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 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 E Yilgarn komatiites Raglan kom. basalts Noril’sk (Mk suite) Sudbury? 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) Noril’sk (Nd suite)

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

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

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)

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)

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

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 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

Isotopic signals – mantle sources vs contamination Zhang et al (2008) Earth Science Reviews 86 145-174 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

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

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)