Tobias Hermansson

Correlation between distribution and shape of VMS deposits and regional deformation patterns, Skellefte district, northern Sweden

The Skellefte district in northern Sweden is host to abundant volcanogenic massive sulphide (VMS) deposits comprising pyritic, massive, semi-massive and disseminated Zn–Cu–Au ± Pb ores surrounded by disseminated pyrite and with or without stockwork mineralisation. The VMS deposits are associated with Palaeoproterozoic upper crustal extension (D1) that resulted in the development of normal faults and related transfer faults. The VMS ores formed as sub-seafloor replacement in both felsic volcaniclastic and sedimentary rocks and partly as exhalative deposits within the uppermost part of the volcanic stratigraphy. Subsequently, the district was subjected to deformation (D2) during crustal shortening. Comparing the distribution of VMS deposits with the regional fault pattern reveals a close spatial relationship of VMS deposits to the faults that formed during crustal extension (D1) utilising the syn-extensional faults as fluid conduits. Analysing the shape and orientation of VMS ore bodies shows how their deformation pattern mimics those of the hosting structures and results from the overprinting D2 deformation. Furthermore, regional structural transitions are imitated in the deformation patterns of the ore bodies. Plotting the aspect ratios of VMS ore bodies and the comparison with undeformed equivalents in the Hokuroko district, Japan allow an estimation of apparent strain and show correlation with the D2 deformation intensity of the certain structural domains. A comparison of the size of VMS deposits with their location shows that the smallest deposits are not related to known high-strain zones and the largest deposits are associated with regional-scale high-strain zones. The comparison of distribution and size with the pattern of high-strain zones provides an important tool for regional-scale mineral exploration in the Skellefte district, whereas the analysis of ore body shape and orientation can aid near-mine exploration activities.

The Skellefte District

Four-dimensional geological modelling has been conducted in the Palaeoproterozoic Skellefte mining district. 3D-modelling of volcanic-hosted massive sulphide deposits and associated host-rocks has been carried out in multiple scales from deposit to regional scale and is based on a combination of geological and geophysical investigations. A conceptual model founded on unravelling the structural control on sedimentation, volcanism and mineralization and the subsequent deformation patterns, acts as a base for geological modelling. The final 3D-model provides a structural framework in which the mineralizations can be studied by improved understanding of the structural evolution in the mine areas, and by comparing the regional structural patterns versus the form and attitude of ore deposits. Additionally, uncertainty and prospectivity models were constructed showing the distribution of data and the potential of discovering new ore deposits. Subsequent 4D-modelling adds the time aspect to the 3D-models and aims at visualizing and understanding the geological history in the district and as a support for ore targeting. Moreover, adding geological time to the modelling helps gaining confidence about both the conceptual models and the 3D-models. The final 3D- and 4D-models provide a regional three-dimensional context for both industrial and academic activities in the Skellefte district, and aid the understanding of large-scale tectonic processes.