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Geology and manganese deposits of the north shore of Cowichan Lake, Vancouver Island, B.C. Fyles, James Thomas 1949

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G E O L O G Y A N D M A N G A N E S E D E P O S I T S O F . T H E  N O R T H S H O R E O F C O W I C H A N L A K E I A Thesis Submitted i n Par t i a l Fulfilment the Requirements for the Degree of Master of Applied Science in the Department of Geology VANCOUVER ISLAND B.C. James Thomas Fyles The University of B r i t i s h Columbia A p r i l , 1949 A& STRAIT* SUMMARY The north side of Cowlchan Lake, Vancouver Island B.C. is underlain by volcanic rocks, sediments, and intrusives of Mesozoic age. The oldest rocks, Jurassic or Triassic flows known as the Sicker andesites,.are conformably overlain by about 3000 feet of cherty tuffs,, coarser pyroclastics, and small lenses of limestone known as the Sicker sediments. Cherty tuffs form the lower . members of the Sicker sediments, felspathic tuffs the central members, and coarser pyroclastics the upper members. About 2000 or 3000 feet of basaltic and andesitic flows conformably overly the Sicker sediments. The Sicker andesites and sediments and younger flows are intruded by large dyke-liko bodies of granodiorite or quartz monzonite, known as the Saanich granodlorite and correlated with the Coast Range intrusives. Upper Cretaceous shales sandstones and conglomerate unconformably overlie the.volcanics and Intrusives. The Sicker series and overlying flows are t igh t ly folded into overturned and esyaesetrical north-westerly trending synclines and ant ic l ines . The Cretaceous rocks are gently folded and dip north along a narrow belt on the north side of a down-faulted block. uanganese deposits occur i n the lower cherty beds of the Sicker sediments as lens shaped bodies pa ra l l e l to the bedding of the sediments.- They are commonly in chert free from felspatnic material, and are always associated with'jasper or jaspery sediments. The main manganese minerals are rhodonite, spessartite, an unidentified yellow manganese s i l i c a t e , and small amounts of rhoaoohrosite. Residual manganese oxides coat the surfaces of the deposits. Several features of the deposits, such as the fact that rhodonite commonly cross cuts and replaces the chert suggest that the deposits are of replacement o r ig in . Other features such os their bedded appearance and the fact that they occur, at about the same horizon In the Sicker sediments indicate a sedimentary o r ig in . Theoretical considerations support trie view thet the deposits are sedimentary and suggest thet tno replacement features were formed by metamorphism. CONTENTS PAGE Introduction 1 Previous l<ork 1 V - • Geography and pysiography 3 • * f> Chapter I General Geology 5 Summary of geology 5 Detailed Descriptions 7 Sicker Andesites 7 Sediments within the Sicker andesltes 9 Sicker sediments 10 Plows Younger than the Sicker sediments 16 Saanich granodiorite 17 Minor intrusives 18 Cretaceous rocks 19 Structure 20 Chapter I I The manganese deposits 24 General Geology 24 Mineralogy 27 Description of deposits 31 Size and grade of the deposits 36 Features i n d i c a t i n g tho o r i g i n of the deposits 38 Chapter I I I Origin of manganese deposits 41 Marine sedimentary deposits 42 Sources of manganese 42 Transportation of manganese 43 r Deposition of manganese i n sea water 45 Manganese of volcanic o r i g i n 46 Summary 48 Metamorphosed manganese deposits 49 Origin of tho Cowichan Lake deposits 51 Conditions of formation of the Sicker sediments 51 Conclusions 53 Bibliography 54 I l lus t ra t ions 57 ILLUSTRATIONS PAGE P i g . l Index map showing the position of the Cowichan Lake map*area. 58 Pig. 2 Sketch map showing the structure of part of the Cottonwood Creek manganese deposit. 59 Pig.3 Sketch map showing the structure of the Shaw Creek manganese deposit. 59 Pig.4 Steeply dipping cherty t u f f s , 60 Pig.5 Tightly folded cherty t u f f s . 60 P i g . 6 Tightly folded cherty t u f f s . 61 Pig,7 Pragmental flow top i n the Sicker andesites 61 Pig.8 Poorly developed columnar j o i n t i n g i n the flows above the Sicker sediments. 62 Pig.9 Photomicrograph of t y p i c a l cherty t u f f . 62 Pig.10 Photomicrograph showing s i l i c e o u s o o l i t e s i n cherty t u f f . 63 Pig.11 Photomicrograph showing s i l i c e o u s o o l i t e s i n manganiferous chert. 63 Pig.12 Photomicrograph showing rhodonite cross-cutting and replacing cherty beds. 64 Pig.13 Polished surfuoe of a specimen from the H i l l 60 manganese deposit. 64 Pig.14 Polished surface of bedded manganiferous chert. 65 Pig.15 Polished surface showing faulted beds of yellow manganese s i l i c a t e i n jasper. 65 Pig.16 X-ray powder photograph of quartz. 66 Pig.17 X-ray powder photograph of quartz and the yellow manganese s i l i c a t e . 66 Pig.18 X-ray powder photograph of spessartite. 66 / Structural sections north of Cowichan Lake ( i n pocket) ^ Geological map of the north side of Cowichan Lake ( i n pocket) A C KWOwLELGMEMIlS The w r i t e r would l i k e to thank Dr. H. Sargent of the B.C. Department of Mines f o r his assistance during the f i e l d season and f o r his help i n making t h i n sections and maps available f o r laboratory work. Thanks are also due to various members of the Department of Geology at tho University of B r i t i s h Columbia, e s p e c i a l l y to Dr^ H.C. Gunning f o r his suggestions, and to Dr. R . l . Thompson f o r his work on the yellow manganese s i l i c a t e . GEOLOGY AND riANOANESE DEPOSITS  OF THE NORTH SHORE OF OOtolCHAN LAKE  VANCOUVER ISLAND, B.C. During the summer of 1948 the write r was employed by the B r i t i s h Columbia Department of Mines to carry on geological mapping and to examine a number of mineral deposits north of Cowichan Lake, Vancouver Island B.C. Geology was mapped on a scale of one inch equal to half a mile and an area (referred to i n th i s thesis as the map-area) along the north shore of Cowichan Lake about 20 miles long and f i v e miles wide was covered. Mineral deposits i n the area include several occurrences of manganese, and the o r i g i n of these deposits has been made the main subject Of t h i s t h e s i s . Although the thesis describes the general geology of the .area, emphasis has been placed on the phases of the general geology that throw some l i g h t on the o r i g i n of the manganese deposits^ Previous Work A preliminary report based on reconnaissance surveying of southern Vancouver Island was published by 2 tho Geological Survey of Canada In 1912 (Clapp, 1912). The report covered Vancouver Island east of Alberni Canal, but was of a preliminary typa and included no de ta i l along the north shore of Cowichan Lake. £ bibliography and description of early exploration and geological work on southern Vancouver Island appears in this report. Later detailed work by C.I1. Clapp and H.,C. Cooke resulted i n the publishing of several memoirs and maps (Clapp, 1913,1914 and Clapp and Cooke, 1917) of the geology of southern Vancouver Island. The western margin of Clapp's Duncan sheet l i e s about three miles east of the east end of Cowichan Lake and i t was possible for the vjriter to make the Cowichan Lake map a direct extension of the Duncan sheet. Detailed descriptions of several mineral deposits of the Cowichan Lake area, including descriptions of the manganese deposits appear i n the annual Reports of the B r i t i s h Columbia Department of foines (1918, 1919, and 1920). During the summer and f a l l of 1939 geological work was done along the north shore of Cowichan Lake by a. party In training under the Dominion-Provincial Mining Training Project. This work consisted mainly of exploration of the known manganese deposits and prospecting for new deposits but some geological mapping was also undertaken. Plans and an unpublished report on the manganese deposits were drawn up by Dr«, H. Sargent of the 3 B.C. Department of Mines, and these have been used extensively by the w r i t e r . Geography and Physiography Cowichan Lake, l y i n g i n a deep northwesterly trending v a l l e y near the center of southern Vancouver Island, averages 1% miles wide, i s 15 miles long, and 550 feet above sea. l e v e l . The V i l l a g e of Lake Cowichan l i e s at the east end of the lake, and the logging town of Youbou i s on i t s north shore about seven miles west of Lake Cowichan. A public highway, tho extension of the Duncan-Lake Cowichan highway, runs as f a r west as Youbou and a branch l i n e of the Canadian National Railway runs along the north shore of the lake as f a r as i t s west end.. Private logging roads extend up most of the valleys trending north from the lake, and the h i l l s i d e s above them have been logged to elevations of about 2500 feet. H i l l s i d e s generally are steep, and outcrops f a i r l y abundant except i n depressions or v a l l e y bottoms, where g l a c i a l d r i f t may reach a depth of 20.or 30 feet. In the larger v a l l e y s , however, creeks have cut through the g l a c i a l d r i f t . a n d exposed the underlying bedrock. The Cowichan Lake map-area l i e s west of the peneplaned surface described by Clapp, (Clapp, 1912), and few of the features of the old peneplane are noticeable i n i t . The nortn shore of the lake i s marked by steep h i l l s i d e s r i s i n g 2000 or 3000 feet above tho lake that at lea s t i n part form a f a u l t l i n e scarp resulting, from post-Cretaceous faul t ing. Northerly-trending ridges, separated by deep re la t ive ly straight valleys extend four or five miles north from tho lake and jo in a northwesterly-trending l ine of h i l l s that marks the divide between the Cowichan Lake and the Chemainus and Nanaimo River drainages. Glacia l erratics and straie indicate that the continental ice sheet covered even the highest h i l l s (elevation 5000 feet above sea level) In tho map-area. Valley glaciers have le f t cirques and cirque lakes. Terraces of g lac ia l t i l l l ine the sides of the larger val leys, and spurs projecting into the valleys are cut by water-worn channels that have been formed by streams flowing from retreating valley glaciers . CHAPTER I GENERAL GEOLOGY Summary of Geology The map-area i a underlain by flows, breccias, sediments and intrusives of Mesozolc age. The oldest rocks are volcanics and tuffaceous sediments of the Vancouver group thought to have been l a i d down i n la t e T r i a s s i c or ear l y Jurassic time. The lower members of the Vancouver group consist of great thicknesses of conformable andesitic flows containing small lenses of sediments. These are conformably overlain by a well doflned band of cherty t u f f s and coarser pyroclastics 2000 or 5000 feet t h i c k known as the Sicker sediments. The Sicker sediments i n turn are conformably overlain by at l e a s t 2000 feet of flows s i m i l a r i n composition to the Sicker andesites. The Vancouver group i s intruded by long, dyke-l i k e bodies of granodiorlte and quartz monzonite correlated with the Saanich granodiorlte, and by inor intrusions mainly of gabbroic and d i o r i t i c composition. Both the Vancouver group and the intrusives are overlain unconforraably by Upper Cretaceous conglomerate, sandstone, and shale. The Vanoouver group has been highly folded into northwesterly trending synclines and a n t i c l i n e s that i n general dip southwest. The Cretaceous sediments ore gently folded, and are exposed i n the map-area only In a 6 down-faulted easterly trending belt just north of Cowichan Lake. TABLE OF FORMATIONS AGE NAME LITHOLOGY Upper Cretaceous Haslam Formation Benson Formation Marine black concretionary shales conglomerate and sandstone Upper Jurassic? Minor Intrusives Saanich Granodiorite gabbro and dior i te dykes s i l l s and irregular bodies granodiorite,quartz monzonite and alaskite Lower Jurassic and/or Upper Tr isss ic 2000 to 3000 feet oi Sicker sediments Sicker andesltes ' basaltic and andesitic flows cherty tuff, fine grained felspathie tuff, coarser pyroclastles and minor limestone. 