Modification of sedimentary barite textures during deformation, Gataga District, NE British Columbia

Abstract

Stratiform BaFeZnPb mineralisation in the Gataga District of northeast British Columbia, is hosted within folded and thrust fine-grained black siliciclastics of the Mid-Upper Devonian Lower Earn Group. Deformation of the barite at 280°–350°C and 2–2.5 kb was associated with the formation of chevron-style folds, an intense pressure solution cleavage and localised development of barite mylonites. Although the barite-rock has retained its gross stratiform characteristics, this textural investigation demonstrates that it has undergone considerable internal remobilisation in that both the primary texture and chemistry have been modified. Low tectonic strains are associated with the development of a grain shape fabric and dissolution seams. With increasing strains, subgrain development and recrystallisation become dominant. Moderate strains are related to the development of microfolds and a polycrystalline aggregate with a strong axial planar grain-shape fabric. Dynamic recrystallisation, extreme grain-size reduction and development of a preferred crystallographic fabric is associated with high shear strains and the formation of barite mylonite containing abundant kinematic indicators. At low to moderate strains remobilisation of barite was dominated by fluid-assisted diffusive mass transfer. Dissolution and diffusion in the presence of a high-salinity basinal brine is proposed as the main cause for the initial alteration of the primary barite texture and chemistry. Remobilisation by intracrystalline plasticity at moderate to high strains was responsible for wholesale recrystallisation and obliteration of primary textures and places doubt on the recognition of ‘graded’ and ‘cross-bedded’ features in polycrystalline barite.