Formation and preservation of a Tertiary to Pleistocene fluvial gold placer in northwest British Columbia

Quaternary International 82 (2001) 33–50

Formation and preservation of a Tertiary to Pleistocene fluvial gold placer in northwest British Columbia Victor M. Levsona, Heather Blythb a

British Columbia Geological Survey, Ministry of Energy and Mines, 1810 Blanshard Street, Victoria, B.C., Canada V8 V 1X4 b Quaterra Environmental Consulting Ltd., 1844 Buena Vista Avenue, Comox, B.C., Canada V9 M 2B7

Abstract A unique longitudinal section through 2 km of a fluvial placer deposit in northwest British Columbia provides an excellent sedimentologic and stratigraphic record of the placer sequence and insight into the Pleistocene history of the region. The section is exposed as a result of long-term, open pit, placer gold mining along Otter Creek. Deposition of the auriferous gravels is believed to have occurred mainly in the Tertiary and Early Pleistocene although parts of the uppermost gravels may have been deposited as recently as the Late Wisconsinan. The gold-bearing strata are mainly coarse-grained gravels that were deposited by high-energy fluvial flows in a narrow bedrock-confined valley. The proposed depositional model shows that ice damming of Otter Creek during an early glaciation, resulted in a dramatic shift from a mainly erosional, fluvial system to an aggrading, glaciofluvial environment. Deltaic foresets can be traced upsection into delta topsets, upvalley into braided stream deposits, and downvalley into deformed proximal and then distal prodelta glaciolacustrine sediments. Late Wisconsinan glaciation appears to be the most erosive glacial event, but a pronounced and widespread unconformity at the base of the till did not penetrate to the placer gravels in the mine area. Preservation of the gravels is attributed to ice-damming and rapid aggradation of the overlying, glaciofluvial and glaciolacustrine sediments. In addition, local glaciers did not flow down the valley prior to the damming event and the valley is oriented oblique to the regional ice-flow direction, further inhibiting erosion. Sites with similar stratigraphic and geomorphic settings and glacial histories are potential exploration targets for gold-bearing paleochannel deposits. Placer gold has been recently recovered from at least one such site in the area where paleoplacer potential was previously inferred using geologic criteria. r 2001 Elsevier Science Ltd and INQUA. All rights reserved.

1. Introduction The distribution of paleoplacer deposits in glaciated regions is generally highly discontinuous due to extensive erosion by ice and subglacial meltwater during the Pleistocene. Deposits that do escape erosion are commonly buried by thick successions of Quaternary sediments (e.g. Clague, 1989; Eyles and Kocsis, 1989; Morison, 1989; Levson and Giles, 1993). Consequently, a detailed understanding of local glacial history as well as the geology of the paleoplacers and their associated overburden sequence is required to identify, evaluate, and potentially mine these deposits (Armstrong, 1983; Pizey, 1991; Levson and Morison, 1996). In this paper, we report on the results of an investigation of the Otter Creek buried placer deposits in a glaciated part of northwestern British Columbia. Stratigraphic and E-mail addresses: [email protected] (V.M. Levson), [email protected] (H. Blyth).

sedimentologic data are provided to illustrate their applicability in investigating paleoplacer deposits and to demonstrate the typically complex relationships between these deposits and overlying Quaternary sediments. The Otter Creek paleoplacer gold deposit is located approximately 20 km east of Atlin townsite (Fig. 1). The area was selected for study because mining along Otter Creek throughout much of the century has produced an excellent longitudinal exposure over 2 km in length, thus providing an unique opportunity to investigate the Quaternary succession associated with the placer gravels. Recorded gold production from Otter Creek during the first half of the 20th century was approximately 700 kg, making it one of highest gold producing areas in the Atlin mining district (Holland, 1950). The Otter Creek mine is one of the largest, open-pit, placer mines in northwest British Columbia. Results of the Otter Creek study, are used to identify, compare and evaluate, from an exploration perspective, other potential buried placer deposits in the region.

1040-6182/01/$ - see front matter r 2001 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 1 ) 0 0 0 0 7 - 6

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Fig. 1. Location of study area and place names mentioned in the text. Details of the Otter Creek study area are shown in Fig. 2.

1.1. Previous work

2. Study area

Regional 1 : 250,000 scale terrain mapping was conducted by Lacelle (1989) and the surficial geology of the Atlin and Surprise Lake areas (NTS 104 N/11, 12) was mapped at a 1 :50,000 scale by Levson and Kerr (1992). Preliminary results of a regional investigation of the Quaternary geology and placer deposits of the Atlin region were presented by Levson (1992a). Levson and Blyth (1993) discussed applications of Quaternary geology to surficial and buried placer investigations using, as a case study example, the placer geology of the Atlin area generalized from several deposits in the region. This paper follows that work and describes in detail the best studied of those examples, the Otter Creek paleoplacer. Other investigations of placer deposits in the Atlin area have been conducted by Black (1953), Proudlock and Proudlock (1976), and Debicki (1984). Anderson (1970) conducted a geobotanical study of Holocene events in the region and Tallman (1975) investigated the Quaternary history of the Fourth of July Creek valley (Fig. 1). Bedrock geology mapping in the study area has been completed by Aitken (1959), Monger (1975), Lefebure and Gunning (1989), Bloodgood et al. (1989a), and Ash and Arksey (1990a).

2.1. Surficial geology The study area is characterized by low mountains rising up to elevations of about 2000 m separated by broad, glaciated valleys that are locally occupied by lakes such as Atlin Lake (B670 m asl) and Surprise Lake (B940 m asl; Fig. 1). Otter Creek is a small tributary of the Surprise Lake valley, and flows northward mainly along a gently sloping U-shaped glacial valley before dropping down into the lake (Figs. 1 and 2). The upper part of the Otter Creek valley is narrow with steep walls and a nearly flat floor. The valley widens substantially where it opens out into the larger Surprise Lake valley near the south end of the Otter Creek mine. The original geomorphology of the Otter Creek channel in this area has been strongly altered by mining activity but a few preserved benches may be isolated remnants of the former valley floor and small valley-side terraces. A thick blanket of glaciofluvial gravels and sands occur in a hummocky icecontact kame complex that extends from Surprise Lake to the south end of the mine. A number of sinuous meltwater channels cross this area (Fig. 2), generally trending easterly to southeasterly. The lower reaches

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Fig. 2. Digital elevation model of the Otter Creek area identifying the location of measured sections shown in Fig. 3a and b. See Fig. 1 for location. The identified surface linear on the east side of the Otter Creek valley is an area with inferred potential for a fluvial paleochannel placer deposit. Meltwater channels at the north end of the valley occur in an area of hummocky moraine. See text for discussion.

of Otter Creek incised a narrow channel through the upper part of the glaciofluvial complex, locally following meltwater channel segments. A gentle gradient fan-delta occurs at the mouth of Otter Creek where it empties into the west end of Surprise Lake. The eastern part of the fan has been greatly enlarged with mine tailings. Mountain areas in the region, above approximately 1500 m elevation, are characterized by bedrock exposures or a thin veneer (less than 1 m thick) of colluvium over bedrock (Levson and Kerr, 1992). Colluvial and morainal veneers dominate most slopes down to elevations of about 1100 m. Lower slopes and valley bottoms throughout the area are blanketed by morainal deposits, generally a few to several meters thick. Erosion was intense during the last glaciation and preservation of older unconsolidated sediments is rare (Levson, 1992a). In valley bottoms, where these sediments might be present, they are obscured by thick deposits of Late Wisconsinan glacial and glaciofluvial deposits. Fluvial deposits are confined mainly to narrow floodplains of streams throughout the area and alluvial fans occur at the mouths of most creeks entering the Surprise Lake valley. Organic deposits occur locally in bogs and marshes, mainly around the shores of Surprise Lake. Glaciolacustrine sediments have been mapped mainly in the area around Atlin Lake (Lacelle, 1989; Levson and Kerr, 1992).

