Stages Of Hydrothermal Alteration Research Articles

Visible-Near Infrared–Short-Wave Infrared Spectroscopy and Mineral Mapping of Hydrothermal Alteration Zones at the Brejuí W-Mo Skarn Deposit, Seridó Mobile Belt, Borborema Province, Brazil

Abstract The Brejuí W-Mo skarn deposit is the main scheelite deposit of the Seridó tungsten province (Borborema province, NE Brazil). It constitutes the largest Brazilian W ore reserve. The orebodies are hosted in the metasedimentary Jucurutu Formation of the Seridó Group (650–610 Ma), close to the margin of the Neoproterozoic Acari pluton (Brasiliano orogeny). Skarns include prograde and retrograde mineral assemblages. Infrared spectral analysis shows that the skarns mainly reflect retrograde mineral assemblages, formed in three different hydrothermal alteration stages with overprinting. Mineral phases spectrally detected include (1) vesuvianite, actinolite, and phlogopite (alteration stage 1), (2) epidote, prehnite, and illite (alteration stage 2), and (3) laumontite, montmorillonite, chlorite, and gypsum (alteration stage 3; main ore zone). Phlogopite chloritization and actinolite recrystallization were observed in the main W-Mo skarn orebodies. Chloritization is marked by a displacement in the Fe-OH–related absorption wavelength (2,246.5–2,250.5 nm). A spectral index using spectral mixtures of laumontite + montmorillonite + actinolite is proposed here to map mineralized skarn layers (WO3 + Mo ≥ 0.1%) in drill cores. It was used to vector to richer mineralized bodies of the Brejuí deposit successfully and may be applied to similar skarn deposits with the same aim.

Geochemistry of hydrothermal and stream sedimentary rutile in the Tiegelongnan porphyry-epithermal Cu (Au) deposit, Tibet: A tool for exploration

Rutile is a common and stable mineral in porphyry Cu deposits, and morphology and geochemistry of rutile are recognized as a potential indicator mineral for exploration. In this study, we investigate physical features and chemical composition of hydrothermal rutile in the Tiegelongnan porphyry-epithermal system, as well as rutile in stream sediments nearby to establish a method to apply rutile as an exploration proxy. Four stages of hydrothermal rutile have been identified. Potassic alteration stage rutile (Rt1) is associated with biotite, with elongated and acicular shape and small grain size (20–50 μm). Rutile from lower temperature phyllic alteration (Rt2), quartz veins (Rt3), and advanced argillic alteration (Rt4), has characteristic sector zoning and larger grain size (50–150 μm). The sector zoning is more commonly present in Rt2, Rt3, Rt4, that are associated with high-grade Cu mineralization, and is associated with incorporation of W, Fe, V, Sn, and Zr. The earlier rutile (Rt1) also has a geochemical signature different from the other three types of rutile, characterized by significantly lower Al, Cu, Zr, Nb, Sn, Hf, and higher Sc and Fe contents. Low-temperature hydrothermal alteration stage rutile (Rt2, 3, 4) share many geochemical similarities, but Rt3 has higher Nb and Ta contents (median = 3371.0 and 176.0 ppm, respectively), whereas Rt4 has higher Sb content (median = 24.3 ppm).Rutile trace element data from this study and literature are integrated to develop a Partial Least Square-Discriminant Analysis (PLS-DA) multivariate statistics plots that can discriminate porphyry Cu deposit rutile from orogenic gold and VHMS deposits, as well as rutile from different alteration stages. The PLS-DA plots predict that stream sedimentary gold-bearing rutile (RtSAu) adjacent to the Tiegelongnan deposit are derived from the deposit, and mainly from low-temperature phyllic and advanced argillic alteration stages. This is consistent with geochemical similarity between the RtSAu and the Tiegelongnan low-temperature hydrothermal rutile (Rt2 + Rt4). This is further confirmed by a U-Pb age of 125.3 ± 4.7 Ma (n = 24, 2σ) for RtSAu, which is consistent with the main porphyry mineralization age of ∼ 120 Ma in the Tiegelongnan deposit. We infer that the RtSAu may have formed during the ∼120 Ma epithermal system at Tiegelongnan, which has already largely eroded. In general, our findings prove the capability of using rutile, by combing the texture, geochemical compositions, geochronology, PLS-DA tools, and their spatial distribution, as a proxy mineral for the exploration of porphyry Cu deposits.

