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Source and character of syntaxial hydrothermal calcite veins in Paleoproterozoic crystalline rocks revealed by fine-scale investigations

Journal article
Authors O. M. Maskenskaya
H. Drake
C. Broman
Johan Hogmalm
G. Czuppon
M. E. Astrom
Published in Geofluids
Volume 14
Issue 4
Pages 495-511
ISSN 1468-8115
Publication year 2014
Published at Department of Earth Sciences
Pages 495-511
Language en
Links dx.doi.org/10.1111/gfl.12092
Keywords brine, fluid inclusions, paleozoic calcite veins, rare earth elements, stable isotopes, FISSION-TRACK THERMOCHRONOLOGY, COMPOSITIONAL DATA-ANALYSIS, RARE-EARTH-ELEMENTS, FLUID-FLOW, FENNOSCANDIAN SHIELD, STATISTICAL-ANALYSIS, GRANITOID FRACTURES, GEOTHERMAL-FIELD, STABLE-ISOTOPES, WATER SYSTEMS, Geochemistry & Geophysics, Geology
Subject categories Geophysics, Geochemistry, Geology

Abstract

Calcite veins in Paleoproterozoic granitoids on the Baltic Shield are the focus of this study. These veins are distinguished by their monomineralic character, unusual thickness and closeness to Neoproterozoic dolerite dykes and therefore have drawn attention. The aim of this study was to define the source of these veins and to unravel their isotopic and chemical nature by carrying out fine-scale studies. Seven calcite veins covering a depth interval of 50-420m below the ground surface and composed of breccias or crack-sealed fillings typically expressing syntaxial growth were sampled and analysed for a variety of physicochemical variables: homogenization temperature (T-h) and salinity of fluid inclusions, and stable isotopes (Sr-87/Sr-86, C-13/C-12, O-18/O-16), trace-element concentrations (Fe, Mn, Mg, Sr, rare earth elements) and cathodoluminescence (CL) of the solid phase. The fluid-inclusion data show that the calcites were precipitated mainly from relatively low-temperature (T-h=73-106 degrees C) brines (13.4-24.5wt.% CaCl2), and the Sr-87/Sr-86 is more radiogenic than expected for Rb-poor minerals precipitated from Neoproterozoic fluids. These features, together with the distribution of C-13 and O-18 values, provide evidence that the calcite veins are not genetic with the nearby Neoproterozoic dolerite dykes, but are of Paleozoic age and were precipitated from warm brines expressing a rather large variability in salinity. Whereas the isotopic and chemical variables express rather constant average values among the individual veins, they vary considerably on fine-scale across individual veins. This has implications for understanding processes causing calcite-rich veins to form and capture trace metals in crystalline bedrock settings.

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