Ca Metasomatism Research Articles

Origin of Erzgebirge ultrahigh‐pressure garnetite: Formation from a basaltic protolith by serpentinization‐assisted metasomatism?

AbstractErzgebirge ultrahigh‐pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite‐hosted eclogite shows the same subalkali‐basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss‐hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high‐pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba‐rich fluids that reacted with associated metabasalt. Finally, subduction to >100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co‐facial garnet peridotite as observed in the Erzgebirge.

Mantle hydration initiated by Ca metasomatism in a subduction zone: An example from the Chandman meta-peridotite, western Mongolia

Fluids in subduction zones contain various aqueous species and cause metasomatic reactions as well as hydration (i.e., serpentinization) at the slab–mantle interface. However, the nature of elemental transfer during fluid infiltration into the dry mantle in the subduction zone is poorly understood. In this study, we describe novel textures related to orthopyroxene (Opx) pseudomorphs and antigorite veins in the Chandman meta-peridotite body in the Alag Khadny accretionary wedge, western Mongolia. This body consists mainly of meta-harzburgite and meta-dunite, and hydration proceeded pervasively within olivine domains associated with the development of antigorite vein networks. Pseudomorphs after orthopyroxene consist of secondary clinopyroxene (S-Cpx) + tremolite (Tr) ± [secondary olivine (S-Ol) + antigorite (Atg)] whereas the primary clinopyroxene (P-Cpx) is almost unaltered. The pseudomorphs after Opx are rarely cut by Atg veins. The mineral occurrences indicate two main stages of mantle alteration: (1) Opx replacement (Ca-metasomatism) and (2) Atg formation induced by Si-bearing fluid infiltration (i.e., the main hydration event), followed by low-temperature (T) alteration that formed lizardite, chrysotile, and brucite. The mineralogical sequence (S-Cpx + Tr ± S-Ol ± Atg) related to Opx replacement can be explained by replacement at 550–650 °C (assuming a pressure of 1.0–2.0 GPa), which is broadly consistent with temperatures experienced by eclogites in the Chandman area during exhumation. Atg in P-Ol domains could have formed at lower temperatures (<500 °C; assuming a pressure of 1.0–2.0 GPa). Mass balance calculations show that Opx replacement was characterized by gains of Ca and a small amount of H2O, whereas Ol hydration was characterized by gains of Si and H2O. A modeled slab-derived fluid, which was calculated to be in equilibrium with basaltic crustal rocks, indicates the time-integrated fluid flux required for Ca metasomatism of the Chandman meta-peridotite body, with an assumption of the alteration thickness of L = 1000 m, was 0.18–1.5 × 106 m3 fluid m−2 rock, whereas the time-integrated fluid flux required for Si metasomatism was 0.3–1.7 × 103 m3 fluid m−2 rock, which is 102–103 lower than that of the Ca metasomatism. The characteristics of the Chandman meta-peridotite body suggest that: (1) the alteration type and trapped elements in the mantle derived from slab-derived fluids vary with temperature and local Si activity; and (2) intense fracture networks facilitated extensive serpentinization related to the Si metasomatism.

Skarns at a peridotite contact, Neyriz, Iran: High-temperature melting and metamorphism in a forearc setting

In the Neyriz area of southern Iran unusual skarns are found above serpentinised peridotites at the contact with crystalline limestones. They have been interpreted as the high-temperature product of intrusion by hot peridotite into limestones, as low-temperature rodingites (the product of calcium metasomatism associated with serpentinisation), or a fortuitous juxtaposition of unrelated rocks. Their age is not known. The skarns are wollastonite-pyroxene-calcite rocks in which dark green pyroxenes are fassaites with high Al, Fe and Ti with high Ca-Tschermak's components. The field relations, textures, mineral assemblages and compositions, and melt inclusions in wollastonite and fassaite indicate the skarns formed by melting at the contact between peridotites and limestones with retrograde reactions during cooling forming garnet and anorthite. There are uncertainties in temperature estimates since pressure, XCO2 and other compositional variables are unknown, but melting temperatures were likely to have been close to 1100°C with garnet formation at approximately 900°C. Later alteration of some skarns and formation of rodingites close to the limestone-peridotite contacts occurred during low-temperature Ca metasomatism, probably after emplacement of the ophiolite during Zagros collision. A hot intrusive origin for the skarns appears incompatible with an arc-related supra-subduction origin of the ophiolite inferred from geochemical studies, but recent work in eastern Indonesia shows that during late Neogene subduction rollback, melts formed above hot mantle that intruded highly extended continental crust in a forearc setting. The scale, timing and temperatures of melting and metamorphism are very similar to those of the Late Cretaceous Neyriz ophiolite.

