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The mineralogy of the Bonanza silver deposit, Great Bear Lake, N.W.T. 1948

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THE MINERALOGY OF THE BQMAEZA SILVER DEPOSIT GREAT BEAR LAKE N.W.'T. A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF APPLIED SCIENCE In the Department of GEOLOGY AND GEOGRAPHY The University of British Columbia Apri l , 1948. JOHN KEITH DIEBEL ACKNOWLEDGMENTS The writer is indebted to Br. C. Riley of Vanoouver, who oolleoted the specimens and contributed valuable information concerning the geology, mineral deposits and general conditions in the area. Special thanks are owing various members of the staff of 'The Department of Geology and Geography for assistance receiv- ed* and particularly to Dr, H.0. Gunning, under whose super- vision the work was carried out. The technical assistance given by Mr. J. A. Donnan is gratefully acknowledged. Mr. R. G. MoOrosaan kindly did a l l the spectrographic analysis, using the Department of Physic's spectrograph. TABLE OF CONTENTS Page ABSTRACT INTRODUCTION by Dr. C. R i l e y GENERAL GEOLOGY 1 Table of Formations. 2 Bono Bay Group. 3 , Cameron Bay Group..... 3 LNTRUSIVES General ., 3 . Gr a n o d i o r i t e . 4 Gr a n i t e s . . . . . . . . . . . . . . . . . . . . . . . . . 4 Bas i c Dykes and S i l l s . . . 5 Giant Quartz Veins. 6 STRUCTURAL GEOLOGY ., 8 GEOLOGY OF THE BONANZA CLAIMS General. 8 Sedimentary Rooks 8 Gr a n o d i o r i t e . . . . . . . . . . . . . . . . . . . . . 9 Minera.// -ia.t/on II MINERAL DEPOSITS General 12 MINERALOGY OF THE BONANZA GROUP General Statement................ 13 Magnetite 14 Hematite • 15 Te t r a h e d r i t e . . , 17 Argent i te 19 Chalc o p y r i t e 21 Unknown 22 Native S i l v e r 22 GANGUE MINERALS Quartz................. 25 S e r i o i t i o Gangue................. 28 Carbonate Gangue 28 ORIGIN OF THE DENDRITIC FORM OF SILVER.... 29 SUMMARY OF MINERALOGY.., 33 PARAGENESIS 34 TABLE OF CONTENTS (float 1 d)- Page TEMPERATURE OF FORMATION , 36 ORIGIN OF THE SOLUTIONS 37 COMPARISON WITH OTHER DEPOSITS .................... 39 GENERAL . 39 COMPARISON WITH THE SILVER DEPOSITS AT COBALT, ONTARIO...... .............. • 40 COMPARISON WITH THE ERZGEBIRGE DEPOSITS OF SAXONY AND CZECHOSLOVAKIA IN CENTRAL EUROPE 41 COMPARISON WITH MISCELLANEOUS DEPOSTS... 43 CONCLUSIONS 44 BIBLIOGRAPHY 46 ILLUSTRATIONS A f t e r F i g u r e I - G e n e r a l i z e d Map of tae Echo Bay page D i s t r i c t , N . W. T . 1 P l a t e 1 . 18 2. 18 3. 18 4 i 18 5 . 19 6. 19 7. 21 8. 21 9. 24 10 . 24 1 1 . 32 12. 32 4 ABSTRACT A study of the mineralogy of a suite of specimens, col- lected "by Dr.C.Riley from the Bonanza silver deposit, has been made, Particular attention is paid to the silver mineraliza- tion and the origin of the dendritic structure, A brief ex- amination of the wall rock alteration i s included. The mineralogy of the deposit is relatively simple, con- sisting of the following metallic minerals in their order of abundance: native silver, magnetite, hematite, tetrahedrite, argentite, chalcopyrite,' and an unknown mineral. Pitchblende and cobalt-nickel minerals are absent. Magnetite and hematite are restricted to the wall rock and are not associated with the other metallic minerals. The magnetite is believed to be of pyrometasomatic origin and related to a granodiorite intrusion, while the other metallic and gangue minerals, m) 1Lli 1.liesaggaafss L1 uii I.I P^s^^^feg, are considered to be of hydrothermal origin. The gangue minerals consist of quartz, sericite, and carbonate. linety-five percent of the native silver occurs as dend- rites and the other five percent as replacement of tetrahed- rite and chalcopyrite. Core replacement by the silver is well developed. The dendritic structure of the silver is inherited from quartz through replacement. In a quartz gangue this structure appears to be controlled by rows of specially orient- ed, doubly terminated, quartz prisms, while in a s e r i c i t i c gangue the euhedral quartz grains, arranged in a rude dendritic pattern, are the controlling factor. The mineral deposits of the Echo Bay area are compared with similar deposits throughout the world. INTRODUCTION by DR. 0. RILEY Introduction. The ores described herewith were collected i"n June, 1932, by 0. Riley from the Bonanza silver prospect. The only work done at that time consisted of one shallow pit on the vein and some stripping. It was therefore not possible to take a complete suite of ores from the deposit. The specimens, how- ever, are probably representative of the main types of vein material. They were taken for the purpose of study and des- cription but opportunity to do such work was not available un t i l Mr. J.K.. Diebel undertook the work during the winter of 1947-48. Location. The Bonanza vein is situated on Dowdell Point, about six miles south of the Eldorado Radium Mine on the eastern shores of Great Bear Lake. The deposit should not be confused with the E l Bonanza prospect which i s about one half mile to the east (see Figure I). History. The Bonanza vein was discovered in June, 1931, by G.A. La Bine and E.C. St.Paul, prospectors working for Eldorado Mines Ltd. A small amount of surface work was done in 1931, and in 1938 some underground work was done but no record of i t has been published. The property has, therefore, been inactive most of the time since its discovery. Accessibility. The vein is less than one half mile on easy grade, from a - i i small Day on Great Bear Lake. Otherwise i t would be serviced in the same manner as the Eldorado Mine - that i s , by river and lake boats, and by a i r . Climate. The climate of the country is severebut not unpleasant. Winters are cold and dry; the snowfall i s from three to four feet. Late winter and spring have long, clear and pleasant days. Summer i s short and f a l l commonly dreary. The lake i s open for navigation from July to October, Topography. 'The Great Bear Lake di s t r i c t has unusually rugged topog- raphy for a pre-Cambrian area. Rocky h i l l s rise about 1200 feet above the lake level. The Bonanza vein is very l i t t l e higher than the lake but the area immediately to the north i s rough and precipitous. General Mining Conditiolis. Exploitation of the Bonanza vein would offer no problems other than those to be expected from the remote location and severe and protracted winter. Timber, however, is sparse and stunted and already made scarcer by the long use of i t "by the Eldorado Mine. Replacement by growth is slow. GENERAL GEOLOGY The general geology of the Eoho Bay area has been desorib- ed by D.F. Kidd (1932$, H.S. Robinson (1933), G.M. F u r n i v a l (1939), B. Murphy (1946) and others. The f o l l o w i n g d e s c r i p - t i o n i s a summary of t h e i r work. The general geology i s shown i n Figure I. The o l d e s t rooks are a complex s e r i e s of v o l c a n i c and sedimentary rocks, and probably i n t r u s i v e porphyries, known as the "Old Complex", On l i t h o l o g i c a l grounds, these e a r l y rooks are d i v i d e d i n t o two groups; the lower, member being c a l l e d the Eoho Bay group and the upper member, the Cameron Bay group.  TABLE OF FORMATIONS 1 Quaternary i i i S i l t , clay, gravel, morainal material Unoonformity Precambrian(?) iBasio dykes and s i l l s ; large quartz veins (Intrusive Contact) Precambrian(?) 'J•Granite and other acid plutonic rocks • i (Intrusive Contact) Preoambrian |Quartz diorite and granodiorite i (Intrusive Contact ?) Precambrian Sedimentary and volcanic complex subdivided into: (a) Cameron Bay group - conglomerate, tuff, ar- g i l l i t e , etc. • unoonformity • •— (b) Eoho Bay group - porphyritic volcanics, tuff, a r g i l l i t e , quartzite, conglomerate, etc. -3 Eoho Bay Group: its In 4U+H-U.