Kaolin weathering crusts on gabbroic rocks at Griva, Macedonia, Greece

Abstract

The hydrothermal and weathering alteration products of the gabbroic rocks at Griva, Macedonia, Greece, are examined using petrographic, mineralogical and chemical data. The kaolin crusts have a thickness of 1 to 8 m and extend over an area of about 5 km 2. Many gabbro-pegmatitic veins crosscut the semi-friable kaolins. The crusts have not been intensively washed and thus the primary minerals exist within the weathered profile in significant amounts. Essential constituents are plagioclase, actinolite, clinopyroxene, clinozoisite, dolomite, kaolinite, corrensite and pyrophyllite, with varying proportions in the different particle size fractions. The < 0.2 μm fraction consists entirely of kaolinite and corrensite. Kaolinite is poorly ordered and decreases in amount from 48% to 16% downwards to the partly weathered gabbroic rock. The same trend, but to a less extent, is noticed for corrensite. The pyrophyllite and dolomite content is almost constant with increasing depth. The Al content in the raw kaolins studied is very low. Calcium, Mg, total Fe and alkali contents are very high. The oxygen isotope values of the clayey fractions (2-0.2 and < 0.2 μm) vary between + 16.93 and + 22.07‰. Initially, hydrothermal fluids, derived from a gabbro-pegmatitic magma, were responsible for the formation of actinolite, chlorite, dolomite and pyrophyllite through the alteration of primary minerals. Hydrothermal activity was responsible for the partial formation of corrensite. Consequently, slightly acid water moving downwards was very important for the formation of kaolinite and the majority of the corrensite through the alteration of plagioclases and actinolites respectively. The low relief and the long-lasting tectonic stability of the area were essential for the development of a significant thickness of the kaolin crust. The high content of Ca, Mg, total Fe and alkali and the predominance of silt- and fine sand-size grains in the kaolins studied confirms a mild intensity of leaching. Features indicative of residual origin of the kaolins are their extensive surficial presence, the gradual transition from the fresh parent rock to the kaolin zone and their δ 18O values. According to the classification of Konta the kaolins studied are of the low grade type and probably only suitable for the ceramic industry.