Ultramafic-Rock-Hosted Vein Sepiolite Occurrences in the Ankara Ophiolitic Mélange, Central Anatolia, Turkey

Abstract

A 2 m thick brecciated zone containing magnesian minerals is present at the contact of tectonites and cumulates. Tectonites below this zone comprise serpentinized orthopyroxenite and serpentinite. An alteration zone with vein-type bedding comprises four different levels; from bottom to top they are: (1) green-brown serpentinite with dolomite (0.9 m), (2) light greenish-white dolomite with serpentine (0.5 m), (3) white dolomite with sepiolite (0.4 m), and (4) greenish-white dolomite with smectite-chlorite (0.2 m). The first level has a mineral association of serpentine + dolomite ± calcite ± aragonite, the second level consists of dolomite + serpentine ± calcite or dolomite + magnesite + serpentine, the third level comprises dolomite + sepiolite, and the fourth level is made up of dolomite + chlorite + smectite + serpentine. Dolomite, the main mineral of the alteration zone, occurs as coarse crystals (microsparitic-sparitic) in fractures and as small crystals (microsparitic-micritic) in the matrix, which includes serpentine and gabbro relics. Sepiolite developed at the edges and surfaces of dolomite and as fibrous forms in voids. Cumulate rocks above this zone comprise uralitic gabbros. The occurrences of magnesian minerals developed in three stages: the first stage was the serpentinization of olivine; the second stage was the dissolution of serpentine by groundwater and/or meteoric water containing carbon dioxide; and the last stage was the synthesis of neoformed minerals.

Study of Serpentinized Ultramafic Rocks of Bela Ophiolite, Balochistan, Pakistan

“…The samples of study area occupies field between average serpentine and talc ( Figure 7A), indicating that initially they were derived from ultramafic rocks. It is also supported by SiO 2 -(Al 2 O 3 +Fe 2 O 3 )-MgO triangular diagram ( Figure 7B), in which the samples of the study area occupied the field of serpentines, as described by Yalcin and Bozkaya (2004). Chemically, the polymorphous of serpentines can be distinguished by the covariation of (Fe 2+ + Mg 2+ )/(Fe 3+ +A1 3+ ) versus H 2 O.…”

“…3) and magnetite (Eq. 4) are also formed during the serpentinization of olivine (Yalcin and Bozkaya, 2004). The reaction mechanism in the study area favours the allochemical type of serpentinization, in which large positively charged ions such as Mg +2 , Ca +2 , Fe +2 and Si…”

An Overview of Authigenic Magnesian Clays

“…Variations in temperature and Mg-concentration of hydrothermal solutions can result in either the formation of talc, which is achieved at higher temperature and Mg-concentration, or sepiolite (achieved at lower temperature and Mg-concentration). Hydrothermal sepiolite has been found both in volcanic-hosted and serpentinite-hosted settings in Turkey; therein, the formation model for sepiolite includes hydrothermal transformation of magnesite, dolomite and/or serpentine [118,119].…”

How phyllosilicate mineral structure affects fault strength in Mg‐rich fault systems

“…They commonly form from reactions involving talc and Mg‐rich smectites (i.e., saponite) which are chemically very similar and coexist under the same activity values of Mg and Si [ Birsoy , ]. The serpentine group minerals are also associated with the formation of fibrous clay minerals, but they are chemically less similar to the minerals in this study due to their significantly lower Si/Mg chemical ratio [ Peters , ; Yalçin and Bozkaya , ]. Fibrous clay minerals commonly occur in fracture and fault zones [ Post and Crawford , ; Sánchez‐Roa et al ., ].…”