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Study of lead isotopes from mineral deposits in southeastern British Columbia and from the Anvil range,… LeCouteur, Peter Clifford 1973

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A STUDY OF LEAD ISOTOPES FROM MINERAL DEPOSITS I N SOUTHEASTERN BRITISH COLUMBIA AND FROM THE ANVIL RANGE, YUKON TERRITORY by PETER CLIFFORD LeCOUTEUR B.Sc, University of Auckland, 1964 M.Sc.(Hons), University of Auckland, 1967 A THESIS SUBMITTED IN THE REQUIREMENTS DOCTOR OF PARTIAL FULFILMENT OF FOR THE DEGREE OF PHILOSOPHY i n the Department o f Geological Sciences We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January, 1973 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of C ^ p ^ o ^ e _ c J L The University of British Columbia Vancouver 8, Canada Date <23^A Kc^rJL l I l X i ABSTRACT The objective of the research was to determine the source and age of emplacement of lead i n mineral deposits in three regions of the Western Cordillera — the East Kootenay d i s t r i c t , the adjoining Kootenay Arc, and the Anvil Range. Measurements were made of the isotopic composi-tion of lead i n 132 samples by gas source techniques and a precision higher than ± 0.16% (2a) was obtained for a l l isotope ratios relative to Pb20**. In the interpretation of these analyses, the reasonable, but unproved, view that "primary" lead i s well-mixed lead from upper crustal rocks was accepted as a working hypothesis. Consequently, correlations of isotopic data with geological occurrence, particularly with stratigraphy, were sought. Galena deposits in the East Kootenay d i s t r i c t f a l l into two distinct isotopic groups; a uniform group and a variable, more radiogenic a group. Deposits of the uniform group, including many of the large deposits and a l l those of stratiform type, are restricted to lower Purcell (Aldridge Formation) rocks and seem to represent a widespread episode of lead-zinc mineralisation 1.2 to 1.4 BY ago. A possible source of this lead i s the Aldridge Formation i t s e l f , and a possible agent of extraction is connate brine. On the assumption that Aldridge sediments were the source, calcula-tions indicate an age of about 2.6 BY for the provenance of these sediments. In contrast to the uniform leads, the variable leads are found i n small veins throughout the Purcell sequence, and probably were emplaced i n Mesozoic or Cenozoic times. The isotopic data suggest that these deposits represent lead scavenged from Purcell rocks, perhaps by f l u i d restricted i i mostly to fracture systems. Lead isotope analyses reported for other parts of the Belt-Purcell basin are similar to those presented here, permitting speculations on the evolution of lead isotopes over a very large region. For Kootenay Arc deposits, a close correspondence can be demonstrated, between lead isotope compositions and geological charac-t e r i s t i c s . Structurally concordant deposits, mostly in Cambrian carbonate rocks, d i f f e r i n lead isotope composition from transgressive deposits. Other workers have suggested on geological grounds that the concordant deposits are significantly older (100 to 500 MY older) than those of transgressive type, and the isotopic data are consistent with this suggestion. Pb 2 0 6/Pb 2 0 1* ratios of samples from Slocan City and Sandon camps f i t a simple concentric zonal pattern, perhaps related to leaching of lead from country rocks by r i s i n g ore-fluid. Lead isotope compositions of four similar stratiform deposits in the Anvil Range area are nearly identical and approximate "primary" leads. The lead may have been derived from the late Proterozoic or Cambrian host rocks at the onset of Cambrian-Ordovician? metamorphism. In conclusion, the writer has demonstrated some close correla-tions between lead isotope data and observable ( i . e . shallow, crustal) geological features. These correlations are regarded as supporting, but not demanding, the conclusion that i n a l l three areas studied the lead (including "primary" leads) is of shallow crustal origin. i i i TABLE OF CONTENTS ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGMENTS CHAPTER 1. INTRODUCTION 1.1 OBJECTIVES OF THIS STUDY 1.2 LEAD ISOTOPE AND GEOLOGICAL MODELS FOR THE GENESIS OF LEAD-ZINC ORES CHAPTER 2. LEAD ISOTOPES FROM THE EAST KOOTENAY DISTRICT, BRITISH cowmiA 2.1 INTRODUCTION 10 2.2 AN OUTLINE OF THE GEOLOGY OF THE EAST KOOTENAY DISTRICT AND ADJOINING PARTS OF IDAHO AND MONTANA 10 Stratigraphy 10 Igneous Activity 15 Regional Metamorphism, Folding and Faulting 16 Geochronology 19 Lead-Zinc Mineralisation 21 2.3 PREVIOUS LEAD ISOTOPE WORK 23 2.4 DISCUSSION OF NEW LEAD ISOTOPE DATA 24 Late Precambrian Leads 27 The mantle as a source 27 The crust as a source 32 The age of the source 41 Mesozoic-Cenozoic Leads 50 i i i i v i v i i x i i 1 3 iv 2.5 COMPARISON WITH GALENA LEADS FROM BELT SUPERGROUP ROCKS IN NORTHWESTERN U.S.A. 52 Late Precambrian Leads 53 Pre-Ravalli leads 56 Ravalli and Piegan leads 62 Mesozoia-Cenozoia Leads 64 2.6 SUMMARY 67 CHAPTER 3. LEAD ISOTOPES FROM THE KOOTENAY ARC, BRITISH COLUMBIA 71 3.1 PREAMBLE 71 3.2 OUTLINE OF THE GEOLOGY AND LEAD-ZINC MINERALISATION OF THE KOOTENAY ARC 71 Geology 71 Lead-Zinc Mineralisation 74 Concordant deposits 75 Transgressive deposits 75 3. 3 PREVIOUS LEAD ISOTOPE STUDIES 76 3. 4 DISCUSSION OF NEW LEAD ISOTOPE DATA 81 Concordant Group 81 Transgressive Group 88 Evolution of Lead Isotopes in the Kootenay Arc 90 3.5 LEAD ISOTOPE ZONING IN SANDON, SLOCAN CITY, AND AINSWORTH CAMPS 95 Slocan City and Sandon Camps 98 Ainsworth Camp 102 3.6 POSSIBLE RELATIONSHIPS BETWEEN KOOTENAY ARC LEADS AND LEAD IN PURCELL ROCKS 102 3.7 SUMMARY • 106 CHAPTER 4. LEAD ISOTOPES FROM THE ANVIL RANGE, YUKON TERRITORY 108 4.1 INTRODUCTION 108 V 4.2 OUTLINE OF TEE GEOLOGY AND LEAD-ZINC MINERALISATION OF TEE ANVIL RANGE 108 Geology 108 Lead-Zinc Mineralisation 111 4.3 DISCUSSION OF LEAD ISOTOPE DATA 113 4.4 SUMMARY 123 CHAPTER 5. SUMMARY 124 5.1 PURCELL LEADS 1 2 4 5.2 KOOTENAY ARC LEADS 1 2 5 5.3 ANVIL RANGE LEADS 1 2 6 REFERENCES APPENDIX NOTES ON SAMPLES, ANALYTICAL METHODS, CALCULATIONS, PRECISION AND CONTAMINATION A.1 SAMPLES A-l A.2 ANALYTICAL METHODS A-2 A.3 CALCULATIONS A-6 A.4 PRECISION A-10 A.5 CONTAMINATION A-10 v i LIST OF TABLES Table 2.1 Isotopic composition of lead in galena from 25 lead-zinc deposits in Purcell rocks, British Columbia Table 2.II Pb 2 0 6/Pb 2 0 l t ratios and total lead contents of a l l 35 minerals of a Precambrian ('vl.l BY) granite studied by Tilton and others (1955) . (The precision of these measurements is low by present-day standards.) Table 2.Ill Recorded production of some metals from a l l 40 significant deposits in Purcell rocks to 1970 (inclusive) Table 2.IV Two-stage model ages for Mesozoic-Cenozoic leads 52 from Purcell Supergroup rocks. Table 2.V Isochron slopes and two-stage ages for Pre-Ravalli 60 leads Table 3.1 Some important lead-zinc deposits of the Kootenay 77 Arc. Taken from Fyles (1967; p. 66, 67). Table 3.II Isotope compositions of lead in galena from 82 concordant deposits, Kootenay Arc Table 3.Ill Isotope compositions of lead in galena from 83 transgressive deposits, Kootenay Arc Table 3.IV Two-stage model ages for concordant deposits, 88 upper anomalous line on fig. 3.4. (Slope 0.119 ± 0.004) Table 3.V Slopes of least-squares lines through transgressive 89 group leads (the lower anomalous lead line on fig. 3.4) Table 4.1 Lead isotope analyses, Anvil Range galenas 114 Table 4.II Age of source rocks of sedimentary host of Anvil 121 Range stratiform deposits (assuming a two-stage model) Table A.I Table A.II Replicate analyses Measurements of Broken H i l l Standard, Set A A-7 A-8 v i i LIST OF FIGURES Fig. 1.1 Location of the two regions studied. Region A i s 1 the East Kootenay d i s t r i c t and Kootenay Arc. Region B is the Anvil Range, Yukon Territory. Major geologic subdivisions are after Douglas (1970). Fig. 1.2 Lead-zinc deposits of British Columbia and neighbour- 2 ing parts of the United States. Fig. 2.1 Approximate outcrop limit of rocks deposited in the 12 Belt-Purcell basin (heavy l i n e ) . Map i s after Harrison (1972, p. 1218). Section AB is taken directly from Gabrielse (1972, p. 526) and shows various formations of the Purcell Supergroup (see f i g . 2.2) overlying Hudsonian? basement. Fig. 2.2 Regional correlation scheme for Belt-Purcell strata 13 proposed by Smith and Barnes (1966, p. 1409). Fig. 2.3 Isopach map (not palinspastic) for the middle Belt- 15 Purcell carbonate unit. Taken from Harrison (1972, p. 1230). Contour interval i s 1000' . Fig. 2.4 Scheme proposed by Harrison for the chronology of 21 some events which have affected Belt-Purcell rocks. Taken directly from Harrison (1972, p. 1236). Fig. 2.5 Generalised geological map, East Kootenay and 26 Kootenay Arc regions. Fig. 2.6 Plot of Pb 2 0 7/Pb 2 0 1 t v. Pb 2 0 6/Pb 2 0 l t for a l l the 28 analysed galena leads i n Purcell rocks (table 2.1). The key identifies the host rock . The growth curve is for (U 2 3 8/Pb 2 0 l t) fc = 9.09. present Fig. 2.7 Plot of Pb 2 0 8/Pb 2 0 l + v. Pb 2 0 6/Pb 2 0 l t for Purcell galenas 29 l i s t e d i n table 2.1. Growth curve is for (Th 2 3 2/Pb 2 0 l t) „ = 39.60. present Fig. 2.8 Plot of Pb 2 0 7/Pb 2 0 1 t v. Pb 2 0 6/Pb 2 0 t t for the "late 30 Precambrian group" of galena leads l i s t e d in table 2.1. The key identifies the host rock of the galena mineralisation. Fig. 2.9 Analyses of the late Precambrian leads (table 2.1) are 37 ar b i t r a r i l y divided above into two sets of different isotopic composition. These samples are plotted on the accompanying map and a correlation of composition with geographic location i s suggested. v l i i Fig. 2.10 Correlation between size of producing mines in 38 Purcell rocks and their lead isotope ratio Pb 2 0 6/Pb 2 0 V Data from table 2.1 and 2.III. Note log scale for tonnages. Fig. 2.11 The results of "frequent mixing" of leads for three- 43 and eleven-stage models. Taken from Kanasewich (1968; p. 169 and 170). Fig. 2.12a Calculated compositions for 50 hypothetical leads at 48 various times in a five-stage history. Small dots, circles and large dots show compositions after 2.7, 1.8, and 1.3 BY (end). Primary growth curve is for (U 2 3 8/Pb 2 0 t t) = 9.09, and numbers indicate billion now ' years. Fig. 2.12b Large circles represent the compositions of 50 hypo- 49 thetical leads after five stages of evolution in various randomly chosen U/Pb environments since 3.0 BY, with fairly thorough mixing at 1.3 BY (see text for details). Solid dots represent measured ratios of Purcell leads of the late Precambrian group. The growth curve shown is the primary growth curve (U238/Pb20l*=9.09) . If more complete mixing is assumed in the calculations, the scatter of the calculated values would more closely resemble the observed scatter. Fig. 2.13 In Model A, the underlying basement is supposed to be 50 a source of radiogenic lead; in Model B, distant basement supplies this component via the sediments. Fig. 2.14 Region studied by Zartman and Stacey (1971). Taken 54 from p. 858 and 859 of their paper. Fig. 2.15 Lead isotope results of Zartman and Stacey (1971). 55 Taken from p. 851 of their paper. Fig. 2.16 Plot of Pb 2 0 7/Pb 2 0 l t v. Pb 2 0 6/Pb 2 0 l t for analyses of 57 Purcell galena leads (table 2.1) and Belt galena leads (Zartman and Stacey, 1971; their tables 1 and 2). The key identifies the host rock. The growth curve is drawn for (U 2 3 8/Pb 2 0 4) ^ = 9.09. present Fig. 2.17 Plot of Pb 2 0 7/Pb 2 0 4 v. Pb 2 0 6/Pb 2 0 £ t for the "late 58 Precambrian group" of galena leads in Purcell rocks (table 2.1) and in Belt rocks (Zartman and Stacey, 1971; p. 852 and p. 854). The symbols identify the host rock of the galena deposit. Fig. 2.18 Belt-Purcell leads of the late Precambrian group plot- 59 ted above are split arbitrarily into three sets and these are plotted on the accompanying map using the same symbols. A correlation between isotopic composi-tion and geographic location is suggested. Data from Zartman and Stacey (1971, p. 852, 854) and table 2.1 (this thesis). i x Fig. 2.19 Large circles represent the compositions of 50 hypo- 61 thetical leads after five stages of evolution in various U/Pb environments since 3.0 BY, with mixing at 1.3 BY. For the large open c i r c l e s , 5% of 2.7 BY lead has been added; for the large striped c i r c l e s , 10% of 2.7 BY lead has been added. The solid dots are measured Purcell leads (table 2.1), the small circles are Belt leads (Zartman and Stacey, 1971). Growth curve i s the primary growth curve of Stacey and others (1969), for (U 2 3 8/Pb 2 0 1 +) = 9.09. now Fig. 2.20 Diagram showing three possible interpretations of the 62 slope for old Ravalli-Piegan leads. Fig. 2.21 Comparison of Rb/Sr dating of Beltian sediments by 64 Obradovich and Peterman (1968) with the lead isotope interpretation of the "old" Belt-Purcell galenas given in this thesis. Fig. 2.22 The pattern of compositions which results from evolu- 66 tion (growth, mixing) from two arrays is shown in the upper sketch. In the lower diagram, actual com-positions from Belt-Purcell rocks are shown. It is suggested that the Mesozoic-Cenozoic leads could be controlled by the compositional pattern of the Precambrian leads. Fig. 3.1 The Kootenay Arc 72 Fig. 3.2 Relationships of formations i n the southern part of 74 the Kootenay Arc. Taken from Monger and Preto (1972, p. 32). Fig. 3.3 Plot of ore and rock leads from the Kootenay Arc, 80 reported by Reynolds and Sinclair (1971, p. 261). Fig. 3.4 Plot of Pb 2 0 7/Pb 2 0 t t v. Pb 2 0 6/Pb 2 0 l + for a l l the 84 Kootenay Arc data l i s t e d in tables 3.II and 3.III. Growth curve is for (U 2 3 8/Pb 2 0 1 t) , = 9.09. present Fig. 3.5 Plot of Pb 2 0 8/Pb 2 0 t t v. Pb 2 0 6/Pb 2 0 l t for a l l the 85 Kootenay Arc data l i s t e d i n tables 3.II and 3.III. Growth curve i s for (Th 2 3 2/Pb 2 0 l f) „ = 39.60. present Fig. 3.6 Sketches of three models which show different "non 91 anomalous" components (A,B,C) for Kootenay Arc leads. Solid triangles represent "concordant" leads, open triangles represent "transgressive" leads. Fig. 3.7 Two examples of zonal patterns. Taken from Orr and Sinclair (1971). 96 Fig. 3.8 Sectional sketch showing a simple model to explain 97 lead isotope zonation in mineral deposits by addition of lead from country rocks to lead i n a circulating f l u i d . Fig. 3.9 Contoured Pb 2 0 6/Pb 2 0 l t ratios for Slocan City and 99 Sandon data (table 3.III). Geology after Cairnes (1934) and L i t t l e (1960). Two samples (901 and 887) do not f i t the contours. Fig. 3.10 Contoured P b 2 0 6 / P b 2 0 4 ratios for analyses of samples 101 from Ainsworth camp (table 3.III). Geology after Fyles (1967). Fig. 3.11 Diagrammatic i l l u s t r a t i o n of two extreme p o s s i b i l - 103 i t i e s for relationships between the three main families of leads (A,B,C). Fig. 3.12 A comparison of isotope ratios of samples from Anvil 105 Range (table 4.1), Pine Point (Cumming and Robertson, 1969, p. 731), and Shuswap deposits (table 3.II). Fig. 4.1 Geological map of Anvil Range. After Tempelman- 109 Kluit (1968, pp. 46-47). Fig. 4.2 Outlines of the three main Anvil Range lead-zinc 110 deposits drawn to a common scale and orientation. Sites of the analysed samples l i s t e d in table 4.1 are indicated by the numbered dots. Fig. 4.3 Vertical cross sections of Faro deposit. Taken 112 from Tempelman-Kluit (1968, p. 50). Sections of Swim and Vangorda deposits are somewhat similar but lack the greenstones and the quartz diorite (Tempelman-Kluit, 1968, p. 51). Fig. 4.4 Plot of Pb 2 0 8/Pb 2 0 t t v. Pb 2 0 6/Pb 2 0 1 + for a l l Anvil 116 Range leads l i s t e d in table 4.1. Growth curve i s for (Th 2 3 2/Pb 2 0 £ t) = 39.60. present Fig. 4.5 Plot of Pb 2 0 7/Pb 2 0 l + v. Pb 2 0 6/Pb 2 0 1 t for a l l Anvil . 116 Range leads l i s t e d in table 4.1. Growth curve i s for (U 2 3 8/Pb 2 0 l +) fc = 9.09 ± 0.06. present Fig. 4.6a Plot of Pb 2 0 7/Pb 2 0 l t v. Pb 2 0 6/Pb 2 0 l f for some early 118 analyses (on an older mass spectrometer) of leads from Swim, Vangorda and Faro deposits. These analyses, which are not normalised to the Broken H i l l Standard, were discarded. Fig. 4.6b Plot of Pb 2 0 7/Pb 2 0 1 t v. Pb 2 0 6/Pb 2 0 t t for the analyses 118 of Swim, Vangorda and Faro samples l i s t e d in table 4.1. xi Fig. 4.7 Some features of Anvil Range deposits which show 119 increases with increasing metamorphic grade (Swim to Faro). Grain size data from Tempelman-Kluit (1970). Isotope data from table 4.1. Tonnage of lead and zinc: Faro, Findlay (1969); Vangorda, Chisholm (1957); Swim, Northern Miner (March 9, 1967, p. 5). Fig. 4.8 Relation of occurrence of base-metal deposits of 122 northern B.C. and southeastern Yukon to facies of Lower Cambrian and Eocambrian strata. Taken from Gabrielse (1969, p. 28). Faro, Vangorda, Swim shown as stars, other deposits as dots. Fig. A. 1 Chart record of spectrum produced by electron A-3 bombardment (about 120 ev) of tetramethyl lead vapour. Principal ions responsible for each set of peaks are identified. Note mass scale below spectrum and the attenuation factors (in brackets). Fig. A.2 Chart record showing detail of peaks at masses 251 A-3 (mainly Pb 2 0 6(CH 3) 3+) and 252 (mainly P b 2 0 7 ( C H 3 ) 3 + ) . Resolution of the mass spectrometer is about 460, peak shape is about 0.67 (10% value). The distance (dispersion) between the centres of the two ion beams which these peaks represent is about 0.05 inches. Fig. A. 3 Typical chart record of trimethyl lead spectrum. A-4 Attenuation factors, atomic mass numbers, time scale, and principal peak related to each of the four isotopes of lead are shown. The height of the largest peak (253 a.m.u.) corresponds to an ion current of about 5*10 - 1 1 amp. As well as being output in chart form, the spectrum is digitised (5 readings/sec) and f i l t e r e d to remove frequencies above about 0.6 hz. Four or five f i l t e r e d readings are obtained on each peak top. Fig. A.4 Plot of mean values obtained by other workers for the A-9 Broken H i l l Standard, and the results of table A.II. The expected (fractionated) ratios were calculated from the two absolute values reported by Stacey and others (1969) and Cooper and others (1969) , and i t was assumed that gas flow into the source was viscous and flow out was molecular (Halsted and Nier, 1950; Whittles, 1964). x i i ACKNOWLEDGMENTS The work of my supervisor, Dr. A.J. Sinclair, first interested me in the use of lead isotopes in the study of ore deposits. Dr. Sinclair suggested the present project and his ready assistance during its progress is sincerely appreciated. Analytical work was carried out in laboratories of the Depart-ment of Geophysics and Astronomy, and I am indebted to many members of this department for support and guidance. Dr. R.D. Russell and Dr. W.F. Slawson helped in various ways, and the generous fashion in which they made facilities available is much appreciated. It is a special pleasure to thank Dr. J. Blenkinsop for his help, advice, and encouragement. Capable technical assistance was provided by E.J. Bellis, R.D. Meldrum, S.N. Newman, K.D. Schreiber and H. Verwoerd (all of the Department of Geophysics and Astronomy), by E.G. Montgomery (Geological Sciences Centre) and E. Williams (Physics Department). The help and guidance of two geologists, Dr. D.J. Tempelman-Kluit and R.G. Gifford, has been of great value to me and is gratefully acknowledged. Others kindly provided me with samples, including Dr. A.E. Aho, R.G. Chaplin, Dr. J.T. Fyles, G. Jilson, Dr. G.B. Leech, A.S. Macdonald, Dr. H.C. Morris, J.F.W. Orr, B. Price, J.B.P. Sawyer, and W.M. Sirola. Members of the committee, and Dr. R.D. Russell, are thanked for reading and commenting on drafts of this thesis. The time spent by Dr. K.C. McTaggart in editing is particularly appreciated. The thesis was typed by R. Rumley. Financial aid provided by a Killam Predoctoral Scholarship is gratefully acknowledged. 1 CHAPTER 1. INTRODUCTION 1.1 OBJECTIVES OF THIS STUDY The writer has determined the isotopic composition of lead in 132 galena samples taken from two regions in the Canadian part of the Western Cordillera (fig. 1.1). In this thesis an attempt is made to relate this isotope data to the regional geology. Fig. 1.1 Location of the two regions studied. Region A is the East Kootenay district and Kootenay Arc. Region B is the Anvil Range, Yukon Territory. Major geologic sub-divisions are after Douglas (1970). The belt containing the Coeur d'Alene district of Idaho and the mining camps of the East Kootenay district and Kootenay Arc in British Columbia is one of the most productive lead—zinc—silver metallogenic W \ FIG. 7-2 Lead-zinc deposits of > British Columbia and \ ^neighbouring parts of BRITISH COLUMBIA ancouver Ruddock Creek the United States V ( o f f e r Fyles and others, , m o p 14-1 C.I.M. Special \f\Volume No. 8 , 1966) WASHINGTON 3 provinces (fig. 1.2) in the world. By studying lead isotopes from galena deposits in the Canadian part of this belt i t was hoped to identify the source of the lead and to account for the persistence of lead-zinc mineralisation from Precambrian to Tertiary times. Three large "stratiform" lead-zinc deposits were recently discovered in the Anvil Range, Yukon Territory (fig. 1.1). It was thought that study of the lead isotope composition of these deposits might reveal the origin and history of the lead. The origin of "strati-form" lead-zinc deposits has been a subject of debate since Stanton and Russell (1959) first suggested that the lead in many deposits of this type came from a uniform U-Th-Pb system, possibly the mantle. 1.2 LEAD ISOTOPE AND GEOLOGICAL MODELS FOE THE GENESIS OF LEAD-ZINC ORES It is widely accepted that "anomalous"* ore-leads are derived from upper crustal rocks. In contrast there has been long-standing dis-agreement as to whether the crust or mantle is the source of the "primary"* leads of many large stratiform lead-zinc deposits, such as Mt. Isa and Broken H i l l in Australia, and Sullivan in Canada. However, in recent years, evidence from lead isotope and other studies has tended to converge in support of a crustal origin for both primary and anomalous lead. Only an outline of this evidence is given here for there are many recently •anomalous" lead; lead that has developed in more than one U-Th-Pb system (= multistage lead) •"primary" lead; lead that appears to have developed in a unique U-Th-Pb system which is world-wide. Primary leads f i t "primary" growth curves on graphs of Pb 2 0 7/Pb 2 0 1 + v. Pb 2 0 6/Pb 2 0 4 and Pb 2 0 8/Pb 2 0 l + v. Pb 2 0 6/Pb 2 0\ (See Stacey and others, 1969; Cooper and others, 1969 0 4 published discussions dealing with the origins of lead in ore deposits (e.g. Krauskopf, 1967; 1971; White, 1968; Doe, 1970; Richards, 1971; and Russell, 1972). Richards (1971) has given a more complete account of many of the arguments mentioned here. Those who suggested the mantle as a possible source of primary lead ores (e.g. Russell, 1956; Stanton and Russell, 1959; Ostic and others, 1967; Kanasewich, 1968) emphasized the apparent uniformity of the U/Pb and Th/U ratios calculated for the source of many stratiform deposits. In contrast they pointed out that these ratios are highly variable in the crust. Others (e.g. Shaw, 1957; Chow and Patterson, 1962; Brown, 1965; Armstrong, 1968) suggested that i f lead was concen-trated from a large volume of crustal rock, sediment for example, and thoroughly mixed, the result could be lead with an apparently uniform U/Pb and Th/U source, like that calculated for primary leads. A conse-quence of this suggestion is that the primary growth curve represents the evolution throughout time of average lead in the upper crust, rather than the mantle. To test this hypothesis, Kanasewich (1962) and Russell and others (1966) investigated by computer-simulation what the effect of mixing heterogeneous crustal leads would be, and they con-cluded that although such mixing might give rise to an apparent primary lead, i t would have to be thorough to escape detection. There is some evidence, however, that isotopes can be thoroughly mixed in crustal environments by natural processes. For example, there is a remarkable homogenisation of sulphur isotopes during migration of oils (Monster, 1972), and sulphur from a single oi l f i e l d can be extremely uniform over a wide area (Thode and others, 1958). Thus, i t is conceiv-5 able that thorough homogenisation of crustal leads could give rise to the narrow range of Th/U values calculated from primary leads, the feature that Kanasewich (1968) considered to be the most convincing evidence for a uniform mantle source. It might be significant that the actual value of the Th/U ratio calculated for the source region of primary leads is about 4 (Stacey and others, 1969; Cooper and others, 1969), similar to the average of measured Th/U values from shales (Rogers and Adams, 1969). Numerous authors have suggested thick shale sequences as possible sources of the lead and zinc in mineral deposits. One measure of the early success of the hypothesis of mantle origin was that single-stage ages calculated from primary leads were reasonably close to independent estimates of ages of mineralisation, generally within 150 MY according to Kanasewich (1968). With a better knowledge of ages of individual ore bodies, however, discrepancies have become more obvious (see Richards, 1971). Although these differences in age can be accounted for to some extent by making the mantle-origin hypothesis more complicated, i t would be more convincing i f these dis-crepancies did not exist. Stanton and Russell (1959) were careful to state that i t was the "stratiform pyritic" type of lead-zinc deposit that they considered might contain lead derived from the mantle. These deposits commonly occur in fine-grained carbonaceous sediments and they are associated consistently with fragmental volcanic rocks. Stanton (1960) concluded that these deposits resulted from the biologic concentration of base-metal and sulphur emitted during volcanic activity, and that they were lenses of sulphide-rich sediment (see also Stanton, 1966). Although i t is true that leads from many deposits of the stratiform type are primary, 6 some leads from other types of deposits where links with volcanism are not obvious also seem to be primary, for example, Pine Point (Cumming and Robertson, 1969) and the Kupferschiefer (see Doe, 1970, p. 47; Wedepohl, 1971). Therefore, i t is suggested that volcanic activity, with the implication of access to the mantle, is not necessary for the formation of a primary lead. There have been many studies of the isotopic composition of trace amounts of lead in rocks (see Doe, 1970) that have some bearing on the sources of lead in mineral deposits. For example, basalts are among the rocks most likely to contain samples of mantle lead and a com-parison of basalt leads and primary leads is therefore of interest. If primary ore leads came from the mantle, i t would be reasonable to expect leads of Cenozoic basalt to have uniform isotopic ratios with values similar to those of a Cenozoic primary ore lead. As analyses of basalt (mostly oceanic) leads have become more numerous and more reliable, i t has been found that they are not in the least like primary ore leads, and they are not uniform (Houtermans and others, 1964; Tatsumoto, 1966; Russell, 1972). This latter fact implies that the mantle has non-uniform U/Pb and Th/Pb ratios, whereas these ratios were thought to be constant for the source of primary leads. Furthermore, values for the present day ratio U 2 3 8/Pb 2 0 1* (y) for the source region of basalt leads are sig-nificantly lower than that for stratiform leads (Gast, 1967). With present analytical accuracy i t has become clear that the primary leads of stratiform ores and basalt leads are products of two different and long-distinct geochemical systems, and that the lead of one of these systems can not be easily derived from the other (Russell, 1972). As 7 Richards (1971) has argued in some detail, the case for a more-or-less direct mantle source for the primary leads found in many large lead-zinc deposits is now weak, i f not untenable. In recent years there has been a shift in geochemical and geological opinion towards a crustal and non-igneous source for the lead of most lead ore deposits. This view, an old one (e.g. see Posepny, 1902) , has been strengthened by several developments. One is the dis-covery of some modern lead-bearing brines, for example in the Red Sea (Degens and Ross, 1969; Ross, 1972), near the Salton Sea in California (White, 1968), and on the Cheleken Peninsula, U.S.S.R. (looms, 1970). Another development includes laboratory studies of brine-filled inclu-sions from ore deposits (e.g. Roedder, 1967), of hydrogen and oxygen isotopes of brines and of some fluid inclusions (e.g. White, 1968; Ohmoto and Rye, 1970), and of sulphur isotopes of sulphides (e.g. Sangster, 1971). A third development includes theoretical studies (e.g. Helgeson, 1967, 1968), and a fourth includes field studies of some ore-deposits such as Pine Point (Jackson and Beales, 1967; Billings and others, 1969)-. As a result of such investigations there is support for the idea that connate* Na-Ca-Cl brines can leach lead and zinc from clays, organic matter, feldspars and other minerals of common fine-grained sediments, and can selectively precipitate these metals, perhaps where the migrating brine encounters H2S-bearing gas, or reacts with wall rocks, or mixes with other waters. Analogies have been made between the generation, migration and accumulation of o i l and of lead and zinc (e.g. •connate brine: this term has many meanings, and i t is used here in a loose sense to refer to interstitial waters of sediments and volcanic rocks (see White, 1968; p. 302). 