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UBC Theses and Dissertations

Geology and petrology of the Troitsa Lake property, Whitesail Lake map area, B.C. Cawthorn, Nigel George 1973

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GEOLOGY AND PETROLOGY OF THE TROITSA LAKE PROPERTY, WHITESAIL LAKE MAP AREA, B.C. by NIGEL GEORGE CAWTHORN B.Sc., University of Aberdeen, 1970. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE. i n the Department of Geology We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA May 1973. In p resen t i ng t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r re ference and s tudy. I f u r t h e r agree tha t permiss ion fo r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed wi thout my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada i ABSTRACTS o ' The T r o i t s a Lake Property i s located at l a t i t u d e 53 32 o > north and longitude 127 2 0 west i n the Whitesail Lake Map area. Lower Jurassic andesitic flows, t u f f s and breccias and int e r c a l a t e d a r g i l l i t e lenses of the Lower Volcanic D i v i s i o n of the Hazelton Group are intruded by a granodiorite stock. A younger s i l l - l i k e r h y o l i t e complex occurs to the northwest of the stock. A vari e t y of northwesterly trending dykes, i n c l u d -ing feldspar porphyries of quartz l a t i t e composition, cut a l l other rocks. The stock i s zoned from a coarse-grained quartz monzon-i t e i n the centre to a r e l a t i v e l y fine-grained granodiorite at the margin.Calculated chemical compositions of the rocks show the stock has followed a c a l c - a l k a l i n e d i f f e r e n t i a t i o n trend. The compositions of plagioclase, a l k a l i feldspar and b i o t i t e systematically vary throughout the stock. The thermal e f f e c t of the stock i s estimated to have produced hornblende hornfels f a c i e s conditions up to 400 feet from the contact. The stock was emplaced i n the epizone at a probable depth of about four kilometres and was subject to a load pressure of a l i t t l e over one kilobar. Compositions of coexisting feldspars allow only a crude estimate to be made of the c r y s t a l l i s a t i o n temperatures. This indicates a temperature of 72 0° to 770°C. Compositions of the b i o t i t e s indicate the stock cry -s t a l l i s e d under conditions of constant or increasing f and °2 the melt may have been water saturated. B i o t i t e compositions and experimental data f o r the 'granite 1 system indicate that the stock c r y s t a l l i s e d under a Pji Q of about one k i l o b a r and at temperatures ranging from 7 3 0 ° to 8 5 0 ° C. The PJJ Q must have approached, and perhaps equalled P ^ o a ( j ' Potassium / Argon data y i e l d an apparent age f o r the stock of 7 5 . 7 i 2 . 3 m i l l i o n years. Several stocks i n the White s a i l Lake Map area have c l o s e l y s i m i l a r ages. The feldspar porphyry dykes have been subject to hydro-thermal a l t e r a t i o n . In one major dyke t h i s has a zonal d i s -t r i b u t i o n pattern; p r o p y l i t i c type a l t e r a t i o n i n the north pas-ses southwards through quartz - s e r i c i t e to b i o t i t e - ortho-clase types. Sulphide mineralisation, also showing a zonal pattern, i s cl o s e l y associated with the hydrothermal a l t e r a t i o n Weak p r o p y l i t i c a l t e r a t i o n and fracture plane sulphide mineral-i s a t i o n a f f e c t the central part of the stock. The dykes appear to have acted as channelways f o r the hydrothermal ore-bearing solutions. i i i LIST OF CONTENTS. CHAPTER PAGE I INTRODUCTION 1. Location and Access 1. Physiography 1. History 3. II REGIONAL GEOLOGY 5» Introduction 5» Metamorphic Strata 5» Hazelton Group 8. Lower Cretaceous Strata 11. Pleistocene and Recent Deposits 11. Intrusive Rocks 11. I l l GEOLOGY OF THE TROITSA LAKE PROPERTY 13. Introduction 13. Hazelton Group 13. The Stock 19-Rhyolite 41. Dyke Rocks 41. Structure 47. IV FELDSPARS 54. Introduction 54* Plagioclase Feldspars 54* A l k a l i Feldspars 60. CHAPTER PAGE V BIOTITE. 6 9 . Introduction. 6 9 . Method of Study. 6 9 . Discussion of Results. 7 5 -VI PETROLOGY OF THE STOCK. 81. Introduction. 8 1 . Contact Metamorphism. 8 5 . Level of Emplacement of the Stock. 9 0 . Chemical V a r i a t i o n of the Stock. 9 2 . Conditions of C r y s t a l l i s a t i o n of the Stock. 9 9 . Summary. 1 0 2 . VII GEOCHRONOLOGY. 1 0 5 -VIII ALTERATION AND MINERALISATION. 1 1 2 . Introduction. 1 1 2 . Feldspar Porphyry Dykes. 1 1 2 . A l t e r a t i o n and mineralisation of the .t Stock. 1 2 1 . Zonal D i s t r i b u t i o n of A l t e r a t i o n and Mine r a l i s a t i o n . 1 2 2 . Sequence of Sulphide Mineralisation. 1 2 8 . IX SUMMARY AND CONCLUSIONS. 131. LITERATURE CITED. 1 3 5 -PAGE APPENDIX I. X-RAY DIFFRACTION INVESTIGATION OF ALKALI FELDSPAR. 140. APPENDIX I I . BIOTITE ANALYSES. 144. Electron Microprobe Analyses. 144. Ferrous Iron Determination. 144-I LIST OF TABLES. v i TABLE PAGE I. Table of Formations - Whitesail Lake Map area. (Modified a f t e r D u f f e l l , 1 9 5 9 ) . 7 . I I . Hazelton Group Succession. 1 0 . I I I . Modal Analyses. 2 6 & 2 7 -IV. Average Mineralogy and Estimated Chem-i c a l Composition of the T r o i t s a Lake Stock. 2 8 . V. Anorthite Content of Plagioclase Feldspar. 5 5 . VI. Oxygen Fugacities of Oxygen Buffers. (From Wones and Eugster, 1 9 6 5 ) . 7 4 . VII. Titanium Content of Magnetite from s e l e c t -ed samples of the Troits a Lake Stock, de-termined from Electron Microprobe Data. 7 4 . VIII. Refractive Indices of B i o t i t e s from the Troitsa Lake Stock. 7 6 . IX. Chemical Analyses of Selected B i o t i t e s from the Tr o i t s a Lake Stock. 7 8 . X. Mineral Compositions used i n c a l c u l a t i n g the Chemical Compositions of rocks from Modal Analyses. 8 2 . XI. Estimated Chemical Composition of rock samples from the Troits a Lake Stock 8 3 & 8 4 . XII. C r i t e r i a , suggested by Buddington ( 1 9 5 9 ) , as in d i c a t i v e of epizonal and mesozonal intrusions, and t h e i r presence or absence i n the Troits a Lake Stock. 9 1 . XIII. Geochronological Data. 1 0 6 . XIV. A l k a l i Feldspar - X-ray D i f f r a c t i o n Data. 1 4 2 . XV. Homogenised A l k a l i Feldspar - X-ray D i f -f r a c t i o n Data. 1 4 3 -XVI. Ferrous Iron Content of Selected B i o t i t e s from the Tro i t s a Lake Stock. 1 4 7 • V l l LIST OF FIGURES. FIGURE PAGE 1. Location of the T r o i t s a Lake Property, B^ 'C. 2 2. Regional Geological Map, Troitsa Lake Area. 6 3. Generalised Stratigraphic Column. 14 4. a) Approximate r e l a t i v e time of c r y s t a l l i s -ation of the major minerals of the T r o i t s a Lake Stock, as indicated by te x t u r a l r e l a t i o n s h i p s ; b) PT projection of the phase boundaries for granodiorite from the Wallowa Batholith, Oregon determined for beginning of melting and for the disappearance of the major mineral phases (Piwinskii & Wyllie, 1968). 32 5. Plot of the Modal Quartz - Orthoclase -Plagioclase of the Troi t s a Lake Stock. 33 6. D i s t r i b u t i o n of Quartz i n the Stock. 36 7. D i s t r i b u t i o n of Mafic Minerals i n the Stock. 37 8. D i s t r i b u t i o n of Plagioclase. i n the Stock. 38 9. D i s t r i b u t i o n of Orthoclase i n the Stock. 39 10. Variation of the Plagioclase/Plagioclase + Orthoclase Ratio i n the Stock. 40 11. Var i a t i o n i n the Mineralogy of Rocks of the T r o i t s a Lake Stock, with respect to the Larsen Index. 42 12. Direction of Lineaments V i s i b l e on A e r i a l Photographs. 50 13. Contoured Plots of Poles to Joint Planes i n the Quartz - Monzonite - Granodiorite of the T r o i t s a Lake Stock. • 51 14. Contoured Plot of Poles to Attitudes of Dykes. 52 v i i i FIGURE PAGE 15. Variation of the Plagioclase and A l k a l i feldspar compositions with respect to the Larsen Index. 57 16. Modal Feldspar constituents of the Tro i t s a Lake Stock. Samples recalculated to 100% Anorthite + A l b i t e + Orthoclase. 58 17. A l k a l i Feldspar from the Troitsa Lake Stock plotted on a (060) - (204) plot, s i m p l i f i e d from Wright (I968). 63 18. Composition of coexisting Plagioclase and A l k a l i feldspars from rocks of the Troitsa Lake Stock. 65 19« Relation between Temperature and the Ratio of D i s t r i b u t i o n of A l b i t e between A l k a l i Feldspar and Plagioclase feldspar, (after Barth, I962). 67 2 0. S t a b i l i t y of B i o t i t e s as a function of f and Temperature at 2070 bars Total 2 Pressure (after Wones & Eugster, 1965). 71 21. The Ternary System KFe^ + A l S i ^ Q(0H ) 2 -KMg2 +AlSi 3 0 1 Q(0H) 2 - KFe^ + A l S i ^ Q ( H _ 1 ) . The compositions of "Buffered" B i o t i t e s are shown,(after Wones and Eugster, 19&5). 71 22. Variation of Refractive Index (- 0.002) of B i o t i t e from rocks of the Troitsa Lake Stock with respect to the Larsen Index. 77 23. Heating of Country Rock adjacent to an in t r u s i v e body of Granodiorite composition calculated according to Lovering (1936,1955)* Jaeger (1957, 1959) and Buseck (1966). 89 24. Variation of the Major Oxides i n the rocks of the Troi t s a Lake Stock, with respect to the Larsen Index. 93 & 94 25. A l k a l i Stock. - Lime Index for the Troitsa Lake 95 i x FIGURE PAGE*. 26. Combined AFM and Na 20 - K 20 - CaO Variation Diagram for rocks of the Tro i t s a Lake Stock, (after Nockolds and Allen,1953). 97 27. An AFM triangular plot for rocks of the Troitsa Lake Stock. Fractionation trends of some other magma series are shown for comparison, (after Yoder and T i l l e y , I962). 98 2 8. Conditions of C r y s t a l l i s a t i o n of the Troits a Lake Stock. 100 29. Phase Relations of the System NaAlSi^Og -KAlSi 30g - S i 0 2 - H 20 at a P H Q of lOOOKg/cm2 2 (after T u t t l e and Bowen, 1958). Compositions of Rocks of the Troits a Lake Stock (Quartz + A l b i t e + Orthoclase) are superimposed on the Phase Diagram. 103 30. Location of Radiometrically Dated Stocks i n the T r o i t s a Lake Area. 108 31. K/Ar age dates of other intrusions near the Tro i t s a Lake Stock. 110 32. Hydrothermal A l t e r a t i o n i n the Feldspar Porphyry Dykes. 113 33. AKF Diagram for minerals common i n hydro-thermal a l t e r a t i o n at porphyry copper deposits with some equilibrium assemblages indicated. ( aft e r Rose, 1970) 120 34- S t a b i l i t y Relations of Ka o l i n i t e , Muscovite and Potassium Feldspar i n Chloride Solutions with respect to Temperature and KC1/HC1 r a t i o . ( a f t e r Rose, 1970). 127 35- T r o i t s a Lake Property Geological Map i n pocket. X LIST OF PLATES *? Plate Page Frontispiece A view looking west towards the Troi t s a Lake Property taken during l a t e May. The Stock outcrops along the ridge i n the centre of the photograph and underlies the v a l l e y and cirque behind. Troitsa Lake i s i n the foreground. 1. A r g i l l i t e of the Hazelton Group with interbedded tuffaceous layers. 18 2. Strata of the Hazelton Group showing i n t r i c a t e interbanding of t u f f and a r g i l l i t e . 18 3. Typical andesite flow breccia of the Hazelton Group - 'Red Volcanic Unit'. 2 0 4. "Coarse grained"quartz monzonite of the core zone of the stock. Note the greenish colour due to p r o p y l i t i c type a l t e r a t i o n . 20 5. "Medium grained" quartz monzonite - granodiorite of the intermediate zpne of the stock. 22 6. "Fine grained" granodiorite from the marginal zone of the stock. 22 7. Photomicrograph (x30). Quartz monzonite. Plagioclase showing resorbed and a l b i t i s e d margins and repeated o s c i l l a t o r y zoning. 30 8. Photomicrograph (x30). Quartz monzonite. Plagioclase with a l b i t i s e d and resorbed margins. Note the i n t e r s t i t i a l nature of the orthoclase. 30 9 . A r h y o l i t e s i l l intruding the "Red Volcanic Unit" of the Hazelton Group. The photograph was taken i n the western part of the property looking towards the northeast. The contact between the r h y o l i t e complex and the quartz monzonite-granodiorite of the stock can be seen i n the background. 43 10. Localised f o l d i n g i s seen i n the Hazelton Group. The photograph shows one limb of an a n t i c l i n a l f o l d i n the "Red Volcanic Unit" i n the southern part of the property near the contact of the stock. The view i s looking towards the southeast. 43 Plate Page 11. Feldspar Porphyry (Specimen 2 5 8 ) . P r o p y l i t i c type a l t e r a t i o n . Note the x e n o l i t h i c i n c l u s i o n of an e a r l i e r l e s s prophyritic phase. 1 1 5 12. Photomicrograph (x30) (Specimen 2 5 8 ) . P r o p y l i t i c type a l t e r a t i o n . Chlorite and epidote pseudo -morphous after amphibole. Groundmass and feldspar phenocrysts (top r i g h t ) are replaced by clay minerals. 1 1 5 1 3 . Photomicrograph (xl25)(Specimen 2 4 8 ) . Secondary b i o t i t e - pseudomorphous after amphibole. 116 1 4 - Feldspar Porphyry (Specimen 2 4 8 ) . Quartz -s e r i c i t e type a l t e r a t i o n . 1 1 7 1 5 . Photomicrograph (x30) (Specimen 210). Intense qu a r t z - s e r i c i t e a l t e r a t i o n , similar to that of Specimen 2 4 8 above. 1 1 7 16. Feldspar Porphyry (Specimen I 8 9 ) . A l t e r a t i o n of the b i o t i t e - orthoclase zone. Note the chalcopyrite replacing the mafic minerals. 118 1 7 - Photomicrograph (x30) (Specimen 1 8 9 ) . B i o t i t e -Orthoclase a l t e r a t i o n zone. Secondary potassium feldspar i n an intensely s i l i c i f i e d groundmass. 118 1 8 . Photomicrograph (x30) (Specimen 1 8 3 ) . Quartz -monzonite showing p r o p y l i t i c a l t e r a t i o n with replacement of o r i g i n a l mafic minerals by c h l o r i t e and minor epidote. 1 2 3 1 9 . F r a c t u r e - f i l l mineralisation and associated a l t e r a t i o n envelope i n quartz monzonite. Quartz and chalcopyrite f i l l a 2mm wide fracture bordered by a t h i n layer of secondary b i o t i t e and a zone up to 3mm wide of secondary potash feldspar. The host rock appears to have been subject to p r o p y l i t i c type a l t e r a t i o n . 1 2 3 x i i ACKNOWLEDGEMENTS. The author wishes to thank a l l members of the teaching and technical s t a f f of the Department of Geology for a s s i s t -ance and advice given during the preparation of t h i s t h e s i s . Thanks go especially to Dr. P.B. Read who supervised the pre-paration of t h i s t h e s i s , and to Dr. H.J. Greenwood who also read the manuscript and made many h e l p f u l suggestions. Thanks are also due to Mr. D.K. Mustard who provided much assistance and advice while the author was employed by Cerro Mining Company of Canada Limited. F r o n t i s p i e c e : A v i e w l o o k i n g west t o w a r d s t h e T r o i t s a Lake P r o p e r t y t a k e n d u r i n g l a t e May. The s t o c k o u t -c r o p s a l o n g t h e r i d g e i n t h e c e n t r e o f t h e p h o t o g r a p h and u n d e r l i e s t h e v a l l e y and c i r q u e b e h i n d . T r o i t s a Lake i s i n t h e f o r e g r o u n d . 1 CHAPTER I  INTRODUCTION Location and Access. The T r o i t s a Lake property i s i n the central part of the Whitesail Lake map-area at la t i t u d e 5 3 0 3 2 ' N. and longitude l z y ^ O ' W., and approximately 9 0 miles south of Smithers B.C. The property consists of a group of 8 0 claims located at the southwest corner of Troit s a Lake. Access to the property i s provided by 4 0 miles of paved road from Smithers to Houston, 7 5 miles of gravel road from, Houston to Tahtsa Lake, and 1 2 miles from Tahtsa Lake to the property by helicopter. The location i s shown i n Figures 1 and 2 . Physiography. The property l i e s i n the western part of the Whitesail Range i n a t r a n s i t i o n a l zone between the rugged Coast Moun-tai n s to the west and the r o l l i n g country of the Nechako Plateau region to the east. The main mountain ranges and valleys trend northeasterly and have a l o c a l r e l i e f of 7 0 0 0 feet. Elevations within the property range from 2 9 4 7 feet at Tro i t s a Lake to a maximum of 6 7 2 0 feet. The property i s drain-ed by a creek fed by three small alpine g l a c i e r s . This flows northeasterly into T r o i t s a Lake, a source area of the Fraser River system. P r e c i p i t a t i o n i s high, averaging 8 9 inches per annum at Kitimat 7 0 miles to the north. Snow remains i n the 2 LOCATION OF TROITSA L A K E PROPERTY, British Columbia. FIGURE I 3 upland valley u n t i l l a te June. Pleistocene g l a c i e r s have l e f t ridge and groove f e a -tures i n d i c a t i n g that the d i r e c t i o n of l a t e s t ice movement i n the Whitesail Lake area was northeastwards along the v a l -l e y s . G l a c i a l d r i f t i s abundant i n the valleys and covers the plateau areas. Alpine g l a c i a t i o n i s s t i l l active and many of the higher peaks have small cirque g l a c i e r s . History. The f i r s t claims staked i n the Whitesail Lake area were recorded i n 1906 and prospectors have been active i n the area ever since. The major discoveries of i n t e r e s t have been the l e a d - z i n c - s i l v e r deposits of Mount Sweeney, and the gold-bear-ing quartz veins and tungsten deposits of Lindquist Peak. The Emerald Glacier property on Mount Sweeney produced a t o t a l of 4,560 tons of l e a d - z i n c - s i l v e r ore i n 1951 and 1952. Explor-ation during the l a s t ten years has resulted i n the discovery of several copper prospects associated with small quartz mon-zonite -granodiorite stocks. Probably the most important of these i s the Berg property of Kennco Explorations staked i n 1961 and located six miles north of Tahtsa Lake and nine miles west of S i b o l a Peak. Ore reserves are estimated at 100,000,000 tons of 0.7 percent copper. The Ox Lake property of S i l v e r Standard Mines and ASARCO staked i n 1968, one mile south of Tahtsa Reach and four miles east of the mouth of Kasalka Creek, has estimated reserves of 25,000,000 tons of 0.3 percent copper and 0.0 8 percent M 0 S 2 . The T r o i t s a Lake property was staked i n I966. S i l v e r Stan-dard Mines Limited obtained an option on the property i n I966 and carried out a l i m i t e d mapping program as well as trench-ing and d r i l l i n g of known showings. S i l v e r Standard r e l i n -quished t h e i r option on the property i n I 9 6 8 and i t was op-tioned i n I 9 6 9 by Aston Resources Limited who carried out an airborne magnetic and electromagnetic survey. In 1971 Cerro Mining Company of Canada Limited obtained an option on the property from Aston Resources. The author carried out a deta i l e d mapping program on the property during the 1971 f i e l d season while employed by Cerro Mining Company, and t h i s provide the basis f o r t h i s t h e s i s . 5 CHAPTER I I  REGIONAL GEOLOGY Introduction. The regional geology of the T r o i t s a Lake area i s shown i n Figure 2. The primary source of information i s G.S.C. Memoir 299, "The Whitesail Lake Map Area" by S. D u f f e l l (1959). Most of the area i s underlain by Lower to Middle Jurassic s t r a t a of the Hazelton Group. An area of metamorphic rocks occurs to the west of Troitsa Lake. The youngest st r a t a i n the area, apart from unconsolidated Pleistocene and Recent deposits, are Lower Cretaceous rocks found to the north of T r o i t s a Lake. The s t r a t a are cut by a variety of Upper Jur-a s s i c and younger int r u s i o n s . The succession i n the Whitesail Lake map area i s shown i n Table 1. Metamorphic Strata . A large area of metamorphic rocks occurs to the west of Tro i t s a Lake. Following D u f f e l l (1959) these have not been d i f f e r e n t i a t e d i n Figure 2. Many of these metamorphic rocks may be correlated with the Hazelton Group and recognisable Hazelton Group rocks can be followed into them; however older metamorphic rocks unconformably ov e r l a i n by metamorphosed Hazelton Group s t r a t a have been found at the western end of Tahtsa Lake (just outside the area shown i n Figure 2) by Stuart (1955) . Palaeontologic evidence (Read, I963) has shown that rocks of Permian or Carboniferous age occur i n t h i s metamorphic 6 I"*'3 S.bo)a Peak REGIONAL GEOLOGICAL MAP, TROITSA LAKE AREA, (after Duffeii, 1959) QUATERNARY JURASSIC (?) AND LATER JURASSIC PLEISTOCENE AND RECENT UPPER JURASSICC?) AND LATER MIDDLE a LOWER JURASSIC I 9 I TILL,SAND,GRAVEL,CLAY, ALLUVIUM | 7 | MOUNT BOLOM STOCK I 6 I SWING PEAK STOCK CRETACEOUS I 5 I GRANODIORITE, DIORITE «le. LOWER CRETACEOUS ( t S U . n g Serlee) EJg} R E D GRANITE ARGILLITE, ARKOSI TUFF , ANDESITE. DIORITE f~2~[ GABBRO | j ~ | H A Z E L T O N GROUP JURASSIC AND OLDER JURASSIC, PERMIAN AND (?) CARBONIFEROUS | A | METAMORPHIC ROCKS (undifferentiated) FAULT ONSET OF GREENSCHIST FACIES 0 FOSSIL LOCALITY (Mid Juraislc) ONSET OF ALMANDINF AMPHIBOLITE FACIES LB FOSSIL LOCALITY , . _ , (Carbonlferoue - Permian) ( Read, 1963) (boundaries approximate, after Read, 1963) FIGURE 2. 