2000 to 3000 feet thick Andesitic flows with small lenses of tuffaceous sediments 7 Detailed Descriptions Tho. Vancouver group of volcanic rocks underlies about 75 percent of the map-area, volcanics being absent only where erosion has exposed la ter intrusives and where small areas have been covered by Cretaceous sediments. Clapp has divided the Vancouver group into the Vancouver Volcanics, the Sutton limestone, and the Sicker series (Clapp, 1912). The Sicker series has been further divided into the Sicker Andesites' and the Sicker sediments (Clapp and Cooke, 1917). Only the Sicker sediments were posi t ively ident i f ied in the map-area, but flows and sediments underlying the Sicker sediments and constituting the oldest rocks exposed in the area, ere probably part of the Sicker andesites. The Sicker sediments form the youngest member of the Vancouver group described by Cooke (Clapp & Cooke 1917), but at least 2000 foet of flows conformably overlie the Sicker sediments i n the Cowichan Lake map-area. While similar l i t h o l o g i c a l sequences have been described at other points on Vancouver Island i t is not possible to correlate the §pper flows at Cowichan Lake witn any that have been previously named. Sicker Andesites The Sicker andesites exposed in the Cowichan Lake map-area are mainly flows, but with them are small discontinuous bands of tuffaceous sediments. Three phases of flow rocks, a l l essential ly of the same composition, can 8 be recognized. Fragmental amygdaloldal phases occur at the tops of thick.flows, massive porphyritic phases occur in ' the central parts, and narrow, often obscure, amyaaloidal phases occur at the bottoms. The massive phases are typ ica l ly dark green, porphyritic andeslte, but occasionally they are l igh t green .or brown. Green hornblende phenocrysts up to three millimeters In length and frequently smaller plagioclase phenocr£f>ts are set i n an aphanitic groundmass composed of a felted moss of nornblende and smaller amounts of plagioclase , . in typica l rocks. In places augite phenocrysts are present and some of the hornblende has formed by al terat ion of augite* Fragments1 phases contain mainly rounded amygdaloldal fragments, from one to four Inches in diameterj surrounded by porphyritic andcsite (F ig . .7) s imilar to that of the matslve phase. In thin section the fragments appear to be of essential ly the same composition as tho surrounding andesite but contain a larger proportion of phenocrysts some of which appear bent or broken. Fragmental phases appear, to represent parts of tho flow near the or ig ina l surface, and within a foot or so of the surface highly amygdaloldal • fragments make up most of the rock and occasionally irregular cavities are present between them. Amygdaloldal bottom phases i n the Sicker andesites are aphanitic and lack structures other than amygdules. The relat ive thicknesses of the three phases varies from flow to flow and probably vuithin the same flow. 9 Tho massive phase constitutes the largest part of the rocks, and because of the large amounts of the massive phase, and because of the widespread al terat ion of the flows, individual flows can be recognized only on good exposures. Some thin flows appear to be made up ent i rely of tne fragmental phase. In widow Creek, for example, d i rec t ly below the Sicker sediments a fragmental flow top 100 feet thick i s overlain by a lens of jaspery sediments and this i n turn i s overlain by a flow made up ent i rely of the fragmental phase. Such fragmental flow tops character is t ica l ly mark the base of the overlying Sicker sediments. The Sicker /ndesites, though rarely schistose, have.been widely altered. They contain abundant epidote and ch lor i te , and probably much of the hornblende Is an al terat ion product. Se r ic i t e , fine grained albi te and quartz can be seen i n some thin sections. The fragments i n the fragmental phases have been more severely altered than the rock enclosing them, and the concentration i n them of epidote and quartz has given rise to epidote nodules characteristic of the Sicker andesites. Sediments within the Sicker Andesites Lenses of fine grained, cherty, and coarser probably tuffaceous sediments occur within the Sicker andesites. The lenses are generally small, averaging 100 feet thick and several hundred feet long, but on the h i l l s east and west of McKay Creek cherty tuffs with few inter* calated flows aggregate 1000 feet i n thickness. The smaller 10 lenses are commonly thin headed, black, rusty weathering and resemble a r g i l l i t e s . Other lenses vary i n grain size and composition both along the str ike and from bed to bed. In Shaw Creek, betweea the north and west forks for example, thin-bedded, l ight-colored, highly cherty tuffs grade Into fine-grained green poorly bedded sediments along the s t r ike , within a distance of less than a quarter of a mile. The cherty rocks are overlain by flows but grade downward into fine grained green sediments within a thick-ness of less than 200 feet of beds. No thin sections of these fine-grained sediments have been studied but in hand specimen they closely resemble coarser types of the Sicker sediments found to be made up of sub-angular fragments of rock s imilar i n composition to the surrounding andesite. In places, especially where they have been highly altered, these sediments closely resemble flow rocks and on the accompanying map undifferentiated areas marked as Sicker andesites may. l o c a l l y contain sediments. Sicker Sediments A thick series of cherty tuffs , coarser sediments, and minor limestones known as the Sicker sedirconts conformably overlies the Sicker andesites. / well defined, though probably not continuous, flow top marks the top of the Sicker andesites and s»t a l l places where the contact was seen the lowest beds of the Sicker sediments l i e para l le l to the surface of the flow. In general, the lower beds of tne Sicker sediments 11 are cherty tuffs, the central beds are fine grained tuffs, while the upper beds are coarser pyroclast ics. The character of the beds, however, varies along the str ike so that there is no well defined sequence. The lowest beds of the Sicker sediments exposed a few hundred feet west of widow Creek, for example, consist of about 15 feet of thin-bedded, black rusty weatnering s la te - l ike tuffs . East of Widow Creek thin bedoed green and white cherty tuffs l i e d i rec t ly on the flow rocks, while south of Sherk Lake a three foot bed of jasper l i e s between the flow top and the overlying cherty tuf fs . Fine-grained green to buff unbedded sediments as much as 50 feet thick form the base of the Sicker sediments in Shaw Creek. Thus, there is considerable variat ion i n the l i thology of the lowest beds, and nowhere i n the series can beds be correlated on l ' i thological s i m i l a r i t i e s . Nevertheless, i n general, cherty tuffs form tho lower beds, felspathic tuffs the central beds, and coarser pyroclastics the upper beds of the series. The lower 200 or 500 feet of beds are mainly l igh t colored, well bedded cnerty tuffs made up of green, red, l igh t grey or black beds alternating with l ighter comcjonly white beds one-half to two inches thick. Some beds grade from one to another by a gradual decrease in grain size or in the amount of coloring matter, but many show sharp breaks between the beds and primary features such as local unconformities are noticeable. In thin section a l l colors of beds appear to be of about the same 12 composition* They are made up of angular fragments of quartz and feldspar, i n the f iner types 0.01 mm across and i n the coarser averaging 0,3 am, (Pig,9) . Where identif iable the feldspar i s always andesine, and andesine probably makes up 80 percent of the grains, although many of the smaller grains are too small to be identif ied as either auartz or feldspar. Green beds contain epidote, green opaaue material, or fine needles possibly of amphibole. Red var ie t ies , both jaspery sediments and jasper, contain hematite and darker varieties contain b io t i te and opaque material. A l l the cherty bods contain cryptocrystalline quartz both as Irregular lenses and as spherical nodules. In the more felspathie varieties the quartz seems to be aost abundant i n certain beds, while in tho more cherty beds quartz i s distributed throughout. The nodules, are moae up of a mosaic of very fine quartz, are s t r ik ing ly spherical and uniform i n s ize , averaging 0,3 mm in diameter. (Pig.10). A poor rad ia l structure can be seen near the edges of a few and semi-opaque material occurs near the centers of others, but i n general they lack Internal structure. Commonly thin bods of opaque material bulge around the nodules and never i s thero any indication of bedding passing through them, although some are lens shaped and grade outward into thin beds of cryptocrystalline quartz. Later quartz-feldspar veinlets tnat cut the rocks also cut *V \ t n o d u l e s through the nodules. Thus, the^ r appear to be syngenotic, and resemble small concretions or ool i tes ; nothing i n them 13 Indicates they are organic remains. The presence of the oolites suggests thet s i l i c a , as a chemical sediment, was being deposited at the some time as the tuffaceous material* Pure cherts made up almost ent i re ly of recrysta l l ized s i l