8 Illlng, 1968; Dozy, 1970), and Jackson and Beales (1967) have argued that the accumulation of many lead-zinc deposits can be viewed as a part of the normal evolution of a sedimentary basin. The many variations on this hypothesis have been discussed by Knight, 1957; Noble, 1963; Davidson, 1967; White, 1968; Beales and Jackson, 1968; Krauskopf, 1971; Dunham, 1970; Tooms, 1970; and E l l i s , 1971, among others. In contrast, accumulating information has given l i t t l e support to theories that the mantle, cooling igneous rocks, or sea-water are significant direct sources for large lead-zinc deposits in sedimentary rocks. The writer, in view of the discussion above, accepts as a working hypothesis that "primary" leads can originate from crustal rocks, in particular from sequences of fine-grained sediments. Interpretation of lead isotope data presented in this thesis was guided by this con-clusion, which was found to be useful in that i t offers a simple explanation for a number of relationships between isotopic data and geology, stratigraphy in particular. * * * Recent discussions on the mathematical models used in inter-pretation of lead isotope data will be found in Hamilton, 1965; Slawson and Russell, 1967; Kanasewich, 1968; Stacey and others, 1968; Doe, 1970; and Russell, 1972. Analyses reported in this thesis were made on a gas-source mass spectrometer in the Department of Geophysics and Astronomy of the University of British Columbia. Isotope ratios are presented as absolute 9 ratios, having been normalised to the absolute ratios of the Broken H i l l Standard as reported by Stacey and others (1969). The precision of the analyses presented is better than ±0.16% (2a sample) for a l l the isotope ratios relative to Pb 2 0 4. Some details of analytical work, including the calculation of precision, are given in the appendix. 10 CHAPTER 2. LEAD ISOTOPES FROM THE EAST KOOTENAY DISTRICT, BRITISH COLUMBIA 2.1 INTRODUCTION The lead Isotope compositions of 47 samples from galena deposits in Precambrian rocks of the East Kootenay district were determined. In this chapter these results are interpreted in relation to regional geology, and are compared with published lead isotope data from galena deposits of northwestern U.S.A. Although this thesis is mainly concerned with Purcell rocks of southeastern British Columbia, i t deals in a general way with the Belt-Purcell rocks exposed on both sides of the Canada-U.S.A. border (fig. 2.1). Harrison (1972) referred to this region as the "Belt Basin", a convenient term for what he suggests was an epicratonic re-entrant of a Precambrian sea that extended along the western edge of North America. 2.2 AN OUTLINE OF THE GEOLOGY OF THE EAST KOOTENAY DISTRICT AND ADJOINING PARTS OF IDAHO AND MONTANA Stratigraphy Sedimentary rocks of the late Precambrian Belt-Purcell succession and their correlatives extend in a discontinuous strip from northern Alaska into California. The fine-grained clastic and carbonate rocks of the Belt-Purcell Supergroup are weakly metamorphosed, are commonly from 15,000' to 35,000' thick, and generally show l i t t l e variation over large areas (Gabrielse, 1972; Smith and Barnes, 1966; Ross, 1970; Harrison, 1972). The base of the Belt-Purcell sequence is rarely exposed, but in southwest-ern Montana Belt sediments rest unconformably on crystalline rocks in 11 several places (e.g. McMannls, 1963; Giletti, 1968). Belt-Purcell and correlative rocks are unconformably overlain by approximately coextensive Precambrian (Windermere and equivalents) to Lower Palaeozoic sedimentary rocks. The basal Windermere rocks are diamictites in many places (Aalto, 1971; Stewart, 1972; Gabrielse, 1972). Belt-Purcell sediments were deposited in an extensive, long-lived, stable marine basin. Modern analogues that have been suggested are the Gulf Coast Geosyncline (Reesor, 1957; Price, 1964) and the continental shelf off eastern North America (Gabrielse, 1972). Attempts have been made to correlate Belt-Purcell rocks through-out the basin, and the scheme of Smith and Barnes (1966) is followed in this thesis. These authors recognised two major cycles in the Belt-Purcell succession, each cycle consisting of a lower carbonaceous and/or carbonate-bearing sequence and an upper reddish or greenish carbonate-poor sequence. They proposed a four-fold division (fig. 2.2), Pre-Ravalli and Ravalli sediments forming the older cycle and Piegan and Missoula Groups the younger cycle. Smith and Barnes (ibid.) suggested that the cycles were related to slow fluctuations in water depth on a large scale. The carbonaceous, "varved", pyritic (pyrrhotitic in Purcell) siltstone deposited near the axis of the basin was considered to imply stable, deep-water reducing conditions, whereas the mud-cracked, reddish (hematitic) and greenish siltstone of the upper half of the cycle suggested a shallow-water, oxidising environment. Pre-Ravalli rocks (Prichard and Aldridge formations) were probably deposited in deeper water than were succeeding strata (Price, 1964). Results of an extensive mineralogic and geochemical study by 12 0 |0 KIlOMITiaS — MIIIS Restored section showing postulated configuration of miogeocline during early Purcell time. Data mainly from Price, 1964. Fig. 2.1 Approximate outcrop limit of rocks deposited in the Belt-Purcell basin (heavy line). Map is after Harrison (1972, p. 1218). Section AB is taken directly from Gabrielse (1972, p. 526) and shows various formations of the Purcell Supergroup (see fig. 2.2) overlying Hudsonian? basement. 13 H 7 ° W CAN. U.S." i *-» 3 MARYSVILLE & DUCK CRK. PASS Greenhorn Mt 50 N 111° 4 0 - " LEGEND Salt—cast-bearing strata Carbonate —bearing, carbonaceous formations Carbonaceous formations Fig. 2.2 Regional correlation scheme for Belt-Purcell strata proposed by Smith and Barnes (1966, p. 1409). 14 Harrison and Grimes (1970) of some Belt rocks from Idaho and Montana probably can be applied to the bulk of Belt-Purcell sediments. Harrison and Grimes concluded that the overall chemical composition, excellent grain-size sorting and the uniformity of mineralogy were the result of very effective homogenisation of clastic components from a granitic source. Evidence that the craton to the east was the important source of Purcell sediment is provided by facies changes and eastward thinning of strata (Reesor, 1957; Price, 1964). There is no evidence in Prichard and Aldridge strata of a basin margin or a sediment source to the west, although there is evidence for sediment transport from a southerly direc-tion, perhaps along the axis of the basin (McMannis, 1963; Bishop and others, 1970; Huebschman, 1972). Facies changes in Ravalli and younger rocks show, however, that these sediments were deposited in a N.W. trending trough (Smith and Barnes, 1966). The carbonate-bearing middle part of the Belt-Purcell Supergroup is apparently the principal correlation marker in the sequence (Smith and Barnes, 1966), and the nature of the basin at this time is suggested by the isopach map (fig. 2.3) of Harrison (1972). Great thickness of strata, uniformity of sediment supplied to the basin, stability of depositional environment, the slow change in facies controlled by water depth, and the long duration of deposition (presumably intermittent) are important features of Belt-Purcell sedimenta-tion. 15 Fig. 2.3 Isopach map (not palinspastic) for the middle Belt-Purcell carbonate unit. Taken from Harrison (1972, p. 1230). Contour interval is 1000' . Igneous Activity . Igneous activity of Precambrian age includes the extrusion of Purcell lava, and the intrusion of the granitic Hell-Roaring Creek Stock, of lamprophyre dikes, and of widespread s i l l s and dikes of diabase. The 16 age of the Purcell lava relative to the sedimentary record (fig. 2.2) is well known, and the Hell-Roaring Creek Stock (1260 MY*) seems reliably dated, but the lamprophyres and diabase s i l l s are not well dated. For example, there seem to be both Precambrian and Tertiary lamprophyre dikes (Hobbs and Fryklund, 1968; Leech and Wanless, 1962), and the extensive tholeiitic diabase s i l l s (Purcell intrusives or Moyie intrusives) are probably of more than one age. In Purcell rocks, these diabase s i l l s are concentrated at several stratigraphic levels, particularly in Aldridge and Kitchener strata (Leech and Wanless, 1962), but they occur in decreasing volume as high as the Mt. Nelson Formation (Rice, 1941). Granodiorite to quartz monzonite batholiths and stocks of Jurassic to Tertiary age intrude and metamorphose Belt-Purcell rocks. The largest of these are the Idaho Batholith, Boulder Batholith, Kaniksu Batholith and White Creek Batholith, a l l of which seem to be Cretaceous in age (McDowell and Kulp, 1969; Yates and Engels, 1968; Wanless and others, 1968). Regional Metamorphism, Folding and Faulting Except near major intrusions, Belt-Purcell sediments are weakly metamorphosed, rarely beyond greenschist facies (Leech and Wanless, 1962; Harrison, 1972). The age of this regional metamorphism of Belt-Purcell sediments is not well known. The principal metamorphism probably is Precambrian in age, although later metamorphism might also be involved. Leech (1962, 1963) was one of the first to recognise the importance of *A summary of geochronology, with references, is given on p. 19 . Units used for ages are MY = million years (106 years) and BY = billion years (109 years). 17 Precambrian (probably Pre-Windermere) metamorphism, a conclusion he reached from K/Ar dating of metamorphic micas in Aldridge rocks, and from the discovery (Leech, 1962) that the Hell-Roaring Creek Stock was Precambrian. Many dates from micas in Aldridge rocks are probably meaningless hybrid ages, but one date of 710 MY might be reliable according to Leech (Wanless and others, 1967). In Idaho, two areas of high grade metamorphic rocks have been dated at about 1500 MY (Reid and others, 1970; Clark, 1971) and these are the earliest ages of metamorphism yet obtained in Belt-Purcell rocks. With the exception of tight E-W folds near Coeur d'Alene, most folds in Belt-Purcell rocks are open and trend NW to NE. The Purcell Geanticline (fig. 2.1) is the major regional structure, a north-trending, north-plunging broad anticlinorium that Leech and Wanless (1962) argued was principally a Mesozoic structure, but which Harrison (1972) suggests could be Precambrian. In Coeur d'Alene region, the major folding is regarded by Hobbs and Fryklund (1968) as being older than a 1250 MY old uraninite vein, and near Kimberley, B.C., the Hell-Roaring Creek Stock (1260 MY) intrudes Aldridge rocks that might have been deformed previously (Leech and Wanless, 1962; Ryan and Blenkinsop, 1971). Leech and Wanless (1962) showed that some folds near Kimberley were cut by lamprophyres with a minimum age of 765 MY. Belt-Purcell sedimentation was terminated by an episode of mild metamorphism and folding that resulted in a slight angular discordance between Belt-Purcell and overlying Windermere rocks (Reesor, 1957). A few apparently reliable ages of between 700-800 MY have been determined from metamorphic minerals in Belt and Purcell rocks (Goldich and others, 1959; Wanless and others, 1967), and these dates could represent the age of this pre-Windermere metamorphism. In any case, 18 this metamorphism and deformation must be younger than about 930 MY, the age obtained by Obradovich and Peterman (1968) for some uppermost Beltian strata. In summary, although the evidence is incomplete and partly unconvincing, i t seems there was metamorphism and folding before 1200 MY (approximately) that affected at least small areas of lower Belt-Purcell rocks, and a later deformation (700-800 MY?) reflected in the slight angular discordance between Windermere and Belt-Purcell sequences. Simi-lar conclusions have been reached by Reid and Greenwood (1968) and by Harrison (1972). High grade regional metamorphism and intense deformation has affected Windermere to Mesozoic rocks west of the Purcell Geanticline and there may have been some mild deformation and metamorphism of Belt-Purcell rocks at the same time(s) (Leech and Wanless, 1962). Although i t is difficult to assess the relative Intensities of the various episodes of Precambrian and Phanerozoic metamorphism and folding, their cumulative effect has not been great. For example, in most exposures, Belt-Purcell sediments s t i l l contain clearly recognisable primary sedimentary structures, such as mud cracks, ripple marks, salt crystal casts. As Gabrielse (1972) observes, the thick Belt-Purcell sedimentary wedge seems to have served as "pseudo-basement" from Windermere times on, although i t was affected by thrust faulting and probably became separated from the craton during Rocky Mountain deforma-tion in late Mesozoic - early Tertiary time (Bally and others, 1966). There is evidence for Precambrian motion on some major faults, including Osburn Fault (Hobbs and others, 1965) and Hope Fault (Harrison and others, 1972) in Idaho, and some small faults near Kimberley (Leech 19 and Wanless, 1962; Freeze, 1966). Repeated movement and complex histories, including later folding, have been suggested for some faults (Hobbs and Fryklund, 1968; Leech and Wanless, 1962; Dahlstrom, 1970). Geoohronology Geochronology of basement rocks, Belt-Purcell sediments, intrusives, and ages of deformation and of mineralisation have been summarised by numerous writers, including Obradovich and Peterman, 1968; Giletti, 1968; Hobbs and Fryklund, 1968; Gabrielse, 1972; and Harrison, 1972. Harrison (1972) made an attempt to unify the available information and reconcile the conflicts (fig. 2.4). Some of these conflicts l i e in the fact that metamorphic and intrusive events that affected lower Belt-Purcell rocks seem to have occurred while sedimentation was s t i l l in progress in the upper part of the sequence, but they are not reflected obviously in the sedimentary record. Especially confusing in this regard are the 1260 MY old stock and ?coeval sillimanite-grade metamorphism near Kimberley (Ryan and Blenkinsop, 1971), and the 1500 MY ages determined on zircons of gneisses in Idaho that are thought to be metamorphosed lower Beltian sediments (Reid and others, 1970; Clark, 1971). As another example, Obradovich and Peterman (1968) concluded from a Rb/Sr study of Belt sediments that there were three distinct periods of sedimentation (>1300, 1100, <s900 MY) separated by breaks of about 200 MY — breaks not yet identified in the field. Other conflicts are probably due to the variety of techniques and materials used — U/Pb on zircon, Rb/Sr on sediments, K/Ar on altered s i l l s , and to interpretations of spurious data. As Harrison (1972) remarks, there are a number of alternative interpreta-tions of present data, none of them completely satisfactory. In the 20 writer's opinion, only a rough sense of the order and probable ages of events can be surmised at present, despite the fairly extensive geo-chronologic work. Geochronological work to date in that segment of the Belt-Purcell basin exposed in British Columbia is summarised below. Reynolds and Sinclair (1971) calculated a single-stage lead age of 1.2 BY for galena from Sullivan Mine. The Hell-Roaring Creek Stock was dated at 1260 ± 50 MY by whole rock Rb/Sr methods (Ryan and Blenkinsop, 1971), after preliminary K/Ar dates of 705 MY and 769 MY were obtained by Leech and Wanless (1962), and by Hunt (1962), respectively. This stock in-trudes Aldridge strata (previously deformed?) and a Moyie s i l l . Nearby regionally-metamorphosed Aldridge rocks have given K/Ar ages of 745 and 790 MY (Leech and Wanless , 1962), and a Rb/Sr age of about 1000 MY (Ryan and Blenkinsop, 1971) and this metamorphism could be the same age as the stock. Moyie intrusions contain amphibole dated by K/Ar methods (6 analyses) at 883 - 1580 MY (Hunt, 1962). Hunt (1962) also dated hornfels of the Purcell lava at 1075 MY by whole-rock K/Ar methods. Leech and Wanless (1962) obtained a K/Ar age of 765 MY (and of 580 MY) on biotite from a lamprophyre dike that cuts Sullivan ore, and they interpreted this as a minimum age for both dike and ore. Leech (e.g. in Wanless and others, 1965; Wanless and others, 1967, 1968) attempted to date mica from Aldridge rocks, but he considered most of the ages (209 to 1310 MY) were meaningless hybrid ages. In his opinion, the age of only one sample (in Wanless and others, 1967) seemed likely to be a true metamorphic age (710 MY), and Leech commented that this age was difficult to reconcile with the prevailing view that the age of Important metamorphism was Mesozoic. 21 GEOLOGIC AGE (nvy.) UNCONFORMITIES SEDIMENTARY OEPOSITS AND THICKNESS MAGMATIC EVENTS TECTONIC EVENTS METAMORPHIC EVENTS 700 800 900 1000 1100 1200 1300 I4O0 1300 1600 1700 McNomoro Bonner Shepord Purcell Lovo Snowiiip H«2jnp-Wolloct_ St. Regis-Spokone Windermere System of Conoda (22,000+ ft) Upper part of Missoulo Group (13,200-+ ft) Upper port of the lower port cf Missoula Group (11,000 ft) Lower part of the lower part of Missoula Group 16,300 ft) Middle Belt carbonate unit (14,500 ft) Rovollf Group (18, SOO ft) Lower Belt (22,000+ ft) Volcanics Gabbroic sills East Kootenay orogeny Purcell anticlinorium East Kootenay event-biotire-grade regional metamorphism at depth Minor folding and tilting along eastern edge Purcell Lava; gabbroic sille Coeur d'Alene lead and uranium veins (calculated ages may be too old) Granodiorite at Hellroaring Creek Gabbroic sills Granitic intrusions, now augen gneisses, in Elk City and Priest River areas, laaho(pre-Balt?) Major change in bosin shape. Questionable faulting and folding in Coeur d'Alene area Warping to form upper Ravalli basin Elk City event(7) (pre-Belt?) Coeur d'Alene event(7)-high grade to south, bio-tite grade in basin Regional metamorphism affecting Prichard near Alberfon, Montana Elk City event!?) (pre-Belt?) Pre-Belt magmafic and metamorphic events Estimated times of some Belt events. Extent of unconformities: basin wide, indicated by solid l ine; local, long dashed l ine; inferred, short dashed line. Fig. 2.4 Scheme proposed by Harrison for the chronology of some events which have affected Belt-Purcell rocks. Taken directly from Harrison (1972, p. 1236). Lead-Zinc Mineralisation Lead, zinc, and silver are the economically important metals of mineral deposits in Belt-Purcell rocks, although there are widespread occurrences of low-grade strata-bound copper mineralisation (Clark, 1971; 22 Harrison, 1972). The largest Pb-Zn-Ag production has come from the Sullivan Mine (B.C.), and from the mines of the Coeur d'Alene district (Idaho). Ore deposits of the Coeur d'Alene are mainly replacement veins along steeply-dipping fracture systems in Prichard, Burke, Revett, St. Regis and,to a lesser extent, Wallace formations (Hobbs and Fryklund, 1968). The Sullivan Mine is an unusually large, gently-dipping strati-form deposit at the boundary of the lower and middle divisions of the Aldridge Formation (Swanson and Gunning, 1945; Leech and Wanless, 1962; Freeze, 1966). A convenient non-genetic classification of lead-zinc* deposits of the East Kootenay district is that of Leech and Wanless (1962), summarised below. (1) Conformable to sedimentary to host-rocks. This group includes Sullivan and some other important mines in Aldridge strata. (2) Small fissure veins within Moyie s i l l s that intrude Aldridge strata. (3) Veins and replacements localised by fractures in Purcell sedimentary rocks. Most veins belong to this class. Deposits in Coeur d'Alene district are of this type (in Belt rocks). Some deposits are known to be Precambrian and others are Tertiary but the ages of most are unknown. Although Hobbs and Fryklund (1968) argue that at Coeur d'Alene the principal mineralisation was Late Creta-ceous, lead isotope and other evidence suggest that the principal mineralisation (Pb-Zn-Ag and Cu) throughout the Belt-Purcell basin is *No distinction is made in this thesis between deposits that contain more zinc than lead (zinc-lead deposits) and those containing less zinc than lead (lead-zinc deposits). 23 Precambrian (Leech and Wanless, 1962; Zartman and Stacey, 1971; Clark, 1971; Harrison, 1972). However, a convincing reconciliation of a l l the isotopic and geologic evidence has not yet been made (see Hobbs and Fryklund, 1968; Zartman and Stacey, 1971; Sorenson, 1972). 2.3 PREVIOUS LEAD ISOTOPE WORK Studies of lead isotopes in Belt-Purcell galena deposits by Long and others, 1960; Cannon and others, 1962; Leech and Wanless, 1962; and Sinclair, 1966; have resulted in two main conclusions. (1) Most of the principal galena deposits are isotopically similar, and contain lead with single-stage ages in the range 1.2 - 1.4 BY. Leech and Wanless (1962) confirmed a Precambrian age of emplacement for Sullivan ore by dating a post-ore lamprophyre dike but there has been argument over the age of the main mineralisation at Coeur d'Alene, where both Precambrian and Late Cretaceous emplacement ages have been suggested. (2) Small deposits generally contain lead of variable, relatively radiogenic composition, except for vein deposits within Moyie intrusions (group 2) which generally contain unradiogenic Pre-cambrian lead (Leech and Wanless, 1962). Many more precise analyses have not changed these conclusions significantly. However, the low precision of most earlier analyses imposes a severe limit to more detailed interpretation. The improved precision of the analyses recently reported by Zartman and Stacey (1971) and of the present study has permitted an evaluation of the earlier con-clusions and the interpretation of small differences in isotopic compositions. 24 Zartman and Stacey (1971) reported a large number of analyses of galena leads from Belt-Purcell rocks in northwestern Montana and northern Idaho, including the Coeur d'Alene district. They confirmed the existence over this whole region of a Precambrian (1.5 - 1.2 BY) group of leads and a variable group of Mesozoic — Cenozoic leads. Assuming a two-stage model for the evolution of the Mesozoic — Cenozoic leads and mineralisation at 100 MY, they calculated beginning ages for the second stages of 2.7 and 2.8 BY for Montana leads and 1.6 and 1.4 BY for Idaho leads. Two principal linear belts of Precambrian mineralisation were identified, one associated with the Osburn Fault system, the other extending northward from Coeur d'Alene to British Columbia. They con-sidered that the Precambrian Coeur d'Alene leads were introduced into Belt rocks during or soon after sedimentation, but they could not com-pletely dismiss the possibility of concentration of disseminated Precambrian lead into vein deposits at a later time. The work of Zartman and Stacey is later considered in more detail (section 2.5) because combining their analyses with those of the present writer permits discussion of the history of leads from almost the entire Belt-Purcell basin. 