Granite Cupola Research Articles

Geology and mineralogy of Li mineralization in the Central Iberian Zone (Spain and Portugal)

AbstractLithium mineralization is common in the Central Iberian Zone and, to a lesser extent, in the Galizia-Trás-OsMontes Zone of Spain and Portugal, occurring along a ∼500 km-long NNW-SSE striking belt. There are different styles of Li mineralization along this belt; they are mainly associated with aplite-pegmatite bodies and, to a much lesser extent, with veins of quartz and phosphate. Lithium mineralization in the Central Iberian Zone may be classified into four types: aplite-pegmatite dykes occurring in pegmatitic fields, Li mineralization associated with leucogranitic cupolas, beryl-phosphate pegmatites and quartz-montebrasite veins. The main Li minerals of these bodies include Li-mica, spodumene and/or petalite in the pegmatitic fields and leucogranitic cupolas; triphylite–lithiophilite in the beryl-phosphate pegmatites, and amblygonite–montebrasite in the quartz-montebrasite veins. The origin of these different styles of mineralization is considered to be related to differentiation of peraluminous melts, which were generated by partial melting of metasedimentary rocks during the Variscan orogeny. On the basis of paragenesis and chemical composition, the pegmatitic fields and Li mineralization associated with granitic cupolas record the highest fractionation levels, whereas the beryl-phosphate pegmatites and quartz-montebrasite veins show lower degrees of fractionation. There are a number of textural and mineralogical indicators for Li exploration in the Central Iberian Zone and in the Galizia-Trás-Os-Montes Zone, with the highest economic potential for Li being in the pegmatite fields.

Drill hole CS-1 penetrating the Cínovec/Zinnwald granite cupola (Czech Republic): an A-type granite with important hydrothermal mineralization

Drill hole CS-1 penetrating the Cínovec/Zinnwald granite cupola (Czech Republic): an A-type granite with important hydrothermal mineralization

Geophysical investigations of the geology and structure of the Pangushan-Tieshanlong tungsten ore field, South Jiangxi, China—Evidence for site-selection of the 2000-m Nanling Scientific Drilling Project (SP-NLSD-2)

The Pangushan-Tieshanlong tungsten ore field, located in the center of southern Jiangxi Province, eastern Nanling region, contains many granitic plutons, both exposed and concealed, and hosts numerous tungsten-polymetallic ore deposits. Despite many previous geological studies, the deep geology and structure of the field are poorly known because few geophysical measurements have been undertaken. In this study, we carried out a variety of geophysical measurements, including gravity, magnetic field detection, magnetotelluric sounding (MT) and high-resolution seismic reflection to better outline the nature of the ore field and to aid in site selection for the 2000-m Nanling Scientific Drilling Project (SP-NLSD-2), to be carried out by the Chinese SinoProbe Program. Four granitic intrusives, the Tangcun, Pangushan, Huangsha and Bai’e plutons, crop out independently at the surface but were found to be parts of a large batholith at depth. The Tangcun composite pluton, which at the surface consists chiefly of Caledonian granite and minor Early Yanshanian intrusive material, was found to be chiefly Early Yanshanian granite concealed beneath the lower part of the Caledonian granite. The Bai’e pluton initially decreases in size with depth and then expands as it merges with the major batholith. The Pangushan and Huangsha plutons are domes on the inferred batholith. Four granitic magma channels and a considerable number of faults have been identified. Due to its favorable geological conditions, the top of the Pangushan pluton is predicted to be the best candidate for ore prospecting. It is basically a granite cupola with many cross-cutting and parallel faults. In addition, it is characterized by a high gravity anomaly, a high magnetic anomaly and locally a low electrical resistivity anomaly.

