Hydrothermal Alteration Around the Zn-Ag Nyborg Volcanogenic Massive Sulfide Deposit, Skellefte District, Sweden

Abstract: The Nyborg deposit is a small mineralization found in the western Skellefte District approximately 8 km NE from the Kristineberg VMS deposit and is classified as a Zn-Ag volcanogenic massive sulfide deposit. The size of the deposit is estimated to be 396 000 tons grading 4.8% Zn, 0.5% Cu, 0.4% Pb, 10.5% S, 36 g/t Ag and 0.2 g/t Au. The deposit is hosted by the upper part of the Skellefte Volcanic Group, close to the contact with the overlying Vargfors Group sedimentary rocks.
A study of the chemical and mineralogical differences between the hanging wall and footwall of the Nyborg Zn-Ag VMS deposit was carried out for this thesis. To further develop the study a regional comparison to the nearby Kristineberg deposit was made.
The hanging wall consists of two units, one more intermediate to mafic and the second a felsic banded rock. Two ore minerals found are ilmenite and pyrrhotite. The footwall rock type is andalusite-sericite schist and the ore mineralogy is dominated by pyrite and minor rutile. The mineralization itself is hosted in a sericite schist. The mineralization consists of iron-poor sphalerite, chalcopyrite, pyrite, galena, pyrrhotite and ilmenite. The iron-poor sphalerite is most likely the result of a replacement process called the “chalcopyrite disease”. The chalcopyrite appears as inclusions in the sphalerite. This iron-poor sphalerite is also observed in some places in the Kristineberg deposit, possibly indicating a similar genesis. This relationship could also be strengthened by a possible fluid pathway presented by Hannington et al. (2003) and similar chemistry (TiO2 vs. Zr) between rock types derived from Nyborg and Kristineberg.
Relationship studies between ilmenite/rutile, pyrrhotite/pyrite, sphalerite/chalcopyrite and distribution of barium was made and the result show that the Nyborg deposit most likely was formed by replacement processes, probably as a result of interaction between colder seawater-derived fluids and hot hydrothermal fluids.