Alteration of chromite during serpentinization of peridotites

Trace element (Zn, Co, Mn, Ni, Ga, V, Ti, and Sc) compositions of chromite in mafic-ultramafic rocks are widely used for addressing their petrogenesis and discriminating tectonic environments, but they can be altered by subsolidus re-equilibration and hydrothermal alteration, and the detailed dynamic processes are not clear. The Jinchang ophiolite in SW China is part of the Tethyan ophiolite, and consists mainly of harzburgite with minor lherzolite. These peridotites are strongly serpentinized, and contain variably modified chromite and magnetite. Two types of chromite are identified in harzburgite, including magmatic and altered chromite. The magmatic chromite occurs as granular or amoeboid minerals, or as cores surrounded by the altered chromite, and they have homogeneous major elemental compositions (Mg0.65Fe0.35(Cr0.46Al0.54)2O4), but their trace elements, such as Ni, Ga, V, Zn, Co, and Mn, are variably changed during subsolidus re-equilibration. The altered chromite consists of ferrous chromite (Mg0.15Fe0.85(Cr0.46Al0.11Fe0.43)2O4) and chromian magnetite (Fe(Cr0.15Fe0.85)2O4), which were formed by two stages of hydrothermal alteration. They are depleted in Ga and Sc, and enriched in Zn, Co and Mn during the alteration, but their Ti, Ni and V are increased and decreased in the first and second stage, respectively. Thermodynamic modeling reveals that: (1) Reaction between magmatic chromite and olivine at the first stage of alteration produces ferrous chromite and chlorite under high temperature (610–470 °C) and increasing oxygen fugacity (ΔFMQ increasing from −1 to 3) and water-rich conditions; (2) Chromian magnetite is generated by further alteration of the ferrous chromite in the second stage alteration under low temperature (< 470 °C) and decreasing oxygen fugacity conditions (ΔFMQ decreasing from 3 to −4). In addition, the first stage alteration significantly changed trace-element compositions of the magmatic chromite compared with the second stage alteration that slightly modified the ferrous chromite. This study clarifies trace element migration of chromite in the silicate mineral-hydrothermal fluid system, indicating that chromite microstructures and trace elemental compositions can record the detailed serpentinization processes.

Mineralogy and geochemistry of hydrothermal alteration of the Mianhuakeng uranium deposit in South China: Implications for mineralization and exploration

The Mianhuakeng deposit is the largest granite-related hydrothermal-vein-type uranium deposit in South China and Jurassic biotite granite serves as its primary host rock. However, the hydrothermal alteration and its constraints on uranium mineralization in this deposit are still poorly understood. To address these issues, we present a detailed study on the petrography, whole-rock geochemistry, and mineral chemistry of altered rocks and ores. Based on the field geological characteristics, these hydrothermal alteration halos can be classified into three alteration zones (zones 1 to 3) and one mineralization zone. Alteration zone 1 is the furthest away from the ore body and consists of extensive illitization and minor chloritization, whereas alteration zone 2 has partial albitization and dequartzification. The pervasive illitization, chloritization, and hematitization are considerable in the alteration zone 3 closest to the ore body. The mineralized zone exhibits widespread silicification, hematitization, and local chloritization. The mineralogical properties of chlorite indicate that at least two stages of hydrothermal alterations occurred within the alteration halos. The first one is the pre-ore hydrothermal alteration, which can be found in alteration zone 1. A weakly alkaline, Fe-rich, and Mg-poor fluid gave rise to this stage of hydrothermal alteration. The second one, known as the syn-ore hydrothermal alteration, appears in the mineralized zone that was created by an acidic, Mg-rich, Fe-poor fluid that overprints on alteration zones 2 and 3. The formation temperatures of chlorite from these two stages of hydrothermal alterations are 350–250 °C and 250–150 °C, respectively. Alterations and uranium mineralization are inextricably linked in the Mianhuakeng deposit. Pre-ore hydrothermal alterations improve the efficiency of fluid circulation and the proportion of uranium leaching from uraninite in granite. Uraninite in granites releases removable uranium through hydrothermal alterations to serve as a major source for mineralization. Syn-ore hydrothermal alterations are responsible for the redox reaction between U6+ in ore-forming fluids and Fe2+ released from reequilibration of Fe/Mg-rich chlorite, which results in uraninite (UO2) precipitation from hydrothermal fluids. Here, we suggest that alteration zone 1 with widespread illitization and uranium loss should be considered as the first-line vector for further uranium exploration in the Mianhuakeng deposit, and alteration zone 3 with significant chloritization and hematitization should be the crucial vector.

Graphite, Sheelite and Cassiterite Mineralized Skarns of Frag Al Ma, Sidi Bou Othmane District, Jebilet, Morocco

Sidi Bou Othmane region in the Central Jebilet is formed by a Devonian schistose series. The polyphase Hercynian deformation in this area is characterised by penetrative, subvertical, synmetamorphic schistosity (S1), accompanied by folding. Syntectonic granitic intrusions, hidden in the Sidi Bou Othmane region, is marked by a contact of metamorphism, which responsible for the replacement phenomena observed essentially in the limestones of the region which are transformed into graphitic Sn-W exoskarns. Syn-tectonic, skarnification is polyphase : (i) prograde stage (T= 500- 750°C) : with graphite, garnets, idocrase, pyroxene, wollastonite, quartz, scheelite and molybdenite, crossed by a network of successive veins (garnet, idocrase, wollastonite, quartz and calcite; (ii) retrograde stage (T= 400-500°C): with garnet, idocrase, pyroxene wollastonite, quartz, calcite, epidote scheelite and cassiterite; (iii) hydrothermal alteration stage (T ≤400°C) : with sericite and clay accompanied by abundant scheelite and cassiterite, favoured by the high oxidation rate. Therefore, they would have formed in a reduced environment, in relation to large batholiths or stocks of quartz diorites and/or granites in syn, late- to post-orogenic to anorogenic contexts, in the absence of any cogenetic volcanism. The economic mineralisation of graphite was formed mainly during prograde stage, under reducing conditions. It was formed by devolatilization of a bituminous origin linked to the reef limestones and organic matter, trapped in the shales according to the following reaction:

