Copper Shales of the Prikolymsky Terrane (Northeast of Russia): Mineralogical and Geochemical Features and Ore-Forming Conditions

Abstract

Copper mineralization in the Magadan oblast (Northeast of Russia) has been established in Middle–Upper Riphean quartz–chloritoid and chlorite shales of the of the Prikolymsky terrane. Compared to the upper crust, copper shales of the Oroek ore occurrence are noticeably enriched in a narrow range of trace elements: Cu, Ag, Au, Rh, and Se, with enrichment factors ranging from 10-fold (Se, Rh) to 100-fold (Ag, Au) and even 1000-fold (Cu) suggesting, apparently, their synchronous involvement in ore formation. In addition, the studied samples are slightly enriched in Cd, Li, Co, Zn, V, U, Sc, Y, and REE. Copper shales show relatively flat rare earth element (REE) patterns similar to the chondrite-normalized pattern and have no distinct positive or negative Eu anomalies and are dominated by light lanthanides. The Ce/Ce* and Eu/Eu* ratios indicate that ore formation proceeded under oxidizing conditions. The geochemical data we obtained show that the Riphean host rocks could serve as a source of trace elements and REE ore-forming fluids. In the ores, covelline and chalcocite are the main copper minerals; less common are roxbyite, idaite, nukundamite, spioncopite, chalcopyrite, silver-bearing (Ag about 1 wt %) bornite, and yarrovite, as well as native gold. Fluid inclusions in quartz suggest that copper mineralization was formed at temperatures of 212–190°C and a pressure of 1 kbar from hydrothermal solutions of medium salinity (from 13.8 to 12.4 wt % eq NaCl) and high density (0.95–0.98 g/cm³) saturated with chlorides of Ca, Mg, and K. The copper shales of the Prikolymsky terrane differ from sedimentary copper and copper shales of the African Copper Belt in low Co and Ni contents and from the Kupfershifer in low Pb and Zn contents. The results we obtained can be used for predicting new deposits.