Hydrothermal Mineral Deposits Research Articles

Geological Structure and Possible Mineralized Zones in Ikungi – Manyoni Area, Tanzania: Delineated from High-resolution Aeromagnetic Data

Understanding geological structures is important in mineral exploration. Delineating subsurface geological structures, structural intersections, and complexities of potential geological settings for the formation of hydrothermal mineral deposits, such as gold requires using magnetic data, particularly, high-resolution aeromagnetic data. To define possible targets for mineral exploration, this study used linear detection and the Centre for Exploration Targeting (CET) techniques on high-resolution aeromagnetic data for structural delineation and interpretation. The study's conclusions indicate that the majority of geological features, such as faults, shears, fault intersections, and fracture systems, trend NE-SW, N-S, and occasionally NW-SE. These structures create potentially large areas that have a strong chance of becoming mineralized. The CET grid analysis technique defined mineralized zones, particularly the structural intersections which are potential targets in mineral exploration. The effectiveness of the CET technique in mineral prospecting was demonstrated by the correlation between the results and existing mineral occurrences (such as Shanta Gold Mine, abandoned pits, and the operating pits of small-scale miners in Muhintiri). Based on the findings reported herein, any sites that share characteristics with those that have demonstrated gold mineralization should be prioritized for ground follow-up

Optimized thermodynamic properties of REE aqueous species (REE3+ and REEOH2+) and experimental database for modeling the solubility of REE phosphate minerals (monazite, xenotime, and rhabdophane) from 25 to 300 °C

Rare earth elements (REE) are critical elements found in monazite, xenotime, and hydrated REE phosphates which typically form in hydrothermal mineral deposits. Accurate predictions of the solubility of these REE phosphates and the speciation of REE in aqueous fluids are both key to understanding the controls on the transport, fractionation, and deposition of REE in natural systems. Previous monazite and xenotime solubility experiments indicate the presence of large discrepancies between experimentally derived solubility constants versus calculated solubilities by combining different data sources for the thermodynamic properties of minerals and aqueous species at hydrothermal conditions. In this study, these discrepancies were resolved by using the program GEMSFITS to optimize the standard partial molal Gibbs energy of formation (ΔfG°298) of REE aqueous species (REE3+ and REE hydroxyl complexes) at 298.15 K and 1 bar while keeping the thermodynamic properties fixed for the REE phosphates. A comprehensive experimental database was compiled using solubility data available between 25 and 300 °C. The latter permits conducting thermodynamic parameter optimization of ΔfG°298 for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 °C requires modifying the ΔfG°298 values of REE3+ by 1–6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 °C requires modifying the ΔfG°298 values of both REE3+ and REEOH2+ by ~15–31 kJ/mol and ~ 2–10 kJ/mol, respectively. For xenotime, adjustments of ΔfG°298 values by 1–26 kJ/mol are only necessary for the REE3+ species. The optimizations indicate that the solubility of monazite in acidic solutions is controlled by the light (L)REE3+ species at <150 °C and the LREEOH2+ species at >150 °C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 °C. Based on the optimization results, we conclude that the revised Helgeson-Kirkham-Flowers equation of state does not reliably predict the thermodynamic properties of REE3+, REEOH2+, and likely other REE hydroxyl species at hydrothermal conditions. We therefore provide an experimental database (ThermoExp_REE) as a basic framework for future updates, extensions with other ligands, and optimizations as new experimental REE data become available. The optimized thermodynamic properties of aqueous species and minerals are available open access to accurately predict the solubility of REE phosphates in fluid-rock systems.

Numerical simulation of ore formation within skarn-type Pb-Zn deposits: Implications for mineral exploration and the duration of ore-forming processes

