GEOLOGY OF THE MOUNT BRENNER STOCK NEAR DAWSON OITY YUKON TERRITORY b y MAURICE BERNARD LAMBERT. B . S c , U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1963 A- T H E S I S SUBMITTED I N P A R T I A L FULF ILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF S C I E N C E i n t h e D e p a r t m e n t o f GEOLOGY We a c c e p t , t h i s t h e s i s a s c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE U N I V E R S I T Y OF B R I T I S H COLUMBIA A p r i l , 1966 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h C o l u m b i a , I agree tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree tha t p e r -m i s s i o n f o r e x t e n s i v e c o p y i n g of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s „ I t i s unders tood that c o p y i n g or p u b l i -c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Lambert) Department of G e o l o g y The U n i v e r s i t y of B r i t i s h C o l u m b i a , Vancouver 8, Canada. Date i-lay 3. 1 9 6 6 •ti 11 ABSTRACT The Mount B r e n n e r s t o c k h a s i n t r u d e d f o l d e d s e d i -m e n t a r y a n d m e t a s e d i m e n t a r y r o c k a t h a t l i e AO m i l e s n o r t h -e a s t o f Dawson C i t y , Y u k o n T e r r i t o r y . • The s t o c k c o n s i s t s o f f o u r m a j o r c o n c e n t r i c z o n e s : ( l ) a n o u t e r z o n e o f f i n e -t o m e d i u m - s r a i n e d a u s i t e - b l o t i t e m o n z o n i t e ; (2) a z o n e o f v e r y c o a r s e - g r a i n e d m o n z o n i t e p o r p h y r y ; (3) a n i n t e r m e d i a t e z o n e o f p o r p h y r i t l c h o r n b l e n d e m o n z o n i t e ; a n d (4) a c e n t r a l z o n e o f c o a r s e - g r a i n e d p i n k q u a r t z m o n z o n i t e p o r p h y r y . E x c e p t f o r t h e o u t e r z o n e , a l l r o c k t y p e s a r e p o r p h y r i t l c a n d t h e a l i g n m e n t o f f e l d s p a r p h e n o c r y s t s g i v e s t h e r o c k s a p r i m a r y f l o w s t r u c t u r e w h i c h c o n f o r m s t o s t e e p l y o u t w a r d d i p p i n g g r a d a t i o n a l i n t e r n a l c o n t a c t s . A l l e x t e r n a l c o n t a c t s a r e s h a r p . The r e g i o n a l s t r u c t u r a l t r e n d i s m o d i f i e d i n t h e v i c i n i t y o f t h e s t o c k so t h a t b e d s a r e g e n e r a l l y c o n f o r m a b l e w i t h t h e i n t r u s i v e c o n t a c t . F rom s t r u c t u r a l e v i d e n c e , i t i s c o n c l u d e d t h a t a t t h e p r e s e n t l e v e l o f e r o s i o n , t h e s t o c k was e m p l a c e d b y f o r c e f u l i n j e c t i o n . The d i f f e r e n t r o c k z o n e s o f t h e s t o c k c a n b e a c c o u n t e d f o r b y d i f f e r e n t i a t i o n o f a n a u g i t e -b i o t i t e m o n z o n i t e magma b y a c o m b i n a t i o n o f c r y s t a l f r a c t i o n -a t i o n , v o l a t i l e a n d a l k a l i d i f f u s i o n , and m u l t i p l e i n t r u s i o n . Pag© mmowGHion 1 • LOO AT I OK AND AGO •iSOl B l i . I X ^ . . . . . . . . . . . . . . . . . . . . . . . 1 TOPOGRAPHY, DAAIiMGA AKD ALACIATION.»,.,«,..,,.• . 1 PREVIOUS GEOLOGICAL <*OHi£. 4 ACKNOWLEDGMENTS.... ......................... 5 Q A K A A A L OSOLOGY« « 6" THK MOUNT D^aLAKER OTOC A............................... 8 GAKEfctAL DLSORIfTIOiS*. .»«..**..•..«• ......... 6 PETROGRAPHY.,,«*»**•..,•.,,,*...»,..,•........... 