Origin of Cretaceous alkaline annular structures in the peri-cratonic terranes of the Reguibat Rise in Morocco: New constraints from carbonatite–silicate pairs and subsolidus processes

We report an integrated field, petrographic, and geochemical study of three major Cretaceous carbonatite annular occurrences from the Oulad Dlim massif (Adrar Souttouf, NW margin of the West African Craton, south Morocco): Twihinate, Lamlaga, and Lahjayra. These complexes are commonly considered isolated carbonatite structures partially to completely overlain by silica breccias and iron oxide rocks. The carbonatites include two calcio‑carbonatites facies with distinct textural features and bulk-rock and mineral geochemistry: the “white” facies present characteristics consistent with a primary magmatic stage, while the “grey” facies likely resulted from hydrothermal alteration of the white carbonatites. We provide evidence that the annular structures include ultrapotassic alkaline syenites, probably derived from the same primary carbonated/silicated melt as the carbonatites, after partial melting of a K-rich lithospheric mantle source. The overlying silica breccias retain sandstone/quartzite-like stratifications, whereas their trace element signature is analogous to those of the carbonatites, suggesting that they were subjected to intense hydrothermal activity during carbonatite magmas emplacement. The iron-oxide rocks are chemically comparable to worldwide references of carbonatite-related laterite profiles, indicating a similar origin. All subsolidus (hydrothermal and weathering) alteration stages were accompanied by significant REE-Nb concentration resulting in high-grade deposits of monazite, synchysite, apatite and pyrochlore. Like other peri-Atlantic alkaline occurrences, the investigated Cretaceous alkaline‑carbonatite occurrences are likely related to the Atlantic Ocean opening and reactivation of pre-existing Pan-African tectonic structures.

Hydrothermal alteration and the remobilization of rare earth elements during reprecipitation of nano-scale apatite in phosphorites

Sedimentary phosphorites enriched in rare earth elements (i.e., REE) are an important reservoir of these critical metals and may have the potential to resolve the global REE supply risk in the future, which is widely distributed around world. However, the occurrence and utilizability of REE in phosphorites remain contentious. In this study, multiple-scale analysis techniques were used to investigate the mineralogical and geochemical characteristics of typical REE-enriched phosphorites from Zhijin, SW China. The mineralization was divided into two stages: early fluorapatite (Fap1) that formed during the sedimentary–diagenetic stage, and late fluorapatite (Fap2) that formed during the hydrothermal alteration stage. The in situ REE geochemical data (i.e., quantitative data and mapping analysis) and scanning electron microscopy images show that Fap1 is the main REE host phase, because it is abundant and high REE concentration. Data obtained by both laser-ablation–inductively coupled plasma–mass spectrometry and high-resolution transmission electron microscopy reveal that most REE are resident in the lattice of nano-scale fluorapatite, which could be remobilized by hydrothermal fluids during the process of dissolution of nano-scale Fap1 and reprecipitation of micro-scale Fap2. In addition, the formation of synchysite-(Y) in the altered phosphorites involved reprecipitation of the mobilized REE. We suggest that F− and SO42− in fluids play a major role in the mobilization of REE in fluorapatite and that CO32− sourced from dolomite alteration contributes to the precipitation of REE. Our results indicate that REE in fluorapatite can be remobilized and further enriched by hydrothermal alteration to REE fluorocarbonates (the current predominant economic source of REE minerals). Overall, our results reveal that REE occur mainly in the lattices of fluorapatite and synchysite-(Y) in phosphorites, and provide new insights into the mineralization mechanism and industrial availability of REE-enriched phosphorites.

Geology and metallogenesis of the Tiegelongnan Cu–Au–Ag deposit, Duolong ore district, Tibet

The Tiegelongnan deposit is a giant deposit (&gt;10 million tons Cu resources with an average grade of 0.53%) located in the west of the Bangong Co‐Nujiang metallogenic Belt, where typical porphyry and epithermal types of alteration and mineralization are developed. A compilation of geological features and mineralogical studies revealed that the Tiegelongnan deposit developed two stages of hydrothermal alteration. The early stage was related to porphyry emplacement, developing potassic, phyllic and propylitic alterations, telescoped by the late advanced argillic alteration in the shallow levels. Correspondingly, two stages of mineralization occurred, the early porphyry‐type mineralization being overlapped by the late high‐sulfidation epithermal mineralization. Due to differences in pH, log fO2, K+ activity and temperature of ore‐forming fluids, distinct sulfide minerals of low to extremely high sulfidation states, such as chalcopyrite, tennantite, enargite and covellite, precipitated throughout the entire hydrothermal process. The Tiegelongnan deposit is a complex porphyry metallogenetic system of ‘multiple structures’. The Early Cretaceous andesitic volcanic rocks of the Meiriqiecuo Formation exposed on the surface act as a cap for the ore body, beneath which an oxidizing‐leaching zone lies. Under the cap and the oxidizing‐leaching zone, a porphyry ore body is remoulded by high‐grade high‐sulfidation epithermal hydrothermal mineralization, which gradually transitions to continuous porphyry‐type mineralization at depth.