Skarn-type deposits are typical hydrothermal mineral deposits distributed throughout the world with multiple metal resources and huge economic value. However, their extremely complex forming processes hinder us from further understanding their forming mechanism and exploring their economic potential. This study builds an integrated and simplified model of skarn-type Pb-Zn deposits and applies numerical simulation approach to quantitatively describe the formation of mineralization, analyzing the continuous dynamic changes of the concentrations of generated garnet, diopside and galena/sphalerite within the system. This numerical model includes chemical reactions, heat conduction, fluid-flow, materials migration and diffusion and their complex coupled relationships. The results show that the alterations at the contact of wall rocks and intrusions have clear zoning of garnet and diopside, and the amount of diopside exceeds that of garnet at the distal end of the generated alteration zone; the generation processes of garnet, diopside and galena/sphalerite follows a strict chronological order, controlling the variation of rock porosity; the cooling process of P3 (Fig. 4b) from 700 °C to 170 °C costs 55,000 years, within which the formation of galena/sphalerite merely lasts 30,000 years, meaning that the effective lifespan of Pb/Zn mineralization takes up only 54.54 % of the whole 55,000 years. All these results provide useful information for metallogenic research of skarn-type Pb-Zn deposits as well as magmatic-hydrothermal system. However, the method applied in this research still has limitations, such as the restriction from the single-period magmatic activity, the law of mass action, and the simplified chemical equations. Future development into both mathematical geosciences and geological analytical tests will definitely provide more appropriate calculation models and testing data with higher precision, improving the accuracy and practical significance of numerical simulation of ore formation, and providing more detailed quantitative answers for remaining hot issues related to economic geology research.

Translation of the function of hydrothermal mineralization-related focused fluid flux into a mappable exploration criterion for mineral exploration targeting

Formation of hydrothermal mineralization is related to the late stages of, or immediate following, orogenic compressional regimes that have been superimposed on arc magmatism. For instance, mineralization-related fluid flux mechanisms of porphyry Cu formation deal with how magmatic fluids are transferred from the corresponding source chambers to the environment of stock emplacement related to porphyry systems. In this paper, firstly, we review the imperative physicochemical processes of mineralizing fluid flux mechanisms, e.g., the function of intrusive, metalliferous compounds, and permeable structures, and then assemble them to translate the mechanism of forming focused fluid flux into 2D mappable criteria for mineral exploration targeting. According to the review and the ensuing knowledge and idea, we propose a simplifying approach by focusing on a geological point feature - inferred sites of strongly concentrated ore fluid flux - that represents a critical, if not the most critical, geological element in mineral exploration targeting. We demonstrate that this point feature can be derived as a mappable criterion and that there are stronger spatial and thermal relationships between the location of hydrothermal mineral deposits and this proposed point feature relative to the existing point and line features (e.g., intrusive contacts, fault density, fault intersections, and proximity to faults), which have been widely used in the procedure of prospectivity analysis for mineral exploration targeting. We illustrate the procedures of generating mineral exploration targets, proposed in this paper, using geochemical and geological data sets of porphyry Cu deposits of Iran.

Fusion of Multispectral Remote-Sensing Data through GIS-Based Overlay Method for Revealing Potential Areas of Hydrothermal Mineral Resources

Revealing prospective locations of hydrothermal alteration zones (HAZs) is an important technique for mineral prospecting. In this study, we used multiple criteria inferred from Landsat-8 OLI, Sentinel-2, and ASTER data using a GIS-based weighted overlay multi-criteria decision analysis approach to build a model for the delineating of hydrothermal mineral deposits in the Khnaiguiyah district, Saudi Arabia. The utilized algorithms revealed argillic, phyllic, and propylitic alteration characteristics. The HAZs map resulted in the identification of six zones based on their mineralization potential, providing a basis for potential hydrothermal mineral deposit assessment exploration, which was created by the fusion of mineral bands indicators designated very low, low, moderate, good, very good, and excellent and covers 31.36, 28.22, 20.49, 10.99, 6.35, and 2.59%. Based on their potential for hydrothermal mineral potentiality, the discovered zones match gossans related to sulfide mineral alteration zones, as demonstrated by previous studies.

AHS mapping for hydrothermal alterations detection and mineral deposits exploration

The Actve Hyperspectral Sensing (AHS) application for a mineral mapping have been tested on outcrops with porphyry-copper style of mineralization in the Vlaykov Vruh and Tsar Assen deposits. As a new tool used in mineral exploration AHS reveals the advantages for mineral detection and targeting as an express new innovative technology and efficient tool for mineral prospecting.

