Magma fertility and ore deposits: lessons from magmatic systems MINERALS DOWN UNDER NATIONAL RESEARCH FLAGSHIP Steve Barnes, CSIRO Earth Science and Resource Engineering GA Mineral Systems June 2012
Jan 18, 2016
Magma fertility and ore deposits: lessons from magmatic systems
MINERALS DOWN UNDER NATIONAL RESEARCH FLAGSHIP
Steve Barnes, CSIRO Earth Science and Resource Engineering
GA Mineral Systems June 2012
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
www.csiro.au
R-factor
Adding sulfide
Olivine
Liquid
• 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
10
100
1000
0 5 10 15 20 25 30
Ni p
pm
MgO wt %
Kom <3GaEmeishanSiberian TrapsDeccan TrapsEast GreenlandHawaiiIcelandKerguelenWest GreenlandBushveld
10
100
0 5 10 15
Ni p
pm
MgO wt %
E Yilgarn komatiites
Raglan kom. basalts
Noril’sk (Mk suite)
Sudbury?
DNi = Ni(sul)/Ni(sil)100-250 for komatiites500-800 basalts MgO~10%>1000 basalts/andesites MgO<10%Also a strong function of fO2, Ni(sul)
Noril’sk (Nd suite)
0.001
0.01
0.1
1
10
30 40 50 60 70 80 90
Pt/
Ti n
Mg# mol%
EmeishanSiberian TrapsDeccan TrapsEast GreenlandHawaiiIcelandKerguelenWest GreenlandBushveldKomatiites
Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
Effect of sulfide liquid extraction
Sulfide –undersaturated mantle melts
Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
0.001
0.01
0.1
1
10
30 40 50 60 70 80 90
Pt/
Ti k
n
Mg#
K-Mun 345, <3GaProt - BushveldPhan - arcPhan - CFBPhan - MORBPhan - ocean plume
Sources of variance• Sulfide retention at
source• Sulfide
fractionation/ extraction in crust
• PGM saturation/ fractionation/ retention
• PGM content of source
PGE-depleted, contaminated basalts associated with Ni-Cu sulfide ores
Mantle melts sulfide-saturated at source
Fiorentini et al Econ Geol 2010
Bushveld magmas
S solubility in magmas – constant pressure
fractionation
S much more soluble as sulphate than sulphide (Jugo et al 2010)
Arc magmas
Implications for magmatic/hydrothermal systems
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 hereNot 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.
Crustal scale Ni mineral systems
Blind Alley
Crustal S source
Ore-body
Orebody
Magma Freeway
Questions:• Is crustal S always necessary?• 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)
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 (subduction-derived), interpreted to be derived by entrainment of sub-continental lithospheric mantle
2. Distinctive differences are detectable between “fertile” and “barren” LIPs.
Crust contam
0.001
0.01
0.1
1
10
30 40 50 60 70 80 90
Pt/
Ti n
Mg# mol%
EmeishanSiberian TrapsDeccan TrapsEast GreenlandHawaiiIcelandKerguelenWest GreenlandBushveldKomatiites
Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
PGE-depleted, contaminated basalts associated with Ni-Cu sulfide ores
Mantle melts sulfide-saturated at source
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
10% partial melting (basalt) – sulfide and most of PGE retained in source
(cartoon from Nick Arndt) Sulfide-enriched sources should produce PGE DEPLETED melts