An X-ray, EPMA, and oxygen isotope study of vermiculitized micas in the ultramafic rocks at Askos, Macedonia, Greece

Extensive metasomatic zones of vermiculite-, tremolite-, chlorite-, and talc-rich rocks have been developed at the contacts of serpentinized ultramafic bodies and surrounding two-mica gneisses in the Askos area, Macedonia, Greece. These zones are probably related to the intrusion of acid magmatic bodies in the area. X-ray and EPMA studies confirmed the formation of vermiculite through a layer-by-layer transformation of original micas. In decreasing abundance, the following mixed-layer and discrete phases were identified: biotite/trioctahedral vermiculite (hydrobiotite), biotite/smectite, trioctahedral chlorite/trioctahedral vermiculite (corrensite), vermiculite/smectite and discrete biotite, vermiculite, chlorite and talc. The 2–20 μm fraction of the vermiculitic samples consists mostly of biotite/vermiculite (>40%) with the biotite percentage dominating in the mixed phase. Lower abundances (20–40%) of biotite/smectite and chlorite/vermiculite occur in both finer fractions (2–20 and <2 μm) of all vermiculitic samples. Vermiculite/smectite is very abundant in the <2 μm fractions of most vermiculitic samples. The DTA curves of the samples analyzed are characteristic of Mg-vermiculites. The electron microprobe analyses show a gradual K leaching from precursor mica with increasing degree of weathering. Oxygen isotope results confirm this assumption. Initially, hydrothermal fluids derived from the neighboring granitic intrusions, were responsible for the micatization of the primary minerals of the ultramafic bodies. Hydrothermal activity was also responsible for the partial formation of corrensite. Consequently, water moving downwards was very important for the formation of the vermiculite and other clay mineral mixed-layer phases through the alteration of micas and chlorite. The low relief and the long-lasting tectonic stability of the area were essential for the development of a significant thickness of the vermiculite zones. These vermiculites have properties desirable in certain building, agriculture, and horticulture products.