Cathodoluminescent, chemical and strontium isotopic characteristics of apatite from Lanniping Fe-Cu deposit, southwestern China: Implications for fluid evolution in IOCG systems

Apatite occurs in diverse types of hydrothermal mineral deposits and has been widely used to constrain the nature and evolution of ore-forming fluid. The origin of ore fluids responsible for formation of iron oxide-copper–gold (IOCG) deposits remains controversial. Here we combine textural observations from optical microscopic cathodoluminescence (OM-CL) and backscattered electron (BSE) images with in situ chemical and strontium isotopic analyses of apatite in the Lanniping Fe-Cu deposit in Kangdian metallogenic province, SW China, to reconstruct the hydrothermal evolution history of this deposit. Paragenetic sequences of the deposit include pre-ore Na-alteration (stage I) of predominant albite, stage II iron-oxide mineralization with the associated Ca-Fe alteration characterized by magnetite and minor apatite, stage III Cu-Fe sulfides and the associated K-Fe alteration, and final stage IV Ca-Mg alteration. Three types of apatite at different paragenetic stages were identified in this deposit. The pristine Type 1 apatite are intergrown with stage II magnetite and show blue colors under OM-CL images. The Type 2 apatite, which are darker than Type 1 apatite under BSE images, show dark yellow CL colors. They commonly occur as metasomatized domains of Type 1 apatite and can contain chalcopyrite mineral inclusions, indicating that they are coeval products during stage III. The Type 3 apatite show bright yellow CL colors, and occur as overgrowth rims of both Type 1 and 2 apatite. The Type 1 apatite have highest REY contents (5342 to 12983 ppm) and primitive 87Sr/86Sr ratios ranging from 0.70630 to 0.71093, which overlap the associated igneous intrusions. In contrast, the Type 2 and Type 3 apatite have gradually lower REY contents (866 to 4255 ppm and 142 to 635 ppm, respectively) but higher 87Sr/86Sr ratios (0.71009 to 0.71698 and 0.71904 to 0.72602, respectively). Our data hence suggest that the ore-forming fluids were mainly magmatic-hydrothermal origin and were gradually enriched in radiogenic Sr isotope by mixing with external fluid. Our case study highlights that texturally constrained apatite could be utilized to effectively trace fluid evolution of IOCG mineralization, and by reference, other hydrothermal mineralizing systems.

Using groundwater in the hyper-arid Atacama Desert to discover buried hydrothermal mineral deposits

ABSTRACT Epithermal deposits occur in many metallogenic belts globally and are economically important sources of precious and base metals. However, post-mineral cover (both transported and younger deposits) also occurs in many prospective districts and hinders exploration efforts. Hydrogeochemistry is a potentially useful exploration tool in these environments, with water–rock interaction shown from other studies to produce large hydrogeochemical footprints. Groundwaters from in and around the Vaquillas (Au-Ag) epithermal deposit in the pre-cordillera of northern Chile are typically Ca-SO4 2- type with meteoric upflow groundwaters interacting with the ore body and producing a significant downflow (down hydraulic head) mobility (up to 3 km) of (oxy)anions of arsenic and molybdenum, which contrasts with proximal trace cation anomalies. Multiple sulphur endmembers contribute to the dissolved sulphate concentrations, including evaporites, primary hypogene sulphide minerals, and a single δ34S(SO4) value of 30.1 ‰ which is potentially associated with disproportionated sulphur in sulphate. This study reports anomalism from one of the most dilute endmember groundwater compositions from Chile reported to date. The sub-alkaline and weakly oxygenated groundwaters are common in many aquifers globally, making the results applicable to many other metallogenic belts. It is also the first hydrogeochemical case study of an intermediate-sulfidation epithermal deposit. Despite the highly dilute groundwaters recharging aquifers at Vaquillas, we conclude hydrogeochemistry to be a highly effective exploration tool for epithermal and porphyry deposits in areas of significant post-mineral cover.

