s recorded by the beginning and end of the time i n t e r v a l s determined by the l e a d - l e a d model. The types of events suggested are r e l a t e d to c o n t i n e n t a l a c c r e t i o n and subsequent development. The sampled lead d e p o s i t s are considered to be a by-product of the i n t r u s i o n s a s s o c i a t e d with u p l i f t . Development w i t h i n t h i s r e p o r t suggests primary type leads may be i n d i c a t i v e that m i n e r a l -o g i c a l f r a c t i o n a t i o n events i n the upper mantle are a s s o c i a t e d with the development of c o n t i n e n t a l m a t e r i a l . The r a d i o g e n i c components of the sampled leads are the product of subsequent r e g i o n a l development i n v o l v i n g orogenic events i n c o n t i n e n t a l c r u s t a l l a y e r s . l a The experimental e f f o r t of the present project is involved with obtaining lead-lead ages in parts of Montana, Wyoming, and Idaho. The general sample locations are shown in Figure 1. It is the intent of the study to determine i f the Superior province as defined by Kanasewich (19651) extends into central and southern Idaho. Precambrian exposures in this region, Figure 2, are primarily sedimentary rocks of the Belt Series, which is consid-erably younger than the Superior province. The major known Precambrian intrusions are located in south-western Montana and in Wyoming. In general, previous attempts to obtain radiometric dates for ancient geological events in the region of Idaho have been frustrated by the presence of the younger Beltian exposures and the "overprinting" by Mesozoic and Cenozoic mountain building events. The use of lead-lead ages to obtain ancient radiometric dates is based on the b e l i e f that a portion of t h e l e a d in present ore deposits had i t s o r i g i n in the basement rocks that would contain the Superior province record, i f this record existed. Isotopic Development of Lead The lead presently incorporated into the t e r r e s t r i a l minerals has been derived from two sources: the lead o r i g i n a l l y incorpor-ated into the earth at the oldest datable event in i t s formation, and the lead which has since been derived from the decay of the natural radioactive elements, uranium and thorium. Table I demonstrates the sources of each of the four isotopes of lead and their r e l a t i v e abundances. The 204 isotope of lead has no radioactive precursor, therefore i t is used to normalize the isotopic abundances of the remaining isotopes. The half lives of 8 *' r i s H c on UMB i A A |L 8 E R T A •Seaff/a Spokane Locait ioni' L « n: d I) Zenith,, Wyoming, 2: Montana 3 Central! Idaho 4 Butte County,, Idahoi 5 Cattia County;,. Idaho-Salmon' « 0 A H ! 0 :J K*tehuini eBur ley C A t ' F 0 R tt I A N E V A OA Idaho: Falle .Salt. Lake City U T A. H F i g u r e 1: L o c a t i o n Map o f the Regions from which Lead Samples were Obtained f o r the Pre s e n t Study.. Figure 2: Location Map Showing Exposures of Precambrian Rocks i n the Northwestern United States.. Table I: ! be L e a d Isotope Sources Lead Isotope 2 0 4 20S 2 0 7 2 0 8 The Lead Originally Incorporated into the Ear th Primordial Abundance Radioactive Ciisin Parent rial? L i f e Longest Lived Intermediate Isotope Haif Life 9 - 5 6 None 10-42 Tha Radiogenic Component 3 0 - i C U - 2 3 S 9 4-5iQxiO yr U - 2 3 4 2 - 4 8 s l 0 yr U - 2 3 5 7- l30it lO yr p c - 2 3 i 3 - 4 3 x 1 0 yr T h - 2 3 2 10 1-39x10 yz R a — 2 2 8 6 - 7 y r Present Day 'Averaga' Abundance* I 13-67 15-80 '••Ostic, R u s s e l l , and Stanton (1967)." 3 9 - 5 5 5 the i n t e r m e d i a t e decay products of uranium and thorium are r e l a t i v e l ) short compared with g e o l o g i c a l processes. The intermediate decay products l e a d i n g to the remaining three lead i s o t o p e s are commonly ignored. A t a b u l a t i o n of the measured t e r r e s t r i a l lead i s o t o p i c abundances i n d i c a t e s the p r o b a b i l i t y that they may be d i v i d e d i n t o two d i s t i n c t groups. R u s s e l l and Farquhar (I960) have summarized the evidence f o r the two groups and placed an i n t e r -p r e t a t i o n on t h e i r s i g n i f i c a n c e . The s i m p l e s t t e r r e s t r i a l l e a d group i s the primary l e a d s . These have been i n v e s t i g a t e d by O s t i c , R u s s e l l and Stanton (1967). They appear to be found on a world-wide s c a l e , most of them coming from l a r g e s t r a t i f o r m d e p o s i t s . They are c h a r a c t e r i z e d by g i v i n g evidence of having developed i n a system with a r e s t r i c t e d v a r i a t i o n i n the uranium-lead and thorium-lead r a t i o s f o r a w e l l d e f i n e d time i n t e r v a l commencing 4550 m i l l i o n years (my) ago and c l o s i n g at the time the l e a d leaves t h i s uniform environment. The age of the primary lead i s the time i n the past at which t h i s i n t e r v a l c l o s e d . The second l e a d group contains the anomalous leads and form a more complex group. In a d d i t i o n to demonstrating a primary-l i k e parentage, they show evidence of an a d d i t i o n a l r a d i o g e n i c component. I t appears that l e a d was o r i g i n a l l y i n c o r p o r a t e d i n t o the parent rock. In a d d i t i o n , a r a d i o g e n i c component evolved i n the rock from the decay of the contained uranium and thorium. The e v o l u t i o n of the r a d i o g e n i c p o r t i o n i s assumed to take plac e i n a c l o s e d system. The amount of r a d i o g e n i c lead a v a i l a b l e f o r mixing with the o r i g i n a l lead depends not only on the r e l a t i v e 6 amounts of. t h o r i u m and. uranium i n the c l o s e d r o c k system but a l s o upon the t i m e i n t e r v a l between the f o r m a t i o n o f the system and the time a t w h i c h the l e a d was e x t r a c t e d and d e p o s i t e d i n the ore body. Anomalous l e a d samples t a k e n from s e v e r a l a d j a c e n t m i n i n g d i s t r i c t s o r even, from-two l o c a t i o n s i n the same ore body, need not have the e same i s o t o p i c c o m p o s i t i o n . However, i f the o v e r a l l g e o g r a p h i c a l r e g i o n has been s u b j e c t e d t o a s i m i l a r g e o l o g i c a l h i s t o r y , c e r t a i n linea-r- t r e n d s i n t h e l e a d i s o t o p e c o m p o s i t i o n s w i l l be a p p arent. Some o f the i m p o r t a n c e time i n t e r v a l s i n the g e o l o g i c a l e v o l u t i o n . o f the r e g i o n may be deduced from t h e s e l i n e a r t r e n d s . * The n o r m a l i z e d l e a d i s o t o p e growth e q u a t i o n s are g i v e n i n F i g u r e 3.. These a r e the u s u a l r a d i o a c t i v e decay e q u a t i o n s t h a t have been, t r a n s p o s e d t o show the growth of the daughter p r o d u c t , and t ime i n t h e p a s t as p o s i t i v e . The f i r s t term on the r i g h t o f each e q u a t i o n shows the i s o t o p i c r a t i o o f the o r i g i n a l l e a d i n c o r p o r a t e d i n t o the c l o s e d system, the second term shows the r a d i o g e n i c l e a d g e n e r a t e d i n the time i n t e r v a l between the f o r -m a t i o n o f t h e r o c k as a c l o s e d system and the e x t r a c t i o n o f the l e a d to; form th e o r e body. To be r i g o r o u s , U q and ftQ i m p l i c i t l y c o n t a i n a f a c t o r t o account f o r the e x t r a c t i o n e f f i c i e n c y f o r o b t a i n i n g l e a d from the rock m i n e r a l s , the o r i g i n a l l e a d and the l e a d d e r i v e d from the decay of uranium and t h o r i u m may be i n t h r e e d i f f e r e n t m i n e r a l s w i t h d i f f e r e n t e x t r a c t i o n e f f i c i e n c i e s . T h i s f u r t h e r f a c t o r i n y Q and QQ-does not. d e t e r the use o f the e q u a t i o n s i n a s c e r t a i n i n g l i n e a r t r e n d s e x c e p t t o produce u n r e a l p Q and ftQ r a t i o s i n some c a s e s . *A g e o l o g i c a l d e m o n s t r a t i o n o f g e n e t i c r e l a t i o n s h i p s would r e i n f o r c e t h i s argument. / The Basic Single Stage Lead Isotope Growth Equations I. P b - 2 0 6 . P b - 2 0 4 P b - 2 0 6 P b - 2 0 4 e 0.15371 0_ 01537 e 2 . P b - 2 07 P b - 2 0 4 P b - 2 0 7 P b - 2 0 4 ^•9722^ e0.9722t," rpb-208i=rpb-2 0 8i + a reo. . [PD-204J LPD-204J. 04 99 tQ_ 0.0499 t," 4 . Slope of 5. Slope of P b - 2 0 7 P b - 2 0 4 P b - 2 0 8 1 o f p b - 2 0 6 ] . _ rjvs Ifb^ loVj p,ot-0.9722U 09722 ti i w — e 0.1537 t, e * e 0.1537 t P b - 2 0 4 il f P b - 2 0 6 l , 0 V S [ P b ^ 2 0 4 J P , 0 t = i l o 0 0499t o 004991, e v — e  ~b~Tsrnz o i537t i e — e ' U — 2 3 8 P b - 2 0 4 ' u - 2 3 8 ; U - 2 35 ' T h - 2 3 2 ' U - 2 3 8 in source rocks , extrapolated to present time extrapolated to present t ime s 137-8 in source rocks , extrapolated to present time time in past (10 yr) at which uranium and thorium commenced decay in a closed system 9 time in past (10 yr) at which the radiogenic lead component was extracted from the closed system, mixed with the initial lead, and deposited in a uranium and thorium free environment (the ore deposit) F i g u r e 3 : T h e B a s i c S i n g l e S t a g e L e a d I s o t o p e G r o w t h E q u a t i o n s , 8 The only i n f o r m a t i o n d i r e c t l y o b t a i n a b l e from the a n a l y s i s of anomalous lead samples from a r e g i o n i s the slope and the 207 206 p o s i t i o n o f the normalized Pb vs Pb l i n e . The slope of t h i s l i n e i s d e f i n e d i n Fig u r e 3, equation 4. I n d i v i d u a l anomalous le a d samples have no g e o c h r o n o l o g i c a l s i g n i f i c a n c e , only the l i n e i s usable i n f o r m a t i o n . S u b s i d i a r y i n f o r m a t i o n , such as other d a t i n g techniques and g e o l o g i c a l evidence, may be employed to d e f i n e T^ and thus TQ , the time at which the c l o s e d rock system was formed, may be determined. The l i n e a r trend 20 7 206 d e f i n e d by the Pb and Pb isot o p e s e x i s t s because these are both decay products of a uranium i s o t o p e , thus the chemistry of the environment i n which they form i s i d e n t i c a l . The primary leads are those which may be d e s c r i b e d by a s p e c i f i c case of the i s o t o p e growth equations. The s i n g l e stage lea d s l i e along growth l i n e s , as shown i n Fig u r e 4, and a l l l i n e s come from a common o r i g i n corresponding to a time 4550 my ago. O s t i c et a l have found that the most probable values of y Q and QQ are 8.99 and 3.89 r e s p e c t i v e l y which correspond to a unique growth curve on each graph. The d e f i n i t i o n of T , U q , and n may be employed as s u b s i d i a r y i n f o r m a t i o n f o r the i n t e r p r e t a t i o n of anomalous l e a d samples, the : values found by O s t i c et a l w i l l be f r e e l y used i n the present r e p o r t . There w i l l be no f u r t h e r use made of the general growth equations of F i g u r e 3, The o r i g i n a l p o r t i o n of the anomalous leads i s sometimes a primary l e a d , i n which event the equations governing the i s o t o p i c abundances are given In F i g u r e 5. These equations i n d i c a t e a lea d which has had a two stage h i s t o r y , the f i r s t stage being 9 Pb 206 I Pb 204 A . Normollied Lead 2 07 vt Lead 206 Common Lead Growth Curves II ? IR »- ~ Y° and lime In the post ore porometers F i g u r e 4: The Common Lead Isotope Growth Curves. " p b - 206' P b - 204 > b ^ 207" P b - 204 > b - 208" P b - 204 f 0 .6993 O.I537t,] [" O . I537t 2 O.I537 t 3 = 9-56 + 8 - 9 9 e - e . +. Mi • ~ e " M f 4 .424 0 . 9 7 2 2 t , ] , M, f = 10-42 + 0 -0652 e - e ' + — 1 0 . 9 7 2 2 t „ 0.9722 t , 2 _ e 3 f 0 . 2 2 7 0 0 . 0 4 9 9 t , " | [ 0 . 0 4 9 9 t » 0 . 0 4 9 9 1 3010 + 37 08 j e - e /ij e - e 9 t, = time in past (10 yr) at which the primary lead is extracted from the l deep uniform source 9 t 2 = time in post (10 yr) at which uranium and thorium commence decay in a closed system 9 t 3 = time in post (10 yr) at which the radiogenic lead is ex t rac ted f r o m the c losed s y s t e m , mixed with t h e primary l e a d , and d e p o s i t e d in a uranium and thorium free system (the ore deposit) F i g u r e 5 : The Simple Two Stage Lead Isotope Growth Equations Assuming the O s t i c - R u s s e l l - S t a n t o n Primary Lead Model 11 e x t r a c t i o n of the primary l e a d from the uniform source (which might be the upper mantle) at the time T^, the second stage being the development of the r a d i o g e n i c l e a d i n a c l o s e d c r u s t a l rock system commencing at time T£. The u n i t i n g of these two leads and the formation of the ore body occurs at time T^. Notice that three d i f f e r e n t events o c c u r r i n g at three d i f f e r e n t times are i m p l i e d by the two stage l e a d e q u a t i o n s 0 The p o s s i b i l i t y t h a t 12 and T j may be e s s e n t i a l l y i d e n t i c a l should not be overlooked, t h i s w i l l produce the s i m p l e s t case of a two stage l e a d . In t h i s i n s t a n c e , the two stage l e a d w i l l c o n s i s t of a primary lead which evolved up to the time T^ and a r a d i o g e n i c component which was generated i n the time i n t e r v a l T. to T^. It Is i n the nature of the equations of Figure 5 t h a t , i f T^ equals T^, the s t r a i g h t l i n e 207 206 * j o i n i n g the Pb vs Pb data crosses the 8.99 growth curve at the times T^ and T^. This s i m p l e s t case i s the only anomalous lead case which w i l l produce a unique s o l u t i o n . In h a n d l i n g a c t u a l anomalous lead i s o t o p e data, i t i s necessary to work backwards to a s c e r t a i n i f the sampled leads appear to be r e p r e s e n t a t i v e of t h i s s i m p l e s t case. The best f i t s t r a i g h t l i n e through the anomalous lea d i s o t o p e data p o i n t s i s p l o t t e d on a graph s i m i l a r to the top of F i g u r e 4. The time T^, which i s assumed to be known from other i n f o r m a t i o n , i s then used as a parameter to determine the p value of the growth curve through the p o i n t of i n t e r c e p t of the time parameter and the anomalous lead l i n e . I t i s now p o s s i b l e to f i n d , where the anomalous lead l i n e r e c r o s s e s t h i s y growth curve to the l e f t , the parametric time at which the r a d i o g e n i c component commenced i t s development. I f the d e r i v e d p value i s c l o s e to 8.99, the lead samples are assumed to be r e p r e s e n t a t i v e 12 .of this simplest two stage case with equal to T£. In this report, the error on obtained from other sources and the un-certainty in the anomalous lead l i n e are such that a derived y value between 8,90 and 9.10 i s considered indicative that the anomalous leads are representative of a simple two stage case. Other derived u values are assumed to indicate more complex cases o There is no a p r i o r i reason to r e s t r i c t the lead isotopes to just two stages, although leads do not normally give evidence of having many components, Russell, Kanasewich and Ozard (1966) have investigated the p o s s i b i l i t y that leads may exhibit a frequently-mixed source and shown that this is incompatible with the good li n e a r trends found in r e a l i t y . The lead analyses given in this report may be interpreted by assuming that there are, at most, two primary components and one radiogenic component. This is s imilar to an interpretation of Utah leads given by Stacey, Zartman, and Nkomo (1968), This writer recognizes that the adequacy of the simplest model to explain observed lead isotope character-i s t i c s does not preclude that the actual history is complex. Linear trends between the Pb^^ and Pb^^ isotopes are not usually observed. The 208 isotope is a daughter product of thorium, which does not have chemical properties