Geology and gravity modeling of the Logrosán Sn–(W) ore deposits (Central Iberian Zone, Spain)

Sn–(W) granite-hosted ore deposits are characterized by the occurrence of pegmatites, stockworks and quartz veins within a granitic cupola and its surrounding host-rocks. A regional exploration tool for these deposits consists of targeting syn- to late-orogenic specialized S-type hidden shallow granites by geophysical methods. However, gravity as a tool for “brownfield type” exploration in a mature granite-related ore setting has not been widely used. The Logrosán area (Central Iberian Zone) provides a perfect case scenario with Sn–W endo- and exogranitic ore deposits within a relatively small-scale realm. We suggest that the understanding of the subsurface of the Logrosán area provides a key tool to develop a more comprehensive geological and metallogenic model for the ore deposits. In addition, a better explanation for the mineral evolution and the Sn–W ore precipitation can be achieved, by comparing the chemical and textural features of ores from several of the Logrosán deposits. Field gravity and geologic data allowed the modeling of the hidden granite bodies. Combining this information with detailed mineralogical, geochemical data and ore deposit modeling has allowed a definition of targets for probable new Sn–(W) deposits, all within a few kilometers of the main intrusion center.

Discussão dos processos de construção do complexo granítico São Sepé, RS: feições geológicas e petrográficas

A construção do complexo granítico São Sepé (CGSS) é discutida a partir de informações de campo, geológicas e estruturais, complementadas com a avaliação dos sistemas tectônicos regionais integrando sensores remotos (MDE-SRTM e imagem ASTER) e mapas aeromagnetométricos, e com a investigação detalhada de texturas e estruturas. O CGSS, localizado no oeste do Escudo Sul-riograndense (Bloco São Gabriel), representa os níveis epizonais (250-160 MPa) de uma coluna vertical de magma-mush com atividade moderada, controlada por zonas de falha pré-existentes N30-35°E e N55-60°W, cuja atividade foi vinculada aos eventos pós-colisionais da Orogênese Dom Feliciano (640-620 Ma). Sua formação ocorreu no final do Neoproterozóico (570-560 Ma), durante evento tectônico transtensivo ao longo de zonas de falha N15-20°E, responsável pelo soerguimento regional e expressivo magmatismo ácido, que é reflexo de orogenias mais jovens ocorridas a norte. É um pluton composto de dimensão moderada (473 km²), consistindo de monzogranitos centrais e sienogranitos na borda, que exibem grande variedade de texturas. A expansão do sistema magmático com acúmulo de voláteis no final de sua formação, como registrado pela cúpula granítica e ampla margem resfriada félsica, teria promovido a reativação do sistema de falhas NW-SE. Isto permite inferir o papel, mesmo reduzido, de mecanismos de alojamento via o soerguimento do teto. De acordo com o modelo proposto, a construção do CGSS decorre de episódios progressivos e cíclicos de recarga com magma máfico na base de uma soleira sienogranítica precursora, o que é registrado por feições diversificadas de hibridismo nos monzogranitos. Isto acarretou a estratificação do magma residente, que inicia a cristalizar separadamente: monzogranitos hibridizados na base, e sienogranitos a partir de líquidos magmáticos residuais coletados no topo, cuja cristalização foi sucessivamente postergada com a concentração de voláteis (H2 O e F). Episódios subsequentes de recarga máfica, em uma intrusão mais fria e parcialmente solidificada, promoveram o reaquecimento e a consequente maturidade textural e erosão termal das fácies cristalizadas; a remobilização de mushes cristalinos, com reintrusão e co-mingling de magmas hibridizados; e o maior subresfriamento termal no polo máfico. Tais feições apontam para o acúmulo de magmas sienograníticos altamente móveis, que sustentaram uma das câmaras magmáticas alimentadoras do vulcanismo coevo (Formação Acampamento Velho).