- upper part, the Eoho Bay group consists mainly of porphyries, at some places extrusives, with minor amounts of a r g i l l i t e and tuff. In the lower part of the group siliceous a r g i l l i t e , tuff, quartzite, conglomerate and a l i t t l e limestone are abundant. The "base of the group has not been seen. The rooks have moderate to steep dips and structurally appear to underlie the Cameron Bay group, though the contact has not been observed. The presence in the Cameron Bay conglomerate of abun-? dant pebbles of rooks similar to those in the Echo Bay group, suggests there is an unconformity between the two. i Cameron Bay Group: The Cameron Bay group shows a uniform deposition of sedi- ments; f i r s t a poorly consolidated cobble conglomerate with some interbedded greywacke, followed by coarse grit, arkose and sandstone. One f e l s i t e flow is interbedded with the conglomer- ate. The total thickness is in the v i c i n i t y of 3000 feet. Vein quartz pebbles and a single granite pebble indicate that there may have been a period of granitic intrusion prior to the formation of the group. Intrusives - General: The Echo Bay and Cameron Bay groups have been intruded by acid and basic rocks. In chronological order from oldest to youngest they are: 1. Granodiorite to diorite -4- 2. Dowdell Poi n t g r a n i t e and Lindsay Bay g r a n i t e 3 . Diabase Dykes (o l d e r ) 4. Diabase Dykes and S i l l s (younger) Gr a n o d i o r i t e : The g r a n o d i o r i t e occurs as i r r e g u l a r elongated stocks i n - t r u d i n g the Echo Bay and Cameron Bay groups. A t y p i o a l s p e c i - men of g r a n o d i o r i t e i s massive, medium-grained, g r a n i t i c t e x - tured and r e d d i s h to greenish-brown i n colo u r . In t h i n s e c t i o n p l a g i o c l a s e ( o l i g o o l a s e to andesine), o r t h o c l a s e , quartz, a u g i t e , b i o t i t e and i r o n oxides are u s u a l l y present. These rocks have caused an unusually great amount of metamorphism round t h e i r borders. At many p l a c e s , f o r a width of one- quarter of a mile or more, the intruded rocks are r u s t y weath- e r i n g , due to disseminated p y r i t e , and contain c h l o r i t e , mag- n e t i t e , b i o t i t e , epidote, a c t i n o l i t e and red f e l d s p a r . The g r a n o d i o r i t e i s o l d e r , at l e a s t , than the Dowdell Point g r a n i t e , because i t i s cut by the g r a n i t e . Granites: The Dowdell P o i n t g r a n i t e and the Lindsay Bay g r a n i t e do not occur i n contact w i t h each other; thus t h e i r r e l a t i v e age i s not known. The Dowdell Point g r a n i t e i s an entiarely massive, f r e s h - l o o k i n g , coarse-grained, u s u a l l y somewhat por- p h y r i t i c , b u f f to pink, crumbly weathering b i o t i t e g r a n i t e . The Lindsay Bay g r a n i t e i s massive, mediums-grained, rather u n i - form i n appearance, wi t h l i g h t brown to buf f to white to pink -5- feldspars, quartz, biotite and, in some places, hornblende. It is occasionally slightly porphyritic near the borders. Aplite dykes are minor differentiates of both granites. Pegmatites are rare. Basic Dykes and S i l l s ; Basic dykes are widespread and cut rocks of a l l the four groups previously'described. A few dykes differ l i t h o l o g i c a l - ly from the more abundant type in having some red feldspar and are provisionally separated as an older type. These older dykes have been termed "gabbro" by Robinson (1933, p. 618). 'They are the youngest intrusive that can be definitely shown to be older than the pitchblende-silver miner- alization. Kidd (1932D, p.140) reports that thin sections from two of these dykes show a rock with a medium even grain, also subhedral plagioclase grains (oligoclase to andesine), a l i t t l e orthoclase, quartz, augite and iron ore. The more abundant type of basic dyke is widespread, gener- a l l y steeply dipping and up to 100 feet in width. The trend is variable. They are medium to fine-grained, and greenish-grey to greenish-black in color. In some instances the medium grained parts have a diabasic texture. A f l a t - l y i n g dyke (or perhaps more than one) outcrops at many places. It i s found as a series of isolated occurrences that may have been one continuous body. It is only 100 to 500 feet thick but of great lateral extent. It is a quartz norite, which in thin section shows conspicuous i n t e r s t i t i a l micrographic i n t e r g r o w t h s of quartz and f e l d s p a r , l i t h o l o g i o a l l y t h i s s i l l , or f l a t - l y i n g dyke, i s s i m i l a r to the -more abundant s t e e p l y - d i p p i n g b a s i o dykes and p r o b a b l y i s p a r t o f the same i n t r u s i v e . S l a n t Quartz Veins; fliWHiimi 1 I r Stookworks o f l a r g e quartz v e i n s are common i n the a r e a . These v e i n s are l o c a l l y known as g i a n t quartz v e i n s . They range i n w i d t h from 50 to 500 f ee t and have lengths up to t e n m i l e s . They c o n t a i n no f e l d s p a r nor any a p p r e c i a b l e quan- t i t y o f m e t a l l i c m i n e r a l s . Their occurrence i n f a u l t s , w h i c h out n e a r l y a l l the known igneous r o c k s , s u g g e s t s that they are not r e l a t e d to i n t r u s i v e s now exposed. STRUCTURAL GEOLOGY In the Eoho Bay d i s t r i c t the rocks of the " O l d Complex" s t r i k e n o r t h and south a t G l a o i e r Bay and swing southeast to s t r i k e about S.60°E. n o r t h of Contact L a k e . They form the western edge of a gent le s y n c l i n e which has "been crumpled a g a i n s t the g r a n i t e a l o n g the shore of Great Bear Lake , n o r t h and south of Echo Bay. Except near the g r a n i t e b o d i e s , the d i p s are u s u a l l y l e s s than 45 degrees; much of the Cameron Bay group i s o n l y s l i g h t l y f o l d e d . C o n s i d e r i n g i t s p r o x i m i t y to so many i n t r u s i v e bodies t h i s i s r a t h e r remarkable . The area i s broken by many f a u l t s , the m a j o r i t y of which have r i g h t - h a n d o f f s e t s and s t r i k e about n o r t h e a s t . L i t t l e evidence has been found of major v e r t i c a l movement. There are l a r g e quar tz v e i n s i n s e v e r a l of the f a u l t s , and the c o i n c i d - ence i n d i r e c t i o n between the system of q u a r t z v e i n s and the major f a u l t s suggests that many of the v e i n s may occupy f a u l t s . As some of the l a r g e quar tz v e i n s cut the g r a n i t e ^ t h e f a u l t i n g p o s s i b l y p o s t - d a t e s the g r a n i t e . The most prominent f a u l t i n the d i s t r i c t i s a t Cameron Bay ( F l g u r e l ) . Contacts meet ing i t appear to be d i s p l a c e d about three m i l e s . A l a r g e quar tz v e i n l i e s i n the f a u l t . In a d d i t i o n to the n o r t h e a s t f a u l t s , there are sugges- t i o n s of o ther s t r u c t u r e s t h a t t r e n d n o r t h and s o u t h . These s t r u c t u r e s may represent j o i n t i n g and f r a c t u r i n g r a t h e r than f a u l t i n g . They are i n d i c a t e d by n o r t h - s o u t h dykes of c o n s i d - - 8 - erab le l e n g t h . Geology of the Bonanza Cla ims G e n e r a l : The m i n e r a l i z a t i o n occurs i n banded sedimentary rocks of the Eoho Bay group, approx imate ly one-quar ter of a m i l e w i d e , bordered on the south by the Dowdel l P o i n t g r a n i t e and on the n o r t h by a body of g r a n o d i o r i t e . Sedimentary Rooks-: The sedimentary rocks s t r i k e west -northwest and d i p n e a r - l y v e r t i c a l l y . They a r e , i n l a r g e p a r t , t h i n l y banded c h e r t s , q u a r t z i t e s and a r g i l l i t e s , now l a r g e l y r e c r y s t a l l i z e d . In t h i n s e o t i o n they c o n s i s t p r i m a r i l y of v e r y f i n e - g r a i n e d , r e - c r y s t a l l i z e d q u a r t z w i t h minor amounts of c h l o r i t e , green mica or hornblende , some c loudy a l t e r a t i o n , p o s s i b l y a c l a y m i n e r a l , Some. and euhedra l g r a i n s of m a g n e t i t e , niiv+.n,* n bands c o n t a i n enough f i n e , d i sseminated hematite to g i v e them a r e d d i s h c o l - o u r . The magnetite c r y s t a l l i z e d l a t e s i n c e i t cuts i n t o a l l the o ther m i n e r a l s . I n d i v i d u a l bands range from l e s s than l / l 6 i n c h to l / 8 i n c h i n w i d t h . The average g r a i n s i z e i s l e s s than 0 .05 mm. Some bands, p a r t i c u l a r l y i n the m i n e r a l i z e d zone, are much s o f t e r , weather d i f f e r e n t i a l l y , and have a dark green c o l o u r . They c o n t a i n a l a r g e amount of f l a k y c h l o r i t e and a t some p l a c e s much hemati te and m a g n e t i t e . They are p r o b a b l y A l t e r e d ca l careous beds. The a l t e r a t i o n i s s i m i l a r to that oaused at other places by the g r a n o d i o r i t e . A study of t h i n s e c t i o n s of these h i g h l y a l t e r e d sedimentary rocks shows widespread development of t a b u l a r to f i b r o u s , green, s t r o n g l y p l e o c h r o i c , pennine c h l o r i t e w i t h anomalous blue i n t e r f e r e n c e c o l o u r s , along w i t h t y p i c a l greenish to c o l o u r l e s s cleno- c h l o r e ( ? ) o h l o r i t e . I r o n - s t a i n e d carbonate predominates i n the groundmass, Replacement of the c h l o r i t e and carbonate by subhedral gr a i n s of magnetite i s common. Rosettes and s t r i n g - ers of hematite are sc a t t e r e d throughout much of the rock.. In one s e c t i o n the groundmass i s composed almost e n t i r