i d e n t i c a l to uranium. The r e l a t i v e behavior of uranium and thorium under various geochemical conditions have been discussed by Ahrens (1965). However, as shown in Figure 3, equation 5, there may be linear trends i f Q, is constant throughout a region of similar geological history. This implies that the thorium to uranium r a t i o i s constant in the source 13 rocks of the lead ores and v i r t u a l l y i m p l i e s that the source of the ore leads are the same rock u n i t . A l i n e a r t r e n d i n the normalized P b ^ ^ vs P b ^ ^ p l o t f o r an extended r e g i o n was f i r s t r e c o g n i z e d by S i n c l a i r (1964) i n the Kootenay Arc of B r i t i s h Columbia. The present study gives two i n s t a n c e s of t h i s l i n e a r trend„ 14 SAMPLE ANALYSES AND INTERPRETATIONS Lead samples d e r i v e d from a t o t a l of f i v e regions have been analyzed f o r t h i s r e p o r t . Three of the r e g i o n s , a l l i n Idaho, have been adequately sampled to a s c e r t a i n the I s o t o p i c t r e n d s . Only one sample was a v a i l a b l e from the southern end of the Wind Ri v e r Mountains of Wyoming. The Montana samples are too few i n number and too s c a t t e r e d g e o g r a p h i c a l l y to i n d i c a t e much more than the e x i s t e n c e of anomalous l e a d s . The r e s u l t s from the l a t t e r two regions can serve only as a p r e l i m i n a r y survey. The p r e l i m i n a r y r e s u l t s of a l l of the regions are given i n Table II and F i g u r e 6. A t l a n t i c C i t y D i s t r i c t , Wyoming. Only one sample was a v a i l a b l e from t h i s d i s t r i c t . I t had been s u p p l i e d to the U.B.C. Geophysics l a b o r a t o r y by Dr. R. S. Cannon of the U.S. G e o l o g i c a l Survey. The i s o t o p i c a n a l y s i s of t h i s , and a l l samples analyzed f o r t h i s r e p o r t , i s , given i n the Appendix. The most immediate i n t e r p r e t a t i o n of t h i s sample i s that of a s i n g l e stage lead \tfhich evolved i n an environment with y Q equal to 9.38 i n the time I n t e r v a l of 4550 my ago (the o l d e s t datable event i n the earth's h i s t o r y ) u n t i l 2750 my ago. With only one sample a v a i l a b l e , I t i s immediately p o s s i b l e to a s c r i b e an JJ value of. 4.05 corresponding to the measured normalized 208 2 0 6 2 0 7 -Pb value and the age d e r i v e d from the normalized Pb vs Pb i s o t o p e s . The obtained s i n g l e stage model age of 2750 my f o r the southern p o r t i o n of the Wind R i v e r Mountains i s c o n s i s t e n t with p r e v i o u s l y measured ages. G o l d i c h et a l (1966) show p r e v i o u s l y measured s u r f a c e sample Rb-Sr ages of 2200 my i n t h i s r e g i o n and a deep w e l l sample from near-by y i e l d e d a whole rock Rb-Sr age of Table I I : T a b u l a t e d R e s u l t s o f the Lead Isotope S t u d i e s i n S e l e c t e d Regions o f Idaho, Montana, and Wyoming. SIODe of l° r i - Slope of .., A g ,? °!- Corresponding commencement Primary Corresponding Region 204 ^06 Z 0 A M'neralizotion ,-value of growth lead age fl-Value vs 20^- line vs £34-line 10 yr 10* yr '0 v r Zenith, Wyo. 9-38 > 2-75 4 05 Central Mont. 0-4 014 005 8-95 2-2 Central Idaho 1001-04 01621-004 0-05 902 2-5 2-5 3-8 Butte County 0-75+ 05 01161008 0 0 914 1-9 2-8 Cassio County 0-1831-008 005 9-34 2-7 ; — Phoneroioic I Late Precambrian I Middle Precambrian I Early Precambrian i— i — i — i — i— f— i — i— I I I t— i—i—i—i— f— i—i—i—i—i—i—i—hi— i—i—i—i—i—i— I—T Cassia County-| Butte County Central Idaho Litte Belt Mtns. Zenith r-* t l t t I I l l l l I I l l r I l I I I I » I I I I 1 I I I I I I I OO 0-2 0-4 0-6 0.6 1.0 I .2 14 16 IB 2 0 2.2 2.4 2 6 2-8 3-0 3 2 3-4 9 Isotopic Age, Billion (10 ) Years F i g u r e 6: G r a p h i c a l Demonstration o f the I n t e r p r e t a t i o n o f the Lead Isotope S t u d i e s i n S e l e c t e d Regions o f Idaho, Montana, and Wyoming. 17 2750 my. Thus the one sample analyzed f o r t h i s r e p o r t y i e l d s ages c o n s i s t e n t with previous measurements, Ac c e p t i n g t h i s one sample as being a s i n g l e stage primary lead denies the work of O s t i c et a l i n showing a uniform primary lead-uranium-thorium system with a uranium-lead r a t i o of 8.99 and a thorium-uranium r a t i o of 3.89. Using the O s t i c r e p o r t as a source of s u b s i d i a r y i n f o r m a t i o n and assuming that the A t l a n t i c C i t y sample i s d e r i v e d from the deep uniform source rocks, the best that can be done with one sample i s to assume that i t represents a s h o r t p e r i o d anomalous l e a d , i . e . i n F i g u r e 5, T^ i s i d e n t i c a l with T^ and T^ approaches T^. In t h i s i n s t a n c e , the A t l a n t i c C i t y sample rep r e s e n t s a minimum age of 3200 my, o l d e r than has p r e v i o u s l y been found f o r t h i s area. T h i s age i s con-s i s t e n t with an age of g r e a t e r than 3100 my found i n the Beartooth mountains to the north by Catanzaro and Kulp (1964) „ This i n t e r -p r e t a t i o n would i n d i c a t e that the cores of the Beartooth Mountains and the Wind R i v e r Range are d i s c l o s i n g an age s i m i l a r i t y . The three l e a d samples from the Beartooth Mountains measured f o r t h i s r e p o r t lend some substance to the c o n t e n t i o n of an age s i m i l a r i t y o f the Beartooth and Wind River ranges. Ages of 3000 my or greater are becoming i n c r e a s i n g l y more common i n the r e g i o n w i t h i n s e v e r a l hundred miles to the north and east of the A t l a n t i c C i t y d i s t r i c t (Heimlich and Banks, 1968, f o r example). Montana Samples. A t o t a l of seven samples from Montana were analyzed. These came from two d i f f e r e n t d i s t r i c t s , three samples from the Bear-tooth Mountains and four samples from the L i t t l e B e l t Mountains. These samples were provided by the Montana Bureau of Mines and 18 Geology and had p r e v i o u s l y been used f o r compiling data f o r t h e i r B u l l e t i n 30 (Young, Crowley and Sahinen, 1962). The s p e c i f i c sample l o c a t i o n s are shown i n Figure 7 and the isotope analyses are shown g r a p h i c a l l y i n Figures 8, 9, 10 and 11. The Beartooth Mountain samples are w e l l s c a t t e r e d geographi-c a l l y and the sample an a l y s e s , whose r e l a t i o n s h i p to the u = 8.99 growth curve i s shown i n Fig u r e 8, i n d i c a t e that the three samples are probably not from the same g e o c h r o n o l o g i c a l p r o v i n c e . The 2 01 206 p o s i t i o n o f the three p o i n t s on the normalized Pb vs Pb and Pb^** vs P b ^ ^ curves of Figures 8 and 9 i s c o n s i s t e n t with lead i n d i c a t i n g a very o l d age, perhaps as o l d as the 3100 my proposed by Catanzaro and Kulp. There i s some p l e a s u r e to be obtained by p l a y i n g with the present data. The 4-Sevens and the Irma mines are both l o c a t e d on the south s i d e of the Beartooth B a t h o l i t h and t h e r e f o r e an assumption may reasonably be made that the two p o i n t s are from the same system. M i n e r a l i z a t i o n may be assumed to be a s s o c i a t e d with the T e r t i a r y i n t r u s i v e s that are abundant i n t h i s area (Perry, 1962). As a f i r s t approximation, m i n e r a l i z a t i o n i s accepted as o c c u r r i n g at 50 my and t h e r e f o r e , 20 7 on the b a s i s of two p o i n t s , the r a d i o g e n i c component of the Pb 206 and Pb commenced development 2950 my ago an a y=9.35 curve. The high y value i s i n d i c a t i v e of a m u l t i s t a g e h i s t o r y . Further sampling of t h i s r e g i o n should prove extremely i n t e r e s t i n g . The normalized P b ^ ^ vs P b ^ ^ isotope p l o t f o r the Beartooth Mountains, Figure 9, does not i n d i c a t e any i n f o r m a t i o n r e l e v a n t to the present r e p o r t . The i s o t o p i c analyses are shown i n r e l a t i o n to theili=3.89 curve. The four samples from the L i t t l e B e l t Mountains have a 19 Scale of M i l e s &—-" - i'6' - " I d i t s ' " '4o Figure 7 : Location Map of Montana Samples. 160 i r Granite Mtn -^—-—Irmg" 4-Sevens 14*0' 140 150 16-0 170 18-0 Pb206/Pb204 190 2O0 F i g u r e 8: Beartooth Mountains, Montana. Normalized Lead 207 vs Lead 206 Isotope R a t i o s . 140 15 0 160 17 0 18 0 19 0 20 0 Pb 206 / Pb 204 F i g u r e 11: L i t t l e B e l t Mountains, Montana. Normalized Lead 208 vs Lead 206 Isotope R a t i o s . 24 better geographical d i s t r i b u t i o n and show a reasonably linear trend. The straight l i n e drawn on the P b 2 0 7 vs P b 2 0 6 portion of Figure 10 is the best f i t line for the four samples and the curved l i n e i s the p=8.99 primary lead growth curve. It i s to be regretted that there i s not a greater dispersion of values among the four samples so that the linear trend could be more accurately ascertained Assuming that the trend exists and accepting T^ as 50 my (Perry, 1962), the L i t t l e Belt Mountain samples indicate an age of 2200 my for the time at which the radiogenic portion of the lead commenced i t s growth. These figures are consistent with a P q value of 8.95, c e r t a i n l y not distinguishable from the 8.99 quoted by Ostic et a l for the uniform primary system. Thus the primary lead portion of the L i t t l e Belt Mountain samples may have been derived from the uniform lead-thorium-uranium environment about 2200 my ago. With the scanty data, the derived age i s reasonable i n view of the age of 2470 my or greater found by Catanzaro and Kulp (1964) . The normalized P b 2 0 8 vs P b 2 0 6 isotope plot for the L i t t l e Belt Mountains, Figure 11, may exhibit a linear trend. The data is not adequate to comment further. Central Idaho Samples. The s p e c i f i c locations for the eight Central Idaho samples are shown in Figure 12 and the normalized isotopic analyses are shown in Figures 13 and 14. Many of these samples were provided by Mr. Norman C. Williams, Federal Resources Corp,, Salt Lake City, Utah. Others were provided by Dr. Edward T. Ruppel of the Denver o f f i c e , United States Geological Survey. The sample locations tend to l i e along the southeastern periphery of the Idaho Batholith. 25 Scale of Miles 6 1 0 do io 4'o. Figure 12: Location Map of Central Idaho Samples. ( S t i p p l i n g Shows Area Covered by Figure 19 ) 140' 14 0 150 160 F i g u r e 13 Phi Kappa --Or Gil more Buttercup Star _OQ/—° Clayton Buttercup Blue Lead Silver I I I I I 1 ' 1 -777-' i s - o T » 0 2 0 * 2 1 - 0 P b 2 0 6 / P b 2 0 4 C e n t r a l Idaho. Normalized Lead 207 vs Lead 206 Isotope R a t i o s . 28 The mines i n the C e n t r a l Idaho group are replacement or v e i n d e p o s i t s i n f o l d e d and f a u l t e d O r d o v i c i a n and Carboniferous s e d i -mentary rock. They are s p a t i a l l y c l o s e to T e r t i a r y i n t r u s i v e rocks and the ore bodies appear to be g e n e t i c a l l y r e l a t e d to these i n t r u s i v e s . This g e n e t i c r e l a t i o n s h i p i s used as an i n d i c a t i o n that the ore bodies were formed during e a r l y T e r t i a r y times, and T j i n F i g u r e 5 i s taken as 50 my f o r these samples. The mines sampled i n the C e n t r a l Idaho group are d e s c r i b e d by Anderson, K i i l s g a a r d , and Fryklund (1950) and by K i i l s g a a r d (1964) . The l a t t e r r e p o r t i s the more i n c l u s i v e and contains an e x c e l l e n t b i b l i o g r a p h y . 207 206 The normalized Pb vs Pb graph gi v e s an i n d i c a t e d age f o r the commencement of growth of the r a d i o g e n i c component as 2500 my. The \i v a l u e , of 9.02, corresponding to the measured p o s i t i o n of the Pb vs Pb l i n e and the assumed T j of 50 my, i s not e x p e r i m e n t a l l y d i s t i n g u i s h a b l e from the U q value of 8.99 found by O s t i c et a l f o r primary l e a d s . Thus the leads from C e n t r a l Idaho might correspond to the s i m p l e s t two stage case p r e v i o u s l y mentioned, wi t h the age of the primary l e a d correspondin to the time at which the r a d i o g e n i c component commenced development The s i n g l e stage parent l e a d , shown by the 207/206 i s o t o p e r a t i o s , e volved 2500 my ago and t h i s i s taken as the maximum age of c o n t i -n e n t a l c r u s t i n t h i s r e g i o n of Idaho. T h e . P b 2 0 8 vs P b 2 0 6 p l o t (Figure 14) f o r the C e n t r a l Idaho samples i n d i c a t e s a reasonable l i n e a r f i t of the data. T h i s i m p l i e t h a t a of the source rocks i s uniform over the e n t i r e r e g i o n of the samples. Using the i n f o r m a t i o n that the r a d i o g e n i c component 29 developed over the time i n t e r v a l 2500 my to 50 my ago, taken 2 07 206 from the Pb vs Pb p l o t , the a value i s c a l c u l a t e d to be 3.8. I t i s i n t e r e s t i n g to observe that the P b 2 0 8 vs P b 2 0 6 l i n e does not i n t e r s e c t the p r i m a r y lead l i n e o f O s t i c et a l , but r a t h e r remains above t h i s l i n e . D i s c u s s i o n of t h i s c h a r a c t e r i s t i c w i l l be expanded. The Triumph mine l e a d a n a l y s i s gives evidence on Figure 13 that i t may not belong to the C e n t r a l Idaho l e a d s u i t e . There i s reason to b e l i e v e that the source rocks of the Triumph mine lea d may not be contiguous with the source rocks of the other l e a d samples comprising the group. This w i l l become more p l a u -s i b l e d u r i n g the d i s c u s s i o n of the nearby Butte County, Idaho samples which are d e c i d e d l y d i f f e r e n t from the C e n t r a l Idaho samples. Butte County, Idaho F i v e samples from Butte County, Idaho, were analyzed. The s p e c i f i c mine l o c a t i o n s are shown i n Figure 15 and the i s o t o p i c analyses are shown i n Figures 16 and 17. Figure 12 best i l l u s t r a t e the p r o x i m i t y of these samples to the C e n t r a l Idaho samples. K i i l s g a a r d r e p o r t s that the ore bodies are replacements along f a u l t s i n O r d o v i c i a n limestone. There i s some q u e s t i o n concerning the time of formation of the ore b o d i e s . T e r t i a r y i n t r u s i v e s and Recent flows both e x i s t i n the v i c i n i t y . I t i s a r b i t r a r i l y chosen to a s s i g n a Recent age to the ore bodies f o r purposes of t h i s r e p o r t , but i t should be noted that S i n c l a i r (1964) has e f f e c t i v e l y demonstrated that the f i n a l r e s u l t s of the study are not very s e n s i t i v e to age assignments of T 3 younger than about 14 0>— 14-0 J L 150 160 170 '8-0 P b 2 0 6 / P b 2 0 4 190 2 0 0 F i g u r e 16 Butte County, Idaho. Normalized Lead 207 vs Lead 206 Isotope R a t i o s . 34 0' 14 0 150 16-0 170 180 Pb206 /Pb204 F i g u r e 17: Butte County, Idaho. Normalized Lead 208 vs Lead 206 Isotope R a t i o s . 3 3 150 my. In the present i n s t a n c e , the p o s s i b l e extreme choices of T 3 , 0 my or 400 my (corresponding to the O r d o v i c i a n limestone f o r m a t i o n s ) , r e s u l t i n c a l c u l a t e d T 2 ages of 1900 my or 1700 my r e s p e c t i v e l y . T h i s i s a spread i n T 2 of only 101. 