13 Au g l t e - b i o t l t e fen&onit®.................... 13 Monaoolte Porphyry 16 Porp h y r t t l c Hornblende Monzonlte............ 18 Pink Quartz Monzonlte Porphyry.»...*...*...« 21 A p l l t e 24 Inclusions. ...» 24 CA:A4IGAL CO.A?OAlTIOft A A L VARIATION* »«.,»».».»#.», 29 COAT •'•Of ;AArA^OAPiiI A«t, ,,.»,,.•,,.,..,.*...*.,•..,, 36 STA JGTURA, »,,..... ......,.«..»..., «... 37 Intern a l Structure® 37 F o l i a t i o n and L i neat Ion 37 J o i n t i n g , 40 Intern a l Contacts. *.... 44 i n t e r n a l Structures Interpretation of Structures .and Mechanics of Sfiplac6ffl@rit•••.•••••••«•••••••••••••»•• As s iiu i. xoti • ••••••••••••••••••••••• • l) .11 X til ' . d i i t X 1 Gli • • • • • • • • • • • • t • Suttuaary of iSaplacement, C r y s t a l l i z a t i o n and l) i £ f c i' on 11 & t jL on • •••••»••••«••••••«#••••••• iii l i ^ l s JL 0 C? h A.t'jri i t « i i i i i « t M « t i t * t * t * M i i « i i i i M « t i i * « -A V i4»p' Cm@lo%lml map o f %b* Ht« 5r»Dn*r 3too*s, In Xufcon "territory ..... pao ;»t 1 led em ;sap »howtag ta® fjfcwaon area Had locution of the Mount Brenner stock*,*** 2 2 lodes of sp#ot;a&$3® f r o * th® Mount 3re»n®r 3 t O O T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3 V a r i a t i o n i n eooipa^itlon of g r a n i t i c roo'*8 with respect to distance froa the s&rgin of «rtcU rack t y p c » . 12 4 Variation dinpras, for ra.-'r *oo*s of the 'ouot r» noer t»toe1«» TU« s.v@rag« %f«laht per cent of O X U U B of «mch rock tan® e"r« plotted ftualnst total silica,»*••*»*•,,« 32 5 Contoured dia^xaa-, of oxide® m » <3,lff«r-c.-.ti . t I O J I index on vcica analysts© of roc ;i,Ht Ion o f pli- ^ i o c l u o e i o a l u a i o n y i n o r t h o s i s s © pnfraocry s t o f t a f c p i - ; ' < qufcr i p n s / j i i i t e p o r p h y r y ( G r o s s e d n l c o l a .•>. X} . . X . : : l a . _ l o o l • a e c r ^ s t u l s w i t hi;; nnX p-..rtl i n c l u d e d i n t n e aargi . - j o f ortaoeles© phen a r y o t p ^ r o a t i c P n i o l s X X) P l a t e s v i l i '•"ollowlaj? Pac e X I . A. Combination twinned plagiooleB© i n c l u s i o n on o r t h o c l a s e p a e n o c r y s t (Crossed n i c o l s X 10} B. rtyrmekite rial on pl«g\ocl«se surrounded by potash f e l d s p a r (li) of the pinfc q u a r t z raonzonite p o r p h y r y . . . . . . . . . A . O s c i l l a t o r y z o n i n g i n p l a g l o c l f s s e of pink quarta monzonite porphyry. (Groused n i c o l s X 1 0 ) . . . . . . . 22 22 B. a l l n a l t e i n nietacaict s t a t e {?) i n -c l u d e d I n h o r n b l e n d e (Hfo). Kobe p l e o -c h r o i c h:;lo end r a d i a t i n g f r a c t u r e s In h o r n b l e n d e . (Pl*.«e l i g h t X 10) . . . . . . . . . XII» A. I n c l u s i o n i n pin k q u a r t z rsoozonit© p o r p h y r y . P o t a s h f e l d s p a r p h e n e c r y s t s • stream* eround t h e I n c l u s i o n (X 1/3).* i i . I n c l u s i o n i n pin k q u a r t z raon.-.onite p o r p h y r y . ftote pheaocrysfcs In b o t h q u a r t z ;flonzonlte and i n the i n c l u s i o n 1 1 ) « . . . . . . . . . . . . . . « . . . . . . . . . . . . . . . . . . X I I I . A. P o r p h y r o b l a s t o f p e r t h l t e i n g r s n o -b l a s t i c m a t r i x , hate l i n e o f t i n y i n c l u s i o n s and I r r e g u l a r b o r d e r . ( G r o s s e d n i c o l s X 5)..... t>. O s c i l l a t o r y z o n i n g i n o r t h o c l u s e raegacryst •. , ( c r o s s e d n i c o l s X 3).... XIV, A , Photomicrograph showing c o r r o d e d p l a g i o o l a s s p h e n o c r y s t s In an I n c l u s i o n , Block: background i s p o t a s h f e l d s p a r . (Grossed 1 n i c o l s X 10) B.