Micro-PIXE Analysis of Trace Elements in Sulfides
D. D. Hickmotta, J. Stimacb, A.C.L. Larocquec, C. Wettelanda, A. Brearleyd
a
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
b
Philippine Geothermal, Makati City, Philippines
Dept. of Geol. Sci., Univ. Manitoba, Winnipeg, MB, R3T 2N2, Canada
d
Dept. Earth and Planet. Sci., Univ. New Mexico, Albuquerque, NM 87131, USA
Abstract Micro-scale Proton-induced X-ray Emission (PIXE) of trace elements (TE) in sulfides provides insights into
geologic processes including magmatic system evolution, ore forming events, and fluid-flow processes. The Los Alamos
nuclear microprobe was used to determine TE concentrations and ratios in sulfides from diverse geologic environments
including hydrothermal ore deposits, coal seams, and metamorphic rocks. Pyrrhotite (Po) from silicic volcanics contains
high Cu and Ni; Po from the Clear Lake volcanic field has higher Mo than does Po from other volcanic fields. Coal pyrites
contain high Cu, As, Se, Mo and Pb, and show high As/Se and Mo/Se in marine influenced sulfides from the Lower
Kittanning coal, but not in other marine-influenced coals. Sulfides are amenable to micro-PIXE studies because of the
difficulties in obtaining the homogeneous standards required for many other TE microanalytical techniques.
‘standardless’ data-reduction system provides accurate
and precise data for trace element analyses of sulfides
with detection limits of a few ppm for most trace
elements heavier than Fe in low-Z matrices [6]. This
method uses a major-element analysis, typically
obtained by electron microprobe (EMP), to calculate
matrix correction factors for trace-element analysis.
Although homogeneous standards for sulfide materials
are not available, the reproducibility and accuracy of
the method was demonstrated in studies of chondritic
meteorites [8], rhyolite glasses [9], and mantle
clinopyroxenes [7]. Analyses of these materials suggest
that most heavy trace elements (Ni – Nb) can be
analyzed to +/- 10% (one sigma) down to a few ppm
sensitivity [7]. The largest source of error in microPIXE analyses of sulfides is overlap of the proton
beam, which penetrates into the sample, into phases
other than sulfide, yielding partially contaminated
analyses. This problem is severe for fine-grained
materials in the samples from magmatic systems. The
key method for minimizing this problem is to monitor
the X-ray spectra of the sulfides for elements that do
not occur in sulfides based on crystal-chemical
constraints (e.g. Rb) and to discard analyses where
these elements are observed at high abundances. Traceelement ratios of slightly contaminated analyses remain
useful in petrogenetic studies, provided the ratioed
elements are not present in the contaminating material
at high abundances.
Sulfides were also analyzed using EMP on a
Cameca SX-50 microprobe. EMP methods are further
described in Stimac and Hickmott [9].
INTRODUCTION
Sulfide minerals are widespread trace components
of terrestrial crustal rocks and common components of
mantle samples and of meteorites. Sulfide chemistry is
extensively used in genetic studies of ore deposits [1],
the mantle [2], and volcanic systems [3]. Trace-element
and stable-isotope microanalysis of sulfides has been
increasingly utilized in such studies. Secondary Ion
Mass Spectrometry (SIMS) [1], laser-ablation
techniques [4], and micro-proton induced X-ray
emission (PIXE) have all demonstrated their utility in
sulfide studies. The advantages of micro-PIXE, when
compared with other microanalytical techniques,
include its ability: 1) to quantify analyses without use
of closely-matrix matched standards and 2) to analyze a
wide range of trace elements rapidly and accurately
with geologically-useful detection limits.
This paper describes petrogenetic investigations of
sulfides performed using the Los Alamos National
Laboratory (LANL) nuclear microprobe. The primary
focus is on: 1) sulfides from felsic to intermediate
volcanic rocks; and 2) sulfides in coal. Additional areas
where studies of sulfides using micro-PIXE may be
valuable will be identified.
ANALYTICAL METHODS
Micro-PIXE analyses were completed in LANL’s
Ion Beam Materials Laboratory (IBML). Typical
analytical protocols and data reduction procedures are
described in detail elsewhere [5-7]. The LANL
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
440
TABLE 1. Representative PIXE analyses of magmatic
sulfides
SULFIDES FROM MAGMATIC
SYSTEMS
Understanding the behavior of ore metals in
magmatic systems and the formation of related ore
deposits requires an understanding of the partitioning of
metals between minerals such as sulfides, liquids
(hydrous, silicate, and immiscible sulfide), and gases
during magmatic evolution [3, 9, 10]. Partitioning
changes during magma ascent and evolution as
pressure, temperature, mineral and melt compositions,
and the nature (and number) of magmatic liquids
change [10]. Trace elements in suites of cogenetic
sulfides, in sulfide melt inclusions, or in zoned sulfides
can provide insights into processes during magmatic
system evolution. Natural system results may more
accurately reflect element distributions during
magmatic evolution, which often reflect disequilibrium
processes, than do data from experimental systems.