Evolution of the Cinovec (zinnwald) Granite Cupola, Czech Republic: Composition of Feldspars and Micas, a Clue to the Origin of W, sn Mineralization

A 1596-m-deep borehole (CS–1) located in the central part of the Cinovec (Zinnwald) granite cupola mineralized in Sn and W intersected zinnwaldite granite (ZG) followed by lithian annite (“protolithionite”) granite (PG). These two types of granite are separated by a transition zone (TZ). We studied the chemical composition of major mineral phases (plagioclase, K-feldspar, micas) by EPMA and LA–ICP–MS analyses. The plagioclase composition corresponds to albite (Ab 99.6–91.1 ) containing 0.09 to 0.18 wt.% Rb 2 O. Albite crystallized from the magma. K-feldspar is perthitic and shows an increase in Rb concentration (up to 0.83 wt.% Rb 2 O) in the apical part of the cupola. The partition coefficient K D–Rb Mi–Kfs is constant throughout the cupola, indicating a systematic re-equilibration of these two minerals with fluid. Lithian annite occurs below –735 m. Its F content decreases with depth and from –940.5 m, OH exceeds F. Magnesium and Ti concentrations show a remarkable positive correlation with depth. The TZ and adjacent area are characterized by strong variations in the chemical composition of micas, reflecting the fluctuation of saturation–oversaturation of residual liquid by a F-rich fluid phase. The Rb concentration in zinnwaldite (up to 2 wt.% Rb 2 O) strongly increases in the apical part of the cupola owing to a significant transfer of volatiles. The chemical composition of mica evolves gradually from the TZ to the apical zone. The discovery of lithian annite included in quartz at –97 m indicates the formation of zinnwaldite by interaction of lithian annite with F-rich fluid. The octahedral site of lithian annite allows the incorporation of Sn, Nb, Ta and W, replacing Ti. Because of the lithian annite → zinnwaldite transformation, these elements with higher ionic charges are expelled from the mica structure, transferred to the fluid phase and concentrated in the apical part of the cupola. The LA–ICP–MS analyses of the micas confirm this process. We envisage the transport of Sn, Nb, Ta and W in the form of fluorides and the precipitation of cassiterite by hydrolysis of SnF 4 . Tungsten was probably transported as H 2 WO 4 , resulting from the reaction of WF 6 with H 2 O. A similar behavior of Nb and Ta is suggested. The hydrolysis of fluorides leads to a strong enrichment of fluid in HF, which induced albite instability and formation of greisens. Calculations show that the amounts of Sn and W released by the transformation lithian annite → zinnwaldite transformation are close to the estimated reserves of these metals within the Cinovec cupola, corroborating the proposed metallogenic model.

Oxide minerals in the granitic cupola of the Jálama Batholith, Salamanca, Spain. Part I: accessory Sn, Nb, Ta and Ti minerals in leucogranites, aplites and pegmatites

*Corresponding author Sn–Nb–Ta–Ti oxides occur as accessory minerals in the granitic facies of the External Unit in the Jalama Batholith (Salamanca, Spain) and in the related LCT pegmatite dikes of the rare-element class. Moreover, abundant cassiterite and columbite-group minerals crystallized in the Cruz del Rayo peribatholithic pegmatite dikes, cutting the pre-Ordovician low-grade metasedimentary rocks of the surrounding Schist-Graywacke Complex. Cassiterite, rutile, ilmenite and tantalite-(Fe) occur disseminated in the border facies of the External Unit, especially throughout the tourmaline-bearing leucogranite and the apical aplites. Additionally, cassiterite, rutile and Ta-rich rutile developed locally in the intragranitic pegmatite dikes. Two types of peribatholithic pegmatites can be distinguished at Cruz del Rayo: (i) granite-like pegmatites, with columbite-(Fe) I and II and cassiterite, in which the influence of metasomatic fluids led to formation of the late albite unit and crystallization of tantalite-(Fe), and (ii) greisen-like bodies, which contain high amounts of columbite-(Fe), columbite(Mn), tantalite-(Mn) I and II as well as cassiterite. The primary oxide assemblage in both types of the peribatholithic pegmatite dikes would have crystallized as a result of magmatic differentiation of the residual melts coming from the Jalama granitic cupola. However, crystallization of the secondary assemblage, richer in Fe and Ta, is interpreted mainly as a consequence of interaction with external fluids coming from the metamorphic host rocks, more or less mixed with meteoric fluids, although partial dissolution and re-precipitation could have played an important role, as well.

