Hydrothermal Chlorite Research Articles

Hydrothermal alteration and its geochemistry of the Xiadian gold deposit, Jiaodong Peninsula, China: Implications for fluid-rock interaction processes and mineral exploration

The interaction between hydrothermal fluids and host rocks, along with associated alteration geochemistry within the Mesozoic Jiaodong gold system, presents significant gaps in our understanding. This study systematically examines the petrographic, mineralogical, and geochemical characteristics of alteration at the Xiadian gold deposit, including comprehensive whole-rock geochemistry analyses, mass balance calculations, and phase equilibria modeling. The alteration history of Xiadian could be divided into four stages. Amid Stage 1, the predominant process involves the replacement of plagioclase by K-feldspar-albite, accompanied by the simultaneous bleaching of amphibolite and associated significant depletion of MgO (ΔCi/CiO = −21.2 %) and Fe2O3 (−27.2 %) owing to the decomposition of mafic minerals. Stage 2a involves sericitization of albite and continued mafic mineral decomposition, resulting in the lowest recorded MgO (avg. 0.23 wt%) and Fe2O3 (avg. 0.92 wt%) contents in all alteration lithologies. Stage 2b is characterized by abundant chlorite vein formation, corresponding to the proximal enrichment of MgO (158.1 %) and Fe2O3 (108.2 %), accompanied by aMg2+/(aH+)2 decreased in the fluid. Stage 3, the main stage of gold mineralization, involves chlorite substitution with sericite during quartz-sericite-pyrite (QSP) alteration, suggesting mild acidification of the ore-forming fluid. Insights from phase equilibrium modeling suggest that sulfidation has contributed to gold precipitation, leading to increased fO2 and reduced fH2S (extending into quartz-carbonate alteration of stage 4). Mass balance calculations indicate gold is enriched compared to protolith at all stages, with the most intense enrichment in stage 3. This suggests that hydrothermal fluids at Xiadian were auriferous throughout their evolution. The process may induce fluid modifications, particularly through the dissolution of mafic minerals, more so in mafic mineral-rich rocks (e.g., amphibolite) than in granite, promoting the incorporation of hydrothermal chlorite into the Au system to contribute to sulfidation. Our results provide insights into lithology controls on gold mineralization at Xiadian, indicating that felsic rocks (Linglong granite and quartz porphyry) with lower initial CaO contents often develop K-metasomatism and Au-bearing QSP alteration, while mafic rocks (amphibolite and lamprophyre) with higher CaO contents develop carbonization. This is likely due to the influence of lower CaO contents on alteration minerals, which promote rock porosity, affecting rock deformation, pore-fluid flow, and reacting with wall rocks. Principal component analysis (PCA) results indicate that elements including Au, Ag, As, Bi, W, and Cu are proxies for Au mineralization, suggesting their mobilization and deposition together during the gold mineralization process. This highlights their significance as a proxy for gold exploration.

Genesis of greenockite (CdS) and associated sulphide‐gold mineralization from Mahakoshal belt, Central Indian Tectonic Zone

The Zn‐Cd‐S critical minerals system is a significant mineralogical indicator of hydrothermal ore‐forming processes. The understanding of greenockite (CdS) formation in natural mineral systems is very restricted due to low crustal abundances (Cd <0.2 ppm). We report the first occurrence of greenockite from the Bagada orogenic gold prospect, Paleoproterozoic Mahakoshal belt, Central Indian Tectonic Zone (CITZ). This provides an ideal opportunity to access the occurrence and its genesis. Greenockite occurs in discrete sheared quartz veins hosted in phyllites. Sulphide mineralization is closely associated with hydrothermal chlorite alterations. Pyrrhotite, arsenopyrite, pyrite, and chalcopyrite assemblages are formed at high temperatures, while sphalerite and galena precipitated when the temperature decreased. Greenockite, native silver and bismuth with gold precipitated during the breakdown of arsenopyrite to scorodite in progressively oxidizing conditions. Greenockite precipitation occurred in a transforming hydrothermal environment from a reducing to an oxidizing one during late‐stage hydrothermal alteration. Sphalerite and chlorite thermometry yielded temperatures in the range from 266 to 366°C for this late‐stage hydrothermal alteration. Textural relations show two modes of occurrence of greenockite, one (Gck‐1) as exsolution within sphalerite (Sp‐2) and the other (Gck‐2) occurring as a discontinuous rim replacing the Cd‐rich sphalerite (Sp‐3). The Gck‐1 contains high Zn and low Cd, while as compared to Gck‐2. Zinc and Cd in greenockite show diadochy relation (Zn2+ ↔ Cd2+). Laser ablation ICPMS analyses of sphalerite (Sp‐2) reveal remarkably high Cd contents (1.9–12 wt%). Cd >0.02 ppm suggests possible Cd derivation from sphalerite (Sp‐2) under variable fS2, T, pH, fO2 conditions, fluid–rock interaction, and change in the Zn/Cd ratio. Sphalerite compositions display a low Zn/Cd (av.22) ratio and high temperature of formation, with characteristics intermediate between volcanogenic massive sulphides (VMS) and magmatic‐hydrothermal Cu‐Pb‐Zn system.

Geochemical Characteristics of Chlorite in Xiangshan Uranium Ore Field, South China and Its Exploration Implication

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.

