The mineral composition of the Burkland Cu-sulphide deposit at Zinkgruvan, Sweden : a supplementary study

Abstract: The Zinkgruvan Mining AB, situated in south-central Sweden in Örebro County, south-east of Åmmeberg, plans to expand their production on a massive, carbonate hosted copper-sulphide ore deposit, the Burkland Cu-mineralization in the eastern part of Knalla. At present mainly Zn-Pb-Ag are mined. The deformed Cu-mineralization belongs to the south-western Bergslagen ore province. It is of Paleoproterozoic age, hosted by supracrustal, metavolcanic- and metasedimentary rocks, mainly serpentine- and dolomitic marble as well as skarn. Hydrothermal ore forming processes are suggested for the formation of the Zinkgruvan deposit. It is described as a Volcanic Massive Sulphide (VMS) and a volcanic SEDimentary EXhalative (SEDEX) type. In order to judge the economic value of the Cu-ore deposit, the mineralogy and possible variation in chemical composition of the opaque minerals as well as the mineral distribution have been investigated. Also the distribution of problematic elements such as Sb, As, Hg and Bi were studied. These can lead to fines during the processing of Cuore. Ten thin sections from the most ore rich 18m section of the drillcore DBH2992 have been analysed under a reflected light and in a scanning electron microscope. This study is a complement to the master thesis of Bjärnborg (2009) on the same topic to test and strengthen her findings through extra samples and analysis. The major ore mineral found in this study is chalcopyrite (CuFeS2), mostly occurring in aggregates. Other common ore minerals are sphalerite, pentlandite and magnetite both in aggregates and as disseminated grains. Cubanite is often found intimately intergrown as lamellea in chalcopyrite or as almost massive grains. Minor ore minerals found are breithauptite, cobaltite and pyrrhotite/mackinawite. The latter two minerals could not definitely be discriminated. The problematic elements Sb and As are found in nine different minerals. None of the major or common ore phases contain Sb or As. The Sb carrying minor ore phase breithauptite (NiSb) occurs in all but one sample. Most of the Sb or As is contained in rare minerals that are found in samples poor in ore. No Hg or Bi is found during this study. The occurrence of Ag is linked to an extensive red to purple-blue tarnish of chalcopyrite. However, tarnishing is common also when no Ag is present. No distinct mineral zoning throughout the studied section is observed. With increasing ore grade, the number of different ore minerals is stable or even decreased. The host rock composition influences the ore grade: Skarn seems to contain the highest quantities of ore minerals. No major variation in chemistry is found in most opaque minerals. Exceptions are pentlandite with varying amounts of (Fe,Co,Ni), sphalerite (a low-Fe and a high-Fe type) and safflorite with a Co-rich and a Fe-riche type.