Sedimentological controls on gold in a late Pleistocene glacial placer deposit, Cariboo Mining District, British Columbia, Canada

Sedimentary Geology, 65 (1989) 45-68 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

45

Sedimentological controls on gold in a late Pleistocene glacial placer deposit, Cariboo Mining District, British Columbia, Canada NICHOLAS

EYLES

and STEPHEN

P. K O C S I S

Glaciated Basin Research Group, Department of Geology, Unioersityof Toronto, Scarborough Campus, Scarborough, Ont. MIC 1A4 (Canada) Received February 7, 1989; revised version accepted June 13, 1989

Abstract Eyles, N. and Kocsis, S.P., 1989. Sedimentological controls on gold in a late Pleistocene glacial placer deposit, Cariboo Mining District, British Columbia, Canada. Sediment. Geol., 65: 45-68. It is a widely perceived notion that glaciation results in dispersal of mineralized bedrock and that sedimentary concentrates of economic minerals (placers) rarely occur in glaciated basins. This paper describes economic gold placers within late Pleistocene glacial and related fluvial sediments of the Cariboo Mining District in central British Columbia, Canada. The area has been defined asa "giant" gold placer; total production since 1858 is over 93,000 kg. The oldest and volumetrically largest placers occur in fluvial gravels and valley-side fan deposits deposited during a long non-glacial interval from as early as 125,000 to 30,000years B.P. The richest placers are found along bedrock "gutters" in the deepest parts of valleys, indicating repeated fluvial reworking of the valley infiUs. Braided and "wandering gravel bed" fluvial facies can be identified. Glacial placers, that overlie the fluvial placers, occur within lodgement till complexes deposited below the late Wisconsin Cordilleran ice sheet after 30,000 years B.P. The basal portions of lodgement tills are commonly enriched in gold as a result of incorporation from older gravels. Subglacial meltwaters created a highly effective sluicing system and left lucrative pay zones along meltwater-cut channels on bedrock benches, within intraformational gravels in lodgement till and within "lee-side" deposits down-ice of bedrock highs. "Lee-side" deposits are essentially water-worked talus slopes that accumulated in subglacial cavities. Finally, postglacial "wandering gravel-bed rivers" have repeatedly reworked older placers resulting in rich pay zones at the base of extensive bar platform deposits. Similar sedimentoiogical controls on gold distribution can be identified in other glacial placers of late Cenozoic and Paleozoic age in North America, southern Africa and Australia. A distinction is drawn between these placers, all characterized by coarse-grained, nuggety gold, and the more weU-known Precambrian and Paleozoic placers where finely-comminuted gold is dispersed through large thicknesses of rock. Episodes of glaciation typically occur after long periods of tropical and subtropical weathering when supergene processes were active and glaciers were able to remove and concentrate coarse gold. In contrast, gold in non-glacial placers of Precambrian and Paleozoic age has been through many cycles of erosion and transport and coarse gold is uncommon.

Introduction E a c h y e a r C a n a d a p r o d u c e s o v e r 100 t o n n e s o f

c i o u s m e t a l s , is r e c o v e r e d in w e s t e r n C a n a d a f r o m a w i d e r a n g e o f Q u a t e r n a r y a n d T e r t i a r y sedim e n t s b u t t h e s t r a t i g r a p h y a n d o r i g i n o f t h e de-

g o l d f r o m lode, b a s e m e t a l a n d p l a c e r s . O f this

p o s i t s is still p o o r l y u n d e r s t o o d

total, p l a c e r g o l d a c c o u n t s for a b o u t 4.5 t o n n e s

history

with the bulk of production shared between British

1983).

Columbia

(e.g. S l i n g e r l a n d a n d S m i t h , 1986) s e d i m e n t o l o g i -

and the Yukon. Gold, and other pre-

0037-0738/89/$03.50

© 1989 Elsevier Science Publishers B.V.

of working In contrast

such

deposits

despite a long (MacDonald,

to p l a c e r d e p o s i t s

elsewhere

46

N. E Y L E S A N D S.P. K O C S I S

cal studies employing facies analysis techniques, have only just started (e.g. Morison and Hein, 1987). This paper decribes the geology and sedimentology of late Pleistocene glacial placers on the western flanks of the Cariboo Mountains in north-central British Columbia (Fig. 1). The Cariboo district is a pre-eminent gold mining area with a long history of production and excellent

prospects for future development. Gold production commenced in 1858 and whereas there is a substantial, largely anecdotal history of mining (Eyles and Kocsis, 1988a) geological details are elusive. The total placer gold production to data is close to 93,000 kg and the area was defined as a "giant" gold placer by Henley and Adams (1979). Prolific gold placers occur in relatively young

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Fig. 1A. Location of study area, in north-central British Columbia, Canada, showing the distribution of Quesnel (Q), Barkerville (B), Slide Mountain ( S M ) and Caribou (C) Terranes. Numbers identify the 35 placer mines studied; mines within the Wells-Barkerville area (inside box) are shown in Fig. 2. Graphic logs of the sedimentology of placer mines 1-21, showing the distribution of payzones, are shown in Figs. 5 and 10. 1 = Alice Creek; 2 = Lightning Creek (east of Mustique Creek); 3 = Williams Creek (Ballarat); 4 = California Gulch; 5 = Lightning Creek (Lookout Point); 6 = Grouse Creek (Heron Channel); 7 = Eight Mile Lake (lake bottom); 8 = Mary Creek (Toope "A"); 9 = Spanish Mountain; 10 = Eight Mile Lake (west side of lake); 11 = Mary Creek (Toope "B"); 12 = Cunningham Creek (MrPherson); 13 = Mount Nelson (Werner); 14 = Pinus Creek (Hatton); 15 = Tregillus Lake (Quesnel Ready-Mix); 16 = Alces Creek; 17= Porter Creek; 1 8 = Lightning Creek (Romano); 19 = Wolfe Creek; 20 = Tregillus Lake (northeast corner); 21 = Nugget Gulch hydraulic pit; 22 = Lightning Creek (west of Moustique Creek); 23 = Sovereign Creek (Allen); 24 = Little Swift River; 25 = Cunningham Creek (VanHalderen); 26 = Pine Creek; 27 = Bullion Pit; 28 = Coulter Creek; 29 = Point Bench and 30 = Ketch Bench along Slough Creek; 31 = Devils Lake Creek (Mount Burns); 32 = Burns Creek (Bjornson); 33 = Fosters Ledge (upper Lightning Creek); 34 = Mosquito Creek (Drinkwater); 35 = Lowhee Creek.

SEDIMENTOLOGICAL

CONTROLS

ON GOLD IN A LATE PLEISTOCENE

GLACIAL

PLACER DEPOSIT

47

Fig. lB. Satellite image at approximately the same scale, of area shown in Fig. 1A from Fraser River in west to Cariboo Lake in east. W = Wells; B = Barkerville. Landsat-5 image number; Band 4, Scene ID: 50515-183009.

Pleistocene sediments ( < 125,000 years B.P.) including subglacial deposits that accumulated below the Cordilleran Ice sheet after 30,000 years

B.P. Eyles and Kocsis (1988a) provide a summary of the evolution of mining methods in the Cariboo district up to the present day. Hydraulic mining methods of the past, which mined large low-grade deposits by sluicing, cannot be employed given stricter environmental controls. M o d e m operators have to be much more selective and a more detailed appreciation of the geology, origin and likely three-dimensional distribution of the Quaternary

deposits is required. This paper identifies the principal sedimentological controls on the distribution of pay zones; an exploration model developed for the Cariboo district can be applied to other glacial placer deposits of Quaternary and Pre-Quaternary age.

Physical setting and bedrock geology The present study area of about 1600 km 2 is centred on the two historic mining communities of Wells and Barkerville and extends southwards to Likely. The principal gold-producing valleys have

48

N. E Y L E S A N D S.P. K O C S I S

been Lightning and Williams Creeks (Figs. 1, 2). The area consists of an undulating but deeply-dissected plateau with mountains to 2100 m a.s.1. Narrow, often deeply-cut canyon-like valleys contain many tens of metres of Quaternary sediments and several buried valleys have been identified (e.g. Clague, 1987; Fig. 3). Elevations above 1700 m a.s.1, are generally drift-free as a result of late glacial mass wasting which has been most intense on south-facing slopes. Along many creeks Pleistocene sequences are preferentially preserved

B~G

along valley floors, on north-facing bedrock benches and down-ice of bedrock highs. The study area lies in an area of complex bedrock geology consisting of allochthonous terranes separated by moderately to steeply east-dipping thrust faults that trend northwest-southeast. Four terranes have been recognized in the area, each with an overlapping relationship as a result of dextral accretion of crustal blocks along the Precambrian and Paleozoic margin of North America (Price et al., 1981; Jones et al., 1983;

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Fig. 2. Distribution of the Downey Creek succession (strippled) relative to placer operations and hardrock gold mines in the Welis-Barkerville area (Fig. 1). Numbers identify location of placer operations noted in the text. Graphic logs from pits 1-21 are shown in Figs. 5 and 10.

