3D modelling and sheath folding at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit and implications for exploration in a 1.9 Ga ore district, Fennoscandian Shield, Sweden

Abstract

Altered and mineralized rocks at the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, situated in the Palaeoproterozoic Bergslagen ore district in the southwestern part of the Fennoscandian Shield, have been metamorphosed at low-pressure, amphibolite-facies conditions and affected by ductile deformation. Using combined surface mapping of lithology and structure, drill core logging and microstructural work, the polyphase (D 1 and D 2 ) ductile deformation is demonstrated and a 3D model for the deposit created. Mineral associations include quartz, biotite, cordierite, anthophyllite, and minor almandine, andalusite and chlorite in silicate-rich altered rock, calcite or dolomite in marble and tremolite-actinolite or diopside-hedenbergite in skarn. The silicate minerals show varying growth patterns during the different phases of the tectonothermal evolution, with considerable static grain growth occurring between D 1 and D 2 , and even after D 2 . F 2 sheath folding along axes that plunge steeply to the SSE, parallel to a mineral stretching lineation and the dip direction of the S 2 foliation, is suggested as a key deformation mechanism forming steeply plunging, cone-to rod-shaped mineralized bodies. This contrasts with a previous structural model invoking fold interference. A major shear zone with talc-chlorite-(quartz-biotite) mineral association separates the northern and southern structural domains at the deposit and bounds the polymetallic massive sulphides to the north.

Time constraints on magmatism, mineralisation and metamorphism at the Falun base metal sulphide deposit, Sweden, using U–Pb geochronology on zircon and monazite

“…The Falun base metal sulphide and precious metal deposit is surrounded by a hydrothermally altered zone (Koark, 1962;Kampmann et al, 2016), which extends over an area of several thousand m 2 (Fig. 1B).…”

“…1B). Hydrothermal alteration and subsequent polyphase ductile deformation and metamorphism resulted in different silicate-rich altered rock types, characterised by variable contents of quartz, biotite, cordierite, almandine garnet and anthophyllite (Kampmann et al, 2016;Fig. 2).…”

“…2). On a local scale in the mine, dolomite and calcite marble, as well as diopside-hedenbergite or actinolite-tremolite skarns occur (Geijer, 1917;Koark, 1962;Kampmann et al, 2016). Transitions between fine-grained felsic rock surrounding the deposit and silicate-rich altered rock have been observed in drill core close to the boundary of the hydrothermally altered zone at the Falun deposit, suggesting that the felsic rock formed the precursor to the silicate-rich altered rocks.…”

“…Mineralisation types at the Falun deposit include a major body of pyritic Zn-Pb-Cu-rich massive sulphides in the central part, which is Cu-rich along its margins (Kampmann et al, 2016;Fig. 2).…”

“…Porphyritic felsic dykes, macroscopically showing no or only weak signs of alteration compared to the silicate-rich altered rocks (Koark, 1962;Kampmann et al, 2016), intruded both the mineralised bodies and the surrounding hydrothermally altered zone at Falun (Fig. 2).…”