Overview of Gold Exploration

Part 1: Geological Characteristics of Gold Deposits

Part 2: Greenfield Exploration Approaches

Epithermal Deposits: Geological Characteristics and Genetic Processes

for Exploration and Discovery

SEG Short Course

Stuart Simmons and M. Stephen Enders
September 21 & 22, 2012

Geologic Characteristics of Gold Deposits

•  Gold Systems
–  Magmatic-Hydrothermal environment
–  Orogenic environment

•  Typical Ore Deposit Styles

–  Epithermal gold deposits
–  Porphyry copper-gold deposits
–  Carlin-style Gold deposits
–  Orogenic gold deposits

1  
Gold Depositional Environments

Magmatic Hydrothermal Environment

Primary magmas ascend from partial

melting of mantle wedge (purple zone).
Pooling of magmas at the base of crust
results in crustal melting and assimilation
in extensional domain. Magmas continue
ascending to upper crustal levels. 20%
may erupt.

Modified from Richards 2005

2  
Orogenic Environment

Large Crustal Structures

Anatomy of a Hydrothermal System

Host
rock
Channelway
(shear zone)

Structural Fluid focussing

permeability

Source
Groves 2004

3  
Epithermal Gold Deposits

Epithermal gold systems develop in extensional environments at high crustal levels where brittle failure prevails
–  Extensional elements - normal faults
–  Tensional fractures generally associated with extensional faulting at district scale

Hydrothermal Fluid Entrapment - Epithermal

4  
Porphyry Copper Deposits

•  Form 2-4 km below the paleo-surface

•  Intrusive rocks typically porphyritic

•  Extensive stockwork fracturing

•  Volumetrically significant alteration

–  Potassic
–  Propylitic
–  Sericitic
–  Advance argillic (near paleo-
surface)

•  Primary mineralization consists of

pyrite, chalcopyrite, /- bornite with
minor molybdenite and gold

•  Supergene mineralization consists of

chalcocite, covelite, and copper oxides

Hydrothermal Fluid Entrapment - Porphyry

Quartz-Sericite Stockwork Veins Quartz-Pyrite-Chalcopyrite-Sericite

Quartz-Chalcopyrite Vein Quartz-Molybdenite Vein

5  
 Carlin-Type Gold Deposits
Sedimentary rock-hosted disseminated gold
•  Occur in argillaceous (“dirty”) calcareous
rocks (limestones)
•  Gold is very fine grained (micron or less
in diameter, i.e. “no seeum” gold)
•  Gold occurs with pyrite or as free gold
where deposits have been oxidized
•  Gold:silver ratios are commonly 1:1 (so
low total silver)
•  Orebodies are related to faults, often
swarms of small offset faults
•  Mineralization associated with
decalcification (i.e. leaching of calcite)
•  Mineralization associated with
silicification (addition of silica) of host
rock. Massive silica zones or jasperoid
bodies are common with or above From Teal and Jackson, 1997,
mineralization SEG Newsletter, no. 31, p. 15.

•  Mineralization contains anomalous

amounts of arsenic, antimony, mercury,
thallium, molybdenum, and/or barium.
Trace to no base metals (Cu, Pb, Zn).

Hydrothermal Fluid Entrapment

Sedimentary rock-hosted disseminated gold

North wall,
Main pit,
Carlin mine
Unoxidized
(“refractory”) ore

Oxidized ore

6  
Orogenic Gold Deposits

Hydrothermal Fluid Entrapment - Orogenic

After Rhys 2005

7  
Greenfield Exploration Approaches

•  Distribution of Gold Deposits

•  Geological Framework Studies
•  Prospectivity Analysis and Area Selection
•  Exploration Toolbox
–  Geochemical Methods
–  Geophysical Methods
•  Exploration Cycles

Distribution of Gold Deposits - statistical

8  
 Distribution of Gold Deposits - spatial

764 / 855 deposits with known geographic coordinates

Distribution of Gold Deposits - environments

From Gold Deposits and their Geological Classification by Robert, Poulson and Dube, 1997

SP EV EH
EI VM
ED
EA
IO
SC
SS
IS

IP

MSV

MI
MP

9  
 Distribution of Gold Deposits – type

Number of Occurrences by Deposit Type

350 341 (> 100,000 oz)

300

250
Number of Occurrences

200

150
109
100
63 59
46
50 36 36
28 25 22 21 21 20 15 13

0
MSV EV IP EH SP MP SC VM ED SS EA MI IO IS EI
Deposit Model

Distribution of Gold Deposits – gold content

Median and Average Total Contained Gold by Deposit Type

35
(> 100,000 oz)
30.9
30
= Median contained gold (Moz)
Total Contained Gold (Moz)

25
= Average contained gold (Moz)

20

15

10.2
10 8.8
6.7
5.5 5.7
5.0
5 3.6 3.5 3.3 3.2 2.8 2.7 2.4 2.1
1.9 1.9 1.3
1.1 1.0 0.7 1.0 1.3 0.8 1.0 0.9 0.7 0.71.3
0.6
0
SP IP EA EI IO SS EH MI SC MP MSV ED EV VM IS
Deposit Model

10  
Global Terrane and Regional Framework
Studies

•  Identification of the most prospective terranes, belts and

districts in the world for the discovery of significant Au
deposits on a technical basis.

