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This article was published in an Elsevier journal. The attached copyis furnished to the author for non-commercial research and
education use, including for instruction at the author’s institution,sharing with colleagues and providing to institution administration.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
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Deposition of a high-sulfidation Au assemblage from a magmatic volatilephase, Volcán Popocatépetl, Mexico
Adrienne C.L. Larocque a,⁎, James A. Stimac b, Claus Siebe c, Karen Greengrass a,Ron Chapman a, Sergio R. Mejia a
a University of Manitoba, Winnipeg, MB, Canada R3T 2N2b Chevron Geothermal Indonesia, Ltd., Sentral Senayan I, Jl. Asia Afrika No. 8, Jakarta 10270 Indonesia
c Instituto de Geofisica, Universidad Nacional Autonoma de Mexico (UNAM), Ciudad Universitaria, C.P. 04510 Coyoacan, Mexico, D.F. Mexico
Accepted 6 September 2007Available online 26 September 2007
The close spatial relationship and similarities in alterationtype between porphyry Cu and high-sulfidation epithermaldeposits have suggested a genetic connection between the twostyles of mineralization (Sillitoe, 1983, 1989; Arribas et al.,1995b). It is widely accepted that magmatic fluids are thesources of ore metals in porphyry environments, and there isisotopic evidence for ascending magmatic volatiles mixing withmeteoric waters to form altering fluids in high-sulfidationepithermal systems (e.g., Giggenbach, 1992; Rye, 1993; Arribas
et al., 1995a). However, the evidence for a magmatic source ofmetals in epithermal systems and the mechanism of theirtransport to higher levels remain subjects of debate (Hedenquistand Lowenstern, 1994; Williams-Jones et al., 2005).
Candela (1989) summarized some of the processes involvedin the separation of a magmatic volatile phase (MVP) duringfinal emplacement of a hydrous felsic magma. During advancedcrystallization of a silicic melt, volatile species such as H2O,CO2, and S gases dissolved in the melt reach saturation, andexsolve to form an MVP. Incompatible elements can be stronglypartitioned into the MVP, although the availability of base andprecious metals for partitioning into the volatile phase dependson the timing of vapor-phase segregation relative to the growthof phenocrysts or separation of immiscible melts that maysequester metals. The MVP is a supercritical fluid which, uponcooling, can separate into a saline aqueous fluid and a vapor.Metals can be variably partitioned between these two phases,
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⁎ Corresponding author. Current address: c/o Chevron-Jakarta, 4800 FournacePlace, Bellaire, TX, USA, 77401.
depending on controls such as their speciation (e.g., Bodnar etal., 1985; Hedenquist and Lowenstern, 1994; Bodnar, 1996).
Studies of fluid inclusions in magmatic–hydrothermalsystems underscore the importance of separation of a vaporphase from a hydrothermal brine as a mechanism for theprecipitation of base and precious metals from aqueous fluids(e.g., So et al., 1995). However, thermodynamic modelling,partitioning experiments, and sampling of volcanic plumesindicate that some elements have a preference for the vaporphase during eruption and shallow degassing of magmas (e.g.,Bernard et al., 1990; Meeker et al., 1991; Symonds and Reed,1993; Williams-Jones et al., 2005). Ascertaining the behavior ofmetals during these processes is critical to understanding theformation of intrusion-related ore deposits. However, therelative importance of each of these processes remains poorlyunderstood, as it is difficult to examine all components of anactive or fossil magmatic–hydrothermal system. In addition, thecontributions of meteoric fluids can be difficult to evaluate, asisotopic evidence of mixing between meteoric and magmaticfluids does not necessarily constrain the sources of metals inthose fluids.
