1. No category

WORKSHOP
Exploration for orogenic
gold deposits – with emphasis on
geochemical exploration in
glaciated Precambrian terrain
Exploration for orogenic gold deposits –
with emphasis on geochemical exploration in
glaciated Precambrian terrain
Workshop, 21 August 2011
25th International Applied Geochemistry Symposium 2011
22-26 August 2011 Rovaniemi, Finland
Pasi Eilu, V. Juhani Ojala and Pertti Sarala
Publisher: Vuorimiesyhdistys - Finnish Association of Mining and Metallurgical
Engineers, Serie B, Nro B92-6, Rovaniemi 2011
Eilu, P., Ojala, V.J. and Sarala, P. 2011. Exploration for orogenic gold deposits – with emphasis on geochemical exploration in glaciated Precambrian terrain. Workshop in the 25th International Applied Geochemistry Symposium 2011 22-26 August 2011 Rovaniemi, Finland. Vuorimiesyhdistys, B92-6, 88 pages.
Layout:
Cover – Irma Varrio
ISBN 978-952-9618-74-3 (Printed)
ISBN 978-952-9618-75-0 (Pdf)
ISSN 0783-1331
© Vuorimiesyhdistys
This volume is available from:
Vuorimiesyhdistys ry.
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02360 ESPOO
Electronic version:
http://www.iags2011.fi or http://www.vuorimiesyhdistys.fi/julkaisut.php
Printed in:
Painatuskeskus Finland Oy, Rovaniemi
Exploration for orogenic gold deposits – with emphasis on
geochemical exploration in glaciated Precambrian terrain
Pasi Eilu1, V. Juhani Ojala2 and Pertti Sarala1
1
2
Geological Survey of Finland, E-mail [email protected]
Store Norske Gull, E-mail [email protected]
Abstract
An orogenic gold deposit is a structurally controlled gold occurrence formed during one of the major stages of an orogeny by orogenic fluids. Any rock type within
a greenstone or schist belt, a metamorphosed supracrustal rock, dyke, or intrusion
within or intrusion bounding such belt may host an orogenic gold deposit. There is
strong structural control of mineralization at a variety of scales but the favoured host
is typically the locally most reactive and/or most competent lithological unit. These
deposits are not present in post- or anorogenic intrusions or unmetamorphosed supracrustal rocks. Most of the Precambrian deposits were formed during 2.70–2.60
Ga and 2.10–1.70 Ga. These epochs appear to be related to rapid crustal growth and
accretionary stages of supercontinents. Mineralisation typically takes place during
the last major stage of an orogeny. The late timing of this deposit type to provide
important geological aids in exploration as the geometry of the geological units has
not significantly changed after the mineralisation. Hence computer aided methods
like geomechanical stress modelling can be utilised to model structurally favourable
sites.
In greenschist facies settings, mineralisation typically takes place slightly
after the metamorphic peak, but in amphibolite facies at the local regional-metamorphic peak. The ore bodies typically have a strongly flattened ellipsoidal shape, are
plate-like, may have a steep or a gentle dip and plunge of ore shoots. An ore body
can be 0.5–50 m wide, 100 m – 2 km long, consisting of a vein network, an en echelon vein swarm, or just of one single large vein. The depth extent of an ore body
may well be much larger than its extent along strike. An individual vein can be 1
cm – 10 m thick and 20–1000 m long. In most cases, gold occurs as native gold, free
in gangue and with main sulphides, and as inclusions and in fractures of gangue and
sulphide grains. In a few cases, most of gold is in the lattice of or submicroscopic
inclusions in pyrite or arsenopyrite.
All deposits are developed by an alteration halo characterised by proximal
to distal carbonatisation and proximal sericitisation or biotitisation. Also, proximal
sulphidation may be distinct if the host rock is iron rich. Elements enriched typically
include As, Au, CO2, K, Rb, S, Sb, Te, W; in some cases also Ag, B, Bi, Co, Cu,
and Se are enriched. The Au/Ag is consistently >1, typically 5–10. Enrichment or
depletion of Ca, Fe, Mn or Mg are non-existent, and Na mobility, if present, is minor
and spatially restricted to the ore itself. Alteration mineral assemblages, alteration
indices based on CO2 and K, and trace (pathfinder) elements enriched in the deposits
can be used in defining exploration targets and vectors to ore in bedrock.
Surficial geological, till geochemical and indicator mineral studies are effective methods in gold exploration in glaciated terrains. By test pit surveys and stratigraphic controlled till geochemical and heavy mineral sampling glacial transport
distances and mechanisms, secondary element dispersions, and ice flow directions
can be estimated. Strong changes in glacial dynamics and erosional and depositional
conditions lead to a variable degree of preservation of earlier deposits and pre-Quaternary regolith, and deposition of complex glacigenic formations.
6
Workshop Program
Sunday, 21 August 2011, Hotel Santa Claus, Rovaniemi
8:30-9:00 am Registration
9:00 am Overview of the orogenic gold deposit type, Juhani Ojala
9:45 am Reference to other types of gold deposits in shield areas, Juhani Ojala
10:30 am Coffee break
10:45 Alteration and geochemical dispersion related to orogenic gold, Pasi Eilu
12:00 Lunch
13:15 Brief overview to the surficial geological and geochemical exploration for gold
in glaciated terrains, Pertti Sarala
14:00 Discussion
14:30 Coffee
14:45-16.00 Ore and alteration zone samples, Pasi Eilu
7
Overview of the orogenic gold
deposit type
Juhani Ojala
Store Norske Gull AS
Heavily based on talks by D.
I. Groves
Talk outline
• Nature and tectonic setting of orogenic gold
• Lithospheric scale energy sources and
processes
• Timing of orogenic gold systems
• Crustal continuum model
• Orogenic gold mineral system
• Structural and host rock controls
• Endowment
• Conceptual targeting and GIS
8
A SIMPLIFIED OROGENIC GOLD
MINERAL SYSTEM
1.
SIMPLIFIED MINERAL SYSTEM
2.
RHEOLOGICAL CONTRASTS AND HOST ROCK
CONTROLS
3.
STRUCTURAL AND GEOMETRICAL CONTROLS
a) Supracrustal Belts
b) Role of Granitoids and Other Rigid Bodies
4.
PRODUCTIVE vs POORLY-ENDOWED TERRANES
Host
rock
Channelway
(shear zone)
Structural
permeability
Fluid focussing
Source
rock
Anatomy of a Hydrothermal System
9
7
Epigenetic Gold Deposits in Orogenic Belts
•
•
•
•
Orogenic gold deposits (e.g. Kalgoorlie, Ashanti)
“Intrusion-related” gold deposits (e.g. Fort Knox)
Overprinted porphyry deposits (e.g. McIntyre,
Boddington?)
Overprinted VMS (Bousquet, Bulyanhulu)
Includes “greenstone-hosted”, “slate-belt hosted”,
“Mother lode-style” etc
8
9
10
Features Common to Majority of
Orogenic Gold Deposits
1. At or near terrane boundaries (or other crustalscale faults/ shear zones).
2. Strong structural control in lower-order structures.
3. Large vertical extent with subtle vertical zonation.
4. Typically K-mica and carbonate alteration at
greenschist facies.
5. Characteristic addition of SiO2, K, Rb, Ba+Na+B.
6. Characteristic ore metals : Au+Ag+As+Sb+Te+W
with low Pb-Zn-Cu.
7. Low salinity H2O-CO2 + CH4 ore fluid.
8. Radiogenic and stable isotope signatures indicate
mixed sources.
10
FACTORS CRITICAL TO FORMATION OF
WORLD-CLASS OROGENIC DEPOSITS
1.
HIGH FLUID FLUX
a)
2.
Thick competent host rocks
b)
Strong contrasts in rock strength (rheology)
c)
High structural permeability in failed units or fault/shear zone
contacts
EFFICIENT GOLD DEPOSITION
a)
Reactive host rocks (high Fe/Mg or C?)
b)
Phase separation (lower H+, CH4 and H2S in residual fluid)
c)
Fluid mixing (note problems of mixing: limited fluid reservoirs;
high fluid pressure; confusion with reaction with previously
altered rocks
CRITICAL CHEMICAL FACTORS FOR LARGE
TONNAGE - OROGENIC GOLD DEPOSITS
1.