7 TABLE I TABLE OF FORMATIONS - WHITESAIL LAKE MAP AREA (modified after Duff e l l , 19.59)-Era Period or Epoch_ Group or Formation Lithology Cenozoic Pleistocene and Recent - -• - - - -T i l l , gravel, sand, clay, alluvium Unconformity " ~ ~ " '. -- - • Oligocene or l a t e r Basalt, t u f f ; gabbro dykes Unconformity - - - . Mesozoic or Cenozoic Upper Cretaceous to Oligocene Ootsa Lake Group Rhyolite,_dacite, andesite, basalt; breccia, t u f f ; minor conglomerate Not i n contact ~ ~ ~ Mesozoic Lower Cretaceous ~ ? Skeena Series A r g i l l i t e , arkose, breccia, t u f f , andesite, basalt Nonconformity, possibly f a u l t contact, with Hazelton group, i n t r u s i v e Contact with part"of Coast "Intrusions Upper Jurassic and l a t e r Coast Intrusions Granodiorite,_diorite, granite, _quartz_.diorite, syenite, quartz monzonite, monzonite, gabbro Intrusive contact ~ ~ ' ~ ~ Middle and and lower Jurassic Hazelton Group Breccia, t u f f , andesite, dacite, r h y o l i t e , . b a s a l t ; a r g i l l i t e , greywacke, chert, conglomerate; miner limestone Partly unconformable, as i n the Nechako River area, and partly a conformable succession Lower Jurassic and Upper T r i a s s i c ;Takla Group Breccia, t u f f , andesite, a r g i l l i t e Jurassic and older.. (?Permo-Carboniferous Metamorphic complex la r g e l y Hazelton group but, _ includes some older rocks (Tahtsa Complex) Greenstone, amphibolite, phyllite., s c h i s t , gneiss, r e e r y s t a l l i s e d limestone, undifferentiated minor granite and d i o r i t e 8 septum. Read ( I 9 6 3 ) , on the basis of mapping and a study of hand specimens, has suggested that the metamorphic grade i n t h i s area increases towards the southwest and has sug-gested tentative f a c i e s boundaries, the approximate p o s i -tions of which are shown i n Figure 2 . Hazelton Group. Most of the northern and eastern part of the Troi t s a Lake area i s underlain by andesitic and b a s a l t i c volcanic rocks, t u f f s and breccias. Some sediments, mainly grey-wackes, also occur. These belong to the Hazelton Group and are predominantly Middle Jurassic i n age. D u f f e l l (1959) has suggested that some st r a t a at the west end of the Whitesail Range, and es p e c i a l l y around T r o i t s a Lake, may be of Lower Jurassic age. The base of the Hazelton Group i s best seen i n the Nechako River area where there i s an erosional unconformity marked by a conglomerate between s t r a t a of the Hazelton Group of Middle Jurassic age and the underlying Takla Group of Lower Jurassic age (Tipper, 1 9 5 5 ) . In h i s r e d e f i n i t i o n of the Hazelton and Takla Groups, Tipper ( 1959) stated that i n parts of the basin there may be no unconformity between Middle and Lower Jurassic s t r a t a and i n t h i s case a l l should be c l a s s i f i e d as Hazelton Group. Lower Cretaceous s t r a t a of the Skeena Series i n the Smithers map area have previously been included i n the Hazel-ton Group (Armstrong, 1 9 4 4 ) ; however i t has now been shown that the Skeena Series i s nonconformable and i s no longer i n -9 eluded with the Hazelton Group. (T. Richards, personal com-munication) . In the T r o i t s a Lake area palaeontologic evidence shows the s t r a t a are predominantly Mid Jurassic i n age. Volcanic rocks conformably overlie lower Middle Jurassic f o s s i l i f e r o u s marine s t r a t a over a large part of the area. These f o s s i l s have been described by Frebold (1951, 1953) and assigned to the Early and Middle Bajocian. The stratigraphy of the Hazelton Group i s summarised i n Table 2 and Figure 3• The Hazelton Group as described by D u f f e l l consists of at l e a s t 1 1 , 5 0 0 feet of interbedded volcanic and sedimentary rocks which can be roughly divided into a lower, mainly v o l -canic d i v i s i o n ; a middle, more sedimentary d i v i s i o n ; and an upper, dominantly volcanic d i v i s i o n . Boundaries between the d i v i s i o n s are a r b i t r a r y and can not be followed across the area as a whole. The lower d i v i s i o n of probable Lower Jur-a s s i c age consists mainly of green and purple andesites with some in t e r c a l a t e d sedimentary material. Above the lower v o l -canic d i v i s i o n i s a succession of sedimentary rocks with minor interbedded t u f f s and flows which i s approximately 2 , 5 0 0 feet t h i c k . I t i s t h i s part of the succession which y i e l d s the marine f o s s i l s . The upper volcanic d i v i s i o n consists of approximately 6 , 0 0 0 feet of volcanic flows, t u f f s , and breccias. I t has been suggested by Tipper (1959) and others that most of the material f o r the Hazelton Group st r a t a was derived mainly from a northwest trending i s l a n d arc on the s i t e of the present Coast Mountains. Some material may have been derived TABLE I I • HAZELTON GROUP SUCCESSION ( i n f o r m a t i o n f r o m D u f f e l l , 1 9 5 9 , and T i p p e r ,1971) AGE DIVISION AND THICKNESS CHARACTER M i d d l e o r Upper J u r a s s i c Upper V o l c a n i c D i v i s i o n . 6 0 0 0 f e e t . V o l c a n i c f l o w s , t u f f s and b r e c c i a s . M i d d l e J u r a s s i c M a r i n e S e d i m e n t a r y D i v i s i o n 2 5 0 0 f e e t . M a r i n e s e d i m e n t s w i t h i n t e r -bedded t u f f s and f l o w s . M i n -o r t u f f a c e o u s g r e y w a c k e , b l a c k a r g i l l i t e , g r e y - g r e e n t u f f , impure l i m e s t o n e , t h i n - b e d d e d c h e r t and a n d e s i t i c f l o w s . Lower a) L a t e Lower J u r a s s i c t o J u r a s s i c e a r l y M i d d l e J u r a s s i c b ) Lower J u r a s s i c c ) E a r l y Lower J u r a s s i c o r Upper T r i a s s i c Lower V o l c a n i c a) Red T u f f D i v i s i o n . U n i t Over 3 0 0 0 f e e t . b) Red V o l -c a n i c U n i t Up t o 3 0 0 0 ' c) Lower G r e e n V o l c a n i c U n i t . R e d and maroon s t r a t i f i e d t u f f s ! and f i n e b r e c c i a s . M i n o r w e l l -bedded s e d i m e n t s w i t h l i m e s t o n e l a y e r s . B r e c c i a s , t u f f s , i g n i m b r i t e s , and v o l c a n i c f l o w s o f v a r i a b l e c o m p o s i t i o n . M i n o r l i m e s t o n e and r e d t u f f . E p i d o t i s e d b a s i c g r e e n v o l c a n i c b r e c c i a s , s t r a t i f i e d t u f f s , and m i n o r f l o w s . 11 from a landmass to the northeast which became emergent i n late T r i a s s i c times and gradually increased i n size u n t i l Late Jurassic and Early Cretaceous time when most of Central B r i t i s h Columbia, including the T r o i t s a Lake area, became em-ergent. Lower Cretaceous Strata . Lower Cretaceous rocks occur i n the northern part of the area around Swing Peak and Laventie Mountain. These consist of 2 , 0 0 0 to 3 * 0 0 0 feet of marine sediments, o v e r l a i n by about 2 , 0 0 0 feet of volcanic t u f f s and flows of andesitic and basal-t i c composition. The beds s t r i k e generally easterly and dip at shallow angles to the south. These st r a t a may belong to the Skeena Series (T. Richards, personal communication). Pleistocene and Recent Deposits. Pleistocene and Recent deposits are abundant i n the v a l -leys and cover the plateau areas. Most of the material i s f l u v i o g l a c i a l i n o r i g i n , consisting mainly of s i l t , sand, gravel and some varved clays. Eskers, drumlins and other g l a c i a l fea-tures occur. Intrusive Rocks. The s t r a t i f i e d rocks of the T r o i t s a Lake area are cut by several types of i n t r u s i v e rocks of Upper Jurassic or l a t e r age. In order of probable increasing age these are:-Mount Bolom Stock - granite (youngest) Swing Peak Stock - d i o r i t e Coast Intrusions and related stocks Red Granite Gabbro (oldest) l i Contacts of the stocks may be c l e a r l y i n t r u s i v e , grad-ation a l through an i r r e g u l a r 'hybrid' zone, or faulted. Small stocks away from the main mass of the Coast Intrusions have sharper contacts with l e s s development of 'hybrid' rock types. Str a t i g r a p h i c evidence indicates that the period of emplacement of the stocks i s l a t e r than the deposition of the Hazelton Group, i . e . Middle Jurassic, but p r i o r to the deposition of Miocene basalts found elsewhere i n the Whitesail Lake map-area. Some stocks are d e f i n i t e l y younger than Lower Cretaceous as they cut st r a t a of t h i s age on Swing Peak and Laventie Mountain. 13 CHAPTER I I I  GEOLOGY OF THE TROITSA LAKE PROPERTY Introduction. The T r o i t s a Lake property i s underlain by volcanic and sedimentary s t r a t a of the Hazelton Group of probably Lower Jur-assi c age (fig.35 i n pocket). These have been intruded by a roughly c i r c u l a r granodiorite - quartz monzonite stock approxi-mately three square miles i n area. A younger r h y o l i t e complex occurs to the northwest of the stock. The stock i s cut by a northwesterly trending series of dykes of variable composition. Hazelton Group. Strata of the Hazelton Group form the country rocks into which the stock i s intruded. These are exposed over a true thickness of approximately 3*500 feet but the base of the group i s not exposed. The stratigraphy i s summarised i n Figure 3-On the Tr o i t s a Lake property s t r a t a of the Hazelton Group probably belong to the lowermost part of the Hazelton succession. D u f f e l l (1959) suggested that rocks of the Hazelton Group at the west end of the Whitesail Range and around T r o i t s a Lake are of Lower Jurassic age. The nearest known f o s s i l l o c a l i t y i s at the 6,500 feet l e v e l on the northwest ridge of Troitsa Peak, twelve miles to the east, and y i e l d s f o s s i l s of Lower Bajocian (early Middle Jurassic) age (Frebold, 1951). At T r o i t s a Lake the strata appear to occupy a lower po s i t i o n i n the Hazelton succession and therefore belong to the 'Lower Volcanic D i v i s i o n ' of D u f f e l l (1959). 14 HAZELTON GROUP (information from Duffell, 1939 and Tipper, 1971 ) TROITSA LAKE PROPERTY 7> a. < ui jS z z — UI 2 Q UJ <f> 5,000-UNIT RED VOLCANIC UNIT LOWER GREEN VOLCANIC UNIT / / Volcanic flows tuffs and breccias. / / / Marine sediments with interbedded tuffs 8 flows. Minor greywacke, argillite, tuff and limestone-/ Tuffs and breccias. Minor sediments with limestone. / Breccias, tuffs, ignimbrites and flows with minor limestone. Epidotised basic volcanic breccias, tuffsS minor flows. A I A .* > r Purple andesite flow breccia. Massive purple andesite flows and breccia units. Massive purple andesite and tuffaceous units. Argillite lenses. Massive purple and green andesite. Epidotised basic green volcanics. GENERALISED STRATIGRAPHIC COLUMN. FIGURE 3. W i t h i n t h e T r o i t s a Lake p r o p e r t y t h e s t r a t i g r a p h i c s u c c e s s i o n c o n s i s t s o f a t l e a s t 500 f e e t o f f ine t o medium-g r a i n e d g r e e n e p i d o t i s e d b a s i c v o l c a n i c s , w h i c h a re o v e r -l a i n by a l m o s t 3 , 0 0 0 f e e t o f m a s s i v e g r e e n and p u r p l e v o l -c a n i c f l o w s w i t h some a r g i l l i t e l e n s e s and m i n o r i n t e r c a l -a t e d t u f f a c e o u s members. I n t h e S m i t h e r s m a p - a r e a , T i p p e r (1971) f u r t h e r s u b -d i v i d e d the ' L o w e r V o l c a n i c D i v i s i o n ' i n t o t h r e e l i t h o l o g i c u n i t s , t h e 'Red T u f f u n i t ( u p p e r m o s t ) ; t h e 'Red V o l c a n i c ' u n i t ; and t h e ' L o w e r G r e e n V o l c a n i c ' u n i t . The ' L o w e r G r e e n V o l c a n i c ' u n i t he d e s c r i b e d a s : -"The u n i t c o n s i s t s o f h i g h l y e p i d o t i s e d b a s i c g r e e n v o l c a n i c b r e c c i a s , s t r a t i f i e d t u f f and m i n o r f l o w s . The t h i c k n e s s o f t h e u n i t i s unknown. I t has y i e l d e d no f o s s i l s b u t as i t u n d e r l i e s t h e 'Red V o l c a n i c ' u n i t i t may be E a r l y J u r a s s i c o r L a t e T r i a s s i c . " He d e s c r i b e d the 'Red V o l c a n i c ' u n i t a s : -"The u n i t i s made up o f b r e c c i a s , t u f f s , i g n i m b r i t e s , m a s s i v e v o l c a n i c f l o w s i n shades o f p u r p l e , r e d and g r e e n , b u t has more g r e e n v o l c a n i c s n e a r t h e base and r e d d i s h v e s i c u l a r l a v a s n e a r the t o p . The t h i c k n e s s o f t h e u n i t i s i n e x c e s s o f 3 , 0 0 0 f e e t . " Based o n t h e s e d e s c r i p t i o n s t h e b a s i c g r e e n v o l c a n i c r o c k s c o n t a i n i n g e p i d o t e w h i c h f o r m t h e l o w e r m o s t 5 0 0 f e e t o f t h e T r o i t s a Lake s e c t i o n may b e l o n g t o t h e ' L o w e r G r e e n V o l c a n i c ' u n i t . The r e m a i n d e r o f t h e s e c t i o n may be c o r r e -l a t e d w i t h t h e 'Red V o l c a n i c ' u n i t . R o c k s o f t h e 'Red T u f f u n i t (Maroon s t r a t i f i e d t u f f s and f i n e b r e c c i a s ) a r e n o t r e -c o g n i s e d on t h e T r o i t s a Lake p r o p e r t y . P a l a e o n t o l o g i c e v i d -ence f r o m t h e S m i t h e r s a r e a i n d i c a t e s a l a t e E a r l y J u r a s s i c t o e a r l y M i d d l e J u r a s s i c age f o r t h e ' R e d V o l c a n i c ' u n i t . T h i s i m p l i e s an E a r l y J u r a s s i c o r L a t e T r i a s s i c age f o r t h e 16 'Lower Green Volcanic' unit . Thus on the T r o i t s a Lake pro-perty s t r a t a of the Hazelton Group are probably of Early Jurassic age. The stratigraphy of the Hazelton Group on the property and i t s c o r r e l a t i o n to the regional s t r a t i g r a p h i c succession i s presented i n Figure 3. 'Lower Green Volcanic Unit'. These rocks are exposed f o r a true thickness of approx-imately 500 feet. They occur at the extreme northwest corner of Tro i t s a Lake and are not exposed elsewhere on the property. They are dark grey-green fine-grained flows i n which epidote i s commonly seen to be developed. Because of t h i s l i t h o l o g y they are thought to correlate with the 'Lower Green Volcanic Unit' described by Tipper (1971) i n the Smithers area. Plagioclase phenocrysts up to 1.5mm. long show well devel-oped a l b i t e and l o c a l l y p e r i c l i n e twinning. It i s unzoned, approximates A n ^ a n d composes 10 to 15 per cent of the rock. Epidote and c h l o r i t e completely replace the o r i g i n a l mafic minerals and usually forms clofcs up to 2mm. i n diameter. Chlor-i t e also occurs as ragged patches i n the groundmass, and as narrow laths i n d i c a t i n g that i t may have replaced o r i g i n a l b i o t i t e . Within some patches of c h l o r i t e are fine idiomorphic flakes of b i o t i t e . The epidote and c h l o r i t e account f o r 10 to 15 per cent of the rock. The remainder of the rock consists of a fine-grained t r a c h y t i c groundmass of plagioclase with some c h l o r i t e . The small amount of quartz present i s intimately associated with the epidote. The rock appears to be of ande-s i t i c composition. 17 The replacement of the o r i g i n a l mafic minerals, apparent-l y b i o t i t e and hornblende, by c h l o r i t e and epidote, and the development of a second generation of b i o t i t e from the c h l o r i t e indicates metamorphism of greenschist f a c i e s grade. The meta-morphism i s l i m i t e d i n extent and i s not found i n other s t r a t a of the Hazelton Group on the property. 'Red Volcanic Unit'. This unit, almost 3,000 feet i n thickness, consists of massive volcanic flows with an increasing number of flow brec-c i a members i n the upper 1,000 feet of the succession and i n -terbedded t u f f s and a r g i l l i t e lenses. I t i s thought to belong to the ^Red Volcanic Unit' described by Tipper (1971) i n the Smithers area as i t o v e r l i e s epidotised green volcanics and because of i t ' s l i t h o l o g i c a l s i m i l a r i t y . Massive volcanic flows dominate the lower two t h i r d s of t h i s u n i t . They are uniform fine-grained rocks, usually deep purple, and i n the lower part of the unit, interbedded with green flows. They are andesitic or d a c i t i c i n composition. Individual flows are r a r e l y distinguishable. Lenses of a r g i l l i t e are common about 1,000 feet above the base of the Red Volcanic u n i t . The best exposed of these outcrops ait an elevation of 5,000 feet at the base of c l i f f s overlooking Blanket Lakes to the west of the property. Only the uppermost part of t h i s lense i s exposed and has a t h i c k -ness of 100 feet and a s t r i k e length exposure of 1,000 fe e t . The a r g i l l i t e commonly contains t h i n (less than four inches) tuffaceous layers (see Plates 1 and 2). 18 P l a t e 2 : S t r a t a o f t h e H a z e l t o n Group s h o w i n g i n t r i c a t e i n t e r b e d d i n g o f t u f f and a r g i l l i t e . S t r a t i g r a p h i c a l l y above the a r g i l l i t e lenses massive volcanic flows dominate with t u f f beds 2 to 3 feet thick. The t u f f s are deep purple i n colour and consist of angular quartz, potash feldspar and plagioclase c r y s t a l fragments l e s s than 0.1mm. and l a p i l l i up to 1cm. i n diameter i n a fine-grained groundmass. In the uppermost 1,000 feet of section i s an increase i n the number of flow breccias among the massive flows. The breccias are purple and consist of angular andesitic and rare r h y o l i t i c fragments i n a fine-grained andesitic matrix. A t y p i c a l andesite flow breccia i s shown i n Plate 3. The Stock. The stock i s roughly c i r c u l a r i n plan with a tongue-l i k e extension to the north, and i s approximately three square miles i n area. I t underlies the main valley area i n the cen-t r a l part of the property, but i s not well exposed there as much of the valley f l o o r i s covered with moraine derived from several small alpine g l a c i e r s . I t i s exposed best along the ridge to the north of the main valley and on the slopes lead-ing up to the ridges to the south and southwest of the main va l l e y , although parts of these are inaccessible. The stock i s found to be t e x t u r a l l y zoned being coarse-grained i n the centre and r e l a t i v e l y fine-grained at the margin. A study of hand specimens shows that the stock i s composition-a l l y zoned. Slabs were cut of the hand specimens, etched with hydroflouric acid and stained with sodium c o b a l t i n i t r i t e s o l -ution to f a c i l i t a t e i d e n t i f i c a t i o n of the feldspar according to the method of Rosenblum (1956). The central part of the stock P l a t e 3: T y p i c a l a n d e s i t e f l o w b r e c c i a o f t h e H a z e l t o n Group - Red V o l c a n i c U n i t . I P l a t e 4: ' C o a r s e - g r a i n e d ' q u a r t z m o n z o n i t e o f t h e c o r e zone o f t h e s t o c k . Note t h e g r e e n i s h c o l o u r due t o p r o p y l i t i c t y p e a l t e r a t i o n . i s coarsest grained (see Plate 4 ) . I t consists of euhedral plagioclase laths forming 4 0 to 5 0 per cent of the rock, a l k a l i feldspar forms a further 2 0 per cent, quartz 2 0 to 3 0 per cent, and mafic minerals about 1 0 per cent. Most of the remainder of the stock i s r e l a t i v e l y medium-grained ( 2 to 3 mm.)(see Plate 5 ) . This appears to have a s l i g h t l y higher plagioclase and mafic mineral content. Near the margin of the stock the rock becomes r e l a t i v e l y fine-grained ( 2 m m . maximum grain size) and i s l i g h t grey i n colour unlike the pinkish colour of the rest of the stock, (see Plate 6 ) . In t h i s rock only about one quarter of the t o t a l feldspar i s a l -k a l i feldspar, the quartz content i s lower ( 1 0 to 1 5 per cent), and the mafic mineral content i s higher ( 1 5 per cent or more) than i n the rest of the stock. This r e l a t i v e l y fine-grained rock i s generally r e s t r i c t e d to within a few hundred feet of the margin of the stock and i s best developed at the northern contacts of the stock around the tongue-like extension. This f i n e r grained variety i s also found i n a small area i n the cen-tre of the stock apparently remote from any contact (specimen 2 4 1 ) and may represent a p a r t i a l l y assimilated x e n o l i t h i c block . In general i t i s seen that the stock i s t e x t u r a l l y and to some extent eompositionally zoned from r e l a t i v e l y coarse-grained quartz monzonite - granodiorite through a l e s s coarse-grained variety to a r e l a t i v e l y fine-grained granddiorite out-wards from the centre of the stock. The v a r i e t i e s are gradat-i o n a l and i n t e r n a l contacts are absent. F i e l d r e l a t i o n s i n d i -cate the stock i s the r e s u l t of one i n t r u s i o n of quartz monzon-22 I P l a t e 5: ' M e d i u m - g r a i n e d ' q u a r t z m o n z o n i t e - g r a n o d i o r i t e o f t h e i n t e r m e d i a t e zone o f t h e s t o c k . P l a t e 6: 1 F i n e - g r a i n e d ' g r a n o d i o r i t e f rom t h e m a r g i n a l zone o f t h e s t o c k . . 2 ? i t e - granodiorite magma which has undergone t e x t u r a l and chemical d i f f e r e n t i a t i o n , probably due to more rapid cooling and c r y s t a l l i s a t i o n at the margins of the stock. The stock contacts andesites of the Hazelton Group ex-cept to the northwest where i t i s i n contact with a younger rh y o l i t e complex. The contacts are generally v e r t i c a l . The stock i s massive throughout with no evidence of any magmatic f o l i a t i o n . Sparse x e n o l i t h i c blocks of Hazelton Group rocks are found i n the stock; these are generally r e s t r i c t e d to with-i n a few feet of the contact, but rare large x e n o l i t h i c blocks up to 5 0 feet by 2 0 feet i n size may be found as f a r as 1 , 5 0 0 feet from the contact. The x e n o l i t h i c blocks have i r r e g u l a r shape and are invaded by tongues and veins of quartz monzonite. A well defined band of g r a n o d i o r i t i c hybrid rock, up to a foot i n width, surrounds the xenoliths. This may indicate some de-gree of ass i m i l a t i o n of the xenoliths. The xe n o l i t h i c blocks have been thermally metamorphosed and are now fine-grained d i o r i t e s . The composition of the x e n o l i t h i c blocks i s more basic than that of the enclosing rock. Bowen ( 1 9 2 8 ) stated that i n such a s i t u a t i o n :-"....saturated g r a n i t i c magma can not dissolve inclusions of more basic rocks. The magma w i l l , however, react with the i n -clusions and e f f e c t changes i n them which w i l l give them a mineral c o n s t i t u t i o n s i m i l a r to that of the granite." In t h i s case the magma i s of quartz monzonite - granodiorite composition but the p r i n c i p l e i s the same. The layer of hybrid rock presumably represents the action of the magma to convert the xenolith to material of mineralogy i n equilibrium with i t -s e l f . This layer of hybrid rock apparently e f f e c t i v e l y 'armour-24 ed 1 the xenolith from further a s s i m i l a t i o n by the magma. The presence of large x e n o l i t h i c blocks i s evidence that the magma may have intruded partly by stopeing. Small a p l i t e veins and dykes, up to a foot wide as-sociated with the stock and wall rocks, may represent the f i n a l residual l i q u i d a f t e r c r y s t a l l i s a t i o n of the major part of the magma. They have no preferred orie n t a t i o n and predate j o i n t i n g of the stock. Petrography. Twenty t h i n sections from a l l parts of the stock, i n c l u d -ing a specimen of an a p l i t e dyke within the stock, were examin-ed. Modal analyses of these t h i n sections involved counting 1000 points over an area of 350 sq. mm., the size of a standard t h i n section. According to the r e l i a b i l i t y chart of Van Der + Plas and Tobi (1965) t h i s should give an accuracy of - 3 per cent at the 95 per cent confidence level f o r the major mineral constituents of the rock. Chayes (1956) has argued that there i s no simple r e l a t i o n between the grain size of a rock and the point distance chosen f o r i t s modal analysis. Van Der Plas and Tobi have stated that to avoid correlated observations be-tween successive points a point distance which i s larger than the largest grain size should be used. The coarse grain size of some of the rocks analysed means that some c r y s t a l s , usually plagioclase laths, are longer than the point distance used even when t h i s i s one millimeter. This may introduce some degree of bias into the r e s u l t s . Some specimens were analysed twice to assess the r e p r o d u c i b i l i t y of the r e s u l t s , and i n a l l rocks these were found to be within the l i m i t s suggested above. The 25 r e s u l t s of the modal analyses are presented i n Table 3 and the locations of specimens are shown on the geological map. Microscopic study confirms that plagioclase, orthoclase and quartz are the major f e l s i c minerals and that hornblende and b i o t i t e are the major mafic minerals. Magnetite i s a ub-iquitous accessory mineral, and sphene and apatite are present l o c a l l y . Chlorite i s a f a i r l y common a l t e r a t i o n product. The average mineralogy of the stock i s presented i n Table 4. This i s the weighted average of 19 modal analyses, and was calculated from the average composition of specimens from the fine-grained marginal zone of the stock (9 per cent of the t o t a l area), the medium-grained major part of the stock (87 per cent of the t o t a l area), and the r e l a t i v e l y coarse-grained central part of the stock (4 per cent of the t o t a l area). This average should ap-proximate the bulk mineralogy of the stock and l i e s within the granodiorite f i e l d as defined by Peterson (i960). Plagioclase i s the major mineral constituent of the rock, forming 45 to 65 per cent of i t . The plagioclase occurs as large euhedral laths which have well developed twinning on the a l b i t e and carlsbad-albite twin laws. P e r i c l i n e twinning i s rare. The maximum anorthite content i s i n the cores of the plagioclase c r y s t a l s and decreases towards the margin. The magnitude of t h i s normal zoning i s variable but i n the larger plagioclase c r y s t a l s i t may be up to 5 per cent anorthite. Os-c i l l a t o r y zoning i s common espe c i a l l y i n the larger plagioclase c r y s t a l s from the central part of the stock (see Plate 7\ The zones represent compositional changes of 1 to 2 per cent an-o r t h i t e . In the f i n e r grained granodiorite the plagioclase has TABLE:III MODAL ANALYSES (VOLUME PERCENT )v CONTACT . ZONE INTERMEDIATE ZONE SAMPLE # -2 01 86 241 144X 212 211 144R 185 225 144W 187 C2 144T PLAGIO-CLASE 63. 