Several different ages of rapidly-cooled tephra from VolcánPopocatépetl, Mexico contain sparse, fine-grained compoundsof base and precious metals on vesicle walls that we interpret ashaving crystallized directly from a trapped and condensedmagmatic volatile phase (MVP). These vesicle coatings areintriguing because they are similar to mineral assemblages thattypically occur in high-sulfidation epithermal Au deposits.High-sulfidation epithermal mineralization (also known as acid-sulfate or alunite–kaolinite-type mineralization) forms inmagmatic–hydrothermal environments adjacent to active vol-canoes; it is characterized by high-sulfidation ore minerals (suchas enargite/luzonite, tennantite and tellurides) and acid-stablealteration minerals (such as alunite, kaolinite and barite). Incontrast, these minerals are rare to absent in low-sulfidationepithermal mineralization characterized by selenide minerals,adularia and calcite (White and Hedenquist, 1995). The vesiclecoatings from Popocatépetl include enargite, tennantite, Autellurides, and Ag sulfides. The presence of these compounds inunaltered, rapidly-cooled tephra provide direct evidence thathigh-sulfidation epithermal-like assemblages can be precipitat-ed from a magmatic volatile phase sourced directly from amagma.
2. Geological and geochemical setting
Volcán Popocatépetl (5452 m) and flanking monogeneticvents are located within the Trans-Mexican Volcanic Belt, aneast–west-trending zone of volcanism extending for over1000 km from the Pacific coast to the Gulf of Mexico. Thebelt consists of cinder cones, maars, shield volcanoes, andstratovolcanoes of Late Tertiary and Quaternary age which arethought to be related to oblique subduction of the Cocos platebeneath North America (Nixon, 1982). The chemical andmineralogical composition of these volcanoes is largely calc-alkaline, although alkaline centers also are present. Popocaté-petl (Popo) and nearby Volcán Iztaccíhuatl (Fig. 1) are located
60 km southeast of Mexico City, and are part of an 80 km-longchain of vents that trends north–south and divides the Valley ofMexico from the Valley of Puebla (Siebe et al., 1995).
The modern stratocone of Popo consists of numerousinterlayered lava and pyroclastic deposits of andesitic to daciticcomposition (Robin, 1984; Robin and Boudal, 1988; Kolisnik,1990; Schaaf et al., 2005; see Fig. 1). Stratigraphic studies ofPopo have shown that the modern cone overlies remnants ofancestral cones that were partially destroyed by gravitationalcollapse (Robin and Boudal, 1987), with the most recentcollapse event occurring about 23,000 yr B.P. (Siebe et al.,1993). During the past 20,000 years, activity has beencharacterized by large plinian eruptions that produced pum-ice-fall and ash-flow deposits. These eruptions produced thinash and pumice layers extending downwind on a regional scale(Macías et al., 1995).
The most comprehensive petrologic studies of the ancestraland modern cones of Popo are those of Boudal (1985), Boudaland Robin (1988), Kolisnik (1990) and Schaaf et al. (2005).Boudal's model of the Popo magmatic system involveddynamic crystal fractionation in crustal magma bodies replen-ished by mafic magma. Kolisnik (1990) studied mineral zoningpatterns in a variety of the younger Popo lavas and pumices andconcluded that zoning patterns in plagioclase and orthopyrox-ene phenocrysts supported a model of repeated mafic recharge,pre-eruptive mixing, and dynamic crystal fractionation indacitic to andesitic magma reservoirs in the middle to uppercrust. Studies of the recent eruptions also provided evidence formagma mixing prior to eruption (Athanasopoulos et al., 1996;Athanasopoulos, 1997; Stimac et al., 1997; Goff et al., 2001).Schaaf et al. (2005) stressed the role of mafic replenishment andmagma mixing in the origin of andesite and dacite at Popo. Goffet al. (2001) and Schaaf et al. (2005) also documentedmineralogical, chemical, and isotopic evidence for assimilationof carbonate basement rocks. Goff et al. (2001) suggested thatshort-term increases in CO2 relative to other gases measured inthe plume in 1998 may have been related to volatilization oflimestone or to preferential degassing of recharging basalticmagma.
The focus of this study – the Pink pumice (1150 yr B.P.) andTutti Frutti pumice (14,000 yr B.P.) – were collected fromplinian units of the Paso de Cortés pyroclastic sequence (Siebeand Macías, 2004), between and west of the edifices of Popoand Izta (Fig. 1). In addition to these older pumices, weexamined samples from the current eruptive activity, includingsamples of the June 30, 1997 ash eruption. The June 30, 1997pumice samples were collected immediately following theeruption, 2.7 km north of the crater.
