Fluid Entrapment Research Articles

The Presence of Carbonic-Dominant Volatiles during the Crystallization of Sulfide-Bearing Mafic Pegmatites in the North Roby Zone, Lac des Iles Complex, Ontario

Mafic pegmatites in the platinum-group element (PGE)-mineralized Roby zone, Lac des Iles Complex, northwest Ontario, Canada, comprise dikes, veins, and irregular pods of coarse-grained magnesiohornblende, pyroxene, and labradorite-andesine with minor biotite, apatite, Fe-Ti oxides and intercumulus quartz that grades into massive quartz or graphic quartz-base metal sulfide-vysotskite [Pd, Ni(S)] intergrowth at their cores. Quartz, apatite, and magnesiohornblende host primary and secondary assemblages of one- or two-phase carbonic fluid (CO 2 ± up to ~10% CH 4 ± minor H 2 O, N 2 ) inclusions that contain ore metals (Ni, Cu, Pd, Bi, Te, Fe). Rare trails of late-stage, high-salinity, aqueous fluid inclusions are secondary in origin and, therefore, unrelated to the crystallization of the pegmatites. Assemblages of primary carbonic fluid inclusions show considerable variation in mode and temperature of homogenization, reflecting large fluctuations in confining pressure at the time of quartz crystallization of as much as ~1 kbar (in single quartz crystals) and ~2.8 kbars (all data). Independent thermobarometric methods constrain conditions for the following two stages of pegmatite formation (and carbonic fluid entrapment): (1) the crystallization of magnesiohornblende-plagioclase intergrowth at T ~650° to 850°C, and P ~1 to 3 kbars; and (2) the crystallization of quartz at T ~535° to 650°C, and P ~0.4 to 3.2 kbars, setting the maximum depth of emplacement of the Lac des Iles Complex North Roby zone magma at 10 to 12 km. The results indicate that aqueous-dominant volatile phases were absent during the crystallization of pegmatitic gabbroic rocks at the Lac des Iles Complex and that water-poor, carbonic fluid entrapment persisted to well below solidus conditions. A role played by carbonic fluid as a potential transport medium for ligands involved in the precipitation and remobilization of the PGE and base metals is strongly suggested and warrants further investigation.

Fluid Inclusion Constraints on the Genesis of Gold in the Darasun District (Eastern Transbaikalia), Russia

Fluid Inclusion Constraints on the Genesis of Gold in the Darasun District (Eastern Transbaikalia), Russia

Internal hydrocephalus, external hydrocephalus, and the syndrome of intracerebral cerebrospinal fluid entrapment: a challenge to current theories on the pathophysiology of communicating hydrocephalus

The natural history of external hydrocephalus (EH) in adults is often marked by conversion into internal hydrocephalus. We describe a complication of this conversion, the ICE (intracerebral CSF entrapment) phenomenon, and demonstrate that both EH and ICE represent a challenge to current theories on the pathophysiology of communicating hydrocephalus (CHC). We propose a new model for CHC where the pattern of CSF distribution is largely determined by the intrinsic compliance of each of the intracranial structures. In this model, failure of distal CSF absorption resulting in an excessive intracranial CSF volume is the common denominator and CSF diversion the common solution to all forms of CHC.

Volcanological constraints on the post-emplacement zeolitisation of ignimbrites and geoarchaeological implications for Etruscan tomb construction (6th–3rd century B.C) in the Tufo Rosso a Scorie Nere, Vico Caldera, Central Italy

