Mass changes in alteration zones of the Petiknäs South volcanic-hosted massive sulfide deposit, Skellefte district, Sweden

Petiknäs South is a producing underground mine in the eastern part of the Skellefte district and contained 6 Mt of pyritic massive sulfide ore grading 5 % Zn, 1 % Cu, 1 % Pb, 2.5 g/t Au and 105 g/t Ag prior to mining. The deposit is hosted within volcanic rocks of the Paleoproterozoic Skellefte Group and consists of several stacked ore lenses (from oldest to youngest: C, B, D and A). The mine sequence comprises coherent and volcaniclastic units of rhyolitic to basaltic andesitic composition, and post-ore andesite sills and mafic dykes. The mine stratigraphy dips subvertically and youngs consistently southwards and the volcanic rocks have been metamorphosed to greenschist facies. Application of immobile-element lithogeochemical methods to 469 samples has allowed classification of the mine sequence into a series of chemostratigraphic units, while the degree of hydrothermal alteration of these units has been quantified using mass change methods. The main alteration minerals are sericite, chlorite, garnet, quartz and locally carbonate. Intense chlorite-garnet alteration occurs immediately below the A and D ore lenses, and in the distal footwall of the C and B ore lenses. A synvolcanic felsic sill was emplaced in the proximal footwall slightly after formation of the massive sulfide lenses. Consequently, a major part of the proximal footwall is only weakly altered and zones of strong alteration are truncated by the sill. The alteration zones are interpreted as hydrothermal upflow or feeder zones. Haloes of serizitization occur around the ore lenses and are wider than the zones of chlorite-garnet alteration and alteration zones with Na2O and CaO depletions occur on a semi-regional scale, but are most intense close to the ores. Alteration zones below and around the ore lenses are characterized by large mass gains of FeO, MnO, MgO and K2O together with large mass gains or losses in silica. The latter alteration zones are approximately three times larger than the actual ore lenses, and consequently could provide a good exploration guide to ore.Godkänd; 2006; 20111007 (andbra