Placer gold mining in alaska

Dylan McFarlane
d.e.mcfarlane@exeter.ac.uk
1

Background – ASM & Placers
Prospecting
Exploration & Development
Mining
Reclamation & Marketing

200 Pans = 1 cubic metre
1 cubic metre = 2 tonnes
1 gram Au = $50 US
10 flyspecks = 1 milligram (dust)
1 A colour = 1 – 2 milligrams (ball point pen)
There are no B or C colours, and no nuggets
> 1 cent
< $1

150 episodes over 5 seasons
5-10 minutes education - 30 minutes drama
Yukon miners better

History
Politics
Economics
Geography

Golden History:
Beauty, Power,Value
Peruvian Chacras -“gold farms”
Las Medulas in Northern Spain, mined 25 a.d.

Serra Pelada, Brazil 1980s
7
Stereotypes of ASM
(Artisanal & Small-
scale Mining)

Artisanal & Small-Scale Mining
Bigger industry than you may think:
• 20,000,000 directly employed
0
5
10
15
20
25
30
ASM Share (%) of Global Mineral
Production (ICMM 2012)
3TG
“Conflict
minerals”
2005 Artisanal Gold
Production (Tonnes)
Columbia 21.6
Mexico 7.4
Ghana 6.9
Brazil 6.1
Tanzania 5.0
Bolivia 3.5
Papua New Guinea 3.2
Rural development, alternative
livelihoods & ‘green gold’

Labour-intensive
Informal
Rudimentary tools & knowledge
Poverty-driven

Green, FairTrade Gold
Artisanal certified ethical standard (ARM) Social & Environmental standards $ Premium
Small mining is beautiful – MarcelloVeiga, UBC Professor

Placer Economics
Commodity Quantity Price ($ US) Value ($ US Million)Note
Alluvial Gold 6,300,000 ounces 1,250per ounce $ 7,875 7% of 90 Moz Hard Rock
Total (mod from USGS, WGC
etc)
Titanium 2,400,000 tonnes 900per tonne $ 2,160 High grade TiO2/FeTiO3 feed
(mod from Credit Suisse)
Zircon 1,200,000 tonnes 1,100per tonne $ 1,320
Diamonds 13,200,000 carats 300per carat $ 3,960 12% of 110 Mct gem (mod.
From Bain)
Tin 100,000 tonnes 25,000per tonne $ 2,500
40% of 250kt est. (mod from
ITRI)
$ 17,815
Also: REEs, critical metals (tungsten,coltan)

Significant Market & Good Prices
333
137
111
26
17
0 50 100 150 200 250 300 350
Iron
Gold
Copper
Silver
Potash
Nickel
Phosphate
Zinc
PGMs
Diamonds
Others
Value of Global Metal Production 2011 ($ US Billion)
Source: ICMM 2012

Rafal Swiecki (http://www.minelinks.com/) 15
19th Century Gold Rushes

Rafal Swiecki (http://www.minelinks.com/) 16
20th/21st Century Gold Rushes

Fairbanks, 8 Moz
Nome, 6 Moz
14 mining regions
67 mining districts
Over 25 Moz historic

2014 est.
230 mines
140 operate
70,000 oz

Gold is whereYOU find it
Prospect Explore & Develop Mining Reclaim

Portable sluice box
Pan the Creek
HistoricWorkings?

Hand-stacked cobbles –YES! High-grade potential
Historic cut-off grade for hand working - ground sluicing 1 opcy

Paystreak Areas
Flood Gold
False Bedrock & Paleochannels

Geophysics? Unlikely…
Ground Magnetometer – variable success, lots of false positives. Metal detecting is
a hobby.
Seismic – successful in simple, homogenous conditions (Valdez Creek 460,000
ounces placer gold) but can be expensive, complex
Ground Penetrating Radar – Same, but less success
LIDAR – Useful in characterizing geomorphology and locating features (channels,
terraces, historic workings) but too expensive. Satellite imagery good enough.
Drilling must be done anyway, so most operators don’t bother with the distraction,
complexity, risk, and high costs.Cheap methods needed.