( P h o t o m i c r o g r a p h showing p o i k i l i t i c h o rnblende ( C r o s s e d n i c o l s ? A: 1 0 ) . *3 23 26 26 26 WJOAflOl M® AOOSaSZBZUTX The map-area lias la west-central Yukon Territory about 40 allee a@rtli.aait o f Dawson City and 5 alias north of tha Tombstone liver (fig» 1 ) . Th© Beateter Highway leaves the Taylor Highway 20 ml%m. east of Dawson city and follows the Korth Klondike River* Both of these gravel highways are kept la excellent condition for summer travel. At mile 4§ of the lesster Highway (in the vicinity of Morth Fork Pass) the north Klondike River swings southweatwari. along a broad valley. fhis valley leads to a low, flat pass into the valley of the $oohaton® River at about 11 miles from the Pewter Highway. the map-area is a two day walk from the highway by way of these valleys. The area Is accessible by helicopter or, with dtff&tt&tar; by horses* V O K X K t A J H X * B8AIM® U t £ a L A O t t X X G V The region lies within th® physiographic unit known as the Ogilvie Mountains (Bostoek, 1948). "The range has a rugged and mountainous aspect and consists of long* branching, knife-edge crests, with sharp and often Figure I. Index map showing the Dawson City area and location of the Mount Brenner stock (lined pattern). p r e c i p i t o u s peaks* separated by deeply out va l leya»• (Coefc -f i e l d , 1919}* fa® map**rea Is e n t i r e l y a b o v e ti«fc-er-lls@ w h i c h l a a t as e l e v a t i o n of approximately 4000 f e o t r •Average r e l i e f I s 1500 f##t and peak® reach & aaxiaua e l e v a t i o n of ?§GQ f e e t * l a s r w s o f grr*»ltio r«ek» the topography hag a u n i q u e a h a m o t o r w h l o t i i»as boot* d o e o r i b o d by - c C o n n e l l ( l y 0 3 ) i a th® f o l l o w i n g word®. This rock Is s t r o n g l y j o i n t e d v e r t i c a l l y «nd wea-t h e r e into r n A n o a a , wed^e-en^eC ricVe--, e a r ->o . : t e d &y 11 nen of r h ' - r p > 1 .nides ..•• *d l o f t y tower-shaped p e a k s * The p i l l a r e d c h a r a c t e r of the r a t i o n i s so remarkable t h s t the pros-pectors n e v e .riven i t t h e oaas© o f t h e t o A s t o n e country. The ares i s d raised by tfere© r l v o r syite.nst i n the west, the south f l o w i n g C h a o d l n d u S i v o r s y s t e m with i t s i s a l n westward flowing t r i b u t a r i e s , the T a s b s t o a o a n d k i t t l e T w e l v e m i l e Rivers j l a the east, the north JClondike l i v e r ay s t e a l and i n the s»o.riht. t h e Blackstoo® E l v e r ay stoat* The O g l i v i e wranialns display o o r t a i d o r a b l e e v i d -ence of g i a e l a t i o n * Main v n l l e y s have U-shaped c r o s s -profile© and the up - . -e rmost-parts o f t r i b u t a r y v a l l e y s g e o e r e l l y t e r m i n e t o I n g l a c i a l c l r a . u a a, w h i ^ h cosaaonly hold tut-nn. - . r o t e s , uor-'«, «: id a • • r t n - : v.t i l e y c a r e co-xiifion f e'it a r c s , ,>a^ii Ice pa tones e t i l l occupy a few of the s i r ; a c a on t ;.e norto. nX< e s of , - a per ^e .ID: t h of '* S y e n i t e Lakes'*,* Valley g l a c i e r s emerged f r o m tae a o u a t a l n