Thermometry of Nickel Bearing Chlorites from the Kolskii Massif (Northern Urals)

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The chemical composition of chlorites from the rocks that are conventionally classified as the residual weathering mantle of the Kolskii ophiolite massif was studied. The chlorites exhibit high Mg/ (Mg + Fe) atomic ratio values (Mg#) of 0.78–0.96 and elevated Si content (2.95–3.74 apfu), but are relatively poor in Al (1.28–2.66 apfu). In terms of octahedral occupancy (RVI is 5.52–5.98 and [R3+]VI is 0.87–2.04 apfu), they are classified as the trioctahedral subgroup. The NiO content in the chlorites varies from 0.2 to 21 wt %; in addition, the tabular low-Ni and high-Ni chlorite grains are often tightly intergrown. There is a pronounced negative correlation between NiO and MgO content. The crystallization temperature estimated using chlorite geothermometers varies widely. The crystallization temperature interval is 125–300°C or higher with a statistical maximum in the region of 175–300°C for the low-Ni chlorites and 50–250°С with a statistical maximum in the region of 75–125°С for the high-Ni chlorites. In addition, the high-Ni chlorites demonstrate a gradual decrease in temperature as the nickel content increases. This correlation indicates the important role of temperature as an ore generation factor during the formation of the oxide–silicate nickel deposits that are associated with the Kolskii massif. These tendencies support the conclusion that the hydrothermal processes not only preceded lateritization, but also played a significant part in the level of nickel concentration in phyllosilicates.