OXYGEN ISOTOPIC STUDIES OF THE ASSIMILATION OF PARAGNEISS XENOLITHS AT THE VOISEY’S BAY CU-NI-CO DEPOSIT, LABRADOR

The Voisey's Bay deposit, located in Labrador, is a recognized example of sulfide mineralization in a plutonic conduit system. Massive Cu-Ni-Co sulfide mineralization occurs in a dike linking two magma chambers. Mineralization also occurs in adjacent troctolitic to gabbroic bodies and breccia zones. The high grade of the mineralization is attributed to the passage of distinct magma pulses and the subsequent extraction of metals by accumulated sulfides. Contamination by both country rock sulfur and silica has been proposed as a key process in the generation of the Cu-Ni ores.

Results of ongoing studies of the mechanisms of xenolith digestion and their effects on sulfide mineralization confirm that plagioclase in the breccia matrix is characterized by d¹⁸O values < 6.6 ‰; at most only slightly anomalous with respect to values typical of magmas of mantle origin. Country rock paragneiss are characterized by d¹⁸ O values of 8-14 ‰. Xenoliths of paragneiss have been converted to plagioclase, corundum, hercynite, biotite, cordierite, hypersthene and magnetite which are frequently pseudomorphous after earlier phases. Rims of xenoliths are enriched in plagioclase with d¹⁸O values of 7.4-13.3 ‰. In the western portion of the deposit, where xenolith digestion has been less intense, plagioclase and corundum-rich cores of xenolith exhibit elevated d¹⁸ O values of 10.5-12.5 ‰. In the eastern portion of the deposit, where xenolith digestion is more advanced, cores show much lower d¹⁸O values of 6.2-7.5 ‰. Hercynite in the xenolith shows the same relative relationship, with d¹⁸O values in excess of 8 ‰ in the western portion of the deposit and values generally in the range of 3-6 ‰ in the east. The lower d¹⁸O values of the xenolith core material where digestion is most complete indicate that a high ¹⁸O melt was removed. d¹⁸ O enrichment in the xenolith margins is not sufficient to account for the high ¹⁸O material lost from the xenolith cores. The absence of anomalous ¹⁸O-enriched troctolitic matrix indicates that magmas have been transported from the sites of xenoliths, and confirms the premise that the passage of magmas through the conduit system was another key process for mineralization. Observed d¹⁸O profiles around the xenoliths are less than ~ 2 cm and have been produced by sub-solidus diffusion in times less than 100 years.