Eocene fault-controlled fluid flow and mineralization in the Paradox Basin, United States

Abstract Sedimentary rocks of the Paradox Basin of the Colorado Plateau (southwestern USA) record widespread manifestations of paleo–fluid flow and fluid-rock reactions including Cu, U-V, and Fe-Mn mineral deposits, Si and Ca metasomatism, hydrocarbon accumulations, and bleached sandstones. Many of these are spatially associated with faults. Here we show evidence for a widespread phase of fault-related fluid migration and mineralization at 41–48 Ma in the Paradox Basin. We measured K-Ar dates of multiple size fractions of clay-rich fault gouge, yielding statistically overlapping dates of authigenic (1Md) illite for the Salt Valley (47.0 ± 3.0 Ma), Kane Springs (47.7 ± 3.8 Ma), Cliffdweller (43.4 ± 4.6 Ma), Courthouse (41.9 ± 2.3 Ma), Lisbon Valley (45.3 ± 0.9 Ma), and GTO (48.1 ± 2.6 Ma) faults. The latter two have an illite Rb-Sr isochron age of 50.9 ± 3.5 Ma, and fault-adjacent bornite has a Re-Os isochron age of 47.5 ± 1.5 Ma. Authigenic illite from a paleo–oil reservoir near the Courthouse fault formed from the interaction of reduced fluids with oxidized red-bed sandstones at 41.1 ± 2.5 Ma. The Moab and Keystone faults have older authigenic illite ages of 59.1 ± 5.7 Ma and 65.2 ± 1.0 Ma, respectively. Our results show a close temporal relationship between fault gouge formation, red-bed bleaching, and Cu mineralization during an enigmatic time interval, raising questions about drivers of Eocene fluid flow.

Uranium mineralization associated with late magmatic ductile to brittle deformation and Na–Ca metasomatism of the Pan-African A-type Zabili syntectonic pluton (Mayo-Kebbi massif, SW Chad)

The Mayo-Kebbi massif (Chad) exposes a Neoproterozoic juvenile crustal segment that has been tectonically accreted in the Central African Orogenic Belt and reworked during the Pan-African orogeny. It comprises a syntectonic high-K magmatic suite including the Zabili A-type granitic pluton. The Zabili pluton is made of a highly differentiated granite generated by fractional crystallization of a magma formed by partial melting of a Neoproterozoic juvenile protolith. Syn- to post-magmatic ductile to brittle deformation of the Zabili pluton is associated with metasomatism and deposition of uranium. Primary magmatic U-bearing minerals are zircon, monazite, and uranothorite. Late-magmatic deformation and Na-metasomatism are marked by the development of R’ antithetic shear zones with high-temperature dynamic recrystallization of K-feldspar and Ca-plagioclase phenocrysts coeval with crystallization of albite along deformation bands and grain boundaries, and crystallization of interstitial amphibole, calcite epidote and albite also affected by intracrystalline deformation. At this stage, U-bearing minerals are monazite, uraninite, brannerite (pseudomorphosed in ekanite). This late-magmatic event is dated by U–Th–Pb on monazite at 599 ± 4 Ma. Brittle deformation and Ca-metasomatism are marked by cataclastic zones and veins containing albite, epidote, calcite, chlorite, apatite, metamict zircon, pitchblende, U-silicates, and iron oxides. U-bearing minerals are altered and/or remobilized in ekanite, kasolite, and uranophane. These data suggest that the uranium mineralization hosted by the Zabili pluton records a superposition of processes and traces extreme crustal differentiation of a Neoproterozoic juvenile crustal segment reworked during the Pan-African orogeny.

REE-enriched skarns in collisional settings: The example of Xanthi's Fe-skarn, Rhodope Metallogenetic Massif, Northern Greece

The garnet-epidote zone in the Xanthi Fe-skarn at the Rhodope Metallogenetic Massif, northern Greece, includes noteworthy cerium mineralization manifested by the presence of epidote-(Ce) containing up to ~130,000 ppm ΣREE and ~ 87,000 ppm Ce. The REE skarn represents a late evolutionary phase of the Xanthi's Fe-skarn in a collisional setting.- The epidote-(Ce) was deposited under pressures of ~1000 bars, at temperatures of ~345° to ~400 °C, from an almost pH neutral, saline, Ce-bearing, heterogeneous trapped, skarn-forming fluids with elevated log[αREE3+/(αH+)3] values. Thermodynamic two-stage modeling suggests that Ca metasomatism (e.g., decrease of the logαCa2+(aq) values) coupled with Ce fractionation (e.g., increase of the logαCe(CO3)+(aq) values) of the skarn-forming fluids led to the formation of epidote-(Ce). The major mechanism for the Ce-enrichment in the metasomatic fluids was the remobilization of the REE from the I-type granodiorite of Xanthi, in a back-arc setting due to subduction retreat.