2.4 DISCUSSION OF NEW LEAD ISOTOPE DATA A l l analyses obtained in this study of galena leads from Purcell rocks are listed in table 2.1. Sample locations are shown on fig. 2.5. The writer's results (table 2.1) are shown in figs. 2.6 and 2.7, and two distinct isotopic groups are apparent. These groups were also noted by earlier workers (Leech and Wanless, 1962). To simplify comparison with the work of Zartman and Stacey (1971), their terminology TABLE 2.1 ISOTOPIC COMPOSITION OF LEAD IN GALENA FROM LEAD-ZINC DEPOSITS IN PURCELL ROCKS, BRITISH COLUMBIA UBC NO. NAME OF MINE OR PROSPECT NTS / UTM SHEET/GRID REF. Pb20B PbZOI Pb2 0 7 Pb20M Pb20 8 Pb204 GEOLOGICAL RELATIONSHIPS LATE PRECAMBRIAN LEAD IN ALDRIDGE SEDIMENTS 844 845 846 847 848 SULLIVAN 82F/NF7206 II It It 16.524 16.531 16.449 16.518 16.519 15.478 15.486 15.460 15.469 15.477 36.191 36.197 36.103 36.162 36.187 Huge (roughly 1 mile x 1 mile * 300'), inverted-saucer shaped stratiform body near lower/middle Aldridge boundary. Gently dipping. Superimposed on weak metamorphism of host a r g i l l i t e - q u a r t z i t e are tourmalinisation, c h l o r i t i s a t i o n , a l b i t i s a t i o n . Mud-flake breccia below. Moyie intrusion below i s pre-ore, lamprophyres are post-ore. The samples analysed are spread over the section of the ore body il l u s t r a t e d on p. 281 of Freeze (1966). 849 It 16.529 15.481 36.169 850 It 16.616 15.464 36.192 Tin-zone fracture, cuts main deposit. 323 " It 16.521 15.484 36.190 778 KOOTENAY KING 82G/PF0209 16.405 15.451 36.151 Replacement of dolomitic a r g i l l i t e and s i l t i t e of mid Aldridge. Mostly conformable. 779 ESTELLA 82G/PF0014 16.393 15.442 36.156 Replacement and fissure veins in sheared a r g i l l i t e and s i l t i t e of mid Aldridge, and in Moyie intrusion. Monzonite nearby. 780 NORTH STAR 82F/NF7103 16.434 15.449 36.052 Oxidised, erosional remnants of conformable body near boundary of lower and middle divisions of Aldridge. Preserved in two shallow syncllnes. 781 STEMWINDER 82F/NF7105 16.444 15.450 36.087 Tabular lens of galena-sphalerite ore in hanging wall of steep, tabular, thick (over 100') pyrrhotite body cutting lower/mid Aldridge in syncline. 761 VULCAN 82F/NF4817 16.339 15.404 35.962 Extensive but weak disseminated galena-sphalerite, mostly along bedding at lower/mid Aldridge boundary. Above mud-flake breccia. 773 FORS 82G/NE8168 16.388 15.421 36.071 Disseminated sphalerite, galena in bleached mid Aldridge strata near (above) No. 808. 808 FORS 82G/NE8168 16.341 15.404 35.981 Lens of disseminated galena, sphalerite in quartzite and a r g i l l i t e of mid Aldridge. 853 FORS 82G/NE8167 16.324 15.401 35.957 Disseminated sphalerite, galena, boulangerite in s i l i c i f i e d ?Aldridge erratic near (below) No. 808. 762 KID CREEK 82F/NE5550 16.332 15.406 35.985 Weak, disseminated galena mostly along bedding. Laterally persistent at several horizons in mid Aldridge quartzites. 832 ST. EUGENE 82G/NE8659 16.340 15.415 36.015 Replacement and fissure veins i n a steep fracture system. Oblique shoots important. In mid and.upper Aldridge. 833 AURORA 82G/NE8460 16.337 15.409 36.024 Veins (to 6') in steep fracture system in mid and upper Aldridge. Extension of St. Eugene? 835 SOCIETY GIRL -82G/NE8759 16.314 15.410 35.996 Veins in zone of shearing and fracturing in uppermost Aldridge. Sub-pa r a l l e l to nearby St. Eugene break. Oxidised upper part. 836 DOMINION 82F/NE5397 16.393 15.429 36.058 Quartz veins (to 1') in fault zone i n middle Aldridge quartzite above Moyie Intrusion. 760 RIMROCK (B+V) 82G/NE7981 16.426 15.430 36.106 Numerous thin (to 1') quartz veins in mid Aldridge quartzite above mud-flake breccia. Near Cretaceous-Tertiary syenite stock. 837 ALICE 82F/NE3542 16.374 15.417 36.073 Veins and lenses of galena-bearing quartz veins i n mid Aldridge sheared a r g i l l i t e and quartzite i n major fault zone. 888 MARYSVILLE D.D.H.* 82G/NE7297 16.603 15.467 36.392 Disseminated sphalerite-galena-pyrrhotite in laminated quartzite near boundary of lower and mid Aldridge ( d r i l l - h o l e sample). 889 MARYSVILLE D.D.H.* 82G/NE7297 16.558 15.467 36.329 (as above). LATE PRECAMBRIAN. LEAD IN MOYIE INTRUSIONS 774 MOYIE TUNGSTEN 82F/NE6869 16.383 15.423 36.066 Coarse grained quartz-calcite-epidote-garnet vein (to 1') with small galena lenses in Moyie intrusion into mid Aldridge. 765 HOPE 82C/NE8272 16.319 15.406 35.996 Quartz ca l c i t e vein (to 1') i n Moyie intrusion into mid Aldridge. 775 PARK 82G/NE7899 16.406 15.436 36.079 Thin quartz-calcite vein in top of Moyie intrusion into mid Aldridge. 769 LONE PINE HILL* 82G/NE7998 16.431 15.433 36.104 Sparsely mineralised quartz-calcite vein (to 3') in top of sheared Moyie intrusion into mid Aldridge. 776 LEADVILLE 82F/NE4852 16.333 15.411 35.997 Quartz-calcite vein (to 18") in thin Moyie intrusion in mid Aldridge quartzite. 777 VULCAN SILL* 82F/NF4818 16.421 15.413 36.072 Lenses of quartz (to 50' long) i n Moyie intrusion into lower middle Aldridge. White Creek Batholith nearby. 851 PEDRO 82G/NE7500. J.6..438 15.433. 3o..082._. • Quartz-calcite, .vein (1.'.-?'). In top. of Moyie intrusion in mid Aldridge. 852 POLLEN BASIN 82F/NE5090 16.542 15.444 36.306 Quartz-calcite veins (to 6') i n top of Moyie intrusion in mid Aldridge. MESOZOIC-CENOZOIC LEADS IN VARIOUS PURCELL ROCKS 759 DAN HOWE 82F/NE5694 17.269 15.532 37.096 Thin quartz veins (to 2') in shear zone i n lower Aldridge. Beneath (20') Moyie s i l l . 736 LEADER (WELLINGTON) 82F/NE6388 18.532 15.612 39.009 Narrow (to 2') banded quartz veins persist over 1000' ln intense shear zone of fault separating Creston and Kitchener f f . Granitic stock nearby. 764 WARHORSE (BOY SCOUT) 82F/NE5991 18.929 15.702 38.805 Very persistent steep mineralised (to 12') shear zone in lower Aldridge. Precambrian stock and pegmatite dikes adjacent. 768 ANDERSON 82F/NE7089 18.792 15.638 38.778 Quartz veins (to 9') i n sheared, altered, argillaceous Creston sediments. In splinter of Perry Creek Fault. 838 MIDWAY 82G/NE8153 17.940 15.564 38.593 Brecciated, recemented quartz vein in shear zone contains sulphides along hanging wall. Cuts middle Aldridge. 839 BIRDIEL 82F/NE7187 18.596 15.624 38.779 Steep quartz veins in shear zone of Perry Creek Fault cut Creston p h y l l i t i c a r g i l l i t e and quartzite. 840 MINERAL KING 82K8W/NF408763 18.503 15.640 38.466 Complexly shaped ore-bodies occur as replacement of fractured Mt. Nelson dolomite in a tightly-folded, fault-bounded syncline. 841 PARADISE 82K8W/NF498911 19.287 15.708 39.892 Pa r t i a l l y oxidised replacement and fissure f i l l i n g deposits in shattered siliceous, magnesian limestone of Mt. Nelson formation in small anticline. 842 LOCKHART CREEK* 82F/NE2384 18.847 15.680 39.231 Thin (about 6") vein of coarse galena i n lower Horsethief Creek Formation. 766 PAL MAY RA 82G/PF0408 19.082 15.674 39.319 Galena in quartz vein (over 10') cutting fractured lustrous paper a r g i l -l i t e of Aldridge and shattered syenite dike. In core of major an t i c l i n e . 767 LILY MAY EXTENSION 82G/PF0406 18.908 15.650 39 . 054 Quartz vein i n shattered syenite dike (to 7') and contorted f i s s i l e a r g i l l i t e s of Aldridge. In core of major anticl i n e . 891 PITT CREEK* 82F/NE7396 18.613 15.618 38.600 Thin quartz veins i n mid Aldridge near St. Mary Fault. 892 POLARIS 82F/NE7298 18.520 15.655 38.657 A 6" quartz vein in lower Aldridge between two Moyie s i l l s . 893 ROSE PASS 82F/NF2712 18.952 15.708 38.870 One or more quartz veins (about 1') in closely-folded black slate of Mt. Nelson formation. Small granitic stock nearby. •Names are those used in the liter a t u r e except where marked by *. Sources: Rice, 1941; Leech and Wanless, 1962; Schofield, 1915; Freeze, 1966; B.C. Dept. Mines Ann. Repta. (consult indexes 3 and 4); R.G. Gifford (pers. comm.). 26 27 is used here, thus the closely clustered group of leads is referred to as "late Precambrian leads", the more radiogenic and variable group as "Mesozoic-Cenozoic leads". These two groups of lead seem to represent two periods of galena mineralisation of very different ages and are therefore treated separately in the following discussion. Late Precambrian Leads Deposits which are known to contain lead of the late Precambrian group are restricted to the Aldridge Formation, or to diabase s i l l s that intrude the Aldridge Formation (table 2.1). On fig. 2.6, 2.7 and 2.8 the compositions of the late Precambrian leads show a small scatter that is not due to experimental error, but to real differences. Because this scatter is in the form of a linear array (;in isochron?), and because of the restricted stratigraphic occurrence of these leads the late Pre-cambrian leads are considered to be genetically related to each other. The source and the age of these leads are discussed below. The mantle as a source Single-stage ages calculated for the late Precambrian leads f a l l within the range 1.2 - 1.4 BY. These single-stage ages appear to be reasonable estimates of the time of mineralisation, but the assumption that any of the leads are in fact products of a single-stage (presumably mantle) development is unreasonable, for the following arguments. Of the whole group, lead of Sullivan composition most closely satisfies the criteria for a primary (= ?mantle) lead (Kanasewich, 1968; Stacey and others, 1969; Cooper and others, 1969). To obtain the other leads substantial contamination of Sullivan-like primary lead with older Late Precambrian group • • • • 164 170 180 19-0 Fig. 2.6 Plot of Pb 2 0 7/Pb 2 0 l + v. Pb 2 0 6/Pb 2 0 l + for a l l the analysed galena leads in Purcell rocks (table 2.1). The key identifies the host rock. The growth curve is for (U 2 3 8/Pb 2 0 1 +) - 9.09. present 15-5 7< 15-4 0 • O 16-3 16-4 16-5 g KEY in Aldridge Formation O Moyie intrusive Sullivan Mine 16-6 1 Fig. 2.8 Plot of Pb 2 0 7/Pb 2 0 1 4 v. Pb 2 0 6/Pb 2 0 4 for the "late Precambrian group" of galena leads listed in table 2.1. The key identifies the host rock of the galena mineralisation. 31 crustal leads would be necessary. According to this argument, the sheer quantity of mantle lead in the Sullivan deposit might have swamped any contaminating crustal lead, but this would not account for the fact that other large lead deposits (notably in Coeur d'Alene district) do not contain lead of Sullivan composition. The converse argument, that small deposits should show least similarity to Sullivan lead, is also untenable. The amount of crustal "contaminant" required for some of the larger ore bodies could easily be a larger fraction of the total lead in the deposit than the original mantle-lead component. Faced with the necessity of a crustal source for at least some of the lead, i t is reasonable to specu-late that the crust could be the source of a l l the lead. The second argument against a mantle origin of any of the Pre-cambrian leads is that, as discussed in section 1.2, lead in modern basalts seems to be too variable and too deficient in Pb 2 0 7 for the mantle to be the source of primary leads (Richards, 1971; Russell, 1972). For this general reason, a mantle origin of Sullivan or the other Late Precambrian leads seems unlikely. Suggestions that Sullivan lead was introduced from the mantle into the crust at the same time as the Moyie intrusions (e.g. Swanson and Gunning, 1945) or Purcell lava, or migrated from the mantle up deep faults (e.g. Kanasewich, 1968a) are therefore not supported by the evidence of lead isotopes. Precambrian granitic rocks could possibly supply lead of the required composition but they are too rare for this idea to be considered seriously. Furthermore, evidence that granitic rocks are the source of metals in lead-zinc deposits in sediments is generally weak or absent both in this area and elsewhere. 32 The crust as a source To the writer, the most plausible explanation for the Late Precambrian leads is that they were leached from lower Purcell sediments (probably Aldridge Formation) by circulating connate brines. Some com-ments on this general hypothesis were made in chapter 1. The connate brine hypothesis accounts for, in a simple way, the uniformity of the old Purcell leads, their slight variation, their wide distribution, and perhaps also for the large size of some deposits which contain old lead. The writer must say, however, that there is very l i t t l e independent evi-dence which might serve to test this hypothesis. At least until studies of geochemistry, fluid inclusions, stable isotopes, etc. are available, the connate brine theory seems acceptable as the most straight-forward explanation for the origin of the "old" Purcell leads. Some details of this explanation follow. If a circulating connate brine did exist in Aldridge strata, presumably i t began its chemical evolution (White, 1965) soon after sedimentation, and may have continued until folding and faulting disrupted the circulation system, and compaction and metamorphism prevented circu-lation by reducing permeability. The widespread occurrence of Late Pre-cambrian galena in fractures in Moyie s i l l s , and sulphides replacing a Moyie intrusion at Sullivan (Freeze, 1966) could mean that the brine was s t i l l actively circulating after this widespread intrusive episode. In fact, the Moyie s i l l s might have played two important roles in mineralisa-tion. They could have supplied thermal energy to drive the circulation system and they could have channeled and restricted brine flow. Control of the path of brine flow is necessary i f large volumes of dilute metal-bearing brine are to result in a large deposit. 33 Isotopic compositions of the late Precambrian group of Purcell galenas are consistent with the model sketched above. Firstly, the compositional similarity of these leads over a vast area could be due partly to the observed mineralogic homogeneity of lower Purcell rocks, and partly to the homogenising effects of a far-ranging brine. Secondly, the small scatter in isotopic compositions could be the result of slight i n i t i a l differences in source lead laterally or vertically in the sedi-mentary column. It is also possible that leads of different composition could be extracted from the same rocks depending on the extraction efficiency of the brine. This suggestion is based on the observation that lead in sediments and other rocks occurs in various physical sites, and the availability and composition of lead in each site can be very different. The principal sites of lead in a fine-grained sediment, such as Aldridge rocks, can be identified as follows. (a) Lead loosely held by clays, organic matter, Fe and Mn hydroxides, or as PbS. According to Wedepohl (1956), the bulk of lead in sediments is contained in the clay mineral fraction. However, a detailed study by Vine and Tourtelot (1969) of black shales showed that, in these sediments at least, lead has various associations with organic material, detrital minerals, carbonates, sulphides, etc. The sorption of lead by clays, organic matter and Fe and Mn hydroxides has been demonstrated to be very effective by laboratory experiments (e.g. Krauskopf, 1956; Weiss and Amstutz, 1966), but just when this sorption occurs in sediments is d i f f i -cult to say. Vine and Tourtelot (1970) suggest that sorption of metals in black shales is an accumulative process. For example, they suggest 34 that metal enrichment in the organic matter component of shales begins during the l i f e of the organism, proceeds during drift to the burial site and can continue until final expulsion of interstitial waters. The isotopic composition of lead in the clay/organic fraction is apparently variable, being chiefly dependent on the mean age of the source rocks (Doe, 1970). A study by Chow and Patterson (1962) of easily-leachable leads from Pacific and Atlantic Ocean sediments indi-cates that where sediments represent well-mixed samples of average crustal rocks, lead in the clay fraction at the time of sedimentation will l i e close to the primary growth curve or its extension. However, in restricted basins draining Precambrian terrains, such as Hudson's Bay (Chowand Johnstone, 1963), leads are likely to be variable and radiogenic. (b) Radiogenic lead in U- and Th-bearing minerals (e.g. zircon, sphene, monazite). Although they contain only a small percentage of the total lead in most rocks, minerals containing uranium and thorium generally have very high lead isotope ratios (relative to Pb20**). For example, zircon commonly has Pb 2 0 6/Pb 2 0 l t ratios in the range 200 to 1,000 (see Doe, 1970), which is extremely radiogenic in comparison with lead compo-sitions in the clay fraction or in feldspars. (c) Lead in silicate lattice sites, mainly in substitution for K + (e.g. in feldspars). The bulk of lead in crystalline rock is in the feldspars. Extensive studies of lead isotopes in feldspars from granitic rocks (e.g. Zartman and Wasserburg, 1969; Doe, 1970) show that these leads do not depart markedly from the primary growth curve, partly because feldspars 35 contain very l i t t l e U and Th. Table 2.II shows some results from a unique study of lead in different mineral phases of a Precambrian granite, and the isotope ratios quoted give a good indication of the sort of isotopic variation that can be expected to exist in the minerals of sediments derived from Precambrian shield areas. Table 2.II Pb 2 0 6/Pb 2 0 t t ratios and total lead contents of a l l minerals of a Precambrian (^ 1.1 BY) granite studied by Tilton and others (1955). (The precision of these measurements is low by present-day standards.) Mineral (acid washed) Mineral wt. % Pb2 0 6/Pb 2 0 1 + Approximate % of total lead in each mineral perthite 52 18.6 57.9 quartz 24 18.6? 15.5 plagioclase 20 18.2 8.9 sphene 0.4 39.1 11.3 magnetite 0.4 36.7 0.1 zircon 0.04 »1,000 ? 2.2 apatite 0.02 31.9 0.3 pyrite 0.02 20.3 0.9 Total 97% acid washed ^ 2 composite (total about 6 ppm) From the above discussion, i t is concluded that the lead released to an interstitial brine depends on the amount, availability and composition of lead in each site. Variations of these quantities, coupled with variations in the extracting efficiency of the brine, say with time, temperature or salinity, could easily be the cause of the vari-ations in composition shown by the late Precambrian group of Purcell leads (figs. 2.6,2.7,2.8). 36 If leaching occurs on a large scale, the amounts of lead nor-mally found in fine-grained sediments (20-25 ppm.; Wedepohl, 1971a) are enough to form large deposits, i f concentrated. Because the brine must therefore leach lead from a fairly large (but not unreasonably large) volume of sediment, the brine must have a fairly large circulation system. It is suggested, therefore, that different deposits in the same general area should have similar isotope compositions because of hcmogenisation in the brine. Figure 2.9 illustrates changes in lead isotope compositions which could be due to regional changes in the composition of lead in connate brine. There is a correlation between lead isotope composition and the size of lead-zinc ore deposits in Purcell rocks. Leech and Wanless (1962) showed that large deposits (and small veins in Moyie intrusives) have similar compositions. They belong to the group referred to here as "late Precambrian leads". Small deposits are more radiogenic, are isotopically variable, and belong to the "Mesozoic-Cenozoic" group. The same generali-sation is apparently true for deposits in Belt rocks (Cannon and others, 1962). The correlation between size and isotopic composition for Purcell galenas is shown in fig. 2.10. A l l the productive mines (table 2.Ill) are in lower Purcell host rocks with the exception of two large mines (Mineral King and Paradise) that are in upper Purcell rocks. These two deposits are also an exception to the generalisation that large deposits contain "late Precambrian lead". A possible explanation for the similarity of lead isotopes in producing mines in Purcell strata is as follows. If the lead deposits were precipitated from fairly dilute metal-bearing brines, large volumes 37 Oo D • Creston Q \2 Fig. 2.9 Analyses of the late Precambrian leads (table 2.1) are arbitrarily divided above into two sets of different isotopic composition. These samples are plotted on the accompanying map and a correlation of composition with geographic location is suggested. 38 10000 LEAD + ZINC 10* lb. 1000 100 10 1 r-0-1 DISTRIBUTION Of ANALYSES I 1 I II I I 164) 17-0 18-0 206 / 19-0 Fig. 2.10 Correlation between size of producing mines in Purcell rocks and their lead isotope ratio Pb 2 0 6/Pb 2 0 l +. Data from table 2.1 and 2.III. Note log scale for tonnages. 39 of fluid are required. Such large volumes would be able to circulate only before induration and metamorphism reduced permeability and caused most of the water to escape from the sedimentary pile. Thus, the brine can only have been effective early in the history of the sediment, so that large scale circulation was possible only in lower Purcell time, before about 1.2 BY. This large-scale circulation could account for the large size of some deposits that contain late Precambrian leads. The later economically unimportant Mesozoic-Cenozoic mineralisation may repre-sent lead scavenged by small volumes of fluid that were mostly confined to fault systems. The writer's work supports the suggestion of Leech and Wanless (1962) that the relationship between size and lead isotope composition may turn out to be useful for prospecting (also see Cannon and others, 1962; Cannon and others, 1971; Zartman and Stacey, 1971). The position of Sullivan leads at the upper end of the short array in fig. 2.8 is curious and i t is tempting to suggest a link between the unusual size and special isotopic composition of this ore body. Sul-livan lead is enriched in Pb 2 0 6 and Pb 2 0 7 relative to the other "old" leads and i f Sullivan lead evolved from any of these other leads, say from a composition like the nearby North Star (or Sullivan 846) , admixture of large amounts of slightly more radiogenic lead, or small amounts of much more radiogenic lead are required. If Sullivan ore was emplaced at about the same time as the local high-grade metamorphism (^ 1.3 BY?; see section 2.2) in the St. Mary River area, the lead released from silicates and from accessory minerals rich in radiogenic lead during recrystallisation of Aldridge rocks could conceivably account for both the large size and slightly more radiogenic composition of the Sullivan. E l l i s (1970) has discussed leaching experiments in the laboratory and analyses of natural 40 brines which show that both salinity and temperature can increase con-siderably the amount of extractable lead and other metals in rocks (also see looms, 1970). It is interesting to note that Harrison and Grimes (1970) report a loss of boron with increasing grade of metamorphism of some Belt sediments in Montana and Idaho, this being the only obvious trend they observed in minor or trace element contents. Possibly the widespread tourmalinisation at the Sullivan represents boron removed from high-grade metamorphic rocks, just as the galena may represent trace leads from the same source. Table 2.Ill Recorded production of some metals from a l l significant deposits in Purcell rocks to 1970 (inclusive) Mine Pb million lb Zn million lb Cd million lb Cu million lb Ag million oz Au thous.oz Sullivan 13,768 11,902 3.24 7.9 229.5 4.6 Kootenay King 1.6 1.9 0.028 Estella 11.4 21.7 0.03 0.17 North Star 48 1 1.3 Stemwinder 2.1 8.8 0.063 St. Eugene 249 32 5.9 3 Aurora 1 Society Girl (0.32) (0.05) (0.004) Alice (0.047) (0.0008) Park 0.024 Midway (0.002) (0.001) (0.0008) (0.39) Mineral King 81 200 0.69 1.1 1.8 Paradise 16 8 (0.01) 0.7 Sources; Min. Mines, B.C., Ann. Repts.; Little (1970); Leech and Wanless (1962). Figures in brackets are minima, rest unrecorded. ? = unknown, but significant. 41 The model proposed above is that a l l the late Precambrian leads were leached from the enclosing Aldridge sediments and were precipitated from connate brine. Although this model accounts, in a simple way, for a number of isotopic and geologic features of Purcell leads, i t is d i f f i -cult to test with independent evidence from Purcell rocks. However, isotopic and other evidence do provide support for a similar origin for the modern lead-bearing brines of the Salton Sea (Doe and others, 1966; White, 1968) and for some lead-zinc deposits such as Pine Point (Beales and Jackson, 1968; Billings and others, 1969), and perhaps the ore deposits of the Pennines, England (Mitchell and Krouse, 1971). The age of the source The late Precambrian type of lead has been interpreted above as being derived from Purcell sediments, but the galena leads could also contain evidence of the age of the source of the sediments themselves. The array of late Precambrian lead compositions resembles a very short anomalous lead line (fig. 2.6) and the slope of a least-squares line through a l l of these points is 0.244 ± 0.009 (la). If, for simplicity, a two-stage model is assumed with termination (by mineralisation) of the second stage at 1.3 BY (=t2) , then t x = 2.62 ± 0.08 BY. This age (2.6 BY) is interpreted to be the minimum age of source rocks of the lower Purcell sediments, which were in turn the source of the lead in the galena deposits. However, because the two-stage model assumed is a gross simplication of the history of the lead in these sediments, a more complicated, multistage history must therefore be considered. The distribution of isotope ratios shown on fig. 2.6 and 2.8 resembles the patterns for theoretical multi-stage leads calculated from 42 the "frequent mixing model" (fig. 2.11) by Kanasewich, 1962, 1968; Russell, 1963; and Russell and others, 1966. The "frequent mixing model" is a multistage model that assumes there is variation in the U/Pb ratio from place to place and that lead is transferred from one U/Pb environment to another at various times, such as during an orogeny. It can be shown (see Kanasewich, 1968) that the isotopic ratios Pb 2 0 6/Pb 2 0 l t and pb 2 0 7/Pb 2 0 l t of a multistage lead after n stages are: ( P b206/ P b204) = ( p b206/p b20i +) + 1 ( U2 38/pb204) ( e X t 0 _ e X t l ) + now primeval now 2(Tj238/pb20^ ( e X t l _ e X t 2 ) + ....