SEDIMENTOLOGICAL

CONTROLS

ON GOLD

IN A LATE PLEISTOCENE

Howell et al., 1987). From west to east, Struik (1986,1988) identified Quesnel, Slide Mountain, Barkerville and Cariboo Terranes separated by the Eureka, Pundata and Pleasant Valley faults. The study area lies almost entirely within the Barkerville Terrane (Fig. 1) which is composed of about 2 km thickness of Late Proterozoic and Paleozoic limestones, pelites, volcanic tufts, quartzites and conglomerates. The Barkerville terrane has been divided stratigraphically into the lower and upper Snowshoe Group and the Sugar Limestone (Struik, 1986). The Lower Snowshoe Group, comprising pelites and minor marbles, tufts and orthoquartzites, is of Precambrian age and is separated unconformably from overlying early Paleozoic upper Snowshoe strata, which in part are of Mississippian age. Upper Snowshoe strata are overlain by the early Permian Sugar Limestone. In the context of the present paper, the Downey Creek Succession, within the upper Showshoe Group, is the most significant rock unit in the Barkerville Terrane because it hosts lode gold and there is an excellent geographic correlation between the outcrop belt of the Downey Creek Succession and placer operations (Fig. 2). Lode gold is associated with interbedded marble and tufts; the rock assemblage suggests sedimentation on a continental margin subject to episodic rifting and volcanism. Preliminary sedimentological observations suggest a deep water setting dominated by turbidity currents. Limestones appear to be of resedimented origin derived from the collapse of littoral carbonate on the coastal margins of volcanic cones. Broadly similar settings, possibly represented at the present day by the Lesser Antilles Arc (e.g. Sigurdsson et al., 1980) have been identified within middle Triassic strata of the adjacent Quesnel Terrane by Struik (1988). Lode gold occurs in two principal associations within the Downey Creek succession which trends northwest through the study area (Fig. 2). First, the Cariboo Gold Quartz, Island Mountain and Mosquito Creek gold mines, located at Wells (Fig. 2), mined "pod-like" or lensoidal structures that are mineralized with fine-grained arsenopyrite. The gold occurs as a film that coats the surfaces of sulphide grains with mineralized lenses restricted

GLACIAL

PLACER DEPOSIT

49

to narrow sheets of limestone that strike northwesterly across the area. Secondly, native gold and tellurides were also mined at the Cariboo Gold Quartz Mine. The precise origin of gold found in placer deposits world-wide is not well constrained (see Boyle, 1979, pp. 381-385; MacDonald, 1983; Wilson, 1984) and the same comments apply to the Cariboo where detailed study of gold geochemistry and origins is only just commencing. In the first regional study of the Cariboo placers Johnson and Uglow (1926, p. 215) argued that coarse gold was derived from the deep weathering and supergene enrichment of quartz veins, containing arsenopyrite and pyrite, during the Tertiary. This model is supported by the present study. Tertiary sediments ranging in age from the Eocene to late Miocene are selectively preserved in a belt 15 km wide along the Fraser River in the Quesnel area, about 80 km west of the WeUs-Barkerville area (Lay, 1941). Paleoenvironmental reconstruction based on spore and pollen assemblages suggest mean annual temperatures over 20 °C during the Eocene with mean annual temperatures no lower than 12°C until the late Miocene (Rous~ and Mathews, 1979, 1988; Wolfe, 1985). Under ithese conditions of enhanced chemical weathering, manganiferous siderite would generate mar~ghnese dioxide which promotes gold solution and transport (Guilbert and Park, 1986). Sulphides could be expected, to produce sulphuric acid which reacts with manganese oxides or goethite to produce the necessary Eh for the dissolution of gold films on the surface of sulphide rains, taken up as a stable auric chloride complex [AuCI4]- to be later precipitated at coarse gold. Cariboo placer gold varies in fineness from about 775 to 950 in contrast to lode gold which varies from 500 to 911. Some gold is crystalline in the form of dodecahedrons, cubes and octahedrons suggesting deposition from solution followed by limited transport, but most is mamillary in character possibly perhaps indicating gradual accretion of gold particles (Fig. 3B). The post-Tertiary history of the Cariboo area has been one of uplift and dissection of the plateau surface and the stripping of gold by cold climate weathering and glacial processes. Each placer creek produces

50

N. EYLES and S.P. KOCSIS

Fig. 3. A. Cariboo Mountains with Cariboo Lake in foreground. Mountain peaks rise to 2210 m a.s.l, above a dissected glaciated plateau (Fig. 1). B. Nugget gold (Romano Claim) from "wandering gravel-bed" river gravels at Lightning Creek (site 18; Fig. 1) showing smooth and rounded forms. These can be contrasted with angular nuggets recovered from valleyside alluvial fan gravels (Fig. 9B). C. The Bullion Pit (site 27; Fig. 1), arrowed at fight, excavated along a buried valley of the Quesnel River and separated from the modern river valley to the left by a bedrock high. Spanish Mountain (Figs. 1, 9A) is arrowed in the background. D. The Bullion Pit, exposing older gravels and late Wisconsin glacial sediments. a distinct t y p e of gold nugget; this, together with a c o m m o n association with pyrite, galena a n d q u a r t z a n d the presence of o n l y slightly w o r n crystal facies, suggest a local origin a n d a t r a n s p o r t distance of less than a few kilometres [Figs. 1,2). G o l d nuggets from valleyside alluvial fan d e p o s i t s show angular p i t t e d edges a n d irregular shapes, i n d i c a t i n g relatively recent d e r i v a t i o n from b e d rock, whereas nuggets from fluvial facies that are k n o w n to have been r e w o r k e d m a n y times, show s m o o t h e d , p o l i s h e d forms (see below).

Late Pleistocene stratigraphy and identification of placers Bedrock valleys in the C a r i b o o area are filled with m a n y tens of metres of late Pleistocene sedi-

m e n t s a n d lucrative placers occur along b u r i e d valleys (Sharpe, 1939; Clague, 1978). W h e r e a s thick T e r t i a r y s e d i m e n t sequences are p r e s e r v e d to the west of the s t u d y area a l o n g the d o w n f a u l t e d F r a s e r River valley, the existence of correlative sediments in the C a r i b o o a r e a is unclear. Several geologists a n d miners have identified the occasional occurrence of g o l d - b e a r i n g ferricrete, or highly i n d u r a t e d a n d oxidized c o n g l o m e r a t e s , as " T e r t i a r y " placers (E.L. F a u l k n e r , p e r s o n a l C o m m u n i c a t i o n 1989). These are d i s t i n g u i s h e d from Pleistocene placers since they require drilling, blasting a n d crushing a n d even then are difficult to process t h r o u g h a c o n v e n t i o n a l wash plant. These s e d i m e n t s are n o t c o m m o n , however, a n d there are no k n o w n current e x p o s u r e s in the s t u d y area. However, other o u t c r o p s of u n c o n s o l i d a t e d

SEDIMENTOLOGICAL CONTROLS ON GOLD IN A LATE PLEISTOCENE GLACIAL PLACER DEPOSIT

and non-cemented gravels that were reported as "Tertiary" by early miners are exposed and can be shown to be glacial in origin; the term "Tertiary" was used simply because the gravels occur below a regionally-extensive till unit (see below). So little is known of the nature, extent and age of the auriferous cemented "Tertiary" conglomerates that they are omitted from consideration in the present paper. The Cariboo area, in contrast to the Fraser

PAY ZONES

3

2

1

SEDIMENTS

51

lowlands, has experienced strong post-Tertiary uplift and rejuvenation and what remains of Tertiary placers is either buried or difficult to distinguish from later Pleistocene gravels. The Pleistocene geology of the area is the subject of ongoing surface and subsurface investigation but a three-fold division into lowermost glacial, middle non-glacial and upper glacial strata is generally observed in north-central British Col-

AGE

POSTGLACIAL FLUVIAL PLACERS

POSTGLACIAL <10,000 YBP

S U B G L A C I A L PLACERS

LATE WISCONSIN <30,000 YBP (FRASER GLACIATION)

O L D E R FLUVIAL PLACERS

OLYMPIA >30,000 YBP NON GLACIAL INTERVAL

EARLY WISCONSIN (about 75,000 YBP)

and/or ILLINOIAN GLACIATION? >12~5,000 YBP

LODE GOLD DEPOSITS

ALLUVIAL FAN SEDIMENTS

DOWNEY CREEK SU~ION

LODGEMENTT1LL & ASSOaATED GLACIALFACIES

BRAIDED/ WANDERING GRAVELBED FLUVIAL FACIES

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Fig. 4. Schematic representation of Pleistocene stratigraphy and associated placer mines in north-central British Columbia. The age of

lowermost glacial sediments, exposed along the Fraser River is poorly constrained. Estimates of the duration of the Olympia non-glacial interval vary from 45,000 years to at least 100,000 years; see text.

