Microstructures of hydrothermal carbonate veins. (a) Botryoidal carbonate with rhombohedral cleavage and rare deformation twins (type 1) collected from Akatore Creek. (b) Botryoidal carbonate, from Bruce Rocks, with radiating grains fracturing parallel to the vein surface, without any sign of deformation twins. (c) Syntaxial veins and small fibrous veins in a quartzofeldspathic layer from the altered greyschist. (d) Ca, Fe, and K element map of a symmetric syntaxial vein in a mica-rich greyschist layer, showing a siderite rim and calcite centre.