+ n (112 38^204) ( e X t n _ 1 - e X t n ) now now (1) (^207/^204) = (p b207/ P b204) + 1 ( U2 3 8 / 1 3 7 . 8 P b204) (e^O-e**1) + v 'now . primeval 'now 2(U238/137.8 Pb204) ( e x / t l _ e x / t 2 ) + .... + n(U 2 38/i37.8 Pb2"1*) v now ' now (e -e ) (2) where: \ t)( = decay constants of U 2 3 8, U 2 3 5 respectively t 0 = 4.55 BY (age of the earth) t l t t 2 , . . , t n = ages of mixing of leads into new U/Pb systems and *»2'** * n(u 2 3 8/Pb 2 0 l t) values vary from place to place. Note these now values are extrapolated to the present day. By assuming a range of values for the ratio (U/Pb) for 60 hypothetical leads and up to 11 stages of development, Kansewich (1962) and Russell (1963) showed that after each stage the lead compositions scattered about a straight line. The slope of this line depends only on the time of commencement of the first stage and the time at the end of 43 2 0 4 p b 16.2 15.8 15.4 6.2 Single - stoge growth curve Anomalous lead line between O and 3 0 0 0 m.y. 18.4 18.8 19.2 206 15.4 Anomalous lead line Between O and 3 0 0 0 m. y. Pb 16.4 18.8 19.2 2 0 6 p t ) 2 0 4 Pb 204 Pb Graph of lead isotope ratios for a theoretical three-stage model. Diastrophism lias caused a random mixing of uranium and lead at 3000 and 1500 m.y. ago. Mineralization occurs at 0 m.y. Graph of lead isotope ratios for a theoretical eleven-stage model. Diastrophism is postulated to occur every 300 m.y. between 3000 m.y. ago and the present time when mineralization takes place. Fig. 2.11 The results of "frequent mixing" of leads for three- and eleven-stage models. Taken from Kanasewich (1968; p. 169 and 170). the last stage, even though evolution in many stages of different U/Pb ratio may have occurred between these two times. For this reason, a two-stage model can be a satisfactory mathematical simplification of the history of multistage leads even though i t is untrue geologically. Another feature of the "frequent mixing model" is that as the number of stages increases, the distribution of points along the line decreases consider-ably, and the lateral scatter increases slightly (see fig. 2.11). The mixing process simulated by Kanasewich (1962) and by Russell and others (1966) was, however, restricted to incomplete mixing of lead in one stage with that of successive stages, and the possibility of mixing leads of the same stage was not considered. If partial or complete mixing of leads of the same stage is considered, there will be further reduction in the length of the anomalous lead line and in the lateral scatter. Richards (1971) has suggested that frequent mixing of crustal leads with more or less complete homogenisation between stages could account for the primary 44 lead ores. The addition of this second mixing process could help remove the objection raised by Kanasewich (1962) and by Russell (1963) that,if primary leads were due to frequent mixing of crustal leads, then evidence of this should be observed. It is suggested here that the Purcell leads could be the result of mixing of crustal leads with partial homogenisation at the end of some stages. The slight scatter of points is taken as evidence that mixing has been incomplete, as Kanasewich (1962, 1968) and Russell and others (1966) expected of crustal materials, whereas the association with a primary lead (Sullivan) appears to support the view of Richards (1971). The source of the leads is considered to be the Belt-Purcell sediments, as discussed previously. These sediments were derived largely from the craton to the east and i t is mixing of lead during the history of these source rocks, including orogenic activity, erosion, sedimentation and brine migration that is considered responsible for the present compo-sition of old Purcell leads. The most important homogenisations are con-sidered to be those effected during Purcell sedimentation and during brine movement. If a reasonable history can be inferred for the lead in Purcell sediments, then the approximate composition of the lead can be calculated from equations 1 and 2. Although some Purcell sediment was transported from the south or southwest, the major source of sediment was the shield to the east (section 2.2). Pre-Purcell K/Ar ages obtained from the North American shield show concentrations at 1.7 BY (Hudsonian Orogeny) and at 2.5 BY (Kenoran Orogeny) according to Stockwell (1970) and King (1969). There is also some evidence that a considerable part of the shield is as old as 3.0 - 3.5 BY, younger ages being the result of 45 successive tectonic overprinting (Slawson and others, 1963; Kanasewich, 1968; Catanzaro, 1968; Giletti, 1968; Goldich and others, 1970). Accord-ing to Price (1964, p. 422) "The large volume of fine terrigenous sediment that constitutes the bulk of the Purcell succession was derived from the older Precambrian rocks that occur in the interior of the continent, far from the site of deposition. It can be compared with the large volume of fine sediment that has been transported to the margin of the continent and discharged into the Gulf of Mexico by the Mississippi River." Thus, Belt-Purcell sediments were probably eroded from as far east as the region where Hudson Bay and the Great Lakes are today and, i f so, the detritus supplied to the basin was therefore from rocks which range in age from about 3.5 BY to 1.7 BY old. For the purposes of calculation, the history of the lead in Purcell sediments is inferred from the above to be as follows. t = 4.55 BY Lead homogenous throughout the Earth. t = 3.0 BY Substantial sialic crust formed, now part of North American Shield. Considerable variation in U/Pb from place to place from 3.0 BY on (previously uniform, (U238/Pb201*) =9.09). now t = 2.7 BY Kenoran Orogeny begins. Leads developed in one U/Pb environment since 3.0 BY mixed into other U/Pb environments. t = 1.8 BY Hudsonian Orogeny begins. Leads developed in one U/Pb environment since 2.7 BY are mixed into other U/Pb en-vironments . t - 1.5 BY Sediment eroded from rocks of Kenoran and Hudsonian age begins to be deposited into the Belt-Purcell basin and forms the Pre-Ravalli strata. Lead is partly homogenised during erosion and sedimentation but different U/Pb environments s t i l l exist in the sediments. 46 t = 1.3 BY Lead leached from Purcell sediments by connate brine is nearly homogenised and is deposited as galena ("late Precambrian group") at about 1.3 BY. No further change occurs in the isotopic composition of this lead. To simulate the above history, the compositions of 50 hypothet-ical leads were calculated from equations 3 and 4 below, which are based on the general equations given previously (1 and 2). (pb206/pb204) = (pb206/pb20t) + l(U238/pb20»») (eXj* • 55_e*3.0) now primeval now + 2«J 2 3 8/Pb 2 0'*) (e X 3- 0-e X 2- 7) + 3(U 2 3 8/Pb 2 0 u) (e* 2• 7-e X l • 8) now now + 1*(U 2 3 8/Pb 2 0 l +) ( e X l - 8 - e X l - 5 ) + 5(U 2 3 8/Pb 2 0 1 +) (e X l- 5-e X l- 3) now now (3) (Pb 2 0 7/Pb 2 0 1 +) = (Pb 2 0 7/Pb 2 0 l +) 4 . + 1(U 2 3 8/137.8Pb 2 0 l t) ( e ^ - S S - e ^ . now primeval now + 2(U 2 3 8/137.8Pb 2 0 1 +) (e^-O-e^2-7) + 3(U238/137.8 Pb 2 0 4) v now now (eX2.7„eX/l.8) + Vu238/137.8 Pb 2 0 1 t) (e*1 • ^  •5) now + 5(U 2 3 8/137.8Pb 2 0 l +) ( e ^ - S - e * ' 1 ' 3 ) (4) now Fifty values of (U 2 3 8/Pb 2 0 l +) were taken from random number J now tables and were drawn from a normal distribution with a mean value of 9.09 and a standard deviation of ± 2.0. This mean value was selected to equal that calculated for primary lead ores, and the assumption here is that the primary lead growth curve represents the evolution of average crustal leads (see chapter 1). Selection of a different standard deviation or a distribution other than a normal one would not significantly affect the results of the calculations. The calculated distribution of isotope 47 compositions at several times is shown i n f i g . 2.12a. In f i g . 2.12b, the leads of the last stage have been averaged i n groups of five to simulate p a r t i a l mixing during erosion, sedimentation and brine movement in the last stage of development. The model shown as equations 3 and 4 assumes that leads of previous stages no longer exist and cannot be added to Purcell sediment at 1.5 BY. Rocks of Kenoran age and possibly older ages do s t i l l exist, however, and the isotope ratios of lead i n feldspar, quartz and other minerals lacking U and Th may be unchanged since these times. As well, analyses of rocks from the Precambrian (2600-1600 MY) Lewisian basement complex of Scotland (Moorbath and others, 1969) suggest that such ancient basement complexes might be depleted in uranium and therefore preserve primitive isotope ratios. To simulate the effect of addition of detrital minerals from rocks of Kenoran age to Purcell sediments 5% of average (U 2 3 8/Pb 2 0 1 + = 9.09) 2.7 BY old lead was included in the calculation of composition of each of the well-mixed leads in f i g . 2.12b. For comparison with the calculated values, the observed compositions of old Purcell leads are also shown on f i g . 2.12b. No claim i s made that there is an exact correspondence between the true history of lead in Purcell sediments and the simple model discussed. The discussion does show, however, that the isotope ratios obtained from calculations based on a f a i r l y simple modified "frequent mixing model" do approximate the observed ratios. In many published interpretations (e.g. Zartman and Stacey, 1971) the calculated age of source rocks i s equated with the age of the base-ment underlying the mineralised region and i t is important to note that in the above interpretation the basement source i s "imported" as a sediment Fig. 2.12a Calculated compositions for 50 hypothetical leads at various times in a five-stage history. Small dots, circles and large dots show compositions after 2.7, 1.8, and 1.3 BY (end). Primary growth curve is for (U 2 3 8/Pb Z o <*) - 9.09, and numbers indicate b i l l i o n years. 49 16-0 16-5 17-0 Fig. 2.12b Large circles represent the compositions of 50 hypothetical leads after five stages of evolution in various randomly chosen U/Pb environments since 3.0 BY, with fairly thorough mixing at 1.3 BY (see text for details). Solid dots repre-sent measured ratios of Purcell leads of the late Precambrian group. The growth curve shown is the primary growth curve (U238/Pb20It=9.09) . If more complete mixing is assumed in the calculations, the scatter of the calculated values would more closely resemble the observed scatter. and no estimate of age of the underlying rocks is made (see fig. 2.13). In effect, the Belt-Purcell sediments are regarded as a large sample of the shield to the east, a sample that s t i l l contains faint lead isotope evidence of its original age. There are several studies of lead isotopes in sediments in which reasonable estimates have been obtained for the age of source rocks. For example, Muffler and Doe (1968) calculated (from lead isotopes) a mean age of 1.7 BY for Cenozoic detritus in the Salton Trough, California. They 50 interpreted this as the age of basement to the east, the ultimate source of the sediments. Hart and Tilton (1966) obtained an age of 2.75 BY from lead in a sample of present-day Lake Superior sediment and they noted that this age corresponded with ages obtained from the Superior Province, the source of the lake sediments. In both these studies anomalous lines that formed the basis of age calculations were obtained by leaching sediments with acid or water and analysing the leaches and residues. The galena analyses that have been interpreted in this thesis could also represent lead leached from sediments, but in this case by natural pro-cesses. Model A Model B Fig. 2.13 In Model A, the underlying basement is supposed to be a source radiogenic lead; in Model B, distant basement supplies this component via the sediments. Mesozoic-Cenozoic Leads Leads of the Mesozoic-Cenozoic group are shown on fig. 2.6 and 2.7 along with the late Precambrian group. Evidence of an exotic source for the Mesozoic-Cenozoic lead is lacking and the simplest explana-tion is that they evolved in the surrounding Purcell sediments. If so, their composition will be the result of addition of radiogenic lead to pre-existing lead, like the widespread Late Precambrian group. The age(s) 51 of emplacement of the variable radiogenic leads is uncertain. Scattered, inconclusive evidence suggests a Mesozoic-Cenozoic age for some mineralis-ation. For example, Leech and Wanless (1962) inferred a Mesozoic age for leads in faults with important Mesozoic movement (e.g. Leader, Pitt Creek). As another example, two minor deposits (Palmyra, Lily May Extension) in the Rocky Mountains occur in quartz veins cutting syenite dikes (Rice, 1937), and these intrusives are probably of the same age as Cretaceous or ?Tertiary intrusives elsewhere in the southern Rocky Mountains (Leech, 1958; Price, 1962). It is not possible to say from lead isotope data whether or not a l l the radiogenic leads are of Mesozoic-Cenozoic age. The data on fig. 2.6 appear related to two near-parallel anomalous lines, both of which intersect the array of Late Precambrian leads. Why the data should f a l l on two apparently distinct lines is not clear, but there is a weak relationship between age of host rock and isotope composition. Leads in Aldridge rocks f a l l on either line; those in Creston rocks l i e on the lower line; those in upper Purcell strata l i e on the upper line. These relationships can not be interpreted confidently. Curiously, the Pb 2 0 8/Pb 2 0 1 + (fig. 2.7) plot also shows two distinct lines, but the mineral deposits show different groupings from those of fig. 2.6 (compare Midway, Palmyra, Leader on both graphs). That the anomalous lines do not pass through the cluster of old leads (fig. 2.7) is evidence that the younger leads are due to growth in various U/Pb environments, rather than to simple mixing of old leads with a single widespread radiogenic lead (see Kanasewich, 1968). The slopes of least-squares lines through Mesozoic-Cenozoic 52 leads are given in table 2.IV with computed ages of commencement of growth of the radiogenic component. These ages were calculated assuming two stage development in which growth was terminated by mineralisation at 100 MY. Table 2.IV Two-stage model ages for Mesozoic-Cenozoic leads from Purcell Supergroup rocks. Deposits Slope ti(calculated) t2(assumed) Lower Line Midway, Leader, Pitt Creek, Birdiel, Anderson, Palmyra, Lily May Extension 0.094 ± 0.011 1,470 ± 230 MY 100 MY Upper Line Dan Howe, Polaris, Rose Pass, Lockhart Creek, Warhorse, Mineral King, Paradise 0.094 ± 0.006 1,470 ± 130 MY 100 MY The computed t j , usually interpreted as the age of the source rocks, is in good agreement with the inferred age of older Purcell strata. The agreement is less impressive than i t appears, however, since selection of lines is somewhat subjective. Further comments on the Mesozoic-Cenozoic leads are made in section 2.4. 2.5 COMPARISON WITH GALENA LEADS FROM BELT SUPERGROUP ROCKS IN NORTHWESTERN U.S.A. Sixty-six lead isotope analyses of galenas in Beltian sediments of N.W. Montana and N. Idaho have been reported recently by Zartman and Stacey (1971). The region studied by Zartman and Stacey is shown as fig. 2.14 and their isotopic results as fig. 2.15. 53 In major aspects, the isotopic compositions of Belt galenas are very similar to those of Purcell galenas, but there are fine-scale differences. Furthermore, there are features of Belt leads which were either not discussed by Zartman and Stacey, or which can be interpreted in alternative ways. The possibility that any interpretable differences between the two sets of analyses are of instrumental origin is remote, since they are of similar precision (la<0.1%) and are referred to a common standard (Broken H i l l Standard). Late Precambrian Leads Zartman and Stacey (1971) stated that the uniformity of the Late Precambrian leads over a large region was a good indication that the calculated single-stage ages of 1.2 - 1.5 BY were reasonable estimates of the time of mineralisation. The absence of this lead from rocks younger than the Belt, and the Precambrian ages of uraninite associated with lead mineralisation in faults at Coeur d'Alene were mentioned as evidence that the deposits probably formed during or soon after sedimentation. The resemblance of the elongate array shown by the Precambrian leads (fig. 2.15) to a two-stage, short-period anomalous lead line was discussed and they calculated an "instantaneous" age (ti=t 2) of 1.3 BY from the slope of this array. As an alternative, a t\ of 1.7 BY was assumed and an age of mineralisation (t 2) of 825 MY was calculated. They stated that the source of the Coeur d'Alene lead remained a matter of speculation. Introduction of lead as a chemical precipitate during sedimentation, or from the mantle/lower crust along major fracture systems were offered as possible explanations of the isotopic similarity I IT* 116* 115* 114" EXPLANATION Foult Boshed where approximately located. Arrows snow relative direction of displacement. Age of mrnerotizotion (based on lead isolopes) Mesozoic or Cenozoic. Numerals refer to groupings m Table 3. Basalt of Columbia Plateau Vtrtcowic rocks end continental sedimentary rocks Lorqety endesite, lotite, and bosalt flows e Lflle Precambrian J < Granitic rocks Largely gronodiorite ond quartz monzonite Sedimentary rocks Largely Combrion quartzite, shale, and limestone; includes some rocks of younger Paleozoic age in Washington Hign-g/oo* met amorphic rocks Age varied ond uncertain. Strotigraphic position uncertain I I Windermere series of Canada Coarse conglomerate, phyllite.ond greenstone Fig. 2.14 Region studied by Zartman and Stacey (1971). Taken from p. 858 and 859 of their paper. Belt Supergroup Subdivided where possible into three units: upper, Missoula Group (Libby and Striped Peok Formation); middle, Waiioce Forma-tion ond Ravalli Group (St. Regis, Revett, ond burke Formations); lower, Prichard Formation 55 40.00 35.001 L — I — 1 — L 15.60 °- 15.40 15.20 i — i — i — i — I — i — i — i — r ceo J i i i_ i i i i J I I L_ I I L _l I I L. 16.00 17.00 Pb 18.00 206 / p b 204 19.00 20.00 Lead isotopic composition "of galenas from Belt Supergroup rocks in northwestern Montana and northern Idaho. Solid symbols: late Precambrian deposits (symbol with star represents Sullivan Mine, British Columbia, from Cooper, Reynolds, and Richards. 1969); open symbols: Mesozoic or Cenozoic deposits. Anomalous lead lines (with numbered arrows) shown for the 4 groupings given in Table 3. Primary growth curves are those of Stacey, Delevaux, and Ulrvch (1969) using their curve A for the P b ^ / P b 2 0 4 vs. P b ^ / P b 3 " diagram. F i g . 2.15 Lead isotope results of Zartman and Stacey (1971). Taken from p. 851 of their paper. 56 of the leads. Although they considered i t unlikely, Zartman and Stacey could not dismiss the possibility that some deposits of the Precambrian group were emplaced in Phanerozoic times by remobilisation of Precambrian galena. The present writer has already argued that the late Precambrian lead of galena deposits in Purcell rocks may have been leached from sur-rounding strata and deposited from connate brines and this explanation is extended to the old Belt leads. This same explanation has been pre-viously argued by others, including Hershey (1913, 1913a), Davidson (1967), and Sorenson (1972), but i t has been less popular than other views, for example that the lead and other metals of the Coeur d'Alene district came from underlying granitic rocks (Ransome and Calkins, 1908) or a deep-seated (sub-crustal?) source (Hobbs and Fryklund, 1968). In contrast to old Purcell leads, which are confined to Aldridge rocks, the old Belt leads are found in Pre-Ravalli, Ravalli and Piegan rocks (fig. 2.16). The leads in Pre-Ravalli rocks are considered separately from the others because, in the U.S.A., Pre-Ravalli leads seem to be different in compositional trend from leads in the younger Precambrian rocks, and because in Canada old Purcell leads are a l l in Pre-Ravalli rocks. Pre-Ravalli leads These leads are identified in fig. 2.17, a composite plot of Belt-Purcell leads of the late Precambrian group. Least-squares lines through Beltian Pre-Ravalli leads, through Purcell Pre-Ravalli leads and through the combined data are shown in table 2.V along with calculated ages of the source rocks. in Fig. 2.16 Plot of Pb 2 0 7/Pb 2 0 l + v. Pb 2 0 6/Pb 2 0 l t for analyses of Purcell galena leads (table 2.1) and Belt galena leads (Zartman and Stacey, 1971; their tables 1 and 2). The key identifies the host rock. The growth curve is drawn for (U 2 3 8/Pb 2 0 l t) - 9.09. present Fig. 2.17 Plot of Pb207/Pb2()l* v. Pb 2 0 6/Pb 2 0 , + for the "late Precambrian group" of galena leads in Purcell rocks (table 2.1) and In Belt rocks (Zartman and Stacey, 1971; p. 852 and p. 854). The symbols identify the host rock of the galena deposit. * A A R— Ravalli host rock _ l _ VO 16-2 16-3 16-4 16-3 ,206/„. 204 Fig. 2.18 Belt-Purcell leads of the late Precambrian group plotted above are split arbitrarily into three sets and these are plotted on the accompanying map using the same symbols. A correlation between isotopic composition and geographic location is suggested. Data from Zartman and Stacey (1971, p. 852, 854) and table 2.1 (this thesis). 60 Table 2.V Isochron slopes and two-stage ages for Pre-Ravalli leads Host Rock Data Points Slope (±la) (York, 1969) t 2 (BY) (calculated) ti (BY) (assumed) Belt (Prichard F.) 14 0.246 ( ±0.023) 2.64 ± .19 1.3 Purcell (Aldridge F.) 33 0.244 (± 0.009) 2.62 ± .08 1.3 Combined Belt-Purcell 47 0.261 ( ±0.006) 2.76 ± .05 1.3 It was suggested in section 2.3 that the immediate source of the late Precambrian leads in Purcell rocks was the lower Purcell sediments and that the "age of source rocks" calculated from the galenas was the minimum age of the source of these sediments. Results of calculations based on a modified "frequent mixing model" were presented to show that this interpretation was a reasonable possibility. The age calculated above (2.6 to 2.8 BY) for the source of the Pre-Ravalli leads is there-fore interpreted in the same way, that is as a minimum age for the source rocks of Pre-Ravalli sediments. There is a small, real,difference between the leads from Purcell and Belt rocks (fig. 2.17), as first suggested by Cannon and others (1962). This difference is shown on fig. 2.18, where the isotopic composition of Precambrian galena leads in Pre-Ravalli and Ravalli rocks is shown to become, in general, less radiogenic to the south. This geographic variation may be due to regional differences in the isotopic composition of extractable lead in the sediments. In section 2.3, a model was proposed for Purcell leads in which 5% of extractable lead in the sediments was supposed to be 2.7 BY old lead in feldspars, quartz, etc. eroded from rocks which were not greatly affected 61 by the 1.7 BY orogeny. If a larger percentage of such lead was present In the source region of Belt sediments than that of Purcell sediments, this would easily account for the regional difference in galena leads. Significantly, basement rocks east of the site of Purcell sedimentation are about 1.7 BY old (Burwash and others, 1962), whereas basement rocks exposed near the southern part of the Belt-Purcell basin (fig. 2.1) are about 2.6 BY old and perhaps older (Giletti, 1968; King, 1969). As an example of the possible effect of this difference in age of the adjacent Fig. 2.19 Large circles represent the compositions of 50 hypothetical leads after five stages of evolution in various U/Pb environ-ments since 3.0 BY, with mixing at 1.3 BY. For the large open circles, 5% of 2.7 BY lead has been added; for the large striped circles, 10% of 2.7 BY lead has been added. The solid dots are measured Purcell leads (table 2.1), the small circles are Belt leads (Zartman and Stacey, 1971). Growth curve is the primary growth curve of Stacey and others (1969), for (Tj238/pb204) = 9.09> now 62 source regions assume the "frequently mixed model" calculated for Purcell leads in section 2.4 is applicable to Belt leads, except for addition of 10% of 2.7 BY old lead for Belt leads instead of 5% for Purcell leads. Isotope ratios calculated on this basis are compared with measured ratios from leads in Purcell and Belt rocks in fig. 2.19 and there is a gross resemblance of the two patterns. It is suggested, therefore, that regional variation in galena leads in Belt-Purcell sediments reflect differences in extractable lead in sediments from one end of the basin to the other. In turn, these differences could reflect differences in the age of the source rocks. Ravalli and Piegan leads Interpretation of Ravalli-Piegan leads is complicated by the fact that some Ravalli leads are indistinguishable from Pre-Ravalli leads. With some reservations, Ravalli leads are included with the Piegan leads, most of which are clearly different from Pre-Ravalli leads. The slope of a least-squares line through the Ravalli and Piegan leads is 0.121 ± 0.008, quite different from leads in the older rocks. There are a number of choices of t j and t 2 that could f i t both this slope and the geological evidence,and three of these are summarised in the diagram below (fig. 2.20) Case (A) Case (B) t1 = 1.7 BY (assumed) t 2 = 0.6 (±0.2) BY ti = 1.3 BY (assumed) t 2 = 1.1 (±0.2) BY Case (C) • t! - 1.2 (±0.1) = t 2 TIME Fig. 2.20 Diagram showing three possible interpretations of the slope for old Ravalli-Piegan leads. 