Towards the fertility trend: Unraveling the economic potential of igneous suites through whole-rock and zircon geochemistry (example from the Tapajós mineral Province, Northern Brazil)

Magmatic hydrothermal mineral deposits axiomatically frame the hosting pluton and the given tectonic setting in which they are inserted. Unsurprisingly, pre-Cambrian porphyry deposits are different than Phanerozoic examples, as collisional or post-collisional deposits are different than arc-related mineralizing systems. As such, this contribution assesses the tectonic framework and the magmatic evolution of the igneous suites present in south-central Amazonian Craton (Tapajós Mineral Province, TMP), and how these parameters translate into metallogenetic potential for the formation of magmatic-hydrothermal copper and gold deposits. For this purpose we gathered whole-rock geochemistry data from selected TMP copper and gold deposits and present new in situ trace-element and U-Pb analyses in zircon grains.The host rocks of the selected mineralizing systems belong either to the older magmatic sequence (OMS, 2.00–1.95 Ga), or to the younger magmatic sequence (YMS, 1.90–1.86 Ga). Whereas OMS presents characteristics of arc-related rocks, YMS represents the evolution towards post-orogenic or collisional setting. OMS might be divided into groups I, II and III granitoids. Group I comprises granites and granodiorites that mark the onset of the arc-magmatism in the TMP at ca. 2011 Ma. In this stage rocks are peraluminous, ferroan, anhydrous, show a strong crustal component and fO2 values close to or below the fayalite-magnetite-quartz buffer (ΔFMQ), yielding metallogenetic unfertile pluton. These rocks correlate with the Cuiú-Cuiú Complex and comprise samples from the Patrocínio and Tocantinzinho deposits. Group II granitoids correlate with the Creporizão Suite and comprise Tocantinzinho, São Jorge, Chapéu do Sol and Patrocínio deposit samples (with extrusive equivalents in the Coringa deposit). Rocks are metaluminous to peraluminous, magnesian to ferroan and mark the evolution of the magmatism towards more oxidizing (0 ≤ ΔFMQ ≤ + 4), hydrous and metallogenetically fertile conditions at ca. 1986 Ma. Mineralized zones are defined by gold bearing sulfide veins and veinlets within the potassic or sericitic alteration halos. Group III comprises rarer high-K, metaluminous, ferroan, anhydrous and reduced syenites and monzonites dated at 1993 and 1974 Ma, generated by decompression melting of metasomatized mantle. By itself, group III rocks should be considered as unfertile batches of magma, however, its interaction with the coeval, more hydrous and oxidized group II melts, confer these magmas their metallogenetic potential. Mineralization in group III rocks is defined by disseminated pyrrhotite – pyrite ± gold within the sericitic, chloritic and/or carbonate alteration zones.YMS comprises intermediate and acid rocks that belong to the Parauari Intrusive Suite and to the Iriri Group. Whereas, intermediate volcanics from YMS evolved on a hydrous and reduced petrologic trend, acid volcanics and the Batalha and Palito granitoids evolved on a dry and reduced trend, reflecting a change on TMP’s tectonic context, from magmatic-arc to a post-orogenic or collisional tectonic setting. Despite the reduced and anhydrous characteristic of the magmas, YMS’s metallogenetic potential is related with remobilization or melting of SCLM or lower crustal Au-rich sulfides formed on the previous magmatic events. Hence, the first pulses of the YMS might be considered as potentially fertile for the formation of Au-rich magmatic-hydrothermal deposits.

Synergy of Remote Sensing Data for Exploring Hydrothermal Mineral Resources Using GIS-Based Fuzzy Logic Approach

The Arabian Nubian Shield (ANS) contains a variety of gold deposits in the form of veins and veinlets formed by hydrothermal fluids. Characterizing potential areas of hydrothermal alteration zones therefore provides a significant tool for prospecting for hydrothermal gold deposits. In this study, we develop a model of exploration for hydrothermal mineral resources in an area located in the ANS, Egypt, using multiple criteria derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Landsat-Operational Land Imager (OLI), and Sentinel-2 data and field work through GIS-based fuzzy logic approach. The hydrothermal alteration zones (HAZs) map extracted from combining mineral indices, spectral bands, and ratios is consistent with observed argillic alteration zones around the mineralized veins. Combining HAZs and lineament density led to identification of six zones based on their mineralization potential, and provides a tool for successful reconnaissance prospecting for future hydrothermal mineral deposits. The detected zones are labeled as excellent, very high, high, moderate, low, and very low, based on their potential for Au production, and the predictive excellent and very high zones cover about 1.6% of the study area. This model also shows that target prospective zones are quartz veins controlled by NNW-SSE trending fracture/fault zones all crosscutting Precambrian rocks of the ANS. Field observations and petrographic and X-ray diffraction analyses were performed to validate the mineral prospective map and revealed that quartz veins consist of gold–sulfide mineralization (e.g., gold, pyrite, chalcopyrite, and sphalerite). Consistency between the high potential hydrothermal alterations zones (HAZs) and the location of gold mineralization is achieved.