REACTIVE HOST ROCK TO PRODUCE DISSEMINATED GOLD RESOURCE (IN
ADDITION TO HIGH-GRADE VEINS)
2.
FLUID IS H2O – CO2 ± CH4 WITH VERY MINOR (<100PPM) H2S
3.
GOLD IS CARRIED AS THIOSULPHIDE COMPLEX SUCH AS Au (HS)2
4.
HENCE GOLD CAN BE PRECIPITATED BY LOW DEGREES OF SULPHIDATION (< 5%
SULPHIDES) OF HOST ROCKS
5.
HENCE HIGH Fe CONTENTS (MORE CORRECTLY HIGH Fe/Fe+Mg+Ca RATIOS,
BECAUSE CO2 IS DOMINANT AND PRODUCES Fe-BEARING CARBONATES) ARE
IMPORTANT
6.
HIGH C CONTENTS MAY REDUCE FLUID OR TRAP Au ON ORGANIC MATERIAL.
7.
HENCE MANY WORD-CLASS OROGENIC GOLD DEPOSITS IN HIGH Fe ROCKS SUCH
AS THOLEIITIC BASALT (TIMMINS) OR CARBONACEOUS ROCKS (BENDIGO)
11
14
15
12
16
Alpine Orogenic Gold: Related to slab
Detachment and mantle thermal anomalies
Slab
Slab
de Boorder et al EPSL’98
17
18
13
A CRUSTAL THICKENING
Accreted
oceanic crust
Hg-Sb
B PLUME IMPACT/SUBDUCTION
Arc
Au
Au
Mantle
plume
head
C SUBDUCTION ROLLBACK
Extension in
continental crust
D OCEANIC RIDGE SUBDUCTION
Extension
Au
Au
Slab
rollback
Asthenosphere
upwelling
Asthenosphere
upwelling
E
EROSION OF MANTLE
LITHOSPHERE
F
DELAMINATION OF MANTLE
LITHOSPHERE
Au
Au
Au
Asthenosphere
upwelling
Asthenosphere
upwelling
Granitoids
Accreted
continental crust
G
A
Greenschistamphibolite
transition
Fault with
movement
vector
19
Stable
continental crust
Oceanic crust
Mantle
plume
Mantle
lithosphere
Asthenosphere
20
21
14
22
23
24
15
25
Youngest rocks hosting gold mineralisation
Granny Smith
Mt Shea
Porphyry
Jundee
Kanowna Belle (pre-main Au event)
Mt Morgans
Ages of gold mineralisation
Eastern
Goldfields
Province
Kanowna Belle (minor)
Golden Mile
Mt Charlotte
Victory, Kambalda
Chalice
U-Pb in zircon
U-Pb in titanite
U-Pb in monazite
Re-Os in molybdenite
Ar/Ar in mica
Sm-Nd in garnet
Pb-Pb in pegmatite
Pb-Pb in sulphide
Lawlers (minor)
Matilda, Wiluna
East Lode, Wiluna
Other
Provinces
Reedys
Marymia
Big Bell
Mt Gibson
Oldest rocks syn to post gold mineralisation
Eastern Goldfields
Province
Scotia
Scotia
Mt Gibson
Other
Provinces
Westonia
Griffin’s Find
Nevoria
Corinthia
2680Ma
2660
2640
2620
2600
2580
2560Ma
26
27
16
28
29
30
17
31
32
33
18
34
35
36
19
37
38
39
20
40
41
42
21
43
44
45
250
1250
1000
750
500
250
kg x 103
1000
500
1250
250
The Situation
in 2000
kg x 103
750
500
250
ABITIBI
Camflo : Val D’Or
Preston : Timmins
Detour : Lake Detour
Pickle Crow : Pickle Lake
Macleod-Cockshunt : Geralton
Agnico-Eagle : Jouiel
Hallnor : Timmins
Hollinger-McIntyre-Coniaurum:Timmins
Dome : Timmins
Kerr Addison : Larder Lake
Campbell : Red Lake
Lake Shore : Kirkland Lake
Williams : Hemlo
Golden Giant : Hemlo
Wright - Hargraves : Kirkland Lake
Lamaque : Val D’Or
Dickinson : Red Lake
Pamour : Timmins
Sigma : Val D’Or
David Bell : Hemlo
Doyon : Bousquet
Macassa : Kirkland Lake
East Malartic : Malartic
Madsen : Red Lake
Bousquet : Bousquet
Aunor : Timmins
Malartic : Malartic
Sylvanite : Kirkland Lake
750
Hallnor : Timmins
500
Emu (Lawlers)
kg x 103
Edna May (Westonia)
Paddington
Reedy
Westralia (Mt Morgans)
New Celebration
Oroya Black Range (Sandstone)
Youanmi
Frasers (Southern Cross)
Bayleys (Coolgardie)
Burbanks
Hannans North (Kalgoorlie)
Cosmopolitan (Kalgoorlie)
Granny Smith (Laverton)
The Situation
in 1990
Camflo : Val D’Or
Preston : Timmins
Detour : Lake Detour
Pickle Crow : Pickle Lake
Macleod-Cockshunt : Geralton
Agnico-Eagle : Jouiel
750
Golden Mile (Kalgoorlie)
Central Norseman
Sons of Gwalia
Mt Charlotte (Kalgoorlie)
Wiluna-Moonlight
Hill 50 - Mt Magnet
Boddington - Hedges
Great Fingall - Golden crown (Day Crown)
Paddys Flat (Meekatharra)
Kambalda - St Ives
Lancefield (Laverton)
Big Bell (Cue)
Lady Shenton - Crusoe - Princess May (Menzies)
Copperhead (Bullfinch)
1000
kg x 103
1000
YILGARN
Golden Mile (Kalgoorlie)
Plutonic
Sunrise/Cleo
Jundee
Wallaby
Kanowna Bell
Bronzewing
Carosue Dam
Central Norseman
Tarmoola
Sons of Gwalia
Mt Charlotte : Kalgoorlie
Wiluna-Moonlight
Hill 50 - Mt Magnet
Boddington - Hedges
Granny Smith : Laverton
Great Fingall - Golden crown : Day Crown
Paddys Flat : Meekatharra
Hollinger-McIntyre-Coniaurum:Timmins
Kambalda - St Ives
Dome : Timmins
Lancefield : Laverton
Kerr Addison : Larder Lake
Thunderbox
Campbell : Red Lake
Big Bell (Cue)
Lake Shore : Kirkland Lake
Lady Shenton - Crusoe - Princess May:Menzies
Williams : Hemlo
Copperhead (Bullfinch)
Golden Giant : Hemlo
Emu : Lawlers
Wright - Hargraves : Kirkland Lake
Edna May (Westonia)
Lamaque : Val D’Or
Paddington
Dickinson : Red Lake
Reedy
Pamour : Timmins
Westralia : Mt Morgans
Sigma
: Val D’Or
New Celebration
David Bell : Hemlo
Oroya Black Range : Sandstone
Doyon : Bousquet
Youanmi
Macassa : Kirkland Lake
Frasers : Southern Cross
East Malartic : Malartic
Bayleys : Coolgardie
Madsen : Red Lake
Burbanks
Bousquet : Bousquet
Hannans : North (Kalgoorlie
Aunor : Timmins
Cosmopolitan : Kalgoorlie
Malartic : Malartic
Sylvanite : Kirkland Lake
22
46
YILGARN
47
ABITIBI
48
23
GIS modelling
Modelling
Digital elevation
model and basemaps
Till geochemistry
Bedrock mapping
Gravity
Satellite images
Quaternary geology
Airborne geophysics
•magnetic
•electro-magnetic
•gamma radiation
Weights of Evidence orogenic gold model,
Combined Empirical/Conceptual WofE
Combined empirical/conceptual weights-of-evidence
model CLGB
Class
Area km2Area %Sites W+
W- Contrast s(C) Confidence
Very high 403
2.2
6 2.1128 -0.1726 2.2854 0.4527 5.0482
High
517
2.8
12 2.5653 -0.4078 2.9731 0.3617
8.22
Moderate 2493
13.4 10 0.7903 -0.2051 0.9954 0.377
2.6404
Low
635
3.4
0
0
0
0
0
0
Very low 14593
78.3
6 -1.4911 1.3379 -2.829 0.4502 -6.2837
total
18642 100.0
24
GIS conclusions
• the models predict areas with high potential
for orogenic Au mineralization in the Central
Lapland Greenstone Belt
-> considerable reductiong of the area to be
explored (less than 1% of the original study
area)
B. Salier (2003)
DISTAL VS PROXIMAL SOURCE MODELS FOR
OROGENIC GOLD DEPOSITS
53
SIMPLE MINERAL SYSTEM MODEL
ARCHAEAN OROGENIC GOLD DEPOSITS
σ1
SEAL
Sub –
Greenschist
σ1
Dolerite
TRAP
Mid Greenschist
Sedimentary Sequence
Volcanic Rock
FLUID PATHWAY
Amphibolite
Metamorphic Fluid
Distal
Magmatic
Fluid
Granite
II
Granulite
SOURCE
Metamorphic Fluid
Granite I
Fluid from Subcreted
Oceanic Crust
54
25
