GEOCHEMISTRY OF MAFIC LAYERED INTRUSIONS
DOS AND DON’TS
James D. Miller
Precambrian Research Center
Department of Geological Sciences
University of Minnesota Duluth
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

GEOCHEMICAL ANALYSES FOR EXPLORATION
PURPOSE 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
GEOCHEMICAL ANALYSES FOR EXPLORATION
ICP-MS/AES
Part. digestion
$20-25/smpl
XRF+ICP-MS
Full digestion
>$60/smpl
2009 Acme Analytical Lab Brochure
Fire Assay
$20-30/smpl
No
Si
THE PROBLEM WITH CUMULATES
The Classic View of Cumulate Rocks is that they are
Mixtures of Primocrysts and a Liquid Component
Primocrysts 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
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 components
Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC)
THE PROBLEM WITH CUMULATES
What 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
rims
Plagioclase – possible if zoning
preserved, but painstaking
Olivine and pyroxene –not precisely,
zoning lost due to subsolidus
re-equilibration
Oxide - ????
THE PROBLEM WITH CUMULATES
What 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
THE PROBLEM WITH CUMULATES
What 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. –
McBirney 2005
From Tegner et al., 2009
MAJOR ELEMENT CHEMISTRY
DO NOT PLOT CUMULATES
ON PLOTS INTENDED FOR
MAGMA COMPOSITIONS!!!
Primocrystic
Plagioclase
fractionation
Liquid Component
MAJOR ELEMENT CHEMISTRY
50.0
liquid
porphyritic
orthocumulate
mesocumulate
adcumulate
liquid
porphyritic
orthocumulate
mesocumulate
olivine
adcumulate
25.0
60.0
plactioclase + olivine
Troctolite Cumulate (PO)
Fo77 olivine, An76 plagioclase, mg#50 liquid
Dunite Cumulate (O)
Fo84 olivine, mg#60 liquid
20.0
40.0
SiO2
15.0
Al2O3
Al2O3
30.0
FeO
FeO
MgO
20.0
MgO
10.0
CaO
CaO
5.0
10.0
0.0
0
20
40
60
80
100
0.0
0
20
40
60
80
100
MAJOR ELEMENT CHEMISTRY
Major and Minor Element
chemistry can serve as
approximate proxies
for abundances of
primocryst phases
e.g.
Al – Plagioclase
Mg – Augite + Olivine
Ti – Ilmenite and Ti-magnetite
From Joslin (2004)
MAJOR ELEMENT CHEMISTRY
Major 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
TRACE ELEMENT CHEMISTRY
Like 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.
Ap?
FC
Liquid
Bio?
Primocrysts
TRACE ELEMENT CHEMISTRY
Compatibility – 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)
Compatible
TRACE ELEMENT CHEMISTRY
Bulk Rock Partition Coefficient of Ce,Yb, and Ni
for 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
Incompatible
D(Ni) = xPl Kd(Ni)Pl + xOl Kd(Ni)Ol
= .7*.01 + .3*25= 7.5
LREE
HREE
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
From Rollinson (1993)
TRACE ELEMENT CHEMISTRY
Rayleigh Distillation: CL/Co = F(D-1)
100.000
100.000
CL/Co
Troctolite
CL/Co
Olivine Gabbro
10.000
10.000
Tr(Yb)
Tr(Ce)
OG(Yb)
OG(Ce)
Ce/Yb
OG(Ni)
Ce/Yb
Tr(Ni)
1.000
1.000
0.100
0.00
0.100
0.00
0.20
0.40
0.60
0.80
F (fraction of liquid remaining)
1.00
0.20
0.40
0.60
0.80
1.00
F (fraction of liquid remaining)
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
TRACE ELEMENT CHEMISTRY
Fractional crystallization of olivine from a komatiitic melt
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 increases
the REE abundance, but has a
neglible effect on the REE pattern
From Rollinson (1993)
From Jirsa and Miller (2006)
TRACE ELEMENT CHEMISTRY
Spidergrams
Negative anomaly
rock/chondrite
rock/chondrite
Positive anomaly
Tectonic Discrimination Diagrams
From Bedard (2001)
Increasing incompatibility
MINERAL CHEMISTRY
Stratigraphic variations in
the compositions of solidsolution cumulus minerals
generally reflect the
progressive differentiation of
the parental magma and the
occurrence of recharge
event...
but not exactly.
From Miller (2004)
MINERAL CHEMISTRY
PLAGIOCLASE
Zoning is preserved and
records a history of
cumulus and postcumulus
crystallization
Strategy 1:
Compare only
An of cumulus
cores...
Problem:
Cores are
commonly
complexly
zoned
White (2009)
Strategy 2: Calculate
An from CIPW norm
Problem:
Integrates cumulus
and postcumulus
components
MINERAL CHEMISTRY
Overcoming
the
Trapped
Liquid Shift
Solution:
Compare only
like types of
cumulates (ortho,
meso, ad)
Problem:
Evaluating the
type of cumulate
is qualitative
OLIVINE AND PYROXENE
Zoning is NOT preserved
and thus integrates
cumulus and postcumulus
compositions
MINERAL CHEMISTRY
TLS
POcf
cumulate
Evaluating the Trapped Liquid Shift
Gradual increase in
incompatible elements;
can assume nearly
constant over limited
stratigraphic thickness
Magma Recharge
From Miller (2006)
MINERAL CHEMISTRY
POcf
cumulate
TLS
From Miller (2006)
Evaluating the Trapped Liquid Shift
Postcumulus mineral abundance are
general proxies for amount of trapped liquid
MINERAL CHEMISTRY
POcf
cumulate
Evaluating the Trapped Liquid Shift
Assuming that well foliated cumulates have
lower porosity – i.e. lower volume of
trapped liquid
TLS
From Miller (2006)
From Meurer & Boudreau (1997)
MINERAL CHEMISTRY
Mineral chemistry also allows the estimation of the magma
composition in equilibrum with that mineral
KD = (XFeOol/XFeOliq)*(XMgOliq/XMgOol) = 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
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. Bedard
Chemical Geology 118 ( 1994 ) 143-153
ASSAY DATA
Variation 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)
Meters above
Cu-Au break
Sonju Lake Intrusion
Precious
Metals Zone
(PMZ)
From Miller (2004)
Greenwood Lake Intrusion
From Joslin (2004)
ASSAY DATA
Dsulf/sil~104-108
Pd is several orders
of magnitude more
compatible in sulfide
melt relative to Cu
Dsulf/sil~102
ASSAY DATA
R = Xsil/Xsulf
after Barnes and others, 1987
ASSAY DATA
Cu/Pd
From Jirsa & Miller (2006)
Pd (ppb)
From Joslin (2004)
ASSAY DATA
Quadrant w/
No Potential
Quadrant w/
Best Potential
Quadrant w/
Potential at Depth
Quadrant w/
Ore Grade
Indicates
duration
since initial
saturation
From Jirsa & Miller (2006)
ASSAY DATA
From Jirsa & Miller (2006)