Search Results (24)

The Yishu Fault Zone (YSFZ) is located in a key tectonic transition zone shaped by the interaction between the Pacific and Tethyan tectonic domains in eastern China. Despite sparse geothermal borehole coverage across this region, the deeply incised fault structures create favorable hydrogeological prerequisites for fault-mediated subsurface heat migration and hydrothermal fluid circulation. This study integrates hydrochemistry, multi-isotope tracing (δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and δ³⁴S), multi-mineral equilibrium modeling, and silica–enthalpy mixing analysis to constrain the evolution process and genetic mechanism of geothermal groundwater in the middle segment of the YSFZ. The geothermal groundwater displays weakly alkaline to alkaline properties and in situ temperatures of 35.2~75.0 °C, which is characterized by the HCO₃–Na, SO₄–Na, and Cl·SO₄–Na type. Stable isotope signatures demonstrate that the geothermal groundwater is recharged by the atmospheric precipitation with elevations of 822~1274 m. The hydrochemical evolution of the geothermal waters is governed by silicate weathering, evaporite dissolution, and cation exchange. The ⁸⁷Sr/⁸⁶Sr ratios indicate mixed solute contributions from silicate and evaporite lithologies, whereas the δ³⁴S signatures suggest that SO₄²⁻ is predominantly derived from gypsum dissolution. Two distinct hydrochemical evolution patterns can be identified in the study area. Samples GG1 and GG5 are characterized by HCO₃⁻ enrichment, whereas GG2, GG3, and GG4 exhibit enrichment in Na⁺ and SO₄²⁻. Reservoir temperatures estimated using multi-mineral equilibrium geothermometry range from 56.7 °C to 92.1 °C, with circulation depths of 1648~2304 m and cold-water mixing ratios of 48%~69%. The results of this study provide geochemical evidence for hidden geothermal resource exploration in deep fault zones. Full article

The Dajing giant Cu–Sn polymetallic deposit is located in the Cu–Sn–Ag–Pb–Zn polymetallic belt of the southern Great Xing’an Range, NE China. Research on its ore genesis is of great significance for understanding Sn polymetallic mineralization in this region. In this study, pyrite, arsenopyrite, and sphalerite were analyzed by electron-microprobe analysis (EMPA) and in situ S–Pb isotope analysis. Previously published fluid-inclusion microthermometric and H–O isotope data were also incorporated to constrain fluid evolution and ore genesis. Both in situ S and Pb isotopic compositions fall within short ranges. The δ³⁴S values suggest a sulfur reservoir with possible magmatic contribution, whereas Pb isotopes indicate a mainly crustal Pb signature in an orogenic setting. Arsenopyrite records variations in As, S, Fe, and Co contents from core to rim. The Co-rich core shows Co enrichment accompanied by Fe depletion, consistent with Co-for-Fe isomorphous substitution. These features indicate changes in local fluid chemistry during arsenopyrite growth. Sulfur isotope geothermometry based on coexisting late-stage pyrite–sphalerite pairs yields 118–233 °C, with an average of 159 ± 49 °C, indicating medium- to low-temperature hydrothermal activity during the late sulfide stage. The Dajing deposit is interpreted as a fault-controlled hydrothermal vein-type Cu–Sn polymetallic deposit formed in a Late Jurassic extensional setting. Ore precipitation was likely promoted by cooling during upward fluid migration away from the magmatic heat source, pressure release, meteoric-water mixing, and fluid–rock interaction with granitic rocks and Linxi Formation wall rocks. This study provides mineral-scale constraints on fluid evolution and ore genesis in the Great Xing’an metallogenic belt. Full article