Footprints of element mobility during metasomatism linked to a late Miocene peraluminous granite intruding a carbonate host (Campiglia Marittima, Tuscany)

The Campiglia Marittima magmatic-hydrothermal system includes a peraluminous granite, its carbonatic host, and skarn. The system evolved generating a time-transgressive exchange of major and trace elements between granite, metasomatic fluids, and host rock. The process resulted in partial metasomatic replacement of the granite and severe replacement of the carbonate host rocks. The fluid activity started during a late-magmatic stage, followed by a potassic–calcic metasomatism, ending with a lower temperature acidic metasomatism. During the late-magmatic stage, B-rich residual fluids led to the formation of disseminated tourmaline–quartz orbicules. High-temperature metasomatic fluids generated a pervasive potassic–calcic metasomatism of the granite, with replacement of plagioclase, biotite, ilmenite, and apatite by K-feldspar, phlogopite–chlorite–titanite, titanite–rutile, and significant mobilization of Fe, Na, P, Ti, and minor HFSE/REE. The metasomatized granite is enriched in Mg, K, Rb, Ba, and Sr, and depleted in Fe and Na. Ca metasomatism is characterized by crystallization of a variety of calc-silicates, focusing along joints into the granite (endoskarn) and at the marble/pluton contact (exoskarn), and exchange of HFSE and LREE with hydrothermal fluids. Upon cooling, fluids became more acidic and fluorine activity increased, with widespread crystallization of fluorite from disequilibrium of former calc-silicates. At the pluton-host boundary, fluids were accumulated, and pH buffered to low values as temperature decreased, leading to the formation of a metasomatic front triggering the increasing mobilization of REE and HFSE and the late crystallization of REE–HFSE minerals.

Coupling restites and mobilizates – Geological and litho-chemical investigations of paired belts of calcsilicate fels and quartzite (SE German Basement) – Quo vadis David London’s pegmatology?

In some areas of the Variscan orogen felsic mobilizates (pegmatitic and aplitic rocks) are closely associated with stratiform and stockwork-like bodies enriched in Ca minerals (e.g. wollastonite, diopside-hedenbergite s.s.s., grossular-spessartine s.s.s., siderite..) and bodies aligned to them similar in structure but abundant in quartz, plagioclase and mica. Geological mapping and lithochemical studies are the tools to decipher the nature of these crystalline rocks which are common to the Hagendorf-Pleystein Pegmatite Province, SE Germany, and present in many ensialic orogens elsewhere. Geological and chemical data suggest paired belts of a restite-mobilization system. The Ca and Si metasomatites are different from calcareous metasediments and quartzites elsewhere in the SE German basement devoid of mobilizates (parent rocks: limestones and cherts). Mobilization conducive to this paired belt of metasomatites involved silica mobilized from a deep level of the crust as a result of metamorphic-metasomatic alteration of Precambrian to Early Paleozoic metagreywackes during retrograde metamorphism from HP to LP metamorphism around 680–600 °C. The arrangement of mobilizates and restites in the field has been denominated as metamorphic differentiation sensu lato. The zone of silica mobilization is transitional into a zone of pegmatoids and aploids that overlaps with another one characterized by rocks derived from Ca metasomatism the footwall facies of which developed in the range 750–400 °C while in the hanging wall metamorphic rocks of rare-element pegmatites 570–430 °C occurred. The intensity of Ca metasomatism diminishes from the footwall to the hanging wall rocks and reflects a subcrustal impact. These investigations call attention among exploration geologists and petrologists to an alternative origin of “metasilica” and “metacarbonate” rocks being encountered in a zoned arrangement with felsic mobilizates (pegmatitic and aplitic rocks). The current study also raises the question “Quo vadis” pegmatology? It is an amendment to the mainstream geoscientific handling of pegmatitic rocks as “..texturally complex igneous rocks” genetically linked to granitic plutons (see review of London (2018) in Ore Geology Reviews). Taking a holistic approach can give us a reality check and prevent pegmatology from converting into a one-way street (granites-only) that eventually ends up in a dead-end street. The field evidence is the litmus test for all our models created in the laboratory and on the PC. There is no ore geology without field geology.

Lithogeochemical Vectors for Hydrothermal Processes in the Strange Lake Peralkaline Granitic REE-Zr-Nb Deposit

