MINERALOGY, GEOCHEMISTRY AND GOLD MINERALIZATION OF Au-Cu SKARNS OF THE EXOCONTACT OF KNYAS’PA DIORITE-GABBRO INTRUSION (NORTHERN URALS)
I.F. Chayka1, S.Yu. Stepanov 2, A.V. Kozlov3, F.D. Sandalov4, R.S. Palamarchuk2, N.I. Baykov1, V.S. Zhdanova5, V.D. Abramova4
1Korzhinskii Institute of Experimental Mineralogy RAS, Chernogolovka, Russia ivanlab211@gmail.com
2Natural science museum of the Ilmeny Reserve, Southern Ural center for mineralogy and geoecology UB RAS, Miass, Russia
3Saint Petersburg Mining University of Empress Catherine II, Saint-Petersburg, Russia
4Institute of geology of ore deposits, petrography, mineralogy and geochemistry RAS, Moscow, Russia
5Zavaritsky Institute of Geology and Geochemistry UB RAS, Ekaterinburg, Russia
Keywords: skarn, Cu-Fe skarn formation, native gold, gabbro, S isotopes, Ural Platinum Belt
Abstract
Skarn-type Fe and Cu deposits enriched in Au are widespread within the Tagil–Magnitogorsk megazone of the Ural Fold Belt and are primarily associated with felsic to intermediate intrusions. In the Northern Urals, Fe–Cu skarn-type deposits and ore occurrences are found in the exocontacts of gabbroic phases of the polyphase intrusions of the Ural Platinum Belt. These systems are of particular interest as they represent the terminal (hydrothermal–metasomatic) stage in the fractionation of chalcophile and noble metals within magmatic systems of young island arcs. The metasomatic rocks developed in the exocontact zone of the Knyaspa intrusion, investigated in this study, contain ore-grade concentrations of Fe and Cu (0.5–5 wt%) and Au (0.2–14 g/t), and are classified as belonging to the Cu–Fe skarn formation of the Ural belt. The geological, mineralogical, and geochemical characteristics of this occurrence are largely typical of Cu–Au skarn deposits and are genetically linked with the gabbroic phase of the intrusion, rather than with the dioritic one. The metasomatic rocks formed after andesibasalts of the Pavda Formation. The following sequence of their formation has been reconstructed: (1) amphibole–plagioclase or clinopyroxene–plagioclase hornfelses (hornfels phase); (2 epidote-bearing assemblages with clinopyroxene or garnet (pre-ore skarn stage of the hydrothermal-metasomatic phase); (3) clinopyroxene–actinolite–epidote associations with magnetite and Cu sulfides (ore skarn stage of the hydrothermal-metasomatic phase); (4) largely zeolite assemblages (late hydrothermal stage of the hydrothermal-metasomatic phase) and (5) supergene phase. The estimated temperatures for the formation of primary sulfide mineralization range from 300 to 400 °C. Formation of native gold, which has exclusively broad compositional range in terms of Cu and Ag admixtures, took place at lower temperatures (approximately 100–250 °C) and probably continued during supergene phase Based on δ34S values in sulfides and the geochemistry of chalcophile elements in skarns, the ore elements were probably orthomagmatic. However, the mobilization of S and Cu from the volcanogenic massive sulfide deposits of the Shemur Formation through their assimilation by the intrusion cannot be ruled out, as well. Regardless of the source, the redox state of sulfur was significantly shifted toward oxidized species (S⁶⁺ or S⁴⁺), resulting in unusually low δ³⁴S values (–6 to –4‰) in the sulfides of the metasomatic rocks.
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