Origin and timing of spilitic alterations in volcanic rocks from Gluszyca Górna in the Intra-Sudetic Basin, Poland.

The formation of spilitic assemblages (i.e. chlorite and albite) has been ubiquitously involved during the evolution of continental early-Permian volcanics from the Intra-Sudetic Basin (ISB). Based on the investigation of laccolith-type and variably-altered trachyandesite exposure in the vicinity of Gluszyca Górna (Lower Silesia, Poland), we have demonstrated that apatite fission-track dating (AFT), coupled with chlorite geothermometry, can be successfully applied to denote the timing of low-temperature alterations within volcanic rocks. The primary magmatic assemblages of the trachyandesites (i.e. augite and andesine-labradorite) have been affected by chloritization and alblitization respectively, followed by the formation of secondary titanite, celadonite, and calcite. The chlorite species have crystallized in the range of 106-170 °C, that exceeds Apatite Partial Annealing Zone (70-110 °C). The secondary, nearly pure albite (Ab ~ 99 mol.%) with weak to dark-brown cathodoluminescence replaces primary plagioclase (~ An37-50Ab47-58Or2-4) along the cleavage and/or twinning planes during Al3+-conservative reaction. The accessory apatite is marked by swallow-tail terminations indicative of rapid cooling formation conditions. It shows homogenous chemical composition, high F- content, and pink to yellow (REE3+ and Mn2+-activated, respectively) cathodoluminescence. Based on the AFT dating, the development of spilitic alterations within the early-Permian (ca 290 Ma) laccolith from Gluszyca could not only span the range of 182-161 Ma (Middle Jurassic), but also occurred prior to large-scale geological events in the ISB, such as burial under late-Mesozoic sediments, as well as tectonic inversion and exhumation. Whole-rock geochemistry of trachyandesites altered to various extent, indicates that original trace elements concentrations, except for i.e. Sr, Cs, and Ba, could be preserved during low-temperature alteration (spilitization). Meanwhile, geochemical fingerprint of the volcanics (i.e. humped-shaped mantle normalized trace element diagrams and positive Zr-Hf anomaly) points to the crustal contamination during magma evolution, combined with the mantle metasomatism in the source via subduction-derived components (i.e. fluids), as shown by i.e. low Nb/Th and Nb/LREE ratios.

Origin and occurrence of gem-quality, skarn-hosted barite from Jebel Ouichane near Nador in Morocco.

Light-blue barite from Jebel Ouichane in Morocco forms blade-like tabular crystals (up to ca. 10 cm) with superb transparency and lustre and represents one of the most spectacular gem-quality worldwide. The barite is hosted by iron-ore-bearing skarns, developed within Jurassic-Cretaceous limestones, and occurs in close spatial association with calcite. The crystals have their cores enriched in Sr and contain abundant monophase (liquid) fluid inclusions of primary and pseudosecondary origin. The barite probably precipitated slowly at a relatively low supersaturation and under the control of a surface reaction precipitation mechanism. However, there were some episodes during its formation with a fast growth rate and the coupled dissolution and recrystallization processes. A combination of fluid inclusion data and stable δ18O value for barite (+ 6.71‰ VSMOW) suggests that low-salinity barite-forming solutions resulted from the mixing of strongly-diluted meteoric waters (enriched in light oxygen isotope) with magmatic-hydrothermal fluids under low-temperature conditions (< 100 °C). Meanwhile, the mineralizing fluids must have been enriched in Ba, Sr, Ca, Mg, and other elements derived from the alteration of carbonate and silicate minerals in sedimentary and igneous rocks. The coupling between sulphur and oxygen isotope data (+ 16.39‰ VCDT and + 6.71‰ VSMOW, respectively) further suggests that barite crystallized in steam-heated environment, where SO42- derived from magmatic-hydrothermal SO2 reacted with sulphates that originate from the oxidation of H2S under near-surface conditions.

Characteristics and origin of agates from Plóczki Górne (Lower Silesia, Poland): A combined microscopic, micro-Raman, and cathodoluminescence study.

Agates from Plóczki Górne hosted by Permian basaltic rocks are predominantly made of length-fast chalcedony, and subordinately megaquartz and quartzine. Moganite occurs in traces mainly in transparent, outer, darker regions of white-grey coloured agates. Silica matrix of agates comprises a wide variety of solid inclusions represented by celadonite, plagioclases, hematite, goethite, barite, calcite, heulandite-clinoptyloite, nontronite-saponite, and Mn-dioxides (ramsdellite). Mineral phases are locally accompanied by black aggregations of carbonaceous matter, which gives a Raman signature of disordered carbon. These organic components were probably deposited from a hydrothermal fluids at low-temperature conditions and originated from sedimentary rocks found in the surrounding area of Plóczki Górne. The abundance of various SiO2 phases, mineral inclusions as well as various micro-textures (colloform, comb, feathery, and jigsaw-puzzle) in agates resulted from physicochemical fluctuations of SiO2-bearing mineralizing solutions at various stages of these gems formation. Agates from Plóczki Górne formed during post-magmatic stage of basaltic host rocks evolution. Not only were the hydrothermal fluids enriched in silica, but also they contained other elements such as Na, Ca, Al, Mg, Mn, Fe, Ba, SO4, and CO2, which were mobilized from host rocks or surrounding area.