2 52.9 5 7 . 9 4 6 . 6 62. 6 4 6 . 4 48.O 49^1 45.3 4 8 . 2 4 4 - 4 43.0 57-1 ORTHO-CLASE 5-7 14 .7 14-9 8.2 13 .4 22.1 18.2 17... 8 2 0. 9 18. 8 2 4 . 4 2 4 . 7 .15'. 5 QUARTZ 12. 6 11. 9 12 .4 22.2 17. 2 19. 8 20. 0 20. 8 20. 6 2 3 . 8 22. 6 20. 0 19 .4 HORN-BLENDE 12. 0 12.1 8 .6 1 .3 5 . 6 8.0 7,3 9-3 0 .6 8 . 7 3-0 BIOTITE 3 . 4 5.1 4 - 5 1 9 . 4 3 . 3 3 . 8 4 . 2 3 .2 1. 8 5.0 1 . 5 3-9 MAGNE-TITE 2 . 9 2 . 3 1 .7 1.6 1 .6 1 .7 1 .6 1 . 6 2.0 2.2 1 . 7 2.0 0. 9 CHLOR-ITE 0. 3 2.0 0. 4 0.1 1.4 4. 8 APATITE 0.2 0. 3 0.3 0.1 0.2 0.1 0. 2 SPHENE 0. 2 0.1 0.1 CALCITE 0. 2 0. 4 HEMATITE 0. 2 PYRITE 0. 2 EPIDOTE 1 . 5 TABLE III CONTINUED"."' CORE ZONE SAMPLE # 1 9 5 1 4 6 1 8 3 2 3 2 1 4 5 1 8 1 144V PLAGIO-CLASE 4 2 . 7 4 8 . 3 4 6 . 4 5 3 . 9 4 9 - 5 4 4 . 7 2 7 . 0 ORTHO-CLASE 2 3 . 7 19.1 1 9 . 8 1 7 . 8 1 6 . 5 1 8 . 3 3 4 . 6 QUARTZ 2 2 . 7 2 5 . 3 2 3 . 7 2 0.1 2 5 - 4 2 6 . 7 3 6 . 9 HORN-BLENDE 4 . 2 5 . 9 5.8 1 . 9 6 . 0 BIOTITE 5 . 4 2 . 9 1 . 0 4 . 3 2 . 8 0 . 9 MAGNE-TITE 1 . 2 1.9 1 . 3 0 . 8 1 . 6 1 . 2 0 . 6 CHLOR-ITE 5 . 4 APATITE 0.1 0 . 2 0 . 2 SPHENE 0 . 4 0 . 6 0 . 3 CALCITE HEMATITE PYRITE EPIDOTE TABLE IV AVERAGE MINERALOGY (VOLUME PERCENT) AND ESTIMATED CHEMICAL COMPOSITION (WEIGHT PERCENT) OF THE TROITSA .LAKE STOCK^ R e l i a b i l i t y at the 95 percent confidence Mineralogy. Volume Percent. l e v e l . (Van Der Plas and Tobi, 1965) Plagioclase 48.9^ ± 3.2% Orthoclase 1 8 . 5% + 2,5% Quartz 21,2% ± 2,6% Hornblende 5.0% ± 1.5% B i o t i t e 3.6% ± 1.1% Magnetite 1.8% ± <l%' Accessory Minerals 1.0% ± Estimated Chemical Composition. Weight Percent. s i o 2 65,2$ A1 20 3 16. 7% F e 2 0 3 2,7% FeO 2.7% MgO 1.1% CaO 4,1% Na £0 4-. 2% K 2 ° 2,7% Minor Constituents 0. 6% no more than three or four zones, i n the medium-grained var-i e t y s i x or seven zones, and i n the coarse-grained variety some of the l a r g e r c r y s t a l s have ten or more zones. There i s a systematic v a r i a t i o n i n the composition of the plagioclase from An^Q i n the marginal part of the stock, through An,^ i n the medium-grained variety to as low as An 2g i n the central part of the stock (see Table 5 )• The compositional v a r i a t i o n of the plagioclase and the cause of the normal and o s c i l l a t o r y zoning i s discussed i n more d e t a i l i n the section on the f e l d -spars . The dominantly euhedral form of the plagioclase i s modi-f i e d by resorption. This i s the most noticeable where a l k a l i feldspar i s i n contact with the plagioclase. The a l k a l i f e l d -spar usually f i l l s embayments i n the plagioclase, and l o c a l l y myrmekite develops at contacts with quartz. Normally a very t h i n rim of a l b i t e occurs around the plagioclase.anThese fea-tures are i l l u s t r a t e d i n Plates 7 and 8. The reasons f o r re-sorption of plagioclase are discussed i n the section on the feldspars. Except near the margins, orthoclase forms 10 to 25 per cent of the rock. Orthoclase forms small subhedral grains and larger anhedral grains which tend to o p h i t i c a l l y enclose plag-ioclase laths and to p o i k i l i t i c a l l y enclose mafic minerals. It i s l o c a l l y m i c r o p e r t h i t i c . The compositional v a r i a t i o n of the a l k a l i feldspar and s t r u c t u r a l state are discussed i n d e t a i l i n the section on the feldspars. Quartz forms 10 to 25 per cent of the rock and i s most abundant i n the coarser grained rocks from the centre of the stock. It i s i n t e r s t i t i a l to a l l other minerals i n the rock. I Plate 7 : Photomicrograph ( x 3 0 ) . i » Quartz monzonite. Plagioclase showing resorbed and a l b i t i s e d margins, and repeated o s c i l l a t o r y zoning. I Plate 8: Photomicrograph ( x 3 0 ) . imm Quartz monzonite. Plagioclase with a l b i t i s e d and resorbed margins. Note the i n t e r s t i t i a l nature of the orthoclase. 31 The major mafic minerals are b i o t i t e and hornblende. These generally form about 10 per cent of the rock but s p e c i -mens near the contact of the stock may have as much as 20 per cent mafic minerals. The coarse-grained central part of the stock tends to be more le u c o c r a t i c and the mafic mineral con-tent f a l l s to about 6 per cent. The r e l a t i v e proportions of hornblende and b i o t i t e vary e r r a t i c a l l y throughout the stock. Both minerals are idiomorphic, and b i o t i t e commonly occurs i n clumps surrounding the hornblende. The above textural relationships suggest that the f i r s t of the f e l s i c minerals to c r y s t a l l i s e was d e f i n i t e l y plagio-clase which was followed a f t e r an i n t e r v a l by orthoclase and quartz which began c r y s t a l l i s i n g at approximately the same time. Both the major mafic minerals appear to have c r y s t a l l i s e d early i n the sequence, at the same time as, or shortly a f t e r , the beginning of plagioclase c r y s t a l l i s a t i o n . Hornblende appears to have begun to c r y s t a l l i s e e a r l i e r than the b i o t i t e . The c r y s t a l l i s a t i o n sequence i s summarised i n Figure 4a. This c r y s t a l l i s a t i o n sequence agrees f a i r l y well with that implied by experiments on the melting relationships of granodiorites from the Wallowa Batholith, Oregon, (Piwinskii and Wyllie 1968) and from the S i e r r a Nevada Batholith, C a l i f o r n i a , (Piwinskii, 1968). These experiments indicate that at pressures of the order of one k i l o b a r plagioclase i s the f i r s t mineral to cry-s t a l l i s e followed shortly by hornblende and then b i o t i t e . Or-thoclase followed rapidly by quartz only begins to c r y s t a l l i s e at the very end of the sequence. A pressure/temperature pro-j e c t i o n of the phase boundaries of a granodiorite from the Wal-32 a) HORNBLENDE PLAGIOCLASE BIOTITE QUARTZ ORTHOCLASE LATE EARLY RELATIVE TIME OF CRYSTALLISATION b) LU cr CO CO UI rr CL 0 I 600 700 800 TEMPERATURE \ 2\ / fe V 900 1000 F I G U R E a) APPROXIMATE RELATIVE TIME OF CRYSTALLISATION OF THE MAJOR MINERALS OF ROCKS OF THE TROITSA LAKE STOCK AS INDICATED BY TEXTURAL RELATIONSHIPS. b) PT PROJECTION OF THE PHASE BOUNDARIES FOR GRANODIORITE FROM THE WALLOWA BATHOLITH, OREGON, DETERMINED FOR BEGINNING OF MELTING AND FOR THE DISAPPEARANCE OF THE MAJOR MINERAL PHASES (PIWINSKII AND WYLLIE, 1968). 33 QUARTZ QUARTZ - \ GRANO- \QUARTZ MONZONITE \DIORITE ^/ORt^E • 144V MONZONITE O R T H O C L A S E PLAGIOCLASE SAMPLE FROM AN APLITE DYKE F IGURE 5. PLOT OF MODAL QUARTZ - ORTHOCLASE - PLAGIOCLASE OF THE TROITSA LAKE S T O C K . 20 Specimens used. (After Peterson, I960) 34 Iowa batholith i s presented, f o r comparison i n Figure 4b. Variations i n the stock. The modal analyses of 19 specimens of the stock and of one specimen of an a p l i t e dyke (specimen 144V), which probably represents the f i n a l stage l i q u i d l e f t a f t e r cry-s t a l l i s a t i o n of the bulk of the magma, are plotted on an orthoclase - plagioclase - quartz ternary diagram (Figure 5). I t can be seen from t h i s that the analyses are not c l e a r l y grouped, but form a b e l t with compositions ranging from with-i n the quartz monzonite f i e l d well into granodiorite f i e l d . I t appears from t h i s diagram that the stock may have undergone d i f f e r e n t i a t i o n . In Table 3 the modal analyses are arranged i n order of increasing s i l i c a content (t h i s was calculated from the modal analyses) from l e f t to r i g h t . The modal analyses show changes i n mineralogy related to the increasing a c i d i t y towards the centre of the stock. The modal percentages of the major min-e r a l phases have been plotted on a series of diagrams of the stock (Figures 6, 7, 8 and 9) to show t h e i r d i s t r i b u t i o n . A s i m i l a r diagram (Figure 10) shows the v a r i a t i o n of the plag-ioclase / plagioclase + orthoclase r a t i o . These diagrams show the increase i n quartz and orthoclase content and the decrease i n plagioclase and mafic mineral content inwards from the mar-gin of the stock. The plagioclase / plagioclase + orthoclase r a t i o shows the trend from g r a n o d i o r i t i c towards monzonitic compositions inwards from the margin of the stock. Figure 11 i s a Larsen d i f f e r e n t i a t i o n diagram f o r the 35 Figures 6 , 7, 8 , and 9 . These diagrams show the modal d i s t r i b u t i o n of the major mineral phases (quartz, t o t a l mafic minerals, plagioclase, and orthoclase) in the stock. These show the r e l a t i v e l y high content of plagioclase and mafic minerals i n the marginal parts of the stock, and of quartz and orthoclase i n the central area. There are exceptions to the general pattern such as specimen 144V which i s from an a p l i t e dyke and specimen 2 4 1 A which i s from what appears to be a large partly assimilated x e n o l i t h i c block. A more regular d i s t r i -bution of specimens was not possible becu/a^se of glacier and moraine cover i n the area. Figure 10. This shows the d i s t r i b u t i o n of the modal feldspar contents of the stock as the r a t i o plagioclase/plagioclase + orthoclase. This shows the higher plagioclase content r e l a t i v e to orthoclase i n the marginal part of the stock. The data used i n t h i s diagram i s given below: Modal % Modal % Orthoclase Total % Plagioclase/ Specimen § Plagioclase Feldspar Plagioclase + Content Orthoclase 2 0 1 86 2 4 1 144X 212 2 1 1 144R 1 8 5 225 144W 187 C2 144T 1 9 5 146 1 8 3 232 145 1 8 1 144V 63 53 58 47 63 46 48 49 45 48 44 43 57 42 48 47 54 50 4 5 27 6 15 15 8 13 22 18 18 21 19 24 25 16 25 19 2 0 18 17 18 35 69 68 73 55 76 68 66 67 66 67 68 68 73 67 67 67 72 67 63 70 . 9 1 . 7 8 . 7 9 . 8 5 . 83 . 68 • 73 . 7 3 . 6 8 . 7 2 . 6 5 . 6 3 . 7 8 . 6 3 . 7 2 . 7 0 . 7 5 . 7 5 . 7 1 . 3 9 56 8 6 0 1 ? % 2 0 l 0 l 3 % I44T 19% 144V 37% l 4 < "< 2 2 % O • | o » O „ I44W 2 4 % \ I 4 4 R 2 0 % \ \ o \ 195 2 3 % \ 146^25% 1 4 5 * 2 5 % 1 8 1 ^ 2 7 % 1 8 3 * 2 4 % \ \ 1 M 0 R A 1 N E 24IA 13% ', O 185021% l • 1 8 7 0 2 3 % 2 3 2 0 2 0 % , ' I G L A C I E R M O R A I N E V « \ \ M I N I \ 2 | 2 O l . 7 % G L A C I E R 225 0 2 1 % Legend \ GLACIER \ I I \ \ \ \ \ \ . Outline of the Stock - - - - Outline of Glacier ft Moraine 2 2 5 0 Sample location and number 0 2 1 % Modal percent of Quartz S C A L E I inch represents 2000 feet I \ ; \ ' ~ C 2 O 2 0 % \ 1000 feet 2000 o o o v _\ 0 " 1 8 % 19 - 23 % > 23 % D I S T R I B U T I O N O F Q U A R T Z IN T H E S T O C K F I G U R E 6. . - - 1 I ' 1 •. 1 I I . ' P i I I 8 6 * 2 0 % \ I •<• * * \ \ o 0 \ I 4 4 R 1 4 % 2 0 1 * 1 8 % ; i 9 5 0 i i ° / < i I44T 8 % 1 4 4 V 2 % | 4 4 X 2 1 % O O *\ 14 4 W 8 % ' » \ » ^ \ I \ I I I I I . | 4 _ 6 Q 2 % 1 4 5 0 8 % I 8 I O I O O / ° 18 3 0 1 0 % / M O R A I N E 2 4 1 / ^ 1 5 % I 8 5 ? I 2 % / 18 7 0 2 % I' '1 I | G L A C I E R * > » \ v 1 212 O 6 % N Legend . / 211 O II % - - - K 2 3 2 0 8 % , ' i G L A C I E R 2 2 5 0 13 % » I \ / l / , \ \ S \ \ "> » v » v \ \ G L A C I E R N / V 2 3 2 O Oe% S C A L E O u t l i n e of me S t o c k ^ Out l ine of G l a c i e r 8 M o r a i n e S a m p l e l o c a t i o n and n u m b e r M o d a l p e r c e n t of M a f i c M i n e r a l s I inch represents 2 0 0 0 f e e t 1000 2000 feat • - - > C 2 0 1 2 % o o o o - io % II - I 4 % > 14 % DISTRIBUTION OF MAFIC MINERALS IN THE STOCK FIGURE 7. ,44 # 57% | 4 4 ^ 2 7 0 / |44X o 47°/c t 201 •S3"/o o w W o I44R 48% I44W 48% , 19 5 042% 1 , / / 1 ) G L A C I E R 146 048% 181045% "14 3 950% l 8 , 047% M O R A I N E I \ 24IA^58% I 8 5049% ' I 8 7 w 4 4 % 23 2 • 54% M O R A I N E I I O 46 % \ \ 2 12 *v63% \ 22 5 0 4 5 0 , Legend 146 O 048% SCALE Outline of StocK 1 * Outline of Glacier S Moraine Sample location and number Modal percent of Plagioclase I inch represents 2000 feet 1 0 0 0 2 0 0 0 feet C 2 n 4 3 % \ \ ^ 1 \ ( * G L A C I E R \ V 0 o o 0 - 45 % 46 - 50 % =» 5 0% DISTRIBUTION OF PLAGIOCLASE IN THE STOCK. FIGURE 8 . I44TI6% | 4 4 v 3 5 % |44X.8% 3 ° • O ° N I44W 19% \ 2 0106% OI44R 18% I95«2 4% I \ V \ J4_6_Q.I9% !8IO l 8 % " 145917% ,8 30 2 0% M O R A I N E I 4 / I ' | G L A C I E R I | \ i \ \ 2^41A 15% l85'Pl8% D2*QJ3% 211 #22 % 187 #24% 232018% ' " i 1 I i 22 5 #21% G L A C I E R I j J I I GLACIER I 1 V 1 V I \ I Legend 2110 022% SCALE \ G L A C I E R ^ Outline ot Stock Outline of Glacier S Moraine Sample location and number Model percent of Orthoclase I inch represents 2000 feet 1000 2000 feet •%C2a25%\ o o 0 - 15 % 16 - 20 % > 20 % DISTRIBUTION OF O R T H O C L A S E IN T H E S T O C K F I G U R E 9. I V ' 8 6 « -78 2 0 1 * -91 I / o • I44R -73 1 9 5 0 - 6 3 \ I 4 4 T - 7 8 | 4 4 V . 3 9 I44X - 8 5 O © « N I44W -72 1 1 \ I \ \ 1 \ \ I / I \ I I 4 6 0 j 7 2 I 4 5 0 . 7 5 l 8 3 0 . 7 I 8 I Q - 7 I MORAINE 2 4 I A . 7 9 1 • I 8 5 0 . 7 3 1 8 7 0 - 6 5 2 3 2 0 - 7 5 , 2 1 1 0 - 6 8 I \ 2 I 2 « « / 1 • / / M ORAINE 1 1 \ 1 > / GLACIER v , 2 2 5 0 - 6 8 1 \ \ \ \ 1 V " ' I ' I ',1 1 , \ GLACIER 1 \ V 1 V \ \ \ V \1 S Legend I GLACIER . ' I - — — — O u t l i n e of t h e S t o c k - — — — O u t l i n e of G l a c i e r 8 M o r a i n e 2 0 1 O S a m p l e l o c a t i o n and n u m b e r * C 2 Q - 6 3 \ O -91 S C A L E R a t i o of M o d a l percent of P l a g i o c l a s e / P l a g i o c l a s e *- A ; Orthoclase I i n c h r e p r e s e n t s 2 0 0 0 feet feet v ' 1 • I 1 • -s -0 < 7 2 0 • 7 2 - • 7 7 • > • 7 7 VARIATION OF T H E P L A G I O C L A S E / PLAGIOCLASE-ORTHOCLASE RATIO IN T H E S T O C K F I G U R E 10. 41 major mineral constituents of the stock. The modal per-centages of the minerals are plotted against the Larsen Index ( l / 3 S i 0 2 + K 2 0 - MgO - FeO - 0.9Fe 2 0 3 - CaO) derived from the calculated chemical composition of the rocks (Larsen, 193 8). The Larsen Index i s taken to represent the degree of d i f f e r -e n t i a t i o n of a rock. The trend of decreasing plagioclase and mafic mineral and increasing quartz and orthoclase contents with d i f f e r e n t i a t i o n of the stock i s shown. Rhyolite. An i r r e g u l a r r h y o l i t e complex approximately half a square mile i n area outcrops to the northwest of the stock. Euhedral pyrite c r y s t a l s up to 0 .5 nun. i n size are i r r e g u l a r l y dissem-inated i n the r h y o l i t e . Flow banding i s l o c a l l y developed. The r h y o l i t e appears to form a thick lensoid mass bapering rapid l y to s i l l - l i k e extremities. (Plate 9)« The r h y o l i t e i s younger than the stock having sharp intrusive contacts with i t , but shows no c h i l l e d margin. A few small fragments of the quartz monzonite - granodiorite are caught up i n the rhy o l i t e near the contact. In t h i n section the rock consists of quartz phenocrysts and pyrite c r y s t a l s i n a mesostasis of quartz, (approximately 50 per cent), a l k a l i feldspar, and some a l b i t e . Secondary muscovite i s scattered throughout the rock. Dyke Rocks. A complex serie s of dykes cuts a l l other rock types on the property. Excluding the a p l i t e dykes and veins, the dykes form four groups. In order of probable increasing age these 42 VARIATION IN THE L A K E S T O C K , WITH MINERALOGY OF ROCKS OF T H E TROITSA RESPECT TO THE LARSEN INDEX. Plate 9 : A r h y o l i t e s i l l intruding the 'Red Volcanic Unit' of the Hazelton Group. The photograph was taken i n the western part of the property looking towards the northeast. The contact between the r h y o l i t e complex and the quartz monzonite - granodiorite of the stock can be seen i n the background. I Plate 1 0 : 44 are : -Quartz porphyry r h y o l i t e (youngest) Lamprophyre Andesite Feldspar Porphyry (Quartz L a t i t e ) (oldest) As most of these dykes have a s i m i l a r northwesterly trend there are few crosscutting relationships to indicate t h e i r r e l a t i v e ages. Quartz Porphyry Rhyolite. This i s s p a t i a l l y and presumably g e n e t i c a l l y related to the r h y o l i t e s i l l complex. These dykes are up to 30 feet wide and generally trend about N.10°W. with a v e r t i c a l dip. They cut the aphanitic r h y o l i t e of the s i l l complex, rocks of the Hazelton Group, and extend into the quartz monzonite -granodiorite of the stock against which they have c h i l l e d mar-gins two to three inches wide. The rock i s creamy coloured and consists of an aphanitic matrix containing about 15 per cent quartz phenocrysts up to 1mm. i n s i z e . The phenocrysts show euhedral hexagonal outline, l o c a l l y with corroded margins and reaction rims. The s e r i c i t i s e d groundmass consists of f i n e -grained a l k a l i feldspar, quartz and some a l b i t i c plagioclase. There are no mafic minerals. Lamprophyre. Lamprophyre dykes are not common. The largest i s 10 feet wide and can be traced f o r a distance of 2 ,000 feet i n the southeastern part of the stock. This dyke trends N.28°E. and p a r a l l e l s other lamprophyre dykes. The rock i s sof t , greenish black, spheroidally weathering, and composed of plagioclase, augite (up to 2 5 per cent), and abundant opaques. A th i n section shows plagioclase phenocrysts (An^) up to 2mm. i n length l y i n g i n a fine-grained groundmass of plagioclase microliths. Augite i s scattered throughout the groundmass as phenocrysts up to 1mm. Some hornblende i s also found. The mafic are heavily altered to c a l c i t e and epidote. Needles of r u t i l e , apatite and fine-grained ilmeno-magnetite are abundant i n the groundmass. The rock appears to belong to the camptonite group of lamprophyres. Andesite. These are the most commonest dykes on the property. Most are only a few feet wide but they can be up to 30 feet wide. They trend generally northwesterly and dip steeply to the south west exploiting the l i n e of weakness of the dominant j o i n t d i r -ection i n the quartz monzonite - granodiorite. The andesite i s green and generally fine-grained but some dykes are porphyritic with hornblende phenocrysts up to 2mm. i n length. Others are amygdaloidal with c a l c i t e and minor epidote f i l l i n g the v e s i c l e s . The rock consists of plagioclase of approximately An^^ i n a fine-grained mafic groundmass. The mafic minerals were mainly b i o t i t e but now are altered completely to c h l o r i t e . C a l c i t e and coarse flakes of s e r i c i t e are a l t e r a t i o n products of the plagio clase. Feldspar Porphyry. This broad term has been used for a suite of apparently 46 g e n e t i c a l l y related dykes ranging i n composition from i n t e r -mediate to a c i d i c . The more important members of the group are quartz l a t i t e s . Minor dark grey prophyritic andesite dykes are also included i n t h i s group. The quartz l a t i t e porphyries are of two textural types; an e a r l i e r sparsely p o r p h y r i t i c variety with 5 per cent plag-ioclase phenocrysts up to 8mm. long, and a l a t e r variety with up to 40 per cent plagioclase phenocrysts. Ac i c u l a r horn-blende c r y s t a l s form up to 10 per cent of both v a r i e t i e s of the rock. Rounded inclusions up to four inches i n diameter of the e a r l i e r sparsely porphyrdttic variety are common i n the l a t e r 'crowded prophyry' var i e t y . Locally the inclusions form a major part of the rock i n d i c a t i n g that perhaps a 'crowded porphyry' dyke has intruded along the same zone of weakness as a previous partly consolidated sparsely porphyritic variety r e s u l t i n g i n a composite dyke. These dykes are usually about 20 feet i n width, but may be up to 40 feet wide. They trend northwesterly and are cut by the younger andesite and lamprophyre dykes. They are most common i n the central area of the quartz monzonite - grano-d i o r i t e , but may be found