Hydrothermal Alteration and Its Superimposed Enrichment for Qianjiadian Tabular-Type Uranium Deposit in Southwestern Songliao Basin

The evolution characteristics of hydrothermal activity and superimposed uranium mineralization in the Qianjiadian ore field in southwestern Songliao Basin are still controversial and lack direct evidence. In this comprehensive study, a detailed identification of dolerite and hydrothermally altered un-mineralized sandstone and sandstone-hosted ore in the Yaojia Formation have been performed through the use of scanning electron microscopy observation, electron probe, carbon-oxygen-sulfur isotope, and fluid inclusion analyses. The results show that the hydrothermal fluid derived from the intermediate-basic magma intrusion is a low-temperature reducing alkaline fluid and rich in CO2, Si, Zr, Ti, Fe, Mg, Mn, and Ca, producing different types of altered mineral assemblages in the rocks, including carbonation, pyritization, sphalerite mineralization, clausthalite mineralization, silicification, and biotitization. Specifically, the carbonate minerals in sandstone are mixed products of deep hydrothermal fluid and meteoric water, with carbon and oxygen isotopes ranging from −5.2‰ to −1.7‰ and −20.4‰ to −11.1‰, respectively. Carbon source of the carbonate minerals in dolerite is mainly inorganic carbon produced at the late stage of intermediate-basic magma evolution, with carbon and oxygen isotopes from −16.1‰ to −7.2‰ and −18.2‰ to −14.5‰, respectively. Various carbonate minerals in the rocks may have been precipitated by the hydrothermal fluid after the magmatic stage, due to the change of its CO2 fugacity, temperature, and cation concentration during the long-term evolution stage. A series of carbonate minerals were generated as calcite, dolomite, ankerite, ferromanganese dolomite, and dawsonite. The precipitation processes and different types of carbonate mineral mixtures identified in this study mainly occur as parallel, gradual transition, interlacing, or inclusion metasomatism in the same vein body, without obvious mineralogical and petrologic characteristics of penetrating relationship. Homogenization temperature of fluid inclusions in calcite is high, in the range of 203–234 °C, with a low salinity of 0.71–4.34% NaCl, and the data range is relatively concentrated. Homogenization temperature of fluid inclusions in ankerite is usually low, ranging from 100 °C to 232 °C, with a high salinity of 4.18–9.98% NaCl. The precipitation processes of carbonate minerals and the results of this study are basically in consistent. Overall, the sandstone-type uranium deposits have a temporal and genetic relationship with hydrothermal activities during Paleogene. (1) Hydrothermal activity was directly involved in uranium mineralization, result in dissolution and reprecipitation of earlier uranium minerals, forming uranium-bearing ankerite and complexes containing uranium, zirconium, silicon, and titanium. (2) Hydrothermal fluid activity provided reducing agent to promote hydrocarbon generation from pyrolysis of carbonaceous fragments and accelerate uranium precipitation rate. (3) Regional water stagnation prolongs reaction time, contributing to huge uranium enrichment. This study provides new petrologic, mineralogical, and geochemical evidence for multi-fluid coupled and superimposed mineralization of sandstone-hosted uranium deposits in the sedimentary basin.

Apatite and Zircon Geochemistry in Yao’an Alkali-Rich Porphyry Gold Deposit, Southwest China: Implications for Petrogenesis and Mineralization

The Yao’an gold deposit is located in the middle of the Jinshajiang-Ailaoshan alkali-rich metallogenic belt, and this belt hosts many porphyry-type Cu-Au-Mo deposits formed at 46–33 Ma. Yao’an porphyry gold-mineralization is intimately associated with biotite syenite porphyry, whereas the contemporaneous quartz syenite porphyry is barren. In this study, we compared the major and trace elements of apatite and zircon and isotopic compositions of zircon from the biotite syenite porphyry and quartz syenite porphyry, to explore their geochemical differences that may affect their mineralization potential. The results show that both porphyries were derived from the partial melting of the thickened lower crust, which has been modified by slab-derived fluids, but has different mineral crystallization sequences, magma fluid activities, and magma oxidation states, respectively. REE contents in apatite and zircon can be used to reveal the crystallization sequence of minerals. A rapid decrease of (La/Yb)N ratio in apatite from both porphyries may be caused by the crystallization of allanite. Large variation of Cl contents and negative correlation between F/Cl and (La/Yb)N in apatite from fertile porphyry indicate that it has experienced the exsolution of Cl-bearing hydrothermal fluid. Higher Y/Ho and lower Zr/Hf in zircon from fertile porphyry indicate a stronger fluid activity than barren porphyry. The high S, V, As contents, δEu, low δCe in apatite, as well as high Ce4+/Ce3+ and log(fO2) estimated from zircon geochemistry from fertile porphyry, indicate high a oxidation state of fertile porphyry, similar to other fertile porphyries in this metallogenic belt. High fluid activity and fluid exsolution are conducive to the migration and enrichment of metal elements, which are very important for mineralization. High oxygen fugacity inhibits the precipitation of metal in the form of sulfide, thereby enhancing the mineralization potential of rock. Therefore, the exsolution of Cl-bearing hydrothermal fluid and high oxygen fugacity are the key factors promoting mineralization in Yao’an area.

