Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.

Modeling Changes in Internal Pressure During the Solidification of Hydrous Granitic Intrusives: Implications for the Crystallization of Miarolitic Pegmatites

ABSTRACT Volatile exsolution is widely recognized as an important trigger for eruptions from shallow magma reservoirs, but relatively few studies quantify the effects of exsolution on internal pressure within deeper-seated intrusive bodies. We present a model to predict internal pressure changes during the crystallization of a haplogranite melt containing 3 and 5 mass % H2O and with an emplacement pressure of 200 MPa. Mass and volume relations between phases are used to determine internal pressure assuming a closed, constant-volume system. The results indicate that initial crystallization of alkali feldspar and quartz causes a decrease in pressure prior to the exsolution of an aqueous fluid from the residual melt (i.e., resurgent boiling). Further crystallization toward the core of the body in the presence of a separate volatile phase results in a sharp increase in internal pressure. Our model shows that in closed, isochoric systems, the crystallization of the H2O-saturated melt will generate internal pressures that greatly exceed emplacement pressures typical of miarolitic pegmatites. Extreme overpressure modifies the physical and chemical properties of the residual melt and coexisting aqueous fluid, which in turn influences crystallization kinetics and the development of primary textures. Primary melt and fluid inclusions in pocket minerals thus likely represent samples trapped at various pressures in a rapidly evolving melt–fluid system. In most pegmatites, increasing fluid pressure and the formation of large pockets is regulated by the permeability and tensile strength of the enclosing rock. This explains why many miarolitic pegmatites occur within rigid host rocks such as granite, gabbro, and gneiss.

Features of Mineral Crystallization at Different Stages of the Magmatism Evolution of the Gorely Volcano (Kamchatka): Data on Melt and Fluid Inclusions

––Studies of melt and fluid inclusions and minerals as well as computational modeling (based on the data on the composition of melt inclusions, clinopyroxenes, and amphiboles) gave an insight into the physicochemical parameters of magmatic systems during the evolution of the precaldera Pra-Gorely Volcano and during the subsequent formation of rock complexes of the Young Gorely Volcano. The estimated temperatures of crystallization of olivine, clinopyroxene, and plagioclase phenocrysts (1115–1260 °С) and amphibole (740–890 °С) are in agreement with the earlier published data on the magmatism of the Gorely Volcano. Computational modeling based on the compositions and homogenization temperatures of melt inclusions showed that the established depth interval of mineral crystallization (21.0–1.5 km) with pressures of 7.0–0.5 kbar can be divided into two ranges, 21–15 km and 9.0–1.5 km. Both the Pra-Gorely and Young Gorely volcanoes have magma chambers in these depth ranges. The Pra-Gorely Volcano is characterized by higher temperatures of mineral crystallization (1240–1190 °С) as compared with the Young Gorely Volcano (1190–1125 °С). The presence of primary fluid inclusions with low-density CO2 and of syngenetic primary melt inclusions in plagioclase of the Pra-Gorely Volcano indicates that the mineral crystallized from a heterophase melt. At the same time, the cores of plagioclase phenocrysts formed from a homogeneous melt. A drastic drop in pressure led to the phase separation of magma throughout the magma column (upper and lower chambers) and to the growth of zones saturated with CO2 fluid inclusions in the plagioclase crystals formed from a two-phase melt. The subsequent closure of the system and the disappearance of CO2 phase resulted in the growth of plagioclase from a homogeneous melt.

Cassiterite crystallization experiments in alkali carbonate aqueous solutions using a hydrothermal diamond-anvil cell