Reference to other types of
gold deposits in shield areas
V. Juhani Ojala
Current organisation:
Store Norske Gull AS
Ouline of the talk
• Evolution of the Fennoscandian Shield
• Gold mineralization types
–
–
–
–
–
–
–
Metamorphosed epitermal
Massive sulphide hosted
Granitoid related (non skarn)
Skarn-Iron Oxide Copper Gold
Orogenic (mesothermal)
Paleoplacer
Supergene and Recent alluvial
Gold mineralisation can occur in nearly all
geological environments
Tectonic settings of gold-rich mineral deposits
Groves et al. (1998, 2000, 2003, 2005)
26
Genetic deposit types
Palaeoproterozoic, 1.92-1.77 Ga
• Multistage rifting during 2.45-1.97 Ga
• Four main orogenic stages:
1.92-1.88 Ga: Microcontinent accretion
1.88-1.85 Ga: Continental extension
1.85-1.79 Ga: Continent-continent collision
1.79-1.77 Ga: Orogenic collapse and stabilisation
27
1.90–1.88
collision II
1.92–1.90
collision
1.86–1.85
1.85–1.79
exension
collision III
Metamorphosed
epithermal
Metamorphosed epithermal:
Pahtavaara
Kutemajärvi/Orivesi
Pasi Eilu 2003
28
Kutemajärvi:
Metamorphosed HS epithermal
Host to ore: quartz rock
Immediately around to ore: Al-rich rocks
quartz-andalusite-pyrophyllite
F-Al(-P) minerals present: topaz, fluorite, apatite
Metal association Au±Ag-As-Te
No potassic or carbonate alteration
Extremely low Na2O + K2O + CaO + MgO
Pasi Eilu 2003
Kutemajärvi, Tampere Schist Belt
After Poutiainen
& Grönholm
(1996)
Pahtavaara
29
Pahtavaara
•Gangue – quartz, baryte, tremolite, dolomite, scheelite
•Ba- and Mn-anomalies
•Au = 99.02% Au, 0.07% Ag, 0.25% Bi
Champagene Pool, New Zealand
White Island New Zealand
30
Pasi Eilu 2006
7 Auriferous massive
sulphide deposits
&
Submarine Au-rich
precipitates
Pasi Eilu 2006
VHMS: back-arc environments
Groves et al. (1998, 2000, 2003, 2005)
Rhyol
intrusion
VHMS: what happens there – ”a bit” simplified view
Pirajno (1992)
31
Precious-metal mineralisation
Pyhäsalmi mine, Proterozoic central Finland
Fahlore?
Electrum
Pyrrhotite
Chalcopyrite
Pyrite
Field of view about 1 mm
Pasi Eilu 2006
All that glitters can be gold!
Haveri ore, SW Finland:
Au-Cu VHMS mineralisation
or a submarine epithermal
overprint on Cu-VHMS?
Pasi Eilu 2006
Field of view about 10 cm
Photo J. Väätäinen
32
Iron oxide-copper
gold
After Lahtinen et al. (2003),
Weihed & Eilu (2003)
IOCG
•Hosted by epigenetic alkaline to alkali-calcic
predominantly subaerial volcanic rocks, ironstones,
skarn-like rocks, albite rocks, graphitic schists, marbles
•Structural control distinct in all cases
•Magnetite-Chalcopyrite-Pyrite-Pyrrhotite-Gold ±
Cobaltite, Co pentlandite, Uraninite association
•Regional extensive albitisation and scapolitisation
•Local multi-stage Fe ± Mg ± K ± Na alteration
Weihed & Eilu 2003
33
IOCG Kolari
HANNUKAINEN
W
0
162
Laurinoja
Kivivuopio
102
79
78
170
71 75 89 42
Kuervaara
39
36
92
90
86
400 m
P
Monzonite
Diorite
Overburden
Skarn
Ironstone
Qz-fsp schist
84
33
E
Quartzite
Mica gneiss
Mafic metavolcanic rock
Modified from Hiltunen (1982)
FeOxCu-Au
Granitoid related
Kopsa
34
Kopsa
•Major opaques Chalcopyrite, arsenopyrite, pyrrhotite
•Minor opaques Loellingite, marcasite, pyrite, sphalerite,
gold, molybdenite, cubanite, bornite, stannite, bismuth and
several Bi-bearing sulphosalts
•The entire intrusion is anomalous in Ag, As, Au and Cu
Granitoid relatedPorphyry gold
Pasi Eilu 2006
Porphyry deposits: arc environments
Groves et al. (1998, 2000, 2003, 2005)
35
Raitevarri till and rock geochemistry
Cu-Au Anomalous zone 3 km long and 1 km wide
2009 drilling
2008 drilling
500 profile Au/Cu
36
Orogenic
Orogenic gold in Fennoscandian Shield
Distribution
Age
Archaean
In every greenstone
belt explored for
gold?
Ilomantsi
Pampalo
After Lahtinen et al. (2003),
Eilu(2003)
2003
Weihed Pasi
& Eilu
37
Orogenic Gold: Archaean
•
Structural control in both regional and local scale
•
The locally most competent ± reactive rock unit as the
main host to ore
•
Au-only deposits
•
Enriched: Ag, As, Au, Ba, Bi, CO2, K, Li, Rb, S, Sb, Te, W
•
Timing: ca. 2.7 Ga, in the latest stage of the
Neoarchaean orogeny (Global control?)
•
Compressional to transpressional deformation
•
Syn-late orogenic TTG, but no indication of fluid or
metals from the granitoids
Weihed & Eilu 2003
Orogenic Gold: Archaean
Pampalo:
Structurally most complicated location,
Locally, the most competent rock types
Orogenic Gold: Palaeoproterozoic
•
Structural control in both regional and local scale
•
The most competent ± reactive rock unit as the main
host to ore
•
Enriched: Ag, As, Au, CO2, K, Rb, S, Sb, Te, W
± Co, Cu, U (Kuusamo, Saattopora)
•
No indication of fluids or metals from granitoids
Weihed & Eilu 2003
38
Central Lapland Greenstone Belt
Gold: Blue and green dots
Keinänen & Eilu 2003
Orogenic Gold: Palaeoproterozoic
Suurikuusikko,
Central Lapland:
In a shear zone,
intense brecciation
Paleoplacer
•No alteration
•No increase of
any other element
than Au
Kumputunturi-Outapää
Kaarestunturi
39
Placers and supergene
Isomaa supergene gold from regolith
Puskuoja (Alhonen)
Miessi (Tapio)
Isomaa-Kittilä
Sotajoki
(Vehviläinen)
Nokia
5 cm
Ruosselkä
Ivalojoki
40
Conclusions
Complex tectonic evolution results diversity of Gold
mineralisation styles
•Orogenic: dominant, in all greenstone or schist belts, all ages
•FeOx-Cu-Au: Palaeoproterozoic, W Lapland,
•Granitoid-related: Palaeoproterozoic, near SW suture
between Archaean and Proterozoic
•Metamorphosed epithermal: Palaeoproterozoic, volcanic arcs
•VHMS: Haveri?, Palaeoproterozoic, volcanic arcs
•Paloeplacer in Lapland – erosion from VHMS and orogenic
•Supergene in regolith remnants weathering started
Neoproterozoic
•Placers reworking until Recent
Pasi Eilu 2003
Thanks:
David Groves
Stephen Gardoll
Raimo Lahtinen
Veikko Keinänen
Pär Weihed
Tero Niiranen
Nicole Patison
Nick Oliver
Pekka Nurmi
Erkki Vanhanen
Peter Sorjonen-Ward
Eelis Pulkkinen
Jukka Jokela
Ilkka Härkönen
Vesa Kortelainen
Helena Hulkki
Esa Sandberg
Heikki Juopperi
Antero Karvinen
Heikki Papunen
jne…
41
Orogenic gold:
alteration
Pasi Eilu
2011
Pasi Eilu
August 2011
1
Factors controlling alteration in orogenic systems
1.Deformation
2.Structure
3 PT
3.PT
4.Primary rock composition
5.Fluid composition
6.Fluid/rock
Pasi Eilu
1.