We examine the role of physical volcanological processes including eruption style (magmatic versus phreatomagmatic) as well as transport and depositional processes (pyroclastic fall versus pyroclastic flow) in promoting the ideal hydrologic conditions necessary for large scale, homogeneous, post-emplacement zeolitisation of ignimbrites, associated with caldera forming eruptions. We consider the Tufo Rosso a Scorie Nere (TRSN) of Vico Caldera (151 ka), in central Italy. The TRSN exhibits pervasive, homogenous alteration of high alkali tephriphonolitic and phonolitic glass to zeolite minerals (chabazite and phillipsite) in all regions of the study area and at all stratigraphic levels with the exception of the basal 1 m. Based on detailed lithological studies, we propose that a large geothermal field around the vent area was destroyed during the closing stages of the Sutri eruption. Subsequent incorporation and entrapment of superheated geothermal fluids into the ensuing pyroclastic flow during eruption column collapse greatly influenced the emplacement temperature and provided the necessary water required for post-emplacement zeolitisation of the TRSN. We suggest that the absence of zeolitisation at the base of the ignimbrite is directly related to transport conditions reflecting cooler regions in the lower portions of the deposit where the flow came into contact with the underlying substrate. We also consider the geoarchaeological implications of enhanced strength and cohesiveness provided by the zeolite rock framework on Etruscan tomb location and burial architecture in the Vico region. The TRSN contains literally hundreds of hypogeum-style Etruscan tombs at a number of sites across the study area. This study focuses on two sites in particular, the Norchia Necropoli and the San Guiliano Necropoli. Considering the enhanced mechanical properties of zeolitised ignimbrites we infer that physically the TRSN would still have been a relatively soft rock, suitable for the carving of tombs. However, we infer the increased strength and cohesiveness provided by the zeolite framework enhanced the conservation potential of these tombs, preserving them for over two thousand years.

A method to synthesize large fluid inclusions in quartz at controlled times and under unfavorable growth conditions

A new synthesis technique allows large fluid inclusions to be produced at conditions under which one normally would obtain only small inclusions, with the additional advantage that the timing of fluid entrapment can be controlled. In the first step, primary fluid inclusions are grown at P-T conditions under which it is relatively easy to produce large inclusions. In the second step, which can be performed either during the same experiment or during a separate experiment after a desired time of pre-equilibration, some of the primary inclusions produced during the first step are re-opened by in-situ fracturing, causing partial replacement of the inclusion content. The sample is then left at high pressure and temperature until the cracks leading to the re-opened inclusions are healed. To test the method and quantify the efficiency of fluid replacement in the re-opened inclusions, we produced primary inclusions at 700 °C/200 MPa and re-opened them during a second experiment in the presence of a compositionally different fluid at 500 °C/70 MPa. Laser-ablation ICP-MS (LA-ICP-MS) analyses of eight primary and 11 refilled inclusions demonstrate that in the latter more than 97% of the original fluid was replaced by new fluid. Thus, the refilled inclusions are representative of the surrounding fluid at the time of in situ fracturing.

Diffusion Weighted Imaging in Heroin-associated Spongiform Leukoencephalopathy

Heroin-associated spongiform leukoencephalopathy (HASL) is a rare complication of heroin abuse. We report a case that highlights the increased sensitivity of diffusion weighted imaging (DWI) compared with T2-weighted imaging in the acute setting of HASL. Single case report. A 36-year-old male with a history of heroin abuse (snorting) presented with a 3-day history of lethargy. MRI brain revealed restricted diffusion involving the globus pallidum and cerebral cortex bilaterally that was not seen on fluid-attenuated inversion recovery (FLAIR) images. The patient was diagnosed with acute HASL. Repeated MRI FLAIR at 3 months confirmed the development of atrophy and T2 hyperintensity in the subcortical white matter, consistent with leukoencephalopathy. Neurological exam at 3-month follow-up was nonfocal. Restricted diffusion, which likely corresponds to electron microscopic findings of fluid entrapment between the myelin lamellae, may be detectable earlier than changes on FLAIR. Clinicians should be aware of the neuroimaging findings of HASL and the increased sensitivity of MRI DWI over T2-weighted images in detecting HASL acutely.