well outside the stock. These dykes do not have a well developed c h i l l e d margin against the quartz manzonite - granodiorite as do the other dykes and were pre-sumably intruded while the stock was s t i l l hot. E s p e c i a l l y i n the central area hydrothermal a l t e r a t i o n and disseminated s u l -phide mineralisation has affected the dykes. These ef f e c t s w i l l be discussed i n d e t a i l i n the section on a l t e r a t i o n and mineralisation. Because of a l t e r a t i o n , i d e n t i f i c a t i o n of the o r i g i n a l 47 mineralogy i s d i f f i c u l t . Plagioclase of indeterminate com-po s i t i o n forms 50 per cent of the rock. Mafic minerals form about a quarter of the rock and appear to have been b i o t i t e and hornblende i n roughly equal proportions. These are up to 1mm. i n size and are now pseudomorphed by c h l o r i t e and epidote. A few rounded phenocrysts of quartz are found. The groundmass i s usually completely altered and the o r i g i n a l min-eralogy i s unknown. Examples of t h i s rock are shown i n Plates 11, 14 and 16. Structure. D u f f e l l (1959) stated that Hazelton Group s t r a t a i n the Whitesail Lake area form a generally f l a t l y i n g blanket warped into open folds with northwesterly trending axes. No major s t r u c t u r a l features are found i n the Hazelton Group rocks of the T r o i t s a Lake property. The s t r a t a usually have f a i r l y shallow dips, e s p e c i a l l y i n the southern part of the property, but the s t r i k e d i r e c t i o n i s variable. The s t r a t a are seen to be l o c a l l y folded (see Plate 10) but t h i s has no o v e r a l l pattern or continuity, and may be due to the influence of the i n t r u s i o n . A f a u l t trending N .25° E. with a near v e r t i c a l dip cuts Hazelton Group s t r a t a i n the western part of the property and passes southwest towards Seel Lake. The f a u l t i s marked on the ground by shearing and f r a c t u r i n g of the rocks, and veining by quartz jasper and magnetite. There i s no evidence to show the degree or d i r e c t i o n of displacement of t h i s f a u l t . Another apparent f a u l t with an easterly trend and approximately v e r t i c a l dip i n the western part of the property can be seen on a i r photographs. The directions of lineaments v i s i b l e .on a i r photographs of the property were measured and are represented by a Rose diagram (Figure 12). The lineaments are seen., on the. ground, to represent small f a u l t s or .shear .zones. The..320. lineaments measured show a dominant. north-northwesterly ..trend. This i s consistent with the regional northwesterly s t r u c t u r a l trend of the Hazelton Group described by. Duff e l l _ (19.59.) • The quartz monzonite,- granodiorite.rocks of the.stock are jointed, with.an average spacing of about..one foot. F i g -ure 13 i s a contoured plot of_67 poles to„joint planes,in the quartz monzonite - granodiorite. This, shows that the. preferred orientation..of the j o i n t s i s N3Q°W. ,dipping__at an angle_of 76° to.the southwest.. I t also shows a subordinate j o i n t set which trends N55^E and has a.._vertical, dip._ _ „ Apart from the few.lamprophyre,dykes which trend.north-easterly most.of the,dykes„on the_property.trend generally northwesterly. The geological map of the property shows a systematic deviation from t h i s trend with dykes forming a crude subradial pattern,.especially within the quartz monzonite -granodiorite stock. The trend of the dykes.ranges from N20°W. to N70°W._ to give the subradial f a n - l i k e pattern which defines an apparent f o c a l centre.for the dykes i n the centre of the stock. Figure 14 summarises the attitudes of..50 dykes.on a contoured equal-area projection of poles. _. This.conf irms the general northwesterly trend .and steep southwesterly dip of the dykes, and indicates..that the. attitude _ of the dykes i s . l a r g e l y c o n t r o l l e d by the.similarly_oriented j o i n t i n g pattern of the stock (Figure 13)• The plot shows two subsidiary maxima i n -49 Figure 12 . 320 measurements of lineaments v i s i b l e on~ a i r photographs. On the ground these are seen to be small f a u l t s and shear zones. The diagram shows the dominant northwesterly trend for these features. FIGURE 12 51 Contoured Plot of Poles to Joint Planes in the Quartz Monzonite - Granodiorite of the Troitsa Lake Stock. FIGURE |3. 52 Contoured Plot of Poles to Attitudes of Dykes IU 4 - 6 % 2 -4 C 2 % % FIGURE 14. d i c a t i n g p r e f e r r e d a t t i t u d e s o f N.56 W. and a d i p o f 81 t o t h e s o u t h w e s t , and o f N.36°W. w i t h a d i p o f 82 degrees t o t h e southwest. T h i s may r e p r e s e n t two s e p a r a t e groups o f dykes, o r a f a n - l i k e arrangement o f the dykes around one main t r e n d . CHAPTER IV FELDSPARS Introduction. The feldspars from the Troit s a Lake stock were i n v e s t i -gated by o p t i c a l and X-ray methods to determine t h e i r compos-i t i o n and s t r u c t u r a l states. This information was used i n the c a l c u l a t i o n of the chemical composition of the rocks. Plagioclase Feldspar. Plagioclase compositions were estimated using o p t i c a l methods. A few determinations were made using the four-axis universal stage method on plagioclase grains from the margin and centre of the stock. The universal stage measurements indicate the plagioclase i s of intermediate s t r u c t u r a l state. I t i s assumed that most plagioclase i s of intermediate s t r u c t -u r a l state and the remainder of the plagioclase compositions of a l l samples from the stock were estimated using flat-stage methods. V a r i a t i o n of the actual s t r u c t u r a l state would not cause an error or more than - 2 weight per cent anorthite i n the estimated composition. The data are presented i n Table 5-The carlsbad-albite twin method was used most commonly, but where i t was not possible to f i n d a suitable grain the XTA(010)J. a method was used. Zoning of the plagioclase l o c a l l y makes i t d i f f i c u l t to estimate the bulk composition. The plagioclase compositions are plotted against the Lar-sen Index po s i t i o n (which i s thought to represent the stage' of TABLE V ANORTHITE CONTENT OF PLAGIOCLASE FELDSPAR SAMPLE NUMBER ESTIMATED PER CENT ANORTHITE (± 2% ANORTHITE) 201 3 9 w £5 3 8 o S ) 8 6 EH 241 3 8 < EH 144X 2 8 O 212 3 7 211 3 8 144R 33 w 125 3 6 o (SJ 1 8 5 w EH 2 2 5 2 9 R Q 144W 3 4 W 187 2 9 EH H C2 33 144T 3 0 1 9 5 3 5 Cd 146 2 8 53 O 183 SI 3 3 ^ o 232 2 8 o 1 4 5 2 8 181 3 5 APLITE DYKE 144V 2 4 56 d i f f e r e n t i a t i o n of the stock) i n Figure 15c. There i s a trend of decreasing anorthite content of the plagioclase, with d i f f e r -e n t i a t i o n of the stock, from An^Q i-n e a r l y d i f f e r e n t i a t e d rocks of the marginal part of the stock through An^Q i - n the central part of the stock to An„. i n the l a t e stage d i f f e r e n t i a t e s r e -z 4 presented by the a p l i t e dykes. Resorbed margins are common i n plagioclase from the central part of the stock. Orthoclase and r a r e l y quartz f i l l the embay-ments i n the c r y s t a l margins. Resorption of plagioclase has been explained by Tuttle and Bowen (1958) i n t h e i r discussion of the system NaAlSi^Og - KAlSi^Og - CaAl 2Si 20g and also by Stewart and Roseboom (I963). This system i s shown i n Figure 16. The bulk feldspar compositions of rocks of the Troitsa Lake Stock are plotted on t h i s system. This diagram i s based on data from volcanic rocks and can not be d i r e c t l y applied to plutonic rocks but does show the general c r y s t a l l i s a t i o n r elationships i n t h i s system. The l i n e ABC represents the approximate l i m i t of f e l d s -par s o l i d solution under dry conditions, and the l i n e DEF i s the boundary between the f i e l d s of potassium feldspar and plag-ioclase feldspar and represents the compositions of l i q u i d s i n equilibrium with two feldspars whose compositions l i e along the curve ABC at one kilobar water vapour pressure (James and Hamil-ton, 1969). Liquids i n the f i e l d BEF c r y s t a l l i s e with early separation of plagioclase followed by an a l k a l i feldspar. The plagioclase reacts with the l i q u i d and f i n a l l y disappears leav-ing an a l k a l i feldspar and l i q u i d . The a l k a l i feldspar c r y s t a l s then react with the l i q u i d and a t t a i n the composition of the i n i t i a l mixture. C r y s t a l l i s a t i o n i n t h i s manner involves r e -57 VARIATION OF THE PLAGIOCLASE AND WITH RESPECT TO THE ALKALI FELDSPAR COMPOSITIONS LARSEN INDEX. 58 ANORTHITE FIGURE 16. MODAL FELDSPAR CONSTITUENTS OF THE TROITSA LAKE STOCK SAMPLES RECALCULATED TO 100% ANORTHITE + ALBITE + ORTHOCLASE. ABC REPRESENTS THE LIMIT OF TERNARY SOLID SOLUTION IN NATURAL FELDSPARS. DEF IS THE BOUNDARY CURVE SEPARATING THE FIELDS OF PLAGIOCLASE AND ALKALI FELDSPAR. M IS THE APPROXIMATE POSITION OF THE TERNARY MINIMUM (ROSEBOOM AND STEWART, 1963 HAVE POINTED OUT THAT THE MINIMUM MAY NOT LIE ON THE ALBITE ORTHOCLASE SIDELINE WHERE IT IS SHOWN IN THIS DIAGRAM.) sorption of the plagioclase. Tuttle and Bowen stated that a l l compositions below the curve from about the position G to C w i l l c r y s t a l l i s e i n t h i s manner, forming a single a l k a l i f e l d s par as an end product although they may i n i t i a l l y c r y s t a l l i s e two feldspars. Fractionated l i q u i d s would also tend to cause resorption of the plagioclase. Zoning commonly observed i n the plagioclase c r y s t a l s indicates that f r a c t i o n a t i o n has oc-curred. I f a magma of a composition above the curve GBC was subject to f r a c t i o n a l c r y s t a l l i s a t i o n the path of the l i q u i d i n equilibrium with the two feldspars would leave the f i e l d boundary DEF and change composition along a curved path t o -wards the ternary minimum, M, and e a r l i e r formed plagioclase c r y s t a l s may be resorbed by t h i s fractionated l i q u i d . The pos i t i o n of the ternary minimum i s shown to l i e on the a l b i t e -orthoclase s i d e l i n e i n t h i s diagram, but i t has been shown by Stewart and Roseboom (I963) that t h i s i s not always the case. Normal and o s c i l l a t o r y zoning are common i n plagioclase from the central part of the stock. Normal zoning r e s u l t s from incomplete reaction between c r y s t a l s i n the a l b i t e -anorthite s o l i d s o l u t i o n s e r i e s and the magma during c r y s t a l l -i s a t i o n i n response to f a l l i n g temperature. There are several theories to explain o s c i l l a t o r y zoning. It may be explained by repeated variations of temperature or pressure; repeated r e l a t i v e movement of the c r y s t a l and magma by convective c i r -c ulation; and by the d i f f u s i o n - supersaturation theory of Harlof f (1927). This l a t t e r theory has been developed by H i l l s (1936) and by Bottinga, Kudo and W e i l l (1966). They suggest that the degree of supersaturation i n the magma immed-l a t e l y adjacent to each l i q u i d / c r y s t a l interface i s related to the mechanism of nucleation and the geometric configurat-ion of the interface. The presence of such a supersaturated layer adjacent to o s c i l l a t o r y zoned bytownite c r y s t a l s i n the glassy matrix of an oceanic basalt has been demonstrated by Bottinga, Kudo and We i l l . This theory regards each c r y s t a l and l i q u i d skin as an i s o l a t e d non-equilibrium system and does not require r e l a t i v e movement of the c r y s t a l s and magma. The o s c i l l a t o r y zoning observed i s usually very f i n e and delicate and the zones may number f i f t e e n or more. Temperature f l u c t -uations s u f f i c i e n t to cause t h i s zoning seem unl i k e l y . F l u c t -uations i n pressure are more a fea s i b l e cause of compositional variations i n the c r y s t a l l i s i n g plagioclase. The number and re g u l a r i t y of the o s c i l l a t o r y zones argue against the cause being r e l a t i v e movement of the c r y s t a l s and magma due to c i r -c u lation by magmatic convection. The d i f f u s i o n - supersatur-ation theory, or the ef f e c t of fluctuations i n pressure are the most l i k e l y explanations of o s c i l l a t o r y zoning. A l k a l i Feldspar. A l k a l i feldspar forms 2 0 to 45 per cent of the quartz monzonite - granodiorite, the percentage being greater i n the more a c i d i c rocks of the central part of the stock. Much of the a l k a l i feldspar i s microperthitic and X-ray d i f f r a c t i o n data show that a l l of i t i s p e r t h i t i c . The a l k a l i feldspars were investigated using X-ray d i f f -f r a c t i o n methods to determine the composition of the p e r t h i t i c phases,the bulk composition and the st r u c t u r a l state.The methods used and the data obtained are described i n d e t a i l i n Appen-dix I . The samples investigated contain a K-rich phase, the composition of which ranges from 83 to 97 weight per cent orthoclase, and an Na-rich phase which ranges from 95 to 100 weight per cent a l b i t e (see Table 14 Appendix I ) . The K-rich phase and Na-rich phase compositions of the a l k a l i feldspar have been plotted against the Larsen index position of the rock (which i s thought to represent the state of d i f f e r e n t -i a t i o n of the rock) to see i f there i s any v a r i a t i o n i n the degree of ordering of the a l k a l i feldspar related to the d i f -f e r e n t i a t i o n of the stock (Figure 1 5 ) . The compositions of the two phases of the unhomogenised feldspar vary randomly throughout the stock and no s t a t i s t i c -a l l y s i g n i f i c a n t trend l i n e can be f i t t e d through the data points (Figure 15a) . The compositions of the potassic phase represent the degree of unmixing of the a l k a l i feldspar and t h i s does not appear to be related to the state of d i f f e r e n t -iationoof the stock. The compositions of the homogenised a l k a l i feldspar do show a s i g n i f i c a n t trend related to the d i f f e r e n t i a t i o n of the stock (Figure 15b) . The composition ranges from up to 87 per cent orthoclase i n the early d i f f e r e n t i a t e d rocks of the marginal part of the stock to 73 per cent i n l a t e r stage d i f f -erentiates from the central part of the stock and to 71 per cent orthoclase i n the f i n a l stage d i f f e r e n t i a t e s represented by the a p l i t e dykes (see Table 15, Appendix I ) . The trend of decreasing orthoclase content of the a l k a l i feldspar (and-also 62 decreasing anorthite content of the plagioclase) i s as expec-ted i n d i f f e r e n t i a t e d igneous rocks. The s t r u c t u r a l state of the potassic phases of the natural perthite has been estimated from the^position o f the (060) and (204) X-Ray d i f f r a c t i o n r e f l e c t i o n s according to the method of Wright (1968). Figure 17 i s a °2Q (060) against °29 (204) plot of the data (after Wright 1968, Wright and Stew-art 1968, and O r v i l l e 1967). A l l the a l k a l i feldspars plot near the orthoclase j o i n and just to the maximum microcline -low a l b i t e side of the j o i n . This plot shows that the a l k a l i feldspars throughout the stock have a s i m i l a r s t r u c t u r a l state and do not vary with composition of the feldspar or with the stage of d i f f e r e n t i a t i o n of the stock. Although T i l l i n g (1968) has observed a general increase i n 'obliquity' with increasing a l b i t i c composition of p e r t h i t i c a l k a l i feldspars from the Rader Creek Pluton, Montana, no such trend exists i n the a l k a l i feldspars of T r o i t s a Lake stock. A l k a l i feldspar i n the margin-a l rocks does not have a higher s t r u c t u r a l state as might be expected. The s t r u c t u r a l state of monoclinic a l k a l i feldspar i s dependent on the degree of A l / S i ordering i n the non-equi-valent Tj and T 2 tetrahddral s i t e s of the structure. The order-ing i s achieved by d i f f u s i o n , i n the s o l i d state, of A l into the more favourable T-^  s i t e s . The degree of ordering depends on the cooling h i s t o r y of the rock, and i n the T r o i t s a Lake stock, the s t r u c t u r a l state indicates a s i m i l a r cooling history throughout the stock. It has been suggested (Smith and MacKen-zie I96I) from t h e o r e t i c a l considerations that as the A l / S i order T 1 1 1 1 1 1 1 1 1 r F I G U R E 17. ALKALI FELDSPARS FROM THE TROITSA LAKE S T O C K PLOTTED ON A (060) - ( § 0 4 ) PLOT SIMPLIFIED FROM WRIGHT (1968). A L L APPEAR TO HAVE AN INTERMEDIATE STRUCTURAL STATE SIMILAR TO THE ORTHOCLASE SERIES. of a p e r t h i t i c a l k a l i feldspar increases so should the purity of the p e r t h i t i c components. The lack of any definable trend i n the compositions of the potassic phases i s consistent with the s i m i l a r i t y of s t r u c t u r a l state of the feldspars. The ternary system KAISi 0 G - NaAlSi^Og - CaAO^Si^Og has been studied at a water vapour pressure of 5 0 0 0 bars by Yoder, Stewart and Smith ( 1 9 5 6 ) . They demonstrated that the composi-tions of coexisting plagioclase and a l k a l i feldspars are r e -lated to the temperature and pressure of t h e i r c r y s t a l l i s a t i o n . Figure 1 8 i s a plot of experimentally determined t i e - l i n e s f o r coexisting plagioclase and a l k a l i feldspars f o r various tem-peratures of formation at 5 0 0 0 bars PH 2 0. T i e _ l i n e s f o r several coexisting feldspars from the Troits a Lake stock are presented f o r comparison. Yoder, Stewart and Smith ( 1 9 5 6 ) found that a plot of analysed coexisting feldspar pairs agreed f a i r l y c lose-l y with the experimentally determined t i e _ l i n e s except f o r feldspars from volcanic rocks which have a steeper slope with respect to the K A l S i „ 0 O - NaAISi 0 O baseline. The authors 3 o 3 o believed that the steeper slope of the t i e - l i n e s i s related to the lower PI^O and higher temperatures. Feldspars from the Troi t s a Lake stock are plotted assuming maximum s o l i d s o l u t i o n of orthoclase i n the plagioclase and of anorthite i n the a l k a l i feldspar. The t i e - l i n e s agree well with those of Yoder, Stewart and Smith, and have a s i m i l a r slope to the experimental t i e -l i n e s f o r 7 2 0 ° to 7 7 0°C. at 5 0 0 0 bars. This indicates that the maximum temperatures of c r y s t a l l i s a t i o n of coexisting f e l d s -pars of the T r o i t s a Lake stock ranged from 7 2 0 ° to 7 7 0°C. As A N O R T H I T E F I G U R E 18. COMPOSITION OF COEXISTING PLAGIOCLASE AND ALKALI FELDSPARS FROM ROCKS OF THE TROITSA LAKE STOCK (•). IN THE DIAGRAM NOT A L L C O -EXISTING FELDSPAR PAIRS ARE SHOWN, BUT ALL PAIRS LIE WITHIN THE RANGE SHOWN BY THE S E L E C T E D DATA. THE OPEN CIRCLES ARE EXPERIMENTALLY DETERMINED COEXISTING FELDSPAR PAIRS AT VARIOUS TEMPERATURES AND A WATER PRESSURE OF 5000 bars BY YODER, STEWART AND SMITH, 1956. the stock probably c r y s t a l l i s e d under a PH 0 of much less than 5000 bars the actual temperatures of c r y s t a l l i s a t i o n of the coexisting feldspars may be more than t h i s . Barth (1962) has proposed a geothermometer based on the compositions of coexisting feldspars. Barth suggested that the p a r t i t i o n of a l b i t e between the two phases was mainly dependent on temperature, so that:-Mole f r a c t i o n of Ab i n a l k a l i feldspar — x = K Mole f r a c t i o n of Ab i n plagioclase X2 the c o e f f i c i e n t of d i s t r i b u t i o n , K , being proportional to the temperature. The r e l a t i o n s h i p between the c o e f f i c i e n t of d i s t r i b u t i o n and temperature proposed by Barth i s shown i n Figure 19. The temperature determined by t h i s method i s not necessarily that of the c r y s t a l l i s a t i o n of the rock, nor even of the feldspars i n i t , but rather the temperature at which the rock reached i t s l a s t equilibrium state. The basis of t h i s method involves several s i m p l i f i c a t i o n s such as: no v a r i a t i o n of s p e c i f i c heat with pressure; anorthite cannot enter into s o l i d s o l u t i o n with a l k a l i feldspar; and feldspars are approximately perfect solutions. O r v i l l e (1972) showed that t h i s l a s t assumption i s v a l i d f o r a l b i t e - r i c h plagio-clase feldspars only, and that a l k a l i feldspars do not form an i d e a l s o l u t i o n ( O r v i l l e , I962). The a c t i v i t y c o e f f i c i e n t of a l b i t e i n a l k a l i feldspar i s not unity and depends on both the composition of the a l k a l i feldspar and the temperature. Data are not available to calculate the f u l l e f f e c t of the a c t i v i t y oh Barth's geothermoneter. The d i s t r i b u t i o n co-e f f i c i e n t s f o r feldspars of the T r o i t s a Lake stock range- f-rom T E M P E R A T U R E 900 700 600 500 400 .50-.40 .30-.25-.20-•15-•10-1 F I G U R E 19. RELATION BETWEEN TEMPERATURE AND THE RATIO OF DISTRIBUTION OF ALBITE BETWEEN ALKALI FELDSPAR AND PLAGIOCLASE FELDSPAR K _ Mol froctjon Ab in olkoli feldspar Mol fraction Ab in plagioclase (After Barth, 1962) 0.21 to 0.41 i n d i c a t i n g temperature values according to the geothermometer from a l i t t l e below 500°C. to over 600°C. Because the a c t i v i t y c o e f f i c i e n t of a l b i t e i n a l k a l i f e l d s -par i s not known, these calculated temperatures are not r e -la t e d to temperatures of c r y s t a l l i s a t i o n . Granodiorite from the marginal part of the stock tends to y i e l d lower temperat ures than quartz monzonite from the central part of the stock which indicates that the a l b i t e a c t i v i t y c o e f f i c i e n t i s more dependent on v a r i a t i o n of composition than of temperature. CHAPTER V BIOTITE. I n t r o d u c t i o n . R e f r a c t i v e i n d i c e s o f b i o t i t e s f r o m t h e T r o i t s a Lake s t o c k have been measured t o f i n d any s y s t e m a t i c v a r i a t i o n . ( T a b l e 8, F i g u r e 22). Three s e l e c t e d b i o t i t e s have been an-a l y s e d t o f i n d any c o m p o s i t i o n a l v a r i a t i o n a s s o c i a t e d w i t h the change i n r e f r a c t i v e i n d e x and t o e s t i m a t e the f l u i d p r e s s u r e o p e r a t i v e d u r i n g c r y s t a l l i s a t i o n o f t h e s t o c k . The c h e m i c a l a n a l y s e s o f the b i o t i t e s and t h e i r c a l c u l a t e d s t r u c t u r a l f o r -mulae a r e p r e s e n t e d i n Table 9-Method o f Study . M i c a s have t h e g e n e r a l f o r m u l a X Y ,Z o0„ r t(OH,F) . . B i o -2 4—0 o 20 4 t i t e m i c a s e x h i b i t a l a r g e range o f p o s s i b l e c h e m i c a l s u b s t i t -u t i o n s w i t h i n t h e c r y s t a l s t r u c t u r e . The Z p o s i t i o n , i n the 4+ 3+ t e t r a h e d r a l l a y e r , i s o c c u p i e d m a i n l y by S i and A l , a l -4+ though some T i may a l s o o c c u r . The i n t e r l a y e r c a t i o n , X, i s m a i n l y K +, but N a + and p o s s i b l y C a 2 + , B a 2 + , and R b + may sub-s t i t u t e f o r t h i s . The most p e t r o g e n e t i c a l l y i m p o r t a n t s u b -s t i t u t i o n s , however, o c c u r i n the o c t a h e d r a l l a y e r , where the 2+ 2+ o c t a h e d r a l l y c o - o r d i n a t e d c a t i o n , Y, may be Mg and Fe w i t h l e s s e r amounts o f F e 3 + , A l 3 + , T i 4 + , and M n 2 + . E u g s t e r and Wones (1958) have shown t h a t f o r s y n t h e t i c b i o t i t e s on the j o i n p h l o g o p i t e - a n n i t e t h e r e i s an i n c r e a s e i n the r e f r a c t i v e i n d e x w i t h i n c r e a s i n g a n n i t e c o n t e n t o f the b i o t i t e . The r e f r a c t i v e indices of b i o t i t e flakes from 20 specimens of the Tro i t s a Lake stock have been measured using r e f r a c t i v e index o i l s to f i n d any systematic v a r i a t i o n which may be related to a v a r i a t i o n i n composition. Wones and Eugster (1965) found that synthetic b i o t i t e s on the j o i n phlogopite - annite react to form a number of assemblages and that the reactions are governed by the i n -dependent intensive parameters of temperature, fugacity of water (fH 2o)» a n d fugacity of oxygen ( f o 2 ) . T h e most common of these assemblages i n natural occurrences are b i o t i t e -hematite; b i o t i t e - sanidine - magnetite; and b i o t i t e -l e u c i t e - o l i v i n e - magnetite. The assemblage b i o t i t e - a l -k a l i feldspar - magnetite i s present i n rocks of the T r o i t s a Lake stock. Figure 20 shows that the composition of the bio-t i t e , s p e c i f i c a l l y the ir o n and magnesium content, i s depend-ent upon fQ and temperature. Wones and Eugster determined the compositions of b i o t i t e s coexisting with sanidine and magnetite f o r a variety of conditions of f j j o £o > a n c * "tern-perature. The compositions l i e i n the ternary system K F e 3 + A l S i 3 0 1 ( ) ( 0 H ) 2 (annite) - KMgg+A16i 0 1 £ ) (0H>2 (phlogopite)-0-f. "I KFe^ AlSi^O-^tH *"') ('oxybiotite T) . Wones and Eugster applied 2+ regular s o l u t i o n theory to KFe^ A l S i ^ 0 ^ 0 ( 0 H ) 2 i n ternary s o l i d s o l u t i o n which y i e l d s the rel a t i o n s h i p : l o g fH,o - 3428 - W 3 < 1 - * ! ) 2 + l o g X i + i l o g f Q + 8 . 