Zeolite Minerals from Wat Ocheng, Ta Ang, Ratanakiri Province, Cambodia – Occurrence, Composition, and Paragenesis

ABSTRACT Located in the southwest corner of the Ratanakiri Volcanic Province, the Wat Ocheng basalt is the first known zeolite locality in Cambodia. The basalt is a fine-grained, vesicular to amygdaloidal, subalkaline to transitional alkaline intraplate tholeiite comprised of 30% lath-like plagioclase (average: Ab51An45Or4), 35% interstitial augite (average: Wo44En35Fs21), 25–30% zeolites after plagioclase and volcanic glass, and minor skeletal ulvöspinel. It contains mineralized amygdales ranging in size from 0.5 × 0.5 cm to 5 × 9 cm. Nine zeolite species occur at Wat Ocheng, including analcime, chabazite-Ca, gonnardite, natrolite, phillipsite-Ca, and thomsonite-Ca, along with clays, aragonite, calcite, and pyrite. All the zeolite species are being described from Cambodia for the first time. The zeolite and secondary mineral assemblages observed at Wat Ocheng are similar to those reported from other alkaline basalt localities, including those in neighboring Vietnam, and are a product of alteration of the primary Ca-Na minerals and volcanic glass as a result of burial metamorphism and infiltration of heated meteoric waters. The mineral assemblage is not homogeneous across amygdales within the exposed lava flow, suggesting localized closed systems, likely the result of early precipitation of clay minerals and fine-grained zeolites. Decreased porosity and differences in fluid geochemistry would account for the diversity in the observed assemblages. Four stages of hydrothermal alteration and zeolitization have been defined based on mineral textures and chemistry. Zeolite formation began with fine-grained Ca- (chabazite-Ca and phillipsite-Ca) and Na- (analcime) dominant, high TSi (Si/Si+Al) species in Stage II following deposition of clay minerals in Stage I. Stage III is characterized by increasing Na+K contents and decreasing TSi. Crystallization of coarse-grained chabazite-Ca and phillipsite-Ca with increasing Na+K contents in the rims of the crystals followed the development of natrolite with a later-stage epitaxial overgrowth of thomsonite-Ca. The final stage of mineralization (Stage IV) included late-stage calcite, pyrite, and termination of growth of acicular sprays of thomsonite-Ca. Post-magmatic cooling and circulation of meteoric water and fluids derived from alluvial sediments overlying the basalts were involved in zeolitization. Thermal sources include an underlying basaltic andesite flow as well as regional deep-seated, extensional pull-apart structures, the result of a thinned lithosphere and injection of fertile mantle following the collision of the Eurasian and Indochina plates during the Himalayan Orogeny.

Morphological and chemical characterization of secondary carbonates in the Toki granite, central Japan, and the evolution of fluid chemistry

Abstract This study describes the: (1) morphological nature of the calcites in the Toki granite, central Japan; (2) the difference in chemical compositions in terms of morphological classification; and (3) the identification of the stages of calcite formation and the corresponding mass transfer between minerals and fluid owing to hydrothermal alterations and groundwater-rock interactions, which reveals the sequential variations in fluid chemistry during the sub-solidus stage. Calcites in the Toki granite were classified into four types: (1) lenticular calcite in the chloritized biotite; (2) granular calcite in the altered plagioclase; (3) intergranular calcite; and (4) fracture-filling calcite. The lenticular, granular, and intergranular calcites contain greater amounts of iron, manganese, and magnesium than fracture-filling calcites. The lenticular calcite in the chloritized biotite, granular calcite in the altered plagioclase, and intergranular calcite formed due to the precipitation of calcium, iron, manganese, and magnesium released from biotite and plagioclase owing to hydrothermal alterations. The fracture-filling calcites formed at a later stage than the lenticular, granular, and intergranular forms. In the hydrothermal fluid, the concentrations of aluminum, iron, manganese, and magnesium gradually decrease, and the concentration of calcium gradually increases as the alteration proceeds. The chemical characteristics of the fluid at the late stage of hydrothermal alteration and those of the subsequent groundwater are consistent with those of fracture-filling calcites, indicating that the fracture-filling calcites precipitated from the fluid at a late stage of hydrothermal alterations and then from the groundwater. Elements released from biotite and plagioclase owing to hydrothermal alterations were incorporated into and fastened to the calcite. Therefore, the calcites influenced the sequential variations in fluid chemistry during the sub-solidus stage.

Tracing the earliest stages of hydrothermal alteration on the CM chondrite parent body

AbstractThe CM carbonaceous chondrites are an important resource in our efforts to understand the role of volatiles in the formation of planetary systems. We report the bulk mineralogy, water abundance, and infrared (IR) reflectance spectra of the CM chondrites LaPaz Icefield (LAP) 04514, LAP 04796, LAP 04565, and LAP 02333. They contain abundant Fe‐ and Mg‐rich serpentines (~70–80 vol%), and based on their phyllosilicate fractions, we classify LAP 04514, LAP 04796, and LAP 04565 as petrologic subtype 1.6 and LAP 02333 as 1.4. This is consistent with estimated water abundances of 9.9 (±1.1) wt% for LAP 04796, 10.4 (±0.1) wt% for LAP 04565, and 11.5 (±0.5) wt% for LAP 02333. However, LAP 04514 contains less water (8.8 ± 0.3 wt%), has a shallower 3 µm band depth, and lacks tochilinite having experienced posthydration temperatures of ~300–400 °C. We conclude that LAP 04514, LAP 04796, and LAP 04565 are among the least altered CM chondrites, which retain primitive features from the initial building blocks of the CM parent body. Finally, we use the IR spectral features of LAP 04514, LAP 04796, and LAP 04565 to identify C‐complex asteroid surfaces that record mild levels of hydration.