Composition and evolution of fluids during skarn development in the Monte Capanne thermal aureole, Elba Island, central Italy

AbstractWe describe the chemistry of the fluids circulating during skarn formation by focusing on fluids trapped in calcsilicate minerals of the inner thermal aureole of the Late Miocene Monte Capanne intrusion of western Elba Island (central Italy). Primary, CH4‐dominant, C‐O‐H‐S‐salt fluid inclusions formed during prograde growth of the main skarn‐forming mineral phases: grossular/andradite and vesuvianite. The variable phase ratios attest to heterogeneous entrapment of fluid, with co‐entrapment of an immiscible hydrocarbon–brine mixture. Chemical elements driving skarn metasomatism such as Na, K, Ca, S and Cl, Fe and Mn were dominantly partitioned into the circulating fluid phase. The high salinity (apparent salinity between 58 and 70 wt% NaCl eq.) and the C‐component of the fluids are interpreted as evidence for a composite origin of the skarn‐forming fluids that involves both fluids derived from the crystallizing intrusion and contributions from metamorphic devolatilization. Oxidation of a Fe‐rich brine in an environment dominated by fluctuation in pressure from lithostatic to hydrostatic conditions (maintained by active crack‐sealing) contributed to skarn development. Fluid infiltration conformed to a geothermal gradient of about 100°C km−1, embracing the transition from high‐temperature contact metamorphism and fluid‐assisted skarn formation (at ca 600°C) to a barren hydrothermal stage (at ca 200°C).

Multifocal desmoplastic noninfantile astrocytoma

This is a report of a case of multifocal desmoplastic astrocytoma in an 11-year-old child in which we describe the MRI findings and discuss the possible mechanism of its development. The MRI appearances in our case support the view that the tumor is primarily of leptomeningeal or superficial cortical origin, with cystic formation secondary to entrapment of cerebrospinal fluid. The question of whether or not the lesions are metastases or metachronous lesions is also discussed. Desmoplastic astrocytoma at a noninfantile age is extremely rare: only four cases have been reported in the literature so far. Even more unusual is the presence of this lesion in multiple locations at the initial presentation.

Fluid entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P– T– d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by ( L ≫ S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO 2 inclusions by preferential loss of H 2O. Those inclusions armoured within garnet retained their primary 3-phase H 2O–CO 2 compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H 2O leakage along dislocations and planar lattice defects which results in the predominance of CO 2 inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO 2-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear. This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P– T conditions, are a welcome supplement for the reconstruction of a rather detailed P– T– d path.

Solubility of tin in (Cl, F)-bearing aqueous fluids at 700 °C, 140 MPa: A LA-ICP-MS study on synthetic fluid inclusions

Synthetic fluid inclusions in quartz were grown from cassiterite-saturated fluids in cold-seal pressure vessels at 700 ° C / 140 MPa / f O 2 ∼ NNO and subsequently analyzed by laser ablation-ICP-MS. Most inclusions were synthesized using a new technique that allows entrapment of fluid that had no immediate contact to the capsule walls, such that potential disequilibrium effects due to alloying could be avoided. Measured Sn solubilities increase with increasing ligand concentration in the fluid, ranging from 100 to 800 ppm in NaCl-bearing fluids (5–35 wt% NaCl), from 70 to 2000 ppm in HF-bearing fluids (0.5–3.2 m HF), and from 0.8 to 11 wt% in HCl-bearing fluids (0.5–4.4 m HCl). Two runs performed with the in-situ cracking method after 1 week of pre-equilibration demonstrate that the speed of hydrogen diffusion through the capsule wall relative to that of fluid inclusion formation is a critical factor in f O 2 -dependent solubility studies. Graphical evaluation of the solubility data suggests that Sn may have been dissolved as Sn(OH)Cl in the NaCl-bearing fluids, as Sn(OH)Cl and SnCl 2 in the HCl-bearing fluids, and as SnF 2 in the HF-bearing fluids. Experiments with NaF-bearing fluids produced an additional melt phase with an approximate composition of 53 wt% SiO 2, 25 wt% H 2O, 14 wt% NaF and 8 wt% SnO, which caused the composition of the coexisting fluid to be buffered at 0.5 wt% NaF and 150 ppm Sn. Fluorine-rich, peralkaline melts may therefore serve as important transport media for Sn in the final crystallization stages of tin granites. Based on the available cassiterite-solubility data in fluids and melts, D Sn fluid/melt in natural granite systems is estimated to be in the order of 0.1–4 (depending on their aluminosity), suggesting that Sn is not easily mobilized by magmatic-hydrothermal fluids. This interpretation is in accordance with the high degrees of Sn-enrichment commonly observed in highly fractionated melt inclusions. D Sn fluid/melt is primarily controlled by the HCl concentration in the fluid, which in turn is a function of the aluminum saturation index of the magma. Compared to HCl, the effect of fluorine on D Sn fluid/melt is subordinate.

