Pb isotopic constraints on the origin of the Husvika Zn-Pb deposit in Nordland,north-central Norway
pp. 43-54

The Husvika Zn-Pb deposit is bosted by rocks of the Helgeland Nappe Complex in the Uppermost Allochthon of the Norwegian Caledonides. It is a small deposit (estimated to be 100,000 metric tonnes, carrying 24% Zn and 10% Pb) which can be followed as longitudinal bodies for about 1.5 km along a fault zone. The deposit occurs in mixed calcareous and pelitic metasedimentary units, which are intruded by the granodioritic Andalshatten Pluton, granitic pegmatites and veins, all of which are parts of the Bindal Batholith. The hosts for the sulphides are metasomatically altered metasedimentary and granitoid rocks. The metasedimentary rocks demonstrate a weak alteration, giving a paragenesis comprising plagioclase, gamet, epidote and quartz, followed by a strong alteration giving a massive rock consisting of coarse-grained gamet, amphibole and quartz, which is in turn altered into a massive griinerite-cummingtonite rock. The altered granitoid contains plagioclase, amazonite, garnet and quartz. The sulphide mineralization is of three different types: (1) massive ore of sphalerite and pyrrhotite with minor galena, chalcopyrite and arsenopyrite, or massive galena in large crystals, sometimes carrying magnetite, (2) disseminations of sphalerite, chalcopyrite, galena and pyrrhotite and/or arsenopyrite in the altered host rocks, and (3) quartz veins and veinlets with pyrrhotite, arsenopyrite and lollingite with accessory minerals. Ranges for Pb-isotope ratios of sulphides from the ore, amazonites from the altered granodiorites and pegmatites and unaltered K-feldspars from the granodiorites are 18.559-18.628, 15.622-15.660 and 38.254-38.373 for the 206Pb/204Pb, the 207Pb/204Pb and the 208Pb/204Pb ratios, respectively. Arsenopyrite from a quartz vein has Pb-isotope ratios of 18.963,15.655 and 37.950. Sm-Nd isotope analyses of the skam minerals indicate contribution from both a magmatic and a sedimentary source of Sm. Field relations and isotope determinations are consistent with a derivation of metals from the granodioritic intrusions and ore deposition simultaneous with, or shortly after, emplacement of the Andalshatten Pluton, pegmatites and granitic veins. Subsequently, sulphides with a UfTh-enhanced metal source (uranogenic lead) have been deposited.

A. Birkeland* & A. Bjørlykke, Institutt for Geologi, PO Box 1047 Blindern, 0316 Oslo, Norway.
*Present address: Norges Geologiske Undersøkelse, PO Box 3006, 7002 Trondheim, Norway