63 For case A, t i was chosen to be 1.7 BY because this is the approximate age of nearby Hudsonian basement rocks. Older ages for ti could be assumed, but t 2 then becomes younger and the scatter of the points could be expected to be greater, a point made by Zartman and Stacey (1971) in a similar calculation. Case B represents a model where radiogenic lead developed in Belt sediments between 1.3 and 1.1 BY is mixed with remobilised lead of the older 1.3 BY mineralisation. If ages younger than 1.3 BY are chosen for tj then t 2 becomes older until, as in case C, tj = t 2 . Case C represents a model where radiogenic lead pro-duced in Belt sediments at 1.2 BY is mixed with leads of the 1.3 BY mineralisation. The age 1.2 BY is the oldest possible time of mineralisa-tion. In both cases B and C upward remobilisation of Pre-Ravalli leads in late Precambrian time is implied. The writer prefers case B because i t is most consistent with the origin proposed for the Pre-Ravalli leads, but there is l i t t l e practical difference between case B and C. There may be a correlation between the two ages (1.3 and 1.1 BY) of mineralisation inferred above from lead isotope data and the two older Rb/Sr ages obtained from Beltian sediments by Obradovich and Peterman (1968). This correlation is shown in a diagrammatic way in fig. 2.21 where ages obtained from whole-rock Rb/Sr isochrons are compared with those obtained by galena lead isochrons. Obradovich and Peterman (1968) interpreted their Rb/Sr data as ages of sedimentation with 200 MY-long periods of non-deposition between them but they were unable to cite field evidence of such long time breaks. The writer's interpretation, based on lead isotopes in mineral deposits, is quite different in viewpoint. The lead ages have been related to min-64 eralisation events involving circulation of connate brine, metamorphism, and Precambrian faulting and these events are believed to have taken place within sediments deposited without major breaks, but probably with numerous diastemic intervals. It seems possible that the Rb/Sr ages are not ages of sedimentation but are instead ages of the same hydrothermal events that the lead is thought to date. Time 0-8 " 0-9 -10 11 1-2 1-3 1-4 1-5 1-6 1-7 1-8 1-9 20 10 9 yr . CL. UJ CO Z o o >-oc U Rb/Sr Pb sedimentation event t^-^-^HKr-I-I! sedimentation t i event Hi! + hydrothermal event hydrothermal event Fig. 2.21 Comparison of Rb/Sr dating of Beltian sediments by Obradovich and Peterman (1968) with the lead isotope interpretation of the "old" Belt-Purcell galenas given in this thesis. Mesozoic-Cenozoic Leads Zartman and Stacey (1971) concluded that the isotopic composition 65 of their Mesozoic-Cenozoic group of leads (fig. 2.15) was largely con-trolled by heterogeneity in the basement rocks. They grouped leads of four different geographic areas (fig. 2.14) and calculated source-rock ages (t^) of 1.6 and 1.5 BY for the two western groups (3 and 4), and 2.7 and 2.8 BY for the two eastern groups (1 and 2), assuming a t 2 of 100 MY. They pointed out that these were the two ages of major tectonism known from the crystalline basement rocks, although they recognised that the slopes they used for the calculations were not firmly established. Zartman and Stacey emphasised a geographic grouping of samples. It is noted here that a crude relationship of composition to stratigraphy also exists. In fig. 2.22 a l l the Mesozoic-Cenozoic leads in Pre-Ravalli rocks are compared to those in Ravalli and younger rocks and a rather ill-defined grouping is seen. Although this grouping is not clear, the distribution Is somewhat like that for the Precambrian leads, suggesting that the older pattern could have influenced or controlled the compositions of the Mesozoic-Cenozoic leads. Fig. 2.22 also illustrates one possible way in which the distribution of old leads could be reflected in the dis-tribution of younger leads. If this sort of evolution has occurred, ages determined from the oversteepened slopes of the lines will be slightly older than they should be. Although i t is speculated from fig. 2.22 that the Mesozoic-Cenozoic leads could be controlled by the older pattern found in the Precambrian leads, more information is needed to test this "stratigraphic" alternative proposal to the very reasonable "basement-control" interpretation of Zartman and Stacey. 66 Pb 307 Pb EVOLUTION FROM A LINEAR ARRAY EVOLUTION FROM TWO ARRAYS KEY Q PRE-RAVALLI 0 RAVALLI, POST—RAVALLI Pbypb™ Fig. 2.22 The pattern of compositions which results from evolution (growth, mixing) from two arrays is shown in the upper sketch. In the lower diagram, actual compositions from Belt-Purcell rocks are shown. It is suggested that the Mesozoic-Cenozoic leads could be controlled by the compositional pattern of the Precambrian leads. 67 2.6 SUMMARY (1) The main conclusions reached by Leech and Wanless (1962) in a study of lead isotopes from galena deposits of the East Kootenay district have been confirmed by the analyses reported in this study. The earlier conclusions are as follows. (a) Two main groups of lead isotopes exist: a uniform group, and a variable, more radiogenic group. Leads of the uniform group have single-stage ages in the range 1.2 - 1.4 BY; the others appear to be Mesozoic-Cenozoic in age. (b) Most of the large lead-zinc deposits of the region, many of them stratiform, contain lead of the old, uniform group. Deposits containing Mesozoic-Cenozoic lead are small and occur in veins. The unimportant veins in Moyie intrusions contain lead of the old, uniform group. (2) The new analyses permit the following additional conclusions. (a) The uniform "late Precambrian leads" form a short elongate array resembling a multi-stage anomalous line. If mineralisation was at 1.3 BY, the source rocks of the lead are 2.6 BY old (two-stage model). (b) A correlation exists between the isotope ratios of the late Pre-cambrian leads and geography, the least radiogenic leads being in the southern part of the East Kootenay district. (c) The Mesozoic-Cenozoic leads appear to be related to two anomalous lines. The groupings could be related to the host rocks, but this relationship is not clear. If mineralisation is assumed to be at 100 MY, the age of source rocks of the anomalous lead is calculated to be about 1.5 BY. It is therefore suggested that Mesozoic-Cenozoic leads came from Purcell sediments and that they are the result of addition of radiogenic 68 lead (developed between about 1.5 BY and 100 MY) to lead resembling the late Precambrian group. (3) The following conclusions can be drawn from a comparison of leads in Purcell rocks with the analyses of leads in Beltian galenas reported by Zartman and Stacey (1971). (a) Both sets of analyses show the same major features — an old (1.2 - 1.5 BY) uniform group of leads and a young (Mesozoic-Cenozoic) variable group. In both regions, the old leads are characteristic of the principal lead-zinc mines. (b) In Beltian sediments, lead of the old group is found in rocks as young as the middle Belt (Piegan Group), whereas in Purcell sediments a l l the deposits known to contain old lead are confined to the lowest Purcell (Aldridge Formation). If isotope ratios of old leads in Aldridge rocks are combined with those of old leads in correlative Prichard rocks an age of about 2.7 BY can be estimated for the source rocks of the lead (assuming mineralisation at 1.3 BY). A less certain anomalous line through old leads in Ravalli-Piegan rocks suggests these leads were emplaced at about 1.1 BY. (c) If a l l the Mesozoic-Cenozoic leads from the Belt-Purcell basin are considered together, a pattern grossly similar (though distorted) to the pattern of the old leads is seen,and i t is suggested that the compo-sitional pattern of the old leads could have influenced or controlled that of the younger. The distortion is that expected by evolution of lead isotopic compositions during the interval since the Precambrian mineralisations. (d) A geographic pattern shown by isotope ratios of old Belt leads 69 is continuous with a geographic pattern shown by old Purcell leads in Canada. (4) In conclusion, the inferred history of the galena leads in Belt-Purcell sediments is as follows. The old lead could have been derived from the Belt-Purcell sediments by circulating connate brines. Thus, because the lead repre-sents an average of a large volume of crustal rock and because i t is well mixed during sediment transport and later during brine movement, the precipitated lead is very uniform in composition and has a single-stage age very close to the age of emplacement (believed to be about 1.3 BY). Because of the slight variability in extractable lead both regional-ly and locally, these old leads retain faint evidence of the age of the 2.7+BY old source-rocks of the Belt-Purcell sediments — the Canadian Shield. The 2.7 BY age is thus not interpreted as the age of underlying basement rocks. There appear to be two groups of old leads, those in Pre-Ravalli rocks and those in overlying Ravalli and Piegan strata. It is the leads in Pre-Ravalli rocks that contain evidence of the ancient source-rocks, and of a mineralisation event at 1.3 BY. The old leads in Ravalli-Piegan rocks could have been derived from growth of radiogenic lead in Belt sediments between 1.3 and 1.1 BY, and by mixing of this lead with 1.3 BY old lead. The large concentrations of lead and zinc that account for the economic importance of the early mineralisation could be due to the opportunity for wide-spread leaching of lead and zinc by a circulating interstitial brine early in the history of the sediments. Later restric-tion of circulation to fault systems, and unavailability of large volumes 70 of fluid could account for the smaller size of the younger mineral deposits. Some of the interpretation offered here is admittedly speculative, and this is partly due to a lack of cr i t i c a l , independent, local evidence of the age and origin of Belt-Purcell leads. However, the general model proposed is consistent with recent investigations elsewhere of modern brines, some lead-zinc deposits and lead isotopes. 71 CHAPTER 3. LEAD ISOTOPES FROM THE KOOTENAY ARC, BRITISH COLUMBIA 3.1 PREAMBLE Analyses from a representative selection of the lead-zinc deposits of the Kootenay Arc have been reported by Sinclair (1966) and Reynolds (1967). Most of this work has been repeated, however, and a brief explanation of why this was done is necessary. Sinclair (1964, 1966) made a detailed study of Kootenay Arc leads and found the isotopic ratios of a l l of them plotted on a single anomalous lead line. Reynolds (1967), in the course of a study of rock leads and ore leads from the Nelson Batholith, reanalysed three of Sinclair's samples. He found that these three samples and two additional samples did not f i t Sinclair's line, but f e l l instead on a parallel anomalous line below i t (see fig. 3.3). Before any new work was done, i t seemed essential to determine whether the slight difference between these lines was real. Accordingly, most of the samples analysed by Sinclair (1964), Reynolds (1967) and one sample of Kanasewich (1962) were reanalysed. These new analyses confirm that both anomalous lines are real. 3. 2 OUTLINE OF THE GEOLOGY AND LEAD-ZINC MINERALISATION OF THE KOOTENAY ARC Geology Ross (1970), Yates (1970) and Fyles (1970) have given recent accounts of the geology of the Kootenay Arc. The following outline is summarised mainly from Fyles (1970). The Kootenay Arc is a curved structural belt (fig. 3.1) bounded 72 FIG. 3'1 The Kootenay Arc mainly after Fyles (1967, fig. 12; 1970, fig. IV-1] I Silver Giant & Badshof-Reeves limestone ==Grnnitir rocks A large deposi'f-more i/ian 50 * JO fc. Pb+Zn ^Mineral King 73 to the east by little-deformed Precambrian sediments of the Purcell Anti-clinorium, and to the west by highly-deformed gneiss and schist of the Shuswap Metamorphic Complex. The arc extends over 250 miles from near Revelstoke (B.C.) where i t merges with the Shuswap terrain, into Washington State where i t is covered by flood basalts of the Columbia Plateau. Rocks involved in this structural belt are complexly deformed and variably metamorphosed sediments and volcanics of late Proterozoic to late Mesozoic age (fig. 3.2). The succession is essentially conformable, although a late Palaeozoic disconformity and an early Mesozoic disconformity appear to be present, and others may exist. An important lithologic marker is the Lower Cambrian Badshot-Reeves limestone which is exposed in a narrow belt in the eastern part of the Arc (fig. 3.1). To the east of this belt rocks are generally older than the limestone unit, whereas to the west there is a thick succession of younger sedimentary and volcanic rocks that includes rocks as young as Jurassic. Major deformation and metamorphism (locally to sillimanite grade) are known to have ceased before or during intrusion of the Nelson Batholith, which has a cooling age of 160 - 170 MY (Gabrielse and Reesor, 1964; Nguyen and others, 1968). The exact timing of the polyphase folding and metamorphism, however, is in dispute (Ross, 1970; Wheeler, 1970). The evidence does suggest that important deformation took place both before and after deposition of the Upper Mississippian-Pennsylvanian Milford Group. The main metamorphism is considered to be early Mesozoic by Wheeler and others (1972), and late Palaeozoic by Ross (1970). Jurassic to Middle Cretaceous granitic intrusions Include the Nelson and Kuskanax batholiths (fig. 3.1). Small intrusions of Tertiary 74 monzonite,syenite and granite are found in the southern part of the Arc. Widespread lamprophyre dikes cut a l l rocks. WEST EAST <1 a. a ft. At) h°na INTERMEDIATE FLOW ROCKS INTERMEDIATE VOLCANIC BRECCIA CONGLOMERATE HI QUARTZITE, SAND-STONE, SILTSTONE SHALE .ARGILLITE. CARBONATES PILLOW LAVA QUARTZ-POOR PLUTONIC ROCKS. QUARTZ-RICH PLUTONIC ROCKS. HIGH GRADE METAMORPHIC ROCK |v\/v\| DISCONFORMITY MIGMATITIC GNEISS rVWv) UNCONFORMITY AND SCHISTS. Fig. 3.2 Relationships of formations in the southern part of the Kootenay Arc. Taken from Monger and Preto (1972, p. 32). Lead-Zinc Mineralisation Fyles (1966, 1967, 1970) has classified lead-zinc deposits of the Kootenay Arc into two broad structural groups, one mainly concordant and the other mainly transgressive. According to Fyles (1967, p. 69) these structural groups are of significantly different ages, the con-cordant deposits being the older. Table 3.1 summarizes many details of 75 the deposits in the Kootenay Arc and is taken from Fyles (1967). Concordant deposits Deposits of the concordant group are the largest and most pro-ductive. They are older than the IJelson Batholith and may be older than the regional metamorphism (Fyles, 1970; Muraro, 1966; Macdonald, 1970, in preparation). In general, deposits of the concordant group are dis-seminations of pyrite, sphalerite and galena, with or without pyrrhotite, in Cambrian carbonate rocks. Fyles (1967) divided the concordant group into three structural types. Metaline type — In unmetamorphosed dolomitised host; faulting and brecciation important; gently folded. Salmo type — In low grade regionally metamorphosed and dolomitised host; complex penetrative deformation. Shuswap type — In calcareous metasediments of medium to high metamorphic grade; extensive and stratiform; complexly deformed. Transgressive deposits The transgressive deposits include fissure and replacement veins, both simple and complex. They are generally richer in silver, lead and zinc than are deposits of the concordant group (Fyles, 1966; Sutherland-Brown and others, 1971) and Sinclair (1964) has shown that there are also distinct differences in the trace element content of galenas (notably Ag, Sb, TI, Sn, Bi, Cu) of the two groups. Three once-important camps where mineralisation was of the transgressive type are Ainsworth (and Bluebell), Slocan City and Sandon, and at these camps mineralisation is younger than a l l facies of Nelson plutonic rocks and younger than most of the widespread lamprophyre dikes 76 (Cairnes, 1934; Fyles, 1967). In the Ainsworth and Bluebell areas there is a close relation between the orientation of veins and lampro-phyric dykes and Fyles (1970) inferred from this that the mineralising process in these areas was connected with the magmatic activity. The lamprophyres have long been considered to be early Tertiary (e.g. Rice, 1941; Fyles, 1970) on geologic grounds, and radiometric ages of about 50 MY for several of these dykes in northeastern Washington (Yates and Engels, 1968) and 49 MY for a dyke near Salmo (Macdonald, in preparation) support this age assignment for most of the lamprophyres. Nguyen and others (1968), however, obtained a K/Ar age of 170 MY for a lamprophyre dike that cues the Nelson Batholith, and this suggests that a few older lamprophyres exist. Regarding a connection between Nelson granitic rocks and trans-gressive deposits, Fyles (1970, p. 52) concluded "The Nelson granitic rocks, which include Nelson and Kuskanax batholiths and a number of related stocks mainly in the southern part of the Arc, as now exposed, seem to have been incidental to the mineralization process. Early workers regarded these granitic rocks as the source of the lead and zinc, but detailed studies have not found evidence that any one deposit or group of deposits is genetically related to a specific pluton." 3.3 PREVIOUS LEAD ISOTOPE STUDIES* Sinclair (1964, 1966) summarised the scattered analytical work on Kootenay Arc galenas prior to his study, the first detailed report of ^Previous workers have used different values for the age of the earth and for primeval ratios. No modification of published ages has been made. Deposit Sulphides In Addition to Galena. Sphalerite, and Pyrite Gangue Form of Ore bodies Structural Control of Orcbodies Formation Rock Types Metamorphic Minerals Grade and Type of Metamorphism Ago Age of Mineralization Classification Met aline district. Dolomite rock, calcite, quartz. Irregular low-dipping tabular, sub bedded. Adjustment related to regional fault-ing and fault zones. Upper Met&llne limestone. Siltcifled dolomite breccia. Chlorite, muscovite. Low.gr ade regional metamorphism. Middle or late Cambrian. Uncertain. Metal Ine type. Reeves MacDonald. Pyrrhotite near lam rrophy re dykes. Dolomite rock, calcite, minor bariie. Elongate, trough-like, irregular in detail. Steeply plunging isoclinal syncline. Reeves limestone. Dolomitized zone in limc>t>me in a sequence of phyl-litcs and arpillites. Muscovite, chlorite, chloritoid. Low regional. Lower Cambrian. Pre-lamprophyre, during regional folding and metamorphism. Salmo type. Jersty. H.B. , Aspen, JackpoL Pyrrhotite. arsenopyrite. Dolomite rock, calcite, tremolite-talc and garnei-diopside alterations. Elongate bodies, low plunpe, irregular in detail. Long dimension parallel to main fold axes. Reeves limestone. Dolomiiircd zones in limestone, in a sequence of phylhte. argillite, and hornfets near granitic : locks. Biotite. garnet diopside, zoisite. Biotite grade regional, with superimposed contact metamorphism. Lower Cambrian. Pre-"granite" but some mineraliza-tion post contact metamorphism (11.B.J Salmo type. Duncan. Pyrrhotite. chalcopyrite. Dolomite rock, cherly dolomite, calcite. Hlonpate bodies, low plunpe, irregular in detail. Lonp dimension parallel to main fold axes. Badshot limestone. Dolomitized and siliceous zones In limestone. Biotite, chloritoid, garnet. Biotite and garnet grades, repional metamorphism. Lower Cambrian. During regional folding and metamorphism. Salmo type. Bannockbum " Shells* Vein." Tetrahedrite. Calcareous quartziie. Elorpate, low plunge. Lonp dimension parallel to main fold axes. Uppermost Hamill Group. Calcareous quaruitc. phyllite, limestone. Clorite. muscovite. Low regional metamorphism. Lower Cambrian. During or after regional folding. Salmo type. Jordan River. Pyrrhotite. Calcareous schist, quartz, minor baiicc. Bedded. Folded with the metamorphic sequence. Shuswap metamor-phic complex. Calcareous schists, maiblc a-id quait.-ite. Garnet sillimanite. Hiizh-grnde regional metamorphism. Uncertain. Pre-metamorphism end folding. Shuswap type. Ains worth Comp. Pyrrhotite, chalcopyrite. arsenopyrite. Quartz carbon-ates, fluorite. Tabular. Veins in normal faults. Milford Group, Limestones, mica and hornblende schists, quartzites. Garnet, staurolite, biotite. Biotite and garnet grades, regional metamorphism. Late Palxozoic. Post r.mprophyre, post foldint: and metnir.orphWrn. . Vein type. Bluebell. Pyrrhotite. chalcoryrite, arsenopyrite. Quartz carbon-ates, limestone. Irregular. Replacement of limestone outward from fractures. Badshot. Crystalline limc-stonej qiartzite, and schii-t. Kyanite-silliminite. High-grade regional metamorphism. Lower Cambrian. Post lamprophyre, post folding and metamorphism. Fracture controlled replacement. San don Camp. Arsenopyrite, tetrahedrite, chalcopyrite, ruby silver. Quartz, sidcrite, calcite. Tabular to irregular. Veins in regional sbear-fauMs. Orrihoots strongly controlled by structure of wallrocks. Slocan Group. Dark-grcy argil-litcs, siliceous and limy argillites. Muscovite, chlorite, seiicite. Low-grade regional metamorphism. Trlasslc. Late stages of folding; post lamprophyre, post Nelson. Complex vein-systems. ZLnctoo—Lucky l im. Calcite, limestone, dolomite rock. Irregular. Replacement of limotoiic outward from fractures. Lucky Jim lime-stone in Slocan Group. Fine-grained limestone. Muscovite. Low-grade regional metamorphism. Tri as sic. Post lamprophyre. Fracture controlled replacement. Cork Province. Chalcopyrite. Sidcrite, quartz, calcite. Lenses within a shear zone with steep plunge. Irregularities In shear zone. Slocan Group. Slates, argillites, thin limestones. Biotite. Low-grade regional and thermal metamorphism. Triassic. Uncertain. Complex shear zone. Whitewater. (1) Tetrahedrite, chalcopyrite. (2) Pyrrhotite, chalcopyrite. (1) Sidcrite, quartz, (2) Magnetite, altered dyke rock. Lenticular, mod-crate plunge. (1) Irregularities within a shear zone. (2) Limestone replacement. (3) Replacement of a lampro-phyre dyke. Slocan Group. (1) and (2) Lime-stone. (3) Lamprophyre dyke. (1) and (2> Chlo-rite • JU: covite. (3) Chrome mica. Low-grade regional metamorphism. Trlasslc. Late stages of deformation(?), post lamprophyre. Complex shear zone. Silver Cup. Tetrahedrite, chalcopyrite, ruby silver. Quartz, sidcrite. Lenticular, iteep plunge. Minor drag on shear zone. Lardeau Group, Triune Formation. Slates. Muscovite, chlorite, sidcrite, chrome mica. Low-grade regional plus Widespread hydro thermal alteration. Mid Paleozoic UD certain. Complex shear zone. O O s CO rn </> 0) 3 Table 3.1 Some important lead-zinc deposits of the Kootenay Arc. Taken from Fyles (1967; p. 66, 67). 