Late Hydrothermal Alteration Research Articles

Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars

AbstractSecondary minerals in martian nakhlites provide a powerful tool for investigating the nature, composition, and duration of aqueous activity in the martian crust. Northwest Africa (NWA) 998 crystallized early from the nakhlite magmatic source and has evidence of minimal signatures of the late hydrothermal alteration event that altered the nakhlites. Using FIB‐TEM techniques to study a cumulus apatite grain in NWA 998, we report the first evidence of a submicron‐scale vein consisting of fluorapatite and an SiO2‐rich phase. Fluorapatite grew epitaxially on the walls of an opened cleavage plane of host F‐bearing chlorapatite and the SiO2‐rich phase filled the center of the vein. The presence of nanoporosity and nanometer‐scale amorphous material and the sharp interface between the vein and the host apatite indicate the vein represents a coupled dissolution–reprecipitation process that generated apatite of a different composition that was more stable with the fluid. Using experimental data and diffusion coefficients of Cl in apatite from the literature, we conclude that the vein was caused by a low temperature (~300°C), slightly acidic, F‐, Si‐rich, aqueous fluid that acted as a closed system. Based on the characteristics of the vein (formation by rapid injection of fluid) and the fluid (composition, temperature, pH), and the lack of terrestrial weathering products in our SEM and TEM images, we infer that the vein is pre‐terrestrial in origin. Our observations support the hypothesis that the heat source triggering a hydrothermal system was a low‐shock velocity impact and rule out a magmatic origin. Finally, the vein could have formed from a late‐stage fluid different from that reported in other nakhlites, but formation during the same magmatic event by, for example, a less evolved fluid might also be plausible.

Mineralogical and geological characteristics of the Nui Phao tungsten deposit and its resource in the Dai Tu area, northeastern Vietnam

Typically, granitic intrusions that document the lengthy and intricate history of the magmatic-hydrothermal system are linked to tungsten deposits. Uncertainty persists about the genetic relationship between tungsten mineralization and magmatic-hydrothermal development. The primary tungsten deposit in the Dai Tu region, known as the Nui Phao deposit, has been the subject of a petrographical and microscopic examination. Tungsten mineralization in the Dai Tu area often occurs in association with the formation of skarn and greisen bodies, and it has drawn much attention from geoscientists. Based on microscopic observations, tungsten ores can be divided into three mineralization stages, namely skarnisation, greisenization, and hydrothermal stage. To examine the geochemical features of the tungsten ores, the SEM-EDS and Microscope analytical methods were performed in this study. Research results indicate that the Nui Phao tungsten deposit was formed due to different tectonic and magmatism episodes. Accordingly, the Nui Phao tungsten deposit is relatively complicated with the multi-sources of ore components. Most of the tungsten ore was accumulated in association with the metasomatism between the Ordovician-Silurian carbonate-terrigenous sedimentary rocks of the Phu Ngu formation and the Cretaceous two-mica granite of the Pia Oac complex. The research results indicate that tungsten resources obtained at levels 122 and 333 are about 227.6 thousand tons. Moreover, the hydrothermal alteration and metasomatism in the study area are influenced by at least three metasomatic episodes, including skarnisation, greisenisation, and the late hydrothermal alteration of medium to a low temperature that is genetically related to fluorite-polymetallic mineralization.

Mineral association in pegmatites from the Djurkovo Pb-Zn deposit, Central Rhodopes: preliminary results

Deformed pegmatites of varying thickness and position are a significant constituent intruded in the metamorphic complex, hosting the Djurkovo Pb-Zn deposit in Central Rhodopes. The mineral composition of the pegmatites consists of plagioclase, K-feldspar, quartz, and minor micas. The main accessory minerals are allanite, titanite, apatite and zircon. Late hydrothermal alteration of pegmatites led to the formation of epidote, adularia, sericite, chlorite, carbonate, quartz and leucoxene. Rare earth carbonate-phosphate assemblage (REE+Y, Th, U), manifested as ˂ 20 µm anhedral grains, is observed along fractures and dissolved zones in allanite and titanite. Because of the limited mobility of REE in fluids, these elements are barely transported during the hydrothermal activity and are incorporated in new phases, precipitated in the frames of the altered accessory minerals. The studied pegmatites contain a significant amount of accessory minerals rich in incompatible elements and therefore could be considered as their potential source.

Robust monazite U-Pb and molybdenite Re-Os ages reveal the magmatic and metallogenic history of a highly evolved granitic system in the Xianghualing deposit, South China