REE mineralization in Siwana peralkaline granite, western India-role of fractional crystallization, hydrothermal remobilization, and feldspar-fluid interaction

Peralkaline granites from the Siwana complex in western India host mineralization of rare earth and other high field strength elements (REE/HFSE). In this study, we use textural relations, major-trace element chemistry and Li-isotope composition of minerals to constrain the magmatic evolution of the granites and the mechanism of hydrothermal REE mineralization. Three facies of peralkaline granites, viz., hypersolvus, transsolvus, and pegmatitic are identified in the Siwana complex. The hypersolvus granites constitute two mineralogical variants: clinopyroxene- and ferrorichterite-bearing. Transsolvus granites contain riebeckite-arfvedsonite and occur as patches/network of dikes in hypersolvus granites. Rayleigh crystal fractionation modelling using incompatible trace element abundances in clinopyroxene and amphibole indicates that the hypersolvus and the transsolvus granites formed after 50-85% and 98% melt crystallization, respectively. Early magmatic clinopyroxenes and ferrorichterites in the hypersolvus granites have fairly heavy δ7Li (+18.5‰ to +20.4‰) suggestive of crystallization from highly evolved magmas. Late magmatic arfvedsonites in the transsolvus granites have even heavier δ7Li (+29.7‰ to +33.8‰). Rayleigh distillation modelling using Li isotopes reveal that the hypersolvus and transsolvus granites crystallized from parental magmas that had undergone >90% fractional crystallization.The Siwana granitoids are hydrothermally altered as evidenced from pseudomorphic re-equilibration of clinopyroxene, alkali feldspar megacrysts, vein and patch perthite formation, replacement of ferrorichterite/clinopyroxene by hydrothermal aegirine, alteration of primary fluorapatite, monazite, aenigmatite, eudialyte, and vlasovite. Three stages of hydrothermal alteration are identified in the granites: stage-I alteration was brought about by highly-saline high-temperature fluid that scavenged REE, HFSE, Ti, Na, and Fe from clinopyroxene and aenigmatite. This was followed by extensive replacement of ferrorichterite and arfvedsonite by aegirine, which decreased the pH and salinity of the fluid. The third stage of alteration led to the replacement of vlasovite by calciocatapleiite, elpidite, and eudialyte and was accompanied by the replacement of alkali feldspar by aegirine, quartz, and secondary REE minerals. The bulk of the REE mineralization occurs in transsolvus granites as cerite associated with aegirine and quartz. Textural relations indicate that these minerals pseudomorph alkali feldspar. Distinct assemblages of secondary REE minerals are identified in the pyroxene- and amphibole-bearing granites-only hydroxyl-dominated REE minerals like cerite are identified in the former while late-stage replacement of cerite by bastnäsite and chevkinite is observed in the latter, the difference controlled by the extent of ferrorichterite to aegirine transformation. The precipitation of secondary REE minerals was largely triggered by pH increase due to the interaction of alkali feldspar with REE-bearing hydrothermal fluids, resulting in the replacement of alkali-feldspar with Fe-phyllosilicates.

Podiform magnetite ore(s) in the Sabzevar ophiolite (NE Iran): oceanic hydrothermal alteration of a chromite deposit

Serpentinite-hosted massive magnetite ore bodies are reported for the first time in the Late Cretaceous Sabzevar ophiolitic belt, northeastern Iran. They show irregular and discontinuous shapes with variable sizes ranging from 30 to 60 cm. Chromian spinel grains are observed within both magnetite ores and host serpentinite. Magmatic chromian spinels, (Cr,Al)-spinel I, with compositions close to (Mg0.6,Fe0.4)(Cr1.2,Al0.75,Fe3+0.05)O4 are preserved in the host serpentinite where they display a porous alteration rim composed of Cr-bearing chlorite and three different spinel-structure minerals: Cr-spinel (Fe0.6,Mg0.4)(Cr1.4,Al0.4,Fe3+0.2)O4, named Cr-spinel II (second generation), magnetite and ferritchromite, nominally FeCr2O4. In the magnetite ore body, no (Cr,Al)-spinel I is found and Cr-spinel II occurs as relict cores surrounded by ferritchromite and magnetite. Detailed X-ray elemental mapping revealed that the 200 μm-thick magnetite rim is composed of two magnetite types with different minor element compositions: the first rim found at the contact with ferritchromite is thin (20 μm; magnetite-I); the thicker outer rim contains numerous Fe-poor and Mg- and Si-rich silicate inclusions (magnetite-II). Observations at the TEM scale allows to identify ferritchromite which occurs as a micrometer-sized rim between Cr-spinel II and magnetite I. Thermodynamic modelling of the phase relationships in the studied Sabzevar serpentinite suggests that Cr-spinel II is produced along with chlorite during a first alteration stage at temperatures between 725 and 575 °C in the course of peridotite-water interactions. A second hydrothermal alteration stage producing ferritchromite and magnetite is inferred from the thermochemical modelling at temperatures < 400 °C under high H2 fugacity. This latter stage corresponds to the serpentinization of the Sabzevar oceanic peridotite and associated podiform chromitite deposit. The two alteration stages are interpreted as the result of the interaction between seawater and oceanic mantle at two different depth ranges in the course of its exhumation. Our thermodynamic calculations and textural relationships revealed that Cr is immobile and Fe is the main element to be transferred to the magnetite ore during alteration processes. Fe possibly originated from direct transport of the Fe2+ produced during olivine dissolution or from the dissolution of nano-sized magnetite grains initially formed in the host serpentinite during early serpentinization. Mass balance calculation reveals significant iron transport at a scale > 10 m during serpentinization.