2 07 206 The normalized Pb vs Pb i s o t o p e l i n e i n d i c a t e s that the r a d i o g e n i c p o r t i o n o f the l e a d i n the samples commenced i t s growth about 1900 my ago. No d i r e c t r e l a t i o n s h i p i s found between these samples and the u=8.99 growth curve. Thus the time at which the r a d i o g e n i c p o r t i o n of the i s o t o p e s commenced growth i s not r e l a t e d to the age at which the primary lead component evol v e d . The l e a s t squares f i t l i n e through the sample data p o i n t s i s tangent to the y=8.99 growth curve at 1200 my. T h i s may be i n t e r p r e t e d as i n d i c a t i n g t h a t a primary l e a d was i n j e c t e d i n t o t h i s r e g i o n 1200 my ago, and 50 my ago t h i s primary l e a d was mixed w i t h a r a d i o g e n i c component which had a g e s t a t i o n p e r i o d 1900 to 50 my ago. A three stage model which d e s c r i b e s the lead i s o t o p e r a t i o s might c o n s i s t of two primary leads mixed with the r a d i o g e n i c component. One of the primary leads could be the 2500 my primary component i n f e r r e d from the C e n t r a l Idaho samples, the other component would be not younger than 1200 my, the p o i n t of tangency of the experimental lead l i n e with the y=8„99 primary l e a d growth curve. T h i s p r o p o s a l may have m e r i t i n view of the Utah l e a d analyses by Stacey et a l that w i l l be d i s c u s s e d l a t e r . The normalized P b 2 0 8 vs P b 2 0 6 p l o t of the Butte County samples, Figure 17, show a good l i n e a r t r e n d as d i d the samples from C e n t r a l Idaho. Again, there i s no i n t e r c e p t with the primary growth curve of O s t i c et a l . C a s s i a County, Idaho. Fiv e samples from C a s s i a County, Idaho and a f u r t h e r sample from the Raft River Range i n Utah that f a l l s i n the same sample group have been analyzed. The l o c a t i o n map of these samples i s g i v e n i n F i g u r e 1 8 and the i s o t o p i c analyses i n Figures 1 9 and 2 0 . The g e n e r a l sample l o c a t i o n i s about 1 0 0 m i l e s due south of the Butte County samples, on the other s i d e of the Snake R i v e r P l a i n . With the e x c e p t i o n of the S i l v e r H i l l s mine, and p o s s i b l y the Old Skoro mine, the samples are from Precambrian rock (Anderson, 1 9 3 1 ) . A more complete g e o l o g i c a l survey has r e c e n t l y been completed by Armstrong ( 1 9 6 8 ) . The source rocks of the ore bodies are a s s o c i a t e d with the metamorphism that ended i n l a t e Cretaceous to e a r l y T e r t i a r y time, an emplacement age of 5 0 my has been used f o r purposes of t h i s r e p o r t . 2 0 7 2 0 6 The normalized Pb vs Pb i s o t o p e graph i n d i c a t e s that the r a d i o g e n i c p o r t i o n of the A l b i o n Range and R a f t R i v e r Range leads commenced development 2 7 0 0 my ago. The corresponding u value i s 9 . 3 4 , b e a r i n g no simple r e l a t i o n to the uniform system of O s t i c et a l . I t i s reasonable to conclude that the o r i g i n a l l e a d i n c o r p o r a t e d i n t o the A l b i o n Range l e a d source system had a complex h i s t o r y . The age found here i s i n good agreement with an age of 2 4 0 0 my or g r e a t e r p r e v i o u s l y found by Armstrong and H i l l s ( 1 9 6 7 ) . The normalized P b 2 ^ 8 vs P b 2 ( ^ i s o t o p e values from C a s s i a County show no l i n e a r t rend and thus no i n f o r m a t i o n i s a v a i l a b l e from t h i s d ata. The l a c k of l i n e a r i t y i n d i c a t e s t h a t the 0 value of the source rocks i s non-uniform. 35 Figure 18: Location Map of Cassia County, Idaho Samples. o so A Q-140 s — 140 ,50 '60 ' ™ i 8 ° Pb206/Pb204 F i g u r e 19: C a s s i a County, Idaho. Normalized g Lead 207 vs Lead 206 Isotope Ra t i o s , 190 200 38 The S i l v e r H i l l s mine sample bears no apparent r e l a t i o n s h i p to the A l b i o n Range leads from i s o t o p i c considerations„ T h i s was the only sample found during a f i e l d t r i p to t h i s r e g i o n . A c t i v e p r o s p e c t i n g i s being pursued and thus f u r t h e r samples may become a v a i l a b l e at a l a t e r date. Summary of the Idaho Analyses. The three regions covered by the Idaho l e a d analyses are w i t h i n a g e o g r a p h i c a l area about 150 miles long and 75 m i l e s wide. I t i s unreasonable to conclude t h a t each r e g i o n sampled has a g e o c h r o n o l o g i c a l h i s t o r y as d i f f e r e n t from the other regions as would be i m p l i e d by the r e s u l t s of the present study. I t i s more reasonable to conclude t h a t , i n the broad g e o l o g i c a l time s c a l e , these regions have a l l been s u b j e c t e d to the same types of events at about the same times i n t h e i r h i s t o r y . Thus the l e a d measurements may be i n d i c a t i n g two p r i n c i p a l ages i n t h i s r e g i o n , one age at about 2500 my as i n d i c a t e d by an i n f e r r e d primary l e a d age i n C e n t r a l Idaho and the other somewhat o l d e r than 1200 my as i n d i c a t e d by the Butte County samples. The ages of 2700 my and 1900 my found i n C a s s i a County and Butte County r e s p e c t i v e l y f o r the r a d i o g e n i c components might then i n d i c a t e the age of formation of sedimentary s t r u c t u r e s now b u r i e d i n the c r u s t a l basement. An i n t e r p r e t a t i o n i n v o l v i n g two s i g n i f i c a n t events i n one g e o g r a p h i c a l r e g i o n i s the same type of c o n c l u s i o n reached by Stacey e t a l f o r l e a d samples from Utah, In f a c t , the Idaho l e a d samples show evidence of the same ages as those found i n Utah, The Utah i n t e r p r e t a t i o n i n v o l v e s c o n s i d e r a t i o n of model ages of 1650 my i n the Oquirrh Mtns and 2415 my i n the Cottonwood-Park 39 City region. The data obtained from these regions are indicative that they are of the simplest two stage anomalous lead case previously discussed. Other age values found in the Utah analyses are 2075 and 1765 my i n the T i n t i c and M i l f o r d regions respectively. The leads from the l a t t e r two regions have a more complex history than the simplest two stage model „ The favored interpretation i s that of a mixing model permitting only 1650 and 2400 my events in the region. The 2400 my system is found in the Uinta Mtns and dips under an increasing thickness of 1650 my rocks as far south as M i l f o r d . These ages are si m i l a r to the ages proposed in the present work for southern Idaho; the 2400 my and 2500 my components found i n the two reports are immediately comparable, the 1650 my and older than 1200 my ages are also comparable. It appears that, to a f i r s t approximation, the leads analyzed by Stacey et al and the leads analyzed for the present study are in d i c a t i v e of a similar geochronology i n southern Idaho and a north-south traverse through central Utah. This information f i l l s i n a region of geochronological uncertainty indicated by Kanasewich (1965) and shown in Figure 21. Some of the reasons for two primary ages i n the same geographical region are discussed l a t e r . This writer believes that the area studied by Stacey and the Idaho region studied i n the present report should be assigned an i n i t i a l age of 2500 my and regarded as a westward extension of the Superior province as defined by Kanasewich, Nevada Samples. Four samples of lead from Nevada mines were i s o t o p i c a l l y measured while accumulating data for this report. The results 40 1 1 Suoerior Ozarkian 3 3 0 0 - 2 3 0 0 m y l 4 5 0 H 2 0 0 m y Slove Grenville 2 9 0 0 - 2 2 0 0 m y 1 2 0 0 - 600my Churchill |: •-,•-•] Innuition 2 3 0 0 - 1 7 0 0 my 6 0 0 - 2 0 0 m y V77A Penokian IV.'4 Appalachian I 9 0 0 - I 4 0 0 m y 5 5 0 - 2 0 0 m y l l l l l l l C o r d i l l e r a n 3 5 0 - 0 my F i g u r e 21: G e o l o g i c a l P r o v i n c e s o f North America. A f t e r Kanasewich (1965). 41 are shown i n the Appendix. These samples are the i n i t i a l measure-ments of a study by Dr. W.F. Slawson to extend the present inves-t i g a t i o n f u r t h e r west. The p r e l i m i n a r y i n v e s t i g a t i o n appears to i n d i c a t e that the leads are short term anomalous of recent age, p o s s i b l y they are primary leads with some a d d i t i o n of r a d i o g e n i c leads a c q u i r e d from deep c r u s t a l s t r u c t u r e s . The p o s s i b i l i t y that a l l primary leads have some contamination with r a d i o g e n i c leads has been d i s c u s s e d by O s t i c et a l . 42 CRUSTAL ACCRETION AND PRIMARY LEADS The p r e v i o u s s e c t i o n s have presented the equations d e s c r i b i n g the time dependent growth of the l e a d i s o t o p e s and i n t e r p r e t e d the e x p e r i m e n t a l l y determined i s o t o p i c abundances from s e v e r a l regions on the b a s i s of these equations and s u b s i d i a r y informa-t i o n . The i n t e r p r e t a t i o n s contained i n f o r m a t i o n about the time of formation o f rock systems and i n v o l v e d the concepts of primary and anomalous l e a d s . The i n t e r p r e t a t i o n s r e l i e d h e a v i l y on the employment of the s i m p l e s t anomalous l e a d case which was d e s c r i b e d . I t i s now e s s e n t i a l to use the l e a d i s o t o p e r e s u l t s to see i f they can be employed i n support of proposed models which d e s c r i b e the earth's present t e c t o n i c c o n f i g u r a t i o n . A number of such models are a v a i l a b l e , l e a d isotope i n f o r m a t i o n i s not d e f i n i t i v e between the models but no model can be considered adequate unless i t may be employed to e x p l a i n observed l e a d i s o t o p i c abundances. I t i s the purpose of t h i s and the next s e c t i o n to c o n s i d e r a model f o r the earth's present t e c t o n i c c o n f i g u r a t i o n and show that t h i s model i s c o n s i s t e n t w i t h the equations d e s c r i b i n g lead i s o t o p e development. The model to be considered i s the one most e x t e n s i v e l y e x p l o r e d by t h i s w r i t e r . The d i s c u s s e d model f o r the earth's t e c t o n i c c o n f i g u r a t i o n represents an e c l e c t i c process using s e v e r a l l i t e r a t u r e sources. The present model i s unique because i t assigns a s p e c i f i c time sequence to p r e v i o u s l y proposed mechanisms. New c o n t i n e n t a l m a t e r i a l i s assumed to be produced by e p e i r o g e n i c events, these are e n v i s i o n e d to take p l a c e i n the upper mantle. For purposes of d i s c u s s i o n , the upper mantle model proposed by Ringwood (1962) w i l l be used as a mechanism f o r the p r o d u c t i o n of c o n t i n e n t a l 43 m a t e r i a l and t h i s p r o d u c t i o n termed a "mantle orogeny". Each s p e c i f i c volume of upper mantle m a t e r i a l i s e n v i s i o n e d as being capable o f only one mantle orogeny„ Subsequent t e c t o n i c develop-ment of a c o n t i n e n t a l r e g i o n r e s u l t s from orogenic events occur-r i n g i n the c r u s t , the " c r u s t a l o r o g e n i e s " are l i k e n e d to the sequence of events d e s c r i b e d by Joyner (1967) . S e v e r a l s u c c e s s i v e c r u s t a l events are proposed to be p o s s i b l e i n each r e g i o n . This s e c t i o n o f the r e p o r t concentrates on a d i s c u s s i o n o f Ringwood's h y p o t h e s i s , the next s e c t i o n amalgamates t h i s hypothesis with Joyner's h y p o t h e s i s . Only the s u r f a c e and near s u r f a c e of the e a r t h i s a v a i l a b l e f o r o b s e r v a t i o n a l study. Any i n f o r m a t i o n r e g a r d i n g m a t e r i a l s and c o n d i t i o n s at depths g r e a t e r than a few k i l o m e t e r s must be obtained by i n d i r e c t methods. On the b a s i s of these i n d i r e c t methods, the e a r t h i s d i v i d e d i n t o three gross r e g i o n s ; the c r u s t , mantle and co r e . Only the c r u s t below the t h i n sedimentary l a y e r s and the upper p a r t of the mantle are of i n t e r e s t i n the present r e p o r t . D i s c u s s i o n w i l l be co n f i n e d to these r e g i o n s . The d e f i n e d boundary between the c r u s t and upper mantle i s the Mohorovicic D i s c o n t i n u i t y (Moho). The Moho i s t h a t depth i n the e a r t h at which s e i s m i c wave v e l o c i t i e s a b r u p t l y i n c r e a s e , g e n e r a l l y P wave v e l o c i t i e s below the Moho exceed 8 km/sec and S wave v e l o c i t i e s exceed 4,6 km/sec. P r i m a r i l y on the b a s i s of the depth to the Moho, but a l s o on the b a s i s o f s e i s m i c wave v e l o c i t i e s above the Moho, the c r u s t i s g r o s s l y d i v i d e d i n t o two types, the oceanic c r u s t found under the ocean basins and the c o n t i n e n t a l crust„ 44 Seismic wave v e l o c i t i e s t e l l us n e a r l y a l l that we know about the i n a c c e s s i b l e regions of the e a r t h . These waves can be used only to p l a c e l i m i t s upon the e l a s t i c p r o p e r t i e s of the m a t e r i a l through which they pass. They do not d e f i n e the c h e m i c a l , p h y s i c a l , or m i n e r a l o g i c a l s t a t e o f t h i s m a t e r i a l . The d e r i v e d e l a s t i c parameters may be used to i n f e r c o n d i t i o n s at depth i f i t i s very reasonably assumed that the m a t e r i a l at depth i s not g r o s s l y d i f -f e r e n t from some of the common m a t e r i a l s found on the s u r f a c e of the e a r t h . T h e r e f o r e , l a b o r a t o r y type experimental i n f o r m a t i o n on the e l a s t i c p r o p e r t i e s of common e a r t h forming m a t e r i a l s i s used to s e t boundaries on the composition and mineralogy of m a t e r i a l s at depth. An example of the use of t h i s method i s given by P a k i s e r and Robinson (1966) . Figure 22 i s adapted from t h i s r e p o r t . More recent use of t h i s method has been by Bateman and Eaton (1967) . Other examples are a l s o a v a i l a b l e and some of them are used l a t e r i n t h i s r e p o r t . Oceanic c r u s t i s t y p i c a l l y t h i n , averaging 5 to 7 km t h i c k and o v e r l a i n by t h i n sedimentary l a y e r s . I t i s r e s t r i c t e d i n extent to the deep ocean basins of the e a r t h , normally under more than 4 km of water. This c r u s t i s c h a r a c t e r i z e d by s e i s m i c wave v e l o c i t i e s of about 6.7 and 3.8 km/sec f o r the P and S type waves r e s p e c t i v e l y . These v e l o c i t i e s may i n d i c a t e a b a s a l t i c composition as d i s c u s s e d by W y l l i e (1963) . The c r u s t now observed i n the ocean basins i s probably not of primeval o r i g i n . There are many i n d i -c a t i o n s that prolonged volcanism has a f f e c t e d a l l of the oceanic c r u s t and v o l c a n i c rocks have accumulated i n t h i n sheets (Menard, 1964) . There i s no evidence of l a y e r i n g of the oceanic c r u s t below the sedimentary l a y e r s . o .tr Increasing Ferro-magnesians-* F i g u r e 22: The V a r i a t i o n o f C a l c u l a t e d S e i s m i c Wave V e l o c i t y w i t h Composition. A f t e r P a k i s e r and Robinson (1966). 46 Continental crust is usually 20 to 60 km thick, averaging about 35 km. It i s r e s t r i c t e d to the continental areas and the adjoining shelves, and comprising about 40% of the surface of the earth. Seismic studies reveal the p o s s i b i l i t y that at least some of the continental crust, below the thin sedimentary layers, consists of two layers separated by the Conrad discont i n u i t y . Seismic wave v e l o c i t i e s above the Conrad discontinuity are about 6.1 and 3.5 km/sec for the P and S waves respectively and about 6.8 and 4.0 km/sec below the discontinuity. Where observed, the Conrad discontinuity is at about 20 km depth. The predominant rock type corresponding to the lower v e l o c i t i e s appears to be gr a n i t i c i n nature, while the higher v e l o c i t i e s may indicate the presence of eclogites (Ringwood and Green, 1966) . The nature of the Moho remains in doubt. It may represent a chemical change i n the constitution of the earth or i t may represent a phase change. The former concept of the Moho is now generally preferred although the l a t t e r is also frequently discussed. Evidence for the Moho i s based e n t i r e l y upon seismic measurements, these cannot conclusively distinguish between an abrupt boundary as would be expected i f the Moho were a chemical t r a n s i t i o n , or a gradual boundary as would be expected i f the Moho were a phase t r a n s i t i o n . General concensus of opinion i s that the Moho is not very thick, evidence has been given for a Moho as thin as 0.1 km (Nakamura and Howell, 1964) and as thick as 6 km (Steinhart et a l , 1962). There is some evidence that the Moho may have a complex layered structure as discussed by Dix C1965) . 