Tin, tungsten, and rare metals are key strategic metals and are regarded as having potentially relationship with highly evolved granites. However, excessive U accumulation with magmatic evolution, complex melt-fluid interaction, and late hydrothermal alteration make it difficult to obtain accurate age information from the highly evolved granitic system. The Xianghualing deposit, located in southern Hunan Province, China, is a giant Sn-Nb-Ta-polymetallic deposit with various types of mineralization spatially associated with the Laiziling and Jianfengling highly evolved granite plutons, and is an ideal target to investigate the magmatic-hydrothermal process of highly evolved magmas and related Sn-Nb-Ta-polymetallic mineralization. Previous studies have reported a relatively wide range of intrusion ages and multi-stage mineralization ranging from Triassic to Cretaceous. However, the timing and genetic link between magma emplacement and Sn-Nb-Ta-polymetallic mineralization remain controversial. Here we employ multiple dating methods, including SHRIMP zircon U-Pb analysis, LA-ICP-MS zircon and monazite U-Pb analysis, and molybdenite Re-Os dating analysis with a view to obtain a precise geochronological framework of the Xianghualing deposit. The SHRIMP zircon U-Pb data from the Laiziling protolithonite granite yielded a weighted 206Pb/238U age of 156.4 ± 1.5 Ma, whereas the LA-ICP-MS monazite U-Pb age for the Laiziling albite granite and the greisenized granite show 207Pb-corrected lower intercept 206Pb/238U ages of 155.5 ± 0.7 and 155.3 ± 0.5 Ma, respectively, which represent the emplacement ages of the Laiziling granite stock. Compared with the scattered SHRIMP and LA-ICP-MS zircon U-Pb age, monazite from the albite granite and greisenized granite yields more precise age information. Our study shows that monazite can be used as a robust tool for dating highly evolved granites. Molybdenite from the Laiziling pegmatite yields a Re-Os isochron age of 157.8 ± 4.2 Ma, which is consistent with the emplacement age of the Laiziling pluton. This result, integrated with previously reported metallogenic age, suggests a temporal and genetic link between the Laiziling granite and Sn-Nb-Ta polymetallic mineralization. Although multi-stage Sn-W metallogenic events have been identified in Nanling Region, it is suggested that the Xianghualing Sn-Nb-Ta mineralization formed during the Late Jurassic within a short duration coeval with the regional large-scale W-Sn mineralization at 160–150 Ma.

Diamictitic iron formation (DIF) deposits of the Neoproterozoic Nova Aurora Iron District (Macaúbas Group, Southeast Brazil)

The best-known Neoproterozoic glaciation-related iron deposits are the Rapitan-type banded iron formation (BIF) formed dominantly by chemical-sedimentary processes. We present a detailed study on a distinct type of glaciation-related iron deposit: the iron-rich metadiamictites of the Nova Aurora Iron District, comprising over 20 billion tons of iron oxide-rich rocks: the diamictitic iron formations (DIF >15 wt% Fe) and ferruginous metadiamictites (FD: 5–15 wt% Fe). Drill core logging, field mapping and analytical data revealed the protoliths, sediment sources, depositional environment, and ore enrichment processes on the iron district, and were used to discuss differences between the studied DIF and classic Rapitan-type BIF. U–Pb age spectra and Lu–Hf data for detrital zircon grains revealed a wide range of sediment sources and a maximum depositional age around 879 Ma. These data support correlations of the Nova Aurora iron deposits with the Cryogenian Macaúbas rift, a basin filled by glaciomarine successions related to the Sturtian glaciation event. The matrices of prevailing hematite-rich metadiamictites comprise (in vol %): hematite (7–55), quartz (17–57), muscovite (2–40), carbonate (<1–30), biotite (<1–18), chlorite (<1–23), epidote (<1–7), magnetite (1–5), and traces of tourmaline (<3), apatite (<3) and zircon (<1), with hematite, mica and stretched quartz outlining the metamorphic foliation. Clasts are free of iron-rich rocks. Hematite reconcentration (>50 wt% Fe) occurs in local mylonitic shear zones and tightly crenulated bands after gangue minerals removal during deformation. Late hydrothermal alteration formed magnetite-rich metadiamictite in high-strain zones. From base to top, the stratigraphic type-section of the Nova Aurora Iron District shows barren metadiamictite with quartz-rich matrix and graded-bedded quartzite lenses, followed by hematite-rich-matrix metadiamictite, then passing to muscovite-rich-matrix metadiamictite gradually richer in magnetite and sulfide, enclosing metapelite lenses. This section presents a fining-up graded-bedded succession with clasts size decreasing upwards and recurrent load structures, typical of debris flows and turbiditic sedimentation. The hematite/mica, Fe/Al, Fe/Ti and Fe/REE ratios, as well as the biotite/muscovite and silicate/sulfide ratios, which accompany variations in the oxidizing conditions, decrease to the top. Accordingly, the early protoliths were quartz-rich diamictites that passed to iron-rich diamictites and then to clay-rich, and under more reducing conditions, sulfide-bearing diamictites. Sedimentological and geochemical evidences suggest ferrous-iron accumulation followed by iron oxide precipitation from seawater when the conditions changed from anoxic to oxidizing by the input of oxygen-rich waters from deglaciation, followed by increasing reducing conditions accompanying marine transgression. These processes took place in a relatively restricted sector of the glaciomarine basin, probably a graben of the Cryogenian Macaúbas rift. Differences in relation to the classic Rapitan-type iron formations are the coarse-grained (ruditic) texture and the massive structure, the high-energy sedimentary environment, and the absence of true BIF in the Nova Aurora DIF deposits.

