Trace Element Mineral Chemistry Research Articles

From melt- to crystal-rich magmatic systems during rift localization: Insights from mineral chemistry in Central Afar (Ethiopia)

Magmatism plays a key role in accommodating and localizing extension during continental breakup. However, how the crustal magmatic systems evolve at the continental-ocean transition is poorly understood. We address these questions by studying the evolution of the magmatic system in the rift of Central Afar (Ethiopia), currently marking the transition from continental rifting to oceanic spreading. We focus on the voluminous and widespread Upper Stratoid Series (2.6–1.1 Ma) and the following Central Afar Gulf Series (1.1–0.6 Ma), the latter corresponding to localization of volcanism in narrow magmatic segments. We carried out the first systematic study of major and trace element mineral chemistry for these two Series and integrated it with geothermobarometry estimates and geochemical modeling, to reconstruct the evolution of the magmatic system architecture during rift localization. The Upper Stratoid magmas evolved by fractional crystallization in a melt-rich, moderately zoned, middle-lower crustal (10–18 km) magmatic system, from where they rose directly to the surface. Polybaric plagioclase convection and dissolution of a plagioclase-rich crystal mush is recorded in the phenocryst texture and chemistry. The Central Afar Gulf magmas evolved at similar depth in a more complex and dynamic storage system, with magma rising and mixing through multiple, relatively small, crystal-rich and interconnected reservoirs. Our study documents the transition during the continental breakup, from an overall stable and melt-rich magmatic system feeding the voluminous and homogeneous Upper Stratoid eruptions to a more dynamic, interconnected and crystal-rich situation feeding small-volume eruption while the rift localizes.

Recognizing Tourmaline in Mineralized Porphyry Cu Systems: Textures and Major-Element Chemistry

AbstractSamples of tourmaline supergroup minerals from seven mineralized porphyry systems (Cu, ±Au, ±Mo), including Casino (Yukon Territory, Canada), Coxheath (Nova Scotia, Canada), Donoso breccia-Los Bronces (Chile), Highland Valley Copper (British Columbia, Canada), New Afton (British Columbia, Canada), Schaft Creek (British Columbia, Canada), and Woodjam (British Columbia, Canada), were examined at a variety of scales to evaluate their relationships with mineralization. Data from paragenetic observations show that tourmaline supergroup minerals are generally early hydrothermal minerals that predate both mineralization and alteration (e.g., overgrown and crosscut by). In general, tourmaline supergroup minerals occur as sub- to euhedral crystals that are black in hand sample and can be found in a variety of mineralized settings (including breccias, veins, and disseminations) and alteration assemblages (including potassic, sodic-calcic, phyllic, propylitic, and argillic). As tourmaline supergroup minerals are physically and chemically resilient and occur throughout a given porphyry system, they are comprehensive recorders of the type and extent of various geochemical processes that exist during the complex genesis of these systems. Data from BSE imaging shows two primary zonation types: concentric and sector. These are interpreted to reflect conditions of rapid crystallization and disequilibrium. Results from SEM-EDS analyses show that most tourmaline supergroup minerals are dravite (∼80% of grains), with the remainder being primarily classified as schorl. Porphyry tourmaline supergroup minerals exhibit remarkably consistent ∼2.0 apfu Mg values (range: 0.69–2.89), with the majority of tourmaline supergroup minerals plotting along the oxy-dravite–povondraite trend, reflecting the predominance of the Al3+ ↔ Fe3+ substitution at constant Mg values. This pattern starts from the povondraite side (reflecting the oxidizing nature of early porphyry mineralizing fluids) and trends toward oxy-dravite as a porphyry system evolves, a feature that can, in turn, be interpreted to reflect relative emplacement depths. In mineralized porphyry systems, tourmaline supergroup minerals exhibit remarkably similar physical and chemical characteristics among the systems examined, suggesting that the source and geological processes must be extraordinarily similar. Unfortunately, these characteristics are not unique to porphyry systems and such observations should be integrated with additional data, such as trace element mineral chemistry, to effectively discriminate tourmaline supergroup minerals that have formed in porphyry systems.