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south-westward over Stikinia in the Middle Jurassic (Monger et al., 1982; Gabrielse, 1991). The Cache Creek Terrane includes a lower accretionary complex of steeply to moderately dipping pelagic metasediments with lesser amounts of metabasalt, limestone and wacke, that is tectonically overlain by an ophiolitic assemblage of metamorphosed oceanic crustal and upper mantle rocks (Ash, 1994). Carbonate-altered ultramafic rocks (listwanites) in the upper ophiolitic assemblage are of particular economic importance because most goldbearing quartz veins in the region occur within or marginal to these rocks (Ash and Arksey, 1990b, c; Ash, 1994). Ultramafic rocks, locally showing listwanitic alteration, underlie much of the Otter Creek mine and outcrop on both the east and west sides of the Otter Creek valley. Otter Creek follows a major structural fault (Lefebure and Gunning, 1989) and heavily sheared and slickensided strata are common in outcrops at the base of the mine. Auriferous quartz veins associated with fault zones and listwanites are believed to be the source of placer gold in the region (Ballantyne and MacKinnon, 1986; Mackinnon, 1986; Bloodgood et al., 1989b; Ash, 1994).

3. Description and interpretation of deposits Gold-bearing strata in the Otter Creek valley are overlain by a complex sequence of Quaternary deposits consisting of over 25 m of non-auriferous gravel, sand, silt and diamicton (Figs. 3 and 4). The stratigraphic relationships of the placer deposits with the Quaternary overburden sequence can be observed readily in the 2 km long mine exposure. Descriptions and interpretations of five main stratigraphic units recognized, are provided below. 3.1. Unit 1: gold-bearing paleochannel gravels

2.2. Bedrock geology

Gold-bearing gravels at the Otter Creek mine are exposed mainly along the active highwall at the south end of the mine where several meters of auriferous, angular, pebble to cobble gravel are interbedded with poorly sorted, pebble to boulder gravel and diamicton (Fig. 4). Reported gold concentrations for the lower 5 m vary from approximately 0.5–2 g per m2 (E. Kolody, Whitehorse, Yukon, personal communication). Recovered nuggets are relatively large, averaging about 4.5 g but occurring up to 900 g (29 oz). The auriferous sequence has particularly high gold contents where large rounded boulders up to a few meters in diameter are concentrated. Several gravel and sand facies occur within the gold-bearing deposits as described below.

The study area is underlain by rocks of the Cache Creek Terrane which consists of remnants of a Late Paleozoic to early Mesozoic ocean that was thrusted

3.1.1. Flood flow gravels Massive, clast-supported, gravel beds, up to a few meters thick, are the most common gold-bearing

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lithofacies (Fig. 5). The gravels are poorly sorted, ranging from granules to boulders, with clasts up to 2 m in diameter. They are disorganized to crudely imbricated. Cobble to boulder clast clusters occur locally with sorted and stratified pebble gravels occurring between the larger clasts. The bedrock surface under these bouldery gravels is well water-worn, undulatory and generally dips towards the north with steep canyon-like walls to the east and west. Very crude bedding is locally evident. Beds, typically 0.5–2 m thick, fine upwards from cobble or boulder gravels to pebble gravels. Clasts in the channel center vary from rounded to well-rounded and angularity generally increases towards the paleochannel margins where subangular to angular clasts dominate. Clasts are mainly locally derived altered mafic volcanics. Weak iron staining and cementation occurs locally. Lower bed contacts are generally sharp and trough-shaped.

These deposits are interpreted as fluvial channel gravels probably deposited during flood flow events. The abundance of large boulders, angular clasts, scoured lower bed contacts and water-sculpted bedrock surfaces, suggest high energy, erosive flows. Deposition of large clasts and mobilization of gold probably occurred during rare flood flow events. Disorganized fabrics, crude normal grading and poor sorting in some beds, suggest deposition from sediment-laden (hyperconcentrated) flood flows (Smith, 1986, 1987; Scott et al., 1992; Gupta, 1999). Subsequent lower-energy flows would have locally remobilized and sorted the finer interstitial gravels, concentrating the gold in bedrock traps and between large boulders. Large boulders, such as are common in this unit, are known to create localized turbulent zones and good placer traps (Spaggiari et al., 1999). During the Pleistocene, large volumes of gravels probably moved through the Otter

Fig. 3. Longitudinal cross-section of Quaternary sediments exposed at the Otter Creek mine: (a) north end, (b) central and southern end. The location of the sections are shown in Fig. 2. Numbers refer to units discussed in text. Vertical lines show locations of measured sections given in Fig. 4.

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Fig. 3. (continued)

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Fig. 4. Stratigraphic sections of the Otter Creek paleoplacer deposit and overlying Quaternary sediments. Section locations are shown on Fig. 3. Symbols and Unit numbers as in Fig. 3. Horizontal scale: (c) clay; (z) silt; (s) sand; (p) pebble; (c) cobble; (b) boulder. Vertical scale in meters above section base (see Fig. 3 for elevations).

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locally derived and incorporated gold from pre-existing auriferous colluvial and alluvial sediments. Their abundance along the channel margins suggests that they originated from slumps and other mass movements along the valley walls, possibly induced by erosion of the steep channel banks.

Fig. 5. Gold-bearing massive gravel facies (Unit 1) with cobble cluster behind 50 cm wide boulder. Paleoflow from right to left.