78 lead isotope analyses from this region. From analyses of 16 representa-tive lead-zinc deposits, Sinclair found a l l were related by a single anomalous lead line on a graph of Pb 2 0 6/Pb 2 0 1 t v. Pb 2 0 7/Pb 2 0 1 t. From the slope of this line, Sinclair calculated that the age of source rocks was 1,700 MY, assuming mineralisation was at about 170 MY. He believed the 1,700 MY age was too young for the source to be Hudsonian basement and suggested that lower Purcell rocks were a possible source of the radiogenic lead. In Sinclair's view, the leads were the result of mixing 1,340 MY old (single-stage age) Sullivan-type lead with radiogenic lead developed in lower Purcell sediments. These mixed leads were considered to have been emplaced during Middle Jurassic (approximately) mineralisa-tion. Sinclair (1966) did not find any correlation between isotopic composition and size of deposit, type or age of wall-rock, or minor element content of galena. Reynolds (1967) analysed four galenas from deposits which he considered were "associated" with the Nelson Batholith, and lead from four samples of feldspar from the batholith itself. All these leads and some older analyses of Bluebell Mine galena (Kanasewich, 1962) plot on an anomalous line below Sinclair's (1966) line. Furthermore, the line determined by Reynolds did not pass through lead of Sullivan composition, for i t was parallel to Sinclair's line. Reynolds suggested that the source rocks for the anomalous leads might be Hudsonian in age, or perhaps a mixture of Hudsonian and lower Purcell rocks. He believed his analyses provided (Reynolds, 1967; p. 65) for the first time, clear evidence of a genetic relationship between ore deposits and granitic rocks." The leads on the lower anomalous line were considered to be the result of 79 mixing of hypothetical 1,600 MY primary lead and radiogenic lead developed between 1,600 MY and 150 MY. This interpretation is similar, although more detailed, to that suggested by Kanasewich (1962), for Bluebell lead. Reynolds and Sinclair (1971) reinterpreted their combined data (fig. 3.3) and related the two anomalous lead lines to two geologically distinct groups of deposits. Leads on the upper line (Sinclair's line) are of the Salmo type, leads on the lower line (Reynold's line) are vein deposits spatially related to Nelson plutonic rocks. The Salmo-type deposits were regarded as the result of simple mixing of Sullivan-type lead and radiogenic lead derived from 1,530 MY old upper-crustal source rocks. The model age of Sullivan lead was calculated to be about 1,200 MY, and i t was suggested i t might be a representative sample of average lead in the surrounding rocks at that time. The magma of Nelson plutonic rocks and the ore fluids of the spatially related "Nelson" deposits were inferred to come from the same U/Pb source, probably the lower crust. Contamination of this ore fluid with radiogenic lead from 1,530 MY old upper crustal rocks was required by their model. For purposes of calcula-tion, Reynolds and Sinclair assumed that Sullivan lead could be included with the Kootenay Arc leads, that both anomalous lines had the same slope, and they used the value (0.0982 ± 0.0052) of the more precisely determined upper line. They also assumed a time of mineralisation of 150 MY for a l l deposits. Ohmoto (1968) noted that a genetic link between lamprophyres and Bluebell ore had been suggested by several authors and he obtained four lead isotope analyses of Bluebell galena, and two analyses of rock leads from lamprophyre dikes for comparison. Because the composition of lead in the ore and the dikes was different, Ohmoto suggested that the 80 Fig. 3.3 Plot of ore and rock leads from the Kootenay Arc, reported by Reynolds and Sinclair (1971, p. 261). Key: O = Salmo type deposits • = Slocan and Ainsworth deposits A = Bluebell analyses (Kanasewich, 1962) • = Feldspar lead, Nelson Batholith ore lead and lamprophyre lead did not share a common source. He concluded (op. cit.., p. 26) "The apparent anomalous age and the large variation of the isotopic composition of Bluebell galena suggest that, with respect to the concentrations of Pb, U and Th and the i n i t i a l Pb isotope ratios, the lead was derived either from a very heterogeneous source or two or more homogeneous but different sources." In a later paper, Ohmoto (1971, p. 99) stated "The sulphur, salts, and metals were probably derived from the thick sedimentary sequences which were present above the Bluebell limestone during Tertiary times and/or from the sedimentary sequences east of the Kootenay Arc." 81 3.4 DISCUSSION OF NEW LEAD ISOTOPE DATA A l l analyses made by the writer are presented in tables 3.II and 3.III. Most sample locations are shown on fig. 2.5. A l l Kootenay Arc samples analysed in the present study are plotted on fig. 3.4 and 3.5. Reynolds and Sinclair (1971) have discussed the fact that leads (fig. 3.3) on their lower line ("Nelson leads") are geologically distinct from those on their upper line ("Salmo leads"). In their view, the isotopic distinction was due to association of Nelson leads with the Nelson Batholith and the lack of such an association ^or the Salmo leads. In contrast, the view advanced in the following discussion is that the distinction is primarily due to a difference in age of emplacement of the concordant and transgressive types of deposit. The new analyses listed in tables 3.II and 3.Ill and plotted in fig. 3.4 clearly show that the geological distinction between deposits on the two anomalous lines is that the "concordant" deposits plot without exception on the upper line, whereas the "transgressive" deposits plot with only one exception (Spider) on the lower line. These two groups are now considered separately, and the results are later integrated. Concordant Group Deposits on the upper line (fig. 3.4) include not only the Salmo-type deposits as already demonstrated by Reynolds and Sinclair (1971), but a l l the analysed Shuswap and Metaline type deposits also, even the two Metaline type deposits from the region east of the Kootenay TABLE 3.II ISOTOPE COMPOSITIONS OF LEAD IN GALENA FROM CONCORDANT DEPOSITS, KOOTENAY ARC UBC No. NAME OF DEPOSIT Pb2 0 6/Pb 2 0 l + Pb 2 0 7/Pb 2 0 1* Pb 2 0 8/Pb 2 0 1 t METALINE TYPE 837 MONARCH 18.165 15.637 38.727 Cathedral F. (Mid Cambrian) (1) 874 SILVER GIANT? 18.258 15.653 38.310 Jubilee F. (Mid, Upper Cambrian) (2) 865 VAN STONE 19.390 15.791 39.890 Metaline F. (mid) (Mid Cambrian-Ord?) (3) 864 PEND OREILLE 19.486 15.789 39.922 Metaline F. (upper) (Mid Cambrian-Ord?)(4) SALMO TYPE 300 JERSEY 19.093 15.736 39.437 Reeves 1st. (L. Cambrian) (5) 285 REEVES-MACDONALD 19.076 15.738 39.386 it I I (5) 286 H.B. 19.089 15.727 39.486 I I ti (5) 866 RAINBOW 19.281 15.756 39.534 I I ii (5) 284 JACKPOT 18.992 15.744 39.304 I I I I (5) 307 DUNCAN LAKE 19.417 15.786 39.915 Badshot 1st. (L. Cambrian) (6) 291 SAL A 19.347 15.767 39.718 I I I I (6) 318 MOLLIE MAC 18.280 15.667 38.275 Index F. (L. Cambrian-Ord.?) (7) SHUSWAP TYPE 854 WIGWAM 18.239 15.630 38.240 Badshot 1st. (L. Cambrian) (8) 855 WIGWAM 18.242 15.630 38.211 856 COTTONBELT 18.417 15.663 38.261 Badshot 1st.? (L. Cambrian) (9) 857 RUDDOCK CREEK 18.468 15.668 38.126 I I I I (9) 858 RIVER JORDAN 18.251 15.630 38.108 I I I I (9) 859 RIVER JORDAN 17.924 15.592 37.884 I I I I (9) HOST ROCK *REF. *REF. (1) Ney (1957) (4) McConnel and Anderson (1968) (7) Fyles and Eastwood (1962) (2) Hedley (1949) (5) Fyles and Hewlett (1959) (8) Thompson (1969) (3) Cox (1968) (6) Fyles (1964) (9) Fyles (1970a) 83 TABLE 3.Ill ISOTOPE COMPOSITIONS OF LEAD IN CALENA FROM TRANSGRESSIVE DEPOSITS, KOOTENAY ARC UBC No. NAME OF DEPOSIT r b ^ / P h2 0 ' * P b ^ / P h 2 0 1 1 P b ^ / P b 2 0 ' ' HOST ROCK **REF. AIliSWOFTH CA!-3> 226 BLUEBELL* 17.481 15.488 37.961 Bluebell 1st.- Badshot 1st. (L. Cambrian) (1) 315 TRIUMPH 17.537 15.506 38.224 Milford Croup (Miss-Penn) (2) 316 NICOLET 17.570 15.502 38.205 (2) 314 HIGHLANDER 17.556 15.511 38.248 " Nelson granite (M. Juras.) (2) 910 NO. 1 17.768 15.530 38.344 if (2) 911 HIGHLAND 17.534 15.498 38.253 " .Kaslo Group (L. Trias.) (2) S08 SILVER HOARD 17.793 15.522 38.298 " (2) 898 SILVER GLANCE 17.489 15.486 37.994 (2) 909 KOOTENAY FLORENCE 17.463 15.488 37.969 " .Kaslo Group (L. Trias.) (2) 901 MONTEZUMA* 17.583 15.507 38.244 Slocan Group (U. Trias-L. Jurassic) (3) SLOCAI1 CTTX CA1-IP 867 SILVER LEAF 18.630 15.614 38.932 Nelson granite (Mid. Jurassic) (3) 869 CORONATION 18.969 15.657 38.965 (3) 868 ENTERPRISE 18.721 15.635 38.970 " (4) 870 OTTAWA 18.878 15.642 39.072 " (4) 871 ARLINGTON 18.851 15.647 39.046 (3) 877 CHAPLEAU 19.141 15.679 38.927 " (4) 295 SCRAKTON* 18.889 15.661 38.715 " , quartzite (4) 903 KALISPELL 18.705 15.632 38.971 Sediment in Nelson granite (3) 902 LITTLE TIM 18.815 15.641 39.155 Nelson granite (4) SAIIDON CAMP 878 RUTH-HOPE 18.703 15.631 39.018 Slocan Group (U. Trias.-L. Jurassic) (5) 879 SILVERSMITH 18.725 15.649 39.071 (5) 880 STANDARD 18.668 15.635 39.026 II (4) 881 KOLLY HUGHES 18.923 15.664 38.747 Nelson granite (4) 882 VAN ROI 18.712 15.647 39.053 Slocan Croup (4) 883 PAYNE 18.744 15.633 39.050 " (5) 884 VULTURE 18.708 15.628 39.032 II (3) 887 NOBLE 5 18.898 15.664 39.045 i t (3) 894 WHITEWATER 18.800 15.656 38.844 n (6) 895 CORK PROVINCE* 18.794 15.635 39.041 t i (4) 896 UTICA 18.790 15.640 39.010 " (4) 899 INDEX* 18.762 15.649 38.959 tt (4) 904 BOSUN 18.741 15.633 39.039 " (4) 905 IVANHOE 18.747 15.644 39.021 i t (5) 906 CALIFORNIA 18.623 15.618 39.055 t i (3) 907 FISHER MAIDEN 18.743 15.634 38.974 Nelson granite (3) 912 LUCKY JIM 18.778 15.633 39.032 Slocan Croup (6) 294 VICTOR 18.719 15.624 38.995 II (5) 900 DUBLIN QUEEN . 18.752 15.639 39.032 i t (3) MISCELLANEOUS 863 RED ROCK 18.733 15.644 38.873 Reeves 1st. (L. Cambrian) (7) 886 YMIR 18.989 15.675 38.776 Ymir Gp. •= Slocan Gp. (U. Tri . -L. Juras.) (8) 317 MOONSHINE 19.225 15.695 39.482 Badshot 1st. (L. Cambrian) (9) 875 SPIDER 19.157 15.732 39.260 7 (10) 897 SILVER CUP 19.322 15.710 39.269 Triune F. (Ord.?) (11) 860 ELSIE 19.114 15.696 38.845 (Carb-Permian?) (12) *Not in the main canp area **References: (1) Shannon (1970) (2) Fyles (1967) (3) Cairnes (1935) (4) Little (1960) (5) Hedley (1952) (6) Hedley (1947) (7) Fyles and Hewlett (1959) (8) Cockfleld (1936) (9) Fyles (1964) (10) Eastwood and Peck (1956) (11) Fyles and Eastwood (1962) (12) Campbell (1963) NOTE: References given are not necessarily the most recent or most complete available. 15-Br Fig. 3.4 Plot of Pb 2 0 7/Pb 2 0 l t v. Pb 2 0 6/Pb 2 0 1 + for a l l the Kootenay Arc data listed in tables 3.II and 3.III. Growth curve is for (U238/Pb201*) . - 9.09. Fig. 3.5 Plot of Pb 2 0 8/Pb 2 0 1 4 v. Pb 2 0 6/Pb 2 0 l + for a l l the Kootenay Arc data listed in tables 3.II and 3.III. Growth curve is for (Th 2 3 2/Pb 2 Q l t) = 39.60. present 86 Arc (Silver Giant, Monarch). All of the concordant deposits are in rocks of known (Salmo type, Metaline type) or inferred (Shuswap type) Cambrian age (Fyles, 1967, 1970a). Although as a group the concordant deposits possess unifying geologic characteristics and appear relatively old (Fyles, 1967), there is no certainty that they are a l l of the same age (Weissenborn and others, 1970). However, the strong isotopic relationship first demonstra-ted by Sinclair (1964) can be regarded as evidence of a common age and i t is assumed for present purposes that a l l the concordant deposits are approximately the same age, although interpretation of the lead isotopes would not be greatly influenced by a range in age of perhaps 200 MY. The absolute age of the concordant deposits is not certain. Salmo type deposits seem to predate the Nelson Batholith (minimum age 170 MY) and are as old as,or older than, metamorphism and deformation of their host rocks (Fyles, 1967). Muraro (1966) argued that in the Salmo type deposits the sulphides have undergone the same deformation as the host rocks and he considered that the deposits of Metaline district represented the closest approach to the original Salmo deposits. From a detailed study of the deformation of the main Salmo type deposits, Macdonald (in preparation) has come to a similar conclusion. Stratigraphic controls for deposition of metals of concordant deposits of every type have been noted by a number of writers and there is the possibility that metal emplacement occurred soon after sedimenta-tion, although not necessarily in the form of ore deposits. Thus Fyles (1970) noted that Cambrian sedimentary processes could have been important in controlling the regional distribution of metals now in Salmo deposits, 87 although in his view the main Salmo mineralisation accompanied and followed folding. Whishaw (1954) suggested from limited geochemical analyses that black argillite of Cambrian age could have been the source of lead in the Jersey deposit, a Salmo type deposit in the underlying Cambrian limestone. In a discussion of regional aspects of Shuswap type deposits, Fyles (1970a, p. 44) concluded "Thus the concordant lead-zinc deposits of the Shuswap Complex appear to have formed along specific stratigraphic horizons at various levels in the sedimentary sequence before intense folding and metamorphism." For the little-deformed deposits of Metaline district, possibly precursors of Salmo and Shuswap type deposits, McConnel and Anderson (1968) suggested that lead might have been introduced during sedimentation and diagenesis and later remobilised to a limited extent, perhaps during intrusion of the Kaniksu Batholith. For the same district Addie (1970) favoured control of pyrite deposition, at the time of sedi-mentation, by facies changes in the carbonate host rocks, but "much later" introduction of lead. Evans and others (1968) studied trace elements and sulphur isotopes from some western Canadian lead-zinc deposits, including the Monarch (table 3.II) and Kicking Horse ore-bodies (both Metaline type). They proposed that the metals in a l l the deposits they studied were precipitated from connate brines, and that the sulphur was derived from H 2 S associated with petroleum and natural gas accumulations. Ney (1957) and Sangster (1970) have commented on the location of the Monarch and Kicking Horse deposits close to a very rapid carbonate-shale facies change — at the "Kicking Horse Rim" of Aitken (1971). From the above i t seems that the age of concordant mineralisation 88 must l i e between the age of sedimentation of the Cambrian host rocks and the age of intrusion of the Nelson Batholith. For this reason, several possible mineralisation ages have been assumed in the calculations sum-marised in table 3.IV. Table 3.IV Two-stage model ages for concordant deposits, upper anomalous line on fig. 3.4. (Slope 0.119 ± 0.004) t i (calculated) t 2 (assumed) 1.86 ± .08 BY 1.78 ± .06 BY 1.72 ± .06 BY 200 MY 350 MY 450 MY Changing the assumed age of mineralisation ( t 2 ) does not have an important effect on the calculated t j , and in a l l cases the source of the anomalous lead can be interpreted to be rocks of Hudsonian age (about 1.6 - 1.9 BY). The slope of the line determined in this study is slightly but significantly steeper than that obtained by Reynolds and Sinclair (1971) , and this accounts for the difference in the calculated ages of source rocks. The age of source rocks calculated by Reynolds and Sinclair (1971) was 1,530 MY, just young enough to be lower Belt-Purcell sediments, but the older ages obtained here suggest these sediments were not involved. Transgressive Group The transgressive deposits are younger than Nelson plutonic rocks and seem to be younger than a widespread suite of lamprophyres (Fyles, 1967; 1970). As previously discussed, most of the lamprophyres 89 seem to be Tertiary. Because the lamprophyre dikes were fractured, o f f -set and altered during the episode of transgressive mineralisation, the transgressive deposits must be Eocene or younger. It i s therefore d i f f i c u l t to see how these leads could bear any relationship to hydro-thermal fluids accompanying Middle Jurassic Nelson plutonic rocks, as Reynolds and Sinclair (1971) inferred. The lower (transgressive) anomalous lead line is made up mainly of samples from three mining camps, Ainsworth (and Bluebell), Slocan City, and Sandon. Slopes of the short lines through these separate data sets, and the longer line through the combined data are given i n table 3.V. Table 3.V. Slopes of least-squares lines through transgressive group leads (the lower anomalous lead line on f i g . 3.4). Mining Camp No. of Samples Slope (± la) Sandon 19 0.124 + 0.027 Slocan City 9 0.115 + 0.019 Ainsworth (+ Bluebell) 9 0.118 + 0.025 Combined data for lower lin e 41 0.116 + 0.003 Slopes of a l l these three sets are the same within the quoted uncertainties and the slope for the combined data is therefore used in the calculation which follows. Assuming a two-stage model and mineralisa-tion (t£) at 50 MY, the beginning of the second stage is calculated to be 1.90 ± 0.05 BY. This age i s interpreted as the age of the source of the anomalous leads and i s consistent with the age of Hudsonian rocks of the 90 Canadian Shield, although i t is older than the mean K/Ar Hudsonian age of 1,735 MY (Stockwell, 1970). Evolution of Lead Isotopes in the Kootenay Arc It is concluded above that the two isotopically distinct groups of deposits correspond to the two structural groups of Fyles (1967). These two groups represent two different periods of mineralisation, apparently separated in time by 100 to 500 MY. From geological relation-ships neither mineralisation seems related to hydrothermal fluids accom-panying intrusion of Nelson plutonic rocks, for one group is too young, the other too old. Since the slopes of the two lines are the same within experi-mental error, both groups of deposits incorporate a variable amount of essentially identical radiogenic lead component, as originally proposed by Reynolds and Sinclair (1971). From the slopes of the lines and assumed ages of mineralisation, the source of the anomalous component is calculated here to be 1.7 - 1.9 BY old source rocks. This anomalous component could have been introduced from underlying Hudsonian basement, but this is not necessary to the interpretation of the lead isotopes, since, i f the Phanerozoic sediments of the Kootenay Arc were derived from Shield rocks of Hudsonian age, the sediments could also supply the anomalous component. Because the lines are parallel, they incorporate the same radiogenic lead component and, for exactly the same reason, they do not incorporate lead of the same i n i t i a l composition. It is not possible to determine the i n i t i a l isotopic compositions with any certainty, and three possibilities are offered instead. A minimum of two leads is required 91 MODEL 2 A A—-A 16 18 P0M6/Pb Fig. 3.6 Sketches of three models which show different "non-anomalous" components (A,B,C) for Kootenay Arc leads. Solid triangles represent "concordant" leads, open triangles represent "transgressive" leads. 92 to form a linear array, and in a l l cases one of these leads is assumed to be of shield derivation as discussed above. Possible compositions for the other lead, loosely termed the "non-anomalous component", are shown on fig. 3.6. Source of non-anomalous component (see fig. 3.6) Model 1. (A) upper line Purcell lead of "late Precambrian group". (B) lower line Unradiogenic lead, possibly from a low U/Pb source (lower crust?). Perhaps associated with Eocene magmatic activity. Model 2. (A) upper line 1.6 - 2.0 BY (approx.) source rocks with "appro-priate" lead composition (e.g. an old "primary" lead). (B) lower line 1.6 - 1.9 BY source rocks with "appropriate" lead composition. Model 3. (A) upper line Average lead in local sediments, extracted at some time between 200 and 450 MY. (B) lower line Average lead in sediments and other rocks in the area at 50 MY. Special explanation needed for Ainsworth (and Bluebell) deposits(C). Model 1 incorporates ideas proposed by Sinclair (1964), Reynolds (1967) and Reynolds and Sinclair (1971), although i t is modified in two ways. Firstly, the Nelson Batholith is not considered directly related to leads on the lower line, both because the batholith is too old, and because some feldspar leads from the batholith which were measured by Reynolds (1967) are more radiogenic than the "transgressive" ore leads (especially Ainsworth lead). Secondly, Sullivan-like lead itself is not considered involved in the upper line, but the general class of old Belt-Purcell leads might be. Reynolds and Sinclair (1971) have discussed the possibil-ity of a low U/Pb source linked with igneous rocks. 93 Model 2 requires hypothetical primary leads or very unradiogenic leads, for which no evidence has been found. Reynolds (1967) postulated that lead of composition B was a component of rock-lead from the Nelson Batholith and lead of some ore deposits. Model 3 is based on the assumption that local country rocks could be the source of the non-anomalous component of the concordant leads. Well-mixed lead from surrounding sediments could provide lead of composition A, similar to a primary lead (see chapter 1). Lead in Shuswap deposits might be representative of this lead component. The same sediments could also be the source of the anomalous component, i f leaching removed vari-able amounts of radiogenic lead. A l l the concordant leads are related by straight lines that pass through the least-radiogenic Shuswap group on both fig. 3.4 and 3.5, providing evidence that concordant leads are the result of simple mixing of variable proportions of a single radio-genic component with a non-radiogenic component (see Kanasewich, 1968). It is supposed that leaching of lead and emplacement of concordant deposits occurred early in the history of the Kootenay Arc, before or during the first deformation in Palaeozoic time. In Eocene times, another average of lead in country rocks, including the Nelson Batholith, Slocan Group sediments, etc., might give composition B. Again the average lead is close to the primary growth curve, and has a single-stage model age of about -300 MY. It is normal for Phanerozoic primary lead ores to have calculated single-stage ages too young by 100 to 200 MY (e.g. Stacey and others, 1969). Imperfect mixing of lead Isotopes could explain the slight scatter of Slocan City and Sandon leads. 94 The origin of Ainsworth (and Bluebell) leads and their relation to Slocan City/Sandon lead must be considered enigmatic at present. The unradiogenic composition of Ainsworth leads probably reflects the unradio-genic composition of their source. The two most obvious sources of unradiogenic lead are high grade metamorphic rocks that have lost radio-genic lead or young intrusions that have penetrated uranium-depleted Precambrian rocks (Doe, 1968; Doe and others, 1968; Zartman and Wasserburg, 1969; Moorbath and others, 1969). In the Ainsworth area both sources are possible. Little can be said about the nearby metamorphic rocks as sources of lead, but there is some evidence against derivation of lead from the same source as the most closely associated igneous rocks, the lamprophyre dikes. For example, Ohmoto and Rye (1970) found that the water of inclusion fluids in late (post-sulphide) minerals of the Blue-bell deposits was meteoric, not magmatic. Also, one analysis of rock-lead in a lamprophyre dike near Bluebell mine is move radiogenic than lead in Bluebell galena (Ohmoto, 1968). The writer has no ready explanation for Ainsworth and Bluebell leads. There are shortcomings in a l l the simple models discussed above, but the writer prefers model 3 for the following reasons. (1) It is consistent with views expressed earlier on the origin of lead-zinc deposits in sediments (see chapters 1 and 2), that i s , the lead could be derived from sediments by connate brines and could approximate a primary lead. Shuswap leads and Slocan City/Sandon leads are regarded in a general way as the Palaeozoic and Cenozoic equivalents, respectively, of the late Precambrian group of Belt-Purcell leads. (2) It is conservative, that i s , only normal crustal processes acting 95 on normal crustal materials are required. (3) Both anomalous and non-anomalous components can be derived from the same local source. (4) The correlation between geology and isotopic composition is clear and the fact that the concordant deposits are older than the transgressive deposits is fully accounted for in this model. If compositions A and B of fig. 3.6 are those of average crustal leads, and i f any significance can be placed on their single-stage ages, the calculated time gap between A and B is about 300 to 600 MY. This gap is regarded as the expression of the difference in age between concordant and transgressive periods of mineralisation. 3.5 LEAD ISOTOPE ZONING IN SANDON, SLOCAN CITY, AND AINSWORTH CAMPS District-wide zonal patterns for the distribution of minerals and metals (e.g. fig. 3.7) have been reported for Sandon, Slocan City and Ainsworth camps (Sinclair, 1967; Orr, 1971; Orr and Sinclair, 1971; Davidson, 1972). An attempt was therefore made to look for district-wide zonation of lead isotopes in these areas. Zonation of lead isotopes has been found elsewhere, both on the scale of a single crystal of galena (Austin and Slawson, 1961; Cannon and others, 1963) and on a regional scale (Heyl and others, 1966; Brown, 1967; Slawson and Austin, 1962). If regional zoning patterns reflect the movement of hydrothermal fluid, perhaps outward from a centre as Sinclair (1967) suggested for Sandon camp, then this fluid could scavenge lead from country rocks dur-ing their passage. The original isotope composition of lead in the fluid 96 an t ZINC X RU / RG Computer contoured plot of average zinc grades for 92 deposits, Computer-contoured map of Au/Ag ratios (multiplied by 1000) Slocan camp. Deposit locations are shown by solid triangles, for 43 deposits, Slocan City camp. Deposit locations are shown by solid triangles. Fig. 3.7 Two examples of zonal patterns. Taken from Orr and Sinclair, (1971). will then be progressively changed en route to depositional sites of the lead. A number of models can be imagined in which leaching and mixing processes will result in a zonal distribution of isotopes, and one of the simplest of these is illustrated in fig. 3.8. 97 ground surface country rock \ O7Pb 2 0 4-?9-5 Ptf ore fluid CIRCULATION SYSTEM contour lines joining deposits of equal Pb isotope ratio \ ^ / I 'feed , deposit "FOSSIL" CIRCULATION SYSTEM Fig. 3.8 Sectional sketch showing a simple model to explain lead isotope zonation in mineral deposits by addition of lead from country rocks to lead in a circulating fluid. The ore fluid (fig. 3.8) is indicated to contain lead with an assumed original Pb 2 0 6/Pb 2 0 l f ratio of 18.6, and during circulation of this fluid, rocks containing lead with a ratio of, say, 19.5 are traversed. Leaching of this rock-lead changes the isotopic composition of the ore fluid, the amount of change depending on the proportions of the original and leached components. The amount of leachable lead may be simply related to the volume of rock traversed, and thus to distance outward from the centre of the circulation system. Since the same quantity of 98 ore fluid may traverse progressively greater volumes of rock for the same radially-outward distance, the rate of change of composition with outward distance will not be linear. If the system does not change greatly with time, lead in galena deposits will show the sort of pat-terns sketched in fig. 3.8. Two important features of the hypothetical sections shown are the concentric pattern of equi-ratio contours and the progressively closer spacing of these contours outwards from the centre. The model shown is not unique, however, and other models relating compositional changes to time or to decreasing availability of leachable lead (e.g. Slawson and Austin, 1962; Sinclair and Walcott, 1966) could be proposed. Considering leads in the three Kootenay Arc camps under dis-cussion, i t is interesting to note first of a l l the remarkable isotopic uniformity of leads within the same camp (table 3.III). This observation implies that the ore fluid was isotopically rather uniform over an area the size of the camp, that i s , the scale on which the ore fluid existed is revealed by the distribution of lead isotopic compositions. Despite this similarity of isotope ratios in deposits of the same camp, i t appears that zoning of isotope ratios is present in the Slocan City, Sandon and Ainsworth areas. In each case, the zoning is best shown by contouring the ratio Pb 2 0 6/Pb 2 0 1 t — that i s , a directly measured quantity. Some details of the patterns obtained follow. Slocan City and Sandon Camps Although Slocan City and Sandon camps are sometimes considered as separate entities, the lead isotope data from these camps suggests that they are parts of a single metallogenic region and are products of 99 Fig. 3.9 Contoured Pb 2 0 6/Pb 2 0 1 + ratios for Slocan City and Sandon data (table 3.III). Geology after Caimes (1934) and Little (1960). Two samples (901 and 887) do not f i t the contours. 