The petrogenesis of chemically zoned, phonolitic, Plinian and sub-Plinian eruptions of Somma-Vesuvius, Italy: Role of accessory phase removal, independently filled magma reservoirs with time, and transition from slightly to highly silica undersaturated magmatic series in an ultrapotassic stratovolcano

A mineralogical, major, LA-ICP-MS trace element mineral chemistry and bulk-rock geochemical study of juvenile samples of the Mercato, Avellino, Pompeii and Pollena eruptions, collected in stratigraphically and volcanologically well-characterized sections of the Somma-Vesuvius stratovolcano (Roman Magmatic Province), along with reference data on the 1944 CE and the Pomici di Base eruptions, highlights the compositional variability of bulk-rock and glass from leucite phonotephrite to garnet-bearing phonolite . The latter products have extreme fractionation of trace elements (e.g., La/Yb n = 126, Zr/Y = 89, Zr/Hf =78, Nb/Ta = 40; Th/U = 2.3), very low Sc, V, Y, HREE and very high As, Tl, Cs, Pb, Th and U. The Pomici di Base products, older than the eruptions described above, range from leucite-bearing shoshonites to trachytes , are devoid of garnet and belong to an independent liquid-line-of-descent, having also different fractionation between trace elements (e.g., La/Yb n = 15; Zr/Y = 12.4, Zr/Hf = 50, Nb/Ta = 15.6; Th/U = 3 in the Pomici di Base trachytes). A marked chemical variability of the observed phases is found. The geochemistry of garnet, amphibole , clinopyroxene and other phases shows wide variations of concentrations and elemental ratios (e.g., La/Yb n up to 520 in the sadanagaite coexisting with garnet). The magmatic evolution is dominated by low-pressure, oxidized, nearly closed-system fractional crystallization of clinopyroxene, plagioclase , leucite, ±magnetite, ±biotite, ±olivine and apatite in the transition tephrite-phonotephrite, and of potassic sanidine (±hyalophane), Fe-clinopyroxene, melanite garnet±Fe-amphibole in the transition tephriphonolite-phonolite. Mineralogic and geochemical evidence and modelling points out the existence of independent, zoned magma batches throughout the activity of the stratovolcano, which possibly started to crystallize at similar depths. The Somma-Vesuvius magmas thus evolved in shallow independent reservoirs with respect to those of the neighbouring volcanic complexes (Campi Flegrei, Ischia) as shown, for instance, by the contrasting compositional trends of clinopyroxene and amphibole, and have very limited evidence of crustal contamination (and/or carbonate assimilation). • Pollena, Pompeii, Avellino and Mercato are zoned eruptions with phonolites • The magmas which formed the various eruptions evolved independently • Different garnets dominate HREE, Y and HFSE; amphibole is strongly HREE-poor • The phonolites and their mineral compositions have no tendency to become peralkaline • F, Cl and H 2 O dominate the volatile budget

Vectors to ore in replacive volcanogenic massive sulphide deposits of the northern Iberian Pyrite Belt: Major and trace element mineral chemistry

As part of a broader characterization and study of vectors to ore in VMS systems of the Iberian Pyrite Belt (IPB, Spain), we have investigated in detail mineral chemistry vectors in a representative replacive deposit hosted in felsic volcanic rocks. At the Aguas Teñidas deposit (northern IPB) the hydrothermal system affected rocks of an originally homogeneous composition extending hundreds of metres beyond its footprint. The major and trace element chemistry of white micas, chlorite and carbonates have been analysed by electron microprobe (EMP) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to characterize geochemical vectors across the extent of the hydrothermal system at Aguas Teñidas.White micas are dominated by muscovite in both regionally altered lithologies and within the hydrothermal system, with coexisting paragonite occurring in a halo beyond the first disseminated pyrite surrounding the stockwork and in the proximal hanging wall. Systematic variations have been observed in FeO/(FeO + MgO) and Na2O/(Na2O + K2O) across the alteration zone.Chlorite is predominantly clinochlore in composition, with chamosite restricted to the centre of the hydrothermal system. In regionally altered lithologied it is characterized by a constantly low Al and AlIV at variable Fe/(Fe + Mg); in rocks influenced by the hydrothermal system it presents higher AlIV and total Al, and a progressive increase in Al, AlIV and Fe/(Fe + Mg) towards its core.Regional carbonates consist of calcite, with additional dolomite forming in proximal rocks. Iron content in dolomite increases towards the centre of the hydrothermal system, with ankeritic compositions becoming dominant.Systematic variations in Ba, Cs, Li, Pb, Rb, Sn, Sr, Tl and Zn have been observed in white micas consistent with a proximal-distal transition (vector). Trends in chlorite have been less well characterized due to analytical limitations, although variations have been observed in the contents of As, Co, Li and Zn.A schematic model has been proposed which accounts for the observed mineralogical and mineral chemistry trends across the hydrothermal footprint of the Aguas Teñidas VMS deposit. Major element signatures in white micas and chlorite are interpreted as having been controlled by the upwards and outwards flow of hot reducing Fe-rich hydrothermal fluids producing a coupled increase in temperature and fluid Fe/(Fe + Mg), and a decrease in fO2, with increasing fluid/rock ratios towards the centre of the stockwork.The data presented herein are not only applicable to VMS exploration in the IPB, but on a broader scale improve our general understanding of vectors to ore in VMS deposits in general.

