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A petrographic study of contact facies of granitic rocks with limestone Gabrielse, Hubert 1950

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A PETROGRAPHIC STUDY OP CONTACT 1 PACIES OP GRANITIC ROCKS WITH LIMESTONE A thesis submitted i n p a r t i a l f u l f i l l m e n t , of the requirements of the course leading to the degree of Master of Applied Science i n Geology at the University of B r i t i s h Columbia. H. Gabrielse UNIVERSITY OP BRITISH COLUMBIA A p r i l 13, 1950 ABSTRACT A petrographic study i s made of g r a n i t i c rocks which are c l o s e l y associated with c r y s t a l l i n e limestone i n north-west B r i t i s h Columbia and south-west Yukon. A description of Contact facies between the two rock types i s emphasized by the author with the aim of showing endomorphic and exomorphic e f f e c t s . It was found that the chief endomorphic ef f e c t on the g r a n i t i c rock was the development of pyroxene as the mafic mineral i n place of the usual b i o t i t e and hornblende. No change i n the composition of the plagioclase feldspar -was noted except i n specimens taken i n close proximity to limestone contacts. In these specimens the plagioclase was found to be abnormally c a l c i c . Considerable quartz, orthoclase, a c t i n o l i t e , and c l i n o z o i s i t e also mark the contact facies of the gr a n i t i c rock. R e c r y s t a l l i z a t i o n , and the development of the common skarn minerals, diopside, garnet, and wollastonite have taken place i n the limestone. The skarn minerals are r e s t r i c t e d to a narrow zone adjacent to gra n i t i c rock and along joints i n the limestone. A number of hypotheses are given with evidence f o r and against i n an attempt to explain the genesis of the various rock types. Assimilation of the schists and gneisses but not limestone is believed by the author to be the most l i k e l y hypothesis to explain the presence of the limestone inclusions in g r a n i t i c rock. It i s postulated that the pyroxene developed as a replacement or metasomatic mineral af t e r the i n i t i a l stages of the intrusion by the g r a n i t i c magma.. In conclusion the introduction of quartz and orthoclase and the development of a c t i n o l i t e and c l i n o z o i s i t e are thought to be related to a late stage of the igneous a c t i v i t y . Vancouver, B.C. A p r i l 13, 1950. Dr. M.Y.Williams Head of Department of Geology and Geography University of B r i t i s h Columbia Vancouver, B r i t i s h Columbia. Dear S i r : It gives me pleasure to submit the following thesis, A PETROGRAPHIC STUDY OP CONTACT FACIES OP GRANITIC ROCKS WITH LIMESTONE, i n p a r t i a l f u l f i l l m e n t of the course leading to the degree of Master of Applied Science i n Geology at the University of B r i t i s h Columbia. Yours t r u l y , H. Gabrielse ACKNOWLEDGMENTS The writer i s indebted to Dr. L. Dolar-Mantuani and Dr. K. Dep. Watson f o r h e l p f u l c r i t i c i s m and suggestions. Dr. R.M. Thompson gave much appreciated assistance i n X-ray and spectrographs work. F i n a l l y the writer i s indebted to Mr. J.A.Donnan f o r the making of t h i n sections used for this t h e s i s . TABLE OF CONTENTS PAGE Introduction 1 Location of Outcrop i n B r i t i s h Columbia 1 General Geology of Area 2 Relationships of Rock Types 4 Detailed Description of Limestone Inclusion 5 Description of Thin Sections "Normal" Granitic Rock Type 8 Altered Quartz Diorite 10 Summary of Rock Types 19 Altered Limestone 21 Feldspar Porphyry Dyke 23 Discussion and Conclusions 25 Hypotheses Direct C r y s t a l l i z a t i o n 30 Metamorphic Origin . 34 Replacement 36 Discussion of Feldspar Porphyry Dyke 42 Description of B r i t i s h Columbia Occurrence 44 Discussion of B r i t i s h Columbia Occurrence . 46 Bibliography 52 APPENDIX Thin Section Page 1 1A IB 2A 4 3A 12 4A 5 5A 5b 6A 12a 7A 12b 9A 12c 10A 14 10A 14a H A 17 12A 19b . 13A 50 13A 13 14A K-530 15A 15 16A 5c 18A 8 . 18A 6 19A 9 1 20A LIST OF ILLUSTRATIONS Page F i g . 1 & 2 Replacement of andeswne by orthoclase with development of myrmekite 48 F i g . 3 Sphene occurring along boundaries and cleavage i n b i o t i t e grains 49 F i g . 4 A c t i n o l i t i c amphibole grains pro-jec t i n g into pyroxene and andesine grains 49 F i g . 5 Inclusions of andesine i n pyroxene 50 F i g . 6 Pyroxene and garnet 50 Photographs of limestone inclusions 51 A PETROGRAPHIC STUDY OP CONTACT FACIES OF IGNEOUS GRANITIC ROCKS WITH LIMESTONE INTRODUCTION Much of southwestern Yukon and neighbouring north-western B r i t i s h Columbia i s u n d e r l a i n by s c h i s t s and gneisses which are i n t r u d e d by g r a n i t i c r o c k s . In p l a c e s , l a r g e i n c l u s i o n s of c r y s t a l l i n e limestone or marble which occur interbedded w i t h the s c h i s t s and gneisses are present i n the g r a n i t i c rock. Specimens from c o n t a c t f a c i e s of c r y s t a l l i n e l i m e -stone and g r a n i t i c rocks o c c u r r i n g i n two areas were examined. The l a r g e r s u i t e of samples was taken i n south-western Yukon, i n the summer of 1949 by the author. The study of these was supplemented by specimens' of a s i m i l a r type taken by K. DeP. Watson i n 1945 about seventeen miles to the south-east, i n northwestern B r i t i s h Columbia (see map i n f o l d e r ) . The Yukon occurrences w i l l be d e s c r i b e d f i r s t . L o c a t i o n of Outcrop: One of the c r y s t a l l i n e limestone i n c l u s i o n s i n q u e s t i o n was examined by the w r i t e r d u r i n g the 1949 f i e l d season and occurs i n the south-west Yukon about ttmee miles east of mile-post 103 on the Haines Road. The limestone out-crop i s at an e l e v a t i o n of about 4300 1 on the n o r t h e r l y face of the mountain b l o c k south of the Unahini r i v e r . The e l e v a t i o n of the Unahini R i v e r i s approximately 2400* i n t h i s v i c i n i t y and the mountains to the south r i s e to an e l e v a t i o n of j u s t over 2 6 0 0 0 ' . The limestone i n c l u s i o n i s one of several such bodies exposed along the northern slope of the mountain. The largest of these bodies, - about 400 feet i n plan length, 300 feet high, and 300 feet wide - occurs at an elevation of 4400 feet and about one quarter of a mile east of the one examined. Numerous smaller inclusions ranging up to t h i r t y feet high by f i f t e e n feet wide occur between elevations of 4000 feet and 4500 feet. Talus and snow makes the determina-t i o n of the relat i o n s h i p of the limestone to the other rock types somewhat d i f f i c u l t but several inclusions are completely surrounded by g r a n i t i c rock. A With regard to the dimensions of these inclusions the f i r s t figure denotes the horizontal outcrop distance, while the second figure r e f e r s to v e r t i c a l distance between the top and bottom of the outcrop. General Geology of the Area: Three main rock types are present i n the area -c r y s t a l l i n e limestone or marble, g r a n i t i c rock, and sch i s t s and gneisses of the "Yukon Group". Rocks assigned by Kindle (1949 - PP» 12-13) to the "Yukon Group" occur extensively to the east of the examined limestone i n c l u s i o n and also as pendants i n the g r a n i t i c rock to the west. These rocks, which are c h i e f l y quartz-biotite schists and quartz-biotite-feldspar gneisses, are i n contact with the g r a n i t i c rock about one and one-half miles east of 3 the area investigated. Beyond t h i s north-south-trending and steeply dipping contact the schists and gneisses outcrop to the east f o r several miles. A f a i r l y extensive pendant of schist and gneiss, the actual extent of which i s not known, occurs about one quarter of a mile west of the largest of the limestone inclusions at an elevation of about 4500 f e e t . The limestone inclusions consist f o r the most part of coarse grains of c a l c i t e , commonly strongly twinned, up to two cm. i n length. In places the limestone appears to be s i l i c i f i e d and l o c a l l y skarn minerals have been developed along j o i n t s . Contorted bedding was seen i n only one of the smaller inclusions. Marble associated with s c h i s t s and gneiss elsewhere i n the "Yukon Group", and the contorted bedding seen i n one of the small inclusions, lead to the conclusion that the limestone inclusions were o r i g i n a l l y members of the "Yukon Group" rocks. The great bulk of the outcrop along the mountain face i s medium to coarse grained, gneissic, oversaturated igneous rock containing about 10% quartz, 60% feldspar, and 30% mafics. The mafics vary from a l l b i o t i t e to a l l hornblende but i n general these minerals are present i n approximately equal amounts. In many cases s t r i a t i o n s are r e a d i l y discernable on the feldspars and from a f i e l d examination the term quartz-d i o r i t e was used f o r the rock type. The grain size of the quartz d i o r i t e varies considerably from place to place. Weathering of the rock has resulted i n the accumulation of a considerable amount of arkosic sand i n the Unahini Valley. 4 Relationships of Rock Types: The main north-south trending contact between the schists and gneisses of the "Yukon Group" to the east and the quartz-diorite to the west i s gradational. On crossing the contact from the g r a n i t i c side to the schists and gneisses the following sequence occurs: 1. Rocks of the normal medium to coarse grained g r a n i t i c type. 2. Gneissic g r a n i t i c rock with mafics well f o l i a t e d sub-parallel to the contact. 3 . Schists, gneisses, and numerous s i l l s and lenses of g r a n i t i c rock. 4 . Normal schists and gneisses. As mentioned previously the c r y s t a l l i n e limestone i s assumed to have been a part of the "Yukon Group" rocks. A limestone i n c l u s i o n on the north side of the Unahini River i s associated with some quartz-biotite-feldspar gneiss and quartz-b i o t i t e s c h i s t . Most of the inclusions on the south side of the r i v e r , however, occur without any apparent associated schist or gneiss. In the immediate v i c i n i t y of the limestone the quartz d i o r i t e has a bleached appearance. The mafic mineral i s l i g h t green and seems to make up l e s s of the bulk of the rock than i n the rock types remote from the i n c l u s i o n s . Skarn minerals are developed along a few j o i n t s i n the lime-stone and i n places a sequence - c a l c i t e , skarn minerals, altered quartz d i o r i t e , - i s evident. Detailed Description of Limestone Inclusion; 5 The limestone i n c l u s i o n examined i s roughly-e l l i p t i c a l being about 230 feet i n plan length, 120 feet high, and 200 feet wide. (See sketch map, Figure 2 ) . The mountain slope i n t h i s v i c i n i t y i s quite steep and i n places precipitous. A conspicuous feature of the outcrop i s a feldspar porphyry dyke which cuts both the limestone and quartz d i o r i t e . The dyke strikes 175 degrees and dips v e r t i c a l l y . I t i s composed of feldspar phenocrysts up to 2 mm. i n length i n a greyish-green aphanitic groundmass. The rock i s heavily rust stained and strongly fractured, weathering to somewhat rounded blocks up to a foot i n length, and i n places to a f i n e rusty sand. The contact between the dyke and the limestone i s undulatory and between the dyke and the quartz d i o r i t e f a i r l y sharp and regular. A shear zone occurs i n the quartz d i o r i t e about f i f t e e n to twenty feet east of the eastern margin of the dyke. This zone, about f i v e feet wide has the same attitude as the dyke and contains deeply weathered and rust stained quartz d i o r i t e . Limestone along the western margin of the shear i s highly brecciated. The quartz diorite-limestone contacts are very i r r e g u l a r with a dyke of quartz d i o r i t e cutting the limestone i n one place as shown on the sketch map. Along the western quartz--diorite-limestone contact skarn minerals, mainly garnet and green pyroxene, occur i n fractures or j o i n t s which s t r i k e 6 o 90 . and dip v e r t i c a l l y . The skarn zones are from three to four inches wide and the t r a n s i t i o n from c a l c i t e to skarn minerals occurs within a width of about one half inch. A t r a n s i t i o n from c a l c i t e to skarn minerals to altered quartz d i o r i t e within a width of four inches was also noted i n one place. The skarn minerals consist c h i e f l y of brown vitreous garnet up to one inch across which occurs mainly as anhedral grains. A green-vitreous f i n e grained mineral resembling diopside and a white vitreous fibrous to lamellar mineral generally occur between the garnet and the c a l c i t e , although the green mineral commonly i s found as grains completely enclosed i n the c a l c i t e or garnet. The limestone near the eastern part of the body, which i s at or near the contact with quartz d i o r i t e and the dyke, ( f i g . 