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. 441 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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Stimac, J.A., and Hickmott D. "Ore metal partitioning in intermediate-to-silicic magmas: PIXE results from natural mineral/melt assemblages", in Giant Ore Deposits 2, edited by A.H. Clark, Queens University, Kingston, Ontario, Canada, 1995, pp. 182-220. 12. Lynton, S.J., Candela P.A., and Piccoli P.M. Econ. Geol. 88, 901-915 (1993). 13. Finkelman, R.B. Modes of Occurrence of Trace Elements in Coal. Ph.D. thesis. University of Maryland (1980) 14. Wiese, R.G., Muir I.J., and Fyfe W.S. Int. J. of Coal Geol. 14, 155-174 (1990). 15. Williams, E.G., and Keith M.L. Econ. Geol. 58, 720-729 (1963). 16. Spiker, E.C., Pierce B.S., Bates A.L., and Stanton R.W. Chem. Geol. 114, 115-130 (1994). 17. Fawcett, J.E. In New Mexico Geological Society Guidebook, Southeastern Colorado Plateau, pp. 303-307 (1989) 18. Hickmott, D.D., and Baldridge W.S. Econ. Geol. 90, 246254 (1995) . 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
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