e l y of. minute shreds of s e r i o i t e (or t a l o ? K l e s s than 0.1 mm. i n le n g t h , S e r i o i t e i s a l s o present to a minor extent i n other t h i n s e c t i o n s of the a l t e r e d rock. The s e r i o i t e has the f o l l o w i n g o p t i c a l p r o p e r t i e s : elongated shreds, p a r a l l e l ex- t i n c t i o n , t h i r d - o r d e r i n t e r f e r e n c e colours and a mean index of r e f r a c t i o n , determined by means of immersion o i l s , of 1.57E. G r a n o d i o r i t e : * The g r a n o d i o r i t e i s a border phase of a comparatively l a r g e i n t r u s i v e that extends about f i v e m i l e s east. A s p e c i - men of t h i s rock, taken c l o s e to the sedimentary contact, was mode.ra.tely a 1 pli a l t e r e d , dark, f i n e - g r a i n e d rook* w e l l mottled w i t h pink f e l d s p a r grains and weathered to a pinkis h - w h i t e c o l o u r f o r a depth of approximately l/32 i n c h on the exposed surf a c e . £ 'The term g r a n o d i o r i t e , as used i n t h i s paper, r e f e r s to ^ a ~ c ^ ' lu,t L ILL 1 u u .ranging i n composition from g r a n o d i o r i t e to d i o r i t e . -10- A t h i n s e o t i o n of the same specimen i n d i c a t e d i t was a gammas* ai u i r l Lu w i t h the f o l l o w i n g mineral composition: M i n e r a l O p t i c a l p r o p e r t i e s by whioh the e s s e n t i a l min- e r a l s were i d e n t i f i e d . Ortho- c l a s e 26 index l e s s than balsam; untwinned; b i a x i a l w i t h l a r g e 2V Horn- blende 15 green - pleoohroio; b i r e f r i n g e n c e * 0.20; maximum e x t i n c t i o n angle i n l o n g i t u d i n a l sec- t i o n s = 13°; b i a x i a l negative w i t h 2V about 70°. B i o t i t e 15 pleochroism; strongest a b s o r p t i o n when the cleavage traces were p a r a l l e l to the v i b r a t i o n plane of the lower n i c o l . P l a g i o - olase (Ande- sine) 15 maximum e x t i n c t i o n angle of a l b i t e twins i n s e c t i o n s normal to 0/0 " 16°; index greater than balsam; b i a x i a l negative w i t h 2V about 80°„ Quartz 10 u n i a x i a l p o s i t i v e C h l o r i t e Magnetite A p a t i t e S e r i c i t e Some c l a y m i n e r a l 10 10 -11- In thin section the rock i s moderately altered. About 80 per- cent of the feldspar, particularly orthoclase, has altered to sericite and a cloudy alteration product, probably one of the clay minerals. Small, fresh remnants of plagioclase, about 0.1 mm. in diameter, are f a i r l y abundant. A l l stages of alteration of hornblende to biotite are evident. Some of the biotite has altered to chlorite. Much of the magnetite i s along cleavage planes of the biotite thus suggesting i t is secondary after the alteration of hornblende. Very l i t t l e magnetite of possible primary origin was seen. Both hornblende and biotite are con- spicuously embayed along their margins by grains of quartz and 'v feldspar, and in addition, commonly contain isolated inclusions of these minerals. The writer has interpreted the above rela- tionships as indicating replacement of the hornblende and bio- t i t e by quartz and feldspar. A few seams of chlorite traverse the rock. The contact between the granodiorite and the sedimentary rocks is very irregular and the intrusive contact appears to dip;.south at a low angle under the sedimentary rocks; this may account for the extensive alteration of the intruded rock. Specimens of the Dowdell Point granite were not available and thus no study could be made of the granite and i t s altera- tion effects. Mineralization: Dr. G. Siley has supplied the following.description of the ore zone: "Bonanza Number 5 showing oocurs in a brecciated and -12- she<x.red z o n e SOme 20 feet wide, striking N. 20° W. and.dipping v e r t i G r a l l y . The zone, lies.within banded sedimentary rocks just at their contact with a granodiorite to the north. The sediment- ary rocks strike N. 20° to 25° W. and vary in dip from 65° S.W. to vertical. They consist of banded oherts, quartzites and a r g i l l i t e s a l l highly metamorphosed and fractured." "The granodiorite i s altered in the same manner." "The chief mineral in the vein is native silver in quartz and calcite lenses and stringers. Locally, the silver forms 25 percent of the lenses and' stringers. Magnetite and hema- tite are abundant gangue minerals and appear to be earlier than the si l v e r . " "Some distance to the south of the zone, coarse-grained, pink granite i s intrusive into the sedimentary rocks along a clearly defined contact." MINERAL DEPOSITS General. The mineralization* of economic interest in the Echo Bay IS d i s t r i c t ss*e silver and pitchblende. The principal deposits are shown in Figure I. Pitchblende occurs in the area at Labine Point and Contact Lake and also at Hottah Lake, 110 miles to the,„south. It is frequently in botryoidal, oolloform or dendritic forms. Mag- netite, cobalt and nickel minerals, native bismuth, argentite, ohalcopyrite, tetrahedrite, galena, and native silver are com- -13- monly associated with the pitchblende. Silver occurs at most of the pitchblende deposits and at Camsell River* 35 miles to the south. It is usually in the native state in the form of dendrites or fine wires. Tetra- hedrite, cobalt and nickel minerals, and argentite are closely associated with the native s i l v e r . Copper is widespread in the area but of no economic im- portance. It is found in a l l deposits, usually as chalcopy- r i t e , bornite, chalcocite, tetrahedrite or covellite. Like copper, the cobalt and nickel minerals are of wide- spread occurrence but of l i t t l e direct economic importance. They are found as smaltite-chloanthite, skutterudite, cobalt- it e , niccolite, and gersdorffite. The more important deposits occur in breociated and shear-, ed zones. The gangue minerals comprise quartz, barite, and calcite. Much of the carbonate is manganiferous. Mineralogy of the Bonanza Group General Statement: Microscopic study was made of 20 polished sections and 16 thin sections from the Bonanza group. Physical and optical characteristics coupled with etch reactions were used for min- eral determinations. A l l the metallic minerals, except for native silver, hematite and magnetite, were considered too small to be isolated into pure samples large enough for spec- troscopic or micro-chemical analysis. Two spectroscopic • - 1 4 - analyses were made of native silver. Magnetite (ffe^O^: • The magnetite is abundant in 1}he altered wall rock and the banded sedimentary rocks but i s conspicuously lacking in the veins or stringers of quartz or calcite. It occurs as euhed- ral to subhedral grains and as irregular stringers or massive patches replacing the country rock. Remnants of carbonate, quartz, ohlorite, and sericite are found in the massive magne- t i t e . In some cases the massive magnetite has irregular dis- seminations of tabular pennine chlorite grains in i t , giving a texture similar in appearance to "diabasic". The magnetite - i s commonly rimmed and cut by stringers of hematite. In thin sections of the. fine-grained, banded sedimentary rocks, magne- tite is found as scattered, small, euhedral grains cutting into the recrystallized siliceous groundmass. In a l l cases examined the magnetite is later than the country rock alteration but i s the earliest metallic mineral present. It i s generally accepted that the majority of the magnetite i s related to the granodiorite intrusion. Dr. C. (1) Riley states that in the Echo Bay d i s t r i c t , and also immediate- ly to the south, i t is more the rule than the exception, to find magnetite as disseminations, patches or veins adjacent to the granodiorite contact but in the intruded rock. (1) Biley, 0.: Oral Communication, 1948. -15^ Hematite (ffegQ<*): Hematite i s widespread i n the d e p o s i t hut not as abun- dant as some of the hand specimens might i n d i c a t e . These specimens have a r e d d i s h tone s u g g e s t i n g a l a r g e p r o p o r t i o n of h e m a t i t e , but when they are examined c a r e f u l l y the r e d d i s h tone i s found to be due to the dominat ing red c o l o u r of a few narrow hemati te s t r i n g e r s whosej.pqwder has covered the sur face o f the specimen. > Hematite commonly occurs as t i n y i r r e g u l a r seams, up to l / l 6 i n c h i n w i d t h , c u t t i n g or rimming the magnet i t e . Where the rook i s b r e c c i a t e d i t i s common f o r the angular p i e c e s to be cemented by carbonate c o n t a i n i n g much f i n e l y - d i v i d e d hema- t i t e . The a l t e r e d r o c k u s u a l l y c o n t a i n s f i n e l y d isseminated h e m a t i t e , or r o s e t t e s of the same m i n e r a l , a l o n g f r a c t u r e s or i n t e r s t i t i a l to the m i n e r a l g r a i n s . Ohert and carbonate v e i n - l e t s commonly c o n t a i n enough disseminated hematite "dust'* to g i v e them a r e d d i s h c o l o u r . The same a p p l i e s f o r some of the s i l i c e o u s bands i n the f i n e - g r a i n e d , banded sedimentary r o c k s . Hematite was not found to be a s s o c i a t e d w i t h any o f the m e t a l l i c m i n e r a l s other than magnet i te ; any cher t or carbonate that c o n t a i n e d hematite was u s u a l l y v o i d of m e t a l l i c s . This f a o t , however, may not have much s i g n i f i c a n c e as K i d d and Hay- cock (1935, p . 