Genesis of the late‐stage base metal sulfide mineralisation and its relationship with the regional deformation in Ambaji–Deri, South Delhi Terrane, North‐West India

Proterozoic stratiform VMS‐type Ambaji–Deri Zn–Pb–Cu deposits in the Aravalli‐Delhi mobile belt of North‐West India are hosted within meta‐sedimentary sequences of the South Delhi Terrane. Several fracture‐filled discordant late‐stage base metal mineralised quartz veins have intruded along with the fractures and foliation at very high Pf (σ2 < Pf < σ1) with typical hydrothermal alteration zones. Temperatures estimated from hydrothermal chlorite vary from 226 to 293°C (mean. 260°C). 40Ar‐39Ar dating of hydrothermal muscovite yielded a plateau age of 625.5 ± 3.3 Ma. Detailed fluid inclusion (FI) analysis reveals three types of primary and four types of secondary FI. Primary H2O‐NaCl‐CO2 ± N2 type 1a FI has medium to high salinity (11.21–21.96 wt% NaCl equivalent) and high density (0.90–1.04 g/cm3), while the secondary type 1b FI with same compositions are characterized by medium salinity (~12.6 wt% NaCl equivalent) and lower density (0.89–0.90 g/cm3). Phase separation/fluid immiscibility is the primary mechanism of mineralisation in the Ambaji–Deri that caused the original fluid to evolve separately as gaseous (CO2) type 2a, b and aqueous type 3a, b (H2O‐NaCl) with similar salinity and density for the aqueous inclusions and low density for the gaseous inclusions (lowering up to 0.58 g/cm3). The ore‐forming fluid with magmatic signatures possibly originated from the cooling G3 granites, remobilized the pre‐existing metals, and precipitated at ~265°C and ~1 kbar at the epizonal depths in the dilated fractures. Later these fluids are mixed with meteoric water and evolved into low saline aqueous fluids that produced supergene alterations.

MAPPING THE MINERAL ZONATION AT THE ERNEST HENRY IRON OXIDE COPPER-GOLD DEPOSIT: VECTORING TO Cu-Au MINERALIZATION USING MODAL MINERALOGY

Abstract Iron oxide copper-gold (IOCG) deposits form in spatial and genetic relation to hydrothermal iron oxide-alkali-calcic-hydrolytic alteration and thus show a mappable zonation of mineral assemblages toward the orebody. The mineral zonation of a breccia matrix-hosted orebody is efficiently mapped by regularly spaced samples analyzed by the scanning electron microscopy-integrated mineral analyzer technique. The method results in quantitative estimates of the mineralogy and allows the reliable recognition of characteristic alteration as well as mineralization-related mineral assemblages from detailed mineral maps. The Ernest Henry deposit is located in the Cloncurry district of Queensland and is one of Australia’s significant IOCG deposits. It is known for its association of K-feldspar altered clasts with iron oxides and chalcopyrite in the breccia matrix. Our mineral mapping approach shows that the hydrothermal alteration resulted in a characteristic zonation of minerals radiating outward from the pipe-shaped orebody. The mineral zonation is the result of a sequence of sodic alteration followed by potassic alteration, brecciation, and, finally, by hydrolytic (acid) alteration. The hydrolytic alteration primarily affected the breccia matrix and was related to economic mineralization. Alteration halos of individual minerals such as pyrite and apatite extend dozens to hundreds of meters beyond the limits of the orebody into the host rocks. Likewise, the Fe-Mg ratio in hydrothermal chlorites changes systematically with respect to their distance from the orebody. Geochemical data obtained from portable X-ray fluorescence (p-XRF) and petrophysical data acquired from a magnetic susceptibility meter and a gamma-ray spectrometer support the mineralogical data and help to accurately identify mineral halos in rocks surrounding the ore zone. Specifically, the combination of mineralogical data with multielement data such as P, Mn, As, P, and U obtained from p-XRF and positive U anomalies from radiometric measurements has potential to direct an exploration program toward higher Cu-Au grades.

Different hydrothermal fluids inducing alteration and uranium mineralisation in the Baquan deposit of the Xiangshan uranium ore field: Constraints from geochemistry of altered rocks and ores