52

N. EYLESand S.P. KOCSIS

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Fig. 5. Top: General stratigraphy of valley infills and Cariboo gold placers. A = older fluvial gravels (greater than 30,000 and possibly as old as old as 125,00 years B.P.; Fig. 4); B = lodgement till complexes (30,000-10,000 YBP); C = postglacial gravels (< 10,000 YBP). Bottom: Sedimentological logs at placer mines working older fluvial gravels. Number identifies locations in Figs. 1 and 2. Au identifies pay-zone. Lithofacies as in Fig. 10. Facies logs from lodgement till complexes and postglacial gravels are shown in Fig. 10.

SEDIMENTOLOGICAL CONTROLS ON G O L D IN A LATE PLEISTOCENE G L A C I A L PLACER DEPOSIT

The age of the pre-Olympia glacial sequences along the Raser River is not well constrained; they may be of early Wisconsin age, belonging to oxygen isotope stage 4 about 75,000 years B.P. (Martinson et al., 1987), but an older, Illinoian age ( > 125,000 years B.P; oxygen isotope stage 6) cannot be ruled out (Clague, 1980; Clague et al., 1988; Fulton, 1984). Correlative glacial sequences have not been definitely identified to date in the Cariboo district (Fig. 5), but well-exposed "Olympia" vaUeyside fan deposits underlying late Wisconsin (Fraser) lodgement tills contain large volumes of poorlysorted gravels and diamict facies that record the reworking of older glacial sediments (Fig. 7B).

umbia. The age of the lowermost glacial units is problematic (Fig. 4). Lower glacial strata are not well-exposed in the Cariboo area and their presence is inferred from thick infill deposits exposed along the Fraser River. These older glacial deposits are dominated by glaciolacustrine sediments extensively deformed by the melt of stagnant ice (Eyles et al., 1987). These are truncated by a major erosion surface on whicli rests fluvial sediments belonging to the Olympia non-glacial interval (Fig. 4; a contact marked by a coarsegrained boulder lag that commonly contains economic values of gold but is difficult to work because it outcrops well above modem river levels.

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54

Consequently the poor exposure of pre-Olympia sediments in the Cariboo area may be the results of the complete or near complete reworking by Olympia rivers. Available evidence therefore suggests that the Olympia non-glacial interval may have lasted from at least 125,000, from the close of Illinoian glaciation, to well after 30,000 years B.P. when late Wisconsin glaciers expanded over the area. This episode was characterized by a cool-temperature climate similar to the climate of the present day but towards the close of this interval, around 30,00 years B.P., the area underwent progressive cooling prior to the Late Wisconsin (Fraser) glaciation. At this time glacier-fed braided rivers were responsible for widespread aggradation along valley floors prior to glacial lake ponding when valleys were dammed, and eventually overrun by ice. The area experienced predominantly westward ice flow during the Fraser Glaciation from an ice centre over the Cariboo Mountains (Tipper, 1971; Fig. 6). Mass-flow activity along valley sides and the construction of alluvial fans was important during deglaciation when large volumes of poorly-sorted, but gold-rich, debris were moved downslope by sediment gravity flow and sheet flood processes (e.g. Eyles and Kocsis, 1988b). Figs. 1 and 2 show the location of 35 placer mines that have been studied to date. Mining records, detailing the geology of past placer operations are largely non-existent since systematic reports of the stratigraphy exposed by the mines has never been collected. Sedimentological study of exposures indicate that placer deposits fall into two broad categories. In the first group are fluvial placers which comprise the largest, by volume, of the Cariboo placer deposits. These were deposited during non-glacial episodes both before (Olympia) and after (Holocene) late Wisconsin glaciation. The second category refers to glacial placers that developed below the late Wisconsin (Fraser) ice sheet. The paper describes the principal facies types and overall depositional settings associated with the "fluvial" and "glacial" placers in the Cariboo emphasizing sedimentological controls on the distribution of gold. This data base provides a frame-

N. E Y L E S and S.P. K O C S I S

work for the discussion of other placer deposits in areas of late Pleistocene, and older, glaciations.

Fluvial placers The sedimentology of pay zones in gravel deposits suggests two fluvial styles have been dominant; braided rivers comprising multiple channels, responsible for very thick, massive and crudelystratified gravel deposits (Fig. 7A) and single channel "wandering gravel-bed rivers" (Church, 1983) where gravels are interdigitated with finegrained overbank and floodplain deposits (Fig. 8A). Cold boreal conditions, with only a restricted distribution of vegetation and glacier ice present in the drainage basins, were associated with braided river systems whereas wandering gravelbed rivers are typical of present-day cool-temperature conditions with densely forested drainage basins and valley floors.

Braided river fluvh~l facies: characteristics and distribution of gold Braided river facies are a major component of valley infill stratigraphies in the Cariboo district. The thickest braided river sequences typically occur below late Wisconsin glacial sediments (e.g. Figs. 7A, C, 8B, C) and record widespread valleyfloor aggradation in a cold climate prior to regional ice expansion. Gravels commonly comprise the lowermost part of infill stratigraphies along the floors of bedrock valleys and contain large volumes of placer gold that were reworked from older deposits. These gravels are volumetrically the most significant placers in the Cariboo area but they are often the most difficult to work given the thickness of overburden. These gravels were, and still are, regarded as "Tertiary" by many placer miners simply because of their stratigraphic position below glacial sediments (e.g. Johnson and Uglow, 1926, p. 25; see above). Coarse-grained and poorly-sorted facies occurring immediately below late Wisconsin lodgement tills (e.g. Fig. 7C) are particularly good exploration targets and were deposited proglacially as the ice sheet margin advanced into valleys. In contrast, postglacial braided

S E D I M E N T O L O G I C A L C O N T R O L S ON G O L D IN A LATE PLEISTOCENE G L A CI A L PLACER DEPOSIT

55

Fig. 7. A. 40 m high section of braided river gravels exposed along the Fraser River, figure arrowed at right. The sequence is capped by late Wisconsin glacial sediments. B. Large rafts of reworked glacial and Tertiary sediments within Olympia river gravels; Fraser River. C. Coarse-grained ice proximal gravels; Pine Creek, adjacent to Cariboo Lake (Fig. 1). D. Section through modem valleyside fan deposit (base shown by dashed line) showing coarse-grained fan debris (large boulder is 2.5 m long) overlying late Wisconsin glaciolacustrine sediments. Mouth of Wolfe Creek along Antler Creek (Fig. 1).

river deposits that overlie late Wisconsin glacial sediments are in general better-sorted and less coarse-grained because of deposition from a retreating ice margin and are not favourable targets for gold. Proximal braided river facies are characterized by units of massive and crudely-bedded boulder gravels often with appreciable matrix. The absence of cross-bedding and the dominance of tabular multi-storey units records accumulation of coarse sediments on longitudinal, or sheet, bars under conditions of shallow water flow (Miall, 1977). Clasts show a-axis imbrication transverse to the valley trend as a result of rolling about their long axis, but in m a n y cases a preferred imbrication is difficult to identify. Large-scale aggradation surfaces dipping downvalley at about 5 o, record

the successive vertical accretion of bar surfaces during rapid valley infilling. Bouldery lag horizons, deposited as a result of shallow and high velocity flow across bar tops, are associated with the highest gold values and are currently being worked at the Ballarat claim on Williams Creek (site 3; Figs. 1, 5). Bar top facies show an " o p e n work" texture, where clasts abut clasts with little or no intervening matrix, resulting from the winnowing of fines on the bar surface. These coarsegrained " b a r top lags" contain very high gold values and in general, the coarsest lags show the highest gold contents. At Alice Creek (site 1; Figs. 1, 5), gold values within bar top lags of correlative age as those at the Ballarat claim (Fig. 8B) range from 0.52 to 4.38 g / m 3 and there is a good relationship between gold values and boulder size.