The Yawan gold deposit, located in the Western Qinling Orogen, contains gold mineralisation that is predominantly controlled by approximately east-west-trending fault systems. This study integrates field geology, petrography, cathodoluminescence imaging, electron probe microanalysis of gold-bearing minerals (pyrite and arsenopyrite), and in situ laser ablation U-Pb dating of calcite to constrain the timing of mineralisation and to elucidate the mechanisms of gold enrichment. This study reveals that the deposit is significantly structurally controlled and comprises two discrete mineralisation stages: a quartz-pyrite (Py1)-arsenopyrite (Apy1)-chalcopyrite assemblage (Stage 1), and a quartz-calcite-pyrite (Py2)-arsenopyrite (Apy2)-stibnite-sphalerite-galena assemblage (Stage 2). Py1 displays distinct zonation, with rim As contents notably higher than core values, while Co and Ni contents gradually decrease from core to rim. Py2 is characterised by high As (0.00%–4.72%), low Fe/S ratios, and a porous texture, containing gold and arsenopyrite inclusions. Invisible gold occurs in lattice-bound form in both Py1 and Py2. The As-Fe-S ternary diagram of pyrite indicates that Au⁺ likely entered the crystal lattice as a solid solution. Arsenopyrite geothermometry yields a mineralisation temperature of 389 ± 44 °C, and sulfur fugacity (ƒS₂) decreased markedly from Stage 1 to Stage 2. Combined with the S and Fe characteristics of pyrite, these features support a medium-temperature metamorphic hydrothermal environment. U-Pb dating of calcite from Stage 2 yields an age of 215.6 ± 7.1 Ma. In summary, the Yawan gold deposit belongs to the orogenic gold system, with its gold precipitation and enrichment controlled by sulfidation triggered by Late Triassic tectono-fluid activity. Full article

Gallium (Ga) enrichment in sphalerite has been widely recognized; however, its enrichment mechanisms remain insufficiently understood. The South Hunan district, located at the intersection of the Nanling Region and the Qin-Hang Metallogenic Belt in South China, is characterized by abundant Jurassic magmatic-hydrothermal Pb–Zn deposits, which typically host Ga-depleted sphalerite. Recently, Ga-enriched sphalerite (up to 385 ppm by LA-ICP-MS) has been identified in newly drilled cores at Zhaojinci, adding complexity to the regional Pb–Zn metallogenic framework. EPMA elemental mapping and LA-ICP-MS time-resolved spectra indicate that Ga is homogeneously distributed within sphalerite, excluding the presence of micron-scale Ga-bearing mineral inclusions. A strong positive correlation between Ga and Cu concentrations suggests that Ga incorporation is facilitated by the coupled substitution of Zn²⁺ by Cu⁺. Sphalerite geothermometry yields formation temperatures of 118–138 °C (average 126 °C for GGIMF is and ~129 °C for SPRFT), accompanied by intermediate sulfur fugacity conditions (lg fS₂ = −22.9 to −21.2), which appear to favor Ga enrichment in sphalerite. The trace element geochemistry of the Zhaojinci sphalerite (Ga-Ge-Cd-enriched and Mn-In-Sn-Co-depleted), combined with its formation under low-temperature (120–180 °C) and intermediate fS₂ conditions (within the pyrite stability field), is consistent with MVT-like mineralization. This interpretation is supported by multiple lines of geological evidence, including the strict confinement of stratabound Pb–Zn mineralization to the Devonian Xikuangshan Formation limestone, structural control by syn-sedimentary normal faults, pervasive dolomitization of the host rocks, and the absence of genetic relationship to magmatic activity. Moreover, the sphalerite geochemical signature, corroborated by an XGBoost-based machine learning classifier, reinforce the MVT-like affinity for the Zhaojinci mineralization. This study not only emphasizes the importance of low-temperature and intermediate-fS₂ conditions in Ga enrichment within sphalerite, but also highlights the significance of discovering MVT-like sphalerite for Pb–Zn resource exploration in the South Hunan district, providing valuable new insights and directions for mineral prospecting in this geologically important region of South China. Full article