Multi-stage precipitation and redistribution of gold, and its collection by lead-bismuth and lead immiscible liquids in a reduced-intrusion related gold system (RIRGS); Dublin Gulch, western Canada

Dublin Gulch reduced intrusion-related gold system (RIRGS), located in the Selwyn Basin area of western Canada, represents one of the best examples of RIRGS mineralization globally and hence can be studied to unravel genesis and evolution of these types of deposits. Based on textural relationships, mineralogy, and trace element mineral chemistry, three auriferous vein stages were identified. The paragenetic sequence for the auriferous vein stages are: 1) Eagle Style (ES), quartz-albite, low sulfide and sulfosalt content (<5% vol.), As-Fe-Mo-W-Pb-Bi-Au-Ag veins; 2) Potato Hills Style-1 (PHS-1), quartz, high sulfide and sulfosalt content (>30% vol.), As-Fe-W-Pb-Bi-Au-Ag veins; and, 3) Potato Hills Style-2 (PHS-2), high sulfide and sulfosalt content (>30% vol.), Fe-Pb-Sb-Zn-Cu-Au-Ag veins. In the ES and PHS-1 veins, Au is present both as native gold (Au, Ag) and as invisible gold in arsenopyrite, whilst in the PHS-2 veins, Au is present as invisible gold in pyrite. Native gold micrograins (individual grains, 1–100’s µm in size) are observed associated with Pb minerals [in anhedral-to-globular cosalite (Pb2Bi2S5) in ES veins, or galena (PbS) in the PHS-1 veins]. Native gold is also observed as micrograins along arsenopyrite margins and in quartz fractures. We suggest a variation on the hydrothermal Bi melt collector model to explain the Au-Pb ± Bi association. The Au, Ag, Pb, and Bi are interpreted to have been locally remobilized from arsenopyrite, which shows textures and trace element distribution patterns consistent with fluid-and-deformation assisted recrystallization. We suggest Pb and Bi were mobilized either as immiscible nanodroplets that coalesced to form larger Pb ± Bi liquid accumulations or into the hydrothermal fluid and subsequently exsolved to form immiscible Pb ± Bi liquids. We propose that remobilized Au and Ag were collected by these Pb+/-Bi immiscible liquids. Subsequent retrograde alteration (sulfidation) is interpreted to have converted the Au-Ag-Pb ± Bi alloys to native gold and cosalite (ES), and native gold and galena mineral assemblages (PHS-1). The similarity of the Au/Ag ratios in native gold and arsenopyrite supports a local source for the native goldmicrograins. Temperatures required to attain liquid Pb0.5Bi0.5 (>145.2 °C) and Pb (>327.5 °C), are consistent with arsenopyrite geothermometry (ES 345–405 °C; PHS-1 ≈ 380 °C). These suggested new variations (Pb-Bi and Pb) on the hydrothermal melt (Bi) collector model are important, given the common association of native gold with Pb ± Bi sulfosalts in many gold deposits, and the relatively low temperatures required to maintain these liquids and collect Au from the hydrothermal fluid.