Using of remote sensing and aeromagnetic data for predicting potential areas of hydrothermal mineral deposits in the Central Eastern Desert of Egypt

This article explored mineral resources and their relation to structural settings in the Central Eastern Desert (CED) of Egypt. Integration of remote sensing (RS) with aeromagnetic (AMG) data was conducted to generate a mineral predictive map. Several image transformation and enhancement techniques were performed to Landsat Operational Land Imager (OLI) and Shuttle Radar Topography Mission (SRTM) data. Using band ratios and oriented principal component analysis (PCA) on OLI data allowed delineating hydrothermal alteration zones (HAZs) and highlighted structural discontinuity. Moreover, processing of the AMG using Standard Euler deconvolution and residual magnetic anomalies successfully revealed the subsurface structural features. Zones of hydrothermal alteration and surface/subsurface geologic structural density maps were combined through GIS technique. The results showed a mineral predictive map that ranked from very low to very high probability. Field validation allowed verifying the prepared map and revealed several mineralized sites including talc, talc-schist, gold mines and quartz veins associated with hematite. Overall, integration of RS and AMG data is a powerful technique in revealing areas of potential mineralization involved with hydrothermal processes.

Stability of aqueous Fe(III) chloride complexes and the solubility of hematite between 150 and 300 °C

Three sets of experiments were performed to test the stability and geologic importance of ferric (FeIII) chloride complexes in acidic, Cl-rich solutions at 150–300 °C, p = psat. Experiment Set A used the change in solubility of AgCl(s) in the presence of FeCl3 or FeCl2 to determine the stoichiometry of Fe(III) and Fe(II) chloride complexes at ΣCl = 0.1–3.0 molal. Results show that FeCl4− and FeCl2(aq) are the dominant Fe(III) and Fe(II) species, respectively, at T > 200 °C and ΣCl ≥ 1 m. Set B experiments used the solubility of elemental gold as a redox sensor to determine the aO2 of the FeCl4−/FeCl2(aq) boundary and log K values of +6.56 and +7.19 for the following reaction at 250 and 300 °C, respectively: FeCl2(aq) + ¼O2(g) + 2Cl− + H+ = FeCl4− + ½H2O(l). Ferric chloride complexes are stable under conditions of aCl−, aO2, and pH where gold is soluble as AuCl2−. Set C experiments measured the solubility of hematite in NaCl-HCl solutions at 200–300 °C by preparing a series of silica tubes with identical matrix chemistry (0.9 m NaCl + 0.1 m HCl) and increasing concentration of FeCl3. Hematite saturation was constrained by the tube with the lowest FeCl3 concentration that precipitated hematite at high temperature. Set C results were used to compute log K values of +3.6, +5.2, and + 7.4 for the following reaction at 200, 250 and 300 °C, respectively: 0.5Fe2O3(s) + 4Cl− + 3H+ = FeCl4− + 1.5H2O.Hematite solubility as FeCl4− is independent of redox state, and increases quickly with an increase in temperature, increase in Cl− concentration (power of 4), and decrease in pH (power of 3). Concentrations in excess of 100 mg/L ΣFe are attainable under geologically realistic conditions. Once formed, FeCl4− is an extremely effective oxidizing agent, capable of destabilizing any sulfide mineral and dissolving Fe, Au, and other metals (e.g., Cu, Pt, Pd) as chloride complexes. In deep hydrothermal systems, FeCl4− is a more viable oxidant than dissolved O2 gas, although its presence requires a source area with abundant hematite and a lack of reductants such as organic carbon, sulfides, or silicate minerals containing ferrous iron. Dissolved ferric chloride could be an important and previously overlooked reactant in the formation of certain types of hematite-rich hydrothermal mineral deposits, including iron oxide-copper–gold (IOCG) deposits.