2) i s altered over a width of about s i x inches to a vitreous brown s i l i c e o u s material. Along contacts with the dyke the. limestone i s altered to a whitish grey s i l i c e o u s rock. As i n the v i c i n i t y of the other limestone inclusions the quartz d i o r i t e has been altered near the limestone con-tacts to a generally l i g h t colored rock. In the macroscopic examination the main difference between the normal quartz d i o r i t e and the altered variety i s the pale green color of the mafic material i n the l a t t e r . The amount of mafic material i n the altered rock appears to be r e l a t i v e l y small but varies considerable and i s not as conspicuous as the black euhedral hornblende or the g l i s t e n i n g b i o t i t e i n the unaltered variety. The o v e r a l l grain size appears to be very l i t t l e d i f f e r e n t from that of the normal igneous.rock. The l a t e r a l extent of a l t e r a t i o n of the quartz d i o r i t e near the limestone contacts i s d i f f i c u l t to determine because of talus,and snow and because of possible i r r e g u l a r i t i e s of the limestone beneath the surface of the outcrop. The a l t e r a t i o n however, i s d e f i n i t e l y less than twenty feet i n width as the unaltered g r a n i t i c rock type occurs i n the shear zone at the eastern margin of the outcrop. The f i n e grain size and the weathered nature of the dyke make i t impossible to determine whether or not i t has been altered. One difference, however, i s the dark grey to black color of the groundmass material i n the limestone contact f a c i e s of the dyke contrasted to the l i g h t grey color.of the groundmass near the centre of the dyke. 8" Description of Thin Sections: Thin sections were made of the various rock types, p a r t i c u l a r l y of the contact f a c i e s . In a few cases i t was possible to obtain the actual contact between two rock types i n a thi n section. For the most part, however, the t h i n sections represent rock types on either side of the contact. A detailed description of the i n d i v i d u a l t h i n sections appears i n the appendix of t h i s report. The ."Normal" Granitic Rock Type (Quartz D i o r i t e ) : Thin Sections 1, l b , 17. The t h i n sections l i s t e d above were made from specimens of the ."normal", g r a n i t i c rock type occurring on the mountain mass which contains the limestone inclusions. Thin sections 1 and 17 are representative of the g r a n i t i c rock i n the immediate v i c i n i t y of the examined outcrop, while t h i n section l b was made from a specimen of rock taken about three miles south-west of the outcrop along the Haines Road. Hand specimens of the g r a n i t i c rock from numerous parts of the area were also examined. Although the grain size and amount of mafic material i s quite variable i n the various l o c a l i t i e s the ove r a l l mineral compositions of the rock remain constant. The rocks show the t y p i c a l g r a n i t i c or medium to coarse grained hypidiomorphic texture, a t y p i c a l feature being perhaps the euhedral hornblende c r y s t a l s which are very well developed i n the coarser grained v a r i e t i e s . The average grain size 9 of the minerals i s approximately 4 mm. but some of the hornblende grains are up to 7 mm. long. The mineralogical composition averages approximately as follows: about 50 - 60% feldspar, 25 - 35% t o t a l mafics (mainly hornblende and b i o t i t e ) and from 15 - 20% quartz. Accessory minerals include t i t a n i t e (sphene), apatite, magnetite, and possibly zircon. Apatite i s f a i r l y abundant and i s c h a r a c t e r i s t i c of the entire group of specimens. The main feldspar i n a l l the examined specimens varies l i t t l e from Ab^An^Q or medium andesine. In most cases the feldspars are zoned to a greater or lesser degree. A few grains show a micr o a n t i p e r t h i t i c texture but t h i s i s not a common feature i n the normal g r a n i t i c rock. Orthoclase i s present i n minor amounts and i s always present i n small i r r e g u l a r grains. The mineral occurs f o r the most part wholly within plagioclase grains but i n some cases cuts across the borders of several plagioclase grains. In t h i n section 1 7 which represents the "normal" g r a n i t i c rock near the limestone i n c l u s i o n orthoclase has extensively replaced the medium andesine feldspar. This feature however appears to be r e s t r i c t e d to the g r a n i t i c rocks close to the limestone. The amount of each of the mafic minerals i s variable with hornblende being dominant i n one case and bio-t i t e i n the next. On the average, however, the two minerals are present i n about equal amounts and show marked euhedral form. 10 A p a t i t e , as p r e v i o u s l y mentioned, i s f a i r l y abundant i n a l l of the u n a l t e r e d igneous r o c k types. The mineral occurs i n euhedral g r a i n s showing the t y p i c a l hexagonal cross s e c t i o n and elongate prisms. Zoning of the p l a g i o c l a s e f e l d s p a r s and the c h a r a c t e r i s t i c hypidiomorphic or g r a n i t i c t e x t u r e of the rock type suggest that i t i s of a normal p l u t o n i c igneous o r i g i n . A c c o r d i n g to Grouty (1932, pp.47-48) c l a s s i f i c a t i o n of the igneous rocks, i n which emphasis i s p l a c e d on the composition of the f e l d s p a r the normal g r a n i t i c r o c k type i n the area should be c a l l e d a quartz d i o r i t e . I t i s l i k e l y t h a t specimens taken i n c e r t a i n areas might show that g r e a t e r than 13^% of the f e l d s p a r i s o r t h o c l a s e . In t h i s case the rock would be termed a g r a n o d i o r i t e . I n ' g e n e r a l , however, i t i s b e l i e v e d t h a t the t h i n s e c t i o n s s t u d i e d are f a i r l y r e p r e s e n t a t i v e so that the rock t y p e - w i l l be termed a quartz d i o r i t e . A l t e r e d Quartz D i o r i t e ; T h i n s e c t i o n s r e p r e s e n t i n g v a r i o u s areas i n the a l t e r e d quartz d i o r i t e a r e : 4, 5, 5(b), 5 ( c ) , 7 ( a ) , 12, 12(a), 14, 14(a), 19, 13. Specimens of a l t e r e d quartz d i o r i t e were taken on or near the limestone c o n t a c t s and a l s o from an i r r e g u l a r dyke i n the l i m e s t o n e . The texture of the a l t e r e d quartz d i o r i t e i s some-what l i k e t h a t of the u n a l t e r e d rock a l t h o u g h most of the g r a i n s i n the former show much l e s s w e l l d e f i n e d c r y s t a l 11 boundaries. Actually a number of t h i n sections show a c r y s t a l l o b l a s t i c texture. Irregular inclusions and shreds of one mineral i n another are a common feature and i n several thi n sections the replacement of one mineral by another i s evident. The rock i s i n general medium grained but i n a few instances aggregates of f i n e r grained material occur. Very narrow f i l l e d fractures are es p e c i a l l y prevalent i n t h i n sections of the actual contact of the quartz d i o r i t e and limestone. In general the mineralogy i s more complex i n the altered g r a n i t i c rock, p a r t i c u l a r l y on the contacts, than i n the unaltered rock type. Description and Occurrence of Minerals: Mafic material: Pyroxene: The chief difference between the mineral-ogy of the altered and uhaltered g r a n i t i c rocks i s the almost complete absence of the mafic minerals hornblende and b i o t i t e i n the former. The mafic mineral i n the a l t e r e d rock i s mainly the pyroxene, diopside and the presence of t h i s mineral accounts to a large extent f o r the l i g h t e r colour of the altered rock i n the hand specimen. The diopside which occurs i n grains up to 3 nun. i n length generally shows anhedral cr y s t a l form and i n many cases contains small grains of plagioclase feldspar and quartz. Most of the grains of plagio-clase feldspar which are enclosed i n pyroxene are euhedral. The boundaries of the pyroxene with bordering plagioclase grains, on the other hand, are i n many places very i r r e g u l a r . The composition of the plagioclase inclusions i n a l l the determinable grains was approximately the same as that of the surrounding plagioclase. The pyroxene i s p a r t l y replaced i n some instances by shreds and f i b r e s of a l i g h t green amphi-bole which i s probably a c t i n o l i t e . The pyroxene i s also altered along v e i n l e t s to an amorphous k a o l i n - l i k e material and to a yellow mineral with low birefringence which i s possibly a serpentine. In some sections the pyroxene i s highly altered to a mixture of carbonate and an a c t i n o l i t i c variety of amphibole. Thin section 13 contains grains of pyroxene i n close association with and included by garnet. Some of the grains have been altered considerably along fractures to a brown, s l i g h t l y pleochroic and highly birefringent mineral. This mineral which i s fibrous i n appearance and shows p a r a l l e l extinction i s possibly t u r g i t e , a secondary iron mineral. An o i l Immersion method was used to determine the approximate r e f r a c t i v e indices of the diopside. The indices ^LU > 1 .6679 and <ri f = 1 . 6 9 0 1 indicate that the pyroxene i s close to diopside i n composition (Winchell, 1947 p. 2 2 6 ) . Maximum extinction angles which i n most cases are about 40° further suggest the mineral i s of a diopsidic composition. Plagioclase Feldspar: The plagioclase feldspar i n the altered g r a n i t i c rock i s l a r g e l y of the same composition as that of the unaltered rock, i e . medium andesine. Many of the plagioclase grains show evidence of r e c r y s t a l l i z a t i o n . Sutured borders are common and the mineral may wholly enclose remnants and small grains of other minerals. Thin sections 5b and 14a have grains of plagioclase feldspar which are considerably more c a l c i c than the normal vari e t y . In general the more c a l c i c feldspar determined as Ab^o^nyo occurs almost d i r e c t l y on the g r a n i t i c r o c k - c r y s t a l l i n e limestone contact. The grains of t h i s material which occur as a decussate aggregate are considerably smaller on the average than the less c a l c i c v ariety. Another c h a r a c t e r i s t i c of the c a l c i c plagioclase i s the r e l a t i v e l y large amount of a l t e r a t i o n as compared to the andesine type. A s l i g h t a l t e r a t i o n of the feldspar to c h i e f l y c l i n o z o i s i t e i s common. In t h i n sections 5b and 14a, however, a large part of the feldspar i s replaced by both c a l c i t e and c l i n o z o i s i t e . Orthoclase Feldspar; Orthoclase i s f a i r l y abundant i n the altered quartz d i o r i t e . This mineral occurs as small anhedral grains which are regarded as a remaining constituent of the unaltered quartz d i o r i t e . I t also occurs as small i r r e g u l a r v e i n l e t s , which appear to have cut through and replaced the plagioclase. The orthoclase commonly i s present along boundaries between plagioclase grains giving r i s e to i r r e g u l a r i t y of grain contacts. An i n s i g n i f i c a n t amount of s e r i c i t e might possibly be an a l t e r a t i o n of the orthoclase. 