To better understand the role of sulfide minerals in
crystal fractionation and magma immiscibility, we have
studied the modes of occurrence and trace-element
compositions of sulfides in a wide-range of silicic to
intermediate volcanic rocks [9, 11]. These are suites of
rocks from volcanic centers that have been investigated
by other researchers, so that the intensive variables and
magmatic processes controlling the distribution of trace
elements in the sulfides can be more easily interpreted.
Sulfide bearing samples studied are described in
Stimac and Hickmott [11]. These suites span a range of
magmatic compositions (rhyolite to andesite), tectonic
settings, and oxygen fugacities. All sulfides described
are FeCu sulfides. Most appear to represent
crystallization products from immiscible Fe-Cu-S(-O)
melts [10]; these are bleb-like globular phases
containing intermixed pyrrhotite and CuFe sulfide
(ISS). More rare are euhedral sulfides that may
represent crystallization products of silicate-rich
liquids, such as the sample from Clear Lake [11]. Note
that uncertainty in our understanding of the
petrogenesis of Fe-Cu-S(-O) globules (immiscible
liquids?) and associated FeCu sulfides may limit the
quantitative applicability of partition coefficient data
derived from these samples. Many of these sulfides
were fine grained (grain sizes on the order to the beam
diameter of 10-20 micrometers) so data for individual
phases should probably be considered to be semiquantitative. However, if these globules/mixed finegrained phases behaved coherently during fractional
crystallization,
magma
ascent,
and
magma
immiscibility, their bulk (mixed) trace-element
compositions are relevant to understanding magmatic
and ore-forming processes. Sulfide globules may be
remobilized during late-stage magmatic/hydrothermal
evolution to produce ore-forming fluids [4].
Sample
Locale
Sample ID
Description
St.
Helens
MSH
10F-1
Po core
Clear
Lake
CL5-1
Po core
Fe
593100
610700
Co
<1870
1600
Ni
8740
10300
Cu
31400
3010
Zn
<169
-As
50
34
Se
42
72
Mo
49
323
All values in parts per million (ppm).
pyrite.
Pinatubo
R-PIN
3B-2
Py
El
Chichon
EC1-B1
Po
476700
586900
---767
-58600
-238
56
-99
153
--Po= pyrrhotite, Py =
Sulfides in silicic to intermediate volcanic rocks
are enriched in Ni, Cu, Zn, As, Se, and Mo (Table 1).
ISS tends to have higher Cu, Zn, and Ag than coexisting pyrrhotite. Partition coefficients between
pyrrhotite and co-existing glass range from 41 to
several thousand for Cu, 6-85 for Zn, 2-3 for As, and >
36 and > 161 for Se and Mo respectively [9, 11].
Partition coefficient values for Cu and other base
metals measured in this study are consistent with results
presented by Lynton et al. [12] in high-silica rhyolite
and Jugo et al. for synthetic haplogranitic melt [10],
although partition coefficient results for natural systems
vary far more than those determined in experimental
systems. Thus, fractional crystallization of either
pyrrhotite or ISS during magmatic evolution will
deplete the fractionating magma in copper and other
associated metals. This is consistent with the general
observation that Cu acts as a compatible element in
intermediate to felsic magmatic systems, with the
lowest observed concentrations in the first eruptive
units in a volcanic edifice.
FIGURE 1: Mo (ppm) vs. Se (ppm) for volcanic sulfides.
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TABLE 2. Analyses of sulfides from coal seams
The pyrrhotites from Clear Lake have distinct traceelement compositions, compared to those from the
other suites. Both total Mo and Mo/Se are higher than
in the other three suites (Figure 1). This may reflect that
the Clear Lake samples represent the most evolved
system (75 wt.% SiO2), or that this system is the least
oxidized (ilmenite-only) [9, 11].
Sulfide phases frequently contain a significant
fraction of a magmatic system’s budget of elements
such as Cu, Ni, Se, and other base and precious metals.
Due to their high density, sulfides will separate
efficiently from magmatic liquids either by gravityinduced fractionation or by filter-pressing of such
liquids to higher crustal levels. Key factors influencing
the behavior of base and precious metals in ore deposits
are: 1) when sulfides precipitate during evolution of
magmatic systems; 2) the order of precipitation and
modal abundance of sulfides; 3) how such sulfides
behave during partial melting and hydrothermal
alteration; and 4) the timing of formation of immiscible
sulfide-rich liquids and aqueous brines relative to the
timing of sulfide formation in magmas. For ore deposits
the key parameter is probably the timing of fluid
exsolution and remobilization relative to crystal (and
immiscible melt) fractionation. Studies of gabbroic
xenoliths in magmas and of the chemical evolution of
intermediate systems suggest that sulfides with their
associated base and precious metals saturate early
during magmatic system evolution and that such phases
are not stable, and associated metals behave
incompatibly, in more evolved magmatic systems.
Sample
Locale/Seam
Description
PIXE Point
Lower
Kittanning
Py
736
Upper
Freeport
Py
956
Menefee
Fruitland
Py
994
Py
870
Ni
--515
----Cu
231
731
141
835
As
592
507
508
1320
Se
54
173
32
240
Mo
15
----743
Pb
173
604
101
959
All values in ppm. For abbreviations see footnote to Table 1
Pyrite from four coal seams was investigated: two
from the Appalachian basin of western Pennsylvania,
the Lower Kittanning coal [15] and the Upper Freeport
coal [16], and two from the San Juan basin of New
Mexico, the Fruitland coal and the Menefee coal [17].