2 3 T 2 - l o g * K A 1 S i 0 - log« F e 0 ( t o . 2 0 ) (1. 3 8 3 4 2"h where : x-, i s the mol f r a c t i o n of KFe~ A l S i o 0 n n (OH) 9 i n the J- o o 10 ^ b i o t i t e . U I o X o o o - I BIOTITE - SANIDINE-HEMATITE - SAS BIOTITE- SANIOINE MAGNETITE - GAS 4 0 0 6 0 0 8 0 0 T E M P E R A T U R E °C 1000 P T 0 T A t = 2 0 7 0 b Q r 9 F I G U R E 20. STABILITY OF BIOTITES AS A FUNCTION OF f0 2 AND TEMPERATURE AT 2070 bars TOTAL PRESSURE. SUBHORIZONTAL LINES REPRESENT CONTOURS OF CONSTANT 100 Fe (Fe + Mg) VALUES. ARROWS LABELLED I AND H REPRESENT OXIDISING AND REDUCING TRENDS IN CRYSTALLISING MAGMAS. (after Wones and Eugster, 1965) F I G U R E 21. THE TERNARY SYSTEM K FB* *A I Si30I0(0H)2 - KMgJ*AI SI,0M(OH), - KFe^AI 9,0lt(H.,). DASHED LINES INDICATE COMPOSITION OF 'BUFFERED' BIOTITES. COMPOSITIONS IN THE SHADED AREA ARE PHYSICALLY IMPOSSIBLE TO OBTAIN. DOTTED LINE IS THAT OF APPROPRIATE COMPOSITION OF BIOTITES THAT WOULD PROVIDE A CONSTANT VALUE FOR THE HEAT OF MIXING, (after Wones and Eugster, 1965) «x^ »-.^ . ~ i s the a c t i v i t y of K A l S i o 0 Q i n the various mineral KAlSi^Og . " 3 o phases. e< i s the a c t i v i t y of Fe 0 i n the various mineral phases. Fe 0 3 4 3 4 Therefore the water pressure operative when the b i o t i t e c r y s t a l l i s e d can be calculated i f the other variables are known. Three b i o t i t e s from the T r o i t s a Lake stock were selected f o r study, one from the fine-grained granodiorite of the con-tact zone of the stock (specimen 86), one from the intermediate zone (specimen 144T), and one from the coarser-grained quartz monzonite of the core zone (specimen 181). These specimens should show the l i m i t s within which the b i o t i t e compositions vary. I f a regular change i n b i o t i t e compositions exists i n the stock these specimens should show i t . Electron microprobe and p a r t i a l wet chemical analyses of these b i o t i t e s yielded t h e i r annite contents. The methods used and results obtained are described i n d e t a i l i n Appendix I I . St r u c t u r a l analyses on a basis of 22 oxygens were calcu-la t e d from the chemical analyses (Table 9). These give the composition of the octahedrallayer of the b i o t i t e and show how much aluminum, i f any, enters i t , and i f any of the titanium i s i n tetrahedral coordination. The mole f r a c t i o n of annite 2+ i n the b i o t i t e ( i . e . the proportion of Fe i n the octahedral layer) can then be calculated. It i s not possible to calculate exactly the oxygen fug-a c i t y that was operative during c r y s t a l l i s a t i o n of the b i o t i t e . However i t i s possible to place l i m i t s on t h i s variable. Wones and Eugster (1965) have synthesised b i o t i t e s at controlled ox-ygen f u g a c i t i e s . The buffers used to control the f'Q were Fe 0 - Fe 0 . NiO - Ni. SiO - Fe SiO - Fe 0 , and 2 3 3 4' ' 2 2 4 3 4 Fe 0 - Fe 0. The compositions of the 'buffered' b i o t i t e s 3 4 1-x 3 + 2 + 3 + synthesised are shown i n Figure 21. The Fe / Fe + Fg r a t i o of the b i o t i t e s i s dependent on the buffer and i s 0.25, 0.10, 0.05 and 0.02 respectively. The oxygen fuga-c i t i e s of these buffers at given temperatures can be c a l c u l -ated from the r e l a t i o n s h i p : log f Q = - ±_ + B• + C (P - 1) ( 2 2 T T The values of the constants f o r the various buffers are given 9+ i+ i n Table 6. The Fe / Fe + Fe J r a t i o f o r the Tr o i t s a Lake b i o t i t e s i s calculated from the analyses. This value w i l l l i e between those f o r two of these 'buffered' b i o t i t e s andvupper and lower l i m i t s can thus be placed on the fp operative when the b i o t i t e c r y s t a l l i s e d . To calculate the fjj Q i t i s necessary to know the a c t i v i t y of KAlSi„0 o and Fe„0.. The a l k a l i feldspars have been homogen-3 o 3 4 ised and the KAlSi^Og content estimated using X-ray d i f f r a c t i o n methods (see ChapterlV). O r v i l l e ' s (1963) data for a l k a l i felds-pars coexisting with aqueous chloride solutions enable the KAlSi^Og a c t i v i t y to be calculated. For orthoclase-rich a l k a l i feldspars t h i s approximates to i d e a l i t y , <=< — x„. ^  „ . „ K A l S i 3 0 g KAlSigOgj. Lindsley's (I963) data demonstrate the nonideal behaviours of Fe A0, - Fe„Ti0. s o l i d solutions, but f o r these calculations 3 4 2 4 ' (following Wones and Eugster, 1965) the assumption of i d e a l i t y i s used, =~ _ v „ n The titanium content of magnetite i n F e 3 0 4 = *Fe 30 4. 74 TABLE VI OXYGEN FUGACITIES OF OXYGEN BUFFERS (from Wones and Eugster, 1 9 6 5 ) -Buffer l o g f = - | + B + C(P - 1 ) T F e 3 + y F e 2 + + F e 3 + or synthesisea b i o t i t e A B C F e 2 ° 3 - F e 3 ° 4 249.12 1 4 , 4 1 0 . 0 1 9 0 . 2 5 NiO - Ni 2 4 7 0 9 8 . 9 4 o. 0 4 6 0 . 1 0 S i 0 o - Fe 9SiO. - FLO, 2 4 2 73.00 1 0 , 30 0 , 0 9 2 0 . 0 5 3 4 F e , 0 , - Fe.. 0 3 2 7 3 0 1 3 . 1 2 0 . 0 8 3 0 . 02 3 4 1-x TABLE VII TITANIUM -CONTENT OF MAGNETITE FROM SELECTED SAMPLES OF THE TROITSA LAKE STOCK, DETERMINED FROM ELECTRON MICROPROBE DATA. Sample # Average total... counts ( 1 0 sees. ) counts ^ % counts W t g T i a — T i ^ Mole f r a c t i o n Fe 0TiO. 2 4 Pure 1 3 2 3 4 2 2 8 5 0 . 2 1 0 0 Titanium Standard - - . . -86 58 7 12 0 . 0 4 4 0 , 0 0 5 0 . 02 144T 38 1 5 4 0 0 , 0 5 4 0 . 0 2 2 0 . 08 1 8 1 72 1 0 14 0 . 0 2 9 0 . 0 0 4 0 . 0 1 the rocks was measured by electron microprobe. The samples used were polished rock slabs and the content was determined by comparision with a pure titanium standard (Table 7). The mole f r a c t i o n of ulvospinel i n the magnetite was calculated from the titanium content. These values for the mole f r a c t i o n of KFe^AlSi^O.^ (OH)^ i n the b i o t i t e , the a c t i v i t i e s of KAlSi^Og and Fe^O^, and ox-ygen fugacity are substituted i n the equation ( l ) allowing the c a l c u l a t i o n of the water pressure at any desired temper-ature . As i t was only possible to calculate the range of possible oxygen f u g a c i t i e s i t i s only possible to calculate upper and lower l i m i t s f o r the water pressure. Discussion of Results. The r e f r a c t i v e indices of the b i o t i t e s (Table 8, and Figure 22) range from 1.635 to 1.613. The 19 b i o t i t e s from the stock show a trend of decreasing r e f r a c t i v e index with d i f f e r e n t i a t i o n i n d i c a t i n g a magnesium-rich trend i n the b i o -t i t e composition with d i f f e r e n t i a t i o n . Specimen 144V, an ap-l i t e , does not f i t t h i s trend. The three chemically analysed b i o t i t e s , specimens 86, 144T, and 181, have r e f r a c t i v e indices of 1.634, 1.620, and 1.613 respectively. These b i o t i t e s are chemically s i m i l a r except f o r the i r o n and magnesium contents (Table 9). Their Fe / Fe 4- Mg r a t i o s change from 0.63 i n specimen 86 through 0.62 i n specimen 144T to O.58 i n specimen 181. This confirms that the systematic change i n r e f r a c t i v e index of the b i o t i t e represents a trend of magnesium enrichment of the b i o t i t e with d i f f e r e n t i a t i o n of the stock. TABLE VIII REFRACTIVE INDICES OF BIOTITES FROM THE TROITSA LAKE STOCK. Specimen 201 w 86 O N 241 EH < 144X Eh 53 212 O o 211 144R w 185 5? O N H 225 EH < ! H 144W P W 187 H EH 55 C2 H 144T 195 146 W 5? 183 O N 232 W o o 145 181 APLITE DYKE 144V Refractive Index (+ 0.002 ) 1-635 1 .634 1, 631 1,631 1. 620 I t 619 1, 624 1. 624 1.619 1,619 1. 62Q 1,615 1. 620 1. 613 1.615 1. 613 1. 615 - I.624 1. 613 1. 626 L A R S E N INDEX — CALCULATED REGRESSION EQUATION FOR THE DATA (excluding specimen 144V from on Aplite dyke ) - 9 5 % CONFIDENCE LIMITS F I G U R E 2 2 . VARIATION OF REFRACTIVE INDEX (10-002) OF BIOTITE FROM ROCKS OF THE TROITSA LAKE STOCK WITH RESPECT TO THE LARSEN INDEX. 78 TABLE IX CHEMICAL ANALYSES OF SELECTED BIOTITES FROM"THE TROITSA LAKE STOCK. (Analyst.: Dr. E. _ Ghent, University of Calgary.) OXIDE s i o 2 A1 20 3 Total Fe as FeO FeO (wet chem.) Fe 0 (by d i f f . ) T i 0 2 MgO MnO CaO Na 2 0 K 2 0 Specimen 181 (core zone) 37.2 13.9 1 6 . 0 14-1 211 3.8 14. 8 0 . 2 0 . 0 5 0.3 -i 9. 6 Specimen 144T (intermediate zone) 3 7 . 0 13.2 17. 6 14-1 3.9 4 . 5 13.7 0 . 2 o; 0 5 0.3 9.3 NUMBER OF IONS ON A BASIS OF 2 2 ( 0 ) . " S i 5 - 5 5 5 . - 5 3 Z - A l 2 . 4 4 2 . 3 3 T i 0 , 0 1 0.14 " A l 0 . 0 0 . 0 T i 0 . 4 2 - 0 . 3 7 Y - F e3 + F e 2 + 0 . 2 4 1 . 7 6 - 5 . 7 4 0 . 4 4 1 . 7 6 Mn 0 . 03 0 . 0 3 Mg 3 . 2 9 3 , 0 5 Ca . 0 . 0 0 . 0 X - Na 0> 0 9 0 . 0 9 . K l;83 1 . 7 7 5 . 6 5 Specimen 8 6 (marginal zone) 3 6 . 8 1 3 . 4 1 7 . 4 1 4 . 5 3 . 2 4 . 3 1 3 . 4 0 . 2 0 . 3 9 . 4 5 - 5 5 2 . 3 8 0 , 0 7 0 . 0 0 . 4 2 0 . 3 6 1 . 8 3 0. 03 3 . 0 1 0 . 0 0 . 0 9 1 . 8 1 5 - 6 5 79 The compositional v a r i a t i o n of synthetic b i o t i t e s cry-s t a l l i s e d at variable oxygen f u g a c i t i e s have been studied by Wones and Eugster (1965). They have shown that b i o t i t e cry-s t a l l i s i n g from a magma may follow either a more i r o n - r i c h trend or a more magnesium-rich trend depending upon f~ con-e-d i t i o n s during cooling. These trends are represented d i a -gramatically i n Figure 20 (after Wones and Eugster, 1965, F i g . 13, p. 1254). The magnesium-rich trend i n b i o t i t e com-p o s i t i o n i s a r e s u l t of constant or increasing f n . This 2 trend may represent saturation of the melt with regard to HO. The more magnesium-rich b i o t i t e from the core zone of the Troi t s a Lake stock indicates i t was probably subject to constant or increasing fg and the melt approached saturation 2 with regard to H 20, however the absence of m i a r o l i t i c c a v i t i e s i n the stock indicates saturation was not achieved. As has been stated previously i t i s possible to make quan-t i t a t i v e estimates of the P H Q operative when the b i o t i t e cry-2 s t a l l i s e d . However i t i s f i r s t necessary to estimate the tem-perature of c r y s t a l l i s a t i o n . Compositions of coexisting f e l d s -pars i n the rocks indicate temperatures 720°C. to 770°C. ac-cording to the data of Yoder, Stewart, and Smith (1956) (see ChapterIV). Buseck (1966) suggests 820°C. as the s o l i d i f y i n g temperature of granodiorite. The temperature of c r y s t a l l i s a t -ion of rocks of the Troi t s a Lake stock probably ranges from over 700°C. to over 800°C. The F e 3 + / F e 2 + + F e 3 + r a t i o of the three b i o t i t e s i s 0.12, 0.20, and 0.17 f o r specimens 181, 144T and 86 respectively. These values indicate that i n a l l b i o t i t e s the l i m i t i n g f values are those defined by the buffers NiO - Ni °2 o and Fe 0. - Fe 0 . At 700 C. the minimum and maximum values 2 3 3 4 of log f n a r e approximately l o " 1 ^ and 10" 1 1 respectively. For 2 a temperature of 800°C. they are approximately 10"""*"4 and 10"9 o —13 respectively. At 850 C. the minimum log fq value i s 10 The annite content of the b i o t i t e s i s 0.31, 0.31 and 0.32 res-pectively. The composition of the coexisting a l k a l i feldspars i s 0 r73* 0 r78' a n d ° r83 r e s P e c t i v e l y . The ulvospinel content of the coexisting magnetite i s 0.02, 0.08, and 0.01 respectively. Applying these values to the relationship : loS fH 9 o = 3428 - 4212(1 - X l ) 2 4- log x, + £ log f Q + 8.23 2 . . -1- 2 T - l o g ~ - l o g ~ ( i 0.20). (1. S KAlSi.Oo S Fe 0 3 o 3 4 f H q at the time of c r y s t a l l i s a t i o n of the b i o t i t e s , and there-2 fore of the stock, y i e l d s minimum values of approximately 100 bars f o r a l l three b i o t i t e s at a temperature of 700°C, approx-imately half a k i l o b a r at 800°C, and at 850°C. the minimum f„ _ indicated i n one k i l o b a r . The maximum values f o r the f„ 2 H 2 ° indicated by the above r e l a t i o n s h i p range from several tens of kil o b a r s to a few hundred k i l o b a r s and are patently unreasonable f o r t h i s epizonal stock. CHAPTER VI 81 PETROLOGY OF THE STOCK. Introduction. Although the data are scanty, the contact metamorphic e f f e c t s of the stock have been examined, and the thermal ef-fec t s of the stock on the country rock have been calculated. The l e v e l of emplacement of the stock has been deduced from the c r i t e r i a of Buddington (1959). The mineralogical v a r i a t i o n of the stock has previously been described (see Chapter III) and the chemical v a r i a t i o n of the stock has been deduced from t h i s . Friedman ( i 9 6 0 ) and Heier (1961) have shown that the chemical composition of rocks can be accurately estimated from modal analyses providing that the composition of the minerals, es p e c i a l l y the plagioclase and a l k a l i feldspar, i s known. The chemical compositions of rocks i n t h i s study, i n terms of oxide weight per cent, have been calculated from the modal analyses using a FORTRAN program according to the method of D i e t r i c h and Sheehan ( I964). The composition of the plagioclase was estimated by o p t i c a l methods and of the a l k a l i feldspar by X-ray methods, as described pre*, viously. For other minerals of variable composition, such as b i o t i t e and hornblende, the compositions used are averages of several analyses taken from the l i t e r a t u r e by D i e t r i c h and Sheehan. The mineral compositions used i n the c a l c u l a t i o n are given i n Table 10. The estimated chemical compositions of the rocks are presented i n Table 11. The average chemical compo-s i t i o n of the stock, calculated i n the same manner as the TABLE X MINERAL COMPOSITIONS (OXIDE WEIGHT PERCENT) USED IN CALCULATING THE CHEMICAL COMPOSITIONS OF THE ROCKS FROM MODAL ANALYSES. MINERAL s±o 2 A 1 2 0 3 Fe 9 0 „ FeO 1 3 MgO CaO OXIDE Na 2 0 K 2 0 H 2 0 ? 2 ° 5 T i 0 2 CaF 2 co 2 FeS 2 K-FELD-SPAR 6 4 . 7 7 1 8 . 31 16 . 92 ALBITE 68 .75 19 .44 11 . 81 ANOR-THITE 4 3 . 2 0 3 6 . 6 5 2 0 . 1 5 QUARTZ LOO.00 HORN-BLENDE 4 6 . 2 7 8 . 5 6 3 . 9 5 1 2 . 7 8 1 2 . 0 9 11 . 71 1 .10 0. 65 1 . 6 8 1 .21 BIOTITE 3 7 - 0 0 1 5 . 2 0 3 .82 1 9 . 1 5 9 . 3 8 0 . 9 6 0 . 6 0 8. 36 2.16 3 . 3 7 MAGNE-TITE 6 8 . 9 7 31.03 CHLOR-ITE 1 7 . 5 8 2 9 . 83 42. 05 1 0 . 54 APA-TITE 50. 04 4 2 . 2 2 7 .74 SPHENE 30.65 2 8 . 60 4 0 . 7 5 CALCITE 56. 02 4 3 . 9 8 EPIDOTE 3 7 . 3 1 21 ; 10 16 .52 2 3 . 2 1 1. 86 HEMA-TITE 1 0 0 . 0 0 PYRITE 1 0 0 . 0 0 TABLE XI ESTIMATED CHEMICAL COMPOSITION OF ROCK SAMPLES FROM THE TROITSA LAKE STOCK. MARGINAL ZONE INTERMEDIATE ZONE SAMPLE NUMBER OXIDE 2 01 8 6 241 144X 212 211 144R 185' 2 2 5 144W 187 C2 144 T s±o 2 58.1 5 8 . 4 6 0 . 5 61. 9 6 3 . 4 64.2 6 4 , 6 6 4 , 6 6 5 . 3 6 5 . 5 6 5 - 5 6 5 . 9 6 6 . 3 A l 0 18 . 6 17. 9 18 . 9 16. 2 19.1 17,1 1 6 , 5 - 1 6 . 7 15 -7 16.8 17-0 15. 3 17 . 7 F e 2 ° 3 4 - 3 7 3 . 7 0 2 . 7 9 2. 8 8 2. 6 9 2 . 6 5 2 . 6 3 2 . 5 6 3.11 3.13 2. 5 7 3 . 0 8 1. 50 FeO 4 . 0 8 4.26 3.19 5.90 1. 8 9 2. 82 3,00 2 , 6 9 2. 9 2 3.11 3 . 31 2; 28... 1. 81 MgO 1. 9 7 2.16 1. 6 5 1 . .97 0. 5 2 1 . 1 7 1 . 5 4 1 . 3 4 1 . 4 8 0. 6 0 1. 3 7 0. 8 3 CaO 6 . 62 6 . 00 5 . 6 4 2. 8 3 5.13 4 . 4 8 4 . 3 4 4.61 3 - 9 8 3 - 5 8 3 . 4 3 3 . - 3 7 4.09 Na 20 4 . 4 9 4.10 4 . 5 7 4 . 0 7 4 . 81 3 . 7 6 4 - 2 3 4,13 4.21 4 , 0 6 4 . 0 5 4 . 31 5 . ©4 K 20 1.16 2 . 4 3 2.26 2 . 7 7 2 . 0 5 3-, 3 4 2. 6 6 2 . 5 0 2. 8 9 2 . 7 9 3 . 2 5 3.41 2 . 3 3 H 20 0. 3 0 0. 3 8 0 . 2 7 0. 6 9 0.12 O . 2 5 0 . 2 5 0. 22 0.22 o. 30 0. 61 0. 20 0.15 P 2 ° 5 0. 0 9 0.14 0.15 0 . 0 5 0.10 0 . 0 5 0.10 T i 0 2 0 . 2 9 0 . 4 5 0.28 0. 71 0.14 0 . 2 7 0.26 0. 22 0 . 2 5 0.19 0.18 0.18 CaF 2 0. 02 0. 03 0. 03 0.01 0.02 0. 01 0. 02 co 2 0. 09 0.18 ' » FeS 2 0 . 3 7 LARSEN INDEX 3 . 9 5 5 . 9 9 9.50 10. 3 13. 3 14. 0 13.1 13 . 3 13. 6 1 4 . 6 16.2 15.2 16 . 4 OD TABLE XI CONTINUED CORE ZONE SAMPLE NUMBER OXIDE 1 9 5 1 4 6 1 8 3 2 32 1 4 5 1 8 1 . 144V s i o 2 6 6 . 4 6 6 . 6 67,;. 1 67...I 6 7 . 9 6 8 . 1 7 6 . 5 A l 2 ° 3 1 6 . 3 1 6 . 9 1 6 ; 1- 1 6 . 9 I5 . .7 1 5 . 6 1 2 . 9 F e 2 ° 3 2 . 01 2 . 51 2 . 1 1 1 . 3 7 2 . 3 8 I . 9 8 O .85 FeO 2 . 4 8 3 . 7 2 2.26 1 . 5 6 2 . 1 4 2.19 0 . 56 MgO 1 . 1 5 1 . 1 3 0 ; 92 0 . 7 1 1 . 1 3 0 . 1 0 CaO 3 . 7 5 2 . 8 7 3 . 9 7 . 4 . 1 8 3 . 4 7 4 . 1 5 1 . 38 Na 2 0 3 . 7 5 4.3-6 4 . 0 4 4 . 9 2 4 . 4 9 3 . 97 3 . 5 8 K 2 0 3 . 6 4 2 . 4 3 2 . 90 2 . 4 5 2 . 5 4 2 . 4 4 4 . 1 1 H 2 0 0 . 2 1 0 . 6 5 0 . 1 8 0 . 1 4 0 . 1 4 0 . 1 8 0 . 0 2 P 2 ° 5 0 . 0 5 . - 0 . 1 0 0 . 1 0 T i 0 2 0 . 26 0 . 1 9 0 . 3 3 0 . 5 0 0 . 35 0 . 0 3 CaF 2 0 . 01 0 . 02 0 . 02 C 0 2 FeS, LARSEN INDEX 1 6 . 7 : 1 5 . 9 1 6 . 1 I M 16. 9 1 6 . 0 26. 