1 4 Quartz: Quartz, l i k e the orthoclase, i s e s p e c i a l l y prevalent i n t h i n sections representing the contact between the g r a n i t i c and c r y s t a l l i n e limestone rocks. In some places quartz encloses grains of c a l c i t e and i n others the c a l c i t e occurs i n small fractures i n the quartz. The mafic minerals are almost completely absent i n the highly s i l i c e o u s f a c i e s . Quartz grains include and cut the plagioclase feldspar and the mafic minerals. In no instance, however, was the quartz noted to have cut the orthoclase. Two ages of quartz are present; the l a t e r type encloses and cuts the e a r l i e r grains which were apparently a part of the o r i g i n a l rock type. Although the quartz i s e s p e c i a l l y prevalent d i r e c t l y on the contact between the al t e r e d g r a n i t i c rock and the limestone i t occurs i n only very minor amounts i n the altered pyroxene-bearing g r a n i t i c rock. Only a few small grains of quartz are present i n the l a t t e r rock type and these are commonly clos e l y associated with the pyroxene. Carbonate: C a l c i t e i s a common mineral i n the examined t h i n sections. On the contacts i t occurs as disseminated grains and as a fracture f i l l i n g . In places the minerals of the 15 g r a n i t i c rock are highly fractured, and. the carbonate forms the cementing material between the fragments. Small veinlets of carbonate have much the same habit as some of the ortho-clase. As might be expected c a l c i t e i s much more common d i r e c t l y on the contact than elsewhere i n the altered g r a n i t i c rock. A small amount of carbonate occurs as an a l t e r a t i o n of the pyroxene and i n the s i l i c e o u s contact facies with c l i n o z o i s i t e as an a l t e r a t i o n of plagioclase. C l i n o z o i s i t e ( Z o i s i t e ) C l i n o z o i s i t e is present i n the altered igneous rock i n variable amounts. In places i t occurs mainly as an a l t e r a t i o n product of the plagioclase feldspar, although minor amounts are associated with the pyroxene. In some instances, however, as p a r t i c u l a r l y exemplified i n thin section 15, the major amount of the c l i n o z o i s i t e occurs associated with a c t i n o l i t e i n a well-defined fracture cutting both plagioclase and pyroxene a l i k e . In general the c l i n o z o i s i t e decreases in abundance i n the specimens taken farther from the contact. The lack of cleavage makes the determination of the mineral d i f f i c u l t since i t i s very s i m i l a r to z o i s i t e . Although a few grains show i n c l i n e d extinction the l a t t e r mineral may also be present. A c t i n o l i t e ; A c t i n o l i t e occurs, as mentioned above, c l o s e l y associated with the z o i s i t e . For the most part the mineral appears to have replaced the diopside. Mutual relations 1 6 of the a c t i n o l i t e and the b i o t i t e are discussed i n the description of the occurrence of the mafic minerals as a group. It should be noted here that very l i t t l e c l i n o z o i s i t e or a c t i n o l i t e occurs;;., i n t h i n sections of the altered g r a n i t i c rock which contain only a minor,amount of quartz. Apatite: Although apatite i s abundant i n the unaltered g r a n i t i c rock i t appears to be even more abundant i n the altered variety. Euhedral grains are very common but never a t t a i n a large s i z e . Sphene: Sphene i s abundant i n a l l of the slide s and commonly occurs as a granular aggregate surrounding grains of magnetite. Some of the grains of sphene, however, show a well developed wedge shape. Both the sphene and magnetite appear to be close l y associated with the mafic minerals. The greater abundance of sphene i n the altered g r a n i t i c rock as contrasted with the unaltered variety i s marked. The relationships of sphene and b i o t i t e w i l l be discussed l a t e r . 1 7 Mafics other than pyroxene and mutual rela t i o n s h i p s ; B i o t i t e and amphibole occur i n a few of the thi n sections of rocks which apparently represent a t r a n s i t i o n zone between the altered and unaltered g r a n i t i c rock types. As a r e s u l t , i n these t h i n sections the three mafic minerals, pyroxene, amphibole, and bi o t i t e , are c l o s e l y associated. The b i o t i t e occurs i n frayed anhedral grains which show a marked reddish brown pleochroism. This pleochroism i s very s i m i l a r to that displayed by the b i o t i t e of the "Yukon Group" schists and i s unlike that of the normal g r a n i t i c variety. The c h l o r i t e , pennine, i s present as an a l t e r a t i o n product of the mica. Abundant small euhedral grains of sphene are associated with the amphibole and b i o t i t e . The mineral i s largel y concentrated along contacts between b i o t i t e grains and along cleavages i n the b i o t i t e . In some instances the sphene appears to have grown by thrusting aside the mica along cleavages. Two v a r i e t i e s of amphibole are present i n the apparent t r a n s i t i o n zone. The f i r s t of these i s a l i g h t green a c t i n o l i t i c v a r i e t y which shows weak pleochroism. The second variety is much l i k e the normal amphibole of the unaltered g r a n i t i c rock showing pleochroism from tans to dark brown. The a c t i n o l i t i c amphibole appears to be l a t e r than the mica. In t h i n section 14 the amphibole contains shreds and remnants of b i o t i t e , many of which are i n o p t i c a l continuity. Gross cutting relationships suggest further that the amphibole is l a t e r than the mica. 1 8 The pyroxene i n the t r a n s i t i o n zone appears t o be more a l t e r e d t h a t that o c c u r r i n g n e a r e r the c o n t a c t . Much of the pyroxene i s a l t e r e d to a mixture of carbonate and a c t i n o l i t i c amphibole. In s e v e r a l i n s t a n c e s assemblages of b i o t i t e , amphibole, and carbonate, occur without a s s o c i a t e d pyroxene. In these areas i t i s p o s s i b l e that complete a l t e r a t i o n has taken p l a c e . In t h i n s e c t i o n 12c the c h i e f mafic minerals are a c t i n o l i t i c amphibole and b i o t i t e . A few a l t e r e d g r a i n s of pyroxene and patches of a c t i n o l i t i c amphibole and carbonate are c l o s e l y r e l a t e d . A few i r r e g u l a r , i s o l a t e d , remnants of pyroxene i n o p t i c a l c o n t i n u i t y seem t o r e p r e s e n t r e l i c l a r g e r g r a i n s t h a t have been r e p l a c e d by p l a g i o c l a s e f e l d s p a r . Needles and l a t h s of a c t i n o l i t i c amphibole p r o j e c t i n t o the pyroxene and i n many cases i n t o the p l a g i o c l a s e . Shreds of the amphibole occur i n much of the pyroxene. As these shreds are i n o p t i c a l c o n t i n u i t y the o v e r a l l appearance suggests at f i r s t t h a t the pyroxene has r e p l a c e d the amphi-bo l e . Th i s type of s t r u c t u r e might be expected, however, as the replacement of the pyroxene by a c t i n o l i t i c amphibole would be c o n t r o l l e d l a r g e l y by the o r i e n t a t i o n of the space l a t t i c e i n the pyroxene. The r e s u l t , t h e r e f o r e , should normally be a common o r i e n t a t i o n of the amphibole g r a i n s . A cl o s e a s s o c i a t i o n of a c t i n o l i t i c amphibole and c l i n o z o i s i t e i s ev i d e n t , e s p e c i a l l y along w e l l - d e f i n e d f r a c t u r e s or v e i n l e t s . The r e l a t i o n s h i p of the pyroxene and b i o t i t e i s 19 d i f f i c u l t to determine, but the evidence suggests t h a t the mica i s contemporaneous with or l a t e r than the pyroxene. In places g r a i n s of b i o t i t e , a lthough very f r a y e d i n appearance, p r o j e c t i n t o pyroxene g r a i n s . T h i s f e a t u r e and the a l t e r a t i o n of the pyroxene suggest that the mica i s l a t e r . One might expect that the b i o t i t e would a l s o be c o n s i d e r a b l y a l t e r e d i f i t were contemporaneous or e a r l i e r than the pyroxene. Summary of Rock Types; A b r i e f summary of the prominent minerals and the s p a t i a l r e l a t i o n s of the v a r i o u s rock types i n the area i s necessary before a d i s c u s s i o n r e g a r d i n g t h e i r genesis i s undertaken. The d e s c r i p t i o n w i l l b egin w i t h the u n a l t e r e d g r a n i t i c rock and then f o l l o w through, the v a r i o u s zones to the c r y s t a l l i n e limestone as f o l l o w s ; (See s k e t c h i n f o l d e r ) - Figure 3 ) 1. Normal g r a n i t i c r o c k 2. T r a n s i t i o n zone 3. Pyroxene-bearing zone 4. S i l i c a r i c h zone 5. Ska rn 6. C r y s t a l l i n e limestone The normal quartz d i o r i t e as d e s c r i b e d p r e v i o u s l y c o n s i s t s of medium andesine f e l d s p a r , q u a r t z , b i o t i t e , hornblende, a p a t i t e , and sphene. In the v i c i n i t y of the limestone i n c l u s i o n andesine i s p a r t i a l l y r e p l a c e d by o r t h o c l a s e . The above-mentioned rock type grades i n t o the t r a n s i t i o n zone v a r i e t y i n which there i s a change of the mafic 2 0 minerals. In this zone the mafics consist of a c t i n o l i t e amphibole, diopsidic pyroxene, and b i o t i t e . The b i o t i t e d i f f e r s from that of the normal g r a n i t i c rock i n i t s frayed appearance and reddish brown pleochroism. A considerable increase i n the amount of apatite and sphene i s evident. Farther towards the limestone, the rock consists e s s e n t i a l l y of medium andesine feldspar and diopside.. Very l i t t l e quartz is present. This i s s i g n i f i c a n t i n that i t represents the only g r a n i t i c rock i n the area that has a low free s i l i c a content. Beyond th i s zone occurs a zone which i s characterized by the predominance of free s i l i c a , the high degree of a l t e r a t i o n of the plagioclase, and the presence of wollastonite. The plagioclase is more c a l c i c than the normal vari e t y . The l a t t e r zone grades into the skarn minerals, garnet, diopside, and wollastonite, and then into c r y s t a l l i n e limestone. It i s d i f f i c u l t to assign d e f i n i t e widths to the zones described above as they appear to be variable. Minerals of the t r a n s i t i o n zone occur i n the centre of the i r r e g u l a r g r a n i t i c dyke approximately ten feet from the limestone. At the western limestone-granitic rock contact the t r a n s i t i o n zone variety occurs within two feet of the contact. The wollastonite, s i l i c a - r i c h zone appears to be not more than s i x inches wide i n most cases. 21 Altered Limestone: - Thin sections 15 and 13. The chief a l t e r a t i o n of the limestone has probably been that of r e c r y s t a l l i z a t i o n . This irsr^ on.!.ys:q.n \*s3WOTLp,t:i.®i*, however, as the entire outcrop i s composed of coarsely c r y s t a l l i n e limestone or marble. Limestone occurring else-where i n the Yukon Group schists and gneisses, however, i s much f i n e r grained. The, development of skarn minerals along f i s s u r -es or joints is another a l t e r a t i o n of the limestone. Minerals determined i n the hand specimen and i n t h i n section are c h i e f l y brown garnet, diopside, wollastonite, carbonate, and quartz. The l a t e r a l d i s t r i b u t i o n of these minerals i s constant with the garnet forming the central part of the skarn bordered by diopside, wollastonite, and then c a l c i t e respectively. The t r a n s i t i o n between these minerals i s not abrupt as grains of diopside occur with and completely enclosed i n garnet. Grains of garnet up to 3 cm. in diameter are present i n the hand specimen while the diopside occurs i n grains up to 2 or 3 mm. Wollastonite bordered by the c a l c i t e on one side and diopside on the other occurs i n lamellar grains up to one cm. i n length. These grains i n general are elongated at r i g h t angles to the walls of the j o i n t s . In this section i5 the wollastonite is very much altered to carbonate giving the mineral a shred-like appearance. Diopside grains are very i r r e g u l a r and are con-centrated along the borders of the garnet grains. The diopside is embayed by and i n places wholly enclosed by the garnet. A l t e r a t i o n of the diopside i n t h i n section has occurred along 22 fractures to a mineral Which i s . possibly t u r g i t e . Aidiscuss-ion of the a l t e r a t i o n and the minerals formed will'be given l a t e r . Quartz occurs as small grains i n the c a l c i t e and i n most cases encloses minute p a r t i c l e s of c a l c i t e . -Small amounts of pyroxene are also enclosed by the quartz. Spe c i f i c gravity, x-ray (Bragg - 1937, p.157) and chemical tests suggest that the garnet i s probably close to grossularite i n composition, '^he approximate r e f r a c t i v e indices of the diopside were determined by an o i l immersion method and according to Winchell (1947 - pp.226) the compos-i t i o n of the mineral probably i s close to that of diopside i n the diopside-hedenbergite s e r i e s . The presence of some ir o n i n the diopside i s suggested i n the hand specimen by a l i m o n i t i c s t a i n associated with grains of this mineral. Thin section thirteen is of interest as i t shows a limestone-altered quartz d i o r i t e contact. This slide shows the minerals ;. occurring i n -this zone Including garnet, wollastonite, quartz, orthoclase, AbgQAnyQ. plagioclase. feldspar, z o i s i t e , and c a l c i t e . The garnet contains a large amount of inclusions of quartz, plagioclase feldspar, and diopside,and appears to have engulfed these minerals. Orthoclase and carbonate i n v e i n l e t s , however, cut across the garnet. The c a l c i c plagioclase feldspar i s fine grained and rather extensively altered to c l i n o z o i s i t e . Quartz i s extremely abundant i n this section.and has replaced the plagioclase feldspar. 