905) , and F u r n i v a l (1939, p.751) found that hemati te was q u i t e commonly a s s o c i a t e d a t Labine P o i n t and at Contact Lake w i t h v a r i o u s oopper and s i l v e r m i n e r a l s . The hematite i s d e f i n i t e l y l a t e r than the magneti te and 16- appears to be earlier than the native silver since the veins containing silver cut hematitio country rock. It is possible that the hematite may be of two generations! the f i r s t one f i l l i n g fractures in the magnetite and altered rock and acting as a cement along with carbonate for the brecciated material, and the second generationuappearing with the chert and carbon- ate stringers. However, since no direct evidence was found substantiating the theory of two generations of hematite, i t i s probably best to assume one generation later than the magnetite and earlier than the silver. The best evidence for hydrothermal origin of the hema- tite in the area is presented by Murphy (1946, p.436) in the following passage: "The alteration of the rocks and veins in the v i c i n i t y of the Eldorado mine 'is prominent, and the 'baked1 appearance of a l l formations, except the later diabase, has been frequently remarked. In both rocks and veins, the alteration has given rise to widespread discolouration by hematite and the obliteration of original textures. This so- called red alteration, undoubtedly related to the quartz- hematite period of mineralization, affects the quartzose rocks most severely, but, where alteration is intense, there i s l i t t l e selectivity. The rocks are then reduced to a dense, reddish 'jasperoid'. The exact nature of the alteration has not been determined, but quartz, hematite, magnetite, s e r i - cite, chlorite, and carbonate are obvious constituents. -17- "Going away from the mine, the degree of alteration f a l l s off, hut the mineralization is so pervasive that in no oase oan i t he said that examination has been carried beyond the zone of alteration. The distribution of the alteration points to the mine as being a centre of mineralization in the d i s t r i c t , and indicates that the veins and alteration have a common hydrothermal source. In a geophysical examination of the area, Brant has noted an unusually high magnetite content of the rocks on Labine Point. The percentage of magnetite de- creases away from the mine, thus giving further evidence as to the locus of mineralization." The above description is comparable to the situation at Bonanza mine and thus i t i s logical to assume that the hema- tite at Bonanza is also hydrothermal. Tetrahedrite (50ugS.3(Ouffe)S.SSbgSgi}: Tennantite (SGugS.SfGuFejS.gAsp.S^): The physical and chemical properties of tetrahedrite and tennantite are so similar that chemical or spec&rographic analysis must be resorted to in order to distinguish between them. Although tetrahedrite (or tennantite) i s widespread in the polished sections, i t was never found in sufficient quantity to enable a spectrographic sample to be isolated. In the remaining part of this paper this mineral shall be called tetrahedrite, with the understanding that i t may be tennantite. Tetrahedrite appears in more than 50 percent of the polish-. -18* ed sections, but only as sparsely disseminated small grains. Usually the mineral could barely be distinguished under low power and was revealed only -by high power or o i l of immersion lenses. The largest single piece of tetrahedrite i s shown in Plate I, where the width of the mineral is about 0.5 mm. Tetrahedrite i s closely associated primarily with native silver and argentite, and to a minor extent with chaloopyrite and an unknown pinkish-cream mineral. With the exception of chalco^ pyrite, i t is older than a l l the metallio minerals with which i t is associated. Argentite consistently replaces tetrahedrite andj invariably, irregular islands of the mineral are found within argentite, as illustrated in Plates II and III. The argentite replacement most commonly starts on the borders of the: tetrahedrite, and advances very rapidly along easiest chan- nels, such as fractures. - • The silver replacement of tetrahedrite generally begins at the core of the mineral and advances towards the rim. In many cases remnant rims of tetrahedrite, partially rimmed by argentite, can be seen bordering silver (Plates I and IV). Edwards (1947, p. 97) cites core replacement by silver as oc- curring at Oobalt, Ontario, KQnigsberg, Norway and Contact Lake in the Eoho Bay d i s t r i c t . Silver i,Ljilm.I.LLII.IIL ol1 Liim Lria-b-jfa- l H H H " l l , n i n n i i i i n l . n I I U J J I M u u l m r n u n r u ) m 1 M n »• n , P r n . n l . u r n M . ^ " " " ' " = 1*1 I.Twi l,Ml.''nlnii1r'1 1,11, i i i i u b i i i i l 1110 Mini I ) nmj> M M ! 1 PJ IIIII will i i l l t l i n ^ ^ ^ t e i ^ ^ M a ^ ^ Occasionally under high power.silver can be seen replacing tetrahedrite from the x 204 Plate I Core replacement of tetrahedrite by silver in a s e r i c i t i c gangue. Plate II Islands of tetrahedrite and silver in argentite. The gangue i s mostly sericite, x 1250 Plate I I I Argentite replacing tetrahedrite. Note t l l e s i l v e r beginning to replace the argentite. x 204 Plate IV S i l v e r rimmed by tetrahedrite (dark gray) and argentite ( l i g h t gray). This i s thought to be an advanced stage o f core replacement. outer borders in much ^he same manner as the-argentite does. A few small remnants of tetrahedrite were f ound in silver in several of the polished sections. Tiny 'snake r-l ike 1 ve inlets of tetrahedrite were found-,! in one polished section, with their borders being replaced by a pinkish-cream, unknown mineral and, occasionally, by argen- tit e (Plate V). Intimate association of chalcopyrite and tetrahedrite was seen in only one section. The chalcopyrite has been definite- ly replaced by tetrahedrite.. The grey mineral rim3 the chalco- pyrite with the replacing front advancing into the chalcopyrite along the easiest channels, very similar t o the replacement of tetrahedrite by argentite. A poor example of this is shown in the upper part of Plate VI. Tetrahedrite occurs in the same gangues as silver, namely* carbonate, quartz and sericite. The following is a summary of the tests used to identify tetrahedrite: Colour (in polished section): Steel grey. Hardness: C *. Crossed Nicols: Isotropic. Etch Tests: HNO g , HCl., FeClg, KOH, K O I , and HgClg a l l negative. Miscellaneous: Triangular pits in some of the mineral. Argentite (AggS): Argentite is intimately associated with, but not quite so • • •  x 1250. Plate V * "Snake-like" stringer of tetrahedrite rimmed by the unknown mineral in a s e r i c i t i c gangue. x 204 Plate VT partly Chalcopyrite be 1 life replaced by silver and tetrahedrite (dark gray). The dark back- ground i s the quartz gangue. - 2 0 - abundant as tetrahedrite. The two minerals have practically the same occurrence^and their relationships have already been described. Silver and argentite have much the same relationships as silver and tetrahedrite except that the silver is more com- monly associated with the latter; however, this may be because of the relative abundance of the two minerals. Plate II shows silver replacing argentite from the core outwards,and Plate III shows i t replacing argentite from the border inwards. Argen- t i t e , with imbedded islands of tetrahedrite, occasionally is completely rimmed by silver. In some instances argentite is found with chalcopyrite. but in these cases tetrahedrite is usually present also. Some replacement of the chalcopyrite by argentite may have taken place but the invariable presence of tetrahedrite as well suggests that a good deal of the argentite replaced tetrahed- rit e and not chalcopyrite. The unknown pinkish-cream mineral was not found associated with argentite. It i s possible that some of the argentite i s supergene though positive evidence on this point was not found. The tests by which argentite was identified are as- f o l - lows: Colour (in polished section): Light grey. Hardness: B - • Crossed Nicols: Isotropic. - 2 1 - Btoh Tests: HNOg - definite bluish-green tarnish HCl - commonly negative; oooasional tarnish KCN - stains brown FeClg- dark stain HgOlg- definite irridesoent stain KOH - negative Ohaloopyrite (OuFeSpj.: Chalcopyrite is rather scarce in the polished sections. It appears in only three out of the 20 sections and in only p o s s / b/<e. one of these is i t i n n n r r i H i i m t . .vim m l i . n n n to determine i t s associations and relative age. Generally the chalcopyrite oocurs as isolated small grains, about 0 . 