3. Methods
Major oxides and trace elements in bulk samples of pumicewere determined commercially at Activation Laboratories Ltd.in Ancaster, Ontario, Canada. The method combined fusion–ICP whole–rock analysis (package 4B for major oxides plus Ba,Sr, Y, Zr, Sc, Be, and V) with ICP-MS for trace elements (4B,option 2). Duplicate samples were analyzed by ICP-MS for Ag,
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Bi, Cu, Ni, Pb, and Zn (package 4B, option 1). Determination ofAu concentrations was done by fire assay. Sulfur wasdetermined by Leco inductive combustion.
Polished thin sections were examined petrographically andwith the scanning-electron microscope (SEM). We selectedsections containing visible sulfides and other metal-bearingcompounds of interest for more detailed study by SEM and forelectron-microprobe (EMP) analysis. SEM analysis of polishedthin sections and freshly broken pumice chips were carried outusing a Cambridge Instruments Stereoscan 120 SEM equippedwith a Kevex 7000 EDS spectrometer. Backscattered-electron(BSE) and secondary-electron (SE) images were processed andstored using a Kontron IBAS image-analysis system. EMPanalysis was carried out using a Cameca SX-50 EMP equippedwith three wavelength-dispersive X-ray (WDX) spectrometers,and one EDX spectrometer. The samples were coated using acarbon coater used exclusively for carbon (not gold) coating, toprevent contamination by precious metals.
The identification of transition–metal-bearing phases involcanic rocks is complicated by the fact that most of the phasesare very fine-grained (generally b10 μm diameter) and scarce(generally b0.01 vol.% abundance). These characteristics makequantitative microanalysis and X-ray diffraction analysisdifficult to impossible because the odds of encountering grainssufficiently large in the plane of a polished thin section areextremely low. A further complication is the inability to detect
light elements using energy-dispersive X-ray spectrometry(EDS), thereby hampering identification of minerals such asoxides, hydroxides, and carbonates. In addition, syn- and post-eruptive phases located in vesicles tend to be delicate (and insome cases, highly water-soluble), making it unlikely that theywill be observed in thin sections unless special methods areused to preserve them. To overcome the latter problem, we alsoexamined freshly broken pumice fragments mounted on stubsusing SEM.
4. Descriptions of pumice samples
4.1. Pre-eruptive mineralogy
Pre-eruptive base–metal-bearing phases are present in allexamined pumice samples from Popo (Larocque et al., 1998).They include: 1) oxide phenocrysts, 2) sulfide globules in glass,and 3) oxide and sulfide inclusions in phenocrysts. The oxidesconsist of magnetite, ilmenite, and chromite (as inclusions inolivine xenocrysts). The sulfides, which commonly are globularor spherical, consist of both pyrrhotite (crystallization productof monosulfide solid solution [MSS]) and rare chalcopyrite(crystallization product of intermediate solid solution [ISS]). Inpumice samples, elongate globules of sulfide (mainly pyrrho-tite) commonly are associated with spongy, Fe-oxide globulesinterpreted as products of degassing of magmatic sulfides
Fig. 1. Map showing local geology and locations of samples studied. Volcán Popocatépetl is located approximately 60 km southeast of Mexico City, in the Trans-Mexican Volcanic Belt.
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(Larocque et al., 2000). Magmatic CuFe sulfide is morecommon in lavas than in tephra (Larocque et al., 1998, Table 2).
The Tutti Frutti pumice is a plinian fall deposit containingmoderately-dense, poorly-vesiculated pumice fragments ofandesite with phenocrysts of hornblende, pyroxene, and FeTioxides. Plagioclase is rarely present, and is xenocrystic.Magnetite and ilmenite are common as phenocrysts in thegroundmass. Abundant sulfide globules up to 20 μm indiameter occur in pyroxene phenocrysts and glass. Theinclusions are elongate and globular, or subhedral. Sulfide
globules exhibit evidence of oxidation along grain boundaries,especially where the enclosing phase is fractured.