The Baquan uranium deposit is one of the hydrothermal uranium deposits in the Xiangshan volcanic uranium ore field and is hosted in subvolcanic granitic porphyry rocks (GPRs). To constrain sources of the hydrothermal fluids and metals for alteration and mineralisation, respectively, we analysed the chemical compositions of chlorites and pyrites formed during alteration and mineralisation processes, in-situ sulfur isotope compositions of pyrites, whole-rock compositions and sulfur and molybdenum isotopes of the least-altered and altered GPRs, and mineralised GPRs (i.e. uranium ores). The results show distinct differences in mineralogical and geochemical features of the hydrothermally altered GPRs and uranium ores, which are as follows: (1) In the altered GPRs, hydrothermal chlorites (chlorite-I) are mainly daphnite with higher Fe/(Fe + Mg) (0.70–0.75) but lower Fe3+/Fe2+ (∼0.014), and hydrothermal pyrites (pyrite-I) have low Co and Ni contents (<0.05% and <0.04%, respectively) and a narrow range of δ34S (9.11–9.87‰). In contrast, hydrothermal chlorites (chlorite-II) in the ores are ripidolite and brunsvigite with lower Fe/(Fe + Mg) (0.43–0.62) but higher Fe3+/Fe2+ (∼0.056), and hydrothermal pyrites (pyrite-II) have high Co and Ni contents (0.02–0.3% and 0.01–0.11%, respectively) and a wide range of δ34S values (2.32–13.8‰). (2) The altered GPRs have Zr and Hf contents (135–187 ppm and 3.39–4.34 ppm, respectively) similar to those of the least-altered GPRs (162–169 ppm, and 3.79–4.01 ppm, respectively), whereas the ores have elevated Zr and Hf contents (184–548 ppm and 4.17–11.6 ppm, respectively). (3) The δ98Mo values of the altered GPRs (0.39–0.99‰) and uranium ores (0.52–1.74‰) show contrasting correlations with K2O or Na2O contents. These differences suggest that hydrothermal alteration and mineralisation in the Baquan deposit may have been caused by two fluids of different origins. Based on these characteristics and comparisons with possible sources, we suggest that the fluid responsible for the alteration of GPRs most likely originated from the subducting slab, whereas the mineralising fluid forming the Baquan uranium deposit likely originated from the brine of a nearby Cretaceous basin. The uranium in the deposit mainly originated through leaching of the volcanic rocks by the brine-derived hydrothermal fluid.

Mineralization and hydrothermal alteration in the Mamão orogenic gold deposit, Andorinhas greenstone belt, Carajás Province, Brazil

The Mamão gold deposit, located in the eastern portion of the Amazonian Craton, northern Brazil, is hosted by mafic rocks of the Mesoarchean Andorinhas greenstone belt. An underground mine was operated in this area by Troy Resources Ltd., which produced approximately 350 koz of gold between 2008 and 2015. The hydrothermal alteration halos are characterized by a distal zone (10- to 80 m wide) with a chlorite-albite-epidote-actinolite assemblage, an intermediate zone (5- to 20 m wide) marked by calcite-albite-chlorite-actinolite-quartz, a proximal zone (0.5–5 m wide) composed of biotite-quartz-calcite-pyrite-(gold) and a quartz vein-hosted orebody zone (0.5- to 6 m wide) composed by quartz-calcite-pyrite-(gold). Geochemical data indicate that the metabasalt, metandesite and metadacite host rocks display a tholeiitic N-MORB signature, suggesting that they were formed in ancient oceanic spreading centers. Hydrothermal chlorite of the Mamão deposit is classified as ripidolite and brunsvigite. Biotite is the main mica and increases in modal content from the distal alteration to the vein zone, displaying compositions within the annite domain. Muscovite occurs as very fine lamellae and is generally located in the intermediate zone of the altered region. Amphiboles are calcium-rich and display compositions mostly varying from hornblende (Mg-hornblende to Fe-hornblende) in the distal zone, to actinolite in the intermediate zone. Sulfidation occurs in all hydrothermal alteration zones, with both fine- and coarse-grained pyrite as the main sulfide phase, and lesser amounts of chalcopyrite, pyrrhotite, cobaltite and sphalerite. Fine-grained pyrite contains many gold inclusions and is typical of the orebody zone preferentially in close contact with the proximal hydrothermal zone, whereas coarse-grained pyrite (without recoverable gold) occurs in the veins. From the edges to the core of the hydrothermal alteration halos, the As, Sb, Ag and Au contents in the pyrite increase, whereas Pb, Co and Mo decrease. Gold occurs as free crystals (2- to 0.1 mm) disseminated within the gangue or as inclusions in the fine-grained pyrite. Temperature estimated via chlorite geothermometer ranges from 284- to 320 °C, with pressure between 1.12 and 1.34 kbar according to biotite geobarometer calculations for the hydrothermal zones. The C–O–S isotope analyses of the calcite and pyrite from the orebody zone yielded δ13C, δ18O and δ34S values ranging from −4.23 to −3.54‰, 8.60–9.77‰ and 3.37–3.52‰ respectively, which suggests that the hydrothermal fluid is predominantly of metamorphic origin. The εNd (3.0) calculated using isotopic analyses of the metavolcanic host rocks varies between +0.11 and + 4.25. A whole-rock Sm–Nd isochron age of 2952 ± 14 Ma was obtained from the host rocks, while a post-mineralized pyrite yielded a Sm–Nd isochron age of 2446 ± 35 Ma. The deposit exhibits mineralogical and isotopic characteristics consistent with those of orogenic type containing gold-bearing, sulfide-rich quartz veins. The Mamão deposit's host rocks are among the oldest rocks described so far in the Carajás Province. The age of the mineralizing event is limited between 2.95 and 2.45 Ga (probably ca. 2.7 Ga), which is the major metallogenic event of gold-deposit formation in several granite-greenstone terrains worldwide.