August 2011
2
August 2011
3
Deformation
Brittle, brittle-ductile or ductile
Events can be episodic, repeated
2.
Structure
-
Located in:
fracture arrays, stockworks, breccia zones,
foliated zones with pressure solution cleavage,
fold hinges, “saddle reefs”, etc.
Pasi Eilu
42
3.
PT
Mineralisation and alteration during peak
regional metamorphism or soon after that
Range: 160–700°C, 0.7–5 kbar
4.
Primary rock composition
Hosted by almost any rock type within a
metamorphic belt
Pasi Eilu
5.
August 2011
4
August 2011
5
Fluid composition
H2O-CO2-NaCl±CH4±N2 fluid
XCO2 typically 0.05–0.30
Low salinity, commonly 2–8 % NaCl eq.
A
Au
+ Ag,
A As,
A CO2, K,
K Rb,
Rb S,
S Sb,
Sb T
Te, W
± B, Ba, Bi, Hg, Mo, Pb, Se
Co, Cu, Fe, Ni, Zn contents normally very low
6.
Fluid/rock
Fluid-dominated to rock-dominated
Pasi Eilu
These factors produce an apparently great variation in the style
and products of alteration
…yet several features of alteration are common to all orogenic gold
deposits
… and many of them can be utilised in exploration
Pasi Eilu
August 2011
6
43
The three main classes of alteration in
orogenic systems
Pasi Eilu
1
August 2011
7
Lateral zoning
- Due to chemical gradients and decreasing
fluid/rock away from the fluid flow channels
- Surrounds all deposits
- Distinct lateral zoning sequence
- Along-strike and -dip variation within a single
rock type is rare
Pasi Eilu
August 2011
8
August 2011
9
Alteration envelope around orogenic gold
mineralisation
Distal alteration
Pasi Eilu
Proximal alteration
Ore
44
2 Variation in primary rock type
- Variation in primary composition =>
variation in alteration mineral assemblage
without a change in w/r or PT
3 Variation in metamorphic grade
- Systematic variation according to
metamorphic grade
- Typical between individual deposits or
deposit groups (gold camps)
- Rare in a single deposit;
only if a T isograd crosses the system
Pasi Eilu
August 2011
10
August 2011
11
August 2011
12
?
Extent of alteration?
Pasi Eilu
Extent of alteration
- Alteration envelope 5 cm - 2 km wide;
length up to 10 km or even more
- Depends on the size and duration of the hydrothermal
system, and the stress field during alteration
- Positive correlation with the size of the
mineralisation (commonly)
- Zone width grows outwards
- Single zone may be 1 mm - 100 m wide,
distal zone even up to 2 km (Golden Mile)
Pasi Eilu
45
Bronzewing,
Western
Australia
Alteration
mapped from
drilling
intercepts at 150
m depth
Pasi Eilu
Eilu et al. (2001)
August 2011
13
August 2011
14
HollingerMcIntyre
Timmins, Abitibi
Alteration at
the present
surface
(simplified)
Smith & Kesler (1985)
Pasi Eilu
Alteration at greenschist facies
Pasi Eilu
August 2011
15
46
0. Unaltered rock
Actinolite + epidote + albite + titanite ±
ilmenite, magnetite (ol, cpx, kfsp, qz)
1. Distal zone
Chlorite + calcite + albite + rutile + quartz
2. Intermediate zone
Chlorite + calcite + ankerite/Fe dolomite +
albite + quartz + rutile ± muscovite
3. Proximal zone
Muscovite + ankerite + quartz + albite + rutile +
pyrite ± arsenopyrite, gold
Pasi Eilu
August 2011
16
August 2011
17
Mafic rock:
Pasi Eilu
Bulletin, Wiluna
Unaltered metabasalt
ActinoliteEpidoteChloriteAlbiteTitanite
TitaniteMagnetite
Pasi Eilu
photo
J. Väätäinen
August 2011
18
47
Distal alteration
ChloriteCalciteAlbiteQuartzQ
RutileMagnetite
2 cm
Pasi Eilu
photo J. Väätäinen
August 2011
19
Intermediate alteration
Chlorite-Calcite-Dolomite-AlbiteQuartz-Rutile-Magnetite
2 cm
photo J. Väätäinen
Pasi Eilu
August 2011
20
Proximal alteration, ore
Dolomite-Sericite-Albite-QuartzRutile-Pyrite-Arsenopyrite
2 cm
photo J. Väätäinen
Pasi Eilu
August 2011
21
48
Proximal alteration, 20 g/t Au
Arsenopyrite
Qz+Dolo+Ab
Sericite
R til
Rutile
Pyrite
Quartz vein
0.5 cm
photo J. Väätäinen
Pasi Eilu
August 2011
22
Bulletin mine, Wiluna, Western Australia
Lower-greenschist facies
Unaltered metabasalt
Proximal alteration, ore
Crossed polarizers, field of wiew 3.2 mm
photo P. Eilu
Pasi Eilu
August 2011
23
Granny Smith, Western Australia
Granodiorite, lower-greenschist facies
Unaltered (Kfsp-Pl-Qz-Biot±Hbl)
+ two mineralisation-related fractures
2 cm
Ore, pervasive proximal alteration (Ab-Qz-Ser-Dol-Py)
photo J. Väätäinen
Pasi Eilu
August 2011
24
49
Sunrise Dam, Western Australia
Komatiite, lower-greenschist facies
Di t l talc-chlorite-dolomite
Distal:
t l hl it d l it
Proximal:
fuchsitequartzankerite
photo P. Eilu
Pasi Eilu
10 cm
August 2011
25
August 2011
26
August 2011
27
Loukinen, Central Lapland
Komatiite, mid-greenschist facies
Pasi Eilu
Loukinen, Central Lapland
Phyllite, mid-greenschist facies
Pasi Eilu
50
Mt Magnet, Western Australia
BIF, upper-greenschist facies
Photo
C. Mathison
Pyrite replacing magnetite; a sulphidation front
Pasi Eilu
August 2011
28
Uppermost greenschist facies
- Biotite instead of muscovite
- Pyrrhotite instead of, or with, pyrite
- Commonly, calcite with or instead of other
carbonates in the proximal alteration zone
- In ultramafic rocks, talc more common
- K feldspar may be stable in proximal alteration
zones (felsic rocks)
Pasi Eilu
August 2011
29
Bronzewing
Yandal Belt, NE Yilgarn
Tholeiitic basalt host rock
Proximal alteration
Chl-Calc-Ank-Ab-Qz → Biot-Calc-Ank-Ab-Qz-Py
photo P. Eilu
Pasi Eilu
August 2011
30
51
Uppermost greenschist facies
If T is above biotite isograd, proximal alteration:
Increasing XCO2 => biotite → sericite
Decreasing XCO2 => sericite → biotite
But no change in sulphides(?)
