placer gold mining in alaska
TRANSCRIPT
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
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Stereotypes of ASM (Artisanal & Small-scale Mining)
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Artisanal & Small-Scale Mining
Bigger industry than you may think:
• 20,000,000 directly employed
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5
10
15
20
25
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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 InformalRudimentary tools & knowledgePoverty-driven
Green, Fair Trade Gold
Artisanal certified ethical standard (ARM) Social & Environmental standards $ Premium
Small mining is beautiful – Marcello Veiga, 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 $ 2,50040% of 250kt est. (mod from ITRI)
$ 17,815
Also: REEs, critical metals (tungsten,coltan)
Significant Market & Good Prices
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137
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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/) 16
20th/21st Century Gold Rushes
Fairbanks, 8 Moz
Nome, 6 Moz
14 mining regions67 mining districts
Over 25 Moz historic
2014 est.
230 mines
140 operate
70,000 oz
Gold is where YOU find it
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Portable sluice box
Pan the Creek
Historic Workings?
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Hand-stacked cobbles –YES! High-grade potentialHistoric cut-off grade for hand working - ground sluicing 1 opcy
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Paystreak Areas
Flood GoldFalse Bedrock & Paleochannels
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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.
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Geochemistry & GeomorphologyAgain – 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
White Channel Gravels in Yukon
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Panning with a Batea
Then, drilling & bulk sampling
Portable Highbanker
Up to 1 m3 samples
1.5” Pump
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Pan Color Count
ColorCategory
Abbreviation
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
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Estimating Particle Mass Using Size ClassificationBlue is Best
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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?
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Camm 1988
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Placer Deposit ClassificationTraditional 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
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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 Southern Tanzania?
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Fluvial, Bench & Terrace: Stream PlacersClassic, 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
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Typical Profile
Waterfalls…Rarely
Gold Rush ‘Glory Hole’ Fail
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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.
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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
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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
• Nome Coastal Plain, Alaska Anglogold Ashanti• 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
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Marine Offshore - Nome
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Other Placer Types
• Desert – wind principal transport & concentration agent although flash flooding can locally enrich• Most scams occur in the desert (stay away from Arizona!)
• Glacial – rare economic concentrations• Unsorted, unstratified, high-clay
• Moraines, tills and shorelines
• Yukon, but nothing significant in Alaska
• Coastal Aeolian – sand dune systems• Richards Bay (ilmenite, rutile, zircon)
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TOP: 20 g gold nugget within 1 tonne gravel sample #1 = 0 g/t sample #2 = 40 g/tUNREPRESENTATIVE
BOTTOM: 20g gold dust within 1 tonne gravel both sub-samples = 20 g/t
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Heterogeneity and Nugget Effect
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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
Low
Hig
h
Low
Pro
po
rtio
n o
f O
re M
ine
ral
Homogeneity
E
C
Fe
B
P
Ni
SS
n
VS
n
VDAD
PC
U
Source: King, McMahon & Bujtor CRA
(Australia) 1980
NuggetyCreeks
Fans, Terraces
Flood-plains,
Offshore
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How Do You 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
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Exploration Methods - Drilling
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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
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Sample Processing - Mini Washplant
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Goldfields Prospector
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LaboratoryDe-mag concentrates, classify Clean, dry and weigh gold (milligrams)
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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
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Channel – Gulch Section
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Alluvial Fan Section
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Methods
Equipment
Washplants
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Underground Placer Mining – 1900s FairbanksThawing Frozen Gravels in Narrow, High-Grade Paystreaks
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1. Remove forest & thaw
2. Strip black muck loess (save for final restoration)
3. Strip overburden gravel
4. Mining paydirt5. Processing plant - discharging into
settling pond(s)6. Restored area
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Hydraulicking – 1.5 Billion cubic yards in California
Blue Babe – baby mammoth
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HydraulickingJohn Miscovich - LegendIntelli-giant – automatic monitor
Baker Family – Mudminers in Tofty
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Dragline Mining
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Don’t Tangle the Cable on the Spool!
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Skyline-Drag System, Nome
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Bucket-Ladder Dredge
Over 100 in AK
Continuous, high volume
Mercury, then trapezoidal jigs
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Problems:
Water
Climate
Davidson Ditch – 90 mile pipeline (1920)
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Steam Fields – Cold Water Saved Alaska
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The Tweet Dredge +110 Years Operation
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Bima Dredge, Nome
1987 – 1990
World’s largest dredge
Marine deposits
118,000 ounces
Built for Malaysian Tin
Failed due to severe conditions – Arctic Storms!!!
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Marine Mining: New Techniques & Technologies All the Time
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Pomrenke Dredge
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The Future: Jack-up Barge (from Namibia Al. D.)
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Other TechniquesRoss-boxSuction DredgingReverse spiralsHydro-clean
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New-Zealand Type Dredge – A Model Mine
1. Strip topsoil2. Push overburden3. Basic equipment:
1. 40t ext x 22. D6 dozer or 920
FEL & 40t truck4. 2-man operation
1. Backhoe Feed2. Ancillary (Tails,
Strip, Fuel, etc.)5. Convey tails6. Place overburden7. Reclaim topsoil8. Natural revegetation
< 2 years9. Farmland restored!
Floating Washplant
Fed from Land
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N.L. Wimmler (1927) – Review of Placer Mining Methods and Costs
Few Good Miners
Time for a Revision!
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Tools of the Trade – Basic Equipment
D11 – 11 tonne push
FEL versatility
40 t Excavator‘backhoe’
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Derocker
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Home-made trammel with hopper, punch plant, forklift/truck tires & sluice boxes
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Double-deck screen Coarse rock conveyorEven feed distribution to sluices
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Sluice Box Riffles
Expanded Metal – Fine Au
Angle Iron – Coarse Au
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Miners Moss – 3M Nomad Carpetting
Ground-breaking Sluice & Riffle Design
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Randy Clarkson
• Riffle Spacing• Feed Rates• Riffle Types (Angle
Iron, Flat Bar, Expanded Metal)
• Matting• Oscillation• Hydraulic Riffles• Punch-Plate• Triple-Run Box• Pre-Screening
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Gold Wheels - Toy
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Jigs, Centrifuges, Bowls & Tables
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Transporting Gold
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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
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Barry Clay with the 294oz Centennial Nugget found whilst on his dozer!!!
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Concurrent Reclamation
Best Practice
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2014 Indictment at Platinum Creek MineI blame the Aussies
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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
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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
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A Great Read – A Mining Legend
Gold in Cornwall (Simon Camm)
University of Leeds Placer Mineral Group (Rob Chapman): http://see-web-01.leeds.ac.uk/misc/miner/97
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Pandora’s Inn
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