23 Petrogra'phic Description of the Feldspar Porphyry Dyke; Thin sections were made representing the material of the central part of the dyke and of the contacts between the dyke with limestone and g r a n i t i c rock. Thin Sections 8, 6, 7b, 9. Thin section 8 represents the rock type from the central part of the dyke. The rock i s composed of approximate-l y 55% plagioclase feldspar phenocrysts, a few hornblende phenocrysts, and a groundmass of plagioclase m i c r o l i t e s . The plagioclase phenocrysts, which average about two mm. i n length, are f o r the most part strongly zoned and i n several instances the feldspar forming the rims is more c a l c i c than that of the central portion. One phenpcryst showing both Carlsbad and A l b i t e twinning was examined which had an outer rim of Ab^AngQ (medium labradorite) plagioclase and an inner portion of AbggAn^g (sodic andesine) plagioclase. Microlites appear to he of a medium andesine composition but the exact determination of these is d i f f i c u l t . (Wahlstrom) The microlites display a c e r t a i n amount of f l u i d a l texture as they are somewhat lineated about the phenocrysts. Hornblende phenocrysts, which are minor i n amount, and small grains of magnetite are also present i n the rock. Much of the groundmass is made up of a brownish-stained, c r y p t o c r y s t a l l i n e to glassy material. Thin section 6 represents the dyke rock near the eastern limestone contact and consists e s s e n t i a l l y of the same 24 minerals as thin section 8. One notable difference however i s the much greater abundance of euhedral hornblende phenocrysts.. Thin section 9 represents a limestone, dyke, gr a n i t i c rock contact near the eastern margin of the outcrop. (See map of outcrop i n f o l d e r ) . • Plagioclase feldspar occurs as microlites and as larger heavily fractured, unzoned, somewhat resorbed grains. The l a t t e r are commonly surrounded by microlites and cr y p t o c r y s t a l l i n e to glassy material apparently related to the dyke rock... Quartz grains are abundant and have replaced some of the larger plagioclase feldspar grains. They are in turn, however, highly fractured with c a l c i t e and dyke material occurring as the f i l l i n g material. Sphene is very abundant i n the section i n close association with grains of carbonate. Contorted shreds of b i o t i t e are quite common and appear to be associated with the dyke material. In several cases an i n c i p i e n t growth of needle l i k e grains is apparent i n the c a l c i t e near the contact with dyke material. The indices of r e f r a c t i o n of the mineral appear to be s l i g h t l y greater than that of the lower index of c a l c i t e . Small grains of s u l f i d e are f a i r l y abundant and the oxidation of this mineral has probably accounted lar g e l y to the rusty color of the weathered dyke. 25 Discussion and Conclusions: The following discussion of the problems concerning. the c r y s t a l l i n e limestone occurrence i n g r a n i t i c rock w i l l deal f i r s t of a l l and p r i n c i p a l l y with, the l o c a l i t y i n the . south-west Yukon. The f i r s t problem is that of the occurrence i t s e l f . As previously stated i t is believed that the c r y s t a l l i n e limestone or marble i s a part of the "Yukon Group", rocks. Contorted bedding seen i n another outcrop of limestone and associated schist and gneiss with similar outcrops i n the area are supporting evidence f o r this theory. Other occurrences of c r y s t a l l i n e limestone associated with gneisses, s c h i s t s , and gr a n i t i c rocks have been described by Kindle (1949, pp. 12-13) i n the Dezadeash Map Area. The question then is why the par t i c u l a r limestone incl u s i o n i n question has no cl o s e l y associated schist or gneiss. Several p o s s i b i l i t i e s concerning the present position of the limestone i n c l u s i o n might be considered. F i r s t of a l l the Yukon Group rocks may have been invaded by a gr a n i t i c magma with large blocks of limestone sinking into the magma. Secondly, the schists and gneisses presumably o r i g i n a l l y associated with the limestone, were r e a d i l y assimilated by the g r a n i t i c magma whereas the limestone, unsuitable chemically was not assimilated. Thirdly, the schists and gneisses may have been granitized. The limestone was l i t t l e affected, on the other hand,because of the great chemical change i t would have to undergo to change i t to the 26 composition of gr a n i t i c rock. The p o s s i b i l i t i e s w i l l be discussed i n the order given. The ov e r a l l s p e c i f i c gravity of the limestone would probably be s l i g h t l y less than that of the g r a n i t i c rock because of the mafic materials i n the l a t t e r . (Calculations based upon the normal g r a n i t i c rock type show this to be the case - sp. gravity of gr a n i t i c rock approx. 2.8 and of the limestone - approximately 2.7). It must be remembered, however that the s p e c i f i c gravity of a magma would be considerably less than that of the c r y s t a l l i n e rock type. Pirsson and Knopf (1946, p. 131) point out that i n the change from glass to granite a decrease i n volume of about 10 percent takes place. They show further that the volume of a magma is some-what greater than when cooled to a glass. ' In the above case, then, allowing f o r a 10 percent decrease i n volume the quartz d i o r i t e magma would have a s p e c i f i c gravity of s l i g h t l y over 2.5. This figure would be increased somewhat i f i t be regarded that the magma contained a small percent of c r y s t a l l i n e material. The difference i n s p e c i f i c gravity of the two rock types does not rule out the p o s s i b i l i t y that the limestone sank into the g r a n i t i c magma. The predominantly upward pre-ssure exerted by the magma would tend to counteract the sinking tendency of the intruded rocks to some extent. This hypothesis does not explain, however, why the limestone would sink into the magma unaccompanied by some schist or gneiss. 2 7 Shand (1943 - pp. 365-366) believes that a large block of dolomite associated with foyaite i n Sekukuniland, South A f r i c a has ac t u a l l y been floated up a distance of approximately three miles from the underlying Transvaal system through the Pretoria series and Bushveld complex to i t s present po s i t i o n . In summary, i t seems probable that the limestone inclusions are not the resu l t of blocks of limestone sinking into an enclosing magma. The other p o s s i b i l i t y is that of the assimi l a t i o n of the schists and gneisses and the chemical u n s u i t a b i l i t y of limestone f o r as s i m i l a t i o n . Lacroix (1943, pp. 103-104) describes an i n t e r e s t i n g example i n the Pyrenees i n which a gr a n i t i c mass has invaded s c h i s t s , quartzites, and limestones. He believes that the massif rose by dis s o l v i n g the sediments away, the schists y i e l d i n g r e a d i l y to di s s o l u t i o n and the limestone being more r e s i s t a n t . The r e s u l t has been that great masses of more or less altered limestone are included i n the granite. He goes on further to describe the change of composition of the granite throojfeasimilation of the limestone with the production of more abundant and more basic mafic materials and i n places more c a l c i c feldspar. The nature of the schists and gneisses i n the general area of the limestone i n the Dezadeash A,rea occurrence lends i t s e l f quite well to the above hypothesis. A study of the gneiss and schist i n the area suggests that i n o v e r a l l composition they are l i k e that of the normal g r a n i t i c rock. 28 The main d i f f e r e n c e i s i n that b i o t i t e i s the c h i e f mafic m i n e r a l . I t might be worthy of note that i n the g r a n i t i c rock near the limestone i n c l u s i o n b i o t i t e i s c o n s i d e r a b l y more abundant than hornblende. Some c o l o r l e s s garnet i s a l s o present i n the metamorphie s c h i s t s and gneisses but appears to be qui t e minor i n the ar e a . The composition of the s c h i s t s and g n e i s s i s t h e r e -f o r e i s that which would c e r t a i n l y have been f a v o u r a b l e f o r a s s i m i l a t i o n without markedly changing the composition of the g r a n i t i c magma. A s s i m i l a t i o n of the c r y s t a l l i n e limestone was a p p a r e n t l y r e s t r i c t e d as the zone of a l t e r a t i o n i n the g r a n i t i c rock around the limestone body i s l e s s than t e n f e e t i n width. Thus the' p o s s i b i l i t y of the limestone i n c l u s i o n having r e s u l t e d from the a s s i m i l a t i o n of the s c h i s t s and gneisses by a g r a n i t i c magma seems reasonable. Some movement of the limestone mass a f t e r i t s i n c l u s i o n i n the g r a n i t i c m a t e r i a l might r e s u l t i f the magma were s t i l l mobile. T h i s c o u l d presumably l e a d to a brea k i n g up of some of the limestone masses and account f o r the number of small i n c l u s i o n s present i n the ar e a . The occurrence of the marble as l e n s e s i n s c h i s t s and gneisses of the Yukon Group c o u l d , however, e x p l a i n t h e i r present d i s c o n t i n u i t y . The f i n a l stage i n v o l v i n g igneous a c t i v i t y i n the area was the i n t r u s i o n of the f e l d s p a r porphyry dyke. I t i s co n c e i v a b l e t h a t the dyke was i n j e c t e d a l o n g a zone of weakness c o n t r o l l e d by the presence of the limestone i n c l u s i o n . 29 F i e l d and p e t r o g r a p h i c evidence do not appear to support the hypothesis i n v o l v i n g g r a n i t i z a t i o n . T h i s hypothesis would i n v o l v e an a d d i t i o n and s u b t r a c t i o n of elements or ions to and from previous rock types w i t h the r e s u l t that they approximate a quartz d i o r i t e i n composition. Since limestone would r e q u i r e a great exchange of components to change i t to a g r a n i t i c rock i t might be expected to be a l t e r e d w i t h d i f f i c u l t y . No p a r t i a l l y g r a n i t i z e d or 'ghost' i n c l u s i o n s were noted i n the quartz d i o r i t e . In places l e n s e s of quartz d i o r i t e i n the s c h i s t have c o n t o r t e d s c h i s t bands adjacent t o them and give the impression t h a t they have been f o r e e f u l l y i n j e c t e d ; A p e t r o g r a p h i c study of the s c h i s t s and gneisses shows that although they are s i m i l a r i n composition to the quartz d i o r i t e the p l a g i o c l a s e i s more s o d i c than t h a t i n the v g r a n i t i c rock. The p l a g i o c l a s e g r a i n s i n the quartz d i o r i t e , moreover, show f a i r l y w e l l - d e f i n e d zoning and those i n the s c h i s t s and gneisses do not. Sequence of Events Leading to A l t e r a t i o n of G r a n i t i c Rock and Limestone: The a l t e r a t i o n of the g r a n i t i c rock has i n v o l v e d c h i e f l y a change of the mafic minerals from b i o t i t e and hornblende to d i o p s i d i c pyroxene. The q u e s t i o n a r i s e s as to whether the pyroxene i s a primary mineral i n the zone of a l t e r a t i o n , whether i t has formed as a r e s u l t of a l t e r a t i o n of b i o t i t e and hornblende, or whether i t i s simply a skarn 30 m i n e r a l . The term skarn as used i n t h i s t e x t r e f e r s to minerals which have formed d u r i n g contact metamorphism of limestone and i n c l u d e s metasomatism. The presence of w o l l a s t o n i t e i n the cont a c t f a c i e s appears to embrace the same p o s s i b i l i t i e s . D i r e c t C r y s t a l l i z a t i o n : In the f i r s t case the o r i g i n of the pyroxene might be a t t r i b u t e d to the magma be i n g e n r i c h e d i n ca l c i u m d e r i v e d from the lim e s t o n e . The a c q u i s i t i o n of lime would probably i n c r e a s e the tendency of a magma to p r e c i p i t a t e pyroxene r a t h e r than