Physicochemical Model of Formation of Gold-Bearing Magnetite-Chlorite-Carbonate Rocks at the Karabash Ultramafic Massif (Southern Urals, Russia)

Abstract

Abstract:We present a physicochemical model for the formation of magnetite-chlorite-carbonate rocks with copper gold in the Karabash ultramafic massif in the Southern Urals, Russia. The model was constructed based on the formation geotectonics of the Karabash massif, features of spatial distribution of metasomatically altered rocks in their central part, geochemical characteristics and mineral composition of altered ultramafic rocks, data on the pressure and temperature conditions of formation, and composition of the ore-forming fluids. Magnetite-chlorite-carbonate rocks were formed by the hydrothermal filling of the free space, whereas chloritolites were formed by the metasomatism of the serpentinites. As the source of the petrogenic and ore components, we considered rocks (serpentinites, gabbro, and limestones), deep magmatogenic fluids, probably mixed with metamorphogenic fluids released during dehydration and deserpentinization of rocks in the lower crust, and meteoric waters. The model supports the involvement of sodium chloride-carbon dioxide fluids extracting ore components (Au, Ag, and Cu) from deep-seated rocks and characterized by the ratio of ore elements corresponding to Clarke values in ultramafic rocks. The model calculations show that copper gold can also be deposited during serpentinization of deep-seated olivine-rich rocks and ore fluids raised by the tectonic flow to a higher hypsometric level. The results of our research allow predicting copper gold-rich ore occurrences in ultramafic massifs.

Formation of Au-Bearing Antigorite Serpentinites and Magnetite Ores at the Massif of Ophiolite Ultramafic Rocks: Thermodynamic Modeling

“…The introduction of the mode of successive passing of a certain amount of portions of the solution allows us to form an equilibrium-dynamic model which can be used for studying the processes of formation and alteration of ore mineralization. A similar approach was earlier used by us for modeling the formation of magnetite-chlorite-carbonate rocks with copper-bearing gold-silver solid solution at the Karabash ultramafic massif [11].…”

“…Modeling was completed for the K-Mg-Mn-Ca-Al-Si-Ti-Fe-Cu-Ag-Au-Cr-Hg-S-Cl-C-H-O system. The thermodynamic properties of minerals, binary, ternary and quaternary solid solutions, aqueous, and gaseous species considered in the models were the same as in [11] (Tables S2 and S3 of the Supplementary Materials from [11]). Thermodynamic constants for chlorites, ilmenites, pyroxenes, carbonates, olivines, and plagioclases, which are natural binary and ternary solid solutions, as well as solid solutions of quaternary system Ag-Au-Cu-Hg, were calculated considering the activity coefficients of end members for the accepted models of solid solutions [33,35,54,56]…”

“…A necessary condition for the formation of hydrothermal rich gold-ore mineralization in magnetite veins is mixing of metamorphic and meteoric fluid at T = 500-400 • C and P = 2-3 kbar and discharge of deep-seated and meteoric fluids in the open space of cracks. The mechanism of mixing of the deep-seated and meteoric fluids was used earlier in modeling of the formation of Au-REE mineralization in magnetite-chlorite-carbonate rocks from the Karabash ultramafic massif [11].…”

Editorial for the Special Issue: Experimental and Thermodynamic Modeling of Ore-Forming Processes in Magmatic and Hydrothermal Systems

“…The reconstruction of the sources of ore components and fluids, physicochemical parameters, and the mechanisms of accumulation and separation of elements during the formation of ore deposits is an important task in addressing the fundamental problems of ore-forming processes. Some of the papers in this issue deal with experimental and thermodynamic modeling [19][20][21][22][23][24][25], while the others are devoted to the analytical geochemistry, geochronology, and genesis of some ore occurrences [26][27][28].…”

“…Murzin and coauthors [22] present a physicochemical model for the formation of magnetitechlorite-carbonate rocks with copper-gold in the Karabash ultramafic massif (Southern Urals, Russia). This massif is located within a belt of ultramafic massifs stretching along the Main Ural fault zone in the Southern Urals.…”