The Pink pumice contains phenocrysts of plagioclase,pyroxene, and FeTi oxides. It is slightly less dense (morevesiculated) than the Tutti Frutti pumice, and slightly moresilicic in composition (see Table 1). It also contains minorolivine phenocrysts with reaction rims of pyroxene andvermicular magnetite, the latter resulting from destabilizationof olivine during magma mixing (see Larocque et al., 1998, Fig.3B). Sulfide globules and euhedral chromite occur as inclusionsin olivine phenocrysts. Magnetite and ilmenite are common asphenocrysts in the groundmass. Magmatic FeCu sulfide is rare,and occurs in olivine phenocrysts.
The phenocryst assemblage in Popo '97 consists dominantlyof plagioclase, with minor clinopyroxene and orthopyroxene.Amphibole is present, as is forsteritic olivine. FeTi-oxidephenocrysts occur as inclusions in pyroxene as well as in thegroundmass glass. Sulfide globules are present in the glass, andare finer-grained than FeTi oxides.
4.2. Mineralogy and chemistry of the volatile-phase assemblage
Vesicles in pumice from Popocatépetl contain phases thatlikely were precipitated during eruption. Little post-depositionalvapor-phase activity is expected since these are b2 m-thickfallout pumice deposits that probably cooled to near ambienttemperatures prior to or shortly after emplacement. Some of thevesicle fillings (some oxides and sulfides) are sufficientlycoarse-grained to be visible under the microscope. Others aremuch finer-grained and were identified by SEM analysis ofpolished thin sections and broken fragments. The pumicesamples contain a varied and complex assemblage of phasesconsisting of oxides, sulfides, sulfates, tellurides, halides andclay minerals (Table 2). In the following description of vapor-phase assemblages, assigned mineral names are based on themajor-element peaks in EDX spectra. Wherever possible, the
Table 1Major- and trace-element compositions of pumice samples a
identification was confirmed by EMP analysis (Table 3).Because of the small size and presence within vesicles ofmany phases, totals for EMP analyzes typically are low;therefore, we used calculated atomic proportions rather thanweight percent to identify some phases. Because of the fine-grained nature of the vesicle fillings, we have not calculatedmodal abundances; thus, the categories in Table 2 arequalitative.
The Pink pumice contains the greatest abundance and varietyof phases (Fig. 2A). The most abundant and coarse-grained ofthe vesicle fillings consist of pyrite and chalcopyrite, and are aslarge as 50 μm in diameter (Fig. 2A; see also Larocque et al.,1998, Fig. 3J). Tennantite is common and ranges up to 50 μm indiameter. Enargite is common, and ranges up to 20 μm indiameter (Fig. 2B). Several examples of calaverite (AuTe2)intergrown with enargite [Cu3(As,Sb)S4] have been observed(Fig. 2C). Low-Fe sphalerite is common, and typically rangesfrom 5 to 20 μm in size (Fig. 2A). Galena is less common, and is10 μm or less in diameter. Magnetite and chromite are present,and typically range from 5 to 10 μm in diameter (Fig. 2D). Raregrains of barite range up to 10 μm in diameter. Alunite andpossible kaolinite were observed in trace amounts. Other phasesthat are present in the Pink pumice but that have not beenobserved in the Tutti Frutti and Popo '97 pumice samplesinclude stannite (Cu2FeSnS4), acanthite (Ag2S; Fig. 2E), AgCusulfide (stromeyerite?; see Larocque et al., 1998, Fig. 3K),AgFe sulfide (argentopyrite or lenaite?), Ag bromide, Agchloride, and AuCu telluride (kostovite?). These phasestypically are quite fine-grained, with a maximum diameter of10 μm.
The volatile-phase assemblage in the Tutti Frutti pumice issimpler, but coarser-grained, than in the Pink pumice, andconsists of sulfides, sulfates, and oxides. There are fewer grainsof magnetite and chromite (Fig. 2F), but sulfides are common.Chalcopyrite is more abundant in the Tutti Frutti pumice than inother samples, and ranges from 30 to 60 μm in diameter. Pyriteis ubiquitous, and ranges up to 60 μm in diameter (Fig. 2G).
