The copper sulphide mineralization of the Zinkgruvan Deposit, Bergslagen, Sweden

Sammanfattning: At Zinkgruvan, Sweden, zinc, lead and silver are mined from a stratiform ore body. It is considered syngenetic and situated in metavolcanic and metasedimentary rocks. A fault, the Knalla fault, separates the Zinkgruvan deposit into two parts. In the western part, Knalla, the ore is severely deformed, whereas in the eastern part, Nygruvan, a distinct layering occurs. In the western part of Knalla, stratigraphically below the Zn-Pb-Ag-ore, a copper sulphide mineralization occurs, predominantly in a marble unit. According to plans mining of the mineralization will start in the coming years. The study is devoted to determining the occurring ore minerals and their distribution. In addition, the host minerals of the elements As, Sb, Bi and Hg are determined. Optical microscopy and scanning electron microscopy including energy dispersive X-ray analyses are the main tools of the study. The study has showed that the dominating ore minerals in the copper sulphide mineralization are chalcopyrite and cubanite. Sphalerite, magnetite, cobalt-rich pentlandite and other Fe-Ni sulphides are common. Breithauptite, cobaltite, valleriite, bornite and a number of smaller arsenic- and antimony containing minerals occur. The ore minerals mostly occur in aggregates of varying size but also as discrete grains. Most ore minerals occur as both. The main exceptions are the Fe-Co-Ni-sulphides which only occur in the aggregates and the As- and Sb– containing minerals (except for breithauptite and cobaltite), which normally occur as intergrowths outside the aggregates. The richer the mineralization is, the larger and more numerous are the aggregates. In the richest part of the mineralization, the amounts of ore minerals exceed the amount of host rock minerals. Most of the ore minerals occur both in the weakly and strongly mineralised samples. The number of different minerals does not increase with increasing degree of mineralization; merely the amounts of the ore minerals increase. Cobaltite is the dominating As-containing mineral, but arsenopyrite and other minor As- and Sb- phases occur. Antimony foremost occurs in breithauptite but also in gudmundite, tetrahedrite and costibite. Bismuth mainly occurs as rounded grains in the host rock but rarely occurs as larger grains of metallic bismuth or as part of the mineral parkerite. Mercury has been proved in silver filled fractures. The two latter elements have a restricted distribution. The chalcopyrite aggregates dominate the copper mineralization in most of the ore body, found in the carbonate horizon. In the underlying quartz-feldspar leptite sequence the degree of mineralization is low. Some parts of the marble have sections with different host-rock mineralogy, such as high content of skarn minerals. There, the ore minerals occurring differ from the normal. Chemically most of the ore minerals are relatively constant in composition; no significant difference can be demonstrated among the three studied cores, irrespective of stratigraphic position. Exceptions are cobalt-rich pentlandite, sphalerite and tetrahedrite, where significant variations in composition have been proved.