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The geology of the Indin "Break", N.W.T. Hodgson, Alexander G. 1948

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&7 #6 Gr 3 Co/*- f THE GEOLOGY OP THE INDIN "BREAK" N.W.T ALEXANDER G, HODGSON A thesis submitted i n partial fulfilment of the requirements for the degree of Master of Applied Science in the Department of Geology and Geography THE UNIVERSITY OP BRITISH COLUMBIA APRIL 194B ABSTRACT This thesis presents the results of a general geological study of a belt of Archean rocks in the Indin Lake d i s t r i c t , N.W.T. The summary results of a summer's f i e l d work are incorporated with a brief petrographical investigation of different rock types. Intermediate to acidic lava flows and pyroclastic rocks overlain conformably (?) by a succession of clastic sediments are invaded by minor acidic intrusives and a plexus of basic dykes and s i l l s . Isoclinal folds in sediments, broader flexures in volcanics, bedded shear zones, and regional foli a t i o n and lineation are a l l attributed to a single system of powerful tangetial forces that affected a l l stratiform rocks in the area. Zones of weakness at contacts are imputed, to differential competence between volcanic and sedimentary rocks, and a. theory of folding of rock units of differing competency is applied in modified form to the origin of the Indin "break" - a gold-bearing shear zone close to a major contact. Evidence is presented to suggest that ncross-faults- clean-cut dislocations that transect the .-regional structural trend - and emplacement of late basic ihtrusives are quasi- contemporaneous events that progressed over a protracted interval of time and may have been consequent upon a single deformational-pattern. An attempt to explain the propinquity, and thus the possible structural relation, of gold mineral- ization to cross-faults is embodied in a theory relating the control of mineralization "ibp channeling of ore-solutions in structures developed during the period of strain accumulation that culminated in cross-faulting. An almost completely reconstituted mineral assemblage is ascribed to a moderate grade of regional metamorphism consisting of dynamic metamorphism during orogeny and relatively.minor effects of superimposed thermal and retrograde metamorphism. Metacrysts of ankeritic carbonate, believed to be hydro- thermal, are discussed, and their superficial genetic or structural association with gold mineralization is offered as a possible guide to future ore'discovery. Rather detailed descriptions are submitted of ankeritic carbonate, an unusual "hornblende" occurring in amphibolite, and'a green-mica, tentatively called phengitic-muscovite, from a carbonate zone. ACKNOWLEDGMENTS The w r i t e r i s g r e a t l y o b l i g a t e d to Trans American Mining C o r p o r a t i o n f o r p r o v i d i n g l i b e r a l access to maps and a e r i a l photographs,.and i n p a r t i c u l a r to Dr. A.P". Beavan of tha t company who guided the w r i t e r i n h i s f i e l d work and c o n t r i b u t e d v a l u a b l e i n f o r m a t i o n p e r t a i n i n g t o d r i l l i n g r e - s u l t s and other g e o l o g i c a l aspects of the a r e a . S p e c i a l indebtedness i s owing Dr. H.C. Gunning, of the Department of Geology and Geography, who gave a comprehensive o u t l i n e of the problem, d i r e c t e d and a s s i s t e d w i t h l a b o r a t o r y i n v e s t i g a t i o n s , and o f f e r e d c o n s t r u c t i v e c r i t i c i s m o f the p r e l i m i n a r y manuscript. Great thanks are a l s o owing Dr. K. DeP. Watson who k i n d l y s u p p l i e d h e l p f u l i n f o r m a t i o n and l i t e r a t u r e r e l a t i n g to the many p e t r o g r a p h i c a l problems encountered by the w r i t e r . CONTENTS PAGE Introduction 1 General Statement 1 Description of the Area • 2 Object of the Field Work 4 Field Methods . 5 General Geology 7 Summary ' 7 Table of Formations 9 Volcanic Rocks 9 Problem of the Volcanic Rocks 9 Description of Volcanic Rocks 11 Sedimentary Rocks 23 Relation of Volcanic and Sedimentary Rocks 29 General 29 Nature of the Contact .Zone 30 Conclusions . 3 1 Intrusive Rocks 31 General 31 "Bird-Porphyry" and Similar Rocks 32 Altered Dykes 35 "Metadiorite" 37 Minor Acidic Intrusives 39 Gabbro and Diorite 41 Diabase 45 Structure 47 General Statement 47 Folding 48 Regional Foliation and Lineation 52 Faulting ' 5 5 Shear Zones 56 The Indin "Break" 57 "Cross-Faults" 61 The Inca Fault 65 Relation of Cross-Faulting and Mineralization 66 Regional Metamorphism 73 Carbonate Alteration 81 ILLUSTRATIONS Figure 1 Sketch Map Showing Location of Indin Lake Figure 2 Generalized Geological Map of Indin Lake District Figure 3-12 Photographs Plates 1-16 Photomicrographs Geological Map INDIN LAKE AE GEOLOGICAL MAP A P P R O X I M A T E S C A L E - I. n - 1800 ft ( a i r P ^ * o b o s , LEGEND / R o c k C l e a v a g e B e d d i n g - D i p '• T o p H Fold R xes - Rnhc lme : S^ncline \ S u r f a c e E l o n g a t i o n o f F r a g m e n t s S e d i m e n t s - Gre^wacke , S la te , e tc . S e d i m e n t s - Q u a r t z i f e , R r k o s e e t c . P h y o l i f e , "Porphyntic "R hyol i t e \ B r e c c i a Q u a r t z - F e l d s p a r "Porphyry j T ^ T p l i t e , e t c ^ ^ F a u l t - D e t e r m i n e d : R s s u m e d C o n t a c t - " T r a c e d '. P r o b a b l e C o n t a c t - R s s u m e d S u r f a c e T r e n c h i n g R n d e s i t e , D a c i t e , Agg lomera te T u ^ [ G o l d O c c u r r e n c e R H c r e d "Rocks , RmpVuhol ' rVe etc Q u a r t z i t e a n d flrkose P r o m i n e n t Bi o t i t i c Z o n e P . t l o w S t r u c 1" u r e THE GEOLOGY OF THE INDIN BREAK » N.W.T. INTRODUCTION GENERAL STATEMENT , The I n d i n Lake g o l d - b e a r i n g a r e a , about 125 miles n o r t h of Y e l l o w k n i f e , - N.W.T., i s i n the southeast c o r n e r of the Ingray Lake Map-Area. The l o c a t i o n and r e g i o n a l g e o l o g i c a l p i c t u r e , summarized on f i g s . 1 and 2, can he found on Map 697 A, Ingray Lake, D i s t r i c t of MacKenzie, N.W-.T., of the G e o l o g i c a l Survey of Canada. P r o s p e c t i n g and e x p l o r a t i o n have been widespread i n the area s i n c e the f i r s t g o l d d i s c o v e r y i n the summer of 1938. No major g o l d p r o d u c t i o n has ensued,but r e c e n t d i a m o n d - d r i l l i n g and underground work by mining companies a c t i v e i n the area have developed s e v e r a l promising gold d e p o s i t s , and the d i s t r i c t c u r r e n t l y a t t r a c t s c o n s i d e r a b l e a t t e n t i o n as a p o t e n t i a l gold-mining camp. I n i t i a l g e o l o g i c a l mapping of the area was conducted by Wilson and Lor d i n 1939 and 1940, and the r e s u l t s of t h e i r work appear i n G e o l o g i c a l Survey of Canada Memoir 235, and Map 697 A s which covers the Ingray Lake Map-Area. The I n d i n Lake s e c t i o n was re-examined i n g r e a t e r d e t a i l by the G e o l o g i c a l Survey d u r i n g the summers of 1946 and .1947, but a t the time of w r i t i n g the r e s u l t s of t h i s work have not been FIGURE: 1 SKETCH MAP SHOWING LOCATION OF INDIN:. LAKE F/GURE 2 p u b l i s h e d . During the f i e l d s-eason of 1947 the w r i t e r was employed by Trans American Mining C o r p o r a t i o n i n the I n d i n Lake d i s t r i c t , and c a r r i e d out g e o l o g i c a l mapping i n the i n t e r e s t s of that company. Inf o r m a t i o n gathered i n the f i e l d ; t o g e t h e r w i t h the r e s u l t s of a b r i e f l a b o r a t o r y study of the rocks,, form the b a s i s of t h i s t h e s i s . The l a b o r a t o r y work i s designed to s u p p l y a d d i t i o n a l , but by no means exhaustive^ i n f o r m a t i o n on the main r o c k types i n the a r e a . R e s u l t s are almost w h o l l y those of the w r i t e r , but some i n f o r m a t i o n was drawn from the scant l i t e r a t u r e a v a i l a b l e on the area when d e a l i n g w i t h problems of a r e g i o n a l n a t u r e . I n a d d i t i o n , .the geology on Inca P e n i n s u l a and p a r t s of Johnson and T a r t a n i s l a n d s was p r e v i o u s l y mapped by Trans American and o n l y p a r t i a l l y r e - examined by the w r i t e r . The geology on the accompanying map embraces a s t r i p of c o u n t r y about 15 miles l o n g from south to n o r t h and 2 to 3 miles wide from west to e a s t . The map-area, which s t r a d d l e s a major c o n t a c t between sedimentary and v o l c a n i c r o c k s , extends from south of Johnson I s l a n d i n I n d i n Lake t o a p o i n t about 'lO miles n o r t h of Lex Bay. The southern p o r t i o n i n c l u d e s the 'gold p r o p e r t i e s of L e x i n d i n Gold Mines, D i v e r s i f i e d Mining I n t e r e s t s , and North Inca Gold Mines. Development of the l a t t e r property, i s i n the hands of Trans American Mining C o r p o r a t i o n . DESCRIPTION.OF THE AREA The area i s s e r v i c e d from Y e l l o w k n i f e , 125 miles to the 3 south, by ..aeroplanes o p e r a t i n g on f l o a t s i n summer and s k i s or wheels i n w i n t e r . A p r o j e c t e d a i r - s t r i p w i l l , when completed, provide year-round l a n d i n g f a c i l i t i e s f o r a i r c r a f t on wheels. I n d i n Lake may a l s o be reached by canoe, v i a Russell. Lake and Snare R i v e r , but the t r i p i s l o n g and arduous, wit h numerous portages; so t h a t a i r c r a f t i s the u s u a l means of e n t e r i n g the a r e a . The topography r e f l e c t s the g e n e r a l c h a r a c t e r of the u n d e r l y i n g r o c k s . Sediments occupy low, f e a t u r e l e s s areas w i t h g e n t l y rounded h i l l s , while v o l c a n i c s u s u a l l y outcrop as r e l a t i v e l y h i g h r i d g e s p a r a l l e l i n g the n o r t h - s o u t h s t r u c t u r e and r i s i n g 200 f e e t or more above a d j a c e n t l a k e s . Many sedim e n t - v o l c a n i c c o n t a c t s are marked by an abrupt s c a r p v i s i b l e on a e r i a l photographs. D r i f t - f i l l e d d e p r e s s i o n s , p a r a l l e l and t r a n s v e r s e t o the no r t h - s o u t h s t r u c t u r a l t r e n d , are occupied by f a u l t s , shear zones, o r b a s i c dykes and s i l l s . I n the n o r t h e r n p a r t of the area topographic e x p r e s s i o n of bedrock geology i s masked c o n s i d e r a b l y , b y widespread d e p o s i t s of g l a c i a l d e b r i s . Rock exposure i s good i n the southern p a r t o f the area, t 'but i s u n d e s i r a b l y scarce i n the c e n t r a l and n o r t h e r n p a r t s . I n many areas i t i s l i m i t e d to broken rubble o r s c a t t e r e d f r o s t - h e a v e s and outcrops of v o l c a n i c s so s m a l l and d i s c o n - tinuous as t o be improbably r e p r e s e n t a t i v e of the e n t i r e v o l c a n i c assemblage. I n d i n Lake i s the l a r g e s t body of water, and no major r i v e r s c r o s s the map-area. Chains of s m a l l l a k e s d r a i n 4 southward i n t o I n d i n Lake through a s e r i e s of s m a l l streams co n n e c t i n g adjacent l a k e s . Although c l o s e to the western f r i n g e of the b a r r e n l a n d s the area i s well-wooded w i t h s t u n t e d spruce and b i r c h up to 12* i n diameter. S m a l l , i s o l a t e d stands of good timber,, up to 2' o r more i n diameter* occur a l o n g some of the stream beds. Caribou are p l e n t i f u l i n the s p r i n g and f a l l , and most of the lakes are w e l l - s t o c k e d with f i s h . ' OBJECT OF THE FIELD WORK The main orebodies of L e x i n d i n , D i v e r s i f i e d and North Inca. l i e , i n t h a t o r d e r from n o r t h to south, along a "shear- zone" i n sediments c l o s e to a.major c o n t a c t w i t h v o l c a n i c s . What i s presumably the s o u t h e r l y e x t e n s i o n of the same s t r u c t u r e was c u t i n a d r i l l - h o l e o f f the n o r t h end of Johnson I s l a n d , g i v i n g an o v e r - a l l p o s s i b l e l e n g t h of approximately 12,000 f t to the s t r u c t u r e . The "shear-zone" i s termed l o c a l l y the I n d i n "break", and has been regarded by some as a major c r u s t a l d i s l o c a t i o n . Map 697 A shows a f a u l t , s t a t e d to have been t r a c e d f o r 46 m i l e s ( L o r d , 1942, p. 43), t h a t r o u g h l y c o i n c i d e s w i t h the I n d i n "break". T h i s , of jcourse, suggests t h a t the orebodies l i e i n , o r are c l o s e l y a s s o c i a t e d w i t h , a major n o r t h - s o u t h t r e n d i n g f a u l t . C o n d i t i o n s are complicated by w e l l - e s t a b l i s h e d , northwest- 's t r i k i n g " c r o s s - f a u l t s " , and the presence, a t North Inca, of . at l e a s t one high-grade orebody, i n v o l c a n i c s r a t h e r than 'sediments : and c o n c e i v a b l y r e l a t e d to " c r o s s - f a u l t i n g " . However, as the I n d i n "break", whatever i t s o r i g i n , i s > a p p a r e n t l y the main s t r u c t u r a l c o n t r o l of most of the ore deposits,, a c l e a r u nderstanding of i t s nature i s of v i t a l importance as an a i d i n d e v e l o p i n g known orebodies and i n the se a r c h f o r new d e p o s i t s . A c c o r d i n g l y , a program of g e o l o g i c a l mapping was i n i t i a t e d i n v o l v i n g the coverage of a s e c t i o n a l o n g the assumed main "break". Emphasis was p l a c e d on s t r u c t u r e , but the a r e a l d i s t r i b u t i o n and r e l a t i o n s h i p s of a s s o c i a t e d rocks were c o n s i d e r e d i n some d e t a i l . FIELD METHODS G e o l o g i c a l o b s e r v a t i o n s were rec o r d e d d i r e c t l y on a e r i a l photographs having a s c a l e of 400 f e e t to 1 i n c h . These were enlargements of o r i g i n a l Royal Canadian A i r f o r c e photos f l o w n at a pproximately 1800 f e e t to 1 i n c h . Two methods were employed to l o c a t e ground p o i n t s on the photographs: (1) R e c o g n i t i o n of ground topographic f e a t u r e s on the photos. (2) Use of b a s e - l i n e s t i e d to. r e c o g n i z a b l e ground f e a t - ures on the photos. The f i r s t method was used whenever p o s s i b l e as i t i s [quick and a c c u r a t e , and c o n s i s t s merely of c o r r e l a t i n g 'prominent ground f e a t u r e s , such as outcrops and lake-margins, w i t h t h e i r p o s i t i o n s on the photo.- Because enlargements tend to be hazy i t proved h e l p f u l to c a r r y the o r i g i n a l s i n t o the f i e l d to a s s i s t i n l o c a t i o n . A p o i n t l o c a t e d on the o r i g i n a l : i s r e a d i l y t r a n s f e r r e d to the enlargement. In a d d i t i o n , o r i g i n a l photographs w i t h c o n s i d e r a b l e o v e r l a p p r o v i d e a ready means of o b t a i n i n g the t h i r d dimension, which f a c i l i t a t e s the l o c a t i o n of ground p o i n t s i n areas of much r e l i e f . Three dimensions may be r e a d i l y o b t ained from d u p l i c a t e photos, e without r e s o r t i n g t o the stereoscope-, once the technique has been mastered. In areas of heavy bush or overburden base- l i n e s were cut and t i e d to p o i n t s r e c o g n i z a b l e on the a e r i a l photographs. Such l i n e s , when p l o t t e d a c c u r a t e l y , a s s i s t m a t e r i a l l y i n l o c a t i n g outcrops on the photographs. So f a r as p o s s i b l e ^ outcrop boundaries were d e l i n e a t e d , but areas of numerous' s m a l l outcrops were d e p i c t e d as a s i n g l e exposure. Observations on c o n t a c t s , a t t i t u d e s of bedding and cleavage, q u a r t z - v e i n i n g e t c . , were recorded d i r e c t l y on the a e r i a l photographs. I n f o r m a t i o n on the photographs was assembled on a 400-scale outcrop map embracing the s e c t i o n from the southern boundary of the area to Negus Lake. The o u t l i n e map and geology-were t r a c e d d i r e c t l y from the photographs. Noteworthy here i s the s l i g h t decrease of s c a l e on photos from centre to margin and the d e s i r a b i l i t y of p r o v i d i n g c o n s i d e r a b l e o v e r l a p between a d j a c e n t photos and cpnfining., mapping to the c e n t r a l p o r t i o n of each photo where I • . ( d i s t o r t i o n i s a minimum. The accompanying map of the e n t i r e area i s a c o m p i l a t i o n of f i e l d data on a s c a l e of about 1800 f e e t t o 1 i n c h , or more e x a c t l y , on the s c a l e of the o r i g i n a l photographs. I n d i v i d u a l outcrops are not shown, minor d e t a i l s are e l i m i n a t e d , and • some inferences'have been made to complete the s t r u c t u r a l and l i t h o i o g i c a l p i c t u r e . F i g . 3 Caribou herd between Tartan I s l a n d and Inca Peninsula (May 1947). Inca Peninsula on l e f t , w i t h Brown veins on extreme p o i n t . The Inca f a u l t passes immediately south of the p o i n t . F i g . 4 A i r c r a f t on -wheels at North Inca. Last a i r c r a f t to land on wheels on the i c e at Indin Lake was on June 7,(1947). F i g . 5 The f i r s t a i r c r a f t on f l o a t s , June 18, 1947, landing at L e x i n d i n camp. F i g . 6 View lo o k i n g east scross Lex Bay. D i v e r s i f i e d camp on r i g h t . F i g . 8 View, l o o k i n g north from .Tartan I s l a n d , of Inca Peninsula. North Inca headframe on p o i n t . Brown veins on l e f t of headframe and main ore zone (I n d i n "break") u n d e r w a t e r o n r i g h t . Inca f a u l t passes south of point, trending northwest. F i g . 9 North Inca camp on Inca Peninsula. Rocks i n background are v o l c a n i c s , contact with sediments (and the Indin "break") under water i n the foreground. P i c t u r e l o o k i n g west. F i g . 1 0 Looking west across the northern end of Cranston Lake. Foreground i n sediments; r e l a t i v e l y h i g h , north-south trending ridge i n background t y p i c a l of much of the v o l c a n i c t e r r a i n . Contact under the lake close to the western shore. 1̂ Crcins+on • i 1 1 **" P i g . 11 Panorama, l o o k i n g n o r t h , of the area around Cranston Lake. Sediments on extreme right*, v o l c a n i c s central', sediments u n d e r l i e much of lake on the l e f t ; and v o l c a n i c s on extreme r i g h t . L i n e a t i o n i n foreground marks fhe locus of a " c r o s s - f a u l t " . P i g . 12 Panorama of area north of Lex Bay; i n d i n L^ke on the extreme l e f t . Water i n f o r e - ground marks approximate volcanic-sediment contact (and probable locus of Indin "break". V o l c a n i c s i n foreground and sediments background. 