Volcanic and Volcano-Sedimentary Facies Analysis of the Viscaria D-zone Fe-Cu Occurrence, Kiruna District, Northern Sweden

The northern Norrbotten area in northern Sweden, is an important mining district and hosts several deposits of Fe-oxide Cu-Au type as well as epigenetic Cu-Au deposits. The Viscaria Cu-deposit is a major sulfide deposit in this area. The ore is hosted by the Paleoproterozoic Kiruna Greenstones. Since 1982, 12.54 Mt of ore with 2.29% Cu has been produced, mostly from A-zone and in less extent B-zone, the C-zone being barren. The Viscaria Cu-deposit has been interpreted as a syngenetic copper ore of exhalative origin (Martinsson, 1997). Recent drilling of the historic deposit by the company Avalon Minerals has identified a potentially economic resource at D-zone which has not yet been studied in detail. The master thesis main objective was to perform a first study of the D-zone Fe-Cu occurrence to help on-going exploration. Facies analysis has been applied to Viscaria D-zone. It comprised 4 steps: 1. Facies identification; 2. Facies associations, interpretation and discussion; 3. Stratigraphy and structural interpretation; and 4. Relationship between alteration and mineralization. The stratigraphy and the petrology of ore and host rocks are based on detailed logging of 1450m of drill cores and geochemical-petrographical study of the ore and associated alterations. The stratigraphy at Viscaria D-zone is fairly simple. Basalt flows that belong to the Pikse Fm. are overlain by an andesitic volcaniclastic sequence including minor carbonate horizons that belong to the Viscaria Fm. Relatively thick basaltic sills and dykes cut the stratigraphy in proximity to ore zone and belong to the Peuravaara Fm. The Viscaria D-zone comprises one steeply dipping ore lense. The host rocks consist of altered basaltic sills and tuffaceous siltstones. The main ore minerals are magnetite and chalcopyrite, magnetite being by far the most common at Viscaria D-zone. The location of the ore follows a NE-SW-oriented linear array. A shear zone appears to be the most important ore-controlling factor. In detail, the position of individual ore types (magnetite ore, magnetite-chalcopyrite ore) appear possibly lithologically controlled by carbonate. Intense fluid-assisted alteration has resulted in variable and overlapping alteration stages in the ore zone. These alterations can be related to the upper greenschist regional metamorphism, to localized syn-tectonic alteration with well defined foliation and shear zone, or more importantly to localized and zoned hydrothermal alteration associated with mineralization. Important to point out is the ore zone is characterized by the chlorite+talc+carbonate+amphibole±scapolite±biotite±quartz alteration assemblage, picturing a long and complex fluid history of the D-zone occurrence. Based on this study, the Viscaria D-zone Fe-Cu occurrence appears similar to epigenetic copper deposits in the region or Fe-oxide Cu-(Au) deposits elsewhere in the world. Most genetically significant for the Viscaria D-zone are: the shear zone and the prominent structural control of the mineralization; the wall-rocks alteration; and the fact the ore consists mostly of magnetite with ore grade chalcopyrite in some parts.Validerat; 20110313 (anonymous