Nickeloan tourmaline from the Berezovskoe gold deposit, Middle Urals, Russia

Nickeloan tourmaline (up to 0.531 pfu Ni) occurs associated with nickeloan talc (0.511-0.582 pfu Ni) or chromian muscovite in magnesite-quartz veins hosted in ultramafic talc + magnesite + or - hematite + or - chromian muscovite hydrothermal alteration in Uspenskaya Gorka, eastern flank of the Berezovskoe Au deposit. The unusual Ni-rich composition of the silicates reflects the environment of formation. The tourmaline forms light to dark green acicular crystals up to 2 cm in length and is strongly pleochroic. The cores of tourmaline associated with talc are magnesiofoitite with an elevated Na content (0.436-0.452 pfu) and a Mg/(Fe (sub tot) + Mg) value of 0.90, or dravite (Na 0.535-0.75 pfu, Mg/(Fe (sub tot) + Mg) 0.87-0.88). In the magnesiofoitite, Ni and Cr contents range from 0.060 to 0.425 pfu and from 0.214 to 0.249 pfu, respectively. Intermediate and rim zones are magnesiofoititic and dravitic and have Ni and Cr 0.020 to 0.531 pfu and 0.188 to 0.448 pfu. The distribution of Mg/(Fe (sub tot) + Mg) within single crystals is oscillatory . Partition coefficients for Ni and Fe (sub tot) between tourmaline and talc are 0.05 and 21.48, respectively. A Moessbauer study shows that about half of the total Fe is Fe (super 3+) in the tourmalines. The Cr and Ni were probably derived from altered ultramafic rocks.