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Location: 45 km SE from Falun, Sweden
Kaspersbo 20GarpenbergSweden776-98
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The Garpenberg supracrustal inlier is situated in the mineralized Palaeoproterozoic igneous province of Bergslagen, south central Sweden. Garpenberg is the largest sulphide deposit in the region and consists of multiple polymetallic deposits hosted within a NE-SW trending tight to isoclinal syncline which is ca. 15 km long and 7 km wide (Vivallo 1985). This syncline is compressed at the southern end and opens to the north with a subvertical axial plane (Allen et al. 2003). The deposits are, for the most part, hosted along the same stratigraphic level within the predominantly rhyolitic sequence.StructureThe complex geometry of the ore-host limestone is due to large-scale folding, shearing, and faulting events. Folding, sub-folding, and shearing are the dominant structural controls on the geometry of limestone and adjacent strata. The resultant structures strongly influence the position, geometry, and metal grade of the ore bodies. The F2 folds have undulating fold axis which locally can grade into cone or sheath shapes due to inhomogeneous stretching strain associated with the folding (Allen et al. 1996). This is also evident on the horizontal plane where sheath folding is evident in interpreted plan views, where the hinges of such structures can be highly prospective.The Garpenberg ore deposits vary somewhat in style depending on their origin and subsequent location within the sequence, from massive in-situ mineralization associated with the altered limestone unit found along the footwall – hanging wall contact, to tectonically remobilized ‘bands’ of ore that run sub-parallel to the dominant footwall foliation. The largest ore bodies are associated with antiform or synform structures, specifically Lappberget and Dammsjön. Even the geometry of the predominantly footwall hosted Huvudmalmen deposit is strongly influenced by parasitic folding of the overlying altered carbonate package.Many of the Garpenberg deposits show extreme isoclinal folding and sub-folding of footwall mica quartzite and schist which can appear almost ‘interbedded’ with the more ductile overlying altered carbonate units (dolomite and skarns) on the hanging wall side. It is also likely that dip-slip faulting and / or shearing have contributed to this repetition / layering. While deposits are well constrained due to tight drilling intercepts, isoclinal folding is common and deposits often comprise of both remobilized and replacement style mineralization.MineralizationMineralization in Garpenberg mainly consists of pyrite, sphalerite, galena and silver-bearing minerals. The ore bodies occur at the heavily skarn- and dolomite-altered contact zone between the limestone and underlying metavolcanic rocks, forming massive to semi-massive sulphides ore lenses. There is also significant mineralization in the footwall metavolcanic rocks (mica quartzites) that are stratigraphically underlying the marble horizon. The footwall mineralization is tectonically controlled, and forms remobilized semi-compact thin veins that are often associated to mica-rich shear zones. Mineralization is mainly of replacement style and is likely to have taken place where metal-bearing fluids penetrated up along synvolcanic, extensional faults and came in contact with reactive limestone to form large, massive sulphide bodies. The initial main stage of mineralization and alteration at all the known Garpenberg ore bodies is interpreted to be essentially syn-volcanic in timing and to pre-date regional metamorphism and deformation (Jansson & Allen 2011).
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