1 mm. in diameter, scattered throughout the gangue and not particularly associated with any of the other metallic minerals. Whenever chalcopyrite does occur with the other metallics i t is decidedly, and in some instances exten- sively, replaced by them, especially by tetrahedrite. Silver occasionally replaces the chalcopyrite as core replacement (see Plate VI). Three irregular grains of chalcopyrite are shown in Plate VII with three patches of unknown pinkish- cream mineral at the bottom of the photomicrograph. Chalcopyrite was identified by the following means: Colour^in polished section): Brass yellow. Hardness: C • x 204 J P l a t e V I I C h a l c o p y r i t e and the unknown m i n e r a l i n a s e r i c i t i c gangue. P l a t e V I I I A t y p i c a l s i l v e r d e n d r i t e i n a quartz gangue. - 2 2 - Grossed Nicols: Slightly anisotropic. Etch Tests: - M O „ - irridesoent tarnish o KCN - very light tarnish HOI, FeClg, KOH and HgClg ~ negative. Unknown Pinkish-Oream Mineral: , An unknown pinkish-oream mineral oocurs sparsely in two of the polished sections. As already mentioned its relation-? ships are: (1) as replacement rims on tetrahedrite (see Plate V). (2) as isolated blebs scattered throughout the section (see Plate VII). A l l that can be said of the unknown's relative age is that i t is younger than tetrahedrite. Properties of the unknown mineral are: Colour (in polished section): Pinkishr-cream. Hardness: C . ;• • . Crossed l i c o l s : Isotropic. Etch Tests: HHOg, FeClg, HgClg - very light tarnish. HCl, KCfi, KOH - a l l negative. lative Silver (Ag.): Native silver is the most abundant and widespread metallic mineral in the polished sections and hand specimens. It i s found in a l l sections of the vein material. With the exception of hematite and magnetite, i t i s the only metallic mineral that can be seen in megascopic examination. Ninety-five per- cent of the silver'occurs either as individual, dendrites or in a dendritic pattern. The other five percent appears as isolated blebs and seams, and as replacements of tetrahedrite, argentite or chalcopyrite. The gangue minerals, in order of abundance, comprise quartz, sericite and carbonate. The dendritic pattern i s most widely developed in a quartz gangue. The individual dendrites*, one of which is i l l u s - trated in Plate VIII, are up to l/4 inch in length with a decided branching tendency. Looally, the dendrites appear to be oriented in one of three directions, intimating rhombic control; however when the specimen is viewed generally the dendrites have a random orientation and no control is suggest- ed. A poor dendritic pattern is ahown in Plate IX. The o r i - gin of the dendritic pattern w i l l be discussed f u l l y with the gangue minerals. Silver i s invariably associated with tetra- hedrite, argentite and to a minor extent with chalcopyrite, whenever these minerals are present. These associations have already been described (see Plates I, II, III, IV and V). Two spectrographic analyses of what was believed to be pure silver gave the following results: Silver : strong Copper : strong Mercury : weak to moderate Bismuth : weak to moderate -24- Cobalt : n i l Nickel : i i i l Arsenic : n i l Antimony: n i l The oopper oould he due to admixed tetrahedrite or chal- copyrite but since no mercury or bismuth minerals were found in any of the polished sections i t is only logical to assume that these elements are alloyed with the silver. It is i n - teresting to note that Furnival (1939, p.763) found mercury alloyed with silver at Contact lake and that Knight (1924» p.35) reported the same at Cobalt. Native silver is the youngest metallic mineral in the deposit and with the possible exception of some of the late barren carbonate veins, is the last mineral to be deposited. Kidd and Haycock (1935, p. 925) state that the silver i s the last hypogene mineral to form at Labine Point (Eldorado). From the small scope that the specimens offer i t i s ra- ther d i f f i o u l t to determine positively whether the silver is supergene or hypogene; however, by comparison with other de- posits in the area and considering the negative evidence i t i s concluded that the greater part of the silver i s hypogene. A summary of the points in favour of hypogene silver follows: (1) The notable absence of supergene mi nw n l a wnnh =ssg- covellite and chalcocite. (2) The presence of mercury and bismuth in the silver. x 65 P l a t e I X S i l v e r r e p l a c i n g d e n d r i t e s o f carbonate i n a q u a r t z gangue. Note t h e specks o f s i l v e r i n t h e c s r b o n s t e . x bo P l a t e X po.\rtly Q u a r t z , I n a rude d e n d r i t i c p a t t e r n , 1»» Inj^ replaced by s i l v e r said carbonate. The ==== i s mostly s e r l c i t e . It is very unlikely that these elements would appear . in supergene silver, especially since no mercury min- eralization has been noted in the area. (3) Furnival (1939, p 763) found a homogeneous* coarse, recrystallized structure in silver at Contact Laket which, according to Edwards (1947, p.6) is indicative of hypogene silver. . "~ . (4) Kidd and Haycock (1935, p.926) state that the silver at Labine Point is older than ^supergene manganese oxides and after careful study concluded that the greater percentage of the silver was hypogene. iurOne point which makes S.jfpergene origin for the silver feasible is the fact that the specimens were taken from within a few feet of the surface. Gangue Minerals: ^ Quartx- Quartz is the most abundant gangue mineral. It constitutes 90 percent of the gangue in 60 percent of the specimens and in the remainder i t is conspicuous as dendritic inclusions. The quartz is a fine to medium grained, white, crystalline variety with interlocking, more or less equidimen- sional, anhedral grains and displays a vuggy character in some of the specimens. The vuggy quartz oontains no silver miner- alization though i t is immediately adjacent to crystalline quartz carrying up to 25 percent silver. This implies that the vuggy quartz i s later than the silver mineralization and could be a result of secondary deposition by circulating ground -26- waters. Bands of "jasper-like" chert, up to l/4 inch wide,-contain- ing cryptocrystalline quartz with finely disseminated, blotohy hematite, frequently occur at the contact of the quartz veins and the country rock. They carry l i t t l e or no silver mineral- ization and are considered to be later than the silver. It is thought that the chert bands, because of their very fine grain- ed .character, were deposited from colloidal suspension near the close of the silver mineralization. On the basis that the hematite in the deposit is of hypogene origin and that i t is not uncommon to get' colloidal s i l i c a deposited in the late stages of mineralization, i t is assumed that the chert is of hypogene origin. The quartz bodies have sharp, frozen contacts. ¥o one specimen showed both walls but from descriptions by Kidd (1932P, (1) p.27c),. Lord (1941, p.49) and Dr. 0. Riley the quartz appears as lenses up to eight inches by 30 inches in brecciated and sheared zones. In thin section the quartz frequently contains i n t e r s t i t i a l fine grained serioite indicating that at least some of i t is replacement of the s e r i c i t i c rock in or near the shear zone. Two &ges of quartz are evident in the s e r i c i t i c gangue. One appears as altered irregular remnants, that have been ex- tensively replaced by serioite; the other is a fresh euhedral (l) Riley, 0.: Oral Communication, 1948. - 2 7 - quartz displaying a poor dendritic pattern, with minor replace- ment by carbonate. The dendritic pattern i s largely inherited by the si l v e r . (See Plate X). In a quartz gangue the silver has a strong tendency to be i n t e r s t i t i a l to the quartz grains and to replace carbonate (Plates IX and XI). ICven in eases where no carbonate appears to be present, slight effervescence can frequently be observed along the margins of silver grains when a drop of dilute hyd- rochloric acid i s applied to the area, suggesting that the silver may have replaced carbonate, In thin section silver dendrites are often surrounded by a narrow rim, less than 1 mm. wide, of cryptocrystalline quartz that is much finer than