Creek valley and gold transported with the gravels would have preferentially accumulated at the base of the fluvial system. The steep undulatory geometry of the bedrock is locally suggestive of paleowaterfall and plunge pool channel features. Narrow bedrock channels and associated scour pools are ideal for creating fixed zones of turbulence where placer minerals can accumulate over time (Levson and Giles, 1993; Jacob et al., 1999; Spaggiari et al., 1999). The local distribution of the gravels along the steep east and west sides of the bedrock canyon indicates that deposition occurred along the channel margins as well as in the paleochannel thalweg. Gold grades are generally lower in such channel margin deposits (Levson and Giles, 1993) and post-mining remnants of these gravels along the channel sides in the lower parts of the mine are evidence of this in the Otter Creek valley. 3.1.2. Gravelly debris-flow deposits Massive, matrix to clast-supported, gravels and gravelly diamictons occur mainly along the paleochannel margins. These gravelly deposits consist of locally derived, angular pebbles and cobbles with a silty sand matrix (Fig. 4). They have a chaotic fabric and they occur as irregular, trough-shaped lenses up to several meters wide. Coarse-tail reverse grading is indicated by the presence of boulders up to 0.75 m in diameter at the top of some beds. High clast contents, poor sorting, matrix support, numerous angular clasts, disorganized fabric and reverse graded bedding in these deposits indicate that they were probably deposited by noncohesive debris flows (Harvey, 1984; Kochel and Johnson, 1984; Nemec and Steel, 1984; Postma, 1986; Scott et al., 1992; Blair and McPherson, 1998). The lack of clay in the matrix precludes a cohesive debris flow origin for most of these deposits (Lowe, 1979). The debris flows were probably

3.1.3. Gravel bar deposits Horizontally bedded and planar cross-stratified pebble gravel and sand facies locally occur in the upper part of this unit, usually overlying massive gravel beds. Gravel beds are typically clast-supported with a sandy matrix. These deposits are moderately to well stratified and moderately well sorted. They are most common in the lee of bedrock knobs and they also infill areas between large boulders and some narrow bedrock crevices (Fig. 6). Strata in beds behind prominent bedrock ridges locally dip upstream. These stratified gravels and sands exhibit characteristics indicative of longitudinal and transverse bar deposits (cf. Hein, 1984; Morison and Hein, 1987) that developed locally as a result of relatively low energy, fluvial reworking of flood flow gravels (see above) and in lower-energy parts of the paleostream channel. Strata dipping upstream in the lee of some bedrock ridges probably formed in gravel bars that developed in eddy currents. Gravel beds infilling bedrock crevices are generally reported to contain relatively high gold concentrations. 3.1.4. Channel-fill gravels Trough-shaped lenses of medium to large pebble gravels, typically about 0.5 m thick and a few meters wide, also occur locally in the upper part of this unit (Fig. 7). The lower contacts of these lenses are commonly marked by cobble concentrations. The gravels are usually open work and commonly stained with iron and/ or manganese oxides. These gravels may be capped

Fig. 6. Horizontally stratified, pebble gravels of Unit 1 infilling depressions in water worn bedrock. Pick at top center is 65 cm long.

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of the unit is, in places, indicated by the presence of rare striated clasts and by a locally conformable contact with overlying glaciofluvial deposits in Unit 2. 3.2. Unit 2

Fig. 7. Trough cross-bedded channel gravel and sand facies in the upper part of Unit 1.

either by fine sand and silt beds, up to 1 m thick, exhibiting broad trough-shaped bedding or by horizontally bedded, sandy pebble gravels. The latter are typically moderately to well sorted with beds 5–25 cm thick. These sand and gravel units are interpreted as channel-fill deposits. Sedimentary characteristics typical of these deposits include trough cross-bedding, finingupwards channel-fill sequences, channel-shaped bed geometries and coarse, channel-lag gravels (Harms et al., 1982; Morison and Hein, 1987; Miall, 1992). 3.1.5. Time of deposition The precise age of the gold-bearing gravels at the Otter Creek mine has not been determined. However, a Late Tertiary to early Pleistocene age for stratigraphically equivalent placer gravels in the region is suggested by palynological data and potassium–argon radiometric dates, ranging from 0.5 to 3.6 Ma, on basalts overlying and interbedded with auriferous gravels in the nearby Ruby Creek valley (Fig. 1; Levson and Blyth, 1993). A preglacial origin for the Otter Creek gravels is suggested by the abundance of locally derived rocks and lack of erratic lithologies. The presence of iron and manganese oxides and the degree of cementation in the auriferous gravels contrasts sharply with unoxidized, noncemented, overlying gravels of glaciofluvial origin. The generally higher permeability and greater age of the gold-bearing gravels probably resulted in their more intense cementation. A radiocarbon date of >41,180 years BP (AECV 1499C) was obtained on charcoal collected during this study from the upper part of a placer gravel sequence at one section on Ruby Creek. Infinite radiocarbon dates also have been obtained on organics, found under till stratigraphically overlying the gravels, at two other sites in the Atlin region (Boulder and McKee creeks; Fig. 1). These dates clearly indicate that the placer gravels minimally predate the Late Wisconsinan glaciation. Although most of the auriferous gravels are believed to be preglacial, a Late Pleistocene age for the upper part

3.2.1. Unit 2a: glaciofluvial deltaic deposits This unit consists of up to 15 m of sandy pebble gravels exhibiting well developed, large scale, planar cross-bedding (Figs. 3, 4 and 8). Beds of clast-supported, matrix-filled gravels, 5–25 cm thick dominate. They are interbedded with oxidized, open work beds, typically less than 15 cm thick, and parallel laminated sand beds up to 20 cm thick. Clasts show a strong b-axis imbrication (a[t], b[i] type of Harms et al., 1982). They vary from angular to well rounded but are mostly rounded to subrounded. Bed contacts are sharp and dip northward at angles of 25–301. Beds generally thicken down dip and they are locally normally graded. The dip of beds decreases to nearly horizontal in the uppermost few meters of the unit. Large-scale, steep, planar cross-bedding in these gravels is interpreted as deltaic foreset bedding. A well-developed b-axis imbrication indicates tractional deposition along the foresets. Normal grading in some beds may reflect the localized development of sediment gravity flows along the steep delta margin. Thickening of beds in the down-dip direction and flattening of strata in the up-dip direction reflects the transition to prodelta and topset deposits, respectively (see Units 2c and 2b). 3.2.2. Unit 2b: glaciofluvial braided stream deposits This unit is dominated by moderately to poorly sorted, imbricated, large pebble to cobble gravels with crude horizontal bedding (Figs. 3, 4 and 9). Beds are mainly clast-supported with a sandy to granular matrix and commonly, 0.5–2 m thick. Weakly iron stained, open-work beds containing little or no matrix also occur. Clasts are mainly subrounded to rounded with less than 5% angular cobbles. Ultramafic and mafic volcanic clasts are common. Approximately 10% of the unit consists of parallel laminated, trough-shaped, fine sand lenses that are typically about 10–50 cm thick and 1–5 m wide (rarely up to 20 m wide). Interbedded small pebble lenses occur within the sands. Flame and load structures are common. Bed contacts are usually sharp with downvalley-dips of up to several degrees. Coarse cobbly strata with crude, trough-shaped bedding and interbedded gravelly diamicton beds, occur locally within this unit (Fig. 10). Large clast clusters with clasts up to about 1 m in diameter occur in these beds and striae are present on a few clasts. Cobble beds locally fine-up through pebble gravels to laminated, fine sands. Interbedded planar cross-bedded gravels also occur rarely. Diamicton beds are massive to crudely bedded, up to 1 m thick and matrix-supported, with a

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Fig. 8. Steeply dipping, deltaic foreset gravels (Unit 2a) overlain by till and debris-flow deposits (Unit 3) and ice-proximal sand and gravel deposits (Unit 4).