i c a are present i n the Sicker sediments and are especially abundant near the manganese deposits. I t i s probable that a l l gradations exist between pure felspatnic tuffs and pure cherts. Above tho cherty members the rocks are more poorly bedaed, coarser grained, and darker colored. Thin sections show them to be made up of angular crystals of feldspar, with some quartz, 0.1 to 0.2 mm across, surrounded by fine grained biot i te and some opaque material. These dark poorly bedoed rocks are i n general very uniform and make up as mueh as 1000 feot of beds, but in places green cherty tuffs and co&rser pyroclastics are interbedded with them. Coarse pyroclastics are present i n small amounts throughout the Sicker sediments but are most abundant in the upper part of the series. Thoy are a l l of essential ly the same composition, but vary from green, well-bedded rocks made up of angular grains less than one mm across to unbedded rocks made up of fragments up to an inch or two across, I'he f iner greinod varieties resemble greywacke and contain angular fragments of porphyritic volcanic rock s imilar in composition to the Sicker andesite. Some of 14 the coarser grained varieties contain angular fragments of o o l i t i c cherty tuff, single quartz and feldspar crys ta ls , and a few contain rounded fragments of limestone. The o r e grains &£i surrounded by opaque dusty-looking material, by epidote and other secondary minerals, and very rarely by ca l c i t e . In specimens containing limestone fragments, calc i te is rarely present i n the cementing material. Wo positive evidence could be found either i n the f i e l d or in thin section as to the or igin of these rocks. However, because they are associated i n many places with cherty tuffs , and because they contain angular grains that lack chemical cementing material, even where the grains themselves are limey, i t is probable that they are of pyroclastic o r ig in . Two small lenses of limestone and calcareous tuff were found within the Sicker sediments. The f i r s t , a lens about 300 feet thick and half a mile long, l i e s on the southeast slope of Mount Franklin between elevations of 3200 and 360U feet. Tnis lens, wnich appears to be less than 300 feet from the base of tne Sicker sediments, i s made up of beds tnat range from almost puro crys ta l l ine limestone to calcareous tuf i i n which limey material is almost absent. Some.of the more coarsely crys ta l l ine beds appear to be c r ino ida l , but only occasionally can cr inoid stems be recognized. In places the limestone contains angular isolated fragments of andesitc up to f of sn inch across. These limestone-ondeslte breccias appear to have forced as beds of limey ooze or groves of crinoids into which volcanic material f e l l before consolidation. P socond small bed of limestone was found on the southwest slope of Jiount Londalt extending north into the east fork of Cottonwood Creek. It Is made up of a series of small lenses the largest of which i s about 1 0 0 feet thick and less than 1 0 0 0 feet long. They are mr.dJ up of f ine ly crys ta l l ine impure white limestone and grade both along end across the str ike into beds of tuf f . The lenses are within 1 0 0 feet of the top of the Sicker sediments,.and hence not i n the same horizon as the f i r s t limestone lens, Th© thickness of the Sicker sediments varies from place to place, and because of the close folding and the fact that both top and bottom of the series are not exposed i n any one section, i t i s d i f f i c u l t to make an accurate estimate of the thickness. The thickest section is exposed i n the Shaw Creek syncline but i n places the thickness of this section has been increased by folding. The top of the Sicker sediments is not oxposod Ixi tho part of Shaw Creek that has been mapped, but several thin flows intercalated with the uppermost beds suggest that the section Is almost complete, Pn average of 3 0 0 0 feet of beds Is oxposod, the easterly limb being thicker than the westerly limb, i 1 thickness of 3 0 0 0 feet i s adequate to explain the exposures and structural relations elsewhere In the map-area and is consistent with estimates made by Cooke. (Clapp and Cooke, 1 9 1 7 ) . 16 Flows Younger than the Sicker Sediments Porphyritic andasites and basalts that conformably overlie the Sickor sediments outcrop on the h i l l s along the northeast odge of the map-area. East of Mount Landalt and west of uount Service the flaws are andesltic and closely resemble the Sicker andesites. Fragmental and amygdaloidal variet ies are common but no otner primary structures were found. On i.lount Landalt and feiount Service the flows are uniform porphyritic basalts, consisting of white-weathering phenocrysts of labradorite up to three millimeters across, set in an aphanitic groundless of hornblende, augite, and plagioclose. Steep c l i f f s on the slopes of E l Capitan and Mount Landalt make i t possible to distinguish poorly developed columnar joint ing in the basalts (Fig . 8) . Amydules and rarely fragments can be distinguished near flow contacts. Cooke found the Sicker sediments to be the youngest members of the Vancouver group exposed i n the Duncan area, (Clapp and Cooke, 1917) but since the flows above the Sicker sediments i n the Cowichan Lake area appear to be conformable with tne Sicker sediments there seems to be no reason to exclude them from the Vancouver group. It i s probable that more complete sections of the upper part of the Vancouver group w i l l be found north of Chemainus River and northeast of Shew Creek ano when tnese sections have been studied i t may be possible to subdivide the Vancouver group more completely. 17 Saanich granodiorite The Vancouver group is intruded by large dyke-l ike bodies of quartz monzonite the most continuous of which i s over 15 miles long i n tho map-area and averages about half a mile wide. Clapp ahous It to extend some eight miles east of the map-area. / second body of quartz-monzonite outcrops less than two miles north of tho f i r s t and the two almost merge west of the head of McKay Creek. By their age rnd l i thology they are correlated with the Coast Range intrusives and are thought to be upward-extending apophyses of a larger intrusive body. Tho ouartz monzonite Is essential ly a medium grained, l igh t colored rock made up of about equal amounts of quartz, andesine, and orthoclase, Bio t i te and hornblende make up about 10 percent of the rock and small amounts of apatite, zircon, magnetite, and pyrite arc commonly present. Dark, me diu.a-grained, rounded inclusions are coamon In the quartz monzonite and the proportion of dark minerals increases toward tho contacts. The quartz monzonite boaies for.a the westward extension of the Saanich granodiorite mapped on the Duncan sheet but only one of several thin sections studied from the Cowichan Lake area was of granodiorite. Clapp points out that the composition of tho granodiorite varies, much of i t being near quartz d io r i t e . I t i s probable, therefore, that some If not 3Juch of th© Saanich granodiorite of the Cowichan Lake area is mor- basic than quartz monzonite. 18 / more acid phase lacking dark minerals i s exposed west of McKay Creek, On the high h i l l s west of S&cKay Creek the intruded volcanic rocks have not been conpletely eroded awry so that the top or end of an apophysis of quartz monzonite i s imposed. About 200 feet below the end of the apophysis, quartz monzonite containing biot i te and hornblonde gracies upward into medium to coarse grained a laski te . / .laskite i s r i c h in quartz and contains less than 10 percent andeslne end lcrge amounts of microperthite and micropegmatite. Alaskite topers upward into dyke-like bodies r i c h i n quartz end commonly containing sulf ides. &inor Intrusives Several types of Igneous rocks intrude the Vancouver group and sojie intrude the quertz monzonite. The largest of these ere irregular bodies of gabbro and d i o r i t e . hornblende, plagioclase, and augite are tho main constituents, andesine occurring i n varieties containing most bornblonde, and lebradorite in those containing augite. ;.iuch of the augite is altered to hornblende, and b io t i t e , ch lor i te , epidote, and ser ic i te have developed i n tho more hiynly altered types. In l&eede Creek canyon, where the contact of the d ior i te with the quartz monzonite is well exposed, tho dior i te intrudes the quartz monzonite i n dyfce-like offshoots from tho main diori te bou-y, and the quartz monzonite appears 19 altered by the intrusion. . i±enco, the a ior i te i s younger than the quartz monzonite, and ".ay correspond to d ior i te porphyrite dykes described by Clapp (Clapp and Cooke, 1917, p.198). Dykes and s i l l s , some f e l s i t i c in composition and others more basic intrude the volcanic rocks, hut very few Intrude both the volcanics and the quartz monzonite. Lifcny of the basic types closely resee»ble the Sicker andesites and era -eften d i f f i c u l t to distinguish from them. Cretaceous Rocks Upper Cretaceous shalos, sandstones, and conglomerates outcrop along a narrow belt on the h i l l s just north of Cowichan Lake extending from Cottonwood Creek to tleade Creek. A basal conglomerate made up of rounded pebbles averaging £ inches in diameter rests with marked unconformity on both the volcanics and the quartz monzonite* Pebbles in the conglomerate ere of loca l or ig in boing mainly of volcanics. Only i n places close to outcrops of quartz monzonite do quartz monzonite pebbles occur i n tho conglomerate, massive greyish-brown sandstone is present as d is t inc t beds throughout the conglomerate and the conglomerate grades upward into pure sandstone. Black shales of the haslam formation conformably overlie the conglomerate and near Uie base of tho shale formation beds of conglomerate occur within the shalos. The shales are unbedued and highly frrctured, breaking on the surface into e l l i p s o i da l clusters of rod-l iko spl in ters . The shales contain hard s i l i c e o u s concretions some of which are over a foot i n diameter and e l l i p s o i d a l i n shape. Poorly preserved f o s s i l s are present i n the shales and concretions, and by means of them i t has been possible to correlate the shales exposed i n tho map-area with the haslam formation found elsewhere i n southern Vancouver Island. (Clapp and Cooke 1917). The conglomerates and sandstones therefore, probably belong to the Benson formation. Structure The Vancouver group has been c l o s e l y folded but only i n tho Sicker sediments has the structure been worked out i n d e t a i l . Since, the Sicker andesites and the flows above the Sicker sediments appear to be conformable vjith the Sicker sediments. F ind 3ince bands of sediments and i n d i v i d u a l flows i n the endesites conform f a i r l y c l o s e l y with the general structure indicated by the Sicker sediments, i t seems probable that the Vancouver group i s folded i n t o synclines and a n t i c l i n e s broadly s i m i l a r to those of the Sicker sediments,n The Sicker sediments i n the main part of the map-area have been clo s e l y f o l d e d i n t o northwesterly trending overturned and asymmetrical synclines. Along the northeast edge of tne map-area tne southwest limb and possibly the crest of an a n t i c l i n e are exposed. 