A genetic linkage between U–Cu–Mo mineralization with feldspar and chlorite minerals and the discrimination of different mineralization events in the hydrothermal and metasomatic system in the Rohil polymetallic uranium deposit in India is presented on the basis of textural relationships and mineral chemistry. Field and EPMA studies reveal that the chlorite formed in two possible modes, viz. (a) replacement of ferromagnesian minerals of the host rock and (b) precipitated directly from hydrothermal solutions. Chlorites follow a distinctive composition from Al-saturated to Al-undersaturated and, in most cases, from Mg- to Fe-rich species as alteration progressed. The chlorites show a wide range of Fe content (1.86–5.06 apfu), high Mg content (3.96–6.28 apfu), and Si contents (5.99–6.90 apfu) with an Fe/(Fe + Mg) ratio (0.23–0.56), leading to their classification as Diabantite/Pycnochlorite. Empirical and thermodynamic geothermometers have been used to determine the temperature of chlorite formation based on chemical composition, which revealed a large variation in temperatures from 130 °C to 260 °C. The feldspar geothermometry reveals a temperature range of 158 to 236 °C, which is in congruence with that of chlorites. Geothermometry by two different methods provides the range of temperature that prevailed in the study area during and succeeding the crystallization of uraninite and associated ore minerals. Mineral chemistry vis-à-vis geothermometry of feldspars and chlorite can provide impetus to geochemical evolution in the North Delhi Fold Belt (NDFB) and similar geological setups in metasomatite-type uranium deposits. Full article

The Kraaipan and Amalia greenstone belts in South Africa occur in the western part of the Kaapvaal Craton. The two belts stretch discontinuously in an approximately north–south orientation over a distance of about 250 km from southern Botswana in the north to the Vaal River near Christiana in the south and are separated by a distance of about 90 km. Gold mineralization is hosted in banded iron formation at both the Kalahari Goldridge deposit (Kalgold) in the Kraaipan greenstone belt in the north and the Amalia deposit in the Amalia greenstone belt in the south, with the mineralization associated with quartz–carbonate veins. The footwalls of these deposits are generally composed of mafic volcanic schist and the hanging walls consisting of graywackes, schist and shale units. The Kalgold and Amalia gold deposits show some variation in the redox condition of the mineralizing system and fluid chemistry. The ore mineral assemblage is characterized by magnetite–pyrrhotite–pyrite at Kalgold, which is indicative of reducing conditions, and a magnetite–hematite–pyrite assemblage at Amalia that suggests a relatively oxidizing environment. Average mineralizing temperatures determined from chlorite geothermometry were relatively higher at the Kalahari Goldridge deposit ranging from 350 to 400 °C compared to the slightly cooler range of 330 to 390 °C at Amalia. The composition of the fluids derived from fluid inclusions is indicative of low salinity H₂O--CO₂±CH₄-rich fluids at Kalgold against relatively H₂O-CO₂-rich fluids at Amalia. Evidence from strontium–carbon–oxygen isotopic ratios from carbonates suggests that differences in redox conditions in the deposits could be attributed to different flow pathways by an evolving fluid from a common source (with minimum ⁸⁷Sr/⁸⁶Sr = 0.70354) to the sites of gold deposition, with a significant ore fluid interaction with a thick sequence of carbonaceous meta-pelitic rock units at the Kalahari Goldridge deposit that is absent in the Amalia deposit. Full article

The concealed Molai Zn-Pb±(Ag,Ge) stratiform deposit in southeastern Peloponnese is hosted in Triassic intermediate tuffs, ignimbrites and subaerial andesitic flows. The host rocks display trace element signatures of a Supra-Subduction Zone (SSZ) setting. Three ore-forming stages are recognized, with stages I and II related to formation of the epigenetic, stratiform, massive-to-semi-massive ore and a late stage III associated with vein-type mineralization. The O and D isotope geochemistry of gangue chlorite and epidote reveal mixing with fresh meteoric water during the weaning stages of the hydrothermal activity of the late stage II due to uplifting of the hydrothermal system. Sphalerite is the major ore phase, with three different varieties formed during stages I (Sp-I) and II (Sp-II and Sp-III). All sphalerite varieties coexist, depicting gradual change in the chemistry of the ore-forming fluids. Molai ores are characterized by elevated Ag and Ge contents. Tetrahedrite is the major Ag carrier, while among the three sphalerite varieties, early Sp-I comprises the highest Ge contents. The Molai Zn-Pb±(Ag,Ge) deposit is characterized by intermediate features between bimodal felsic massive sulfides and subaerial epithermal systems based on the shallow formation depth, the presence of hydraulic breccias associated with phase separation, the ore formation along high-angle faults, the relatively low ore-forming temperatures below 250 °C obtained from geothermometry, and the absence of the typical structure of bimodal felsic type ores. Full article