s a n d o c c u p i e d t h e valleys of t h e North K l o n d i k e a n d ' Chandindu R i v e r s b a t e x t e n d e d o n l y a s h o r t dlataoee beyond t h e m o u n t a i n s (Green a n d R o d d l c k # 1962).. S i n c e g le . e l a t i o n * s t r e a m s have cut narrow t r e n c h e s a n d canyons i n t o o l d e r v a l l e y o o t t o m s . PREVIOUS QmimiQAL mm Early i n v e s t i g a t i o n s In t h e T o m b s t o n e a r e a were mad® by M c C o n n e l i (1903) a n d C o e k f l e l d (1916 a n d 1929). The work c o n s i s t e d of r e c o n a o l t e r l n g th® c o u n t r y a n d examining a l n e r a l d e p o s i t s * G r e e n a n d R o d d i c k (1961) mapred th® area a t a s c a l e of 1 i n c h t o 4 a i l e s a s p a r t of a p r o j e c t known as O p e r a t i o n Ogilvte. Vernon a n d Hughes (1961) s a p p e d the a u r f l e t a l geology* Teapelman-fflult (1964 a n d 1965) c a r r i e d out m a p p i n g i n t h e T o m b s t o n e :uap-a r e a a t a s c a l e of 1 Inch t o 1/2 ; a i l e f o r e v e n t u a l p u b l i c -a t i o n at 1 I n c h to 1 a i l © * The p r e s e n t f i e l d work was c a r r i e d o u t by t h e w r i t e r d u r i n g t h e months o f J u l y a n d A u g u s t , 1964, while employed by the G e o l o g i c a l Survey of C a n a d a , AQ.i * ) are massive and obaracterlstloally have thin partings of slate and p h y l l i t e (commonly grsphitic) between thicker beds* The. maximus thickness of t h i s unit has been est Lias ted to be 45,000 feet (Green and Roddick, 1962)* Resent work, however, suggests that the roc its are Iso-o l i n a l l y folded with steeply dipping axial plc.ies and that 1 the true Va.iokne.sa i s l a the order of 5000 f e e t , The contact between the quartsite and th® underlying rooka " l a considered to be a gently southeast-dipping thrust surface upon which the quartzit® has aoved to the northwest.* (Teajpeliaun-Kluit, 1965). Two large stocks that intrude th® sedimentary and aetamorphia roc&e In the Tombstone map-area range i n composition froxn d l o r i t e to syenite. Textures of these rocks range froa fine-grained end equlgranulor, to coarse-grained and porphyritic. f o l i a t i o n i n th© rock i a shown by alllament of tabular feldspar pbenocrysts. Inclusions are re.ro. Age of the g r a n i t i c rocks I s not known, though they are thought to be co;ate.tiparsneoue with granitic rooks far to the southeast which have been dated as Middle Cretaceous (Green and 'Roddick, 1962) . 1 T e m p e l m a n - K l u i t , p e r s o n a l c o m m u n i c a t i o n DESCRIPTION Mount Brenner i a the highest peak of the moat northern granitic body in the To.&b stone asp-orea* In t h i s thesis the g r a n i t i c body i s referred, to as th© Mount Brenner stocs* Th© i n t r u s i o n o^eupies s roughly e l l i p t i c a l area with i t s lo/ig axis trending northeastward. It crops out over an area of about 1? square mllee and exhibits a more or less, concentric arrangement of i t s various subdivisions* A segment of auglt#-*blotlte laonaonite l i e s around the western part of the etoeW A orescent* shaped* narrow band of ,» Quartz occurs i n two formsj (l) i r r e g u l a r , i n t e r s t i t i a l , fractured grains w i t h undulatory extinction; and (2) euhedral, unfractured grains, with even e x t i n c t i o n , that are included i n the unzoned margins of pe r t h l