8 average mineralogy, i s given i n Table 4. The data have been used to estimate the pressure and temperature conditions under which the stock c r y s t a l l i s e d . Contact Metamorphism. The stock intrudes andesitic and d a c i t i c volcanic rocks of the Hazelton Group which have been hornfelsed f o r a d i s -tance of 200 feet or more from the contact. At the contact rocks of the Hazelton Group are r e c r y s t a l l i s e d to form a f i n e -grained l i g h t grey rock of d i o r i t i c appearance. Farther from the contact the rock i s a hard r e c r y s t a l l i s e d andesite or dacite with a t y p i c a l hornfelsio texture and has a bleached colour compared to the unaltered rock, especially f o r a d i s -tance of 2mm. on eit h e r side of the many fractures which cut t h i s rock. Poor exposure of the contact zone prevented the c o l l e c t -ion of an adequate suite of samples f o r a detailed t h i n sec-t i o n study of the thermal metamorphic e f f e c t s of the stock. A t h i n section of a specimen (221) immediately adjacent to the contact shows the rock to consist of 10 to 15 per cent b i o t i t e , which i s commonly altered to c h l o r i t e , i n a fine-grained i n t e r -locking matrix of quartz, untwinned plagioclase,and a l i t t l e a l k a l i feldspar. The plagioclase composition was estimated from the separation of the 20(131) - 20(131) and the 29(132) -20(131) r e f l e c t i o n s using the X-ray d i f f r a c t i o n method of Smith (1956) and i s An . This method involves the assumption that 27 the plagioclase i s of low s t r u c t u r a l state; i f t h i s i s v a l i d an accuracy of _2 per cent anorthite i s claimed by Smith. The mineralogy of t h i s rock i s not that expected from an andesite or dacite and may be derived from an a r g i l l i t e i n the Hazelton Group succession. A specimen (144Y) c o l l e c t e d 2 0 0 feet from the contact consists of i r r e g u l a r quartz grains and a l i t t l e plagioclase (An 2g) i n a heavily s e r i c i t i s e d matrix. A c t i n o l i t e i s developed along fractures i n the rock. It i s not possible to estimate the grade of thermal meta-morphism from these l i m i t e d data, however the mineralogy i s consistent with metamorphism of a l b i t e - epidote - hornfels or of hornblende - hornfels fa c i e s grade, which would be expected from an epizonal stock of t h i s type. Lovering ( I 9 3 6 , 1 9 5 5 ) and Jaeger ( 1 9 5 7 , 1 9 5 9 ) have sugges-ted models f o r c a l c u l a t i n g the heat flow from an intrusive body The contact temperature of an int r u s i v e body can be calculated using the expression developed by Jaeger ( 1 9 5 7 ) and modified by Buseck ( I 9 6 6 ) to contain a correction f o r the geothermal grad-i e n t . The expression does not take into account the e f f e c t of lat e n t heat of c r y s t a l l i s a t i o n , but i t has been suggested (Lovering, 1 9 3 6 ) that t h i s may be negated by the cooling e f f e c t of water vapour released from the magma which i s also not taken into account. The expression states that the contact temper-ature, Tc, i s : ( K k i ) :LiL_ (Tm - T o ) ( K 0 k l > + T C ( ICjk* ) - + 1 ( K k 5 ) v 0 1 ' where k Q and k 1 are the d i f f u s i v i t i e s and K Q and are the con d u c t i v i t i e s of the intruded rock and the intrusive respect-i v e l y ; T in i s the s o l i d i f y i n g temperature of the magma and To i s the temperature of the intruded rocks. I t must be noted that the above model assumes that there i s no convection i n the magma, which i s probably not the case. Convection would have the e f f e c t of increasing the contact temperature. Assuming a country rock temperature (Tc) of 120°C. (the expected temperature i n the central part of the epizone assum-ing normal geothermal gradients), a s o l i d i f y i n g temperature o of the i n t r u s i o n of 820 C. (the temperature f o r granodiorite used by Buseck), and using the constants l i s t e d by Lovering (1936) (using the values f o r d i o r i t e f o r the stock and f o r pyroxene d i o r i t e f o r the country rock) the calculated tem-perature at the contact of the Troitsa Lake stock i s 6l5°C. It i s emphasised that because of assumptions made i n the model t h i s figure can only be taken as an approximate i n d i c a t i o n of temperature conditions at the contact of the stock; however i t suggests that thermal metamorphic conditions at the contact of the stock were of upper hornblende hornfels f a c i e s , and may have approached pyroxene hornfels f a c i e s . The heating of country rocks away from the contact has been calculated f o r an i n t r u s i o n with the form of an i n f i n i t e plate of uniform thickness assuming that heating of the country rock i s e n t i r e l y due to conduction. (Lovering, 1936, 1935; Jaeger, 1957, 1959). According to t h i s model the temperature of the country rock i s increased by 50 per cent of the temper-ature of the i n t r u s i o n at a distance equal to one tenth the thickness of the in t r u s i o n . At a distance equal to half the thickness of the i n t r u s i o n the temperature of the country rock i s increased by one t h i r d the temperature of the i n t r u s -ion (the heating e f f e c t of a plate of thickness 1 0 , 0 0 0 feet i s shown gra p h i c a l l y i n Figure 2 3 ) . This model can not be d i r e c t l y applied to the T r o i t s a Lake stock as i t i s l i k e a v e r t i c a l cylinder i n form and not a plate. An approximation of the heating e f f e c t of an i n t r u s i o n i n the form of a c y l i n -der can be obtained i f i t i s assumed that the same volume of magma i n a cylinder w i l l heat the same proportional volume of country rock to the same temperature as i t would i n the form of a plate. In t h i s case the country rock would be heated by 50 per cent of the temperature of the i n t r u s i o n at a distance equal to one tenth the radius of the cylinder and by one t h i r d of the temperature of the i n t r u s i o n at a distance equal to two f i f t h s the radius of the cylinder. The approximate heating e f f e c t of a cylinder of radius 5 , 0 0 0 feet - the size of the Tro i t s a Lake stock - i s shown graphically i n Figure 2 3. Ac-cording to t h i s , at a pressure of one k i l o b a r , hornblende hornfels f a c i e s conditions would be expected f o r a distance of approximately 4 0 0 feet from the contact of the stock, and a l -bite - epidote - hornfels conditions would be expected f o r a further 1 3 0 0 feet. The time during which the country rocks are sustained at nearly maximum temperature i s proportional to the square of th thickness of the i n t r u s i o n i f i t i s i n the form of a plate ( 1 0 0 , 0 0 0 years f o r a plate of thickness 1 0 , 0 0 0 f e e t ) , but heat 5 300 P Y R O X E N E H O R N F E L S F A C I E S H O R N B L E N D E H O R N F E L S F A C I E S Jo., A L B I T E - E P I D O T E X . H O R N F E L S F A C I E S PRESSURE - 1000 bars T c 1000 2000 3 0 0 0 DISTANCE FROM THE CONTACT IN F E E T F I G U R E 2 3 . HEATING OF COUNTRY ROCK ADJACENT TO AN INTRUSIVE BODY OF GRANO-DIORITE COMPOSITION, C A L C U L A T E D ACCORDING TO LOVERING (1936,1955), JAEGER (1957, 1959) AND BUSECK (1966). INITIAL TEMPERATURE OF THE COUNTRY ROCK (Tc) IS TAKEN AS I 2 0 ° C . TEMPERATURE OF CONTACT METAMORPHIC FACIES BOUNDARIES IS SHOWN BY SOLID LINES, ALTHOUGH THIS IS APPROXIMATE AND MAY VARY BETWEEN LIMITS SHOWN BY THE DASHED LINES. l o s s from a stock i n the form of a cylinder would be more rapid and the period of heating caused by the T r o i t s a Lake stock must have been much shorter than t h i s . Level of Emplacement of the Stock. Buddington (1959) has c l a s s i f i e d the l e v e l of emplacement of g r a n i t i c stocks into three main categories : the epizone, the mesozone, and the catazone. The epizonal plutons are thought to be emplaced at depths of l e s s than four or f i v e miles, the mesozonal plutons at depths between f i v e and nine miles, and the catazonal plutons at depths greater than seven miles and as deep as twelve miles. Buddington has emphasised that the c h a r a c t e r i s t i c s of the plutons are dependent on other environmental factors as well as depth and there can be no s t r i c t depth c o r r e l a t i o n f o r the zones. A l l evidence indicates that the Troi t s a Lake stock was emplaced at a shallow l e v e l i n the crust and belongs to the epizonal class of plutons, as defined by Buddington. The fea~ tures c i t e d as being t y p i c a l of epizonal and mesozonal plutons by Buddington are summarised i n Table 12 and t h e i r presence or absence i n the T r o i t s a Lake stock i s indicated. The table shows that a l l features of the T r o i t s a Lake stock are t y p i c a l of the epizone. Features t y p i c a l of the mesozone, such as the presence of a primary magmatic f o l i a t i o n or l i n e a t i o n , are lacking. Not a l l epizonal features are developed i n the stock; f o r example, no m i a r o l i t i c c a v i t i e s were observed, but these develop only when the magma i s saturated with f l u i d s . The features present c l e a r l y indicate that the T r o i t s a Lake stock was intruded into 91 TABLE XII C r i t e r i a , suggested, by Buddington-(1959), as i n d i c a t i v e of  epizonal and mesozonal intrusions, and t h e i r presence (y)  or absence (X) in, the T r o i t s a Lake__stQck. _ Epizonal Features. . a) Largely or wholly d i s -cordant to the country rock. Of composite character. Roof pendants common. Absence of primary f o l -i a t i o n or l i n e a t i o n . Associated ge n e t i c a l l y r e l a t e d volcanics i n stocks of the upper part of the epizone. Country rocks not regi o n a l l y metamor-phosed. g C h i l l zones at the contact. Presence of late-stage aphanitic or porp h y r i t i c dykes. Associated lamprophyre dykes. A p l i t e veins and dykes present but not abundant. Occurrence of a l a s k i t e . M i a r o l i t i c structure. Occurrence of granophyre Presence of explosion breccia. y X X y X y y J J X X X Mesozonal Features. a) Degree of .metamorphism of country rocks not more__intense than green- J c h i s t or epidote-amphibolite f a c i e s . b) Stocks always composite. X c) Complex emplacement. . X relationships.to the country rock - i n part discordant, and i n part concordant.. d) Planar f o l i a t i o n often X well developed. . e) Assimilation may be X s i g n i f i c a n t i n border or roof zones... f ) Wallrocks at the.contact X may have a s c h i s t o s i t y p a r a l l e l to. the contact. g) Country rocks may be X deformed by outward pressure from the pluton. h) Contact metamorphic X aureoles may be well developed. the epizone. Chemical V a r i a t i o n of the Stock. The chemical composition of the rocks from the stock has been calculated from the modal analyses as described i n the introductory section. The major oxide constituents of the rocks (SiO , A l 0 , CaO, Na_0, K 0, and MgO + FeO + 0.9Fe 0 ) z z 3 z z 2 3 have been plotted on Larsen v a r i a t i o n diagrams (Larsen, 193 8) and are presented i n Figure 24- The Larsen v a r i a t i o n diagram plots the chemical constituents against the Larsen Index (l/3Si0 + K 0 - MgO - FeO - 0.9Fe 0 - CaO) which i s taken 2 2 2 3 to represent the stage of d i f f e r e n t i a t i o n of the rocks. This index number ranges from 4 to 17 for the rocks from the stock and i n the a p l i t e dykes, presumably the f i n a l stage d i f f e r e n -t i a t e s of the stock, i t i s 27• This shows quite a large com-po s i t i o n a l v a r i a t i o n i n the stock and the v a r i a t i o n diagrams indicate that t h i s i s systematic. S i l i c a shows a steady i n -crease from 58 to 76 per cent with increasing d i f f e r e n t i a t i o n of the stock, and potash shows a s l i g h t tendency to increase, from two to three per cent. Magnesia and i r o n show a sharp decrease from a t o t a l content of over 10 per cent to less than 2 per cent, and lime and alumina also show a steady decrease. Soda appears to remain constant. These trends are as expected i n a d i f f e r e n t i a t e d acid igneous rock. The stock has an a l k a l i - lime index of 56.5 (see Figure 25) which places i t i n the calc - a l k a l i group (Peacock, 1931). The d i f f e r e n t i a t i o n trends of lime and the a l k a l i e s can be seen i n t h i s diagram. 93 F I G U R E 24 . continued -s2 " 0 C A L C U L A T E D REGRESSION EQUATIONS FOR T H E DATA 9 5 % CONFIDENCE LIMITS FIGURE 24. VARIATION OF THE MAJOR OXIDES IN THE ROCKS OF THE TROITSA LAKE STOCK WITH RESPECT TO THE LARSEN INDEX. ALKALI CA L CIC CALC -ALKALI C A L C I C FIGURE 25. ALKALI - LIME INDEX FOR THE TROITSA L A K E S T O C K . The data have also been plotted on an FMA ternary d i a -gram (a f t e r Nockolds and A l l e n , 1953) showing the r e l a t i o n -ship between lime, potash, and soda; and also the r e l a t i o n -ship between magnesia, t o t a l i r o n (FeO + 0.9Fe 0 ) and t o t a l a l k a l i content (Figure 26). The plot shows that i n the course of the chemical d i f f e r e n t i a t i o n of the stock CaO decreases r e l a t i v e to K 0 while Na 90 remains constant. It also shows that the t o t a l i r o n content decreases markedly as the a l k a l i content increases and that magnesia tends to follow i r o n show-ing a steady decrease with increasing a l k a l i content. Figure 27 i s another FMA plot f o r the Troi t s a Lake rock seri e s on which trends of other d i f f e r e n t i a t e d rock ser i e s are presented f o r comparison (after Yoder and T i l l e y , 1962). The d i f f e r e n t i a t i o n trend of the Troi t s a Lake stock i s one of a l k a l i enrichment t y p i c a l of the calc - a l k a l i trend and p a r a l l e l s other calc - a l k a l i series shown i n the diagram. It must be pointed out that since average analyses were used f o r the ferromagnesian minerals i n c a l c u l a t i n g the chemical com-position of the rock there i s no good control on the FeO, Fe 0 , and MgO content. The true composition of the b i o t i t e 2 3 and hornblende may d i f f e r s i g n i f i c a n t l y from the average com-po s i t i o n used, and also, as i s seen i n other d i f f e r e n t i a t e d plutons, the i r o n and magnesium contents of these minerals may vary systematically as d i f f e r e n t i a t i o n proceeds. Chemicals analyses of some b i o t i t e s from the stock indicate that they are i n f a c t r i c h e r i n magnesium and poorer i n i r o n than the average compositions used and the Mg/Mg + Fe increases i n the l a t e r VARIATION DIAGRAM FOR ROCKS OF THE TROITSA LAKE STOCK, (offer Nockolds and Allen, 1953 ) 1 TROITSA LAKE 2 — SKAERGAARD LIQUIDS 3 THOLEIITE SERIES 4 CASCADES 5 — • CALC ALKALI SERIES 6 HAWAIIAN ALKALI SERIES 7 OSLO DISTRICT F I G U R E 27. AN F.M.A. TRIANGULAR PLOT FOR ROCKS OF THE TROITSA LAKE STOCK. FRACTIONATION TRENDS OF SOME OTHER MAGMA SERIES ARE SHOWN FOR COMPARISON . (after Yoder and Tilley, 1962) d i f f e r e n t i a t e d rocks. The other ferromagnesian minerals ( i . e . hornblende) probably also have a s i m i l a r high Mg/Fe r a t i o . The true d i f f e r e n t i a t i o n trend, therefore, probably l i e s a l i t t l e to the right of where i t i s shown i n the d i a -gram, however the general trend indicated s t i l l holds true. Conditions of C r y s t a l l i s a t i o n of the Stock. The compositions of selected b i o t i t e s from the stock (see Chapter V) appear to be more magnesium-rich i n the l a t e r d i f f e r e n t i a t e s of the central part of the stock, which i n d i -cates constant or increasing oxygen fugacity and that the melt approached saturation with regard to HO. The data f o r the Troit s a Lake stock can therefore be compared with the ex-perimental data f o r the granite system of Tuttle and Bowen ( 1 9 5 8 ) . Minimum water vapour pressures operative during cry-s t a l l i s a t i o n of the b i o t i t e (and hence the stock) have pre-viously been calculated f o r a reasonable range of temperatures (see Chapter V). These are shown diagramatically i n Figure 28 as curve a). Tuttle and Bowen ( 1 9 5 8 ) have presented a serie s of i s o b a r i c phase diagrams f o r a variety of water vapour pres-sures f o r the 'granite' system (NaA3LSi„0O- K A l S i „ 0 O - SiO - H 0) 3 8 3 8 2 2 based on experimental data and these allow the liquidus temper-atures of g r a n i t i c rocks to be estimated i f the PJJ Q conditions during t h e i r c r y s t a l l i s a t i o n are known. The liquidus temper-atures of the three specimens from which the b i o t i t e was analy-sed (specimens 86, 1 4 4 T and 1 8 1 ) have been estimated from these diagrams f o r a variety of pressures and these are also shown diagramatically on Figure 28 (curves b), c), and d), respect-100 < tr hi s P R E S S U R E bars FIGURE 28. a) CONDITIONS OF CRYSTALLISATION OF THE TROITSA LAKE STOCK. CALCULATED MINIMUM P H 2 0 OPERATIVE DURING CRYSTALLISATION OF BIOTITE FROM THE STOCK FOR A REASONABLE RANGE OF TEMPERATURES. , c) a d) INDICATED LIQUIDUS TEMPERATURES FOR THE THREE SPECIMENS FROM THE STOCK FOR WHICH THE BIOTITE WAS ANALYSED -ESTIMATED FROM THE EXPERIMENTAL DATA OF TUTTLE AND BOWEN ( I958). b) SPECIMEN 86 (FROM THE MARGINAL ZONE) c) SPECIMEN I44T (FROM THE INTERMEDIATE ZONE) d) SPECIMEN I8I (FROM THE CORE ZONE ). SEE TEXT FOR EXPLANATION 101 i v e l y ) . The i n t e r s e c t i o n of curve a) - the minimum water pressure indicated, by the b i o t i t e - and curve b) - the l i q -uidus temperature of specimen 8 6 from the marginal zone of the stock - indicates that the minimum water pressure of the stock when t h i s rock c r y s t a l l i s e d was about one kilobar. The intersections of curves c) and d) - the li q u i d u s temperatures of specimens 1 4 4 T and 1 8 1 from the intermediate and core zones of the stock respectively - indicates the minimum P when these rocks c r y s t a l l i s e d : however the magnesium enrichment of the b i o t i t e towards the centre of the stock indicates constant or increasing f and hence constant or increasing P so i t ° 2 H 2 ° i s probable that the P u n of approximately one kilobar i n d i -H 2 ° cated by specimen 8 6 was maintained during c r y s t a l l i s a t i o n of the stock. The maximum P„ „ operative during c r y s t a l l i s a t i o n of the H 2 ° stock can not have exceeded the load pressure. A l l evidence indicates that the stock was fcrmplaced i n the epizone. Budding-ton ( 1 9 5 9 ) has suggested a maximum depth of the epizone of 7 to 1 0 kilometres which implies a maximum load pressure of 2 to 3 kilobars. However the thickness of the stratigraphic succes-sion l i m i t s t h i s further. The stock i s emplaced i n the 'Red Volcanic Unit 1 of the Lower Volcanic D i v i s i o n of the Hazelton Group. Above t h i s l e v e l was deposited the 'Red Tuff Unit' (about 1 0 0 0 feet t h i c k ) , the Marine Sedimentary D i v i s i o n and the Upper Volcanic D i v i s i o n ( t o t a l thickness 8 , 5 0 0 f e e t ) ; t h i s succession i s s t i l l preserved at Chikamin Mountain about 12 miles south east of Troitsa Lake. Above t h i s were deposited up to 5 * 0 0 0 feet of Lower Cretaceous rocks (3000 feet of these 102 rocks occur at Swing Peak about 8 miles north of T r o i t s a Lake). Therefore the maximum thickness of s t r a t a overlying the stock when i t was intruded i n Upper Cretaceous times was j u s t over 14,000 fe e t . This i s equivalent to a maximum load pressure of about one and one quarter k i l o b a r s , and may have been somewhat les s i f any of the Upper Jurassic rocks of the Hazelton Group were eroded p r i o r to deposition of the Lower Cretaceous, or any of the Lower Cretaceous s t r a t a were eroded p r i o r to i n t r u s i o n of the stock. The maximum pos-s i b l e load pressure i s also shown on Figure 28. The above evidence considered together indicates the stock c r y s t a l l i s e d under P _ conditions of one to one and one quar-2 t e r k i l o b a r s and the water pressure must have approached or possibly equalled the load pressure. Figure 29 i s a reproduction of the 1000 Kg/cm i s o b a r i c phase diagram f o r the 'granite' system of Tuttle and Bowen (1958) on which the compositions of the rocks of the T r o i t s a Lake stock ( as orthoclase + a l b i t e + quartz normalised to 100 per cent) have been superimposed. This indicates that the tem-peratures of beginning of c r y s t a l l i s a t i o n of the rocks ranges o o from 850 C. at the margin of the stock to about 780 C. Specimen 144V, the a p l i t e , plots i n the area of the ternary minimum, as would be expected of a f i n a l stage d i f f e r e n t i a t e , and has an o indicated liquidus temperature of 730 C. The d i f f e r e n t i a t i o n trend of the stock i s also apparent on t h i s diagram. Summary. The T r o i t s a Lake stock was emplaced i n the epizone, thermal-QUARTZ FIGURE 29. PHASE RELATIONS OF THE SYSTEM Na Al Si 0 - K Al Si 0 -3 8 3 8 S i O , - H,0 AT A P u OF 1 0 0 0 Kg / c m 2 2 2 H20 3 (AFTER T U T T L E and B O W E N , 1958) COMPOSITIONS OF ROCKS OF THE TROITSA L A K E STOCK (QUARTZ • ALBITE * O R T H O C L A S E ) ARE SUPERIMPOSED ON THE P H A S E DIAGRAM 104" l y metamorphosing the country rocks and probably a t t a i n i n g hornblende hornfels f a c i e s conditions for a distance of ap-proximately 400 f e e t . The calculated chemical composition of the rocks i n d i c a -tes that the stock i s d i f f e r e n t i a t e d following a normal c a l c -a l k aline trend. The stock was probably emplaced at a depth of not more than four and a h a l f kilometres and was subject to a maximum load pressure of about one and one quarter k i l o b a r s . The composition of the b i o t i t e i n the stock indicates that the melt approached saturation with regard to H 20. The composition of the b i o t i t e and the experimental data on the granite system together indicate the stock c r y s t a l l i s e d at a water vapour pressure of about one k i l o b a r . The d i f f e r e n t i a t -ion trend of the stock i s also seen on the one k i l o b a r i s o b a r i c phase diagram f o r the granite system and indicates a range of c r y s t a l l i s a t i o n temperatures from 780°C. to 850°C. (with the a p l i t e dyke c r y s t a l l i s i n g at 730°C). During c r y s t a l l i s a t i o n of the stock P H 0 approached and perhaps equalled P, ,. CHAPTER VII GEOCHRONOLOGY. A sample of fresh unaltered quartz-monzonite weighing approximately 40 pounds was coll e c t e d from talus slopes i n the central part of the stock. A Potassium/Argon determin-ation was made on b i o t i t e from the sample. The sample was crushed and b i o t i t e was separated from the other material so that the f i n a l concentrate contained more than 95 per cent b i o t i t e . Samples from t h i s area show l i t t l e c h l o r i t i s a t i o n of the b i o t i t e (this would lower the potassium content). Four analyses were made of the potass-ium content of the b i o t i t e using flame photometric techniques. The argon content of the b i o t i t e was measured by mass spect-rometer using isotope d i l u t i o n techniques. The potassium and argon a n a l y t i c a l methods used at the University of B r i t i s h Columbia have been described i n d e t a i l by Northcote (1967). The r e s u l t s are presented i n Table 13. The analyses indicate an apparent age f o r the stock of + 75-7 - 2.3 m i l l i o n years, or Upper Cretaceous (Campanian) age according to the geological time scale of Kulp (1961). Radiometric age determinations have been made on several other stocks i n the Whitesail Lake map area and just to the north of i t and the Tr o i t s a Lake stock can be compared with these. The locations of radiometrically dated stocks i n the v i c i n i t y of Troits a Lake are shown i n Figure 30. These ages TABLE XIII GEOCHRONOLOGICAL DATA. Potassium/Argon determination on b i o t i t e from the Troits a Lake sto ck:-K% *40 Ar -"-40 Ar *40 Ar 40 Total Ar -5 (10 cc STP/gm) 40 K 7107 0.94 2.169 -3 4.520 x 10 7-12 7-07 7-11 x = 7.09 <ft 0.03 Constants used i n c a l c u l a t i o n of the radiogenic age K 4 0/K = 1.81 x 10 ~ 4 *e = 0.585 x 10~ 1 0 y r " 1 *fi = 4-72 x 10 -10 -1 yr Apparant age ~ -"-40 K 40 ( ) Apparent age = 75.7 - 2.3 m i l l i o n years. Mineralogy of the rock from which the b i o t i t e was extracted (modal per cent) Plagioclase (An ) 46% Hornblende Orthoclase ( 0 r g l ) 20$ B i o t i t e 3% Quartz 24% Magnetite 1% are represented diagramatically i n Figure 31 and i t appears that the stocks f a l l into two separate age groups. Several stocks, notably the i n t r u s i v e s of the Berg property f i f t e e n miles north of the T r o i t s a Lake property, have ages between 44 and 54 m.y. t y p i c a l of the small high l e v e l i n t r u s i v e s of the eastern flank of the Coast Range dated by White, Hara-jkal, and Carter (1968). Other stocks, including that of the Troitsa Lake property, appear to form a d i s t i n c t older group of i n t r u s i v e s with ages ranging from 7 0 to 85 m.y. Hutchison (1970) summarised published radiometric data on plutonic rocks of the Coast C r y s t a l l i n e Belt. The data suggest emplacement of plutonic rocks i n pulses becoming i n -creasingly frequent from Palaeozoic through Mesozoic culmin-ating i n Cretaceous and early T e r t i a r y time. Hutchison stated that K/Ar ages of the Prince Rupert - Skeena region and the Central Coast Mountains display a marked regional pattern form ing a western zone with ages of 84 - 140 m.y., a median zone with ages of 64 - 7 9 m.y., and an eastern zone with ages of 4 7 - 4 m.y. Hutchison suggests these zones may r e f l e c t the r e l a t i v e times of emplacement of the plutonic bodies but also points out that i t may r e f l e c t u p l i f t and unroofing of the zones sequentially from west to east ( i . e . sequential u p l i f t and cooling through the c r i t i c a l K/Ar 'clock s e t t i n g ' isotherm S i m i l a r episodic emplacement of plutonic rocks has been des-cribed from north central B r i t i s h Columbia and the adjacent Yukon and from south-eastern B r i t i s h Columbia by Gabrielse and Reesor (I964); from south-eastern Alaska by Loney, Brew and 1 LOCATION OF RADIOMETRIC ALLY DATED STOCKS IN THE TROITSA L A K E AREA. 109 Figure 31 s-K/Ar age dates of stocks near the Troit s a Lake stock are presented diagramatically. The age ranges of the three pluton-i c zones defined by Hutchison (1970) i n the Central Coast Moun-tai n s of the Prince Rupert region are presented f o r comparison. The age range of small high-level plutons of the east flank of the Coast Mountains determined by White e t . a l . (1968) i s also shown. K/Ar ages of stocks shown i n the diagram are as follows:-No . Name K/Ar age m.y. Location Source 1 Berg - La t i t e por-phyry dyke. 44 ± 2 53°48'N.127°45tW White(1968) 2 Berg - Quartz monzon-i t e prophyry. 