Composition Of Fluid Inclusions Research Articles

Timing and Mechanism of Ore Precipitation in Porphyry Cu Systems: Insight from LA-ICP-MS Analysis of Fluid Inclusions and In Situ Oxygen Isotope Analysis of Hydrothermal Quartz at Zhunuo Porphyry Cu Deposit, China

Abstract The timing and mechanism of ore precipitation in porphyry copper systems are hot topics and remain controversial. The large Miocene collision-related Zhunuo porphyry Cu deposit in the Gangdese belt of southern Tibet, China, was produced by multistage quartz veining and hydrothermal alteration, accompanied by Cu sulfide precipitation. In this study, we have combined cathodoluminescence (CL) petrography with in situ oxygen isotope analysis and fluid inclusion microthermometry and laser ablation-inductively coupled plasma-mass spectrometer microanalysis to constrain the growth history of individual quartz veins, the source and evolution of the hydrothermal fluids, and the timing and mechanism of ore precipitation at Zhunuo. Early quartz A veins associated with potassic alteration are composed of quartz, K-feldspar, biotite, Cu-Fe sulfides, and pyrite. Quartz B veins are composed of quartz, Cu-Fe sulfides, molybdenite, and pyrite. CL imaging shows that quartz grains in the A and B veins consist of abundant early generation of bright-luminescent quartz (QA and QB) with volumetrically minor later generation of dull-luminescent quartz (QA-crack and QB-crack) occurring in the voids or at the margins of the QA and QB veins with embayed contacts. Cu-Fe sulfides are generally in contact with the dull-luminescent quartz and locally in contact with the bright-luminescent quartz and K-feldspar in the A and B veins or occur as disseminations in the potassic-altered porphyries that have been overprinted by chlorite ± muscovite alteration. QA and QB contain single-phase intermediate-density inclusions and abundant brine and vapor-rich inclusions. A boiling assemblage in QA has a homogenization temperature of ~560°C and trapping pressure of ~530 bar. Two boiling assemblages in QB have homogenization temperatures of ~440°C with trapping pressures of ~230 and ~250 bar, indicating a transition from lithostatic to hydrostatic conditions at a paleodepth of ~2.0 to 2.5 km. QA-crack and QB-crack contain aqueous inclusions with lower homogenization temperatures of 340° to 400°C and salinities of 6 to 12 wt % NaCl equiv. In situ oxygen isotopes indicate that QA and QB have δ18O values of 7.6 to 11.4‰, whereas QA-crack and QB-crack have δ18O values of –7.2 to 6.7‰. Combined with fluid inclusion compositions, we propose that condensation of vapors into brines and mixing with 25 to 60% meteoric water can produce the salinity and oxygen isotopes of fluids that caused the dissolution of the early bright-luminescent quartz and the precipitation of the later dull-luminescent quartz. Zhunuo Cu-Fe sulfides are in contact with the bright-luminescent quartz and cut the growth bands. The dull-luminescent quartz in contact with the Cu-Fe sulfides has oscillatory growth banding. In addition, Cu-Fe sulfides in the A and B veins or in the potassic-altered rocks coexist with chlorite ± muscovite alteration minerals. These geologic observations, together with evidence that there is an abrupt drop in the Cu/(Na + K) ratio by more than one order of magnitude in the aqueous fluids within the dull-luminescent quartz compared to the brines within the bright-luminescent quartz, indicate that precipitation of Cu-Fe sulfides occurred after the potassic alteration stage and during the chlorite ± muscovite alteration stage. Ore precipitation was associated with mixing of magmatic fluids (brines + vapors) with meteoric water, accompanied by fluid cooling, salinity decrease, and retrograde quartz solubility as the rocks transitioned from lithostatic to hydrostatic conditions. During continued cooling in the latest stage, the hydrothermal fluids precipitated D veins that contain abundant pyrite with minor quartz. Quartz in the D veins (QD) contains aqueous inclusions with much lower temperatures and Cu/(Na + K) ratios, indicating that most of the Cu-Fe sulfides have been precipitated during this stage.

High-precision analysis of carbon isotopic composition for individual CO2 inclusions via Raman spectroscopy: Addressing issues arising from the laser-heating effects

The carbon isotopic composition of carbon dioxide fluid inclusions trapped within minerals reflects the origin and evolution of the CO2-bearing fluids and melts. Recently, we developed a highly accurate method using Raman spectroscopy to determine the carbon isotopic composition of CO2 fluid inclusions, by simultaneously collecting 13CO2 and hot bands at a controlled temperature, and using the peak height ratio of 13CO2 and one of hot bands near 1410 cm−1 (HR) to determine δ13C value. This microanalytical technique enables precise measurements of the δ13C values of different CO2 fluid inclusions within zoned mineral crystals to reveal the multiple-stages evolution of CO2-bearing fluids and melts. However, when studying fluid inclusions hosted within high laser heating coefficient minerals such as olivine and pyroxene in mantle xenoliths, local temperature near the inclusions will be higher than the temperature controlled by heating-cooling stage. To address this issue, we examined the effects of laser heating on CO2 isotopic standard samples sealed in the silica capillary, and developed a novel procedure to determine the δ13C values of natural fluid inclusions by using two CO2 standard samples with known carbon isotopic compositions. The proposed workflow successfully determines the δ13C values of natural CO2 inclusions, with an average deviation of 0.58 ‰ between the measurements and the model.