100 a single episode of mineralisation. This conclusion is supported by the pattern of contours drawn for Pb 2^ 6/Pb 2 0 1 t ratios of samples from the two camps (fig. 3.9). This pattern is like the idealised sketch of fig. 3.8 and a correspondence with the simple model already outlined is therefore suggested. The westerly trough of low Pb 2 0 6/Pb 2 0 t t, the more closely spaced contours towards the periphery, and the assymetry are noteworthy features. An interpretation according to the simple model proposed is that ore fluids rising near the western trough carried lead with Pb 2 0 6/Pb 2 0 1* ratio of about 18.6 (or less) and that this fluid leached lead from Slocan Group sediments and from Nelson plutonic rocks. The leachable lead composition is assumed to be more radiogenic than the ore fluid lead, but its composition is unknown. However, leads in four samples of feldspar from the Nelson Batholith (Reynolds, 1967) have p b 2 0 6 / P b 2 0 » t r a t i o s i n the range 18.93 - 19.33, and the existence of some leachable lead more radiogenic than the least radiogenic galena lead measured (18.6) is therefore established. In any case, acid-leaching of rocks and feldspars in the laboratory (e.g. Doe and others, 1966; Catanzaro and Gast, 1960) often yields a more radiogenic lead in the leach solution than the total lead and this could also account for an increase in radiogenic lead in the ore fluid. One consequence of the interpretation outlined above is that i t suggests that the difference in metal values (Cairnes, 1934; Little, 1960) between Sandon camp (Pb - Ag, Ag - Pb - Zn) and Slocan City camp (Ag - Pb - Zn, Ag - Au, Au - Ag) are the result of different proportions of elements being leached from different wall rocks by a single wide-spread ore fluid. 101 Nelson granite Slocan Group Kaslo Group Milford Group • Lardeau Group Hamill Group Fig. 3.10 Contoured Pb 2 0 6/Pb 2 0 1* ratios for analyses of samples from Ainsworth camp (table 3 .III). Geology after Fyles ( 1 9 6 7 ) . 102 Ainsworth Camp It can be seen from fig. 3.9 that leads of Ainsworth camp do not f i t the pattern for Slocan City and Sandon leads. Only nine samples were analysed from this camp, insufficient to define any pattern that may exist but enough to suggest a weak correlation between the isotope contours (fig. 3.10) and the strike of the three major vein systems described by Fyles (1967). The major vein systems shown give a good indication of the changing attitude of mineralised fractures from south to north in Ainsworth camp (Fyles, 1967; his fig. 8). It is suggested, therefore, that the isotope contours simply reflect movement of ore fluid preferentially along, rather than across, the vein systems. 3.6 POSSIBLE RELATIONSHIPS BETWEEN KOOTENAY ARC LEADS AND LEAD IN PURCELL ROCKS As has been shown, there are three distinct families of lead isotopes in southeastern British Columbia (a) Leads in Purcell rocks (b) Leads in "concordant" ore deposits of the Kootenay Arc (c) Leads in "transgressive" ore deposits of the Kootenay Arc Leads of the same family seem to be related, but there is no evidence of a direct isotopic relationship between families. In particular, the leads in Purcell rocks cannot be shown to have any genetic connection with the concordant or transgressive deposits of the Kootenay Arc. In fig. 3.11, the three families noted above are represented by compartments and two extreme possible relationships between the families are shown. In one case, the whole system is open to lead, in the other, each compart-103 ment is closed. In the absence of contrary evidence, i t seems that the closed compartment scheme is most applicable to leads of southeastern B.C. open system Cambrian — Jurassic sequence Purcell -Windermere sediments crystalline basement Fig. 3.11 Diagrammatic illustration of two extreme possibilities for relationships between the three main families of leads (A,B,C,) The above conclusion has an important bearing on the question of the source of lead in deposits of Precambrian to Tertiary age in southeastern British Columbia. There are three main explanations for the sources of this lead: (a) There was an enrichment of lead in Purcell sediments during sedi-mentation in Precambrian times. This lead was remobilised from Purcell rocks during deformation, erosion or igneous activity and formed the galena deposits of the Kootenay Arc. (b) There is an enrichment of lead in the mantle or lower crust in southeast B.C. which has persisted since Precambrian times. This deep-seated enrichment has supplied most of the metals in deposits of Precam-104 brian to Tertiary age in both the East Kootenay district and the Kootenay Arc. (c) The number of lead-zinc deposits in southeastern B.C. could be due to a repetition of processes capable of extraction and concentration of lead from surrounding rocks. Sediments might be the source, connate waters (including water evolved during metamorphism) the agent of extrac-tion and concentration. No absolute regional enrichment in crust or mantle (as in (a) or (b)) is necessary. To the writer, the evidence for (a) is not convincing, but this explanation must remain a possibility. The second explanation, (b), is difficult to assess because of lack of data, but from previous dis-cussion i t seems unlikely that the mantle was a source of lead. The writer favours the third explanation, which has already been discussed in chapters 1 and 2. Thus, the source of lead in the abundant lead-zinc deposits ih southeastern B.C. and adjoining parts of the northwestern U.S.A. is considered to be the thick successions of fine-grained sediments derived from the shield. Major lead-zinc deposits in other parts of the world, for example, Australia (King, 1965), are also associated with sequences of fine-grained sediments, and the idea that such sediments could be the source of lead in ore deposits certainly is not new. There may, however, be an indirect relationship between the three "families" of leads. If the leads come from sediments, and i f the sediments come from the shield, leads of the three families should show some sign of a common source. In fact, ages calculated for these source rocks from the lead isotope compositions of Kootenay Arc and 105 Purcell galenas do coincide with important ages known from the shield. Although the age of source rocks calculated from Purcell data (2.6 - 2.7 BY) is different from that calculated from Kootenay Arc data (1.7 - 1.9 BY), this difference is not inexplicable. For example, the age difference could reflect simply a difference in provenance of sediments, or a difference in level of erosion of the shield, the deeper levels exposed in Palaeozoic and later times having been more thoroughly homogenised by the 1.7 BY Hudsonian Orogeny. According to the above argument, large lead-zinc deposits in thick fine-grained sediments eroded from the Canadian Shield could be 15C-KEY it Shuswap # Pine Point V Anvil 160 170 Fig. 3.12 A comparison of isotope ratios of samples from Anvil Range (table 4.1 ), Pine Point (Cumming and Robertson, 1969, p. 731), and Shuswap deposits (table 3.II). 106 indirectly related throughout western Canada. Furthermore, distant deposits of the same age should have a close isotopic resemblance, i f they have the same source. For example, a resemblance of lead isotope ratios might be expected for the concordant deposits of the Kootenay Arc, the Anvil Range deposits (chapter 4), and the Pine Point ore-body (Beales and Jackson, 1968; Cumming and Robertson, 1969). A compositional simi-larity between some of these deposits is shown in fig. 3.12. Only further work will determine, however, whether lead isotope relationships of the sort suggested above do exist throughout the Canadian Cordillera. 3.7 SUMMARY Analyses by Sinclair (1964) and by Reynolds (1967) of lead . isotopes in mineral deposits of the Kootenay Arc were repeated and the earlier results found to be essentially correct. In particular, the repeated analyses and analyses of new samples confirm the existence of two parallel anomalous lines which seem related to two geologically different types of deposits. The slopes of the anomalous lines are slightly steeper than previously reported, however, and i t is therefore suggested that the source of the anomalous lead was 1.6 - 1.9 BY Hudsonian rocks, not 1.53 BY old rocks as suggested by Reynolds and Sinclair (1971). The two geological groups of Kootenay Arc lead-zinc deposits proposed by Fyles (1967) correspond in detail to the two isotopic groups of leads; the concordant deposits plot on the upper line, transgressive deposits plot on the lower. It is proposed that the two anomalous lines represent two periods of mineralisation that differ in age by 100 to 500 MY, an interpretation that is consistent with geological evidence. The 107 older deposits are those of the concordant type and these are interpreted to be average lead from lower Palaeozoic sediments mixed with variable amounts of an anomalous component derived from the same sediments, or perhaps from underlying Hudsonlan? basement. The transgressive deposits probably were emplaced in Tertiary times and their-lead isotope composi-tion could also be the result of mixing of "average crustal lead" with an anomalous component from surrounding country rocks. There is insuf-ficient information to satisfactorily account for the origin of lead in deposits of Ainsworth camp (and Bluebell). The Pb 2 0 6/Pb 2 0 1 t ratios of galena leads from Sandon and Slocan City camps show a zonal pattern. This pattern is somewhat elongated in a NE-SW direction, is assymetric and shows an increase in Pb 2 0 6/Pb 2 0 1 t outward from a centre east of Slocan Lake. It can be accounted for by changes in the composition of lead in an ore fluid due to progressive leaching of lead of a different composition from country rocks during fluid circulation, although this explanation is not unique. 108 CHAPTER 4. LEAD ISOTOPES FROM THE ANVIL RANGE, YUKON TERRITORY 4.1 INTRODUCTION Three large lead-zinc deposits were discovered in the Anvil Range area of central Yukon Territory (fig. 1.1) between 1953 and 1966, and the largest of these (Faro) is now an important producing mine. A l l three occurrences are stratiform pyritic lead-zinc deposits in Protero-zoic or Cambrian phyllite and schist. Eighteen drill-core samples from these deposits were analysed for their lead isotope composition, and the most interesting feature of this work is the small but regular variation in composition between deposits. This variation could be related to progressive regional metamorphism of the host rocks. 4.2 OUTLINE OF THE GEOLOGY AND LEAD-ZINC MINERALISATION OF THE ANVIL RANGE Geology The following is summarised mainly from a preliminary account (Tempelman-Kluit, 1968) of a detailed study of the ore deposits and geology of the Anvil Range. Except for sampling the ore deposits, the writer has not worked in the field in this area and discussions with Dr. Tempelman-Kluit on his unpublished findings have been invaluable. Anvil Range consists of an axial granitic core (Anvil Batholith) of mid-Cretaceous age flanked by late Proterozoic and Palaeo-zoic strata (fig. 4.1). The stratigraphic sequence includes two regional unconformities, one beneath Devonian-Mississippian strata and the other beneath Pennsylvanian-Permian volcanic rocks. Principal rock-types and L E G E N D Isograd Biotile Garnet S'auroMe U - Y - ' Foliation -q— Bedding Fault «•* Sulphide deposit 4 : #i76.75» U.B.C. sample numbers t N K E Y K ] Granodiorite. TR ] Conglomerate. ±2000' P. PN | S{ Volcanics. chert, serpent.+3B0Q" DM ] Clastic rocks. ±1000' SD Limestone, sandstone. 50-150' OS | 1 Slate 400'+ | | PhJL^)'^ffon^nist. marble. 6000'+? i? <JWf. ? F/G. 4-7 Geological map of Anvil Range. After Tempelman-Kluitf 1968. pp. 46-47 ) Milts 110 lead-zinc deposits drawn to a common scale and orientation. Sites of the analysed samples listed in table 4.1 are indicated by the numbered dots. I l l thicknesses of units are indicated in fig. 4.1, and i t may be noted that sediments and metasediments dominate the pre-Pennsylvanian succession, whereas basic volcanics dominate the Pennsylvanian-Permian sequence. Movement on faults in the Tintina Trench, a regional lineament that here forms the valley of the Pelly River, occurred in Early Triassic and later times in this area (Tempelman-Kluit, 1972). Lead-Zina Mineralisation A l l three of the major sulphide deposits (Swim, Vangorda, Faro) occur in the lowest part of a thick phyllitic to schistose unit that probably is of Cambrian or late Proterozoic age. These metasediments, which are intensely deformed, are overlain at one locality by Ordovician-Silurian graptolitic slate which is only weakly deformed. Because de-formation of the ore deposits is similar to that of the host rock, i t can be argued that the mineralisation is older than the weakly deformed Ordovician-Silurian slates — that i s , the ore deposits were probably emplaced in late Proterozoic to Ordovician time. Each of the main deposits consists of about 50% sulphides, characteristically with a granular texture. Quartz and pyrite are the main constituents, other important minerals being sphalerite, galena, pyrrhotite, chalcopyrite and marcasite, in decreasing order of abundance. The sulphide bodies are tabular (fig. 4.2, 4.3), roughly conformable to a pervasive transposition foliation, and are elongate parallel to linear structural features in the host rocks. Envelopes of bleached phyllite up to 300' thick surround the deposits. According to Tempelman-Kluit (pers. comm.) the form, setting, mineralogy and metal content of the deposits are very similar to deposits of the "stratiform type" discussed 112 by Stanton (1958, 1960). Unpublished analyses of sulphur Isotopes (D*F. Sangster, pers. comm.) are similar to the sulphur isotope ratios reported for a number of stratiform lead-zinc deposits by Stanton and Rafter (1966) VERTICAL LONGITUDINAL SECTION THROUGH CENTRE PARO DEPOSIT J V O C Scale of feet VERTICAL CROSS-SECTION NEAR CENTRE OF FARO DEPOSIT VERTICAL CROSS-SECTION NEAR KV END OF FARO DEPOSIT NE 4 0 0 0 Massive and banded sulphide* |V.1 Disseminated sulphides fry?J Qusrtx diorite Locally graphitic biotit. chlorite cuartx phyllite 'Bleached** phyllite I * ' J\ Chlopitic tuffaceoua greenstone |A* . [ Brecciated tuffaceoua greenstone VERTICAL CROSS-SECTIONS FARO DEPOSIT Fig. 4.3 Vertical cross sections of Faro deposit. Taken from Tempelman-Kluit (1968, p. 50). Sections of Swim and Vangorda deposits are somewhat similar but lack the greenstones and the quartz diorite (Tempelman-Kluit, 1968, p. 51). The metamorphic grade of the host rock increases (from Swim to Faro) as follows: Swim phyllite Vangorda .... schistose phyllite quartz-muscovite-chlorite-graphite quartz-muscovite-chlorite-biotite Faro schist quartz-muscovite-biotite (andalusite, granet, staurolite) 113 Tempelman-Kluit (1970) has shown that accompanying this increase in meta-morphic grade are increases in grain size of both silicates and sulphides, a relationship that he suggests is a result of progressive regional metamorphism of the sulphide deposits. Stratigraphic relationships fix the age of metamorphism as definitely pre-Devonian, and probably pre-Ordovician. In Tempelman-Kluit's view, the parallel increases in degree of deformation, in metamorphic grade, and in sulphide grain size are evidence that metamorphism and deformation affected pre-existing sulphide bodies. He suggests that the deposits were emplaced in approximately their present concentrations during or soon after deposition of the Proterozoic-Cambrian host rocks. These rocks were originally carbonaceous sil t s with intercalated tuffs of intermediate composition. Tempelman-Kluit (pers. comm.) emphasises the strong similarities between the three deposits, the evidence that they are older than deformation and metamorphism, and the variation in the effects these processes have had upon them. 4.3 DISCUSSION OF LEAD ISOTOPE DATA A l l the isotope analyses reported in table 4.1 are plotted on figs. 4.4 and 4.5. Sample locations are shown on plans of the ore deposits (fig. 4.2) or on the regional geological map (fig. 4.1). Leads from Faro, Vangorda and Swim deposits approximate primary lead (for criteria see Kanasewich, 1968; Stacey and others, 1969). Single-stage model ages calculated from the average isotopic composition of each deposit range from 150 MY for Faro to 175 MY for Swim (225 MY for Sea 754), an insignificant difference. It is well known (e.g. Stacey and others, 1969; Richards, 1971) that such single-stage ages are unreli-114 TABLE 4.1 LEAD ISOTOPE ANALYSES, ANVIL RANGE GALENAS UBC LOCATION No. D r i l l hole (footage) Pb206/Pb 20k Pb207/Pb 20** Pb 208 /Pb 201+ FARO DEPOSIT (NTS/105K 62°22*N, 133°22'W) 772 66-10 (571-577) 18.349 15.662 38.326 753 66-15 (307-311) 18.354 15.672 38.332 752 66E1 (182-191) 18.374 13.672 38.374 809 66-33 (370-384) 18.365 15.668 38.345 810 67-34 (400-408) 18.370 15.667 38.358 811 67-4 (464-465) 18.368 15.657 38.312 812 67-26 ( 78- 80) 18.346 15.670 38.321 813 67-27 (115-117) 18.401 15.683 38.409 755 Coarse galena in pod 18.392 15.672 38.401 h mile E. of mine VANGORDA DEPOSIT (NTS/105K 62°14'N, 133°13'W) 818 33 (245-270) 816 81 (413-415) 771 72 ( 38- 42) 815 56 ( 95-115) 757 26 (220-235) 18.365 18.346 18.351 18.356 18.351 15.676 15.668 15.662 15.662 15.668 38.334 38.273 38.272 38.315 38.309 SWIM DEPOSIT (NTS/105K 62°13'N, 133°02'W) 822 820 819 821 A30 (480) A28 (255) A24 (176) A29 (141) 18.336 18.338 18.346 18.341 15.659 15.666 15.664 15.669 38.273 38.293 38.297 38.309 SEA DEPOSIT (NTS/105K 62°11'N, 132°54'W) 823 754 2 (222) (Late vein) 2 (232) 18.486 18.224 15.668 15.635 38.457 38.142 VEINS IN ANVIL BATHOLITH 758 876 NTS/105K 62°19'N, NTS/105K 62°22'N, 133°5'W 133°8'W 19.230 19.217 15.725 15.724 39.325 39.293 Galena samples were taken from typical massive or banded pyritic ore, except for 755, 823 and the two veins in the Anvil Batholith. 115 able in detail, especially for deposits of Phanerozoic age. Emplacement ages of Phanerozoic stratiform leads are consistently older than their single-stage model lead ages, by about 100 to 400 MY (e.g. see table 6 of Stacey and others, 1969). On this empirical basis, i t seems likely thai: the age of emplacement of the Anvil ore bodies is also older than their single-stage ages indicate. From the age discrepancies mentioned above i t is suggested that their true emplacement age may l i e between the approximate limits 250 to 600 MY. As previously stated (section 4.2) geological relationships seem to indicate emplacement of the sulphide deposits between 450 and 800 MY ago. Analyses of samples from Swim, Vangorda and Faro deposits are very similar, but there is a slight change in Pb 2 0 6/Pb 2 0 1* ratios from Swim (lowest ratio) to Faro (highest). This pattern was first suggested by some early imprecise analyses (fig. 4.6a) and was later confirmed by more numerous and more precise measurements (fig. 4.6b). The slight progressive change in isotopic composition parallels the changes in metamorphic grade, in sulphide and silicate grain size, and in degree of deformation referred to earlier (section 4.2). Some of these relation-ships are shown in fig. 4.7 along with the change in tonnage of combined lead and zinc of the three deposits. Because the only obvious feature to which the isotopic variation can be related is the increase in intensity of deformation and metamorphism, i t is suggested that a genetic connection could exist between the metamorphism and the ore deposits. A possible explanation of the nature of this connection follows. As previously argued (chapters 1 and 2) primary lead might be simply average lead from thick sequences of fine-grained sediments. The easily FIG. 4-4 Fig. 4.4 Plot of Pb 2 0 8/Pb 2 0 l + v. Pb 2 0 6/Pb 2 0 1 t for a l l Anvil Range leads listed in table 4.1. Growth curve is for (Th 2 3 2/Pb 2 0 l +) „ = 39.60. present Fig. 4.5 Plot of Pb 2 0 7/Pb 2 0 1 + v. Pb 2 0 6/Pb 2 0 l + for a l l Anvil Range leads listed in table 4.1. Growth curve is for (U 2 3 8/Pb 2 0 l») fc - 9.09 ± 0.06. present 117 removable lead is leached from the sediments by connate brine, from which i t can then be deposited by a variety of mechanisms. If the evolv-ing connate brine s t i l l exists during the onset of regional metamorphism i t might be capable of leaching greater amounts of lead from the rocks as the temperature of the brine rises and recrystallisation proceeds (see chapter 2). In the laboratory, leaching of sediments with hot water (Hart and Tilton, 1966) or hydrochloric acid (Doe and others, 1966), and volatilisation of lead in feldspars (Sinha, 1969) usually, but not invariably, releases a more radiogenic lead than the total lead. These experiments suggest the possibility of removing different compositions of lead depending on the conditions under which leaching occurs. As discussed in chapter 2, the reason for this probably is that leads of different compositions occupy sites of different availability in the same rock. From the above discussion i t is proposed that the Anvil leads could represent lead stripped from surrounding fine-grained sediments by a connate brine at the onset of regional metamorphism in Cambrian or early Ordovician times. The amount and isotopic composition of the leached lead might have been dependent on both the character of the brine (e.g. temperature, salinity) and the availability of lead in the sediments. Where metamorphism of the rocks was most intense, the greatest amount and the most radiogenic lead would have been released, thus accounting for the increase in size and in radiogenic lead content of the deposits with increasing metamorphic grade. The isotopic pattern shown (fig. 4.6) for the three large deposits suggests that in rocks of lower metamorphic grade than at Swim deposit, an even less radiogenic lead than Swim might 118 FIG. 4-6a 757) / s 15-64 KEY /O F A R O VANGORDA \J SWIM 18*35 18*37 18*39 18-41 18*43 18*45 Pb 15*68 207 i 304 Pb 15*66 15-64 FIG. 4-6b 18*32 I 18-34 18-36 18*38 18*40 18*42 Pb Pb 204 Fig. 4.6a P l o t of P b 2 0 7 / P b 2 0 l t v. P b 2 0 6 / P b 2 0 l + f o r some e a r l y analyses (on an ol d e r mass spectrometer) of leads from Swim, Vangorda and Faro d e p o s i t s . These analyses, which are not normalised to the Broken H i l l Standard, were discard e d . F i g . 4.6b P l o t of P b 2 0 7 / P b 2 0 t t v. P b 2 0 6 / P b 2 0 1 t f o r the analyses of Swim, Vangorda and Faro samples l i s t e d i n t a b l e 4.1. 119 6-0 1 0 6 tons COMBINED Pfa + Zn TONNAGE 4-0-20-3-0-mm. MEAN GRAIN SIZE OF COARSE PYRITE 20 1 0 -mm. 0 2 MEAN GRAIN SIZE OF ORE SULPHIDES 0 1 0-3 mm. MEAN GRAIN SIZE OF HOST-ROCK SILICATES 0-2 ^Pb 01 206 204 18-360 MEAN LEAD ISOTOPE RATIO 18-350 18-340 — + SWIM • increasing' • metamorphic -VANGORDA grade-FARO Fig. 4.7 Some features of Anvil Range deposits which show increases with increasing metamorphic grade (Swim to Faro). Grain size data from Tempelman-Kluit (1970). Isotope data from table 4.1. Tonnage of lead and zinc: Faro, Findlay (1969); Vangorda, Chisholm (1957); Swim, Northern Miner (March 9, 1967, p. 5). 120 be expected. A sample of disseminated galena from the stratiform Sea prospect was analysed, where the host rocks are perhaps of slightly lower grade than at Swim deposit. Significantly, this lead is the least radiogenic of the four deposits. Perhaps disseminated lead from the Sea prospect is similar to the average composition of easily leach-able lead in the sediments prior to their modification by progressive metamorphism. If lead in the four deposits mentioned above is derived from surrounding sediments, and i f compositional differences are due to addition of variable amounts of radiogenic lead to an i n i t i a l composition like that of the Sea deposit, i t should be possible to calculate the age of the source of radiogenic lead. This age should be either the i n i t i a l age of the source rocks for the sediments, or the age of last thorough isotopic homogenisation of the source rocks. It was suggested in chapter ? tht.t parts of the Canadian Shield, the main source of the late Protero-zoic-Cambrian sediments of the Anvil Range, were as old as 3.5 BY. Ages of 2.7 BY were calculated from Precambrian lead in Purcell sediments, and ages of 1.7 - 1.9 BY from Kootenay Arc deposits. Therefore, ages similar to either of these results might be expected from Anvil Range leads, since the sediments are believed to have the same eastern shield source. In table 4.II, the model age of the source rocks of Anvil Range deposits is calculated to be 2.7 to 2.9 BY. The line is obviously very short, and the uncertainty in age is large. Two analyses (758, 876) from small veins in the roof of Anvil Batholith are interesting because the lead isotope ratios are very dif-ferent from those of lead of the main deposits (see figs. 4.4, 4.5). 