7 • GENERAL GEOLOGY SUMMARY The area i s u n d e r l a i n by Precambrian rocks of the Archean Y e l l o w k n i f e group. A heterogeneous v o l c a n i c assemblage of a n d e s i t e , d a c i t e , r h y o l i t e , p y r o c l a s t i ' c s and a l t e r e d rocks are o v e r l a i n conformably (?) by a monotonous s u c c e s s i o n of c l a s t i c sediments comprised mainly of interbedded greywacke, s l a t e and impure q u a r t z i t e . V o l c a n i c s and sediments are l o c a l l y i n t e r c a l a t e d a t c o n t a c t s . One band of predominantly c o a r s e - .grained sediments may be o l d e r than some of the v o l c a n i c s . V o l c a n i c t e r r a i n i n c l u d e s some a c i d porphyry and d i o t i t i c r ock probably, i n p a r t , i n t r u s i v e . V o l c a n i c and sedimentary rocks have been i n t r u d e d by a c i d i c to b a s i c dykes and s i l l s o f . s e v e r a l ages, i n c l u d i n g f r e s h diabase dykes of p o s s i b l e • F r o t e r o z o i c age. G r a n i t i c i n t r u s i v e s , which surround the r e g i o n , are not exposed i n the map-area. Rocks of the Y e l l o w k n i f e group have been d i s r u p t e d by i n t e n s e f o l d i n g and f a u l t i n g , and have been s u b j e c t e d to a moderate degree of metamorphism. Intense f o l d i n g s r e f l e c t i n g ! c r u s t a l adjustments t o g r e a t l a t e r a l s t r e s s e s , may have culminated i n the development of shear-zones i n incompetent rocks, w i t h p o s s i b l e attendant.' t h r u s t - f a u l t i n g a long some of the shears. F o l d i n g was succeeded by a t l e a s t one p e r i o d of major f a u l t i n g accompanied by the f o r m a t i o n of s m a l l , s t e e p l y - p l u n g i n g d r a g - f o l d s . F a u l t i n g p r o b a b l y extended over a l o n g ' time, and was quasi-contemporaneous w i t h i n t r u s i o n s of b a s i c 8 dykes. Dynamic metamorphism accompanying folding, with relatively minor effects of superimposed thermal and retrograde metamorphism, have modified or obliterated primary structures and reconstituted most rocks to an assemblage of new minerals. No record exists for the deposition of Paleozoic or Mesozoic strata, and since Precambrian time the history of the area may have been one of erosion down to the present level, with slight modifications attributable to the Pleistocene period of glaciation. Intrusive Rocks Sedimentary Rocks: Volcanic Rocks: TABLE OF FORMATIONS Diabase dykes Diorite and gabbro dykes and s i l l s "Metadiorlte" Altered dykes (carbonate and chlorite) Acidic dykes and s i l l s "Bird-Porphyry" and similar rocks. Greywacke, slate, arkose, quartzite Graphitic slate, slaty greywacke Tuffaceous sediments. • Agglomerate and tuff Rhyolite', porptiyritic rhyolite, rhyolite breccia Quartz-feldspar porphyry Dacite, porphyritic dacite, fragmental dacite Andesite-massive, fragmental, pillowed, amygdaloidal Amphibolite Carbonate, serieite, and chlorite schists The units l i s t e d i n the foregoing table do not necessarily read i n the correct age sequence. Too much remains to be learned about the structural set-up and age relationships to permit such a tabulation. VOLCANIC ROCKS Problem of the Volcanic Rocks Volcanic rocks proved d i f f i c u l t to deal with in the f i e l d ;as regards both structural relationships and petrographic classification, and microscopic study does l i t t l e towards \solving the problem. To.begin with, rocks in the area have been subjected to great deformation and intense alteration. Volcanic rocks proved particularly susceptible to alteration, and* in many places,, have been converted to serieite, car- bonate, and chlorite schists to which i t is impossible to assign any precise original composition. Relatively fresh-looking specimens show an almost completely reconstituted mineral assemblage that is strikingly similar in many apparently dissimilar rock types i n the f i e l d . Primary minerals, with the exception of quartz, are rare or entirely lacking. Feldspar can seldom be positively identified- i f so i t is usually a sodic-variety- and ferromagnesian minerals have been largely converted to chlorite. Another problem is the separation of intrusive from extrusive. Certain d i o r i t i c intrusives closely resemble some facies of massive, recrystallized andesite, and much rock classed as porphyritic rhyolite is very similar to bodies of quartz-feldspar porphyry believed to be, in part, intrusive. In addition, structural c r i t e r i a in volcanics are rare i and unreliable. Definite horizons can seldom be traced with any degree of certainty beyond a single outcrop, and primary flow structures are either obliterated or so highly deformed as to require cautious interpretation as structural indicators. Flow rocks were named in the f i e l d mainly on the basis of color and quartz content. Typical basaltic lavas were not encountered in the map-area, so that flows range from andesite to rhyolite; the former dark green and quartz-free, the latter light shades of cream or grey and quartz-rich. Intermediate facies are dacites with light-green or grey color and aphanitic or porphyritic texture. Pyroclastics are agglomerates i f fragments are numerous and over one inch and tuffs i f fragments are less than one inch. However, one aggravating feature of a l l the volcanic rocks is their imperceptible gradation from one type to another, and since divisions are arbitrary to begin with, i t is impossible: in practice, to adhere to any rigid system of classification. Such rocks can only be described, and grouped as large units for mapping purposes. until they are studied in much greater detail. The writer does not propose to enter into any detailed descriptive or petrographic discussion of rock types. Innumerable accounts of similar rocks appear in the literature, and an exhaustive treatise here would be largely repetition and of doubtful merit i n connection with the present problem. Moreover, a complete petrographic study., to be of any value, would demand consideration of many more than the few typical rock types chosen by the writer for microscopic study. Description of Volcanic Rocks: Volcanic rocks are a heterogeneous assemblage of flows, pyroclastics, and altered equivalents. At one place or another most types of volcanics can be seen to grade along or across the strike into another type of the assemblage. Various members of the volcanic pile convey the impression of being lenticular and discontinuous, as i f no particular horizon had spread over a wide area of the volcanic f i e l d . Save for one band of "rhyolite" that may be intrusive, not a single marker horizon was traced more than a few thousand feet. Andesites are brown to green weathering, aphanitic to 12 f i n e l y c r y s t a l l i n e r o c k s . They are l i g h t to dark green or almost b l a c k on the f r e s h s u r face,and f r e q u e n t l y s t r e a k e d w i t h elongate patches of c h l o r i t i c m a t e r i a l darker than the main rock mass. Well-formed p i l l o w s are l o c a l l y abundant, and amygdules are widespread but seldom r e s t r i c t e d to any p a r t i c u l a r h o r i z o n i n a f l o w . P i l l o w s have t h e i r l o n g axes p a r a l l e l with the f o l i a t i o n and average 2 to 5 f e e t , a l t h o u g h "mattress" p i l l o w s up to 16 f e e t l o n g are l o c a l l y developed. The present p i l l o w alignment and accentuated e l o n g a t i o n may be due as much to d e f o r m a t i o n a l s t r e s s e s as to o r i g i n a l a t t i t u d e i n the approximate plane of the f l o w . L a r g e s t p i l l o w s are n o t i c e a b l y r e s t r i c t e d to the more b a s i c a n d e s i t e s . P i l l o w s f r e q u e n t l y e x h i b i t a f i n e - g r a i n e d , brown to b l a c k weathering, p e r i p h e r a l selvage l / 4 to 1 inchwLde. I n t e r - p i l l o w f i l l i n g s are of the same composition as the p i l l o w s . Amygdules show no c o n s i s t e n t p a t t e r n w i t h i n p i l l o w s and may occur i n g r e a t e s t abundance a t the p e r i p h e r y , a t the c o r e , or towards the top. of i n d i v i d u a l p i l l o w s . Amygdaloidal f i l l i n g s are c h i e f l y carbonate and quartz w i t h l e s s e r amounts of c h l o r i t e , epidote or c l i n o z o i s i t e , and hematite. Where carbonate amygdules are abundant the r o c k weathers wi t h a p i t t e d s u r f a c e . Andesites are i n f r e q u e n t l y fragmental; i f so fragments are of about the same composition as the main rock mass and the rock i s presumably a f l o w - b r e c c i a . A n d e s i t e s are composed almost e n t i r e l y of secondary m i n e r a l s . C h l o r i t e i s the most abundant c o l o r e d m i n e r a l - present i n f e l t e d aggregates, i r r e g u l a r f l a k e s and minute, 13 p r i s m a t i c g r a i n s . I t e x h i b i t s anomalous " B e r l i n - B l u e " and p e c u l i a r , d i r t y - g r e e n i s h , i n t e r f e r e n c e c o l o r s . Carbonate i s p l e n t i f u l a long w i t h v a r y i n g amounts of green amphibole, e p i d o t e , c l i n o z o i s i t e , b i o t i t e , and q u a r t z . O r i g i n a l f e l d s p a r s are o c c a s i o n a l l y evident, but are so h i g h l y shot with i n c l u s i o n s of o ther minerals t h a t t h e i r d e t e r m i n a t i o n i s i m p o s s i b l e . Numerous v e r y s m a l l , c l e a r g r a i n s are probably quartz and a l b i t e , but the r e l a t i v e abundance of the two minerals has not been determined. Where b i o t i t e i s present i t i s always i n c l o s e a s s o c i a t i o n w i t h c h l o r i t e . Most minerals show ragged o u t l i n e s and numerous i n c l u s i o n s of o t h e r m i n e r a l s . H i g h l y c h l o r i t i c a n desites'grade i n t o rocks w i t h c o n s i d e r a b l e amounts of f i b r o u s amphibole. These rocks are c l a s s e d as amphibolites and are developed mainly i n the n o r t h e r n p a r t of the map-area. They may be r e c r y s t a l l i z e d p o r t i o n s of t h i c k flows but, as a l r e a d y s t a t e d , are d i f f i c u l t to separate from some i n t r u s i v e , d i o r i t i c r o c k s . In the f i e l d they are massive to s c h i s t o s e , dark green rocks w i t h a green o r brown weathered s u r f a c e . A s e c t i o n from a t y p i c a l specimen r e v e a l s a rock composed of about 60 percent amphibole and the remainder q u a r t z , e p i d o t e , a l t e r e d f e l d s p a r , carbonate, magnetite, and b i o t i t e . The amphibole i s i n f i b r o u s aggregates and i d i o b l a s t i c c r y s t a l s w i t h l o c a l s i e v e s t r u c t u r e . I t i s v e r y s t r o n g l y p l e o c h r o i c w i t h a b s o r p t i o n Y>Z)X. The p l e o c h r o i c scheme i s : X - p a l e yellow Y - deep o l i v e - g r e e n Z - blue or g r e e n i s h - b l u e Individual grains show variations in color hut no correspondiig variations in optic properties. The interference figure is biaxial negative and 2V = 45° . Elongate grains are length- slow; the optic plane is parallel with 010.; and the angle Z Ac is approximately 17°. The mean refractive index (p) is about 1.67. The amphibole resembles common hornblende i n some respects but has a smaller extinction angle, smaller optic angle, and different pleochroism. Undoubtedly, the proportions of the various oxides in the hornblende molecule effect its optic properties, and the writer hoped to estimate the composition of the hornblende using the foregoing optic data. However, no suitable diagram was found for estimating the composition from the optic properties obtained for this particular hornblende, and a cursory review of writings by Deer (1938), MacCarthy (1926), Graham (1926), Billings (1928), and Winchell (1924) leads the writer to conclude that no satisfactory relations, between optic properties and chemical composition of hornblende have been established. Thes-e writers mention such factors as Fe 20 3-Pe0 ratio, MgO-PeO ratio Alg0 3-Si02 ratio, total iron tenor, and valence state of titanium as affecting optic properties of hornblende, but show no general agreement as to how these effects are manifest in the optic properties. They make no mention that soda content has any influence on optic properties; an idea entertained by the writer as a possible explanation of the unusual blue pleochroism of his hornblende. Rice (1935) and Hutton (1938) describe amphiboles with, for the most part, 1 5 optic properties similar to the amphibole described here. Rice believes his is an uncommon type of hornblende and Hutton styles his an act i n o l i t i c amphibole and shows, by chemical analysis, that the formula is nearly that of tremolite. Both amphiboles are high in iron but the ratios peo0„-Pe0 do not correspond. The writer concludes that his amphibole is probably a variety of highly ferruginous hornblende with unusual optic properties attributable to abnormal proportions of constituent oxides, but before pursuing the subject any further i t would be desirable to obtain a complete analysis and carry out a more exhaustive optical examination of the mineral. Dacites are porphyritic to aphanitic, streaked rocks that weather pale-green to grey with lighter-colored streaks and patches. On the weathered surface they may show white feldspar phenocrysts that usually disappear on the fresh fracture. They locally exhibit pillows and scattered amygdules of quartz and carbonate. Dacites are frequently fragmental and grade i n this direction to rocks classed as agglomerate. Where undeformed, fragments are round to angular and average 3 or 4 inches. They are occasionally of the same composition as the main rock mass, but more often are light, cream-weathering lenses of f e l s i t i c material; usually resembling common facies of rhyolite in the area, and occasionally amygdaloidal or containing prominent quartz phenocrysts. Undoubtedly, some of these gragmental rocks were produced by brecciation during the advance of the flow, 16 but the m a j o r i t y of fragments, q u i t e f o r e i g n to the matrix, must have been i n t r o d u c e d by some means d u r i n g vulcahism. The u n i f o r m s i z e and d i s t r i b u t i o n of fragments through l a r g e volumes of l a v a seems d i f f i c u l t t o e x p l a i n . Most d a c i t e s are so f i n e - g r a i n e d t h a t few minerals are r e c o g n i z a b l e under the microscope. They are s t r o n g l y f o l i a t e d , w i t h abundant lenses and s t r e a k s of a white, opague m a t e r i a l and l o n g , narrow shreds of p l e o c h r o i c c h l o r i t e p a r a l l e l t o f o l i a t i o n . The s i l i c e o u s - l o o k i n g matrix shows zones of d i f f e r i n g g r a i n - s i z e t h a t may be a r e f l e c t i o n of primary f l o w - banding. Most s e c t i o n s show abundant carbonate and some b i o t i t e and s m a l l f l a k e s of s e r i e i t e . A p o r p h y r i t i c specimen shows numerous grain s of s o d i c a l b i t e h a v ing a p r e f e r r e d o r i e n t a t i o n . Round to e l l i p t i c a l amygdules, which d e f l e c t planes of f o l i a t i o n , are f i l l e d w i t h q u a r t z , c h l o r i t e , carbonate, b i o t i t e , and c l i n o z o i s i t e . One s e c t i o n r e v e a l s s m a l l , amygdaloidal fragments w i t h a c l o u d y " r e a c t i o n - r i m " ; amygdules are round or e l l i p t i c a l ( p l a t e 1), some show c o n c e n t r i c banding of minerals or dusty i n c l u s i o n s , and are, f i l l e d w i t h q u a r t z , carbonate, c l i n o z o i s i t e , and a mineral w i t h g r i d - t w i n n i n g , low r e l i e f , and low b i r e f r i n g e n c e . Carbonate appears as metacrysts i n one a p h d n i t i c specimen, ( p l a t e 2 ) . Metacrysts c o n t a i n numerous s m a l l i n c l u s i o n s , and some have remarkably sharp boundaries. They tend to be concentrated somewhat alo n g l i n e s p a r a l l e l to the f o l i a t i o n and, although they show some evidence t h a t t h e i r o r i e n t a t i o n was i n f l u e n c e d by the d i r e c t i o n of f o l i a t i o n , they o b v i o u s l y V\ V-a\\aY\ov\ ve*\\ccvV HoVe. c * ^ " 1 7 grew a f t e r r e l a x a t i o n of the s t r e s s e s t h a t developed the f o l i a t i o n . A c i d i c v o l c a n i c s vary from a p h a n i t i c to h i g h l y p o r p h y r i t i c and from massive t o i n t e n s e l y sheared. They weather l i g h t shades o f yellow, cream, b u f f , grey and green, and to a minor extent dark green to almost b l a c k . Darker v a r i e t i e s resemble some d a c i t e s and a n d e s i t e s , b u t can u s u a l l y be d i s t i n g u i s h e d by the presence of prominent quartz tteyes". Dense, c h e r t y , a p h a n i t i c v a r i e t i e s were termed r h y o l i t e i n the f i e l d to d i s t i n g u i s h them from h i g h l y p o r p h y r i t i c types c l a s s e d as quartz-porphyry; but the d i v i s i o n of many int e r m e d i a t e f a c i e s i s p u r e l y a r b i t r a r y . R h y o l i t e s l o c a l l y show primary flow s t r u c t u r e s i n the form of b r e c c i a t i o n , amygdules, and sinuous, d i s c o n t i n u o u s f l o w - l a y e r s . Much r h y o l i t e i s fragmental and grades i n t o rocks composed almost e n t i r e l y o f fragments. Fragments are rounded to s h a r p l y a n g u l a r , f e l s i t i c to p o r p h y r i t i c , and u s u a l l y of about the same composition as the m a t r i x . In one place flow- l a y e r s were observed to curve around fragments. o P o r p h y r i t i c rocks tend to break to a dar k e r shade of the same c o l o r as the weathered s u r f a c e . V i s i b l e phenocrysts are u s u a l l y rounded quartz "eyes" up to l / 4 i n c h and ave r a g i n g 2 mm., but l o c a l l y grey, f r a c t u r e d f e l d s p a r s are conspicuous on the weathered-surface. A l l a c i d i c v o l c a n i c s s t u d i e d under the microscope are' p o r p h y r i t i c , i n c l u d i n g V a r i e t i e s t h a t appear a p h a n i t i c i n the hand specimen. F e l d s p a r phenocrysts predominate i n the 18 fine-grained ones, but i n coarser facies quartz becomes' abundant u n t i l phenocrysts constitute over 20 percent of the rock (plates 3 and 4 ) . Feldspar phenocrysts are stubby, unzoned, euhedral to subhedral crystals of coarsely-twinned albite charged with small inclusions of serieite, Quartz phenocrysts are rounded to irregular grains with local embayments and inclusions of the matrix. Some of them are fractured, and they usually show undulatory extinction. The matrix is fine-grained, allotriomorphic„ quartz and feldspar impregnated with s e r i c i t i c mica. Serieite shows strong directive orientation, except when deflected by phenocrysts, and constitutes up to nearly half of the matrix. The matrix of some specimens exhibits, in addition, irregular patches of green chlorite that imparts the dark green color to the rock in the f i e l d . Carbonate, as anhedral patches and euhedral rhombs, has an exceptionally high r e l i e f and well-developed cleavage. Some euhedral crystals, or metacrysts, show, a brown alteration around margins of crystals and working along cleavage cracks (plate ]5) . The largest metacrysts have .developed in' the matrix but some small, perfectly euhedral rhombs were observed within feldspar phenocrysts. The foregoing acid rocks have been described under the head of volcanics, but the writer is not convinced they are a l l extrusive. Contacts are usually sheared or under over- burden so that f i e l d relations are obscure and inconclusive. Coarsely porphyritic phases are locally less foliated than P l a t e 3 ; P o r p h y r i t i c R h y o l i t e flow or q u a r t z - f e l d s p a r porphyry i n t r u s i v e . Phenocrysts are a l b i t e and quartz i n a f i n e - g r a i n e d , h o l o c r y s t a l i i n e matrix of quartz ana f e l d s p a r . Grossed n i c o l s . P l a t e 4: P o r p h y r i t i c r h y o l i t e . Quartz (<j) and a l b i t e phenocrysts i n a f i n e - g r a i n e d , h i g h l y - s e r i c i t i c matrix. Note round shape of quartz phenocrysts. Grossed n i c o l s . 