Magma fertility and ore deposits: lessons
from magmatic systems
Steve Barnes, CSIRO Earth Science and Resource Engineering
GA Mineral Systems June 2012
MINERALS DOWN UNDER NATIONAL RESEARCH FLAGSHIP
Fertility (as applied to magmas)
Do particular kinds of
magmas have a tendency to
give rise to ore deposits?
Lessons from orthomagmatic
Ni-Cu-PGE sulphide deposits
R-factor
Liquid
Adding sulfide
Olivine
www.csiro.au
•
•
Two orders of magnitude
variability in Ni and Pt in
magma
One order (Ni), >5 orders
variability in Pt in ore
deposits
MgO and Ni contents of parent magmas to major ore deposits
E Yilgarn komatiites
1000
Kom <3Ga
Emeishan
Siberian Traps
Deccan Traps
East Greenland
Hawaii
Iceland
Kerguelen
West Greenland
Bushveld
100
Noril’sk (Mk suite)
Sudbury?
100
10
0
5
10
15
MgO wt %
20
DNi = Ni(sul)/Ni(sil)
100-250 for komatiites
500-800 basalts MgO~10%
>1000 basalts/andesites MgO<10%
Also a strong function of fO2, Ni(sul)
25
30
Ni ppm
Ni ppm
Raglan kom. basalts
Noril’sk (Nd suite)
10
0
5
10
MgO wt %
15
Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
Emeishan
Siberian Traps
Deccan Traps
East Greenland
Hawaii
Iceland
Kerguelen
West Greenland
Bushveld
Komatiites
10
Pt/Ti n
1
Sulfide –
undersaturated
mantle melts
0.1
Effect of
sulfide liquid
extraction
0.01
0.001
30
40
50
60
Mg# mol%
70
80
90
Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
PGE-depleted,
contaminated
basalts associated
with Ni-Cu sulfide
ores
Fiorentini et al Econ Geol 2010
10
K-Mun 345, <3Ga
Prot - Bushveld
Phan - arc
Phan - CFB
Phan - MORB
Phan - ocean plume
1
Pt/Ti kn
Sources of variance
• Sulfide retention at
source
• Sulfide
fractionation/
extraction in crust
• PGM saturation/
fractionation/
retention
• PGM content of
source
Bushveld magmas
0.1
Mantle melts
sulfidesaturated at
source
0.01
0.001
30
40
50
60
Mg#
70
80
90
Implications for magmatic/hydrothermal systems
S solubility in magmas – constant pressure
Arc
magmas
fractionation
S much more soluble as sulphate
than sulphide (Jugo et al 2010)
Sulfide saturation in arc magmas – Pual Ridge, Manus Basin
(Jenner et al 2010 J Pet)
Onset of magnetite crystallisation
Effect of magnetite saturation:
Reduces sulfate to sulfide
Lowers FeO content of melt
(major control on sulfide
solubility)
A lesson for felsic hosted Cu-Au systems? Be on the right side of the
magnetite/sulfate reaction
Be here
Not here
Fertile porphyry (and VHMS?)
systems likely to be
Oxidised
Magnetite undersaturated
Check out papers by
Sillitoe, Richards, Mungall,
Botcharnikov et al et al
Conclusions
• “Fertility” in magmatic sulfide systems is a bit of a myth
– process dominates over source
• But not entirely – high PGE contents in magmatic ores
require lack of previous sulfide extraction – a little goes
along way
• Same probably applies to Cu and Au in felsic systems
• Fertile felsic magmas favoured by reducing conditions.
Magnetite saturation can tip the balance.
Questions:
• Is crustal
S always
necessary?
Crustal
scale
Ni mineral systems
• Does the magma source matter? Is the SCuM involved?
• How to distinguish magma freeways? (e.g. use of resistate detrital
minerals such as Ti-rich chromite)
Orebody
Blind Alley
Orebody
Crustal S source
Magma Freeway
Isotopic signals –
mantle sources vs
contamination
Zhang et al (2008) Earth
Science Reviews 86 145-174
Zhang et al claim that:
1. Signals of variability within
continental LIPs basalts can’t all
be explained by crystal
contamination, require
component of variance from
mantle plume – specifically the
“EM1” component (subductionderived), interpreted to be
derived by entrainment of subcontinental lithospheric mantle
2. Distinctive differences are
detectable between “fertile” and
“barren” LIPs.
Crust contam
Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
Emeishan
Siberian Traps
Deccan Traps
East Greenland
Hawaii
Iceland
Kerguelen
West Greenland
Bushveld
Komatiites
10
Pt/Ti n
1
0.1
PGE-depleted,
0.01
contaminated
basalts associated
with Ni-Cu sulfide
ores
Mantle melts
sulfidesaturated at
source
0.001
30
40
50
60
Mg# mol%
70
80
90
Sulfide in the mantle?
PGE content of mantle melts at source depends on whether
or not sulfides are retained
>30% partial melting
(komatiite) – all sulfide
gone, all PGE in melt
(cartoon from Nick Arndt)
10% partial melting (basalt) –
sulfide and most of PGE
retained in source
Sulfide-enriched sources should produce PGE
DEPLETED melts