Sabina Strmić Palinkaš

The role of magmatic and hydrothermal processes in the evolution of Be-bearing pegmatites: Evidence from beryl and its breakdown products

Abstract Beryl and euclase crystals from the Mina do Santino and the Jacú pegmatites in the Borborema Pegmatite Province in northeastern Brazil contain several generations of melt and fluid inclusions, which allow interpretation of P-T-X conditions responsible for beryl crystallization and for alteration of a primary pegmatitic mineral assemblage to a mixture of hydrothermal minerals (euclase, bertrandite, kaolinite, and quartz). Primary melt and fluid inclusions hosted by beryl were trapped simultaneously. However, their homogenization temperatures are significantly higher (870-900 °C) than the values previously reported for pegmatitic systems (<712 °C) and should be treated with caution. An isobaric drop of temperature resulted in the exsolution of a fluid. A low-salinity CO2-enriched phase and a saline water-rich phase were trapped in pseudosecondary inclusions in beryl at a pressure of 2.1-2.7 kbar and temperature of 390-480 °C. Cooling of the country rocks below 400 °C caused a ductile-to-brittle transition and allowed infiltration of cold groundwater, which further decreased the temperature in the system to 190-240 °C. At the same time, the pressure dropped from a lithostatic (2.1-2.7 kbar) to a hydrostatic value (0.57-0.73 kbar). Consequently, minerals deposited under magmatic conditions (feldspars and beryl) became unstable and a newly formed hydrothermal mineral paragenesis (euclase, bertrandite, kaolinite, and quartz) overprinted the earlier one. The hydrothermal fluids responsible for the alteration differ from the earlier-exsolved fluids in having a lower salinity, lower homogenization temperature, the absence of CO2, and the presence of CH4.

Permian–polysulphide-siderite–barite–haematite deposit Rude in Samoborska Gora Mts., Zagorje–Transdanubian zone of the Inner Dinarides

Samoborska Gora Mts. is situated within the westernmost part of the Zagorje–Mid–Transdanubian zone of the Internal Dinarides. The Samoborska Gora Mts. predominantly consists of Permian unmetamorphosed siliciclastic sediments and evaporites, overlain by Lower Triassic sediments. Rude mineralisation is hosted by Permian siliciclastic sediments, below gypsum and anhydrite strata. The central part of the deposit consists of a 1.5 km long stratabound mineralisation, grading laterally into ferruginous sandstone and protruding vertically into a gypsum–anhydrite layer. Siderite–polysulphide–barite–quartz veins are located below the stratabound mineralisation. The stratiform part ofthe deposit is situated above the stratabound and consists of haematite layer with barite concretions and veinlets. Late stage galena–barite veins overprint earlier types of mineralisation. The Rude ore deposit was generated by predominantly NaCl±CaCl 2 –H 2 O solutions. Detrital quartz from stratiform mineralisation contains fluid inclusions with salinities between 7 and 11 wt. % NaCl equ., homogenizing between 150°C to 230°C. Stratabound/siderite–polysulphide–barite–quartz vein type mineralisation was derived from solutions with salinities between 5 and 19 wt. % NaCl equ., homogenizing between 60°C and 160°C, while late stage galena–barite veins were precipitated from solutions with salinities between 11 and 16 wt. % NaCl equ., homogenizing between 100°C to 140 °C. Fluid inclusion bulk leachate chemistry recorded Na + >Mg 2+ >K + >Ca 2+ >Li + and Cl - >SO 4 2- ions. Sulphur isotope composition of barites and overlying gypsum stems from Permian seawater sulphate, supported by increased Br - content, which follows successively the seawater evaporation line. The sulphur isotopic composition of sulphides varies between –0.2 and +12.5 ‰, as a result of thermal reduction of Permian marine sulphate. Ore–forming fl uids were produced by hydrothermal convective cells (reflux brine model), and were derived primarily from Permian seawater, modified by evaporation and interaction with Permian sedimentary rocks. Rude deposits in Samoborska Gora Mts. may be declared as a prototype of the Permian siderite–polysulphide–barite deposits (products of rifting along the passive Gondwana margin), in the Inner Dinarides, and their equivalents extending northeastward into the Zagorje–Mid–Transdanubian Zone and the Gemerides, and southeastward to the Hellenide–Albanides.