WEST AFRICAN OROGENIC GOLD DEPOSITS: DO THEY FIT THE GLOBAL PARADIGM?
PETER J TRELOAR
KINGSTON UNIVERSITY
DAVID LAWRENCE, JAMES LAMBERT-SMITH, DIAMACOUR SENGHOR,
KEN KING, MICHAEL WIEDENBECK AND ADRIAN BOYCE
WHAT ARE OROGENIC GOLD DEPOSITS?
• The most abundant producer of gold on Earth
• Mesothermal rather then epithermal epigenetic ore deposits
• Temporally linked to periods of crustal growth in accretionary or collisional orogens
• Struturally controlled, steeply dipping ore lodes commonly located in allochthonous terranes characterised by strike slip faults
• Regional first order strike-slip structures provide the main control of deposit distribution with greatest ore fluxes in higher order shears, faults, and folds
• Generated in metamorphic terrains and hosted in greenschist facies mafic rocks, volcanic or intrusive, or clastic sediments
• The standard model suggests that they form from deep seated low salinity H2O - CO2 + CH4 + N2 metamorphic fluids that carry Au-Ag + As + B + Bi + Sb + Te + W (low base metal)
Herrington after Groves et. al. (2003. Econ Geol)
Tectonic Settings of Epigenetic Gold Deposits
GROVES & GOLDFARB 2015
QUESTIONS
1. Where have models for orogenic
gold mineralisation taken us?
2. Are models that imply a
metamorphic origin for orogenic
gold fluids correct?
3. What are the potential roles of
magmatic fluids in orogenic gold
deposits?
Regional geology of the KKI
Diale-Dalema Series
Kofi Series
Falémé Series
Loulo
Segala
Tabakoto
Sadiola
Yatela
Massawa
Sabodala
GARA - ORE MINERALOGY
Loulo, West Mali: Gara stockwork in quartz-tourmaline rock Quartz and carbonate veins with pyrite
MULTI-STAGE TOURMALINE GENERATION AT GARA
400 µm
A B
C D
200 µm
400 µm
E-stage
tur
Py
PyM-stage tur
Py
Py
AnkE-stage tur
150 µm
L-stage
tur
Ank
Qtz
Ank
L-stage tur
MINERAL PARAGENESIS
Py
Cb
50 µm 50 µm
Gf
Cb
Py
Py
NiPy
Ni-Co sulphides at Yalea North
Mineral Paragenesis at Gara
YALEA MINERAL PARAGENESIS
Albitic QR alteration Ductile textures (rich in carbonate)
YALEA – KEY ORE MINERALOGY
Asp-I
Py-II
Asp -I
Py-II
LOULO, LOW SALINITY FLUID INCLUSIONS
Type 1 mono- or bi-phase CO2 inclusions (60-90%) Type 2 rare l+v H2O inclusions (<5%) Type 3 mixed CO2-H2O inclusions (5-12%)
0%
20%
40%
60%
80%
100%
Ashan
ti (la
te T
e m
in.)
Ashan
ti M
ine
Prest
ea
Prest
ea
Konon
go
Dam
ang
Abawso
Sansu
Burkina
Fas
o ov
erview
Lara
fella
Dia
bato
u
Tapar
ko
Angov
ia
Min
eralised
gra
nito
ids
Banke
t Ser
ies
CH4-rich
H2O-CO2
H2O
CO2
LOW SALINITY FLUIDS
• This range of fluids is seen elsewhere in the Birimian
• Fluid immiscibility (Coulibaly et al., 2008) +/or post-entrapment modifications (e.g. Klemd et al., 1997)
• Low salinity H2O-CO2 fluids common for orogenic Au deposits and generally suggested to represent a metamorphic fluid source???