Melt-rock interaction and fractional crystallization in the Moho transition Zone: Evidence from the cretaceous Naga Hills Ophiolite, North-East India

The Moho Transition Zone (MTZ) within the suboceanic lithosphere records an episode of multiple magmatic exchanges between the underlying mantle and the overlying oceanic crust. This study investigated a fossilized MTZ from the Naga Hills Ophiolite (NHO) by characterizing its lithology and describing the petrogenesis of various rock types. Like the other well-studied MTZs around the world, the NHO MTZ has an association of dunite, wehrlite, clinopyroxenite and gabbroic rocks. The dunite formed by crystal fractionation from an olivine-normative basaltic melt that successively crystallized olivine-gabbro, gabbro and oxide-gabbro. Our study of the wehrlite provides evidence of melt-mantle interactions that lead to the formation of replacive dunite, later impregnated with clinopyroxene saturated melt. The melt percolated through the replacive dunite and re-equilibrated with the olivine to some extent. The clinopyroxenite formed simultaneously with the wehrlite as a crystal mush in pockets from clinopyroxene saturated melt. In summary, the major and trace element mineral chemistry of the NHO MTZ rocks documents their evolution through melt-rock interaction and fractional crystallization. Clinopyroxene played an important role in the evolution of NHO MTZ, particularly the formation of wehrlite and clinopyroxenite. There is no direct evidence for the involvement of water in the formation of the MTZ rocks. However, the presence of primary hydrous phases in the evolved rock types and the signatures of early crystallization of clinopyroxene prior to plagioclase in the initial cumulates, discernible from a positive Sr anomaly, suggest the hydrous nature of the parent melt. Therefore, the NHO possibly represents a fragment of oceanic lithosphere that had its origin above a subduction zone and was later incorporated within a mountain belt during terminal closure of the Neo-Tethys Ocean.

Water in orthopyroxene from abyssal spinel peridotites of the East Pacific Rise (ODP Leg 147: Hess Deep)

Abyssal spinel peridotites from Hess Deep, East Pacific Rise (ODP Leg 147) were investigated concerning their major, minor, and trace element mineral chemistry and the incorporation of structural water in orthopyroxene.The rocks are partially serpentinized harzburgites containing primary minerals of olivine, orthopyroxene, clinopyroxene, and spinel. Orthopyroxene is enstatitic with Mg# (Mg/(Mg+Fe)) between 0.90 and 0.92 and Al2O3 from 0.5 to 2.9wt.%. The residual harzburgite experienced high degrees of melt removal in the spinel peridotite stability field. The average degree of partial melting was calculated to be 17.5% (range: 16.4–17.8%). Trace element data of ortho- and clinopyroxenes reflect this strong depletion, characteristic for the restitic nature of abyssal peridotites. Mantle re-equilibration temperatures around 1000°C indicate that, after melt extraction and before exhumation to the ocean floor, the rocks experienced significant cooling in the spinel peridotite facies. Water contents of orthopyroxene range from 86 to 233wt. ppm H2O with an average concentration of 142wt. ppm H2O. These results represent the first data on water contents in the sub-pacific mantle obtained by direct measurements of sub-oceanic peridotite. The water contents are not related to mineral chemistry, stratigraphy, melting degree, mantle equilibrium conditions or oxidation state. Calculated post-melt peridotite water contents vary between 40 and 100wt. ppm H2O.Compared to Mid-Atlantic Ridge peridotites, the East Pacific Rise samples of Leg 147 contain somewhat lower water concentrations than samples from Leg 153 and considerably higher contents than those of Leg 209 (Gose et al., 2009; Schmädicke et al., 2011). In Leg 147, the strongest OH absorbtion band occurs at 3420cm−1, wheras orthopyroxene from MAR peridotite (Legs 153 and 209) has its strongest absorbtion band at 3566 and 3522cm−1. The mantle equilibrium temperature of Leg 147 peridotites is lower than that of Leg 209 (~1250°C) but close to that of Leg 153 samples (950–1000°C). The high degree of partial melting of Leg 147 peridotite samples overlaps with the Leg 209 samples. In accordance to this data we conclude that in order to obtain relatively high water contents of up to 233wt. ppm in depleted peridotite, after melt removal water must have re-entered the orthopyroxene structure. We suggest that re-equilibration of water contents took place under spinel-facies conditions before exhumation of the tectonite, since elevated temperatures enhance diffusion and elevated pressures facilitate hydrogen uptake. The extended time span between melt removal and uplift inferred from significant isobaric cooling of at least 200K at spinel-facies depth facilitates re-equilibration of water contents and may explain the high concentrations. Exhumation from spinel-facies depth was fast and accompanied by further cooling such that re-equilibration to lower pressure assemblages and decompression-induced water loss were prevented.