Ferrous hydroxychlorides hibbingite [γ-Fe2(OH)3Cl] and parahibbingite [β-Fe2(OH)3Cl] as a concealed sink of Cl and H2O in ultrabasic and granitic systems

Abstract Ferrous hydroxychlorides are geochemically important but less recognized mineral species due to their extreme sensitivity to oxidation and hydration in contact with air {typically they convert to akaganéite [Fe3+(O,OH,Cl)]}. Only the γ-form was previously known as the orthorhombic mineral hibbingite, associated with altered mafic intrusive rocks. In this study, we describe the β-polymorph of Fe2(OH)3Cl as a new mineral parahibbingite that was found in pyroxenite from the Karee platinum mine in the Bushveld Complex, South Africa. The two minerals were distinguished by a combination of Raman spectroscopy and FIB-SEM-TEM analytical techniques (TEM-EDX and TEM-SAED). They can be easily recognized by their distinct Raman spectra. Parahibbingite has two very strong vibration bands at ~3550 and 3560 cm–1, accompanied by much weaker bands at ~124 and 160 cm−1, while the Raman spectrum of hibbingite has a sharp, strong band at 3450 cm−1 and two moderate bands at 199 and 385 cm−1. Parahibbingite was found as fine-grained reaction rims at the contact of orthopyroxene phenocrysts and talc inside a drill core. It has a trigonal space group [R3m, a = 6.94(5) Å; c = 14.5(2) Å], with an empirical formula (Fe1.982+Mn0.012+Ca0.01)(OH)3.08Cl0.92. The origin of this mineral in the Bushveld Complex is most likely related to a late hydrothermal alteration of pyroxenite. Hibbingite forms as an abundant daughter mineral hosted by fluid inclusions and salt melt inclusions in hydrothermal quartz associated with granitic systems during cooling under reducing conditions. Such inclusions are common in Au-porphyry mineralization worldwide, such as the Biely Vrch (Slovakia) deposit studied in detail in this work. The lattice parameters obtained by TEM-SAED are a = 6.30 Å, b = 7.12 Å, and c = 9.89 Å. Hibbingite was recognized as the only phase that carries “water” (as a hydroxyl group) in otherwise water-free, salt melt inclusions. Furthermore, both hibbingite and parahibbingite should be considered as reservoirs for Cl and H2O in large volumes of altered basic and ultrabasic rocks. They can transport volatiles to shallow levels of subduction zones. Alternatively, their dissolution can fuel remobilization, transport, and deposition of sulfidic ores in saline fluids. Their detection, however, is difficult because of their sensitivity to oxidizing atmospheres. For example, in natural outcrops exposed to air, they may vanish, thus distorting estimates of their abundance and role in many processes that involve mineral-derived volatiles.

Major types and occurrences of platinum-group minerals in the Jinchuan Ni-Cu-(PGE) sulfide deposit: Insights for PGE enrichment during hydrothermal alteration

金川铜镍硫化物矿床是我国最主要的铂族元素（PGE）资源产地，其矿石受热液蚀变作用影响明显，并产出多种铂族矿物（PGM）。岩浆演化和热液蚀变过程中PGE的迁移富集机制和PGM的成因，一直是研究PGE地球化学行为非常关注的问题。本文对金川铜镍硫化物矿床中PGM的研究发现，其主要类型包括含PGE的硫砷化物（硫砷铱矿）和砷化物（砷铂矿），Pd的铋化物、碲化物和硒化物，以及少量其他铂族矿物。其中，硫砷铱矿可包裹于各种贱金属硫化物（镍黄铁矿、磁黄铁矿和黄铜矿）中，表明硫砷铱矿可能结晶于早期的含As硫化物熔体，随后被包裹于硫化物熔体冷凝分异产生的单硫化物固溶体（MSS）和中间硫化物固溶体（ISS）中。硫化物熔体中的As可能主要通过地壳混染作用加入幔源岩浆。大量铋钯矿（PdBi）呈微细乳滴状包裹于黄铜矿中，为晚期ISS冷凝形成黄铜矿过程中出溶的产物。少量铋钯矿（PdBi₂）呈不规则状充填于矿物裂隙，与次生磁铁矿脉紧密共生，并随矿石的蚀变程度增加，铋钯矿的化学成分由PdBi逐渐向PdBi₂转变，表明这部分铋钯矿为后期热液蚀变产物。铋碲钯矿和钯的硒化物则主要产出于镍黄铁矿裂隙且与次生磁铁矿紧密共生，指示明显的热液成因。钯的硒化物的出现表明，岩浆期后酸性、高盐度、高氧逸度的富Cl^-流体对金川铜镍硫化物矿床中Pd的迁移和富集起到了关键控制作用。;The Jinchuan Ni-Cu-(PGE) sulfide deposit hosts the major resource of platinum group elements (PGE) in China. Most Ni-Cu sulfide ores of the deposit have been altered by hydrothermal fluids and contain diverse types of platinum group minerals (PGM). The formation of PGM and enrichment of PGE are likely controlled by the partitioning behaviors of PGE during magmatic processes and late-stage hydrothermal overprints. In this study, main PGM types discovered in the ores include PGE-bearing arsenide (irarsite and sperrylite) and Pd-bearing bismuthide, telluride and selenide, as well as a few other types of PGM. Irarsite (IrAsS) is enclosed within pentlandite, pyrrhotite and chalcopyrite, indicating that irarsite may have crystallized earlier from the As-rich sulfide melt. During the fractionation of sulfide melts, the irarsite can be trapped in the initial monosulfide solid solution (MSS) or residual intermediate solid solution (ISS). Arsenic was likely incorporated into the mantle-derived mafic magmas by crustal contamination and then concentrated in sulfide melts. Most of the Pd bismuthide (PdBi) grains enclosed within chalcopyrite show obvious emulsion texture, indicating that they were exsolved from the ISS during subsolidus cooling. Some irregular Pd bismuthide (PdBi₂) grains are associated with secondary magnetite veins along micro-fractures in the ores. The Pd bismuthides have chemical compositions varying from PdBi to PdBi₂ with increasing degrees of hydrothermal alteration, indicating that they were likely produced by late hydrothermal alteration. Michenerite (PdBiTe) and Pd selenides grains mainly occur in the fractures throughout pentlandite and are associated with secondary magnetite, indicating a hydrothermal origin. Abundant Pd selenides in the ores indicates that the fluids are of acidic, saline and highly oxidized, which may play a key role in the PGE enrichment of the altered ores in the Jinchuan Ni-Cu sulfide deposit.

