Fluid-rock interaction constraints from geochemical modelling of the mineral paragenesis and system of the Viscaria Cu-Fe deposit, Kiruna district, Northern Sweden

The previously mined Viscaria Cu-Fe deposit consists of three stratabound orebodies, named the A, B and D zone. Historic copper mining focused on the highest-grade A zone. Sulfide mineralization is distributed within several ore zones of a 1 km thick sequence of steeply tilted volcano-sedimentary Paleoproterozoic greenstones. The deposit was originally interpreted as a syngenetic exhalative deposit, which was partially enriched and altered during subsequent footwall alteration (Martinsson, 1997). Recent exploration with deep drilling by Gruvaktiebolaget Viscaria AB was undertaken with the purpose of restarting copper production and unlocking the exploration potential of the broader mineral system. This work has shone a new light on the deposit by redefining the spatial relations of the alteration zones and uncovering new mineralized lenses at depth, including a mineralized body between the A and B zone, aptly named the ABBA zone. In combination with the proximity to the world-class Kiirunavaara Fe deposit, and its location within a broader metasomatic iron alkali-calcic (MIAC) mineral province, the Viscaria Cu deposit has been reinterpreted by some as an epigenetic IOCG-style deposit (Imaña et al., 2023).

This study investigated the recently discovered ABBA zone to chime in on this ongoing debate. The evolution of the mineral system was first constrained through detailed study of drill core, petrography, mineral geochemistry and lithogeochemistry. These results provided the boundaries in which the geochemical modelling work was fitted by first constructing a detailed mineral paragenesis and conceptual fluid evolution model. Alteration zones in and around the ABBA zone are dominantly replacive, developing into more vein-hosted mineral assemblages over time. The replacive alteration assemblages are well-suited to geochemical modelling, as they indicate pervasive fluid-rock interaction.

Geochemical modelling was performed using the Gibbs Energy Minimization Selektor (GEMS) code package with the MINES 2023 thermodynamic database. A combination of titration, flush and flow-through model set-ups were used to constrain both the influence of the fluid-rock ratio and fluid evolution through fluid-rock interaction. Physiochemical fluid conditions were derived from previous work on the Viscaria deposit, from regionally similar deposits and further constrained by equilibrium with the Viscaria alteration assemblage. The influence on alteration of the diverse volcano-sedimentary host rock sequence, consisting of black schists, basic tuffs and carbonates, was tested. Geochemical fluid-rock interaction modelling shed light on some key ingredients of the Viscaria Cu-Fe mineral system, including host rock composition, physiochemical fluid characteristics, fluid-rock ratios and fluid evolution. The results of this study support an epigenetic origin by fluid-rock interaction with MIAC-style fluids for the Viscaria Cu-Fe deposit.