Paradigms Lost: Geologic Thought as Driver of Exploration Success - - PowerPoint PPT Presentation

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Paradigms Lost: Geologic Thought as Driver of Exploration Success - - PowerPoint PPT Presentation

Feb 19, 2024 28 likes •226 views

Paradigms Lost: Geologic Thought as Driver of Exploration Success for Nevada Gold Mike Ressel, Nevada Bureau of Mines and Geology University of Nevada, Reno 1 Top 10 Gold Producers - 2014 Nevadas size: 2.9% of U.S total area 2.9% of

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Paradigms Lost: Geologic Thought as Driver of Exploration Success for Nevada Gold

Mike Ressel, Nevada Bureau of Mines and Geology University of Nevada, Reno

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211 t Nevada 153 t 270 t 245 t 150 t 150 t 160 t

COUNTRY TONNES Moz

  • 1. China

450 14.5

  • 2. Australia

270 8.7

  • 3. Russia

245 7.9

  • 4. Canada

160 5.1

  • 5. Nevada

153 4.9

  • 6. South Africa

150 4.8

  • 7. Peru

150 4.8

  • 8. Uzbekistan

102 3.3

  • 9. Mexico

92 3

  • 10. Ghana

90 2.9

Top 10 Gold Producers - 2014

U.S. Geological Survey Commodity Summaries, 2015

Nevada’s size: 2.9% of U.S total area 2.9% of Canada 3.7% of Australia 102 t 92 t 90 t 450 t

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Nevada = 185 Moz 1981-present Carlin 1st pour

  • Calif. Mother Lode ~102 Moz

Sources: Nevada Bureau of Mines and Geology Minerals Industry report, 2013

Annual Gold Production (millions of troy ounces)

Gold Production in the U.S. and Nevada

Mother Lode and the 1st Gold Rush Sustained CA production + new mines in the West Current boom started modestly with the Carlin discovery Real trigger for new boom: Fixed Au price removed in 1971 Yesterday’s waste became today’s ore Carlin discovery drove more discovery & innovation

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Windows and Invisible Gold

John Livermore Ralph Roberts

Elko Wells Winnemucca

50 km N

Roberts Mtns Thrust

Major innovations:

  • 1. Recognition of the Antler orogeny and the

regional Roberts Mountains thrust (Roberts, 1950s)

  • 2. Recognition of mineral trends (Roberts, 1960)
  • 3. “Invisible” Au at Carlin coincide

with lower-plate windows

  • 4. Heap leaching and bulk mining
  • 5. Reverse-circulation drilling
  • 6. Advent of rapid, low-detection

geochemistry

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1 2 3 4 5 6 7 8 9 10 200 400 600 800 1,000 1,200 1,400 1,600 1,800 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 Price Production

2014 Nevada Gold Production

4.94 Moz >30% from UG >refractory <oxide 1989 level

Gold Price, $/troy ounce Gold Production (millions of troy ounces)

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Eocene Gold Belt Production Carlin: 91 Moz Battle Mtn-Eureka: 35 Moz Getchell: 22 Moz Jerritt Canyon: 7 Moz Bald Mountain: 3.5 Moz Miocene Epithermal Belt Northern Nevada Rift: ~5 Moz Total: ~163 Moz produced

Eocene Gold Belt Miocene Au-AgBelt Deposit Eocene Intrusion

Jerritt 50 km Bald Mtn

0.3% of U.S. landmass! 125 x 225 km

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What’s so special about the Eocene?

Ludington et al., 1996, NBMG Open-file 96-2

Basalt-rhyolite (Mid-Miocene; rift-related) OLIG-MIO- Rhyolite-dacite pyroclastics arc-related)

EOCENE- Andesite-dacite-rhyolite

lava/intrusions

Cenozoic Igneous Rocks of Nevada

Andesite-dacite (Mid-Miocene arc-related)

  • Carlin-type deposits coincide

spatially and temporally with Eocene magmatism

  • Heat source, possible metal

source

  • Eocene magmatism followed a

~40-50 Ma hiatus

  • Characterized by rapid SW

migration: 30-70 km/m.y.

  • Later magmatism related to

slower rollback

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Eocene: Sweet Spot for Carlin-type

  • Eocene metallogeny: Au-dominant

Carlin-type Au (major) Distal disseminated Au Au skarn

  • Other periods have more typical

magmatic-hydrothermal deposits: epithermal, porphyry

  • What’s special about Carlin-types?

Trends/clusters=large systems Replacement style=large tonnage But, often high grade too

Green = Eocene Blue = Oligocene/E. Miocene Pink = mid-Miocene Red = L. Miocene/Pliocene

Intrusion-related Au-Ag Carlin-type Epithermal Au-Ag

Cenozoic Deposit Types

Ludington et al., 1996, NBMG Open-file 96-2

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10 20 30 40 50 60

Gold Quarry 1980 Pit Goldstrike 1987 Pit+U.G. Twin Creeks 1987 Pit Pipeline 1988 Pit Getchell 1988 U.G. Leeville 2006 U.G. Cortez Hills 2009 Pit+U.G. Goldrush (NEW) U.G. (Resource)

Growth History of Nevada Carlin Gold Giants Production+Reserve vs Initial Reserve

Initial Reserve Mined Mined + Reserve Endowment*

Production data from NBMG Mineral Industry 2013 report and corporate financial reports *Endowment estimates from historic data, corporate presentations, publications; includes unclassified, mineralized material

Gold (Millions, Ounces) 24X 25X 18X 9X 2X 3X 2X 4X

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  • 1. Eastside – Columbus Gold-Cordex (2011-13)
  • 2. N. Bullfrog – Redstar-Corvus Gold (2011-13)
  • 3. Long Canyon – Pittston/AuEx (1999-2003)
  • 4. Kinsley – Pilot Gold (2012-13)
  • 5. Leeville corridor – Newmont (1994 initial)
  • 6. Arturo – Barrick-Meridian (1998-2005)

7.

