Geochemistry of the proterozoic wolframite-bearing greisen veins and the associated granite at Rostberget, northern Sweden

Abstract

At Rostberget in northern Sweden, a swarm of thin, undeformed wolframite-bearing greisen veins occurs within the 1.78-Ga-old Joran granite intrusion. This is a microcline-rich porphyritic granite probably generated with sediments as important source materials. When the Joran granite crystallized, an aqueous fluid, rich in volatiles and W, was active in the central and apical parts of the intrusion. This fluid escaped through fractures caused by tectonic rupturing, the immediate surroundings of the fractures became altered, and the greisen veins were formed. The contents of SiO 2, Na 2O, Sr, V, Zr and LREE decreased during this greisenization, while the contents of Fe 2O 3, MnO, MgO, CaO, H 2O, W, Sn, Mo, Cu, Rb, Sc, Pb, Be, Nb, HREE and probably Ag increased. K 2O, TiO 2, P 2O 5, Ba, Zn and Y were not immobile, but no consistent trends of gain or loss were found. Neutralization of the acid vapour by alteration reactions was probably important for controlling the precipitation of wolframite. Accessory minerals contain most of the REE in all sample types. The gain of HREE during the greisenization was caused mainly by deposition of fluorite (and xenotime), and the loss of LREE was the result of dissolution of monazite (and allanite). No change in Eu content was observed. The enrichment of HREE and the presence of fluorite and minor topaz, indicate alteration from a F-rich fluid, but do not necessarily imply that W was transported as F − complexes.