Pasi Eilu
August 2011
31
Py-Po-Mgt-Hm at greenschist facies:
effect of temperature
Rock: average
interflow
sediment at
Kambalda, WA
p = 2 kbar
kb
constant w/r
p(ox) =
proportion of
oxidised sulphur
Evans (2010) Mineralium Deposita 45, 207-213
Pasi Eilu
August 2011
32
Py-Po-Mgt-Hm in greenschist facies:
effect of fluid-rock ratio
Evans (2010) Mineralium Deposita 45, 207-213
300°C
350°C
No need
for another,
p = 2 kbar,
constant w/r,oxidising or reducing fluid, if you see magnetite
p(ox)or= proportion
of oxidised
sulphur
haematite
in a system,
or Po±Py±Mgt±Hm zoning
Pasi Eilu
August 2011
33
52
Amphibolite facies
Pasi Eilu
August 2011
34
Contrasts to greenschist facies
- Bleaching is not a characteristic feature
- Calcic plagioclase is stable
- Potassic
P
i alteration
l
i (biotite
(bi tit ± K-feldspar)
K f ld
) hhas
a wider extent than carbonation
- Calcite is, normally, the only carbonate present
Pasi Eilu
August 2011
35
August 2011
36
Contrasts to greenschist facies
- Calc-silicates characterise proximal alteration:
diopside, hornblende, tremolite-actinolite, garnet
- Pyrrhotite is the dominant Fe sulphide
- Löllingite may be present
- Rutile gives way to ilmenite and titanite as
stable Ti-minerals, and magnetite may be stable
Pasi Eilu
53
Lower-amphibolite facies
Regional metamorphic mineral assemblage
plagioclase + hornblende
→ Distal alteration zone (1 cm - 40 m)
plagioclase + hornblende + biotite ± pyrrhotite
→ Proximal alteration zone (1 mm - 20 m)
plagioclase + biotite + calcite + quartz + pyrrhotite
± arsenopyrite, actinolite
Pasi Eilu
August 2011
37
Ore, proximal alteration in lower-amphibolite facies
Pasi Eilu
August 2011
38
Flin Flon, Canada
BIF: sulphidation front also here
Pasi Eilu
photo C. Mathison
5 mm
August 2011
39
54
Mid-amphibolite facies and higher
metamorphic grades
Regional metamorphic mineral assemblage
plagioclase + hornblende
→ Distal alteration zone (0 cm - 20 m)
plagioclase + hornblende + biotite
→ Proximal alteration zone (1 mm - 10 m)
diopside ± grossular, almandine, actinolite, hornblende, biotite/Kfeldspar, plagioclase, calcite, quartz, pyrrhotite,
arsenopyrite/löllingite
Pasi Eilu
August 2011
40
Orogeeninen Au
Ore, proximal alteration
Mid- to upper-amphibolite facies
Basalt, Polaris South, Southern Cross, Yilgarn
Di--Hbl
Di
Di-HblDiHblBiot
Gar
Gar--Qz
mm scale
Pasi Eilu
photo C. Mathison
August 2011
41
Alteration as exploration tool
Pasi Eilu
August 2011
42
55
Alteration as exploration tool
1. Alteration envelope can be used to define potential exploration
targets
Easy to identify
Targets detected are much larger than if only gold mineralisation
was used in the target identification
2. Once an alteration halo is recognised, the sequence of alteration
zones can be used as a rough vector towards the potential ore
It is most important to recognise these features in the
early stages of exploration
Pasi Eilu
August 2011
43
August 2011
44
General trends
In all rocks:
K metasomatism + carbonation
( Ca-silicates)
(±
C ili
) + sulphidation
l hid i + quartz veins
i
Pasi Eilu
Greenschist facies
Carbonate-free → calcite → calcite-dolomite → dolomite/ankerite;
ilmenite / magnetite / titanite → rutile;
sericitisation + bleaching
BIF: sulphidation front
Pasi Eilu
August 2011
45
56
Unaltered
Distal
Intermed
Proximal,
ORE
Pasi Eilu
August 2011
46
Amphibolite facies and uppermost greenschist facies
Biotitisation and brown colour
Amphibolite
p
facies
Banded proximal alteration characterised by
diopside and intense green colour
Pasi Eilu
August 2011
47
Sources of confusion 1, 2
Sheared felsic rocks within a sequence dominated by mafic or
ultramafic rocks
=> apparently bleached zones:
Check primary chemical characteristics
Spilites metamorphosed at amphibolite facies conditions
=> diopside ± quartz, garnet, calcite, pyrite
between pillows, fractures, etc.:
Check primary volcanic structures
Pasi Eilu
August 2011
48
57
Sources of confusion 3
Carbonatisation related to VMS-style mineralisation
Differences to orogenic gold systems:
- Distinct gains in Ca, Fe, Mg
- At greenschist facies, no mineralogical gradients defined
by carbonates
- Silicate assemblages commonly contain Al-rich, alkali-deficient
minerals (also true in metamorphosed epithermal systems)
- Typically stratiform, unrelated to late faults or shear zones
Pasi Eilu
August 2011
49
Sources of confusion 4
Amphibolite facies: Skarns
Differences to orogenic gold systems:
- Intimate association with an (granitoid) intrusion +
- No potassic alteration associated with calc-silicate formation
- Gold in retrograde gangue association, typically distal
to intrusion and to high-T skarn mineral association
- Radiometric dating shows synchronous mineralisation
and intrusion
Pasi Eilu
August 2011
50
Next:
Geochemical
haloes
Pasi Eilu
August 2011
51
58
Orogenic gold:
geochemical features
Pasi Eilu
2011
Pasi Eilu
April 2010
1
Geochemical anomalies
- Discriminate between gold-related and unmineralised
structures
- Expand the target
- Define vectors to ore
Pasi Eilu
April 2010
2
Elements enriched in orogenic gold systems
Very little difference regarding:
- Metamorphic grade
- Host rock
- Craton or greenstone belt
Pasi Eilu
April 2010
3
59
Bulk ore samples, Yilgarn Craton
All concentrations in ppm
Deposit
Host
Sub-greenschist
Wiluna
Mafic
Bulletin
Mafic
Wiluna
Felsic
Greenschist
Mt Pleasant
Mafic
Nth Kalgurli Mafic
Mt Charlotte Mafic
Ora Banda
Mafic
Paddington
Mafic
SOG
Mafic
Golden Crown Mafic
Moyagee
Umaf
Lawlers
Felsic
Au Ag
As
Bi
Sb
Se
8.1 <1 12000 <0.2 33.0
20.2 0.3 20800 0.02 39.0 0.20
1.5 0.1 5833
1
<1
12.0
14.0
7.6
5.9
3.5
2.4
51.0
116.0
2.0
2.0 380
2.0 320
2.0
66
<1 9500
<1 7800
<1 190
2.0 12000
5.5 1401
<1
14
Te
W
3.6
2.0
<1
16
13
10
4.00 2.6
1.4
<0.2 9.0
10.0
<0.2 1.3
3.0
<0.2 2.0
0.2
<0.2 3.0
0.8
<0.2 0.8
<0.2
0.40 2.6
5.0
0.04 2.1 0.86 0.1
0.60 0.4
1.0
65
95
950
24
40
9
2
4
110
Pasi Eilu
April 2010
4
Bulk ore samples, Yilgarn Craton
All concentrations in ppm
Deposit
Greenschist
Granny Deeps
Granny Deeps
Twin Peaks
Hill 50
Amphibolite
Kings Cross
Corinthian
Edward’s Find
Hopes Hill
Marvel Loch
Westonia
Mt Morgans
Nevoria
Host
Felsic
Sedim.
Sedim.
BIF
Au
12.3
6.3
6.0
5.8
Mafic 6.9
Mafic 6.9
Mafic 34.0
Umaf 13.0
Umaf 8.4
Felsic 3.2
BIF
3.6
BIF
4.9
Ag
As
Bi
2.4
5 1.90
1.2
68 2.10
8.2 5573 0.53
0.4
25
Sb
Se
Te
W
1.0
2.2
3.2
9
0.30
1.04
0.32
0.34
1.1
55
2.1
39
0.2
10
0.48 200
0.4 6330 0.10 2.3 1.02 0.5
6
<1
32 25.0
0.6
1.8
25
<1 1200 0.40 18.0
<0.2 2200
1
4
<0.5
5
2
6.0 2200 0.40 18.0
0.4 160
<1
12 3.80 0.2
1.0
35
<1
4 1.00 0.6
1.2
8
<1
280 1.00 1.0
0.8
8
Pasi Eilu
April 2010
5
April 2010
6
All elements enriched in ore are potential anomalyforming parameters around a deposit
But: which ones really have been mobile?