t e phenocrysts ( p i . VII 13), Zoned green hornblende that has a pleochrolo scheme that i s very s i m i l a r to hornblende i n porphyritlc hornblende monzonite (see p.20 ) f p o l k i l l t l c a l l y encloses i n c l u s i o n s of plagloclase, auglte, a p a t i t e , z i r c o n , and magnetite* Trace amounts of auglte occurs either as small grains rimmed by hornblende or a s tiny grains p a l k i l t I c a l l y enclosed i n hornblende* h l l s n i t e , spheae and zircon are very common accessory miner-Is in t h i s rock, AllanIte f-d i r r e g u l a r un zoned margins enclose randomly o r i e n t a t e d ailo- r a l s o f t h e m a t r i x but do not e n c l o s e p l a g i o c l a s e s i m i l a r t o that found In the c o r e , Aohedral o s o i l l a t o r i l y zoned p l a g i o c l a s e ph..nocrysts (•*'ng..1- )^ t h a t average 1 am. by 3 airs,, are corroded and embayed by c o t e s h f e l d s p a r of t h e m a t r i x ( p i . XIV), Subbedral t o enhedrel h o r n b l e n d e ( p i . XIV c ) , t h a t ra > e s up t o 5 mm. i n long e s t d i x e n s i n, c o l k l l i t i c a l l y • n cloae g r a i n s o f a u ^ i t e , plfigiocl*»se and p e r t h l t e . The m a t r i x h-s a .". vbla t i e s i m i l a r t o t h a t o f the en :. oalng cry stallXsing .ma» ma, Ho 1 1 i U l d , saturated with a certain ;;»muer of a reaction series, can aelt I n c l u s i o n s consist 1 i g of minerals fcelo ging to an 2c earlier s t ^ e of t.-.e re-.ction series (bowen, 192o, p. 221) • ?aus, augite would be oat of equilibrium In a ai^aw of quarts monzonite composition where bornble.-de Is the stable a-.flc mineral pb^se. The presence of abundant augite in the Inclusions Indicates eltber non»equllibriu3» or, perhaps*, that the xenolith was relatively anhydrous ard impermeable, Pletributtoo of inclusions In the porphyrltio horn, lends ttonsohltc and in the pink quartz monaorjlte por'hyry but not in the au<;..Ite biotite monzonite, the presence af ahundnat magma by a s s i m i -l a t i o n of country rock has not been a fac t o r In causing compositional v a r i a t i o n of the stock. D i f f e r e n t i a t i o n The systematic compositional v a r i a t i o n within the stock (See p.30 : n (' the following figures) l a compatible with the concept of d i f f e r e n t i a t i o n - of a ma:: ma as i t c r y s t a l l i z e d inward from the margin. The a u g i t e - b i o t i t e monzonite i s conride red to represent th© ^imposition of the 52 parent f l u i d because I t was the f i r s t rock zone to be eopl cad and because i t has & composition from which a l l other rock types can be formed by r e l a t i v e l y simple processes of d i f f e r e n t i a t i o n . That the main rock types of the stock pass from under-saturated, through saturated to oversaturated types (fl&. 5ai seems to suggest that the thermal b t r ^ l e r of the system b l 0 2 - nahlblC^ - S A I 0 I O 4 (Bowen 1937) has been crossed. T l l l e y (1957) suggests, however, that the thermal b a r r i e r which prevents the transition from syenite to granite in toe experimental melts '"say be "Inoperative i n a natural melt where i t s composition Is modified by the presence of nafic components, and fu r t h e r , notable d i s p a r l t In v o l a t i l e content". The trend from undersaturated to ovsrseturatec" magaa can ce accompli oh ed by various processe of e l i t e ; t i o n of undersaturated map-mas, louring, c r y s t a l l i z a t i o n of trie magma, the residual l i q u i d can be enriched i n s i l i c a by reaction or inc.