The first finds of cuprous gold in the Riphean deposits of the Bashkir meganticlinorium (Southern Urals)

Abstract

Introduction: The article describes finds of native gold with a high content of copper (32.9-38.2 wt. %), first discovered in the Riphean deposits of the Bashkir meganticlinorium. Methods: Electron microscopy with energy dispersive microanalysis; atomic emission and X-ray fluorescence analyses were used in the study. Results and discussions: In the south-western part of the Bashkir meganticlinorium cuprous gold was discovered in three places significantly remote from each other: in the West Aktash ore zone, in the Suran sellaite-fluorite deposit, and in the Katav limestones on the Zilim River. Cuprous gold in all localities is characterized by a similar rare chemical composition corresponding to the composition of Cu 3 Au 2, with impurities of Ag (4.7–7.4 wt. %) and Zn (~1 wt. %), sometimes Fe (0.8 wt. %). Cuprous gold is always associated with chalcopyrite, and its host rocks are enriched in Cu, less often in Ba, Zn, Pb, and As. The temperature of its formation is estimated at 400–450°C. In the West Aktash zone and the Suran deposit, cooper gold is confined to endo- or exocontacts of gabbrodolerites, which, in terms of petrochemical features, are similar to gabbrodolerites of the Late Riphean Inzer complex and differ from gabbrodolerites of the Late Paleozoic Yurmatinsk complex in higher alkalinity, lower calcareous content, higher contents of V and Zn, and lower contents of Cr, Ni, and Cu. Conclusions: Cuprous gold in the Riphean deposits of the Bashkir meganticlinorium is genetically related to the gabbrodolerites of the late Riphean Inzer complex and the activity of postmagmatic fluids. The formation of cuprous gold in the West Aktash zone and in the Katav limestones on the Zilim River at the retrograde stage of the skarn process is not excluded.