Geochemical characteristics of Upper Jurassic - Lower Cretaceous platform carbonates in Hazine Magara, Gumushane (northeast Turkey): implications for dolomitization and recrystallization

摘要

The Upper Jurassic - Lower Cretaceous Berdiga Formation of the Eastern Pontides, Turkey, represents a carbonate platform succession composed of pervasively dolomitized intra-shelf to deep-shelf facies. In this area, polymetallic deposits occur as veins and lenses within the Berdiga Formation in close proximity to its upper contact with the overlying formation. Three different types of replacive dolomites occur in the formation: (i) microcrystalline dolomite, (ii) fabric-preserving dolomite, and (iii) fabric-destructive dolomite. Replacive dolomites are Ca rich and nonstoichiometric (Ca56-58Mg42-44) and are characterized by a pronounced negative shift in oxygen (-11.38 parts per thousand to -4.05 parts per thousand Vienna Pee Dee Belemnite (VPDB)), delta C-13 values of 0.69 parts per thousand to 3.13 parts per thousand VPDB, radiogenic Sr-87/Sr-86 ratios (0.70753 to 0.70884), and extremely high Fe (2727-21 053 ppm) and Mn (1548-27 726 ppm) contents. All dolomite samples have low Y/Ho ratios (23-40), and they also contain highly variable contents of rare earth elements (REE) (7-41). REE patterns of dolomites normalized to Post-Archean Australian shale show a distinct positive Eu anomaly (1.3-2.1) and slightly flattened Ce anomalies (0.8-1.1). Integration of petrographic and geochemical studies reveals the history of a variety of diagenetic processes highly affected by hydrothermal alteration, which include dolomitization, recrystallization, dissolution, silicification, and pyrite mineralization associated with the emplacement of the polymetallic mineralization.