Porphyry copper deposit formation in arcs: What are the odds?

Arc magmas globally are H2O-Cl-S–rich and moderately oxidized (ΔFMQ = +1 to +2) relative to most other mantle-derived magmas (ΔFMQ ≤ 0). Their relatively high oxidation state limits the extent to which sulfide phases separate from the magma, which would otherwise tend to deplete the melt in chalcophile elements such as Cu (highly siderophile elements such as Au and especially platinum-group elements are depleted by even small amounts of sulfide segregation). As these magmas rise into the crust and begin to crystallize, they will reach volatile saturation, and a hydrous, saline, S-rich, moderately oxidized fluid is released, into which chalcophile and any remaining siderophile metals (as well as many other water-soluble elements) will strongly partition. This magmatic-hydrothermal fluid phase has the potential to form ore deposits (most commonly porphyry Cu ± Mo ± Au deposits) if its metal load is precipitated in economic concentrations, but there are many steps along the way that must be successfully negotiated before this can occur. This paper seeks to identify the main steps along the path from magma genesis to hydrothermal mineral precipitation that affect the chances of forming an ore deposit (defined as an economically mineable resource) and attempts to estimate the probability of achieving each step. The cumulative probability of forming a large porphyry Cu deposit at any given time in an arc magmatic system (i.e., a single batholith-linked volcanoplutonic complex) is estimated to be ~0.001%, and less than 1/10 of these deposits will be uplifted and exposed at shallow enough depths to mine economically (0.0001%). Continued uplift and ero­sion in active convergent tectonic regimes rapidly remove these upper-crustal deposits from the geological record, such that the probability of finding them in older arc systems decreases further with age, to the point that porphyry Cu deposits are almost nonexistent in Precambrian rocks. A key finding of this paper is that most volcanoplutonic arcs above subduction zones are prospective for porphyry ore formation, with prob­abilities only falling to low values at late stages of magmatic-hydrothermal fluid exsolution, focusing, and metal deposition. This is in part because of the high threshold required in terms of grade and tonnage for a deposit to be considered economic. Thus, the probability of forming a porphyry-type system in any given arc segment is relatively high, but the probability that it will be a large economic deposit is low, dictated to a large extent by mineral economics and metal prices.

The Diagenetic Alteration of the Carbonate Rocks from the Permian Qixia Formation as Response to Two Periods of Hydrothermal Fluids Charging in the Central Uplift of Sichuan Basin, SW China

The hydrothermal fluid–carbonate rock reaction is frequently regarded to occur in deep-burial diagenesis, and the hydrothermal dissolution is usually distributed and takes place along the faults. Previous studies have suggested that there was hydrothermal fluid activity locally in the Permian Qixia Formation in Sichuan Basin, likely related to the Emeishan basalt eruption. However, the effect of hydrothermal fluids on the carbonate rocks of the Qixia Formation in the central uplift of Sichuan Basin is still unclear. Based on the characteristics and geochemical parameters of the diagenetic minerals, this study aims to reveal the diagenetic alteration related to the hydrothermal fluid–rock reaction in the Qixia Formation and reestablish the diagenetic evolution by using the timing of diagenetic mineral precipitation. The methods include petrographic observation; trace and rare earth element (REE) analysis; C, O and Sr isotope measurement; fluid inclusion temperature measurement and cathodoluminescence analysis. According to the petrographic characteristics, the dolostones are mainly of crystalline structure, namely fine-medium crystalline dolostone, meso-coarse crystalline dolostone, and coarse crystalline dolostone, with the cathodoluminescence color becoming brighter in that order. The limestones from the Qixia Formation are of the bioclastic limestone type, with no cathodoluminescence color. Compared with dolostones, limestones have higher Sr content, lower Mn content, and heavier oxygen isotopes. With the crystalline size of dolostone becoming coarser, the oxygen isotopes of dolostones tend to become lighter. The meso-coarse crystalline dolostone has the highest Mn content and negative carbon isotope. Both limestones and dolostones have an obvious positive Eu anomaly in the Qixia Formation. However, the REE patterns of fine-medium crystalline dolostones are very different from those of meso-coarse crystalline dolostone. It is credible that there were two periods of hydrothermal fluid charging, with different chemical compositions. The first period of hydrothermal fluids could laterally migrate along the sequence boundary. Fine-medium crystalline dolostones were almost completely distributed below the sequence boundary and were dolomitized during the shallow burial period. As products of the hydrothermal fluid–dolostone reaction, the saddle-shaped dolomites in the meso-coarse crystalline dolostones were the evidence of the second period of hydrothermal fluids. As a result, the dolomitization model was established according to the timing of diagenetic mineral precipitation, which can improve the geological understanding of the effect of hydrothermal fluid activities on the carbonate rocks in the Qixia Formation.