48 I 2 i t t i 3 Berg - Hornfels. 53 4 3 i t 11 4 5 6 7 8 Berg - Quarz d i o r i t e . Sam Goosly. Bone Lake . Nadina Lake. Rocher de Boule. 54 ± 3 48.8^1.9 49 t 2 52.9-2.2 71.9-3.1 t i 54°ll tN.126°20»W 53°l7,N.127°00»w 53°50'N.127°00'W 55°10'N.127°35,W i t J.Harakal N. Carter (personal communi-cations) 11 9 G l a c i e r Creek. 73.3±3.4 55°46'N.127°48'W 11 10 T r o i t s a Lake. 75-7±2.3 53°32tN.127°20tw 11 Bergette. 76.7^2.5 53°46'N.127°20'W i t 12 13 Hucklebe rry. Laura Mines 82 i 3 82.4^3.1 53°41'N.127°09'W 54°2 6'N. 12 7°21 I'W 11 i t 14 Ox Lake 83.4-3.2 53 o38'N.127 o05tW i t 15 Lucky Ship 49.9-2.3 54°02TN.127°30'W i t 16 Nadina 7 4 - 2 54°05,N.126°43,W 11 110 P L U T O N I C Z O N E S O F T H E C E N T R A L C O A S T M O U N T A I N S IN T H E P R I N C E R U P E R T R E G I O N A S D E F I N E D BY H U T C H I S O N ( I 9 7 0 ) A N D T H E E A S T E R N F L A N K Z O N E O F WHIT E ( I 9 6 8 ) . A G E O F T H E T R O I T S A L A K E S T O C K A N D O T H E R S T O C K S IN T H E W H I T E S A I L L A K E M A P A R E A . a. a. < I D >-50- AST - I ERNI ONE N z LU LU z Z l - < o N < LL LU 1 OT z o 100- OT O UJ o < H Ul LT O a> CL a. o Ltr\ TROITSA LAKE STOCK 75.7 i 2.3 m.y. • 5 0 OT CC < UJ OT z o -100 UJ o < 150 -150 F I G U R E 31. Lanphere (1967); and from south central Alaska by Reed and Lanphere (1969). The age and geographical p o s i t i o n of the T r o i t s a Lake stock does not allow i t to be c l a s s i f i e d with any of the groups described by Hutchison. The T r o i t s a Lake stock and others of a s i m i l a r age i n t h i s area appear to form a separ-ate group of i n t r u s i v e s from those described by Hutchison. The age of hydrothermal a l t e r a t i o n and mineralisation of the Troi t s a Lake stock must be younger than the age of the stock, as must be the age of the feldspar prophyry dykes. Data from the Berg property shows that the age of a l t e r a t i o n and mineralisation (related to the quartz-monzonite plug) and of the porphyry dyke i s only a few m i l l i o n years younger than the age of the oldest intrusive of the property. I f a p a r a l l e l can be drawn with the T r o i t s a Lake property i t may be suggested that the age of the a l t e r a t i o n and mineralis-ation and of the feldspar porphyry dykes may be only a few m i l l i o n years younger than the stock. It was not possible to c o l l e c t an adequate sample of rock with u n c h l o r i t i s e d hydro-thermal b i o t i t e to obtain a K/Ar age date f o r the hydrothermal a l t e r a t i o n . I CHAPTER VIII 112 ALTERATION AND MINERALISATION. Introduction. Hydrothermal a l t e r a t i o n , l o c a l l y intense has affected parts of the T r o i t s a Lake property. Copper and minor moly-bdenum sulphide mineralisation i s associated with the hydro-thermal a l t e r a t i o n . A l t e r a t i o n i s mainly associated with the feldspar por-phyry suite of dykes, as are the most s i g n i f i c a n t 'showings' of copper and molybdenum mineralisation. P r o p y l i t i c a l t e r a t -ion has affected the central part of the quartz monzonite -granodiorite stock. Feldspar Porphyry Dykes. The feldspar porphyry dykes are abundant i n the proper-ty, e s p e c i a l l y i n the central part of the stock, and are probably a l l of quartz l a t i t e composition. Hydrothermal a l t e r a t i o n has affected most of them to some degree. One of these dykes, which contains copper and molybdenum s u l -phide mineralisation over part of i t ' s length, i s f o r t y feet wide and has been traced over a distance of 7 0 0 0 feet. A suite of samples from t h i s dyke (samples 2 5 8 , 1 4 4 Q , 2 4 8 , 1 9 1 and I 8 9 ) show that i t has been subject to varying de-grees of a l t e r a t i o n . The l o c a t i o n of these and other sam-ples of altered feldspar porphyry dykes are shown i n Figure 3 2 . Samples from the northern end of t h i s long dyke ( 2 5 8 QUARTZ SERICITE ALTERATION 1 9 1 < ^ BIOTITE, ORTHOCLASE ALTERATION N. QUARTZ SERICITE ALTERATION V I — f •A GLACIER / \ I * \ \ \ * PROPYLITIC ALTERATION \ \ r C \ \ \ I ) L e g e n d I GLACIER V . ^ / Outl ine o f the S t o c k — • Outline of G l a c i e r and M o r a i n e O S a m p l e locat ion and number • F e l d s p a r Porphyry D y k e s _ . A p p r o x i m a t e b o u n d a r y of h y d r o t h e r m a l a l t e r a t i o n — ' types in the f e l d s p a r porphyry dyke A. I \ S C A L E I inch represents 2000feet E===^^^o5!F='>l''^000 ft HYDROTHERMAL ALTERATION IN FELDSPAR PORPHYRY DYKES. FIGURE 32 and 144Q, see plates 11 and 12) show i t has been sub-j e c t to p r o p y l i t i c a l t e r a t i o n . The o r i g i n a l mafic minerals, b i o t i t e and hornblende, have been replaced and pseudomorphed by c h l o r i t e and epidote. To the south secondary b i o t i t e pseudomorphs the o r i g i n a l amphibole (sample 248, Plate 13). The groundmass and plagioclase phenocrysts are altered to a fine-grained brownish mater-i a l . Further south along t h i s dyke (sample 248, Plates 14 and 15) the rock i s l i g h t grey rather than pinkish i n colour. Epidote i s no longer present and the o r i g i n a l mafics are replaced by secondary b i o t i t e and an almost colourless c h l o r i t e . The plagioclase phenocrysts are now altered to s e r i c i t e and there i s an increase i n the amount of quartz i n the groundmass. Pyrite i s common and may form two to three per cent of the rock by weight. This part of the dyke apparently belongs to the quartz s e r i c i t e a l t e r -ation type as defined by Rose (1970). Further south along t h i s dyke (samples I89 and 191, Plates 16 and 17) the rocks belong to the potassic zone of a l t e r a t i o n . Secondary pot-ash feldspar occurs, often p a r t i a l l y enclosing the plagi o -clase phenocrysts. The plagioclase has a l b i t e composition. Coarse s e r i c i t e i s developed. The mafic minerals are re-placed by c h l o r i t e and secondary b i o t i t e . Pyrite, c h a l -copyrite, and minor bornite and molybdenite may occur. The clay f r a c t i o n was separated from crushed samples of the above rocks and the clay minerals present were i d e n t i f i e d using the X-ray d i f f r a c t i o n methods of C a r r o l l (1970). Several smear mounts were made from each sample. One mount 115 I Plate 1 1 : Feldspar porphyry (specimen 2 5 8 ) - P r o p y l i t i c type a l t e r a t i o n . Note the xe n o l i t h i c i n c l u s i o n of an e a r l i e r l e s s porphyritic phase. I Plate 1 2 : Photomicrograph (specimen 2 5 8 ) . ( x 3 Q ) . 1 m m P r o p y l i t i c type a l t e r a t i o n . Chlorite and epidote pseudomorphous afte r amphibole. Groundmass and feldspar phenocrysts (top r i g h t ) are replaced by clay minerals. 116 P l a t e 13: 117 I Plate 14: Feldspar porphyry (specimen 248). Quartz -s e r i c i t e type a l t e r a t i o n . Plate 15: Photomicrograph (specimen 210) (x30). 1 m m Intense quartz - s e r i c i t e a l t e r a t i o n , similar to that of specimen 248 above. I Plate 17: Photomicrograph (specimen 189) £x30). 1 m m B i o t i t e - orthoclase a l t e r a t i o n zone. Secondary potassium feldspar i n an intensely s i l i c i f i e d ground-mass. of each sample was glycolated and another was heated at 3 50 c. f o r two hours. Diffractometer traces of treated and untreated smear mounts of a l l samples were made from c° o b and 40 20 using Ni f i l t e r e d Cu radiation. The clay minerals present were i d e n t i f i e d using the data of C a r r o l l (1970). The diffractometer traces show the presence of k a o l i n i t e i n the sample from the biotite-orthoclase zone. Mica i s present i n a l l samples but i s most abundant (judg-ing from the r e l a t i v e peak i n t e n s i t i e s ) i n samples from the bio t i t e - o r t h o c l a s e zone. A l t e r a t i o n at porphyry copper deposits (Figure 33, a f t e r Rose 1970) indicates that c h l o r i t does not form i n equilibrium with the assemblage muscovite -b i o t i t e - potash feldspar which i s present i n these rocks and probably formed under l a t e r deuteric conditions. The suite of samples from t h i s dyke indicate that hydro thermal a l t e r a t i o n has greatly changed the o r i g i n a l minerals ogy of the rock r e s u l t i n g i n a sequence of zones from prop-y l i t i c through quartz - s e r i c i t e to b i o t i t e - orthoclase a l t e r a t i o n types from north to south. The source of the hydrothermal f l u i d s was apparently to the south of sample s i t e 189. Sulphide mineralisation i s associated with the hydro-thermal a l t e r a t i o n of the feldspar porphyry dykes. The most s i g n i f i c a n t 'showing' i s i n the dyke exposed between samples 189 and 191 i n the central part of the stock. This miner-a l i s e d section of the dyke i s 40 feet wide and i s exposed f o r a length of 450 feet. Chalcopyrite grains up to 0.25 A K F DIAGRAM FOR MINERALS COMMON IN HYDROTHERMAL ALTERATION AT PORPHYRY COPPER DEPOSITS, WITH SOME EQUILIBRIUM ASSEMBLAGES INDICATED. THE D O T T E D LINE S E P A R A T E S T H E BIOTITE - ORTHOCLASE AND QUARTZ - SERICITE ALTERATION T Y P E S . ALTERATION A S S E M B L A G E S P R E S E N T IN THE FELDSPAR PORPHYRY DYKES OF T H E TROITSA L A K E PROPERTY INDICATE A TREND FOR THE H Y D R O T H E R M A L FLUID IN THE MANNER SHOWN BY T H E ARROW IN T H E DIAGRAM. (After Rose, 1970) inch i n size with pyrite and minor bornite and molydbenite are disseminated throughout the rock. Chip samples taken over ten feet lengths on three traverses across the dyke indicate an average grade of 0.53 per cent copper and 0.009 per cent MoS9 over the exposed section of the dyke. The sulphide mineralisation i s r e s t r i c t e d to the dyke and does not extend f o r more than a few inches into the adjac-ent quartz monzonite. Samples taken i n the quartz monzonite f o r a distance of ten feet on either side of the dyke give an average grade of only 0.07 per cent copper and a trace of M0S2 confirming the abrupt termination of the sulphide mineralisation. Chalcopyrite occurs i n several other f e l d s -par porphyry dykes, notably a dyke i n the southern part of the stock (sample s i t e 226) which returned assays of 0.22 per cent copper over a 40 feet width. A l t e r a t i o n and mineralisation of the stock. A l t e r a t i o n of the granodiorite - quartz monzonite of the stock i s l i m i t e d . Most of the rock i s fresh and shows only a l i t t l e deuteric a l t e r a t i o n . However a l i m i t e d area i n the centre of the stock has undergone some degree of hydrothermal a l t e r a t i o n of p r o p y l i t i c type (see Plate 18). The rock i s greenish rather than pink i n colour due to the complete replacement of the mafic minerals by c h l o r i t e and some s a u s s u r i t i s a t i o n of the plagioclase. Epidote and c a l -c i t e occur along fractures i n the rock. Secondary b i o t i t e occurs along fractures i n the central part of the stock. This can not be attributed to late stage f l u i d s derived from the c r y s t a l l i s a t i o n of the stock as fracture planes with secondary b i o t i t e cut a p l i t e veins. The secondary b i o t i t e must be due to a separate l a t e r event probably associated with the hydrothermal a l t e r a t i o n and mineralis-ation of the feldspar porphyry dykes. No disseminated sulphide mineralisation occurs within the stock but sulphide minerals are found along fracture planes i n the central part of the stock i n an area 5000 feet north - south by 3 500 feet east - west. Samples of the stock excluding fracture plane mineralisation returned assay r e s u l t s of around 0.03 per cent copper. Samples with fracture f i l l m i neralisation return assay values of around 0.1 per cent copper throughout t h i s area. The mineralised fractures are usually several feet apart and are most abundant near the felds par porphyry dykes. The fracture f i l l material i s usually quartz, py r i t e , chalcopyrite and/or molybdenite forming veins up to one inch wide. Many of these veins have t h i n a l t e r a t -ion envelopes (Plate 19). Chalcopyrite and molybdenite also occur as coatings on 'dry' fractures. Some small veins con-t a i n i n g galena are found i n the central part of the stock. Other minerals found along fractures are sphalerite, c a l c i t e , rhodochrosite, epidote, c h l o r i t e , tourmaline, and s t i b n i t e . Hematite occurs along fractures near the margin of the stock. The fracture plane mineralisation follows the dominant north-northwesterly j o i n t d i r e c t i o n . Zonal D i s t r i b u t i o n of A l t e r a t i o n and Mineralisation. Many papers have been published on the hydrothermal a l -1 2 3 L Plate 1 8 : Photomicrograph (specimen l 8 3 ) . . ( x 3 0 ) . Quartz monzonite. P r o p y l i t i c a l t e r a t i o n - replace-ment of o r i g i n a l mafic minerals by c h l o r i t e and minor epidote. I 0 I 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 c m Plate 1 9 : F r a c t u r e - f i l l mineralisation, and associated a l t e r -ation envelope i n quartz monzonite. Quartz and chalcopyrite f i l l a 2mm. wide fracture bordered by a t h i n layer of secondary b i o t i t e and a zone up to 3mm. wide of secondary potash feldspar. The host rock appears to have been subject to p r o p y l i t i c type a l t e r a t i o n . t e r a t i o n associated with copper deposits of the 'porphyry copper' type, one of the most recent of these being that of Rose (1970). Rose stated that the most common hypogene min-e r a l assemblages i n porphyry copper deposits area:-1) Orthoclase - b i o t i t e - quartz. 2) Orthoclase - b i o t i t e - s e r i c i t e - quartz. 3) Orthoclase - s e r i c i t e - quartz. 4) S e r i c i t e - quartz. 5) S e r i c i t e - k a o l i n i t e - quartz. These assemblages are shown on an AKF diagram i n Figure 33-Rose c l a s s i f i e s these assemblages into two a l t e r a t i o n types. The b i o t i t e - orthoclase a l t e r a t i o n type i s characterised by the presence of b i o t i t e or orthoclase and includes assemblages 1, 2, and 3- The c r i t i c a l chemical feature of t h i s a l t e r a t -ion type i s the presence of a K /H r a t i o i n the hydrothermal f l u i d high enough to stabalise and form b i o t i t e and/or ortho-clase; and, i n the case of b i o t i t e , a Mg/H"*" rat i o high enough to avoid removal of magnesium. The quartz - s e r i c i t e a l t e r a -t i o n type includes assemblages 4 and 5- This i s characterised by the a l t e r a t i o n of potash feldspar and plagioclase to s e r i -c i t e and k a o l i n i t e . The c r i t i c a l chemical feature i s almost complete leaching of calcium, magnesium, and sodium; and l i m -i t a t i o n of the potassium content to muscovite. Rose also describes a p r o p y l i t i c a l t e r a t i o n type which i s characterised by the presence of various combinations of c h l o r i t e , epidote, c a l c i t e and a l b i t e , with l o c a l montmorillonite, s e r i c i t e , and quartz. The distinguishing chemical feature of t h i s a l t e r a t -ion type i s the addition of R^O and CO with no strong leaching of bases. The a l t e r a t i o n assemblages of the feldspar porphyry dykes can be c l a s s i f i e d according to the above c r i t e r i a (Figures 32 and 33). Because of the presence of secondary potash feldspar samples 189 and 191 belong to the b i o t i t e -orthoclase zone, sample 248 to the quartz - s e r i c i t e zone, and samples 258 and 144Q to the p r o p y l i t i c zone. This main dyke therefore shows a c l a s s i c zonal a l t e r a t i o n sequence decreasing i n grade from south to north. The inner b i o t i t e orthoclase zone i s exposed f o r a distance of approximately 500 feet, the quartz - s e r i c i t e a f f e c t s the dyke f o r approx-imately 3500 feet, and t h i s grades northwards into propy-l i t i c type a l t e r a t i o n . The approximate boundaries of the a l t e r a t i o n zones are shown on Figure 32. The hydrothermal a l t e r a t i o n generally r e s t r i c t e d to the dykes and the stock shows only l i m i t e d p r o p y l i t i c a l t e r a t i o n which i s r e s t r i c t e d to the central area. Rose (1970) and Lowell and Guilbert (1970) have des-cribed zoning of the sulphide minerals i n a s i m i l a r manner to the a l t e r a t i o n sequence. Molybdenite and bornite occur only i n the biotite-orthoclase zone, and the p y r i t e / c h a l -copyrite r a t i o i s low (e.g. 1:2 at San Manuel - Kalamazoo). The centre of t h i s zone tends to barren of sulphide minerals The quartz - s e r i c i t e zone has a greater amount of t o t a l sulphide minerals but there i s an increase i n the p y r i t e / chalcopyrite r a t i o (e.g. 10:1 at San Manuel - Kalamazoo). This zone may contain as much as 15 weight per cent of py r i t e . The 'ore s h e l l ' usually occurs i n the outer part of the b i o t i t e - orthoclase zone and i n the inner part of the quartz - s e r i c i t e zone. The p r o p y l i t i c zone has up to 5 weight per cent pyrite which may contain up to 500 p.p.m. copper. This zonal d i s t r i b u t i o n of sulphide minerals also occurs on the Troits a Lake property. Rocks i n the b i o t i t e -orthoclase zone grade over 0.5 per cent copper with ch a l -copyrite being dominant over p y r i t e . In the quartz -s e r i c i t e zone pyrite i s abundant and form 2 to 3 per cent of the rock by weight. Pyrite also occurs i n the propy-l i t i c zone, and malachite rims around some pyrite grains indicate i t does contain copper. The mode of occurrence of the sulphides i s also seen to vary. The sulphides i n the higher grade parts of the dykes are disseminated, whereas ,in the quartz monzonite they occur along fracture planes. Lowell and Guilbert (1970) state that dissemination i s t y p i c a l of the inner zones, and the vein type occurrence i s t y p i c a l of the outer zones, Low-e l l and Guilbert mention that the outermost veins also con-t a i n galena and sphalerite as i s l o c a l l y seen i n the centre of the stock. The formation of hydrothermal a l t e r a t i o n assemblages has been investigated experimentally by Creasey (1959), Hemley (1959), and Hemley and Jones (1964) .' The a l t e r a t i o n assemblage developed i s found to be dependant on the temper-ature and the K /H r a t i o (Figure 34). The zonal sequence 127 — i L o g K / H STABIL ITY R E L A T I O N S of KAOLINITE, MUSCOVITE and POTASSIUM F E L D S P A R in CHLORIDE SOLUTIONS with respect to temperature and KCI / H C I ratio, (after R o s e , I970 ) FIGURE 34. for the system K 20 - A l 2 0 g - S i 0 2 "* H 2 ° i s e s s e n t i a l l y a change outwards i n mineralogy from potash feldspar through s e r i c i t e to k a o l i n i t e r e f l e c t i n g a decreasing temperature and/or K /H r a t i o . S i m i l a r relationships hold f o r the system Na 20 - Alr^O.