UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Lead isotope study of ores and adjacent rocks Reynolds, Peter Herbert 1967

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1968_A1 R48.pdf [ 4.75MB ]
Metadata
JSON: 831-1.0053358.json
JSON-LD: 831-1.0053358-ld.json
RDF/XML (Pretty): 831-1.0053358-rdf.xml
RDF/JSON: 831-1.0053358-rdf.json
Turtle: 831-1.0053358-turtle.txt
N-Triples: 831-1.0053358-rdf-ntriples.txt
Original Record: 831-1.0053358-source.json
Full Text
831-1.0053358-fulltext.txt
Citation
831-1.0053358.ris

Full Text

The U n i v e r s i t y of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of PETER HERBERT REYNOLDS B.Sc, The Univ e r s i t y of Toronto, 1963 IN ROOM 301, HENNINGS BUILDING (PHYSICS) MONDAY, OCTOBER 30, 1967, AT 2:00 P 0M„ COMMITTEE IN CHARGE Chairman: R.W. Wellwood R.D. Russell J,A. Jacobs MoW. Ovenden A„J„ S i n c l a i r R. Nodwell WoF. Slawson External Examiner: D. York Department of Physics Uni v e r s i t y of Toronto Research Supervisor: R.D. Russell A LEAD ISOTOPE STUDY OF ORES AND ADJACENT ROCKS ABSTRACT The purpose of this thesis is to investigate the isotopic relationships between lead in orebodies and lead in adjacent igneous rocks. Past studies of rocks and ores have been largely uncorrelated, and, in addition, much of the published data is unreliable because of large experimental uncertainties and ina-dequate interlaboratory calibrations„ These data have, however, suggested that most rock-leads originated in a system with a distinctly lower U/Pb ratio than the one associated with certain ore-leads,, Samples were obtained for the present study from four selected areas. Both rock-leads and ore-leads were analyzed from Balmat, N.Y,, and from Nelson, B.C. Rock-leads from Broken H i l l , Australia and from West-Central New Mexico were also studied,, The iso-topic abundances of ore-leads from these latter two areas have already been determined in this laboratory. Identical experimental techniques were used throughout for both rocks and ores, and a precision of better thai O o l O per cent (one standard deviation) in the measure-ment of isotope ratios with respect to lead-204 was achieved„ A systematic difference in isotopic compo-sition was observed between certain of the above leads In particular, rock-leads from Balmat and from the . Nelson batholith, ore-leads from deposits associated with this batholith, and ore-leads from New Mexico (analyzed by J. Blenkinsop) were apparently derived from a primary system characterized by a present-day value of the U 2 3°/Pb 2 0^ ratio (they^-value) equal to 8 . 7 to 8 . 8 5 0 The significance of the isotopic compo-sition of rock-lead from New Mexico was not revealed by the one sample studied. On the other hand, ore-lead from one of the s t r a t i f o r m deposits at Balmat was apparently derived from a primary system with a /""-value of 9.0, This value agrees with the one ob-tained in t h i s laboratory by R.G. Ostic for s t r a t i -form deposits selected by R,L, Stanton i n accordance with geological c r i t e r i a . . It i s also consistent with A . J o S i n c l a i r ' s i n t e r p r e t a t i o n of the i s o t o p i c abundances of lead from the S u l l i v a n mine and from deposits located i n the Kootenay arc north and south of the Nelson b a t h o l i t h . In addition, both rock-leads and ore-leads from Broken H i l l appear to re-f l e c t the existence of this higher/^-system. The present study has therefore provided, for the f i r s t time, d e f i n i t i v e evidence r e l a t i v e to the existence of two d i s t i n c t d i s t r i b u t i o n s of p r i -mary/*-- values. Several geophysical models are d i s -cussed i n an attempt to reconcile this difference, and explain i n general terms the evolution of lead isotope r a t i o s in the earth. Also, for the f i r s t time, analyses of Nelson rocks and ores have provided clear evidence of a genetic r e l a t i o n s h i p between ore deposits and g r a n i t i c rocks. At the same time, lead isotope abundance patterns in plutonic rocks of b a t h o l i t h i c dimensions were investigated. GRADUATE STUDIES Field of Study: Geophysics Advanced Geophysics Radioactive and Isotopic Processes in Geophysics Modern Aspects of Geophysics Applied Geophysics Principles of Earth Science Seismology J.A. Jacobs: R.D. Russell R .D. Russell G o P o Erickson M.A. Chinnery W.F. Slawson R.D. Russell Related Studies: Fluid Dynamics Waves Electromagnetic Theory R.W. Stewart J.C. Savage G . B . . Walk-*-PUBLICATIONS Ostic, R.G., Russell, R.D. and P.H. Reynolds. A new calculation for the age of the earth from abund&u ces of lead isotopes. Nature, 199, 1150 (1963) Russell, R.D. and P.HL Reynolds. The primary lead growth curve and the age of the earth. (Abstract' Trans. Amer. Geophys. Union, 45, 111 (1964) Russell, R.D. and P.H. Reynolds. The age of the earth Problems in Geochemistry, Acad. Sci., U,S.S.R., Vinogradov Jubilee Volume, 37 (1965) Ulrych, T.J. and P'uH. Reynolds. Whole-rock and mineral leads from the Llano Uplift, Texas. J. Geophys. Res., 71^ 3089 (1966) Russell, R.D., Slawson, W.F., Ulrych, T.J. and P.H. Reynolds. Further applications of concordia plo*-' to rock:"lead isotope abundances. Submitted for publication. A LEAD ISOTOPE STUDY OP ORES AND ADJACENT ROCKS by PETER HERBERT REYNOLDS B.Sc, The University of Toronto, 1 9 6 3 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of GEOPHYSICS We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1 9 6 7 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e 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 t h e 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 t h e Head o f my Depar tment o r 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 n o t 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 . D e p a r t m e n 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 V a n c o u v e r 8, Canada Date / f / o U ^ / ? £ / i i ABSTRACT The purpose of t h i s thesis i s to investigate the isotopic relationships between lead i n orebodies and lead i n adjacent igneous rocks. Past studies of rocks and ores have been largely uncorrelated, and, i n addition, much of the pub-lished data i s unreliable because of large experimental un-ce r t a i n t i e s and inadequate interlaboratory c a l i b r a t i o n s . These data have, however, suggested that most rock-leads originated i n a system with a d i s t i n c t l y lower U/Pb r a t i o than the one associated with cert a i n ore-leads. Samples were obtained for the present study from four selected areas. Both rock-leads and ore-leads were analyzed from Balmat, N.Y. and from Nelson,, B.C. Rock-leads from Broken H i l l , A u s t r a l i a and from West-Central New Mexico were also studied. The isotopic abundances of ore-leads from these l a t -t er two areas have already been determined i n t h i s laboratory. Identical experimental techniques were used throughout for both rocks and ores, and a precision of better than 0.10 per cent (one standard deviation) i n the measurement of isotope ra t i o s with respect to lead-204 was achieved. A systematic difference i n isotopic composition was observed between certain of the above leads. In p a r t i c u l a r , rock-leads from Balmat and from the Nelson b a t h o l i t h , ore-leads from deposits associated with t h i s b a t h o l i t h , and ©re-leads from New Mexico (analyzed by J . Blenkihsop) were apparently derived from a primary system characterized by a present-day value of the I i i U238/ P b204 r a t i o (the y-value) equal to 8.7 to 8.85. The significance of the isotopic composition of rock-lead from New Mexico was not revealed by the one sample studied. On the other hand, ore-lead from one of the stratiform deposits at Balmat was apparently derived from a primary system with a y-value of 9-0. This value agrees with the one obtained i n t h i s laboratory by R.G. Ostic for stratiform deposits selected by R.L. Stanton i n accordance with geological c r i t e r i a . It i s also consistent with A.J. S i n c l a i r ' s i n t e r p r e t a t i o n of the isotopic abundances of lead from the Sul l i v a n mine and from deposits located i n the.Kootenay arc north and south of the Nelson ba t h o l i t h . In addition, both rock-leads and ore-leads from Broken H i l l appear to r e f l e c t the existence of t h i s higher y system. The present study has therefore provided, for the f i r s t time, d e f i n i t i v e evidence r e l a t i v e to the existence of two d i s -t i n c t d i s t r i b u t i o n s of primary y-values. Several geophysical models are discussed i n an attempt to reconcile t h i s difference, and explain i n general terms the evolution of lead isotope ratios i n the earth. Also, for the f i r s t time, analyses of Nelson rocks and ores have provided clear evidence of a genetic relationship be-tween ore deposits and g r a n i t i c rocks. At the same time, lead isotope abundance patterns i n plutonic rocks of b a t h o l i t h i c dimensions were investigated. i v TABLE OF CONTENTS ABSTRACT i i LIST OF FIGURES v LIST OF TABLES v i ACKNOWLEDGEMENTS v i i CHAPTER 1 INTRODUCTION 1 Objectives of the Present Investigation 1 Scope of the Present Investigation 6 CHAPTER 2 EXPERIMENTAL TECHNIQUES 8 Extraction of Lead from Potassium Feldspars 8 Preparation of Tetramethyllead 13 Gas Chromatographic P u r i f i c a t i o n of Tetramethyllead 17 Contamination 21 Mass Spectrometry 24 Sample Size, Precision, and Reproducibility 26 CHAPTER 3' ANALYSES AND INTERPRETATIONS 36 Choice of Samples 36 Nelson, B r i t i s h Columbia 36 Balmat, New York 38 Broken H i l l , New South Wales, A u s t r a l i a 40 West-Central New Mexico 42 Balmat, N.Y. 43 Nelson, B.C. 53 West-Central New Mexico 65 Broken H i l l , A u s t r a l i a 70 CHAPTER 4 CONCLUSIONS 80 BIBLIOGRAPHY 89 V LIST OP FIGURES FIGURE 2-1 SCHEMATIC DIAGRAM OF MICRO-LEAD APPARATUS 14 FIGURE 2-2 TRIMETHYLLEAD SPECTRUM FROM APPROXIMATELY 500 MICROGRAMS OF TETRAMETHYLLEAD 30 FIGURE 3-1 ISOTOPIC COMPOSITION OF BALMAT LEADS 45 FIGURE 3-2 ISOTOPIC COMPOSITION OF KOOTENAY ARC AND NELSON LEADS 55 FIGURE 3-3 APPROXIMATE SAMPLE LOCATIONS, NELSON AREA, BRITISH COLUMBIA 57 FIGURE 3-4 ISOTOPIC ABUNDANCES OF NEW MEXICO LEADS 68 FIGURE 3-5 SURFACE GEOLOGICAL PLAN BROKEN HILL MINE AREA AND APPROXIMATE SAMPLE LOCATIONS 73 FIGURE 3-6 ISOTOPIC ABUNDANCES OF BROKEN HILL LEADS 74 FIGURE 3-7 ISOTOPIC COMPOSITION OF LEAD FROM THE MUNDI MUNDI GRANITE 77 v i LIST OF TABLES TABLE 2-1 REPRODUCIBILITY OF ANALYSES ON THE BASIS OF LOOP CLOSURE ERRORS 33 TABLE 2-2 REPLICATE ANALYSES OF BALMAT 500 GALENA 34 TABLE 3-1 ISOTOPIC COMPOSITION' OF BALMAT LEADS 44* TABLE 3-2 LEAD ISOTOPE , RATIOS, URANIUM' AND LEAD CONCENTRATIONS OF BALMAT ROCKS AND MINERALS 51 TABLE 3-3 ISOTOPIC COMPOSITION OF NELSON LEADS 56 TABLE 3-4 ' ISOTOPIC COMPOSITION OF LEADS FROM THE KOOTENAY ARC AND FROM SULLIVAN MINE 58 TABLE 3-5 ISOTOPIC COMPOSITION OF NEW MEXICO LEADS 66 TABLE 3-6 OBSERVED ISOTOPIC COMPOSITION OF BROKEN HILL LEADS 71 TABLE 4-1 APPARENT URANIUM/LEAD RATIOS FOR LEADS FROM CERTAIN STRATIFORM DEPOSITS 83 v i i ACKNOWLEDGEMENTS The writer thanks Dr. R.D. Russell for his help i n formulating the present t h e s i s , and also for his guidance during the course of the entire i n v e s t i g a t i o n . Technical achievements f i n a l l y attained would not have been possible without the advice and encouragement at various times of Dr. W.P. Slawson, Dr. T.J. Ulrych, Dr. A.J. S i n c l a i r and Mr. J . Blenkinsop. The l a t t e r shared with the writer the joys and sorrows of mass spectrometry during the past two years, and, i n addition, helped Dr. Russell successfully reduce Invaluable isotopic data stored on paper and magnetic tapes. The writer also wishes to acknowledge the many help-f u l discussions with Dr. J.R. Richards of the Australian National University. Discussions began during the l a t t e r ' s v i s i t to this university i n 1965 and were continued i n l e t t e r s , exchanged a f t e r his return to Canberra. The writer i s grateful to a l l who helped supply samples for t h i s study; i n d i v i d u a l acknowledgements are included i n the thesis proper. Mineral separation f a c i l i t i e s were pro-vided by Dr. W.H.-White, Dr. R.E. Delavault, and Dr. S i n c l a i r of the Department of Geology. The general co-operation of t h i s department throughout the investigation was appreciated. Tech-n i c a l assistance i n the art of glass-blowing was provided i n many emergencies by Mr. J. Lees and by Mr. E. Williams; Miss S. Newman kindly prepared three of the Nelson ore-leads for the writer; Mr. J.M. Ozard c r i t i c a l l y reviewed a draft of the present v i i i thesis which was subsequently typed by Miss J . Kalmakoff. The present research was supported by National Research Council of Canada grants to R.D. Russell, W.P. Slawson and A.J. S i n c l a i r , and to a lesser extent, by a National Science Foundation grant to W.F. Slawson. Personal f i n a n c i a l a s s i s t -ance for three years i n the form of a National Research Council Studentship was also appreciated. CHAPTER 1 INTRODUCTION Objectives of the Present Investigation The study of the variations of lead isotope abundances i n natural minerals has long been of interest to geophysicists and geochemists i n t h e i r attempts to understand the chemistry and history of the outer parts of the earth. Lead occurs i n trace quantities i n p r a c t i c a l l y a l l types of rocks and, i n -addition, Is found In massive concentrations i n the form of the ore mineral galena (PbS). During the past seven years geophysicists at t h i s university have been studying isotopic variations i n ore-leads, and have postulated two basic models. It appears that some ore-leads have developed from a common i n i t i a l time (the 'age' of the earth) i n a single system closed with respect to the transfer of uranium, thorium and lead and that they have sub-sequently had r e l a t i v e l y l i t t l e i s o topic a l t e r a t i o n i n c r u s t a l rocks. Leads from most of the stratiform* deposits o r i g i n a l l y selected by Stanton and studied here by Ostic and Russell (Stanton and Russell, 1 9 5 9 ; Ostic, et a l , 1 9 6 7 ) seem to belong in t h i s c l a s s i f i c a t i o n . The measured lead isotope r a t i o s , P b2 0 6 / P b 2 0 > t j p ^ o y / p ^ o i * t a n d P b 2 0 8 / P b 2 0 ' + f o r these samples have been used to calculate the present-day uranium/lead and . * In t h i s t h e s i s , for brevity, the term 'stratiform' w i l l always re f e r to t h i s p a r t i c u l a r group, of deposits. It i s recognized that this i s a narrower d e f i n i t i o n than i s normally accepted. 2 thorium/uranium rat i o s of thi s primary lead systemj y = U 2 3 8 / Pb 2 0 1 t = 8.99 1 0 . 0 5 , Th/U = 3.89 * 0 .04 (Ostic, et a l , 1967)**. Apparently t h i s source material i s remarkably uniform on a world-wide scale with respect to these three elements, and i t has been suggested that the leads from these stratiform deposits come from a deep (probably subcrustal) source and are transported to the crust by volcanic action. The second basic model used to explain' ore-lead var-iations postulates a single-stage history i n the primary system as described above, and subsequent h i s t o r i e s i n cru s t a l systems. Hence radiogenic lead produced by the. decay of uranium and thorium i n cr u s t a l rocks i s added to the primary lead and an H anomalous lead suite i s produced. Kanasewich (1962) and others have shown that i n many instances the leads have apparently seen only one or two c r u s t a l systems and hence straight l i n e relationships are often observed when P b 2 0 7 / P b 2 0 4 i s plotted as a function of Pb 2 0 6/Pb 2 0 1*. The slopes of these lines can be interpreted to give age and geochemical Information. I p i |< Galena >,s i n g l e - s t a g e model t j H 2 Galena- >,two-stage model , y l , y 2 ) Y 3 <—Galena—». three-stage model 1 0 . 1 1 • 1 2 1 3 0 ** In thi s t h e s i s , the primeval r a t i o s , a 0 and b 0 , and the age of the earth, t 0 , are assigned the values 9.56, 10.42, 4.55 x 10 9 yr respectively (Murthy and Patterson, 1962). 