47 The models a v a i l a b l e to d e s c r i b e the r e l a t i o n s o c c u r r i n g between the oceanic c r u s t and mantle on the one hand, and the c o n t i n e n t a l c r u s t and mantle on the other hand, f a l l i n t o three b a s i c groups. There are model v a r i a t i o n s a v a i l a b l e i n each of the three groups. These b a s i c groups are: 1) the c o n t i n e n t a l and oceanic c r u s t a l m a t e r i a l s are c o n t i n u a l l y r e c y c l e d but, on balance, the r a t i o of c o n t i n e n t a l m a t e r i a l to ocean b a s i n m a t e r i a l has remained unchanged throughout most of the earth's h i s t o r y . T h i s concept has been most r e c e n t l y d i s c u s s e d by Armstrong (1968b) and by I s a c k s , O l i v e r , and Sykes (1968). 2) the o r i g i n a l o v e r a l l crust-mantle r e l a t i o n s h i p may have been s i m i l a r to that p r e s e n t l y observed i n the c o n t i n e n t s . The ocean basins have been d e r i v e d from a roughly time-independent c o n v e r s i o n of c o n t i n e n t s i n t o oceanic b a s i n s , as d i s c u s s e d by Beloussov and Kosminskaya (1968). 3) the o r i g i n a l o v e r a l l crust-mantle r e l a t i o n s h i p may have been s i m i l a r to t h a t p r e s e n t l y observed i n the ocean b a s i n s . The c o n t i -nents have been d e r i v e d from a roughly time-independent conversion of oceanic i n t o c o n t i n e n t a l m a t e r i a l . Each of the above s t a t e d model groups have merit depending upon the o r i g i n a l assumptions which are made. A d e t a i l e d d i s c u s s i o n of the r e l a t i v e m e r i t s of each group i s out of p l a c e i n t h i s t h e s i s ; r a t h e r , three of the reasons f o r the author's s e l e c t i o n of the l a t t e r group ( c o n t i n e n t a l a c c r e t i o n ) w i l l be g i v e n and then the d i s c u s s i o n l i m i t e d to t h i s group of models. An argument f a v o r i n g the growth o f c o n t i n e n t s by a c c r e t i o n i s based upon the q u a n t i t y of v o l a t i l e s now i n the hydrosphere and atmosphere. Rubey (1951) presented evidence that the v o l a -t i l e s are mantle d i f f e r e n t i a t e s t h a t have escaped from " s e l e c t i v e 48 fusion of lower melting fractions from deep-seated, nearly an-hydrous rocks beneath the unstable continental margins and geo-synclines". Evidence i s thus available that upper mantle d i f f e r -e n t iation occurs, Menard (1964) demonstrates that there i s no compelling reason to assume other than that the oceans have ac-cumulated at a f a i r l y constant rate throughout geological time, thus the crust-mantle processes giving r i s e to the earth surface forms have been occurring throughout geologic time. It i s inter-esting to note that the information used by Menard can account for the quantity of water in the oceans only i f about half of the water came from mantle d i f f e r e n t i a t e s that did not y i e l d continental material. This l a t t e r would be consistent with the previously mentioned seismic observation that the oceanic crust is volcanic material and leads to speculations that the presently observed oceanic crust i s evolving. The physical c h a r a c t e r i s t i c s of the cru s t a l sections under the small ocean basins, as the Aleutian basin, Black Sea, etc., have been investigated by Menard (1967) . He has reached the conclusion that these areas of intensive sedimentation demonstrate cr u s t a l sections intermediate between oceanic and continental material. The major portion of the evidence indicates that they represent modern day sites of the development of continents from an oceanic type mantle-crust re l a t i o n s h i p . The t o t a l area of these ocean basins is approximately 5% of the area of the earth's surface, thus the t r a n s i t i o n of crust-mantle r e l a t i o n types may not be uncommon. Further evidence favoring continental accretion i s to be gained from compiling the results of radiometric dating studies 49 i n North America, Engel (1963) , Kanasewich (1965) and Muehlberger et a l (1967), among o t h e r s , have drawn i s o t o p i c age maps of North America showing that there i s a c e n t r a l core from which the c o n t i -nent grew outwards. F i g u r e 21 i s m o d i f i e d from Kanasewich's paper and shows t h a t the c e n t r a l core of the c o n t i n e n t i s about 10 times o l d e r than the margins. T h i s map i s drawn by p l o t t i n g r a d i o m e t r i c dates i n t h e i r proper l o c a t i o n s and s e l e c t i n g the o l d e s t a v a i l a b l e dates i n each r e g i o n as the age of t h a t r e g i o n . I t i s a p p r o p r i a t e at t h i s p o i n t to endeavor to d e f i n e what i s meant by the age of a r e g i o n . Consider the general r e g i o n under d i s c u s s i o n i n t h i s r e p o r t . The g e o l o g i c a l evidence i n d i -cates t h a t t h i s was a mountainous r e g i o n d u r i n g Cretaceous time, about 90 m i l l i o n years ago. T h i s was f o l l o w e d by a p e r i o d during which the topography was reduced by e r o s i o n and i n f i l l i n g . Then, i n T e r t i a r y time (about 50 my ago) the present Rocky Mountain system commenced i t s e v o l u t i o n . P r i o r to these two mountain systems there i s evidence that f o l d b e l t s e x i s t e d i n the r e g i o n i n middle Precambrian time. The evidence of these i s c o n t a i n e d i n the Cherry Creek s t r a t a found i n southwestern Montana (Perry, 1962) . The e x i s t e n c e o f at l e a s t three f o l d b e l t s at v a r i o u s times i n the h i s t o r y o f the r e g i o n makes i t obvious that d a t i n g the o r o g e n i c events does not date the time at which c o n t i n e n t a l type m a t e r i a l was f i r s t present i n the r e g i o n . Rather, a more fundamental event than mountain b u i l d i n g or age of i n d i v i d u a l rock s t r a t a i s r e q u i r e d to date a r e g i o n . Within the concept that c o n t i n e n t s are formed by f r a c t i o n a t i o n from primeval mantle m a t e r i a l , the time of t h i s f r a c t i o n a t i o n serves to date the r e g i o n . R e c a l l i n g 50 the l e a d equations given i n Figure 5, i t i s the time which i s sought i n the l e a d - l e a d g e o c h r o n o l o g i c a l s t u d i e s . This time i s i n t e r p r e t e d as the time of o r i g i n f o r the re g i o n of the c o n t i n e n t . The p r o p o s a l of c o n t i n e n t s a r i s i n g from f r a c t i o n a t i o n of primeval mantle m a t e r i a l r e q u i r e s s p e c u l a t i o n concerning the composition and p h y s i c a l environment of the upper mantle. Only a guess may be made at the present time, but t h i s guess i s bounded by i n f o r m a t i o n a t t a i n e d from study of the e a r t h . Three obse r v a t i o n s which p l a c e bounds on the c o n s t i t u t i o n of the upper mantle are: 1) the c o n c e n t r a t i o n of r a d i o a c t i v e isotopes as d i s c l o s e d by the measured heat flow at the earth's s u r f a c e . 2) the angular moment of i n e r t i a of the e a r t h . 3) the v e l o c i t y of s e i s m i c waves through the upper mantle. Two a d d i t i o n a l items may be taken i n t o account, these being the composition of stony meteorites and the composition of m a t e r i a l s found on the earth's s u r f a c e which are of suspected upper mantle o r i g i n . These l a t t e r i n c l u d e u l t r a m a f i c x e n o l i t h s found i n b a s a l t s , garnet p e r i d o t i t e found i n k i m b e r l i t e p i p e s , and the u l t r a m a f i c rocks themselves. The measured heat flow from the earth's s u r f a c e and the sei s m i c data i n d i c a t i n g no wide-spread m e l t i n g i n the mantle p l a c e a boundary on the p o s s i b l e abundance and l o c a t i o n of the earth's r a d i o a c t i v e heat producing elements. T a y l o r (1964) presents i o n i c chemical evidence that a l l of the heat producing elements must l i e above 400 km depth. T a y l o r proposes that over 801 of the earth's uranium and thorium and 62% of the potassium are i n the c o n t i n e n t a l c r u s t . Other models have been 51 proposed (see MacDonald, 1965, for example) but they a l l lead to e s s e n t i a l l y the same conclusion. The radioactive elements and t h e i r location must figure prominently in any fractionation process taking place i n the earth's mantle, since they w i l l provide energy for endothermic chemical and physical processes. The angular moment of i n e r t i a of the earth has been used by Bullen (1963) to test a proposed density d i s t r i b u t i o n for the earth's i n t e r i o r . The d i s t r i b u t i o n proposed by Bullen was i n i t i a l l y based upon the observed broad co r r e l a t i o n between b a s i c i t y , density and seismic v e l o c i t i e s in rocks and upon high pressure laboratory experiments on rocks. The best estimate 3 of the density of material just below the Moho is 3.32 g/cm , " 3 and that this density has increased to 3.64 g/cm at 413 km depth. Any proposal concerning the chemical and mineralogical composition of the upper mantle should be consistent with these density determinations. Seismic waves are the basic media through which we know anything regarding the earth's upper mantle. As indicated in the previous paragraphs concerning heat flow and moment of i n e r t i a , seismic evidence is always used for study of the earth's i n t e r i o r either by i t s e l f or in conjunction with other informa-ti o n . The corr e l a t i o n of calculated seismic wave v e l o c i t i e s with rock composition made by Christensen (1965) indicates that dunite and eclogite are among the possible materials composing the upper mantle. Dunite is indicative of a probable chemical change at the Moho; eclogitej the high pressure phase of basalt, is i n d i c a t i v e of a probable phase change of the Moho. A model of the earth with element abundances based upon 52 t h o s e o f s t o n y m e t e o r i t e s h a s b e e n f r e q u e n t l y d i s c u s s e d . The m o s t c o m p l e t e a p p l i c a t i o n o f e x t r a t e r r e s t r i a l d a t a t o t h e p r o b l e m o f t h e e a r t h i s g i v e n b y R i n g w o o d ( 1 9 6 6 ) t w h o c o n s i d e r s t h e h y p o -t h e s i s o f f o r m i n g t h e t e r r e s t r i a l p l a n e t s f r o m m a t e r i a l s i m i l a r t o t y p e I c a r b o n a c e o u s c h o n d r i t e s . H a r r i s e t a l ( 1 9 6 7 ) p o i n t o u t some o f t h e o b j e c t i o n s t h a t h a v e b e e n r a i s e d t o t h e c h o n d r i t i c e a r t h m o d e l a n d p r o p o s e a m o d e l b a s e d u p o n m i n e r a l s f o u n d a t t h e e a r t h ' s s u r f a c e . I t m u s t b e r e - e m p h a s i z e d t h a t t h e r e i s no d i r e c t a c c e s s t o e a r t h m a t e r i a l s b e l o w a f e w k i l o m e t e r s , s o t h a t a n y i n f e r e n c e s c o n c e r n i n g t h e c o m p o s i t i o n o f t h e e a r t h i s c o n -j e c t u r a l a n d o b j e c t i o n s c a n be r a i s e d t o a n y h y p o t h e s i s . T h i s i s n o t t o i m p l y t h a t a h y p o t h e s i s s h o u l d n o t be r a i s e d . E a c h h y p o t h e s i s r e p r e s e n t s a s t a r t i n g p o i n t f r o m w h i c h t o f o r m u l a t e e x p e r i m e n t s t o f u r t h e r i n v e s t i g a t e t h e s u b j e c t a t h a n d . I n t h e p r e s e n t i n s t a n c e , t h e h y p o t h e s i s c o n c e r n i n g t h e c o m p o s i t i o n o f t h e u p p e r m a n t l e i s e s s e n t i a l f o r e x p e r i m e n t a l s t u d i e s on magma g e n e r a t i o n b y p a r t i a l m e l t i n g o f u p p e r m a n t l e m a t e r i a l s a n d f o r t h e s t u d y o f t h e e a r t h ' s t h e r m a l h i s t o r y . T h e r e h a v e b e e n t w o b a s i c a l l y d i f f e r e n t h y p o t h e s e s p r e s e n t e d f o r t h e a c c r e t i o n o f c o n t i n e n t s . The e a r l i e r o f t h e s e was b y K e n n e d y ( 1 9 5 9 ) a n d r e q u i r e d a p h a s e c h a n g e Moho a n d a n u n u s u a l g e o t h e r m b e l o w t h e o c e a n b a s i n s . Some o f t h e p o i n t s made b y K e n n e d y c o n t i n u e t o b e w i d e l y d i s c u s s e d , b u t i n g e n e r a l t h e p r o p o s a l h a s b e e n r e p l a c e d b y a h y p o t h e s i s due t o R i n g w o o d ( 1 9 6 2 ) . T h i s l a t t e r r e q u i r e s a c h e m i c a l c h a n g e Moho a n d . e m p l o y s g e o t h e r m s w h i c h a r e m o r e i n k e e p i n g w i t h t h e b e s t e s t i m a t e s o f t h e a c t u a l g e o t h e r m s . R i n g w o o d ' s h y p o t h e s i s , a s p r o p o s e d , w i l l be o u t l i n e d 53 and d i s c u s s e d as a s t a r t i n g p o i n t f o r the d i s c u s s i o n of le a d isotopes i n r e l a t i o n to orogenic processes. T h i s l a t t e r d i s -c u s s i o n w i l l suggest some m o d i f i c a t i o n s to the o r i g i n a l h y p o t h e s i s . Ringwood's Hypothesis. Ringwood formulated h i s hypothesis of c o n t i n e n t a l a c c r e t i o n i n an e f f o r t to account f o r the presence of the low v e l o c i t y s e i s m i c l a y e r . His model was unique from previous proposals i n that he e l e c t e d to f i n d a c h e m i c a l - m i n e r a l o g i c a l system which would permit a phase change to a lower s e i s m i c v e l o c i t y media at a depth of about 80 to 150 km. Previous attempts to account f o r t h i s l a y e r had been based upon a mutual c a n c e l i n g of the pressure and temperature e f f e c t s on sei s m i c wave v e l o c i t y . The e x t r a p o l a t i o n of the pressure-temperature concept p r e d i c t e d broad s c a l e m e l t i n g of mantle m a t e r i a l at about 200 km. No such m e l t i n g i s i n d i c a t e d by the seismic evidence. According to Ringwood's model, the sei s m i c wave v e l o c i t y at a given depth should depend upon temperature, p r e s s u r e , and chemical composi-t i o n . The low v e l o c i t y l a y e r was accounted f o r by assuming that i t r epresented a broad t r a n s i t i o n r e g i o n between two s t a b l e phases of a proposed mantle m a t e r i a l . The hypothesis s p e c i f i e s p o s s i b l e mantle m a t e r i a l s which are compatible with the d e n s i t y , temperature, and seismic i n f o r m a t i o n r e l e v a n t to the upper mantle. The d i s c u s s i o n of Ringwood's model w i l l employ the mineral assemblages proposed by him, but the s p e c i f i c a t i o n o f these assemblages i s not of primary impor-tance to t h i s r e p o r t . The major i n t e r e s t i n the hypothesis i s i n the p h y s i c a l environment and response which i s an important 54 p a r t of the package. A change i n mineral assemblage as a r e s u l t of f u t u r e study would not, of n e c e s s i t y , s i g n i f i c a n t l y a l t e r the p h y s i c a l environment and response of the upper mantle as proposed by Ringwood. Ringwood's hypothesis proposes that the mean chemical compo-s i t i o n of the c r u s t and upper mantle i s uniform over any ext e n s i v e r e g i o n of the e a r t h , whether the