Zones of PGE–Chromite Mineralization in Relation to Crystallization of the Pados-Tundra Ultramafic Complex, Serpentinite Belt, Kola Peninsula, Russia

Autor: Andrei Y. Barkov, Andrey A. Nikiforov, Vladimir N. Korolyuk, Larisa P. Barkova, Robert F. Martin
Jazyk: angličtina
Rok vydání: 2021
Předmět:
Zdroj: Minerals
Volume 11
Issue 1
Minerals, Vol 11, Iss 68, p 68 (2021)
ISSN: 2075-163X
DOI: 10.3390/min11010068
Popis: The lopolithic Pados-Tundra layered complex, the largest member of the Serpentinite belt&ndash
Tulppio belt (SB&ndash
TB) megastructure in the Fennoscandian Shield, is characterized by (1) highly magnesian compositions of comagmatic dunite&ndash
harzburgite&ndash
orthopyroxenite, with primitive levels of high-field-strength elements
(2) maximum values of Mg# in olivine (Ol, 93.3) and chromian spinel (Chr, 57.0) in the Dunite block (DB), which exceed those in Ol (91.7) and Chr (42.5) in the sills at Chapesvara, and (3) the presence of major contact-style chromite&ndash
IPGE-enriched zones hosted by the DB. A single batch of primitive, Al-undepleted komatiitic magma crystallized normally as dunite close to the outer contact, then toward the center. A similar magma gave rise to Chapesvara and other suites of the SB&ndash
TB megastructure. Crystallization proceeded from the early Ol + Chr cumulates to the later Ol&ndash
Opx and Opx cumulates with accessory Chr in the Orthopyroxenite zone. The accumulation of Chr resulted from efficient cooling along boundaries of the Dunite block. The inferred front of crystallization advanced along a path traced by vectors of Ol and Chr compositions. Grains and aggregates of Chr were mainly deposited early after the massive crystallization of olivine. Chromium, Al, Zn and H2O, all incompatible in Ol, accumulated to produce podiform segregations or veins of chromitites. This occurred episodically along the moving front of crystallization. Crystallization occurred rapidly owing to heat loss at the contact and to a shallow level of emplacement. The Chr layers are not continuous but rather heterogeneously distributed pods or veins of Chr&ndash
Ol&ndash
clinochlore segregations. Isolated portions of melt enriched in H2O and ore constituents accumulated during crystallization of Ol. Levels of fO2 in the melt and, consequently, the content of ferric iron in Chr, increased progressively, as in other intrusions of the SB&ndash
TB megastructure. The komatiitic magma vesiculated intensely, which led to a progressive loss of H2 and buildup in fO2. In turn, this led to the appearance of anomalous Chr&ndash
Ilm parageneses. Diffuse rims of Chr grains, abundant in the DB, contain elevated levels of Fe3+ and enrichments in Ni and Mn. In contrast, Zn is preferentially partitioned into the core, leading to a decoupling of Zn from Mn, also known at Chapesvara. The sulfide species display a pronounced Ni-(Co) enrichment in assemblages of cobaltiferous pentlandite, millerite (and heazlewoodite at Khanlauta), deposited at &le
630 °
C. The oxidizing conditions have promoted the formation of sulfoselenide phases of Ru in the chromitites. The attainment of high degrees of oxidation during crystallization of a primitive parental komatiitic magma accounts for the key characteristics of Pados-Tundra and related suites of the SB&ndash
TB megastructure.
Databáze: OpenAIRE
Externí odkaz:
Nepřihlášeným uživatelům se plný text nezobrazuje