Summary:
The Dannemora deposit is contained within the regional host rocks of the Leptite Formation, an assemblage of Svecofennian (of Lower Proterozoic age ca. 1.8 to 1.9 Ga) marbles, metavolcanics and subsidiary metasediments. The Dannemora deposit is hosted by Svecofennian volcanic rocks (c. 1.9 Ga). The volcanic rocks are part of a sequence seen in most of the Bergslagen area, also called the leptite formation.
The principal units of the Leptite Formation are steeply dipping along the limbs of northeasttrending (ca. 030° NE) isoclinal fold structures, and occur between large granitoid intrusive bodies. These rocks were later deformed in the Svecokarelian orogen by granitoids, leading to the steeply dipping syncline-anticline structures seen today. The lithologies have undergone varying degrees of metamorphism related to both regional and local thermal alteration activity. The metamorphism is low to high grade in low pressure conditions, and the original structures are well-preserved.
The volcanic sequence is divided into a lower and an upper formation. The lower formation consists only of volcaniclastic rocks and lacks iron ore. The basal unit of the upper formation consists of volcaniclastic rocks with iron ore clasts. The primary rocks of the upper unit of the upper formation are volcaniclastic rocks and carbonate rocks. These primary rocks have been altered diagenetically and hydrothermally to form chlorite fels, skarn (Fe-MnCa-Mg-silicate rocks), sulphides, and iron ore.
In the Dannemora Mine area, the metavolcanic and metasedimentary rocks are shaped into two parallel synclines separated by an isoclinal anticline which together plunge gently to the northeast. The principal mineralisation is confined to the easternmost syncline, with exploration to date only identifying smaller bodies associated with the westernmost structure.
Mineralisation
The iron ore is strata-bound to dolomitic limestone and is thickest in the central part of the sedimentary basins, thinning outwards. The magnetite mineralisation is mostly restricted to the upper unit of the upper formation at Dannemora, and is normally strata bound within dolomitic units. Previous exploration work has identified about 25 individual mineralised bodies situated along a 3 km strike length of the syncline and surrounding structures at varying depths from surface. Due to these primary features and later tectonic movement the iron mineralisation is separated into approximately 25 different bodies at varying depths. The bodies occur within a ca. 400 - 800 m wide stratigraphic thickness and display a thinning and thickening, which is commensurate with their positions relative to the primary structures, i.e., thinning and fragmenting within the limbs of the syncline and thickening towards the keel of the Dannemora Syncline. The limbs dip at ca. 85° near surface and shallow to ca. 60° at the 350 m level of the mine.
The mineralisation has been commonly categorised as a manganese-rich skarn iron ore (with an iron content of 30 to 50%, and a manganese content of 1 to 6%), and a manganese-poor skarn iron ore (with an iron content of 30 to 50% and a manganese content of 0.2 to 1%).
The iron mineralisation is present as magnetite, which is found in aggregates with other minerals such as knebelite, dannemorite, diopside, actinolite, chlorite, and serpentine as well as carbonate minerals such as calcite, dolomite, siderite, and rhodochrosite. In terms of ore-related parageneses, magnetite is commonly associated with calcite and dolomite, and to a lesser extent with amphiboles (tremolite-actinolite), albite, andesine, arsenopyrite, biotite, chalcopyrite, chlorite, diopside, epidote, garnet, muscovite, oligoclase, orthoclase, phlogopite, pyrite, pyroxene, pyrrhotite, quartz, rhodochrosite, scapolite, serpentine, siderite and tremolite.
Rare, accessorial phases - often only observed in magnetite ore by electron microprobe - include apatite, arsenopyrite, bismuth, bismuthinite, cassiterite, chrysotile, cobaltite, dannemorite, galena, knebelite, löllingite, monazite, sphalerite, titanite and zircon.
Dimensions
The Mineral Resource extends for a length of approximately 2.5 km and is approximately 100 m wide corridor. Diamanten 2 and Myrmalmen are both located outside of this corridor.
The Mineral Resource extends approximately 400 m in depth starting between 0 and -400 m. The individual domains range in width from 3 m to 263 m, in length from 38 m to 325 m, and in depth from 26 m to 461 m. The general strike of the deposit is N30°E.