Extreme enrichment and postmagmatic hydrothermal mobilization of the rare earth elements (REE), Zr, and Nb have been reported for a number of anorogenic peralkaline intrusions, including the world-class REE-Zr-Nb deposit at Strange Lake, Quebec, Canada. Establishing lithogeochemical vectors for these types of deposits is a challenging task because the effects of hydrothermal processes on element distribution are poorly known and the relationships of alteration types to mineralization stages have not been well documented. Here, we present results of a detailed mineralogical and geochemical investigation involving a dataset of over 500 mineral and bulk-rock analyses of a northeast-southwest section through the potential ore zone at Strange Lake. Based on these data, we develop a model that explains the role of hydrothermal processes in concentrating metals in peralkaline granitic systems and identify lithogeochemical vectors for their exploration. The B zone, located along the northwestern margin of the Strange Lake pluton, contains a lens-shaped, pegmatite-rich domain comprising subhorizontal sheets of pegmatites hosted by granites with a total indicated resource of 278 million tonnes (Mt) grading 0.93 wt % total rare earth oxides (TREO), of which 39% are heavy rare earth elements (HREE). Within this resource, there is an enriched zone containing 20 Mt of ore grading 1.44 wt % TREO, of which 50% are HREE. The pegmatites are characterized by a core enriched in quartz, fluorite, and light rare earth elements (LREE) fluorocarbonates, and a granitic border enriched in zirconosilicates and granitic minerals. The pegmatite sheets and surrounding granites evolved in three essential stages: a magmatic stage (I), a near-neutral hydrothermal stage involving their interaction with NaCl-bearing orthomagmatic fluids (II), and an acidic hydrothermal stage (III, comprising high-[IIIa] and low-temperature [IIIb] substages) that resulted from their interaction with pegmatite-sourced HCl-HF–bearing fluids. Stage IIIa led to pseudomorphic mineral replacement reactions (e.g., Na-Ca exchange during replacement of zirconosilicates) and formation of an aegirinization/hematization halo around the pegmatites. In contrast, stage IIIb, which was responsible for the hydrothermal mobilization of Zr and REE, is manifested by fluorite and quartz veins, zircon spherules, gadolinite-group minerals, gittinsite, ferriallanite-(Ce), and a pervasive replacement of the granite by these minerals. The distribution of REE, Zr, Nb, and Ti was controlled by the competition between hydrothermal fluids and the stability of primary REE-F-(CO 2 ) minerals (e.g., bastnasite-(Ce) host to LREE), zirconosilicates (i.e., Na zirconosilicates and zircon host to HREE and Zr), and Nb-Ti minerals (i.e., pyrochlore host to Nb and narsarsukite host to Ti), and the stability of secondary LREE silicates (i.e., ferriallanite-(Ce)), HREE silicates (i.e., gadolinite-(Y)), zirconosilicates (i.e., gittinsite and zircon), and Nb-Ti minerals (i.e., titanite and pyrochlore). Lithogeochemical vectors were identified to distinguish between the high-temperature acidic alteration (IIIa), using CaO/Na 2 O (indicator of Ca metasomatism) and Fe 2 O 3 /Na 2 O ratios (indicator of aegirinization/hematization), and the low-temperature acidic alteration (IIIb), using the CaO/Al 2 O 3 ratio (indicator of Ca-F metasomatism). Bulk-rock compositional data show that alteration was accompanied by an enrichment in heavy rare earth oxides (HREO) and ZrO 2 at the deposit scale, whereas there was no selective enrichment in the light rare earth oxides (LREO). A 2-D geochemical model of the deposit indicates that the LREO are more dispersed, whereas HREO and ZrO 2 are selectively distributed. These variations in LREE/HREE are also reflected in the mineral chemistry, especially in hydrothermal zircon crystals showing an unusual LREE enrichment and HREE depletion, contrasting with pseudomorphs, which are enriched in HREE. Hydrothermal ferriallanite-(Ce) and gadolinite-group minerals also show a clear trend of REE depletion with Ca enrichment. Controlling factors for the hydrothermal mobilization of LREE, HREE, and Zr were temperature, pH, and the availability of fluoride ions (F − ) in the fluid for the dissolution of zircon, and chloride ions (Cl − ) for the complexation of the REE. The study of rare hydrothermal minerals in conjunction with field observations and the evaluation of variations in bulk-rock composition allowed us to develop a new model for the hydrothermal evolution stage of Strange Lake.

Magnetite from the Cogne serpentinites (Piemonte ophiolite nappe, Italy). Insights into seafloor fluid–rock interaction

