Iuliu Bobos

The Al-bearing scorodite from the Hemerdon Ball Mine (HBM) was studied using electron microscopy and microprobe analysis, single-crystal X-ray diffraction, infrared, and Raman spectroscopy. The crystal chemistry formula of Al-bearing scorodite is expressed as Fe³⁺_0.87Al³⁺_0.16(As_0.97O₄)·H₂O. The calculated d-spacings and unit-cell parameters of Al-bearing scorodite are slightly affected by the substitution of Al for Fe in the octahedral sites. The Al-bearing scorodite HBM crystalizes in the Pbca space group with the following unit-cell lattice parameters: a = 8.92882(14) Å; b = 10.02217(14) Å; c = 10.30525(15) Å; V(Å) = 922.18(2) and Z = 8. The lattice structure becomes slightly distorted by the formation of the Fe,Al-OH octahedron, which leads to a compression of the newly formed octahedron along the a* ^ b* direction and an expansion of the Fe-OH octahedron along the c* direction. The incorporation of Al³⁺ has a strong effect on the tilting angle of the Fe,Al-OH octahedron in the b* ^ c* crystallographic direction. The refined structure suggests that Al³⁺ occupies the octahedral sites alongside Fe³⁺, leading to a distortion of the Fe,Al-OH octahedron. Infrared and Raman spectroscopy exhibit a doublet at 820 and 800 cm⁻¹, and at 810 and 800 cm⁻¹ ascribed to the Fe,Al-O-OAsO₃ group. The 799–800 cm⁻¹ Raman region is assigned to the Fe–O–As group (at 798 and 803 cm⁻¹), whereas the 810–814 cm⁻¹ region is ascribed to a band resulting from the AsO₄³⁻ [υ₁ (A₁) symmetric stretching vibrational modes], indicative of the Fe,Al–OH–As group in both Al-bearing scorodite and mansfieldite.

Apuseni Mts. represent the biggest exhumed area of two major tectonic blocks of the Inner Carpathians: the Tisza and Dacia Mega-Units, with the Biharia Nappe System (NS) thrusted over the Codru NS, which in turn is thrusted over the Bihor Unit. Alpine metamorphism was previously detected in the Bihor Unit and the Biharia NS, but not in the Codru NS. This study is aimed at explaining this difference, using deformed Permian volcanic rocks from the Codru NS. Illite/mica is the dominant metamorphic mineral in the deformed felsic rocks, while chlorite dominates in the deformed mafic rocks. Illite/mica is a mixture of 1 M and 2M₁ polytypes (also 1M _d in one sample), perfectly non-swelling, and with the 0.31–0.35 Δ° 2θ range of the standardized Kübler index. Electron probe microanalysis (EPMA) documented two varieties of illite/mica: illite/phengite and illite/muscovite. Combined mineral metamorphic indicators (illite/mica characteristics, presence of epidote, and the absence of metamorphic biotite) restrict the maximum metamorphic temperatures to the 250–310 °C range, explaining the lack of Alpine ages in the earlier studies: temperatures below resetting of the old coarse-grained micas, which were used for dating. Our new K–Ar data constrain the cooling ages of illite/mica in the Codru NS to 117—112 ± 3.2 Ma in the Dieva and Moma nappes, and 102 ± 3 Ma in the Arieseni nappe. These ages correspond to the denudation during the eastward advancing of the thrust wedge during the “Austrian” tectonic phase. Graphical Abstract

Low-temperature geochemical processes are ubiquitous in geological areas related to weathering, diagenesis, very low-grade metamorphism, and post-magmatic processes, and provide an explanation of the P–T–X history of the Earth’s crust

Organic carbon and nitrogen fixed in illite (I) clays were identified in a hydrothermal breccia structure from the Harghita Bãi area of the Neogene volcanism of the East Carpathians. The potassium-illite (K-I) alteration related to an early magmatic-hydrothermal event (9.5 ± 0.5 Ma) was later replaced by ammonium-illite (NH₄-I) (6.2 ± 0.6 Ma) owing to circulation of an organic-rich fluid. Several textural evolutions of breccia structures were recognized, such as ‘shingle’, ‘jigsaw’, ‘crackle’ and hydraulic in situ fractures. The high-field-strength element behaviours of the K-I and NH₄-I argillic altered andesite are close to chondritic ratios, indicating no contribution of hydrothermal fluid, especially on NH₄-I andesite alteration and the CHArge-and-RAdius-Controlled (CHARAC) behaviour within silicate melts. The rare earth element normalized patterns show two distinct trends, one with a Eu* anomaly (K-I) and the other with a Nd* anomaly (NH₄-I), indicating a boundary exchange with the organic-rich fluid. The strongly depleted δ¹³C (V-PDB) measured for the NH₄-I clays reflects values (−24.39 to −26.67 ‰) close to CH₄ thermogenic oxidation, whereas the δ¹⁵N (‰) from 4.8 to 14.8 (± 0.6) confirmed that the N₂ originated from post-mature sedimentary organic matter. The last volcanism (6.3 to 3.9 ± 0.6 Ma) and simultaneous volcano-induced tectonics in the proximity of the eastern Transylvanian basin basement increased the heat flow, generating lateral salt extrusion, diapirism and increased pressure in the gas reservoir. New pathways were opened that provided new circulation routes for basinal fluids to new and old permeable zones and expelled seeps from the biogenic petroleum system.