Oxide minerals in the granitic cupola of the Jálama Batholith, Salamanca, Spain. Part II: Sn, W and Ti minerals in intra-granitic quartz veins

*Corresponding author The mineralized quartz veins hosted by the distal granitic facies of the Jalama Batholith were generated by successive processes of the fractures opening and filling by hydrothermal fluids enriched in volatiles. Cassiterite I, wolframite I and rutile I crystallized during an early stage, followed – after a ductile deformation – by a main Fe–Zn sulphide deposition, composed initially by subordinate cassiterite II and wolframite II. Later during this main sulphide deposition, ixiolite and Nb–Ta-rich rutile II filled fractures. Finally, the late open spaces and cavities were sealed by quartz and carbonates. The crystallization of wolframite II indicates enrichment in Mn of the fluid, compared with wolframite I, suggesting a normal evolutionary trend from the early Sn–W deposition to the first stage of the main Fe–Zn sulphide precipitation in the quartz veins. The scarcity of Sn during the latest stages of the ore deposition prevented the crystallization of cassiterite, favoring instead the incorporation of this element into the ixiolite and rutile II structure. This hydrothermal evolution from the crystallization of wolframite II to ixiolite and rutile II responded to enrichment in Ti and Fe of the late fluid, thus favoring a partially reversed trend in the Nb–Ta–Ti oxides composition. The late enrichment in Ti and Fe of the hydrothermal fluid could be explained by contamination by external metamorphic and/or meteoric fluids en riched in these elements. The incorporation of Ti, Nb and Ta into the ixiolite and rutile structure was favored by high F concentrations in the hydrothermal fluids, inherited from the residual melts of the Jalama Batholith.

Fluid evolution of the Hub Stock, Horní Slavkov–Krásno Sn–W ore district, Bohemian Massif, Czech Republic

The Horni Slavkov–Krasno Sn–W ore district is hosted by strongly altered Variscan topaz–albite granite (Krudum granite body) on the northwestern margin of the Bohemian Massif. We studied the fluid inclusions on greisens, ore pockets, and ore veins from the Hub Stock, an apical expression of the Krudum granite. Fluid inclusions record almost continuously the post-magmatic cooling history of the granite body from ∼500 to <50°C. Rarely observed highest-temperature (∼500°C) highest-salinity (∼30 wt.% NaCl eq.) fluid inclusions are probably the result of secondary boiling of fluids exsolved from the crystallizing magma during pressure release which followed hydraulic brecciation of the gneissic mantle above the granite cupola. The greisenization was related to near-critical low-salinity (0–7 wt.% NaCl eq.) aqueous fluids with low amount of CO2, CH4, and N2 (≤10 mol% in total) at temperatures of ∼350–400°C and pressures of 300–530 bar. Crush-leach data display highly variable and negatively correlated I/Cl and Br/Cl values which are incompatible with both orthomagmatic and/or metamorphic origin of the fluid phase, but can be explained by infiltration of surficial and/or sedimentary fluids. Low fluid salinity indicates a substantial portion of meteoric waters in the fluid mixture that is in accordance with previous stable isotope data. The post-greisenization fluid activity associated with vein formation and argillitization is characterized by decreasing temperature (<350 to <50°C), decreasing pressure (down to ∼50–100 bar), and mostly also decreasing salinity.

MINERALOGICAL RESPONSES TO SUBSOLIDUS ALTERATION OF GRANITIC ROCKS BY OXIDIZING As-BEARING FLUIDS: REE ARSENATES AND As-RICH SILICATES FROM THE ZINNWALD GRANITE, EASTERN ERZGEBIRGE, GERMANY