3 Hence, o r e - l e a d s are e i t h e r s i n g l e - s t a g e primary leads or m u l t i - s t a g e anomalous leads and the l a t t e r can o f t e n be r e c o g n i z e d by doing a s u f f i c i e n t number o f p r e c i s e i s o t o p i c analyses from a given d i s t r i c t . I n v e s t i g a t o r s i n other l a b o r a t o r i e s have been s t u d y i n g r o c k - l e a d s and, i n g e n e r a l , have observed r a t h e r l a r g e v a r i a -t i o n s i n i s o t o p i c composition. These v a r i a t i o n s can be i n t e r -p r e t e d to'mean that r o c k - l e a d s i n the earth's c r u s t are products of heterogeneous systems open with respect t o l e a d , uranium and thorium. G e o l o g i c a l a c t i v i t y i n the c r u s t f o r over two-t h i r d s of the earth's l i f e t i m e may account f o r these open systems. On the other hand, observed i s o t o p i c v a r i a t i o n s may be due to experimental e r r o r s and to c a l i b r a t i o n u n c e r t a i n t i e s . P r e s e n t l y a v a i l a b l e r o c k - l e a d data have been obtained from a number of d i f f e r e n t mass spectrometers that have not been r i g o r o u s l y c a l i b r a t e d i n order t o remove p o s s i b l e i n t e r l a b o -r a t o r y d i f f e r e n c e s . In a d d i t i o n , samples have been analyzed by s o l i d - s o u r c e techniques on v a r i o u s f i l a m e n t m a t e r i a l s ( u s u a l l y rhenium or tantalum), and i t has been shown (Doe, et a l , 1 9 6 5 ) that the i s o t o p i c composition of the l e a d depends to -some extent on the type of f i l a m e n t m a t e r i a l used. Much of the e a r l i e r data are a l s o i n q u e s t i o n because of u n c e r t a i n adjustments f o r mass d i s c r i m i n a t i o n e f f e c t s i n e l e c t r o n m u l t i p l i e r s . Although present s o l i d - s o u r c e techniques are capable of good i s o t o p i c analyses of only a few micrograms of 4 lead, Doe, et a l (1967) have pointed out that mass spectrometer fractionation i s always present. The effect of t h i s f r a c t i o n a -t i o n cannot be quantitatively removed from the isotope ratios and may not remain constant from one run to the next. Hence the r e p r o d u c i b i l i t y of the data i s poor. A recent modifica-tion of the solid-source technique by Catanzaro (1967) seems to have solved the fractionation problem. However, the new method has not as yet been f u l l y tested and no new rock-lead data have been produced. The solid-source data do, however, suggest that the rock-leads originated i n a system that has a d i s t i n c t l y lower U/Pb r a t i o than the one associated with the st r a t i f o r m ores (y = 9 - 0 ) . For example, Cooper and Richards (1966) have analyzed the lead extracted from 25 modern volcanic extrusives. Single-stage u-values calculated for these samples range between 8 . 4 7 and 8 . 9 1 and the mean value i s 8 . 7 0 . The average u-value calculated from Zartman's (1965a) analyses of 12 microclines from igneous and metaigneous rocks from the Llano U p l i f t , Texas, i s 8 .55 ( 8 . 4 7 - 8 . 6 5 ) . The low lead content of most rocks (^15 ppm for igneous rocks) has been one of the factors that has so far prevented the study of rock-leads i n t h i s laboratory. However, i t i s of p a r t i c u l a r interest at this time to obtain more r e l i a b l e rock-lead data i n order to c l a r i f y t h i s apparent discrepancy. I f the above difference i s not due to experi-mental uncertainties.or to lack of interlaboratory c a l i b r a t i o n 5 (cf. Richards, 1967), but i s a r e a l phenomenon, then a funda-mentally d i f f e r e n t source material i s required to account for the isotopic abundances of trace leads i n rocks. In other words, the lead i n these strati f o r m ores i s not genetically related to lead i n rocks. The present research originated i n the b e l i e f that i t was desirable to make a study of the relationships between ore-leads and t h e i r associated rocks. There are only two published examples of correlated studies of rocks and ores. In the e a r l i e r of these, Murthy and Patterson (1961) found that the lead i n the ore deposits of Butte, Montana was i s o t o p i c a l l y s i m i l a r to that i n potassium feldspars of the Boulder bath o l i t h , the host rock of the ore. However, the lead i n one quartz sample from an associated igneous intrusion was di f f e r e n t and the authors therefore concluded that the feldspar leads had been contaminated by ore-leads and that there was not a parental relationship between the hydrothermal ores and the host rock. The second investigation was carried out by Doe (1962) on ore deposits and rocks near Balmat, New York. The study of Balmat leads was continued by the present writer (see Chapter 3)• The purpose of this t h e s i s , then, i s to report new precise analyses that help c l a r i f y rock-lead relationships and also relationships between ores and adjacent rocks. 6 Scope of the Present Investigation The present study i s r e s t r i c t e d to gas-source mass spectrometer analyses of ore-leads and of leads extracted from potassium feldspars. The ore mineral galena contains 'common lead', lead which has not been i n contact with Its parent elements uranium and thorium since the time of c r y s t a l l i z a t i o n , and hence has not changed i t s isotopic composition since that time. In order to make a comparative rock-lead study, one must know the isotopic composition of the lead that existed i n the rock at the time of i t s l a s t c r y s t a l l i z a t i o n - the common lead component. The mineral potassium feldspar (KAlSi30 8) has the highest lead/uranium r a t i o of any of the common rock-forming minerals ( t y p i c a l l y ^ 100 i n g r a n i t i c rocks), and hence uranium decay i n this mineral w i l l be least e f f e c t i v e i n a l t e r i n g the common lead isotope r a t i o s . It i s believed (cf. S o r r e l l , 1962) that the lead (ionic radius 1.20) substitutes for the potassium (radius 1.33) In the c r y s t a l l a t t i c e forming i n effect a lead feldspar ( P b A l 2 S i 2 0 8 ) . In order to obtain the exact common lead component i t i s necessary to calculate the amount of radiogenic lead produced by the i n s i t u decay of uranium and thorium i n the feldspars from the time of c r y s t a l l i z a t i o n to the present. To do this one needs to know the lead, uranium and thorium concentrations as well as the present-day isotopic • abundances. This correction i s usually small for potassium feldspars and can be safely neglected for rocks less than 7 500 m.y. old. Even i n the case of the 1000 m.y. old igneous rocks studied by Zartman ( 1 9 6 5 a ) , the correction applied to the Pb 2 0 6 / P b 2 0 1 + r a t i o ranged from only 0 . 2 5 to 0 . 5 0 per cent; corrections to the Pb 2 0 7/Pb 2 0 1* r a t i o were less than 0 . 1 0 per cent. In addition to having a low uranium/lead r a t i o , potassium feldspars also contain more lead (-v 35 ppm) than any of the other common minerals. Hence i t i s possible to obtain from about 15 grams of sample the 500 micrograms of lead that are required f o r a sat i s f a c t o r y gas-source mass spectrometer analysis. In addition, the time spent on mineral separations.is-kept to a minimum. Lead isotope abundances were determined by means of gas-source mass spectrometry of tetramethyllead to ensure a consistency with ore-lead data obtained i n t h i s laboratory. Gas-source measurements here have been refined to the extent that the techniques are well understood and capable of produc-ing highly precise data (see, for example, Ostic, 1 9 6 3 , for a detailed discussion). Whittles (1964) has claimed that mass spectrometer f r a c t i o n a t i o n , the major uncertainty i n the s o l i d -source work, i s not a problem with gas sources providing one maintains a sample l i n e pressure s u f f i c i e n t to produce viscous flow through the c o n s t r i c t i o n (or 'leak') leading into the source region of the spectrometer." A sample l i n e pressure of at least 10 mm i s recommended for the present leak. 8 CHAPTER 2 EXPERIMENTAL TECHNIQUES Extraction of Lead from Potassium Feldspars Potassium feldspars were separated from the whole-rock samples by the following methods. A small portion of each sample was i n i t i a l l y passed through the entire crushing appa-ratus to clean the system. Clean hand specimens of the sample were then crushed, washed thoroughly, and s i f t e d through clean brass screenso For the majority of the samples, the - 5 0 , + 7 0 and - 7 0 , + 1 0 0 mesh fractions were found to be the most con-venient to use, and these were subsequently passed through a Franz Isodynamlc separator which removed most of the dark min-er a l s . The K-feldspars were then floated i n an acetone-bromoform solution adjusted to a s p e c i f i c gravity of approximately 2 . 5 8 . -The- pu r i t y of the K-feldspar concentrate was estimated by staining with sodium-cobaltinitrite solution and was found to be t y p i c a l -l y 8 5 - 9 5 per cent. Impurities remaining were mainly plagioclase and quartz. Doe and T i l l i n g ( 1 9 6 7 ) have shown that quartz con-tains much less lead than.either K-feldspar or plagioclase, and that plagioclase usually contains less than one-half as much lead as coexisting K-feldspar. In addition, these authors have shown that the leads from coexisting feldspar pairs are, i n general, grossly s i m i l a r i n isotopic composition. Hence, contamination due to impurities i n the K-feldspar concentrates should be inconse-quential. The feldspar sample was ground to a very fine powder i n an agate mortar and was subsequently immersed i n warm 9 6 N HCl for about 30"minutes followed by 30 minutes i n warm 8 N HN0 3. This step was adopted from the procedures followed by Doe ( 1 9 6 2 ) , Zartman ( 1 9 6 5 a ) , and others, to remove lead and uranium loosely held on the surface of the mineral grains. "In general, the leachable lead i s more radiogenic than that more t i g h t l y bound i n the c r y s t a l l a t t i c e . A considerable increase i n the Pb/U r a t i o and a better agreement of isotopic data re s u l t a f t e r the acid pre-treatment" (Zartman, 1 9 6 5 a ) . In addition, the acid-wash w i l l remove any lead contamination due to handling of the mineral separates. Doe and T i l l i n g (1967) have recently pointed out i n t h i s regard that the lead content of bromoform may be as high as several hundred ppm and furthermore, that It Is very d i f f i c u l t to remove a l l traces of t h i s from the sample by merely washing repeatedly with acetone'. The quantitative extraction of lead from rocks and minerals i s generally accomplished by one of two methods. In the most popular procedure, the sample i s dissolved i n hydrofluoric and perchloric acids and the lead i s subsequently i s o l a t e d and p u r i f i e d by means of ion exchange columns and dithizone extractions. Marshall and Hess ( 1 9 5 8 ) , Masuda ( 1 9 6 2 ) , Cooper and Richards ( 1 9 6 6 ) , Tatsumoto (1966) and others, however, have extracted lead by heating the sample either i n a vacuum or i n a stream of hydrogen at temperatures of the order of 1000 °C. The v o l a t i l e material i s collected on a cold finger or on a cool portion of the quartz furnace tube. 10 The lead can then be p u r i f i e d by the usual chemical techniques„ The second method was chosen for the present research as i t avoids excessive handling of the sample i n a laboratory not designed for work with small quantities of lead, and also the necessity of obtaining quantities of lead-free chemicals. One major problem that could arise with t h i s technique i s the p o s s i b i l i t y of iso t o p i c fractionation during the vola-tilization,,, Starik and co-workers (Starik, et a l , 1957) have demonstrated such a frac t i o n a t i o n when uranium minerals or granites were sublimated i n vacua. Their data indicate, however, that there are di f f e r e n t modes of lead occurrence i n minerals and rocks, rather than an actual fractionation of the lead e x i s t i n g within a single mode. Khlopin ( 1 9 5 6 ) , on the other hand, did not observe isotopic fractionation with granites at -v 1000°C. The acid-washed feldspar samples v o l a t i l i z e d i n the present study should contain only the lead held within the c r y s t a l l a t t i c e of a single mineral, hence fr a c t i o n a t i o n should not be a problem. Finely powdered graphite (Fisher S c i e n t i f i c Co., Grade # 3 8 ) , previously p u r i f i e d i n a hydrogen stream at 1000°C, was thoroughly mixed with the feldspar sample. The weight r a t i o of rock powder to graphite was approximately 3 0 : l o The sample was then loaded into a quartz combustion boat which was, i n turn, inserted into a quartz tube approximately 30 cm long, 15 mm i n diameter. This tube was car e f u l l y placed inside a larger diameter (25 mm) thick-walled quartz tube which forms 11 part of the heavy-duty, resistance furnace. Combustion boats not more than one-half the length of the furnace element were used to ensure uniform heating of the sample. The furnace reached a temperature of 1000°C i n about one hour and was held at t h i s temperature for f i v e hours. A hydrogen flow of approximately 150 cm3 per minute was maintained through the furnace during the heating period. The v o l a t i l e material i n the sample appears as a th i n metallic mirror about 1 cm long on the inside of the inner quartz tube which extends into the cooler region of the furnace. The mirror was allowed to cool i n a hydrogen atmosphere before the quartz tube was removed from the furnace. The boat size l i m i t e d the amount of feldspar that could be v o l a t i l i z e d on a single loading to about 6 or 7 grams; hence, for most samples, several runs were required. Baskova and Novikov (1957) v o l a t i l i z e d rock samples i n vacua for 1.5 to 2.5 hours at 1050-1100°C. They used a weight r a t i o of rock powder to carbon of 1:1, believing that the reduction to metallic lead by the carbon increased the y i e l d of the process. An 88 per cent y i e l d at the ten ppm l e v e l (chemical methods = 100 per cent) was obtained i n the case of feldspars. Masuda (1962) reduced th i s r a t i o to 30:1, pointing out that the vapour pressure of lead (II) oxide i s i n fact higher than that of the metal. The main purpose of the carbon, he believes, i s to prevent the rock p a r t i c l e s from cohering to form a hard mass. Both Masuda and the present writer have found that without carbon the sublimation process 12 i s impeded and hence the y i e l d i s reduced* Masuda heated his samples i n vacua for about one hour at 1000-1100°C and obtained yie l d s comparable to those of Baskova and Novikov= Cooper and Richards (1966) have recently used th i s technique to extract lead from b a s a l t i c rocks by heating f i n e l y powdered rock-graphite mixes i n vacua for five hours at 1000°C. The methods employed i n the present study as described e a r l i e r should give yie l d s comparable to the above. Whittles (1964) was troubled by the formation of a lead oxide s i l i c a t e of the form xPbO.Si0 2, a black compound which could not be reduced to free lead i n the hydrogen furnace or dissolved i n n i t r i c acid. In some cases over 50 per cent of the lead In the sample was l o s t . Whittles solved t h i s problem by keeping his lead mirrors at a l l times i n an oxygen free environment. Fortunately, during the present research, t h i s s i l i c a t e was never observed to form, despite the fact that the lead mirrors were exposed to the atmosphere for b r i e f periods. It may be that i t s formation required some trace element i n the sulfides studied by Whittles that was not present i n the feldspars of t h i s study. A l t e r n a t i v e l y , i t may r e f l e c t s l i g h t differences i n the properties of the quartz tubing used i n the two studies. Because of the small sample size and a large lead background i n the laboratory, i t was necessary to take pre-cautions to avoid contamination. Feldspar samples were a c i d -washed i n new pyrex beakers which had been thoroughly cleaned 13 i n hot 3 N HN0 3 and rinsed with d i s t i l l e d water p r i o r to use. The samples were rinsed with d i s t i l l e d and d o u b l y - d i s t i l l e d water to remove a l l traces of the acid, covered with a clean beaker and dried at 250°F. They were kept covered u n t i l used to prevent contamination from lab dust, etc. The inner quartz tubing was wholly immersed i n 6 N HN0 3 for at least twelve hours, flushed with d i s t i l l e d water and dried. I f there remained a residue on the tube from a previous run, the inside of the tube was cleaned for 5-10 minutes with 10% HF„ The quartz boats were s i m i l a r l y cleaned with n i t r i c acid. The heavy furnace tubing was cleaned b r i e f l y i n d i l u t e n i t r i c acid, flushed with d i s t i l l e d water and d r i e d o The large quartz tubing and sample cold trap of the micro-lead apparatus (described i n the following section) were cleaned each time for several hours i n 3 N HN0 3, flushed and dried. A l l stopcocks that come i n contact with the t e t r a -methyllead were cleaned and regreased. Disposable p l a s t i c gloves were used to handle the clean glassware. Preparation of Tetramethyllead The reader i s referred to the thesis of Whittles (1964) for a detailed description of the 'micro-lead apparatus' used to prepare tetramethyllead. A schematic diagram of the apparatus i s shown i n Figure 2-1. Only a b r i e f description of the free r a d i c a l technique and the main departures from Whittles' procedure are given here. FIGURE 2-1 SCHEMATIC DIAGRAM OF MICRO-LEAD APPARATUS Mercury Diffusi o n Pump t Lead Mirror Furnace m w a r Hydrogen Exhaust Sample Capsule Hydrogen Di-t-butyl peroxide closed open -»—w Quartz Tubing Viton A Seal Liquid Nitrogen 15 After the v o l a t i l i z a t i o n , the small quartz tube was inserted into the large tubing of the micro-lead apparatus and the system was pumped down with a mercury d i f f u s i o n pump. Hydrogen was then admitted to the system at atmospheric pressure and the flow rate adjusted to approximately 150 cm3/min. The mirror was moved progressively down the tube by heating at 750°C for about 15 minutes i n each p o s i t i o n . Eventually the lead was i s o l a t e d from the other v o l a t i l e material and formed a pure mirror near the end of the furnace on the inside of the outer quartz tube. In addition to th i s lead mirror, with some of the feldspars a faint bluish mirror was also observed. In p r a c t i c e , the mirror had to be moved only four or fiv e times i n order to obtain s u f f i c i e n t purity and i t was found that i t was not necessary to move It at a reduced hydro-gen pressure (1 to 2 cm of mercury). This step i n the pro-cedure was i n i t i a t e d by Ulrych ( 1 9 6 2 ) In order to remove iron contamination from