A continuum between structurally controlled Cu-Au and Au-only systems in northern Sweden

Abstract

<p>Northern Sweden is dominated by supracrustal and related intrusive rocks formed and deformed during the Svecofennian orogeny (1.90-1.78 Ga). The orogeny comprises several phases of crustal extension and shortening, resulting from subduction and repeated arc-accretion events. An early-Svecofennian extensional phase (c. 1.90-1.88) Ga results in the formation of iron oxide-apatite deposits (IOA, e.g. Kiruna and Malmberget) in the back-arc region and volcanogenic massive sulfide deposits (VMS, e.g. Kristineberg) in the arc. These deposits have been subjected to deformation and regional metamorphism resulting in transposition and re-mobilisation of ore bodies. The latest phase of the Svecofennian orogeny (1.80-1.78 Ga) is characterized by a distinct E-W-directed crustal shortening at high crustal levels representing the last continental assembly resulting from late arc-accretion and followed by a post-orogenic collapse. This leads to folding of early metamorphic fabrics and mainly brittle to brittle-ductile shear zone re-activation. A general N-S gradient from deeper-crustal conditions in Northern Norrbotten (north) to higher-crustal conditions in Västerbotten (south) is observable. The emplacement of syn-tectonic, felsic and mafic intrusive bodies causes high temperature conditions while pressures remain low. Such conditions are favorable for driving fluids and leading to the formation of structurally controlled, Au-bearing deposits in 3<sup>rd</sup> and 4<sup>th</sup> order structures of large, crustal-scale, re-activated shear zone complexes. Deposits in Northern Norrbotten that are related to this late-Svecofennian phase show both ductile and brittle features and host Cu and Au, hence often assigned to the IOCG group of deposits. Deposits that formed during the same phase in Västerbotten are typically characterized by Au-bearing quartz veins that formed as lower-order structures, hence often classified as orogenic Au-deposits. Weather these deposits are entirely newly formed or just represent various grades of re-mobilization of older mineralization remains an open question. Looking at these deposits on a crustal-scale, they represent structurally-controlled deposits, hosted by lower-order structures in re-activated shear zone complexes whereas differences in mineralogy and hydrothermal alteration assemblages are an effect of different crustal levels during formation and differences in host rock compositions.</p>