21 The a x i a l plane of the most easterly syncllne s t r i k e s about north 70° west and dips southwest at about 50°., The rooks of t h i s syncllne outcrop continuously from the east edge of the map-area to Sherk Lake and toward the west the structure trends more nearly northwest, west of Cottonwood Creek the a x i a l plane of a poorly defined syncllne s t r i k e s north of northwest and dips steeply southwest, and i n Shaw Creek the a x i a l plane s t r i k e s nortn 15° west and dips steeply west. Thus, the axes of f o l d i n g trend more nearly north toward the west and more noarly east toward the east. The most easterly syncllne and the syncline west of Cottonwood Creek are overturned, most of the. rocks dipping southwest, but the Shaw Creek syncline Is asymmetrical and not as t i g h t l y folded as those farther east. The structure of the a n t i c l i n e along the northeast edge of the map-area i s not w e l l exposed, and i t s r e l a t i o n to the synclines i s complicated by post-Cretaceous f a u l t i n g . In general however, the rocks of the a n t i c l i n e are f l a t -l y i n g and appear to form tne gently aipping southwest limb of an overturned a n t i c l i n e corresponding to the syncllne on the southwest. In many places, e s p e c i a l l y i n the synclines the sediments are highly contorted by minor folds ( P i g , 5) p a r a l l e l to the regional trend, and by others across the regional trend. In the Shaw Creek syncline, e s p e c i a l l y i n the lower cherty beds, pronounced minor folds with axes 22 trending about north 70° east are superimposed on minor f o l d s , resembling drag f o l d s , that trend north 20° west. The cross folds generally produce only gentle plunges In the regional structures, commonly to the northwest or sharp warps i n the drag f o l d s ^ , ^ . 'Locally, however, regional structures plunge at angles up to 20° f o r distances of h a l f a mile owing to the eff e c t of cross^Folding. Cross f o l d i n g along axes at about 45° to the regional trend and minor f a u l t i n g have produced the complicated outcrop pattern of the Sicker sediments between widow Creek and Sherk Lake. The structure of the Sicker sediments has been complicated by post-Cretaceous f a u l t i n g . Two continuous, almost v e r t i c a l f a u l t s trend northwesterly through most of the length of the map-area. The southerly f a u l t , near the shore of Cowichan Lake, forms the north contact of the small area of Cretaceous sediments between Youbou and Meade Creeks. The thickness of the Cretaceous rocks indicates that the rocks on the southwest side of the f a u l t have moved down more than 1000 feet i n r e l a t i o n to the rocks northeast of the f a u l t . The horizontal displacement has probably been small, but the position of the lower contact of the Sicker sediments i n Meade Creek south of tho f a u l t suggests that rocks southwest of the f a u l t moved west In r e l a t i o n to those northeast of tho f a u l t . Near the northeast side of the map-area a major f a u l t nas displaced the rocks i n the opposite d i r e c t i o n . Rocks northeast of t h i s f a u l t have 23 moved down, probably botwoen 2000 and 3000 feet i n r e l a t i o n to those southwest of the f a u l t * The horizontal displacement appears to have been small. The Cretaceous rocks s t r i k e east and dip north at angles of about 10° i n the west and of 40° i n the east. The northern contact of the sediments i s marked by a f a u l t , while the southern contact marks the unconformity between the sediments and tne underlying c r y s t a l l i n e rocks. The lower contact of the sediments aouth of Meade Creek i s p a r a l l e l to the slope of the h i l l so that i r r e g u l a r i t i e s i n the surface on which the conglomerate was l a i d down have produced a complicated outcrop pattern. 24 CHAPTER I I THE 1MGAMESE DEPOSITS The discovery i n 1918 of manganese ore at H i l l 60 l e d to intensive prospecting f o r manganese along the north shore of Cowichan Lake and the discovery of a number of small deposits. The largest of these was i n Shaw Creek, but three other deposits were staked and since the o r i g i n a l stakings several small occurrences have been found* Ore reported to t o t a l 1117 tons was shipped i n 1919 and 1920 from the H i l l 60 deposit and a l i t t l e work i n the form of trenching and t e s t i n g was done i n 1918 and 1919 on the Shaw Creek deposit. Very l i t t l e xvork was done on the other deposits and since 1920 no work other than that of the Mining Training Project has been done on any of the deposits. The H i l l 00 manganese deposit i s east of the eastern edge of the Cowichan Lake map-area. The main showings are between 2500 and 2700 feet above sea l e v e l , four miles due east of the V i l l a g e of Lake Cowichan. The location of the other f i v e deposits Is shown on the accompanying map. In the summer of 1948 o i l the deposits were less than a mile from a private logging road or public highway, but although favorably Iocsted none of the deposits i s of economic value. General Geology The manganese deposits occur i n the cherty 25 Sicker sediments that, owing to t h e i r resistance to weathering and erosion form r e l a t i v e l y numerous moss-free outcrops. Near the manganese deposits, the over-burden and outcrops contain conspiouous black oxides that obscure the form and size of the primary mineralization At the H i l l 60 deposit a large open p i t and shaft extendi ten or f i f t e e n feet beaeath tho oxidized layer, but only shallow prospecting trenches have be^n made at the other deposits and several of them are poorly exposed. The deposits are a l l In the lower cherty members of the Sicker sediments, the Shaw Creek, wardroper Creek* Sherk Lake and Cottonwood Creek deposits being within 200 feet of the base of the s e r i e s . The other two large deposits are possibly within the lower 200 feet of the s e r i e s ; the position of the Meade Creek deposit i s obscured by f a u l t i n g , and by the f a c t that the base of the series i s not exposed, and that of the H i l l 60 deposit i s obscured by large bodies of intrusive rock. Several less important occurrences of manganiferous beds that have been found are a l l In the same lower members of the Sicker sediments. The deposits nave the form of lens shaped bodies with long axes p a r a l l e l to the bedding of tne sediments. In the more t i g h t l y folded beds the r e l a t i o n of the manganese-bearing lenses to the bedding i s often obscure but i n the s l i g h t l y folded beds tne lenses containing manganese 26 minerals are obviously related to the bedding. The lenses vary from an eighth of an inch to several feet In thickness and from a few inches to 30 or 40 feet in length. Larger lenses are a l l short i n relat ion to their thickness and appear to be made up of closely spaced smaller lenses. Small lenses are either bedded or massive, rhodonite tending to be massive, the other s i l i ca tes tending to be bedded. In the t igh t ly folded deposits individual manganese-bearing beds appear to follow the folding and are thinneu snd displaced i n the same way as the surrounding beds. Within the manganese beds, however, rhodonite forms irregular masses and occasionally well defined velnlets cross-cutting the enclosing rock. The deposits are always associated witn massive jasper or jaspery sediments, fiiiassive jasper occurs i n beds s ix inches to two or tnree feet thick that near the manganese deposits are commonly cut by wel l defined quartz veinlets . Jaspery sediments are pink to brick red and generally well bedded; they owe their color to f ine ly divided hematite. Beds of massive jasper and substantial thickness® of jaspery sediments that contain no manganese are common throughout the lower members of the Sicker sediments. Thin sections of rocks associated with manganese deposits show them to be mainly quartz. The quartz occurs as a mosaic of grains less than 0.1 mm across, Tnus these grains are larger than those of the quartz in si l iceous lenses of the normal cherty tuffs . Outlines of si l iceous 27 oolites can be seen i n some thin sections of manganese bearing rocks. (F ig . 11) and those too are more coarsely crystal l ine than those in the nort.al cherty tuf fs . The bedding is often obscure i n thin sections, but may be marked by bands of opaque material or by fine-grained manganese s i l i c a t e s . (F ig . 12). Rocks of the manganese deposits thus differ from the normal cherty tuffs i n two respects. F i r s t , they consist mainly of quartz and smaller amounts of manganese minerals and contain no recognizable tuffaceous material. Second they are coarser grained and resemble recrysta l l ized cherts. Mineralogy The following manganese minerals were found i n the Cowichan Lake deposits: . Rhodonite, Bin (Ca,B*e) SiO ' o Qaraet, probably spessartite, Ito„ A l Si , . 0 , o O Ci '* O 12 Unidentified yellow manganese s i l i c a t e Weoctoclte, manganiferous opal Khodochroslte, £2n COg Undifferentiated oxides Rhodonite i s the most abundant manganese s i l i c a t e occurring mainly as Irregular masses i n chert or i n chert containing other manganese minerals. Light pink rhodonite i s the commonest type but red-brown, wel l -c rys ta l l i zed rhodonite was found i n the H i l l 60 deposit. No difference i n 28 optic properltes between tiie pink and red-brown rhodonite could be detected, As seen i n thin section, much of the brown variety is euhedral and occurs within large crystals of quartz in what might be referred to as a p o i k i l i t i c arrangement. The red-brovai rhodonite i n quartz appears to have been recrysta l l izod by the intrusion of an andesitic dyke to which i t i s spa t ia l ly related. The rhodonite i s massive and fine grained; only i n the largest crystals can cleavage he distinguished. Rhodonite occurs as masses with vague outlines and as short lonse3 pa ra l l e l to the bedding of tho rocks. (P ig . 