Fault–valve action and vein development during strike–slip faulting: An example from the Ribeira Shear Zone, Southeastern Brazil

Fluid inclusion microthermometry and structural data are presented for quartz vein systems of a major dextral transcurrent shear zone of Neoproterozoic–Cambrian age in the Ribeira River Valley area, southeastern Brazil. Geometric and microstructural constraints indicate that foliation–parallel and extensional veins were formed during dextral strike–slip faulting. Both vein systems are formed essentially by quartz and lesser contents of sulfides and carbonates, and were crystallized in the presence of CO 2–CH 4 and H 2O–CO 2–CH 4–NaCl immiscible fluids following unmixing from a homogeneous parental fluid. Contrasting fluid entrapment conditions indicate that the two vein systems were formed in different structural levels. Foliation–parallel veins were precipitated beneath the seismogenic zone under pressure fluctuating from moderately sublithostatic to moderately subhydrostatic values (319–397 °C and 47–215 MPa), which is compatible with predicted fluid pressure cycle curves derived from fault–valve action. Growth of extensional veins occurred in shallower structural levels, under pressure fluctuating from near hydrostatic to moderately subhydrostatic values (207–218 °C and 18–74 MPa), which indicate that precipitation occurred within the near surface hydrostatically pressured seismogenic zone. Fluid immiscibility and precipitation of quartz in foliation–parallel veins resulted from fluid pressure drop immediately after earthquake rupture. Fluid immiscibility following a local pressure drop during extensional veining occurred in pre-seismic stages in response to the development of fracture porosity in the dilatant zone. Late stages of fluid circulation within the fault zone are represented dominantly by low to high salinity (0.2 to 44 wt.% equivalent NaCl) H 2O–NaCl–CaCl 2 fluid inclusions trapped in healed fractures mainly in foliation–parallel veins, which also exhibit subordinate H 2O–NaCl–CaCl 2, CO 2–(CH 4) and H 2O–CO 2–(CH 4)–NaCl fluid inclusions trapped under subsolvus conditions in single healed microcracks. Recurrent circulation of aqueous–carbonic fluids and aqueous fluids of highly contrasting salinities during veining and post-veining stages suggests that fluids of different reservoirs were pumped to the ruptured fault zone during faulting episodes. A fluid evolution trending toward CH 4 depletion for CO 2–CH 4–bearing fluids and salinity depletion and dilution (approximation of the system H 2O–NaCl) for aqueous–saline fluids occurred concomitantly with decrease in temperature and pressure related to fluid entrapment in progressively shallower structural levels reflecting the shear zone exhumation history.

