Mineralization controls in the sandstone lead-zinc deposits at Vassbo, Sweden

Abstract

The sandstone-lead-zinc mineralization in the Vassbo region of west-central Sweden is concentrated in two deposits, Vassbo and Guttusjoe. The ores occur in Lower Cambrian quartz sandstones in a thin sedimentary sequence, underlain by a Precambrian basement and overlain by Caledonian nappes. Although the ores are stratabound, mineralization occurs in suitable arenaceous horizons throughout the sedimentary sequence.The Precambrian basement is crossed by a number of diabases dated at 1,100 to 1,350 m.y. Erosion of these diabases and their associated joints led to the formation of basement depressions. This is reflected in thickening in the overlying sediments. The distribution of mineralization in the area is found to be strongly controlled by the location of the basement diabases and, consequently, the sediment thickness. The recognition of this relationship and the fact that the subsurface location of the diabase dikes could be traced by aeromagnetics led to the discovery of the Guttusjoe deposit.Within the ores, mineralization is strongly controlled by sedimentary structures. However, the ore minerals infill the interstices between the sand grains, and thus high ore concentrations are independent of sedimentary structures. The spatial relationships suggest that the mineralization originated through penetration of the sedimentary sequence by solutions, preferentially flowing along the low pressure conduits provided by the thicker sandstones overlying the diabases.