Fig. 9. Section along Otter Creek exposing bedded braided stream gravels (Unit 2a) unconformably overlain by basal till deposits (Unit 3). Rod behind person in center-left is 4 m high. Note the sharp, nearly horizontal and laterally continuous unconformity at the base of the till.

silty sand matrix (Fig. 10). Gravel content in these beds ranges from approximately 30–50%. Clasts are subangular to subrounded, many are striated and they exhibit a chaotic fabric. The lower contact of this unit over Unit 2a is locally marked by cobble concentrations and often truncates bedding in the underlying gravels. The contact is clear and planar, generally dipping 3–51 downvalley to the northwest (3101). This unit was only observed to directly overlie Unit 1 at the southern most end of the mine (Fig. 3b). The lower contact there is sharp and horizontal but no truncation of bedding or obvious erosional unconformity was observed. Poor sorting, imbrication and crude horizontal bedding in these gravels are representative of longitudinal bar deposits in shallow, gravelly, braided streams (cf. Hein, 1984; Miall, 1992) particularly in proximal glaciofluvial environments (Boothroyd and

Fig. 10. Proximal glaciofluvial gravels and diamicton in Unit 2b. Exposure is 3.5 m high.

Ashley, 1975; Rust and Koster, 1984). Sand lenses interbedded with the gravels probably were deposited in channels during periods of decreased flow, variable flow rates being typical of proximal glaciofluvial streams (Church and Gilbert, 1975). Channelized flow is locally indicated by crude trough-shaped bedding. Cobble gravel beds, fining upwards into pebble gravels and laminated sands, represent typical channel-fill sequences. Imbrication and clast clusters indicate the dominance of tractional flow and planar cross-bedding suggests local development of transverse gravel bars. Poor sorting, large clast size, a high proportion of striated clasts and interbedded diamicton deposits in some parts of this unit suggest nearby ice (Fig. 10). Diamicton beds are interpreted as glacially derived, debris flow deposits as indicated by matrix support, crude bedding, the abundance of striated clasts and lithologic variability. The presence of laterally extensive interbeds of laminated fine sands and abundant load and flame

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structures at the north end of the mine (Fig. 3a) suggests local deposition in ponded water. Towards the south end of the mine where this unit overlies gravels of Unit 2a (Fig. 3b), the delta topset beds grade upvalley into braided stream deposits that overlie Unit 1. The erosional lower contact presumably resulted from the downvalley migration of the braided stream gravels over the foreset beds during delta progradation. 3.2.3. Unit 2c: proximal glaciolacustrine deposits This unit consists of up to 10 m of parallel laminated, fine sand and silt (Figs. 3a, 4 and 11). Sand beds are up to 1 m thick and locally show trough cross-bedding and some climbing ripple-bedding. Localized clay beds up to 25 cm thick, occur throughout the unit and minor gravel interbeds occur in the upper few meters. Large clasts (dropstones) commonly deform laminae in underlying beds and are draped by overlying strata. They occur throughout the unit but are most abundant in clay-rich beds. Most beds thicken towards the north (downvalley) and the unit rises and pinches out towards the south (Fig. 3a). As a consequence, the dip of sandy strata at the base of the unit increases towards the south to as much as 251 but bedding rapidly becomes more horizontal up-section and down-valley. Large-scale, intraformational, overturned folds and convoluted bedding are common in the lower half of this unit (Figs. 3a and 11). Axial planes in large overturned folds dip southerly. Upwards-narrowing clay injection structures, up to 10 cm wide and 2 m or more long, occur in the upper 2 m of the unit where it is overlain by massive diamicton of Unit 3. Unit 2c is exposed only at the northern end of the mine where the Otter Creek valley issues into Surprise Lake (Fig. 3a). Up-valley, this unit is laterally interbedded with stratified diamictons of Unit 3 (Fig. 3a). The dominance of parallel laminated, fine grained, sediments and abundant dropstones suggests that

sediments in this unit were deposited in a glacial lake. The increase in abundance of fine-grained strata and thickening of beds towards the north suggests that the lake was ponded in the Surprise Lake valley. Similarly the rise and pinching out of the unit towards the south reflects the shallowing of the lake in that direction. Horizontally stratified, clay, silt and fine sand beds represent deeper water deposits and the increase in abundance of these sediments downvalley and upsection reflects both expansion and deepening of the lake with time. The presence of coarse sand, diamicton and gravel interbeds with fine-grained sediments along the southern margin of this unit is interpreted to reflect sedimentation in shallower water from prodelta debris flows and underflows. Ripple bedding and trough crosslaminae in sand beds throughout the unit also indicate the influence of underflow currents that extended out into the lake bottom. Instability along the delta front is recorded by the presence of highly disrupted bedding. The orientation of large overturned folds indicates that the disruption of bedding probably took place as a result of slumps moving northward off the Otter Creek delta front. The intraformational nature of the deformation indicates that the slumping was episodic. The upper limit of ponded water in the Otter Creek valley occurs at about 1060 m above sea level, as indicated by the level of delta topset beds. Lake level was probably controlled by the elevation of the pass north of Surprise Lake which would have acted as a northern outlet to a raised level of the lake, if the lower Pine Creek valley was blocked by ice. The modern elevation of this pass is approximately 1010 m above sea level and it is probable that the 50 m difference between that elevation and the maximum level of the lake can be accounted for by glacial deepening of the outlet valley. 3.3. Unit 3: glacial deposits Two diamicton lithofacies are recognized in the overburden sequence at the Otter Creek mine (Figs. 3 and 4). These include crudely stratified diamicton with interbedded clay, silt, sand and gravel (debris flow deposits) and over-consolidated, massive diamicton (basal meltout and lodgment till deposits).

Fig. 11. Intraformational folds in Unit 2c reflecting syndepositional subaqueous slumping. Note the abundant dropstones. Measuring rod is 4 m high.