In the Cogne area (Aosta Valley, Western Italian Alps), magnetite mineralization occurs associated with different types of serpentinite (pseudomorphic, rodingitic and magnetite-rich serpentinites), which are also heterogeneous in terms of bulk chemical composition, mineral assemblage and mineral chemistry. The mineralization is hosted by a lizardite-, ±chrysotile-bearing serpentinite showing a pseudomorphic texture after oceanic peridotite. It contains fine-grained magnetite showing oscillatory zoning with Cr-poor cores (~0.4 wt.% Cr2O3), which are surrounded by Cr-rich inner euhedral rims (up to 17.0 wt.% Cr2O3), mantled in turn by partially resorbed Cr-bearing outer rims (~2.0 wt.% Cr2O3). This dramatic variation of Cr content in magnetite is described by the exchange vector Fe3+ Fe2+ Cr3+−1 (Mn, Zn, Ni) 2+−1. The absence of Cr-rich spinel cores suggests that the source of chromium was the Cr-Tschermak component of the orthopyroxene, made available during the serpentinization-related breakdown. In these rocks, lizardite, the primary serpentine phase, is partially overgrown by prograde antigorite, indicating that serpentinization occurred under relatively low- T conditions in a preorogenic ( i.e. , oceanic) setting, and was followed by an Alpine metamorphic re-equilibration. Rodingitic serpentinite and serpentinite rich in magnetite are composed of, in highly variable amounts, diopside (locally with inclusions of partially disordered graphite), calcite, magnetite, andradite/hydroandradite, prograde olivine, brucite and serpentine (mostly antigorite but also relict lizardite), with minor chlorite and accessory apatite. Magnetite content can be very high. In these rocks, magnetite is exceptionally Cr-poor (almost Cr-free). Such almost Cr-free magnetite is characterized by significant Fe2+ ↔ Mg substitution (2–15 mol.% MgFe2O4). Crystal-chemical investigations confirm that Mg, as in magnesioferrite, shows a strong preference for the octahedral site. Due to Fe2+ ↔ Mg substitution, the Fe2+ content available for Fe2+ – Fe3+ electron hopping in the octahedral site decreases leaving Fe3+ in excess and giving rise to a charge increase. Additionally, a small Fe3+ - Fe2+ disorder is present at the tetrahedral site (Fe2+T =0.07 apfu). The bulk-rock compositions and the mineral associations of rodingitic serpentinite and serpentinite rich in magnetite point to a strong metasomatism and iron mobilization sustained by CO2-bearing fluids, developed at the expense of the host serpentinite. These processes likely occurred on the Tethyian seafloor, mediated by fluids sourcing from hydrothermal vents. Thermodynamic modelling suggests that mineral assemblages observed in the Cogne area are consistent with a process of serpentinization coupled with Ca (±Al) metasomatism at 300–360°C. This might have been driven by carbon-saturated C-O-H fluids characterized by CO2 contents comparable to present-day seawater, capable of fixing the redox potential of rocks close to the fayalite–magnetite–quartz buffer. Nevertheless, magnetite and associated Fe-Ni sulphides suggest slight variations in f O2 (and f S2) conditions recorded, ranging from relatively reducing (magnetite–heazlewoodite–pentlandite ±pyrrhotite) towards rather oxidizing (magnetite–millerite–pyrite) assemblages.

Metasomatism of ferroan granites in the northern Aravalli orogen, NW India: geochemical and isotopic constraints, and its metallogenic significance

The late Palaeoproterozoic (1.72–1.70 Ga) ferroan granites of the Khetri complex, northern Aravalli orogen, NW India, were extensively metasomatised ~900 Ma after their emplacement, at around 850–830 Ma by low-temperature (ca. 400 °C) meteoric fluids that attained metamorphic character after exchanging oxygen with the surrounding metamorphic rocks. Albitisation is the dominant metasomatic process that was accompanied by Mg and Ca metasomatism. A two-stage metasomatic model is applicable to all the altered ferroan intrusives. The stage I is represented by a metasomatic reaction interface that developed as a result of transformation of the original microcline–oligoclase (An12–14) granite to microcline–albite (An1–3) granite, and this stage is rarely preserved. In contrast, the stage II metasomatic reaction front, where the microcline-bearing albite granite has been transformed to microcline-free albite granite, is readily recognisable in the field and present in most of the intrusives. Some of them lack an obvious reaction interface due to the presence of stage II albite granites only. When studied in isolation, these intrusives were incorrectly classified and their tectonic setting was misinterpreted. Furthermore, our results show that the mafic mineralogy of metasomatised granites has a significant impact on the characterisation of such rocks in the magmatic classification and discrimination diagrams. Nevertheless, the stage I metasomatised granites can be appropriately characterised in these diagrams, whereas the characterisation of the stage II granites will lead to erroneous interpretations. The close spatial association of these high heat producing ferroan granites with iron oxide–copper–gold (IOCG), U and REE mineralisation in the region indicates a genetic link between the metasomatism and the mineralisation. World-class IOCG, U and REE deposits are associated with metasomatised ferroan granites, suggesting that such a relationship may act as a critical first-order exploration target for undiscovered mineral deposits.

In situ LA-ICP-MS U–Pb titanite dating of Na–Ca metasomatism in orogenic belts: the North Pyrenean example

In the Pyrenees, in association with the rotation of the Iberian plate around Europe during the Mid-Cretaceous, a Na–Ca metasomatism is recognized as a complementary record of the hydrothermal activity that led to Na-metasomatism (albitization) and talc–chlorite mineralization. It affected metasedimentary rocks as well as Hercynian granites. In situ laser ablation ICP-MS U–Pb analyses of titanite grains formed in albitites during metasomatism date the Na–Ca metasomatism between 110 and 92 Ma. The temperature of the Na–Ca metasomatism is estimated to be approximately 550 °C. Both the time constraints and temperature estimates suggest that the Na–Ca metasomatism is related to the low-P high-T North Pyrenean metamorphism.

