Feldspar replacement reactions by interface-coupled dissolution-precipitation: a case study from the Larvik Plutonic Complex, SE-Norway

Abstract

Hydrothermal fluids can migrate through large amounts of impermeable crustal rock and are often associated with element fluxes and heat. This can change structural rock properties and locally result in ore deposits. To interpret fluid-rock interaction correctly, understanding their driving mechanisms is of vital importance. A stepwise insight in replacement processes is provided by the hydrothermal alteration of feldspar rocks in the Larvik Plutonic Complex in SE-Norway, which acts as a perfect natural laboratory.

Fluid-rock interactions are visible as a colour change: the blue larvikite rock is (partly) replaced by red tønsbergite. It started as a red discolouration around grains but at pervasive alteration, mineral replacement occurs and porous crystals surround the larvikite feldspar relicts. The degree of alteration varies throughout the area of interest. Textural and chemical analysis was done using scanning electron microscopy, X-ray tomography, an electron microprobe and XRF; trace elements were measured using an ICP-MS. The alteration is found to be zoned, from non-porous larvikite feldspar at the core to porous albite and orthoclase at the rim. Isocon diagrams are used to quantify element mobilisation. Surprisingly, major and minor element analysis show no large elements flux related to fluid flow. Instead, element redistribution has taken place when water entered the rock. This is possible when the infiltrating fluid was already nearly saturated in feldspar, co-precipitation of albite and orthoclase is the result of a changing fluid activity for Na+ and K+. Mineral textures were changed because of energy reduction, facilitated by the fluid. Trace elements have remained constant throughout the reaction. Feldspar re-equilibration with immobility of REE limits fluid temperatures to 250-300 °C and a neutral pH, and formed minor phases such as hematite and pumpellyite constrain the fluid pressures to 1-3 kb and slightly oxidizing conditions. The absence of element mobility rules out the process of albitisation and makes a link to the nearby Kodal ore deposits very unlikely.

Fluid flow is controlled by an interaction between grain boundary diffusion and reaction front migration through an interface-coupled dissolution-reprecipitation process. It involves atomic scale bond-breaking and dissolution of the primary mineral followed by pseudomorphic precipitation of the new, porous phase. The reaction proceeds by this porosity development, enabling fluid to remain in contact with the old phase at the reaction interface.