Structural Controls on Alteration Stages at the Chuquicamata Copper-Molybdenum Deposit, Northern Chile

AbstractAll existing bench and tunnel vein and fault structural data with identified mineral infill, acquired in Chuquicamata, were georeferenced, digitized, and, according to their mineralogy, assigned to one or more of the major alteration events developed between 35 and 31 Ma. Veins and faults were separated into two main stages: (1) the late magmatic and potassic stage that comprises the background potassic and the propylitic alteration and (2) the hydrothermal stage composed by early (intense potassic), main (principal and late sericite; hydrothermal stages H1 through H4), and late (advanced argillic alteration) hydrothermal events. The spatial distribution of the propylitic to late-hydrothermal events that plotted within the major fault framework indicate these had either permeable or impermeable (±barrier) behavior through time. The area of the deposit was divided into 600 square grids measuring 100 × 100 m, and a stress orientation analysis was carried out for every propylitic to late-hydrothermal alteration event. The analysis indicates that the local principal horizontal stress (σH) trajectories are nonlinear and noncoaxial through the successive alteration events, differing from the previous and following stages, and in the majority of cases do not coincide with the approximate east-northeast orientation of the inferred tectonic far-field stress orientation. The differences between the stress trajectories, away from the far-field stress orientation throughout the evolution of the system, are considered to be principally related to the dynamic variations experienced by the stress components, such as thermal-magmatic stresses linked to temperature fluctuations due to cooling or heating by progressive igneous/hydrothermal activity and/or elastic, overburden-related stresses associated with reaccommodations developed during uplift and erosion. The estimated stresses resulting after erosional unroofing and decreasing temperature indicate that the maximum horizontal stress varied as the system evolved from the commonly accepted depth of emplacement of ~6 km. During the late magmatic, background potassic, and intense potassic stages, the calculated differential stress was contractional, decreasing to an isotropic state at the contraction-extension stress reversal that hosted the main hydrothermal H1 through H3 events, to finally become extensional at the shallow late-hydrothermal event. The most significant mineralization occurred at the time of stress reversal, coincidental with the sericite and quartz-sericite events (H1-H4), associated with hydrothermal fluid accumulation, overpressuring, and multiple-orientated hydraulic fracture development.The Chuquicamata study suggests that the local stress control involved in the emplacement of porphyry copper systems is fundamentally related to variable and progressive heat energy release, associated with igneous and hydrothermal activity, and to the elastic stresses derived from uplift and unloading, rather than to a constant far-field tectonic stress. The continuous local stress fluctuations led to bulk stress readjustments and cyclical stress-fluid interactions for local fault reactivation, damage zone modification, brecciation, permeability creation/destruction, and fluid focusing, as well as the discharge of hydrothermal fluids throughout the evolution of the system.

Early evidence of pyrometamorphism and a late hydrothermal alteration related to Albian volcanism in the Pernambuco sedimentary basin, northeast Brazil

Pyrometamorphic aureoles and late hydrothermal alterations linked to Albian alkaline volcanism of the Ipojuca Magmatic Suite are described for the first time in the Pernambuco Basin, northeast Brazil. Field features, petrography, scanning electron microscopy (SEM-WDS), reflectance spectroscopy and X-ray diffraction data were used to characterize the mineral assemblage related to these two geological features. The pyrometamorphic aureoles occur when trachybasalts and arcosian sandstones (upper Cabo Formation) are in contact and were classified as buchites, due to the mineral assemblage composed of mullite, spinel (hercynite), tridymite, pyrrhotite and montmorillonite. The sanidinite facies (high temperature and lower pressure) is attributed to the mineral assemblage described above. Regarding the late hydrothermal alteration, two main events occurred in shallow conditions (up to 1 km). The first event is marked by an argillic alteration along NE-SW and NW-SE faults and fractures. The mineral assemblage is composed of kaolinite, quartz and minor magnetite. This alteration is also characterized by a reflectance spectral signature demonstrated by the high crystallinity of kaolinite that could also indicate its hypogene origin, in temperatures ranging from 100 to 250 °C. The second event is characterized by amygdales and veinlets in basalts lava flows that are filled with smectite (e.g montmorillonite), the formation of which is attributed to alkaline hydrothermal fluids with temperatures between 75 and 150 °C. Both events have been associated with the same fluid source. However, the first event occurred at higher temperatures and with acidic fluids, while the second presented lower temperatures and alkaline fluids.