Water-Containing Defects in Variously Deformed Milky-White Vein Quartz of the Larino Deposit (South Urals)

Abstract ––The methods of Infrared (IR) and Raman spectroscopy, as well as gas chromatography, were used to assess the distribution, content and composition of water-containing defects in variously deformed milky-white vein quartz of the Larino deposit. Weakly deformed quartz and quartz with intensive polygonization and recrystallization, in which water is present in molecular form, in fractures, channels, intergranular space, as well as in the composition of fluid inclusions, are analyzed. The content of water-containing defects, according to IR spectroscopy and gas chromatography, decreases in a series from weakly deformed primary granular coarse-grained quartz to blocked and recrystallized. The obtained results indicate the release of water during recrystallization, along the newly formed grain boundaries by diffusion and further homogenization to achieve an equilibrium state. Gas content also depends on the degree of deformation changes in samples and decreases from large coarse-grained differences to intensely deformed quartz with a high content of recrystallized grains.

The genesis of the Tianbaoshan Pb[sbnd]Zn deposit in Sichuan, SW China: Insights from sphalerite and fluid inclusion compositions

The Tianbaoshan deposit, located in the Sichuan–Yunnan–Guizhou (SYG) metallogenetic belt, is one of famous PbZn deposits of Southwest China, with an established reserve of 2.6 Mt. Pb and Zn. According to the detailed field geological survey and microscopic petrography observation, three distinct ore fluid evolution stages can be divided. In this contribution, sphalerite and associated quartz from above diverse stages were carefully chosen for systematic fluid inclusion (FI) petrography, microthermometry, individual FI LA-ICP-MS analysis, Raman microspectroscopy, and in situ trace element investigation of sphalerite. The sphalerite only defines a two-phase aqueous FI type, while the FIs hosted in quartz exhibit diverse types, including two-phase aqueous FIs, vapor-rich FIs, and pure aqueous fluid inclusions. Our microthermometry results indicate that the homogenization temperature (Th) and salinity of FIs in sphalerite from the stage 1 are 109.6‐182.2 °C and 12.51–21.61 wt% NaCl equiv., respectively. The Th of the sphalerite-hosted fluid inclusions in stage 2 ranges from 97 to 158.3 °C, with a salinity range of 14.46–19.76 wt% NaCl equiv. The FIs trapped in sphalerite exhibited medium-high salinity and low Th, and Raman spectrum analysis shows that vapor composition of the FI is mainly H2O, reflecting the typical characteristics of basin brine. Measurements on quartz-hosted FIs yield homogenization temperatures from 134.3 to 233.1 °C, with a salinity from 4.49 to 12.62 wt% NaCl equiv. The gas phase composition of these inclusions is complex and consists of several components, such as CH4, N2, CO2, and H2O. The results indicate that the FIs present in quartz are dissimilar from those in sphalerite, and the sphalerite mineralization fluid has typical characteristics of basin brine and represents the ore-forming fluid. The significantly high Na and Ca of the mineralizing fluid is revealed, suggesting that the mineralizing fluid may be of the NaCa system. Combined with Ca/K, K/Na and Rb/Na ratio, typical basinal brines origin was constrained by fluid composition analysis. The gradual enrichment of Ca, Mg, and Sr in the mineralizing fluid emphasizes the positive effect of fluid-rock reactions on the precipitation of Zn. The trace element in sphalerite of the Tianbaoshan deposit is distinct from SEDEX-, skarn-, and volcanogenic massive sulfide (VMS)-type deposits and notably similar to MVT deposits. Combining with fluid inclusion and sphalerite trace elements feature, we define the Tianbaoshan deposit as a typical MVT PbZn deposit.