56

N. E Y L E S and S.P. K O C S I S

Fig. 8. A. "Wandering gravel-bed" bar platform gravels (Gin) (Fig. 11) overlain by floodplain sands (Sh) and silts (Fm) at Lightning Creek (site 2, Figs. 1, 5). Gravels produce rounded gold nuggets as a result of frequent reworking (Figs. 3B, 11). Section is truncated by late Wisconsin lodgement till (base shown by dashed line). B. Braided river gravels (at base of section) being worked at Alice Creek (site 1, Figs. 1, 5). Gravels are overlain by glaciolacustrine sediments recording ice-damming of the valley, that are truncated by late Wisconsin lodgement till (base arrowed). C. Auriferous braided river gravels (at base of section) overlain by late Wisconsin lodgement till; California Gulch (site 4, Figs. 1, 5). D. Large boulder "cluster" produced by the partial reworking of valleyside alluvial fan debris by modem wandering gravel-bed rivers. The clusters contain coarse angular gold (e.g. Fig. 9B) indicating local derivation and limited transport. Burns Creek (site 32, Figs. 1, 2).

Exceptionally coarse-grained horizons, containing "outsize" clasts over 1 m in diameter, suggest the incidence of large floods. Glacier-fed outwash fans are k n o w n to be affected by substantial outburst floods which entrain and sort considerable sediment volumes (Ostrem, 1975; Maizels, 1987). Because of the wide exposure of " O l y m p i a " aged sediments in placer mines, tentative paleogeographic reconstructions are possible for this long non-glacial interval of cool climate. Braided outwash rivers along the valley floors, flanked by large valley-side fans and talus slopes, surrounded by tundra vegetation, appear to have been the d o m i n a n t landscape elements during this

time period. Several properties are currently working alluvial fan deposits of O l y m p i a age. At California Gulch, about 7 km southeast of Barkerville (site 4; Figs. 1, 5) a thick valley-side alluvial fan deposit, consisting of crudely-bedded downslopedipping sheets of gravel interbedded with debris flows, occurs below a late Wisconsin lodgement till. Poorly-sorted gravels contained within crosscutting channels, up to 10 m wide and 8 m deep, produce coarse, angular gold particles indicating local derivation, with grades ranging from 0.67 to 8.18 g / m 3. The gravels contain m a n y angular clasts and slabs derived from nearby phyllite and limestone and are p r o b a b l y representative of the "slide

S E D I M E N T O L O G 1 C A L C O N T R O L S ON G O L D IN A LATE PLEISTOCENE G L A CI A L PLACER DEPOSIT

57

Fig. 9. A. Late Wisconsin lodgement till (base shown by dashed line) draped over large alluvial fan deposit at Spanish Mountain (site 9, Figs. 1, 5). Recovered gold from fan deposit is shown in B. B. Irregular and pitted gold nuggets recovered from alluvial fan at Spanish Mountain indicating local and probably recent derivation from surrounding plateau. C. 4 m thick late Wisconsin lodgement till (between arrows) exposed at Mount Nelson. Gold is recovered from intraformational gravels (outlines) deposited by subglacial meitwaters (site 13; Figs. 1, 2, 10). D. Bedrock notch cut by subglacial meltwaters at the McPberson claim, Cunningham Creek (site 12; Figs. 1, 2). E. Washing lodgement till where it rests on bedrock; upper part of Lowhee Creek, 1930's (site 35; Figs. 1, 2). F. Auriferous lodgement till along lower bedrock bench at Thistle's Pit (site 10; Figs. 1, 2, 10) adjacent to Eight Mile Lake. rocks" described by early placer miners (Johnson a n d U g l o w , 1926). A s i m i l a r d e p o s i t o c c u r s o n the w e s t e r n s l o p e of S p a n i s h M o u n t a i n (site 9; Figs. 1,

5) w h e r e a l a r g e a l l u v i a l f a n (1 km2), b l a n k e t e d b y l a t e W i s c o n s i n l o d g e m e n t till, is c u r r e n t l y e x p o s e d b y p l a c e r m i n i n g (Fig. 9A). T h e u p p e r s u r f a c e of

58

N. EYLES

10 EIGHTMILELAKE

11

MARY CREEK 0

,

5

12

CUNNINGHAMCREEK 1 3 MOUNT NELSON ~_L.is

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PINUS CREEK

~ o!Omo C

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!: :!::;i:i om ....... "°°i

Au~Gm

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0

Au

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15

~ "°c'

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PORTER CREEK L o

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LIGHTNING CREEK c

20 TREGILLUS

Au 19

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Dmm

s s G

:I~',2::"2Gc

Dmm

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-18

TREGILLUS LAKE C S S

Au ~

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4 Au

:.o.:

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-I

20.

20 Au 16

Dram

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I Au

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C S S G

C S S G

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" " c : ) ~ ,s

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~

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Fig. 10. Sedimentological logs through placer mines working lodgement till complexes (sites 10-15) and postglacial gravels (sites 16-21). Numbers identify location in Figs. 1 and 2; for overall stratigraphy see Figs. 4 and 5. Au identifies the pay streak. Lithofacies codes from Eyles et al. (1983).

SEDIMENTOLOGICAL

CONTROLS

ON GOLD IN A LATE PLEISTOCENE

the fan gravels has been deformed by glacial overriding and is underlain by crudely-bedded gravels that dip downvalley and provide a profitable low yield but high volume operation. The area is well known for its good lode gold prospects and the fan may have received gold stripped from the surrounding plateau by cold climate weathering processes because gold nuggets are irregular and pitted indicating a local source and limited transport (Fig. 9B; see Discussion). The same comments apply to the rich pay zone at the Toope property, located at Mary Creek (sites 8, 11; Figs. 1, 5, 10), which consists of orange-colored oxidized fan gravels preserved within a channel cut into siltstones, quartzites and locally auriferous black argillites. Very coarse, angular gold nuggets are recovered at this site and it also appears that the gold is of local origin derived by the erosion of supergene-enriched bedrock. Two placer operations along the lower portion of Lightning Creek (sites 5 and 22; Fig. 1) are working fan gravels of a similar character and genesis. Braided river and fan gravels, deposited during deglaciation of the Cariboo creeks, commonly obscure subglacial and older gravel placers at depth (Fig. 5, 10). Along several valleys, postglacial gravels extend to low elevations and can be assumed to have completely reworked pre-existing placers because they contain rich pay zones at their base. Coarse-grained proximal outwash gravels are being worked in the vicinity of Tregillus Lake (northeast comer), along Lightning Creek (Romano Mine), at Nugget Gulch, Porter Creek, Sovereign Creek, Alces Creek, Mary Creek ("Toope" B) and at Wolfe Creek (Fig. 1). In general, gold values are low, but mining costs are reduced substantially in the absence of overburden. The early postglacial saw widespread masswasting throughout the Cordillera (e.g. Church and Ryder, 1972; Eyles et al., 1988; Eyles and Kocsis, 1988b) when large volumes of heterogeneous and often coarse-grained glacial debris resting on steep valley sides, were moved downslope as sediment gravity flows to be reworked by braided rivers along valley floors. The distal, downvalley, portions of these gravels are better sorted and generally are uneconomic. Bouldery

GLACIAL

PLACER DEPOSIT

59

proximal gravels containing many outsized clasts are the best prospects and the highest gold values occur toward the base of the gravel sequences such as at Mary Creek (site 8; Figs. 1, 5) where postglacial gravels show gold values averaging 0.5 g / m 2.