Behavior of a Viscous LNAPL Under Variable Water Table Conditions

An intermediate-scale experiment in a 1.02-m-long, 0.75-m-high, and 0.05-m-wide flow cell was conducted to investigate the behavior of a viscous LNAPL under variable water table conditions. Two viscous LNAPL volumes (0.4 L) were released, one week apart, from a small source zone on top of the flow cell into a partly saturated, homogenously packed porous medium. Following a redistribution period of 30 days after the second release, the water table was increased 0.5 m in 50 minutes. After the water table rise, viscous LNAPL behavior was monitored for an additional 45 days. Fluid saturation scans were obtained periodically with a fully automated dual-energy gamma radiation system. Results show that both spills follow similar paths downwards. Within two hours after the first LNAPL arrival, the capillary fringe was reduced across the cell by approximately 0.04 m (22%). This reduction is directly related to the decrease in the air-water surface tension from 0.072 to 0.057 N/m. LNAPL drainage from the unsaturated zone was relatively slow and a considerable residual LNAPL saturation was observed after 30 days of drainage. Most of the mobile LNAPL moved into the capillary fringe during this period. After a rapid 0.5 m water table rise, the LNAPL moved up in a delayed fashion. The LNAPL used the same path upwards as it used coming down during the infiltration phase. After 45 days, the LNAPL had moved up only approximately 0.2 m. Since the LNAPL had only moved up a limited amount, nonwetting fluid entrapment was limited. The experiment was simulated using the STOMP multifluid flow simulator, which includes entrapped and residual LNAPL saturation formation. A comparison indicates that the simulator is able to predict the observed phenomena well, including residual saturation formation in the vadose zone, and limited upward LNAPL movement after the water table rise. The results of this experiment show that viscous mobile LNAPL, subject to variable water table conditions, does not necessarily float on the water table and may not appear in an observation well.

Linking drainage front morphology with gaseous diffusion in unsaturated porous media: A lattice Boltzmann study

The effect of drainage front morphology on gaseous diffusion through partially saturated porous media is analyzed using the lattice Boltzmann method (LBM). Flow regimes for immiscible displacement in porous media have been characterized as stable displacement, capillary fingering, and viscous fingering. The dominance of a flow regime is associated with the relative magnitudes of gravity, viscous, and capillary forces, quantifiable via the Bond number Bo, capillary number Ca, and their difference, Bo-Ca . Forced drainage from an initially saturated two-dimensional (2D) porous medium was simulated and the resulting flow patterns were analyzed and compared with theoretical predictions and experimental results. The LBM simulations reproduced expected flow morphologies for a range of drainage velocities and gravitational forces (i.e., a range of capillary and Bond numbers). Furthermore, measures of drainage front width as a function of the dimensionless difference Bo-Ca correspond well with scaling laws derived from percolation theory. Effects of flow morphology on residual fluid entrapment and gaseous diffusion were assessed by running LBM diffusion simulations through the partially saturated domain for a range of water contents. The effective diffusion coefficient as a function of water content was estimated for three regimes: stable drainage front, capillary fingering, and viscous fingering. Significant reductions in gaseous diffusion coefficient were found for viscous fingering relative to stable displacement, and to a lesser extent for capillary fingering, indicating that wetting phase distribution with a high degree of fingering in the 2D domain severely restricts connectivity of gas diffusion pathways through the medium. The study lends support for the use of LBM in design and management of fluids in porous media under variable gravity, and enhances the understanding of the role of dynamic fluid behavior on macroscopic transport properties of partially saturated porous media.

Implications of super dense carbonic and hypersaline fluid inclusions in granites from the Ranchi area, Chottanagpur Gneissic Complex, Eastern India

A combined fluid inclusion and mineral thermobarometric study in groups of synchronous inclusions in quartz within weakly foliated granites from the Chottanagpur Gneissic Complex, India, reveals super dense carbonic (CO 2 with minor CH 4 and H 2O) inclusions and hypersaline (H 2O–NaCl ± NaHCO 3) inclusions, with halite- and nahcolite daughter phases. This study documents the highest density (1.115 g cm − 3 ) CO 2 fluids ever reported in granites. Fluid isochores, constructed from CO 2 (± CH 4) and halite-bearing inclusions, coupled with two-feldspar thermometry constrain the minimum P–T at 8 kbar/∼ 750 °C for fluid entrapment in granites. By contrast, the carbonic inclusions in quartz from granite-hosted metapelite enclaves contain substantial CH 4 (up to 30 mol%), and the entrapment pressure (∼ 4.3 kbar/600 °C) is considerably lower compared to those in the granites. By implication, the sillimanite-free granites were not derived from the metapelitic enclaves, and instead were formed by partial melting of fluid-heterogeneous lower crustal protoliths, with fluid entrapment at magmatic conditions.

Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)

The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones.

Studies of the entrapment of non-wetting fluid within nanoporous media using a synergistic combination of MRI and micro-computed X-ray tomography

Three-dimensional magnetic resonance imaging (MRI) and micro-computed X-ray tomography (micro-CXT) have been combined to study the entrapment of mercury within nanoporous silica materials following porosimetry. MR images have been used to construct structural models of particular porous media within which several simulations of mercury intrusion and retraction have been performed with variations in the mechanism for the ‘snap-off’ of the mercury menisci. The simulations gave rise to different predictions for the pattern of the macroscopic ( > 10 μ m ) spatial distribution of entrapped mercury, depending on ‘snap-off’ mechanism, which were then compared with corresponding experimental data obtained from micro-CXT images of real pellets containing entrapped mercury. The information obtained from the micro-CXT images, and also from the porosimetry curves themselves, was then used to constrain a model for the microscopic mercury retraction mechanism. Additional predictions of the retraction model were then subsequently confirmed using scanning loop experiments. The simulations showed that the overall level of entrapment of mercury was determined by the close interaction between the pellet macroscopic structure (particularly pore size spatial correlation), and the microscopic mercury retraction mechanism. Hence, it was subsequently possible to explain fully why high mercury entrapment occurred within one particular type of sol–gel silica material, while only low entrapment occurred in another batch of superficially similar material.

Mineralizações de Sn do Maciço Granítico Serra Branca, Goiás: evolução do sistema hidrotermal e fonte dos fluidos

The Serra Branca granitic massif occurs in the Goias tin province and is part of the Paleo- to Mesoproterozoic within-plate granites suite, which is rich in F, Sn, Rb, Y, Th, Nb, Ga and REE. The Serra Branca granitic massif hosts tin mineralization and comprises: a) pink to gray, porphyritic, medium- to coarse-grained granite (g1a); b) pink to gray, heterogranular, medium- to coarse-grained granite (g1b); c) gray, heterogranular, medium- to coarse-grained granite (g1c); and d) gray, heterogranular, medium-grained granite (g1d), which is restricted to the eastern part of the massif. Greisens and mica veins also occur in the massif. Important post-magmatic processes have been identified: albitization, greisenization (the most prominent) and microclinization. A fluid inclusion study in K-feldspar from the (g1a) and (g1c) granites and in quartz, topaz and fluorite from the (g1d) granite, greisen and quartz + fluorite and quartz + topaz veins revealed the presence of two fluid inclusions types: i) type 1, interpreted as primary and modeled by the H 2 O-NaCl-CO 2 ( ± CH 4 ) system; and ii) type 2, composed of H 2 O-NaCl ± (KCl-MgCl 2 -FeCl 2 ) two-phase fluid inclusions, with petrographic characteristics of primary fluid inclusions in the hydrothermal rocks and of secondary ones in quartz from the (g1a) granite. The low T H (~240oC) and salinity (< 2 wt. % NaCl equiv.) values of fluid inclusions entrapped in K-feldspar from the granites suggest that this mineral doesn’t contain the original magmatic fluid, but fluids derived either from progressive cooling or from late hydrothermal alteration. The petrologic characteristics of the Serra Branca massif together with the fluid inclusions data are consistent with predominant magmatic source for the fluids rich in CO 2 ( ± CH 4 ). The petrographic and microthermometric data suggest that the aqueous fluids were entrapped later. It is proposed an evolution model in which an initially magmatic fluid has been released and mixed with external lower-temperature fluid. After the hydrothermal alteration and mineralization, there was entrapment of late fluids in fractures of the hydrothermal minerals, probably due to the deformation associated with the Brasilano orogenic event.