Isotopic Signatures of Carbon in the ‘Los Pobres’ Graphite Mine, Ronda, Spain

AbstractGraphite formation temperatures in the ‘Los Pobres’ mine within the Ronda peridotite, Spain, previously reported to be between 770 and 820 °C, have been reinterpreted based on new temperature measurements using Raman spectroscopy. Additional in situ and bulk stable carbon isotopic measurements and fluid inclusion studies contributed to improved understanding of parts of the graphite formation process. Raman spectroscopy revealed that the formation of the ‘Los Pobres’ graphite extends to temperatures as low as 500 °C, indicating a broader temperature range than previously reported. Stable carbon isotopes and temperature estimates suggest two different crystallization events, followed by a late hydrothermal alteration of the host rock. The first event occurred at temperatures higher than ∼600°C, in which crystalline graphite was formed with a mixed 13C composition as a result of the mixing of two different carbon-bearing sources. The second graphite formation event took place below ∼600°C, within the same system, but with lower purity and crystallinity of the graphite. In the third event, the temperature decreased to less than 550 °C, and hydrothermal fluids altered the host rock, precipitating silica and iron oxides in veins penetrating both the host rock and the deposited graphite.

Pyrochlore-Group Minerals in the Granite-Hosted Katugin Rare-Metal Deposit, Transbaikalia, Russia

Pyrochlore group minerals are the main raw phases in granitic rocks of the Katugin complex-ore deposit that stores Nb, Ta, Y, REE, U, Th, Zr, and cryolite. There are three main types: Primary magmatic, early postmagmatic (secondary-I), and late hydrothermal (secondary-II) pyrochlores. The primary magmatic phase is fluornatropyrochlore, which has high concentrations of Na2O (to 10.5 wt.%), F (to 5.4 wt.%), and REE2O3 (to 17.3 wt.%) but also low CaO (0.6–4.3 wt.%), UO2 (to 2.6 wt.%), ThO2 (to 1.8 wt.%), and PbO (to 1.4 wt.%). Pyrochlore of this type is very rare in nature and is limited to a few occurrences: Rare-metal deposits of Nechalacho in syenite and nepheline syenite (Canada) and Mariupol in nepheline syenite (Ukraine). It may have crystallized synchronously with or slightly later than melanocratic minerals (aegirine, biotite, and arfvedsonite) at the late magmatic stage when Fe from the melt became bound, which hindered the crystallization of columbite. Secondary-I pyrochlore follows cracks or replaces primary pyrochlore in grain rims and is compositionally similar to the early phase, except for lower Na2O concentrations (2.8 wt.%), relatively low F (4 wt.%), and less complete A- and Y-sites occupancy. Secondary-II pyrochlore is a product of late hydrothermal alteration, which postdated the formation of the Katugin deposit. It differs in large ranges of elements and contains minor K, Ba, Pb, Fe, and significant Si concentrations but also low Na and F. Its composition mostly falls within the field of hydro- and keno-pyrochlore.

40Ar-39Ar age constraints on timing of a late hydrothermal event in the Hutti greenstone belt, Karnataka, India

Obtaining constraints on the timing of orogenic gold mineralization hosted in shear zones in greenstone belts is often difficult because of the lack of easily datable minerals and problems of overprinting of earlier events of mineralization by later hydrothermal events. In the orogenic Hutti gold deposit the U-Pb SHRIMP date obtained from a monazite grain within arsenopyrite that gave an age of 2547 ± 10 Ma, is generally accepted as the age of early hydrothermal mineralization. Nevertheless, other work has indicated that hydrothermal mineralization may have occurred in later episodes as well. This study reports new 40Ar-39Ar ages for hydrothermal biotite, amphibolite and hornblende in granites that occur along the northern and southeastern boundary of the greenstone belt. The 40Ar-39Ar ages of biotite samples are inferred to be the result of re-setting of the ages of alteration biotite by the last phase of hydrothermal activity in this area at ∼2332 Ma. The study indicates that the effects of the thermal event are more significant in areas of greater hydrothermal access such as within the Hutti shear zone. This late hydrothermal alteration event may be related to the D3 deformational event of Roy (1991). The results of the present work are important because it demonstrates a significant hydrothermal event much after the major 2.58–2.52 Ga accretionary event and accompanying TTG and felsic magmatism in the Eastern Dharwar Craton.

Evolution of Hutti-Maski greenstone belt of the Eastern Dharwar Craton: Evidence for metamorphic and hydrothermal phases from the Hira-Buddini deposit, Raichur district, Karnataka, India

The Hira-Buddini gold deposit is located along the steeply dipping ENE trending sheared contact of felsic and mafic rocks of strike length of about 600 m with mylonitic foliation parallel to the S1 schistosity in amphibolites. Second-generation open folds with axial planes (S2) marked by fractures that are often filled by later calcite veins are observed in surface and underground exposures. Garnetiferous amphibolites occur in patches on the footwall side of the shear in the western part of the deposit. This rock shows garnet porphyroblasts, coarse second-generation hornblende and large grains of biotite that grow over an early S1 fabric which is made up of early hornblende, plagioclase, ilmenite and retrograde first-generation chlorite. Second-generation hornblende and biotite grains make high angles to S1 schistosity and are sub-parallel to S2. Late hydrothermal alteration is marked by an albite-epidote-chlorite-zoisite assemblage. Geothermometric estimates based on garnet-biotite, and garnet-hornblende pairs, as well as Ti in biotite, show that temperatures during D2 deformation that led to the growth of the porphyroblasts were \(530{\pm }20^{\circ }\hbox {C}\). The fabric and mineralogy of the rock indicate that porphyroblastic growth of garnet, hornblende and biotite was preceded and succeeded by stages of hydrothermal alteration. Primary gold mineralization is inferred to be associated with the early stage of hydrothermal ingress.

