Rock Creek Gold Mine Near Nome, Alaska
Geotechnical Investigation/Studies for Open Pit, Tailings
Impoundment, Waste Dumps, and Plant Facilities
Thomas G. Krzewinski, P.E.1, Travis E. Ross, P.E.2 and Doug Nicholson3
ABSTRACT
NovaGold Resources, Inc./Alaska Gold Company is advancing feasibility studies
and moving towards a construction start up in 2005 for the Rock Creek Gold Mine. The
mine is 13 km north of Nome, Alaska and will take advantage of its proximity to Nome
for infrastructure and support once production starts.
In the fall of 2003 a preliminary geotechnical exploration work was initiated and
data was gathered to use during the required design work for the mine civil works and
facilities. The geotechnical field program focused on defining subsurface conditions in
the footprints of the planned open pit, tailings impoundment and dam, required waste
dumps, and plant facilities. The field investigation included deep rock coreholes, soil
borings, test pits, and logging of exposed rock faces in the development area. Samples of
encountered soils and rock were obtained for subsequent laboratory testing. Coreholes
were instrumented with submersible pressure transmitters and thermistor strings. The
field program needed to determine the presence and extent of permafrost, the location of
the water table, and geotechnical properties of the thawed and frozen soil and rock.
This technical paper documents the geotechnical field investigation, the first
months of instrumentation results, and the results of the laboratory testing program. The
design implications of the data obtained are also summarized.
INTRODUCTION
The City of Nome was built along the Bering Sea, on the south coast of the
Seward Peninsula, facing Norton Sound. Nome lies 870 km by air northwest of
Anchorage and 160 km south of the Arctic Circle. The Rock Creek project is located
approximately 13 km north of Nome, Alaska, and accessed via gravel roads (Figure 1).
Reportedly, the average annual air temperature at Rock Creek is about -3°C, with
summer temperatures ranging from +8°C to +15°C, and winter averages of -15°C. The
site is generally south in latitude of regional continuous permafrost (Ferrians, 1965).
Even though people have been mining the beaches of Nome for over a century,
Rock Creek stands to be the first hard rock gold mine to go into production in the region.
1
Senior Geotechnical Engineering Consultant/Associate, Golder Associates, Inc., 1750
Abbott Road, Suite 200, Anchorage, Alaska 99507
2
Project Geotechnical Engineer, Golder Associates, Inc., 1750 Abbott Road, Suite 200,
Anchorage, Alaska 99507
3
Project Manager, NovaGold Resources, Inc./Alaska Gold Company, P.O. Box 82430,
Fairbanks, Alaska, 99708
PROJECT
LOCATION
ROCK
CREEK
VICINITY MAP (NTS)
ARCTIC OCEAN
Seward
Peninsula
ALASK A
A
GULF OF
ALASKA
TRUE NORTH
17º
PACIFIC OCEAN
NO
RT
H
Anchorage
MA
GN
ET
IC
Fairbanks
Norton
Sound
A
SE
NG
I
R
BE
CANAD
NOME
APPROXIMATE MEAN
DECLINATION, 1950
0
2km
SCALE (km)
4 km
NOME
Figure 1
PROJECT LOCATION MAP
The deposit will be mined by open pit methods, and an approximate 1 km long by 0.5 km
wide, northeast/southwest trending pit will be excavated to a depth of approximately 160
meters (Figure 2). Mine development will require construction of a tailings dam and
impoundment area, a milling plant area with administrative offices, waste dumps for both
rock and soil waste, and an open pit area for mining (Figures 2, 3, & 4). The tailings
storage facility will consist of sub-aqueous deposit tailings, covered by a water cap
(Norwest, 2003). Rock Creek intends to be a “no discharge” facility. Based on the
anticipated quantity of material to be mined, the estimated final elevation of the waste
rock is up to 30 m above existing grades. The expected life of the mine is 6 to 9 years, at
which point most of the facilities will be decommissioned (Norwest, 2003). Freeze-back
of the tailings impoundment is being considered for closure planning.