Unravelling the hydro-mechanical evolution of a porphyry-type deposit by using vein structures

The Permian San Pedro Cu-(Mo) deposit consists of a porphyry system with associated polymetallic veins in the San Rafael Massif, Argentina. This deposit resulted from the emplacement of a dioritic - tonalitic subvolcanic stock into the Early Permian Choiyoi volcanic rocks. Veins and veinlets linked to the hydrothermal alteration stages (potassic, phyllic and carbonitization) show different attitudes and are predominantly steeply dipping; sequentially, they fill four fracture systems: stockwork (potassic), dilatant WNW- & ENE-trending shear-compressional fractures (phyllic), NNE- & ~NS-striking tensional and shear-extensional fractures (phyllic), and NW/NNW- & WNW-trending tensional and shear-extensional fractures (carbonitization). Because of mechanical anisotropies of the host-rock, alteration telescoping, presence of hydrothermal fluids, thermodynamic constraints, and waning orogenic conditions, vein/veinlet overprinting is conspicuous. Focused hydrothermal fluid flow promoted episodic hydraulic reactivation and hydraulic fracturing, and reveals that differences in vein fabrics do not necessarily represents veins emplaced under different stress regimes. Based on the study of the structure of the area (including available geophysical maps) and its kinematics, the control of structures during vein emplacement, the vein/veinlet overprint sequence, and the thermo and physical conditions at the time of vein emplacement (using fluid inclusion data), it was possible to identify, characterize and track the changing hydro-mechanical evolution of this deposit as well as the progressive exhumation of the area during the time of vein emplacement.

Metallogenesis of the orosirian epithermal coringa gold-silver (Cu-Pb-Zn) deposit, southeastern Tapajós mineral Province, Amazonian craton, Brazil

The Coringa gold-silver (Cu-Pb-Zn) deposit, in southeastern of Tapajós Mineral Province, is hosted in felsic volcanic rocks the Vila Riozinho Formation (1996 ± 21 Ma) and in the Serra Alkali Feldspar Granite (1999 ± 25 Ma), and has been interpreted to have formed in a post-collisional volcano-plutonic setting. The mineralized shear-vein system follows the NNW-SSE trend of the main regional structures and is coincident with a magnetic trend that can represent a crustal discontinuity at the boundary zone between a magnetic pyroclastic facies of the Vila Riozinho Formation and the pluton of the Serra Alkali Feldspar Granite. The hydrothermal alteration stages comprise: (a) proximal chlorite-hematite alteration, (b) intermediate-proximal sericite-pyrite alteration, with an estimated temperature of 400 °C (white mica chemistry), and (c) late, distal Mn-carbonate-chlorite-epidote alteration, with an average temperature of 234 °C (chlorite chemistry). There are discontinuous massive barren quartz veins (veins type 1), and two types of quartz-sulfide veins (veins types 2 and 3). The veins type 2 are sulfide-poor, gold-bearing, open-space filling veins with pyrite, galena, and hematite, with an average temperature formation of 243 °C (fluid inclusions in quartz). The veins type 3 are sulfide-rich, and gold-bearing quartz veins with sulfide phases represented by pyrite, galena, sphalerite, and chalcopyrite and presented a minimum temperature of 340 °C (fluid inclusions in quartz). Chemical analyses of the sulfides detected significant Mo, Bi, Ni, and Co, with decreasing contents according to the crystallization sequence of the sulfide phases (pyrite-chalcopyrite-sphalerite-galena-gold). The mineralizing fluids are H2O-rich and CO2-poor, low salinity, and the stable isotope compositions of the alteration minerals are mostly consistent with a magmatic source that mixed with a diluted and colder meteoric fluid. The geological setting, the timing of mineralization being nearly contemporaneous with the host rocks (given by previous works), the genetic characteristics, and the local presence of adularia and Mn-carbonate led us to classify Coringa as formed by an intermediate-sulfidation epithermal system.

