Lazulite and Ba, Sr, Ca, K-rich phosphates–sulphates in quartz veins from metaquartzites of Tribeč Mountains, Western Carpathians, Slovakia: Compositional variations and evolution

The phosphate–sulphate mineralization occurs in quartz veins in Lower Triassic metaquartzites of the Tribeč Mts., Central Western Carpathians, Slovakia. The mineralization comprises of lazulite, Ba, Sr, Ca, K-rich phosphates–sulphates and barite in an association with muscovite, hematite, locally rutile, zircon, chlorite and tourmaline. The most widespread lazulite forms up to 10 cm large pale to deep blue aggregates in massive quartz. Electron-microprobe analyses show a relatively uniform composition with Mg/(Mg + Fe) =0.85 to 0.93. The Mössbauer spectroscopy reveals 11–30% Fe3+/Fetotal. Possible primary goedkenite–bearthite binary s.s. shows the highest known Sr contents worldwide: Sr/(Sr + Ca) = 0.67–0.71; Mg, Ba and REE contents are negligible. The lazulite is replaced by a secondary association of Ba, Sr, Ca, K-rich phosphates–sulphates: gorceixite, rarely goyazite, crandallite, svanbergite, jarosite and a rare phase, close to (Ba,K,Sr)(Fe3+,Al)3[(OH,H2O)6(PO4)(SO4)] composition (Ba,Fe,S,P-phase). Gorceixite exhibits more restricted compositional variations between gorceixite–goyazite and gorceixite–crandallite s.s.: Ba/(Ba + Sr) = 0.73–0.99, Ba/(Ba + Ca) = 0.78–0.99 and (P − 1)/[(P − 1) + S] = 0.84–0.99. On the contrary, the secondary Sr, Ca-dominant phosphates–sulphates of the crandallite and beudantite groups show wide compositional variations and complex quarternary solid–solution series between goyazite–crandallite and svanbergite–woodhouseite with Sr/(Sr + Ca) = 0.16 to 0.99 and (P − 1)/[(P − 1) + S] = 0.07 to 0.97. The K, Ba-dominant phosphates–sulphates of the alunite and beudantite groups occur along jarosite–Ba,Fe,S,P-phase s.s. line with Ba/(Ba + K) = 0.07 to 0.56, Fe/(Fe + Al) = 0.55 to 0.99, P/(P + S) = 0.14 to 0.57 and elevated Sr and Ca (up to 0.24 and 0.12 apfu, respectively). The compositions indicate a close relationship and mutual substitutions between the crandallite, beudantite and alunite groups. Unlike to analogous phosphate-bearing assemblages in the Alps, investigated phosphate–sulphate association doesn't contains REE, Y and Sc minerals but it is rich in Ba-phases (barite, gorceixite). The peak metamorphic conditions of the host rocks estimated using the Kübler index of phyllosilicates point to anchizone/epizone boundary, i.e. ca. 270–350 °C. Fluid inclusions study constrained the minimum formation temperatures of the lazulite to 144–257 °C and of the superimposed sulphate–phosphate mineralization to 175–289 °C. Lazulite crystallized from brines of the system H2O–Na–Mg–Cl–CO2 with a salinity of 17.2 to 19.8 wt.% NaCl eq. We propose, that the studied mineralization originated from fluids enriched in elements from breakdown of feldspars, biotite, apatite and other phosphates in underlying Hercynian granites. The fluids passed upwards into the metaquartzites and precipitated discrete minerals, due to absence of any suitable sink for the elements among rock-forming minerals.