PHYSICAL AND CHEMICAL CONDITIONS FOR THE FORMATION OF THE ORLOVKA OROGENIC GOLD DEPOSIT IN THE SOUTHERN URALS (ACCORDING TO THE DATA OF A CHLORITE
 GEOTHERMOMETER AND REE DISTRIBUTION IN CALCITE)

Abstract

The chemical composition of chlorite from ore-bearing albite-biotite-quartz-chlorite-epidote-actinolite schists and the REE content in calcite of sulfide-carbonate-quartz veinlet ores of the Orlovsky orogenic gold deposit located in the zone of the Main Ural Fault in the Southern Urals have been studied. The chemical composition of chlorite was determined on a Tescan Vega Compact scanning electron microscope with an Oxford Instruments Xplorer 15 energy-dispersive analyzer. The REE content in calcite was studied using inductively coupled plasma mass spectrometry (ICP-MS) on a PerkinElmer ELAN 9000 instrument. It has been established that chlorite, represented by a variety with a transitional chemical composition from clinochlore to pycnochlorite (5.78–6.0 Si f.u.; 0.27–0.28 Fetot/(Fetot + Mg)) was formed at a temperature of 295.6–266.2°C. The REE distribution spectra in calcite, normalized to C1 chondrite, are characterized by the accumulation of light lanthanides ((La/Yb)N=1.28–7.18), the value of the ratio (La/Lu)N=1.1–6.58, indicating the predominance of rare earth sorption processes in the fluid, small negative anomalies of cerium (Ce/Ce*=0.71–0.92) and positive anomalies of europium (Eu/Eu*=1.05–2.38). Negative Ce anomalies in calcite may be due to the interaction of the fluid with limestones, as well as the presence of meteoric water in its composition in a small amount. Positive Eu anomalies indicate low-temperature conditions for calcite crystallization (<250°С). A model of deposit formation is proposed, which provides for the generation of mineral-forming fluid at the progressive stage of greenschist dynamometamorphism of host rocks and the formation of calcite and gold-bearing mineralization at the regressive stage. Data on REE geochemistry confirm the hydrothermal-metamorphogenic genesis of the gold-bearing sulfide-carbonate-quartz mineralization of the Orlovsky deposit. Keywords: Southern Urals, orogenic gold deposit, chlorite geothermometer, REE distribution, europium anomaly.