REGIONAL GEOLOGY
The Seward Peninsula is underlain by bedrock consisting of several metamorphic
rock sequences (the Nome group), each of which has undergone at least two tectonic
events. The protoliths are believed to be Cambrian to Devonian sediments consisting of
shales, siltstones, sandstones, marls and limestones, deposited in a shallow continental
shelf setting (Norwest, 2003). The bedrock is overlain by a variety of unconsolidated
sediments. If frozen, these sediments are mostly thaw-unstable, especially on steeper
slopes.
The Nome group appears locally as a progression consisting of four major units.
The basal unit is complexly deformed pelitic schist, with a mixed unit consisting of
mafic, pelitic, and calc schists and marble forming the next unit. The third unit is a mafic
dominated schist, and the top unit consists of impure marble.
There are three units that are closest to the ground surface in the area. These are a
carbonaceous schist, a calc schist, and a mixed schist, with the carbonaceous schist being
present in the stream valley, and the mixed schist generally to the east and the calc schist
generally to the west.
Quartz muscovite schist (QMS) is the dominant rock unit in the project area, but
other types of schist are also present.
Three major structures are recognized in the project area. The northeast-striking,
steeply dipping Albion East and Albion West shear zones are the dominant structural
trends in the project area. The Sophie Gulch fault strikes west-northwest and dips steeply
north-northeast, and a relatively shallow fault dips approximately 30° to the northnortheast in the south part of the deposit.
At Rock Creek, gold occurs in quartz veins in replacement bodies, tension veins,
and in the Albion shear zone (Norwest, 2003). The replacement bodies are small and
irregular and occur in low angle structures and the noses of small folds. Tension veins
are generally less than 10 cm thick and extend from approximately 10 to 100 m. Veins in
the shear zones exhibit episodic fracturing, and contain quartz banding and quartzcemented breccias. Gouge zones one to three meters thick that reflect postmineralization faulting are evident in the core.
PROPOSED
PIT LIMITS
Oriented
Core Drilling
TAILINGS
IMPOUNDMENT
Figure 3
Photograph showing pit and waste dump sites
PROPOSED
OPEN PIT
Figure 4
Photograph showing proposed pit limits and tailings impoundment (looking SSW)
WASTE ROCK
DUMP SITE
NO
RT
H
Oriented
Core Drilling
FIELD INVESTIGATION
Pit Walls
Four oriented coreholes were drilled in the vicinity of the proposed pit, as shown
on Figures 2, 3, & 4. Drilling was performed using a track-mounted Boyles Brothers
CS1000 drill rig. One corehole was drilled in the north part of the northwest wall, one in
the south part of the southwest wall, one in the south wall, and one in the south part of
the southeast wall. The coreholes ranged in length from 68.3 m to 159.7 m, and were
drilled with a nominal inclination of 55º (from horizontal). Typically, the inclination
grew slightly steeper with depth, up to about 61º.
The coreholes were continuously cored using a complete triple-tube HQ3 (61.1
mm I.D. x 96.0 mm O.D) core system and polymer drilling fluid. The core was oriented
using the clay impression method, and the core was retained in the split during data
collection. Geotechnical data was collected concurrently with the orientation data during
logging of each corehole. Data from a total of 473 discontinuities were collected from
the oriented core. Most of the discontinuities reflected foliations, but also consisted of
joints, shears, veins, and faults.
Tailings Dam / Impoundment
Four vertical coreholes (each to 32 m) were drilled in the vicinity of the proposed
tailings dam. These coreholes were also drilled using a triple tube system with HQ3 bits.
Geotechnical data collected from the core included: core recovery, rock quality
designation (RQD), rock strength, number of discontinuities per run, joint condition
rating (JCR), and weathering index, as well as planarity, roughness, infill, and dip with
respect to core axis for the discontinuities.
