| Summary: | In this project a quartz-andalusite-sericite alteration zone in close vicinity to the Udden massive sulphide deposit in the Skellefte District in northern Sweden was studied. The objective was to find any correlation between the geological, mineralogical and geochemical character of the alteration zone and the associated Au-Te-Bi mineralization. The Udden area is situated in the Paleoproterozoic supracrustal rocks of the Skellefte Group in the central part of the Skellefte district. Most of the rocks in the Udden area are interpreted to be volcaniclastic, overlaid by a thick package of basinal sediments. The composition of the volcaniclastic rocks ranges from rhyolitic to mafic. The area has been subjected to several deformation events and regional metamorphism of upper greenschist facies. This, together with local deformation, has created a complex structural environment.A laterally extensive alteration zone is situated about 600 m south of the Udden open pit. It is characterized by strong leaching of most major elements, leaving a residue of mainly SiO2 and Al2O3. The mineral assemblage of the alteration zone mainly consists of quartz, andalusite and sericite with locally occurring biotite, chlorite, topaz, fluorite and minor rutile. Just above the alteration zone occurs a layer of strongly foliated quartz and sericite. The highest concentrations of Au, Te and Bi seem to be spatially related to this layer, although the concentrations of these metals are generally elevated in the whole alteration zone, sometimes together with Hg and Sb. In close vicinity of the Udden area the Kedträsk and Åsen massive sulphide deposits can be found. They are both related to similar types of alteration as occur in the alteration zone in Udden. In other parts of the Skellefte district similar types of alteration are known to occur as well, often related to mineral deposits. For example in the gold-rich Boliden massive sulfide deposit, in Mångfallberget telluride mineralization not far from Boliden, and in Kristineberg massive ...
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