Low-Fe sphalerite also is common. Minor phases include barite(Fig. 2H), enargite, and tennantite (Fig. 2I). Trace grains ofgalena were observed (Fig. 2J).
The Popo '97 pumice has the simplest assemblage ofminerals in vesicles. Vesicle fillings are abundant, althoughfewer than in other samples, and those present are finer-grained(usually less than 20 μm in diameter). The most abundantphases are sulfides; FeTi oxides are intermediate in abundancerelative to the Pink and Tutti Frutti pumices. As with the othersamples, chalcopyrite, pyrite, low-Fe sphalerite (Fig. 2K) andgalena (Fig. 2L) are present; however, sphalerite and galena aremore abundant than chalcopyrite in Popo '97 pumice.Aggregates of kaolinite up to 30 μm in diameter are present.Several small grains of Bi chloride (b5 μm) were observed.Barite ranges up to 10 μm in diameter. Small cubes of halite(b5 μm) were observed.
Simple salts were not observed in the older pumice samples.This may reflect an observational bias, as compounds contain-ing transition metals tend to appear bright in backscattered-electron SEM images, whereas salts would appear darker andless obvious. Alternatively, simple salts may have been leachedby groundwater in the older samples, whereas they arepreserved in the very fresh Popo '97 pumice. Larocque et al.(1998) documented minor evidence of post-depositionaloxidation of magmatic sulfides in the Tutti Frutti pumice.
5. Discussion
5.1. Metal-residence sites in volcanic rocks
In volcanic rocks, base and precious metals may be hosted bypre-eruptive, syn-eruptive, or post-eruptive mineral phases, aswell as glass (Larocque et al., 1998). Pre-eruptive minerals formduring fractional crystallization of parental magma, and occuras phenocrysts, microphenocrysts, and inclusions in pheno-crysts. Segregation of immiscible Fe–S(–O) melts also givesrise to pre-eruptive phases hosted by phenocrysts and matrix
Table 3Atomic proportions of elements in vesicle fillings a
tennantite pyrite sphalerite enargite calaverite intergrown with enargite galena acanthite tennantite pyrite chalcopyrite sphalerite enargitea Determined by electron microprobe.
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glass. The presence of CuFe-sulfide globules in maficphenocrysts and andesite matrix glass in pumice samplesfrom Popo indicates that source magmas typically containedsufficient sulfur to become saturated with an immiscible Fe–S(–O) melt at an early stage of the crystallization history(Larocque et al., 2000). The timing of sulfide saturation relativeto volatile saturation and eruption is important, because muchhigher concentrations of volatile metals such as Cu are likely toremain in the melt in the absence of sulfide melt separation or ifimmiscible melts are destabilized and become resorbed by themelt (Larocque et al., 2000). Free concentrations of these metalsin the silicate magma would allow significant partitioning of themetals into a volatile phase prior to and during eruption.
Syn-eruptive phases form by precipitation of additional mi-nerals from a cooling magma or from precipitation of volatilemetal-rich phases on tephra during eruption. They also mayoccur due to gas expansion and adiabatic cooling during vesi-culation. Post-eruptive phases occur in vesicles, along fractures,and in segregation vesicles (Moore and Calk, 1971; Yeats andMathez, 1976; Larocque et al., 1997, 1998). Crystallization of
post-eruptive metal compounds related to emplacement oferuptive units results from condensation of gases that are trappedin vesicles or generated during devitrification (Stimac et al.,1996). It can be difficult to distinguish syn-eruptive phases frompost-eruptive phases, as metal compounds in vesicles may haveprecipitated at any time between the first formation of vesicles inthe melt and final emplacement, depending on the temperature-range of stability of the phase. Also, alteration of pre-existingphases may be caused by percolating meteoric fluids (Larocqueet al., 1998). In the samples that we studied from Popocatépetl,there was no mechanism to form post-eruptive phases related todevitrification because the rocks were rapidly quenched.Therefore, we are confident that the compounds in vesicles inour samples formed by crystallization directly from a magmaticvolatile phase during or soon after eruption.