14). The lenses in themselves are massive even though bedding may be well developed i n the surrounding chert and manganese s i l i c a t e s . A few small, well-defined quartz veinlets also contain rhodonite. Hence, in a l l respects the rhodonite appears to replace chert and other manganese s i l i c a t e s . Garnet was found i n several thin sections of the manganese-bearing rocks but i s too fine grained to be recognised i n hand specimens. Luhedral grains of garnet about 0.1 mm across occur i n bands lying para l le l to the bedding of the sediments. Garnet is generally closely associated with the yellow manganese s i l i c a t e , and brown bands that commonly alternate with pink or yellow bands in the well-bedded deposits contain abundant garnet. Because of the close association of garnet with tho manganose-bearing' rocks, and because of the fact that the surrounding 29 rocks have not been subjected to a high grade of metamorphism the garnet i s probably spessartite. Garnet i s too fine grainod, however, for i t s chemical composition or many of i t s physical properties to be determined. A. yellow i-anganese s i l i c a t e i s abundant i n the H i l l 60 deposit and was found in small amounts at the Meade Crook and ^ardroper Creek deposits. In hand specimens tho mineral appears to bo closely associated with quarts of the cherty rocks in the form of very fine fibrous often closely spaced sheets ly ing para l le l to the bedding of the sediments (Pig, 13). Thin sections of oven, the most massive material contain as much as 50 percent quartz. The mangan-ese mineral occurs with tho quartz i n equidimensional grains of high r e l i e f and loss than 0.01 mm across. By comparing the index of the crushed mineral with o i l s of known refractive index, the index of the manganese mineral was found to be less than 1.765 but greater than 1.750. I t ivas not possible to separate the M a n g a n e s e mineral from quartz by means of "heavy" l iqu ids , but spoctrographic and x-ray analyses were made of material composed only of quartz and the yellow manganese mineral. In addition to s i l i c a and manganese, considerable amounts of iron and magnesia were founo to be present. This composition, together with the refractive index suggest that the mineral is. a manganese pyroxene such as sobral i te , but x-ray data to confirm this are not avai lable. The x-ray powder photograph of quartz 30 ana the yellow mineral ia shown in F i g . 17, and that of pure auartz i s shown i n F i g . 16. Assuming that none of tne l ines of quartz are exactly the same as those of the yellow mineral, the extra l ines of F i g . 17 represent the x-ray pattern of the yellow mineral. The pattern i s r e la t ive ly simple and was thought to possibly correspond to that of spessartite. (F ig . 18) but several alotangoo can be noticed. Thus, the yellow mineral remains unidentified, Rhodochrosite i s present in small amounts in most of the deposits and occurs as disseminated grains and veinlets cutting the manganese s i l i c a t e s . I t may be pink, white, or brown and i s generally fine grained. The highest concentration of rhodochrosite found occurs in a small lens In the Cot.onwood Creek deposit. S\ th in sections of the brown carbonate from tnis lens shows i t to replace rhodonite, which forms half the specimen. In other places where the age of the rhouochrosite in re la t ion to., rhodonite could be determined, rhodochrosite i s always la ter than the rhodonite. Small amounts of neoctocite occurring i n well defined veinlots i n rhodonite and rhodochrosite, were V<s i n I e t & ident i f ied i n two thin sections. The fi-pptures appear to be related to the surface, and the neoctocite i s probably a product of weathering. Black, generally hydrous manganese oxides coat th© surface of the deposits and form branching veinlets along fractures i n the s i l i ca tes for several feet below tne surface. 31 The oxides have been produced by weathering of the manganese s i l i c a t e s , but residual concentrations of oxides are small. Tho oxides wore not ident i f ied . Description of Deposits The manganese deposits vary from simple lenses or manganese-bearing beds to complexly folded and metamor-phosed deposits in which the re la t ion of the manganese lenses to the bedding is obscure. The Wardroper Creek and Meade Creek deposits occur i n re la t ive ly undeformed beds but the Shaw Creek and Cottonwood Creek deposits are i n highly folded beds, and the H i l l 60 deposit has been complicated by intrusion. The more complex deposits w i l l be described In de t a i l , but the simpler deposits need only be described i n general. Small lenses, or beds of manganese bearing material an inch or two thick and two or three feet long occur southwest of the main h i l l 60 deposit i n sediments otherwise free from manganese. Similar beds an eighth of an inch thick and a foot or two long occur in the jaspery sediments east of the north fork of Shaw Cre.ek. The 22eade Creak and wardroper Creek deposits appear to be made up of s imilar larger and more closely spaced lonses separated by beds re la t ive ly free of manganese minerals. At the Meade Creek deposit the small lenses are re la t ive ly closely spaced and form discontinuous bodies two to three feet thick and extending along the str ike of the beds for 200 to 300 feet. 32 The wardroper Creek deposit i s similar but the manganese bearing lenses are much narrower* Internally rhodonite lenses are commonly massive but other oanganese s i l i ca tes are always well bedded (Pig.14). On a small scale, bods containing garnet and the yellow manganese mineral grade along the strike into lenses of massive rhodonite. The main H i l l 60 deposit from which ore hrs been shipped occurs in jaspery sediments that s tr ike on the average north 65° to 70° west and dip southwest generally at angles greater than 55°. /lthougn steeply dipping, the rocks are not contorted by minor folas . The ma i i i concen-trat ion of manganese minerals occurs as a lens-shaped body some 80 to 100 feet long and 30 or 40 feet thick with i t s long axis para l le l to the bedding of the sediments. Jaspery sediments containing small manganese-bearing lenses extend east and wost along the s tr ike of the sediments for over 400 feet from tho main deposit. Ore was probably shipped from a lerge open pi t some &u feet long 30 foet wide, end 15 or 20 feet deep. An ndit crosscuts beneath the p i t and a shaft extends below the main f loor of the p i t . The adit is caved and the best exposures of the manganese-bearing material are i n the walls of the p i t ana smaller open cuts beside i t . Two types of primary manganese-bearing material occur In the main deposit. Brown to red-brown massive rhodonite rock occurs on the hanging-wall side of the large p i t and along Its west end. In thin section some of this 33 dark rhodonite appears rs euhedral grains i n quartz, hut more commonly rhodonite i3 anhedral, occurring i n i r r e g u l a r masses intimately associated with quartz. Some specimens contain rounded brown fragments about three eighths of an inch i n diameter surroundod by dark red rhodonite. In t h i n section, the fragments appear to be mainly very f i n e -grained carbonate, possibly rhodochrosite with small amounts of quartz. These fragmental types rosemble the coarser pyroclastics of the upper part Of the Sicker sediments and may be of the same o r i g i n , although pyroclastics ?>s coarse grained as these have not beon found elsewhere i n the lower cherty members of the Sicker sediments. Most of the broken rock i n the bottom of tho p i t Is made up of pink rhodonite and a yellow manganese s i l i c a t e . Pink massive c r y s t a l l i n e rhodonite, commonly associated with the yellow s i l i c a t e outcrops along the north side of the big p i t . The yellow s i l i c a t e nearly everywhere occurs as microscopic grains i n quartz. The grains form interleaved 1 , laroelae, probably representing the o r i g i n a l bedding.(Fig.13) Rhodonite, on the other hand, occurs as cr y s t a l s up to several millimeters long and forms i r r e g u l a r masses and occasionally well defined veinlets c u t t i n g the yellow mineral. In the oast end of the p i t massive pink rhodonite contains i r r e g u l a r masses of carbonate, veinlets of ouartz, and lonses of Chalcopyrite. Th® quartz veinlets and sulfides do not form a w e l l defined zone, although they seem to be confined to the east end of the p i t . They do not extend across the s t r i k e beyond the normal l i m i t s of the manganese lens Into the cherty sediments. However, the rocks immediately to the east of the p i t are covered by overburden so that the east end of the manganese-bearing lens i s not exposed. Toward the west the outcrops show a gradual decrease i n manganese content u n t i l a l i t t l e over a hundred feet wost of tho p i t the rocks are greyish-whlte very cherty sediments containing no manganese minerals. The Cottonwood Crook deposit, less than h a l f a mile west of the head of Widow Creek, occurs i n t i g h t l y folded jaspery and chorty sediments probably within 200 feet of the base of tho Sicker sediments. The deposit, exposed i n several prospecting trenches seovers an area not more than 100 feet from north to south and 50 foet from east to west. I t i s made up of several small, t i g h t l y folded lenses containing manganese minerals widely separated by jospery sediments r e l a t i v e l y f r e e of manganese. P diagrammatic sketch of tho structure, deduced from a knowledge of the shape of surrounding minor f o l d s and f r o n c o r r e l a t i n g the attitudes of beds exposed i n tho trenches and outcrops near the deposit, i s shown i n P i g . 2. In addition to secondary oxides, rhodonite and rhodochrosite are the only abundant manganese minerals In tho Cottonwood Creek deposit, "within tho manganeso-bearing beds, some of tho rhodonite end quartz s t i l l shovi/s the outlines of bedding, but more commonly rhodonite occurs 3 5 as p o o r l y defined masses throughout the q u a r t z . In t h i n s e c t i o n the manganese-bearing rock appears as a mosaic of f i n e auartz grains c o n t a i n i n g masses of rhodonite that f o l l o w no w e l l defined v e i n l e t s or zones of replacement. Brown carbonate-boaring m a t e r i a l i n which bedding can be d i s t i n g u i s h e d occurs i n one l e n s , massive jasper present near the manganese lenses i s cut by i r r e g u l a r , but w e l l d e f i n e d , quartz veins and i n places contains p y r i t e . The Shaw Creek deposit i s h a l f a mile west of the n o r t h f o r k of Shaw Creek and 2% miles up the n o r t h f o r k from i t s j u n c t i o n w i t h the east f o r k . The d e p o s i t i s s i m i l a r to the Cottonwood Creek dep o s i t i n t h a t i t l i e s w i t h i n h i g h l y f o l d e d jaspery sediments near the base of the S i c k e r sediments. / n o r t h w e s t e r l y t r e n d i n g b e l t of s c a t t e r e d outcrops and trenches c o n t a i n i n g manganese minerals covers an area about 300 f e e t long and 100 f e e t wide. Llinor f o l d s t r e n d i n g n o r t h 20° west and plunging northwest at angles up t o 30° tend to obscure the r e l a t i o n of the manganese bodies to the sediments ( f i g . 