Four test pits and two testholes were also completed to further explore the
overburden. Each of the testholes was completed to depths ranging from 3 m to 8 m with
a Mayhew drill rig, employing air rotary techniques with a tri-cone bit. Drive samples
were typically taken at 1.5 m intervals. The test pits ranged in depth between 1.4 m and
4.7 m, and were excavated using a Caterpillar 320 excavator. The test pits either
extended down to the weathered bedrock surface in thawed soils or met refusal within
permafrost.
All soil samples were individually sealed in double plastic bags prior to being
transported to the laboratory for further examination, classification, and testing. All
frozen samples were maintained as such in route to the laboratory.
Waste Dump and Plant Sites
Five test pits were excavated around the footprint of the plant site, and seven test
pits completed in the vicinity of proposed waste dump sites. Five of the test pits were
within thawed soils and extended down to the weathered bedrock strata (1.5 m to 5 m
depths). In cases where test pit excavation was limited by frost, accompanying test holes
were drilled adjacent to the test pits. Depth of the testholes ranged between 2.7 m and
5.4 m, and typically penetrated into weathered bedrock 1.5 meters or less.
Instrumentation
Submersible pressure transmitters and thermistor strings were installed in two of
the oriented coreholes drilled at the pit. The two thermistor strings in the pit indicate
absence of permafrost.
Thermistor strings were also installed in two of the coreholes drilled at the
tailings impoundment. Permafrost within corehole RKGT03-220 extends greater than 31
m depth, and ranges in temperature between -0.1° and -0.7° Celsius. Corehole RKGT03219 was completely thawed below the active layer. The pressure transmitters and
thermistor strings were each attached to the exterior of a 3 cm diameter PVC pipe and fed
down to the bottom of the open drillhole. After placing the instruments, the holes were
backfilled by pumping a cement/bentonite grout. All of the instruments are continuing to
be read throughout the change of seasons.
LABORATORY TESTING
Rock Testing
Representative samples of core from the oriented coreholes were collected for
triaxial compressive strength testing (ASTM D-2664), unconfined compressive strength
testing (UCS, ASTM D-2938), and direct shear strength testing on open fractures (ISRM
1981, pgs. 135-137). Samples selected for triaxial and UCS testing had a length at least
three times the core diameter and were relatively free of defects. Samples chosen for
direct shear testing on open fractures were relatively free of defects on each side of the
fracture.
Soil Testing
Laboratory tests were performed to measure selected index properties of the
samples. Moisture content tests were run on each sample and generally conducted
according to procedures described in ASTM D-2216. In addition, numerous samples
were tested to determine Atterberg Limits (ASTM D-4318), grain size distribution
(ASTM C-117), percent passing the No. 200 sieve (ASTM D-1140), and organic content
(ASTM D-2974).
A composite sample of highly weathered bedrock was prepared from several test
pit samples near the tailings impoundment. The composite sample was tested to
determine grain size distribution, maximum dry density by modified proctor method
(ASTM D-1557), and permeability. One permeability test was performed in a “loose
state” (averaging 65% of the max dry density) by constant head methods (ASTM D2434), and a second test by falling head methods targeting 95% of the maximum dry
density. Results show a “loose state” permeability of 1.3 E-02 cm/s and 7.8 E-05 cm/s for
95% of max dry density. The intent of the two permeability tests is to provide a
bounding range of values.
A second composite sample was prepared combining fine-grained overburden
soils encountered near the tailings impoundment. The sample was tested to determine
Atterberg Limits, grain size distribution including hydrometer analysis (ASTM D-422),
maximum dry density (modified), specific gravity (ASTM D-854), and permeability.
One permeability test was performed in a “loose state” (averaging 73% of max dry
density) by constant head methods (ASTM D-2434). The second test was run targeting
about 89% of may dry density, using a flexible wall permeameter (ASTM D-5084).
Results indicate a range of permeability values between 1.1E-04 and 1.6E-05 cm/s. This
“loose state” was the lowest density we could effectively test for permeability.