Conversion of Organic Matter of Carbonate Deposits in the Hydrothermal Fluid

This present paper investigates the conversion of organic matter in carbonate rocks of the Semiluksko-Mendymsky horizon Volga-Ural Basin under hydrothermal conditions with excess water in subcritical conditions at 613–653 K and 170–200 Bar. It has been found that the content of hydrocarbon extracts increases (relatively) which indicates the oil-generating potential of the studied deposits. Besides, it has been shown that organic matter undergoes oxidation reactions under hydrothermal conditions, as indicated by the presence of oxidized structures in the extracted hydrocarbons. Moreover, our results indicate that increasing the temperature and pressure of the used hydrothermal fluid leads to an increase in the content of n-alkanes C25-C30 and asphaltenes in the extracts. On the other hand, it has been found that saturated, aromatic hydrocarbons and resins content decreases at this stage. The obtained data about the geochemical parameters dependencies of the hydrocarbons obtained from the studied carbonate rock on temperature and pressure associated to hydrothermal effect show possible pathways of migration, genesis, and formation of hydrocarbon deposits.

Influence of Hydrothermal Recharge on the Evolution of Eruption Styles and Hazards During the 2018–2019 Activity at Kuchinoerabujima Volcano, Japan

The activity of the 2018-2019 eruption of Kuchinoerabujima Volcano in Japan changed from continuous ejection of ash-laden plumes between October 21 and the middle of December, to intermittent explosive activity accompanied by several pyroclastic density currents until January 2019. To understand the behaviors of magma and hydrothermal fluid that controlled the eruptive sequence, we carried out component analysis, X-ray diffractometry, and leachate analysis for ash samples. The proportion of non-altered volcanic ash particles is ~15 % in the earlier phase, then it decreased to less than 10 % in the later explosive phase. Accordingly, the mineral assemblage of the volcanic ash samples changed from plagioclase-dominant to sulfate minerals-dominant. Concentration of SO42- and Cl/SO4 values of the ash-leachates decreased toward the later activity. These results indicate that the proportion of fresh volcanic rocks decreased and sulfuric acid fluid-derived sulfate minerals increased toward the later activities. Consequently, the 2018-2019 eruption at Kuchinoerabujima Volcano changed from magmatic activity to phreatomagmatic activity. Weak glowing of the crater was observed during the magmatic activity, indicating the volcanic conduit was hot enough to dry up the subvolcanic hydrothermal system. The following phreatomagmatic activity indicates that the hydrothermal fluid recharged after the magmatic eruption phase. Recharge of the hydrothermal fluid likely caused the variation of the eruption style, and is a process that may control the evolution of hazards during future eruption scenarios at similar active volcanoes in Japan and worldwide.

Three-dimensional inversion of audio-magnetotelluric data acquired from the crater area of Mt. Tokachidake, Japan

Subvolcanic hydrothermal systems can lead to hydrothermal eruptions as well as unrest phenomena without an eruptive event. Historical eruptions and recent unrest events, including ground inflation, demagnetization, and a gradual decrease in the plume height, at Mt. Tokachidake, central Hokkaido, Japan, are related to such a subvolcanic hydrothermal system. This study investigates the three-dimensional (3-D) resistivity structure of Mt. Tokachidake to image its subvolcanic hydrothermal system. A 3-D inversion of the magnetotelluric data, acquired at 22 sites around the crater area, was performed while accounting for the topography. Our resistivity model was characterized by a high-resistivity layer at a shallow depth (50–100 m) and two conductors near the active crater and dormant crater. The high-resistivity layer was interpreted to be composed of dense lava, which acts as a caprock surrounding the conductor. The high conductivity beneath the active crater can be explained by the presence of hydrothermal fluid in fractured or leached zones within the low-permeability lava layer, as the sources of ground inflation and demagnetization were identified within the conductive zone immediately beneath the resistive layer. The resistivity structure was used to estimate the volume of hydrothermal fluid within the pore space. The minimum volume of hydrothermal fluid beneath the active crater that can explain the resistivity structure was estimated to be 3 × 106 m3. This estimate is comparable to the water volume that was associated with the long runout and highly fluidized lahar in 1926. The resistivity structure and volume of hydrothermal fluid presented in this study can be used as a reference for further numerical simulations, which aim to reveal the mechanisms of recent unrest events and assess the risk of hazards, such as lahar.

Morphology of Barite Synthesized by In-Situ Mixing of Na2SO4 and BaCl2 Solutions at 200 °C

Barite is an abundant sulfate mineral in nature. Especially, the variety of morphologies of barite is often driven by the mixing of Ba-bearing hydrothermal fluid and sulfate-bearing seawater around hydrothermal chimneys. In order to better understand the factors affecting the morphology and precipitation mechanism(s) of barite in seafloor hydrothermal systems, we synthesized barite by a new method of in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C while varying Ba concentrations and ratios of Ba2+/SO42−, and at room temperature for comparison. The results show that barite synthesized by in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C forms a variety of morphologies, including rod-shaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped morphologies, while room temperature barites display relatively simple, granular, or leaf-like morphologies. Thus, temperature affects barite morphology. Moreover, dendritic barite crystals only occurred at conditions where Ba2+ is in excess of SO42− at the experimental concentrations. The dendritic morphology of barite may be an important typomorphic feature of barite formed in high-temperature fluids directly mixing with excess Ba2+ relative to SO42−.