hornblende or b i o t i t e . The reason f o r t h i s l i e s i n the f a c t that the r a t i o of Mg : Ca i n pyroxene which contairs the two elements i s commonly about 1:1 whereas i n amphiboles the Mg : Ca r a t i o i s much h i g h e r or approximately 3:1. Thus an in c r e a s e i n lime would seem to fa v o u r the c r y s t a l l i z i n g of pyroxene from a magma. A study of the t h i n s e c t i o n s throws l i g h t on the nature of the pyroxene. Most of the pyroxene g r a i n s have i r r e g u l a r boundaries although some remarkably euhedral cross s e c t i o n s are prese n t . In a l l of the t h i n s e c t i o n s i n which pyroxene i s present s m a l l remnants and g r a i n s of subhedral to euhedral medium andesine f e l d s p a r and quartz occur e n c l o s e d w i t h i n pyroxene g r a i n s . R e l a t i o n s h i p s of the pyroxene to the p l a g i o c l a s e f e l d s p a r and quartz suggest that i f the d i o p s i d e were a product of magmatic c r y s t a l l i z a t i o n i t c r y s t a l l i z e d l a t e r 3 1 than other two minerals. If this were not the case i t would be d i f f i c u l t to explain the subhedral to euhedral grains of plagioclase and quartz included i n the pyroxene. A number of euhedral cross-sections of diopside, however, do not suggest that the mineral has formed i n i n t e r s t i c e s l e f t by the c r y s t a l l i z a t i o n of the other minerals. PL Diag>> 1 is a ternary equilibrium diagram a f t e r Bowen, i n which the three components are diopside, anorthite, and -albite.' Although the diopside i n the altered g r a n i t i c rock ac t u a l l y contains some of the hedenbergite molecule, a small percentage of thi s molecule would not probably a l t e r the equilibrium diagram to any great extent. Any l i q u i d having an o r i g i n a l composition l y i n g within the hatched area of the diagram w i l l c r y s t a l l i z e plagioclase feldspar before diopside regardless of the f i n a l feldspar composition. Thus a 32 r e l a t i v e l y late c r y s t a l l i z a t i o n of the diopside might he explained. Olivines \ Calcic Plagioclase Mg Pyroxenes / \ c a l c i - a l k a l i c plagioclase Mg.Ca pyroxenes / \ a l k a l i - c a l c i c plagioclase Amphiholes / \ a l k a l i c plagioclase Biotites -/ Potash Feldspar Muscovite Quartz D i a. g. Diag. 2, also a f t e r Bowan, is termed Bowen's reaction s e r i e s . This sequence of mineral formation i s that which might be expected to normally take place i n magraatic c r y s t a l l i z a t i o n . From this diagram i t can be seen that pyroxene should form before quartz. Thus even though i t might be argued that the pyroxene could have c r y s t a l l i z e d l a t e r than the plagioclase there does not appear to be the same p o s s i b i l i t y concerning the quartz. Another feature which seems to be d i f f i c u l t to explain i f this hypotheses is true i s the composition of the pyroxene'. Diopside is c h a r a c t e r i s t i c a l l y a metamorphie mineral and one might expect an aluminous pyroxene rather than the non-aluminous diopside to be precipitated from a 35 magma. The presence of wollastonite occurring i n a similar manner along the limestone contact suggests that the mineral has a s i m i l a r o r i g i n as i t is also a t y p i c a l metamorphie mineral. If the c r y s t a l l i z a t i o n of the pyroxene and diopside is the re s u l t of a quartz-diorite magma being enriched i n lime the question arises as to why the plagioclase feldspar i s not much more c a l c i c i n the contact f a c i e s . Admittedly a small amount of c a l c i c labradorite occurs almost d i r e c t l y on the limestone contact but the major portion of the feldspar associated with the diopside is medium andesine. This' plagio-clase appears to have an almost i n d e n t i c a l composition with that of the normal quartz d i o r i t e . G.D.Osborne (1932, pp. 227-232) has described an occurrence of limestones and contaminated igneous rocks of the Carlingford d i s t r i c t i n Ireland. In this area - pegma-t i t e s consisting normally of microperthite, a l b i t e , oligoclase, quartz, magnetite, pyrite, b i o t i t e , and hornblende, have: invaded and are contaminated by limestone. The contaminated or altered pegmatites consist of wollastonite, andradite, hedenbergite, labradorite, orthoclase, sphene, apatite, magnetite, and p y r i t e . Osborne concluded that the pegmatites reacted with the limestone and took Ca 0 into solution. Before this reaction, however, metasomatic replacement of the lime-stone by solutions derived from the approaching pegmatite resulted i n the formation of skarn minerals. The lime enriched 3 4 l i q u i d began to c r y s t a l l i z e wollastonite and labradorite f e l d -spar while at the same time the mechanical action of the l i q u i d separated garnet and pyroxene from the neighboring skarn. F i n a l l y Osborne attributed the euhedral habit of the garnet and pyroxene to a r e c r y s t a l l i z a t i o n due to the influence of the magma. In the above instance i t i s clear that the magma assimilated lime because of the composition of the feldspar i n the altered rock. It i s not clear, however, why Osborne chose to attribute the wollastonite and hedenbergite to di f f e r e n t modes of o r i g i n . R e c r y s t a l l i z a t i o n of the minerals due to metamorphism could presumably explain the present r e l a t i o n s h i p between the pyroxene and plagioclase grains. The composition of the pyroxene and of the plagioclase as previously mentioned, does not seem to favour a primary c r y s t a l l i z a t i o n of these minerals from a magma enriched i n lime. Wollastonite and Ab^QAn^Q plagioclase occurring at the contacts of limestone and g r a n i t i c rock may possibly be products of c r y s t a l l i z a t i o n from a magma enriched i n lime. Metamorphie Origin of Pyroxene; If the diopside is not a product of normal c r y s t a l l i z a t i o n i t may have formed as a metamorphie mineral.. In the l a t t e r case two processes might be examined. F i r s t of a l l the diopside and wollastonite formed as metamorphie minerals as the magma approached the limestone. These 35 minerals were l a t e r engulfed by the magma and as a r e s u l t attained t h e i r present relationships. Secondly the normal mafjc minerals o r i g i n a l l y c r y s t a l l i z e d i n this area and the diopside formed l a r g e l y as a replacement of these minerals. The fore-going hypotheses w i l l be discussed i n the order given above. Wdlastonife and Diopside as Skarn Minerals: The f i r s t hypothesis involving diopside and wollastonite as skarn minerals, is si m i l a r to that proposed by Osborne, as mentioned previously. The wollastonite is associated with a plagioclase feldspar which is more c a l c i c than the normal vari e t y . It might be postulated that both minerals c r y s t a l l i z e d from a lime enriched l i q u i d . Since the diopside, however, is not associated with an unusually c a l c i c plagioclase i t does not seem that the zone In the magma i n which diopside occurs was enriched i n lime. The order of the zones occupied by the wollastonite and pyroxene near the limestone contact has the same s p a t i a l r e l a t i o n to the limestone as those which occurred i n the bands of skarn minerals along the joints i n the in c l u s i o n . If the diopside and wollastonite were included by a magma after t h e i r formation, a considerable amount of associated garnet would be expected, as i t is the predominant skarn mineral. No garnet occurs i n the altered g r a n i t i c rock excepting d i r e c t l y on the limestone contact. This l a t t e r feature appears to be the major objection to the formation of the skarn minerals p r i o r to inclus i o n by the magma. An 3 6 i n c l u s i o n of skarn minerals by a magma not enriched i n lime also does not explain why b i o t i t e and hornblende are not present i n the pyroxene-bearing g r a n i t i c rock. Another fact to be explained i s the occurrence of subhedral to euhedral grains of quartz and plagioclase i n the pyroxene. This apparently could be brought about only by a considerable r e c r y s t a l l i z a t i o n of the minerals involved. Pyroxene (and Wollastonite) as a replacement: A replacement o r i g i n of the diopside and wollastonite would involve a d i f f u s i o n of lime across the contact from the limestone to the g r a n i t i c rock. The constant composition of the plagioclase suggests that very l i t t l e lime was assimilated by the magma pri o r to i t s i n i t i a l c r y s t a l l i z a t i o n . If this were the case there seems to be l i t t l e reason why the normal mafic minerals, b i o t i t e and hornblende, should not have formed. Thus i n the early stages of magmatic intr u s i o n the gr a n i t i c rock may have been similar to the present normal variety. A minor amount of c a l c i c plagioclase occurring along the limestone contact might represent c r y s t a l l i z a t i o n from a l o c a l l y lime-enriched portion of the magma. Later, as the magma completely engulfed the inc l u s i o n the temperature could have increased. During this stage a d i f f u s i o n of lime from the limestone into the gr a n i t i c rock promoted a replacement of the o r i g i n a l mafic minerals by pyroxene. This change would involve a release of 3 7 H20, aluminum, and probably some iron and magnesium. The l a s t three elements are present i n the diopside and garnet of the skarn zones. In this way the exchange of elements which has apparently taken place between the limestone and g r a n i t i c rock might be explained. The a c t i n o l i t i c amphibole of the t r a n s i t i o n zone may also represent a replacement of e a r l i e r formed rock minerals. In the formation of a c t i n o l i t e the Mg; Ca r a t i o would be considerably greater than that f o r diopside. This zone then might represent a dropping o f f i n the amount of available lime as the lime diffused f a r t h e r and farther from the limestone. In the pyroxene zone the a l t e r a t i o n has resulted i n a c e r t a i n amount of d e s i l i c a t i o n . Possibly much of the free s i l i c a i n this area moved into the skarn zone to form part of the skarn minerals. At this period of maximum mobility of the various elements a minor r e c r y s t a l l i z a t i o n of the plagioclase feldspar would r e s u l t i n the sutured i r r e g u l a r borders displayed by this mineral. R e c r y s t a l l i z a t i o n of the b i o t i t e could also have occurred at this time. Metamorphism of a titano-bi.ot.ite with the introduction o f lime from the limestone may account for the large amount of sphene occurring along cleavages i n the b i o t i t e and along the contacts o f b i o t i t e grains. It has been noted that aggregates of sphene grains occur around grains of magnetite. This suggests that the magnetite may 38 have been a ti t a n i f e r o u s variety. Harker (1932, p 281) has a diagram showing an albite-epidote-hornblende schist i n which abundant sphene has formed, around grains of iron ore. Pro-bably the available titanium was c l o s e l y related i n both of the above cases to the iron-bearing mineral. Whether or not a titanium-bearing magnetite i s unstable i n a lime-rich environment i s questionable. Large bodies of titanium-bearing magnetite occur associated with anorthosites. On the other hand large masses of magnetite related to contact metamorphism of limestones appear to contain l i t t l e titanium. Now l e t us assume a normal cooling of the area associated with the introduction of a considerable amount of highly s i l i c e o u s material now represented l a r g e l y by quartz and orthoclase. One suggestion is that this f l u i d represented a late d i f f e r e n t i a t e of the parent magma s i m i l a r i n composition to normal pegmatitic f l u i d s r e l a t e d commonly to large g r a n i t i c masses. This migration of this l i q u i d was r e s t r i c t e d mainly to the limestone-granitic rock contact. In the highly s i l i c e o u s environment pyroxene and medium c a l c i c plagioclase would be unstable. As a r e s u l t s a u s s u r i t i z a t i o n of the feldspar and u r a l i t i z a t i o n of the pyroxene took place. Turner (1946 - p. 121) states that s a u s s u r i t i z a t i o n and u r a l i t i z a t i o n are sometimes intimately connected, with the two processes taking place simultaneously. In f a c t , one equation suggested by Turner to represent the 39 s a u s s u r i t i z a t i o n of