Pasi Eilu
60
First, define the
primary rock
types
Pasi Eilu
April 2010
7
April 2010
8
April 2010
9
Define also the primary
variation within the
rock types
Blue: unaltered sample
Red: altered sample
Based on data from
Stanley & Madeisky (1995)
and Eilu (1996)
Pasi Eilu
Then, evaluate the mass transfer
Pasi Eilu
61
Greenschist facies
Eilu & Mikucki (1998)
Pasi Eilu
April 2010
10
Quartz porphyry, sericitis
sation zone I, potential ore
Qz-porphyryy, intense alteration
30
0.004xBi
0.09xCu
0.04xPb
4xAu
0.4xCd
25
Enriched
0.05xCr
0.2xZr
Ga
Isocon
0.02xTe
Mataralampi
prospect
0.05xSb
20
0.2xV
0.1xW
10xS
0.01xBa
0.1xRb
Ni
Al2O3
0.25xSiO2
La
0.005xZn
15
50xP2O5
2xK2O
10xCO2
10
FeO*
Li
0.2xCl
Host rock:
quartz porphyry
Ag
Y
5
10xTiO2
0.05xSr
100xMn
MgO
Depleted
CaO
Na2O
0
Pasi Eilu
Archaean
Kuhmo
greenstone belt,
Finland
0
5
10
15
Qz-porphyry,
least 20
altered
Least altered quartz porphyry
25
30
April 2010
11
April 2010
12
Elements shown to be enriched
in orogenic gold deposits,
no of cases
no.
Pasi Eilu
62
Pasi Eilu
April 2010
13
April 2010
14
April 2010
15
Pathfinders and alteration indices
Major components:
+ No significant problems with detection limits
or analytical methods
+ Easy to relate with mineral assemblages
+ Alteration indices can be very useful parameters
- Host rock effect can be large
- Mass balance evaluation needed
Pasi Eilu
Pathfinders and alteration indices
Pathfinder trace elements:
+ Enrichment up to 1000-10000 x
+ Host rock effect generally very minor
- Very low detection limits may be needed
- Background thresholds needed to be
defined for each area
Pasi Eilu
63
Extent of an anomaly
1. The threshold between background
and an anomaly
Pasi Eilu
April 2010
16
April 2010
17
April 2010
18
Background threshold
Trace-element
contour map
and a cross
section
Optimal background
threshold
h h ld somewhere
h
between 60 and 80 ppm
for the element depicted
Pasi Eilu
Statistical methods: histogram
an empiric method
Concentration in log(ppm)
Pasi Eilu
64
Concentration (pppm)
Statistical
methods:
cumulative
frequency plot
After Sinclair
(1974, 1976, 1991)
Cumulative frequency (%)
Pasi Eilu
April 2010
19
April 2010
20
The background thresholds achieved
must be checked with geology:
are the results of the statistical analysis
analysis,
whatever used, reasonable?
Pasi Eilu
Background thresholds defined
Background I:
igneous host rocks
Au
Ag
As
Sb
Se
Te
W
Granny
Smith
Granodiorite
8.5
140
5
0.9
0.10
10
6.0
Bulletin
Basalt
6
80
28 (6)
2.0 (0.6)
10
0.6
KX
BW
Madsen- Moyagee
Stratt-O.
Basalt Basalt Basalt,
Komat.
Komat.
5
4
6
130
160
110
4
50
5
0.9
0.4
2
0.45
0.30
0.15
37 (10) 44 (10)
10
1.3
1.9
0.5
Au, Ag, Te in ppb
ppb,, others in ppm
Pasi Eilu
April 2010
21
65
Background thresholds defined
Background II:
felsic to intermediate igneous host, Mataralampi, Finland
Au
Bi
Cu
Pb
S
Sb
Te
W
Zn
0.045
0.10; 0.41
42
90
270
0.07
0.065
14; 80
57; 160
All data in ppm
Pasi Eilu
April 2010
22
Background thresholds defined
Background III:
metasedimentary host rocks
Au
Ag
As
Bi
Sb
Se
Te
W
Granny
Smith
Twin
Peaks
Sunrise
Dam
Hattu belt
(Finland)
2
180
40
0.20
1.0
1.0
125
2.3
2
100
6
0.20
0.8
0.17
50
3.0
6
110
15
0.07
2.8
0.08
12
8.5
5
150
15
0.02
0.5
0.10
75
3.0
BendigoBallarat
10
50
Au, Ag, Te in ppb,
ppb, others in ppm
Pasi Eilu
April 2010
23
Detection limits needed for pathfinder elements
At least, during the early stages of exploration,
to define the local background levels
Ag
10 ppb
ppm
As
0.2-1 pp
Au
0.2-1 ppb
B
1 ppm
Bi
10 ppb
Cd 0.01-0.1 ppm
Pasi Eilu
Hg
3-5 ppb
Mo
0.1 pp
ppm
Sb 0.02-0.1 ppm
Se
10 ppb
Te
1-2 ppb
W
0.1 ppm
April 2010
24
66
Extent of an anomaly
Pasi Eilu
April 2010
25
April 2010
26
Form of an
anomaly
Suurikuusikko
N Finland
Au at the
till-bedrock interface
Härkönen (1992)
Pasi Eilu
Form of an
anomaly
Bronzewing,
Central zone
Pathfinder
elements
l
t I
Dispersion after data
available in June 1994
Ore as realised
(1999)
Pasi Eilu
27
Eilu etAprilal.2010(2001)
67
Form of an
anomaly
Bronzewing,
Central zone
Pathfinder
elements
l
t II
Dispersion after data
available in June 1994
Ore as realised
(1999)
Pasi Eilu
28
Eilu etAprilal.2010(2001)
Form of an
anomaly
Bronzewing,
Central zone
Alteration indices
Dispersion after data
available in June 1994
Ore as realised
(1999)
29
Eilu etAprilal.2010(2001)
1000 m
Pasi Eilu
100 m
Arsenic
Pasi Eilu
Sunrise Dam,
Western Australia
Section across
host rocks and
mineralised shear zones
April 2010
30
68
AsForm of an
Au
anomaly
100 m
As and Au, values along a mineralised shear zone
(= within a fault plane)
April 2010
31
Pasi Eilu
April 2010
32
Pasi Eilu
April 2010
33
Pasi Eilu
How far from ore an
anomaly can extend?
(in bedrock)
69
Pasi Eilu
April 2010
34
Size of primary anomaly in plan
Archaean
Fennoscandia
Hattu schist belt
Canada
Stratt-Olsen–
Madsen
Hollinger-McIntyre
Hollinger
McIntyre
Yilgarn
Golden Mile
Moyagee
Bronzewing
Tanzania
Golden Pride
Parameter
Across
Along strike
Te, As
1-4 km
60 km
As, Au, B,
K, Sb, Na
As
CO2
200-600 m
800 m
>2 km
As, Au, Te
As, Au, Se, Te, W
Te
Au, CO2, K,
Rb, Sb, W
>1.5 km
Sb, Li
>500 m
>400 m
100-300 m
9 km
>3 km
>3.5 km
>1.2 km
>800 m
>800 m
Pasi Eilu
April 2010
35
Size of primary anomaly in plan
Proterozoic
Fennoscandia
Pahtavaara
Suurikuusikko
Saattopora camp
Vesiperä camp
Pasi Eilu
Parameter
Across
Au
As, Au
As, CO2
As, Au, K
50-100 m
>200 m
100-200 m
Along strike
>66 km
>10 km
>20 km
>8 km
April 2010
36
70
Beyond the gold anomaly,
into
unaltered rock
Pasi Eilu
April 2010
37
April 2010
38
April 2010
39
Beyond bleaching,
locally into unaltered
rock, no rock type effect
Background
threshold: 4 ppb Au
Pasi Eilu
Relative lateral extent
Pasi Eilu
71
Vectors towards the ore?
Shkolnoe, Kolyma, Russia
Pasi Eilu
April 2010
40
April 2010
41
April 2010
42
Any pathfinder or alteration index may
define a trend towards ore
In orogenic gold systems, this is not always
easily seen – why?