The granite cupola at Zinnwald (Germany) or Cinovec (Czech Republic) consists of highly evolved, weakly peraluminous and variably altered topaz – albite – Li mica leucogranites of A-type affiliation. In the apical part of this cupola, As-bearing REE–Y–Th–U–Zr minerals, including arsenoflorencite-(Ce), chernovite-(Y), hydrated chernovite-(Y) – xenotime-(Y) solid solutions and As-bearing thorite, coffinite and zircon, formed locally. Accompanying minerals comprise As-poor bastnasite-(Ce), synchysite-(Ce), synchysite-(Y), fluocerite-(Ce), and columbite-(Fe). Electron-microprobe analyses show that arsenoflorencite-(Ce) contains low to moderate concentrations of P, Pb, and particularly Sr (4.0–8.3 wt% SrO), reflecting substitution of florencite, arsenoflorencite, goyazite, and kintoreite components. Relative to published data, chernovite-(Y) is the most REE-rich yet recorded, with up to 0.51 apfu REE substituting for Y. All REE arsenates display a variably kinked REE CN pattern and a negative Eu anomaly. Thorite, coffinite, and zircon accommodated maximum concentrations of As of 14.4, 5.3, and 1.3 wt% As 2 O 5 , respectively. The results of this study demonstrate that As-rich hydrothermal fluids are susceptible to dissolve and severely alter primary magmatic REE–Y–Th–U–Zr minerals.

A geochemical study of the Sweet Home Mine, Colorado Mineral Belt, USA: hydrothermal fluid evolution above a hypothesized granite cupola

Deposition of quartz–molybdenite–pyrite–topaz–muscovite–fluorite and subsequent hübnerite and sulfide–fluorite–rhodochrosite mineralization at the Sweet Home Mine occurred coeval with the final stage of magmatic activity and ore formation at the nearby world-class Climax molybdenum deposit about 26 to 25 m.y. ago. The mineralization occurred at depths of about 3,000 m and is related to at least two major fluid systems: (1) one dominated by magmatic fluids, and (2) another dominated by meteoric water. The sulfur isotopic composition of pyrite, strontium isotopes and REY distribution in fluorite suggest that the early-stage quartz–molybdenite–pyrite–topaz–muscovite–fluorite mineral assemblage was deposited from magmatic fluids under a fluctuating pressure regime at temperatures of about 400°C as indicated by CO2-bearing, moderately saline (7.5–12.5 wt.% NaCl equiv.) fluid inclusions. LA-ICPMS analyses of fluid inclusions in quartz demonstrate that fluids from the Sweet Home Mine are enriched in incompatible elements but have considerably lower metal contents than those reported from porphyry–Cu–Au–Mo or Climax-type deposits. The ore-forming fluid exsolved from a highly differentiated magma possibly related to the deep-seated Alma Batholith or distal porphyry stock(s). Sulfide mineralization, marking the periphery of Climax-type porphyry systems, with fluorite and rhodochrosite as gangue minerals was deposited under a hydrostatic pressure regime from low-salinity ± CO2-bearing fluids with low metal content at temperatures below 400°C. The sulfide mineralization is characterized by mostly negative δ34S values for sphalerite, galena, chalcopyrite, and tetrahedrite, highly variable δ18O values for rhodochrosite, and low REE contents in fluorite. The Pb isotopic composition of galena as well as the highly variable 87Sr/86Sr ratios of fluorite, rhodochrosite, and apatite indicates that at least part of the Pb and Sr originated from a much more radiogenic source than Climax-type granites. It is suggested that the sulfide mineralization at the Sweet Home Mine formed from magmatic fluids that mixed with variable amounts of externally derived fluids. The migration of the latter fluids, that were major components during late-stage mineralization at the Sweet Home Mine, was probably driven by a buried magmatic intrusion.