19 aphanitic varieties hut, in general, a l l have been subjected to the same degree of metamorphism. A l l of these roe&s studied under the microscope are composed of the same essential minerals, and textural differences could be attributed to rate of cooling as a function of, say, thickness of flows or crystallization at depth or on the surface. Briefly, coarsely porphyritic facies could be the central portions of very thick flows., or intrusive bodies emplaced at some distance below the surface and derived from the same magma chamber as the surface flows. Of significant importance in connection with the problem is the mineralogical and text- ural similarity of coarsely-porphyritic facies to an acid stock, mapped by the writer, to the south of the map-area. The acid stock invades sedimentary strata and i s , therefore, much younger than the volcanic rocks. Evidence in favor of an extrusive origin for these rocks could be summarized as follows: (1) Primary flow-structures locally developed definately establish some acidic rocks as lava flows. (2) Elongate bodies conform with the regional structure- does not preclude the possibility of s i l l - l i k e intrusives. Evidence for an intrusive origin i s ; (1) Lack of f l u i d a l structures and textures in coarsely- porphyritic facies. (2) Some small masses of dense, cherty rock, superficially resembling fine-grained "flows", show intrusive relations. (3) Similarity of porphyries to intrusive stock of quartz- feldspar porphyry south of the map-area. (4) Coarse, noncrystalline porphyries indicate slower cooling than expected in normal flows, unless very thick. Obviously, more work i s necess-ary to u n r a v e l the true nature of these a c i d i c r o c k s , and present evidence suggests they i n c l u d e rocks of more than one o r i g i n , and p o s s i b l y more than one age. P y r o c l a s t i c rocks probably c o n s t i t u t e n e a r l y h a l f of the exposed v o l c a n i c s t r a t a i n the map-area. They grade from coarse, h i g h l y fragmental agglomerates to dense, very f i n e - g r a i n e d t u f f s . They i n c l u d e numerous, d i v e r s e , n o n d e s c r i p t rocks c l a s s e d as p y r o c l a s t i c s f o r want of a b e t t e r name. Some may be fragmental or b r e c c i a t e d f l o w s ; others may be d e v i t r i f i e d and a l t e r e d p o r t i o n s of flow-banded l a v a s ; but the m a j o r i t y are probably accumulations of v o l c a n i c d e b r i s i n t e r m i x e d , i n p a r t , w i t h flow m a t e r i a l . Close to c o n t a c t s w i t h sediments, l o c a l l y , a t r a n s i t i o n a l zone g r a d i n g from f i n e - g r a i n e d t u f f through t u f f a c e o u s sediments to normal c l a s t i c beds, i s so h i g h l y sheared, s i l i c i f i e d and otherwise a l t e r e d t h a t s e p a r a t i o n of t u f f from sediment i s i m p o s s i b l e . Whenever f e a s i b l e , the w r i t e r used the presence of numerous, rounded, quartz g r a i n s on the f r e s h s u r f a c e as i n d i c a t i v e of sedimentary m a t e r i a l . Much rock c l a s s e d as agglomerate c o n s i s t s of s c a t t e r e d , r h y o l i t i c fragments-?- weathering l i g h t - cream, grey or greenish-r- i n a dark-green, c n l o r i t i c matrix. Many fragments break to show s m a l l , green patches; some have quartz phenocrysts. A few are amygdaloidal. These rocks shear to a green and cream, s t r e a k e d rock l o c a l l y resembling some v a r i e t i e s of s t r e a k e d r y h o l i t e . Fragments inc r e a s e i n number u n t i l they c o n s t i t u t e 50 percent or more. 2 1 of the rock. They locally occur in crude, discontinuous bands with intervening rock relatively poor in fragments. Other agglomerates exhibit fragments set in a very coarse "white- flecked", tuffaceous matrix. Occasionally, fragments become so abundant that they comprise most of the rock. Such rocks usually appear to have been s i l i c i f i e d and some may be highly brecciated, rhyolite lavas. Fragments are angular to rounded or bomb-shaped, and very rarely show rims that weather lighter than cores. The majority of fragments 8re acidic, and many resemble the rhyolite and acid porphyry in the area. Tuffs are extremely variable rocks, and range from coarse, crystal tuffs with occasional fragments over one inch to very fine-grained, sometimes slaty rocks with no discernible minerals. Crystal tuffs weather shades of grey, green, or brown, and show'nume'rous, ragged, grey-weathering grains of feldspar on the exposed surface. These phenocryst- like feldspars render the rock d i f f i c u l t to distinguish from porphyritic volcanics. A very few tuffs are finely laminated, or are intercalated with narrow bands or lenses of grey to black, cherty material, but the majority are unstratified and show l i t t l e evidence of sorting. Good contacts with flows are seldom visible.. At sediment-volcanic contact- . zones tuffs are extremely variable - probably as a result of concurrent vulcanism and sedimentation - and are usually sheared to a rusty, greenish, grey, or black, slaty rock impossible to distinguish from fine-grained sediments. 22 A typical crystal tuff is composed of 20 to 60 percent twinned feldspar grains, averaging several millimeters, set in a dirty-looking, tuffaceous groundmass containing patches, lenses, and veinlets of a quartz mosaic with i n t e r s t i t i a l chlorite and carbonate (plate 5). The feldspar is albite highly charged with tiny flakes of serieite. Feldspar grains almost disappear in the matrix at extinction,and many are bent or fractured and f i l l e d with chlorite and carbonate. In more-altered .crystal tuffs the feldspar is largely converted to an aggregate of secondary serieite and clinozoisite (?) and other alteration products. Elongate zones of chlorite and associated muscovite or biotite. are invariably present. A fine-grained tuff (?) examined reveals a very finely- divided roGk with plentiful minute shreds and prisms of chlorite and serieite, and a few scattered, sub-rounded grains of quartz and feldspar of probable clastic origin. Before leaving the subject of pyroclastics the writer would point out the preponderance of acidic, pyroclastic deposits, and the common occurrence of-acidic fragments in flows of a more basic composition. If the area mapped by the writer is representative of the entire volcanic f i e l d i t appears that during vulcanism- much acidic magma was expelled from volcanic vents with explosive violence while more basic material -poured out as flows.' This exemplifies the relative high viscosity of acidic as compared to basic magmas, and the consequent release with explosive violence, of volatile constituents from acidic melts. P l a t e 5: Photomicrograph of c r y s t a l t u f f . C r y s t a l s are s l b i t e , h i g h l y f r a c t u r e d and bent, i n a very f i n e - g r a i n e d , a l t e r e d matrix. A l b i t e i s w e l l - twinned>and charged w i t h minute i n c l u s i o n s of s e r i c i t e . 2 3 Volcanic terrain includes numerous bands of schistose rocks impossible to relate with any assurance to their unaltered equivalents. Original structures and textures have been obliterated by intense deformation, and primary minerals almost completely replaced by secondary products. These altered rocks are principally chlorite, carbonate, and serieite schists. Generally speaking, basic rocks have altered to chlorite schists and acid rocks to serieite schists; carbonate shows no preference for any particular kind of rock. However, rhyolites do become chloritized, dark-green rocks . distinguishable from altered andesites only by the presence of primary quartz. Numerous highly carbonatized rocks weather with a deeply-pitted or ribbed surface, others have crumbled to a rusty rubble. Many narrow bands of very f i s s i l e , light-colored, sericite-carbonate schist,, locally termed "brown-paper schist", are probably highly sheared and altered rhyolite flows or acidic tuffs. Although chlorite, serieite, and carbonate schists appear throughout the volcanics they are probably most abundant close to major contacts with sediments. Many bands of schist are the l o c i of concentrated shearing, and some may be zones of movement- with the total displacement distributed over a large number of closely spaced slip-planes. SEDIMENTARY ROCKS A prolonged period of sedimentation produced a monotonous succession of clastic sediments consisting of alternating beds of predominantly greywacke, slate, and impure quartzites with minor interbeds of arkose, slaty-greywacke, 24 and g r a p h i t i c s c h i s t . Primary s e d i m e n t a t i o n f e a t u r e s are r a r e w i t h the e x c e p t i o n of q u i t e widespread g r a i n - g r a d a t i o n , which i s l o c a l l y e v ident i n a l l kinds of beds except some pure s l a t e s and uniform q u a r t z i t e s and arkoses. Many beds grade from a l i g h t - c o l o r e d , sandy base to a d a r k - c o l o r e d , muddy top. The d i f f e r e n t types of sediments are interbedded,and although one type may predominate i n a c e r t a i n l o c a l i t y o t h e r types are u s u a l l y w e l l r e p r e s e n t e d . Most beds v a r y from 6 inches to 2 or 3 f e e t t h i c k , but widths up to 30 f e e t were observed i n some q u a r t z i t e beds. Despite the p e r s i s t e n c e of i n d i v i d u a l beds and the c o n t r a s t between contiguous beds, the o v e r - a l l s i m i l a r i t y ' of the sedimentary assemblage and the l a c k of d i s t i n c t i v e h o r i z o n markers seldom permits a p o s i t i v e c o r r e l a t i o n of sedimentary s t r a t a between adjacent o u t c r o p s . The w r i t e r might mention that the s o - c a l l e d " r u s t y arkose" and c e r t a i n , wide zones of massive q u a r t z i t e s o f f e r p o s s i b i l i t i e s as marker h o r i z o n s , but t h e i r u t i l i z a t i o n as such would r e q u i r e more d e t a i l e d study than was e x e r c i s e d i n the present work. Greywackes are medium to f i n e - g r a i n e d rocks t h a t weather dark-grey to g r e e n i s h - g r e y or b u f f . M i n e r a l s are d i f f i c u l t to i d e n t i f y m e g a s c o p i c a l l y , but some of the c o a r s e r - g r a i n e d v a r i e t i e s show s m a l l , rounded quartz and white f e l d s p a r g r a i n s i n an a l t e r e d , g r e e n i s h to dark-grey, l i m o n i t e - s p e c k e d matrix. Dark, f i n e - g r a i n e d greywacke,' s t i l l somewhat g r i t t y , i s c l a s s e d as s l a t y greywacke t o d i s t i n g u i s h i t from t y p i c a l s l a t e s . A v e r y few greywackes show dark, i n d i s t i n c t spots on 25 the fresh-surface. Slates are very fine-grained, compact, usually black rocks with a well-developed, slaty-cleavage. Slate occurs as individual beds and as the fine-grained tops of some sandy beds. A few slate beds are finely laminated, but most are uniform in color and texture. A very few slate beds are "dappled" or spotted with small nodules of uncertain composition and never over 3 mm. in diameter. They locally appear as tiny protuberances on the weathered surface. Dappled slates were noted in two places very close to the contact with volcanic rocks. At several places delicately dappled slates very close to contacts of basic dykes may be a fortuitous relationship, but the writer suspects they are the product of very local, thermal metamorphism associated with the emplacement of the dykes. The dappled slates are reminiscent of the incipiently-nodular "hot sediments" outside the map area., that are the products of thermal metamorphism related to granitic intrusives. Graphitic horizons are locally developed i n slate beds that have been the locus of concentrated shearing. One such horizon, up to 15 feet wide, was traced at intervals for 800 feet along the strike. Graphitic zones usually have rusty- weathering patches. They are local l y veined by quartz or brown carbonate, and are frequently heavily mineralized with pyrite. In many places they exhibit peculiar, white or grey weathering circular to e l l i p t i c a l patches of siliceo.us material that resemble quartz pebbles. Shear-planes curve 26 around the patches, which have their longest visible axis parallel with the direction of shearing. Small, irregular patches, less than l/8 inch in diameter are composed of fine-grained, sugary quartz, but other larger, compact patches up to several inches• have a smooth periphery and suggest pebbles of quartz. These quartzose patches could conceivably be deformed quartz pebbles; but are more l i k e l y secondary quartz, possibly injected under pressure as rods and kidneys along planes of weakness in highly sheared slate beds. Quartzites and impure quartzites are medium to coarse- grained, well-cemented rocks that weather very dark grey to almost black, light-grey, greenish-grey, or buff. They are characterized by prominent, rounded, highly vitreous, dark, quartz fragments on the fresh fracture. The thickest beds are the coarsest-grained, and show no recognizable stratification and l i t t l e or no grain variation. Quartzites are typically massive, and have tended to fracture rather than shear. Fractures are usually f i l l e d with quartz, resulting in a haphazard network of reticulating veinlets, bedded veins, or enjschelon quartz lenses approximately paralleling the regional fo l i a t i o n . Zones of fine, pebble- conglomerate occur locally in some quartzite beds. They consist of rounded pebbles, predominantly white quartz, averaging l / l 6 to l/8 inch but up to l/4 inch JLn diameter and constituting 25 to 75 percent of the rock. Conglomeratic zones are discontinuous and appear to be restricted, intraformational lenses and pockets in the quartzite beds. 2 7 In some places wide bands of massive quartzite beds can be followed up to nearly a mile. They may grade along the strike into other sedimentary types but their scattered occurrence over most of the map-area-suggests that they represent a f a i r l y extensive period of deposition of coarse, detrital material. Individual beds in such bands average 8 to 10 feet wide, are usually coarse-grained, massive, light-weathering, and well-fractured. Bands as a whole are more resistant to erosion than surrounding sediments and stand up as low, rounded, elongate outcrops. Rocks termed "rusty arkose" in the f i e l d weather buff with a rusty hue. Beds average 5 to 10 feet wide and are massive with l i t t l e perceptible stra t i f i c a t i o n . On the broken surface they are grey or bleached-green, veined by tiny, rusty seams, and show a few small, vitreous quartz grains in a dense, ashy-looking matrix. They locally show indistinct color-banding that may be a reflection of stratification. These rocks are not abundant, but are conspicuous when encountered i n the f i e l d , and very careful work might prove them to be valuable horizon markers. Mention must be made here of the band of sediments lying between volcanics and extending from west of Lex Bay to Cranston Lake, wita possible faulted extensions both north and south. On straight lithological grounds these rocks are markedly different than the main belt of sediments to the east. They are comprised mainly of coarse, massive, frequently ferruginous, quartzite and arkose beds, with minor 28 s l a t e s and f i n e l y f r a g m e n t a l beds. Coarse rocks weather v a r i e g a t e d shades of green, brown, r e d d i s h , b u f f , and grey. S l a t e s are greenish-grey, tan, brown, and b l a c k , the l a t t e r o c c a s i o n a l l y p u r p l i s h - r e d on water-worn s u r f a c e s . Some unusual, l i g h t - c o l o r e d , "buff-beds" are l a r g e l y r e p l a c e d by carbonate and s i l i c a ; and may be a c i d t u f f s o r a c i d i c , s i l l - l i k e i n t r u s i v e s . Close to v o l c a n i c s beds l o c a l l y get v e r y coarse, and may c a r r y v a r i o u s kinds of round to ang u l a r fragments up to 1 i n c h i n diameter. T h i s band of sediments may r e p r e s e n t an i n t e r v a l of aqueous d e p o s i t i o n before the c l o s e of vnlcanism, or may be a b e l t of i n - f o l d e d sediments of the same age as those to the e a s t . The problem w i l l be d i s c u s s e d l a t e r under s t r u c t u r e . Sedimentary rocks were not s t u d i e d i n any d e t a i l under the microscope and the f o l l o w i n g o b s e r v a t i o n s , recorded as a b r i e f a djunct to f i e l d d e s c r i p t i o n s , were obtained from the study of a v e r y few t h i n - s e c t i o n s . No sediments examined show profound r e c r y s t a l l i z a t i o n , but a l l have been a l t e r e d and deformed. Slates- show a s t r o n g f o l i a t i o n , or s l a t y cleavage, accentuated, by o r i e n t e d shreds and f l a k e s of micaceous m i n e r a l s . They c o n s i s t of v e r y f i n e l y - d i v i d e d , sub-microscopic m a t e r i a l impregnated w i t h white-opaque a l t e r a t i o n , c h l o r i t e , and s e r i e i t e . Abundant minute c r y s t a l s with very h i g h r e l i e f and y e l l o w i s h c o l o r are probably r u t i l e . Dappled s l a t e s show a maculose s t r u c t u r e , w i t h round to o v o i d patches ( p l a t e 6), mainly of quartz ( ? ) , much c o a r s e r - g r a i n e d than the r e s t of the r o c k . These P l a t e 6: Photomicrograph of "dappled" s l a t e . Note r e l a t i v e coarse-grain and l a c k of f o l i a t i o n e x h i b i t e d by the "spot". Dark, p e r i p h e r a l rim i s apparently due to a d i f f u s i o n outwards, and concentration around the circumference of c h l o r i t e w i t h i n the "spot". Main rock mass i s h i g h l y impregnated with minute, shreds of c h l o r i t e and s e r i o i t e p a r a l l e l w i t h the f o l i a t i o n ( h o r i z o n t a l ) . Very l i t t l e s e r i c i t e appears w i t h i n the "spot". Grossed n i c o l s 29 recrystallized patches are unfoliated and practically devoid of serieite. Chlorite is coarser and has apparently diffused outwards from the centre of the patch to give a peripheral zone enriched in chlorite. Greywackes show rounded to, angular grains-, mostly quartz with some sodic plagioclase, set in a fine gromdmass containing abundant chlorite and serieite. Quartz and feldspar grains are original clastic constituents l i t t l e affected by processes that have altered the fine-grained groundmass. Very minor tourmaline,, in elongate crystals- may be recrystallized, allogenic tourmaline, or may have been introduced. Spots, which are rare in greywackes and probably analogous to "dappling" of slates, are indefinate zones ' enriched in chlorite and depleted of serieite. RELATION OF VOLCANIC, AND SEDIMENTARY ROCKS General; Though very unsatisfactory, the evidence for the structural relationship between volcanic and sedimentary rocks provides no sound basis for any marked angular discordance. Accordingly, volcanics and sediments are tentatively regarded as constituting a conformable succession of rocks. Beds in sediments close to volcanic contacts always face away from, and nearly always strike parallel to, the contact zone. Sediments and volcanics show the same degree of metamorphism,and both possess a folia t i o n with the same general trend. Traceable horizons in volcanics, contact zones, and beds in sediments are essentially parallel. 