"'Wandering gravel-bed" fluvial facies," characteristics and distribution of placer gold "Wandering gravel-bed rivers", as defined by Desloges and Church (1987) commonly show a single sinuous channel, often with large medial bars and occasional braided reaches (Fig. 11). The modern Bella Coola River, which drains over 5000 km 2 of the Coast Ranges west of the Cariboo area is typical of this type and its deposits have been described recently by Desloges and Church (1987). Deposits are dominated by massive and weaklystratified cobble gravel facies, often very poorlysorted with vague imbrication. Bar surfaces are especially coarse-grained, often forming a bouldery surface lag. Sand facies are limited and typically occur within stable vegetated areas of the bar, or downstream of log piles, where they comprise thin (< 50 cm) sheets composed of horizontally-bedded or planar cross-stratified facies. In areas of stable channels, well-defined lateral and point bars develop. Point bars are coarsegrained and traversed by gravel-floored chute channels that are active only during floods. Bars have bouldery surfaces with sand facies forming a downstream thickening wedge on the trailing edge of the bar. Lateral migration of the main channel results in lateral accretion of massive gravels and the development of an extensive "bar-platform" across the entire valley. During large floods sands and silts may accumulate as thin ( < 1 m) sheets across the platform. These sediments are rapidly vegetated and channel avulsion, by undercutting of channel banks, incorporates large amounts of forest debris within channel gravels. Sections through floodplain deposits and underlying bar platforms show a simple sequence of massive cobble gravels overlain by thin (< 1.5 m) sands interbedded with mats of detrital organic debris. Wandering gravel-bed gravel facies occur below late Wisconsin lodgement till at Lightning Creek (site 2; Figs. 1, 5). The mined succession (Fig. 8A)

N.EYLESandS.P.KOCSIS

60

~

ANDERING GRAVEL-BED RIVER

/ / i ,',', ,, ,, ~f,,',"

JJ

~ *

',

,,

(

~\

/

~i

I

'?/~

', ,

d)

~

Overbank/floodplain ~ sand/siltfacies

Bar platforrn gravels

~

payzone

Fig. 11. Depositional model portraying payzones within wandering gravel-bed river deposits characteristic of present day drainage systems in the Cariboo Mining District. Note restruction of payzones to the base of gravel beds as a result of repeated reworking of the bar platform by stream channels. 1) ="Older" fluvial gravelsand subglacial placers (Figs. 4,5); 2) = Coarse-graineddebris from modern valleysidefan (Figs. 7D, 8D) contributing local gold from the surrounding plateau (see Fig. 9B).

shows laterally-extensive horizontally-laminated fine sands resting on cobble gravels with the main pay zone limited to bouldery horizons at the base of the exposure. Repeated migration of channels and the multiple reworking of the bar platform gravels results in the continued ' t u r n over' of the entire gravel body producing a gold-rich basal horizon characterized by well-rounded gold nuggets. Desloges and Church (1987) estimated that the bar platform and flood plain deposits of the Bella Coola River Valley are reworked every 150 years; this may give some indication of the number of times that the postglacial rivers have re-

worked valleyfloor gravels during the last 10,000 years in the Cariboo area. Along most creeks, active alluvial fans and steep tributary streams continue to contribute coarse-grained debris from the surrounding plateau to the valley floor. Precipitation in the Cariboo district is substantial ( > 150 c m / y r ) and Cariboo rivers experience recurring floods as a result of spring and autumn snow melt. Clusters of angular boulders within and on bar platforms suggest only partial digestion of recently introduced debris which may only be completely reworked i n t o the bar platform by the larger floods. On the lower

S E D I M E N T O L O G I C A L C O N T R O L S ON G O L D IN A LATE PLEISTOCENE G L A CI A L PLACER DEPOSIT

portion of Wolfe Creek (site 19; Figs. 1, 10) which drains into Antler Creek, bouldery clusters within wandering-gravel bed facies (Fig. 7D) show gold values up to 0.7 g / m 3. These clusters are composed of imbricated boulders up to 1.5 m in diameter suggesting the accretion of large rolling boulders against obstructions during floods (Brayshaw, 1984). The source of the debris is a small fan at the mouth of a side valley (Wolfe Creek). Similar boulder "dusters" are described by Morison and Hein (1987) from the braided "White Channel" gravels of the Klondike placer mining district in the Yukon Territory. In contrast to the rounded gold particles from the bar platforms of wandering gravel-bed rivers, gold nuggets from valleyside alluvial fans and boulder "clusters", show a pitted, irregular form indicating a local source area and limited reworking (Fig. 9B).

Subglacial placers Grey-coloured lodgement till, deposited after 30,000 years B.P., is a widespread stratigraphic marker horizon throughout the Cariboo district. It rests with a marked erosional contact on underlying facies, often characterized by glaciotectonic deformation structures, and varies from a few, to as much as 20 m in thickness. It is a stiff, overconsolidated, massive and clast-rich diamict and frequently shows deformed "rip-up" rafts, composed of underlying gravel deposits, in its basal parts (e.g. Cunningham Creek; site 12, Figs. 1, 10). The genesis of lodgement till and the characteristics of lodgement till sequences have been described by Boulton (1982), Eyles et al. (1982) and Drewry (1986). Lodgement till accumulates by the repeated release or "smearing" of englacial basal debris as wet-based ice flows over the underlying substrate. The process by which englacial debris is released is essentially one of frictional retardation and pressure melting against the underlying bed as dirty basal ice slides over the substrate. Debris particles, or aggregates of particles, with the basal debris layer of the glacier, are lodged against the substrate when ice velocities are generally less than about 50 m/yr; at higher ice velocities the bed is swept clear and erosion becomes dominant (Boulton, 1982). Deposition rates of about 2 cm/yr

61

have been reported from modem subglacial settings (Boulton, 1975). Successive increments of till deposition result in heavily overconsolidated diamict that shows a pronounced structural fabric created by shear stresses applied by the overriding glacier both during and after deposition. As a result of seasonal, or longer term variations in ice velocity, subglacial deposition is commonly punctuated by erosional episodes and the final till sequence shows overlapping and cross-cutting diamict beds, each in erosional contact with underying units (Fig. 12B). Changes in ice flow direction may accompany erosional breaks, resulting in changes in dominant bedrock lithology from one diamict unit to another. Commonly, the basal portions of lodgement till sequences contain locally-derived material with more far travelled lithologies represented upward in the section. Clasts within lodgement tills show a preferred long-axis orientation parallel to ice flow with an up-ice imbrication, as a result of high shear stresses applied during deposition. A distinctive "bullet"shape frequently evolves as a result of abrasion of clasts lodged on the underlying bed by dirt-rich basal ice (Boulton, 1978). Because of enhanced plastic deformation of basal ice around large obstacles on the bed, large boulders are preferentially deposited against the obstacle (see Boulton, 1982). Laterally extensive pavements composed of nested boulders jammed together are a common feature and record subglacial "traffic jams" involving the repeated emplacement of boulders around an obstacle. These can provide problems of interpretation during drilling programs (the "false bedrock" of early miners) but are good gold placer prospects because they have trapped coarse gold. Lodgement till is a characteristic subglacial deposit of wet-based ice masses, characterized by abundant subglacial water which assists the sliding process. Intraformational gravel bodies (6, Fig. 12) occur within lodgement till sequences and record erosion and deposition by subglacial meltwater rivers. Gravels are typically poorlysorted and frequently contain "armoured" till balls as a result of fluvial reworking Of cohesive diamict fragments slumped from the channel walls. Subglacial fluvial activity is highly dynamic, with epi-

62

N. EYLES and S.P. KOCS|S

5mI

////..,-

/

Fig. 12. Depositional model portraying pay zones in lodgement till complexes. 1 = Bedrock "gutter", 2 = glaciotectonic structures and incorporation of gold-rich "older" gravels (site 12; Figs. 1, 10); 3 = bouldery lee-side deposits (site 15; Figs. 1, 10); 4 = bedrock notches and vertical shafts, "moulins", cut by meltwaters (site 12; Figs. 1, 9D); 5 = boulder pavements; t = intraformational channel fills (site 13; Figs. 1, 2, 9C, 10); 7 = proximal braided river facies (site 17; Figs. 1, 10). Arrows and dashed lines show trend of intraformational gravel bodies.

S E D I M E N T O L O G I C A L C O N T R O L S ON G O L D IN A LATE PLEISTOCENE G L A C I A L PLACER DEPOSIT

sodes of low or non-existent flow alternating with flood discharges; abandoned subglacial channels, cut into the till bed and buried by subsequent till deposition, are preserved as "shoe-string" gravel bodies within the till sequence, e.g. Mount Nelson (site 13, Figs. 1, 2, 9C, 10, 12). The term "lodgement till complex" was employed by Eyles et al. (1982) to identify multistorey units of till containing intraformational subglacial gravel bodies (e.g. Fig. 12) deposited during a single glacial episode. The erosive ability of subglacial meltwaters under high hydrostatic pressures and armed with abrasive sediments is well illustrated in the Cariboo area. Bedrock surfaces, particularly on the lower benches, show meltwater-cut "notches" (e.g. Cunningham Creek and Fosters Ledge along upper Lightning Creek; sites 12, 33 respectively; Figs. 1, 10) which vary from small sinuous grooves less than a centimetre deep to larger channels several metres deep and wide. The walls of the notches are typically smooth, regardless of lithology, and often striated; their long profiles show an upand-down character with a sinuous form in plan. These together with vertical shafts, up to 1.5 m deep and a metre wide, have formed very effective gold traps, akin to a large-scale subglacial sluice box (Fig. 9D; 4, Fig. 12). Fig. 12 shows the principal gold pay zones that are associated with lodgement till complexes. Basal enrichment of lodgement tills, as a result of the incorporation of older gravels, has been the most widespread subglacial placer forming mechanism in the Cariboo. Gold is either dispersed fairly uniformly within the basal till or is contained within "rip-up" rafts of gravel displaced from the underlying substrate by glaciotectonic processes (e.g. Cunningham Creek; site 12, Fig. 10; see below). In these cases, the glacier has simply reworked underlying placers; the higher stratigraphic portions of the till sequences contain uneconomic gold values as a result of dispersion. A critical consideration in accessing the economic viability of these deposits is the thickness of the till section and the volume of overburden to be removed. A hydraulic mine at Coulter Creek (site 28; Figs. 1,2) worked the basal portions of lodgement till but experienced poor returns when the overburden thickness of lodgement till became too