the lead, and was continued by Whittles who believed that i t tended to give more consistent results because of increased mirror purity. An advantage of working at atmospheric pressure i s that the lead does not become too dispersed on the tube and hence i s more eas i l y observed. The lead mirror i s removed by the action of free methyl r a d i c a l s produced from the thermal decomposition of d i - t e r t i a r y - b u t y l peroxide. This source material was 16 o r i g i n a l l y chosen by Ulrych and was also used by Whittles a f t e r the l a t t e r had ca r e f u l l y considered other p o s s i b i l i t i e s . It i s quite suitable from the point of view of high r a d i c a l y i e l d , absence of Interfering r a d i c a l s , freedom from unde-sira b l e impurities, and the r e l a t i v e ease with which the decomposition products (methane, ethane, acetone, higher b o i l i n g ketones) can be separated from the tetramethyllead„ A decomposition temperature of approximately 700°C was chosen. At t h i s temperature a l l of the peroxide i s decomposed and the removal of the mirror seems most e f f i c i e n t . The peroxide was admitted to the system by means of a glass leak calibrated to give a pressure of one mm of mercury i n the region of the furnace. The tetramethyllead along with the decomposition products were collected In a l i q u i d nitrogen trap and l a t e r transferred to a small glass capsule. Lead mirrors were usually removed i n from 1 to 4 minutes; the best results were obtained when the mirror was s l i g h t l y hot to the touch and the separation between the mirror and furnace was about one cm. In some cases there appeared to be i n s u f f i c i e n t mass flow towards the cold trap, and hence some of the t e t r a -methyllead would diffuse back into the hot region of the furnace and decompose to form a second mirror'. This backflow was e a s i l y removed by r e t r a c t i n g the furnace several c e n t i -metres and cooling the new mirror with a blower„ In order to.be certain that a l l of the lead had been removed, hydrogen 17 was re-admitted to the system and any remaining lead was moved to a fresh position further down the tube. This f i n a l mirror was then removed as described above. It i s believed that this procedure consistently recovers greater than 90 per cent of the lead. The problem of backflow can be at least p a r t i a l l y eliminated by increasing the mass flow i n the system, that i s , by admitting more of the peroxide source material. This i s readily accomplished by using an adjustable needle valve instead of a glass leak. The disadvantage i s the greater d i f f i c u l t y i n separating by gas chromatography a larger quantity of the decomposition products. The glass leak was normally used i n t h i s research. Gas Chromatographic P u r i f i c a t i o n of Tetramethyllead The methods here were developed from the work of Ulrych ( I 9 6 0 , 1962) who was the f i r s t to successfully apply gas chromatographic techniques i n the p u r i f i c a t i o n of t e t r a -methyllead for isotopic analysis. Ulrych was able to produce samples containing a few milligrams of tetramethyllead of s u f f i c i e n t purity to give precise mass spectrometric analyses. He employed a pyrex column five feet long with an i n t e r n a l diameter of 10 mm. This column was packed with 40-60 mesh ground f i r e b r i c k coated with 25 per cent by weight white p a r a f f i n o i l . Use of f i r e b r i c k as a s o l i d support i s not recommended by authorities i n t h i s f i e l d ( J . Cornelius, Varian Aerograph, personal communication) because of i t s 18 large adsorptive surface area. Ulrych, therefore, probably lost considerable lead by adsorption on the s o l i d support. However, his samples were r e l a t i v e l y large so that he could afford s i g n i f i c a n t loss without losing mass spectrometer sensitivity» Whittles, working with about one order of magnitude less lead than Ulrych, f e l t that losses i n the column would be too severe, and hence developed a vacuum d i s t i l l a t i o n tech-nique. He found, however, that when he pumped on his samples at dry ice temperatures, considerable lead was l o s t so that i n practice he had to reach a compromise between desired purity and size of sample. This meant that for many of the samples, the pressure inside the mass spectrometer was several times higher than normal, and hence larger and probably more uncertain corrections were required. Neither method as i t existed was believed adequate for the present work. Gas chromatography, however, seemed to be capable of producing the more consistent results i f only the lead loss In the column could be reduced to a tolerable l e v e l . This can be accomplished by minimizing the losses on (a) the s o l i d support and (b) the l i q u i d phase. A support i s required that has a small and non-adsorptive surface area. The support chosen was 45-60 mesh Chromosorb G, an oyster white, very hard, dense diatomaceous earth with a surface area of only 0„5 square metres per gram, A column packed with this support w i l l have a t o t a l surface 19 area approximately three to four times smaller than that of a t y p i c a l f i r e b r i c k column. Chromosorb W i s a highly recommended support that would give a s t i l l smaller area, but i t i s con-siderably more expensive. The Chromosorb was acid-washed by the manufacturer (Varian Aerograph) and was also coated with dimethyldichlorosilane (DMCS) to reduce surface active s i t e s i n the diatomaceous earth material, and hence reduce adsorption on the supporto Others have found that the Chromosorb supports are very suitable from t h i s point of view. B o n e l l i and Hartman ( 1 9 6 3 ) , for example, using a column packed with Chromosorb W were able to detect lead alkyls i n the 1 0 ~ 1 0 gram range. Blenkinsop (personal communication) has recently shown that there i s n e g l i g i b l e cross-contamination of large (^  100 mgm) tetramethyllead samples; hence column losses are probably small (since cross-contamination has been found to be associated with large column losses) . His tests were made with a column packed with Chromosorb W coated with 15 per cent by weight dinonyl phthalate. Adsorption on the l i q u i d phase can be minimized by using the l i g h t e s t possible loading. The maximum recommended loading for Chromosorb G i s five per cent. When greater l i q u i d loadings are used, column e f f i c i e n c y i s reduced; that i s , the peaks on the chromatogram become broader. I n i t i a l l y , the present writer attempted to use a five per cent loading but found that the samples passed through the column too rapidly (in about seven minutes), and the separation from the decomposition 20 products was not complete. A 10-15 per cent loading proved to be s a t i s f a c t o r y and was used throughout the present research. Ethane and methane are both very v o l a t i l e and hence ea s i l y separated from tetramethyllead i n a chromatographic column. The c r i t i c a l separation i s between the large quantity of acetone and the much smaller quantity of tetramethyllead„ Type 2044 white p a r a f f i n o i l has a high solvent e f f i c i e n c y for these two substances (that i s , the two peaks are well resolved) and was chosen as the stationary l i q u i d . A pyrex column f i v e feet long with an i n t e r n a l diameter of 10 mm was used. A column temperature of about 75°C was maintained by means of a heated o i l bath. The c a r r i e r gas, helium, was passed through an activated charcoal trap held at l i q u i d nitrogen temperatures at a flow rate of approximately 70 cm3/min. Although th i s trap was apparently not capable of removing a l l traces of water vapour from the helium, r e s u l t -ing contamination of samples was i n s i g n i f i c a n t . The thermal conductivity detector has been described i n d e t a i l by Ulrych (1962). The sample was admitted to the column by crushing the glass capsule i n a section of thick-walled tygon tubing. This method of introduction i s non plug-like and hence not i d e a l , but does not i n practice seem to adversely affect the o v e r a l l performance of the column. The use of tygon tubing i n a chromatographic system i s not recommended as i t tends to break down readily and may cause contamination. It has, 21 however, been used successfully i n this laboratory for several years and was found to be s a t i s f a c t o r y . The tetramethyllead was subsequently collected i n a l i q u i d nitrogen trap and trans-ferred to a break-seal tube for analysis on the mass spectro-meter. The column packing was changed for each sample as an added precaution against cross-contamination. The column glassware, cold trap, etc. were cleaned each time by the methods described e a r l i e r . Ulrych's chromatographic procedures were therefore adapted to meet the requirements of the present study. In p a r t i c u l a r , special attention was given to the problem of lead loss i n the column, and hence a better quality s o l i d support _and a smaller percentage of stationary l i q u i d were used. A quantitative determination of the amount of lead actually lost was not made; however, It would appear that this revised chromatographic technique i s capable of produc-ing larger mass spectrometer ion beams from a given sample than the vacuum d i s t i l l a t i o n method of Whittles. In addition, the mass spectrometer operating pressures are much lower. Contamination In order to check the e f f i c i e n c y of the cleaning procedure, a 'blank' sample (consisting of a small amount of the p u r i f i e d graphite i n a quartz boat) was prepared. No peaks were detectable i n the mass range 248-255 above the 22 usual background i n the mass spectrometer. This background, due to residual amounts of tetramethyllead i n the machine, i s always present to some extent, but can be minimized by thor-oughly cleaning the source assembly and sample l i n e of the instrument. The background spectrum Is no larger than 0.5 per cent of the smallest sample beam intensity and causes n e g l i g i b l e contamination. A considerably larger background spectrum can, however, be produced by flushing the instrument with d i -t e r t i a r y - b u t y l peroxide. Apparently t h i s i s due to the formation of tetramethyllead i n the source region of the spectrometer by the action of free methyl radicals on metallic lead deposited from previous samples. The heat supplied by the filament would be s u f f i c i e n t to decompose both t e t r a -methyllead and d i - t - b u t y l peroxide. Fortunately, a strong 'masking' effect was observed, since the isotopic composition of t h i s background could be changed to a value close to that of a new sample by simply flushing t h i s sample through the machine for 1 - 2 hours before beginning the analysis. This flushing technique was employed with a l l samples, and, i n addition, samples of s i g n i f i c a n t l y d i f f e r e n t composition were not analyzed i n succession. This procedure e f f e c t i v e l y pre-vented cross-contamination of samples. Lead contamination r e s u l t i n g from the preparation procedure i s therefore less than that due to the machine back-ground. The l a t t e r can be controlled by periodic cleaning of 23 source and sample l i n e and by flushing before each analysis. Whittles (1964) reported a contaminant that was characterized by three peaks i n the 280 mass range and by a series of peaks at a l l masses between 240 and 248. In addition, there were peaks at masses 249 and 250 i n the trimethyllead spectrum. He believed that this contaminant resulted i n some way from the use of hydrofluoric acid i n the cleaning of the quartz tubing, and found that i t apparently did not form i f the tubing was baked i n an oxygen stream at 950°C after the cleaning. Although this contaminant was not observed during the present study, as a precaution, the quartz tubing was baked i n oxygen whenever t h i s acid was used. An unknown material was sometimes produced i n the fr e e - r a d i c a l process that was eluted from the chromatographic column at about the same time as the tetramethyllead. This substance did not have any mass fragments i n the trimethyllead spectrum, so a small quantity of i t i n a sample was not serious. Both Ulrych (1962) and Whittles (1964) were troubled by the presence of trimethyl bismuth i n some of t h e i r samples. This material has the same b o i l i n g point as tetramethyllead and hence would not l i k e l y be removed i n the column p u r i f i c a -t i o n . It gives r i s e to an increased abundance of mass 2 5 4 , a peak which i s measured to correct for the presence of C 1 3 . In p r a c t i c e , the C 1 2 / C 1 3 r a t i o for the p a r t i c u l a r methyl r a d i c a l source i s s u f f i c i e n t l y well known so that the mass 254 peak cou be ignored i f bismuth were found to be present. 24 The only other metal a l k y l that could contaminate the trimethyllead spectrum i s trimethylthalium which would give peaks at masses 248 and 250. Up and down mass ' t a i l i n g ' from these peaks (including the effects of C 1 3 and hydrogen loss) would contaminate the very important 249 peak. The size of the mass 250 peak (measured to correct for the loss of a hydrogen atom from one of the methyl groups) depends only on the mass spectrometer source potentials. Hence, the presence of thalium would be c l e a r l y revealed by abnormally large mass 250 peaks. Throughout the present work, neither bismuth nor thalium was observed i n any of the samples. Mass Spectrometry The mass spectrometer used i n t h i s study was the 90°-sector 12-inch radius, gas-source instrument designed and b u i l t by P. Ko l l a r and-R.D. Russell and subsequently used by Ulrych (1962), Whittles (1964) and others. The routine mass spectrometer operating procedures have been described i n d e t a i l by Ostic (1963). A b r i e f summary here emphasizes the procedures that are important when the sample size i s small. To determine the difference i n iso t o p i c composition between two samples A and B, sample A i s f i r s t analyzed, then sample B i s analyzed, and f i n a l l y sample A i s reanalyzed. The -.verage of the two A analyses i s then compared with the 2 5 B analysis. When possible, two samples of si m i l a r isotopic composition were chosen for comparison i n order to minimize the effects of any cross-contamination. Ostic and others have shown that i t Is very important to maintain the same mass spectrometer operating conditions throughout a given comparison (usually one day) In order to obtain a meaningful difference i n isotopic composition between the two samples. Hence source p o t e n t i a l s , operating pressures, etc. must be held approximately constant during the day. This i s only possible with two samples of about the same size and pur i t y . While t h i s i s r e l a t i v e l y easy to achieve with large, pure samples (^  100 mgm of tetramethyllead), a special e f f o r t must be made with small, r e l a t i v e l y impure samples (^  1 mgm of tetramethyllead). A check on the precision of the isotopic comparison of two samples i s made by a routine intercomparison or loop-ing technique (for a detailed discussion, see Ostic, et a l , 1 9 6 7 ) . Three samples, A, B and S are included i n each loop and over three days each sample i n turn i s compared with the other two samples i n the manner described above. The isotopic differences (A-B, B-S, S-A) are added alg e b r a i c a l l y to determine the loop closure error, e , for each isotope r a t i o . These t o t a l errors are then d i s t r i b u t e d evenly among the analyses of the three days. In this way, the is o t o p i c compositions of the two samples, A and B are obtained r e l a t i v e to a standard sample, S. In addition, i t can be shown that 26 the quantity e//~3~approximates the standard deviation of a measured isotope r a t i o and hence provides an estimate of the r e p r o d u c i b i l i t y of the analyses on a day-to-day basis. A l l samples were ultimately intercompared with the laboratory standard, Broken H i l l #1, a s p l i t of the o r i g i n a l University of Toronto sample T1003 from the main lode ore-body at Broken H i l l . Isotope r a t i o s for thi s sample were given by K o l l a r , et a l ( i 9 6 0 ) : Pb 2 0 6/Pb 2 0 1 + = 1 6 . 1 1 6 , p b20 7 / P b 2 0 t = 15.52(2, Pb 2 0 8/Pb 2 0 l t = 3 6 . 0 6 8 . Sample Size, Precision, and Reproducibility The s u l f i d e samples which were analyzed by Ulrych ( 1 9 6 2 ) generally produced several milligrams of t e t r a -methyllead and he was able to claim a precision of approxi-mately 0 o 0 5 per cent i n the measurement of isotope ra t i o s r e l a t i v e to l e a d - 2 0 4 . In other words, the sum of the per-centage standard deviations of the mean trimethyllead peak heights< / z ( x 1 - x ) 2 l i s 0 . 