Thaw strain tests were ran on three drive split spoon samples gathered near the
tailings impoundment and plant site. Two of the samples, comprised mostly of silt, had
unit thaw settlements near 33% (thaw strain/frozen height). The third sample (Sandy
Silty Gravel) tested as having unit thaw settlement of 12%.
RESULTS
Pit Walls
The four coreholes drilled at the pit were found to be free of permafrost. Much of
the area had been previously disturbed, and also encompasses the thaw bulb of the creek.
Overburden ranges from 1 m to 20 m thick, but typically less than 7 m.
A geologic model was developed; incorporating geologic units, thermal regime,
water pressures, testing results, discontinuity data, rock mass quality, and major faults.
Units within the pit were identified where rock fabric and mechanical characteristics
were found to be similar. The model was used to assess bench scale stability, overall
stability of the slopes, and establish recommended configurations.
The results of the stability investigation indicate that in general the pit will be
excavated in competent schists that contain steeply dipping northeast/southwest trending
shear zones and veins (Golder, Mar. 2004). For the most part, the most common rocks in
the deposit will exhibit similar strengths and structural fabric. The rocks also contain a
flat-lying, generally southeast dipping foliation, and two orthogonal joints that are steeply
dipping and strike northeast/southwest and northwest/southeast, respectively. The
foliation is generally pervasive throughout, and typically dips shallower than 30 degrees.
Structurally controlled failure (such as plane, wedge, and toppling failures) in rock occurs
as a result of sliding along these pre-existing geologic discontinuities.
Several continuous, widely spaced faults that dip toward the north-northeast have
also been identified, and will control overall rock mass stability.
Tailings Impoundment
The variable conditions within the impoundment footprint are characterized
according to four areas, where similar geologic deposits and thermal regime were found.
•
Northern Third (near RKGT03-217, -218, & TP/BH-04). These explorations
were frozen, and soils consisted of 0.3 m to 0.6 m of organics, 0.8 m to 5.2 m of
Silt or Clay (ML or CL), overlying 0.3 m to 1.5 m of Silty Gravel (Glacial Till,
GM to ML). Slightly weathered bedrock was encountered in -217 & -218 at 1.7
m & 1.5 m, respectively, and highly weathered bedrock was found at 7.3 m depth
in BH-04. Moisture contents typically ranged between 11% and 39%, however a
few ice-rich clay samples had 100% to 240% moisture. Figure 5 shows ice lenses
discovered in Test Pit TP-04. Bedrock was still frozen at the base of corehole
RKGT03-217 (32 m depth), and the bottom of permafrost wasn’t reached. A
monitoring well 250 m north indicates the bottom of permafrost near 18 m depth.
•
Within Creek Floodplain (near RKGT03-219 & TP-03). This area is within the
thaw bulb of the creek. Areas outside of the active floodplain may have a few
feet of organics and Silt. Otherwise, subsurface conditions consisted mostly of
Gravel and Sand with minor silt and with fragments of Schist (cobble & boulder),
3.7 m to 4.6 m thick. Weathering of the upper strata of bedrock varied: highly
weathered bedrock was encountered at 4.6 m depth at TP-03, but conversely,
fresh bedrock is present at 3.7 m depth near RKGT03-219. Groundwater is
expected near the elevation of the creek.
•
Southwestern Portion (near RKGT03-220 & BH-B4). Permafrost was found in
each of these explorations. Corehole RKGT03-220 was drilled within an existing
fill work pad, and contained 1.8 m of fill. On the other hand, testhole BH-B4 was
undisturbed and had 0.3 m of organics. At depth, there was 0.6 m to 1.1 m of
Silty Sand and Silt (SM to ML), underlain by 0.9 m to 2 m of Silty Gravel
(Glacial Till, GM to ML). A thick stratum of highly weathered bedrock is present
below 2.3 m to 4.4 m. Moisture contents typically ranged between 18% and 58%,
but a few ice rich samples exceeded 100% moisture. Frozen soils were ice rich
and contained 10% to 60% visible ice content by volume. Thermistor data from
corehole RKGT03-220 shows that the active layer is nearly 3 meters deep, and
that the temperature of the permafrost typically ranges between -0.2° and -0.7°
Celsius.