Weathering products at the historic Qulaan gold prospect, Eastern Desert, Egypt: Implication on the mobility and distribution of arsenic, gold and silver

Gold mineralization of the historic Qulaan prospect is hosted in hydrothermally altered rhyolite. A variety of supergene minerals that includes arsenic, gold and silver phases were formed as a consequence of superimposed weathering on primary mineralization hosted in the hydrothermally altered rhyolite at the prospect. The arsenic-bearing phases are represented mainly by arsenic-rich ferric oxyhydroxides and Ca–Fe arsenates (arseniosiderite). Gold occurs in the form of native platelet and irregular grains that are commonly associated with ferric oxyhydroxide, while the Ag-phase, represented by iodargyrite, occurs in sericite-rich cavities. The formation of these minerals is related to oxidation of primary arsenian pyrite during the weathering process. Although pyrite oxidation results in an acidic condition, the presence of arseniosiderite as well as iodargyrite and the association between ferric oxyhydroxide and gold indicate an increase in the pH during the weathering process. Relict feldspar as well as hydrothermal chlorite and calcite in the host rocks are responsible for neutralizing the earlier acidic condition during the weathering of rhyolite.

Chemistry of rock-forming silicate and sulfide minerals in the granitoids and volcanic rocks of the Zefreh porphyry Cu–Mo deposit, central Iran: Implications for crystallization, alteration, and mineralization potential

Abstract Cu–Mo mineralization and associated hydrothermal alteration zones in the Zefreh porphyry Cu–Mo deposit are a result of the emplacement of the early Miocene granodioritic-to-granitic porphyritic intrusions into the Eocene volcanic rocks (dacites, pyroclastic rocks, and andesitic lavas) during active subduction along the Urumieh–Dokhtar Magmatic Belt (UDMB) of Iran. The compositions of rock-forming minerals, such as plagioclase, amphibole, and biotite, in the granitoids of the Zefreh magmatic suite are used to determine temperature and oxygen fugacity in the magma during crystallization and emplacement. Plagioclase is locally unaltered and ranges widely in composition with reverse, normal, and oscillatory zoning, but there is no evidence of excess aluminum. Primary calcic amphiboles in the granodiorite porphyry are mainly magnesio-hornblende with high Mg#’s. The crystallization temperature of the granodiorite is in the range of 765–867 °C (average 821 ± 28 °C for amphibole–plagioclase geothermometer) and 706–861 °C (average 808 ± 35 °C for amphibole alone). Whereas, the zircon saturation temperature for the Zefreh granodiorite is lower and ranges from 724° to 733 °C (average 729 ± 4 °C). Biotites in the least-altered granodiorite and in the potassic alteration are Mg-rich and Ti-poor, and mainly fall within the fields of phlogopite and re-equilibrated primary biotite, because of their low TiO2 contents (2.7–4.5 wt%, average 3.5 wt%). Copper and molybdenum in the Zefreh porphyry deposit appear to have been transported by Cl-rich hydrothermal fluids as chloride complexes, based on the calculated halogen fugacity ratios of the fluid in equilibrium with hydrothermal biotite that range from 5.0 to 5.4 for log (ƒH2O)/(ƒHF), 4.3 to 4.8 for log (ƒH2O)/(ƒHCl), and –2.0 to –1.4 for log (ƒHF)/(ƒHCl). The calculated fugacity ratios indicate that the biotites equilibrated with hydrothermal fluids that varied in composition, including oxygen, sulfur, and halogen fugacities. The intercept values for biotites from the Zefreh porphyry Cu–Mo deposit [IV(F) = 1.9 to 2.2 and IV(Cl) = –4.8 to –4.3] correspond to those from other well-known porphyry Cu–(Mo–Au) deposits worldwide [IV(F) = 1.5 to 2.8 and IV(Cl) = –5.0 to –3.5]. On the other hand, hydrothermal chlorite and epidote in the propylitically altered granodiorite of the Zefreh deposit are more Fe-rich and Mn-poor than those in some mineralized porphyry systems. The calculated temperatures of chlorite formation by the geothermometer of Cathelineau (1988) are all similar (300–330 °C), suggesting the potassically altered granodiorite was also affected by later, cooler hydrothermal fluids associated with the distal propylitic alteration zone. Disseminated chalcopyrite in the matrix of the potassically and propylitically altered granodiorite hosts as much as 1.3 wt% Zn and 0.1 wt% As. Based on whole-rock and mineral compositions, we conclude that the Zefreh magmatic system is similar in many regards to subduction-related ore systems, but it is unlikely to host a major porphyry Cu–Mo deposit.

Chlorite thermometry and fluid inclusion studies on vein-type Tintini copper deposit, Eastern Dharwar Craton, India: Ore genetic implications