the fine to medium-grained groundmass. This could be due to, either excessive quartz entering solution at the time of silver replacement and being redeposited in situ as a dryptocrystal-\ line rim around the silver, or to an introduction of cherty quartz with the silver. Of the two proposals the f i r s t one seems to be more plausible as i t does not involve introduction of siliceous material and as the cryptocrystalline quartz has a tendency to grade into the quartz gangue. The fact that some of the rims have a slightly cloudy appearance somewhat resembling some of the chert bands may support the latter proposal. A l l the quartz is definitely older than the silver miner- alization, excepting possibly of course, the chert bandstand the rims around the silver dendrites. -28- Ser i c i t i c Gangue - S e r i e i t e i s next in abundance to quartz as a gangue mineral in the specimens studied. It i s composed of a mass of small fibres or flakes of serieite less than 0.3 mm. in length, with minor amounts of carbonates, and a few remnant grains of quartz that have been largely replaced^ by the serieite. Euhedral grains of quartz in a poor dendrit- ic pattern, some carbonate and the metallic ore minerals are a l l younger than the gangue. The serieite, as determined by means of immersion oils, has a mean index of about 1,572, thus indicating that i t is close to pure muscovite in composition (Wlnchell, 1946, p.268). Carbonate Gangue - Most of the carbonate in the specimens is present in barren caleite stringers up to l/4 inch wide, that cut indiscriminately through the altered rock. These stringers commonly have narrow seams of chert along their con- tacts^and are usually devoid of any mineralization. The only carbonate which i s consistently related to the silver is that replaced by the silver and associated with quartz (Plates X ana XI). Kiaa (1932P, p.27c) ana Lora (1941, p.49) give the impres- sion that most of the silver is in carbonate veins or lenses rather than quartz; thus, i t appears that carbonate is con- siderably more important as a gangue in the aeposit than the available specimens inaicate. Kiaa (1936, p.40) reportea the following analysis of white carbonate with silver from the Bonanza aeposit: Percent Insoluble - 10.96 Metals - 1.14 F e2°3 " 3 , 0 6 Alg0 3 - 8.76 FeO - 0.29 MnO - 3.18 OaO - 41.50 MgO - 0.56 C0„ - 35.38 98.83 Equivalent to FeCOg - 0.61 Mn003 - 5.51 OaOO, - 74.11 o MgG03 - 1.18 A l l the carbonate effervesces readily with cold dilute hydrochloric acid indicating that i t is largely calcite. Origin of the Dendritic Form of Silver: There i s l i t t l e doubt that the dendritic form of the silver has been inherited from either the quartz or carbonate. Plate X shows partial replacement of dendritic quartz by carbonate and silver and though i t is not illustrated the dendrite of silver in Plate VIII changes abruptly to carbonate outside the fie l d of the photomicrograph. The problem then i s : "the origin of the dendrites of carbonate and quartz". -30- In the ease of the carbonate dendrites in the quartz gangue (Plates IX and XI) the carbonate i s mainly i n t e r s t i t i a l to the grains of quartz with the silver following the pattern of the carbonate when i t i s present. Thin section study of this material was rather disheartening in that very l i t t l e carbonate was seen in the sections; nevertheless, i t is ap- parent that the silver dendrites have replaced a dendritic structure in the quartz gangue caused by a special arrangement of certain quartz grains. For example, in one thin section" several doubly terminated quartz prisms l i e side by side in a row with their ends terminating in rude pyramids. Silver has replaced these prism? resulting in the dendritic pattern. In Plate VIII the arms of the silver dendrite could easily represent replacement of rude prisms of quartz lying side by side; as a matter of fact, under the existing circumstances i t i s d i f f i c u l t to explain them by any other means. In pol- ished section, silver i s seen to have frequently replaced dendrites of carbonate in a quartz matrix. In this case the carbonate evidently has replaced the dendritically arranged quartz and Jin turn >is replaced by silver. The doubly terminated quartz prisms must have formed in a medium that did not seriously obstruct their growth and, in addition, permitted them to grow in two directions. This type of medium might be provided by: (1) replacement of a gangue, such as a carbonate, which offers l i t t l e resistance to the replacing solutions and the -31- growth of crystals. The growth of the doubly terminated quartz prisms could have started along minute fractures or joints in the carbonate. This would explain the random orientation of the dendrites and in addition would help to explain the rhombic control which is suggested by some of the dendritic patterns. The objection to this theory i s that there is no evidence that the quartz is a replacement of carbonate. (2) the quartz crystallizing in an open fissure. A l - though i t might be possible to have doubly terminated quartz prisms developed in open fissures, i t is d i f f i c u l t to visual- ize how the decided random orientation of the dendrites could be produced by this means. In addition, there i s no evidence in the deposit of fissure f i l l i n g . In summary, no definite conclusion has been arrived at as to the origin of the rows of doubly terminated quartz prisms, Two theories have been suggested but both of these lack sube stantiating evidence. The origin of the dendritic structure certainly warrants further study and as an approach to this the writer suggests the study of thin sections cut to give various orientations of the quartz prisms. At the same time the prob- lem of the selective replacement of the rows of quartz prisms by the silver, or carbonate,in preference to the unoriented quartz gangue might be investigated,- In the s e r i c i t i c gangue the silver inherits i t s dendritic fomm from fresh euhedral quartz arranged in a dendritic pattern (Plate ,'Xi). The explanation here i s not so d i f f i c u l t . -32- The quartz in an individual dendrite a l l has the same orienta- tion and from i t s fresh appearance, as compared to the corrod- ed remnants of older quartz in the same gangue, and i t s strong- ly euhedral shape i t is believed that i t has replaced the s e r i c i t i c gangue. Here again there is discordance between the polished and thin sections. In the polished sections one gets the; impression that silver invariably replaced carbonate after quartz, while in thin section carbonate is less abundant and the majority of the silver replaced quartz directly, in the absence of carbonate. In view of the fact that approximately twice as many polished sections were studied containing dendrit- ic s i l v e r as thin sections, the writer concludes that the majority of the silver is a replacement' of carbonate after quartz,but that direct replacement of quartz by silver is more prevalent than indicated by polished section. There is l i t t l e doubt that the silver prefers carbonate to quartz as indicated by the conspicuous specks of silver in the carbonate and the equally noticeable absence of silver in the quartz in Plates I X , £ndanX /XII. Ho detailed study was made of dendritic silver in a car- bonate gangue but one instance was noted of a^dendritic struc- ture in carbonate due to differential polishing (Plate ill)'.' similar to that reported by Kidd and Haycock (1935, p.956). Kldd and Haycock interpreted this as being two carbonates of different hardness with the silver preferring the softer. x 204 ' P l a t e XI Carbonate being r e p l a c e d by s i l v e r i n a quartz gangue. x 204v P l a t e X I I D i f f e r e n t i a l p o l i s h i n g i n a carbonate gangue. -33- Summary of Mineralogy: The mineralogy of Bonanza i s very similar to that des- cribed by ICidd and Haycock (1935) and Furnival (1939) at Labine Point and Contact Lake, except that far fewer minerals were identified at Bonanza; however, i t must be kept in mind that no more than 15 specimens were examined by the writer compared to the hundreds examined by Kidd, Haycock, and Furnival.. It is interesting to note that Kidd and Haycock (1935, p.888) identified chalcopyrite, bornite, sphalerite, tetrahedrite, covellite, and erythrite on the Bonanza claims, (1) and R.B. Scott found several radioactive specimens in the Bonanza dump in 1945* (1) Scott, R.B.: Oral Communication. -34- PABA GENESIS Kidd and Haycock (1935, p. 931) have divided the mineral- ization of the Echo Bay di s t r i c t into three main stages: (1) pyrometasomatic stage (2) hydrothermal stage (3) supergene stage. This same classification is well suited for the Bonanza depos- i t . The pyrometasomatic stage, at Dowdell Point, is connected with the granodiorite intrusion, and is concerned primarily' with the introduction of magnetite and the alteration of the handed sedimentary rocks, characterized by widespread develop- ment of chlorite, serieite and carbonate. It is the earliest of the three stages. The hydrothermal stage is the most important as far as the economic aspects of the deposit are concerned. The quartz and carbonate gangues and a l l the metallic minerals, excepting magnetite, are confined to the hydrothermal period. l i t t l e information was obtained concerning the relative age of the hematite; i t is undoubtedly early, as i t cements part of the brecciated zone. On the other hand, i t occurs in minor amounts in the chert bands which are regarded as being late. Murphy '(1946, p.432) ties the major part of the hematite in with the quartz and i t i s reasonable to assume that the same relationship holds at Dowdell Point, with minor surges -35 of hematite following later. Where carbonate i s present in direct association with silver i t is unquestionably the older of the two, but in the case of the barren carbonate stringers some uncertainty arises. These stringers contain no mineralization and are often assoc- iated with chert bands. The fact that the stringers are in a mineralized zone and are barren and, in addition, are assoc- iated with late chert bands suggests that they are late. Purnival (1939, p.768) recognized much the same phenomena at Contact Lake but in his case the carbonate was mineralized. Though the-sulphide mineralization does follow a definite sequence i t is very sparse. In no instance was i t abundant enough to be seen with the nakea eye. These sulphides appear to be the forerunners of the silver mineralization with the early tricklings being copper-iron sulphides, followed by copper, and antimony or arsenic sulphides, then by the silver sulphide and f i n a l l y by the great surge of silver mineraliza- tion. At the Bonanza property the supergene stage i s negligible in that no supergene minerals were identified. Table I gives the suggested paragenesis for the mineral- ization at Bonanza. -36- TABLE I . Magnetite • Hematite Quartz Carbonate - ^ ^ m ^ P ^ - <t^l- > Chalcopyrite Tetrahedrite Argentite — Silver <^kfm\\\+ 7 Unknown < 0 TEMPERATURE OF FORMATION No definite criterion could be found in the suite of speci- mens studied that might be used as a geological thermometer. The occurrence of chert bands might indicate' epithermal tempera- tures, but is not conclusive. The vuggy quartz suggests low pressure but, as was mentioned, i t could be supergene. None of the other minerals, textures or structures can be considered diagnostic of any particular temperature. At other deposits in the Eoho Bay di s t r i c t however, more definite evidence of the temperature of deposition of the s i l - ver has been found. Furnival (1939, p.770) found a coarse, homogeneous, recrystallized texture in the silver indicating i t had been deposited above i t s recrystallization temperature of 200°C; Furnival (1939) and Zidd and Haycock (1935) identi- fied native bismuth, which is earlier than the silver, suggest- -37- ing a temperature of silver deposition of less than 271°C, Kidd and Haycock (1935, p.932) remark that f i e l d evidence indicates a low pressure "but give no further explanation. Although direct supporting evidence i s lacking, the Bonanza deposit is provisionally placed in the epithermal to mesothermal group in view of the following points: (1) The investigations of Furnival, Kidd and Haycock at Contact Lake and Labine Point (Eldorado) suggest a silver deposition temperature of between 200 and 271°C possibly accompanied by a low pressure. (2) Silver is a mineral typically deposited under epi- thermal to mesothermal conditions. OBI SIN OF THE SOLUTIONS Much has been written on the origin of the si l v e r bearing solutions in the Echo Bay d i s t r i c t , with the result that two different hypotheses have developed regarding their origin. Kidd and Hayoook (1935), after their detailed work on the E l - dorado Ores, prefer the granite as the origin for the silver, while Furnival (1939) and Murphy (1946) show preference for the diabase s i l l . A brief summary of the evidence supporting both hypotheses is made below. -38 I. Evidence favouring the granite as the origin of the s i l - ver mineralization. (1) The areal distribution of the known silver deposits is adjacent to the Dowdell Point granite or i t s equivalent. (2) The close association of the pitchblende and silver mineralization, with the pitchblende believed to be genetically related to the granite. (3) The presence of a l i t t l e fluorite in both the Bonanza deposit and the Dowdell Point granite. None of the other intrusives in the area have been found to con- tain fluorite. I I . Evidence supporting the diabase s i l l as the origin of the silve r mineralization. (1) Silver has been found in the s i l l . (2) The silver is later than the youngest acid intrusive. (3) The close areal association of diabase s i l l s or dykes and the silver deposits. , (4) The silver veins at Oobalt, Ontario, both above and below the Mpissing diabase s i l l , are generally re- garded as being genetically related to the parent magma of the s i l l . As can be seen, the evidence on both sides is rather flimsy and certainly far from conclusive. Associations are strongly emphasized, but in no single case i s anything approach- ing positive evidence cited. The writer feels he is in no -39- ppsition to choose either hypothesis and is of the opinion that the silver may he genetically related to either the dia- base s i l l or the granite. COMPARISON OF THE BONANZA DEPOSIT WITH OTHER DEPOSITS GENERAL In the comparison of the Bonanza deposit with other de- posits, i t should be kept in mind that only a limited number of specimens were examined, and thus i t is possible that the complete picture of the mineralization of the Bonanza property has not been determined. In order to make the comparison more complete the writer has f i r s t compared the Bonanza- deposit with the other deposits in the Echo Bay di s t r i c t and has then com- pared these as a unit with similar deposits throughout the world. The major discrepancies between the Bonanza mineralization and the Echo Bay di s t r i c t as a unit are: (1) The pitchblende and cobalt-nickel minerals are absent at Bonanza. In addition, other minerals such as native bismuth, bornite, molybdenite, stromeyerite and jalpaite, which are common In the d i s t r i c t , have not been identified in the specimens examined. (2) Dendritic silver i s more abundant at Bonanza and the dendritic structure is inherited from the quartz, whereas at the other properties the dendritic structure -40 of the silver is .commonly inherited from dendritic pitchblende or dendritic cobalt-nickel minerals. In summary, the Bonanza deposit is similar to the other deposits of the Echo Bay di s t r i c t only in the silver mineral- ization, wall rock alteration, and relationship to intrusive ' bodies. The absence of pitchblende and cobalt-nickel miner- als make the deposit unique in the area. A summarization of the main characteristics of the Echo Bay d i s t r i c t is as follows: The mineralization consists primarily of early pitchblende, followed by cobalt-nickel minerals, some sulphides, and f i n - a l l y native silver. Oopper minerals are widespread and abun- dant. Characteristic minerals found are magnetite, hematite, pitchblende, cobalt-nickel arsenides and sulpharsenides, native bismuth, chalcopyrite, galena, sphalerite, argentite, native silver, and mercury alloyed with the silver. Gangue minerals comprise quartz, calcite, manganiferous carbonate, and barite. The pitchblende is thought to be genetically related to a granite intrusive while the silver i s believed related either to the granite or to a diabase s i l l . The deposits are classified as lower epithermal to upper mesothermal and are believed to be of Pre-Cambrian age. COMPARISON WITH THE SILVER DEPOSITS AT COBALT, ONTARIO "is The assemblage of minerals at Cobalt, Ontario sass very -41- similar to those of the Eoho Bay d i s t r i c t . The cobalt-nickel arsenides and sulpharsenides, the character and manner of re- placement of the silver, the gangues, and the presence of mer- cury in the silver are practically identical with the Great Bear Lake deposits. On the other hand, no pitchblende, hema- t i t e , or magnetite has been reported at Gobalt and in addition, chalcopyrite is not so widespread nor so abundant at the Ont- ario locality. At Cobalt the deposits are generally accepted as being genetically related to a diabase dyke whereas in the Echo Bay di s t r i c t there is some doubt whether the silver min- eralization is related to a diabase. Both deposits are thought to be late Pre-Cambrian age. COMPARISON WITH THE ERZGEBIRGE DEPOSITS OP SAXONY AND CZECHOSLOVAKIA IN CENTRAL EUROPE The deposits of pitchblende, cobalt-nickel/minerals, and silver in the Erzgebirge area most, closely resemble those of the Eoho Bay d i s t r i c t . Bastin (1917,pp.121-122) has summar- ized these deposits from the original works of Milller (1860); Step, Josef and F.Becke (1904), and Viebig (1905). The Erzgebirge deposits occur as veins in and near in- trusive masses of late Palaeozoic granite, with which the de- posits may be genetically connected. Milller (1860) has c l a s s i - ^ fied the deposits as follows: A. Older ore forming period. 