It is likely that such phases are widespread in pumices, butgenerally go unreported due to their small size and location onvesicle walls. Vesicles “spotted” or “decorated” with spherulesof Fe(NiCu) sulfide have been reported previously from studiesof seafloor basalt (Moore and Calk, 1971; Yeats and Mathez,
Fig. 2. A–M. Scanning-electron microscope (SEM) images of metal compounds in tephra. A to L) Backscattered-electron (BSE) and scanning-electron (SE) images ofvesicle fillings from Popo. A) BSE image of Pink pumice showing abundant white grains in vesicles (grey shades represent glass and phenocrysts, black shadesrepresent empty vesicles). This field of view shows pyrite, chalcopyrite, sphalerite, magnetite, chromite and enargite. Scale bar is 200 μm. B) BSE image of enargite inPink pumice. Scale bar is 20 μm. C) BSE image of calaverite (Clv) rimmed by enargite (En) in Pink pumice. Scale bar is 10 μm. D) BSE image of octahedron ofmagnetite in Pink pumice. Scale bar is 20 μm. Henriquez and Martin (1978) proposed that euhedral magnetite in vesicles in unusual flows in El Laco, Chile,crystallized from a gas phase. E) SE image of acanthite in Pink pumice. Scale bar is 10 μm. F) BSE image of chromite in Tutti Frutti pumice. Scale bar is 20 μm. G) SEimage of pyrite in Tutti Frutti pumice. Scale bar is 50 μm. H) BSE image of barite in Tutti Frutti pumice. Scale bar is 10 μm. I) BSE image of tennantite in Tutti Fruttipumice. Scale bar is 20 μm. J) BSE image of galena in Tutti Frutti pumice. Scale bar is 10 μm. K) BSE image of sphalerite in Popo '97 pumice. Scale bar is 50 μm. L)BSE image of galena in Popo '97 pumice. Scale bar is 100 μm. M) BSE image of “degassed”magmatic sulfide (Larocque et al., 2000) in Tutti Frutti pumice. Scale baris 200 μm.
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1976). Strand et al. (2002) reported Cu–Sn–Co alloys and FeCuoxides in vesicles from quenched mafic lavas and lava lakesamples. Yang and Scott (2002) documented vesicle fillings inmelt inclusions and matrix glass in submarine volcanic rocks;the vesicle fillings contained Ni, Cu, Zn and Fe in mafic rocks,Cu, Zn and Fe in intermediate rocks, and Fe, Zn and possibly Pbin felsic rocks. The compounds in vesicles in the samples fromPopocatépetl are hosted by dacitic and andesitic rocks (Schaaf etal., 2005); the presence of elements (e.g., Ni and Cr) commonlyenriched associated with more mafic rocks is consistent withmafic inputs that likely triggered eruptions (Athanasopouloset al., 1996; Athanasopoulos, 1997; Stimac et al., 1997; Goffet al., 2001; Schaaf et al., 2005).
5.2. Metal speciation in magmatic fluids
Concentrations of Au in condensates of high-temperaturefumaroles are typically in the range of 0.01 to 10.0 μg/kg (seesummary by Hedenquist, 1995), and large fluxes of preciousmetals can be attained during periods of vigorous degassing(e.g., Goff et al., 1994). Gold has been observed occurring innatural mineral encrustations around high-temperature fumar-oles (Kavalieris, 1994), as particles in aerosols (Meeker et al.,1991), and as crystals in silica tubes inserted into high-temperature fumaroles (Taran et al., 2000). Kavalieris (1994)documented Au and Ag concentrations of 100 and 60 ppm,respectively, in high-temperature fumarolic encrustations at
Fig. 2 (continued).
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Merapi Volcano, Java, that were deposited during a period ofnew magma intrusion. They noted a close similarity of the metalenrichment in the Merapi samples and trace-element enrich-ments documented in high-sulfidation ore deposits (White andHedenquist, 1995). Our observations of trace minerals invesicles in fresh pumice samples are consistent with descrip-tions of high-temperature fumarolic sublimates (e.g., Fulignatiand Sbrana, 1998; Kavalieris, 1994) and their trace-metalenrichments. The advantage of our study is that the role ofmeteoric fluids which may have mixed with magmatic fluidsprior to exhalation (in the case of fumarolic sublimates) neednot be considered, confirming that the metals and S in vesicleshad a direct magmatic source.
