The magmatic-hydrothermal transition in S-type, peraluminous granites: Examples from Cornwall, UK, and Helgeland, Norway

Abstract

The magmatic-hydrothermal transition is the transition from silica melt to aqueous fluid. This process is fundamental for magmatic-hydrothermal ore deposits, and direct evidence of the process can be observed in fluid and melt inclusions in minerals, and by chemical and textural analyses of minerals. This thesis tracks the magmatichydrothermal transition in the Sn (-Cu) mineralized Land’s End granite, SW England, and investigates the hydrothermal overprint in a regional metamorphic terrane associated with W (-Mo) mineralizations in the Bjellatinden area, Northern Norway. Analyses of fluid inclusions, isotopic, major and trace element compositions of minerals have been the key methods in this study. The first part of the study investigates the crystallization and emplacement history of the Land’s End granite. Trace elements and cathodoluminescence textures in quartz, combined with fluid inclusions are used to constrain the pressure and temperature during crystallization. Phenocrysts grew in a magma chamber at ca. 18 – 20 km at water undersaturated conditions, and was transported together with the magma to the emplacement depth at ca. 5 – 9 km. The magma experienced water saturation and exsolved an aqueous phase prior to and at the emplacement level. Multiple recharges of new magma and fluids into the shallow magma chamber created a chaotic fluid inclusion chronology. In the late stages of crystallization, a borosilicate melt separated from the silicate melt, migrated through the crystal-silicate melt mush and coalesced to form the quartztourmaline orbicules observed in the Land’s End granite. Tourmaline from these orbicules has similar major and trace element chemistry as tourmaline in the granite matrix. A slight change in boron isotopic composition, from the granite matrix to the inner parts of the orbicules, can be explained by Rayleigh fractionation. A generation of blue tourmaline associated with hydrothermal quartz textures overgrows the main brown generation, and is interpreted to represent the hydrothermal side of the transition. Tourmaline associated with cassiterite in the mineralized veins has not formed from the same fluids as the blue tourmaline generation in the orbicules, and orbicule formation is not directly related to ore formation. The final part of the thesis investigates fluid inclusions and mass transfer in the regional metamorphic Bjellatinden area. Three types of fluids are observed, and are interpreted to represent metamorphic fluids produced during exhumation and contact metamorphism by a proposed, not observed, underlying granitic intrusion. The earliest fluid generations caused scapolitization of hornblende schist along a vein set penetrating the schist. A high salinity fluid derived from the underlying magma overprinted the earlier alteration and caused K-metasomatism of the hornblende schist. The area experienced rapid exhumation, and a near isothermal pressure drop, before being heated by the intrusion, and subsequently cooled along a near isobaric P-T path.