EGU25-523, updated on 14 Mar 2025
https://doi.org/10.5194/egusphere-egu25-523
EGU General Assembly 2025
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
Poster | Wednesday, 30 Apr, 08:30–10:15 (CEST), Display time Wednesday, 30 Apr, 08:30–12:30
 
Hall X1, X1.168
Effect of a partial melting on the development of ore-forming fluid: a case study from the Varena Iron Ore Deposit, SE Lithuania
Laurynas Šiliauskas and Grazina Skridlaite
Laurynas Šiliauskas and Grazina Skridlaite
  • Nature Research Center, Bedrock geology, Vilnius, Lithuania (laurynas.siliauskas@gamtc.lt)

The development of high-grade Cl brines is linked to some of the major mineral deposits, such as IOCG and skarns, that are often characterized by wide aureoles of scapolitization. In terranes of enhanced metallogenesis, the prograde track scapolitization can be obscured by the later intense hydrothermal reworking, making it difficult to reconstruct the prograde fluid development. For these reasons, studying the unmineralized country rocks in a deposit vicinity could provide otherwise erased information on deposit formation. The augen orthogneisses from scapolitization aureoles, located beneath the Varena Iron Ore Deposit (VIOD), in the East European Craton (Lithuania) were chosen for this study. The rocks are composed of Cpx-Amf-Bt-Pl-Kf-Scp-Qtz-Ilm-Mag-Ttn. They range from domain-structured gneiss with Pl phenocryst remains (partly or entirely replaced by Scp) in distal parts (samples D8 and D9), to almost pure scapolite rock at contact with the iron ore (sample V987), suggesting re-equilibration at different temperature and varying fluid composition and fluid/rock ratios.

Scapolite replacing Pl phenocrysts (D8 and D9) from the prograde metamorphic assemblage of Cpx+Pl+Kf+Bt+Qtz+Ilm in the mafic domains has a Cl content increasing from 0.44 apfu in the core to 0.87 apfu in the rim.  The matrix scapolite forming channel-like patchwork in the felsic matrix has Cl content up to 1 apfu and is in equilibrium with the peak-temperature Mg-Hst (750 °C, sample D9). Similar Ap-Bt temperatures of 694-766 °C were obtained in the sample D8. Biotite is thinning out towards the mafic domain centre, where it completely disappears, suggesting a partial melting of the biotite and formation of the Pl+Kf+Qtz+Scp felsic matrix around the mafic domains.

An inverse relationship in scapolite chlorinity was observed in samples with high fluid/rock ratios (V987). Here, the Cl content of 0.64-0.89 apfu is recorded in the blocky scapolite, surrounded by an analcime-scapolite (Cl content of 0.32-0.76) matrix with minor calcite and anhydrite. The Cl content in matrix scapolite is decreasing towards the contact with the ore. This indicates a change in fluid regime and its chemistry during the retrogression, with decreasing chlorinity and increased oxygen fugacity.

High Cl content in scapolite at the estimated peak conditions suggests the presence of a fluid with high aCl and low aH2O. Domain structure, dehydrated biotite and dark-CL metamorphic zircon rims are in favour of partial melting at the peak temperatures. Water is highly partitioned into the silicate melt, whereas chlorine solubility in silicic melts is very limited and is usually retained in the fluid phase. Thus, in a rock-buffered fluid, partial melting could shift fluid composition in the ternary system H2O-CO2-NaCl towards or into the “halite” stability field, producing molten salts, capable of mobilizing elements such as Fe, REE, U and Th.

How to cite: Šiliauskas, L. and Skridlaite, G.: Effect of a partial melting on the development of ore-forming fluid: a case study from the Varena Iron Ore Deposit, SE Lithuania, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-523, https://doi.org/10.5194/egusphere-egu25-523, 2025.