3 ) . Small lenses c o n t a i n i n g manganese minerals l i e p a r a l l e l t o , and seuia to form beds w i t h i n the sediments. Larger bodies are made up of b l a c k s i l i c e o u s m a t e r i a l cut by I r r e g u l a r masses of pink rhodonite. Two such bodies exposed over wifiths of two to f o u r f e e t and over lengths Of 20 to 30 f e e t show no i n t e r n a l s t r u c t u r e . Bedding cannot be d i s t i n g u i s h e d i n hand specimens and t h i n s e c t i o n s of the black m a t e r i a l are opaque. The i r r e g u l a r masses of 36 rhodonite become more numerous beneath the surface and i t appears that bodies of black sil iceous .aaterial are mainly rhodonite and quartz that have weathered to dense si l iceous material containing black oxidos of manganese. The Sherk Lake deposit differs i n several respects from the others. I t occurs within a bod of Jasper two or three feot thick that l i e s at the base of tne Sicker sediments d i rec t ly on the surface of the upper flow of the Sicker andesites. The jasper is cut by well defined, irregular quartz, veinlets about % of an inch th ick . The manganese minerals occur as irregular masses in the Jasper the largest of which is about l g inches wide and s ix or eight inches long. The bed of jasper extends over 1000 feet along the contact but only in places does i t contain manganese minerals. Rhodonite, rhodochrosite and quartz are the main constituents of the manganese-bearing masses. In thin section the rhodonite appears to replace jasper and rhod'ochrosito replaces rhodonite. Size and Grado of the Deposits Ore shipped from the H i l l 60 deposit averaged, over 50 percent manganese and less than 20 percent s i l i c a . The low s i l i c a content indicates that.the ore was probably obtained from the oxidized surface of the deposit. | Analyses reported i n the minister of Mines Report (1918, 1919, and 1920) range from about 16 to 55 percent manganese and from 6 to over 60 percent s i l i c a . About the same range 37 i n manganese and s i l i c a content i s reported from the Shaw Creek deposit. Many of the samples highest i n manganese are lowest i n s i l i c a and i t Is probable that they were taken from residual oxides on the surface. In general the depth of residual oxide rarely exceeds two or three feet, although oxides are present i n joints i n tne bed rock to greater depths. Picked samples of s i l i ca tes from below th« layer of oxides taken by Dr». Sargent In an attempt to determine the probable tenor of the primary mineralization ranged from about 14 to 30 percent manganese and were probably high i n s i l i c a . At the H i l l 60 deposit the lens of highest grade material i s about 30 feet wide and somewhat over 60 feet long on the surface. 0pa>pit work and crosscutting have shown that the depth extent of the main lens ' i s comparable to i t s length on surface. The Shaw Creek deposit i s of about the same dimensions as the H i l l 60 deposit but i s more highly folded. The other deposits are a l l of much smaller s i ze . Thus, while some of the more highly mineralized material i s of commercial grade, none of the deposits so far discovered i s large enough to be of e economic value. Attempts to f ind residual deposits in the valleys and depressions below known primary s i l i c a t e deposits have been unsuccessful. Glacial erosion has probably destroyed any residual concentrations that may have accumulated i n pre-Glacial times* 38 Features Indicating the Origin of the Deposits Many features of the manganese deposits indicate that they "are of sedimentary or ig in and were formed at the same time as the enclosing sediments. Other features indicate that they were formed la ter than the sedicients by replacement of certain beds by manganese-bearing solutions. The most significant features of tho deposits suggesting their or igin are the following. (1) The deposits are confined to tne lower cherty beds of the Sicker sediments and although small bodies of s imilar cherty sediments occur elsewhere i n tne series, none of them contain manganese. The fact that no manganese deposits have been found i n the upper members of the series suggests that the manganese had been incorporated in the lower beds before the upper ones were l a i d down, or that for some unknown reason, the lower beds were the only ones suitable for mineralization. (2) The general shape of the larger bodies could have resulted from either replacement of the sediments or from sedimentary deposition of the manganese. In many places the manganese i s confined to beds that show primary structures, minor faults and, small folds ident ica l witn those of the surrounding sediments. ( F i g . l b ) . In other places, especially where beds containing small lenses of manganese minerals occur i n s l i gn t ly folded rocks, the conformable re la t ion of the manganese-bearing beds to the 39 surrounding sediments i s very marked. Such features suggest that the manganese deposits^formed by sedimentary processes. ^ (3) The rocks of the manganese deposits are recrysta l l ized chert containing l i t t l e beside quartz and manganese minerals. The fact that they grade along str ike into normal cherty tuffs suggests that they have formed by s i l i c i f i c a t l o n of cherty tuffs . On the other hand, the manganiferous cherts may have formed as lenses of s i l i c a into which no tuffaceous material was deposited. The abundance of si l iceous oolites and the absence of any evidence, either textural or mineralogical, of angular feldspar grains might suggest a sedimentary or ig in for the s i l i c a . (4) The cross-cutting and replacing features of the rhodonite within the manganese lenses strongly suggest a replacement or igin for the deposits. Passive rhodonite either in lenses or Irregular masses replaces surrounding chert and manganese-bearing beds i n nearly a l l the deposits and i n places, small woll-defined quartz-carbonate-rhodonite veinlets are present* Rarely, as at the h i l l 60 deposit sulfides also are present. On the other hand, replacing and cross-cutting relations are seen only within the manganese-bearing lenses. With the possible exception of the Shark Lake deposit, no manganese-bearing veinlets or replacements have been found either along edges of larger lenses or by themselves in 40 bods free from manganese. Small veinlets similar to those in the manganese deposits occur in the tuffs away from the deposits,' In rocks rich in plagioclase the veinlets contain quarts; and plagioclase and i n cherty rocks they are mainly quartz, Tho material in these veinlets seems to have been derived from the surrounding rocks, and It seems possible that the rhodonite-quartz veinlets In the manganese lenses formed In a similar way. Similarly, the irregular masses of rhodonite may hove formed by reconstltutlon of the manganiferous cherts. 41 CHAPTER I I I ORIGIN OF MEKQ/HESE DEPOSITS Fie ld and laboratory studies-indicate two possible methods of formation of the Cowichan Lake manganese .deposits. They may havo formed either as replacements of cherty tuff by hydrothermal manganese-bearing solutions or as manganese oxides or carbonates tnat were l a i d down with the cherty tuff end wore lf>ter converted to manganese s i l i c a t e s . In an attempt to determine the characteristic features and relat ive importance of these processes, and the transformations involved in tho formation of manganese deposits by them, a study was made of the published writings on the genesis of snangnnose deposits, lifenganese deposits may be syngenetic or epigenetic. Syngenetlc deposits Include those of sedimentary either marine or t e r re s t r i a l o r ig in , while epigenetic deposits include hydrothermal replacements and residual deposits formed by weathering and concentration of manganese from any of the other types. A l l deposits may be metamorphosed so that their ultimate or ig in i s obscure. The Cowichan Lake deposits show no s imi la r i t i e s in primary mineralization to rosidual or s t r i c t l y hydrothermal deposits but closely resemble metamorphosed marine sedimentary deposits. The following chapter b r ie f ly summarizes the method of formation of manganese deposits i n oceans, describes some features of metamorphosed deposits, and shoivs how the Cowichan Lflko occurrences may have formed by metamorphism of sedimentary deposits. 42 Marino Sedimentary Deposits  Sources of Manganese Manganese deposits of sedimentary or ig in are formed either by concentration of the small amounts of manganese or ig ina l ly present i n igneous and sedimentary rocks or by tho deposition of manganese from volcanic sources. Most igneous and sedimentary rocks contain small amounts of manganese; i n igneous rocks i t comroonly occurs as s i l i ca tes and i n sediments as carbonates and oxides. Hanson (Hanson 1952) has tabulated the manganese content of igneous rocks given by Daly (Daly,1914) and Clarke (Clarke,1920). Clarke f s analyses vary from.0.02 percent Mn 0 i n granites and rhyolites to u.15 percent . Mn 0 i n basic igneous rocks. Analyses given by Daly vary from 0*14 percent i n acid types to 0,25 percent En 0 in basis types, there being a regular increase i n the manganese content with a decrease i n the ac id i ty of the rock. This may be due to tho fact that minerals abundant i n basic rocks, such as pyroxene and ol ivine tend to contain more manganese, either as impurities or as essential constituents than do minerals abundant i n acid rocks such as a l k a l i feldspar and quartz. Thus igneous rocks, which form about 95 percent of tho llthosphera, contain small but significant amounts of manganese. The manganese content of 3euimontary rock3 varies, but i n general shales and sandstones contain less manganese 43 then limestones and dolomites. S ix analyses of sandstone and chert given by Wells (Wells 1937) i n which manganese i s reported average 0.03 percent Mn 0 and two analyses of shale average 0.01 percent. In over twenty samples of normal limestone i n which manganese i s reported, some contain no manganese and others as much as 6 percent MnO. tfanganese may be present i n c l a s t i c sediments as carbonates or oxides In the cementing