Whether from the near-surface or at great depths, geothermal energy aims to harness the heat of the Earth to produce energy. Herein, emphasis is put on geothermal reservoirs and their cap rock in crystalline rocks, in particular, the basements of sedimentary basins and volcanic islands in the context of subduction. This study is based on a case study of three examples from around the world. The aim of this paper is to show how the study of newly formed minerals can help the exploration of geothermal reservoirs. The key parameters to define are the temperature (maximum temperature reached formerly), fluid pathways, and the duration of geothermal events. To define these parameters, numerous methods are used, including optical and electronic microscopy, X-ray diffraction, microthermometry on fluid inclusions, chlorite geothermometry, and geochemistry analysis, including that of isotopes. The key minerals that are studied herein are phyllosilicates and, in particular, clay minerals, quartz, and carbonates. They are formed because of hydrothermal alterations in fracture networks. These minerals can have temperatures of up to 300 °C (and they can cool down to 50 °C), and sometimes, they allow for one to estimate the cooling rate (e.g., 150 °C/200 ka). The duration of a hydrothermal event (e.g., at least 63 Ma or 650 ka, depending on the site) can also be established based on phyllosilicates. Full article

The Ruwai skarn deposit is located in the Schwaner Mountain complex within the central Borneo gold belt and is currently considered the largest Zn skarn deposit in Indonesia. The deposit has been known to host Zn-Pb-Ag mineralization in the form of massive sulfide ore bodies; however, the occurrence of Ag-bearing minerals has not been identified yet. This study documents the mineralogical characteristics of several Bi-Ag sulfosalts and sulfoarsenides, as well as their chemical compositions. Ten Bi-Ag sulfosalts were identified, including native bismuth, tetrahedrite, cossalite, tsumoite, bismuthinite, joseite-B, Bi₆Te₂S, Bi-Pb-Te-S, Bi-Ag-S, and Bi-Te-Ag. Three sulfoarsenides were identified, including arsenopyrite, glaucodot, and alloclasite. The occurrence of Bi-Ag sulfosalts is typically associated with massive sulfide mineralization, although tsumoite can also be found associated with massive magnetite. In terms of sulfoarsenides, both arsenopyrite and glaucodot are associated with massive sulfide mineralization, whereas alloclasite is associated with massive magnetite mineralization. The Bi-bearing minerals are characterized by irregular, bleb-like texture or patch morphology, and occur either as free grains or inclusions within sulfides, such as galena or pyrite. Tetrahedrite typically has an anhedral shape with a rim or atoll texture surrounding sphalerite or galena. In contrast, sulfoarsenides are typically found as euhedral–subhedral grains where glaucodot typically is rimmed by arsenopyrite. Both Bi-Ag sulfosalt and sulfoarsenides were formed during the retrograde stage under high oxidation and a low sulfidation state condition. The ore-forming temperature based on arsenopyrite geothermometry ranges from 428 °C to 493 °C. Full article

Estimating deep geothermal reservoir temperatures is an essential mission of geothermal exploration and development. The thermal reservoir temperature estimated directly using geothermometry without comparative analysis is often far from the actual temperature. In this paper, taking the typical geothermal systems in the Xiamen Island–Zhangzhou area of southeastern China as an example, different methods such as a water–rock equilibrium analysis, SiO₂ geothermometer, multi-mineral equilibrium diagram, and silica-enthalpy mixing model are used to make a quantitative and qualitative analysis of the chemical equilibrium of minerals and fluids in the geothermal system. Finally, the applicability of different methods was compared and analyzed, and the geothermal reservoir temperature was estimated using the appropriate method. The results show that the calculated results of the Si-enthalpy mixing model of a typical geothermal system in southeastern China are significantly high. At the same time, the SiO₂ geothermometer (without vapor loss), which is closest to the results of the multi-mineral equilibrium diagram, was chosen as the geothermal reservoir temperature of the geothermal system in the study area. This study can provide a reference for the future selection of methods of deep geothermal reservoir temperature estimation in similar areas. Full article