0 5 per cent for each of the isotope J n ( n - l ) J r a t i o s . Ulrych"s precision on a single analysis compares then quite favourably with that obtained by Ostic ( 1 9 6 3 ) who used sample sizes about two orders of magnitude larger. Ostic, however, found that i n order to obtain a r e p r o d u c i b i l i t y of measured isotope r a t i o s on the basis of r e p l i c a t e analyses that was comparable with the above prec i s i o n , he had to use the intercomparison technique described above. Ulrych's ar-' ses, on the other hand, were not intercompared i n thi s 27 i n t h i s manner and i t i s not clear what r e p r o d u c i b i l i t y he obtained. Whittles (1964) subsequently analyzed samples containing less than 100 micrograms of lead to a precision of about 0.40 per cent. Intercomparison techniques were not used i n t h i s study either and, although Whittles claims to have obtained a comparable r e p r o d u c i b i l i t y , t h i s was not convincingly demonstrated. In t h i s study, i t i s necessary to make highly repro-ducible measurements. Doe's (1962) analyses of Balmat rocks and ores are at least 0.8 per cent deficient i n the Pb 2 0 7/Pb 2 0 t t r a t i o i n comparison with the locus of the stratiform ores studied by Ostic, et a l (1967). Since his Pb 2 0 7/Pb 2 0 t t ratios are reproducible to within only 0.8 per cent at the 95 per cent confidence l e v e l , more accuracy i s c l e a r l y needed to v e r i f y this difference. The ore-leads appear to have, on the average, s l i g h t l y higher Pb 2 0 7/Pb 2 0 1 t ratios than the feldspar leads. However, the maximum range i n this r a t i o for fiv e of the seven samples amounts to only 0.5 per cent. Since the mass spectrometer operating conditions must not be changed during a day's analyses, the t e t r a -methyllead samples prepared for the present study must be s u f f i c i e n t l y large so that the ion beams produced i n the mass spectrometer can be measured to a high precision (say ^ 0.05 per cent) and, i n addition, there must be s u f f i c i e n t residue of unused sample to be recovered and analyzed a second time under the same conditions. I f the sample i s too small, 28 the ion beam in t e n s i t y decreases very rapidly during the f i r s t analysis and the source potentials must then be adjusted i n order to produce a s a t i s f a c t o r y beam Intensity for the second analysis. The minimum sample size required to f u l f i l the above conditions was determined to a large extent by the c a p a b i l i t i e s of the data reduction system. The output of the mass spectrometer measuring system consists of the shaft rotation of a ten-turn high-precision potentiometer. Ostic used a three inch recording d i a l mounted on the end of t h i s shaft, and the trimethyllead ion currents were measured by reading t h i s d i a l . Weichert ( 1 9 6 5 ) , following a suggestion made by K o l l a r ( i 9 6 0 ) , modified the system by replacing the d i a l with a ten-turn, 1000 counts per turn encoder (Perkin Elmer Model 1 0 / 1 0 0 0 ) . The encoder provides input to a four decimal d i g i t memory; twice a second, on command, the contents of the memory along with the scan d i r e c t i o n and 'shunt' selector value are recorded on punched paper tape. Weichert also programmed an IBM 7040 computer to f i l t e r and reduce the data. Recently, t h i s automatic reduction system has been modified to incorporate a Precision Instrument incremental tape recorder and the reduction program has been revised and streamlined by Russell and Blenkinsop (Weichert, et a l , 1967) , so that a complete analysis can be reduced i n about 40 seconds. This automatic reduction system i n i t s present form greatly assisted the writer i n his attempts to make precise 29 analyses of small samples of tetramethyllead. The standard deviations of the i n d i v i d u a l peak measurements are approxi-mately a factor of two lower than those obtained with the dial-recording system. In addition, human errors i n the data reduction calculations are eliminated and the time taken to obtain the f i n a l i s o t o p i c abundances i s greatly reduced. A main asset of the system i s i t s a b i l i t y to calculate meaningful pressure scattering c o e f f i c i e n t s . This ' t a i l i n g ' i n the mass spectrum i s probably due to the presence of gas molecules of various kinds i n the analyser tube and accumulated corrections, t y p i c a l l y of the order of 0 . 5 per cent of the lead Isotope r a t i o s , are required. Whittles (1964) found that he could not measure the actual c o e f f i c i e n t s for each run from the chart recorder spectrum because the peak amplitudes become too small when the sample size i s reduced. Ultimately, he had to rely on an empirical correction based on the reading of the i o n i z a t i o n gauge. The d i f f i c u l t y with this method i s that the gauge i s not uniformly sensitive to a l l gases and hence true pressure comparisons are only possible with samples of uniform pur i t y . Therefore, c o e f f i -cients obtained by the above method may not be consistent. The auto-reduction program calculates one and two mass pressure c o e f f i c i e n t s from the d i g i t a l l y recorded data according to the formulas given by Ostic ( 1 9 6 3 ) . The half-mass factor i s obtained by measuring the 'valley' between the mass 251 and mass 252 peaks. The values obtained for small samples 30 GURE 2-2 TRIMETHYLLEAD SPECTRUM FROM APPROXIMATELY 500 MICROGRAMS OF TETRAMETHYLLEAD 31 during the course of the present research compare favourably with t y p i c a l values obtained for large amplitude spectra. The data therefore should.not be s i g n i f i c a n t l y i n error because of t h i s correction factor. At very low ion beam i n t e n s i t i e s , even the automatically computed factors become .unsatisfactory. The present writer found that i n order to s a t i s f y the requirements given above a minimum of 500 micrograms of tetramethyllead were required. The sample preparation system was quantitatively calibrated from the preparations of the Balmat rock samples since t h e i r lead contents have been deter-mined (Doe, 1962). Five hundred micrograms of tetramethyllead give r i s e to a mass spectrometer ion current of approximately 2 x 10~ 1 3 amperes for the least abundant isotope at mass 249 (that i s , lead-204). I f the feedback voltage* i n the measur-ing system i s 3 v o l t s , t h i s current corresponds to a peak height on the chart recorder approximately 20 per cent of the f u l l scale value (Figure 2-2). The precision of measure-ment of isotope ra t i o s with respect to lead-204 on the basis of a single analysis ranged between 0.05 per cent and 0.10 per cent for samples of t h i s s i t e . The ore-lead analyses were generally somewhat more precise, i n order to obtain at least 500 micrograms of sample, a maximum of 30 grams of feldspar was processed. In other words, feldspars containing less than about 20 ppm lead were not analyzed. This p r a c t i c a l *The current s e n s i t i v i t y of the measuring system can be increased by lowering t h i s voltage, but to the detriment of servo c h a r a c t e r i s t i c s . 32 lower l i m i t did not exclude any of the chosen samples. A r e p r o d u c i b i l i t y of analyses on a day-to-day basis was achieved that was comparable with the above precision of a single analysis. The quantity, e/V~3~» where e i s the loop closure error, has an average value of about 0.08 per cent for the intercompared samples of the present study (Table 2-1). Some of the smaller samples were not f u l l y intercompared and hence loop closure errors are not available. This Is due to the fact that some of these samples were too small aft e r they had been analyzed once or twice, and thus i t was not possible to analyze them successfully a t h i r d time. However, the abun-dances as determined from a single day's analyses are probably not i n error by more than 0.10 per cent. Another estimate of the day-to-day r e p r o d u c i b i l i t y can be obtained from the r e p l i c a t e analyses of a standard sample over a s i g n i f i c a n t l y long time i n t e r v a l . In t h i s study, the Balmat 500 galena.was chosen as a secondary standard and approximately 10 milligrams of tetramethyllead were prepared using the free r a d i c a l technique. This sample was analyzed eleven times during a f i v e month period, and towards the end of t h i s time i t had become comparable i n size 1 mgm) to most of the rock-lead samples. The Isotopic r a t i o s , means and standard deviations for these analyses are given i n Table 2-2. The, standard deviations.of the three isotope ra t i o s range between 0.10 per cent and 0.15 per cent, and are only s l i g h t l y higher than values quoted by Ostic TABLE 2-1 REPRODUCIBILITY OF ANALYSES ON THE BASIS OF LOOP CLOSURE ERRORS Percent Standard Deviation ( e / 3) Loop Sample Pb 2 0 6/Pb 2 0 1 + Pb 2 0 7/Pb 2 0 , + Pb 2 0 8/Pb 2 0 1 t 1 1 Broken H i l l 500 Balmat 0.07 0.03 0.06 84 Balmat 0.07 0.03 0.06 1 Broken H i l l 500 Balmat 0.03 0.04 0.02 qbg Balmat 0.03 0.04 0.02 500 Balmat ccg Balmat 0.07 0.05 0.05 ccgp Balmat 0.Q7 0.05 0.05 500 Balmat Rl4 Nelson 0.33 0.12 0.12 R9 Nelson 0.33 0.12 0.12 Blue Star, Nelson Scranton 0.10 0.08 0.07 Victor 0.10 0.08 0.07 Means 0.12$ 0.06% 0.06% 34 TABLE 2-2 REPLICATE ANALYSES"OF BALMAT 500 GALENA Date of Observed Isotope Ratios ' Feedback Analysis Pb 2 0 6/Pb 2 0 , + Pb 2 0 7/Pb 2 0 1* Pb 2 0 8 / P b 2 0 1 + Voltage Nov. 29, 1966 16.830 15.528 36.614 12 16.833 15.537 36.624 12 Dec . 5, 1966 16.817 15.499 36.544 3 16.829 15 .510 36.498 3 Jan. 9, 1967 16.840 15.534 36.659 12 Mar. 19, 1967 16.827 15.503 36.547 3 Apr. 4, 1967 16.806 15 .491 36.499 3 Apr. 9, 1967 16.808 15.484 36.496 3 Apr. 13, 1967 16.845 15.487 36.541 3 Apr. 19, 1967 16.860 15.497 36.576 3 Apr. 23, 1967 16.849 15.481 36.512 3 Means and Standard Deviations*: A l l data 16 .831 15 .505 36 .555 * 0 .017 * 0 .020 ± 0 .056 (0 .10$) (0 .13?) (0 .15%) 3-volt data 16 .830 15 .494 36 .527 * 0 .020 * 0 .010 * 0 .030 (0 .1236) (0 .07*) (0 .08*) 12-volt data 16 .834 15 • 533 36 .632 * 0 .005 * 0 .005 * 0 .024 (0 .03%) (0 .03?) (0 .07*) 35 ( 1 9 6 3 , p. 32) for re p l i c a t e analyses of large samples over a one month period. The above estimate of r e p r o d u c i b i l i t y i s probably pessimistic as the data i n Table 2-2 suggest that the measured rat i o s are a function of the feedback voltage in the measuring system. The 3-volt data and the 1 2-volt data treated separately give s i g n i f i c a n t l y lower deviations for the Pb 2 0?/Pb 2 0 t t and Pb 2 0 8/Pb 2 0 1* r a t i o s . In summary, small samples (less than one milligram of lead) have been precisely analyzed for the f i r s t time. In spite of a difference i n sample size of at least two orders of magnitude, the rock-lead data are only s l i g h t l y less precise than the best ore-lead data from t h i s laboratory. 36 CHAPTER 3 ANALYSES AND INTERPRETATIONS Choice of Samples With one exception, the rock samples that were chosen for t h i s study are associated with lead ores that have been analyzed i n t h i s laboratory. In the one case (Balmat, N.Y.) where t h i s i s not true, both ore-leads and rock-leads were analyzed. In addition, these ore studies are a l l c l a s s i c examples of 'type' interpretations of lead isotope abundances. That i s , the data support the two basic models described i n an e a r l i e r chapter. The ores at Balmat and at Broken H i l l (N.S.W., Australia) belong to the special class of stratiform deposits defined i n Chapter 1 and by Stanton ( i 9 6 0 ) . Anomalous lead suites have been found i n the Broken H i l l d i s t r i c t (Thackaringa-type deposits), near Nelson, B r i t i s h Columbia (Kootenay arc deposits), and i n the West-Central New Mexico area. The Balmat area i s one of a very few from which there have been published ore-rock studies. Nelson, B r i t i s h Columbia Samples were coll e c t e d by the writer from the Nelson b a t h o l i t h , a large and r e l a t i v e l y young (^  150 m.y. old) g r a n i t i c body located i n south-eastern B r i t i s h Columbia. For these.young rocks i t .is not necessary to correct for i n situ.uranium and thorium decay i n the potassium feldspars. Evidence for a.metasomatic o r i g i n of at least some of the Nelson plutonic rocks ( L i t t l e , I960, p. 98) has been 37 discredited as a result of detailed f i e l d mapping by J ...V. R o s s and A . J . S i n c l a i r (personal communication, .1967). However, . • this does not preclude derivation from melting of pre-existing rocks, as opposed to mechanical i n j e c t i o n of new'material from a deep source. Low i n i t i a l S r 8 7 / S r 8 6 r a t i o s (0.705 - 0.710) for rocks from this area (Pairbairn, et a l , 1964) do, however,"'" imply that the g r a n i t i c magma has not been derived from t y p i c a l rubidium-enriched c r u s t a l material. The Nelson granites should, i n any case, contain a common lead component c h a r a c t e r i s t i c of the parent rocks and, i n addition, uranium and thorium i n these rocks should have produced a radiogenic component. I f complete mixing of lead isotopes occurred when the batholith was formed, potassium feldspars from d i f f e r e n t regions w i l l then contain lead of uniform isotopic composition. On the other hand, p a r t i a l mixing of isotopes w i l l be revealed by a l i n e a r relationship (a two-stage anaomalous lead line) between the isotope r a t i o s p b20 7/p b20«t a n d p b20 6/p b20' + < second j< f i r s t stage > | < stage > | I age of age.of age of t = 0 the earth parent rocks granite Rock-lead is o t o p i c analyses for part of the southern C a l i f o r n i a batholith (Patterson, et a l , 1956) indicated a uniform composition throughout. Doe (1967), however, has recently found is o t o p i c variations as high as 8 per cent among the d i f f e r e n t rock-types that make up the Boulder ba t h o l i t h . The present research continues the study of lead isotope 38 abundance p a t t e r n s i n p l u t o n i c rocks of b a t h o l i t h i c dimensions. An important reason f o r a n a l y z i n g l e a d i n the Nelson g r a n i t e s was to i n v e s t i g a t e a p o s s i b l e genetic r e l a t i o n s h i p between these rocks and s p a t i a l l y r e l a t e d m i n e r a l d e p o s i t s . M i n e r a l i z a t i o n i n the Nelson area i s known to have occurred a f t e r emplacement of the b a t h o l i t h , and a g e n e t i c r e l a t i o n -s h i p between the two has been suggested by s e v e r a l authors ( S c h o f i e l d , 1 9 2 0 ; C a i r n e s , 1934; I r v i n e , 1 9 5 7 ) . The present study was f a c i l i t a t e d by the f a c t t h a t the m i n e r a l d e p o s i t s a s s o c i a t e d with the Nelson b a t h o l i t h occur i n the s t r u c t u r a l b e l t known as the Kootenay a r c . The f i r s t l e a d Isotope analyses r e p o r t e d from t h i s area are giv e n by R u s s e l l and Farquhar ( I 9 6 0 ) . S i n c l a i r (1966) has sub-sequently r e p o r t e d more p r e c i s e analyses of o r e - l e a d s from t h i s r e g i o n . H i s data suggest that a l l leads from the Kootenay arc have had g r o s s l y s i m i l a r i s o t o p i c h i s t o r i e s t h a t can be ex p l a i n e d by a m o d i f i e d two-stage anomalous le a d model. Balmat, New York A more p r e c i s e study of the l e a d i n Balmat rocks and ores was made p o s s i b l e by Dr. B.R". Doe who k i n d l y s u p p l i e d the f o u r f e l d s p a r and two galena samples that were analyzed In h i s own i n v e s t i g a t i o n . Prom Doe's study, i t appeared that a g e n e t i c l i n k between the ores and at l e a s t some of the rocks i n the area 39 was possible, but radiogenic lead (that i s , primarily lead - 2 0 6 ) must have been added to the potassium feldspars subsequent to the formation of the ores for t h i s to be so. The ore deposits at Balmat are of the stratiform type studied by Ostic, et a l (1967). However, on the basis of Doe's analyses, the ore-leads were derived from a primary system characterized by a u-value of about 8.7 rather than 9 . 0 , the value calculated from Ostic's data. A c l a r i f i c a t i o n of th i s apparent difference w i l l be a main contribution of the present thesis. Doe's analyses of Balmat rock-leads suggest that they too are related to a primary system with a p-value of 8.7 or lower. With the exception of one highly anomalous sample, these leads have a rather small range i n isotopic composition and Doe (1962) therefore states that " i t i s very l i k e l y that these bodies were formed with lead from a common source". Within the a n a l y t i c a l uncertainty (95 per cent confidence l i m i t s are approximately 0.8 per cent of isotope rat i o s with respect to lead -204 ) , Doe's results also suggest that the Balmat ore-leads were derived from t h i s same source material. Doe (1962) concludes that "the data from both minerals ( i . e . galena and potassium feldspar) f i t the normal growth curves equally well". More precise isotopic analyses of the Balmat rocks and ores are required i n order to ve r i f y t h i s conclusion. 