* of reaction of th© l i q u i d with minerals that have c h a r a c t e r i s t i c s of lneon-gruent melting,, T l l l e y (1957) suggests that " p r e c i p i t a -t i o n of mwgnetlte In the place of aenerlna w;-uld tnrow excess s i l i c a i n t o the l i q u i d i f t h i s oxide pnase f a i l e d to resorb". The formation of hornblende by reaction of l i q u i d with pyroxene w i l l also r e s u l t i n s e t t i n g free of 53 quartz, arid thus the possible quantity of a .auartzose d i f f e r e n t i a t e w o u l d be a u g m e n t e d (Bowes, 1923, p, 9 0 ) , The change o f t h e SlOg c u r v e ( f i g . 5a) from tae u n d e r -s a t u r o t e d Into the s a t u r a t e d areas of the diagram corresponds with the a p p e a r a n c e of hornblende as the m a j o r mafic mineral. The 5 l 0 o c o n t e n t Increases toward t h e ovc-rsaturated area w i t h ino easing amojot of hornblende (compare p o i n t s 3 end C with the corresponding, modes i n T a b l e I ) . Thus, the formation o f hornblende may h o v e played a signifies.*at role I n s i l l c a t l o n of t h e r e s i d u a l l i q u i d . The foregoing processes w i l l a f f e o t t h e compostlon of the f i n a l r e s i d u a l l i q u i d s and t h e corresponding rocks, b u t can only cause d i ffer t - n tia11 on i f r e l a t i v e movement b e t w e e n c r y s t a l s and residual l i q u i d i s brought about by p r o c e s s e s such a a c r u v l t y separation or " f i l t e r p r c s s l r g " . C e r t a i n o x i d e s o f the h o r n b l e n d e - c l o t I t e p y r o x -e n l t e show s i g n i f i c a n t d e p a r t u r e s from t h e emooth v a r i a t i o n curves ( f i g s , 4 and 6 ) . T h i s s o r t of departure i s a t t r i b u t e d by bowen il92a) to c r y s t a l a c c u m u l a t i o n . I f t h e p o r p h y r l t i c hornblende monzonite i s to be f o r m e d from t h e a u g l t e - b l o t i t e m o n z o n i t e by t h e s e p a r a t i o n of components o f b l o t i t e - h o r n b l e n d e p y r o x e u l t e alone, the r a t i o of volumes of the p y r o x e n l t e to components of t h e m o n z o n i t e 54 should be approximately 1 to 3 (calculated fro a f i g . 4 ) . Migration of water and a l k a l i s can a l s o c o n t r i -bute to d i f f e r e n t i a t i o n of a. -magma. Water w i l l diffuse and distribute i t s e l f in a magiaa so that the chemical potential of the water Is the sane throughout the magma chamber. Al k a l i s and certain metals w i l l co-ordinate with the water and w i l l be concentrated In the regions of lowest pressure and temperature (Kennedy, 1355)# A hypothetical sequence of events that i s prop-osed to explain the development of rock zones, i s Illustrated d l a g r a a a t l c a l l y In Figure 10, The processes of v o l a t i l e and a l k a l i d i f f u s i o n and of f r a c t i o n a l c r y s t a l -l i z a t i o n , which effected differentiation of the parent magma, were Interrupted by Intrusion. During the f i r s t ©tag© of Intrusion ( f i g , 10a) the upper and outer parts of the magsaa which formed a cupola of a larger zaagiaa chamber, cooled very rapidly to form the fine-grained auglte-blotite monzonite. .Effects of the hot magma