The mineral chemistry of illite/mica and chlorites, together with the evaluation of textural data of low-temperature metaclastic rocks, plays an important role in determining their origin and metamorphic grade. This study aimed to investigate the chemical properties of phyllosilicates in early Paleozoic metaclastic rocks in the Eastern Tauride Belt, Türkiye. The textural (electron microscopy) and chemical (mineral chemistry analysis) analyses were performed on the samples representing different grades of metamorphism. The illites/micas and chlorites are observed as detrital (chlorite–mica stacks) and neoformation origin. Trioctahedral chlorites (chamosite) exhibit different chemistry for detrital and neoformed origin as well as the metamorphic grade. Tetrahedral Al and octahedral Fe + Mg increase, whereas octahedral Al decreases together with the increasing grade of metamorphism. The detrital chlorites have higher tetrahedral Al and Fe contents than their neoformed counterparts. Chlorite geothermometry data (detrital: 241–≥350 °C; neoformed: 201–268 °C) are compatible with the texture and illite Kübler index data. Illite/white-mica compositions display muscovite and Na-K mica. Tetrahedral Al and interlayer K + Na contents of illites/micas increase with metamorphic grade. Na-K mica and paragonite are observed as replacement-type developments within the detrital CMS. The obtained data indicate that phyllosilicate chemistry can be used effectively for determining the geological evolution of low-grade metamorphic sequences. Full article

Sulfide and fluoride mineralization in the NNE Achemmach (NNE-A) area is located in the NE of Central Hercynian Morocco. In veins or when disseminated, it is hosted either in Visean sedimentary formations or in the magmatic bodies, described for the first time in this article and corresponding to pillow-lavas, dolerites and olivine-bearing gabbros. The mineralization is multiphase and results from the succession of the following three events: (i) an early high-temperature hydrothermal event (T ≈ 350–420 °C) associated with a simple primary sulfide paragenesis composed of pyrrhotite, pyrite, chalcopyrite, sphalerite and galena with gangue of quartz.(ii) The second event corresponds witha low temperature fluorite hydrothermal one (T≈ 120–160 °C), whereas the (iii) third is marked by, the deposition of a late sulfide paragenesis in a carbonate gangue within a moderate temperature (T≈ 200–250 °C). The temperatures of the paragenetic stages (350–400 °C) are estimated on the basis of the geothermometry constraints of the mineralogical assemblages, particularly the pyrrhotite-sphalerite equilibrium, in which the FeO content varies from 9.23 to 14.42 Wt%, and in the full study of their corresponding fluid phases. They are in perfect agreement with the fluid inclusion data of the first event. Full article

Chlorite is one of the most important hydrothermal minerals in many hydrothermal uranium deposits worldwide and is commonly closely associated with the uranium mineralization. Trace elements in chlorite have been extensively applied to fingerprinting the hydrothermal fluid evolution and indicating the concealed ore bodies in porphyry Cu (-Au) deposits and skarn-related Pb-Zn deposits. However, this approach was rarely attempted on hydrothermal uranium deposits to date. Xiangshan uranium ore field, located in the southeast part of Gan-Hang Metallogenic (or Volcanic) Belt (GHMB), is the largest volcanic-related ore field in the whole country. In this study, the focus was placed on the petrographic characteristics and trace elements in hydrothermal chlorite from two typical deposits (Zoujiashan and Yunji) at Xiangshan. Four types of chlorites were identified, i.e., Chl1-Y and Chl2 from Yunji deposit, and Chl1-Z and Chl3 from Zoujiashan deposit. The pre-ore Chl1-Y and Chl1-Z are formed through replacing the original magmatic biotite. Chl2 and Chl3 occur as veinlets or disseminated, and are closely associated with early-ore U mineralization and main-ore U mineralization, respectively. All the four types of chlorites are typically trioctahedral chlorite. Vein-type/disseminated Chl2 and Chl3 in ore veins were precipitated directly from the hydrothermal fluids through dissolution-migration-precipitation mechanism, whereas the replacement-type chlorite was formed by the dissolution–crystallization mechanism. Empirical geothermometry indicates that the chlorite from Yunji and Zoujiashan were crystallized at 179~277 °C, indicating a mesothermal-epithermal precipitation environment. EPMA and LA-ICP-MS results show that the replacement-type chlorite has relatively consistent compositions at Yunji and Zoujiashan. Both Chl2 and Chl3 are enriched in U, Th but depleted in Mn and Ti. Compared with the Chl2 related to early-ore U mineralization, Chl3 that formed at main-ore stage has higher concentrations of Fe, U, Th, REEs, Mn and Ti, as well as higer Fe/(Fe + Mg) ratios. Such compositional differences between Chl2 and Chl3 are mainly attributed to the formation temperatures and fluid compositions/natures. Combined with petrology and chemical compositions of different types of chlorite, we propose that the presence of vein-type/disseminated chlorite with high U and Fe/(Fe + Mg) ratio but relatively low Mn, Ti and Pb contents can be regarded as an effective vector toward the most economic (high U grade) mineralized zone, whereas the occurrence of Chl2 is likely to indicate the subeconomic U mineralization and less potential exploration for uranium at depth. Full article