Quintinite-1M from the Mariinsky Deposit, Ural Emerald Mines, Central Urals, Russia

The paper describes the first finding of quintinite [Mg4Al2(OH)12][(CO3)(H2O)3] at the Mariinsky deposit in the Central Urals, Russia. The mineral occurs as white tabular crystals in cavities within altered gabbro in association with prehnite, calcite, and a chlorite-group mineral. Quintinite is the probable result of late hydrothermal alteration of primary mafic and ultramafic rocks hosting emerald-bearing glimmerite. According to electron microprobe data, the Mg: Al ratio is ~2: 1. IR spectroscopy has revealed hydroxyl and carbonate groups and H2O molecules in the mineral. According to single crystal XRD data, quintinite is monoclinic, space group C2/m, a =5.233(1), b = 9.051(2), c = 7.711(2) A, β = 103.09(3)°, V = 355.7(2) A3. Based on structure refinement, the polytype of quintinite should be denoted as 1M. This is the third approved occurrence of quintinite-1M in the world after the Kovdor complex and Bazhenovsky chrysotile–asbestos deposit.

40 Ma of hydrothermal W mineralization during the Variscan orogenic evolution of the French Massif Central revealed by U-Pb dating of wolframite

We present U-Pb thermal ionization mass spectrometer (TIMS) ages of wolframite from several granite-related hydrothermal W±Sn deposits in the French Massif Central (FMC) located in the internal zone of the Variscan belt. The studied wolframite samples are characterized by variable U and Pb contents (typically <10 ppm) and show significant variations in their radiogenic Pb isotopic compositions. The obtained U-Pb ages define three distinct geochronological groups related to three contrasting geodynamic settings: (i) Visean to Namurian mineralization (333–327 Ma) coeval with syn-orogenic compression and emplacement of large peraluminous leucogranites (ca. 335–325 Ma), (ii) Namurian to Westphalian mineralization (317–315 Ma) synchronous with the onset of late-orogenic extension and emplacement of syn-tectonic granites (ca. 315–310 Ma) and (iii) Stephanian to Permian mineralization (298–274 Ma) formed during post-orogenic extension contemporaneous with the Permian volcanism in the entire Variscan belt. The youngest ages (276–274 Ma) likely reflect the reopening of the U-Pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration (e.g. ferberitization). Our results demonstrate that W(±Sn) mineralization in the FMC formed during at least three distinct hydrothermal events in different tectono-metamorphic settings over a time range of 40 Ma.

Chromatite and its Cr3+- and Cr6+-bearing precursor minerals from the Nabi Musa Mottled Zone complex, Judean Desert

Chromatite (CaCrO4, tetragonal) is mainly known from Cr6+-contaminated soils associated with chromium ore processing residue. This extremely rare mineral was found at the Nabi Musa locality (Judean Desert, Israel), in a peculiar rock complex of the Mottled Zone. We have explored the possible mechanisms responsible for leaching Cr6+ from natural rocks, by means of field observations, batch experiments, thermodynamic modeling, and mineralogical analyses of the Nabi Musa rocks (XRPD, EMPA, SEM, FTIR, and optical microscopy). A remarkable feature of the Mottled Zone rocks is a broad occurrence of high- and ultrahigh-temperature combustion metamorphic rocks, with Cr3+ accumulated mainly in opaque minerals such as Fe-spinel, brownmillerite, and perovskite. Another feature of the Mottled Zone sequence is abundant Cr3+ (bentorite and volkonskoite) and Cr6+ mineralization (Cr6+-bearing ettringite and baryte-hashemite solid solution) in low-temperature hydrothermal veins. Field, mineralogical, and thermodynamic modeling data suggest that Cr was leached from Cr3+-bearing opaque minerals during late hydrothermal alteration of combustion metamorphic rocks by unusual hyperalkaline waters (pH up to 12). The Cr3+ was then oxidized to Cr6+, and subsequently partially immobilized in Cr6+-bearing ettringite. As a consequence of the highway construction across Nabi Musa hill in 2006, the buried veins filled by Cr6+-substituted ettringite were exhumed and exposed to supergene alteration. The ensuing decomposition of Cr6+-bearing ettringite was followed by Cr6+ release into pore waters in rainy seasons, and then by precipitation of chromatite on the evaporation barrier under the hard desert insolation in dry seasons. The chromatite formation has been due to both unique rock and water chemistry of the Mottled Zone sequence and to the arid climate of the Judean Desert.