Taran et al. (2000) concluded that native gold precipitated at550–600 °C in silica tube experiments fed by high-temperaturefumarolic gas from Colima Volcano, Mexico, because the gascontained a large amount of oxygen from entrained air. Theyproposed that in more reduced volcanic gases uncontaminatedby air, stable complexes such as AuH(g) and AuS(g) are morelikely to transport gold. In contrast, Symonds and Reed (1993)proposed that Ag, Cu, Fe and Zn would be transporteddominantly as simple chlorides under magmatic conditions,and that precipitation of sulfides resulted from reactionsbetween metal–chloride complexes and H2S gas. Theirconclusions were based on multi-component chemical equilib-ria calculated for volcanic gases collected from Mount St.Helens (their model also indicates that Au is transported as asulfide complex, but the model lacks data for Au–chloridecomplexes). Popo is similar to Mount St. Helens in that both aresubduction-related stratovolcanoes constructed mainly of calc-alkaline andesite and dacite. Moreover, the chemistry, pheno-cryst assemblage, eruption temperature, and fO2 of the 1996–1998 eruptions at Popo are similar to those of the 1981 MountSt. Helens dacite-dome sample used in thermodynamicmodelling by Symonds and Reed (1993), although thetemperatures of recent Popo eruptions were higher (Athanaso-poulos, 1997; Goff et al., 2001).
Electron-microprobe data for silicate melt inclusions inolivine from the 1996 dacite dome and primitive lavas eruptednearby in the Valley of Mexico (Schaaf et al., 2005) indicate thatprimitive mafic magmas recharging Popo contained up to2600 ppm dissolved S and 1800 ppm Cl during olivinecrystallization. Cervantes and Wallace (2003) found similar Sconcentrations in melt inclusions in olivine from primitive lavasof the Valley of Mexico. They documented S and Clconcentrations up to 6000 ppm and 1900 ppm, respectively,in melt inclusions trapped at relatively high pressure (althoughthe average S and Cl contents of their samples are similar tosamples from Popo). The high concentrations of both S and Clin melt inclusions indicate that both elements would have beenavailable to form complexes with base and precious metals inmagmatic gases at Popocatépetl. Obenholzner et al. (2003)documented Mo, Cu, Zn and Pb occurring with S and Fe, Cu,Na, Mg, and Ca with Cl in poorly-crystallized particlescollected in the plume of Popocatépetl during a period ofpassive degassing in March 1997; the samples were analyzed bySEM and field-emission gun SEM.
5.3. Implications for metal transport in magmatic–hydrothermal systems
The presence of metal sulfides, oxides, tellurides, and halidesin vesicles in pumice from Popocatépetl demonstrates that metalpartitioning into a magmatic volatile phase was importantduring eruptive events. The vesicle coatings may have sublimeddirectly from a supercritical fluid (Hedenquist, 1995); alterna-tively, the MVP may have exsolved to form a metal-poor vaporand a metal-rich, hypersaline brine (e.g., Bodnar et al., 1985;Hedenquist and Lowenstern, 1994; Shinohara, 1994), with theepithermal assemblage precipitating from the latter. Based onfluid-inclusion analysis, Bodnar (1996) proposed that Fe, Znand Pb in porphyry systems tend to be partitioned into the liquidphase and Cu (and likely Au and Ag) into the vapor phase,forming stable complexes with sulfur.
The MVP assemblage in the Pink pumice is very similar toassemblages found in high-sulfidation epithermal mineraliza-tion (e.g., Arribas, 1995; White and Hedenquist, 1995). TheTutti Frutti pumice contains a similar assemblage which,although lacking in Au minerals, does contain tennantite andenargite. However, both pumice samples contain significantamounts of Zn and Pb sulfides, which typically are minorminerals in high-sulfidation epithermal deposits. Furthermore,the vesicle assemblage in Popo '97 pumice is dominated bybase–metal sulfides, with no observed Au- or Ag-bearingphases. These observations are not entirely unexpected. If thevesicles trapped an unexsolved volatile phase, it would containZn and Pb as well as Cu, Au and Ag. As long as the vesiclesremained closed systems, compounds containing all of thesemetals would be precipitated. The importance of base–metalsulfides, the low abundance (relative to the older pumices) ofchalcopyrite, and the absence of Au- and Ag-bearing phases inthe Popo '97 sample suggest that the vesicles in that pumice didnot remain closed. The vapor (enriched in Cu, Au and Ag)escaped, and Zn and Pb sulfides were precipitated from anaqueous solution that remained.