•
Southeastern Portion (near TP-01 & TP-02). Thawed soils were encountered,
consisting of 0.2 m to 0.6 m of organics, 1.2 m to 2.7 m of Sandy Silt (SM to
ML), underlying 0.9 m to 1.8 m of Sandy Gravel (GP with cobbles). Highly
weathered bedrock was encountered below 2.7 m & 4.6 m in Test Pits TP-01 &
TP-02, respectively. The range of moisture content within the Sandy Silt strata
was 15% to 39%, and close to 15% in the Sandy Gravel.
Plant Site
Subsurface conditions were fairly consistent across the plant site, with the
exception that Test Pits TP-06 & -10 (east side of the footprint) were thawed while the
remaining test pits encountered permafrost. Soil conditions consisted of 0.15 m to 0.3 m
of organics, 1.2 m to 3 m of Silt to Clayey Silt (ML to ML-CL), overlying highly
weathered bedrock below 1.4 m to 3.2 m below ground surface. The highly weathered
bedrock zone is generally greater than 1.5 m thick. Moisture contents in the thawed silt
strata ranged between 12% and 22%. Frozen Silts had moisture content between 25%
and 88%, with an estimated 5% to 20% visible ice by volume. Figure 6 shows
segregated ice encountered in Test Pit TP-09.
Waste Dump Sites
Frozen soils were found within about half of the dump site area (near Testholes
BH-12, -13, & -15). Subsurface conditions found in those testholes consist of 0.3 m of
organics, 1.2 m to 4.3 m of Silt and Clay (ML & CL), overlying highly weathered
bedrock. Moisture content of the frozen Silt/Clay ranged between 14% and 176%, with
an estimated 5% to 50% visible ice.
Soil conditions found in the remainder of the explorations (Test Pits TP-11, -14, 16, & -17) consisted of about 0.3 m of organics, 0 m to 1.5 m of Silty/Clayey Sand (SPSM to SC), overlying 1.2 m to 3 m of Silt and Clay. Highly weathered bedrock was
encountered below 1.4 m to 4.9 m depths. Moisture contents generally ranged between
15% and 34%, except for two Clay samples that had 57% and 124% moisture.
Figure 5
View of 3 cm thick ice lenses found in Test Pit TP-04 (Tailings Dam), at 1.2 m depth
Figure 6
View of segregated ice found in Test Pit TP-09 (Plant Site), 1.1 m depth at base
SUMMARY AND CONCLUSIONS
Pit Walls
The results of the overall stability analyses indicate that the proposed walls are
expected to exhibit adequate stability with respect to deep-seated overall instability, and
to deep-seated block sliding along the foliation (Golder, Mar. 2004). Localized planar,
wedge, and toppling failures may occur and instability is dependant upon the cut angle
relative to the foliations or joints. Bench catchments can minimize the impacts.
Additional stability measures and slope reducing can be done for portions of the pit that
intersect faults.
The four coreholes drilled within the proposed pit were found to be free of
permafrost, partly a result of the disturbed ground and the proximity of the creek channel.
Although, isolated pockets could be present outside of the exploration area. Seepage
flows can build up pressure behind frozen (perennial or seasonal) cut faces and leads to
icing or other safety concerns.
Tailings Impoundment
The conceptual dam configuration includes a core, which is made up mostly of
rock fill but is mixed with low-permeability borrow material, and an up-stream liner and
filter. Fine-grained overburden stripped from the pit site may suffice for some of the
construction material, but quantities will likely dictate finding another source. The upstream liner and dam core should be keyed through the overburden and highly weathered
bedrock zone. All permafrost soils (including thaw-unstable weathered bedrock)
underlying the tailings dam should be removed in order to prepare the foundation area.