Pasi Eilu
Golden Mile
Kalgoorlie, WA
Dolerite-hosted
Dolerite
hosted
Alteration vs. carbonation
index
After data from Phillips (1986)
Pasi Eilu
72
Mataralampi section 7149710N
KU
/H
K3
#
#
H
CM 19/4412/2003/4/10
Geological Survey of Finland
Pasi Eilu
4
K-
#
#
K1
H
H/
2
KH
H/
/H
KU
H
KU
KU
Antimony
Gold
grdr
doler
Au
Sb
fsp-por
qz-por
mvolc
20
0
20
40 Meters
mdyke
Pasi Eilu
April 2010
43
April 2010
44
April 2010
45
HJB SZ
Bulletin, Wiluna
Northern Yilgarn
Tholeiitic-basalt hosted
Section across the ore,
shear
h
zone and
d hanging
h
i
wall
Eilu & Mikucki (1998)
Pasi Eilu
Harbour Lights
Central Norseman-Wiluna
Belt, Yilgarn
Section across the ore
and wallrocks
Based on data from
Skwarnecki (1990)
Pasi Eilu
73
Surficial geological and geochemical exploration for
gold in glaciated terrains – brief overview
Pertti Sarala
Geological Survey of Finland
Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
1
Outline
•
•
Introduction
Surficial geology in glacigenic environment
–
–
–
–
–
•
Glacial dynamics
Ice flow indicators
Till stratigraphy
Bedrock vs. pre-glacial weathered bedrock vs. till
Glacigenic formations and deposition processes
Surficial exploration methods
– Till geochemistry
– Indicator minerals
– Prospectivity modelling
•
Exploration case studies
Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
2
Introduction
•
Indicator (usually heavy) mineral studies are the oldest methods in
gold exploration
•
•
Nuggets were concentrated using water and gravity (e.g. panning)
First ideas and observations of glacial transportation from 18th
and
d 19th century
t
– Glacial erratics -> boulder fans -> tracing the source
•
Theory of ice ages and glaciations with glacial transportation was
largely accepted in the beginning of 20th century
Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
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74
Introduction
•
Use of soil in exploration started after development of chemical
analysis methods
– References from antiquity of the relation of geology and chemistry
– Techniques of modern geochemical prospecting in the Soviet Union
and Scandinavia in 1930s; in Northern America in 1940s
– Trace elements in gold exploration
•
Soil and till samples were used largely since 1950s in exploration
in glaciated terrains
–
–
–
–
Analysis methods developed to ppm and ppb levels
Easy and effective sampling
Development of sampling techniques
Costs reasonable for large sampling projects
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Glacial dynamics
•
Geomorfological systems
- Cold-based (Dry
bed)
-no erosion and
deposition
- Warm-based (Wet bed)
-erosion and deposition
- Marginal meltwater
zone
-eskers and end
moraine
complexes
Kleman & Borgström 1995
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Glacial dynamics
•
Glacial processes and their variations key issues=> time-transgressive and spatially
chancing events like cold - warm basal conditions and ice stream network
Punkari 1997
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Punkari 1997
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75
Ice flow indicators
•
•
Erosion marks; striae, grooves etc.
Till fabrics
Hirvas et al. 1973-1977
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Ice flow indicators: Surficial boulder fans
Salonen, V-P. 1986.
Sector of ore boulder observations
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Ice flow indicators: Single boulder transportation
The longest transport distance of single ore boulder from the known source
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76
Ice flow indicators: Glacial morphology
•
Morphological interpretation key point for estimating ice-flow direction but
also distance and deposition processes
N
Perpendicular ribbed-moraine ridges in
Petäjäskoski, S-W Rovaniemi
Streamlined drumlins in Kuusamo (two fields overlapping)
Pertti Sarala,
Sarala 24.11.2008
25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
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Example: Relief/landforms in southern Lapland
Hummocky
ribbed
Ribbed moraines
and drumlins
moraines
Drumlins in Kuusamo
N-S oriented drumlins and mostly till-covered esker chains
drumlin field
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Example: Glacial morphology as an indicator for
glacial dynamics in southern Lapland
Kuusamo Ice lobe
Ribbed
moraines
Tervola
Younger
drumlins
Older
drumlins
Ranua Interlobate
area
Glacial flow
direction
younger
Oulu Ice lobe
Sarala 2005
Pertti Sarala,
Sarala 24.11.2008
25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
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77
Example: Glacial dispersion and transport
distance in southern Lapland
Sarala et al. 2007
Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
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Till stratigraphy
•
•
Till stratigraphy includes till beds and interlayers
Two different types stratigraphy: Straight (simple and straightforward; 1-2 till
beds) and complex (multiple till beds with different ice-flow directions)
70 ± 5 ka
93 ± 10 ka
99 ± 11 ka
102 ± 11 ka
107 ± 13 ka
?
?
Till stratigraphy in Rautuvaara, Kolari, Drawing H. Kutvonen
Pertti Sarala 24.11.2008
14
Pre-Quaternary weathered bedrock
•
•
Pre-glacial weathered bedrock surface has been preserved beneath glacial
deposits in many areas in northern Finland.
Remnants of weathered bedrock are up to tens of meters thick mainly in the last
ice divide zone in Central Lapland
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78
Till vs. weathered
bedrock
Washed
surface
Till
Weathered
W
th d
bedrock
Fresh
bedrock
•
Sometimes difficult to distinguish
different stratigraphical units
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Glacial erosion
and deposition
•
Simple transportation
– Glacier erodes the bedrock
and deposits material into
some distant down-stream
• Complex transportation
– Till units include material
deposited and transported by
various glaciations
– Tracing the source of
mineralized material needs good
knowledge of the till
stratigraphy
Hirvas & Nenonen 1990
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Erosion
and deposition - drumlins vs. ribbed moraines
• Short transportation in the
ribbed moraines is seen in the
surficial parts, i.e. mineralized
surficial boulders indicate
local source in the bedrock (if
the qquarrying
y g was reached the
bedrock surface)
• Deposition process at the
warm-based conditions (for
example in drumlins) is
different and transport
distance longer
Pertti Sarala
Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
Pertti Sarala 14.2.2007
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79
Surficial geochemical exploration methods
•
Soil surveys
– Most widely used in geochemical exploration methods
– Based on secondary dispersion of weathered and leach material or elements
from the buried source
– Samples from the soil horizons (A and B) or fresh material (C horizon)
– Sampling
S
li using
i drilling
d illi or test
t t pits
it
•
Rock surveys (also lithogeochemical survey)
– Sampling of unweathered bedrock
– An idea to find favourable host rocks for mineralization
– Samples from the ourcrops or drill cores
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Surficial geochemical exploration methods
•
Stream-sediment surveys
– Used in reconnaissance i.e. in regional scale
– Based on secondary dispersion from the upstream in drainage basins
– Panning is a good example of this survey method
•
Water
W
e su
surveys
veys
– Both ground water and surface water sampling
– Usually used in detailed surveys (ground water)
– Contents low, and adsorption causes difficulties for interpretation
•
Biogeochemical surveys
– Vegetation used as a test medium
– Plants concentrate elements in themselves or in humus
– Animals can also collect mineral or organic material in reservations or nests
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Surficial geochemical exploration methods
•
Gas surveys
– Used in detailed scale to find buried deposits
– Based on detection of different gases (hyrdorgen sulphide, mercury, iodine, radon)
or ions in gases (hydrocarbons)
•
Mobile metal surveys
y
– Nowadays widely tested and used method
– Based on the analyse of weakly bounded metal ions on the surface of mineral soil
or organic particles at the top of the soil
– Weak acidiferous solutions were used for leaching ions without dissolving minerals
•
Radiation surveys
– Total radiation and/or spectrums were measured usually from airplanes
– Can use for lithogeochemical purpose and for soil geology
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Till geochemistry
•
Till geochemistry is the most commonly used method for estimating transport
distance of mineralized material in glaciated terrain.