3.3.1. Debris flow deposits These deposits consist of crudely stratified diamicton with numerous lenses and intrabeds of clay, silt, sand and gravel. Diamicton beds contain 30–50% clasts and are mostly matrix-supported but clast-supported beds also occur. Beds often occur as broad, trough-shaped, lenses that taper and eventually pinch-out down valley. Matrix textures range from silty clays to sands. Clasts vary from angular to rounded and are randomly oriented. They are often striated and of erratic lithology. Carbonate cementation is locally strong. Silt, sand and

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clay beds are laminated and up to 20 cm thick. Gravelly beds are massive, matrix or clast-supported, matrix filled and poorly sorted. Large clasts in clay intrabeds commonly protrude into and are draped by overlying diamicton beds. Diamicton beds are interpreted to be debris flow deposits as indicated by poor sorting, crude stratification, and chaotic pebble fabrics. Variations in clast content, matrix texture and sorting probably mainly reflect differences in the composition of source materials and water content of the flows. Intrabeds of clay, silt, sand and gravel indicate that minor lacustrine and fluvial sedimentation occurred periodically between debris flows. The abundance of striated and erratic clasts indicate that the flows were probably derived from nearby glaciers. Similar deposits are well documented from both modern and ancient ice-marginal environments (e.g. Lawson, 1979, 1981a, b, 1982; Levson and Rutter, 1988; Bennett and Glasser, 1996). Lateral interbedding of gravelly diamicton and matrix-supported gravel beds with silts and clays at the north end of the minesite (Fig. 3a) indicates that these beds may have originated as subaqueous, pro-delta slump deposits. 3.3.2. Basal till deposits These deposits consist mainly of massive, overconsolidated diamicton. The only structures apparent in the diamicton are weak horizontal fissility or rare, vertical variations in color or total clast content. The diamicton typically contains about 10–30% clasts, supported in a sandy-silt matrix (Fig. 12). Clasts are subangular to subrounded and commonly striated. They generally are up to small-cobble size, and boulders are rare or absent. The a-axis of elongated clasts show a strong preferred north to northwesterly orientation (Fig. 4). The lower contact of diamicton beds of this type over sands or gravels is invariably sharp and nearly horizontal (Fig. 9). The underlying sands and gravels commonly have well-developed, compressional, deformation structures including chevron folds, overturned folds and small-scale thrust faults (Fig. 13). Horizontal shear planes with northeasterly (0501) trending, slickensided surfaces occur at the base of diamicton beds of this type at the northern end of the minesite. Rare, discontinuous, cobble-rich layers contain up to 50% clasts supported in a silty-sand matrix. At one locality, two such layers, 10–20 cm thick, are separated by 2 m of diamicton with a silty-clay matrix. Clasts are subrounded to angular and up to 0.5 m in diameter. Striated and faceted clasts are common and horizontal shear planes are locally abundant. Fabric data also show a strong preferred orientation of clast long-axes parallel to the valley orientation. Massive diamicton sequences can locally be divided into subunits by subtle variations in color and texture. At the south end of the mine, two 4–5 m thick subunits

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are recognized (Fig. 4), the upper usually being lighter in color and having a higher total clast content than the lower. The contact between the two subunits varies from clear to diffuse. The upper subunit is moderately compact and more permeable than the lower as indicated by the presence of water seeps at the contact. The abundance of striated clasts, strong unimodal pebble fabrics, fine matrix textures and over-consolidation indicate that these diamictons are lodgement or melt-out tills (Boulton, 1976; Kruger, 1979; Haldorsen, 1982, Dreimanis, 1988, Bennett and Glasser, 1996). Features indicative of lodgement include a high proportion of glacially abraded and embedded clasts, strong valley-parallel fabrics, erosional basal contacts, and compressional deformation of the underlying deposits (Boulton, 1976, 1978, 1996; Kruger, 1984; Levson and Rutter, 1988; Dreimanis, 1993). Variations in color in massive diamicton beds probably reflect changes in the provenance of the source materials. The dark coloring of the diamicton near the base of the till sequence at the south end of the mine, for example, may reflect erosion

Fig. 12. Matrix-supported diamicton of Unit 3, interpreted as basal till, unconformably overlain by trough-shaped sand and gravel lenses in Unit 5. Measuring rod is 4 m high.

Fig. 13. Small-scale glaciotectonic faults and compressional folds in sand lense underlying Unit 3.

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and incorporation of local dark bedrock, whereas the lighter coloration of the overlying diamicton subunit may result from the addition of more distally derived material and less erosion of local bedrock. Color banding and the presence of rare clast-rich layers in a few otherwise massive diamicton units may also indicate deposition by meltout processes (Haldorsen and Shaw, 1982; Levson and Rutter, 1988). This interpretation is supported by the lower compaction, slightly coarser matrix texture and higher permeability of these deposits compared to other massive diamicton units. The movement of ice, up the northeast-trending Surprise Lake valley into the Otter Creek valley, is first recorded by northeasterly trending slickensides in the lower part of this unit. The orientation of overturned folds and thrust faults in sands and gravels underlying the tills also record the same sense of ice movement. Fabric data from the main part of the unit (Fig. 4) indicate a general northwesterly to northerly paleoflow. A dominantly west–northwest, regional flow at the last glacial maximum is indicated by the trend of flutings on high mountain peaks in the region (Levson and Kerr, 1992). Changes in fabric orientations from northwesterly in the main part of the till sequence to northerly near the top of the sequence (Fig. 4) may reflect late shifts in ice flow direction. This may have resulted from the influence of topographic control, towards the end of the last glaciation when ice in the area had thinned substantially (resulting in a more northerly flow in the Otter Creek valley). 3.4. Unit 4: ice-contact deposits This unit consists of interbedded diamicton, gravels and sands (Figs. 3a, b, 4 and 14). It is thickest and most commonly occurs in areas of hummocky topography (Fig. 3). Diamicton beds are massive or crudely stratified. Beds may be matrix or clast-supported. Matrix textures vary from silty to sandy. Similarly, maximum clast-size in diamictons varies from large pebbles to boulders. Beds are commonly lens shaped and typically about 1–2 m thick. Clasts include both angular rocks of local origin and many, subangular to well rounded, distally derived erratics. Angular intraclasts, up to 20 cm in diameter, of unconsolidated sand with preserved internal stratification, often dipping at high angles, are locally common. Bed contacts are sharp to gradational. Interbedded, pebble gravel beds are typically massive, clast-supported and poorly sorted. They vary from matrix-filled gravels to gravelly sands. Some open work lenses with a granular matrix also occur. Cobble beds are uncommon. Beds are up to 1 m thick but are usually less than 50 cm. Interbedded, parallel laminated and trough cross-laminated fine to medium sands are common. Gravels are usually trough cross-bedded but planar cross-bedding and horizontal bedding also occur.

Fig. 14. Interbedded gravels, sands and diamicton of Unit 4, erosionally overlain by coarse gravels of Unit 5.