30 Nature of the Contact Zone>. ' ' Sediment-volcanic contacts are usually obscured by overburden, and the contact zone is commonly marked by a d r i f t - f i l l e d depression that rises in an abrupt scarp to volcanics. At many places contacts are sheared. Shearing was probably initiated during folding, with possible later adjustments as witnessed by local shearing in a basic dyke where i t traverses a rhyolite-sediment contact on the southwest shore of Lex Bay. Contapts appear to be steeply- dipping, but surface observations on such poorly-defined features as contact zones are seldon reliable. i. In some placessediments and volcanics are intercalated at contacts. In other places, as southwest of S l i t Lake, there is a transition from fragmental volcanics, through tuffaceous sediments and peculiar beds of intermixed sandy and argillaceous material, to normal sediments. In s t i l l other places, as on the central-east side of Johnson Island, the rock grades upwards from volcanicsj through a twelve-foot zone with sub-angular to rounded boulders 9 up to 12 inches or more acrossr. in a tuffaceous or sandy matrix; through rusty beds; and f i n a l l y into interbedded slates and greywacke. Boulders are mostly of volcanic rocks, but include some grey chert. Thus, there is evidence on the one hand to suggest an erosional interval between vulcanism and sedimentation, and on the other hand to suggest a progressive change from vulcanism, through a transitional period of intermittent and 31 concomitant vulcanism and sedimentation, to sedimentation. Other features observed commonly,- but by no means universally, at contacts are: (1) Black to brown, pulverulent "gouge". (2) Abundant, coarsely-crystalline pyrite. (3) Garbonatization, as disseminated carbonate and metacrysts of a ferruginous carbonate. (4) Ferruginous breccia of diverse, round to angular, volcanic and sedimentary fragments - probably recently consolidated debris cemented by iron salts provided by oxidation of pyrite in the contact-zone. (5) Minor acidic intrusions (?) injected along contacts, usually pyritized and sheared to a light-colored, rusty, very-fissile schist of uncertain composition. ( 6 ) Basic intrusives injected along contact zone. Conclusions; The foregoing' data supply no evidence, of a major stratigraphic break between volcanic and sedimentary rocks. Field evidence indicates contacts have acted as zones of weakness along which shearing, and probably movement, have taken place, and hydrothermal solutions have moved; so that they may be the l o c i of major structural breaks. The problem wi l l be reconsidered in connection with the Indin "break" which, as already mentioned, is postulated to coincide approximately with a main sediment-volcanic contact. INTRUSIVE ROCKS General: No large intrusive masses are exposed in the area, but numerous, small, hypabyssal dykes and s i l l s invade sedimentary 32 and volcanic strata. They range in composition from acidic to basic and in age from pre-sediments (?) to Proterozoic (?) Relative ages of the different intrusives are poorly known<,; as clear-cut relations are discouragingly rare in the f i e l d . Relative age can. in most cases,, be assessed only on the basis of the degree of alteration. The oldest intrusives probably include some small, highly-altered, chlorite- carbonate dykes and s i l l s . Others of these altered intrusives probably belong to- a later period. Certain types of acidic rocks are probably intermediate in age, and the youngest consist of a plexus of basic dykes and s i l l s that cut a l l other rocks, and probably include dykes of several different ages. The possibility of quartz-feldspar porphyry intrusives has already been considered. In addition, "Bird-Porphyry" and similar rocks,, easily mistaken for crystalline flows, are of uncertain origin and age. Late basic dykes are divided'into two types - altered gabbro and diorite and relatively unaltered diabase. Since these basic dykes may shed some light on the problem of age of faulting in the area they w i l l be described here in some, detail. "Bird-Porphyry" and Similar Rocks; Under this head are included massive to schistose, crystalline rocks with d i o r i t i c texture and composition. Their areal distribution is limited to volcanic rocks i n the northern quarter of the map-area, with the exception of one dyke, in volcanics, on the southwest side of Negus Lake. \ 33 Within these limits.* they increase in abundance from south to north. These rocks were not recognized in sediments although, admittedly, some rocks in sedimentss classed as late diorites, are very similar. Over wide areas in the north the "porphyries" constitute nearly half of the rock exposure, but their apparent abundance may be a reflection of superior resistance to erosion over enclosing strata. Some show definate chilled margins and apophyses against volcanics, but most occur as isolated outcrops that t e l l nothing of their relation or origin. Meagre structural data imply that at least some' bodies of this rock conform with the general structure in volcanics. The rock may be, in part, the intrusive counterpart of andesite flows, intruded along structure planes in the volcanic p i l e f and contemporaneous with outpourings of andesite lava. However, the freshness of these rocks compared to most volcanics suggests they belong to a much later period of igneous activity; their confinement to volcanics is due to structural control rather than contemporaneity. Exceedingly poor exposure permitted no f i e l d separation of these rocks on maps so they are grouped with volcanics, but the writer feels they are largely intrusive and should be described under that heading. They are mottled, green or brown weathering rocks frequently characterized by anhedral, light-weathering, f e l s i c patches,, or smalls quartz "eyes". They are dark- green on the fresh surface, massive to schistose, noncrystalline, with a poorly developed diabasic texture.. - 34 The f e l s i c patches are white, cream, or. greenish, altered feldspar, average l/4 to 1 inch across, and probably are large, partly a'dsorbed phenocrysts. They are erratically distributed, never constituting more than 5 or 10 percent of the rock and. usually very widely scattered or almost entirely absent. Rocks with these f e l s i c patches were called "Bird-Porphyry" in the f i e l d because of marked similarity to rocks of that name seen by the writer from the area around Yellowknife. Quartz "eyes", which occur with or without f e l s i c patches, are rounded blebs of opalescent, bluish quartz that average about 1 mm. in diameter. Occasionally these rocks show abun- dant, s c i n t i l l a t i n g flakes of black mica. Outcrops are featured by lenticular fractures f i l l e d with white, usually barren, quartz. Quartz veins are up to 2 feet wide,but seldom more- than 20 feet long. . Microscopic examination reveals the rock is composed mainly of hornblende, altered feldspar, chlorite, and quartz. Hornblende is green to almost colorless, moderately pleochroic, and shows extensive alteration to chlorite. Original lathy feldspars with random orientation remain as opaque masses of secondary products. Clear quartz, as irregular grains up to 1 mm.,, shovi/ing modulatory extinction, and scattered smaller grains, composes 15 percent or more of the rock. Some quartz is intergrown with altered sodic (?) feldspar and resembles i n t e r s t i t i a l granophyre. Locally, however, quartz is seen penetrating the margins of feldspars, suggesting that the intergrowths may be of replacement origin and unrelated to 35 late-stage crystallization of the rock." Some large quartz grains show parallel trains of minute, colored inclusions. Much of the quartz has probably been introduced, or possibly released from the breakdown of primary constituents. Chlorite, as aggregates and scattered flakes, usually shows a dark, purplish-blue or. purplish-red, interference color. A seim-opaque material, white in reflected light, has except- ionally.high r e l i e f and constitutes over 5 percent of the- rock. It shows almost total reflection, so that no optical properties are obtainable. Some well-defined, wedge-shaped, cross-sections suggest that i t may be sphene partially altered to leucoxene. It frequently carries small kernels of a black, opaque mineral. Other minerals present in varying proportions are carbonate,•epidote, pyrite and apatite. The f e l s i c patches show definate, but very irregular outlines, and consist of feldspar almost entirely converted to a semi-opaque aggregate of secondary minerals including clinozoisite or epidote, carbonate, sericite and chlorite, and possibly some kaolin minerals. ' • Varieties containing biotite show less hornblende, more chlorite-showing a- dirty-greenish- interference color - and an abundance of the leucoxene-like material. Biotite, constituting around 5 percent of the rock, is strongly pleochroic and invariably lies within, or closely associated with chlorite. • • Altered Dykes; Numberless highly altered, usually small, intrusive 3 6 bodies are included under this heading. Their original composition is unknown for they are now composed almost entirely of secondary minerals. They probably include rocks of different compositions and ages. Some are known-to cut acidic intrusives in sediments, and some are cut by late basic dykes. Others resemble altered phases of late basic dykes, and may themselves belong to that same period of igneous . activity. They weather brown, reddish-brown, or green, are infrequently cut. by veinle.ts of a green, fibrous mineral, and usually show sharp contacts against the confining rock. On freshly-broken surfaces they are grey, green, or brownish, and aphanitic or studded with small, glistening-, cleavage- faces of carbonate. Others show well-developed, euhedral . - crystals of ferruginous carbonate up to several millimeters across, and e t i l l others are almost completely carbonate. In thin-section they consist of a structureless aggregate of carbonate and pale-green chlorite, with lesser amounts of muscovite, quartz, and a plagioclase believed to be .albite. Pyrite is usually present, along with iron-ores and brownish- white, opaque, alteration material. Muscovite ia secondary and shows a sieve structure. One variety exhibits evenly- distributed, rounded clots of deep-green- chlorite with minor quartz and carbonate. Opaque minerals are very abundant in the main rock mass- but practically absent in the clots of chlorite. Since carbonate metacrysts are linked with metamorphism and alteration, their discussion will be withheld u n t i l a later 37 page. In passing, the writer would mention that dykes.and s i l l s of the present-considered rocks have apparently offered .very permeable "channels" for the passage of carbonate solutions, : .-• and frequently they have been highly carbonatized while thê adjacent rocks have been but slightly affected. This suggests that carbonatization is controlled, at least to some extent, by the composition of the dykes and s i l l s ; a composition probably considerably more basic than' the adjoining rocks. "Metadjogite" Intrusive rocks classed as "metadiorite" are found only in sediments, and-thes'e mainly in that narrow stretch of land separating Indin and Float lakes. None of these rocks were recognized in the northern half of the "area. "Metadiorite" occurs almost invariably as sill-like.bodies intruded along bedding-planes .and conforming in strike and dip with the enclosing strata. Numerous small s i l l s from one to three feet wide are d i f f i c u l t to prove as intrusive. They possess sharp contacts with no appreciable c h i l l i n g or other intrusive c r i t e r i a . "However, one such small body was observed trending at right angles to bedding along the axial plane of a small syncline in thinly-bedded sediments, so other similar rocks were concluded to be intrusive and correlated with larger-, somewhat similar bodies of typical "metadiorite". S i l l s of "metadioii?ite" occasionally attain widths of 30 feet or more, but are usually exposed along edges of d r i f t - f i l l e d depressions so that true widths are indeterminate. Some have been intensely cracked and f i l l e d by a network of smoky 3 8 to milky quartz veins that rarely penetrate the enclosing rocks. Others are highly schistose, or show a* poorly- developed augen structure, .parallel to the regional foliation. Thus these rocks have yielded to stress conditions by both fracturing and shearing, and appear to have been subjected to at least some of the forces that caused folding. The weathered surface of larger bodies is usually reddish brown or greenish, with myriads of tiny, white flecks that project slightly above the surface. Least deformed varieties show, on the fresh surface, a granitoid texture and about equal amounts of felsic and mafic constituents. Schistose varieties break with a waxy sheen along irregular cleavage, surfaces, and show lenses of light-colored minerals i n an altered, greenish-grey matrix specked with brown alteration. Under the microscope unsheared varieties (plate 7) are a medium-grained, granular rock composed mainly of plagioclase feldspar and chlorite. ' Chlorite is i n t e r s t i t i a l to feldspar, and was probably derived from the breakdown of primary, ferromagnesian minerals. It is in large, optically-continuous masses loca l l y intergrown with coarse serieite. Plagioclase is a twinned, ,very-sodic albite, host to numerous coarse inclusions of carbonate, serieite, and chlorite. Some untwinned feldspar grains showing good cleavage may be orthoclase. Feldspar grains are locally fractured or bent, with arcuated twin lamellae. Strained quartz constitutes less than, ten percent of the rock. /.Apatite is very abundant as prismatic, cross-fractured crystals up to l/2 ram. Carbonate, P l a t e 7; Unsheared "meta d i o r i t e 1 1 . Note l a c k of any d i r e c t i v e t e x t u r e . Rock composed mainly of a l b i t e (white) and C h l o r i t e ( b l a c k ) , w i t h minor quartz, carbonate, s e r i c i t e and a p a t i t e . Crossed n i c o l s . P l a t e 8 : s t r u c t u r e accentuated by sinuous zones of c h l o r i t e that curve around l e n t i c u l a r patcnes of a l b i t e , quartz and carbonate. Much micropegmatite and minor s c h i l l e r - s t r u c t u r e s probably were developed during shearing. Grossed n i c o l s . 39 comprising about.15 percent of the rock, magnetite, and skeletal frameworks of white, opaque material mainly in chlorite complete the mineral assemblage. Sheared varieties (plate 8) show increased amounts of carbonate and chlorite and a lesser amount of serieite. Schistosity is accentuated by long, sub-parallel strings of shredded chlorite that curve around lenticular zones of feldspar, quartz, and carbonate. Quartz and feldspar have been crushed to granular aggregates that have been drawn out in the direction of schistosity and impregnated with carbonate and chlorite. Some quartz is in graphic intergrowth with a •feldspar of much lower refringence. Irregular grains of opaque, brownish material are abundant in chlorite. Delicate, acicular to fi l i f o r m crystals appearing as inclusions in feldspar and chlorite are arranged locally in a crude sort of schiller-structure. Apatite is abundant and serieite is absent, except as a few minor-inclusions in feldspar. The name nmetadiorite" was adopted in the f i e l d for the foregoing rock, and has been retained in this report. Obviously, the rock has been altered and deformed,but probably i n i t i a l l y approached a diorite in composition - with subsequent albitization of the feldspar, conversion of ferromagnesi<an constituents to chlorite, and introduction of carbonate (or at least of C O 2 ) . Minor Acidic Intrusives: With the exception of quartz-feldspar porphyry, already discussed^ and very numerous quartz veins, many of them 40 feldspathic, acidic intrusives in the area are limited to a few small, s i l l - l i k e bodies in sediments. These bodies are exposed as a single s i l l on the small island off the southeast end of Johnson Island, and again close to the eastern shore of Indin Lake as several, discontinuous s i l l s extending from a point directly across the lake from North Inca to half way up the shore of Lex Bay. It is not known whether these exposures are parts of the same intrusive system. Other so- called "buff-beds u i n sediments west of Lexindin may be similar intrusives. In addition, some volcanic-sediment con- tacts are locally marked by a light-colored rock, usually pyritized and sheared to a rusty, very-fissile schist that was thought to represent minor acidic intrusives injected into contact-zones. Clear-cut relations of these rocks were not encountered in the f i e l d , and petrographic similarity, under the microscope, to sericitized rhyolite leads to the idea that they are sheared acidic lavas rather than minor intrusives. The s i l l - l i k e acidic intrusives in sediments usually conform with the attitude of the enclosing strata, but locally cut the beds at a small angle, and very infrequently swing at right angles to bedding for a few inches. Along strike they pinch and swell from less than a foot to more than eight feet in width. They are locally mineralized and cut by irregular stringers of quartz. In a few places they are almost completely replaced by quartz. Smooth slickeh- sides covered by a serieite film, at contacts with sediments 41 and within the rock i t s e l f , give evidence of movement along the s i l l s after their emplacement. The rock is aphanitic, white to light-grey, dense to sugary, and in some places speckled with limonite or patches of green chlorite. In thin-section the rock is comprised of an inequi- granular, interlocking, allotriomorphic aggregate of albite and quartz, with possibly some orthoclase. Ferruginous carbonate altering to limonite is abundant. The rock is veined by coarsely-crystalline quartz associated with a lesser amount of albite. Quartz is fractured and highly strained. Serieite occurs as i n t e r s t i t i a l films between auartz grains, as fracture-fillings in quartz, and as a few small inclusions in feldspar grains. Coarse lines of dusty inclusions traverse the rock and quartz veinlets, with no regard for crystal boundaries. Gabbro and Diorite; These are the most widespread and abundant intrusive rocks in the area. They have been intruded in several well- defined sets, and their emplacement probably took place over a long period of time. One set trends in a northwesterly direction, approximately parallel to "cross-faults", and another set trends, in general, a few degrees east of north, approximately parallel to the regional structure. Some northwesterly-trending dykes appear .to follow, and be younger than, "cross-faults"; and other northerly-trending dykes are offset by, and older than, "cross-faults". Dykes usually show weak resistance to erosion, and weather to d r i f t - f i l l e d 42 depressions, but in some places, in contact with the same rocks. they stand well above the surrounding strata. Dykes, which attain widths of 100 feet and average 20 to 30 feet, vary considerably in thickness along the strike, and in places become quite irregular, with numerous off-shoots and ramifications. They show indistinct contacts against fine- grained sediments,and. against a l l rocks, a chilled, marginal facies frequently lighter-weathering than the interior. Pine- grained sediments close to the contact are usually baked to a cherty, buff, grey, or greenish hornfelsic rock in which a l l signs of folia t i o n are obliterated - signifying that these intrusives are later than the development of folia t i o n . The location of a dyke can be detected frequently by the presence of baked sediments on the edge of a line a l depression. Coarse grained sediments and volcanic rocks have not been appreciably altered by the d7/ke-intrusions. In the f i e l d different dykes, or'specimens from the same dyke taken at different l o c a l i t i e s , vary somewhat in detail; but in general these rocks a l l show sufficient s i m i l a r i t y to be classifi e d under the same head. For that matter, some of these rocks are so like the previously described "Bird Porphyry" and a l l i e d rocks that the validity of creating the two divisions is questionable, and can be justified with some degree of certainty only by seeing the two types of rock in the f i e l d . The rock, noticeably heavy in the hand, is massive, fresh-looking, green to brov/n weathering, and has a mottled 43 d i o r i t i c to diabasic texture. It is dark-green on the fresh surface, and shows varying amounts of mafic and f e l s i c constituents, the former usually predominant. Pyrite is usually visible in the hand specimen. A few dykes are cut by veinlets of salmon-pink feldspar, or contain irregular segregations of coarsely-crystalline quartz and pinkish or white feldspar. Others are fractured and f i l l e d with white to grey, vitreous quartz, occasionally well-mineralized with chalcopyrite and pyrrhotite. S t i l l others are cut by small, seldom-persistent, stringers of white, vuggy quartz. Large, irregularly-distributed, yellowish-green to cream patches appear in some of the dykes. They may constitute up to 25 percent of the rock at one exposure and be entirely absent on the next exposure of the same dyke. They never show crystal outlines, and for the most part are very irregular in shape. In thin-section the freshest rocks are composed mainly of amphibole and altered plagioclase with lesser, varying amounts of chlorite, quartz, epidote, clinozoisite, biotite, carbonate, magnetite, pyrite, leucoxene (?) and apatite (plate 9). Amphibole, which probably includes hornblende and tremolite-actinolite, is brown, green,, and colorless. It is weakly to strongly pleochroic in tints of green and brown. Variegated shades of green and brown appear locally in a single grain. Much amphibole, reminiscent of a secondary origii, exists in fibrous aggregates with or witnout chlorite. Other optically continuous, occasionally twinned, subhedral P l a t e 9: Late d i o r i t e dyke, c o n s i s t i n g mainly of secondary green to brown amphibole (dark) and andesine f e l d s p a r ( l i g h t ) . Compare'with p l a t e s 10 and 11. 