Thermometry of Ni-bearing chlorites in the Kolskiy massif (Northern Urals)

R. K. Ilalova, Yu. L. Gulbin

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2018. V. 147. N 5. P. 1-17

https://doi.org/10.30695/zrmo/2018.1475.00

Full text is available on eLIBRARY.RU

Language: Russian

Abstract

The Kolskiy massif is located in the eastern slope of the Northern Urals and belongs to the northern branch of the Ural Ophiolite Belt. This massif extends for over 15 km in the meridional direction and is hosted by middle Paleozoic metavolcanic and metasedimentary rocks. In the south-west it is associated with porphyrite and quartz diorite intrusions. During Paleozoic and Mesozoic tectonic movements, the massif was broken into three blocks, two of which were lowered and covered by continental sediments of Jurassic, Cretaceous and Paleogene age. Beneath Mezozoic and Cenozoic sediments, the ancient Triassic—Jurassic weathering rinds are developed on serpentinite. The rinds are divided into two groups: the residual rinds and the transformed infiltrative metasomatic rinds. The most complete alteration profile of the residual rind occurs in the southern part of the massif. It consists of three zones which include from bottom to top: disintegrated serpentinites, leached serpentinite, and oxidized ser pentinite. Among supergene minerals, the main role is played by rock-forming Ni-bearing phyllosilicates: Ni-serpentines and Ni-chlorites, Ni-talc, «kerolite», Ni-vermiculite. A study of the mineralogy in this profile including electron microprobe analysis (EPMA) and micro-Raman lazer spectroscopy showed that chlorites are enriched in Mg (with Mg/[Mg+Fe] atomic ratio is 0.78— 0.96), Si (2.95—3.74 apfu), and depleted in Al (1.28—2.66 apfu). Many of the values for octahedral occupancy (R^VI 5.52—5.98, [R³⁺]^VI 0.87—2.04 apfu) are similar to those needed for trioctahedral chlorites. NiO content in chlorites varies from 0.19 to 21 wt % with intergrowths between tabular grains of low-Ni and high-Ni chlorites. There is a strong negative correlation between NiO and MgO contents. Temperatures of chlorite formation calculated with the geothermometers of Cathelineau (1988) and Bourdelle et al. (2013) lie within a wide range. In case of low-Ni chlorites, the estimated temperatures vary from 125 to over 300 °Ñ with a maximum in the region of 175—300 °Ñ. In case of high-Ni chlorites, the same temperatures change from 50 to 250 °Ñ with a maximum in the region of 75—125 °Ñ, while the temperature decreases when the Ni content increases. These results give evidence that hydrothermal processes not only preceded the lateralization but were essential for Ni enrichment in weathering rinds.

Key words: Ni-bearing weathering rind, chlorite thermometry, Kolskiy massif, ophiolites, Northern Urals.