a c a l c i c plagioclase involves an addition of lime, s i l i c a , and .alumina, a l l of which could be the by-products of u r a l i t i z a t i o n . Swanson (1924 - P.124-A) shows a similar type of a l t e r a t i o n i n a t o n a l i t e which intrudes limestone rocks on Texada Island in B r i t i s h Columbia. Replacement of the medium andesine of the ''normal" feldspar by orthoclase was probably contemporaneous with the sa u s s u r i t i z a t i o n and u r a l i t i z a t i o n . Fractures i n the altered g r a n i t i c rock containing c l i n o z o i s i t e and a c t i n o l i t e also contain orthoclase. The myrmekitic structure displayed by the plagioclase feldspar represents a somewhat unusual type of occurrence. Small i r r e g u l a r blebs of quartz i n numerous instances occur within plagioclase grains similar i n composition to the plagioclase of the normal g r a n i t i c rock. In a few places these blebs protrude into the bordering orthoclase and some blebs occtr wholly "within orthoclase grains. The close r e l a t i o n s h i p of the orthoclase and the myrmekite suggests that the structure i s connected to the replacement of the medium plagioclase. The potash-rich l i q u i d probably contained a high percentage of s i l i c a and the corrosive e f f e c t of this l i q u i d resulted i n the resorption of some of the plagioclase a f t e r the i n i t i a l formation of a myrmekitic structure. Thus the isolated blebs of quartz i n orthoclase probably represent the remnants of a previous plagioclase 40 myrrnekite. Resorption of the plagioclase would then result i n the presence of the hlebs i n orthoclase. A s i m i l a r occurrence i s c i t e d by T. Strand (1949, p. 21-22) i n which potash feldspar appears to have replaced plagioclase. The formation of myrrnekite i n this manner i s the reverse of the process generally ascribed to the normal format-ion of myrrnekite. This l e t t e r process involves the replacement of orthoclase by a plagioclase feldspar. The released s i l i c a forms blebs which appear around the borders of the orthoclase. T y r r e l l (1942 - pp.94) attributes the structure i n most cases to a thermal metamorphism under uniform pressure. The type of replacement involving the a l k a l i and medium andesine feldspars appears to be d i f f e r e n t from that involving the replacement of diopside by a c t i n o l i t e . In the l a t t e r case the relationships of the two minerals suggests an ion f o r ion substitution during which neither of the minerals were ever i n a l i q u i d form during the replacement period. The r e l a t i o n s h i p of the two v a r i e t i e s of feldspar suggests, however, that replacement took place at a s u f f i c i e n t l y high temperature as to allow a c e r t a i n degree of f l u i d i t y . This is shown by the bulbous protruberances of the medium andesine into the orthoclase and vice versa. Serpentinization of the pyroxene as mentioned previously may have taken place before the formation of the metamorphie b i o t i t e . This r e l a t i o n s h i p is suggested by the lack of a l t e r a t i o n of the b i o t i t e to any great extent, in the zones containing p a r t i a l l y serpentinized pyroxene. The 41 p o s s i b i l i t y e x i s t s , however, that b i o t i t e , being a hydrous mineral, would not be as r e a d i l y a l t e r e d as' the pyroxene. A d e f i n i t e weakness i n the above hypothesis is that no r e l i c s of amphibole and b i o t i t e occur i n the pyroxene. A preservation of these r e l i c structures would be expected, however, to occur i n the t r a n s i t i o n zone, i f at a l l . Admittedly the relationships of the minerals i n this zone are complex and c e r t a i n l y anything but c l e a r . The hypothesis seems to f i t the evidence more s a t i s f a c t o r i l y than those discussed previously and as a r e s u l t i s favoured by the author. 42 Discussion of Feldspar Porphyry Suffice A l t e r a t i o n caused by the intrusion of the feldspar porphyry dyke appears to be r e s t r i c t e d to a narrow border of s i l i c i f i c a t i o n where the dyke is i n contact with the lime-stone. No apparent a l t e r a t i o n of the dyke by the limestone was noted i n the thi n sections. Many phenocrysts of plagioclase, as noted i n the description of the thin sections, show a reversal of normal zoning. This is brought about by the presence of a more c a l c i c plagioclase near the margins of the grains than i n the central portions. The presence of an i l l - d e f i n e d trachytic texture suggests that the l i q u i d has been i n motion a f t e r the formation of some of the c r y s t a l l i n e material. Movement of a plagioclase c r y s t a l into an area of higher temperature than that at which i t was formally in equilibrium would r e s u l t i n a reversal of normal zoning. Lack of s i g n i f i c a n t a l t e r a t i o n with the exception of minor s i l i c i f i c a t i o n , along the dyke-limestone contact suggests that the dyke was intruded at a temperature*lower than that at which the main igneous mass was intruded. looot d E o ,^000 /0.000 At-43 Diagram 3 shows the P-T diagram f o r c a l c i t e , auartz, and wollastonite. It can he seen that even at a r e l a t i v e l y great pressure the temperature at which wollastonite would form i s approximately 800° C. This seems to rule out the p o s s i b i l i t y that the plagioclase phenocrysts c r y s t a l l i z e d i n the v i c i n i t y i n which they are now found. It appears probable then that the plagioclase phenocrysts c r y s t a l l i z e d i n an environment of higher temperature and were injected as ready-formed cr y s t a l s into t h e i r present position. Further evidence to support this theory i s that there appears to be very l i t t l e difference i n grain size of the plagioclase phenocrysts at the centre of the dyke as compared to those along the margin. It might be noted that an i n c i p i e n t c r y s t a l l i z a t i o n of fibrous to l a t h - l i k e mineral occurs i n some of the c a l c i t e grains. The re f r a c t i v e index of this mineral appears to be of the same order as that of wollastonite. Possibly, these grains represent the i n i t i a t i o n of wollastonite formation. In any case, with the exception of minor s i l i c i f i c a t i o n no evidence i s present of endomorphie' or exomorphic effects produced by the intrusion of the dyke into the limestone incius ion. 44 Description of B r i t i s h Columbia Occurrences; A second location of g r a n i t i c rock i n contact with c r y s t a l l i n e limestone i s in B r i t i s h Columbia about eighteen miles southeast of the f i r s t described occurrence. The rock types i n the area as described by K. DeP. Watson (1948, pp.27-28) are very similar to those which have already been described, consisting of quartz, d i o r i t e and granodiorite with inclusions of gneiss, s c h i s t , and lenses of grey marble. The schists and gneiss are c h i e f l y rusty-weathering staurolite-bearing v a r i e t i e s which contain s i l l s of g r a n i t i c material. The grey marble i s i n places metamorphosed to form wollastonite skarn, brown garnetite, and skarn consisting of green pyroxene and brown garnet. These metamorphosed rocks are mixed with abundant metamorphosed g r a n i t i c rock which consists of white feldspar, green pyroxene, and which l o c a l l y contains bands and nodules of brown garnet. In general the unaltered g r a n i t i c rock types i n this v i c i n i t y are s i m i l a r to those which occur to the north-west. A somewhat higher percentage of orthoclase was noted i n some specimens, however, and some of the rock types are properly classed as granodiorite. Description of t h i n sections 530 and 50. Thin section 530 is a s l i d e made of a specimen which might be regarded as being of the t y p i c a l altered " g r a n i t i c " rock type of the area. 45 The mafic material which is a diopsidic pyroxene occurs as porphyroblasts up to two cm. i n length i n the f e l d -spar. The porphyroblasts include small grains of feldspar i n a few places. Orthoclase and medium andesine plagioclase feldspar are present i n approximately equal amounts with the former c l e a r l y cutting and including remnants of the l a t t e r . The feldspar i s i n general equigranular occurring in grains up to three mm. i n length. No quartz i s present and the only other mineral noted i n thin section was apatite which occurs i n small euhedral grains. Thin section 50 which contains both quartz and ortho-clase shows both these minerals cutting and containing remnants of plagioclase feldspar and pyroxene. Also i n the t h i n section a small amount of a c t i n o l i t i c amphibole has replaced portions of the pyroxene grains around the grain boundaries. In the hand specimen some of the garnetiferous a l t e r -ed g r a n i t i c rock i s made up of brown garnet, creamy feldspar, and large crystals of orthoclase and carbonate. Other specimens contain a considerable amount of c l e a r quartz. In general, then, the mineralogical composition of the c r y s t a l l i n e limestone and altered g r a n i t i c rock in the v i c i n i t y is s i m i l a r to that of the f i r s t mentioned l o c a l i t y . The main difference appears to be the greater amount of orthoclase which i n places occurs as euhedral grains up to three cms. i n length at the B r i t i s h Columbia l o c a l i t y . The minerals z o i s i t e and c l i n o z o i s i t e were not noted i n the thin sections representing the above occurrence. 46 Discussion and Conclusions: No t r a n s i t i o n zone has been described i n the l o c a l i t y between the contaminated or hybrid igneous rock and the "normal" g r a n i t i c rock of the area. The close association of the pyroxene-bearing d i o r i t e with limestone, however, suggests that the genesis of t h i s rock type i s s i m i l a r to that of altered g r a n i t i c rock type previously described. There i s a remarkable s i m i l a r i t y not only in the mineral assemblage pres-ent but also i n t h e i r paragenesis. Porphyroblasts of diopside are well developed and as a result the mineral is more conspicuous i n the hand specimen than i t is i n the other occurrences. In th i n section the diopside does not seem to contain many inclusions but those that do occur are subhedral to euhedral. A metamorphie o r i g i n of the pyroxene seems to be the most l i k e l y postulate. The s i m i l a r i t y in composition of the plagioclase feldspar i n the "hybrid" rock type and that of the unaltered g r a n i t i c rock, to which the former rocks are presumably related, does not suggest an assimilation of much lime by the magma. Thus a theory involving the c r y s t a l l i z a t i o n of pyroxene from a lime enriched magma hardly seems j u s t i f i a b l e . Whether the pyroxene has formed as a replacement of the o r i g i n a l rock-forming minerals or was included as an already-formed skarn mineral i s con t r o v e r s i a l . The l a t t e r hypothesis has some support in that garnet and wollastonite also occur i n parts "of the altered g r a n i t i c rock. 47 R e c r y s t a l l i z a t i o n could then produce relationships of the plagioclase grains including grains of plagioclase. Certainly no evidence i s present that pyroxene has replaced amphibole or b i o t i t e in these specimens. It is questionable, however, whether the lack of r e l i c fragments of the other mafic minerals i s s u f f i c i e n t proof to rule out a hypothesis involving the replacement of these minerals by the pyroxene. The o r i g i n a l altered " g r a n i t i c " rock i n this area contained l i t t l e i f any quartz. The quartz which is present has been introduced as a replacement of e a r l i e r minerals with the orthoclase. Presumably the free s i l i c a of the o r i g i n a l magmatic material combined with lime and other available elements to form the skarn minerals. Some of the large, subhedral to euhedral grains of orthoclase have obviously formed i n vugs or c a v i t i e s allowing a more o r l e s s unrestricted growth. A myrmekitic structure involving quartz blebs i n plagioclase feldspar has resulted from the replacement of the plagioclase by the orthoclase and quartz. The o r i g i n of this structure appears to be s i m i l a r to that of the myrrnekite previously described. In conclusion the remarkable s i m i l a r i t y of the altered g r a n i t i c rock types i n the two l o c a l i t i e s and a corresponding s i m i l a r i t y between the unaltered types suggests that t h e i r genesis is probably c l o s e l y r e l a t e d . 