Otherwise, differential thaw settlement and instability could compromise the liner
integrity, especially in light of the discontinuous permafrost along the impoundment
perimeter. Seepage flows may be concentrated along the thaw bulb of the creek, and
down-stream seepage collection will likely be required.
As the detailed design of this project progresses, one issue that needs to be
addressed is what will happen to the thermal regime of the tailings impoundment over
time. Especially if freeze-back is stipulated into the closure plan. Thermal modeling can
be utilized in order to predict permafrost degradation (or aggradation) during operation
and closure.
Once the pond has been fully discharged upon closure, latent heat of the tailings
will be relatively low. However, unless special provisions are made, re-establishing the
permafrost within the impoundment is improbable. This is anticipating a relatively high
surface thaw factor (i.e. nt) of the “disturbed” ground. The down-slope face of the dam
should freeze over time, and maybe even during production (since it will be away from
the conductive heat of the tailings slurry, and contingent upon low seepage). But, full
aggradation of the dam into permafrost is unlikely, especially considering the southern
exposure of the slope and lack of natural insulation.
Plant Site
Permafrost underlies most of the plant site, and segregated ice was found in
higher quantities here compared to the remainder of the proposed mine. The thickness of
the overburden ranged between 1.4 m and 3.2 m, overlying highly weathered bedrock.
Facilities at the plant site can be characterized as either light structures that can withstand
some settlement (i.e. trailer type administration buildings) or heavier structures where
allowable settlement is limited to a couple centimeters (i.e. mill buildings and bulk fuel
storage tanks). Considering the sensitivity of some of the structures, and the relatively
thin, yet ice-rich, overburden, the best technique will be to excavate all soils down to
bedrock. The weathered bedrock zone should have adequate bearing capacity for the
planned structures, but should be allowed to thaw (and drain) down to competent bedrock
prior to placing foundations. Continued thaw of the competent bedrock is expected, but
corresponding settlement will be limited.
Waste Dump Sites
Rock dump sites will be used for disposal of waste rock stripped during the
mining operation. Permafrost underlies most of the footprint of the dump sites, but
thawed soils are present on the northern and eastern fringes. Overburden ranges between
2m and 4m thicknesses, and is ice-rich where frozen. Excavating the overburden is one
alternative for preparing the subgrade, which would provide the best trafficability for the
initial lifts. However, stripping the overburden is not entirely necessary in this case, and
leaving the organic mat has the benefit of slowing the degradation of permafrost. Design
implications of leaving the frozen overburden in-place include; slope instability upon
thaw, thaw settlement, and down slope creep. The existing ground surface (and
underlying rock surface) is at 7% grade at its steepest, so global slope instability will not
be much of a concern. Local instability is likely to manifest in the form of sloughing and
cracking at the edge of the waste dump berms/dykes, and possibly some ponding at the
toe. The magnitude of down slope creep should be small, and thaw consolidation will
likely occur before creep becomes much of an issue. No special provisions for protecting
the permafrost are anticipated for the waste dump sites. Again, thermal analyses can be
used during the detailed design to predict permafrost behavior over time.
Access Road Embankments
Gravel haul and access roads are planned over both thawed and permafrost areas.
Although the overburden is ice rich, it is generally less than 3 m thick, and therefore thaw
induced settlement should be within acceptable maintenance limits for gravel roads.
ACKKNOWLEDGEMENTS
Acknowledgement is made to NovaGold Resources, Inc./Alaska Gold Company,
for giving permission to present this paper.
REFERENCES
Norwest Corporation (2003), Preliminary Economic Study – Rock Creek Project, Nome,
Alaska.
Ferrians, Oscar J. Jr. (1965), Permafrost Map of Alaska, published by Department of
Interior, United States Geological Survey.
Golder Associates Inc (Mar. 2004), Draft Report on Pit Slope Stability Assessment of the
Proposed Rock Creek Open Pit - Nova Gold Resources Inc., Alaska.
Golder Associates Inc. (Feb. 2004), Draft Report on Geotechnical Investigation Proposed Rock Creek Mine Development Near Nome, Alaska