Geochemistry & Geomorphology
Again – not much use yet…
Geochemistry
Defining gold signatures, lode mineralization types,
spatial and temporal evolution
 Identify hard-rock lode sources
Geomorphology & Grain Morphology
Morphology: Particle Size & Shape = Distance
Magadan success  Arc GIS today?
Flume tank modelling to understand deposition
Improve knowledge of geological controls, better models
WhiteChannel Gravels inYukon

Panning
with a
Batea
Then,
drilling &
bulk
sampling

Portable Highbanker
Up to 1 m3 samples
1.5” Pump

Pan Color Count
Color
Category
Abbreviati
on
Mass (mg) Description
Micro-
flyspeck
Mf 0.03 (30 = 1 mg) Cannot see 30cm away, must squint close-up;
fine dust; flat and flaky
Flyspeck F 0.1 (10 = 1mg) Specks visible 30cm away; flat or flaky
A Colour A 1-2 ~1mm diameter, ball-point pen sized
B Colour B 2-5 Up to ~2mm, elongate or spherical in 3
dimensions
C Colour C 5-25 Up to 3mm, smaller than pea grain, clunk
sound
Nugget Nug +25, weigh
individually
Heft in hand
31
Estimating Particle Mass Using Size Classification
Blue is Best

Grades and Resources
• Measured in volume not
tonnage
• Square ft. of bedrock
• LOW GRADE:
• 0.3 g/m3……………0.01 oz/yd
(equivalent to 0.1 – 0.2 gpt ore)
• $10 per yard
• Fineness (850? 950? 700?)
• Cut-off pan: 2 mg
• 20 flyspecks, or:
• 1 – 2 ‘A’ colours, or:
• 1 ‘B’ colour, or:
• 60 micro-flyspecks
• 500 – 500,000 ounces
• 10,000 – 100,000 oz
• Depth?
• How much overburden-muck?
• 0 – 200 feet, 1000 ft.???
• Frozen or thawed?
• Multiple paystreaks?
• Old workings?
• Clay? Boulders?
• Fine grained or nugget, flat or
round grains?

Camm 1988

Placer Deposit Classification
Traditional classification based on environmental process nearest lode source:
• Eluvial & Coluvial
• Fluvial, Bench &Terrace
• Buried Channels
• Alluvial Fan, Floodplains
• Beach Strandline & Marine
• Rare: Desert, Glacial, Coastal Aeolian
Russian Classification: Gradient-Geomorphological & Aggradational-Degradational
(energy systems)  + 25 depositional categories
34Prospect Explore & Develop Mining Reclaim

Eluvial
• Weathering/Leaching in-situ on hill
slopes and outcrops
• Rain, wind, percolating waters, heat,
chemical and biological degradation
• Upgrading through removal of
soluble minerals or sheet flow
Colluvial
• Located at base of hill slopes
• Downslope movement of weathered
rock, wind, rain, freeze-frost action
• Poorly sorted and erratic grade and
size distribution
• Rarely economic
• Mkuvia placers in SouthernTanzania?

Fluvial, Bench &Terrace: Stream Placers
Classic, most common, and most important
River, stream, creek, gulch and relicts (bench or terrace)
Lighter particles winnowed away while heavy minerals
concentrate on bedrock, or
• inside bends and areas of lower velocity-pressure -gradient
• Crevices, boulders and other natural traps
Slatey, vertical bedrock good; rough & irregular good.
Usually formed within few kilometers of source rock
• Grain morphology and inclusions to relate placer to lode (R.
Chapman, Mortenson, etc.)
With distance, particle sizes reduce and sorting improves,
UNLESS it is re-worked, glaciated, or disturbed

Typical Profile

Waterfalls…Rarely
Gold Rush ‘Glory Hole’ Fail

Deep, Buried Channels
• Can be High Grade!!!
• Valdez Creek, Alaska 1981
• Most productive in N.America
• 100,000 oz Au/year
• 9 year mine life
• 10 oz/yd3
• 15 ft. pay sections
• Formed by Alaska Range uplift
causing southward stream
migration
• Seismic survey and drilling
discovered channel at 165 ft.