Scheelitization of Mn-bearing wolframite, scheelite, quartz, and Fe,Mn-chlorite veins was identified in the W, (Cu,Mo) ore deposits of Borralha, by optical microscopy, electron-microprobe analysis, and stable isotope geochemistry. Fluid inclusions derived scheelite crystallization temperature was compared with the oxygen isotope temperature estimated. Scheelite was formed mainly during stage I from a low salinity aqueous-carbonic fluid dominated by CO₂, where the homogenization temperature (T_h) decreased from 380 °C to 200 °C (average of 284 °C). As temperature decreased further, the aqueous-carbonic fluid became dominated by CH₄ (Stage II; (average T_h = 262 °C)). The final stage III corresponds to lower temperature mineralizing aqueous fluid (average T_h = 218 °C). In addition, salinity gradually decreased from 4.8 wt.% to 1.12 wt.%. The δ¹⁸O_Fluid values calculated for quartz-water and wolframite-water fractionation fall within the calculated magmatic water range. The ∆_{quartz-scheelite} fractionation occurred at about 350–400 °C. The ∆_{chlorite-water} fractionation factor calculated is about +0.05‰ for 330 °C, dropping to −0.68‰ and −1.26‰ at 380 °C and 450 °C, respectively. Estimated crystallizing temperatures based on semi-empirical chlorite geothermometers range from 373 °C to 458 °C and 435 °C to 519 °C. A narrower temperature range of 375 °C to 410 °C was estimated for Fe,Mn-chlorite crystallization.

Acid-sulfate alteration is comprised by clays, sulfate, sinter and native sulphur minerals crystallized as neoformation products from dissolution of primary minerals during water-rock interaction. Smectite, kaolinite, halloysite-7 Å and opal-A occur in assemblages with alunite. Smectite represents a mechanical mixture between two (propylitic and acid-sulfate) alteration types. High amounts of high-field strength elements (HFSE) and rare earth elements (REE) were measured in acid-sulfate rocks. The Nb vs. Ta and Zr vs. Hf show a positive trend and a widely scattered relationships, suggesting a large fractionation during acid-sulfate alteration. Higher ∑REE amounts (up to 934.5 ppm) were found in clay-sulfate assemblages and lower ∑REE amounts in sinter (opal-A ± sulfate, 169.05 ppm) than to fresh rocks (up to 751.2 ppm). The acid-sulfate rocks reveal a distinctive gull-wing chondrite-normalized pattern with a negative Eu anomaly and light- and heavy-REE “wings” similar to the gull-wing pattern of fresh rocks. The Eu/Eu* shows a large fractionation of acid sulfate rocks from 0.16 to 0.78 with respect to fresh trachyte products (0.10 to 0.38). Variation of (La/Sm)_N and (La/Yb)_N ratio show a large fractionation of light-REE and heavy-REE. The Y vs. Dy and Y vs. Ho show a very good positive correlation coefficient and a large Y fractionation in acid-sulfate rocks with respect to fresh rocks.

Nitrogen influx was identified in the Harghita Bãi area, where the mechanism of NH₄⁺-fixation in illitic clays is relevant for the N-input budget estimation. The nanotextural features of K-illite (K-I), NH₄,K-I and NH₄-illite-smectite (NH₄-I-S) mixed layers observed in argillic-altered andesitic rocks from the hydrothermal area of Harghita Bãi (East Carpathians) were studied by X-ray diffraction, infrared spectroscopy and transmission and analytical electron microscopy (TEM-AEM). The texture of undisturbed argillic-altered andesite rocks exhibits chaotic intergrowths of randomly oriented and curved illitic packets with abundant pore spaces and high porosity between packets. The TEM images of K-I and NH₄,K-I intergrowths show subparallel packets with clear contacts, exhibiting a diffuse contrast across layers. The thicknesses of K-I and NH₄,K-I packets range from 150 to 500 Å, and 1M_d and 1M polytypes were identified by selected area electron diffraction patterns. Crystal chemistry of K-I, NH₄,K-I and NH₄-I-S was carried out by AEM. A third interlayer cation Na⁺ beside K⁺ was detected in several NH₄,K-I packets. The NH₄,Na,K-I packets interleaved with NH₄,K-I or NH₄-I-S (12% smectite layers) packets were also identified by TEM. The thicknesses of NH₄,Na,K-I packets range from 300 to 1200 Å, with abundant lenses and lenticular layer separation along the boundaries between them. The 1M_d polytype dominates the NH₄,Na,K-I packets. Straight and parallel packets, continuous 00l layers and collapsed swelling layers at the boundary of individual NH₄-I (5% smectite layers) packets with thicknesses ranging from 20 to 95 Å were observed. The nanotextural observations indicate direct crystallization of NH₄-I crystals within a NH₄-I-S series from a pore fluid, where NH₄-I packets occupy void spaces previously occupied by fluids.