– Is based on secondary dispersion of the indicator elements from the
mineralized sources
•
Based on the sampling
p g with
different intervals and
variable depths
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Sampling methods – percussion drilling
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Test pit excavations –
also in winter
Test pit surveys in Petäjäselkä at
the end of Marsh in 2007
- Temperature -15°C
- Snow depth 1 m
Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
Gold grains in till at Misi
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Collected datasets
•
Test pits and trenches for:
– Till stratigraphical observations and sampling
– Till fabrics and striae
– Till and weathered bedrock sampling (incl. geochemistry (ICPAES, ICP-MS, GAAS, etc.) indicator minerals, rock
composition, grain size distribution)
– Bedrock observations
– Bedrock and boulder sampling
•
Percussion drilling:
– Till and weathered bedrock sampling for chemical analyses
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Pertti Sarala 18.6.2007
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Till geochemical datasets
•
Small-scale till geochemical
datasets (1 sample/4 or16 km2)
are used for identifying
regional geochemical
characteristic
– Anomaly patterns are also
reflecting general ice flow
directions
•
For targeting and target-scale
examination more detailed till
geochemistry is required
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Example of sampling densities in Au exploration
•
Central Lapland; Suurikuusikko deposit
Till sampling: 10 m interval
Till sampling: one sample / 4 km2
N-S structure
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Example: Till and weathered bedrock
geochemistry in the Suurikuusikko deposit
Au ppb
4m
140 m
Indicator elements: Au, As, Sb, K, Mn
Sb ppb
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Rovaniemi
Case study: Petäjävaara
• Investigations for
tracing the Au-Cu
mineralized surficial
boulders in ribbed
moraine area
B d k
Bedrock:
metasedimentary and
metavolcanic rocks of
the Peräpohja Schist Belt
•
• Two
till units: lower
lodgement till and upper
melt-out till,
representing advance
and retreat phases
Sarala & Rossi 2006
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Rovaniemi
Case study: Petäjävaara
• Many hydrothermally altered Cu-Au mineralized boulders found on the
top of ribbed moraine ridges
Quartzite boulders
Au 0.1-0.6 ppm
Cu 0.7-2.4 %
Banded amphibolite boulders
Au 0.1-6.9 ppm
Cu 0.1-3.2 %
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83
Rovaniemi
Case study: Petäjävaara
•
Sampling: percussion drilling and test pits
•
Distinct metal anomalies in upper till (e.g. < 0.06 mm fraction)
a
GFAAS
ICP-AES
Glacial flow direction
Known Cu-Au mineralization
(Sarala & Rossi 1998)
Glacial flow direction
Fresh chalcopyrite
grain in till
(SEM photo)
Indicator elements: Au,
Co, Cu, Te, S
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Other methods for Au exploration
•
Weak leach methods
– Based on the analyse of weakly bounded metal ions on the surface of
mineral soil or organic particles at the top of the soil
– Weak acidiferous solutions were used for leaching ions without dissolving
minerals
– The used methods were Mobile Metal Ion (MMI), Enzyme leaching and Soil
Gas Hydrogen analyses of which results were compared to conventional
partial leaching (aqua regia) and total leaching (four acid) analyses
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Weak leach methods
•
Mobile Metal Ions (MMI) + several other commercial methods
– Ions are moving from the bedrock through the overburden
– Mobilization, movement and enrichment
of the ions are the sum of many factors.
The main reasons are capillary
action,, difference of electrochemical
charge, and biogeochemical
processes
– MMI is the first commercial
method; SGS Minerals is
the patentee
– Other: Ammonium acetate,
entzyme leach, soil gas etc.
Cameron et al. 2004
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84
Sampling
•
Easy and fast sampling
– The sampling depth 10-25 cm under the
contact of humus and mineral soil
– Small test pits or soil drills in sampling
– Samples from the lines, frequent
10 50 m => 25-30
10-50
25 30 samples/day/
two-people sampling group
– Sampling procedure same for all the weak
leach methods
Sample
Sample
•
Sample
Other methods are for example humus
and Ah samplings that can be used
beside the weak leach methods
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Examples - Case Lauttaselkä, Au
Au
Lauttaselkä target is Au
exploration target in Kittilä, ca.
10 km from the Agnico-Eagle’s
Kittilä Mine to the NE
• Bedrock is composed of
hydrothermally altered mafic
volcanic and sedimentary rocks
of which contact zones are
enriched of Au, As and Te
• Conventional till and weathered
bedrock sampling supported by
MMI sampling revealed
potential zones for Au
exloration in the bedrock
•
Zn
As
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Mobile XRF analysis
•
XRF analyzers can also be used in Au
exploration in the field
– Portable equipments have been
developed a lot during the last ten years
– Automated scanners can be used for
drill cores but also for the till and PreQuaternary weathered bedrock samples
•
•
•
Measurement direct in the field => no
sampling or
Sampling as separate samples or
continuous sample series of till and
weathered bedrock along test trences,
In Au exploration indicator elements or
indication of suitable alteration in the
bedrock is useful
Continuous weathered bedrock
sampling in Lauttaselkä, Kittilä
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Mobile XRF analysis (cont.)
•
•
Portable XRF analyzers very useful
A mobile laboratory for on-line elemental
XRF analysis technology new application
Portable XRF analyzer
tested during the
percussion drilling for
the till sample
Scanmobile (Mine On-line
Services Ltd)
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Indicator mineral methods
•
Indicator minerals (particularly heavy minerals) are largely used in exploration
– Straight indication of mineral potentiality
– Information for interpreting stratigraphy and determining the provenance of a
sediment
•
Till and weathered bedrock samples,
p
but also stream sediments
•
Au, sulphide minerals, Fe-minerals, garnets and pyroxene and phosphate
minerals most common, also PGE-minerals
– Demand increasing also for light indicator element separation and research due to
increased high-tech metal exploration
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Indicator mineral methods
Panning, Knelson concentrator and spiral separator most used field equipments
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GIS-based
prospectivity
mapping
4. Evaluation
3. Spatial analysis
2. Data preprosessing
1. Data
Nykänen et al. 2006
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Pertti Sarala 18.6.2007
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Example: Prospectivity in Central Lapland
•
Several new areas potential for Au mineralization found
Nykänen & Salmirinne (2006)
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Example: Prospectivity in Central Lapland
•
Weight of evidence
Four test targets
on very high
prospectivity
areas :
Vuomanperänmaa
Nuttiot
• Petäjäselkä
P j lk
• Lauttaselkä
•
•
•
Situate near the
modern mines of
Pahtavaara and
Kittilä, and
several known Au
occurrences
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87
Case: Petäjäselkä
•
Bedrock is composed of Mg and Fe tholeitic metabasalts and minor BIF
• NNW
trending magnetic anomalies are seen on a high resolution
aeromagnetic map. NE-SW oriented faults are breaking them.
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Pertti Sarala 18.6.2007
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Pertti Sarala 18.6.2007
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Case: Petäjäselkä
• Several anomalous Au(CoAs-Cu) mineralized zones have
been defined
• Till geochemistry highlights
the multimetal anomaly in the
target area
• Anomalies are clearly relating
to SW-NE trending faults
Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
Petäjäselkä (contin.)
Au in till (<0.06 mm)
Pertti Sarala, 25th IAGS 2011, WS 5: Exploration for orogenic gold deposits , 20.8.2011
Au grains in heavy mineral samples
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88
Case: Petäjäselkä
•
Mineralization
hosted by sheared
graphitic chert
and clastic
sedimentary rocks
between Mg and
Fe tholeitic
metabasalts
•
The best drill
intersection is
12g/t Au over 1m,
and this lode is
exposed in the
test trench.
0.5 mm
Pertti Sarala
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Conclusions
• Till
geochemistry is useful method for estimating transport distance of
mineralized material in glaciated terrain. Ore indicators – mineralized
boulders and till, and indicator heavy minerals are useful in tracing the
mineralized bedrock
New applications like weak leach methods and portable XRF developed
for exploration.
exploration Effective,
Effective low sampling and analyzing costs,
costs low
low-impact
impact to
the nature particularly in sensitive areas
•
• Examples from northern Finland shows different kind of glacial
transportartion from short and sharp dispersals of for examples Au and its
pathfinder elements
• The
study of moraine formations, ice flow directions, till structures and
stratigraphy is essential before planning sampling and analysing till
geochemistry, and interpreting the results; i.e. successing in till
geochemical exploration in glaciated terrains
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ISBN 978-952-9618-70-5 (Printed)
ISBN 978-952-9618-71-2 (Pdf)
ISSN 0783-1331