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GEOCHEMISTRY OF MAFIC LAYERED INTRUSIONS
DOS AND DON’TSJames D. Miller
Precambrian Research CenterDepartment of Geological Sciences
University of Minnesota Duluth
Workshop on Nickel -Copper-Platinum Group Element Mineralization Thunder Bay, Ontario
January 14, 2011
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OUTLINE Geochemical Analyses for Exploration The Problem with Cumulates Major Element Chemistry Trace Element Chemistry Mineral Chemistry Assay Data for Cu-Ni-PGE Mineralized
Intrusions
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GEOCHEMICAL ANALYSES FOR EXPLORATIONPURPOSE OF GEOCHEMICAL ANALYSES OF
MLI ROCKS IN EXPLORATION (IN ORDER OF IMPORT)
ESTABLISH GRADE OF ORE DEPOSIT EVALUATE THE POTENTIAL FOR MINERALIZATION EVALUATE THE COMPOSITION OF THE PARENTAL
MAGMA (SOURCE OF METALS) AND POSSIBLE CONTAMINANTS (COMMONLY THE SOURCE OF S)
EVALUATE THE CRYSTALLIZATION AND DIFFERENTIATION HISTORY OF THE MAGMA
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GEOCHEMICAL ANALYSES FOR EXPLORATION
XRF+ICP-MSFull digestion>$60/smpl
ICP-MS/AESPart. digestion$20-25/smpl
Fire Assay$20-30/smpl
No Si
2009 Acme Analytical Lab Brochure
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THE PROBLEM WITH CUMULATESThe Classic View of Cumulate Rocks is that they are
Mixtures of Primocrysts and a Liquid ComponentPrimocrysts are:
Enriched in high-T solid solution components (Mg in mafic phases, Ca in plagioclase)Enriched in compatible trace elements (e.g. Ni in Ol, Sr in Pl)
Liquid component is:Enriched in low-T solid solution components (Fe in mafic phases, Na,K in plagioclase)Enriched in incompatible minor and trace elements
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THE PROBLEM WITH CUMULATES
The concentration (X) of any element (a) in a cumulate rock (WR) is dependent on:
• The relative proportions of primocrysts (PC) and the liquid component (LC)
• The compositions of those componentsXa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC)
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THE PROBLEM WITH CUMULATESWhat parts of this mass balance can we know?
Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC)
Modes of Primocrysts? Problem is that cumulus phases continue to crystallize post-cumulus rimsPlagioclase – possible if zoning preserved, but painstakingOlivine and pyroxene –not precisely, zoning lost due to subsolidus re-equilibrationOxide - ????
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THE PROBLEM WITH CUMULATESWhat parts of this mass balance can we know?
Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC)
Compositions of Primocrysts? Problematic because most primocrysts are solid solutions phases
Plagioclase – possible, but cumulus cores can be very complexly zoned
Olivine and Pyroxene – Ease of re-equilibration leads to “trapped liquid shift”
Oxides – easily re-equilibrated and oxy-exsolved
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THE PROBLEM WITH CUMULATESWhat parts of this mass balance can we know?
Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC) Compositions of the “Trapped” Liquid Component.... and might this be representative of the Parent Magma? Problem is ...the amount of liquid in the cumulate changes over time due to compaction driven by bouyancy and crystal accumulation, and ....the composition of the liquid in the cumulate changes over time due to fractional crystallization of the intercumulus liquid.
