The Ulandryk and related iron oxide-Cu-REE(-Au-U) prospects in the Russian Altai: A large emerging IOCG-type system in a Phanerozoic continental setting

The Ulandryk and related prospects in the Altaid orogenic belt (near the Russian-Mongolian border) comprise intense iron oxide (hematite), Cu, REE, and locally U mineralization and are estimated to contain in excess of 240 Mt iron oxide ores and 3.7 Mt copper. These prospects bear many signatures of the classic IOCG (-REE, U) deposits typical of Precambrian terranes but, in contrast, are situated in a Phanerozoic (Paleozoic) setting. They are related to the Early Devonian subvolcanic potassic granite stocks with transitional shoshonitic to A-type granite signatures suggesting a post-orogenic (post-collisional) setting, with some anorogenic (intracontinental to within-plate) affinity. The stocks have been intruded into a coeval local volcanic structure (a volcanic dome complicated by the central caldera) composed of Devonian trachyandesite, trachydacite, quartz latite, rhyolite, rhyolite-dacite, and dominant trachyrhyolite lavas, tuffs and subovolcanic porphyry intrusions. The granitic stocks are accompanied by magmatic and multi-staged hydrothermal breccias, zones of sodic (albite-quartz), potassic (quartz-K-feldspar), propylitic (quartz-albite-chlorite-amphibole) and dominant phyllic to carbonate-phyllic (quartz-sericite to quartz-sericite-carbonate) to possibly argillic (quartz-kaolinite) alteration, with intense development of hematite. Fluorite is also ubiquitous and locally abundant. The mineralization occurs in several wide (typically 60-100 m, up to 200 m) and extended (1.4-6 km) linear to arc-like paralleling steeply dipping zones of breccias surrounding the granite stocks, with the host rock and quartz fragments cemented by fine-grained quartz-hematite and coarser hematite (specularite) material. Copper-gold, REE, and locally U mineralization overprints hematite-rich zones. Wider quartz-sulfide stockworks with Cu-Au mineralization zones containing minor to trace hematite are also present. Quartz is generally abundant forming quartzhematite veins, stockworks, and breccias (locally with crustiform texture), and zones of quartzite-like pervasive silicification (typically with sericite and clay minerals).Fluid inclusions in quartz contain daughter halite crystals and homogenize by halite dissolution after vapor bubble disappearance indicating a homogenous, high-salinity (35-45 wt%), Na-Ca-K-chloride, aqueous mineralizing fluid. The relatively low homogenization temperatures (-200-260 degrees C) and generally subvolcanic-level pressure (-0.2-0.5 kbar) conditions suggest a shallow level of mineralization possibly corresponding to an upper (to the uppermost) part of a vertically extended magmatic-hydrothermal system.

Automated summary

The Ulandryk and related prospects in the Altaid orogenic belt are characterized by intense iron oxide, copper, rare earth elements, and uranium mineralization. These deposits exhibit similarities to IOCG deposits but are found in a Phanerozoic setting instead of a Precambrian one. The mineralization is associated with Early Devonian subvolcanic granite and has both post-orogenic and anorogenic characteristics.