63

great. Another problem is that of disaggregating cohesive clumps of till during sluicing; the operation is not completely successful and gold may be lost. Several placer operations along Cunningham Creek have mined auriferous lodgement tills resting on older gravel or bedrock. At one site, about 5 km from the Creek mouth (site 25; Fig. 1) a large-volume/low-yielding deposited is being worked; gold values average 0.34 g / m 3 and are greatest on the lowest bedrock benches as a result of the proximity to older gravels along the valley floor. On the higher bedrock benches gold is too dispersed to be worked. In several localities, lodgement till rests on glaciolacustrine silts and clays that blanket older gravel placers. These fine-grained sediments, termed "slum" by early miners, were deposited in short-lived lakes dammed by encroaching late Wisconsin ice and appear to have prevented glacial erosion of the underlying auriferous gravels. Overlying tills are generally barren. At Pinus Creek and Eight Mile Lake (sites 14, 10; Figs. 1, 2, 10) gold values of 1.4 g / m 3 are found within the basal 3 m of lodgement till where it rests on older gravels or bedrock. The till is an orange-brown colour, as a result of the weathering of ankeritic limestone clasts, and shows rafts of oxidized interstadial gravels. At Mount Burns, adjacent to Devils Lake Creek (site 31; Figs. 1, 2) gold is being worked from lodgement till resting on bedrock. Tyrrell (1919) describes the working of auriferous "boulder clays" at Mosquito Creek where current mining operations (site 34; Figs. 1, 2) report angular "crystal" gold from the lower 2-3 m of lodgement till resting on limestones and tufts of the Downey Creek Succession; at this site bedrock shows gold values of up to 69 g/tonne. The volume of gold contained in lodgement till in the Cariboo district can be gauged by the Devils Lake Canyon where auriferous lodgements tills that rest directly on the Downey Creek Succession, were worked extensively until the close of the Second World War. More than 9300 kg of gold was recovered from lodgement tills between 1880 and 1946 principally at the Ketch Bench near Burns Creek and the Point Benches near Nelson Creek (sites 30, 29; Figs. 1, 2). Along

64

Cunningham Creek at the McPherson pit (site 12; Fig. 1), up to 20 m of lodgement till is draped over a series of bedrock benches up to 25 m above the modem river. The till contains large rafts, up to 4 m long, of auriferous gravels stained by manganese oxide. Gravels contain coarse gold, with nuggets weighing up to 5 g, and were derived by glacier erosion of underlying braided river deposits. Elsewhere in section, gold is often preferentially associated with intraformational boulder pavements of aligned boulders showing long axes parallel to ice flow direction. These pavements as related above result from the preferential lodgement of large clasts (Boulton, 1975, 1982) which acted as a "rough bed" in and around which coarse gold paticles could be trapped. Intraformational gravel bodies, deposited by subglacial waters draining the ice sheet base, are being worked at several sites. Observations of pit walls and discussions with mine operators show that gravel bodies have an elongate, shoestring geometry aligned parallel to former ice flow, and record erosion and deposition by meltwaters flowing on the lodgement till surface; they are genetically related to eskers and may have formed "feeders" to larger subglacial conduits. A placer mine on the eastern slope of Mount Nelson, 12 km west of Barkerville, is working intraformational gravels containing nuggets as coarse as 12.8 g (site 13; Figs. 1, 2, 10). Clast imbrication studies indicate that the channel is orientated north/south and there is the possibility that the same gravel body was worked earlier this century at the Point Bench some 800 m to the north. The Point Bench site produced over 730 kg of gold from intraformational gravels within lodgement till between 1906 and the early 1970's. The erosional effects of subglacial meltwaters have also been of significance in generating placers. Meltwater-cut notches on bedrock ("gutters") offer excellent prospects. Along Cunningham Creek, for example, about 6.5 km upstream of the Creek mouth, under a cover of about 20 m of auriferous lodgement till, 18.6 kg of gold was recovered from a narrow bedrock channel less than 1.5 m deep and 10 m long (Fig. 9D). Similar bedrock notches are exposed in a hydraulic pit situated at Fosters Ledge on the upper part of Lightning Creek (site 33; Fig. 1).

N. E Y L E S a n d S.P. K O C S I S

The Heron Channel at Grouse Creek, mined in the 1920's, is another good example (site 6; Figs. 1, 2, 5), and was described at the time as a 3 m wide by 3 m deep, gravel-filled gutter cut into bedrock (see Johnson and Uglow, 1926). The auriferous gravels were covered by up to 20 m of late Wisconsin lodgement till and over 1000 kg of gold was recovered from the 125 m long gutter. The Quesnel Ready-Mix property, at Tregillus Lake, currently exposes a bedrock knob about 20 m high forming a large "crag and tail" structure oriented parallel to ice flow. The tail is composed of sheet-like units of crudely-stratified bouldery diamict and gravels dipping away from bedrock (site 15; Figs. 1, 10). Lodgement till caps the sequence and buries the bedrock high. The deposit is a "lee-side" accumulation typical of areas of moderate relief where large subglacial cavities develop down ice of bedrock knobs (Hillefors, 1973; Fig. 12). Observations within modem subglacial cavities show that cavity size is a function of ice velocity. Cavity closure commonly occurs during the winter as a result of lowered ice velocity (Boulton et al., 1979; Boulton, 1982). Lee-side deposits that accumulate in such cavities can be regarded essentially as subglacial talus slopes fed by debris falling from the ice roof and by meltwaters entering the head of the cavity. The site of Tregillus Lake shows several intraformational unconformities recording partial cavity closure and erosion. The highest gold values ( > 5 g / m 3) are associated with bouldery horizons that have been reworked by meltwaters; the subglacial cavity operates essentially as a subglacial gold trap. Large volumes of sediments were probably moved through the cavity during many cycles of filling and reworking by meltwater leaving residual coarse-grained lag deposits enriched in gold. Discussion

Topography, tectonic setting, and climate have played a critical role in the formation of lucrative fluvial and glacial placers in the Cariboo district. The area consists topographically, of a deeply-dissected plateau crossed by narrow, entrenched valleys (Fig. 1) and the absence of a well-integrated drainage network has contributed to the local

S E D I M E N T O L O G I C A L C O N T R O L S ON G O L D IN A LATE PLEISTOCENE G L A CI A L PLACER DEPOSIT

preservation of substantial volumes of gold along these valleys. The richest payzones occur on bedrock at the base of the Pleistocene valley fills and were mined last century by driving adits and shafts (Eyles and Kocsis, 1988a, c). The volume of gold found along the gutters of several creeks far exceeds that recovered from other pay zones within overlying sediments (Johnson and Uglow, 1926), suggesting that valley infills have been reworked repeatedly down to bedrock during the Pleistocene, thereby concentrating gold along the lowest points of valley floors. It is possible that pre-existing Tertiary placers (see above) were reworked in a similar fashion as a result of uplift and rejuvenation. Each placer creek produces a distinct type of nugget, which, together with the common occurrence of crystalline gold, is good evidence of a local source and a transport distance of less than 1 or 2 km. A corollary of these arguments is that the gold found higher up the stratigraphic sequence, within subglacial and postglacial placers, has only recently been released from bedrock and introduced into the valleys. The occurrence of rich pay zones, showing coarse and angular gold particles, within postglacial alluvial fans draining the surrounding plateau supports this argument. With regard to tectonic setting, Henley and Adams (1979) drew attention to the common presence of young (i.e. post-Tertiary) giant placers around the margins of the Pacific Ocean (of which the Cariboo is one). They argued that plate margin convergence, crustal shortening and uplift were the primary controls on the release and accumulation of detrital gold in rich placer deposits in Siberia, Alaska, British Columbia, California, South America and New Zealand. Most of the fields discussed by Henley and Adams occur in fluvial deposits (alluvials) but they recognized that glaciation also played a key role in accelerating bedrock erosion rates. The ocean oxygen isotope record provides a proxy record of late Cenozoic climate change (Martinson et al., 1987) and suggests that the Cariboo area experienced alternating subarctic and cool-temperature conditions during the Quaternary. Substantial volumes of supergene-enriched bedrock may have been stripped from the surrounding plateau by coldclimate weathering processes such as freeze-thaw