s u r f a c e form be oceanic or c o n t i n e n t a l . This mean chemical composition approximates that obtained by mixing one p a r t b a s a l t with three p a r t s dunite (Green and Ringwood, 1963). This composition i s s e l e c t e d on the b a s i s of the r e l a t i v e p r o p o r t i o n s of MgO, CaO, and A^O^ i n b a s a l t s , d u n i t e s , and c h o n d r i t e s . The name " P y r o l i t e " i s s e l e c t e d to d e s c r i b e t h i s p r i m i t i v e m a t e r i a l . F r a c t i o n a l m e l t i n g of u n d i f f e r e n t i a t e d p y r o l i t e i n the upper mantle i s proposed to r e s u l t i n a b a s a l t i c composition magma r i s i n g to form the c r u s t a l r o c ks, l e a v i n g behind a r e s i d u a l d u n i t e . Thus the co n t i n e n t s are v i s u a l i z e d as having formed by v e r t i c a l s e g r e g a t i o n with the b a s a l t i c composition above and the dunite below, the dunite grading i m p e r c e p t u a l l y to p r i m i t i v e p y r o l i t e at about 150 km depth. The nature of seismic t r a n s m i s s i o n s through the oceanic paths i n d i c a t e s that water d e f i c i e n t p y r o l i t e may extend upwards to the Moho under the oceans, or perhaps there i s a l a y e r of dunite about 25 km t h i c k between the Moho and the p r i m i t i v e p y r o l i t e (Ringwood, 1962b). This w r i t e r p r e f e r s the l a y e r of dunite concept, which would be compatible with the sub-oceanic mantle d i f f e r e n t i a t i o n and present oceanic c r u s t development d i s c u s s e d by Menard (1964). The depth composition of the major 55 earth s u r f a c e forms as proposed by Ringwood are shown i n Fig u r e 23. Two s t a b l e phases of p y r o l i t e are employed to e x p l a i n c o n t i n e n t a l u p l i f t and oceanic trough subsidence. There i s some experimental evidence to support the e x i s t e n c e of these two phases with a broad t r a n s i t i o n r e g i o n between them, as shown i n the i d e a l i z e d pressure-temperature diagram, Figure 24. The more dense g a r n e t - p y r o l i t e phase i s s t a b l e i n the low temperature-high p ressure p o r t i o n of the diagram, the l e s s dense p l a g i o -c l a s e - p y r o l i t e i s s t a b l e i n the high temperature-low pressure p o r t i o n of the diagram. The r e l a t i v e volume change between the two phases i s 3%. The t r a n s i t i o n - p y r o l i t e c o n s i s t s of o l i v i n e , pyroxene, p l a g i o c l a s e , and garnet. The proposed p e t r o l o g i c a l model and the c h a r a c t e r i s t i c geotherms are shown i n Figure 25. The r e g i o n of f r a c t i o n a l m e l t i n g of the garnet phase and p l a g i o c l a s e phase are i n d i c a t e d , as i s the t r a n s i t i o n zone. The i n d i c a t e d temperature gr a d i e n t s show the s t e e p e s t g r a d i e n t to be under the oceans and the s h a l -lowest g r a d i e n t s under the Precambrian s h i e l d s i n accordance with the most probable estimates. These temperature g r a d i e n t s form a major f e a t u r e of Ringwood's h y p o t h e s i s . The orogenic geotherm passes through the p y r o l i t e phase change zone f o r a c o n s i d e r a b l e depth, thus orogenic regions may be expected to be unstable toward s l i g h t temperature and pressure v a r i a t i o n s . The oceanic geotherm remains i n the p l a g i o c l a s e - p y r o l i t e phase f o r a c o n s i d e r a b l e depth, moving across the t r a n s i t i o n zone j u s t under the f r a c t i o n a l m e l t i n g curve. The shallow Precambrian 56 P r e -C a m b r i a n Oceanic Orogenic S h i e l d ii u F i g u r e 23: Assumed Chemical Model f o r the Upper Mantle. A f t e r Ringwood (1962). Pressure F i g u r e 24: P o s s i b l e M i n e r a l Assemblages of P y r o l i t e . A f t e r Ringwood (1962), CD Qi E 6> 58 s h i e l d geotherm does not enter the phase t r a n s i t i o n r e g i o n at a l l , thus accounting f o r the s t a b i l i t y of the s h i e l d s . The proposed model f o r the upper mantle w i l l now be employed to demonstrate a p l a u s i b l e mechanism f o r the growth and s t a b i l i t y o f c o n t i n e n t s . Consider f i r s t an oceanic r e g i o n near a c o n t i -n e n t a l margin. The pressure on the upper mantle i s i n c r e a s e d while the geotherm undergoes an apparent downward m i g r a t i o n as sedimentary m a t e r i a l s are deposited i n the r e g i o n (note that the r a d i o g e n i c heat source i n the upper mantle i s not great, t h e r e f o r e the temperature of a p o i n t w i l l not g r o s s l y change while the accumulating sediments are i n c r e a s i n g the depth of the p o i n t ) . As the geotherm moves downwards, some of the p l a g i o c l a s e -p y r o l i t e phase w i l l be converted to the more dense garnet-p y r o l i t e and the r e s u l t i n g e f f e c t w i l l be a s i n k i n g of the over-r i d i n g sediments about as r a p i d l y as they are being d e p o s i t e d . The small ocean basins are not observed to r a p i d l y f i l l i n s p i t e of the l a r g e q u a n t i t i e s of m a t e r i a l being washed i n t o them. E v e n t u a l l y , the geotherm w i l l l i e w i t h i n the t r a n s i t i o n p y r o l i t e r e g i o n f o r a c o n s i d e r a b l e depth range and an era of i n s t a b i l i t y w i l l ensue. I t i s p o s t u l a t e d that the m a t e r i a l w i t h i n the t r a n s i -t i o n r e g i o n may now be subjected to inhomogeneous r a d i o g e n i c h e a t i n g and l o c a l h e a t i n g due to energy d i s s i p a t i o n along f a u l t p l a n e s . There w i l l be v e r t i c a l d e n s i t y i n s t a b i l i t y as a r e s u l t of phase changes i n the p y r o l i t e which w i l l i n t u r n cause con-v e c t i v e c u r r e n t s . Some p a r t i a l m e l t i n g i s to be expected, l e a d i n g to magma r i s i n g to form new c o n t i n e n t a l c r u s t a l m a t e r i a l beneath the d e p o s i t e d sediments. The deposited sediments are pushed up, out of the way of the r i s i n g magmas and form a p l a t e a u r e g i o n . 59 The base of the c r u s t may now be subjected to a v a r i e t y o f s t r e s s e s from the changes t a k i n g p l a c e i n the upper mantle, l e a d i n g to the deformation observed i n a l l orogenic b e l t s . The r i s i n g b a s a l t from the upper mantle p y r o l i t e w i l l c a r r y with i t the r a d i o a c t i v e elements. As a f i n a l stage i n the c r u s t a l formation, the upper mantle i s deprived of i t s r a d i o g e n i c heat source. C o o l i n g commences and an era of s t a b i l i t y i s s t a r t e d . The geotherm e v e n t u a l l y drops to that shown f o r the Precambrian s h i e l d s and se i s m i c a c t i v i t y d i m i n i s h e s . Comments on Ringwood's Hypothesis. Information concerning the v a l i d i t y o f the p h y s i c a l c h a r a c t e r -i s t i c s o f the model proposed by Ringwood i s i n d i r e c t but none-t h e l e s s p r e s e n t . Evidence of an intermediate o c e a n i c - c o n t i n e n t a l type of c r u s t o c c u r r i n g p r e s e n t l y over about 5% of the earth's s u r f a c e has been p r e v i o u s l y c i t e d (Menard, 1967). A d d i t i o n a l evidence that some shallow sea basins are u n d e r l a i n by a t r a n s i t i o n a l type of c r u s t i s obtained from a study of the low v e l o c i t y l a y e r under the western Mediterranean b a s i n by Berry and Knopoff (1967). Evidence which c o r r e l a t e s h i g h e r average heat flow data with a more pronounced low v e l o c i t y l a y e r i s c i t e d by Archambeau et a l (1968) f o r the Basin-Range pr o v i n c e of the Un i t e d S t a t e s . This type of c o r r e l a t i o n i s an inherent p a r t of Ringwood's model. Accor d i n g to Ringwood's model, the sei s m i c wave v e l o c i t y at a given depth should depend upon temperature, p r e s s u r e , and chemical composition. It i s suggested that the appearance of p l a g i o c l a s e as a primary phase i n the p y r o l i t e i s r e s p o n s i b l e 60 f o r the low v e l o c i t y l a y e r . Reference to Figure 25 shows that there should be no p l a g i o c l a s e - p y r o l i t e under the Precambrian s h i e l d s , t h e r e f o r e i t i s p r e d i c t e d that there i s no low v e l o c i t y s e i s m i c l a y e r under the s h i e l d s . B o l t , Doyle, and Sutton (1958) found no evidence of a low v e l o c i t y s e i s m i c l a y e r under the A u s t r a l i a n s h i e l d . Brune and Dorman (1962) have found evidence of a low v e l o c i t y l a y e r under the Canadian S h i e l d , but s t a t e that i t appears to be d i s c o n t i n u o u s and may represent a r e g i o n of l a t e r a l inhomogeneity. I t seems reasonable that the geotherm under the Canadian S h i e l d i s not yet completely below the t r a n s i t i o n ' p y r o l i t e r e g i o n and some p l a g i o c l a s e continues to e x i s t t h e r e . A d d i t i o n a l support to the hypothesis that the major e a r t h s u r f a c e forms may r e s u l t from developing inhomogeneities i n the upper mantle may be obtained from se i s m i c s t u d i e s i n orogenic r e g i o n s . Cook (1962) has summarized the s e i s m i c data from a number of orogenic regions i n c l u d i n g mid-ocean r i d g e s , i s l a n d a r c s , and c o n t i n e n t a l p l a t e a u s . He notes t h a t , i n each case, the Moho does not appear as a d i s t i n c t boundary but that i t i s d i f f i c u l t or impossible to d e t e c t . The s e i s m i c v e l o c i t y p a t t e r n as a f u n c t i o n of depth i n d i c a t e s that these orogenic regions are u n d e r l a i n by a m a t e r i a l with d e n s i t y intermediate to c r u s t a l and mantle m a t e r i a l . Cook concludes that t h i s i n t e rmediate m a t e r i a l r e p r e s e n t s mantle s t r u c t u r e being converted to c r u s t a l s t r u c t u r e . The m a n t l e - c r u s t mix which Cook proposes i s i n agreement with the t r a n s i t i o n - p y r o l i t e and u p w e l l i n g magma proposed by Ringwood. The concept that the major earth s u r f a c e forms are i n i s o s t a t i c e q u i l i b r i u m leads immediately to the c o n c l u s i o n that the c r u s t i s t h i c k e r under the orogenic regions than under the 61 s h i e l d s . There i s now evidence that the reverse i s g e n e r a l l y t r u e , the c r u s t i s t h i c k e r under the s h i e l d s than under the orogenic regions ( P a k i s e r and S t e i n h a r t , 1964, f o r example). I t appears that i s o s t a t i c compensation i n orogenic regions i s a t t a i n e d i n the mantle r a t h e r than at the mantle-crust boundary. The evidence that the mantle-crust r e l a t i o n s h i p i s d i f f e r e n t i n orogenic and s h i e l d regions i s an inherent f e a t u r e of Ringwood's h y p o t h e s i s . The b a s i c and important r e s u l t of the a p p l i c a t i o n of Ringwood's hypothesis i s the p r e d i c t i o n that s i a l i c m a t e r i a l w i l l be pro-duced i n and removed from the upper mantle. This s i a l i c m a t e r i a l c a r r i e s with i t the uranium and thorium that had p r e v i o u s l y been i n the upper mantle. The removal of the uranium and thorium from an environment i n which they p r e v i o u s l y e x i s t e d u n i f o r m l y with lead i s c o n s i d e r e d to be synonymous with the formation of primary leads as d e f i n e d by O s t i c et a l (1967). Each of the primary leads c o n t a i n s an i s o t o p i c composition i n d i c a t i v e of the time at which the r e g i o n a l c o n t i n e n t a l c r u s t a l m a t e r i a l was evolved from the upper mantle. I t i s t h i s i n t e r p r e t a t i o n of the s i g n i f i c a n c e of the primary lead i s o t o p i c abundances which i s important. Regardless of the uranium-thorium system or systems i n which the lead may l a t e r f i n d i t s e l f , the i s o t o p i c abundance acq u i r e d by the lead at the time of i t s removal from the upper mantle i n t o the lower c r u s t can never be removed as a c h a r a c t e r -i s t i c of the l e a d . The subsequent h i s t o r y of a primary l e a d can modify the i s o t o p i c abundance p a t t e r n and produce anomalous l e a d s , t h i s process w i l l be d i s c u s s e d i n the next s e c t i o n . ' 6 2 CRUSTAL DEVELOPMENT AND ANOMALOUS LEADS This r e p o r t i s p r e s e n t l y endeavoring to demonstrate that measured l e a d i s o t o p e abundances are compatible with a model f o r c r u s t a l e v o l u t i o n . In p a r t i c u l a r , the measured i s o t o p i c abundances from southern Idaho are d i s c u s s e d i n r e l a t i o n to t h i s model. The e f f o r t r e s t s upon the premise that the o r i g i n of the sampled lead ore m a t e r i a l s l i e s below the p h y s i c a l l y a c c e s s i b l e r e g i o n of the c r u s t . The lead d e p o s i t s i n v o l v e d i n the present r e p o r t are commonly acknowledged to be of hydrothermal o r i g i n , t h e r e f o r e the f o l l o w i n g b r i e f d i s c u s s i o n i s l i m i t e d to the genesis of d e p o s i t s of t h i s type. The p r e c i s e mechanism l e a d i n g to the formation of hydrothermal ore d e p o s i t s i s not important f o r present purposes, the model mechanisms proposed f o r t h i s w i l l not be reviewed. I t i s a premise of the present r e p o r t that hydrothermal lead ore bodies represent a sampling of the lead contained i n the source rock. The source rock of hydrothermal ore d e p o s i t s are here accepted to be i n t r u s i v e b o d i e s . T h i s assumption i s i n agreement with the g r e a t e r p a r t of present day ore genesis hypotheses. The ore d e p o s i t s represent minerals which have been removed from the i n t r u s i v e by hydrothermal a c t i o n and emplaced along openings i n the rock s t r a t a above and around the i n t r u s i v e body. The lead ore d e p o s i t s represent lead removed from the source rock, t h i s l a t t e r i s d i s t i n c t from the host rock of the ore. The bulk of the hydrothermal ore d e p o s i t s are found i n the immediate v i c i n i t y of known i n t r u s i v e b o d i e s . T h i s i s the most c o n c l u s i v e evidence a v a i l a b l e f o r a s s o c i a t i n g i n t r u s i v e s as the 63 source rock of the ore m i n e r a l s . The c o n c e n t r a t i o n of the ore metals i n i n t r u s i v e s i s low, but the enormous s i z e of the i n t r u s i v e s r e q u i r e s only i n e f f i c i e n t c o n c e n t r a t i o n processes to produce l a r g e ore d e p o s i t s . There i s a general tendency f o r l e a d c o n c e n t r a t i o n s to i n c r e a s e with i n c r e a s i n g a c i d i t y o f the i n t r u s i v e s , as shown by the c a l c u l a t i o n s of Turekian and Wedepohl (1961). Perhaps l e a d ore de p o s i t s may be more r e a d i l y expected around the a c i d i n t r u s i v e s that are i n d i c a t i v e of re-worked c r u s t a l m a t e r i a l , i . e . g r a n i t i c g n e i s s . There appear to be two sources f o r magma and each source appears to y i e l d a d i s t i n c t i v e type of magma. Basic magmas are g e n e r a l l y acknowledged to come from great depth, below the Moho. Evidence f o r t h i s deep o r i g i n i s contained i n : 1) the depth of earthquake f o c i below volcanoes (Eaton and Murata, 1960) implying that movement of m a t e r i a l i s o c c u r r i n g at depth. 