MINERALIZAÇÕES DE Sn DO MACIÇO GRANÍTICO SERRA BRANCA, GOIÁS: EVOLUÇÃO DO SISTEMA HIDROTERMAL E FONTE DOS FLUIDOS

The Serra Branca granitic massif occurs in the Goiás tin province and is part of the Paleo- to Mesoproterozoic within-plate granites suite, which is rich in F, Sn, Rb, Y, Th, Nb, Ga and REE. The Serra Branca granitic massif hosts tin mineralization and comprises: a) pink to gray, porphyritic, medium- to coarse-grained granite (g1a); b) pink to gray, heterogranular, medium- to coarse-grained granite (g1b); c) gray, heterogranular, medium- to coarse-grained granite (g1c); and d) gray, heterogranular, medium-grained granite (g1d), which is restricted to the eastern part of the massif. Greisens and mica veins also occur in the massif. Important post-magmatic processes have been identified: albitization, greisenization (the most prominent) and microclinization. A fluid inclusion study in K-feldspar from the (g1a) and (g1c) granites and in quartz, topaz and fluorite from the (g1d) granite, greisen and quartz + fluorite and quartz + topaz veins revealed the presence of two fluid inclusions types: i) type 1, interpreted as primary and modeled by the H 2 O-NaCl-CO 2 (±CH 4 ) system; and ii) type 2, composed of H 2 O-NaCl ± (KCl-MgCl 2 -FeCl 2 ) two-phase fluid inclusions, with petrographic characteristics of primary fluid inclusions in the hydrothermal rocks and of secondary ones in quartz from the (g1a) granite. The low TH (~240ºC) and salinity (&lt; 2 wt. % NaCl equiv.) values of fluid inclusions entrapped in K-feldspar from the granites suggest that this mineral doesn’t contain the original magmatic fluid, but fluids derived either from progressive cooling or from late hydrothermal alteration. The petrologic characteristics of the Serra Branca massif together with the fluid inclusions data are consistent with predominant magmatic source for the fluids rich in CO 2 (± CH 4 ). The petrographic and microthermometric data suggest that the aqueous fluids were entrapped later. It is proposed an evolution model in which an initially magmatic fluid has been released and mixed with external lower-temperature fluid. After the hydrothermal alteration and mineralization, there was entrapment of late fluids in fractures of the hydrothermal minerals, probably due to the deformation associated with the Brasilano orogenic event.

Contrasting fluid inclusion characteristics of staniferous and non-staniferous pegmatites of Southeast Bastar, Central India

Tin and rare metal-bearing granitic pegmatites in the Bastar–Malkangiri pegmatite belt of Central India are hosted by metabasic and metasedimentary country rocks. Fluid inclusion studies were conducted in spatially associated two-mica granite and the staniferous and non-staniferous pegmatites to characterize the physicochemical environment of mineralization, to distinguish different pegmatites in terms of their fluid characteristics and to envisage a possible genetic link between the pegmatites and spatially associated granite. Three different types of primary inclusions were identified. The type-I, aqueous bi-phase (L+V) inclusions are the most abundant and ubiquitous. Type-II polyphase (L+V+S) inclusions are rare. Type-III, monophase (L) and metastable aqueous inclusions, though less abundant than type-I inclusions, are ubiquitous. The fluid evolution trends indicate that mixing of two different fluids of contrasting salinities, one of high salinity (20–30 wt% NaCl equivalent) and another of low salinity (0–10 wt% NaCl equivalent), was responsible for precipitation of the bulk of the cassiterite. This mixing is the single most important characteristic that distinguishes the staniferous pegmatites from their non-staniferous counterparts. The non-staniferous pegmatites, on the other hand, are typified by the presence either of a high saline or a low saline fluid that evolved through simple cooling. The minimum pressure–temperature of entrapment, estimated from the intersections of the halide liquidus with the corresponding inclusion isochores of type-II inclusions, range between 2.1–2.2 kb and 300–325 °C. The similar P– T range of fluid entrapment of the staniferous and non-staniferous pegmatites indicates that they were possibly emplaced within a similar physical environment. Type-I inclusions from granite recorded only the high salinity fluid, the salinity of which compares well with that of the highly saline fluid component of type-I inclusions in the pegmatites. This is a possible indication of a genetic link between the pegmatites and spatially associated granite.