Chlorine Solubility in Silicate Melts: New Experiments and Thermodynamic Mixing Model

We present new experimental data on Cl solubility in model basalt melts of eutectic compositions diopside (Di)–albite (Ab) and Di–anorthite ± quartz (Qtz). The starting glasses were equilibrated with aqueous NaCl–CaCl2 fluid at 4 kbar in the temperature range of 900–1200°C. The experiments show that Cl solubility decreases with increasing NaCl in the fluid. Ca–Na partitioning between melts and fluid is weekly temperature dependent and resembles that of the plagioclase–fluid system. The newly obtained experimental data, along with previously published results on the model granite melting in the presence of (Na,K)Cl brines (Aranovich et al., 2013), are used to calibrate an empirical thermodynamic model for salt species (NaCl, KCl, and CaCl2) in silicate melt. Calculations show that Cl solubility in haplogranite melt decreases with increasing K/Na ratio in the fluid (and correspondingly, melt). The data acquired on Ca and Na partitioning between melt and fluid make it possible to model the evolution of the Ca/Na ratio in the crystallization course of basalt melts. At a high pressure (10 kbar), Cl solubility in model granite increases with increasing Н2О content. The calculated phase diagram for a simple pseudo-ternary system Ab–H2O–NaCl demonstrates complex phase relations and, correspondingly, evolution of the Н2О and NaCl concentrations in the melt. This complex evolution is illustrated by data on the composition of quartz-hosted melt and fluid inclusions from granites in the Verkhneurmisskii massif in the Badzhal volcano-plutonic zone.

Mechanism of carbonate assimilation by intraplate basaltic magma and liquid immiscibility: example of Wangtian’e volcano (Changbaishan volcanic area, NE China)

The balance of CO2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO2-rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ18О and δ13С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO3, 24 wt% MgCO3 and 3 wt% CaCO3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.

Determination of the pressure and composition of wet gas fluid inclusions: An in situ Raman spectroscopic approach

Carbonaceous fluid within mineral-hosted inclusions provides important information for carbon cycle in deep Earth. In addition to CH4 and CO2, heavy hydrocarbons (e.g., C2H6 and C3H8) are frequently observed in carbonaceous fluid inclusions (i.e, wet gas inclusions with C1/∑Ci < 0.95). However, determination of the composition of such complex volatiles is difficult based on traditional microthermometric measurements. Here we carried out experimental calibrations on Raman spectroscopic measurements of the pressure (P) and composition of the CH4 ± C2H6 ± C3H8 ± H2S system at room temperature and 0.1–130 MPa. We confirmed that the C–H symmetric stretching vibration band of CH4 [ν1(CH4), ∼2917 cm−1] shifted to lower wavenumber with rising pressure, thus the P-ν1(CH4) relationship could be applied to calculate the pressure of wet gas. It should be noted that the presence of C2+ and/or H2S will shift the [ν1(CH4)] to lower wavenumber at constant pressure (with the order of C3H8 ≥ H2S > C2H6). Obviously, the P-ν1(CH4) relationship derived from pure CH4 system could not be simply applied to wet gas inclusion, otherwise the pressure would be overestimated. To avoid the overlap of the C–H vibrations of CH4, C2H6 and C3H8, the peak areas and peak heights of the overtone vibration of CH4 [2ν4(CH4), ∼2580 cm−1], C–C symmetric stretching vibrations of C2H6 [ν3(C2H6), ∼995 cm−1] and C3H8 [ν8(C3H8), ∼868 cm−1], and S–H symmetric stretching vibration of H2S [ν1(H2S), ∼2612 cm−1] were fitted using Gaussian + Lorentz functions. The obtained peak areas and peak heights were then used to calculate the Raman quantification factors (F factor and G factor, respectively) of C2H6, C3H8 and H2S relative to CH4, respectively. Both the F factor and G factor increased with rising pressure, whereas the FC2H6, FC3H8 and GH2S kept nearly constant at ∼5.69, 6.39 and 153.8, respectively in high pressure gas mixtures (e.g., >30 MPa). Therefore, for inclusions with higher internal pressure, the molar ratio of CH4, C2H6, C3H8 and H2S could be determined by the aforementioned F and G factors. This method was applied to the calcite-hosted single-phase gas inclusions in the Upper Permian Changxing Formation carbonate reservoir from the eastern Sichuan Basin (South China). Our results indicated that the trapping pressure would be obviously overestimated if the presence of heavy hydrocarbons was not taken into account.