From Tegner et al., 2009
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THE PROBLEM WITH CUMULATESRare Solution to Estimating the Parent Magma of an MLI – Chill Zone
Tamarack Intrusion, Minnesota
(Goldner, UMD MS thesis, in progress)
Basal Chill Zone – 30% Ol phenocrysts in a fine gabbroic groundmass
~1cm
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MAJOR ELEMENT CHEMISTRY
A B D
Fo 89Olivine
Chill Zone 4-334.2
4-334.2-30% Fo89
SiO2 39.0 47.8 51.5Al2O3 8.73 12.5FeOT 10.8 10.9 10.9MnO 0.16 0.17 0.17MgO 49.8 23.3 12.0CaO 0.25 5.66 7.98Na2O 1.14 1.63K2O 0.41 0.59TiO2 0.82 1.17P2O5 0.08 0.11
H2O + CO2 1.05 1.50Total 100.0 100.0 100.0
mg# (.9FeOT) 89.1 81.0 68.5
PELE Crystallization Models (Boudreau, 2005)
Equilibrium Crystallization QFM buffer, P=1 Kb
Fractional Crystallization QFM buffer, P=1 Kb
Tamarack Parent Magma Calculation
(Goldner, UMD MS thesis, in progress)
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MAJOR ELEMENT CHEMISTRYDO NOT PLOT CUMULATES ON PLOTS INTENDED FOR MAGMA COMPOSITIONS
PrimocrysticPlagioclase
Liquid Component
fractionation
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MAJOR ELEMENT CHEMISTRY
0 10 20 30 40 50 60 70 80 90 1000.0
10.0
20.0
30.0
40.0
50.0
60.0
SiO2Al2O3FeOMgOCaO
0 10 20 30 40 50 60 70 80 90 1000.0
5.0
10.0
15.0
20.0
25.0
Al2O3FeOMgOCaO
Dunite Cumulate (O)
adcu
mul
ate
mes
ocum
ulat
e
orth
ocum
ulat
e
porp
hyrit
ic
liqui
d
oliv
ine
adcu
mul
ate
mes
ocum
ulat
e
orth
ocum
ulat
e
porp
hyrit
ic
liqui
d
plac
tioc
lase
+ o
livin
e
Troctolite Cumulate (PO)Fo77 olivine, An76 plagioclase, mg#50 liquidFo84 olivine, mg#60 liquid
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MAJOR ELEMENT CHEMISTRY
Major and Minor Element chemistry can serve as approximate proxies for abundances of primocryst phasese.g.
Al – PlagioclaseMg – Augite + OlivineTi – Ilmenite and Ti-magnetite
From Joslin (2004)
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MAJOR ELEMENT CHEMISTRYMajor and Minor Element chemistry (that includes accurate SiO2) can be used to calculate CIPW Norms.
Helpful for metamorphosed / altered cumulates (assuming a closed system)
Helpful for calculating average An content (Ca/(Ca+Na+K)) of complexly zoned plagioclase
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TRACE ELEMENT CHEMISTRYLike major elements, the absolute concentration of a trace element in a cumulate rock is typically dependent on the relative proportions AND compositions of the primocrysts and the liquid component.
Primocrysts
Liquid
Cpx?
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TRACE ELEMENT CHEMISTRYCompatibility – degree to which an element prefers to partition into the solid over the liquid phase .
Kd(i)1 – Mineral-Liquid Partition Coefficient for element i in mineral
1
Kd(i)1
= C(i)mineral 1/ C(i)
liquid (C(i) - concentration of element i in wt. %)
Kd(i)1
> 1 – Compatible, Kd(i)1
< 1 – Incompatible
D(i) – Bulk Rock Partition Coefficient for element i
D(i) = x1 Kd(i)1
+ x2 Kd(i)2
+ x3 Kd(i)3
+ .... (x1 – proportion of mineral 1)
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TRACE ELEMENT CHEMISTRY
From Rollinson (1993)
Com
patib
leIn
com
patib
le
Bulk Rock Partition Coefficient of Ce,Yb, and Nifor Crystallization of:
1) Troctolite (70% Pl, 30% Ol)
D(Ce) = xPl Kd(Ce)Pl
+ xOl Kd(Ce)Ol
= .7*.103 + .3*.007 = 0.092
D(Yb) = xPl Kd(Yb)Pl
+ xOl Kd(Yb)Ol
= .7*.07 + .3*.065 = 0.069
D(Ni) = xPl Kd(Ni)Pl
+ xOl Kd(Ni)Ol
= .7*.01 + .3*25= 7.5
2) Olivine Gabbro (63% Pl, 12% Ol, 25% Cpx)
D(Ce) = xPl Kd(Ce)Pl
+ xOl Kd(Ce)Ol + xCpx Kd(Ce)
Cpx
= .63*.103 + .12*.007 + .25*.09 = 0.088
D(Yb) = xPl Kd(Yb)Pl
+ xOl Kd(Yb)Ol + xCpx Kd(Yb)
Cpx
= .63*.07 + .12*.065 + .25*.09 = 0.074
D(Ni) = xPl Kd(Ni)Pl
+ xOl Kd(Ni)Ol + xCpx Kd(Ni)
Cpx
= .63*.01 + .12*25 + .25*8 = 5
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0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.000.100
1.000
10.000
100.000
Tr(Yb)Tr(Ce)Ce/Yb
0.000.100.200.300.400.500.600.700.800.901.000.100
1.000
10.000
100.000
OG(Yb)OG(Ce)Ce/YbOG(Ni)
TRACE ELEMENT CHEMISTRY
F (fraction of liquid remaining)
Rayleigh Distillation: CL/Co = F(D-1)
Conclusions: Fractional crystallization of mafic magmas gradually increases the concentrations of similarly incompatible elements, but has a minimal effect on their ratios; and strongly decreases the concentrations of compatible elements
F (fraction of liquid remaining)
CL/Co CL/CoTroctolite Olivine Gabbro
