Placer gold mining in alaska

Dylan McFarlane

d.e.mcfarlane@exeter.ac.uk

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

Stereotypes of ASM (Artisanal & Small-scale Mining)

Artisanal & Small-Scale Mining

Bigger industry than you may think:

• 20,000,000 directly employed

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

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

0 50 100 150 200 250 300 350

Copper

Silver

Potash

Nickel

Phosphate

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 regions67 mining districts

Over 25 Moz historic

2014 est.

230 mines

140 operate

70,000 oz

Gold is where YOU find it

Prospect Explore & Develop Mining Reclaim

Portable sluice box

Pan the Creek

Historic Workings?

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

Paystreak Areas

Flood GoldFalse 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 & 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

Panning with a Batea

Then, drilling & bulk sampling

Portable Highbanker

Up to 1 m3 samples

1.5” Pump

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

Estimating Particle Mass Using Size ClassificationBlue 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 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

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 Southern Tanzania?

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

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

• 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

Marine Offshore - Nome

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)

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

Heterogeneity and Nugget Effect

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)

Homogeneity

Source: King, McMahon & Bujtor CRA

(Australia) 1980

NuggetyCreeks

Fans, Terraces

Flood-plains,

Offshore

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

Exploration Methods - Drilling

Hammer & RAB

Double Cased

Hammer

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

Sample Processing - Mini Washplant

Goldfields Prospector

LaboratoryDe-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

Optimize

Channel – Gulch Section

Alluvial Fan Section

Methods

Equipment

Washplants

Underground Placer Mining – 1900s FairbanksThawing 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 paydirt5. Processing plant - discharging into

settling pond(s)6. Restored area

Hydraulicking – 1.5 Billion cubic yards in California

Blue Babe – baby mammoth

HydraulickingJohn Miscovich - LegendIntelli-giant – automatic monitor

Baker Family – Mudminers in Tofty

Dragline Mining

Don’t Tangle the Cable on the Spool!

Skyline-Drag System, Nome

Bucket-Ladder Dredge

Over 100 in AK

Continuous, high volume

Mercury, then trapezoidal jigs

Problems:

Climate

Davidson Ditch – 90 mile pipeline (1920)

Steam Fields – Cold Water Saved Alaska

The Tweet Dredge +110 Years Operation

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!!!

Marine Mining: New Techniques & Technologies All the Time

Pomrenke Dredge

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

Other TechniquesRoss-boxSuction DredgingReverse spiralsHydro-clean

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

N.L. Wimmler (1927) – Review of Placer Mining Methods and Costs

Few Good Miners

Time for a Revision!

Tools of the Trade – 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 conveyorEven 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

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

Gold Wheels - Toy

Jigs, Centrifuges, Bowls & Tables

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 MineI 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

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

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

Pandora’s Inn

Placer gold mining in alaska

Science

Transcript of Placer gold mining in alaska

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