Ore-forming fluids enriched in alkali carbonate are commonly observed in natural melt and fluid inclusions associated with tin mineralization, particularly in granitic pegmatite. However, the roles of alkali carbonates remain unclear. Hence, to investigate the roles of alkali carbonate, herein, cassiterite (SnO2) crystallization experiments in SnO2–Li2CO3–H2O and SnO2–Na2CO3–H2O systems were conducted using a hydrothermal diamond-anvil cell. The results showed that SnO2 could dissolve into the alkali carbonate aqueous solution during heating, and long prismatic cassiterite crystals grew during the subsequent cooling stage at average rates of 0.61 × 10–6 to 8.22 × 10–6 cm/s in length and 3.40–19.07 μm3/s in volume. The mole fraction of cassiterite crystallized from the SnO2–Li2CO3–H2O system ranges from 0.03 to 0.41 mol%, which depends on the Li2CO3 content dissolved in the aqueous solution. In situ Raman analysis of the alkali carbonate-rich aqueous solution in the sample chamber suggests that the dissolution of SnO2 can be attributed to the alkaline conditions produced by hydrolysis of alkali carbonate in which Sn(OH)62− may be a potential tin-transporting species. The cassiterite crystallization conditions obtained in our SnO2–alkali carbonate–H2O systems primarily fell within the 400–850 °C and 300–850 MPa temperature and pressure ranges, respectively; furthermore, cassiterite crystallization ended in rare metal pegmatite-forming conditions. These crystallization features of cassiterite are similar to those formed in tin-mineralized granitic pegmatites. It indicates that an alkali carbonate-rich aqueous solution or hydrous melt can work as a favorable transport medium for tin and provides the necessary conditions for cassiterite crystallization in granitic pegmatite, bearing the roles in decreasing the viscosity of hydrous melts and enhancing the solubility of SnO2 in ore-forming melts or fluids. These roles of alkali carbonate can also be extended for the mineralization of other rare metals (e.g., Li and Be) in granitic pegmatite.

Features of heat and mass transfer processes under the Avachinsky volcano (Kamchatka)

<p>We investigated the processes beneath the Avacha volcano using mantle peridotite xenoliths  the with the EPMA, electronic microscope and ICP methods and  numeric modeling of the mass transfer accounting the melt fluid reactions with peridotites</p><p>The decompression melting processes  in peridotites beneath Avachinsky volcano (Kamchatka) are associated with seismic events. After the reactions with the Si, Ca, Na, K  from partial  melts associated  with  the  subduction related fluids the spinel and orthopyroxene were melted and essentially clinopyroxene veins were formed. Secondary crystals growth in the mantle xenoliths (with melt and fluid inclusions) are associated possibly with  the fluids appeared  due to retrograde boiling of the magma chamber beneath the volcano.</p><p>The processes of sublimation and recrystallization of  Avacha harzburgites was investigated at the facility in the Institute of  Nuclear Physics (Novosibirsk, Russia), which generates high-density electron beams and makes it possible to obtain boiling ultrabasic and basic liquids and condensates of magmatic gas on the surface of  harzburgite.</p><p>Results of  experiments provides a satisfactory explanation for the observed local heterophase alterations within ultramafic rocks that have experienced multistage deformation beneath volcanoes of the Kamchatka volcanic front.</p><p>Mathematical model of convective heating and metasomatic reactions in harzburgites were modeled using the  Selector PC thermodynamic software. The obtained virtual dynamic patterns of metasomatic zoning across the mantle wedge show   how   composition   variations   of   fluids   and  PT  conditions   at   their   sources   influence   the   facies   of   metasomatized   mantle   wedge harzburgite.   Such processes are apparently common to seismically  deformed   permeable   lithosphere   above   magma   reservoirs.  </p><p>There are two regions fluid filtration conditions under the Avachinsky volcano which are regulated by the tectonic conditions. The lower field where compression conditions prevail. And the upper field, where the prevailing tensile conditions and intense seismic destruction of the rocks of the crust and upper mantle. The heat flux distribution shows the manifestation of the convective heating mechanism in the earth's crust over the most permeable fault zones.</p><p>The study of the composition of the gas phases and melt inclusions suggests that the partial melting of metasomatized ultrabasites occurs in the range of 1150 ° C <T <1200 ° C.</p><p>In accord with the composition of the glassy phase in the melt inclusions of spinel crystals, the harzburgite metasomatism in the local melting sites is associated with brine melts that bringing Ca, K, Na, Si. C. The work was financially supported by the Russian Foundation for Basic Research, Grants No. 16-29-15131, 16-01-00729.</p><p>References</p><p>Arai S., Ishimaru S. Insights into Petrologycal Characteristics of the Lithosphere Mantle Wedge beneath Arcs through Peridotite Xenoliths: a Review.// J. Petrol., 2008. V.49(4), 359-395.</p><p>Tomilenko A.A., Kovyazin S.V., Sharapov V.N., Timina T.Yu., Kuzmin D.V. Metasomatic recrystallization and melting of ultrabasic rocks of mantle wedge beneath Avacha Volcano, Kamchatka // ACROFI III and TBG XIV Abstracts Volume / SB RAS IGM, Novosibirsk: Publishing House of SB RAS, 2010, p. 248-249.</p>