material, as grains of oxides i n very f i n e c l a s t i c sediments, or as grains of carbonates or s i l i c a t e s i n coarser sediments. Transportation of Manganese In the normal processes of rock weathering and erosion, the manganese i n igneous and sedimentary rocks i s released. According to Clarke manganese Is taken Into solution as the carbonate or su l f a t e . Dale notes (Dale 1915) that manganese In r i v e r water result s from the solu t i o n of manganiferous s i l i c a t e s , the manganese being converted f i r s t to the carbonate or oxide and l a t e r carried i n solution as the bicarbonate. Experimental work by Savage (Savage,1936) showed that "manganese i n primary d i s t r i b u t i o n i n rocks i s taken into solution c h i e f l y by the action of percolating carbon-ated waters", but carbonic a c i d , formed by decaying organic matter i s also e f f e c t i v e . The experiments also showed that manganese Is oarried i n solution c h i e f l y as the bicarbonate.. In a l k a l i or neutral solutions the manganese bicarbonate 44 sp l i t s up and unstable manganous hydroxide forms. Oxidation and precipitat ion of Mn Og may be prevented by the presence of organic matter i n the manganese-bearing waters. Under these conditions manganese might be transported as a mixture of the bicarbonate and of oxides. Manganese i s present i n most r ivers and spring waters i n small amounts. Analyses of fifteen samples of Miss iss ippi r iver water taken by weib (Hanson,1932) show the manganese content to range from 0.044 to 0.128 parts per m i l l i o n . Prom these analyses Hanson has shown that some 40,000 tons of manganese are discharged into the Gulf of Mexico every year by the Mississ ippi River, ulunielpal water supplies are commonly contaminated by manganese and much work on the precipi tat ion of manganese c has been done by ghemists interested in purifying domestic water supplies (see Twenaofe1,1926, and Zapffe 1931). Spring waters i n general contain more manganese than r iver waters, Twenhofel states that "the content (of spring waters) ranges from a trace to 117 parts per m i l l i o n ; commonly i t ranges from 0,5 to 5 parts per m i l l i o n . " (Twenhofel,1926). According to Clarke (Clarke,1924) carbonate and sulfate waters are r e l a t ive ly high in manganese while chloride solutions are generally low, Dittmar (Clarke 1924) states that manganese can readily be detected i n sea water but gives no analyses, and hence the change i n the manganese content of r ive r waters as they pass into the sea cannot be estimated. 45 Deposition of fi3angariese in Sea Water Savage has carried out experiments on the precipitat ion of manganese and concludes that "a bicarbonate solution of manganese must become neutral or s l i gh t l y alkaline before precipi tat ion can tako place,"and manganese may be precipitated f i r s t as hydrated manganous oxide (Mn 0.0H) that may later be oxidized to Mh Og, The following equations may represent the reactions: Ravage 1936) 2 Mn (HC0 3) 2 • 4H20 - 2 Mn (0H) 2 * 4H20 * 4Ce2 2 Mh (OH)g 4 4H20 * 0 » 2 Mn 0.0H * 5H20 2 Mn 0.0H * 5H20 •» 0 - 2 Eta Og * 6H20 Thread bacteria and the catalyt ic action of MnOg are Important i n increasing the rate at which Mn Og is precipitated (Zapffe,1931). Manganese in deep sea sediments is well known but the processes by which the manganese Is precipitated are not well understood. I t is possible that ita Og, either from shells and bodies of marine animals that have sunk to the sea bottom or from rocks along the sea f loor , may act as a catalyst i n aiding the precipitat ion of manganese oxides suspended or dissolved i n the deep sea waters. The manganese may occur as coloring matter i n the sea sediments, as coatings on the sea f loor , or as manganese nodules (Twenhofel,l926) ,x Finely divided coloring matter 46 i n deep sea sediments Is found to be raaae up of manganese oxides and hydroxides. Samples of sediments from the north At lant ic Ocean contain from a trace to over four percent manganese. (Carrens 1939). Red muds contain most, but blue muds and globegerina ooze also contain manganese. Coatings of manganese oxides on shells and stones on the sea bottom are reported from the Clyde Sea (Twenhofel 1926) and manganese oxides coating the rocky sea bottom are reported along the shores of the East Indian /rchipelago (Carren.1939). Manganese nodulos are common in many of the worlds largest manganese deposits and are found to be forming in the present seas. Manganese appears to be precipitator mainly as the oxide in marine deposits, but manganese carbonate may be precipitated f*om bicarbonate solutions in the presence of limestone. The following equation represents the reaction (Savage 1936) Mn (HC0 3) 2 * Ca C0 3 * Ca (HC0 3 ) 2 + Ufa C0 3 Twenhofel points out that manganese carbonate i s not precipitated except by replacement of pre-existing rock, generally a carbonate (Twenhofel 1926) Manganese of Volcanic Origin Volcanic springs have been suggested as the source of several manganese deposits such as tho ss of Cuba and H a i t i , California (Taliaferro and Hudson 1943) and of the Olympic Peninsula (Pardee 1927), Analyses of hot-spring waters, whioh might reveal the importance of a volcanic source of manganese, rarely report the aanganese content. 47 Sixteen analyses of hot-spring waters from the St. Helena Range i n C a l i f o r n i a vary from zero to 2.2 percent MnO and are high i n s i l i c a and sulfates (Allen and Day, 1924). A l l e n and Day note, i n addition that "manganese i n small quantities i s no doubt present i n many (hot) spring waters where i t has not been looked f o r , " Analyses of hot spring waters of Yellowstone National Park reported by Oooch and Whitfield (Oooch and Whitfield 1888) are low i n manganese. A l l 38 samples were tested f o r manganese, four contained only traces the remainder contained no manganese. Analyses of volcanic gases and hot-spring waters from Lassen. National Park (Allen and Day 1925) do not report the manganese content but soluble s a l t s , e s s e n t i a l l y s u l f a t e s , that are products of fumerolic a c t i v i t y , contain from 0.01 to 0.07 percent metallic manganese. Shipley reports that manganese, i n places i n considerable q u a n t i t i e s , i s present i n many encrustations around fumeroles i n the Katmai region i n Alaska. One encrustation contained 17.3 percent MnO (Shipley 1920). Thus, although the data are few, i t seems probable thet small amounts of manganese are present i n many volcanic springs and that large quantities of these waters might constitute a source of s i g n i f i c a n t amounts of manganese. Tweiihofel points out the close association between volcanic a c t i v i t y and .nanganese deposits, Manganese nodules 48 are most.abundant on the shores of volcanic islands end there appears to be a genetic relationship between volcanic material and manganese i n marine deposits. I t seems probable that conditions of deposition of manganese of volcanic or ig in would be s imilar to those . controll ing the deposition of manganese derived from r iver waters. Volcanic a c t i v i t y may be important not only in providing a source of manganese, but also in bringing about conditions favorable for the precipi tat ion of manganese. Volcanic ac t iv i t y may increase the temperature of the sea ' water, leading to a growth of bacteria and an increased rate of chemical reaction, or may provide agitation leading to a loss of carbon dioxide or to oxidation. Such conditions would favor the precipitat ion of manganese oxides. The presence of limestone, possibly precipitated by volcanic ac t i v i t y , may aid in the formation of manganese carbonate. Summary Thus, manganese or ig ina l ly present i n igneous and sedimentary rocks may be released and transported by rivers as manganese bicarbonate or as oxides, and on reaching the see be deposited as oxides or as the carbonate. S imi la r ly , manganese entering the sea froiij volcanic sources may be deposited as oxides or as the cerbonato. Theoretically shallow basins near shore would be well suited to the deposition of manganese. Grains of oxides formed by oxidation in r ivers , by change of a l k a l i n i t y on reaching the sea, or by oxidation in shallow sea waters, would be 49 deposited along with fine sediments near shore, while manganese i n solution might react with limey beds to form manganese carbonate. Shallow,warm, agitated waters of seas i n volcanic regions would be par t icular ly favorable for the deposition of manganese either oxides or the carbonate. Manganese either as suspended oxides or dissolved bicarbonates that become carried farther out to sea might be precipitated as oxide encrustations, nodules, or fine coloring matter i n deep soa oozes. Metamorphosed Manganese Deposits Manganese deposits of s t r i c t l y hydrothermal origin that show features similar to those of Cowichan Lake seem to be rare or non-existant. Several deposits that are thought to nave formed by the metamorphism of sedimentary deposits, however, show features that closely resemble those of the Cowichan Lake deposits. Deposits of the Olympic Peninsula, Washington are s imilar in some respects to those of Cowichan Lake. (Pardee 1921, 1927, Park 1942, and Green 1945). They occur i n a series of Eocene rocks made up of interbedded basaltic flows, pyroclastics, c las t ic sediments, and some limestone. The manganese deposits are lens shaped or tabular and l i e pa ra l l e l to the bedding of the surrounding sediments. They are usually in red si l iceous limestone and are always associated with basaltic flows. The main minerals are bementite (hydrous manganese s i l i ca t e ) and 50 hausmannite (Mn3 0 4 ) . Rhodonite and rhodochrosite are rare hut jasper i s abundant. Tho deposits are. thought to have formed as sedimentary manganese carbonate deposited with limestone and la ter converted to s i l i ca t e s and oxides by hydrothermal solutions and i n part by volcanic a c t i v i t y . Hausmannite probably formed under special oxidizing ' conditions while s i l ica tes formed by reactions with s i l i c a -r ich solutions. The manganese deposits of California are s t r ik ing ly s imilar to those of Cowichan Lake. (Talioferro and Bas£dson,1943) • Thoy occur i n Jurassic chert and quartzite as pods and lenses lying para l le l to the bedding of the sediments. Orebodles i n the Coast Ranges contain mainly rhodochrosite cut by quartz veinlets , but i n some bementite, hausmannite, and neoctocite are abundant. In deposits of the Sierra Nevada the pr incipal primary manganese mineral is rhodonite, but spessartite is usually present and In places i s abundant. Rhodochrosite and