65

and solifluction and by glaciation. This is borne out by the presence of gold-rich fan deposits, containing gold particles of local origin and limited transport, preserved below late Wisconsin lodgement till (e.g. Spanish Mountain, Fig. 9A, B). Subsequently, when glaciers overrode pre-existing placer gravels, powerful meltwater streams were able to rework and redeposit gold into lucrative placers within lodgement till complexes and associated proglacial gravel facies. This process has probably been repeated during each Quaternary glaciation.. Boyle (1979) drew an important distinction between late Cenozoic placers, characterized by coarse nugget gold, and placer deposits of Precambrian and Paleozoic age typified by very finegrained gold dispersed through many thousands of metres of sandstones and conglomerates as a result of continued reworking (e.g. Vos, 1975; Minter, 1978; Smith and Minter, 1980; Kingsley, 1984; Slingerland and Smith, 1986). In contrast, because of the relative youth of late Cenozoic placers, there has been less opportunity for gold to be reworked. Glacial placers are typified by coarse gold because glaciation and regional uplift commonly followed a long period of warm climate in which widespread supergene enrichment could take place. The very coarse nuggety character of the Cariboo gold placers is essentially the result of a new cool-climate erosion cycle following a long period of supergene chemical activity during the Tertiary; a similar model can be applied to PreQuaternary glacial placers. For example, the Clermont goldfield (350 km 2) of central Queensland, Australia, produces about 50 kg of gold each year from basal tillites and associated braided river facies of early Permian age. The detrital gold, commonly coarser than 30 g, is characterized by its high fineness (gold to silver ratios vary from 16:1 to 20:1) whereas lode gold in the area has a considerably lower fineness. Basal tillites rest on schists and contain intraformational lenses of poorly-sorted conglomerate, similar to those of the Cariboo district (site 13; Fig. 9C, 6, Fig. 12) with grades up to 35 g/tonne. I'Ons (1983) argued that supergene enrichment of gold in quartz vein systems occurred under tropical climates of the Silurian and Permian prior to erosion by north-

66

ward flowing ice sheets. The advancing ice sheet repeatedly cannibalized and reworked its own outwash gravels thereby concentrating heavy minerals, such as gold, diamonds, cassiterite and gem quality corundum. In South Africa, comparable diamond placers occur in the correlative Permian "Dwyka Tillite" as a result of glacial erosion of kimberlite pipes and the subglacial reworking of rich preglacial diamond placers. Diamonds contained in relatively modern (Tertiary/Quaternary) placers that occur along the Vaal and Orange Rivers, and offshore on the continental shelf of western South Africa, are thought to have been derived from the dissection and reworking of diamond paleoplacers in the Dwyka Tillite (M.J. de Wit, personal communication, 1988).

Future prospects The same sedimentological controls on gold distribution identified in the Cariboo appear to obtain in the glaciated placer gold districts in northern British Columbia (e.g. Atlin, Cassiar and Omineca), the Yukon, Alaska and eastern Canada but studies on which detailed comparisons can be made are few (see Cobb, 1973; Boyle, 1979; Cook, 1983). In the Chaudiere Valley area of Quebec, buried valleys, plugged by glacial sediments, host small-scale placers that have produced about 3100 kg since 1830 (Boyle, 1979). Work is ongoing to reconcile the distribution of gold with modern investigations of Quaternary stratigraphy in the area (Shilts and Smith, 1986a, b) and initial results suggest that the buried placers are the result of the glacial reworking of "preglacial" gold-bearing colluvium derived from mass-wasting of surounding slopes. Maurice (1986a, b) showed in the Gaspesie area of Quebec, that gold-bearing debris could be glacially transported over 100 km from source areas and still produce placer deposits when reworked by postglacial rivers. Other occurrences of late Pleistocene glacial placers are known from the Westland and Nelson provinces of New Zealand where gold contained in late Pleistocene glacial sediments has been worked extensively, but again geological details are few (Williams, 1974; Boyle, 1979). Clearly, a comprehensive sedimentological

N. EYLESand S.P. KOCSIS

study of late Cenozoic glacier placers would be a valuable exercise. To summarize, data from areas of Quaternary and pre-Quaternary glaciation indicate that glacial erosion of pre-existing placers or bedrock enriched supergenetically during a preceding phase of warm climate, can generate lucrative placer deposits. The sedimentological controls identified in this paper regarding the distribution of gold in the placers of the Cariboo Mining District, British Columbia, may form the basis for further exploration of placer deposits in other areas of Quaternary and pre-Quaternary glaciation.

Acknowledgements This work is funded by grants from the British Columbia Geosciences Research Grant Program and the Natural Sciences and Engineering Research Council of Canada to the senior author. We thank Mike J. de Wit of DeBeers, Kimberley, South Africa, Clinton Foster of the Western Mining Corporation, Australia, and Ted Faulkner and Brian Grant of the Ministry of Energy, Mines and Petroleum Resources, British Columbia for stimulating discussions and for critical reviews of previous drafts of the manuscript. The manuscript was reviewed for the journal by two anonymous referees. We thank John Clague of the Geological Survey of Canada for ongoing discussions of the glacial history of British Columbia.

References Boulton, G.S., 1975. Processes and patterns of glacial erosion. In: Glacial Geomorphotogy, Publications in Geomorphology. State University of New York, Binghampton, N.Y., pp. 41-87. Boulton, G.S., 1978. Boulder shapes and grain-size distributions of debris as indicators of transport paths through a glacier and till genesis. Sedimentology, 25: 773-799. Boulton, G.S., 1982. Subglacial processes and the development of glacial bedforms. In: R. Davidson-Arnott et al. (Editors), Research in Glacial, Glaciofluvial and Glacio-lacustrine System. Geo-Books, Norwich, pp. 1-31. Boulton, G.S., Morris, E.M., Armstrong, A.A. and Thomas, A., 1979. Direct measurement of stress at the base of a glacier. J. Glaciol., 22: 3-24.

SEDIMENTOLOGICAL CONTROLS ON GOLD IN A LATE PLEISTOCENE GLACIAL PLACER DEPOSIT Boyle, R.W., 1979. The Geochemistry of Gold and its Deposits. Geol. Surv. Can. Bull., 280. Brayshaw, A.C., 1984. Characteristics and origin of cluster bedforms in coarse-grained alluvial channels. In: E.H. Koster and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Soc. Pet. Geol., Mem., 10: 77-86. Church, M., 1983. Pattern of instability in a wandering gravel bed channel. In: J.D. Collinson and J. Lewis (Editors), Modern and Ancient Fluvial Systems. Int. Assoc. Sedimentol., Spec. Publ. 6. Blackwell, Oxford, pp. 169-80. Church, M. and Ryder, J.M., 1972. Paraglacial sedimentation: a consideration of fluvial processes conditioned by glaciation. Geol. Soc. Am., Bull., 83: 3072-3095. Clague, J.J., 1980. Late Quaternary geology and geochronology of British Columbia, I. Radiocarbon Dates. Geol. Surv. Can., Pap., 80-13: 1-28. Clague, J.J., 1987. A placer exploration traget in the Cariboo district, British Columbia. Geol. Surv. Can., Pap., 87-1A: 177-80. Clague, J.J., Easterbook, D.J., Hughes, O.L. and Mathews, J.V., 1988. Was there an ice-sheet in the cordillera during the early Wisconsin? A re-examination of the geological record. Geol. Soc. Am., Abstr. Progr., 20: A4. Cobb, E.H., 1973. Placer deposits of Alaska. U.S. Geol. Surv. Bull., 1374:213 pp. Cook, D.J., 1983. Placer mining in Alaska. Miner. Ind. Res. Lab., Univ. Alaska, Rep., 65:157 pp. Desloges, R.J. and Church, M., 1987. Channel and floodplain facies in a wandering gravel bed river. In: Recent Developments in Fluvial Sedimentology. Soc. Econ. Paleontol. Mineral., Spec. Publ., 39: 99-110. Drewry, D., 1986. Glacial Geologic Processes. Edward Arnold, 276 pp. Eyles, N. and Kocsis, S.P., 1988a. Placer gold mining in Pleistocene glacial sediments of the Cariboo district, British Columbia, Canada, 1858-1988. Geosc. Can., 15: 291-299. Eyles, N., and Kocsis, S.P., 1988b. Sedimentology and clast fabric of a glacially-influenced alluvial fan: Fraser River, British Columbia. Sediment. Geol., 59: 15-28. Eyles, N. and Kocsis, S.P., 1988c. Gold placers in Pleistocene glacial deposits; Barkerville, British Columbia, Canada. Can. Min. Metall. Bull., 81: 71-79. Eyles, N., Sladen, J.A. and Gilroy, S., 1982. A depositional model for stratigraphic complexes and facies superimposition in lodgement tills. Boreas, 11: 317-333. Eyles, N., Eyles, C.H. and Miall, A.D., 1983. Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences. Sedimentology, 30: 393-410. Eyles, N., Clark, B.M. and Clague, J.J., 1987. Coase-grained sediment gravity flow facies in a supraglacial lake. Sedimentology, 34: 193-216. Eyles, N., Eyles, C.H. and McCabe, A.M., 1988. Late Pleistocene subaerial debris flow facies of the Bow Valley, near Banff, Canadian Rocky Mountains. Sedimentology, 35: 465-480.