Using hyperspectral imagery for identification of pyrophyllite-muscovite intergrowths and alunite in the shallow epithermal environment of the Yerington porphyry copper district

Hydrothermal mineral deposits are the primary source of many mineral commodities of global importance. Since hydrothermal alteration minerals associated with the formation of these mineral deposits are active in the visible and infrared range, the analysis of spectral absorption features can be used to identify the mineralogy associated with different alteration events. Some hydrothermal events are responsible for the occurrence of mineral commodities, while other events create hydrothermal alteration unrelated to the introduction of base and precious metals. Therefore, it is crucial to develop a mineral exploration strategy to rapidly identify and map the indicator minerals linked to a mineralising event. The separation of minerals of different alteration events which are spectrally active in the same overlapping range of the spectrum, is the challenge addressed in this study. High spatial resolution airborne and laboratory-based hyperspectral images are combined to detect and visualise textures of muscovite replacing pyrophyllite in the shortwave infrared (SWIR) imaging spectroscopy survey over the Buckskin Range, the volcanic-hosted lithocap part of the Yerington porphyry district, Nevada (USA).Spectral wavelength maps in different SWIR ranges are used to map the hydrothermal alteration mineralogy at both laboratory (26 µm) and airborne (1 m) scales. The airborne spectral data define outward zoning from alunite ± pyrophyllite to muscovite characterized by variable wavelength positions of its Al-OH absorption feature. The wavelength range of 1650–1850 nm is used to differentiate zones of pyrophyllite predominance over alunite within the inner domain. The laboratory data improves the characterisation of the hydrothermal alteration mineralogy, which includes alunite, pyrophyllite, muscovite, dickite, chlorite, topaz and zunyite. The textural relationship of muscovite replacing pyrophyllite is addressed through the development of a novel spectral index, the pyrophyllite-muscovite index (PMI). The characterisation of the intergrowths of pyrophyllite and muscovite at the laboratory scale is based on two aspects: (1) the definition of pervasive versus veinlet-controlled textures and (2) a subtle shift detection in the wavelength position of the Al-OH absorption feature of muscovite from 2189 to 2195 nm. The combination of the spatial patterns with the textural relationship of the pyrophyllite-muscovite association allows the identification of areas which contain the muscovite replacement of pyrophyllite. The recognition of a late muscovite replacement of pyrophyllite suggests that advanced argillic alteration reflecting intense acid leaching is followed by late near-neutral pH magmatic-hydrothermal fluids, adding K+ and potentially other alkali elements and metals in the epithermal environment. As a result of this study, we document the hydrothermal muscovite-pyrophyllite intergrowth relationships in the study area, thus contributing to an improved understanding of the lithocap epithermal system and a better assessment of its exploration potential for Au, Ag and Cu mineralisation.

Mapping hydrothermal alteration mineral deposits from Landsat 8 satellite data in Pala, Mayo Kebbi Region, Southwestern Chad

The different image attribute extracting techniques, spectral enhancement ability, pixel classification methods have been applied on the Landsat 8 OLI satellite image data to map geological features and spatial distribution of hydrothermal alteration zones, which have been recorded to be associated with gold mineralization in the Pala, Mayo Kebbi region of southwestern Chad. Radiometric calibrated, noise filtered and image enhanced Landsat 8 OLI data were subjected to colour composite, band rationing and crosta analyses in order to enhance spectral reflectance and compositional information, create colourful multispectral images which enable the mapping of desired spectral bands into principal component images for the purpose of accentuating salient information needful to delineate hydrothermal alteration zones within the study area.Composite ratios 4/2-6/7-6/5 indicate relative spatial distribution of iron-oxide in the southeast while clays and hydroxyl minerals occur in the west and northwestern part of the study area. The eigenvectors and eigenvalues for hydroxyl minerals from the crosta analysis indicate highest loadings of opposite signs (-0.721498 & 0.649465; bands 6 & 7) in PC3, representing ‘’H’’ image. The eigenvectors and eigenvalues for iron-oxides on the other hand present highest loading of opposite signs (0.895721 & -0.443283; bands 2 & 4) in PC4, representing the “F” image. The white coloured pixels within the bright reflections generated from Crosta composite image of H, H+F & F in RGB delineate hydrothermal alteration zones. The plot of delineated zones of hydrothermal alteration on the Landsat 8 true colour indicate spatial distribution of the alteration areas within the host greenstone belt belonging to the Precambrian Goueygoudoum series and they plot mainly along major structural features, suggesting structural control. Field control revealed that some areas delineated by band rationing as hydrothermal alteration zones appear to be the result of intense weathering. Accordingly, we recommend that remote sensing delineation of hydrothermal alteration zones should be checked by a relevant ground-observation based control procedure.

