The mode of occurrence and mineral paragenesis of quartz calcite veins in Caledonian metamorphics around Trondheim fjord are described and discussed. Quartz and calcite are the major minerals; quartz occurring in all veins, calcite in the majority of them. Chlorite, albite, biotite, muscovite, tourmaline, epidote and a few ore minerals are present in some veins in decreasing abundance. The localization of the veins is in part controlled by the mechanical behavior of the country rocks in relation to the stresses which have prevailed during the regional metamorphism of the area.
It is concluded that the veins have been emplaced at a very slow rate contemperaneous with the regional recrystallization in the area, partly as tension fracture fillings, partly as volume-by-volume replacement bodies.
The vein minerals appear to have grown in pace with the dilation of the fractures so that empty or fluid-filled openings were hardly existing at any stage during the evolution of the veins.
Some of the elements of the vein minerals are extracted from the immediate adjacent rocks; the source of other elements is less certain.
The typical vein paragenesis in greenstone host rocks is quartz, calcite, chlorite and albite. It is shown by thermodynamics that such a mineral assemblage would tend to develop at sites of tensile - or minimum compressive - stress provided C02 is a very mobile constituent and the mineral assemblage in the host rock in bulk is actinolite, albitic plagioclase and quartz.