67

Fulton, R.J., 1984. Quaternary glaciation, Canadian Cordilleran. In: R.J. Fulton (Editor), Quaternary Stratigraphy of Canada. Geol. Surv. Can., Pap., 84-10: 39-48. Guilbert, J.M. and Park. F.P., Jr., 1986. The Geology of Ore Deposits. W.H. Freeman, New York, N.Y., 985 pp. Henley, R.W. and Adams, J., 1979. On the evolution of giant gold placers. Trans., Inst. Min. Metall., Sect. B, 88: 41-00. Hillefors, A., 1973. The stratigraphy and genesis of stoss and lee-side moraines. Bull. Geol. Inst. Uppsala Univ., 5: 139-154. Howell, D.G., Moore, G.W. and Wiley, T.J., 1987. Tectonics and basin evolution of western North America--an overview. In: D.W. Scholl, A. Grantz and J.G. Wedder (Editors), Geology and Resource Potential of the Continental Margin of North America and Adjacent Ocean Basins. Cirnm-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 6. American Association of Petroleum Geologists, Tulsa, Okla., pp. 1-16. I'Ons, M.E., 1983. Some aspects of the Clermont goldfields. In: G. Hoffman and C.B. Foster (Editors), Permian Geology of Queensland. Geological Society of Australia, pp. 379-384. Johnson, W.A. and Uglow, W.L., 1926. Placer and vein gold deposits of Barkerville, Cariboo District, British Columbia. Geol. Surv. Can., Mem., 149. Jones, D.L., Howell, D.G., Coney, P.J. and Monger, J.W.H., 1983. Recognition, character and analyses of tectonostratigraphic terranes, Western North America. In: M. Hasimoto and S. Uyeda (Editors), Accretionary Tectonics in the Circum-Pacific Regions. Terra, Tokyo, pp. 21-35. Kingsley, C., 1984. Daghreek fan-delta: An alluvial placer to prodelta sequence in the Proterozoic Welkon Goldfield, Witwatersrand, South Africa. In: E.H. Koster and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Soc. Pet. Geol., Mere., 10: 321-330. Lay, D., 1941. Fraser River Tertiary drainage-history in relation to placer-gold deposits, 2. B.C. Dep. Mines, Bull., 11: 75 pp. Maizels, J.K., 1987. Large-scale flood deposits associated with the formation of coarse-grained, braided terrace sequences. In: Recent Developments in Fluvial Sedimentology. F.G.. Ethridge, R.M. Flores and M.D. Harvey (Editors), Soc. Econ. Paleontol. Mineral., Spec. Publ., 39: 135-148. MacDonald, E.H., 1983. Alluvial Mining; The Geology, Technology and Economy of Placers, Chapman and Hall, 580 pP. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C. and Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: development of a high resolution 0 to 300,000-year chronostratigraphy. Quat. Res., 27: 1-29. Maurice, Y.T., 1986a. Distribution and origin of alluvial gold in southwest Gaspesie, Quebec. Part B. Geol. Surv. Can., Pap., 86-1B: 785-795. Maurice, Y.T., 1986b. Interpretation of a reconnaissance geochemical heavey mineral survey in the eastern Townships of Quebec, Part A. Geol. Surv. Can., Pap., 86-1A: 307-317.

68 Miall, A.D., 1977. A review of the braided-fiver depositional environment. Earth Sci. Rev., 13: 1-62. Minter, W.E.L., 1978. A sedimentological synthesis of placer gold, uranium and pyrite concentrations in Proterozoic witwatersrand sediments. In: A.D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Mem., 5: 801-829. Morison, S.R. and Hein, F.J., 1987. Sedimentoiogy of the White Channel Gravels, Klondike area, Yukon Territory; Fluvial Deposits of a Confined Valley. In: F.G. Ethridge, R.M. Flores and M.D. Harveys (Editors). Soc. Econ. Paleontol. Mineral., Spec. Publ., 39: 205-216. Ostrem, G., 1975. Sediment transport in glacial meltwater streams. In: A.V. Jopling and McDonald (Editors), Glaciofluvial and Glaciolacustrine Sedimentation. Soc. Econ. Paleontol. Mineral., Spec. Publ., 23: 101-122. Price, R.A., Monger, J.W.H. and Muller, J.E., 1981. Cordilleran cross-section--Calgary to Victoria. In: R.I. Thompson and D.G. Cook (Editors), Field Guides to Geology and Mineral Deposits. Geological Association of Canada, Calgary, Alta., pp. 261-334. Rouse, G.E. and Mathews, W.H., 1979. Tertiary geology and palynology of the Quesnel area, British Columbia. Bull. Can. Pet. Geol., 27: 418-445. Rouse, G.E. and Mathews, W.H., 1988. Paleontology and geochronology of Eocene beds from Cheslatta Falls and Nazko area, central British Columbia. Can. J. Earth Sci., 25: 1268-1276. Sharpe, R.F., 1939. The Bullion hydraulic mine. The Miner, 12: 37-40. Shilts, W.W., 1976. Glacial till and mineral exploration. In: R.F. Legget (Editor), Glacial Till. R. Soc. Can., Spec. Publ., 12: 205-224. Shilts, W.W. and Smith, S.L., 1986a. Stratigraphy of placer gold deposits; overburden drilling in Chaudiere Valley, Quebec, Part A. Geol. Soc. Can., Pap., 86-1A: 703-712.

N. EYLESand S.P. KOCSIS Shilts, W.W. and Smith, S.L., 1986b. Stratigraphic settings of buried gold-bearing sediments. Beauceviile Area, Quebec, Part B. Geol. Surv. Can., Pap., 86-1B: 271-278. Sigurdsson, H., Sparks, R.S.J., Carey, S.N. and Huang, T.C., 1980. Volcanogenic sedimentation in the Lesser Antilles Arc. J. Geol., 88: 523-540. Slingerland, R., and Smith, N.D., 1986. Occurrence and formation of water-laid placers. Annu. Rev. Earth Planet. Sci., 14: 113-147. Smith, N.D., and Minter, W.E.L., 1980, Sedimentological controls of gold and uranium in two Witwatersrand palaeoplacers. Econ. Geol., 75:1-14. Struik, L.C., 1986. Imbricated terranes of the Cariboo gold belt with correlations and implications for tectonics in southeastern British Columbia. Can. J. Earth Sci., 23: 1047-1061. Struik, L.C., 1988. Regional imbrication within Quesnel Terrane, central British Columbia, as suggested by conodont ages. Can. J. Earth Sci., 25: 1608-1617. Tipper, H.W., 1971. Multiple glaciation in central British Columbia. Can. J. Earth Sci., 8: 743-752. Tyrrell, J.B., 1919. Notes on the placer mines in the Cariboo, British Columbia. Econ. Geol., 14: 335-345. Vos, R.G., 1975. An alluvial plain and lacustrine model for the Precambrian Witwatersrand deposit of South Africa. J. Sediment. Petrol., 45: 480-493. Williams, G.J., 1974. Economic Geology of New Zealand. Aust. Inst. Min. Metall,, Monogr. Ser., 4:490 pp. Wilson, A.F., 1984. Origin of quartz-frec nuggets and supergene gold found in laterites and soils--a review and some new observations. Aust. J. Earth Sci., 31: 303-316. Wolfe, J.A., 1985. Distribution of major vegetation types during the Tertiary; In: E.T. Sunquist and W.S. Broecker (Editors), The Carbon Cycle and Atmospheric CO2: Naturai Variations Archean to Present. Am. Geophys. Union, Geophys. Monogr., 32: 357-375.