2) evidence that b a s i c magmas come from a reasonably constant environment on a world-wide s c a l e as shown by the r e l a t i v e constancy of the 87 86 o r i g i n a l Sr /Sr r a t i o s r e g a r d l e s s of the time or l o c a t i o n of emplacement of the magmas (Hurley et a l , 1962). The o r i g i n a l 8 7 86 Sr /Sr r a t i o corresponds to the i n t e r c e p t of the is o c h r o n at the o r d i n a t e . Such a constant environment i s i n c o n s i s t e n t w i t h the mechanisms of change observed on the earth's s u r f a c e . 3) l a r g e q u a n t i t i e s of m a t e r i a l with a b a s a l t i c composition may be formed from m a t e r i a l s thought to comprise the upper mantle (Ringivood, 1962). A c i d i c magmas, on the other hand, are more l i k e l y d e r i v e d from reworked c r u s t a l m a t e r i a l s and thus evolve at shallower depths. Evidence f o r t h i s shallow depth i s contained i n the i m p r o b a b i l i t y that l a r g e volumes of a c i d i c ( s i l i c e o u s ) 64 rocks c o u l d have been d e r i v e d from d i f f e r e n t i a t i o n of mantle m a t e r i a l (Yoder and T i l l e y , 1962). There i s some evidence to i n d i c a t e that b a s i c magmas have i n s u f f i c i e n t water content to produce s o l u t i o n s f o r forming hydrothermal ore d e p o s i t s . A reasonable guess may be made that b a s i c magmas produce ore forming s o l u t i o n s only i f some s i l i c e o u s m a t e r i a l s are f i r s t engulfed by the magmas to provide water, as de s c r i b e d by Krauskopf (1967). Thus hydrothermal d e p o s i t s may be expected to show, contamination with leads which have had a c r u s t a l h i s t o r y p r i o r to t h e i r i n c o r p o r a t i o n i n t o the source rocks of the present ore d e p o s i t s . I f t h i s reworked c r u s t a l l e a d i s pre s e n t , the simple two stage equations given i n Figure 5 cannot be expected to h o l d r i g o r o u s l y i n a l l i n s t a n c e s . The degree to which the equations w i l l be a p p l i c a b l e i n a s p e c i f i c case w i l l depend upon the r e l a t i v e p r o p o r t i o n of reworked c r u s t a l l e a d that combines with the lead d e r i v e d from below the Moho and upon our a b i l i t y to d i s t i n g u i s h the p o s s i b l e presence of t h i s reworked l e a d . This type of contamination has been p r e v i o u s l y d i s c u s s e d by S i n c l a i r (1965). The contamination of hydrothermal d e p o s i t s i s i n c o n t r a s t to the s i t u a t i o n with conformable ore bodies d e s c r i b e d by Stanton (1960) and s e l e c t e d as the primary l e a d type d e p o s i t showing but l i t t l e c r u s t a l lead contamination as d i s c u s s e d by O s t i c et a l . Based on the previous d i s c u s s i o n , i t i s proposed that the i s o t o p i c r a t i o s of hydrothermal leads c o n t a i n a primary component i n d i c a t i v e of the time at which the c r u s t a l m a t e r i a l of the re g i o n was d e r i v e d from the upper mantle. This primary l e a d component was removed from the upper mantle with the r i s i n g 65 b a s a l t i c m a t e r i a l . The primary lead r i s i n g from the mantle may have become contaminated with leads that were i n c o r p o r a t e d i n t o the o v e r l y i n g sediments and d e r i v e d from adjacent c o n t i n e n t a l masses. An i n s p e c t i o n of the data of t h i s r e p o r t d i s c l o s e s that the c e n t r a l Idaho leads may have a n e a r l y pure primary l e a d component d e r i v e d from an environment with a y r a t i o of about 8.99 ( O s t i c et a l , 1967), whereas the leads of Butte County and ' C a s s i a County do not appear to have a s i n g l e primary component. The c y c l e e n v i s i o n e d f o r the growth of lead i s o t o p e s i n hydrothermal d e p o s i t s i s as f o l l o w s : a c o n t i n e n t a l mass adjacent to a s m a l l ocean b a s i n p r o v i d e s eroded m a t e r i a l which tends to f i l l the b a s i n . As the b a s i n f i l l s , the mantle below responds by c o n v e r t i n g to the denser g a r n e t - p y r o l i t e phase and s i n k i n g occurs. The t h i c k n e s s of sediments over the mantle continues to i n c r e a s e and p r o v i d e thermal i n s u l a t i o n of the mantle, so that the mantle temperature i n c r e a s e s . E v e n t u a l l y , p a r t i a l m e l t i n g of the upper mantle ensues and lenses of magma are formed, as t e n t a t i v e l y i d e n t i f i e d by A k i (1968). These lenses r i s e , o b l i t e r a t i n g the c o n v e n t i o n a l crust-mantle i n t e r f a c e as d e f i n e d by the Moho. The lenses a l s o a s s i m i l a t e some of the sediments that had formed e a r l i e r i n the ocean b a s i n s . The magmatic lenses would o r i g i n a l l y c o n t a i n only primary l e a d d e s c r i p t i v e of the time at which the lenses were formed. These leads could become contaminated with anomalous c r u s t a l leads as soon as the magmatic lenses reached and engulfed some of the sediments. The degree to which conta-mination occurs would depend upon the r e l a t i v e amount of sediments engulfed by the r i s i n g magmas. This phase of e v o l u t i o n of c r u s t a l m a t e r i a l w i l l be termed a "mantle orogeny". 66 The l a t e r h i s t o r y of the r e g i o n i s considered to be as-s o c i a t e d with orogenic a c t i v i t y i n the c r u s t . The heat producing r a d i o a c t i v e m a t e r i a l s c a r r i e d from the mantle are assumed to be depo s i t e d i n i t i a l l y i n the lower c r u s t ; with each subsequent orogenic event that produces i n t r u s i v e s these m a t e r i a l s are c a r r i e d c l o s e r to the s u r f a c e . I t i s during these subsequent c r u s t a l orogenies that the b a s i c magmas from the mantle become mixed with s i l i c e o u s m a t e r i a l s and adequate water i s obtained by the magmas to produce hydrothermal d e p o s i t s . The previous few paragraphs i n d i c a t e that the use of lead from s t r a t i f o r m d e p o s i t s may be of s u p e r i o r value to le a d from hydrothermal d e p o s i t s f o r the determination of age of c o n t i n e n t a l e v o l u t i o n . However, the s i t u a t i o n r e g a r d i n g hydrothermal d e p o s i t s i s not hopeless. S i n c l a i r (1964) found that the leads o f the Kootenay Arc of B r i t i s h Columbia contained only one observable primary l e a d and a r a d i o g e n i c component which had commenced development at an e a r l i e r time. The primary lead parent was found i n the proposed s t r a t i f o r m d e p o s i t at the S u l l i v a n Mine. In the present cases, no primary l e a d parent has been found. In the case of the Butte County and C a s s i a County samples, a s i n g l e primary l e a d parent may not e x i s t . The C e n t r a l Idaho samples, on the other hand, may have a primary lead parent i n d i c a t i v e of about 2500 my. The evidence i s not as c o n c l u s i v e as S i n c l a i r ' s 208 206 because the normalized Pb vs Pb p l o t does not i n d i c a t e a primary lead parent at the same age and the primary lead i t s e l f was not s p e c i f i c a l l y found. The case f o r le a d i s o t o p e analyses i n d i c a t i n g two primary components has been p r e v i o u s l y proposed. The Utah case r e p o r t e d 67 by Stacey et a l has a l r e a d y been d i s c u s s e d . An e a r l i e r re-p o r t i n g was of the Cobalt-Noranda leads by Kanasewich and Farquhar (1965). In both mentioned e a r l i e r cases, the leads were proposed to r e s u l t from the p r o x i m i t y of c o n t i n e n t a l c r u s t evolved at the two s p e c i f i e d times and the unknown c h a r a c t e r of the p h y s i c a l boundaries between g e o l o g i c a l p r o v i n c e s . The g e o l o g i c a l e x p l a n a t i o n of the Utah leads was that they represent a 2400 my system found at the U i n t a Mtns and that dips under an i n c r e a s i n g t h i c k n e s s of 1650 my rocks as f a r south as M i l f o r d . The present Idaho analyses show that a younger component i s a l s o present north of the U i n t a Mtns and t h i s younger component has been i d e n t i f i e d as s i m i l a r to the younger component found by Stacey et a l . Thus a p i c t u r e emerges of a n e a r l y pure 2400 or 2500 my primary l e a d present i n C e n t r a l Idaho and the U i n t a Mtns. Between and south of these l o c a t i o n s there i s evidence of younger le a d s . A 2500 my age f o r t h i s e n t i r e r e g i o n of the c o n t i n e n t has been claimed. The younger component i n Utah may have come from the south and e a s t , a d d i t i o n a l data on t h i s p o i n t i s d e s i r a b l e . A primary age of about 1340 my f o r the leads from southern B r i t i s h Columbia has been claimed by S i n c l a i r (1964). I t i s not unreason-able to presume that t h i s younger age component i s a l s o present along the e a s t e r n s i d e of the Idaho B a t h o l i t h and the e f f e c t s of i t present i n southeastern Idaho. An a l t e r n a t e e x p l a n a t i o n o f two primary l e a d components i n one r e g i o n c o u l d employ the concept of mantle c u r r e n t s and the c o n t i n e n t a l d r i f t h y p o t h e s i s . At t h i s time i n the development of t h e o r i e s r e g a r d i n g c o n t i n e n t s , no paper of t h i s nature would be complete without mentioning the d r i f t p o s s i b i l i t y . Using t h i s 68 p o s s i b i l i t y , t h i s p a r t i c u l a r r e g i o n o f the c o n t i n e n t evolved about 2500 my ago by exhausting the r a d i o a c t i v e heat producing elements i n the mantle below. The mantle below then became rejuvenated i n the heat producing elements and lead as a r e s u l t of mantle c o n v e c t i o n c u r r e n t s and the wandering of the c o n t i n e n t s . This s et the stage f o r another mantle orogeny about 1600 my ago. The p o s s i b i l i t y of a more recent mantle r e j u v e n a t i o n under t h i s r e g i o n w i l l be mentioned l a t e r . There are two f u r t h e r problems to be considered concerning the present a n a l y s e s . The f i r s t to be d i s c u s s e d w i l l be the long q uiescent p e r i o d i n d i c a t e d by the time span between T^ and T^. 208 The second problem i s th a t of why some of the normalized Pb vs 206 Pb l e a d l i n e s d e r i v e d i n the present r e p o r t e x h i b i t l i n e a r i t y and do not i n t e r c e p t the lead growth curve of O s t i c et a l . Unique answers to these questions are not forthcoming i n t h i s r e p o r t but r a t h e r ideas concerning each w i l l be d i s c u s s e d . Ringwood's model, which has been s e l e c t e d as the b a s i s of t h i s r e p o r t , p r e d i c t s that a mantle orogeny removes the r a d i o -a c t i v e heat producing elements from the mantle and pla c e s them i n t o the c r u s t . The formation of the magmas which do t h i s removal may be assumed to be s i m i l a r to the s o f t lenses suggested by A k i (1968). These r a d i o a c t i v e elements are presumably emplaced i n t o the lower c r u s t and, under proper p h y s i c a l c o n d i t i o n s , a v a i l a b l e to supply energy f o r a c r u s t a l orogeny. I t i s assumed th a t a c i d i c magmas would be produced i n the lower c r u s t during the c r u s t a l orogenies. The removal of the r a d i o a c t i v e m a t e r i a l s from the upper mantle deprives i t of a heat source, t h e r e f o r e the geotherm through the mantle w i l l tend to f a l l to that 69 d e f i n e d as the Precambrian s h i e l d geotherm i n Figure 25. The mantle i s i n c a p a b l e of f u r t h e r orogenic a c t i v i t y . C r u s t a l Development. A model f o r c r u s t a l orogenic a c t i v i t y on a la r g e s c a l e , such as i s r e q u i r e d to e x p l a i n the u p l i f t qf the present C o r d i l l e r a , has been d i s c u s s e d by Joyner (1967) . This model i s now proposed as a p o s s i b l e mechanism f o r the continued develop-ment of a c o n t i n e n t a l r e g i o n a f t e r the mantle orogeny has produced the c o n t i n e n t a l m a t e r i a l . Joyner's p r o p o s a l r e q u i r e s that the Moho under the c o n t i n e n t s be a phase change and b r i n g s us to adapt s e c t i o n s from the.model of c o n t i n e n t a l a c c r e t i o n d i s c u s s e d by Kennedy (1959). T h i s r e p o r t r e q u i r e s a phase change f o r subsequent c o n t i n e n t a l r e g i o n a l development. A phase change at the Moho w i l l be employed f o r convenience o n l y . W y l l i e (1963b) has p r e v i o u s l y suggested t h a t the Moho under the co n t i n e n t s may rep r e s e n t a phase change while that under the oceans represents a chemical change. The development of t h i s c h a r a c t e r change i n the Moho i s as f o l l o w s : the o r i g i n a l chemical change Moho was o b l i t e r a t e d by the r i s i n g magmatic m a t e r i a l during the mantle orogony.. A f t e r the e x t r u s i o n of s i a l i c m a t e r i a l from the upper mantle i n t o the lower c r u s t , a new form of Moho develops i n the s i a l i c m a t e r i a l . T h i s new Moho marks the l o c a t i o n of the pressure-temperature induced phase change between b a s a l t and e c l o g i t e . (This same phase change was p r e v i o u s l y mentioned i n t h i s r e port ' as a p o s s i b i l i t y f o r e x p l a i n i n g the Conrad d i s c o n t i n u i t y . T h i s type o f i n c o n s i s t e n c y r e p r e s e n t s the present, s t a t e of the a r t . ) Joyner proposes t h a t the v e r t i c a l l o c a t i o n of t h i s new Moho changes i n response to the sedimentation and e r o s i o n t a k i n g place 70 on the s u r f a c e above, The high pressure phase, e c l o g i t e , i s accepted as having a d e n s i t y of 3,4 gm/cc while the d e n s i t y of b a s a l t i s 2.9 gm/cc. Thus c o n s i d e r a b l e v e r t i c a l motion i s a v a i l a b l e through the phase change process. One of the models s t u d i e d by Joyner demonstrated a c y c l i c behavior of i n t e r m i t t e n t d e p r e s s i o n and e l e v a t i o n not u n l i k e the s e v e r a l stages of u p l i f t t h a t have been observed i n the present C o r d i l l e r a . The i n i t i a t i o n of the phase change b r i n g i n g about an u p l i f t -ing process e n v i s i o n e d by Joyner i s caused i n p a r t by the r a d i o -n u c l i d e heat p r o d u c t i o n i n i n d i v i d u a l rock u n i t s which l i e at depth. The rock s t r a t a e n t e r i n g i n t o t h i s process may have r e s u l t e d p r i m a r i l y from the new s i a l i c m a t e r i a l p r e v i o u s l y generated from the upper mantle, or may have r e s u l t e d from b u r i a l of sediments. In e i t h e r event, the r a d i o g e n i c component of the anomalous leads now sampled i n the hydrothermal d e p o s i t s may be e n v i s i o n e d as being the r e c o r d of formation and l a t e r temperature induced metamorphism of s p e c i f i c lower c r u s t a l rock systems. In the two stage lead equations given i n Figure 5, 12 corresponds to the time of formation of the s p e c i f i c c r u s t a l rock system and T^ corresponds to the time at which t h i s system was metamorphosed. The meta-morphism corresponds to the r e g i o n a l u p l i f t i n Joyner's model. The rock s t r a t a r e s p o n s i b l e f o r t h i s u p l i f t are thought to be i n t r u d e d i n t o higher c r u s t a l horizons and i n c i d e n t a l l y become the source rocks of the hydrothermal lead ores. The experimental e f f o r t of t h i s r e p o r t i n d i c a t e s that the rock bodies which are the source rocks f o r the lead ores i n the three sampled Idaho regions must be d i f f e r e n t i n each case. The three Idaho regions are a l l i n the same sedimentary b a s i n and 71 there i s no reason to suspect but that the sub c r u s t a l rocks are