Occurrence of phlogopite in the Finero Mafic layered complex

Abstract Phlogopite-bearing lithologies are the main constituent of the Phlogopite-Peridotite unit of the Finero sequence and the result of pervasive migration of metasomatizing melts/fluids. Conversely, the presence of phlogopite within the associated Finero Mafic Complex, a mafic-ultramafic pluton intruded into the metamorphic basement of the Adria plate, is mentioned in literature as rare. Recent detailed fieldwork has evidenced the presence of two distinct phlogopite-rich ultramafic lithologies within the Amphibole-Peridotite unit of the Finero Mafic Complex, where phlogopite is always associated with amphibole. Field and petrographic features of these occurrences, as well as major- and trace-element mineral chemistry, are here presented to i) place constraints on the nature of the parent melt from which they have been generated and ii) to address their relationship with the other lithologies of the Finero Complex. We find that these rocks were formed by late melt migrations along shear zones under high-T conditions. The geochemical affinity of these lithologies is different to the tholeiitic-transitional affinity reported in literature for the Finero Mafic Complex. The enrichment in LREE, Th, U and Sr of the associated amphibole possibly suggests that these phlogopite-bearing lithologies are genetically related to the metasomatic events that have affected the Finero mantle massif.

Basaltic diversity at the Apollo 12 landing site: Inferences from petrologic examinations of the soil sample 12003

AbstractA detailed petrologic survey has been made of 17 basaltic chips (sized between 1 and 10 mm) from the 12003 soil sample as part of an ongoing study of basaltic diversity at the Apollo 12 landing site. An attempt has been made to classify these samples according to the well‐established grouping of olivine, pigeonite, ilmenite, and feldspathic basalts. Particular attention has been paid to variations in major, minor, and trace element mineral chemistry (determined by electron microprobe analysis and laser ablation ICP‐MS), which may be indicative of particular basaltic suites and less susceptible to sampling bias than bulk sample characteristics. Examples of all three main (olivine, pigeonite, and ilmenite) basaltic suites have been identified within the 12003 soil. One sample is identified as a possible new addition to the feldspathic suite, which currently consists of only one other confirmed sample. Identification of additional feldspathic basalts strengthens the argument that they represent a poorly sampled basaltic flow local to the Apollo 12 site, rather than exotic material introduced to the site by impact mixing processes. Three samples are identified as representing members of one or two previously unrecognized basaltic suites.

Sulfides and Sulfarsenides from the Rosie Nickel Prospect, Duketon Greenstone Belt, Western Australia

Recent exploration in the Duketon greenstone belt, Yilgarn craton, Western Australia, led to the discovery of a new occurrence of high-grade Ni-PGE (platinum group element) sulfide mineralization associated with komatiite; this is referred to as the Rosie Ni Prospect. The mineralization consists predominantly of disseminated and brecciated semimassive to massive base metal sulfide with 0.5 to 5 cm thick sulfarsenide-bearing lenses. This pilot study focuses on the petrology, mineralogy, and trace element mineral chemistry of sulfides and sulfarsenides, and the mineralogy of minor PGE-rich minerals (sperrylite, melonite, and bismuthotel-lurides) in selected samples representing different parts of the orebody, with a particular emphasis on the sulfarsenide-rich lenses. Our mineral chemistry and mineralogical studies indicate that As-rich phases (either as a melt or as primary minerals) played a critical role in collecting and concentrating PGEs from the komatiitic magma. The concentrations of trace elements within the sulfarsenides and sulfides from the different mineralization types reflect the interaction between the silicate and sulfide liquids. The concentration of PGEs in the As-rich minerals is a function of the volume of sulfide melt with which they have interacted. The smaller the proportion of the sulfarsenide relative to sulfide in the rock is, the higher the PGE concentration in the sulfarsenide will be. In situ Se analysis of the base metal sulfides from the different ore types indicates that Se concentrations in pentlandite and pyrrhotite from sulfarsenide-rich lenses are an order of magnitude higher than those of sulfides found in As-poor samples. This correlation between the Se concentrations in the sulfide minerals and the As concentration in the whole rock indicates that the processes which led to As enrichment at Rosie also contributed to Se enrichment. The particular As-Se enrichment is inferred to have been triggered by the erosion and assimilation of sulfidic sediments enriched in organic matter (now observed as shales and/or black shales) by the komatiitic magma flows, leading to the formation of immiscible S-As-rich melt, where PGEs partition preferentially into the As-rich phases.