An Assessment of Deep Borehole Disposal Post-Closure Safety

Around the world, deep borehole disposal is being evaluated for intermediate level-waste (ILW), high-level waste, and spent nuclear fuel. To facilitate a disposal concept options analysis for ILW in Australia, desktop and lab-based geoscientific investigations, together with generic post-closure safety assessments of deep borehole disposal of long-lived ILW, have been undertaken. This paper reports on geoscientific data obtained on crystalline rock and rock salt as model rocks for geological disposal. Petrophysical and mineralogical properties for these rocks have been investigated to provide realistic data for evaluation and input to post-closure safety assessments. For crystalline rock samples originating from depths between 700 to 1900 m, very low hydraulic conductivity (2 × 10−12 to 3 × 10−11 m/s) and very low porosity (0.02% to 1.2%) were obtained. The noble gas isotopic composition of fluid inclusions from the same depth interval confirmed the rock had been devoid of recent interaction with meteoric water, thus providing potentially suitable conditions for geological disposal. Rock salt from a 802-m (heterogeneous sample with 40% halite) and a 1100-m (sample with 98% halite) depth also had a low hydraulic conductivity (5 × 10−10 to 5 × 10−9 m/s at 802 m and 10−11 to 2 × 10−10 m/s at 1100 m) and very low porosity (~0.8% for the heterogenous sample and ~0.2% for the pure halite sample). Post-closure safety assessments based on numerical modeling provided bounding conditions around the thermal evolution of the disposal environment in crystalline rock for low heat generating ILW (50 W per 180-L vitrified waste canister), including exploring the sensitivity of temperature evolution within the borehole and rock environment to parameters such as heat load, borehole depth, geothermal gradients, and rock thermal conductivity. The coupling of heat transport with radionuclide migration to account for buoyancy-driven transport was shown to have a limited impact on radionuclide migration. For a disposal borehole in crystalline rock, the radionuclide concentrations and annual dose rates from key radionuclides (99Tc and 79Se) for a 500-m, 1000-m, or 3000-m deep borehole were negligible (i.e., many orders of magnitude smaller than the threshold dose the International Atomic Energy Agency considers insignificant for humans, 0.01 mSv/year). For disposal in rock salt, a suite of numerical model scenarios explored the effectiveness of the engineered barriers, including the glass matrix, primary package, and overpack, assuming diffusion-dominated transport. These scenarios illustrated that the performance of the disposal system was insensitive to the presence or absence of engineered barriers, as dose rates at late time (>105 years) were nearly identical for all scenarios. These results indicate that the natural barrier provided by the salt is very effective at containing radionuclides, while the engineered barriers serve mainly to delay the arrival of the peak dose. While the results are preliminary, the post-closure safety assessments, supported by measured data from crystalline rock and rock salt, give confidence that deep borehole disposal of long-lived ILW would result in dose rates considered insignificant for humans within a few meters from the borehole.

The noble gas signature of the 2021 Tajogaite eruption (La Palma, Canary Islands)

Here, we characterize the temporal evolution of volatiles during the Tajogaite eruption by analyzing the elemental (He-Ar-CO2-N2) and isotopic (He-Ar-Ne) composition of fluid inclusions (FI) in phenocrysts (olivine+pyroxene) identified in erupted lavas. Our 2021 lava samples identify substantial temporal variations in volatile composition. We show that, during the 2021 Tajogaite eruption, the He-CO2-N2 concentrations in FI increased since October 15th; this increase was accompanied by increasing 40Ar/36Ar ratios (from ∼300 to >500), and paralleled a major shift in bulk lava chemistry, with increasing Mg contents (Mg#, from 47 to 52 to 55–59), CaO/Al2O3 (from 0.65 to 0.74 to 0.75–0.90), Ni and Cr, and decreasing TiO2, P2O5 and incompatible elements. The olivine core composition also became more forsteritic (from Mg# = 80–81 to Mg# = 84–86). Mineral thermobarometry and FI barometry results indicate that the eruption was sustained by magmas previously stored in at least two magma accumulation zones, at respectively ∼6–12 km and 15–30 km, corroborating previous seismic and FI evidence. We therefore propose that the compositional changes seen throughout the eruption can be explained by an increased contribution - since early/mid-October - of more primitive, less-degassed magma from the deeper (mantle) reservoir. Conversely, Rc/Ra values (3He/4He ratios corrected for atmospheric contamination) remained constant throughout the whole eruption at MORB-like values (7.38 ± 0.22 Ra, 1σ), suggesting an isotopically homogeneous magma feeding source. The Tajogaite He isotope signature is within the range of values observed for the 1677 San Antonio lavas (7.37 ± 0.17Ra, 1σ), but is more radiogenic than the 3He/4He values (>9 Rc/Ra) observed in the Caldera de Taburiente to the north. The 3He/4He ratios (6.75 ± 0.20 Ra, 1σ) measured in mantle xenoliths from the San Antonio volcano indicate a relatively radiogenic nature of the mantle beneath the Cumbre Vieja ridge. Based on these results and mixing modeling calculations, we propose that the homogeneous He isotopic signatures observed in volatiles from the Tajogaite/San Antonio lavas reflect three component mixing between a MORB-like source, a radiogenic component and small additions (6–15%) of a high 3He/4He reservoir-derived (>9Ra) fluid components. The simultaneous occurrence of high 3He/4He (>9Ra)- and MORB-like He signatures in northern and southern La Palma is interpreted to reflect small-scale heterogeneities in the local mantle, arising from spatially variable proportions of MORB, radiogenic, and high 3He/4He components.