48 Pig. 1. Replacement of andesine by orthoclase with development of myrrnekite Pig. 1 x80 F i g . 2 x80 Pig. 2 Same as above 49 F i g . 4 A c t i n o l i t i c amphibole grains projecting into pyroxene and andesine grains F i g . 5 F i g . 5 ' x26 Inclusions of andesine i n pyroxene F i g . 6 Concentration of pyroxene along border of garnet grain! A few i s o l a t e d grains of pyroxene occur i n carbonate 51 Photos snowing inclusions of c r y s t a l l i n e limestone (white weathering) i n ouartz d i o r i t e on the north siae of the unahini Valley 52 BIBLIOGRAPHY Bc-wen, N.L. Bragg, W.L. Grout, P.P. Harker, A. Kindle, E.D. Lacroix, A. Osborne, G.D. Pirsson and Knopf Shand, S.J. Strand, T. "The Evolution of the "Igneous Rocks." Princeton University Press, 1937, p. 47 and p. 60. "Atomic Structure of Minerals". Cornell University Press. 1937, p. 157 "Petrography and Petrology." McGraw-Hill 1st Ed. 1932, pp. 47-48. "Metamorphism". Methuen, 2nd Oct. 1939, p. 281 "Dezadea.sh Map-Area, Yukon". 2nd P r e l . Account, G.S.C. 1949, p. 12-13. See Shand - pp. 103-104 "Metamorphosed Limestones and Associated Contaminated Igneous Rocks of the Carlingford D i s t r i c t " . Co. Louth, Geol. Mag. Vol. 68, 1932, pp. 222-231. "Rocks and Rock Minerals". Wiley, 3rd Ed. 1946. p. 131. "Eruptive Rocks". Murby, 1943, 2nd Ed. pp. 365-366, pp.103-104 "On the Gneisses From a Part of the North-Western Gneiss Area of Southern Norway." Norges Geologishe Undersokelse, No. 173 1949, pp. 21-22. 53 Swanson, CO. !iThe Genesis of the Texada Island Magnetite Deposits." G.G.S. Summ. Rept. 1924, part A. pp. 106A-144A. Turner, P.J. "Mineralogical and Structural Evolution of the Metamorphie Rocks". G.S.A. Mem. 30, 1948 Wahlstrom, E.E. "Igneous Minerals and Rocks". Wiley and Sons, 1947, p. 73 Watson, K. dep. "The Squaw Greek-Rainy Hollow Area, Northern B r i t i s h Columhia". B.C. Dept. Mines, B u l l . 25, 1948, pp. 27-28. Winchell, A.N. "Elements of Optical Mineralogy" Part 11 3rd ed. J. Wiley and Sons, pp. 224-226, 174-182. Introduction to Appendix: The appendix includes a detailed description of the more important t h i n sections studied f o r this report. The method of i d e n t i f i c a t i o n of the i n d i v i d u a l minerals is given only for the thin sections i n which a p a r t i c u l a r mineral was f i r s t encountered. Thin sections which were examined but which are si m i l a r to other t h i n sections are not included. IA Thin Section 1 Quartz D i o r i t e 50' east of eastern Is. contact Minerals Present: 1. Med. Andesine feldspar (Ab 6 0An 4 0) - (55-60)$ Optical properties A l b i t e twinning N'S > balsam low birefringence low r e l i e f symmetrical e x t i n c t i o n angles = 23° Z A 001 - Sect, cut IX - 25° 2. quartz (15-20$) Optical properties low b i r e f N'S > balsam no twinning Uniaxial © 3. B i o t i t e (15$) N'S >balsam high birefringence = .030 good cleavage i n one d i r e c t i o n p a r a l l e l extinction pleochroic - browns - l i g h t tan maximum absorption p a r a l l e l to P P. 4. Hornblende (10$) N'S >balsam Medium r e l i e f Good cleavage at 56° Birefringence = .020 Pleochroic - greens to l i g h t brown maximum extinction 20° 5. Orthoclase (very minor amount) low r e l i e f N'S ^  balsam no twinning low birefringence pink tinge i n t h i n section. 6. Accessory minerals, Apatite Sphene Texture Medium to fine grained, hypidioraorphic; plagioclase feldspar shows weak zoning; i r r e g u l a r i t y of some of feldspar 2A grains suggests minor r e c r y s t a l l i z a t i o n . Inferences The i r r e g u l a r plagioclase-quartz contacts and oriented inclusions of plagioclase i n quartz indicates a minor amount of replacement or resorption of the plagioclase has taken place. The mineral assemblage, texture, and zoning of the plagioclase' feldspars, suggest that the rock i s a normal Plutonic igneous type. According to Grout's c l a s s i f i c a t i o n of igneous rocks, the rock type i s a quartz d i o r i t e . Thin Section I B Quartz-Diorite - 3 miles east of Ls..on Haines Rodd. Minerals Present: 1. Medium Andesine ( A b f i n A h , n ) - 50$ ' B i o t i t e 20$ ^ ou 4tu Hornblende 15$ Quartz 15$ Orthoclase (minor) 2. Accessory minerals Apatite Sphene Magnetite Texture Medium.to coarse grained hypidioraorphic average grain size approximately 4 mm. Feldspars show f a i r l y well-defined zoning. A very minor,amount of myrrnekite is present. Inference The rock type represented appears to be a normal Plutonic quartz-diorite. The minor amount of myrrnekite around 3A the borders of a plagioclase grain appears to be related to i r r e g u l a r grains of orthoclase which contact the plagioclase. Thin Sections 4 and 12 Altered Quartz D i o r i t e . 'T.S. 4 Minerals Present 1. Medium Andesine (Ab 6 0An 4 0) - 35$ 2. Quartz - 30$ 3. Diopside - 20$ 4. C l i n o z o i s i t e (possibly some z o i s i t e ) N'S ^balsam medium high r e l i e f anomalous blue birefringence - also yellowish green cleavage 1 d i r e c t i o n extinction commonly p a r a l l e l but i n one instance - 7° 5. A c t i n o l i t e 6. Orthoclase Accessory minerals Apatite Sphene Texture and Relationship of Minerals The texture i s c r y s t a l l o b l a s t i c with the average grain size being approximately 3 mm. As i n t h i n section 5 the diopside grains contain numerous inclusions. A l t e r a t i o n A c t i n o l i t e and c l i n o z o i s i t e have formed as a l t e r -ation products of diopside. The c l i n o z o i s i t e also occurs as 4A an a l t e r a t i o n of the plagioclase. Both the'"clinozoisite and a c t i n o l i t e are concentrated along a fracture which cuts both plagioclase and pyroxene. The a c t i n o l i t e is r e s t r i c t e d to that part of the fracture crossing the pyroxene taut the c l i n o z o i s i t e occurs throughout. Orthoclase i s also present i n the v e i n l e t . Inferences The close association of a c t i n o l i t e and c l i n o z o i s i t e suggests that these minerals were produced contemporaneously and thus i n the same environment. The presence of orthoclase i n the fracture indicates that the a l t e r a t i o n is c l o s e l y related to the period when the orthoclase was introduced. Thin Section 12 Altered Quartz D i o r i t e Thin section twelve i s i d e n t i c a l with t h i n section four i n most respects but does not contain c l i n o z o i s i t e or a c t i n o l i t e . In places plagioclase feldspar seems to have replaced plagioclase. This i s thought to be the re s u l t of intensive r e c r y s t a l l i z a t i o n which has also l e f t i s o l a t e d grains of pyroxene i n o p t i c a l continuity surrounded by plagioclase. Thin Section 5 Typical altered Q.uartz-diorite . 5A Minerals Present: Medium Andesine (Ab 6 QAn 4 0) - 70% Diopside - 15-20$ (Indices of r e f r a c t i o n - o i l immersion method) . > . 1.6679 n r= 1.6901 Carbonate - 10$ Quartz - 5$ Orthoclase (minor) A c t i n o l i t e (minor) Accessory Minerals Apatite Sphene - not abundant. Texture The texture i s predominantly c r y s t a l l o b l a s t i c with the average grain size being about 1.5 mm. In general the . pyroxene grains are quite i r r e g u l a r but a number of almost euhedral grains are present. Subhedral to euhedral inclusions of plagioclase are present i n grains of diopside. In a few cases quartz inclusions also occur i n the pyroxene. A very i l l - d e f i n e d zoning i s displayed by a few plagioclase grains. A l t e r a t i o n Minor a l t e r a t i o n of the pyroxene has resulted i n a CA amorphous k a o l i n - l i k e material along fractures, and a minor amount of a c t i n o l i t e . Inferences Inclusions of plagioclase and quartz i n the diopside indicate that the diopside grew l a t e r than the former minerals. The small amount of sphene and the lack of b i o t i t e seems to 6A be evidence to support the view that the t i t a n i t e i s c l o s e l y related to the b i o t i t e i n o r i g i n . Thin Section 5b Altered Quartz Di o r i t e close to limestone contact. Minerals present: Quarts - 45$ Wollastonite - 20$ Colorless i n thi n section N'S > balsam ^ 1.6 birefringence ~' .012 nearly p a r a l l e l e x t i n c t i o n good cleavage p a r a l l e l to length of grain on cross section cleavage i n three directions -two nearly at right angles B i a x i a l © 2V 30° Optic plane at right angles to length of grain and cleavage length slow and length f a s t Diopside - 5$ Calci c Labradorite - 20$ Carbonate Zois i t e Accessory minerals Apatite Sphene Texture and Relationship of Minerals The texture displayed by this s l i d e i s controlled l a r g e l y by the replacement of the plagioclase by z o i s i t e and carbonate. As a result most of the grains are anhedral and remnants of the various minerals are common. Some of the wollastonite grains are subhedral to euhedral but a l t e r a t i o n to carbonate has also resulted i n shreds of this mineral being present. Wollastonite occurs i n grains up to 0.5 mm. whereas the diopside i s present only as very small anhedral grains. 7A A l t e r a t i o n Much of the plagioclase has al t e r e d to z o i s i t e and carbonate. The carbonate has also formed as an a l t e r a t i o n of the wollastonite. Inference The presence of wollastonite appears to be r e s t r i c t e d i n this suite of specimens to the near-contact facies of the altered g r a n i t i c rock. Thin Section 12a Transition zone - altered 'granitic' rock Minerals Present Medium Andesine (Ab 6 0An 4 0) 50$ B i o t i t e - 15$ Quartz - 5$ Orthoclase - minor A c t i n o l i t e - 15$ Pleochroic - l i g h t greens N'S "> balsam medium r e l i e f b i r e f * .025 max. extinction - 15$ max. absorption p a r a l l e l to length of grain length slow i . e . Vibration d i r e c t i o n making smaller angle to cleavage good cleavage at 56° o p t i c a l l y negative Diopside - 15$ N'S > balsam med. high r e l i e f birefringence - .028 good cleavage at right angles maximum extincti o n angle - 42° B i a x i a l © 2V == 60° Accessory Minerals Magnetite Sphene Apatite Carbonate 8A Texture The o v e r a l l texture displayed by the s l i d e might be termed c r y s t a l l o b l a s t i c . Almost a l l of the grains are anhedral and only laths of a c t i n o l i t e show any tendency to regular grain boundaries. B i o t i t e , e s p e c i a l l y , occurs i n extremely anhedral grains. Some of the plagioclase feldspar grains show a very f a i n t zoning. A l t e r a t i o n Minor carbonatization has occurred i n the plagioclase along small fractures and has also affected pyroxene to some extent. The a l t e r a t i o n of pyroxene to a c t i n o l i t e is evident i n a number of cases. Inference The a c t i n o l i t e has evidently formed as an a l t e r a t i o n of the pyroxene. This fact i s indicated by the fresh-looking laths of a c t i n o l i t e which project into the pyroxene grains and also project from the ends of pyroxene grains into the plagioclase. The relationships of the three mafic minerals, diopside, b i o t i t e and a c t i n o l i t e is s p a t i a l l y very close. It is d i f f i c u l t to determine, however, the paragenetic r e l a t i o n -ship of the b i o t i t e to the pyroxene. That the b i o t i t e is a metamorphie variety i s suggested by i t s strong reddish pleochroism. Sphene is extremely abundant along cleavages of 9A b i o t i t e and along borders of this mineral. This close s p a t i a l relationship indicates that the formation of sphene may be related to the metamorphism involving the b i o t i t e . Thin Section 12b Transition zone - altered g r a n i t i c rock. Minerals present Sodic andesine (Ab An ) - 70$ Hornblende - 15%' b ° * 3 5 Quartz - 10$ Orthoclase ) A c t i n o l i t e ) Minor Carbonate ) Pennine ) Accessory minerals Sphene Apatite Texture The texture i s s i m i l a r to that of t h i n section 12a. A l t e r a t i o n The b i o t i t e has p a r t l y altered to the chlorite,pennine. Inference The minor amount of a c t i n o l i t e and pyroxene i n this t h i n section serves to indicate further that a c t i n o l i t e In section 12a has formed l a r g e l y by a replacement of the pyroxene. Patches of carbonate and shreds of a c t i n o l i t e may represent a l t e r a t i o n of pyroxene grains but no remnants remains. 