44 g r a i n s may be primary. F e l d s p a r i s l a t h y p l a g i o c l a s e , l o c a l l y f r e s h , w i t h r e g u l a r , p o l y s y n t h e t i c twins; but u s u a l l y a l t e r e d and charged w i t h i n c l u s i o n s of c l i n o z o i s i t e or epidote and c h l o r i t e . C h l o r i t e l o c a l l y r e p l a c e s f e l d s p a r a l o n g zones p a r a l l e l to l a m e l l a t i o n , and o c c a s i o n a l l y f e l d s p a r i s " c l e a r e d up" where i n c o n t a c t w i t h c h l o r i t e . P o s i t i v e i d e n t i f i c a t i o n of f e l d s p a r i s not easy but the index of r e f r a c t i o n and maximum e x t i n c t i o n angle f a v o r an andesine of composition about Abtjg. E p i d o t e or c l i n o z o i s i t e and c h l o r i t e are always p l e n t i f u l , and p y r i t e , magnetite and leucoxene (?) are u s u a l l y p r e s e n t . Other minerals noted i n some s l i d e s are b i o t i t e , carbonate, a p a t i t e , q u a r t z , and micropegmatite. Quartz, when present, i s never over 10 percent,, and i n some se c t i o n s appears to have been i n t r o d u c e d . B i o t i t e , which i s absent i n most s e c t i o n s but q u i t e p l e n t i f u l i n a few, occurs as s m a l l f l a k e s a s s o c i a t e d w i t h amphibole and c h l o r i t e . I t i s s t r o n g l y p l e o c h r o i c from t a n to almost c o l o r l e s s . The r e l a t i o n of b i o t i t e to amphibole i s not c l e a r , but i n a few places amphibole appears to be a l t e r e d to b i o t i t e through an intermediate stage i n which green amphibole turns brown,to a f i n a l stage i n which the brown mineral takes on the o p t i c a l p r o p e r t i e s of b i o t i t e . Both b i o t i t e and amphibole are a l t e r e d t o c h l o r i t e . L i g h t patches, which were o r i g i n a l l y composed of p l a g i o c l a s e , now c o n s i s t of a mass of secondary products and a very few cloudy remnants of the o r i g i n a l f e l d s p a r . The secondary mass c o n s i s t s p r i m a r i l y of an e q u i g r a n u l a r aggregate 4 5 of one or more of the epidote minerals (probably mainly clinozoisite) with minor amounts of chlorite, and could probably be termed "saussurite". The original feldspar was not identified, but could be expected to have been quite calcic. Two sections taken from altered portions of these rocks, one from close to a mineralized zone and the other from the sheared portion of a dyke where i t crosses a sediment-volcanic contact, show greatly increased amounts of chlorite and carbonate,and diminished amphibole. and clinozoisite or epidote. Diabase; Diabase dykes are probably the youngest rocks in the area, although nowhere were they found intersecting gabbros and diorites. They are not abundant and in no case was a single dyke traced any great distance. In several lo c a l i t i e s they are intruded parallel with, and only several tens of feet away from, a diori.te or gabbro dyke; in a l l instances both dykes are in sediments and strike approximately parallel with bedding. A very short section of a diabase dyke was found in a s i l i c i f i e d breccia-zone believed to be the locus of a "cross-fault' 1 1. The dyke was perfectly fresh4, and apparently post-dated faulting and s i l i c i f i c a t i o n . Typical diabase has a smooth, brown, wea the red-surf s c e, «<v<\ sharp, regular contacts with a chilled, marginal selvage. The rock is nearly black on the broken surface, with myriads of tiny feldspar needles that sparkle when the rock is rotated in the hand. Less typical varieties weather greenish, are dark- 46 green on the fresh surface, and d i f f i c u l t to separate from fresh diorite and gahbro in the f i e l d . The rock has a diabasic texture, and consists of about equal amounts of pyroxene and feldspar, and abundant accessory magnetite, (plate 10). Marginal facies are porphyritic (plate 11), with phenocrysts of pyroxene and feldspar in a fine- grained, menocrystalline matrix of the same minerals and magnetite. Pyroxene, with an optic angle of about 40 degrees, is a pigeonitic augite. It locally shows incipient uralitization or alteration to chlorite. Feldspar is labradorite, occasionally zoned, and replaced by chlorite (?) along narrow seams predominantly transverse to the length of the feldspar grain. Magnetite is evenly scattered through the rock in anhedral, highly corroded grains. Small flakes of biotite partly altered to chlorite probably belong to a late, aeuteric stage of crystallization. Biotite is brown, strongly pleochroic, and usually surrounded by chlorite. The diabase- as a w h o l e i s a fresh rock, and apparently has not been exposed to the same vicissitudes as the/vbasic intrusives in the area. P l a t e 10: Fresh diabase, c o n s i s t i n g of p i g e o n i t i c pyroxene (grey ) , l a b r a d o r i t e (white) and magnetite (black) Compare with p l a t e 9. Crossed n i c o l s . P l a t e 11: Fresh diabase, showing c r u d e l y - p o r p h y r i t i c , contact f a c i e s , with phenocrysts of pyroxene and f e l d s p a r i n a f i n e matrix of the same minerals and magnetite. Some f e l d s p a r i s zoned. Crossed n i c o l s . 47 STRUCTURE General Statement When dealing witn such regional features as structure the fragmentary data gathered from the study of a small area ^is inadequate to formulate a sound analysis of the structural set-up. Apart from this, the general character of the rocks and thepaucity of rock exposure in some areas, imposed severe restrictions on the number of reliable structural determinations available, and consequently, conclusions must be based on undesirably few observations. The scale of the accompanying map of the entire map-area precludes the inclusion of numerous minor details of structure recorded on the 400-scale map. The writer offers here a few ideas of structure in the area, with the realization that the theoretical phases may have several equally valid interpretations, and in the hope that additional f i e l d work w i l l lead to a clearer understanding of the structural pattern in the area. The ensuing discussion is not intended as an exhaustive treatise of structure in the area, but rather ss a preliminary consideration of a topic that necessitates considerable additional information before i t may be dealt with in any great detail. Structure in the area is complex. Sedimentary rocks have been thrown into a series of tight, isoclinal folds with axial planes trending, in general, a few degrees east of north. A l l sedimentary strata dip from 70 degrees to vertical^and in many places beds ere overturned as much as 10 or 15 degrees. Folding appears to have been less complex in volcanics. They have resisted intense buckling by the development of zones of shearing in incompetent members of the assemblage and, in general, probably behaved as a competent unit under the influence of tectonic forces to which the rocks were subjected. Movement, i f any, along shear zones was probably mainly vert- i c a l , and took place as a very late-stage adjustment to stresses; consequent upon the relaxation of stresses; or even during a later period of deformation. Many small-scale folds in sediments may be minor flexures "related to the main period of folding, but some, and probably the majority, are related to later earth movements that developed steeply- plunging dragfolds and imposed crenulations upon regional .foliation produced during the main period of folding. Regional stresses,, subsequent to folding,, culminated i n major northwest-trending "cross-faults" of considerable a magnitude. "Cross-faults" effect a l l rocks in the area with the possible exception of some .late.basic dykes. FOLDING Apparent structural conformity between volcanics and sediments leads to the assumption that these rocks have been • subjected to a single period of regional folding. Discussion of the nature and origin of the forces that caused this folding w i l l be withheld from this report except for saying that the fold pattern suggests compressional stresses operating oin a general east-west direction perpendicular to 4 9 the trend of fold axes,and incidentally, perpendicular to the long axes of granitic intrusives and a belt of rocks of the Snare group (Proterozoic) to the west. Generally speaking, the map area lies astride a volcanic- sediment contact, with volcanics on the west and sediments on the east. To the east, and out of the map area, a second band of volcanics is exposed. The belt of sediments is about a mile wide at its narrowest part. The contact strikes a few degrees east of north over most of the area, but swings to about north 30 degrees east above Cranston Lake. Lord (1942,' p. 41) regards the volcanic bands as, in part, the cores of complex, anticlinal structures. If so, the sediments, flanked on either side by older volcanics, would occupy the trough of a synclinal structure, and the map area would l i e essentially on the western limb of this syncline. Field observations, while not vi t i a t i n g the fundamentals of this concept, introduce many complications. That the sedimentary strata do not occupy a simple, synclinal trough is shown by numerous reversals of tops- best explained, in such steeply-dipping beds by tight, isoclinal folding. Divergence in strike of beds from the regional trend may reflect'plunging folds, or later flexing or dragging of strata already standing on edge as a result of isoclinal folding. In many places beds can be followed around the noses of small anticlines and synclines. In some, regional foli a t i o n is perpendicular to bedding on the crest or trough, and in others the f o l i a t i o n follows bedding around V 50 the nose or has been crumpled into a series of closely- spaced crenulations or tiny foldlets. Some of these folds are probably drag-folds contemporaneous with major folding, and others are probably associated with one or more later periods of deformation. Since the two types of folds cannot be distinguished with confidence they are of l i t t l e value as a guide in interpreting the major structure. The axes of some of these folds are shown on the accompanying map. In no case were they traceable for any great distance, and in most cases could be followed only a very few tens of feet. The axis of the major structure is indeterminate, and its plunge is a matter of some uncertainty. Most small folds plunge steeply (usually to the north), but as already stated, the majority are probably genetically unrelated to major folds. For .reasons given later, secondary linear features were not considered reliable indicators of regional plunge. However, the essential parallelism of sedimentary and volcanic belts suggests gentle regional plunge. The belt of volcanic rocks along the eastern edge of the map-area appears to plunge to the north and south under younger sedimentary rocks, and suggests an elongate dome structure, with gentle northward plunge in the northern part of the area, reversing to a southward plunge in the soutnern part of the area. These relations, however, are not duplicatedfas to be readily apparent by volcanics in the western part of the map-area. To what extent the picture is complicated by unrecognized faults, especially strike-faults, is not known. 51 The narrow, band of sediments in volcanics west of the main contact zone deserves some consideration here. That portion extending from west of Lexindin to Cranston Lake, on the basis of structural c r i t e r i a , appears to be the trough of a narrow syncline. Stratigraphic tops on the west side face east, and on the east side face west. In addition, a rusty-weathering, "buff-bed", either an acidic s i l l or a tuff- * aceous bed, is exposed west of Lexindin on each side of the sedimentary belt at approximately the same stratigraphic horizon, as i f i t were being repeated on each limb of a syncline. On the other hand, as already discussed, sediments constituting this band are not similar, lithologically, to the main belt of sediments on the east. Also, rhyolite lying on the western side of the sediments does not appear on the eastern side as i t should i f the structure were a simple syncline, and discounting any complications possible through discontinuity of the rhyolite horizon or unrecognized faulting. Thus, there is on one hand evidence to suggest a shallow syncline or infolded band of sediments, and on the other hand evidence to suggest the sedimentary band is the result of a major interval of sedimentation before the close of vulcanism, and thus a wide sedimentary interband within the succession of volcanics. The writer is extremely dissatisfied with the fold picture established by mapping and summarized bri e f l y on the foregoing pages. However, when we consider that the rocks now exposed at the surface are the cores .of what were presumably 52 great mountainous belts produced by crustal orogeny and modified by later disturbances, i t is not surprising that the structural constitution of the rocks is complicated to such an extent that structural information accumulated in the course of a single f i e l d season leaves much to be desired. Regional Foliation and Lineation Compressional stresses to which the region was subjected imposed a schistose or foliate structure upon most rocks in the area. The writer applies the term "regional fol i a t i o n " in a broad sense to include the approximately parallel disposition., in layers, lenticules,or undulating lamellae, of rock elements and minerals. In general, i t includes schistosity, flow- cleavage, and augen-like structures - secondary features probably developed during folding by stresses transcending the limits of the map-area, and consequently conforming to a regional "grain" or trend of such structures that compares in extent to the extent of the causal stresses. The degree of development of regional foliation varies i n different rocks, and in separate bodies of the same rock. Pre-folding intrusives are frequently somewhat less foliate than the enclosing rocks. Some bodies of one rock type may be highly foliate,and other bodies of the same rock be quite massive. Coarse-grained sediments and some intrusives have fractured rather than sheared. Slate beds lying between relatively competent greywackes and quartzites. are usually more highly sheared than adjacent formations. Late basic 53 dykes, i n t r o d u c e d a f t e r most or a l l of the f o l d i n g , have not been a f f e c t e d except where l a t e movement, u n r e l a t e d to s t r e s s e s causing r e g i o n a l f o l i a t i o n has developed some l o c a l s h e a r i n g . F o l i a t i o n s t r i k e s between northwest and n o r t h e a s t , predominantly a few degrees east of n o r t h , and dips v e r y s t e e p l y to v e r t i c a l l y . In sediments i t i s u s u a l l y p a r a l l e l to beds, or cuts bedding-planes a t a small a n g l e . Cleavage planes p a s s i n g from coarse-bottomed to f i n e - t o p p e d beds are r e f r a c t e d towards c l o s e r p a r a l l e l i s m w i t h bedding upon e n t e r i n g the f i n e - g r a i n e d top. Secondary l i n e a r f e a t u r e s are developed i n some pa r t s of the area, u s u a l l y i n rocks where f o l i a t i o n i s s t r o n g l y developed. L i n e a r f e a t u r e s are elongate elements whose long axes are o r i e n t e d p a r a l l e l w i t h f o l i a t i o n , and i n c l u d e p i l l o w s , fragments, and streaks of c h l o r i t e i n v o l c a n i c s , and a few • pebbles i n sediments, and "quartz patches" i n g r a p h i t i c beds. Fragments i n agglomerates are l o c a l l y deformed i n t o i r r e g u l a r , e l l i p s o i d a l b odies, or squeezed and f l a t t e n e d i n t o "pancake- shaped" bodies that may reach a f o o t i n l e n g t h and l e s s than an i n c h i n width on the s u r f a c e of an exposure. Such extreme deformation, which i s of r a r e occurrence, must have taken plac e when the rocks were i n a very p l a s t i c c o n d i t i o n as there i s no v i s i b l e evidence of c a t a c l a s t i c s t r u c t u r e s a s s o c i a t e d w i t h the deformation. P i l l o w s i n v o l c a n i c s i n v a r i a b l y have t h e i r l o n g e s t v i s i b l e a x i s p a r a l l e l w i t h f o l i a t i o n , and the same can be s a i d f o r pebbles i n coarse sediments and "auartz patches" i n g r a p h i t i c s l a t e s . However, p i l l o w s may owe t h e i r e l o n g a t i o n 5 4 / and o r i e n t a t i o n as much to primary shape and d i s p o s i t i o n i n the flow as to secondary processes, and "quartz patches" i n g r a p h i t i c beds probably owe t h e i r shape and o r i e n t a t i o n to c o n t r o l by p r e - e x i s t i n g s t r u c t u r e s r a t h e r than to deformation subsequent to t h e i r f o r m a t i o n . Fragments i n agglomerate and numerous, elongate lenses and streaks of dark-green, c h l o r i t i c m a t e r i a l i n v o l c a n i c s , when viewed i n the plane, of f o l i a t i o n , u s u a l l y show a very steep to v e r t i c a l i n c l i n a t i o n on t h e i r l o n g e s t a x i s . In the f i e l d t h i s was i n t e r p r e t e d as i n d i c a t i n g a s t e e p l y plunging r e g i o n a l s t r u c t u r e . However, s i n c e such elongate elements may be conceived to have developed by more than one mechsnism, e i t h e r as a s t r e t c h i n g p e r p e n d i c u l a r to f o l d axes or as a squeezing or r o l l i n g - o u t p a r a l l e l w i t h f o l d axes, the i n t e r p r e t a t i o n reached 6n the b a s i s of l i n e a t i o n depends upon which mechanism i s adopted by the i n t e r p r e t e r . I f s t r e t c h i n g p e r p e n d i c u l a r to f o l d axes i s presupposed, s t e e p l y i n c l i n e d l i n e a l elements i n d i c a t e gentle plunges; i f squeezing or " r o l l - i n g - o u t" p a r a l l e l w i t h f o l d axes i s adopted, s t e e p l y i n c l i n e d l i n e a l elements i n d i c a t e steep plunges. In an area of i n t e n s e f o l d i n g there i s c o n s i d e r a b l e l e n g t h e n i n g along the f o l d e d b e l t p a r a l l e l w i t h the f o l d axes. Such l e n g t h e n i n g i s r e l i e v e d by the development of s a l i e n t s or plunging f o l d s , the l a t t e r p robably the u s u a l method of r e l i e f . I f the apparent l e n g t h e n i n g of a few observed fragments be taken as a q u a n t i t a t i v e index of the t o t a l l e n g t h e n i n g along the f o l d e d belt., the r e s u l t a n t plunge would be' c o n s i d e r a b l e , 55 assuming, of .course, s t r e t c h i n g of fragments to be p a r a l l e l w ith f o l d axes. However, extreme s t r e t c h i n g i