HELIUM ISOTOPIC COMPOSITION IN ALKALINE INTRUSIONS OF THE HOVSGOL AREA, NORTHWESTERN MONGOLIA

The helium isotope composition of fluid inclusions has been studied in magnetite from alkaline and subalkaline intrusions of the Hovsgol area in NW Mongolia. The measured content of 4He is in the range from 6.6 × 10–7 to 114 × 10–7 см3/г. The 3He/4He isotopic ratio of most samples varies within 0.23–0.59 Ra and may indicate the presence of helium from different sources in the magmatogenic fluid. The largest amount of mantle He (2.51 Ra) is hosted by the magnetite of the subalkaline gabbro. During the generation of primary magma, which probably involved either SCLM-type material or a plume-like reservoir less enriched in 3He, the mantle component accounted for approximately 40–60%. In the evolution of foid and alkali-syenite melts, its share did not exceed ~10–15% due to mixing with crustal radiogenic He. We assume that the intrusions interacted with fragments of Precambrian accretionary-collisional complexes of the Tuva-Mongolian terrane. Such mixing of He isotopes in melts and fluids may be regarded as circumstantial evidence supporting the development of mantle magmatism at the active continental margin.

He Isotopic Composition of Alkaline Intrusions of the Hovsgol Area, Northwestern Mongolia

The He isotopic composition of fluid inclusions in magnetite of alkaline and subalkaline intrusion of the Hovsgol region, Northwestern Mongolia, is studied. The measured 4He content varies from 6.6 × 10–7 to 114 × 10–7 cm3/g. The 3He/4He isotope ratio of most samples is 0.23–0.59 Ra indicating the presence of He from different sources in magmatic fluid. The maximum of mantle He (2.51 Ra) is observed in magnetite of subalkali gabbro. The mantle He component was ~40–60% during the generation of parental magma with possible involvement of subcontinental lithospheric mantle material or a plume-like reservoir less enriched in 3He. The mantle He component did not exceed 10–15% during the evolution of the foidoitic and alkali-syenitic melts because of mixing with crustal radiogenic He. We suggest the interaction of intrusions with fragments of the Precambrian accretionary–collision complexes of the Tuva–Mongolian Terrane. This mixing of He isotopes in melts and fluids can serve as indirect evidence of the development of mantle magmatism at the active continental margin.

Fluid properties and ore-forming process of the giant Jiajika pegmatite Li deposit, western China

The world class Jiajika lithium deposit in the Songpan-Ganze orogenic belt, western China is related to Late Mesozoic S-type granitic pegmatite. More than 500 pegmatite veins have been discovered in the Jiajika deposit and they show well defined zonal mineral pattern, including microcline pegmatite zone (Ⅰ), microcline-albite pegmatite zone (Ⅱ), albite pegmatite zone (III), albite-spodumene pegmatite zone (IV), and albite-lepidolite pegmatite zone (Ⅴ) from the center outward. Pegmatite Li orebodies are primarily hosted in zones III, IV and V. Barren samples of zones I and II were collected from the recent Jiajika Scientific Drilling (JSD) project performed in the years of 2020–2021 and ore samples of zones III, IV and V were collected from outcrop pegmatite veins No. 308, No. 134, and No. 528, respectively. Fluid inclusions in quartz from the Jiajika deposit include intermediate density CO2-bearing inclusions (type A), high density aqueous inclusions (type B), and low density CO2-rich inclusions (type C). Type A inclusions are predominant in zones I and II, whereas coexisting type B and C inclusions are prevailing in zones III, IV and V. Fluid inclusion microthermometry indicates that type A inclusions in zones I and II show homogenization temperatures ranging from 328 to 362 °C and salinities from 1.2 to 4.5 wt%. Type B inclusions and coexisting type C inclusions in the zones III, IV and V show homogenization temperatures of 278 to 336 °C and 301 to 358 °C, respectively. Their salinity values are 8.1 to 11.6 wt% and 0.8 to 3.6 wt%, respectively. The estimated pressures of type A inclusions are 310 to 424 Mpa, and those of immiscible type B and C inclusions are 151 to 239 Mpa. The change of fluid types with decreasing pressure indicates that extensive fluid immiscibility occurred during the formation of Li pegmatite veins in zones III, IV and V. Moreover, the LA-ICP-MS microanalysis demonstrates that chemical composition of fluid inclusions in this deposit is relatively simple, mainly containing Li, Na, K, Rb, Cs, B, and As. Such enrichment of alkaline elements (Li, Na, K, Rb and Cs) and volatile elements (B and As) is a typical geochemical index for lithium pegmatite. We suggest that the high-degree fractionated of parent magma and strong fluid immiscibility promoted the formation of the giant Jiajika Li deposit. This study highlights that immiscible fluid inclusions and strong enrichment of alkaline elements in inclusions can be an excellent tool for explorations of lithium pegmatites like Jiajika.