Physicochemical Parameters of the Origin of Hydrothermal Mineral Deposits: Evidence from Fluid Inclusions. VI. Fluorite and Barite Deposits

Authors’ original continuously updated database, which currently comprises data from 22 300 publications on mineral-hosted fluid and melt inclusions, is used to generalize data on the physicochemical parameters of processes that produced fluorite and barite deposits. The paper presents data on the following parameters of the fluids: temperature, pressure, density, salinity, and gas composition. The average composition (Н2О, CO2, CH4, N2, H2S, and CnHm) of fluids at fluorite–barite deposits is calculated. The paper reports the average composition of major gas components (CO2, CH4, N2, and H2S) of natural fluids at these deposits on the basis of Raman spectroscopic analyses of individual inclusions. Average F, S, and Ba concentrations are calculated for silicate magmatic melts and natural mineral-forming fluids.

Li and B isotopic fractionation at the magmatic-hydrothermal transition of highly evolved granites

The isotopic compositions of Li and B have great potential for the tracing of fluid sources and for the characterization of ore-forming processes. The fractionation of Li and B isotopes during the magmatic-hydrothermal transition in evolved granites, however, is not well understood as these rocks commonly show multiple late hydrothermal overprints. Tourmaline is sensitive to changes in melt and fluid compositions during its growth and resistant to alteration by later fluids, which makes it a good recorder of Li and B isotopic signatures and fractionation at late evolutionary stages of magmatic systems. To characterize the evolution of a Sn-ore forming granitic intrusion, we analyzed magmatic, hydrothermal, and detrital-sedimentary tourmaline from the Yuanbaoshan granite, associated cassiterite-tourmaline-quartz veins, and disseminated tourmaline-bearing cassiterite-sulfide ore in the sedimentary wall rocks.The Li and B isotopic compositions of tourmaline from these rocks do not covary, largely due to mineral-controlled decoupling during magmatic and hydrothermal processes. Because of the dominant role of tourmaline on the B budget of the rock, magmatic and hydrothermal tourmalines show little variation in δ11B (−12.5 to −8.2‰), a signature reflecting the metasedimentary protoliths of the melt. In contrast, several rock-forming minerals contribute to the budget of Li, which shows significant variation in δ7Li (−0.3 to +6.9‰). The isotopic fractionation of Li in tourmaline from rocks of the Yuanbaoshan area is controlled by three major processes: (i) fractional crystallization results in magmatic tourmaline having progressively higher δ7Li values and higher Li contents, (ii) the magmatic-hydrothermal transition leads to higher δ7Li values at lower Li contents in hydrothermal tourmaline, and (iii) extensive fluid-rock interaction adds isotopically light Li to hydrothermal tourmaline in the wall rocks. Extensive fluid-rock interaction also leached HREE, Mg, isotopically light B, and probably Sn from the granites and/or wall rocks. Tourmaline seems to robustly record progressive changes in Li isotopes in evolving magmatic-hydrothermal systems and hydrothermal mineral deposits.

Shannon Entropy as an Indicator for Sorting Processes in Hydrothermal Systems.

Hydrothermal processes modify the chemical and mineralogical composition of rock. We studied and quantified the effects of hydrothermal processes on the composition of volcanic rocks by a novel application of the Shannon entropy, which is a measure of uncertainty and commonly applied in information theory. We show here that the Shannon entropies calculated on major elemental chemical composition data and short-wave infrared (SWIR) reflectance spectra of hydrothermally altered rocks are lower than unaltered rocks with a comparable primary composition. The lowering of the Shannon entropy indicates chemical and spectral sorting during hydrothermal alteration of rocks. The hydrothermal processes described in this study present a natural mechanism for transforming energy from heat to increased order in rock. The increased order is manifest as the increased sorting of chemical elements and SWIR absorption features of the rock, and can be measured and quantified by the Shannon entropy. The results are useful for the study of hydrothermal mineral deposits, early life environments and the effects of hydrothermal processes on rocks.

