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

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 50-420 m 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 ( 87 Sr/ 86 Sr, 13 C/ 12 C, 18 O/ 16 O), trace-element concentrations (Fe, Mn, Mg, Sr, rare earth elements) and cathodoluminescence (CL) of the solid phase. The fluidinclusion data show that the calcites were precipitated mainly from relatively low-temperature (T h = 73-106°C) brines (13.4-24.5 wt.% CaCl 2 ), and the 87 Sr/ 86 Sr is more radiogenic than expected for Rb-poor minerals precipitated from Neoproterozoic fluids. These features, together with the distribution of d 13 C and d 18 O 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.

Re-Evaluating the Age of Deep Biosphere Fossils in the Lockne Impact Structure

“…wt% CaCl 2 corresponding to brine, and homogenization temperatures in the range of 77-218 • C [33], which at least for the salinities correspond to fluid inclusions in similar fracture coating calcite in Oskarshamn and Forsmark in Sweden, from southern Finland, and to Caledonian mineralizations [19,53,[69][70][71]. The δ 18 O values of calcite and quartz [33,72] at Lockne are also in line with compositions of the fracture-filling calcite reported from other parts of the Fennoscandian shield [19,53,[69][70][71]73]. Most of the fluid inclusion homogenization temperatures in the previous study [33] were in the 100-180 • C range, which is generally too high for microbial activity.…”

Syn- and postkinematic cement textures in fractured carbonate rocks: Insights from advanced cathodoluminescence imaging

“…Although optical-CL studies of opening-mode fractures in sedimentary carbonate rocks yield important insights (e.g., Trave et al, 1998;Hood et al, 2003;Holland and Urai, 2010;Vandergiste et al, 2013;Lavenu et al, 2013;Kotta, 2014), compared to SEM-CL, the low spatial resolution, magnification, and sensitivity of optical-CL means that only broad textural distinctions are possible rather than the fine resolution needed to document crack-seal gap deposits that track increments of fracture widening that may be only a few to tens of microns wide. Optical-CL studies of carbonate minerals reveal coarse zoning patterns between red-, blue-, and bright-and dull-luminescent areas from walls to inner parts of fractures in many deformed sedimentary (Trave et al, 1998;Hood et al, 2003;Berryman, 2013;Lavenu et al, 2013) and crystalline rocks (Blyth et al, 2004;Maskenskaya et al, 2014 Figure 2 are the first example of this type of structure in calcite fracture cement. Simplified diagrams of the basic textural types identified in this study are shown in Figure 9.…”

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb carbonate geochronology: strategies, progress, and limitations

“…Fracture mineralisation in the crystalline basement of southern Sweden has been investigated extensively to evaluate potential geological nuclear waste repository facilities. Several studies have shown that most trace element concentrations vary over an order of magnitude within calcite samples (at the thin section scale), and up to several orders of magnitude across individual fractures (Drake et al, 2012(Drake et al, , 2014Maskenskaya et al, 2014;Milodowski et al, 2018). These authors suggest that (1) trace element chemistry does not trace the source rock of the metals; (2) the co-variation in most trace elements implies changing metal / Ca ratios in the fracture waters; and (3) in situ factors affect trace element incorporation, such as microbial activity, metal speciation, crystal habit, water type, and coprecipitation of other phases such as barite and pyrite.…”