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