Sedimentary record for exhumation of ultrahigh pressure (UHP) rocks in the northern Cordillera, British Columbia, Canada

We use field relationships, paleoflow indicators, petrography, and major and trace element mineral chemistry to examine the protolith and provenance of detrital mantle-derived ultrahigh pressure (UHP—2.8 GPa) minerals in immature clastic sediments of an Early Jurassic basin (Laberge Group) in northwestern British Columbia, Canada. Our results show that fresh mantle detritus in the Laberge Group was derived from mantle lithosphere that equilibrated at >2.8 GPa and temperatures of 850–1100 °C, exhumed in orogenic massifs and quickly deposited over a restricted time interval. Two models are proposed for the exhumation and denudation of the UHP rocks in either (1) an arc-continent collision between the Stikinia and Yukon Tanana terranes, or (2) an exposed forearc and accretionary melange in a convergent margin in the Cache Creek terrane. A collision between the Stikinia and Yukon Tanana terranes has not previously been documented, but this scenario is the most consistent with widespread evidence for rapid exposure of deep-seated rocks at the required time period for deposition of UHP detritus ca. 185 Ma.

Garnet-field Melting and Late-stage Refertilization in 'Residual' Abyssal Peridotites from the Central Indian Ridge

The role of residual garnet during melting beneath mid-ocean ridges 1967; Bottinga & Allegre, 1978; McKenzie, 1984). Petrohas been the subject of many recent investigations. To address this logical, geochemical and experimental evidence as well as issue from the perspective of melting residues, we obtained major thermodynamic considerations suggest that this adiabatic and trace element mineral chemistry of residual abyssal peridotites decompression melting is a polybaric process (O’Hara, from the Central Indian Ridge. Many clinopyroxenes have ratios 1985; Klein & Langmuir, 1987; McKenzie & Bickle, of middle to heavy rare earth elements (MREE/HREE) that are 1988). The depth at which melting begins, however, is too low to be explained by melting in the stability field of spinel intensely debated (Klein & Langmuir, 1987; McKenzie peridotite alone. Several percent of melting must have occurred at & Bickle, 1988; Langmuir et al., 1992; Shen & Forsyth, higher pressures in the garnet peridotite stability field. Application 1995), as this has direct implications for the temperature of new trace element partitioning models, which predict that HREE of the upwelling mantle and the thickness of the generated are compatible in high-pressure clinopyroxene, cannot fully explain crust. Several geochemical characteristics of MORB are the fractionation of the MREE from the HREE. Further, many attributed to melting in the presence of residual garnet, samples show textural and chemical evidence for refertilization, such suggesting that the depth of onset of melting exceeds as relative enrichments of highly incompatible trace elements with >85 km (Green & Ringwood, 1970; O’Hara et al., 1971; respect to moderately incompatible trace elements. Therefore, highly Takahashi, 1986): incompatible elements, which are decoupled from major and mod(1) high middle to heavy rare earth element ratios erately incompatible trace elements, are useful to assess late-stage (MREE/HREE) occur in melt inclusions in magnesian processes, such as melt entrapment, melt–rock reaction and veining. olivines from Mid-Atlantic Ridge basalts (Sobolev, 1996; Moderately incompatible trace elements are less affected by such Shimizu, 1998). late-stage processes and thus useful to infer the melting history of (2) Examination of combined Lu–Hf and Sm–Nd abyssal peridotites. systematics by Salters & Hart (1989) and Salters (1996) revealed subchondritic Lu/Hf values in MORB, but a long-term Lu/Hf value that is higher than chondritic. These workers explained this so-called Hf paradox by a