In this paper, we present a new point of view on the deposit geology and genesis of the largest gold deposit in Eastern Sayan, Zun-Kholba. Quartz–sulfide replacement ores and shallow quartz veins are of great economic importance. At the deposit, quartz–pyrite ore bodies are dominant, whereas paragenetically late base metal-bearing quartz veins occur only at shallow levels. The study of the fluid inclusions, stable isotopes (C–O–S), and mineral geothermometry allowed us to determine the P–T conditions of ore deposition. It is established that the temperature of ore formation changed from 380 to 433 °C at the deepest levels of the deposit to 316 °C at the shallowest levels. The pressure estimates for gold deposition at 570–950 bar were calculated for the 1490-m level in the center of the deposit. The determined P–T conditions can help estimate the average depth of deposit formation at about 3.6 km. Ore mineral assemblages were formed from homogeneous fluids of low to moderate salinity (2.4–7.9 wt.% eq. NaCl). The sulfur isotope composition of sulfide minerals (δ³⁴S = 0–4.6‰), as well as pyrite geochemistry, corresponds to that of older volcanogenic massive sulfide (VMS) occurrences, which are widespread within the Eastern Sayan ophiolitic belts. Lens-like fragments of metamorphosed VMS-type sulfide ores are also present in the Zun-Kholba deposit. The oxygen isotope data indicate a metamorphic origin for the ore-forming fluids. Migrating metamorphic fluids may have become enriched in gold, sulfur, and other metals during the flow through the complex ore-hosting lithologies, including ophiolitic bodies with sulfide-rich VMS fragments that are characterized by a relatively high content of gold and related ore elements. The obtained data allow us to suggest a metamorphic devolatilization model for the genesis of the Zun-Kholba gold deposit. Full article

Mg-Fe²⁺ diffusion patterns in olivine and chromite are useful tools for the study of the thermal history of ultramafic massifs. In the present contribution, we applied the exponential modeling of diffusion patterns to geothermometry and geospeedometry of chromitite ores from two different ophiolite contexts. The Iballe ophiolite (Northern Albania) hosts several chromitite pods within dunites. Primary and re-equilibrated Mg#, estimated by using an exponential function, provided re-equilibration and primary temperatures ranging between 677 and 996 °C for chromitites and between 527 and 806 °C for dunites. Cooling rates for chromitites are higher than for dunites, suggesting a different genesis for the two lithologies, confirmed also by spinel mineral chemistry. Chromitites with MORB affinity formed in a SSZ setting at a proto-forearc early stage, explaining the higher cooling rates, while dunites, with boninitic affinity, were formed deeper in the mantle in a more mature subduction setting. At the Nea Roda ophiolite (Northern Greece) olivine in chromitites do not show Mg-Fe variations, and transformation into ferrian chromite produced “fake” diffusion patterns within chromite. The absence of diffusion patterns and the low estimated temperatures (550–656 °C) suggest that Nea Roda chromitites were completely re-equilibrated during an amphibolite-facies metamorphic event that obliterated all primary features. Full article