Alluvial Fan
•Aggradational placer - depositing
high-grade pay into thick section of
low-grade pay (100 ft. +)
•Patchwork of lenses and highly
erratic local concentrations  Bulk
•Little Squaw, Alaska Canyon, Bench
and Alluvial Fan
• 200,000 oz Au @ 0.02 oz/yd3
• 200 ft. thick but 100 ft. till overburden

Beach Strandline & Marine
• Sorting action of waves, tides, currents and winds concentrating heavies along lines
• Offshore deposits scoured by glaciers, sorted by tides, concentrated in storms and re-worked by
fluvial processes
• NomeCoastal Plain, Alaska AnglogoldAshanti
• Discovered 1899, hand-miners 1-5 oz/day from beach
• 5-6 M oz historical production
• Current resource: .5 - 2.5 M oz @ 0.002 oz/yd3

43
Marine Offshore - Nome

Other PlacerTypes
• Desert – wind principal transport & concentration agent although flash flooding
can locally enrich
• Most scams occur in the desert (stay away fromArizona!)
• Glacial – rare economic concentrations
• Unsorted, unstratified, high-clay
• Moraines, tills and shorelines
• Yukon, but nothing significant in Alaska
• CoastalAeolian – sand dune systems
• Richards Bay (ilmenite, rutile, zircon)

TOP: 20 g gold nugget within 1 tonne gravel  sample #1 = 0 g/t  sample #2 = 40 g/t
UNREPRESENTATIVE
BOTTOM: 20g gold dust within 1 tonne gravel  both sub-samples = 20 g/t

Heterogeneity and Nugget Effect
47
Proportion of Ore Mineral versus Homogeneity
E= Evaporite; C= Coal; Fe= Bedded Iron Ore; P=Phosphate; B=Bauxite; PbZn=Stratiform lead-zinc;
Ni=Stratiform Ni; SSn=Stratiform tin; PC=Porphyry Copper; VSn=Tin veins; V=Gold, Silver veins; U=Uranium;
D=Diamonds; AD=alluvial diamonds (After Haddon King et al 1980)
High
LowHigh
Low
ProportionofOreMineral
Homogeneity
E
C
Fe
B
P
Ni
SS
n
VS
n
VD
AD
PC
U
Source: King, McMahon & Bujtor CRA
(Australia) 1980
Nuggety
Creeks
Fans,
Terraces
Flood-
plains,
Offshore

How DoYou Raise Finance? Ex: Petropavlovsk
Equivalent ‘ore’ grades: 0.1 - 0.18 g/tonne
Disposed assets for $25 M b/c high alluvial cash costs  $1,319/oz

Exploration Methods - Drilling
49
Banka
Auger
Churn
Hammer & RAB
RC
Double Cased
Hammer
Sonic
Which one do you choose?
• 3in. – 12in. (8in. Minimum in >60 mesh)
• $5/ft - $90/ft
• Penetration rate – gold loss/migration –
recovery -
Boart Longyear Sonic
Mini-Auger
Banka

Sample Processing - MiniWashplant
50
Goldfields Prospector

Laboratory
De-mag concentrates, classify Clean, dry and weigh gold (milligrams)

Mobile Lab
Scale in
mg
Special
clean-up
pans

Bulk Sampling – Pilot Production
Representative samples (1 to 5,000 m3)
Detailed geology, stratigraphy, mineralogy
Simple model
PSD
Optimize

Channel – Gulch Section

Alluvial Fan Section

Methods
Equipment
Washplants

Underground Placer Mining – 1900s Fairbanks
Thawing Frozen Gravels in
Narrow, High-Grade Paystreaks

1. Remove forest & thaw
2. Strip black muck loess
(save for final restoration)
3. Strip overburden gravel
4. Mining paydirt
5. Processing plant - discharging into
settling pond(s)
6. Restored area

Hydraulicking – 1.5 Billion cubic yards in California
Blue Babe – baby mammoth

Hydraulicking
John Miscovich - Legend
Intelli-giant – automatic monitor

Baker Family – Mudminers inTofty

Dragline Mining

Don’tTangle the Cable on the Spool!