Age and Petrogenesis of the Sondalo Gabbroic Complex (Central Alps, Northern Italy): Implications for the Origin Of Post-Variscan Magmatism

The Late Carboniferous-Early Permian gabbroic complexes of the Alpine belt are commonly ascribed to the extensional tectonics which postdate the Variscan orogeny (Dal Piaz, 1993). The best examples are provided by the Main Gabbro from the Ivrea-Verbano Zone (Quick et al., 1994) and by the Malenco Gabbro (Hermann et al., 1997). However, both Main and Malenco Gabbro have a widespread granulite-facies metamorphic overprint, which does not allow a complete petrological characterization of their igneous features. Very little is known about the Sondalo gabbroic complex of the Languard-Campo nappe (Austroalpine nappe system of Central Alps), although it is volumetrically important (it occupies an area of about 40 km 2) and almost devoid of metamorphic recrystallization. There are no recent studies on its petrogenesis and a determination of the intrusion age is lacking, despite geochronological determinations on adjacent granitoid masses suggesting that the Sondalo gabbroic complex was emplaced in Permian times (Del Moro and Notarpietro, 1987). The Sondalo gabbroic complex is a stock intrusive into high-grade metapelites of the Languard-Campo nappe (Austroalpine nappe system of the Central Alps). The pluton broadly consists of norites and minor troctolites at the core, and diorites and subordinate granodiorites at the rim. The pluton frequently contains xenoliths of granulite-facies restitic rocks and is crosscut by muscovite-bearing pegmatitic to aplitic dykes. This work aims to verify whether the Sondalo gabbroic complex is related to the Alpine gabbroic complexes which postdate the Variscan orogeny. For this purpose, we carried out a petrological study (petrography, major and trace element mineral chemistry, and Nd-Sr-O isotopic compositions) of the troctolite/norite association, thus unraveling the intrusion age, the affinity of the parental liquids and the role of crustal contamination. Combining the data presented with the literature information on the Late Carboniferous-Early Permian gabbroic complexes from the Alpine belt, we give some insights into the origin of the postVariscan magmatism. Troctolitic rocks commonly consist of euhedral to subhedral plagioclase and olivine (Fo = 73-78 mol %), and minor poikilitic clinopyroxene. The variation trend of anorthite content of plagioclase versus the forsterite content of olivine differs from that of arcrelated troctolitic rocks (Beard, 1986) and is similar to that of troctolitic rocks from slow spreadingridges. Noritic rocks are usually composed of euhedral to subhedral plagioclase and orthopyroxene, and minor poikilitic clinopyroxene. Moreover, noritic rocks may contain relatively high percentages of interstial ilmenite and biotite. Both troctolitic and noritic rocks have up to 15 % of titanian pargasite, which occurs as discontinuous rims around mafic minerals or as poikilitic grains. Pyroxenes and Tipargasite from troctolitic rocks have higher Mg, A1 and Cr than those from noritic rocks, whereas plagioclase does not show systematic compositional variations (An about 65 mol %). Plagioclase, pyroxene and Ti-pargasite were analysed for REE and selected trace elements by ion microprobe. Plagioclase shows a regular decrease from LREE to HREE and a marked positive Eu anomaly. Orthopyroxene contains low REE, with a steady increase from LREE to HREE. Clinopyroxene has depleted LREE, negative Eu anomaly and slightly depleted to almost fiat HREE. The REE pattern of titanian pargasite mimics that of the associated clinopyroxene, but at higher abundance levels. Chondrite normalization of incompatible trace elements in Ti-pargasite reveals that K, Ba and Sr (LILE) are slightly depleted relative to LREE, and Nb is commonly enriched relative to K and Ba. The trace element compositions of parental liquids,