Badzhal tin magmatic-fluid system (Far east, Russia): the transition from the granite crystallization to the hydrothermal ore deposition

With a view to reveal special characteristics of the transition stage from granite crystallization to rare-metal ore deposition it is studied Badzhal tin-bearing magmatic-fluid system of eponymously-named volcano-plutonic zone of the Middle Priamyrie. For that end the detail research of melt, fluid-melt and fluid inclusions and oxygen isotopes from minerals of granitoids from Verkne-Urmi massif from Badzhal volcano-plutonic zone and also minerals of Sn-W deposits Pravo-Urmi and Blizhnee have been carried out. The formation of greisens and hydrothermal veins were caused by the development of the integrated system associating with establishing of Verkne-Urmi granite massif which is one of a dome fold of Badzhal cryptobatholith. For the first time for tin deposits it has been followed up the transition from the magmatic phase of granite crystallization to the hydrothermal ore formation stage and the evolution of magmatic fluid from its separation from magmatic melt to Sn-W ore deposition. The direct evidence of tin-bearing fluid separation under melt crystallization is combined fluid-melt inclusions. Glass composition in inclusions shows that granites and granite-porphyry were crystallizing from acid and from limited to high-aluminous melts, that is value ASI changes from 0.95 to 1.33 and a content of alkalies varies from 6.02 up to 9.02 mass.%. Cl and F concentrations in glasses are according 0.03–0.14 and 0.14–0.44 mass.% and turned out to be higher of same in the total composition of rocks (0.02 and 0.05–0.13 mass.% in accordance). These differences indicate that Cl and F could be separated from granite melt under its crystallization and degasation. H2O content made from total deficiency electron microprobe analysis is 8–11 mass.%. This evaluation was made inclusive of a probable effect of “Na loss” (Nielsen, Sigurdson, 1981) under aqueous glass crystallization. Considering a high error of a such estimation (Devine et al., 1995), it should take to obtained values as a very approximate evaluation and consider that examined melts contained about 9,5–10,0 mass.% of H2O. The results of melt inclusion examination show that at any rate a part of melt forming magmatic rocks of Badzhal Ore Magmatic System are crystallizing at about T = 650 °C. These melts were acid, limited fluoride and meta- and high aluminous. The reason of low temperatures of its crystallization are likely a high pressure of aqua and also a increased content of F. Most likely that examined inclusions characterize the final stage of establishing of the massif, herewith at the system crystals, residual liquor and magmatic fluid phase coexist. The fluid from which greisens of Pravo-Urmi deposit formed is similar in properties to the supercritical fluid absorbing by magmatic minerals. The salinity of this fluid varying from ~9 to 12 mass.% equiv. NaCl, maximal T = 550 °C (with consideration for the temperature correction of T gom on a pressure ~1 кbar) are similar to such of magmatic fluid, which permit to connect its origin with pluton cooling. The formation of greisens and quartz-topaz veins of Pravo-Urmi deposit is related to fall of temperature of magmatic fluid from 550–450 up to 480–380 °C. The evolution of fluid deposited quartz-cassiterite veins of Blizhnee deposit, which based upon oxygen isotope composition (d18ОН2О ≈ 8.5‰) also separated from magma, was going at more subsurface conditions under much lesser pressure. That led to the gas separation of a fluid with salinity ~13 mass.% equa. NaCl under T = 420–340 °C on thin low salinity vapour and brine with concentration 33.5–37.4 mass.% equiv. NaCl. The research of oxygen isotope system testifies that oxygen isotope composition of ore-forming fluid controlled by equilibrium with granites at wide interval tempera­tures (from ~700 °С up to the beginning of greisen crystallization). Correspondence of measured and calculation data of the offered model indicates that the considerable volume of external fluid with other isotope characteristics which did not reach the isotope equilibrium with Verkhne-Urmi massif did not come into the magmatic isotope system. The discovered differences of physico-chemical conditions for two studied deposits are not “critical” and support an idea about their formation as the single magmatic-fluid system.