40 Broken H i l l , New South Wales, Aus t r a l i a The writer i s grateful to Mr. J.L. Liebelt and to Mr. H.F. King of the Zinc Corporation, Limited, for supplying samples of Mundi Mundi granite and lode pegmatite. The lode pegmatite sample, consisting primarily of green potassium feldspar (amazonite), came from within the B-lode orebody (No. 9 i n the. Mine Sequence of Carruthers and Pratten, 1961), on the No. 15 l e v e l , New Broken H i l l Consolidated Limited. This orebody i s one of the zinc lodes and t y p i c a l l y grades about 5% lead, 18% zinc. The sample of Mundi Mundi granite was collected from an outcrop 18 miles northwest of Broken H i l l . This outcrop, an e l l i p t i c a l mass measuring 1 mile x % mile, i s one of a number of small g r a n i t i c bosses which intrude the Willyama Complex, the metamorphosed sediments which are the host rocks for the Broken H i l l orebodies. The writer i s also grateful to Dr. S.E. Shaw of the Geology Department, University of New South Wales at Broken H i l l f o r supplying samples of the Upper Granite and "Potosi" gneisses (numbers 13 and 5 respectively i n the Mine Sequence). Mineral separations for six of the above samples were carried out by H. Berry and R. Rudowski under the d i r e c -t i o n of Dr. J.R. Richards at the Australian National University, Canberra. Ore-leads from the Broken H i l l area have been used on many occasions to i l l u s t r a t e the two basic models employed i n lead isotope interpretations. The lead deposits can be 4 1 d i v i d e d i n t o two typ e s , Broken H i l l or s t r a t i f o r m type and Thackaringa or v e i n type. The Broken H i l l type i s represented by the massive main lode at the c i t y o f Broken H i l l and by other s m a l l e r d e p o s i t s i n the area. O s t i c , e t a l (1967) have sug-gested that these s t r a t i f o r m d e p o s i t s c o n t a i n a p r i m a r y - l i k e l e a d . On the b a s i s of a s i n g l e - s t a g e model, t h i s l e a d has an apparent age of 1600 m i l l i o n years and has developed i n a source m a t e r i a l c h a r a c t e r i z e d by a y-value of 9 - 0 . The Thackaringa d e p o s i t s c o n t a i n l e a d that i s g e n e t i c a l l y r e l a t e d to l e a d o f the main lode type ( R u s s e l l , et a l , 1 9 6 1 ) . Kanasewich ( 1962) f u r t h e r suggested that the r a d i o g e n i c component of the Thackaringa leads was generated i n source rocks between the times 1600 m.y. ago and 500 m.y. ago, and that at t h i s l a t e r time mixing o f the r a d i o g e n i c and primary components occ u r r e d and the Thackaringa d e p o s i t s were formed. The rock types chosen f o r the present study are l o c a t e d at d i f f e r e n t d i s t a n c e s from the main orebody. For example, the lode pegmatite sample came from w i t h i n an ore format i o n , while the samples of Mine Sequence gneisses were c o l l e c t e d at d i s t a n c e s o f 1000 f e e t or more from the main ore zone. The sample of Mundi Mundi g r a n i t e came from an outcrop about 18 m i l e s from Broken H i l l . Hence, rock-ore r e l a t i o n -s h i p s i n the Broken H i l l area can be examined as a f u n c t i o n of d i s t a n c e from the main orebody. K-A and Rb-Sr age determinations (Richards and Pidgeon, 1 9 6 3 ; Pidgeon, 1967) suggest that the Mine Sequence gneisses and the Mundi Mundi g r a n i t e have had a c h r o n o l o g i c a l h i s t o r y s i m i l a r to that of the Thackaringa-type o r e s . For example, the 500 m.y. ages f o r b i o t i t e s from these rocks can be c o r r e l a t e d with the event which caused the emplacement of the Thackaringa d e p o s i t s . I t i s of i n t e r e s t to see i f the t r a c e l e a d i n the rocks has had a s i m i l a r h i s t o r y . West-Central New Mexico The sample of Precambrian Bosque d e l Apache g r a n i t e which was analyzed i n t h i s study was c o l l e c t e d by Dr. C.F. A u s t i n . A s s o c i a t e d with t h i s g r a n i t e i s a s m a l l , contact pyrometasomatlc ore d e p o s i t . The i s o t o p i c abundances of l e a d from t h i s d e p o s i t have been r e p o r t e d by Slawson and A u s t i n (1962) as p a r t of t h e i r study of o r e - l e a d s a s s o c i a t e d with the Zuni lineament. R e c e n t l y , Blenkinsop.and Slawson (1967) have r e p o r t e d more p r e c i s e i s o t o p i c analyses f o r a number of these New.Mexico l e a d s . The above s t u d i e s have suggested t h a t the anomalous leads i n t h i s r e g i o n are the product of a s i n g l e c r u s t a l system and, moreover, t h a t they are g e n e t i c a l l y r e l a t e d to a l e a d which has an i s o t o p i c composition s i m i l a r to t h a t of the Bosque ore. S i n c l a i r (1964, p. 44) has suggested t h a t ore-rock r e l a t i o n s h i p s can be simply and d i r e c t l y s t u d i e d by comparing o r e - l e a d from such a contact metasomatic d e p o s i t with the common l e a d i n t h e adjacent 'parent' r o c k s . 43 Balmat, New York A detailed description of the Balmat samples has been given by Doe (1962). Results from the present study are given i n Table 3-1 along with Doe's e a r l i e r data. Pb 2 0 7/Pb 2 0 1 t and Pb 2 0 8/Pb 2 0 i t versus Pb 2 0 6/Pb 2 0 1 t diagrams for a l l of the data are shown In Figure 3-1. The new analyses.presented i n t h i s thesis show cl e a r l y relationships between the samples that could not be shown by Doe's measurements. Both ore-leads have s i g n i f i c a n t l y higher P b 2 0 7 / P b 2 0 h ratios than the feldspar leads. Therefore the p o s s i b i l i t y that the ores were derived d i r e c t l y from any of the igneous rocks i s precluded since a negative amount of lead-207 would have had to have been added to the f e l d -spars subsequent to the formation of the ores i n order to account for the difference. Doe (1962a) concluded that rocks similar to the marbles surrounding the orebodies are an unlikely source of the r e l a t i v e l y i s o t o p i c a l l y uniform ore-lead since a large difference i n isotopic composition was observed between two marble samples. Hence, the ores do not appear to be related to any of the associated rocks so far examined. The ore-lead from the no. 2 mine (Pb 84, F19) was apparently derived from a similar primary system as the lead i n stratiform deposits studied by Ostic, et a l (1967). The P b 2 0 7 / P b 2 0 6 single-stage model age for t h i s sample i s 1100 m.y. A u-value (see page 1) of 9.0 was calculated for TABLE 3-1 ISOTOPIC COMPOSITION OF BALMAT LEADS Observed Lead.Isotope Abundances Sample fLead. ppm Pb Pb 'This 206 20H i Study P b 2 0 7 Pb 2 0-Pb Pb 20 8 2 0 4 Doe P b 2 0 6 Pb 2 0 1* (1962. : P b 2 0 7 P b 2 0 4 1962a) Pb Pb 208 20 <+ 84, F19 galena 17 .019 15.677 36 • 857 16 .96 15.55 36 • 54 500, 26N galena 16 .864 15.605 36 .733 16.82 15.51 36 .55 ccg K-feldspar 30 17 .054 15.520 36 .857 16.94 15.31 36 .43 ccgp.K-feldspar 25 17 .260 15.599 36 .962 17.26 15.54 36 .74 qbg K-feldspar 50 17 .320 15.551 36 .685 17.14 15.47 36 .55 hpg K-feldspar 113 16 .997 15.527 36 .641 17.10 15.51 36 .64 cdm 14-p K-feldspar 9 18.99 *17.76 **17.8l 15.63 *15.54 **15.64 37 .12 fd 7-2 marble 3.2 17.98 *15-95 **16.00 15.46 *15-32 **15.42 36 .32 fd 7-3 marble 0.48 19 .22 *17.43 **17.48 15.60 *15.47 **15.57 36 .64 T Data from Doe (1962, 1962a) * corrected for 1000. m.y. i n s i t u uranium decay ** adjusted to the UBC- standard as.explained, i n text F I G U R E . 3 - 1 I S O T O P I C COMPOSITION OF BALMAT L E A D S fd 7 - 2 1 204 e r r o r - l i n e 1 6 . 0 p b206/p b204 1 7 . 0 46 the source material of the lead, i n precise agreement with Ostic's value. If the two ore-leads were derived at the same time from the same source they should, according to the model described i n Chapter 1, have the same isotopic composition. But, the lead from the no. 3 mine (Pb 500, 26N) could only have developed i n a sin g l e , closed system with a p-value d i s t i n c t l y lower than 9-0. It i s more l i k e l y that there was one primary mineralization 1100 m.y. ago and that the i s o -topic differences between the mines are explained by sub-sequent contamination of the lead i n one of them. Doe (1962a) has shown that the no. 3 mine has a much lower lead concentra-t i o n (< 10 ppm) than the no. 2 mine (> 5000 ppm) and hence that the former was more l i k e l y to show s i g n i f i c a n t contamina-t i o n from lead "released from the carbonate minerals replaced by the s u l f i d e s " . In addition, Doe's data suggest that the no. 2 mine has the more uniform lead isotopic composition. Contamination of the lead i n the no. 3 mine by older lead from a d i f f e r e n t source i s consistent with the following in t e r p r e t a t i o n of the rock-lead data. On the basis of Doe's analyses, i s o t o p i c . v a r i a t i o n s among the four feldspar leads appear to be primarily i n a d i r e c t i o n corresponding to errors i n the measurement of the lead-204 abundances (that .is,• l i n e s passing through the o r i g i n i n Figure 3-1). The :apparent Pb 2 0 7/Pb 2 0 1* variations among these four samples have been considerably reduced by 47 the present a n a l y s e s . Model ages and u-values c a l c u l a t e d on the b a s i s of a s i n g l e - s t a g e . model range from 730 m.y. to 950 m.y. and from 8.7 to 8.8 r e s p e c t i v e l y . These model ages would be i n c r e a s e d i f c o r r e c t i o n s f o r i n s i t u uranium decay were made. However, the c o r r e c t i o n s must be small and the ages should s t i l l be younger than the Rb-Sr and K-Ar. m i n e r a l ages 1050 m.y.). In a d d i t i o n , there would s t i l l be a s i g n i f i c a n t v a r i a t i o n i n i s o t o p i c composition among the samples. Hence, a s i n g l e - s t a g e model i s not. adequate and a m u l t i - s t a g e model must.be adopted to e x p l a i n the observed abundances. I t has been shown ( R u s s e l l and Farquhar, i960) t h a t the slope R of a l i n e a r r e l a t i o n s h i p between/the r a t i o s P b 2 0 7 / P b 2 0 l t and P b 2 0 6/Pb 2 0 1* f o r common leads i s given by: R = - ^ T T fx 1 ... -..(3-D 0 ( e x t i - e X t 2 ) where a = ( U 2 3 8 / U 2 3 5 ) t = Q X' = 0.9722 x 10~ 9y.- 1 X = 0.1537 x 10~ 9y.- 1 These anomalous leads c o n t a i n a mixture of r a d i o g e n i c l e a d developed between times t j and t 2 , with a s i n g l e common l e a d . I s o t o p i c a l t e r a t i o n a f t e r time t 2 ( f o r example, as a r e s u l t of i n s i t u uranium decay) i s assumed n e g l i g i b l e . The Balmat data are most r e a d i l y i n t e r p r e t e d by means of a s h o r t - p e r i o d anomalous l e a d model (Kanasewich, 1962a). In terms of t h i s model, the d u r a t i o n of the second (or most r e c e n t ) stage of development In uranium-rich c r u s t a l environments i s only a few hundred m i l l i o n y e a r s . The l i m i t i n g 48 case occurs when t x -*- t 2 = t 1 . Equation (3-1) then .reduces to: R = 11 e ( X ' " A ) t i (3-2 AX The least-squares l i n e (a weighted f i t as suggested by York (1966)) through the four feldspar lead points i n Figure 3-1 has a slope of 0.19 1 0.12. From equation (3-2), the time t i i s 1740 ^J^QQ m«y- T h e sample, ccgp, has a higher Pb 2 0 7/Pb 2 0 1* r a t i o than the others . I f t h i s sample i s omitted, the slope becomes 0.10 ± 0.03, and the time t i i s 900 ± 450 m.y. A better estimate of t h i s time can be obtained by in c l u d -ing i n the above c a l c u l a t i o n the two marble samples and the one feldspar sample (cdm 14-p) which were analyzed by Doe but were not included i n the present study. Two corrections were f i r s t applied to the observed abundances: (a) a correction for i n s i t u uranium decay to 1000 m.y. ago, and (b) a correction to compensate for interlaboratory differences. The l a t t e r correction was made by comparing the present analyses of the Balmat rocks and ores with Doe's e a r l i e r data and hence ca l c u l a t i n g the mean displacement vector. The isotopic abundances of the samples were then adjusted by t h i s amount. This correction changed the Pb 2 0 7/Pb 2 0 1* ra t i o s by about 0.6 per cent and the Pb 2 0 6/Pb 2 0 1* ratios by 0.3 per cent (Figure 3-1). The slope of the least-squares l i n e through the five feldspar leads and two marble leads i s 0.12 ± 0.03 and the corresponding time i s 1200 ± 300 m.y. 49 The s h o r t - p e r i o d model adequately e x p l a i n s the observed i s o t o p i c v a r i a t i o n s ( p r i m a r i l y i n the P b 2 0 6 / P b 2 0 l t r a t i o s ) among the f e l d s p a r l e a d s . I t i s suggested that t h i s short-term c r u s t a l development took p l a c e i n a whole-rock system of age t1 which was subsequently metamorphosed at time t 2 when mixing of l e a d i s o t o p e s o c c u r r e d among the v a r i o u s m i n e r a l phases. Zartman's (1965) Rb-Sr study of whole rocks and mi n e r a l s from the Llano U p l i f t , Texas p r o v i d e s an a p p r o p r i a t e comparison. He has shown that there was a primary event i n that area o f Texas about 1120 m.y. ago fo l l o w e d by a metamorphic event about 100 m.y. l a t e r . U l r y c h and Reynolds (1966) subsequently showed t h a t the whole-rock and m i n e r a l l e a d Isotope abundances could be i n t e r p r e t e d to give the same age i n f o r m a t i o n . On the b a s i s of the Rb-Sr and K-Ar mi n e r a l ages, the l a s t metamorphism ( t 2 ) i n the Balmat area o c c u r r e d about 1050 m.y. ago. The age of the whole-rock system has not been measured by the Rb-Sr technique; however, t h i s age can be estimated i f i t i s assumed t h a t the l e a s t r a d i o g e n i c common l e a d i s the product o f a s i n g l e - s t a g e system. The s i n g l e -stage model age and the apparent u-value f o r -the marble sample, f d 7-2 ( c o r r e c t e d f o r 1000 m.y. of i n s i t u decay and ad j u s t e d to compensate f o r i n t e r l a b o r a t o r y d i f f e r e n c e s ) , are 1590 m.y. and 8.76 r e s p e c t i v e l y . This age, based on a s i n g l e marble sample, seems unacceptable when compared with the uranium-l e a d age c a l c u l a t e d by the method of U l r y c h ( i n press) which 50 i s described i n the following section. Incorrect adjustment for i n s i t u decay i s a l i k e l y explanation for t h i s discrepancy. Ulrych ( i n press) and Russell, et a l (in press) have recently shown how the measured lead isotope ra t i o s and uranium and lead concentrations for a U/Pb system can be applied to an assumed two-stage model i n order to calculate age and geochemical information. In t h i s model i t i s assumed that the lead now observed In a mineral or rock existed i n a primary system (y = yj) u n t i l the time t x when i t was trans-ported to i t s present environment (y = y 2, as observed). The Balmat leads, however, are apparently the product of three environments: a primary system u n t i l t l s a short-term exposure to a c r u s t a l system u n t i l t 2 , and the presently observed system. But, i f i t i s assumed that the second stage i s very short (that i s , t j = t 2 ) , the Ulrych model i s a reasonable approximation and the age, t i and the primary y-value, y x can be calculated using the observed isotopic abundances and the U/Pb r a t i o s . The uranium and lead con-centrations have been determined by Doe for the two marbles (dolomites) and for two of the feldspars at Balmat and are given i n Table 3-2 along with the observed lead isotope abundances. The isotopic ratios for the three samples not analyzed i n the present study were adjusted by the method described above i n order to compensate for interlaboratory differences. An age of 970 ± 300 m.y. and a primary y-value of 8 . 8 0 ± 0 . 5 3 have been calculated for these four samples TABLE 3-2 LEAD ISOTOPE RATIOS, URANIUM AND.LEAD CONCENTRATIONS OF BALMAT ROCKS AND MINERALS Sample Observed Isotope Ratios U Pb p b206/p b20«» p b207/p b^04 p b208/ P b20'» p p m p p m V = (U^.s/Pb2.01*) obs cdm-l4-p* (K-feldspar) 19-04 hpg(K-feldspar) 17-00 fd 7-2 (marble)* 18.03 fd 7-3 (marble)* 19-27 15-73 15-53 15-56 15.70 37-33 1-17 9-9 36.64 0.324 113-36.53 0.64 3-2 36.85 0.083 0.48 0.054 0.0013 O.O885 0.0786 V J l *Isotope Ratios from Doe (1962, 1962a), adjusted-as explained.in text. A l l chemical determinations of U and Pb by Doe. 5 2 9 using the parameters a 0 = 9.56, b 0 = 10.42, t D = 4.55 x 10 y. This c a l c u l a t i o n i s less sensitive to the U/Pb r a t i o of any pa r t i c u l a r sample and hence i s more r e l i a b l e than the one described above that i s based on a single marble sample. I f i t i s assumed that the ore-lead from the no. 3 mine was contaminated by lead from th i s rock system, a minimum age of the system can be calculated from the i n t e r -section of the anomalous lead l i n e i n Figure 3-1 with the li n e through the two ore-leads. These l i n e s intersect on the 1230 m.y. old primary isochron at a point corresponding to a y-value of 8.7; that i s , t ! = 1230 m.y. Hence, despite high standard deviations, the average age of about 1200 ± 300 m.y. obtained above for the rock system from the slope of- the anomalous lead l i n e i s consistent with other age information. Evidence given above suggests that on the basis of a single-stage model the common lead which was incor-porated into the Balmat rocks at the time of t h e i r formation had developed i n a system characterized by a y-value of 8.75 * 0.05. This i s s i g n i f i c a n t l y lower than 9.0, the value associated with the stratiform ores studied by Ostic. It i s , of course, possible that t h i s common lead did not develop i n a sing l e , closed system but rather that i t has had a multi-stage h i s t o r y . However, i t i s not possible to suggest a multi-stage model based on the primary (y = 9»0) growth curve which would y i e l d the observed isotopic 53 abundances without at the same time p o s t u l a t i n g the e x i s t e n c e of very much o l d e r (at l e a s t 2000 m.y.) rock systems i n the Balmat area. There i s no evidence to suggest that there are rocks o f t h i s age; hence, the e x i s t e n c e o f a primary system with a lower y-value i s p o s t u l a t e d . T h i s s u b j e c t i s developed i n the f o l l o w i n g chapter. Summary and P r e f e r r e d I n t e r p r e t a t i o n On the b a s i s o f the new analyses presented i n t h i s t h e s i s , the Balmat ores formed from l e a d i s o t o p i c a l l y of the type observed by O s t i c and others (1967) i n c e r t a i n s t r a t i f o r m d e p o s i t s . In a case where there i s a low c o n c e n t r a t i o n of l e a d i n the ore, contamination of the o r e - l e a d by lead from a rock system i s apparent. The r o c k - l e a d s (from potassium f e l d s p a r s and from marbles) form a s u i t e o f s h o r t - p e r i o d anomalous leads a p p a r e n t l y d e r i v e d from a source m a t e r i a l with a y-value o f about 8 . 