Isotopic composition of lead from dikes and ores of the Vorontsovkoe gold deposit (Northern Urals)

Research subject. The Vorontsovskoe deposit belongs to gold deposits, in which gold is disseminated finely across carbonate-terrigenous rocks (Carlin-type). This study was aimed at identifying the sources of lead in various ore types (in skarn, jasperoid, tuff-sandstone and limestone breccia) and evaluating the role of magmatic rocks (apophises of Auerbakh intrusions and dykes of medium-basic composition) in the mineralization.

Materials and methods. Ore and rock samples for isotopic analysis were collected in the northern quarry of the deposit, as well as were retrieved from exploration holes. The lead isotopic composition  was studied in the analytical centre Geoanalytic, Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences, using a mass spectrometer Neptune Plus on the basis
of the TLN MC ICP MS method.

Results. The lead isotopic composition of ores and igneous rocks indicates mixed mantle-crustal  sources. The primary lead isotopic composition of igneous rocks corrected for the age of 400 Ma is characterized by a linear trend with wide variations in 207Pb/204Pb (15.6824–15.5182) and 206Pb/204Pb (18.1973–17.7707). The lamprophyre dykes feature the least radiogenic lead located between the evolutionary curves for the mantle and orogen in the Doe–Zartman model. The most radiogenic lead is typical of the granodiorite of the Auerbakh intrusion apophises. The analysis points of diorite porphyrite, gabbro-diorite and diorite dykes are located between the orogen and upper crust curves. The sulphide ores are characterized by lower variations in the primary lead isotopic composition compared to igneous rocks – 207Pb/204Pb (15.6009–15.5421) and 206Pb/204Pb (18.0434–17.8404). The most productive ores in tuff-sandstones and limestone breccias are  characterized by a significant predominance of the crustal component over the mantle component. Their points in the diagram 207Pb/204Pb–206Pb/204Pb are located between the curves for the orogen and the crustal source with µ2 (238U/204Pb) = 9.74.

Conclusion. Mantle lead in the dykes under study appears to reflect the nature of their parent magmas and may have been partially extracted by melts from the footwall mafic rocks containing pyrite ores. However, crustal lead is likely to have resulted from assimilating host rocks when magmatic melts  reached the upper crust level. Crustal lead migrated with the metamorphic fluid formed by the heat action of the Auerbakh intrusion on sulphide-bearing volcano-sedimentary rocks. Mantle lead might have been transported by the fluid flow from a deepseated magmatic chamber.

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