Transition of granite to quartz-kyanite rock at Hålsjöberg, southern Sweden: Consequence of acid leaching and later metamorphism

Abstract

Small deposits of rare Al-phosphates, rutile and abundant kyanite occur at Hålsjöberg, Diksberg, Hökensås and Västanå in southern Sweden. Gradual transitions between magmatic rocks and high alumina rocks consisting of quartz, kyanite, rutile and Al-phosphates have been noted at Hålsjöberg and Hökensås. These transitions reveal that hydrothermal leaching formed the high alumina assemblage in a process where alkalis and other rock forming elements have been leached, leaving a residual rock mainly composed of high alumina minerals and quartz. A partly amphibolitised dolerite cuts the high alumina rock at Hålsjöberg postdating this hydrothermal event. Textural relations, character of alteration and considerations of the origin of acid fluids suggest that hydrothermal leaching driven by magmatic activity occurred at brittle conditions before metamorphism. Alteration of intrusive rocks and the structurally controlled character of alteration furthermore suggest a magmatic hydrothermal environment. The occurrence of rare Al-phosphates in high alumina rocks at Hålsjöberg and Hökensås suggests a temporally related origin of hydrothermal leaching and formation of Al-phosphates, although subsequent regional metamorphism to a large extent determined the present assemblages.

Keywords:

• Hydrothermal alteration
• advanced argillic alteration
• Protogine Zone
• high alumina rocks