dc.description.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. | nb_NO |