1. Veins of t i n type 2. Veins of pyritic lead-zinc type -42- B. Younger ore forming period. 3. Veins of the Gobalt-rSilver type with pitchblende 4. Veins of the iron and manganese type. It is with the deposits of type B3 that we are particularly concerned. Notable deposits of this type occur at Jaachims- thal in Bohemia, and at Schneeberg, Annaberg, and iiohannge- orgenstadt in Saxony. Held (1932, p 65) has competently des- cribed the mineralogy in the following quotation: "Veins of type 3 vary somewhat in different l o c a l i t i e s , but in general carry the following minerals: pitchblende, silver, both native and as sulphosalts, arsenides and sulpharsenides of cobalt, nickel and iron, native bismuth, antimonial minerals, includ- ing tetrahedrite, stibnite, and berthierite, sulphides, pyrite, chalcopyrite and sphalerite. Gangue minerals comprise quartz, siderite, manganosiderite, calcite, dolomite, barite, and fluorite. Pitchblende i s later than the cobalt-nickel miner- als but earlier than the silver ores. In the Annaberg d i s t r i c t of the Saxon Erzgebirge, the cobalt-silver veins in many places cut and displace the earlier tin, copper and pyritic lead-zinc veins." In addition to Reid's summary i t is worth noting that at Schneeberg bismuth is prevalent enough to be- come the major ore mineral. From the above description, i t i s evident that the Erzge- birge deposits and those of the Echo Bay d i s t r i c t , i f consid- ered collectively, are similar both geologically and minerai- logically. The major discrepancies are: -43- (1) The association of the Erzgebirge deposits with tin minerals. (2) At Erzgebirge the cobalt-nickel minerals are consid- ered to be older than the pitchblende while at Great Bear Lake the pitchblende i s considered the older. COMPARISON WITH MISCELLANEOUS DEPOSITS There are other deposits that deserve brief mention in this comparison. These are: (1) Ten deposits of Cornwall (Pearce, 1875). (2) Silver Islet property near Port Arthur, Gnt.dngall, 1887). (3) Native Silver Ores near Wickenburg, Arizona (Bastin, 1922). (4) Silver deposits at Sabinal, Mexico (Krieger, 1935). (5) Silver deposits at KtJnigsberg, Norway (Beyschlag, Vogt and Krusch, 1915). 'The deposits of Cornwall are notably t i n and copper lodes a associated with granitic intrusives. Pitchblende has been fecund at several l o c a l i t i e s associated with copper, cobalt, nickel, bismuth, and lead ores; at only one locality was s i l - ver found. According to Pearce (1875), the pitchblende and associated minerals, commonly occur as small veins crossing the tin lodes. The deposits at Silver Islet, Ontario, Sabinal, Mexioo, and Wickenburg, Arizona are similar enough to the cobalt de- - 4 4 - posits to warrant no further comparison. The deposits at Konigsherg, Norway are mainly silver, believed to be supergene, and contain no pitchblende or cobalt- niokel minerals. They can hardly be compared with the Echo Bay deposits, CONCLUSIONS The suite of specimens studied i s rather, a limited collec- tion and may not be a genuine representation of the Bonanza deposit; therefore, in evaluating the following conclusions this point should be borne in mind. 1 . The magnetite is of pyrometasomatic origin and is related to the granodiorite intrusion, while a l l the other metallic and gangue minerals, excepting serieite, are of hydrothermal origin. S . The order of deposition of the minerals is believed to be magnetite, quartz, hematite, carbonate, chalcopyrite, tetra- hedrite, argentite, and native silver. Late hematite, and car- bonate may also be present in minor amounts. The exact rela- tive age of the unknown mineral could not be determined. 3. Evidence suggests that the native silver i s of hypogene origin. It occurs primarily as dendrites, but occasionally core replacements, by silver, of tetrahedrite, argentite, and chalcopyrite are also well developed. 4 . The dendritic structure of the silver is inherited from rows of doubly terminated quartz prisms in a quartz gangue^or -45- from a rude dendritic pattern of euhedral quarta grains in a ser i c i t i o gangue. Two hypotheses have been advanced for the origin of the doubly terminated quartz prisms but both lack sustaining evidence. The rude dendritic pattern of euhedral e quartz grains is thought to be replacment of the s e r i c i t i o gangue. 5. The Bonanza deposit is similar to other deposits in the Echo Bay d i s t r i c t only in the silver mineralization, wall rock alteration and relationship to intrusive bodies. The absence of pitchblende and cobalt-nickel minerals differentiates i t from other deposits in the area. -46" BIBLIOGRAPHY 1. Bastin,E.S.(1932): Contributions to Economic Geology. U.S. Geol. Surv. Bull. 735, pp. 131-155. 2. Bastin.E.S. and'Hill, J.M.(1917): Economic Geology of Gilpin County and Adjacent Parts of Clear Creek and Boulder Counties, Colorado. U.S. Geol. Surv., Professional Paper, 94. 3. Beyschlag, Vogt and Krusch (1916): Ore Deposits, Vol,II, translated by S.J. Truscott. Maomillan and Co. lt d . , london. 4. Edwards, A.B.(l947): Textures of Ore Minerals, Aust. I, M.M, 5. : • Furnival, G.M.(1934): Silver Mineralization at Great Bear lake, C.M.J., Vol. 55, No. 1, pp. 5-8. 6 (1939): A iilver-Pitohblende Deposit at Con- tact lake.. Econ. Geol., Vol. 34, No. 7, pp. 739-776. 7. Kidd, £.]?. (1931): Great Bear lake - Coppermine River Area, Mackenzie Distriot, N.W.T., Geol. Surv. Canada, Summ. Rept., Part C., pp. 47-69. 8. (1932 A): The Great Bear lake - Coppermine River District, C.M.J. Vol. 53, No.l, pp.5-12, 9. (1932 B): A Pitchblende-Silver Deposit, Great Bear Lake, Canada, Econ. Geol. Vol. 27, No. 2, • p p . . 145-159. 10. (1932 C): Geology and Mineral Deposits of Great Bear Lake - Coppermine District, C.I.M.M., Bull. 245, pp. 512-523. 11. (1932 D): Great Bear Lake Are'a, N.W.T., Geol. Surv. Canada, Summ. Rept., P a r f 0. pp. 1-36. 12. .......... (1936): Rae to Great Bear Lake, Mackenzie Distri c t , N.W.T., Geol. Surv. Canada, Mem. 187. 13. Kidd, D.P. and Haycock, M.H. (1935): Minerography of the Ores of Great Bear Lake. G.S.A. Vol. 46, pp. 879-960. 14. Knight, C.W. (1922):: Geology and the Mine Workings of Cobalt and South Lorraine Silver Areas. Ont. Dept. Mines, Vol. 31, Part II. 15 (1930): Pitchblende at Great Bear Lake, C.M.J. Vol, 51, No, 41, pp, 962-965. 16. Erieger, P. (1935): Primary Silver Mineralization at Sabinal, Chihuahua, Mexico. Econ. Geol. Vol. 30, No, 3, pp. 242-259. 17. Lindgren, W. (1933): Mineral Deposits. McGraw-Hill Book Co. New York, N.Y. . 18. Lord, C.3.(1941): Mineral Industry of the Northwest Territories. Geol. Surv. Canada, Mem. 230. 19. Mtlller, H. (1860): Der Erzdistrikt von Schneeberg in Erzgebirge, in B. von Cotta's Gangstudien, Vol. 3, pp. 129-138. -47- BIBLIOGRAPHY (cont'd) 20. Murphy, 2. (1946): Geology and Mineralogy at Eldorado. G.I.M.M. Bull. 413, pp. 426-435. 21. Pearce, R. (1875): Notes on Pitchblende in Cornwall. Roy. Geol. Soc. Cornwall Trans. Vol. 9, pp. 103-104. 22. Reid, J.A. (1932): The Minerals of Great Bear Lake, C.M.J. Vol. 53, No. 2, pp. 61-66. 23. Riley, C. (1933): Some Mineral Relationships in the Great Bear Lake Area. C.M.J. Vol. 54, No.4, pp. 137-141. 24. Robinson, H.S. (1933): Notes on the Echo Bay District, N.W.T. C.I.M.M. Bull. 258, pp.609-628. 25. Step, Josef and Becke (1904): Das Vorkommen des Uran- pecherzes zu St. Joachimsthal. X. Akad, Wiss. Wien Sitzunglier. Vol. 113, pp. 585-618. 26. Spenoe, H.S. (1931): The Pitchblende and Silver Discov- eries at Great Bear Lake, N.W.T. Can. Dept. Mines, Mines Branch, Invest. Min. Res. and Min. Ind. No. 3, pp. 55-92. 27 (1932): Radium and Silver at Great Bear Lake, Min. Met.-, Vol. 13, No. 303, pp. 147-151. 28. Viebeg, W. (1905); Die Silber-Wismutgfinge von Johannge- orgenstadt in Erzgebirge. Zeitschr. prakt. Geologie Vol. 13, pp. 89-115. 29. Winohell, A.1*. (1946): Elements of Optical Mineralogy, Part II, Third Edition. John Wiley & Sons, • New York.


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