The Archaean Dharwar Craton was intruded by mafic dyke swarms during early Paleoproterozoic and some of the E-W trending dykes show evidence of copper mineralisation. In the present study, we have conducted detailed mineralogical, fluid inclusion and C-O isotopic study of the Tintini copper deposit (TCD), which is hosted by a mafic dyke (ca. 2.21 Ga) in the Yelagatti granitoid near the Hutti-Maski greenstone belt. The main copper mineralisation is hosted in steeply dipping quartz-calcite veins that cut across N85°E trending diabase dyke. The TCD shows a typically zoned alteration pattern akin to magmatic-hydrothermal deposits, with a narrow potassic alteration zone at the centre and a peripheral wide propylitic alteration halo. Formation of hydrothermal K-feldspar and amphiboles indicate high temperature (>450 °C) of alteration during potassic alteration stage. The proximal central alteration zone was affected by later pulses of cooler fluids that caused extensive propylitisation of the early potassic alteration assemblage. Geothermometry based on mineral chemistry of hydrothermal chlorites (type-I trioctahedral) of propylitic stage show 260–300 °C temperature of formation. The main sulfide ores were precipitated during a later stage after propylitisation, at a temperature (180–200 °C), much lower than the formation temperature of the early chlorite (260–300 °C). The pressure of emplacement of ores is calculated as 1.2–2 kbar. It is inferred that copper was carried as aqueous chloride complex and that ore precipitation took place because of change in fluid pH due to fluid-rock interaction. The δ13Cfluid(VPDB) (–6.38 to –6.03‰) and δ34SH2S(VCDT) (1.0 to 3.7‰) compositions of hydrothermal fluid are consistent with a magmatic source for the hydrothermal fluid. The alteration assemblage and sulfide mineralisation of TCD show evolution and cooling of hydrothermal system from high temperature (>450 °C) potassic alteration stage, to intermediate temperature (260–300 °C) chlorite stage, to low temperature (180–200 °C) main sulfide stage. The oxygen fugacity of ambient fluid shows wide variation. TCD can be classified as intrusion related, fracture-controlled, vein-type epithermal deposit.

Fitting Iron and Magnesium into the Hydrothermal Chlorite Geothermometer

Two versions of a useful chlorite geothermometer calibrated in the Los Azufres and Salton Sea geothermal fields have been published. The first applies the linear correlation between measured tetrahedral site aluminum (AlIVM) in the chlorites and the measured in situ temperatures. In the second, a correction for Fe/(Fe+Mg) is made before applying the AlIV-T°C correlation. However, while Fe and Mg do affect AlIVM independently of temperature and should be incorporated, the published Fe/(Fe+Mg) normalization was not isothermal. For an isothermal correction, the field data are contoured for temperature in AlIVM – Fe/(Fe+Mg) space, with the resulting isotherms being linear with consistent negative slopes. Except for two Salton Sea temperatures, data from both sets are compatible, supporting the isothermal contours. For structural formulae based on 14 oxygens, normalizing along the isotherms to Fe/(Fe+Mg) = 0 yields: AlIVC = AlIVM + 0.10 [Fe/(Fe+Mg)], and T°C = 318.5 AlIVC - 68.7. Temperatures from this isothermal Fe-Mg modified chlorite geothermometer are compatible with independent temperature estimates in ore deposits and with one geothermal field. Within its limits, it provides an alternative to existing theoretical and field-based versions. Detailed petrography for composition and paragenesis is necessary before the chlorite geothermometer can be applied with confidence.

Modeling the hydrothermal impact on the Jurassic deposits of the West Siberian sedimentary basin (series with H2O and H2O2)

The results of hydrothermal modeling of the formation of epigenetic mineral associations and secondary reservoirs in various types of sedimentary rocks of the Jurassic age in Western Siberia in the H2O-H2O2 system are considered. The experiments were carried out at a temperature of 390°C, a pressure of 70 MPa and H2O2 concentrations varying from 0 to 37.5%. The duration of the experiments was 10 days. In all experiments, a pyrite mixture was added as a buffer. It was found that in experiments in which there was no H2O2, that is, the hydrothermal fluid consisted only of water, there was an intense formation of bitumen and volatile components from the organic matter present in the initial samples. The process of generation of bitumen and volatile components was accompanied by fluid fracturing of rocks and the formation of secondary porosity. Under these conditions, carbonates (calcite, siderite), plagioclases, and kaolinite turned out to be unstable. On the contrary, the formation of hydrothermal ferruginous chlorite was noted. Dissolution of unstable mineral phases by hydrothermal fluid led to the formation of additional secondary porosity. As the concentration of H2O2 and, accordingly, oxygen in the composition of the hydrothermal fluid increased, more and more complete oxidation of the organic matter present in the composition of the sediments was noted until its complete disappearance. With an increase in the oxygen concentration in the composition of the hydrothermal fluid, the appearance of first magnetite and then hematite, native sulfur, and anhydrite was observed. Under hydrothermal conditions with a maximum oxygen concentration, the formation of kaolinite and various sulfates in addition to anhydrite, represented by millosevichite, alum, as well as phases with the participation of elements that make up autoclaves, nickel and chromium sulfates, was noted. Intensive silicification of the sample surface is noted, as well as the constant presence of amorphous silica, which, along with various sulfates, is a quenching phase. These mineral phases are separated from the supersaturated hydrothermal fluid due to its rapid cooling and do not have time to form well-faceted crystals, but are present in the form of loose microporous aggregates. Along with kaolinite, the formation of hydrothermal film illite is noted. The results obtained indicate that the presence of bitumen in the Jurassic sediments is an important exploratory feature, indicating that in this place they were subjected to hydrothermal action, which means, firstly, secondary reservoirs with high filtration and capacity properties were formed, and second, there was an intensive generation of hydrocarbons and, as a consequence, the formation of hydrocarbon deposits.

