Volcanic stratigraphy, chemical stratigraphy, and hydrothermal alteration of the Petiknäs South Volcanic-hosted massive sulfide deposit, Sweden
Sammanfattning: The main exploration methods which have been used in the Skellefte district are prospecting for ore boulders in young glacial deposits and electrical geophysical methods. The discovery of new ores which likely are located at deep levels (>200 m), requires improved exploration methods, which in turn require a better understanding of the geology and alteration patterns of the known VMS deposits. This thesis seeks to improve the understanding of the geology and alteration patterns of VMS deposits in the Skellefte district by providing new data and new interpretation of the geology and alteration system of the Petiknäs South VMS deposit. The Skellefte district is an important ore province, containing more than 80 pyritic Zn-Cu-Au-Ag massive sulfide deposits within volcanic rocks of the Paleoproterozoic Skellefte Group. The Petiknäs South volcanic-hosted massive sulfide deposit is a producing underground mine in the eastern part of the Paleoproterozoic Skellefte district. The deposit consists of several ore lenses (A, B, C and D) and prior to mining contained 6 Mt of ore grading 5 % Zn, 1 % Cu, 1 % Pb, 2.5 g/t Au and 105 g/t Ag. The deposit occurs on the southern limb of a tight, steeply plunging, upright, anticline. The mine stratigraphy dips subvertically and youngs consistently southwards. A major thrust fault truncates the downdip portion of the deposit at the 700 m level of the mine. The volcanic rocks of the mine sequence generally have one penetrative tectonic foliation and have been metamorphosed to greenschist facies, although garnet-biotite bearing rocks are present locally. 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. From oldest to youngest, the main units are: The footwall complex (Unit 1) consists of rhyolite B and minor rhyolite C, B/C, and dacite feldspar-porphyritic rhyolitic cryptodomes, sills and volcaniclastic rocks, and is overlain by rhyolitic sandstone-siltstone beds, an up to 20 m thick unit hosting the B/C ore lens (Unit 2). The B/C ore lenses formed a single lens prior to intrusion of the late andesite sills. Unit 2 is overlain by unmineralized basaltic andesite volcaniclastic rocks (Unit 3). Above this, Unit 4 is host to the D and A ore lenses. This unit comprises mainly rhyolitic volcaniclastics with several thin andesitic intercalations. A WNW-ESE trending fault separates Unit 4 from Unit 5, which comprises feldspar-quartz porphyritic rhyolite sills and sandstone- siltstone volcaniclastic rocks. The lower part of Unit 5 is a mass flow deposit of distinctive low-Ti mafic composition that can be traced for 700 m eastwards from the proximal part of the mine. Unit 6 comprises the post- ore andesite sills and mafic dykes. A portion of the chemostratigraphic sequence (Units 1 and 2) has also been recognized on the northern limb of the Petiknäs South anticline. In summary, 6 chemostratigraphic units can be identified at the Petiknäs South mine and these can be correlated throughout and beyond the mine, even where they are strongly altered. Two good chemostratigraphic marker horizons were identified: one is of basaltic andesite and andesite composition (unit 3) and one is of low-Ti mafic composition (unit 5a) and both markers can be correlated on the mine-scale and semi-regional scale. The chemostratigraphic results show that ore lenses (B/C, D and A) occur at different stratigraphic positions within clastic stratigraphic intervals of dominantly rhyolitic composition. The most favorable exploration horizon (the B/C ore horizon) is located at the contact between a felsic clastic and a basaltic-andesitic clastic unit. Based on identification of magmatic affinity, chemostratigraphy revealed that the volcanic succession at Petiknäs South comprises calc-alkaline to transitional footwall rocks representing a proximal facies association and this succession is overlain by tholeiitc juvenile volcaniclastic rocks which were derived from a different and relatively distal volcanic center. In terms of ore location chemostratigraphy revealed that the main ore body at Petiknäs South is located at the contact between rhyolitic calc-alkaline to transitional and basaltic-andesitic tholeiitic rocks. 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. These extensions of the Petiknäs South ore horizon are potential drill targets for new VMS ore lenses. The main alteration minerals are sericite, quartz, chlorite, garnet, and locally carbonate. Intense chlorite-garnet alteration occurs immediately below the A and D ore lenses, and in the distal footwall of the B and C ore lenses. These 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. Carbonatization occurs in narrow zones throughout the mine sequence. Sericitic alteration in felsic footwall rocks that show only small mass changes in K and Si is explained by hydration at low water/rock ratios. By contrast sericitic altered felsic rocks immediately below the B/C ore lens show large gains in K and Si and are attributed to alteration by hot fluids. Restoration of the Petiknäs South stratigraphy and alteration system prior to deformation indicates that the Petiknäs South deposit originally contained three stacked ore lenses: from oldest to youngest, the B/C, D and A lenses. The B/C and the A ore lens were formed at the seafloor above a zone of hydrothermal upflow and the D ore lens is interpreted to have formed in the footwall of the A ore lens via replacement. Alteration zones below the B/C and the A ore lens and around the D ore lens 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 around 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. The proximal part of the footwall complex, which is dominated by synvolcanic felsic intrusions, is only weakly altered, which suggests that the intrusions were emplaced slightly after formation of the massive sulfide lenses. The results of this study can be used to help identify ore horizon and favorable alteration zones in other parts of the Skellefte district and elsewhere.
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