7 5 . T h i s short-term development took p l a c e between approximately 1050 m.y. ago and % 1230 m.y. ago. I t i s c l e a r that the. primary system p o s t u l a t e d f o r the r o c k - l e a d s (y = 8 . 7 5 ) i s d i s t i n c t l y d i f f e r e n t from the one a s s o c i a t e d with the ores (y = 9 - 0 ) ; hence the o r e - l e a d Is not g e n e t i c a l l y r e l a t e d to the l e a d i n the rock system. Nelson, B.C. Four rock samples from d i f f e r e n t l o c a l i t i e s i n the Nelson b a t h o l i t h were chosen f o r the present study. In a d d i -54 t i o n , the writer analyzed four galena samples from ore de-posits located i n the Nelson granite or i n adjacent sedimen-tary rocks. The sample locations are shown i n Figure 3 - 3 and isotopic abundances, approximate lead contents of the potassium feldspars, and sample descriptions are given i n Table 3 - 3 = The galena sample from the Blue Star mine was collected by the writer. Samples from the Lakeshore showing and from the Scran-ton and V i c t o r mines were collected by S i n c l a i r . S i n c l a i r (1966) has also reported preliminary isotopic analyses for these three samples. S i n c l a i r (1966) made precise intercomparison anal-yses of eight samples from seven mines located i n the Kootenay arc nqr-%h and south qf- t,he Nelson batholith. The present writer r.e=eal§ulat;ed t;he isq^qpic, ratiips fpqm the raw data (see Table 3-4} and alsp the leas,t=squares l i n e (York, 1J6>6) through the points i n Figure 3-2. The slope pf this l i n e i s 0.1022 * Q.Q0§5 when weights inversely pr-oppr-tional to the squares of- the anal= y t i c a l uncertainties are assigned to each c o o r d i n a t e , and 0.1024 * Q . O O 8 3 when a l l co-ordinates are given equal weight, The ls§to= pic abundances of lead from the S u l l i v a n mine (East Kpotenay dis= t r i c t ) , re-calculated from S i n c l a i r ' s data (Table 3 - 4 ) , plot on the anomalous lead l i n e i n Figure 3 - 2 . Hence, a genetic r e l a t i o n -ship between Kootenay arc leads and Sullivan-type lead Is suggested. S i n c l a i r ( 1 9 6 6 ) s i m i l a r l y concluded that "Kootenay arc leads were formed by addition of variable amounts of radio-genic lead to lead of Sul l i v a n isotopic composition". FIGURE 3-2 ISOTOPIC COMPOSITION OF KOOTENAY ARC AND NELSON LEADS TABLE 3-3 Sample Rl4 R 3 RIO R 9 Blue S t a r Mine 182 Lakeshore 294 V i c t o r 295 Scranton B l u e b e l l 226 228 229 230 237 ISOTOPIC COMPOSITION OF NELSON LEADS Pb 2 0 6 / P b 2 0 1 t D e s c r i p t i o n K - f e l d s p a r from p o r p h y r i t i c g r a n i t e K - f e l d s p a r from p o r p h y r i t i c quartz monzonite K - f e l d s p a r from pegmatite a s s o c i a t e d w i t h . f o l i a t e d d i o r i t e K - f e l d s p a r from p o r p h y r i t i c g r a n i t e Galena Galena Galena Galena B l u e b e l l galena data from Kanasewlch (1962a) ppm lead. 50 40. 30 40 19 .466 19.076 19 .287 19.066 17.528 17.682 18.802 19.000 17.59 17.57 17.52 17.55 17.62 p b 2 0 7 / p b 2 0i» 15.812 15.756 15.815 15.711 15.641 15.649 15.757 15.798 15.62 15.61 15 .60 15.59 15.63 P b 2 0 8 / P b 2 39.348 39.043 39 .486 39.078 38.188 38.631 39.368 39.090 38.42 38.38 38.26 38.31 38.48 57 FIGURE 3-3 APPROXIMATE SAMPLE LOCATIONS, NELSON AREA, BRITISH COLUMBIA New Denver Castleg T r a i l 0 L. 10 * miles 20 [Kpotenay Lake U.S.A. 1. S u l l i v a n Mine 2. Bluebell Mine R3. Quartz Monzonite 4. Blue Star Mine 5. Lakeshore Showing 6. Scranton Mine 7. Sal Showings 8. Duncan Mine R9. Granite RIO. Pegmatite 11. Mollie Mac Showing 12. Reeves Macdonald Mine 13• Jersey Mine Rl4. Granite 15. Vi c t o r Mine V 16. H.B. Mine 17. Jackpot Showings 58 TABLE 3-4 ISOTOPIC COMPOSITION* OP LEADS PROM THE KOOTENAY ARC AND FROM SULLIVAN MINE Sample p b206/p b204 p b 2 0 7 / p b 2 0 t p b20 8/p b2 318 Mollie Mac 18.495 15.836 38.70 300 Jersey Mine 19 .266 15.918 39.87 288 Jersey Mine 19.258 15.921 39 .82 286 HB Mine 19.250 15.915 39.85 284 Jackpot 19.134 15.886 39.62 293 Reeves Macdonald 19.192 15.907 39-72 291 Sal A Zone 19.529 15.937 40.15 282 Duncan Lake 19.555 15.944 40.26 321 Sul l i v a n 16.638 15.658 36.623 323 Sull i v a n 16 .639 15.653 36.582 *re-calculated from raw data of S i n c l a i r 59 The o r i g i n of the Sull i v a n orebody i s a subject that i s continually debated. S i n c l a i r (1964) suggested an epigenetic o r i g i n and a probable genetic relationship of the Sullivan ore f l u i d s with the source magma of the Precambrian Moyie Intrusions. Others have suggested that a syngenetic o r i g i n more readi l y explains the conformable nature of the orebody. On the basis of analyses by Leech and Wanless (1962) and by S i n c l a i r (1966), t h i s large deposit contains lead of apparently uniform isotopic composition. In addition, the isotopic r a t i o s are on the locus defined by leads from the stratiform deposits studied by Ostic, et a l (1967). A s i n g l e -stage model age of 1340 m.y. Is calculated for the Sul l i v a n lead. According to th i s model, t h i s age gives the time of ex-tra c t i o n of the lead from the source material, and, i n addition, the probable time of mineralization at Sul l i v a n . Some of the Kootenay arc leads, however, do not f i t the model proposed by S i n c l a i r ( 1 9 6 6 ) . In p a r t i c u l a r , the new analyses of the four galenas from deposits associated with the Nelson batholith plot s i g n i f i c a n t l y below S i n c l a i r ' s anomalous lead l i n e (Figure 3-2). The Blue Star and Lakeshore samples appear to have s l i g h t l y lower Pb 2 0 7/Pb 2 0 1* rat i o s and s l i g h t l y higher Pb 2 0 6/Pb 2 0 l t r a t i o s than S u l l i v a n lead. Hence, a direct genetic relationship with the l a t t e r i s precluded. In comparison with the present r e s u l t s , the isotopic abundances reported by S i n c l a i r (1966) for the Lakeshore, Scranton, and Victor samples appear to be i n error by about 0.75 per cent. The 60 least-squares l i n e through these four ore-lead points has a slope of 0.1031 ± 0.0084. This value i s i n good agreement with that obtained from S i n c l a i r ' s data. Therefore, ore-leads associated with the Nelson batholith apparently have had a chronological history s i m i l a r to that of leads i n the Kootenay arc north and south of the batholith. However, the batholith leads do not incorporate a Sullivan-type component. Si g n i f i c a n t variations i n isotopic composition (^ 2 per cent for the P b 2 0 6/Pb 2 0'* ra t i o ) were obtained for the feldspar leads from the b a t h o l i t h . Hence, complete mixing of lead isotopes did not occur when the batholith was formed. However, there i s some evidence that p a r t i a l mixing did take place. Rock samples R3 and R9, for example, are very s i m i l a r i n isotopic composition and are from the same area of the b a t h o l i t h . Sample Rl4, on the other hand, from the op-posite side of the b a t h o l i t h , has a d i s t i n c t l y d i f f e r e n t isotopic composition. Moreover, i t i s s i g n i f i c a n t that the general trend of the rock-lead points i n Figure 3-2 i s i n a d i r e c t i o n approx-imately p a r a l l e l to the S i n c l a i r anomalous lead l i n e . Five samples from the Bluebell mine, located on the east shore of Kootenay Lake about 2% miles east of Lakeshore, were analyzed i n t h i s laboratory by Kanasewich (1962a). Their isotopic compositions are s i m i l a r to those obtained i n the present study for the Blue Star and Lakeshore samples (Figure 3-2, Table 3-3). There are, however, s i g n i f i c a n t P b 2 0 7/Pb 2 0 l t variations among these seven samples. 61 It seems l i k e l y that the leads i n the Nelson rocks and the leads i n the associated orebodies (including Bluebell) have developed i n the same source rocks during the same time i n t e r v a l . The slope of the weighted least-squares l i n e through the 13 points i n Figure 3-2 (4 rock-leads, 4 ore-leads, 5 Blue-b e l l leads) i s 0.1028 ± 0.0094. This i s i n excellent agree-ment with the value obtained above (0.1031) when only the four, ore-leads were considered, and also with the value calculated from S i n c l a i r ' s data •( 0 .1022) . The P b 2 0 7/Pb 2 0 l t variations among the ore-leads are again observed i n the rock-lead data. The resultant scatter about the least-squares l i n e suggests that the leads contain an i s o t o p i c a l l y heterogeneous common lead component. Several authors (for example, Schofield, 1920 , p.36) have suggested that the ore deposits i n the Nelson area are genet-i c a l l y related to g r a n i t i c bodies. Irvine (1957, p.97), for ex-ample, concerning the o r i g i n of Bluebell ore states: "Ore deposition i s probably related to one or another of the g r a n i t i c bodies which surround the mine, but so far no direct evidence l i n k s the deposits to any one body". Lead isotope data now suggest that the Bluebell ore-leads and the Nelson rock-leads were derived from the same source rocks. Cairnes (1934) believed that mineral deposits i n the Slocan d i s t r i c t (for example, Victor and Scranton) were formed from ore solutions derived from the Nelson b a t h o l i t h . Hedley and Fyles (1956), however, point out that "no evidence has been 62 found that any one deposit i s genetically related to a s p e c i f i c part of the [Nelson] batholith". Lead isotope abundances suggest that i n d i v i d u a l deposits cannot be related to a s p e c i f i c part of the batholith on a geo-graphical basis. Por example, lead from Scranton mine i s i s o t o p i c a l l y s i m i l a r to rock-leads R3 and R9 (Table 3-3), but Scranton i s geographically closer to rock sample RIO which has a d i s t i n c t l y d i f f e r e n t isotopic composition. This re s u l t was not unexpected since a given ore deposit was not formed u n t i l the corresponding part of the batholith had consolidated, and therefore the ore-lead and nearby rock-lead probably did not originate i n the same region of the source rock system. Mineralization i n the Nelson area i s believed to have occurred shortly after the emplacement of the Nelson batholith (Hedley, 1952). The batholith was emplaced approximately 150 m i l l i o n years ago on the basis of the K-Ar age determinations recently carried out by Khanh ( i n preparation) on b i o t i t e s and hornblendes from the Nelson rocks. Prom equation (3-1) the time t i i s 1620 ± 170 m.y. when R = 0.1028 ± 0.009^ and t 2 = 150 m.y. Hence the source rocks of the anomalous leads are at least 1600 m i l l i o n years old. S i n c l a i r (1966) has suggested that either Lower Pur c e l l (Beltian) rocks or rocks of the Ch u r c h i l l geological province could have been the source of the radiogenic 63 component of Kootenay arc leads. He points out that Lower Pur c e l l strata are at least 1500 m i l l i o n years old since they are intruded by Moyie s i l l s , dikes, and stocks which have K-Ar ages of about 1500 m.y. (Hunt, 1961). Recently, however, Obradovich and Peterman (1967) have reported Rb-Sr whole-rock ages for B e l t i a n rocks from Montana which suggest that there were three d i s t i n c t episodes of sedimenta-t i o n at approximately 1300, 1100 and 900 m.y. ago. These sediments rest on a metamorphosed basement that has been dated at 1600-1850 m.y., apparently the time of major C h u r c h i l l orogeny (Goldich, et a l , 1966). I f the K-Ar age of 1500 m.y. for the Moyle Intrusions i s not accepted, then the maximum age of the Lower Pu r c e l l series may be only about 1300 m.y. Therefore, the source rocks of the anomalous leads may be the metamorphic rocks of the C h u r c h i l l province or perhaps a mixture of C h u r c h i l l and Lower Purcell rocks. A l l of the Kootenay arc leads contain a single radiogenic component developed between 1600 m.y. ago and 150 m.y. ago, possibly i n rocks of the Churchill province. Then, about 150 m.y. ago, ore deposits were formed north and south of the Nelson batholith that apparently contain mix-tures of t h i s radiogenic lead and Sullivan-type lead. At the same time the Nelson batholith and associated ore deposits were formed which, however, did not incorporate s i g n i f i c a n t amounts of Sullivan-type lead. They did incorporate a common lead component (perhaps derived from various mineral phases 64 i n the Chu r c h i l l rocks) that i s d i s t i n c t l y d i f f e r e n t i n isotopic composition from the Sullivan lead. Figure 3-2 shows clear evidence for a dire c t genetic association between the trace lead i n the batholith and the ores i n i t s immediate proximity. The i s o t o p i c composition of the common lead com-ponent i n the rocks of the batholith can be calculated i f i t i s assumed that t h i s lead i s the product of a single, closed system. The lower anomalous lead l i n e i n Figure 3-2 intersects the 1600 m.y. isochron at the point ( 1 6 . 0 5 , 15-47) which corresponds to a u-value of 8 . 8 5 . It i s possible, of course, that t h i s common lead component i s not a si n g l e -stage lead, but rather was produced i n a number of uranium-lead systems (for example, In reconstituted crustal systems). This anomalous lead l i n e intersects the locus of the Ostlc-Russell-Stanton stratiform ore-leads (u = 9 . 0 ) at a time greater than 2000 m.y. Hence, any multi-stage model based on t h i s growth curve requires leads older than 2000 m.y. The presence of rocks of t h i s age i n the Nelson area appears less l i k e l y than the presence of rocks with an average age of about 1600 m.y. Therefore, the existence of a 1600 m.y. old primary lead i s postulated. The scatter about the lower anomalous lead l i n e (Figure 3 -2) may be due to small v a r i a -tions i n the iso t o p i c composition of the 1600 m.y. old common lead. It may be, however, that some of the scatter i s also due to contamination of these rocks and ores by small amounts 6 5 of Sullivan-type lead. Summary and Preferred Interpretation A l l of the Kootenay arc leads contain a radiogenic component which developed between l 6 0 0 m.y. ago and 1 5 0 m.y. ago. At least some of the ore-leads north and south of the Nelson batholith are genetically related to Sullivan-type primary lead (y = 9 . 0 ) . On the other hand, rock-leads from the Nelson batholith and ore-leads from deposits closely associated with the batholith contain a d i f f e r e n t common lead component. This common lead could be derived from a s i n g l e -stage system characterized by a y-value of about 8 . 8 . The analyses reported i n t h i s thesis provide, for the f i r s t time, clear evidence of a genetic relationship between ore deposits and g r a n i t i c rocks. Incomplete mixing of lead isotopes occurred when the Nelson batholith formed. West-Central New Mexico The i s o t o p i c r a t i o s of the Bosque rock-lead are given i n Table 3 - 5 along with sample descriptions and i s o -topic abundances for the 8 ore-leads studied by Blenklnsop and Slawson ( 1 9 6 7 ) . These data are plotted i n F i g u r e '3-4 • The slope o f the weighted l e a s t - s q u a r e s l i n e ( Y o r k , 1 9 6 6 ) through 7 of the ore-leads (omitting the Bosque sample) TABLE 3-5 ISOTOPIC COMPOSITION OF NEW MEXICO LEADS Sample 501 La Bonlta 504 Hansonburg 502 Box Canyon 508 Kelly 507 Linchburg 532 Orogrande 530 Modoc 505 Bosque del Apache ore-lead Bosque del Apache rock-lead Sample Description vein deposit i n Precambrian granite deposit associated with s i l l c i f i e d limestone vein deposit i n f a u l t contact of Precambrian granite with Paleozoic limestone vein and replacement deposits i n limestone adjacent to a stock contact pyrometasomatic deposit i n limestone Observed Isotopic Abundances* Pb 2 0 6/Pb 2 0 J t Pb 2 0 7/Pb 2 0 l t Pb 2 0 8 ^ 2 0 " contact pyrometasomatic deposit 2 5 . 2 7 2 2 . 2 6 20 .64 18.74 1 8 . 5 5 1 8 . 9 2 1 8 . 2 1 16.19 potassium feldspar from granite, -v 20 ppm lead 24 .60 16.34 16 .05 1 5 . 8 9 15.73 15.74 15.74 1 5 . 6 8 15.46 16 .01 42.53 40.50 39 . 8 0 38.82 38.72 39-19 39.07 36.03 41.31 * 0 r e data from Blenkinsop and Slawson (1967) 67 i n Figure 3-4 i s 0 . 0 9 2 3 * 0 . 0 0 2 1 . I f the time of anomalous lead mineralization, t 2 , i s 30 m.y. (on the basis of K-Ar ages by Weber and Bassett, 1 9 6 3 ) , the time t x from equation ( 3-D i s 1480 ± 40 m.y. This value for the age of the source rocks of the anomalous leads i s i n good agreement with con-cordant U/Pb ages of 1450 m.y. recently reported by T i l t o n and Grunenfelder (1967) for rocks from the Sandia Mountains to the northeast. The slope of the l i n e i n Figure 3-4 i s 0 . 0 9 4 1 * 0 . 0 0 2 2 i f the Bosque sample i s included i n the c a l c u l a t i o n . Therefore, a genetic relationship between the Bosque sample and the other ore-leads Is not precluded despite the fact that the "mineralogy and appearance of t h i s deposit suggest a d i s t i n c t l y d i f f e r e n t condition of mineralization from that which resulted i n the other deposits sampled" (Slawson and Austin, 1 9 6 2 ) . The single-stage model age and the apparent y-value calculated for the Bosque ore-lead are 1500 m.y. and 8 . 