Root Zones of Porphyry Systems: Extending the Porphyry Model to Depth

The root zone of a porphyry system is a specific region beneath a porphyry orebody that was a site of focused fluid flow, as evidenced by abundant quartz veins, widespread wall-rock alteration, or porphyry dikes merging downward into a porphyritic granite cupola. These zones constitute an important source region of ore fluids and other components, and in certain geologic terrains the characteristics of root zones may point to previously undiscovered deposits. The root zones of four Laramide porphyry copper systems in Arizona recently have been characterized at a reconnaissance level: the Miami Inspiration system associated with the Schultze Granite, the Sierrita-Esperanza system associated with the Ruby Star Granodiorite, the Ray system associated with the Granite Mountain pluton, and the Kelvin-Riverside system associated with the Tea Cup pluton. The two well-studied root zones related to the Jurassic Yerington batholith in Nevada, and associated with the Yerington mine and the Ann-Mason deposit, provide a basis of comparison. All six systems occur in areas with unusually large exposures in both lateral and vertical paleodirections, locally to paleodepths of >10 km, because of postore extensional faulting and associated tilting. No two systems are alike, but many share the presence of the following hydrothermal characteristics: quartz veins and potassic alteration, sodic-calcic and sodic alteration, calcic alteration, and relatively coarse grained muscovite-quartz (greisen). Quartz veins and potassic alteration are focused centrally, directly above related cupolas; sodic-calcic and sodic alteration, calcic alteration, and evidence for leaching of silica are observed on the deep flanks of certain systems; and greisen occurs directly beneath ore within and beneath coeval cupolas in many systems. Certain systems exhibit evidence of multiple cycles of release of magmatic fluid followed by incursion of saline ground waters, which are analogous to the biological cycle of exhale-inhale, respectively. The characteristics of the root zones provide important constraints on the exsolution and transport of the magmatic aqueous phase that leads to ore formation, the variable incursion of external fluids into the hydrothermal system, and the degassing of magmatic volatiles that may not be related directly to porphyry ore formation. The most robust conclusions are drawn from the localities that offer the greatest quality of exposure and degree of continuity (including compelling structural reconstructions) between the roots and the ore deposit, from the studies that identify timelines linking processes in the roots with those in the mineral deposit, and from systems in which the deposit itself is well characterized.

Modeling of geochemical processes occurred during the formation of the Schlema deposit, Erzgebirge. II. Formation of pre-uranium veins

By means of computer modeling, it was shown that the pre-uranium quartz and quartz-sulfide veins of the Schlema deposit with their typomorphic assemblages could be formed at the expense of cooling of hot pore solutions from the Aue granite cupola underlying the mineralized zones. As was shown previously [1], the ore metal content of these pore solutions changes with decreasing temperature and pressure in the granite-water system and an increase in the intensity of granite “flushing” with a water phase. The dependence of the composition of precipitated material on the proportions of chlorides and carbon dioxide in the pore solutions was evaluated in the models. It was shown that silica mobilization from the granite and redeposition were accompanied by the oxidation of ferrous iron in the granite and extraction of sulfide sulfur in molal amounts similar to or even higher than those of silica. This provided conditions for uranium migration after these stages and formation of hydrothermal uranium ores.

Metals, arsenic, and sulfur in the Aue and Eibenstock granites, Erzgebirge

Average concentrations of Pb, Zn, Cu, and other metals, as well as S and As, were calculated for the Aue granitic cupola, the contact aureole of which hosts the large vein-type uranium deposit of Schlema-Alberoda and the Schneeberg uranium-base metal deposit (Erzgebirge, Germany). The cupola was exposed by mine workings and boreholes, which provided an opportunity to evaluate variations in the abundances of metals in the granites over a vertical interval of more than 2.5 km and estimate their losses in the upper oxidized part of the investigated volume of the cupola (coefficient of iron oxidation, KO Fe, increases in the granites from bottom to top from 7 to 70%) compared with the lower unaltered and unoxidized part (with a KO Fe plateau at about 5%). The average concentrations of metals in the upper part of the cupola are lower than those in the lower part by a factor of 2.5 for Pb, 1.56 for Zn, 1.45 for Cu, 1.3 for Co, etc. A similar decrease in the abundances of ore elements along the vertical section associating with the relative epigenetic alteration and oxidation of the granite was previously described by us for U and Th and for the components of high-temperature ores, W, Sn, and Mo. The removal of ore elements from the granite was accompanied by a decrease in the bulk contents of sulfur and arsenic by a factor of 1.35 and 1.65, respectively. The leaching of trace metals from the granites of the upper part of the Aue cupola was followed by their partial redeposition above the cupola in the ore veins of the Schlema and Schneeberg deposits.

