Volcanic stratigraphy and hydrothermal alteration of the Petiknäs south Zn-Pb-Cu-Au-Ag volcanic-hosted massive sulfide deposit, Sweden

Abstract: 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 studies show that the thick andesite and dacite breccias, and rhyolites of the lower footwall (Units 1a, 1b) belong to a high-TiO2 fractionation trend, whereas rocks of the upper footwall (Unit 1d), the host unit to the B-C ore lenses (Unit 2) and most of the felsic rocks higher in the stratigraphy belong to a low-TiO2 trend. This change from a high-TiO2 to a low TiO2 series corresponds closely to the change from an early andesite-dacite-rhyolite shield volcano to a later rhyolitic cryptodome-sill-tuff volcano. The B-C ore horizon is considered the most favorable stratigraphic horizon for mineral exploration and lies between calc-alkaline to transitional, felsic volcanic rocks (Unit 2) and tholeiitic, basaltic-andesitic and andesitic juvenile mass-flow deposits (Unit 3) that were derived from a different and relatively distal volcanic center. This ore horizon can be traced along strike for several hundred meters and its position has also been located on the northern limb of the regional anticline, 700 m north of Petiknäs South. Furthermore, the Petiknäs South ore horizon can be correlated with the ore horizon of the Renström mine which is located 2 km to the east. These extensions of the Petiknäs South ore horizon provide potential drill targets for new VMS ore lenses. Application of immobile-element lithogeochemical methods has allowed definition of a series of chemostratigraphic units, which can be correlated throughout and beyond the mine, even where the rocks are strongly altered. At the Kristineberg and Rävliden mines in the western part of the Skellefte district it has been shown that lithogeochemical techniques can also be applied to intensely altered rocks that have been metamorphosed to amphibolite facies. The chemostratigraphic correlations at Petiknäs South support and verify the volcanic-stratigraphic correlations based on lithology and facies. The results show that the ore lenses occur at different stratigraphic positions within clastic stratigraphic intervals of dominantly rhyolitic composition. Six alteration types have been distinguished based on mineral assemblage. Intense chlorite-sericite-quartz-garnet±feldspar alteration is found immediately below the ore lenses and locally in the distal footwall and represents the strongest alteration. These zones are interpreted to represent the upflow zones of hot hydrothermal fluids. Haloes of sericite alteration occur around the ore lenses and are wider than the zones of chlorite-sericite-garnet. Mass change calculations show that alteration zones below 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 about three times larger than the actual ore lenses, and consequently could provide a good exploration guide to ore. Other alteration zones with Na2O and CaO depletions occur on a semi-regional scale, but are most intense close to the ores. Surprisingly, the proximal part of the footwall complex, which is dominated by syn-volcanic felsic intrusions, is only weakly altered, which suggests that these intrusions were emplaced slightly after formation of the massive sulfide lenses at the waning stage of hydrothermal alteration. The Manus basin in Papua New Guinea is a back-arc basin where VMS ores are presently forming and represents a good modern analogue to the Petiknäs South ores. The rocks from the Manus basin are basalt, basaltic-andesite, andesite and rhyodacite, similar to rocks from the Petiknäs South mine sequence, and the ores are located just above a felsic proximal facies association. The results of this study can be used to help identify (1) felsic proximal facies associations, (2) ore horizons and (3) favorable hydrothermal alteration zones, in other parts of the Skellefte district and elsewhere.