•  A ranked / prioritized source of exploration pipeline

opportunities.

•  Example Ranking Criteria

•  Structural architecture
•  Metamorphic grade
•  Gold endowment
•  Exploration maturity
•  Metallogenic zoning
•  Magmatic suites

Global Terrane Studies – prospective terranes

Orogenic Terranes

11  
 Prospective Terranes – area selection

Ahafo

Atla Akyem
ntic
O cea
n

Regional Framework Studies – area selection

Data Compilation & Interpretation Prospectivity Analysis

Regional Framework Studies

Focus On Prospectivity
Landsat
Geophysics
Geochemistry
Geology
Topography

•  Goal : 85% Of Prospectivity In 10% Of The Terrain

12  
Exploration Toolbox – exploration geochemistry
Stream sediment sampling and BLEG geochemistry
BLEG Orientation Survey

BLEG
(Au ppb)
>8
2.5 – 8.0
< 2.5
Bill Griffin: BLEG inventor
•  Rapid reliable reconnaissance tool
•  Ultra-low gold detection and precision
•  Many notable discoveries

Exploration Toolbox – exploration geochemistry

Soil and rock sampling and the interpretation of results

•  Mapping regolith distribution + profile

•  Knowing which data to trust

•  Improved geochemical interpretation

•  Promotes better drill targeting

13  
Exploration Toolbox – exploration geophysics

Airborne EM Surveys Aeromagnetic Surveys

Resistivity Depth Slice At 200m: •  Lithologic and Structural interpretations
<2.8 ohm meters,
>3.5 ohm meters Nassau
•  Resistivity depth
slice mapping

•  Conductivity
mapping

•  District scale
reconnaissance
tool

Exploration Toolbox – exploration geophysics

Induced Polarization & Resistivity Gradient Array Apparent Resistivity

Gradient Array Interpretation

570000
570000
570000mN
570000
570000
570000mN
mN
mN
mN
mN

568000
568000
568000mN
568000
568000
568000mN
mN
mN
mN
mN

566000
566000
566000
566000
566000
566000mN
mN
mN
566000mN
566000
566000 mN
mN
mN
mN
mN

564000
564000
564000mN
564000
564000
564000mN
mN
mN
mN
mN

•  Used to assist deeper drill targeting 562000

562000
562000mN
562000
562000
562000
562000
562000
562000mN
mN
mN
mN
mN
mN
mN
mN

•  Provides additional understanding of

lithology and alteration patterns 560000
560000
560000mN
560000
560000
560000mN
mN
mN
mN
mN

•  Integrated with district data portfolio

mE
mE
mE

mE
mE

mE
mE
mE

mE
mE

mE
mE
mE

mE
mE
mE
mE
mE

mE
mE

mE
mE
mE

mE
mE

mE
mE
mE

mE
mE
768000mE
mE
mE

770000mE
mE

772000mE
mE
mE

774000mE
mE

776000mE
mE
mE

778000mE
mE
768000
768000

770000

772000
772000

774000

776000
776000

778000
768000
768000
768000

770000
770000

772000
772000
772000

774000
774000

776000
776000
776000

778000
778000
768000
768000
768000

770000
770000

772000
772000
772000

774000
774000

776000
776000
776000

778000
778000

558000
558000
558000mN
558000
558000
558000mN
mN
mN
mN
mN

14  
 Exploration Toolbox – drill testing
Drilling Logging

Social Responsibility

Modeling

Exploration Cycles – 13 year discovery timeframes

Yanacocha: Discovery and Initial Reserve Reporting* History
20 10

9 Years to Initial
Reserve Reporting
8
15

Reserve &
Expenditures ($M)

Resource
6
Ounces (M)
Cumulative

Definition
10
~4 Years to 0.310
Discovery 0.178 4
Resource 0.048
Definition & 1.632
0.51
5 Exploration 1.436 1.509
2

2.338 1.572 1.601 1.967

Discovery
0 0
1982 84 86 88 90 92 94 96
Year
Resources Reserves Product Ounces
Recalculated to Reserves
*Newmont Equity Expenditures and Ounces Shown

15  
Exploration Strategies:
Exploration Cycles – classic boom & bust cycles
Counter Cyclical

Discovery is Roller Coaster Ride…

16