bementite are minor. A l l the eal i fornia deposits In Jurassic chert are thought to have formed as sedimentary manganese carbonate that accumulated i n marine basins. Deposits of the Coast -Ranges are only s l igh t ly metamorphosed and contain mainly or ig inal manganese minerals. Rhodonite-spessartite deposits of the Sierra Nevada occur only i n metaraorphle rocks and hence are thought to be metamorphosed sedimentary carbonate deposits of the same ultimate origin as those of the Coast 51 ranges. The source of tho manganese in both the Washington and California deposits i s thought to have been related to volcanic a c t i v i t y . The reactions involved i n the formation of rhodonite and spessartite may be similar to those involved i n the formation of other pyroxenes and garnets by metamorphic processes. Under heat and pressure the sil iceous carbonate may broak down to form rhodonite. Mn C0 3 • SlOg s Mn SCOg * COg Impurities other than s i l i c a may give r ise to other s i l i c a t e s . Spessartite readily forms by thermal or regional metamorphism of sediments containing either large or small amounts of manganese carbonate or oxides. Manganese-bearing garnet i s found- i n the chlorite and b io t i te zones of regional metamorphism and differs i n this respect from other garnets that are found only in zones of higher grade metamorphism. Origin of the Cowichan Lake Deposits  Conditions of Formation of.the Sicker Sediments Clapp has outlined the conditions of formation of the Vancouver group (Clapp and Cooke,1917) and many of his theories have been substantiated by the work i n the Cowichan Lake d i s t r i c t . The long period of volcanic ac t iv i t y during the Triassic and Jurassic began with outpourings of great quantities of lava. The fact that the volcanics are closely related to tho Sutton limestone, which formed at least i n part by the accumulation of marine 5 2 organisms, suggests thet the flows say have been sub?nsrlne. Mo d i r e c t evidence, nosever, f o r tho submarine o r i g i n of the Sicker andesites was found i n the Coadchon Lake pres. The occurrence of angular and u n s t r a t i f i e d t u f f s In the Duncan area l e d Clapp to believe that volcanic vents above sea l e v e l formed volcanic islands on the shores of which limey beds accussuloted. In the Cowichan Lake area there i s abundant evidence that the Sicker sediments are water l a i n and probably of aarlne o r i g i n . The cherty t u f f s are w e l l bedoed; in d i v i d u a l beds contain sorted grains and show gradations In grain s i z e ; bedding planes are marked by l o c a l unconformities possibly poorly preserved r i p p l e marks; and some l l s e y t u f f s ere cross-bedded. The presence of s i l i c e o u s o o l i t e s i n the f u f f a indicates thet they aire water l a i n f>nd probably asrlne, end that chemical p r e c i p i t a t i o n was taking ylace P t the some time as pyroclastic grains wore being deposited. Further evidence of the marine o r i g i n Is provided by the close association of limestone lenses and the t u f f s . Limestone lenses rsngc. from limey t u f f s containing only o small proportion of llsaey Material to r e l a t i v e l y pure limestone.. The Sicker sediments may thus have forced i n r e l a t i v e l y shallow sees around volcanic islands or near ao:oe larger center of volcanic a c t i v i t y . Volcanic a c t i v i t y may have added manganese to the s«as, end favorable conditions of p r e c i p i t a t i o n may have resulted i n tho deposition of 5 3 manganese-along with other chemical sediments, Conclusiona F ie ld and laboratory evidence substantiated by theoretical considerations suggests that the Cowichan Lake manganese occurrences formed as sedimentary deposits. Manganese, possibly derived from volcanic springs, may have been deposited as oxides, or under special conditions as the carbonate. Manganese seems to have accumulated in regions where s i l i c a v/as being deposited most rapidly, such as regions close to volcanic springs supplying abundant s i l i c a and manganese or regions in which conditions of chemical deposition were most favorable. Subsoauent bu r i a l , folding, end metamorphism, possibly aided by hydrothermal solutions converted the oxides or carbonates to s i l i ca tes and produced the replacement and cross cutting relations seen in many of the deposits, upl i f t and erosion exposed the manganese s i l i ca tes to the atmosphere, and weathering has formed the coating of black oxides, the stringers of neoctocite, and possioly some of the carbonate tnat. replaces the s i l i c a t e s . 54 BIBLIOGRAPHY A l l e n , E . T . , and Day; A . L . (1925) The volcanic a c t i v i t y e and hot springs of Laaaen Peak, Carnagi# Institute of Washington. (1925) Steam wells and other  thermal ac t iv i t y at "The Goyaers" California Carnegie Institute of Washington. Garrens, C a r l . W. (1939) Recent marine sediments, American Association of Petroleum Geologists, Synposium Clapp, C.H. (1912) Southern Vancouver Island. Geol. Surv. Canada, Memoir 13. (1913) Geology of the Victor ia and Saonich T.tap  areas, Geol. Surv. Canada Memoir 36. (1914) Geology of the Nanaitno map-area, Geol. Surv. Canada ivlemoir 51. Clapp, C.H. and Cooke M.C. (1917) Sooke and Duncan map-areas, Vancouver Island, Geol. Surv, Canada Memoir 96. Clarke, F.W. (1920) The oata of geochemistry, U.S. Geol. Surv. Bul l No. 695. (1924) The data of geochemistry, U.S, Geol, Surv. Bul l No, 770. Dale, N.C. (1915) The Cambrian manganese deposits of Tr in i ty and Conception Bays, Newfoundland, Amer, P h i l . Soc. Vo l . 54 pp 371-456. 55 Daly, R.A.. (1914) Igneous rocks and their o r ig in , McGraw-Hill. Gooch, F . A . , and Whitfield, J . E . (1888) Analyses of waters of the Yellowstone National Park, U.S. Geol. Surv. Bul l No. 47 Green, S.H. (1945) Manganese deposits of the Olympic Peninsula, Washington, Washington Divis ion of Mines and Mining, Report of Investigations No. 7 Hanson, G. (1932) Manganese deposits of Canada, Geol. Surv Canada Econ. Ser. No. 12. Pardee, J .T. (1921) Deposits of manganese ore In Montana, Utah, Oregon, ana Washington, U.S. Geol. Surv. B u l l . No. 725 C (1927) Manganese hearing deposits near Lake Crescent and humptullps Washington, U*k* Geol* Surv. B u l l . No. 7 9 5 / . Park.-C.F. J r . (1942) langanese resources of the Olympic Peninsula Washington, a preliminary report. U.S. Geol. Surv. B u l l . 931 R • Savage, W.C. (1936) Solution, transportation and precipitat ion of manganese. Econ. Geol. V o l . 31 pp 278-297* Shipley, J.w. (1920) Volcanic emanations in Alaska, fmer. Jour. S c i . 3rd series, p 146. Taliaferro, N.L. and Hudson, F*S. et a l (1943) Manganese i n Cal i forn ia , State Division of Mines, B u l l . No. 125. 56 Twenhofel, W.H. (1926) Treatise on Sedimentation, Williams and Wllkins Co. Baltimore. Wells, R.C. (1937) Analyses of rooks and minerals from the laboratory of the U.S. Geological Survey 1914-1936, U.S. Geol. Surv. Bul l No* 878. Zapffe, Car l . (1931) Deposition of manganese, Econ. Geol, V o l . 26 p 799. 58 Vig.l Index map showing the petition of the cowichan Lake amp are*.. ( croe* hatched nrea) 5 9 £» v ^ j,N \ S v N I I J « > < > f n 4 ED f\A A N ( AM K S C O w T C B O P S ^ x • * • s / t • / j s / / SCALE I ' - ' E O '1 Fig.2 Sketch map showing the structure of part of the Cottonwood creek manganese deposit. N \ V \ V s \ \ "\ \ \ N * v >o-3o* \ • S l D I M I N T S V - \ S ••.';.*."'.';• O U T C R O P S • -y; Pig.3 Sketch map showing the structure of the Shaw creek manganese deposit. F i g . 4 Steeply-dipping cherty tuffs (east of widow Greek.) F i g . 5 Tightly-folded cherty tuffs (west of miaow Creek.) F i g . 7 Fragments1 flow top near the top of the Sicker andesites (east siae of widow Creek) F i g . 8 Poorly developed columnar joint ing in the flows above the Sicker sediments (west slope of M i Landalt) F i g . 9 Photomicrograph of typical cherty tuff showing lens of angular grains of nndesine i n fine cherty material (xl3 crossed nicols) 63 •frig. 10 Photomicrograph snowing si l iceous oolites i n cherty tuff (x l3 , crossed nicols) F i g . 11 Photomicrograph showing sil iceous oolites in manganiferous chert (xl3) 64 Fig* 12 P h o t o n i c r o g r s p h s h o w i n g r u o u o n i t e c r o s s -c u t t i n g end r e p l a c i n g c h e r t y beds* R h o d o n i t e (anu manganese o x i d e s ; b l a c k , b e d d i n g a l m o s t h o r i z o n t a l ( x l 3 p l a n e l i g h t ) F i g * 15 P o l i s h e d s u r f a c e of a s p e c i m e n from tne h i l l 60 d e p o s i t s n o w i n g banded y e l l o w manganese s i l i c a t e and i r r e g u l a r masses o l r n o d o n i t e . (1) y e l l o w s i l i c a t e rind q u a r t z . {&) r h o u o n i t e ana q u a r t z . l » ) Quarts v e i n l e t . 65 F i g . 14 rolishfco. surrace of bedoea mangsniferous cuert from Meade Creek deposit. (1) lens of rhodonite. (2) garnetif erous bed, (3) lens containing siliceous ool i tes . (4) manganese oxides related to surface fractures. F i g . lb Faulted beds of yellow manganese s i l i c a t e i n jasper from near the u i l l 60 ueposit. (1) fault (2) yullow s i l i ca t e cor ted witu oxides (3) Jasper. * F i g 16.X-ray powder-p h o t o g r a p h of q u a r t s F i g 17.X-ray powder-p h o t o g r a p h of q u a r t z and y e l l o w manganese s i l i c a t e F i g 18X-ray powder-p h o t o g r a p h of s p e s s -a r t i t e . G-en 6U> S T R U C T U R A L SECTIONS N O R T H O F C O W I C H A N L A K E B O H O R I Z O N T A L S C A L E \ l W = 2 6 4 0 F T V E R T I C A L S C A L E 2 5 0 0 " cc u \ i 1 in 4000 3 0 0 0 2 o o o SSAUVSU 4 0 0 0 3 0 0 0 2 0 0 0 , 0 0 0 SEA U v e u S e CT \ o t>» A A ' S E C T I O N B - D ' 4000 3000 2 0 0 0 1000 u 0 L I z 0 y r 0 u 4 0 0 0 -3 0 0 0 — 2000 \-A \ o o o l O S E C T I O N D - D ' P P E R P P E R C R E T A C E O U S ""] H a s l a m F o r m a i i o n -» S h a l e ~~| B e n s o n F o r m a t i o n —' Conglomerate £ Sandstone J O R A S S V C M i n o r I n t r u s i v e s M a i n l y G a b b r o £ Diori te . 5 a a m c h G r a n o d i o r i t e G r a n o d i o r i t e Q u a r t z M o n z o n i t e L O W E R J U R A S S I C and/or- U P P E R ( " R I A S S I C A n d e s i t e £ B A S A L T ] S i c k e r S e J i m e n t s C K e r t x y T u f f s i J C o o r s e r P j r r o c l a s T i C S S i c k e r A n d e s i t e s M o m l y F l o w s b^t vrvdudirvg m i n o r S e d \ m « n t s c * i n 1 r u s \ v e s d O O O 3 0 0 0 Z O O O l O O O SUA L e v e l . S E C T I O N E ~ E ' S E C T I O N F" — F " 1 F ' 3 o o o E O O O I O u t S6.A L e v t s i H S E C T I O N H - H ' H' 4C L K~ tf7 74/ 63,9 7537 e?7 ."535 4g r^r-. TL3346.P 

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