Summary:
Kankberg Geology
The Kankberg Mine lies within the eastern part of the Skellefte mining field, one of the most important mining regions in Sweden, where Boliden has been active since the 1920s. It’s significance in relation to 52 other known deposits in the field is shown in Figure 11 from a paper by Allen et al (1996) that describes the marine volcanic arc setting of these Zn-Cu-AuAg polymetallic massive sulfide deposits, vein Au deposits and porphyry Cu-Au-Mo deposits.
The host rock in the Kankberg area is dominated by volcanic rocks of primarily dacitic and rhyolitic compositions forming quartz-feldspar porphyritic, rhyolitic and dacitic rock types. The felsic magmas forming these volcanics intruded as shallow (subvolcanic) dykes and sills and extruded as lavas at the surface where they mixed with sediments and mass flows derived from volcanic slopes. The volcanism initiated a convection of solutions through the rocks. These solutions dissolved and transported minerals and metals to sites of deposition.
After the major volcanic period had ended the area was subsequently deformed and folded. This resulted in a dominantly vertical trend of the rocks and structures. At a later stage, brittle deformation took place. Fractures and fissures were intruded by mafic magma forming basaltic and andesitic dykes, which are common in the Kankberg area.
Mineralization
Gold mineralization is hosted in a complex mix of volcanic rocks consisting primarily of quartz-feldspar porphyry, volcaniclastics and breccias. The host rocks are strongly altered by silicification, andalusite ± topaz alteration and to a varying degree sericitization. The strong alterations form a highly competent body, which is surrounded by dacites. The contact zone is characterized by sericite ± chlorite alteration associated with pyrite ± pyrrhotite.
The economic mineralization is contained in ‘metallic’ minerals primarily located within the quartz-andalusite ± topaz alteration. It includes fine-grained native gold alloyed with silver at proportions of between 0 to 20%. More commonly, gold occurs as gold-tellurides including petzite (Ag3AuTe2), calaverite (AuTe2) and sylvanite (AuAgTe4). Another common telluride is tellurobismuthite (Bi2Te3). Several more telluride minerals have been identified through microscopy.
Renstrom Geology
The rocks in the Skellefte district were formed approximately 1.9 Ga during a period of active volcanism. The felsic magmas intruded as shallow (subvolcanic) intrusions (dykes and sills) at and close to the surface, where they mixed and mingled with wet sediments and mass-flows derived from volcanic slopes resulting in hyaloclastic brecciation and peperites. The active volcanic region also initiated a convection of solutions within the deposited package which enabled the dissolution and transportation of metals and minerals. These solutions also altered the rocks both physically, through (hydro-) brecciation and fragmentation, and chemically resulting in the heavily altered rocks present today. After the main volcanic period, regional deformation took place within the Skellefte district. The brittle deformation accommodated for fractures and fissures, which would be filled by mafic magmas forming andesitic and basaltic dykes.
The Renström area is located 15km west of Boliden, in the eastern part of the Skellefteå district. The Renström area has a volcanically complex and multiply deformed rock sequence. Rock types include a large range of basaltic andesite to rhyolite volcanic facies. Juvenile basaltic andesite, dacite and rhyolite volcanoclastic facies are particularly abundant and these have been intruded by numerous basaltic, andesitic dacitic and rhylitic sills and doms. The area has two main generations of folding with a complex interference pattern, and several generations of faults and intrusions.
The Renström area is one of the most intensely mineralized parts of the Skellefte district and the Renström deposit is one of the most important deposits due to its size (>10 million tonnes), grade (high Zn, Au, Ag values) and metallurgical characteristics (medium grained; low arsenic and antimony contents). The ores in the Renström deposit are associated with strong chlorite, dolomite, sericite and silica alteration.
Mineralization
The Renström mineralization consists of several smaller lenses, which are all characterized by massive to semimassive pyrite-sphalerite dominated ores with subordinate massive to semimassive pyrite-chalocopyrite ore and local stringer-type pyrite-chalocopyrite±pyrrhotite mineralization. The main ore minerals are pyrite, sphalerite, galena, chalcopyrite, pyrrotite and arsenopyrite with minor tetrahedrite-tennantite, other sulphosalts, electrum and amalgam (Helfrich, 1971; Kläre, 2001). Ores in the Renström area have higher zinc, gold, silver and lead contents and lower sulphur and arsenic content than most volcanic-hosted massive sulfide ores in the Skellefte district.
Kristineberg Geology
The Kristineberg Camp is located on the western extent of the Skellefteå district. The Skellefte district is a Paleoproterozoic (1.89 Ga) Volcanic sedimentary area Located in Västerbotten, northern Sweden. The area stretches roughly 100 km from the village of Kristineberg in the west to the village of Boliden in the east. The Skellefte district hosts more than 85 VHMS deposits, of which 26 have been, or are currently hosting mining operations.
The VHMS deposits of the area are mostly hosted in the upper parts of a volcanic sequence of intermediate to felsic juvenile volcanoclastic rocks, sub volcanic intrusions and lavas. These rocks together form the Skellefte group, which in turn is the lowest stratigraphic sequence in the Skellefte district. (Allen, Weihed, & Svenson, 1996).
Mineralisations
Mineralisations of the Kristineberg Mine are hosted in steeply-gently dipping Chlorite Schist lenses, with a gentle plunge towards the SW. The mineralisation generally appears as two “arms”, the southern arm consisting of the B-, E-, J-, K-, M-,and Ag-Zones as well as the Raimo and Tommy mineralisations. On the northern “arm” lies the L-Zone and A-Zones. Mineralisations can be generally split into two types:
• Chlorite Schist hosted mineralisations, and
• Ag-Pb “remobilised” mineralisation.
Chlorite schist hosted mineralisation generally contains sulphide mineralisation that is semimassive to massive in nature with variable abundances of economically important minerals: chalcopyrite (CuFeS2), sphalerite ((Zn, Fe)S) and galena (PbS), with minor silver and gold. The schists themselves contain variable amounts of muscovite, quartz, chlorite, phlogopite, biotite, cordierite, andalusite, pyrite and magnetite. The chlorite schists appear as lenses within colloquially named “quartzites” which are hypothesised to be highly altered rhyolitic to dacitic rocks (Barrett & MacLean, 2000). Chlorite, cordierite, sericite and andalusite as well as quartz overprint the original rock textures making primary rock identification difficult.
The “remobilised” Ag-Pb type is hosted within silicified cordierite and chlorite quartzites. Five silver bearing minerals are present within the Ag-Zone; freibergite ((Ag, Cu, Fe)12(Sb, As)4S13) being the dominant one with minor amounts of hessite (Ag2Te) often present. High silver grades are often present in narrow zones associated with galena veins or fracture fillings.