8 respectively. I f the Bosque sample i s not considered, the least squares l i n e i n Figure 3-4 intersects the primary isochron ti = 1480 m.y. at a. point which cor-responds to a y-value of approximately 8 . 8 ( 5 ) . Hence, i f the lead i n the source rocks 1480 m.y. ago was the product of a single-stage system, t h i s system i s characterized by a y-value lower than 9 . 0 , the value associated with certain stratiform ores. The Bosque rock-lead and the Bosque ore-lead are FIGURE 3-4 ISOTOPIC ABUNDANCES OF NEW MEXICO LEADS o 41 p D 2 0 8 P F 2 0 1 17 P b 2 0 7-P b 2 0 t t 37 •16 O o o o CP 20 Pb 2 0 6/Pb 2 0 t t 24 ' t i = 1480 m.y O ore-lead data from Blenkinsop and Slawson (1967) (Q . 0 9 2 3 ) Bosque rock-lead = 9 . 0 Bosque | ore-lead 17 19 21 p b 2 0 S / p h 2 0 t 23 25 69 very d i f f e r e n t i n isotopic composition. In addition, the rock-lead does not plot on the anomalous lead l i n e i n Figure 3-4; hence i t s isotopic composition cannot be explained by the two-stage model postulated above for the ore-leads. It i s , however, not possible to preclude the p o s s i b i l i t y that the Bosque ore-lead Is genetically related to the Bosque rock-lead; that i s , that they were derived from the same source material. The isotopic composition of the rock-lead was subsequently altered by the addition of large quantities of radiogenic lead from the crust, while the ore-lead appears to be r e l a t i v e l y uncontaminated by crustal lead. It i s noteworthy i n t h i s regard that the Bosque rock-lead and the ore-lead i n a nearby vein deposit (La Bonita, 501) are similar i n isotopic composition (Table 3-5)• The New Mexico ore-leads that were studied by Blenkinsop and Slawson are genetically related to a 1500 m.y. old common lead. I f interpreted by a single-stage model, t h i s lead has an isotopic composition similar to that of the Bosque ore and i s characterized by a y-value lower than the one associated with the stratiform ores studied by Ostic. The Bosque rock-lead does not belong to the above suite of ore-leads. However, the true significance of the isotopic abundances of t h i s lead cannot be determined without analyses of additional samples. 70 Broken H i l l , A u s t r a l i a The lode pegmatites at Broken H i l l appear concor-dant with the orebodies and with the surrounding gneisses and are, i n general, closely associated with the main lode galena. It i s therefore very d i f f i c u l t to obtain potassium feldspar separates that are e n t i r e l y free of ore-lead. The feldspar analyzed i n the present study was f i r s t hand-picked from cleavage fragments and ground to a very fine powder i n an agate mortar. The sample was then washed and the K-feldspar was floated i n an acetone-bromoform solution. This feldspar was subsequently leached for 24 hours i n warm 6 N HC1. Since at t h i s stage a microscopic examination revealed no v i s i b l e galena, the remaining lead In the sample was separated by the v o l a t i l i z a t i o n technique and analyzed on the mass spectrometer. The sample contained approximately 2000 ppm lead of i s otopic composition i d e n t i c a l to that of the main lode galena (Table 3 - 6 ) . This res u l t was not unexpected since previous authors (Andrews, 1 9 2 2 ; S t i l l w e l l , 1959) have suggested that a close genetic relationship exists between the lode pegmatites and the orebodies. Rb-Sr analyses by Pidgeon (1967) of whole rocks and potassium feldspars from the lode pegmatites give an approximate age of 1650 m.y. for these rocks (A = 1 .39 x 1 0 - 1 l y , - 1 ) } i n good agreement with the model lead age of 1600 m.y. 71 TABLE 3-6 OBSERVED ISOTOPIC COMPOSITION OF BROKEN HILL LEADS Sample Description 1* Main lode galen? 70 ore-leads from 45 Thackaringa-68 type deposits 67 69 66 65 Mundi Mundi K-feldspar granite BH6, BH9 K-feldspar from Upper granite gneiss Lode pegmatite K-feldspar Pb 20 6 p b207 P b 2 0 8 Lead Pb 20k pb204 Pb 2 0 1 + ppm 16 .116 15 .542 36.068 17 .40 15.69 38.54 17 .81 15.71 38.62 17 .89 15.72 38.87 17 .95 15.74 39.09 17 .97 15-75 38.93 18 .02 15.75 38.93 19 .55 15.96 39 .85 39 .709 18.221 51-967 15 17 .029 15.642 37-328 30 16 .108 15 .542 36.011 2000 * Isotopic Abundances from K o l l a r , et a l (I960). **Isotopic Abundances from Russell, et a l (1961). / 7 2 Since the samples from the Mine Sequence gneisses were not d i r e c t l y associated with the lode material, routine mineral separation procedures were adequate. It was not possible for the writer to analyze the feldspar samples i n d i v i d u a l l y since only a limited amount of each ( 6 - 1 1 grams) was available. In order to obtain s u f f i c i e n t lead for one adequate mass spectrometer analysis, two samples from the Upper Granite Gneiss (BH 6 and BH 9 ) were combined and the isotopic abundances of the composite sample were determined. Sample BH 9 was collected from a bore core at a depth of 146 feet and at a location given by the mine coordinates 9 5 0 0 ' S, 1 0 0 0 ' E. BH 6 was co l l e c t e d at the surface at 9 5 0 0 ' S, 3 0 0 ' E . Hence the separation of the two samples i s only about 7 0 0 feet, a n e g l i g i b l e distance compared to the scale of the Broken H i l l mine area (Figure 3 - 5 ) . The isotopic r a t i o s for t h i s composite sample and for the Broken H i l l ore-leads are given i n Table 3 - 6 and are plotted i n Figure 3 - 6 . The feldspar lead plots on the anomalous lead l i n e defined by the ore-leads, or, i n other words, the lead from the gneiss has had a history s i m i l a r to that of the Thackaringa-type ores. The rock-lead i s therefore genetically related to lead of the main lode type and, i n addition, contains a radiogenic component generated i n source rocks between 1 6 0 0 m.y. ago and 5 0 0 m.y. ago. FIGURE 3-5 SURFACE GEOLOGICAL PLAN BROKEN HILL MINE AREA* AND APPROXIMATE SAMPLE LOCATIONS *from Lewis, Forward, and Roberts ( 1 9 6 5 ) FIGURE 3-6 ISOTOPIC ABUNDANCES OF BROKEN HILL LEADS Pb208 38 P b 2 0 4 P b 2 0 7 P b 2 0 4 1 6 -15 40 16 36 16 O o ore-lead data from R u s s e l l , et a l (1961) 17 18 206 /PK2 0 k 19 20 p B 2 0 6/p b ( .122) Upper Granite Gneiss main lode; lode pegmatite 17 18 p b206/p b20t 19 20 75 Pidgeon (1967) has reported a mean whole-rock Rb-Sr age of 1640 ± 40 m.y. (X = 1.39 x 1 0 _ 1 1 y . _ 1 ) for three of the gneiss rock units ( s i l l i m a n i t e , Potosi, Alma) from the Mine Sequence. The Alma Gneiss i s located about a mile away from the main lode but has suffered the same high-grade (granulite facies) metamorphism. In addition, Pidgeon has calculated an age of 1750 m.y. for the Upper Granite Gneiss. This age i s based on only four whole-rock samples and he believes that the Upper Granite may not be s i g n i f i c a n t l y older than the other three types. Pidgeon further suggests that the gneiss whole rock age of 1640 ± 40 m.y. gives the time of the high-grade metamorphism i n the Broken H i l l area and that "almost.complete r e d i s t r i b u t i o n of strontium i s o -topes occurred within a rock unit at t h i s time". In Palaeozoic time, these rocks were affected by a r e l a t i v e l y mild regional metamorphism as indicated by the 500 m.y. K-Ar and Rb-Sr ages reported by Richards and Pidgeon (1963) and by Pidgeon (1967). Pidgeon (1967) has, i n addition, interpreted the i n i t i a l S r 8 7 / S r 8 6 r a t i o s calculated for the gneiss units to give the o r i g i n a l age of sedimentation or emplacement of the ".Willyama rocks. The maximum age of deposition was estimated to be 1820 ± 100 m.y. However, th i s r e s u l t i s not very r e l i a b l e p r i n c i p a l l y because i t depends on the assumption that each of the gneiss units has remained a closed system with respect to rubidium and strontium since the time of o r i g i n a l deposition. It i s 76 therefore concluded that when a l l of the assumptions and inherent uncertainties i n the lead and strontium models are considered (for example, i n the age of the earth and i n the rubidium decay constant), there are no demonstrable d i f f e r -ences among the model lead age of 1600 m.y., the 164,0 m.y. whole rock Rb-Sr age of the gneisses and the 1820 m.y. maximum age of deposition. On the assumption that the one composite sample from the Upper Granite Gneiss i s broadly representative of the whole body, the re s u l t s from the present study suggest that the lead that was incorporated into these rocks at the time of t h e i r formation (approximately 1600 m.y. ago) had an isotopic composition very s i m i l a r to that now observed i n the main lode orebody. Radiogenic lead generated i n the uranium-rich phases of the gneiss was mixed with t h i s primary lead and incorporated into the potassium feldspars at the time of the 500 m.y. metamorphism. In s i t u uranium decay i n the potassium feldspars during the past 500 m.y. should be n e g l i g i b l e . The genetic relationship observed between trace lead i n the gneiss and the lead from the main lode suggests that the Broken H i l l orebody had a syngenetic o r i g i n . I f , for example, barren sediments were metamorphosed to form the gneiss, and the ore-lead was introduced at a l a t e r time, one would not expect to observe a large ore-lead component In the gneissic rocks unless s i g n i f i c a n t isotopic exchange 77 occurred between the gneiss and the ore solutions. The observed isotope ra t i o s for the potassium feldspar from the Mundi Mundi granite are given i n Table 3-6. Pb 2 0 7/Pb 2 0 l t and Pb 2 0 6/Pb 2 0 , + ratios are plotted i n Figure 3-7-The whole-rock Rb-Sr age for these granites i s 1520 * 40 m.y. (Pidgeon, 1967; X = 1.39 x 1 0 - 1 1 y . - 1 ) - , and on the basis of K-Ar and Rb-Sr b i o t i t e ages (Richards and Pidgeon, 1963; Pidgeon, 1967) these rocks were also disturbed by the mild regional metamorphism about 500 m i l l i o n years ago. FIGURE 3-7 ISOTOPIC COMPOSITION OF LEAD FROM THE MUNDI MUNDI GRANITE 20 P b 2 0 7 Pb 2 0 1* (1600, 1520 m.y.) 15 Main Lode, t = 1600 m.y. 20 30 40 p b2 0 6/pb2 0»t 78 A three-stage model Is postulated to explain the observed lead isotope abundances of t h i s sample. (The above two-stage model i s considered less probable since i t would not r e f l e c t the well established 1520 m.y. event.) The i n i t i a l stage began about 1600 m.y. ago i n a rock system that contained lead of the main lode type, uranium and thorium. The lead isotope r a t i o s grew i n t h i s system for about 80 m.y. to points along the l i n e defined by the two times 1600 and 1520 m.y. (Equation 3 - 1 , Figure 3 - 7 ) . Then, about 1520 m.y. ago the Mundi Mundi granite was formed. Radiogenic lead was produced by the decay of uranium and thorium i n the granite between the times 1520 m.y. ago and 500 m.y. ago, and was incorporated into the potassium f e l d -spars at the time of the 500 m.y. metamorphism. The t h i r d stage i n the development, the i n s i t u uranium decay i n the feldspars during the past 500 m.y., i s assumed to be incon-sequential. The precursor of the granite, according to the model i l l u s t r a t e d i n Figue 3 - 7 , apparently had an average y-value of approximately 1 0 0 . The average value i n the granite during the second stage was about 1 2 0 . The model proposed to explain the observed lead isotope abundances i n the Mundi Mundi granite i s consistent with information provided by previous lead isotope studies and by K-Ar and Rb-Sr geochronology. Since only one sample of granite was ava i l a b l e , analyses of samples from other l o c a l i t i e s are desirable. 79 It i s concluded from t h i s study that the lode pegmatites and the orebodies have had a common o r i g i n . Probably a l l of the g r a n i t i c and gneissic rocks within 20 miles of the main orebody contain a common lead component that i s i d e n t i c a l to the 1600 m.y. old lead of the main lode type. In addition, these rocks contain a radiogenic component generated between about 1600 m.y. ago and 500 m.y. ago. These two components were mixed when the rocks were mildly disturbed at the time of the 500 m.y. regional metamorphism. The above interpretation i s consistent with previous lead isotope, Rb-Sr and K-Ar studies. 80 CHAPTER 4 CONCLUSIONS The objective of this thesis was to provide de-fi n i t i v e evidence relative to the existence.of systematic differences between the isotopic abundances of lead from rocks and lead from certain stratiform ores. Interlaboratory , differences and large analytical uncertainties have pre-viously precluded a clear answer to this question. Using an identical experimental technique for both rocks and ores, the writer has analyzed samples from Balmat (New York), Nelson (British Columbia), Broken 'Hill (New South Wales), and from West-Central New Mexico. The precision of analyses (approximately 0 . 1 5 per cent at the 95 per cent confidence level) was sufficient to resolve the differences in question. A clear separation of rock-lead and ore-lead patterns was resolved in the cases of Balmat and Nelson. In both local-i t i e s , some ores were apparently derived from a primary system for which the calculated present-day value of the U 2 3 8/Pb 2 0 l f ratio (the p-value) is 9 . 0 . This agrees identically with the value for the stratiform deposits studied by Ostic, et al (1967) . However, a different primary system with a u-value of 8.7 to 8 . 8 5 is required to explain the observed abundances of feldspar leads from Balmat and Nelson and ore-leads from New Mexico. , • •.T~h,e^$4^ about five times the analytical uncertainty, and must Do 81 explained by any v a l i d geophysical model. However, the absolute numerical values quoted may be two or three per cent high because no correction for mass spectrometer d i s -crimination has been made. For a gas-source mass spectro-meter, the discrimination can be estimated from the laws of gas flow, and i s constant i f operating conditions are reproduced. A l l of the rock-leads and ore-leads from the Broken H i l l d i s t r i c t r e f l e c t only the existence of the higher u system. Lead from t h i s system (that i s , i s o t o p i c a l l y of the main lode type) was apparently incorporated into the g r a n i t i c and gneissic rocks at the time of t h e i r formation. Lead c h a r a c t e r i s t i c of the lower u system was not detected, even in the case of rocks 20 miles distant from the main orebody. For each of the Balmat, Nelson, and New Mexico suites, i t i s assumed that a two-stage model adequately describes the observed isotopic v a r i a t i o n s . (For the f e l d -spar leads, a t h i r d stage corresponding to i n s i t u uranium decay i s neglected.) In other words, i t i s assumed that the observed anomalous lead l i n e s can be interpreted as the addition of radiogenic lead to lead produced i n a si n g l e , closed environment. Adequate independent age information from these three areas might help to v e r i f y the above model as i t has i n the case of the Broken H i l l leads. However, the generally good f i t of the data to the anomalous lead lines suggests that the two-stage model i s a reasonable 82 approximation. Since the calculated y-values are rather i n s e n s i t i v e to small deviations from the assumed model, they are unlikely to be s i g n i f i c a n t l y i n error. It i s therefore the conclusion of t h i s thesis that the primary lead system required to explain the observed lead isotope abundances of certain stratiform ore deposits i s not applicable to rock-leads and to ore-leads In general. Apparent y-values for the stratiform deposits studied by Ostic, et a l (1967) are given i n Table 4-1 along with the values obtained i n the present study for the ore-leads from Balmat, N.Y. and from the Sullivan mine, B.C. The average y-value i s 8.96 ± 0.13. There i s evidence to suggest, however, that the samples from Manitouwadge and from Read Rosebery have been s i g n i f i c a n t l y contaminated by radiogenic c r u s t a l lead (Ostic, et a l , 1967). I f these samples are omitted from the c a l c u l a t i o n , the average y i s 9.00 ± 0.05. It i s perhaps s i g n i f i c a n t that while the average y-value for these deposits i s s i g n i f i c a n t l y higher than the average obtained i n t h i s study for rock-leads (that I s , about 8.75), the range i n values for each group (8.70 - 8.85 for the rock-leads and 8.93 - 9-09 for the stratiform ore-leads) suggests that the two d i s t r i b u t i o n s may overlap. An acceptable geophysical model must, however,explain the apparent existence of two d i s t i n c t d i s t r i b u t i o n s of y-values. Ostic, et a l (1967), following the e a r l i e r work of Stanton and Russell (1959), suggest that the leads from 83 TABLE 4-1 APPARENT URANIUM/LEAD RATIOS FOR LEADS FROM' CERTAIN STRATIFORM DEPOSITS* Sample u = ( U 2 3 8 / P b 2 0 M t = 0 Mount Isa Sul l i v a n mine Cobar, C.S.A. Bathurst Balmat Broken H i l l Captain's Flat Cobar, lower horizon Hall's Peak Read Rosebery Manitouwadge 9-09 9-05 9 .00 9 .00 9 .00 8.98 8.98 8.97 8.93 8.93 8.61 Mean ± std dev. 8.96 ± 0.13 a l l samples 9.00 ±0.05 excluding Manitouwadge, Read Rosebery * A 1 1 data, except Balmat-and Su l l i v a n from Ostic, et a l ( 1 9 6 7 ) . 