Mineralogy, fluid regime and potential ore content of the post-accretion Ivdel gabbrodolerite-monzonite-granodiorite complex (Northern Urals)

Abstract

Research subject. Hypabyssal intrusive bodies of a gabbro-dolerite-monzonite-granodiorite composition, common among the Devonian island-arc sequences of the East Tagil structural-formational zone in the Northern Urals. These bodies are dated as the Late Devonian-Early Carboniferous and attributed to the post-accretionary continuously differentiated Ivdel complex. Material and methods. The composition of rock-forming and accessory minerals of these intrusive bodies was studied using an SX-100 microanalyzer. This made it possible to obtain new data on the conditions of their formation, features of the fluid regime, and potential ore content. In addition, the geodynamic setting of this period was clarified, along with the composition, possible position of the primary mantle source, and the features of its fluid regime. Results. It was established that the mafic rocks (dolerites) in the Ivdel complex correspond to derivatives of the depleted mantle, with a primary source close to N-MORB basalts. The connection between the source and the separation of the slab and the “mantle window” located under the frontal zone of the former East Tagil island arc was substantiated. Such a position of the melting chamber determines the primary weak water saturation of dolerite magmas and their redox regime. According to mineralogical data, this feature is further manifested in the formation of the entire series of rocks of the Ivdel complex (with an increase in the iron index of clinopyroxenes and amphiboles, an increase in titanium contents in titanomagnetite, and other data). Further evolution of magmas (from dolerites to monzonites, quartz diorites, and granodiorites) occurred as they moved upward to the Earth’s surface and reflected the increasing contamination of differentiated melts by suprasubduction island-arc matter. Another important petrological feature of this complex is its formation with the participation of a chlorine-rich magmatogenic fluid. Depending on the composition of intruding melts, their contents of water, chlorine and oxygen activity, a number of autonomous degassing fluid-magmatic systems are formed. These systems are characterized by noticeable features in the processes of post-magmatic autometasomatosis, with the participation of an iron-chloride fluid. According to the regime of volatiles (halogens, water, and oxygen), the common affiliation of such autonomous fluid-magmatic systems to titanomagnetite ferrofacies (according to G.B. Fershtater), determines both their general metallogenic specialization and potential ore content. The removal of volatiles from such fluid-magmatic systems could be accompanied by extraction and transfer of chlorophyll ore elements from crystallizing melts to exocontacts of intrusive bodies. This, in the presence of migration zones favorable for fluids (faults, crushing zones, etc.) and geochemical barriers (especially in the presence of favorable enclosing sulfide-bearing island-arc strata), could have contributed to the formation of hydrothermal-metasomatic ore mineralization (Cu, Zn, Au, Mo and etc.). Such ore-bearing zones during erosion could be the primary source of gold for the known placers of this region. Large bodies of gabbrodolerites of the Ivdel complex may be promising for high-Ti magnetite-ilmenite mineralization.