^ - S i 0 2 ~ H^O and the zonation from a l -bite through paragonite or montmorillonite to pyrophy-l l i t e or k a o l i n i t e r e f l e c t s decreasing temperature and/or Na +/H + r a t i o (Hemley and Jones, 1964). The zonal a l t e r a t -ion sequence as a whole therefore r e f l e c t s decreasing tem-perature and/or increasing a c i d i t y of the hydrothermal f l u i d . The s t a b i l i t y of potash feldspar and b i o t i t e r e -quires a high K +/H + r a t i o . The development of the quartz -s e r i c i t e zone involves the s e r i c i t i s a t i o n of feldspar which consumes H ions and tends to keep the K /H r a t i o high and i n the s t a b i l i t y f i e l d of the b i o t i t e - orthoclase zone. The development of the zonal sequence therefore probably involves a reduction i n temperature as well as of the K"^/H+ r a t i o . Rose (1970) and Toulmin and Clark (I967) use mathe-matical models to show that cooling of the hydrothermal f l u i d by heat exchange with the wall rock i s un l i k e l y to be the cause of the a l t e r a t i o n sequence. Rose suggests adia-bati c expansion or mixing with cool groundwater, as possible methods of cooling the hydrothermal f l u i d , e s pecially as oxygen isotope data indicates a large meteoric component to the hydrothermal f l u i d . Sequence of sulphide mineralisation. There are four distinguishable stages of sulphide min-129 e r a l i s a t i o n of the Tro i t s a Lake property. These are assoc-iated with s t r a t a of the Hazelton Group, the stock, the ap-l i t e dykes, and the feldspar porphyry dykes. The e a r l i e s t stage of sulphide mineralisation i s the disseminated pyrite and pyrrhotite i n the lower green v o l -canic un i t of the Hazelton Group. This i s presumably syn-genetic i n o r i g i n . Elsewhere i n the Smithers and Whitesail Lake map areas copper sulphide mineralisation has been found at the base of the red volcanic unit and i n the lower green volcanic unit of the Hazelton Group (Tipper 1 9 7 1 ) . The second stage of sulphide mineralisation i s related to the emplacement of the stock. Molybdenite i s l o c a l l y found coating fractures and to some extent disseminated i n thermally metamorphosed rocks immediately at the contact of the stock, and i s presumably related to i t s emplacement. The t h i r d stage of mineralisation i s occasional flakes of molybdenite associated with a p l i t e veins and dykes and presumably derived from l a t e stage f l u i d s of the stock. The fourth and f i n a l stage i s p o t e n t i a l l y the most econo-mically important. This i s the pyri t e , chalcopyrite, bornite and molybdenite disseminated i n the feldspar porphyry dykes. This mineralisation i s s p a t i a l l y and apparently g e n e t i c a l l y related to the hydrothermal a l t e r a t i o n of the dykes. Hemley et. a l . (I967) have s o l u b i l i t i e s f o r ZnS of thousands of p.p.m. i n solutions at 300 to 500°C. i n equilibrium with quartz, muscovite, and potash feldspar. S o l u b i l i t i e s i n t h i s range have been suggested by other data (Barnes and Czamanske I967). Rose (1970) suggested that a copper content of 2000 p.p.m. f o r hydrothermal solutions causing t h i s type of a l -t e r a t i o n would not be unreasonable. The close r e l a t i o n s h i p and zonal p a t t e r n of the hydrothermal a l t e r a t i o n and s u l -phide minerals has been described above. The p r o p y l i t i c a l t e r a t i o n of the quartz monzonite and the f r a c t u r e f i l l type m i n e r a l i s a t i o n of p y r i t e , chalcopy-r i t e , molybdenite, and galena i n the c e n t r a l p a r t of the stock i s probably caused by the same processes which caused the a l t e r a t i o n and m i n e r a l i s a t i o n of the dykes. The dykes appear to have acted as channelways f o r the hydrothermal ore bearin g s o l u t i o n s and the zonal p a t t e r n of a l t e r a t i o n and m i n e r a l i s a t i o n i s more pronounced i n them at the present l e v e l of e r o s i o n . CHAPTER IX SUMMARY AND CONCLUSIONS The T r o i t s a Lake property i s located i n the Whitesail Range at the south west corner of T r o i t s a Lake at l a t i t u d e 53°32 t north and longitude 127°20' west, ninety miles south of Smithers, B.C. The regional geology i s mainly 12,000 feet or more of Jur a s s i c volcanic and sedimentary s t r a t a of the Hazelton Group and t h e i r metamorphosed equivalents. These are i n t r u -ded by a variety of stocks and plutons. The property geology consists of a g r a n o d i o r i t i c stock intruded into s t r a t a of the Lower Volcanic D i v i s i o n of the Hazelton Group. The Hazelton Group rocks are probably of Lower Jurassic age. These are red and green andesite and dacite flows with in t e r c a l a t e d a r g i l l i t e lenses and an i n -creasing number of flow-breccia and tuffaceous members i n the upper part of the succession. A younger r h y o l i t e complex, half a square mile i n area, occurs to the northwest of the stock. A l l rocks on the property are cut by a variety of dykes; quartz porphyry r h y o l i t e , lamprophyre, andesite, and feldspar porphyry (quartz l a t i t e ) . Most of the dykes trend northwesterly following the dominant j o i n t pattern of the stock. The stock i s roughly c i r c u l a r i n form and i s three to four square miles i n area. The stock i s t e x t u r a l l y and com-p o s i t i o n a l l y zoned from a coarse-grained quartz monzonite i n the centre to a r e l a t i v e l y fine-grained granodiorite at the margin. The plagioclase feldspar of the stock varies systemat-i c a l l y i n composition from i n the granodiorite to A.n^Q i n the quartz monzonite and to An 2^ i n a p l i t e dykes which presumably represent the f i n a l stage d i f f e r e n t i a t e of the stock. The a l k a l i feldspar, when homogenised, show a s i m i l -ar systematic compositional change from Org^ at the margin to Or on i n the centre and less than 0r r t r. i n the a p l i t e . The oo 75 a l k a l i feldspar has an intermediate s t r u c t u r a l state, s i m i l a r to that of the orthoclase series, and t h i s shows no v a r i a t i o n throughout the stock. The compositions of the coexisting f e l d s pars indicate c r y s t a l l i s a t i o n temperatures of 720°C. to 770°C. Lack of information on the a c t i v i t y c o e f f i c i e n t of a l b i t e i n a l k a l i feldspar creates inaccuracies i n estimating c r y s t a l l i -sation temperatures from the feldspar compositions, es p e c i a l l y by Barth's feldspar geothermometer. Limited information on the b i o t i t e compositions indicate that these also vary i n composition, being more magnesium r i c h and i r o n poor i n the c e n t r a l part of the stock. The stock has thermally metamorphosed the country rocks. The temperature at the contact of the stock i s calculated to have been about 6l5°C. and conditions of hornblende hornfels f a c i e s are calculated to have affected rocks f o r a distance of about 400 feet from the contact. The stock was emplaced i n the epizone at a maximum depth of about four kilometres and was subject to a maximum load pressure of a l i t t l e over one k i l o b a r . The chemical composition of specimens from the stock have been calculated from the modal analyses and there i s seen to be a systematic v a r i a t i o n of most oxide constituents of the rocks. The stock i s shown to follow a normal c a l c - a l k a l i n e d i f f e r e n t i a t i o n trend. The compositions of the selected b i o t i t e s indicate the stock c r y s t a l l i s e d under conditions of constant or increasing fQ and that the melt approached saturation with regard to HO. The fir n calculated from the annite content of the b i o -2 n 2 t i t e for a variety of temperatures when compared with the ex-perimental data f o r the granite system indicates that the stock c r y s t a l l i s e d at a water vapour pressure of about one k i l o b a r and at temperatures of 7 3 0°C. to 8 5 0°C. At the time of cry-s t a l l i s a t i o n P H 2 0 a P P r o a c h e d and may have equalled **-]_ o a (|* Potassium / Argon data i n b i o t i t e indicate a radiometric age f o r the stock of 7 5 . 7 _ 2 . 3 m i l l i o n years ( i . e . Upper Cre-taceous) . Several other stocks i n the Whitesail Lake map area have s i m i l a r ages, forming a d e f i n i t e group with ages from 7 0 to 8 5 m i l l i o n years. Other stocks on the east flank of the + Coast Range have a d i s t i n c t younger grouping of ages of 5 0 - 6 m i l l i o n years. The porphyry dykes, and to some extent the stock, have been subject to hydrothermal a l t e r a t i o n and sulphide mineral-i s a t i o n . One major dyke shows a c l a s s i c zonal a l t e r a t i o n pat-tern from b i o t i t e - orthoclase through quartz - s e r i c i t e to p r o p y l i t i c a l t e r a t i o n types. The central part of the stock appears to have been subject to weak p r o p y l i t i c type a l t e r a t -ion. Copper and molybdenum sulphide minerals are disseminated i n the dykes i n the b i o t i t e - orthoclase a l t e r a t i o n zone and pyrite i s disseminated i n the quartz - s e r i c i t e a l t e r a t i o n zone. Pyrite, chalcopyrite, molybdenite, and other sulphide minerals are found along widely spaced fractures i n the central part of the stock. The a l t e r a t i o n and mineralisation i s thought to be caused ore bearing hydrothermal solutions with an i n i t -i a l l y high K+/H*** r a t i o apparently radiating out from the cen-t r a l area of the stock. The dykes appear to have acted as channelways f o r these solutions. The changing character of the a l t e r a t i o n i s a r e s u l t of decreasing temperature and/or + / + decreasing K /H r a t i o outwards from the source of the hydro-thermal f l u i d s . 1 3 5 LITERATURE CITEDi Armstrong, J.E. 1 9 4 4 "Smithers B.C." G. S...C... Paper 4 4 - 2 3 . _ . ... . Barnes, H.L. & Czamanske, G.K. .. 1 9 6 7 " S o l u b i l i t i e s and Transport of Ore Minerals. " i n : "Geochemistry of Hydrothermal Ore.Deposits." Ed. H.L.Barnes. Holt,..Rinehart .& Winston; New Yorkj p. . 3 3 4 - 3 8 1 . Barth, T.F.W. 1 9 6 2 "The Feldspar Geologic Thermometers." Norsk. Geol., Tidsskr. 4_2 II_.p. 330.-339.. . . . Bottinge, Y. , Kudo,. A. & Weill., D.. 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The soda r i c h p l a g i o c l a s e s . « M i n . Mag. 3_1 p . 47 - 68. ... . . . . S m i t h , J. V . & M a c K e n z i e , W. S. 1961 " A t o m i c , c h e m i c a l and p h y s i c a l f a c t o r s t h a t c o n t r o l t h e . s t a b i l i t y o f a l k a l i f e l d s p a r s . " I n s t . . " L u c a s M a l l a d a " C u r s i l l o s . C o n f . ...8 p . 39-S t e w a r t , D. B. & Roseboom, E . ...H... .1962 "Lower t e m p e r a t u r e t e r m i n a t i o n s o f t h e t h r e e - p h a s e r e g i o n P l a g i o c l a s e - A l k a l i F e l d s p a r - L i q u i d . » J. P e t . .3 p . 280 - SIS. . . . -S t u a r t , R. A . 1955 " G e o l o g i c a l s e t t i n g o f t h e A l c a n t u n n e l . " P r o c . G e o l . A s s n . Can . 7_-P.10_2 - 112. _ ... T i l l i n g , R. I . 1968. "Zonal d i s t r i b u t i o n s o f . v a r i a t i o n s i n . t h e s t r u c t u r a l s t a t e o f a l k a l i f e l d s p a r w i t h i n t h e Radar C r e e k P l u t o n , B o u l d e r B a t h o l i t h , M o n t a n a . " J. P e t . 9 p 331 - 357 139 Tipper, H. W. 1955 "Nechako River, B.C." G. S. C. Paper 54-11. .. . Tipper, H. W. 1959."Revision of the Hazelton and Takla groups of central B.C." G.S.C. B4S0.1. 47. . ._ Tipper, H. W. 1971 "Lower Jurassic volcanic rocks of the west half of the Smithers map area B.C." . G.S.C«.Information Release., Toulmin, P. & Clark,. S.p. . 19-67. "Thermal aspects.of Ore Formation." i n : '.'Geochemistry of Hydrothermal Ore Deposits. " Ed. H. L. Barnes. Holt, Rinehart & Winston. New York. ... , _. _. Tuttle, 0. F. & Bowen, N. L. 1958 "Origin of Granite i n the l i g h t of experimental studies i n the system NaAlSi.Oo KAlSi»0 s - S i 0 2 - H20." 6 G.S.Ar Hem. 74. .... . . ... . Van Der Plas, L. & Tobi, A.C. .19.65 "A. chart for judging the r e l i a b i l i t y of point counting r e s u l t s . " .A.J. S. 263 p. 87 - .90. . . White, W. H. , Harakal, „J._ E. & Carter, N. C. 1968 "Potassium-Argon ages of some ore deposits i n B r i t i s h Columbia." -Trans. C.I.M.M. 71_ p. 363...-_371 • . ' _ _ Wones, D. R. & Eugster,. H, p'. 1965 " S t a b i l i t y of B i o t i t e : Experiment, Theory, and Application".**-" Am. Min. 50...p. 1228..- 1272. „ _ ... . . Wright, T. L. 1968 H.X-Ray"and.optical study of .alkali feldspar. I I . An.X-Ray method for..determining the compos-i t i o n and ,structural state from measurement of 2 9 values for three r e f l e c t i o n s . " Am. Min. 53 p.1228 - 1272. Wright, T. L. & Stewart, D. B. 1968 "X-Ray and.optical study of a l k a l i feldspar. I. Determination of composition and s t r u c t u r a l state from refined unit c e l l parameters and 2V.» Am. Min. 53. p. .3.8.. - 87, Yoder, H. S. , Stewart, D. B. & Smith, J. R.. 195.6 "Feldspar Investigations." . • Ann. Rept. Dir. Geoph. ..Lab. Year. Book 55-Yoder, H. S. & T i l l e y , C..E. 1962 "Origin of Basalt Magmas: An experimental study of natural and synthetic systems." J. Pet. 3. 140 APPENDIX I X-RAY DIFFRACTION INVESTIGATION OF ALKALI FELDSPAR The a l k a l i feldspars were investigated using X-ray d i f -f r a c t i o n methods. The rock was crushed and sieved and the a l -k a l i feldspar was separated from the 80 - 120 mesh f r a c t i o n using a combination of electromagnetic and heavy l i q u i d methods. Magnetite and other strongly magnetic minerals were f i r s t re-moved from t h i s size f r a c t i o n with a hand magnet. The remain-ing material was passed through a Franz electromagnetic sep-arator set at maximum amperage to remove mafic minerals and any strongly altered feldspar. The remaining f r a c t i o n was then pro-cessed by separation i n bromoform diluted with acetone to a s p e c i f i c gravity such that an orthoclase c r y s t a l f l o a t e d and a quartz and a plagioclase c r y s t a l sank i n the mixture. The ' f l o a t ' f r a c t i o n from the f i n a l separation consisted of a l k a l i feldspar with minor amounts of adhering quartz. The 'sink' f r a c t i o n consisted mainly of discrete quartz and plagioclase grains, composite grains and non-magnetic mafic minerals. Smear mounts were made of each sample and X-ray d i f f r a c t -ion patterns were run f o r each using potassium bromide (KBr) as an i n t e r n a l standard. The runs were made using N i - f i l t e r e d Cu rad i a t i o n from a 26 angle of 57° to 20° at a scan speed of l°2ft per minute and a chart speed of 2cm. per minute. The X-ray d i f f r a c t i o n data show that the separation procedures described above give a clean separation of a l k a l i feldspar; no plagioclase peaks were found although a l i t t l e quartz was present i n a few samples. The feldspar peaks present were measured, and indexed 141 using the tables of Wright and Stewart (1968). Following Wright and Stewart the peaks have been measured at the p o s i -t i o n of maximum peak height except i n uncertain or ambiguous peaks where the peak was measured at the half width at two th i r d s peak height p o s i t i o n . A l l samples were found to be p e r t h i t i c and to possess a K-rich and a subordinate Na-rich phase. For a l l samples the position of the (201) r e f l e c t i o n s of both phases and the (060) and (204) r e f l e c t i o n s were mea-sured and the orthoclase and a l b i t e content of the two phases has been estimated from the position of the r e f l e c t i o n s accord-ing to the method of Wright and Stewart (1968). The r e s u l t s are presented i n Table 14)• To estimate the bulk composition of the a l k a l i feldspar each sample was homogenised by heating i n a s i l i c a glass tube at 950°c. f o r two days. Two diffractometer traces were made for each sample between 24° and 20° 26 using N i - f i l t e r e d Cu rad i a t i o n . A l l samples showed only a single (201) r e f l e c t i o n on the d i f f r a c t i o n patterns i n d i c a t i n g that homogenisation was complete. The po s i t i o n of the (201) r e f l e c t i o n was measured using potassium bromide (KBr - ( i l l ) r e f l e c t i o n at 23.35°26 CuK=*) as an i n t e r n a l standard. The orthoclase content of the a l k a l i feldspar was estimated from the pos i t i o n of the (201) r e f l e c t -ion. The X-ray d i f f r a c t i o n data f o r the homogenised a l k a l i feldspar i s given i n Table 15. The maximum deviation of the measured position of the (201) r e f l e c t i o n of the two traces i s o 0.02 26, i n d i c a t i n g that the estimated composition i s accurate + to within „2 weight per cent orthoclase. TABLE XIV 142 ALKALI FELDSPAR - X-RAY DIFFRACTION DATA. o H <s E-i O W o W H H w iz; o si o o APLITE DYKE 1 1 i POTASSIC PHASE SODIC PHASE 1 SAMPLE Est.Or DEGREES 29 Est.Ab. DEGREES NUMBER Wt. % (201) (060) (204) Wt. % 20(201) 201 92.7 21.00 41.655 50.68 95.2 21.985 86 92.7 21.00 41.68 50.72 99.0 22.03 241 90.0 21.03 41.685 50.715 98.2 22.02 144X 87-9 , 21.0 55 41.70 50.73 97.8 22.015 212 91-0 21.02 41.715 50.745 96.9 22 .005 211 86.8 21.065 41.76 50.78 97.5 22.01 144R 83-5 21.10 41.74 50.685 100.0 22.07 185 93-4 20.99 41.73 50.75 98.2 22.02 225 96.6 20.95 41.70 50.69 99-9 22.04 144W 87.2 21.06 41.74 50.715 98.2 22.02 187 86.2 21.07 41.74 50.715 100.0 22.05 C2 86.2 21.07 41.72 50.73 100 .0 22.11 144T 87.9 21.055 41.68 50.695 100.0 22.045 195 90.0 21.03 41.715 50.685 98.2 22.02 146 89.0 21.045 41.73 50.735 96.4 22.00 183 91.0 21.02 41.74 50.685 96.9 22.005 232 93-4 20 .99 41.655 50.695 94.7 21.98 145 85.0 21.09 41.745 50.80 100 .0 22.085 181 89.0 21.045 41.695 50.69 100.0 22.065 144V 90.0 21.03 41.69 50.69 99.4 22.035 TABLE XV HOMOGENISED ALKALI FELDSPAR - X-RAY DIFFRACTION DATA. SAMPLE ESTIMATED 26 (201) NUMBER Wt. % Or. 201 21.065 87-1 w 82.8 tz; 86 21.11 w N EH 241 21.163 78.1 O •IS 144X 21.15 79.4 O 81.5 o 212 21.125 211 21.095 84.4 w 144R 21.175 77.2 o 185 21.185 76.3 tsi 225 21.14 80.2 w EH 144W 21.163 78.1 n Q 187 83.5 1 21.103 W EH C2 21.115 82.1 & H 144T 21.165 78.2 195 21.115 82.1 146 21.15 79.4 Cd o 183 21.125 81.4 Si 232 21.135 80.7 o 145 21.140 80.3 o 181 21.22 73.1 144V 21.245 71.0 APLITE DYKE APPENDIX II BIOTITE ANALYSES. Electron Microprobe Analyses. The microprobe analyses of the three selected b i o t i t e s were done by Dr. E. Ghent of the University of Calgary. The b i o t i t e samples were prepared as polished t h i n sections. I t was also attempted to mount b i o t i t e f lakes (hand picked from the +40 mesh f r a c t i o n of the crushed and seived rock) i n epoxy r e s i n cylinders as t h i s allows a more representative sample to be examined, however i t was not possible to obtain an adequate p o l i s h or f l a t surface on the f l a t l y i n g b i o t i t e flakes and the results obtained were not s u f f i c i e n t l y con-s i s t e n t . The electron microprobe analyses (see Table 16) show the amount of a l l major components of the b i o t i t e (ex-cluding water) as oxide weight per cent; however i t i s not possible to d i s t i n g u i s h between ferrous and f e r r i c i r o n and the r e s u l t s are given with t o t a l iron as FeO. Ferrous Iron Determination. Introduction. In order to calculate the oxygen fugacity operative during the c r y s t a l l i s a t i o n of the b i o t i t e i t i s necessary to know the f e r r i c i r o n content. To obtain t h i s the ferrous i r o n content was measured by wet chemical methods and the f e r r i c i r o n content was calculated by difference from the t o t a l i r o n content determined by electron microprobe. The main problem i n the determination of the, ferrous i r o n content of b i o t i t e i s the prevention of a e r i a l oxidation of the ferrous i r o n . To achieve t h i s the solution must not come into contact with a i r during decomposition of the sample Other possible sources of error are impurities i n the sample (for example, grains of magnetite included i n the b i o t i t e ) , incomplete decomposition of the sample, and i n t h i s case the small quantity of sample - a few tens of milligrams only -available f o r two of the determinations. The method used i s a modification of the Pratt t i t r a t i o n method described by Maxwell (1968, p.416). Method. The reagents used are: Sulphuric acid, 50% V/V. Hydroflouric acid, 40$, A.R. Boric acid, A.R. Diphenylamine in d i c a t o r . Standard potassium dichromate sol u t i o n . The diphenylamine i n d i c a t o r i s prepared by d i s s o l v i n g 0.2 gm. of barium diphenylamine sulphonate i n 1 l i t r e of d i s t i l l e d water. This i s mixed with 1 l i t r e of phosphoric acid. The standard potassium dichromate s o l u t i o n i s prepared by d i s s o l v i n g 1.3649 gms. of dried potassium dichromate A.R. i n 2 l i t r e s of d i s t i l l e d deionised water. 1 ml. of t h i s s o l -ution i s equivalent to 1.0 mg. FeO. The procedure i s as follows: 50 mgm. of the sample i s weighed into a Teflon test-tube The sample i s moistened with a few drops of water and 5 mis. of hydroflouric acid - sulphuric acid s o l u t i o n i s then added, the hydroflouric acid and sulphuric acid having previously been mixed i n equal proportions. The tube i s closed by a Teflon stopper which i s pierced by a hole 1/10 inch i n d i a -meter. The s o l u t i o n i s brought quickly to the b o i l by plac-+ o ing the tube i n a sand-bath at a temperature of 130 - 10 C. The vapour of the b o i l i n g acid drives the a i r out of the tube preventing oxidation of the ferrous i r o n . The s o l u t i o n i s boiled at t h i s temperature fo r 30 minutes a f t e r which time the sample i s almost completely decomposed. The only residue i s a l i t t l e s i l i c a which i s occasionally l e f t ; no magnetite i s seen i n the residue, i n d i c a t i n g that there i s l i t t l e con-tamination from t h i s source (magnetite i s extremely r e f r a c t -ory and i s u n l i k e l y to have been completely decomposed). Af t e r decomposition of the sample the test-tube i s immediat-ely plunged i n a 600 ml. beaker containing a previously boiled and cooled s o l u t i o n of 10 gms. of boric acid i n 400 mis. of deionised water, the stopper i s removed under the surface of the boric acid solution, and the hydroflouric acid - sulphuric acid mixture i s emptied into the boric acid s o l u t i o n avoiding contact with the a i r . 10 mis. of the diphenylamine in d i c a t o r i s immediately added to the solution. I t i s then t i t r a t e d with standard potassium dichromate s o l u t i o n (using a micro-burette with a capacity of 10 ml.) u n t i l the f i r s t permanent tinge of purple occurs (although the s o l u t i o n must be kept well s t i r r e d or a premature end point may occur). The end 147 TABLE XVI FERROUS IRON CONTENT OF SELECTED BIOTITES FROM THE TROITSA LAKE STOCK. Sample Numbe r, Sample weight T i t r a t i o n 100 t %FeO i n milligrams. i n ml. w 181 44.2 6.275 627.5 14.20 44.2 18.1 49.2 6.90 690 14.02 49.2 181 50.2 7-10 710 14.14 50.2 144T 33.0 4.65 465 14.09 33-0 86 25.8 3-75 375 14-53 25.8 BLANK 0 NIL 0 point may also be measured potentiometrically as described by Maxwell (1968, p.419). The ferrous i r o n content of the sample i s calculated as follows :-l o o t %FeO = w where t = volume of standard potassum dichromate used, i n ml. w = weight of sample i n mgm. Results. The r e s u l t s are presented i n Table 16. The accuracy of the r e s u l t s , where such small samples are used, i s l i m i t e d by the accuracy of the burette readings. This means that f o r a 50 mgm. sample the ferrous i r o n content can only be measured + to an accuracy of - 0.1 percent FeO, and f o r a 25 mgm. sample the accuracy i s 0.2 per cent FeO. There was only s u f f i c i e n t b i o t i t e f o r repeat analyses of one sample (specimen 181), but the r e s u l t s f a l l within these l i m i t s . Blank samples were also run, and there was seen to be no blank consumption of potassium dichromate. 

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