84 these stratiform deposits originated i n a primary system that i s probably subcrustal. The leads are transported to the crust by isla n d arc vulcanism and deposited syn-genetically i n associated sediments without s i g n i f i c a n t contamination by radiogenic leads from cr u s t a l rocks. Recent geophysical observations have given strong support to theories that postulate a spreading of the sea flo o r s as a r e s u l t of convection currents i n the upper mantle (see, for example, Vine, 1967)• Armstrong (1967) has suggested a steady-stage model based on these ideas to explain the evolution of lead isotope ra t i o s i n the earth. In t h i s model, s i a l i c material continuously eroded from the continents forms oceanic sediments that are subsequently carried down into the mantle by means of convection currents. This down-welling of c r u s t a l material takes place along volcanic isla n d arcs and marginal trenches. Por example, Oliver and Isacks ( 1967) have shown from seismological studies that i n the Tonga-Fiji region intermediate and deep earth-quake f o c i occur i n a section of the lithosphere (about 100 km thick) that has been thrust 600-700 km into the mantle. Armstrong suggests that a degree of isotopic e q u i l i b r a t i o n takes place between t h i s c r u s t a l section and the upper mantle before the material i s returned to the surface i n the form of volcanic matter. Crustal material has been continuously r e c y c l e d i n t h i s manner through the upper mantle, and the continental volumes have therefore remained constant since 85 the time very early i n the earth's history (probably less than 100 m.y. af t e r formation) when d i f f e r e n t i a t i o n Into core, mantle, and crust was completed. The continents themselves, however, are frequently broken up, moved and fused together by the convection currents In the mantle. Armstrong defines the upper mantle as the mantle above 500 km, the depth at which convective mixing ceases. Recent calculations on the v i s c o s i t y of the mantle by McConnell ( 1 9 6 7 ) suggest that con-vection must be n e g l i g i b l e below about 200 km. This correc-t i o n , i f v a l i d , w i l l not, however, have a bearing on the q u a l i t a t i v e aspects of the Armstrong model. From abundance data for various rock types believed to be representative of c r u s t a l and upper mantle material, Armstrong has calculated an apparent p-value of 8.5 for the mixed crust-plus-upper-mantle system. This value, derived from only a semi-quantitative model, Is i n remarkably good agreement with values determined by lead isotope methods. Therefore, such a long-term mixing of crustal and upper mantle material appears to be a possible mechanism to explain the evolution of lead isotopes i n the earth. The rather special status of certain stratiform ore deposits i s not, however, immediately explained by t h i s mixing model. Armstrong ( 1 9 6 7 ) believes that the lead from these deposits represents the "best mixed crust plus upper mantle lead". It i s of interest to point out here that ore-leads from South A f r i c a , recently analyzed i n this laboratory 86 by J. Blenkinsop and reported by Ulrych, et a l (1967), apparently originated i n a primary system with a p-value of about 9.5. This value, when averaged with the lower value obtained i n the present study for rock-leads (= 8.8) gives some support to t h i s 'best mixed' model for stratiform ores. (It should be noted that p-values calculated for these very old, > 3000 m.y., South African leads are sensitive to the uncertainties i n the values of the primeval r a t i o s , a 0 and b Q.) Armstrong considers that the lead In modern pelagic sediments (including manganese nodules) plots on an extension of the stratiform ore-lead growth curve (y = 9.0). This lead, produced by the chemical weathering of continental crustal rocks, i s well mixed and transported to marine sedi-ments by sea water. It therefore approximtes an average crustal-plus-upper-mantle lead according to the Armstrong model. Isotopic analyses of lead i n pelagic sediments have been-reported by Chow and Patterson (1959, 1962), but u n t i l more precise analyses of such samples are made and i n t e r -laboratory differences removed, one cannot be sure that they l i e on an exact extension of the stratiform growth curve. Therefore, the mixing model as proposed by Armstrong to account for the o r i g i n of these stratiform deposits requires further experimental v e r i f i c a t i o n . I f a 'best mixed' model for these leads i s rejected, then either a 'preferentially mixed model' or a 'homogeneous 87 source' model must be considered. Ostic, et a l (1967) propose the l a t t e r and suggest a subcrustal source. This model would be rejected on the basis of the abundance data compiled by Armstrong since the y-value for the upper mantle alone (= 6 . 4 ) i s much too low. However, Armstrong's model for the upper mantle i s based on the observed abundances of uranium and thorium i n oceanic t h o l e i i t e s , and Ulrych ( i n press) has argued that these rocks are the result of a f a i r l y recent d i f f e r e n t i a t i o n ( i n general, less than 250 m.y. ago), and hence are not t r u l y representative of upper mantle material. Therefore, the homogeneous source model cannot be ruled out. A p r e f e r e n t i a l but thorough mixing i n an island arc, marginal trench environment of s i a l i c c r ustal material (y = 1 0 . 4 ; Armstrong, 1967) with material from the upper mantle also appears to be a possible mechanism capable of producing the observed abundances of the stratiform ores. It i s of intere s t to note that lead from White Island, New Zealand has an apparent y-value of 8 . 8 9 (Ostic, et a l , 1 9 6 7 ) • White Island i s an active andesitic volcano located on the margin of a trench that i s associated with many of New Zealand's deep-focus earthquakes. This possible mixture of stratiform ore-lead (y = 9 . 0 ) and g r a n i t i c rock-lead (y = 8 . 8 ) can be interpreted to give support to the above mixing model. Zartman (personal communication, 1967) has recently suggested a model that attempts to explain the observed 88 abundances of 1000 m.y. old leads solely on the basis of c r u s t a l phenomena. In terms of t h i s model, a more or less complete melting of c r u s t a l material down to a depth of about 35 km occurred at t h i s time and the lead isotope ratios were p a r t i a l l y homogenized. Zartman would produce the s t r a t i -form ore deposits by p r e f e r e n t i a l melting and homogenization of the upper r e l a t i v e l y uranium-rich sections of t h i s c r u s t a l block. Previous authors (Alpher and Herman, 1951; Shaw, 1957) have agreed that an e f f i c i e n t mixing of continental material can provide the apparently uniform source that i s required to explain the observed abundances. Calculations carried out by Russell, et a l (1966) emphasize the fact that a very thorough mixing of heterogeneous crustal systems i s required i n order to produce leads that can be interpreted by means of a single-stage model.. It i s clear from the above discussion that there exists a wide choice of possible models and mechanisms to explain the evolution, of lead isotopes in the earth. On the basis of data presently a v a i l a b l e , none of these appear to be e n t i r e l y s a t i s f a c t o r y . The contribution of t h i s thesis i s to prove for the f i r s t time that there i s a s i g n i f i c a n t difference between the primary lead system associated with certain stratiform ore deposits and the one apparently associated with g r a n i t i c rocks, and to suggest that t h i s finding has important geophysical consequences. 89 BIBLIOGRAPHY Alpher, R.G. and R.C. Herman. The primeval lead isotope abun-dances and the age of the earth's crust. Phys. Rev., 84, 1111 ( 1 9 5 D Andrews, E.C. Geology of the Broken H i l l d i s t r i c t . Mem. Geol. Surv. N.S.W., 8, 1 (1922) Armstrong, R.L. A model for Sr and Pb isotope evolution i n a dynamic earth. Privately d i s t r i b u t e d manuscript, Geology Department, Yale University (1967) Baskova, Z.A. and G.I. Novikov. The evolution of small amounts of Pb by a reducing ca l c i n a t i o n i n vacuum. Geochemistry, 678 (1957) Blenkinsop, J . and W.P. Slawson. Geophysical evidence of the Zuni Lineament. Earth and Planetary Science Letters, accepted for publication. B o n e l l i , E.J. and H. Hartmann. Determination of lead a l k y l s by gas chromatography with the electron capture detector. Paper presented to Gulf Coast Spectroscopic Group, Baton Rouge, Louisiana (1963) Cairnes, C.E. Slocan mining camp, B r i t i s h Columbia. Geol. Surv. Can. Mem. 173 (1934) Carruthers, D.S. and R.D. Pratten. Stratigraphic succession and structure i n Zinc Corp. Ltd. and New Broken H i l l Consolidated Ltd., Broken H i l l , N.S.W. Econ. Geol., 5 6 , 1088 (1961) Catanzaro, E.J. Triple-filament method for solid-sample lead isotope analysis. J. Geophys. Res., 7_2, 1325 (1967) Chow, T.J. and C C . Patterson. Lead isotopes i n manganese nodules. Geochim. et Cosmochim. Acta, 1_7, 21 (1959) Chow, T.J. and C C . Patterson. The occurrence and significance of lead isotopes i n pelagic sediments. Geochim. et Cosmo-chim. Acta, 2(5 , 263 (1962) Cooper, J.A. and J.R. Richards. Lead isotopes and volcanic magmas. Earth and Planetary Science Letters, 1_, 259 (1966) Doe, B.R. Relationships of lead isotopes among granites, peg-matites, and su l f i d e ores near Balmat, N.Y. J. Geophys. Res., 6 7 , 2895 (1962) 90 Doe, B.R. D i s t r i b u t i o n and composition of sulf i d e minerals at Balmat, New York. Geol. Soo. Amer. Bull., 73., 833 (1962a) Doe, B.R., T i l t o n , G.R. and C.A. Hopson. Lead isotopes i n feldspars from selected g r a n i t i c rocks associated with regional metamorphism. J. Geophys. Res., 70_, 1947 (1965) Doe, B.R. The bearing of lead isotopes on the source of gr a n i t i c magma. J. Petrol., 8, 51 (1967) Doe, B.R. and R.I. T i l l i n g . The d i s t r i b u t i o n of lead between coexisting K-feldspar and plagioclase. Amer. Mineral., 52, 805 (1967) Doe, B.R., Tatsumoto, M., Delevaux, M.H. and Z.E. Peterman. Isotope-dilution determination of 5 elements i n G-2 (granite), with a discussion of the analysis of lead. U.S. Geol. Surv. Prof. Paper 575B (-1967) Fair b a i r n , H.W., Hurley, P.M. and W.H. Plnson. I n i t i a l S r 8 7 / S r 8 6 and possible sources of g r a n i t i c rocks i n southern B.C. J. Geophys. Res., 6_9, 4889 (1964) Goldich, S.S., Lidiak, E.G., Hedge, C.E. and F.G. Walthall. Geochronology of the midcontinent region, United States. J. Geophys. Res., 71, 5389 (1966) Hedley, M.S. Geology and ore deposits of the Sandon area, Slocan mining camp, B r i t i s h Columbia. B.C. Dept. Mines Bull., 29 (1952) Hedley, M.S. and J.T. Fyles. L o c a l i z a t i o n of lead-zinc ore i n the Kootenay arc. Paper presented to P a c i f i c Northwest Regional Conference, Amer. Inst. Min. Eng., Seattle, Washington (1956) Hunt, G.H. The Purcell eruptive rocks. Ph.D. Thesis, Univ. of Alberta (1961) Irvine, W.T. The Bluebell mine. Structural geology of Canadian ore deposits, Can. Inst. Min. Met., 2_, 95 (1957) Kanasewich, E.R. Approximate age of tectonic a c t i v i t y using anomalous lead Isotopes. Royal Astron. Soo. Geophys. J., 7, 158 (1962) Kanasewich, E.R. Quantitative interpretations of anomalous lead isotope abundances. Ph.D. Thesis, Univ. of B.C. (1962a) 91 Khanh, N.K. M.Sc. Thesis, Univ. of B.C.(in preparation) Khlopin, V.G. Determination of isotopic composition of lead i n rocks. Doklady Akademii Nauk, SSSR, 111, 395 (1956) K o l l a r , F. The precise intercomparison of lead isotope r a t i o s . Ph.D. Thesis, Univ. of B.C. (I960) Ko l l a r , F., Russell, R.D. and T.J. Ulrych. Precision i n t e r -comparisons of lead isotope r a t i o s : Broken H i l l and Mount Isa. Nature, 187, 754 (i960) Leech, G.B. and R..K. Wanless. Lead-isotope and potassium-argon studies i n the East Kootenay d i s t r i c t , B r i t i s h Columbia. Geol. Soc. Amer., Buddington Volume, 241 (1962) Lewis, B.R., Forward, P.S. and J.B. Roberts. Geology of the Broken H i l l lode, reinterpreted. Geology of Australian  Ore Deposits, 1, Eighth Commonwealth Mining and Metal-l u r g i c a l Congress, 319 (1965) L i t t l e , H.W. Nelson map-area, west h a l f , B r i t i s h Columbia. Geol. Surv. Can. Mem., 308 (I960) Marshall, R.R. and D.C. Hess. Lead from some stone meteorites. J. Chem. Phys., 2J3, 1258 (1958) Masuda, A. Experimental method for determination of isotopic composition of lead i n volcanic rocks. Earth S c i . Nagoya Univ., 10, 117 (1962) McConnell, R.K. J r . Earth's equatorial bulge and v i s c o s i t y of the mantle. (Abstract) Trans. Amer. Geophys. Union, 48, 212 (1967) Murthy, V.R. and C C . Patterson. Lead isotopes i n ores and rocks of Butte, Montana. Boon. Geol., 56, 59 (1961) Murthy, V.R. and C C Patterson. Primary isochron of zero age for meteorites and the earth. J. Geophys. Res., 67, 1161 (1962) Obradovich, J.D. and Z.E. Peterman. Geochronology of the Belt Series, Montana. Paper presented to conference on geo-chronology of Precambrian s t r a t i f i e d rocks. Geology De-partment, Univ. of Alberta, Edmonton (1967) Oliver, J . and B.L. Isacks. Some evidence on the structure of the mantle near an island arc. (Abstract) Trans. Amer. Geophys. Union, 4_8, 219 (1967) 92 Ostic, R.G. Isotopic inv e s t i g a t i o n of conformable lead de-pos i t s . Ph.D. Thesis, Univ. of B.C. (1963) Ostic, R.G., Russell, R.D. and R.L. Stanton. Additional measurements of the isotopic composition of lead from stratiform deposits. Can. J. of Earth Sciences, 4_, 245 (1967) Patterson, C.C., S i l v e r , L. and C. McKinney. Lead isotopes and magmatic d i f f e r e n t i a t i o n . (Abstract) Intern. Geol. Congr., 2 0 , 221 (1956) Pidgeon, R.T. A rubidium-strontium geochronologlcal study of the Willyama Complex, Broken H i l l , A u s t r a l i a . J. Petrol., 8 , 283 (1967) Richards, J.R. and R.T. Pidgeon. Some age measurements on micas from Broken H i l l , A u s t r a l i a . J. Geol. Soc. A u s t r a l i a , 1 0 , 243 (1963) Richards, J.R. Lead isotopes at Dugald River and Mount Isa, A u s t r a l i a . Geochim. et Cosmochim. Acta, 3_1, 51 (1967) Russell, R.D. and R.M. Parquhar. Lead Isotopes i n Geology, Interscience Publishers Inc., New York (I960) Russell, R.D., Ulrych, T.J. and P. K o l l a r . Anomalous leads from Broken H i l l , A u s t r a l i a . J. Geophys. Res., 6 6 , 1495 (1961) Russell, R.D., Kanasewich, E.R. and J.M. Ozard. Isotopic abun-dances of lead from a "frequently-mixed" source. Earth and Planetary Science Letters, 1_, 85 (1966) Russell, R.D., Slawson, W.P., Ulrych, T.J. and P.H. Reynolds. Further applications of concordia plots to rock lead isotope abundances. Submitted for publication. Schofield, S.J. Geology and ore deposits of the Ainsworth mining camp, B r i t i s h Columbia. Geol. Surv. Can. Mem., 117 (1920) Shaw, D.M. Comments on the geochemlcal implications of lead isotope dating of galena deposits. Econ. Geol., 52. > 570 (1957) S i n c l a i r , A.J. A lead Isotope study of mineral deposits i n the Kootenay,arc. Ph.D. Thesis, Univ. of B.C. (1964) S i n c l a i r , A.J. Anomalous leads from the Kootenay arc, B r i t i s h Columbia. C.I.M. Special Volume No. 8 , 249 (1966) 93 Slawson, W.F. and C F . Austin. A lead isotope study defines a geological structure. Econ. Geol., 5 7 , 21 (1962) S o r r e l l , CA. S o l i d state formation of Ba, Sr, and Pb feldspars i n clay-sulfate mixtures. Amer. Mineral., kj_, 291 (1962) Stanton, R.L. and R.D. Russell. Anomalous leads and the em-placement of lead su l f i d e ores. Econ. Geol., 5_4, 588 (1959) Stanton, R.L. General features of the conformable " p y r i t i c " orebodies. Trans. C.I.M., 6_3, 22 ( I 9 6 0 ) Starik, I.E., Sobotovich, E.V., Lovtsyus, G.P., Lovtsyus, A.V. and G.V. Avdzeiko. Modes of lead occurrence i n nature. Geochemistry, 683 (1957) S t i l l w e l l , F.L. Petrology of the Broken H i l l lode and i t s bearing on ore genesis. Proc. Aust. Inst. Min. Met., 1 9 0 , 1 (1959) Tatsumoto, M. Isotopic composition of lead i n volcanic rocks. J. Geophys. Res., 7 1 , 1721 (1966) T i l t o n , G.R. and M.H. Grunenfelder. Isotopic lead ages of sphene. (Abstract) Trans. Amer. Geophys. Union, 4_8_, 2^3 (1967) Ulrych, T.J. The preparation of lead tetramethyl for mass spectrometer analysis. M.Sc. Thesis, Univ. of B.C. (I960) Ulrych, T.J. Gas source mass spectrometry of trace leads from Sudbury, Ontario. Ph.D. Thesis, Univ. of B.C. (1962) Ulrych, T.J. and P.H. Reynolds. Whole-rock and mineral l e a d s from the Llano U p l i f t , Texas. J. Geophys. Res.,, 7J-, 3089 (1966) Ulrych, T.J., Burger, A. and L.O. Nicolaysen. Least radiogenic t e r r e s t r i a l leads. Earth and Planetary Science Letters, 2 , 179 (1967) Ulrych, T.J. Oceanic basalt leads: a new int e r p r e t a t i o n and an independent age for the earth. Science, accepted for publication. Vine, F.J. Spreading of the ocean f l o o r : new evidence. Science, 1_5_4, 1405 (1966) Weber, R.H. and W.A. Bassett. K-Ar ages of Tertiary volcanic and int r u s i v e rocks i n Socorro, Catron, and Grant Counties, New Mexico. New Mexico Geol. Soc. Guidebook, Fourteenth F i e l d Conference, 220 (1963) 9 4 Weichert, D.II. D i g i t a l a n a l y s i s of mass s p e c t r a . Ph.D. Thesis, Univ. of B.C. ( 1 9 6 5 ) Weichert, D.H., Russell, R.D. and J . Blenkinsop. A method for d i g i t a l recording for mass spectra. Can. J. Phys., 4_5, 2609 (1967) Whittles, A.B.L. Trace lead isotope studies with gas source mass spectrometry. Ph.D. Thesis, Univ. of B.C. (1964) York, D. Least-sauares f i t t i n g of a straight l i n e . Can. J. Phys., 4 4 , 1079 ( 1 9 6 6 ) Zartman, R.E. Rb-Sr age of some metamorphic rocks from Llano U p l i f t , Texas. J. Petrol., 6 , 28 (1965) Zartman, R.E. Isotopic composition of lead i n microcllnes from Llano U p l i f t , Texas. J. Geophys. Res., 7_0_, 965 (1965a) 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0053358/manifest

Comment

Related Items