The Origins of Yakutian Eclogite Xenoliths

Owing to the association with diamonds, eclogite xenoliths have received disproportionate attention given their low abundance in kimberlites. Several hypotheses have been advanced for the origin of eclogite xenoliths, from the subduction and high-pressure melting of oceanic crust, to cumulates and liquids derived from the upper mantle. We have amassed a comprehensive data set, including major- and trace-element mineral chemistry, carbon isotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os, and oxygen isotopes in ultrapure mineral and whole-rock splits from eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia. Furthermore, eclogites from two other Yakutian kimberlite pipes, Mir and Obnazhennaya, have been studied in detail and offer contrasting images of eclogite protoliths. Relative to eclogites from southern Africa and other Yakutian localities, Udachnaya eclogites are notable in the absence of chemical zoning in mineral grains, as well as the degree of light rare earth element (LREE) depletion and unradiogenic Sr; lack of significant oxygen, sulfur, and carbon isotopic variation relative to the mantle; and intermineral radiogenic isotopic equilibration. Several of these eclogites could be derived from ancient, recycled, oceanic crust, but many others exhibit no evidence for an oceanic crustal protolith. The apparent lack of stable-isotope variation in the Udachnaya eclogites could be due to the antiquity of the samples and consequent lack of deep oceanic and biogenically diverse environments at that time. Those eclogites that are interpreted to be non-recycled have compositions characteristic of Group A eclogites from other localities that also have been interpreted as being directly from the mantle. At least two separate and diverse isotopic reservoirs are suggested by Nd isotopic whole-rock reconstructions. Most samples were derived from typical depleted mantle. However, two groups of three samples each indicate both enriched mantle and possible ultra-depleted mantle present beneath Yakutia during the late Archean and early Proterozoic. The vast majority of eclogites studied from the Obnazhennaya pipe also exhibit characteristics of Group A eclogites and are probably derived directly from the mantle. However, the eclogites from the Mir kimberlite are more typical of other eclogites world-wide and show convincing evidence of a recycled, oceanic crustal affinity. We concur with the late Ted Ringwood that eclogites can be formed in a variety of ways, both within the mantle and from oceanic crustal residues.

Petrology, mineral chemistry, and exploration significance of Fe-sulfides from the metal dispersion halo surrounding the Cadjebut Zn [sbnd]Pb MVT deposit, Western Australia

The Cadjebut MVT mineralization is hosted by carbonate-evaporite units in the Givetian lower dolomite sequence of the Pillara platform, northern Western Australia, and occurs as 2 ore types: (1) An early Zn-rich, stratiform and rhythmically banded ore and its laterally equivalent halo of banded marcasite, barite ± calcite; and (2) a later cross-cutting, breccia-fill, Pb-rich ore. The 2 ore types are exploited through 2 mining lenses; an upper lens 1 and lower lens 2. The orebody is sited N of the normal, basin-side down, Cadjebut Fault. Rhythmically banded marcasite completely envelopes the orebody in both lenses, whereas banded barite occurs N of the lower lens 2 only where the barite is partially to completely replaced by late cloudy calcite + marcasite. Barite replacement by marcasite and cloudy calcite increases northwards. Petrographic and trace element data indicate 3 types of Fe-sulfides: (1) low-Pb Fe-sulfides with Pb contents <5300 ppm; high-Pb Fe-sulfides with Pb contents <30 000 ppm and, extremely high-Pb Fe-sulfides with > 30 000 ppm Pb. Only the ‘colloform’ pyrites exhibit Pb contents above 30 000 ppm, and SEM images show these elevated Pb contents are due to micron-scale galena inclusions. Mineral chemical data from sulfides in the stratiform Fe-sulfide and barite halos indicate that main ore-stage fluids of the rhythmically banded ore event were enriched in Zn, Pb, Cd, As and, to a minor extent Ni and Cu. Late-stage fluids of the rhythmically banded ore event were not saturated with respect to galena, precipitating low-Pb marcasites along the very outer fringes of the orebody, and replacing rhythmically banded barite in lens 2. Only the low-Pb marcasites show various stages of pyrite inversion. Petrographic and trace element mineral chemistry of the 2 stratiform halos are best explained by a model in which main ore stage fluids of the early rhythmically banded ore event emanated from below the Cadjebut orebody along an inferred antithetic reverse fault. The presence of a laterally zoned alteration halo with a pattern of proximal higher Pb and trace element-bearing Fe-sulfides provides a vector towards the ore zone, and has important exploration significance for this style of base-metal mineralization.