Iron‐rich olivine in the unequilibrated ordinary chondrite, MET 00526: Earliest stages of formation

AbstractIn order to understand the effects of the earliest stages of hydrothermal alteration and fluid‐assisted metamorphism on the matrices of unequilibrated ordinary chondrites (UOCs), we have investigated the fine‐grained matrix of MET 00526 (L3.05) using multiple electron microscope techniques. Iron‐rich olivines (Fa50‐100) are present in all four representative fine‐grained matrix regions analyzed in this study. This study shows for the first time the occurrence of FeO‐rich olivines in distinct submicron veins that crosscut regions of matrix consisting of amorphous silicates and phyllosilicates, providing evidence for elemental mass transport in a hydrothermal fluid. Our detailed transmission electron microscopy study reinforces the idea that FeO‐rich olivines are formed on asteroidal parent bodies by the interaction between a hydrothermal fluid and the pristine solar nebular materials that may be the product of condensation processes in the protoplanetary disk, that is, amorphous silicates. We propose that the FeO‐rich olivines currently observed in MET 00526 matrix are the products of three possible reaction mechanisms, (1) replacement of amorphous silicates, (2) precipitation from an SiO‐rich fluid, and (3) replacement of phyllosilicates; all these mechanisms take place in the presence of an iron‐rich fluid. The chemical evolution of the hydrothermal fluid could trigger the formation of either fayalite or phyllosilicates depending on the Si/Fe ratios. A low Si/Fe ratio is required to form FeO‐rich olivines, rather than phyllosilicates, which form at high Si/Fe ratio. Although MET 00526 records the effects of secondary alteration processes, its fine‐grained matrix still preserves some evidence of its pristine, solar nebular characteristics.

Mineralogical and geochemical properties and genesis of kaolin and alunite deposits SE of Aksaray (Central Turkey)

The kaolin and alunite deposits, situated approximately 20 km southeast of Aksaray in Central Anatolia, were formed by hydrothermal alteration related to extensional tectonics from the Upper Miocene to Quaternary, mostly from rhyolitic volcanoclastic rocks. The mineralogical and geochemical properties of the deposits were investigated to explain the stages of hydrothermal alteration and the origin of the deposits. X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical and stable isotope (O, H, and S) studies were carried out on the alteration minerals (e.g., kaolinite, gypsum, alunite, native sulfur and smectite) of the deposits.Kaolinite/halloysite minerals were determined along with mostly alunite- and silica-group minerals (α-quartz, opal-CT, and tridymite), feldspar, native (elemental) sulfur, and partial hematite, goethite, gypsum, cinnabar and realgar. The δ34S isotope values for alunite, gypsum and native sulfur are in a fairly narrow range and vary from 5.03 to 6.62‰, 4.5–5.24‰ and 5.97–7.49‰, respectively. The δ18O values of the kaolinites and smectites vary from +1.8 to +13.3‰ and from −98 to −152‰, respectively, and their δD values range from −4.2 to 15.7‰ and from −93 to −191‰, respectively, showing a spread that is very close the primitive magmatic water line. From the δ18O values, the calculated formation temperatures of the kaolinites and smectites range from 38 to 135 °C and from 29 to 189 °C, respectively. The δ18O values of the alunites and amorphous silica vary from 4.4 to 8.9‰ and from −1.1 to 8.4‰, respectively, and the δD values vary from −72 to −87‰, and from −104 to −191‰, respectively. In addition, the formation temperatures of alunite and amorphous silica minerals range from 57 to 100 °C and from 139 to 281 °C, respectively. Hydrothermal alteration progressed in different stages between 29 and 281 °C according to the calculated modal formation temperatures. The isotopic mineral data suggest that the original hydrothermal waters were a mixture of magmatic and meteoric waters. Oxygen and deuterium analyses indicate that kaolinite and alunite formed at lower temperatures than smectite and silica minerals. The determined stable isotopic compositions of the minerals indicate that hydrothermal solutions at different compositions and temperatures played important roles in the formation of minerals. The determined neoformed minerals may have been formed by steam-heated hydrothermal solutions sourced from rhyolitic magma and are mostly of hypogene origin.

Hydrothermal alteration characteristics of the Chating Cu-Au deposit in Xuancheng City, Anhui Province, China: Significance of sericite alteration for Cu-Au exploration

