Origin of gem-quality barite at Jebel Ouichane district in Nador, Morocco: implications from integrated fluid inclusions, stable isotopes, and geochemistry

Light-blue barite from Jebel Ouichane in Morocco forms blade-like tabular crystals (up to ca. 10 cm) with superb transparency and lustre. It represents one of the most spectacular gem-quality specimens in the world. The barite is hosted by iron-ore-bearing skarns, developed within Jurassic-Cretaceous limestones, and occurs in close spatial association with calcite. Although it exhibits simple chemical composition, some irregular sectorial zoning, maintained by elevated contents of Sr in various crystal domains, were found. Barite from Ouichane is abundant in one phase (liquid or gas) or two-phase (liquid-gas) fluid inclusions of primary, pseudosecondary, and secondary origin. A combination of fluid inclusion microthermometry and stable isotope data suggest that δ 18 O value of barite-forming solutions, resulting from mixing of meteoric waters with hydrothermal fluids, could be in the range from -3,5‰ to +2,7 ‰ (VSMOW), whereas the main barite crystallization stage falls in the compounds. The combination of δ 34 S and δ 18 O values of barite (+16.39‰ and 6.71‰, respectively) indicate that its formation occurred in a steam-heated (near-surface) environment.

thin veins intersecting the rock. Mn-oxides, chalcopyrite, and pyrite are accessory components of the rock.

Microscopic observations
Barite crystals analysed both in parallel and perpendicular sections host an abundance of fluid inclusions exhibiting various shapes that slightly resemble flags, feathers, or wings.
Most of the inclusions are one-phase and composed of either liquid or gas (Fig. 4A). Only some of them exhibit characteristic tails (Fig. 4B) triggered by the necking down process 14 .
Rarely, two-phase (liquid and vapour) inclusions were observed. The size of inclusions usually ranges from a few μm to 30 μm, although larger inclusions were also found. Moreover, the numerous groups of rectangle-shaped and violet-coloured inclusions were also observed (Fig.4D). They are one phase inclusions composed of liquid. Their geometry and arrangement may suggest pseudosecondary or secondary genesis. A concentration of the solid phase with a brown colour resembling an organic substance was also observed within the host barite.
They occur the most often in the central part of calcite crystals in the form of densely packed individuals (Fig.5A). The size of the inclusions range from ~5 to ~50 µm and their shape is irregular. In the outer part of the crystals, the solid inclusions form zones and disappear completely in the outermost parts of the crystals. Fluid inclusions in calcite crystals are very rare. They form very small (up to ~2 µm) individuals filled with liquid phase (Fig.5B).

Microthermometry of fluid inclusions
Primary inclusions were distinguished based on their mode of occurrence following the rules provided by Roedder 14 . They mostly form randomly distributed groups, followed by scarce, isolated individuals. In general, one-phase inclusions predominate over two-phase inclusions.
During heating experiments, all inclusions underwent homogenization to the liquid phase at temperatures ranging from 143.3 to 243.7°C (Tab.1). If a few highest temperatures (above 200°C) are not considered with the other results, the narrow range of Th might be representative for conditions of barite precipitation.
During freezing experiments, the initial ice melting took place at temperatures from the range of -35.9 to -41.5 ºC (Tab.1). It means that the chemical composition of inclusion is more complex with some divalent cations such as Ca 2+ and Mg 2+ . The melting temperature represents the little diversified values ranging from -0.2 to 0.0°C and corresponding to the very low salinity, which spans from 0.00 to 0.35 % NaCl eq. 15 .

Discussion
The

Temperature condition of barite precipitation
Barite, as a soft and cleavable mineral, provides a very challenging material for microthermometric analysis due to the high susceptibility of its inclusions to stretching or decrepitation. As concluded by Ulrich and Bodnar 23 , the appearance of internal pressure required to initiate stretching of inclusions might result from samples overheating during the microthermometric experiments. Despite those methodological difficulties a lot of barite crystals representing various geological systems were successfully examined using microthermometry with the conclusions on their origin and metallogenesis [24][25][26][27][28] . Barite-hosted inclusions might provide valuable data on p-T conditions of host crystal growth; however, the measurements must be performed with special care. In the studied case no stretching was observed as it was supposed from the chemical composition of inclusions revealed by Raman micro-spectroscopy to be dominated by H2O. Ulrich and Bodnar 23  The predominance of monophase inclusions in barite is considered as an indicator of lowtemperature conditions of crystals growth; however microthermometric data indicate a rather low-to a medium-temperature hydrothermal environment of its precipitation. Hence, the precipitation temperature of barite was estimated also using the isotope fractionationtemperature equation proposed by Kusakabe and Chiba 29 , i.e.: It was assumed that the oxygen isotopic composition of barite-forming fluid corresponds to the value for meteoric waters, i.e. -7.0‰ (VSMOW) 30 . As a result, the determined temperature was calculated at 106°C, which is lower than the values obtained from microthermometry. Thus, the discrepancy between temperatures obtained from isotopic data and fluids inclusion studies could be explained by the mixing of meteoric and hydrothermal fluids of unknown δ 18 O characteristics. To obtain a similar temperature range of barite crystallization as from microthermometry, δ 18 O characteristics of mineralizing fluid should be assumed to follow the range of -3,5‰ to +2,7 ‰ (VSMOW). The interpretation of our results stays in agreement with data obtained by Bouabdellah et al. 13 , who concluded that sulphides and calcite-barite assemblages hosted in skarn of Ouichane deposit were deposited due to mixing of hydrothermal fluids with external dilute solutions. Much earlier, under the temperature range of 500-400 ºC, iron oxides (magnetite-hematite) had crystallized 13 .

