Hydrothermal fracture mineralization in the stable cratonic northern part of the Baltic Paleobasin: sphalerite fluid inclusion evidence

Abstract

Calcite–sphalerite-bearing veins in a Silurian carbonate sequence in the Kalana quarry, central Estonia were studied to characterize the properties of the mineralizing fluids in the northern Baltic Paleobasin. This part of the basin has traditionally been considered to be devoid of any major tectonic and/or hydrothermal activity since the beginning of Paleozoic, although occurrences of Pb–Zn mineralization accompanying secondary dolomitization have earlier been described in relation to fracture zones that intersect the Paleozoic sedimentary sequences in the basin. Syntaxial vein fillings in the Kalana quarry occur as blocky to elongate-blocky calcite aggregates, sphalerite-pyrite and minor galena. Primary two-phase liquid–vapor inclusions in the sphalerite suggest a NaCl–CaCl₂–H₂O fluid composition, with the salinity of the fluids ranging from 24.3 to 27.9 wt% CaCl_2 eq. The homogenization temperatures for the sphalerite two-phase inclusions vary between 64 and 200 °C. The mineralization style, fluid characteristics and tectonic setting of these calcite–sphalerite veins and the Pb–Zn mineralization in the northern part of the Baltic Paleobasin is mostly similar to those of Mississippi Valley-Type ores that can be constrained by cross-cutting relationships to the early Middle Devonian (Eifelian). They coincide with the fluid-driven illitization of K-bentonite beds in the northern Baltic Paleobasin. The veins are thus younger than the Pb–Zn mineralization event at the Scandinavian Caledonian front.

Keywords:

• Caledonides
• MVT
• Devonian
• Estonia

Acknowledgment

Authors are thankful to the staff of the Kalana (Otissaare) quarry for their full support and Mart Gaškov for XRF analysis, and Associate Editor Magnus Ripa and anonymous reviewers for their constructive criticism.