s probably c o n f i n e d t o r e l a t i v e l y small areas of g r e a t e s t s t r e s s i n t e n s i t y , so t h a t h i g h l y deformed elements i n these r e s t r i c t e d areas are not r e p r e s e n t a t i v e of deformation of the e n t i r e a r e a . Since there i s no unanimity r e g a r d i n g the genesis of e l o n g a t e , l i n e a l elements, and s i n c e proof to the c o n t r a r y i s l a c k i n g , the w r i t e r p r e f e r s to t h i n k of the major f o l d axes as being • g e n t l y u n d u l a t i n g l i n e s r a t h e r than l i n e s t h a t stand n e a r l y v e r t i c a l l y , as would be i n d i c a t e d by the a t t i t u d e of the m a j o r i t y of l i n e a l elements i n the planes of f o l i a t i o n . In c o n c l u s i o n , l i n e a l f e a t u r e s are elongated e s s e n t i a l l y i n the plane-of r e g i o n a l f o l i a t i o n , and l i n e a t i o n , f o l i a t i o n , and f o l d i n g are probably contemporaneous f e a t u r e s developed by a s i n g l e system of t e c t o n i c f o r c e s . FAULTING F a u l t s i n the area may be d i v i d e d i n t o two main c l a s s e s ; (1) Shear zones, u s u a l l y p a r a l l e l i n g the r e g i o n a l s t r u c t u r e . (2) N o r t h w e s t - s t r i k i n g " c r o s s - f a u l t s " , o b l i q u e to the r e g i o n a l s t r u c t u r e . A l though the shear zones are c l a s s e d as f a u l t s no p o s i t i v e evidence e x i s t s of a p p r e c i a b l e displacement along them, and a l l that can be s a i d i s they are zones of intense s h e a r i n g r o u g h l y p a r a l l e l w i t h the r e g i o n a l s t r u c t u r e and u s u a l l y accompanied by c a r b o n a t i z a t i o n and other kinds of a l t e r a t i o n . Included w i t h them are the I n d i n "break" and o t h e r zones of s h e a r i n g 56 l o c a l i z e d a t major c o n t a c t s . " C r o s s - f a u l t s " are d e f i n a t e c r u s t a l breaks attended by c o n s i d e r a b l e displacement but p r a c t i c a l l y no intense s h e a r i n g . Shear Zones: Exposed shear zones i n v o l c a n i c s are i l l - d e f i n e d bands of s c h i s t t h a t grade l a t e r a l l y i n t o l e s s - s c h i s t o s e r o c k s . Pronounced shear zones are not abundant i n sediments, and when they do occur there are f r e q u e n t l y l o c a l i z e d i n s l a t y h o r i z o n s between c o a r s e r beds of a more competent nature. The a t t i t u d e of the s c h i s t o s i t y i n shear zones seldom shows much d e v i a t i o n from the a t t i t u d e of r e g i o n a l f o l i a t i o n i n the surrounding rocks, and f o r this, reason are thought to be a l a t e - s t a g e adjustment to s t r e s s e s that caused f o l d i n g , o r a t l e a s t to have been c o n t r o l l e d by zones of weakness i n i t i a t e d d u r i n g f o l d i n g . Shear zones are f r e q u e n t l y accompanied by q u a r t z - v e i n i n g , e s p e c i a l l y i n sediments, and by carbonate i n v e i n s , i n t e r l a c i n g s t r i n g e r s , and c a r b o n a t i z e d w a l l - r o c k . H i g h l y c a r b o n a t i z e d shears, o r "carbonate-zones", are widespread i n v o l c a n i c s , a n d although sometimes 1 m i n e r a l i z e d , seldom c o n t a i n a p p r e c i a b l e p r e c i o u s - m e t a l v a l u e s . Many outcrops weather r u s t y from the decomposition of the carbonate. Other shear zones i n v o l c a n i c s are composed of h i g h l y s c h i s t o s e s e r i c i t i c and c h l o r i t i c r o c k s . The w a l l s are p o o r l y d e f i n e d , and the shear zones g r a d u a l l y l o s e t h e i r i d e n t i t y i n the r e g i o n a l f o l i a t i o n . C h l o r i t e and s e r i c i t e p robably developed mainly as a r e s u l t of dynamic metamorphism, and t h e i r presence and abundance tee dependent 57 upon the composition of the original rock, hut as already discussed, certain chloritized rhyolites attest to the fact that, during deformation, some of the constituents necessary for the formation of chlorite, and for that matter other minerals generated by dynamic processes, must have been introduced. No attempt has been made to show shear zones on the accompanying map. For the most part they are obscured by d r i f t and muskeg and their delineation, to be of any value, would nave required more time than was available in the present work. The Indin "Break" . Paradoxically, the Indin "break", which is economically the most significant structural feature in the area, is not indicated on the accompanying map. There are several good reasons for this. In the f i r s t place, the only positive evidence within the map-area to support the existence of such a "break" is to be found in the results of diamond-drilling at and around North Inca, and since d r i l l i n g results were not incorporated with f i e l d studies this record is lost for the purpose of this report. Tne writer does not intend, at this point, to disagree with the results of mapping set forth on map 697 A showing, at intervals,.a north-trending fault that coincides, at North Inca and vicinity, witn the position of the ore-bearing shear zone or "break" established by d r i l l i n g . Tnis fault has been postulated from f i e l d work having a much wider scope than the 58 writer's investigations. Nevertheless, mapping along the assumed position of the fault produced l i t t l e evidence to support the existence of a fault having a displacement to the right even approaching the magnitude indicated by the offset sediment-volcanic contact south of S l i t Lake. Careful mapping at this point revealed the contact to maintain essential continuity, with l i t t l e likelihood of being displaced, i f at a l l , more than a few tens of feet. This, of course, s t i l l admits the possibility of a fault having a large vertical component of movement - a possibility enhanced by a deep, lineal depression extending from S l i t Lake to Leg Lake, and poor correlation between volcanic rocks on adjacent sides of the depression. Several obscure exposures of late basic intrusive along the depression, together with numerous ex- posures of the same rock to the south, and approximately in line with the depression, suggest that i t may be the result of differential erosion of a basic dyke, since basic intrusives m \W axeot are known to offer relatively weak resistance to weathering. Turning to positive evidence of the "break11 . one cannot deny the existence of the zone of shearing established by d r i l l i n g to extend from its intersection with the Inca fault northwards almost to Lexindin camp, a distance of about 9000 feet. This zone of shearing, hitherto termed the Indin "break? would probably be better described as the Indin "shear" u n t i l positive evidence of movement, i f any, is obtained. Of course, the most important structural problem of the area is whether this zone of shearing is a comparatively small structure, the 59 l i m i t s of which have been p r e t t y w e l l d e f i n e d , o r whether i t i s a major s t r u c t u r e that continues a c o n s i d e r a b l e d i s t a n c e n o r t h and south of where i t has been detected by d r i l l i n g . F i e l d work was extremely d i s a p p o i n t i n g i n s o f a r as r e s u l t s that have any b e a r i n g on the problem, and no d i r e c t evidence was obtained e i t h e r f o r or a g a i n s t extending the s t r u c t u r e beyond i t s p r e s e n t l y - d e f i n e d l i m i t s . A t Worth Inca the shear i s i n sediments c l o s e to t h e i r c o n t a c t with v o l c a n i c r o c k s , and i s b e l i e v e d to d i p s t e e p l y to the west w i t h the c o n t a c t , which l i e s a few f e e t west of the shear. I f i t be assumed to maintain t h i s e s s e n t i a l l y bedded c h a r a c t e r , and e s p e c i a l l y i f movement along the shear i s not great, i t might continue f o r a great d i s t a n c e without being d e t e c t e d on the s u r f a c e . I t s s o u t h e r l y e x t e n s i o n would l i e under the waters of I n d i n Lake and c o u l d be l o c a t e d o n l y by d r i l l i n g . In t h i s c o n n ection, a hole d r i l l e d from the n o r t h e a s t shore of Johnson I s l a n d , about 3000 f e e t south of the Inca f a u l t , i n t e r s e c t e d a zone of m i n e r a l i z e d s c h i s t i n sediments a t about the same h o r i z o n as the main shear at North Inca. This suggests t h a t the main shear extends south o f , and i s d i s p l a c e d by, the Inca f a u l t . Going n o r t h of where the shear was l a s t i d e n t i f i e d by L e x i n d i n i t s path e a s i l y c o u l d f o l l o w the low ground that marks the volcan i c - s e d i m e n t c o n t a c t from Lex Bay to south of Cranston Lake. Here the shear would presumably be d i s p l a c e d to the northwest by a " c r o s s - f a u l t " , and extend northwards along Cranston Lake. The s t r u c t u r a l c o n d i t i o n s n o r t h of Cranston Lake are not at a l l c l e a r . I t i s not known whether the b e l t 60 of v o l c a n i c s between Cranston Lake snd the lake to the west i s i n f a u l t e d contact a g a i n s t , or plunges beneath, the younger sediments on the n o r t h , so i t would be f o l l y t o attempt even an assumed p o s i t i o n of the shear beyond Cranston Lake. As a matter of f a c t , sediments are h i g h l y sheared over widths up to a mile n o r t h of Cranston Lake, and i t i s u n l i k e l y that any concentrated zone of s h e a r i n g does e x i s t . I t may be that t h i s marks the t r a n s i t i o n from a more or l e s s w e l l - d e f i n e d shear zone i n t o a wide b e l t of d i s t r i b u t e d s h e a r i n g i n which m i n e r a l i z a t i o n , i f present, would be so w i d e l y spread that i t s c o n c e n t r a t i o n i n economic amounts would be u n l i k e l y . Before l e a v i n g the s u b j e c t , the w r i t e r would p o i n t out the obvious f a c t that a d d i t i o n a l i n f o r m a t i o n i s s o r e l y needed before the nature and o r i g i n of the I n d i n "break" may be e v a l u a t e d . However, the none too v a l i d s uggestion i s made th a t the I n d i n "break", l o c a l i z e d as i t seems at a major c o n t a c t , may have been i n i t i a t e d d u r i n g f o l d i n g , and r e f l e c t adjustment to s t r e s s e s c a u s i n g f o l d i n g of rocks of d i f f e r e n t competency. P r i o r to f o l d i n g the rock assemblage can be thought of as a competent v o l c a n i c basement o v e r l a i n by a r e l a t i v e l y incom- petent s u c c e s s i o n of sediments. Shor t e n i n g across the f o l d e d b e l t , a f t e r the p o s i t i o n of major f o l d s was determined, was accomplished by i n t r i c a t e b u c k l i n g of the sediments and broader warping and s h e a r i n g i n the v o l c a n i c basement. Such deformation, i n an i n t e r m e d i a t e stage, would probably n e c e s s i t a t e s l i p p a g e or s h e a r i n g along, o r c l o s e t o , major- contacts between v o l c a n i c and sedimentary rocks which, i n an . 61 advanced stage, might develop into high angle thrust faults along steeply inclined or overturned limbs of major folds. At any rate, adjustments during folding l i k e l y develop zones of weakness at contacts between rock units of different competency which, in themselves, may prove favorable structures for ore deposition. Or, zones of weaknessAat con- tacts iacuguratoeV during folding may determine the position of later faults unrelated to the stresses causing folding. Carrying this idea further, a fault-, controlled in its location by structures developed during folding, may be an aftermath of, and indirectly related to, folding. With the relaxation of lateral compression and the cessation of folding., gravity may attempt to compensate for the crustal shortening during folding by active crustal extension in the form of gravity faults. Such faults would tend to follow zones of weakness pre-determined. during folding. './hether the Indin "break" is merely a zone of shearing or a fault of some magnitude is not known.o It i s , however, closely related in space to a major contact between rocks of different competency that have been intensely folded. The foregoing hypothesis, relating the origin of such a structure, or at least its control, to' folding, is entirely speculation, and i t s application to the genesis of the Indin "break" may be found invalid when more information,is obtained. "Cross-Faults;" "Cross-fault" is the term used to denote those structural breaks that cross the map-area i n the northwest quadrant and 62 intersect the regional strike at a large angle. Gross-faults invariably possess a left-hand offset, or in other words, rocks on the northeast side of a fault have moved northwest relative to those on the southv/est side. L i t t l e is known of vertical movement along cross-faults, but the greater part of the displacement is probably i n a horizontal direction. Fault-planes, or more correctly fault zones, are seldom visible in the f i e l d . In the southern part of the area cross-faults are marked by narrow, lineal depressions usually apparent on aerial photographs. Cross-faults appear to be clean-cut breaks. Pronounced shearing is not evident, and indications of movement are limited to a local crumpling of regional f o l i a t i o n and dragging of beds in the immediate vicinity of a fault. Drag-folds and axes of crumples usually pitch steeply. North of Negus Lake , the surface expression cross-faults may have had is masked by. a heavy mantle of overburden, and . although faulting may be indicated by displaced horizons the delineation of the surface trace of a fault is largely a matter of inference. At one place, north of Corner Lake, what is believed to be a short section along a cross-fault zone is exposed i n sedimentary strata. The sediments are brecciated, intensely s i l i c i f i e d , and veined by quartz over a width of more than 100 feet. A breccia-zone cemented by quartz, is probably the locus of most of the movement along the fault. Breccia fragments are- sub-angular, highly s i l i c i f i e d , and show no 63 evidence of folia t i o n that presumably existed prior to the introduction-of s i l i c a . On both sides of the breccia-zone rocks are highly s i l i c i f i e d and cut by reticulating stringers and veins of white to grey quartz. Although quartz veins trend in every direction one set of north-trending veinlets is cut and offset to the right by another set trending about northeast. Quartz loc a l l y exhibits depositional banding and vuggy structure that may be attributable to low pressures prevailing at the time of deposition. S i l i c i f i e d wall-rock varies from a grey to black, f l i n t y rock composed almost entirely of cryptocrystalline s i l i c a to a rock that, though s i l i c i f i e d , maintains i t s essential sedimentary .character. Quartz veins are barren, but s i l i c i f i e d wall- rock is local l y mineralized with finely disseminated pyrite. Farther northwest along the probable extension of the same zone small, irregular quartz stringers exposed on the edge of several outcrops of sediments are probably related to the same cross-fault. Brecciation and s i l i c i f i c a t i o n , i f present, are obscured by overburden. The zone just described is similar in many respects to the "giant quartz veins" occurring in various parts of the Northwest Territories. Descriptions by Furnival (1935) and Kidd (1936) of giant quartz veins around Great Bear Lake reveals them to be zones of quartz and ouertz-cemented breccia flanked by stockworks of quartz stringers. Many occupy large faults, and there is evidence of two or more generations of quartz, separated by long time intervals end deposited in an 64 environment of low pressure and temperature. The zone described by the writer, and believed to occupy a cross-fault, exhibits features vastly different from cross-faults i n the southern part of the area where wall-rock alteration is negligible and brecciation is largely supplanted by drag. However, i f its origin be ascribed to the same stresses that produced other cross-faults, and i f i t be considered an analogue of giant quartz veins, the conclusions of Furnival and Kidd, that giant quartz veins represent a very prolonged period of deformation and s i l i c a deposition, might offer some support for the contention to be put forth later that cross- faulting represents adjustment to stresses applied over a very extended interval of time. In the sout-iern part of the area three cross-faults, the Inca, Aztec, and Leta faults, are f a i r l y well established. The Inca fault, which is closely associated spacially with gold mineralization at Nortn Inca, w i l l be considered later i n some detail. The Aztec and Inca faults are inferred to converge at the southern end of Float Lake and to continue southeast as a single zone of movement. Betv/een Lexindin and Cranston Lake several probable cross-faults are indicated by displaced sediment-volcanic contacts that, in some cases, may be irregularities in the contact rather than genuine offsets. North of Cranston Lake the faulting pattern is not clear, probably several cross-faults are responsible for the apparent left-hand displacement of the main sediment-volcanic contact, but unusually scarce exposure, heavy overburden, and poorly 65 understood structural conditions, render their delineation impossible. The Inca Fault The Inca fault is the best understood cross-fault in the area. It is under water except where i t crosses the narrow neck of land separating Indin and Float lakes, and again on the mainland west of Inca Peninsula. In the f i e l d an offset of approximately 1000 feet (a horizontal displacement along the fault of approximately 1800 feet) along the Inca fault is indicated by two dykes, one of "metadiorite" and the other of "gabbro", that intersect the south side of the fault on the southern tip of Float Lake and reappear on the north side of the fault on the southwest shore of Float Lake. On both sides of the fault the dykes are essentially parallel, and on the north side the two dykes strike parallel with north-striking sediments, and both sediments and dykes are dragged to the east upon approaching the fault zone. The two dykes are approximately the same distance apart on both sides of the fault which, unless they have identical dips, would imply that the vertical component of movement along the fault is very 0 small. However, the attitudes of both dykes are probably controlled by steeply-dipping beds, or in other words they are s i l l - l i k e bodies with almost identical dips. If this is . so, and assuming no pronounced rotation along the fault, a vertical component of movement would have l i t t l e effect on the relative positions of the two dykes on opposite sides of the fault. A rough measure of the offset along the Inca fault is 86 afforded by a shift to the l e f t of the east shore of Indin Lake where i t crosses the fault. That the lake margin represents approximately the same horizon in the sediments is indicated by a series of very closely-spaced fold axes along the shore for some distance on both sides of the fault. The fault traverses Indin Lake, where i t presumably displaces the Indin "break", passes close to the southwest shore of the Inca Peninsula, and enters the western „shore of Indin Lake in sediments. Close to the shore the sediments show pronounced drag on both sides of the fault zone. Going inland, the locus of the fault is marked by a poorly defined lineation that disappears when the fault approaches volcanics. From here, as far as the fault was followed, rocks on the south side are dominantly sediments and those on the north volcanics, leading to the conception that