Chlorite chemistry of Tongshankou porphyry-related Cu–Mo–W skarn deposit, Eastern China: Implications for hydrothermal fluid evolution and exploration vectoring to concealed orebodies

Recent years, the depletion of shallow mineral resources has drawn increasing attention to the exploration of hidden ore bodies in deep of skarn type deposits. Chemical composition of chlorite is an effective tool for tracing the hydrothermal fluid evolution, and has been increasingly used for deep-level exploration of porphyry Cu (–Au) deposits (PCDs). However, the technique was rarely applied onto (porphyry-related)-skarn deposits. The Tongshankou Cu–Mo–W deposit (0.55 Mt Cu @ 0.86%, 0.01 Mt Mo @ 0.10% and 0.12 Mt WO3 @ 0.19%) is one of the largest skarn–porphyry deposits in the Middle–Lower Yangtze River Metallogenic Belt (MLYRB) in Eastern China. In this study, new petrographic and trace element data of the hydrothermal chlorites from Tongshankou were presented to constrain the ore-fluid evolution, and to explore the vectoring potential of the concealed orebodies at Tongshankou. Four types of chlorite have been identified, i.e., G-type chlorite that replaced the original magmatic biotite or hornblende, V-type chlorite in chlorite–quartz–sulfides veins in the granodiorite porphyry (alteration zones I and II), S-type chlorite that replaced garnet in the intensive skarn alteration zone (III) and M-type chlorite–quartz–calcite–sulfides veins that cut the (dolomitic)-marble in the distal alteration zones IV and V.All the four types of chlorites are typically trioctahedral chlorite. The Fe-Mg, Tschermark and di-trioctahedral substitution mechanisms likely control the geochemical variations of the Tongshankou chlorites. Empirical thermometer indicates that the Tongshankou chlorites were crystallized at 169–322 ℃. Compared with typical metamorphic chlorites, the higher Zn and Sr contents but lower Al content in the Tongshankou chlorites can potentially distinguish the skarn-type hydrothermal chlorites from typical metamorphic ones. Compared with the chlorites in the typical PCDs, the Tongshankou G-type chlorite has higher Fe, Zn, As and Sn contents, but lower Al, Mg, Co and Cu contents, which likely reflect its protolithic and/or fluid geochemical inheritance. The Tongshankou V-, M- and S-type chlorites have high contents of mobile elements (such as Sr, Ca, Sn and B) and metals (such as Fe, Cu, As, Zn and Pb), indicating that they were directly precipitated from the hydrothermal fluids. From the granodiorite porphyry to the marble wallrocks, the spatial chlorite geochemical variations at Tongshankou indicate that the increasing Zn, Mn, Ga, Ge and Pb contents, and decreasing V, Sc and Ti contents are potential vectors toward concealed mineralized porphyries at Tongshankou. Our study highlights that the chlorite geochemistry is a potential tool for ore-fluid tracer and vectoring tool toward concealed skarn–porphyry orebodies in the MLYRB.

Geochemistry and isotopic geology of the Lagoa Seca gold deposit in the Andorinhas greenstone-belt, Carajás Province, Brazil

The Lagoa Seca is a gold deposit located in the Mesoarchean Andorinhas greenstone belt in the eastern portion of the Amazonian Craton, northern Brazil. It consists of clastic metasedimentary (metagreywacke and metasiltstone) and metavolcanic (metadacite and metaultramafic) rocks. It was operated by Troy Resources Ltd. as an open pit mine from 2008 to 2014, and it produced approximately 150 koz of gold. The hydrothermal alteration halo is directly associated with the internal zonation of the Lagoa Seca shear zone, and it includes i) a distal zone composed of chlorite-epidote-carbonate, ii) an intermediate zone with minerals from the distal alteration plus moderate silicification, iii) a proximal zone composed of biotite-magnetite (carbonate-amphibole-sulfide-gold), and iv) a gold-rich ore zone that has been affected by sulfidation and potassification. Whole-rock geochemistry and mineral chemistry studies on the hydrothermally altered host rocks show that Al2O5 and Na2O increase with silica content, while Fe2O3, MnO, MgO, CaO and the LOI decrease. The minor and trace elements are characterized by very good correlations among Zr, Nb, Ti and LREEs. The REE patterns of hydrothermal rocks show high to moderate fractionation with the La/Lu(cn) of the LREEs and HREEs towards the center of the hydrothermal conduit and are accompanied by a weak negative europium anomaly. The trace-element geochemistry is consistent with that of the subduction related calc-alkaline magmas in continental arcs. In addition, the tectonic discriminant diagrams show a calc-alkaline affinity and classify the host rocks as being derived from subduction processes. The hydrothermal chlorites are classified as brungsvite; mica is phlogopite and amphiboles range from actinolite, tremolite to Mg-hornblende. The main sulfide is pyrite, which is accompanied by minor amounts of chalcopyrite, galena, pyrrhotite and traces of millerite. The EPMA analysis of the pyrite shows that As, Co and Cu increase towards the center of the hydrothermal zone, as well as Zn, Ni and Au decrease in the opposite direction. Gold is present predominantly as inclusions in pyrite/chalcopyrite, with a high Au content. A chlorite geothermometer indicates that the temperatures ranged from 275 °C to 351 °C in the hydrothermal zones. The oxygen and carbon stable isotope data from the calcite of the ore zone return values of 8.12‰ SMOW and −6.36‰ PDB, respectively, which are compatible with a metamorphic fluid source. However, sulfur stable isotope displays 1.92‰ V-CDT, which may suggest a magmatic sulfur source. The composition of the fine-grained pyrite indicates that the hydrothermal fluid presents a deep crustal source of metamorphic origin, and it is accompanied by a mixed magmatic contribution, which was probably generated during the dominant TTG evolution of the area. The whole-rock Sm-Nd isotopic analyses produced a TDM of 2.92–3.04 Ga and an εNd (2.9) of +0.39 to +1.67 for metaultramafic rocks, a TDM of 2.78–2.92 Ga and an εNd of (2.9) +1.61 to +2.92 for metasedimentary rocks, and a TDM of 2.79–2.83 Ga and an ƐNd of (2.9) +2.83 to +3.02 for metadacite rocks. An Sm–Nd isochron age of 2867 ± 40 Ma indicates a Mesoarchean age for the host rocks.