Modeling of geochemical processes that occurred during the formation of the Schlema deposit, Erzgebirge. I. Synmineralization processes in the Aue granites

The geochemical events that occurred within the outlines of the Schlema ore field during the formation of vein bodies cannot be understood without considering the processes that occurred synchronously in the footwall zone, in the Aue granite cupola, from where mineral-forming thermal solutions ascended into the deposit. It is known that the upper part of the cupola experienced hydrothermal alterations, which had a minor influence on the appearance of the rocks, but caused a number of important geochemical consequences: an increase in the degree of iron and sulfur oxidation in grantie, the replacement of accessory sulfides by oxygenbearing salts, etc. These changes were analyzed using computer models for a decrease in T and P of the granitewater system from 400°C (3.0–1.5 kbar) to 140°C (2.0–0.2 kbar). The role of these changes in the fate of disseminated ore elements of granite and in the extraction of ore elements from the changing rock into the equilibrium pore solution was evaluated.

Accessory minerals of the C�novec (Zinnwald) granite cupola, Czech Republic: indicators of petrogenetic evolution

A fully cored drillhole was drilled to 1596 m by the Czech Geological Survey in 1961–1963 in the central part of the Cinovec (Zinnwald) granite cupola. Two types of granite were intersected: zinnwaldite granite (ZG), observed down to a depth of 730 m, and protolithionite granite (PG), occurring to the end of the hole. The core was used to study the distribution and chemistry of: zircon, thorite, xenotime, monazite, bastnasite, synchysite, REE oxyfluorides and hydroxyfluorides. Zircon occurs throughout the drillcore; it is strongly hydrated and fluorinated with about 18.5 wt.% H2O content in the apical part of the cupola. Its F-content reaches 2.41 wt.%. Within the PG, the F concentration in zircon is low. Zircon is poor in Th and U and its HfO2 contents vary from 1.01 to 5.24 wt.%. Thorite is common in the PG, becoming rare in the ZG. It is strongly hydrated (up to 14 wt.% H2O) and fluorinated (up to 2.04 wt.% F). Extensive solid solution between ThSiO4 and YPO4 was observed. Xenotime is strongly hydrated (up to 16 wt.% H2O), but its F content is low (<0.31 wt.%). Two types of monazite were identified: Th-rich (up to 9.3 wt.% ThO2) in the ZG, and Th-poor (<2.5 wt.% ThO2) in the PG. Monazite remained stable during the hydration and fluorination process. Its REE chondrite-normalized distribution patterns show negative anomalies for La and Nd and a pronounced negative anomaly for Eu. Chemical compositions of several REE oxyfluorides and hydroxyfluorides were studied. REE fluorocarbonates are represented by bastnasite and synchysite. Bastnasite is abundant in the ZG. Its chondrite-normalized REE patterns are characterized by an important negative Eu anomaly and downward kinks at La and Nd. Synchysite-(Ce) and synchysite-(Y) are particularly well developed in the deeper parts of the cupola, and exhibit REE distribution patterns characterized by a weak negative Eu anomaly (synchysite-(Ce)), or a weak positive Eu anomaly (synchysite-(Y)).

Petrological and geochemical evolution of the Kymi stock, a topaz granite cupola within the Wiborg rapakivi batholith, Finland

The 6×3 km Kymi monzogranite stock represents the apical part of an epizonal late-stage pluton that was emplaced within the 1.65 to 1.63 Ga Wiborg rapakivi batholith. The stock has a well-developed zonal structure, from the rim to the center: stockscheider pegmatite, equigranular topaz granite, porphyritic topaz granite. The contact between the two granites is usually gradational within a few centimeters, but local inclusions of the porphyritic granite in the equigranular granite indicate that the latter solidified later. Hydrothermal greisen and quartz veins, some of which contain genthelvite, beryl, wolframite, cassiterite, and sulfides, cut the granites of the stock and the surrounding country rocks. The equigranular granite contains 1 to 4 vol.% topaz, and its biotite is lithian siderophyllite; the porphyritic granite has 0 to 3 vol.% topaz, and the mica is siderophyllite. The equigranular granite is geochemically highly evolved with elevated Li, Rb, Ga, Ta, and F, and very low Ba, Sr, Ti, and Zr. The REE patterns show deep negative Eu anomalies and tetrad effects indicating extreme magmatic fractionation and aqueous fluid–rock interaction. The zonal structure of the stock is interpreted as a result of differentiation within the magma chamber. Internal convection in the crystallizing magma chamber and upward flow of residual melt as a boundary layer along sloping contacts resulted in accumulation of a layer of highly evolved, volatile-rich magma in the apical part of the chamber. Crystallization of this apical magma produced the stockscheider pegmatite and the equigranular granite; the underlying crystal mush solidified as the porphyritic granite. Much of the crystallization took place from volatile-saturated melt, and episodic voluminous degassing expelled fluids into opened fractures where they or their derivatives reacted with country rocks and caused alteration and mineralization.