Timing and origin of barite mineralization
The sulphur isotope data of barite (+16.39 ‰VCDT) could be used as a premise for the nature of Ba and SO4-rich fluids. Such high isotopic value is probably associated with the migration of barite-forming solutions within rocks enriched in the sulphur of organic origin 31-33 , as it is generally accepted that organosulphur compounds are enriched in heavy S isotope relative to the coexisting sulphides 34 . Additionally, the sulphur isotopic composition of the barite is not only consistent with the values adopted for evaporates of the Mesozoic age 35 , but also covers the range of hydrothermal sulphates described by i.e. Jurkowić et al. 36 . For the oxygen delta, the isotopic value found at +6,71 VSMOW ( Fig. 10) is characteristic of meteoric water infiltration 37 , but also stays in agreement with the composition of fluids related to volcanic activity (Fig. 10). To determine the source of sulphate necessary for barite crystallization three potential alternatives were discussed: (1) direct dissolution of evaporites, (2) oxidation of sulphide minerals, or (3) mineralization of organic substance 41 . In Nador, evaporite deposits are not known to exist in the rock sequences of that area, so they could not be considered as a possible source of the sulphate. The abundant amount of pyrite was found within the Jurassic-Cretaceous limestones that occupy the study area 13 . Hence, the production of sulphate as a result of sulphide oxidation might seems to be probable in this geological setting. On the other hand, both pyrite and barite are low-temperature minerals that could both form contemporaneously, as a result of hydrothermal activity. Finally, the enrichment in heavy S isotope data supports the organic origin of sulphur, which could be derived from i.e. thermal decomposition of organosulphur compounds in the limestones 33 Samples devoid of two-phase inclusions at room temperature were previously cooled to induce the vapor nucleation. Such an approach has been previously presented by many authors 24,25 with no stretching effects on inclusions.

X-ray Fluorescence spectrometry
The Energy-dispersive micro X-ray Fluorescence Spectrometry analysis was

Raman micro-spectroscopy
Raman spectra of barite and inclusions hosted in them were recorded with a Thermo Scientific DXR Raman microscope featuring 10x, 50x, and 100x magnification objectives.
The samples were excited with a 532 nm high-power laser. Laser power was from 5 to 10 mW, the exposure time was 3 s, the number of exposures-10 times. The laser focus diameter was approximately 2.1-0.7 mm. The spectra were corrected for background by a method of a sextic polynomial using Omnic software. Raman analyses were made both on clean cleavage surfaces and doubly polished wafers. Raman studies were performed in the same analytical spots of barite, for which chemical analyses were carried out using the EMPA method to trace the differences in the position of individual Raman bands in the points differing in the Sr contents.

Isotope analyses
The isotope ratios of barite (δ 34 S and δ 18 O) were determined by measuring the isotopic composition of SO2 and CO2 gases on a dual-inlet and triple collector mass spectrometer.
Sulphur in the form of SO2 gas was quantitatively extracted from the BaSO4 sample by thermal decomposition at 850 °C in a Cu boat in the presence of Na2PO4 reagent 43,44 . CO2 gas was prepared by graphite reduction with the conversion of CO to CO2 by glow discharge 45 .
Nearly quantitative CO to CO2 conversion was attained using a magnetic field in the conversion unit 46  For the accompanying calcite, the δ 13 C and δ 18 O values were determined as well. CO2 gas was extracted from calcite at 25 °C by reaction with H3PO4 47 and measured on an isotope-ratio mass spectrometer with a dual-inlet system. The standard deviations of measurements for the NBS19 international standard were better than 0.1‰. Delta values were normalized to the Vienna Pee-Dee Belemnite (VPDB).

Acknowledgments
We are grateful to Tomasz Praszkier for providing samples for the study and data on the Tab.2. Chemical composition of barite in weight % (wt. %), measured by Energy-dispersive μX-ray Fluorescence Spectrometry, recalculated to fixed SO3 content and a total of 100 (wt. %). Al2O3, MgO, MnO, and Na2O contents were below the detection limit. Error is 0.00 for all (in wt. %, 1 Sigma).