Scour-and-fill structures up to 1 m deep and 5 m wide are common. Convoluted bedding is locally common (Fig. 14) with beds dipping in various directions at angles up to 601. Clasts vary from angular to rounded and some highly weathered clasts occur. Lenses of fine sand, up to 20 cm thick and 5 m wide, are common and also exhibit post-depositional deformation structures such as overturned folds and convoluted bedding (Fig. 14). Deformed beds are common adjacent to large cobbles and boulders (up to about 2 m in diameter) that occur rarely within the pebble gravel beds. Angular mud intraclasts occur in some beds. Laminated clay beds up to 20 cm thick are locally common in this unit. Diamicton beds in this unit have characteristics indicative of debris flow deposits including matrixsupport, crude stratification, lens shaped geometries, abundant sand intraclasts and frequent interbeds of sand and gravel. Variations in matrix texture and clast content probably reflect differences in rheology between cohesive and noncohesive debris flows. The abundance of clasts of erratic lithology indicates a glacial derivation. Interstratified sands and gravels probably were deposited in shallow, ice-marginal, braided streams between debris flow events. The abundance of scourand-fill structures in gravel beds and intraclasts of mud and fine sand, interpreted as rip-up clasts, indicates that stream flows were highly variable and erosive. Intraclasts of unconsolidated sand probably were frozen prior to being eroded. Sharp vertical and lateral changes in grain size and sorting, the presence of isolated large cobbles and boulders in pebble gravel beds and highly variable bedding strike and dip directions are suggestive of deposition in proximity to an ice-margin. Overturned folds, convoluted bedding and steep bedding dips in this unit probably reflect slumping and collapse of sedments in contact with ice. An ice-contact origin for this unit is supported by its association with hummocky topography.

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3.5. Unit 5: post-glacial fluvial channel gravels This unit consists mainly of interbedded, medium to large pebble and cobble gravels (Fig. 4). The unit shows horizontal bedding and large scale, trough cross-bedding with beds typically a few meters thick and 5–30 m wide (Fig. 12, top and Fig. 14). Beds locally fine upwards and usually have trough-shaped (Fig. 12) or subhorizontal (Fig. 14) lower contacts. The gravels are massive to crudely imbricated, moderately to poorly sorted, clast-supported and matrix filled. Iron and manganese stained, open-work beds occur at the base of some fining-up sequences. Pebble beds have a fine to coarse sand matrix. Cobble beds generally are about 1 m thick, laterally traceable for at least 30 m and have a coarse sand matrix. Boulders are rare. Trough-shaped, fine to coarse, sand lenses up to 0.5 m thick and 10 m wide are common (Fig. 12). The sands are trough crosslaminated and commonly grade upwards into parallel laminated silts. The lower contact of this unit is generally sharp and undulatory and is locally marked by small cobble and boulder concentrations (Fig. 12). This unit occurs as small terrace-like deposits that occur at several topographic levels. At its lowest level observed in the mine, this unit overlies deposits of Unit 3. These gravels are locally gold-bearing, particularly where they occur near the modern stream level, but grades are generally uneconomic. Gravels and sands of this unit are interpreted as Holocene stream channel deposits. Fining-up sequences with erosional lower contacts and basal gravel lags are interpreted as scour-and-fill deposits. Initial sedimentation at higher levels along the valley may have been influenced by the proximity of retreating glaciers as indicated by poor sorting, crude bedding and weak clast imbrication in these deposits. However, the general lack of out-sized clasts and relatively low (near modern) level of many of the terrace deposits suggests that deposition occurred mainly in nonglacial streams during the postglacial development of the Otter Creek valley. Channel abandonment presumably occurred as a result of episodic valley incision forming progressively lower terraces. Gold in these deposits presumably was eroded and reconcentrated from the Quaternary overburden and consequently is low grade in comparison with the older buried channel gravels (Unit 1).

4. Discussion 4.1. Reasons for preservation of the Otter Creek paleoplacer The Otter Creek valley is narrow and is oriented obliquely to the regional, west–northwest, ice-flow direction that dominated at the last glacial maximum.

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Preservation of the Otter Creek paleochannel gravels is attributed to this setting since northwesterly flowing glaciers probably would not have been able to erode deeply into a narrow, north-trending, bedrock walled valley. Shilts and Smith (1986, 1988) came to a similar conclusion after a study in the Appalachian Mountains where it was found that preservation of preglacial placer deposits occurred in valleys oriented transverse to the regional ice flow direction but many other deposits were destroyed during glaciation. Similarly, paleochannel placer deposits in the Cariboo region in central British Columbia and in the Livingstone Creek area in central Yukon Territory are preserved along tributary streams oriented obliquely to the former ice flow direction of the main valley glaciers in those areas (Levson, 1992b; Levson and Giles, 1993). Preservation of the Otter Creek paleoplacer gravels is also attributed to rapid aggradation, induced by icedamming, of the overlying glaciofluvial and glaciolacustrine deposits. Since local glaciers did not occupy the valley prior to the damming event, erosion of the placer deposits by glacial ice or meltwater did not occur. During the last glaciation, the Otter Creek valley was subjected to erosion by topographically controlled, valley glaciers only during deglaciation, but the ice at that time apparently did not erode to the depth of the paleochannel gravels. In addition, the clay-rich glaciolacustrine sediments that were deposited in the icedammed lake probably inhibited ice erosion of the underlying gravels. Paleochannel placer deposits preserved in comparable settings (i.e. in tributary stream valleys dammed by main valley glaciers) have been described from numerous glaciated areas including the Cariboo region in central British Columbia (Levson and Giles, 1993), the Livingstone Creek area in Yukon Territory (Levson, 1992b) and the Valdez Creek area in Alaska (Reger and Bundtzen, 1990). 4.2. Regional implications for exploration Gold-bearing gravels, believed to be stratigraphically equivalent to the Otter Creek paleoplacer deposits, occur in other valleys in the region including the Ruby, Boulder, Spruce, McKee and Pine Creek valleys (Fig. 1). The gold-bearing gravels at Ruby Creek are conformably overlain by a sequence of Pleistocene columnar basalts which are in turn locally overlain by till and rock avalanche deposits (Levson and Blyth, 1993). Since the complex overburden sequence has inhibited exploration, the area still has high potential for the discovery and exploitation of new deposits. Currently mined gravels are similar to the Otter Creek paleochannel deposits. They consist of clast-supported, mainly matrix filled, poorly to well sorted, cobble and boulder gravels with some pebble beds and they exhibit horizontal stratification, clast clusters and crude imbrication. They are