volcanics are in faulted contact against the sediments. However, one small exposure of rhyolite was seen south of where the fault is believed to pass. Relation of Cross-Faulting and Mineralization: The age of cross-faulting, or at least of the f i n a l movement along cross-faults, cannot be defined more exactly than post-Archean and pre-Pleistocene. Cross-faults appear to bear no relation to the system of forces that caused folding, and are undoubtedly later than the bedded shear zones developed during the f i n a l stages of folding. On map 697 A cross-faults are shown penetrating for three miles the main granite mass to the west, so that i f they be assumed to have formed as a result of stresses activated by the intrusion of 67 the g r a n i t e a s u f f i c i e n t time l a g between emplacement of g r a n i t e and f o r m a t i o n of c r o s s - f a u l t s must be allowed f o r a t h i c k s h e l l of g r a n i t e to c r y s t a l l i z e and become s u f f i c i e n t l y b r i t t l e to f a i l by r u p t u r i n g . I t i s l i k e l y t h a t c r o s s - f a u l t i n g r e s u l t e d from a system of f o r c e s u n r e l a t e d t o , and much l a t e r than, e i t h e r f o l d i n g or i n t r u s i o n of g r a n i t e . L i k e w i s e , the age of go l d m i n e r a l i z a t i o n i n the area i s not known. No i n t r u s i v e bodies are exposed w i t h which m i n e r a l i z a t i o n can be a s s o c i a t e d w i t h any degree of assurance, y i t h the e x c e p t i o n of b a s i c dykes, the most proximate i n t r u s i v e s are a few s m a l l , a c i d i c s i l l s that are pro b a b l y much o l d e r than g o l d m i n e r a l i z a t i o n . However, i t i s i n t e r e s t i n g to note t h a t many of the known g o l d occurrences i n the area, i n c l u d i n g those a t North Inca and v i c i n i t y , are c l o s e l y r e l a t e d i n space to c r o s s - f a u l t s , and although c r o s s - f a u l t s a p p a r e n t l y d i s p l a c e the o r e - b e a r i n g s t r u c t u r e s and c a r r y no a u r i f e r o u s lodes themselves, i t i s not easy to d i s p e l the n o t i o n that c r o s s - f a u l t s played some r o l e i n the l o c a l i z a t i o n of ore. The s p a c i a l r e l a t i o n s h i p between gold m i n e r a l i z a t i o n and c r o s s - f a u l t s c e r t a i n l y lends support to the i d e a , but the t i m i n g f a c t o r presents a s e r i o u s o b s t a c l e to any the o r y r e l a t i n g g o l d d e p o s i t i o n to c r o s s - f a u l t i n g . S t r u c t u r a l c o n d i t i o n s a t North Inca, although not n e c e s s a r i l y r e p r e s e n t a t i v e of the e n t i r e area, w i l l be e l a b o r a t e d upon here to present some of the d e t a i l s , so f a r as known, concerning the r e l a t i o n between c r o s s - f a u l t s and gold m i n e r a l i z a t i o n . The main o r e - b e a r i n g shear o r Indi n "break?', 68 in sediments under' the waters of Indin Lake, strikes about north-south, roughly parallel to the eastern shore of Inca Peninsula. Its possible origin and relation to the contact have already been discussed. In addition, the gold-bearing Brown veins are exposed in the volcanics on the southern tip of Inca Peninsula. The main, or No. 1, Brown vein is approximately parallel with shearing along the contact, and thus with the main shear, but appears to dip steeply in the opposite direction, i.e. to the east. The main shear is probably offset by the Inca fault, certainly the reverse is not true. The No. 1 vein is offset by several minor parallel faults, notably close to the best ore shoot in the vein, but d r i l l i n g south of the Inca fault has thus far failed to pick up the faulted extension of the vein. This suggests that the vein may be confined to the north side of, and possibly genetically related to, the Inca fault. If so, i t would not be the normal gash-fracture type of v e i n - f i l l i n g frequently associated with faulting, as such a structure related to a left-hand, northwest-trending fault should strike in an approximate east-west, rather than north-south direction. The Brown veins are more l i k e l y controlled by subsidiary structures related to the formation of the main shear-zone. Mineralization and quartz are similar in the Brown veins and the main shear zone, so that they were both probably mineralized by hydrothermal solutions having a common origin. The Inca fault i t s e l f shows practically no quartz, mineralization, or wall-rock alteration, and evidence 69 of movement is restricted to drag, minor brecciation, and crumpling of f o l i a t i o n in the wall-rock. To recapitulate briefly, the north-south, ore-bearing structures (Brown veins and main shear or Indin "break") are intersected by the northwest-trending Inca fault and, although the ore appears to be pre-f aulting, its disposition in relation to the cross-fault suggests that its localization may have been influenced by the cross-fault. As already noted, drag is considerable along the fault zone - an indication that the present level of erosion truncates the fault zone where plastic deformation and bending were considerable prior to any actual rupture along the fault. In other words, the strain, whi-sh culminated in faulting, may have accumulated over a long period of time before the advent of rupture. During the accumulation of strain the effect on steeply-dipping strata may have been to develop steeply-pitching, highly- appressed drag folds that might act, in some manner, as channels for ascending hydrothermal solutions; and orebodies might be developed wherever such "channels" intersected structures favorable for the deposition of ore. This, of course, necessitates coincidence in time and space between the formation of ore-solutions and adequate strain accumulation to develop permeable channels for the movement of the ore-solutions. Vi/'ith the consummation*ore deposition strain accumulation would not necessarily cease, and might continue u n t i l the elastic limit of the rock was reached and dislocation occurred along the line of maximum strain. Such a 70 c o n c e p t i o n r e q u i r e s a l o n g time i n t e r v a l between the i n i t i a l a p p l i c a t i o n of s t r e s s and the f i n a l movement along the f a u l t , but the time f a c t o r i s probably commensurate with modern views on r a t e s of f a u l t i n g movement and a p p l i c a t i o n of f o r c e s t h a t produce the f a u l t i n g . The r e l a t i o n of c r o s s - f a u l t s to b a s i c dykes i n the area suggests that dyke i n t r u s i o n s and c r o s s - f a u l t s are q u a s i - contemporaneous events t h a t proceeded over a l o n g p e r i o d . Dykes and c r o s s - f a u l t s c o n c e i v a b l y c o u l d r e p r e s e n t adjustments to a s i n g l e s e t of s t r e s s c o n d i t i o n s . In some p l a c e s , as e x e m p l i f i e d by the Aztec f a u l t , i n t r u s i o n s of d i o r i t e and gabbro a p p a r e n t l y f o l l o w c r o s s - f a u l t s . In the p r e v i o u s l y d e s c r i b e d s i l i c i f i e d f a u l t - b r e c c i a zone n o r t h of Corner Lake f r e s h diabase i s exposed i n s i l i c i f i e d w a l l - r o c k , i t s e l f a p p a r e n t l y u n a f f e c t e d by the a l t e r a t i o n . Other dykes show a c o n s i d e r a b l e change i n t r e n d as they approach the c r o s s - f a u l t to which the s i l i c i f i e d b r e c c i a i s b e l i e v e d a s s o c i a t e d , as i f there- i s l a r g e - s c a l e drag connected w i t h the f a u l t . These same dykes a p p a r e n t l y are o l d e r and younger than the c r o s s - f a u l t . In t h i s c o n n ection, Kidd (1936) f i n d s l a t e b a s i c i n t r u s i v e s o l d e r and younger than g i a n t quartz veins which, i f the analogy a l r e a d y made between g i a n t quartz v e i n s and the s i l i c i f i e d b r e ccia-zone i s genuine, and the br e c c i a - z o n e occupies a c r o s s f a u l t , c o u l d be o f f e r e d as a d d i t i o n a l support to the c l o s e time r e l a t i o n s h i p assumed between c r o s s - 71 f a u l t i n g and dyke i n t r u s i o n . A gabbroid dyke n o r t h of Cranston Lake i s c h i l l e d a g a i n s t f e r r u g i n o u s goug© that may, or may not, be r e l a t e d to c r o s s - f a u l t i n g . The same dyke, a few hundred f e e t south of the above l o c a l i t y , i s h i g h l y c a r b o n a t i z e d and a l t e r e d where i t passes a few f e e t from the f o o t - w a l l of a m i n e r a l i z e d zone c a r r y i n g minor g o l d v a l u e s . B a s i c dykes are d e f i n a t e l y o f f s e t i n the southern p a r t of the area by the Inca and Leta f a u l t s . No s h e a r i n g or a l t e r a t i o n were observed i n the dykes where they cross the f a u l t zones, the a c t u a l i n t e r s e c t i o n s being obscured by d r i f t , and there i s always the remote p o s s i b i l i t y they are post- f a u l t i n g and occupy s t r u c t u r e s that were d i s p l a c e d by the f a u l t s before being f i l l e d by the dyke-rock. However, a more l o g i c a l e x p l a n a t i o n i s to assume more than one age of dyke- i n t r u s i o n , or f a u l t i n g , or both; or- as a l r e a d y s t a t e d , t h a t dyke emplacement and f a u l t i n g are more or l e s s contemporaneous events t h a t took plac e over a p r o t r a c t e d time i n t e r v a l . The f o r e g o i n g d i s c u s s i o n does l i t t l e to e l u c i d a t e the problem of the r e l a t i o n between c r o s s - f a u l t i n g and m i n e r a l i z a t i o n , but might serve to s t r e n g t h e n the idea that c r o s s - f a u l t i n g r e f l e c t s a very slow adjustment to r e g i o n a l s t r e s s e s a n d h e l p j u s t i f y the t h e o r y that o r e - s o l u t i o n s may have been channeled by s t r u c t u r e s developed d u r i n g the accumulation of s t r a i n t h a t culminated i n c r o s s - f a u l t i n g ; and t h a t . between i n i t i a l a p p l i c a t i o n of s t r e s s and f i n a l rupture a s u f f i c i e n t time i n t e r v a l e l a p s e d f o r m i n e r a l i z a t i o n to be 7 2 e f f e c t e d . Such a conception, to advance beyond a proposed theory, r e q u i r e s a great d e a l more i n f o r m a t i o n than-at the w r i t e r ' s d i s p o s a l , and d r i l l i n g must be r e l i e d upon t o supply much of t h i s i n f o r m a t i o n . R e s o l u t i o n of the problem would n e c e s s i t a t e an extremely d e t a i l e d examination of c r o s s - f a u l t zones and any s u b s i d i a r y s t r u c t u r e s . In p a r t i c u l a r , the v a l i d i t y of the theory would be enhanced by evidence f o r the c o n t r o l of m i n e r a l i z e d lodes by s t e e p l y - p l u n g i n g s t r u c t u r e s , such as drag f o l d s , r e l a t e d to c r o s s - f a u l t s ; and evidence f o r the c o n t r o l , i n m i n e r a l i z e d lodes (such as the I n d i n "break") i n t e r s e c t e d by c r o s s - f a u l t s , o f m i n e r a l i z a t i o n by f r a c t u r e s (say e a r l i e r quartz f r a c t u r e d and veined by m e t a l l i c s ) whose p a t t e r n c o u l d be r e l a t e d to the assumed s t r e s s p a t t e r n oper- a t i v e d u r i n g s t r a i n accumulation, m i n e r a l i z a t i o n , and f a u l t i n g . Undoubtedly, as d r i l l i n g and underground work proceed i n the area^. and more i s l e a r n e d of the geology, many of the present ideas w i l l be r e v i s e d , and any c o n c l u s i o n s based on evidence a v a i l a b l e at present should be regarded as t e n t a t i v e and of use o n l y i n s o f a r as they prove to be c o r r e c t . REGIONAL METAMORPHISM The w r i t e r c l a s s e s under " r e g i o n a l metamorphism" those changes induced i n the rocks by the i n t e r a c t i o n of heat and pressure o p e r a t i v e on a s c a l e t r a n s c e n d i n g the l i m i t s of the map-area; and excludes such e f f e c t s as hydrothermal a l t e r a t i o n a s s o c i a t e d with m i n e r a l i z a t i o n , thermal metamor- phism r e l a t e d to minor i n t r u s i v e s , and weathering r e l a t e d to the present l e v e l of e r o s i o n . The present mineralogy and i n t e r n a l s t r u c t u r e of the rocks r e f l e c t an attempt to a t t a i n p h y s i c a l and chemical e q u i l i b r i u m w i t h a new environment r e s u l t i n g from changes i n heat and p r e s s u r e . The tremendous t a n g e n t i a l f o r c e s to which the r e g i o n was su b j e c t e d d u r i n g f o l d i n g developed powerful d i f f e r e n t i a l pressures or s h e a r i n g s t r e s s e s t h a t , i n c o n j u n c t i o n w i t h any f r i c t i o n a l heat evolved by s h e a r i n g and c r u s h i n g , c o n s t i t u t e the p r i n c i p a l agents of metamorphism. Any f r i c t i o n a l heat generated i n s i t u by s h e a r i n g may have been augmented by heat a s s o c i a t e d w i t h the i n v a s i o n of g r a n i t i c b a t h o l i t h s that surround, and presumably u n d e r l i e , .the map-area. Changes e f f e c t e d by metamorphism c o n s i s t of m i n e r a l o g i c a l t r a n s f o r m a t i o n s and i n t e r n a l s t r u c t u r a l rearrangements or mechanical changes. The l a t t e r have a l r e a d y been d i s c u s s e d , and i n c l u d e r e g i o n a l f o l i a t i o n (flow cleavage, s c h i s t o s i t y , and augen-like s t r u c t u r e s ) and the development of l i n e a r e elements such as elongated fragments and lenses of mafic m i n e r a l s , as w e l l as permanent s t r a i n e f f e c t s and c a t a c l a s t i c deformation of i n d i v i d u a l minerals as e x e m p l i f i e d by s t r a i n e d 7 4 q u a r t z , bent o r f r a c t u r e d f e l d s p a r , g r a i n s , and twinned carbonate. Most rocks have been p a r t i a l l y o r w h o l l y r e c o n s t i t u t e d to a new min e r a l assemblage that i s r e l a t i v e l y simple f o r a s u i t e of such d i v e r s e rock types. (Quartz, c h l o r i t e and carbonate are almost u n i v e r s a l , and are u s u a l l y supplemented by c l i n o z o i s i t e , zoismte (?) or epidote and amphibole i n b a s i c v o l c a n i c s , and s e r i c i t e i n a c i d v o l c a n i c s and sediments. F e l d s p a r , except i n l a t e b a s i c i n t r u s i v e s , when i d e n t i f i a b l e i s almost i n v a r i a b l y a l b i t e . In most rocks a l b i t e occurs as s m a l l , c l e a r , untwinned g r a i n s intergrown w i t h other minerals and d i f f i c u l t to d i s t i n g u i s h from q u a r t z , and primary phenocrysts i n f l o w s , fragments i n t u f f s , and d e t r i t a l g r a i n s i n sediments. The former a l b i t e was pro b a b l y e i t h e r i n t r o d u c e d or r e l e a s e d by the break-down of primary m i n e r a l s ; and the l a t t e r were probably p l a g i o c l a s e s of v a r y i n g composition that were a l b i t i z e d by m o l e c u l a r replacement t h a t f a i l e d to d e s t r o y the form and l a m e l l a t i o n of the o r i g i n a l f e l d s p a r g r a i n . The w r i t e r does not f e e l q u a l i f i e d to t h e o r i z e on the b a s i s of the few s e c t i o n s s t u d i e d under the microscope, but suggests that the apparent prevalence of a l b i t e f e l d s p a r may denote e i t h e r t h a t the rocks belong to a s o d i c p r o v i n c e , o r the " s p i l i t i c s u i t e * of B r i t i s h p e t r o l o g i s t s , or that- subsequent to t h e i r f o r m a t i o n t h e rocks were "soaked" by s o d a - r i c h emanations, p o s s i b l y concomitant w i t h i n t r u s i o n s of g r a n i t i c magmas, t h a t converted a l l f e l d s p a r to s o d i c v a r i e t i e s . At any r a t e , such an a s s o c i a t i o n of minerals as c h l o r i t e , a l b i t e , 75 epidote and s e r i e i t e are t y p i c a l of a f a i r l y low grade of metamorphism i n which the dominant f a c t o r was d i f f e r e n t i a l s t r e s s somewhat a b e t t e d by e l e v a t e d temperatures and accompanied by m a t e r i a l i n t r o d u c e d from meteoric, and p o s s i b l y a b y s s a l , sources. C h l o r i t e , s e r i e i t e , and a l b i t e (?) are d i a g n o s t i c s t r e s s m i n e r als whose abundance, coupled w i t h the absence of abundant t y p i c a l a n t i - s t r e s s and h i g h temperature m i n e r a l s , a t t e s t s to the pre-eminence of dynamic f o r c e s of metamorphism under low or i n t e r m e d i a t e temperature c o n d i t i o n s . Up t o t h i s p o i n t no mention has been made of b i o t i t e , which' i s widespread and i n some -places abundant, and which i s most l i k e l y of metamorphic o r i g i n . In the f i e l d , b i o t i t e became conspicuous i n v o l c a n i c s i n the nor t h e a s t c o r n e r of the map-area, but u n f o r t u n a t e l y mapping was suspended here so that the extent and s i g n i f i c a n c e of the " b i o t i t e zone" were not pursued to completion. In the f i e l d ' , the sudden appearance of b i o t i t e suggested a d i s c o n t i n u i t y i n the r e g u l a r d i s p o s i t i o n of metamorphic grade as might r e s u l t from: (1) unrecognized f a u l t i n g b r i n g i n g a zone of h i g h e r grade metamorphism t o the s u r f a c e . (2) a f o r t u i t o u s i r r e g u l a r i t y i n i s o g r a d s u r f a c e s as c o u l d ' r e s u l t from an unexhumed i n t r u s i v e ( p o s s i b l y s a t e l l i t e to the main g r a n i t i c i n t r u s i v e s ) or a summit c^upola i n the r o o f of a deep-seated g r a n i t i c basement. A l s o s i g n i f i c a n t i s the s p a c i a l r e l a t i o n between the " b i o t i t e - z o n e " and t o u r m a l i n i z e d s u l f i d e d e p o s i t s and quartz 76 v e i n s , as w e l l as the o n l y p e g m a t i t i c v e i n observed i n the map-area. Thus the h i g h e r metamorphic grade i n d i c a t e d by the presence of b i o t i t e i s accompanied by a h i g h e r temperature type of v e i n i n g and the apparent d e r i v a t i o n of boron from a source not f a r removed from the same l o c a l i t y . F i e l d c o n c l u s i o n s , however, are somewhat complicated by m i c r o s c o p i c examination of the r o c k s . In t h i n - s e c t i o n : b i o t i t e was observed i n most rock types - exceptions being minor a c i d i n t r u s i v e s , some of which may post-date the f o r m a t i o n of b i o t i t e , and a c i d v o l c a n i c s . where the necessary c o n s t i t u e n t s .for i t s f o r m a t i o n probably were not p r e s e n t . B i o t i t e i n b a s i c i n t r u s i v e s may be primary or the a l t e r a t i o n product of ferromagnesian c o n s t i t u e n t s , but i n o t h e r rocks i t s f e a t u r e s are c h a r a c t e r i s t i c of a metamorphic o r i g i n . I t i s the brown, f e r r u g i n o u s v a r i e t y of b i o t i t e , s t r o n g l y p l e o c h r o i c from brown to' pale tan or almost c o l o r l e s s . . The o p t i c angle i s v e r y small) approximately zero f o r most g r a i n s t e s t e d . In some pla c e s b i o t i t e i s r e l a t i v e l y f r e e of i n c l u s i o n s and possesses r e g u l a r margins, but i n o t h e r places i t e x h i b i t s a w e l l - developed p o i k i l o b l a s t i c s t r u c t u r e and sutured margins. The one f e a t u r e i n the h a b i t of b i o t i t e t h a t seldom v a r i e s i s i t s i n t i m a t e a s s o c i a t i o n with c h l o r i t e . I t i s almost i n v a r i a b l y surrounded by, or i n c o n t a c t w i t h c h l o r i t e . The problem ' a r i s e s of which i s the o l d e r m i n e r a l and whether one i s an a l t e r a t i o n product d e r i v e d from the o t h e r . Contacts between b i o t i t e and c h l o r i t e v a r y from very sharp to r e l a t i v e l y g r a d a t i o n a l across the c o n t a c t , and from l i n e a r to i r r e g u l a r 77 along the contact. Various relationships between the minerals may be summed up as follows: (1) irregular, diffuse patches of chlorite within biotite grains. (2) small "residuals" of biotite surrounded by chlorite (plate 12) (3) chlorite "eating into" the margin of biotite grains (,4) parallel intergrowths of alternate bands of biotite and chlorite. (5) wedges of chlorite penetrating biotite grains from the margin and tapering out within the biotite grains (plate 13). The writer interprets the foregoing as evidence favoring the formation of chlorite after biotite. In many sections are minute, prismatic grains of a greenish mineral with parallel extinction and very low birefringence. The habit suggests biotite but the optic properties indicate chlorite or a mineral closely a l l i e d to chlorite; so that these small prismatic grains may be pseudomorphs of chlorite after biotite. In one section grains of chlorite showing an anomalous "Berlin-Blue" interference i color are quite definitely pseudomorphs of chlorite after biotite. The chlorite maintains the same form and cleavage orientation as the parent biotite (p'late 12). Therefore, although none too well founded on the basis of such a limited scope of work, the writer concludes that • biotite is of metamorphic origin, formed at the expense of P l a t e 12: Pseudomorphs of c h l o r i t e (cl} a f t e r b i o t i t e . S l i g h t l y above centre of photomicrograph a small " r e s i d u a l " of b i o t i t e (dark grey) appears i n c h l o r i t e (c ) . P l a t e 13: B i o t i t e (t>) a l t e r e d to c h l o r i t e (cl ) along bands and "wedges" p a r a l l e l w i t h cleavage i n b i o t i t e (best seen i n l a r g e s t b i o t i t e g r a i n . 