1. 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

2. 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

3. R-factor
Adding sulfide
Olivine
Liquid

4. Two orders of magnitude variability in Ni and Pt in magma
One order (Ni), >5 orders variability in Pt in ore deposits

5. MgO and Ni contents of parent magmas to major ore deposits
E Yilgarn komatiites
Raglan kom. basalts
Noril’sk (Mk suite)
Sudbury?
DNi = Ni(sul)/Ni(sil)
for komatiites
basalts MgO~10%
>1000 basalts/andesites MgO<10%
Also a strong function of fO2, Ni(sul)
Noril’sk (Nd suite)

6. Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
Sulfide –undersaturated mantle melts
Effect of sulfide liquid extraction

7. Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
Fiorentini et al Econ Geol 2010
Sources of variance
Sulfide retention at source
Sulfide fractionation/ extraction in crust
PGM saturation/ fractionation/ retention
PGM content of source
Bushveld magmas
Mantle melts sulfide-saturated at source
PGE-depleted, contaminated basalts associated with Ni-Cu sulfide ores

8. S solubility in magmas – constant pressure
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)

9. Onset of magnetite crystallisation
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)

10. A lesson for felsic hosted Cu-Au systems
A lesson for felsic hosted Cu-Au systems? Be on the right side of the magnetite/sulfate reaction
Fertile porphyry (and VHMS?) systems likely to be
Oxidised
Magnetite undersaturated
Be here
Not here
Check out papers by Sillitoe, Richards, Mungall, Botcharnikov et al et al

11. 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.

12. Crustal scale Ni mineral systems
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)
Crustal scale Ni mineral systems
Blind Alley
Crustal S source
Ore-body
Orebody
Magma Freeway

13. Isotopic signals – mantle sources vs contamination
Zhang et al (2008) Earth Science Reviews
Zhang et al claim that:
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
Distinctive differences are detectable between “fertile” and “barren” LIPs.
Crust contam

14. Pt/Ti (mantle norm) vs Mg# - mafic magmas +komatiites
Mantle melts sulfide-saturated at source
PGE-depleted, contaminated basalts associated with Ni-Cu sulfide ores

15. 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
Sulfide-enriched sources should produce PGE DEPLETED melts
(cartoon from Nick Arndt)