Behoite and mimetite from the Saharjok alkaline intrusion, Kola Peninsula

The detailed study of the mineral composition of the nepheline syenite pegmatite from the Saharjok Intrusion has resulted in the finding of behoite and mimetite, a mineral species identified in the Kola region for the first time. The pegmatite body at the contact between nepheline syenite and essexite is unusual in textural and structural features and combination of mineral assemblages including unique beryllium mineralization. Behoite Be(OH)2 is an extremely rare beryllium mineral. It occurs as powderlike aggregates in the leaching cavities between euhedral pyroxene crystals. Behoite was identified by comparison of X-ray powder diffraction data of the studied mineral phase and behoite from the Be-bearing tuff in the type locality of this mineral (Utah, United States). Mimetite was found in the same pegmatite of the Saharjok intrusion. It forms unusual parallel-fibrous aggregates with individual fibers as long as ∼1 mm and only ∼1 μm across. X-ray powder diffraction data and the chemical composition characterize the mineral as hexagonal phase Pb5[AsO4]3Cl. Both behoite and mimetite are the products of late hydrothermal alteration of primary minerals (meliphanite, galena, arsenopyrite, and loellingite). The secondary phases freely crystallized in the cavities remaining after the leached nepheline.

Mineralogical and geochemical characteristics of scheelite-bearing skarns, and genetic relations between skarn mineralization and petrogenesis of the associated granitoid pluton at Sargipali, Sundergarh District, Eastern India

Skarn rocks occur at the contact between calcite-bearing dolomitic marbles and granitoids (massive varieties with pegmatites) in close spatial association with the mica schist-hosted Proterozoic Pb–Cu–Ag sulfide deposits at Sargipali, Sundergarh District, Eastern India. The exoskarn (pyroxene–garnet) of variable width (1 to 30 m) occurs in marble proximal to the granitic intrusion, and endoskarn (pyroxene–epidote) is variably developed (< 1 to 10 m). Molybdenum-free scheelite with minor pyrrhotite (0.2%) is found only in late garnet–clinopyroxene exoskarn assemblages. In the Sargipali area early regional and contact metamorphism converted impure carbonate lithologies to calc-silicate hornfels. Subsequent contact metasomatism formed the skarn rocks, which are well zoned geochemically, mineralogically, and texturally in the sequence pyroxene–garnet–amphibole from the calcite-bearing dolomitic marble to granitoid contact. The presence of zoning relative to igneous contacts indicates that skarn-forming fluids originated from the crystallizing magma. The skarns are composed of clinopyroxene, garnet, calcic amphiboles (K-rich ferropargasite, ferrohornblende, hastingsite, tschermakite, magnesiohornblende, and actinolite), wollastonite, plagioclase, potash feldspar, epidote, titanite, and quartz. The skarns are notably enriched in Al, Mg, and Fe. Garnets are grossularite–almandine with 9 to 10 mol% spessartine, whereas pyroxenes are hedenbergitic to diopsidic in composition. The variable Mg:Mn:Fe proportions in the skarn clinopyroxene suggest the formation of clinopyroxene compositions from relatively homogeneous fluids, which experienced local variations in their Mg:Mn:Fe proportion instead of from successively different compositions. The earliest hornfels assemblage (Stage I) formed initially above 500 °C. This was overprinted by prograde anhydrous skarn (Stage II) at about 500 °C–600 °C and of 3–4 kbar pressure in a mildly reducing environment under X(CO 2) = ~ 0.18. With increasing fluid/rock interaction, epidote, green amphibole ± quartz-bearing retrograde skarn (Stage III) formed as temperature decreased to approximately 480 °C at X(CO 2) = 0.05. Late hydrothermal alteration (Stage IV) caused the formation of actinolite. There is a correlation between intrusion composition and the metal contents of associated skarns. Calc-silicate mineral compositions in the Sargipali skarns are similar to those in other W skarn systems. This granitic complex is comprised of reduced, highly evolved, and metallogenically specialized S-type leucogranites, comparable to those commonly associated with Mo-poor W skarns. A syn-collisional tectonic setting is proposed, based on field evidence, the relative timing of the intrusions with respect to metasedimentary and carbonate rocks, and empirical trace-element geochemical evidence. Based on field evidence and geochemistry, two main intrusive phases have been recognized in the Sargipali granitoid pluton: (1) an undeformed massive granite in the west, and (2) a foliated granite along the eastern margin. Porphyritic granites are also recognized locally, which are older than the other units. A genetic link exists between granite magmatism, and the formation of pegmatites in the region. The granite–pegmatite system is highly peraluminous (Al-saturation index ranges from 1.2 to 1.8). The peraluminous character increases from the foliated and porphyritic granite through massive granite to pegmatite. The foliated granite has higher FeO t, TiO 2, MgO, Ba, Sr, Zr, Th, ∑ REE (~ 200 ppm), and lower SiO 2 contents than massive granite (∑ REE = ~ 22 ppm). Both of these granite phases are highly evolved, while the massive variety is more evolved, and is mostly dominated by variable source rock composition. The minimum crystallization temperature of granite magma is at 628 °C–695 °C and greater than the 3 kbar pressure. The granites may have been generated by partial melting of metasedimentary rocks of the Gangpur Group that might have been enriched in W. The exsolving W-rich magmatic fluids interacted extensively with carbonate rocks and formed disseminated scheelite mineralization. New CHIME U–Th–Pb monazite dates for foliated granites of Sargipali are 960 ± 10 Ma, while massive granite shows two ages; the U-poor population and U-rich population yielded 955 ± 15 Ma and 997 ± 13 Ma, respectively. These ages indicate that the deformed foliated granite is slightly older than the undeformed massive granite, while there exists a still older intrusion (porphyritic granite) that is syn- to post-kinematic to the major phase of deformation (D 1) of the enveloping country rock at ~ 1 Ga. Overall, the Sargipali granitoid pluton post-dates the syngenetic Pb–Cu–Ag sulfide mineralization in the region at ~ 1.69 Ga.