Defining active, inactive, and extinct seafloor massive sulfide deposits

Hydrothermal activity results in the formation of hydrothermal mineral deposits, including seafloor massive sulfide deposits, at oceanic spreading ridges, arcs, and back-arcs. As hydrothermal systems age, the mineral deposits eventually become severed from the heat source and fluid-flow pathways responsible for their formation and become extinct. The timescales and processes by which this cessation of activity occurs, and the resultant distinction between hydrothermally active and inactive deposits has recently taken on policy implications related to the potential issuance of exploitation leases for seafloor massive sulfide deposits by the International Seabed Authority in Areas Beyond National Jurisdiction. Here, we discuss the scientific rationale behind designating hydrothermal systems as active, inactive, or extinct, with the aim of applying a scientific underpinning to ongoing policy discussions, which often lack a common set of criteria and use the same descriptions for opposing phenomena. We apply the simple definition that active vent fields currently exhibit fluid flow above ambient seawater temperatures, inactive vent fields are not currently exhibiting fluid flow but may potentially become active again, and extinct vent fields are not expected to become active again. We suggest these terms can only be correctly applied at the vent field scale and define a vent field as a geologically continuous entity that may include both actively and formerly venting hydrothermal deposits. Finally, we propose criteria and techniques for determining activity and reasonably bounding the extent of a vent field for classification purposes.

Reconstructing Deformation History by Using Microstructural and Petrographic Analysis of Sorobo, Konso Area, Southern Ethiopia

The deformation history of Sorobo, Konso area (southern Ethiopia) within the Mozambique Belt, is described using microstructural and petrographic analysis. The area is characterized by folds and metamorphic fabrics that trend between NNE and NNW and consist of high-grade, amphibolite- to granulite-facies rocks. It is affected by five deformational phases (D1 to D5) and two metamorphic events (M1 & M2). During shortening deformation events (D1 to D3) the development of gneissosity (D1), the formation of tight to isoclinal and recumbent folds (D2) and the superposition of secondary (upright) fold (D3) on earlier recumbent fold resulting in type-3-fold interference pattern are formed respectively. During fourth deformation phase (D4) most of the rock units of the area are affected by shearing or shows east and west vergence. The area is affected by both sinistral and a dextral sense of shearing but dextral shear-sense appears dominant. Brittle type of deformation phase (D5) was developed and resulting in different types of faults varies in orientation. Peak progressive metamorphism of the granulite facies (M1) and retrogression (M2) are also associated with the deformational phase. An equal-area plot and rose diagram shows the orientation of all fabric, which indicates multiphase deformation. Geological structures in the study areas like faults, joints & fissures, shear zones and folds are important depositional sites for different hydrothermal economic mineral deposits.

Alb-Cenomanian granitoid magmatism and copper ore genesis of the Sikhote-Alin

Late Albian-early Cenomanin epoch of Au-Cu porphyry mineralization has been distinguished within the Sikhote-Alin. It is associated with the Alb-Cenomanian granitic rocks which emplacement coincided with the processes of orogeny and neoformation of continental lithosphere caused by compressive stress in the setting of transform continental margins of that time. The intrusion of the granitic magma into the crust of Jurassic accretionary wedge terranes and Early Cretaceous terrane of epicontinental turbidite basin provoked development of Au-Mo-Cu and Cu-Au-W ore genesis, respectively. U-Pb dating of zircons from host granites of the Malmuzh Au-Cu deposit yielded Alb-Cenomania age of 100-95 Ma. This age harmonizes with the age data reported by other researchers on the granitic rocks of East and Southeast Asia which are productive for hydrothermal mineral deposits of copper, gold, tin and other metals. Such age consistency suggests that there is Albian-Cenomanian metallogenic megabelt extending throughout the entire East Asian continental margin.