continuous under each. The f a c t that the obtained lead-l e a d ages i n d i c a t i v e of the time of formation of the source rocks are d i f f e r e n t i n each r e g i o n suggests that d i f f e r e n t rock bodies are r e s p o n s i b l e f o r the c r u s t a l orogenic events i n each case. I t i s not intended to give the impression that the w r i t e r b e l i e v e s that the s p e c i f i c c r u s t a l orogenic events l e a d i n g to ore formation, which a l l o c c u r r e d about 50 my ago or l e s s , are u n r e l a t e d , the r e l a t i o n s h i p between them has to do with some mechanism other than c o n t i n u i t y of rock u n i t s . The obvious ex t e n s i o n of Joyner's model leads to the eventual c o n c e n t r a t i o n of the r a d i o n u c l i d e s i n the upper c r u s t . The heat f l u x to the s u r f a c e from t h i s l o c a t i o n i s s u f f i c i e n t l y high to prevent temperatures reachi n g those l e v e l s r e q u i r e d f o r orogenic a c t i v i t y . The geotherm under the r e g i o n must slowly drop through-out the c r u s t and upper mantle u n t i l i t reaches the Precambrian s h i e l d geotherm. The pressure i n the lower c r u s t w i l l be reduced as e r o s i o n continues at the s u r f a c e . The b a s a l t - e c l o g i t e Moho w i l l respond to t h i s e r o s i o n and some compensating u p l i f t continue to occur. E v e n t u a l l y the c o n t i n e n t a l r e g i o n must be eroded to sea l e v e l and t h i s w i l l mark the f i n a l stage i n the development of the c o n t i n e n t a l r e g i o n . I t i s tempting to s p e c u l a t e that no f u r t h e r s i g n i f i c a n t development of the Canadian and A u s t r a l i a n S h i e l d s w i l l take p l a c e . The hypothesized mantle c u r r e n t s would i n v a l i d a t e the r a t h e r glum f o r e c a s t of f u t u r e dreary s e a - l e v e l c o n t i n e n t a l masses spe c u l a t e d i n the previous paragraph. There i s seismic evidence i n d i c a t i n g an abnormally t h i c k c o n t i n e n t a l c r u s t a l l a y e r under 7 2 the Himalayan mountain system, the i n f e r e n c e being drawn that mantle c u r r e n t s are s l i d i n g the Indian S h i e l d under the A s i a n c o n t i n e n t a l mass and thereby causing renewed u p l i f t as d e s c r i b e d by Holmes (1965). The A f r i c a n R i f t system may be i n d i c a t i v e that mantle c u r r e n t s are f l o w i n g under the A f r i c a n S h i e l d and that the mantle m a t e r i a l p r e s e n t l y there has not always been under the s h i e l d . The r e j u v e n a t i n g of mantle m a t e r i a l under c o n t i n e n t s through a mantle c o n v e c t i o n c u r r e n t system could cause a renewal of the mantle orogenic c y c l e proposed by Ringwood. I f the mantle c u r r e n t hypothesis i s c o r r e c t , then the present u p l i f t i n the C o r d i l l e r a could be the r e s u l t of mantle r e j u v e n a t i o n . T h i s p o s s i b i l i t y was p r e v i o u s l y d i s c u s s e d regarding the i m p l i e d presence of two primary l e a d components i n the same r e g i o n . There i s evidence implying that the P a c i f i c Ocean f l o o r i s now s l i d i n g under the western North American c o n t i n e n t . It may be that new m a t e r i a l s are being f o r c e d i n t o the s u b c r u s t a l or mantle regions under the area of study and renewed mantle type orogenies are r e s p o n s i b l e f o r the r e j u v e n a t i o n of the area. The lead i s o t o p e abundances i n southern Idaho have been i n t e r p r e t e d i n terms of a c o n t i n e n t a l a c c r e t i o n model that was f o l l o w e d by a c r u s t a l development model and then i n t e r p r e t e d i n terms of a mantle con v e c t i o n h y p o t h e s i s . The former combined models were e x p l a i n e d i n more d e t a i l because they represent the conceptual mechanism p r e f e r r e d by t h i s w r i t e r . Three reasons f o r p r e f e r r i n g t h i s mechanism were p r e v i o u s l y presented. They i n v o l v e d c o n s i d e r a t i o n of the v o l a t i l e s p r e s e n t l y on the earth's s u r f a c e , the s e i s m i c s t u d i e s of small ocean b a s i n s , and the geo-c h r o n o l o g i c a l map of North America. The a c t u a l t r u t h concerning 73 c o n t i n e n t a l a c c r e t i o n and development probably l i e s i n a combi-n a t i o n of the two extremes presented here: processes s i m i l a r to those d e s c r i b e d by Ringwood and Joyner take p l a c e and occas-i o n a l r e j u v e n a t i o n occurs through mantle c u r r e n t s . 208 Normalized Pb L i n e a r Trends. The second problem to be considered concerning the present l e a d analyses was the l i n e a r t rend observed i n the normalized P b 2 ^ 8 vs P b 2 ( ^ isotope p l o t s . T h i s l i n e a r t rend had p r e v i o u s l y been observed by S i n c l a i r while studying the Kootenay Arc l e a d s . The general experience o f t h i s l a b o r a t o r y had been, however, that there was l i t t l e i f any e x p e c t a t i o n of a c o r r e l a t i o n i n the normal-i z e d P b 2 ^ 8 vs P b 2 ^ lead i s o t o p e s from a given r e g i o n . Thus the l i n e a r trends found i n the c e n t r a l Idaho and Butte County samples of the present study were not expected. The l i n e found by S i n c l a i r i n t e r c e p t e d the growth curve d e s c r i b e d by O s t i c et a l at the 20 7 206 c o r r e c t time as p r e d i c t e d by the normalized Pb vs Pb isotope p l o t . The present l i n e s , however, do not i n t e r c e p t the growth curve at a l l , but r a t h e r l i e above i t . I n s p e c t i o n of equation 5 of Figure 3 i n d i c a t e s that l i n e a r trends i n the normalized 208 vs 206 le a d p l o t s may be expected under c e r t a i n c o n d i t i o n s . The experimental i n f o r m a t i o n concerning the e x i s t e n c e of the l i n e a r trends i n some regions has been a v a i l a b l e f o r some time, see Farquhar and R u s s e l l (1957) as an example of e a r l y experimental evidence. The amount of e x p e r i -mental evidence now a v a i l a b l e appears, to t h i s w r i t e r , to be unequivocal i n favor of the e x i s t e n c e of the trend under c e r t a i n c o n d i t i o n s . I t remains only to s p e c i f y what are those c o n d i t i o n s . 74 The c o n d i t i o n f o r the e x i s t e n c e of a l i n e a r t r e n d i n the normalized P b 2 ^ 8 vs P b 2 ^ r a t i o s from a r e g i o n i s that the n value be constant throughout the source rocks or that nearly-complete homogenization occur during the events r e s p o n s i b l e f o r the formation of the ore bodies. Evidence w i l l s h o r t l y be presented to i n d i c a t e that the homogenization process does not occur, at l e a s t i n some cases. I t appears that a uniform value i n the source rocks i s the one c o n d i t i o n f o r l i n e a r i t y i n 208 206 the Pb vs Pb r a t i o s . T h i s does not appear to be a very r e s t r i c t i v e c o n d i t i o n . The l a c k of l i n e a r trends may be i n d i c a t i v e that more than one source rock u n i t i s i n v o l v e d i n the process l e a d i n g to m i n e r a l i z a t i o n and the homogenizing processes are not o p e r a t i v e . Stacey et a l have observed the same l i n e a r trend and e l e v a t i o n of the normalized P b 2 ^ 8 vs P b 2 ^ l i n e i n the lead i sotope s t u d i e s of Utah. T h e i r e x p l a n a t i o n of the e l e v a t i o n phenomena was to i n t e r -pose a short time term anomalous lead with a high Q, v a l u e . There i s not, however, any other reason f o r i n t e r p o s i n g the short term anomalous leads. F u r t h e r , the r e q u i r e d n value i s of the order of 10^, about three orders of magnitude too l a r g e to be r e a l i s t i c . T h i s w r i t e r favors an e x p l a n a t i o n which w i l l produce a 208 isotope enrichment through a more n a t u r a l p r o c e s s , perhaps r e l a t e d to e r o s i o n and sedimentation. Such a process should add 208 a time independent step f u n c t i o n to the Pb equation given i n 206 20 7 Figure fi without a l t e r i n g the Pb and Pb equations. 208 I n d i c a t i o n that a r e g i o n a l e f f e c t may produce a high Pb 206 vs Pb l i n e i s obtained from the Sudbury area leads analyzed by U l r y c h (1962). The normalized P b 2 0 7 vs P b 2 0 6 p l o t i s 75 reproduced here as F i g u r e 26 and demonstrates that a l l of the leads have been s u b j e c t e d to the same h i s t o r y . , The normalized P b 2 ^ 8 ;vs';Pb 2^ p l o t reproduced, i n F i g u r e 27 shows two d i s t i n c t trends i n the Pb i s o t o p e c h a r a c t e r i s t i c s . The lower l i n e , c o n s i s t i n g of f i v e samples, i n t e r c e p t s the growth curve pf O s t i c 207 206 et a l at the time p r e d i c t e d by the Pb vs Pb l e a d l i n e . This, l a t t e r i s the s i t u a t i o n as observed by S i n c l a i r i n the Kootenay A r c . The upper l i n e of F i g u r e 27 c o n s i s t s o f , o n l y three p o i n t s and these, u n f o r t u n a t e l y , represent only two mines./' In'...; h i s r e p o r t , U l r y c h gives f u r t h e r p o i n t s i n h i s F i g u r e 20 that were obtained from U n i v e r s i t y of Toronto analyses and these f u r t h e r s u b s t a n t i a t e the evidence f o r two d i s t i n c t t r e n d s . There i s no immediately apparent change i n the l o c a l g e o l o g i c a l s t r u c t u r e , t o s i g n i f y the d i f f e r e n t source rocks that are respond slble'••'for the d i f f e r e n t l e a d l i n e s ; the Frood mine, f o r example, has a p o i n t on each l i n e , the samples being .obtained from d i f -f e r e n t l e v e l s i n the mine. /The f a c t that one-mine contains such .".•.'•.•'• '•'•208 d i f f e r e n t , ores when c o n s i d e r i n g the Pb i s o t o p e .must i n d i c a t e that there has been r e l a t i v e l y l i t t l e homogenization of i .e./perhaps the ores were not/ t r a n s p o r t e d f a r from the source rocks-. /;/ • .•' F r u i t f u l ground f o r e x p l a i n i n g the s h i f t i n the normalized Pb'-.^;! ys - P b 2 ^ l e a d i s o t o p e line;'probably; l i e s i n the r e l a t i v e c r u s t a l mineralogy of uranium and thorium. For example,, i f • a ; - / s i g n i f i c a n t q u a n t i t y ' o f the thorium i n a rock u n i t i s contained i n a m i n e r a i o g i c a l system with, a lower, mechanical and chemical ."";••' s t a b i l i t y than the m i n e r a i o g i c a l systems c o n t a i n i n g the bulk of the ..uranium, then weathering of the minerals w i l l tend, to; •;.;,/; Pb206/Pb204 F i g u r e 26: Sudbury, O n t a r i o , Canada. Normalized Lead 207 vs Lead 206 I s o t o p i c R a t i o s . A f t e r U l r y c h (1962). 77 Figure 27: Sudbury, O n t a r i o , Canada. Normalized Lead 208 v s Lead 206 I s o t o p i c R a t i o s ; . A f t e r U l r y c h (1962). 78 20 8 p r e f e r e n t i a l l y remove the Pb isotope that has formed i n the rock u n i t . The sedimentary s t r a t a r e s u l t i n g from t h i s weathering 2 0 8 w i l l be e n r i c h e d i n the Pb i s o t o p e . F i e l d evidence f o r such a phenomenon i n o p e r a t i o n has been c i t e d by T i l t o n et a l (1955). They found that a g r a n i t e from O n t a r i o appeared to have been a c l o s e d system to uranium and i t s decay products, but an open system with r e s p e c t to thorium and i t s decay products. In the 20 8 case of t h i s g r a n i t e , at l e a s t , the Pb isotope could be pre-f e r e n t i a l l y removed. Furth e r f i e l d evidence f o r n a t u r a l l y o c c u r r i n g lead isotope f r a c t i o n a t i o n has been obtained by Robinson et a l (1963) while s t u d y i n g d i s c o r d a n t ages of euxenite from a G r e n v i l l e pegmatite. The d i s c o r d a n t ages were a t t r i b u t e d t o , among other items, a 208 206 207 g r e a t e r l o s s of Pb than of Pb and Pb . Laboratory study of the m a t e r i a l d i s c l o s e d that the lead removed from the mineral 208 while h e a t i n g i n a vacuum tended to be e n r i c h e d i n the Pb i s o t o p e . This i s o t o p i c enrichment of the removed le a d was at-t r i b u t e d to a tendency on the p a r t of thorium to be near the s u r f a c e of the mineral c r y s t a l s as compared to the l o c a t i o n of the uranium. The lead l o s t by the c r y s t a l s can go i n t o rock u n i t s which l a t e r become source rocks f o r hydrothermal ore bodies. These leads w i l l c a r r y with them the h i s t o r y of the i s o t o p i c f r a c t i o n a t i o n which occurred at the time of removal from the euxenite c r y s t a l s . Further study of the r e l a t i v e m i n e r a l o g i e s of thorium and uranium and t h e i r decay systems i s r e q u i r e d to a s c e r t a i n the importance of i s o t o p i c f r a c t i o n a t i o n that can occur during weathering and d e p o s i t i o n p r o c e s s e s . Such s t u d i e s should y i e l d v a l u a b l e i n f o r m a t i o n r e g a r d i n g the mechanisms of formation 79 of l e a d ore b odies. F u r t h e r a l t e r n a t i v e s f o r producing an apparently e l e v a t e d P b 2 ^ 8 vs P b 2 ^ lead l i n e may be proposed by e n v i s i o n i n g areas of high n value i n the lower c r u s t , say below the Conrad d i s -c o n t i n u i t y . There does not, however, appear to be any b e n e f i t to proposing such mechanisms. 80 SUMMARY The i s o t o p i c analyses of leads from s e l e c t e d regions of Montana, Wyoming and south c e n t r a l Idaho have been obtained and i n t e r p r e t e d i n the present study. Adequate sampling f o r d e f i n i t i v e age determinations was done f o r three of the r e g i o n s , a l l i n south c e n t r a l Idaho. The r e s u l t s of the study i n d i c a t e that t h i s i s a r e g i o n of complex geochronology, the present work i s considered to be the beginning of an e f f o r t that the w r i t e r wishes to see pursued to a more complete end. The south c e n t r a l Idaho r e g i o n concentrated upon has been p r e v i o u s l y c onsidered a r e g i o n of unknown ancient geochronology. The r e s u l t s of the study imply that c o n t i n e n t a l m a t e r i a l f i r s t evolved i n t h i s r e g i o n 2500 my ago. There appears a l s o to be a lead component demonstrating a primary age of g r e a t e r than 1200 my, other r e g i o n a l evidence i n d i c a t e s that t h i s component i s 1400 to 1600 my o l d . Radiogenic components of the Idaho leads commenced development i n a c l o s e d system at 2700 my ( C a s s i a County), 2500 my (southeastern f l a n k of the Idaho B a t h o l i t h ) and 1900 my (Butte County). The r e s u l t s of t h i s study are c o n s i s t e n t with the r e s u l t s of a s i m i l a r study of Utah leads that was c a r r i e d on simultaneously at the U.S. G e o l o g i c a l Survey Denver o f f i c e . P r e l i m i n a r y l e a d - l e a d ages were a l s o found f o r the southern end of the Wind River Mtns, Wyoming (3200 my), and f o r the L i t t l e B e l t Mtns of Montana (2200 my). The present i n t e r p r e t a t i o n of the g e o c h r o n o l o g i c a l h i s t o r y t o l d by the i s o t o p i c analyses i s based upon the presumption that a l l of the i n d i v i d u a l samples are anomalous leads and the parent rock s t r a t a l a y at depth. Samples were grouped i n t o s u i t e s and 81 meaningful g e o c h r o n o l o g i c a l r e s u l t s obtained only by c o n s i d e r i n g the groups, i n d i v i d u a l sample analyses have no s i g n i f i c a n c e i n the present r e p o r t . The important i n f o r m a t i o n obtained from anomalous lead s u i t e s i s the slope and p o s i t i o n of the normalized Unicpe g e o c h r o n o l o g i c a l i n f o r m a t i o n i s obtained from anomalous lea d l i n e s only by using s u b s i d i a r y i n f o r m a t i o n . In the present r e p o r t , s u b s i d i a r y i n f o r m a t i o n i s obtained by assuming that the time of m i n e r a l i z a t i o n of the sample d e p o s i t s i s known and that the unique primary lead growth curve d e f i n e d by v equal to 8.99 i s v a l i d . I t has been demonstrated that the anomalous lead isotope r e s u l t s are compatible with reasonable models re g a r d i n g c o n t i -n e n t a l a c c r e t i o n and development. The model e x t e n s i v e l y explored i n the present r e p o r t represents an e c l e c t i c process from the l i t e r a t u r e , care being exerted to maintain c o m p a t i b i l i t y . The p r i n c i p a l components of the model were previous suggestions by Ringwood and by Joyner. The former e n v i s i o n e d a chemico-physical process f o r e v o l v i n g c o n t i n e n t a l c r u s t a l m a t e r i a l from the upper mantle. The l a t t e r e n v i s i o n e d a p h y s i c a l process f o r r e p e t i t i v e c r u s t a l u p l i f t . The l i n k i n g of the two concepts i n t o a s i n g l e c o n s i s t e n t model f o r c o n t i n e n t a l a c c r e t i o n and development was accomplished by a s c r i b i n g a chemical change to the Moho under the oceans and a phase change to the Moho under the c o n t i n e n t s , f o l l o w i n g a suggestion by W y l l i e . An a l t e r n a t i v e mechanism f o r the development of anomalous lea d i s o t o p e abundances has been b r i e f l y d i s c u s s e d . T h i s i n v o l v e d the use^of mantle c u r r e n t s and c o n t i n e n t a l d r i f t hypotheses. Pb 207 vs Pb 206 l i n e d e f i n e d by the samples comprising the s u i t e s . 