Mineralizações de Sn do Maciço Granítico Serra Branca, Goiás: evolução do sistema hidrotermal e fonte dos fluidos

The Serra Branca granitic massif occurs in the Goias tin province and is part of the Paleo- to Mesoproterozoic within-plate granites suite, which is rich in F, Sn, Rb, Y, Th, Nb, Ga and REE. The Serra Branca granitic massif hosts tin mineralization and comprises: a) pink to gray, porphyritic, medium- to coarse-grained granite (g1a); b) pink to gray, heterogranular, medium- to coarse-grained granite (g1b); c) gray, heterogranular, medium- to coarse-grained granite (g1c); and d) gray, heterogranular, medium-grained granite (g1d), which is restricted to the eastern part of the massif. Greisens and mica veins also occur in the massif. Important post-magmatic processes have been identified: albitization, greisenization (the most prominent) and microclinization. A fluid inclusion study in K-feldspar from the (g1a) and (g1c) granites and in quartz, topaz and fluorite from the (g1d) granite, greisen and quartz + fluorite and quartz + topaz veins revealed the presence of two fluid inclusions types: i) type 1, interpreted as primary and modeled by the H 2 O-NaCl-CO 2 ( ± CH 4 ) system; and ii) type 2, composed of H 2 O-NaCl ± (KCl-MgCl 2 -FeCl 2 ) two-phase fluid inclusions, with petrographic characteristics of primary fluid inclusions in the hydrothermal rocks and of secondary ones in quartz from the (g1a) granite. The low T H (~240oC) and salinity (< 2 wt. % NaCl equiv.) values of fluid inclusions entrapped in K-feldspar from the granites suggest that this mineral doesn’t contain the original magmatic fluid, but fluids derived either from progressive cooling or from late hydrothermal alteration. The petrologic characteristics of the Serra Branca massif together with the fluid inclusions data are consistent with predominant magmatic source for the fluids rich in CO 2 ( ± CH 4 ). The petrographic and microthermometric data suggest that the aqueous fluids were entrapped later. It is proposed an evolution model in which an initially magmatic fluid has been released and mixed with external lower-temperature fluid. After the hydrothermal alteration and mineralization, there was entrapment of late fluids in fractures of the hydrothermal minerals, probably due to the deformation associated with the Brasilano orogenic event.

MINERALIZAÇÕES DE Sn DO MACIÇO GRANÍTICO SERRA BRANCA, GOIÁS: EVOLUÇÃO DO SISTEMA HIDROTERMAL E FONTE DOS FLUIDOS

The Serra Branca granitic massif occurs in the Goiás tin province and is part of the Paleo- to Mesoproterozoic within-plate granites suite, which is rich in F, Sn, Rb, Y, Th, Nb, Ga and REE. The Serra Branca granitic massif hosts tin mineralization and comprises: a) pink to gray, porphyritic, medium- to coarse-grained granite (g1a); b) pink to gray, heterogranular, medium- to coarse-grained granite (g1b); c) gray, heterogranular, medium- to coarse-grained granite (g1c); and d) gray, heterogranular, medium-grained granite (g1d), which is restricted to the eastern part of the massif. Greisens and mica veins also occur in the massif. Important post-magmatic processes have been identified: albitization, greisenization (the most prominent) and microclinization. A fluid inclusion study in K-feldspar from the (g1a) and (g1c) granites and in quartz, topaz and fluorite from the (g1d) granite, greisen and quartz + fluorite and quartz + topaz veins revealed the presence of two fluid inclusions types: i) type 1, interpreted as primary and modeled by the H 2 O-NaCl-CO 2 (±CH 4 ) system; and ii) type 2, composed of H 2 O-NaCl ± (KCl-MgCl 2 -FeCl 2 ) two-phase fluid inclusions, with petrographic characteristics of primary fluid inclusions in the hydrothermal rocks and of secondary ones in quartz from the (g1a) granite. The low TH (~240ºC) and salinity (&lt; 2 wt. % NaCl equiv.) values of fluid inclusions entrapped in K-feldspar from the granites suggest that this mineral doesn’t contain the original magmatic fluid, but fluids derived either from progressive cooling or from late hydrothermal alteration. The petrologic characteristics of the Serra Branca massif together with the fluid inclusions data are consistent with predominant magmatic source for the fluids rich in CO 2 (± CH 4 ). The petrographic and microthermometric data suggest that the aqueous fluids were entrapped later. It is proposed an evolution model in which an initially magmatic fluid has been released and mixed with external lower-temperature fluid. After the hydrothermal alteration and mineralization, there was entrapment of late fluids in fractures of the hydrothermal minerals, probably due to the deformation associated with the Brasilano orogenic event.