Proterozoic hydrothermal alteration and mineralization along the Protogine Zone in southern Sweden

Abstract: Occurrences of high alumina rocks constituted by quartz, kyanite, rutile and Al-phosphates are spatially related with the Protogine Zone, a system of ductile and brittle deformation zones within the Proterozoic crust of southern Scandinavia. A hydrothermal origin has been proposed for high alumina rocks at Västanå, based on the chemistry of these rocks, while gradual transitions between magmatic rocks and high alumina rocks at Hökensås and Hålsjöberg suggest a similar origin. These rocks are characterised by very low alkali/aluminum ratios, caused by hydrolytic leaching whereby 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 and clearly postdates this hydrothermal event. Textural relations and considerations of the ductile-brittle transition in the crust furthermore suggest that hydrolytic leaching occurred before or during prograde metamorphism at a temperature below c. 400°C. Character of alteration and geological setting indicate that solutions causing hydrothermal alteration derived from a magma of acid or intermediate composition, which also served as heat source for convection in a brittle, near surface 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 some of the Al-phosphates. Rare Al-phosphates at Västanå; on the other hand, seem to be restricted to hematite mineralization, indicating a simultaneous origin of hydrothermal leaching and Fe-, Mn-mineralization. Comparison of high alumina rocks world over and consideration of plate tectonic setting favour formation in a subduction related environment rather than in an intracontinental rift setting.