胶东半岛夏甸金矿床热液绿泥石矿物学特征及其对流体演化的指示意义

胶东半岛夏甸金矿床热液绿泥石矿物学特征及其对流体演化的指示意义

Overprinting by episodic mineralization in the Dongyaozhuang gold deposit, Wutai Mountain, China: Constraints from geology, mineralogy, and fluid inclusions

The Archean volcanic‐sedimentary rocks (greenstones) hosting the Dongyaozhuang (DYZ) gold deposit are situated in the Wutai Mountain, north‐central China. Field geology and mineral assemblages show at least two episodes of mineralization. The first episode features deformed quartz‐pyrite veins filling in the regional foliation (MI, Paleoproterozoic) and can be divided into two stages (MIa and MIb) based on whether the mineralization is accompanied by tourmalinization. The mineral assemblages are mainly quartz + carbonate + pyrite + chalcopyrite (+ tourmaline for MIb); both are cut by deformed barren quartz veins, which represent the final stage of MI. The second episode features undeformed stockworks/veinlets with a mineral assemblage of quartz + carbonate + pyrite + chalcopyrite + arsenopyrite ± native gold (MIIa), crosscutting MI; these ore bodies are also cut through by undeformed barren carbonate and/or quartz veins (MIIb), which represent the end of metallogeny. By integrating the crustal evolution history with ore geology, we suggest that the DYZ gold deposit is a product by overprinting of episodic mineralization rather than a single metallogenic event. For further understanding and evidence in support of this speculation, fluid inclusions and hydrothermal chlorite and biotite were studied to reveal the physicochemical conditions of fluids from various stages, and C‐H‐O isotopes were used to trace the source(s) of fluids. Temperatures recorded in chlorite from various stages C1 (from the MIa), C2 (MIb), and C3 (MIIa) generally agrees with the homogenization temperatures of aqueous type fluid inclusions (FIs) of corresponding stages. Components of FIs from each stage are dominated by water with low salinities (0–12.6 wt% NaCl equiv). The Ib‐1, Ib‐2, and Ib‐s type CO2‐H2O FIs observed from the same deformed quartz vein of the MIb can be attributed to overprinting of hydrothermal fluids. The primary Ib‐1 and Ib‐2 FIs (209–404°C) were trapped at pressures of approximately 2.2–3.7 kbar, representing the formation pressure of the MI stage, whereas the secondary Ib‐s FIs (240–349°C) were trapped at pressures of approximately 0.8–1.5 kbar, which indicates overprinting induced by MII. The C‐H‐O isotope compositions of quartz and carbonates from MIa, MIb, and MIIa can be interpreted to indicate that the fluids were derived from the same source with multistage evolution or to reflect a homogeneous signature induced with overprinting mineralization. According to the geology, mineral assemblages, component of fluid inclusions, and C‐H‐O isotopes, the DYZ gold deposit is likely to be a product of overprinting of an orogenic gold deposit.

Characteristics of hydrothermal chlorite from the Niujuan Ag-Au-Pb-Zn deposit in the north margin of NCC and implications for exploration tools for ore deposits

The Niujuan Ag-Au-Pb-Zn deposit is located in the northern Hebei Province of North China, which is hosted by Permian coarse-grained granite. The orebodies are controlled by a NE striking fault. Chloritization is the most extensive alteration with increasing alteration intensity from the distal to proximal end of the orebody. Chlorite and biotite from twenty-one altered samples from different depths of four drill holes were analyzed using electron microprobe (EMPA) and laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). Empirical geothermometry shows that chlorite formed at 206–340 °C. Through comparing with compositions of unaltered biotite, two biotite converted to one chlorite (2Bt → 1Chl) is the main chloritization mechanism in weakly and moderately altered granites during the hydrothermal alteration in the Niujuan deposit. During the alteration, hydrothermal fluid mainly brought in Fe, Sr, Th, Pb and CO2 and took out Mg, Ti, Si, F, K, Li, Zn, Sc, Ga, Co from the biotite, which was accompanied by the formation of quartz, rutile, calcite and fluorite during the chloritization. Chlorite in quartz-chlorite veins was precipitated directly from the hydrothermal fluids. Veined chlorite in intensely altered granites formed by the dissolution-transport-precipitation mechanism. Under the high fluid/rock conditions, veined chlorite has nearly the same compositions as chlorite in quartz-chlorite veins. According to the compositions of chlorite from intensely altered granites and quartz-chlorite veins, we infer that the hydrothermal fluid had low HFSEs and transition elements, such as Ti, Ga, Sc and V, and low fO2. From weak to intense chloritization granites, FeO, Sr, Th and Pb contents of chlorite increase, while MgO, TiO2, Li, Zn, Sc, Ga, V, Co and Nb contents decrease. The composition variations of chlorite are controlled by the composition of biotite, the compositions of fluids and the intensity of fluid-rock interaction during alteration. This indicates that the chemical variations of chlorite could be as a tool for mineral exploration in the moderate-low temperature Ag-Au-Pb-Zn deposit. Comparison with chlorite of different genesis shows that compositions of chlorite mainly differ in Mg, Fe and AlIV, which are mainly controlled by the composition of host rocks and the formation temperature.