Skyline-Drag System, Nome

Bucket-Ladder Dredge
Over 100 in AK
Continuous,
high volume
Mercury, then
trapezoidal jigs
Problems:
Water
Climate

Davidson Ditch – 90 mile pipeline (1920)

Steam Fields – ColdWater Saved Alaska

TheTweet Dredge +110Years Operation

Bima Dredge, Nome
1987 – 1990
World’s largest dredge
Marine deposits
118,000 ounces
Built for MalaysianTin
Failed due to severe
conditions – Arctic
Storms!!!

Marine Mining: New
Techniques &Technologies
All theTime

Pomrenke Dredge

The Future: Jack-up Barge (from Namibia Al. D.)

OtherTechniques
Ross-box
Suction Dredging
Reverse spirals
Hydro-clean

New-ZealandType Dredge – A Model Mine
1. Strip topsoil
2. Push overburden
3. Basic equipment:
1. 40t ext x 2
2. D6 dozer or 920
FEL & 40t truck
4. 2-man operation
1. Backhoe Feed
2. Ancillary (Tails,
Strip, Fuel, etc.)
5. Convey tails
6. Place overburden
7. Reclaim topsoil
8. Natural revegetation
< 2 years
9. Farmland restored!
Floating Washplant
Fed from Land

N.L.Wimmler (1927) – Review of Placer Mining
Methods and Costs
Few Good Miners
Time for a Revision!

Tools of theTrade – Basic Equipment
D11 – 11 tonne push
FEL versatility
40 t Excavator
‘backhoe’

Derocker

Home-made trammel with hopper, punch plant,
forklift/truck tires & sluice boxes

Double-deck screen
Coarse rock conveyor
Even feed distribution to sluices

Sluice Box Riffles
Expanded Metal – Fine Au
Angle Iron – Coarse Au

Miners Moss – 3M Nomad Carpetting

Ground-breaking Sluice & Riffle Design
85
RandyClarkson
• Riffle Spacing
• Feed Rates
• Riffle Types (Angle
Iron, Flat Bar,
Expanded Metal)
• Matting
• Oscillation
• Hydraulic Riffles
• Punch-Plate
• Triple-Run Box
• Pre-Screening

GoldWheels -Toy

Jigs, Centrifuges, Bowls &Tables
88

Transporting Gold

Advice
1. Maintenance, Maintenance, Maintenance!
2. Do Not Fire Assay Placer Gold
3. Washplant Design – K.I.S.S.
4. Coarse Gold ≠ Fine Gold
5. Sampling Evidence and Production History
Barry Clay with the 294oz
Centennial Nugget found
whilst on his dozer!!!

Concurrent
Reclamation
Best Practice

2014 Indictment at Platinum Creek Mine
I blame the Aussies

Closure & Marketing
• Turbidity – Settling Ponds Required!
• Keep it Safe & Stable –Wetlands & Moose Pasture
• Mercury unnecessary; retort  Green Gold, Ethical Jewelry
• Heritage &Tourism
• Nugget prices 3x for jewelery
94

Resources – thank you
Websites
• Alaska Gold Forum
http://bb.bbboy.net/alaskagoldforum
• AMDS Prospecting Forum
http://www.akmining.biz/forums/forum.html
• ICMJ Prospecting & Mining Journal Forum
http://forum.icmj.com/
• Canadian Prospector’s Forum
http://gpex.ca/smf/index.php
• World Placer Journal http://www.mine.mn/
• New 49’ers Forum http://www.goldgold.com/
Books
• Wells, John. Placer Examination
• MacDonald, Eoin. Alluvial Mining
• IMM. Alluvial Mining Conference Proceedings
1990
• M.I.R.L. Reports & Alaska Placer Conference
Proceedings
95
A Great Read – A Mining Legend

University of Leeds Placer Mineral Group (Rob Chapman): http://see-web-01.leeds.ac.uk/misc/miner/97

101
Pandora’s Inn

Placer gold mining in alaska

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