10A Thin Section 12c and 14 'Transition' zone - altered quartz d i o r i t e . Minerals present 1. Medium Andesine (Ab 6QAn 4 0) - 50% 2. A c t i n o l i t e - 15% 3. B i o t i t e - 10$ 4. quartz - 20% 5. Orthoclase (minor) 6. Pyroxene (minor) Accessory Minerals Apatite Sphene Texture The texture i s s i m i l a r to that shown by t h i n section 12a. Many inclusions of plagioclase are present i n the amphibole. Replacement involving the introduction of quartz and minor orthoclase i s evident. Inference On the whole a rather extensive r e c r y s t a l l i z a t i o n is suggested by the i r r e g u l a r grains and numerous inclusions. Inclusions of plagioclase by amphibole indicate that the l a t t e r was the l a t e r mineral to grow. Thin Section 14 'Transition' zone - altered quartz d i o r i t e . Thin section 14 is similar i n almost every respect to thin section 12c. The importance of t h i s section, however, l i e s i n the f a c t that good relationships showing the replacement of b i o t i t e by a c t i n o l i t e are present. 11A Thin Section 14a Altered Quartz-diorite close to the limestone contact. Minerals Present 1. Quartz - 55% 2. Diopside - 15$ 3. Sodic Bytownite N'S ^ balsam f a i r r e l i e f low birefringence A l b i t e twinning maximum symmetrical extinction angle - 41° Exti n c t i o n - "7'A Angle from trace of 001 A to X i n section cut normal to Z - 49° in same section 010 A to X - 41 4. Z o i s i t e Similar properties as c l i n o z o i s i t e Shows anomalous blue but not yellowish green birefringence 5. Orthoclase Accessory Mineral Sphene Texture and Relationships of Minerals In places the plagioclase grains occur i n a c r y s t a l l -o b lastic decussate arrangement-the i n d i v i d u a l grains averaging about 0.2 mm. Larger grains of plagioclase up to 2mm. occur but appear t o have been fractured considerably. Orthoclase i s present along most of these fract u r e s . Quartz has extensively replaced the feldspar and occurs i n grains up to 3 mm. A l t e r a t i o n Z o i s i t e is the major a l t e r a t i o n product and has almost completely replaced some of the larger plagioclase grains. 12A Inference The composition of the plagioclase indicates a de f i n i t e increase i n lime content i n th i s p a r t i c u l a r area. The high quartz content i s obviously the re s u l t of a r e l a t i v e -l y l a t e r introduction of this mineral. Thin Section 17 S l i g h t l y altered Quartz D i o r i t e 50* west of Ls. inc l u s i o n . Minerals Present Sodic Andesine (Ab 6 5An 3 5) 30$ Hornblende 20$ Bi o t i t e 15$ Quartz 10$ Orthoclase 20$ (Determined by K. DeP. Watson with use of universal stage) Accessory Minerals Apatite Sphene Texture The texture i s l a r g e l y medium grained, hypeirdiomorphic but i n numerous cases highly i r r e g u l a r grains have formed by the replacement of plagioclase by orthoclase. Myrrnekite is present near the borders of plagioclase grains where they are i n contact with orthoclase. Inference An introduction of potash feldspar and s i l i c a has resulted i n resorption of some of the plagioclase and apparently has been a f a c t o r i n the o r i g i n of the myrrnekite. 1 3 A Thin Section 19b Typical gneiss of 'Yukon Group' (Quartz-feldspar-biotite-garnet-gneiss) Minerals Present (r)h70/?n3O) Sodic Andesine (Ab-e-An-c-) 45$ Quartz - 30$ b 0 ^ B i o t i t e - 20$ Garnet ) Magnetite ) Apatite ) Minor Chlorite ) Texture C r y s t a l l o b l a s t i c Structure Gneissose The minerals display the t y p i c a l mosaic arrangement c h a r a c t e r i s t i c of metamorphie rocks. Only the mica, apatite, and garnet, show any c r y s t a l form with the mica giving the rock i t s d i r e c t i o n a l properties. Inference The rock type seems to be the product of regional metamorphism. An abnormally high content of soda, as might be noted by the mineralogical composition, is present. Thin Section 50 Altered g r a n i t i c rock east of Blanchard Lake. Minerals present Quartz - 40$ Sodic Andesine 20$ Orthoclase - 20$ Diopside - 15$ . Apatite 14A Texture and relationships of minerals A l l of the minerals occurring i n t h i s t h i n section are anhedral with the average grain size being approximately 2jg mm. Replacement of plagioclase and diopside by orthoclase and quartz i s evident. The pyroxene i s remarkably fresh and shows no a l t e r a t i o n . Thin Section 15 Altered Quartz D i o r i t e - limestone contact Minerals present Garnet - 20% High r e l i e f - N'S greater than balsam Colorless i n t h i n section i s o t r o p i c Diopside - 10% Quartz - 30% Plagioclase (Ca Labradorite - 25% (Na Bytownite Carbonate C l i n o z o i s i t e Orthoclase Accessory Minerals Apatite. Texture and Relationships of Minerals The texture i s t y p i c a l l y c r y s t a l l o b l a s t i c with much of the feldspar, diopside,and considerable quartz occurring i n grains averaging about .4 mm. Irregular grains of quartz up to 3 mm. are present but the largest grains are those of garnet which display a sieve structure. The garnet contains grains of quartz, feldspar, quartz,and plagioclase. 15A Orthoclase and carbonate veinlets cut a l l the other minerals present i n the section. Quartz has replaced, much of the plagioclase. A l t e r a t i o n C l i n z o i s i t e and. carbonate have formed, as a l t e r a t i o n products of the plagioclase and of diopside. Diopside has altered along fractures to a dark brown, fibrous material which is possibly an iron-bearing mineral. Inference The c a l c i c plagioclase appears to be r e s t r i c t e d to areas close to the contact. It i s apparent that the garnet has formed l a t e r than the diopside, plagioclase, and at least some of the quartz. The c r y s t a l l o b l a s t i c texture and decussate arrangement of the minerals indicates the influence of predominantly thermal metamorphism. Thin Section K-550 Altered g r a n i t i c rock east of Blanchard Lake Minerals Present Diopside - 10$ Med. Andesine ( A b 6 0 A n 4 o ) 4 5 $ Orthoclase 45$ Apatite Texture Porphyroblastic - i n the hand specimen porphyro-blasts of diopside occur up to 2 cm. i n length. Structure - Granulose The orthoclase has replaced the plagioclase and i n a few cases myrrnekite has developed near the borders of the plagioclase being replaced. Very l i t t l e diopside occurs i n the t h i n section and i t is d i f f i c u l t to determine i t s rela t i o n s h i p to the other minerals. Thin Section 15 Skarn occurring along joints i n limestone. Minerals Present 1. Cal c i t e - 15$ 2. Wollastonite - 10$ N'S o i l immersion method n d * 1.6111 n r near 1.6190 low birefringence b i a x i a l 0 2 v = 30° length fast and length slow as optic plane almost perpendicular to length good cleavage i n one d i r e c t i o n extinction p a r a l l e l - not i n a l l cases (up to 26°) 3. Diopside - 20$ N'ST- balsam - o i l immersion N«f'' 1.6901 high birefringence ^ r * 1.7290 good cleavage at right angles B i a x i a l ® 2- * *• 65° maximum extinction 44° polysynthetic twinning s l i g h t greenish tinge i n t h i n section 4. Garnet - 50$ Colorless i n t h i n section high r e l i e f i s o t r o p i c N balsam f 1.75 - o i l immersion contains Ca, Al , Pe, Mn, Mg, SiOg - spectrograph chemical tests show only trace of Pe c e l l edge - 11.80 X-ray pattern 5. Quartz. 1 7 A Texture and Relationships of minerals The garnet occurs i n large grains up to one inch i n <i.ic£m%tdK. i n the hand specimen. The mineral which i s r e s t r i c t e d to the centre of the fracture encloses a l l the other minerals present i n the s l i d e . The diopside is concentrated i n a narrow hand on either side of the garnet hand but many small inclusions are present i n the garnet. The diopside grades into wollastonite which i n turn grades into c a l c i t e . The l a t t e r two minerals occur i n grains up to 2 mm. in length with the diopside tending to smaller grains of the order of about 1 mm. Wollastonite grains are elongate at r i g h t angles to the banding. A l t e r a t i o n Wollastonite has altered considerable to carbonate with the result that only shreds of the former mineral remain. Inference The indices of r e f r a c t i o n of the pyroxene suggest that i t i s diopside containing some of the hedenbergite molecule. (Winchell, 1947, pp.226) Various tests on the garnets indicate that this mineral i s made up l a r g e l y of the grossularite molecule. The order of formation of the skarn minerals appears to be related to t h e i r s p a t i a l d i s t r i b u t i o n . Thus the order seems to have been wollastonite, diopside,and garnet respectively. 18A Thin Section 5c Altered, quartz d i o r i t e near dyke Minerals present Medium andesine - 70$ Quartz - 10$ Carbonate - 15$ Accessory minerals Muscovite (a few shreds) Sphene Apatite Texture and relationships of minerals Subhedral plagioclase grains occur up to 3 mm. in length but have been considerably fractured. The angular fragments are cemented by carbonate which has also replaced plagioclase to a certain extent. Inference The f r a c t u r i n g of the feldspar grains is apparently the r e s u l t of the int r u s i o n of the dyke. The subsequent f i l l i n g by carbonate suggests the l a t t e r mineral was f a i r l y mobile at the time of the emplacement of the dyke. Thin Section 8 Centre of Feldspar Porphyry dyke Minerals Present Plagioclase phenocrysts - strongly zoned - 30$ vary from A b g 5 A n 3 5 to A b 4 5 A n 6 5 Plagioclase microlites - A b 6 0 A n 4 0 - 40$ Hornblende phenocrysts (minor) Cryptocrystalline material - N'S> balsam - 20$. Magnetite - 5$ 19A Texture and relationships of minerals The texture i s d i s t i n c t l y porphyritic with plagio-clase phenocrysts up to 2 mm. occurring i n a groundmass of microlites and c r y p t e c r y s t a l l i n e material. Most of the plagioclase phenocrysts show marked zoning and i n a number of cases the borders are more c a l c i c than the centres. The microlites shoYi/ a trachytic texture which is e s p e c i a l l y marked around the phenocrysts. Some of the plagioclase phenocrysts have been resorbed considerably by the groundmass material. Inference The reversed zoning of plagioclase-phenocrysts indicates a change i n environmental conditions during t h e i r c r y s t a l l i z a t i o n . Apparently they were moved into a warmer area a f t e r having p a r t i a l l y c r y s t a l l i z e d . Thin Section 6 Dyke near eastern limestone contact Thin section 6 although representing a border facies of the dyke is i d e n t i c a l with t h i n section 8 except f o r an approximate 15$ content of remarkably euhedral hornblende phenocrysts. Thin Section 9 'western limestone dyke contact 20A Minerals present Ca l c i t e - 40% Plagioclase - 20$, microlites and. a few larger grains - medium andesine AbgQAn,^ Quartz - 30$ Zoi s i t e ) Bi o t i t e ) Hornblende ) minor Sphene ) Unidentified small needle-like grains i n c a l c i t e N'S 1.55 - 1.6 ? Texture and Relationships of minerals With the exception of the microlites the minerals present show very i r r e g u l a r outlines. Much of the quartz occurs as a mosaic of anhedral grains up to 1 mm. Fragments of c a l c i t e vary greatly i n size with the largest being about 1 mm. Carbonate stringers cut a l l minerals and have apparently f i l l e d fractures i n these minerals. The dyke material occurs between the c a l c i t e and quartz grains. The small needle-like grains i n the c a l c i t e occur where the c a l c i t e i s i n contact with dyke material. Inference This thin section indicates that the dyke was intruded at a temperature s u f f i c i e n t l y low to prevent the formation of skarn minerals. 13(^50' I3t>"50' 4 V. v • \ r / « i-1 r / 3 4 ° J O GEOLOGIC M A P S H O W I N G YUKON GROUP ROCKS A N D ASSOCIATED o P A N l T I C R >GKS IN SOUTH-WEST YUKON AND NORTH-WEST B P l ~ S H COLUMBIA 435C : 4 / r \ u r 1 i — v. -—, — V r-A SKETCH MAP OF LIMESTONE INCLUSION IN SOUTH-WEST YUKON TER* L E G E N D eg \ r > \ C R Y S T A L L I N E L IMESTONE U N A l ERED A N D A . V L R E D Q' JARTI O i O R i ' E D Y K E - F ELDSPAR PORPHYRY TALUS SNOW Scale: / JC C2 rysia Hi d<-L i rn <t J lo n >c i j h i y Z. O n cc E B ' ' E5 B 1 — v — : - 7 -D iof>»««« 1 1 1 W Huston,'* Mu<~h u out 1^ Minor <-/<<->psidl**-rf<iq to ** ' < J ^ > * : -atf^r^tj fo c lino £>/ A»_ iZyj/L'O'ra.r «i III ^ r o v _ / c / » t > A buo sS"-*o P i * 4 «. ifCj/tC ^yi4k<Zt7* wilt h " a J V <ll\f I n / fi uj i 7 f- & h, <u o / / / / - I * K <£• Ill <-( f <2- r ta ( <JL f7 <~t S r p*. ft * cj.n cJ <2. c 11 n o 11 f <. o c_ c /-CL tort a i, v «_ // . i Y t. / v. cV ^ / 3 //be. o /////,_/ /'<?<-/« a «2 { // <^l <t u i ox • d-tof**' I • « ru* U / f/ f-> I CCC * b >^y C<U l~ h Off CI / C£ D I A G R A M M A T I C S K E T C H O F Z O N I N G 1 Quartz &,ut;t< 

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