Morphology and Composition Changes in Fluid Inclusions from Quartz under Progressive Deformation: Case Study of a Vein System in the Western Kelyan-Irokinda Fold Zone (Western Transbaikalia)

Abstract —The behavior of fluids during plastic deformation is studied from the morphology and distribution of fluid inclusions in quartz grains of different microstructure types from a vein system controlled by thrusting and strike-slip faulting in the eastern Sayan–Baikal fold area. The analytical work includes electron backscatter diffraction (EBSD) for quartz microstructure and crystallography, as well as Linkam heating-and-freezing analysis and Raman spectroscopy for the composition of fluid inclusions. The studied fluid inclusions are of seven types that differ in morphology and position in the deformed quartz structure. A model is suggested to describe successive structural changes of quartz aggregates during dislocation sliding and subsequent creep-related recrystallization associated with redistribution of fluid. Fluid inclusions undergo qualitative and quantitative changes due to water leakage at all stages of plastic deformation. The changes occur by two main mechanisms: (i) mass transfer during dislocation sliding at medium temperatures and strain rates and (ii) diffusion creep at low strain rates and high temperatures. The contribution of creep increases gradually with temperature, which maintains the interaction of inclusions with migrating grain boundaries.

A comparative study of fluid characteristics of W and Cu mineralization in the Shiweidong deposit, giant Dahutang ore field, South China: Evidence from LA-ICP-MS analysis of fluid inclusion

The Shiweidong W-Cu deposit, located within the giant Dahutang W-Cu ore field, contains quartz vein type W-Cu and veinlet-disseminated type W mineralization. These ore veins are mainly hosted in the Jinningian biotite granodiorite. Two distinct mineralization stages were identified in association with quartz, namely wolframite-quartz veins and chalcopyrite-scheelite-quartz association. The deposit represents an example of a complex of economically valuable W and Cu, with resources of 0.3 Mt WO3 and 0.24 Mt Cu. The characteristic and origin of the W and Cu mineralizing fluids remain debated. In our study, homogenization temperature (Th) and salinity were determined for fluid inclusions (FIs) in ore or associated gangue minerals. Additionally, we use the trace element compositions of fluid inclusions obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to clarify fluid characteristics responsible for W and Cu mineralization. Primary fluid inclusions (FIs) trapped in wolframite display high Th ranging from 305 to 352 °C and have moderate salinity from 3.2 to 5.0 wt% NaCl equiv., quartz associated with chalcopyrite exhibit a close syngenetic relationship, and a significant low-temperature (248 ∼ 271 °C) fluid was recorded by FIs in quartz. Wolframite-hosted FIs were characterized by high concentrations of incompatible lithophile element Cs and volatile elements including Li and B, indicating that W mineralization fluid was derived from highly fractionated granitic systems. All FIs in wolframite display the absence of Cu. In contrast, FIs hosted in quartz coexisting with chalcopyrite have the high Cu concentrations with average value of 1033 ppm. Giving the lower Cs content in FIs from quartz coexisting with chalcopyrite, we suggest that the Cu-rich ore-forming fluid released from a less fractionated magma. These results indicate that W and Cu mineralization at Shiweidong was introduced by distinct fluids. Fluid simple cooling is likely a major driving factor in wolframite formation, while fluid boiling accompanied chalcopyrite deposition. Our new findings illuminate that the existence of independent Cu mineralization events in the W deposit in northern Jiangxi.

The genesis of hydrothermal veins in the Aukam valley SW Namibia– A far field consequence of Pangean rifting?

High purity fluorite veins (acid grade > 97 % CaF2) are generally rare in a global context. The fluorite-(±quartz-calcite) veins in the Aukam Valley mining district in SW Namibia have not previously been studied but due to their excellent exposure they are an ideal natural laboratory to study such mineralization. These veins cross-cut Mesoproterozoic high-grade gneisses and granites (Namaqua-Natal Metamorphic Province) close to the unconformity with the overlying late-Neoproterozoic Nama Group sedimentary rock cover. Based on the regional context, their genesis has been assumed to be related to nearby fluorite-bearing pegmatites. However, the present contribution provides new microthermometry data from vein fluorite (with 25.0–25.6 wt% NaCl and Th of 270–300 °C) that indicate precipitation from a mixed fluid with two chemically contrasting end-members for the early paragenetic fluorite stage. However, it was not possible to identify the chemical composition of the endmembers. Based on the chemical composition of the fluid mixture and the regional geological context, we propose that these end-members are F-rich brines sourced from the Namaqua basement and Ca-rich limestone-derived formation fluids of the Nama Group. A fluorite-overgrowing quartz precipitation is likely the effect of post-mixing fluid cooling as the fluid inclusions hosted in quartz show lower homogenization temperatures (Th 230–260 °C) with similar salinity. Locally, late-stage calcite is recognized with fluid inclusion compositions (0.2–2.7 wt% NaCl and Th of 60–92 °C) different from those observed in the fluorite and quartz. With significantly lower homogenization temperatures, they likely represent precipitates formed during the shutdown and collapse of the hydrothermal system. The fluorite veins are hosted in post-Cambrian N-trending brittle structures that probably formed during the opening of the South Atlantic in late Mesozoic times. Interestingly, similar massive fluorite mineralization on both sides of the Atlantic Ocean is also likely associated with Pangea rifting and known from numerous mining districts operating on unconformity-related hydrothermal veins in Central and North America, northern Africa and Europe.