121 This dissimilarity can be interpreted as evidence that the Anvil Batholith has no genetic relationship with the principal stratiform sulphide deposits, the conclusion reached on geological grounds (section 4.2) by Tempelman-Kluit. Table 4.II Age of source rocks of sedimentary host of Anvil Range stratiform deposits (assuming a two-stage model). Deposits Slope t i (calculated) t£ (assumed) Sea 754, Swim, Vangorda, Faro (19 analyses in all) 0.202 ± 0.036 2.9 ± 0.3 BY 2.7 ± 0.4 BY 0 MY 500 MY A "modified sedimentary" origin for Anvil Range leads is not a new idea. Gabrielse (1969) discussed geologic relationships of base-mstal deposits in the northern Canadian Cordillera and showed that the principal occurrences (many of them vein deposits) were almost invariably in Lower Cambrian strata. Furthermore, the mineralisation in these strata seemed restricted to the westerly fine-grained facies (fig. 4.8). Gabrielse pointed out the significance of this relationship both to exploration and to concepts of ore genesis, and suggested two different explanations. (1) Derivation of metals from late Proterozoic-early Cambrian sedi-ments, the last important contribution of elastics to the Cordilleran Geosyncline from the craton. (2) Deposition of metals from thermal springs associated with deep fractures of early Cambrian age. 122 The interpretation offered for lead isotope data from the Anvil Range is in accord with Gabrielse's first explanation. Fig. 4.8 Relation of occurrence of base-metal deposits of northern B.C. and southeastern Yukon to facies of Lower Cambrian and Eocambrian strata. Taken from Gabrielse (1969, p. 28). Faro, Vangorda, Swim shown as stars, other deposits as dots. 123 4.3 SUMMARY (1) Lead isotope ratios of the known stratiform lead-zinc deposits of the Anvil district are nearly identical. Because this lead approxi-mates "primary" lead, the source could be the surrounding late Proterozoic - Cambrian metasediments. (2) The surrounding sediments were derived from the Canadian Shield and i t is possible to calculate the age of this sediment source from the lead now in the ore deposits. This (imprecise) age is about 2.7 -2.9 BY, which suggests that the source area was affected (lead homogenised?) during the Kenoran Orogeny. (3) A slight variation in Pb 2 0 6/Pb 2 0 l f ratios of the four analysed deposits appears to be related to variation in metamorphic grade of the host rocks, higher ratios being associated with higher grade. It is Therefore suggested that mineralisation was going on during the earliest stages of metamorphism. Continuing metamorphism and deformation subse-quently modified the sulphide deposits prior to Ordovician-Silurian time, according to Tempelman-Kluit (pers. comm.). 124 CHAPTER 5. SUMMARY 5.1 PURCELL LEADS New lead isotope analyses confirm the findings of Leech and Wanless (1962) that isotope ratios of galena leads i n Purcell rocks f a l l into two distinct groups; a uniform, less radiogenic group and a variable, more radiogenic group. The uniform group, which includes many of the large stratiform deposits, seems to represent a widespread and economically important episode of lead-zinc mineralisation 1.2 to 1.4 BY ago (single-stage age). The variable leads were probably emplaced during the Mesozoic or Cenozoic and they are found i n small vein deposits. On a plot of Pb 2 0 7/Pb 2 0 1 t v. Pb 2 0 6/Pb 2 0 l\ the uniform leads form a short elongate array, lik e that expected from imperfect averaging of crustal leads, and i t i s suggested, therefore, that they are average leads leached from the enclosing lower Purcell strata by coiiiiate brine. If this lead was deposited as galena 1.3 BY ago, the source rocks of the uniform group of leads are about 2.6 BY old (two-stage model). The Purcell sediments, considered to be the immediate source rocks of the lead, are derived from the Canadian Shield, where ages as old as 2.6 BY and older are known. Although extraction of lead i n lower Purcell sediments by con-nate brines i s a reasonable p o s s i b i l i t y , convincing independent evidence either for or against i t i s lacking. In the absence of such independent evidence, the uniformity of the Precambrian leads, and correlations between isotopic composition and factors such as geography, stratigraphy and size of deposit are considered to support the hypothesis of a "modified 125 sedimentary" origin for the lead. The simplest explanation for the isotopic composition of the Mesozoic-Cenozoic leads is that they were formed in the Purcell sediments by addition of radiogenic lead to trace leads of the uniform Precambrian group. Calculations based on a two-stage model with mineralisation at 100 MY indicate that the source rocks for these leads are about 1.5 BY old. This age is consistent with the age of lower Purcell sediments. The writer proposes that the uniform, late Precambrian leads of the Belt Supergroup (Zartman and Stacey, 1971) could have had a similar origin to the uniform, late Precambrian Purcell leads. 5. 2 KOOTENAY ARC LEADS Lead isotopes from throughout the Kootenay Arc plot on two parallel anomalous lines, as first discussed by Reynolds and Sinclair (1971). In this thesis, these two isotopic groups are shovr. to correspond In detail to the two geological groupings of Kootenay Arc lead-zinc deposits proposed by Fyles (1967); structurally concordant deposits plot on the upper line, transgressive deposits plot on the lower line. The separation of the lines can be explained as the result of a difference in age of 100 to 500 MY (approximately) between the two principal mineralisa-tions, and this explanation is consistent with geological observations. Both groups of deposits contain lead derived from 1.7 - 1.9 BY old rocks, and i t is suggested that sediment eroded from Hudsonian (1.7 BY) basement or perhaps underlying Hudsonian basement are the most likely sources of this lead. Contours of Pb 2 0 6/Pb 2 0 t t ratios of samples from Slocan City and 126 Sandon camps show a simple zonal pattern in which ratios increase outward from a single centre. A model that could account for this pattern is one in which the isotopic composition of lead in hot rising fluid is progres-sively changed by leaching more radiogenic rock-lead during circulation. It is suggested that the repeated lead-zinc mineralisation in southeastern B.C. could be partly due to recurrent deposition of thick sequences of fine-grained sediment that served as source rocks for the lead and zinc now in the ore deposits. 5.3 ANVIL RANGE LEADS The composition of lead isotopes from four geologically alike stratiform deposits of the Anvil Range (Y.T.) are very similar, and approximate primary leads with single-stage model ages of 150 - 225 MY. The late Proterozoic to Cambrian metasediments which are the host rocks for the deposits are suggested as a possible source of the metals. On this assumption, the age of the source of the sediments can be calculated from the galena leads to be 2.7 - 2.9 BY (two-stage model). This age is consistent with the oldest ages reported from the Canadian Shield, the probable source of the sediments. 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Guidebook for excursion A-01 and X-01, 24th International Geological Congress, Montreal, Quebec. 118 p. Whishaw, Q.G. (1954) The Jersey lead-zinc deposit, Salmo, B.C. Econ. Geol. 49, 521-529. White, D.E. (1965) Saline waters of sedimentary rocks. In Fluids in Subsurface Environments (editors A. Young and J.E. Galley), pp. 342-366. Am. Assoc. Pet. Geol. Memoir 4, 416 p. White, D.E. (1968) Environments of generation of some base-metal ore deposits. Econ. Geol. 63, 301-335. Whittles, A.B.L. (1964) Trace lead isotope studies with gas-source mass spectrometry. Ph.D. thesis, Univ. of B.C. 204 p. Williamson, J.H. (1968) Least-squares fitting of a straight line. Can. J. Phys. 46, 1845-1847. 142 Yates, R.G. (1970) Geologic background of the Metaline and Northport mining districts, Washington. In Lead-zinc deposits in the Kootenay Arc, northeastern Washington and adjacent British Columbia (editor A.E. Weissenborn), pp. 17-39. State of Wash. Dept. Nat. Resources, Div. Mines & Geol. Bull. 61, 123 p. Yates, R.G. and Engels, J.C. (1968) Potassium-argon ages of some igneous rocks in northern Stevens County, Washington. In Geological Survey Research 1968, pp. D242-D247. U.S. Geol. Surv. Prof. Paper 600-D, 268 p. York, D. (1969) Least squares fitting of a straight line with correlated errors. Earth Plan. Sci. Letters .5, 320-324. York, D. and Farquhar, R.M. (1972) The Earth's Age and Geochronology. Pergamon Press. 178 p. Youden, W.J. (1951) Statistical Methods for Chemists. John Wiley and Sons, Inc., New York. 126 p. Zartman, R.E. and Stacey, J.S. (1971) Lead isotopes and mineralization ages in Belt Supergroup rocks, northwestern Montana and northern Idaho. Econ. Geol. 66_, 849-860. Zartman, R.E. and Wasserburg, G.J. (1969) The isotopic composition of. lead in potassium feldspars from some 1.0-b.y. old North American igneous recks. Geochim. Cosmochim. Acta ^3, 901-942. APPENDIX NOTES ON SAMPLES, ANALYTICAL METHODS, AGE CALCULATIONS, PRECISION AND CONTAMINATION A . 1 SAMPLES The writer spent two summers (1968, 1969) in the field in the East Kootenay district of British Columbia, and during this time most of the analysed galenas from deposits in Purcell Supergroup rocks were collected. Discussions with R.G. Gifford were helpful in selection of these samples. Several samples were obtained from R.G. Gifford, Dr. G.B. Leech and from the collection of the Geological Sciences Centre of the University of British Columbia. Drill-core samples from Sullivan Mine were made available by Dr. H.C. Morris and were selected with the assistance of F.R. Edmunds. The writer has not worked in the field in the Kootenay Arc or in the Anvil Range. Samples from the Kootenay Arc were obtained from Dr. A.J. Sinclair, Dr. J.T. Fyles, A.S. Macdonald, J.F.W. Orr, B. Price and from the collections of the UBC Geological Sciences Centre. A l l samples from the Anvil Range were collected by the writer except for one obtained from R.G. Chaplin and another supplied by G. Jilson. Most of the Anvil Range samples were taken from drill-core at Faro Mine, and at the Vangorda, Swim and Sea prospects. These samples were made available by Dr. A.E. Aho (Faro, Sea) and W.M. Sirola (Vangorda, Swim). The writer was assisted in making these collections (in 1968) by Dr. D.J. Tempelman-Kluit. A-2 A.2 ANALYTICAL METHODS A l l lead isotope analyses, including sample preparations, were made by the writer, using a gas-source mass spectrometer and other facilities of the Department of Geophysics and Astronomy of the University of British Columbia. Samples were introduced as tetramethyl lead and the trimethyl spectrum was measured (see Ostic and others, 1967). Examples of typical spectra are shown in figs. A.l, A.2 and A.3. The tetramethyl lead was prepared by a Grignard reaction (Pb^+CR^MgBr) and was purified by a gas chromatographic method (see Ulrych, 1960; Sinclair, 1964). The mass spectrometer used in this study incorporates the analyser tube, magnet and Nier-type source of an older 90°-sector, 12 inch radius Instrument described in detail by Kollar (1960), but entirely new electronic supplies (Russell and Bellis, 1971). Some details of this instrument are: ion accelerating voltage 5 KV; spectrum is scanned by variation of the magnetic field; tungsten ribbon filament; electron energy about 120 ev; source s l i t 0.004", collector s l i t 0.020"; Faraday cup collector. The instrument is connected to an Interdata model 4 computer (Russell and others, 1971; Blenkinsop, 1972) which filters digital ion-current information and stores the filtered data and switch positions on magnetic tape using a program written by Dr. J. Blenkinsop. j Final processing of data on the tape is done on an IBM Duplex 360/67 computer by a program written by Weichert (Weichert and others, 1967) and modified by Dr. R.D. Russell and Dr. J. Blenkinsop (all of the Department of Geophysics and Astronomy). Electronic equipment for the mass spectrometer and the interface was constructed by E.J. Bellis, H. Verwoerd, and R.D. Meldrum, a l l of the FIG. Al Pb(CH3J4 Fig. A.l Chart record of spectrum produced by electron bombardment (about 120 ev) of tetramethyl lead vapour. Principal ions responsible for each set of peaks are identified. Note mass scale below spectrum and the attenuation factors (in brackets). Fig. A.2 Chart record showing detail of peaks at masses 251 (mainly Pb 2 0 6 (CIi 3) 3 +) and 252 (mainly Pb 2" 7(CH 3X 3 +). Resolution of the mass spectrometer is about 460, peak shape is about 0.67 (10% value). The distance (dispersion) between the centres of the two ion beams which these peaks represent is about 0.05 inches. FIG. A3 j P Fig. A. 3 Typical chart record of trimethyl lead spectrum. Attenuation factors, atomic mass numbers, time scale, and principal peak related to each of the four isotopes of lead are shown. The height of the largest peak (253 a.m.u.) corresponds to an ion current of about 5*10" amp. As well as being output in chart form, the spectrum is digitised (5 readings/sec) and filtered to remove frequencies above about 0.6 hz. Four or five filtered readings are obtained on each peak top. A-5 technical staff of the Department of Geophysics and Astronomy. Dr. R.D. Russell was responsible for circuit design of the mass spectrometer; D.L. Mitchell and R.D. Meldrum designed the interface and associated control circuits. Isotope ratios were calculated from raw analyses in the follow-ing way. Samples were analysed in the sequence shown below, where , Bi, etc. etc. represent separate analyses of sample A,B, etc. and S i , S 2 , etc. represent separate analyses of Broken H i l l Standard. Each analysis is based on measurements from five pairs (up- and down-mass) of spectra and took about one hour. Day 1 ^ 1 ^ 2 * » • • • • • • • > A j A 2 . . . . . . . . . . B j B 2 Calculate mean of S 1S 2SsSu, calculate (S-A), (S-B) Day 2 8 3 8 ^ CC DD Calculate mean of SsS^Se,, calculate (S*-c") , (S-D) Day 3 S 5S 6 EE FF etc. Absolute values were calculated for each sample (A,B,C, etc.) by addition of the true differences (S-A)1, (S-B)', etc. to the absolute value of the Broken H i l l Standard as reported by Stacey, Delevaux and Ulrych (1969) , that i s : ^ 2 0 6 ^ 2 0 4 = 1 6 > Q 0 7 p b 2 0 7 / p b 2 0 i 4 = 1 5 < 3 9 7 p b 2 0 8 / p b 2 0 < + = 35^75 A-6 A.3 CALCULATIONS For calculations of lead isotope ages, the decay constants in common use (e.g. see York and Farquhar, 1972) were employed (A238 = 0.1537 x 10 _ 9y - 1, X 2 3 5 = 0.9722 x 10" 9y - 1. *232 = 0.0499 x 10~ 9y - 1). The primeval ratios Pb 2 0 6/Pb 2 0 t t = 9.346, Pb 2 0 7/Pb 2 0 l + = 10.218, Pb 2 0 8/Pb 2 0 I\ = 28.963 (Oversby, 1970) and an "age of the earth" of 4.55 BY (Murthy and Patterson, 1962) were used where necessary. These parameters are the same as those used by Zartman and Stacey (1971), permitting the direct comparison of results in section 2.6. Selection of a different "age of the earth" (e.g. Cooper and others, 1969; 4.58 BY) will significantly change some ages calculated in this thesis but should have l i t t l e effect on the interpreta-tions made. It is not clear at the present time what the most appropriate value for the age of the earth is (e.g. Gale and others, 1972). Slopes of anomalous lead lines were calculated by computer (program of Dr. R.D. Russell) using the least-squares method of York (1969), and assuming perfect positive correlation of errors in x and y. As partial justification for using correlated errors, the correlation coefficient for the data of table A.II (n=39) is +0.83 (95% limits 0.70, 0.91). For slope calculations, analytical errors were uniformly assumed (from inspection of data in table A.I) to be 0.1% (la) for a l l lead isotope ratios, and standard errors of slopes (method of Williamson, 1968) which are quoted and used in age calculations are the 68% confidence level values. The goodness of f i t of lines was tested by determining the value of (S/n-2), where S is the minimised quantity and n the number of points plotted (York, 1969). In a l l cases (12), the value of (S/n-2) was less than 1.4, in 9 cases was less than 1.0, and no adjustment of slope errors was made. A-7 TABLE A. I RF.n.ICATE ANALYSES UBC No. rb 2 0 l 7 r b " * T V ' / I V * U B C No. rb 2 0'/rb : o i- P b ' ^ / P b 2 0 " P b 2 o e / P b 2 0 " 765 16.313 15.409 35.993 855 18.240 15.634 38.207 16.325 15.402 35.999 18.244 15.625 38.215 775 16.412 15.447 36.129 859 17.926 15.589 37.900 16.396 15.428 36.050 17.921 15.595 37.867 16.404 15.435 36.070 16.413 15.435 36.066 284 18.991 15.742 39.287 18.992 15.746 39.320 769 16.427 15.432 36.103 16.435 15.434 36.105 295 18.889 15.661 38.710 . 18.888 15.660 38.720 851 16.432 15.432 36.072 16.443 15.433 36.101 226 17.485 15.481 37.964 17.476 15.494 37.957 852 16.536 15.435 36.281 16.546 15.454 36.327 772 18.356 15.650 38.319 16.543 15.443 36.311 18.353 15.671 38.340 18.345 15.652 38.319 8A6 16.450 15.465 36.116 18.345 15.660 38.325 16.447 15.455 36.090 18.354 15.668 38.333 18.341 15.668 38.320 847 16.523 15.466 36.182 16.512 15.472 36.142 754 18.213 15.626 38.131 18.230 15.634 38.152 849 16.525 15.477 36.168 18.235 15.643 38.171 16.532 15.484 36.170 18.220 15.630 38.116 778 16.399 15.447 36.130 809 18.366 15.674 38.337 16.411 15.454 36.172 18.354 15.653 38.326 18.374 15.678 38.374 761 16.332 15.397 35.939 16.346 15.410 35.984 810 18.362 15.662 38.323 18.371 15.664 38.365 773 16.391 15.435 36.104 18.379 15.684 38.401 16.383 15.414 36.047 18.369 15.659 38.347 .16.392 -15.425 36.063 818 18.369 15.684 38.347 808 16.338 I D . A U D 35.970 18.361 15.668 38.321 16.344 15.407 35.991 822 18.336 15.655 38.253 762 16.327 15.401 35.968 18.336 15.662 38.293 16.336 15.411 36.002 819 18.342 15.655 38.285 759 17.275 15.529 37.085 18.350 15.672 38.308 17.263 15.534 37.106 820 18.341 15.666 38.299 300 19.093 15.743 39.454 18.335 15.666 38.287 19.092 15.792 39.419 821 18.337 15.664 38.301 768 18.788 15.638 38.778 18.347 15.673 38.316 18.795 15.637 38.777 0 t 0.007(0.04%) • 0.008(0.05%) ± 0.022(0.06%) 2a ± 0.014(0.09%) ± 0.017(0.11%) ± 0.045(0.13%) Note 1. Standard dev ia t ions were c a l c u l a t e d i n the fo l lowing way (Youden, 1951) Set Samples Mean C a l c u l a t e D i v i s o r (n-1) W W , , W 2 , W* (W)-W*) 2 + (Wj-W*) J 1 X X , , X 2 , X , X* ( X i - X * ) 2 + ( X 2 - X * ) 2 + ( X 3 - X * ) 2 2 Y Y I . Y J Y n Y * ( Y i - Y * ) 2 + (Y 2 -Y* ) 2 + ( Y n " Y * ) 2 n-1 T o t a l of above - "sum of squares" T o t a l of above « "degrees of freedom" sum of squares legrees of freedom Note 2. Of a t o t a l of 79 analyses done as checks of p r e c i s i o n , a l l but 6 are given above; these being re jec ted as unacceptab le . Note 3. Each a n a l y s i s reported above i s c a l c u l a t e d from the mean of 2 separate , consecutive analyses as compared with the mean of 4 separate consecut ive analyses of the standard. Note 4. The percentage standard dev ia t ions were ca lcu la ted by comparing the standard devia t ions with the mean values obta ined for the Broken H i l l Standard, the sample with the lowest r a t i o s measured l n t h i s study ( i . e . , these are maximum Z e r r o r s ) A-8 TABLE A.II MEASUREMENTS OF BROKEN HI LI. STANDARD, SET A. P b J C 6 / P b J 0 " p b 2 0 7 / p b 2 0 » P b ' c ' / P b 2 0 * tober 8, 1971 16.046 15.447 35.84 3 16.055 15.456 35.919 16.048 15.448 35.914 16.055 15.443 35.907 16.051 15.450 35.899 16.060 15.467 35.947 16.068 15.466 35.945 16.069 15.472 35.961 16.056 15.456 35.922 16.054 15.459 35.926 16.057 15.457 35.924 16.057 15.461 35.910 16.056 15.467 35.956 16.068 15.470 35.963 16.054 15.456 35.919 16.049 15.446 35.897 16.055 15.452 35.904 16.051 15.448 35.892 16.039 15.443 35.866 16.049 15.456 35.898 16.053 15.448 35.919 16.054 15.463 35.938 16.054 15.459 35.916 16.058 15.455 35.919 16.055 15.459 35.927 16.062 . 15.466 35.951 16.063 15.469 35.917 16.065 15.471 35.965 16.064 15.467 35.961 16.058 15.455 35.935 16.066 15.463 35.955 16.060 15.457 35.928 16.066 15.455 35.952 16.071 15.470 35.989 16.064 15.456 35.951 16.064 15.465 35.956 - 16.059 15.458 35.939 16.068 15.470 35.940 c i l 1, 1972 16.062 15.460 35.940 H^MEAN 16.058 15.459 35.928 0 ± 0.007(0.05%) ± 0.008(0.05%) ± 0.029(0.08%) 20 ± 0.015(0.09%) ± 0.017(0.11%) ± 0.059(0.16%) Note 1. Above are a l l the analyses of UBC 001 done during the interval October 8, 1971 to Apri l 1, 1972. Nearly a l l analyses interpreted in this thesis were run between these dates. The l i s t ing is consecutive. Note 2 . Calculation of standard deviation A (X-X) 2 V (n-1) Note 3. Each analysis above is the mean of two consecutive analyses. In calculating sample results, consecutive results shown above were paired, and this mean value of the standard was compared with the unknown sample analysis ( i tself a mean of two analyses). Note 4. The above analyses show a change with time and the distribution of values is not normal (is bimodal). As might be expected, the replicate sample analyses do not show this effect. Note 5. Because isotope ratios determined on a gas-source instrument depend on source conditions (e.g. electron voltage, magnetic f ie ld , etc.) as well as viscous/molecular flow frac-tionation, the mean value quoted is not particularly significant. For example, another 6et of nine analyses gave the result r P b 2 0 6 / P b 2 0 " =- 16.078 ?b207/?b20h = 15.487 P b 2 0 8 / P b 2 0 1 * = 36.008 (±0.07%) (±0.12%) (±0.021%) ecu i where the errors are 2a values (corrected for the small number of samples). Nevertheless, published values for the Broken H i l l Standard and the two values given here are reasonably close to the ratios calculated (assuming fractionation) from the absolute ratios reported for this standard. 15-500 Pb 207 Pb 204 15-400 KEY (CRK Cooper «nd others (1969) K Kol lar (1960) R Richards (1962) Sm S c u l l (1968) SDU Stacey and others (1969) S>K Stacey and others (1967) F Farquhar (TorontoT Ka Kanasewich (rue) Reported ln 0 Ost lc (UBC) Slawson and SI S i n c l a i r (UBC) Russell (1967) 0 Ulrych (UBC) _ A-9 O Individual r e s u l t s , set~A •y^rr.ean, set A •yk^ iiean, note 5 This thes is , p. A-8 Fractionated ratios Ct3 )SMR / iSm ±o-iz Absolute ratios I 16-000 I 16-100 Fig. A.4 P l o t of mean values obtained by other workers f o r the Broken H i l l Standard,and the r e s u l t s of t a b l e A . I I . The expected ( f r a c t i o n a t e d ) r a t i o s were c a l c u l a t e d from the two absolute values r e p o r t e d by Stacey and others (1969) and Cooper and others (1969), and i t was assumed that gas flow i n t o the source was vis c o u s and flow out was molecular (Halsted and N i e r , 1950; W h i t t l e s , 1964). A-10 A.4 PRECISION The precision of measurements reported in this thesis is fairly-indicated by the two estimates of analytical uncertainty based on the replicate analyses in table A.I and A.II. Because i t is difficult to justify, very l i t t l e data were rejected (about 20 analyses out of 450), and then only where really necessary (e.g. blips on peak tops, circuit failures, etc.). The replicate analyses were run concurrently with the rest of the analyses. A.5 CONTAMINATION The precautions taken against contamination during purification and analysis of samples are mentioned below, with some evidence to suggest these ^ere effective. Contamination in the gas chromztograph The chromatographic column used to separate tetramethyl lead from ether after the Grignard reaction was a pyrex helicoil (278 cm. x 7 mm. I.D.) packed with 45/60 mesh Chromosorb W (AW, DMCS) coated with 20% dinonyl phthalate (HETP for Pb^s)!, = 0.5-1.1 cm.). In normal use the temperature was maintained at 75° and flow rates (N2) were 85-110 ml/min. The materials used are relatively inert, the temperature is low relative to the decomposition temperature of tetramethyl lead (265°C; Simons and others, 1932), large samples (0.3-1.0 ml) containing about 0.1 ml, Pb(CH3)it were injected, and the column was flushed for a minimum of 4 hours at a flow rate of about 50 ml/min (12 litres of N 2). Samples that A - l l were known or suspected to be similar in composition were processed in groups and a separate column was used exclusively for samples of the Broken H i l l Standard. The following experiment was done to test these precautions. Two samples of a highly radiogenic galena lead (UBC 642 from the Siljan Ring, Sweden) and two samples of Broken H i l l Standard (001) were prepared and put through the column in the same way as other samples and in the sequence 642A, 001A, 642B, 001B. If small amounts of 642 contaminate the succeeding 001 samples then the difference between these 001 samples and "uncontaminated" 001 will give two estimates of the extent of contamination. Sample Pb 2 0 6/Pb 2 0 1 t Pb 2 0 7/Pb 2 0 1 t Pb 2 0 8/Pb 2 0 1 t Check 1. 001A 16.081 15.477 35.964 Check 2. 001B 16.085 15.456 35.952 Preceding analysis of "uncontaminated" 001* 16.062 15.460 35.940 standard sample (001A - 001*) + 0.019(0.12%)+ 0.017(0.88%)+ 0.022(0.16%) (001B - 001*) + 0.023(0.14%)- 0.004(0.21%)+ 0.012(0.09%) Assuming that the differences (001A - 001*) represent an incre-ment due to contamination by 642, the bracketed figures express the amount of contamination as a percentage of the difference in ratios between 642 and 001. Because the difference between normal samples was always less than h of the difference between 642 and 001, the experiment demonstrates A-12 that contamination in the chromatographic column should be negligible. Contamination in the mass spectrometer Because larger amounts of sample are handled in the inlet system of the mass spectrometer than are admitted to the ion source, contamina-tion is most likely to arise in the inlet system - by sorption, or by trapping on reservoir walls by mercury or in valves (which are greaseless). Another possibility is that lead deposited in the source or elsewhere by the thermal degradation of a previous sample of tetramethyl lead wil exchange with lead in a new sample (Leigh-Smith and Richardson, 1935; Duncan and Thomas, 1967). Flushing the system with a portion of the new sample before analysis, keeping the source hot by leaving the filament on, allowing adequate pump-down time (normally more than 2 hours) between analyses, and running samples of similar composition in groups are a l l useful precautions against contamination and were normal practice. The following test was done. Radiogenic sample 642 and the cur-rent standard (001) sample were analysed in the following sequence: 642(a), 001(a), 642(b), 001(b), with the results shown below. Analysis Pb 2 0 6/Pb 2 0 t* Pb 2 0 7/Pb 2 0 J t Pb 2 0 8/Pb 2 0 1 + 642(a) 32.568 17.395 49.598 001(a) 16.069 15.458 35.927 642(b) 32.575 17.380 49.619 001(b) 16.055 15.461 35.953 Difference between the two 642 analyses: 0.007 (0.02%) 0.015 (0.09%) 0.021 (0.04%) Difference between the two 001 analyses: 0.014 (0.09%) 0.003 (0.02%) 0.026 (0.07%) A-13 The results suggest that contamination in the mass spectrometer can be kept to a negligible level. The analyses of the Broken H i l l Standard shown in table A.II show no effects of contamination during routine work, and confirm this conclusion. 

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