Timing of Uralian orogenic gold mineralization at Kochkar in the evolution of the East Uralian granite-gneiss terrane

Abstract

Gold mineralization at Kochkar (Urals, Russia) is hosted mainly by quartz lodes, which developed at lithological contacts between mafic dikes and granitoids of the Plast massif during late Carboniferous to early Permian, regional E–W compression in the East Uralian Zone (EUZ). The alteration mineralogy in mafic dikes comprises biotite, actinolite, albite, K-feldspar, quartz, epidote, tourmaline, sericite, pyrite, arsenopyrite, chalcopyrite, sphalerite, fahlores, galena, bismuthinite, and gold, and in Plast granitoids quartz, sericite, calcite, epidote, and ore minerals. Geochemically, an enrichment of Si, K, Rb, Ba, S, base metals, W, and Au can be observed. The ore fluid had δ¹⁸O values between 8.2‰ and 9.5‰ typical for metamorphic or deep magmatic fluids. The tectonometamorphic evolution of the EUZ is marked by peak metamorphic conditions at 635±40°C and 5–6 kbar through 500±20°C during gold mineralization, and 300–350°C and 2–3 kbar. The last event was dated on a late, barren quartz vein formed during greenschist facies metamorphism at 265±3 Ma by the Rb–Sr method. Fluids related to this overprint had a δ¹⁸O value of 5.2‰ and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70685 indicating that they are largely equilibrated with metamorphic lithologies of the EUZ. The Plast granitoids and the adjacent Borisov granite, which was dated at 358±23 Ma (U–Pb zircon age), have an adakitic character. This, together with the arc-signature of the mafic dikes, supports the setting of the EUZ within the Valerianovsky continental arc. Eastward subduction of the Uralian Ocean below this arc began during the late Devonian to early Carboniferous. Between 320 and 265 Ma, the oblique closure of the ocean resulted in doming of granitoid massifs in a sinistral transpressional regime, subsequent retrograde gold mineralization during E–W compression and a later greenschist facies overprint. This long-lasting retrograde evolution of the EUZ was caused by the lack of postcollisional collapse. Heat for a “deep-later" type of metamorphism and triggering the auriferous fluid system was supplied by radiogenic heating of an overthickened crust. The greenschist facies overprint at Kochkar and coeval crustal melting in the EUZ was additionally initiated by local external heating of the terrane. This could have been caused by syn- to postcollisional slab rollback or delamination resulting in magmatic underplating of the EUZ, which postdates orogenic gold mineralization at Kochkar. The tectonic interpretation of the EUZ indicates that gold mineralization at Kochkar formed in a mid-crustal environment of a continental magmatic arc at the cessation of active subduction predating post orogenic plutonism.

Appendix

Analytical procedure

The composition of amphibole, plagioclase, biotite, and tourmaline was determined by a JEOL JXA-8900R electron microprobe on polished thin sections. Operating conditions were 15 kV and 2 mA. Amphibole, plagioclase, and tourmaline were measured with a focused beam, whereas biotite was analyzed by a defocused beam of 10 μm diameter.

Handpicked quartz from five quartz lodes and one late quartz vein were analyzed for their ¹⁸O/¹⁶O ratios in order to obtain information on the fluid source and characteristic variations in δ¹⁸O values. The data are given in the usual δ-notation versus SMOW (Table 4). Mass spectrometric measurements have been performed on a SIRA 9 triple collector instrument of VG-Isogas at the University of Bonn, Germany. Ni-autoclaves were loaded with 8–10 mg of powdered samples, which subsequently reacted with F₂ at 2 bars and a temperature of 650°C. Reaction usually occurred overnight within 12-15 h. The fluorine was cleaned according to the method described by Asprey (1976). The rest of the analytical procedure (conversion to CO₂) is adapted from the classical method described by Clayton and Mayeda (1963). The CO₂ pressure was used to determine reaction yields, which were between 98 and 100% in most cases. Since sulfide contamination in silicate samples is known to cause fractionation in oxygen isotope compositions, several samples had to be treated with diluted HCl prior to isotope analysis. The data presented in Table 3 are mean values of at least two analyses of the same sample. The accuracy for the analyses lies within±0.2‰.

For Rb–Sr isotope analyses mineral fractions were produced with the help of a magnetic separator, heavy liquids, and final handpicking. Bulk samples and silicate mineral fractions were dissolved with a HF–HNO₃ mixture and HCl (Table 6). Carbonate mineral fractions were leached from powdered samples in HCl. Elements were separated using standard cation exchange techniques. Rb–Sr-isotope ratio measurements were performed on a multicollector Finnigan MAT 261 (Institut für Geowissenschaften und Lithosphärenforschung, Gießen, Germany) and a VG Sector 54 (Mineralogisches Institut, Münster, Germany) solid source mass spectrometer (Table 6). For Sr the mass fractionation corrections were based on ⁸⁶Sr/⁸⁸Sr=0.1194. Repeated analyses of the ⁸⁷Sr/⁸⁶Sr ratio of the NBS 987 Sr standard in the period of analytical activity yielded 0.71023±0.00003 (n=23, 2σ of all analyses) in Gießen and 0.71028±0.00003 (n=29, 2σ of all analyses) in Münster. Total blanks are Rb < 60 pg and Sr < 100 pg. All decay constants are taken from Steiger and Jäger (1977).

Digestion of zircon crystals is performed in Teflon liners designed for the dissolution of six single crystals simultaneously using 24 N HF at 180°C. The digestion is completed after 4–10 days. HF is evaporated on a heating plate. After spiking with a mixed ²⁰⁵Pb–²³³U solution, sample and spike are homogenized in 6 N HCl for 24 h at 80°C. The excess HCl is evaporated finally. The Pb standards as well as the spiked samples are loaded without element separation on single Re filaments into a silica gel bed with 2 μl of a loading solution containing 90 vol.% of 6 N HCl and 10 vol.% of 0.1 N H₃PO₄. Isotope measurements are performed on a VG sector 54 mass spectrometer at the Zentrallabor für Geochronologie, Münster University, equipped with a Daly multiplier (Table 5). The Pb composition is determined at 1,300 – 1,400°C followed by the U measurement at 1,400 – 1,450°C. In most cases signals are only analyzed by the Daly multiplier leading to long times of analysis. At maximum the 2σ error margin obtained for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁶Pb, and ²³³U/²³⁸U is 0.5, 0.5, and 0.1%, respectively. The obtained intensities range mostly at 1 – 2 mV for ²⁰⁶Pb and 0.5 – 2 mV for ²³³U. Isotope ratios are corrected for mass fractionation (0.11%±0.02/a.m.u.), blank (<10 pg total Pb, ²⁰⁶Pb/²⁰⁴Pb=17.7±0.5, ²⁰⁷Pb/²⁰⁴Pb=15.5±0.1, 1 pg U) and initial common Pb estimated after the model of Stacey and Kramers (1975). The isotope ²⁰⁸Pb is not analyzed. This has no impact on the U–Pb dating. However, the total amount of Pb in the sample solution cannot be calculated from the data. Only the sum of ²⁰⁴Pb, ²⁰⁶Pb and ²⁰⁷Pb can be given.