The former two are representative of eastern, moderateto high-sulfur coals and the latter are representative of
western, low-sulfur coals.
The trace elements most frequently found in the
coal pyrites were Ni, Cu, As, Se, Sr (possibly due to
overlap with associated macerals), Mo, and Pb (Table
2). All trace elements exhibited heterogeneous
distributions. Eastern coal sulfides tended to have
higher Ni and lower Hg than the western coals. The
sulfides from the Navajo Mine (Fruitland) coal seam
were high in Mo when compared to samples from other
seams. However, the range of abundances for most of
the seams overlapped for most of the investigated trace
elements such as As and Se (Figure 2).
The Lower Kittanning seam provides an opportunity
to evaluate the effects of varying depositional
environments of overlying sedimentary sequences on
trace elements in coal sulfides [18]. The Lower
Kittanning coal is overlain by sediments with
associations ranging from marine to brackish to
freshwater [15] and many of the properties of the coal
including sulfur content, mineral content, ash fusion
temperature, and trace-element content are related to
the nature of the overlying sediments [15]. As/Se and
Mo/Se in sulfides from samples with marine or brackish
overburden are higher than those with fresh-water
overburden. This is probably due to the higher Mo and
As in the marine or brackish fluids that may have
produced epigenetic coal sulfides in the Lower
Kittanning sulfides with marine associations.
However, this association between high As/Se and
Mo/Se and marine influences does not manifest itself in
coal sulfides from the other seams (Figure 2). The
Upper Freeport coal has high As/Se in some samples,
and yet is associated with fresh-water sediments.
SULFIDES FROM COAL SEAMS
Understanding the distribution of trace metals in
coal seams is important in order to develop
beneficiation systems for removal of trace metals from
coal prior to combustion, to understand the behavior of
coal-derived metals in combustion systems, and to
develop genetic models for coal seams [13]. Sulfides
are a ubiquitous trace component within coals and
sulfides in coal contain a significant fraction of the
inventory of a number of metals of interest (e.g. As, Se,
Hg, Pb) to the coal and utilities industries.
Both bulk methods and microanalytical methods
have been used to determine the distribution of metals
in coals [13, 14]. Microanalytical methods are useful
due to the heterogeneous distribution of metals in coals
on a bed scale, on a regional scale, and even on a grain
scale. Micro-PIXE has advantages in coal sulfide
studies because 1) small individual sulfide grains can be
analyzed; and 2) zoning in sulfide grains can be
identified. This allows heterogeneities that may affect
the performance of beneficiation systems to be evaluated as well as facilitates petrogenetic interpretations of
coal seams.
442
techniques, because it is a ‘standardless’ method, and
does not require closely matrix matched standards.
In order to model the behavior of sulfides and their
associated trace elements in geologic environments, it is
vital to measure mineral/mineral, mineral/fluid, fluid/
fluid, and mineral/melt partition coefficients both
experimentally [10] and using natural samples. MicroPIXE is ideally suited to determination of these key
transport parameters. Only when trace-element partition
coefficients are well constrained, will the full potential
of trace-element micronanalysis of sulfides as a
petrogenetic indicator be realized.
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FIGURE 2: As (ppm) vs. Se (ppm) for coal sulfides
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.
Similarly, the Menefee coal has high Mo (and Mo/Se)
and is associated with non-marine sediments. This
apparent dichotomy exists because trace elements in
coal sulfides reflect a wide range of syngenetic and
epigenetic processes: 1) composition of materials in the
original peat swamps; 2) the composition of epigenetic
fluids associated with overlying sediments; 3) the
location on the flow path of epigenetic fluids where the
sulfides precipitate; 4) the fluid-coal ratios during
epigenetic coal sulfide formation; and 5) any sulfide
mobilization processes during coalification. However,
detailed micron-scale trace-element and isotopic studies
of coal sulfides represent a possible method of
interpreting this myriad of competing complex
processes. Such genetic information may also aid in
refining beneficiation methods for coal combustion
systems.
SULFIDES - OTHER ENVIRONMENTS
Sulfides from metamorphic rocks, sulfides from
‘black smokers’ located on oceanic ridges, and sulfides
from meteorites have all been analyzed in the LANL
IBML. In each environment the goal is to understand
the dynamic processes that occurred within a sample or
suite of samples from the variations of the traceelements and trace-element ratios in sulfides.
CONCLUSIONS
Micro-PIXE analyses of sulfides can provide
important information on geologic processes in
environments in which sulfides are present. Sulfides are
a trace phase in a wide-range of geologic environments,
so trace-element investigations of them can augment
petrogenetic studies of a wide range of rocks types.
Two relevant environments in which sulfides are
abundant are in ore deposits and their associated
volcanic or hydrothermal systems and in coal seams.
Micro-PIXE has advantages for trace-element analysis
compared to SIMS and laser-based microanalytical
443