Mineralogy, Geochemistry and Localization of Regional Pyritization Zones – Constraints from Early Mesozoic Deposition in the Chay–Yureya Fault of the Kular-Nera Terrane, NE Russia

The Early Mesozoic clastic rocks the Kular-Nera terrane in the Verkhoyansk-Kolyma orogenic belt, northeast Russia, exhibit regional sulfidation zones. The most abundant mineral of the zones is pyrite. However, its origin (sedimentary-diagenetic, metamorphogenic-hydrothermal, metasomatic) and contribution to the formation of economically important gold deposits remain debatable. The localization of the sulfidation zones is still poorly understood. We have studied geological-structural and mineralogical-geochemical features of pyritization zones in bedrock outcrops on Khara-Yuryakh creek (a right-side tributary of the Nera river) within the Kular-Nera terrane, Verkhoyansk-Kolyma folded area. The pyritization zones are distant from the known gold deposits, and they extend along the Chay-Yureya regional fault. The deformation structure of the clastic rocks of the Chay-Yureya fault is defined by different-aged fold-thrust and strike-slip elements. The early deformations are isoclinal and tight folds up to a few hundred meters wide, with rounded and sharp crests and subhorizontal hinges. The early folds were refolded so that their crests may be seen on the limbs of later folds of the same NW strike. These structures were formed in the conditions of progressive deformation D1 in course of a single continuous (non-stop) tectonic regime during the frontal convergence of the Kolyma-Omolon superterrane with the eastern margin of the Siberian craton. These are commonly inclined or, more rarely, recumbent folds. Also present are late folds associated with dextral and sinistral strike-slip motions on the Chay-Yureya fault. The sulfide mineralization of the Chay-Yureya fault is represented by the disseminated idiomorphic pyrite crystals 1 to 10 mm in size. Metapyrites exhibit a cataclastic microtexture complicated by later corrosion processes. They contain zircon, rutile, and monazite microinclusions entrained in the process of growth. Microinclusions of galena, sphalerite, chalcopyrite, and other late sulfides are confined to defects in the pyrite grains. The chemical composition of pyrite was determined on the Camebax-Micro microanalyzer, using a standard X-ray spectral analysis (DPMGI, SB RAS, Yakutsk). Most of the grains have a stoichiometric composition. 30 percent of the analyses showed excess Fe. Metacrystals of pyrite demonstrate variations in the concentration of trace elements, which often leads to chemical zonality. Our investigations showed that pyritization zones of the rocks are localized in the trans-crustal Chay-Yureya fault which served as a transit path for ascending regional fluid flows. Typomorphic trace elements found in pyrites are Co, Ni, As, Sb, and Cu, with the total amount of 0.1 to 0.4%. They are characterized by the low concentrations and nonuniform distribution. Zonality in the distribution of trace elements is attributable to the poly-