Other factors that may have caused variations in theassemblages of vesicle fillings between the pumice samplesinclude variations in the timing and degree of separation ofimmiscible sulfide or oxide liquids (e.g., Larocque et al.,2000), or different trace-metal and volatile compositions inthe bulk magmas. The latter depends on many factors,including the timing of S-rich mafic recharge and thedegree of contamination by crustal rocks (Goff et al., 2001;Schaaf et al., 2005). Other factors contributing to variationsin metal concentration include variations in degassinghistory of the magma, before or during eruption, and thedynamics of the eruptive plume. Plume dynamics controlthe location of the fragmentation surface, the extent offragmentation, and the resulting particle size distribution.Witham et al. (2005) presented a more detailed discussionof the importance of these factors.
In addition to variations in metal speciation between thedifferent pumice samples, we observed variations in speciationwithin individual samples. In the Pink pumice for example,Ag occurs in sulfide, bromide, chloride and telluride minerals,
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and Cu is hosted by sulfides, sulfosalts and tellurides.The variations in metal speciation within individual samplesmay reflect inhomogeneity in the distribution of metals andcomplex species between vesicles, which acted as closedsystems.
Regardless of the specific transporting complexes andprecipitation mechanisms involved, it is clear that sourcecomponents of the vesicle fillings in pumice from Popo had amagmatic origin. The partitioning of metals, S and Cl intomagmatic volatile phases was important, as well as efficient.The occurrence of calaverite in the Pink pumice is significantbecause the bulk sample is not highly anomalous with respect toAu content (Table 1). While the calculation is poorlyconstrained because of the tendency of Au to exhibit thenugget effect, a spherical grain of calaverite (10 μm in diameter)in a 1-g sample of andesite would yield a concentration of Au of2.13 ppb. For a bulk Au content of 3 ppb, this implies that atleast 71% of Au originally present in the magma would havebeen stripped from the melt and partitioned into the MVP,subsequently precipitating as calaverite. This value does notaccount for additional Au which may have escaped to theatmosphere.
Based mainly on experimental evidence, Williams-Joneset al. (2005) proposed that vapor is the principal agent oftransport of Cu, Ag and Au in some porphyry–copper systemsand that high-sulfidation epithermal mineralization also isdeposited from vapor. McPhail (1995) predicted that vapor-phase transport of Te may be important in the formationof telluride-bearing epithermal mineralization. Our resultssupport their conclusions and provide direct evidence that ahigh-sulfidation epithermal assemblage can originate directlyfrom a melt and precipitate directly from a magmatic volatilephase.
6. Summary
The presence of metal-bearing phases in vesicles in pumicefrom Popocatépetl demonstrates that metal partitioning intomagmatic volatile phases (MVP) during eruptive events wasimportant and efficient. Furthermore, the MVP assemblage inthe Pink pumice is very similar to assemblages found in high-sulfidation epithermal mineralization, consistent with a mag-matic source of metals and S in that type of deposit. Largevariations in the trace-mineral assemblage present in vesicles offresh pumice are related to a number of factors including:partitioning of metals between vapor and brine in vesicles thatbehaved as open or closed systems; changes in magma trace-metal and volatile compositions as a function of fractionation;and recharge and degassing history.
Acknowledgements
This research was supported by grants from the NaturalScience and Engineering Research Council of Canada(NSERC) awarded to Larocque and Stimac. Sample collectionby Siebe was funded by CONACYT 50677-F and DGAPA-UNAM-IN-101006-3. We are grateful to Jake Lowenstern and
Nigel Cook for thoughtful and constructive reviews thatimproved the manuscript.
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