Summary of FI work from Ghana, Burkina Faso & Cote d’Ivoire
GARA HYPERSALINE FLUID INCLUSIONS (LAWRENCE ET AL. 2013
0
5
10
15
20
25
25 30 35 40 45 50 55 60 65 70
TDS (w.t. %)
Fre
qu
en
cy
GARA CRACKED HYPSERSALINE FLUID INCLUSIONS
GARA VS YALEA
Gara
• Tourmaline rich
• Fe-REE-As-Ni-Co-Au+Cu+W
• Pyrite dominant sulfide
• Aspy, cpy, gersdorffite, pentlandite, pyrrotite, cobaltite, monazite, xenotime, scheelite
• Qtz-cc veins
• Abundant High salinity fluid inclusions
Yalea
• Tourmaline poor
• Fe-As-Cu-W-Pb-Sb-Au-Ag
• Pyrite-arsenopyrite rich
• Asp + py + scheelite + late Cu minerals + Pb minerals (absence of tourmaline, REE and Ni-Co sulphides)
• Carb – phyllic alteration
• No high salinity fluid inclusions
Massawa
Diale-Dalema Series
Kofi Series
Falémé Series
Loulo
Segala
Tabakoto
Sadiola
Yatela
Massawa
Sabodala
Massawa deposit is hosted in: A. Volcaniclastic agglomerate B. Greywackes C. Graphitic schists D. Gabbro E. Feldspar Porphyrys some
mineralised and some not F. Ductile, carbonate-sericite-
quartz alteration zone
Three mineralising events 1. Ore stage 1 (replacive qtz, ser,
ank, pyr, aspyr, Au 2. Barren qz- molybdenite veins 3. Ore stage 2 (quartz-stibnite-
Au)
Main Ore Phase Stage 1:
A: Ankerite-sericite-quarz alteration; B: Asp after Pyr; C: Pyr after Asp; E: Asp after Pyr; F: Au in Asp; H: Au in qtz.
MINERAL PARAGENESIS
Qtz-Molybenite veining at Massawa: Stage 2
A: Qtz intergrown with stibnite; B: stibnite-chalcostibite-quartz; C: stibnite associated with aurostibite and native gold; D: Stibnite intergrown with sphalerite, gold and zinkerite
Dominant fluid inclusions in both qtz-stibnite veins (A and C) and quartz associated with stage one ore (B) are: Type 1 CO2-CH4 and Type 2 H2O-NaCl
0
5
10
15
20
25
-65 -64 -63 -62 -61 -60 -59 -58 -57 -56
Fre
qu
en
cy
Qtz-py-asp-Au
Qtz-stibnite-Au
Barren qtz-moly
0
4
8
12
16
20
-10 -5 0 5 10 15 20
Fre
qu
en
cy
Qtz-py-asp-Au
Qtz-stibnite-Au
Barren qtz-moly
0
5
10
15
20
25
30
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
Fre
qu
en
cy
Qtz-py-asp-Au
Qtz-stibnite-Au
Barren qtz-moly
Barren qtz
0
5
10
15
20
25
30
35
100 125 150 175 200 225 250 275 300 325 350
Fre
qu
en
cy
Qtz-py-asp-Au
Qtz-stibnite-Au
Barren qtz-moly
Barren qtz
n = 99
n = 112
(°C) (°C)
n = 65 n = 65
(°C) (°C)
(A) (B)
(C) (D)
NB Typical Metamorphic Fluids Type 1. Ore stage 1 15 – 33% Mol% CH4
Ore Stage 2 10 – 15% Mol% CH4
Type 2 2 – 8% NaCl
Type 1. T m(CO2) Type 1. T h(CO2)(L)
Type 2. T m(ice) Type 2. T h(L))
LOULO STABLE ISOTOPES: LAWRENCE ET AL 2013
Sulphur Isotope data overlap for all stages.
(NB diagenetic sulfides in the KKI have δ34S values of 6 – 15 ‰)
MASSAWA SULPHUR ISOTOPE DATA
0
2
4
6
8
10
-4 -3 -2 -1 0 1 2 3 4 5
Fre
qu
en
cy
δ34SV-CDT (‰)
LE stibnite
NZ quartz-sulphide veins
NZ disseminated sulphides
CZ Disseminated sulphides
CZ Molybdenite
CZ Stibnite
0
1
2
3
4
5
6
7
8
9
0 1 2 3 4 5 6 7 More
Fre
qu
en
cy
∂34S
Morila Sulphur Isotope Data Morila Interpreted as a Reduced Intrusion Related Gold System (RIRGS) McFarlane et al. 2011
Variation in sulfide S isotope compositions in sediment hosted orogenic gold deposits through time and seawater sulfate curve (Chang, 2008, Geology)
BORON ISOTOPE DATA – LAMBERT-SMITH 2014
CONCLUSIONS
• West African Orogenic Gold deposits do not have a one size fits all model – different mineralogies, fluid chemistries and isotope signatures
• Hypersaline fluids can be derived from metamorphic reactions
• Boron does not necessarily mean magmatic
• Not all orogenic gold deposits are sourced from metamorphic fluids
• The Gara and Yalea deposits at Loulo (bizarrely given the B and Cl levels) are likely sourced from metamorphic fluids
• S isotope data at Massawa, together with field relations, speak of magmatic fluids
• Morila is a RIRGS