Open-system Behavior during Pluton-Wall-rock Interaction as Constrained from a Study of Endoskarns in the Sierra Nevada Batholith, California

Crustal xenoliths (pyroxenites and plagioclase + quartz + pyroxene lithologies) from the Quaternary Big Pine volcanic field on the eastern flank of the Sierra Nevada Batholith in California (USA) represent the products of metasomatic reaction between the margins of a Cretaceous granodioritic pluton and Paleozoic marbles, possibly at mid-crustal depths based on the equilibration temperatures recorded by Ti-in-quartz geothermometry. This interpretation is based on the presence of plagioclase showing relict plutonic textures, pyroxenite characterized by nearly pure diopside clinopyroxene, recrystallized plagioclase with anomalously high anorthite content, textures indicating replacement of plagioclase by clinopyroxene (and vice versa), ‘ghost’ plagioclase rare earth element signatures in some clinopyroxenes, and the presence of phlogopite endmember micas at the contact between clinopyroxene-rich and plagioclase-rich zones. These observations suggest that the xenoliths represent fragments of an ‘endoskarn’, the outer sheath of a pluton that chemically reacted with carbonate country-rock. Mass transfer between the carbonate country-rock and the pluton involved transfer of Ca and Mg from the carbonate into the pluton and transfer of Na, K, Al and Si from the pluton to the carbonate, the latter generating extensive endoskarns. The Ca metasomatism of the pluton converted alkali feldspar components into anorthite-rich plagioclase, releasing Na and K, which left the plutonic system. K, in particular, migrated towards the carbonate and precipitated phlogopite upon entering clinopyroxene-rich lithologies. Mass-balance calculations, based on theory and residual enrichments in immobile elements such as Ti, suggest that the pluton experienced net mass loss (>15%) in the form of Si, Al, Na and K to the surrounding country-rock, but a net gain in Ca and Mg.

Early Palaeozoic metasomatism of the Archaean Napier Complex, East Antarctica

Abstract Emplacement of post-tectonic Early Palaeozoic pegmatites on Tonagh Island, Napier Complex, East Antarctica, was accompanied by the introduction of aqueous low-salinity fluids at mid- P upper-amphibolite facies conditions ( c. 8 kbar, c. 680 °C). Fluid–wall-rock interaction resulted in the development of spectacular alteration selvedges, in the immediate vicinity of the pegmatites, in adjacent Archaean orthogneisses. Archaean wall-rocks affected by the infiltration of aqueous fluids show contrasting patterns of K, Na and Ca metasomatism, which we demonstrate was fundamentally controlled by disequilibrium of invading fluids with wall-rock feldspars, rather than fluid flow up or down regional pressure or temperature gradients. Other species, such as the rare earth elements (REE; except Eu), Y, P, Rb, Th, U and Pb, for example, show significant enrichment in the metasomatized wall-rock in both examples studied. Enrichment of P (and Y), sympathetic with that of REE enrichment, is consistent with recent suggestions that REE transport in fluids may be enhanced by complexing with dissolved P and Y compounds. Dehydration and partial melting of previously unmetamorphosed sedimentary rocks, underthrust beneath the SW Napier Complex, has been long considered a viable source for felsic pegmatites (and associated fluids) observed in that region. Our results are consistent with the hypothesis that underplated sedimentary rocks were a viable source for pegmatite melt and aqueous fluids. Furthermore, as our study demonstrates a plausible relationship between Early Palaeozoic ( c. 500–530 Ma) pegmatites and fluid infiltration, we suggest that dehydration and prograde partial melting of the underthrust sedimentary rocks beneath the Napier Complex occurred, at least in part, by convergent Early Palaeozoic tectonism.

Noble gas and halogen constraints on regionally extensive mid-crustal Na–Ca metasomatism, the Proterozoic Eastern Mount Isa Block, Australia

