Structural controls, mineralogy of gold-silver ores, and a proposed formation model for the Arkharly deposit (Kazakhstan)

Purpose. To determine the role of structural-tectonic factors in the localization of gold-silver mineralization at the Arkharly deposit, characterize the mineralogical composition of the ores, and substantiate a conceptual model of deposit formation. Methods. The study is based on an integrated analysis of geological exploration materials, geological maps, data on the spatial distribution of faults and orebodies, mineralogical descriptions, and published sources. The structural analysis involved comparing the orientations, morphologies, and spatial relationships of faults, quartz veins, brecciation zones, and zones of vein-disseminated mineralization. Mineralogical data were systematized with consideration of quartz generations, the composition of sulfide assemblages, the modes of occurrence of gold and silver, and hydrothermal and supergene alteration of the rocks. Findings. The principal factor controlling the localization of mineralization is a system of approximately east-west- and northwest-trending oblique-slip faults with normal and strike-slip components formed under a dextral strike-slip regime. Four types of ore zones were distinguished: right- and left-stepping en echelon fracture zones, deformation and ductile shear zones, and extensional fracture zones within subvolcanic rocks. Northwest-trending right-stepping en echelon fracture zones are the most productive. Ore shoots and zones with elevated Au and Ag grades are confined to fault-intersection nodes and segments characterized by changes in vein orientation. The ores comprise several generations of quartz, pyrite, galena, sphalerite, chalcopyrite, native gold, and silver-bearing phases. The principal economic mineralization is associated with late generations of gray and dark-gray quartz. Based on the combined evidence, the deposit is provisionally classified as a low-sulfidation epithermal system. The proposed model comprises structural preparation of the ore field, early quartz formation, the principal stage of quartz-sulfide mineralization, and supergene redistribution of precious metals. Originality. A conceptual model is proposed that integrates the structural evolution of the volcanic dome, the types of ore-hosting faults, orebody morphology, and the stages of mineralization. The role of repeated fault reactivation in the formation of ore shoots is demonstrated. Practical implications. The obtained results can be used to predict analogous mineralization in South Junggar and other volcano-plutonic belts of Central Asia with similar geological settings. The principal exploration indicators include right-stepping en echelon fracture zones, fault-intersection nodes, brecciation zones, silicification, and hydrothermal alteration of the host rocks.