Gold-Bearing Mesothermal Veins from the Gubong Mine, Cheongyang Gold District, Republic of Korea: Fluid Inclusion and Stable Isotope Studies

The Gubong deposit consists of five massive, gold-bearing mesothermal quartz veins that fill fractures oriented northeast and northwest along fault shear zones over an area of 14 km2 in Precambrian metasedimentary rocks of the Gyeonggi massif. The veins are divided into three groups, based on their orientation and location. They have a ribbon texture that is interpreted to result from repeated hydraulic fracturing events. Mineral deposition was associated with hydrothermal fluid overpressuring within and below the active fault shear zones. The vein mineralogy and paragenesis of the veins allow two separate discrete mineralization episodes to be recognized, separated by a major faulting event. The ore minerals are contained within quartz and calcite associated with fracturing and healing of veins that occurred during both mineralization episodes. Wall-rock alteration minerals during stage I, the main ore stage, include sericite, chlorite, and minor pyrite and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, and argentite. Electrum also was deposited during stage II mineralization in the one group of veins, along with pyrite, sphalerite, chalcopyrite, and galena, but the second stage of deposition in other veins was barren. Petrographic examination of textural relationships among sulfides, fluid inclusions, microfracturing, and quartz shows chronological and genetic relationships between gold deposition and fluid entrapment. Systematic studies of fluid inclusions in stage I vein indicate two contrasting events: a relatively high temperature (203°–432°C) and pressure (943–2,098 bars) event related to early sulfide deposition and associated with H2O-CO2-CH4-NaCl ± N2 fluids (less than about 13.4 wt % NaCl), and a lower temperature (202°–399°C) and pressure (670–850 bars) late sulfide depositional event involving H2O-NaCl fluids (3.9–17.3 wt % NaCl). The H2O-NaCl fluid involved in ore-related stage II mineralization had a salinity of 0.4 to 4.2 wt percent NaCl and a homogenization temperature of 201° to 378°C. The calculated sulfur isotope compositions of hydrothermal fluids from the stage I veins (δ34SH2S = 3.5–10.5‰) indicate that ore sulfur was derived mainly from a magmatic source but also in part from sulfur contained in the host rocks. The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (stage I: δ18OH2O = 1.1–9.0‰, δD = −92 to −21‰; stage II: δ18OH2O = −0.1 to +0.3‰, δD = −95 to −93‰) indicate that the fluids were derived from magmatic and/or deep-seated metasedimentary rocks (stage I) and evolved by mixing with local meteoric water (stage II), by limited water-rock exchange and by degassing during mineralization in uplift zones. The H2O-NaCl fluids involved in the stage I development of these veins represent fluids that evolved either through unmixing of H2O-CO2-CH4-NaCl ± N2 fluids following a decrease in fluid pressure or through mixing with deeply circulating meteoric waters, possibly as a result of uplift and/or unloading during mineralization. The H2O-NaCl fluid involved in stage II was derived from meteoric water. Early deposition of gold in stage I was caused by a decrease in sulfur fugacity (H2S loss) that accompanied the immiscible separation of carbonic vapor from H2O-CO2-CH4-NaCl ± N2 fluids. Gold in late stage I and stage II veins was precipitated from H2O-NaCl fluids by cooling and by dilution caused by mixing with meteoric water.