Extensive fracture systems in the Skaergaard intrusion, including veins, sills and dikes, constitute channels for the flow of magmas and hydrothermal solutions during the subsolidus cooling and deformation of the intrusion. The abundance, orientation, transgressive relations and mineralogy of these fracture systems provides a record of fluid transport and chemical reactions during the Skaergaard's subsolidus cooling history.
The earliest mineralized fractures in the Layered Series are gabbroic pegmatites that occur as veins, sills and irregular patches. These structures are crosscut by leucocratic granophyres. Veins filled with hydrothermal minerals occur throughout the Skaergaard intrusion and its host rocks. Hydrothermal minerals in veins are similar to mineral assemblages found in metabasalts ranging in grade from upper amphibolite to zeolite facies. The earliest hydrothermal veins contain upper amphibolite facies assemblages. They are <1-2 mm wide, continuous for tens of metres and occur with frequencies of 0.2 to > 1.5 and 1.4 to >20 fractures/metre in the Layered Series, and Marginal Border Group and contact metamorphosed basalts, respectively. These early veins are inferred to have formed at temperatures of >500° to 750°C and <925° to 960°C. Later vein types contain mineral assemblages characteristic of lower amphibolite, greenschist and sub-greenschist facies and occur as more localized fracture systems than the early, high temperature vein types. Compositions of secondary minerals from all of these hydrothermal vein types show systematic changes relative to both the overall fractionation trend and local modal variations in the igneous mineralogy of the Skaergaard intrusion.
Observed phase relations and fracture abundances are, in general, consistent with isotopic and transport characteristics of the Skaergaard magma-hydrothermal system reported by Taylor & Forester (1979) and Norton & Taylor (1979).
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