DIAMOND CRYSTALLIZATION AND PHASE COMPOSITION IN THE FeNi – GRAPHITE – CaCO3 SYSTEM AT 5.5 Gpa

An experimental simulation of diamond crystallization in the system FeNi - graphite - calcium carbonate at a pressure of 5.5 GPa and a temperature of 1400℃ was carried out. Two sample assembly configurations were used. In the first one – the starting materials were put layer by layer, and in the second one - the components were mixed. It has been established that calcium carbonate, when interacting with the FeNi-melt, decomposes with the formation of Ca,Fe oxides and the release of CO2. Magnetite may be present as an accessory phase. Due to the formation of solid reaction products (Ca,Fe oxides) during layer-by-layer filling of the growth volume, the presence of calcium carbonate between graphite and FeNi-melt prevents diamond crystallization in the graphite layer and carbon transport to diamond seed crystals. When the components are mixed in the growth volume, diamond synthesis and growth onto seed crystals occur. The phenomenon of segregation of diamond crystals together with calcium carbonate and oxide phases, the products of the reaction in the bulk of the metal, has been discovered. Aliphatic, cyclic, and oxygenated hydrocarbons, including heavy compounds (C13-C17), CO2, H2O, nitrogen- and sulfonated compounds, were identified in the fluid phase captured by diamonds in the form of inclusions during growth. The composition of the fluid phase in the studied diamonds is more oxidized compared to the composition of fluid inclusions in diamonds grown in the FeNi – graphite system without carbonate. The results obtained correlate with the data on natural diamonds, among which there are crystals with “essentially carbon dioxide” compositions of fluid inclusions, which indicates the possible participation of crustal carbonate matter in the processes of diamond formation during subduction into the deep mantle.

Metamorphic reactions and their implication for the fluid budget in metapelites at seismogenic depths in subduction zones

In order to determine fluid-mineral reactions and the fluid budget in subduction zones, we have examined rocks from two accretionary complexes, the Kodiak (Alaska) and Shimanto (Japan), that were buried at the temperature conditions of the seismogenic zone. Maximum burial temperatures based on Raman spectroscopy of carbonaceous material in both examples range from 230 to 350 °C, whereas pressures reached up to ⁓3 and ⁓4kbars (based on multi-equilibrium modelling) in the higher-grade units of the Japanese and Alaskan complexes, respectively. At 230–260 °C, chlorite is interpreted as a coproduct of the lower-grade smectite-to-illite transformation. Chlorite content increases as temperature increases to 330–350 °C, as a result of illite-to-chlorite transformation. Other reactions include pyrite oxidation, dissolution of titanite and precipitation of anatase. Whole-rock chemistry points to the absence of systematic chemical variations in the temperature range between 250 and 350 °C and only local transport of quartz and albite from the matrix, to veins nearby, is observed. Qualitative analyses of the composition of fluid inclusions using decrepitation method show that the composition of the fluid is significantly different from the original seawater trapped in pores. In addition, the salinity of fluid inclusions in the quartz veins at 230–260 °C, analyzed using the Raman spectrum of water, is lower than seawater and interpreted as dilution of the original seawater by the fresh water released by the smectite-to-illite transformation. In contrast, veins formed at 330–350 °C contain fluid inclusions with a salinity on the order of or higher than that of seawater. The increase in salinity for temperatures of 250–350 °C reflects hydration reactions, which are mostly controlled by chlorite crystallization. This study demonstrates an uptake of pore fluid by mineral reactions at temperature corresponding to the base of the seismogenic zone, which is in contrast to the general trend of dehydration reactions occurring along rock burial in subduction zones.

Geochronology, Geochemistry, Fluid Inclusion and C, O Isotope Compositions of Calcite Veins in the Paleogene of the Jiangling Basin, South China: Implications for Fluid Evolution and Brine Potash Mineralization

AbstractDeep‐seated potassium‐rich brines were identified in the Jiangling Basin, South China. Although magmatic‐hydrothermal sources have been proposed, the relationship between brine‐type potash mineralization and volcanism remains unclear. In this study, U‐Pb geochronology, geochemistry, fluid inclusion and C‐O isotopic compositions of hydrothermal vein minerals in the Jiangling Basin are examined. Laser ablation U‐Pb dating of calcite veins indicates that the ages are slightly younger than the formation age of the Balingshan basalt. Fluid inclusions in hydrothermal minerals show medium–low homogenization temperatures (160–220°C) and low salinities (0.14 to 4.9 wt% NaCl eqv.) and densities (0.882–0.944 g/cm3). The liquid compositions of fluid inclusions in calcite veins from sedimentary strata have higher contents of potassium, compared with those from basalt. The coupled negative δ13CPDB (–10.3‰ to −8.0‰) and positive δ18OSMOW (17.4‰ to 20.7‰) values imply that calcite precipitation resulted from CO2 degassing of the basaltic magmatic fluids, as indicated by the gas composition of these inclusions in hydrothermal minerals. Rare earth element patterns indicate that water‐rock interaction between hydrothermal fluids and sedimentary wall rocks contributed to the calcite precipitation in sedimentary strata. It is proposed that high‐temperature water‐rock interaction between magmatic fluids and sedimentary strata resulted in the potassium enrichment in fluids, interpreted as one of the sources of potassium‐rich brines in the Jiangling Basin.