Fluid inclusions in late-Isan quartz veins associated with regional Na–Ca alteration (albitisation), in the Mary Kathleen Fold Belt and the Cloncurry District of the Eastern Mt Isa Block, have been analysed for naturally occurring and neutron produced isotopes of Ar, Kr and Xe. The noble gases have been extracted using a thermal decrepitation procedure that enables partial deconvolution of the different fluid inclusion types, including variably saline aqueous, liquid carbon-dioxide and mixed aqueous-carbonic varieties. The variably saline (<5–65 wt%) aqueous fluid inclusions dominate and have 40Ar/ 36Ar values of less than 2700 in most of the samples from across the region. These fluid inclusions have extremely variable molar Br/Cl values of 0.3–4 × 10 −3 and I/Cl values of 0.2–35 × 10 −6, and the fluids are interpreted to represent sedimentary formation waters derived from the upper crust that have dissolved variable quantities of halite (or scapolite) to achieve their ultra-high salinity. Fluid inclusions in a sample from the Snake Creek anticline in the Cloncurry District have the highest 40Ar/ 36Ar value of ∼25,000 demonstrating a deep magmatic or metamorphic fluid origin in this case. A magmatic origin is favoured because this fluid is very similar to a fluid involved in Iron-Oxide-Copper-Gold (IOCG) mineralization at Ernest Henry that was also interpreted to have a magmatic origin, and because the aqueous fluid inclusions have Br/Cl of ∼1–2 × 10 −3 and I/Cl of ∼10 × 10 −6 that are similar to mantle-derived igneous rocks. Carbon-dioxide fluid inclusions dominate samples from the Knobby Quarry in the Mary Kathleen Fold Belt and have a maximum 40Ar/ 36Ar value of 6000–7000. These fluid inclusions are estimated to have a 36Ar concentration of 1–4 ppb, that is similar to the range determined for aqueous fluid inclusions in all the other samples. In addition, aqueous and carbonic fluid inclusions in all the samples have similar 129Xe/ 36Ar plus 84Kr/ 36Ar values that are unfractionated, and close to the air—Air Saturated Water (ASW) range. These data are interpreted to indicate an independent and dominantly metamorphic origin for CO 2, and the presence of mixed aqueous-carbonic fluid inclusions are attributed to fluid mixing and/or mingling, rather than fluid unmixing. However, the data do not preclude the presence of a minor magmatic CO 2 component in the samples from the Knobby Quarry. Fluid inclusions in most of the samples from the Mary Kathleen Fold Belt have higher Br/Cl and I/Cl values, higher 36Ar concentrations and lower maximum 40Ar/ 36Ar values than fluid inclusions in samples from the Cloncurry District. This suggests Na–Ca alteration in these different parts of the Eastern Mt Isa Block occurred independently. However, fluid inclusions associated with Na–Ca alteration in the Cloncurry District have a very similar composition to fluid inclusions in IOCG mineralization-stage quartz veins from Ernest Henry. These data are therefore compatible with a genetic relationship between regional Na–Ca alteration and IOCG mineralization in the Cloncurry District. In both cases the ultra-saline hydrothermal fluids had a dominant origin from sedimentary formation water, but are interpreted to contain a magmatic component sourced from the late-Isan Williams-Naraku Batholiths which may have driven fluid convection.

PLAGIOCLASE-QUARTZ ROCKS OF METASOMATIC ORIGIN AT THE EXPENSE OF GRANITIC ROCKS OF THE KOMAKI DISTRICT, SOUTHWESTERN JAPAN

In the Komaki district of southwestern Japan, small bodies composed mostly of intermediate plagioclase and quartz occur in a Late Cretaceous granitic batholith. The properties of the plagioclase–quartz rock indicate that the rocks were formed by calcium metasomatism. They are found in a lenticular body 50 m thick and a dome-shaped body 80 m in diameter. The plagioclase–quartz rock formed along fractures and shear zones by hydrothermal replacement of the host granitic rocks. The upper parts of the plagioclase–quartz rock bodies were strongly kaolinized. Greisenized portions are distributed sporadically in surrounding granitic host-rocks. On a 87Sr/86Sr versus 87Rb/86Sr diagram, the regression line formed by the plagioclase–quartz rock and a small stock of leucogranite 1.5 km southwest of the plagioclase–quartz occurrences can be regarded as an isochron, whereas other granitic rocks yield an errorchron. The Rb–Sr isochron age agrees well with the oldest K–Ar age of muscovite from a Mo deposit within the leucogranite. The plagioclase–quartz rock shows significantly lower δ18O compositions relative to the leucogranite and the host granitic batholith, suggesting the contribution of meteoric water. The O and H isotope compositions of clay minerals from the plagioclase–quartz rock suggest that the kaolinization was caused by low-temperature (<150°C) hydrothermal alteration. The geological and geochemical features suggest that the Ca metasomatism was caused by hydrothermal fluids that also produced Mo mineralization. Subsequent to the Ca metasomatism, kaolinization of the plagioclase–quartz rock took place by low-temperature hydrothermal alteration in combination with supergene processes.