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interpreted as high energy, fluvial, channel gravels and hyperconcentrated flood-flow deposits. Gold nuggets are typically about 2 mm in diameter and subrounded to angular suggesting short distances of transport (Knight et al., 1999). The largest recently recovered nugget was 180 g (5.75 oz) but nuggets up to 1.37 kg (44 oz) have been reported. Gold grades in the lower few meters of the auriferous gravels vary from 15 to 150 g (0.5–5 oz) per m2 (Levson, 1992a). As in the Otter Creek valley, bedrock rises sharply on the valley walls and follows low gradient benches along the valley-bottom margins which slope approximately 2–31 down valley. By comparing the geological setting of the Otter Creek mine with the largely unexplored Wright Creek valley (Figs. 1 and 2), Levson (1992a) suggested that there was also high potential for a large, buried channel, placer deposit along Wright Creek. The two valleys are geomorphologically similar, each being relatively narrow and deep, and both are oriented obliquely to the west–northwest, regional, ice-flow direction. Gold-bearing Holocene gravels have been recovered from both valleys and post-glacial alluvial fan sediments have been mined on upper Wright Creek (Levson, 1992a). Although less extensive along Wright Creek than Otter Creek, potential host rocks, particularly altered ultramafics, have been mapped in both valleys (Lefebure and Gunning, 1989). Till deposits mantle the surface in both areas and it is probable that auriferous paleochannel gravels, stratigraphically equivalent to the Otter Creek deposits, underlie the till in the Wright Creek valley. This interpretation was substantiated by evidence of an underground operation along lower Wright Creek that exploited a rich gold-bearing gravel at approximately 30 m depth (Levson, 1992a). The broad alluvial flat on Wright Creek, downstream from the point where it bends from a northwest to a northerly trend (Figs. 1 and 2), was identified by Levson and Kerr (1992) as the area with the best potential for a buried placer deposit. The valley there is narrow and oriented obliquely to the regional ice-flow direction and therefore may have escaped deep glacial erosion. These interpretations have been confirmed in recent years by a deep, open-pit, placer-mining operation exploiting paleoplacer deposits along Wright Creek (D. Flynn, Smithers, British Columbia, personal communication). A photo of a nugget recovered from this placer operation is illustrated in Fig. 15. A comparison of the Otter Creek paleoplacer with modern drainage patterns in the area suggests that there is also some potential for buried, tributary channel, deposits along both sides of the Otter Creek valley (Figs. 1 and 2). For example, buried fluvial gravels have been mined on Snake Creek which drains a low pass on the east side of Spruce Mountain (Figs. 1 and 2). Recovered gold is relatively coarse with nuggets commonly 2–3 g, but up to 30 g, in weight. Bedrock

Fig. 15. Gold nugget weighing approximately 900 g (29 oz) recovered from a recent placer mining operation on Wright Creek, adjacent to the Otter Creek mine. Note smoothed and rounded edges and corners of nugget, typical of high energy, fluvial placer nuggets (photo courtesy of D. Flynn).

in the area is overlain by 4.5 m of poorly sorted, large-cobble gravels interbedded with pebble gravels and horizontally laminated sands. The gravels are clastsupported, matrix-filled, horizontally bedded and crudely imbricated. They are overlain by a massive, matrix-supported, sandy silt diamicton. Gold has been recovered mainly from the Holocene Snake Creek channel but the possibility of a deeper buried channel, tributary to the Otter Creek paleochannel, is indicated at one site along the present creek where a bedrock depression, at least several meters deep, apparently crosses the creek obliquely. East of the creek, towards the Otter Creek valley, the possible paleochannel is buried by an additional several meters of glacial and glaciofluvial deposits. There is also potential for buried channel deposits along the east side of Otter Creek where the valley widens. The location of one such possible buried channel in this area is indicated by a linear surface depression that extends northeasterly from the Otter Creek valley (Fig. 2). The depth of burial of gold-bearing strata under till in the lower Otter Creek valley, and inferred extensions of those deposits, suggest that buried paleochannel deposits probably also exist in the Surprise Lake/Pine Creek valley (Figs. 1 and 2). Although the stratigraphy in the lower Otter Creek valley indicates that thick glacial and glaciofluvial overburden would inhibit exploration along Pine Creek, there may be some potential close to the valley sides. The area between the Birch Creek confluence and Surprise Lake (Fig. 1) has not been mined and, given the historical productivity of upstream tributaries such as Otter, Boulder and Ruby creeks, it seems probable that paleochannels in that area would also be gold-bearing. Depth of ice erosion and thick

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overburden are the main factors limiting the location and exploitation of these deposits. More documentation of these factors is required before the placer potential of this area can be further evaluated. The geometry of the Otter Creek paleoplacer, suggests that locations of undiscovered paleochannel deposits in the region will be constrained largely by the bedrock topography. For example, the narrow width of the bedrock canyon containing the Otter Creek deposit suggests that any unexploited channel gravels along the sides the valley in this area or in other similar settings in the region will be small. Other narrow and deep valleys, particularly those oriented obliquely to the northwest regional, ice-flow direction, are considered to be good targets. The upper limit to mining in high gradient, paleochannels like Otter Creek is controlled by the depth of ice erosion in the upper reaches of the valleys.

5. Depositional model for the Otter Creek valley The stratigraphic and sedimentologic study of the succession of deposits exposed at the Otter Creek mine suggests the following sequence of events. Placer gold in the mine was concentrated during an extensive period of fluvial valley incision, probably during the Tertiary. Gold was derived from auriferous quartz-veins associated with fault zones and carbonate-altered ultramafic rocks. Gold-bearing gravels in Unit 1 were deposited in high energy, erosive, fluvial flows typical of preglacial and interglacial paleogulch environments. Relatively rare, hyperconcentrated flood flow events transported coarse gravels and gold into the system. Subsequent, lower-energy flows reworked the finer gravels and concentrated the gold in areas of increased turbulence caused by large boulders and bedrock irregularities. Channel scour-and-fill events occurred periodically and the development of typical channel bar gravels was relatively uncommon except in the upper parts of the unit. Episodic, mass movements along the valley walls and channel banks resulted in local deposition of debris flow deposits along the channel margins. Auriferous gravels are overlain by a complex sequence of crudely bedded, gravels, sands, silts and diamicton interpreted as proximal glaciofluvial and glaciolacustrine deposits (Unit 2). The proximity of ice is indicated locally by the poor sorting, presence of striated clasts and interbedded glacigenic debris flow deposits. The complexity of this overburden succession generally increases from the south to the north. A progressive downvalley facies change from large scale, planar crossbedded, delta foresets to interbedded delta front sand, diamicton and gravel and finally to horizontally bedded silts, clays and sands deposited in deeper water is recorded. Deltaic foreset gravels near the mine center (Unit 2a) are unconformably overlain by a topset gravel