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TRACE ELEMENT CHEMISTRY
From Rollinson (1993)
Fractional crystallization increases the REE abundance, but has a neglible effect on the REE pattern
Since incompatible elements are 2-3 orders of magnitude greater in abundance than primocrysts, the REE pattern of cumulates (especially orthocumulates and adcumulates) will approximate that of their parental magmas and the magma source
Fractional crystallization of olivine from a komatiitic melt
From Jirsa and Miller (2006)
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TRACE ELEMENT CHEMISTRYSa
mpl
e/Pr
imiti
ve M
antle
(Goldner, UMD MS thesis, in progress)
Proposed Tamarack Parent Magma Trace Element Composition compared to Early MCR Volcanics
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TRACE ELEMENT CHEMISTRYSpidergrams Tectonic Discrimination Diagrams
rock
/cho
ndrit
ero
ck/c
hond
rite
Increasing compatibilityFrom Bedard (2001)
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MINERAL CHEMISTRY
From Miller (2004)
Stratigraphic variations in the compositions of solid-solution cumulus minerals generally reflect the progressive differentiation of the parental magma and the occurrence of recharge event...but not exactly.
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MINERAL CHEMISTRYPLAGIOCLASE
Zoning is preserved and records a history of
cumulus and postcumulus crystallization
White (2009)
Strategy 1: Compare only An of cumulus cores...Problem:Cores are commonly complexly zoned
Strategy 1: Calculate An from CIPW normProblem:Integrates cumulus and postcumulus components
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MINERAL CHEMISTRYOLIVINE AND PYROXENE
Zoning is NOT preserved and thus
integrates cumulus and postcumulus compositions
Overcoming the Trapped Liquid Shift
Solution: Compare only like types of cumulates (ortho, meso, ad)
Problem: Evaluating the type of cumulate is qualitative
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MINERAL CHEMISTRYEvaluating the Trapped
Liquid Shift
Gradual increase in incompatible
elements;can assume nearly
constant over limited stratigraphic
thicknessMagma Recharge
POcfcumulate
From Miller (2006)
TLS
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MINERAL CHEMISTRYEvaluating the Trapped
Liquid ShiftPOcfcumulate
From Miller (2006)
TLS
Postcumulus mineral abundance are general proxies for amount of trapped liquid
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MINERAL CHEMISTRYEvaluating the Trapped
Liquid ShiftPOcfcumulate
From Miller (2006)
TLS
Assuming that well foliated cumulates have lower porosity – i.e. lower volume of trapped liquid
From Meurer & Boudreau (1997)
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MINERAL CHEMISTRYMineral chemistry also allows the estimation of the magma composition in equilibrum with that mineral
A procedure for calculating the equilibrium distribution of trace
elements among the minerals of cumulate rocks, and the
concentration of trace elements in the coexisting liquids I
Jean H. BedardChemical Geology 118 ( 1994 ) 143-153
KD = (XFeOol/XFeO
liq)*(XMgOliq/XMgO
ol) = 0.3 (Roedder and Emslie, 1970)
which translates in determining the mg# of the liquid as:
mg# liq = 100 / (3.333(FeO/MgO)ol + 1)
This assumes no trapped liquid shift. Therefore, one should apply this only to adcumulates
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ASSAY DATA
Precious Metals Zone (PMZ)
Meters above Cu-Au break Sonju Lake IntrusionVariation in the Cu/Pd is one
of the best monitors of sulfide saturation in magmatic systems, but need high precision Pd analyses (<2 ppb DT)
From Joslin (2004)From Miller (2004) Greenwood Lake Intrusion
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ASSAY DATADsulf/sil~104-108
Dsulf/sil~102
Pd is several orders of magnitude more compatible in sulfide melt relative to Cu
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ASSAY DATA
after Barnes and others, 1987
R = Xsil/Xsulf
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ASSAY DATA
Pd (ppb)
Cu/Pd
From Joslin (2004)From Jirsa & Miller (2006)
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ASSAY DATA
From Jirsa & Miller (2006)
Quadrant w/Best Potential
Quadrant w/Ore Grade
Quadrant w/No Potential
Quadrant w/Potential at Depth
Indicates duration since initial saturation
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ASSAY DATA
From Jirsa & Miller (2006)