  • N. Bullion – Gold Standard (2010-11)
  • 8. Cortez Hills – Cortez JV (2002-2003)
  • 9. Iceberg – NuLegacy Gold (2012)
  • 10. Goldrush – Barrick (2011)
  • 11. Helen/2201 – Victoria Gold/Premier (2007-14)
  • 12. Spring Valley – Echo Bay/Midway (1999-2003)

Selected Discoveries

Miocene epithermal Eocene Carlin-type Cretaceous orogenic

2 1 4 9 3 6 8

Eocene belt 42-34 Ma Miocene belt <24 Ma

Au Belts

5 7 10 11 12

100 km

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When Were Nevada Discoveries Made?

  • Helen/2201

2007 Victoria (Junior) Exploration Res

  • Goldrush

2009/10 Barrick Prefeasibility

MII

  • North Bullion

2010/11 Gold Standard (Jr) Exploration No Res

  • North Bullfrog

2011/12 Corvus Gold (Junior) Exploration M+I+I

  • Kinsley

2012/13 Evolving Gold (Jr) Early Exploration Ind+Inf

  • Iceberg

2012 NuLegacy (Junior) Early Exploration No Res Late Boom/Early Bust 2011-2013

  • Leeville corridor

post-1994 Newmont Mining

  • South Arturo

1999-2005 Barrick Mining

  • Spring Valley

1999-2000 Echo Bay Prefeasibility?? Resource

  • Long Canyon

2000 Pittston (Junior) Mining

  • Cortez Hills

2002-2003 Cortez JV (majors) Mining Late Bust/Early Boom 1994-2003

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2 4 6 8 10 12 14 16 18

Recent Discoveries and Resource Expansion Initial Resource/ Reserve Current Resource/ Reserve Gold (millions of troy oz)

  • 5 of 6 notable resources in the

recent past are from Carlin- type deposits

  • 1 low-sulfidation epithermal
  • Most show the desirable trend of

rapid increase in resource

  • unces despite mining

downturn

  • Newer discoveries discovered

after the ‘financial meltdown’

  • Expect similar increases in

resources to established deposits 2009-14 2005- 2015 2009- 2015 2007-11 2010- 2014 2015 2006- 2015

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2 1 4 9 3 6 8

Eocene belt 42-34 Ma Miocene belt <24 Ma

Au Belts

5 7 10 11 12

100 km

North-central Nevada

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Barrick’s Goldrush Discovery: The Right Address

Source: Barrick corporate presentation, Sep. 2011 Jerritt

50 km

Bald Mtn Goldrush 3 Subtrends 6 km wide

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  • Most significant discovery

since Cortez Hills in 2002

  • Original Red Hill discovery by

Placer Dome (61m @ 8.2 g/t)

  • Goldrush was discovered

2km south of Red Hill in 2009

  • Headframe exploration: 6

km from Cortez Hills

  • 3 parallel subtrends, 6 km

wide

Source: Barrick corporate presentation, Sep. 2014

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Cortez 3 km

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Barrick’s Goldrush Discovery: The Right Stuff!

2009 discovery: 70 ft @ 0.89 opt Au (21.3 m @ 30.5 g/t) 2011 discovery announcement with ~3.5 Moz MII 2012: ~7 Moz MII 2015: 15.5 Moz MII; pre-feasibility Key Points Leading to Discovery:

  • Aggressive exploration program – elephant country,

Placer Dome discovery of Cortez Hills

  • Recognition of parallel structural corridor – curiously

subparallel to edge of Northern Nevada Rift

  • Structural & stratigraphic expertise: distinction of upper

and lower plates, recognition of receptive host rocks

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6 km Cross Section (SW-NE) Looking northwest

Long Section (NNW-SSE) Looking eastward

Key observations:

  • Re-analysis of stratigraphy and recognition of prospective lower-plate lithologies at

shallow depth

  • Recognition of another parallel structural trend: Cortez Hills, Horse Canyon, Goldrush
  • Roberts Mountains thrust in hanging wall

6 km

Source: Barrick corporate presentation, Sep. 2015

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NuLegacy Gold - Iceberg

35m@3.4g/t Au 34m@2.5 g/t Au 35m@3.9g/t Au 24m@3.4 g/t Au

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Conclusions on Innovation in Nevada

Nevada represents a huge step change in geologic thinking that contributed directly to the current long-lived production boom Generating and testing geologic concepts are the engine of exploration and innovation Discoveries and innovations came in many different forms, and these continue to redefine the exploration model for Nevada deposits Economic discoveries often come during down cycles: desperation is a mechanism

  • f creativity

New technologies are very important especially when applied to situations where a good geologic framework already exists. Technologies should be ‘fit for purpose’.

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