Fluid extraction from quartz in sheeted leucogranites as a monitor to styles of uranium mineralization: an example from the Rössing area, Namibia

The Rössing Mine area in Namibia contains uranium mineralization primarily within crustally derived post-collisional sheeted leucogranites. Adjacent to the mine in the SH area, a small 1-km long granite cupola of coalesced sheets is also characterized by elevated U levels. In the Rössing pit, in the SJ area, U is hosted by uraninite and secondary U minerals including beta-uranophane. Conversely, in the SH area, U is hosted principally by the pyrochlore-group mineral, betafite. Manometric determination of the proportions of H 2 O, CO 2 and non-condensables (mostly methane) has been carried out on inclusions within quartz from leucogranite samples from both the SH and SJ anomalies. This method has been able to distinguish between leucogranites of the SH and SJ areas which has not been possible with thermometric fluid inclusion studies or by major and trace element geochemical investigations. Fluid geochemistry of sheeted leucogranites from the SH area exhibit lower absolute H 2 O contents, lower H 2 O/CO 2 ratios and low total fluids compared to that from the SJ area. Fluid geochemistry and large ion lithophile (LIL) element data suggest that the occurrence of the SJ anomaly as a major uranium deposit is linked to the high H 2 O and total fluid contents. This study illustrates the usefulness of this technique to distinguish between economic mineralization and the sub-economic SH anomaly.

Les micas de la coupole granitique de Cı́novec (Zinnwald), République tchèque : un nouvel aperçu sur la métallogenèse de l'étain et du tungstène

Abstract The zonation of Cinovec granite cupola results from a transformation protolithionite → zinnwaldite due to the interaction with a F-rich fluid. This is a major metallogenic process leading to the release of Ti, Sn, Nb, Ta and W from the protolithionite crystal structure and to the transfer of these elements into the F-rich fluid phase. By analogy with REE-oxyfluorides discovered at Cinovec, the transportation of Sn as SnF4, whose hydrolysis leads to SnO2 (cassiterite) and of W as WF6, which reacts with H2O yielding H2WO4, is suggested. Accessory mineral phases included in protolithionite (zircon, thorite, xenotime) are strongly hydrated and fluorinated. Thorite, monazite, xenotime and rutile become unstable in the zinnwaldite granite.

Tantalum mineralization in the apical part of the Cínovec (Zinnwald) granite stock

Granite-hosted tantalum mineralization was studied in the apical part of the Cinovec (Zinnwald) granite stock (eastern Krusne hory, Czech Republic). Samples of granite and greisen from drill hole CS-1 (1596m deep), situated in the centre of the granite body were analysed chemically and examined by electron microprobe. The apical, mainly lepidolite granite with 24 to 212 ppm Ta contains Ta-rich Mn columbite, uranmicrolite and strueverite (tantalian rutile). During greisenization the contents of Ta decreased or remained the same, but the newly formed cassiterite has elevated contents of Ta, probably due to remobilization from albitized granites. Niobium and tantalum mineralization can be linked with both magmatic and postmagmatic stages of granite evolution. Ta-rich columbite formed as a result of albitization dominant at the top of the granite cupola, whereas magmatic Nb-rich columbite is present throughout the entire profile of the granite stock.