The Chating Cu-Au deposit in Xuancheng City, Anhui Province, is characterized by porphyry-style mineralization with intensive hydrothermal alteration. Short-wave infrared reflectance spectroscopy was used to systematically analyze the alteration of drill cores from two cross sections. Eleven alteration minerals—sericite, smectite, kaolinite, dickite, chlorite, epidote, gypsum, carbonate, iron oxide, saponite, and prehnite—were identified, and the relative abundances of individual minerals at various depths were calculated by depth of absorption peak. In these minerals, iron oxides and gypsum are probably related to supergene process and other alteration minerals are hydrothermal alteration minerals. The mineral assemblages and their spatial distributions indicate that the alteration patterns in this Cu-Au deposit were controlled mainly by water–rock interaction of the magmatic–hydrothermal process, resulting in a typical porphyry-style hydrothermal mineralization system. A saponite–smectite–K-feldspar–anhydrite (gypsum) alteration assemblage is located mainly in the host quartz diorite porphyry, and is minor in the wall rocks of limestone and a post-mineralization diorite porphyry. A sericite–chlorite alteration assemblage was widely developed in the quartz diorite porphyry, and the intensity of alteration decreases gradually outward from the center to the margins of the deposit. A kaolinite–calcite alteration assemblage is mainly located on the top of the quartz diorite porphyry and in the surrounding limestone. The physiochemical conditions of the hydrothermal fluid, such as temperature and pH, control the compositional variations of specific alteration minerals. The most remarkable feature is a subtle change in octahedral Al cation content of sericite, as identified by the change in wavelength of sericite Al-OH absorption peak position. Muscovite is the main component of sericite, with a trace amount of phengite. The Cu-Au grades are negatively correlated with the wavelength of sericite Al-OH absorption peak position, because high ore grades are associated with relatively shorter Al-OH wavelengths, corresponding to a high Al/Si ratio in sericite (i.e. towards muscovite). This correlation suggests that the ore metal tends to be enriched and precipitated in an acidic environment of the alteration system. In addition, a lateral variation across the deposit in Al-OH wavelength indicates that the acidity of the hydrothermal fluids decreases gradually from the center of the deposit outward to the wall rocks, whereas the variation of sericite Al-OH wavelength with depth suggests an increase in fluid pH with decreasing depth. The alteration patterns imply that during mineralization the ore-forming fluids probably migrated from the southwest in the deep part of the porphyry system towards the northeast near the surface. Based on the spectral alteration mapping, it is concluded that there were at least three stages of hydrothermal activity in the Chating mining district: an early potassic–skarn hydrothermal alteration stage, an intermediate propylitic–sericite medium-low temperature alteration stage, and a late kaolinite–dickite epithermal stage. The sericite alteration stage is temporally related to Cu-Au mineralization, leading to the formation of the main orebody at Chating. As the current exploration is still focused in the sericite alteration zone, it is inferred that deep exploration has great potential in the Chating district.

Understanding seismic reflectivity across hydrothermal alteration zones associated with gold: Example of the Karari gold deposit, Western Australia

In this study, we examined whole core samples from a gold deposit in Western Australia, the Karari mine, located south of the Carosue Dam processing plant. By analyzing the mineralogy and performing petrophysical and acoustic measurements on 21 core samples representing different stages of hydrothermal alteration, our goal was to document the values of acoustic impedance as a function of the degree of alteration and identify which parameters (e.g., mineralogy, porosity, foliation) affect the seismic reflectivity across the alteration zones. To do so, we measured the ultrasonic velocities and bulk density at ambient conditions for all samples as well as at confining pressures for two samples. The acoustic measurements were conducted in the directions perpendicular and parallel to the foliation of the samples. A subset of samples was selected to identify the mineral phases, perform automated mineral mapping, measure grain density, and compute porosity using the bulk density–grain density relation and backscattered electron (BSE) images. Our results show that, regardless of the direction of acquisition, the acoustic impedance in the background samples is consistently smaller than that in the altered samples. This difference in acoustic impedance is attributed to a porosity difference of around 2% between the background and altered samples and is not owing to the presence of alteration minerals. Seismic reflectivity at the background/hydrothermal alteration interface decreases with increasing pressure owing to the closure of microcracks. For the Karari site, with a targeted exploration depth between 200 m and 800 m, the altered zones can be differentiated from the host rock by their acoustic impedance, as the reflection coefficients are expected to vary between 0.05 and 0.13, depending on the direction of seismic propagation and depth.

Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

Chromite deposits hosted in a layered ultramafic–mafic intrusion in the Archean Chithradurga greenstone belt are part of a suture that divides the Western and Central Dharwar craton blocks in southern India. Serpentinised ultramafic rocks including dunite, peridotite, and phlogopite-rich clinopyroxenite occur with massive chromitite. The olivine compositions show two distinct groups, one with Fo ~91–92 olivines in the massive chromitite, and another group with olivine Fo values of ~95–96, in the serpentinised dunite. The chromite occurs as massive chromitite and as an accessory phase in dunite. The former is characterized by high Cr# (Cr/(Cr + Al) × 100 = 67–70) and moderate to high Mg# (Mg/(Mg + Fe) = 52–66); The latter shows a larger variation in Cr which is likely related to sub-solidus re-equilibration of the chromite during the serpentinization event. In the massive chromitites, sub-solidus re-equilibration of chromite is less significant or absent. Bulk-rock major and trace element analyses of the peridotites reveal a composition less magnesian than depleted mantle rocks, with low TiO2, Na2O, CaO and high Cr, Ni content, a general depletion in Sr and enrichment in Th, U. The chromitites contain variable PGE abundances (ΣPGE = 8-244 ppb) with enrichments of Os, Ir and Ru, relative to Pt and Pd . These features suggest that the chromitites are cumulates formed from a highly magnesian parental magma with komatiitic affinity, derived by large degrees of partial melting, , which underwent crustal contamination. The ReOs isotope data demonstrate a minimum Re-depletion model age (TRD) of 2895 ± 24 Ma for the serpentinized dunite (MY11/3), and a more robust age of 3120 ± 12 Ma for two chromitite samples, which we interpret as the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains also occur within these ultramafic rocks with UPb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a dominant age population in the range 2.8–2.6 Ga. We interpret the upper intercept age at ca. 2.9 Ga to represent the initial crystallization of zircon in the ultramafic rocks, potentially as a result of an early stage of metamorphism and hydrothermal alteration. The 2.5–2.4 Ga zircon ages reflect a late-stage high-grade metamorphic overprint.