AbstractsEngineering

The Skellefte district is one of the most important mining regions in Sweden and Europe. The district contains more than 85 pyritic Zn-Cu-Au-Ag massive sulfide deposits. The discovery of new ores, which likely are located >200 m below surface is a challenge and requires improved exploration methods, which in turn require a better understanding of the geology and alteration patterns of known volcanic-hosted massive sulfide (VMS) deposits in the district. The Petiknäs South ore deposit lies in the eastern part of the Skellefte district. Between 1992 and 2006, 5.3 Mt of ore grading 4.9% Zn, 0.9% Cu, 0.9% Pb, 31% S, 2.4 g/t Au and 102 g/t Ag were mined in an underground operation by Boliden Mineral AB. The ores are pyritic massive sulfides, hosted within altered, deformed and metamorphosed volcanic rocks of the Paleoproterozoic Skellefte Group. The deposit contained several ore lenses at three stratigraphic levels. This study indicates that the Petiknäs South ore deposit occurs on the southern limb of a steeply plunging, tight, upright, F2-anticline. The mine stratigraphy dips subvertically and youngs consistently southwards. A major thrust fault truncates the down-dip portion of the deposit at the 700 m level. The rocks generally show one penetrative tectonic foliation and have been metamorphosed to greenschist facies. The following six mine-scale units occur from north to south (footwall to hanging-wall): diffusely stratified andesite breccias (Unit 1a), thick dacitic pumice breccia (Unit 1b) overlain by feldspar-porphyritic rhyolitic domes and sills with hyaloclastitic and peperitic contacts (top of Unit 1b and Unit 1d), the B-C ore lens enveloped by a thin screen of reworked rhyolitic volcaniclastic rocks (Unit 2), thick andesitic and basaltic andesitic pumiceous mass-flow deposits (Unit 3), volcanic sandstones and siltstones containing large quartz-phyric pumice clasts, and strongly altered volcanic rocks enveloping the D-, and A-ore lenses (Unit 4), and finally the hanging-wall complex comprising feldspar-quartz porphyritic rhyolite sills (Unit 5). The andesite breccias, dacite pumice breccia and high-Ti rhyolites (Units 1a and 1b) represent an andesite-dacite-rhyolite shield volcano whereas the feldspar-porphyritic rhyolite (Unit 1d) corresponds to a cryptodome-sill-tuff volcano with peperitic margins. The rhyolitic sandstones (Unit 2) were emplaced via turbidity currents under subaqueous conditions below wave base and the main B-C ore lens formed at the sea-floor within this facies. The intermediate mass-flow breccias (Unit 3) were rapidly emplaced in subaqueous conditions following a pyroclastic eruption and the host unit of the D-, and A-ore lenses (Unit 4) was deposited by turbidity currents below wave base. The hanging-wall fault causes repetition of Units 3 and 4. The rhyolitic sills of the hanging-wall complex (Unit 5) were intruded into wet volcaniclastic sediments. Post-ore and post-alteration andesite and felsic sills and mafic dykes (Unit 6) intruded most of the mine sequence. Lithogeochemical…