Cathodoluminescence, fluid inclusions, and trace element data for the syntaxial quartz cementation in the sandstones of the Ora Formation, northern Iraq

Quartz cements of the quartz arenitic sandstones from the Chalky Nasara and Ora sections of the (Devonian-Carboniferous) Ora Formation in northern Iraq have been studied. A combination of hot cathodoluminescence, LA-ICP-MS, and fluid inclusion microthermometry revealed three syntaxial quartz cement generations (Q1, Q2, and Q3). The early Q1 cementation has gray to slightly brown luminescences, postdated compaction, and reduced intergranular porosity associated with illite formed during eogenesis. Q2 is characterized by dark brown luminescence overgrowths and is more voluminous in the thinly bedded sandstones than in the thickly bedded sandstones filling most of the remaining pore space during mesogenesis. Q3 was formed during the early telogenesis stage fully cementing the sandstones and the fractures were filled by hydrothermal chlorite and sulfides. Significant amounts of trace elements Al, Li, Ge, and Fe have been detected in quartz overgrowths. Al varies consistently between each cement with averages of 7125, 4044, and 2036 ppm for the Q1, Q2, and Q3 generations, respectively. A strong linear correlation between Al and Li in the three quartz cements with an average Li/Al of ~0.02 in Q1 and Q2 indicates sufficient availability of both Al and Li where Li is most likely to be found in high-saline pore waters. Illite is the most probable origin of Li since high salinities favor the mobilization of Li during diagenesis. Germanium concentrations in quartz cements are slightly less than that in the detrital quartz of the Ora Formation, indicating that the pressure dissolutions of quartz and feldspar are the dominant sources of cementation in the Ora Formation. Homogenization temperatures of fluid inclusions indicate precipitation of the Q1, Q2, and Q3 cement generations at temperature ranges of 155-160 °C, 160-166 °C, and 168-178 °C, respectively, with salinities ranging between 5.0 and 6.4 wt.% NaCl equiv., as an indication of hydrothermal burial conditions for Q3 cement, which was affected by the major Zagros Thrust Zone faulting.

The origin of hydrothermal chlorite- and anhydrite-rich sediments in the middle Okinawa Trough, East China Sea

During the Integrated Ocean Drilling Program (IODP) Expedition 331, five sites were drilled into the Iheya North Knoll hydrothermal system in the Okinawa Trough (OT) — a back-arc basin characterized by thick terrigenous sediment. Following up on the previous study by Shao et al. (2015), we present new mineralogical, geochemical, and Sr-Nd isotope data to investigate the origin of the hydrothermal sediments and characterize the hydrothermal system. The substrate at the Iheya North Knoll is dominated by pumiceous sediment and other volcanoclastic materials interbedded with hemipelagic (terrigenous and biogenous) sediments. Impermeable layers separate the hydrothermal sediments into distinct units with depth that are characterized by various assemblages of alteration materials, including polymetallic sulfides, sulfates, chlorite- and kaolinite-rich sediments. The rare earth elements (REEs) and Nd isotope data suggest that the chlorite-rich and kaolinite-rich layers primarily resulted from the alteration of pumiceous materials in different chemical and physical conditions. Kaolinite-rich sediment likely reflects low pH and low Mg concentration fluids, while chlorite-rich sediment formed from fluids with high pH and increased Mg contents, likely at higher temperatures. The Sr isotopic compositions of subsurface anhydrite reflect high seawater/hydrothermal fluid ratios in the mid-OT hydrothermal area. Compared with chlorite-rich sediments from other sediment-covered or felsic-hosted hydrothermal systems, the chlorite-rich sediments in the mid-OT are characterized by lower concentrations of Al and Fe but much higher Y, Zr, Hf, Th and REEs, indicative of the distinct nature of the precursor rocks in this region.

A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure, Finland

AbstractField investigations in the eroded central uplift of the ≤30 km Keurusselkä impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone‐bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding ~8–10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. 40Ar/39Ar dating of dark and clast‐poor whole‐rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 ± 10 Ma [± 11 Ma] (2σ; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new 40Ar/39Ar age is incompatible with ~1.88 Ga Svecofennian tectonism and magmatism in south‐central Finland and probably reflects the Keurusselkä impact, followed by impact‐induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic 40Ar/39Ar age of ~1150 Ma should be treated as a minimum age for the impact. The new 40Ar/39Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselkä, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.