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sequence that grades vertically and upvalley into braided stream deposits (Unit 2b), reflecting the down-valley migration of braided channel, topset gravels during delta progradation. Downvalley, the deltaic foreset gravels grade into prodelta glaciolacustrine silts and sands (Unit 2c). The transition of foreset gravels to prodelta deposits, is indicated by thickening of foreset beds in the down-dip direction and by lateral interbedding of glaciolacustrine sands, silts and subaqueous slump deposits. Proximal prodelta deposits in Unit 2c include coarse sand, diamicton and gravel interbeds interpreted as subaqueous debris flows and underflow deposits. Deeper water deposits in Unit 2c, represented by parallel laminated, fine-grained, sediments with abundant dropstones, increase in abundance and thicken northwards towards the center of the lake that was dammed in the Surprise Lake valley. Instability along the delta front as a result of slumping and icemarginal debris flows is recorded both by the presence of the interbedded diamictons in the shallow water deposits and highly disrupted bedding in some of the deeper water sediments. Glaciofluvial and glaciolacustrine deposits are unconformably overlain by compact, massive, matrixsupported, diamicton (Unit 3) interpreted as basal till. At the south end of the mine, a thick sequence of lodgement and meltout tills, are locally overlain by glacially derived debris-flow deposits. At the north end of the mine, debris flow deposits also underlie basal tills and are interbedded with Unit 2c. Less extensive glacial erosion at the north end of the valley probably reflects the higher proportion of subaqueous, clay-rich facies there, inhibiting erosion. It is inferred that glacier ice initially flowed up the Otter Creek valley from the Surprise lake valley. This flow was superseded at the last glacial maximum by a west-northwesterly, regional-flow and later by topographically controlled northerly flow down the valley. Basal till deposition in most areas followed a period of erosion by ice that produced a sharp, planar, basal unconformity. As a result, the age of Unit 2 cannot be determined. It is conceivable that Unit 2 was deposited during an earlier glaciation, possibly even the first glaciation to cause ice damming in the Otter Creek valley. In this case, subsequent glaciations would have eroded down to the level of the aggraded glaciofluvial and glaciolacustrine deposits of Unit 2. Alternatively, if Unit 2 was deposited at the beginning of the last glaciation, then all evidence for earlier Pleistocene glaciations in the valley must have been removed by fluvial erosion prior to deposition of Unit 2. There is a sharp and horizontal contact at the base of Unit 2 where it is observed to overlie Unit 1 but evidence for a significant unconformity there is lacking. We suggest that it is unlikely, although not impossible, erosion prior to deposition on Unit 2 would remove all evidence of

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older Pleistocene glacial and glaciofluvial deposits and not also remove the preglacial placer deposits. Consequently, we infer that Unit 2 may have been deposited during an earlier glaciation and that the widespread erosional unconformity at the base of Unit 3 represents the deepest level of Pleistocene erosion in the valley. Thus, either the Late Wisconsinan glaciation was the most erosive in the area or, less likely, Unit 3 is a compound till deposited during more than one event. During deglaciation, an area of stagnant ice topography developed along the lower part of the Otter Creek valley. Debris flow and glaciofluvial sedimentation (Unit 4) occurred along the ice margin. Subsequent melting of the ice resulted in slump and collapse structures, juxtaposition of highly variable sediment types and steeply dipping beds. Finally, horizontally bedded and trough cross-bedded stream channel gravels (Unit 5) were deposited in the valley, temporarily under the influence of retreating ice, but mainly as post-glacial terrace gravels.

6. Conclusions The antiquity and localized occurrence of productive placer gravels in the study region suggests that identification of the geological conditions required for their formation and preservation would be useful in exploration for buried placer deposits in other areas. The most obvious geological factor, the presence of a narrow, bedrock-walled valley at the site, was conducive not only to the development of a rich placer deposit but also to its preservation. The valley geometry resulted in focusing of flood waters and the development of sedimentologic traps where gold accumulated during valley erosion in the Tertiary and Early Pleistocene. The narrow and deep shape of the valley and its orientation, oblique to the regional ice-flow direction, probably also prevented the glaciers from eroding deeply into the paleoplacers. The onset of glaciation in the Otter Creek valley resulted in a dramatic change in the sedimentary regime, from a long-lived erosional river to a dominantly aggrading system, choked with glacial and glaciofluvial sediments. During glaciation, ice occupied the main Surprise Lake valley before the Otter Creek valley, causing the formation of an ice-dammed lake along the lower parts of Otter Creek, and probably also other tributary creeks in the area. In the lower part of the Otter Creek valley a glaciofluvial delta began to form in the lake and in the upper part of the valley; the sudden rise in base level resulted in a rapid change in sedimentation, with an erosional fluvial channel changing to an aggrading braided stream. Preservation of the Otter Creek paleoplacer gravels is also attributed to rapid aggradation of these glaciofluvial deposits as well as to glaciolacustrine sediments that accumulated in the

ice-dammed lake in the lower part of the valley and inhibited ice erosion there. Stratigraphic and geomorphologic evidence obtained from study of the Otter Creek paleoplacer deposits, suggests that other areas with buried placer potential in the region may be identified. Stratigraphically equivalent gravels occur in several other valleys including the Snake, Ruby, Boulder, Spruce, McKee and Pine Creek valleys. In addition, similar geomorphic and geological settings in some valleys with the Otter Creek valley suggests that they may also have potential for deeply buried placer deposits. For example, geomorphic similarities between the Wright and Otter creek valleys, were previously used to infer that the latter also had high potential for a large, buried channel, placer deposit (Levson, 1992a). The two valleys are relatively narrow and deep, and both are oriented obliquely to the regional ice-flow direction. Recent mining in the Wright Creek valley has yielded coarse placer gold and has confirmed the presence of a high energy paleoplacer deposit there. Geomorphic and stratigraphic data also suggest that buried paleochannel deposits are present in the lower Otter Creek valley and probably extend into the Surprise Lake/Pine Creek valley. Likewise, icedammed lakes, similar to the one that developed in the Otter Creek valley and contributed to placer preservation, probably also formed in the lower parts of other tributaries to the Surprise Lake valley such as Wright, Snake, Birch and Boulder Creeks. In placer exploration programs, application of geological criteria similar to that described here from the Otter Creek study, may also be useful for identifying prospective buried placer targets in other regions. Finally, excellent exposures of Quaternary sediments afforded by extensive mining in the Otter Creek valley, illustrate how rapid lateral and vertical facies changes occur in Quaternary sequences. Interpretation of any of several smaller isolated exposures in the area, would have resulted in a much less complete picture of the Quaternary history. For example, the placer gravel sequence is locally not exposed on bedrock highs, glaciolacustrine sediments are exposed only at the north end of the valley, delta gravels do not occur at the south end of the valley, and, although till is widespread in the area, there are several locations where till is absent, due either to erosion or nondeposition. The obvious implication of this is that interpretations based on small Quaternary exposures will be incomplete or in error and, as a result, caution is advised when developing sedimentary models in areas with limited exposure.

Acknowledgements This study was funded by the British Columbia Ministry of Energy and Mines as part of a research

V.M. Levson, H. Blyth / Quaternary International 82 (2001) 33–50

project of the Geological Survey Branch to investigate the geology of economically important placers in British Columbia. Figs. 1 and 2 were made by Travis Ferbey. The authors acknowledge improvements in this paper resulting from earlier comments by Drs. F. Hein, L. Jackson (reviewers) and D. Liverman (Guest Editor) on a previous QI paper on the Atlin region (Levson and Blyth, 1993). The suggestion by D. Liverman to follow our previous work with this paper on the sedimentology and stratigraphy of the Otter Creek placers is also appreciated. The authors are indebted to the Atlin Placer Mining Association and all of the miners and property owners in the region including B. Berg, B. Bonnell, M. Bonnell, S. Connelly, A. Diduck, A. Ellis, J. Harvey, E. Kolody, S. Kyle, M. Russo, G. Schmidt and Queenstake Resources Limited, for providing information and access to properties.

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