Re-evaluation of equilibrium relationships involving U6+/U4+ and Fe3+/Fe2+ in hydrothermal fluids and their implications for U mineralization

Uranium (U) and iron (Fe) are generally considered to have contrasting aqueous geochemical behavior, with U dissolution being favored in oxidizing fluids as oxidized U (U6+) and Fe in reducing fluids as reduced Fe (Fe2+) species. However, high concentrations of both U and Fe have been reported for the ore fluids of uranium deposits, suggesting that the two elements can be co-transported in the same fluid. In this study, geochemical modeling, based on recent thermodynamic and experimental data for U and Fe species and fluid inclusion compositions from two types of hydrothermal U deposits (volcanic-related and unconformity-related), was conducted to investigate the nature and conditions of UFe co-transportation. The results of the modeling indicate that high concentrations of U (>10, up to >100 ppm) and Fe (>1000 ppm) can be dissolved in acidic aqueous fluids (pH < ∼3.3) of moderate to high chlorinity (∼ 55 to 250 g/L Cl), for a wide range of logfO2 above and below the magnetite-hematite (MH) buffer (from ∼ MH-5 to MH + 30), at temperatures from 150 to 250 °C. There are four different combinations of dominant dissolved U and Fe species depending on the pH and redox conditions: U6+ and Fe3+ species (UO2Cl20 and FeCl30) at relatively high fO2, U4+ and Fe2+ species (UCl40 and FeCl42−) at relatively low fO2, and U6+ and Fe2+ species (UO2Cl20 and FeCl42−) or U4+ and Fe3+ species (UCl40 and FeCl30) at intermediate fO2 conditions. Because the most important U mineral in hydrothermal uranium deposits is uraninite (UO2), in which U is present as U4+, a necessary condition for U mineralization is that U is either transported as U4+ species or, if transported as U6+ species, precipitation of uraninite is induced by a reducing agent. The recognition that U6+ and Fe2+ species can be co-transported in the same fluid indicates that uraninite can be precipitated without the need to invoke a reducing agent external to the ore fluid. The driver of uranium mineralization in this case is an increase in pH due to fluid-rock interaction that leads to the alteration of the rocks by clay minerals and associated coupled sorption-reduction. These findings provide for a more extensive range of conditions for U transport and deposition than previously imagined and require that interpretation of the conditions of uranium mineralization consider both oxygen fugacity and pH. This study underscores the potential for uranium deposits to occur in geological settings where reducing agents are either absent or insufficiently abundant to account for the observed mineralization, and the possibility for the transport of U in environments that might otherwise be considered unfavorable.

Fluid evolution and petroleum accumulation in the precambrian gas reservoirs of the Sichuan Basin, SW China

Deep marine carbonates comprise one of the most significant reservoir types globally. Reconstruction of Precambrian oil reservoirs and the complex tectonism that affected Precambrian carbonate reservoirs demonstrate the complexity of the natural gas enrichment mechanism. A comparison between the fluid evolution of the well-preserved reservoirs in the central Sichuan Basin and the poorly preserved reservoirs in the southeastern Sichuan Basin offers insights into the factors that control the preservation of ancient reservoirs. We conducted geochemical analysis of the multistage cementation sequence, Raman quantitative analysis on fluid (gas) inclusions, and rare earth element (REE) analysis of diagenetic fluid environments. Hydrocarbon migration can cause variations in the diagenetic environment. Quantitative Raman measurements of the trapping pressure, trapping temperature, and composition of fluid (gas) inclusions are also useful in defining periods of gas accumulation. In situ U–Pb dating results of dolomite veins indicate that extensive dolomitization in the Sichuan Basin began during the early Cambrian. In the central Sichuan Basin, reservoir pressure evolved from normal during the Middle Jurassic to strongly overpressured during the mid-Cretaceous. Overpressure generated before Himalayan tectonism appears to be positively correlated with gas preservation. Overpressure within inclusions bearing hydrogen sulfide indicates that thermochemical sulfate reduction (TSR) does not produce intense dissolution that increases the porosity of carbonate reservoirs and reduces pore pressure. Poorly preserved reservoirs were intruded by exogenous and hydrothermal fluids but remained normally pressured. Secondary methane-rich gas inclusions record normal pressure conditions during the early Paleogene.