82 The normalized Pb 208 vs Pb 206 l e a d i s o t o p e data obtained from two of the three Idaho regions d i s p l a y e d l i n e a r t rends; the l i n e s d i d not i n t e r s e c t the a = 3.89 primary l e a d curve 20 7 of O s t i c et a l at the parametric time i n d i c a t e d by the Pb t i v e l y e x p l a i n e d i n terms of the d i f f e r e n t mineralogy of the thorium and uranium p r e c u r s o r s of the 208 l e a d i s o t o p e and the 206, 207 l e a d i s o t o p e s r e s p e c t i v e l y . Such an i s o t o p i c c o n c e n t r a t i o n could occur through chemical and mechanical a c t i o n during the formation of sedimentary s t r a t a which sub-sequently became source rocks f o r the r a d i o g e n i c component of anomalous le a d s . vs Pb 206 i s o t o p e data. This f a i l u r e to i n t e r s e c t i s t e n t a -83 BIBLIOGRAPHY Ahrens, L.H. 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Marketing problems of small business e n t e r p r i s e s engaged i n l e a d and z i n c mining. Montana Bureau of Mines and Geology B u l l e t i n 30 (1962). 89 APPENDIX SAMPLE MEASUREMENT The r e s u l t s of the measurements of the 34 samples comprising the data of t h i s r e p o r t are given i n the accompanying t a b l e A-I. The samples were prepared i n the t e t r a m e t h y l lead form f o l l o w i n g the general chemical technique o u t l i n e d by S i n c l a i r (1964). The i s o t o p i c abundances of the samples were obtained using the U.B.C. MS-1 which had been designed and c o n s t r u c t e d at the d i r e c t i o n of Dr. R.D. R u s s e l l . The mass spectrometer data was recorded d i g i t a l l y and the i s o t o p i c r a t i o s were c a l c u l a t e d by an automatic data re-d u c t i o n computor program. This program had been i n i t i a l l y w r i t t e n by Dr. D.H. Weichert and l a t e r s t r e a m l i n e d and m o d i f i e d by Dr. R.D. R u s s e l l and Mr. J . Blenkinsop (Weichert et a l , 1967). The i s o t o p e r a t i o s given i n the present r e p o r t are normal-i z e d to the Broken H i l l , A u s t r a l i a , l e a d a n a l y s i s i n accordance with the general procedure of t h i s l a b o r a t o r y . The Broken H i l l l e a d sample was measured r e p e a t e d l y on the mass spectrometer du r i n g the time that the t e s t samples were being analyzed. From the Broken H i l l measurements, n o r m a l i z i n g c o r r e c t i o n f a c t o r s were found f o r c o r r e c t i n g the t e s t sample measurements to be compatible 206 with the t r a d i t i o n a l l y accepted values f o r Broken H i l l (Pb / P b 2 0 4 = 16.116, P b 2 0 7 / P b 2 0 4 = 15.542, P b 2 0 8 / P b 2 0 4 = 36.068). The l a r g e s t c o r r e c t i o n f a c t o r obtained i n the present measurements a l t e r e d the measured value by only 0.121. The c o r r e c t i o n f a c t o r s were found to change each time a new f i l a m e n t was i n s t a l l e d i n the source, a t o t a l of four f i l a m e n t s were r e q u i r e d f o r the data given i n t h i s r e p o r t . The c o r r e c t i o n f a c t o r s were, to a l i m i t e d e xtent, a f u n c t i o n of the p a r t i c u l a r mass being measured. Table A - I : L i s t o f Lead Isotope Analyses R e s u l t s . U BC ample Na Mine Name State County Town T R 206/204 - Analysis 207/204 208/204 Central Idaho Samples 572 Clayton Silver Idaho Custer Clayton 1 IN I7E 19 024 15-884 40-423 574 Phi Kappa Idaho Blaine Ketchum * 7N I8E 18-372 15-797 39- 394 575 Triumph Idaho Blaine Ketchum 4N 18 E 20-655 16-238 4 I- 3 37 576 577 Buttercup Star Idaho Idaho Camas Blaine Blaine Hailey 2N 2N 16 E I8E 18- 936 18- 675 19- 288 15-878 15- 823 15- 91 1 40-289 40- 1 17 40 422 578 Silver Star Queen Idaho Blaine Hailey 2N I8E 20-762 16- 172 41-77 5 579 Silver Star Queen Idaho Blaine Hailey 2N I8E 20-5 17 16-128 4 1- 730 580 Silver Star Queen Idaho Blaine Hailey 2N I8E 20-737 16- 169 41-843 581 590 SilverStar Queen Gilmore Idaho Idaho Blaine Lemhi Hailey Gilmore 2 N I3N I8E 27E m%% 20-413 18 069 II?8T 16- 087 15- 702 %\m 41-653 + 39- 17 3 591 Blue Lead Idaho Lemhi Leadore I7N 26E 18-429 15- 790 39-556 Butte County, Idaho Samples 607 Wilber Idaho Butte Howe 7N 29E 20-833 16- 107 41-365 634 Ella Idaho Butte Arco 3 N 24 E 19-296 15-968 40075 638 (prospect) Idaho Butte Arco 2 N 24E 18-031 15-775 39 122 639 Hub Idaho Butte Arco 2 N 24E 17-990 15-783 39-099 640 Hanni 2 Idaho But te Arco 2 N 24E 17-408 15-726 38-932 Cassio County, Idoho Samples 556 Melchoir Idaho Cassia Conner I3S 25E 19-948 \ m i 16-174 16192 15-975 42-287 42-288 557 Balgor Idaho Cassia Connor 13 S 25E 42 323 620 Old Dominion Idaho Cassia Albion IIS 25E 621 Walton's Idaho Cassia Conner I3S 25E 18-355 15-921 4116 1 622 (prospect) Idaho Cassia Conner I3S 25E 18-116 15-853 41-787 623 Sliver Hills Idaho Cassia Strevell I5S 29E 19-296 15-832 39-215 681 Old Skoro Utah Box Elder Almo(ldaho) I4N I7W 17-490 15-718 39074 + Measured at USGS Denver by Dr. J. S. Stacey Table A-I—-Continued U B C Mine Name • Location - Analysis — Sample No. State County Town T R 206/204 207/204 2 0 8 / 2 0 « Montana Samples 594 4 - Sevens Montana Park Cooke City 7S I2E 17 563 15-728 38-880 595 Irmo Montana Park Cooke City 9S I4E 16- 283 15-460 37-173 598 Block V Montana Judith Basin Nelhart I5N 9E 16- 686 15-525 37-740 600 Lexington Montana Cascade Nelhart I4N 8E 17- 124 15-589 37 926 601 Boss Montana Cascade Nelhart I4N 8E 17- 099 15-562 3 7-824 603 Tiger Montana Judith Basin Neihart I5N 9E 16 794 15-526 37 766 604 Granite Mtn. Montana Sweet grass Cooke City (unknown) 15 •768 15-460 36 658 Wyoming Sample 582 Z enith Wyoming Fremont Atlantic City 29N I00W 14 113 15 09 7 34-212 Nevada Sompleo 685 Mitchel Nevada Elko Wells 43N 68E 19 •719 15 899 39•997 691 Delno Nevada Elko Wells 4 4N 68E 19 •719 15-902 40-025 701 Looney Nevada Pershing Lovelock 26N 3 4E 18 791 15-803 39-056 703 Pershing Nevada Pershing Lovelock 26N 34E 18 •791 15-801 39 069 92 A l l sample p r e p a r a t i o n s were measured at l e a s t twice. The re p o r t e d i s o t o p i c r a t i o s are the c o r r e c t e d average of the s e v e r a l measurements made on each p r e p a r a t i o n . Some samples were prepared more than once, where t h i s occurred the r e s u l t s from the d i f f e r e n t p r e p a r a t i o n s are l i s t e d i n d i v i d u a l l y . In one inst a n c e (sample 581 S i l v e r S t ar Queen Mine), there i s a comparable measurement from the U.S. G e o l o g i c a l Survey Denver l a b o r a t o r y which has been c o r r e c t e d to the Broken H i l l sample and shows good agreement with the r e s u l t s from t h i s l a b o r a t o r y . Two sets of data are presented to show that the reproduc-i b i l i t y of the present measurements i s about 0.1%. Figure A-1 shows the frequency d i s t r i b u t i o n of the percent d i f f e r e n c e between d u p l i c a t e measurements of 32 d i f f e r e n t samples. The histograms i n t h i s f i g u r e show that there i s no p a r t i c u l a r d i f f e r e n c e i n the a b i l i t y to reproduce the measurement of the three i s o t o p i c r a t i o s . In g e n e r a l , 2/3 of the sample measurements were reproduced to 0.11 or b e t t e r . A second i n d i c a t i o n of the r e p r o d u c i b i l i t y of the mass spectrometer i n f o r m a t i o n i s shown i n Table A-11..- Here i s given the s t a t i s t i c a l a n a l y s i s of twelve measurements of the Broken H i l l l e a d sample (UBC 1). Th i s a n a l y s i s i s based upon the co n v e n t i o n a l s t a t i s t i c a l methods f o r small sample numbers. The 50% probable e r r o r of the samples i s about 0.1%, the 50% probable e r r o r of the 207 mean i s about 0.03%. The normalized Pb measurements i n d i c a t e b e t t e r r e p r o d u c i b i l i t y than the remaining two i s o t o p e s . The data o f Table A-II i s a c t u a l l y p e s s i m i s t i c concerning the r e p r o d u c i b i l i t y of the mass spectrometer toward an i n d i v i d u a l sample. Fi g u r e A-2 shows g r a p h i c a l l y the c h r o n o l o g i c a l values given i n Table A - I I . In a l l except one case (consecutive measure-93 8-7-6 • 5 • 4 3 2 I 0 3£\ 206/204 Ratios I' I I I I I I I I I I I I I I I 1 I I I 0 ) I:•.••V..-;>'.'.-:«:i.«s>;.; T'TT r e 207/204 Ratios I I I I I I I I I I I I I I 206/204 Ratios S i I S I S l S i ' ? Is? NI ^1 =?IS I S I CM • 8 | S | o tO 2 O CM * ID 00 O P ~ - ~ ~ - to 00 o ty w m CM ro I S CJl Tj-I <0 I 00 10 00 to ro O cvi ^ co co * t * * * Percentage Difference Range Percent Difference = Maximum Measurement-Minimum Measurement x 100 Average Measurement F i g u r e A-1: Frequency D i s t r i b u t i o n o f Percent D i f f e r e n c e Between D u p l i c a t e Measurements o f 32 D i f f e r e n t Samples. 94 ment 7) the preceeding sample i n the mass spectrometer had h i g h e r i s o t o p i c r a t i o s than does sample 1. Figure A-2 shows t h a t the normalized P b 2 0 6 and P b 2 0 8 i s o t o p i c r a t i o s f o r t h i s p r e p a r a t i o n of sample 1 are g e n e r a l l y i n c r e a s i n g through nine c o n s e c u t i v e measurements. Reference to Table A-I or to F i g u r e 4 w i l l show 207 that there i s only a l i m i t e d v a r i a t i o n i n the normalized Pb i s o t o p i c r a t i o s , t h e r e f o r e no s i g n i f i c a n t change i s expected i n t h i s r a t i o i f the changes observed are a t t r i b u t e d to an i n t e r -sample contamination phenomena i n the mass spectrometer. A rough check to determine i f the apparent i s o t o p i c growth observed i n the normalized P b 2 ^ a n d P b 2 ^ 8 r a t i o s of t h i s p r e p a r a t i o n was due to intersample contamination from one run to the next or due to a change i n spectrometer o p e r a t i n g c h a r a c t e r i s t i c s was performed by using a new sample 1 p r e p a r a t i o n f o r c o n s e c u t i v e measurements 10, 11, and 12. Figure A-2 shows that the measured i s o t o p i c r a t i o s r e turned to near the o r i g i n a l values achieved f o r c o n s e c u t i v e measurement 1, i n d i c a t i n g t h a t intersample contamination and not changing o p e r a t i n g c h a r a c t e r i s t i c s were r e s p o n s i b l e f o r the i s o t o p i c changes. Since each of the sample p r e p a r a t i o n s used f o r data i n the present r e p o r t were not p l a c e d i n the mass spectrometer more than three times, t h i s source of e r r o r i s h o p e f u l l y reduced i n the measurements. No p o i n t of blame i s attached to the source of the i n t e r -sample contamination. A l l of the p r e c a u t i o n s p r e v i o u s l y c o n t r i v e d at t h i s l a b o r a t o r y to prevent t h i s occurrence were employed. Some of the data obtained i n the present study was i n d i c a t i v e of the contaminant being impressed while the sample was being removed from the machine r a t h e r than while i n the machine. This l a t t e r Table A - I I : S t a t i s t i c a l A n a l y s i s o f Twelve Measurements o f U.B.C. Sample 1. No S o u r c e Magnet and D e f l e c t o r P la tes S h o r t e d and Grounded Date P r e p a r a t i o n No. 2 0 6 / 2 0 4 2 0 7 / 2 0 4 2 0 8 / 2 0 4 J u n e 21 , 6 7 21 2 2 2 4 26 26 28 J u l y 4 4 6 7 8 2 2 2 A v e r a ge 5 0 % P r o b a b l e E r r o r 5 0 % P r o b a b l e E r r o r of Mean 16 16 16 16 16 16 16 16 16 16 16 16 16 0 0 6 8 0 9 9 103 101 120 130 139 134 1 6 0 0 9 0 I 0 9 I I 7 I 14 016 ( 0 1 % ) 0 0 5 ( 0 ' 0 3 % ) 15 4 6 4 15 4 8 0 1 5 - 4 7 7 1 5 - 4 7 3 1 5 - 4 9 2 1 5 - 4 8 7 1 5 - 4 8 5 1 5 - 4 7 4 1 5 - 4 8 4 1 5 - 4 8 2 1 5 - 4 7 9 1 5 - 4 8 6 1 5 - 4 8 0 0 ' 0 0 5 ( 0 0 3 % ) 0 0 0 2 (0 01 %) 35- 9 3 0 3 6 - 0 2 3 3 6 - 0 4 9 3 6 0 4 0 3 6 - 0 7 4 3 6 - 0 9 7 3 6 0 8 6 3 6 - 0 6 9 3 6 - 0 5 4 3 5 - 9 7 7 35 9 8 9 36 0 2 9 3 6 0 3 5 0 0 3 2 (0 0 9 % ) 0 0 0 9 ( 0 0 2 % ) I D I O f JO I 3 4 5 6 7 8 9 Consecutive Measurement Number 10 II 12 5 - O 0- -0-Consecutive Measurement Number 10 II 12 05 -36-95 -36- 90*- I I 3 4 5 6 7 8 9 Consecutive Measurement Number 10 11 12 Figure A-2: Chronological Values Obtained.for Twelve Measurements of U.B.C. Sample 1. 9 7 would i n d i c a t e contamination of the new break s e a l s i n t o which samples were r e t r i e v e d . The degree of contamination was not s u f f i c i e n t to warrant a b i g study of the mechanism, i t i s suf-f i c i e n t to recognize i t s presence and p l a n operations around i t . The l i n e a r trends found i n the lead i s o t o p e data from the va r i o u s g e o g r a p h i c a l regions were determined with a computer r o u t i n e which allowed l e a s t squares f i t t i n g f o r each of two v a r i a b l e s . This program was o r i g i n a l l y d e s c r i b e d by York (1966) and has s i n c e been m o d i f i e d by Dr. R.D. R u s s e l l . The one sigma e r r o r on the slopes of the l i n e s as determined by the Y o r k - F i t program i s given i n Table I I . The u n c e r t a i n t i e s i n the slopes may be converted i n t o u n c e r t a i n t i e s i n the computed ages of commencement of growth of the r a d i o g e n i c l e a d component. In the present i n s t a n c e s , i t i s f e l t t h a t g e o c h r o n o l o g i c a l u n c e r t a i n t i e s obtained by the e r r o r s i n the slope are c o n s i d e r a b l y l e s s than the u n c e r t a i n t i e s which are inherent i n the p o s s i b l e i n e x a c t i t u d e s i n the assumptions u n d e r l y i n g the general l e a d - l e a d model. Thus us i n g u n c e r t a i n t i e s i n the e x p e r i m e n t a l l y determined slopes of the l i n e s leads to u n r e a l i s t i c a l l y high p r e c i s i o n s i n the geo-c h r o n o l o g i c a l d e t e r m i n a t i o n s . The u n c e r t a i n t i e s to be pl a c e d on the ages gi v e n i n Table II must be based upon the i n d i v i d u a l assessment of the v a l i d i t y of the assumptions employed i n the present r e p o r t . 

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