Structural controls of lode-gold mineralization by mafic dykes in late-Paleozoic granitoids of the Kochkar district, southern Urals, Russia

The Kochkar gold district in the East Uralian Zone of the southern Urals is located in late-Paleozoic granite gneisses of the Plast massif. Gold miner alization is associated with tabular quartz lodes that are preferentially d eveloped along the margins of easterly trending mafic dykes. Fabric develop ment indicates that dykes had a profound influence on the development of sh ear zones in granitoids. ENE- and SE-trending dykes have been reactivated a s dextral and sinistral oblique strike-slip shear zones, respectively, form ing a set of approximately conjugate shear zones related to the Permian, re gional-scale E-W directed shortening. Dyke-shear zone relationships in the Plast massif are the result of strain refraction due to the presence of bio tite-rich, incompetent dykes in more competent granite-gneisses. Deformatio n and the formation of associated gold-quartz lodes occurred close to peak- metamorphic, upper-greenschist to lower-amphibolite facies conditions. Stra in refraction has resulted in partitioning of the bulk strain into a compon ent of non-coaxial mainly ductile shear in mafic dykes, and a component of layer-normal pure shear in surrounding granitoids where deformation was bri ttle-ductile. Brittle fracturing in granitoids has resulted in the formatio n of fracture permeabilities adjacent to sheared dykes, that together with the layer-normal dilational component, promoted the access of mineralizing fluids. Both ore-controlling dykes and gold-quartz lodes were subsequently overprinted by lower greenschist-facies, mainly brittle fault zones and ass ociated hydrothermal alteration that post-date gold mineralization.