on the relatively cool country rocks were to produce a zone of cordlerlte h o r n f e l s In c e l l t i c roc'ith the Feldspar Pyroxene F i g u r e 10a Figure 10c Figure 10b LEGEND A p l i t s Pink quartz monzonite porphyry P o r p h y r i t l c hornblende monzonite Monzonite porphyry Hornblende-blotite pyroxenlte A u g i t e - b i o t l t e monzonite Country rocks Present l e v e l of erosion Figure 10. Diagrammatic sketch showing a h y p o t h e t i c a l development of rock zones of the Mount Brenner 3tock„ I n i t i a l emplacement of the aagaia, temperature and pressure gradients were e s t a b l i s h e d i n the magma chamber* Water and alkali® tended to d i f f u s e In the magma and they grad-u a l l y c o l l e c t e d and became concentrated In the upper and outer regions of the cupola, the regions of lowest pressure and temperature. Large potassium feldspars c r y s t a l l i s e d from the higaly f l u i d sikali-rleh magma thus produced. Early formed pyroxene c r y s t a l s sank In the fluid l i q u i d and tended to accumulate oa the more viscous magma Immediately"below this region* The magma below t h i s region was a l s o beginning to c r y s t a l l i z e , and processes e f f e c t i n g d i f f e r e n t i a t i o n of the magma were constantly taking place, that l e , d i f f u s i o n of volatlles and of alkalis, and s i l i c a t i o n of r e s i d u a l fluids by the mechanisms previously mentioned (See page 52 ), and gravity se-pa ration, Following c o n s o l i d a t i o n of the outer find up er parts of the cupola, the saagiaa continued to surge upward, and i t intruded the augite-blotlte monzonite (fig* 10b), The accumulated pryoxene and the large orthoclase crystals, that l i e immediately below tue outar zone, were dragged up and spread along the wall of the chamber by the upward, and outward force of the i n t r u d i n g magiaa. This cryatal 57 aiush l a t e r cooled to form toe hornblende-biotite pyroxen-It® and the monzonite porphyry* The newly Intruded magma continued to c r y s t a l l i z e inward i n a zone which la now the porphyritlc hornblende aoazonlts (th© intermediate zone), 'Che medium- to coarse-grained texture of this zone suggests that the magma cooled ©ore slowly than d i d the auglte-blotite monzonite, p o s s i b l y because the country rooks were preheated by the e a r l i e r stag© of i n t r u s i o n , The mobile core continued to surge upward, C r y s t a l s suspended In the f l u i d were aligned by magma-tic flow. The magma reacted with the p a r t i a l l y consolidated Inner margin of th© Intermediate zone to form the t r a n s i t i o n zone of glomeroporphyrltio hornblende monzonite, Th© lenses of monzonite porphyry, which occur In some places at the contact between the core and the porp~.y r i t i c horn-blende monzonite, may have formed by processes s i m i l a r to those which formed the main band of monzonite porph...-orth -merica| Geol, Soc. Amer,, B u l l . , v* 70, p. 671-747. C o c k f l e l d , W.ia, l i l h : S i l v e r - l e a d Deposits of" the L i t t l e Twelvemile Area; Geol, Surv, Canada, hum. nept., lj>lu, pt. a, p , 1 and p t , B, p , 1-17. 1913: Explorations In the O g i l v i e iiangej Geol. Surv,, Canada, Sum, Rept., 1919, ft. A, :.. 1, and pt. a, P. 1 - 7 . Co aptan, a » a « C a l i f o r n i a ; Geol. .3sc. 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