POSTMAGMATIC MINERAL ASSEMBLAGES IN RHYOLITES OF THE DUKAT VOLCANO-PLUTONIC STRUCTURE AND THEIR RELATION TO ORE MINERALIZATION

Abstract

Postmagmatic mineral assemblages were studied within the volcanic sequence 400 m thick at the Dukat volcano-plutonic complex. Its largest surface exposure (around 15 km2) northern part of the complex includes the ore bodies of the unique Au-Ag Dukat deposit. Postmagmatic minerals fill in a system of vugs and veinlets predominantly in fluidal rhyolites. The preore postmagmatic the quartz-clinozoisite-chlorite assemblage occupies the outer alteration zone, and the quartz-chlorite-phengite one constitute the inner zone. The carbonaceous matter is typical especially in the tuffisite bodies intruding rhyolites. Propylitic minerals were formed at a depth of 800-1200 m under reduced conditions at 250-350°C according to the composition of coexisting minerals. Postore minerals belong to the todorokite-vernadite-kaolinite and vernadite-goethite-montmorillonite assemblages. They were deposited later, when the ore-bearing block was rised and involved in the circulation of the low-temperature hydrothermal solutions. The former of these assemblages is observed in the inner zone, while the later-is observed in the outer one. The quartz-chlorite-sulfide-phosphate assemblage occurs in some local zones (<0.5 km2) within the Early Creataceous potassic rhyolites. It can relate to the earliest cycle of the Early Creataceous Au-U ore deposition. Epidote-quartz-chlorite assemblage of postore minerals composes some linear propylitic zones including the latest Sn-Ag-base metal ore mineralization. The Cr-containing mica (fuchite) is common in some local zones, and so it is possible that basic rocks were strongly influenced by preore hydrothermal solutions.