A Cl-Dominant Analogue of Annite Occurs at the Eastern Edge of the Oktyabrsky Cu-Ni-PGE Deposit, Norilsk, Russia

A Cl-rich annitic mica is present in zones in taxitic gabbro–dolerite enriched in base metal sulfides in the eastern portion of the Oktyabrsky deposit in the Norilsk complex (Russia). Other Cl-enriched minerals in the assemblage include hastingsite (4.06 wt.% Cl), ferro-hornblende (2.53 wt.%), and chlorapatite (>6 wt.%). New wavelength-dispersive electron probe analyses reveal compositions with up to 7.75 wt.% Cl, corresponding to the formula K0.742Na0.047Ca0.007)Σ0.796 (Fe2+2.901Mg0.078Mn0.047Ti0.007Cr0.003)Σ3.036 (Si3.190Al0.782)Σ3.972O10 (Cl1.105OH0.854F0.041)Σ2.000 based on 22 negative charges per formula unit, in which OH(calc.) = 2 − (Cl + F). Unfortunately, the grain size of the Cl-dominant mica precluded a single-crystal X-ray diffraction study even though its EBSD pattern confirms its identity as a member of the Mica group. We present results of a refinement of a crystal from the same mineralized sample containing 0.90(6) apfu Cl [R1 = 7.89% for 3720 unique reflections]. The mica is monoclinic, space group C2/m, a 5.3991(4), b 9.3586(6), c 10.2421(10) Å, β 100.873(9)°, V = 508.22(7) Å3, Z = 2. We also describe physical properties and provide a Raman spectrum. Among the mica compositions acquired from the same sample, a high Cl content is correlated with relative enrichment in Si, Mn, and Na and with a depletion in Al, Mg (low Mg#), K, Cr, and Ti. The buildup in Cl in the ore-forming environment is ultimately due to efficient fractional crystallization of the basic magma, with possible contributions from the Devonian metasedimentary sequences that it intruded.