78 minerals developed mainly by dynamic processes during folding, and that., since its formation, much has partly or wholly reverted to chlorite. The writer is not prepared to say how much of the chlorite now present in the rocks was 'developed by metamorphism accompanying folding, and how much was derived from biotite. Presumably, the biotite formed largely at the expense of chlorite, which would exert considerable influence on the abundance and disposition of biotite in the rocks. It is unlikely that the chlorite, which would provide the starting-point for biotite, and the chlorite derived from biotite^ would vary markedly i n optical properties; and i t cannot be assumed that a l l of the later chlorite presents i t s e l f as pseudomorphs after biotite. In short, the extent, to which the rocks were biotitized previous to retrogression to chlorite is d i f f i c u l t to evaluate, and the appearance' of more biotite in some areas than others might be a reflection of irregularities in the degree of biotitization, differences in the degree of 'retrogression to chlorite, or be dependent on other factors set forth earlier i n this discussion. Turning to the probable genesis of biotite, some consideration must be given its relation to regional fo l i a t i o n . Where developed in considerable quantity- biotite occurs,, with chlorite, in lenticular zones elongated parallel with fo l i a t i o n . Individual grains of biotite rarely show evidence of bending or contortion and have grown at a l l -angles to foli a t i o n . Admittedly, a great many biotite flakes l i e closely parallel to the direction of fol i a t i o n , but an almost P l a t e 14: Metamorphic b i o t i t e that has grown at a large angle to r e g i o n a l f o l i a t i o n (approx. h o r i z o n t a l ) , i n d i c a t i n g the b i o t i t e was generated a f t e r r e l a x a t i o n of the st r e s s e s causing f o l i a t i o n . 7 9 equal number lying athwart the f o l i a t i o n lends support to the contention that most of the biotite developed subsequent to foliation, i.e. after relaxation of the stresses causing folding (plate 14). The extent to which biotite orientation conforms with fo l i a t i o n might be the combined effects of the influence of chlorite,- oriented with the f o l i a t i o n and provid- ing a nucleus for biotite; of any recurrent stresses con- current with the growih of biotite; and possibly of early growth of biotite, before cessation of folding and before the main spread of biotitization, in local zones of above- average heat intensity. Biotite. may be attributable to a rise in temperature during the ebbing stages of folding when mechanical and f r i c t i o n a l heat developed by folding would l i k e l y reach i t s maximum. Or, i t may be related to a rise in temperature accompanying the invasion of large, plutonic masses of granitic rocks whose emplacement was soon after, or much later than, the consummation of folding. To summarize, regional metamorphism in the area may be divided into three categories; (1) Dynamic metamorphism associated with folding and accompanied by the generation of stress minerals. (2) Thermal metamorphism due to a temperature rise and tending to substitute anti-stress minerals for stress minerals. (3) Retrograde metamorphism - a degradation process including the reversion of biotite to chlorite. 8 0 The writer is aware that the foregoing is an exceedingly incomplete treatise of such a complex mechanism as regional metamorphism,. particularly where more than one kind of .metamorphic process is involved.. Along with the transformations described, analogous changes would take place in other members of the mineral assemblage. The changes that have been described are intended only asAindex to.the types and intensity of the metamorphic process involved. Then, too, some of the changes noted could not have obtained without the intervention .of some element of _. metasomatism, probably involving additions of some material from both abyssal and atmospheric sources, and partial elimination of other material. Any changes in the bulk composition of the rocks have been omitted in the foregoing discussion on metamorphism. 81 CARBONATE ALTERATION Widespread carbonate alteration of rocks in the. area is of sufficient interest to warrant a brief consideration here. Its relation to gold mineralization is not known, so the following discussion w i l l be mainly descriptive. As already stated, in the f i e l d "carbonate-zones" are abundant in volcanic rocks, usually associated with zones of intense shearing. They vary from rocks almost completely replaced by carbonate to rocks highly impregnated with disseminated carbonate but maintaining their essential original characteristics. Carbonate zones'usually weather rusty, and locally break down to a rusty gossan. Carbonate also appears in veins, with or without quartz. Vein-carbonate is usually in coarsely-crystalline, cleavable masses, and is white, buff, or brown and to a lesser extent pinkish or grey. The most unusual occurrence of carbonate, and the one on which the greater part of this discussion w i l l be centred, is. as metacrysts. Carbonate metacrysts, although most abundant in volcanic terrain, are not limited in distribution to any particular rock type. They were observed only in the southern half of the map-area, where they are localized, to some extent,, in rocks close to volcanic-sediment contacts. They reach their, maximum development in altered dykes and s i l l s , but were observed in a l l kinds of volcanic rocks and in fine-grained sediments. The altered, so-called "carbonate-dykes* have apparently proved the most favorable site for the development of carbonate metacrysts. The "dykes" were probably originally 82 quite basic in composition, but are now composed•essentially of chlorite and carbonate. They are definately intrusives that have been subsequently carbonatized^ rather than regular zones of alteration; as they frequently traverse the structural trend and show intrusive relations against the adjacent rocks. They vary from rocks that are almost entirely carbonate, through rocks that show large metacrysts set in a green or grey groundmass, to metacryst-bearing rocks that resemble some varieties of diorite and gabbro dykes. The latter occasionally have metacrysts concentrated i n the marginal zone of the' dyke. It is not knovaawhether "carbonate-dykes" represent one kind of dyke, or whether they include dykes of several kinds and ages. In many places rocks adjacent to dykes carrying carbonate metacrysts exhibit metacrysts that diminish in size and peter out a few inches away from the dyke. It is as i f the agents that formed the metacrysts ascended along the dyke fissure, affecting the dyke-rock and at the same time exerting a minor influence upon the wall-rock. In many lo c a l i t i e s , however, metacrysts in volcanic rocks show no particular spacial relation to "carbonate-dykes", but these same loc a l i t i e s are usually not far removed from a major volcanic-sediment contact. . The metacrysts vary from a fraction of a millimeter to nearly l/4 inch across. They usually weather to form a square or rhombic pit partially f i l l e d with indigenous limonite. Where metacrysts are small and abundant their tendency to weather out imparts a porous appearance to the 83 surface of the rock. Carbonate metacrysts are light brown to pale tan when fresh, but weather dark brown or rusty for l/4 to 1 inch below the tsurface of an exposure. Metacrysts are revealed under the microscope in rocks where they are quite, unapparent megascopically. They usually show prominent cleavage and an extremely high birefringence and pronounced change of r e l i e f , and occasionally exhibit very coarse twinning or brown alteration at crystal margS/ns and working along cleavage planes (plate 15). Some have sutured boundaries and a sieve structure (plate 16), while others, from which a l l inclusions have been expelled, have sharp, regular boundaries, Metacrysts usually have grown in the fine-grained portion of rocks, but many were observed completely enclosed in feldspar grains. With the exception of twinning, metacrysts show no evidence of deformation, have not been affected by or exerted any effect upon fo l i a t i o n , and appear to have grown passively after the development of f o l i a t i o n . The metacryst-forming carbonate exhibits an idiomorphic tendency not displayed by normal calcite. Since the optical: properties of the various members of the calcite group are not distinctive, the writer attempted to ascertain the composition of the carbonate by specific gravity,and chemical methods. A' series of. nine specifie gravity determinations were made on fragments of -the mineral picked as clean as possible under binoculars. The results gave a value of 2.87* .03 (with an average of 2.88) for the specific gravity of the carbonate. F l a t s 15: Plat e 16: Carbonate ( a n k e r l t e ) metacrysts i n r h y o l i t e f l o w. Note pronounced cleavage and brown a l t e r a t i o n (black) around margin and penetrating cleavage cracks of the metacryst ( f o l i a t i o n approximately h o r i z o n t a l ) . P o i k i l o b l a s t i c metacrysts of a n k e r i t e (high r e l i e f ) i n carbonate zone with green, phengitic-muscovite («*). Note v a r i a t i o n i n r e l i e f between d i f f e r e n t grains of carbonate. F o l i a t i o n i n N.S. - S.'.,<'. quadrant. However, undue emphasis cannot be placed upon specific gravity measurements owing to inevitable contamination by minute inclusions of other minerals. Extremely slow effervescence of the carbonate in cold acid is accelerated rapidly by heat. A fragment heated to red heat becomes strongly magnetic upon cooling. Chemical tests confirmed the presence of Ca,Pe,Mg, Mn, and CCv>. Tests for Mg and Mn were not as conclusive as might be desired, but the writer feels certain the two elements are present in considerable amounts. The results of the foregoing tests indicate the carbonate is a type of dolomite intermediate between magnesiodolomite and ferrodolomite, with some manganese probably replacing magnesium in the molecule. Its composition could be represented by the formula Ca CO^. (Mg,Pe,Mn) COg, and i t would probably be correctly termed an ankerite. The specific gravity obtained by the writer (2.87 ) agrees with that of dolomite (2.8-2.9) rather than the accepted value for ankerite (2.95-3.1), as given by Dana (1932, pp. 516 and 517) The discrepancy could probably be imputed to minute inclusions of feldspar, serioite, and quartz that, incidentally, were observed in a thin-section of the same rock from which'the carbonate for specific gravity determinations was . obtained. Inclusions of such minerals having a lower specific gravity than the carbonate would tend to lower its average specific weight, and yet i f uniformly distributed would yield f a i r l y consistent results to specific gravity determinations. Ankerite is by no means limited to euhedral crystals, or 85 metacrysts, but exists rather commonly as highly sutured, poikiloblastic grains (plate 16) and mosaic patches in various kinds of rocks. It is abundant in acid ssrolcanics, where i t has a marked tendency to replace feldspar grains. On the southwest shore of Negus Lake an exposure of volcanics reveals a highly carbonatized zone with abundant flakes of a b r i l l i a n t green, micaceous mineral not observed elsewhere in the area. In the f i e l d the green mineral was thought to be a chromian mica of the mariposite-fuchsite- muscovite series, especially in view of the fact that chromium-bearing micas frequently accompany zones of ferruginous carbonate alteration adjoining prominent faults.. Rock from the carbonate zone exhibits inherent regional foliation, has a rusty, finely-pitted, weathered surface, and a variegated grey to buff fresh surface that shows myriads of tiny, sparkling, cleavage-faces of carbonate. The green mineral occurs in narrow streaks elongated parallel with the f o l i a t i o n . The microscope reveals the rock is composed of over 50 percent ankerite as euhedral rhombs, mosaic patches, and irregular grains with a very pronounced sieve structure. Quartz and pale-tan, opaque, alteration material are plentiful along v/ith varying amounts of colorless chlorite (?), fine crystals of rutile (?), sulfides, and minute, prismatic crystals of an unidentified mineral with very low birefringence The green mineral constitutes about 5 to 10 percent of the rock. Its optical properties are; 86 (1) one perfect cleavage; parallel extinction; length-slow (2) pale green color; very weakly pleochroic (3) prominent change of re l i e f (4) biaxial (-); 2V = 35° (5) birefringence = .035 (6) 1.599> (i > 1.588 The optical properties indicate that the mineral belongs to the mica group of minerals, and more particularly, to the potash micas. The green color is not typical of normal muscovite. The moderate optic angle and mean index practic- a l l y eliminate the possibility of mariposite. The optic properties correspond closely to those of. fuchsite, reputed to contain from .27 to 4.81 percent C^gOg (Whitmore, Berry and Hawley, 1946, p.15), although fuchsite carrying only a trace of CrgOg has been noted (Whitmore, Berry and Hawley, 1946, p.13). In testing for chromium, the writer, cognizant of the inseparability of ankerite from the green mineral and the consequent adulteration with iron, f e l t that the inconclusive positive bead-test obtained for chromium could not be relied upon. A check was made by spectrographic analysis and yielded negative results for chromium, practically eliminating a l l possibility of the mineral being chromiferous. The writer then concluded that the mineral is probably a member of the muscovite-phengite system, the small optic angle attributable to a high tenor of phengite and the unusual green color possibly the result of an appreciable iron content contributed by the phengite molecule (Winchell 1932, P 268). 87 In c o n c l u s i o n , the w r i t e r might add that the problem of carbonate a l t e r a t i o n i n the area might l e a d to s i g n i f i c a n t r e s u l t s i f pursued to completion. Carbonate-zones and carbonate metacrysts should probably be d i v o r c e d from the minor c a r b o n a t i z a t i o n of a l l rocks as a r e s u l t of the break- down of primary minerals d u r i n g r e g i o n a l metamorphism. The former are the r e s u l t of hydrothermal a c t i o n , probably oper- a t i v e a f t e r r e g i o n a l dynamic processes, and p o s s i b l y q u i t e l a t e as i n d i c a t e d by the presence of metacrysts of carbonate i n some of the di o r i t e - g a b b r o . i n t r u s i v e s . A l s o worth r e i t e r a t i n g i s the f a c t t h a t some of these same b a s i c dykes have been a f f e c t e d by c a r b o n a t i z a t i o n (not as metacrysts) a s s o c i a t e d w i t h s u l f i d e m i n e r a l i z a t i o n accompanied by v e r y minor g o l d v a l u e s . Proof i s l a c k i n g of whether carbonate metacrysts r e p r e s e n t an e a r l y stage i n the c o n v e r s i o n of rocks to carbonate-zones. C e r t a i n l y the carbonate of both i s the same a n k e r i t i c v a r i e t y . On one hand, abundant metscrysts i n a l t e r e d b a s i c dykes Cbarbonate-dykes") suggests t h a t the dyke f i s s u r e s , a f t e r they were f i l l e d by dyke-rock, proved i n some manner permeable conduits f o r the ascent of carbonate- bearing s o l u t i o n s . On the oth e r hand, the apparent c l o s e • r e l a t i o n of carbonate metacrysts i n other kinds of rock to the main sediment-volcanic c o n t a c t suggests that the phenomenon i s a phase of a l t e r a t i o n c o n t r o l l e d i n some measure by the c o n t a c t , and not r e s t r i c t e d to any p a r t i c u l a r rock type. The gen e r a l p r o x i m i t y of metacrysts to the best, known ore-bodies i n the area, i e , their, r e s t r i c t i o n to the southern 88 p a r t of the map-area, i f genuine, suggests that the metacrysts are i n some v/ay g e n e t i c a l l y a s s o c i a t e d with g o l d m i n e r a l i z - a t i o n or c o n t r o l l e d by the same s t r u c t u r e s t h a t have l o c a l i z e d m i n e r a l i z a t i o n . I f carbonate a l t e r a t i o n , p a r t i c u l a r l y meta- c r y s t s , could be r e l a t e d s p a c i a l l y o r g e n e t i c a l l y to g o l d m i n e r a l i z a t i o n , i t c o u l d c o n c e i v a b l y become a u s e f u l i n d i c a t o r of g o l d p o t e n t i a l i t i e s i n other p a r t s of the r e g i o n . A d m i t t e d l y , i t would not be d e c i s i v e evidence and i t s a p p l i c a t i o n would be on a broad s c a l e , but i t might l e n d a s s i s t a n c e i n the s e l e c t i o n of f a v o r a b l e p r o s p e c t i n g and t e s t i n g ground. Billings, M.P. 1942 Billings, M. 1928 Cloos, E. 1946 Dana, E.S. 1932 Deer, W.A. 1938 Furnival, G.M. 1935 Graham, W.A.P. 1926 Harker, A. 1932 Hutton, G.O. 1938 Kidd, D.F. 1936 Lord, C.S. 1942 MacCarthy, G.A 1926 Nevin, C.M. 1936 Rice, H.M.R. 1935 Whitmore, D.R.E Berry, L.G. and 1946 Hawley, J.E. V/inchell, g.N. 1924 Wlnohell, A.N. 1932 REFERENCES Structural Geology Amer. Min., Vol. 13, p. 292 Lineation-Geol. Soc. Am. Mem. 18 Dana's Textbook of Mineralogy. Amer.Min., Vol. 25, p. 56 The large Quartz Veins of Great Bear Lake - Ec. Geol., vol. 30, No. 8 Amer. Min., Vol. 11, p. 121 Metamorphism Min. Mag. Vol. 25, Nos 160-171 pp. 207-211 Rae to Great Bear Lake, Mackenzie District N.'w.T. - Geol. Surv. Canada Mem. 187, pp 18-19 Snare River and Ingray Lake Map- Areas, N.i/.T. Geol. Surv. Canada, Mem. 235 Amer. Min., Vol. 11, p. 321 Structural Geology Amer. Min.,, Vol. 20, p. 308 Chrome Micas - Amer.- Min. Vol.31 pp 1- 21 Amer. Journ. Sci., ser.5, vol.7 p 303 Elements of Optical Mineralogy.

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