瀬戸内領家帯小大下島に産するエメリー

Limestone is accompanied with widely distributed Ryoke metamorphic rocks in islands of the Seto Inland Sea. Ko-Oge Island is situated in this zone and mainly composed of pelitic hornfels and crystalline schist rarely interbedded with dark color and massive Al-Fe rich sediments, so called emery.Emery from Ko-Oge Island is charactarized by high TiO2 contents and by relict of primary texture such as pisolitic one. Emery can be divided, according to the mineral assemblage and texture of rocks: 1. zone of emery, 2. metasomatic zone of emery, 3. metasomatic zone of emery with limestone, and 4. metamorphic zone of another emery. The main constituent minerals of Zone1 are corundum, hercynite, ilmenite, and plagioclase, which occur as granoblastic aggregates. Perovskite, grossular, and vesuvianite occur in Zone 3. It was concluded that emery of Zone 3 is formed by Ca metasomatism in later stage of the prograde and retrogrede stages of metamorphism. Original rock of emery is probably sediment such as laterite, rich in the TiO2, Al2O3, and Fe oxide. The rock is composed of gibbsite, Fe-hydrooxide, anatase, kaolinite, and calcite.

Mantle Xenoliths from Tenerife (Canary Islands): Evidence for Reactions between Mantle Peridotites and Silicic Carbonatite Melts inducing Ca Metasomatism

Mantle Xenoliths from Tenerife (Canary Islands): Evidence for Reactions between Mantle Peridotites and Silicic Carbonatite Melts inducing Ca Metasomatism

Relationships among zirconosilicates: examination by cathodoluminescence and transmission electron microscopy .

The origin, whether primary (magmatic) or secondary (Ca-metasomatic) after elpidite, of the Ca-rich zirconosilicates armstrongite and gittinsite in the subsolvus granite of the Strange Lake peralkaline complex (Quebec-Labrador border, Canada) has been a well debated topic. These zirconosilicates, along with zircon and amphibole, account for most of the Zr in the associated Zr-, REE-, Y-, Nb-, and Be-rich ore deposit. In this study, cathodoluminescence (CL) examination and transmission electron microscopy (TEM) have been used to shed light on the relationships among and origin of these zirconosilicates. Optical and CL observations indicate that elpidite undergoes epitaxial replacement by armstrongite and, subsequently, by gittinsite. This gittinsite is generally associated with quartz and variable amounts of hematite. TEM results suggest the bulk replacement of elpidite by poorly crystalline armstrongite, followed by gittinsite and quartz, or zircon and quartz. The Ca-rich compositions of elpidite and its structural similarity with armstrongite imply the existence of a solid solution with armstrongite. Moreover, the enrichment of gittinsite in Mn and Fe (and in some cases Y), and armstrongite and zircon in Y, as determined by analytical electron microscopy (AEM), correlates with the presence of these elements in the Ca-metasomatic fluid. These structural and compositional relationships indicate that the primary, magmatic elpidite is replaced by armstrongite and, subsequently, by gittinsite and quartz during Ca metasomatism of the complex.

Rare earth element metasomatism in hydrothermal systems: the Willsboro-Lewis wollastonite ores, New York, USA

Wollastonite ores and garnet-pyroxene skarns in the Willsboro-Lewis district, New York, USA were formed in a complex hydrothermal system associated with the emplacement of a large anorthosite pluton. Contact-metamorphic marbles were replaced by wollastonite, garnet, and clinopyroxene during infiltration metasomatism involving large volumes of water of chiefly meteoric origin. Rare earth elements (REE) in these rocks show large departures from the protolith REE distribution, indicative of substantial REE mobility. Three types of chondrite-normalized REE distribution patterns are present. The most common, found in ores and skarns containing andradite-rich garnet, is convex-up in the light REE (LREE) with a maximum at Pr and a positive Eu anomaly. Europium anomalies and Pr/Yb ratios are correlated with X Ad in garnet. This pattern (type C) results from uptake of REE from hydrothermal fluids by growing crystals of calcsilicate minerals, principally andradite, with amounts of LREE controlled by the difference in ionic radius between Ca ++ and REE 3+ in garnet X sites. The Eu anomaly results either from prior interaction of the fluids with plagioclase-rich, Eu-positive anorthositic rocks in and near the ore zone, or by enrichment of divalent Eu on growth surfaces of garnet followed by entrapment, or both. Relative enrichment in heavy REE (type H) occurs in ores and skarn where calcsilicates, including grossularitic garnet, in contact-metamorphic marble have been concentrated by dissolution of calcite. In most cases a negative Eu anomaly is inherited from the marble protolith. Skarns containing titanite and apatite exhibit high total REE, relative light REE enrichment, and negative Eu anomalies (type L). These appear to be intrusive igneous rocks (ferrodiorites or anorthositic gabbros) that have been converted to skarn by Ca metasomatism. REE, sequestered in titanite, apatite, and garnet, preserve the approximate REE distribution pattern of the igneous protolith. Post-ore granulite facies metamorphism homogenized zoned mineral grains without causing complete intergranular reequilibration and does not appear to have significantly affected the whole-rock REE distributions. These results demonstrate that extensive REE metasomatism can occur in hydrothermal systems at shallow to intermediate depths and that REE geochemistry may be useful in discerning the origin of skarns and skarn-related ore deposits.